Defects in MetalâOrganic Frameworks: Challenge or Opportunity

Perspective pubs.acs.org/JPCL

Defects in Metal−Organic Frameworks: Challenge or Opportunity? David S. Sholl and Ryan P. Lively* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, United States ABSTRACT: Metal−organic framework (MOF) materials are nanoporous materials whose crystalline character has made them attractive targets for synthesis of new materials and potential use in a diverse set of applications. The vast majority of studies of MOFs envision these materials as having ideal crystal structures. This Perspective gives an overview of the current understanding of defects in MOFs. Compared to related materials such as zeolites, the ability to detect and control defects in MOFs is nascent. Nevertheless, it is likely that defects will play a vital role in a number of contexts where MOFs are of widespread interest, so advancing our understanding of these structural features will be important in coming years. Potential origins of point defects, plane defects, and surface defects are discussed. The difficulty of defect detection in metal−organic frameworks is discussed and useful paths for future work are provided.

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describing and controlling defects in MOFs is at a nascent stage. The short-term and long-term stability of ordered porous framework materials such as MOFs are emerging as a major issue in their development as successful materials in a wide range of applications.5 Some MOF materials that show good initial performance in target separation or catalysis applications are found to degrade upon exposure to water or acid gases, for example. In some instances, lack of stability can be understood from a thermodynamic perspective.6 In a large number of examples, however, stability (or lack thereof) appears to be dominated by kinetics.7 It seems likely that in many cases, the degradation of materials in the presence of adsorbed species such as acid gases is driven by the reactivity of defects within the porous framework. The large numbers of distinct MOFs that exist have led to effort aimed at systematizing information on their structures and using this information in computational screening of materials for targeted applications.8−11 A common feature of computational efforts to predict properties of MOFs is the assumption that MOFs can be modeled as defect-free materials represented by crystal structures determined experimentally. There has been little work to date to understand how defects in MOFs may affect the predictions of calculations of this kind. The comments above have highlighted two topics where defects in MOFs could diminish their performance or complicate methods to describe them. There are instances, however, where defects in MOFs could have beneficial effects. Deliberate introduction of point defects in nanoporous frameworks can be a useful way to impart catalytic activity into a

OFs are crystalline nanoporous materials synthesized by self-assembly of organic ligands and inorganic clusters. MOFs have been the target of intense activity in the academic chemistry community, leading to the development of a diverse range of materials.1−3 The wide variety of metal precursors and organic ligands that exists allows tuning particular properties of these unique nanomaterials and designing them to have a specific surface area, pore size, and chemical functionality. The flexibility of incorporating functional groups and open metal sites into MOFs allows exceptional tailoring and tuning of the chemical and physical properties of the materials for targeted adsorption interactions.

The likely ubiquitous nature of defects in MOFs implies that efforts to systematically understand them will yield considerable benefits in controlling material properties. As was aptly captured by Colin Humphreys, “Crystals are like people: it is the defects in them which tend to make them interesting!”4 Although the vast majority of the rapidly growing literature on MOFs and related materials focuses on the ordered crystal structure of these materials, even the most carefully synthesized MOFs must contain a variety of defects. In some applications, these defects may be of little consequence. However, in other situations defects may control the overall performance of a material. The likely ubiquitous nature of defects in MOFs implies that efforts to systematically understand them will yield considerable benefits in controlling material properties. Despite this observation, work on © 2015 American Chemical Society

