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.

高压下共进料氢气可延长甲醇制烯烃（methanol-to-olefin, MTO）反应中沸石催化剂的寿命，同时维持较高的烯烃-烷烃比值；然而，氢气共进料防止催化剂失活的原子级作用机理与被氢化的物种仍未得到阐明。本研究采用周期性密度泛函理论（periodic density functional theory, DFT），探究了MTO反应中与催化剂失活相关的产物烯烃及生成物种的氢化机理与反应速率：包括C4与C6二烯烃、甲醛以及苯。针对这些物种的氢化反应，我们在氢型ZSM-5（MFI骨架）、氢型SSZ-13与氢型SAPO-34（均具有CHA骨架）的模型中进行了考察。单步与两步氢化机理在所有沸石的所有反应物上具有相近的能垒，其中氢气解离（氢负离子转移）是这类机理的决速步骤。氢化能垒与碳正离子的稳定性呈现良好的相关性，生成氧鎓碳正离子或烯丙基碳正离子的物种，其氢化速率高于经由烷基碳正离子进行反应的物种。因此，相较于烯烃，MTO过程中二烯烃与甲醛会被优先氢化，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，主要产物为2-丁烯；而α,β路径则生成1-丁烯——两类路径均经由烯丙基碳正离子进行，表明碳正离子的稳定性并非选择性二烯烃氢化的唯一驱动因素。己二烯氢化的能垒低于丁二烯，说明更长的碳链可稳定中间体碳正离子。与之相反，苯的氢化能垒高于C2~C4烯烃，尽管其反应经由稳定的苯鎓阳离子进行，这是由于非芳香族氢化产物的稳定性较差。氢型SSZ-13与氢型ZSM-5的氢化能垒相差不超过12 kJ·mol⁻¹，表明二者的氢化速率相近。氢型SAPO-34的氢化能垒比氢型SSZ-13（二者均为CHA骨架）高12~38 kJ·mol⁻¹，且与硅铝酸盐沸石不同，SAPO型沸石更倾向于甲醛氢化而非二烯烃氢化。水可提升氢气共进料的效果，但不会直接参与氢化路径；相反，它通过烷基水化反应增加表面质子浓度，从而提升氢化速率。