模型反应表征长链烷烃异构化催化剂的研究 Ⅰ.乙醇在载体上的脱水反应

Model reactions for characterizing long-chain hydrocarbon hydroisomerization catalysts Ⅰ. Ethanol dehydration reaction on supports

以氧化铝(Al2O3)、ZSM-5、Beta和SAPO-11分子筛粉体为原料制备出不同的载体,在不同温度下考察了探针分子乙醇脱水转化反应.根据NH3-TPD方法测定载体的酸量和酸分布,以及载体负载贵金属Pt后C14～24长链烷烃异构化评价结果,探讨了载体表面酸、乙醇转化和异构化性能三者之间的关系.结果表明,载体表面酸与乙醇转化和异构化性能没有直接的关系,但乙醇转化和异构化性能之间有一定关联.载体上乙醇转化的稳定性较差、乙烯选择性低,则对应的贵金属催化剂异构化性能也较差;乙烯选择性高且稳定性好,可制备出性能优异的异构化催化剂.因此,作者提出区分固体酸的酸性和酸性质的必要性.酸性包含酸量、酸强度及分布等概念,酸性质指酸中心在催化反应中的行为,简称为酸功能.前者可由常规的表面酸方法表征,后者只能在反应条件下获得.数据表明了临氢异构化剂的酸功能可以采用乙醇转化反应进行表征.用乙醇转化作为模型反应表征载体的酸作用行为即酸功能,可以为异构催化剂载体的选择提供指导.

Transformation of the probe molecular ethanol over the selected isomerization catalyst supports,which were prepared from Alumina,ZSM-5 and SAPO-11 powders,was investigated at different reaction temperatures.Combined with the results of acidity data by NH3-TPD,and the results of long-chain C14~24 hydrocarbon hydroisomerization,the relationships among the acidity,ethanol conversion as well as the hydroisomerization performance were investigated.It was shown that neither the ethanol conversion nor the hydroisomerization could correlate with acidity.However,a good relationship between ethanol conversion and hydroisomerization was observed.When a support presented low ethylene selectivity and inferior stability during ethanol conversion,the corresponding hydroisomerization catalyst had bad performance.While a support shown stable activity and high ethylene selectivity for ethanol conversion,a hydroisomerization catalyst with better stability and high isomerization selectivity might be obtained after loading precious metal onto the support.After detailed analysis of these data,we found that it was necessary to discriminate between acidity and acidic property.Acidity includes the acid amount,acid distribution as well as acidic strength,while acidic property,temporally named as acid function,is the acidic functional behavior during a reaction.The former can be characterized by traditional method,but the later can only be probed by a catalytic transformation.The obtained data strongly suggests that it is possible to characterize the acid function of hydroisomerization catalyst by a model reaction of ethanol conversion.The model reaction using ethanol as probe molecule could be used to characterize the acid function of a support,and thus provide useful instructions in selecting hydroisomerization catalyst supports.