Catalysis and photocatalysis by metal organic frameworks

Amarajothi Dhakshinamoorthy *a, Zhaohui Li *b and Hermenegildo Garcia *c
aSchool of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamil Nadu, India. E-mail: admguru@gmail.com
bResearch Institute of Photocatalysis, State Key Laboratory on Photocatalysis, Fuzhou University, Fuzhou 350002, People's Republic of China. E-mail: zhaohuili1969@yahoo.com
cDepartment of Chemistry and Instituto de Tecnología Química, Consejo Superior de Investigaciones Científicas-Universitat Politecnica de Valencia, Universitat Politecnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain. E-mail: hgarcia@qim.upv.es

Received 3rd April 2018

First published on 13th July 2018


Metal organic frameworks (MOFs) are a class of porous crystalline materials that feature a series of unique properties, such as large surface area and porosity, high content of transition metals, and possibility to be designed and modified after synthesis, that make these solids especially suitable as heterogeneous catalysts. The active sites can be coordinatively unsaturated metal ions, substituents at the organic linkers or guest species located inside the pores. The defects on the structure also create these open sites. The present review summarizes the current state of the art in the use of MOFs as solid catalysts according to the type of site, making special emphasis on the more recent strategies to increase the population of these active sites and tuning their activity, either by adapting the synthesis conditions or by post-synthetic modification. This review highlights those reports illustrating the synergy derived from the presence of more than one of these types of sites, leading to activation of a substrate by more than one site or to the simultaneous activation of different substrates by complementary sites. This synergy is frequently the main reason for the higher catalytic activity of MOFs compared to homogeneous catalysts or other alternative solid materials. Besides dark reactions, this review also summarizes the use of MOFs as photocatalysts emphasizing the uniqueness of these materials regarding adaptation of the linkers as light absorbers and metal exchange at the nodes to enhance photoinduced electron transfer, in comparison with conventional inorganic photocatalysts. This versatility and flexibility that is offered by MOFs to optimize their visible light photocatalytic activity explains the current interest in exploiting these materials for novel photocatalytic reactions, including hydrogen evolution and photocatalytic CO2 reduction.


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Amarajothi Dhakshinamoorthy

Amarajothi Dhakshinamoorthy obtained his PhD, degree in 2009 from Madurai Kamaraj University, Tamil Nadu, India. Later, he worked as a postdoctoral researcher with Prof. Hermenegildo Garcia at the Technical University of Valencia for four years. Then, he returned to India and joined as a UGC-Assistant Professor in June 2013 at School of Chemistry, Madurai Kamaraj University. His research interests include catalysis by metal–organic frameworks and graphene-related materials. He is the recipient of the Young Scientist Award 2014 for Chemical Sciences by The Academy of Sciences, India. He was also awarded an INSA-DFG international bilateral exchange award to visit Germany in 2015. He has co-authored over one hundred publications, two book chapters and one international patent.

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Zhaohui Li

Zhaohui Li received her PhD degree from the National University of Singapore in 2000. She stayed as a postdoctoral research associate at the University of Notre Dame from 2000–2002. She joined the faculty of Fuzhou University in 2003 and is currently a full professor of State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, P. R. China. Her current research interest centers on the development of porous/nanostructured materials for catalysis and photocatalysis.

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Hermenegildo Garcia

Hermenegildo Garcia is a full Professor at the Instituto de Tecnologıa Quimica of the Technical University of Valencia, a joint center of the Technical University of Valencia and the Spanish National Research Council and Honorary Adjunct Professor at the Center of Excellence in Advanced Materials Research of King Abdulaziz University. He made a postdoctoral stay at the University of Reading with Professor Andrew Gilbert and several sabbatical leaves in the group of Professor J. C. Scaiano at the University of Ottawa. Prof. Garcia has been active in the field of heterogeneous catalysis working with porous catalysts and nanoparticles, has published over 700 papers and has filed over 25 patents. Prof. Garcia is Doctor Honoris Causa from the University of Bucharest and the recipient of the 2011 Janssen-Cilag award given by the Spanish Royal Society of Chemistry and the 2008 Alpha Gold of the Spanish Society of Glass and Ceramics.


Introduction

Metal organic frameworks (MOFs) are one type of micro-/mesoporous crystalline solid in which the lattice is generated by connecting metallic nodes constituted by metal cations or clusters of a few metal ions with rigid organic linkers having two or more coordination positions.1–5 Due to the nature of the interaction between the metallic nodes and organic linkers these types of materials are also known as porous coordination polymers (PCPs).6 There are examples of MOFs of almost all di-, tri- or tetrapositive metal ions of the Periodic Table. In addition, there is also a large diversity in the structure and binding groups of the organic linker, although the most common ones are aromatic polycarboxylates, nitrogenated heterocycles and organophosphorous compounds. Fig. 1 illustrates the concept of MOFs and the constructive elements.
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Fig. 1 Concept of MOFs by combining metal ions or clusters with multipodal rigid organic linkers forming a crystalline network through metal–ligand coordination bonds.

It frequently occurs that one or more of the coordinative positions around the metallic elements in the ideal crystal structure of the MOF is not compromised with the construction of the lattice. These positions are typically occupied by solvent molecules such as water from the ambient environment or any ligand that could be present during the preparation of the material. These coordinative exchangeable positions not occupied by the multipodal linker can be frequently evacuated upon heating the MOF at moderate temperatures under vacuum, generating Lewis acid sites and coordinatively unsaturated sites (CUS) around the metal ions. In other cases, imperfections and defects in the structure may also introduce some CUS around the metal centres.

Based on the above considerations, MOFs can be used in different cases as solid Lewis acids and, therefore, they can act as general catalysts for those reactions promoted by these types of sites.

In another case, the ligand can be used to anchor to the lattice some active sites such as Brönsted acid sites7 (the case of sulfonic acid groups), basic sites8 (the case of amino groups) or additional binding centres that can participate in the formation of metallic complexes at peripheral positions respect to the solid lattice that can introduce some specific catalytic activity.

Besides the catalytic activity derived from metal nodes and the linkers, MOFs can also act as hosts to include guests exhibiting catalytic properties. A typical case that will be covered in this review is the case of metal NPs (MNPs) that have intrinsic catalytic activity in oxidations and couplings. These MNPs can be occluded inside the pores of the MOFs, becoming stabilized against particle growth. Fig. 2 illustrates the various possibilities where the active sites can be located in MOFs.


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Fig. 2 Various possibilities that MOFs offer to locate active sites for catalysis.

The importance of MOFs in heterogeneous catalysis derives from the consideration of different structural features of MOFs that are very positive from the point of view of their activity. One of these general properties of MOFs is their large specific surface area and high porosity that ranks this type of solid among the materials with the largest values of internal area and lowest framework density.9 Related to surface area and porosity, one of the unique features of MOFs is the possibility to design isostructural materials and to predict the pore geometry and dimensions of a given composition by analogy with the structure of related MOFs and the relative dimensions of the ligands.10,11 Thus, maintaining the metallic nodes, if one ligand having a certain directionality is replaced by another one with the same directionality, but larger dimension, it is generally observed that the two MOFs exhibit the same structure with pore dimensions related to the relative size of the organic linkers.12Fig. 3 illustrates the possible ways to prepare isostructural MOFs with the same topology but different compositions depending on the substitution on the linkers.


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Fig. 3 Preparation of isostructural MOFs with the same topology but different dimensions or substitution at the linkers. Reproduced from ref. 13 with permission from the Royal Society of Chemistry, copyright 2009.

Another positive feature of MOFs with regard to catalysis is the high content of active sites that, as previously commented, can be related to the percentage of transition metal or linker in the unit formula of the material. To put into context this unique characteristic of MOFs with respect to other related porous materials, it has been difficult in the case of zeolites to graft on the aluminosilicate walls other metallic elements and, in any case, the content of these metallic elements that would be acting as catalytic centres cannot be higher than a few weight percents without undergoing agglomeration and becoming segregated from the structure of the silicate. Besides, as commented earlier, another advantage of MOFs is the large diversity of metallic elements for which MOFs can be prepared. Furthermore, it will be covered in this review that besides a single metallic element, certain MOFs offer the opportunity to have more than one metal, the preparation of mixed-metal MOFs being possible.14–16 Also, mixed-linker MOFs having more than one linker are easily obtained and, therefore, the diversity in the composition of MOFs, the flexibility in the modification of these compositions and the design of the pore dimensions make MOFs incomparable with any other crystalline porous solid known so far.

The above reasons have led to extensive use of MOFs as heterogeneous catalysts since these types of materials were reported. Myriads of papers have appeared since 2000 presenting the use of MOFs as heterogeneous catalysts in reactions that require Brönsted, Lewis, redox and other types of sites. There are excellent reviews in the literature covering the various relevant properties of MOFs in heterogeneous catalysis, particularly for liquid-phase reactions,9,17–23 homochiral MOFs for enantioselective catalysis,24–26 industrial based applications,27 supramolecular catalysis,28 hosts for metal nanoparticles,29–32 biomimetic catalysis,33,34 environmental applications,35 polyoxometallate based MOFs for catalysis,36 synthesis of heterocycles,37 oxidation reactions,38,39 tandem reactions,40 preparation of fine chemicals,41 condensation reactions42 and C–C coupling reactions,43 among others.

However, in spite of the interest of MOFs in heterogeneous catalysis, these materials have encountered a limitation due to their poor thermal and chemical stability compared to zeolites and other inorganic porous materials. The low thermal stability observed in MOFs makes it generally not possible to use these materials for typical gas-phase reactions and the use of reactivation procedures typically employed for zeolites that consist of thermal treatment under air to produce the combustion of the organic material acting as a poison by blocking the active sites is not possible in the case of MOFs. Thus, reactivation of deactivated MOF catalysts is considerably more problematic than in the case of zeolites. With respect to chemical stability, the nature of the coordinative bonds between metal nodes and linkers also limits the type of solvents and reagents that can be used. Generally, MOFs are not stable using reagents that can also act as good ligands for metals, such as thiols, amines and even carboxylic acid derivatives. Also, the stability of MOFs in certain solvents, particularly water, is poor, especially at extreme pH values, either acidic or basic. For these reasons, the structural stability of MOFs under reaction conditions is always an important issue that has to be demonstrated convincingly when using these materials in catalysis. However, as a general rule of thumb it can be assumed that due to the strength of coordinative bonds, MOFs formed by the interaction of cations with high charge (+4 or +3) are more stable than those with ions with lower charge density. In the present review, we will discuss, commenting in a critical way, stability data. The issue of stability is related to deactivation of the material and also to the truly heterogeneous nature of the process. Complete catalytic studies should include also a mention of the possible leaching of the components from the solid to the solution, accurate analytical data of the liquid phase being necessary together with conventional hot-filtration tests to support that the catalytic reaction takes place on the pores of the solid.

Besides catalysis, MOFs are also gaining importance in electrocatalysis and photocatalysis. The present review also covers the use of MOFs as photocatalysts highlighting how these materials offer features characteristic of molecular organic photocatalysts, like tunability of absorption maxima by introducing substituents on the aromatic linker and that of inorganic semiconductors of a small particle size in the nanometric and subnanometric length scale. In this way, MOFs offer unique properties in photocatalysis that have no comparison with other types of photoresponsive materials, bridging the two, until now, separate types of photocatalysts. These properties have opened promising opportunities particularly in the field of solar photocatalysis applied to hydrogen generation and CO2 reduction. It will be shown that this remarkable photocatalytic activity derives from the intimate interaction between the organic linker and the metallic nodes occurring in these materials, one of the most general mechanisms being photoinduced single electron transfer from the linker to the metal, resulting in charge separation. Linkers can be substituted to increase their visible light absorption and nodes can also be modified by ion exchange to favour this electron transfer. In addition, the intracrystalline voids can include MNPs that can act as co-catalysts, increasing the dark redox processes occurring after light absorption. Following a parallel approach to that used to overview the field of catalysis, the purpose of the present review is to illustrate novel strategies based on the tuning of MOF components and on the multifunctionality to increase the photocatalytic activity.

Concept and scope of the review

Considering the vast number of studies that have already appeared describing the use of MOFs as heterogeneous catalysts for organic reactions, whose number is continuously increasing at a very large pace, as well as available existing reviews in the area, it would be difficult to provide a comprehensive coverage of the use of MOFs as heterogeneous catalysts32,39 and photocatalysts44,45 without overlapping with the articles already published. For this reason, rather than trying to be exhaustive on the existing reports on every MOF described as a heterogeneous catalyst and comment on each single reaction, the present review is aimed at describing recent trends and novel strategies that have been developed to apply MOFs in heterogeneous catalysis.

In this way, rather than organizing the review based on the type of organic reaction or the type of MOFs, the following sections are grouped according to work reported on MOFs as catalysts based on the nature of the active sites, their topological location with respect to the lattice and the modification of the parent MOF structure to introduce or increase their catalytic activity and the origin of the photocatalytic activity and strategies to improve it by MOF tuning. The purpose is to illustrate the tools that have been developed to adapt and apply MOFs in heterogeneous catalysis, starting from the initial paradigm that the catalytic activity of MOFs resides in the presence of CUS around the metal nodes that makes such MOFs behave as Lewis acids. This is the case, for instance, for HKUST-1 (HKUST: Hong Kong University of Science and Technology) or MIL-100 (MIL: Materials Institute Lavoisier) in which each Cu2+ or two-thirds of the Fe3+ ions, respectively, are coordinated with solvent molecules or other ligands that can be removed by thermal treatment without altering the lattice of the materials. In the following sections, it will be shown that the presence of structural defects produced purposely in a controlled way is gaining increasing importance to introduce or enhance the catalytic activity of MOFs even if their ideal structure does not contain initially CUS.

Besides the existence of CUS or defects, another strategy that has become well-established in this area and is becoming increasingly used is the use of mixed-linker MOFs. In these materials, two different linkers are employed, one having the wanted substituent exhibiting the catalytic activity and a second innocent linker completing the structure of the MOF. In that way, the density of the active sites can be controlled to the optimal value, avoiding the overcrowding of these sites in the limited pore volume of the structure.

Besides MOFs playing an active role in the reaction mechanism, another alternative is to use these porous materials as hosts to embed MNPs, metal oxide NPs or other guests that could be responsible for the catalysis. In this strategy, the role of MOFs is mainly to increase the stability of these active species and to transform the process from homogeneous to heterogeneous, while providing a large surface area. It will be commented that even in this case recent studies have shown that the intrinsic activity of the guests in MOFs can be complemented with that of the active sites to develop tandem reactions and even to control the reactivity of the embedded guest by favouring some of the available pathways in the mechanism with respect to others. In fact, multifunctionality by combining different active sites is one of the main advantages that MOFs can offer with respect to other conventional solid catalysts and can be simply implemented by engineering different sites at different locations of the MOF structure. For instance, it is being increasingly recognized that bifunctional acid/base sites that can be present in MOFs due to, for instance, acidity at metal nodes and basicity of the ligand substituents can be one of the main reasons for the remarkable activity observed in MOFs in some cases, particularly in condensation reactions, with respect to other catalyst types.

Thus, the concept of our review is to compile the existing literature by classifying them according to the nature of the active sites and the modifications of the MOF structure to adapt the material to its role as a heterogeneous catalysts or photocatalysts. In the last case light absorption and subsequent charge separation are events that can be also favoured by the close proximity and interaction of chromophores and the hole/electron trapping sites, as it will be commented on below.

Catalysis by metal nodes

Metal nodes having CUS or with some exchangeable coordination positions not compromised with the crystal structure can act as catalytic centres, analogously to molecular metallic complexes used in homogeneous catalysis.46 More specifically, this type of metal ion exhibits Lewis acid character and, therefore, this type of node can promote Lewis acid catalyzed reactions.

Besides the above mechanism, in which no changes in the formal oxidation state of the metal ion occurs during the course of the mechanism, metal nodes can also catalyze other reaction types involving a redox mechanism in which a swing in the metal oxidation state between two different states occurs. This possibility is not obvious if one considers the change in the coordination number and geometry that is associated with variations of the oxidation state of a metal ion. For instance, while Pd2+ ions form tetracoordinated complexes with square planar geometry, Pd0 complexes are more generally dicoordinated and exhibit a linear geometry.47 This implies a remarkable structural stress in each turnover if Pd2+ ions would be forming part of the nodes for those reactions that require cycling between Pd2+ and Pd0, such as C–C cross-coupling reactions. C–C bond forming cross-coupling reactions are among the most useful and versatile transformations in modern organic synthesis and Pd salts, complexes and NPs are the most general catalysts for these processes.48

In one of the pioneer examples on the use of MOFs as heterogeneous catalysts, a MOF having Pd2+ ions in nodal positions was tested as a solid catalyst for representative Pd catalyzed reactions, such as Suzuki coupling, alcohol oxidation and selective hydrogenation of alkenes. Specifically, the activity of [Pd(2-pymo)2]n (2-pymo: 2-hydroxypyrimidinolate) was tested in the Suzuki–Miyaura cross coupling reaction between phenylboronic acid and 4-bromoanisole.49 Under the optimized reaction conditions, 85% conversion with 99% of the cross coupling product was achieved at 150 °C in o-xylene as a solvent. A TOF value of 1230 h−1 was measured for this coupling reaction working at a Pd concentration of 0.25 mol%. The Pd-MOF catalyst was found to be reusable for this coupling reaction for four cycles working at room temperature and long reaction times. This Pd-containing MOF was also reported to catalyze the oxidation of cinnamyl alcohol to cinnamaldehyde with a selectivity of 74% at 99% conversion at 90 °C in toluene using air as an oxidant.49 Considering the relevant catalytic activity of Pd in many important reactions widely employed in modern organic synthesis,50 it would be of large interest to develop many other examples of Pd2+-containing MOFs at nodal positions, addressing the issue of their structural stability and comparing their catalytic activity with that of other homogeneous Pd2+-complexes.

A large number of MOF structures with different transition metals have been reported in the literature with the inherent Lewis acidity around the metal centres, generally after adequate thermal activation of the sample, and their activity tested in cyanosilylation reactions. These catalysts have been tested as solid Lewis acid catalysts for the cyanosilylation of aldehydes and ketones using trimethylsilyl cyanide (TMSCN) as a reagent.51 This nucleophilic addition was often employed as a model reaction to rank the performance of the as-synthesised MOF catalysts and also to compare the activity of MOF catalysts with that of other solid catalysts like zeolites. In most of the cases, the employed reaction conditions consist of mild reaction temperatures and the use of solvents compatible with the stability of the crystal structure. The readers are referred to the exhaustive existing literature through the series of review articles published52,53 for a comprehensive coverage of this reaction.

