Characterization And Kinetics Of Calcination And Crystalization In Synthesis Of Mesoporous Zeolites With Al And P

The mesoporous AlPO₄ molecular sieves with new crystal structure and performance are built up from alternating AlO and PO tetrahedron by sharing oxygen atom. The modified AlPO₄ molecular sieves, such as Me-AlPO and Si-AlPO, can be applied in hydrogenation, dehydrogenation, reformation, crack, isomerization, cyclization and oxidation, etc., and have tremendous potential applications. Presently, mesoporous AlPO₄ molecular sieves mainly contain hexagonal and lamellar shapes. Compared with the lamellar mesoprous AlPO₄, the hexagonal AlPO₄ has more regular channels and thicker walls of pores, which can not only improve diffusion of reactants and products, but also improve thermal stability of molecular sieves. So the hexagonal meosoporous AlPO₄ is more suitable as adsorbent and catalyst at high temperature. However, the main synthetic method of hexagonal mesoporous AlPO₄ molecular sieves is solvethermal synthesis. Due to many influence factors in such kind of synthetic method, hexagonal shape is easily conversed to lamellar shape, but few available reaction mechanism and kinetics can be used to solve the problems, such as empirical and difficult control and prediction in synthesis. In addition, unique synthesis method for hexagonal meosoporous AlPO₄ limits the further development and application.The mesoporous AlPO₄ molecular sieves were synthesized by solvethermal method. Based on the experiments about crystallization time, crystallization temperature, pH, P/Al molar ratio, different templates and templates removal methods etc., the effects of methods on shape and structure of AlPO₄ molecular sieves were carefully researched. Moreover, the optimum synthesis condition was determined with thermogravimetric and differential thermal analysis (TG-DTA), X-ray powder diffraction (XRD), Fourier transform infrared spectrophotometery (FT-IR), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), transmission electron microscope (TEM), nitrogen sorption isotherms, magnetic angle spinning nuclear magnetic resonance (MAS-NMR), etc. Thus, the transition metal elements and additive (PEG) influences on structure of mesoporous AlPO₄ were investigated. The results show that the Me-AlPO (Me=Mn, Ni, Ti) molecular sieves still keep hexagonal structure and thicker wall of pores, which can improve the thermal stability of molecular sieves. The PEG can forbid the conversion of hexagonal to lamellar shape, and the obtained hexagonal mesoporous AlPO₄ molecular sieves have regular shape of crystal and ordered pore channel. Moreover, the action scheme of PEG was put forward according to characteristic solvents and characterization of high polymer in aqueous solution. Because of undesirable thermal stability and irregular pore channel for hexagonal meosoporous AlPO₄ molecular sieves by hydrothermal synthesis, another solvethermal synthesis was investigated by using ethanol and water as the reaction media, and the results of experiments showed that AlPO₄ molecular sieves have thicker wall of pores and very regular porous channels, which can enhance the diffusion of reactants and products and kinetic description when this zeolite is applied to reaction.According to the tests of FI-IR, NMR and elementary analysis, the interaction of organic templates with inorganic matters was stated by the electric charge equilibrium. The apparent activation energies for nucleation and crystal growth were calculated by interpreting the relative crystallinity of the mesoporous AlPO₄ molecular sieves XRD diffraction peak in different conditions. The calculated results showed the transition metal element can reduce the apperant activation energies. The processes of template removals were monitored by in situ XRD, FT-IR, and TG-DTA. Results showed the templates was decomposed, oxidized to COx and NOx, and removed. High calcination temperature can produce the crack of pillar morphology. According to the analysis data of TG-DTG at different heating rates, average apparent activation energy of template removal was calculated.The solid state method was attempted to synthesize the mesoporous AlPO₄ zeolites. Utilizing TG-DTA, in situ XRD and FT-IR, the influence factors on shapes and structures of zeolites, such as calcination time, calcination temperature and different aluminous sources, were investigated. Moreover, the optimum reaction condition was obtained. In the end, the synthesis kinetics of mesoporous AlPO₄ molecular sieve was calculated.