甲醇转化制丙烯反应机理
|
摘要
以杂化轨道理论和关于含Si、Al元素催化剂酸中心的认识为理论依据和研究方法,从反应物与催化剂B酸中心相互作用为切入点,分析相互作用过程中甲醇与B酸中心分子结构变化,揭示甲醇制烯烃反应机理本质。研究结果表明甲醇转化制丙烯反应过程可分为3步:(1)B酸中心释放H~+攻击甲醇分子中C-O键,C-O键断裂生成甲基碳正离子(CH~+_3)和H_2O,CH~+_3中C原子外层有个空轨道,释放了H~+后的B酸中心失去活性。(2)失活B酸中心从CH~+_3中获得H~+,反应生成了一种新中间体H-form,H-form中C原子外层有1对孤对电子(e~-)和1个空轨道,结构式为C_(↑↓)~〇H_2;失活的B酸中心获得H~+后恢复活性。第3步,有3个H-form结构的C原子,第1个C原子上的孤对e~-占据第2个C原子上空轨道,第2个C原子上的孤对e~-占据第1个C原子上空轨道,彼此共用2对e~-,形成C=C键;第3个C原子上的空轨道与第1个C原子上C、H原子重叠轨道再次重叠,形成C、H和C三原子共用1对e~-结构,结果是第1个C原子与第2个C原子形成C=C键,第1个C原子与第3个C原子形成C-C键,3个C原子形成C=C-C结构,即生成丙烯。
Based on the hybrid orbital theory, knowledge of acid center of Silicon and Aluminum-containing catalysts and the mutual interaction between the reactant and the catalyst B-acid center, the molecular structure of methanol and B-acid center in the interaction process are investigated. Through the above studies, the nature of reaction mechanisms of methanol to olefins are understood. It was found that the methanol-to-propylene(MTP) reaction process could be divided into three steps: At the first step, the B-acid center provides hydrogen proton(H~+) to attack the C-O bond in the methanol molecule, and the C-O bond breaks to form methyl carbocation(CH~+_3) and H_2O. The outer layer of the carbon atom of the CH~+_3 has an empty orbit, but the B-acid center after the release of the hydrogen proton(H~+) loses its activity. At the second step, the inactivated B-acid center obtains hydrogen proton(H~+) from CH~+_3, and the reaction produces a new intermediate H-form. H-form has a pair of lone pairs of electrons(e~-) and an empty orbit. The structural formula is C_(↑↓)~〇H_2, where the double arrow represents the lone pair e~-, and the circle represents the empty orbit. Thus, the inactive B-acid center recovers the activity after obtaining hydrogen proton(H~+). At the third step, there are three carbon atoms of H-form structure, the lone pair e~- on the first carbon atom occupies the an empty of the second carbon atom, and the lone pair e~- on the second carbon atom occupies an empty orbit of first carbon atom, then first carbon and second carbon atoms share two pairs e~- to form a C=C double bond; the empty orbit on the third carbon atom overlaps with the overlapping orbit of the carbon and hydrogen atoms on the first carbon atom, forming a pair e~- structure which carbon, hydrogen and carbon atoms share with each other. As a result, the first carbon atom forms a C=C bond with the second carbon atom, and the first carbon atom forms a C-C bond with the third carbon atom. Then the three carbon atoms of H-form structure lead to a C=C-C structure, which is propylene.
Based on the hybrid orbital theory, knowledge of acid center of Silicon and Aluminum-containing catalysts and the mutual interaction between the reactant and the catalyst B-acid center, the molecular structure of methanol and B-acid center in the interaction process are investigated. Through the above studies, the nature of reaction mechanisms of methanol to olefins are understood. It was found that the methanol-to-propylene(MTP) reaction process could be divided into three steps: At the first step, the B-acid center provides hydrogen proton(H~+) to attack the C-O bond in the methanol molecule, and the C-O bond breaks to form methyl carbocation(CH~+_3) and H_2O. The outer layer of the carbon atom of the CH~+_3 has an empty orbit, but the B-acid center after the release of the hydrogen proton(H~+) loses its activity. At the second step, the inactivated B-acid center obtains hydrogen proton(H~+) from CH~+_3, and the reaction produces a new intermediate H-form. H-form has a pair of lone pairs of electrons(e~-) and an empty orbit. The structural formula is C_(↑↓)~〇H_2, where the double arrow represents the lone pair e~-, and the circle represents the empty orbit. Thus, the inactive B-acid center recovers the activity after obtaining hydrogen proton(H~+). At the third step, there are three carbon atoms of H-form structure, the lone pair e~- on the first carbon atom occupies the an empty of the second carbon atom, and the lone pair e~- on the second carbon atom occupies an empty orbit of first carbon atom, then first carbon and second carbon atoms share two pairs e~- to form a C=C double bond; the empty orbit on the third carbon atom overlaps with the overlapping orbit of the carbon and hydrogen atoms on the first carbon atom, forming a pair e~- structure which carbon, hydrogen and carbon atoms share with each other. As a result, the first carbon atom forms a C=C bond with the second carbon atom, and the first carbon atom forms a C-C bond with the third carbon atom. Then the three carbon atoms of H-form structure lead to a C=C-C structure, which is propylene.
