Functional Kaolinite

Abstract

The world resources of all clays are extremely large. Among the various types of clays, the world mine production of kaolin in 2016 was 37.0 Mt, the largest mined clay. Kaolin is traditionally used in ceramics, refractories and as paper coating and filling. But kaolin, as it is demonstrated in this paper, has a bright potential for use in non-traditional, high value-added, applications. This is particularly true for its principal component: the mineral species kaolinite which has a chemical structure allowing its functionalization, leading to a variety of potential applications. Kaolinite is a layered 1 : 1 clay mineral, the layer being made of two different sheets, a tetrahedral silica sheet and an octahedral alumina sheet. Large dipole-dipole interactions, in addition to a network of H-bonds, link the siloxane surface of a layer to the aluminol surface of another layer, making intercalation of guest species in kaolinite challenging. There is however a limited number of molecular units (molecules or salts) that can directly intercalate in kaolinite to form "pre-intercalates". Once intercalated these molecular units can be exchanged by a large number and variety of guests, providing access to the interlayer space of kaolinite, and to its reactive aluminol internal surfaces. The intercalation of molecules of pharmacological interest showed the potential of kaolinite to act as a slow-releasing agent for drugs, and the intercalation of polymers resulted in the creation of intercalated nanocomposites. The intercalation of ionic liquids gave materials with ionic conductivity properties in the solid-state. Intercalates are however unstable in water. One needed to make these organo-inorgano nanohybrid materials resistant to hydrolysis and more thermally stable. The network of aluminol groups on the internal surfaces of kaolinite offers the opportunity to design and create controlled organo-inorgano nanohybrid materials, taking advantage of their reactivity, in particular with hydroxyl groups of organic compounds, to form Al-O-C bonds. A functional, two-dimensional, spatially restricted, environment can be created with controlled nanoarchitecture. The grafting of organic groups on the aluminol internal carpets has allowed applications in catalysis, in sensing, in heavy metals adsorption, in exfoliated nanocomposite, in luminescence, and in structural modifications to form nanoscrolls or nanorolls. This paper shows how the future of the use of kaolinite will shift from its traditional uses in ceramics, tiles and paper coating to more sophisticated, high value-added, uses. In particular, research should amplify in the years to come to design an efficient and cost-effective method to produce kaolinite with nanotubular morphology. One can foresee also that efficient, easy-to-use, electrochemical devices based on modified kaolinite, will be created to quantify selectively a variety of pollutants in waste waters.

Keywords: Kaolinite; catalysis; grafting; intercalation; nanocomposites; nanohybrid materials; sensing.