Study On Side-chain Alkylation Of Toluene With Methanol And The Catalysts

Producing styrene from side-chain alkylation of toluene with methanol is of potential industrial application.In this work,a series of catalysts,cesium ion exchanged zeolite X(CsX),alkali metal oxides modified CsX zeolite and their composite catalysts containing B2O3/SiO2,were employed to study the side-chain alkylation of toluene with methanol or formaldehyde,and some valuable results are illustrated below:Firstly,a series of CsX zeolites with different cesium ion exchange degrees(CsED)were prepared and characterized,and their catalytic activities for side-chain alkylation of toluene with methanol were studied.Combining with the reaction behaviors of side-chain alkylation of toluene with formaldehyde and the conversion of styrene in different atmosphere,the influences of base properties of CsX on the formation of styrene and the formation path of ethylbenzene over CsX were discussed.Results showed that the cesium ion exchange degrees of CsX zeolites play an important role in the side-chain alkylation of toluene with methanol.When the CsED of CsX was lower than 32%,the crucial step of side-chain alkylation of toluene with methanol was the activation of methyl group of toluene;while the CsHD of CsX was higher than 32%,the crucial step was the methanol dehydrogenation to formaldehyde.Additionally,the results demonstrated that the formation of ethylbenzene was related to hydrogen transfer and transfer hydrogenation processes in the side-chain alkylation of toluene with methanol.The ethylbenzene formed mainly through hydrogen transfer process between styrene and carbon deposition when the CsED of CsX was lower than 22%;while transfer hydrogenation of styrene with methanol and formaldehyde(generated fro:methanol dehydrogenation)took control of the formation of ethylbenzene when CsED of CsX was higher than 46%.Secondly,a series of alkali metal oxides modified CsX were prepared by changing the kinds of alkali metal oxides,alkali metal oxides loaded amounts and the CsED of the parent CsX.The base sites provided by alkali nethal oxide itself and the oxygen with negative charge(O?-)in the zeolite framework were distinguished by CO2-TPD,FT-IR spectra of adsorbed CO2 and XPS,and the reaction behaviors of side-chain alkylation of toluene with methanol over those alkali metal oxides modified CsX were further studied.It was found that the base properties of CsX were greatly increased by alkali metal oxides modification,thus enhancing the yield of side-chain alkylation products.The base properties of the oxygen with negative charge(O?-)in the CsX framework was enhanced by electron donating effect of the loaded alkali metal oxides,which further promoted the dehydrogenation of methanol to formaldehyde,and accounted for the improved yield of side-chain alkylation products over the alkali metal oxides modified CsX.However,the base properties enhanced Ueu sites also promoted the transfer hydrogenation of styrene with methanol and formaldehyde(generated from methanol dehydrogenation),resulting in a low styrene/ethylbenzene in the products.Thirdly,boric acid supported SiO2(B2O3/SiO2)was mixed with CsX in different modes,and the resultant composites toward side-chain alkylation of toluene with methanol were investigated.CsX impregnated boric acid was also prepared for comparison.The results showed synergistic catalysis between the boron sites and CsX In the side-chain alkylation of toluene with methanol.The synergistic catalysis increased the utilization of formaldehyde(generated from methanol dehydrogenation)in side-chain alkylation as well as the molar ratio of styrene to ethylbenzene in the products.The combination of B2O3/SiO2 with CsX(CsED=45.9%)in a mass ratio of 1:5 through grinding treatment demonstrated a better catalytic performance,the molar ratio of styrene to ethylbenzene in the products was increased from 2.03 to 7.57,in comparison to that of CsX impregnated boric acid.Finally,several binary composite catalysts(CsX and B2O3/SiO2)and ternary composite catalysts(CsX,alkali metal oxides modified CsX and B2O3/SiO2)were prepared,and their catalytic activities for side-chain alkylation of toluene with methanol were studied.It was found that the ternary composite catalyst,prepared by grinding CsX(CsED=45.9%),cesium oxide modified CsX(8CsOx/CsX4)and B2O3/SiO2 in a mass ratio of 5:5:2,showed the best catalytic performance.The yield of side-chain alkylation products reached 39.45%and the molar ratio of styrene to ethylbenzene was 1.81.