Hollow sodalite spheres synthesized in a first-closed then-open system from the synthesis gels aged under ultrahigh pressures

Hollow sodalite spheres with a hole on the shell were synthesized in a first-closed then-open system from the synthesis gels aged under ultrahigh N₂ pressures. The effect of the synthesis time and the aging pressure was examined. The formation process was characterized by scanning electronic microscopy and X-ray diffraction. The results indicated that high pressure aging and synthesis in the open system were crucial for forming the hollow structure. By changing the synthesis time in the closed system, the size of the hollow sodalite spheres and the diameter of the hole as well as the fine structure of the sodalites could be adjusted. The formation process of the hollow sodalites was observed to follow a surface-to-core crystallization mechanism, i.e., formation of cubic zeolite A crystals in the closed system, growth of small sodalite spheres on the zeolite A crystal, wrap of the zeolite A by sodalite sphere aggregate and dissolution of zeolite A in the open system. Compared with those methods for synthesis of hollow zeolite crystals, this method is simple, effective and without using any template or additives.