Received: May 29, 2015 Accepted: August 13, 2015 Published: August 13, 2015 3437

DOI: 10.1021/acs.jpclett.5b01135 J. Phys. Chem. Lett. 2015, 6, 3437−3444

The Journal of Physical Chemistry Letters

Perspective

MOF.12,13 There are limited examples where framework defects allow facile adsorbate diffusion in materials where onedimensional pores might otherwise strongly limit pore access.14,15 In these cases and others, detailed knowledge of the mechanisms controlling formation and stability of defects would aid use of these phenomena in a useful way. In this Perspective, we offer a brief review of the current understanding of defects in MOFs and use this review to suggest some useful paths for future work. One aim of this article is to draw analogies between defects in MOFs and well-known defect structures in zeolites. By identifying these analogies, we hope to point out that the zeolite research community has developed a deep understanding of defects that should benefit the nascent area of defects in MOFs. This article also seeks to provide a snapshot of what is currently known about MOF defect structures, methods to detect defects in these materials, and the ramifications of these defects in MOF applications. Throughout this Perspective, we use the term defect in the broadest possible sense to encompass any deviations from an ideally ordered crystal structure. Defects in Zeolites: A Roadmap for Understanding Defects in MOFs. Before considering the topic of defects in MOFs, it is useful to briefly examine what is known about defects in zeolites, a topic that has been studied for many years. Thinking about defects in MOFs in this context is helpful for making several points that are perhaps obvious to people that have worked on zeolites for many years but are not always widely appreciated in the MOF community. First, defects in MOFs, as in zeolites, are ubiquitous. Second, multiple classes of defects exist in these materials. Third, the presence of defects in zeolites and MOFs can have a decisive impact on material performance. Finally, understanding of defects in these materials requires careful characterization, often with multiple techniques, and is a topic with a great deal of depth and potential for impact. The catalytic properties of zeolites can be viewed as a direct consequence of point defects, because the presence of acid sites and other catalytic sites stems from the existence of framework substitutions of Al or related atoms for Si atoms in the silica frameworks that make up zeolites. In “traditional” aluminosilicates (such as Type A, X, and Y zeolites), Al atoms are not considered point defects because Al sites are part of the stoichiometric composition of the material. Partial dealumination during processing, however, certainly creates point defects in these structures. However, in high-silica zeolites, Al and other p-block metal atoms (e.g., Ga) can be reasonably described as point defects in the framework. An important direction in recent work on zeolite synthesis has been to control the spatial distribution of framework substitutions among the inequivalent sites that exist in high-silica zeolite crystals.16,17 The distribution of point defects in a zeolite (high-silica or otherwise) does not need to be uniform. Recent synchrotron-based micro-XRD analysis of large high-silica MFI crystals showed that the Al density in these crystals varied by approximately a factor of 15 as a function of depth in the crystal.18 Aluminum zoning, which can be thought of as “compositional” disorder, of this kind could have important implications for the catalytic efficiency of materials in which it occurs. Point defects in zeolites can also exist in the form of vacancies. The zeolite catalyst SSZ-74, for example, has been found to have ordered Si vacancies in its structure,19 and Si vacancies have been implicated in the catalytic activity of TS-1.20 Oxygen vacancies in zeolites have been shown to be photoluminescent.21 Vacancy complexes can

also exist, as in the formation of ordered Ge4O4 vacancies has been demonstrated in the germinate zeolite PKU-1.22 These kinds of vacancy defects are perhaps more relevant to MOFs, because they have direct analogies in MOFs in the form of missing linkers or metal sites. The existence and implications of extended defects in zeolites have also been extensively studied. A particularly simple example of an extended defect is a grain boundary, a 2D interface that forms between two crystals of a material that are each (nominally) ideal crystals but have different spatial organization. Grain boundaries are known to form in large zeolite crystals that appear to be single crystals from a macroscopic perspective. An especially clear example of this phenomenon is the large “coffinshaped” crystals of silicalite that are known to contain multiple distinct grains separated by grain boundaries.23,24 Extended defects can also form in zeolites due to coexistence of multiple local crystal structures. At a fundamental level, it should not be surprising that these defects exist. Each zeolite structure is one of many polymorphs of silica, so crystallization of zeolites can in principle allow coexistence of multiple crystalline phases, in addition to the possibility of distinct grains within a crystal separated by grain boundaries, dislocations, and so forth. It is worth noting that the polymorphic nature of silica is not restricted to simple crystalline phases; a quasicrystal form of silica was reported by Xiao et al. in 2012.25 Intergrowths of zeolite phases have been observed with transmission electron microscopy (TEM) for over 30 years, and early examples of this phenomenon were shown to have important implications for the diffusion reactions and products in zeolite L when it was used as a catalyst.26 TEM remains a vital tool in probing extended defects in zeolites. To give several recent examples, TEM has been used to probe intergrowths of ZSM-12, SSZ-59, and SSZ-61, which have the same layered structure but differ in the connections between layers,27 to understand the stacking disorder of three polymorphs that complicate the structure of ITQ-39 (Figure 1),28 and to examine the role of intergrowths of MFI and MEL in the synthesis of hierarchical zeolites based on the MFI structure.29 Atomistic modeling of extended defects

