Synthesis Of Aluminophosphate Molecular Sieves With Special Aggregative States And Investigation Of Their Growth Mechanism

Zeolites and related microporous materials that have a large surface area and well-defined pore structures are important due to their wide applications in catalysis, ion-exchange, adsorption, chemical separation, and host/guest chemistry. Aluminophosphate molecular sieve (AlPO4-n) is one of the important families of microporous materials. In contrast to the traditionally anionic aluminosilicate zeolite frameworks, their structures are typically built up from strict alternation of AlO4 and PO4 tetrahedra through corner sharing of an oxygen atom to form a neutral open framework. The lattice Al and/or P atoms can be partially replaced by silicon and/or other elements to form frameworks with Br?nsted acid sites as well as catalytically active metal sites. In the recent years, aluminophosphate molecular sieves have also been developed for new applications, such as optics devices and membranes. The morphology control is important for the special property and application of AlPO4-n. And it depends on the better understanding of the growth mechanism of AlPO4-n. Whereas, because of the limit of characterization method, the growth mechanism of AlPO4-n is not well clear. So the growth mechanism research of AlPO4-n faces a big challenge.In this thesis, systematic researches have been done on the preparation of aluminophosphate with special aggregative states. The pEG molecules have been used as a co-solvent to control the crystal growth of transition metal substituted AFI crystals. The results demonstrate that the crystal growth of AFI crystal along c-axis can be significantly inhibited by using pEG molecules as the crystal growth inhibitor. Varying the volume ratio of pEG/H2O, different morphologies of AFI crystals can be obtained, such as prismlike, tabletlike and flowerlike Cr-AFI, and kiwi fruitlike Ti-AFI crystals depending on the type of transition metal ions in the reaction system. Our work suggests that the different isomorphic substitution of metal ions may be responsible for the different growth behavior of Me-AFI crystals with the presence of pEG in the reaction system. The flowerlike and nanosheetlike morphology significantly reduce the length of the [001] pore of Me-AFI crystal, whch is important for the diffusion of guest molecular in the pores of host molecular sieves.Additional, the combination of two single-site heterogeneous catalysts of Cr-AFI and Ti-AFI was achieved for the first time by forming a core-shell composite via solvothermal epitaxial growth. The sheetlike Ti-AFI microcrystals were epitaxially grown on the surface of the pre-synthesized tabletlike Cr-AFI crystals core. The growth process of the Ti-AFI microcrystals on the surface of core of Cr-AFI crystal was detailedly studied by the SEM characterization. The results demonstrate that the Ti-AFI microcrystals nucleate preferentially on the (100) basal face of the Cr-AFI crystals core.The N2 adsorption analysis shows that the channel systems of the core and shell have good communication, which may provide a good mass transport pathway for the catalytic reaction. The core-shell composite of Ti-/Cr-AFI makes it possible to combine two catalytic sites in one catalyst, which may benefit some special catalysis reactions. The morphology control of AlPO4-5 crystal was also investigated by microwave irradiation in mixed-solvents system. The results show that the aspect ratio of the AlPO4-5 (c/a) crystals decreases significantly with the increase of the volume ratio of EG/H2O. Interestingly, the dumbbell shape AlPO4-5 crystals can be obtained when the volume ratio of n-butanol/H2O reaches to 5:1 in the mixed-solvents of n-butanol and H2O. With the addition of the crystal seeds, the size of the AlPO4-5 crystals decreases with the increase of the amount of crystal seeds and the induction time.Based on the understanding of the epitaxial growth mechanism of aluminophosphate molecular sieve, we designed and prepared the SAPO4-34 core-shell material. The N2 adsorption analysis shows that the channel systems of the core and shell have good communication. The NH3-TPD results show that the strong acid sites are fewer in the core-shell composite than that in SAPO4-34, which is caused by the lower Si content in the shell. The catalytic property is evaluated by the MTO reaction. The core-shell material shows good ethylene selectivity and lower propane by-product. The SAPO4-34 core-shell material may provide a new way for the development of new catalyst for MTO reaction.In conclusion, the work of preparing aluminophosphate molecular sieves with special morphology and aggregative states provides new methods and ideas to realize the controlled synthesis of AlPO4-n, and can help us to better understand their growth mechanism.