Green Chemistry: Renewable Feedstocks and Biological Processes in Industrial Chemistry
Exploring the 7th principle of Green Chemistry focusing on the use of renewable feedstocks in industrial processes. Examples include transforming waste from seafood and paper industries into valuable materials, utilizing lipids and carbohydrates as feedstocks, and leveraging biological processes to produce biopolymers. The importance of reducing reliance on depletable resources and embracing sustainable practices in chemical synthesis is highlighted.
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B.Sc (Industrial Chemistry) Semester IV SEC Paper- GREEN CHEMISTRY Twelve Principles of Green Chemistry By RITU MALIK Assistant Professor Rajdhani College University of Delhi
7th principle of green chemistry Use of renewable feedstocks: A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable. A feedstock is considered green if it satisfies the following conditions: It should impose least demands on the earth's resources. It s acquisition and refining should be safe. It should be non-hazardous and relatively safe. If possible, feedstock should always be renewable for example feedstocks from biomass that can be grown repeatedly should be given preference to depletable petroleum feedstocks. Reusing of waste from bio-industries, if possible, as raw material or feedstock. Various examples are listed below: 1. Chitan, a waste from of seafood industry can be transformed into Chitosan by Deacetylation that has many applications as in water purification, biomedical and in other industries. It would definitely replace the petroleum feedstocks. 2. Lignin, a waste from paper and pulp industry, is used as feedstock for manufacture of Annelyn venelin DMSO
Continued. 3. Lipids and hydrocarbon terpenes are the oils, greases and waxes which have same properties as that of petroleum products and are used to synthesize the synthetic liquid fuels and also can be used directly as diesel fuel. 4. For chemical synthesis, the fatty acids from natural sources is advantageous because of the carboxylic acid group and the presence of reactive double bond between two carbon atoms in the carbon chain. For example oleic acid from sunflower, linolenic acid from soya bean and linseed, rinoleic acid from Castor oil etc. 5. The carbohydrates as feedstocks have advantages as they have many hydroxyl functional groups that may provide sites for the attachment of other functionalities or to initiate chemical reactions. 6. Cellulose another bio feedstock is mostly employed as plant biomass suppliants thus diminishing the need of petroleum feed stocks.
Biological processes Biological processes produce a variety of biopolymers like cellulose. hemicellulose, lignin, starch, proteins, lipids, monosaccharides (glucose), disaccharides (sucrose), waxes, fats, oils, terpene hydrocarbons etc. The three main steps in acquisition of a feedstock and converting it into useful product are : 1. Source (lifetime methods and environmental impacts of extraction ) 2. Separation of desired components from the waste or byproduct matter 3. Conversion of isolated feedstock material to desired product Each of the above three steps have environmental effects and can benefit from the application of the Principles of green chemistry
This Photo by Unknown Author is licensed under CC BY Conventional synthesis of Adipic acid from Cyclohexane
This Photo by Unknown Author is licensed under CC BY-NC Green method for the synthesis of Adipic acid from D-Glucose
Continued . The separation, processing and purification of biological feedstocks are usually carried out in biorefineries which follow the principles of green chemistry. For example using fermentation produced ethanol or supercritical carbon dioxide as solvents/the use of biocatalyst/high efficiency of energy utilization etc. There are many advantages of using biological feedstocks: The biological feedstocks are more complex and therefore we start with the material in which few steps of the synthesis of the desired product have already being done by the living plant. The biological feedstocks have bound oxygen and this avoids the usually employed step involving conversion of petroleum based hydrocarbon feedstock to an oxygenated compound which may need harsh conditions or hazardous oxidizing agents or troublesome catalysts. Examples:
8th Principle of Green Chemistry Reduce Derivatives. Unnecessary derivatization (use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste. Covalent derivatization is all pervasive technique employed for organic synthesis /analytical chemistry. But, employing green chemistry, Warner developed a way to use little energy and less material to achieve chemical variations from the conventional system. In the early 1990s, an innovative concept called non-covalent derivatization was introduced, a derivatization that does not involve covalent bonding but is based on intermolecular interactions. An example of non-covalent derivatization is demonstrated by the controlled diffusion and solubility of hydroquinones used in Polaroid films.Researchers at Polaroid wanted to release hydroquinones at high pH. Instead of using conventional base-labile covalent protecting groups, a non-covalent protecting group in the form of a co-crystal between hydroquinones and bis-(N,N- dialkyl)terephthalamides was considered (Figure on next slide). This approach was successful and feasible for the industrial process and solved the problem without changes in the original hydroquinone structures, minimizing waste material and energy.
