Catalysis by Metal Organic Frameworks: Perspective and Suggestions

Perspective Cite This: ACS Catal. 2019, 9, 1779−1798

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Catalysis by Metal Organic Frameworks: Perspective and Suggestions for Future Research Dong Yang and Bruce C. Gates*

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Department of Chemical Engineering, University of California, Davis, California 95616, United States ABSTRACT: Metal organic frameworks (MOFs) have drawn wide attention as potential catalysts, offering high densities of catalytic sites in high-area porous solids, some with stabilities at high temperatures. The field is at an early stage, characterized by numerous discoveries and novel demonstrations of catalytic properties associated with the crystalline structures of MOFs, but applications of MOFs as catalysts are still lacking. In this perspective we summarize advantages and limitations of MOFs as catalysts and fundamental issues to be addressed about their potential applications. MOF framework compositions and pore structures can strongly influence catalytic performance, allowing, for example, shape-selective and bifunctional catalysis, but research is needed to quantify reaction/transport processes in MOFs, identify catalytic sites, and determine intrinsic catalytic reaction rates. Progress is hindered by the lack of understanding of the heterogeneity of MOFs, with catalytic sites sometimes being in structures such as defects not determined by X-ray diffraction crystallography. Determination of the dynamics of MOFs and their catalytic sites, as well as the intrinsic kinetics of catalytic reactions, will help to advance guidelines for synthesizing optimum catalysts. Further, MOFs present challenges related to stability and regeneration as catalysts, some associated with the unique nature of MOFs, such as the node−linker bonds, which can break during catalysis. There are opportunities to understand these matters in depth and to find conditions of catalytic operation that minimize the processes leading to deactivation. KEYWORDS: metal organic framework, catalytic sites, defects, transport limitations, catalytic kinetics, stability

1.1. Advantages of MOFs as Practical Catalysts. MOFs have many properties that commend them as catalysts:11,12,14,25 • high internal surface areas and active site densities for high catalytic reaction rates per unit volume. • opportunities for synthesis and postsynthesis modification of MOFs with complementary catalytic groups, allowing catalysis by numerous functional groups and also bifunctional or tandem catalysis • opportunities for tailoring of MOF pore structures to allow • pores large enough for rapid transport of large reactant and product molecules • shape-selective catalysis • confinement effects for stabilizing encapsulated catalytic species and placement of different catalytic groups next to each other or in separate, nearby environments The rapid growth of research on MOF catalysis is explained in part by these attractive properties and the potential for largescale catalytic applications, although to our knowledge none has yet emerged. We suggest that part of the motivation for research on MOFs as catalysts is also related to researchers’

1. INTRODUCTION Metal organic frameworks (MOFs)reticular solids consisting of inorganic nodes (such as metal atoms) and organic linkerswith high internal surface areas (up to 7000 m2 g−1) are among the materials drawing the most attention by researchers today.1−3 Tens of thousands of MOFs have been described, with new ones being discovered at a high rate.1,4 Many reports of MOFs provide statements of practical motivations for investigating them, often with an emphasis on the prospects for application in gas storage and separations.5−10 Researchers also frequently mention potential catalytic applications of MOFs,11−20 and we address them in this Perspective. The prospects for large-scale applications have motivated industrial work on the scale-up of MOF synthesis, with BASF having played a leading role by developing large-scale syntheses of a number of MOFs, often with methods (such as mechanochemical synthesis) different from those of the common laboratory syntheses carried out with solvents.21,22 Challenges in the development of economical large-scale MOF synthesis remain, but we do not address them here, because they are well reviewed.23,24 We focus instead on what we regard as fundamental, sometimes overlooked, issues regarding MOFs as catalysts, attempting to place them in perspective and pointing out research opportunities. © XXXX American Chemical Society

Received: November 9, 2018 Revised: January 3, 2019 Published: January 15, 2019 1779

DOI: 10.1021/acscatal.8b04515 ACS Catal. 2019, 9, 1779−1798

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characterization and that are often important in catalysis by solids generally. Our goals are to highlight such issues and point out research opportunities. 1.4. Open Questions about MOFs as Catalysts. Here are our key points, which constitute an outline of this perspective: • Quantitative representations of the influence of transport limitations within MOF pores are largely lacking. • Repeatable synthesis of some MOFs is challenging, and for most MOFs there is a lack of methodology to make large batches with nearly identical properties, hindering systematic experimentation to provide an understanding of transport effects and kinetics. • In catalysis by MOFs, minority (possibly unrecognized) structural features may be of dominant importance in catalysis, and an understanding of them is crucial to quantifying catalytic activities of MOFs by reporting them as turnover frequencies. • Some stability limitations that are intrinsic to MOFs have been given less attention than they deserve: for example, many MOFs lack of stability in water, and some catalytic reactants and products induce reactions that lead to destruction of MOFs. • There is a lack of fundamental reaction kinetics of MOFcatalyzed reactions. • MOFs as catalysts are sometimes compared with zeolites, but such comparisons lack a broad fundamental foundation, and more appropriate comparisons may be with metal oxides and solid organic polymer catalysts, but these are mostly lacking; • There is a lack of prototypical MOF catalysts to allow comparisons of catalyst performance between laboratories; • MOFs have been investigated as catalysts for many reactions, but there is a lack of clear guidelines for research that might lead to catalytic applications of MOFs.

