INTRODUCTION:

The global economy has been growing at a tremendous speed during the twentieth century, and the standard of living in developed countries continued to rise. Because of the world's increasingly competitive economic perspective and the planet's diminishing natural resource graph, it is critical to reduce both energy consumption and trash output. One of the key drivers of innovation is sustainability, which allows technological industries to work for the well-being of customers in a safe and healthy environment¹. The most alluring concept for achieving sustainability is "Green Chemistry," which is defined as the application of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture, and application of chemical products and was coined by Anastas and Warner at the United States Environmental Protection Agency. Physical (e.g. explosive, flammable), toxicological (e.g. carcinogenic, mutagenic), and global (e.g. ozone depletion, climate change) elements are all included in the phrase "hazardous." Alternative feedstock, solvents and reagents, and catalytic vs. stoichiometric processes are all techniques used in green chemistry².

Principle of green chemistry:

1. Waste prevention —- it is far more important to take care to avoid generating rubbish materials than it is to clean up the waste after it has been generated ^3,4,5

2. Atom economic development - new ways and methods for incorporating each and every material to be used in various processes for manufacturing the final product should be developed.

3. Use of less hazardous chemical synthesis processes —- if practicable, synthetic procedures should be used to synthesise compounds that are safe and non-toxic to human health and the environment.

4. Safe chemical design and development —- nontoxic chemical products should be created and developed to improve their function while producing very low levels of toxicity.

5. Usage of safe solvents and auxiliary substances --- the usage of auxiliary substances should be discarded and they must only be used when it becomes necessary.

6. Energy efficiency design —- the energy required by chemical processes must be recognised, their environmental impact must be assessed, and financial costs must be kept to a minimum. Synthetic methods should only be used under ideal conditions of temperature and pressure.

7. Use of renewable raw resources —- whenever practicable and feasible under technical and economic conditions, renewable raw materials should be used, and their use should be promoted.

8. Derivatives reduction —- derivatization should be reduced and used only when absolutely necessary because it necessitates the use of extra chemicals, which generate a lot of hazardous waste that is harmful to the environment.

9. Promote the use of catalytic reagents —- catalytic reagents are a superior product to stoichiometric reagents, and their use should be encouraged.

10. Designed to degrade —- towards the end of their life cycle, chemical products must disintegrate into harmless by products that will not poison the environment on their own.

11. Pollution prevention analysis —- analytical methods for analysing the importance of pollution should be established and developed. a review of pollution monitoring and the many procedures taken to prevent the production of hazardous materials

12. Safe chemistry for preventing unintentional damage —- chemical materials should be utilised in such a way that accidental damage, such as gas leaks, explosions, and fires, is minimised.^3,4,5

1) Green Nano-catalyst:

a) Rhodium nanoparticles:

The hydrogen reduction method in water as a solvent, the ethanol reduction method in an ethanol–water mixture in which the ethanol can be rotovaped, and numerous other green approaches can all be used to make rhodium nanoparticles. The solvent used to make rhodium nanoparticles adsorbed on titanium dioxide supports is water. They're also excellent catalysts for the hydrogenation of arenes in water. Various types of ionic liquids, which are green solvents 20, are used to stabilise the rhodium nanoparticles. The hydrogenation of acetophenone was aided by the introduction of rhodium nanoparticles as catalysts. Using rhodium nanoparticles produced in water, hydrogenations of several nitrogen-, oxygen-, and sulfur-heterocyclic aromatic compounds were catalysed. The rhodium nanoparticles of various sizes produced with water as the solvent. The reaction is carried out at room temperature and atmospheric pressure with mild reaction conditions.^{6, 7}

b) Titania nanoparticles:

Metallic and metal oxide nanoparticles (NPs), such as titanium dioxide NPs, are becoming highly significant due to their potential use in novel medical therapies, such as liposomes, micelles, quantum dots, dendrimers, or fullerenes, among polymeric NPs, liposomes, micelles, quantum dots, dendrimers, or fullerenes. Titanium dioxide (titanium (IV) oxide, titania, TiO2) is an inorganic chemical that has received a lot of attention recently due to its photoactivity. TiO2 produces a variety of reactive oxygen species after being exposed to UV radiation in aqueous solutions (ROS). The capacity to produce reactive oxygen species (ROS) and consequently induce cell death has been used in photodynamic therapy (PDT) to treat a variety of ailments ranging from psoriasis to cancer. Titanium dioxide nanoparticles have been investigated as photosensitizing agents in the treatment of malignant tumours and in the photodynamic inactivation of antibiotic-resistant bacteria. In PDT, TiO2 NPs can be employed as photosensitizers on their own, as well as in composites and mixtures with other chemicals or biomolecules. Furthermore, different chemical molecules can be grafted onto TiO2 nanoparticles to create hybrid materials. These nanostructures can show higher light absorption, allowing them to be used in medicine for focused therapy⁸

c) Iron nanoparticles:

