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

Abstract

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 SiO 2/Al 2O 3 molar ratio is as high as 80.0, higher Na 2O 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 SiO 2/Al 2O 3 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 Na 2O 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 2O–100SiO 2– xAl 2O 3–2500H 2O ( x=1.5, 2 and 3) was conducted at 423, 443 and 463 K, respectively, and the results suggest that the higher the Al 2O 3 content is, the higher the temperature is preferred, enhancing the rate of crystallization with the increase in temperature.