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

Abstract

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-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 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 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 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). H 2 O 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.

MAPO-18 Catalysts for the Methanol to Olefins Process: Influence of Catalyst Acidity in a High-Pressure Syngas (CO and H2) Environment

“… 52 Experimental studies further suggest that cracking reactions are promoted by high acid strength 63 − 65 and so are formaldehyde, diene, and alkene hydrogenation reactions. 55 The above results provide confidence that this is indeed the case, but absolute conclusions could not be made due to the differences in heteroatom uptake despite identical synthesis gel composition and the lack of control in specific heteroatom location. Therefore, a series of SAPO-18 and MgAPO-18 with varying heteroatom loadings were prepared to elucidate the acidity–performance relations.…”

“… 34 According to DeLuca et al , the reaction barrier to alkene hydrogenation decreases with an increasing chain length and stabilization of the intermediate carbocation-like species. 55 Comparison of the SSZ-13 zeolite to the isostructural SAPO-34 material further showed a positive relation between the acid strength and hydrogenation rates, and this acid strength effect is more significant than topology (ZSM-5 vs SAPO-34). 55 In the current case, the preferred hydrogenation of ethene suggests that the heteroatoms are preferably located in the 8-ring windows of the AEI topology: strong confinement effects for small-molecule conversion have recently been demonstrated for C 2 = –C 4 = methylation in 1D 10-ring ZSM-22 (TON) 57 and for methanol carbonylation reactions in the 8-ring pockets of MOR.…”

“… Mechanistic insights of the interaction between ethene and CO. (a) Ethene hydrogenation over SAPO-18 with N 2 or CO co-feeding at 350 °C, 10 bar, 0.25 bar ethene, and WHSV = 1.0 h –1 . (b) Illustration of ethene hydrogenation precursor states in SAPO-18, adapted from Hibbitts et al ( 55 ) Optimized electron cloud distribution in (c) SAPO-18 and (d) MgAPO-18 with ethene, showing the more polar character of the Mg–O bond compared to the Si–O one. The atom positions are the same as in Figure 3 .…”

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

“…Density functional theory (DFT) calculations are increasingly employed to gain insight into the reactivity of zeolites [16][17][18][19][20][21], for methanol dehydration [22][23][24][25] as well as subsequent steps in the MTO process involving the reaction of olefins and aromatics [26][27][28][29][30][31][32][33][34]. Kinetic models were developed to study initiation, step-wise methylation via surface methoxy species (SMS), concerted methylation, cracking and deactivation through coking revealing many features of the process [35][36][37][38][39].…”

Parallel Alkane Dehydrogenation Routes on Brønsted Acid and Reaction-Derived Carbonaceous Active Sites in Zeolites

“…Thus, we interpret the initial transient decrease in dehydrogenation rates after O 2 pretreatments and in the absence of co-fed H 2 (Figures a and a; Figure S21, SI) to arise from the formation of organic residues that evolve in composition as steady-state carbon and hydrogen chemical potentials are attained, to forms that become less effective at catalyzing alkane dehydrogenation. The more gradual increases in dehydrogenation rates at initial time-on-stream with H 2 pretreatments and co-feeds (Figures S4–S8, SI) appear to reflect the slower buildup of unsaturated carbonaceous deposits because the high hydrogen chemical potentials result in hydrogenation of unsaturated organic residues. , This interpretation is consistent with our findings that the removal of propane from propane/H 2 mixtures results in the removal of unsaturated organic residues to restore H-form zeolites to their states before propane exposure (Figure c). Thus, we conclude that H 2 co-feeds mitigate the formation and reactivity of carbonaceous deposits at initial time-on-stream, allowing for measurement of protolytic reaction events.…”