Figure 1. HRTEM structure projection of ITQ-39 highlighting the stacking disorder of three ITQ-39 polymorphs (“A”, “B”, “C” in the image). The inset is the corresponding selected-area electron diffraction pattern. Reproduced with permission from ref 28. 3438



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in zeolites is challenging,23 but modeling of this kind has been useful in aiding analysis of powder XRD patterns to assess the density of stacking faults in intergrowths of SAPO 18 and SAPO 34.30 We emphasize that this extremely brief overview of defects in zeolites serves primarily to indicate that much work has been done on this topic and to illustrate several types of defects that are commonly observed in zeolites. Below, we comment on the analogies between these well-studied phenomena and the appearance of defects in MOFs. Classes of Defects in MOFs. The discussion of zeolites above has already introduced two broad classes of defects that can exist in nanoporous crystals: (i) point defects and (ii) extended defects. Point defects in MOFs can arise from vacancies associated with either metal centers or organic ligands. Extended defects are one or two-dimensional defects associated with imperfections in crystal structure. Although extended defects can exist in even the simplest of nature’s crystal structures (e.g., elemental metals), the observation that many MOFs can exist in multiple polymorphs with similar overall energies creates an additional means by which extended defects can form.31,32 Point Defects in MOFs. There are two main varieties of point defects in metal−organic frameworks: vacant ligands and vacant metal centers. These defects are analogous to oxygen vacancies and metal/metalloid vacancies, respectively, in zeolites. These sites can be introduced intentionally to improve an aspect of the MOF’s performance (e.g., catalytic activity),12 but they likely also occur naturally in neat MOF crystals. Whether occurring naturally or induced intentionally, there are several probable points of origin for these vacancy defects. Rapid crystallization times (e.g., occurring in nonsolvent-induced crystallization) likely “lock in” crystallization products with incomplete assembly that are intrinsically more defective than crystals synthesized and quenched more slowly.33 Ligand vacancies can also occur when crystallization modulators are employed during MOF synthesis.34 Using inelastic neutron scattering, Wu et al. demonstrated that modulators can bind preferentially with metal centers in nascent UiO-66 crystals, resulting in random ligand vacancy defects.35 These defect-inducing modulators can be removed postsynthesis via thermal activation to expose open metal sites that are Lewis acids.36 It is important to note that not every modulator interacts with metal centers. Metals or metal centers with low coordination numbers seem especially resilient to the formation of ligand vacancy defects (a zirconium metal center in UiO-66 is connected to 12 benzene-1,4-dicarboxylate ligands, whereas a copper metal center in Cu-BTC is only connected to four benzene-1,3,5-tricarboxylic acid ligands).35 Missing ligand defects can be intentionallyif somewhat randomlyremoved via postsynthetic acid or base treatments of the MOF, increasing the reactivity, surface area, and porosity of the material but also typically introducing extraframework cations.37 Binding errors between ligand and metal can also result in the formation of missing metal site or metal center defects, although these defects have proven more difficult to quantitatively detect. Indeed, the origin of missing metal center defects is somewhat of an enigma relative to missing linker defects. The most prevalent hypothesis for these defects is the presence of ligand fragments (e.g., a ditopic ligand instead of a tritopic ligand) in the crystallization liquor; undercoordinated metal centers or missing metal centers can result depending on the ligand fragment concentration.12,38−40 In a detailed study of UiO-66(Hf), Cliffe, Goodwin, and co-workers attributed diffuse scattering profiles in powder X-ray diffraction patterns

