Effect of alkalinity on the crystallisation of silicalite-1 precursors

Abstract

The crystallisation of (Na, TPA)-silicalite-1 from the reaction mixtures xNa₂O–2TPABr–20SiO₂–1000H₂O (0.25 ⩽x⩽ 6.5, TPA = tetrapropyl-ammonium) at 95 °C has been studied by pH and X-ray powder diffraction measurements. The yield decreases with x and extrapolates to zero when x= 6.7. The changes in pH that occur during the crystallisation are interpreted in terms of the incorporation of base into the molecular sieve and the relative solubilities of the amorphous gel solid and the crystalline product. Incorporation of sodium ions increases with x, and for high values the composition of the unit cell approaches (NaOH)₄(TPAOH)₄(SiO₂)₉₆. The size of the crystals and their aspect ratio decrease with x. Crystallization of some related reaction mixtures is also reported and the results are compared with those for silicalite synthesis in the Na–TPA system.