Introduction of kaolin clay and its application as a heterogeneous catalyst in organic syntheses: a review

Rocks rich in kaolinite and halloysite are known as kaolin or china clay. In some sedimentary deposits, kaolin exists as mixed with quartz and feldspar particles, which can be separated via wet processing methods. Kaolinite is a pure clay mineral crystal, composed from one part of alumina and two parts of silica. Most clay minerals are derived from kaolin. Kaolin has formed of the chemical weathering of rocks in hot, moist climates and exists in abundance in soils. This clay mineral is a raw material in industries and has medical, cosmetics, archeology and geotechnical engineering applications. Kaolin is simple chemically and structurally compared with other clay minerals and are usually synthesized at high temperatures in laboratories. Under thermal treatment in air at atmospheric pressure, a series of phase transformations occurs in kaolin structure. Various organic reactions are catalyzed in the presence of kaolin or modified kaolin as an acid or base. This review article reviews the various synthetic methods for the preparation of catalysts based on kaolin and has been used for the synthesis of organic compounds during the years 1993–2023, especially since 2009. The catalytic activity of these catalysts has been evaluated.