Study Of Hydrolysis Carbonyl Sulfide Over Mixed Oxides From Hydrotalcite-like Compounds

The off-gas of yellow phosphorus containing a large portion of CO, produced in the process of yellow phosphorus, is a high quality material gas for C1 (one carbon) chemical industry. COS is a harmful gas which can pollute the environment and affect the catalysts in C1 chemical industry. If yellow phosphorus tail gas can be used as raw material gas to synthesize chemical product, it's necessary to remove COS. According to the characters of yellow phosphrus tail gas, the COS and product of H2S can be removed simultaneously by catalytic hydrolysis.The hydrotalcite-like compounds (HTLCs) and their derivatives, mixed oxides by calcining HTLCs, are one of important potential materials in the catalysis and adsorption fields. In the thesis, Ni-Mg-Al, Ni-Mg-Fe, Co-Ni-Al, Co-Mg-Al and Ni-Mg-Al HTLCs were prepared by coprecipitation method, from the corresponding nitrate solutions and mixed precipitants (NaOH and Na2CO3). The mixed oxides were obtained by calcining the corresponding HTLCs. The effects of metal combination, pH value, n(MⅡ)/n(MⅢ), ratio(mol) of divalent metal, calcination temperature, temperature of hydrothermal treatment on catalytic hydrolysis of COS were investigated by means of Orthogonal Design Method. The catalysts prepared under following condition exhibited superior catalytic activity:metal combination of Co-Ni-Al, pH value of 9, M(Ⅱ)/M(Ⅲ) at 3, ratio of divalent metal at 1, calcination temperature at 350℃, and temperature of hydrothermal treatment at 50℃.Based on the results of orthogonal design method, the mixed oxides catalysts derived from CoNiAl hydrotalcite-like compounds which were prepared by coprecipitation method were used for catalytic hydrolysis of COS. The effects of n(Co)/n(Ni), calcination temperature, temperature of hydrothermal treatment, n(MⅡ)/n(MⅢ), pH value, synthetic temperature, M(Ⅲ) and addition of rare earth on catalytic hydrolysis of COS were investigated by means of Single-Factor-Experiment Method. The catalysts were characterized by BET,FT-IR,SEM,XRD and XPS and the possible causes from the various factors on the differences of catalytic activity were proposed. At the same time, the method of regeneration was systemetally studied. Besides, according to the characters of yellow phosphorous tail gas, the influence factors of COS hydrolysis kinetics were analyzed; including the reaction temperature, the gas hourly space velocity(GHSV), relative humidity, the inlet concentration of COS, the content of O2 and the atmosphere of CO. The fresh and exhausted catalysts were characterized by various characterizations and the reaction mechanism of COS hydrolysis was also proposed.The activity test results in the process of screening showed that the catalysts could be prepared by precipitating solution of (Co+Ni)/Al with n(Co)/n(Ni) of 0.25～0.5 and n(MⅡ)/n(MⅢ) of 2 at pH=9.0, synthetic temperature at 25℃, and hydrothermal treatment at 50℃. The products were calcined at 350℃finally to get the mixed oxides catalyst. The catalysts prepared following above conditions exhibited super activity and there was NiO and part of Co3O4 in the catalysts. The catalytic activity decreased when the Al3+ was substituted by Fe3+ and there were mainly NiO and CoO in the CoNiFe catalyst. Therefore, the trivalent metal ions affected the form of divalent metal oxides and therefore affected the catalytic activity. When the n(Al):n(Ce/La) was 50, the catalysts modified by rare earth gave higher capacity of sulfur and activity. The addition of rare earth decreased the surface areas indicating that the effects of surface area and pore volume on the hydrolysis of COS activity were little, while the pore size distribution at the range of 70～80A played a role in the catalytic activity; The rare earth affected the formation and aggregation of sulfate species and element sulfur, which resulted the deactivation of catalysts.The regeneration experiments showed that the deactivated catalysts could be regenerated by immersing alkali solution and thermal decomposition treatments. After the regeneration process of immersing Na2CO3 solution and under air flow at 350℃, the activity could be restored effectively. The effect of regeneration cycles was little on the activity and after three cycles the catalytic capacity decreased slightly compared with fresh catalyst. The alkali solution didn't only play a role in the reconstruction of structure but also provided basic sites. The element sulfur and sulfate on the surface contributed to catalysts poisoning.The kinetics of hydrolysis COS was studied and the results showed that the catalyst had good stability under the condition of the temperature 60～70℃, gas hourly space velocity of 1000～3000h-1, relative humidity of 0.64～4.74%, COS concentration of 1500～2100mg/m3. The increase of O2 promoted the oxidation of H2S and accelerated catalyst poisoning. Besides, the catalysts prepared in the above conditions are not affected by the atmosphere of CO. The reaction kinetics of catalytic hydrolysis of COS over mixed oxides derived from CoNiAl HTLCs was fitted by MATLABR2009a surface fitting tool kit version and expressed as follows:The reaction mechanism of COS hydrolysis over mixed oxides derived from CoNiAl HTLCs was proposed based on the characterization results, some literatures and calculation. There were two steps on the surface of catalyst in the reaction process: hydrolysis of COS process and oxidation of H2S from the hydrolysis. The rate of COS adsoption was the control step of COS hydrolysis. The adsorptive COS was continuously hydrolysised to H2S on the active center with the reaction and the H2S was oxidized to elemental sulfur and sulfate on the oxidation center finally.