Blog Green Catalyst for Chemical Synthesis
Green Catalyst for Chemical Synthesis
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Introduction
Over the past 18th century, there is an incremental growth in global economy and steadfast improvement in the chemical industry. The prominent rise in the competitive outlook and the significant decline of the green resources has lead to the potential threat of unavailability of many renewable sources forthcoming. As a result, the chemical industry has been continuously coming across substantial confrontation regarding the hazardous impact on the environment conservation and sustainable development due to the massive utilization of chemical technology. In this context, there are several methods that have emerged in contributing to the new alternatives to have a steep rise in the efficiency of the chemical transformation. Among the various green methods available, enzymes are the most promising green catalyst to bring the change in reducing the hazardous waste and accompanying the chemical industry move closer to sustainable and environmental friendly future. Fig. 1 illustrates the connecting routes of the enzymatic bio-catalysis process with green chemistry to overcome the challenge of non-sustainability in the productive manner. It generally uses green methods to develop the chemical compounds replacing the environmentally risky raw materials and processes, into efficient and eco-friendly approach with high prolific and selective enzymatic pathways, while reducing the required additional purification steps and generation of transitional harmful waste.
Application with benifited outcome.
| S.No. | Products | Green approach | Outcome |
|---|---|---|---|
| 1 | Green fuels | Lipases enzyme can be considered for esterification and transesterification reaction for bio-fuel production resulting into high purity products | • Milder reaction conditions |
| • Preventing oil oxidation | |||
| • Low energy cost | |||
| 2 | Bio-lubricants | Alternative to petroleum derivatives | • Reduces the acid wastewater |
| • Lower the reaction temperature | |||
| • Higher selectivity | |||
| • Doesn't generally manufacture the undesired by-product that causes corrosion of equipment | |||
| 3 | Bio-Insecticides | Chitanase enzyme act as potential agents for biological pest controls | • Cost effectiveness |
| Protease enzyme act as natural inhibitor of insect reproducibility | • Target specificity | ||
| Lipases are responsible for fractionation of lipids on the surface of insect resulting into pest control | • Being safe for non target organisms | ||
| • Easy production | |||
| • Environmental friendly | |||
| 4 | Bio-detergent | Enzymes like amylases, cellulases, lipases, and proteases are prominently used in the formulation | • Bio-friendly alternative to non- biodegradable detergents |
| • Enhances cleaning and efficiency of the final material due to the hydrolytic degradation |
Industrial perspective of enzymes in green chemistry. Above are the fascinating products produced by enzymatic catalysis presented to show the significance of biotechnology in aligning the green approach to the benefit of the environment and developing more sustainable product and processes. As mentioned, the utilization of the bio-catalyst in the chemical revolution allows the acquisition of the eco-friendly environment along with enhancing the processes by performing environmental benign chemical practices, following the world-wide requirement and the principles of green chemistry.
Organic synthesis
Organic reactions are the center of synthesis and processes of transforming matter. When conducting a holistic analysis, all processes and applications already mentioned in this text involve organic reactions of different complexities and for different purposes. Organic synthesis is the central science behind most of today's industrial processes and thus is also an important source of pollutants and toxic wastes. Therefore, the green chemistry idea calls for changing the way organic synthesis is done today by developing processes that obey green principles (Bevan and Franssen, 2006). The ideology of green chemistry generally demands that syntheses be sought that have greater energy efficiency, selectivity, simplicity, and safety for nature and human health (Li and Trost, 2008). The main characteristics of a green organic synthesis are:
(i) economy of atoms, in which the greatest possible amount of reagents must be incorporated in the product (Trost, 1995)
(ii) direct conversion of C–H bonds to C–C bonds (Trusova et al., 2016)
(iii) no use of protective groups, since this increases the number of steps in the synthesis (Young and Baran, 2009)
(iv) cascade reactions which can incorporate a series of steps for the synthesis of the product in only one process (Webb and Jamison, 2010)
(v) the use of biocatalysts, which are more selective and efficient, while at the same time being non-toxic (Reetz, 2013)
(vi) use of less toxic solvents such as water, supercritical CO2 and ionic liquids (Sheldon, 2005).
In this way, the already defined "white biotechnology" has great importance within organic synthesis. In many cases, the use of biocatalysts as enzymes or microorganisms can be a better alternative route to traditional synthesis processes, generally allowing greater selectivity and yield and in most cases being compatible with the use of aqueous media. In addition, the advances in metagenomics4 have allowed the production of extremely specific enzymes that cannot be obtained through culturing microorganisms. The applications of enzymes and microorganisms as alternative synthetic routes are a topic apart, due to the numerous applications that have been developed, mainly for the synthesis of chiral compounds and amino acids.
