Synthesis And Catalytic Properties Of Novel Mesoporous Materials By Using Citric Acid Route

Mesoporous materials with high specific surface areas and narrow pore size distributions are of great interest for their important potential applications in many fields such as separation, catalysis, biology and electronics. Recently, more attention has been focused on the synthesis of various non-silica mesoporous materials, such as mesoporous aluminophosphates, mesoporous carbon materials, etc, for developing new functional materials.So far, surfactant templating route, which is an extension of the synthesis of mesostructured silica, is the most popular way to synthesize lamellar or hexagonal mesoporous AlPO. However, one main drawback related to this route is that most of these materials are thermally unstable and therefore collapse upon removal of the surfactants by calcinations. Recently, a few relatively successful examples of thermally stable mesoporous AlPO materials have been reported. For example, ordered and stable (up to 823 K) mesoporous AlPO material can be obtained by using a so-called"acid-base pairs"route in the presence of a block copolymer as template. However, these methods usually require strict control of the synthetic conditions and/or carrying out complicated postsynthesis treatments. Besides, it is difficult to obtain porous AlPO with an adjustable P/Al ratio close to or above 1:1 (a usual ratio for microporous aluminophosphates). Generally, it is well-known that the P/Al molar ratio is a critical parameter for adjusting the acidic-basic property, which can greatly influence the catalytic performance of the aluminophosphate materials. Therefore, it is still a rewarding and challenging subject to search simple and effective synthesis route for obtaining thermally stable mesoporous AlPO with a controllable P/Al ratio.As for mesoporous carbon materals, silica template route is widely used for the synthesis of such materials. This method generally involves the infiltration of carbon precursor into the template pores, carbonizaion (or directlly carbonize nanocomposites containing carbon precursors and silica templates) and template removal. However, a main drawback of the silica template route is that the pore structures of the carbon materials are usually generated via removing the silica template by using hydrofluoric or caustic base. This disposal process is harmful for the environment and the sacrificed silica material cannot be recycled easily. Besides, the tuning of porous characteristics (e.g. proe size) of carbon materials is also difficult by using silica template route. Therefore, it is still an attractive subject to develop novel, economical and environment friend method to synthesize mesoporous carbon with desirable physical-chemical property.In this thesis, thermally stable mesoporous aluminophosphate materials with uniform pore size distribution and controllable P/Al ratio have been synthesized from the precursors of Al(NO3)3/H3PO4/NH4OH in the presence of citric acid. We denote this method as"citric acid route". The formation mechanism of porous structures is also discussed from the viewpoints of the interaction between citric acid and the AlPO framework. Based on these studies, a series of mesoporous carbon materials are synthesized via treating the composite of as-synthesized AlPO. By extending this citric acid route, we also obtained mesoporous aluminophosphates containing heteroatom and mesoporous titania-silica mixed oxides. O-methylation of catechol, epoxidation of cyclooctene and CO oxidation reactions are carried out to investigate the catalytic performances of these materials. The relationship between the nature of catalysts and their catalytic performances are also discussed.In the section of mesoporous aluminophosphates, we found that the obtained materials exhibit relatively high specific surface areas, narrow pore size distributions and excellent thermally stabilities (1173 K). The composition (P/Al molar ratios) of AlPO can be adjusted in the range of 0.8¹.15. The characterization studies show that the change in P/Al ratio could affect the structure, texture, thermal stability, and surface acid-base properties of AlPO. Samples with a relatively low P/Al ratio (≦1.0) exhibit uniform amorphous mesoporous character and high thermal stability. Partial crystallization of the AlPO framework easily occurred on the sample with higher P/Al ratio (≧1.1), thus leading to significant decrease of surface area. Both weak acid and weak base sites can be detected on the surface of AlPO materials, and the amount of acid-base sites can be effectively controlled by adjusting the P/Al ratio. Vapor phase selective O-methylation of catechol with methanol reaction is carried