Interior Decoration of Stable MetalâOrganic Frameworks - Langmuir

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Interior Decoration of Stable Metal-Organic Frameworks Christina Tori Lollar, Jun-Sheng Qin, Jiandong Pang, Shuai Yuan, Benjamin Becker, and Hong-Cai Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00823 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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Interior Decoration of Stable Metal-Organic Frameworks Christina Tori Lollar, Jun-Sheng Qin, Jiandong Pang, Shuai Yuan, Benjamin Becker, and HongCai Zhou* Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States

ABSTRACT: Metal-organic frameworks (MOFs) are a diverse class of hybrid organic/inorganic crystalline materials comprised of metal-containing nodes held in place by organic linkers. Through discerning selection of these components, many properties such as internal surface area, cavity size and shape, catalytic properties, thermal properties, and mechanical properties may be manipulated. Due to this level of tunability, MOFs have been heralded as ideal platforms for various applications including gas storage, separation, catalysis, and chemical sensing.1-8 Regrettably, these theoretical possibilities are limited by the reality of constraining conditions for solvothermal synthesis, which typically include high temperatures (usually over 100 oC), the use of specific solvents, and necessary exposure to acidic or basic conditions. In order to incorporate more delicate functionalities, postsynthetic decoration methods were developed. This feature article focuses on developed interior decoration methods for stable MOFs and the dynamic relationship between such methods and MOF stability. In particular, methods to transform organic, inorganic, and organometallic MOF parts, as well as combination techniques, the generation of defects, and the inclusion of enzymes are addressed.

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1. INTRODUCTION MOFs with excellent performances often struggle to realize their full potentials due to issues of chemical or thermal instability. MOF instability typically descends from two main problems. The first is lability in the metal cluster-organic linker connection. While this can be resolved by changing the metal-containing unit or the organic linker to adjust the hardness/softness match or by adding bulky substituents to protect the labile bonds, this essentially changes the entire MOF and risks the loss or alteration of properties as well. The second reason is associated with the collapse of the MOF structure upon evacuation of solvent molecules from the pores. In this way, although a MOF may show immense promise in a certain application, if it lacks sufficient chemical and mechanical stability, it is not likely to see utility. The procurement of robust MOF frameworks would be a boon to their applicability. For our purposes, stable MOFs are considered those that possess thermal and chemical stability. These MOFs retain their structural integrity in the presence of air, water, reasonable pH ranges, and elevated temperatures. Although it is possible to employ de novo alterations to address the issue of MOF instability,9 these approaches often require changing key MOF features. If these compromises to the structure are not made, MOF components may not survive solvothermal conditions of elevated temperatures and high acidities. Therefore, it can be easier and more economical to start with a stable MOF and to add properties postsynthetically, which will be the focus of this review. 2. Decoration of stable MOFs with organic species: 2.1 Ligand Exchange Ligand exchange, also commonly referred to as solvent-assisted ligand exchange (SALE) or postsynthetic ligand exchange, is a versatile interior decoration method whereby organic linkers can be replaced by different organic linkers that may not have been easily incorporated by de


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novo synthetic methods. Ligand exchange is typically accomplished by saturating a pre-formed MOF crystal with a solution of excess linker. The linker in excess replaces the initial linker while the parent MOF topology is retained. This can circumvent difficulties associated with pinpointing ideal solvothermal conditions, particularly when delicate ligands are employed, and provide a facile route to tuning porosity and aperture size. Although our group showed that organic linker replacement could be accomplished in metal organic polyhedra (MOPs),10 it wasn’t until 2011 that Choe and coworkers exchanged linkers in a Zn-dicarboxylate MOF structure and showed that previously unattainable multidimensional MOF structures could be made using this new method of ligand exchange.11 The process of ligand exchange has since been investigated and some of its intricacies have been elucidated. For one, a control of linker concentration is important when inducing exchange on 2D MOFs in order to regulate whether linker exchange or a conversion from 2D to 3D will preferentially occur.12 Additionally, although initial ligand exchange utilization had been predominantly found in MOFs which were believed to be especially susceptible to it due to relatively weak metal cluster-organic ligand connections, opinions began to change when it was demonstrated that weak SBU connections were not a requirement. This technique could in fact be performed on MOFs which were believed to be rather inert. In 2012, Cohen and coworkers described a route to exchange the structural organic linkers in UiO-66, which is considered to be an inert framework, composed of Zr6O4(OH)4 clusters and 1,4-benzene dicarboxylate linkers.13 Zr(IV)-carboxylate MOFs such as UiO-66 are known to be chemically stable, and most notably water stable, due to the strength of the metal-ligand bonds achieved by a favorable interaction between high oxidation state metal ions (Zr(IV)) with charge dense carboxylate-terminated ligands.14 Using ligand exchange, they achieved exchange of benzene dicarboxylate (BDC) in an


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intact UiO-66 framework with N3-BDC, NH3-BDC, OH-BDC, and 2,5-(OH)2-BDC linkers. In addition, they were able to observe ligand exchange between two intact UiO-66 derivative frameworks (UiO-66-NH2 and UiO-66-Br) when they were physically mixed together. While these results suggest ligand exchange to be universally compatible with most MOFs, Cohen’s group found that some restrictions do exist.13 They examined ligand exchange in four MOFs of different levels of robustness: MIL-53(Al), MIL-68(In), dichloro-substituted ZIF-71, and MIL-101(Cr). However, successful ligand exchange was observed only in the cases of the dichloro-substituted ZIF-71, MIL-53(Al), and MIL-68(In), while it was resisted in MIL-101(Cr). They posit that this finding can be explained by the slow kinetics of ligand exchange of Cr(III) metal ions, which determine its capability to participate in such substitution. In cases such as this, they posit that linker basicity, often viewed in terms of pKa, can be helpful to predict whether successful exchange will occur and to what degree the original linker will be replaced (this can be anywhere from 1% to 100%) since thermodynamics play a key, but not the only, part in determining resulting structures from SALE attempts. This concept has been found and supported by authors previously as well.15-18 The supplanting of linkers deepens the potential MOFs possess for various applications by enriching the scope of ligands that are incorporable.19 The installation of ligands with anchors for further functionalization, ligands with innate fluorescence or catalytic properties, ligands with flexibility, etc. all hold promise to develop into interesting properties when inserted into a framework. For these reasons, our group discerned the need for a systematic study on the SALE of functionalized ligands. We decided to look at ligand exchange on PCN-333(Fe) and PCN333(Sc), robust mesoporous MOFs constructed from triazine-2,4,6-triyl-tribenzoate (TATB) organic linkers.20 Although an analogous structure of PCN-333 with BTB organic linkers has


