Sara Navarro-Jaén

Mirella Virginie

Julien Bonin

Marc Robert
email: robert@u-paris.fr

Robert Wojcieszak

Andrei Y Khodakov
email: andrei.khodakov@univ-lille.frtoc

Highlights and challenges in the selective reduction of carbon dioxide to methanol

Keywords: CO2 reduction, methanol, catalysis, photocatalysis, electrocatalysis

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Atmospheric carbon dioxide (CO2) concentration has reached 1 417 ppm in May 2020. This gas is definitely the main greenhouse player, responsible for climate changes through transport, industry and household emissions. CO2 is mainly generated by the combustion of nonrenewable fossil fuels and by industrial activities such as iron, steel or cement production and petrochemical refining. Currently, there are two strategies to address the continuously increasing CO2 content in the biosphere: carbon capture and storage (CCS) [START_REF] D'alessandro | Carbon dioxide capture: Prospects for new materials[END_REF] and carbon capture and utilization (CCU) [START_REF] Aresta | Catalysis for the valorization of exhaust carbon: From CO2 to chemicals, materials, and fuels. technological use of CO2[END_REF] . CCS relies on the CO2 capture, including its separation, compression, transport, for permanent storage in a geological layer. However, such strategy relies on putting the dust under the carpet, while its technological and economic feasibility at larger scale has not been demonstrated. CCU involves either direct technological utilization of CO2 (in soft drinks or fire extinguishers for example) or its chemical and/or biological conversion into valueadded products and fuels [START_REF] Aresta | Catalysis for the valorization of exhaust carbon: From CO2 to chemicals, materials, and fuels. technological use of CO2[END_REF][START_REF] Goeppert | Recycling of carbon dioxide to methanol and derived products-closing the loop[END_REF][START_REF] Ordomsky | Challenges and Role of Catalysis in CO2 Conversion to Chemicals and Fuels[END_REF] .

The CO2 chemical conversion could contribute to lower consumption of hydrocarbon-based fuels (gasoline, diesel, jet fuels), which are currently manufactured from fossil feedstocks [START_REF] Centi | Opportunities and prospects in the chemical recycling of carbon dioxide to fuels[END_REF] and which are major sources of greenhouse gas emissions. The synthesis of fuels and chemicals from CO2 can also enhance carbon-and energy-efficiency of existing technologies and elaboration of new sustainable "green" industrial processes [START_REF] Centi | Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries[END_REF][START_REF] Artz | Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment[END_REF] . However, CO2 is a very stable molecule, having the most oxidized state of carbon. The linear geometry and the strength of the double C=O bond (ca. 750 kJ mol -1 ) require external energy for the CO2 activation and conversion, which can come from either energy-dense feedstocks (e.g. hydrogen, reduced nucleotides), thermal energy (heat), light irradiation or electricity. In recent years, solar energy and renewable electricity have gradually become more and more economically competitive compared to the conventional fossil-based energy sources [START_REF] Wang | CO2 Hydrogenation to Formate and Methanol as an Alternative to Photo-and Electrochemical CO2 Reduction[END_REF] .

To address the perspective of an on-going shift from fossil to renewable energy supplies, development of chemical catalytic processes for the CO2 reduction to methanol (CH3OH) that Among the various reduction products possibly obtained from CO2 reduction, methanol is a major target, having numerous applications in chemistry (Fig. 1). It can account alone for about 30% of the most widely used chemicals: dimethyl ether, dimethyl carbonate, formaldehyde, aromatics, ethylene, propylene, methyl tertiary butyl ether, methylamine, methyl methacrylate and acetic acid. Methanol is also used for the production of fatty acid methyl esters (FAME), which are the main components of biodiesel, and it can be used for direct blending of gasoline. Moreover, methanol is liquid at ambient temperatures and is thus easy to store and to handle.

Last but not least, it can be directly used to feed fuel cells.

Worldwide, the methanol production capacity is of ca. 110 million metric tons per year [START_REF]The methanol Industry[END_REF] .

Methanol production already involves utilization of 2 million tons of CO2 [START_REF] Alper | CO2 utilization: Developments in conversion processes[END_REF] . The CO2 reduction to CH3OH is often considered as the core of the "methanol economy" [START_REF] Goeppert | Recycling of carbon dioxide to methanol and derived products-closing the loop[END_REF] . Since 2012, a demonstration plant [START_REF]Carbon Recycling International (CRI)[END_REF] in Iceland is producing about 4000 tons of methanol/year from CO2 and renewable H2 over a heterogeneous Cu-ZnO catalyst, and is thus recycling 5500 tons of CO2/year. Other industrial facilities for the CO2 conversion to CH3OH are under development in Germany [START_REF]Methanol from CO2[END_REF] and China [START_REF] Norskov | Research needs towards sustainable production of fuels and chemicals[END_REF] .

In this review, we draw a comparative qualitative and quantitative analysis of the CO2 reduction routes to methanol via heterogeneous, homogeneous, enzymatic, photo-and electrocatalysis.

Analysis of current trends is mainly based on the literature published in the last 5 years.

Heterogeneous hydrogenation catalysis.

On an industrial scale, methanol is mostly produced from syngas, a mixture of CO, H2 and some traces of CO2, which is generated from fossil fuels (e.g. coal or natural gas). Therefore, the emerging environmental paradigm envisages anthropogenic CO2 and "green" H2 as ideal feedstock to produce "green" methanol in the near future [START_REF] Ali | Recent development in catalytic technologies for methanol synthesis from renewable sources: A critical review[END_REF][START_REF] Zhong | State of the art and perspectives in heterogeneous catalysis of CO2 hydrogenation to methanol[END_REF][START_REF] Prieto | Carbon Dioxide Hydrogenation into Higher Hydrocarbons and Oxygenates: Thermodynamic and Kinetic Bounds and Progress with Heterogeneous and Homogeneous Catalysis[END_REF][START_REF] Jiang | Recent Advances in Carbon Dioxide Hydrogenation to Methanol via Heterogeneous Catalysis[END_REF] : CO2 + 3H2 ↔ CH3OH + H2O ΔH298 K, 5 MPa = -40.9 kJ•mol -1 (Eq. 1)

However, the CO2 hydrogenation to methanol is more challenging than that of CO due to the thermodynamic and chemical stability of the CO2 molecule [START_REF] Zhong | State of the art and perspectives in heterogeneous catalysis of CO2 hydrogenation to methanol[END_REF] , whereas the competitive reverse water-gas shift (RWGS) reaction (Eq. 2) can also take place, in particular at higher reaction temperatures:

CO2 + H2 ↔ CO + H2O ΔH298 K, 5 MPa = 49.8 kJ•mol -1 (Eq. 2)
Taking these aspects into consideration, design of appropriate stable catalysts able to activate the CO2 molecule and to favour its conversion into CH3OH is essential.

Cu-based catalysts. Although numerous systems have been studied for the conversion of CO2 into methanol, major effort has been put on the Cu-based catalysts. Copper alone presents low activity for methanol synthesis, making the addition of promoters necessary to boost its performance [START_REF] Valant | The Cu-ZnO synergy in methanol synthesis from CO2, Part 1: Origin of active site explained by experimental studies and a sphere contact quantification model on Cu + ZnO mechanical mixtures[END_REF][START_REF] Dang | A review of research progress on heterogeneous catalysts for methanol synthesis from carbon dioxide hydrogenation[END_REF] . The traditional Cu-ZnO catalyst (50-70% mol CuO, 20-50% ZnO, 5-20% Al2O3, ZrO2, …) achieves conversion efficiencies of about 30% and methanol selectivity between 30-70% at 220-300°C and < 50 bar [START_REF] Ye | CO2 hydrogenation to high-value products via heterogeneous catalysis[END_REF] . Its low price, high activity and possibility to operate at mild temperature and pressure [START_REF] Bansode | Impact of K and Ba promoters on CO2 hydrogenation over Cu/Al2O3 catalysts at high pressure[END_REF][START_REF] Álvarez | Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes[END_REF] have attracted the attention of numerous research groups.

