Selectivity and kinetics of methyl crotonate hydrogenation over Pt/Al2O3

The hydrogenation of gas-phase methyl crotonate (MC) over Pt/Al2O3 was investigated with the aim to understand C=C hydrogenation in unsaturated methyl esters. Three Pt/Al2O3 catalysts with different Pt dispersions were prepared by varying calcination temperature and evaluated for MC hydrogenation. The main products were found to be methyl butyrate (MB) and methyl 3-butenoate (M3B), resulting from hydrogenation and shift of the C=C bond in MC, respectively. The measured activity for both hydrogenation and shift of the C=C in MC was found to depend on the Pt dispersion where higher Pt dispersion favors the C=C hydrogenation reaction. The effect of reactant concentrations on the activity and selectivity for MC hydrogenation over the Pt/Al2O3 catalyst was examined in detail. Under the investigated conditions, the C=C hydrogenation was found to have a negative reaction order with respect to MC concentration but a positive H-2 order. Further understanding of the MC hydrogenation was provided from H-2 chemisorption experiments over the catalyst with and without pre-adsorbed MC and from transient experiments using alternating MC and H-2 feeds. Based on the present experimental results, a reaction pathway was proposed to describe gas-phase MC hydrogenation over Pt/Al2O3. In order to gain more insight into the reaction, a kinetic analysis of MC hydrogenation was performed by fitting a power-law model to the kinetic data, moreover, dissociative H-2 adsorption on the catalyst was found to be the rate-determining step by comparing the power-law model with the overall rate expressions derived from mechanistic considerations.