Preparation Of Modified β Zeolite And Its Catalytic Performance In Hydrocracking Of 1-methylnaphthalene To BTX

Benzene,toluene,and xylene（BTX）are important petrochemical raw materials,the production of BTX is insufficient in China and has been decreasing year by year.China has a large diesel production capacity,with the continued decline in diesel-gas consumption ratio,diesel capacity becomes excessive,the proportion of FCC diesel which is rich in aromatic hydrocarbons in diesel is about 30%.It is an ideal raw material to produce BTX by hydrocracking.Hydrocracking technology is an important oil reprocessing technology,which plays an increasingly important role in the modern petrochemical industry,the choice of catalyst is an important factor for the selectivity of target product.Beta zeolite has been widely studied in diesel hydrocracking due to its unique three-dimensional twelve-membered ring pore channel and good intermediate product selectivity.However,the conventional Beta zeolite are too acidic and often easily coke and cause reduced activity during hydrocracking reactions.In order to meet industrial requirements,they need to be modified.In this study,Beta zeolites were modified with zinc（Zn）and gallium（Ga）heteroatom metals by using impregnation,ion exchange and hydrothermal synthesis.The structure of the modified zeolites was studied.The results showed that the framework structure of Beta zeolites is not destroyed by the introduction of Zn or Ga,and the crystallinity of synthesized zeolite only slightly decreased.The Zn introduced by the ion exchange method has a better dispersion.The heteroatom Ga（DS）/Beta zeolite was successfully hydrothermally synthesized,and the optimal synthesis raw material molar ratios were:OH^-/Si O₂=0.35,TEAOH/Si O₂=0.2,and the crystallization time was 3 days.The effects of different modification methods on the metal morphology and acid distribution on the surface of zeolite were discussed.The results showed that the Zn is easily combined with B acid to form Zn OH⁺in Beta zeolites,which weakens the strength of B acid.During the growth of Ga（DS）/Beta zeolite,Ga³⁺is more distributed in the outer layer of zeolite crystal due to the larger hydrated ion radius.The strong acid of Beta zeolites decreases with the increase of Zn and Ga content,and the weak acid increases first and then decreases,and the overall acid content and acid strength decrease.Ga（DS）/Beta has a higher proportion of medium strong acid,but when the Ga₂O₃ content exceeds 2 wt%,the crystallinity is decreased because the structure is destroyed and the amount of acid sites is greatly reduced.Finally,1-MN was used as a model reactant,the effects of different content of modified metals and acids on the performance of the catalyst in hydrocracking reaction and coking reaction were investigated.The results showed that the modified catalyst has higher BTX yield and selectivity than the unmodified catalyst,2%Zn（IM）/Beta,3%Zn（IE）/Beta,1%Ga（IM）/Beta and 1%Ga（DS）/Beta showed the best balance of hydrogenation and acid centers under the same modification method,and the maximum BTX yield and selectivity increased much compared with the unmodified catalyst.Since the hydrogenation active center and acid center of the modified catalyst are more balanced,the reaction stability is better.At the same time,the amount of coke generated was reduced,and the rate of coke was reduced obviously.