Despite tremendous efforts in understanding the catalyst structure and reactivity, the nature of the active site is still a matter of intense debate [START_REF] Jiang | Recent Advances in Carbon Dioxide Hydrogenation to Methanol via Heterogeneous Catalysis[END_REF][START_REF] Kattel | Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts[END_REF][START_REF] Mota | Highly active Cu/ZnO-Al catalyst for methanol synthesis: Effect of aging on its structure and activity[END_REF] . The active site is commonly thought to result from the intimate Cu-ZnO synergy, i.e. formation of a Cu-Zn alloy or presence of oxygen vacancies generated by means of a junction of both metals (Fig. 2a) [START_REF] Jiang | Recent Advances in Carbon Dioxide Hydrogenation to Methanol via Heterogeneous Catalysis[END_REF][START_REF] Valant | The Cu-ZnO synergy in methanol synthesis from CO2, Part 1: Origin of active site explained by experimental studies and a sphere contact quantification model on Cu + ZnO mechanical mixtures[END_REF][START_REF] Dang | A review of research progress on heterogeneous catalysts for methanol synthesis from carbon dioxide hydrogenation[END_REF][START_REF] Zabilskiy | The unique interplay between copper and zinc during catalytic carbon dioxide hydrogenation to methanol[END_REF] . Thus, improving the Cu-ZnO interaction and maximizing the Cu surface area, which can be achieved by carefully controlling the synthesis and activation conditions [START_REF] Álvarez | Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes[END_REF][START_REF] Kondrat | Preparation of a highly active ternary Cu-Zn-Al oxide methanol synthesis catalyst by supercritical CO2 anti-solvent precipitation[END_REF] are determinant factors to obtainthe enhanced catalytic performance. In this sense, co-precipitation of metal hydroxycarbonates produced via the reaction of the metal precursors and a basic precipitating agent is the most explored synthesis method [START_REF] Álvarez | Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes[END_REF] . Temperature, pH and aging time are crucial factors that determine the hydroxycarbonate phase. The formation of zincian malachite [(Cu1-xZnx)2(OH)2CO3, x ≤ 0.27] is generally associated to a greater Zn incorporation in the catalyst structure, which drives the enhanced catalytic behaviour [START_REF] Zwiener | Evolution of zincian malachite synthesis by low temperature coprecipitation and its catalytic impact on the methanol synthesis[END_REF] . Recently, it has been found that supercritical CO2 allows the formation of stable amorphous georgeite [Cu7(CO3)5(OH)4•5H2O], which can further improve the activity and stability of the Cu-ZnO system [START_REF] Kondrat | Stable amorphous georgeite as a precursor to a high-activity catalyst[END_REF] .

Al2O3 has been the most popular catalyst promoter for the methanol synthesis from syngas [START_REF] Dasireddy | The role of copper oxidation state in Cu/ZnO/Al2O3 catalysts in CO2 hydrogenation and methanol productivity[END_REF] , but its hydrophilicity seems to be detrimental for methanol synthesis from CO2. Production of large amounts of water indeed induces catalyst sintering and subsequent deactivation [START_REF] Behrens | Promoting the Synthesis of Methanol: Understanding the Requirements for an Industrial Catalyst for the Conversion of CO2[END_REF][START_REF] Guil-López | Methanol Synthesis from CO2: A Review of the Latest Developments in Heterogeneous Catalysis[END_REF] . ZrO2 has been widely explored due to its weaker hydrophilic character, its basicity, which favours the CO2 adsorption, enhanced activity and selectivity in the methanol synthesis from CO2 (Fig. 2b) [START_REF] Álvarez | Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes[END_REF][START_REF] Guil-López | Methanol Synthesis from CO2: A Review of the Latest Developments in Heterogeneous Catalysis[END_REF][START_REF] Wang | Exploring the ternary interactions in Cu-ZnO-ZrO2 catalysts for efficient CO2 hydrogenation to methanol[END_REF] . Another strategy aims at improving the Cu-ZnO/ZrO2 catalytic and mechanical properties by adding different mesoporous supports. For instance, addition of SBA-15 allows obtaining highly dispersed metal particles as well as confinement of the active phase inside the SBA-15 channels. Methanol space-time-yield of 376 mgCH3OH•h -1 •gcat -1 was obtained, significantly larger than the 10 mgCH3OH•h -1 •gcat -1 measured in the absence of support [START_REF] Mureddu | Highly efficient CuO/ZnO/ZrO2@SBA-15 nanocatalysts for methanol synthesis from the catalytic hydrogenation of CO2[END_REF] . Hydrotalcites enhance methanol selectivity to 78.3% at 250°C and 30 bar, in contrast to 68.6% reached with CuZnO/ZrO2 only, by virtue of the hydrotalcite CO2 adsorption capacity, which increases the CO2 concentration near the active sites [START_REF] Fang | Improved methanol yield and selectivity from CO2 hydrogenation using a novel Cu-ZnO-ZrO2 catalyst supported on Mg-Al layered double hydroxide (LDH)[END_REF] . A series of amorphous mesoporous aluminosilicates has been also studied, evincing the possibility of controlling specific area of metallic copper by adjusting the Si/Al ratio and increasing methanol production with the increase in the Al content [START_REF] Jiang | Tuning the highly dispersed metallic Cu species via manipulating Brønsted acid sites of mesoporous aluminosilicate support for CO2 hydrogenation reactions[END_REF] .

Noble metal and bimetallic catalysts.

Due to their well-known activity in the CO2 hydrogenation reactions, Pd catalysts have also been explored for methanol synthesis [START_REF] Álvarez | Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes[END_REF] . The selectivity is generally directed by the support and promoters [START_REF] Bahruji | Pd/ZnO catalysts for direct CO2 hydrogenation to methanol[END_REF] . The Pd/ZnO catalysts can convert 11% CO2 with 60% selectivity towards methanol at 250°C and 20 bar. The enhancement of the catalytic performance has been attributed to the formation of a PdZn alloy.

In this sense, CO2 is adsorbed on ZnO and H2 is dissociated on the surface of Pd, giving rise to the formation of surface formate species. If the alloy is not formed, formate species directly decompose instead of producing methanol, due to their low stability on Pd. However, the formation of PdZn is thought to passivate the surface and stabilize the formate intermediate, favouring methanol production. Methanol production is promoted by small particle sizes, whereas isolated Pd particles foster the competitive RWGS reaction [START_REF] Bahruji | Pd/ZnO catalysts for direct CO2 hydrogenation to methanol[END_REF] .

Ga2O3 is also a promising solid support. Upon association with Pd, it leads to the 20% CO2 conversion and 51.5% methanol selectivity. In this case, it is assumed that the active metal species for methanol production involve Pd n+ (0 < n < 2). The excellent catalytic activity explained by the presence of an optimal amount of Pd n+ stabilized by GaxOy on the surface of Pd [START_REF] Fujitani | Development of an active Ga2O3 supported palladium catalyst for the synthesis of methanol from carbon dioxide and hydrogen[END_REF] . However, the RWGS reaction occurs as well, giving rise to the undesired formation of CO. Addition of Ga2O3 as promoter to Pd/SiO2 led to a 500-fold increase in methanol production relative to the parent catalyst [START_REF] Bonivardi | Enhancement of the catalytic performance to methanol synthesis from CO2/H2 by gallium addition to palladium/silica catalysts[END_REF] .

Although scarcely studied, Au has also been considered as a potential catalytic material, and due to its relative inertness in the hydrogenation reactions, selection of the support is crucial.

As an example, Au/TiO2 and Au/CeOx/TiO2 have been compared, showing that lower CeOx coverages improve the selectivity towards CH3OH even at low pressure [START_REF] Yang | Low pressure CO2 hydrogenation to methanol over gold nanoparticles activated on a CeOx/TiO2 Interface[END_REF] .

Bimetallic systems have been widely investigated and exhibited enhanced catalytic properties.

