Halloysite-based hybrids/composites in catalysis

Abstract Heterogeneous catalysis benefits from some merits such as facile catalyst separation and recyclability. In this context, use of halloysite that is a naturally occurring clay with tubular morphology received growing attention and in recent decade many halloysite –based heterogeneous catalysts have been developed. Halloysite is a promising candidate for the formation of composites and hybrids. Mostly, halloysite preserved its structure in the course of hybridization and the properties of the resulting hybrids/composites depend on some factors such as the content of halloysite, preparation procedure and the hybrid/composite components. To date, halloysite based hybrids/composites have been applied for promoting various chemical and photochemical transformations. Mostly, this class of compounds exhibited promising catalytic performances that were superior to their individual components, suggesting the synergism between halloysite and the composite/hybrid component(s). Considering the outstanding catalytic performance of these composites/hybrids as catalysts or catalyst supports, this review article targets disclosing the catalytic utility of the halloysite nanocomposites/hybrids. The classification presented in this review is based on the types of the components that were hybridized with halloysite. In each section, it is tried to discuss the catalytic performance of the hybrid/composite and the effects of incorporation of halloysite on the catalysis.