(previously attributed to guest solvent molecules) to metal center vacancies with short-range correlation.41 A key finding of this work is the propensity for one metal-center vacancy to apparently propagate a series of vacancy defects in a nanosized region. Quantum mechanical and density functional theory calculations suggest that under-coordinated metal centers can form a complementary but miscible UiO-66(Hf) phase; the modulator may assist in the formation of these undercoordinated metal center phases. This separate phase can crystallize within nascent MOFs over nanosized length scales, thus instilling the final UiO-66 bulk crystal with metal center vacancies. These under-coordinated binding defects have also been detected in MIL-47(V), where certain vanadium sites exhibit square pyramidal binding as opposed to octahedral binding.42 Point defects in MOF structures can have pronounced effects on the catalytic activity and separation performance of these materials. There are several examples in the literature of missing linker vacancies boosting the reactivity of CO inside MOFs.40,43 Increased CO2 uptakes and mesopore formation have also been noted from linker vacancies; both of these have implications for membrane and sorption processes as the mesopores can provide low-resistance diffusion pathways through MOF crystals.44,45 In general, guiding principles for the nature and concentration of defect sites and their relationship to deviations in performance from ideal MOF crystals have not yet been developed.

In general, guiding principles for the nature and concentration of defect sites and their relationship to deviations in performance from ideal MOF crystals have not yet been developed. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs that exhibit excellent chemical and thermal stability.46 ZIF apertures are generally smaller than typical MOF materials and can exhibit high diffusive selectivities for molecules of similar size (e.g., propylene and propane). 3 Considering the strong analogies that exist between the crystal structures of ZIFs and the structure of zeolites,46,47 they are a particularly useful subclass of MOFs with which to consider the analogies that exist between defects in zeolites and defects in MOFs. Point defects in ZIFs, although not directly detected to date, can be expected to be either missing metal atoms or missing imidazolate ligands. The former will likely result in termination of open metal sites with radicals or modulators in the crystallization liquor. The latter will likely result in “amide nests” (analogous to “silanol nests” in silicalite-1 synthesized in alkaline media48,49) from four protonated imidazolate ligands. Generally, there is indirect evidence for the existence of point defects in ZIFs, but unlike the parent MOF materials, no quantitative description of the chemical nature or structural location of the defects has been discovered. The strongest evidence to date for the existence of defects in ZIFs comes from measurements of water adsorption in ZIF-8. To interpret these experiments, it is helpful to first consider the adsorption of water in pure silica zeolites. Pure silica zeolites are expected to be highly hydrophobic, and measurements of the isosteric heat 3439



Perspective

unit cells at the crystal surface.55 Although the implications of this collapse have not been quantitatively shown, it is easy to imagine the deleterious effects collapsed surface pores would have on catalytic, membrane, and adsorption processes using ZIFs. Extended Defects in MOFs. We noted above in our overview of defects in zeolites that extended defects are common in these materials. Considering this, it is reasonable to assume that extended defects exist and perhaps are ubiquitous in MOFs, even though there are currently only a few experimental reports that support this assumption. Much of the information available regarding extended defects in zeolites has come from TEM imaging. Unfortunately, this technique is of limited value for many MOFs because of their propensity to be severely damaged by the electron beams from typical TEM instruments.56 A small number of studies, however, have used other experimental techniques to reveal the existence of extended defects in MOFs. Xiao et al. synthesized a MOF they called Cu-SIP-3 that was shown to have interesting properties for uptake of NO.56 Although the hydrated version of this material was highly susceptible to electron beam damage, an isostructural variant with some water replaced by pyridine, Cu2(OH)(C8H3O7S)(C5H5N)·H2O, could be readily imaged with HRTEM. A HRTEM image of this material is shown in Figure 3. The

