Jerome Canivet

Florian M Wisser

Metal-Organic Framework Catalysts for Solar Fuels: Light-Driven Conversion of Carbon Dioxide into Formic Acid

The direct conversion of carbon dioxide into formic acid as renewable feedstock for chemicals or as fuel using sunlight as sole energy source would provide a crucial step towards a more sustainable industrialized society. While crucial advances have been made using molecular catalyst, their heterogenization within porous framework can provide additional features resulting in enhanced efficiency. Thanks to their high affinity towards carbon dioxide and high versatility in composition, Metal-Organic Frameworks (MOF) already offered appealing opportunities for the design of photocatalysts active in the light-driven carbon dioxide reduction into formic acid.

In this Spotlight, we look back on a decade of discovery and understanding in the light-driven MOF-catalyzed formic acid production form carbon dioxide leading to a 1000-fold enhancement in productivity and, moreover, opening perspectives for the design of novel porous hybrid photoactive platforms.

Introduction

The direct transformation of small molecules into more valuable synthetic intermediates is of paramount importance especially dealing with carbon dioxide (CO2) as a practical and costless C1 reservoir to increase the sustainability of chemical processes. [START_REF] Artz | Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment[END_REF] In our industrialized society, reducing both the carbon footprint of the human activity and the level of greenhouse gases in the atmosphere using photons as sole energy source appears as an highly desirable, impactful and environmentally friendly process. [START_REF] Vooradi | Energy and CO2 Management for Chemical and Related Industries: Issues, Opportunities and Challenges[END_REF] In this sense, converting carbon dioxide produced from industry and transportations into fuel or combustible using sunlight is a change in paradigm by transforming one of the main causes of climate change into a renewable resource towards a more sustainable society. [START_REF] Le | Light-Driven Reduction of Carbon Dioxide: Altering the Reaction Pathways and Designing Photocatalysts toward Value-Added and Renewable Fuels[END_REF] Indeed, tremendous achievements have been reached for the CO2 photoreduction towards CO, through an easily accessible 2 electrons reduction, and CH4, through a more challenging electrons reduction process. [START_REF] Ulmer | Fundamentals and Applications of Photocatalytic CO2 Methanation[END_REF] Formic acid (HCOOH) is another accessible 2 electrons reduction product from CO2 considered as a promising renewable feedstock for fine chemical syntheses. [START_REF] Mura | Formic Acid: A Promising Bio-Renewable Feedstock for Fine Chemicals[END_REF] Furthermore, the development of fuel cells based on formic acid as hydrogen storage molecule is emerging due to its easy transportability, high volumetric energy density and hydrogen content. 6- 8 Thus the direct and efficient photoreduction of carbon dioxide into formic acid will give access to sunlight conversion into chemical and electrical energy in a virtuous carbon cycle. [START_REF] Cauwenbergh | Photochemical Reduction of Carbon Dioxide to Formic Acid[END_REF] While finely tunable homogeneous catalysts already demonstrated the efficiency of this photoreduction process, the controlled heterogenization of well-defined single-site catalysts on various supports, such as carbon nitride, metal oxides or porous hybrids, is an appealing way to increase the sustainability of molecular catalytic processes by saving energy, with an easier catalyst isolation from the product, and precious resources, with an increased catalyst stability thanks to single-site isolation. [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF][START_REF] Copéret | Homogeneous and Heterogeneous Catalysis: Bridging the Gap through Surface Organometallic Chemistry[END_REF][START_REF] Copéret | Surface Organometallic Chemistry: Paving the Way Beyond Well-Defined Supported Organometallics and Single-Site Catalysis[END_REF][START_REF] Conley | State of the Art and Perspectives in the "Molecular Approach" Towards Well-Defined Heterogeneous Catalysts[END_REF][START_REF] Gao | Coordination Chemistry in the Design of Heterogeneous Photocatalysts[END_REF][START_REF] Perazio | Light-Driven Catalytic Conversion of CO2 with Heterogenized Molecular Catalysts Based on Fourth Period Transition Metals[END_REF][START_REF] Korzyński | Single Sites in Heterogeneous Catalysts: Separating Myth from Reality[END_REF][START_REF] Rana | Development of Heterogeneous Photocatalysts via the Covalent Grafting of Metal Complexes on Various Solid Supports[END_REF][START_REF] Morikawa | Solar-Driven CO2 Reduction Using a Semiconductor/Molecule Hybrid Photosystem: From Photocatalysts to a Monolithic Artificial Leaf[END_REF] Beyond the advantageous easy separation of solubilized product from the solid catalyst and the possible recycling of the later, the heterogenization of a single-site molecular complex within a porous host allows to control the catalytic activity by the design of the host framework. [START_REF] Jeoung | Pore Engineering of Metal-Organic Frameworks with Coordinating Functionalities[END_REF] Thanks to their hybrid organic-inorganic nature, Metal-Organic Frameworks (MOF), as platform for the heterogenization of single-site catalysts, allow to adjust structure-properties relationships from molecular design of the catalyst local environment towards the solid's surface interactions with both substrates and light. [START_REF] Luo | From Molecular Metal Complex to Metal-Organic Framework: The CO2 Reduction Photocatalysts with Clear and Tunable Structure[END_REF] Indeed, among other heterogeneous sytems, [START_REF] Bhowmik | Review of Graphitic Carbon Nitride and Its Composite Catalysts for Selective Reduction of CO2[END_REF][START_REF] Zhao | Progress in Catalyst Exploration for Heterogeneous CO2 Reduction and Utilization: A Critical Review[END_REF][START_REF] Pachaiappan | A Review of Recent Progress on Photocatalytic Carbon Dioxide Reduction into Sustainable Energy Products Using Carbon Nitride[END_REF][START_REF] Lu | Engineering Graphitic Carbon Nitride (g-C3N4) for Catalytic Reduction of CO2 to Fuels and Chemicals: Strategy and Mechanism[END_REF] MOF are highly promising carbon dioxide photoconversion candidates thanks to their tunable affinity with carbon dioxide and high storage capacity, their high versatility in composition as photoactive solid from either their organic or inorganic building units, their possible post-synthetic modification for introducing additional light harvesting unit and/or catalytic sites, and finally thanks to their ordered network allowing the structuration of photosystems components to get long lasting excited states. [START_REF] Dhakshinamoorthy | Metal-Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production[END_REF][START_REF] Li | Metal-Organic-Framework-Based Catalysts for Photoreduction of CO2[END_REF][START_REF] Ezugwu | Engineering Metal-Organic Frameworks for Efficient Photocatalytic Conversion of CO2 into Solar Fuels[END_REF] Here we aim at reviewing a decade of discovery and advances in the ever-growing field of MOF photocatalysis while shining light towards formic acid production as sustainable energy vector, with a 1000-fold improvement in productivity (Table 1) since the first report on this topic. The increased sophistication in the design of multifunctional MOF frameworks and the subsequent structure-activity relationships open also perspectives for the development of other porous solid hybrid photocatalysts like porous organic polymers (POP) and metal-organic polyhedra (MOP).

In this article, the product of carbon dioxide reduction reaction is referred to as formic acid, as at least under the standard conditions used in most of the literature studies which are mixtures of acetonitrile and triethanolamine (Table 1), this amine is not basic enough to deprotonate formic acid, which may however not be the case under other conditions. Additionally, catalytic activities are always compared as the catalyst productivity 𝑅𝑅 defined as 𝑅𝑅 = 𝑛𝑛 formic acid 𝑚𝑚 solid •𝑡𝑡 , with 𝑛𝑛 formic acid the amount of formic acid produced in mmol, 𝑚𝑚 solid the total amount of solid catalyst used in g and 𝑡𝑡 the reaction time in h. We used this nomenclature, as for some catalytic systems the precise nature of the active site is still under debate, avoiding the use of turnover frequencies or turnover numbers to compare catalysts. Indeed the comparison of the catalytic activity per gram of solid material has been stated to allow for direct comparison between different systems and reports. [START_REF] Luo | Efficient Photocatalytic Reduction of CO2 Catalyzed by the Metal-Organic Framework MFM-300(Ga)[END_REF] However, as for homogeneous catalysis, [START_REF] Martín-Sómer | Influence of Light Distribution on the Performance of Photocatalytic Reactors: LED vs Mercury Lamps[END_REF][START_REF] Marx | Addressing the Reproducibility of Photocatalytic Carbon Dioxide Reduction[END_REF] often precise information about the reactor design and the light source (position from the reactor, constant radiant flux or irradiance) are not reported, making the direct comparison of turnover numbers / frequencies or quantum efficiency, but also productivities between different research laboratories, challenging. Also, the reproducibility of photocatalytic experiments remains scarce, adding another challenge for reproducing and comparison of data.

MOF photocatalysts with pure organic linkers

In the following examples, the MOF described are themselves light sensitive and photoactive for the reduction of carbon dioxide without use of any external photosensitizer or catalyst, mainly thanks to light harvesting amino-functionalized linkers and catalytic metal-oxo nodes.

(Ti/Zr/Fe) amino-MOF

In the case of amino-functionalized linkers, the strategy takes advantage of the light harvesting properties of the amino-functionalized linker and ability for transfer charges towards the inorganic node acting as catalyst for the CO2 transformation.

