Pt/HBeta For N-Pentane Hydroisomerization:Structure-activity Relationship And Reaction Kinetics

With the rise of environmental awareness,the demand for clean fuels is soaring.Due to this,producing clean gasoline through isomerization of n-alkanes has attracted much attention.Pt/HBeta has outstanding isomerization activity and high selectivity to branched alkanes,thus making this catalyst highly potential for industry use.In this thesis,n-pentane hydroisomerization over Pt/HBeta was selected as a model reaction system,since this reaction is representative in producing clean gasoline.Focusing on this model reaction system,the structure-activity and reaction kinetics were investigated.This work could provide theoretical and practical guidance for industrial development of Pt/HBeta for gasoline isomerization.Firstly,effects of Beta zeolite particle size on n-pentane hydroisomerization were probed.Four Beta zeolites with different particle sizes were synthesized by hydrothermal method and their morphology,structure and acidic properties were characterized;Pt/HBeta catalysts were synthesized by incipient-wetness impregnation method.Then,these catalysts were evaluated for n-pentane hydroisomerization under different reaction temperatures.The results show the zeolite particle size affects the catalyst performance importantly;the activity decreases with the increase of particle size,because of the increased diffusion path.Besides,at elevated temperature,the overall reaction rate transfers from reaction controlled region to diffusion controlled region.By performing zero length column experiments on the Beta zeolites with pentene as probe molecules,their characteristic diffusion time was determined,which further proved the important effects of particle size on diffusion and subsequently catalyst performance.Then,effects of Beta zeolite acidity on n-pentane hydroisomerization were probed.Four Beta zeolites with different Si/Al were synthesized by hydrothermal method and their morphology,structure and acidic properties were characterized;Pt/HBeta catalysts were synthesized by incipient-wetness impregnation method.Then,the relationship between catalyst activity and amount of acid as well as strength of acid were experimentally determined.The results show that the amount of Br(?)nsted acid significantly affects catalyst performance;strong Bronsted acid has more acute activity,especially at low reaction temperature.Besides,the catalyst prepared by single crystal zeolites was far more active than the one prepared by polycrystalline zeolites,since additional diffusion resistance could be generated on the grain boundaries.Lastly,the kinetics model of n-pentane hydroisomerization over Pt/HBeta was deduced on the basis of reaction mechanism.Model parameters were determined according to experimental data.Activation energy(104.86 kJ/mol)and orders of hydrogen(-0.78)and n-pentane(0.63)were measured by linear fitting of Arrhenius equation.