The effects of synthesis operation conditions on the properties of modified γ-alumina nanocatalysts in methanol dehydration to dimethyl ether using factorial experimental design

The catalytic performance of a series of gamma-Al2O3 nanocatalysts prepared by precipitation method and modified with various amounts of silica (0-15 wt.%) were studied in dehydration of methanol to dimethyl ether. The nanocatalysts were characterized by TGA, ICP, BET, XRD, FE-SEMandNH(3)-TPD techniques. The screening and performance tests were carried out in an isothermal fixed-bed reactor at 300 degrees C, 16 atm and LHSV of 15.6 mL/(h g(cat)). It was found that the gamma-Al2O3 nanocatalyst containing 2 wt.% of SiO2 exhibits the highest activity among the modified ones. The number of acidic sites of the modified nanocatalysts was increased by the silica modification up to 2 wt.% of SiO2 and afterward decreased. The effects of five preparation parameters including precipitation/aging temperature (50,80 degrees C), aging time (2,6 h), pH of precipitation step (6.5, 7.5), mixing rate (300,600 rpm) and the type of precipitant (NaOH, NH4HCO3) on surface area, particle size, number of acidic sites and catalytic performance were investigated using 2((5 1)) fractional factorial experimental design method. The main effects of pH, precipitant agent and mixing rate were concluded to be the key factors influencing the catalytic activity and physico-chemical properties of nanocatalysts. The optimum conditions for preparation of 2 wt.% SiO2-modified gamma-Al2O3 nanocatalyst with maximum yield of dimethyl ether were obtained as T-(pre./aging) = 50 degrees C, t((aging)) = 2 h, pH((pre.)) = 7.5, mixing rate = 600 rpm and precipitant agent = NH4HCO3. The sample prepared under optimized conditions showed a favorable surface area of 251.5 m(2) g(-1), pure gamma-alumina crystallite phase and a methanol conversion of 78.2%. (C) 2014 Elsevier Ltd. All rights reserved.