fascination with novel MOF chemistry and materials properties and the corresponding opportunities to advance the fundamental understanding of catalysis. 1.2. Limitations of MOFs as Practical Catalysts. Here are some limitations of MOFs as practical catalysts: • Because they include organic components, MOFs have limited stabilities in operationtypically much less than stabilities of inorganic solids. • Some reactants and products react with MOFs to cause them to unravel. • The organic components of MOF catalysts limit the opportunities to regenerate them by selectively burning off organic deposits without destroying the MOF. • Impurities in feedstocks may modify MOFs without being removable in regeneration processes. 1.3. Prospects of MOFs as Catalysts. The limited stabilities and limited possibilities for regeneration of MOFs at the high temperatures needed for most large-scale catalytic applications in fuel processing, vehicle and power plant emissions abatement, petrochemical synthesis, and commodity chemical synthesis likely severely limit applications of MOFs in these technologies. Consequently, the literature reflects a seeming consensus that the best prospects offered by MOFs as practical catalysts involve the synthesis of specialty and fine chemicals11,26 and specialty applications that do not require high temperatures or catalyst regeneration. The latter are exemplified by potential applications of MOF catalysts in gas masks for destruction of nerve gas.27,28 Beyond potential applications, MOFs have attracted the attention of researchers as candidate catalysts because they offer opportunities to extend fundamental understanding and facilitate catalyst design. With their wide ranges of compositions and structures, MOFs offer extensive opportunities to (a) incorporate catalytically active groups, (b) tailor pore structures, and (c) tailor environments around catalytic sitesboth individual catalytic sites and those working in concertall in combination. MOFs offer opportunities to investigate catalysts that have a subtlety in structure and function beyond that of almost all inorganic solids, and one could even suggest they are a preliminary step toward enzyme mimics. Thus, MOFs offer a fascinating playing field for testing new concepts in catalysis, and recent progress has attracted numerous researchers to the catalysis field, notably those with strong synthesis skills. A substantial literature of MOF catalysis has already emerged, and it is growing rapidly; it is well reviewed. These reviews address issues of synthesis, including postsynthetic treatments,29−32 structure,33−36 defects,37−40 multiplicity of functions,25,41 individual active sites and their characterization,42 encapsulated nanoclusters/particles,43 water stability, Brønsted acidity, activities for CO2 conversion and biomass conversion, among others;44−49 and scale-up of syntheses.23,24,50 Because these points have been well addressed, we do not elaborate on them here. Rather, we proceed from the thesis that much of the reported work on MOFs as catalysts largely overlooks some fundamental issues that are essential for potential application, and we attempt to summarize and clarify them. We posit that much of the MOF catalysis literature has been focused more on the properties of MOFs inferred from the standard MOF characterization techniquesX-ray diffraction (XRD) crystallography and surface area-pore volume measurementsand less on properties that require additional

2. MOFS INVESTIGATED AS CATALYSTS Tens of thousands of MOFs are known, but only a small fraction of them have been investigated as catalysts. These prominently include UiO-66/67, NU-1000, and MOF-808 (with Zr6O8, Ce6O8, Hf6O8, or mixed oxide nodes, which are metal oxide clusters),51−62 MOF-5 (with Zn4O or mixed oxide nodes),63−66 MIL-101 (with Cr3O or Al3O nodes),67−71 MIL53 (with Al(OH) or Fe(OH) nodes),72−74 MIL-100 (with Al 3 O, Fe 3 O, or Cr 3 O nodes), 75−78 ZIF-8 (with Zn 2+ nodes),79−82 and HKUST-1 (with Cu2 nodes).83−87 These MOFs incorporate a variety of linkers: for example, the ditopic carboxylate ligand BDC2+ (derived from benzene-1,4-dicarboxylic acid), the tritopic carboxylate ligand BTC3+ (derived from benzene-1,3,5-tricarboxylic acid), and the tetratopic carboxylate ligands TBAPy4+ (derived from tetratopic 1,3,6,8tetrakis(p-benzoate)pyrene).1,36,88 Some of these MOFs offer the potential advantages of metals that exist in various oxidation states (e.g., Fe, Ce) for redox catalysis, some have been modified by addition of groups to the linkers and the nodes, expanding the potential for applications in catalysis, and some of these catalysts are multifunctional.35,42,89 Groups that have been incorporated in MOFs include basic groups such as NH2,68,90−92 acidic groups such as OH and SO3H,93−97 metal complexes,98−103 and metal clusters.104−109 In prospect, 1780


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do not determine quantitative effectiveness factors except for those that are virtually zero for excluded reactants. Zhang et al.114 tested the activity of platinum in UiO-66 (Pt/UiO-66) for hydrogenation of olefins of various sizes. After 24 h of catalytic hydrogenation, the conversions of cyclooctene (minimum diameter ∼5.5 Å), trans-stilbene (5.6 Å), trans-phenylethylene (5.8 Å), and tetraphenylethylene (6.7 Å) were 66%, 35%, 8%, and 0%, respectively. These results demonstrate the shape selectivity of UiO-66, which has an average pore aperture diameter of 6.0 Å. Fischer’s group115 compared the activity of palladium clusters in the pores of UiO-66 (Pd/UiO-66) and Pd/UiO-67 (having 6 and 8 Å aperture diameters, respectively) for hydrogenation of acetophenone (AP) and benzophenone (BP). The much lower activity of Pd/UiO-66 was attributed to transport limitations. The results are not sufficient to determine values of the effectiveness factors. 3.2. Product Shape Selectivity. An example of product shape selectivity was reported by Farrusseng’s group,125 whose results demonstrate that a zinc-containing IRMOF has a high selectivity (95%) for the alkylation of biphenyl with tert-butyl chloride to form p-2-tert-butylbiphenyl, whereas selectivity for o-2-tert-butylbiphenyl and di-tert-butylbiphenyl is much lower ( UiO-67 > NU-1000 having average particle diameters of 200 nm, 800 nm, and about 3 μm, respectively.110 Again, the data were not sufficient for a quantitative representation in terms of the Thiele model. 3.4. Pore Structure Modification. To facilitate transport within MOF pores, some researchers developed synthesis/ treatment methods (by using mixed linkers,139,140 adding modulators,111,141 adding supramolecular species (such as micelles) as templates142,143 in the syntheses, or treating the

Figure 4. Schematic illustration of the synthesis of HP-MOFs with adjustable porosity using UiO-66 as an example. Reproduced with permission from ref 111., Copyright John Wiley and Sons, 2017. 1783


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Figure 5. Illustration depicting the structural and compositional differences between the ideal UiO-66 unit cell and those with missing cluster/ missing linker defects. Trifluoroacetate ligands compensate for the defects in the examples above; however, the compensating ligand can vary depending on the modulator used during the MOF synthesis. Reproduced with permission from ref 146. Copyright American Chemical Society, 2016.

by linkers, and they may be occupied by other ligands that are formed, for example, from modulators present during syntheses.53,151 In some MOFs, modulators generate defects by bonding to nodes in competition with linkers.146 Further, there may be defects where nodes are missing (as determined by XRD crystallography);152 then the linkers that would bond to the nodes in a defect-free structure may also be missing (and linkers dangling in a MOF may be replaced by ligands formed from modulators, thereby creating other atypical sites in MOFs; Figure 5). The Kieslich and Fischer group37 wrote that “...it is important to realize that a missing node defect is a result of a critical concentration and spatial distribution of missing linker defects, or in other words is the result of an unequal distribution of linker defects within the framework.” Vacant sites on nodes that expose metal atoms are present in some defect-free MOFs and in MOFs that are missing linkers. These are Lewis acid sites that can bond to reactants and lead to catalysis. Progress has been made in understanding the nature of such defects and counting them, so that the catalytic activity can be correlated with the number of defects. An early attempt was made by the De Vos group,112 who used CD3CN chemisorption to probe the number of Lewis acid sites generated by high-temperature vacuum treatment of UiO-66. These researchers found that the catalytic activity for cyclization of citronellal to isopulegol (measured by conversions of reactants) correlated well with the number of Lewis acid sites. Other examples illustrating this point were reported by the group of Farha and Hupp,153 who tested a broad range of MOFs with various numbers of defect sites on Zr6O8 nodes (Table 2). The numbers of vacancies on nodes that were defect sites in UiO-type MOFs were counted by a potentiometric acid-base titration method which considers the protons (M-OH) present in the nodes of defective UiOtype MOFs and uses NaOH as a titrant to quantify them; the numbers of vacancies on Zr6O8 nodes in the structurally related MOFs NU-1000 and MOF-808 were calculated from their crystal structures (because the vacancies are part of the structures). The results demonstrate that the activity (measured as conversion of reactant in an epoxide ringopening reaction) increased linearly for each reaction with an increasing number of vacancies on Zr6O8 nodes in the MOFs given in Table 2. These results provide clear evidence that these are active sites for these reactions. These Lewis acid sites are among the best understood catalytic sites in MOFs. However, an understanding of how reactants interact, bond, and react with MOFs is still incomplete, and so is evidence of catalytic reaction

