Synthesis of millimetre-scale HZSM-5 single crystals by secondary hydrothermal treatment

Abstract

Millimetre-scale HZSM-5 single crystals were successfully synthesized by a two-step hydrothermal method. In the first step, millimetre-sized silicalite-1 single crystals were prepared by hydrothermal synthesis using quartz tube as siliceous source. In the second step, framework Al atoms were inserted into the single crystals of silicalite-1 by another hydrothermal treatment involving NaAlO2 as aluminium source. Through the characterizations of power and single crystal X-ray diffraction, optical microscope, ICP, 27Al MAS NMR, 29Si MAS NMR, and n-propylamine temperature-programmed decomposition (NPA-TPD), it was confirmed framework Al atoms with uniform spatial distribution can be inserted into giant single crystals of silicalite-1 with no obvious effects on the crystal structure, size and morphology. Such giant HZSM-5 single crystals are a promise candidate for the probe adsorption investigations, which can be applied in the measurement of framework Al spatial distribution. The strategy of insertion of aluminium or other heteroatoms of pure siliceous zeolite through secondary hydrothermal method provides a possible route to prepare giant single crystals that is difficult to synthesize directly.