A Quantum Study On The Mechanism Of Skeletal Isomerization Of1-Butene Over FER,MTT And MRE Zeolties

Based on the density functional theory (DFT), this paper investigated the distribution and acid strength of Bransted acid (B-acid) in FER, MTT and MRE zeolites by ONIOM method. According to the results of the acidity calculations, acid sites of which both stability and acid strength were fairly good were chosen as the active centers in the investigation on the mechanisms of skeletal isomerization and double-bond migration reactions of1-butene. Through the calculations of the activation energies of these reactions, the selectivity of isobutene and catalytic performance of the three zeolites were compared.The results of Br(?)nsted acidity of FER, MTT and MRE zeolites show that: the most stable site for B-acid in FER is A14-O6-Si2on the6-membered ring, and the acid strength of A13-O7-Si4is the strongest; A14-O6-Si4and A12-O1-Sil are the most prone to form B-acids, and A14-O3-Si3is the strongest in acid strength; the most stable site for B-acid in MRE is A12-O7-Si3, and the acid strength of A12-O6-Si2is the strongest. Besides, the distributions of two or more Al atoms in the three zeolites were also studied. The results also indicate that the acid strength of B-acid in zeolite with higher Si/Al molar ratio is stronger. What’s more, four clusters were utilized to descript the pore structure of FER to different extents. And the results show that the completeness of the pore structure have significant effects on the distribution and acid strength of B-acids.On the basis of the investigation on the acidity of FER, MTT and MRE zeolties combined with the possible steric hindrance from the pores, six B-acid sites which were relatively stable and strong in acid strength were involved in the study of isomerization of1-butene as the adsorption and active centers. The adsorption structures of1-butenen, isobutenen and trans-2-butenen on zeolties indicate that the shape and size of pores of the three zeolites have great effects on the adsorption performance. Usually, the adsorption enery of molecules on zeolites with large pore is relatively low. As we calculated, the skeletal isomerization can proceed via four unstable carbenium ion transition states and three intermediates on all of the three zeolites. The first and second steps are reversible but the last step is irreversible. However, the rate-determine steps on the three zeolites are the third step on the zeolites except FER, on which the second step is the rate-determine. The apparent activation enegy on FER zeolite is the highest, and on MTT it is the lowest of all. What’s more, we also studied the mechanism of a side reaction which started from1-butene and ended with trans-2-butene. This reaction is reversible in the whole process. So trans-2-butene can also changed to isobutene through a similar molecular pathway like1-butene. Both the main and side reactions are exothermic.