(741d) Elucidating the Effects of ReOx Promoters and Water during Metal-Catalyzed C–O Hydrogenolysis of Alcohols

The selective C–O cleavage of biomass-derived alcohols is critical to their transformation into value-added chemicals (while complete deoxygenation is desired for fuels). Oxophilic metal promotors such as ReO_x, WO_x, MoO_x, RuO_x, have been shown to increase C–O hydrogenolysis rates and alter product distributions for a variety of hydrogenolysis reactions.^1–4 Most studies have focused on aqueous phase reactions, which complicate kinetic studies and mechanistic analysis because of the ubiquitous role of H₂O in chemical reactions (as a solvent, reactant, and co-catalyst). Here, we have examined the effects of ReO_x-promotors on Pt and Au catalysts for C–O hydrogenolysis of glycerol (a bio-diesel byproduct) and ethanol (a volatile fermentation product suitable for gas and liquid phase comparisons). These reactions are studied in aqueous and gas-phase flow and batch reactors with and without co-fed water.

Pt (5 wt. %) and Pt-ReO_x (5 wt. % of both metals) were impregnated on activated carbon (Norit SG-1), additionally, Au (1 wt. %) and Au-ReO_x (1 wt. %, 3 wt. %) were prepared on CeO₂ supports. Aqueous phase reactions are carried out in a batch reactor (1 wt. % alcohol, 200 °C, 40 bar H₂). For glycerol reactions on Pt-based systems, the ReO_x promoter increased rates by a factor of ~10 (normalized by Pt-surface atoms) and shifted selectivity from 1,2-propanediol (72%) and ethylene glycol (19%) to 1,3-propanediol (18%), 1,2-propanediol (20 %), and 1- and 2-propanol (61 %). These selectivity and rate shifts indicate that ReO_x facilitates C–O activation and enables C–O activation at secondary positions of the C₃ backbone. This result is consistent with prior studies^2,4 and consistent with their work suggesting that ReO_x promotors incorporate Brønsted acid sites into the catalyst. These Brønsted acid sites catalyze dehydration reactions (with a preference towards secondary C–O activation because of enhanced carbenium ion stability) and any unsaturated compounds formed via dehydration are rapidly converted to saturated alcohols on Pt at these high H₂ pressures (40 bar). For ethanol, ReO_x increases Pt-based reaction rates by ~3 and shifts selectivity away from diethylether (formed by condensation) and towards gas-phase ethane formation. Preliminary data gathered in a gas-phase flow reactor (neat ethanol feed, no water) shows rates ~10-times higher on ReO_x-Pt than the unpromoted Pt catalyst and (as was observed in aqueous phase) shifts selectivity away from diethylether and towards C–O cleavage products. This indicates that liquid H₂O is not required to enable ReO_x to promote C–O hydrogenolysis rates, additional studies which will co-feed water and ethanol vapor will further elucidate the role of H₂O in this reaction.

Au-based catalysts, in contrast to Pt, are ineffective at hydrogenation; thus, dehydration chemistry on Brønsted acid sites (formed by ReO_x) should yield unsaturated compounds and avoid complete deoxygenation (as C₃ dienes are very unstable). Such hypotheses were confirmed here, with glycerol showing no reactivity on mono-functional Au/CeO₂ catalysts while Au-ReO_x/CeO₂ produced prop-2-en-1-ol at high (> 90%) selectivity, indicating that C–O cleavage can occur solely on bifunctional Au-ReO_x sites (ReO_x alone is not reactive) and that it forms, as primary intermediates, unsaturated compounds (indicative of Brønsted acid reactivity). The consequence of phase and the presence of H₂O on these Au-catalyzed reactions is currently being examined.

References

(1) Nakagawa, Y.; Tamura, M.; Tomishige, K. Res. Chem. Intermed. 2018, 44, 3879–3903.

(2) Daniel, O. M.; DeLaRiva, A.; Kunkes, E. L.; Datye, A. K.; Dumesic, J. A.; Davis, R. J. ChemCatChem 2010, 2, 1107–1114.

(3) Hibbitts, D.; Tan, Q.; Neurock, M. J. Catal. 2014, 315, 48–58.

(4) Chia, M.; Pagán-Torres, Y. J.; Hibbitts, D.; Tan, Q.; Pham, H. N.; Datye, A. K.; Neurock, M.; Davis, R. J.; Dumesic, J. A. J. Am. Chem. Soc. 2011, 133, 12675–12689.