Ga-ZSM-5催化MTO生成初始烯烃反应机理研究

DFT Investigation on the Formation of Initial Olefins Catalyzed by Ga-ZSM-5

采用密度泛函理论广义梯度近似方法(DFT-GGA)研究了Ga-ZSM-5分子筛上MTO反应过程中第一个C—C键及初始烯烃生成的反应机理。选用周期性模型,系统地研究了生成初始烯烃的整个反应体系,确定了各个反应物、过渡态及产物的最稳定构型,同时也确定了各个反应所涉及的反应能垒和反应热。结果表明:二甲醚生成反应可按照分步和协同机理进行。在生成初始C—C键过程中,理论上证实了一个关键性的反应中间体,即甲氧基甲基阳离子,1,2-二甲氧基乙烷是第一个包含C—C键的反应中间体,生成的初始烯烃为乙烯。

DFT-GGA w as ap p lied to stu d y the m ech an ism s of the fo rm atio n of the first C-C bon d and the initial o lefin s over Ga-Z SM-5 in the MTO p ro c ess.We determ in ed the sta b le co n fig u ratio n s of r e a c ta n ts,in te rm e d ia te s,and p ro d u cts a s w ell a s the co rresp o n d in g activ atio n b a rrie rs and reaction en ergy of the n u m erou s reactio n s involved in su ch a p ro c ess.H e r e,tw o d ifferen t kin d s of m ech an ism s w ere p ro p o se d fo r the fo rm atio n of dim eth yl e th e r,one in volvin g an a sso c ia tive in teraction and an oth er occurrin g via a d isso ciativ e path w ay.A critical in term ed iate of the m eth o xy m eth y l cation w as th eo retically verified.F u rth e rm o re,1,2-dim eth oxy eth an e is the first in term ed iates con tain in g a C-C bon d and ethene is the first alken e p ro d u ct in the MTO p ro cess.