Ga/HZSM-5分子筛上乙烯二聚反应的理论研究

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

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.

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 [Bronsted] 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.