Critical role of formaldehyde during methanol conversion to hydrocarbons

Liu, Yue; Kirchberger, Felix M.; Müller, Sebastian; Eder, Moritz; Tonigold, Markus; Sanchez‐Sanchez, Maricruz; Lercher, Johannes A.

doi:10.1038/s41467-019-09449-7

Cited by 129 publications

(250 citation statements)

References 44 publications

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“…The main mechanism of lifetime improvement is through limiting the formation of deactivation precursors-dienes and CH2O-as C6H6 hydrogenation occurs with high barriers (> 200 kJ mol −1 in MFI and CHA) because breaking aromaticity of these species is unfavorable. Direct hydrogenation barriers of butadiene are relatively low (140 kJ mol −1 in MFI and 122 kJ mol −1 in CHA), as are those of hydrogenation of CH2O, which plays a role in diene formation 24,42 (140 kJ mol −1 in MFI and 142 kJ mol −1 in CHA). The limited hydrogenation of alkenes with dramatic increases in catalyst lifetime in MTO studies suggest that deactivation precursors must be selectively hydrogenated (i.e., hydrogenated at a higher rate) than the desired alkene products, and this is proven here as diene and formaldehyde hydrogenation occurs with barriers 20-30 kJ mol −1 lower than those for propene or butene hydrogenation and 60-70 kJ mol −1 lower than those for ethene hydrogenation.…”

Section: Discussionmentioning

confidence: 99%

“…Previous literature has implicated CH2O as a precursor to dienes and aromatics and as a significant contributor to catalyst deactivation. 24,42,[64][65][66] Kinetic studies have demonstrated that co-feeding H2 and CH2O (4 bar H2, 0.13 mbar CH2O, 1.3 bar CH3OH, 673 K) increases catalyst lifetimes by 2.1-fold compared to identical co-feeds of He and CH2O. This indicates that H2 may limit polyaromatic formation by intercepting CH2O diene precursors.…”

Section: Hydrogenation In H-mfimentioning

confidence: 99%

“…41 Overall barriers (relative to the Z-H state) of concerted and sequential hydrogenation are less than 10 kJ mol −1 different for C2H4, 41 indicating that the two mechanisms are likely competitive. Additionally, overall potential energy barriers of CH2O hydrogenation are facile (60 kJ mol −1 ) and are over 100 kJ mol −1 lower than those of ethene, indicating that hydrogenation of CH2O, 41 a diene precursor, 8,24,42 may contribute to the experimentally observed increases in catalyst lifetime.…”

Section: Introductionmentioning

confidence: 96%

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Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

DeLuca

Janes²,

Hibbitts³

2019

Preprint

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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.

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Section: Discussionmentioning

confidence: 99%

Section: Hydrogenation In H-mfimentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

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Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

DeLuca

Janes²,

Hibbitts³

2019

Preprint

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“…7), consistent with previous studies suggesting that barriers of CH2O hydrogenation are low compared to those of ethene hydrogenation. 42 Despite the shorter chain length, these barriers are comparable to those of butadiene likely because of the relative stability of oxocarbenium ions (Fig. 2) coupled with hydrogen bonding between the framework and -OH of the transition state.…”

Section: Hydrogenation In H-mfimentioning

confidence: 96%

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

DeLuca¹,

Janes²,

Hibbitts³

2019

Preprint

View full text Add to dashboard Cite

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 co-feeds 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.

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“…Therefore, the surface acetyl species and/or ketene might represent the first C−C bond formation toward ethane over Zn/ZSM‐5 in syngas conversion. Although acetate species were also reported in MTH, they were generally accepted to form over Brønsted acid sites upon carbonylation of surface methoxy species . In comparison, CO activation and ethane formation, reported herein, are related to Zn sites.…”

Section: Figurementioning

confidence: 99%

C−C Bond Formation in Syngas Conversion over Zinc Sites Grafted on ZSM‐5 Zeolite

et al. 2020

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Despite significant progress achieved in Fischer–Tropsch synthesis (FTS) technology, control of product selectivity remains a challenge in syngas conversion. Herein, we demonstrate that Zn2+‐ion exchanged ZSM‐5 zeolite steers syngas conversion selectively to ethane with its selectivity reaching as high as 86 % among hydrocarbons (excluding CO2) at 20 % CO conversion. NMR spectroscopy, X‐ray absorption spectroscopy, and X‐ray fluorescence indicate that this is likely attributed to the highly dispersed Zn sites grafted on ZSM‐5. Quasi‐in‐situ solid‐state NMR, obtained by quenching the reaction in liquid N2, detects C2 species such as acetyl (‐COCH3) bonding with an oxygen, ethyl (‐CH2CH3) bonding with a Zn site, and epoxyethane molecules adsorbing on a Zn site and a Brønsted acid site of the catalyst, respectively. These species could provide insight into C−C bond formation during ethane formation. Interestingly, this selective reaction pathway toward ethane appears to be general because a series of other Zn2+‐ion exchanged aluminosilicate zeolites with different topologies (for example, SSZ‐13, MCM‐22, and ZSM‐12) all give ethane predominantly. By contrast, a physical mixture of ZnO‐ZSM‐5 favors formation of hydrocarbons beyond C3+. These results provide an important guide for tuning the product selectivity in syngas conversion.

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Critical role of formaldehyde during methanol conversion to hydrocarbons

Cited by 129 publications

References 44 publications

Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

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

C−C Bond Formation in Syngas Conversion over Zinc Sites Grafted on ZSM‐5 Zeolite

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