Molecular catalysts and polyoxometalates, immobilization in Metal-Organic Frameworks
We mentioned previously the need of a catalyst in order to reduce CO2 photochemically. The nature of these catalysts can vary a lot from oxides, metal nanoparticles to organometallic complexes. Organometallic complexes have been widely studied as photocatalysts, or photocatalytic complexes, for CO2 reduction reactions (CO2RR) due to their tunable structures.3–5 The possibility to tune these catalysts at a molecular level can indeed allow a better control on the different CO2RR key points such as the efficiency, the selectivity or stability of the catalyst. Polyoxometalates represent another family of molecular catalysts, recently identified as potentially interesting for CO2RR, amongst others (see below). They can be functionalized with a wide variety of metals proven to be active for CO2RR, including Mn, Fe, Cu, Co. Moreover, organometallic complexes can be linked covalently to polyoxometalates and bring additional properties to the hybrid species, such as electron relay.6 Nonetheless, these homogeneous catalysts suffer from two main drawbacks, i) their stability is often limited, with the catalyst deactivating after few hours and ii) the complexes being dissolved in solution cannot be easily recovered. Immobilizing these molecular catalysts in porous materials, namely metal-organic frameworks, is a commonly used approach in order to circumvent the above drawbacks. Indeed, MOF porosity and versatility regarding their organic building units enable the incorporation of molecular catalysts via covalent grafting or supramolecular interactions, thus increasing the stability of the catalyst as a result of its heterogenization. In this chapter we will first describe, in a non-exhaustive way, the commonly used organometallic complexes for CO2RR and POMs. We will then present the main metal-organic frameworks used to heterogenize these molecular species. Finally, we will discuss the different synthetic methods used to immobilize catalysts in MOFs and go through the usual characterization methods for such composite materials.
Homogeneous molecular catalysts
Before introducing molecular catalysts, we will briefly present the different components needed for photocatalytic reduction: a solvent, a sacrificial electron donor for reduction reactions, a photosensitizer, light and lastly, a catalyst.
– Choosing the solvent is the first important step as gas concentration differs depending on the solvent and pH. For example, CO2 concentration in acetonitrile is almost ten times higher than in water at pH 7 (0.28 M vs. 0.033 M respectively).7 Moreover, the molecular catalyst as well as the sacrificial agent have to be soluble and stable in the chosen solvent.
– For light driven CO2 reduction, the photosensitizer should be able to be excited by photons preferably in the visible region. It must possess properties such as light-harvesting ability, long excited-state lifetime and strong oxidizing/reducing power in its excited state in order to receive an electron from the sacrificial electron donor or reduce the substrate. Two possible quenching routes of the excited state of the photosensitizer are possible: oxidative quenching or reductive quenching as illustrated in Figure 1.3.8 In the case of oxidative quenching, the excited state of the photosensitizer reduces the substrate before being reduced by the sacrificial electron donor. In the reductive quenching mechanism, the excited state of the photosensitizer is first reduced by the electron donor before being oxidized by the substrate.
– The last key component is the catalyst. The catalyst performances can be evaluated by four main factors: the catalyst’s selectivity, turnover number, turnover frequency and quantum yield. The highest the catalyst’s selectivity, the better as it avoids having a mixture of products. Turnover numbers (TONs) can be calculated as follows and correspond to the number of reductions occurring per catalytic site: Turnover frequencies (TOFs) are generally measured at the initial stage of the reaction when the product formation is still linear. It can be calculated with the following formula and is usually expressed in s-1, min-1 or h-1: Apparent quantum yield (Φ) is the amount of product in respect to the number of induced photons used for the reaction. The number of induced photons can be calculated with the Plank-Einstein relation where Etot is measured beforehand with a photodiode power sensor.
The nature of the catalyst, including both its metal center and organic ligands, will impact greatly its performances. In the following subsection, we will present briefly several families of catalysts for CO2 photoreduction. The catalyst performances will not be detailed as they can be found in recent reviews.4,9
Molecular catalysts for CO2RR
In this subsection we separate the molecular complexes in two categories, where the first one gathers noble metal polypyridyl molecular catalysts for carbon dioxide reduction, and the second one, noble-metal-free polypyridyl catalysts. Noble metal catalysts described in this paragraph contain ruthenium and iridium. We will also consider rhenium complexes, even if rhenium is not usually reported as noble metal. Rhodium cyclopentadiene based photocatalytic systems will be treated in chapter 2.
