Synthesis And Catalytic Properties Of Bifunctional Mesoporous SiO₂ With Isolated Acid And Base Sites

Since the report of MCM-41, ordered mesoporous materials had attracted much attention. In the field of catalyst, ordered mesoporous material was the ideal support of heterogeneous catalyst because of their outstanding structure. In order to catalyze multistep reactions, different catalytic active sites were incorporated into the same mesoporous material, which made it the multifunctional catalyst. However, the acidic and basic sites must be spatially separated to avoid the mutual neutralization in the synthesis of acid-base bifunctional catalyst.In this paper, double-shelled hollow mesoporous SiO₂ nanosphere was fabricated as acid-base bifunctional catalyst with polystyrene latexes and CTAB as dual-template via a two-step co-condensation method. The polystyrene acted as the core template to generated hollow void, while the CTAB served as a co-template to structure the formation of mesopore in the shell. Moreover, the design of double-shelled structure made it possible for the acidic site and the basic site to locate in different shells, which made them isolated in space. And the hollow void within the mesoporous sphere enlarged the specific surface area and shortened the pathway of mass transfer for reactants and products out/in the catalyst. Then the samples were characterized by XRD, SEM, TEM, FT-IR spectroscopy, N2 adsorption-desorption isotherms measurement, TGA-DSC analysis and solid NMR. The measurement results confirmed that the as-prepared SiO₂ nanosphere possessed well-defined hollow structure, the wall were consist of two different mesoporous shells and acid-base active sites were successfully incorporated into the mesoporous shells. Furthermore, the catalysts with different mole ratios of silicon and aluminum and monofunctional catalysts were also prepared by the similar method.In order to evaluate the catalytic performance, the as-prepared mesoporous SiO₂ were used to catalyze the Deacetalization-Knoevenagel cascade reaction at 110 ℃ and the action of H2 O in cascade reaction was also investigated. The catalytic results showed that only bifunctional mesoporous SiO₂ could effectively catalyze the cascade reaction and the existence of H2 O could promote the acid-catalyzed reaction and had less affection on the base-catalyzed reaction. The sample named Al-SiO₂@SiO₂-NH2-25 showed the highest catalytic activity under the condition mentioned above. The benzaldehyde dimethylacetal was all converted into benzaldehyde in 20 minutes and the yield of final product reached to 97.1 % in 40 minutes. Meantime, the results of leaching and recycling experiments indicated that the catalytic performance was very stable. In addition, the cascade reaction of Deacetalization-Henry between benzaldehyde dimethylacetal and nitromehane was also demonstrated over various bifunctional catalysts at 90 ℃ under nitrogen. The reactant was all converted into the target product and no intermediate was detected with Al-SiO₂@SiO₂-NH2-25 as catalyst in 14 h, which is the best result up to now.