Molekulare Simulation der Wechselwirkung von Diffusion und Reaktion in porösen Medien
Zusammenfassung der Projektergebnisse
In the field of heterogeneous catalysis, a long standing goal of theoretical research has been the development of methods to predict the effects of catalyst composition and structure on the rates of catalyzed reactions, as well as the distribution of products formed from a specific set of reactants. Impressive progress towards this goal has been demonstrated in recent years showing that theoretically determined overall kinetics, selectivities, or conversions of reactions are described correctly by theoretical approaches. The results reported in references and in the present work contribute to these efforts by providing a multiscale approach for the theoretical investigation of reactions in zeolites. It was demonstrated how detailed atomistic simulations can be used to generate data that can then be combined with continuum modeling to achieve superior predictability compared to empirical kinetic rate expressions. However, especially for zeolite catalyzed reactions there is still considerable room for improvement on all levels of the multiscale approach. Whereas predictions of activation enthalpies for hydrocarbon conversion reactions with near chemical accuracy are possible, these calculations cannot be routinely applied due to high computational costs. Consequently, there is need to speed up calculations or to extend the capabilities of density functional theory to describe correctly van-der-Waals interactions and barrier heights. Moreover, the estimation of entropy beyond the harmonic approximation is necessary to improve the calculation of pre-exponential factors. Progress in this direction has been achieved recently by applying advanced simulation approaches such as free energy techniques based on ab-initio molecular dynamics or transition path sampling. On the force field level there is the need for force fields that can describe interactions between hydrocarbons and Brønsted acid sites. Such force fields have to be parameterized against accurate quantum mechanical calculations because experimental data are hard to obtain. These force fields should also be applied in MD simulations. Moreover, reliable methods are required to calculate diffusion coefficients for slow diffusing molecules such as aromatics which are difficult to obtain directly from classical MD simulations. In this context, there is also the need for accurate experimental diffusion data as results obtained with different methods often deviate from each other. On the continuum level, the description of multicomponent diffusion can be improved by accounting for strong adsorption sites by using for example the effective medium approximation proposed by Coppens and co-workers. Moreover, mass transfer coefficients accounting for transport barriers between the gas phase and the zeolite pore mouths might be included. Especially for small particle sizes, diffusion can be considerably controlled by surface effects. Mass transfer coefficients under the condition of sorption equilibrium can be calculated from MD simulation data. Finally accurate experimental kinetic data are required to assess the reliability of modeling approaches for predicting effective reaction rates for zeolite catalyzed reactions. With ongoing improvements in all of these fields, multiscale approaches such as the one presented in the present work are expected to become increasingly important in computer-aided catalyst design. It is therefore anticipated that theoretical investigations of the combined effects of adsorption thermodynamics, diffusion, and reaction kinetics on the rates of catalyzed reactions will continue to be a fertile area for future research.
