Study On The Synthesis Of Ti-MWW And The Catalytic Properties In The Epoxidation Of Allyl Alcohol

Ti-MWW is more attractive by a comparison with those conventional titanosilicates (TS-1,Ti-MCM-41,et al ) in the epoxidation of linear functionalized alkenes, on which the epoxidation of allyl alcohol to glycidol is one of the most industrial promising processes for benign reaction conditions, high yields of the product and environment friendly. Up to now, the synthesis of Ti-MWW is costly and the preparation method is not convenient. Besides, the synthesis of B-MWW and Ti-MWW by the static crystallization method has not been reported. Based on the reasons mentioned above, the synthesis and catalytic performance of Ti-MWW have been studied. B-MWW was synthesized under static hydrothermal crystallization conditions using hexamethyleneimine(HMI) or piperidine(PI) as a template. The effect of boron to silica molar ratio, basicity, alkali cations, seeds, aging and reaction time on the crystallization was investigated. It was found that B-MWW with good crystallinity was obtained when the SiO₂/B₂O₃ molar rate was between 1.5～6 and pH value was between 9.0～10.3 in the gels. Formation of B-MWW was not affected by the alkali cations. Adding the seeds shortened the crystallization time and prolonging the aging time improved the purity of products. Ti-MWW was prepared by the acid treatment and the calcination on a corresponding lamellar precursor which was synthesized under dynamic and static hydrothermal crystallization conditions using HMI or PI as a template, and boric acid as a stucture-supporting agent. When the Si/B molar ratio of the gel was maintained at 0.75, the Ti-MWW precursor was obtained to have a Si/Ti molar ratio as low as 10 by the dynamic synthesis method, but as low as 20 by static one. The crystallization time was shortened by adding ERB-1 seeds and increasing the stirring rate. It was indicated by UV-Vis spectra characterization that the precursor always contains the tetrahedral Ti species and the octahedral Ti species regardless of the Ti content. Calcination of the precursor resulted in a partial condensation of the octahedral Ti species to form the anatase phase. The octahedral Ti species belong to a kind of extraframework Ti species, which was removed easily by the acid treatment in a short time. The calcined Ti-MWW contained the anatase when the acid-treated time was too long. The characteristic IR band at 960cm-1 was not observed for the Ti-MWW precursor, but the acid-treated and calcined samples shown the characteristic one. The catalytic properties of Ti-MWW and the reaction conditions in epoxidation of allyl alcohol to glycidol have been studied. It was found that Ti-MWW-PI synthesized under dynamic crystallization conditions has more effective catalytic performance. Low concentration of allyl alcohol helped to increase the reaction rate and high concentration of H2O2 could decrease the selectivity to glycidol using water as a solvent. Ti-MWW exhibited a conversion of 97% for allyl aolcohol and a selectivety of above 99% for glycidol when the allyl aolcohol epoxidation was carried out at 333K for 30 min with Ti-MWW of 0.05g, allyl aolcohol of 10mmol and H2O of 20ml.