Ga/HZSM-5分子筛上乙烯二聚反应的理论研究

应用理论计算方法研究了Ga/HZSM-5及Al/HZSM-5分子筛上乙烯二聚生成1-丁烯的反应历程,比较了分子筛酸性对反应能量的影响.计算采用分为两层的76T簇模型,应用量子力学和分子力学联合的ONIOM2（B3LYP/6-31G（d,p）：UFF）方法.乙烯二聚过程可按照分步机理和协同机理进行,均得到表面丁基烷氧中间产物.计算结果表明,与在Al/HZSM-5分子筛上的反应过程相比,乙烯在Ga/HZSM-5分子筛上的吸附能低20.62kJ·mol-1,但质子化反应的活化能只高出1.26kJ·mol-1;而乙基烷氧中间体与乙烯分子结合过程的活化能高出62.55kJ·mol-1,原因是Ga原子半径...

Full description

Saved in:
Bibliographic Details
Published in物理化学学报 Vol. 27; no. 6; pp. 1393 - 1399
Main Author 李惊鸿 周丹红 任珏
Format Journal Article
LanguageChinese
Published 辽宁师范大学化学化工学院,功能材料化学研究所,辽宁大连116029 2011
Subjects
Ga/HZSM-5
乙烯二聚
反应机理
密度泛函理论
Ga/HZSM-5
乙烯二聚
反应机理
密度泛函理论
Online AccessGet full text
ISSN1000-6818

Cover

More Information
Summary:应用理论计算方法研究了Ga/HZSM-5及Al/HZSM-5分子筛上乙烯二聚生成1-丁烯的反应历程,比较了分子筛酸性对反应能量的影响.计算采用分为两层的76T簇模型,应用量子力学和分子力学联合的ONIOM2（B3LYP/6-31G（d,p）：UFF）方法.乙烯二聚过程可按照分步机理和协同机理进行,均得到表面丁基烷氧中间产物.计算结果表明,与在Al/HZSM-5分子筛上的反应过程相比,乙烯在Ga/HZSM-5分子筛上的吸附能低20.62kJ·mol-1,但质子化反应的活化能只高出1.26kJ·mol-1;而乙基烷氧中间体与乙烯分子结合过程的活化能高出62.55kJ·mol-1,原因是Ga原子半径大,降低了六元环过渡态的稳定性.若按协同机理,质子转移和C―C键聚合同时进行,在Ga/HZSM-5分子筛上的活化能较Al/HZSM-5的高16.44kJ·mol-1.因此乙烯二聚按照协同机理有利.研究还表明,表面丁基烷氧中间体脱质子,生成1-丁烯并吸附在复原的分子筛酸性位上.该反应在两种酸中心上的活化能几乎相同,但明显高于其他各步的活化能,因此成为整个反应的速度控制步骤.
Bibliography:11-1892/06
Ga/HZSM-5; Ethylene dimerization; Reaction mechanism; Density functional theory
We studied the reaction mechanisms of ethylene dimerization to 1-butene on Ga/HZSM-5 and Al/HZSM-5 zeolite catalysts by theoretical calculations and investigated the influence of zeolite acidity on the reaction energetics. The calculations were performed using the hybrid ONIOM2 （B3LYP/6-31G（d, p）：UFF） method based on the two-layered 76T cluster model. Ethylene dimerization may proceed along two different pathways： either a stepwise or a concerted mechanism, and both produce a surface butoxide intermediate. Our results indicated that with respect to the reactions on Al/HZSM-5, the adsorption energy of ethylene on Ga/HZSM-5 was 20.62 kJ·mol-1 lower, and the activation energy for the protonation process was only 1.26 kJ·mol-1 higher. Additionally, the activation energy for a combination of ethoxide intermediate with ethylene was 62.55 kJ·mol-1 higher because of the larger atomic radius of Ga, which led to an unstable six-mem
ISSN:1000-6818