3.2. Effect of Variable-Temperature/Time Method
The microstructure of the ZSM-5 zeolite layer on the surface of tubular
alumina supports was controlled by changing the synthesis
temperature/time profile. Based on the fact that low and high
temperatures favor nucleation and crystal growth, respectively [11,
20], experiments on secondary growth of the best prepared membrane
(sample M9) under the six different temperature/time profiles were
conducted in order to identify optimum synthesis conditions for
obtaining high quality ZSM-5 zeolite membranes.
According to the literature [15, 21, 22] the growth rate of ZSM-5
crystal is too low to form the compact zeolite layer on the support
surface at temperatures less than 130°C. Even though prolonging the
crystallization time can increase the degree of crystal growth
reactions, the membranes synthesized at low temperatures are still
defective and show low selectivities. Li et al. [23] showed that to
achieve only 60% zeolite yield in the synthesis of the ZSM-5 zeolite
membrane, the synthesis time should be prolonged up to 70 h and 31 h for
synthesis temperatures of 80 and 100˚C, respectively. Contrarily, Zhang
et al. [24] showed that the permeance of ZSM-5 zeolite membranes
synthesized at temperatures higher than 180°C is drastically reduced.
They suggest that ZSM-5 membranes synthesized at 180°C show the highest
separation performance for separation of organic/water mixture. The
synthesis temperature can also affect the crystallographic planes of
zeolite crystal. The growth of ZSM-5 crystal is anisotropic with each
crystal orientation exhibiting different growth rates. Wong et al.
[25] showed that the highest intensity ratio of (1 0 1)/(2 0 0)
peaks forms in the range of 135-150˚C. Consequently, more oriented
zeolite layers may be formed. In two other works, Kong et al. [15,
26] found that the best temperature range for the synthesis of ZSM-5
zeolite membranes is 130-170˚C. Therefore, the synthesis temperature
range of 140-180˚C was chosen to investigate the effects of
varying-temperature/time method in this study. Table 5 represents the
synthesis conditions and performances of membranes prepared by the
variable-temperature/time methods.
Table 5: Effect of using the variable-temperature/time profiles on the
performance of ZSM-5 membrane (the M9 membrane were selected as the
control sample)