Metal organic framework based catalysts for CO₂ conversion

Abstract

Metal organic frameworks (MOFs) are hybrid crystalline materials, exhibiting high specific surface areas, controllable pore sizes and surface chemistry. These properties have made MOFs attractive for a wide range of applications including gas separation, gas storage, sensing, drug delivery and catalysis. This review focuses on recent progress in the application of MOF materials as catalysts for CO₂ conversion through chemical fixation, photocatalysis and electrocatalysis. In particular, this review discusses the co-relationship between the physicochemical properties of MOF materials including their catalytic performance as well as their stability and recyclability under different reaction conditions, relevant to CO₂ conversion. Current modification techniques for improving MOF performance are highlighted along with the recent understanding of their electronic properties. The limitations of MOF based catalysts are also discussed and potential routes for improvement are suggested.

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1. Introduction

Increased emission of carbon dioxide (CO₂) from the combustion of fossil fuels is the primary cause of global warming, leading to adverse climate changes and ocean acidification.^1,2 Carbon capture and storage (CCS) technologies, whereby CO₂ is captured from a point source or the atmosphere, prior to being stored or injected into an underground reservoir, have been proposed as potential pathways to reduce anthropogenic emissions.^2–5 Although storage in porous materials such as metal organic frameworks (MOFs)⁶ and zeolites has also been proposed, long-term storage in underground reservoirs is currently the leading proposal, due to their large capacity for holding this greenhouse gas. However, the high costs associated with CO₂ transport and injection to the geologically suitable storage sites render such a process energy-intensive and cost prohibitive.³ In addition, to ensure that the large amounts of CO₂ stored underground do not leak back to the atmosphere, complex infrastructures and frequent monitoring are required, which currently have limited feasibility and require extremely long term planning.⁷

The utilization of CO₂ as a chemical feedstock to produce valuable chemicals and fuels is an alternative sustainable approach for mitigating the adverse effect of this greenhouse gas,^8,9 offering a more sustainable business model for the CCS industry. Utilization technology may be employed directly to emission sources using appropriate materials that are able to selectively capture CO₂ and facilitate catalytic conversion, or maybe used in conjunction with other CO₂ capturing technologies.^10–13 This greenhouse gas may be converted into valuable products through chemical fixation,^14,15 hydrogenation,¹⁶ photocatalysis^9,17 and electrocatalysis.⁸ However, CO₂ utilization is limited by the high chemical stability of the C=O bond (bond enthalpy +805 kJ mol⁻¹), thus requiring high energy input for bond cleavage.⁹ Significant efforts are therefore currently being devoted to developing catalytic materials with a high surface area, large adsorption capacity for CO₂, and both high and stable catalytic activity to facilitate the energy efficient conversion of CO₂.

Amongst potential candidates, metal organic frameworks (MOFs) are a class of hybrid materials composed of metal ions or clusters coordinated by organic ligands.¹⁸ These materials exhibit exceptionally high specific surface area, tunable pore size distribution and surface chemistry, which make them attractive not only in gas and liquid surface driven capture and separation applications and heterogeneous catalysis, but also in sensing and drug delivery.^1,18–21 The chemical composition and microstructure of some MOF materials that have been investigated for CO₂ reduction are summarized in Table 1. Over the last two decades, MOF materials have been extensively studied for selective gas adsorption, where they have been shown to outperform other materials, such as zeolites, silica and metal oxides, in terms of adsorption capacities for CO₂, as reviewed in a number of articles.^1,22–26 These high adsorption capacities are attributed to their high porosity and surface area, as well as the chemical interaction between CO₂ molecules and metal centers or organic ligands within MOFs.¹ This high affinity for CO₂ chemisorption has inspired research on MOF based catalysts for CO₂ fixation and conversion.^27,28 The catalytic properties of MOF materials can be readily controlled by the judicious selection of metal ions and organic ligands, by modification with catalytically active functional groups,¹⁸ and by tuning the morphology to maximize the surface area and/or enhance charge separation.²⁹

Table 1 The chemical composition and structure of some investigated MOF materials

Entry	MOF	Metal	Organic ligand	SABET (m^2 g^−1)	Structure	Catalytic cycles	Ref.
1	MOF-74	Mg, Zn, Co	2,5-Dihydroxybenzene-1,4-dicarboxylic acid	393–1027		3	38–40
2	HKUST-1	Cu	1,3,5-Benzenetricarboxylic acid	1055		1	41 and 42
3	MOF-505	Cu	1,3-Benzendicarboxylic acid	2713		1	43 and 44
4	MMCF-2	Cu	Custom designed azamacrocycle	450		1	43
5	Hf-NU-1000	Hf	1,3,6,8-Tetrakis(p-benzoic acid)pyrene (H4TBAPy)	1780		1	45
6	MIL-101	Cr, Al, Fe	1,4-Benzodicarboxylic acid	4100		3	49 and 42
7	Ru-MOF	Cd, Ru	2,2′-Bipyridine-4,4′-dicarboxylate	8.08		4	29
8	PCN-222	Zr	Tetrakis(4-carboxyphenyl)-porphyrin (H2TCPP)	1728		1	46
9	Ni-TCPE1	Ni,	Tetrakis(4-carboxyphenyl)ethylene	N/A		20	47
10	Ni-TCPE2	Ni	Tetrakis(4-carboxyphenyl)ethylene	N/A		10	47
11	ZIF-8	Zn, Co	2-Methylimidazole	1200–1400		3	48 and 49
12	MOF-5 (IRMOF-1)	Zn	1,4-Benzodicarboxylic acid	3800		1	50 and 42
13	MIL-53	Sc, Fe, Al, Cr	1,4-Benzodicarboxylic acid	1100		3	51 and 52
14	MIL-68	In, Ga, V	1,4-Benzodicarboxylic acid	603		2	53–55
15	Cu4[(C57H32N12)(COO)8]n	Cu	Octcarboxylate	2436		5	56
16	[Zn6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O	Zn	4,4′,4′′-s-Triazine-1,3,5-triyl-tri-p-aminobenzoic acid, and (1,4-diazabicyclo[2.2.2]-octane)	61.4		6	57
17	UMCM-1	Zn	Terephthalic acid (H2BDC), 1,3,5-tris(4-carboxyphenyl)benzene (H3TBT)	4160		5	58
18	ZIF-68	Zn	Benzimidazolate, Nim	791.13		3	59−61

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This review focuses on the recent progress in the application of MOF based catalysts in the fixation and conversion of CO₂. Although there have been a number of recent reviews on this topic,^{17,27,30–37} there is a need for a comprehensive review to highlight the co-relationship between the physicochemical properties of MOF materials and not only their catalytic activities, but also their stability and recyclability, under different reaction conditions relevant to CO₂ conversion. This review also discusses recent modification techniques for improving the catalytic performance of MOF materials, and highlights recent understanding of their electronic properties. The review discussion is divided into four sections, where the first section gives a brief introduction on the stability of MOF based catalysts. The second section presents MOF catalysts for the chemical fixation of CO₂ with epoxides, while the third part covers MOF materials that have been reported to exhibit photocatalytic activity towards CO₂ conversion. The fourth section reviews electrocatalytic MOF materials. Particular challenges of MOF based catalysts as well as research opportunities for further development are highlighted and critically discussed.

2. Chemical stability of MOF catalysts

Due to the potential degradation of MOF catalysts during catalytic reactions,^62–64 detailed characterization to compare their properties before and after catalysis is necessary. The bond between the metal ion and the organic linker is the weakest point in the structure, since the MOF crystals may be degraded via hydrolysis or displacement of organic ligands with solvent molecules.^63,64 Depending on the properties of the metal centers such as the coordination number and the hydrophobicity or chemical structure of the organic ligand, MOF materials exhibit different levels of stability in the presence of solvents.^63,65,66 Metals with high coordination numbers exhibit higher stability because they can coordinate multiple ligands, which can hold the structure together even after the cleavage of some bonds.⁶³ In addition, the strength of the metal ligand bond varies depending on the nature of metal ions. For example, the Al–O bond across MIL-53 exhibits a higher strength of ∼520 kJ mol⁻¹ as compared to the Zn–O bond across MOF-5, which has a bond strength of 365 kJ mol⁻¹.^62,66 Consequently, a higher activation energy (180 kJ mol⁻¹) is required for the displacement of linkers in MIL-53 by water, as compared to MOF-5 (50 kJ mol⁻¹).⁶² MOF structures with a highly hydrophobic ligand, on the other hand, exhibit high stability in hydrophilic solvents, since the solvent molecules cannot access the metal centers.⁶⁴

XRD analysis after the exposure to the reaction conditions may reveal details about the integrity of the crystal structure, but may not reveal partial collapse of the pore structure, or surface degradation.⁶² Comparison of the morphology of the MOF catalysts before and after reactions reveals any surface degradation, while a decrease in the BET surface area maybe an indication of the partial collapse or blockage of pores.⁶² TGA and FTIR analysis, on the other hand, could shed light on any change in chemical composition following the catalytic reactions.