linker strategy to create large pores by burning off those linkers having low stabilities, thereby making UiO-66 with pores having diameters of even 10 nm. In both cases, the catalytic activity for large reactants increased as a result of incorporation of larger pores in the MOF. Again, the results fall short of assigning the transport limitations in a fundamental, quantitative way.

4. IMPORTANCE OF MOF SYNTHESIS AND THE CHALLENGE OF PREPARING MOFS WITH IDENTICAL STRUCTURES AND DIFFERENT PARTICLE SIZES Some MOFs present a challenge for the determination of particle size effects in catalysis, because the MOF structure (e.g., the density of defects and the degree of crystallinity of MOF particles) may change during the synthesis as particle sizes increase. Thus, various samples of a given MOF may not have the same structures and intrinsic catalytic activities. For example, a method for modulation by monocarboxylic acids or HCl for defective MOFs such as UiO-66 was reported to give samples with lower densities of defects and larger crystals when the MOFs were synthesized at 220 °C rather than at lower temperatures.145 It was also reported that larger crystals with more defects could be generated when more modulators were added to the synthesis mixture at 120 °C.146 Recall from Table 1 that defect sites are the catalytic sites for some reactions, exemplified by ethanol dehydration catalyzed by UiO-66.110 There is a need for synthetic methods to make MOFs with equivalent compositions and structures and particles of various sizes; such syntheses may be challenging, and we recommend work to evaluate them. 5. MINORITY AND DEFECT STRUCTURES IN MOFS AND THEIR IMPORTANCE IN CATALYSIS Identification and counting of active sites are central challenges in catalysis, and the challenge extends to MOFs because of the nonuniformities in their structures. The major catalytic sites in some MOFs are straightforwardly determined, as in MOFs with metal atom nodes, exemplified by copper and iron (HKUST-1 and MIL-101)147,148 that are coordinatively unsaturated.149 However, some MOFs have catalytic sites that are defects in the crystalline structures: for example, vacancies such as node sites that are not bonded to linkers (Figure 5). In addition, linkers may be missing (as determined in early research on the basis of BET surface area measurements150). Some MOFs (e.g., NU-1000 and MOF-808) intrinsically have node bonding sites that are not occupied 1784


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ACS Catalysis Table 2. Estimated Missing Linkers in UiO-Type MOFs As Determined by Potentiometric Acid−Base Titration and Activities for Styrene Oxide Ring-Opening Reaction with Isopropyl Alcohola MOF catalyst

no. of vacancy sites (per node)

rel reaction rate

conversion (%)

Zr-PCN-57 Zr-UiO-66 Zr-UiO-67 Zr-NU-1000 Zr-MOF-808 Hf-PCN-57 Hf-UiO-66 Hf-NU-1000 Hf-MOF-808

0 1.75 1.75 4 8 0 1.8 4 6

∼0 1 0.9 1.3 1.9 ∼0 1.0 1.3 2.0

5 40 34 90 100 5 41 86 100

Figure 6. Removal of the trifluoroacetate ligands on node defect sites by thermal activation. Reproduced with permission from ref 112. Copyright American Chemical Society, 2013.

structure, as shown by IR spectra, XRD crystallography data, and BET surface areas.110,157 Such modifications of MOF node defects can markedly influence the catalytic properties. Hupp, Farha, and coworkers117 showed that, after treatment under high vacuum at 300 °C, the activity of NU-1000 for hydrolysis of the nerve agent simulant DMNP became 10 times greater than that of the original MOF. A possible explanation for the observation is that ligands were removed from the nodes in the treatment, opening bonding sites for reactants. Accordingly, it is inferred that acetate ligands on UiO-66 nodes temporarily blocked the active sites for ethanol dehydration, because as they were removed by reacting with ethanol to form ethyl acetate, the catalytic activity increased (Scheme 1).110 Methods for quantifying the number of defects in MOFs include the aforementioned TGA and analysis of digested MOFs by 1H NMR spectroscopy.110,146 These methods account for both missing linker and missing node defects, but distinguishing them quantitatively is still challenging. Results from Lillerud’s group146 show that the number of defect sites generated by modulation is controlled by the pKa of the modulator and its concentration. The lower the pKa and the greater the concentration of modulator, the higher the number of defect sites. Recent results from Lamberti’s and Bordiga’s groups further demonstrate that there might be an upper limit of the number of defect sites in UiO-66. They showed that when the samples were synthesized with various amounts of benzoic acid (BC) modulator, with 1,4benzenedicarboxylic acid (BDC) or 1,4-naphthalenedicarboxylic acid (1,4-NDC) as a linker, the maximum BC/BDC or BC/1,4-NDC ratio determined by 1H NMR spectroscopy corresponded to 3.4 and 2.3 defect sites per Zr6O8 nodeand attempts to make the UiO-66 samples with even higher defect site densities failed to form precipitates of MOF powders because the MOF structure was not stable.155,158 Understanding of these sites facilitates the quantification of catalytic performance in terms of turnover frequencies. A few researchers have reported turnover frequencies of MOFcatalyzed reactions,159 but often questions have remained about whether the sites were identified and counted accurately, whether they were all equivalent, and whether they were all fully accessible. Typical MOF nodes are metal ions or small metal oxide clusters, and these can be modified and functionalized to give new catalytic sites. Sulfate groups have been added to the vacancies of metal oxide cluster nodes: for example, to generate strong Brønsted acid sites, which have high activity for acidcatalyzed reactions such as isomerization of methylcyclopen-

a

The conversion of styrene oxide was after 24 h reaction in a batch reactor. Reproduced with permission from ref 153. Copyright Royal Society of Chemistry, 2016.

