EXPLORING THE ROLE OF TRANSITION METAL CATALYSTS IN GREEN CHEMISTRY

 

EXPLORING THE ROLE OF TRANSITION METAL CATALYSTS IN GREEN CHEMISTRY

Aiswarya Lakshmi S

St. Joseph College of Teacher Education for Women, Ernakulam

 

ABSTRACT

Transition metal catalysts play a pivotal role in driving the principles of green chemistry by enabling sustainable synthetic pathways with minimized environmental impact. This article provides a comprehensive overview of the multifaceted contributions of transition metal catalysts in promoting green chemistry practices. It discusses the fundamental principles underlying the design and application of transition metal catalysts in various green chemical transformations, highlighting their ability to facilitate atom-economical processes, improve reaction selectivity, and reduce energy consumption. Moreover, the article explores recent advancements in the field, including the development of novel transition metal catalysts with enhanced activity and stability under environmentally benign conditions. Case studies illustrating the application of transition metal catalysts in key green chemistry reactions, such as C-C bond formation, C-H activation, and asymmetric catalysis, are presented to underscore their significance in sustainable synthesis. Furthermore, challenges and future directions in harnessing the full potential of transition metal catalysts for advancing green chemistry principles are discussed, aiming to inspire further research efforts towards the development of efficient and sustainable catalytic systems.

Key Terms: Catalysts, Green Chemistry, Synthesis, Carbon nano tubes, Membrane Filter, Fluoride Remediation, Transition metal carbide, Catalysis, biomass, Glycols, Lignin, Hydrodeoxygenation

Introduction

Transition metals have emerged as indispensable tools in the realm of green chemistry, revolutionizing the landscape of chemical synthesis towards sustainability. Green chemistry, also known as sustainable chemistry, embodies the principles of designing chemical processes and products that minimize environmental impact, conserve resources, and prioritize human health and safety. At the heart of this paradigm shift lie transition metal catalysts, which serve as molecular architects orchestrating transformations with precision, efficiency, and environmental consciousness. The exploration of transition metal catalysts in green chemistry is a journey of innovation, driven by the urgent need to address global challenges such as climate change, pollution, and resource depletion. Transition metals, occupying a strategic position in the periodic table with their unique electronic structures and versatile bonding capabilities, exhibit a remarkable array of catalytic properties that can be harnessed to drive sustainable chemical transformations. This article embarks on a comprehensive exploration of the multifaceted role of transition metals in green chemistry, traversing the vast landscape from fundamental principles to cutting-edge applications. We delve into the fundamental aspects of transition metal catalysis, elucidating the underlying mechanisms and principles governing their efficacy in driving sustainable chemical processes. From catalytic C-C bond formations to selective oxidations and hydrogenations, transition metal catalysts have become indispensable allies in the quest for greener chemical transformations. Furthermore, this article delves into the pivotal role of transition metals in enabling atom-efficient processes, wherein catalytic cycles are designed to minimize waste generation and maximize the utilization of renewable feedstocks. By harnessing the power of transition metal catalysts, chemists can unlock pathways that lead to cleaner, more efficient syntheses of pharmaceuticals, agrochemicals, and fine chemicals, thereby mitigating the environmental footprint associated with traditional synthetic routes. Moreover, the exploration extends beyond conventional homogeneous catalysis to encompass emerging frontiers such as heterogeneous and bioinspired catalysis, where transition metal catalysts continue to catalyze innovation in green chemistry. Through a series of case studies and illustrative examples, this article showcases the diverse applications of transition metal catalysts in driving sustainable chemical transformations across various sectors. In summary, this article serves as a compass guiding researchers, practitioners, and enthusiasts through the intricate landscape of transition metal catalysis in green chemistry. By unraveling the underlying principles, highlighting key advancements, and envisioning future prospects, we aim to inspire a collective endeavor towards harnessing the transformative potential of transition metals for a greener, more sustainable future.

 

Transition metal carbide catalysts for biomass conversion

The increasing demand for sustainable energy resources has initiated the investigation of biomass conversion over a wide range of catalysts. Among those, transition metal carbides have been extensively studied and demonstrated distinct reactivity and or selectivity from transition or noble metals in a variety of chemical reactions. catalytic conversions of (hemi)cellulose, lignin and some typical platform chemicals to fuels or fine chemicals in the aspects of C-C, C-O-C and C-O-H bonds cleavages. Great progresses have been made for both the synthesis and application of transition metal carbides in biomass conversion. A variety of bottom-up construction strategies have been developed to control the structure and size of metal carbides, which play an essential role to their applications in this fields. As such, the catalytic conversion of biomass to fuels and chemicals over carbide catalysts has attracted more and more attentions. Different types of carbides including tungsten and molybdenum carbide have been widely used in (hemi)cellulose, lignin and platform chemicals conversion, and some carbides have been demonstrated to be highly selective in hydrodeoxygenation reactions. However, despite of the potential of transition metal carbides in the upgrading of biomass and/or biomass derived chemicals, challenges still exist in both the catalysts synthesis and the application in catalytic biomass upgrading.