For instance, PdCu is an interesting system considering the outstanding performance of both metals separately. The SiO2-supported PdCu catalysts with Pd/(Pd+Cu) ratios ranging from 0.25 to 0.34 convert from 5 to 7% of CO2 with a 20-30% methanol selectivity, whereas ca. 3% conversion is obtained with Pd or Cu alone. The bimetallic promoting effect is explained by the formation of PdCu and PdCu3 alloys, which may provide a source of chemisorbed hydrogen, thus favouring methanol formation [START_REF] Jiang | Bimetallic Pd-Cu catalysts for selective CO2 hydrogenation to methanol[END_REF] . The SiO2-supported InPd catalysts [START_REF] Snider | Revealing the Synergy between Oxide and Alloy Phases on the Performance of Bimetallic In-Pd Catalysts for CO2 Hydrogenation to Methanol[END_REF] have also shown promising results, with higher CO2 conversion and better methanol selectivity than the single metals, thanks to the formation of an InPd alloy and to a synergistic effect between indium oxide and bimetallic alloy. Such enhancement has also been observed with InNi catalysts, suggesting that these results can be expanded to other alloy/oxide systems [START_REF] Snider | Revealing the Synergy between Oxide and Alloy Phases on the Performance of Bimetallic In-Pd Catalysts for CO2 Hydrogenation to Methanol[END_REF] . The SiO2supported NiGa intermetallic catalysts are as well suitable for methanol production even at low pressure (10 bar), their performance being enhanced by doping with Au or Cu [START_REF] Duyar | Low-pressure methanol synthesis from CO2 over metal-promoted Ni-Ga intermetallic catalysts[END_REF] .

Metal oxide catalysts.

There has been a recent surge in the use of metal oxides as catalysts for methanol synthesis. For example, the ZnO-ZrO2 solid solution has emerged as a highly selective and stable catalyst, reaching more than 10% conversion and around 90% selectivity during 500 hours at 320°C and 50 bar [START_REF] Wang | A highly selective and stable ZnO-ZrO2 solid solution catalyst for CO2 hydrogenation to methanol[END_REF] . In a similar way, MaZrOx (Ma = Cd, Ga) solid solutions can achieve from 4.3 to 12.4% CO2 conversion and 80% selectivity, values that stand significantly higher than those obtained with the individual oxides [START_REF] Wang | High-Performance MaZrOx (Ma = Cd, Ga) Solid-Solution Catalysts for CO2 Hydrogenation to Methanol[END_REF] . In both cases, the results were attributed to the synergy between the catalytic components. The cubic phase of In2O3 has been investigated, but although the methanol selectivity reaches almost 100%, low conversion was achieved [START_REF] Dang | Rationally designed indium oxide catalysts for CO2 hydrogenation to methanol with high activity and selectivity[END_REF] . In order to overcome this problem, the promotion with other components is envisaged. For instance, Pd enhanced the In2O3 performance through its H2 splitting capacity [START_REF] Frei | Atomic-scale engineering of indium oxide promotion by palladium for methanol production via CO2 hydrogenation[END_REF] .

The results reported so far demonstrate that the Cu-based catalysts remain the catalytic family par excellence for the hydrogenation of CO2 into methanol. However, the oxidation and sintering of Cu species together with the low water tolerance constitute significant drawbacks in terms of activity and stability. Consequently, the research is still focused on the search for appropriate promoters and hydrophobic components able to improve the catalyst stability.

Noble metal, bimetallic and oxide catalysts have been studied to a lower extent, understanding of their characteristics and components interaction being wide paths to be explored.

On the other hand, the reaction mechanism is still a matter of debate. Although it is generally accepted that the catalytic formulations must be constituted by metals and/or oxides able to adsorb CO2 and dissociate H2, the identification of active sites, understanding of the interaction between components and dynamic behaviour of the participant species remain unclear and need to be clarified in order to pave the way to new catalytic formulations.

Regarding the industrial application of the heterogeneously catalysed methanol synthesis from CO2, some additional aspects need to be considered. A greater amount of hydrogen is required for the hydrogenation of CO2 with respect to that from syngas. Thus, for the process to be sustainable, hydrogen needs to be produced from renewable sources, such as, for example, from water electrolysis. On the other hand, the CO2 hydrogenation is thermodynamically limited. For instance, the methanol yield from CO2 at 200°C is about 50% lower with respect to that obtained from syngas. In order to overcome these problems, optimization of operating conditions and reactor design is mandatory [START_REF] Zhong | State of the art and perspectives in heterogeneous catalysis of CO2 hydrogenation to methanol[END_REF] . Furthermore, methanol synthesis from CO2 competes starting from 240°C, with the production of CO through the undesired RWGS reaction. Despite these necessary improvements, heterogeneous catalysis for methanol synthesis is already a relatively mature technology.

Molecular catalysis

Molecular-based catalysts, such as enzymes or molecular complexes, have noticeable advantages, such as fine tuning of the steric, electronic, acid-basic and electrostatic properties at the catalytically active site and in its close environment.

Enzymatic processes. Catalytic transformations using enzymes include a possibility to perform the reactions in mild conditions, with high efficiency and low energy consumption, taking advantage of the enzyme selectivity and with almost no deleterious impacts on the environment [START_REF] Shi | Enzymatic conversion of carbon dioxide[END_REF] . The CO2 reduction using enzymatic catalysis has been intensively studied in the last 5 years [START_REF] Kuk | Photoelectrochemical Reduction of Carbon Dioxide to Methanol through a Highly Efficient Enzyme Cascade[END_REF][START_REF] Oliveira | Toward the Mechanistic Understanding of Enzymatic CO2 Reduction[END_REF][START_REF] Katagiri | Double-Electron Reduced Diphenylviologen as a Coenzyme for Biocatalytic Building Carbon-Carbon Bonds from CO2 as a Carbon Feedstock[END_REF][START_REF] Zhang | Artificial Thylakoid for the Coordinated Photoenzymatic Reduction of Carbon Dioxide[END_REF][START_REF] Cai | Chloroplast-Inspired Artificial Photosynthetic Capsules for Efficient and Sustainable Enzymatic Hydrogenation[END_REF][START_REF] Ma | Enzyme Mediated Increase in Methanol Production from Photoelectrochemical Cells and CO2[END_REF][START_REF] Schwarz | Hydrogenation of CO2 at ambient pressure catalyzed by a highly active thermostable biocatalyst[END_REF] . Direct CO2-to-methanol transformation remains however a challenge. An alternative consists in developing multi-cascade enzymatic systems [START_REF] Zhang | Efficient ionic liquid-based platform for multi-enzymatic conversion of carbon dioxide to methanol[END_REF][START_REF] Singh | Insights into Cell-Free Conversion of CO2 to Chemicals by a Multienzyme Cascade Reaction[END_REF] . To date, such multienzymatic systems only showed a relatively low methanol productivity, with the yield of less than 1 mmol•L -1 after hours of reaction [START_REF] Singh | Insights into Cell-Free Conversion of CO2 to Chemicals by a Multienzyme Cascade Reaction[END_REF] . In addition, high concentration (up to 100 mmol•L -1 ) of cofactor is necessary, which makes such process quite costly.

Multi-enzymatic systems. Catalytic conversion of CO2 to methanol involves three dehydrogenases and presence of a cofactor (typically NADH, reduced nicotinamide adenine dinucleotide) [START_REF] Singh | Insights into Cell-Free Conversion of CO2 to Chemicals by a Multienzyme Cascade Reaction[END_REF] . The nature of cofactor strongly affects the selectivity of CO2 reduction [START_REF] Kuwabata | Electrochemical conversion of carbon dioxide to methanol with the assistance of formate dehydrogenase and methanol dehydrogenase as biocatalysts[END_REF] , i.e.

it can improve the rate of the reaction towards a target compound.

