Research On Low Temperature Catalytic Hydrolysis Of CS₂ On Nano-carbon Based Catalyst And The Reaction Mechanism

Phosphorus chemical industry is produced maximumly in China, and its yield is first in the wolrd. In the process of one ton yellow phosphorus, the off-gas of yellow phosphorus is 2500-3000m3, the off-gas is containing a large amount of CO, and the CO content is up to 85-95%. Therefore, the off-gas of yellow phosphorus could be used as chemical raw materials and secondary energy due to the CO is a high quality material gas for C1 (one carbon) chemical industry. However, the carbon disulfide (CS2) in the yellow phosphorus off-gas is a kind of harmful gas which can affect the catalysts in C1 chemical industry. Thus, the CS2 must be removed to the yellow phosphorus off-gas could be effectively used again.The characters of yellow phosphtus tail gas is taken to a consideration, the CS2 can be removed by low temperature catalytic hydrolysis method in this research. In this work, a series of nano-active carbon (Nano-AC) based catalysts were prepared by Liquid Phase Deposition method for CS2 hydrolysis. The main research contents include:the eddects of the preparation conditions included the konds and amount of metal oxides, ratio of F:metal ions, solution pH and calcination temperatures were studied; the influence of operation conditions such as: reaction temperature, relative humidity (RH), space velocity, CS2 inlet concentration and O2 content were also dicussed, respectively; and the detialed hydrolysis mechanism of CS2 was studied using the theory method. The main research contents and conclusions as follows:The results show that catalysts with 0.2 mol/L Fe2+, F:Fe2+(mole ratio)= 3:1, solution pH= 5.1 after calcining at 500℃ for 2 h have superior activity for the low temperature catalytic hydrolysis of CS2, and above 90% CS2 conversion is observed for about 540 min. What’s more, the following order of activity was obtained:Fe/Nano-AC>Nano-C-Zn/Nano-AC>Mn/Nano-AC ≈KCu/Nano-AC>Ni/Nano-AC; In addition, the CS2 removal efficiency over the catalysts initially increased and then decreased with the amount of Fe concentration, the molar ratio of F:Fe2+, solution pH and the calcination temperatures gradual increasing.At the same time, the influences of operation conditions on catalytic hydrolysis activity of Fe/Nano-AC were investigated. The results showed that:the CS2 removal efficiency over the catalysts initially increased and then decreased with the reaction temperature gradual increasing. And the catalytic hydrolysis activity over the CS2 and the sulfur capacity was highest when the reaction temperature was 50℃. The sulfur capacity of Fe/Nano-AC catalyst was also highest (21.5 mgS/g) when the RH was 11.8%, and the lower RH is beneficial to enhanced catalytic hydrolysis activity over CS2. The catalytic hydrolysis activities of Fe/Nano-AC were relatively stable in the space velocity range of 15000 h-1～20000 h-1. And the catalytic hydrolysis activities of catalyst were relatively suitable when the O2 content was 0.6～2.5%, and when the O2 content was 2.5%. the the catalytic hydrolysis activitiy of catalyst and sulfur capacity achieved maximum (28.88 mgS/g). In addition, the results also showed that when the CS2 inlet concentration gradual increased from 50 ppm to 110 ppm, the CS2 removal efficiency over the Fe/Nano-AC catalyst decreased evidently, but the sulfur capacity shaply increased.On account of above the hydrolysis mechanism of CS2 and according to the experimental results, the possible six reaction paths were reasonably designed and explored using the density functional theory at B3LYP/6-311+G* levels. It can be concluded that the hydrolysis mechanism of CS2 occurs via two mechanisms, leading to the formation of the intermediate COS; in the all process of CS2 hydrolysis,20 intermediates and 9 transition stated were formed, and the H migration plays an important role. The results also showed that the favorable reaction path was from Path B to Path 1. What’s more, the reaction kinetics of hydrolysis of CS2 was studied, and it can be found that CS2 hydrolysis is an exothermic reaction, so increasing the reaction temperature is disadvantageous for this reaction, which is in agreement with experimental observations (reaction temperature); it also can be found that the six reaction paths are parallel and competitive in the process of CS2 hydrolysis.Meanwhile, it also was studied that the CS2 is adsorped on the surface of Fe/Nano-AC catalyst. The results showed that:the adsorption of CS2 and H2O on the catalyst surface is parallel and competitive, which is in agreement with experimental observations (relative humidity), and adsorption of CS2 is more easy than the H2O. Thus, the CS2 catalytic hydrolysis maybe can be concluded:CS2 is adsorped firstly on the catalyst surface, then react with H2O, and finally formed the sulfates. The detailed reaction path following as:at the first, CS2+H2Oâ†'IM1 â†'S1â†'IM2-1â†'S2’â†'IM3’-1â†'S3’â†'IM3â†'COS+H2S; secondly, COS+H2Oâ†'IM4â†'TS4â†' IM5-1â†'IM6â†'CO2+H2S; finally, H2S, as hydrolysis products, reacted with catalyst leading to sulfates. In the process of CS2 hydrolysis, the catalyst palys two roles:on the one hand, the catalyst could be decrease the reaction barriers of CS2 hydrolysis so that CS2 hydrolysis reaction is smooth at low temperature conditions; on the other hand, reacted with hydrolysis products forming sulfates.