Compositional and kinetic study on the rapid crystallization of ZSM-5 in the absence of organic template under stirring

The MFI type high silica zeolite, ZSM-5 with well-defined crystal morphology was successively synthesized in the absence of organic template within a short reaction time under stirring at 200 rpm and 463 K. The results clearly show that Na+ cation plays a structure directing role as well as a charge balancing role. In addition, it is interesting to notice that when the SiO2/Al2O3 molar ratio is as high as 80.0, higher Na2O content seems to be required to obtain pure ZSM-5. Otherwise, the crystallization path is more likely to direct toward the formation of a dense phase, keatite rather than ZSM-5, indicating the structure directing role of Na+ cation. On the other hand, as the SiO2/Al2O3 molar ratio decreases to lower than 30, the crystallization tends to direct toward the formation of mordenite, which is more aluminous than ZSM-5. In this case, the Na2O content should be as low as possible to direct the crystallization toward the formation of pure ZSM-5. The NMR study suggests that organic-free ZSM-5 with tetrahedrally coordinated Al only can be easily crystallized within 8 h of reaction. The kinetic study based on 14Na(2)O-100SiO(2)-xAl(2)O(3)-2500H(2)O (x = 1.5, 2 and 3) was conducted at 423, 443 and 463 K, respectively, and the results suggest that the higher the Al2O3 content is, the higher the temperature is preferred, enhancing the rate of crystallization with the increase in temperature. (C) 2004 Elsevier Inc. All rights reserved.