M El Doukkali
email: mohamed.eldoukkali@univ-lille.fr

M Bahlouri

S Paul

F Dumeignil

Bioglycerol-to-propylene routes: from fundamental catalysis to process design and marketing

Keywords: Bioglycerol-to-propylene catalysis, catalyst design, active sites requirement, mechanisms, reaction conditions, process configurations, thermodynamics-kinetics, market potentialities

Bioglycerol-to-propylene (GTP) routes are undergoing major developments in terms of both fundamental catalysis and process design on the way to becoming a connecting bridge between the biorefinery and polyolefin industries. This review starts introducing some GTP routes-related market potentialities and continues discussing significant mechanistic, kinetic, and engineering developments in GTP catalysis involving high-temperature, multi-step, tandem, and single-step hydrodeoxygenation strategies. It highlights the main advances made in the design of efficient catalysts and in the elucidation of their active sites, thereby shedding light on state-of-the art preparation, functionalization, and characterization methods. The GTP mechanisms are also assessed over versatile metallic, acid, and bifunctional catalysts' surfaces to discover which C-O bond is removed and which C=C bond is formed. GTP configurations are discussed as a function of the thermodynamic and operating conditions affecting catalysts' reactivity, selectivity, and stability. They are also compared using various qualitative and quantitative criteria such as process configuration, severity of operating conditions, energy consumption, sustainability assessment, and propylene production. We thus intend to provide a broad overview of GTP catalysis for inducing new opportunities in the biorefinery-toolefins field.

Introduction

Background

Light olefins, especially propylene and ethylene, are the most widely used feedstocks in the chemical industry. [1][2][START_REF] Corma | Crude Oil to Chemicals: Light Olefins from Crude Oil[END_REF] They are currently produced mainly from fossil heavy-oils, naphtha fractions, and propane/ethane using petroleum-based technologies (Scheme 1). [2][START_REF] Corma | Crude Oil to Chemicals: Light Olefins from Crude Oil[END_REF][4][START_REF] Akah | Maximizing Propylene Production via FCC Technology[END_REF] Although they are well-developed industrial processes used worldwide, the production of propylene usually experiences a continuous shortage in availability and price fluctuations, which are mainly due to excess of market demand, changes in industrial manufacturing methods and transportation rules, political conflict, and social restrictions (i.e., the ongoing war in Ukraine and COVID -19). 1,[START_REF] Company | Global Ethylene Market and Propylene Industry Research Analysis by TBRC[END_REF][7][8] This prompts a need for feedstock diversification to introduce some flexibility into the production of propylene and its logistical supply and distribution.

In turn, the traditional light olefins production industry is now considered, together with steel and glass manufacturing, as the most polluting and energy-intensive one. [START_REF] Ren | Olefins from Conventional and Heavy Feedstocks: Energy Use in Steam Cracking and Alternative Processes[END_REF][START_REF] Amghizar | Sustainable Innovations in Steam Cracking: CO2 Neutral Olefin Production[END_REF] The environmental issues stemming from excessive energy consumption and the use of fossil-based resources pose real threats to environmental preservation. [START_REF] Amghizar | Sustainable Innovations in Steam Cracking: CO2 Neutral Olefin Production[END_REF][START_REF] Zhao | Low-Carbon Roadmap of Chemical Production: A Case Study of Ethylene in China[END_REF][12] The conflation of these environmental-energy issues together with the instability of markets and the upcoming shortage of fossil-fuel reserves are driving an increasing number of countries to adopt more sustainable policies for consolidating socio-economic progress. For example, the European Union is actively following the guidelines of Directives 2018/2001/EC 13 and 2009/28/EC, [START_REF]European Parliament and of the Council of 11 December 2018 on the Promotion of the Use of Energy from Renewable Sources (Recast)[END_REF] designed to achieve a 20% global share in bio-based products and reduce CO2 emissions by 30%, on the way to achieve carbon neutrality by 2050. [START_REF]Stratégie à long terme à l'horizon[END_REF] In turn, the United Nations has recently adopted the new "2030 Agenda for Sustainable Development", [START_REF]The Sustainable Development Agenda[END_REF] aimed at promoting a circular economy with a low-carbon footprint. [START_REF] Zhao | Low-Carbon Roadmap of Chemical Production: A Case Study of Ethylene in China[END_REF][START_REF] Vollmer | Beyond Mechanical Recycling: Giving New Life to Plastic Waste[END_REF] Numerous academic and industrial collaboration programs are funding research projects for advancing the recovery and upgrading of carbon sources co-generated by the production chains in olefin industries and their byproducts. The aim is to comply with global targets for future CO2-neutral plastic manufacturing, thereby reducing its harmful impacts on health, environment, and security.

Research context

Within this aforementioned broad context, numerous specific studies have focused on the development of more eco-friendly technologies for producing sustainable C2-C4 olefins as an alternative to fossilbased ones or ultimately to introduce further flexibility in traditional technologies. There are such examples as the pyrolysis of industrial-urban wastes, [START_REF] Yuan | Superstructure Optimization of Integrated Fast Pyrolysis-Gasification for Production of Liquid Fuels and Propylene[END_REF][START_REF] Ramamurthy | Conversion of Waste Plastic to Propylene and Cumene[END_REF][START_REF] Babor | Bioethylene and Biopropylene Production from Waste Fat and Rapeseed Oil via Catalytic Hydrodeoxygenation and Hydrocracking Followed by Pyrolysis[END_REF] biomass fermentation to bioethanol followed by subsequent dehydration, dimerization and metathesis to propylene, [START_REF] Gotro | Bio-Based Polypropylene; Multiple Synthetic Routes Under Investigation[END_REF][START_REF] Reznichenko | Next Generation of Polyolefin Plastics: Improving Sustainability with Existing and Novel Feedstock Base[END_REF][START_REF] Machado | Bio-Based Propylene Production in a Sugarcane Biorefinery: A Techno-Economic Evaluation for Brazilian Conditions. Biofuels Bioprod[END_REF] biomass liquification to crude bio-oil followed by hydrotreatment and cracking to olefins, [START_REF] Babor | Bioethylene and Biopropylene Production from Waste Fat and Rapeseed Oil via Catalytic Hydrodeoxygenation and Hydrocracking Followed by Pyrolysis[END_REF][START_REF] Reznichenko | Next Generation of Polyolefin Plastics: Improving Sustainability with Existing and Novel Feedstock Base[END_REF][START_REF] Zhang | Liquefaction of Biomass and Upgrading of Bio-Oil: A Review[END_REF][START_REF] Gayubo | Olefin Production by Catalytic Transformation of Crude Bio-Oil in a Two-Step Process[END_REF] or biomass gasification to syngas followed by Fischer-Tropsch (FT) synthesis, and methanol-to-propylene (MTP) or methanolto-olefins (MTO). [START_REF] Reznichenko | Next Generation of Polyolefin Plastics: Improving Sustainability with Existing and Novel Feedstock Base[END_REF][START_REF] Tian | Methanol to Olefins (MTO): From Fundamentals to Commercialization[END_REF][START_REF] Chieregato | Olefins from Biomass. In Chemicals and Fuels from Bio-Based Building Blocks[END_REF][START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Teketel | Co-Conversion of Methanol and Light Alkenes over Acidic Zeolite Catalyst H-ZSM-22: Simulated Recycle of Non-Gasoline Range Products[END_REF] More recently, novel bio-based routes such as bioglycerol-to-propylene (GTP) catalysis (Scheme 1) are attracting a great deal of research interest focusing on the following: i) finding a mass application for the widely available crude bioglycerol, which is nonetheless largely considered a waste byproduct in the global biorefinery industry, [START_REF] Tan | Glycerol Production and Its Applications as a Raw Material: A Review[END_REF][START_REF] Hájek | Treatment of Glycerol Phase Formed by Biodiesel Production[END_REF][START_REF] Kumar | A Review on Variation in Crude Glycerol Composition, Bio-Valorization of Crude and Purified Glycerol as Carbon Source for Lipid Production[END_REF] and ii) introducing a new biofeedstock for the global polyolefin manufacturing industry, which is heavily exposed to the so-called "propylene gap"-excess of market demand over its real production. Thus, the topic of GTP catalysis dovetails perfectly with global efforts to mitigate climate change and the future depletion of fossil resources. For example, it may contribute to lowering biodiesel costs, diversifying propylene feedstock, and avoiding the reduced availability of ethylene (reference for olefin price). It may also open new perspectives for integrating GTP routes with the well-developed technology of renewable Power-to-Hydrogen, particularly, in countries where there is an excess production of solar energy, wind, electricity, etc. Indeed, GTP routes is undergoing advances in research in terms of both fundamental catalysis and process design [START_REF] Chieregato | Olefins from Biomass. In Chemicals and Fuels from Bio-Based Building Blocks[END_REF][START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF][START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF][START_REF] Gujar | A Review on Catalytic Conversion of Biodiesel Derivative Glycerol to Bio-Olefins[END_REF] by exploring different transformation strategies from both heterogeneous and homogeneous catalysis. GTP routes are being investigated through high-temperature processes, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Zakaria | Thermodynamic Analysis of Glycerol Conversion to Olefins[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF][START_REF] Gujar | A Review on Catalytic Conversion of Biodiesel Derivative Glycerol to Bio-Olefins[END_REF] multi-steps reactions, including triple-stage reactors, [START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] separate double reactors, [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF][START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] tandem or dual-bed catalysts, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] and single-step conversion strategies. 37,43,44,46- 48,52,54,55,57-63 These studies conclude that in the future GTP catalysis may become a sustainable bridging route between the biorefinery and polyolefin industries. Nonetheless, integrating GTP routes in a unified network, from biomass to sustainable propylene, raises intrinsic challenges, such as the nature of the most efficient GTP configurations, control of the interoperability of multi-steps and/or combined bed reactors, influenced by contact states between catalyst beds, operating conditions, reaction mechanisms, and so forth.

Scope of the review

We have recently published new insights to fill some of the gaps in GTP catalysis. [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] Indeed, despite the important advances made in this field over the past decade, no attempt has been made accordingly to comprehensively examine GTP catalysis in a single review. So far, and to the best of our knowledge, the following studies are the only ones that have in some way addressed GTP routes, albeit some time ago: three related chapters [START_REF] Chieregato | Olefins from Biomass. In Chemicals and Fuels from Bio-Based Building Blocks[END_REF][START_REF] Bakhtyari | 4 -Light Olefins/Bio-Gasoline Production from Biomass[END_REF][START_REF] Vasiliadou | Production of Biopropylene Using Biomass-Derived Sources[END_REF] and tow related mini-reviews. [START_REF] Zacharopoulou | Olefins from Biomass Intermediates: A Review[END_REF][START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF] These studies are very similar and focus more on a phenomenological description of certain sustainable processes and biofeedstocks (e.g., ethanol, butanol, bio-oils, and general biomass) for producing light olefins (C2-C4). Moreover, the information reported on specific GTP catalysis is very succinct, of a general nature, and incomplete. These studies do not cover the most recent research findings (2018-2023) or the patented information.

Our review project is therefore designed to provide a balanced assessment of the current state-of-theart by covering contributions from the earliest discoveries (5-10 years ago) through to the most recent GTP-related publications and patents. The objective is to provide the research community with an allcomprehensive analysis of the main developments in the field of GTP catalysis. The aim is to go beyond a general description and provide a timely, critical, and systematic discussion of the mechanistic, kinetic, and engineering advances in GTP routes and their market potential.

As illustrated in Scheme 2, this review is organized into interconnected sections to directly introduce the contexts and potential of producing sustainable propylene from crude bioglycerol. Thus, the main advances made in the design of efficient catalysts and in the elucidation of their active sites for GTP catalysis will be critically addressed, highlighting cutting-edge preparation, functionalization, and characterization methods. Our latest report on the single-step hydrodeoxygenation (HDO) of bioglycerol-to-propylene [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] are compared with the current state-of-the art in GTP routes for filling existing gaps and postulating future trends in the re-modulation of the target catalytic properties. The discussion focuses on how to overcome catalyst instability in aqueous media, which is still a huge challenge, and the need for cutting-edge, in situ, and operando techniques, allowing more precise elucidation of active sites and mechanisms. GTP catalysis is also assessed in terms of reaction pathways under versatile metallic, redox, acid, and bifunctional catalysts' surfaces to understand when the C-O bond is removed and when the C=Cone is formed, and why certain systems perform better than others. A critical comparison is made between all the GTP configurations as a function of the thermodynamic limits and operating conditions affecting catalyst reactivity, selectivity, and stability.

Other qualitative and quantitative indicators, such as severity, energy consumption, cost, viability, and/or sustainability aspects (CO2 emission) have also been considered for the GTP routes. By considering all these aspects, we aim at identifying existing research gaps in GTP catalysis and discuss novel approaches, allowing for further progresses to be made towards the development of efficient strategies for producing sustainable propylene, thereby presenting new openings in the biorefinery-toolefins field.

Setting the scene for GTP

The following two sub-sections provide a comprehensive analysis of the technological issues and market considerations behind the need for developing novel GTP routes. The related data were gathered from open literature sources and databases to provide an overview of current GTP routes technology.

The need for sustainable propylene production: The current situation

Propylene belongs to the group of alkenes with three carbon atoms, two of which are double bonded (C=C-C). The relative weakness of this latter bond explains its easy reactivity through different processes such as (co)-and polymerization, [START_REF] Buehler | Process for Polymerizing Propylene Using a Silica-Supported Catalyst[END_REF][START_REF] Fink | Propene Polymerization with Silica-Supported Metallocene/MAO Catalysts[END_REF][START_REF] Kulajanpeng | Multiscale Modelling of Multizone Gas Phase Propylene (Co)Polymerization Reactors-A Comprehensive Review[END_REF][START_REF] Nakano | Copolymerization of Propylene and Polar Monomers Using Pd/IzQO Catalysts[END_REF] oxidation, [START_REF] Ghosh | Selective Oxidation of Propylene to Propylene Oxide over Silver-Supported Tungsten Oxide Nanostructure with Molecular Oxygen[END_REF][START_REF] Khatib | Direct Oxidation of Propylene to Propylene Oxide with Molecular Oxygen: A Review[END_REF] halogenation, [START_REF] Martin | Process for Preparing Halogenated and Sulfohalogenated Copolymers of Lower Olefins[END_REF][START_REF] Lin | Halogen-Mediated Conversion of Hydrocarbons to Commodities[END_REF] epoxidation, [START_REF] Teržan | Propylene Epoxidation Using Molecular Oxygen over Copper-and Silver-Based Catalysts: A Review[END_REF][START_REF] Lu | Kinetics of Propylene Epoxidation Using H2 and O2 over a Gold/Mesoporous Titanosilicate Catalyst[END_REF] alkylation, [START_REF] Degnan | Alkylation of Aromatics with Ethylene and Propylene: Recent Developments in Commercial Processes[END_REF][START_REF] Prokešová | Catalytic Activity of Micro/Mesoporous Composites in Toluene Alkylation with Propylene[END_REF] oligomerization, and dimerization, [START_REF] Li | Propene Oligomerization to High-Quality Liquid Fuels over Ni/HZSM-5[END_REF][START_REF] Olivier-Bourbigou | Nickel Catalyzed Olefin Oligomerization and Dimerization[END_REF] etc. This useful reactivity and properties (Table 1 82,83 ) mean that propylene is one of the main platform molecules for several sectors such as the petrochemical, plastics, textile, and automotive industries. Fig. 1 A shows that propylene is used mainly for the production of polypropylene (64 %), acrylonitrile (10 %), oxo/iso alcohols (9 %), and sundry other products (17 %). [84][START_REF]Propylene Market Size, Share, Growth, Analysis, Price, Segmentation, & Industrial Trends By[END_REF][START_REF] Ltd | Global Propylene Market to Boost In Coming Years -Projected To Reach 132.1 Metric Tons In[END_REF] This is prompting a clear growth in global economy and demand regarding the effective production capacity and the supply chain (Fig. 1 B-C).

