Synthesis Of Mesoporous Nano-sized Beta Zeolites And Its Application In Tetralin Hydrocracking

The heavy and inferior crude quality and the implementation of the National V and VI gasoline and diesel standards have posed new challenges to the refining industry.Light cycle oil(LCO)is limited by its composition characteristics and can no longer be used directly as fuel,but due to its highly aromatic chemical properties,the route of converting it into light aromatics by partially hydrogenating and hydrocracking has attracted more and more attention.Previous studies have obtained more tetralin by controlling the degree of hydrogenation,and the problem of low yield of the target product in the second step needs to be solved urgently.At present,bifunctional catalysts are mostly used in this process,nickel/tin is often used as an active metal component due to proper hydrogenation activity,and Beta zeolites with unique pore structure are often used as the carrier acid component.In the application of zeolites for many years,it has been fully realized that reducing the crystal size and introducing some mesopores can optimize its diffusion performance and acidity,which is an effective way to improve the catalyst effect.First of all,Aspen Plus analysis of the tetralin hydrocracking shows that the equilibrium constant of each reaction is much greater than 1,and the process is thermodynamically feasible;At the same time,it is found that high feed ratio and low pressure are beneficial to the formation of the target product BTX,and the effect of temperature is two-sided;The equilibrium conversion rate of BTX calculated based on the real operating conditions is 87.01%,and the control step of the reaction is judged to be the conversion of tetralin isomerization to methylindane.Hydrothermal synthesis of Beta zeolites finds that when the amount of water in the precursor is relatively less,the zeolites follow a non-classical growth path including directional attachment growth and self-assembly of nanoparticles,increasing its ratio tends to be layered of the zeolite morphology.Secondly,the mesoporous nano-sized Beta zeolites are synthesized using ball-milling and recrystallization treatment combined with the idea of soft template method.The influence of ball-milling conditions is systematically investigated,and it is found that when the mass ratio of zeolites to water is 3:20,the ball-milling speed is 150 rpm,and the ball-milling time is 8 h,the crystal size obtained is small and uniform,with clear boundaries and moderate relative crystallinity.On this basis,the changes of samples synthesized with different media in the recrystallization process are studied.Compared with the parent zeolite,although the crystallinity is reduced and some acidity is lost,the crystallite dimension is significantly reduced,and some mesopores are introduced at the same time.The acid sites have also been redistributed.In the tetralin hydrocracking,it also shows higher conversion rate,stability and BTX selectivity.Finally,a new strategy for synthesizing mesoporous nano-sized Beta zeolites in a two-phase system of toluene and water combined with hydrothermal method and growth modifiers is proposed.It is found that there is an optimal stirring time during the induction period,and the increase in the ratio of toluene to water and the decrease in the stirring temperature are conducive to obtaining Beta zeolites with higher crystallinity,smaller size,larger surface area,more mesopores,and stronger acidity.The introduction of HTS in the toluene phase and CTAB in the water phase will produce more mesoporous structures,and the particle size will be further reduced.However,the modification of silane will cause the crystallinity and the acidity of the zeolites to decrease,and The Beta-C zeolite optimizes acid properties under the condition of unchanged crystallinity.The tetralin hydrocracking performance of the samples obtained by the new method has been significantly improved.Among them,Ni-Sn/Beta-H shows the longest service life;Ni-Sn/Beta-C shows a BTX selectivity of 75.02%,which is attributed to its best pore structure and more medium-strong Br(?)nsted acid sites.