Figure 2. Isosteric heat of adsorption of water in ZIF-8 with three different crystal sizes and nominally point-defect-free silicalite-1. The water loadings for the materials shown here are for relative pressures ranging from p/p* = 0.02 to p/p* = 0.98. The dashed line is the heat of condensation of pure water. Reproduced with permission from ref 49.

of adsorption of water in silicalite-1 synthesized via a fluoride route known to give essentially no silanol defects bear out this expectation.50 As shown in Figure 2, the heat of adsorption of water in silicalite is very low at low pore loadings and increases considerably at higher pore loadings (as pH2O/p* → 1.0) where nucleation of water clusters has occurred. In contrast, silicalite-1 synthesized by a different technique that leads to incorporation of silanol defects shows strong adsorption of water even at very low pore loadings due to the hydrophilic nature of these defects.50 Theoretical calculations for water adsorption on defect-free ZIF materials support the hypothesis that sorption enthalpies of water at low loadings in ZIF-8 should be significantly more endothermic than water−water interactions.51 Experimental measurements, however, show strong adsorption of water at the lowest measurable loadings and a significant decrease in the heat of adsorption as the water loading increases (see Figure 2).49 This effect is consistent for measurements of three samples with crystal sizes that vary by almost 4 orders of magnitude, indicating that this phenomena is associated with the internal pores of the material rather than being driven by the crystal’s external surface area. Although these experiments provide strong evidence that hydrophilic defects exist in this ZIF, nothing can currently be said definitively about the structure of these defects. Hydrophobic ZIFs have attracted significant attention for biofuel recovery from dilute aqueous streams.49,52 As noted above, however, the presence of hydrophilic point defects can significantly detract from their separation performance relative to nominally “defectfree” all-silica zeolites. It is important to note that the isosteric heats of CO2 and CH4 in ZIF-8 and silicalite-1 trend similarly to each other with loading,53,54 indicating that defects do not necessarily have major impacts on adsorption when the adsorption process is primarily driven by dispersion forces. Structural defects in ZIFs can potentially be induced via postcrystallization removal of solvent molecules. Cai, Choi, and co-workers showed that removal of DMF from ZIF-7 resulted in an apparent structural change in the ZIF material; this structural change was tentatively linked to the collapse of ZIF-7

Figure 3. HRTEM image of MOF Cu-SIP-3-pyridine·H2O highlighting extended antiphase plane defects. The top left inset is created by computationally removing the noise from the fast Fourier transform (bottom right inset) diffraction pattern. Reproduced with permission from ref 56.

crystal is highly ordered on length scales of tens of nanometers, but there is clear evidence of an antiphase-like defect (highlighted by the white circle in Figure 3). Shöaê é and co-workers used atomic force microscopy (AFM) to image the external surfaces of large crystals of CuBTC (also known as HKUST-1).57 These images showed a variety of spirals of steps on the surface that are characteristic of the existence of screw dislocations in the bulk material. Experiments of this kind have also shown similar defects in zeolite crystals.58 Subsequent atomistic modeling by Walker and Slater suggested a possible structure for a screw dislocation in CuBTC.59 This model suggests that the dislocation may 3440



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the planes separating the regions in this image are formed by [111] crystallographic planes. This result allowed direct visualization of the fractures that were observed externally in the AFM images of Shöaê é et al. The experiments by Ameloot et al. have several interesting implications. First, the extended defects associated with the fracture planes are catalytically active due to the presence of acid sites. Second, the images imply that the planes define significant barriers to diffusion of the reaction products. Finally, it is likely that the screw dislocations inferred from the AFM images of Shöaê é et al. also exist in the crystals examined by Ameloot et al. The confocal fluorescence images from the latter work do not provide any evidence that these screw dislocations are catalytically active or that they impede molecular diffusion inside the crystal. Taken together, these two sets of experiments illustrate the concept that extended defects of various kinds are likely to be common in MOFs and also the equally important observation that whether these defects will have an important influence on the performance of the crystals is dependent on the application that is envisioned for the materials.