Despite the slightly negative impact the COVID-19 pandemic has had on the demand for olefins, 1,[START_REF] Company | Global Ethylene Market and Propylene Industry Research Analysis by TBRC[END_REF]87 particularly between 2020-2021, when the demand for propylene shrank by 1.8 % (Fig. 1B), the global market for propylene was worth around $97.6 billion in 2022, and it is expected to grow to $117.9 billion in 2026, at a compound annual rate of 4.8 %. [START_REF]Propylene Market Size, Share, Growth, Analysis, Price, Segmentation, & Industrial Trends By[END_REF]88 This high demand is being driven primarily by the increasing use of polypropylene and the extensive application of polymers and fibers in electronic devices and vehicles, which is being further stimulated by the growing economic development of emerging countries. [84][START_REF]Propylene Market Size, Share, Growth, Analysis, Price, Segmentation, & Industrial Trends By[END_REF][START_REF] Ltd | Global Propylene Market to Boost In Coming Years -Projected To Reach 132.1 Metric Tons In[END_REF]88,[START_REF]Propylene Market Capacity and Capital Expenditure (CapEx) Forecast by Region, Top Countries and Companies, Feedstock, Key Planned and Announced Projects[END_REF] Polypropylene is therefore projected to continue to be the dominant outlet for propylene, and its overall share among the global propylene market will increase even further through to 2029. For example, the global market volume for thermoplastic from polypropylene alone in 2022 amounted to approximately 76 million metric tons (MmT), and it is forecast to grow to almost 100 MmT by 2029, increasing at an annual rate of 3.6 %. 90 Consequently, global propylene demand has doubled over the past twenty years, 8,[START_REF]Propylene Market Size, Share, Growth, Analysis, Price, Segmentation, & Industrial Trends By[END_REF] and is expected to reach 158.1 MmT/year in 2027/28, 91 at an annual growth rate of more than 5.2-6.1 %. [START_REF]Propylene Market Size, Share, Growth, Analysis, Price, Segmentation, & Industrial Trends By[END_REF][START_REF] Ltd | Global Propylene Market to Boost In Coming Years -Projected To Reach 132.1 Metric Tons In[END_REF][START_REF]Global Propylene Market to Reach 158.4 Million Metric Tons by[END_REF] Global industrial output is expected to rise from 144. 16 MmT in 2021 to approximately 209.18 MmT by 2030, [START_REF]Propylene Market Capacity and Capital Expenditure (CapEx) Forecast by Region, Top Countries and Companies, Feedstock, Key Planned and Announced Projects[END_REF] mostly due to new plants that are scheduled to come online in Asia and the Middle East. [START_REF]Asia dominates global propylene capacity additions[END_REF] Nonetheless, the true production of propylene remained below 125 MmT in 2022. This was due to manufacturing problems (i.e., the shift towards shale gas fracking 87 ), among other supply and geopolitical issues, such as trade disputes between the US and China, the war in Ukraine, and Covid-19 uncertainties. 1,[START_REF] Company | Global Ethylene Market and Propylene Industry Research Analysis by TBRC[END_REF]7,[START_REF] Najari | Mixed Matrix Membranes for Hydrocarbons Separation and Recovery: A Critical Review[END_REF] Moreover, these problems are ongoing and overlap, whereby the demand for propylene will continue outpacing its production, which means that the current supply cannot keep up. This gap in propylene may be further aggravated by the emergence of fracking, which is likely to lead to a shortage in some geographical areas, specifically in North America and Europe. [START_REF] Company | Global Ethylene Market and Propylene Industry Research Analysis by TBRC[END_REF][START_REF] Najari | Mixed Matrix Membranes for Hydrocarbons Separation and Recovery: A Critical Review[END_REF][START_REF] Peplow | How Fracking Is Upending the Chemical Industry[END_REF] As a result of this uncertain scenario, the average price of propylene has increased over the past three years by more than 45 %, rising from 778 $/metric ton (mT) in 2020 to approximately 1133 $/mT in 2022. [START_REF]Global propylene prices[END_REF] Against this backdrop, the international propylene market keeps growing yearly at an average rate of 4-6.5 %, 84 which will put an additional burden on the supply of this key resource for the global economy.

A further aggravating problem is the fossil origin of the propylene available on international markets. [START_REF] Ren | Olefins from Conventional and Heavy Feedstocks: Energy Use in Steam Cracking and Alternative Processes[END_REF][START_REF] Amghizar | Sustainable Innovations in Steam Cracking: CO2 Neutral Olefin Production[END_REF][START_REF] Zhao | Low-Carbon Roadmap of Chemical Production: A Case Study of Ethylene in China[END_REF][12] At least 80-90 % of propylene is currently produced by environmentally harmful technologies such as Steam cracking (SC), Fluidized catalytic cracking (FCC), and Propane dehydrogenation (PDH), which involve intensive energy consumption and high ecological and health hazards. [2][START_REF] Corma | Crude Oil to Chemicals: Light Olefins from Crude Oil[END_REF][4][START_REF] Akah | Maximizing Propylene Production via FCC Technology[END_REF][START_REF] Ren | Olefins from Conventional and Heavy Feedstocks: Energy Use in Steam Cracking and Alternative Processes[END_REF][START_REF] Amghizar | Sustainable Innovations in Steam Cracking: CO2 Neutral Olefin Production[END_REF]12,[START_REF] Bakhtyari | 4 -Light Olefins/Bio-Gasoline Production from Biomass[END_REF] The largest source of propylene (~42 %) is the conventional SC of light oil fractions (naphtha, light paraffins, and middle distillate fuels) and the shale-gas used for producing ethylene, 2,[START_REF] Shafei | Naphtha Catalytic Cracking to Olefins over Zirconia-Titania Catalyst[END_REF][START_REF] Sadrameli | Thermal/Catalytic Cracking of Hydrocarbons for the Production of Olefins: A State-of-the-Art Review I: Thermal Cracking Review[END_REF] where propylene is a byproduct with a yield of 1.6-18%. [START_REF] Bakhtyari | 4 -Light Olefins/Bio-Gasoline Production from Biomass[END_REF][START_REF] Zacharopoulou | Olefins from Biomass Intermediates: A Review[END_REF] SC is a highly endothermic process, which is carried out without a catalyst under steam at high temperatures of ~800-1200 °C. The process energy consumption in a typical SC plant generally accounts for more than 70 % of production costs, [START_REF] Ren | Olefins from Conventional and Heavy Feedstocks: Energy Use in Steam Cracking and Alternative Processes[END_REF] while CO2 emissions may amount to 15 mT/ton of ethylene. [START_REF]Enhancing the Production of Light Olefins by Catalytic Cracking of FCC Naphtha over Mesoporous ZSM-5 Catalyst[END_REF] The second largest supply source (~39 %) is the FCC of heavy crude or vacuum gas oils to produce gasoline and light distillates, where propylene is co-produced with a yield < 15 %. [START_REF] Corma | Crude Oil to Chemicals: Light Olefins from Crude Oil[END_REF]4,[100][101][102][103][104][105][106] This process provides certain flexibility operating at 500-650 °C; however, the yield is generally small at low temperatures. Hence the reason it is mainly combined with PDH plants 87,107,108 to improve propylene yield up to 25 %. This combination involves huge facilities that consume enormous amounts of energy because the endothermic PDH reaction involves volume expansion, 87 and requires increasing the temperature to enhance the dehydrogenation rate. In turn, bespoke olefin technologies such as PDH, 107,108 MTO, [START_REF] Tian | Methanol to Olefins (MTO): From Fundamentals to Commercialization[END_REF]109,110 and MTP,111,112 make a smaller contribution due to their lower economic performance compared to the two first SC and FCC technologies. 108 Overall, all these processes involve i) significant energy consumption, ii) a high carbon footprint (problematic CO2 recovery), and iii) fast catalyst deactivation (e.g., coke formation, particle attrition, and poisoning). Some olefin conversion technologies (i.e., metathesis [113][114][115] ) provide more flexibility, but their efficiency requires further development and their viability also depends on the supply of raw feedstocks (fossil-based ethylene and butylene) , which limit their contribution to bridging the gap between propylene supply and demand.

Synthetic Summary:

As mentioned above, the factors destabilizing the traditional propylene market mainly involve logistical-management concerns, with the intensive energy consumption of production prompting environmental concerns. The future depletion of fossil resources and the rising costs of commodities due to political or social issues add further problems. This uncertain scenario renders it necessary to find other sustainable processes to overcome these problems.

Targeting crude bioglycerol as a renewable feedstock for propylene

The rapid technological and industrial development of the biorefinery has exponentially increased the global production of crude bioglycerol. 116,117 Processes such as transesterification, saponification, hydrolyzation, and even the fermentation of biomass can co-produce up to 10 wt%. of crude bioglycerol, depending to the target product (i.e., biodiesel, fatty acids, bioethanol, etc.). [START_REF] Tan | Glycerol Production and Its Applications as a Raw Material: A Review[END_REF][118][119][120] All these bio-based resources are directly related to the market for fossil feedstocks that set the initial prices and market tendencies. 121,122 Fig. 2 A illustrates the evolution of the actual figures (2010-2022) and predicted ones (2023-2030) for the world production of the main two biofuels; biodiesel and bioethanol, with which crude bioglycerol is usually co-produced. These data, taken from the recent OECD-FAO Agricultural Outlook, 123,124 indicate an enormous potential for growth. Thus, output of crude bioglycerol rose from 8.3 MmT in 2010 to more than 12 MmT in 2022. It is expected to grow at a rate of 4.5 wt% per year, with a projected value of 15 MmT by 2030, due mainly to the increasing use of second and third generation biomass (i.e., non-edible oils, [125][126][127] waste animal fats, 127,128 Jatropha, 129,130 and algae. 131,132 ). This global glut in crude bioglycerol, of which the treatment as a waste is currently more of a burden than an economic opportunity, impairs the value of the entire biofuel chain. Furthermore, this ever-growing trend 121,126 is also causing serious environmental problems. Despite glycerol's benign properties (Table 1 118 ,133,134 ), the management of crude glycerol is challenging because of its abundance and impurities (e.g., methanol, water, fatty acids, unreacted glyceride, salts, sulfurs,). [START_REF] Hájek | Treatment of Glycerol Phase Formed by Biodiesel Production[END_REF][START_REF] Kumar | A Review on Variation in Crude Glycerol Composition, Bio-Valorization of Crude and Purified Glycerol as Carbon Source for Lipid Production[END_REF]121,135 Many smaller biorefinery plants therefore assign it zero value, and simply dispose of it as a waste to avoid costly multiprocessing. 118,136,137 Medium-sized plants directly burn crude glycerol to produce energy. [START_REF] Kumar | A Review on Variation in Crude Glycerol Composition, Bio-Valorization of Crude and Purified Glycerol as Carbon Source for Lipid Production[END_REF][137][138][139][140] These practices are not environmentally friendly due to the formation of highly toxic substances (i.e., acrolein) and CO2 emissions that increase the process's carbon footprint. Besides, the net amount of energy produced is very modest (~18-25.3 MJ/kg 140,141 ) due to glycerol's high ignition temperature (Table 1) and because crude bioglycerol contains a lot of water. Early alternatives involved long-term storage of crude bioglycerol in abandoned mines, but various studies [141][142][143] have revealed that its impurities (i.e. methanol, wastewater with high salinity or alkalinity, etc.) can lead to an unacceptable level of groundwater contamination and the inhibition of microbial growth. In turn, the largest biorefineries seek to refine crude bioglycerol as a treated commodity to be sold for $0.6-$9 per kg, depending on the target sectors. [START_REF] Tan | Glycerol Production and Its Applications as a Raw Material: A Review[END_REF][START_REF] Kumar | A Review on Variation in Crude Glycerol Composition, Bio-Valorization of Crude and Purified Glycerol as Carbon Source for Lipid Production[END_REF]118,121,[144][145][146] In 2020, refined bioglycerol amounted to around 2.5 MmT, 147 which accounted for less than 23 % of the total crude glycerol generated in the world, 121 but the market of main uses (Fig. 2 B) is tending to saturation. In addition, the costly multi-steps of refining are undermining the commercial viability of these strategies. [START_REF] Hájek | Treatment of Glycerol Phase Formed by Biodiesel Production[END_REF][START_REF] Kumar | A Review on Variation in Crude Glycerol Composition, Bio-Valorization of Crude and Purified Glycerol as Carbon Source for Lipid Production[END_REF] The price of unrefined bioglycerol does not usually exceed ~$0.11-0.33 per kg, 118 depending on the content of glycerol and impurities.

The combination of low prices and market saturation means that around 65-77% of all crude bioglycerol is currently treated as waste. 121,148 This situation is expected to remain unchanged at least until 2050, given the high number of new biorefinery facilities that will be opening over the coming years, particularly in East Asia and South America. 126,146 The availability of crude bioglycerol will therefore continue to grow annually, driven by the use of non-standard bioresources for the production of second and third generation biofuels. This boom may help to repeat the success of first-generation biofuels, so bioglycerol co-generation will continue to grow annually. 50 MmT/year of biodiesel are expected to be produced from these renewable sources, meaning the availability of an even larger amount of bioglycerol in the future. 126,146 The combination of abundant crude bioglycerol with problematic mis-management issues is driving research to develop new upgrading strategies. Fig. 3 reveals the high number of studies published each year. These studies have used thermochemical and catalytic strategies designed to re-demodulate the high oxophilicity of glycerol. 116,117,121,136,146 Scheme 3 shows that reactions such as esterification, 149- 151 reforming, 152,153 oxidation, 154-156 dehydration, 157-159 hydrogenolysis, 160,161 and hydrodeoxygenation, [START_REF] Zacharopoulou | Olefins from Biomass Intermediates: A Review[END_REF][161][162][163] have been explored in an attempt to convert bioglycerol into useful intermediates (i.e., mono-alcohols, hydrogen, syn-gas, acrolein, ally-alcohol, diols, and light olefins, among others). The initial idea was to convert glycerol into a platform molecule as a sustainable-based equivalent of propylene for petrochemistry.

These approaches involve mainly the activation and reactivity of C-O, O-H, C-C, and/or C-H bonds, depending on the target product. For example, breaking C-H and C-C bonds necessarily leads to CO2 emissions. 164,165 Even though that this bio-based CO2 is less problematic than that the fossil-based ones, however, its capture, storage and transport are still difficult to be immediately harness in form of useful end products.

In turn, oxidation, esterification, and dehydration require a high selectivity towards the desired liquid products in order to avoid the costly multi-distillations of liquid phases. However, only a few of these processes may be economically viable (i.e., glycerol to propylene glycerol, 116,166 glycerol to acrolein 116,157 ) if the costly separation process is solved. Furthermore, the viability of these early technologies will also depend on the market prices of refined bioglycerol directly linked to the policies adopted by biofuel producing countries. 116,117 This list of issues is clearly not exhaustive, but only indicative of the need for continuing the search for efficient upgrading strategies to directly convert crude bioglycerol into an industrial-scale intermediate. The target process should operate under moderate temperatures and pressure conditions without CO2 emissions to maintain a low carbon footprint. These factors are important for reducing processing costs, increasing income, and minimizing the environmental impact.

Synthetic conclusion:

Design an efficient GTP technology will improve not only biorefinery viability and the supply for short-chain olefins, but will also mitigate the sustainability issues affecting the global environment, thereby contributing to a greener economic future. For example, simple calculus indicates that the HDO of all surplus crude bioglycerol, which currently amounts to more than 11 MmT/year (Fig. 2 A), to sustainable propylene may theoretically replace at least ~10.5 wt% of preexisting fossil-based propylene or provide an additional volume to the international market, thereby, contributing to mitigate "propylene gap". In addition, the current price of propylene is ~seven times higher than crude bioglycerol, [START_REF]Global propylene prices[END_REF]118 which could boost the competitiveness of the biorefinery sector.

For example, the major drawback with biodiesel is still the high cost of its industrial production, which ranges from $0.2/kg to $0.4/kg, without including logistical and managements issues. 167,168 Moreover, several biorefinery plants in certain European countries (i.e., France, Spain, and Italy) are not operating to their full capacity due to the low economic competitiveness of biodiesel compared to fossil fuels and foreign imports. 169

The state-of-the-art of GTP catalysis

Latest developments in active sites identification and catalyst design

There is still no commercialized GTP catalyst that can convert crude bioglycerol under hydrothermal conditions to produce a high propylene yield without excessive H2 consumption. Thus, cutting-edge preparation and functionalization strategies are being explored to improve the catalyst properties for the GTP reaction in both a homo-and heterogeneous catalysis medium. [START_REF] Chieregato | Olefins from Biomass. In Chemicals and Fuels from Bio-Based Building Blocks[END_REF][START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF][START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] These efforts are focusing on controlling the nature and population of active sites for efficiently eliminating C-O and controlling C-C hydrogenation.

In a homogeneous medium, the catalyst is used as a liquid or gas in full contact with the reactant, [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF][START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] whereas in a heterogeneous catalysis, the approach involves dispersing an actives species over a solid support to improve its functional characteristics. [START_REF] Chieregato | Olefins from Biomass. In Chemicals and Fuels from Bio-Based Building Blocks[END_REF][START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Zacharopoulou | Olefins from Biomass Intermediates: A Review[END_REF][START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] Impregnation is the most widely used method for developing a GTP catalyst because of its ease of use; however, it may lead to low metal dispersion and easy leaching in an aqueous medium, particularly when with a high loading of metals. [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] Other synthesis strategies, such as pH-assisted, 63 slurry 63 or sequential impregnation, [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] modified impregnations, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] co-precipitation , [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] homogeneous dissolution, and/or post-recrystallization, [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF][START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] melt-infiltration, [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] physical vapor deposition (PVD), [START_REF] Wan | Controlling Reaction Pathways of Selective C-O Bond Cleavage of Glycerol[END_REF][START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] physical mixtures, [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF] or include combined methods. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] The pre-activation treatment of the catalyst, which depends on the active sites required for the target GTP reaction, involves manly in situ or ex situ thermo-treatments under a controlled atmosphere, such as calcination, [START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF] reduction, [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Xiao | Conversion of Glycerol to Hydrocarbon Fuels via Bifunctional Catalysts[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] carburization, [START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] sulfidation, [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF] or include the co-feed of acids or reductive agents or hydrogen donor molecules [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] With these strategies, the type and population of target active centers are subject to mainly physicochemical modulations by controlling the interactions between the centers and the environment, chemical states, site-regeneration, geometrical exposures, and dispersion, for example.