Among the first example of MOF-catalyzed CO2 photoreduction towards HCOOH, Li and coworkers reported the use of Ti-MIL-125-NH2 as efficient photocatalyst (Figure 1). [START_REF] Fu | An Amine-Functionalized Titanium Metal-Organic Framework Photocatalyst with Visible-Light-Induced Activity for CO 2 Reduction[END_REF] This Tibased MOF is made from titanium oxo-cluster linked by 2-amioterephthalate linkers, formulated as Ti8O8(OH)4(bdc-NH2)6 (bdc-NH2 = 2-amino-benzene-1,4-dicarboxylate). Being highly porous towards nitrogen with an apparent surface area of 1300 m 2 /g, Ti-MIL-125-NH2 show superior CO2 adsorption uptake with 132 cm 3 /g at 273 K compared to its non-functionalized parent Ti-MIL-125 with an uptake of 99 cm 3 /g. More importantly, the absorbance is shifted into the visible region, the absorption edge is around 550 nm. The strong absorbance in the visible part of the spectrum has been attributed to an O to Ti ligand to metal charge transfer (LMCT) in the TiO5(OH) cluster. For the photocatalytic reduction of CO2, the Ti-MIL-125-NH2 was suspended into a CO2-saturated acetonitrile solution using triethanolamine (TEOA) as electron donor in a 5:1 volume by volume ratio and irradiated using a Xe lamp equipped with filters to produce light in the range 420-800 nm. Under these conditions the productivity reached 0.016 mmolHCOOH/gcat/h and isotope labelling experiment using [START_REF] Conley | State of the Art and Perspectives in the "Molecular Approach" Towards Well-Defined Heterogeneous Catalysts[END_REF] CO2 confirmed that CO2 was the unique source of HCOOH. As for other TiO2 related photocatalyst, [START_REF] Dan-Hardi | A New Photoactive Crystalline Highly Porous Titanium(IV) Dicarboxylate[END_REF][START_REF] Kameneva | New Photoactive Hybrid Organic-Inorganic Materials Based on Titanium-Oxo-PHEMA Nanocomposites Exhibiting Mixed Valence Properties[END_REF] a photochromic behavior (color transition from yellow to green) was observed upon light excitation and in the presence of an electron donor and under exclusion of O2 and CO2. Combining UV Vis and electron spin resonance (ESR) spectroscopy, the authors attributed the color change to intervalence electron transfer from Ti(III) to Ti(IV) sites in the titanium oxo-clusters. The Ti(III) sites are supposed to be generated by a long-lived excited charge separation state also occurs by transferring a photo excited electron from bdc-NH2 linker to Ti(IV)

site. The linker is reductively quenched by electron transfer from TEOA, while the Ti(III) sites are stable under N2 atmosphere (Figure 1). Following a similar methodology, the same group studied Zr-based MOF analogue as photocatalyst candidates for CO2 photoreduction. [START_REF] Sun | Studies on Photocatalytic CO2 Reduction over NH2-Uio-66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal-Organic Frameworks[END_REF] They reported in this study the UiO-66-NH2 composed of hexameric [Zr6O4(OH)4] 12+ units linked by bdc-NH2 and formulated as Zr6O4(OH)4(bdc-NH2)6. Similar to previous reports, the substitution of the organic terephthalate linker in UiO-66 by bdc-NH2 results in the visible-light-responsive UiO-66-NH2. The photocatalytic CO2 reduction was performed under the exact same conditions the productivity reached 0.026 mmolHCOOH/gcat/h. Combining photoluminescence (PL) and ESR spectroscopy the authors speculated that a similar mechanism than in Ti-MOF occurred, involving bdc-NH2 as photosensitizer and charge transfer to potential Zr 3+ /Zr 4+ at the MOF node. However, the precise mechanism and in particular the occurrence of a LMCT and the role of the node or leached Zr species as catalytic active centers is still under debate (see below). [START_REF] Bhattacharya | The Role of Leached Zr in the Photocatalytic Reduction of CO2 to Formate by Derivatives of UiO-66 Metal Organic Frameworks[END_REF][START_REF] Benseghir | Unveiling the Mechanism of the Photocatalytic Reduction of CO2 to Formate Promoted by Porphyrinic Zr-Based Metal-Organic Frameworks[END_REF] Furthermore, the authors took advantage of the high stability and high versatility of UiO MOF to partly substituted bdc-NH2

in UiO-66-NH2 by 2,5-diaminoterephthalic acid (bdc-(NH2)2) to obtain an aminated UiO-type MOF with mixed bdc-NH2 and bdc-(NH2)2 linkers. The mixed-linker aminated UiO-66 presented similar textural properties than its analogues with a slightly higher CO2 uptake. Diffuse-reflectance UV/Vis spectroscopy showed also a stronger absorption in the visible region than for pure monoaminated UiO-66-NH2. Under the same catalytic conditions, the amount of HCOOH formed reached about 20.7 µmol in 10 h (0.041 mmolHCOOH/gcat/h), more than 50% higher than over the mono-aminated UiO-66-NH2. The improved photocatalytic performance was attributed to the enhanced light absorption and CO2 adsorption resulting from the tuning of MOF's linker properties thanks to additional functional group, here the second amino group. [START_REF] Hendon | Engineering the Optical Response of the Titanium-MIL-125 Metal-Organic Framework through Ligand Functionalization[END_REF] Another variation around diamino-functionalized UiO-type MOF was more recently reported by Xing and coworkers. [START_REF] Sun | Enhancement of Visible-Light-Driven CO2 Reduction Performance Using an Amine-Functionalized Zirconium Metal-Organic Framework[END_REF] They combined amino-functionalized conjugated linker (2,2′-diamino-4,4′-stilbenedicarboxylic acid, H2SDCA-NH2) with ZrCl4 to prepare a novel UiO-type MOF named as Zr-SDCA-NH2 (Figure 2). Being isoreticular to UiO-type MOF with the fcu topology, the Zr-SDCA-NH2 has the same framework structure as UBMOF-8 [START_REF] Naeem | Mixed-Linker Approach in Designing Porous Zirconium-Based Metal-Organic Frameworks with High Hydrogen Storage Capacity[END_REF] with an apparent surface area of 2550 m 2 /g. Notably,

Zr-SDCA-NH2 presents a strong absorption band between 400-600 nm. The Zr-SDCA-NH2 photocatalyst allowed to reach the production of 18.8 μmol of HCOOH after 12 hours of continuous visible light irradiation, corresponding to a productivity of 0.096 mmolHCOOH/gcat/h.

Importantly, the activity of the H2SDCA-NH2 linker alone in solution reached 19.6 µmol in 12 h, 29 mg linker (0.097 mmol; 16.8 µmol/h/mmol linker) when aminoterephthalic acid was found to be inactive in CO2 photoreduction under the same conditions. According to UV-Vis and PL studies, two parallel pathways were postulated to take place within the MOF photocatalyst. As found for other aminated MOF, the SDCA-NH2 can act as light harvesting unit which transfer charges to Zr6O4(OH)4(COOR)12 cluster though ligand-to-metal-charge-transfer (LMCT) process, the CO2 transformation taking place at Zr-O sites, but the SDCA-NH2 linker could also directly react with CO2 to generate HCOOH upon excitation. However, the precise mechanism of all Zr-based catalysts is still under debate and the role of leached species requires careful consideration. Saouma and co-workers observed for series of UiO-66 type catalysts with varying functionalization of the linker or particle size that an important parameter to be considered is the amount of Zr leached during the photocatalysis under basic conditions. In their study they considered TEOA or diethylamine (DEA) as sacrificial electron donor. [START_REF] Bhattacharya | The Role of Leached Zr in the Photocatalytic Reduction of CO2 to Formate by Derivatives of UiO-66 Metal Organic Frameworks[END_REF] To stimulate the leaching that may occur under photocatalytic conditions, they suspended the UiO-66-NH2 catalysts in an ACN/electron donor suspension for 24h under N2 atmosphere.

Afterwards, the particles were separated from the supernatant and redispersed in a fresh solution of electron donor in ACN. Both, the supernatant (~0.1 to 0.3 % of Zr leached) and the pre-treated MOF catalyst were then used in photochemical CO2 reduction reaction. The authors observed a much higher HCOOH productivity over the leached Zr species than over the MOF, indicating that the leached species are catalytically active. As during catalysis with the MOF, Zr leaching still occurred, a clear statement on the catalytic activity of the nodes in the MOF could not be given.

Separation of the supernatant and the spent MOF catalyst showed that also the species leached during catalysis are catalytically active in a second cycle. The precise nature of the leached Zr species is still under investigation. Similarly, the amount of HCOOH produced increased with increasing amount of leached Zr species within a series of UiO-66-NH2 MOFs. Note that, when the production of HCOOH is normalized to the mass of MOF used and the reaction time, very similar apparent activities were observed as for other studies using the same MOF (Table 1). [START_REF] Sun | Studies on Photocatalytic CO2 Reduction over NH2-Uio-66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal-Organic Frameworks[END_REF] However, when also other parameters such as the functionalization of the bdc-linker were changed, no direct correlation between leached amount of Zr and catalytic activity could be given. Another important observation made by Saouma and co-workers is that for DEA as sacrificial electron donor a constant and stable evolution of HCOOH over the course of the reaction was observed. In contrast, for TEOA as sacrificial electron donor the catalytic activity levels off after a few hours, in line with other reports in literature. Thus, for most UiO-66 derivatives investigated, the use of DEA as sacrificial electron donor yields higher HCOOH concentrations.

