Methanol to Olefins over H-MCM-22 Zeolite: Theoretical Study on the Catalytic Roles of Various Pores

H-MCM-22 zeolite bears three types of pores, supercages, sinusoidal channels, and pockets, and exhibits excellent catalytic performance in the process of methanol to olefins (MTO); however, the catalytic role that each type plays in MTO is still unclear. In this work, density functional theory considering dispersive interactions (DFT-D) was used to elucidate the contributions of various pores in H-MCM-22 to MTO. The results demonstrated that these three types of pores are different in their catalytic action on MTO, because of the large differences in pore size and shape that determine the space confinement and electrostatic stabilization effects. The formation of propene is predicted to take place in the supercages, where propene can he effectively produced through both polyMB and alkene cycles, with a relatively low free energy barrier as well as low enthalpy barrier and entropy loss for the rate-determining steps. In the sinusoidal channels, the free energy barrier of the methylation and cracking steps is elevated due to the space confinement and the reactivity of alkenes is also markedly depressed in the narrow channels, in comparison with those in the supercages; as a result, the contribution of the sinusoidal channels to the entire propene formation is minor. Meanwhile, the pockets are probably detrimental to MTO, as certain large intermediates such as 1,1,2,6-tetramethyl-4-isopropylbenzenium cations are easily formed in the pockets but are difficult to decompose due to the lack of an electrostatic stabilization effect from the zeolite framework, which elevates the total free energy barrier and may lead to a rapid deactivation of these active sites. In comparison with the difference in pore size and structure, the difference of various pores in the acid strength of the active sites exhibits an insignificant effect on their catalytic behaviors in MTO. The theoretical insights in this work are conducive to a subsequent investigation on the MTO mechanism and the development of better MTO catalysts and reaction processes.