低碳烯烃(乙烯、丙烯等)是重要的基本有机原料,一般通过蒸汽裂解或催化裂解生成得到。基于中国的资源结构特点,发展非石油资源路线合成低碳烯烃具有重要的战略意义.其中从煤、天然气等资源出发,通过甲醇合成低碳烯烃就提供了这样一条可...低碳烯烃(乙烯、丙烯等)是重要的基本有机原料,一般通过蒸汽裂解或催化裂解生成得到。基于中国的资源结构特点,发展非石油资源路线合成低碳烯烃具有重要的战略意义.其中从煤、天然气等资源出发,通过甲醇合成低碳烯烃就提供了这样一条可替代的路线.因此分子筛催化甲醇制烯烃(MTO)反应在过去几十年获得了广泛的关注和研究.为了获得高的产物选择性,一般要求MTO分子筛催化材料具有较小的孔道结构以及合适的笼结构,H-SAPO-34和H-SAPO-18分子筛就具有这样的空间结构特点.但是MTO催化反应产物分布多样复杂,因此需要深入认识MTO催化反应机理,从而优化设计分子筛结构和反应条件.目前已经形成的共识认为,MTO催化反应沿着烃池反应机理进行,但是烃池活性中心的结构还存在很多争议.我们曾系统研究了H-SAPO-18分子筛中多甲基苯的分布,以及催化MTO反应的芳烃循环路线,指出多甲基苯路线的总吉布斯自由能垒高于200 k J/mol(673 K).本文以四甲基乙烯(TME)作为代表性的烯烃烃池活性中心,系统研究了H-SAPO-18分子筛催化MTO反应的烯烃循环路线.TME循环路线的总吉布斯自由能垒不大于150 k J/mol,远小于芳烃循环的总能垒.因此,烯烃本身有很大可能是H-SAPO-18催化MTO反应的烃池活性中心.我们也指出了芳烃循环和烯烃循环路线的相似性,这包括基元反应的相似性和中间体结构的相似性.或者可以说,芳烃循环和烯烃循环路线机理上没有区别,关键是为了得到具有烷基(侧)链的裂解前驱体,最后通过裂解生成低碳烯烃.在烯烃循环路线中,产物选择性与裂解前驱体(高碳烯烃、碳正离子等)的分布以及裂解动力学有关.计算发现生成乙烯和丙烯的裂解基元反应能垒与裂解前驱体的碳数之间存在线性关系.本文进一步强调了分子筛催化MTO反应中烯烃活性中心的重要性,并且清楚指出了烯烃循环和芳烃循环的机理相似性.展开更多
Summaryof main observation and conclusion Understanding and optimizing structure of active sites is of significance in zeolite catalysis.Benzene ethylation is an industrially important process catalyzed by H-MCM-22 ze...Summaryof main observation and conclusion Understanding and optimizing structure of active sites is of significance in zeolite catalysis.Benzene ethylation is an industrially important process catalyzed by H-MCM-22 zeolite;while the active sites still remain elusive.In this work,density functional theory(DFT)calculations were employed to investigate the benzene ethylation at two different types of Br?nsted acid sites(BAS)in H-MCM-22 zeolite,namely the internal cavity(IC)acid site and the external surface(ES)acid site.Both the stepwise and concerted pathways were addressed.The compari-son of the calculated energetics of two pathways indicates that the benzene ethylation reaction primarily proceeds via the concerted pathway at both the IC and ES acid sites of H-MCM-22.The calculated overall Gibbs free energies at reaction condition(473 K and 3.5 MPa)on the IC and ES acid sites are 90 and 86 kJ/mol,with the rate constants of 1.20×10^3and 2.92×10^3s^-1,respectively.It indicates that benzene ethylation could occur both on the IC and ES acid sites,with the catalytic activity of IC acid site being slightly lower than that of ES acid site.Furthermore,we theoretically reveal that the acid strength at the ES site is slightly weaker than that at the IC site via the frequency shift after the adsorption of CO.The differences in dispersion interaction between ES and IC sites are also quantified by the adsorption of base molecules with different sizes.The calculated results in this work demonstrate that the acid sites at the external surface of H-MCM-22 zeolites are suitable for benzene ethylation to produce ethylbenzene,providing theoretical implications for tailoring the distribution of active sites in H-MCM-22 zeolite.展开更多
基金supported by the National Key Research and Development Program of China (2016YFB0701100, 2017YFB0702800)the National Natural Science Foundation of China (21673295)~~