out to investigate the catalytic performances of AlPO materials with different P/Al ratios. Among them, AlP1.1O shows the highest activity (88.4% conversion of catechol) and the highest yield of guaiacol (74.3%). The presence of suitable weak acid-base pairs may play an important role on the title reaction. The effects of different synthetic conditions on the formation of mesoporous structure of AlPO have been systemically investigated. The formation mechanism of porous structures is also discussed from the viewpoints of the interaction between citric acid and the AlPO framework. The results showed that the presence of citric acid in the precursor of AlPO is important for the formation of uniform mesoporous structure. The mesoporous texture with narrow pore size distribution can be obtained in a wide pH range from 3.5 to 10.0, whereas a lower pH value (pH=2) is unfavorable for the formation of uniform mesopores. The presence of suitable interaction between citric acid and AlPO framework is critical for the formation of mesoporous structures. Both CA and PO4 units are considered to be ligands to coordinate with aluminum ions, forming relative uniform complexes (such as CA-Al-PO4), which can inhibit the fast hydrolysis and condensation reaction of Al and/or P species to form separate phase in the solution. The mesoporous structure of AlPO materials is thus formned after the rapid decomposition of citric acid by calcining the as-synthesized AlPO. The presence of carbon on the surface of AlPO material, after citric acid is partly decomposed at ca.673 K, can inhibit the rapid condensation reaction between AlPO particles, thus preventing pore collapse upon heating and permitting the regular condensation reaction for the formation of Al–O–P linkages at higher calcination temperatures. This unique pore formation process of AlPO materials in the presence of citric acid can provide an opportunity to control the P/Al composition of the resulting AlPO materials. Moreover, mesoporous aluminophosphates with adjustable pore volume and pore size can also be obtained by using a series of organic additives with relatively low coordination ability. The nature of organic additives and the special interaction between organic molecular and aluminophosphate should play an important role on the changing porous characters of aluminophosphates.In the section of mesoporous carbon, we report a simple and efficient environment benign route to synthesize mesoporous carbon material via direct carbonizing composite of as-synthesized AlPO. The result carbon exhibits graphitic phase characteristic and electronic conductivity. The characteristics of porous carbon materials (e.g. proe size, pore volume, etc.) can be easily tuned by changing the synthesis conditions, such as adjusting P/Al ratio, or introducing a part of sugar. The mesostructured AlPO, which is in-situ formed during the carbonization process, can be regarded as a hard-template for the formation of porous carbon material. Besides, an interesting concept of"mutual template"was put forward in this work concerning the framework formation mechanism of mesoporous carbon and mesoporous AlPO materials. To the best of our knowledge, this is the first report of the synthesis of mesoporous carbon through aluminium phosphate route. One merit for this route is that the inorganic template can be easily recycled as the aluminium and phosphous resources (i.e. Al(NO3)3 and H3PO4) for the synthesis of carbon materials. This process are more economical and environmental benign in comparison with general silica template route.By extending citric acid route, we find that heteroatom such as Ti, Cr, Mn, Fe, Co can be effectively introduced into the framework of mesoporous aluminophosphate. The liquid- phase epoxidation of cyclooctene is carried out to investigate the catalytic performances of MAlPO materials. Among them, Ti_0.04Al_0.96PO shows the highest activity and the selectivity to epoxycyclooctene is almost 100%, showing much potential application in large-molecule catalytic oxygenation reaction. Additionally, we find that mesoporous titania-silica mixed oxides with relatively high Ti contents (Si/Ti=4) can also been obtained by using citric acid route. The characterizations show that nano TiO₂ particles are highly dispersed on the surface of silica. The mesostructure and nano-titania particles of the titania-silica mixed oxides exhibit high thermal stability even up to 1173 K. As a support, the Au/TiO₂-SiO₂ catalysts show high catalytic activity and stability for the CO oxidation reaction. We proposed that the properties of high specific surface areas and high dispersant of nano TiO₂ particles of titania-silica prevent the sinter of Au clusters, thus leading to the high stability.