result in pore blockage in the vicinity of the dislocation core. Although this kind of blockage would affect diffusion of adsorbates close to the dislocation, it would be unlikely to strongly affect net diffusion in the material because of the threedimensional nature of the pores in CuBTC. The impact of this kind of defect on adsorbate diffusion could potentially be more severe in materials with less fully connected pores (e.g., materials with 1D pores). Shöaê é et al. also noted from their AFM images the appearance of oriented fractures on the surface of CuBTC, where the height difference across a fracture could be as large as 5 nm, which corresponds to multiple lattice spacings.57 The steps observed on the crystals external surface were continuous across these fractures, leading Shöaê é et al. to the conclusion that these fractures occurred during postsynthesis treatment crystal (i.e., solvent removal). The fractures observed by Shöaê é et al. also form the basis of fascinating experiments by Ameloot et al. to probe the implications of extended defects in CuBTC.60 These experiments take advantage of the observation that oligomerization of furfuryl alcohol on acidic sites yields a fluorescent species that can be directly observed with confocal microscopy. This effect has also been used to probe spatial heterogeneities in zeolite crystals.61 Two facts make this reaction a useful probe for CuBTC. First, furfuryl alcohol readily fits inside the pores of CuBTC, so it can probe the entire internal structure of a crystal. This differs from related experiments with polycrystalline zeolite films where probe species were chosen specifically so they could not penetrate into the crystalline pores of the zeolite.62 Second, defect-free CuBTC is not expected to catalyze reactions involving furfuryl alcohol. The presence of a signal from the reaction products therefore is evidence of defects in the material. Ameloot et al. performed elegant experiments that compared two sets of crystals that are essentially identical when characterized by standard macroscopic techniques (e.g., XRD powder patterns and BET surface area). The crystals synthesized with relatively short crystallization times showed essentially no evidence of furfuryl alcohol oligomerization. The crystals with longer synthesis times, however, showed spatially inhomogeneous zones of strong fluorescence from reacted molecules. One example of these results is shown in Figure 4. Analysis of the results showed that

These two sets of experiments illustrate the concept that extended defects of various kinds are likely to be common in MOFs and also the equally important observation that whether these defects will have an important influence on the performance of the crystals is dependent on the application that is envisioned for the materials. “Surface Barriers” and Other Interface Defects. In any application of MOFs or other porous crystals that involves adsorption of guest molecules, the adsorbing species must cross an external surface of a MOF crystal before they can access the ̈ description of the material’s internal pores. The most naive surface of any crystal is the ideal termination of bulk crystal structure. Even the surfaces of simple crystalline materials under pristine conditions can be structurally complex due to surface reconstructions and more complex surface layers can form when a surface is in contact with ambient or other environments.63,64 The external surfaces of MOFs are likely to be a rich source of deviations from ideal crystal structures, which within the extremely broad definition of defects we offered at the beginning of this Perspective therefore can broadly be classified as defects. As is the case with internal defects, little detailed information is available about the kinds of defects that can exist on MOF external surfaces, in large part because of the experimental challenges associated with characterizing this environment. A pair of studies by Kärger and co-workers gives a fascinating example that hints at the potential impact of surface defects in MOFs and in turn sheds light on the existence of internal defects.65,66 These experiments used IR microimaging to visualize the uptake of molecules into a large crystal of Zn(tbip), a MOF with one-dimensional pores. The observed uptake profiles imply the existence of strong resistances to diffusion at the external surface of the crystal that would not be

Figure 4. Confocal fluorescence microscopy of Cu-BTC single crystal. The bright bands indicate a fluorescent signal from the products of furfuryl alcohol oligomerization over acidic sites present in defect structures. The structure of the bright bands shows strong evidence of diffusion barriers that exist for these reaction products due to extended defects. Reprinted with permission from ref 60. 3441



