Combining noble and oxophilic metals is known to improve selectivity in hydrogenation of carboxylic acids to alcohols, but the corresponding catalyst structure and influence on reaction mechanism remains poorly understood. In this work, we investigated RuSn for the selective reduction of propionic acid (PA) to 1-propanol (1-PrOH). Initial experiments confirmed the high yield and stability of RuSn, while Ru alone resulted in extensive cracking under identical conditions. Extended X-ray absorption fine structure spectroscopy was used to identify Ru0 nanoparticles with a near-surface bimetallic Ru0Sn0 phase and small SnOx domains capable of cycling between 2+ and 4+ oxidation states. The presence of SnOx under operating conditions was further supported experimentally by inhibiting reactions with phenyl phosphonic acid. The catalytic role of Ru0, Ru0Sn0, and SnOx was then investigated with density functional theory (DFT) for: (i) PA hydrogenation to 1-PrOH, (ii) over-reduction of 1-PrOH to propane, and (iii) PA cracking via decarbonylation and decarboxylation. All paths investigated along PA hydrogenation to 1-PrOH were found to be kinetically accessible over Ru0. In contrast, these pathways exhibit increased barriers over Ru0Sn0 and only the direct C-OH scission path to 1-PrOH was found to be feasible over SnO/Ru, with the SnO cluster facilitating C-OH scission and Ru enabling hydrogen activation. The computed barriers for over-reduction and PA cracking followed the trend Ru0 &lt; Ru0Sn0 &lt; SnO/Ru, showing that non-selective pathways are likely over Ru0 but practically inaccessible over SnO/Ru. The Sn0 poisoning effect was attributed to lower density of reaction-ready Ru sites and the destabilization of adsorbate fragments on Ru next to Sn. The discovery that SnO can perform selective hydrogenation was linked to its acidity, which was evident by Brønsted-Evans-Polanyi (BEP) relations for C-H scission. As a result, we propose a reaction mechanism that involves both metallic and oxidic sites to provide new insight into the selective reduction of carboxylic acids over RuSn.

Active surface and mechanism for propionic acid reduction over RuSn

256th National Meeting (2018)

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