文摘低碳烯烃(乙烯、丙烯等)是重要的基本有机原料,一般通过蒸汽裂解或催化裂解生成得到。基于中国的资源结构特点,发展非石油资源路线合成低碳烯烃具有重要的战略意义.其中从煤、天然气等资源出发,通过甲醇合成低碳烯烃就提供了这样一条可替代的路线.因此分子筛催化甲醇制烯烃(MTO)反应在过去几十年获得了广泛的关注和研究.为了获得高的产物选择性,一般要求MTO分子筛催化材料具有较小的孔道结构以及合适的笼结构,H-SAPO-34和H-SAPO-18分子筛就具有这样的空间结构特点.但是MTO催化反应产物分布多样复杂,因此需要深入认识MTO催化反应机理,从而优化设计分子筛结构和反应条件.目前已经形成的共识认为,MTO催化反应沿着烃池反应机理进行,但是烃池活性中心的结构还存在很多争议.我们曾系统研究了H-SAPO-18分子筛中多甲基苯的分布,以及催化MTO反应的芳烃循环路线,指出多甲基苯路线的总吉布斯自由能垒高于200 k J/mol(673 K).本文以四甲基乙烯(TME)作为代表性的烯烃烃池活性中心,系统研究了H-SAPO-18分子筛催化MTO反应的烯烃循环路线.TME循环路线的总吉布斯自由能垒不大于150 k J/mol,远小于芳烃循环的总能垒.因此,烯烃本身有很大可能是H-SAPO-18催化MTO反应的烃池活性中心.我们也指出了芳烃循环和烯烃循环路线的相似性,这包括基元反应的相似性和中间体结构的相似性.或者可以说,芳烃循环和烯烃循环路线机理上没有区别,关键是为了得到具有烷基(侧)链的裂解前驱体,最后通过裂解生成低碳烯烃.在烯烃循环路线中,产物选择性与裂解前驱体(高碳烯烃、碳正离子等)的分布以及裂解动力学有关.计算发现生成乙烯和丙烯的裂解基元反应能垒与裂解前驱体的碳数之间存在线性关系.本文进一步强调了分子筛催化MTO反应中烯烃活性中心的重要性,并且清楚指出了烯烃循环和芳烃循环的机理相似性.
基金This work was supported by the National Natural Science Foundation of China(No.92045303)the China Postdoctoral Science Foundation(No.2020M681444).The computational resources from Sinopec Geophysical Research Institute are acknowledged.
基金The authors acknowledge the financial support from the Na-tional Key Research and Development Program of China(No.2017YFB0702800)the National Natural Science Foundation of China(No.21673295).The computational resources from Sinopec Geophysical Research Institute are acknowledged.
文摘Summaryof main observation and conclusion Understanding and optimizing structure of active sites is of significance in zeolite catalysis.Benzene ethylation is an industrially important process catalyzed by H-MCM-22 zeolite;while the active sites still remain elusive.In this work,density functional theory(DFT)calculations were employed to investigate the benzene ethylation at two different types of Br?nsted acid sites(BAS)in H-MCM-22 zeolite,namely the internal cavity(IC)acid site and the external surface(ES)acid site.Both the stepwise and concerted pathways were addressed.The compari-son of the calculated energetics of two pathways indicates that the benzene ethylation reaction primarily proceeds via the concerted pathway at both the IC and ES acid sites of H-MCM-22.The calculated overall Gibbs free energies at reaction condition(473 K and 3.5 MPa)on the IC and ES acid sites are 90 and 86 kJ/mol,with the rate constants of 1.20×10^3and 2.92×10^3s^-1,respectively.It indicates that benzene ethylation could occur both on the IC and ES acid sites,with the catalytic activity of IC acid site being slightly lower than that of ES acid site.Furthermore,we theoretically reveal that the acid strength at the ES site is slightly weaker than that at the IC site via the frequency shift after the adsorption of CO.The differences in dispersion interaction between ES and IC sites are also quantified by the adsorption of base molecules with different sizes.The calculated results in this work demonstrate that the acid sites at the external surface of H-MCM-22 zeolites are suitable for benzene ethylation to produce ethylbenzene,providing theoretical implications for tailoring the distribution of active sites in H-MCM-22 zeolite.
