Surface and structural effects in the hydrogenation of citral over RuCu/KL catalysts

Four 2 wt% Ru/KL catalysts, doped with copper, were prepared by successive impregnation of a KL zeolite with Ru3(CO)12 and Cu(NO3)2 · 3H2O aqueous solutions in an adequate concentration to obtain 0, 0.5, 1 and 1.5 wt% of copper. The samples Ru/KL, RuCu0.5/KL, RuCu1.0/KL and RuCu1.5/KL were characterized by X-ray diffraction, temperature-programmed reduction, chemisorption of CO and H2, BET surface area, infrared spectroscopy of adsorbed CO and microcalorimetry of CO adsorption. Catalytic activity measurements in the selective hydrogenation of citral, at 323 K and 5 MPa in the liquid phase, showed that copper inhibits the overall hydrogenation activity of Ru/KL catalysts, the higher the copper concentration the larger the activity reduction, without significant modification of the hydrogenation turnover frequencies. Selectivity toward unsaturated alcohols decreases as the copper content increases in favor of that of citronellal. The presence of bimetallic entities on the doped catalysts was suggested by the TPR measurements and the occurrence of hydrogen spillover phenomenon evidenced by the results of the chemisorption measurements of hydrogen and CO. The catalyst behavior is consistent with both, decoration/encapsulation of ruthenium by copper particles and occlusion of Ru inside the zeolite pores. This is suggested by the strong decrease of CO chemisorption capacity of the catalysts in the order Ru/KL > RuCu0.5/KL > RuCu1.0/KL > RuCu1.5/KL and strengthened by the CO-FTIR and microcalorimetry results.