Selective CO hydrogenation of cinnamaldehyde over Ir-based catalysts and its comparison with C–O hydrogenolysis of polyols†

Ir-based bimetallic catalysts supported on rutile TiO₂ with low surface area were applied to the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. Ir–FeO_x/rutile (Ir: 3 wt%, Fe/Ir = 0.1 (molar basis)) was particularly effective. The Ir–FeO_x/rutile catalyst could be applied to the selective hydrogenation of various unsaturated aldehydes (crotonaldehyde, furfural, 2-hexenal and citral) to unsaturated alcohols (≥95% selectivity, ≥81% yield). Since Ir-FeO_x/rutile catalysts have also been reported to be 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 a lower Ir loading and a lower Fe/Ir ratio than that for C–O hydrogenolysis. In addition, high-temperature reduction of the catalyst decreased the activity in cinnamaldehyde hydrogenation, while the effect of reduction temperature was reported to be small in C–O hydrogenolysis. Characterization using transmission electron microscopy (TEM), CO adsorption, XPS and FT-IR suggested a similar structure for the optimized catalysts in the two reactions: Ir–Fe alloy and Fe^m+ species modifying the alloy surface. However, a higher Fe^m+/Fe⁰ ratio was observed for the optimized catalyst in cinnamaldehyde hydrogenation. From the kinetic studies (first-order with respect to H₂ pressure and zero-order with respect to cinnamaldehyde concentration, similar to C–O hydrogenolysis), the nucleophilic attack of the hydride species on the adsorbed cinnamaldehyde was considered the rate-determining step. The Ir–Fe alloy imparted the hydride nature to the adsorbed hydrogen species, and the Fe^m+ modifier served as the adsorption site. In cinnamaldehyde hydrogenation, supplying adsorption sites for cinnamaldehyde was more effective.