Extending the series of amino-MOF, Li and coworkers studied iron-based MOF series with different topologies, namely Fe-MIL-101-NH2, Fe-MIL-53-NH2 and Fe-MIL-88B-NH2. [START_REF] Wang | Fe-Based MOFs for Photocatalytic CO2 Reduction: Role of Coordination Unsaturated Sites and Dual Excitation Pathways[END_REF] reaching productivity of 0.45 mmolHCOOH/gcat/h in acetonitrile/TEAO mixture using a 300 W Xe lamp (400-800 nm). A reported 3-fold productivity enhancement from Fe-MIL-101 to aminated analogue was attributed to a postulated synergy between two photocatalytic pathways (Figure 3).

Indeed, in addition to the direct excitation of FeO cluster, a second pathway, similar to other amino-MOF photocatalysts, considered the excitation of the amino linker followed by charge transfer from the excited linker to the metal node. , 4254-4260. [START_REF] Wang | Fe-Based MOFs for Photocatalytic CO2 Reduction: Role of Coordination Unsaturated Sites and Dual Excitation Pathways[END_REF] Copyright 2014 American Chemical Society.

Doping amino-MOF with transition metals

Li and coworkers also demonstrated that doping Ti-MIL-125-NH2, using either [PtH2Cl6] • 6 H2O

or [AuHCl4] • 4 H2O as precursor, affect the catalytic activity of the MOF in the photocatalytic CO2 reduction. [START_REF] Sun | Noble Metals Can Have Different Effects on Photocatalysis Over Metal-Organic Frameworks (MOFs): A Case Study on M/NH2-MIL-125(Ti) (M=Pt and Au)[END_REF] After impregnation and reduction, the Pt and Au were found in the MOF both as isolated atoms, clusters and small nanoparticles of 30 Å and 25 Å, respectively, with however an ill-defined location. Under the same conditions than those used for Ti-MIL-125-NH2, Au doping in Au/Ti-MIL-125-NH2 induced a slight decrease in HCOOH acid production as well as a significant production of H2, not reported for Ti-MIL-125-NH2. In contrast, Pt/Ti-MIL-125-NH2

shows an important production of H2 of 0.58 mmolH2/gcat/h and a slight increase in HCOOH production to 0.032 mmolHCOOH/gcat/h (compared to 0.025 mmolHCOOH/gcat/h using pure Ti-MIL-125-NH2). This increase in activity for both water and CO2 visible light-driven reduction was attributed to hydrogen spillover from Pt particles to bridging oxygen linking Ti sites, thus favoring the formation of Ti 3+ active species.

Wang and coworkers also studied the effect of amino-MOF doping, UiO-66-NH2, by impregnation with Ir salt inside defective MOF. [START_REF] Hao | Metal-Organic Framework Membranes with Single-Atomic Centers for Photocatalytic CO2 and O2 Reduction[END_REF] The design of defective UiO MOF, named A-UiO-66-NH2, is based on the missing linkers approach, leading to accessible hydroxyls site on the Zr6O4(OH)4(COOR)12 node to which Ir atoms are grafted using K2IrCl6 precursor. Iridium isolated atoms (Ir1) were found highly dispersed in the UiO-66-NH2 after annealing with loadings of 0.7 wt%. Extended X-ray absorption fine structure (EXAFS) analysis evidenced Ir-O bonds with an average coordination number of 3.8 and the absence of Ir-Ir bonds confirming the isolated characters of the Ir atoms. For higher loadings, (ca. 1.4 to 2.7wt%) the formation of small Ir clusters was also reported. The Ir1/A-UiO-66-NH2 served as the photocatalyst for the CO2 reduction. The authors followed the same methodology using Pd atoms for O2 reduction reaction. In a water/isopropanol mixture under irradiation of a 300W Xe lamp equipped with a 420 nm longpass filter, the Ir1/A-UiO-66-NH2 produced HCOOH with a rate of 0.24 mmolHCOOH/gcat/h with a high selectivity of 96.3%, with H2 being the only by-product. Increasing the iridium loading to 1.4 wt% boosted the HCOOH productivity to 0.51 mmolHCOOH/gcat/h with a near full selectivity. The difference in activity was attributed to difference in electronic structure between isolated atoms and clusters. As revealed by X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) analysis, Ir atoms were more positively charged than Ir clusters and particles. A two-steps mechanism for the production of HCOOH was postulated involving CO2 adsorption at the MOF's surface, generating carbonate species, followed by hydrogenation at Ir site using photogenerated electrons provided by the excited amino-MOF and protons from water.

The authors also showed a similar trend for Ir single atoms on Ti-MIL-125-NH2, with an increase from 0.013 mmolHCOOH/gcat/h (same range than Li and coworkers [START_REF] Fu | An Amine-Functionalized Titanium Metal-Organic Framework Photocatalyst with Visible-Light-Induced Activity for CO 2 Reduction[END_REF][START_REF] Sun | Noble Metals Can Have Different Effects on Photocatalysis Over Metal-Organic Frameworks (MOFs): A Case Study on M/NH2-MIL-125(Ti) (M=Pt and Au)[END_REF] ) to 0.27 mmolHCOOH/gcat/h for pristine Ti-MIL-125-NH2 and Ir-doped Ti-MIL-125-NH2, respectively. Finally, the Ir1/A-UiO-66-NH2 particles were deposited as a membrane onto commercially available porous polytetrafluoroethylene (PTFE) films following a layer-by-layer protocol. [START_REF] Hao | Metal-Organic Framework Membranes with Single-Atomic Centers for Photocatalytic CO2 and O2 Reduction[END_REF] By shaping the MOF catalyst as a membrane, they showed that CO2 adsorption was facilitated and the contact between the gaseous CO2 and the catalytic sites was enhanced. Importantly the use of a gaseous feeds allowed to use only water vapor as proton and electron donor, thus avoiding the need of any sacrificial electron donor. Indeed Ir1/A-UiO-66-NH2 membrane was found to catalyze the CO2to-HCOOH photoreduction with productivity of 3.38 mmolHCOOH/gcat/h, largely surpassing the activity of the analogous powder catalyst. However, the crucial effect of the MOF support nature, including the topology and node composition, remained elusive.

More recently, Serre and coworkers incorporate ultrasmall Cu nanocrystals (Cu NCs, diameter 1.6 nm) into two Zr-MOFs, MOF-801 (also called Zr-fumarate) and UiO-66-NH2 (Figure 4), [START_REF] Dai | Ultrasmall Copper Nanoclusters in Zirconium Metal-Organic Frameworks for the Photoreduction of CO2[END_REF] their strategy is based on growing the MOF shell around pre-synthesized Cu NCs. The Cu NCs@UiO-66-NH2 composite showed similar crystallinity and porosity as the pristine UiO-66-NH2, with a loading of 2.85 at% of Cu as determined by ICP-MS. TEM analysis of the Cu NCs@MOF-801 and Cu NCs@UiO-66-NH2 composites show a decent distribution of ultrasmall Cu nanocrystals within the MOF. The CO2 photoreduction experiments were carried out using TEOA as a sacrificial electron donor under UV irradiation (385 nm) at room temperature for 18 h.

The Cu NCs@MOF-801 was found to produce mainly CO (22.5%) and HCOOH (64.9%) with rates of 0.032 mmolCO/gcat/h and 0.094 mmolHCOOH/gcat/h, respectively. In contrast, Cu NCs@UiO-66-NH2 showed an enhanced reactivity for HCOOH production with a rate of 0.128 mmolHCOOH/gcat/h. This enhancement was attributed to a higher affinity of UiO-66-NH2 for CO2 as evidenced by CO2 sorption isotherm measurements. Other by-products (methanol, CO and formaldehyde) were observed in low amounts, their production being comparable for both catalysts. Ga-MFM-300. Very recently, Schröder, Yang and coworkers reported a Ga-based MOF photocatalyst. [START_REF] Luo | Efficient Photocatalytic Reduction of CO2 Catalyzed by the Metal-Organic Framework MFM-300(Ga)[END_REF] Ga-MFM-300 was made of [GaO4(OH)2]n chains linked by biphenyl-3,3′,5,5′tetracarboxylates and showed an apparent surface area of 1060 m 2 /g and a CO2 uptake of 5 mmol/g at 298 K and 1 bar. In a CO2-saturated acetonitrile solution containing triethanolamine under irradiation at 350-780 nm the photoreduction of CO2 over Ga-MFM-300 produced HCOOH with a productivity of 0.5 mmolHCOOH/gcat/h and excellent selectivity and stability. The activity of Ga-MFM-300 was attributed to the ability of bridging OH sites within the pore to bind and activate CO2 followed by the generation of CO2 •-radical anions intermediate as postulated based on electron paramagnetic resonance spectra of trapped radical intermediates (Figure 5).