Hydroquinones protected by non-covalent interactions with bis-(N,N-dialkyl)terephthalamides.
Traditional versus green method for the synthesis of semi synthetic penicillin Traditional versus green method for the synthesis of semi synthetic penicillin: Traditionally penicillin G is first protected as it s silyl ester and then reacted with phosphorus pentachloride to form an enol ether- chlorimidate that on hydrolysis give 6-APA (an intermediate for the synthesis of semi-synthetic penicillins) while the green method involves just the reaction with the enzyme pen-acylase. hydroquinone This Photo by Unknown Author is licensed under CC BY-SA This Photo by Unknown Author is licensed under CC BY-SA
9th Principle of Green Chemistry Catalysis. Catalytic reagents as selective as possible are superior to stochiometric reagents Usually, the waste generation may be attributed to the traditional use of a stoichiometric amount of reagents.Therefore, greener approach will be to usage of catalysts methodologies instead of stochiometric methodologies,to improve the eficiency of a reaction and avoid waste generation. Catalysis enhance the efficiency of a reaction by lowering the energy intake, by avoiding the use of stoichiometric amount of reagents, and by greater product selectivity. This involves less energy, less feedstock and less waste. In addition, new innovative chemical reactions and unconventional solutions to traditional chemical challenges are being introduced. Examples are the oxidation and reduction (DIBAL-H as the hydride donor) reactions (Figure on next slide) Catalytic hydrogenation like the Noyori hydrogenationeliminates the need for stoichiometric reagents and in consequence decreases the amount of feedstock needed and the amount of waste generated (figure on next slide)
Comparison between stoichiometric and catalytic reduction.
Unknown Author is licensed under CC BY-SA This Photon by Unkown Author is licensed under CC BY Stochiometric versus catalytic reactions in the reduction of acetophenone: W Stochiometric versus catalytic reactions in the reduction of acetophenone: When acetophenone is reduced using stochiometric amounts of sodium borohydride, Atom economy is 81% and when the reaction is proceeded in the presence of catalyst, atom economy is 100% Hydrogen, Pd/C
Green Catalysis/Bio Catalysis-Enzymes act as catalysts In general, the catalysts used for the practice of green chemistry are usually Heterogenous catalyst as homogeneous catalyst may lead to the formation of byproducts as they intimately mix with the reagents involved in the chemical reactions. Green catalyst employed today, are the enzymes which carry out biochemical processes in the organisms. Biocatalysis refers to the use of living (biological) systems or their parts to speed up (catalyze) chemical reactions. Enzymes are the biological catalysts which regulate the rate at which the chemical reactions proceed. Enzymes increase the rate of biochemical reactions by factor ranging from 106 to 1012. The enzymes are highly specific and an average living cell contains more than 3000 different enzymes each catalyzing a specific reaction. Today, natural or modified enzymes to perform organic synthesis is termed chemoenzymatic synthesis; the reactions performed by the enzyme are classified as chemoenzymatic reactions
Advantages of Bio-catalysis Very highly enantioselective, chemoselective and regioselective. Very high transformation under under mild conditions Solvent often water Advantages of chemoenzymatic synthesis 1. Enzymes are environmentally benign, being completely degraded in the environment. 2. Most enzymes typically function under mild or biological conditions, which minimizes problems of undesired side-reactions such as decomposition, isomerization, racemization and rearrangement etc.. 3. Enzymes can be affixed on a solid support, demonstrating high stability and re-usability and can be used to conduct reactions in continuous mode in microreactors. 4. Advancements in protein engineering, mutagenesis and evolution; may lead non-natural reactivity of the enzymes. Variations may enhance reaction rate or catalyst turnover and allow a wider substrate range.
