Enhancing the performance of SBA-15-supported copper catalysts by chromium addition for the chemoselective hydrogenation of trans-cinnamaldehyde

Abstract

SBA-15-supported copper–chromium mixed oxide nanoparticles (CuCr/SBA-15) were prepared by incipient wetness impregnation followed by mild drying at 25 °C and calcination. The Cu : Cr weight ratios were 1 : 1, 5 : 1, and 10 : 1, at a constant total loading of 5 wt%. Monocomponent SBA-15-supported Cu–oxide (Cu/SBA-15) and Cr–oxide (Cr/SBA-15) were prepared as reference samples. The materials were systematically characterized by XRD at low and high angles, N₂ physisorption, DR UV-vis, FT-IR, and XPS spectroscopies, and TPR. XRD at low angles and N₂ physisorption confirmed the preservation of the mesoporous structure of the SBA-15 support after impregnation and calcination. In the case of monocomponent samples, CuO appeared poorly dispersed, while the Cr species (Cr₂O₃, mono- and polychromates) appeared highly dispersed on the surface of the SBA-15 support. The progressive addition of chromium to copper had positive effects on the average crystallite size of CuO, which decreased from ∼28 nm (Cu/SBA-15) to ∼3 nm (CuCr/SBA-15; Cu : Cr = 1 : 1), and on reducibility, as well. Metallic active phases were obtained by reducing of the oxide phases under a hydrogen flow at 350 °C. By comparison to the monometallic catalysts, the reduced CuCr/SBA-15 materials were active in the hydrogenation of cinnamaldehyde and chemoselective towards cinnammyl alcohol (>50 mol%). The activity can be correlated with the particle size of copper, whereas the high selectivity to unsaturated alcohol can be associated with the presence of dual Cu⁰–Crⁿ⁺ sites.