The nodes of some MOFs closely resemble small pieces of metal oxides, exemplified by the Zr6 nodes of UiO-66, UiO-67, and NU-1000. Understanding the precise structures of the nodes, including their vacancies and surface sites, is crucial for the application of MOFs as catalyst supports. Structural defect sites of UiO-66/67 and NU-1000 have been inferred on the basis of IR spectra and density functional theory (DFT) to consist of pairs of terminal OH and OH2 groups (called site 1). An open challenge is to tune these sites as a basis for tuning the catalytic properties of metal complexes bonded to them. We developed a treatment process to convert site 1 into a new proton topology incorporating methoxy groups. IR and DFT results led to the identification of an intermediate methoxy-Zr structure incorporating structural vacancies on the node surface which facilitated the conversion of site 1 to a new dehydro-monohydroxo site, which provides new opportunities for anchoring of metal-containing catalytic groups to the MOFs. The results show how the combination of DFT calculations and IR spectra can be used to determine the identity and chemistry of the functional groups on the defect sites on MOF nodes. We investigated species formed by chemisorption of Rh(C2H4)2(acac), I, (acac is acetylacetonate) on the MOF nodes as catalysts. Bonding of the rhodium to three separate sites gave species identified by IR, EXAFS, and DFT results as mononuclear Rh(C2H4)2 complexes. Conversion of these to rhodium gem-dicarbonyls resulted from reaction with CO; the νCO values distinguish electron-donor properties of the support sites and thereby provide a basis for comparison of various MOFs as catalyst supports. The catalytic activity and selectivity of the species initially present as Rh(C2H4)2 for ethylene hydrogenation and dimerization were investigated experimentally and computationally, and the results show that the catalyst performance is strongly influenced by the node sites.

Ideal catalysts beyond single crystals: MOFs with metal oxide-like nodes

253rd National Meeting (2017)

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