MOF with photocatalytic metalolinker

In this section, the MOF were synthesized using metalolinker to be all-in-one photocatalyst where the inorganic nodes acted mostly as structuring agent. The metalolinker being a well-defined molecular photocatalyst, the heterogenous MOF-catalyzed carbon dioxide photoreduction proceeded without any external photosensitizer.

Iridium and ruthenium polypyridyl linkers

Luo and coworkers reported an Y-based coordination polymer (CP) using Ir(ppy)2(dcbpy) (ppy :

2-phenylpyridine, dcbpy : 2,2′-bipyridine-4,4′-dicarboxylate as metalolinker. [START_REF] Li | Effective Visible-Light Driven CO2 Photoreduction via a Promising Bifunctional Iridium Coordination Polymer[END_REF] The Ir-CP solid was made by chains of hydroxy-bridged Y(III) linked by two Ir(ppy)2(dcbpy) metalolinkers. The neighboring chains packed with each other through π-π interactions to form a three-dimensional supramolecular framework. The Ir-CP was not found porous but its light adsorption range was found to be the same than that of the isolated Ir unit. In an acetonitrile-triethanolamine mixture under visible-light irradiation (Xe lamp with filter) the Ir-CP was found to produce HCOOH with a rate up to 0.119 mmolHCOOH/gcat/h. The Ir-CP was reported to be recyclable for up to five runs with however a slight decrease I productivity and an observable leaching of Ir of 3% after one catalytic run. The Ir-Cp was found more stable than its homogeneous Ir(ppy)2(H2dcbpy) counterpart. This superiority was attributed to the structuration of Ir sites within CP framework with increased rigidity, resulting also in longer-lived excited states.

Later, the same group described light-harvesting Ru-MOF made by combination of Ru(H2dcbpy)3 linker (dcbpy : 2,2′-bipyridine-4,4′-dicarboxylate) and cadmium perchlorate (Figure 6). [START_REF] Zhang | Hierarchical Metal-Organic Framework Nanoflowers for Effective CO2 Transformation Driven by Visible Light[END_REF] This Ru-MOF, formulated as {Cd2[Ru(dcbpy)3] • 12 H2O}n, resulted in a six-connected pcu network structure. Moreover, the authors showed that Ru-MOF was easily tuned from sub-millimeter scale flakes to micro-scale flakes (about 10-30 µm) and nanoflowers. In an acetonitrile-triethanolamine mixture under visible-light irradiation (Xe lamp with filters 420-800 nm) the Ru-MOF was found to produce HCOOH with a rate up to 0.077 mmolHCOOH/gcat/h, being stable and recyclable.

Comparing the Ru-MOF with its constitutive molecular Ru units, the luminescence lifetime of the Ru-MOF was more than one order of magnitude higher than that of the molecular Ru complex.

The longer luminescence and thus excited-state lifetime have been stated to be crucial for an efficient photocatalytic process. The authors also demonstrated the advantage of three-dimensional hierarchical nanostructured crystals with increase productivity found for nanoflowers compared to micro-flakes and bulk crystals (respectively 0.077, 0.053 and 0.031 mmolHCOOH/gcat/h). The superior efficiency of Ru-MOF nanoflowers was attributed to their larger surface-to-volume ratio compared to bulk crystals as well as their high energy transfer efficiency, indicated by the highest quantum yield observed for the nanoflowers in the series. Reprinted with permission from reference 41 with permission from the Royal Society of Chemistry.

Metalated catecholate linker

Cohen and coworkers reported chromium and gallium catecholate derivatives as linker in the UiO-66 framework (Figure 7). [START_REF] Lee | Photocatalytic CO2 Reduction Using Visible Light by Metal-Monocatecholato Species in a Metal-Organic Framework[END_REF] The Zr-based MOF was subjected to post-synthetic linker exchange with 2,3-dihydroxyterephthalic acid (H2catbdc) to produce UiO-66-CAT which was metalated at the catechol site using Cr 3+ or Ga 3+ salts. XPS analysis of the as-prepared UiO-66-CrCAT and UiO-66-GaCAT confirmed the trivalent oxidation state of Cr and Ga in the MOFs. In this case, the catbdc organic linkers were responsible for visible light absorption and the metalation by Cr 3+

and Ga 3+ was found to facilitate the electron transfers within the MOFs. Using a mixed solution of TEOA and 1-benzyl-1,4-dihydronicotinamide (BNAH, 0.1 M) in acetonitrile under visible light irradiation (300 W Xe arc lamp equipped with cut-off filters 420-800 nm), the HCOOH productivity of UiO-66-CrCAT and UiO-66-GaCAT reached 1.72 mmolHCOOH/gcat/h and 0.56 mmolHCOOH/gcat/h, corresponding to turnover numbers of 11 molHCOOH/molCr and 6 molHCOOH/molGa, respectively. It was noteworthy that UiO-66-CAT prior to metalation as a photocatalyst did not produce HCOOH. The stability and reusability of the metalated catechol-based MOF photocatalysts after three cycles of photocatalysis were assessed using PXRD and SEM measurements to evaluate their structural and chemical integrity.

Metalated porphyrin linkers

Mialane, Morris, Mellot-Draznieks and coworkers explored the metalation using various cations (M = Mn 3+ , Fe 3+ , Co 2+ , Cu 3+ , Zn 3+ ) of the Zr-based porphyrinic MOF-545. [START_REF] Benseghir | Unveiling the Mechanism of the Photocatalytic Reduction of CO2 to Formate Promoted by Porphyrinic Zr-Based Metal-Organic Frameworks[END_REF] The MOF-545 was built of tetrakis(4-carboxyphenyl) porphyrin (TCPP) linkers connecting Zr6O8(H2O)4(COOR)8 nodes using 1,2-dichloroacetic acid as a modulator. In this study, the synthesis of nanocrystalline MOF-545(M) was achieved via a new microwave protocol giving access to particle size of ca. 3000, 200 and 150 nm for MOF-545, NanoMOF-545 and NanoMOF-545(Fe), respectively. The activity of the MOF-545(M) series was evaluated in acetonitrile using TEOA as sacrificial electron/proton donor under visible light irradiation with a 280W Xe arc lamp. Only HCOOH was formed and no other product, such as H2, CO or CH4, could be observed. Amongst the MOF-545(M) series, the HCOOH production increased with the order Cu < Co < Mn < Zn < Fe, reaching a maximum of 0.57 mmolHCOOH/gcat/h with the MOF-545(Fe) photocatalyst. In the case of nanosized MOF catalyst, NanoMOF(Fe), the visible light-driven HCOOH production reached 1.5 mmolHCOOH/gcat/h significantly boosted with respect to the conventional micron-sized crystals.

This was attributed to an increased density of accessible catalytic sites. By combining spectroscopic and photophysical techniques with Density Functional Theory (DFT)-level calculations (Figure 8), an unconventional mechanism of the MOF-545(Fe)-catalyzed CO2 photoreduction was postulated to involve (i) the photo-oxidation of TEOA at the porphyrinic linkers followed by deprotonation to yield TEOA • radicals; (ii) the thermally-activated CO2 reduction at the Zr6O8(H2O)4(COOR)8 clusters using TEOA • as hydride donor yielding to its dehydrogenated form (ald • ); and (iii) the radical quenching between the reduced linker (TCPPc •-) and the dehydrogenated donor (ald • ). As for UiO-type catalysts (section 2.1), the impact of leached Zr species might also play a role, in particular as the calculations were performed for isolated Zr6O8(H2O)4(COOR)8 model clusters and a decrease in the crystallinity was observed after catalysis. Note that for similar isolated Zr6O4(OH)4(COOR)12 clusters the catalytic activity in CO2

