Tetramethylguanidine-templated synthesis of aluminophosphate-based microporous crystals with AFI-type structure

Crystalline microporous aluminophosphates with AFI-type structure have been hydrothermally synthesized by the use of low-cost and biological template of tetramethylguanidine (TMG), which were characterized by X-ray powder diffraction (XRD), scanning electron micrograph (SEM), nitrogen isotherms, infrared, elemental analysis, thermogravimetry (TG), temperature-programmed desorption of NH3 (TPD-NH3), and photoluminescent (PL) techniques. XRD experiments show that TMG-templated synthesis has high crystallization rate, compared with conventional organic template of triethylamine. Observation of SEM images indicates that the crystalline aluminophosphate products are spheres ranged in 5–30 μm. The results obtained from nitrogen isotherms show that these spherical particles of AlPO-5 crystals have mesoporosity, which are due to the presence of intercrystalline voids. The formation of hierarchical mesoporous and microporous crystalline spheres would be favorable for mass transport in catalysis and adsorption, compared with the small and sole microporous crystals. The combination of infrared, TG, and elemental analysis indicates that there are about 1.4 TMG molecules per unit cell of AlPO-5. Furthermore, TMG-templated synthesis are extended to prepare heteroatom-substituted AlPO-5 crystals such as SAPO-5, MnAPO-5, and CoAPO-5, and acidic measurements of TPD-NH3 show the order of sample acidity is as follows: CoAPO-5 > MnAPO-5 > SAPO-5 ≫≫ AlPO-5. The determination of PL shows that as-synthesized AlPO-5 templated by TMG has 38 nm red-shift for emission peak, compared with TMG liquid. Importantly, in this work, the use of TMG might open a door to use the guanidine-type compounds as templates for “green” preparation of aluminophosphate-based microporous crystals in the future.