Synthesis, Characterization Of Acid-base Bifunctional Mesoporous Materials And Catalytic Application In Aldol Condensation

Many fine chemicals are synthesized through the two-step or multi-step chemicalreactions, most of which require acidic or basic catalysis. If mono-functionalizedacidic or basic catalysts are used alternately in the chemical reactions, the processes offeeding, separating and recovering will be carried out repeatedly in each step, and it istime-consuming and tedious. Therefore, bifunctional or multi-functional catalysts aremuch more desirable in many reactions, since the combined functionalities may act ina cooperative way to improve the reactivity of the catalysts. With the acid-baseactivity centers being fixed on the surface of the solid firmly, the acid-base center cancarry out the acid-base cooperative catalysis in the heterogeneous catalytic system.However, the acid-base bifunctional catalysis can not be achieved in thehomogeneous system, because of losing catalytic activities by neutralizing reaction. Inaddition, in some organic reactions, even if the strength of the acid-base bifunctionalcatalyst is much lower than the acid or base strength of the acidic or basicmono-functionlized catalyst, the cooperative catalysis of acid-base centers showsbetter catalytic performance.As a very important type of organic reactions, the aldol reaction is widelyregarded as an important carbon-carbon bond forming reaction in organic synthesis.Acid, base or acid-base bifunctional catalyst can all be used in the aldol condensationreaction. In the aldol reaction between acetone and a variety benzaldehyde, theacetone transforms the enol structure in the acid-catalyzed conditions, and thehydrogenation of benzaldehyde as positive ions attack the enol structure; acetone lostα hydrogen as the anion and attacks the aldehyde group of the benzaldehyde in thebase-catalyzed conditions. The product of aldol dehydrates to α, β-unsaturatedcarbonyl product through the acidic or basic catalysis. Therefore, it indicates that theacid-base cooperation effect should be an important research topic in the aldolreaction.Under this background, a series of acid-base bifunctional mesoporous materialshave been successfully synthesized. The mesoporous structure and acid-base properties of the samples were studied by various characterization methods of X-raydiffraction, N₂adsorption-desorptio, transmission electron microscopy (TEM),scanning electron microscopy (SEM), FT-IR spectra, X-ray fluorescence spectroscopy,and so on. The acid-base bifunctional mesoporous catalytic materials have made greatprogress, but the acid-base mechanism of the cooperative catalysis has not yet beenclear. Therefore, the effect of strength matching of acid-base, basic sites protonated,the acid-base space distance and the surface silanols of the acid-base bifunctionalmesoporous materials on the catalytic performance and acid-base cooperativecatalysis have been investigated.In the third chapter, some immobilized carriers (SBA-15, Al-SBA-15,SBA-15-COOH and SBA-15-SO₃H) with different acid strength were used tosynthesize a series of acid-base bifunctional mesoporous catalytic materialsSBA-15-NH₂,Al-SBA-15-NH₂,NH₂-SBA-15-COOH and NH₂-SBA-15-SO₃H bygrafting basic group. Comparing with SBA-15-NH₂, the samples of Al-SBA-15-NH₂,NH₂-SBA-15-COOH displayed excellent catalytic activity value in the aldolcondensation reaction. It indicates that the selection of the strong acidic immobilizedcarriers to grafting basic groups can improve the catalytic properties of the acid-basebifunctional materials. However, the sample of NH₂-SBA-15-SO₃H is poorer thanNH₂-SBA-15-COOH because the strong acidic group and basic group neutralizedreadily in the same reactor. Therefore, the catalytic results showed that the acid-basestrength matching gave a great effect on catalytic performance.In the fourth chapter, a method is described for successful cohabitation oforganic acids and organic amines on MCM-41through oxidation and thermolysis. Forobtaining bifunctional material, the novel method can reduce mutual destructionbetween sulfonic acids and amines and protonation of amino group. Furthermore, themethylcyclohexyl group can provide more steric hindrance to turn the acid-basedistance. All these could be critical factors that make bifunctional material possesshigh activity for aldol condensation reaction.In the fifth chapter, acid-base bifunctional heterogeneous catalysts containingcarboxylic and amine groups, which were immobilized at the defned distance fromone another on the mesoporous solid were synthesized by immobilizing lysine ontocarboxyl-SBA-15. Proximal-C-A-SBA-15with a proximal acid-base distance wasmore active than maximum-C-A-SBA-15with a maximum acid-base distance in aldol condensation reaction between acetone and various aldehydes. It appeared that thedistance between acidic site and basic site immobilized on mesoporous solid shouldbe an essential factor for catalysis optimization.In the sixth chapter, amino-functionalized hollow nanospheres of silica(HNS-NH₂and HNS-NNH₂) were synthesized by using F127as surfactant in theneutral synthetic conditions. The samples of HNS-NH₂and HNS-NNH₂demonstratedexcellent catalytic activity in aldol condensation reaction between4-nitrobenzaldehyde and acetone in comparison to amino-functionalized MCM-41and SBA-15mesoporous silicas. The novel amino-functionalized hollow nanospheresof silica (HNS) with high surface area, large pore size and short diffusion lengths canfacilitate the molecule diffusion and improve catalytic reactivity. In addition, thecapping of silanols with trimethylsilyl groups resulted in the reduction of catalyticactivity, indicating the existence of cooperative effect between amine and silanol inthe condensation reaction.