| 1. |
- Simons, Matthew C., et al.
(author)
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Beyond Radical Rebound : Methane Oxidation to Methanol Catalyzed by Iron Species in Metal–Organic Framework Nodes
- 2021
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:31, s. 12165-12174
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Journal article (peer-reviewed)abstract
- Recent work has exploited the ability of metalorganic frameworks (MOFs) to isolate Fe sites that mimic the structures of sites in enzymes that catalyze selective oxidations at low temperatures, opening new pathways for the valorization of underutilized feedstocks such as methane. Questions remain as to whether the radical-rebound mechanism commonly invoked in enzymatic and homogeneous systems also applies in these rigid-framework materials, in which resisting the overoxidation of desired products is a major challenge. We demonstrate that MOFs bearing Fe(II) sites within Fe-3-mu(3)-oxo nodes active for conversion of CH4 + N2O mixtures (368-408 K) require steps beyond the radical-rebound mechanism to protect the desired CH3OH product. Infrared spectra and density functional theory show that CH3OH(g) is stabilized as Fe(III)-OCH3 groups on the MOF via hydrogen atom transfer with Fe(III)-OH groups, eliminating water. Consequently, upon addition of a protonic zeolite in inter- and intrapellet mixtures with the MOF, we observed increases in (CHOH)-O-3 selectivity with increasing ratio and proximity of zeolitic H+ to MOF-based Fe(II) sites, as methanol is protected within the zeolite. We infer from the data that (CHOH)-O-3( g) is formed via the radical-rebound mechanism on Fe(II) sites but that subsequent transport and dehydration steps are required to protect (CHOH)-O-3( g) from overoxidation. The results demonstrate that the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, that the selectivity-controlling steps involve both chemical and physical rate phenomena, as well as offering a strategy to mitigate overoxidation in these and similar systems.
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| 2. |
- Simons, Matthew C., et al.
(author)
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Structure, Dynamics, and Reactivity for Light Alkane Oxidation of Fe(II) Sites Situated in the Nodes of a Metal–Organic Framework
- 2019
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 141:45, s. 18142-18151
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Journal article (peer-reviewed)abstract
- Metal organic frameworks (MOFs), with their crystalline, porous structures, can be synthesized to incorporate a wide range of catalytically active metals in tailored surroundings. These materials have potential as catalysts for conversion of light alkanes, feedstocks available in large quantities from shale gas that are changing the economics of manufacturing commodity chemicals. Mononuclear high-spin (S = 2) Fe(II) sites situated in the nodes of the MOF MIL-100(Fe) convert propane via dehydrogenation, hydroxylation, and overoxidation pathways in reactions with the atomic oxidant N2O. Pair distribution function analysis, N-2 adsorption isotherms, X-ray diffraction patterns, and infrared and Raman spectra confirm the single-phase crystallinity and stability of MIL-100(Fe) under reaction conditions (523 K in vacuo, 378-408 K C3H8 + N2O). Density functional theory (DFT) calculations illustrate a reaction mechanism for the formation of 2-propanol, propylene, and 1-propanol involving the oxidation of Fe(II) to Fe(III) via a high-spin Fe(IV)=O intermediate. The speciation of Fe(II) and Fe(III) in the nodes and their dynamic interchange was characterized by in situ X-ray absorption spectroscopy and ex situ Mossbauer spectroscopy. The catalytic relevance of Fe(II) sites and the number of such sites were determined using in situ chemical titrations with NO. N-2 and C3H6 production rates were found to be first-order in N2O partial pressure and zero-order in C3H8 partial pressure, consistent with DFT calculations that predict the reaction of Fe(II) with N2O to be rate determining. DFT calculations using a broken symmetry method show that Fe-trimer nodes affecting reaction contain antiferromagnetically coupled iron species, and highlight the importance of stabilizing high-spin (S = 2) Fe(II) species for effecting alkane oxidation at low temperatures (<408 K).
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| 3. |
- Babucci, Melike, et al.
(author)
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Atomically Dispersed Metals on Well-Defined Supports including Zeolites and Metal–Organic Frameworks: Structure, Bonding, Reactivity, and Catalysis
- 2020
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In: Chemical Reviews. - : American Chemical Society (ACS). - 0009-2665 .- 1520-6890. ; 120:21, s. 11956-11985
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Journal article (peer-reviewed)abstract
- When metals in supported catalysts are atomically dispersed, they are usually cationic and bonded chemically to supports. Investigations of noble metals in this class are growing rapidly, leading to discoveries of catalysts with new properties. Characterization of these materials is challenging because the metal atoms reside on surfaces that are typically nonuniform in composition and structure. We posit that understanding of structures and catalytic properties of these materials is emerging most strongly from investigations of structurally uniform catalysts (metal atoms dispersed on crystalline supports) which can be characterized incisively with atomic-resolution electron microscopy, X-ray absorption spectroscopy, and infrared spectroscopy, bolstered by density functional theory. We assess the literature of such catalysts supported on zeotype materials, metal-organic frameworks, and covalent organic frameworks. Assessing characterization, reactivity, and catalytic performance of catalysts for oxidation, hydrogenation, the water-gas shift reaction, and others, we consider metal-support interactions and ligand effects for various metal-support combinations, evaluating the degree of structural uniformity of exemplary catalysts and summarizing structure-reactivity and structure-catalytic property relationships.
