| With the rapid development of the automobile industry and the increasingly strict environmental protection requirements,many countries around the world have established strict fuel emission standards.In order to meet these strict emission requirements,the production of clean gasoline has become a worldwide trend.The new emission standards further reduce the levels of sulfur compounds,aromatics and olefins in gasoline,which will inevitably lower the octane number of the gasoline.Hydroisomerization of linear alkanes is considered to be the most effective and economical method to solve this problem.However,since n-heptane and noctane are easily cracked,at present,no commercial catalysts are available for the complete hydroisomerization of n-heptane and n-octane.Therefore,it is necessary to develop a highly efficient catalyst,which can isomerize long-chain alkanes with little or no occurrence of cracking reactions.The hydroisomerization reaction requires the catalyst to possess an open pore structure that can eliminate diffusion resistance of large reactant molecules in the pores.At the same time,the catalyst is required to have suitable acidity to improve the hydroisomerization performance.Therefore,the development of supports having suitable acidity and open pore structure is the key to designing a highly efficient catalyst.In addition,establishing the relationship between the support structure and the catalyst performance with the aim of realizing the rational design of novel support materials is the key scientific problem that needs to be solved.Facing with these crucial challenges,a series of meso-microporous composite Beta-MCM-41(BM-41),were prepared by nano-assembly method using alkali solution treated Beta zeolite as the silicon-alumina source.Pt/HBM-41 catalysts were prepared using the composite materials(BM-41)as supports.The results showed that Pt/HBM-41 had better catalytic performance than Pt/HBeta and Pt/MCM-41,and the yield of iso-heptane was significantly improved.This is mainly due to the fact that the composite has interpenetrating micropores and mesopores,which is beneficial for improving the mass transfer efficiency of the isoparaffin molecules.At the same time,the composite material BM-41 has an appropriate amount of medium-strength acid sites,which can effectively reduce the occurrence of cracking reactions.An indexed isomerization factor(IIF)was proposed by correlating the physicochemical properties of the support(mesopore volume,total acid density and strong acid density).Results showed that IIF value has a strong linear relationship with isoheptanes yield-the higher the IIF,the higher the iso-heptane yield.Pt/HBM-41(1.5)had the highest iso-heptane yield(48.01%),while its support HBM-41 also had the highest IIF value(0.28).With these results,therefore,IIF can be used as a performance descriptor for predicting the catalytic performance of hydroisomerization catalysts and as a theoretical foundation for the precise design of novel meso-microporous composite support materials.Compared with the two-dimensional hexagonal straight channels,the three-dimensional cubic channel structure helps improve mass transfer efficiency.The meso-microporous composite material Beta-MCM-48(BM-48)was prepared by using nano-assembly method with Beta zeolite as a precursor.The composite material(BM-48)has a three-dimensional cubic phase mesoporous pore structure,and the mesopore size is concentrated at about 2.4 nm.Compared to BM-41,BM-48 has a larger IIF value(0.32).And the isoheptanes yield of Pt/HBM-48 was 66.34%.The mesopore size distributions of BM-41 and BM-48 are concentrated between 2-3 nm,which still imposes diffusion resistance to certain isoparaffins.To further increase the mesopore size of the material,Beta-SBA-15(BS)was prepared under the acidic condition using Beta zeolite as a precursor by the nano-assembly method.The BS has a two-dimensional hexagonal mesoporous channel with a pore size of 6.4 nm.Compared to BM-41 and BM-48,BS has a larger IIF value(0.35).And Pt/HBS has higher hydroisomerization activity than those of Pt/HBM-41 and Pt/HBM-48.The isoheptanes yield of Pt/HBS was 68.04%.In order to further improve the catalytic performance of the catalyst.The mesomicroporous materials of Beta-KIT-6(BK)with large pore size(6.4 nm)and three-dimensional cubic phase mesostructures was prepared from the Beta zeolite crystallites using nonionic surfactants P123 and n-butanol as structure-directing agents by self-assembly method.Compared with BM-41,BM-48 and BS,BK has a larger IIF value(0.43).And Pt/HBK has better hydroisomerization performance than those of Pt/HBM-41,Pt/HBM-48 and Pt/HBS.The isoheptanes yield of Pt/HBK was 72.46%. |
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