The optimum synthesis conditions for zeolite 13X were examined in terms of synthesis temperature and synthesis time, and the corresponding results are shown in Fig. 7. Hydrothermal reaction for 36 h at 85 ◦C was the optimum set of synthesis conditions; the characteristic XRD pattern for 13X was detected after 12 h synthesis and steadily grew in intensity until a maximum crystallinity was obtained after 36 h.