Hydrothermal synthesis of pencil-like SAPO-5 and observation of its reversed crystal-growth process

SAPO-5 with a novel hexagonal pencil-like morphology was hydrothermally synthesized from hydrogels that contain triethylamine and high concentrations of acetic acid at 180 °C for 48 h. The effect of the acetic acid concentration was examined and indicated that usage of a high concentration of acetic acid is crucial to the synthesis of SAPO-5 with a pencil-like morphology. The time-dependent growth process of novel SAPO-5 was observed by scanning electron microscopy with the aid of acid treatment to remove the amorphous materials for clearer observation. The samples were also characterized by X-ray diffraction and Fourier-transform infrared spectroscopy. The results show that the crystal growth at the early stage follows the reversed crystal-growth route. First, the crystallization occurs on the surface of the aggregated amorphous ellipsoidal particles to form a hexagonal prism crystal shell with the encapsulation of amorphous materials. Then, the amorphous materials wrapped inside start to grow to a hexagonal prism inside the hollow larger hexagonal prism shell. Finally, the interior hexagonal prism continues to grow to the two ends with its length beyond that of the larger one by means of the Ostwald ripening process, thus forming the pencil-like crystal.