Nicola Young

Ni- and Zn-promotion of γ-Al2O3 for the hydrolysis of COS under mild conditions

Abstract The hydrolysis of COS using γ-Al2O3 as catalyst at 30°C is described. The effect of additives (ca. 3% Na, Fe, Co, Ni, Cu, Zn) is described in detail. The additives reduce the surface area of the γ-Al2O3 markedly but have no effect on the powder X-ray diffraction pattern. All significantly enhance the initial intrinsic activity (mol COS hydrolysed/m2/h) when compared with unpromoted γ-Al2O3. However, this initial performance rapidly decays for the Na-, Fe-, Co- and Cu-modified catalysts. During this initial reaction period, of up to 5 h, the catalyst retains sulphur. In terms of specific activity (mol COS hydrolysed/g/h) only Ni- and Zn-promoted catalysts give a significant increase that is stable throughout the time-scale of these experiments.

Low temperature hydrolysis of carbonyl sulfide using γ-alumina catalysts

The hydrolysis of COS using alumina as catalyst in the temperature range 10–80°C is described in detail. The rate of COS hydrolysis is found to be approximately first order in [COS] but is significantly inhibited with increasing [H2O]. Addition of CO2 is also found to have an inhibiting effect on the rate of hydrolysis, but no marked effect is observed when additional H2S is present in the feedstock. The reaction in this temperature range is contrasted with the previous studies at higher temperatures and it is suggested that the reaction proceeds via reaction of adsorbed COS with surface hydroxyl groups on the alumina. Supporting evidence for this is provided from experiments in which water was not co-fed with COS, or the flow of water vapour was interrupted. In both cases, an initial increase in the rate of COS hydrolysis is observed. The rate of COS hydrolysis can be significantly enhanced by the addition of Fe, Co, Ni, Cu and Zn to the alumina.

Ambient Temperature Hydrolysis of Carbonyl Sulfide Using γ-Alumina Catalysts: Effect of Calcination Temperature and Alkali Doping

The hydrolysis of COS using γ-alumina as catalyst at 20 °C is described and discussed. In particular, the effect of calcination on the catalyst activity is investigated. Catalysts calcined at 100 and 500 °C are found to give the highest catalyst activities, in terms of both specific (mol COS converted/g catalyst/h) and intrinsic (mol COS converted/m2 catalyst/h) activity. Calcination at other temperatures leads to diminished catalyst activity. The effects are discussed in terms of the known surface chemistry of γ-alumina involving physisorbed water, surface dehydroxylation and defect formation. The addition of alkali additives (Li+, Na+, K+, Cs+, Mg2+, Ca2+, Ba2+, Si2+) is also reported. Only K+ and Cs+ give a sustained enhancement in catalyst activity, whereas all the other additives act as catalyst poisons for the steady-state performance measured following 5 h time-on-stream. Interestingly, addition of Na+ and Mg2+ leads to a very high initial activity (>95% COS conversion) but the effect is very short-lived and, after 5 h time-on-stream, a much lower steady-state activity (∼15-30% COS conversion) is observed.

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

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.