A Study On Chemical Modification Of Active Sites In Ti-MWW And Allyl Chloride Epoxidation To Epichlorohydrin

Epichlorohydrin（ECH）is a volatile colorless oily liquid.As an important organic raw material and fine chemical,it has been widely applied in the production of epoxy resin,glycerol,and chlorohydrin rubber.At present,the main methods for producing ECH in industry are the chlorination of propylene at high-temperature,the glycerol method and the allyl alcohol epoxidation method.However,these processes have the problems of co-production of pollution,and low atomic economy.Thus,the green process to produce ECH is highly needed.The epoxidation of allyl chloride（ALC）to ECH directly catalyzed by Ti-MWW/H₂O₂ system has the advantages of no pollution,high selectivity and mild reaction conditions.It is one of the potential ways to effectively solve the above problems encountered in traditional processes.Herein,a slurry-bed reaction process using piperidine（PI）modified Ti-MWW as the catalyst and H₂O₂ as the oxidant was fabricated for the preparation of ECH via ALC epoxidation.The effects of reaction parameters on H₂O₂ conversion and ECH selectivity were explored in detail by simulating the operating conditions in industrial process.And the causes of catalyst deactivation were further explored.The strategy of adding ammonia was adopted to inhibit the occurrence of side reactions and thus prolong the durability of the catalyst according to the causes of deactivation.Zn-Ti-MWW was prepared successfully via modification of Ti-MWW with divalent zinc ion(Zn²⁺).The effects of different synthetic strategies,different ratios of silicon to titanium,and types of zinc salts on the performance of the ALC epoxidation were explored,and the influencing factors of the reaction were investigated in a batch kettle.Finally,the reasons for the improved performance of the Zn-Ti-MWW catalyzed by the ALC epoxidation were systematically explored.The main content and results were as follows:1.Ti-MWW-PI containing six-coordinated Ti species（Ti O₆）was successfully obtained via treating of Ti-MWW with PI aqueous solution.On this basis,Ti-MWW-PI/H₂O₂/TBA catalytic reaction system for the epoxidation of ALC showed excellent catalytic performance,and had been successfully applied to the slurry bed continuous reaction process.The slurry bed reaction process for the ALC epoxidation was investigated systematacially.The optimal conditions for the slurry-bed process of the Ti-MWW-PI/H₂O₂/TBA catalyzed ALC epoxidation were:the catalyst was 7 g,the temperature was s 313-323 K,the amount of ammonia was 4 ppm,and the molar ratio of ALC/H₂O₂ was 3;the mass ratio of TBA/ALC was 2;the WHSV of H₂O₂ was0.26 h^-1.And the reaction continued for 244 h with high ECH selectivity（>99.8%）and high H₂O₂ conversion（>97.0%）.The physicochemical characterization of the deactivated catalyst showed that coke deposition was the main reason for the catalyst deactivation.And the loss of PI molecules and the leaching of Ti species during the reaction also accelerated the catalyst deactivation.The activity of deactivated catalyst could be recovered obviously after a combination of calcination and PI treatment.2.Zn²⁺modified Ti-MWW greatly improved the activity and product selectivity of ALC and the catalytic stability was significantly improved by more than three times compared with the pristine Ti-MWW in the slurry bed reactor.The characterization technologies(XRD,SEM,UV-vis,FT-IR,Py-IR,N₂ adsorption,²⁹Si MAS NMR)were adopted to determine the reasons for the improvement of catalytic performance.Zn²⁺modification increased in the amount of Lewis acid sites from framework Ti and the acid strength,which was a main reason for the improvement of catalytic performance.Meanwhile,the amount of Si-OH in the zeolite was obvious decreased after the Zn²⁺modification,and the hydrophobicity of Zn-Ti-MWW was improved,which was another reason for the improvement of the catalytic performance of the catalyst.