Mechanism of Methanol Dehydration Catalyzed by Al₈O₁₂Nodes Assisted by Linker Amine Groups of the Metal-Organic Framework CAU-1

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 Al₈O₁₂nodes 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 S_N2-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 Al₈O₁₂node adsorbs methanol strongly but, because of geometric constraints, renders the TS for methanol dehydration energetically unfavorable.