The species studied for GTP reactions are noble metals (e.g., Ir, [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Ru, [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Pt, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Xiao | Conversion of Glycerol to Hydrocarbon Fuels via Bifunctional Catalysts[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Pd, [START_REF] Xiao | Conversion of Glycerol to Hydrocarbon Fuels via Bifunctional Catalysts[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Rh, [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] and Re, [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] ), functionalized systems based on non-noble transition metals (e.g., MoOx, [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] β-Mo2C, [START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] η-MoC, [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] NiMoSx, [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF] WOx, [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] Ni, [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Cr, [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Fe, [START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Zn, [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] V, [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] Nb, [START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Cu, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] ), non-metals (e.g., Li,[START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF] Ca, [START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF] Mg, 38 HBO3, 64 I, 65 P, [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] Br, [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] and Carbon, [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] ), oxides such as Al2O3, [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] SiO2, [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] SiO2-Al2O3, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] TiO2, [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] ZrO2, [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] and zeolites (e.g., Beta, Y, USY, Mordenite, and ZSM-5). [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] The physiochemistry of these systems (i.e., element loading, geometry, location, chemical states, and acidity) have been systemically characterized by numerous techniques (see Table 2 and abbreviations list in the supplementary information file).

Density-functional theory (DFT) [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Wan | Controlling Reaction Pathways of Selective C-O Bond Cleavage of Glycerol[END_REF] and microkinetic modeling [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF] have also been combined to support the design of the target catalysts. Table 2 provides a summary of the preparation and activation conditions for the catalysts studied and their most relevant physiochemistry for the GTP reaction.

Acid systems catalyzing GTP at high temperature

For the acid surface catalysis, Corma et al. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF] have been the first to use acid aluminosilicates for converting glycerol to light olefins at high temperatures. Among these systems, HZSM-5 records a greater affinity toward C2-3 olefin formation and less tendency to coke (< 20 %), which were attributed to its significant Brønsted acid sites (Si-OH-Al) and uniform micro-porosity ~0.5 nm. The other systems tend to yield more coke (30-50 %), causing fast deactivation; USY ∼ FCC > γ-Al2O3 > ECat.

They might also undergo dealumination, mainly under hot steam. 170,171 To attenuate deactivation, they have used a moving-bed FCC reactor for the in situ regeneration of coked zeolite or for replacing dealuminated catalyst. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF] In addition, physically mixing ZSM-5 with FCC systems improve C2-3 olefin formation from glycerol cracking because of ZSM-5's good properties and resistance to coke.

Blass et al. [START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF] have studied the effect of pore shape on GTP selectivity over combined acid H-zeolites with hydrogenating catalysts (i.e., Pt or Pd-based Al2O3). Increasing the pore size from 10-MR in HZSM-5 to 12-MR in HBEA (Fig. 4 A) shifts the formation of ethylene and propylene to heavy C4+ olefins, as larger pores seem to accommodate compounds with longer carbon-chains.

Zakaria [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF] has tailored the HZSM-5 surface by impregnating ~30 wt% of different elements such as Al, Ca, Cr, Mg, Cu, Li, Ni on HZSM-5. A direct relationship has not been found between their texture and olefin formation. However, the incorporation of metal seems to balance ZSM-5 acidity, thereby improving the yield of C2-4 olefins. The Cu/ZSM-5 catalyst, which has Cu 2+ /Cu 0 species alongside a higher amount of moderate and strong acid sites, leads mainly to ethylene, while the Cr/ZSM-5 catalyst produces the highest TOFpropylene (Fig. 4 C). More recently, Lima et al. [START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF] have adopted a similar approach by impregnating ~2.5 wt% of Mo, Fe or Nb on HZSM-5. Despite the low loading of metals, there is a major change in the total acidity of ZSM-5 and its distribution. The Fe/ZSM-5 catalyst, which exhibits the highest increase in both the population and strength of medium and strong acid sites, leads to propylene and ethylene (selectivity ~33 % and ~19 %, respectively). However, significant surface acidity favors competitive C-C breaking events, which decrease GTP yield after a few hours. In turn, Wu et al. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] have controlled the acidity of aluminosilicate materials by varying their Si/Al ratio, while their bi-functionalities are improved by impregnating transition metals.

Pt/ZSM-5 boosts the initial hydrogenolysis of glycerol to propanol. Balancing the ratio of weak to strong acid sites in HZSM-5 by setting Si/Al at 127 significantly improves the subsequent dehydration of propanol to propylene selectivity (~76 %). [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] This rationalization minimizes coke formation, extending the lifetime of the catalyst to ∼500 h, which can also be associated with the H2-rich medium used instead of an N2 atmosphere. These findings are consistent with those reported by Xiao et al. [START_REF] Xiao | Conversion of Glycerol to Hydrocarbon Fuels via Bifunctional Catalysts[END_REF] and Murata et al. [START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF] whereby noble metal sites entail an additional hydrogenation bifunction for HZSM-5, increasing its pre-existing acid dehydration sites for efficient glycerol to olefins and avoids aromatization trends. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Xiao | Conversion of Glycerol to Hydrocarbon Fuels via Bifunctional Catalysts[END_REF] 1.6.2. Metallic and/or multifunctional systems catalyzing GTP under typical HDO conditions In 2009, Taher et al. [START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF] have been the first in reporting the formation of propylene during catalytic deoxygenation of glycerol under typical HDO conditions using Ru complexes based catalyst, and since 2011, an increasing number of noble and non-noble metals and oxides have been studied as catalysts for the GTP reaction. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] These systems include combined bifunctional Ir/ZrO2 53 or MoO3-Ni2P/Al2O3 49 with acid HZSM-5 in tandem mode, with a good synergy between H2-dissociation, hydrogenolysis, and dehydration events (Fig. 5 A). This cooperative effect raises propylene selectivity to 88 %, which is comparable to that obtained under simulated crude glycerol. However, H2-transfer from methanol to glycerol leads to excessive coke deposition, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] as corroborated by comparing the spent HZSM-5 sample in the GTP reaction with the catalysts usually used in industrial MTP process, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Teketel | Co-Conversion of Methanol and Light Alkenes over Acidic Zeolite Catalyst H-ZSM-22: Simulated Recycle of Non-Gasoline Range Products[END_REF]111,172 Sun et al. [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF] have also adopted a similar dual-bed approach inspired by an early tandem process [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] to combine metallic species with acid aluminosilicates catalysts. NH3-TPD shows (Fig 5 B) that the population of acid sites depends heavily on the temperature of the calcination process and the content of impregnated species on the support, while H2-treatment at 250 ºC (Fig 5 C) suffices to fully reduce the Cu precursor to Cu 0 . This synchronization of metals with acid species greatly boosts the hydrogenolysis and dehydration events in glycerol and intermediates, yielding 84.8 % of propylene. However, such combination of complex catalyst mixtures with less defined active sites hinders the in situ regeneration of the spent catalysts because acid sites require mainly specific oxidative agents, whereas metallic or redox sites involve a reducer atmosphere.

Among the first patents, Hulteberg et al. [START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] have used impregnation to re-modulate the properties of various metals and non-metallic systems, from which a combination of three, WO3 (10 wt%)/ZrO2, Cu (10 wt%)/ZnO2 and (10 wt%)/ZrO2 catalysts yield 46 % propylene. Fadigas et al. [START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] have combined modified-pH and slurry impregnations to study a vast array of metals from group VIII-B (M1= Fe, Ni, etc.) and group VI-B (M2= Mo, W, Cr, etc.). Specific mixed phases (with atomic ratios M1/(M1+M2) = 0.2-0.4) have been reported as active sites for propylene and propane formation (with a selectivity up to ~90 %). [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF] A similar synthesis approach has been explored by Shi et al. [START_REF] Shi | Converting Glycerol to Propylene[END_REF] to patent hydrotreating catalysts based on bulk and supported mono-, bi-and tri-metals from groups VI-B and VIII-B or from the first three rows of the periodic table, including nobles metals and single sulfides. The main GTP catalysts are bimetallic Co-Mo, Ni-Mo and Ni-W impregnated on aluminosilicates and sulfided in situ. In certain cases, bulk systems are recommended, as the population of active sites is much higher with the absence of metals/support interactions, which endows them with an intrinsic HDO activity.

Deshpande et al. [START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] have patented various halogen elements (i.e., I2, HI, HIO3, Lil, and their combinations ), as potential homogeneous catalysts for the complete dehydroxylation of crude glycerol in two steps. Both catalytic systems require external H2 pressure, acetic acid, and additional solvents to dissolve the halogen formed as intermediate in the catalytic cycle, and then act as the main leaving group to start partial dehydroxylation in the first step and facilitate the final dehydroxylation in the second one. All these patents [START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF][START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] report catalyst pre-activation under a reducer atmosphere, sometimes including in situ synthesis; however, the physiochemistry of the catalysts has still not been examined.

More recently, Janssens et al. [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] have explored a homogeneous approach to boost one-step GTP catalysis. They have combined in situ dissolution synthesis and final recrystallization to manipulate RuBr3 (as catalyst precursor), HBr-doped ionic-liquid type Bu4PBr -tetrabutylphosphonium bromide (as a dehydrating co-catalyst) and formalin (as an additive) for enhancing the formation of Ru-CO bonds that act as hydrogenation sites. Similar Ru-C active sites have previously been reported [START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF] for similar homogeneous acidic conditions. They suggest initial glycerol dehydration over a Brønsted cosite, followed by further hydrogenation either via i) the insertion of carbonyl in the Ru-H bond and subsequent release of the substrate formed through the protonation of the resulting Ru-O or Ru-C bonds, or ii) direct transfer of a hydride ligand from Ru to a H + -substrate. Janssens et al. [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] have pretreated the RuBr3 salt with 1 bar of CO pressure, instead of formalin, to increase the population of Ru-CO bonds, thereby reaching a 82 % propylene yield from crude glycerol (Fig. S1 A). Comparative FTIR spectra of fresh and recycled Ru-based complex catalyst (Fig. S1 B) reveal good catalyst recyclability up to eight cycles without any significant loss in propylene selectivity. However, working with expensive noble Ru metal homogeneous catalysis poses a real challenge for the use of GTP beyond lab tests.

Di Mondo et al. [START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF] have found that the corroded surface of a 316 Stainless Steel -316SS reactor (with HOTf acid in a reducer medium) catalyzes GTP in one-step. SEM/EDX, XRF, and ICP-MS analyses reveal the formation of a thin layer (< 10 nm) of highly active mixed Cr 3+/5+ states, which with Fe or Ni metals contribute to HDO activity through H2-spillover phenomena. The GTP activity is confirmed by attacking different commercial 316SS powders with an increasing concentration of an HOTf acid/glycerol mixture, which is comparable to those of the 316SS reactor itself. The reported insights may open new perspectives for designing innovative metallic reactors that can play the role of both low-cost catalyst and host reactor. For example, studying a 316SS reactor with a spongy aspect may allow overcoming its low surface per volume ratio, and so improve the geometrical exposure to the high GTP rate.

Zacharopoulou et al. [START_REF] Zacharopoulou | Olefins from Biomass Intermediates: A Review[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] have merged impregnation and co-precipitation methods to support high loading of Mo/Fe elements over carbons, albeit with low metal dispersion, which facilitates their sintering and leaching during the liquid HDO of glycerol. [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] Both effects seem to have been induced by the low interaction of these metals with a practically neutral carbon surface. They suggest that the partial substitution of Mo 4+ ions by Fe in the MoO2-based catalysts reduces Mo 6+ to form Mo 5+ and Mo 4+ species with oxygen vacancies. [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] The arrangement of these sites within redox cycles seems to contribute to the complete removal of oxygen from glycerol and the creation of the C=C bond in propylene, according to the reverse Mars-van Krevelen cycle (Fig. S1 C). [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] Mota et al. [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF] pressure and 300 ºC This catalyst is highly active and more selective than those used by Zacharopoulou et al. [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] that require a high H2 pressure ~80 bar to reach ∼76 % propylene selectivity and ∼88 % glycerol conversion. Mota et al. [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF] attribute the best GTP performance to the formation of MoC phases and partially reduced Mo 4+ species with an oxygen vacancy. A recent DFT calculation (Fig. S1 D) [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF] indicates that the formation of a Mo oxygen vacancy is thermodynamically favored under an H2 atmosphere. However, the exact nature of these Mo δ+ species, hypothesized by both groups based on ex situ characterization, [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] remains unclear.

More recently, El Doukkali et al. [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] have combined hydrothermal, melt-infiltration, in situ topotacticand non-topotactic reductive carburization (Fig. 6 A) to fine-tuning the HDO properties and protect highly reactive sub-2 nm Mo particles in hydrophobic-inert silica against leaching and sintering, which prevail under an H2O-rich reaction medium. [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF] A combination of TEM-HAADF imaging (Fig. 6 B), in situ XRD, and in situ XPS analyses with kinetic GTP test data has revealed the existence of MoOx, β-Mo2C, and η-MoC phases with partially reduced Mo 5+ , Mo 3+ , and Mo 2+ -C species that play a critical role in HDO events both in glycerol and in intermediates without causing significant C-C breaking.

The single-step GTP reaction seems to effectively take place over Mo oxides via successive redox cycles of Mo 5+ /Mo 6+ and/or Mo 3+ /Mo 4+ states, while over Mo carbides it occurs mostly on a carbidic and oxycarbide surface. The re-oxidation of partially reduced Mo δ+ states either to inactive Mo 6+ or Mo 4+ species hinders deoxygenation-hydrogenation events, which is attributed to the surface accumulation of oxygen that is related to the co-adsorption of certain oxygenated compounds (i.e., CxHyOz). This finding complements those of Lee et al. 173 and Schaidle et al.,174 attributing Mo deactivation instead to the deposition of graphitic carbon during the HDO of anisole or acetic acid. Anderson et al.,[START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF] in turn, control the GTP activity-stability of NiMo/Al2O3 by in situ sulfidation. The partially reduced catalyst records a similar initial activity in C-O breaking and C-C hydrogenation to that of the sulfided one, but the latter provides a significant improvement in terms of stability under in situ H2S co-feeding (Fig. S2 A-B). This improved stability, which is attributed to the persistence of a high number of MoSx edge sites acting as active sites, is surprisingly accompanied by significant C-C cleavage activity. As revealed by the TPO and N2 adsorption of spent NiMo/Al2O3 catalysts, deactivation is attributed to NiMo sintering and coke formation, resulting in a loss of surface area and fewer active sites.

Other authors [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Wan | Controlling Reaction Pathways of Selective C-O Bond Cleavage of Glycerol[END_REF][START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] use DFT calculation, sometimes coupled with kinetic micro-modeling or TPD-MS and HREELS analyses, to provide a better understanding of the role of the Mo surface in GTP activity. Rellán-Piñeiro et al. [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF] conclude that C-O scission and C-C hydrogenation take place effectively through the formation of oxygen defects. This process starts with initial H2 adsorption and dissociation of the H atom that reacts with the oxygen surface to form water and an oxygen vacancy (Fig. S1 D). Subsequently, glycerol (or oxygenated intermediates) is directly adsorbed on the Mo site with an undercoordinated O-environment, whereupon a selective cleavage of the C-O bond produces partial or fully deoxygenated compounds. The snatched oxygen atom remains bound to the Mo, filling the vacancy. The latter is immediately regenerated by H2 reduction with low energy barriers, whereby H2 pressure or an H2-rich medium is required for completing the HDO cycle. In turn, Wan et al. [START_REF] Wan | Controlling Reaction Pathways of Selective C-O Bond Cleavage of Glycerol[END_REF][START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] use a glycerol-statured H2 flow to compare the HDO reactivity-stability of Mo2C, Cu/Mo2C, Fe/Mo2C, and Pt/Mo2C surfaces by evaluating the number of C-O, C-C, and C-H bonds broken during the reaction (Fig. S3 A-D). Fe/Mo2C and Mo2C surfaces are highly active in the complete GTP cycle, while Pt/Mo2C records significant activity in C-C and C-H bonds breaking to syngas products.

Cu/Mo2C has the greater hydrogenolysis ability of only one C-O bond of glycerol to hydroxyacetone.

