Modification Of Beta Zeolites For Tetralin Hydrocracking To Light Aromatics

There is excess production of light cycle oil（LCO）in China,and hydrocracking it into light aromatics（Benzene（B）,Toluene（T）,and Xylene（X））in short supply is economical.Tetralin is often chosen as a model compound for LCO to study the reaction mechanism and test catalyst performance due to the complex composition of LCO.The main reaction of tetralin hydrocracking to BTX is a series reaction that includes the isomerization of tetralin to methylindan,the further ring-opening cracking of methylindane to BTX,and the deep hydrocracking of BTX to low-carbon alkanes.Obviously,because BTX is an intermediate product of this series reaction,BTX deep hydrocracking definitely occurs on the catalyst,resulting in reduced BTX yields.To solve this problem,we carried out the following research works.Firstly,because the hydrogenation reaction is a strongly exothermic process,the thermodynamic analysis showed that the deep hydrogenation of BTX is weakened by increasing the reaction temperature（the industrial reaction temperature is lower than420℃）.In this study,the effects of different reaction temperatures（420°C,450°C,and480°C）on the catalytic performance of tetralin over catalyst for hydrocracking of BTX were investigated.The catalytic performance results showed at the weight hourly space velocity（WHSV）corresponding to the maximum BTX yield at 420℃was 2.5 h^-1,while the WHSV corresponding to the maximum BTX yield at 480℃was 8 h^-1.It was shown that the production cycle at 480℃was shortened by more than 2 times in comparison with 420℃.In addition,the maximum BTX yield increased from 44%（at420℃）to 49%（at 480℃）.Therefore,increasing the reaction temperature is a simple and effective way to improve the activity and selectivity of the hydrocracking catalyst.Secondly,the calcination temperature of Beta zeolite was controlled to partially remove the template agent in the sample and then acid treatment of it was used to selectively remove the aluminum on the external surface of Beta zeolite.The surface Si O₂/Al₂O₃on selectively dealuminated Beta samples was higher than that of conventionally dealuminated samples under the same bulk Si O₂/Al₂O₃,and the hydrogenation activity of the selectively dealuminated Beta zeolite catalysts was decreased compared with conventionally dealuminated samples after the hydrocracking catalysts was prepared by loading WO₃.The catalytic performance showed that the highest yield of BTX over selectively dealuminated zeolite catalysts was about 5 wt%higher than that of conventionally dealuminated zeolite catalysts.Besides,because the low Al content on the external surface of the selectively dealuminated Beta samples decreased the amounts of acid sites on the external surface of the Beta zeolite,the carbon deposition on selectively dealuminated zeolite catalysts was 3 wt%less than that on the conventionally dealuminated zeolite catalysts.Thirdly,a series of Ga-Al-Beta zeolites with different Ga contents were prepared with the isomorphous substitution method.It was found that the acid strength and acid content of the samples gradually decreased with the increase of Ga content,while the crystalline phase and pore properties of Ga-Al-Beta zeolites did not change significantly.Furthermore,their catalytic performance for hydrocracking of tetralin was investigated,and the results showed that the highest BTX yield over the Ga-Al-Beta catalyst was about 5 wt%higher than that over the Al-Beta catalyst.The reduction of Ga-Al-Beta zeolite acid strength and acid content led to the cracking activity of the catalyst being moderately weakened,and the conversion of alkylbenzene with side chain into benzene（the final product of dealkylation of monocyclic aromatic hydrocarbons）is suppressed,and the yields of toluene and xylene are increased.Therefore,the yield of BTX is increased under the same depth of reaction.In addition,the carbon deposition on the Ga-Al-Beta sample is less than that on the Al-Beta sample under the same reaction time.Finally,two hierarchical Beta zeolites were prepared with the surfactant-templated method（cetyltrimethylammonium bromide（CTAB）as surfactant）,and the mesopore volume of hierarchical Beta zeolites increased by at least 1 time.WO₃was loaded on the two hierarchical Beta zeolites to prepare hydrocracking catalysts.The performance of the catalysts in the hydrocracking of tetralin was investigated.Based on the experimental results,the following two conclusions were obtained.（1）The activity and BTX yield of the catalysts prepared from the hierarchical Beta zeolites are higher than those of the parent zeolites.（2）The hierarchical Beta zeolites only contain disordered mesopores,the activity and the selectivity were improved with the increase of the amount of mesopores.Furthermore,the BTX yield of the hierarchical Beta zeolite containing ordered mesopores was 4 wt%higher than that of the hierarchical zeolite only containing disordered mesopores,which indicates that improving the mesoporous ordering can further increase the activity and selectivity of the catalysts.