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been reported, functionalized BTB linkers cannot be incorporated during solvothermal synthesis.21 The attempted direct synthesis of a PCN-333 analogue with OH-BTB resulted in a different structure, PCN-262. For these reasons, the TATB in original PCN-333 structures were exchanged with BTB and BTB derivatives. Although PCN-333 is a stable MOF with strong M-L bonds, ligand exchange proved to be successful using a solid-liquid exchange set up due in part to the large pore size of PCN-333 allowing for facilitated diffusion of ligands into and out of the structure, the bent conformation of the TATB ligand, and the large size/length of the TATB ligand. Non-functionalized BTB incorporation percentages up to ~45% were attained depending on the conditions used, with higher temperatures (up to about 85 oC), longer incubation times (up to about 24 hours), and BTB stock replacement (best results from replacing stock solution after 24 hours during 48 hours of exchange) supplying higher exchange ratios. Functionalized BTB linkers were then installed with exchange ratios ranging from 18% to 39.4%. Although this incorporation did marginally decrease the pore volume, the new ligand-exchanged material could then be poised for further click functionalization. 2.2 Solvent-assisted ligand incorporation (SALI) Solvent-assisted ligand incorporation (SALI) is a mild interior decoration method that replaces aqua/hydroxo pairs on metal cluster nodes with structural or nonstructural organic ligands featuring charge-compensating groups. Free –OH/OH2 sites are necessary for the utilization of this method. After incorporation of the nonstructural organic ligand, secondary modifications on this ligand may be conducted in order to access otherwise unattainable functionalities. SALI offers another, less apparent benefit concerning the use of single-crystal xray diffraction (SCXRD) without necessitating the procurement of single crystals. This is especially beneficial in the structure elucidation of molecules that struggle to crystallize, since the immobilization of


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free carboxylate-terminated molecules onto the metal nodes of a MOF may result in a host-guest system that retains enough of the MOF’s initial order to enable successful SCXRD. Fujita and coworkers began work on the concept of a ‘crystalline sponge’ in 2013 when they immobilized a variety of organic guests into MOFs and solved their structures using SCXRD.22 They found that the MOF inclusion method succeeds for chiral compounds, works well when only trace amount of sample is available, and can be used in combination with HPLC as a powerful characterization tool. Additionally, Yaghi’s group found that this coordinative alignment (CAL) method could be applied using an innately chiral MOF, MOF-520, to successfully crystallize and identify via SCXRD the structures of 16 molecules, ranging from simple achiral species to a molecule with 8 chiral centers.23 With these inspiring works and seeking to help push these innovations and further understand the adsorption modes, we examined the adsorption capability of a stable, flexible MOF composed of Zr6O4(OH)8(H2O)4 clusters and biphenyl-4,4′-dicarboxylic acid (BPDC) linkers, PCN-700.24 The flexibility of this framework allows for the incorporation of a number of different organic dicarboxylate compounds, including squaric acid (H2SA), 2,5dihydrozyterephthalic acid (H2DOBDC), muconic acid (H2MA), 3,3’-dihydroxy-[1,1’-biphenyl]4,4’-dicarboxylic acid (H2DOBPDC), and (E)-4,4’-(diazene-1,2-diyl)dibenzoic acid (H2AZDC), which insert into the framework at –OH/H2O occupied sites on the Zr6 cluster. Each dicarboxylic acid guest was introduced by soaking PCN-700 in a solution of the guest and settled into the structure in different ways, shown in Figure 1. The guests may dangle from two adjacent Zr6 clusters in the case of SA, which repel each other, increasing the distance between the now occupied Zr6 clusters. Alternatively, as in the case of MA, the guest may bridge two adjacent Zr6 clusters. DOBPDC stiffens the structure by adopting a double bridge mode due to its size. DOBDC assumes a similar mode of binding but it is shorter and so both ends cannot bind to Zr


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sites. Lastly, AZDC as a guest binds similarly to the structural Me2-BPDC ligands. In addition, guest introduction may induce physical changes commonly referred to as MOF ‘breathing’.25-27 With an interest in MOF breathing and the potential this brings to switchable catalysis, we continued a similar vein of research into the incorporation of other organic dicarboxylic acids, including fumarate (FA), 1,4-benzenedicarboxylate (BDC), 2,6-napthalene dicarboxylate (NDC), and 4,4’-biphenyldicarboxylate (BPDC), included in Figure 1.28


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Sequential linker installation (SLI) is a kinetically controlled subcategory of SALI that accomplishes precise placement of organic functionalities resulting in regular multivariate MOFs. This expansion of the MOF toolkit may be used to meticulously introduce multiple linkers to a single MOF structure. Attempts to simply include multiple organic linkers during a one-pot solvothermal synthesis often results in a MOF with separate domains dominated by one

Figure 1. Encapsulation modes of a series of dicarboxylates including (a) squaric acid, (b) fumarate, (c) 1,4-benzenedicarboxylate, (d) 2,5dihydrozyterephthalic

acid,

(e)

muconic

acid,

(f)

2,6-napthalene

dicarboxylate, (g) 4,4’-biphenyldicarboxylate, (h) 3,3’-dihydroxy-[1,1’biphenyl]-4,4’-dicarboxylic

acid,

and

(i)

(E)-4,4’-(diazene-1,2-

diyl)dibenzoic acid. Reprinted with permission from ref 25. Copyright 2017 American Chemical Society


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linker rather than a true mixed-linker structure.29-30 Although a few successful instances of MOFs with multiple linkers in controlled positions were known, such as in the case of MUF-7,31-32 the methods uncovered were usually not universally applicable, especially when one of the organic linkers contains functional groups sensitive to solvothermal conditions. We were able to consistently obtain true mixed-linker MOFs by using SLI on the highly stable MOF, PCN-700.33 Obtaining the Zr6 cluster as an 8-connected node as opposed to the more thermodynamically stable 12-connected node was accomplished by selection of BPDC linkers with substituents in the 2- and 2’- positions to produce out-of-plane phenyl groups. These 8-connected metal nodes with coordination sites occupied by pairs of OH-/H2O ligands are situated forming ‘pockets’ of different sizes capable of accepting linear organic linkers of two different lengths. BDC was successfully installed in the smaller pocket, followed by Me2-TPDC (TPDC is p,p′terphenyldicarboxylic acid) installation in the larger pocket to produce the multi-linker MOF PCN-703 shown in Figure 2.