Additional HREELS data [START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] indicate that specific sites from Mo2C are responsible for the activity in C-O scission. This is explained by the fact that the high oxophilic Mo2C surface, which has previously been revealed by DFT calculation, [START_REF] Wan | Controlling Reaction Pathways of Selective C-O Bond Cleavage of Glycerol[END_REF] interacts more strongly with the oxygen of glycerol than over Fe/Mo2C and Cu/Mo2C. The coverage of Cu species, and to a lesser extent Fe, seems to slightly reduce the oxophilicity of Mo2C. Moreover, Fe coverage also seems to improve the stability of the carbidic Mo2C surface during the GTP reaction. [START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] Synthetic Summary: Different catalytic systems may efficiently accelerate the kinetics of the GTP reaction. They require specific preparation, operating, and activation treatments to condition the chemical environment of active sites for the reactants, depending on the target reaction conditions and process configuration. Acid catalysts such as zeolites seem suitable for the high-temperature dehydration/cracking of glycerol to ethylene and to a lesser extent propylene; however, they still experience severe deactivation. Adjusting the zeolite acidity type and starching, as well as pore reopening by post-treatments (e.g., desilication and delamination) or nanoscale synthesis (e.g., soft/hard templating) may also improve coke diffusion. Synergic catalysis involves combined metal and acid catalytic systems, with the tandem mode providing promising GTP rates. However, further studies should be conducted for controlling the kinetics of the GTP reaction over tandem bed zones to extend the useful life of the multi-functional catalysts used. The best homogeneous catalyst for the one-step GTP route involves Ru-complexes, providing an extremely high selectivity towards propylene and good stability. [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] The best heterogeneous catalyst, in turn, is based on Mo species, featuring unprecedented rates of C-O removal and C-C hydrogenation without excessive CO2 emissions. [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF] These systems are still affected by the excessive surface accumulation of oxygen and oxygenates, which seems to be related to unbalanced deoxygenation-hydrogenation rates. Controlling in situ the coverage of oxygen on the overlayer of Mo-based catalyst during the HDO reaction or exploring structural single-atom catalysis (e.g., Mo 5+ or Mo 3+ ) into specific 3D structure; selectively orienting Mo nanoparticles or doping few amounts of highly hydrogenating metals (i.e. Ir, Pd, Ru, Ni, V, Co, etc.) may improve catalytic stability by balancing H2 activation vs. C-O elimination. The selective in situ poisoning of Mo active sites during the reaction, using specific oxygenates as probe molecules, could provide useful insights into the evolution of the most active species and how to better control the oxygen rate over the Mo δ+ surface. While as the in situ tracking of the real active sites seems difficult under practical HDO conditions (e.g., high pressure and a complex reactant mixture, etc.), the operando EPR, NAP-XPS and 3D-NMR analyses should be preferred to ex situ explorations. This is suitable for further studies on the most efficient Mo-based catalysts because of i) the high oxophilicity of their Mo δ+ species (e.g., Mo-Ox, Mo-Cx, and Mo-Sx), and ii) their rich chemistry (i.e., multi-oxidation states) that can change radically upon contact with an air-containing environment. These cutting-edge operando studies can help to accurately identify catalytic active sites, while further theoretical calculations (ab initio, DFT, etc.) may support in-depth research on the reaction mechanism and predict the most suitable architecture for robust GTP catalysts.

Reaction mechanisms for GTP routes

There are currently major discussions on the pathways through which glycerol is converted to propylene, concluding that GTP routes encompass sequential/parallel competitive vs. cooperative reactions, including H2 dissociation-formation, dehydration, hydrogenolysis, deoxygenation, and hydrogenation, among others. The kinetic arrangement of these reaction intermediates to form a complete GTP route differs from one catalyst to another, depending on the reaction conditions and process configuration.

Acid-catalyzed mechanisms

For high-temperatures (> 400 ºC), Corma et al. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF] have been the first ones to build a complex mechanism for converting glycerol to olefins and aromatics over acid surfaces without an external H2 supply (Scheme 4). The integral pathways towards gas products (i.e., olefins, COx and paraffins) involve the catalytic dehydration of glycerol, cracking, and hydrogen producing-consuming reactions on acid sites at temperatures mainly higher than 450 ºC. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF][START_REF] Gujar | A Review on Catalytic Conversion of Biodiesel Derivative Glycerol to Bio-Olefins[END_REF] H2 is formed mainly through C-C and C-H cleavages, water-gas shift (WGS), decarbonylation, and dehydrogenation reactions, and is then transferred to dehydrated intermediate molecules by hydrogenation and hydrogen-transfer events, thereby producing light olefins; mainly ethylene and to a lesser extent propylene. [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF][START_REF] Gujar | A Review on Catalytic Conversion of Biodiesel Derivative Glycerol to Bio-Olefins[END_REF] The latter are involved mainly in further aldol-condensation and Diels-Alder rearrangements to form heavier paraffins or even aromatics, which negatively affects the formation of propylene as target product. An optimal balance between all these reactions is therefore required to improve propylene yield. Zakaria et al. [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF] have simplified this mechanism by hypothesizing that an acidic surface leads to initial glycerol dehydration to 3-hydroxypropanal, which in turn follows two major routes, as illustrated in Scheme S1. They have discarded glycerol dehydration to hydroxyacetone, considering that the formation of 3-hydroxypropionaldehyde is a more feasible option because the latter has a major tendency to break C-C bonds for the further formation of acetaldehyde and formaldehyde, and vinyl alcohol and formaldehyde, which have been experimentally detected during the reaction. [START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF] Acetaldehyde therefore undergoes carbonyl bond dissociation (ethylidene oxo-species) to form propylene. At the same time, protonated acetaldehyde combines to undergo reductive C-C coupling to form butylene, while vinyl alcohol releases oxygen to ethylene. A high amount of coke is observed resulting from C-C bond cleavages, which affect the C2-4 olefins formation. Lima et al. [START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF] have experimentally evaluated products distribution as a function of reaction temperature for glycerol conversion over acid Fe/ HZSM-5 to coincide on some sentences with Zakaria et al. [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF] and Corma et al. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF] The proposed mechanism (Scheme S2) has only briefly been discussed, but they consider that the -OH group of glycerol is initially dehydrated on H-acid sites to form hydroxyacetone and 3hydroxypropanal. The first compound is then hydrogenated to acetone via H2 production and transfer, whereas the second one is further dehydrated to acrolein or cracked to acetaldehyde and formaldehyde.

Subsequently, acetone and acetaldehyde undergo decarbonylation and dehydration, respectively, to produce CO, H2O, and ethylene. The latter is in turn oligomerized to propylene, but its selectivity (17-33%) is remarkably inferior to that of ethylene (up to ~ 46%). Besides, there is further surface hydrogen-transfer from acrolein to ally-alcohol and then to propanol, while the hydrogenation of formaldehyde leads to methanol that is dehydrated to methane. The simultaneous decarbonylation of acrolein, which is a highly unstable intermediate, [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF] is fully possible over an acid surface, but there is no explanation as to why it has been discarded. In addition, propanol can be easily dehydrated to propylene over an acid surface such as HZSM-5, as evidenced by Croma et al. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF] and Zakaria et al. [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF] In contrast to these two studies, Lima et al. [START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF] propose an additional ethylene-to-propylene pathway via simultaneous oligomerization and cracking steps (Scheme S2), which have not been kinetically proven.

Mechanisms involving metallic and/or multi-functional sites

For a moderate temperature ≤ 400 ºC and an H2 atmosphere, Taher et al. and Di Mondo et al. [START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF] have been the first ones to propose pathways (Scheme 5a) for glycerol to propylene and/or propane, catalyzed in acid medium by Cr or Ru-C sites submitted to an H2 treatment. These conditions constitute a thermodynamic sink for these sites because the C-O cleavage in glycerol seems to begin with an initial H2 dissociation, which is prompted by spillover phenomena that favors the hydrogenation events both in glycerol and its intermediates. [START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF] It then progresses through glycerol dehydration and hydrogenation to 3-hydroxypropionaldehyde. The latter undergoes a secondary Brønsted acidcatalyzed dehydration to acrolein, which is then hydrogenated to n-propanol. A third dehydration takes place immediately to convert n-propanol to propylene that can be excessively hydrogenated to propane. The presence of i-propanol in the aqueous medium is hypothesized to be the result of water reacting to propylene via the Markovnikov mechanism instead of n-propanol isomerization. This early mechanism is overly simple and does not consider the other possible C3 intermediates whose formation is also kinetically possible under the typical HDO conditions studied. Hydroxyacetone and C3-diols can be formed in parallel from 3-hydroxypropionaldehyde, which is less kinetically stable, as reported in due course by Mota et al. [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF] (Scheme 5b). This route has been experimentally identified as the most probable one for propylene because both 1,2-propanediol and i-propanol have a much higher propylene yield over the Mo-based catalyst (~57 % and ~64 %, respectively) than their respective isomers: 1,3propanediol (~0 %) and n-propanol (~ 4.3 %), under identical experimental conditions. [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF] The formation of hydroxyacetone and 1,2-propanediol intermediates during the GTP reaction formed over bifunctional sites and under a low temperature has been reported. [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] However, the proposed GTP pathway (Scheme 5b) is still incomplete because propionaldehyde and n-propanol can also establish an isomerization equilibrium with allyl-alcohol and i-propanol, respectively. [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] Thus, Sun et al. [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF] have proposed a new GTP mechanism over metals/acid sites taking into account that partial C-O and C-C breaking to additional intermediates such as propionaldehyde, propionic acid, methanol, acetaldehyde, ethanol, ethylene-glycol, ethylene, propane and CO2 have been included in the new pathway (Scheme S3). Some of the reported intermediates, whose formation becomes prominent under an H2 atmosphere, has been experimentally confirmed. [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] Zacharopoulou et al. [START_REF] Zacharopoulou | Olefins from Biomass Intermediates: A Review[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] have, in turn, proposed four pathways for GTP (as gathered in Scheme 6), which differ in the main intermediates formed over a Mo-based catalyst: a) allyl alcohols, b) acrolein, c) hydroxyacetone, and d) C3-diols. The reported data [START_REF] Zacharopoulou | Olefins from Biomass Intermediates: A Review[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] indicate that all these pathways start with the cleavage of one or two C-O bonds of glycerol either by dehydration or hydrogenolysis followed by sequential hydrogenation-deoxygenation of C3 intermediates up to propylene.

According to the first pathway (Scheme 6 a), glycerol directly undergoes the simultaneous removal of two vicinal -OH groups to form allyl-alcohol, as primary intermediate. This step is derived either by the use of a H2-rich atmosphere or H2 transfer events where glycerol itself is acting as H2 donor, as reported by other groups. 175,176 The allyl-alcohol formed is then transformed via the following steps: i) a direct hydrogenolysis of C-O bond to propylene, and ii) an isomerization to propionaldehyde as secondary intermediate that is hydrogenated to 1-and 2-propanol equilibrium and then to propylene.

Further ex situ HDO tests with the oxygenates involved [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] indicate that the second step contributes little to propylene formation considering the very low rate of 1-propanol to propylene (< 0.6 mmol.gcat - 1 .h -1 ). Nevertheless, the rate of allyl-alcohol to propylene is the highest (~23 mmol.gcat -1 .h -1 ). [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] This might be due to the high reactivity of the C=C bond that facilitates H2 adsorption and probably affects the dissociation energy of the C-OH bond, enabling its cleavage and simultaneous C=C isomerization.

However, the proposition for simultaneous removal of two vicinal OH groups from glycerol should be re-contextualized because according to the principle of microscopic reversibility, 177,178 the transition state for a hypothetic single step of direct glycerol-to-allyl alcohol must be also the same for the reverse incorporation of two simultaneous water into allyl alcohol at overall equilibrium. This would involve simultaneous collision of three chemical species to form the corresponding transition state, which is unlikely from a statistical point of view. Therefore, it is more probable that the microscopic pathway of glycerol to allyl alcohols would involve more than one transient state, which means simultaneous removal of two OH groups is less likely.

Acrolein is formed as an intermediate through another totally independent (Scheme 6 b), via the protonation of the central -OH group of unstable 3-hydroxypropionaldehyde (formed by tautomerism). 179,180 The subsequent hydrogenation of acrolein through propionaldehyde to 1-or 2propanol equilibrium results in propylene via final dehydration. However, acrolein is a highly unstable intermediate [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] that is detected in a small amount in these studies, [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] particularly when an H2 atmosphere is used. This suggests that this route does not make a significant contribution to propylene formation.

A third GTP pathway (Scheme 6 c) takes place through hydroxyacetone, which is formed through an initial protonation of terminal -OH of the glycerol and a rapid isomerization of an unstable enol intermediate. 181,182 The dehydration of hydroxyacetone to acetone over acid sites is then favored more than the parallel hydrogenation to 1,2-propanediol. Nonetheless, the hydrogenation of acetone to propanol upon dehydration of the latter contributes to some extent to the formation of propylene. This pathway has also been corroborated by ex situ propanol HDO tests, where a significant propylene rate (~14 mmol.gcat -1 .h -1 ) is observed. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] A fourth GTP pathway (Scheme 6 d) has also been proposed, justified by the fact that the removal of primary OH in glycerol under these typical HDO conditions is easier than the secondary one. 183,184 It involves the formation of 1,2-propanediol and to a lesser extent 1.3-propanediol, which undergoes dehydration to propionaldehyde, further hydrogenation to propanol, and then to propylene.

Based on their experimental findings, Rellán-Piñeiro et al. [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF] have theoretically evaluated the energy parameters affecting both the adsorption of glycerol and H2 dissociation over MoO3. As gathered in Scheme S4, they have found that the GTP reaction seems to start by creating undercoordinated Mo atoms with an oxygen defect, induced by H2 reduction, over which sequential and simultaneous dehydration, keto-enol equilibria, and hydrogenation events in glycerol and oxygenated intermediates can take place. The latter step fills the Mo oxygen vacancy and closes the catalytic cycle. This means H2 is used as a mandatory reductive agent to re-launch the catalytic cycle. The proposed sequences occur in accordance with the reverse Mars-Van Krevelen model, thus corroborating the experimental findings already reported by Zacharopoulou et al. [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] As illustrated in Scheme 7 a, glycerol is first dehydrated to either 1,2-or 1,3-enols in an identical way to the mechanism of ethanol dehydration over MoO3 clusters. 185 It is adsorbed on the uncoordinated Mo-O bond, where its H + is transferred to a terminal oxygen, and the β-hydrogen is stripped by oxygen and, concomitantly, the C-O bond is cleaved. This step eventually breaks C-H and C-O bonds with a weaker barrier, forming double bonds; Mo=O on the catalyst surface and C=C in the 1,2-or 1,3enols. These highly unstable enols OH quickly transit to their respective keto tautomers, [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] depending on whether the C-O cleavage is taking place from primary or secondary. Keto-enols are thus interconverted via adsorbed enolic -OH on a Mo vacancy (Scheme 7 b). This step is followed by -OH dissociation, with an H atom reacting with an O-Mo bond. This new OH-Mo bond transfers H* either to the H2=C group of 1,2-enol to form hydroxyacetone, or to the =CH-group of 1,3-enol to form 3hydroxypropionaldehyde. Subsequently, the ketone group of hydroxyacetone or hydroxypropionaldehyde interacts with the vacancy site to absorb the H* atom from the neighboring OH-Mo bond, leading to deprotonated 1,2-or 1,3-propanediol (Scheme 7 c). In parallel to this step, 3hydroxypropionaldehyde can also dehydrate to acrolein, which is not feasible for hydroxyacetone because its -OH group no longer has any β-hydrogen to be cleaved. Further dehydration of 1,2propanediol via its primary OH group forms allyl alcohol, while the dehydration via a secondary OH group lead to 1-propenol (or allyl alcohol again), depending on whether the β-hydrogen of C1 or of C3 is eliminated. 1,3-propanediol can only be dehydrated to allyl alcohol, but this step is less likely to occur experimentally. [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] These modeling data also indicate that acrolein can quickly hydrogenate to unstable enols, 1-propenol or 2-propenol (allyl alcohol), supporting the experimental findings of Zacharopoulou et al., [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] discarding propylene formation via an acrolein intermediate. 1-and 2enolates mainly enter into an equilibrium with their respective keto-tautomers, acetone and propionaldehyde. Both compounds undergo hydrogenation to form 2-propanol and 1-propanol, respectively, which are finally dehydrated to propylene.

The allyl alcohol formed by the dehydration of C3 diols and the hydrogenation of acrolein may undergo further hydrogenolysis over undercoordinated Mo sites (Scheme 7 d). This route involves the simultaneous hydrogenation of the H2=C group and C-O cleavage to directly form propylene. This last route -glycerol → allyl alcohol → propylene -has been experimentally identified, [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] but the modeling data discussed 37 consider the pathway glycerol → hydroxypropionaldehyde→ acrolein → 1-propenol → propionaldehyde→ 1-propanol → propylene to be the main GTP route. The modeling approach used 37 has a major drawback by considering gas-phase GTP conditions without addressing lateral effects, which provide a high conversion rate and a wide variety of products, among which 3hydroxypropionaldehyde and hydroxyacetone are the main intermediates. However, the GTP experiments, [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] which report 1,2-propanediol and allyl alcohol as the main intermediates, have been conducted under practical liquid or mixed-phase GTP conditions. Wan et al. [START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] have complemented these calculations by considering the lateral interactions of reactants over the Cu/Mo2C catalyst. These data reveal that propylene is formed specifically over the Mo2C surface and not on the Cu one or on the Cu-Mo interface. As indicated in Scheme S5 (in green) for propylene formation, each oxygen atom of glycerol strongly interacts with a Mo site located above the second carbon layer on the Mo2C(0001) surface, forming an alkoxide through a binding energy of - These findings increase our understanding of the GTP route over a Mo-based catalyst; nevertheless, this approach still has some drawbacks, such as these calculations only considers the cleavage of one C-H bond at a terminal and central C atom without any C-C bonds breaking. Furthermore, it requires the complete scission of all C-O and O-H bonds and relevant hydrogenation events to form only four C3 compounds: hydroxyacetone, allyl alcohol, propionaldehyde, and propylene. However, in our last study, [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] we experimentally identified around twenty partially and/or fully C1-3 deoxygenated compounds formed over Mo-O or Mo-C based catalysts. Some of these intermediates were also detected over a Mo-Sx-based catalyst, [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF] tested under similar hydrotreating GTP conditions. At a high conversion rate, glycerol was mainly hydrodeoxygenated to propylene, propane, and 1-propanol, or cracked to ethylene, methane, COx, methanol, and ethanol. However, at a low conversion rate, it was mainly converted to partially deoxygenated liquid oxygenates, such as hydroxyacetone, allyl alcohol, acrolein, and acetaldehyde. The integral networks for glycerol conversion to propylene/propane and ethylene/ethane, which include two main C3 and C2 pathways, are summarized in Scheme 8.