CO2 is firstly reduced into formate (Fig. 3a) by FateDH (formate dehydrogenase), followed by the two-electron reduction of formate to formaldehyde catalysed by the formaldehyde dehydrogenase enzyme (FaldDH). In the final step, methanol is produced by the alcohol dehydrogenase enzyme (ADH). The active sites of relevant enzymes are displayed in Fig. 3be. The overall methanol yield achieved in this three-step process is ca. 44% [START_REF] Zhang | Efficient ionic liquid-based platform for multi-enzymatic conversion of carbon dioxide to methanol[END_REF] . This relatively low conversion could be explained by the fact that because of unfavourable thermodynamics, the CO2 reduction to formic acid catalysed by FateDH is 30 times slower than the reverse oxidation of formate to CO2 [START_REF] Zhang | Efficient ionic liquid-based platform for multi-enzymatic conversion of carbon dioxide to methanol[END_REF][START_REF] Rusching | CO2 Reduction to Formate by NADH Catalysed by Formate Dehydrogenase from Pseudomonas oxalaticus[END_REF] . In other words, the formate formation is the bottleneck of the catalytic process. In order to increase the conversion to methanol, one may operate at higher CO2 concentration, which could be achieved by using ionic liquids compatible with the enzymes as a CO2 co-solvent [START_REF] Zhang | Efficient ionic liquid-based platform for multi-enzymatic conversion of carbon dioxide to methanol[END_REF] allows stabilizing proper conformation of the enzyme and also decreases the binding free energy of CO2 to the enzyme [START_REF] Zhang | Efficient ionic liquid-based platform for multi-enzymatic conversion of carbon dioxide to methanol[END_REF] . In addition, the yield of methanol strongly depends on the [CH][Glu] concentration. From 10 to 20% increase in the concentration shifts the reaction equilibrium towards higher methanol production due to the larger concentration of CO2 at the enzymatic catalytic site [START_REF] Zhang | Efficient ionic liquid-based platform for multi-enzymatic conversion of carbon dioxide to methanol[END_REF] . However, when the [CH][Glu] concentration increases from 20% to 60%, an opposite effect is observed, which may be explained by higher ionic concentrations (electrostatic charges) in the enzyme. Pressure is an additional mean to increase the CO2 concentration in the system. The methanol formation rate could be enhanced from 1.20 to 2.17 × 10 -3 μmol•min -1 , when CO2 pressure was raised from 0.2 to 0.5 MPa. Further increase to 1.0 MPa 63 has no beneficial effect on the reaction rate as NADH availability became the rate limiting factor of the process.

Multi-enzymatic processes are highly pH-sensitive [START_REF] Baskaya | Thermodynamic Feasibility of Enzymatic Reduction of Carbon Dioxide to Methanol[END_REF] . They usually require low pH values and elevated temperatures. These conditions are not compatible with the stability of most enzymes, thus leading to their denaturation and/or inactivation. A strategy to prevent such deactivation lies in stabilizing the enzyme, for example through immobilization. It was shown [START_REF] Obert | Enzymatic conversion of carbon dioxide to methanol: Enhanced methanol production in silica sol-gel matrices[END_REF] that the methanol productivity could be significantly enhanced by encapsulation of the three dehydrogenases in a porous silica sol-gel matrix. Both immobilization and confinement favourably affect the reaction thermodynamics. Different carriers and immobilization approaches were applied to create nature-mimicking microenvironments for enzymes, for example with alginate-silica composites [START_REF] Xu | Efficient Conversion of CO2 to Methanol Catalyzed by Three Dehydrogenases Co-encapsulated in an Alginate-Silica (ALG-SiO2 ) Hybrid Gel[END_REF] . Specific and suitable microenvironment is obtained in these cases, namely higher hydrophilicity, short diffusion distance, moderate rigidity and flexibility. These synergistically combined factors being suitable for the activity and the stability/reusability of the enzymes [START_REF] Xu | Efficient Conversion of CO2 to Methanol Catalyzed by Three Dehydrogenases Co-encapsulated in an Alginate-Silica (ALG-SiO2 ) Hybrid Gel[END_REF] . The green spheres represent Zn 2+ ions. [START_REF] Netto | Carbon dioxide/methanol conversion cycle based on cascade enzymatic reactions supported on superparamagnetic nanoparticles[END_REF] f) NADH cofactor photo-regeneration mechanism using semiconductor photocatalyst with simultaneous triethanolamine (TEOA) oxidation and NAD + reduction on the material surface (one-pot process). g) NADH cofactor photo-regeneration mechanism using semiconductor photocatalyst involving TEOA oxidation to glycolaldehyde (GA), an intermediate able to reduce NAD + after the light and photocatalyst removal (two-stage process). [START_REF] Kinastowska | Photocatalytic cofactor regeneration involving triethanolamine revisited: The critical role of glycolaldehyde[END_REF] .

Cofactor regeneration. As already mentioned, use of NADH as a cofactor for dehydrogenasecatalysed reduction steps, is a drawback for the economic feasibility of enzymatic processes [START_REF] Beller | CO2 fixation through hydrogenation by chemical or enzymatic methods[END_REF] .

For the synthesis of one mole of methanol, three moles of NADH are consumed. This obviously hampers large scale application of enzymatic CO2 conversion due to high cost of NADH [START_REF] Rusching | CO2 Reduction to Formate by NADH Catalysed by Formate Dehydrogenase from Pseudomonas oxalaticus[END_REF] . A way to circumvent this problem is to implement a regeneration/reuse strategy. In situ cofactor regeneration is viewed as an ideal solution, for example by using photochemically induced regeneration (Fig. 3 f,g). The methanol production from CO2 using four immobilized enzymes including FateDH, FaldDH, ADH and glutamate dehydrogenase (GDH) was recently reported. [START_REF] El-Zahab | Particle-tethered NADH for production of methanol from CO2 catalyzed by coimmobilized enzymes[END_REF] NADH was immobilized on a separate carrier and was regenerated by dehydrogenation of glutamate to 2-oxoglutarate. Even if the methanol yield was lower than in the case of the freely diffusing enzymes, the immobilized system showed largely improved stability and good recyclability. After seven cycles, more than 80% of original productivity was still maintained [START_REF] El-Zahab | Particle-tethered NADH for production of methanol from CO2 catalyzed by coimmobilized enzymes[END_REF][START_REF] Shi | Enzymatic conversion of carbon dioxide[END_REF] . This led to a cumulative methanol yield of 127% (based on the amount of cofactor). The choice of the NADH regeneration system also plays a crucial role in reaching high efficiency and stability of the cascade enzymatic system in the CO2 reduction to methanol.

For example, phosphate dehydrogenase (PTDH) was found to be the most efficient agent for the NADH regeneration, with the activity 55 times higher than that of the free multi-enzymes FateDH, FaldDH, ADH and PTDH system [START_REF] Cazelles | Reduction of CO2 to methanol by a polyenzymatic system encapsulated in phospholipids-silica nanocapsules[END_REF] .

Homogeneous hydrogenation catalysis.

The works reporting direct CO2 hydrogenation to methanol using homogeneous metal complexes are not numerous [START_REF] Schieweck | Structurally Versatile Ligand System for the Ruthenium Catalyzed One-Pot Hydrogenation of CO2 to Methanol[END_REF][START_REF] Kar | Advances in catalytic homogeneous hydrogenation of carbon dioxide to methanol[END_REF][START_REF] Kar | Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads[END_REF][START_REF] Liu | Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations[END_REF][START_REF] Sen | Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol[END_REF][START_REF] Chu | Acid-Assisted Hydrogenation of CO2 to Methanol in a Homogeneous Catalytic Cascade System[END_REF][START_REF] Schneidewind | Low-Temperature Hydrogenation of Carbon Dioxide to Methanol with a Homogeneous Cobalt Catalyst[END_REF][START_REF] Liu | Third-generation biorefineries as the means to produce fuels and chemicals from CO2[END_REF][START_REF] Li | Synergetic interaction between neighbouring platinum monomers in CO2 hydrogenation[END_REF][START_REF] Balaraman | Efficient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO2 and CO[END_REF] . Typically, a three-step cascade catalysis process is pursued: (i) formic acid formation from CO2 hydrogenation; (ii) functionalization of formate to an activated ester, such as alkyl formate or carbonate or carbamate; and finally (iii) CH3OH formation upon hydrogenation of the previous intermediate.

