醇原位催化转化制备芳烃及机理

Study on in-situ catalytic conversion of alcohols to aromatics and its mechanism

以不同的醇为原料,HZSM-5分子筛为催化剂,在微型固定床与气相色谱-质谱联用装置上进行了醇催化转化制备芳烃的研究,考察了醇的碳链长度以及反应温度的影响。结果表明,醇的碳链长度对产物的分布有明显的影响,400℃下芳烃产率随着醇的碳链增长先增加,在正丙醇处获得最大的芳烃产率39.6%,而后降低并趋于稳定。随着温度的升高,甲醇、乙醇和正丙醇3种醇的产物分布变化趋势相同,芳烃产率均先增加后减小,在500℃获得最大值,分别为26.0%、36.5%和42.5%;随着温度的升高,CO、CO_2和烯烃的产率逐渐升高,但烃类产物总收率逐渐下降。高温下大分子芳烃逐渐向小分子芳烃转化,使得苯和甲苯的选择性逐渐增大。最后,结合原位红外表征和实验结果探讨了醇芳构化的反应机理:短碳链醇通过脱水得到醚和低碳烯烃,低碳烯烃再经过聚合得到长链烯烃,长碳链醇则可直接脱水得到长链烯烃,长链烯烃再通过环化反应生成环烷烃,最后环烷烃经过脱氢芳构化或分子间氢转移得到芳烃。

The effects of carbon chain length of alcohol and reaction temperature on catalyticconversion of alcohols to produce aromatics over HZSM-5were studied in a micro fixed-bed reactorcoupled directly to GC/MS system.The results showed that the carbon chain length of alcohol had asignificant effect on the product distribution.At400℃,with the increase of carbon chain length ofalcohol,the yield of aromatics increased firstly up to the maximum of39.6%(1-propanol),thendecreased and finally stabilized at a lower yield.As the temperature increased,the productsdistributions for methanol,ethanol,1-propanol showed the same tendency,and the yields of aromaticsincreased firstly,then decreased and the maximum values reached26.0%,36.5%and42.5%at500℃respectively.As the temperature increased,however,the yield of CO,CO2and olefins increased gradually while the total yield of hydrocarbons decreased.Higher temperature helped the conversion of largeraromatics to lighter aromatics,which contributed to the higher selectivity for benzene and toluene.Finally,the reaction mechanism of alcohol aromatization was discussed by the combination of in-situFTIR and experimental results.It is supposed that the light olefins and ethers were produced bydehydration of short carbon chain alcohols,while long carbon chain olefins were produced bypolymerization of the light olefins or direct dehydration of long carbon chain alcohols.The obtainedlong carbon chain olefins were then converted into cycloalkanes by cyclization.At last,aromatics wereformed by dehydrogenation-aromatization or intermolecular hydrogen transfer of the cycloalkanes.