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Contrasting Arene, Alkene, Diene, and Formaldehyde Hydrogenation in H-ZSM-5, H-SSZ-13, and H-SAPO-34 Frameworks during MTO
ACS Catalysis ( IF 12.9 ) Pub Date : 2020-03-10 , DOI: 10.1021/acscatal.9b04529
Mykela DeLuca 1 , Christina Janes 1 , David Hibbitts 1
Affiliation  

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.

中文翻译:

在MTO期间在H-ZSM-5,H-SSZ-13和H-SAPO-34框架中形成对比的芳烃,烯烃,二烯和甲醛加氢

在高压下共同进料H 2可以延长甲醇/烯烃(MTO)反应期间沸石催化剂的寿命,同时保持高烯烃/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理和物质仍未确定。这项研究使用周期性密度泛函理论(DFT)来检查MTO产物烯烃和MTO期间形成的与催化剂失活有关的物质的加氢机理和加氢速率:C 4和C 6二烯,甲醛和苯。在H-ZSM-5(MFI框架),H-SSZ-13和H-SAPO-34(CHA框架)模型中检查了这些物种的氢化反应。对于所有沸石上的所有反应物,单步和两步加氢机理都以相似的屏障发生,其中H 2离解(氢化物转移)是这些机理中的难点。氢化障碍的形成与碳稳定有关,并且形成羰基碳或烯丙基碳正离子的物种的氢化速率高于通过烷基碳烯生成的物种。因此,与烯烃相比,二烯烃和甲醛在MTO过程中被选择性氢化,发生的势垒比C 2 –C 4低10–85 kJ mol –1。烯烃加氢,甲醛化加氢平均比二烯加氢低10 kJ mol –1。α,δ质子化和氢化方案也促进了丁二烯的氢化反应,形成2-丁烯作为主要产物,与形成1-丁烯的α,β途径相反,这两种途径都是通过烯丙基碳正离子发生的,这表明碳正离子稳定性不是唯一的选择性二烯氢化的驱动力。己二烯氢化的壁垒比丁二烯低,表明更长的碳链可以稳定中间碳正离子。与二烯和甲醛相比,苯的氢化反应比C 2 –C 4的屏障更高由于非芳族产物的不稳定性,尽管烯烃通过稳定的苯阳离子进行反应,但仍具有烯烃。H-SSZ-13和H-ZSM-5中的加氢壁垒相互之间在12 kJ mol –1之内,这表明两者都显示出相似的氢化速率。H-SAPO-34的加氢壁垒比H-SSZ-13(均为CHA)高12-38 kJ mol –1,SAPO分子型似乎也比二烯加氢更有利于甲醛加氢(与硅铝酸盐相反)。H 2 O可提高H 2共进料的效率,但不能直接协助氢化途径。相反,它通过烷基水合反应增加表面质子的浓度来提高氢化速率。
更新日期:2020-04-23
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