intermediates. We foresee many opportunities to investigate these points by spectroscopy complemented by DFT calculations in experiments done with MOFs having welldefined structures. In recent work with the MOFs UiO-66, UiO-67, and NU1000 incorporating Zr6O8 nodes for ethanol dehydration catalysis, DFT calculations were used to infer that the reaction involved adjacent ethoxy-capped sites on nodes, but the number of these sites was not determined, because missing linker sites were randomly distributed in the MOF particles, and there is insufficient information to determine how many of them were adjacent to each other.110 We posit that there is much to learn about the interactions of various reactants with the node vacancies and the mechanisms of reactions on them. Methods of controlling the numbers of defect sites in MOFs include modification of the MOF syntheses by use of modulators. For example, in the synthesis of MOFs with Zr6O8 nodes, a monocarboxylic acid (e.g., formic acid, acetic acid, benzoic acid, and trifluoroacetic acid) or HCl is added as a modulator to compete with organic linkers to form ligands that bond to the nodes to form defects.90,146,154,155 Organic solvents used in the syntheses (e.g., dimethylformamide) and their decomposition products (e.g., formic acid) can also bond to the nodes as ligands and thereby contribute to the formation of defect sites.110,146 The molecular structures at these defect sites can sometimes be tuned by postsynthesis treatments. One method is a treatment at a high temperature, 300−320 °C, under high vacuum, which leads to removal of the original ligands at these sites as the defect sites on MOF nodes are opened to form Lewis vacancy sites (Figure 6).112,117,156 However, the chemistry of the conversions is not simple, and they can lead to changes in node structures and possibly even crystal structures of the MOFs. For example, Zr6O8 nodes have been shown by EXAFS spectroscopy to be converted to Zr6O6 on treatment under high vacuum at 300 °C, but the properties of the new nodes remain to be elucidated.156 Other methods for modification of these sites require only mild conditions. For example, the ligands bonded on defects on these nodes can be switched from carboxylates to alkoxy groups and further to hydroxyls by applying appropriate treatments;157 the conversions can be done at temperatures less than 120 °C, and they do not lead to substantial changes in the local molecular 1785


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Scheme 1. Topology Change on Missing Linker Site of UiO-66 Nodes with the Initial Ligands on the As-Synthesized Samplea

a

Reproduced with permission from ref 110. Copyright American Chemical Society, 2018.

tane.122 The OH groups on node defects are sites at which reactive metal complexes can be bonded to form supported metal complexes: for example, of vanadium,118,160 copper,161 molybdenum,120 nickel,119,162 rhodium,121 iridium,100,163,164 and others. The metal sites are catalytic sites for reactions such as hydrogenation,100,119,157,163,164 dehydrogenation,165 and oligomerization,99,166 and they are easily counted atomically dispersed catalysts. Debate continues about the compositions and structures of Zr6O8 nodes incorporating hydroxyl groups. Many researchers still work from the hypothesis that these hydroxyl groups are terminal, with water molecules hydrogen-bonded to them (Scheme 2).167−169 However, recent results110 have provided

metal complexes can be bonded) and sites for Brønsted acid catalyzed reactions.157 There is no doubt that these terminal OH groups on the node vacancies are Brønsted acids. However, there is a lack of reports clearly demonstrating that these sites are catalysts for specific reactions. Complicating matters, there are other types of hydroxyl groups on the MOF nodes that exist to balance charge: for example, μ2-OH groups in MIL-53 (Al) and μ3-OH groups in UiO-66. They are Brønsted acids as well, but we have found no examples showing that specific OH groups are catalytic sites for any reactions. Little research has been done to target these groups and their catalytic properties. The accessibilities of these hydroxyl groups to reactants (or to metal complexes to bond to them to make anchored catalysts) are not yet understood. Further, the ion-exchange properties of these groups remain to be explored. Thus, there are ample opportunities to elucidate the chemistry of these nodes in more depth. The sites mentioned in the preceding paragraphs can be counted in various ways: for example, by the aforementioned TGA and 1H NMR methods and by adsorption of molecular probes. We posit that it is important to quantify the nature and accessibility of these sites before reporting TOF values. In addition, because there are usually ligands bonded to Lewis acid node defect sites, it is also important to determine how these ligands affect catalytic reactions on the nodes. In addition to the sites that are present on nodes, catalytic species in MOFs include those included in the pore structure that are not part of the MOF itself. For example, small metal clusters, Pd4 and Pt2, have been formed within MOF frameworks during synthesis and found to be catalytically

Scheme 2. Originally Proposed Defect Structure for Zr6O8 MOFs, Which Recent Work Has Shown To Be Different, As Discussed in the Main Text

evidence that the IR band at 2745 cm−1 should not be attributed to the hydrogen-bonded structure but instead to formate groups on the defects, remaining from the synthesis. Other results provide evidence that the hydroxyl groups are single terminal OH groups on defect sites (i.e., sites where

Figure 7. (left) Conversion (%) in a flow reactor (TOS is time on stream) observed for platinum (mostly present as nanoparticles) in UiO-67. Pt/ UiO-67 catalyst under reference conditions after preactivation at 350 °C (1 h) in flowing argon with 0, 3, and 10% H2 (50 mL min−1 total flow rate). (right) Respective CO and CH4 selectivities. The conversion and CO selectivity observed for Pt/SiO2 are denoted by green triangles. Reproduced with permission from ref 116. Copyright American Chemical Society, 2017. 1786


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Figure 8. (A) Activities of various catalysts for ethanol dehydration to form diethyl ether at 473 K and 1.0 bar in a flow reactor: UiO-66 (HCl), black; UiO-66 (acetic acid), red; γ-Al2O3, green; ZrO2, blue. (B) XRD patterns characterizing samples of UiO-66 (HCl) and UiO-66 (acetic acid) (black) and these samples after ethanol dehydration catalysis at 473 K in a flow reactor for 20 h (red). Reproduced with permission from ref 110. Copyright American Chemical Society, 2018.

