Conversion of methanol to dimethyl ether over silicoaluminophosphates: Isolated acid sites and the influence of silicon islands. A DFT-ONIOM study

ONIOM(DFT:PM3) calculations were analyzed here in order to examine if, and how the isolated Brønsted acid sites, the silicon islands and the pore diameter affect the activation energies in the dimethyl ether formation from methanol for a series of silicoaluminophosphate (SAPO) molecular sieves, SAPO-5, SAPO-11, SAPO-34 and SAPO-41. Three density functionals were employed: B3LYP, ωB97X and ωB97X-D. The results revealed that the CH3OH adsorption energy increases with the type of functional used, according to the trend B3LYP < ωB97X < ωB97X-D. In general, the height of the transition state barriers depends on the acid site type. In the case of the formation of methoxy species, the energy barriers are lower in the silicon islands than in the isolated Si sites. On the contrary, for the dimethyl ether formation from methoxy species, the energy barriers are higher at silicon islands than at isolated sites.