物理化学学报 >> 2011, Vol. 27 >> Issue (06): 1393-1399.doi: 10.3866/PKU.WHXB20110631

理论与计算化学 上一篇    下一篇

Ga/HZSM-5分子筛上乙烯二聚反应的理论研究

李惊鸿, 周丹红, 任珏   

  1. 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
  • 收稿日期:2011-01-14 修回日期:2011-03-19 发布日期:2011-05-31
  • 通讯作者: 周丹红 E-mail:dhzhou_lnnu@163.com
  • 基金资助:

    国家自然科学基金(20073058)资助项目

Theoretical Study of Ethylene Dimerization on the Ga/HZSM-5 Zeolite

LI Jing-Hong, ZHOU Dan-Hong, REN Jue   

  1. Institute of Chemistry for Functionalized Materials, College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, Liaoning Province, P. R. China
  • Received:2011-01-14 Revised:2011-03-19 Published:2011-05-31
  • Contact: ZHOU Dan-Hong E-mail:dhzhou_lnnu@163.com
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (20073058).

摘要:

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 20.62 kJ·mol-1, 但质子化反应的活化能只高出1.26 kJ·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 62.55 kJ·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16.44 kJ·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.

关键词: Ga/HZSM-5, 乙烯二聚, 反应机理, 密度泛函理论

Abstract:

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-member ring transition state. For the concerted mechanism, protonation and C―C bond formation proceeded in one step and the activation energy on Ga/HZSM-5 was 16.44 kJ·mol-1 higher than that on Al/HZSM-5. Therefore, the ethylene dimerization reaction proceeded according to the concerted mechanism. The surface butoxide intermediate was transformed to 1-butene by deprotonation and adsorbed on the recovered Brönsted acid sites. The corresponding activation energy on Ga/HZSM-5 was similar to that on Al/HZSM-5 but it was obviously higher than that in the other steps. Therefore, it was the rate-determining step for this reaction.

Key words: Ga/HZSM-5, Ethylene dimerization, Reaction mechanism, Density functional theory

MSC2000: 

  • O641