Design And Preparation Of Hierarchical Beta Zeolites And Study On The Catalytic Performance Of Alkylation Of Benzene With 1-Dodecene

Linear alkylbenzene(LAB),an indispensable chemical intermediate in industry for the production of linear alkylbenzene sulfonate(LAS)with superior characteristics such as good biodegradability and solubility,is generated normally from alkylation of benzene with long-chain olefins.LAS is an essential synthetic surfactant with the largest output worldwide,which is employed extensively in household detergents,industrial wetting and detergency.However,currently the production of LAB in industry is still dominated by the traditional HF process,which exhibits poor selectivity toward LAB,especially the most desired and environmentally favorable one 2-LAB that has the best biodegradability and highest solubility among the several LAB positional isomers.At the same time,there some serious and obvious problems exhibit toward HF catalysts,such as equipment corrosion and separation difficulties,which do not meet the requirements of green chemistry.Design and develop highly efficient,stable and easily separated environment-friendly catalysts for preparation of LAB by alkylation of benzene with long-chain olefins is an important research goal.Zeolite,a regenerable solid acid with remarkably structural stability,high specific surface area and selectivity toward molecular size and shape,etc,is one of the most potential candidates for industrial application,which is widely used in adsorption,separation and petrochemical fields at present.This article is dedicated to develop new,efficient and stable zeolite catalysts for alkylation of benzene with 1-dodecene.This research is mainly about the design and synthesis of hierarchical Beta zeolites,to reveal the intrinsic relationship between catalytic performance and the characteristics of Beta zeolite about structure,content and distribution of acid sites and so on.The main research contents include the following three aspects:(1)For these problems that it is easy to fabricate and deposit coke in the inherent micropores when the alkylation catalyzed by zeolites involves macromolecules such as benzene and 1-dodecene,resulting in the deactivation of zeolite catalysts.And at the same time,alkylation of benzene with 1-dodecene catalyzed by zeolites commently requires harsh conditions(higher temperature and pressure,and high molar ratio of benzene toward1-dodecene).In this thesis,a series of hierarchical Beta zeolite catalysts were successfully prepared by a simple one-step hydrothermal method with organosilane TPOAC as the mesoporous structure directing agent.The research result showed that the obtained Beta zeolites exhibited excellent 2-LAB selectivity and high 1-dodecene conversion under mild reaction condition.Among these catalysts,the hierarchical zeolite Beta-5 showed the best catalytic activity.Under 80 ^oC and atmospheric pressure,when the ratio of benzene to1-dodecene was 8.75,the conversion of 1-dodecene reached 73%and the selectivity of2-LAB was 82%.It was also found that the micro-mesoporous hierarchical structure could improve the diffusion performance and catalytic stability of Beta zeolite,at the same time,improving the utilization of active sites in the pores.(2)The introduction of mesoporous structure in zeolites is a common and effective method to shorten the intracrystalline diffusion path and reduce the diffusion limitation.The structural characteristics of mesopores are closely related to molecular diffusion and catalytic performance.However,the mesopores introduced in zeolite by conventional methods are usually disordered and irregular,and the connectivity between mesopores and mesopores to catalyst surface is relatively poor.Aiming at this problem,a series of highly open-connected hierarchical Beta zeolites were fabricated by secondary-crystallization method.The result showed that this method could not only introduce a highly open-connected intracrystalline mesoporous structure,but also could adjust the content of Br(?)nsted acid sites regularly in these Beta zeolites,and the relevant experiment result proved that the Br(?)nsted acid in the Beta zeolite was the main active sites in this alkylation.In addition,the obtained Beta zeolite samples exhibited excellent mass transfer diffusion characteristics and catalytic stability.Moreover,these samples still kept superior 2-LAB selectivity.The catalytic activity of these Beta zeolite samples enhanced with the gradual increase of their porosity and Br(?)nsted acid content.Besides,the intrinsic factors of the decreased activity during cycling experiment were revealed visa series of relevant characterizations.(3)In order to further improve the catalytic activity of Beta zeolites obtained by secondary-crystallization method,a series of hierarchical Beta zeolites with different Si/Al molar ratios were prepared by increasing the aluminum source content in the mixed solution during the secondary-crystallization process,while keeping the concentration of organic base unchanged.The result showed that this method could adjust the content of Br(?)nsted acid sites in Beta zeolites,and the obtained samples maintained excellent pore structure characteristics.The total surface area of the obtained Beta zeolites gradually increased,whlie the mesopore pore size was not obviously affected by the different aluminum concentration during the secondary-crystallization process.Meanwhlie,the content of Br(?)nsted acid in Beta zeolites was increased to a certain extent,thus improving the catalytic activity and still exhibiting excellent 2-LAB selectivity.