to CO photoreduction has been demonstrated recently. 57 Zhang, Lin and coworkers reported two new chemically stable metalloporphyrin-bridged metalcatechol frameworks able to perform CO2 to HCOOH photoreduction using water instead of TEAO, BIH or BNAH as electron donor (Figure 9). [START_REF] Chen | Energy Band Alignment and Redox-Active Sites in Metalloporphyrin-Spaced Metal-Catechol Frameworks for Enhanced CO2 Photoreduction[END_REF] Theses MOF were made of metal-phenate building unit linked with photosensitive metalloporphyrins. Typically, In(NO3)3 or FeCl2 reacted Further strategies relayed on the use of MOF as platform for the heterogenization of molecular CO2 reduction catalysts, mainly based on bipyridyl linker, and thus was envisioned as a porous macroligands, ie a functional porous solid acting like the organic ligand in the case of a molecular complexes. [START_REF] Wisser | Porous Macroligands: Materials for Heterogeneous Molecular Catalysis[END_REF] Rh-MOP. Furukawa and coworkers studied dinuclear Rh paddlewheels, found in molecular cages and their porous self-assemblies, in the visible-light driven CO2 to HCOOH photoreduction (Figure 10). 46 The controlled arrangement of Rh2 paddlewheels was reported by Furukawa and coworkers in metal-organic cages, also called metal-organic polyhedra (MOP), which are discrete molecular structures, self-assembled from bent ligands and metal precursors. The MOP [Rh2(bdc)2]12 was found to self-assembled into supramolecular structures by coordination on the Rh open metal sites of 1,4-bis(imidazole-1-ylmethyl)benzene (bix) as ditopic N-donor linker, giving access to colloidal particles (Rh-CPP) or aerogels (Rh-SAG). [START_REF] Wang | Multiscale Structural Control of Linked Metal-Organic Polyhedra Gel by Aging-Induced Linkage-Reorganization[END_REF][START_REF] Legrand | Understanding the Multiscale Self-Assembly of Metal-Organic Polyhedra towards Functionally Graded Porous Gels[END_REF] For comparison, they also synthesized the Rh3btc2 MOF (btc = 1,3,5-benzenetricarboxylate), an HKUST analogue made with Rh paddlewheel nodes, first reported by Fischer and coworkers. [START_REF] Heinz | Mixed Precious-Group Metal-Organic Frameworks: A Case Study of the HKUST-1 Analogue [RuxRh3-x(BTC)2[END_REF] The photocatalytic CO2 reduction was performed in a CO2-saturated acetonitrile-TEOA solution using Ru(bpy)3Cl2 as photosensitizer and irradiation by an ABA-class solar simulator without a filter for 2 hours. The Rh3btc2 MOF catalyst showed a turnover frequency (TOF) of 5 molHCOOH/molRh2/h (10 mmolHCOOH/gcat/h). Rh-CPP and Rh-SAG Soc. 2022, 144 (8), 3626-3636. [START_REF] Ghosh | Rhodium-Based Metal-Organic Polyhedra Assemblies for Selective CO2 Photoreduction[END_REF] Copyright 2022 American Chemical Society.

Mn@UiO-67-Bpy. Cohen and coworkers pioneered the post-synthetic metalation of robust Zrbased MOF made with open 2,2′-bipyridine metal-chelating linkers to achieve the site-isolation of Mn(CO)3Br sites in the MOF resulting in UiO-67-bpyMn(CO)3Br catalyst. [START_REF] Fei | Photocatalytic CO2 Reduction to Formate Using a Mn(I) Molecular Catalyst in a Robust Metal-Organic Framework[END_REF] The UiO-67-bpy was prepared using an equimolar mixture of 4,4′-biphenyldicarboxylic acid and (2,2′-bipyridine)-5,5′dicarboxylic acid with ZrCl4 and acetic acid as modulator. The metalated material, UiO-67-bpyMn (CO)3Br, was obtained by infiltration of Mn(CO)5Br and isolated as a red microcrystalline powder with an apparent surface area of 1430 m 2 /g. ICP-OES and EDX analyses suggested that ca. 76% of the bpy sites were metalated. The CO2 photoreduction proceeded in a DMF/TEOA solvent mixture in the presence of [Ru(dmb)3](PF6)2 (dmb = 4,4′-dimethyl-2,2′-bipyridine) as photosensitizer and BNAH as sacrificial reducing agent under 470 nm LED irradiation (Figure 11). The UiO-67-bpyMn(CO)3Br catalyst reached a HCOOH productivity of 5 mmolHCOOH/gcat/h and a turnover number of 110 molHCOOH/molMn after 18 h. The system produced low yields of CO and H2 with TON of 4.5 and 1, respectively, after 18 h. The robust nature of the UiO-type MOF and the site-isolation inhibited the deactivation of the Mn molecular catalyst by dimerization, enabling a however limited reusability over three catalytic cycles. PXRD, FT-IR and ICP-OES data indicated that the main sources for the loss of catalytic activity were both the loss of CO ligands at the Mn sites and the slight degradation of the MOF framework. Ru(terpy)@UiO-67-Bpy. Using the same MOF hosting framework, Kitagawa and coworkers designed UiO-67 MOF functionalized with a polypyridyl ruthenium molecular catalyst. [START_REF] Kajiwara | Photochemical Reduction of Low Concentrations of CO2 in a Porous Coordination Polymer with a Ruthenium(II)-CO Complex[END_REF] From pristine UiO-67, they applied a post-synthetic linker exchange methodology using new metalated linker derivative [Ru II (H2bpydc)(terpy)(CO)](PF6)2 (H2bpydc = 2,2′-bipyridine-5,5′-dicarboxylic acid, terpy = 2,2′;6′,2′′-terpyridine) to synthesized the UiO-67-bpyRu(terpy)(CO) catalyst (Figure 12). PXRD analysis of UiO-67-bpyRu(terpy)(CO) indicating the retention of the UiO-67-type framework structure during the post-synthetic linker exchange process. For a loading of 5.5 mol% of the molecular complex within the MOF framework, the apparent surface area reached 2170 m 2 /g. Interestingly, CO2 isotherms recorded at 273 and 298 K revealed an increased uptake for MOF catalyst compared to the parent UiO-67, attributed to an increase affinity for CO2 upon loading with Ru(CO) species. The CO2 photoreduction proceeded under visible light irradiation (385-740 nm, 67 mW/cm 2 ) in the presence of [Ru(bpy)3] 2+ as photosensitizer and TEOA as an electron donor. HCOOH was found as the main product however with H2 and CO as byproducts.

The UiO-67-bpyRu(terpy)(CO) catalyst produced a larger amount of H2 than its molecular counterpart suggesting that the Zr6O4(OH)4 clusters were active as proton reduction catalysts, a side reaction however not observed for pure MOF based catalysts (see section 2.1 and 2.2). The dilution of CO2 into the gaseous feed was found to impact the productivity of the molecular catalyst (TON decreased from 60 to 30 molHCOOH/molRu(CO) for 100%vol to 5%vol CO2) but did not affect that of the MOF catalyst (TON decreased from 55 to 50 molHCOOH/molRu(CO) for 100%vol to 5%vol CO2), also more selective towards formic acid, reaching a productivity of 1.07 mmolHCOOH/gcat/h. Cp*Rh@UiO-67-Bpy. Fontecave and coworkers identified a novel Rh-based selective molecular catalyst for the reduction of CO2 into HCOOH and incorporated the catalyst into UiO-67 MOF as part of the framework. [START_REF] Chambers | Photocatalytic Carbon Dioxide Reduction with Rhodium-Based Catalysts in Solution and Heterogenized within Metal-Organic Frameworks[END_REF][START_REF] Hendon | A Simple and Non-Destructive Method for Assessing the Incorporation of Bipyridine Dicarboxylates as Linkers within Metal-Organic Frameworks[END_REF] Following post-synthetic ligand exchange methodology, UiO-67 reacted with the synthesized molecular complex Cp*Rh(bpydc)Cl2 (Cp* = pentamethyl cyclopentadienyl) in deionized water at room temperature for 24 h (Figure 13). Ligand exchange rate varied from 5 mol% to 35 mol% with retention of the UiO-67 structure. Cp*Rh@UiO-67-bpy samples showed a linear correlation between decreased in apparent surfaces area and increased rhodium incorporation, from 1750 to 500 m 2 /g for 5 mol% to 35 mol% linker exchange.

Photocatalytic CO2 reduction reactions were performed using a mixture of acetonitrile and TEOA as both an electron and proton donor in the presence of Ru(bpy)3Cl2 as the photosensitizer. For of different host materials on the activity of the same Cp*Rh site. [START_REF] Wisser | Hammett Parameter in Microporous Solids as Macroligands for Heterogenized Photocatalysts[END_REF] Also, they used a slightly different setup as Fontecave and co-workers (see Table 1), they observed similar activity for the Cp*Rh@UiO-67 catalyst. When replacing the UiO-67 host by MOF-253 (Error! Reference source not found.Error! Reference source not found.b), the catalytic activity drops to approx.

