Study On Improve Acid Sites And Hydrothermal Stability Of Mesoporous Molecular Sieves

According to the IUPAC, porous materials can be classified into three types by pore size: microprous materials are characterized by pore sizes of < 2 nm, mesoporous materials are characterized by pore size in the range of 2–50 nm, and the pore size > 50 nm can vest in macroporous materials. The microporous materials have been widely used in many fields such as adsorption and separation, ion-exchange, and industrial catalysis, because of their unique pore structures and characters. However, microporous materials can only be applied in the area related to small molecules; it can not effectively deal with large molecules due to the limitation of the pore size. Therefore, investigating porous materials with larger pores becomes very necessary.Mesoporous materials have large surface area, uniform pore channels, high thermal stability and ability to be recycled, and thus they are widely used as catalysts or catalyst supports for many catalytic reactions. Since the discovery of M41S molecular sieves, varied kinds of mesoporous materials have been synthesized. Because conventional mesoporous aluminosilicates commonly possess weak acidity and poor hydrothermal stability due to amorphous pore walls, their catalytic applications are limited. Therefore, the preparation of new mesoporous aluminosilicates with strong acidity and high hydrothermal stability becomes very important. Based on this background, this paper carries out research of improving acidity and hydrothermal stability of mesoporous materials.The acid strength of zeolites materials is higher than conventional mesoporous aluminosilicates. According to literatures, making the secondary structure units of zeolites into the framework can enhance the acid strength of mesoporous materials. In the third chapter, using zeolite MCM-22 precursor as silica and aluminium sources, through two-step crystallization procedure in the presence of cetyltrimethylammonium bromide (CTAB), a KIT-1-like structural mesoporous aluminosilicate (denoted as MM-22) has been synthesized under basic conditions. The characterized results show that MM-22 is pure mesoporous phase, and constructed of secondary structure units of zeolite MCM-22. Compared with classic Al-MCM-41 and ZSM-5, MM-22 has stronger acidity and higher hydrothermal stability. In the alkylation reaction of phenol with tert-butanol, MM-22 shows higher catalytic activity than Al-MCM-41, MCM-22 and ZSM-5.In addition, destroying the crystal structure of zeolites also can obtain the secondary structure units of zeolites. All of the MCM-22 zeolite family is samdwich structure. Their crystal structure can be destroyed easily in the alkaline solution, producing large numbers of secondary structure units. In the chapter four, zeolite MCM-22 is treated by sodium hydroxide solution firstly, then cetyltrimethylammonium bromide (CTAB) is combined as mesoporous template to synthesize mesoporous material denoted as M-MCM-22 with enhanced acidity. Analytic results show that M-MCM-22 has ordered hexagonal p6mm mesostructure and its framework contains the secondary structure of zeolites MCM-22, which makes the acid sites of M-MCM-22 stronger in comparison with conventional Al-MCM-41. 27Al MAS NMR study confirms that aluminum in M-MCM-22 is exclusively in tetrahedral coordination. The catalytic performance of M-MCM-22 is tested in alkylation of phenol with tert-butanol and M-MCM-22 shows highly steady catalytic properties. The highest conversion of phenol can be achieved at 418 K, while the highest selectivity to 2, 4-di-TBP is obtained at 398 K. It is found that high temperature is advantageous to form 4-TBP, whereas low weight hourly space velocity (WHSV/h-1) is helpful for both conversion of phenol and selectivity to 2,4-DTBP. It is also shown that high ratio of tert-butanol/phenol is beneficial for obtaining high conversion of phenol and selectivity to 2,4-di-TBP.In chapter five, by treating zeolite MCM-56 and MCM-49 with a sodium hydroxide solution for obtaining secondary structure units of zeolite as silicon and aluminum sources, using cetyltrimethylammonium bromide (CTAB) as template and adjusting the pH value to neutral, mesoporous aluminosilicat with hexagonal structure denoted as MM-56 and MM-49 are synthesized. Compared with conventional mesoporous aluminosilicates materials, MM-56 and MM-49 have enhanced acid sites and show higher catalytic activity for the alkylation of phenol with tert-butanol.Less Si–OH groups can reduce mesoporous structure damage conducted by hydrolysis in hot water, thus the hydrothermal stability can be enhanced. It is well known that high temperature can make the number of Si–OH groups decreased, and the hydrothermal stability can be correspondingly reinforced. However under the high temperature condition, the mesoporous structure is easily destroyed. In chapter six, super-microporous materials with enhanced hydrothermal stability are synthesized by a novel method. We make sucrose dip into the pore of pure silica MCM-41 and MCM-48 to keep the pore structure firstly. Afterwards, by treating the materials at high temperature under nitrogen condition and then removing the carbon filled in pore channel, super-microporous silica with high hydrothermal stability denoted as MCM-41-T and MCM-48-T were obtained. Up to now, less attention has been developed to the synthesis of materials in the super-microporous (pore size 1–2 nm) range. The materials in this pore size range are very important since they may bridge the gap between microporous zeolites and mesoporous materials. Such materials exhibit the potential of size and shape selectivity for those organic molecules which are too large to access into the pores of microporous zeolites and zeolite-like materials. Analytic results show that after treatment by high temperature, the pore sizes of MCM-41 and MCM-48 decrease from mesoporous area to super-microporous range. MCM-41-T and MCM-48-T have high Q4/Q3 ratio and enhanced hydrothermal stability. Refluxed in boiled water for 96 h, the pore structure still remains. Making the furfural dip into the pore of mesoporous aluminosilicates and then treating at high temperature do not induce aluminium out from the mesoporous framework, and therefore the Bronsted acid sites only slightly decreases. We also apply this method to synthesize lager pore size mesoporous materials KIT-6 and it is found that hydrothermal stability of the material is considerably enhanced after high temperature treatment.