The Performance Studies Of Alkane Hydroisomerization Over Composite Molecular Sieves

The hydroisomerization of n-alkanes is one of the most effective means of producing high quality gasoline.Nowdays composite molecular sieves have become a hot subject because of their superior catalytic performance and good application prospects.In addition,the use of non-precious metals instead of precious metals to modify the composite molecular sieves can reduce costs while improving the catalytic performance.Therefore,in this thesis,on the basis of the synthesis of MCM-41/A,MCM-41/B and MCM-41/C composite molecular sieves,the performance of the composite zeolite catalysts which loaded non-precious metals were investigated in n-hexane hydroisomerization reaction,and preliminary researched the hydroisomerization reaction mechanism.This thesis researched that the MCM-41/A?MCM-41/B ? MCM-41/C composite molecular sieves were hydrothermally synthesized respectively by using the alkali-treated A,B,C seriflux as part of silica-alumina source.The samples were characterized by XRD,BET,FT-IR,Py-IR,TEM and so on.The results show that the three kinds of composite molecular sieves obtained all have microporous and mesoporous dual pore structures,and compare to common zeolites,they have larger specific surface area,pore volume and average pore size.The Py-IR results show that the three kinds of composite molecular sieves have both L acid sites and B acid sites,and the load of metal makes the amount of B acid decreased but the amount of L acid and total acid increased obviously in the composite molecular sieves.This acid distribution is conducive to improve the hydroisomerization reaction performance.In addition,the composite samples have good thermal and hydrothermal stability.The catalytic performance of metal modified catalysts on the hydroisomerization of n-hexane under the same reaction conditions was researched and compared.The results show that M-MCM-41/A catalyst has high catalytic activity and selectivity due to its larger pore size and more acid sites.Otherwise,the conversion of n-hexane and the isomerization selectivity were maintained at 40 % and 60 % respectively within 72 h,which indicates that M-MCM-41/A has good reaction stability.The influence of reduction temperature,reaction temperature,LHSV and metal loading on the n-hexane hydroisomerization reaction performance of M-MCM-41/A catalyst was also investigated.The experimental results indicate that when the reduction temperature was 450?,the reaction temperature was 260?,LHSV was 3 h-1,and the catalyst metal loading was 3 wt%,M-MCM-41/A catalyst had best n-hexane hydroisomerization reaction performance.The conversion of n-hexane was 48.58 %,i-C6 selectivity was 64.45 %,and Liquid yield was 91.10 %.In the end of the thesis,the n-hexane isomerization performance of the composite molecular sieves catalysts under the condition of hydrogen and non-hydrogen was compared.The results show that compared with non-hydrogen condition,the presence of hydrogen has no significant influence on n-hexane conversion,but i-C6 selectivity was increased by 10 %~20 %,cracking selectivity was reduced by about 15 %,and liquid yield was greatly increased.Therefore,hydroisomerization has better performance than non-hydroisomerization.The preliminary study of the reaction mechanism shows that hydroisomerization reaction of n-hexane on composite molecular sieve catalyst follows the carbenium-ion mechanism,and its activity center is composed of the metal ion and the acid sites of the molecular sieves.