Synthesis Of Beta Zeolite From Natural Minerals And Its Catalytic Performance In Hydrocracking Of Diesel Oil

In recent years,China's economy is developing rapidly in the trend of economic globalization and auto sales continue to increase.Therefore,the demand for gasoline will also become more vigorous.It is expected that the consumption of gasoline will continue to be higher than that of diesel by 2020,so diesel fuel ratio will continue to go down.Converting fluidized catalytic cracking?FCC?diesel,a relatively poor-qulity diesel in the diesel pool to gasoline via hydrocracking process will become a promising method for increasing gasoline product.The hydrocracking catalyst is one of the core of hydrocracking process.Hydrocracking catalysts are bifunctional catalysts,which are composed of zeolites with acid sites,such as Y-zeolite,Beta-zeolite,ZSM-5 and mordenite,and metal components with hydrogenation activity,such as Pt,Ni,Mo and W.Beta-zeolite is the only high-silicon Beta-zeolite with a three-dimensional twelve-membered ring pore system,which facilitates the diffusion of ring-opened products and reduces the chance of occurrence of secondary cracking reactions.And its unique topology strcture gives it good thermal stability,acid resistance and coking resistance.Therefore,hydrocracking catalysts prepared from Beta-zeolite have the advantages of good catalytic performance,high product quality,strong nitrogen resistance and slow deactivation rates.However,the traditional Beta-zeolite using chemical reagents as raw materials have only microporous structures.And its acidity is too strong,which may leads to excessive cracking of reactants and intermediate products,thereby reducing the selectivity and yield of middle distillates and deactivating the catalyst due to coking.In this paper,the synthetic method of Y-zeolite and ZSM-5 by using natural silica-alumina minerals as raw materials in our laboratory was used to prepare Beta-zeolite using natural silica-alumina minerals as raw materials,such as rectorite and diatomite.The effects of synthetic conditions and different silicon sources on the physical and chemical properties of pore structure and acidity of as-obtained zeolites were investigated.We prepared a bifunctional catalyst Ni-W/Beta-zeolite to catalyze tetrahydronaphthalene,the model compound of diesel,and evaluated its hydrocracking reaction performance.The results of the study are as follows.?1?The as-obtained Beta-zeolites were prepared by using rectorite and diatomite as as the source of all silica and aluminum.The effects of synthetic conditions on the physical and chemical properties of the obtained Beta-zeolites,such as the crystallinity and morphology,were investigated.The NiW/Beta-zeolites catalysts were prepared by NiW-doping to evaluate the performance of the hydrocracking reaction.The synthesized Beta-zeolite prepared via a hydrothermal reaction with the material ratio of7Na₂O:1K₂O:1Al₂O₃:50SiO₂:6.5?TEA?₂O:750H₂O at 150 ^oC for 60 h,has high crystallinity,and its relative crystallinity is up to 106%of the commercial Beta-zeolite.The crystal grains are spherical and grain size have an uniform distribution.The crystal grains have a large external specific surface area and a meso-micro-composite cell structure,as well as a higher amount of Lewis?L?acid and a lower Br?nsted?B?acid amount.When it is loaded with Ni and W and used in hydrocracking reaction,compared with the catalyst prepared by commercial Beta molecular sieve,it has higher conversion of tetralin and liquid yield,lower catalyst coking amount and gasoline yield.?2?The acidity of the Beta zeolite was modified by changing the feed ratio of Si to Al and the performance of the hydrocracking reaction was evaluated.The results show that when the molar ratio of SiO₂/Al₂O₃ is 40-60,a pure phase Beta zeolite can be synthesized using natural silica-alumina minerals as raw materials.And with the increase of feed Si to Al ratio of the Beta molecular sieve synthesized,the amount of L acid gradually decreased and the amount of B acid increased;when used in the hydrocracking reaction,the conversion of tetralin,the total liquid yield,and the yield of gasoline were gradually improved with the increase of the ratio of Si to Al in the feed.Especially when the Beta molecular sieve prepared with a molar ratio of SiO₂/Al₂O₃ of60 in the feed was used as a catalyst carrier,the conversion rate of tetralin and total liquid yield were respectively 78.8%and 91.3%,and the gasoline yield was 50.4%,which was increased by 9.2 percentage points than that of commercial catalysts.?3?The mesoporous templating agent cetyltrimethylammonium bromide?CTAB?was introduced into the Beta molecular sieve synthesis system to modify the pore structure of the Beta zeolite,and this Beta molecular sieve has higher specific surface area and mesoporous pore volume.The results showed that the BET specific surface area,external surface area,total pore volume and mesopore pore volume of the obtained Beta molecular sieves were gradually increased with the increase of the mesoporous templating agent dosage,while the mesopores pore size was gradually decreased.The results of hydrocracking evaluation showed that the conversion rate of tetralin and liquid yield of the obtained Beta zeolite catalyst increased significantly with the increase of the dosage of mesoporous templating agent compared to the sample without the addition of the mesoporous templating agent.While the gasoline yield increased slightly and amount of carbon deposit was obviously reduced indicating that the increase of the mesopore pore volume facilitates the mass transfer of the material in the reaction system and reduces the secondary cracking and polymerization of the intermediate product.?4?The Beta zeolites were synthesized using white carbon black,silica sol and silochrom as the supplemental silicon source instead of diatomite.The results show that compared with the Beta molecular sieve synthesized with diatomite as the supplemental silicon source,the Beta molecular sieves synthesized with the above three materials as the silicon source have lower relative crystallinity,lower BET specific surface area and external surface area,and belong to a typical microporous molecular sieve;while the amount of L acid is reduced and B acid is increased.The evaluation results of the hydrocracking reaction performance of the NiW/Beta catalyst prepared by using them as a carrier show that the catalyst of the Beta molecular sieve prepared with white carbon black as a silicon source has the conversion rate of tetralin and total liquid yield reached 76.7%and 88.9%respectively,and the gasoline yield reached 45.7%,which was 4.5%higher than the commercial catalyst.