Selectivity and kinetics of methyl crotonate hydrogenation over Pt/Al2O3

Abstract

The hydrogenation of gas-phase methyl crotonate (MC) over Pt/Al₂O₃ was investigated with the aim to understand CC 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 CC bond in MC, respectively. The measured activity for both hydrogenation and shift of the CC in MC was found to depend on the Pt dispersion where higher Pt dispersion favors the CC 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 CC hydrogenation was found to have a negative reaction order with respect to MC concentration but a positive H₂ order. Further understanding of the MC hydrogenation was provided from H₂ chemisorption experiments over the catalyst with and without pre-adsorbed MC and from transient experiments using alternating MC and H₂ 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 H₂ 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.