whether impurities accumulated or deposits formed in the catalyst that would be challenging to remove and could limit catalyst lifetime. In another investigation,126 a Ni@MOF-5 catalyst was tested for CO2 hydrogenation in experiments with 200 mg of catalyst operated at 1 atm and 280 °C with a feed reactant ratio of H2:CO2 = 4:1 (molar) and a gas hourly space velocity of 2000 h−1. The conversion of CO2 and the selectivity for methane formation were both stable for 100 h on stream. However, the authors did not report whether the catalyst structure was changed after this period of operation. Hupp and Farha’s group119 reported a Ni/NU-1000 catalyst synthesized by atomic layer deposition that was stable for ethylene hydrogenation for 20 days at 100 °C, as shown by the lack of change in the crystal structure. The results verify that ethylene and H2 are reactants that do not react with the MOFs and cause them to unravel. Thus, one could anticipate that numerous MOFs would offer stabilities for various hydrocarbon reactions exemplified by isomerization and hydrogenation, whereas, in contrast, oxidations are not expected to offer comparable opportunities because of the susceptibility of the linkers to destruction by burning. Compounds that react to break node−linker bonds are also expected to jeopardize MOF structures in catalysis. This point was tested in the reaction of ethanol to form diethyl ether and water catalyzed by UiO-66, UiO-67, and NU-1000 (Figure 8).164 These catalysts all gradually decomposed in the presence of ethanol at 250 °C. UiO-66 was found to be the most stable among these catalysts, and lowering the reaction temperature to 200 °C slowed the decomposition. The disintegration of the MOFs resulted from the reaction of ethanol with the node− linker bonds, breaking them and forming esters. In general, reactants and products that reverse the reactions that form node−linker bonds in MOF syntheses are expected to lead to MOF decomposition. In summary, the stabilities of MOFs as catalysts are influenced not only by their intrinsic resistance to decomposition but also by their reactivities with reactants such as O2 and compounds that break metal−linker bonds. Compounds such as hydrocarbons, H2, and inorganic gases seem to be some of the best reactants in prospect for stable MOF catalyst

active and selective for carbene-mediated reactions, HCN production, CO2 methanation, etc.170,171 Nickel oxo clusters having the structure Ni4OxHy were reported to be generated and to bridge the Zr6O8 nodes of NU-1000 as a result of atomic layer deposition, drawing the zirconia nodes closer together and increasing the resistance to sintering of these clusters during the catalytic hydrogenation of ethylene.172 Keggin polyoxometalates (POM), for example, H3PW12O40 and H4SiW12O40, have been incorporated in MOF-101 and HKUST-1 to add Brønsted acid sites to catalyze 5hydroxymethylfurfural (HMF) synthesis and styrene oxide alcoholysis.127,128,173 Such modified MOFs markedly expand the catalytic properties of the materials, but the structures and surroundings of the included structures are for the most part not fully known.

6. STABILITY LIMITATIONS INTRINSIC TO MOFS THAT ARE IMPORTANT IN CATALYSIS The stabilities of MOFs under nonreaction conditions have been investigated extensively and reviewed.44,174 Especially important for this essay are the stabilities of MOFs during catalysis. There are only a few examples addressing this topic. Olsbye and co-workers116 synthesized samples of platinum supported on UiO-67 and tested them as catalysts for CO2 hydrogenation to form CO and methane (with the water-gas shift reaction taking place) with a H2/CO2 molar ratio of 6 and an inverse space velocity of 0.01 g of catalyst min mL−1 at 240 °C. Conversion data (Figure 7) show that the catalyst was stable for 8 h under these conditions, and XRD and BET data characterizing the used catalyst do not show significant changes in the MOF structure. Extensive characterization of the catalysts with spectroscopic and microscopic methods showed that the state and particle size of the platinum in the MOF pore structure changed during catalyst activation and operation, but an important point is that the MOF framework stood up to the relatively high temperature. This work is a valuable demonstration of the potential applicability of UiO66/67 under rather severe conditions and commends it as a candidate catalyst and catalyst support. However, the period of testing was far shorter than would be needed for most industrial applications, and there is a lack of evidence of 1787


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Ferey’s group179,180 compared the Brønsted acidity of OH groups formed from various grafted species (CH3OH, H2O, CF3CH2OH, (CF3)2CHOH) on MIL-101(Cr) with that of silica and various zeolites in terms of the CO adsorption (Figure 9). The results demonstrate that the Brønsted acidity

operation. The dynamics of MOF structure changes during catalysis are not well understood, and there is a need for work to investigate how various reactants and products interact with the node−linker bonds in various MOFs to change the MOF structures and limit their lifetimes in catalysis.

7. KINETICS OF MOF-CATALYZED REACTIONS Understanding of catalysis often benefits from knowledge of the kinetics of the reaction, and the kinetics data that are of most fundamental value are not influenced by catalyst deactivation or transport limitations. Because only little stability testing of MOF catalysts under reaction conditions has been reported and because only little information is available for quantitative assessment of intraparticle transport on rates of MOF-catalyzed reactions, it is not surprising that little in the way of fundamental kinetics of MOF-catalyzed reactions is available. In the work mentioned above in the context of catalyst stability, Olsbye and colleagues116 investigated the kinetics of CO2 hydrogenation to form methane catalyzed by Pt/UiO-67. This work is exceptional in that the authors considered catalyst stability and transport issues and made kinetics measurements; as the field of MOF catalysis emerges, we may expect more investigations of this sort. The data do not indicate a decrease in catalytic reaction rate as the average particle diameter was increased from 0.15 to 2 μm, consistent with the inference that the effectiveness factors were essentially unity for this MOF with pore apertures of 8 Å (although the dependence of rate on particle size is not fully understood). Thus, the authors determined an apparent activation energy for the MOFcatalyzed reaction to be 50 ± 3 kJ mol−1 for the MOFsupported platinum, in good agreement with the value they observed for a Pt/SiO2 catalyst. Thus, this work is one of the few examples of determination of intrinsic kinetics of a MOFcatalyzed reaction, but it is important to realize that the MOF was perhaps only an inert support. In this work, TOF values were not determined, but they would have pertained to the platinum and not the MOF. The authors also determined reaction orders for the CO formation, with the orders in CO2 being 0.45 and 0.5 at 240 and at 260 °C, respectively, and the order in H2 being 0.3 at each temperature. In the aforementioned investigation of ethanol dehydration, the MOFs containing Zr6O8 nodes were found not to be stable enough in 20 h of continuous operation to allow measurement of the kinetics.110

Figure 9. Brønsted acid strength of OH groups from various grafted species on MIL-100(Cr) measured by CO adsorption: correlation between the νOH shifts and the H0 values and the ν(CO) position. Reproduced with permission from ref 179. Copyright American Chemical Society, 2007.

