synthetic zeolites do not belong to the group of nanomaterials - scientific assessment

X-Ray powder diffraction analysis first of all shows that zeolites are highly crystalline substances. Industrial zeolites are polycrystalline materials with a crystal size of 1 – 10 µm. The crystal lattice contains defective sites, leading to intergrown crystals that are fixed together by covalent chemical bonds.

Depending on the pore structure and the ion exchange status of the zeolite, a BET surface area of a few m2/cm3 up to a few hundred m2/cm3 can be measured. As mentioned above, the BET method is not an adequate tool to determine whether zeolites are nanomaterials or not. Using the t-plot method, all commercially produced zeolites have an outer surface area in the range of 20 m2/cm3. The outer surface area is dependent on the crystal size. A small outer surface area is a strong indication, that the zeolite crystal size is not in the nanometer range.

Laser diffraction is a widely used tool in industry to determine the particle size of a crystal. The more modern instruments use a dual wavelength laser system and can determine the particle sizes between 20 nm and 2 mm. The results obtained with laser diffraction were compared with results obtained by other, more labor intensive methods, such as scanning electron microscopy (SEM) and aerosol analysis. All of the studied zeolites have hardly any particles below 1 µm and thus do not fall under the definition of nanomaterials. The coincidence of the different measuring methods is sufficiently good.

Further, the zeolites were treated with various dispersing systems to disintegrate potential agglomerates. Ultrasonic treatment for different times, rotating brush generators and sedimentation shafts were used at different energy levels. All these treatments did not lead to a disintegration of the zeolite crystals.

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