In one of the seminal contributions, the inherent Lewis acidity in Cu3(btc)2 (btc: 1,3,5-benzenetricarboxylate) due to the presence of coordinative exchangeable positions around each Cu2+ ion was used to catalyze the cyanosilylation of benzaldehyde by TMSCN (Scheme 1).54 Similarly, in another example, Mn3[(Mn4Cl)3(btt)8(CH3OH)10]2 (btt = 1,3,5-benzenetristetrazol-5-yl) was reported to exhibit Lewis acidity after solvent removal through coordinative exchangeable positions around Mn2+ and these acid sites were assumed to be responsible for the size-selective cyanosilylation of aromatic aldehydes (Scheme 1).55


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Scheme 1 Cyanosilylation of benzaldehyde with TMSCN using MOFs as catalysts.

In another of the earlier studies on the use of MOFs as solid Lewis acids, Cu3(btc)2 was shown to be a highly selective solid Lewis acid catalyst for the isomerization of terpene derivatives, such as the rearrangement of α-pinene oxide to camphonelal (Scheme 2).56 The issue of the Brönsted or Lewis nature of the active sites in Cu3(btc)2 promoting the isomerization was addressed by employing ethylene ketal of 2-bromopropiophenone as a test substrate and by analyzing the product distribution. This cyclic acetal can give different products including 2-bromopropiophenone, 5-methyl-6-phenyl-2,3-dihydro-1,4-dioxin and 2-bromoethyl 2-phenylpropanoate depending on the nature of the acid site57 (Scheme 3). Based on the product distribution, it was concluded that Cu3(btc)2 behaves as a hard Lewis acid.


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Scheme 2 Isomerization of α-pinene oxide to camphonelal.

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Scheme 3 Determination of the nature of acid sites in Cu3(btc)2 using ethylene ketal of 2-bromopropiophenone as a test substrate.

Later, a series of iron based MOFs were also tested as solid catalysts for the isomerization of α-pinene oxide to camphonelal that is also a rearrangement that requires Lewis acid sites.58 In the previously commented case of 2-bromopropiophenone acetal rearrangement, Cu3(btc)2 was used as a solid Lewis acid, but, Cu3(btc)2 is not among the most robust MOFs due to the weaker strength of the interaction of dipositive Coulumbic charge of Cu2+ with carboxylate anions compared to tripositive metal ions. In this regard, the series of structurally more stable porous metal(III)-benzenetricarboxylates such as MIL-100(Al, Fe and Cr), MIL-110(Al) and MIL-96(Al) has also been investigated as solid Lewis acids in the rearrangement of α-pinene oxide to camphonelal.59 The Lewis acidity of these materials was indirectly determined by EPR and IR spectroscopy using for each type of measurement 2,2′,6,6′-tetramethyl-1-piperidinyoxyl radicals and benzonitrile as the probe molecules, respectively. In the case of Al based MOFs, both methods showed a decrease in the amount of Lewis acid sites in the order MIL-100 > MIL-110 > MIL-96, this correlation being in good agreement with the reaction rate and selectivity toward camphonelal, indicating that Lewis acidity plays a major role in this rearrangement, as expected.

Besides being a versatile organic functional group transformation giving rise to a series of α-functionalized alcohols used as synthetic intermediates, ring opening of epoxides by nucleophiles is another favourite model reaction to assess the catalytic activity of acid MOFs. Specifically, one of the most studied reactions is the styrene oxide opening by methanol or ethanol and in some cases by aniline (Scheme 4). A wide range of MOFs60–64 have been reported to exhibit high activity and stability for this reaction in terms of recyclability in many consecutive runs with no significant drop in their activity. In this case, the predominant product is the regioisomer corresponding to an SN1-type mechanism and the molecular dimensions of the reagents and steric encumbrance can exert a remarkable influence on the rate and yield of the final substituted alcohol, implying that this reaction occurs within the internal voids of the porous MOF.


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Scheme 4 Ring opening of styrene epoxide catalyzed by MOFs.

Epoxide ring opening, being a favorite reaction to test the acidity of catalysts, has also been used to establish how substitution on the organic linker with electron donor and withdrawing groups influences the acid strength65 and, therefore, the activity of the metal nodes. This issue will be discussed in detail in the section dealing with the role of linkers in catalysis by MOFs.

Ni2(bdc)2(DABCO) (DABCO: 1,4-diazabicyclo[2.2.2]octane) was found to be an efficient heterogeneous catalyst for the cross-coupling reaction between phenylboronic acid and phenylacetylene in the presence of air using molecular oxygen as the stoichiometric oxidant.66 Ni2(bdc)2(DABCO) exhibited 86% conversion to afford the desired 1,2-diphenylacetylene with very high selectivity in DMF at 120 °C. Moderate to high conversions of the cross-coupling products were achieved under identical conditions with no observation of homocoupling products. Furthermore, Ni2(bdc)2(DABCO) exhibited significantly higher catalytic activity than other analogous Ni-MOFs, such as Ni3(btc)2 and Ni(btc)(bpy) (bpy: 2,2′-bipyridine). In contrast, Zn2(bdc)2(DABCO) and Co2(bdc)2(DABCO) were inactive for this transformation under identical conditions. Ni2(bdc)2(DABCO) was recycled with no significant drop in its activity in four consecutive cycles. Also, powder XRD showed that the reused Ni2(bdc)2(DABCO) retained its crystalline nature.

Other examples in the literature referring to the use of MOFs as solid Lewis acids due to the presence of coordinative exchangeable positions around the metal ions not compromised with the structure include CO2 addition to epoxides,67–70 among others.53

As it has been already mentioned in the introduction and although it will also be commented on in other sections of the present review, particularly when discussing the catalytic activity of MNPs incorporated inside MOFs in which more than one type of active centre is present in the material, MOFs are especially suited catalysts to promote tandem reactions. In tandem reactions, two or more individual reactions are performed in a single step without changing the reaction conditions. Thus, Corma and coworkers have reported the three component (secondary amine, aldehyde and terminal acetylene) coupling to form propargylamines.71 Due to the presence of highly reactive functional groups in the propargylamine, this primary product can be even further transformed in one step by cyclization. In another example Monge and coworkers have studied the reaction of a carbonylic compound, a primary amine and cyanosilane. These authors have shown that it is possible to control the product selectivity either to the α-cyanoamine or silane by controlling the proportion of In in a Ga-MOF.72

Oxidation reactions

This section provides an overview of the recent developments in the field of liquid phase oxidation of (cyclo)alkanes by using MOFs as heterogeneous solid catalysts in the presence of suitable oxidizing agents. The reader is referred to the existing reviews dealing specifically with oxidation reactions for an exhaustive coverage of the literature in this area.38,39

Aerobic oxidation of tetralin (Scheme 5) has been reported using Cu(2-pymo)2 and Co(PhIM)2 (PhIM = benzoimidazolate) as heterogeneous catalysts. The aerobic oxidation of tetralin using Cu(2-pymo)2 as a solid catalyst afforded 52% conversion mainly to the ol/one mixture at the benzylic position accompanied by some tetralin peroxides that are the precursors of the corresponding ol/one mixture.73 In contrast, Co(PhIM)2 reached lower tetralin conversion under the optimized conditions (23%), but this transition metal has the ability to decompose completely hydroperoxides that were present in the reaction mixture in much lower percentage using Co(PhIM)2 as a catalyst in comparison to Cu(2-pymo)2. Due to complimentary activity of the two MOFs, the Cu2+ promoting high conversion of tetralin and the Co2+ exhibiting high selectivity toward ol/one, it was postulated and confirmed that a cocktail of the two MOFs, Cu(2-pymo)2 and Co(PhIM)2 in appropriate proportions, is a better performing catalytic system. Considering the current interest in developing mixed metal MOFs and applying them to catalysis, it appears logical that a single material having the two types of metal ions, Cu2+ and Co2+, could be ideal to enhance further catalytic activity. A comparable activity to Cu(2-pymo)2 has been also observed for [CuII2(1,3-bdpb)(OCH3)2] (CFA-5: Coordination Framework Augsburg University-5; H2-1,3-bdpb: 1,3-bis(3,5-dimethyl-1H-pyrazol-4-yl)benzene) in the aerobic oxidation of tetralin at 90 °C after 30 h.74


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Scheme 5 Oxidation of tetralin catalyzed by MOFs.

Besides molecular oxygen, hydroperoxides are also suitable oxidizing reagents from the point of view of green chemistry, avoiding the use of stoichiometric amounts of transition metals. In another precedent of the use of MOF for hydrocarbon oxidation, commercially available Fe(btc) (Basolite F300), a MOF presumably having a distorted framework similar to MIL-101(Fe), was reported as a solid catalyst for the oxidation of xanthene (99% conversion) to xanthol/xanthone (99%) at 70 °C after 24 h using tert-butylhydroperoxide (TBHP) as an oxidizing reagent. Furthermore, Fe(btc) resulted in 62% conversion of tetralin with 71% selectivity of tetralone at 75 °C after 24 h.75

MIL-101(Cr) and MIL-100(Fe) as well as commercial Fe(btc) have been used as oxidation catalysts employing TBHP as an oxidant.39 It was, however, observed that these MOFs can also promote, after an induction period, the aerobic oxidation of benzylic hydrocarbons. MIL-101(Cr) and MIL-101(Fe) can promote the aerobic oxidation of indane to the ol/one mixture at 120 °C with 87% selectivity at 30% conversion (MIL-101(Cr)) and 71% selectivity at 30% conversion (MIL-101(Fe)).76

In another precedent, oxidation of cyclohexane has been reported using MIL-101(Cr) or MIL-101(Fe) as reusable catalysts and TBHP as an oxidant. MIL-101(Cr) resulted in 36% conversion with the combined selectivity to ol/one of 83% and the presence of cyclohexane peroxide being not observed. Under identical conditions, MIL-101(Fe) exhibited 27% conversion with 44% ol/one selectivity, along with 50% of the peroxide.77 These results illustrate again that the nature of the transition metal plays an important role in the product distribution of radical oxidations due to the different ability to decompose unwanted, hazardous peroxides.

Ahn and coworkers have reported that MIL-101(Cr) is a reusable catalyst for the oxidation of tetralin (68% conversion) to 1-tetralol (2.5%) and 1-tetralone (85.5%) either using TBHP or oxygen/trimethylacetaldehyde as oxidants at 80 °C after 2 h.78 The combination of aldehydes and transition metals such as Ni2+ and Co2+ under aerobic conditions is a general procedure for oxidation of alcohols and hydrocarbons.79 The main limitation is, however, that the system requires the presence of aldehydes in stoichiometric or larger excess with respect to the substrate undergoing oxidation.

Several other MOFs, including {[Cd(L)(H2O)]·3H2O} (L-H2 = 4,4′-(9,10-anthracenediyl)dibenzoic acid) have also been reported to oxidize alkylbenzenes to the corresponding ketones using TBHP as an oxidant.80

A copper-based MOF having a trinuclear triangle cluster, namely [Cu33-OH)(μ-pz)3(EtCOO)2(H2O)] (Hpz = pyrazole) (see structure in Fig. 4), has been reported as a catalyst for the peroxidative oxidation of cyclohexane using hydrogen peroxide as an oxidant in combination with nitric acid in an acetonitrile/water medium, with selectivity towards cyclohexanol (25.1%) and cyclohexanone (2.8%).81 As previously commented, the structural stability of Cu MOFs is generally not very high and the presence of nitric acid in the reaction mixture could be detrimental for catalyst stability particularly considering that MOFs exhibit generally low stability at extreme low and high pH values.


image file: c8cs00256h-f4.tif
Fig. 4 Structure of [Cu33-OH)(μ-pz)3(EtCOO)2(H2O)]. Reproduced from ref. 81 with permission from the American Chemical Society, copyright 2015.

Metallated porphyrins and phthalocyanins82 are well established general oxidation catalysts82 that mimic the active centres of oxygenases and P450.83 Polycarboxylated porphyrins have been used as linkers in the preparation of MOFs. In one of these examples, a zinc metallated (MnIII) porphyrin MOF, namely, [(CH3)2NH2][Zn2(HCOO)2(MnIII-tcpp)]·5DMF·2H2O (tcpp: tetrakis(4-carboxyphenyl)porphyrin) was used as a solid catalyst for the oxidation of cyclohexane reaching 20.6% conversion using iodosobenzene as an oxidant in dichloromethane at room temperature.84 Furthermore, Zr-PCN-221(Fe) (PCN: porous coordination network) having a porphyrin linker was explored as a heterogeneous catalyst for cyclohexane oxidation by TBHP resulting in cyclohexanone (86.9%) and cyclohexanol (5.4%) at 65 °C after 11 h (TON 18).85

Recently, an interesting example was reported by Long and co-workers, who systematically studied the effects of the local hydrophobic environment on the product selectivity of cyclohexane oxidation by iodosobenzene catalysed by expanded analogues of Fe-MOF-74 with exposed Fe2+ sites (Fig. 5).86 A three-fold enhancement of the ol-to-one product ratio and an order of magnitude increase in TON could be achieved by altering the pore size and installing nonpolar hydrophobic functional groups near the iron centers (Fig. 5). In this way, apolar cyclohexane will be preferentially adsorbed around the Fe porphyrin, while polar cyclohexanol could be more easily desorbed.


image file: c8cs00256h-f5.tif
Fig. 5 Oxidation of cyclohexane to cyclohexanol and cyclohexanone catalyzed by Fe-based MOFs having increasing pore size and hydrophobicity. Reproduced from ref. 86 with permission from the American Chemical Society, copyright 2016.

Similarly, engineering of hydrophobic pore walls in an iron-porphyrinic MOF, PCN-222(Fe), enhances its catalytic performance in the cyclohexane oxidation.87 PCN-222(Fe) exhibited 20.5% conversion of cyclohexane with a selectivity to the ol/one oil of 81% in acetonitrile using TBHP and AgBF4, while more hydrophobic PCN-222(Fe)-F7 (F7 stands for the number of fluorine atoms in the linker; Fig. 6) showed 50.2% conversion with a selectivity to the ol/one mixture of 90.1% under identical conditions.87 These data suggest that grafting perfluorinated alkyls onto the pore walls significantly increases the hydrophobicity and improves the interaction with cyclohexane, resulting in enhanced conversion and selectivity to the ol/one oil mixture. In contrast to the remarkable activity of PCN-222(Fe)-F7, the homogeneous iron porphyrin is almost inactive for this reaction. Interestingly, it is proposed that the introduction of AgBF4 creates a weak coordination between the BF4 and Fe(III) sites that facilitates the formation of the Fe(IV)–oxo active species, leading to a very high percentage (>90%) of cyclohexanone in the ol/one mixture. Furthermore, the optimized catalyst, PCN-222(Fe)-F7, was recycled for three cycles and found to be heterogeneous in nature. Other oxidation reactions that have been reported using MOFs as catalysts include oxidation of benzyl amine, alcohol, thiophenol and thiols and are covered in the existing literature.22,38,39,88,89


image file: c8cs00256h-f6.tif
Fig. 6 (a) Time-conversion plots for cyclohexane oxidation over PCN-222(Fe) and PCN-222(Fe)-Fn (n = 3, 5, and 7); (b) proposed mechanism for cyclohexane oxidation. Reproduced from ref. 87 with permission from the Royal Society of Chemistry, copyright 2017.

Interestingly, also in the case of oxidation reactions, an influence of the presence of electron donor/withdrawing substituents on the catalytic activity of the metal nodes in the aerobic oxidation of benzyl amines to N-benzylidene benzyl amines by MIL-101(Cr) has been observed.90 This issue will be discussed in detail in the section dealing with the influence of the linkers in catalysis.

Catalysis by defective MOFs

One of the general methodologies to generate purposely CUS around the metal ions, so that these metal ions can act as active centres in catalysis, is the creation of defects in the MOF structure.91 Two possibilities, either submitting the synthesised MOF to further treatment to produce partial structural damage or performing the synthesis of the material using defect-inducing ligands, have been developed to generate structural defects.

Defective MOFs have encountered an immediate application in catalysis due to their larger density of active sites. For this reason, their interest is growing in catalysis. In one of the reported examples, a new crystalline material denoted as MOF-5(Oh) (Oh: octahedral) (Fig. 7) has been obtained by reacting Zn(II) with bdc (bdc: 1,4-benzenedicarboxylate) in the presence of small amounts of 1,3,5-tris(4-carboxyphenyl)benzene (btb). MOF-5(Oh) is nearly identical to the structure of MOF-5, but it has an octahedral morphology and a number of defect sites associated with the presence of btb that are uniquely functionalized with dangling carboxylates.92 The BET surface area values of 5 and 10% btb-loaded materials were 3070 and 2850 m2 g−1, respectively, whereas pure MOF-5 synthesized under identical conditions exhibits a somewhat larger value of 3470 m2 g−1. Uncoordinated carboxylic acids can be used to attach additional metal ions through these groups. In this way, highly dispersed Pd(II) ions were loaded on this defective MOF-5(Oh). The Pd content in Pd(II)/MOF-5(Oh) determined by inductive coupled plasma-optical emission spectroscopy was 2.3 wt%. The activity of Pd/MOF-5(Oh) was tested in the C–H phenylation of naphthalene using diphenyliodonium tetrafluoroborate (Scheme 6). Under the optimized reaction conditions, the phenylation of naphthalene catalyzed by Pd(II)/MOF-5(Oh) (5 mol% Pd) afforded 64% yield with a regioselectivity α[thin space (1/6-em)]:[thin space (1/6-em)]β of 3[thin space (1/6-em)]:[thin space (1/6-em)]1. In contrast, a control experiment with unmetallated MOF-5(Oh) resulted only in 7% yield, even at longer times with α[thin space (1/6-em)]:[thin space (1/6-em)]β selectivity of 5[thin space (1/6-em)]:[thin space (1/6-em)]1. On the other hand, the use of MOF-5 in combination with a homogeneous reaction with Pd(OAc)2 slightly improved the yield to 21% with a α[thin space (1/6-em)]:[thin space (1/6-em)]β selectivity of 8[thin space (1/6-em)]:[thin space (1/6-em)]1 with respect to the activity of Pd(OAc)2 without any MOF-5 additive. These data highlight the potential of generating defects in MOFs to create additional catalytic sites that can exhibit increased selectivity towards the target products in a given reaction. It should be, however, commented that MOF-5 is probably not the most appropriate structure to test this methodology due to its poor structural stability. Other examples exploiting this strategy for more robust MOFs should be expected.


image file: c8cs00256h-f7.tif
Fig. 7 (a) Synthesis of btb-incorporated MOF-5 crystals by addition of btb to the reaction mixture of bdc and Zn-(NO3)2·6H2O. (b) Photographs of crystals showing the dependence of the morphology upon the percentage of btb in the feed (scale bar: 100 μm). Reproduced from ref. 92 with permission from the American Chemical Society, copyright 2011.

image file: c8cs00256h-s6.tif
Scheme 6 Pd-catalyzed phenylation of naphthalene by Ph2IBF4 using Pd(II)/MOF-5(Oh).

