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Polyoxometalates, fully inorganic molecular catalysts
Polyoxometalates (POMs) are anionic molecular metal oxides of the early transition metals, which have been recognized for their remarkable redox activity and their ability to undergo reversible multi-electronic reduction processes. They have accordingly found numerous applications in electrocatalysis, energy storage, and photochemical processes, assorted with a wide variety of structures, as described in more detail below. POMs have been discovered in 1826 by Berzelius,40 but structurally characterized only a century later by Keggin, in 1993.
Polyoxometalate structures
POMs are constituted of MOx polyhedra, mainly octahedra (x = 6), but polyhedra with x = 4 or 5 are also encountered, with M being mainly W(VI/V), Mo(VI/V) and V(V/IV). The polyhedra are linked by their vertices, edges or faces with bridging oxygen atoms noted (µ-O) for vertices, (µ-O)2 for edges and (µ-O)3 for faces (Figure 1.9). Polyoxometalates are exclusively anionic species, their charge is compensated either by protons for their acidic form, or by alkaline cations. These counterions can be exchanged by organic counter cations, for example tetrabutylammonium (TBA) in order to increase the POM’s solubility in organic solvents,42 or to form ionic liquids.
Figure 1.9: Polyoxometalates building bricks and connectivity by their vertices (μ-O), edges (μ-O)2 or faces (μ-O)3. POMs can include heteroatoms from group p- or d-, such as phosphorus, silicon, boron for the p-group or cobalt for the d-group. This divides the POMs in two categories, isopolyoxometalates, on the one hand, with the general formula [MxOy]n-, without heteroatoms at their center and, heteropolyoxometalates, on the other hand, with the general formula [XxMyOz]n- with X being a heteroatom. The most studied POMs are represented in Figure 1.10: for the isopolyoxometalate family, the Lindqvist [M6O19]n, the decavanadate [V10O28]6- and the paratungstate [H2W12O42]10- and for the heteropolyoxometalate family, the Keggin [X M12O40]n- and the Dawson [X2M18O62]n-.
Metal-Organic Frameworks
Metal-organic frameworks, also called porous coordination networks (PCN), are crystalline and porous solids. MOFs are hybrid materials composed of organic linkers coordinated to metal ions or metal oxoclusters. The diversity of possible metal precursors and organic linkers leads to a huge variety of MOFs in terms of structure and chemical composition. Moreover, the same {metal ion, organic linker} combination can give several different metal-organic frameworks, making their phase diagram particularly complex and sensitive to synthetic conditions.69 In order to differentiate these unique structures, they are labeled with a prefix (usually MOF, PCN or the university where the MOF has been discovered) and a number. Nonetheless, the same structure can sometimes be discovered by two different teams at the same moment resulting in two different names for the same MOF. To date, more than a hundred thousand MOFs have been synthesized and more than half a million predicted.
MOFs present several advantages such as thermal and chemical stability, high surface area and the possibility to accommodate the molecular catalytic guest into their cavities, with, however, some variations from one MOF to the other. Looking at a recent review we published,71 it can be concluded that the Cr-based MIL-101 MOF has been by far the most studied one for molecular catalyst’s immobilization. We can propose several reasons: MOF-101 can be synthesized in water with high yields, and its Cr-based version is very stable while possessing large cavities and windows capable of accommodating a large range of bulky species (see below). However, its main drawback is that it contains chromium ions which are considered as one of the most toxic heavy metal ions. MIL-100(Fe) and NH2-MIL-101(Al) have thus also been largely investigated as alternative platforms. Zr-based MOFs also appear more eco-friendly and among them, UiO-66 and UiO-67 are the most commonly used but the porphyrinic Zr-based MOF-545 has also been encountered. Finally, among the other MOFs, HKUST-1 and ZIF-8, copper- and zinc-based MOF respectively, seem to be the most popular. A representation of the main MOF hosts with their formulas and the dimensions of their cavities is given in Figure 1.14 and a short description of their structure as well as their stability is given below. Both the structure and stability of the MOF are obviously important parameters to take into account when choosing a MOF as a platform to immobilize a catalytic species.
MOF structural description
In this subsection, we will introduce the main metal-organic frameworks used as immobilization platforms for catalysts. Then we will emphasize on zirconium-based MOFs, which are the main focus of this PhD thesis.
HKUST-1, also called Cu-BTC, is one of the oldest MOF, first synthesized in 1999 by Chui et al..72 HKUST-1 is composed of dimeric cupric tetra-carboxylate units. Each copper dimer is linked to four different benzenetricarboxylate through 8 oxygen, Cu ions complete its coordination with aqua ligands. Each BTC linker is connected to three Cu dimers leading to a paddle-wheeled framework. Cu-BTC has 18.6 Å diameter hexagonal cages and 13 Å diameter square pore windows.72 This MOF exhibits a relatively low surface area of 690 m²·g-1. Cu-BTC is known to be unstable to water and steam leading to a partial transformation after a short period of time but is stable to air and oxidative conditions (5% H2O2) for a few days.
