Research On Synthesis, Characterization And Catalytic Applications Of Acid-base Bifunctional Mesoporous Materials

Materials with only one type of active center usually just could be used in onecatalytic reaction for one time. However, many reactions in industry are finishedthough twice or multiple steps. Obviously, the design and synthesis of materialscontaining more than one type of active centers is necessary. A large number ofresearch groups have pursued new and creative approaches toward incorporatingmultiple functional groups onto heterogeneous catalysts, which can catalyze multistepreaction cascades in one system or work in a cooperative manner to alter thecharacteristics of a single reaction. Interest in bifunctional catalysts synthesized bycontrolling spatial arrangement of the functional groups has been growing recently.Particularly nanomaterials with coexisting two antagonist functions, such as acid andbase, are attracting chemists’ great attention. The reason is that these functionalitiesmay act in a cooperative way to provide reactivity and selectivity superior to thosecan be obtained over monofunctionalized materials or perform several steps in areaction sequence. Post-grafted route and “one-pot” synthesis are two major methodsused in preparation of acid–base bifunctional materials.Throughout the last decades, ordered mesoporous materials have received muchattention owing to their potential applications in catalysis and are usually chosen assupport to functionalize. Large-pore three-dimensional （3D） mesoporous silicas areamong the most interesting mesoporous materials discovered in recent years, and theyhave attracted much attention for potential applications requiring easily accessible, uniform, large pores. Recently, Kleitz et al. reported a new cubic mesoporous silicamaterial named KIT-6, which was prepared using triblock copolymer EO20PO70EO20（P123） as the mesopore-directing agent and n-butanol as additive under acidicconditions. KIT-6was reported to possess bicontinuous cubic mesostructure withlarge pore diameter and thick pore walls of about8and3.5nm, respectively. So,acid-base bifunctional mesoporous catalysts Al-KIT-6-NH₂in three dimensional （3-D）pore system containing Al and organic amine functional groups have been synthesizedand used as catalyst support for the Knoevenagel reaction.Among the mesoporous materials, SBA-15materials possess a high surface area（600-1000m2/g-1） and are formed by a hexagonal array of uniform tubular channelswith tunable pore diameters in the range of5-30nm. Owing to its thick walls （3-6nm）, SBA-15provides a good thermal and hydrothermal stability. Zeidan et al. havedescribed the formation of bifunctionalized mesoporous SBA-15containing primaryamine and different acid centers. Our group has also reported a series of acid–basebifunctional mesoporous materials, such as benzyl-APS-S-SBA-15andanthracyl-APS-S-SPA-15by controlling steric hindrance, and Al-MCM-41-NH₂. Theresultant materials exhibit excellent acid-base properties in various reactions such asMichael addition, aldol condensation, Henry condensation and Knoevenagelcondensation. However, one major drawback presented in acid-base bifunctionalmaterials is the potential mutual destruction of antagonist functions. One of thesolutions is to tune the strength and relative position of acid-base pairs. So, we use aneffective and convenient method （denoted “pH-adjusting”） to introduce high contentheteroatom Al into ordered mesoporous silica materials in strongly acidic media.Their catalytic activities for one-pot deacetalization-Knoevenagel reaction, one-potdeacetalization-Henry reaction and nitroaldol reaction were investigated.In order to explore the influence of acid-base strength matching on the acid-basecooperative catalysis, a series of acid-base bifunctionalized materials containingpyrrolidine or primary amine paired with various acidic supports were prepared withpost-grafted approach in the last chapter. These materials were used as catalysts in theone-pot deacetalization-Knoevenagel reaction, one-pot deacetalization-Henry reaction, Knoevenagel reaction and nitroaldol reaction. The catalytic results showed that theacid-base strength matching has a great impact on the catalytic behavior of acid-basefunctionalized materials in the reactions.