Acta Phys. -Chim. Sin. ›› 2011, Vol. 27 ›› Issue (06): 1393-1399.doi: 10.3866/PKU.WHXB20110631

• THEORETICAL AND COMPUTATIONAL CHEMISTRY • Previous Articles     Next Articles

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).

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