3. Chemical fixation of CO₂ with epoxides to form cyclic carbonates

3.1. Introduction

CO₂ captured from power plants may be utilized as a raw material for the synthesis of cyclic carbonates, which have important applications as precursors in the synthesis of polymers, and as electrolytes in lithium ion batteries and aprotic solvents.^15,67–69 This is accomplished through cycloaddition reaction, a process first disclosed in 1943 by Vierling et al.,^70,71 where CO₂ is reacted with epoxides, in the presence of a Lewis acid catalyst and a Lewis base co-catalyst, at elevated temperature or pressure, to form a cyclic carbonate. A schematic illustration for the cycloaddition reaction is presented in Scheme 1.⁷²

Scheme 1 Cycloaddition of CO₂ to epoxides.

As recently reviewed by Martin et al. (2015)⁶⁹ most catalysts for the cycloaddition reaction require high temperature (above 100 °C) and a CO₂ pressure higher than 9.9 atm to achieve a sufficient yield. Among the different catalysts, homogenous catalysts, such as metal complexes of porphyrin, salen, and salphen, display the highest activity, with high turnover numbers (TONs, i.e. moles of product formed per mole of catalyst) and turnover frequency (TOF, i.e. TON per time).^15,69 For example, bifunctional Mg^II porphyrin with 8 tetraalkylammonium bromide groups has been reported as being among the most efficient homogeneous catalysts to date, with a TON of 138 000 and a TOF of 19 000 h⁻¹.⁷³ However this impressive performance was achieved at a high CO₂ pressure of 16.8 atm and a temperature of 120 °C, which may be an impediment to an economical and environmentally friendly conversion process. Another limitation of homogenous catalysts is their high solubility in the organic solvents used during cycloaddition reactions, complicating the purification procedures and recyclability.⁴⁵ Although the issue of recyclability could be addressed by mobilizing the homogeneous catalyst on solid supports such as silica⁷⁴ and polymers^75,76 to make heterogeneous catalysts, high temperature and/or pressure was still necessary (Table S1, ESI†), and their product yield and selectivity were lower due to the inhomogeneous distribution of the catalytic sites.¹⁴ Consequently, the development of more efficient and recyclable catalysts that can operate near ambient pressure and temperature is a topic of great interest and will be hereafter discussed.

3.2. MOF materials as heterogeneous catalysts for the cycloaddition of CO₂ to epoxides

Owing to their crystalline nature, chemical tunability, exceptionally high surface area, and regular pore structure, MOF materials have recently gained attention as potential recyclable alternatives to homogeneous catalysts.²⁷ Vacant or partially coordinated metal ions are readily available Lewis acid sites to activate epoxides while the organic ligand may be functionalized with Lewis basic sites or additional metal ions to enhance the catalytic activity.²⁷

As summarized in Table 2, a broad range of MOF catalysts for cycloaddition reactions have been reported. Herein the MOFs are divided into three categories, namely (i) MOFs with active Lewis acid metal sites, (ii) MOFs with both Lewis acid and Lewis base active sites, and (iii) MOFs with active defect sites. The cycloaddition reactions may be carried out not only in the presence of solvents such as chlorobenzene and acetonitrile, but also in the absence of solvents for substrates that are in the liquid state such as propylene oxide and epichlorohydrin.^77,78 The solvents play a critical role in facilitating the efficient transport of the substrates to the catalytic sites, as well as extraction of catalytic products from the catalyst surface. However, the solvents may also retard the catalytic reactions via competitive binding at the metal centers,²⁷ and also contribute to the degradation of the MOF catalysts.^63,64 MOF materials that are able to exhibit high catalytic efficiency and recyclability in the absence of a solvent are therefore of great interest.

Table 2 Application of MOFs in the cycloaddition of CO₂ to epoxides

Entry	MOF catalyst	Pore (Å)	Substrate	Reaction conditions	% Yield per cycle			Ref.
					1st	2nd	3rd
1	Mg-MOF-74	12	Styrene oxide	100 °C, 19.7 atm, 4 h	95	95	95	40
2	Co-MOF-74	12	Styrene oxide	100 °C, 19.7 atm, 4 h	96	—	—	82
3	Hf-NU-1000	13 & 29	Styrene oxide	RMT, 1 atm, 56 h	100	—	—	45
4	MMCF-2	—	Propylene oxide	RMT, 1 atm, 48 h	95.4	—	—	43
5	MOF-505	8.3	Propylene oxide	RMT, 1 atm, 48 h	48	—	—	43
6	[Zn6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O	—	Propylene oxide	100 °C, 1 atm, 16 h	99	99	99	57
7	BIT-103	—	Propylene oxide	160 °C, 30 atm, 24 h	95.2			83
8	BIT-102	—	Propylene oxide	160 °C, 30 atm, 24 h	89.4			83
9	BIT-101	—	Propylene oxide	160 °C, 30 atm, 24 h	84.7			83
10	[Cd2(Ni-L)2(H2O)4]·3DMF	—	Chloropropylene oxide	80 °C, 19.7 atm	80	—	—	84
11	Ni-TCPE1	21	Styrene oxide	100 °C, 9.9 atm, 12 h	>99	97	91	47
12	Ni-TCPE2	—	Styrene oxide	100 °C, 9.9 atm, 12 h	86.2	—	—	47
13	In2(OH)(btc)(Hbtc)0.4(L)0.6·3H2O	—	Propylene oxide	80 °C, 19.7 atm, 4 h	93.9	93.9	93.9	85
14	Cu4[(C57H32N12)(COO)8]n	1.11	2-Methyloxirane	RMT, 1 atm, 48 h	96	96	96	56
15	Cu4[(C57H32N12)(COO)8]n	1.11	2-Ethyloxirane,	RMT, 1 atm, 48 h	83	—	—	56
16	Cu4[(C57H32N12)(COO)8]n	1.11	(Chloromethyl)oxirane	RMT, 1 atm, 48 h	85	—	—	56
17	Cu4[(C57H32N12)(COO)8]n	1.11	2-(Bromomethyl)oxirane	RMT, 1 atm, 48 h	88	—	—	56
18	MIL-68(In)	—	Styrene oxide	150 °C, 7.9 atm, 8 h	39	—	—	55
19	MIL-68-NH2	—	Styrene oxide	150 °C, 7.9 atm, 8 h	74	53	—	55
20	UiO-66-NH2	—	Styrene oxide	100 °C, 19.7 atm	96	95	95	86
21	UMCM-1-NH2	17–28	Propylene oxide	RMT, 11.8 atm, 24 h	90	90	90	77
22	MIL-101-N-(n-Bu)3Br	13–27	Propylene oxide	80 °C, 19.7 atm, 8 h	98.6	—	98.5	42
23	MIL-101-P(n-Bu)3Br	—	Propylene oxide	80 °C, 19.7 atm, 8 h	99.1	—	98.1	42
24	HKUST-1	10	Propylene oxide	80 °C, 19.7 atm, 8 h	5.4	—	—	42
25	MOF-5	4–6	Propylene oxide	80 °C, 19.7 atm, 8 h	2.5	—	—	42
26	ZIF-8	3.4	Epochlorohydrin	80 °C, 6.9 atm, 4 h	43.7	22.7	—	78
27	ZIF-8-NH2	—	Epochlorohydrin	80 °C, 6.9 atm, 4 h	73.1	30.6	—	78
28	ZIF-68	3.4–8.6	Styrene oxide	120 °C, 9.9 atm, 12 h	93.3	88.3	80.9	61 and 87
29	ZIF-8	3.4	Styrene oxide	80 °C, 6.9 atm, 5 h	39.4	37	37	49
30	PCN 224 (Co)	19	Propylene oxide	100 °C, 19.7 atm, 4 h	42	33	39	88
31	Cr-MIL-101	14.5 × 16	Styrene oxide	25 °C, 8 atm, 48 h	95	64	52	81 and 89
32	Cr-MIL-101	14.5 × 16	Propylene oxide	25 °C, 8 atm, 48 h	91	92	82	26 and 43
33	ZIF-22	3	Epochlorohydrin	120 °C, 11.8 atm, 2 h	99	—	—	90
34	ZIF-22	3	Propylene oxide	120 °C, 11.8 atm, 2 h	99	—	—	90
35	ZIF-67	—	Epichlorohydrin	100 °C, 8 atm, 8 h	98	98	—	91

