Characterization and fluorine-free microwave hydrothermal synthesis of AlPO4-5 molecular sieves as adsorbents

With a characteristic of S-shaped water sorption isotherms, AlPO4-5 molecular sieves have been considered as up-and-coming adsorbents for the utilization in adsorptive cooling and heating systems. In order to avoid toxicity and corrosion of fluoride, this paper introduced a fluorine-free microwave hydrothermal synthesis strategy of pure AlPO4-5 crystals. The effects of hydrothermal conditions including template agent, crystallization temperature and time on the performances of AlPO4-5 adsorbents were systematically investigated. Fourier transform infrared spectroscopy, X-ray diffraction (XRD), Al-27 and P-31 solid state magic angle spinning nuclear magnetic resonance (MAS NMR), pore size analyzer and Scanning electron microscopy (SEM) were used to determine the chemical structure, crystalloid phase, framework, pore structure and the morphology. Adsorption and desorption performances were measured by static adsorption, thermogravimetry and temperature programmed desorption. XRD results indicated that both triethylamine and tetraethylammonium hydroxide as templates, AlPO4-5 crystals could quickly be synthesized within 30-45 min under fluorine-free microwave irradiation. By comparison, the adsorption and desorption performance of the former (recorded as AlPO4-5A) was superior to that of the latter (recorded as AlPO4-5H). FT-IR, MAS NMR and SEM results revealed that AlPO4-5 crystals have the frameworks of alternating AlO4 and PO4 units and typical hexagonal rod-like morphologies.