| 1. |
- Wang, Yang, et al.
(author)
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A Tunable Multivariate Metal-Organic Framework as a Platform for Designing Photocatalysts
- 2021
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:17, s. 6333-6338
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Journal article (peer-reviewed)abstract
- Catalysts for photochemical reactions underlie many foundations in our lives, from natural light harvesting to modern energy storage and conversion, including processes such as water photolysis by TiO2. Recently, metal–organic frameworks (MOFs) have attracted large interest within the chemical research community, as their structural variety and tunability yield advantages in designing photocatalysts to address energy and environmental challenges. Here, we report a series of novel multivariate metal–organic frameworks (MTV-MOFs), denoted as MTV-MIL-100. They are constructed by linking aromatic carboxylates and AB2OX3 bimetallic clusters, which have ordered atomic arrangements. Synthesized through a solvent-assisted approach, these ordered and multivariate metal clusters offer an opportunity to enhance and fine-tune the electronic structures of the crystalline materials. Moreover, mass transport is improved by taking advantage of the high porosity of the MOF structure. Combining these key advantages, MTV-MIL-100(Ti,Co) exhibits a high photoactivity with a turnover frequency of 113.7 molH2 gcat.–1 min–1, a quantum efficiency of 4.25%, and a space time yield of 4.96 × 10–5 in the photocatalytic hydrolysis of ammonia borane. Bridging the fields of perovskites and MOFs, this work provides a novel platform for the design of highly active photocatalysts.
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| 2. |
- 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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| 3. |
- 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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