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3.2.1. MOFs with active Lewis acid metal sites. Unsaturated metal centers within MOFs serve as Lewis acid sites to activate epoxides towards cycloaddition of CO₂.¹⁴ Examples of MOFs with catalytically active metal centers include, M-MOF-74, HKUST, MOF-505, Hf-NU-1000, Fe-MIL-101, and Ni-TCPE1^{40,45,47,79–81} (Table 1, entries 1 to 9). The catalytic efficiency of Mg-MOF-74 in the cycloaddition of CO₂ to styrene oxide was investigated at 100 °C and 19.7 atm of CO₂.⁴⁰ An optimal yield of 95% was achieved after 4 h with 30 mg of the catalyst in chlorobenzene solvent. While most catalytic systems require a Lewis base co-catalyst, there was no co-catalyst necessary for Mg-MOF-74. The oxygen atoms from the organic linkers acted as Lewis base sites, while unsaturated Mg atoms acted as Lewis acid sites. The stability of the MOF materials was also confirmed after three repeated runs which showed no decrease in catalytic activity, while powder XRD showed no change across the diffraction patterns suggesting that the crystal structure of the MOF was stable even after the catalytic reactions.

An Hf based MOF, Hf-NU-1000 (Table 1, entry 5), has also been investigated for the cycloaddition of CO₂ to epoxides under mild conditions, in the presence of a tetrabutylammonium bromide co-catalyst, with acetonitrile as the solvent.⁴⁵ The MOF achieved a yield of 100% in the cycloaddition of CO₂ to propylene oxide, following 26 h of reaction at 1 atm pressure and at room temperature (Table 2, entry 3). This efficiency was shown to be higher than that of other MOFs such as HKUST-1, MOF-505, MMPF-9, and MMCF-2, which achieved a yield of 49, 48, 87, and 95%, respectively, under similar experimental conditions but longer reaction durations of 48 h.⁴³ The superior performance of the Hf-NU-1000 MOF was attributed to the high density of Hf Lewis acid sites, and the relatively large pore size (13–29 Å).

Aiming at increasing the density of Lewis acid sites, Gao et al.⁴³ designed a metal macrocycle framework (MMCF-2), by crystal engineering of MOF-505 (Table 1, entry 3). The 3,3′,5,5′-biphenyltetracarboxylate ligands, typically present in MOF-505, were substituted with a custom designed azamacrocycle ligand (1,4,7,10-tetrazazcyclododecane-N,N′,N′′,N\-tetra-p-methylbenzoic acid), to yield a new MOF (Table 1, entry 4), where each of the six faces of the cuboctahedral cage is occupied by a Cu^II metallated azamacrocycle, leading to 18 Cu²⁺ Lewis acid sites per cage, 50% higher than its parent MOF-505, which has only 12 Cu²⁺ sites. In addition, the Cu²⁺ sites of MMCF-2 are also more accessible, since they are located towards the center of the cage, as opposed to MOF-505 where the Cu²⁺ sites are located at the corner of the octahedral cages. These features resulted in improved catalytic efficiency with MMCF-2 having a yield of 95.4% in the conversion of propylene oxide into propylene carbonate, which was almost twice as efficient as compared to MOF-505 (48% yield), at room temperature and 1 atm of CO₂, after 48 h of reaction in a solvent free environment. The yield was also higher than those of a homogenous Cu catalyst (tactmb) and a benchmark Cu based MOF, HKUST-1, which had yields of 47.5% and 49.2%, respectively.⁴³ The recyclability and stability of the MMCF-2 MOF were, however, not reported.

Very recently, a dual wall caged MOF, having a high density of Lewis acid sites, has been reported.⁵⁷ The MOF was prepared by interpenetration of two independent but similar cages, Zn₂₄-A and Zn₂₄-B, leading to a higher number of Zn Lewis acid sites, with 48 Zn²⁺ per cage (Fig. 1a–c). Consequently, the MOF exhibited high efficiency for the cycloaddition of CO₂ with propylene oxide, with up to 99% yield, at 100 °C and ambient pressure after 12 h of reaction, in the absence of a solvent and a co-catalyst (Table 2, entry 6). This yield was higher than those of zinc-trimesic acid (Zn-BTC) MOFs, BIT-103, BIT-102 and BIT-101 (Table 2, entries 7–9), which had a yield of 95, 89.4 and 84.7%, respectively, despite being tested at a higher temperature of 160 °C and a pressure of 30 atm, for a duration of 24 h.⁸³ In addition, the MOF could be recycled up to 6 times, with only a 3% decrease in yield after the 6th run (Fig. 1d). Other than the high density of Lewis catalytic sites, the good catalytic performance was also attributed to the presence of amine groups across the organic ligand, which may increase the affinity for CO₂, and facilitate the activation of the CO₂ molecules upon adsorption.⁵⁵

Fig. 1 (a) View of the Zn₂₄-A cage (the distance between opposite vertices is ca. 34.4 Å). (b) View of the Zn₂₄-B cage (the distance between opposite vertices is ca. 30.2 Å). (c) View of the dual-walled cage. (d) The catalytic cycles for the cycloaddition of CO₂ to propylene epoxides using the dual-walled cage MOF. Reproduced from ref. 39 with permission from the Royal Society of Chemistry.

Another approach for increasing the density of Lewis acid sites is the use of metal–organic complexes as ligands for constructing MOFs.^18,23,24 Ren et al. in 2013⁸⁴ used a nickel salphen complex, Ni–H₂L, as a bridging metalloligand to construct MOFs with Cd metal centers. The presence of Cd²⁺ and Ni²⁺ was found to provide synergistic and additive activation effects, resulting in an enhanced catalytic activity to give 80% yield, in the synthesis of propylene carbonate in the presence of an ammonium salt co-catalyst (Table 2, entry 10). This yield was significantly higher than the 38% yield obtained when Ni–H₂L was used as a homogeneous catalyst under the same experimental conditions. The MOF could also be used for three cycles, with only a 2% decrease in the product yield in the third cycle, while XRD analysis after the catalytic reactions showed no noticeable change across the diffraction patterns.

The pore size has also been shown to play an important role in governing the efficiency of MOFs in cycloaddition reactions.^14,47 MOFs having relatively large pores facilitate the reaction by enabling the efficient diffusion of the reactants and products, while small pores may retard reaction by hindering diffusion. Zhou et al.⁴⁷ designed single walled metal organic framework nanotubes (Ni-TCPE1), using the tetrakis(4-carboxyphenyl)ethylene (H₄TCPE) ligand with the Ni metal ion (Table 1, entry 9). The Ni-TCPE1 MOF exhibited a large cross-sectional pore structure, enabling superior catalytic performance in cycloaddition of CO₂ to epoxides, in the presence of a tetrabutylammonium bromide (TBABr) co-catalyst. Catalytic studies were carried out for 12 h at 373 K, 9.9 atm CO₂ and in the absence of a solvent, to obtain almost complete conversion of styrene oxide into styrene carbonate (>99%) (Table 2, entry 11). Under these conditions, the MOF had a turnover number of 2000, which was more efficient than other MOFs such as Hf-Nu-1000, Nu-1000 and Cr-MIL-101, which had TONs of 25, 11.5, and 177.6, respectively, under the same experimental conditions. Furthermore, the MOF maintained its catalytic activity for over 20 cycles (70 h), achieving a TON value of 35 000. XRD analysis following the catalytic reactions confirmed that the crystal structure of the MOF was maintained. The excellent performance was attributed to the large cross-section of the nanotube channels that enabled the efficient transport of the substrate and products. The high stability was attributed to the nonplanar configuration of the H4TCPE ligand, which facilitated the formation of highly connected frameworks.

