Metal oxide promoters generally induce structural or electronic effects or create a new type of active site to improve catalytic performance. Manganese oxide is an efficient promoter for the Cu-catalyzed hydrogenation of ethyl acetate (EtOAc) to ethanol, however yet little is known about its exact nature. We use a chemically inert carbon support to facilitate characterization and fundamental studies of intrinsic MnOx promoter effects. We prepared series of model catalysts of highly dispersed CuMnOx nanoparticles on graphitic carbon (~500 m2/g) via facile co-impregnation of aqueous solutions of Cu(NO3)2 and Mn(NO3)2. Careful tuning of the synthesis parameters led to catalysts with a constant particle size around 6 nm over a wide range of Mn loadings. The performance of these catalysts was investigated for hydrogenation of EtOAc, at 30 barg and 180–210 °C. The selectivity towards ethanol was ≥99.5%. Adding 11 mol% Mn resulted in a 7-fold increase in TOF compared to unpromoted Cu/C (Fig. 1b). MnOx strongly limited the Cu particle growth (Fig. 1a). The addition of 11 mol% Mn led to a remarkable decrease in activation energy (Ea) from 104 (Cu/C) to 54 kJ/mol (11_MnOx-Cu/C). Adding water (from 50 to 400 ppm) in the EtOAc feedstock strongly decreased the catalytic activity. For Cu/C the presence of extra water caused severe particle growth (from 5.0 nm to 16.4 nm) compared to 33_MnOx-Cu/C (from 5.5 nm to 6.2 nm). The reaction order in H2O was –2.24 for Cu/C, showing the inhibiting effect of increased H2O concentrations. In contrast, the reaction order in 33_MnOx-Cu/C was –0.52, mitigating the reversible catalyst deactivation. Large amounts of MnOx caused redispersion of the surface-mobile MnOx species. Especially for &gt;11 mol% Mn, a fraction of the MnOx was not in contact with Cu, thus not contributing to a further increase in activity. These findings may contribute to implementing MnOx as an efficient promoter for various hydrogenation reactions.

Manganese oxide promoter effects in the copper-catalyzed hydrogenation of ethyl acetate

ACS Spring Meeting 2021

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