Insight into seed-assisted template free synthesis of ZSM-5 zeolites

Abstract

Synthesizing nanocrystalline ZSM-5 using a templating method is considered to be time, energy, and materials inefficient. In addition, the production of CO2 from the calcination process has a negative impact on the environment. Therefore, finding another method to synthesize nanocrystalline ZSM-5 without using an organic template would be beneficial. Here, nanocrystalline ZSM-5 was synthesized successfully in high yield by using a seed-assisted method and without using the organic template. In addition, the effects of synthesis temperature, synthesis time, pH, amount of seeds, size of seeds, aging time, and use of calcined and uncalcined seeds were investigated. The synthesized nanocrystalline ZSM-5 materials were characterized by using X-ray diffraction (XRD), gas adsorption isotherm (BET/BJH), inductively coupled plasma optical emission spectroscopy (ICP/OES), and transmission electron microscopy (TEM). It was found that the synthesis temperature and time have a strong influence on the crystallinity of the synthesized nanocrystalline ZSM-5. Under the conditions reported here, the ideal synthesis temperature and time to synthesize fully crystalline ZSM-5 crystals are 165 °C, and 14–24 h, respectively. Controlling the pH and seed amount facilitated the synthesis of ZSM-5 with high crystallinity. A basic environment with Na2O/SiO2 = 0.4, and a seed amount of 0.35 wt % were necessary to form fully crystalline ZSM-5 particles. In addition, ZSM-5 with high crystallinity and yield could be obtained using calcined seeds. Finally, seeds with large complex aggregates showed a strong influence on the size and aggregation of the synthesized particles.