The Influence Of Microwave Irradiation On Microstructure Of Precursor And Catalytic Performance Of The Cuo/zno/al₂o₃ For Slurry Methanol Synthesis

The liquid-phase methanol synthesis process has many advantages such as isothermal temperature, high reaction heat transfer rates, suitable for CO-rich syngas, high conversion per pass, replacement of catalyst on-line, so it always get much attention. Although the slurry phase synthesis of methanol is superior to the gas phase synthesis, the deactivation of the catalyst in slurry phase methanol synthesis is still a bottleneck for the industrial application. In order to obtain a catalyst with high activity and stability, previous work mainly focused on the study of methanol synthesis mechanism, catalyst activity site and deactivation mechanism of the commercial CuO/ZnO/Al2O3 catalyst, while few studies were carried out on the preparation of the catalyst for the liquid phase process. The previous studies in our laboratory showed that requirement for the microstructure of catalyst between liquid-phase and gas phase methanol synthesis process is different. The precipitation and aging process have crucial effects on the crystal composition of the precursors and then determine the catalyst microstructure and performance. Recently, there has been a rapid increase of using Microwave technology in the preparation of catalytic materials to reduce the preparation time and to optimize the crystallization. In this paper, CuO/ZnO/Al2O3 catalysts with Cu/Zn/Al ratios of 6/3/1 were precipitated and aged by conventional and microwave heating methods and tested in the slurry phase reactor for methanol synthesis. The effect of precipitation and aging temperature under microwave irradiation on the catalytic performance was investigated to get the optimum condition of preparation. X-ray diffraction (XRD), FT-IR spectroscopy (FT-IR), Derivative thermogravimetry (DTG), H2 temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) were used to examine the influence of microwave irradiation on precursor and catalyst microstructure of the CuO/ZnO/Al2O3. The results as fellow:(1)The microwave irradiation during precipitation process has little effect on the crystal composition of precursors. During precipitation process, the microwave irradiation could improve the activity of the catalyst, but had little effect on the stability. Under microwave irradiation, the methanol STY of catalyst increases firstly and decreases subsequently with the increasing of precipitation temperature. The methanol STY of the catalyst precipitated at 70℃is highest, which is 296.98 mg/(g·h).(2) The microwave irradiation during aging process promoted substitution of Zn2+in (Zn)5(CO3)2(OH)6 compound by Cu2+and increased the content of (Cu, Zn)5(CO3)2(OH)6 phase in the precursors, the calcinated catalysts had strong interaction between CuO-ZnO, well dispersed CuO crystal particle and high surface CuO content. It exhibited the highest methanol STY and more stable catalytic activity of the CuO/ZnO/Al2O3 catalyst aged at 80℃for 1h under microwave irradiation. Compared with the catalyst prepared without microwave irradiation, the methanol Y and deactivation of catalyst aged at 80℃for 1 h under microwave irradiation improve 10.1% and 31.2% respectively.(3) The content of (Cu, Zn)5(CO3)2(OH)6 phase in the precursors precipitated and aged under microwave irradiation is higher, the calcinated catalysts also had strong interaction between CuO-ZnO, well dispersed CuO crystal particle and high surface CuO content. The activity and stability of the catalyst had got a further improvement, and the optimal condition for the catalyst precursor preparation was precipitated at 60℃and aged at 80℃under microwave irradiation. The catalyst had the average methanol STY of 312 mg/(g·h) and the lowest deactivation of 0.05 %/d.(4) The effect of polarity of solvent on the composition of precursors and microstructure and performance of the catalyst aged under microwave irradiation was investigated. The results show that the content of (Cu, Zn)2(CO3) (OH)2 and (Cu, Zn)5(CO3)2(OH)6 phase become higher with the increasing of polarity of solvent, the interaction between CuO-ZnO become stronger, the stability and activity become better.