(18b) Intrinsic and Apparent Kinetics for the Alkylation of Benzene Over H-ZSM-5 – A Multiscale Investigation From the Molecular Level to the Reactor

Gas-solid-reactions in porous media are taking place via a number of elementary steps that are (i) entrance of the reactants in the pore, (ii) multicomponent diffusion towards the active site, (iii) adsorption on the active site, (iv) reaction, (v) desorption of the products from the active site, and (vi) multicomponent diffusion of the products back to the bulk phase. In zeolite catalysis the kinetic diameter of reactants is often of similar size as the pore diameter. This leads to strongly confined reactants and/or products. When modeling these processes special care has to be taken of a proper description of the multicomponent diffusion combined with reaction kinetics. Both of these rate phenomena are strongly dependent on the pore topology and are unique for a certain zeolite.

In the present study highly accurate intrinsic activation energies are calculated for the alkylation of benzene with ethene over H-ZSM-5. This is achieved by employing a MP2:DFT multilevel approach [1,2] which includes first the optimization of all stationary points within the full unit cell using plane-wave DFT, second the calculation of the dispersion interaction between zeolite framework and adsorbates, and third the extrapolation of activation energies to the complete basis set limit. Rate coefficients for all elementary processes are then obtained from transition state theory.

The simulation of the apparent kinetics requires information about multicomponent diffusion inside the zeolite channels as well as the knowledge of the multicomponent adsorption equilibrium. The necessary diffusivity data is obtained from molecular dynamics simulation while adsorption isotherms are calculated using Monte Carlo simulations in the grand canonical ensemble [3].

A continuum approach, based on the Maxwell-Stefan (M-S) equations adapted to diffusion in zeolites [4], is then used to simulate the overall reactivity of a zeolite crystal as function of the external conditions. Moreover, the continuum approach is implemented into a reactor model to study the alkylation of benzene with ethene and ethane under various process conditions and to illustrate the ability to perform reactor simulations based on purely theoretical input.

[1] C. Tuma, J. Sauer, Phys. Chem. Chem. Phys. 8 (2006) 3955-3965.

[2] S. Svelle, C. Tuma, X. Rozanska, T. Kerber, J. Sauer, J. Am. Chem. Soc. 131 (2009) 816-825.

[3] N. Hansen, R. Krishna, J. M. van Baten, A. T. Bell, F. J. Keil, J. Phys. Chem. C 113 (2009) 235-246.

[4] R. Krishna, J. M. van Baten, Microporous Mesoporous Mater. 109 (2008) 91-108.