A sulfone-functionalized MOF, USTC-253 (University of Science and Technology China), was synthesized purposely adding trifluoroacetic acid (TFA) during the synthesis of the USTC-253 material to afford a defect-containing MOF sample, namely USTC-253-TFA, with exposed metal centers (Fig. 8).93 The DRIFT spectra of CO adsorption revealed the presence of both Brönsted and Lewis acidity in USTC-253-TFA. The catalytic activity of USTC-253-TFA was examined in the CO2 cycloaddition to propylene oxide at room temperature with 1 bar pressure. USTC-253-TFA afforded 81.3% conversion in the presence of TBAB as a cocatalyst at room temperature. The catalytic performance of USTC-253-TFA was higher than that of homogeneous Al(NO3)3 (72.3% yield) and 4,4′-dibenzoic acid-2,2′-sulfone (29.5% yield) under identical conditions. A series of control experiments revealed that the activity of USTC-253-TFA is also much superior or comparable to that of USTC-253, EL-MIL-53 (EL: elongated), and MOF-253 affording 74.4, 57.4 and 81.9% yield of the cyclic propylene carbonate, respectively. These data indicate that the activity of USTC-253-TFA is higher than that of defect-free USTC-253, a fact that has been mainly ascribed to the presence of defect-induced CUS in some metal centres in USTC-253-TFA. These unsaturated metal ions will behave as Lewis acid sites and would be responsible for the boost in the conversion. The catalytic activity of USTC-253-TFA was retained for three cycles. The powder XRD pattern of USTC-253-TFA shows its intact framework after the reaction. On the other hand, the performance of USTC-253-TFA was also higher than that observed for highly stable MOFs such as MIL-101, UiO-66, ZIF-8 and MIL-53 that reached conversion values of 30.7, 54.8, 18.7 and 54.4%, respectively, under identical conditions, reflecting again the positive influence that introduction of defects may play in the catalytic activity of MOFs.


image file: c8cs00256h-f8.tif
Fig. 8 Synthesis of USTC-253 (left) and defect-engineered USTC-253-TFA (right) of TFA by addition. Reproduced from ref. 93 with permission from Wiley, copyright 2015.

A related strategy was employed to obtain Ru-DEMOFs (DEMOFs: defect-engineered MOFs)94 by utilizing a mixture of tritopic btc linker and different functionalized ditopic isophthalate (ip) denoted 5-X-ipH2, where X = OH, H, NH2 and Br, as defect-generating linkers to achieve a mixed-valent ruthenium analogue of [Cu3(btc)2]n having the empirical formula [Ru3(btc)2−x(5-X-ip)xYy]n (Y: Cl, OH, OAc; 0 < y ≤ 1.5) (Fig. 9). Characterization data confirmed the existence of two kinds of structural defects (A and B) induced by framework incorporation of 5-X-ip. On one hand, defective paddlewheel units (defect type A) would introduce in the materials additional sites around the partially reduced Ru centres. On the other hand, the absence of the entire Ru paddlewheel cluster (defect B) can affect strongly the adsorption properties, especially when the X functional group at the defect-generating linker is as small as H. The relative abundances between defects A and B depend on the choice of the functional group X on the defect linkers and three different materials with different A/B ratios were prepared. The activity of the series of Ru-DEMOFs was tested for the condensation reaction between 2,5-hexadione and aniline to form dimethylphenyl-1H-pyrrole in toluene at 90 °C (Scheme 7).


image file: c8cs00256h-f9.tif
Fig. 9 Synthesis of Ru-DEMOFs by the mixed-component/fragmented-linker solid-solution approach and the proposed models of the two possible paddlewheel-related defects. Reproduced from ref. 94 with permission from Wiley, copyright 2016.

image file: c8cs00256h-s7.tif
Scheme 7 Ru-DEMOF catalyzed synthesis of 2,5-dimethyl-N-phenyl-1H-pyrrole.

Among the tested materials, Ru-DEMOFs having H and OH substitutions in ip exhibited the highest catalytic activity, although an increase in the percentage of defective linkers over an optimal value can be detrimental for the catalytic activity. The influence of the nature of the substituent and the percentage of the defective linker on the catalytic activity was explained as reflecting variations among the two types of defects A and B and a decrease in the steric hindrance. When Ru-DEMOFs using 5-OH-ip as defect linkers with different weight proportions 8, 20, 32 and 37% were employed as catalysts, the yield of pyrrole increased from 48% (parent Ru-MOF) to 77% for the defective Ru-DEMOF with the optimal 5-OH-ip proportion. This is due to the presence of reduced Ru centres (defects A) in these materials. The reduced, softer Ruδ+ binding sites can easily undergo coordination to the O atom of the carbonyl group, which favours nucleophilic attack by the lone-pair of an aniline amino group. However, upon increasing the proportion of 5-OH-ip beyond the optimal value (8%), the gradual dominance of defects B in the resulting samples decreases the number of reactive metal centres, and thus diminishes the catalytic activity of the defective Ru-DEMOFs. On the other hand, Ru-DEMOF with ip shows a higher yield of pyrrole compared to the parent Ru-MOF (82 vs. 48%), regardless of the proportion and density of metal centres (defects B). This higher activity of Ru-DEMOFs with H-ip is suggested to be the result of the smaller steric hindrance surrounding the Ru2+ sites as a consequence of the presence of the H-ip defect linkers. Overall these results exemplify the possibility to introduce more than one type of defect on the MOF structure, each defect site having a distractive influence in the catalysis.

UiO-66 is one of the preferred MOFs to determine the influence of defects on the catalytic activity. The advantages of UiO-66 in these studies derive from a combination of reasons, including high structural stability, large pore dimensions and surface area and the fact that conventional synthesis can generate materials with a large density of defects. Defects can even be increased by post-synthetic treatment. In comparison, there are well-established synthetic methods that produce samples with considerably high crystallinity. In this regard, a series of Zr/Hf-based MOFs (UiO-66, UiO-67 and PCN-57) with different degrees of defects were prepared by modulation of the synthesis conditions.95 The number of missing linkers in these MOFs was determined by acid–base titration. Interestingly, a quantitative correlation was established between the number Zr6 defective nodes and the catalytic activity of the defective UiO-type sample in the acid-catalyzed styrene oxide ring-opening reaction by 2-propanol (Scheme 8). To prove the influence of defects on the catalytic activity, two UiO-67 samples, one without defects obtained using benzoic acid to improve crystallinity (UiO-67-BA) and another treated with HCl to generate defects (UiO-67-HCl) were prepared. These two samples exhibited 34% and 4% conversions of styrene oxide that correlate with 1.75 and 0 number of missing linkers per Zr6 cluster for UiO-67-HCl and UiO-67-BA, respectively. Similarly, Zr-NU-1000 and Hf-NU-1000 were synthesised with four different percentages of missing linkers per Zr6 or Hf6 cluster, observing 90 and 86% conversions of styrene oxide, respectively, for the samples with the highest density of missing linkers. On the other hand, 8-connected Zr-NU-1000 and Hf-NU-1000 were prepared with six different proportions of missing linkers per M6 cluster and showed quantitative conversions under identical conditions (Fig. 10). These MOFs exhibited high SN1-like regioselectivity (Scheme 8), giving the primary alcohol as the sole product as evidenced by GC and 1H-NMR spectroscopy.


image file: c8cs00256h-s8.tif
Scheme 8 Ring opening of styrene oxide by 2-propanol using defective UiO-67 based catalysts.

image file: c8cs00256h-f10.tif
Fig. 10 Crystal structure of (a) defect-free UiO-67 and (b) defective UiO-67 due to missing linkers. (c) Zr6 cluster in defect-free UiO-67, and (d) defective Zr6 cluster with terminal –OH and –OH2 groups. Reproduced from ref. 95 with permission from the Royal Society of Chemistry, copyright 2016.

The influence of both linker vacancies (defective MOFs) and the hydration state of the UiO-66 framework was studied in the Oppenauer oxidation of prenol by furfural (Scheme 9).96 Two UiO-66 materials containing a different amount of linker vacancies were synthesized by varying the molar ratio of bdc to ZrCl4. Powder XRD indicated the formation of UiO-66 frameworks in both cases. However, thermogravimetric analysis determined that the average number of linkers per inorganic Zr6 node was 9.9 and 11.6, respectively, for the materials prepared at linker/Zr molar ratios of 1/1 and 2/1. Due to the higher defect degree in UiO-66-9.9, a lower thermal stability was observed for this sample compared to UiO-66-11.6, that has a –COOH/Zr6 ratio closer to the ideal 12 value. The activity of the more defective UiO-66-9.9 for the Oppenauer oxidation was higher than that of UiO-66-11.6, owing to an increased number of coordinatively unsaturated Zr sites, regardless of the hydration state of the framework. For both tested MOFs, the hydrated inorganic building units yield higher prenol conversions (Fig. 11), reminiscent of the case of hydrous zirconia,97,98 even though the hydrated materials are expected to have a lower density of Lewis acidic sites. The higher catalytic activity of hydrated Zr6 nodes was attributed to the cooperative effect between Zr sites and μ3-OH groups. If these were the case, it would be important to provide more examples of the occurrence of this synergy between different structural components of a MOF in catalysis.


image file: c8cs00256h-s9.tif
Scheme 9 Oppenauer oxidation of prenol by furfural.

image file: c8cs00256h-f11.tif
Fig. 11 Conversion profiles of prenol in the Oppenauer oxidation by furfural, using UiO-66-9.9 (red) and UiO-66-11.6 (black) as catalysts (hydrated UiO-66 as closed circles, dehydrated UiO-66 represented with open triangles). Reproduced from ref. 96 with permission from Wiley, copyright 2017.

With regard to the synergy between two different sites in a MOF, cooperating to the catalytic activity, theoretical models have revealed a remarkable case in the activation of malononitrile by Cu3(btc)2. This MOF is one of the most-frequently used MOFs in condensation reactions42 and it was believed that its activity was solely due to the presence of one unsaturated position per Cu2+ ion. However, on the basis of computational investigations it has been proposed that the high catalytic activity and selectivity observed experimentally in the Knoevenagel reaction between benzaldehyde and malononitrile by Cu3(btc)2 as a heterogeneous catalyst could be due to the generation of temporary defects generated by malononitrile causing protonation of btc.99 According to these calculations, the various factors responsible for the high activity of Cu3(btc)2 would be: (i) the role of the btc linkers as a base deprotonating the active methylene reactant, whereas a temporary defect in the framework is formed; (ii) the thus-formed defect, a Brönsted acid site, activating the aldehyde; and (iii) the cooperation of two adjacent Cu2+ sites (Lewis acid sites) located in the same cavity and separated by 8.2 Å, just the distance matching in malononitrile the separation between the two cyano groups. What would be, then, unique in the malononitrile/Cu3(btc)2 combination is the matching between the reagent dimensions and shape and lattice of the Cu MOF that, eventually, results in a double activation of each malononitrile molecule by two adjacent Cu2+ ions, together with the role of the carboxylic/carboxylate groups.

The catalytic activity of solids, even those that have pores, depends considerably on the external surface. The external surface in the case of MOFs can be considered also as defects, since coordinative exchangeable positions around the metal nodes or linkers having unsaturated positions have to be present on this surface. One general way to increase the external surface area is to decrease the average crystallite size. In the specific case of MOFs, diminution of the crystallite size is a way to increase the relative contribution of the external vs the internal surface and, indirectly, to increase the proportion of defective sites, As one illustrative example of this approach, the defect engineering and fabrication of a highly stable nano-BIT-58 (BIT: Beijing Institute of Technology) was recently reported using a modulation method100 from Ce-btb (btb: 1,3,5-tris(4-carboxyphenyl)benzene) (BIT-58; (C27H15CeO6)·3.71H2O·1.16DMF).101 Ce-btb MOF was synthesised by using Ce3+ as the metal centre and btb as the ligand. This synthesis method has resulted in the preparation of nano-BIT-58 with the crystallite size reduction from micrometers (∼25 μm) to nanometers (∼30 nm), accompanied by a concomitant 10 times increased acid site amount and 7 times higher mesopore volume compared to micrometric BIT-58. The catalytic activity of BIT-58 and nano-BIT-58 has been evaluated for the Knoevenagel condensation between benzaldehyde and malononitrile. The catalytic efficiency of nano-BIT-58 reached 100% which was superior to that of micrometric BIT-58 under the same conditions (78%). Furthermore, the difference in activity was more significant with 1-naphthaldehyde and 9-anthraldehyde as substrates, in which nano-BIT-58 has 2.9 and 14 times higher conversion efficiency, respectively, than BIT-58. This enhanced activity of nano-BIT-58 with respect to BIT-58 of micrometric particle size is ascribed to the enhanced external surface, resulting in more Lewis acid sites (unsaturated metal sites) and Brönsted acid sites (uncoordinated carboxyl groups) exposed to the reagents, as evidenced by NH3-TPD curves. Although these nanometric catalysts exhibit higher activity, the stability of these solids in liquid phase reactions needs to be demonstrated. In principle, nanometric particles have an enhanced tendency to undergo leaching and dissolution in liquids.

Recently, a versatile modulator-induced defect-formation strategy based on the use of monocarboxylic acids as modulators and employing an insufficient amount of the required organic ligand was adopted for the controllable synthesis of hierarchically porous MOFs (HP-MOFs) with high stability and tailorable pores.102 The modulator has a chemical structure such that it can play a dual role. On one hand, the carboxylic acid coordinates to the metal ion for the formation of metal–carboxylate clusters typical of many MOF structures. On the other hand, the presence of an alkyl chain creates lattice defects and increases the pore space as shown in Fig. 12a–c. Moreover, using this approach, the pore diameter can be systematically tuned via altering the chain length and concentration of the modulator (Fig. 12d–f). To determine the advantages of the combination of defects and hierarchical porosity, the catalytic activity of phosphotungstic acid (HPW, H3PW12O40·nH2O) loaded on HP-UiO-66 (HPW/HP-UiO-66) and HP-UiO-66 was studied in the ring opening of styrene oxide with methanol as a model reaction. The conversion of styrene oxide using HPW/HP-UiO-66 was quantitative in 20 min, whereas when using HP-UiO-66, UiO-66, and HPW/UiO-66 as catalysts, styrene oxide conversion was less than 10% under identical conditions. This enhanced activity was related to a higher content of active HPW guest loaded into the HP-UiO-66 host. Furthermore, HPW/HP-UiO-66 exhibited superior catalytic performance to HPW-loaded UiO-66, even at similar HPW loadings. In addition HPW/HP-UiO-66 was recyclable without remarkable activity drop, indicating a good stability under reaction conditions. The superior activity of HPW/HP-UiO-66 compared to related materials shows the importance of creating defects and the advantages of the hierarchical porosity in MOFs. It should be, however, made clear that the active sites in this example are likely to be HPW and the defects are not expected to play an active role in the reaction mechanism.


image file: c8cs00256h-f12.tif
Fig. 12 Schematic illustration of the synthesis of HP-MOFs with adjustable porosity and UiO-66 structure. The modulator plays a dual role, namely the carboxylic acid coordinates to the metal ion for the formation of metal–oxo clusters, while the alkyl chain creates structural defects and additional pore space (a–c); the pore diameter can be systematically tuned via altering the length and concentration of the modulator (d–f). Reproduced from ref. 102 with permission from Wiley, copyright 2016.

A mixed-linker approach has been also employed to introduce structural defects into the mixed-valence RuII/III analogue of [M3(btc)2] MOFs (M = Cu, Mo, Cr, Ni, Zn), with partly missing carboxylate groups at the Ru2 paddle-wheels (Fig. 13).103 The incorporation of pyridinedicarboxylic acid (pydc), which has a similar size as btc, but carries a lower charge, as a second defective linker resulted in the mixed-linker isoreticular analogue of M3(btc)2 [Ru3(btc)2]. In addition to the creation of additional coordinative exchangeable positions in the crystal structure, incorporation of pydc induces the partial reduction of RuIII to RuII. The MOFs with modified Ru sites were highly efficient catalysts in the hydrogenation of 1-octene using molecular hydrogen. The parent [Ru3(btc)2Cl1.5] MOF exhibits low hydrogenation activity, reaching only 12% conversion of 1-octene. On the other hand, the use of [Ru3(btc)1.4(pydc)0.6Cl] (ca. 30 mol% of pydc) under identical conditions increased conversion to 50%. Besides the expected product, octane, a mixture of C[double bond, length as m-dash]C bond isomerization products, namely (E,Z)-2-octene, (E,Z)-3-octene, and (E,Z)-4-octene, was also observed, accounting roughly for 30% of the 1-octene converted. The occurrence of C[double bond, length as m-dash]C bond isomerization indicates a certain π-acid character in the sample which is in good agreement with CO adsorption data on parent [Ru3(btc)2Cl1.5] monitored by UHV-FTIR spectroscopy. Furthermore, the defect-engineered samples showed a slightly higher percentage of 1-octene isomerization products than [Ru3(btc)2Cl1.5], in line with the increased π-binding ability of the defective engineered materials.


image file: c8cs00256h-f13.tif
Fig. 13 RuII/III paddle wheel nodes of defect-engineered [Ru3(btc)2−x(pydc)xXy] (X = Cl, OH, OAc) (left) and parent [Ru3(btc)2Cl1.5] (right). Reproduced from ref. 103 with permission from Wiley, copyright 2014.

