Synthesis, Characterization And Catalytic Proreties Of Aluminosilicate With Hierarchical Porous Structure

Zeolites are widely used in catalysis as well as in the separation and purification fields due to their uniform, small pore size, high internal surface area, flexible frameworks, highly hydrothermal and chemical stability. The major drawback of zeolites is that the small size of the pores (<2 nm) imposes diffusional limitations on reactions that can cause high back pressure on flow systems. Therefore, mesoporous silica materials with lager pore size (2 ~ 50 nm) have been attracted much attention. Mesoporous materials have large surface area, uniform pore channels, high thermal stability and easily 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. However, conventional mesoporous aluminosilicates commonly possess weak acidity and poor hydrothermal stability due to amorphous pore wall; their catalytic applications are also limited. Generally, the macroporous materials cannot sieve molecules, since their pore size exceeds 50 nm. Based on this background, synthesize of new materials to combine advantages of microporous, mesoporous and macroporous materials are very interesting, and it is expected to provide great opportunity to overcome the difficulty in bulk molecular catalytic conversion and separation. In this thesis, the hierarchical porous materials were successfully synthesized and their structures were also carefully characterized. Physic-chemical characters of each material and their applications were also investigated.According to the literatures, a generally method is crystallization of pore wall of mesoporous materials into microporous material with MFI structure. In the third chapter, order aluminosilicates with hierarchical porosity were synthesized by recrystallizing the wall of SBA-15 and JLU-1 with TPAOH in a short period of recrystallization time. The effect of the crystallization time on the properties of the composite materials was studied. The structures of these materials were characterized by X-ray diffraction, FT-IR, nitrogen adsorption-desorption, SEM, TEM. The results revealed that these materials were ordered mesoporous aluminosilicates with crystalline zeolite wall structure and the distribution of mesopore and micropore of materials were changed with the increasing of crystallization time. The nature and the strength of the acid sites of these materials were discussed by IR spectroscopy of pyridine adsorption and NH3-TPD techniques, which indicated that all materials have better acidic property than that of the mesoporous substrate. The hydrothermal stability of the obtained JF-n and M-In materials were also quiet good. And they have good performance in alkylation reaction of phenol with tert-butanol.It is well known, carbon material is not easy to be participated in the reaction of the molecular sieve synthesis, and it is easy to be removed during calcinating. Therefore, many researchers are very interesting in using carbon as template to synthesize hierarchical porous zeolite attracts. In the fourth chapter, we synthesized two carbon xerogels firstly, then they were impregnated in ZSM-5 precursor solution, finally two hierarchical ZSM-5 (denote as m-ZSM-5-1 and m-ZSM-5-2) were successfully obtained under static hydrothermal conditions. The characterized results show that the samples have the typical structure of MFI zeolite. Many bumps were found on the surface of the samples which can be ascribed to the mesoporous or macroporous holes after removing carbon template. The mesoporous pore volumes of two samples are all larger than that of the conventional ZSM-5. The difference of two samples is that m-ZSM-5-2 particles stick together while m-ZSM-5-1 does not, which results in smaller mesopore surface area of m-ZSM-5-2 than that of m-ZSM-5-1. The catalytic performance of m-ZSM-5-1 and m-ZSM-5-2 were tested in alkylation of phenol with tert-butanol. And comparing with the conventional ZSM-5, these two samples show higher conversion of phenol and selectivity to 2,4-DTBP. We also prepared a combined SiO2-C material by adding TEOS in the synthesis process of carbon xerogels. Using zeolite SiO2-C as template and silicate source, through vapor phase transport (VPT) of the TPAOH in the different period, a series of hierarchical ZSM-5 (denote as M-ZSM-5) were synthesized. The characterized results show that M-ZSM-5 has the typical structure of MFI zeolite. After removing the template, it is clearly to see that there are many mesopores and macropores on the surface of M-ZSM-5 and the irregularly morphology of SiO2-C templates were well preserved. With different crystallization time, the samples show different micropore, mesopore and macropore distribution. The NH3-TPD results show that the acidity of the samples varies with different crystallization time. This is because of the different amount of aluminum entering into the structure framework with the different crystallization time. The catalytic performance of M-ZSM-5 was tested in alkylation of phenol with tert-butanol. It also shows a higher conversion of phenol and selectivity to 2,4-DTBP when compared with the conventional ZSM-5,.Due to the similar advantages to carbon material, polystyrene microsphere was used as template to synthesize hierarchical zeolite. In the fifth chapter, Silicalite-1 microsphere with uniform diameter about 2.5μm was prepared by using polystyrene microsphere (PSD) with hierarchical structure as template. In this method, the PSD particles were impregnated firstly with tetraethoxysilane (TEOS) and then with tetrapropylammonium hydroxide (TPAOH). After crystallization and calcination, the hierarchical Silicalite-1 microsphere was charactered by XRD, FT-IR, SEM and TEM. The results show that Silicalite-1 possesses a regularly spherical morphology similar to the polystyrene microsphere and have the typical structure of MFI zeolite. More importantly, the Silicalite-1 microspheres have large secondary pores in the range of 40 to 120 nm and high pore volume up to 0.7 cm3g-1. Silicalite-1 with a lot of defect holes (denote as q-Silicalite-1) was also synthesized by using cyano-functioned polystyrene microsphere as a template. The formation mechanism of such material was also discussed. The characterized results show that the samples have the typical structure of MFI zeolite and with an average size of 1-2μm. The results of the nitrogen adsorption-desorption show that such material have large pore volume and more secondary pores. We believe that such hierarchical materials will be widely used in fields such as catalyzing, gas separation, sewage treatment and fine chemicals.