Selective Reduction of Carboxylic Acids to Aldehydes with Promoted MoO₃ Catalysts

Abstract

Selective activation of renewable carboxylic acids on promoted molybdenum oxides to form alcohols and aldehydes is reported. A combination of reaction kinetics, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) reveals that the activity scales with the concentration of Mo5+ active sites and is a strong function of surface hydrogen coverage. The addition of a very small loading (0.05 wt %) of Pt drastically increases rates of selective deoxygenation at lower temperatures (<350 °C) but diminishes rates at elevated temperatures due to over-reduction of the support. Here, it is reported that the incorporation of Pt clusters on MoO3 decreases the apparent activation barrier for acid conversion by over 32 kJ/mol, which highlights the significant role of site regeneration facilitated by hydrogen splitting and spillover. Our findings suggest that the rate-determining step for converting pentanoic acid shifts upon introducing Pt clusters from formation/regeneration of oxygen vacancies to H addition to the carbonyl carbon.

Revealing the Mechanistic Details for the Selective Deoxygenation of Carboxylic Acids over Dynamic MoO3 Catalysts

“…Figures S3 and S4 illustrate the selectivity profiles of both catalysts at 350 °C under low and high partial pressures of PA, respectively. At this temperature, it is anticipated that the presence of Pt as a promoter would lead to a reduction in selectivity toward PAL, as we have previously reported . This reduction in selectivity can be attributed to the occurrence of side reactions facilitated by the presence of the Pt metal, leading to a shift in the overall selectivity.…”

“…The strong adsorption of PA over an oxygen vacancy might cause the inhibition effect observed at high partial pressures of PA over both MoO 3 and 0.05 wt % Pt/MoO 3 . As previously reported, DFT calculations of the PA adsorption over MoO 3 indicate that the adsorption of PA through the carbonyl-group oxygen (CO) at a defect site is 57 kJ/mol stronger than the adsorption through the hydroxyl group (-OH) . Although the adsorption through the CO group is stronger, the activation energy required to produce the aldehyde is significantly higher compared to the activation energy through the hydroxyl group, suggesting that the carboxylic acids adsorbed via the CO group are only present as spectator species .…”

“…This result supports the hypothesis that H 2 dissociation is rate-limiting at a high partial pressure of PA due to the strong inhibition by PA. Previously, diminished activation barriers have been reported when comparing the Pt/MoO 3 catalyst to MoO 3 alone . Comparing the activation barrier for Pt/MoO 3 at higher partial pressures of acid co-fed reveals an identical activation barrier, as shown in Figure S2, indicating that the Pt sites still influence the chemistry at high PA partial pressures.…”

“…As previously reported, DFT calculations of the PA adsorption over MoO 3 indicate that the adsorption of PA through the carbonyl-group oxygen (C�O) at a defect site is 57 kJ/mol stronger than the adsorption through the hydroxyl group (-OH). 19 Although the adsorption through the C�O group is stronger, the activation energy required to produce the aldehyde is significantly higher compared to the activation energy through the hydroxyl group, suggesting that the carboxylic acids adsorbed via the C�O group are only present as spectator species. 19 Hence, we hypothesize that these strongly binding species are responsible for the inhibition behavior observed at 350 °C.…”

Cell-free metabolic engineering enables selective biotransformation of fatty acids to value-added chemicals

“…In sunflower oil, the composition (%) of stearic acid is 4.5 ( Aransiola et al, 2014 ). While chemical methods have been established for fatty acid conversion, these processes often require harsh reaction conditions and rare-metal catalysts with low efficiency and poor selectivity ( Ford et al, 2012 ; Gomez et al, 2022 ; Witsuthammakul and Sooknoi, 2016 ).…”