A triazole containing MOF (Cu₄[(C₅₇H₃₂N₁₂)(COO)₈]_n) for the cycloaddition of CO₂ to epoxides under ambient conditions has been reported.⁵⁶ The cycloaddition reactions were carried out at 1 atm CO₂ pressure and room temperature, for 48 h, with a tetra-n-tertbutylammonium bromide co-catalyst in the absence of a solvent. The reaction yields were found to be dependent upon the substrate size. For example, propylene oxide with a molecular weight of 58 Da leads to a product yield of 96%, while larger substrates 1,2-epoxyoctane (M_w = 128.21 Da) and 2-ethyl-hexy glycidyl ether (M_w = 186.29) lead to dramatically lower yields of 8% and 5%, respectively. The low yield implied that the larger substrate could not enter the pore framework and that the cycloaddition reactions were therefore only limited on the surface of the MOF crystals. Under similar conditions, the MOF crystals were found to be more efficient than HKUST-1, in the cycloaddition of CO₂ to 2-methyloxirane, 2-ethyloxirane, 2-(chloromethyl)oxirane, and 2-(bromomethyl)oxirane, with a yield of 65, 54, 56 and 57%, respectively, as compared to the 96, 83, 85 and 88% yields obtained with the triazole containing MOF. The recyclability of the MOF was confirmed by conducting 5 repeated cycles, with 95% yield of propylene carbonate being achieved in the 5th cycle. Powder X-ray diffraction (PXRD) analysis also demonstrated that the MOF maintained its crystal structure following the catalytic reactions.

By increasing the density of Lewis acid sites, the catalytic activity of MOFs towards the cycloaddition of CO₂ to epoxides may also be enhanced. MOF crystals with large pore channels enhance the accessibility of the Lewis acid sites and facilitate the efficient diffusion of substrates and products, leading to improved catalytic performance.

3.2.2. MOFs with both Lewis acid and Lewis base active sites. MOFs may also be constructed with linkers exhibiting Lewis base functionalities to eliminate the need for a co-catalyst.⁵ To this end, several MOFs containing both Lewis acid metal centers and Lewis base rich organic linkers have been designed for the cycloaddition of CO₂ to epoxides.^{55,86,92–94} The concurrent presence of the Lewis acid site and the basic acid site within the MOFs results in better performance as compared to MOFs without Lewis basic functionality.

For example, amine functionalized MIL-68(In) demonstrated an improved catalytic efficiency for the synthesis of styrene carbonate, as compared to pristine MIL-68 without such amine functionality (Table 2, entries 18 and 19).⁵⁵ The reaction was carried out for 8 h at 150 °C and 7.9 atm of CO₂, and in the absence of a co-catalyst with dimethylformamide (DMF) as the solvent, to obtain a conversion efficiency of 74%, which is significantly higher than the 39% conversion obtained in the absence of an amine functionality. XRD analysis after the catalytic reaction showed that the diffraction patterns of the powder remained unchanged. However, the catalytic efficiency decreased from 74% to 53% upon recycling, while BET analysis revealed a decrease in surface area from 1100 to 720 m² g⁻¹, following the catalytic reactions, potentially due to blockage of pores and catalytic sites by the products.

Amine functionalized UiO-66 crystals were also shown to be more efficient in the cycloaddition of CO₂ to styrene oxide compared to pristine UiO-66.⁸⁶ The reaction was carried out at 100 °C and 19.7 atm of CO₂ in chlorobenzene solvent, in the absence of a co-catalyst. Under these experimental conditions, the amine functionalized MOF achieved a conversion of 70%, which was 22% higher than the conversion obtained with a similar amount of non-functionalized UiO-66, under similar experimental conditions.

Babu et al.⁷⁷ very recently designed dual porous amine functionalized MOFs (UMCM-1-NH₂,), combining meso- and microporous structures for room temperature CO₂ fixation (Table 2, entry 21). The MOF materials exhibited a high product selectivity greater than 99% and a yield of 90%, in the cycloaddition of propylene oxide to propylene carbonate, in the presence of a TBAB co-catalyst at room temperature and 11.8 atm of CO₂, and under solventless conditions. These MOF crystals were found to outperform other reported MOF materials tested, such as MOF-5, which gave a yield of 93% at 50 °C and 4 atm, and Cr-MIL-101 which gave a yield of 82% at room temperature and the pressure of 8 atm.⁴⁵ The excellent performance was attributed to the dual porous structure, where mesoporosity allows easy accessibility and efficient transport of substrates and products, while micropores modulate the catalytic reactions. The UMCM-1-NH₂ MOF also maintained its catalytic activity upon recycling, with a yield of 89% in the 5th cycle. XRD analysis of the MOF powder revealed that the crystal structure was maintained after the catalytic reactions.

Ma et al. in 2015⁴² used post-synthesis modification to prepare quaternary ammonium and quaternary phosphorous ionic liquid functionalized MOFs, MIL-101-N-(n-Bu)₃Br and MIL-101-P(n-Bu)₃Br (entries 24 and 25). Owing to the synergistic effect of Br⁻ Lewis acid sites from the ionic liquids and coordinatively unsaturated Cr³⁺ sites, the MOFs exhibited superior catalytic activity in the cycloaddition of propylene oxide (PO), with product yields of 98.6 and 99.1 for MIL-101-P(n-Bu)₃Br and MIL-101-N-(n-Bu)₃Br, respectively, at 80 °C and 19.7 atm in the absence of a solvent and a co-catalyst, after 8 h of reaction. This was significantly higher than the yield obtained with Mg-MOF-74, MIL-101, HKUST and MOF-5, which gave a yield of 23.2, 20.9, 5.4, and 2.5%, respectively, under the same experimental conditions. The functionalized MOFs could also be recovered and re-used up to 3 times, with less than 1% decrease in the product yield in the third run.

Incorporating Lewis base functionalities within MOFs leads to improved performance due to their ability to act as co-catalysts. Functionalization with amine groups also results in enhanced CO₂ adsorption, further leading to an enhanced yield. Among the different amine functionalized MOFs, UMCM-1-NH₂⁷⁷ outperforms most of them such as amine functionalized MIL-68⁵⁵ and UiO-66,⁸⁶ due to its unique dual porous structure, in addition to its Lewis base functionality. The relatively large meso-pores enable the efficient transport of substrates and products across the porous frameworks, while micropores, owing to their smaller size, shorten the product retention duration within the reaction sites, thus reducing the probability of the formation of the secondary product, and leading to improved product selectivities.⁷⁷

3.2.3. MOFs with active defect sites. Defects in MOFs, arising from crystal imperfections such as vacant metal or linker sites, may also serve as active sites for catalytic reactions.^95,96 MOFs with catalytic defect sites include ZIF-8, ZIF-68 and MOF-5.^14,61,78 For example, Miralda et al. in 2012⁷⁸ utilized ZIF-8 for the cycloaddition of CO₂ to chloropropene oxide, in the absence of co-catalysts and solvents, between 70 °C and 100 °C, and obtained a maximum carbonate yield of 44% at 80 °C (Table 2, entry 26). The conversion efficiency could be improved up to 73% after functionalization of the ZIF-8 surface with amine functional groups. However, the catalysts offered poor recyclability and the particles lost their crystalline structure and catalytic performance upon recycling. In a separate work, Zhu et al.⁴⁹ used ZIF-8 for the conversion of styrene oxide into styrene carbonate and obtained a conversion efficiency of 54% at 100 °C and 6.9 atm, without the need for a co-catalyst or solvent (Table 2, entry 29). XRD analysis demonstrated that the crystal structure of ZIF-8 particles could be maintained under these reaction conditions, at least for three subsequent cycles. The cycloaddition reaction is expected to take place on the surface due to the small aperture of ZIF-8 (0.34 nm), where low coordinated Zn atoms and active defect sites are located.^97,98

The catalytic performance of ZIF-68 in cycloaddition reactions has also been reported, where 93.3% yield of styrene carbonate was achieved at 120 °C and 9.9 atm in the absence of a solvent and co-catalyst.⁶¹ Similar to ZIF-8, ZIF-68 crystals exhibit small pore apertures of 0.55 nm, which may prevent the reaction from taking place within the pore structures. The reaction thus takes place on the surface of the crystals where unsaturated coordinative metal ions and catalytically active structural defects are present. XRD analysis revealed that the MOF crystals were stable for at least three cycles, though there was a decrease in product yield with each subsequent use, from 93.3% for fresh MOFs to 88.3% in the second cycle and 80.9% in the third cycle. The fourth cycle recorded an even bigger drop in the yield down to 66.4%, highlighting the potential blockage or poisoning of catalytic sites.