An interesting example that serves as a case study of organic synthesis performed through microorganisms was published in 2013 by Jiang and co-workers in Biosource Technology. They proposed the synthesis of succinic acid through Actinobacillus succinigeses. The biotechnological synthesis of succinic acid has been a recurrent focus of research, and there are several methods for obtaining it through microorganisms. These studies were reviewed and published in Current Opinion in Biotechnology by Lee et al. The reasons for large number of studies of succinic acid is the fact that it is an organic molecule with strong industrial interest, since it serves as a precursor for several other products such as tetrahydrofuran, γ-butyrolactone, 1,4-butanediol, and others. Industrially, synthesis of succinic acid is accomplished by the hydrogenation of maleic acid, oxidation of 1,4-butanediol alcohol and carbonylation of ethylene glycol. The objective of Jiang et al. was to obtain succinic acid through cellobiose obtained through enzymatic hydrolysis (enzymolysis) of sugarcane bagasse by the metabolism of Actinobacillus succinogenes, which is a gram negative bacterium found in bovine rumen and is one of the most promising bacteria for the production of succinic acid through sugar sources. Therefore, several production routes through a fermentative process have been proposed using industrial and agricultural wastes as starting material for organic synthesis, since they are renewable sources of carbon. Therefore, this work presented a biotechnological route to obtain succinic acid, using sugars obtained through the hydrolysis of sugarcane bagasse, making this synthesis renewable and in line with several concepts of green chemistry. Another example of alternative synthesis development is the socalled solar-to-chemical synthesis via microorganisms. In this chemical route, a microorganism with photosynthetic behavior – native or artificially introduced to its biological system – is used in the high-efficiency synthesis of several compounds, using CO2 as the carbon source. An interesting example of the implementation of such a system was published in Science by Yang and co-workers in 2016 . The conversion of solar energy into chemicals has a similar scheme to that of the fourth-generation fuels mentioned in the previous topic. But in this work, the authors developed a hybrid system that combined the high light capture efficiency of inorganic semiconductors with the ability of selective and efficient synthesis of biocatalyst microorganisms. They induced the photosensitization in an acetogenic non-photosynthetic bacterium, Moorella thermoacetica, through the biological precipitation of cadmium sulfide nanoparticles that aggregate to the bacterial cells, resulting in the photosynthetic synthesis of acetic acid from CO2, through the metabolism of this microorganism. Although it is a trivial molecule, the approach of this work has great relevance and innovation, since it can be used for the synthesis of other more complex molecules in future studies. The authors emphasized that there is a need to improve the capture of solar energy for sustainable chemical synthesis, and that some major advances have been made with the development of solid-state inorganic semiconductors, which generally do this more efficiently than photosynthetic processes. However, they do not have the same performance in transforming this energy into new carbon bonds (mainly in the conversion of CO2 into more complex molecules), that is, forming new C–C bonds for organic synthesis, something that photosynthetic systems do with high efficiency. Therefore, the approach used in their work aimed to combine the advantages of these two mechanisms: the ability to capture solar energy of semiconductors and the high catalytic performance of biological systems. One interesting aspect of this paper is the use of microorganisms to induce the precipitation of semiconductor nanoparticles.
This approach has some advantages because the semiconductors are, therefore,biocompatible. It also reduces the costs of their production, since all the chemical and technological refinement for the synthesis of semiconductors is not necessary in this case. Living organisms have the endogenous ability to perform the synthesis of inorganic materials, such as nanomaterials. One advantage of this is that they generally can very precisely control the shape and crystallinity of a developing inorganic material. In this respect, Iverson and co-workers published the first report of semiconductor nanocrystal synthesis in bacteria in 2004. They showed that E. coli, when incubated with cadmium chloride and sodium sulfide, synthesized intracellular cadmium sulfide (CdS) nanocrystals. The CdS nanoparticles were apparently being formed inside bacterial cells following transport of Cd2+ and S2- ions, and this formation is strongly dependent on the bacterial growth phase and strain. The precipitation of semi conductive nanoparticles is mainly performed by photosynthetic microorganisms. However, the authors of the solar-to-chemical approach argued that their metabolic pathways are sometimes less interesting than the metabolic pathways of non-photosynthetic microorganisms, because they offer simpler ways of obtaining a larger diversity of products through the simple reduction of CO2 to new molecules. In addition, there is a thermodynamic advantage of photosensitizing non-photosynthetic bacteria to reduce CO2 to other organic compounds, caused principally by the metabolic mechanisms present in these bacteria, such as the called Wood-Ljungdahl pathway.5 In this mechanism, CO2 is reduced to acetyl-CoA, which is a fairly common biosynthetic intermediate – and possibly to acetic acid – both of which can be biotransformed into other more complex organic molecules through the metabolism of engineered microorganisms. As previously mentioned, the authors then developed a hybrid system containing Moorella thermoacetica, a non-photosynthetic bacteria reducing CO2, and CdS as biologically precipitated semiconductor nanoparticles.
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