Polymers as capping agents in water as a green solvent, as well as other green approaches, have been used to make iron nanoparticles. Tea polyphenols were used to create a green synthesis of iron nanoparticles that did not require the addition of additional polymers or surfactants. For the treatment of organic pollution, iron nanoparticles are utilised to catalyse hydrogen peroxide. For alkene and alkyne hydrogenations, iron nanoparticles have been employed as environmentally friendly catalysts^9.

d) Palladium Nanoparticle:

Palladium is a silvery white valuable metal with a high density. Anogeissus latifolia, Cinnamom zeylanicum, Cinnamomum camphora, Curcuma longa, Doipyros kaki, Gardenia jasminoides, Glycine max, Musa paradisica, Ocimun sanctum, Pinus resinosa, and Pulicaria glutinosa have all been used to make biogenic palladium and platinum nanoparticles. The properties of palladium nanoparticles generated from various plant sections. Because of their huge surface-to-volume ratio and high surface energy, they are used as both heterogeneous and homogeneous catalysts. They are employed in a variety of medical diagnosis without causing DNA damage. The heterogeneous catalytic activity of palladium nanoparticles produced from herbal extracts was investigated in the Suzuki–Miyaura coupling reaction. It can be carried out in an aqueous solution in the open without danger of dissociation because it is a ligand-free catalytic reaction¹⁰.

e) Silica nanoparticles:

For the production of highly substituted pyridines, moderate and environmentally friendly silica nanoparticles were utilised as catalysts. After three reuses, these catalysts retained the majority of their catalytic activity. Catalysts made of silica nanoparticles for the production of highly substituted pyridines. The Stoeber synthesis process may be used to easily manufacture silica colloids and nanoparticles, which can then be redisposed in water, an environmentally benign solvent. For the synthesis of thioethers, thioesters, vinylthioethers, and thio-Michael adducts, environmentally benign silica nanoparticles have been utilised as catalysts 14. For the synthesis of 4H-pyrans and polysubstituted aniline derivatives, very mild, neutral, and reusable silica nanoparticles have been utilised as catalysts. Environmentally benign and gentle conditions are used to create tubular silica nanostructures. In the microwave-assisted homocoupling of terminal alkynes, these catalysts are found to be exceedingly active and selective. Mild, effective, and ecologically friendly silica nanoparticles were used in the three-component synthesis of highly substituted pyridines¹¹.

f) Zinc oxide nanoparticles:

Because of their high stability, inherent photoluminescence properties that can be useful in bio sensing applications, and wide band-gap semiconductor properties that can be useful in photo catalytic systems and the promotion of reactive oxygen species generation, ZnO nanoparticles have gained interest in biomedical applications. Recently, ZnO nanoparticles have been employed in cholesterol biosensors, dietary modulators for hydrolase activity in diabetes and hyperglycaemia, and cell imaging. ZnO nanoparticles have also showed potential in controlling allergy reactions by inhibiting mast cell degranulation. As a result of the diversity of these activities, ZnO nanoparticles have become important in interdisciplinary research communities involving physicists, chemists, and biologists. The intrinsic preferential cytotoxicity against cancer cells in vitro is one of the key advantages for evaluating ZnO nanoparticles for treatment in cancer. It is claimed that engineering design to limit adverse effects on normal body cells, which has been found at very high concentrations of ZnO nanoparticles, particularly those in the smaller size range of 4 - 20 nm, could increase their cancer cell selectivity even more. In this sense, the surface chemistry of ZnO nanoparticles lends itself to functionalization with targeting proteins or chemical groups, which could be a key to making them non-toxic to normal cells while keeping their cancer-targeting and killing properties¹².

g) Nickel–platinum nanoparticles:

The ethanol reduction method, hydrogen reduction method, and other green approaches have all been used to make bimetallic nanoparticles. Using a green colloidal approach 41, nickel encapsulated by Pt (NiPt) has been produced. As electrocatalysts, platinum nanoparticles are quite expensive, so the solution is to minimise the cost by synthesising NiPt bimetallic nanoparticles¹³

h) Iron nanoparticles:

i) Gold nanoparticles:

The creation of colloidal solutions containing metallic nanoparticles is one of the most important topics of current research. Gold nanoparticles and their uses are among the most researched materials in a variety of fields, including optoelectronics and catalysis. Nano biotechnology, biosensor investigations, imaging of cell architecture, and targeted medication delivery all involve gold and silver nanoparticles. Colloidal nanoparticles are now being explored because of their distinct physicochemical features compared to "bulk". In all nanotechnology applications, the shape or size of the nanoparticle is critical. Metal nanoparticles have unique features that lead to applications such as fuel cells and environmental protection¹⁵

j) Nickel nanoparticles:

Due to its superior ferromagnetic features such as magneto-crystalline anisotropy, strong coercive forces, and chemical stability, nickel nanoparticles (NiNPs) have been researched for a variety of potential applications during the last decade. As a result, NiNP synthesis techniques, postulated reaction mechanisms, and applications have all improved dramatically¹⁶

k) Silver nanoparticles:

