Mass transport effects in gas-phase selective hydrogenation of 1,3-butadiene over supported Pd

Selective hydrogenation reactions are essential in the purification of light olefins by removal of polyunsaturated hydrocarbon impurities (alkynes/alkadienes). Pd-based catalysts are typically used because of their high activity at ambient temperatures. Unfortunately, retaining high selectivity at high conversion using a Pd catalyst is challenging, resulting in more undesired alkane formation, which is often ascribed to intrinsic properties of the Pd metal. However, in this work we show that heat and mass transport effects strongly impact the catalytic activity and selectivity of Pd nanoparticles on carbon catalysts (Pd/C) in the selective hydrogenation of butadiene. By systematically varying the Pd loading and catalyst grain size, we show that higher loadings and larger grains strongly decrease the butene selectivity. This is ascribed to an effect of internal diffusion limitations, arising from butadiene depletion in the core of the catalyst grains, and not by intrinsic properties of Pd. The comprehensive assessment of heat and mass transport phenomena is essential to reliably relate experimental observations to catalyst properties such as Pd particle size, support or promoter effects. It contributes to the understanding and rational design of catalysts for selective hydrogenation of butadiene and can be extended to other reactions and/or supported metal catalysts. The performance of selective hydrogenation reactions is affected by mass transfer limitations, induced by higher local metal loading and larger catalyst grain size.