60% from a turnover frequency (TOF) of ~7 h -1 (1.78 mmolHCOOH/gcat/h) to ~4 h -1 (0.78 mmolHCOOH/gcat/h). In order to rationalize this observation, the authors designed a series of porous organic polymers (POPs) containing the 2,2′bipyridine binding site and different substituents and compared the catalytic activities of all materials to the activities for molecular model catalysts. Interestingly, the activity of the solid catalyst followed the same trend as the activity of the homogeneous model catalysts. For the latter it is well established that changes in the activity can be described by the so-called Hammett correlation. In molecular chemistry, the electronic effects of substituents are usually described using Hammett's constant, with tabulated values allowing a direct comparison of the influence at the molecular level of different functional groups on the rate of reaction. The theory shows that the electronic effects of the substituent are additive and therefore offer a wide latitude of variations. The correlation between activity and Hammett's constant in UiO-67, MOF-253 and POPs highlighted the crucial impact of the local electronic environment surrounding the active catalytic center compared to the structure on a larger scale of the porous support (porosity, dimensionality, conjugation). In this case, the POP-based catalysts showed a much smaller Hammett value compared to MOF, making them more suitable as macroligands for the Rh-catalyzed CO2-to-HCOOH reduction, with TOF of up to 29 h -1 (4.2 mmolHCOOH/gcat/h). Most importantly this correlation rules out any impact of diffusion limitation, as the material with the largest pore opening (MOF-253) showed the lowest activity. PW12/Cp*Rh@UiO-67-Bpy Mellot-Draznieks and coworkers further studied the previous Cp*Rh@UiO-67-bpy catalytic system by exploring the potential impact of the co-immobilization of polyoxometalates (POM) and Cp*Rh catalytic complexes on the photocatalytic performances for CO2 conversion. [START_REF] Benseghir | Co-Immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO 2 Reduction[END_REF] The PW12@UiO-67 composite was obtained by solvothermal self-assembly of ZrCl4, H2bpdc and H3PW12O40 (H3PW12). Post-synthetic linker exchange with the molecular complex Cp*Rh(bpydc)Cl2 yielded the (PW12, Cp*Rh)@UiO-67. The percentage of the bpdc linkers exchanged in the MOF and the amount of POM and Rh immobilized within the UiO-67 cavities were evaluated by energy dispersive spectroscopy and elemental analyses, leading to the following proposed formula Zr6O4(OH)4(bpdc)5.45(Cp*RhCl(bpydc))0.44(PW12O40)0.22. From 31 P NMR spectroscopy, the authors suggested that two POM environments coexisted within the pristine (PW12, Cp*Rh)@UiO-67 material, some being close to the Cp* fragment of the Cp*Rh complex and some being closer to the organic bpdc linker, also supported by DFT-level calculations. The activity of Cp*Rh@UiO-67 and (PW12, Cp*Rh)@UiO-67 materials were compared in an acetonitrile-TEOA mixture with Ru(bpy)3Cl2 as external photosensitizer under irradiation by a 280 W Xe lamp equipped with a 415 nm cut-off filter. After 3 hours of reaction, the productivity in H2 was enhanced by a factor of 2 using (Cp*Rh, PW12)@UiO-67 composite (0.83 mmolH2/gcat/h; TON = 20 molH2/molRh) compared to the POM-free Cp*Rh@UiO-67 (0.48 mmolH2/gcat/h; TON = 7.7 molH2/molRh). The production of HCOOH followed the same trend with similarly enhancement when using POM composite (0.52 mmolHCOOH/gcat/h, TON = 14 molHCOOH/molRh) compared to the POM-free system (0.44 mmolHCOOH/gcat/h, TON = 7 molHCOOH/molRh). The beneficial role of immobilizing POMs near catalytic sites for boosting their photocatalytic activity was later elucidated by a comprehensive approach combining DFT calculations and microkinetic modeling techniques, supplemented by photo-physical and spectroscopic experimental measurements. [START_REF] Solé-Daura | Origin of the Boosting Effect of Polyoxometalates in Photocatalysis: The Case of CO 2 Reduction by a Rh-Containing Metal-Organic Framework[END_REF] The PW12 POM immobilized inside the UiO-67 MOF was postulated to act as an efficient electron reservoir, accepting electrons from the photo-reduced Ru(bpy)3 photosensitizer and transferring them to the Rh catalyst. The increase in H2 production was attributed to concomitant increase in activity and higher concentration of protons from the reductive quenching of the photoexcited photosensitizer by TEOA.

Critical assessment on parameters driving catalysis efficiency

As mentioned in the introduction, the lack of precise information about the reactor design, the light source used (position from the reactor, constant radiant flux or irradiance) and information on the reproducibility of photocatalytic system remains a major challenge for comparing the activity in photochemical CO2 reduction reaction (CRR) and thus designing better performing catalysts. As the same problems are also reported in homogeneous photochemical CRR, also the recommendation made by Beller and co-workers for homogeneous catalysis [START_REF] Marx | Addressing the Reproducibility of Photocatalytic Carbon Dioxide Reduction[END_REF] should be acid catalysis in series of isostructural UiO-66 MOFs and the Hammett constant of the linker. [START_REF] Vermoortele | Electronic Effects of Linker Substitution on Lewis Acid Catalysis with Metal-Organic Frameworks[END_REF][START_REF] Vandichel | Active Site Engineering in UiO-66 Type Metal-Organic Frameworks by Intentional Creation of Defects: A Theoretical Rationalization[END_REF] We would also expect an impact of the linker functionalization not only on the light harvesting capability of such MOFs (i.e., Section 2) but also on the electron density and thus on the catalytic activity of the node. Understanding those parameters will allow in the future the design of even better performing catalysts.

Another parameter which is seldom addressed in detail is the stability of the MOF under the conditions of catalysis. The often-performed powder X-ray diffraction analysis allows studying the crystalline fraction of the material. However, even if changes in the relative peak intensities, a significant increase in full width at half maximum of the peaks or a non-negligible background are observed after catalysis, the material is stated to be stable. This observation does not take into account the leaching of metal centers, which may account for a significant fraction of the catalytic activity, as demonstrated for Zr based materials by Saouma and co-workers, [START_REF] Bhattacharya | The Role of Leached Zr in the Photocatalytic Reduction of CO2 to Formate by Derivatives of UiO-66 Metal Organic Frameworks[END_REF] or the formation of an amorphous phase. To this end, Taheri and Tsuzuki recently reported a detailed investigation of the photo accelerated decomposition of ZIF-8 in water. [START_REF] Taheri | Photo-Accelerated Hydrolysis of Metal Organic Framework ZIF-8[END_REF] Also ZIF-8 is supposed to be amongst the most stable MOFs, [START_REF] Park | Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks[END_REF] it undergoes significant decomposition in water when irradiated with UV or near-UV visible light. The decomposition directly correlates with the absorbance spectrum of ZIF-8, underlining that formed ZnO is not responsible for the degradation. [START_REF] Taheri | Photo-Accelerated Hydrolysis of Metal Organic Framework ZIF-8[END_REF] In light of the work of Saouma et al. as well as Taheri and Tsuzuki it is thus recommended to perform more systematic studies on stability of the MOFs used, in particular those described in section 2, 3.1 and 3.3, as metal oxides which might be derived from decomposition of the modes such as titania are wellknown photocatalysts. For systems where the MOF acts as host (section 3.2 & 4) the focus should be set on hot filtration test, that is removing the solid catalyst after a given time and conducting the reaction solely with the supernatant. If a further increase in product formation (i.e., its concentration) is observed, leaching of catalytically active species cannot be excluded and requires further investigation. Reporting of recycling test alone is not considered to be sufficient, as the reaction might be conducted at full or equilibrium condition. In such a scenario, no reliable conclusion on the stability, recyclability and/or catalytic performance can be drawn. [START_REF] Kramm | Pitfalls in Heterogeneous Thermal, Electro-and Photocatalysis[END_REF] Understanding of the stability of MOFs under photocatalytic conditions will also be of utmost importance for designing catalytic systems which can be used in pure water or aqueous buffer solutions with water as electron and proton source. Ideally such catalysts will evolve oxygen as the sole by-product, thus avoiding the use of sacrificial electron donors (e.g. TEOA, TEA) and organic solvents (e.g. acetonitrile). [START_REF] Hao | Metal-Organic Framework Membranes with Single-Atomic Centers for Photocatalytic CO2 and O2 Reduction[END_REF][START_REF] Chen | Energy Band Alignment and Redox-Active Sites in Metalloporphyrin-Spaced Metal-Catechol Frameworks for Enhanced CO2 Photoreduction[END_REF] In such scenario, the full benefit of CO2 reduction to convert atmospheric CO2 as an urgent demand to mitigate climate change can be realized. In order to replace the sacrificial electron donors by water, however a precise understanding of the role of the amine is required. In particular as the amine might be involved in activating CO2 by forming e.g. a carbamate or an hemi-ester, [START_REF] Sampaio | Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO2 to Formate by Ruthenium Complexes[END_REF] or in the electron transfer.

Conclusions

In conclusion, the last decade has seen a tremendous evolution of hybrid heterogenous photocatalytic systems for the direct and selective light-driven production of formic acid from carbon dioxide. The design of sophisticated MOF-based and related catalysts has led to a thousandfold increase in productivity using solar light as unique energy source. Notwithstanding the boost in activity provided by electron donors stronger than TEAO, such as BIH or BNAH, it appeared from the above-described literature that highest productivity arose from well-defined isolated noble metal sites (Ru, Rh, or Ir) within the MOF framework. In addition to potentially intrinsic catalytic site activity, the ability of MOF to favorably adsorb CO2 was postulated as a key feature to reach high formic acid production. The efficient recyclability of such heterogenous catalytic systems was mainly limited by the degradation of widely used Ru-based organometallic photosensitizer, especially for long-term reactions, which was advantageously circumvented by the use of more stable organic chromophores. The structuration of both light harvesting and catalytic units into a single framework allowed for both site-isolation and efficient charge transfer and separation at the origin of the superior stability and efficiency of MOF-and POP-based catalysts compared to analogous homogeneous systems. In the case of MOF made with photoactive linkers and/or nodes, the mechanisms at play during photocatalysis remained however often not clear or with contradicting conclusions, with a need to experimentally unravel (i) the real active site nature, in particular for materials where the node is supposed to be catalytically active, (ii) the charge transfer mechanism and (iii) the CO2 activation mechanism, including also reaction intermediates. Also evidence for true heterogeneity of the catalyst including hot filtration and recycling test are often missing, but would help to understand the true nature of the catalyst. Some of these issues were addressed on the basis of molecular chemistry mechanisms in the case of heterogenized molecular catalysts. Indeed, these well-defined hybrid solid systems at the interface between homogeneous and heterogeneous catalysis represent ideal candidates for structureactivity relationship elucidation thank to the combination of computational chemistry, transient spectroscopy and advanced diffraction techniques. Furthermore, the key role of the sacrificial electron donors, in particular of triethanolamine, as unavoidable additive has been questioned and addressed under various aspects for its ability to be at the same time electron, proton or hydride donor. Albeit being used for decades, the precise role of TEOA seen in the different mechanism needs still to be elucidated.

