The Research On The Synthesis Of Acid And Base Bifunctional Mesoporous Materials

According to International Union of Pure and Applied Chemistry (IUPAC) definition, porous materials can be classified into three types according to pore size: microporous materials are characterized by pore size of <2.0 nm, mesoporous materials are characterized by pore size in the range of 2-50nm, and porous materials with pore size> 50.0 nm can vest in macroporous materials. In general,"molecular sieve"is a genetic terms of the micro-and mesoporous materials, and macroporous materials have lost the ability of sieving capacity because their pore sizes are too large. Molecular sieves can be divided into another three types according to pore arrangement and ordering of framework: (1) microporous crystalline material. For example, zeolite is microporous crystalline material with rules of pore structure and ordering of skeleton atoms; (2) MCM-41 and SBA-15 have high surface area, ordered mesoporous pore size distribution and large pore volume, but their skeleton atoms was disorderly arranged; (3) HMS-type,MSU-type materisl have uniform pore size distribution, channel array and composition of the the skeleton atoms arranged disorderly, such type of material is conducive to catalysis, adsorption process of mass transfer because its one-dimensional pore channel has crossed into three-dimensional structure each other.Microporous materials can only be applied in the area related to small molecules; they can not effectively deal with large molecules due to the limitation of pore size. Therefore, investigating porous materials with larger pores becomes very necessary. Since the M41S family of mesoporous molecular sieves was synthesized, many types of mesoporous molecular sieves have been developed. Mesoporous materials with high specific surface areas and narrow pore size distributions are of great interest for potential applications in many fields such as separation, catalysis, biology and electronics.Mesoporous SBA-15 was firstly synthesized using polyoxyethylene -polyoxypropylene-polyoxyethylene triblock copolymer (P123) as template agent under acidic condition by zhao et al. in 1998. Triblock copolymers are an interesting class of structure-directing agents and attracted much attention in recent years because template agent with large PEO segments display excellent interfacial stabilization properties and are low-cost, nontoxic, biodegradable, and adapt to the needs of economy. Solid catalysts can provide numerous opportunities for recovering and recycling catalysts from reaction environments.Surface-modified mesoporous materials with various active sites have been extensively investigated in recent years because functional modification permits tailoring of the surface properties for numerous potential applications in the fields of heterogeneous catalysis and adsorption–separation. A great of modification techniques such as element-substituted, loading, inorganic-organic hybrid method has become paramount for its application in the field of mesoporous molecular sieves. Among the known mesoporous materials, hydrothermal stable SBA-15 is highly ordered mesoporous silica with two-dimensional p6mm symmetry, which possesses hexagonal arrays of uniform pores, large special surface area, and high pore volume, which will be conducive to heterostructure self assembly. Therefore, a series of organic functionalities such as alkyl, amino, thiol, carboxyl, vinyl etc were incorporated into the channel of mesoporous materials. The synthesis of modified mesoporous materials is very important for application in acid catalysis reaction or base catalysis reaction. Generally, the modification of mesoporous materials typically employs two approaches: (1) the direct synthesis wherein a silane containing the desired functional group is added to the synthesis mixture and the functional group is incorporated during synthesis, and (2) post-synthetic grafting, i.e. the functional group is incorporated after the surfactant/polymer template is removed from the mesopores. However, the distribution of the functional groups on the surface of the pore wall is likely not uniform and the organic groups are grafted mainly on the external surface of the mesoporous particles or near the pore mouth due to mass transfer limitation. Comparatively, the advantage of direct synthesis is that it produces materials with high loading and relatively uniform distribution of the functional groups.Among the known organic modified mesoporous materials, many monofunctionalized SBA-15 mesoporous silica catalysts have been reported and shown to have unique properties. Monofunctionalized material was restricted for its application