Selective C=O hydrogenation of cinnamaldehyde over Ir-based catalysts and the comparison with C-O hydrogenolysis of polyols
Abstract
Ir-based bimetallic catalysts supported on rutile TiO2 with low surface area were applied to selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. Ir-FeOx/rutile (Ir: 3 wt%, Fe/Ir = 0.1 (molar basis)) was particularly effective. The Ir-FeOx/rutile catalyst can be applied to selective hydrogenation of various unsaturated aldehydes (crotonaldehyde, furfural, 2-hexenal and citral) to unsaturated alcohols (≥ 95% selectivity, ≥ 81% yield). Since Ir-FeOx/rutile catalysts have been reported to be also effective in C-O hydrogenolysis of 1,2-diols to 2-monoalcohols, the structure-performance relationship was closely compared between hydrogenation and C-O hydrogenolysis. The optimum catalyst for hydrogenation had lower Ir loading and lower Fe/Ir ratio than that for C-O hydrogenolysis. In addition, high-temperature reduction of catalyst decreased the activity in cinnamaldehyde hydrogenation, while the effect of reduction temperature was reported to be small in C-O hydrogenolysis. Characterization with transmission electron microscope (TEM), CO adsorption, XPS and FT-IR suggested similar structure of the optimized catalysts in the two reactions: Ir-Fe alloy and Fem+ species modifying the alloy surface; however, higher Fem+/Fe0 ratio was observed for the optimized catalyst in cinnamaldehyde hydrogenation. From the kinetic studies (first order towards H2 pressure and zero order towards cinnamaldehyde concentration, which are similar to C-O hydrogenolysis), the nucleophilic attack of the hydride species to the adsorbed cinnamaldehyde was considered as the rate-determining step. The Ir-Fe alloy can give the hydride nature to the adsorbed hydrogen species, and the Fem+ modifier can be the adsorption site. In cinnamaldehyde hydrogenation, supplying adsorption site of cinnamaldehyde is more effective.