Modulating The Morphologies Of Beta Zeolite Crystals For The Catalytic Application In Friedel-Crafts Acylation Reaction

Aromatic ketones are important intermediates in the field of fine chemical synthesis,and are widely used in the production of pharmaceuticals,fragrances,dyes,pesticides and special engineering plastics.Friedel-Crafts acylation reaction is an important method for the production of aromatic ketones,which is usually catalyzed by homogeneous catalysts like AlCl₃.The main disadvantage of the AlCl₃-catalyzed process is that more than the stoichiometric amount of catalyst is required in order to obtain a desirable yield,and multi-step treatments including washing with water and alkaline solution are required for product separation,which can bring about serious problems like higher consumption of catalysts and producing a large amount of liquid waste.Therefore,considerable efforts have been devoted to developing solid acid catalysts with high activity and recyclability for the purpose to replace the traditional homogeneous catalysts.Among the investigated solid acid catalysts,Beta zeolite with unique three-dimensional 12-membered ring channels and strong acidity shows relatively high catalytic activity and recyclability for various Friedel-Crafts acylation reactions.However,Beta zeolite deactivates rapidly during the acylation reaction process,thus requiring frequent regeneration by high-temperature calcination and leading to serious limitation for industrial application.Some recent literature works revealed that Beta zeolites with nano size and/or hierarchical porosity showed enhanced ability in mass transportation efficiency.This is the key to reducing the adsorption capability of the ketones and avoiding the rapid accumulation of carbonaceous deposits on the the surface/channel of the zeolites,and thereby improving their catalytic activities and lifetimes.However,some problems are still present,such as a large amount of expensive organic structure directing agents or mesoporous templates are often required for the successful synthesis of nanosized Beta zeolites and hierarchical Beta zeolites.Moreover,for the nanosized zeolite crystals,they commoly meet serious trouble in the sepration process with products upon using in a slurry reaction system;while the drcease in the structure stability of the hierarchical zeolites usually occurs due mainly to the presenece of some structure defects in zeolite crystals derived from the formation process of mesopores and macropores.In this dissertation,we devoted to the design and preparation of zeolite Beta catalysts with excellent catalytic performance for Friedel-Crafts acylation,and systematically studied the synthesis of Beta zeolites featured with different morphologies and porosities,and their catalytic performance.The morphology,crystal size,porosity and acidity of the zeolites were tuned by introducing a suitable amount of the easily available additives,and by adopting different synthesis method and optimizing the synthesis parameters.The catalytic performance of the resultant Beta zeolites were studied for the Friedel-Crafts acylation of anisole with acetic anhydride.Based on a variety of characterization results,the influence of synthesis parameters on the structure and physicochemical properties of the zeolites were investigated,and the relationship between these physicochemical properties and their catalytic activities/stabilities were also studied.The main research results of this dissertation are as follows:（1）A common non-ionic surfactant of Triton X-100（TX）was used as an additive in the hydrothermal synthesis of Beta zeolites with small crystal size（Beta-nTX,n represents the molar ratio of TX/SiO₂）.Compared with the conventional Beta-C zeolite（2μm）,the addition of a very small amount of TX can reduce the crystal size to about 300～600 nm,and the resultant Beta-TX zeolites possess relatively high crystallinity.Investigation of the crystallization process reveals that a suitable amount of TX can effectively improve the nucleation,and shorten the induction period,and consequently reduce the crystallization time from 96 h（for Beta-C without TX）to 24 h.The H-form Beta-0.005TX zeolite shows much higher catalytic activity and stability in the acylation of anisole with acetic anhydride than the conventional Beta-C zeolite.The excellent catalytic performance should be mainly related to the relatively small particle size of Beta-nTX zeolites,which can reduce the diffusion limitation and improve the accessibility of catalytically active acid sites to the reactents.（2）Beta nanozeolite aggregates with hierarchical porous structure were synthesized via steam-assisted conversion（SAC）of dry gel in presence of seeds and a cationic polymer of polydiallyldimethylammonium chloride（PDAD）.The addition of suitable amounts of PDAD and Beta zeolite seed could facilitate the fast nucleation and the formation of ultra-small nanocrystals（around 10 nm）,which are further assembled into micron-sized Beta zeolite aggregates（S-Beta）.By optimizing various synthesis parameters,such as the alkalinity,the contents of PDAD and the zeolite seeds,and the crystallization conditions（temperature,water/dry gel ratio）,the formation of impurity MOR phase could be inhibited,while the generation of Beta zeolite aggregates with micron size could be achieved within 48 h.Compared with the conventional Beta zeolite,S-Beta aggregates show much higher catalytic activity and lower deactivation rate in the Friedel-Crafts acylation of anisole with acetic anhydride.The excellent catalytic performance of S-Beta should be mainly attributed to the enhanced mass transport ability for reactants,as well as the mild acidity that can effectively inhibit the formation of carbonaceous deposits resulting from the multi-acylation reactions.（3）Micron-sized Beta zeolite crystals（～2μm）with hierarchical porosity（Beta-H）were hydrothermally synthesized by introducing a certain amount of polydiallyldimethylammonium chloride（PDAD）into the initial synthesis system.By optimizing the synthesis conditions,the resultant Beta-H zeolite possesses high crystallinity,interconnected intracrystalline mesopores and relatively lower density of strong acid sites.Compared with the conventional Beta zeolite,the Beta-H zeolite exhibits much higher catalytic activity and durability（against deactivation）for the Friedel-Crafts acylation of anisole with acetic anhydride in a fixed bed reactor.The conversion of acetic anhydride reaches 71%after 10 h on stream,much higher than that of the conventional Beta-C zeolite.The 4 consecutive catalytic runs（regenerated by calcination after each run）show that the catalytic activity of Beta-H is well maintained,demonstrating the excellent structural stability and good reusability of the zeolite.More significantly,the deactivation rate of Beta-H could be further inhibited after steam treatment of Beta-H zeolite at 700 ℃ for 2 h.The steamed Beta-H exhibits 94%conversion of acetic anhydride after 10 h on stream,much better than the nanocrystal Beta zeolites or hierarchical Beta zeolites reported in literatures.The excellent catalytic activity and stability of Beta-H zeolite could be mainly assigned to the existence of abundant structurally stable intracrystalline mesopores,which facilitate the diffusion of reagents,and may also be correlated to the relatively low strong acid density which can effectively inhibit the formation of bulky multi-acylation products.