A facile green method to obtain in a short reaction time under solvent-free conditions defective UiO-66, namely UiO-66-free has been reported (Fig. 14) and its catalytic activity has been compared with that of UiO-66-solvent synthesised by hydrothermal/solvothermal methods.104 Powder XRD patterns of UiO-66-solvent and UiO-66-free confirmed the formation of a UiO-66-structure in both cases. An acid–base titration method revealed that UiO-66(Zr)-solvent shows 1.53 missing linkers in the structure, while the number of missing linkers per unit cell in UiO-66(Zr)-free is 2.16, which is even higher than those in some reported UiO-66(Zr) materials with defect sites prepared by the addition of monocarboxylic acid. Furthermore, analytical data suggested that these defect sites in UiO-66(Zr)-free mainly consist of Zr-OH sites. The catalytic activity of these solids was examined in the oxidative desulfurization (ODS) reactions of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The use of UiO-66(Zr)-solvent as a catalyst resulted in 80.5% DBT removal, while UiO-66(Zr)-free as a catalyst enhanced DBT removal to 99.6% under identical conditions. Interestingly, the difference in catalytic activity in favour of UiO-66(Zr)-free for ODS with respect to UiO-66(Zr)-solvent was even higher for the removal of 4,6-DMDBT reaching a value of 98.1%, which is about two times higher than that achieved using UiO-66(Zr)-solvent (52.5%). These results indicate the superiority of defective UiO-66(Zr) also in the field of deep ODS of fuel oil. On the other hand, UiO-66(Zr) without defects was almost inactive for the ODS reaction under identical conditions. The UiO-66-free catalyst retained its initial activity for four cycles and the results suggest the heterogeneous and reusable nature of these defective MOFs. The beneficial influence of defects on the catalytic activity of UiO-66(Zr) can be rationalized considering that the ideal Zr6(OH)4O4 node should not have any CUS around the Zr4+ ions and its activity should derive from the OH and O atoms on the surface of the octahedral nodal clusters. However, the presence of defects would generate some coordinative exchangeable positions around the Zr4+ ions introducing Lewis acidity and also some redox activity that should boost the performance of the defective material with respect to the ideal, defectless structure.


image file: c8cs00256h-f14.tif
Fig. 14 Synthetic conditions of two UiO-66(Zr) samples under different conditions. Reproduced from ref. 104 with permission from the American Chemical Society, copyright 2017.

In another study, comparison of the catalytic performance and the nature of the active sites of two porous iron trimesates, namely, commercial Basolite F300, having molecular formula of Fe(btc) with an unknown structure and synthetic MIL-100(Fe) also with Fe(btc) formula, but a well defined crystalline structure, for styrene oxide ring opening reaction and aerobic oxidation of thiophenol was reported.105 The interest of this study lies in the consideration that large scale, industrial synthesis of commercial MOFs can produce defective materials, while small scale production can be more suitable for the preparation of highly crystalline samples. This is the case of commercial Basolite F 300 compared to MIL-100(Fe). It was found that commercial Basolite F 300 exhibits higher activity for catalytic reactions requiring strong Lewis acid sites, while, on the other hand, MIL-100(Fe) is the best performing catalyst for oxidation reactions requiring redox pairs.105 These conclusions were reached by combined in situ infrared and 57Fe Mössbauer spectroscopic characterization techniques. Characterization of the Basolite F 300 sample indicated the presence of extra Brönsted acid sites, which are responsible for the enhanced activity observed in acid catalyzed reactions. These catalytic results illustrate the importance of structural defects introducing weak Brönsted acid sites (case of Basolite F 300) and the structural stability in the case of MIL-100(Fe) that makes possible the swing between Fe3+ and Fe2+ and vice versa required in the mechanism of some redox reactions. Spectroscopic evidence has shown that MIL-100(Fe) is stable upon annealing at 280 °C where a spontaneous Fe3+ to Fe2+ reduction is observed without being accompanied by much lattice disturbance. No similar behaviour was observed for defective Basolite F 300, which probably collapses at this relatively high temperature required to promote spontaneous Fe3+ to Fe2+ reduction. This study highlights how large scale production of a particular MOF can result in materials that, even if the chemical compositions are the same, exhibit contrasting catalytic behaviour due to the presence of a different density of defects and different structural stability. Structural defects on MOFs are attracting increasing interest in catalysis and the reader is referred to the existing recent reviews on this subject.106–109

Catalysis by functionalized linkers

In a seminal paper, De Vos and coworkers showed that the presence of functional groups on the aromatic linkers can strongly influence the intrinsic catalytic activity of the metal nodes through inductive effects.110 At the moment this effect has been tested in UiO-66 and MIL-101 that contain terephthalate linkers, although it would be important to expand the strategy to other different ligands.65,90,110–112 As a general rule, the presence of electron withdrawing groups such as –NO2 or –SO3H enhances the Lewis acidity of the metal nodes, while electron donor groups can decrease this activity. Over one order of magnitude enhancement is frequently achieved, but an up to three orders of magnitude increase in the catalytic activity has been reported. It should be commented that this effect of substituents on the terephthalate influencing the activity of the metal nodes has also been observed for aerobic oxidations and, therefore, seems to be more general than plain Lewis acidity.90 A linear relationship between the logarithm of the relative initial rate measured for the MOF having a substituent at the linker and an unsubstituted parent MOF with respect to the Hammett σmeta constant has been observed. In the few examples in which this linearity is not followed, such as in the case of –NH2 substitution for condensations, it has been proposed that it is because the reaction mechanism should involve for this substituent other pathways than just acidity of the metal nodes.65 It could occur that the NH2 condenses with the carbonyl group and, in this way, it forms an intermediate in which the reagent is close to the active metal site. This mechanism is not possible when the substituents are halogens or alkyl groups. In any case, it appears that this concept of increasing the activity of metal nodes by substituents on the ligands will be further exploited to design even more active MOFs.

Besides metal nodes or the influence of the linkers on the activity of these metal nodes, the catalytic activity of MOFs can also reside on the functional groups7,8 present at the organic linkers that typically can introduce activity as acid,7 base8 or organocatalysts.113,114

Among the preferred reactions to test the catalytic activity of basic MOFs,8,42 the Knoevenagel condensation42 is one of the most widely used due to its interest in organic synthesis and experimental simplicity.

Amino groups on the aromatic linker are the preferred substituents to produce MOFs with basic sites. In one of the examples illustrating the activity of NH2-functionalized MOFs, the Knoevenagel condensation reaction between benzaldehyde and malononitrile (Scheme 10) was reported using Fe-MIL-101-NH2 and Al-MIL-101-NH2, observing 90% yield of the expected condensation product at 80 °C.115 In contrast, the catalytic activity of CAU-1-NH2 [Al4(OH)2(OCH3)4(bdc-NH2)3] was significantly lower than the amino functionalized MIL-101 catalysts under the same conditions. Since in order to observe a significant catalytic activity, the reaction has to take place inside the crystal structure of the MOF and not exclusively on the external surface, it is clear that pore dimension is an important issue that has to be addressed prior to the selection of the MOF structure. Thus, the enhanced activity of MIL-101-based catalysts may be due to the large pore size of the MIL-101 structure compared to CAU-1. This rationalization is in agreement with the general assumption that the reaction takes place inside the pores of the MOF.116–119 MIL-101 has mesoporous cages with diameters of 2.9 and 3.4 nm respectively, which are accessible by windows of 1.2 and 1.6 nm, respectively.120 On the other hand, MIL-101 type catalysts exhibit also better activity than amorphous aluminophosphate oxynitrides and nitridated zeolites,121 two of the most widely used solid bases. This enhanced performance of aminated-MIL-101 is probably due to the higher density of basic sites in MIL-101 compared to the other basic solids.


image file: c8cs00256h-s10.tif
Scheme 10 Knoevenagel condensation reaction between benzaldehyde and malononitrile or ethyl cyanoacetate catalyzed by MOFs.

It should be commented that although the activity of CAU-1-NH2 was lower than that of Fe-MIL-101-NH2 and Al-MIL-101-NH2, CAU-1-NH2 can also catalyse the Knoevenagel condensation reaction between aldehydes and malononitrile, as reported by Stock and co-workers.122 In the case of benzaldehyde, this reaction afforded complete conversion and selectivity in ethanol at 40 °C. A series of other aldehydes was also tested with malononitrile to obtain the desired condensation products in moderate to high yields under the optimized reaction conditions. The catalyst maintained its activity and structure for two cycles as evidenced by catalytic data and powder XRD.

In another precedent, UiO-66-NH2 was also studied as a heterogeneous catalyst for Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate or malononitrile.123 UiO-66-NH2 afforded 94% conversion for the reaction of benzaldehyde with ethyl cyanoacetate at 80 °C in ethanol. The catalytic performance was also screened for various aromatic aldehydes, reaching in all cases conversions higher than 90% under mild conditions. The catalyst was recycled for three cycles without losing its framework integrity and catalytic activity. The high activity of UiO-66-NH2 was attributed to the site-isolated bifunctional acid–base character, in such a way that acidic Zr sites at the nodes are in close proximity to the basic amino groups on the linker, thus, activating aldehydes to promote the formation of aldimine intermediates from the aldehydes and the amino group. This is an interesting concept worthy to be further exploited since the crystal structure of MOFs with acidic metallic nodes and basic organic linkers allows a certain degree of control of the distance at molecular dimensions between the separate acidic and basic sites, to match certain substrate dimensions and also some tuning of their relative acid–base strength. By optimization of both distance and strength, high substrate specificity and optimal activity could be achieved with MOFs.

The catalytic activity of IRMOF-3 for Knoevenagel condensations was one of the first pioneer studies describing the catalytic activity derived from the functional groups present in the linker. These –NH2 groups were introduced during the synthesis of the MOF.124 The condensation between benzaldehyde and ethyl cyanoacetate was selected as a test reaction to assess the catalytic activity of IRMOF-3DEF (DEF meaning that the sample was prepared using diethylformamide) and IRMOF-3DMF (DMF meaning that the sample was prepared using dimethylformamide) (2.5 mol%) at 40 °C in DMF as a solvent. The higher activity of the IRMOF-3DEF achieving 75% yield compared to IRMOF-3DMFSB (65% yield) (SB: small batch) under the same conditions correlates with their larger specific surface area, indicative of a better accessibility to reach the amino groups inside the structure.124 The catalyst stability in three consecutive uses was supported by the absence of leaching and a minor decrease in the activity that was assumed to be due to a loss of catalyst mass during the recovery workup. In contrast, NH2-MIL-53(Al) resulted in no activity under these conditions either at 40 or 60 °C. The poor performance of NH2-MIL-53(Al) was due to the one-dimensional topology of its pore structure together with its smaller pore diameter (7.3 Å) compared to IRMOF-3 that has a tridirectional pore geometry and much larger pore diameter (15 Å).

Recently, Corma and co-workers have studied the catalytic activity of IRMOF-3 (2.5 mol%) in the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate at 60 °C.125 The structure of MOF-5 and IRMOF-3 differs in the nature of the organic ligand as the former contains bdc, while the latter contains 2-NH2-bdc. Under the optimal conditions, 65 and 90% conversions of ethyl cyanoacetate were observed using MOF-5 and IRMOF-3 catalysts, respectively, suggesting that the catalytic activity observed in IRMOF-3 does not solely reside on the uncoordinated amino groups, but other sites should also be responsible for this activity. On the other hand, the catalytic activity observed using MOF-5 was ascribed with the existence of defects, either generated by the partial hydrolysis of the framework or by the formation of embedded ZnO nanoparticles during the synthesis of MOF-5. In this regard, determination of the elemental analysis of a MOF catalyst is always a good practice to assess or rule out the possible presence of metal oxide formed during the synthesis of the MOFs under basic conditions. These metal oxide NPs would be embedded within the pores and could play a substantial role in the catalysis.

Similarly, an amino-containing MIL-101(Al) material, namely, NH2-MIL-101(Al) was synthesized and its catalytic activity was evaluated for the Knoevenagel condensation between benzaldehyde with ethyl cyanoacetate.126 The use of 2.5 mol% of NH2-MIL-101(Al) in toluene can lead to 28% conversion. A comparable ethyl cyanoacetate conversion of 30% was obtained using a much stronger base, like 1,5,7-triazabicyclo-[4.4.0]dec-5-ene. However, in contrast to the soluble organic base, NH2-MIL-101(Al) was reused without noticeable activity decay, while the recovery of a homogeneous nitrogenated base is difficult. The TOF values for NH2-MIL-101(Al) based on the total number of amine moieties at 40 °C were 15.4 and 1.8 h−1 in DMF and toluene as the solvent, respectively. Although NH2-MIL-101(Al) and IRMOF-3 showed similar TOFs in DMF, NH2-MIL-101(Al) exhibited a much higher TOF in toluene, probably reflecting an appropriate hydrophilicity/hydrophobicity balance in NH2-MIL-101(Al) with respect to IRMOF-3. The catalyst showed no leaching and could be reused for several runs without any deactivation.

The previous examples have the amino groups present as substituents of the aromatic linker. In another alternative approach aliphatic amino groups have been attached to the metal nodes taking advantage of the existence of CUS. In one of the reports illustrating this strategy, 3-aminopropylsilane (APS) and ethylenediamine (ED) were grafted on the Cr33O nodes of MIL-101(Cr) to prepare two modified MIL-101 materials exhibiting high activity in the Knoevenagel condensation.127 The use of ED-MIL-101(Cr) resulted in 97.7% conversion with 99.1% selectivity, while the catalyst APS-MIL-101(Cr) showed 96.3% conversion with 99.3% selectivity under the same conditions. In contrast, the parent MIL-101(Cr) exhibited 31.5% conversion with the same selectivity as that of amine-grafted MIL-101(Cr). The activity of the aminated-MIL-101(Cr) was compared with that of an analogous mesoporous silica, namely APS-SBA-15. Aminopropylsilicas having a periodic mesoporous structure such as MCM-41 and SBA-15 were intensively studied as CO2 adsorbents and basic catalysts.128,129 Interestingly, the TOFs observed for ED-MIL-101(Cr) and APS-SBS-15 were 328 and 32 h−1, respectively, thus, reflecting a notably higher activity for the MOF catalyst with respect to related mesoporous silica. ED-MIL-101(Cr) was reused for three runs without any loss in its activity.

Later, different loadings of ED moieties were grafted onto MIL-101(Cr) and the activity of the resulting ED-MIL-101(Cr) was studied in the Henry reaction between benzaldehyde and nitromethane (Scheme 11).130 A complete conversion was obtained using ED-MIL-101 (1.97 mmol g−1 ED) in toluene at 110 °C with 100% selectivity to β-nitrostyrene. Under identical conditions, MIL-101(Cr) showed only 6% conversion of benzaldehyde with 95% selectivity of β-nitrostyrene illustrating again the influence of amino groups anchored to the MOF structure as organocatalysts. Reusability experiments revealed that the conversion of benzaldehyde was 100, 98 and 76% for the first, second and third cycles, respectively. This decrease in activity can be correlated with the gradual loss of ED, as evidenced by elemental analysis of the nitrogen content decreasing from 1.97 to 1.29% from the first to the third run, respectively. Since no activity decay was observed for ED-MIL-101(Cr) for the Knoevenagel condensation, it could be of interest to understand the reasons why ED becomes detached under the conditions of the Henry condensation.


image file: c8cs00256h-s11.tif
Scheme 11 Henry reaction of benzaldehyde with nitromethane using ED-MIL-101(Cr) as the catalyst.

Using a similar strategy of anchoring through the CUS, but for attaching a Pd complex, a bifunctional MOF catalyst containing palladium grafted over Cu3(BTC)2 [Cu3(BTC)2-Pd] was prepared by anchoring 4-aminopyridine ligands through the CUS to some nodal Cu2+ sites (Fig. 15). The catalytic activity of Cu3(BTC)2-L2-Pd (L2: (E)-2-(tert-butyl)-4-methyl-6-[(pyridin-4-ylimino)methyl]phenol) was performed in the tandem Sonogashira/click reaction starting from 2-iodobenzylbromide, sodium azide and phenylacetylene (Scheme 12) and observing 59% conversion and 94.8% selectivity to produce 8H-[1,2,3]triazolo[5,1-a]isoindoles.131 However, the recyclability of this catalyst was limited and the activity decayed in the second cycle and practically disappears in the third cycle. This deterioration of the catalytic activity was accompanied by a gradual destruction of the structure as evidenced by powder XRD analysis, something that is in general agreement with the poor stability of Cu3(btc)2.


image file: c8cs00256h-f15.tif
Fig. 15 Functionalization of Cu3(BTC)2 with palladium complexes. Reproduced from ref. 131 with permission from Wiley, copyright 2012.

image file: c8cs00256h-s12.tif
Scheme 12 Synthesis of polycyclic condensed nitrogen heterocycles via tandem reactions catalyzed by Cu3(BTC)2-L2-Pd.

In general the approach of attaching functional groups to CUS or defects has been underexplored compared to the introduction of substituents on the aromatic linker. However, the simplicity of the experimental procedure to attach catalytically relevant groups to CUS makes this strategy highly appealing and straightforward to implement. There are in the literature many other examples of –NH2 as basic sites promoting condensation reactions.8,42,122,132

Amino groups on the organic linker can, on the other hand, be used to attach in framework satellite positions both types of active sites, such as for instance, metallic complexes. In one of these examples, post-synthetic modification of IRMOF-3 in a multistep process leads to a modified material IRMOF-3-PI-Pd (PI: pyridinimine) anchoring Pd(II) ions using a Schiff base ligand. The activity of IRMOF-3-PI-Pd was evaluated in the Suzuki–Miyaura cross coupling reaction (Scheme 13).133 The process of preparation of IRMOF-3-PI-Pd is shown in Fig. 16. The cross coupling reaction between bromobenzene and phenylboronic acid was effectively promoted at 80 °C by IRMOF-3-PI-Pd to achieve 98% yield with a TOF value of 3056 h−1 using K2CO3 as a base in an aqueous–ethanol mixture. The use of chlorobenzene instead of bromobenzene afforded, however, only 19% yield with a TOF value of 197 h−1 under identical conditions, ranking this catalyst among the Suzuki–Miyaura catalysts of medium activity. The catalyst was recycled for five runs without any decay in its activity. Comparison of XRD patterns, IR spectra and gas adsorption studies of the pristine sample with the recovered catalyst indicates the stability and maintenance of the structural integrity under reaction conditions.


image file: c8cs00256h-s13.tif
Scheme 13 Suzuki–Miyaura cross-coupling reaction catalyzed by MOF-based catalysts.

image file: c8cs00256h-f16.tif
Fig. 16 Post-synthesis modification of IRMOF-3 to obtain a modified IR-MOF-3-PI-Pd containing the Pd(II) Schiff-base complex. Reproduced from ref. 133 with permission from the American Chemical Society, copyright 2013.

Catalysis by mixed linkers

One of the current strategies to increase the catalytic activity of MOFs is to prepare isoreticular materials of mixed linkers, in which the presence of appropriate loadings of the second linker can result in optimal catalytic performance in comparison with analogous MOFs having exclusively either of the two linkers. Several reasons could be responsible for this optimal performance of mixed-ligand MOFs, but the most general one is the higher porosity of the material for one linker and the higher intrinsic activity of the sites for the other linker, there being a compromise in the composition for the highest activity.

In this regard, the catalytic performance of stable mixed-ligand MOFs with the molecular formula, Zn2(tpt)2(2-atp)I2 (tpt = tris(4-pyridyl)triazine, 2-atp = 2-aminoterephthalate) was evaluated in the Knoevenagel condensation reaction.134 This catalyst exhibited 37 and 99% condensation product yield in ethanol at 60 °C for the reactions of benzaldehyde with ethyl cyanoacetate and with malononitrile, respectively. This difference in activity was attributed to the different sizes of the reactants and the difficulty encountered by the bulky reactants to reach active sites in MOFs. Although the powder XRD of the recovered catalyst after the reaction did not show any difference in the crystalline structure, reusability data along with analytical and catalytic leaching tests are essential to further ascertain the stability of the catalyst.

