| The M41s mesoporous materials synthesized by using supramolecular surfactant assemblies as template, which were first reported by Mobil researchers in 1992, have attracted considerable attention because of their tunable pore size and ordered mesopore channels, large specific surface area, rich surface functional groups, high thermal stability and great potential in various applications such as catalysis, adsorption and separation, environmental protection and so on. However, the relatively weak acidity and poor hydrothermal stability of M41s, as compared to zeolites, limit their potential in petroleum refining and the fine-chemistrys industry, and hinder them from further developing. Many research efforts have therefore been focused on how to improve them. The research presented in this thesis is mainly aimed at these aspects, and it basically consists of two parts: firstly, as far as the two members of the M41s, MCM-48 and MCM-41, are concerned, MCM-48 containing two independent three-dimension pore systems provides more favorable mass transfer kinetics in catalysis, catalytic substrate, adsorption and separation applications than that of MCM-41 that has one-dimension pore system. In order to adequately utilize the superiority of MCM-48, it is important to study the synthesis procedure of MCM-48 by combining the concepts of surfactant chemistry, sol-gel chemistry and colloid science, then to extend the region of MCM-48 in the synthetic phase diagram, to decrease the cost of template and to synthesize high quality cubic MCM-48 with high thermal and hydrothermal stability. So we have designed and developed a new approach to synthesize MCM-48 with thick wall by using the ternary cationic -anionic-nonionic mixed surfactant template route S~+s~-s~0I~-, and introduced a novel method for synthesizing MCM-48 with high thermal and hydrothermal stability by using buffering solution instead of distilled water as the synthetic media. Secondly, mesoporous materials containing secondary building units (SBUs) of zeolite Beta in their frameworks are first.synthesized in several binary mixed surfactant systems, their acidity and hydrothermal stability have been greatly enhanced and improved, which provides some new catalysts for acid catalytic and bifunctional acid catalytic reactions.The research work in this thesis includes the following three sections: In Section 1, according to the recognition in theory of surfactant chemistry and the formation mechanism of MCM-48, we have designed and developed a simple,low-cost and environmentally compatible pathway to synthesize MCM-48 based on the ternary cationic (CTAB)-anionic (SL)-nonionic (OP-10) mixed surfactant template route S+s"s°F. We have confirmed that the fixed location and configuration of the benzene ring of nonionic surfactant OP-10 in the micelles by combining the results of 'H NMR and the theory of colloid science, and have proposed a model for the possible formation process of micelle in the new ternary surfactant synthesis system. In addition, true vesicles have been first found in the new ternary surfactant system for the synthesis of MCM-48 through vacuum freeze drying experiment at liquid nitrogen temperature, which can be used to explain the formation of hollow particles in the new synthesis system and enriches the synthesis chemistry of MCM-48. The experimental results of surface tension measurement verify that synergistic effects enhancing surface activity are present in the ternary mixed surfactant system; especially the ability to considerable reduction in "cmc" and surface tension is stronger than that in the binary mixed surfactant system. Up to now, there are no any literatures concerning this aspect, which will increase new contents for surfactant chemistry and colloid chemistry, and also open up a new way for the study of mixing behaviors and applications of surfactants. The new synthetic route adequately makes use of this kind of synergistic effects, and helps to adjust more effectively the local effective surfactant packing parameter "g" value to the range of 1 /2~2/3, and favors the formation of cubic MCM-48 at extremely low Surf/SiO2 (0.05:1-0.07:1) and extremely low surfactant concentration (1.76-2.4%).Moreover, this new ternary surfactant template route can also synthesize hexagonal MCM-41 and lamellar MCM-50, and even MCM-48 single crystal consisting of 6 well-defined crystal facets and belonging to the cubic Ia3d space group of m3m point group. This is the first synthesis of periodic mesoporous material possessing such less crystal facets. The cubic single crystals of MCM-48 may have potential applications in physics and chemistry, and also provide a good case to study the relationship between microscopic structure and macroscopic morphology of mesoporous materials.In Section 2; hexagonal MCM-41 materials with SBUs of zeolite Beta in their frameworks have been first synthesized in three different binary mixed surfactant templating systems (CTAB/OP-^ CTAB/SL, CTAB/DDA). Our research shows that this kind of mesoporous sieve materials can be synthesized under the conditions of very low Surf/Si (corresponding to 0.146:K 0.163:1 and 0.293:1,respectively.) and surfactant concentration. These synthetic methods are not only economical but also friendly to environments. The effects of different synthesis systems on the structure and performance of hexagonal MCM-41 with SBUs of zeolite Beta are different. In the same synthesis system, hexagonal MCM-41 with different crystallization degree can be obtained through adjusting the concentration of surfactant, the ratio of two kinds of surfactants, pH of reaction mixture, crystallization temperature and time, which eventually affects their catalytic performance. The results of catalytic performance indicate that the catalytic activities at 350°C for cumene cracking over MCM-41 with SBUs synthesized in CTAB/OP-10 and CTAB/DDA system are higher (79.2% and 85.9 %) than that of corresponding hexagonal MCM-41 materials with SBUs (60.4%)reported previously using single surfactant as template under similar synthetic conditions. While the hexagonal MCM-41 with SBUs synthesized in CTAB/SL system also shows high catalytic activity at 350°C for cumene cracking (65.3%), indicating that the synthesized materials possess very strong acidity and may be widely applied in various fields such as petroleum processing industry, Fine chemicals and environment protection. Moreover, a macropore/mesopore hierarchical structural and reticular MCM-41 material can be synthesized in CTAB/SL system under basic conditions by adding NO3" ion to the synthetic system, and the corresponding material also shows high catalytic activity at 350°C for cumene cracking.Cubic MCM-48 materials with SBUs of zeolite Beta have also been prepared in the above three different binary mixed surfactant templating systems by optimizing the synthesis conditions. The characteristic results show that the MCM-48 materials containing SBUs of zeolite Beta synthesized in CTAB/OP-10 and CTAB/SL system have high thermal and hydrothermal stability. The calcined MCM-48 samples treated with 100% steam at 600 °C for upwards of 4h can still retain its cubic structure. The results of catalytic performance indicate that the catalytic activity at 350°C for cumene cracking over MCM-48 with SBUs synthesized in CTAB/SL system was much higher (50.5 %) than that of corresponding mesoporous Al-MCM-48 synthesized directly in the same synthesis system (20.1%) . In addition, Spherical siliceous MCM-48 material with high thermal and hydrothermal stability has been synthesized by mixed cationic-nonionic surfactant templating pathway on the basis of the study on the above mesoporous materials, the calcined Spherical MCM-48 sample treated with 100% steam at 600 °C for 6h or calcined in air at 900 °C...
|
PDF Full Text Request