Pair sites on Al3O nodes of the metal-organic framework MIL-100: Cooperative roles of defect and structural vacancy sites in methanol dehydration catalysis

Abstract

The metal–organic framework (MOF) MIL-100, which incorporates vacancy sites on Al3O nodes, was synthesized with various densities of missing-benzene-1,3,5-tricarboxylate (BTC3−)-linker defect sites by using HNO3 or acetic acid as a modulator. We report data that resolve the effects on reactivity of the node vacancies and defects that are neighbors of each other. For example, extra-framework BTC3− ligands were bonded to one vacancy site per node, and modulator-derived NO3− and acetate ligands were bonded in various ratios to defect sites. In reactions with formic acid, these ligands were replaced with monodentate and bidentate formates. The ligands on defects were found to be more reactive than those on vacancies. The MOF was used to catalyze methanol dehydration at 220 °C and 1 bar in a flow reactor, and the catalytic activity increased with increasing numbers of defects and with replacement of extra-framework BTC3- with formate. Catalysis started as methanol reacted with formate ligands to form methyl formate, which desorbed, leaving methoxy groups on the nodes—and these became catalytic reaction intermediates. IR spectra and catalyst performance data show that that catalysis proceeds as bidentate methoxy bonded to a defect site reacts with monodentate methoxy bonded to a neighboring vacancy site. Having a high density of these paired sites, MIL-100(Al) is markedly more active catalytically than the MOF MIL-53, in which these essential sites are mostly isolated. Deactivation of MIL-100 during catalysis resulted from self-inhibition, as bidentate methoxy ligands were replaced with μ2-OH ligands formed from water produced in the dehydration reaction. The results highlight the role of pair sites on MOF nodes for catalyzing a reaction that involves a bimolecular step, indicating how to control the catalytic sites. We thus foresee opportunities to tailor MOF catalytic properties for reactions involving various numbers of catalytic sites on the nodes.