ZIF-874 is a zeolitic imidazolate framework where two zinc tetrahedra are bridged with an imidazole forming a sodalite-type network. ZIF-8 has a unique pore type of 12 Å diameter and a window made of six zinc clusters with an aperture diameter of 3.4 Å. The empty ZIF-8 BET surface area is equal to 1630 m²·g-1. ZIF-8 has been claimed to be highly stable to air, water, steam showing no degradation over a few days in those conditions. It can withstand acid and basic condition (from ~ pH 4 to pH 12) for several days and oxidative conditions for the same duration.73
MIL-101(M) (M = CrIII, FeIII, AlIII)75 is built on MIII octahedra trimers connected with benzenedicarboxylate (BDC) linkers giving the general formula M3O(BDC)3(H2O)2X (X = F, Cl, NO3, CH3COO-, C6F5COO-).76 The structure displays two types of mesoporous cages, the first one with a free diameter of 29 Å and pentagonal windows of 12 Å, and the second and larger cage with a free diameter of 34 Å and both pentagonal windows of 14×14 Å and hexagonal windows of 16×16 Ų.75 MIL-101(M) displays a gigantic cell volume of 702 000 Å3 and a huge BET surface area of around 4500 m²·g-1 allowing the hosting of large molecules. Moreover, MIL-101(M) is easily functionalized via post synthetic modification or via direct pre-synthetic modification of the BDC linker (adding NH2, NO2, SO3 or other functions)77 which allows great control and uniformity of the MOF. MIL-101(M) stability is highly linked to the nature of the metal and the BDC functionalization. MIL-101(Cr) is highly stable to water (as solvent, moisture or even steam) and can last several days in boiling water without any structural degradation. The MIL-101 Fe and Al homologues show much lower stability toward hydrolysis, with for example NH2-MIL-101(Al) transforming into the more thermodynamically stable NH2-MIL-53(Al) after only 5 min exposure to water.78 MIL-101(Cr) is stable to both acidic and basic conditions for several weeks, and stable to oxidative conditions (5% H2O2) for a few days. Again, Al and Fe homologues show poor stability toward acidic or basic condition with partial transformation into MIL-53 and high loss of porosity.
MIL-10080 is built on metal octahedra trimers connected by their summit and interconnected by BTC linkers giving the general formula M3O(BTC)2(H2O)2X (X = F, Cl, SO4).81 Four metal trimers are linked together by BTC forming a supertetrahedron (ST), the STs being linked to each other by BTC forming a pentagonal mesoporous cage (20 STs) or a hexagonal mesoporous cage (28 STs). The smallest cage has a diameter of 25 Å and pentagonal windows of 4.8×5.8
Å. The largest cage displays a diameter of 29 Å and has hexagonal windows with an aperture of 8.6×8.6 Ų. Those two cages lead to a huge surface area of 3100 m²·g-1. MIL-100 MOFs display high water stability and no degradation after 24h at 323K82 and are stable in most solvents. However, again, the thermal stability is highly linked to the nature of the metal node, being stable up to 270, 325 and 370°C for MIL-100(Fe), MIL-100(Cr) and MIL-100(Al) 34 respectively.83 MIL-100 MOFs are mainly stable in acidic pH and start degrading at pH 7 as shown for MIL-100(Fe) by Bezverkhyy et al..84
Zirconium-based MOF
Zirconium-based MOFs are made of zirconium oxoclusters. The zirconium building bricks can either be consisted of 6 or 8 Zr(IV) ions.85 The MOFs studied in this PhD thesis are solely composed of Zr6 oxoclusters with formula Zr6O4(OH)4. The Zr6 cluster can be coordinated to either 6, 8 or 12 ligands, being fully coordinated with 12 ligands (Figure 1.15). When the Zr6 cluster is unsaturated (coordination less than 12), the free sites are occupied by OH or H2O groups. In this thesis, we will focus on two MOFs, namely UiO-67 (12 coordinated Zr6) and MOF-545 (8 coordinated Zr6). We will thus describe the UiO MOF family and the Zr6-porphyrin MOF family in more detail.
