Contrasting Arene, Alkene, Diene, and Formaldehyde Hydrogenation in H-ZSM-5, H-SSZ-13, and H-SAPO-34 Frameworks during MTO
Contrasting Arene, Alkene, Diene, and Formaldehyde Hydrogenation in H-ZSM-5, H-SSZ-13, and H-SAPO-34 Frameworks during MTO
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DOI：
10.1021/acscatal.9b04529
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发表时间：
2020-04-17
期刊：
影响因子：
12.9
通讯作者：
Hibbitts, David
中科院分区：
文献类型：
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作者：
DeLuca, Mykela;Janes, Christina;Hibbitts, David
在高压下共进料H-2增加了甲醇至烯烃（MTO）反应期间的沸石催化剂寿命，同时保持高的烯烃与烷烃比;然而，通过H-2共进料氢化以防止催化剂失活的原子机制和物质仍然不确定。本研究采用周期性密度泛函理论（DFT）研究加氢MTO产品烯烃和物种形成的MTO过程中，已链接到催化剂失活的机制和速率：C-4和C-6二烯，甲醛和苯。在H-ZSM-S（MFI骨架）、H-SSZ-13和H-SAPO-34（CHA骨架）模型中考察了这些物种的氢化。单步和两步氢化机制发生类似的障碍，所有反应物在所有沸石，与H-2解离（氢化物转移）是这些机制的困难部分。氢化障碍的趋势以及与碳稳定性，和物种，形成氧代碳或烯丙基碳阳离子以更高的速率比那些进行通过烷基碳。因此，与烯烃相比，二烯烃和甲醛在MTO期间被选择性地氢化，以比C-2-C-4烯烃氢化低10-85 kJ mol（-1）的势垒发生，甲醛氢化平均比二烯烃氢化低10 kJ mol（-1）。丁烯氢化也通过α，δ质子化和氢化方案促进，其形成2-丁烯作为主要产物，与形成1-丁烯的α，β路线相反，两种路线都通过烯丙基碳阳离子发生，表明碳阳离子稳定性不是选择性二烯氢化的唯一驱动因素。己二烯氢化的势垒低于丁二烯，表明较长的碳链可以稳定中间体碳阳离子。与二烯和甲醛相反，苯的氢化具有比C-2-C-4烯烃更高的势垒，尽管由于非芳族产物的不稳定性而通过稳定的苯阳离子进行。H-SSZ-13和H-ZSM-5中的氢化势垒彼此在12 kJ mol（-1）内，表明两者表现出相似的氢化速率。H-SAPO-34中的氢化势垒比H-SSZ-13（均为CHA）中的氢化势垒高12-38 kJ mol（-1），并且SAPO沸石型似乎也比二烯氢化更有利于甲醛氢化（与铝硅酸盐相反）。H2O提高了H-2共进料的效率，但不直接有助于氢化途径;相反，它通过烷基水合反应提高表面质子的浓度来提高氢化速率。
Co-feeding H-2 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 H-2 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: C-4 and C-6 dienes, formaldehyde, and benzene. Hydrogenations of these species are examined in models of H-ZSM-S (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 H-2 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 C-2-C-4 alkene hydrogenation, with formalde hydehydrogenation on average 10 kJ mol(-1) lower than diene hydrogenation. Butadiene hydrogenation is also facilitated by alpha,delta protonation and hydridation schemes, which form 2-butene as primary products, in contrast to alpha,beta 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 C-2-C-4 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 H-2 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.