Modifications of the citral hydrogenation selectivities over Ru/KL-zeolite catalysts induced by the metal precursors

Three Ru/KL-zeolite catalysts containing 2 wt% of Ru were prepared from Ru3(CO)12, RuNO(NO3)3 and Ru(C5H7O2)3 precursors. The samples named Ru(c)/KL, Ru(n)/KL and Ru(a)/KL were studied by temperature-programmed reduction (TPR), by microcalorimetry of CO adsorption, by volumetric hydrogen chemisorption, by X-ray diffraction (XRD) and also by infrared spectroscopy of the CO adsorbed species (CO-FTIR). The catalytic activities and selectivities were evaluated in the hydrogenation of citral at 323 K and 5 MPa, in a stirred batch reactor. Metal dispersion follows the order Ru(c)/KL > Ru(n)/KL > Ru(a)/KL. The CO-FTIR spectra show a set of different ruthenium species on the support, suggesting the presence of large metal particle outside the zeolite and small crystallites inside the channels, the latter being majority in the Ru(c)/KL sample. Hydrogenation activity per surface metal atom (TOF) was found to be independent on the metal dispersion and, consequently, on the precursor used. The selectivity towards unsaturated alcohols (geraniol + nerol) is in the order Ru(c)/KL < Ru(a)/KL = Ru(n)/KL, while the citronellal selectivity is maximum over the Ru(c)/KL catalyst. This latter effect can be correlated to the special catalytic properties of the smaller Ru particles located inside the zeolite cavities.