Study On The Behavior Of Alkanes Hydroisomerization Reaction On Composite Molecular Sieve Catalysts

In recent years,due to the increasing awareness of environmental protection of people,the quality requirements for automotive fuels have become higher,and gasoline has developed toward low olefins,low aromatics,and high octane numbers.Isomerization of light alkanes is one of the most effective methods to produce high octane gasoline.The composite molecular sieve has a suitable pore structure,acidic distribution and good hydrothermal stability.The catalyst supporting non-noble metal with the composite molecular sieve as a carrier exhibits excellent reaction performance in the light alkanes isomerization reaction.Therefore,in this paper,the non-precious metal was modified on the basis of preparing MCM-41/Beta composite molecular sieve.The n-hexane hydroisomerization performance of the composite molecular sieve catalyst was studied,and the mechanism of hydrogen-isomerization reaction was preliminarily explored.The MCM-41/Beta composite molecular sieve was synthesized by hydrothermal synthesis using alkali-treated beta molecular sieve slurry as part of silica-aluminum source and cetyltrimethylammonium bromide as template.The synthesized composite molecular sieves were characterized by XRD,BET,TEM,FT-IR,NH₃-TPD,Py-IR,H₂-TPR and so on.The results showed that the synthesized molecular sieves had both micropores and mesoporous dual pore structures,and the specific surface area,pore size and pore volume of the composite molecular sieves were higher than that of the Beta microporous molecular sieves,with a specific surface area of 862.40 m²·g^-1,an average pore size of 3.62 nm and a pore volume of0.78 cm³·g^-1.The total acid content of composite molecular sieve MCM-41/Beta was761μmol·g^-1,which obviously improved the problem of poor acidity of mesoporous MCM-41molecular sieve.And the synthesized composite molecular sieve has good thermal stability and hydrothermal stability.The isomerization of n-hexane over different supported catalysts was investigated in a fixed-bed hydrogenation reactor and their acidity was analyzed separately.The results showed that the catalytic activity of n-hexane isomerization was related to the acidity of the catalyst.The stronger the acidity was,the higher the conversion rate of n-hexane was,and the more suitable acidic distribution was more favorable to increase the selectivity of isoparaffin.Compared with the isomerization reaction data of different catalysts,the effect of n-hexane isomerization of 2%Y-Ni-M41βcatalyst was the best,the conversion of n-hexane reached61.60%,and the selectivity of isoparaffin was 78.77%.The effects of reaction conditions such as reduction temperature,reaction temperature,reaction pressure and volume-space velocity on the isomerization of n-hexane were investigated.The optimal reaction conditions were as follows:reduction temperature was 400℃,reaction temperature was 300℃,reaction pressure was 3MPa,The volumetric airspeed is 2h^-1.At the same time,the isomerization performance of the catalyst under hydrogen and non-hydrogen conditions was comparatively examined.Under the condition of hydrogen,the conversion rate and selectivity of the isomerization reaction increased.Therefore,the catalyst exhibited better catalytic isomerization performance under the condition of hydrogen.A preliminary exploration of the isomerization reaction mechanism of n-hexane showed that the isomerization reaction mechanism of n-hexane follows the mechanism of the normal carbon ion reaction.