(394g) A DFT Study of Peroxide Decomposition Over Copper Paddlewheels within Metal-Organic Frameworks

Autooxidation is an important industrial process for converting raw hydrocarbon materials to more useful oxidized products. Hydrocarbons react with dioxygen in the air to produce hydroperoxide intermediates, which then decompose to give oxidized products such as alcohols and ketones. These oxidized products, such as cyclohexanone, are then used as precursors for many pharmaceuticals, dyes, and polymer processes. In this work, we investigated metal-organic frameworks (MOFs) as catalysts for these processes. Recently, Corma and coworkers (Xamena et al., J. Catal., 2008, 255, 220) showed that cobalt- and copper-containing MOFs were active for peroxide decomposition during tetralin autooxidation. We studied the two MOFs studied by Xamena et al., as well as a copper paddlewheel structure which is common to other MOFs. Using density functional theory (DFT), we studied decomposition reactions over different metal clusters that are representative of the repeating metal sites in each MOF. Our DFT results suggest that the tetrahedral cobalt site in Co-MOF is too sterically protected to catalyze hydroperoxide decomposition. Additionally, carboxyl groups of the linkers in Cu-MOF seem to shield the copper sites from hydroperoxide species and hinder catalysis. For these two cases, we believe catalysis observed by Xamena et al. likely occurs on the outer surface of the MOF crystals. We have successfully mapped a decomposition pathway of hydroperoxide molecules over a copper paddlewheel, which is found both in the HKUST-1 material and the series of NOTT materials. Our results indicate the copper paddlewheel stabilizes the ?OH radical after oxygen-oxygen bond cleavage of the ROOH species, and this radical can further react with another ROOH molecule to yield a peroxy radical species (ROO?) and a bound water molecule. After removal of water, the catalytic cycle is complete and the paddlewheel is free to react again. Our results suggest that catalysis appears to be possible over copper paddlewheels in MOFs, and this work may lead to new, improved oxidation catalysts in the future.