1) Synthesis of highly dispersed and stable metal carbide catalysts. Great efforts have been made to improve the synthesis method, including the control of the particle size, phase structure, and stability of carbide catalysts. However, the synthesis of highly disperse transition metal carbides nanoparticles (i.e., 1-5 nm) is still limited to special synthesis protocols such as using confinement of nonporous supports. Preparing highly dispersed metal carbide with high activity and stability is still challenging. Currently, the prevailing carbide catalysts are still prepared at high temperatures, which caused the irreversible particle aggregation. The solid-solid phase reaction has a long history, but it is conducted at high temperatures, and it is very difficult to control the carburization process. Although the method was improved to some extent by changing the metal precursors and employing high surface area carbon precursors, the particle size of most metal carbides reported is still larger than 5 nm. In contrast, the liquid derived solid-solid phase reaction shows a great potential to control the particle size of finally carbides Especially, the liquid phase provides a versatile media for the controllable synthesis of the precursors, which significantly improved the dispersion of carbides to 1-5 nm. The solid-gas phase reaction, i.e., temperature program reduction method, is relatively mature, which has been widely used for preparing different carbides. Nevertheless, it still has the drawback of carbon depositions.

2) Synthesis of high purity carbide catalysts. The carbide prepared by traditional methods always have the problems of surface carbon deposition, and even a mixture of different carbide phases, making the correlation of structure-functionality of metal carbide catalysts difficult. Extensive efforts have been being made to control the phase and surface structure of metal carbides. One of the promising ways is exposing the clean metal surfaces to unsaturated hydrocarbons and then annealing at certain temperatures. Pure carbides with controllable carbon/metal ratios could be synthesized, which builds a solid foundation for the reaction mechanisms study. Additionally, besides the commonly synthesized carbides, some non-stoichiometric carbide with special defects, or unique structure carbides have been proved to have special activities in biomass conversion [170-172]. These achievements will guide the transition metal carbide catalysts synthesis for the biorefinery process.

3) Stability of metal carbide in biomass conversion. The stability of metal carbide is crucial for its applications. Many factors, such as oxidation, particle aggregation, coke formation and leaching, affect the stability of carbides, as previously discussed by. Transition metal carbides are very sensitive to oxygen containing compounds. The presence of high-pressure water inhibited the adsorption of the reactant, and induced the irreversibly deactivation. Similarly, the introduction of oxygen, water and carbon dioxide significantly decreased the toluene formation rates in m-cresol hydrodeoxygenation over Mo2C catalysts. To maintain the high activity of carbides, different strategies have been proposed. For instance, some researches modified the carbide with noble or transition metals and operated the reaction under high pressure hydrogen, which promotes the activation of the hydrogen with the increased H*-site and prevents the carbide from oxidation as proved by experimental results and DFT calculations .Besides, many new methods such as modification of the carbide surface to be hydrophobic and spatial confined carbides have been proposed to stabilize carbide catalysts, which shows high potential for biomass conversion.

4) Transition metal carbides for some novel reactions in biomass conversion. As we discussed, carbide catalysts demonstrated unique reactivity in cellulose, lignin and platform chemicals conversions. Tungsten carbides selectively converted carbohydrates to glycols (i.e., EG and PG), which had a high selectivity to EG (WCx 75% EG yield). Moreover, it shows promising selectivity in the hydrodeoxygenation of lignin derived phenolics. Different from most of metals showing high activity for aromatic ring saturation, the transition metal carbide catalysts selectively cleavage the Csp2-O without consuming extra hydrogen. In spites of these achievements, the application of carbides catalysts in biomass is still needs to be exploited. For instance, more traditional carbide catalysts such as Re, Ti, V or Zr carbides could be explored in the hydrodeoxygenation of bio-chemicals, and some novel structure of carbide like 2D transition metal carbides with preferential exposed facet (MXene) could be helpful in fundamental understanding the catalysts in biomass conversion. Additionally, more attempts may be made on the application of carbide in new reactions, especially the reactions concerning weak hydrogen bonds cleavage such as ethanol and alcohols upgrading.

