The closer the better? Theoretical assessment of the impact of catalytic site separation for bifunctional core–shell catalyst particles

Abstract

One-pot heterogeneous catalysis with different active centers offers great potential for increasing yield and selectivity. In this field, the distance between the different catalytically active centers starts playing a role and its influence as well as its control is an open question. Here, porous core–shell particles provide the opportunity to control the distance on a mesoscopic scale, where the centers are placed on different shells and are separated by an inert porous matrix. We present a continuum-mechanical model of such particles and exploit symmetry to arrive at a computationally efficient reduced model. Using methanol synthesis from CO2 on the first kind of center followed by a dimethylether synthesis on a second kind of center as an example, we investigate the influence of the distance between these two centers. In particular, we consider three simple backcoupling mechanisms and address the question whether it is best to place the centers as close as possible or at a non-zero optimal distance. We find that this question cannot a priori be answered but the answer depends largely on the employed backcoupling mechanism.