Hierarchical Beta Zeolite

Beta zeolite,a large apertures zeolite with three-dimensional architecture composed of interconnected 12-ring channels,is a promising catalyst for petrochemical industries.Beta zeolite has shown excellent properties in petroleum refining and petrochemical process because of its unique properties,such as strong Br?nsted acidity amount,high Si/Al ratio,favorable(hydro)thermal stability and high surface area.However,the utilization of conventional microporous Beta zeolites as catalyst is limited by steric hindrance and severe diffusion limitation that prevent macromolecules from passing through their internal pores and therefore deactivate easily by the carbon deposits.An effective way of solving this problem is the conception of hierarchical Beta zeolite which allows the transformation of macromolecules and reduces diffusion pathlength.Although there are many methods for preparing hierarchical Beta zeolites,they are still limited to laboratory scale and difficult to be realized in industrial scale,because of complicated processes or exorbitant materials.Therefore,to truly realize the application of hierarchical Beta zeolite in the future,it is necessary to develop an economical method with the requirements of stable synthesis results,low cost,and simple procedure to perform to prepare hierarchical Beta zeolite with superior performance.In this paper,the hierarchical Beta zeolites were synthesized in a hydrothermal system by the addition of cationic surfactant CTAB(Cetyl Trimethyl Ammonium Bromide)into the gel of conventional microporous Beta zeolites synthesis.The method to prepare hierarchical Beta zeolite with CTAB as template was established by exploring of the effects of pre-crystallization time and ethanol,resulting in a method with a simple procedure that does not require pre-crystallization and ethanol.Hierarchical Beta zeolites that present ellipsoid aggregates composed of single crystal nanoparticles and abundant intergranular pores were synthesized within specified limits by changing the parameters of H₂O/(SiO₂+Al₂O₃),Na₂O/(SiO₂+Al₂O₃)and TEAOH/(SiO₂+Al₂O₃)or using alkyl trimethyl ammonium bromide with different chain lengths.With the retention of high microporous specific surface area as a precondition,the external specific surface area in the range 80 m²·g^-1 to 290 m²·g^-1 and the mesoporous volume in the range 0.23 cm³·g^-1 to 0.54 cm³·g^-1 of hierarchical Beta zeolites were adjusted by controlling the parameters,meanwhile,the size of single crystal nanoparticles and ellipsoid aggregates were also adjustable.Consequently,the purpose to regulate the pore structure and morphology of hierarchical Beta zeolites was realized.Notably,the method above is easy to operate with less expensive materials to obtain the products with adjustable properties,basically meets the demand of premise for industrial production.XRD,N₂ adsorption,SEM and TG were used to analyze the function of CTAB during the crystallization process of hierarchical Beta zeolite by investigating their crystallization kinetics and varying the Si/Al ratios and CTAB concentrations in the gel.During the nucleation period,positively charged CTA⁺existing as the form of spherical micelles in solid attracted by negatively charged Al(OSi)₄^-and Si-O^-,therefore,the interconnection of tetrahedrons were impeded resulting in the generation of more crystal nucleus due to steric hindrance.In the process of crystallization,Si-O^-gradually formed electroneutral silicon oxygen tetrahedron in the skeleton,a large number of CTAB micelles separated from the solid,the residual CTAB partly attracted with Si-O^-on the outer surface of the zeolite in the form of CTA⁺,and the other part bonded with Al(OSi)₄^-.The CTA⁺bonded with Al(OSi)₄^-and Si-O^-on the surface of zeolite hindered the aggregation of nano particles which formed intergranular mesopores after calcination at high temperature,therefore,mesopores derived from the accumulation of zeolite nanoparticles.The other part of CTA⁺combined with internal Al(OSi)₄^-in the direct channel of Beta zeolite in monomer form,and competed with TEA⁺replacing part of TEA⁺to play as template.The value of external specific surface area has a close association with the Si/Al ratio and CTAB concentration in the gel and is determined by the amount of adsorbed CTAB micelles.Therefore,the pore structure and morphology of zeolite synthesized with different Si/Al ratios could be regulated by changing the CTAB concentration in a certain range.The effect of CTAB in its suitable range on the properties of hierarchical Beta zeolites were investigated in the range of Si/Al ratios 10 to 200 in the gel.As the Si/Al ratios in the gel are in the range of 10 to 100,the adjustable range of CTAB/SiO₂ decreases with the increase of