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.

高压共进料氢气（H₂）可在甲醇制烯烃（methanol-to-olefin, MTO）反应过程中延长沸石（zeolite）催化剂的使用寿命，同时维持较高的烯烃与烷烃比值；但目前仍未明确氢气共进料阻止催化剂失活所涉及的原子级机制，以及被氢气氢化的靶向物种。
本研究采用周期性密度泛函理论（density functional theory, DFT），探究MTO反应中与催化剂失活相关的产物烯烃及中间物种的氢化机制与反应速率，涉及的物种包括C4、C6二烯烃、甲醛与苯。研究在H型ZSM-5（MFI骨架）、H型SSZ-13与H型SAPO-34（CHA骨架）的模型体系中开展上述氢化反应的考察。
单步与两步氢化机制在所有沸石的所有反应物上具有相近的反应能垒；其中氢气解离（氢负离子转移）是该类机制的决速步骤。氢化能垒与碳正离子（carbenium）稳定性呈现显著正相关：形成氧鎓碳正离子（oxocarbeniums）或烯丙基碳正离子（allylic carbocations）的物种，其氢化速率高于经由烷基碳正离子的物种。
因此，相较于烯烃，二烯烃与甲醛在MTO过程中会被选择性氢化，其氢化能垒比C2~C4烯烃的氢化能垒低10~85 kJ·mol⁻¹；其中甲醛氢化的平均能垒比二烯烃氢化低约10 kJ·mol⁻¹。
丁二烯的氢化还可通过α,δ质子化与氢负离子转移路径进行，主产物为2-丁烯；而α,β路径则生成1-丁烯——两类反应路径均经由烯丙基碳正离子（allylic carbocations），表明碳正离子（carbenium）稳定性并非调控二烯烃选择性氢化的唯一驱动因素。己二烯的氢化能垒低于丁二烯，表明更长的碳链可更好地稳定中间碳正离子（carbenium）。与之相反，苯的氢化能垒高于C2~C4烯烃：尽管其经由稳定的苯鎓阳离子（benzenium cations）中间体，但非芳香性氢化产物的热力学稳定性较差，导致能垒升高。
H型SSZ-13与H型ZSM-5的氢化能垒差值不超过12 kJ·mol⁻¹，表明二者的氢化速率相近。H型SAPO-34的氢化能垒比H型SSZ-13（二者均为CHA骨架）高12~38 kJ·mol⁻¹；且与硅铝酸盐（aluminosilicates）沸石不同，该SAPO型分子筛更倾向于对甲醛进行氢化，而非二烯烃。
水（H₂O）可提升氢气共进料的效果，但并不会直接参与氢化路径；相反，其通过烷基水化反应增加表面质子浓度，从而提升氢化反应速率。