Modelling the adsorption of short alkanes in protonated chabazite: The impact of dispersion forces and temperature

Abstract

The adsorption of alkanes in a protonated zeolite has been investigated at different levels of theory. At the lowest level we use density-functional theory (DFT) based on semi-local (gradient-corrected) functionals which account only for the interaction of the molecule with the acid site. To describe the van der Waals (vdW) interactions between the saturated molecule and the inner wall of the zeolite we use (i) semi-empirical pair interactions, (ii) calculations using a non-local correlation functional designed to include vdW interactions, and (iii) an approach based on calculations of the dynamical response function within the random-phase approximation (RPA). The effect of finite temperature on the adsorption properties has been studied by performing molecular dynamics (MD) simulations based on forces derived from DFT plus semi-empirical vdW corrections. The simulations demonstrate that even at room temperature the binding of the molecule to the acid site is frequently broken such that only the vdW interaction between the alkane and the zeolite remains. The finite temperature adsorption energy is calculated as the ensemble average over a sufficiently long molecular dynamics run, it is significantly reduced compared to the T=0K limit. At a higher level of theory where MD simulations would be prohibitively expensive we propose a simple scheme based on the averaging over the adsorption energies in the acid and in the purely siliceous zeolite to account for temperature effects. With these corrections we find an excellent agreement between the RPA predictions and experiment.