The ability to tailor the geometry and ensemble size of the metal nodes in MOFs allows for unprecedented control of the active sites and could lead to significant advances in rational catalyst design. (CuxRh1-x)3(BTC)2 (abbreviated CuRhBTC, where BTC3-=benzenetricarboxylate) is prepared from Rh3+ and CuBTC (HKUST) by post-synthetic ion-exchange . Using a combination of X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) techniques, the oxidation state of Rh in CuRhBTC is identified as 2+. These Rh2+ sites are active for the catalytic hydrogenation of propylene to propane at room temperature, and catalytic activity is stable over a 24 hour reaction period. In situ and post-reaction XPS studies show that the Rh2+ is stable under reaction conditions and over extended reaction times. Density functional theory (DFT) calculations suggest a mechanism in which hydrogen dissociation at the Rh2+ sites is facilitated by Rh-O bond scission. The experimental apparent activation energy (6.3 kcal/mol) is in general agreement with the barrier height predicted by DFT for the rate-limiting step.

Metal nodes in metal-organic frameworks as active sites for gas-phase catalytic hydrogenation

2019 Fall National Meeting

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