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defects into MOF crystals to enable a variety of new applications. In the longer term, we expect that the atomicscale structure and synthetic origin of these defects will be elucidated, allowing for improved synthesis of MOF materials in their “defect-free” form for applications where defects are deleterious to system performance. Detection of defects in MOFs will remain a challenge relative to zeolites due to the major differences in electron beam sensitivity between the two materials. The development and implementation of other quantitative tools, including diffraction-based methods, will be important in making progress in this area.

expected if the surfaces were simply ideal terminations of the bulk crystal. This phenomena is known as a “surface barrier”, and it has been invoked in many experiments with zeolites and similar materials.67,68

The external surfaces of MOFs are likely to be a rich source of deviations from ideal crystal structures.

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Heinke and Kärger explored a range of models to explore possible mechanisms that could account for the surface barriers observed with Zn(tbip).66 This work gave compelling (albeit indirect) evidence that a large fraction of the pore mouths that should be accessible on the surface of Zn(tbip) were inaccessible to molecules because they were blocked in some way. Heinke and Kärger’s analysis implies that this is a very strong effect, with only about one pore mouth in every 2000 being available for molecules to enter. The molecular-scale origin of this blocking remains unclear. Although it is not possible to know how common surface barriers of this kind are, or whether the MOF in this study is unusual in some way, these results are a striking indication that the external surfaces of MOFs in real materials may differ in important ways from simple conceptions based on ideal defect-free terminations of crystal structures. A surprising implication of the surface barrier deduced to exist on Zn(tbip) is that the internal pores of the material must also have many defects.15,65,66 The microimaging experiments mentioned above show uniform filling of the Zn(tbip) crystal in a plane normal to the one-dimensional internal pores. If these pores were defect-free, it would not be possible for molecules to transfer between pores. If the great majority of pores are blocked at the surface, then molecular diffusion would only occur in a tiny fraction of the one-dimensional pores in a material with defect-free internal pores. These observations provide indirect evidence that many defects of molecular dimensions must exist inside the Zn(tbip) crystal used experimentally. Outlook. Developing an understanding of the origin and nature of defects in metal−organic frameworks will almost certainly lead to new opportunities for these multipurpose materials. As in essentially all materials, it should be anticipated that defects are a ubiquitous feature of MOFs rather than existing only in special cases. We anticipate that the nature and concentration of defects have important implications for the long-term stability of MOFs in the complex environments demanded by many practical applications, a topic that remains an unresolved challenge for the majority of MOF materials. The number of studies that provide direct information about defects in MOFs is currently quite small. Our objective with this Perspective has been to highlight analogies that can be productively drawn between MOFs and better studied materials such as zeolites to help lay groundwork for an area that is likely to develop rapidly in the near future. In the short term, and similar to the trajectory of zeolites, controlled insertion of defects into the MOF will be utilized to endow the crystal with specialized properties (e.g., catalytic activity, chemical sensing, etc.). Indeed, a very recent review by Fischer et al. published after this Perspective was submitted highlights a variety of strategies to controllably introduce

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies David Sholl is Chair of the School of Chemical & Biomolecular Engineering at the Georgia Institute of Technology, where he is also the Michael Tennenbaum Family Chair and GRA Eminent Scholar in Energy Sustainability. He is active in using quantum chemistry and atomistic calculations for materials screening in a range of applications. Ryan Lively is an Assistant Professor of Chemical & Biomolecular Engineering at the Georgia Institute of Technology. His research group focuses on materials-inspired energy efficient separation processes. His group primarily investigates the use of microporous materials such as zeolites, MOFs, and polymers for adsorptive and membrane separation applications

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ACKNOWLEDGMENTS This work was financially supported by the Center for “Understanding and Control of Acid Gas-induced Evolution of Materials for Energy”, an Energy Frontier Research Center funded by DOE, Office of Science, BES under Award #DESC0012577. Stimulating discussions with Prof. Krista Walton, Prof. Sankar Nair, Prof. Chris Jones, and other members of the Center are gratefully acknowledged.

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REFERENCES

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