The advantage of this cascade approach is that the rate and selectivity of each individual step can be individually tuned. However there is usually incompatibility for the catalysts in terms of reaction rate and reaction conditions, which is a major difficulty to achieve direct, one pot hydrogenation of CO2 to methanol [START_REF] Chu | Acid-Assisted Hydrogenation of CO2 to Methanol in a Homogeneous Catalytic Cascade System[END_REF] . Conversion of CO2 to formic acid derivative is favoured in basic medium in the presence of amines and amino alcohols, while subsequent reduction to methanol is favoured under acidic conditions [START_REF] Kar | Advances in catalytic homogeneous hydrogenation of carbon dioxide to methanol[END_REF] . The first CO2-to-methanol reaction under basic conditions was successfully achieved 83 using a combination of dimethylamine and ruthenium catalyst and, so far, the best results in terms of activity were obtained using noble metal complexes and acidic conditions. Recently, a three-component catalyst made of a Ru catalyst, an Ir catalyst and a scandium salt (Sc(OTf)) was able to convert CO2 to methanol with 402 TON at 155°C in ethanol as a solvent under high hydrogen pressure (ca. 83 bar). species [START_REF] Kar | Manganese-Catalyzed Sequential Hydrogenation of CO2 to Methanol via Formamide[END_REF] . The nature of ligand plays an important role in the catalytic cycle, through both steric and electronic effects [START_REF] Schieweck | Structurally Versatile Ligand System for the Ruthenium Catalyzed One-Pot Hydrogenation of CO2 to Methanol[END_REF] . For example, tuning of the triphos ligand to cis,cis-1,3,5-tris-(diphenylphosphino)cyclohexane (tdppcy) gave high efficiency in direct methanol synthesis (Fig. 4c). Even if both ligands have comparable coordination geometries the tdppcy system is more rigid, which strongly alters the flexibility of the substituents on the phosphorus [START_REF] Liu | Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations[END_REF] . This rigidity allows a rotation along the P-C(cy)-axis, which at the same time reduces the conformational space and degrees of freedom of the phenyl groups. [START_REF] Liu | Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations[END_REF] The increase in the catalytic activity observed for tdppcy as compared to the triphos ligands (Fig. 4a) is then due to this specific rigidity of the P-substituents. [START_REF] Liu | Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations[END_REF] A highly active system with a TON close to 2000 using n((PPh3)4Ru(H)2) complex was obtained using Lewis acid aluminum tristriflate as a cocatalyst and ethanol as a solvent. The modulation of the substituent on the ligands allowed to adjust to the polarity of various alcoholic solvents. High TON (ca. 2100) could be obtained in mixture alcohols [START_REF] Schieweck | Structurally Versatile Ligand System for the Ruthenium Catalyzed One-Pot Hydrogenation of CO2 to Methanol[END_REF] .

Noble metal complexes (Ru

Non-noble metal complexes (Co, Mn, Fe).

A breakthrough in the direct methanol synthesis from CO2 using non noble homogenous catalysts was achieved with cobalt/triphos-based systems [START_REF] Schneidewind | Low-Temperature Hydrogenation of Carbon Dioxide to Methanol with a Homogeneous Cobalt Catalyst[END_REF] , which can catalyse the reaction at 100°C with good activity. A high turnover number (TON) of 125 could be obtained after 96 hours of reaction. An inner-sphere mechanism was proposed, involving catalysis by the complex formed after the acetylacetonate ligands removal [START_REF] Schneidewind | Low-Temperature Hydrogenation of Carbon Dioxide to Methanol with a Homogeneous Cobalt Catalyst[END_REF] . High temperature NMR studies showed that the initially formed cobalt-phosphine species were slowly transformed into various new catalytically active complexes [START_REF] Liu | Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations[END_REF] . The specific coordination of the triphos ligand is responsible for the remarkable reactivity of these catalysts, as it was already observed with the noble Ru complexes. However, novel versatile ligand structures are necessary for the development of more efficient homogeneous catalysts [START_REF] Schieweck | Structurally Versatile Ligand System for the Ruthenium Catalyzed One-Pot Hydrogenation of CO2 to Methanol[END_REF][START_REF] Liu | Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations[END_REF] . They should allow adapting the molecular catalyst for the multiphase and multistep reactions as well as permit the catalyst immobilization to facilitate the recovery and recycling. Recently, other transition metal (Mn [START_REF] Sen | Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol[END_REF][START_REF] Ribeiro | Carbon dioxide-tomethanol single-pot conversion using a C-scorpionate iron(ii) catalyst[END_REF] and Fe 86 ) complexes showed good activity as compared to the Co complexes described above. In both cases, formation of an intermediate (formamide and formaldehyde, respectively) was a key step of the reaction. In the case of a Mn-pincer complex (Fig. 4d), the first step is the N-formylation of the amine and then its hydrogenation to methanol. High yield of methanol could be obtained (up to 84% with respect to amine), but the reactions were performed at high pressure (60-80 bar of H2), high temperature (150°C) and the final TON was quite low (36) [START_REF] Ribeiro | Carbon dioxide-tomethanol single-pot conversion using a C-scorpionate iron(ii) catalyst[END_REF] . The challenge in the processes carried out on the transient metal complexes is to decrease the H2 pressure and temperature with the simultaneous TON increase. Lower temperature (80°C) was applied in the reaction carried out with C-scorpionate iron(II) catalyst [START_REF] Ribeiro | Carbon dioxide-tomethanol single-pot conversion using a C-scorpionate iron(ii) catalyst[END_REF] . A remarkable very high TON of 2283 (comparable to Ru complexes) was obtained with a TOF of 167 h -1 after 12 h of reaction. In this case, the reaction proceeds via hydride Fe complexes formation and CO2 insertion into Fe-H bond to form the intermediate Fe-OC(O)H complex [START_REF] Ribeiro | Carbon dioxide-tomethanol single-pot conversion using a C-scorpionate iron(ii) catalyst[END_REF] .

Photocatalysis. Photocatalysis refers to the acceleration of a light-induced chemical process in the presence of a catalyst. Water or molecular hydrogen usually are the reducing agents. They provide necessary protons, fuel electron transfers [START_REF] Yu | Design of core-shell titaniaheteropolyacid-metal nanocomposites for photocatalytic reduction of CO2 to CO at ambient temperature[END_REF] and are oxidised to molecular oxygen and water, respectively. The term "artificial photosynthesis" [START_REF] Dau | Artificial Photosynthesis: Beyond Mimicking Nature[END_REF][START_REF] Butburee | Beyond Artificial Photosynthesis: Prospects on Photobiorefinery[END_REF] is used when the photocatalysis simultaneously involves the CO2 reduction and water oxidation. Most commonly, photocatalytic reduction of CO2 with water takes place at ambient temperature and pressure, while the hydrogen relevant processes necessitate more severe conditions [START_REF] Yan | Polymorph selection towards photocatalytic gaseous CO2 hydrogenation[END_REF][START_REF] Jia | Heterogeneous catalytic hydrogenation of CO2 by metal oxides: Defect engineering-perfecting imperfection[END_REF][START_REF] Yan | Nickel@Siloxene catalytic nanosheets for high-performance CO2 methanation[END_REF][START_REF] Jia | Visible and Near-Infrared Photothermal Catalyzed Hydrogenation of Gaseous CO2 over Nanostructured Pd@Nb2O5[END_REF][START_REF] Wang | A highly selective and stable ZnO-ZrO2 solid solution catalyst for CO2 hydrogenation to methanol[END_REF][START_REF] Guan | Reduction of carbon dioxide with water under concentrated sunlight using photocatalyst combined with Fe-based catalyst[END_REF][START_REF] Wang | Photocatalytic Hydrogenation of Carbon Dioxide with High Selectivity to Methanol at Atmospheric Pressure[END_REF] and can be often considered as a combination of photocatalysis and thermo-catalysis. Regarding this review, we will focus on the CO2 photocatalytic reduction using water as a green and renewable reducing agent. Photocatalytic conversion of CO2 is strongly affected by both thermodynamics and kinetics in such a way, that several criteria must be fulfilled. First, the band gap energy must be higher that the variation of the overall free energy (Fig. 5a), taking into account the thereafter reactions (Eqs 3-6) involved in the CO2 reduction by water: 100

