Synthesis, Characterization And Catalytic Activity Of Acid-base Bifunctional Mateirals

Some research groups have been interested in the design and synthesis of catalysts thatcontain multiple types of active centers. Particularly nanomaterials coexistence of organicacid and base functionalities are attracting chemists' great attention because thesefunctionalities may act in a cooperative way to provide reactivity and selectivity superior towhat can be obtained from monofunctionalized materials or could perform one-pot cascadereactions. Post grafting and direct co-condensation procedures have been used for thepreparation of these bifunctionalized catalysts. SBA-15materials are usually chosen assupport to functionalize because of its relatively good hydrothermal stability and larger porediameter permitting easy diffusion of bulky reactants and products. As the earliestmesoporous silica materials, MCM-41materials were also functionalized to get acid-basebifunctionalized catalysts in direct synthesis. In1995, Tanev and Pinnavaia havesuccessfully synthesized mesoporous material denoted as hexagonal mesoporous silica(HMS) which possesses worm holelike or sponge-like framework structures, large surfacearea, narrow pore size distribution and can be prepared by neutral assembly pathwaysbetween neutral primary amine surfactant and neutral inorganic precursor. Compared withthe other kinds of mesoporous siliceous materials possessing long-range hexagonalframework structures, HMS silicas possess smaller particle sizes, along with defects inchannel packing of HMS silica molecular sieves, which result in complementary texturalmesoporosity providing better transport channels for reactants to the active sites and better diffusion channels for products to move out. In addition, HMS silicas exhibit moreextensively cross-linked frameworks and thicker framework, contributes to superior thermalstability. Besides these advantages, HMS can be synthesized by using cheaper surfactanttemplate under mild synthesis condition and the template can be simply removed by usingsolvent such as acidified water and ethanol. These properties make HMS attractive forapplication in catalysis.Grafting method has been used for the preparation of these bifunctional materials. In thethird chapter, we prepared two samples of aminopropyl-functionalized silica (APS) bygrafting an organosilane precursor3-aminopropyl-triethoxysilane (APTES) onto the SBA-15in ethanol or toluene and investigated their catalytic activity for two importantCarbon–Carbon Bond Forming Reactions: Knoevenagel condensation and Claisen–Schmidtcondensation. Silylation and H/D isotopic substitution technique are employed to investigatethe role of surface silanol groups in the catalytic process. The material grafted in ethanolwith less organoamines showed higher catalytic activity for Knoevenagel condensation andClaisen–Schmidt condensation than the material grafted in toluene. The surface silanolgroups were found to be responsible for the enhanced catalytic activity in Carbon–CarbonBond Forming Reactions.Direct co-condensation has been used for the preparation of these bifunctional materials.In order to avoid mutual destruction of antagonist functions, we reported a method tosynthesize MCM-41or HMS that contains organic amines and organic acids throughprotection of amino group in the fourth chapter. The resultant catalysts have excellentacid-basic properties exhibiting high activities for tandem C-C bond forming reactions. Thefree acid and base centers, the proper acid-base distance, along with better transport channelsfor reactants and products in HMS type catalysts account for the excellent catalyticproperties. We expect that this bifunctionalized catalysts can be applied to other variousone-pot incompatible systems and believe that a further detailed understanding of thebehavior of the coexisting acidic and basic sites in a single material can open a door todesign efficient catalysts for multifunctional applications. We also expect that thisbifunctional material can serve as an efficient heterogeneous catalyst for other variousone-pot incompatible systems. We believe that a further detailed understanding of thebehavior of the coexisting acidic and basic sites in a single material can open a door to newroutes for the rational design of morphology-controlled novel nanomaterials formultifunctional applications. Our aim in the last chapter is to describe a simple and effective method for thepreparation of efficient acid-base bifunctional materials possessing large special surface areaand high pore volume. The materials were used as catalysts in one-pot cascade reactions andthe acid-base reaction sequences were successfully promoted. An acid-base cooperativemanner to catalyze nitroaldol reaction is also found. We expect that this bifunctional materialalso can serve as an efficient heterogeneous catalyst for many other various one-potincompatible systems and catalytic nucleophilic reactions, especially including largemolecules.