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

Synthetic zeolites, CAS No. 1318-02-1, comprise a group of synthetic, crystalline aluminosilicates made out of silicon dioxide (SiO2) and aluminum oxide (Al2O3) in various compositions, together with metal oxides. They are normally produced by mixing the starting materials (an aluminate, a silicate, sodium hydroxide and optionally further metal oxides and templates) in an aqueous solution and crystallized at elevated temperature to form a slurry of crystals. These crystals are separated by filtration, washed with water and dried. Through partial ion exchange (surface modification) the initial product may be modified. 

Synthetic zeolites were produced and marketed for many decades without significant changes in its physical properties. The range of synthetic zeolites has become wider, as over time further metal oxides were used for synthesis. Synthetic zeolites are used as detergent additives in the form of dried white powders, in the field of adsorption and catalysis as dehydrated white powders or as formed bodies. In addition, synthetic zeolites are also approved as food contact additives, and as food additives, although with restrictions. 

As zeolites are porous substances, containing defined pores (channels) up to 1 nm, they are affected by the evolving discussion on nanomaterials. The purpose of this statement is to provide information that industrially produced zeolites are not considered to be nanomaterials. The statements given are mainly based on the “EU Commission Recommendation 2011/696” (published on 18th October 2011) intended to be applied as an overarching framework with regard to other EU regulatory definitions. In this recommendation nanomaterials are defined as “a natural, incidental or manufactured material containing particles in an unbound state or as aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more of the external dimensions is in the range 1 nm to 100 nm, or where the specific surface area by volume of the material is greater than 60 m2/cm3.”

It must be highlighted that the long standing success of synthetic zeolites is not related to the particle dimensions but to the internal pores (channels), which are typically in the range of 0.3 – 1.0 nm (10 Angstrom) in size and thus in the sub-nanometer range. These pores are responsible for the extraordinary performance of zeolites and not the crystal size of the zeolites. Decreasing the size of the crystals does not change the chemical properties and the specific surface of the zeolites. The pores are structure inherent and independent of the crystal size. They are of the size of small molecules. As the porous structure of the zeolites is independent on the crystal size, the surface area using the BET method cannot be used to describe whether zeolites are nanomaterials or not. BET measurement may provide false results based on the pore size. Further, the pores are in the sub-nanometer range and thus not covered by the above mentioned definition.

A few commercially available zeolites were analyzed in different ways to show that the definition of nanomaterials does not apply to zeolites.