of MIL-101(Cr) results from OH groups of the grafted species on the open sites of Cr3O nodes, and the acid strength is strongly influenced by organic ligands in these structures. Figure 9 shows that MIL-101(Cr) has a slightly higher acid strength than the silica but a much lower acid strength than zeolite HY. The data are reported in terms of H0 values (H0 is the Hammett acidity function), however, and numerous authors have questioned whether such values provide meaningful measures of the acid strengths of solids.181 Vimont’s group182 further characterized the acid sites in MIL-100(Al) with the probe molecule CO. The Lewis acidities of open sites on Al3O nodes of dehydrated MOF samples were characterized by a νCO value of 2183 cm−1, indicating a lower acid strength of the MOF sites than of Lewis acid sites on alumina. Similarly, the Brønsted acid strength of MIL-100(Al) that was measured when water was adsorbed on the open sites of the Al3O nodes is not high, as indicated by the νCO value of 2154 cm−1. The MOF UiO-66, with Zr6O8 nodes, has been characterized by the vibrational frequency of CO adsorbed on its Lewis acid vacancy sites and on Brønsted acid sites consisting of μ3-OH groups, observed at 2180 and 2154 cm−1, respectively,183 and these values are close to those reported for MIL-101(Al). The νCO value characterizing CO adsorbed on Lewis acid sites and on Brønsted acid sites on the zeolite HZSM-5 were reported to be 2222 and 2175 cm−1, respectively, in comparison with values of 2183 and 2158 cm−1, respectively, for t-ZrO2.184,185 The data suggest that the acid strengths of Zr6O8 MOF nodes are close to those of bulk ZrO2 but much less than those of zeolites. 8.2. Electron-Withdrawing Properties and Uniformity of Bonding Sites for Metal Complexes. We probed the electron-withdrawing properties of Zr6O8 MOF nodes by anchoring Ir(CO)2 and Rh(CO)2 complexes to the node sites.121,163 The electronic properties of the site-isolated metal

8. COMPARISON OF MOF CATALYSTS WITH METAL OXIDE CATALYSTS Because of the similarity between MOFs and zeolites in terms of regular pore structures and (to a small degree) their tunable compositions, MOFs are frequently compared with zeolites.175−178 However, in our view, the available data suggest that the properties of at least the MOFs with metal oxide clusters as nodes and to a degree those with metal atoms as nodes are more nearly comparable to metal oxides than to zeolites. 8.1. Acidity. The open sites or defect sites on MOF nodes are typically Lewis acid sites. CO has been used as a probe to determine the acid strengths of these sites. They can be converted to Brønsted acid sites: for example, by reaction with water. 1788


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Figure 10. Correlation between (A) the turnover frequency (TOF) for ethylene hydrogenation and (B) selectivity for ethylene dimerization catalyzed by iridium complexes initially in the form of supported Ir(C2H4)2 with νCO of the Ir(CO)2 complexes formed from the catalysts by replacement of the ethylene ligands with CO (a higher νCO,as frequency indicates that the iridium complex is more electron-deficient). Reproduced with permission from ref 163. Copyright American Chemical Society, 2016.

complexes were indicated by the νCO values. The results demonstrate that the MOF nodes withdraw electrons to a degree that is similar to that observed with ZrO2, and they are much weaker electron-withdrawing agents than zeolite HY (Figure 10). The breadths of the carbonyl bands indicate the uniformity of bonding sites on the materials.121,163 The results show that the bonding sites on the MOF nodes are not as uniform as those on HY zeolite but are similar in uniformity to those on ZrO2. 8.3. Structural Modification. As mentioned above, the ligands on the vacancy sites of MOF nodes can be tuned by postsynthesis modifications; for example, alcohols tend to replace the monocarboxylate ligands originating from modulators (they are desorbed as esters) and form alkoxide species that can be easily replaced by terminal OH groups upon exposure to water vapor.110,157 Formate ligands are reported to be replaced by sulfide ligands in MOF-808 and, upon oxidation, to form −SO4 groups that show high acid strengths (Figure 11).122 We note that similar structures and properties of adsorbed species have been reported for ZrO2,186−188 but there are more bonding modes on the metal oxides than on MOF nodes because of the greater complexity of the surface sites on the metal oxides.

The similarities in chemistry of these MOF nodes and the comparable metal oxides are unsurprising in view of the similar coordination environments of the metal centers. For example, Zr atoms are 8-coordinated in both an ideal crystal of ZrO2 and in the aforementioned MOFs having zirconium-containing nodes; Al atoms are 6-coordinated in octahedral environments in both alumina and the comparable MOFs (as has been determined by 27Al solid-state NMR spectroscopy).189 However, Al is 4-coordinated in a tetrahedral environment in zeolites, and so the properties of zeolites are markedly different from those of the MOFs. There is much more to learn about the comparisons between MOF nodes and metal oxides.

9. COMPARISON OF MOF CATALYSTS WITH SOLID ORGANIC POLYMER CATALYSTS Functional groups on MOF linkers provide excellent opportunities for controlling the catalytic properties of MOFs. They may be catalytic sites (e.g., −SO3H groups on bdc linkers96) or they may influence catalysis on nearby groups: for example, by controlling the compositions of solventlike spheres and the affinity for reactants, products, or other components in a reaction mixture or, alternatively, by being engaged in bifunctional catalysis. Further, some functional groups, such as porphyrins, catechol, and bipyridine, are bonding sites for metal complexes that provide catalytic sites.190−192 Various metals have been bonded to these sites to form MOF-bound single-metal-atom sites that catalyze many reactions.42 These supported metal complexes are analogues of metal complexes in homogeneous catalysis, and they may have greater activities and stabilities when they are supported because their site isolation prevents dimer formation, for example. These functional groups are similarly anchored to organic chains of many porous organic polymers (POPs).193 There are many types of such polymers, including polymers with intrinsic microporosity (PIMs), 194 porous organic frameworks (POFs),195 conjugate microporous polymers (CMPs),196 and porous aromatic frameworks (PAFs).197 These materials, synthesized by various routes from various monomers, are typically less crystalline than MOFs.193

Figure 11. Sulfated MOF-808 with strong Brønsted acidity. Reproduced with permission from ref 122. Copyright American Chemical Society, 2014. 1789