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| 4. |
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| 5. |
- Babucci, Melike, et al.
(author)
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Characterization of a Metal–Organic Framework Zr6O8 Node-Supported Atomically Dispersed Iridium Catalyst for Ethylene Hydrogenation by X-ray Absorption Near-Edge Structure and Infrared Spectroscopies
- 2021
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:31, s. 16995-17007
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Journal article (peer-reviewed)abstract
- The catalytic properties of atomically dispersed supported metals depend on the supports as ligands. We report metal-organic frameworks in the UiO-66 family, synthesized with various ligands that influence the electron-donor properties of the Zr6O8 nodes, including benzene-1,4-dicarboxylate linkers (some with substituents) and formate, acetate, benzoate, and trifluoroacetate. Catalytically active iridium species on the nodes were made by chemisorption of Ir(CO)(2)(acetylacetonato), giving Ir(CO) 2 groups, identified by infrared (IR) and extended X-ray absorption fine structure spectroscopies. The electronic properties of the iridium centers, which are sensitive to the supports as ligands, were characterized with high-energy-resolution fluorescence detection X-ray absorption near-edge spectroscopy (HERFD XANES), distinguishing the supports and giving results correlated with the nu(CO) IR spectra and catalytic activities of partially decarbonylated iridium sites for ethylene hydrogenation at 313 K and atmospheric pressure. The IR spectra of the working catalyst incorporating linkers with NH2 substituents show an initial induction period as reactants changed the iridium ligand environment, after which the catalyst operating in a once-through flow reactor underwent no measurable deactivation for 48 h. Among the results, we emphasize the value of HERFD XANES spectroscopy for the sensitive assessment of the effects of supports as ligands that determine the catalytic properties of atomically dispersed metals.
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| 6. |
- Babucci, Melike, et al.
(author)
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Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports
- 2019
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In: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 10:9, s. 2623-2632
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Journal article (peer-reviewed)abstract
- Single-site Ir(CO)(2) complexes bonded to high-surface-area metal oxide supports, SiO2, TiO2, Fe2O3, CeO2, MgO, and La2O3, were synthesized by chemisorption of Ir(CO)(2)(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts.
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| 7. |
- Babucci, Melike, et al.
(author)
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Tuning the Selectivity of Single-Site Supported Metal Catalysts with Ionic Liquids
- 2017
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In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 7:10, s. 6969-6972
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Journal article (peer-reviewed)abstract
- 1,3-Dialkylimidazolium ionic liquid coatings act as electron donors, increasing the selectivity for partial hydrogenation of 1,3-butadiene catalyzed by iridium complexes supported on high-surface-area gamma-Al2O3. High-energy-resolution fluorescence detection X-ray absorption near-edge structure (HERFD XANES) measurements quantify the electron donation and are correlated with the catalytic activity and selectivity. The results demonstrate broad opportunities to tune electronic environments and catalytic properties of atomically dispersed supported metal catalysts.
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| 8. |
- Babucci, Melike, et al.
(author)
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Unraveling the individual influences of supports and ionic liquid coatings on the catalytic properties of supported iridium complexes and iridium clusters
- 2020
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 387, s. 186-195
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Journal article (peer-reviewed)abstract
- Supported iridium complexes were synthesized by the reaction of Ir(CO)(2) (acac) (acac = acetylacetonato) with SiO2, gamma-Al2O3, and MgO. Extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectra demonstrate that the iridium was present as atomically dispersed species anchored to each support. The samples were treated in flowing H-2 at 673 K to form supported iridium clusters. EXAFS spectra and highangle annular dark-field scanning transmission electron microscopy images demonstrate that the average diameter of the iridium clusters on each support was approximately 1.2 nm. The catalysts before and after cluster formation were coated with each of a family of 1,3-dialkylimidazolium ionic liquids (ILs) having varying electron-donor tendencies probed by their nu(C2H) frequencies determined by IR spectroscopy. The coated and uncoated samples were tested as catalysts for partial hydrogenation of 1,3-butadiene in a flow reactor at 333 K, with turnover frequencies determined from differential conversions. The individual influences of the IL coatings and supports on the catalyst performance were found to depend strongly on whether the iridium was site-isolated complexes or present in clusters. The IL coatings as ligands exerted dominant effects on the clusters as catalysts, whereas the supports exerted dominant effects on the isolated iridium atoms. The results indicate how to tune the effects of metal nuclearity, IL coatings, and supports on the electronic environments and catalytic properties of the metals
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| 9. |
- Kurtoğlu, Samira F., et al.
