Synergistic Catalysis Of CuZnAl And ZSM-5 In CO₂ Hydrogenation To Produce Low-carbon Olefins

Catalytic hydrogenation is an important way of chemical conversion and utilization of CO₂.In this thesis,the copper-zinc-aluminum catalyst（Cu Zn Al）and ZSM-5 molecular sieve are packed in layers in a fixed-bed reactor to investigate the direct preparation of low-carbon olefins from CO₂.The preparation method of Cu Zn Al catalyst,Si/Al ratios of ZSM-5,metal modification effects of ZSM-5,catalyst loading mode and reaction conditions are demonstrated according to their effects on the performances.Detailed research work includes:1)A series of Cu Zn Al catalysts were prepared by sol-gel method（citric acid,tartaric acid,EDTA）and coprecipitation method（sodium carbonate,oxalic acid,urea）.Combined with ZSM-5,these Cu Zn Al catalysts were utilized in CO₂ hydrogenation to obtain low-carbon olefins.The Cu Zn Al catalysts obtained by different preparation methods were characterized by N₂ physical adsorption,X-ray diffraction（XRD）,hydrogen temperature programmed reduction（H₂-TPR）and SEM.The influence of different preparation methods were compared by the reaction performances.The results show that different preparation methods can significantly affect the Cu O particle size,specific surface area and Cu component dispersion in the catalyst,which in turn affects the CO₂ hydrogenation performance.The CO₂ conversion rate of the catalyst prepared by citric acid complexation through the sol-gel method was 20.17%,and the selectivity of low-carbon olefins in hydrocarbons was 50.48%.Under the same reaction conditions,the CO₂ conversion rate of the catalyst prepared by coprecipitation method（Na₂CO₃）was 29.14%,the CO₂ selectivity was 68.67%,and the low carbon olefin selectivity in hydrocarbons was 54.57%.2)On the basis of the sdium carbonate precipitation method,the hydrogen plasma technology was introduced to reduce the catalyst precursor,and the catalyst was characterized by XRD and SEM.Compared with the catalysts obtained by calcination or high temperature reduction,the ones directly treated by hydrogen plasma has a smaller particle size,a slightly improved CO₂ conversion rate and a decreased CO selectivity.The catalytic performance of the catalyst prepared using oxalic acid precipitation-hydrogen plasma is greatly improved.The Cu Zn Al catalysts were prepared by sodium carbonate precipitation-hydrogen plasma method were modified by metal Ce,Zr.Though there is little effect on the conversion rate of CO₂,the selectivity of by-products such as CO was improved,and the selectivity of CO was improved over 5%.3)Combined with the characterization of XRD,N₂ physical adsorption and NH₃-TPD,ZSM-5 molecular sieves with different Si-Al ratios were investigated.When the Si-Al ratio was 200,a better catalytic performance was obtained.Furthermore,metal nitrate was introduced to modify ZSM-5.The metal Y modified catalyst contributed to the improvement of the selectivity of light olefins.The carbon dioxide conversion rate is 32.19%,and the selectivity of light olefins reaches up to 57.51%.4)The effects of reaction conditions,reduction conditions,catalyst ratio and packing method of the catalytic system on the reaction performance of CO₂ hydrogenation to low-carbon hydrocarbons were systematically investigated.The results show that when the reduction temperature is 500℃,the reaction temperature is 320℃,the pressure is 2.0 MPa,the hydrogen-carbon ratio is 3:1,the space velocity is 3600 m L/g_cat·h,the mass ratio of CZA-Na and ZSM-5 is 1:1,shows a better performance in CO₂ hydrogenation.