| Waste gases containing cyanogen are mainly derived from petroleum chemical and carbon fiber industry. The low concentration can cause serious damage to human health, even life-threatening. Catalytic combustion technology has the obvious industrial advantage compared with other treatment methods. The catalyst is the core technology of catalytic combustion, and is still facing low catalytic activity and selectivity, expensive and short life.One side, the mesoporous zeolites (SBA-15, SBA-16, KIT-6) were synthesized using the hydrothermal synthesis method and further were modifiezed using impregnation by transition metal ions (Cu, Co, Fe, V, Mn). The noble metals of (Pt, Pd, Ag) over SBA-15 or traditional supported of SiO2 and Al2O3 as the contrast were also prepared catalysts. These catalyses were applied to purify the containg cyanide exhast gases (CH3CN, C2H3CN, HCN) by selective catalytic combustion under the existing condition of oxygen. The results show that the CH3CN conversion of the investigated catalysts follows a trend of Pt/>Pd/> Cu/> Co/>Fe/> V/> Ag/> Mn/>SBA-15. Among the prepared M/SBA-15 samples, Cu/SBA-15 exhibited a nearly complete CH3CN conversion associated with a N2 selectivity around 80% at T> 350℃, significantly better than the Pt and Pd/SBA-15. The corresponding activities and selectivities were verified to be associative with both redox properties and chemical natures of the loaded metals. Comparingdifferent mesoporous material Cu/SBA-15, Cu/SBA-16 and Cu/KIT-6, the physicochemical properties of Cu/SBA-15 have revealed highly dispered Cu2+ ions and large quantity, causing the best of activity and N2 selectivity. But for the Cu/ SiO2 and Cu/Al2O3 samples, copper element is existed wih CuO, and CuO is confirmed low catalytic activity and selectivity of N2. So the Cu2+ ions is the active center for the selective catalytic combustion acrylonitrile-containg exhaust gases. In different cyanide containg waste gas were removed by catalytic combustion over Cu/SBA-15,the results reveal the catalys activity and N2 selectivity are related to the hydrocarbon band togerther with cyanide (-CN), the activity and N2 yield gradually increased associated with decreasing carbon number of nitrile (C2H3-< CH3-< H-). Furthermore, the CH3CN or C2H3CN catalytic combustion mechanism was studied by DRIFTS. Four kinds of mechanisms were proposed to be essentially dependent on the physicochemical nature of the loaded metals and their supports, as well as the reaction temperature, with a priority formation of N2 or NH3 or N2O or NO, respectively. The isocyanate (NCO) could be generated over Cu/SBA-15 through the oxidation of-CN; this NCO can be readily oxidized into N2. While acylamino (CONH2) was formed over Fe/SBA-15 by the hydrolysis of CN, the CONH2 could be further hydrolyzed into NH3, and NH3 further was oxidized into N2.On the other hand, the different structures of materials (ZSM-5, MCM-29, MCM-22, Y, Beta) were selected as the catalyst carriers and further were modified with transition metal ions (mainly copper ions solution) by impregnation method. These catalysts were used for selectivity catalytic combustion of containing cyanide waste gas under the same conditions. Based on basic laboratory research and pilot pattern amplification, one of these catalyst samples was ultimatelty realized the industrial scale application.First obtained in laboratory studies, the conversion of acrylonitrile and N2 yield were close to 100% over 5% Cu-ZSM-5(SiO2/Al2O3=26) under an atmosphere of 0.3 vol% C2H3CN+8% O2 and with the gas hourly space velocity of 37000h-1 in the temperature region of 350-550 ℃. The result was better than other microporous zeolites and other metal elements. By physicochemical properties analysis of XRD, H2-TPR, XPS, and XRF,5% Cu-ZSM-5 with excellent performance mainly caused by the low silicon aluminum ratio of ZSM-5, which can provide suitable ion exchange sites and Cu mainly existed in the high activity of Cu2+. Meanwhile, too much copper oxide was proofed low selectivity of N2. The presence of a small amount of H2O help improve the activity and selectivity of the catalyst and promote mutual conversion between Cu2+ and Cu+. Mechanism study of samples revealed that intermediate product NCO was the key factor for formating N2. Under different conditions, the machanisms of NCO forming N2 over Cu-ZSM-5 are also different. Only in the presence of O2 can NCO be directly decomposed into N2. But under the presence of H2O and O2, the NCO can be firstly hydrolyzed into CONH2 and CONH2 can be further decomposed into NH3, while NH3 is easy to be oxided into N2 on the surface of the catalytic active center finally.Secondly, based on the excellent catalytic performanceof power samples,5%Cu-ZSM-5(SiO2/Al2O3=26) was further used for the pilot studies. Firstly, the binder, water, acid and other substances were combined with the Cu-ZSM-5-26 and were ultimately prepared particles or honeycomb monolithic catalysts (marked for BHAN-1). BHAN-1 was tested up to 4000 hours in the simulated real conditions of contain cyanide waste gases. Furthermore, the optimized process conditions and the reason of activity decreasing of BHAN-1 were also optimized and studied. After the long period testing, the activity and selectivity of nitrogenalso maintained a high rate and the emission index of waste gases were lower than the national emission standards. The best optimum conditions are obtained for the reactor bed entrance temperature should be greater than 350℃, the highest catalyst temperature tolerance should be lower than 650℃, volume space velocity is less than 20000 h-1, and the oxygen concentration is more than 7.5%. So the waste gases containing cyanogen and nitrogen conversion yield were both higher than 98%. Carbon deposition, sulfur poisoning, high temperature and active substrate lasting cannot case the activity of BHAN-1 decreasing or deactivation. The main factors are the metal state changing of activity center of copper with the long time and repeated transformation and eventually result in the divalent copper ions change to copper oxide, and the BHAN-1 specific surface area reduces.Lastly, this paper focuses on how to realize the process of BHAN-1 and how to apply it to the industrial treatment device of containing cyanide exhaust gases. From the view of industrial production, it was illustrated description of the honeycomb monolithic catalyst preparation process and got products. Through the cooperation with enterprises, the demonstration project includes replacing the noble metal catalyst with the BHAN-1 catalyst, simplifying the process of AOGC system. The original two section types (catalytic oxidation+ catalytic reduction of nitrogen oxides) are adjusted to one section (selective catalytic oxidation), achieving a "one step" remove all pollutants. The exhaust gas processing capacity of demonstration project ups to 100 thousand cubic meters/hour, volume space velocity is 19000 h’1. The acrylonitrile conversion rate is as high as 95%, nitrogen selectivity is higher than 90%. Acrylonitrile, HCN and NOX emissions are lower than the current national emission standards. |
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