Abstract
Background: Specifically, this review includes: (i) a brief overview of basic structure of clay (tetrahedral and octahedral sheet); (ii) types of clays (anionic/ cationic) and their modified forms; (iii) properties of clays such as ion exchange, swelling, intercalation and cation-exchange, acidity; (iv) as well as overview of different clays like montmorillonite clay, pillard clay, organoclay and basic clay with their catalytic applications in various organic transformations.
Methods: The objective to write this review was to provide introductory information to those young researchers who are neophytes in the area of clay science and clay catalysis. In the first part of the review, we discussed the basic structure, properties and applications of natural clays. Montmorillonite clay has great importance in the area of catalysis due to their environmental compatibility, low cost, high selectivity, reusability and operational simplicity. Porosity and stability of montmorillonite clay are improved by pillaring, which leads to materials known as pillared clays (PILC). These materials show increased surface area, pore volume, thermal and mechanical stability and, depending on the pillars, improved catalytic activity compared to the parent clays, making them suitable catalysts and adsorbents. Pillaring of clay include the addition of inorganic polycations of nanodimension to the clay interlayer and following thermal initiation. Clay pillaring can be done with a variety of inorganic polycations of Al, Zr, Ti, Cr, Fe etc. Synthesis of these polycations is mostly carried out by controlled hydrolysis of the corresponding metal cations in solutions. The metals incorporated in the pillared clay structure are crucial and make them suitable for a number of different applications, most of them belonging to the named “green chemistry”. This review shows an overview of the different reactions like oxidative cleavage of olefins, allylic oxidation of cyclic and acyclic alkenes, oxidation of phenol, benzylic oxidation of alkyl arenes, hetero-diels–alder reaction, etc. performed using pillared clays as catalysts. The second half of the review covers Organoclays, is an organically modified phyllosilicate, derived from a naturally occurring clay mineral and by exchanging the original interlayer cations from organocations (typically quaternary alkylammonium ions) an organophilic surface is generated, consisting of covalently linked organic moieties. The lamellar structure remains analogous to the parent phyllosilicate. Organoclays are very useful in the removal of oil from water, as a component in paint formulations or as a viscosifier for oil-based drilling fluids. Organoclays have been used in various studies aimed at environmental cleanup application and also utilized as nucleating agents in polymer chemistry. The aromatic pollutants can be remove with the help of organoclays. Clays which are exchanged with hexadecyltrimethylammonium (HDTMA) had shown ability to adsorb benzene, nitrobenzene, chlorobenzene, carbon tetrachloride and trichloroethylene. The review ended with the discussion on properties and various applications of the basic clay i.e. Hydrotalcite, also plays an important role in the area of catalysis not only from an economical viewpoint, but also due to ease of handling, simple separation and reusability.
Results: In this review, we discussed the types as well as the properties of different clays (natural or synthetic) along with their basic and important applications to provide a fundamental understanding of clays to the young readers. The interlayer modification of clay generates effective heterogeneous catalytic materials for application in the field of organic synthesis and the catalyst produced with clays are recyclable, which makes them environment friendly. The clay based catalyst synthesis is simple step process as compared to others and it is easily available material.
Conclusions: In future, the clay will play important role in the area of organic synthesis due to their physico-chemical properties and it will be helpful to researcher in order to achieve the high selectivity, activity, durability and recyclability in organic transformations.
Keywords: Montmorillonite, hydrotalcites, organoclays, heterogeneous catalysis, organic reaction, catalysis.
Graphical Abstract
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