Si/Al ratio in the gel,and the maximum external specific surface area of the prepared hierarchical Beta zeolites also gradually decreases,moreover,the morphology changes from the aggregates of nano particles to a dense octahedral structure.When the Si/Al ratio in the gel is 150,the hierarchical Beta zeolite shows dense configuration of particles and appears amorphous lamellar structure.The change of Si/Al ratio in the gel causes the changes of Si/Al ratio and Al distribution for hierarchical Beta zeolite,and consequently the change of its acidity.As the initial Si/Al ratio in the gel increases,the bulk Si/Al ratio of product increases,then the total acid amount decreases.The gradual raise of framework Si/Al ratio for product gives the results of a decrease in the total Br?nsted acid amount and an increase in the strong Br?nsted acid proportion.For hierarchical Beta zeolite prepared with the Si/Al ratio of 100 in gel,a lot of four-coordinate framework aluminum Al(IVb)appeared,which provides acid finitely,following a sharp decrease in the of Br?nsted acid amount.The decrease of non-framework aluminum content with the increase of Si/Al ratios from 10 to 100,particularly for the five-coordinate non-framework aluminum Al(V),brings the result of a gradual reduction in the L acid amount.Three probe molecules mesitylene,1,2,3,4-tetrahydronaphthalene and 1,3,5-triisopropylbenzene of different molecular sizes were selected to study the adsorption and diffusion properties of hierarchical Beta zeolites.Compared with the conventional Beta zeolite,three probe molecules all revealed higher adsorption capacity and faster diffusion rate in the hierarchical Beta zeolites in the adsorption/desorption experiments.Three probe molecules,methyl cyclohexane,mesitylene and 1,3,5-triisopropylbenzene of different molecular sizes were selected to investigate the acidity of the hierarchical Beta zeolites.The investigation of the catalytic conversion of methyl cyclohexane and mesitylene are effective methods to characterize the acidity and acid distribution in or outside the pore of hierarchical Beta zeolites,respectively.Besides,the cracking of 1,3,5-triisopropylbenzene can be used to estimate the properties of accessible acids.With the introduction of mesopores into Beta zeolites,in the catalytic reaction of methyl cyclohexane,the increases of conversion,yields of cracking products and especially for isobutane,indicate that the enhance of accessible acid,Br?nsted acid and strong Br?nsted acid amounts in hierarchical Beta zeolites,respectively.In the catalytic reaction of mesitylene,the improvement of conversion,and the reduction of I/D value and 1,2,4/1,2,3-TMB value illustrate that the increase in the accessible acid amount,the acid density of reaction space and pore or pore mouth,and the diffusion ability,respectively.In the cracking of 1,3,5-triisopropylbenzene,the improvements of conversion and selectivity of deep cracking products elucidate the greater quantities of accessible acid and the accessible strong acid.In a pilot-scale reactor,hierarchical Beta zeolite was successfully synthesized in a dynamic hydrothermal system,demonstrating that the industrialization of hierarchical Beta zeolite is achievable.The one that synthesized in pilot scale shows high external area and mesoporous volume without distinct loss of micropores which is approximate to that prepared in laboratory scale.The gratifying results confirm that the method of synthesizing hierarchical Beta zeolite is suitable for industrial production because it is easy to operate,and the properties of product are immutable in the process of scaling up.The properties of hierarchical Beta zeolite prepared on a pilot scale were studied by diffusion test and hydrocracking of 9,10-dihydroanthracene.The introduction of mesopores is favorable for shortening the diffusion path length and increasing the diffusion rate,thus increasing the mass transfer rate of adsorbate molecules.The hydrocracking of 9,10-dihydroanthracene of hierarchical Beta zeolites shows that the additional mesopores are beneficial for the accessibility of acid sites and the mass transfer of products.In the VGO hydrocracking on a simulated industrial unit,catalyst based on hierarchical Beta zeolite synthesized in pilot-scale shows enhanced activity and middle distillate yield compared with the reference catalyst,because the presence of mesopores enhance the accessibility of acid sites.In addition,the improved ring-opening ability of hierarchical Beta zeolite leads to increases in the smoke point of jet fuel and the freezing point of diesel fuel and decrease in the BMCI value of tail oil.In conclusion,hierarchical Beta zeolite is demonstrated to be a greatly potential candidate in acid-catalyzed reactions involving bulky molecules.