Hwang et al.⁹⁰ evaluated the performance of ZIF-22 in the cycloaddition of CO₂ to epoxides, in the absence of a solvent and co-catalyst (Table 2, entry 33). The reactions were carried out using the epichlorohydrin (ECH) substrate, at 120 °C and a pressure of 11.8 atm. The MOF achieved a TOF of 155 h⁻¹, which was significantly higher than other Zn containing MOFs, ZIF-8, ZIF-67 and ZIF-8-NH₂, which had TOFs of 12, 22 and 17, respectively,^61,78,99 under similar experimental conditions. The higher efficiency of ZIF-22 in comparison with other ZIFs was attributed to the presence of the extra non-coordinated N atom in the 5-azabenzimidazolate linker, which acted as a Lewis base co-catalyst and facilitated the cycloaddition reactions. XRD analysis revealed that the crystal structure of the MOFs could be maintained even after recycling 3 times.

The defective sites present across the MOF surface may act as active sites for cycloaddition reactions. However, these MOFs required high temperatures and pressure, suggesting that the catalytic activity of the defect sites is not sufficient. A potential route for increasing the catalytic performance of these MOFs is by engineering MOFs with a high density of defect sites.^96,100,101 One effective strategy to increase the density of defect sites is to use fast precipitation synthesis methods during the preparation of MOFs, which denies the MOF building blocks sufficient time to adhere to the growing crystal lattice at the right place, leading to a higher density of defect sites.^96,100 This technique has been used successfully to increase the density of defect sites across MOF-5.¹⁰⁰ Another approach is to use the isostructural mixed linker, but with different side functionalities to coordinate extra metal ions, resulting in a high density of defect sites while preserving the framework topology.¹⁰⁰ Post-synthetic treatment of MOF crystals with acid or base has also been used to introduce defect sites across MOFs,^100,101 but these approaches should be employed cautiously, since they may lead to the degradation of the MOF materials, by exposing the metal centers to solvents.

3.2.4. Conclusions and prospects. Lewis acid metal centers and defect sites across metal organic frameworks act as active sites for the cycloaddition of CO₂ to epoxides. However, Lewis acid sites appear to play the most important role, and their density and accessibility are critical for efficient catalytic performance. The chemical versatility of MOF materials allows the density of these active sites to be enhanced directly by employing organic ligands having multiple coordination sites for the metal ions,¹⁰² or by mixing two or more similar ligands during MOF synthesis, to lead to an interpenetrated structure, such as the dual walled cage MOF reported by Han et al.⁵⁷ The rational choice of organic ligands can also be used to tune the pore size and architecture,⁴⁷ which in turn influence the accessibility of the active sites as well as the diffusibility of substrates and catalytic products across the porous frameworks. Post-functionalization with Lewis base functionality such as amine groups^55,103 and halide ions (e.g. Br⁻ and I⁻)^42,104 also leads to improved catalytic performance due to their ability to act as a co-catalyst, besides enhancing the CO₂ adsorption capacity of the MOF materials.

Recyclability tests have also been reported, where most MOF catalysts were shown to maintain their activity and crystal integrity for 3 to 5 cycles. However, to fully assess the potential MOF catalysts, a higher number of cycles is required, while monitoring not only the changes in the crystal structure, but also the changes in the morphology, surface area and chemical composition over time. Tailoring the porous structure and optimizing the density of Lewis acid and base sites across MOF materials are expected to lead to enhanced catalytic activity towards cycloaddition reactions. However, significant effort should also be devoted to designing MOF catalysts with a high degree of connectivity and hydrophobic pores, which result in more tolerant structures under harsh reaction conditions.^63,64,105

4. Photocatalytic conversion of CO₂

4.1. Introduction

Another attractive approach for utilizing CO₂ is its conversion into valuable fuels and chemicals via photocatalysis.¹⁷ This greenhouse gas may be converted into valuable products such as methanol, formic acid, methane and carbon monoxide, in the presence of a photocatalyst.^9,17 Since photocatalysis can be accomplished using renewable solar energy, with water as the proton source, the production of valuable liquid fuels such as methanol using this approach would be particularly attractive, since the current industrial synthesis approach using the Fischer–Tropsch process is energy intensive,¹⁰ which makes the process expensive and contributes further to CO₂ emission. Using methanol produced from the photocatalytic conversion of captured CO₂ as fuel is also not expected to lead to net CO₂ emission and will reduce the reliance on fossil fuels, further mitigating the adverse effects of global warming.

The most common photocatalysts are inorganic semiconductors such as TiO₂, ZnO, CdS, and ZnS,^9,17 with TiO₂ being the most popular due to its ready availability, non-toxicity and long term stability. However, the catalytic performance of these materials is limited due to the rapid recombination of photo-generated electrons and holes, and low adsorption capacities for CO₂.^17,46,106 As recently reviewed by Ola et al. the benchmark TiO₂ photocatalysts are only active under UV irradiation.⁶ Modifying TiO₂ with organic or inorganic materials and photosensitization with organic dyes resulted in photocatalytic activity under visible light, but the product yield of most of the modified systems was low, with the product yield lower than 100 μmol per gram of catalyst.⁶ N doped TiO₂ nanotubes exhibited the highest activity among different modified TiO₂ systems, with 12 475.8 μmol of formic acid per gram of catalyst,^9,107 after visible light illumination for 12 h. Significant effort is currently being devoted to the development of alternative photocatalytic materials able to facilitate more efficiently the conversion of CO₂ under visible light activation.⁵²

Recently, MOF materials have generated interest as visible light active photocatalytic materials in (i) degradation of organics, (ii) chemical synthesis and (iii) reduction of CO₂.^19,28,108 By judicious selection of metal ions and organic ligands, the photocatalytic properties of MOFs may be modulated.¹⁹ The organic ligands may serve as antennae to harvest light and generate electrons, which are then transferred to the metal centers via linkers by metal cluster charge transfer.^19,109,110

4.1.1. Electronic properties of MOF materials. The efficiency of a photocatalytic material is determined not only by the width of the bandgap but also by the absolute positions of the band edges.^12,111,112 For a MOF material to be active towards CO₂ conversion, the lowest unoccupied crystal orbital (LUCO) must be above the redox potential for the CO₂ reduction half reaction,^112,113 which depends on the product formed. For example the redox potential for the conversion of CO₂ into formic acid is −3.42 eV while those for methanol and methane are −3.65 and −3.79 eV, respectively.¹¹² While both bridging metal ions and organic ligands have been shown to have an influence on the electronic properties,^109,113,114 the organic ligand has been shown to play the most important role,^112,115,116 and the electronic properties of MOFs may be predicted based on the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the organic ligand.¹² Grau-Crespo et al. demonstrated through computational studies that the band gap of zeolitic imidazolate frameworks (ZIFs) could be varied from 5.2 eV when a pure imidazole ligand was used to 3.3 eV when a nitro-imidazole ligand was used. The use of mixed linkers of furanimidazole/nitroimidazole (fIm/nIm) and methylimidazole/nitroimidazole (mIm/nIm) even led to a further reduction in the band gap to 1.9 eV and 2.5 eV, respectively, while the incorporation of Co and Cu ions into the Zn/mIm/nIm MOF led to band gaps of 2.3 eV and 2.1 eV, respectively.

The metal centers should be selected such that the empty d orbitals overlap with the lowest unoccupied molecular orbital (LUMO) of the organic ligand, to facilitate efficient ligand to metal charge transfer (LMCT),^117–119 and long lived charge separation. LMCT enables the MOF to exhibit a longer charge separation time, as compared to isolated organic ligands,¹¹⁹ and explains why some MOF materials exhibit a longer charge separation time as compared to typical semiconductors. For example MOF-5 was shown to have a charge separation time of 15 μs which was 3 times higher than that of P25 TiO₂.¹⁰⁹

The following section discusses specific examples of MOF materials that have been utilized in the photocatalytic conversion of CO₂.

