On the effects of confinement within a catalyst consisting of platinum embedded within nanoporous carbon for the hydrogenation of alkenes

In heterogeneous catalysis, pore size exerts an influence on reaction pathway, selectivity, equilibrium and adsorption constants. This effect can in principle cause noticeable changes in selectivity. Here, we present an analysis of a diffusion-reaction process in the pores of a catalyst comprised of platinum nanoparticles embedded within a molecular sieving carbon. When the alkenes are hydrogenated over this catalyst, the reaction takes place within the ultramicropores of the carbon. Experimental data for the liquid phase hydrogenation of different alkenes over platinum supported on the carbon versus platinum embedded within the same carbon were collected. From these data kinetic parameters and diffusion coefficients for reactions were evaluated. The forward rate constant for 2-methyl-1-pentene hydrogenation was found to be almost one order of magnitude larger within the embedded platinum catalyst versus the supported platinum catalyst. The variation in pore size and reactant molecule dimension, were also found to affect the adsorption equilibrium constants and diffusion coefficients. For 2-methyl-1-pentene molecule with the highest steric hindrance, K increased to 500 g/mol in embedded catalyst compared to 100 g/mol on supported catalyst. At the same time the diffusion coefficient for 2-methyl-1-pentene was one order of magnitude smaller than 1-hexene.