Mechanistic insight into the formation of aromatics during the MTO process in H-SSZ-13 from DFT calculations

Aromatics play a key role in the methanol-to-olefins process, both as a co-catalyst for the production of short chain olefins and as a key reaction intermediate towards coke formation, but mechanistic details on how aromatics form are still scarce. Here, we investigate all reaction steps from olefins towards benzene in H-SSZ-13 using density functional theory (DFT) calculations. We find that the rate determining step is likely given by the oxidation of an olefin through hydrogen transfer (HT) reactions, while the subsequent reaction steps of isomerization, cyclization and further oxidation of cyclic olefins are for the most part associated with lower reaction barriers, with the exception of the cyclization of hexadiene, for which cyclization and initial diene formation have similar barriers. Importantly, the key step of oxidation of olefins either through the formaldehyde route or via HT of longer chain olefins has barriers on the order of 180 kJ mol−1 at 673 K and is thus comparable to those typically found for the methylation of olefins as well as cracking of larger olefins, suggesting that the extent to which the olefin cycle branches towards aromatics formation depends on a delicate balance of the type of olefins formed as well as the Brønsted acid site, but also on the availability of formaldehyde.