In the examples presented below, the mixed linker strategy has been used to obtain Pd2+ complexes anchored on the MOF lattice and used as Suzuki–Miyaura catalysts. These reports likely illustrate that a single ligand would not be suitable to obtain efficient solid catalysts due to the lack of anchoring positions for Pd2+ complexes or, on the other side, the undesirably high density of Pd2+ complexes that would decrease substantially the material pore volume and would produce overcrowding of the Pd2+ complexes.

Thus, for example, UiO-66 and UiO-67-mixed linker MOFs have been synthesised with different ratios of bdc/2-aminoterephthalic acid, and bpdc/2-amino-biphenyl-4,4′-dicarboxylic acid, respectively. In both cases, the pendant amino groups were subjected to the condensation with pyridine-2-carboxaldehyde. Later, Pd metal centres were anchored onto the MOFs by coordination to the PI moieties (Fig. 17).135 Among the various catalysts examined for the Heck cross coupling between bromobenzene and styrene, UiO-67-3-PI-Pd (1[thin space (1/6-em)]:[thin space (1/6-em)]5 ratio of amino linker and parent linker) afforded 100% conversion with complete selectivity at 80 °C using K2CO3 as a base, reaching a TOF value of 1852 h−1. A series of olefins were synthesised using this catalyst with substrates containing electron withdrawing and donating groups with the yield ranging between 87 and 100% under identical conditions. The catalyst was recycled for 10 cycles without significant leaching or loss of catalytic activity.


image file: c8cs00256h-f17.tif
Fig. 17 Post-synthetic modification of UiO-66-Mix and UiO-67-Mix to yield UiO-66-Mix-PI-Pd and UiO-67-Mix-PI-Pd, respectively. Reproduced from ref. 135 with permission from Wiley, copyright 2016.

In another study, Cohen et al., have compared alternative strategies involving pre-synthetic and post-synthetic approaches to synthesise UiO-67 MOFs with open bpy sites which can be complexed with Pd(II) salts to immobilize Pd(bpy)Cl2 species on the MOF strut (Fig. 18).136 These Pd-containing UiO-67 were used as heterogeneous Suzuki–Miyaura catalysts, concluding that both direct synthesis and post-synthetic linker exchange can lead to suitable Pd-containing UiO-67 catalysts. Among the various conditions screened, UiO-67-Pdbpydc0.5/bpdc0.5 (bpydc: 2,2′-bipyridine-5,5′-dicarboxylic acid; bpdc: 4,4′-biphenyldicarboxylic acid) as the catalyst afforded 89% yield of 4-methylbiphenyl at 95 °C. In contrast, pristine UiO-67 and UiO-67-bpydc0.5/bpdc0.5 gave no conversions under identical conditions. On the other hand, homogeneous controls with PdCl2 and Pd(OAc)2 gave 54 and 51% yields under identical conditions. A commercial catalyst, namely 10% Pd/C, gave only 63% yield. These activity data clearly demonstrate the uniqueness of the activity of UiO-67 containing the polypyridyl Pd2+ complex, in which the active sites are uniformly anchored on the MOF struts to achieve better activity. The recovered catalyst was used in three successive runs without observing a decrease in the product yield (>85%).


image file: c8cs00256h-f18.tif
Fig. 18 Synthesis of UiO-67-bpydc using either direct synthesis or post-synthetic ligand exchange methods. Reproduced from ref. 136 with permission from the Royal Society of Chemistry, copyright 2014.

In another related precedent, the Pd complex Pd(H2bpydc)Cl2 was immobilized on a porous UiO-67 MOF, Zr6O4(OH)4(bpdc)6, by employing a mixed-linker strategy.137 This method allowed a porous UiO-67 framework with isolated Pd single active sites (Pd/UiO-67) to be obtained in a uniform way, directly in the synthesis of the material (Fig. 19). Among the various reaction conditions, Pd/UiO-67 afforded 97% yield for the reaction between 4-nitrochlorobenzene and phenylboronic acid in a DMF–EtOH mixture as a solvent and K2CO3 as a base at 100 °C under a nitrogen atmosphere. The catalyst was reusable exhibiting identical activity for at least five cycles. Pd/UiO-67 exhibits a wide scope for the Suzuki–Miyaura coupling and was successfully employed as a solid heterogeneous catalyst for the synthesis of a series of biphenyl derivatives in high yields using chloroarenes under the optimized conditions.


image file: c8cs00256h-f19.tif
Fig. 19 Schematic representation of the synthesis of Pd(II) doped UiO-67. Reproduced from ref. 137 with permission from the Royal Society of Chemistry, copyright 2014.

A series of methyl substituted mixed-linker bipyridyl UiO-67 MOF-supported Pd(II) complexes (Fig. 20) were employed as catalysts for the Suzuki–Miyaura cross-coupling reaction between bromobenzene and phenylboronic acid, reaching the conclusion that the electronic and steric effects of the bipyridyl substitution influences the catalytic activity of the material.138 This is a well-established fact in homogeneous Pd catalysis, where the stereoelectronic effects on the Pd centre and the biting angle with which Pd coordinates to the substrates can accelerate considerably the reaction rate and enlarge the scope of the Pd complex as a catalyst of low reactivity substrates.139 In the case of Pd(II) complexes attached to the MOF framework, PdCl2/m-6,6′-Me2bpy-MOF exhibited 110- and 496-fold enhancements in activity compared to non-functionalized PdCl2/m-bpy-MOF and PdCl2/m-4,4′-Me2bpy-MOF, respectively. These results clearly illustrate that the stereoelectronic properties of metal-binding linker units are critical in determining the catalytic activity in single-site organometallic catalysts in MOFs. The heterogeneity of the catalytic process using these MOFs was supported by hot-filtration experiments.


image file: c8cs00256h-f20.tif
Fig. 20 Schematic structures of m-MOF-PdCl2 precatalysts. Reproduced from ref. 138 with permission from the American Chemical Society, copyright 2016.

There are many other examples describing the synthesis of mixed-ligand MOFs in which one of the ligands is used to build a metallic complex that exhibits catalytic activity promoting organic reactions including cross-coupling reactions.14,43,140

MOFs as hosts of MNPs and metal complexes

Micro-/mesoporous materials have been frequently explored as hosts to embed MNPs. Since one of the major causes of deactivation of MNPs is agglomeration and growth, it has been proposed that confinement of these small-sized MNPs inside the restricted voids of porous solids can lead to more stable catalysts due to steric constraints to the growth imposed by the host lattice.

Initial studies on this area of embedded MNPs were performed with zeolites141,142 and later with mesoporous periodic silicas143 and after the synthesis of MOFs, there was an obvious interest in exploiting their unique properties such as higher porosity, pore dimensions in the range of 1–2 nm, framework flexibility and low framework density among others for their use as hosts. Some catalytic tests described below have provided evidence that certain MOFs can effectively host MNPs minimizing their tendency to grow and providing a highly active and stable catalyst based on MNPs as active sites. Examples in cross-coupling reactions rank the MNPs/MOFs among the most active heterogeneous catalysts compared to other alternatives.43

One of the key points in MNPs/MOFs is the preparation procedure and particularly, how MNPs are formed inside the MOF pores.32 This is a crucial point since it is frequently observed that MNPs larger than the pore dimension can be formed on the material implying that these MNPs of a relatively large size cannot be accommodated inside the MOF pores. Characterization of MNPs/MOFs to provide convincing evidence of the internal location of the MNPs is another important issue to be addressed in this research field.

A palladium-catalyzed carbon–carbon coupling reaction between aryl halides and phenylboronic acids is a very important and versatile route to construct biaryl units in organic synthesis.144–147 Homogeneous palladium complexes have been extensively employed for this reaction and can exhibit very large TON values. However, large scale industrial applications of this coupling reaction remain challenging mainly due to the expensive cost of the ligands, impossibility to recycle the catalyst, and the difficulty to avoid the presence of palladium in the final product,148–150 thus posing an issue in the synthesis of pharmaceutical products. One of the convenient ways to overcome the limitations of homogeneous Pd catalysts is to develop efficient heterogeneous Pd-based catalysts that could promote the coupling under mild reaction conditions. As it will be shown below, Pd/MOFs could be possible heterogeneous catalysts for this cross-coupling reaction.

Suzuki–Miyaura cross-coupling

Pd NPs have been supported on MIL-101(Cr) by impregnation and its activity was investigated in a Suzuki–Miyaura cross coupling reaction.151 The Pd loading on the material was 1 wt% and the TEM images showed a uniform Pd NP distribution within the MIL-101(Cr) crystallites with a mean diameter of (1.9 ± 0.7 nm) that are smaller than any of the two MIL-101(Cr) cavity dimensions (2.9 and 3.4 nm). The reaction of 4-chloroanisole with phenylboronic acid catalyzed by Pd/MIL-101(Cr) afforded 82% of 4-methoxybiphenyl using NaOMe as the base in water at 80 °C under a nitrogen atmosphere. To rank the performance of Pd/MIL-101(Cr), a series of catalysts were prepared with 0.9 mol% of Pd loading such as Pd2+/MIL-101(Cr) (<3% yield), Pd/C (35% yield) and Pd/ZIF-8 (16% yield) and their activity was compared with that of Pd/MIL-101(Cr) under identical conditions. Interestingly, it was observed that Pd/MIL-101(Cr) exhibits much superior activity to any of the above tested catalysts. This enhanced activity of Pd/MIL-101(Cr) was proposed to be due to the surface Lewis acidity127 on MIL-101(Cr) which would favour the adsorption of the chloroarenes, thus increasing the concentration of this reagent near the active Pd NPs. This type of interaction has already been claimed to play a role in the catalysis as, for instance, in the case of Al-MCM-41 in which the Al3+ ions in Al-MCM-41 could serve as Lewis-type acid sites that may adsorb iodoaryl molecules.152

Besides impregnation, more elaborated preparation procedures have been reported to obtain MNPs/MOFs. In this regard, Xu and co-workers have developed the so-called double solvent method (DSM), derived from incipient wetness impregnation techniques, which minimizes the deposition of metal precursors on the outer surface.153 In this method, activated MOF was suspended in a large amount of anhydrous n-hexane, followed by a dropwise addition of metal precursor in aqueous solution with a volume less than the pore volume of the MOF under vigorous stirring. It is believed that the presence of the excessive hydrophobic hexane can spread on the external surface, which ensures the diffusion of the hydrophilic metal precursor into the pores of the MOF via capillary force. Using this approach, highly dispersed Pd NPs over MIL-101(Cr) were prepared and the activity of the material was tested in the Suzuki–Miyaura coupling reaction.154 The TEM images show that the average size of the Pd NPs was 2.4 nm, which is small enough to be accommodated in the mesoporous cavities of MIL-101. The as-prepared Pd/MIL-101(Cr) (2.7 wt%) exhibited 95% yield of biphenyl for the room temperature reaction between iodobenzene and phenylboronic acid using K2CO3 as a base. Although this catalyst provided higher yields for a series of substrates containing bromo- and iodobenzenes, it was inactive for chloroarenes as reactants under identical conditions. This higher activity of Pd/MIL-101(Cr) was attributed to the good dispersion of Pd NPs in the pores of MIL-101(Cr) and the confinement effect, but, however, this deserves further studies to prove the location of Pd NPs inside the pores.

In fact the external surface of MOFs can also be used to support MNPs. In one of these examples, Pd NPs were supported on the surface of a Co-containing MOF (MCoS-1) to obtain Pd/MCoS-1 and its activity evaluated in the Suzuki–Miyaura cross coupling reaction.155 TEM analysis revealed the existence of Pd nanospheres of dimensions ranging between 5 and 10 nm distributed uniformly throughout the material. XPS analysis showed that Co is present in the +2 oxidation state and Pd in its metallic state. Under the optimized reaction conditions using Pd/MCoS-1 as the catalyst, the cross coupling reaction between 4-bromobenzaldehyde and phenylboronic acid resulted in 96% yield of 4-formylbiphenyl with K2CO3 as a base at 70 °C in water. A series of substituted biphenyls with electron donating and electron withdrawing groups were prepared in high yields (88 to 97%) using this catalyst under the optimized reaction conditions. Pd/MCoS-1 was recycled for six consecutive runs with no noticeable decay in its activity. Table 1 compares the activity of this catalyst with a series of other Pd catalysts reported in the literature. It is clear from these data that Pd/MCoS-1 exhibits higher catalytic activity compared to other catalysts. In the case of Pd2+-sepiolite, this material showed higher TOFs than Pd/MCoS-1, these reactions were conducted at much higher temperature and it is always appropriate to compare TON/TOF values under identical conditions. In any case using Pd/MCoS-1, the Suzuki–Miyaura cross-coupling reaction was conducted in water which is environmentally friendly compared to conventional organic solvents.

Table 1 Comparison of the catalytic activity of Pd/MCoS-1 with other catalysts reported from the literature for the reaction of bromobenzene with phenylboronic acid155
Catalyst Conditions Yield (%) TOF (h−1) Ref.
a Pd@poly-Sty-co-diOH-Cl: Pd NPs supported on a co-polymer of 3-(2,3-dihydroxypropyl)-1-vinylimidazolium chloride and styrene.
Pd2+-sepiolite K2CO3, DMF, 0.5 μmol of a Pd catalyst, 100 °C 81 2098 156
CNT@PCOOH@Pd Na2CO3, 10 mg catalyst, 100 °C, 2 h 77 48.1 157
Pd–CoFe2O4 MNP Na2CO3, EtOH, 2 mg catalyst, reflux, 12 h 79 4.1 158
Pd@poly-Sty-co-diOH-Cla K2CO3, EtOH/H2O, 0.05 mol% Pd, 70 °C, 6 h 92 306.6 159
Pd(0)/MCoS-1 K2CO3, H2O, 20 mg catalyst, 70 °C, 5 h 96 96 155


Substituents in the ligand can be used to further stabilize the embedded MNPs by establishing interactions between them. In one of these examples, the catalytic activity of Pd/MIL-101(Cr)-NH2 with four different loadings of Pd NPs was examined in the Suzuki–Miyaura cross-coupling reaction.160 As it has been commented, MIL-101(Cr) is among the structurally most robust MOFs, and, therefore, it is one the best options to be used as a host of MNPs, particularly considering its large porosity in the mesopore range. Among these catalysts, the 8 wt% Pd/MIL-101(Cr)-NH2 catalyst showed a quantitative yield of 4-methylbiphenyl using K2CO3 as a base in a water–ethanol mixture at room temperature. The catalyst was recycled for ten runs with no significant decay in its activity. This enhanced activity of this catalyst at room temperature may be due to the homogeneous distribution of small size Pd NPs (2.6 nm) over MIL-101(Cr)-NH2 and the effective stabilization of Pd NPs by amino groups. This catalyst is efficient to promote the reaction with a wide range of substrates, achieving in many cases high yields under remarkably mild conditions (water, aerobic conditions, room temperature and 0.15 mol% of catalyst loading). A series of biphenyl derivatives were synthesised using 8 wt% Pd/MIL-101(Cr)-NH2 as a catalyst in moderate to high yield. Notably, 3-bromothiophene and 3-bromopyridine afforded their respective biphenyl with heteroatoms in 89 and 92% yield in water after 6 h. The use of –NH2 substituents at the linker to establish interactions with MNPs resulting in an effective stabilization under reaction conditions is a common tool well-described in the literature to develop efficient catalysts based on MOF encapsulated MNPs.32,161–163

Amino groups can also stabilize Pd NPs located on the external surface of a MOF. Thus, another structurally robust MIL structure is MIL-53(Al) whose pore geometry defines parallel, flexible rhomboidal channels of 1.4 × 1.6 nm dimension with linkers containing –NH2 groups. MIL-53(Al) has also been used to host Pd NPs. Pd NPs supported on MIL-53(Al)-NH2 (0.97 wt%) exhibit a mean diameter of Pd NPs of 3.12 nm. The activity of the resulting Pd/MIL-53(Al)-NH2 was tested in the Suzuki–Miyaura cross coupling reaction.164 The XPS analysis of Pd/MIL-53(Al)-NH2 indicated that most of the Pd atoms in the NPs were in the reduced form. The Pd NPs in Pd/MIL-53(Al)-NH2 were well dispersed on the external surface and no aggregation was observed. This uniform NP distribution on a large area surface is proposed to be formed and stabilized by electrostatic attraction. In agreement with this proposal, Pd NPs in Pd/MIL-53(Al) (0.19 wt%) lacking amino groups undergo aggregation. The reaction of bromobenzene and phenylboronic acid with Pd/MIL-53(Al)-NH2 resulted in 92% yield (TON: 194) using Na2CO3 as a base at 40 °C. This activity was much superior to that of Pd/MIL-53(Al) (45% yield, TON: 98), 0.5 mol% Pd/C (37% yield, TON: 88) under identical reaction conditions. These data clearly indicate the beneficial influence of amino groups at the organic linker for stabilization of Pd NPs with uniform distribution and the better activity, even with regard to stabilization, of Pd NPs located on the external surface. The Pd/MIL-53(Al)-NH2 catalyst was recycled for five reuses without observing a drop in its activity for this coupling reaction.

Again exploiting the role of amino groups to stabilize Pd NPs, another precedent reported the catalytic activity of Pd/UiO-66-NH2 in the Suzuki–Miyaura cross coupling reaction between iodobenzene and phenylboronic acid, achieving a TOF of 2190.5 h−1 under the optimized reaction conditions.165 On the other hand, the activity of Pd/UiO-66-NH2 was much superior to that of Pd/MIL-53(Al)-NH2 (396 h−1) and Pd/MIL-53(Al) (98 h−1). This enhanced activity of Pd/UiO-66-NH2 was believed to derive from the presence of –NH2 groups on the organic linker dispersing Pd NPs uniformly on the support and the favourable large pore dimensions and geometry of the UiO-66 lattice enhancing reagent and product diffusion.

There are many other examples in the literature reporting the use of incorporated MNPs within MOFs as excellent catalysts in C–C and C–heteroatom bond forming reactions. The reader is referred to the existing literature for in-depth details.43,166–168

Sonogashira cross-coupling

Pd NPs incorporated on MIL-101(Cr) [Pd/MIL-101(Cr)] were synthesized and their activity was investigated in the Sonogashira cross-coupling reaction of phenylacetylene and 4-nitrobromobenzene (Scheme 14).169 The average particle size of Pd in Pd/MIL-101(Cr) was 3–4 nm as evidenced by TEM analysis. Under the optimized reaction conditions, Pd/MIL-101(Cr) afforded 90% yield of 4-nitro-1,2-diphenylacetylene at 130 °C using CH3COOK as a base. Pd/MIL-101(Cr) promoted the formation of a series of 1,2-diphenylacetylene derivatives in moderate to high yields. The stability of Pd/MIL-101 was proved by reusing it in four consecutive cycles without any significant loss in its activity.
image file: c8cs00256h-s14.tif
Scheme 14 Sonogashira cross-coupling reaction catalyzed by Pd/MIL-101(Cr).