引文
[1] 刘中明,齐越.甲醇制取低碳烯烃(DMTO)技术的研究开发及工业性试验[J].中国科学院院刊,2006,21(5):406-408.(LIU Zhongming,QI Yue.Research and development and industrial testing of DMTO technology[J].Bulletin of Chinese Academy Sciences,2006,21(5):406-408.)
[2] 张玉卓.煤清洁转化工程[M].北京:煤炭工业出版社,2011.
[3] 许磊,杜爱萍,魏迎旭,等.骨架富含Si(4Al)结构的SAPO-34分子筛的合成及其对甲醇制烯烃反应的催化性能[J].催化学报,2008,29(8):727-732.(XU Lei,DU Aiping,WEI Yingxu,et al.Synthesis of SAPO-34 with rich Si(4Al) coordination environment in the framework and its catalytic performance in methanol-to-olefins reaction[J].China Journal of Catalysis,2008,29(8):727-732.)
[4] 吴秀章.煤制低碳烯烃工艺与工程[M].北京:化学工业出版社,2015.
[5] DAHL I M,KOLBOE S.On the reaction-mechanism for hydrocarbon formation in the MTO reaction over SAPO-34[J].Catal Lett,1993,20(3/4):329-336.
[6] DAHL I M,KOLBOE S.On the reaction mechanism for hydrocarbon formation from mechanism studies[J].Catal,1994,149:458-464.
[7] OLSBYE U,BJORGEN M,SVELLE S,et al.Mechanistic insight into the methanol to hydrocarbons reaction[J].Catal Tod,2005,106:108-111.
[8] HEREIJGERS B P C,BLEKEN F,NILSEN M H,et al.Product shape selectivity dominates the methanol-to-olefin (MTO) reaction over SAPO-34 catalysts[J].J Catal,2009,264:77-87.
[9] 严志敏.甲醇催化转化反应的固体核磁共振研究[D].大连:中国科学院大连化学物理研究所,2003.
[10] 唐宏青.现代煤化工新技术[M].北京:化学工业出版社,2009:10.
[11] LI JINZHE,WEI YINGXU,CHEN JINGRUN,et al.Observation of heptamethylbenzeniumcation over SAPO-type molecular sieve DNL-6 under real MTO conversion conditions[J].J Am Chem Soc,2012,134:836-839.
[12] XINQIANG WU,SHUTAO XU,WENNA ZHANG,et al.Direct mechanism of the first carbon-carbon bond formation in the methanol to-hydrocarbons process[J].Angewandte Chemie International Edition,2017,129 (31):9039-9043.
[13] WU Xinqiang,XU Shutao,WEI Yingxu,et al.Evolution of C-C bond formation in the methanol-to-olefins process:From direct coupling to autocatalysis[J].ACS Catal,2018,8:7356-7361.
[14] 贺振富,代振宇,龙军.硅-铝催化剂酸中心形成及其结构[J].石油学报(石油加工),2011,27(1):11-19.(HE Zhenfu,DAI Zhenyu,LONG Jun.Formation and structural characteristics of acidic center of silicon-alumina catalyst[J].Acta Petrolei Sinica (Petroleum Processing Section),2011,27(1):11-19.)
[15] 贺振富,刘俊,龙军.硅-铝催化剂酸中心失活与再生的分析[J].石油学报(石油加工),2011,27(6):845-851.(HE Zhenfu,LIU Jun,LONG Jun.Analysis for deactivation and regeneration of acidic centers of silica-alumina catalyst[J].Acta Petrolei Sinica (Petroleum Processing Section),2011,27(6):845-851.)Reaction Mechanisms of Methanol to PropyleneHE ZhenfuB-acid center releases H+ to attack the C-O bond in the methanol molecule,and the C-O bond cleaves to form the methyl carbon cation (CH+3).The outer layer of the carbon atom of the CH+3 has an empty orbit,and the B-acid center after the release of H+ loses its activity.The inactivated B-acid center obtains H+ from CH+3,and the CH+3 produces a new intermediate H-form.The H-form has a pair of lone pairs of electrons (e-) and one empty orbital,and the structural formula can be expressed as C↑↓〇H2.The inactivated B-acid center can recover activity after obtaining H+.For carbon atoms of the three H-form structures,the first carbon atom and the second carbon atom share two pairs e- to form a C=C bond;the first carbon atom also shares a pair e- with the third carbon atom to form C-C bond.As a result,the three carbon atoms of H-form structure can form a C=C-C structure,which is propylene.
