The Theoretical Research On Reaction Mechanism Of Directional Conversion Of Methanol To Styrene

In this thesis,the reaction mechanism of directional conversion of methanol to styrene over alkali-exchanged Y zeolite was investigated by theoretical calculation, whose pivotal link is a side-chain alkylation of toluene with formaldehyde. Many experimental results show that the major ity of the alkaline earth metal cations are located at site SII. In this paper, we used an isolated 6-membered-ring(6MR) cluster to describe the zeolite framework. The main conclusions are summarized as follows:1. The adsorption energies of formaldehyde adsorbed on a 6MR cluster are larger than those of toluene obvious ly. The former numerical value range from 11.71 to 15.83 kcal mol-1, while the latter ’s is from 8.38 to 12.62 kcal mol-1.2. We constructed two different co-adsorption types, 6mr-M-T-F and 6mr-M-F-T, to further understand the interaction between formaldehyde and toluene and the effect of the two components on the entire co-adsorption system. 6mr-M-T-F denotes that the zeolite will first adsorb toluene on a cluster(6mr-M-T) then continue to adsorb formaldehyde(F); 6mr-M-F-T denotes that the zeolite will adsorb formaldehyde on a cluster first(6mr-M-F) then continue to adsorb toluene(T). The comparative results show ed that zeolite preferentially adsorbed formaldehyde, the ability of continuing to adsorb toluene would become weak; if zeolite preferentially adsorbed toluene, the system still kept a good adsorption performance for formaldehyde. Bes ides, adsorbing formaldehyde preferentially led the methyl carbon atom of toluene to gather more negative charge, enhancing the activity of the methyl carbon atoms, which is conducive to a better side-chain alkylation reaction.3. The reaction mechanism showed that side-chain alkylation of toluene to prepare styrene contained two reactions: toluene and formaldehyde generat ing phenylethanol; phenylethanol’s intramolecular dehydration giving styrene and water. So there were two extremely high energy barriers, and phenylethanol dehydration had a higher energy barrier; thus, it was the rate-controlling step in this whole reaction mechanism. There were three transition states and two intermediate products in this reaction process. The activation energy of the rate-controlling step is 63.09 kcal mol–1 in Rb+-modif ied zeolite, which is higher than 62.20 kcal mol–1 in Cs+-modified zeolite. The desorption energy of the product is 82.35 kcal mol–1 in Rb+-modified zeolite, which is also higher than 80.75 kcal mol–1 in Cs+-modified zeolite. In summary, Cs+-modified zeolite is a suitable catalyst during the entire side-chain alkylation process of toluene.