Pd/MCoS-1 described earlier in the Suzuki–Miyaura reaction has also been reported to catalyze the Sonogashira cross coupling between phenylacetylene and iodobenzene at 80 °C using triethylamine as a base in water to achieve 94% yield of 1,2-diphenylacetylene.155 A series of 1,2-diphenylacetylene derivatives containing electron donating and withdrawing groups were prepared with yields ranging between 70 and 95% under identical reaction conditions. Pd/MCoS-1 was recycled in six consecutive cycles with a slight decay in its activity.

As it has been presented earlier, MOFs can also act as hosts to encapsulate metal complexes, transforming the catalysis promoted by them from homogeneous to heterogeneous. In the most abundant preferred strategy, the ligands to build these complexes are covalently attached to the linker. Cross-couplings are among the reaction types that can be generally promoted by some of these metal complexes. As one of these examples, recently, a novel and highly efficient UiO-67-bpy-Pd(II) was reported to catalyze the carbonylative Sonogashira coupling of aryl iodides with phenylacetylenes without the requirement of phosphine ligands (Scheme 15).170 UiO-67-bpy-Pd(II) was able to promote the carbonylative coupling reaction under atmospheric pressure of CO to afford the corresponding aryl α,β-alkynyl ketones in more than 85% yields using Cs2CO3 as a base in DMF at 100 °C. Furthermore, the catalyst showed negligible Pd leaching and it could be reused five times without significant decay in catalytic activity and selectivity.


image file: c8cs00256h-s15.tif
Scheme 15 Sonogashira type carbonylation of iodobenzene and phenylacetylene catalyzed by UiO-67-bpy-Pd(II) under a CO atmosphere.

Other C–C cross-coupling reactions

A series of MOF-5 materials, Zn4O(bdc-NH2)n(bdc)3−n have been synthesized with different proportions of bdc and bdc-NH2 and the resulting amino-functionalized MOF-5s were further modified by covalent post-modification with salicylaldehyde to form a salicylimine Schiff base that was used to anchor active Pd(II) ions (Fig. 21).171 The activity of these materials was tested in the Heck coupling between 4-methoxystyrene and 3,5-dimethoxybromobenzene to give resveratrol trimethyl ether (Scheme 16), an important pharmaceutical precursor. Among the various catalysts tested a complete conversion with trans-isomer selectivity was achieved at 120 °C using Zn4O(bdc-NH2)0.6(bdc)2.4-Pd and TEA as a base. Interestingly, the Zn4O(bdc-NH2)0.6(bdc)2.4-Pd catalyst retained its activity and selectivity for ten consecutive cycles and furthermore, ICP-AES indicated no Pd leaching under the experimental conditions.
image file: c8cs00256h-f21.tif
Fig. 21 Post-synthetic modification of MOF-5. Reproduced with permission from Elsevier.171

image file: c8cs00256h-s16.tif
Scheme 16 Heck cross-coupling between 4-methoxystyrene and 3,5-dimethoxybromobenzene to give resveratrol trimethyl ether catalyzed by Zn4O(bdc-NH2)0.6(bdc)2.4-Pd.

Pd(II) complexes were covalently attached to the satellite positions to the post-synthetically modified MIL-53(Al)-NH2 with maleic anhydride (Fig. 22).172 The catalytic activity of the resulting MIL-53(Al)-NH-Mal-Pd was studied in the Heck coupling between bromobenzene and styrene to afford trans-stilbene with 88% yield (TON 8236) at 140 °C using sodium acetate as a base. On the other hand, the use of chlorobenzene gave 17% yield when tetrabutylammonium bromide (TBAB) is used as a co-catalyst at 160 °C. MIL-53(Al)-NH-Mal-Pd was reused in three consecutive cycles, observing that the conversions in the second and third run were significantly lower (22%) than that in the first run (about 80%). The activity loss was believed to be due to the blocking of the pores by sodium acetate base, which is not completely dissolved in the reaction mixture.


image file: c8cs00256h-f22.tif
Fig. 22 Functionalization of MIL-53(Al)-NH2 by maleic anhydride followed by Pd(II) acetate. Reproduced from ref. 172 with permission from the Royal Society of Chemistry, copyright 2013.

The catalytic activity of Pd/MIL-101(Cr) was also evaluated in the Heck cross-coupling between iodobenzene and styrene, observing 98% yield of the coupling product in DMF using K2CO3 as a base and TBAB as a co-catalyst at 120 °C.154 The Pd/MIL-101(Cr) catalyst exhibited large scope promoting the coupling of a variety of substituted styrenes to yield the corresponding coupling products in percentages ranging between 80 and 98% under identical reaction conditions. The catalyst retained it activity for five consecutive cycles. ICP-AES analysis indicated the absence of Pd leaching under the experimental conditions.

IRMOF-3-PI-Pd was also reported to catalyze the Stille cross-coupling reaction between bromobenzene and tributylphenyltin (Scheme 17) in 88% yield with a TOF value of 2195 h−1 at 80 °C in ethanol.133 Furthermore, this protocol was further extended to synthesise a variety of biphenyl derivatives containing electron donating and withdrawing substituents in very high yields. A low yield of 10% was observed for the reaction between chlorobenzene and tributylphenyltin under identical conditions, suggesting that further progress in the field should aim at developing efficient catalysts for this type of coupling reaction using chloroarenes as a reactant.


image file: c8cs00256h-s17.tif
Scheme 17 IRMOF-3-PI-Pd catalyzed Stille coupling.

Alcohol oxidation

Besides coupling reactions, MNPs are also general catalysts for oxidation and reduction reactions. In the case of oxidations, selective aerobic oxidation of organic compounds has attracted continuous interest, motivated by application of the green chemistry principles and minimization of the environmental impact of this type of organic transformation. Particularly, selective aerobic oxidation of alcohols to carbonylic compounds is a chemical reaction of large economic and industrial importance that can be catalyzed by noble MNPs.32,88 One logical type of catalyst to be tested was MNPs/MOFs.

In one of the examples in this area, highly dispersed Pd NPs were deposited on MIL-101(Cr) using a colloidal method consisting of impregnation of the porous solid with preformed Pd NPs. In the case of Pd/MIL-101(Cr), TEM images showed the existence of Pd NPs with uniform size distribution of 2.5 ± 0.5 nm over MIL-101(Cr). The resulting Pd/MIL-101(Cr) solid catalyst exhibited high activity in the liquid-phase aerobic oxidation of alcohols (Scheme 18) under base-free conditions.173 Typically, aerobic oxidation of alcohols requires strong basic conditions to generate sufficient concentration of alcoholate to bind to the surface of the MNPs, which is the first step in the alcohol oxidation mechanism. The aerobic oxidation of benzyl alcohol using Pd/MIL-101(Cr) showed 99% conversion and selectivity after 1.5 h at 80 °C in toluene. Remarkably, the oxidation of benzyl alcohol afforded a high TOF value of 16[thin space (1/6-em)]900 h−1 under the optimized reaction conditions. Interestingly, the catalytic activity for benzyl alcohol oxidation was significantly suppressed to 45% yield using an ED grafted catalyst, Pd/ED-MIL-101(Cr). This catalytic behaviour suggests that the open Cr sites might play an important role in promoting the oxidation of alcohols under the present experimental conditions. This hypothesis was verified by performing a control experiment with Pd/AC (AC: active carbon) which resulted in 23% conversion. Pd/MIL-101(Cr) was also highly active for the selective aerobic oxidation of cinnamyl alcohol to cinnamaldehyde even in five consecutive recycles.


image file: c8cs00256h-s18.tif
Scheme 18 Oxidation of benzyl alcohol to benzaldehyde using MOF-based catalysts.

Similarly, Au NPs incorporated on MIL-101(Cr) (Au/MIL-101(Cr)) by colloidal deposition using PVP as a protecting agent exhibited extremely high catalytic activity for the aerobic oxidation of a variety of alcohols under base-free conditions in liquid-phase conditions.174 TEM images revealed the presence of highly dispersed Au NPs on MIL-101(Cr) with mean diameters of 2.3 ± 1.1 nm, which are in good agreement with the cage diameters of MIL-101(Cr), suggesting that Au NPs can be accommodated within the lattice of MIL-101(Cr) as a host. One of the noticeable features of Au/MIL-101(Cr) as a catalyst is that it can promote the aerobic oxidation of 1-phenylethanol with a notably high TOF value of 29[thin space (1/6-em)]300 h−1 at 160 °C. Secondary alcohols are frequently reluctant to undergo aerobic oxidation by MNPs due to steric hindrance and the high TOF value reached for 1-phenylethanol oxidation ranks this Au/MIL-101(Cr) as an excellent oxidation catalyst. In the aerobic oxidation of 4-methoxybenzyl alcohol, the catalyst maintained its activity for six cycles. This enhanced activity of Au/MIL-101(Cr) was believed to be due to the high dispersion of Au NPs as well as the electron donation effects of aryl rings to the Au NPs in the large cages of the support.

In a seminal report, a series of catalysts were prepared by embedding Au clusters on MOF-5, MIL-53(Al), and Cu3(BTC)2 using a volatile organogold complex, [Me2Au(acac)] (acac: acetylacetonate), as a precursor to form Au NPs and their activity was examined in the oxidation of benzyl alcohol in methanol and subsequent esterification of benzoic acid.175 Tandem reactions in which two or more individual reactions are performed in a single step are under much current investigation, since they represent a case of process intensification with less work-up and separation steps. The use of Au/MOF-5 as a catalyst in the presence of a base resulted in 82% conversion of benzyl alcohol with a selectivity of 66% to methyl benzoate, whereas in the absence of a base 69% conversion was observed with 23% selectivity to methyl benzoate. Furthermore, Au/MIL-53(Al) in the presence of a base afforded an even higher conversion of benzyl alcohol of 98% with a somewhat improved selectivity to methyl benzoate of 77%. Due to the known poor structural stability of MOF-5 to water and other solvents, the issue of integrity after the catalytic tests has to be always convincingly addressed when using this MOF.

In another precedent, Au NPs were embedded via gas phase infiltration of volatile [Au(CO)Cl] followed by thermal hydrogenation to form the Au NPs into ZIF-8 and ZIF-90 (both ZIFs posses pore sizes of 1.2 nm and pore windows of 0.35 nm) [Zn(ICA)2; ICA = imidazolate-2-carboxyaldehyde] and the activity of these solids was tested in the tandem aerobic oxidation followed by esterification (Scheme 19).176 Au NPs were distributed throughout the ZIF matrix with sizes ranging between 1 and 5 nm. Some of these Au NPs are obviously too large to be encapsulated inside the pore of ZIFs that, in addition, have too small pore windows to allow diffusion of substrates through the internal voids. The parent MOF hosts ZIF-8 and ZIF-90, showed no activity for the oxidation of benzyl alcohol, while Au/ZIF-8 and Au/ZIF-90 exhibited 81 and 13% conversion with methyl benzoate selectivity of 98 and 50% in methanol, respectively.


image file: c8cs00256h-s19.tif
Scheme 19 Au/MOF-5 catalyzed oxidation of benzyl alcohol to methylbenzoate.

Recently, a heterogeneous tandem catalyst was reported for the oxidation of benzyl alcohol and the subsequent acetalization of benzaldehyde with ethylene glycol catalyzed by Pd NPs with a mean diameter less than 1.2 nm supported on UiO-66-NH2 (Pd/UiO-66-NH2).177 This bifunctional (oxidizing and acid sites) Pd/UiO-66-NH2 catalyst showed quantitative conversion of benzyl alcohol with high selectivity to the acetal (Scheme 20) product at 90 °C via a tandem oxidation–acetalization reaction. In addition, Pd/UiO-66-NH2 was reused five times without any loss in activity and selectivity. The beneficial role of the amino groups at the linkers was again illustrated by comparing the Pd/UiO-66-NH2 performance with that of Pd/UiO-66, the latter showing deactivation at the third recycle run and more severe aggregation of Pd NPs.


image file: c8cs00256h-s20.tif
Scheme 20 Pd/UiO-66-NH2 catalyzed conversion of benzyl alcohol to the acetal product.

Also Pt NPs have been incorporated inside MOFs. In one of the earlier studies, Pt NPs supported on [Zn4O(btb)2]8 (MOF-177; pore dimension 2.3–2.5 nm) (Pt/MOF-177) were reported to catalyze the aerobic oxidation of alcohols at room temperature under base-free conditions.178 These experimental conditions are notably mild and convenient with considerable environmental advantages. TEM images showed the NP size is in the range of 2–5 nm and the crystal structure of MOF-177 was not altered upon gas-phase loading of Pt NPs as evidenced by powder XRD. The catalytic activity of Pt/MOF-177 in the aerobic oxidation of benzyl alcohol under solvent- and base-free, room temperature conditions afforded 50% conversion with a TON value of 968. However, Pt/MOF-177 loses its activity after the first cycle due to the breakdown of the structure of the MOF host under the reaction conditions. An attempt to carry out a second cycle showed no activity under identical conditions, implying the catalyst instability under these conditions. It would be, however, worthwhile to revisit the catalytic performance of these Pt NPs embedded in other MOFs with sufficient structural robustness and test their activity to determine if they are superior to that of other analogous noble MNPs hosted on MOFs or other supported Pt catalysts.

Although there are a large number of other reports describing the catalytic activity of MNPs encapsulated within or located on the external surface of MOFs as oxidation catalysts,32,168 the paragraphs above serve to illustrate some of the key points concerning preparation of these materials, the use of substituents at the linkers to stabilize these NPs, the preferred MOF structures and other issues with respect to the use of these materials.

MOFs as photocatalysts

The ever-increasing global demand for energy has stimulated extensive research activities on the efficient utilization of solar energy. As a unique type of heterogeneous catalysis, photocatalysis can transform solar energy to chemical energy and therefore has attracted extensive research interest during the past several decades.179–181 Ever since the first demonstration of UV-light-driven water splitting to generate hydrogen over semiconducting TiO2 reported by Fujishima and Honda in 1972,182 photocatalysts based on both inorganic as well as metal-free organic semiconductors have been developed for a variety of applications, including those most relevant for energy and environmental concerns, such as CO2 reduction, water reduction/oxidation, pollutant degradation as well as organic syntheses.183–188

In addition to the well-studied semiconductors, MOFs have recently emerged as a new type of prospective photocatalytic materials. As a class of three dimensional crystalline micro-/mesoporous hybrid material constructed from metal nodes interconnected with multi-dentated organic linkers, the modular structure of MOFs enables them to be facilely immobilized with photoactive sites for photocatalysis. As it has been commented for the general thermal catalysis, the three components in the MOF materials, i.e., the metal nodes, the organic linkers and the guests encapsulated inside the cavities of MOFs, can simultaneously be tailored to tune the material for optimal performance.20,189 The metal nodes of MOFs can be regarded as isolated semiconductor quantum dots, which can be excited directly upon light irradiation or activated by the organic linkers acting as the light absorption antenna.190–193 Thanks to the availability of a large diversity of organic linkers and the rich coordination chemistry of metal cations, the light absorption properties of the MOFs can be adjusted for efficient utilization of the solar light via a judicious selection of the ligands and the metal ions. Such a huge structural tunability of MOFs, usually a characteristic of molecular catalysts as opposed to catalysis by materials, is unmatched by the semiconductor-based photocatalysts. In addition to the metal nodes, photoactive ligands or dyes can be used directly as building blocks to fabricate the MOF materials or these chromophores can be grafted on the organic ligands as a photoactive part via a post-synthetic method.194,195 Moreover, the cavities in the MOFs can be very appropriate hosts for encapsulating photoactive metal complexes or dyes that have shown high activity in homogeneous solutions and can become immobilized within an insoluble solid by encapsulation. These MOF-supported or encapsulated metal complexes or dyes have all the advantages of heterogeneous catalysts, including easy separation and recycling, without sacrificing their superior activity observed as homogeneous catalysts.196,197 With all these advantages, MOFs have been successfully applied in photocatalytic hydrogen evolution,198 CO2 reduction,199 and pollutant degradation200–202 as well as to promote photocatalytic organic transformations.203–206

The past several years have witnessed a remarkably fast development of MOF-based photocatalysis and a proof of the interest in this field is the unprecedented number of reviews that have appeared over the past several years in this specific area.44,140,198–212 However, despite some advances in this field, the general performance of most already reported MOF-based photocatalysts is still far from satisfactory and the application of MOFs in photocatalysis is still in its emerging stage, as compared with the semiconductor-based photocatalysts.

Photocatalytic pathways

When semiconductors are irradiated with energy equal to or greater than their band gap, charge separated states are generated, with the electrons on the valence band (VB) excited to the conduction band (CB), leaving positive holes (h+) in the VB. These photogenerated charge carriers (electrons and holes) migrate separately to the surface of the semiconductor particles and react with surface-adsorbed substrates to promote photocatalytic reactions. Several de-excitation pathways which lead to the inferior performance also exist for these photogenerated charge carriers (Fig. 23). Since MOFs can be regarded as an organized, ordered assembly of metal nodes and the organic linkers, which act as isolated semiconductor quantum dots and light antenna respectively,190–193,213 light excitation on the metal nodes or the organic linkers can also generate charge separation states in the MOFs. To be effective to promote photocatalysis over the MOFs, these photogenerated charges not only should be long-lived enough to diffuse to the reactive sites, but also should possess sufficient redox potential for driving the desired chemistry on the substrates. Therefore, to achieve efficient photocatalysis over MOF materials, MOFs with strong light absorption and being able to undergo fast charge separation and migration to the catalytic active sites to react with the substrates are highly desirable. Besides tunability and flexibility in the selection of components, what makes MOFs particularly appealing in photocatalysis lies in the presence of different types of catalytic active sites in a single MOF material, which enables the MOFs to behave as multifunctional catalysts for light induced all-in-one or light-assisted cascade/tandem catalytic reactions. Yet research in this field is still in its infancy.
image file: c8cs00256h-f23.tif
Fig. 23 Schematic photoexcitation in a semiconductor followed by de-excitation events. Reproduced from ref. 179 with permission from the American Chemical Society, copyright 1995.