Silver nanoparticles have been created utilising numerous environmentally friendly processes, including seed-mediated growth, synthesis in the presence of ionic liquids, and other reduction methods like hydrazine reduction and sodium borohydride reduction. A green photo catalytic approach for producing silver nanoparticles has been developed, with the synthesis taking place in water. A photochemical green synthesis approach 36 is used to make calcium-alginate stabilised silver nanoparticles. The 4-nitrophenol reduction process is catalysed by these nanoparticles⁷

l) Aluminium oxide nanoparticles:

Aluminium oxide nanoparticles have been synthesised using solvent-free methods as well as methods that employ water as a solvent. Water is used to create aluminium oxide nanoparticles, making it a green nanocatalyst. In mild circumstances, it is utilised as a catalyst for the production of 1, 5-benzodiazepine and 1, 5-benzothiazepine derivatives. For the solvent-free synthesis of 2-aryl-2, 3-dihydroquinazolin-4(1H)-ones, green aluminium oxide nanoparticles are utilised, resulting in greater yields and shorter reaction times¹⁸

Importance of green reactions in industry:

Green chemistry isn't just a good challenge in the lab; it's about making the world a better place. Green approaches established by academic and industrial researchers are becoming more widely available, allowing businesses to market these concepts. Green chemistry ideas have already been adopted by industry, ranging from tiny firms to huge corporations, as a strategic move toward sustainability. The development of less hazardous processes and commercial products, the shift from inefficient chemical routes to bio-based synthesis, and the replacement of oil-based feed stocks with renewable starting materials are just a few examples of major decisions that will have far-reaching implications for global chemical markets.

Some important reaction:

· There is huge industrial importance of reduction reaction in Greener synthesis, as published by Merck, involves a novel cinchonidine based PTC-catalyzed Aza-Michael reaction for configuring the single stereocenter. Also, there is an increase in overall yield by 60%, reduction in raw material cost by 93% and reduction in water usage by 90%. It has been estimated that, once operational, this optimized process will lead to reduction of more than 15,000 MT of waste over the life time of Letermovir. Life-Cycle Assessment reveals that the green process is expected to decrease the carbon foot-print by 89%. It is quite evident from the green synthesis of Letermovir that the Green Chemistry is not only environmentally friendly but also economically lucrative.

· In 2008, Pfizer improved the classical route for the synthesis of Pregabalin by adopting bio-catalysis as a key step which led to 90% reduction in solvent usage, 50% reduction in the requirement of raw materials besides energy savings. Solvent and energy saving in the process is equivalent to reducing 3 million tons of CO₂ emissions which is actually equivalent to taking 1 million Indian cars off the road for a year.

· Palladium nanoparticles with perfluoro tags were utilized as catalysts for the alkylation of aryl halides with water as the solvent. In environmentally friendly settings, multi-walled carbon nanotubes coated with palladium nanoparticles have been employed as catalysts for hydrogenation and alkylation processes.

· 2-hydroxypropyl—cyclodextrin (-HPCD) is used as both a reluctant and a stabiliser in the preparation of palladium nanoparticles in water. The Heck cross-coupling reaction in water is catalysed by these palladium nanoparticles. Under hydrothermal circumstances, nickel nanoparticles were discovered to be excellent catalysts in an environmentally friendly Heck reaction^19,20

· For environmentally friendly lactonization of diols 8, gold, silver, and copper nanoparticles supported on hydrotalcite have been employed as catalysts. Copper nanoparticles supported on hydrotalcite are a highly efficient heterogeneous Nan catalyst for lactonization of different diols without the use of oxidants 83. It's worth mentioning that these findings were acquired under environmentally favourable circumstances.

· Methyl esters were made by employing oxygen as a green oxidant and gold nanoparticles supported on silica 87 in a one-step catalytic esterification of primary alcohols. This method is more environmentally friendly than traditional methods since it does not employ environmentally unfavourable oxidants, strong acids, or an excess of reactants. A TEM image and size distribution histogram of gold nanoparticles utilized as catalysts for the esterification of primary alcohols.

· Without the need of a solvent, zinc aluminate nanoparticles have been employed as catalysts for the acetylation of amines, alcohols, and phenols. It is an environmentally friendly, safe, and cost–effective method 82. Furthermore, when nanoparticles were used instead of a bulk catalyst, the catalytic activity was greater²¹

CONCLUSION:

Molecular scientists have invented the chemicals, materials, and industrial techniques that have enabled economic and societal progress for millennia. Green Chemistry ensures that all of the inventiveness that has long been associated with the discipline of chemistry is applied in a way that considers the impact on people and the environment as a design criterion. Green Chemistry has demonstrated that through innovation, businesses may become more economically lucrative while also being more environmentally friendly.

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Received on 26.02.2022 Modified on 10.04.2022

Research J. Science and Tech. 2022; 14(3):188-192.

DOI: 10.52711/2349-2988.2022.00031