H2O → H2 + 1/2 O2 ΔG0 = 237 kJ•mol -1 (Eq. 3) CO2 → CO + ½ O2 ΔG0 = 514.2 kJ•mol -1 (Eq. 4) CO2 + 2 H2O → CH3OH + 3/2 O2 ΔG0 = 689 kJ•mol -1 (Eq. 5) CO2 + 2H2O → CH4+2O2 ΔG0 = 800 kJ•mol -1 (Eq. 6)
All of these reactions indeed correspond to the increase in the Gibbs free energy. Thus, without an external energy supply, the CO2 reduction as well as water oxidation are thermodynamically unfavourable at ambient temperature. Second, the energy position of the conduction band minimum (CBM) of the semiconductor must be higher than the energy required for the CO2 reductive reaction and most importantly higher than the energy levels of the reduction catalyst, whereas the position of valence band maximum (VBM) must be lower than the energy of the corresponding oxidative reactions (e.g. water oxidation (Fig. 5a)). 101 The apparent standard potentials vs. NHE at pH = 7 for CO2 reduction reactions into the C1 products (Eqs. 7-12), water splitting (Eq. 13) and proton reduction (Eq. 14) are outlined below [START_REF] Habisreutinger | Photocatalytic reduction of CO2 on TiO2 and other semiconductors[END_REF]102,103 :

CO2 + e -→ CO2 • - -1.90 V (Eq. 7)

CO2 + 2H + + 2e -→ HCOOH -0.61 V (Eq. 8)

CO2 + 2H + + 2e -→ CO + H2O -0.53 V (Eq. 9)

CO2 + 4H + + 4e -→ HCHO + H2O -0.48 V (Eq. 10)

CO2 + 6H + + 6e -→ CH3OH + H2O -0.38 V (Eq. 11)

CO2 + 8H + + 8e -→ CH4 + 2H2O -0.24 V (Eq. 12)

2H2O → O2 + 4H + + 4e - +0.81 V (Eq. 13)

2H + + 2e -→ H2 -0.42 V (Eq. 14)

The single electron transfer to CO2 is the most energy demanding process due to the need of bending the CO2 linear geometry (-1.9 eV, Eq. 7). The proton-assisted transfers of two electrons to CO2 with formation of CO or formic acid are more favourable (Eqs. 8, 9), but they involve multi-particle transfer and thus, may face kinetic limitations. Further reactions requiring the simultaneous transfer of 4 to 8 electrons and concomitant 4 to 8 protons (Eqs. 10-12) are even less probable in a single photochemical step, despite their favourably low potentials. The thermodynamic control of the selectivity of CO2 reduction to methanol is rather difficult, since several reactions may occur with rather similar potentials. Kinetic challenge for the reduction to methanol lies in the competition the reduction of protons (Eq. 14) and various CO2 reduction products such as CO 104 , formic acid, formaldehyde or methane. Separation of methanol from O2 simultaneously produced by water oxidation is an additional hurdle, the produced oxygen being able to initiate the reverse methanol oxidation reaction. Because of low methanol yield in most of photocatalytic processes, isotope labelling studies using 13 CO2 are mandatory to prove that methanol is indeed, produced from CO2. Any study not reporting such control experiment should be taken with extreme caution. The photocatalysts for the CO2 selective reduction to methane contain semiconductors and cocatalysts (Figure 5b).

Semiconductors.

The following semiconductors have been most commonly used: titanium dioxide (TiO2) 105 , bismuth vanadate (BiVO4) 106 , copper oxide (Cu2O) 107 , zinc oxide (ZnO) 108 , zinc sulfide (ZnS) 109 , hematite (α-Fe2O3) 110 , tungsten oxide (WO3) 111 , strontium titanate (SrTiO3) 112,113 , cadmium selenide (CdSe), gallium oxide (Ga2O3) 114 and zirconium dioxide (ZrO2) 115 , carbon nitride (C3N4) 116,117 . Narrow band gap semiconductors usually are preferred, since they can absorb visible light. However, the energy position of both CBM and VBM in these semiconductors may not allow simultaneous oxidation of water and reduction of CO2.

Thus, engineering of the semiconductor band structure is an extremely important task, 101 which can be achieved by doping [START_REF] Xie | Photocatalytic and photoelectrocatalytic reduction of CO2 using heterogeneous catalysts with controlled nanostructures[END_REF] , electronic promotion 118 , isomorphous substitution 119 , sensitization 120 , variation of crystallite size, morphology changes [START_REF] Yan | Polymorph selection towards photocatalytic gaseous CO2 hydrogenation[END_REF]121,122 and heterostructuring 123 . Recombination of the photoexcited electron-hole pairs must be retarded, for example using traps for holes or electrons 124 . The trapping of charge carriers can be achieved by doping with different elements or small semiconductor nanoparticles (e.g. quantum dots 125- 127 ). Heterojunctions 128 with semiconductors, metals and carbon-based materials also are efficient to promote charge separation.

Reduction cocatalysts. The selectivity of CO2 reduction is mainly affected by the active sites present provided by a co-catalyst, rather than by the semiconductor. If the latter contains copper 129-134 , zinc [START_REF] Wang | A highly selective and stable ZnO-ZrO2 solid solution catalyst for CO2 hydrogenation to methanol[END_REF]135,136 , copper-zinc 137-144 , silver 145,146 , bismuth 147 and indium 136,148 , methanol generally has been proposed to be the main reaction product. The presence of metals with strong hydrogenating and deoxygenating properties such as Pt, Ni 149 usually results in higher selectivity to methane. Additionally, the selectivity of CO2 reduction is also affected by the photoreactor characteristics, whose optimization can result in a more than ten-fold increase in the methanol yield 150 . Photoreactors can be divided into classes according to their operation mode (batch or flow reactors) or phase composition (gas-solid or gas-solid-liquid). Higher activity is usually observed in the gas-solid mode [START_REF] Xie | Photocatalytic and photoelectrocatalytic reduction of CO2 using heterogeneous catalysts with controlled nanostructures[END_REF]151,152 thanks to a better separation of charge carriers, a less prominent competition from the water reduction and an easier separation of the small amount of the reaction products. Gaseous phase CO2 reduction in the absence of sufficient concentration of water usually results in the production of CO and methane, while conducting the CO2 reduction in the water liquid phase is more favourable for methanol synthesis [START_REF] Habisreutinger | Photocatalytic reduction of CO2 on TiO2 and other semiconductors[END_REF] .

Electrocatalysis. The electroreduction reaction (CO2ERR) 153 proceeds under mild reaction conditions (no need for hydrogen feeding, ambient temperature and pressure) and possesses excellent compatibility with clean renewable energy sources (electricity generated by solar and wind can be used to activate the CO2). The reduction to methanol requires transfers of 6 electrons (Eq. 11) and as in other methods, it needs an active and selective catalytic material 154,155 .

Catalytic process is typically conducted in aqueous solutions containing inorganic salts (mostly constituted of bicarbonate 153,156,157 ) at a close-to-neutral pH or in alkaline solution 158 . The electrolyte can also be an organic solvent or ionic liquids, which show interesting FE in methanol production 159,160 . Generally speaking, such media are known for higher CO2 solubility and can stabilize charged CO2 intermediates, facilitating CO2 reduction 161,162 .

Heterogeneous electrocatalysts. A wide range of metal based electrocatalysts for the CO2ERR have been investigated. Copper based catalysts have been mostly used for converting CO2 with acceptable activity and efficiency into methanol 162,163,164,165 . As already mentioned for photocatalysis, few articles evidenced, upon using labelled [START_REF]Methanol from CO2[END_REF] CO2 experiment, that methanol really comes from CO2.

A series of PdxCuy aerogels bimetallic electrocatalysts 160 Recently, incorporation of metal in carbon structure has been explored 158,172,173 . High surface area and well-conducting carbon materials are capable of stabilizing high metal loading, promoting transfer of electrons on the catalyst without impairing the electrical conductivity.

Supported molecular electrocatalysts.

