Contrasting Arene, Alkene, Diene, and Formaldehyde Hydrogenation in H-ZSM-5, H-SSZ-13, and H-SAPO-34 Frameworks during MTO

Co-feeding H2 at high pressures increases zeolite catalyst lifetimes during methanol-to-olefin (MTO) reactions while maintaining high alkene-to-alkane ratios; however, the atomistic mechanisms and species hydrogenated by H2 co-feeds to prevent catalyst deactivation remain undetermined. This study uses periodic density functional theory (DFT) to examine mechanisms and rates of hydrogenating MTO product alkenes and species formed during MTO that have been linked to catalyst deactivation: C4 and C6 dienes, formaldehyde, and benzene. Hydrogenations of these species are examined in models of H-ZSM-5 (MFI framework), H-SSZ-13 and H-SAPO-34 (CHA framework). Single-step and two-step hydrogenation mechanisms occur with similar barriers for all reactants on all zeolites, with H2 dissociation (hydride transfer) being the difficult part of these mechanisms. Hydrogenation barriers trend well with carbenium stabilities, and species that form oxocarbeniums or allylic carbocations hydrogenate at higher rates than those proceeding via alkylcarbeniums. As such, dienes and formaldehyde are selectively hydrogenated during MTO compared to alkenes, occurring with barriers 10–85 kJ mol–1 lower than C2–C4 alkene hydrogenation, with formalde hydehydrogenation on average 10 kJ mol–1 lower than diene hydrogenation. Butadiene hydrogenation is also facilitated by α,δ protonation and hydridation schemes, which form 2-butene as primary products, in contrast to α,β routes forming 1-butene—both routes occur via allylic carbocations, indicating that carbocation stability is not the only driver towards selective diene hydrogenation. Barriers of hexadiene hydrogenation are lower than those of butadiene, indicating that longer carbon chains can stabilize the intermediate carbocations. Benzene, in contrast to dienes and formaldehyde, is hydrogenated with higher barriers than C2–C4 alkenes despite proceeding via stable benzenium cations because of the instability of the nonaromatic product. Hydrogenation barriers in H-SSZ-13 and H-ZSM-5 are within 12 kJ mol–1 of one another indicating both demonstrate similar hydrogenation rates. Hydrogenation barriers in H-SAPO-34 are 12–38 kJ mol–1 higher than those in H-SSZ-13 (both CHA) and the SAPO zeotype also seems to favor formaldehyde hydrogenation over diene hydrogenation (in contrast to the aluminosilicates). H2O increases the efficacy of H2 co-feeds but does not directly assist in hydrogenation pathways; instead, it increases hydrogenation rates by increasing the concentration of surface protons through alkyl hydration reactions.