Optimization of CO2/CO Ratio and Temperature for Dimethyl Ether Synthesis from Syngas over a New Bifunctional Catalyst Pair Containing Heteropolyacid Impregnated Mesoporous Alumina

Dimethyl ether (DME) is a promising nonpetroleum diesel fuel alternate. Synthesis of DME was achieved directly from synthesis gas having different compositions, over bifunctional hybrid catalysts. Silicotungstic acid impregnated mesoporous alumina (STA@MA) was shown to be an excellent catalyst to be used as the dehydration component of the hybrid catalyst combination. Results proved that feed composition, as well as reaction temperature had important influence on product distributions, as well as on DME yield. Results obtained with different CO/CO2 ratios in the feed stream proved the positive effect of CO2 on both overall conversion of CO + CO2 and DME yield. Overall fractional conversion of CO + CO2 and DME yield values were shown to increase with an increase in CO2/CO ratio and also with an increase in reaction temperature, reaching to the values of 0.70 and 0.55, respectively, at 275 °C, with the feed stream composition of H2/CO/CO2 = 50/10/40. However, from the DME selectivity point of view, a DME selectivity value of about 0.9 was obtained with a feed stream composition of H2/CO/CO2 = 50/40/10, at 275 °C. Further increase of CO2/CO ratio in the feed stream caused some decrease in DME selectivity, due to increased contribution of reverse water gas shift reaction, which caused formation of higher amounts of water. Increase of water concentration in the product stream has a negative effect on the dehydration reaction of methanol.