Study On Dehydration Of Ethanol To Ethylene Over Modified HZSM-5Catalysts

The reaction of ethanol dehydration to ethylene was studied over modified catalyst in this thesis. HZSM-5/SAPO-11composite was synthesized by embedding methods. Micro/mesoporous HZSM-5zeolites was prepared using dealumination and desilication method. The activity and stability of the catalysts were tested in a continuous flow fixed-bed reactor. The modified catalysts were characterized by XRD、SEM、ICP-AES、NMR、BET、NH3-TPD and IR. The reason of improved stability of modified catalysts was investigated by catalytic performance. The main conclusions are as following:1、A new catalyst HZSM-5/SAPO-11composite for ethanol dehydration to ethylene was synthesized. The influence of ratios of raw materials, template and crystallization on catalytic performance was studied. The main conclusions are as following:Micro/microporous HZSM-5/SAPO-11melecular sieve was synthesied in the best synthesis conditions. HZSM-5/SAPO-11composite molecular sieve exhibted better catalytic performance for ethanol dehydration to ethylene. Ethanol conversion and ethylene selectivity were99.8%and99.5%. They were above95%after150hours run. The optimum synthesis condition was determined by varing the synthesis parameters. The optimum synthesis condition was as follow:ratios of raw materials was DPA:HZSM-5:Al2O3=8:4:1. DPA as template agent, crystalliza-tion time was24h and temperature was185℃Characterizations indicated that the composite zeolites existed in a form of a core-shell structure, with the HZSM-5phase as the core and the SAPO-11as the shell. Results indicated that the improved catalytic performance was related not only to the pore structure, but to the catalyst acid number.2、The modified catalysts were characterized and the catalytic performance were tested. The effect of hydrothermal and high temperature treatment on HZSM-5catalysts was analyzed during dehydration of ethanol to ethylene. The main conclusions are as following:Modified zeolites showed good catalytic performance. Ethanol conversion and ethylene selectivity of hydrothermal teated catalyst were99.5%and99.4%. they were94.4%and96.0%after350hours run at280℃. High temperature treated zeolite suppressed the formation of carbon deposit and improved ethylene selectivity. Ethanol conversion and ethylene selectivity were99.8%and99.5%and were still above95%after300hours run at260℃.Hydrothermal and high temperature treatment adjusted surface acidic distribution of HZSM-5zeolites. The total acidic amount decreased and the strength of acid sites weakened. BrOnsted acid sites and the strong acid sites were decreased; more Lewis acid sites were produced. Some of A1was removed from the framework of zeolite and extracted out of the zeolite channels after hydrothermal and heat treatment. Mesopores were formed after high temperature heat and hydrothermal treament. The improvement of catalytic performance can be attributed to suitable acidity and new complex meso/microporous structure.Moderate hydrothermal and high temperature treatment improved the catalytic performance. However, the severe treatment led to partial destruction of the zeolite framework, which was disadvantageous not only for the micropores but also for the formation of mesopores.3、A series of mesoporous HZSM-5were synthesized by desilication and the effects of the alkali treatment conditions on catalytic dehydration of ethanol to ethylene were investigated. The main conclusions are as following:The experiment of catalytic dehydration of ethanol showed that alkali-treated HZSM-5zeolites improved not only the catalytic activity but the stability. Ethanol conversion and ethylene selectivity were99.6%and the ethylene selectivity remained above95%after350hours run.The experiment of catalytic dehydration of ethanol indicated improved catalytic activity and excellent catalytic stability of alkali-treated HZSM-5catalyst was related not only to the catalyst acid number and strength, but to the created mesopores by desilication.The complexed meso/microporous HZSM-5zeolites were formed owing to alkali-treatment desilication. The amount of strong acid sites of modified zeolites decreased and weak acid sites increased. The coke produced in the reaction procedure tended to deposit in the mesopores of the zeolites. So, the newly created mesoropores by desilication led to the improvement of catalytic stability.The effects of several alkali-treatment parameters on the catalytic performance of HZSM-5zeolite were investigated by orthogonal and dehydration of ethanol. The results was concentration> alkali treatment temperature> alkali treatment time.4、A series of modified HZSM-5were prepared by hydrothermal treatment, high temperature treatment and desilication. The effects of the treatment conditions on pore structure, surface acidity and catalytic dehydration of ethanol to ethylene were investigated. Regeneration of alkali-treated catalyst was studied. The main conclusions are as following:Modified zeolites showed good catalytic performance. Ethylene selectivity of modified catalysts was improved. The lifetime of hydrothermal treated catalyst, high temperature treated catalyst and alkali-treated catalyst was350h,300h and350h.The improvement of catalytic performance can be attributed to suitable acidity and new complex meso/microporous structure.Temperature programming regeneration would obtain a better coke burning and microporous recovery than constant temperature regerneration. The regenerated catalyst showed good catalytic stability. Ethanol conversion and ethylene selectivity remained above95%after250hours run. Considering the activity, stability and regerneration of alkali-treated HZSM-5zeolites catalyst comprehensively, the alkali-treated mesoporous HZSM-5zeolites was the suitable catalyst in the dehydration of ethanol to ethylene.