(180e) Metal-Organic Framework Catalysts for Chemical Upgrading Reactions

Metal-organic frameworks (MOFs), structurally well-defined porous materials built from metal oxide nodes connected by organic linkers, present unique isolated active sites and catalytic environments for performing chemical reactions. MOFs can exhibit catalytic activity through their metal oxide nodes and linkers, or they can be used as supports to graft catalytically active low-nuclearity metals or nanoparticles introduced through post-synthetic chemical modifications. Using density functional theory (DFT) in conjunction with experimental activity-selectivity measurements, we demonstrate the various opportunities for designing MOF and MOF-supported catalysts by elucidating their active sites and reaction mechanisms—illustrated for alcohol dehydration and alkyne conversion reactions.

We probed the catalytic activity of the Al₈O₁₂ nodes of the MOF CAU-1 for methanol dehydration to dimethyl ether. Using methanol conversion data measured in plug-flow reactor, infrared spectra of the MOF, and DFT studies on cluster models of the Al₈O₁₂ nodes to which methanol and water were bonded, we found that methanol dehydration occurs on catalytic sites involving methoxy ligands as reaction intermediates that bridge paired aluminum atoms of the node, and the reaction is assisted by nearby amine groups of the linker that allow for a near-linear geometry of the S_N2 transition state. In contrast, the dehydration of methanol strongly adsorbed in the central cavity of the node was found to be energetically unfavorable owing to the geometric constraints of the transition state.

Cu grafted on the Zr₆O₈ nodes of the MOF NU-1000 through post-synthesis modifications was found to be highly active for propyne dimerization to hexadienes. DFT studies of the single-atom Cu-NU-1000 catalyst suggest that the dissociation of hydrogen adsorbed on Cu-NU-1000 is the rate-limiting step, in agreement with the observed dependence of the hexadiene selectivity on the hydrogen partial pressure. DFT calculations are also carried out to compare the catalytic properties of single-atom Cu and Cu nanoparticles in NU-1000.