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Inspired by the success of our first SLI, we sought to comprehensively study the potential SLI holds for precise installation of different functional moieties within a MOF.34 Me2-BPDC was again chosen to form the base PCN-700 structure since it possesses enough steric hindrance to avoid competing UiO-67 formation but not enough to significantly clog the pores. Using Me2BPDC generates a length of 18.9 Å (labeled α) between connected clusters in the base structure.

Figure 2. (a) Two open ‘pockets’ present in PCN-700. The addition of (b) BDC produces PCN-701 and the addition of (c) Me2-TPDC produces PCN-702. (d) Sequential addition of BDC followed by Me2-TPDC produces PCN-703 while the sequential addition of Me2TPDC followed by BDC does not. Reprinted with permission from ref 30. Copyright 2015 American Chemical Society


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Three potential ligand installation pockets were envisioned along the x, y, and z directions. Due to the relative flexibility of PCN-700, the lengths of linkers can be estimated compatible or incompatible using the equation 4α2 = x2 + y2 + z2, where x, y, and z are the lengths of 3 linear linkers among a pool of 6 linear linkers: fumarate (FA), BDC, 2,6-napthalenedicarboxylate (NDC), BPDC, TPDC, or 4,4’-(1,3-butadiyne-1,4-diyl)bis(3-methylbenzoate) (BDDC). Combinations of these linkers result in 11 new multi-linker MOFs shown in Figure 3 composed of up to 3 different linkers. The potential applications of such controllable structures were then validated by the 57% increase in H2 adsorption capacity of PCN-700-(NO2)2-BPDC over the mono-linker structure. Additionally, a size-selective aerobic alcohol oxidation catalytic site was

Figure 3. The structures of 11 different MOFs that arise from sequential linker installation on PCN-700 with its two open pockets. Reprinted with permission from ref 31. Copyright 2016 American Chemical Society


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built into PCN-700 through the installation of a BPYDC(Cu) (BPYDC is 2,2’-bipyridine4,4’dicarboxylate) center in conjunction with a TPDC-R2 size-excluding moiety. These results indicate that SLI may prove useful for the installation of, and possibly the improvement of, other functionalities. 3. Decoration of stable MOFs with metal ions/clusters: 3.1 Cluster metalation Atomic layer deposition (ALD) is a layer-by-layer-type method employed in various scientific and engineering fields to generate films from gaseous species. This is often accomplished by consecutive surface reactions of different gases generating a material with a relatively reliable, uniformly-sized alternation of layers. Atomic layer deposition in MOFs (AIM) was first conceived by Hupp and coworkers upon considering the potential of applying gas phase ALD to MOFs.35 They were successful in depositing Zn and Al in the stable, Zr-based NU-1000 network through sequential precursor exposure and water exposure cycles. In contrast to conventional ALD which forms a layered, continuous film of material, AIM instead tends to form an array of uniform multiple-metal-containing nodes. Although AIM is a versatile method of pore modification, a few requirements for a MOF to be ALD-suitable exist, including the need for relatively good stability and labile sites for replacement. Solvothermal deposition in MOFs (SIM) is another method of cluster metalation that emerged, which entails soaking a MOF in a solution of the modifying species. If desired, partial AIM or SIM can be accomplished by modification of the added precursor concentration, decreasing the reaction time, or by usage of a less reactive precursor. AIM, in addition to its solvent-utilizing complement, SIM, has since been used in a variety of other MOFs, including MOF-5, MIL-53, and MIL-101, have been


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functionalized with deposited metals, and other metals have been deposited onto the stable NU1000 framework, including Ni, In, Co, Cu, and more.36-46 In compliment to the Ni-AIM on NU-1000 reported Farha,38 Chapman and coworkers sought to further understand the deposition of the new metals by studying the specifics of these ALDdeposited NiOxHy clusters within the Zr6-oxide-based NU-1000 framework.47 They probed the local and long-range electron density mapping to ascertain how deposited Ni4 clusters bound to the structural Zr6 nodes using density functional calculations as well as extended xray absorption fine structure (EXAFS), differential pair distribution function (PDF), and differential envelope density (DED) analyses. While it was initially proposed that Ni was installed equally at all available –OH/-OH2 sites on the [Zr6(µ3-O)4(µ3-OH)4(OH)4(OH2)4]8+] cluster, resulting in a maximum of 4 deposited metals at each original cluster, it was found that Zn deposition occurred selectively within the smallest pores of the Nu-1000 structure. In this way, the Ni moieties form a catalytic nanowire of alternating Zr-oxo and Ni-oxo pieces.47 Further studies are needed to ascertain the more comprehensive details of AIM deposition. 3.2 Postsynthetic metal exchange (PSME) Postsynthetic metal exchange (PSME), also known as metal metathesis (MM), metal exchange (ME), postsynthetic ion metathesis (PSIM), or cation exchange, is the inorganic conceptual equivalent of ligand exchange. In this technique, a MOF is synthesized with a labile metal to then be replaced postsynthetically in a solution of a less labile metal. As in many other methods, the degree of completion depends on various properties, including the solubility of the metals, the relative concentration of the less-labile metal, the stability of the new metal in the original coordination geometry, the time allotted for the system to reach the thermodynamic product, and the hardness/softness match between the metal and the ligand, among others. Replacement of


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metals via metal metathesis can result in MOFs with altered physical and chemical properties, including framework stability, porosity, surface area, catalytic capabilities, and luminescence.4849