4.2. MOFs as photocatalysts for CO₂ conversion

Li et al. investigated the MIL-125(Ti) MOF and its amine functionalized derivative, NH₂-MIL-125, for the photocatalytic conversion of CO₂ under visible light (Table 3, entry 1). NH₂-MIL-125(Ti) exhibited an enhanced catalytic activity towards the conversion of CO₂ into formate, in acetonitrile with triethanolamine (TEOA) as an electron donor, as compared to the MIL-125(Ti) without the amine functionality.¹²⁰ The accumulated HCOO⁻ content, over a 10 h period, was 8.14 μmol, while no HCOO⁻ ions were formed in the absence of the amine functionality. Upon excitation, excited electrons are transferred from the amine functionalized ligand to Ti⁴⁺ clusters, reducing them to Ti³⁺, which subsequently transfers the excess electrons to the CO₂ molecule. The valence transition between Ti⁴⁺ and Ti³⁺ was witnessed by the observation of color change from bright yellow to green upon excitation with visible light, signifying the reduction of Ti⁴⁺ clusters to Ti³⁺. However, upon introduction of CO₂, the color reverted from green to the original bright yellow, as Ti³⁺ was oxidized to Ti⁴⁺, upon transfer of the electrons to the CO₂ molecule. XRD patterns taken after the photocatalytic reaction revealed that the scattering pattern of the parent MOF was preserved, suggesting that the crystal structure of the MOF catalyst was not compromised during the photocatalytic reactions.

Table 3 Application of MOFs in the photocatalytic conversion of CO₂

Entry	MOF	BET	Light source	Proton donor	Product	Yield	Ref.
1	NH2-MIL-125(Ti)	1302	Visible	TEOA	Formate	16.3 μmol g^−1 h^−1	120
2	MIL-88B(Fe)	—	Visible	TEOA	Formate	67.5 μmol h^−1	52
3	NH2-MIL-88(Fe)	—	Visible	TEOA	Formate	225 μmol g^−1 h^−1	52
4	MIL-53(Fe)	—	Visible	TEOA	Formate	222.75 μmol g^−1 h^−1	52
5	NH2-MIL-53(Fe)	—	Visible	TEOA	Formate	348.75 μmol g^−1 h^−1	52
6	MIL-101(Fe)	—	Visible	TEOA	Formate	442.5 μmol g^−1 h^−1	52
7	NH2-MIL-101(Fe)	—	Visible	TEOA	Formate	1335 μmol g^−1 h^−1	52
8	Re doped UiO-67	1092	UV	TEA	CO	0.545 TOF	123
9	UiO-66-CrCAT	—	Visible	TEOA	Formate	20 692 μmol g^−1 h^−1	121
10	UiO-66-GaCAT	—	Visible	TEOA	Formate	11 512 μmol g^−1 h^−1	121
11	Ru-MOF	8.08	Visible	TEOA	Formate	77.2 μmol g^−1 cat h^−1	29
12	NH2-UiO-66(Zr)	778	Visible	TEOA	Formate	26.4 μmol g^−1 h^−1	129
13	Porphyrin based MOF (Al)	1187	Visible	Water	Methanol	37.5 ppm g^−1 h^−1	124
14	Porphyrin based MOF (Al, Cu)	932	Visible	Water	Methanol	262.6 ppm g^−1 h^−1	124
15	10%-Cp*Rh@UiO-67	1650	Visible	TEOA	Formate	7.4 TOF	130
16	PCN-222	1728	Visible	TEOA	Formate	60 μmol g^−1 h^−1	46
17	ZIF-8/Zn2GeO4	319.5	—	Water	Methanol	0.2218 μmol g^−1 h^−1	106
18	TiO2 doped HKUST-1	756	UV	Water	CH4	2.64 μmol g^−1 h^−1	128
19	Co-ZIF-9	1428	Visible	TEOA	CO, H2	179 TOF	131 and 132
20	Zr6(O)4(OH)4-[Re(CO)3Cl(bpydb)]6	—	Visible	TEOA	CO	1.073 TOF	133

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MOFs composed of Fe metal centers, owing to the wide availability of Fe and the visible light responsiveness of Fe based photocatalysts, were also investigated.⁵² The study compared MIL-101(Fe), MIL-53(Fe) and MIL-88B(Fe), and their amine functionalized derivatives (Table 3, entries 2–7).⁵² Unlike the Ti–O clusters, the Fe–O clusters in the Fe based MOFs exhibited visible light activity towards the conversion of CO₂ into formic acid, even in the absence of an amine functionality, with a yield of 59 μmol, 29.7 μmol, and 9 μmol, for MIL-101, MIL53 and MIL88, respectively, after 8 h of reaction. MIL-101(Fe) exhibited the best photocatalytic activity towards the conversion among the non-functionalized MOFs, due to the presence of uncoordinated unsaturated Fe sites in the structure. Amine functionalization was found to boost the photocatalytic activity of all the Fe based MOFs, leading to 178 μmol, 46.5 μmol, and 30 μmol, for MIL-101, MIL53 and MIL88, respectively, over the same reaction duration, due to dual excitation pathways of the NH₂ functionality and Fe–O clusters. The Fe based MOFs could be used for three cycles without any noticeable loss in the yield of formic acid. The crystal structure, chemical composition and porosity were also preserved as XRD, IR, TGA and N₂ adsorption of the crystals recovered after the reactions matched those of fresh samples.

MOF materials may also be modified with homogenous catalysts or photosensitizer molecules,^19,121,122 to enhance their catalytic properties. Wang et al. (2011)¹²³ prepared MOF UiO-67 doped with catalytically active Ir, Re, and Ru complexes H₂L₁–H₂L₆, with dicarboxylic acid functionalities (Table 3, entry 8). The study found [Re¹(CO)₃(dcbpy)Cl] (H₂L₄) to be the most active catalyst in the selective reduction of CO₂ to CO, under visible light irradiation with trimethylamine (TEOA) as an electron donor. The catalyst was found to be 3 times more effective than homogenous catalyst Re-H₂L₄. The activity was attributed to the [Re¹(CO)₃(dcbpy)Cl] (H₂L₄) moiety, since UiO-67 demonstrated no activity under the same experimental conditions. The doped UiO-67 catalyst was, however, found to have poor stability as 43.6% of Re leaked into the solution during 20 h of photocatalysis. Liu et al. in 2013¹²⁴ investigated the photocatalytic reduction of CO₂ using Cu modified porphyrin based MOFs, with Al metal centers. The study compared the efficiency of the Cu modified MOF (S_Cu), with that of the parent MOF (Sp), and found S_Cu to be up to 7 times more efficient than Sp (262.6 ppm g⁻¹ h⁻¹vs. 37.5 ppm g⁻¹ h⁻¹), in the photo-conversion of CO₂ into methanol. The improved performance was attributed to the enhanced chemical adsorption and activation of CO₂ due to the presence of Cu²⁺. The recyclability of the catalyst was, however, not reported.

Lee et al.¹²¹ post-synthetically modified UiO-66(Zr) with a catechol functionalized organic linker (catbdc, 2,3-dihydroxyterephthalic acid), to produce UiO-66-CAT, followed by the incorporation of trivalent metal ions, Cr and Ga, at the catbdc sites. Photocatalytic reactions were carried out under visible light irradiation, where formic acid was found to be the major product, with 51.73 ± 2.64 μmol for UiO-66-CrCAT and 28.78 ± 2.52 μmol for UiO-66-GaCAT. The catbdc organic linker served as a visible light absorber to generate excited electrons, while Cr and Ga metal ions facilitated electron transfer within the framework. The use of UiO-66-CAT in the absence of the Cr and Ga metal ions did not produce any formic acid, due to the high redox potential of the Zr metal centers across UiO-66 relative to the lower unoccupied molecular orbital (LUMO) of the catbdc linker, which hinders the transfer of photo-generated electrons.^117,118 Although there was no noticeable drop in the product yield for three consecutive cycles, ICP-MS analysis indicated that a small amount of Cr and Ga ions leached out after every cycle.

