Higher-alcohol synthesis reaction study V. Effect of excess ZnO on catalyst performance Original Research Article

A nonpromoted, K-promoted, and Cs-promoted 1 : 1 Zn/Cr spinel as well as K- and Cs-promoted Zn/Cr spinel catalysts containing excess ZnO were tested for higher alcohol synthesis (HAS) using a syngas feedstream (1 : 1 CO : H2). Two reactor operating pressures, 1000 and 1500 psig, and two reactor bed temperatures, 400 and 440°C, were used in order to determine the influence of these operating parameters, and the effects of promotor concentrations and excess ZnO on the product stream composition. Of the catalysts tested, the 3 wt% Cs/Zn/Cr catalyst with excess ZnO yields the highest isobutanol production rate of 171 g/kg h at 440°C and 1500 psig. Although lower methanol-to-isobutanol mole ratios were attained, this catalyst yields a ratio of 1.4 which is nearly ideal for downstream synthesis of methyl tertiary-butyl ether (MTBE). Superior performance is achieved using Cs as the promotor on the Zn/Cr spinels in comparison to the K promotor. Lower hydrocarbon production rates and corresponding increased selectivities to total alcohols, however, are obtained using K as the promotor of the 1 : 1 Zn/Cr spinel catalysts while Cs is more effective in reducing the hydrocarbon production in the presence of excess ZnO. The addition of excess ZnO to the Cs-promoted catalyst enhances not only the isobutanol rate but also results in increased selectivity, lower hydrocarbon production rates and lower methanol-to-isobutanol mole ratios at the higher Cs loadings. Higher isobutanol production rates are also obtained over the K-promoted Zn/Cr spinels containing excess ZnO in comparison to the K-supported 1 : 1 Zn/Cr spinels if 3 wt% or greater K addition is employed, but higher hydrocarbon production rates result in lower selectivities to the desired alcohol products.