Rhenium polypyridyl catalysts were first reported in 1993 by Hawecker et al.10 working on Re(CO)3(bpy)X (X = Cl) (Figure 1.4). The catalyst could reduce CO2 in CO as the sole product, in DMF with TEOA as an electron donor and could reach a TON of 48 after 4h. The solution was irradiated with visible light ( > 400 nm) without the use of external photosensitizer. The same catalyst with other X groups (X = CN, SCN, Br) was studied by different groups.11–14 It was demonstrated that with X = SCN, the rhenium catalyst exhibits the best CO2RR properties, due to the easy dissociation of the SCN- group after the first one-electron reduction of the complex.13 Indeed, to reduce CO2 the rhenium is first excited, then quenched by TEOA leading to the reduced state of the complex. At this step, the X linker is lost and CO2 can bind to the rhenium atom (Figure 1.5). Besides, Kurz et al.15 showed that the modification on the aromatic group (bipyridine-4,4’-dicarboxylic acid, phenanthroline, azopyridine, etc.) induced a decrease of their fluorescence lifetime, thus of their catalytic activity.
The efficiency and catalytic conditions of the different catalysts presented in this subsection are summarized in Table 1.2.
The first report of CO2 photoreduction with a ruthenium polypyridyl catalyst was published in 1985 by Hawecker et al.16 where they studied [Ru(bpy)3]2+. The ruthenium catalyst, now widely used as a photosensitizer, is able to reduce CO2 to CO and formate in a dimethylacetamide/water mixture with benzyl-dihydronicotinamide (BNAH) as an electron donor. During the process, [Ru(bpy)3]2+ loses a bipyridine ligand which is replaced by two solvent molecules leading to [Ru(bpy)2(solvent)2]2+ which is the actual catalyst for CO2 reduction. To facilitate the formation of the active form of ruthenium polypyridyl complexes, [Ru(bpy)2R1R2] complexes (Figure 1.4) were investigated (where R = H, Cl, CO, MeCN, DMF). No clear trend on which R substituent was the most efficient for CO2 reduction, but the difference between the catalysts is outstanding. When R1 = R2 = Cl a TON of 51 could be achieved, when substituting a Cl for a CO, the TON was increased to 326 after 2h.17 Besides, the selectivity of Ru(bpy)2 complexes towards CO and formate production is highly dependent on the pH. For example, in presence of TEOA (pH ~10) the only product observed was HCOOH. In contrast, when using BNAH, the formation of both HCOOH and CO could be obtained at a neutral or acidic pH. The main issue with Ru(bpy)2-type catalysts is their poor stability. Indeed, after a few hours a black precipitate could be observed inducing a slowdown in the catalyst performances.18,19 Finally, ruthenium catalysts bearing only one bipyridine and two CO ligands were studied with different bipyridine functionalizations (Figure 1.4).20,21 The various catalysts have shown similar properties as the previously described ones, with a mixture of CO and HCOOH as reduction products.
Iridium complexes have been barely studied as photocatalysts for CO2 reduction. Two complexes are presented in Figure 1.4: [Ir(R2-tpy)(R1-ppy)Cl] and [Ir(tpy)(bpy)] (tpy = terpyridine and ppy = phenylpyridine). These complexes can reduce CO2 selectively in CO in acetonitrile with TEOA as an electron donor. When R1 = R2 = H, the catalyst could exhibit TONs up to 38,22 functionalization on the aromatic rings with methyl groups could slightly increase the catalyst’s efficiency with turnover numbers up to 50. Genoni et al.23 have shown that functionalizing the tpy linker with an electron donating group, an anthryl, could drastically increase the TONs that can reach 310.
Another emerging strategy is the covalent grafting of photosensitizer on molecular catalyst to form a diade. This strategy allows an easier electron transfer from the photosensitizer to the catalyst thus increasing impressively the system efficiency, often reaching TONs of several thousands.4
As we have seen, noble metal-based molecular catalysts present many advantages: their high selectivity, efficiency and light adsorbing properties. They generally do not need the addition of external photosensitizer. The main critical issue is their rare nature and thus the high price of the metal needed to design these catalysts. When considering a large-scale production, the metal prices would be a huge drawback with prices ranging from ~20 k€/kg for ruthenium to almost 500 k€/kg for rhodium. Developing noble-metal-free catalysts is thus a necessity for considering sustainable photocatalytic systems. For comparison, the price of nickel is ~20 €/kg and that of cobalt ~50 €/kg which makes these more abundant metals a thousand times less expensive.
Metalloporphyrins are well known for their electron transfer roles in a myriad of redox systems in nature (chlorophyll, heme, vitamin B12, cytochrome P450, etc.) as well as highly effective photocatalysts thanks to their strong absorption in the 400-450 nm region (Soret band) and weak absorption in the 500-700 nm region (Q-bands). Metalloporphyrins and related derivatives are strong light absorbers with extremely large extinction coefficients in the visible spectral region. However, yields of active catalysts formed after light absorption are very low, mainly due to the short (picosecond) lifetimes of their excited states. Metalloporphyrins and related derivatives studied for CO2 reduction include metallocorrins, metallophthalocyanines and metallocorroles. These different systems have been largely studied by Savéant and his group in the 1990s. Among the metalloporphyrins, they have shown that iron porphyrins are highly selective for CO in protic solvents in presence of a weak Brønsted acid (phenol, trifluoroethanol) or a Lewis acid. In early stages of the development of the field, the porphyrin efficiency was low and photodecomposition of the catalyst could be observed.24 Addition of external photosensitizer can, sometimes, be beneficial for the reduction reaction. Dhanasekaran et al25. have shown that the addition of p-terphenyl in solution with iron tetrakis(phenyl)porphyrin (FeTPP) could promote the reduction of FeII to FeI and FeI to Fe0. The presence of p-terphenyl could thus multiply by ten the CO production. The same study was performed with CoTPP and also showed a higher CO production, both photo and electrochemically.26 Conditions and TONs are detailed in Table 1.3.