to industry because of it has only a single catalytic active sites. Preparation of chemicals must be completed through the two-step or multiple step reaction, These catalytic reactions need multifunctionality of active sites with cooperative effect, e.g. the combination of acid and base sites, thus required one-pot reaction manner and a catalyst with multi-centers. However, the previous multi-center catalytic materials are mostly homogeneous catalyst so that it is not conducive to recycling and reuse. In recent years, many materials with dual-avtivation have been prepared and applied through co-condensation or post-synthetic grafting method. However, only ten odd literatures were reported and applied with acid-base dual-avtivation centers. Based on this background, this paper carries out research of the synthesis of acid-base bifunctional mesoporous materials.It is difficult to synthesize alkali amino-functionalized mesoporous materials under acid conditions. In the third chapter, a series of amino-functionalized mesoporous materials were successfully synthesized by a direct synthesis method at first. Then, SBA-15 mesoporous silica has been functionalized with sulfonic acid groups through a simple co-condensation approach by tetraethyl orthosilicate (TEOS) and 3-mercaptopropyltrimethoxy(3-MPTS) silane using amphiphilic block copolymers as template under acidic conditions. Various methods were employed to characterize the synthesized mesoporous materials. The experimental results indicate that it is difficult to introduce the amount of over 10% organosilane. The obtained amino-functionalized materials have greatly promising applications in base catalysis (Claisen-Schmidt condensation reaction). The obtained sulfonic acid- functionalized materials have the enhanced acid sites and show high catalytic activity for alkylation of phenol with tert-butanol.In the fourth chapter, two solid mesoporous materials containing both sulfonic acid and secondary amine were synthesized. Adjustment of the volume of secondary amine can control steric hindrance and thus prevents amine-sulfoacid interaction. The two solid materials were used as bifunctional heterogeneous catalysts for the aldol condensation of acetone with 4-nitrobenzaldehyde. Compared with Benzyl-APS-SBA-15, Anthracyl-APS-SBA-15 displays higher catalytic performance, showing excellent conversion for the aldol condensation reaction.Many research efforts have focused on functionalization of amino group onto the surface of the mesoporous silica materials by the direct incorporation of organic groups through co-condensation of siloxane precursors due to their potential applications in the field of enzyme catalysis, selectively catalytic reactions and separation. Mesoporous silica materials MCM-type and HMS-type functionalized with amino groups have been prepared continually using cetyltrimethylammonium bromide (CTAB) as a template agent under basic or neutral conditions. Mesoporous material SBA-15 has been synthesized by using triblock copolymer surfactant as a template under acidic conditions, and superseded other mesoporous materials because of its high special surface area, narrow pore size distribution and thicker pore walls, much larger pore size and better stability. Larger pores enable immobilization of larger biomolecules which cannot be accommodated in small pores. Up to now, a great deal of research effort has been made duo to applications of amino-functionalized mesoporous SBA-15. However, it was't ignored that the protonated amine groups would interfere with the selfassembly of the copolymer pore-directing agent and the silica precursor under the strong acidic condition. In chapter five, a method of synthesizing acid-base bifunctional material through protection of amino group is described. Firstly, tert-butyloxycarbonyl(aminopropyl)trimethoxysilane (NHBoc) was prepared by mixing 3-aminopropyltrimethoxysilane (APTMS) and Di-tert-butyl dicarbonate (Boc acid anhydride) in anhydrous ethanol. The synthesis of SH-SBA-15-NH2 was performed by co-condensation of the corresponding NHBoc, 3-MPTS and TEOS in the presence of P123 under weak acidic conditions. The resulting material was then subsequently treated with H2O2 solution at room temperature for 3 h. The carbamate deprotection was completely achieved by thermal treatment of NHBoc-SBA-15-SO3H at 185°C under vacuum during 4 h and obtained bifunctional mesoporous silica material containing aminopropyl groups and sulfonic acid groups, named NH2-SBA-15-SO3H. Compared with SBA-15-SO3H and SBA-15-NH2, bifunctional material SO3H-SBA-15-NH2 exhibit excellent acid-basic properties and high activity for aldol condensation of acetone with 4-nitrobenzaldehyde.