(author)
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Electronic Structure of Atomically Dispersed Supported Iridium Catalyst Controls Iridium Aggregation
- 2020
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In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 10:21, s. 12354-12358
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Journal article (peer-reviewed)abstract
- Supported iridium complexes, Ir(C2H4)(2)/support, were characterized by X-ray absorption spectroscopy during a temperature ramp to 120 degrees C in flowing H-2. Iridium in complexes bonded to weak and moderate electron-donor supports, SiO2 and gamma-Al2O3, underwent aggregation, forming nanoparticles and clusters, respectively. When the support was a strong electron-donor (MgO), iridium remained site-isolated. Density functional theory calculations confirm the dependence of iridium-support bond strength on the support's electron-donor character. Coating the SiO2-supported complexes with 1-n-ethyl-3-methyl-imidazolium acetate enhanced electron density on the iridium, hindering its aggregation. These results demonstrate opportunities for stabilizing atomically dispersed supported noble metals under reducing conditions by choice of support/ionic liquid sheath combinations.
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| 10. |
- Qi, Liang, et al.
(author)
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Dehydrogenation of Propane and n-Butane Catalyzed by Isolated PtZn4 Sites Supported on Self-Pillared Zeolite Pentasil Nanosheets
- 2022
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In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 12:18, s. 11177-11189
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Journal article (peer-reviewed)abstract
- Propene and 1,3-butadiene are important building-block chemicals that can be produced by dehydrogenation of propane and butane on Pt catalysts. A challenge is to develop highly active and selective catalysts that are resistant to deactivation by Pt sintering and coke formation. We have recently shown (Qi , J. Am. Chem. Soc. 2021, 143, 21364-21378) that these objectives can be met for propane dehydrogenation (PDH) using atomically dispersed Pt anchored to neighboring SiOZn-OH groups bonded to the framework of dealuminated zeolite BEA. In the present study, we demonstrate that significantly superior performance can be achieved using self-pillared pentasil (SPP) zeolite nanosheets as supports. Following catalyst reduction in H-2, atomic resolution, scanning transmission electron microscopy (STEM), and X-ray absorption spectroscopy (XAS) indicate that Pt is stabilized in structures well approximated as ( Si-O-Zn)(4-5)Pt. These species are highly active, selective, and stable for PDH to give propene and for n-butane dehydrogenation (BDH) to give 1,3-butadiene. No catalyst deactivation was observed after 12 days of time on stream, and the selectivity remained at nearly 100% for PDH conducted at 823 K and a weight hourly space velocity (WHSV) of 1350 h(-1). The apparent rate coefficient for PDH on this catalyst is significantly higher than that reported previously for Pt-containing catalysts. For BDH at 823 K and a WHSV of 3560 h(-1), the selectivity to butene isomers and 1,3-butadiene is 98.9%, and the selectivity to 1,3-butadiene is 45%. We propose that the high catalyst stability observed during PDH and BDH is a consequence of a large fraction of the Pt-containing centers being located on the external surface of the zeolite nanosheets, where nascent coke precursors can desorb before condensing to form coke.
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| 11. |
- Wang, Zhengyan, et al.
(author)
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Dialing in Catalytic Sites on Metal Organic Framework Nodes : MIL-53(Al) and MIL-68(Al) Probed with Methanol Dehydration Catalysis
- 2020
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In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 12:47, s. 53537-53546
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Journal article (peer-reviewed)abstract
- Many metal organic frameworks (MOFs) incorporate metal oxide clusters as nodes. Node sites where linkers are missing can be catalytic sites. We now show how to dial in the number and occupancy of such sites in MIL-53 and MIL-68, which incorporate aluminum-oxide-like nodes. The methods involve modulators used in synthesis and postsynthesis reactions to control the modulator-derived groups on these sites. We illustrate the methods using formic acid as a modulator, giving formate ligands on the sites, and these can be removed to leave μ2-OH groups and open Lewis acid sites. Methanol dehydration was used as a catalytic reaction to probe these sites, with infrared spectra giving evidence of methoxide ligands as reaction intermediates. Control of node surface chemistry opens the door for placement of a variety of ligands on a wide range of metal oxide cluster nodes for dialing in reactivity and catalytic properties of a potentially immense class of structurally well-defined materials.
