HZSM5分子筛上乙烯二反应机理的理论研究.docx

H-ZSM一5分子筛上乙烯二聚反应机理的理论研究Abstract H-ZSM一5分子筛上乙烯二聚反应机理的理论研究 Abstract In the present work,the dimerization of ethylene on the Br6nsted acid site in H-ZSM一5 zeolite has been investgated by using theoretical calculations．We mainly focus on the following topics：(1)the mechanisms for C—C bond formation and chain propagation in ethylene oligomerization；(2)the effect of zeolite acid sites；(3)the effect of zeolite framework and pore structure Off reaction selectivity．A 40T cluster model was adopted to mimic the Br6nsted acid site located at the channel intersection of ZSM-5 zeolite．All calculations were carded out by the ONIOM method，one of the widespread QM／MM hybrid methods．Two different reaction mechanisms，stepwise and concerted pathways were examined．The natural bond orbital analysis Was used to explore the electronic properties for兀一hydrogen bonding interaction in adsorption complexes and the molecular orbital features at the transition states． The results indicated that in the stepwise mechanism，ethylene firstly adsorbs on the acid site through 7c—hydrogen bonding and then proceeds protonation to form the ethoxide intermediate；then the intermediate reacts、丽廿l the second ethylene molecule to give the surface butoxide product．In which the ethylene protonation shows the highest energy barrier and is confirmed as the rate-limiting step．In the concerted mechanism，two ethylene molecules CO—adsorb on the acid site．The reaction proceeds via simultaneous protonation and formation of C—C bond to give the butoxide product． The corresponding activation energy is slightly higher than that of the rate—limiting step in the stepwise mechanism．It is proposed that there should be a competition between two reaction mechanisms．The natural bond orbital analysis revealed that the ethoxide intermediate reactes、^，ith another ethylene by a transition state of carbonium． The confinement effect of zeolite micropores plays important role in the physisorption of ethylene molelcule，which also facili