In addition to their work on organic linker exchange, Cohen’s group conducted investigation into PSME of metal cations within ‘inert’ MOFs.13 They observed cation exchange when MIL53(Al)-Br and MIL-53(Fe)-Br were placed in contact with one another for 5 days at 85 oC resulting in approximately 40% MIL-53(Al/Fe). Aerosol time-of-flight mass spectrometry (ATOFMS) in addition to scanning electron microscopy and energy-dispersed xray spectroscopy (SEM-EDAX) confirmed the presence of both metal cations while PXRD assured the maintained crystallinity of the particles before and after postsynthetic exchange (PSE). This work demonstrated that, like SALE, PSME is a technique that can be nearly universally applied to MOFs, even in the presence of stable, high oxidation state metal-carboxylate bonds. Uranyl carboxylate MOFs can also possess resistance from degradation under a wide pH range and in aqueous systems, rationalized by the strong bonds formed between uranyl ions and carboxylate oxygens.50 In 2017, an exploration into building actinide MOFs found that U6Me2BPDC-8 could be converted to Th5.65U0.35-Me2BPDC-8 (94% metal exchanged) through soaking the Uranium MOF in ThCl4 in DMF at room temperature for three days.51 The conversion could even be visually monitored since the solution transforms from a green color to white. As might be expected, the resulting MOF shared the same topology as the parent MOF. Recently, Dinca’s group has also demonstrated the catalytic benefits that partial PSME provides when they substituted a Mn2+ center into the relatively weak ligand field produced by the remaining zinc-carboxylate SBU pieces.52 This Mn2+ center is capable of engaging in redox


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chemistry by epoxidizing cyclic alkenes with >99% selectivity. PSME is actively being investigated due to its ability to strongly influence MOF stability and application.53-55 4. Decoration of stable MOFs with metalloligands: The incorporation of new, nonstructural metal-organic functionalities into existing MOFs, usually by stepwise sequences, can permit the installation of useful, uniformly distributed catalytic centers while providing benefits such as inter-catalyst deactivation prevention. Generally, the construction of these is accomplished by the utilization of structural organic ligands that offers metal chelation sites or by structural organic ligands that possess clickable sites for the incorporation of preformed metalloligands. Using the first type of metalloligand installation, Lin and coworkers began by adding a bidentate β–Diketiminate (NacNac) into a Zr-dicarboxylate UiO-type base framework.56 NacNac may then chelate to metal ions, Fe, Cu, and Co, forming catalytic centers within the MOF interiors. The Fe-NacNac incorporated complex showed efficient transformation of various alkyl azides to α–substituted pyrrolidines, the Cu-NacNac incorporated complex catalyzed the amination of cyclohexene, and the Co-NacNac delivered alkene hydrogenation turnover numbers up to 700,000. In the second fashion, Jerome Canivet et al. took advantage of an amine anchor to postsynthetically install a preformed organometallic nickel moiety, Ni(PyCHO)Cl2, into a (Fe)MIL-101(NH2) framework.57 (Fe)MIL-101(NH2) is a stable MOF with a high pore volume composed of trimeric iron(III) clusters connected by 2-aminoterephthalates. The diimino nickel complex MOF was installed by soaking (Fe)MIL-101(NH2) in a solution of Ni(PyCHO)Cl2 in methanol. Depending on whether 1 or 3 equivalents of catalyst per –NH2 were used, they were able to obtain 10Ni@(Fe)MIL-101 (10% of amino groups reacted) or 30Ni@(Fe)MIL-101


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(~30% of amino groups reacted), respectively. They then tested these immobilized catalysts in the ethylene dimerization reaction and found them to almost exclusively form 1-butene (>95% product selectivity). Compared to their molecular analogue Ni(bipy)Cl2, 10Ni@(Fe)MIL-101 and 30Ni@(Fe)MIL-101 both showed increased recyclability (up to 3 uses) and activity improvements over coordination polymer catalysts as well as a more than 10x increase over its single molecule analogue catalyst Ni(bipy)Cl2. Similar results were found by Farha and coworkers in 2015 using a nickel catalyst inserted into a NU-1000 framework to form NU-1000(bpy)NiII to selectively form 1-butene from the ethylene dimerization reaction.58 However, rather than soaking the preformed catalysts with the preformed MOF, they used the catalyst precursors in a methanol solution with NU-1000-bpy to form the catalyst/MOF composite. Notably, the immobilized organometallic catalyst actually benefited from an increase in activity during its second cycle of reuse, which they hypothesized to be due to the formation of a polyethylene layer around the MOF crystals that helps to solubilize ethylene. Subsequent cycles result in further buildup of polymer which begins to block the windows of the MOF and decrease access to the internal active sites. Loading a MOF with ruthenium catalysts requires relatively large pore spaces to accommodate these decently large catalysts and so the MIL-101 family was chosen owing to its large pore size and impressive stability.59 Specifically, (Cr)MIL-101 and (Al)MIL-101-NH2 were selected. Seven ruthenium catalysts shown in Figure 4 were chosen to be absorbed into the model MOFs using either toluene (catalysts 1-3) or dichloromethane (catalysts 4-6) as solvent with absorption efficiencies ranging from 78.5-99.9%. These absorbed catalysts were resistant to leaching and maintained their activity after treatment with HCl. Some other acids, including CH3SO3H and CF3SO3H, entirely deactivated the immobilized catalysts and therefore the Cl- anions were


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Figure 4. – The structures of 7 ruthenium catalysts incorporated into MIL-101 family MOFs. Reprinted with permission from ref 57. Copyright 2016 American Chemical Society. presumed to be important in activity retention. Indeed, although sodium chloride in water affected the crystallinity of (Al)MIL-101-NH2, it also managed to preserve its catalytic activity. It is hypothesized that the Cl- anions replace original F- and OH- anions in the case of (Cr)MIL101 or OH- anions in the case of (Al)MIL-101-NH2 which pose the risk of poisoning olefin metathesis catalysis. Replacing these anions before catalysis has the added benefit of preventing the activated MOF support from filling its vacancies with chloride anions from the bound ruthenium catalysts. The final system functioned as a ring-closing olefin metathesis catalyst with an increased stability and turn over numbers of up to 8,900 without requiring a glovebox. 5. Sequential organic/inorganic techniques 5.1 Postsynthetic metathesis oxidation (PSMO) Postsynthetic metathesis and oxidation (PSMO) refers to a procedure by which successful PSE is followed by oxidation of the new clusters. This is sometimes done in order to further increase the disparity of energies between the old and new structures, often by changing coordination environment or by improving the hard/soft match of the ligand and the metal ions. PSMO is in


