Green chemistry catalysts

The main components of a chemical process can be divided into the categories of catalysts, media, feedstocks, and reagents, all of which are important in the practice of green chemistry. Catalysts and catalysis are addressed in this section, and the other aspects of chemical production are addressed in Section 14.12. 

By definition all catalysts must be heterogeneous or homogeneous within and across these two categories there are other classifications that are important to green chemistry. Most important among these are ... 

The most widely used homogeneous catalysts are simple acids and bases which catalyse well-known reactions such as ester and amide hydrolysis, and esterification. Such catalysts are inexpensive enough that they can be neutralized, easily separated fi om organic materials, and disposed of. This, of course, is not a good example of green chemistry and contributes to the huge quantity of aqueous salt waste generated by industry. 

Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3. 

Born in Oban, Argyll, in 1960, Duncan Macquarrie studied Pure and Applied Chemistry at the University of Strathclyde, graduating with a first class degree in 1982 and a PhD in 1985. He then moved to York, where he carried out research in Phase Transfer Catalysis. He subsequently spent time in industry, where he worked in the UK and abroad, mostly in synthetic chemistry, but always with an interest in method development and catalysis. He returned to York in 1995 to take up a Royal Society University Research Fellowship, and has developed a range of novel catalysts for green chemistry. He is Associate Editor of Green Chemistry, and a National Member of Council with the Royal Society of Chemistry. 

Eissen, M., Hungerbuhler, K., Dirks, S., Metzger, J. (2003) Mass Efficiency As Metric for the Effectiveness of Catalysts. Green Chemistry, 5, G25-G27. 

Gladysz, J.A. (2001) Recoverable Catalysts. Ultimate Goals, Criteria of Evaluation, and the Green Chemistry Interface. Pure and Applied Chemistry, 73, 1319-1324. 

In pursuit of green chemistry, Mr. Clean aqueous H2O2 leaves only water. Thus readily available aqueous 30% H2O2 has been used for oxidation of cyclohexene to adipic acid using a PTC and a noble metal catalyst (Sato, 1998). 

The multi-component systems developed quite recently have allowed the efficient metal-catalyzed stereoselective reactions with synthetic potential [75-77]. Multi-components including a catalyst, a co-reductant, and additives cooperate with each other to construct the catalytic systems for efficient reduction. It is essential that the active catalyst is effectively regenerated by redox interaction with the co-reductant. The selection of the co-reductant is important. The oxidized form of the co-reductant should not interfere with, but assist the reduction reaction or at least, be tolerant under the conditions. Additives, which are considered to contribute to the redox cycle directly, possibly facilitate the electron transfer and liberate the catalyst from the reaction adduct. Co-reductants like Al, Zn, and Mg are used in the catalytic reactions, but from the viewpoint of green chemistry, an electron source should be environmentally harmonious, such as H2. 

Rossi, L.M., Silva, F.P., Vono, L.L.R., Kiyohara, P.K., Duarte, E.L., Itri, R., Landers, R. and Machado, G. (2007) Superparamagnetic nanopartide-supported palladium a highly stable magnetically recoverable and reusable catalyst for hydrogenation reactions. Green Chemistry, 9 (4), 379-385. 

Polshettiwar, V., Baruwati, B. and Varma, R.S. (2009) Nanoparticle-supported and magnetically recoverable nickel catalyst a robust and economic hydrogenation and transfer hydrogenation protocol. Green Chemistry, 11 (1), 127-131. 

Kotani, M., Koike, T., Yamaguchi, K. and Mizuno, N. (2006) Ruthenium hydroxide on magnetite as a magnetically separable heterogeneous catalyst for liquid-phase oxidation and reduction. Green Chemistry, 8 (8), 735-741. 

Zheng, X.X., Luo, S.Z., Zhang, L. and Cheng, J.P. (2009) Magnetic nanoparticle supported ionic liquid catalysts for CO2 cydoaddition reactions. Green Chemistry, 11 (4), 455 158. 

Catalyst nanoencapsulation is an excellent fit to the concepts of green chemistry [2] in the area of process intensification - enabling incompatible catalysts to function in the same reactor, thereby achieving what otherwise simply cannot be done. 

Nonetheless, a wide variety of potential methods are available to achieve the goal of nanoencapsulation for the purpose of facilitating the use of two or more incompatible catalysts in cascade reactions. The many multistep reactions that are of importance in the fine chemicals industry are prime targets for the application of the principles of nanoencapsulation and, therefore, of green chemistry. 

Ease of separation of tritiated products from a reaction medium is an important feature in the choice of labeling procedure. Sometime ago we used polymer-sup-ported acid and base catalysts [12, 13] to good effect and with the current interest in Green Chemistry one can expect to see more studies where the rate accelerations observed under microwave-enhanced conditions are combined with the use of solid catalysts such as Nafion, or zeolites. 

The progress of Green Chemistry impacts the established scenarios for the chemical industry (98,99). Future catalysts will likely become more enzyme-like in the sense that they will more efficiently balance the properties of effectiveness and fragility. An overarching environmental consideration holds a synthetic catalyst should simply go away by some mechanism when its work is done. Nature has developed enzymes with limited lifetimes. After their task is completed, their constituent molecules and atoms become recyclable to serve as useful resources for the... 

Lewis acids as water-stable catalysts have been developed. Metal salts, such as rare earth metal triflates, can be used in aldol reactions of aldehydes with silyl enolates in aqueous media. These salts can be recovered after the reactions and reused. Furthermore, surfactant-aided Lewis acid catalysis, which can be used for aldol reactions in water without using any organic solvents, has been also developed. These reaction systems have been applied successfully to catalytic asymmetric aldol reactions in aqueous media. In addition, the surfactant-aided Lewis acid catalysis for Mannich-type reactions in water has been disclosed. These investigations are expected to contribute to the decrease of the use of harmful organic solvents in chemical processes, leading to environmentally friendly green chemistry. 

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