Synthesis, characterization and catalytic application of MCM-41 mesoporous molecular sieves containing Zn and Al

摘要

Mesoporous silica molecular sieves MCM-41 containing zinc and aluminum ion (Zn-Al-MCM-41) with Si/(Zn + Al) ratio equal to 340 and 380 and only aluminum ions with Si/Al ratio equal to 93 and 104, respectively, were synthesized under hydrothermal conditions using cetyltrimethylammonium (CTMA~+) surfactants as tempalte in the absence of auxiliary organics. The mesoporous materials, viz. Zn-Al-MCM-41 and Al-MCM-41 were characterized using several techniques, e.g. powder X-ray diffraction (XRD), nitrogen adsorption measurements, Fourier transform-infra red spectroscopy (FT-IR) and thermogravimetric-differential thermal analyzer (TG-DTA). XRD studies indicated that the calcined materials had the standard MCM-41 structure. Nitrogen adsorption was used to determine specific surface area, pore volume, and pore size distribution in the Zn-Al-MCM-41 (340), Zn-Al-MCM-41(380), Al-MCM-41(93) and Al-MCM-41(104) mesoporous molecular sieves. FT-IR studies showed that Zn and Al ions were incorporated into the hexagonal mesoporous structures of Zn-Al-MCM-41 and Al-MCM-41. The thermal stability of the as-synthesized materials was studied using TG-DTA. The effect of reaction temperature, WHSV and isopropanol: toluene ratios on the selectivity of p-cymene were studied. The Zn-Al-MCM-41(340) catalyst containing larger pore size and higher number of Bronsted acid sites was found to be more effective in the alkylation of alkyl aromatics, for example, in the production of p-cymene from toluene, using 2-propanol as the alkylating reagent. The selectivity of p-cymene under varying experimental condition, via. effect of reaction temperature, WHSV and ratio of isopropanol: toluene for the various catalysts follows the sequence Zn-Al-MCM-41 (340) > Al-MCM-41(93) > Zn-Al-MCM-41(380) > Al-MCM-41(104).