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essence a combination of postsynthetic techniques commonly used in conjunction to form MOFs that may be unattainable from one-pot solvothermal methods. Essential to PSMO is the concept of hard and soft acid and base (HSAB) theory. Since most MOFs employ carboxylate-terminated (a hard Lewis base) linkers, a MOF’s stability may be tuned by oxidizing or reducing the metals within the metal-containing SBU, thereby increasing or decreasing the HSAB match, respectively. The stability of hard Lewis acid-hard Lewis base bonds limits the reversibility of bond formation and hence the procurement of crystalline versions of these stable MOFs through one-pot methods can be regrettably difficult. This is where PSMO shines. We performed PSMO on PCN-426-Mg, a MOF containing relatively labile Mg-O bonds that can be easily replaced with Cr(II) or Fe(II), since the (II) oxidation state of Cr and Fe have accelerated exchange rates compared to (III) counterparts. These Cr(II) or Fe(II) cations were subsequently oxidized to the more stable Cr(III) or Fe(III) versions, respectively.60 Although this method delivered otherwise unattainable crystalline forms of PCN-426-Fe(III) and PCN-426-Cr(III), the knowledge obtained from Cohen’s work13 showing that even ‘labile’ MOFs may undergo PSME urged us to enact this strategy on MOFs containing more stable metal-ligand bonds. We turned to PCN-333-Fe(III), a generally stable MOF except for when exposed to alkylamine solution. We posited that replacing these Fe(III) atoms with the more stable Cr(III) atoms could increase the framework’s stability. This was an especially attractive option since our attempts to synthesize PCN-333-Cr(III) via one-pot synthesis failed to deliver crystals. Attempts at direct metathesis of PCN-333-Fe(III) to PCN-333-Cr(III) also proved ineffective. More complete conversions at lower temperatures and shorter times were accomplished using a reductive labilization-metathesis technique whereby a redox reaction reduces Fe(III) in the framework to Fe(II), Cr(II) replaces Fe(II) within the framework, and


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Cr(II) is oxidized to Cr(III), forming a structure with superior stability. The primary requirements for this method are that the redox potential of the oxidant far exceed the redox potential of the reductant and that the reaction does not alter the environment harshly enough to destroy the framework. In this case, the resulting PCN-333-Cr(III) boasted improved stability in alkylamine solutions followed by a high CO2 adsorption capacity and a higher pH tolerance than PCN-333-Fe(III).


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Since HSAB theory predicts increased stability between carboxylate linkers and high valent metals, it is interesting that Ti(IV) MOFs have not received the same amount of attention in literature that Zr(III)-MOFs have. While some titanium MOFs have been reported, including MIL-125,61 MIL-91,62 PCN-22,63 MOF-902,64 and COK-69,65 among others, titanium salt precursors to MOF synthesis are often water-sensitive and therefore difficult to use.


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Additionally, strong Ti(IV)-carboxylate bonds reduce bond association/dissociation potential, hindering the crystallinity of potential MOF products. Inspired by this problem along with the interesting photocatalytic potential of previous Ti-MOFs, our group sought to employ PSMO in order to reliably obtain Ti-MOFs with predictable topologies.66 We were able to acquire Ti(IV) analogues of / by metathesis of the framework’s starting metal with Ti(III) followed by oxidation


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to deliver the Ti(IV) MOF. A schematic of these transformations can be found in Figure 5. The resulting Ti-MOFs were able to degrade methylene blue in the presence of a 300 W xenon lamp, reinforcing the concept that new Ti-MOFs may have intriguing photocatalytic applications. PSMO as a way to incorporate high oxidation state metals expands the scope of accessible MOFs and in turn, the applicability of MOFs.

Figure 5. – (a) PSME on MIL-100(Sc) or PCN-333(Sc) with Ti(III) sources leads to MIL101(Sc)-Ti(III) or PCN-333(Sc)-Ti(III) and subsequent oxidation in air leads to MIL-101(Sc)-Ti or PCN-333(Sc)-Ti, respectively and (b) PSME on MOF-74(Zn) and MOF-74(Mg) with Ti(III) sources leads to MOF-74(Zn)-Ti(III) or MOF-74(Zn)-Ti(III) and subsequent oxidation in air leads to MOF-74 (Zn)-Ti or MOF-74(Zn)-Ti, respectively. Reprinted with permission from ref 64. Published by The Royal Society of Chemistry.


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5.2 Other technique combinations Other techniques have been applied in conjunction to carefully design a MOF with metal clusters and organic ligands which would not be feasible through de novo techniques. In particular, AIM followed by PSME, ‘dual-exchange’ using ligand exchange plus metal incorporation, and organometallic installation followed by oxidation have proven valuable in obtaining elusive MOF structures and will be mentioned below. AIM in combination with PSME promises novelty in MOF structures by enabling the inclusion of metal atoms which lack AIM precursors or do not possess economically viable AIM precursors. Farha and coworkers used this combination technique to install Cu, Co, or Ni atoms into NU-1000.67 They began by installing Zn atoms onto the Zr6 clusters’ open sites using the cost-effective ZnEt2 AIM precursor followed by PSME using CuCl2· 2H2O, NiCl2· 6H2O, or CoCl2· 6H2O. In addition, they were able to produce bimetallic combinations but they did not systematically study the production and potential applications of these materials. In 2015 our group performed dual exchange of structural ligand and metal on PCN-333(Fe) to construct the functionalized L-PCN-333(Cr)68. It is necessary in this process to perform ligand exchange from non-functionalized to functionalized linker before the metal metathesis from iron to chromium in order to avoid the high activation energy associated with Cr(III)-carboxylate bond dissociation. The dual exchange is easy to enact and provides access to a resulting MOF with maintained crystallinity and porosity along with an overall increase in chemical stability due to the strength of Cr(III)-carboxylate over Fe(III)-carboxylate bonds. Not only have attempts to directly synthesize crystalline PCN-333(Cr) proven unsuccessful, the incorporation of functionalized ligands into Cr-MOFs has always been a difficult pursuit. This work shed light on


