Hydrothermal synthesis of galloaluminosilicate (MFI) zeolite crystals having uniform size, morphology and Ga/Al ratio

Abstract

The hydrothermal synthesis of galloaluminosilicate (MFI) zeolite from a gel containing a source of silica (Na-trisilicate), a source of gallium (Ga-nitrate or Na-gallate), a source of aluminum (Al-nitrate or Na-aluminate), a templating agent (TPA-Br), deionized water, and sulfuric acid has been thoroughly investigated to know the influence of various synthesis parameters (viz., the source of Al and Ga, gel composition, pH of the gel, and crystallization temperature) on the size and morphology of the zeolite crystals formed and also on the Al and Ga content (or Ga/Al ratio) of individual crystals. The zeolite samples were characterized for their bulk composition by chemical analysis, for their crystalline nature by XRD, for crystal size and morphology by SEM, and for the Ga/Al ratio of the individual crystals by EDS analysis. The morphology and Ga/Al ratio of zeolite crystals are strongly influenced by the source of Ga and also by the pH of the synthesis gel. The use of Na-gallate (as a source of Ga) with Na-aluminate or Al-nitrate (as a source of Al) and pH of gel = 7-9 results in zeolite crystals with a uniform shape/morphology (sphericalhexagonal) and Ga/Al ratio. The use of Ga-nitrate as a source of Ga, however, results in zeolite crystals with two different shapes, spherical crystals with a high Ga/Al ratio and hexagonal crystals with a low Ga/Al ratio.