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ethylene,202 but issues of catalyst stability and regenerability remain. 11.2. Dehydrogenation. Dehydrogenation of alkanes is a valuable industrial process for the conversion of shale gas components. In industrial application, direct dehydrogenation of propane takes place at temperatures >500 °C on catalysts such as Pt-Sn/Al2O3 (normally with H2O as a feed component to remove coke formed during the reaction). Such conditions exclude MOFs as candidate catalysts. However, the Hupp− Farha group reported that NU-1000-supported cobalt clusters catalyze propane dehydrogenation at 230 °C in the presence of oxygen,165 with the catalyst demonstrating stable operation for 20 h. This is an encouraging step forward, but XRD and N2adsorption data showed some decomposition of the NU-1000 in operation, and so challenges remain. 11.3. Epoxidation. Epoxidation is another important industrial process. Hupp and Farha’s group120 reported that NU-1000-supported molybdenum oxide catalyzed epoxidation of cyclohexene with O2 at 60 °C, with the activity of the MOFsupported catalyst being similar to that of a ZrO2-supported catalyst. Dinca’s group64 reported that a Mn-exchanged MOF5 catalyzed cyclopentene epoxidation at room temperature with tBuSO2 PhIO as the oxidant. Industrial ethylene epoxidation with O2 is catalyzed by promoted Ag/Al2O3 at temperatures of 220−280 °C, but we do not know of any MOF-supported silver catalysts that can catalyze this reaction. 11.4. Oligomerization. Olefin oligomerization processes catalyzed by soluble or supported metal complexes or acids are important: for example, for making monomers, polymers, and motor fuel components. Some MOFs are active for these reactions. Canivet et al.203 reported that MIL-101(Fe)anchored nickel complexes are selective for ethylene dimerization to form 1-butene, and the selectivity was higher than that of a molecular nickel diimino complex (the reactions were performed in n-heptane in the presence of Et2AlCl under 15 atm of ethylene at 10 °C). Dinca’s group99 showed that single Ni sites could be incorporated in MFU-4l (MFU-4l = Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b], [4′,5′-i])dibenzo[1,4]dioxin) by cation exchange, giving a catalyst having high selectivity for ethylene dimerization to 1butene (96.2%, which the authors claimed to be higher than those for current industrial dimerization catalysts); the reaction was performed at 0 °C and 50 atm. Yaghi’s group204 reported MOF-808-SO4-catalyzed dimerization of isobutylene to isooctene at 80 °C, showing that this strongly acidic MOF is more active, selective, and stable than sulfated zirconia in a 240-h long run. Klet et al.166 reported that Hf-NU-1000anchored organozirconium complexes catalyze ethylene and 1hexene polymerization. These reactions take place under such mild conditions that MOFs may be found to be viable catalysts for them; long-term catalyst testing would be helpful. 11.5. Dehydration/Esterification/Condensation/Hydrolysis. These reactions involve water and are catalyzed by acidic sites, finding numerous industrial applications (e.g., for manufacture of olefins and ethers), with typical catalysts being alumina and sulfonic acid ion-exchange resins. As mentioned above, UiO-66 catalyzes the dehydration of ethanol to diethyl ether at 200 °C. However, formation of olefins by dehydration of alcohols normally requires higher temperatures (>300 °C with Al2O3 or zeolites), and as mentioned above, MOFs lack stability at such temperatures. On the other hand, MOFs have high activities for esterification at lower temperatures (60−80 °C),122,205 and they catalyze condensation206,207 and hydrol-

Notwithstanding their similarities, these two classes of porous materials seem to be developing on separate paths. We propose that comparisons of these types of materials with the same functional groups or metal centers in terms of their catalytic properties may provide insights into optimizing them as catalysts: for example, in terms of placement of complementary catalytic groups near each other and/or accessible to each other.

10. POTENTIAL BENEFITS OF PROTOTYPE MOF CATALYSTS Catalysis science has benefitted from the availability of standard samples to allow comparisons among laboratories and benchmarking of results. The MOFs commercialized by industrial organizations are readily available from well-known suppliers and are good candidates for development as prototype catalysts. However, it is difficult to tell from the web sites of suppliers how consistently prepared the samples are, because the reported BET surface areas cover large ranges. If would be helpful if the suppliers would provide other characterization data besides BET surface areas and XRD patterns, such as, for example, TGA, IR, and 1H NMR data. It would also be helpful for researchers to share samples from large, well-prepared batches of MOF catalysts. 11. REACTIONS CATALYZED BY MOFS AND PROSPECTS FOR APPLICATIONS MOFs have been shown to catalyze many reactions; some are summarized in Table 1. A number of these reactions take place under mild conditions but lack potential for large-scale applications because the reactants are more valuable than the products; examples include ethylene/hexene/octene hydrogenation and epoxide ring-opening reactions with CO2 or alcohols. These reactions have been valuable as test reactions used to determine correlations between catalyst structure and performance. Although the MOF literature has raised hopes of large-scale catalytic applications of MOFs, enticing members of the research community with reports of MOFs catalyzing challenging reactions, enthusiasm in the industrial sector is muted. Here, we mention some industrially important and tantalizing reactions that have been catalyzed by MOFs, pointing out some hurdles that need to be cleared before applications emerge. 11.1. Hydrogenation. 11.1.1. CO/CO2 Hydrogenation. CO hydrogenation is extremely important in technology, producing hydrocarbons, alcohols, and other oxygenates; CO2 hydrogenation lacks industrial applications but is a hot topic in catalysis research because economical processes for CO2 conversion could help reduce accumulation of this greenhouse gas in the atmosphere. We mentioned MOF catalysts for this reaction above,116,126 and there are others,198−200 but demonstrations of sufficient catalyst stability and economic viability are lacking. 11.1.2. Alkyne Hydrogenation. Selective hydrogenation of alkynes to alkenes is an important industrial process to purify ethylene and propylene. Lu’s group201 reported that Rh-Ga bimetallic sites supported in NU-1000 catalyzed a family of acyclic alkynes to alkenes with high stereoselectivity (>99%), and NU-1000 encapsulated Cu nanoparticles were reported to be active and selective for acetylene hydrogenation to form 1790