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the potential that postsynthetic modification techniques have as powerful tools to overcome MOF design obstacles when used in carefully planned concert. Thompson and colleagues showed how they installed a Co-Al oxide complex onto the Zr6 nodes of NU-1000.69 They began by anchoring an organometallic complex, (py3tren)-AlCoMe, onto the open sites of the structural metal nodes. They did this by stirring together a solution of the Co-Al species in benzene with microcrystalline NU-1000 at 25 oC for 3 hours. The installation of the organometallic complex was followed by heating the material in air at 300 oC to selectively remove the non-structural organic ligands attached to the organometallic, resulting in the formation of the immobilized heterometallic binuclear Co-Al oxide moiety. Even before the conversion to the Co-Al oxide, the organometallic Co-Al also showed 7.5 times higher activity per Co atom than Co on NU-1000 alone. The final structure was able to catalyze the oxidation of benzyl alcohol to benzaldehyde, although with tert-butyl hydroperoxide as opposed to the more commonly used hydrogen peroxide, with a 7.5 times increase in activity per Co atom compared to only Co added to the Zr6 cluster using AIM. The post catalysis sample of the organometallic-grafted MOF showed maintained porosity as compared with the pure bimetallic oxide sample. They posited that this is due to the Co-Al oxide analogue producing more benzoate that proceeds to bind to other Zr6 nodes, blocking the pores of the MOF for future substrates. This work revealed how the use of preformed bimetallic organometallic complexes can facilitate the installation of two different metals onto already existing metal nodes within MOFs. 6. Decoration of stable MOFs with enzymes: The primary limitations of enzymatic catalysis, including environmental instability, low catalytic yield, and poor recyclability, can be circumvented or at least reduced by incorporating


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the enzyme catalyst into a mesoporous MOF. For these reasons, the MOFs used for these purposes must themselves be stable to different chemical and thermal environments in order to protect their cargo. The act of stationing an enzyme into a MOF’s pores is generally referred to as encapsulation, internalization, or incorporation. The methods that accomplish this include de novo incorporation, postsynthetic modification, and co-precipitation. Since this review focuses on postsynthetic modifications of stable MOFs, we direct interested readers to reviews which touch more on other methods not discussed.70-72 Although enzyme immobilization onto other material platforms, such as mesoporous silica, carbon nanotubes, and sol-gel matrices, have been successful, these options may still suffer from unwanted leaching, slow loadings, vulnerability of the enzyme catalyst, and/or limited diffusion of substrates and products.70, 73-77 In contrast, MOFs offer vast enzyme-framework interaction tunability by prudent selection of metal cluster and organic linker parts, pores large enough to encapsulate the enzyme guests and allow for diffusion of substrates, and protection from unfavorable environmental conditions which could denature the enzymes, thereby impairing their catalytic efficiency. One key point to keep in mind is that the MOF chosen should have channels that are small enough to properly interact with and trap the enzyme but large enough to not constrict the enzyme and hinder its catalytic ability. Lastly, room should remain within the MOF to allow for ready diffusion of reactants and products into and out of the structure. Postsynthetic enzyme incorporation is typically accomplished by soaking a preformed MOF in a solution containing the desired enzyme at a pH (and buffer system) in which both the enzyme and framework are stable (often ~7.4). In this way, horseradish peroxidase (HRP), cytochrome c (Cyt c), and microperoxidase (MP-11) may be successfully incorporated into PCN-333, a MOF consisting of M3(µ3-O)(OH)(H2O)2 clusters adjoined by TATB ligands.78 Unoccupied PCN-333


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survives solvent removal and exposure to water and is compatible in typical enzyme-friendly environments. Hierarchically porous MOFs are especially attractive foundations for enzyme encapsulation since the presence of smaller, secondary pores can act as channels for substrate/product diffusion into/out of the MOF. If a MOF is comprised of 3 or more pore types, multiple enzymes may be incorporated, creating a tandem catalyst system. PCN-333 mentioned above contains 3 cage sizes. In the case of HRP, Cyt c, and MP-11, it was found that HRP could be loaded strictly into the largest cage, Cyt c could reside in the large or medium cage (leaving the smallest cage open for substrate diffusion), and MP-11 could fill any cage. All three enzymes immobilized within PCN-333 managed to maintain catalytic activity extremely well over 5 cycles as compared to the same enzymes immobilized onto a common mesoporous silica platform, SBA-15. While the free enzymes may have high initial rates of reaction, they quickly aggregate and lose this advantage over their MOF-immobilized counterparts. The design and synthesis of a tandem enzyme reactor was accomplished by our group using the hierarchically porous MOF PCN-888.79 The three distinct pore types with varying shapes and sizes are ideal for the installation of two enzymes while still allowing for efficient substrate diffusion through the smallest pore. Using a rational installation order (glucose oxidase, GOx, first followed by HRP second) ensures selective installation of GOx into the largest cages and HRP installation into the medium sized cages, resulting in a structure with efficient bi-enzyme coupling. In addition to an increased catalytic activity, the enzyme-loaded PCN-888 minimizes enzyme leaching and increases catalyst reusability. 7. Decoration of stable MOFs with defects: The expansion of MOF pore sizes may potentially increase gas uptake properties and allow for the incorporation of larger guests. This is an active goal in the MOF community that is ordinarily


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pursued through increasing the length of organic linkers. Regrettably, this may also make the MOF more prone to structural collapse upon activation or increase the chance of MOF interpenetration, which can actually decrease pore size.9, 80-86 Another way to effectively increase a MOF’s average pore size while attempting to avoid previously mentioned issues is by defect generation. The intentional inclusion of defects at the point of de novo synthesis or by the removal of a few structural features can be executed to expand the pore size. This is, perhaps less intuitively, also a type of stable MOF interior decoration. Oftentimes, low stability structures cannot survive solvothermal synthesis and only stable MOFs can accommodate for defects without collapsing or otherwise compromising their crystallinity and structure. The stability of Zr-carboxylate MOFs makes them exceptionally tolerant to defects and as such, defects within these types of MOFs are the most commonly studied.87-92 A one-pot route to defect-containing MOFs is attractive due to its ease of execution. This route has been employed with variation, most notably through auxiliary linker occupancy tuning and metal-ligand-fragment coassembly.93-96 However, a method to postsynthetically introduce defects may elude the difficulties in getting a de novo defective MOF to properly crystallize.


