Multiscale Modeling of Reaction and Diffusion in Zeolites: From the Molecular Level to the Reactor

A hierarchical multiscale approach is presented to calculate effective rates of reaction for zeolite catalyzed reaction systems. The first step in this approach involves the determination of intrinsic rate coefficients for all elementary reactions by means of quantum chemical calculations combined with transition state theory. The second step is the calculation of adsorption isotherms and diffusion coefficients of all species by means of Monte Carlo and molecular dynamics simulations. The third step comprises a continuum description of a zeolite crystal based on the reaction-diffusion equation. For coupling the intrinsic rate coefficients obtained in the first step to the continuum variables, a model is proposed that calculates the local concentration of reactants at the catalytically active centers from the total concentrations of adsorbed species. The adsorption isotherms and diffusivities are coupled to the continuum variables by analytical theories such as the ideal adsorbed solution theory and the Maxwell-Stefan formulation of multicomponent diffusion. The fourth step involves fixed-bed reactor simulations based on the continuum description of single zeolite crystals. The approach is illustrated by means of the alkylation of benzene with ethene over zeolite H-ZSM-5.