Design And Self-assembly Synthesis Of Porous Silica Materials And Investigation Of Their Adsorption, Catalytic Properties

Porous materials (including microporous and mesoporous materials) with high surface areas and pore structures are a kind of important materials with vast potential applications in adsorption, separation, ion exchange, and catalysis. Microporous zeolites with pores less than 1.3 nm are usually crystallized from templating-synthesis with organics acting as structure-directing agents (SDAs) in alkaline conditions, and mesoporous materials with the pores in the range of 2 to 30 nm are prepared from self-assemblies of supramolecular surfactant micelles with silica species. Recently, the route of self-assembly has been extended to fabricating macroporous and supermicroporous materials. Self-assembly process is suitable for both alkaline and acidic conditions, which is obviously different from the crystallization of microporous zeolites in alkaline conditions. Notably, there is still no successful example for obtaining microporous silica-based materials in acidic media by the self-assembly route yet, due to the limitation of amphiphilicity and large diameterof surfactant micelles.In the present project, we have studied the status of small quarternary ammonium salts in aqueous solution, and investigated new routes to designed synthesize porous silica materials by self-assembly of organic templates with silica species. Further more, we elucidated the possible formation mechanism in the synthetic system and investigated the adsorption and catalytic properties of the obtained porous materials.It is well known that organic template plays an important role in the synthesis of molecular sieves, and has been widely studied. The most frequently used templates are quaternary ammonium compounds, such as tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH), tetrapropyl ammonium hydroxide (TPAOH), etc.Nuclear magnetic resonance (NMR) technique is a powerful tool in structural analysis of compounds. In recent years, new development in NMR such as diffusion ordered spectroscopy (DOSY) has been widely used to analyze the structure of materials. One advantage of this technology is that the diffusional properties of objects in solution could be investigated, which has been successfully applied in many systems, including analysis the self-assembling interaction of molecular species, their aggregation in solution,and the sizes of molecules or compounds.We investigated here the status of several quarternary ammonium cations which were usually used as templates in zeolite synthesis in aqueous solutions by NMR DOSY technique. And we found that tetraethyl ammonium cations (TEA+) could self-aggregate with each other and form particles with relatively larger diameter. They behaved as monodispersed at low concentration (< 6 wt%) and partially aggregated at relatively high concentration (6 wt% ~ 40 wt%). Moreover, this was a unique phenomenon for TEA+ cations in acidic or neutral media. Such aggregations could not be observed in basic condition or for tetramethyl ammonium (TMA+) cation and tetrapropyl ammonium (TPA+) cation in acidic aqueous media. The unique self-aggregating property of TEA+ cations might provide new route for fabrication of porous materials.On the basis of the NMR results, a series of silica materials with micropores or hierachical micro/mesopores have been rationally synthesized in acidic media by simply controlling the concentration of TEA+. Interestingly, N2 adsorption results showed that the mesopores in the samples are uniform, indicating that the particle size of the aggregated TEA+ particles were similar. Further analysis on organic vapor adsorptive properties showed that the products have excellent adsorptive capacities for some volatile organic compounds (VOCs) especially at low relative pressure, compared with conventional zeolites (zeolite Y and ZSM-5) and mesoporous materials (MCM-41 and SBA-15), which may be important for the potential applications in the removal of VOCs in the atmosphere.We also prepared hybrid organic/inorganic mesoporous materials. The mesoporous materials with organic functional groups in the pores or within the frameworks would have certain special properties. Here we prepared a dipropyl amine modified organic/inorganic hybrid mesoporous material in the presence of citric acid as a template and a combination of bis[γ-(triethoxy silyl)propyl] amine (KH402) and tetraethoxysilane (TEOS) as precursor species. Through the self-assembly of the organic-inorganic species and the neutralization of citric acid with the amine group in the silica source, an organic group bridged hybrid mesoporous silica material was obtained. This amine bridged hybrid mesoporous silica exhibited good catalytic performance in the Knoevenagel condensation reaction of benzaldehyde and malononitrile due to the presence of amine group in the mesoporous framework.