Despite crucial advances and appealing productivities reached by the above-reviewed catalytic systems, few prerequisites remained unsolved in order to apply such systems for large-scale CO2 abatement and solar fuel production. Indeed, the economic viability of these MOF-based systems might be questioned and the need to get rid of sacrificial electron donors, yielding huge amounts of organic waste without any further use, will drive the next developments in the field. Already large-scale MOF production has reached a level of maturity allowing already their application in real-life [START_REF] Cordova | The 'Folklore' and Reality of Reticular Chemistry[END_REF][START_REF] Schoedel | The Role of Metal-Organic Frameworks in a Carbon-Neutral Energy Cycle[END_REF][START_REF] Almassad | Environmentally Adaptive MOF-Based Device Enables Continuous Self-Optimizing Atmospheric Water Harvesting[END_REF][START_REF]Frameworks for Commercial Success[END_REF] and recent examples showed the use of MOF catalyst for the carbon dioxide-to-formic acid reaction combined with water oxidation without any sacrificial organic molecule. [START_REF] Hao | Metal-Organic Framework Membranes with Single-Atomic Centers for Photocatalytic CO2 and O2 Reduction[END_REF][START_REF] Chen | Energy Band Alignment and Redox-Active Sites in Metalloporphyrin-Spaced Metal-Catechol Frameworks for Enhanced CO2 Photoreduction[END_REF] Thanks to the combined effort of molecular and material science communities to tackle actual limitations, new breakthrough photosystems using MOF and related porous compounds for carbon-based solar fuels production will certainly appear in a near future.

Figure 1 .

 1 Figure 1. Postulated mechanism for the light-induced CO2 reduction catalyzed by Ti-MIL-125-NH2. Reprinted with permission from reference 31. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 2 .

 2 Figure 2. Postulated mechanism for the light-induced CO2 reduction catalyzed by SDCA-NH2. Reprinted

  Fe-MIL-101-NH2 has a three-dimensional crystal structure and is formed of octahedral trimeric iron (III) clusters linked by 2-aminoterephthalate linkers generating two types of mesoporous quasispherical cages of 29 Å and 34 Å diameter. In Fe-MIL-53-NH2, chains of iron (III) centered cornersharing octahedra, joined along the chain through hydroxyl oxygens, are linked perpendicular to the chains to four chains by 2-aminoterephthalate linkers, thus generating one-dimensional flexible diamond-type channels of 3.5 to 8 Å diameter. Fe-MIL-88B-NH2 is made of trimeric Fe(III) shared by µ3-O and linked by 2-aminoterephthalate linkers giving access to flexible three-dimensional cages with diameter form 6 to 16 Å. As found in the case of Ti and Zr-based amino-MOF, the amino-functionalized Fe-based MOF materials show enhanced absorption in the visible light region compared to non-aminated parents. Also, amino-functionalized MOFs show enhanced activity for photocatalytic CO2 reduction. The best catalyst reported, Fe-MIL-101-NH2, allowed

Figure 3 .

 3 Figure 3. Postulated synergetic reaction mechanism for the light-induced CO2 reduction catalyzed by Fe-MIL-101-NH2. Reproduced from Wang et al. ACS Catal. 2014, 4 (12), 4254-4260.[START_REF] Wang | Fe-Based MOFs for Photocatalytic CO2 Reduction: Role of Coordination Unsaturated Sites and Dual Excitation Pathways[END_REF] Copyright 2014

Figure 4 . 2 . 3

 423 Figure 4. Synthesis of Cu NCs@MOF composites as CO2 photoreduction catalysts under UV irradiation. Reprinted with permission from reference 38. Copyright 2022 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 5 .

 5 Figure 5. Light-driven CO2 photoreduction catalyzed by Ga-MFM-300. Reprinted with permission from reference 28 with permission from the Chinese Chemical Society (CCS).

Figure 6 .

 6 Figure 6. Postulated mechanism for the light-driven CO2 photoreduction catalyzed by Ru-MOF.

Figure 7 .

 7 Figure 7. Synthesis of metalated catecholate-UiO-66 as CO2 photoreduction catalysts. Reprinted with permission from reference 42 with permission from the Royal Society of Chemistry.

Figure 8 .

 8 Figure 8. Gibbs free-energy profile for the reduction of CO2 to HCOOH calculated for Zr6O8(H2O)4(COOR)8 clusters as molecular model of the MOF-545 catalyst. Reprinted with permission

with 5 ,

 5 10,15,20-tetrakis(3,4-dihydroxyphenyl)porphyrin-Co (3,4-TDHPP-Co) to produce In/FeTCP-Co as a three-dimensional framework connected by one-dimensional indium-oxo/ironoxo coordination rods. Similarly, In/FeTCP-OH-Co were prepared using 5,10,15,20tetrakis(2,3,4-trihydroxyphenyl)porphyrin-Co (TCP-OH-Co) having hydroxyl moieties attached to the 2-position of the catechol. Surface areas of InTCP-Co and FeTCP-Co reached 430 and 380 m 2 /g, respectively, and their hydroxylated analogues InTCP-OH-Co and FeTCP-OH-Co reached 130 m 2 /g and 140 m 2 /g, respectively.

Figure 9 . 4 .

 94 Figure 9. Metalloporphyrin-based MOF for CO2 photoreduction using water instead of organic sacrificial

  showed higher catalytic activity (TOF = 56 -59 molHCOOH/molRh2/h), slightly higher than their molecular building block Rh-MOP (TOF = 45 -52 molHCOOH/molRh2/h). The significant lower activity of the Rh3btc2 MOF compared to all other catalysts was explained by a significant lower electron density on the Rh as deduced from XPS. In terms of productivity, the Rh-SAG reached 54 mmolHCOOH/gcat/h and Rh-CPP surpassed all previous catalysts with 76 mmolHCOOH/gcat/h using a solar simulator. Both recycling and long-term experiments highlighted the stable catalytic activity. Pair distribution function (PDF) analysis evidenced that no structural change occurred at the Rh2 active site upon catalysis.

Figure 10 .

 10 Figure 10. Rh-MOP structure as CO2 photoreduction catalyst. Adapted from Ghosh et al., J. Am. Chem.

Figure 11 .

 11 Figure 11. Proposed mechanism for the formation of HCOOH from the photocatalytic reaction with UiO-67-bpyMn(CO)3Br. Reproduced from Fei et al., Inorg. Chem. 2015, 54(14), 6821-6828.[START_REF] Fei | Photocatalytic CO2 Reduction to Formate Using a Mn(I) Molecular Catalyst in a Robust Metal-Organic Framework[END_REF] Copyright 2015 American Chemical Society.

Figure 12 .

 12 Figure 12. Synthesis of UiO-67-bpyRu(terpy)(CO) catalyst by post synthetic linker exchange. Reprinted

  comparison, the molecular complexes Cp*Rh(bpy)Cl2 and Cp*Rh(bpydc)Cl2 were also evaluated.The two molecular catalysts showed good activity for CO2 photoreduction into HCOOH under homogeneous conditions with TON of 125 molHCOOH/molRh and 42 molHCOOH/molRh for Cp*Rh(bpy)Cl2 and Cp*Rh(bpydc)Cl2, respectively, after 10 h reaction. H2 was the only other product of the reaction with TON of 55 molH2/molRh and 38 molH2/molRh for for Cp*Rh(bpy)Cl2 and Cp*Rh(bpydc)Cl2, respectively, after 10 h reaction. The Cp*Rh@UiO-67-bpy heterogeneous catalyst showed TON similar to its homogeneous counterpart Cp*Rh(bpydc)Cl2, including carboxylate side-groups, with TON = 47 molHCOOH/molRh, corresponding to 0.3 mmolHCOOH/gcat/h, and TONH2 = 36. Interestingly, for similar rhodium loading within catalytic system, the MOFheterogenized catalyst was slightly more selective to HCOOH than the corresponding homogeneous counterpart.

Figure 13 . 2 -

 132 Figure 13. Synthesis of Cp*Rh@UiO-67-bpy for CO2 photoreduction. Adapted from reference 49. Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 14 .

 14 Figure 14. a) structure of molecular catalysts, b) structure of MOF and POP based catalysts and c) Structure-activity correlation bridging homogeneous and heterogenized Cp*Rh(bpy) catalysts within MOF and POP for CO2 photoreduction. Adapted from Wisser et al., ACS Catal. 2018, 8 (3), 1653-1661.[START_REF] Wisser | Hammett Parameter in Microporous Solids as Macroligands for Heterogenized Photocatalysts[END_REF] Copyright 2018 American Chemical Society.

