Tetraethyl silicate has been hydrolysed under
controlled conditions to produce "clear" silicate sols.
An investigation of these sols by both
trimethylsilylation and reaction with molybdic acid
solution did not reveal any major difference between
these sols and those produced from fumed silica. However
these sols remain clear when heated to temperatures
normally used in the synthesis of high silica molecular
sieves (353K to 473K) and do not separate into solution
plus solid gel as is the case with sols normally used in
molecular sieve synthesis. New methods to follow the
growth of zeolite crystals have been developed. Since
there is no solid gel phase the zeolite crystals can be
recovered by filtration and then weighed. Thus
crystallization can be followed directly by mass growth
measurements. Crystallization from the "clear" sols can
also be followed by silicate analysis of the solution
phase. These methods are not appropriate for crystallization from gels. Reaction mixtures have been sealed
inside glass capillary tubes and the crystals grown at
different temperatures. The crystal growth of individual
crystals can then be followed by optical microscopy. The
effects of temperature and chemical composition on the
crystallization of ZSM -5 type zeolites has been examined.
Apparent activation energies of growth for the different
crystal faces have been calculated from Arrhenius plots.
Factors which influence the size and shape of the crystals
have been determined. Conditions for the growth of relatively large crystals of ZSM -5 have been established.
Similar conditions are found to produce larger crystals
of some other high silica molecular sieves (e.g. EU -1,
ZSM -39 and ZSM -48). The crystallization of silicalite -1
at 368K from reaction mixtures with the composition 1Na20
20SiO2 1960H2O 80Et0H 2TPABr has been studied in detail,
using thermal gravimetric analysis, scanning electron
microscopy, X -ray powder diffraction, pH measurements
and crystal mass measurements. The rate of linear crystal
growth has been determined by measurement of the largest
crystals at various stages of the crystallization.
Nucleation curves have been calculated from the final
crystal size distribution and the crystal growth curve.
The crystal mass growth curve has also been calculated
from the crystal growth curve data and the final crystal
size distribution. The calculated mass growth curve is
found to be in close agreement with the actual mass
growth curve obtained experimentally