5) The synergistic effect between transition metal carbides and metals. Other than directly used as catalysts, transition metal carbides could serve an important support for metals, which has been shown promising in other catalytic reactions due to the synergistic effect between metals and metal carbides. For example, atomically dispersed Pt1 (single atom Pt) on α-MoC has been shown highly active for hydrogen production in the aqueous reforming of methanol. Due to the synergistic effect of Pt1 and the maximized active interface of Pt1/α-MoC and α-MoC, the reaction temperature can drop to as low as 423–463 K with an average turnover frequency of 18,046 moles of hydrogen per mole of platinum per hour. In another attempts, the synergistic effect between Au1 (single atom Au) and α-MoC resulted in the high activity of Au1/α-MoC catalysts in water-gas shift reaction at low reaction temperatures (393 K). These promising results will shed a light on the synthesis and application of the novel carbide in biomass conversion.

6) In situ investigations of reaction mechanisms. The conversion of biomass to fuels or chemicals always conducts in the liquid phase, which increases the difficulty in in situ characterizations and reaction mechanisms study. With the development of state-of-the-art characterization tools, some characterizations could be conducted under operando or reaction conditions. Recently, in situ Attenuated Total Reflection Infra-Red (ATR-IR) spectroscopy has been used in monitoring the reaction process of biomass conversion, which can be conducted at 623 K and 200 bar, covering most reaction conditions in biomass conversion. Moreover, sum-frequency generation vibrational spectroscopy (SFG-VS) has been proved as a surface/interface sensitive spectroscopic probe for characterizing the properties of molecular surfaces and interfaces. It will also provide a broad range of information in the catalytic conversion of biomass due to its multi-phase reactions. Some routine instruments have been modified or combined to operando characterizations based on the improved reaction conditions, including the X-ray absorption, FT-IR, Uv-Vis, Mössbauer spectroscopy. Very recently, Murugappan et al. used operando near-ambient pressure X-ray photoelectron spectroscopy to unveil the role of MoO3 and Mo2C in hydrodeoxygenation of anisole. Different from the MoO3 catalysts with 5+ and 6+ oxidation states, the Mo2C showed negligible oxidation state changes and maintained constant 2+ states under the hydrodeoxygenation reaction conditions. Additionally, DFT has been used to model the catalysts based on the in-situ characterizations, which in turn directed the catalysts design and characterizations. All these efforts will promote the transition metal carbide catalysts design and reaction pathways understanding.

Green Synthesis of Carbon Nano tubes and Application as a membrane for fluoride remediation

In most African rural communities, ground water is the primary source of drinking water. Fluoride isa harmful substance found in ground water that causes serious health problems. The only answer to the fresh and clean water crisis is water treatment. Sustainable, cost-effective and efficient, membrane filtration technologies are becoming progressively vital to solve the scarcity. Owing to their unique tunable physico-chemical properties, carbon nanotubes (CNTs) were used in water treatment. This paper thoroughly reviews the health effects of consuming excess fluoride ions. The recent progress on the synthesis of CNT was also highlighted. A special emphasis was given to green synthetic routes for its preparation. Green catalysts provide defined size and morphology than conventional materials and also prevent metal leaching. The effects of incorporation of CNTs as filler in the matrix have been discussed in detail. The large-scale production of CNTs and their growth mechanisms; for water purification purposes are explored. It has been observed that CNTs have got an excellent filtration performance of fluoride, low biofouling activities and high-water flux capacity. Besides, this review attempt to provide a clue for the functionalized CNTs based membranes as potential solutions in water purifications of fluoride ions in the future.

                                   Eastern Shoa zone of Oromia regional state, Ethiopia lies totally in the rift valley and people of this region are at the extreme risk of lack of pure drinking water. Their life is totally relying on underground water, whereas more than 90% of the sample contain fluoride ion above WHO standard level 1.5 mg/l. It was also reported that many villages are totally paralyzed due to fluoride problems. The Oromia regional state government employed some traditional water treatment strategies to rescue the people but it is not yet efficient. The CNTs with remarkable physico-chemical properties exhibit a myriad of applications, including supercapacitors, batteries, catalysts, and adsorption. Thus, in this review, we realized that CNTs membrane filter will be the most effective and efficient nanomaterials to remove fluoride ion from the drinking water by reverse osmosis mechanisms. Even if the conventional membrane filtration mechanisms are expensive, require skilled manpower and time taking, with this review we identified the efficient and effective material, CNT, for the simple, eco-friendly and cheap way of removing fluoride from aqueous media due to the presence of dangling bond-forming capacity and the vertical alignment with the available inner and outer surface of CNT. The toxicity of conventional CNTs can be reduced by the application of green catalysts in the place of metal catalysts. Hence, we thoroughly reviewed the green synthesis, growth mechanism and indicated the promising potential applications of CNTs towards the waste water treatment potential in this review article. In addition, exploration of the degradation mechanisms for CNTs in the presence of diverse contaminants is a very challenging and indispensable activity.