Table 1 .

 1 Overview over MOF-, POP-and MOP-based catalyst for CO2 to HCOOH photoreduction.

	support	postulated catalyst photosensitizer	R mmol/h/g	/	t / h	conditions	ref .
	Ti-MIL-125-NH2	Ti-O node	amino linker	0.016		10	420 nm ACN-TEOA, 500 W Xe lamp, λ >	31
	UiO-66-NH2 UiO-66-(NH2)2	Zr-O node	amino linker	0.026 0.041		10	ACN-TEOA, 500 W Xe lamp, λ > 420 nm	32
	Zr-SDCA-NH2	Zr-O node and amino linker	amino linker	0.096		12	420 nm ACN-TEOA, 300 W Xe lamp, λ >	33
	UiO-66-NH2 UiO-66	leached Zr species linker	0.032 0.010		12 ACN-TEOA, 5 : 1, 200 W Xe lamp	34
	UiO-66-NH2	leached Zr species linker	0.056		12 ACN-DEA, 5 : 1, 200 W Xe lamp	34
	Fe-MIL-101-NH2	Fe-O node	amino linker	0.448		8	420 nm ACN-TEOA, 300 W Xe lamp, λ >	35
	Pt/Ti-MIL-125-NH2	Ti-O node	amino linker	0.032		8	420 nm ACN-TEOA, 300 W Xe lamp, λ >	36
	Ir1/A-UiO-66-NH2 Ir1/Ti-MIL-125-NH2	Ir sites	amino linker	0.51 0.27		10	water-isopropanol, 300 W Xe lamp, λ > 420 nm	37
	Ir1/A-UiO-66-NH2 membrane	Ir sites	amino linker	3.38			Gaseous CO2 feed saturated with water vapours, 300 W Xe lamp, λ > 420 nm
	Cu NCs/UiO-66-NH2	Cu NCs	amino linker	0.128		18	385 nm DMAc-TEOA, 300 W Xe lamp, λ =	38

Table 1 .

 1 Overview over MOF-, POP-and MOP-based catalyst for CO2 to HCOOH photoreduction. (Continued)

	Support	catalyst	photosensitizer	R mmol/h/g	/	t / h	conditions	ref .
	KGF-9	not determined	thiadiazole linker	1.15		5	DMSO-BIH, LED, 370-500 nm	39
	Ga-MFM-300	not determined	not determined 0.50		4	nm ACN -TEOA, 300 W Xe lamp, 350-780	28
	Ir-CP	Ir site	Ir site	0.119		6	475 nm ACN-TEOA, 500 W Xe lamp, λ >	40
	Ru-MOF powder Ru-MOF nanoflower	Ru site	Ru site	0.031 0.077		8	ACN-TEOA, 500 W Xe lamp, λ > 420 nm	41
	UiO-66-catechol	Cr 3+ Ga 3+	catechol linker	1.72 0.56		6	ACN -TEOA-BNAH, 300 W Xe lamp, λ > 420 nm	42
	MOF-545-Fe nano-MOF-545-Fe	Zr-O node	porphyrin linker	0.57 1.52		4	ACN-TEOA, 280 W Xe lamp, λ > 420 nm	43
	In-TCP-Co			0.001
	Fe-TCP-Co In-TCP-OH-Co	In/Fe-catecholate node	porphyrin linker	0.004 0.006		6	water, 300 W Xe lamp, λ > 420 nm	44
	Fe-TCP-OH-Co			0.017
	Ni/Mg-MOF-74	Ni/Mg-O node	Ru(bpy)3Cl2	0.53		0.5	ACN -TEOA, 300 W Xe lamp, λ > 420 nm

Table 1 .

 1 Overview over MOF-, POP-and MOP-based catalyst for CO2 to HCOOH photoreduction. (Continued)

	Support	catalyst	photosensitizer	R mmol/h/g	/	t / h	conditions	ref .
	CPP	C12Rh2-CPP HRh2-CPP		58.6 75.7		2	ACN-TEOA, ABA solar simulator
	SAG CPP SAG	C12Rh2-SAG C12Rh2-CPP HRh2-CPP C12Rh2-SAG	Ru(bpy)3Cl2	53.7 29.9 32.7 25.1		4	ACN-TEOA, 200 W Xe lamp, λ > 420 nm	46
	UiO-67-bpy	Mn(CO)	Ru(dmb)3(PF6)2 5.03		18 DMF-TEOA-BNAH, LED, λ = 470 nm 47
	UiO-67-bpy	Ru(terpy)	Ru(bpy)3Cl2	1.07		6	740 nm ACN-TEOA, 300 W Xe lamp, λ = 385-	48
	UiO-67-bpy	Cp*Rh(bpydc)	Ru(bpy)3Cl2	0.34		10	420 nm ACN-TEOA, 200 W Xe lamp, λ >	49
	MIL-101-NH2	Cp*Rh(bpydc)	Ru(bpy)2(bpydc )Cl2	0.026		5	425 nm ACN-TEOA, 280 W Xe lamp, λ >	50
	PW12/Cp*Rh@UiO-67-bpy	Cp*Rh(bpydc)	Ru(bpy)3Cl2	0.52		3	415 nm ACN-TEOA, 280 W Xe lamp, λ >	51
	MOF-253 BpyMP-1 BpyMP-1-NH2	Cp*Rh(bpy)	Ru(bpy)3Cl2	0.78 3.73 4.18		4	ACN-TEOA, 200 W Xe lamp, λ > 420 nm	52

  However, under 5% CO2 atmosphere, the CO production was reduced to 0.1 mmolCO/gcat/h, four-fold lower than under pure CO2, and the HCOOH production reached at 0.53 mmolHCOOH/gcat/h, corresponding to 80% of the production under pure CO2. Furthermore, simulated flue gas containing NOx, H2S and SOx did not alter the productivity. Such experiments are hardly provided in the literature but are of tremendous significant for the application of CO2 reduction catalysts under realistic conditions, as NOx, H2S and SOx represent potent catalyst poisonings. Supported by experimental CO2 physisorption and photocurrent response measurements, theoretical calculations revealed that both strong binding affinity of Mg sites for CO2 and lowered energy barrier for the formation of the formate radical intermediate by the cooperation of Ni 2+ and Mg 2+ were at the origin of the high HCOOH productivity.

25

-MOF-74 exhibited the highest HCOOH production with a rate of 0.64 mmolHCOOH/gcat/h which is higher than that of Ni-MOF-74 (0.29 mmolHCOOH/gcat/h).

Significant CO production was reported under pure CO2 atmosphere with a rate of 0.4 mmolCO/gcat/h.

considered for scientist working on heterogeneous photochemical CRR alongside recommendations for classical heterogeneous photocatalysis. [START_REF] Kisch | Best Practice in Photocatalysis: Comparing Rates or Apparent Quantum Yields?[END_REF] Beside the detailed description of the reactor setup and the light source, one should consider the following points to ensure the accurate analysis of the catalytic performances obtained:

Stating the number of independent experiments conducted;

(ii) Performing at least two independent measurements on independent samples;

(iii) Reporting of increased catalytic activity by reducing the catalyst loading and/or solid loading should prerequisite insurance that the reproducibility is also given at the lowest, and thus optimum, catalyst / solid loading;

(iv) Reporting quantum efficiencies for catalyst loadings within the catalyst loading of the so-called "optimal reaction rate". The optimal reaction rate is defined as the reaction rate which increases linearly with increasing catalyst/solid loading. In case the experiments are conducted with monochromatic light, an apparent optimal quantum yield might be reported.

When comparing parameters such as quantum yield, one should also consider that particle size and shape also dramatically affect light scattering, which might count for 13 to 76% loss of incoming photons in heterogeneous photocatalysis. [START_REF] Kisch | Best Practice in Photocatalysis: Comparing Rates or Apparent Quantum Yields?[END_REF][START_REF] Kisch | On the Problem of Comparing Rates or Apparent Quantum Yields in Heterogeneous Photocatalysis[END_REF] For the comparison of catalytic activities considering the same active site within different MOF supports, the Hammett correlation has been demonstrated to be a useful descriptor also for heterogenized single site catalyst. [START_REF] Wisser | Hammett Parameter in Microporous Solids as Macroligands for Heterogenized Photocatalysts[END_REF] But such comparison is so far only feasible within one laboratory, due to the issues discussed above. However also for MOF photocatalysts with pure organic linkers (discussed in section 2) similar considerations should be considered, as De Vos, Van Speybroeck, and co-workers reported correlation between the relative rate constants of Lewis

AUTHOR INFORMATION

Corresponding Author * jerome.canivet@ircelyon.univ-lyon1.fr; florian.wisser@fau.de

ORCID

Jérôme Canivet: 0000-0002-0458-3085

Florian M. Wisser: 0000-0002-5925-895X ACKNOWLEDGMENT J.C. thanks the ANR project FLIPS (ANR-21-CE07-0028) for funding.