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Toward these issues, the Zhou group founded a new process, coined ‘linker labilization’.97 Linker labilization can be thought of as the reverse of SALI, in that a stable ligand and a labile ligand are simultaneously incorporated into a MOF structure followed by the cleavage and removal of the labile linker to produce defects (Figure 6). In contrast to the high temperatures that solvothermal synthesis employs to incorporate defects, linker labilization may occur under

Figure 6. – The process of linker labilization with stable ligands shown in grey. Reprinted with permission from ref 94. Copyright 2017 Nature Communications. mild conditions. In one particular example, the labile linker 4-carboxybenzylidene-4-aminobenzate (CBAB) was successfully installed postsynthetically, from 5.5 to 65% incorporation, into PCN-160, made up of azobenzene-4,4’-dicarboxylate (AZDC) and Zr6 clusters.97 CBAB within the MOF could then dissociate into 4-amino benzoic acid and 4-formylbenzoic acid with acid treatment. This


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generated both missing-linker and missing-cluster defects, with the presence of defects propelling the production of other defects around them. The extent of defect generation may be detected using a combination of techniques, including thermogravimetric analysis (TGA), UVvis after MOF decomposition, SEM and TEM, SCXRD, and diffuse reflectance infrared spectroscopy Fourier transform spectroscopy (DRIFTS) before and after acid treatment. The amount of defects present in PCN-160 was shown to be related to the percent of CBAB installed and the amount of acetic acid used to etch the structure. The creation of a hierarchical porous structure was confirmed to have beneficial effects on the adsorption and catalytic properties compared to the defect-free MOF. 8. Conclusions and Outlook Herein have been summarized interior decoration methods that may be applied to stable MOFs and the applications they afford. Existing moieties may be replaced in even ‘inert’ MOF using ligand exchange and PSME approaches; new organic, inorganic, and organometallic, nonstructural moieties may be inserted using methods such as SALI, SLI, AIM, and SIM; defects may be generated through ‘linker labilization’; and enzymes may be incorporated within stable MOFs via enzyme encapsulation. With the primary advantages of MOFs residing in their tunability, the expansion of the MOF interior decoration toolkit is perhaps the fastest way to enhance their practicality and advance the field. Much has been accomplished in the pursuit of methods to replace or include linkers in structural positions, bypassing concerns arising from solvothermal synthesis and offering an extension of possible physical and chemical properties. One interest that requires more work however, concerns the precise placement of multiple, structural functionalities. Attempts to accomplish multi-linker MOFs through one-pot means often result in segregated linker domains.


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SLI is a great first step toward solving this problem, however it requires diligent foresight to match the open space’s length with linker lengths. The realization of a strategy to precisely and uniformly place multiple linkers with less need for planning would greatly benefit the MOF field. On this current trajectory, the continued pursuit of novel modification techniques will doubtlessly lead to captivating, previously unobtainable MOFs with capabilities to solve a multitude of problems relating to energy, catalysis, gas storage, and more.


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AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation Graduate Research Fellowship to C.T.L. under Grant No. DGE: 1252521 and the Welch Foundation through a Welch Endowed Chair to H.J.Z. (A-0030). S.Y. acknowledges the Texas A&M Energy Institute Graduate Fellowship funded by ConocoPhillips and the Dow Chemical Graduate Fellowship. Additionally, this work was supported as part of the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DESC0001015. REFERENCES 1. Zhou, H.-C.; Long, J. R.; Yaghi, O. M., Introduction to Metal-Organic Frameworks. Chem. Rev. 2012, 112 (2), 673-674. 2. Li, H.; Wang, K.; Sun, Y.; Lollar, C. T.; Li, J.; Zhou, H.-C., Recent advances in gas storage and separation using metal–organic frameworks. Mater Today 2017. 3. Gu, Z.-Y.; Park, J.; Raiff, A.; Wei, Z.; Zhou, H.-C., Metal-Organic Frameworks as Biomimetic Catalysts. ChemCatChem 2014, 6 (1), 67-75. 4. Furukawa, H.; Cordova, K. E.; O'Keeffe, M.; Yaghi, O. M., The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341 (6149). 5. Wang, H.; Zhu, Q.-L.; Zou, R.; Xu, Q., Metal-Organic Frameworks for Energy Applications. Chem Rev 2017, 2 (1), 52-80. 6. Zhu, L.; Liu, X. Q.; Jiang, H. L.; Sun, L. B., Metal-Organic Frameworks for Heterogeneous Basic Catalysis. Chem. Rev. 2017, 117 (12), 8129-8176. 7. Li, J. R.; Sculley, J.; Zhou, H. C., Metal-organic frameworks for separations. Chem Rev 2012, 112 (2), 869-932.