The C3 pathway involves glycerol dehydration either to 3-hydroxypropionaldehyde or to hydroxyacetone without involving 1,2-or 1,3-enols, previously reported as possible starting intermediates. [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] Their absence may be due to the fast-kinetic reaction at a higher temperature ~400 ºC. Hydroxyacetone was identified as the major intermediate, while a smaller amount of 3hydroxypropionaldehyde was detected. As suggested by previous DFT calculations, [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Wan | Vibrational Spectroscopic Characterization of Glycerol Reaction Pathways over Metal-Modified Molybdenum Carbide Surfaces[END_REF] hydroxyacetone can be hydrogenated to 1,2-propanediol, which may undergo further dehydration to acetone or allyl alcohol, while 3-hydroxypropionaldehyde can be hydrogenated to 1,3-propanediol or dehydrated to acrolein. Further hydrogenation events in the last three intermediates (acetone, allyl alcohol, and acrolein) can form propanols, which subsequently dehydrate into propylene or saturate to propane. In parallel, the C2 pathway, which was probably started by the initial C-C scission in the first adsorbed glycerol, as C-C cracking in the H2-transfer reaction of alcohols has been experimentally proven. 184,188 Thus, acetaldehyde was detected as the major C2 intermediate, while a smaller amount of hydroxyacetaldehyde was observed. Both compounds could be formed through the hydrogenolysis of C-C bonds, undergoing subsequent deoxygenation-hydrogenation events until the formation of ethylene and ethane.

Synthetic Summary:

The proposed mechanisms for the GTP reaction are reported according to the following two concepts; i) high-temperatures catalysis (T > 400 ℃, atmospheric pressure, and acid catalyst), and ii) typical mild condition catalysis (T ≤ 400 ℃ , H2 atmosphere, and metallic or bifunctional catalysts). The first GTP mechanisms involve mainly complex pathways including glycerol dehydration/cracking, H2 producing-transfer-consuming events, and further decarbonization and/or oligomerization to C2-4 olefins; mainly ethylene and to a lesser extent propylene. Further pathways towards coke, aromatics, or paraffins may be involved via parallel and/or sequential reactions (e.g., C-C coking, Diels-Alder, and aldol-condensation), which require adjusting catalyst properties and operating conditions to improve propylene formation. The secondary GTP pathways occur mainly through sequential deoxygenation-hydrogenation events in the glycerol and intermediates, including initial dehydration or hydrogenolysis and subsequent hydrogenation, isomerization, or further dehydration to propylene. The most probable route for the formation of a C=C bond in propylene is through the hydrogen transfers from the surface -OH group to the H2C= group, formed in allyl alcohol via previous hydrogenolysis and isomerization, which leads to eventual C-O breaking, according to the reverse Mars-van Krevelen mechanism. [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] Excessive hydrogenation events lead mainly to C=C saturation in propylene and to C-C in propane. [START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF]

GTP process operability and the effect of operating conditions

Our aim here is to address the influence of key operating conditions according to different GTP configurations. The most relevant GTP data, which are gathered from lab-scale experiments and/or thermodynamic simulations, are summarized in Table 3.

Effect of temperature

Various studies [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Zakaria | Thermodynamic Analysis of Glycerol Conversion to Olefins[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] report that temperature is a crucial factor that influences both glycerol conversion and product distribution, and which also depends on process configuration and the nature of the catalyst. Indeed, total glycerol conversion is usually obtained at high temperatures (> 400 ºC), [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Bakhtyari | 4 -Light Olefins/Bio-Gasoline Production from Biomass[END_REF][START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF][START_REF] Zakaria | Thermodynamic Analysis of Glycerol Conversion to Olefins[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF] , as an eventual result of the reactions reported in GTP mechanisms. Furthermore, the selectivity of propylene vs. ethylene is highly sensitive to the type of reaction involved at each temperature. Corma et al. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF] have increased the temperature in micro-reactors from 500 ºC to 700 ºC (with acid HZSM-5 and FCC1, for example) to promote glycerol cracking up to a 31 % yield of C2-C4 olefins. This temperature increase has also promoted CO and H2 formation (up to 62 %) due to the decomposition of C-C/C-H bonds into syngas. The observed tendencies can be attributed to thermodynamic effects, predicting a smaller improvement in exothermic H2-transfers and oligomerization at a higher temperature. In contrast, lower temperatures usually favor the parallel formation of aromatics and coke, [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Xiao | Conversion of Glycerol to Hydrocarbon Fuels via Bifunctional Catalysts[END_REF] leading to fast catalyst deactivation. To resolve this conundrum, Corma et al. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF] recommend a high-temperature FCC reactor, allowing for homogeneous mixing, better energy dissipation, and the in situ regeneration of catalysts.

Lima et al. [START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF] report that ethylene selectivity increases proportionally with temperature from 450 ºC to 600 ºC (over metals/HZSM-5), while propylene decreases inversely. This change in C2-3 olefin composition is due to the entropically favored ethylene formation via endothermic glycerol cracking. Indeed, C-H bonds decomposed to H2 are partially used for the hydrogenation events in ethylene and CH4 formation. This effect becomes more pronounced at certain temperatures (> 550 °C), when the selectivity for CH4 increases sharply. [START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF] Similar findings [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] highlight the increasing selectivity of ethylene (up to 22 %) and propylene (up to 14 %) over ZSM-5 (Si/Al ~15-127) within a temperature range of 400-600 ºC (Fig. S4 A). COx selectivity (~ 31 %, mainly CO) is also higher at 600 ºC and catalyst deactivation also occurs, which is consistent with the observations reported by Corma et al. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF] It may be concluded that the integrated in situ extraction of COx or the WGS reaction could be efficient strategies for producing additional H2, and thus improving the GTP yield at high temperatures.

Other experimental data coupled with a multi-objective genetic algorithm [START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF][START_REF] Zakaria | Thermodynamic Analysis of Glycerol Conversion to Olefins[END_REF] reveal similar trends, indicating also that increasing temperature favors ethylene yield over acid catalysts. An optimal yield of C2-3 olefins (17.7 %) is achieved at 715.9 ºC, which is supported by further thermodynamic analysis, [START_REF] Zakaria | Thermodynamic Analysis of Glycerol Conversion to Olefins[END_REF] indicating that the highest ethylene rate peaks at temperature higher than 700 ºC (Fig. S4 B).

In a multi-step GTP reaction, Blass et al. [START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF] have tweaked the distribution of olefins and byproducts by varying the temperature of three-stage fixed-bed reactors. Operating the first dehydration reactor (i.e., HZSM-5) at 400-500 ºC produces mainly a mixture of acrolein (70 %) and acetaldehyde (20 %).

Acrolein is then hydrogenated in the second stage (over Pt or Pd/Al2O3) into propionaldehyde between 100 ºC and 300 ºC, where a selectivity between 70-99 % was attained at 115 ºC. Temperatures near to 300 ºC simultaneously favor oxygen removal and hydrogenation rates, leading to all the deoxygenated intermediates being fully converted to paraffins. Furthermore, the reverse endothermic dehydrogenation of the formed paraffins to C2-3 olefins under H2-atmosphere is thermodynamically limited. 189,190 Consequently, the conversion of glycerol to C2-3 olefins cannot proceed in two steps through paraffin intermediates. The integration of a third reactor for separate propionaldehyde conversion over zeolites (e.g., HBEA) increases C2-3 olefin yields; 10 % and 46 %, are obtained at 400 and 500 ºC, respectively (Table 3). The direct connection of the three reactors in series for continuous glycerol-to-olefins route produces only a 6% yield of ethylene + propylene at 400 ºC. However, increasing the temperature of the third reactor to 500 ºC improves C2-3 olefin yield (from 6 % to 15 %, Table 3), while that of C4-5 olefins decreases (from 13 % to 9 %, Fig. 7 A). This trend confirms that C2-3 olefins are effectively produced from the cracking of C4-5 olefins, formed initially by the condensation of propionaldehyde. Similar trends have been observed in the conversion of acetone, which is an isomer of propionaldehyde, to C3-6 compounds. [START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF]191,192 Hulteberg et al. [START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] have also patented a similar three-step route: glycerol dehydration over WO3/ZrO2, acrolein hydrogenation over Cu/ZnO2, and propanol dehydration over WO3/ZrO2, to produce propylene and propane. The following are the optimal temperatures for the three steps: 280 ºC, 270 ºC and 208 ºC, respectively, yielding ~ 46 % propylene and ~10 % propane (Fig. 7 B).

Deshpande et al. [START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] have studied the homogeneous dehydroxylation of crude glycerol in two batch reactors to boost propylene yield vs. byproducts. The first reactor initiates partial dihydroxylation at a temperature below 120 ºC, which require reductive conditions and a molar ratio of halogen/crude glycerol between 1:4 and 1:8. However, slightly higher temperatures between 180 ºC and 210 ºC in the second reaction are necessary to attain 80 % propylene and 4 % propane at full conversion. The exact temperatures values depend also on other operating conditions (e.g., H2 pressure, H2/glycerol/ ratio, and glycerol/halogen ratio). Operating under inert conditions produces only 38 % propylene at 59 % glycerol conversion, even when lengthening time-on-stream (TOS) to 6.5 h.

For the heterogeneous two-step GTP route, Yu et al. [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] increase temperature to 250 ºC to boost the initial hydrogenolysis of glycerol over Ir/ZrO2 to attain ~94 % 1-propanol yield (Fig. S5 A), because this reaction is kinetically suppressed below 200 ºC. The subsequent 1-propanol dehydration in a second step (over ZSM5, Si/Al=30) leads to a propylene selectivity of 99 % (Table 3). Similar twostep approaches, which integrate glycerol hydrogenolysis over WOx/T317, [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF] Pt/ZSM-5, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] and MoO3modified Ni2P/Al2O3, [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] and subsequently 1-propanol dehydration over acid aluminosilicates, have also been studied by Sun et al. [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF] and Wu et al. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] who found 242 ºC and 250 ºC as appropriate temperatures for the GTP reaction. Temperatures below these values shift the reaction to liquid oxygenates, while a further increase in temperatures favors the decomposition of byproducts.

Furthermore, a separate two-step strategy still reduces the efficiency of the GTP process. A compact tandem GTP configuration closely integrating double-beds (Ir/ZrO2+HZSM-5, [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] 9.3%WO3/T317+ amorphous SiO2-Al2O3, [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF] Pt/ HZSM-5 + HZSM-5, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] or MoO3-modified Ni2P/Al2O3 + HZSM-5, [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] ) has been validated at the optimal temperatures identified, recording ~88 % propylene selectivity at ~73 % glycerol conversion, 53 ~84.8 % propylene yield, [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF] and ~76 % 28 and ~71-88 % 49 propylene selectivity at full conversion (Fig. S5 B-D, Table 3). The slight discrepancy in the values of propylene selectivity and glycerol conversion reported in these four studies carried out at comparable temperatures range may be due to the difference in the composition and partial pressure of glycerol, H2 and/or solvent, space-velocity, molar ratio of H2/glycerol, and the nature of the catalysts used. When methanol is used as solvent, Wu et al. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] recommend a temperature of 500 ºC for the reactor zone of the acid dehydration catalyst, which is similar to that usually used in the MTP/MTO process, 111,172 with a view to the following: i) avoiding the excessive alkylation of C2-3 olefins with methanol to long C4+ olefins through a dual-cycle mechanism, [START_REF] Teketel | Co-Conversion of Methanol and Light Alkenes over Acidic Zeolite Catalyst H-ZSM-22: Simulated Recycle of Non-Gasoline Range Products[END_REF]110 and ii) enhancing the cracking of the C4+ compounds already formed to C2-3 olefins.

The influence of temperature on a single-step GTP under typical HDO conditions has also been studied. [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Di Mondo et al. [START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF] have increased temperatures from 175 ºC to 250 ºC to boost glycerol and H2 conversion to propylene yield (96 %) over the 316SS alloy and HOTf co-catalyst, which occurs via the mechanism in Scheme 5a. Fadigas et al. [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] recommend a range from 250 ºC to 350 ºC for the full HDO of glycerol/water (30-100 wt%) to propylene and propane. A promising yield of propylene ~ 90 % is obtained over MoFe/AC at an optimal temperature of 300 ºC and space-velocity of 5.4 h -1 , although a high molar ratio of H2/glycerol (120/1) is required. [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF] Similar GTP findings have been patented by Shi et al. [START_REF] Shi | Converting Glycerol to Propylene[END_REF] for directly hydrotreating glycerol to propylene at temperatures between 175 ºC and 550 ºC (most efficiently, 200 ºC and 300 ºC) and under a limiting H2/glycerol molar ratio but higher total pressure (up 200 bar). Decreasing temperature from 500 ºC to 316 °C or even further to 254 °C greatly improves propylene selectivity (up to 82.1 %) despite the use of a low molar ratio for H2/glycerol ~2.3:1 (Table 3). Consequently, this decrease leads to incomplete glycerol conversion (from 0 % to almost 15 %). However, this un-converted glycerol could easily be separated in the liquid phase because the inlet temperature (254 ºC) is lower than that of the fusion of glycerol ~290 °C (Table 1). It may therefore be recycled in commercial GTP facilities thanks to excellent propylene selectivity at a lower temperature of 254 ºC without detecting any appreciable content of propane, C1-2, and COx compounds. The reported data require further reproducibility and validation tests.

Zacharopoulou et al. [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] and El Doukkali et al. [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] have investigated temperatures of 240 °C, 264 °C, 280 °C, 300 °C and 318 °C, which are close to those reported by Shi et al. [START_REF] Shi | Converting Glycerol to Propylene[END_REF] A comparable GTP performance requires a high molar ratio of H2/glycerol (53:1-100:1), a low glycerol/water mixture (2-10 wt%), and lower H2 pressure (30-80 bar).

According to Zacharopoulou et al.,[START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] glycerol HDO over MoFe/C increases progressively with temperature from 240 ºC to 300 ºC, when propylene selectivity begins to noticeably improve over 280 ºC to reach 71 % of propylene selectivity at glycerol conversion ~ 50 % (Fig. 8 A). [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] Over this temperature, ally-alcohol is considered the main intermediate (Fig. 8 B), which is consistent with previous studies. 175,176 This ally-alcohol is then directly converted to propylene or isomerized to propionaldehyde and then hydrogenated to 1-propanol, which in turn is dehydrated to propylene, as recently reported (Fig. 8 C-D). [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] A propylene yield of up to ~84.1 % was thus obtained, depending on the reaction temperature (264 ºC to 318 ºC), space-velocity (1.7 h -1 to 4.7 h -1 ), a molar ratio of H2/glycerol (78/1 to 98/1), and catalyst type. [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] A higher propylene yield may potentially be achieved at around 300-318 ºC, as this range seems to be ideal for balancing the exothermic hydrogenation of ally-alcohols to 1-propanol (𝛥𝛥𝛥𝛥 propionaldehyde, which forms an isomerization with ally-alcohols. This explains the higher amounts of ally-alcohols and propionaldehyde observed at low temperatures (264 ºC, Fig. 8 D). Both compounds are unable to undergo further hydrogenation due to thermodynamic and/or kinetic restrictions, whereby propylene yield does not exceed 8.8 %. Under these conditions, both water and glycerol are fully in the liquid phase as indicated by Aspen data in Fig. 8 E, where the condensed phase seems to prevent H2 solubility in the medium required for the hydrogenation events. This H2 shortage favors the dehydration and/or hydrogenolysis of glycerol, leading to partially deoxygenated compounds.

In a homogeneous catalysis medium, Janssens et al. [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] have also highlighted the importance of temperatures (210-220 ºC) in balancing dehydration with the hydrogenating rate of crude glycerol over a Ru-complex catalyst to improve propylene yield (~76%, Fig. 8 F). However, an optimal value of temperature needs to be found because an excessive increase in temperature risks favoring other pathways, while the presence of noble metals mainly leads to an over-reduction of the desired propylene to propane under excess H2. [START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF] Therefore, at 400 ºC and 18 bar of H2-pressure, single-step GTP provides only ~26 % propylene selectivity over a Mo-based catalyst (Table 3), which is attributed to the excessive formation of propane, C-C cracking in glycerol, and coke. [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF] Synthetic Summary: In the high-temperatures GTP route, a better compromise between propylene and ethylene yield requires optimal temperatures of 500-700 ºC to balance glycerol dehydration, C-C cracking, and H2 transfer events, while suppressing aromatics favored below 500 ºC and excessive COx formation at severe temperatures. Propylene yield cannot exceed the theoretical value of ~77.7

% 41 because a part of the C-C/C-H bonds in glycerol are parallelly cracked for H2-transfer events to form C2-3 olefins. Multi-step GTP routes, including tandem processes, require relatively lower temperatures for the hydrogenolysis step (≤ 250 ºC), while the catalyst dehydration beds should be at a slightly higher temperature (~500 ºC) to shift the equilibrium towards C2-3 olefins. For the singlestep GTP route, a high propylene yield can be obtained at milder temperatures; whose exact values depend on operating in a homogeneous or heterogeneous catalysis medium (Table 3). However, further optimization studies of not only temperature of reaction but should address also H2-pressure, H2/glycerol molar ratio, spaces-velocity, and glycerol/water mixture, etc.

