Aromatic-based hydrocarbon pool mechanism for methanol-to-olefins conversion in H-SAPO-18: A van der Waals density functional study

王传明,; 王仰东,; 刘红星,; Guang, Yang; 杜钰珏,; 谢在库,

doi:10.1016/s1872-2067(15)60891-9

Cited by 27 publications

(23 citation statements)

References 48 publications

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“…The main identified soluble species are monocyclic, bicyclic, tricyclic and tetracyclic aromatics, in agreement with the observations reported for SAPO-18 catalysts [67] and similar catalyst topologies. [23,25,68] It is expected to find olefins and monocyclic aromatics within the retained species that act as hydrocarbon pool species in SAPO-18 and SAPO-34 catalysts, whereas other polycyclic aromatics found are deactivating species (coke), [17,18,29,35,67,69] though bicyclic aromatics may also be hydrocarbon pool species in SAPO-34 catalysts depending on the temperature. [34] Figure 10a shows the abundance of these species for each experiment.…”

Section: Analysis Of Soluble Speciesmentioning

confidence: 99%

“…The difference in the cavities, which affects the diffusion of molecules, may be responsible for improving the selectivity of propylene and butylenes. [14,15] The MTO reaction on SAPO-18 catalysts follows the well-established dual-cycle mechanism, [16][17][18][19] comprising an olefins and aromatics cycle. The hydrocarbon pool of olefinic and aromatic species undergoes reactions of methylation of olefins or aromatics, dealkylation of alkylaromatics, oligomerization of olefins, cracking of olefins, cyclization of olefins and hydrogen transfer between aliphatic and cyclic olefins.…”

Section: Introductionmentioning

confidence: 99%

“…[22][23][24][25][26][27][28] The insitu or ex-situ analyses of retained species reveal the presence of mono-, bi-, tri-and tetracyclic aromatics remaining in the catalyst during or after the MTO reaction (which are soluble in organic solvents and therefore these species are often referred to as soluble species). [6,17,24,[29][30][31][32][33][34][35] There are many research works on the MTO reaction using in-situ or operando spectroscopies to study the retained species in the catalyst, particularly ultraviolet-visible (UV-vis) spectroscopy [24,33,34,[36][37][38][39] and Fourier transform infrared (FTIR) spectroscopy. [24,33,39] The challenge is to apply these techniques together with the analysis of products to correlate both active species with kinetics and deactivation, which could be instrumental to solve the fate of the hydrocarbon pool species.…”

Section: Introductionmentioning

confidence: 99%

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Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

et al. 2021

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The dynamics of retained and deactivating species in a SAPO-18 catalyst during the methanol-to-olefins reaction have been followed using a combination of ex-situ and in-situ techniques in differential and integral reactors.The retained species were analyzed using extraction, in-situ FTIR and in-situ UV-vis spectroscopies combined with online product analysis (gas chromatography and mass spectrometry). The composition of the extracted soluble species was determined using gas chromatography-mass spectrometry and that of the insoluble species using high-resolution mass spectrometry. We observe a decrease in the formation and degradation rates of retained species when co-feeding water, whereas the extent of the decreases is the same across the entire spectrum of retained molecules. This indicates that co-feeding water unselectively quenches the formation of active and deactivating species. At the same time, the catalyst has an extended lifetime when cofeeding water due to the diffusion of species (particularly olefins) out of the SAPO-18 crystals, and subsequent growth of heavy polycyclic aromatic structures that imply less deactivation. These conclusions can be extrapolated to other MTO catalysts with relatively similar pore topology such as SAPO-34 or SSZ-13 structures.

