Nanocrystalline zeolites in supercritical water. Part A: Opportunities and challenges for synthesis using organic templates

• First synthesis of various zeolite-nanocrystals in supercritical water.
• Effect of operational parameters were investigated on synthesized powder.
• Two-step synthesis is more rapid than single step procedure.
• One step synthesis of desired zeolites from a fresh precursor seems impossible.

Nanocrystals of various zeolitic framework types, i.e. LTA, FAU, MFI, and ANA as well as smectite nanoclay (montmorillonite) with narrow size distribution and mean particle sizes smaller than 50 nm have been synthesized rapidly in supercritical water (SC-H2O) medium by two approaches, namely, single and two-step procedures. The influence of the synthesis parameters on the particle size, shape and crystallinity was studied in a wide range of batch composition of reaction mixture, reactant sources, aging time, crystallization temperature and time in the presence of organic template. The samples were comprehensively characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and inductively coupled plasma optical emission spectrometry (ICP-OES). The results suggest that contrary to the low temperature hydrothermal synthesis; there are strong competitions for the formation of different zeolites in SC-H2O. Hence, fine-tuning of all above-mentioned parameters is important to attain a pure product as desired. The aging time for induction step and establishment of initial nuclei, necessitate the aging time to be more than 24 h at ambient temperatures in one-step process and 2 h at 80 °C in a two-step procedure. According to our results and previous studies of zeolite synthesis in hydrothermal method, a mechanism of zeolite formation in SC-H2O medium was suggested.

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