Bifunctional Silicotungstic Acid and Tungstophosphoric Acid Impregnated Cu–Zn–Al & Cu–Zn–Zr Catalysts for Dimethyl Ether Synthesis from Syngas

Abstract

In the present study, a set of methanol synthesis catalysts with different Cu/Zn/Al or Cu/Zn/Zr molar ratios were synthesized by a co-precipitation method. Silicotungstic acid and tungstophosphoric acid were impregnated into these materials to synthesize new bifunctional catalytic materials to be used in direct synthesis of dimethyl ether (DME) from syngas. In the case of methanol production, synthesized Cu/Zn/Al catalysts exhibited quite high methanol selectivity, reaching to approximately 87%. These results indicated that the dispersion of copper particles profoundly affected the selectivity of methanol. Direct synthesis of DME was investigated in the presence of heteropoly acid impregnated copper-based novel hybrid type bifunctional catalysts. Results revealed that product distributions were strongly influenced by the reaction temperature, pressure, heteropoly acid content, and type. Results proved that silicotungstic acid (STA) impregnated bifunctional catalysts showed much better catalytic performance than the tungstophosphoric acid (TPA) impregnated ones in DME synthesis from syngas. Very high dimethyl ether selectivity values were achieved in direct synthesis of dimethyl ether over the 25%, and 30% STA impregnated methanol synthesis catalysts, at 275 °C and 50 bar. DME selectivity presented an increasing trend due to the increase in heteropoly acid content over methanol synthesis catalysts. The silicotungstic acid incorporated (30%) Cu/Zn/Al catalyst, having a composition of 6/3/1 (30STA@CZA:631), showed the highest CO conversion and DME selectivity. However, coke formation over this catalyst was much more than the catalyst containing 25% STA. This is mainly due to the higher Bronsted acidity of 30STA@CZA:631 than 25STA@CZA:631. Low coke formation, together with quite high DME selectivity values achieved with 25STA@CZA:631, is a worthy distinction of this catalyst for the direct synthesis of DME from synthesis gas. Novel hybrid type catalysts are successfully synthesized for direct synthesis of dimethyl ether process. Heteropoly acid incorporated methanol synthesis catalysts are very active and stable in the direct synthesis of dimethyl ether process. Silicotungstic acid (STA) impregnated copper-based catalyst exhibits a superior performance than the tungstophosphoric acid impregnated catalyst.