Synthesis, Characterization And Catalytic Activity Of Bi-Si Mesoporous Molecular Sieve

Bi-Si mesoporous molecular sieves (Bi-MCM-41) were synthesized firstly by hydrothermal method used TEOS as silica source, Bi(NO3)3·5H2O as metal source, cetyltrimethylammonium bromide as template, the oxidation of styrene with hydrogen peroxide to benzaldehyde was investigated on it. The nature characterization of the Bi-Si mesoporous molecular sieves with different Si/Bi ratios was discussed based on the results of X-ray diffraction (XRD),nitrogen adsorption-desorption(BET),scanning electronmicroscopy(SEM), Fourier transform infrared spectroscopy(FT-IR), Ultraviolet-visible spectroscopy (UV-Vis) and thermogravimetric method(TG). It was indicated that Bi-Si mesoporous molecular sieves had high ordering structures of Mobile Crystalline Material-41 and Bismuth was high dispersed in the mesostructure frameworks.With the Bismuth content increasing, the value of d100 increased and the crystallite of Bi-Si mesoporous molecular sieves became disorderly. Bi-Si mesoporous molecular sieves had catalytical active in the oxidation of styrene to benzaldehyde with hydrogen peroxide as oxidant and methanol as solvent. The conversion of styrene increased with the increase of Bi content, while the selectivity of benzaldehyde decreased. The main effect of conversion was Bi content and the main effect of selectivity was the ordering of mesostructure. The effect of synthetic conditions, water concentration ,template concentration,pH value,crystallizing time,crystallizing temperature, on the structure and properties of Bi-MCM-41 were studied in detail. The molecular sieve sample showed higher crystallinity and catalytic activity in the conditions as follows, H₂O/SiO₂=150,CTAB/SiO2=0.2,pH=9,crystallizing temperature:120℃,crystallizing time:48h. The relation of the synthesize conditions to the crystallinity and catalytic activity of Bi-MCM-41 was also discussed. Bi-MCM-41 had high active in oxidation of styrene with hydrogen peroxide to benzaldehyde. The effect of solvents, reaction time, reaction temperature, oxidant concentration and catalyst concentration on oxidation of styrene to benzaldehyde were studied. It was found that in the reaction conditions as follows, catalyst 0.075g, styrene 1.5ml, methanol 25ml,H2O2(30%) 4.5ml,T=80℃,t=2h,the conversion of styrene was 43.1%, the selectivity of benzaldehyde was 73.5%, the yield of benzaldehyde reached 31.7%. The possible reaction mechanism was also speculated that one styrene molecular reacted with two hydrogen peroxide moleculars that contacted on two joined active sites alone. Bi-MCM-41 with different silica source were also synthesized. The samples which used Na2SiO3·9H2O and TEOS as silica source had higher catalytic activity than the sample with colloidal silica as silica source based on the characterization results of XRD,UV-Vis and the reaction of oxidation of styrene with hydrogen peroxide to benzaldehyde.