In such systems, the catalysts are transition-metal complexes immobilized in thin conductive films deposited at a conductive surface. They may benefit from both high selectivity at the catalytic site and good stability of catalytic materials. 179,181 . The latter is highly modular and can be attractive for decentralized operation [START_REF] Norskov | Research needs towards sustainable production of fuels and chemicals[END_REF] . Other routes such as homogeneous catalysis, photo-179 and enzymatic catalysis are still at the level of the experimental proof-of-concept reached at the laboratory scale (ca. TRL 2-3).

The TRL levels for the various CO2 reduction routes are closely linked to the methanol cost which is significantly higher, for all the discussed paths, than the current market price (Table 1). Heterogeneous CO2 hydrogenation catalysis 180-185 and CO2 electrocatalysis 186,187 provide methanol with a cost most closely approaching the market values. In the former, the methanol price is affected, up to 80-90%, by the price of sustainable hydrogen [START_REF] Ordomsky | Challenges and Role of Catalysis in CO2 Conversion to Chemicals and Fuels[END_REF]179,188 . To reduce the methanol cost in electrocatalysis, the catalyst stability must be significantly increased (up to several thousand hours), the same being true regarding both cathodic and anodic sides. 187 At the current scientific and TRL levels, methanol produced via photocatalysis has an extremely high cost (Table 1). To manufacture economically competitive methanol, a 15% solar-to-fuels efficiency is required 189 and the photocatalytic activity must be raised by several orders of magnitude (from ca. 0.02-0.5 to 10 mmol gcat -1 •h -1 ), which can only be envisioned in a longterm perspective. It seems difficult to fairly evaluate the methanol cost produced via homogeneous catalytic hydrogenation or enzymatic catalysis. The high price of enzymes and large quantities of cofactors limits the economic feasibility and scale up. Regeneration of metal complexes and their separation from methanol are major challenges for the cost reduction in homogeneous catalysis.

Conclusion.

Among the different routes for the CO2 reduction that have been discussed in this review, heterogeneous catalysis and direct electrocatalysis stand out of the pack.

Methanol synthesis from CO2 hydrogenation using heterogeneous catalysts has currently reached high scientific and technological maturity, while the CO2 electrocatalytic approach seems to be very promising as a mid-term perspective. It is important to emphasize that sustainable electricity is a key prerequisite in both approaches, especially in the former case, since it must employ "green" hydrogen produced by water electrolysis.

Because of thermodynamic limitations, the state-of the-art CO2 hydrogenation to methanol via heterogeneous catalysis requires high pressures (30-50 bar). In addition, water, which is a reaction by-product, inhibits the catalytic activity. The single pass methanol yield can be increased by selective water removal from the reactor using a combination of methanol synthesis and water adsorption/absorption or by using water selective membranes. Another target would be the development of new low-temperature catalysts operating at lower pressure, which could enable higher CO2 conversion, higher methanol selectivity and zero CO production. In the conventional CO2-to-methanol process, Cu-based catalysts remain the catalytic family par excellence. However, deactivation of copper catalysts constitutes an important drawback. Identification of active sites, understanding of the interaction between components and dynamic behaviour of the participant species need to be clarified. The cost and availability of sustainable hydrogen can also become an obstacle for implementation of larger scale CO2 hydrogenation processes.

In the electrocatalytic approach, future trends may focus on the design of nanostructured highperformance catalysts and molecular supported catalysts with well controlled micro-environment at the active site, so as to reach higher methanol selectivity. Following the strategy reminiscent of enzymatic catalysis, electrocatalysis could benefit from a sequential approach upon combining two successive reactions, first CO2 to CO and then CO to methanol, with optimization of each step, as it has been recently proposed 174 for a Co based molecular supported catalyst. Strategies to avoid poisoning and thus catalytic material degradation is of great importance. Currently, most CO2RR electrocatalysts were tested only over few hours (up to 50 h typically), while for industrial application, thousand hours per year are expected. New developments in cell design (gas-diffusion cells, liquid-flow cells, solid polymer electrolytes), electrode structures and catalyst robustness are still required to improve performances under various operating regimes.

Fig. 1 .

 1 Fig. 1. Carbon cycle of methanol, a platform molecule for chemical industry.

Fig. 2 .

 2 Fig. 2. (a) Transmission electron micrograph of a Cu-ZnO/Al2O3 catalyst during exposure to H2 at 300 °C, showing (1) arrangement of the catalyst nanoparticles and (2) exposed Cu nanoparticles surfaces with direct gas accessibility 34 . (b) Comparison of catalytic performance (CO2 conversion: red bars, CH3OH selectivity: green bars) for selected heterogeneous catalysts[START_REF] Li | Ga-Pd/Ga2O3 Catalysts: The Role of Gallia Polymorphs, Intermetallic Compounds, and Pretreatment Conditions on Selectivity and Stability in Different Reactions[END_REF][START_REF] Angelo | Study of CuZn MOx oxides (M = Al, Zr, Ce, CeZr) for the catalytic hydrogenation of CO2 into methanol[END_REF][START_REF] Bahruji | Pd/ZnO catalysts for direct CO2 hydrogenation to methanol[END_REF][START_REF] Dang | Rationally designed indium oxide catalysts for CO2 hydrogenation to methanol with high activity and selectivity[END_REF] .

Fig. 3 .

 3 Fig. 3. a) Reaction pathway in enzymatic cascade for methanol synthesis from CO2 using nicotinamide adenine dinucleotide hydrogenase (NADH), dependent formate dehydrogenase (FATEDH), formaldehyde dehydrogenase (FALDDH) and alcohol dehydrogenase (ADH) enzymes. 60 b) Representation of the active sites of the FALDDH. c) Representation of the active sites of the ADH, (d) Representation of the active sites of the FATEDH e) Representation of the active sites of the glutamate dehydrogenase (GDH). The green spheres represent Zn 2+ ions.[START_REF] Netto | Carbon dioxide/methanol conversion cycle based on cascade enzymatic reactions supported on superparamagnetic nanoparticles[END_REF] f) NADH cofactor photo-regeneration mechanism using semiconductor photocatalyst with simultaneous triethanolamine (TEOA) oxidation and NAD + reduction on the material surface (one-pot process). g) NADH cofactor photo-regeneration mechanism using semiconductor photocatalyst involving TEOA oxidation to glycolaldehyde (GA), an intermediate able to reduce NAD + after the light and photocatalyst removal (two-stage process).[START_REF] Kinastowska | Photocatalytic cofactor regeneration involving triethanolamine revisited: The critical role of glycolaldehyde[END_REF] .

  ). Typically, the catalytic reduction of CO2 to methanol requires addition of an alcohol or amine to promote transformation of CO2 to an intermediate species (alkyl formate, carbonate, urea). In a representative work, a highly active system including a Ru triphos (1,1,1-tris(diphenylphosphinomethyl)ethane) complex, [Ru(triphos)(TMM)] (TMM = trimethylenemethane), was shown to catalyze direct reduction of CO2 to methanol (Fig.4ac)[START_REF] Wesselbaum | Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium-Triphos catalyst: from mechanistic investigations to multiphase catalysis[END_REF] reaching a TON of 603 in the presence of 1 equivalent of bis(trifluoromethane)sulfonimide (HNTf2), under high CO2 pressure (60 bar) and ambient temperature[START_REF] Wang | CO2 Hydrogenation to Formate and Methanol as an Alternative to Photo-and Electrochemical CO2 Reduction[END_REF] . The initial TOF of 70 h -1 was comparable to the results obtained with the most efficient heterogeneous catalysts. Moreover, the experiments indicated that the formato complex, [Ru(triphos)(η 2 -O2CH)(solvent)] + was the active catalyst for this reaction, while ([Ru(triphos)(H)(CO)2] + , [Ru2(μ-H)2(triphos)2] and [Ru(triphos)(H)(CO)(Cl)]) were inactive. In the proposed mechanism, hydride migration from Ru to the formic acid ligand in [Ru(triphos)(H)(H2)(HCO2H)] + is a key step in the formation of a Ru hydroxymethanolate

Fig. 4 .