Some metalloporphyrins bear functional groups with negative or positive charges at the meso substituents, designed to enhance efficiency of their catalytic properties. Bonin et al.27 reported in 2014 the functionalization of FeTPP phenyl groups with alcohol (CAT) or fluorine (FCAT) (Figure 1.6). The introduction of OH groups on the phenyl rings allows stabilizing the CO2 adduct owing to the internal hydrogen bonding between –OH groups and the CO2 molecule bound to the metal center. FeTPP could reach TONs of 5.5 for H2 and less than 0.5 for CO after 1h irradiation but was poorly selective, with only 8% selectivity for CO. In contrast, CAT and FCAT were highly selective for CO, with a product selectivity reaching 93% for CAT and 85% for FCAT. All the catalytic tests were conducted in acetonitrile with TEA as an electron donor, 22 with or without trifluoroethanol and without external photosensitizer. The production rate of CO is linear for a few hours before decaying, showing the limited stability of the molecular catalyst.
The same group reported a few years later the catalytic activity of CAT with the addition of [Ir(ppy)3]+ or 9-cyanoanthracene as photosensitizer.28 The addition of a photosensitizer not only increases the TONs of the system but also its stability, with no decaying production even after 50h. A last example of porphyrin design was reported by the same group in 2017. Rao et al.29 reported the catalytic activity for CO2 reduction of a trimethylammonium substituted FeTPP on the p-position in water with purpurin as a photosensitizer. The catalyst is 95% selective for CO and reaches TONs of 120 after 47h. Interestingly, when working with [Ir(ppy)3]+ as a photosensitizer the catalyst is highly selective for CH4 (82%). They propose that the high reduction property of [Ir(ppy)3]+ might be necessary to reduce the carbonyl from the FeII-CO adduct.30
Table of contents :
Chapter 1: Molecular catalysts and polyoxometalates, immobilization in Metal-Organic Framework
1. Homogeneous molecular catalysts
1.1 Molecular catalysts for CO2RR
1.2 Polyoxometalates, fully inorganic molecular catalysts
2. Metal-Organic Frameworks
2.1 MOF structural description
2.2 Zirconium-based MOF
3. Encapsulation strategies
3.1 Synthetic encapsulation
3.3 Ship-in-a bottle
3.4 Charge compensation
4. Characterizations of Cat@MOF composites
4.1 Determination of the composition (ICP, solution NMR, UV-vis, TGA, EDX)
4.2 IR and Raman spectroscopy
4.3 BET measurements
4.4 Solid-State NMR
4.5 X-ray diffraction
Chapter 2: Co-immobilization of a Keggin POM and a Rh- catalyst in UiO-67 MOF: in-depth characterization and CO2 photosensitized reduction
1. State of the art for Rhodium bipyridine complexes for CO2 reduction
2. Synthesis and Characterizations of the (PW12,Cp*Rh)@UiO-67 composite
3. In-depth Characterizations of the (PW12,Cp*Rh)@UiO-67 composite
3.1 2D Solid-state NMR
3.2 DFT calculations
3.3 Pair Distribution Functions analysis
4. Photocatalytic reduction of CO2
4.1 Photocatalytic activity
4.2 Post-catalysis characterization
4.3 Mechanistic investigation
5. Conclusions and perspectives
6. Experimental Section
Chapter 3: Photocatalytic reduction of CO2 by Porphyrinic Metal-Organic Framework
1. State of the art of Zr6-TCPP MOFs for CO2 reduction
2. Synthesis and characterizations of metalated and nanosized MOF-545
3. Photocatalytic reduction of CO2
4. Reaction mechanism for CO2 reduction to formate in MOF-545
4.1. DFT Calculation
4.2. Complementary photocatalytic tests
5. Co-catalyst immobilization in MOF-545
5.1. Synthesis, characterization and photocatalytic activity of the cat@MOF composites
5.2. Synthesis and characterization and photocatalytic activity of POM@MOF composites
7. Experimental section
Chapter 4: P4Mo6-based polyoxometalates for CO2 heterogeneous photoreduction
1. Polyoxometalates for CO2 photoreduction
2. Synthesis and characterization
2.2 Structure description
3. Photocatalytic properties
4. Mechanistic study
6. Experimental section