Synthesis of Benzimidazoles From o-Phenylenediamine via Nanoparticles and Green        Strategies Using Transition Metal Catalysts

Benzimidazole is a heterocyclic moiety of immense importance as it acts as a primary “biolinker” in diverse synthetic routes to obtain bioactive compounds. Substituted benzimidazoles are known to possess a varied range of pharmacological applications, namely, anti-cancer, anti-diabetic, anti-inflammatory, and antiviral like anti-HIV and anti-fungal. A number of reviews covering the important aspects of benzimidazoles such as pharmacological activities, SAR studies, and well-known methods of synthesis have appeared in the literature. However, green synthetic methods particularly using transition metal (TM) catalysts and their nanoparticles, although being more viable and extensively applied by researchers in the present scenario,have not been exclusively and expansively reviewed. Besides this, the vital precursors required for knitting the skeleton of benzimidazole are mainly o-aryldiamines. The conventional synthesis generally involved the condensation of these diamines with carbonyl/carboxylic acid derivatives either via high temperature heating or via adding strong acids, mostly resulting in poor yields or mixtures. However, recent trends are replacing these conditions by mild and green conditions through TM catalysts. Therefore, the current review emphasizes on the recent trends adopted in the synthesis of benzimidazoles using condensation reaction of o-phenylenediamines and various aldehydes/ester/amide/alcohols with TM in a catalytic role in nanoform and under environmentally benign green conditions.

                                            Emphasizing on the use of TM catalysis, this review summarizes numerous one-pot highly efficient and eco-friendly synthetic methodologies that have been applied for the synthesis of benzimidazole moiety. Thus, long reaction hours in classical reactions proposed by Phillips–Ladenburg and Weidenhagen are replaced with minutes and conventional requirement of strong acidic condition has been overcome by employing TM catalysts in minimal amount. The spectrum of the methods presented, including Cu/Fe/Zn/Mn/Co/Ni/Sc/Mo/Ti/Ag/Ce/La catalyzed processes, encompasses the utility of nano and green TM catalysts in the reactions. Reusability of all these catalysts was remarkably high proving them a competent practice for the synthesis.Diverse synthetic procedures have been precisely demonstrated with mechanistic detail wherever needed. Although synthesizing a nanocatalyst was a tedious task, but quick reaction and minimal chemical waste provided the impetus to make it a popular trend. About 43 different nano-TM catalysts were explored and are proven to be highly efficient in providing a maximum yield of the product. Regioselective synthesis of either 2- or 1,2-disubstituted benzimidazoles were alsoachieved in many reactions. Likewise, the review provides comprehensive details of green TM catalysis used for the said synthesis, emphasizing the efficacy and increased usage of eco-friendly methodologies. Hence, this study provides numerous significant clues on synthetic protocols towards benzimidazoles and would be of potential utility in the process of designing superior catalysts in the future. This would certainly suffice the never-ending urge and need of fetching new benzimidazole nucleus, owing to its biological importance and leading role in the synthesis of potent bioactive molecules.

Conclusion

In conclusion, the exploration of transition metal catalysts in green chemistry represents a transformative journey towards sustainable chemical synthesis. Throughout this article, we have traversed the diverse landscape of transition metal catalysis, from fundamental principles to cutting-edge applications, showcasing their pivotal role in driving sustainable solutions. Transition metal catalysts, with their unique electronic structures and versatile bonding capabilities, offer unparalleled opportunities to design and execute green chemical transformations with precision and efficiency. By harnessing the catalytic prowess of transition metals, chemists can unlock pathways that minimize waste generation, conserve resources, and prioritize environmental and human health. Moreover, the exploration has underscored the critical importance of transition metal catalysts in enabling atom-efficient processes, wherein catalytic cycles are designed to maximize the utilization of renewable feedstocks and minimize environmental impact. From catalytic C-C bond formations to selective oxidations and hydrogenations, transition metal catalysts continue to catalyze innovation across various sectors, including pharmaceuticals, agrochemicals, and fine chemicals. Looking ahead, the future of transition metal catalysis in green chemistry holds immense promise, with emerging frontiers such as heterogeneous and bioinspired catalysis opening new avenues for sustainable synthesis. As researchers, practitioners, and enthusiasts continue to unravel the intricacies of transition metal catalysis, it is imperative to foster interdisciplinary collaborations and leverage technological advancements to accelerate the development and adoption of greener catalytic processes. In essence, the exploration of transition metal catalysts in green chemistry is not merely a scientific endeavor but a collective commitment towards building a more sustainable future. By embracing the transformative potential of transition metals and integrating green chemistry principles into chemical synthesis, we can pave the way for a cleaner, healthier, and more resilient planet for generations to come.

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