Effect of Pore Structure and Acidic Property of MCM-22 Zeolite on Product Distribution of Benzene Alkylation with Propylene
LI Yingxia 1, CHEN Biaohua 1*, MENG Weijuan 1, LI Chengyue 1, WANG Wenxing 1, CAO Gang 2(1 The Key Laboratory of Controllable Chemical Reactions of Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China; 2 Beijing Yanshan Petrochemical Corporation, Beijing 102500, China)
MCM-22, having MWW topology, is a zeolite which has unique and unusual pore structure and acidic property on its external surface. In this paper, the total acid sites of MCM-22 ( n(Si)/ n(Al)=15) were measured by means of TG and FT-IR. The B and L acid sites were 375 and 450 μmol/g respectively, and the external surface acid sites were 45 μmol/g. The ratio of initial reaction rate of benzene alkylation over β zeolite (n(Si)/n(Al)=15) to that over MCM-22 was better consistent with the ratio of B acid sites on β zeolite to that on the MCM-22 external surface. It was showed that the benzene alkylation took place on the external surface of MCM-22 and in the 12 MR pore channels of β zeolite, respectively. The distribution of products, including isopropyl benzene(IPB), diisopropyl benzene(DIPB), tri-isopropyl benzene(TIPB), oligomers(Olig) and n-propyl benzene(NPB), was studied under the conditions of n(C 6H 6)/ n(C 3H 6)=2～10, WHSV(C 3H 6)=2.5～30 h -1, p=3.0 MPa and θ=140～210 ℃. On the basis of the diffusion coefficients of the reactants and the energy barriers of the products in MCM-22 and β zeolites, it was found that: (1) owing to that the benzene alkylation took place on the MCM-22 external surface, the selectivity for IPB was lower by 2%～4%, and the amount of DIPB and TIPB was higher over MCM-22 than over β zeolite; (2) the B acid sites located in the 10 MR sinusoidal channels of MCM-22 may not take part in the benzene alkylation owing to the steric effect, but it may take part in oligomerization of propylene, so the oligomers formed over MCM-22 were higher than those over β zeolite; (3) the amount of NPB formed over β zeolite is 3～4 times as that over MCM-22.