Flower-like hierarchical Ru-MOFs, {Cd₂[Ru(dcbpy)₃]·12H₂O}_n (dcbpy = 2,2′-bipyridine-4,4′-dicarboxylate), have also been recently reported by Zhang et al. (2015) (entry 11).²⁹ Photocatalytic reactions were carried out in the presence of the sacrificial agent TEOA under visible light. The Ru-MOF demonstrated superior catalytic properties towards the conversion of CO₂ into formate, under visible light in the presence of TEOA, with a product formation rate of 77.2 μmg⁻¹ h⁻¹. This efficiency was higher than those of other MOF based catalysts such as NH₂-MIL-125(Ti), NH₂-UiO-66(Zr) and mixed NH₂-UiO-66(Zr), whose product formation rates were 16.3, 26.4 and 41.4 μmol g⁻¹ h⁻¹ of catalyst per hour, respectively. The value is also higher than some TiO₂ based catalysts such as iodide doped TiO₂ and nitrogen and nickel co-doped TiO₂¹²⁵ (product formation rates of 2.4 and 15.1 μm g⁻¹ of catalyst).

The improved properties were attributed to the high visible light harvesting capacity of the MOF and the long lasting excited state. The MOF exhibited a long-lived excited state, with a luminescence lifetime of 5.49 μs, which was significantly higher than that of Ru units (483 ns), and TiO₂ particles (0.16–0.55 ns)¹²⁶ and TiO₂ nanorods (12.02 ns),¹²⁷ suggesting a low recombination rate of the photo-generated electrons and holes. The superior charge separation properties were facilitated by the unique hierarchical morphology, which enabled the efficient diffusion of generated electrons to the catalyst surface. Catalytic studies using microcrystals and bulk crystals of the same MOF produced lower formate yields of 52.7 and 30.6 μmol per g, respectively (Fig. 2), highlighting the critical role of the morphology in photocatalytic activity. The MOFs could be recycled up to 4 times with no noticeable change in the photo-catalytic activity. Inductively coupled plasma (ICP) analysis found that only 0.43% of Ru ions leaked after the reaction, while PXRD showed no apparent change in the crystal structure after the reactions.

Fig. 2 The role of the morphology of Ru-MOFs in the generation of HCOO (a) nanoflowers, (b) microcrystals, and (c) bulk crystals. The hierarchical morphology of the nanoflowers results in more enhanced charge separation, leading to improved HCOO⁻ generation as compared to microcrystals or bulk crystals of the same MOF. Reproduced from ref. 29 with permission from the Royal Society of Chemistry.

The composition of metal centers, the organic ligand and the morphology of MOFs all play an important role in the catalytic activity. For example, MOFs with Fe–O metal centers exhibit visible light activity as opposed to MOFs with Ti–O clusters. Similar to cycloaddition reactions described in the previous sections, the modification of MOFs with amine functionality led to enhanced photocatalytic activity, due to the ability of the amine group to generate excited electrons following irradiation with light, and its high affinity for CO₂. MOF catalysts with a hierarchical morphology were also shown to result in improved performance, due to the efficient transfer of photo-generated electrons to the reaction sites.

Another approach for imparting or modifying the photocatalytic properties of MOF materials is the integration of inorganic semiconductors into MOFs, to make hybrid catalyst meta-materials.

4.3. Inorganic semiconductor MOF hybrid catalysts

The integration of MOF materials with inorganic catalysts to make hybrid catalysts has the potential to result in enhanced catalytic activity, due to the synergistic effect arising from the photo-excitation properties of the inorganic semiconductors, and the higher CO₂ adsorption capacities for MOFs.¹²⁸

For example, with the aim of boosting the photocatalytic activity of Zn₂GeO₄ nanorods, Liu et al. deposited ZIF-8 nanoparticles on the nanorods, and obtained improved efficiency towards the photocatalytic conversion of CO₂ into methanol, in aqueous medium, as compared to Zn₂GeO₄ nanorods alone (Table 3, entry 17).¹⁰⁶ The hybrid catalysts led to a yield of 2.32 μmol g⁻¹ over a 10 h period, which was 62% higher than 1.43 μmol g⁻¹ obtained with Zn₂GeO₄ nanorods alone. The enhanced performance was attributed to improved CO₂ adsorption capacity, which increased by 3.8 times following the incorporation of ZIF-8 (25 wt%) into the Zn₂GeO₄ nanorods. No product was detected by using ZIF-8 particles alone, clearly demonstrating the synergistic role of combining the two different materials.

Li et al., on the other hand, prepared TiO₂ coated Cu₃(BTC)₂ MOFs, and demonstrated that charge transfer can occur between the photoexcited TiO₂ and the MOF, leading to improved catalytic properties under UV irradiation (Table 3, entry 18). Methane was found to be the only product generated for the TiO₂ doped MOFs, with a yield of 2.64 μmol g⁻¹ h⁻¹, which was significantly higher than 0.52 μmol g⁻¹ h⁻¹ obtained with bare TiO₂.¹²⁸ The Cu₃(BTC)₂ MOF crystals alone did not exhibit any catalytic activity under similar experimental conditions. The TiO₂ doped MOF could be used for three subsequent cycles without any change in photocatalytic efficiency. TEM imaging and XRD analysis revealed that the morphology, crystal structures and composition were maintained during the photocatalytic reactions.

The improved catalytic efficiency of the ZIF-8/Zn₂GeO₄ and the TiO₂/Cu₃(BTC)₂ MOF hybrid systems clearly highlights the benefits of combining these two classes of materials, to make more efficient photo-catalysts. Further work should investigate hybrid systems based on MOFs with inherent photocatalytic activity, such as MIL-101-NH₂ and Ru-MOFs, which may lead to more enhanced performance due to additive photocatalytic activity between the two materials. In addition, understanding the influence of the inorganic nano-material–MOF interface on charge transfer will facilitate the design of hybrid systems with optimized charge transfer, leading to more efficient photocatalysts. The influence of the inorganic materials on the structure and long-term stability of MOFs should also be investigated.

The products formed during the photocatalytic conversion of CO₂ are dependent on the number of electrons transferred and reaction pathways, which are in turn influenced by the type of catalyst used, as well as the reaction environment.^9,134 For example, the formation of carbon monoxide and formic acid requires the transfer of two electrons, while methanol and methane require 6 and 8 electrons, respectively. Consequently, the formation of CO and formic acid is more thermodynamically favorable. As can be seen in Table 3, most MOF catalysts led to high product selectivity, producing only one product, as opposed to typical photocatalysts which lead to a mixture of products.⁹ As discussed previously, the high product selectivity of MOF based catalysts may be attributed to the confined reaction spaces that shorten the product retention duration within the reaction sites, and reduce the probability for the formation of secondary products.⁷⁷ The high selectivity is also attributed to the uniform distribution of catalytic sites and microspores that ensure the uniform supply of protons to the catalytic sites.¹³⁵

4.4. Conclusions and prospects

Unlike inorganic semiconductors, MOF based catalysts exhibit catalytic activity under visible light, allowing CO₂ conversion with product yields similar to or better than most state of the art TiO₂ based systems. The chemical composition of the MOF materials plays a critical role in the extent of their photocatalytic properties, and careful selection of the MOF building blocks is necessary for MOFs to exhibit photocatalytic properties. Organic ligands should be selected from those which have chromophores, which are responsible for absorbing light to generate excited electrons.¹³⁶ The metal centers, on the other hand, should have a lower redox potential relative to the lowest unoccupied molecular orbital (LUMO) of the organic ligand to be able to accept the photo-generated electrons.^117,118

The main challenge, however, will be balancing the photocatalytic properties with MOF stability, both of which are strongly influenced by the composition of the MOF building blocks. The stability in the presence of water, in particular, is of critical importance given that water is commonly used as a proton donor in the photocatalytic reduction of CO₂, and also due to the possible presence of moisture in the CO₂ stream, which could be significantly high if the CO₂ is sourced from brown coal flue gas. The presence of other contaminants in flue gas streams, such as SOx and NOx, should also be taken into account, since they have been shown to accelerate the degradation of MOF materials.^137–139 For example, MIL-125 was found to lose its structure and N₂ adsorption capacity upon exposure to an aqueous environment containing SO₂, although amine functionalized MIL-125 was found to be stable under similar conditions.¹³⁸ While using hydrophobic ligands and metal with a high coordination number may enhance the stability in water,^63,64 the building blocks may not necessarily be suited for photocatalytic activity. A potential approach is catenation, whereby two or more identical but independent frameworks are interpenetrated, by using mixed linkers and/or metal ions during MOF synthesis. By using this approach building blocks that elicit photocatalytic properties could be mixed with those that lead to high stability, to possibly result in robust photocatalytic MOFs. Catenation has also been shown to lead to more stable frameworks by providing steric hindrance to ligand displacement,⁶³ locking labile ligands across the framework in their place.

