Studies On Preparation And Catalysis Performance Of The CuO/ZnO/Al₂O₃ Catalyst For Liquid-phase Methanol Synthesis System

The liquid-phase methanol synthesis process has many advantages such as high reaction heat transfer rates, suitable for CO-rich syngas, high conversion per pass, no or less circulation of tail gas, replacement of catalyst on-line, flexible on operational load, etc., is suitable for large-scale production of methanol and especially for being involved in the coal-based integrated gasification combined cycle. But the catalyst used was mostly the crashed commercial CuO/ZnO/Al₂O₃ catalyst for fixed-bed methanol synthesis process, and has not been commercially popularized until now because of its poor stability. On the researches of the predecessors, the co-precipitation process of the CuO/ZnO/Al₂O₃ catalyst for liquid-phase methanol synthesis was systematically studied in this dissertation. Spray-drying process was introduced in co-precipitation process for the first time and powdery CuO/ZnO/Al₂O₃ catalyst with both of high stability and activity was obtained. The structures of the catalyst and its precursor were characterized and analyzed, and relations between structure and performance of the catalyst were discussed. The main results were as follows:1. The precipitation and aging procedures were the main influencing factors of the phase composition and crystalinity of the precursor and the micro structure of the CuO/ZnO/Al₂O₃ catalyst. Along with the temperature increment of precipitation and aging and the time extending of aging, the crystalinity of phases in the precursors enhanced gradually. The precursor was mainly composed of malachite (Cu₂(CO₃)(OH)₂) and zinc malachite ((Cu, Zn)₂(CO₃)(OH)₂) under the conditions investigated in this dissertation. The precursor obtained under the conditions of precipitating at 70℃and aging at 80℃for 1.0 hr had proper crystalinity, and the mean diameter of CuO crystallite (4.50 nm) in the resulted CuO/ZnO/Al₂O₃ catalyst was smaller, both of its BET surface (98.6 m²/gcat) and proportion of pore diameter (53.8 %) between 58 and 200 ? were the largest among the catalysts prepared in the investigation of precicitation and aging precedure. The space time yield and deactivation rate of the catalyst had reached up to 153.3 g/kgcat·h and 1.44 %/d respectively under the reaction conditions of 240℃,4.0 MPa,H₂/ CO /CO₂ = 92/36/7 and space velocity 810 ml/gcat·h.2. The calcination procedure will influence the micro structure of the CuO/ZnO/Al₂O₃ catalyst as well. Along with the temperature increment and time extending of calcination, The mean diameter of CuO crystallite increased gradually, and both of the BET surface and proportion of pore diameter between 58 and 200 ? increased at first and then decreased. Under the conditions of precipitating at 70℃, aging at 80℃for 1.0 hr and calcining at 350℃for 2.0 hrs, the mean diameter of CuO crystallite (3.41 nm) in the catalyst was smaller, both of the BET surface (122.2 m²/gcat) and proportion of pore diameter (67.6 %) between 58 and 200 A of the catalyst were the largest among the catalysts prepared in the investigation of calcination precedure. The space time yield and deactivation rate of the catalyst had reached up to 172.2 g/kgcat·h and 0.43 %/d respectively under the reaction conditions of 240℃,4.0 MPa,H2/ CO /CO₂ = 92/36/7 and space velocity 810 ml/gcat·h.3. The spray drying procedure was beneficial for the stability of the CuO/ZnO/Al₂O₃ catalyst. Under the conditions of spray drying at 225℃and the other preparation conditions same as conventional drying, the mean diameter of CuO crystallite (3.47 nm) in the catalyst was the smallest, both of the BET surface (114.5 m2/gcat) and proportion of pore diameter (68.1 %) between 58 and 200 A of the catalyst were the largest among the catalysts prepared in the investigation of spray drying precedure. The space time yield and deactivation rate of the catalyst had reached up to 166.7 g/kgcat·h and 0.16 %/d respectively under the reaction conditions of 240℃,4.0 MPa,H₂/ CO /CO₂ = 92/36/7 and space velocity 810 ml/gcat·h. Comparing with the catalyst obtained with conventional drying procedure, the space time yield of the two catalysts are equivalent, but the stablility of the catalyst that spray-dried is much better, and the deactivating rate of the catalyst is lower than the lowest value (0.18 %/d) reported in the published literatures.4. The reason for the catalyst spray dried with better stability was that its morphology was different from the catalysts conventionally dried. Comparing the catalyst spray dried at 225℃with the catalyst conventionally dried, their micro structures such as mean diameter of CuO crystallite, BET surface and proportion of pore diameter between 58 and 200 A were equalent with each other, but the morphology of the two catalysts were different. The catalyst spray dried was homogeneous sphere with mean diameter 71μm and the catalyst conventionally dried was irregular shape with mean diameter 121μm.5. The XPS measurement shows that the stability and activity of the catalyst will be better when the binding energy of Cu element shifts more to lower value, and the binding energy of Zn and Al element shift less to lower value.6. In the atmosphere of syngas for methanol synhesis, the state of the Cu element in the Cu/ZnO/Al₂O₃ catalyst prepared in this study was metal copper, and the activity and stability of the catalyst will be better when the mean diameter of the metal copper crystallite within the used catalyst was smaller.