Dispersion energy effects on methane interaction within zeolite straight micropores: a computational investigation

The interaction of methane with an aluminium-free zeolite (ZSM-5) porous substrate has been investigated by means of DFT and DFT-D calculations. We observe no charge transfer between host-guest species and, most interestingly, the energetic balance appears to be reasonably linked to the volume size of individual internal cavities. In fact, the gaseous-molecule is loosely bound only in larger 10MR pores while, inside the narrow 6MR ring, on because of the proximity of individual electronic clouds, the chemical interaction is repulsive. From a comparison with DFT approach it is evident that dispersion energies are crucial for a correct energetics and that long range forces drive the adsorption processes. Similar results are obtained for other small species, like hydrogen (atom and molecule) and CH3 radical species, considered in our current, although not exhaustive, investigation as products of hypothetical methane dissociative adsorptions.