Abstract
We herein report a rapid (30min) and effective route for the synthesis of ZSM-5 nanocrystals with well-defined morphology (rounded shapes and hexagonal forms) in supercritical water (SCW) by single and two-step procedures where in case of the latter, the precursor was aged for 120min in 80°C before carrying to supercritical state (T=450°C). The effect of parent composition with different ratio of NaOH/Al2O3 and SiO2/Al2O3 were investigated comprehensively on crystallinity and morphology of product through powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emotion scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) analyses. Compared with the low temperature hydrothermal (LTH) synthesis method with high duration (two days), the product obtained from SCW exhibited considerable increase in BET surface area. To the best of our knowledge, this development was attributed to the modification in thermodynamic (solvation and dispersion effects) and transport properties of solution in supercritical state. Considering the advanced liquid-base mechanism of zeolites formation at low temperature, such modifications in properties of SCW can increase the liquid-bulk gel interface and transport of nutrients (e.g., Na+) through the amorphous gel network, which they have significant effect on the establishment of zeolite building, namely protozeolite. The high nucleation and diffusion rates of protozeolite nuclei in SCW, from the amorphous gel network to the bulk liquid, then lead to the high efficient formation of fine ZSM-5 nanoparticles with the large BET surface area.
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