Fig. 1 shows a series of X-ray diffraction (XRD) patterns for the
products obtained by conventional heating for various times. It can
be seen that after hydrothermal synthesis by conventional heating
for 24 h at 190 C, binary mixtures of mordenite (Na8[Al8Si40O96]$
24H2O) crystals as major phase and P-type (Na8[Al8Si8O32]$16H2O)
zeolite crystals as minor phase were obtained. After hydrothermalsynthesis for 48 h, small quantities of analcime (Na16[Al16Si32O96]$
16H2O) zeolite crystals also appeared in the product in addition to
mordenite and P-type zeolite crystals. When the reaction time
lasted for 72 h, P-type zeolite crystals disappeared while analcime
zeolite crystals, though in a trace quantity, were still present
together with mordenite crystals. These results indicate that pure
mordenite crystals could not be obtained even after hydrothermal
treatment of 72 h at 190 C by conventional heating. Using
a hydrogel with the same chemical composition as the one used in
this study, Kim and Ahn [13] had obtained pure mordenite crystals
after hydrothermal treatment of 20 h at 170 C by conventional
heating. This might result from the difference in the silica source
used (Zeosil in Ref. [13] versus silica gel herein).