Purification of chemical feedstocks by the removal of aerial carbonyl sulfide by hydrolysis using rare earth promoted alumina catalysts

Abstract

The effect of rare earth doping of alumina catalysts is investigated for the carbonyl sulfide (COS) hydrolysis reaction (COS + H₂O = CO₂ + H₂S). The effect of the catalyst preparation method is described and discussed, and three methods are compared, namely: impregnation by incipient wetness, coprecipitation and deposition precipitation. The most effective catalysts are prepared using the incipient wetness impregnation method. The addition of rare earth oxides, namely Y₂O₃, Gd₂O₃, Nd₂O₃, La₂O₃, increases the basicity of the material as shown by pulsed CO₂ chemisorption and the basicity increases with the amount of rare earth oxide added. CO₂ TPD shows that the La₂O₃-doped alumina has the strongest basic sites. The promoted catalysts are all effective for the COS hydrolysis reaction and the best results are obtained with Y₂O₃-doped materials, as these have the most pronounced promotion of activity over the reaction timescale we have examined. The combination of the results for COS conversion with the H₂S selectivity data and the effects of H₂S pre-treatment shows that a highly active catalyst also has a high H₂S selectivity. The La₂O₃-doped materials deactivate rapidly and have poor H₂S selectivities, and we propose that the higher basicity of this material leads to reaction with the acidic COS and H₂S leading to the formation of the less basic lanthanum sulfide. This study has presented results for the first time showing that an alumina catalyst for COS hydrolysis can be promoted by the addition of rare earth oxides, and this is related to the enhanced basicity of the promoted catalyst.