Mechanism of Methanol Dehydration Catalyzed by Al₈O₁₂ Nodes Assisted by Linker Amine Groups of the Metal–Organic Framework CAU-1

Abstract

Elucidation of catalytic sites and mechanisms of reactions on metal oxides is hindered by the complexity and heterogeneity of the surfaces. In contrast, the metal oxide cluster nodes of metal–organic frameworks (MOFs) have well-defined, essentially molecular structures that provide excellent platforms for understanding catalytic reactions in depth. We report an experimental and density functional theory (DFT) investigation of methanol dehydration on the Al8O12 nodes of the MOF CAU-1, presenting catalyst performance data obtained with vapor-phase methanol contacting MOF particles in a flow reactor at temperatures in the range of 180–250 °C. These data and infrared spectra of the MOF characterizing its reactivity with methanol and water show that the reactivities of node μ2-OH groups, influenced by the coordination environments of the node aluminum atoms, react with methanol to form catalytic sites incorporating methoxy ligands that bridge paired aluminum atoms, with the dehydration mechanism engaging nearby MOF linker amine groups that bond to methanol. DFT investigations of the mechanism suggest that the SN2-type methanol dehydration reaction is energetically favored to occur at MOF linker amine groups, which allow for a near-linear transition state (TS) geometry; in contrast, the central cavity of the Al8O12 node adsorbs methanol strongly but, because of geometric constraints, renders the TS for methanol dehydration energetically unfavorable.

Pressure-induced postsynthetic cluster anion substitution in a MIL-53 topology scandium metal–organic framework

“…The presumably associative ligand exchange mechanism could be a proxy not only for solvent-induced structural breakdown of MOFsfor example certain Zr MOFs containing m 2 -OH ligands at their SBUs are known to be sensitive to CH 3 OH [80][81][82] but also a potential mechanism for small molecule activation, as similar bridging methoxide units at the SBUs of related MOFs have been implicated in catalytic mechanisms. 83 In addition, formation of terminal Zr-OCH 3 units at defect sites in Zr MOFs is implicated in their CH 3 OH activation for further functionalisation, 84 while the presence of m 2 -OCH 3 in the SBUs of GUF-1 could explain changes in physical properties observed when related MIL-53 analogues are directly synthesised in CH 3 OH. 85,86 Our bulk scale pressure measurements identied a low level (∼2%) pore-located impurity in the samples, which we have not currently identied, but whose isotopic enrichment conrms its origin in the 13 CH 3 OH used to induce reactivity.…”

Computational Insights into the Mechanism of Nitric Oxide Generation from S-Nitrosoglutathione Catalyzed by a Copper Metal–Organic Framework

“…A variety of computational studies of chemical processes catalyzed by MOFs have provided structural, thermodynamic, and mechanistic insights. − Herein, we present theoretical calculations that address the following five fundamental questions relevant to the catalytic mechanism of NO release from GSNO at an active site of a Cu-BTTri MOF catalyst. First, what constitutes a minimal functional model for the MOF, the GSH, and the GSNO that will enable meaningful computational studies?…”

2-Propanol Dehydration on the Nodes of the Metal–Organic Framework UiO-66: Distinguishing Catalytic Sites for Formation of Propene and Di-isopropyl Ether

“…The dehydration reactions of methanol, ethanol, and tert -butyl alcohol , have been used to characterize the catalytic sites of MOFs that incorporate Zr 6 O 8 nodes, a large family typified by the well-investigated UiO-66. − This MOF is appealing in prospect for catalytic applications because it has (relatively) high stability (being reported to retain its crystalline structure at temperatures as high as 400 °C) , and includes members offering a wide range of pore sizes. − ,, The results of experiments characterizing the catalytic properties of UiO-66 have shown that changes in the composition of the node ligands, including hydroxyl, aqua, and alkoxides, ,,, and even the linkers, affect the catalyst performance.…”

“…14−16 This MOF is appealing in prospect for catalytic applications because it has (relatively) high stability (being reported to retain its crystalline structure at temperatures as high as 400 °C) 15,17 and includes members offering a wide range of pore sizes. [14][15][16]18,19 The results of experiments characterizing the catalytic properties of UiO-66 have shown that changes in the composition of the node ligands, including hydroxyl, aqua, and alkoxides, 5,6,12,13 and even the linkers, 7 affect the catalyst performance.…”

“…Alcohol dehydration reactions are valuable probes of the active sites of many catalysts that incorporate acidic and basic groups, including metal oxides, , zeolites, functionalized carbons, and metal–organic frameworks (MOFs). , Common probe reactions are methanol dehydration, , which gives only one dehydration product, dimethyl ether, at low temperature, and tert -butyl alcohol dehydration, , which often gives only isobutene as a dehydration product. 2-Propanol dehydration is especially helpful because it often gives both the olefin and ether products. , The selectivity of this reaction is valuable in providing insights into subtle changes in the nature of the catalytic sites.…”