Effect of pressure and reaction atmosphere

The influence of the reaction atmosphere, which is directly related to the variation in the total and partial pressure of reactants and products, has also been evaluated for GTP routes. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF][START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF] Inert atmospheric pressure is mainly preferred for high-temperature GTP because H2 is generated in situ by C-C/C-H cracking, thereby forming mainly ethylene and to a lesser extent propylene. [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF][START_REF] Zakaria | Thermodynamic Analysis of Glycerol Conversion to Olefins[END_REF] Moreover, a high H2 pressure is not necessary at temperatures of more than 400 ºC because excess H2 can shift the reaction to saturated molecules, becoming particularly pronounced over noble metals (i.e., Pt or Pd) modified acid supports. [START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF] For example, ethylene and propylene hydrogenation to ethane and propane, respectively, involve a decrease in volume and in moles (example in Eq. 1) so they will be favored by high H2 pressures according to Le Chatelier's principle. 87,193 C 3 H 6 + H 2 → C 3 H 8 ; (Eq. 1), ∆H 298 0 ≈ -124.3 kJ/mol) Unsaturated C=C bonds and allylic C-H bonds are known to be thermodynamically unstable and highly reactive under excess H2, rapidly evolving to form alkanes, aromatics, or polymerized coke, depending on the reaction temperature. [START_REF] Hoang | Conversion of Glycerol to Alkyl-Aromatics over Zeolites[END_REF][START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF] For multi-step GTP routes, Blass et al. [START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF] use a continuous H2-flow and glycerol in three separate reactors to reach a C2-3 olefin yield of 46 %. However, when connecting these reactors in series, only a 15 % yield of C2-3 olefins was obtained (Table 3). The last configuration requires an increase in total pressure to improve the hydrogenation rate. According to the Antoine Equation, 133 the temperature of this step should also be proportionally increased over 140 ºC so that the vapor pressure of glycerol is always higher than its partial pressure to avoid glycerol condensation. Adopting similar three steps approach, Hulteberg et al. [START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] have reached a better compromise between H2 flow and total pressure (from 2 bar to 10 bar) to balance hydrogenation and dehydration rates in three successive reactors, obtaining ~46 % propylene yield at 5 bar and 400 ml/min of H2 flow (Fig. 7 B). However, a separate control of H2 pressure for each reactor needs to be carefully considered because H2 benefits hydrogenation, while not necessary for the dehydration steps. Adapting it accordingly can help to reduce H2 wastage. Also, integrate an additional reactor involving simultaneous catalytic reforming of bioglycerol to H2-rich gas, which can be carried out under similar operating conditions, 194 may also help in letting go of external H2 use or at least minimize its utilization.

Deshpande et al. [START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] have evaluated the influence of H2 and N2 partial pressure on the reductive and non-reductive dehydroxylation of crude glycerol in two steps. Reductive dehydroxylation under H2 pressure (3.45-34.5 bar), which involves a molar ratio of glycerol/halogen-catalyst (4:1-27:1), temperature (120-210 ºC), and TOS (6-6.5 h), improves GTP performance, attaining a propylene selectivity from 38 % to 96 % at a conversion from 24 % to 100 %. Yu et al. [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] have adjusted total H2 pressure between 5 and 50 bar to promote the GTP route, according to separate two-step and tandem processes (Ir/ZrO2+ZSM-5). Working at 50 bar and 250 ºC records a propylene selectivity of 67 % at full glycerol conversion (Table 3), although a high amount of propane is also detected. To overcome this drawback, total pressure is optimized at 5 bar, with a propylene selectivity of 88 %, although glycerol conversion unfortunately decreases to 73 %. Wu et al. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] have also studied the effect that the presence or absence of H2 has on the HDO of simulated crude glycerol (glycerol/methanol molar ratio =1/1.28 and oxygenates/H2O = 1/1) over double-bed catalysts under typical MTP process conditions. [START_REF] Tian | Methanol to Olefins (MTO): From Fundamentals to Commercialization[END_REF]109 Increasing partial H2 pressure from 0 to 95.6 kPa therefore increases propylene selectivity from 38 % to 63 %, while decreasing the optimal temperature for the highest GTP yield from 600 ºC to 500 ºC (Fig. 9 A-B). This H2-rich medium probably renders it unnecessary for C-C/C-H cracking at higher temperatures to promote hydrogenation events. Sun et al. [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF] have modulated H2 partial pressure by increasing atmospheric H2 flow from 30 ml.min -1 to 180 ml.min -1 to promote GTP selectivity from 57.8 % in a single bed (9.3 wt% WO3/T317) to 84.8 % in a double bed (9.3 wt% WO3/T317+Al2O3-SiO2, Fig. S5 B, Table 3). This enriched H2 medium shifts the equilibrium in favor of propylene formation through the initial hydrogenation of hydroxyacetone to 1,2-propanediol and propionaldehyde to 1-propanol, 161 which seems to be intensified by the final dehydration of propanols over acid aluminosilicates catalyst placed in the down-bed . [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] Indeed, they recommend a slightly higher H2 pressure than an atmospheric one to balance the pressure drop in the double powdered beds. This configuration should be adjusted to better control mass transport restrictions, kinetic reaction rates and reactor obstruction.

The effect of H2 or N2-pressure on a single-step GTP route has also been experimentally studied in the range of 0-200 bar. [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] This produces a propylene yield of ~ 90 %, [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] a propylene selectivity of ~71 % at a glycerol conversion of ~ 50 %, [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] propylene selectivity of ~82.1 % at a glycerol conversion ~ 85.8 %, [START_REF] Shi | Converting Glycerol to Propylene[END_REF] and a propylene yield up to ~ 84.1 %, [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] which depend to the target optimal conditions; 98 bar (H2/glycerol molar ratio ∼120 and 300 ºC), [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] 80 bar (H2/glycerol molar ratio ∼53 and 300 ºC), [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] 83 bar (H2/glycerol molar ratio ∼2.3 and 254 ºC) [START_REF] Shi | Converting Glycerol to Propylene[END_REF] and 50 bar (H2/glycerol molar ratio ∼98 and 318 ºC), [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] respectively, using a transition metal-based catalyst (Table 3, Fig. 9 C-F).

Increasing H2 pressure or the H2/glycerol molar ratio generally enhances propylene selectivity, most likely by suppressing the formation of partially deoxygenated intermediates (i.e., ally-alcohols, propionaldehyde, acetone), which has also been corroborated experimentally and by modeling. [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] However, an excessive increase can saturate C=C, instead forming propane. [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF] For example, when SHI et al. [START_REF] Shi | Converting Glycerol to Propylene[END_REF] increase H2 availability by raising the H2/glycerol molar ratio from 2.3:1 to 6.8:1 (at constant total pressure ~ 83 bar and 316 ºC), the selectivity of products shifts significantly from ~59.2 % of propylene to ~ 75.7 % of propane (Table 3). Nevertheless, no difference in glycerol conversion is observed when N2-pressure ( ~ 30 bar) is used instead of H2, but propylene selectivity decreases significantly due to the formation of oxygenates (Fig 9 D). [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] An inert or weak H2 atmosphere mainly favors the isomerization of ally-alcohols to unstable propionaldehyde that becomes actively involved in the subsequent polymerization and condensation with other C3 compounds to yield undetected heavier products in CG and/or coke deposition in the catalyst surface, [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] which is manifested as a low carbon balance. Anderson et al. [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF] co-fee-d H2S (ppm) in situ with a glycerol/water mixture under 18 bar of H2 pressure and 400 ºC, allowing them to eradicate the formation of oxygenates and improve the catalyst stability by 6-12 times for deoxygenation-hydrogenation activities.

Fig. 9 C-E shows that lowering H2 pressure and the H2/glycerol molar ratio also negatively affects glycerol HDO by providing lower propylene selectivity. [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] This lower level of hydrogenation and deoxygenation can be primarily ascribed to low H2 solubility in the medium. Insufficient H2 dissolved in the medium leads to poor contact between H2 and glycerol on the redox or metallic site, which instead forms oxygenates over acid site. Thermodynamics may have an unfavorable effect. According to Le Chatelier's rules, when the total pressure is decreased, the reactions shift towards more molecules in the gas phase. This will favor the pathways that repress H2 consumption, or at least those that consume less H2 (used in excess) to maintain a high number of moles of gas in the overall process, partially rewarding the effect of decreasing the pressure. Specifically, the HDO of glycerol theoretically needs two or three moles of H2 to be fully deoxygenated in one mole of propylene or propane, respectively. However, certain partially dehydrated compounds (e.g., acrolein, hydroxyacetone, C3 diols, and allyl-alcohol) can be formed via dehydration without external H2 consumption or via hydrogenolysis with just one mole of H2, maintaining a high number of moles in the gas-phase by lowering the pressure of the overall process. Consequently, the single-step GTP route is blocked when partial H2 pressure decreases.

Synthetic Summary:

An inert atmospheric pressure is used mainly during the conversion of glycerol to C2-3 olefins at high temperatures. However, an H2 atmosphere plays a critical kinetic and thermodynamic role in the GTP reaction under typical HDO conditions (mild temperature et pression, neutral aqueous medium, etc.). An excessively H2-rich medium is undesirable, as it favors the further hydrogenation of propylene to propane, which becomes particularly abundant over a high amount of hydrogenating metals (e.g., Ir, Pt, Pd, Ni, Mo. [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] ). At low H2 pressure, and has been corroborated by recent experimental data, [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] numeric calculations [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF] indicate weak H2 solubility in the reaction medium and insufficient H2 adsorption-dissociation over active site, which negatively affects C-O cleavage and C-C hydrogenation events in glycerol and oxygenates, becoming particularly pronounced when the reaction is occurring in liquid phase. Therefore, an optimized compromise between total pressure, H2/glycerol (mol.) ratio and the partial pressure of the other reactants (i.e.

glycerol, H2O, solvent, etc.) can provide acceptable GTP rates, albeit ultimately depending on other parameters such as temperature and space-velocity, among others.

Influence of feed mixture and/or space-velocity

Given that the crude bioglycerol recovered in industry contains solvents and impurities (e.g., H2O, methanol, heavy organic fractions, and inorganic salts), 121,135,137 the effect of such additives on GTP catalysis has also been studied by different authors. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Xiao | Conversion of Glycerol to Hydrocarbon Fuels via Bifunctional Catalysts[END_REF][START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] Corma et al. [START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF] have injected a glycerol/vacuum oil mixture (molar ratio ~1/7-1/31) to simulate GTP conditions (WHSV ~20-60 h -1 , 500 ºC, FCC1 catalyst) in near operational industrial FCC process. 195 The co-feeding of oils with glycerol seems to have a diluting effect because the initial calculations of heat requirements indicate that a mixture of glycerol/oil higher than 20 wt% (WHSV at 20 h -1 ) increases the yield of C2-C4 olefins to 13.3 %, which is better than those yields obtained with pure glycerol or pure vacuum oil. Furthermore, the co-injection of glycerol (before mixing with oil) with extra water (~50 wt%), which acts as a heat sink in the FCC unit due to its high vaporization temperature, increases the formation of small C2-C4 olefins to 31 %. These findings suggest that it may be suitable to combine GTP with MTP processes in tandem to intensify the production of sustainable propylene.

Hoang et al. [START_REF] Hoang | Conversion of Glycerol to Alkyl-Aromatics over Zeolites[END_REF] have adjusted the weight of catalyst (HZSM-5) per mass of glycerol feed (W/F~ 0.05 to 8) under N2-flow at 400 ºC, finding that W/F ~8 yields almost ~40 % propylene, while W/F ≤ 4

shifts the reaction toward heavy C6-C12 aromatics (60 %). Xiao et al. [START_REF] Xiao | Conversion of Glycerol to Hydrocarbon Fuels via Bifunctional Catalysts[END_REF] report an inverted tendency for the GTP route over Pd/HZSM-5 when using an H2/N2 flow instead of pure N2 at the same temperature of 400 ºC. A W/F of more than ~2 yields constant rates of C2-C3 oxygenates and C2-C3 olefins (~30%), although the yield of aromatics increases proportionally as W/F increases. This might be related to the fast conversion of oxygenates and light olefins to aromatics, which can be induced by the dissociation of excess H2 over Pd element impregnated over HZSM-5 catalyst to generate hydrogenating sites adjacent to Brønsted and/or Lewis sites.

Yu et al. [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] inject refined glycerol and simulated crude glycerol (containing H2O ~5-50 wt% and NaCl/methanol mass ratio ~4:1-15:1) to study the effect of these impurities on two-step GTP (Ir/ZrO2 and HZSM-5 ). No change in GTP performance is observed with a low content of NaCl or methanol of ≤ 4 wt%: 92-94 % of 1-propanol in the first hydrogenolysis step and 99 % of propylene in the second dehydration step. However, when a higher amount of NaCl (15 wt%) is used, the amount of Ir used as hydrogenating element in the hydrogenolysis catalyst should be increased from 0.12 mol% to 0.2 mol%, which is equivalent to dividing WHSV by 1.67, to maintain a comparable GTP performance. A similar co-feeding strategy has been used by Wu et al. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] over a tandem GTP system by injecting WHSV~ 1 h -1 of simulated crude glycerol (glycerol/methanol = 1/1.28 and catalyst loading to report similar GTP findings. As expected, glycerol conversion and propylene selectivity improve with an initial increase in catalyst loading, which is evidently related to the availability of additional active sites. However, a further increase in catalyst loading does not lead to a proportional increase in conversion and selectivity (Fig. S6 C-D), which suggests there may be diffusional restrictions. The amount of catalyst in a batch reactor should be carefully adjusted to the mass of reactants in order to both improve transport aspects and avoid fast catalyst deactivation. [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] Anderson et al. [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF] report that a short contact time (~3 s) for the single-step HDO of a higher glycerol/water mixture (80 wt%) leads to a very low HDO activities and alkenes formation (Fig. S7 A-B), producing mainly unsaturated C3 oxygenates, while a longer contact time (~360 𝑠𝑠) improves glycerol conversion to yield ~ 58% alkanes/alkenes (i.e., propylene ~ 26%). These HDO tendencies have recently been corroborated, [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] whereby a low WHSV (1.7 h -1 ) of glycerol/water (10 wt%) seems to balance the deoxygenation-hydrogenation rates over MoOx supported on SBA-15, with a high GTP yield (up to 84.1 %). However, increasing WHSV to 4.7 h -1 significantly decreases the GTP yield to 21 % (Table 3). Similar reaction trends, which seems to be mostly related to the availability of active sites, as corroborated in our recent research, [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] were also observed over bulk MoOx catalyst (Fig. S7 C-D).

Synthetic Summary: Feed composition, space-velocity, and catalyst amount are important parameters that affect not only the HDO events in glycerol and intermediates to form the target propylene, but also catalyst stability along TOS. GTP catalysis in a homogeneous medium seems to provide more resistance to impurities, although recycling issues are still a challenge. In turn, heterogeneous GTP routes should be further studied to reach a balanced compromise between the population of active sites and the extent of deoxygenation and hydrogenation in glycerol and intermediated oxygenates, thereby avoiding the formation of partially deoxygenated compounds (i.e., CxHyOz) or fully hydrogenated propane. Combine high-throughput experimentation with predictive modeling and design of experiments methodologies could be powerful tools in adjusting the partial pressure of oxidizing (CxHyOz, H2O) and reducer (i.e., H2) agents at optimal temperature of reaction with the HDO properties of the used catalysts. This can help to overcome catalyst instability in an aqueous medium, which is due to the excessive accumulation of oxygen on the catalyst surface, as we have been confirmed. 60

Thermodynamic-kinetic considerations and GTP routes comparison

As highlighted in this review, different GTP strategies (Scheme 9 a-e) have been or are being explored to link the biorefinery and polyolefin industries. We therefore compare these routes using qualitative reactor configurations, target operating conditions, catalysts type, etc. Nonetheless, the viability of the multi-step mode also remains questionable because it requires the following:

i) Expensive separation of value-added C3 such as acrolein, propionaldehyde, or 1-propanol, which cannot be viably converted into propylene. For example, the difference in the price of propylene and 1-propanol (spread of $400 per ton on average, 2022). 8

ii) Extensive H2 consumption because hydrogenolysis or hydrogenation steps involve mainly noble metal-based catalysts (i.e., Ir, Pt, Pd, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] ). The use of these noble metals is very expensive, reducing the financial benefits. In addition continuous H2 use for some cascade reactions should be carefully rationalized, as the dehydration step does not require an H2 feed. [START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF] From an integration point of view, the efficiency of the multi-step approach could be improved if a continuous process combines various catalysts and active species in tandem or in close contact. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] Various dual-bed systems have been explored accordingly: Ir (0.12 mol%)/ZrO2 +HZM5, 53 WO3(9.3 wt%)/T317 + SiO2-Al2O3, 42 Pt(0.3 wt%)/ZSM-5 +ZSM-5 [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] and MoO3/Ni2P (20 wt%)/Al2O3

+ ZSM-5. [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] Propylene selectivity up to 88 % has been reported for TOS between 0.5 and 24 hours. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] Given this short TOS, the in situ regeneration of spent catalysts seems difficult over such a complex mixture of active sites, where some ones require oxidative calcination and other need reducer treatment. Besides, catalysts working in tandem reaction-bed zones require an optimal kinetic control of the involved reactions for the overall GTP because the route occurs through successive reactions steps (i.e., first hydrogenolysis and then dehydration) with different kinetic rates. If the GTP reaction is extended for a longer period, some active sites may be deactivated before others. Thus, successive stops of the process to unload spent catalyst and load new ones would greatly increase the cost of operation. These drawbacks would negatively affect the inter-operability of this tandem strategy.

