H‐ZSM‐5 zeolite is a typical catalyst for methanol‐to‐olefins(MTO)conversion.Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framewor...H‐ZSM‐5 zeolite is a typical catalyst for methanol‐to‐olefins(MTO)conversion.Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framework,the catalytic roles of the acid sites in different pore channels of the H‐ZSM‐5 zeolite are not well understood.In this study,the MTO reaction network,involving the aromatic cycle,alkene cycle,and aromatization process,and also the diffusion behavior of methanol feedstock and olefin and aromatic products at different acid sites in the straight channel,sinusoidal channel,and intersection cavity of H‐ZSM‐5 zeolite was comparatively investigated using density functional theory calculations and molecular dynamic simulations.The results indicated that the aromatic cycle and aromatization process occurred preferentially at the acid sites in the intersection cavities with a much lower energy barrier than that at the acid sites in the straight and sinusoidal channels.In contrast,the formation of polymethylbenzenes was significantly suppressed at the acid sites in the sinusoidal and straight channels,whereas the alkene cycle can occur at all three types of acid sites with similar energy barriers and probabilities.Consequently,the catalytic performance of H‐ZSM‐5 zeolite for MTO conversion,including activity and product selectivity,can be regulated properly through the purposive alteration of the acid site distribution,viz.,the location of Al in the zeolite framework.This study helps to elucidate the relation between the catalytic performance of different acid sites in the H‐ZSM‐5 zeolite framework for MTO conversion,which should greatly benefit the design of efficient catalyst for methanol conversion.展开更多
The reaction mechanism of zeolite- or zeotype-catalyzed methanol-to-olefins（MTO） conversion is still a subject of debate. Employing periodic density functional theory calculations, the olefin-based cycle was studied...The reaction mechanism of zeolite- or zeotype-catalyzed methanol-to-olefins（MTO） conversion is still a subject of debate. Employing periodic density functional theory calculations, the olefin-based cycle was studied using tetramethylethene（TME） as a representative olefinic hydrocarbon pool in H-SAPO-18 zeotype. The overall free energy barrier at 673 K was calculated and found to be less than 150 kJ/mol in the TME-based cycle, much lower than those in the aromatic-based cycle（〉 200 kJ/mol）, indicating that olefins themselves are the dominant active hydrocarbon pool species in H-SAPO-18. The similarity of the intermediates involved between the aromatic-based cycle and the olefin-based cycle was also highlighted, revealing that both cycles were pattern-consistent. The selectivity related to the distribution of cracking precursors, such as higher olefins or carbenium ions, as a result of the olefin-based cycle for the MTO conversion. The enthalpy barrier of the crack-ing step scaled linearly with the number of carbon atoms of cracking precursors to produce ethene or propene with ethene being much less favored than propene for cracking of C7 and higher pre-cursors. This work highlighted the importance of the olefin-based cycle in H-SAPO-18 for the MTO conversion and established the similarity between the olefin-based and aromatic-based cycles.展开更多
文摘H‐ZSM‐5 zeolite is a typical catalyst for methanol‐to‐olefins(MTO)conversion.Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framework,the catalytic roles of the acid sites in different pore channels of the H‐ZSM‐5 zeolite are not well understood.In this study,the MTO reaction network,involving the aromatic cycle,alkene cycle,and aromatization process,and also the diffusion behavior of methanol feedstock and olefin and aromatic products at different acid sites in the straight channel,sinusoidal channel,and intersection cavity of H‐ZSM‐5 zeolite was comparatively investigated using density functional theory calculations and molecular dynamic simulations.The results indicated that the aromatic cycle and aromatization process occurred preferentially at the acid sites in the intersection cavities with a much lower energy barrier than that at the acid sites in the straight and sinusoidal channels.In contrast,the formation of polymethylbenzenes was significantly suppressed at the acid sites in the sinusoidal and straight channels,whereas the alkene cycle can occur at all three types of acid sites with similar energy barriers and probabilities.Consequently,the catalytic performance of H‐ZSM‐5 zeolite for MTO conversion,including activity and product selectivity,can be regulated properly through the purposive alteration of the acid site distribution,viz.,the location of Al in the zeolite framework.This study helps to elucidate the relation between the catalytic performance of different acid sites in the H‐ZSM‐5 zeolite framework for MTO conversion,which should greatly benefit the design of efficient catalyst for methanol conversion.
基金supported by the National Key Research and Development Program of China (2016YFB0701100, 2017YFB0702800)the National Natural Science Foundation of China (21673295)~~
文摘The reaction mechanism of zeolite- or zeotype-catalyzed methanol-to-olefins（MTO） conversion is still a subject of debate. Employing periodic density functional theory calculations, the olefin-based cycle was studied using tetramethylethene（TME） as a representative olefinic hydrocarbon pool in H-SAPO-18 zeotype. The overall free energy barrier at 673 K was calculated and found to be less than 150 kJ/mol in the TME-based cycle, much lower than those in the aromatic-based cycle（〉 200 kJ/mol）, indicating that olefins themselves are the dominant active hydrocarbon pool species in H-SAPO-18. The similarity of the intermediates involved between the aromatic-based cycle and the olefin-based cycle was also highlighted, revealing that both cycles were pattern-consistent. The selectivity related to the distribution of cracking precursors, such as higher olefins or carbenium ions, as a result of the olefin-based cycle for the MTO conversion. The enthalpy barrier of the crack-ing step scaled linearly with the number of carbon atoms of cracking precursors to produce ethene or propene with ethene being much less favored than propene for cracking of C7 and higher pre-cursors. This work highlighted the importance of the olefin-based cycle in H-SAPO-18 for the MTO conversion and established the similarity between the olefin-based and aromatic-based cycles.
