Evaluation of Promoters for Rhodium-Based Catalysts for Mixed Alcohol Synthesis

Abstract

Pacific Northwest National Laboratory (PNNL) and National Renewable Energy Laboratory (NREL) are conducting research to investigate the feasibility of producing mixed alcohols from biomass-derived synthesis gas (syngas). PNNL is tasked with obtaining commercially-available catalysts or preparing promising mixed-alcohol catalysts and screening them in a laboratory-scale reactor system. Commercially-available catalysts and the most promising experimental catalysts are provided to NREL for testing using a slipstream from a pilot-scale biomass gasifier. A total of 28 tests were conducted to evaluate 22 different promoters as well as an unpromoted catalyst. The following general trends were observed for the test results: • The highest carbon selectivity to C2+ oxygenates occurred at the lowest reaction temperatures and accompanying lowest space time yields (STYs). • The lowest carbon selectivity to C2+ oxygenates occurred at the highest reaction temperatures because of high carbon conversion to hydrocarbons. • The highest C2+-oxygenate STYs occurred between 300°C and 325°C, with the gas hourly space velocity (GHSV) adjusted when necessary to maintain carbon conversion ranges between ~ 30 and 40 percent. Higher carbon selectivity to hydrocarbons at higher temperatures resulted in lower C2+-oxygenate STYs. • When catalysts were heated to between 300°C and 325°C the catalysts showed evidence of some deactivation with respect to C2+ oxygenate productivity, accompanied by reduced chain growth for the hydrocarbon products. The degree of deactivation and the temperature at which it occurred varied between the different catalysts tested. Of all of the catalysts evaluated, the Li-promoted catalysts had the highest carbon selectivity to C2+ oxygenates (47 percent) under the conditions at which the maximum C2+-oxygenate STYs were obtained.