In turn, a single-step GTP route carried out in gas, liquid or mixed phases is undergoing substantial advances. [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] It may become a truly real ecofriendly route for producing sustainable propylene, particularly if the production of H2 from fossil-based resources is replaced by, for example, in situ H2 generation from renewable sources (i.e., biomass, water electrolysis, etc.). There are therefore numerous aspects to consider for continuing working on improving single-step GTP route efficiency:

i) Oxygen elimination and hydrogen introduction, through which the GTP route takes place, are thermodynamically favorable mainly under mild HDO conditions: T ≤ 400 ºC and P ≤ 83 peak GTP yield is wholly possible by optimizing both the HDO abilities of the catalysts and the operating conditions (i.e., temperature, pressure, space-velocity, feed, and composition). These propositions provide some guidance on the single-step GTP reaction, but the severity of the unselective mass/heat transfer conditions should also be considered, as the overall process is relatively exothermic (Eq. 5, -128 kJ/mol).

Relevance to techno-economic and environmental efficiency

To the best of our knowledge, there are no techno-economic or environmental studies specifically focused on the evaluation of GTP routes technology. The current literature deals instead with the assessment of routes integrating olefin production from industrial and urban organic wastes or general biomass, as a wider context. [START_REF] Machado | Bio-Based Propylene Production in a Sugarcane Biorefinery: A Techno-Economic Evaluation for Brazilian Conditions. Biofuels Bioprod[END_REF][196][197][198][199][200] This might be related to the major progress in research and technology made in the field of these processes. [START_REF] Yuan | Superstructure Optimization of Integrated Fast Pyrolysis-Gasification for Production of Liquid Fuels and Propylene[END_REF][START_REF] Ramamurthy | Conversion of Waste Plastic to Propylene and Cumene[END_REF][START_REF] Gotro | Bio-Based Polypropylene; Multiple Synthetic Routes Under Investigation[END_REF][START_REF] Reznichenko | Next Generation of Polyolefin Plastics: Improving Sustainability with Existing and Novel Feedstock Base[END_REF][START_REF] Machado | Bio-Based Propylene Production in a Sugarcane Biorefinery: A Techno-Economic Evaluation for Brazilian Conditions. Biofuels Bioprod[END_REF][START_REF] Zhang | Liquefaction of Biomass and Upgrading of Bio-Oil: A Review[END_REF][START_REF] Gayubo | Olefin Production by Catalytic Transformation of Crude Bio-Oil in a Two-Step Process[END_REF][START_REF] Tian | Methanol to Olefins (MTO): From Fundamentals to Commercialization[END_REF][START_REF] Chieregato | Olefins from Biomass. In Chemicals and Fuels from Bio-Based Building Blocks[END_REF][START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF]197,[201][202][203][204] Some of these concepts have already entered the testing stage or industrial development; i) SABIC technology (the Netherlands and Saudi Arabia) is based on the co-feeding of "second generation" animal fats and vegetable oils with petroleum feedstocks, 205,206 ii) The BRASKEM approach (Brazil) involves sugarcane fermentation to bioethanol followed by dehydration to bioethylene and further dimerization and metathesis to biopropylene, [START_REF] Gotro | Bio-Based Polypropylene; Multiple Synthetic Routes Under Investigation[END_REF][START_REF] Machado | Bio-Based Propylene Production in a Sugarcane Biorefinery: A Techno-Economic Evaluation for Brazilian Conditions. Biofuels Bioprod[END_REF] iii) The MITSUI chemical route (Japan) 207 involves second generation biomass fermentation to biobutanol, followed by dehydration to biobutylene and metathesis with ethylene to produce green propylene, for example. Furthermore, the most developed concept, which is employed by many companies (e.g., ExxonMobil, TotalEnergies, and Shell Global Solutions) involves the initial gasification of second-generation biomass and urban wastes to produce syngas, followed by FTS of methanol. Conventional MTO/MTP technology is then applied to obtain relatively sustainable propylene and/or ethylene. 196,197,205,208 As described, these technologies integrate multi-stage reactions, sometimes the co-feed of petroleum-based feedstocks, and the manufacturing chain involves unrecovered CO2 emissions, 196,197,208 which reduces its sustainability.

The development of a novel GTP route such as that involving only a single-step HDO of crude bioglycerol to biopropylene, where the required H2 is produced in situ via simultaneous catalytic reforming of bioglycerol to H2-rich gas, or H2O-electrolysis or splitting using surplus electricity, solar energy or wind-powered systems with closed carbon emission cycles can convert this GTP approach into a truly ecofriendly green technology. Thus, producing sustainable propylene will obviously help to comply with the global targets for future CO2-neutral polypropylene production, whereby the

In turn, single-step GTP catalysis progresses mostly via sequential C-O deoxygenation and C-C hydrogenation in the glycerol and oxygenated intermediates to form a C=C bond in propylene through H2 transfer events, according to the reverse Mars-van Krevelen mechanism. It seems to be the most promising strategy that can provide ~100 % propylene yield without thermodynamic limits, if mild operating conditions (e.g., temperature, H2-pressure, feed, space-velocity) and highly selective catalysts (i.e., homogeneous Ru-complex and heterogeneous Mo-based systems) have been chosen

accordingly. Further advances in optimizing GTP operating conditions is still required for balancing deoxygenation-hydrogenation events, minimizing H2 consummation, and improving the stability of catalysts in a hydrothermal medium.

In brief, implementing an efficient ecofriendly GTP route technology will require a balanced synergy between the design of a robust HDO catalyst and the successful optimization of GTP operating conditions, which depend on the process configuration chosen for the most viable conversion strategies.

Supporting

REFERENCES:

(1) Alpha Olefin Market-Global Industry Analysis and Forecast (2022-2029) 82,83,118,121,133,134 Homogeneous halogen-based catalysts (i.e., I2, HI, HIO3, Lil or a combination thereof). [START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] In situ dissolution, reductive treatment (H2), non-reductive treatment (inert gas) NA HI catalyst, formed in situ from I2 and H2 co-feeding, displays promising dehydroxylation functions Homogeneous Ru-complex; RuBr3.xH2O (4.0 mg), HBr (2 mg), Bu4PBr (577 mg) mixtures [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] In situ dissolution (CO or CH2O), in situ RED (H2 ~40 bar, stirring, 180 °C, 0.5 h), recrystallization FTIR CO-rich medium forms more Ru-C carbonyl-bonds, acting as a hydrogenation site. Homogeneous complex: Ru (H2O)3 (4'-phenyl-terpyridine)] (OTf)2, Ru(H2O)3 (terpy)](OTf)2 Brønsted acid co-catalyst; HOTf 57 Suspension, PR, filtration and vacuum evaporation, reductive manipulation in dry-box and direct transfers to reaction medium Elemental analysis, MALDI-TOF, 1 H-NMR, 13 C-NMR HOTf acts as a Brønsted site, while Ru-C bands act as a hydrogenating site. [START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF] Equilibrium Gibbs conditions [START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF][START_REF] Zakaria | Thermodynamic Analysis of Glycerol Conversion to Olefins[END_REF] 511.8-688.2 ºC, 1 bar, N2~40 mL/min, gly. in H2O ~12. 4 -47.6 wt%, WHSV~ 25.6-184.4 --Two separate steps: hydrogenolysis in batch Ir (1 wt%)/ ZrO2 (0.12 mol) and dehydration in fixed-bed HZSM-5 (2 g) [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] Combined batch and fixed-bed; Ir (1 wt%)/ ZrO2 (2 g) + HZSM-5 (2 g). [START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] T~ 250 ºC, gly. in H2O ~ 30 wt%, PH2~ 50 bar, H2:gly. (mol.)~100:1, TOS~4 h, T~ 250 ºC, propanol in H2O ~ 10 wt%, PN2 ~ 1 bar, N2~20 ml/min, WHSV~1 h -1 , TOS~2 h T~ 250 ºC, H2:gly. (mol.)~100:1, gly. in H2O ~ 30 wt%, WHSV~1 h [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] T~ 240-300 ºC, P~ 30-80 bar, H2: gly (mol.) ~53:1, gly. in H2O ~ 2 -10 wt%, CT~7. [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF] Copyright 2016 Elsevier. from ref. [START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF] Copyright 2009 Wiley Online Library. from ref. [START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF] Copyright 2011 American Chemical Society. Reproduced with permission from ref. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF] Copyright 2016 American Chemical Society. C-D) Effect of N2 or H2 atmosphere on glycerol conversion and product selectivity over FeMo/carbon (300 °C, glycerol/H2O~ 2 wt%, catalyst/Substrate ratio~ 0.83, TOS ~2 h). Reproduced with permission from ref. [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] Copyright 2015 Royal Society of Chemistry. from ref. [START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] Copyright 2018 Wiley Online Library. E) Effect of H2 partial pressure on propylene yield as a function of H2 to glycerol partial pressure ratio over Mo (8 wt%)-based Carbon catalyst (at 280 °C, glycerol/H2O~10 wt%, H2/glycerol molar ratio ~22-250, total pressure ~70 bar, W/F ~195 gcat/molgly.h, TOS ~4.8 h). Reproduced with permission from ref. [START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] Copyright 2022 Elsevier. F) effect of total H2 pressure on the variation of total glycerol conversion (TGC) by deoxygenation (GC_C-O) and by hydrogenation (GC_C-C), carbon balance (CB), and on the yield of propylene and main byproducts over MoOx (14.8 wt%)-based SBA-15 (318 °C, glycerol/H2O~10 wt%), H2/glycerol molar ratio ~98, WHSV~1.7 h -1 , TOS ~4.8 h). Reproduced with permission from ref. [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] Copyright 2023 Elsevier.

Characteristics

A B

Step-1: Hydrogenolysis

Step-2: Dehydration

  used the same methods to support a similar loading of NiMo, ZnMo, CuMo and FeMo on activated carbon (AC), comparing them with commercial Ru/AC and Pd/AC catalysts. The noble metals and Ni/Mobased catalysts lead to C1-3 alkanes, whereas ZnMo and CuMo-based catalysts instead form oxygenates. A FeMo/AC catalyst features a propylene yield of up to ∼90 % under atmospheric H2

  2.13 eV. The energy barriers indicate that the cleavage of O-H bonds in the two alkoxides formed over Mo2C (with 0.03 eV and 0.28 eV) is more favorable than the cleavage of their C-O bonds (with 1.11 eV and 0.98 eV). The dissociation of the third O-H bond is also favorable against the C-O bond because the activation barriers of the O-H bond is just 0.21 eV, while that of the C-O ones is in the order of 0.68 and 0.73 eV. Such preferential O-H cleavage has been reported for ethanol and ethyleneglycol conversion over carbides. 186,187 The adsorbed CH2O-CHO-CH2O* intermediate may subsequently undergo the successive cleavage of the three C-O bonds, whose respective activation barriers are 0.48 eV (for the middle carbon), 0.31 eV and 0.15 eV (for the two terminal carbons) to form an CH2-CH=CH2* intermediate. Propylene is therefore finally formed after protonation at a terminal carbon of the last intermediate with an activation barrier of 0.56 eV. This pathway requires the least energy (-10.36 eV), with a more favorable energy performance than those based on the prior cleavage of the C-H bond or the protonation of C-C to acrolein or saturated C-C bonds, which have higher activation barriers and lower energy (-9 eV and -8.77 eV, respectively).

  This study is part of the CatGTP project, which is a Marie Skłodowska-Curie Actions (MSCA) grant (agreement No. 101030636), funded by the EU H2020 program. The authors are also grateful to the following for their financial supports: the Franco-Moroccan research program of PHC Toubkal 2022 (project ref. TBK22/150, 47338YM), the Moroccan CNRST's program for excellence fellowship (No. 2USMS2022) granted to the PhD student (M. Bahlouri), the FRAPPE program of the "Hauts-de-France" region, and Centrale Lille Institut.

  Fig. 1 A) Contribution of the main sectors for global propylene use (in %), 84-86 B) current and projected compound annual growth rates (CAGR) for the propylene market (in billion dollars -B$)and C) evolution of the gap between real production of propylene and its market demand during the period 2009-2022 (statistical data) and 2023-2028 (predicted data). 8,[START_REF]Propylene Market Size, Share, Growth, Analysis, Price, Segmentation, & Industrial Trends By[END_REF][START_REF] Ltd | Global Propylene Market to Boost In Coming Years -Projected To Reach 132.1 Metric Tons In[END_REF][87][88][START_REF]Global Propylene Market to Reach 158.4 Million Metric Tons by[END_REF][START_REF]Propylene Market Capacity and Capital Expenditure (CapEx) Forecast by Region, Top Countries and Companies, Feedstock, Key Planned and Announced Projects[END_REF]

Fig. 2 . A )Fig. 3 .

 2A3 Fig. 2. A)Evolution of the global production of crude bioglycerol; mainly co-regenerated with biodiesel and to a lesser extent with bioethanol, for the period 2010-2022 (statistical data) and 2023-2030 (predicted data),123,124 and B) share of the main application sectors for bioglycerol.

3 .Scheme 4 .

 34 Scheme 3. Illustration of the most promising strategies for upgrading bioglycerol into added-value chemical intermediates. In green, the expected route for biomass to propylene.

6 .

 6 Scheme 6. Four main reaction pathways for glycerol conversion to propylene over an MoOx based catalyst via the following intermediates: a) allyl-alcohol, b) acrolein, c) hydroxyacetone, and d) C3diols. Gathered with permission from ref. 59 Copyright 2018 Wiley Online Library

Fig. 8 .Fig. 9 .

 89 Fig.8. Effect of temperature on glycerol conversion and propylene selectivity(A) and on oxygenate selectivity (B) over the FeMo/Carbon (80 bar, H2/glycerol (mol.) ratio~ 53:1, glycerol/H2O ~ 2 wt%, TOS~ 2 h). Reproduced with permission from ref.[START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] Copyright 2015 Royal Society of Chemistry. Total glycerol conversion (TGC), deoxygenation (GC_C-O), hydrogenation (GC_C-C), and the yield of propylene and byproducts over an β-Mo2C@SBA-15 catalyst tested at 318 °C (C) and 264 ºC (D), glycerol/H2O ~10 wt%, 50 bar, H2/glycerol~ 78 and WHSV~ 2.35 h -1 . Existence of gas-liquid phases for glycerol/H2O mixture ~10 wt% (E). Reproduced with permission from ref.[START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] Copyright 2023 Elsevier. Yield a,b,c and Selectivity d provided by the single-step HDO of glycerol/H2O mixture (50-100 wt%) over an Ru-based catalyst at 210 °C and 220 °C (F). Reproduced with permission from ref.[START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] Copyright 2021 Royal Society of Chemistry.

  Scheme 9 (a-e). Main configuration strategies for converting glycerol to propylene

(

  Based on transition metals;Mo, Ru, etc.)

Table 1 .

 1 Fakhroleslam, M.; Sadrameli, S. M. Thermal/Catalytic Cracking of Hydrocarbons for the Production of Olefins; a State-of-the-Art Review III: Process Modeling and Simulation. Fuel 2019, 252, 553-566. https://doi.org/10.1016/j.fuel.2019.04.127. Main characteristics of glycerol and propylene molecules

	. MAXIMIZE

Table 2 .

 2 Summary of the catalysts and their main catalytic properties for the GTP reaction

		Glycerol			Propylene
	IUPAC name	Propane-1,2,3-triol		Propene
	Synonyms	glycerin,	1,2,3-Trihydroxy	propane,	1-Propene, prop-1-ene, Methyl-
		propanetriol, 1,2,3-Propanetriol, etc.		ethylene, etc.
	CAS Number	56-81-5			115-07-1
	Short formula	C3H8O3			C3H6
	Semi-structure
	Molecular weight	92.09 g.mol -1			42.08 g.mol -1
	Description, security &	viscous, colorless, odorless, nontoxic, and	gas, colorless, nearly aromatic,
	toxicity	sweet tasting			irritant,	highly	flammable,
					explosion limit; 1.8-11.2 vol%
	Refraction index	1.47			1.35
	Melting point	17.8 °C			-185.2 °C
	Boiling point	290 °C			-47.6 °C
	Solubility in water 20 °C, 1	Totally miscible		384 mg.l -1
	atm)
	Density (25 ºC, 1 atm)	1.26 g/cm³			1.74 kg/m 3
	Viscosity	1.49 Pa.s (25 ºC, 1.013 10 5 Pa )		87.8 10 -7 Pa.s (25 ºC, 1.27 10 5 Pa)
	Enthalpy, ∆ 𝑓𝑓 𝛥𝛥 0 (25 °C) 1.8.6. Heat	liquid phase; -669.6 kJ.mol -1 liquid phase; 221.9 J.mol -1 .K -1		gas phase; 20.41 kJ.mol -1 gas phase; 63.79 J.mol -1 .K -1
	capacity,
	𝑪𝑪 𝒑𝒑 𝟎𝟎 (25 °C)
	Auto-ignition temperature	370 °C			485 °C
	Flash point	177 °C			-108 °C
	Saturated vapor pressure	0.0263 bar (183.25 °C)		11.68 bar (25 °C)

Table 3 .

