Effect Of High Pressure Heat Treatment On Structure And Performance Of Cuznal Catalysts Prepared By Complete Liquid Phase Technology

Conventional preparation methods of catalyst include precipitation,impregnation, mixing, ion exchange and sol-gel method, etc., which have theirown advantages, disadvantages and applications. However, differentpreparation methods and conditions have a significant impact on catalyststructure, and thus also determine the performance of catalyst.To solve the deactivation of the catalyst in slurry reactor for one-stepsynthesis of DME from syngas, our group invented complete liquid-phasetechnique for preparing catalyst. Due to the difference of physical and chemicalchanges that occurred in complete liquid-phase technique and conventionalmethods, the catalyst prepared by complete liquid-phase method displayedexcellent stability in slurry reactor.In our previous study, the catalyst precursor was heat-treated underatmospheric pressure, and so prepared slurry catalyst was directly used in slurryreactor. In order to enrich complete liquid-phase technique, heat treatment under high pressure was adopted in this paper. The effect of heat treatmentprocess conditions on CuZnAl catalyst structure and performance wasinvestigated by changing the heat treatment conditions (pressure, temperature,time), heat treatment methods (closed, flow) and heat treatment medium underhigh pressure conditions. The performance of the catalyst was attempted toevaluate in a fixed bed for CO hydrogenation in order to expand the applicationfield of complete liquid-phase method. The main conclusions were as follows:1. Compared with the catalyst heat-treated under atmospheric pressure, thecatalyst heat-treated under high pressure (closed) was conducive to form thecatalyst with more complete and higher crystalline. Moreover, the spinel phasewas appeared and the catalyst showed larger specific surface area, differentpore structure. However, the dispersibility of all phases was decreased. Theamount of reducible species and acid sites of catalysts were reduced followingthe weakening of the acid strength. It was showed that Cu species were reducedto Cu0under high pressure, but the heat treatment with atmospheric pressurethen high pressure could suppress the reduction of Cu species to some extent.2. By replacing with fresh liquid paraffin, the heat treatment withatmospheric pressure then high pressure (closed) could suppress the reductionof Cu species and the reducible species existed in more forms. The structure ofpore was changed and many larger pores were generated. Moreover, the weakacid sites increased and the activity was improved.3. The pressure of heat treatment had a significant impact on the species of Cu under a flowing gas. Increasing pressure would favor the improvement ofCu2O crystallinity. It was also found that heat treatment under higher pressurecould broaden the distribution of pore size. When decreasing the pressure, thepore structure was regular and pore size distribution was more concentrated4. With raising the temperature of heat treatment under a flowing gas, thereducible species were decreased and could be easily reduced. The high valueof Cu species were gradually reduced to Cu0and the average pore diameter ofcatalysts increased.5. With prolonging the time of heat treatment under a flowing gas, thecrystallinity of catalyst was more complete. Moreover, Cu species were moreclosely combined with other components, resulting in Cu species were difficultto be reduced and a variety of Cu2O were formed.6. When water was used as heat medium in sealed autoclave, the coke incatalyst surface was decreased and other elements such as Cu, Zn and Al wereelevated. Cu species mainly existed as CuO and the degree of crystallinity ofeach phase was improved. However, the catalyst was easily to form a densephase, leading to the specific surface area and pore volume decreased rapidly.7. The activity of the catalysts evaluated in a fixed bed for COhydrogenation was low. But it was significantly improved by charcoaltreatment. The reason was that C content of the surface decreased and Cu, Zn,Al content increased. Meanwhile，the catalyst particles became smaller and itsdispersibility was improved.