In this section, we do not intend to summarize again all the reports of using MOFs for different photocatalytic reactions, for which the readers can refer to the numerous already reported reviews.198–200,203 Instead special emphases were placed on the strategies to improve the performance of MOF-based photocatalysts and expand their applications in photocatalysis. Future efforts that can be carried out in this field are also outlined. One of the aims of this section is to encourage more future efforts to be devoted to the design and development of more efficient MOFs for solar fuel productions.

Strategies to improve the light absorption of MOFs

Previous studies have revealed that the metal nodes of MOFs can be regarded as isolated semiconductor quantum dots, while the organic linkers can be considered as light antenna to absorb light and activate these quantum dots.203 Therefore, the light absorption of the MOF materials depends both on the metal nodes as well as the organic linkers, although the photochemical relaxation pathways after excitation can be different depending on the chromophore absorbing light. For the efficient use of solar energy, it is ideal that the photocatalyst be responsive to visible light, which accounts for 43% of the entire solar energy. However, except for some Fe-based MOFs which consist of extensive fused Fe–O metal clusters responsive directly to visible light, most of the already reported Ti4+, Zr4+, Zn2+ and Cu2+ based MOFs do not contain visible light directly excitable metal nodes. Strategies to obtain MOFs with superior light absorption in the visible light region include the fabrication of MOFs using ligands with broad light absorption, ligand functionalization, designing larger SBU and dye sensitization.

Ligands with broad light absorption

The most powerful strategy to develop MOFs with intensive absorption in the visible light region is to use ligands with broad light absorption to directly fabricate the MOFs. Usually ligands with specific substituents (–NH2, –NO2, –SH and –OH),214 ligands with high π-conjugation215 and photosensitizers like porphyrins/metalloporphyrins216,217 and some metal complexes218 are responsive in the visible light region, and they can reach high absorptivity and, therefore, they can be used in the fabrication of visible light responsive MOFs.

A red Ti-containing MOF NTU-9, which absorbs visible light up to 750 nm, has been obtained based on 2,5-dihydroxyterephthalic acid as the organic linker.214 The calculated band gap of NTU-9 (1.72 eV) is much smaller than that of MIL-125(Ti) (3.6 eV) and NH2-MIL-125(Ti) (2.6 eV), which consist of bdc and NH2-bdc respectively. Density functional theory (DFT) study reveals that in addition to the VB, the p atomic orbitals of carbon also contribute to the CB in NTU-9, indicating the possibility of fabricating MOFs with a small band gap using organic linkers with strong adsorptivity in the visible light region.

A stable Zr-based MOF (VNU-1) with the absorption edge extending to approximately 540 nm has been obtained using a highly π-conjugated 1,4-bis(2-[4-carboxyphenyl]ethynyl) benzene (cpeb) as the linker.215 DFT calculations reveal that both the VB and CB are primarily composed of deprotonated cpeb ligands with little contribution from Zr6O4(OH)4(CO2)12 secondary building units (SBUs), indicating the important role of the cpeb ligand in tuning the band gap. Owing to its excellent light absorption properties, VNU-1 shows superior performance for photocatalytic degradation of methylene blue (MB) and methyl orange (MO). The activity of VNU-1 is significantly higher than that of Degussa P-25 TiO2 as well as the isoreticular UiO-66(Zr) and UiO-67(Zr).

In contrast to the case of VNU-1 in which the photochemistry is localized in the linker, the large advantage of a MOF structure is the close contact between the organic linker and the metal nodes that makes possible a fast quenching of the ligand excited states. In this regard, sensitizers like porphyrins, metalloporphyrins and metal complexes have also been used to construct MOFs with intensive visible light absorption. A dark red porphyrinic MOF (PCN-22) with a broad range of absorption from 200 nm to 640 nm has been obtained by reacting preformed Ti–O carboxylate clusters with tetra-kis(4-carboxyphenyl)porphyrin (tcpp).216 The calculated band gap of PCN-22 (1.93 eV) is much smaller than that of NH2-MIL-125(Ti) (2.6 eV). When acting as a photocatalyst, the tcpp ligand in PCN-22 injects electrons upon excitation to the Ti–O clusters to achieve a charge separation state and realize efficient photocatalytic alcohol oxidation under visible light irradiation.

A Ru complex bis(4′-(4-carboxyphenyl)-terpyridine)Ru(II) (Ru(tpy)2) has been used to replace NH2-bdc in NH2-MIL-125(Ti) for the construction of Ti-MOF-Ru(tpy)2.218 The as-formed Ti-MOF-Ru(tpy)2 shows a wider absorption band in the visible-light region, which enables the photocatalytic hydrogen evolution over Ti-MOF-Ru(tpy)2 to be realized under light with a longer wavelength and in the presence of more moderate sacrificial agents like ethylenediaminetetraacetic acid (EDTA) and methanol.

In some cases when the direct synthesis of MOFs from the organo/metal–organic chromophores may not be successful, a “mix-and-match” synthetic strategy, first demonstrated by Lin et al., can be adopted in the fabrication of doped photoactive MOFs. By partial substitution of H2bpdc with either Ir(ppy)2(5,5′-dcbpy)Cl (ppy: 2-phenylpyridine) or Ru(bpy)2(5,5′-dcbpy)Cl2 (dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid), highly stable and robust Ir(ppy)2(5,5′-dcbpy)Cl and Ru(bpy)2(5,5′-dcbpy)Cl2 doped Zr6O4(OH)4(bpdc)6 frameworks (UiO-67) have been obtained, which show photocatalytic activities for a series of organic reactions under visible light irradiation.194 The mix-and-match strategy can be applied in the fabrication of photoactive MOFs using steric bulky metalloligands. However, the limitation of this strategy lies in the fact that its effectiveness as well as the concentration of the photoactive chromophores incorporated into the resulting mixed-ligand MOFs strongly depends on the degree of similarity between the original linker and the functionalized photoactive metallo-ligand.

Ligand functionalization

Generally, electron-donating substituents can red shift the light absorption of the MOFs and thus have a promoting effect on the MOF-based photocatalysis. Actually substituents with different electron donating and withdrawing capability in the organic linkers have been introduced into the MOFs and their influence on the light absorption capability as well as the photocatalytic performance of the resultant MOFs have been studied both experimentally and theoretically219–224 (Fig. 24). MOFs with strong visible light absorption can also be fabricated based on existing MOFs using ligands with two substituents following a “push–pull” principle. Theoretical studies reveal that at least one of the frontier bands of the MOFs is centered on the ligand, usually the conjugated organic molecule. The “push–pull” effect of the substituents significantly influences the light absorption of the MOFs since the energies of these bands can be influenced by the electron donating/withdrawing capability of the additional substituents.225 These MOFs with desired functionality can be synthesized directly or using a post-synthetic modification (PSM) strategy.226,227
image file: c8cs00256h-f24.tif
Fig. 24 Experimental UV/Vis absorption spectra of different ligands used for the synthesis of UiO-66 frameworks. Reproduced from ref. 220 with permission from the American Chemical Society, copyright 2015.

The effect on the light absorption capability of the MOFs induced by the substituents in the organic ligands was for the first time demonstrated on a series of isoreticular IRMOFs.228 The semiconducting properties of a series of isoreticular MOFs with different organic linkers, namely bdc (IRMOF-1), 2-bromoterephthalic acid (IRMOF-2), 2,5-dibromoterephthalic acid, bpdc (IRMOF-9), 1,4-naphthalenedicarboxylic acid (IRMOF-7) and 2,6-naphthalenedicarboxylic acid (IRMOF-8) are found to depend strongly on the resonance effects in the organic linkers. By using the photoinduced gas-phase oxidation of propylene as a model reaction, it was observed no appreciable activity over MOF-5 with a band gap of 4.0 eV, while MOFs with biphenyl, and naphthalenedicarboxylic acids with 1,4- and 2,6-substitution with band gaps of 3.8 eV, 3.3 eV and 3.3 eV respectively, are active photocatalysts. Among them, 2,6-naphtha-MOF, with a band gap energy close to that of ZnO and TiO2 is found to be the most active. This was probably the first experimental demonstration that the light absorption wavelength of isoreticular MOFs can be successfully tuned by changing the organic linker.

Among all the investigated functionalities, amine functionality has been demonstrated to be extremely powerful to shift the light absorption of various MOFs towards visible light. For example, amino functionalization on MIL-125(Ti) and UiO-66(Zr) brings the light absorption of these UV absorbing MOFs to the visible light region. As a consequence of this light absorption, the resultant amine-functionalized NH2-MIL-125(Ti) and NH2-UiO-66(Zr) exhibit photocatalytic activity for CO2 reduction,219,223 hydrogen evolution229,230 and organic transformations231,232 under visible light illumination. It is believed that the nitrogen lone pair of the amino substituent on the ligand introduces a new electronic transition populating the π*-orbitals of the benzene ring, thus donating electron density to the anti-bonding orbitals.233

In addition to the UV light responsive MIL-125(Ti) and UiO-66(Zr), the introduction of amino functionality into the visible light responsive Fe-based MOFs can also enhance their light absorption in the visible light region.193 These amino functionalized Fe-based MOFs show superior photocatalytic performance for CO2 reduction as well as Cr(VI) reduction under visible light, due to the existence of the dual excitation pathways, i.e., the direct excitation of the Fe–O clusters and the excitation of the amino functionality followed by an electron transfer from the excited organic linker to the metal center.193,234 However, a different enhanced degree of the photocatalytic performance for CO2 reduction over NH2-MIL-101(Fe), NH2-MIL-53(Fe) and NH2-MIL-88B(Fe) in comparison to their non-functionalized counterparts may indicate that the efficiency of the electron transfer from the excited organic linker to the metal clusters in these three Fe-based MOF materials could be different.193 A more in-depth study of these systems to elucidate the generated active species and intermediates is still required to understand the electron transfer processes occurring in these Fe containing MOFs.

In addition to the mono-amino substitution, incorporation of a second amino functionality into the MOFs can further shift the light absorption of the MOFs toward longer wavelength, as demonstrated theoretically and experimentally on MIL-125(Ti)221 and UiO-66(Zr).223

Although the presence of the amino functionality into the MOFs enhances the light absorption of the resultant MOFs, it reduces the oxidation potential of the MOFs. A theoretical study on three MIL-125(Ti) analogues with a different number of amino groups carried out by Walsh et al. reveals that the presence of an amino functionality on MIL-125(Ti) results in the elevation of the VB by 1.2 eV, while the CB remains unchanged. The increase of the VB implies that the resultant NH2-MIL-125(Ti) possesses a lower oxidation power.221

These studies clearly illustrate the current level of understanding of the capability of engineering the functionality and highlight the possibility of tuning the optical properties of MOFs through rational substitution on the organic linker. However, the light absorption and the redox potential should be balanced during the fabrication of MOFs for photocatalysis based on ligand functionalization. Since after light absorption, quenching of the excited states of the ligand by the metal nodes are assumed to be fast, due to their close interaction, some of the photocatalytic events indicated in Fig. 23 may be very efficient in MOFs and further studies should focus on the subsequent deactivation and catalytic steps to achieve the optimal MOF photocatalysts.

Designing larger secondary building units (SBUs)

As mentioned above, in one simplistic view that is frequently useful, MOFs can be regarded as sensitized semiconductors, in which the SBUs of the meal oxide clusters can be considered to be discrete quantum dot analogues, while the conjugated organic linkers are acting as the photon antenna. Besides the fact that most of the already reported d0 (Ti4+, Zr4+) or d10 (Zn2+, Cu2+) transition metal ions based MOFs do not contain visible light excitable metal nodes, the band gap of these quantum dots can be very large (≥4 eV), meaning a direct absorption deep in the UV region. It is well known that the band gap of the quantum dots can be altered by adjusting their sizes, with the band gap decreasing with increasing size of the quantum dots. It is, therefore, anticipated that by designing larger SBUs to assemble the MOFs, MOF-based semiconductors with smaller node-localized band gaps, even responsive in the visible region can be obtained.

A previous study on three Zn-based MOFs, IRMOF-9, Zn5-bpdc, and CPO-7, which contains the same ligand of bpdc, reveals that their band gaps decrease with the increasing size of the SBUs.235 IRMOF-9 has a roughly cubic structure of double interpenetration, with bpdc ligands linking 6-connected Zn4O13 SBUs. Zn5-bpdc consists of a 3D framework with an expanded diamondoid topology constructed from double 4-connected Zn5O22 SBUs linked by bpdc ligands. In CPO-7, the inorganic zinc hydroxide layers serving as infinite SBUs are pillared by bpdc ligands, leading to a stairway-like configuration along the [001] direction. Among these three Zn-based MOFs, the size of the Zn based SBU clusters follows the order of CPO-7 > Zn5-bpdc > IRMOF-9. As a result, IRMOF-9 has a band gap of 3.42 eV, while the band gap of Zn5-bpdc red shifts to 3.36 eV, and that of CPO-7 decreases further to 3.26 eV, close to that of bulk ZnO with a value of 3.27 eV. Since these MOFs contain a similar bpdc ligand, which should contribute similarly to the light absorption of these MOFs, the changes in the band gaps of these three MOFs should mainly be attributed to their different SBU cluster sizes. Although the light absorption of these MOFs still falls in the UV region, it is anticipated that visible light responsive MOF-based photocatalysts can be obtained via a rational design of large SBUs for other transition metals.

Actually the influence of the SBUs on the MOF's band gap could be very complicated since many factors including cluster size, electronic effects, shape effect, vacancies and so on, can all contribute to the band gap shift. However, such complexity also offers more opportunities for further tuning the optical properties of MOFs via rational SBU design.

Dye sensitization

The use of dyes to sensitize wide band gap semiconductor based photocatalysts is a widely adopted strategy to develop photocatalytic systems with light absorption into the visible light region. This strategy can also be used to obtain visible light responsive MOF-based photocatalysts. In 2013, Gascon et al. demonstrated that NH2-MIL-125(Ti) can be modified with a dye-like molecular fragment to obtain methyl red-MIL-125(Ti) (MR-MIL-125(Ti)) via a post-synthetic modification strategy using –NH2 in the ligand as the anchoring point (Fig. 25).236 As compared with parent NH2-MIL-125(Ti), a clear red shift with the absorption edge to almost 700 nm is observed for MR-MIL-125(Ti) due to the extended conjugation of the ligand system. The stronger optical absorption makes MR-MIL-125(Ti) exhibit superior activity, as compared with pristine NH2-MIL-125(Ti), for photocatalytic oxidation of benzyl alcohol to produce benzaldehyde under visible light irradiation.
image file: c8cs00256h-f25.tif
Fig. 25 Diffuse reflectance spectra of MIL-125(Ti) (grey), NH2-MIL-125(Ti) (orange) and MR-MIL-125(Ti) (red). Reproduced from ref. 236 with permission from the Royal Society of Chemistry, copyright 2013.

A more convenient method to realize dye sensitization is to add a dye directly to the reaction system containing the MOF material.237 Although UiO-66(Zr) does not absorb visible light, the direct addition of rhodamine B (RhB), a coumarin dye, to UiO-66(Zr) extends the absorption to ca. 600 nm. As compared with bare Pt@UiO-66(Zr), the resultant Pt@RhB/UiO-66(Zr) exhibits a 30 fold increase of photocatalytic activity for hydrogen evolution under visible light in the presence of TEOA as a sacrificial agent. In this reaction system, RhB adsorbed on the surface of Pt@UiO-66(Zr) is excited when irradiated with visible light. An electron is transferred from the excited RhB to UiO-66(Zr), which further migrated to Pt NPs due to the existence of the Schottky barrier at the interface of UiO-66(Zr) and Pt NPs. Pt NPs act as the active site for hydrogen evolution, while TEOA donates electrons to recover the neutral RhB dye.

Strategies to improve the separation of charge carriers and promote the reaction

It should be noted that enhancing the sunlight absorption of MOF-based photocatalysts is meaningful only when the photogenerated charge carriers are long-lived enough to migrate to the reactive sites to become able to take part in the photocatalytic reactions. Therefore, as commented earlier, in addition to enhancing the light absorption of the MOF based materials, the suppression of the recombination of the charge carriers and favouring their mobility as well as facilitating the catalytic reaction is important. Advanced spectroscopy techniques like ultrafast spectroscopy can elucidate the kinetics of the photoexcited states in MOFs and can provide important information in the development of highly efficient MOF-based photocatalysts.

Ligand functionalization

Although most of the modifications on the ligand of the MOFs aim at the enhancement of its absorption toward solar light, there are a few reports that show that the presence of functional groups on the organic ligand also influences the efficiency of charge separation. A recent study by van der Veen and co-workers demonstrated that the amine functionality also influences the decay profile and topography of the excited states in the MIL-125(Ti).238 The results obtained based on spectro-electrochemistry and ultrafast spectroscopy demonstrate that upon photoexcitation in NH2-MIL-125(Ti), the photogenerated electrons locate in the Ti–oxo clusters, while the holes reside on the aminoterephthalate unit, specifically on the amino group. NH2-MIL-125(Ti) shows markedly longer lifetimes of the charge-separated state as compared with MIL-125(Ti), which results in improved performance in photo-conversion by the suppression of competing decay mechanisms. The results highlight the role of the amino group in NH2-MIL-125(Ti) to extend the lifetime of the photo-excited state.

Another study by Huang and co-workers also reported that the introduction of a halogenated ligand into NH2-UiO-66(Zr) to form the mixed-ligand MOFs enhances its photocatalytic activity for benzyl alcohol oxidation.239 It is proposed that the presence of a halogenated ligand stabilizes the active species ˙O2 on Zr3+, suppressing the recombination of the charge carriers and thus facilitating the reaction. Clearly more studies are needed to gain deeper understanding on the role of ligand substituents beyond light absorption.

Metal ion substitution

The doping of a metal ion is an efficient strategy in inorganic semiconductor-based photocatalysts to promote light absorption, as well as the separation and mobility of the photo-excited charge carriers.240,241 Similar to the inorganic semiconductor-based photocatalysts, the photocatalytic performance of the MOF-based photocatalysts can also be promoted via the introduction of dopant metal ions on the inorganic clusters of MOFs.

A Ti-substituted NH2-UiO-66(Zr) (NH2-UiO-66(Zr/Ti)) has been obtained via a post-synthetic strategy and explored for photocatalytic CO2 reduction and hydrogen evolution.242 The mixed NH2-UiO-66(Zr/Ti) exhibits obviously higher photocatalytic performance as compared with the parent NH2-UiO-66(Zr). DFT and ESR studies reveal that the introduced Ti moieties act as a mediator to facilitate the electron transfer from the excited linker NH2-bdc to the Zr–O oxo-clusters to form the catalytically active species Zr3+, resulting in the enhanced photocatalytic performance. The role of substituted Ti as a mediator to facilitate electron transfer from the excited ligand to the (Zr/Ti)6O4(OH)4 nodes in mixed NH2-UiO-66(Zr/Ti) was later confirmed by transient absorption spectroscopy (TAS) (Fig. 26).243 The introduction of the substituted Ti as a mediator is similar to the creation of the mid-gap metal centered states to promote the LMCT transition in the MOFs.


image file: c8cs00256h-f26.tif
Fig. 26 Proposed mechanism to rationalize the enhanced performance over mixed NH2-UiO-66(Zr/Ti). Reproduced from ref. 243 with permission from the American Chemical Society, copyright 2017.

