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-buteneboth 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.
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 mechanisms and species hydrogenated by H2 cofeeds to prevent catalyst deactivation remain unknown. This study uses periodic density functional theory (DFT) to examine hydrogenation mechanisms of MTO product C2-C4 alkenes, as well as species related to the deactivation of MTO catalysts such as C4 and C6 dienes, benzene, and formaldehyde in H-MFI and H-CHA zeolite catalysts. Results show that dienes and formaldehyde are selectively hydrogenated in both frameworks at MTO conditions because their hydrogenation transition states proceed via allylic and oxocarbenium cations which are more stable than alkylcarbenium ions which mediate alkene hydrogenation. Diene hydrogenation is further stabilized by protonation and hydridation at α,δ positioned C-atoms to form 2-butene from butadiene and 3-hexene from hexadiene as primary hydrogenation products. This α,δ-hydrogenation directly leads to selective hydrogenation of dienes; pathways which hydrogenate dienes at the α,β-position (e.g., forming 1-butene from butadiene) proceed with barriers 20 kJ mol -1 higher than α,δ-hydrogenation and with barriers nearly equivalent to butene hydrogenation, despite α,β-hydrogenation of butadiene also occurring through allylic carbocations. Hydrogenation of formaldehyde, a diene precursor, occurs with barriers that are within 15 kJ mol -1 of diene hydrogenation barriers, indicating that it may also contribute to increasing catalyst lifetimes by preventing diene formation. 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 thermodynamic instability of the product which has lost aromaticity. Carbocation stabilities predict the relative rates of alkene hydrogenation and in some cases shed insights into the hydrogenation of benzene, dienes, and formaldehyde, but cation stabilities alone cannot account for the poor hydrogenation of benzene or the facile hydrogenation of dienes, boosted by stabilization conferred by a,δ-hydrogenation. This work suggests that the main mechanisms of catalyst lifetime improvement with high H2 co-feeds is reduction of diene concentrations through both their selective hydrogenation and hydrogenation of their precursors to prevent formation of deactivating polyaromatic species.
<p>Co-feeding H<sub>2</sub> at high pressures increases zeolite catalyst lifetimes during methanol-to-olefin (MTO) reactions while maintaining high alkene-to-alkane ratios; however, the mechanisms and species hydrogenated by H<sub>2</sub> co-feeds to prevent catalyst deactivation remain unknown. This study uses periodic density functional theory (DFT) to examine hydrogenation mechanisms of MTO product C<sub>2</sub>–C<sub>4</sub> alkenes, as well as species related to the deactivation of MTO catalysts such as C<sub>4</sub> and C<sub>6</sub> dienes, benzene, and formaldehyde in H-MFI and H-CHA zeolite catalysts. Results show that dienes and formaldehyde are selectively hydrogenated in both frameworks at MTO conditions because their hydrogenation transition states proceed via allylic and oxocarbenium cations which are more stable than alkylcarbenium ions which mediate alkene hydrogenation. Diene hydrogenation is further stabilized by protonation and hydridation at α,δ positioned C-atoms to form 2-butene from butadiene and 3-hexene from hexadiene as primary hydrogenation products. This α,δ-hydrogenation directly leads to selective hydrogenation of dienes; pathways which hydrogenate dienes at the α,β-position (e.g., forming 1-butene from butadiene) proceed with barriers 20 kJ mol<sup>-1</sup> higher than α,δ-hydrogenation and with barriers nearly equivalent to butene hydrogenation, despite α,β-hydrogenation of butadiene also occurring through allylic carbocations. Hydrogenation of formaldehyde, a diene precursor, occurs with barriers that are within 15 kJ mol<sup>-1</sup> of diene hydrogenation barriers, indicating that it may also contribute to increasing catalyst lifetimes by preventing diene formation. Benzene, in contrast to dienes and formaldehyde, is hydrogenated with higher barriers than C<sub>2</sub>–C<sub>4</sub> alkenes despite proceeding via stable benzenium cations because of the thermodynamic instability of the product which has lost aromaticity. Carbocation stabilities predict the relative rates of alkene hydrogenation and in some cases shed insights into the hydrogenation of benzene, dienes, and formaldehyde, but cation stabilities alone cannot account for the poor hydrogenation of benzene or the facile hydrogenation of dienes, boosted by stabilization conferred by a,δ-hydrogenation. This work suggests that the main mechanisms of catalyst lifetime improvement with high H<sub>2</sub> co-feeds is reduction of diene concentrations through both their selective hydrogenation and hydrogenation of their precursors to prevent formation of deactivating polyaromatic species.</p>
<p>Co-feeding H<sub>2</sub> at high pressures increases zeolite catalyst lifetimes during methanol-to-olefin (MTO) reactions while maintaining high alkene-to-alkane ratios; however, the mechanisms and species hydrogenated by H<sub>2</sub> co-feeds to prevent catalyst deactivation remain unknown. This study uses periodic density functional theory (DFT) to examine hydrogenation mechanisms of MTO product C<sub>2</sub>–C<sub>4</sub> alkenes, as well as species related to the deactivation of MTO catalysts such as C<sub>4</sub> and C<sub>6</sub> dienes, benzene, and formaldehyde in H-MFI and H-CHA zeolite catalysts. Results show that dienes and formaldehyde are selectively hydrogenated in both frameworks at MTO conditions because their hydrogenation transition states proceed via allylic and oxocarbenium cations which are more stable than alkylcarbenium ions which mediate alkene hydrogenation. Diene hydrogenation is further stabilized by protonation and hydridation at α,δ positioned C-atoms to form 2-butene from butadiene and 3-hexene from hexadiene as primary hydrogenation products. This α,δ-hydrogenation directly leads to selective hydrogenation of dienes; pathways which hydrogenate dienes at the α,β-position (e.g., forming 1-butene from butadiene) proceed with barriers 20 kJ mol<sup>-1</sup> higher than α,δ-hydrogenation and with barriers nearly equivalent to butene hydrogenation, despite α,β-hydrogenation of butadiene also occurring through allylic carbocations. Hydrogenation of formaldehyde, a diene precursor, occurs with barriers that are within 15 kJ mol<sup>-1</sup> of diene hydrogenation barriers, indicating that it may also contribute to increasing catalyst lifetimes by preventing diene formation. Benzene, in contrast to dienes and formaldehyde, is hydrogenated with higher barriers than C<sub>2</sub>–C<sub>4</sub> alkenes despite proceeding via stable benzenium cations because of the thermodynamic instability of the product which has lost aromaticity. Carbocation stabilities predict the relative rates of alkene hydrogenation and in some cases shed insights into the hydrogenation of benzene, dienes, and formaldehyde, but cation stabilities alone cannot account for the poor hydrogenation of benzene or the facile hydrogenation of dienes, boosted by stabilization conferred by a,δ-hydrogenation. This work suggests that the main mechanisms of catalyst lifetime improvement with high H<sub>2</sub> co-feeds is reduction of diene concentrations through both their selective hydrogenation and hydrogenation of their precursors to prevent formation of deactivating polyaromatic species.</p>
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