Crystallisation of zeolitic molecular sieves: direct measurements of the growth behaviour of single crystals as a function of synthesis conditions

Abstract

A technique is described for the milligram-scale synthesis of zeolitic molecular sieves in sealed capillary tubes. The method is ideally suited to the measurement of the linear growth rates of individual crystals. Spectroscopic characterisation of individual crystals within the sealed capillaries has been demonstrated. Detailed results for growth rates as a function of synthesis temperature and composition are presented for the isostructural zeolite ZSM-5 and silica molecular sieve silicalite (MFI framework). Length growth rates follow the Arrhenius law over a wide temperature range and have an apparent activation energy of 80 ± 3 kJ mol^–1. Width growth rates show more complex behaviour and yield apparent activation energies in the range 62–81 kJ mol^–1, depending upon the temperature range chosen and the synthesis conditions. The crystal aspect ratio is found to increase with increasing temperature and also when the crystals are growing under chemical constraint. The maximum value observed for the length growth rate {0.5 dl/dt=(1.9–2.0)× 10^–2µm h^–1} at 368 K agrees well with that found in earlier studies, and it is suggested that this represents a rate limit dependent upon the integration of growth species into the crystal surface. Addition of aluminium compounds, tetramethylammonium salts or ethanol reduces the growth rate, and a number of possible mechanisms are suggested. Some observations have also been made on zeolites EU-1 (EUO), ZSM-39 (MTN) and ZSM-48.