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Section: Analysis Of Soluble Speciesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

et al. 2021

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“…This indicates that arenes either escape zeolite domains as aromatic products of MTH or become extensively substituted as C10-C12 species that are trapped within MFI intersections. These highly substituted arenes can only serve as co-catalysts in the aromatic cycle 18,20,70,71 and ultimately grow to polyaromatic species [75][76][77] in deactivation mechanisms when trapped in zeolite pores. Figure 15.…”

Section: Mechanisms Of Methylbenzene Methylation At Methanol-to-hydromentioning

confidence: 99%

Mechanism and Kinetics of Methylating C₆–C₁₂ Methylbenzenes with Methanol and Dimethyl Ether in H-MFI Zeolites

et al. 2019

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This study uses periodic density functional theory (DFT) to determine the reaction mechanism and effects of reactant size for all 20 arene (C6-C12) methylation reactions using CH3OH and CH3OCH3 as methylating agents in H-MFI zeolites. Reactant, product, and transition state structures were manually generated, optimized, and then systematically reoriented and reoptimized to sufficiently sample the potential energy surface and thus identify global minima and the most stable transition states which interconnect them. These systematic reorientations decreased energies by up to 50 kJ mol −1 , demonstrating their necessity when analyzing reaction pathways or adsorptive properties of zeolites. Benzene-DME methylation occurs via sequential pathways, consistent with prior reports, but is limited by surface methylation which is stabilized by co-adsorbed benzene via novel cooperativity between the channels and intersections within MFI. These co-adsorbate assisted surface methylations generally prevail over unassisted routes. Calculated free energy barriers and reaction energies suggest that both the sequential and concerted methylation mechanisms can generally occur, depending on the methylating agent and methylbenzene being reacted-there is no consensus mechanism for these homologous reactions. Intrinsic methylation barriers for step-wise reactions of benzene to hexamethylbenzene remain between 75-137 kJ mol −1 at conditions relevant to methanol-to-hydrocarbon (MTH) reactions where such arene species act as co-catalysts. Intrinsic methylation barriers are similar between CH3OH and CH3OCH3 suggesting that both species are equally capable of interconverting between methylbenzene species. Additionally, these methylation barriers do not systematically increase as the number of methyl-substituents on the arene increases and the formation of higher methylated arenes is thermodynamically favorable. These barriers are significantly lower than those associated with alkene formation during the aromatic cycle, suggesting that aromatic species formed during MTH reactions either egress from the catalyst-depending on that zeolite's pore structure-or become trapped as extensivelysubstituted C10-C12 species which can either isomerize to form olefins or ultimately create polyaromatic species that deactivate MTH catalysts.

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“…[8][9][10][11][12][13] Two complementary cycles form C2-C4 alkenes in methanol-to-olefins (MTO) processes. [14][15][16][17][18][19] Olefins methylate and grow to a size capable of cracking into C3-C5 compounds in the olefin cycle. 1,17,20,21 These olefins can undergo hydride transfer reactions to form alkanes and dienes-either via formaldehyde-assisted routes [22][23][24] or through alkene disproportionation 25,26 -which can cyclize to form aromatic species.…”

Section: Introductionmentioning

confidence: 99%

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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Aromatic-based hydrocarbon pool mechanism for methanol-to-olefins conversion in H-SAPO-18: A van der Waals density functional study

Cited by 27 publications

References 48 publications

Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

Mechanism and Kinetics of Methylating C₆–C₁₂ Methylbenzenes with Methanol and Dimethyl Ether in H-MFI Zeolites

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

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Aromatic-based hydrocarbon pool mechanism for methanol-to-olefins conversion in H-SAPO-18: A van der Waals density functional study

Cited by 27 publications

References 48 publications

Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

Implications of Co‐Feeding Water on the Growth Mechanisms of Retained Species on a SAPO‐18 Catalyst during the Methanol‐to‐Olefins Reaction

Mechanism and Kinetics of Methylating C6–C12 Methylbenzenes with Methanol and Dimethyl Ether in H-MFI Zeolites

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

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Mechanism and Kinetics of Methylating C₆–C₁₂ Methylbenzenes with Methanol and Dimethyl Ether in H-MFI Zeolites