Preparation, Characterization And Catalytic Performance Of Bifunctional Catalyst Au/Ti-HMS

Gold catalysis gradually becomes a focal research all over the world. Gold nanoparticles supported on mesoporous molecular sieves MCM-41 and SBA-15 have been prepared successfully. But there are few reports on HMS supported gold catalysts. Oxidative desulfurization of bulky sulfur compounds in diesel over Ti-HMS/H2O2 has been made a considerable progress. However, the continuous decomposition of commercial H2O2 during heated reaction seriously cuts down its effective availability. To resolve the above problems, a bifunctional catalyst Au/Ti-HMS is synthesized by a series of methods, and is used in the oxidative desulfurization by in-situ-H2O2 directly synthesized from H2/O2. The main contents of this paper are as follows.Au/TS-1 and Au/Ti-HMS prepared by deposition-precipitation (DP) method were used in the direct synthesis of H2O2 from H2/O2 to investigate the similarities and differences of their catalytic performance. It is shown that Au/TS-1 obtains a higher productivity of H2O2 in water solvent, whereas Au/Ti-HMS is more suitable to synthesize H2O2 in methanol solvent. The Au0 species act as the active sites during synthesis of H2O2, which is confirmed by H2-TPR technique. Both Au/TS-1 and Au/Ti-HMS obtain lower decomposition rate and hydrogenation rate of H2O2 in methanol solvent than in water. The hydrogenation of H2O2 occurs much more spontaneously than the decomposition.The ordered mesoporous structure of the support was seriously influenced during the DP preparation of Au/Ti-HMS. Therefore, Au/Ti-HMS was synthesized by an in situ method, and the effects of different preparation method on the structure of support and the catalytic performance of catalyst were investigated. It is shown that Au/Ti-HMS (In situ) maintains the wormlike mesoporous structure and the framework Ti successfully. It achieves effective in-situ-H2O2-oxidation of bulky sulfur compounds in model oil and especially obtains 93% removal rate for 4,6-dimethyldibenzothiophene. Au/Ti-HMS (NH3 DP) still possesses intact mesopores of HMS in spite of the decrease of mesopores ordering. In addition, it possesses smaller gold nanoparticles and achieves more effective removal of benzothiophene and dibenzothiophen than Au/Ti-HMS (In situ). For Au/Ti-HMS (Urea DP), the mesoporous structure of the support is seriously damaged in hydrothermal condition. It can only achieve the removal of benzothiophene and dibenzothiophen due to the existence of framework Ti, whereas, it has no catalytic activity for the removal of 4,6-dimethyldibenzothiophene. The formation of H2O2 and in-situ-H2O2 oxidation occur as a simultaneous process. Compared with Ti-HMS/commercial H2O2 method, the in-situ-H2O2-ODS technique achieves effective utilization of H2O2, zero emission and process simplification.Au/Ti-HMS(In situ) showed a poor reusability due to the loss of gold active sites during reactions. Therefore, an in-situ-reduction method was adopted to synthesize Au/Ti-HMS. It is shown that the sample with 1.25:1 of NaBH4/Au molar ratio and addition of HC1 possesses mesoppres with high order, as well as small and uniformly dispersed gold nanoparticles which were strongly embedded between the channels of the support. The catalyst exhibits attractive stability and recycled performance in the direct synthesis of H2O2 and in the in-situ-H2O2 oxidation of benzothiophene and dibenzothiophen.