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64. Nguyen, H. L.; Vu, T. T.; Le, D.; Doan, T. L. H.; Nguyen, V. Q.; Phan, N. T. S., A Titanium–Organic Framework: Engineering of the Band-Gap Energy for Photocatalytic Property Enhancement. ACS Catalysis 2016, 7 (1), 338-342. 65. Bueken, B.; Vermoortele, F.; Vanpoucke, D. E. P.; Reinsch, H.; Tsou, C. C.; Valvekens, P.; De Baerdemaeker, T.; Ameloot, R.; Kirschhock, C. E. A.; Van Speybroeck, V.; Mayer, J. M.; De Vos, D., A Flexible Photoactive Titanium Metal–Organic Framework Based on a [TiIV3(µ3‐O)(O)2(COO)6] Cluster. Angewandte Chemie International Edition 2015, 54 (47), 13912-13917. 66. Zou, L.; Feng, D.; Liu, T.-F.; Chen, Y.-P.; Yuan, S.; Wang, K.; Wang, X.; Fordham, S.; Zhou, H.-C., A versatile synthetic route for the preparation of titanium metal–organic frameworks. Chem. Sci. 2016, 7 (2), 1063-1069. 67. Klet, R. C.; Wang, T. C.; Fernandez, L. E.; Truhlar, D. G.; Hupp, J. T.; Farha, O. K., Synthetic Access to Atomically Dispersed Metals in Metal–Organic Frameworks via a Combined Atomic-Layer-Deposition-in-MOF and Metal-Exchange Approach. Chemistry of Materials 2016, 28 (4), 1213-1219. 68. Park, J.; Feng, D.; Zhou, H. C., Dual Exchange in PCN-333: A Facile Strategy to Chemically Robust Mesoporous Chromium Metal-Organic Framework with Functional Groups. J Am Chem Soc 2015, 137 (36), 11801-9. 69. Thompson, A. B.; Pahis, D. R.; Bernales, V.; Gallington, L. C.; Malonzo, C. D.; Webber, T.; Tereniak, S. J.; Wang, T. C.; Desai, S. P.; Li, Z. Y.; Kim, I. S.; Gagliardi, L.; Penn, R. L.; Chapman, K. W.; Stein, A.; Farha, O. K.; Hupp, J. T.; Martinson, A. B. F.; Lu, C. C., Installing Heterobimetallic Cobalt-Aluminum Single Sites on a Metal Organic Framework Support. Chemistry of Materials 2016, 28 (18), 6753-6762. 70. Lian, X.; Fang, Y.; Joseph, E.; Wang, Q.; Li, J.; Banerjee, S.; Lollar, C.; Wang, X.; Zhou, H. C., Enzyme-MOF (metal-organic framework) composites. Chem Soc Rev 2017, 46 (11), 3386-3401. 71. Majewski, M. B.; Howarth, A. J.; Li, P.; Wasielewski, M. R.; Hupp, J. T.; Farha, O. K., Enzyme encapsulation in metal–organic frameworks for applications in catalysis. CrystEngComm 2017, 19 (29), 4082-4091. 72. Doonan, C.; Ricco, R.; Liang, K.; Bradshaw, D.; Falcaro, P., Metal-Organic Frameworks at the Biointerface: Synthetic Strategies and Applications. Acc Chem Res 2017, 50 (6), 14231432. 73. Mohamad, N. R.; Marzuki, N. H.; Buang, N. A.; Huyop, F.; Wahab, R. A., An overview of technologies for immobilization of enzymes and surface analysis techniques for immobilized enzymes. Biotechnol Biotechnol Equip 2015, 29 (2), 205-220. 74. Gill, I.; Ballesteros, A., Encapsulation of Biologicals within Silicate, Siloxane, and Hybrid Sol−Gel Polymers: An Efficient and Generic Approach. J Am Chem Soc 1998, 120, 8587-8598. 75. Sheldon, R. A., Enzyme Immobilization: The Quest for Optimum Performance. Advanced Synthesis & Catalysis 2007, 349 (8-9), 1289-1307. 76. Lee, K. Y.; Yuk, S. H., Polymeric protein delivery systems. Progress in Polymer Science 2007, 32 (7), 669-697. 77. Hudson, S.; Cooney, J.; Magner, E., Proteins in mesoporous silicates. Angew Chem Int Ed Engl 2008, 47 (45), 8582-94. 78. Feng, D. W.; Liu, T. F.; Su, J.; Bosch, M.; Wei, Z. W.; Wan, W.; Yuan, D. Q.; Chen, Y. P.; Wang, X.; Wang, K. C.; Lian, X. Z.; Gu, Z. Y.; Park, J.; Zou, X. D.; Zhou, H. C., Stable


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Christina T. Lollar received her B.A. degree in French and her B.Sc. degree in Chemistry in 2015 from SMU. She joined Prof. Hong-Cai Zhou’s research group in 2017 at TAMU. Her research interests focus on stimuli responsive molecular organic frameworks with potential applications in gas storage, seperation, and sensing. Jun-Sheng Qin received his B.Sc. degree in chemistry in 2008 and M.S. degree in physical chemistry in 2011 from Northeast Normal University. In 2014, he obtained his Ph.D. degree under the supervision of Prof. Zhong-Min Su from Jilin University. Since the fall of 2014, he worked as a postdoctoral research associate in Prof. Hong-Cai Zhou’s group (Texas A&M University, TAMU). His current research interests focus on the development of porous materials for applications in gas capture and catalysis. Jiandong Pang received his Ph.D. degree in chemistry in 2016 under the supervision of Prof. Maochun Hong from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. He then joined Prof. HongCai Zhou’s group in the same year as a postdoctoral research associate at TAMU. His current research interests focus on the design and synthesis of flexible metal-organic frameworks with intriguing structures and potential functions in gas adsorption and separation.

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Shuai Yuan received his B.Sc. in chemistry from Shandong University in 2012. He then joined Prof. Hong-Cai Zhou’s research group in the same year at TAMU. He is now a Ph.D. candidate at Department of Chemistry (TAMU). His research is focused on the synthesis of stable metal-organic frameworks for energy conversion and storage. Benjamin Becker is currently an undergraduate student pursuing a degree in Biomedical Sciences in 2019 from Texas A&M University. In the fall of 2017, he joined Prof. Hong-Cai Zhou’s research group, where he assists with research on potential metal organic framework applications in gas storage, separation, and sensing. After graduation, he plans on attending medical school. Hong-Cai ‘‘Joe’’ Zhou obtained his PhD in 2000 from TAMU under the supervision of F. A. Cotton. After a postdoctoral stint at Harvard University with R. H. Holm, he joined the faculty of Miami University, Oxford, in 2002. He moved back to TAMU and became a full professor in 2008. He was promoted to a Davidson Professor of Science in 2014 and a Robert A. Welch Chair in Chemistry in 2015. He was recognized as a Thomson Reuters ‘‘Highly Cited Researcher’’ in 2014 − 2016. His research focuses on the discovery of synthetic methods to obtain robust framework materials with unique catalytic activities or desirable properties for cleanenergy-related applications.


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Interior Decoration of Stable MetalâOrganic Frameworks - Langmuir

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