Determination of the composition (ICP, solution NMR, UV-vis, TGA, EDX)
A combination of several techniques can be used to determine the composition of the Cat@MOF materials and give access to the Cat loading. The control of the loading is important in view of catalytic applications. A material with only a small amount of immobilized catalyst will have low performances. On the other hand, overloading can block the cages and windows and thus the accessibility of the reactants to the catalytic species. This has for example been described for PW12@MOF-808X (X = F, A, and P where the coordinated monocarboxylate group is formic, acetic or propionic, respectively).138 The precise knowing of the composition of the Cat@MOF is also essential for the determination of the turn over numbers (TONs) in catalytic studies. The most common and also the most precise technique of determination of the composition is the ICP analysis of digested solutions of Cat@MOFs. For example the Co/In ratio of a composite [Cp2Co]@[R4N]3[In3(BTC)4] (R = Et, nPr, nBu) digested in diluted HNO3 (1/10 v/v) was measured by ICP-OES and allowed to quantify the amount of immobilized complex.137 Note that ICP is also a powerful technique to monitor uptake of catalysts during impregnation reactions by an analysis of the impregnation solutions and has for example been used for PW12@NU-1000.132 This analytic method also allowed to follow POM leaching when the same material was dispersed in acidic aqueous solutions.
Several other techniques can also be used to apprehend the composite composition. UV-vis spectroscopy can thus allow monitoring the incorporation of the catalyst in the MOF during the impregnation and to estimate the amount of Cat immobilized in the MOF. For example, the decrease of the bands around 250 nm (assigned to the charge transfer from the bridging O-atoms to W-atoms) allowed to follow qualitatively the immobilization of PW9,139 PW11, 140 SiW11,140 PW11Ti141 and PW11Co141 in MIL-101(Cr). A detailed UV-vis study was also performed to measure the catalyst concentration in the supernatant after the impregnation experiment of various Ru catalysts in NH2-MIL-101(Al) and MIL-101(Cr) MOFs142 and of a Rh catalyst and a Ru photosensitizer in NH2-MIL-101(Al).113 This study allowed to determine the catalyst loading and to find the best solvent for their immobilization. The analysis of digested solutions of the composite instead of the supernatant solutions should allow a more accurate determination of the catalyst loading. Indeed, due to Cat species adsorbed at the surface of the MOF crystallites that go out after careful washing, the amount of immobilized Cats is overestimated by an analysis of the supernatant solutions.
Thermogravimetric analysis (TGA) can also be used to confirm the composition of a Cat@MOF. A typical example is represented in Figure 1.21 for a PW11Zn@MIL-101(Cr) composite.143 The first weight loss is attributed to the departure of solvent molecules, the second weight loss to the decomposition of the framework. The total weight loss for the POM@MOF is lower than for the parent MOF due to the presence of POM guests which decompose into oxides. Thus, the comparison of the weight loss calculated from the results of elemental analysis with the experimental weight loss allow confirming the formula of the composite and in particular the guest loading. TGA also allows studying the thermal stability of the composites. It can be noted that the temperature of decomposition of POM@MOF composites is usually very close to that of the MOF, which indicates that the insertion of POMs only slightly affects their thermal stability. This is also the case for the rare examples of TGA curves reported for Complex@MOFs. A few exceptions can be noticed. For example, the SiW12@HKUST-1 (also named NENU-1) composite, after thermal treatment (180°C under vacuum) to remove TMA+ cations and H2O molecules (NENU-1a material), is stable up to 300°C while the POM-free MOF is stable only up to 240°C. The authors attributed this difference to physicochemical interactions between the POM and the MOF framework.
IR and Raman spectroscopy
In all the reported Cat@MOF studies, IR spectra are recorded to check whether the catalyst and MOF structures are preserved. This technique is also routinely performed to study the stability of the composites after the catalytic experiments. For example, for the PW12 catalyst, characteristic P-O, W=O and W-O-W bands are observed around 1000, 980 and between 800 and 900 cm-1 respectively. Red shifts have sometimes been described between the POM precursor and the encapsulated POM. A red shift of about 30 cm-1 was thus observed for the P-O and W=O bands of PW12 immobilized in MOF-808X.138 The P–O vibration was also shifted from 1030 to 1048 cm−1 from P2W18Co4 to P2W18Co4@MIL-101(Cr) (Figure 1.23 a).112 These shifts were attributed to strong interactions between the POM and the MOF. IR spectroscopy can also evidence the catalyst transformation after its immobilization in the MOF.
Table of contents :
Chapter 1: Molecular catalysts and polyoxometalates, immobilization in Metal-Organic Frameworks
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
3.2 Impregnation
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
5. References
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
5.1 PMo12@UiO-67
5.2 PW10Ti2@UiO-67
5.3 (PW12,Cp*Rh)@POP
6. Experimental Section
7. References
8. Appendix
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
6. Conclusions
7. Experimental section
8. References
9. Appendix
Chapter 4: P4Mo6-based polyoxometalates for CO2 heterogeneous photoreduction
1. Polyoxometalates for CO2 photoreduction
2. Synthesis and characterization
2.1 Synthesis
2.2 Structure description
2.3 Characterization
3. Photocatalytic properties
4. Mechanistic study
5. Conclusion
6. Experimental section
7. References
8. Appendix
General Conclusion