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ACS Catalysis ysis reactions117 at room temperature. Thus, MOFs offer intriguing prospects for these reactions, but there are not enough data to evaluate their stabilities under potentially realistic conditions. 11.6. Isomerization/Alkylation. These reactions are important in industry, catalyzed by acids. Skeletal isomerization of olefins and paraffins, for example of 1-butene to isobutylene, typically takes place at temperatures too high for MOFs (e.g., >400 °C). However, cyclic hydrocarbons take place at temperatures that are lower, and MOFs offer some prospects; for example, MOF-808-SO4 catalyzes methylcyclopentane conversion into various hydrocarbons at 150−200 °C, but the stability of the catalyst was not determined.122 Further, NU-1000-supported Keggin polyoxometalate clusters catalyze the isomerization of o-xylene to form m-xylene at 250−300 °C, but the catalyst is not stable.208 Double-bond isomerization is more facile, and MOF-808-SO4 exhibited high activity for limonene isomerization at 60 °C, but data were obtained only for short times.122 In industry, alkylation of paraffins with olefins (or with benzene) is catalyzed by strong acids, such as H2SO4 and HF (in liquid-phase reactions), or zeolites (with gas-phase reactants at temperatures typically >320 °C, which evidently exclude MOFs as candidate catalysts). However, alkylations involving alcohols and benzene take place at lower temperatures, and MOFs such as MIL-53(Al), MIL-88(Fe), and MIL-101(Fe) were shown to have high activity and selectivity, with stability at 100−200 °C for limited periods (14 h).209,210 11.7. Challenging Reactions. Catalysis researchers have turned to MOFs as candidate catalysts for some challenging reactions that could have great technological impact, such as methane conversion to methanol. Some researchers have thought of MOFs as potential mimics of enzymes for such reactions. Progress in this direction was reported by the group of Lercher,161 who used NU-1000 supported Cu-oxo clusters to convert methane to methanol with selectivities of up to 60%. The conversion was achieved in three steps: (a) the MOF was activated in O2 at 150 °C for 1 h, (b) the MOF was loaded with CH4 at 150 °C for 3 h, and (c) the MOF was exposed to water, leading to the desorption of methanol at 135 °C for 30 min. Much work would be needed to translate these results into an economical catalytic process. The group of Farha211 reported that iridium complexes anchored in UiO-67 are active for methane borylation with high selectivity at 150 °C and 34 atm of methane (the number of catalytic turnovers was 67). Long’s group149,212 reported the oxidation of ethane to ethanol (and acetaldehyde) by N2O with the MOF Fe2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate) and, alternatively, its magnesium-containing analogue, Fe0.1Mg1.9(dobdc). This reaction was also achieved in three steps: (a) the MOF was treated with N2O at 60 °C to form a high-spin iron(IV)-oxo species, (b) this was exposed to a stream containing N2O and ethane at 75 °C, and (c) products were extracted with a solvent. These examples give indications of the rich catalytic chemistry of MOFs; they are encouraging but only preliminary, and extensive work would be needed to discover whether there are catalytic cycles involving MOFs that are active, selective, stable, and regenerable for these challenging reactions; the prospects motivate fundamental research to better understand the catalytically active sites and methods to design and synthesize them. We also note that the products of these

reactions, methanol and ethanol, tend to dissociate some MOFs, as discussed above. 11.8. Summary. MOFs offer rich new catalytic chemistry, already having been shown to catalyze a broad class of reactions including many of industrial importance. Because the reaction conditions, such as temperature and the nature of the reactants and products, are crucial to determining the viability of MOFs as practical catalysts, it is evident that not only their activities and selectivities but also their stabilities are important. The issues of MOF catalyst stability need to be addressed going forward. The highest temperature reported for catalysis in the examples mentioned in this section is 280 °C, which we might suggest to be an approximate upper limit of what MOFs can tolerate under reaction conditions. The effects of reactants and products, such as oxygen and oxygencontaining compounds, have not been determined in any depth, and they are essential for assessment of the stabilities of MOFs as catalysts. Coking is a general problem for all of the hydrocarbon conversion reactions (among others)and it will lead to deactivation of MOF catalystsespecially in reactions of unsaturated hydrocarbons and reactions requiring strong acidity and those involving highly reactive metal species. Because MOFs cannot be regenerated at high temperatures, deactivation caused by coking will be fatal. In our opinion, applications of MOFs as catalysts are likely to be restricted to low-temperature applications, to reductive rather than oxidative conditions, and to hydrocarbons rather than oxygen-containing compounds.

12. CONCLUSIONS AND RECOMMENDATIONS FOR RESEARCH Notwithstanding the enormous opportunities afforded by MOFs for new catalysts with subtle, multifunctional catalytic properties, there are numerous open questions about what the MOFs are and how they work as catalysts. Although many MOFs have well-defined bulk structures, the sites in them that are catalytically active sometimes remain to be identified: such as, for example, defect sites. There are opportunities to identify such sites and to learn how to control their synthesis. Determination of these sites is part of the foundation needed for quantitative representations of MOF catalyst performance in terms of intrinsic kinetics and turnover frequencies. Moreover, MOFs present unique challenges in terms of stability in catalysis. Although some MOFs have been shown to withstand temperatures as high as 350 °C, it is nonetheless difficult to find applications of MOFs as catalysts at such temperatures, because of their stability limitations and the challenges of regenerating them associated with their organic components, the linkers. In addition, MOFs are held together by node−linker bonds that may be unstable under some catalytic reaction conditions, so that MOFs may decompose during catalysis. There are opportunities to understand the destabilization phenomena better and to find guidelines for synthesis of MOFs that resist decomposition during catalysis. Further, the influence of intraparticle transport phenomena on catalysis by MOFs is usually not understood on a quantitative foundation, and there is a need for research to resolve reaction/transport processes in MOFsand such resolution is also needed for determination of intrinsic kinetics of MOF-catalyzed reactions. There are still needs to discriminate between reactions on the outer surfaces of MOFs and those occurring within the pore structures. Investigations of transport effects in catalysis by MOFs with 1791


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uniform structures should be done with MOF particles of various sizes, pore structures, and compositions to determine how well the reaction/transport processes are represented with simple reaction/diffusion models and how to predict rates. Assessments of how defects affect the rates of transport would also be of value. Investigations of defects in catalysis by MOFs have been addressed mostly in the context of catalysis by Lewis acid sites on MOF nodes. There are opportunities for investigation of other kinds of defect sites, including OH groups (Brønsted acid sites) on defect sites, for example. Moreover, much remains to be understood about the properties of OH groups in various surroundings as catalytic sites in MOFs. In addition, it would be helpful to understand why various single-site metal complex catalysts anchored in MOFs are nonuniform and how to control the degree of uniformity. We foresee opportunities for characterization of MOFs as catalysts (even in the working state) by spectroscopic methods combined with density functional theory. It would be helpful to do in-operando catalysis experiments combined with longterm testing not only to elucidate reaction intermediates and identify catalytic sites but also to gain evidence of how catalyst deactivation takes place. Specifically, experiments are recommended to investigate the defects and catalytic sites as they undergo changes during catalysis. We also recommend further investigations of MOF catalysts for which confinement effects are significant; these would include investigations of smaller-pored MOFs. Related to this are investigations of bifunctional MOFs and the roles of placement of the functions and how they interact with reactants and products (and possibly each other) under reaction conditionsspecifically, how these effects influence rates of catalytic reactions. Moreover, there are opportunities to better understand how MOFs interact with encapsulated catalytic species such as metal clusters; it would be helpful to investigate samples in which the encapsulated species were uniform in size, structure, and placement within the MOF pore structure, but making them presents a challenge.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail for B.C.G.: [email protected]. ORCID

Dong Yang: 0000-0002-3109-0964 Bruce C. Gates: 0000-0003-0274-4882 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We thank our colleagues Christopher Cramer and Laura Gagliardi of the University of Minnesota, Omar Farha and Joseph Hupp of Northwestern University, and Johannes Lercher of the Technical University of Munich and the Pacific Northwest National Laboratory for helpful discussions and collaborations as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (DE-SC0012702), which supported this work. 1792


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