Insight into the synthesis and characterization of zeolite millimeter-sized crystals

Abstract

The effect of time, composition and structure-directing agents on the synthesis of silicalite-1 and octadecasil zeolite millimeter-sized crystals has been studied. Crystals up to about 3 mm size range were obtained and special attention was paid to their aspect ratio, thermal behavior, monocrystallinity and textural properties by means of electronic and optical microscopy, powder and single-crystal X-ray diffraction (XRD), thermogravimetry and Ar adsorption. For the silicalite-1 crystals obtained after 34 days of hydrothermal synthesis, more than one thousand specimens from two repeated batches were measured along a, b and c axes. Reproducibility was thus assessed simultaneously with a statistical analysis to obtain cumulative and differential distributions of crystal dimensions. Single-crystal XRD evidenced a high degree of monocrystallinity, while thermal analyses suggested that crystals with sizes above 125 μm have serious difficulties in activation, giving rise to high external specific surface area values upon Ar adsorption.