Synthesis, Characterization And Catalytic Properties Of Stable Mesoporous Materials

Since the discovery of ordered mesoporous materials, they have received numerous attentions due to their large pore size, high BET surface area and uniform pore array, which make them potentially important in many fields such as ion-exchange, adsorption and separation, bulky molecule catalysis, host-guest assembly. In the last decade, mesoporous materials have become a fresh field gradually, and the composition of mesoporous materials has been developed pure silica into various non-silica systems. However, with the deeper research, many scientists found that lots of aspects should been improved before their practical applications. Among of them, stability is the most important problem.Firstly, compared with conventional zeolite, poor hydrothermal stability is primary problem for mesoporous silica-based materials. To solve this problem, many well-known scientific groups have dedicated themselves to improving the hydrothermal stability of mesoporous silica-based materials. Based on previous reports, we realized that wall thickness and silica condensation are two critical factors for hydrothermal stability of mesoporous silica-based materials. Furthermore, improving the degree of silica condensation in mesoporous walls is a direct and effective method for increasing hydrothermal stability. In the end of last century, there was a report about enhancement of the crystallization rate of zeolites in the presence of inorganic anions. In other words, some inorganic anions can accelerate the rate of silica condensation. Subsequently, couples of groups try to use inorganic anions as additives in the synthesis under alkaline media to increase hydrothermal stability of ordered mesoporous materials. However, it is regrettable that these researchers did not study a relationship between the degree of silica condensation in mesoporous materials and these anions. In this paper, we systemically study anionic effect on the degree of silica condensation under various conditions including acidic, neutral, and alkaline media. Then, we found inorganic anions of SO42- and CO32- were suitable promoters for silica condensation under neutral and alkaline condition. Finally, based on the experimental results, we rationally synthesize mesoporous silica-based materials. For example, hydrothermally stable and well ordered hexagonal mesoporous SBA-15 materials with very high degree of silica condensation have been successfully synthesized in the presence of inorganic anions such as sulfate and carbonate species.Additionally, many results indicate that there are several advantages for synthesizing mesoporous silica-based materials under strongly acidic medium. Nevertheless, it is difficult to prepare heteroatom-incorporated mesoporous silica-based materials under strongly acidic medium. Although postsynthetic grafting are common methods, it is relatively multistep. It is expected direct synthesis of heteroatom-incorporated mesoporous silica-based materials under strongly acidic medium. Recently, Prof. Xiao et al. in Jilin Univ. have developed a novel method ("pH-adjusting") for the synthesis of heteroatom-incorporated mesoporous silica-based materials in strong acidic media. However, this method is easily influenced by final pH value and rate of dropping ammonia, improper operation will make theses samples lose their ordered mesopores. In this paper, we have successfully prepared highly ordered mesoporous silica-based materials with very high heteratoms (Al, Zr) loading by using urea as a pH-adjustor. Particularly, the samples of Zr-SBA-15 from this method are highly hydrothermally stable, after sulfation, they exhibit good catalytic activities in esterification and transesterification. A series of results indicated that there are several advantages of this method including simple manipulation, high efficiency, highly ordered products and general applicability. Meanwhile, we also take a novel semi-fluorinated surfactant (FSO-100) to synthesize Fe-containing mesoporous silica-based materials, the resultant products have high Fe-incorporation efficiency and exhibit unusual catalytic activities in Friedel-Crafts alkylations.On the other hand, it is found that mesoporous nonsilica-based materials (here is major for metal oxides and metal phosphates) had very poor thermal stability. During the removal of surfactant by high-temperature calcination, the mesostructure always suffers collapse. Actually, people have studied mesoporous metal phosphates since the discovery of M41S, but there are much less successful examples about mesoporous metal phosphates than mesoporous silica-based materials due to their poor thermal stability. Thus, the synthesis of thermally stable mesoporous metal phosphates is still challenging. Recently, Prof. Zhao et al. in Fudan Univ. have fabricated mesoporous metal phosphates with highly thermal stability by an novel"acid-base pair"route, which opens a door to multicomponent mesostructured minerals. Faultily, these mesoporous metal phosphates generally exhibit relatively low surface area (BET, 140~260 m2/g) due to thick mesoporous walls, which is not favorable for practical application. Herein, by using a template of semi-fluorinated surfactant and"acid-base pair"route, we have synthesized thermally stable mesoporous aluminophosphates (MAP) and Fe-aluminophosphates (Fe-MAP) with large surface area (430-580 m2/g). The catalytic tests of phenol hydroxylation with hydrogen peroxide show that Fe-MAPs are very catalytically active. Moreover, the combination of metal phosphates with mesoporous silica can, to a certain extent, not only improve its thermal stability, but also enhance its catalytic properties. However, up to now, there are few successful examples. In this paper, we demonstrated a facile method for preparing ordered mesoporous materials of functionalized SBA-15 with various contents of titanium phosphates. These samples retained well ordered mesostructure like SBA-15, besides they exhibit high catalytic activity and well regenerated ability in dehydration of cyclohexanol.Transition metal oxides have always been popular in catalysis area for its broad applications as industrial catalysts and catalyst supports. Particularly, when some of them are treated in dilute sulfuric acid or ammonium sulfate solution, it will generate strong acidic sites on their surface, which can transform n-butane to i-butane even at room temperature. In contrast, conventional solid acidic catalysts, such as ZSM-5, are inactive in the same condition. However, it is found that this kind of catalysts has two fatal disadvantages: low surface area (70～100 m2/g) and quick deactivation, which restrict their practical application. In order to solve low surface area, many researchers have tried to prepared mesostructured sulfated metal oxides. Presently, sulfated zirconia, as a typical example, has been studied adequately. Nevertheless, several researchers have pointed out that sulfated tin oxides have better catalytic activity than sulfated zirconia in many reactions. Unfortunately, papers concerning on sulfated tin oxides have been quite few. Here, we have prepared successfully mesostructured sulfated tin oxides (MST) with strong acidity and high surface area by using the tri-block copolymer (P123) as a surfactant. The sample shows high catalytic activity on the esterification and Friedel-Crafts acylation. Alternatively, we also prepared sulfated silica-doped tin oxides from hydrothermal synthesis, followed by sulfation and calcination. Very interestingly, these samples have much high BET surface areas (113-188 m2/g) and well catalytic activity in transesterification. Noticeably, the presence of tiny SiO2 is very important for these samples, which suppresses the agglomeration of SnO2 nanoparticles and improves the stability of these samples during calcination and reaction.Moreover, there is the same problem, the thermal stability, for some functional mesoporous materials. For example, some magnetic mesoprous materials in previous reports will encounter fatal challenge under condition of high temperature in air, because this can lead to transformation of usual magnetic species (such as Co,Fe3O4,γ-Fe2O3), giving drastically decrease of magnetism. In this paper, we have successfully synthesized highly ordered mesoporous silica materials with good heat-resistant magnetism and open pore system from a combination of SBA-15 with CoFe2O4.