[2] 张玉卓.煤清洁转化工程[M].北京:煤炭工业出版社,2011.
[3] 许磊,杜爱萍,魏迎旭,等.骨架富含Si(4Al)结构的SAPO-34分子筛的合成及其对甲醇制烯烃反应的催化性能[J].催化学报,2008,29(8):727-732.(XU Lei,DU Aiping,WEI Yingxu,et al.Synthesis of SAPO-34 with rich Si(4Al) coordination environment in the framework and its catalytic performance in methanol-to-olefins reaction[J].China Journal of Catalysis,2008,29(8):727-732.)
[4] 吴秀章.煤制低碳烯烃工艺与工程[M].北京:化学工业出版社,2015.
[5] DAHL I M,KOLBOE S.On the reaction-mechanism for hydrocarbon formation in the MTO reaction over SAPO-34[J].Catal Lett,1993,20(3/4):329-336.
[6] DAHL I M,KOLBOE S.On the reaction mechanism for hydrocarbon formation from mechanism studies[J].Catal,1994,149:458-464.
[7] OLSBYE U,BJORGEN M,SVELLE S,et al.Mechanistic insight into the methanol to hydrocarbons reaction[J].Catal Tod,2005,106:108-111.
[8] HEREIJGERS B P C,BLEKEN F,NILSEN M H,et al.Product shape selectivity dominates the methanol-to-olefin (MTO) reaction over SAPO-34 catalysts[J].J Catal,2009,264:77-87.
[9] 严志敏.甲醇催化转化反应的固体核磁共振研究[D].大连:中国科学院大连化学物理研究所,2003.
[10] 唐宏青.现代煤化工新技术[M].北京:化学工业出版社,2009:10.
[11] LI JINZHE,WEI YINGXU,CHEN JINGRUN,et al.Observation of heptamethylbenzeniumcation over SAPO-type molecular sieve DNL-6 under real MTO conversion conditions[J].J Am Chem Soc,2012,134:836-839.
[12] XINQIANG WU,SHUTAO XU,WENNA ZHANG,et al.Direct mechanism of the first carbon-carbon bond formation in the methanol to-hydrocarbons process[J].Angewandte Chemie International Edition,2017,129 (31):9039-9043.
[13] WU Xinqiang,XU Shutao,WEI Yingxu,et al.Evolution of C-C bond formation in the methanol-to-olefins process:From direct coupling to autocatalysis[J].ACS Catal,2018,8:7356-7361.
[14] 贺振富,代振宇,龙军.硅-铝催化剂酸中心形成及其结构[J].石油学报(石油加工),2011,27(1):11-19.(HE Zhenfu,DAI Zhenyu,LONG Jun.Formation and structural characteristics of acidic center of silicon-alumina catalyst[J].Acta Petrolei Sinica (Petroleum Processing Section),2011,27(1):11-19.)
[15] 贺振富,刘俊,龙军.硅-铝催化剂酸中心失活与再生的分析[J].石油学报(石油加工),2011,27(6):845-851.(HE Zhenfu,LIU Jun,LONG Jun.Analysis for deactivation and regeneration of acidic centers of silica-alumina catalyst[J].Acta Petrolei Sinica (Petroleum Processing Section),2011,27(6):845-851.)Reaction Mechanisms of Methanol to PropyleneHE ZhenfuB-acid center releases H+ to attack the C-O bond in the methanol molecule,and the C-O bond cleaves to form the methyl carbon cation (CH+3).The outer layer of the carbon atom of the CH+3 has an empty orbit,and the B-acid center after the release of H+ loses its activity.The inactivated B-acid center obtains H+ from CH+3,and the CH+3 produces a new intermediate H-form.The H-form has a pair of lone pairs of electrons (e-) and one empty orbital,and the structural formula can be expressed as C↑↓〇H2.The inactivated B-acid center can recover activity after obtaining H+.For carbon atoms of the three H-form structures,the first carbon atom and the second carbon atom share two pairs e- to form a C=C bond;the first carbon atom also shares a pair e- with the third carbon atom to form C-C bond.As a result,the three carbon atoms of H-form structure can form a C=C-C structure,which is propylene.