Preparation Of CuO-ZnO-Al₂O₃/Modified HZSM-5Bifunctional Catalyst And Its Performance Studies In Catalytic Synthesis Of Dimethyl Ether From Hydrogenation Of Carbon Dioxide

Carbon dioxide is one of the main greenhouse gases and is the richest C1resources in the nature. Emission and utilizing of carbon dioxide have attractedwidespread attention. Catalytic hydrogenation of CO2to synthesis methanol anddimethyl ether is an significant way, which not only can effectively reduce the contentof carbon dioxide in the atmosphere, but also can obtain important chemical products.This synthetic technology surely brings a remarkable environmental and economicefficiency, especially to those chemical plants producing a large number ofby-products of CO2and H2. In this dissertation, the catalyst for the hydrogenation ofcarbon dioxide to methanol and dimethyl ether was studied, and the following workshad been done:1. The mixed acid of H2SO4/HNO3-pretreated multi-walled carbon nanotubeswere employed as supports and ultrasound-assisted co-precipitation method wasdesigned to prepare multi-walled carbon nanotubes supportedCuO-ZnO-Al2O3/HZSM-5catalyst. The catalyst was characterized by means of x-raydiffraction spectrum (XRD), scanning electron microscope (SEM), transmissionelectron microscopy (TEM), thermal analysis (TG) and Brunauer-Emmett-Teller(BET). The catalyst activity for the preparation of dimethyl ether from hydrogenationof CO2was investigated in a fixed-bed reactor, which shows that multi-walled carbonnanotubes could promote the catalytic activity of CuO-ZnO-Al2O3/HZSM-5. Underthe reaction conditions of temperature at262℃, pressure at3.0MPa, H2/CO2=3(volume ratio) and space velocity (SV)=1800mL·gcat-1·h-1, the conversion per passof carbon dioxide is46.2%. The yield and selectivity of dimethyl ether are20.9%and45.2%, respectively.2. Imidazole modified HZSM-5zeolite was prepared usingtetrapropylammoniuhydroxide as template and imidazole as additive by hydrothermalsynthesis method. The samples were characterized by the methods of XRD, SEM,EDS, BET, TG and NH3-TPD. The results show that adding a small amount of imidazole can produce uniform and smaller HZSM-5zeolite particles with higheracidity and perfect high temperature resistance performance. TheCuO-ZnO-Al2O3/imidazole-modified-HZSM-5bifunctional catalyst was prepared byimpregnation method and its catalytic activity for carbon dioxide hydrogenation todimethyl ether was investigated. Under reaction pressure at3.0MPa, temperature at270℃, space velocity (SV) at1800mL gcat-1h-1, CO2/H2volume ratio at1:3,CuO/ZnO molar ratio at2:1and CuO-ZnO-Al2O3/imidazole-modified-HZSM-5weight ratio at2:1, the conversion of CO2and selectivity of dimethyl ether is46.5%and37.8%, respectively.3. The honeycomb-type HZSM-5zeolite (HZSM-5N) was synthesized withspan-80and tween-80as the surface active agents using two steps synthetic method.The samples were characterized by the methods of XRD, SEM, BET and NH3-TPD.The influence of hydrothermal synthesis temperature and time on relative degreecrystallinity of honeycomb-type HZSM-5zeolite was also discussed. TheCuO-ZnO-Al2O3/HZSM-5Nbifunctional catalyst was prepared by mechanical mixingmethod and its catalytic activity for carbon dioxide hydrogenation to dimethyl etherwas investigated. Under reaction pressure at3.0MPa, temperature at270℃, spacevelocity(SV) at1800mL gcat-1h-1, CO2/H2volume ratio at1:3andCuO-ZnO-Al2O3/HZSM-5N=1, the conversion of CO2and yield of dimethyl ether is45.4%and20.1%, respectively.