By doping Ce into the framework of MIL-101(Cr), Yamashita et al. fabricated a Ce-doped MIL-101(Cr) (CeMIL-101) which is photoactive for the hydrolysis of ammonia borane under visible light.244 CeMIL-101(Cr) was obtained from Ce(NO3)3 and Cr(NO3)3via a direct hydrothermal treatment process. The structural, textural or optical properties of MIL-101(Cr) do not change obviously after doping with Ce3+ ions. By deposition of Pd NPs to CeMIL-101(Cr), the Pd/CeMlL-101(Cr) exhibits much greater catalytic activity than Pd/MIL-101(Cr) under similar reaction conditions. The superior performance of Pd/CeMIL-101(Cr) can be ascribed to the existence of the doped Ce3+ ions in the MOFs, which helps to promote the transportation of photo-generated electrons from the excited MIL-101(Cr) to the Pd NPs via the Ce4+/Ce3+ redox pair and suppresses the recombination of the photo-generated charge carriers. Also Ni-doped NH2-MIL-125(Ti) has been found to exhibit a significantly enhanced photocatalytic activity for the selective oxidation of aromatic alcohols, as compared with parent NH2-MIL-125(Ti), attributable to the enhanced charge separation and the improved visible-light harvesting.245 The facile realization of metal substitution in MOFs and the diversified MOF structures would allow for developing a variety of bimetallic MOF-based photocatalysts with optimal performance.

Deposition of noble MNPs

The deposition of noble MNPs in semiconductor photocatalysts is a widely adopted method to promote the separation of the photogenerated electrons and holes due to the formation of a Schottky barrier between the semiconductor and NPs and to enhance the photocatalytic reaction by acting as co-catalysts. The promoting effect introduced by MNPs also exists in MOF-based photocatalysis, since the migration of photogenerated electrons from the excited MOF to MNPs acting as a reservoir of electrons can also result in an improvement of the charge separation efficiency. As compared with semiconductors, MOFs show more advantages as hosts for MNPs attributed, as commented on in previous sections, to the high porosity and tunable cavity dimensions, which not only provide a confined space to avoid the growth of the NPs, but also allow an even distribution of the MNPs in the MOFs. In addition, the close proximity between the MOFs and the encapsulated NPs and the fixed spatial arrangement also favor the charge transfer and lead to superior photocatalysis.

Lin and co-workers first reported that the introduction of Pt NPs inside the cavity of a MOF material derived from [Ir(ppy)2(bpy)]+ dicarboxylate ligands and Zr63-O)43-OH)4(carboxylate)12 SBUs promotes its photocatalytic activity for hydrogen evolution.246 The superior photocatalytic performance of the resultant Pt@MOFs can be attributed to the efficient electron transfer from the excited Ir-complex to the incorporated Pt NPs, which not only results in an efficient charge separation, but also stabilizes the Ir complex from decomposition. Similarly, Pt/MIL-100(Fe) obtained via photo deposition of Pt NPs on the surface of MIL-100(Fe) shows activity for water reduction to produce hydrogen in a methanol–water mixed solution under visible light.247 In addition to water reduction reaction, metal NPs/MIL-100(Fe) nanocomposites (M = Au, Pd and Pt) have also been reported to show improved photocatalytic performance for degradation of dyes248 as well as several types of organic pollutants, including theophylline, ibuprofen and bisphenol A, using H2O2 as an oxidant.249

Interestingly, the contribution of different noble MNPs to MOF-based photocatalysis might be quite different. Li et al. reported that both Au and Pt NPs promote hydrogen evolution over NH2-MIL-125(Ti), however Au and Pt NPs have different effects on CO2 reduction.250 The deposition of Pt NPs promotes the photocatalytic CO2 reduction to formate, whereas Au NPs show a negative effect on this reaction. ESR combined with DFT calculations reveal that for Pt/NH2-MIL-125(Ti), the generated hydrogen dissociates on Pt to form hydrogen atoms and spillsover to the Ti–O oxo-clusters of NH2-MIL-125(Ti) to form active Ti3+ for CO2 reduction, while such a pathway cannot occur over Au/NH2-MIL-125(Ti). Due to the efficient activation of molecular hydrogen, Pt/NH2-MIL-125(Ti) shows improved formate generation, as compared with Au/NH2-MIL-125(Ti) and parent NH2-MIL-125(Ti). This suggests that an appropriate selection of MOFs and MNPs is vital in the design of MNP/MOF photocatalytic systems.

Moreover, the position where the MNPs locate in the MOF influences also its photocatalytic performance. Jiang et al. reported that Pt NPs of ca. 3 nm incorporated inside or supported on UiO-66-NH2 (denoted as Pt@UiO-66-NH2 and Pt/UiO-66-NH2, respectively) show different photocatalytic activity for hydrogen evolution.251 The Pt@UiO-66-NH2 greatly shortens the electron-transport distance, which favors the electron–hole separation and thereby yields higher efficiency than Pt/UiO-66-NH2. The involved mechanism was further unveiled by means of ultrafast transient absorption and photoluminescence spectroscopy. This study highlights the relationship between the photocatalytic efficiency and the Pt location relative to the MOF material.

Encapsulation of catalytic active sites

Most of the photocatalytic reactions require cocatalysts to suppress the recombination of the photogenerated charge carriers as well as providing active sites for interaction with the substrate and activation of the required chemical reactions. To improve the photocatalytic activity of the MOFs, it is important to encapsulate the catalytic active site on the MOFs.

In 2015, Gascon et al. reported the facile synthesis of a CoII complex within a MOF (Co@MOF) using a ‘ship-in-a-bottle’ method (Fig. 27).252 The introduction of a catalytically active molecular CoII complex leads to a 20-fold enhancement in photocatalytic hydrogen evolution as compared to the pristine NH2-MIL-125(Ti), demonstrating the synergy between a stable, photo-active MOF structure and a Co-based proton reduction catalyst for photocatalysis. Similarly, Jiang et al. rationally encapsulated a Co(II) molecular photocatalyst, [CoII(tpa)Cl][Cl] (tpa: tris(2-pyridylmethyl)-amine) inside the cages of NH2-MIL-125(Ti) for visible-light-driven hydrogen production.253 In both systems, the photo-induced electrons migrate from NH2-MIL-125(Ti) to the Co(II) complexes, which act as a catalytic active center for hydrogen evolution and promote the spatial charge separation, thus significantly boosting the photocatalytic efficiency of hydrogen production. A biomimetic heterogeneous photocatalyst ([FeFe]@ZrPF) has also been synthesized through the incorporation of homogeneous [(i-SCH2)2NC(O)C5H4N][Fe2(CO)6] into the highly robust zirconium-porphyrin based metal–organic framework (ZrPF) for photocatalytic hydrogen evolution.254 The modular design behind the cooperative action of a photoactive matrix and a catalytically active encapsulated guest would also allow the related composites for application in other photocatalytic reactions.


image file: c8cs00256h-f27.tif
Fig. 27 ‘Ship-in-a-bottle’ synthetic strategy for assembling Co@MOF. Reproduced from ref. 252 with permission from the Royal Society of Chemistry, copyright 2014.

Modular optimization of the MOFs has also been realized by incorporating coordinative unsaturated single atoms in a MOF matrix. Ye et al. reported that the introduction of catalytically active Co to coordinate with the porphyrin in a porphyrin containing MOF can greatly boost the electron–hole separation efficiency in porphyrin units. The directional transfer of the photogenerated excitons from porphyrin to Co centers supplies long-lived electrons for CO2 reduction on catalytic active Co centers.255 As a result, a porphyrin MOF comprising atomically dispersed Co in the porphyrin ligand exhibits significantly enhanced photocatalytic conversion of CO2 as compared with the parent MOF. A 3.13-fold improvement in CO evolution rate as well as a 5.93-fold enhancement in CH4 generation rate is achieved over the Co modified MOF.

Coupling MOFs with semiconductors and carbon materials

The coupling of two semiconductors with matched band gaps has been proven to be a feasible method to suppress the recombination of the photogenerated charge carriers, leading to improved performance of the heterojunction with regard to the independent semiconductor photocatalysts.256 This strategy has also been applied to MOFs, as demonstrated in a number of recent publications. A variety of MOF-semiconductor heterojunctions have already been developed, including CdS-UiO-66(NH2),257 Fe3O4@MIL-100(Fe) core–shell microspheres,258 UiO-66/carbon nitride nanosheet,259 HKUST/TiO2260 and CPO-27-Mg/TiO2,261 which have been proven to be beneficial for photocatalytic reactions. As compared with conventional semiconductor nanocomposites fabricated by two semiconductors, the additional advantages provided by the use of MOFs to build the MOF/semiconductor nanocomposite derive from the consideration of (1) the facile tuning of the light absorption of the MOF-based materials that enables matching the band position of the MOF with the semiconductor; (2) MOFs can provide extra pathways for migration of the photo-generated charge carriers due to the presence of ordered porosity in its structure; (3) MOFs can provide more catalytic sites for reactions due to their large specific surface area.

Besides organic semiconductors, the integration of carbon materials (in particular reduced graphene oxide (RGO)) with MOFs is another strategy to promote the mobility of the photogenerated charge carriers due to the high conductivity of RGO. For example, RGO-UiO-66(NH2)262 and RGO-MIL-53(Fe)263 have been developed and were found to show enhanced photocatalytic activity for Cr(VI) reduction.

The interfacial contact between MOFs and semiconductors, MOFs and RGO in these nanocomposites, influences their photocatalytic performance. However, currently the assembly of MOF/semiconductor and MOF/RGO nanocomposites to achieve their full potential is still a big challenge and new strategies are required to form heterojunctions with intimate contact, strong interaction and large interfacial area.

Multifunctional MOFs outperforming a simple photocatalyst

In addition to being used as a simple photocatalyst for hydrogen evolution, CO2 reduction, degradation of organics and photocatalytic organic redox reactions, the application of MOFs as multifunctional catalysts for light induced all-in-one or light-assisted cascade/tandem catalytic reactions shows great potential, due to the existence of different isolated catalytic active sites in MOFs which can function independently in combination with the photo-response. It is anticipated that the merging of the metal/ligand/guest based catalysis with the MOF-based inherent photocatalysis would enable some complex multi-component reactions or tandem reactions to be realized over MOF materials under light irradiation, although research in this area is still at an initial stage.

In 2015, Li et al. reported the introduction of amine functionality to the organic linker of MIL-101(Fe), which enables it to act as a multifunctional catalyst to realize a tandem photo-oxidation/Knoevenagel condensation reaction between different aromatic alcohols and active methylene compounds under visible-light irradiation.264 Also by coupling photocatalysis with Lewis acidic Fe3+ sites in MIL-100(Fe) and MIL-68(Fe), these two Fe-based MOFs act as a multifunctional catalyst to successfully promote the tandem reactions between o-aminothiophenols and alcohols to produce benzothiazoles under visible light.265 Another example is the coupling of the photocatalysis of Fe–O clusters with a Fenton-like route over MIL-100(Fe) to enable photocatalytic benzene hydroxylation to from phenol using H2O2 as an oxidant under visible light (Fig. 28).266


image file: c8cs00256h-f28.tif
Fig. 28 Possible reaction mechanism for the photocatalytic benzene hydroxylation over MIL-100(Fe). Reproduced from ref. 266 with permission from the American Chemical Society, copyright 2015.

By incorporating photoactive polyoxometalate [SiW11O39Ru(H2O)]5− within the pores of a copper(II)-bipyridine MOF, Duan and co-workers developed 3D porous CR-BPY1 MOFs as a dual catalyst for a light induced C–C coupling reaction between N-phenyltetrahydroisoquinoline and nitromethane by merging photocatalysis and Cu-based catalysis.267 Li et al. also reported the encapsulation of small Pd NPs inside the cavity of Fe-based MOFs to achieve N-alkylation of amines with alcohols under visible light irradiation via a successful coupling of the photocatalytic oxidation and Pd-catalyzed hydrogenation.268 MIL-100(Fe) with small Pd NPs encapsulated inside the cavity of the MOF shows significantly superior catalytic performance, as compared with Pd deposited on the external surface of MIL-100(Fe). The enhanced performance can be attributed to the influence of the particle size with the existence of more catalytically active unsaturated Pd atoms in the small sized encapsulated Pd NPs than in the surface supported larger Pd NPs. In addition, the confinement of both Pd NPs and the substrates in the cavity, which acts as a nanoreactor, ensures a rapid electron transfer from the excited Fe–O clusters to the Pd NPs for the reaction. Similarly, the encapsulation of ultrafine Pd nanoclusters smaller than 1.2 nm inside the cavity of NH2-UiO-66(Zr) has also been reported to act as an efficient catalyst for visible light driven Suzuki coupling.269

The key to design such versatile multifunctional MOF materials is to choose a right combination of structural components (linkers, nodes and guests) in one MOF material so that these components not only can maintain their individual functions, but also work synergistically. This strategy is promising considering the high design flexibility and wide framework diversity of MOFs arising from the large possibilities in the selection of metal nodes and organic linkers as well as the tunable pores.

Outlook and prospective

The use of MOFs as heterogeneous catalysts is an area that has experienced an increasing growth in the last decades since the initial reports on the synthesis of these materials. We have outlined the main reasons explaining the suitability of these porous materials as solid catalysts. Besides versatility in the synthesis and design, the review has shown the various possibilities that MOFs offer to incorporate active sites in their structure and the more recent strategies to increase the density of these sites, either during their synthesis or by post-synthetic treatments. The use of modulators during the synthesis or partial damage after material preparation have been commented on as complementary strategies to generate and increase the density of sites, even in MOFs that do not have coordinatively unsaturated positions around the metal sites.

Emphasis has been made on the importance of proving MOF stability under catalytic conditions, since poor structural stability compared to zeolites and other purely inorganic porous materials is one of the major drawbacks of these materials. This stability has to be confirmed by a combination of catalytic recyclability tests, characterization of the exhaustively used sample and chemical analysis of the liquid phase.

This review has highlighted how MOFs are specially suited as multifunctional catalysts by comprising more than one active site. This multifunctionality can result in the case of MOFs in a synergy between different complementary sites cooperating to a reaction mechanism. This cooperativity can lead to an overall activity much higher than expected compared to homogeneous catalysts or with solid catalysts having only one type of site. Among the various possibilities that have been proposed, it could be that a single reagent can be activated by two sites in the MOF structure or each substrate reacting can undergo independent activation.

In the future, it can be expected that multifunctionality is going to be the key for the use of MOFs in the development of tandem reactions, in which more than one reaction is performed using only one MOF as a catalyst. Tandem reactions present many advantages from the point of view of process intensification, diminishing workup and product purification. For these tandem reactions, the location of the required density of sites in specific places in the catalyst structure appears to be very important and MOFs offer several possibilities in this regard.

The section dealing with photocatalysis summarized the strategies to improve the performance of MOFs to promote light assisted reactions and expand their applications. It should be noted that factors influencing the performance of the MOF-based photocatalysts are generally complex. Different strategies should be combined to realize an efficient MOF-based photocatalysis.

In our opinion, some of the open issues that research should address in the future regarding the use of MOFs as photocatalysts are the following. First, the charge transfer process significantly influences the efficiency of photocatalysis. Yet the charge mobility in most photocatalytic MOFs is low. Improving the conductivity of the MOFs is expected to allow for a higher level of electron/hole separation and suppress their recombination resulting in improved photocatalytic efficiency. Therefore, the conductive MOFs should be encouraged to be used in photocatalysis in the future.270,271 Moreover, to shorten the transport path of the charge carriers and to maximize the light penetration as well as to provide more exposed active catalytic sites, it is also encouraged that 2D ultrathin MOF nanosheets as well as MOF films should be fabricated for use as photocatalysts. Several reports have already revealed that 2D MOF nanosheets show superior performance in catalysis and electrocatalysis.272,273 However none of them has been reported for photocatalysis.

Second, the design and development of efficient MOF-based photocatalysts should rely more on the knowledge of the electronic structure of MOFs and the kinetics of the photoexcited states in MOFs. To drive the desired chemical reactions, MOFs should possess sufficient redox potentials. Such information can be obtained via the electrochemical technique. Even with poor electron conductivity, MOFs deposited on the conducting materials like ITO can be submitted to electrochemical characterization or alternatively, the redox potentials of the MOFs can be estimated based on their molecular components.238 The kinetics of the photoexcited states in MOFs, including the generation of the transient states and their lifetimes, significantly influences the photocatalytic efficiency. For an efficient photocatalysis, the lifetime of the photoexcited transient states should be sufficiently long to ensure that the catalytic reaction can compete with the decay of the charge separated states. Time-resolved absorption spectroscopy can provide information on the formation, decay, recombination, and transfer processes of photogenerated charge carriers,274 all these kinetic data being important for the understanding of the photocatalytic reaction mechanisms and for a better design of MOF photocatalytic systems.

Most of the MOFs are built up from carboxylate linkers and do not possess enough oxidation potential for water oxidation. Therefore, currently MOF-based photocatalysis like hydrogen evolution and CO2 reduction only involves the reductive half reaction, while the oxidative half reaction is accomplished by the oxidation of a sacrificial agent. It would be ideal if the sacrificial agent could be replaced with recyclable electron donors275 or the MOF photocatalytic system could be coupled with a water oxidation catalyst to realize sustainable solar fuel generation. Great efforts will be made in the future for the development of multifunctional MOFs to promote cascade/tandem catalytic reactions, one of the most promising future applications of MOFs in catalysis. Finally, the fabrication of multifunctional MOF-based nanocomposites is another important trend for the use of MOFs in catalysis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

AD thanks the University Grants Commission, New Delhi, for the award of an Assistant Professorship under its Faculty Recharge Programme. AD also thanks the Department of Science and Technology, India, for the financial support through Extra Mural Research Funding (EMR/2016/006500). Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa SEV2016-0683 and CTQ2015-69563-CO2-1) and Generalitat Valenciana (Prometeo 2017-083) is gratefully acknowledged. Financial support from the National Science Foundation of China (NSFC U1705251) and Ministry of Science and Technology of China (973 Program 2014CB239303) is acknowledged. Z. Li also thanks the Award Program for Minjiang Scholar Professorship for financial support.

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