 3 Main GTP performance as function of reactor configurations, operating conditions, and most promising catalysts.

	Bed	configuration,	relevant	Main operating conditions	Conv. (%) Sel. (%)	Yield (%)
	catalyst, ref.
	High-temperature MAT reactor;	500-700 ºC, 1 bar, N2~40 mL/min:	-
	-HZSM-5(SiAl ~50, binder ~ 15	-gly. in H2O ~ 50-100 wt, cat./feed (mass ratio) ~1.5	100	-C2-4 olefins~
	wt%)			-6	-	31 %.
	-FCC1 (Y, Si/Al~13, metals ~1	-gasoil:glycerol (mol.) ~31:1 -7:1, WHSV~ 20-60 h -	100	-C2-4 olefins
	%) 41,66			1, TOS~ 30 min		~13.3 %
	High-temperature	fixed-bed;
	Cu/ZSM-5

oxygenates/H2O = 1), corresponding to 1 kPa glycerol, 1.2 kPa methanol, 2.2 kPa water, 95.6 kPa H2, over dual-system Pt/ZSM-5+ HZSM-5, which provides better propylene selectivity (~63.7 %) than that usually obtained in a typical MTP process (~47.9 %) under similar conditions (Fig. S6 A-B). A similar GTP selectivity (~ 71 %), which remains constant for ~500 h, is obtained when feeding a similar composition of reactants over the tandem system: MoO3-Ni2P/Al2O3+ZSM-5 (Fig. S5 C). [START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF] Sun et al. [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF] varied the amount of glycerol in water (20-60 wt%) over the increased mass (1-4 g) of the double-bed system; 9.3 wt% WOx/T317 + Al2O3-SiO2. A low glycerol/water mixture (20 wt%), an increase in the mass of the first hydrogenolysis catalyst (from 1 g to 4 g) improves the formation of propylene and propane (from 48 % to 54%), while that of 1-propanol decreases proportionally from 38 % to 31 %. This suggests that increasing the contact time over catalytic sites favors hydrogenation events, as corroborated recently. [START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] However, when the amount of glycerol is increased to 60 wt%, the selectivity towards more hydrogenated compounds (propylene, propane, and 1-propanol) decreases significantly, whereas the selectivity of more oxygenates (hydroxyacetone, 1,2-propanediol) increases proportionally. This kinetic tendency suggests the reduced availability of active-sites, which are suppressed by increasing the amount of the second dehydration catalyst (Al2O3-SiO2, 1 g to 3 g), reaching a stable propylene selectivity of 84.8 % at total conversion for about 24 h (Fig. S5 B).

Di Mondo et al. [START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF] have correlated the single-step HDO of glycerol/water (10 wt%) with the contact time of the homogeneous co-catalyst to show that increasing the concentration of HOTf compound (up to 60 mmol/L) improves GTP yield (up to 96 %) after just 5 h TOS under 55 bar of H2 pressure, instead of 24 h as reported for the 40 mmol/L of co-catalyst. However, when water content is increased to 50 wt%, the HDO of glycerol forms mainly 1-propanol, including when the hydrogenating Ru-based catalyst is coupled with the HOTf co-catalyst and a high H2 pressure ( ~75.8 bar) is applied. [START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF] More recently, Janssen et al. [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] have used glycerol/water mixtures (50-100 wt%) to compare GTP performance with crude glycerol under similar homogeneous conditions, (Table 3, Fig. 8 F, Fig. S1 A). For high water content, a significant decrease in both dehydration and hydrogenation rates has been observed at 210 ºC, thus decreasing GTP yield from 57 % to 35 %. This has been circumvented by improving glycerol and H2 mixing at a slightly higher temperature ~ 220 ºC, which yields 76 % propylene (Fig. 8 F). Under the same operating conditions, the use of crude glycerol yields 82 % of propylene (Table 3, Fig. S1 A).

For heterogeneous GTP conditions, Fadigas et al. [START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] have studied space-velocity between 2 h -1 and 20 h -1 for the single-step HDO of glycerol/water (30-100 wt%) to propylene and propane. A high molar ratio of H2/glycerol up to 120/1 is required to obtain a high GTP yield (~ 90 %), increasing the spacevelocity to 5.4 h -1 and the glycerol/water mixture to 90 wt%. Zacharopoulou et al. [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] have adjusted and/or quantitative indicators (e.g., process configuration, severity of conditions, energy consumption, propylene yield, and sustainability aspects) to identify the most suitable route that can potentially produce a high yield of green propylene with minimal H2 consumption and byproducts, as well as reducing workup facilities and investments.

High-temperature GTP [START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Zakaria | Catalyst Screening for Conversion of Glycerol to Light Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Lima | Catalytic Conversion of Glycerol to Olefins over Fe, Mo, and Nb Catalysts Supported on Zeolite ZSM-5[END_REF][START_REF] Zakaria | Optimization of Catalytic Glycerol Steam Reforming to Light Olefins Using Cu/ZSM-5 Catalyst[END_REF][START_REF] Zakaria | Thermodynamic Analysis of Glycerol Conversion to Olefins[END_REF][START_REF] Corma | Biomass to Chemicals: Catalytic Conversion of Glycerol/Water Mixtures into Acrolein, Reaction Network[END_REF][START_REF] Gujar | A Review on Catalytic Conversion of Biodiesel Derivative Glycerol to Bio-Olefins[END_REF] exploits simultaneous glycerol dehydration and cracking under inert atmosphere to form propylene and ethylene at temperatures higher than 400 °C, whereas H2 is generated in situ and directly activated on the catalytic sites. [START_REF] Bakhtyari | 4 -Light Olefins/Bio-Gasoline Production from Biomass[END_REF][START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF] Nevertheless, the viability of this strategy is questionable because it involves the following drawbacks: i) Intensive energy spending on sensible and latent heats for vaporizing glycerol and water mixture, ii) Huge volume expansion induced by the vaporization of glycerol and water, which greatly increases the investment cost.

iii) Low experimental yield of propylene, which in the best of cases cannot exceed the theoretical values between ~33.3 % and ~77.7 % 41 because part of the glycerol is converted at high temperatures through H2 transfer reactions and cracked in C, COx emission and H2O, according to the following reactions (Eq. 2-4):

3 C3O3H8 → C3H6 + 6C + 9H2O, Eq. 2 3 C3O3H8 → 2 C3H6 + 3CO + 6H2O, Eq. 3 C3O3H8 → 7/9 C3H6 + (6/9) CO2 + (15/9) H2O, Eq. 4 iv) Rapid deactivation of the acid catalysts by stream dealumination or coke formed at high temperatures v) Instability of C=C bonds that undergo complete saturation to C1-3 alkanes, which become more pronounced when external H2 is used instead of inert gas, particularly over noble metals modified zeolites. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Zakaria | A Perspective on Catalytic Conversion of Glycerol to Olefins[END_REF][START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF] Multi-step GTP strategies, which apply external H2 source either in separate steps or through cascade reactors, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF][START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] whereby the equilibrium can be shifted to form propylene under moderate operating conditions. Each reactor normally contains a specific hetero-or homogeneous catalyst for accelerating the kinetics of the target intermediate reactions: dehydration, 50,64 hydrogenolysis, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF] hydrogenation, [START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] aldol condensation to C4-5, 50 cracking to C2-3 olefins, [START_REF] Blass | Conversion of Glycerol to Light Olefins and Gasoline Precursors[END_REF] double dehydroxylation, [START_REF] Deshpande | Dehydroxylation of Crude Alcohol Streams Using a Halogen-Based Catalyst[END_REF] and/or further dehydration to propylene. [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Wu | MoO3 Modified Ni2P/Al2O3 as an Efficient Catalyst for Crude Glycerol to Propylene[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Hulteberg | Process for Preparing Lower Hydrocarbons from Glycerol[END_REF] The articulation of these reactions depends on bar, as reviewed in this work. These less-intensive energy conditions favor a better technoeconomic performance and environmental impact. For example, working in liquid or mixed phases can further reduce the consumption of energy due to the unnecessary evaporation of reactants or solvents (i.e., ∆H Vap 0 (H 2 O)~2257 kJ/kg, C p 0 (H 2 O)~4.18 J/g/°C), and reduce installation size and cost. The bioglycerol degradation to C1-2 co-products and the CO2 emissions prevailing at severe temperatures can also be minimized.

ii) No thermodynamic limits on the full HDO of glycerol to 100 % propylene yield from mild temperatures ~ 200 ºC and at any H2 pressure (Fig. 10 A). The pressurized unit systems could be used in situ for the subsequent separation of propylene and H2 using welldeveloped membrane technologies or pressure swing adsorption, while water can easily be recovered by condensation.

iii) Propylene hydrogenation to propane (eq. 1) is an exothermic reaction and favored at even lower temperatures (Fig. 10 B). The formation of oxygenates (i.e., hydroxyacetone and C3 diols)

is kinetically favored under a H2-poor medium and low temperature. [START_REF] Rellán-Piñeiro | A Coupled Density Functional Theory-Microkinetic Modeling for the Hydrodeoxygenation of Glycerol to Propylene on MoO3[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] The formation of these byproducts may be reduced by moderately increasing temperature and adjusting H2 partial pressure accordingly, obtaining a high GTP yield over an appropriate catalyst.

iv) Under mild HDO conditions, the one-pot HDO of glycerol theoretically requires just two moles of H2 to produce one mole of propylene and co-generate only H2O (eq. 5), which allows maintaining a zero carbon footprint. [START_REF] Wan | Controlling Reaction Pathways of Selective C-O Bond Cleavage of Glycerol[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] C 3 H 8 O 3 (g) + 2H 2 (g) → C 3 H 6 (g) + 3H 2 O(g); (Eq. 5, ∆H 298 0 ≈ -128 kJ/mol) However, this ideal state is still very difficult to achieve experimentally. As we have been reviewed, kinetically, and instead of completely removing oxygen from glycerol to selectively form propylene, the reaction may only contribute to partial C-O scission to form C3-oxygenates at a low temperature, [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] breaking C-C and C-H bonds at a higher temperature to produce H2 + shorter carbon chain compounds, [START_REF] Wu | Selective Conversion of Glycerol into Propylene: Single-Step versus Tandem Process[END_REF][START_REF] Corma | Processing Biomass-Derived Oxygenates in the Oil Refinery: Catalytic Cracking (FCC) Reaction Pathways and Role of Catalyst[END_REF][START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF] or at the most, saturate the C=C bound of propylene to yield propane under high H2 pressure. [START_REF] Murata | Propane Formation by Aqueous-Phase Reforming of Glycerol over Pt/H-ZSM5 Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] However, recent literature [START_REF] Sun | Efficient Production of Propylene in the Catalytic Conversion of Glycerol[END_REF][START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Yu | Propylene from Renewable Resources: Catalytic Conversion of Glycerol into Propylene[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] reports that it may draw very close to this ideal state through the application of appropriate conditions and catalysts (homogeneous Ru-complex, [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] and heterogeneous Mo-based catalysts [START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF] ). Although GTP in a discontinuous batch reactor [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF][START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Taher | Acid-, Water-and High-Temperature-Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] is less suited to the mass production of propylene, it provides initial insights, such as proof of the single-step GTP concept. Operating in continuous systems [START_REF] Anderson | The Hydrodeoxygenation of Glycerol over NiMoSx: Catalyst Stability and Activity at Hydropyrolysis Conditions[END_REF][START_REF] Mota | Green Propene through the Selective Hydrogenolysis of Glycerol over Supported Iron-Molybdenum Catalyst: The Original History[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF][START_REF] Shi | Converting Glycerol to Propylene[END_REF][START_REF] Ioannidou | Optimization of Reaction Conditions for Hydrodeoxygenation of Bio-Glycerol towards Green Propylene over Molybdenum-Based Catalyst[END_REF][START_REF] Fadigas | Préparation de catalyseurs hétérogènes utilisés dans l'hydrogénation sélective de glycérine en propène et procédé pour l'hydrogénation sélective de glycérine en propène[END_REF] provides an interesting GTP yield of up to 90 %. Nevertheless, this strategy still requires further improvements to run for a longer TOS, as catalyst instability in aqueous media is a major challenge. [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] El Doukkali | New Insights in Single-Step Hydrodeoxygenation of Glycerol to Propylene by Coupling Rational Catalyst Design with Systematic Analysis[END_REF] Therefore, stabilizing the polyolefin industries may become more ecofriendly. This viewpoint is backed by the high availability of crude bioglycerol that will continue to grow annually, as highlighted in section 2.2.

Based on the above, it seems reasonable to advise other researchers to conduct pioneering technoeconomic and environmental assessments of GTP routes. This review reports sufficient data (i.e., Table 2-3) in terms of feed composition, reactant conversion, product distribution, operating conditions, and catalyst efficiency for comparative studies. As described in section 3.3.5, the GTP route in a single step seems to have promising operating characteristics, which will reduce the investment in equipment and operating costs.

Comparing the efficiency of a single-step GTP route carried out in, for example, liquid or mixed phases with those undertaken fully in a gas phase via a high-temperature GTP process, multi-step reactions, or a tandem GTP process, may provide valuable insights about their technical feasibility, energy gains, and environmental benefits. For example, light may be shed on how much energy can be saved by operating in the liquid phase, and whether it would be suitably acceptable to compensate what is lost in propylene selectivity and yield, in comparison to a GTP process fully operated in the gas phase.

Such insights will encourage further research into overcoming the reported GTP-related challenges, forming a sufficiently consolidated concept to move on to the next upscaling tasks.

Conclusions

In view of the stimulating market prospects for propylene and crude bioglycerol, GTP catalysis should boost research and technological progress to become an alternative for producing sustainable polypropylene. This review of the state-of-the-art of GTP catalysis provides a useful source to guide researchers towards further developments for advancing this technology.

High-temperature GTP catalysis, which involves effective teamwork between glycerol dehydration, C-C/C-H cracking to H2 producing-transfer-consuming reactions, and further decarbonylation over acid catalysts such as zeolites, instead forms ethylene and, to a lesser extent, propylene. The multi-step GTP process, including the tandem mode that involves various successive reactions (e.g., dehydration, hydrogenolysis, aldol-condensation, and further cracking) over complex multi-functional systems, is proposed to shift the conversion of glycerol and oxygenated intermediates at a low temperature to propylene instead, avoiding further alkylation or Diels-Alder rearrangements. An optimal compromise is required between acidity, size-shape diffusion, active sites synergy and kinetic reaction rates for the process configurations used for improving GTP efficiency and avoiding constant catalysts regenerations or replacements.

316SS walls/alloy powder, HOTf co-catalyst, Commercial Ru (5 wt%)/AC (EVONIK) [START_REF] Di Mondo | Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium[END_REF] chemical corrosion, in situ acid attack (HOTf, H2 ~55 bar, 250 ºC) centrifugation, ICP-MS, SEM/EDX and XRF Cr 3+/5+ act as hydrogenation sites, while Fe or Ni prompt H2 spillover. Fe ~2.7 wt% and Mo~19.3 wt% (Mo/Fe atomic ratio ~4:1)/BC or AC [START_REF] Zacharopoulou | Novel Process for Propene Production from Biomass: Catalytic Glycerol Hydro-Deoxygenation[END_REF][START_REF] Zacharopoulou | One-Step Propylene Formation from Bio-Glycerol over Molybdena-Based Catalysts[END_REF][START_REF] Zacharopoulou | Exploring the Reaction Pathways of Bioglycerol Hydrodeoxygenation to Propene over Molybdena-Based Catalysts[END_REF] IMP, CP, ex situ CAL (N2~100 ml/min, 200 ºC, 2 h), in situ RED (H2~40 ml/min, 500 °C, 0.5 h) N2-Ads., XRD, NH3-TPD, He-TPD, TPR, TGA, XPS Fe substituted Mo 4+ promotes Mo 6+ reduction to Mo 5+ /Mo 4+ states; responsible for HDO events.

Mo-M (i.e., Ni, Fe, Cu or Zn)/AC; atomic ratio M/(M+Mo) = 0.4. Mo-Fe/AC, atomic ratio Fe/(Mo+Fe) from 0 to 1. Pd (5 wt%)/AC and Ru (5 wt%)/AC

One-pot batch; homogeneous Rucomplex (583 mg) [START_REF] Janssens | From Crude Industrial Waste Glycerol to Biopropene via Ru-Mediated Hydrodeoxygenation in Ionic Liquids[END_REF] T~ 220 ºC, P H2~40 bar, PCO~10 bar, H2 excess, CT~ 285 𝑠𝑠, TOS ~ 1 h: -pure gly. in H2O (50-100 wt%), -crude gly. including H2O, CH3OH, NaCl, etc.

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Scheme 1. Expected eco-friendly GTP route (green) versus current fossil-based technologies (red). Abbreviations: see supplementary information.

Scheme 2. Overview of the approach for reviewing the topic of GTP catalysis.