A computational study on the formation mechanism of naphthalenic species under MTO conditions in H-SSZ-13

A mechanistic pathway for the formation of naphthalene species in the MTO reaction network is proposed, starting from polymethylbenzenes, formaldehyde and an olefin as reactants. The mechanism is investigated computationally for di-, tri- and tetramethylbenzene and n-butene in the H-SSZ-13 zeolite. The highest free energy barriers at 400 °C are 202, 201 and 236 kJ mol$^{−1}$, respectively. After the second ring is formed, the free energy profile of the substrate with the highest degree of methylation is clearly the least favorable, due to space restrictions in the CHA cell. The free energy barriers are higher than olefin production barriers from the aromatic cycle, but still in an accessible range under MTO conditions, which makes them relevant as deactivation reactions.