Search

Abstract: Selective synthesis of p-xylene by vapor phase alkylation of toluene with dimethyl carbonate, over MgO-modified MCM-22, was carried out to demonstrate the relationship between acidity and catalytic performance. Characterization results show that the number of Brønsted acid sites on MCM-22 decreased significantly after MgO modification, which resulted in a decrease in activity. Selectivity for p-xylene was improved greatly over MgO-modified MCM-22, which can be attributed to effective suppression of xylene isomerization by the reduction in external surface Brønsted acid sites. The additional Lewis acid sites generated after MgO modification had no apparent effect on the catalytic performance. [Copyright &y& Elsevier]

Abstract: Acidity adjustment of MCM-22 was carried out by hydrothermal treatment of parent MCM-22 with flowing pure steam at high temperature. XRD, SEM and N2 adsorption/desorption results indicated that the crystallinity and pore properties of MCM-22 were affected slightly by hydrothermal treatment at different temperatures. NH3-TPD and FT-IR with pyridine adsorption showed only a slight decrease in Brønsted acid sites, compared with parent MCM-22, with hydrothermal treatment temperature in the range 773–873K. The strength of Brønsted acid sites was reduced severely at hydrothermal treatment temperatures of 973 and 1073K. The Lewis acid sites were only slightly affected by hydrothermal treatment. A substantial improvement in ethylbenzene selectivity along with a slight decrease in benzene conversion was achieved in catalytic synthesis of ethylbenzene by alkylation of benzene with diethyl carbonate over MCM-22 hydrothermally treated at 873K. A sharp decrease in benzene conversion was detected over MCM-22 hydrothermally treated at 973K. The results indicate that alkylation of benzene with diethyl carbonate occurred mainly on Brønsted acid sites, and the effect of Lewis acid sites on the alkylation process is negligible. Reduction of Brønsted acid sites of the catalyst can suppress side reactions and improve selectivity for ethylbenzene. It was found that a particular acid strength of catalyst was required to maintain high benzene conversion and ethylbenzene selectivity. [Copyright &y& Elsevier]

Abstract: The catalytic synthesis of ethylbenzene by alkylation of benzene with diethyl carbonate was carried out over HZSM-5 with Si/Al ratios from 50 to 250. The catalysts were characterized by XRD, SEM, 29Si and 27Al MAS-NMR, NH3-TPD and pyridine IR techniques. HZSM-5 with a Si/Al ratio of 200 exhibited the best benzene conversion and ethylbenzene selectivity because of its ideal acid strength. Strongly acidic sites may lead to an excessive decomposition of diethyl carbonate. The effects of temperature, feed ratio, WHSV and time-on-stream on catalytic activity and product selectivity were studied, and a possible mechanism is presented. [Copyright &y& Elsevier]

A series of B 2 O 3 /HZSM-5 catalysts were prepared by impregnation of HZSM-5 zeolites with triethyl borate, trimethyl borate and boric acid. The selective synthesis of para -diethylbenzene by alkylation of ethylbenzene with diethyl carbonate was carried out over the B 2 O 3 /HZSM-5 catalysts. The physicochemical properties of the catalysts were characterized by X-ray diffraction, N 2 adsorption–desorption, Fourier-transform infrared spectroscopy with pyridine adsorption and NH 3 temperature programmed desorption. The characterization results indicated that the 15% B 2 O 3 /HZSM-5 catalyst prepared by using triethyl borate as the precursor exhibited an outstanding shape-selectivity along with a high catalytic activity in alkylation of ethylbenzene with diethyl carbonate. This might be ascribed to the large molecular size of triethyl borate, which would lead to the formation of B 2 O 3 on the external surface of HZSM-5 zeolite and preserve the acid sites in the micropores of HZSM-5 zeolite. By contrast, the B 2 O 3 /HZSM-5 catalysts prepared by using trimethyl borate or boric acid led to the severe reduction in catalytic activity, which was attributed to the decrease in the amount of the total acid sites caused by the blockage of the partial pores of HZSM-5 zeolite. [ABSTRACT FROM AUTHOR]

Abstract: A series of catalysts were prepared by impregnation of MCM-41 mesoporous silica with TiO2. Alkylation of phenol with diethyl carbonate was carried out over the TiO2-modified MCM-41 catalysts for the first time, and the reaction had high ortho-selectivity and a high phenol conversion. The physicochemical properties of the catalysts were characterized by X-ray diffraction, X-ray fluorescence, N2 adsorption–desorption, Fourier-transform infrared spectroscopy with pyridine adsorption, and CO2 and NH3 temperature programmed desorption. Long-range regularity of the hexagonal arrays of MCM-41 was sustained after TiO2 modification. As the TiO2 mass fraction in the catalyst increased, the number of acidic and basic sites also increased, which suggests that TiO2 modification affects the acid–base properties of MCM-41. The high ortho-selectivity and the high phenol conversion for alkylation of phenol with diethyl carbonate over TiO2-modified MCM-41 could be ascribed to catalytic interaction between adjacent acidic and basic sites on the catalysts. The effects of temperature, feed ratio, reactant contact time and time-on-stream on the conversion of phenol and the selectivity for various products over TiO2-modified MCM-41 catalysts were also investigated. [Copyright &y& Elsevier]