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| 12. |
- Yang, Dong, et al.
(author)
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Synthesis and characterization of tetrairidium clusters in the metal organic framework UiO-67 : Catalyst for ethylene hydrogenation
- 2020
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 382, s. 165-172
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Journal article (peer-reviewed)abstract
- Clusters that are well approximated as tetrairidium were synthesized from Ir(C2H4)2 complexes anchored to the Zr6O8 nodes of the metal organic framework (MOF) UiO-67 by treatment in H2 at 353 K. The conversion taking place within the porous MOF structure was monitored by infrared and X-ray absorption spectroscopies, which provide evidence of tetrairidium clusters formed from the mononuclear precursors. The supported clusters were tested in a flow reactor as catalysts for ethylene conversion at 298 K and atmospheric pressure with a 1:1 M feed ratio of H2 to ethylene. The results show that the turnover frequency (per Ir atom) characterizing the clusters is twice that of the mononuclear iridium complexes, with both catalysts being active for hydrogenation and dimerization and the clusters being less selective than the complexes for dimerization. Density functional theory calculations of the reaction energetics are in good accord with experiment, showing that the rate-determining step for the hydrogenation on the isolated iridium complexes is the H2 activation on iridium, whereas the hydrogenation of an iridium-bound ethyl ligand is rate determining for the cluster.
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| 13. |
- Yang, Dong, et al.
(author)
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The Surface Chemistry of Metal Oxide Clusters : From Metal–Organic Frameworks to Minerals
- 2020
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In: ACS Central Science. - : American Chemical Society (ACS). - 2374-7943 .- 2374-7951. ; 6:9, s. 1523-1533
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Journal article (peer-reviewed)abstract
- Many metal–organic frameworks (MOFs) incorporate nodes that are small metal oxide clusters. Some of these MOFs are stable at high temperatures, offering good prospects as catalysts—prospects that focus attention on their defect sites and reactivities—all part of a broader subject: the surface chemistry of metal oxide clusters, illustrated here for MOF nodes and for polyoxocations and polyoxoanions. Ligands on MOF defect sites form during synthesis and are central to the understanding and control of MOF reactivity. Reactions of alcohols are illustrative probes of Zr6O8 node defects in UiO-66, characterized by the interconversions of formate, methoxy, hydroxy, and linker carboxylate ligands and by catalysis of alcohol dehydration reactions. We posit that new reactivities of MOF nodes will emerge from incorporation of a wide range of groups on their surfaces and from targeted substitutions of metals within them.
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| 14. |
- Yang, Dong, et al.
(author)
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Tuning Catalytic Sites on Zr6O8 Metal–Organic Framework Nodes via Ligand and Defect Chemistry Probed with tert-Butyl Alcohol Dehydration to Isobutylene
- 2020
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 142:17, s. 8044-8056
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Journal article (peer-reviewed)abstract
- Metal–organic frameworks (MOFs) have drawn wide attention as candidate catalysts, but some essential questions about their nature and performance have barely been addressed. (1) How do OH groups on MOF nodes act as catalytic sites? (2) What are the relationships among these groups, node defects, and MOF stability, and how do reaction conditions influence them? (3) What are the interplays between catalytic properties and transport limitations? To address these questions, we report an experimental and theoretical investigation of the catalytic dehydration of tert-butyl alcohol (TBA) used to probe the activities of OH groups of Zr6O8 nodes in the MOFs UiO-66 and MOF-808, which have different densities of vacancy sites and different pore sizes. The results show that (1) terminal node OH groups are formed as formate and/or acetate ligands present initially on the nodes react with TBA to form esters, (2) these OH groups act as catalytic sites for TBA dehydration to isobutylene, and (3) TBA also reacts to break node–linker bonds to form esters and thereby unzip the MOFs. The small pores of UiO-66 limit the access of TBA and the reaction with the formate/acetate ligands bound within the pores, whereas the larger pores of MOF-808 facilitate transport and favor reaction in the MOF interior. However, after removal of the formate and acetate ligands by reaction with methanol to form esters, interior active sites in UiO-66 become accessible for the reaction of TBA, with the activity depending on the density of defect sites with terminal OH groups. The number of vacancies on the nodes is important in determining a tradeoff between the catalytic activity of a MOF and its resistance to unzipping. Computations at the level of density functional theory show how the terminal OH groups on node vacancies act as Brønsted bases, facilitating TBA dehydration via a carbocation intermediate in an E1 mechanism; the calculations further illuminate the comparable chemistry of the unzipping.
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