High performances of Pt/ZnO catalysts in selective hydrogenation of crotonaldehyde

Hydrogenation of crotonaldehyde in the gas phase, at atmospheric pressure and 353 K over Pt/ZnO catalysts, was studied. Two types of precursor, (Pt(NH3)(4)(NO3)(2) and H2PtCl6, referred to as A and B catalysts, respectively, were used for catalyst preparation. Before the catalytic experiments the catalysts were reduced at different temperatures. The reducibility of the support and the catalysts was followed by TPR. Catalysts were also analysed by XPS and XRD. Rapid deactivation during time on stream was observed. The A and B catalysts showed different dependence on the reduction temperature. Thus, the A catalyst had the highest activity when reduced at 473 K; a further increase in the reduction temperature led to a decrease in the activity, but at 673 K both catalysts A and B showed nearly the same activity. On the B catalyst, the crotyl alcohol selectivity reached a value as high as 75-80%, whatever the reduction temperature. The B catalyst was better dispersed than the A catalyst and formed a PtZn alloy at low reduction temperature (473 K). It contained about 5 wt% chloride, whatever the reduction temperature. In contrast, Pt metal particles were only formed on the A catalyst, reduced at 473 K, and then showed low selectivity in crotyl alcohol. However, when the reduction temperature was increased, activity decreased and crotyl alcohol selectivity increased parallel to Pt-Zn ahoy formation. One can speculate that Pt sites, when alloyed to Zn, formed Ptdelta--Zndelta+ entities, on which the crotonaldehyde adsorbed by the carbonyl group rather than by the C=C double bond. On the B catalyst, the high selectivity observed, whatever the reduction temperature, led us to assume that besides the alloying effect, chlorine has an important promotor effect by increasing the polarity of Zndelta+ in the PtZn catalytic sites and facilitating the carbonyl adsorption. A reaction network and mechanism were put forward. Kinetic models, developed from the proposed elementary step mechanisms, were used to discuss the influence of support and promoters on reaction selectivity. (C) 1999 Academic Press.