Textural properties of ZSM-5 nanocrystals prepared in alkaline potassium fluoride medium

• The fluoride route leads to highly crystalline ZSM-5.
• The external surface of the optimized ZSM-5 is strongly favoured.
• Ageing more than 20 h proves useless.

In this paper, we optimized the fluorine route in hydrothermal conditions under atmospheric pressure for the synthesis of ZSM-5 crystals of controlled size. Several experimental parameters were varied for obtaining particles in the nanoscale range in the shortest time with the highest crystalline as well as textural properties. The 100% crystalline ZSM-5 crystals were obtained for a temperature of reaction of 100 °C and a crystallization time of 7 days. The concentration of the organic template was found to greatly influence the crystallization process. XRD showed that the best result was obtained for a TPABr/SiO2 ratio of 0.4. Below this ratio, a slowdown of the crystallization process was observed. The increase of the molar ratio template/silica above 0.4 induced an unfavourable increase of the alkalinity of the mixture. This resulted in a decrease of the crystal growth and consequently a decrease of the size of the zeolite crystals. In these conditions, the average diameter of the crystals was extended from 20 to 47 nm. Beyond 20 h, the ageing time had a little effect on the crystallinity of the final products, whereas the crystal size was considerably reduced from 52 nm to less than 25 nm when the ageing time was increased from 20 to 44 h.

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