Journal
MICROPOROUS AND MESOPOROUS MATERIALS
Volume 292, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.micromeso.2019.109732
Keywords
ONIOM; SAPO; omega B97X-D; DME; NH3 adsorption
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ONIOM(DFT:PM3) calculations were analyzed here in order to examine if, and how the isolated Bronsted 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, omega B97X and omega B97X-D. The results revealed that the CH3OH adsorption energy increases with the type of functional used, according to the trend B3LYP < omega B97X < omega B97X-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.
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