The amount of waste generated in cashew nut processing factories has caused serious problems for a long time. However, this situation is about to change because they are being turned into an opportunity by a variety of bio-based chemicals. Today, cashew nut shells (CNS) have proven to be among the most versatile renewable resource as they produce cashew nut shell liquid (CNSL). CNSL which is a dark reddish brown viscous liquid (ca. 30–35 wt%) is extracted from the soft honeycomb of the CNS. The shells have been regarded as a by-product of the cashew industry though now it is a cheaper source of natural unsaturated phenols. CNSL offers a multitude of interesting possibilities for the synthesis of speciality chemicals, high value products and polymers. Our recent research efforts have demonstrated that its constituents can be transformed into diverse functional chemicals. This paper reports some key results on how cashew nut shells (an agro waste from cashew nut processing factories) have been employed to produce several functional materials and chemicals. The materials that are highlighted include the synthesis of 3-propylphenol from cardanol and anacardic acid, some polymers prepared from CNSL components, heterogeneous catalysts prepared using CNSL as a templating agent and anacardic acid capped chalcogenide nanoparticles.
Green chemistry, renewable energy and waste management have recently become a recurring trifactor in the forefronts of innovative research. Due to this, derivatives of by-products or wastes prepared from environmentally-friendly protocols, intended for use in various applications such as renewable energy, are increasingly receiving a lot of attention since they not only provide an alternative source of chemicals but also provide an alternative waste disposal approach. In cashew nut processing factories, cashew nut shells (CNS) which are about 25% of the cashew weight are normally released to the environment and discarded as waste. However, inside the soft honeycomb (Fig. 1) of these shells, there is a valuable viscous liquid called cashew nut shell liquid (CNSL). This reddish-brown liquid can be extracted from the soft honeycomb structure located inside the cashew nut shell by solvent, thermal or by mechanical means. Depending on the mode of extraction, CNSL is classified into natural and technical CNSL where natural CNSL is obtained by cold solvent extraction while thermally extracted is classified as technical CNSL. Cashew nutshell liquid (CNSL) constitutes up to about 30–35 wt% of the nut shell depending on the method of extraction
A cross-section of a cashew nut showing the honeycomb part of the shell.
The worldwide annual production of raw cashew nuts stand at approximately 2.1 million tons; the top five producers being Vietnam (1.1 million tons), Nigeria (0.95 million tons), India (0.75 million tons), Coted’Ivoire (0.45 million tons%) and Brazil (0.11 million tons). CNSL has many industrial applications such as the manufacturing of: (a) Friction modifying materials for brake lining, clutch facing and industrial belting, (b) Various types of coatings (e.g., industrial and marine coatings, varnishes, lacquers, and enamels), (c) Rubber products of improved performance, (d) Adhesives used in the manufacturing of ply-wood and particle boards, and (e) Laminating materials for reducing brittleness and improving the flexibility of the laminates. Furthermore, it has been reported that chlorinated and copper derivatives of CNSL have pesticidal effects while components of CNSL have medicinal potential as anticancer, antioxidant and antibacterial agents. The presence of a benzene ring and the side chain (C15H31-n), of varying degrees of unsaturation located at a meta position relative to the phenolic group of CNSL components has made them excellent renewable starting materials in the synthesis of benzene ring-containing chemicals. Furthermore, the presence of double bond(s) in the side chain presents additional advantage for generating useful bio-based chemicals. For instance, the synthesis of cardanol-based porphyrins have been possible by taking the advantage of double bonds present in the side chain of cardanol through ruthenium-catalyzed self and cross-metathesis reactions via olefin metathesis. The olefin metathesis reaction on cardanol has also afforded the synthesis of new olefins that are sometimes impossible to prepare via other methods and approaches .
Although there are many reviews on CNSL-related topics, very few have focused on the derivatisation and/or transformation of CNSL components into fine chemicals and new hybrid materials. In this contribution, the use of CNSL components as key renewable starting material(s) in the synthesis of fine chemicals and some hybrid functional materials prepared via catalytic reactions such as metathesis, isomerisation, hydrogenation, epoxidation and polymerization is reported.
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