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)