5. Enzymes exhibit extreme selectivity towards their substrates. Typically enzymes display three major types of selectivity: Chemo-selectivity: As an enzyme acts on a single type of functional group, other sensitive functionalities will not react to generate waste. As a result, biocatalytic reactions are cleaner and labour, energy and time required for purification of product(s) from impurities formed through side reactions, is saved. Regioselectivity and diastereo-selectivity: Due to their complex three-dimensional structure, enzymes may distinguish between functional groups which are chemically situated in different regions of the substrate molecule. Enantioselectivity: Since almost all enzymes are made from L-amino acids, enzymes are chiral catalysts. As a result, any type of chirality present in the substrate molecule is "recognized" upon the formation of the enzyme-substrate complex. Thus a prochiral substrate may be transformed into an optically active product and both enantiomers of a racemic substrate may react at different rates. These reasons, and especially the latter, are the major reasons why synthetic chemists have become interested in biocatalysis. This interest in turn is mainly due to the need to synthesize enantiopure compounds as chiral building blocks for Pharmaceutical drugs and agrochemicals.
Use of biocatalysis to obtain in enantiopure compounds The use of biocatalysis to obtain enantiopure compounds can be divided into two different methods: Kinetic resolution of a racemic mixture Biocatalyzed asymmetric synthesis In kinetic resolution of a racemic mixture, the presence of a chiral object (the enzyme) converts one of the stereoisomers of the reactant into its product at a greater reaction rate than for the other reactant stereoisomer. The stereochemical mixture has now been transformed into a mixture of two different compounds, making them separable by normal methodology.
Scheme 1. Kinetic resolution Biocatalyzed kinetic resolution is used in the purification of racemic mixtures of synthetic amino acids. Most of amino acid synthesis, such as the Strecker Synthesis, result in a mixture of R and S enantiomers. This mixture can be separated by acylating the amine using an anhydride followed by selective diacylation of only the L enantiomer using enzymes (hog kidney acylase). These enzymes are extremely selective for one enantiomer, leading to very large differences in rate, permitting for selective deacylation.Finally the two products are now separable techniques, such as chromatography.
The maximum yield in such kinetic resolutions is 50%, since a yield of more than 50% means that some of wrong isomer also has reacted, giving a lower enantiomeric excess. Such reactions must therefore be terminated before equilibrium is reached. If it is possible to perform such resolutions under conditions where the two substrate- enantiomers are racemizing continuously, all substrate may in theory be converted into enantiopure product. This is called dynamic resolution. In biocatalyzed asymmetric synthesis, a non-chiral unit becomes chiral in such a way that the different possible stereoisomers are formed in different quantities. The chirality is introduced into the substrate by influence of enzyme, which is chiral. Yeast is a biocatalyst for the enantioselective reduction of ketones.
Scheme 2. Yeast reduction The Baeyer Villiger oxidation is another example of a biocatalytic reaction. In one study a specially designed mutant of Candida antarctica was found to be an effective catalyst for the Michael addition of acrolein with acetylacetone at 20 C in absence of additional solvent.[10] Another study demonstrates how racemic nicotine (mixture of S and R-enantiomers 1 in scheme 3) can be deracemized in a one-pot procedure involving a monoamine oxidase isolated from Aspergillus niger which is able to oxidize only the amine S-enantiomer to the imine 2 and involving an ammonia borane reducing couple which can reduce the imine 2 back to the amine 1. In this way the S-enantiomer will continuously be consumed by the enzyme while the R-enantiomer accumulates. It is even possible to stereoinvert pure S to pure R.
