The role of the dissolution of silicic acid powders in aluminosilicate synthesis mixtures in the crystallization of large mordenite crystals

The role of the dissolution of silicic acid powders in alkaline aluminosilicate synthesis mixtures in the crystallization of mordenite was investigated. Three different as-received lots of silicic acid (Aldrich lots 01807PW (lot A), 04720KX (lot B), and 18913LY (lot C)) were used to synthesize mordenite from the same initial batch composition (4.32 Na2O:Al2O3:19 SiO2:293.6 H2O). Different overall crystallization times (lot A approximate to 10 h, lots B and C approximate to 28 h), conversion rates at the 50% point (lot A approximate to 44% h(-1), lots 8 and C approximate to 12% h(-1)) and maximum crystal sizes (lot A=10-15 mu m, lot B=35-40 mu m, lot C=30-35 mu m) were observed. The prismatic crystal morphologies were the same in all cases. Heat treatment of as-received lots A and B in air at 550 degrees C before syntheses resulted in about 6- and 2-fold decreases of mordenite conversion rates, respectively; and in about 6- and 2-fold increases of mordenite crystal sizes, respectively. Heat treatment of asreceived lot C in air at 850 degrees C resulted in about a 6-fold decrease of mordenite conversion rate and in about a 6-fold increase of mordenite crystal size. Induction periods and overall crystallization times also increased in these cases. Large crystals of mordenite with sizes of up to 250 mu m were obtained using lot C heat treated at 850 degrees C. These results reflect different silica dissolution rates that were correlated with the variation of specific surface area and average pore diameter of silicic acid powders. The dissolution of silicic acid in the investigated crystallization of mordenite intervened in the rate-determining step for nucleation and growth of mordenite crystals.