Understanding the coupling between reactions and the mass transport in a catalytic system is crucial to predict its observed functionality. By aiming for a first-principles description, the selectivity and lifetime of the catalyst can be rationally optimized. In the reactor volume outside the catalyst pellets, the mass transport is described as a fluid flow, whereas inside the catalyst pellets, it is diffusive. The diffusivity in the catalyst pellets, provides a link between the reactions at the active sites and the fluid flow in the reactor. A catalytic process that will be important in a fossil-free society is the methanol to hydrocarbon (MTH) reaction. While being promising, a drawback of the MTH reaction is a rapid catalyst deactivation through coking. The catalyst considered in this work is a H-ZSM-5 zeolite with a pellet size of 212 – 250 µm consisting of a conglomerate of single crystallites of size 2 – 6 µm. Thus, the catalyst pellet consists of macropores (size ~ 1 µm) between the single crystallites and micropores (0.5 nm) within them (Figure 1). In this communication, we will present effective diffusivities in the catalyst pellet obtained from first-principles modelling. A model of the porous structure of the pellet is obtained from a digital reconstruction. In the macropores of the resulting model, we compute diffusivity through computational fluid dynamics (CFD) simulations; while in the micropores, we obtain it by use of molecular dynamics simulations (MD). The effective diffusivity is derived by coupling the MD and CFD diffusivities. The micropore diffusivity of relevant species decreases with increasing kinetic diameter. When coupled with the reaction network, the faster diffusion of propene (11.2 10-9 m2 s-1) compared to propane (8.05 10-9 m2 s-1), may have implications on the equilibrium of propene hydrogenation. These results open a pathway to rational catalyst design where all processes in the catalytic reactor are considered.

Diffusion and reaction from first-principles: An application to multiscale modelling of the methanol to hydrocarbon reaction

ACS Spring Meeting 2021

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