5. Electrocatalytic conversion of CO₂

5.1. Introduction

Electrocatalysis is another process by which CO₂ maybe converted into useful products, such as carbon monoxide, formic acid, methanol, and methane, in the presence of an electrocatalyst.⁸ Similar to photocatalysis, electrocatalysis can be carried out at room temperature, and has low energy requirements, which can be directly obtained from renewable sources.³ Metal electrodes are to date the most investigated electrocatalysts for CO₂ conversion due to their wide availability and suitability in electrochemical processes. Hori et al. (1994)¹⁴⁰ studied the electrocatalytic conversion of CO₂ using metal electrodes in aqueous media, and categorized them as CO forming electrodes and formate forming electrodes. Formate forming electrode materials include Pb, Hg, In, Sn, Cd, and Tl, while CO forming electrode materials include Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt. Cu was found to be unique and the most promising electrode, since it may facilitate the formation of multi-electron products, such as methane and methanol.^{135,141–148} The key determinant to the product selectivity has been attributed to the binding energy of the metal electrodes to CO, which is the main intermediate for CO₂ conversion.¹⁴⁹ Metals that bind CO strongly exhibit a low product yield, as the surface of the catalyst is poisoned by the CO. Metals with weak binding strength for CO, on the other hand, produce CO as the main product, since the molecules are desorbed from the catalyst surface before further reduction. Cu has an intermediate binding energy for CO, which gives it the unique ability to produce hydrocarbon products. However, Cu based electrocatalysis yields a mixture of products, and it is difficult to obtain a single product with high purity.

The product selectivity may be improved by using porous catalytic materials, which can provide confined reaction spaces for the catalytic reactions, while maintaining the uniform supply of protons.¹³⁵ Potential candidates are proton conducting MOF materials, due to their high porosity and the uniform distribution of the catalytic sites across the nano-sized pores, which may restrict reaction pathways and enhance product selectivities.¹⁵⁰ This section reviews MOF materials that have been applied in the electrocatalytic conversion of CO₂. The product selectivity may be expressed by the purity of the product obtained or by faradaic efficiency, which is the mole of product formed on an electrode per charge consumed.

5.2. MOF materials as electrocatalysts for the conversion of CO₂

MOF materials have been explored in a number of electrochemical processes, including electrocatalytic water splitting to produce hydrogen and oxygen evolution,^151,152 and in electrochemical sensors,¹⁵³ lithium ion batteries¹⁵⁴ and the electrocatalytic reduction of CO₂.^150,155,156 Hinogami et al.¹⁵⁰ first evaluated the electrocatalytic potential of copper rubeanate metal organic frameworks (CR-MOFs), for the electroconversion of CO₂. By using a CR-MOF coated on carbon paper to form a working electrode, formic acid was found to be the only by-product. Using the same set up, the use of Cu metal as the working electrode in the absence of the MOF led to a mixture of products, including formic acid, methane, ethylene and ethane. The use of a CR-MOF also led to a 13 fold increase in formic acid production as compared to the Cu electrode at 1.2 V vs. SHE, probably due to the higher CO₂ adsorption within the MOF crystals. The high selectivity of Cu based MOFs was also demonstrated by Kumar et al.,¹⁵⁶ and oxalic acid with 90% purity was obtained using a Cu₃(BTC)₂ MOF film, supported on a glassy carbon electrode.

Cobalt-porphyrin MOF films supported on a carbon substrate have also been reported.³¹ The MOF materials achieved a faradaic efficiency of 76% and a turnover number of 1400 (assuming that every Co atom was catalytically active), in the conversion of CO₂ into CO. In situ spectroelectrochemical analysis showed that most of the Co metal centers were accessible, and the Co(II) was reduced to Co(I) during the electrocatalysis, which subsequently reduced CO₂. Although the recyclability of the MOF was not tested, XRD analysis and SEM imaging taken after 7 h of reaction revealed that the films retained their morphology as well as their crystalline structure.

Recently, Hod et al. (2015) used Fe_MOF-525 as a support to immobilize the Fe-porphyrin molecular catalyst, and obtained a high faradaic efficiency of 100% for H₂ and CO (54 ± 2% for CO and 45 ± 1% for H₂).¹⁵⁷ The catalytic efficiency could be enhanced by the addition of a trifluoroethanol (TFE) proton donor, resulting in up to a 7 times increase in CO generation, with a TON of 1520, after 3.2 h. However, the turnover frequency (TOF) of the MOF was 16 times lower than that of the Fe-porphyrin homogeneous catalyst, potentially due to the limited rate of charge diffusion across the MOFs.

5.3. Conclusions and prospects

Electrocatalytic conversion of CO₂ using MOF catalysts is a new area of research, where a lot is yet to be discovered. However, the few examples discussed above highlight the potential of MOF based electrocatalysts in facilitating CO₂ conversion with high product selectivity. The high porosity and well defined pore channels facilitate the uniform supply of protons to catalytic sites, while also modulating the reaction pathways. Unlike cycloaddition and photocatalysis where MOF crystals can be employed directly, for electrocatalytic application MOF materials need to be deposited on the surface of an electrically conductive substrate. This brings further complexity, since the nature of the interface between the substrate and the MOF may have an influence on charge transfer, and ultimately on their catalytic performance. The nature of the interface and its influence on catalytic performance should therefore be critically investigated in future studies.

Although the reported MOF materials demonstrated high product selectivity, none of the reports discussed the recyclability of the MOF based electrocatalysts. Given the vulnerability of most MOF materials in a wide range of solvents,⁶⁴ their recyclability in the electrochemical solution is critical and should be thoroughly investigated to assess their limitations. A potential means of enhancing their stability is by carbonization. Carbonized MOFs have been shown to preserve the key properties of parent MOFs such as surface area and CO₂ adsorption capacities, while exhibiting higher stability.^158,159 For example, the carbonized MOF MIL-88B-NH₃ outperformed the commercial Pt/C electrocatalysts in the oxygen reduction reaction (ORR), in terms of stability and performance when tested in alkaline direct fuel cells, where it achieved a power density of 22.7 mW cm⁻², which was 1.7 times higher than the commercial Pt/C catalyst.

6. General conclusions

In this review, the application of MOF based catalysts in the conversion of CO₂ through chemical fixation, photocatalysis and electrocatalysis has been discussed. Compared to other catalytic materials used for CO₂ conversion, MOF based catalysts stand out, due to their chemical versatility, higher adsorption capacity for CO₂, uniform distribution of catalytic sites and confined reaction spaces. A major problem in the area of CO₂ conversion is poor product selectivity, since most catalytic systems lead to a mixture of products, making the process expensive by increasing the cost of product separation. Across all three areas discussed in this review, MOFs consistently demonstrate higher product selectivity due to the uniform distribution of catalytic sites and restricted reaction spaces. Just by manipulating the composition of MOF materials, key properties relevant to CO₂ conversion, including the density of active sites, pore architecture and electronic properties, can be optimized, explaining why MOFs feature prominently across all three different areas of CO₂ utilization.

However, while tailoring the chemical functionality and crystal structures of MOF materials can lead to enhanced catalytic activity, their stability under different reaction conditions should be taken into account. Cross-comparison of crystal structures, chemical composition and nitrogen adsorption properties before and after catalytic reaction is critical, to assess their long-term stability. Highly interconnected frameworks should be targeted while designing new MOF catalysts, by using metal ions with a high coordination number or organic ligands with a high number of coordination sites. Since it may be challenging to get building blocks that combine both high connectivity and catalytic activity, a mixed ligand or metal ion approach should be employed to bring different functionalities into a single unit. In electrocatalysis, carbonized MOFs should be explored as they may exhibit superior performance, due to their higher stability in electrochemical solutions. Other areas for exploration should include photoelectrocatalytic conversion of CO₂ using MOF based catalysts, since photoelectrocatalysis has been shown to lead to a higher yield and selectivity for methanol,^160,161 as compared to either photocatalysis or electrocatalysis alone.

Abbreviations

TON	Turnover number
TOF	Turnover frequency
MOFs	Metal organic frameworks
CCS	carbon capture and storage

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6mh00484a

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