 4 Fig. 4. a) Molecular structure of the triphos active cation (H2O as a solvent) in the solid state as derived from single crystal X-ray diffraction (hydrogen atoms are omitted for clarity). Some selected bond lengths (A ): Ru-P1 = 2.245(9); Ru-P2 = 2.255(3); Ru-P3 = 2.253(0); Ru-O1 = 2.171(2); Ru-O2 = 2.208(6); Ru-O3 = 2.204(7) 84 , b) Basic catalytic cycle for the transformation of CO2 to methanol at the Ru-Triphos fragment via formic acid and formaldehyde. P3Ru denotes the Triphos-Ru(II) fragment comprising additional ligands to fill the coordination sphere, c) Comparison of defined precatalysts (triphos)Ru(tmm) and (tdppcy)Ru(tmm). Reaction conditions: nRu = 5 μmol, nAl = 5 μmol (1 equiv), VSolvent = 4 mL, p(H2/CO2) [bar/bar] = 90/30, T = 120°C, t = 20 h. [a] 8 equiv of Al(OTf)3 used 73 , d) Structure and reaction pathway for the manganese-catalysed sequential CO2 reduction to CH3OH. In the first step of the reaction, N-formylation of an amine occurs in a mixture of CO2:H2 (1:1) at a pressure of 60-70 bar in THF at 110°C. This intermediate formamide is then hydrogenated to methanol at high pressure of hydrogen (80 bar) and T = 150°C.

Fig. 5 .

 5 Fig. 5. Photocatalytic CO2 reduction: (a) conduction band, valence band potentials and band gap energies of various semiconductor photocatalysts of compounds involved in CO2 reduction relative to the redox potentials at pH 7 (adapted from 97-99 ) (b) simplified structure and main components of photocatalyst. Thermodynamics, mechanisms and kinetics of the CO2 photoreduction. Photocatalysis typically involves photon absorption, which promotes the electrons (e -) transfer from the valence band (VB) to the conduction band (CB), creating holes (h + ), followed by charge separation, transport and catalytic reactions. Holes participate in the oxidation of water, while electrons reduce CO2. The electron-hole recombination can drastically lower the amount of useful charge carriers reaching the interface.

Fig. 6 .

 6 Fig. 6. a) Total current density over Cu1.63Se(1/3) catalyst at different applied potentials. 162 b) Plots of particle size vs. FE of methanol on different Cu1.63Sex catalysts. Data were obtained at ambient temperature and pressure with CO2 stream of 10 sccm with 5 h electrolysis. 162 c) Partial current density for CO2 electroreduction catalysed by CoPc-NH2/CNT. Error bars represent one standard deviation from three measurements. 171 d) Faradaic efficiencies for CO2RR using CoPc-NH2/CNT as catalyst. 171

Table 1 )

 1 However, very few molecular catalysts are able to electroreduce CO2 to methanol, and in fact only two examples, both related to Co phthalocyanines, are available in the literature. is based on several characteristics: reducing agent, maximum selectivity, productivity, stability, temperature, pressure, technological maturity and methanol cost. Molecular hydrogen is generally used as a reducing agent in homogeneous or heterogeneous catalysis, whereas water is employed in photo-or electro-catalysis. Enzymatic catalysis requires a cofactor (usually NADH) for the CO2 reduction, which is expensive and must be regenerated for a subsequent cycle. Methanol selectivity above 90% can be reached in homogeneous, enzymatic, photocatalysis and 70% in heterogeneous catalysis and electrocatalytic processes. Higher selectivity is often either obtained at low CO2 conversion, for example in photocatalysis, or by using multi-step processes as in homogeneous and enzymatic approach. In heterogeneous catalysis, CO is the main by-product. The methanol selectivity can be further improved, for example by increasing the total pressure up to 331 bar.[START_REF] Prieto | Carbon Dioxide Hydrogenation into Higher Hydrocarbons and Oxygenates: Thermodynamic and Kinetic Bounds and Progress with Heterogeneous and Homogeneous Catalysis[END_REF] Productivity is another important figure of merit of any catalytic process and it can be expressed, depending on the process Surprisingly, the methanol productivity obtained in heterogeneous and homogeneous catalytic hydrogenation of CO2, enzymatic and electrocatalysis are of the same order of magnitude, with exception of photocatalysis, for which it is one to two orders of magnitude less productive. Catalyst stability is commonly considered the determining parameter for any chemical process and in particular, for the synthesis of relatively cheap commodities such as methanol. Regarding this criteria, heterogeneous catalysis stands at the forefront of the various methods. For example, stable operation for methanol synthesis from CO2 can surpass 3000 h 175 . In contrast, when conducting the reaction in a discontinuous mode (homogeneous or enzymatic catalysis), the catalyst stability is often limited to a few cycles or to a few tens of hours in the best cases for continuous operation in CO2 photo-and electrocatalysis. Moreover, the regeneration of deactivated homogeneous, electro-and photo-catalysts as well as enzymes is often difficult if not impossible.Both heterogeneous and homogeneous hydrogenation catalytic processes require the harshest conditions for methanol synthesis. Reaction typically occurs at 180-300°C and pressure up to 50-80 bar. Slightly lower temperature in homogeneous catalysis is due to the batch operation and longer CO2 residence time compared to heterogeneous conditions, for which a flow reactor is used. Quasi ambient temperature and moderate pressure are major advantages of the enzymatic, photo-and electro-catalysis.

	Recently, cobalt phthalocyanine (CoPc) supported on carbon nanotubes has showed remarkable
	activity and selectivity in CO2RR methanol 171,174 . In a first study, unsubstituted cobalt
	phthalocyanine (CoPc) was supported onto multi-wall carbon nanotubes (MWCNT) and Nafion
	resin 174 . A global Faradaic efficiency of 19.5% and chemical selectivity of 7.5% for CH3OH
	could be obtained in a sequential process converting first CO2 to CO, then using CO as a
	substrate to generate methanol in alkaline conditions (pH 13). The fact that the methanol
	originates from CO2 was carefully assessed by 1 H NMR. It was further shown that
	formaldehyde was also produced during electrolysis as a by-product. It is currently not known
	if this formaldehyde is an intermediate in the process or if on contrary, methanol comes from
	direct reduction of CO, with HCHO being only produced along a parallel pathway. Long term
	electrolysis led to deactivation of the catalyst through partial reduction of the ligand. In a
	subsequent study, amino groups were introduced at the phthalocyanine ligand (CoPc-NH2),
	and the catalysts were highly dispersed in the conductive ink containing carbon nanotubes and
	Nafion, leading to higher stability over time and larger currents 171 . In 0.1 M KHCO3 aqueous
	electrolyte at a potential of -1.00 V vs. RHE, methanol production could be sustained for 12 h
	with a FE of ca. 28 %, with corresponds to a partial current density of ca. 8.5 mA cm -2 (Fig. 6
	c, d) 171 . It was not demonstrated that the methanol is issued from CO2, but the results obtained
	in Ref 176 validate those obtained in this study. The mechanism proceeds via CO2 reduction to
	CO intermediate and then methanol production, as with the non-substituted Co
	phthalocyanine 174 .
	Comparative assessment of the CO2 reduction routes to methanol. Various routes have been
	explored for the selective CO2 reduction to methanol. Their overview and comparison (

(homogeneous, heterogeneous, photo-, electro-enzymatic catalysis), in various units (such mmol•gcat -1 •h -1 , TON or µmol•m -2 , etc.). In order to compare different CO2 reduction routes, we recalculated the productivities in a common unit, namely in the mol quantity of methanol produced per gram of catalyst per hour (Table

1

).

Acknowledgements S. N.-J., M. V., R. W. and A. Y. K. acknowledge financial support from the European Union ("Electron to Value-Added Chemicals (E2C)" Interreg 2 Seas Project). Partial financial support to M. R. from the Institut Universitaire de France (IUF) is gratefully acknowledged.

Author contributions

All authors contributed to writing, editing and reviewing the manuscript before submission.

Competing interests

The authors declare no competing interests.