Ecofriendly Epoxidation of Olefins Using Supported Au NPs Catalyst

Epoxides are highly used as the intermediates for the production of pharmaceuticals, drug intermediates, food additives, perfumes, agrochemicals and polymers. It is very imperative to develop a greener and a sustainable reaction protocols on the continuing demand of the epoxides.

Propylene oxide (PO) is widely used for the production of propylene glycols and polyurethane. It is found to be one of the most significant members of epoxide family and its total production has been reached nearly about 10 million tons per annum [1, 2]. Hydrogen peroxide (H₂O₂) based process was industrialized in 2008, for the commercial production of PO through EVONIK and it is selectively produced PO with H₂O as the only by-product [3]. But, it can be seen that, H₂O₂ is found to be comparatively expensive and gets decomposition quickly during the chemical reaction and also it is very difficult for the transportation and the search for an improved process are continued. Various transition of the metal catalysts are explored, however, these homogeneous catalytic processes required a tedious workup in addition to the thermal instability of the catalysts for the generation of wasteful alcohols as the by-products [4].

As per the available reports, the various homogenous catalysts for the epoxidation of olefins possess many advantages like better yields, high selectivity (chemo-, regio- and enantio) and easy optimization for the catalytic systems. All the catalytic sites are easily accessible due to its soluble metal complex by modifying the deployed ligands as well as the metals [5]. But, the major drawback in terms of their separation was the homogenous catalysts have been contributing to tedious work up and recycling [6]. On the other hand, nanocatalysts, exhibited the typical advantages with heterogeneous catalysts with its efficiencies compared with homogeneous counterparts and were highly prized because of its excellent recyclability, high selectivity, good chemical and thermal stability [7].

Numerous supported metal nanoparticles was developed on keeping with green chemistry principles, and are applied for the epoxides synthesis, greener and mild under various sustainable conditions. The deploying reagents like H₂O₂, TBHP and O₂ as oxidants, and the latter being the one of the most abundant, and environmentally benign of inexpensive oxidant.

Pertaining to many reports, Haruta et al. have investigated the Au NPs/Ti-SiO₂ can bring out of 8.5% conversion with 91% selectivity of propylene for PO [8].The large increase in the conversion can be mainly attributed to the presence of 3D interconnected porous structure with their earlier experiments, which will enhance the diffusion of reactants and products via the catalyst. Furthermore, the PO molecules can easily oligomerize for the formation of low-molecular weight polymer compounds which can lead to the gold nanoparticles (AuNPs)deactivation.

Trimethylsilylation of Ti-SiO₂ have decreased the polymerization and enhance the catalyst stability through the reduction in the hydrophilicity of the support (Figure 1). Further they have reported that the Au clusters, are deposited on the alkali-treated TS-1 with its diameters smaller than 2.0 nm. Hence, this can catalyze the propylene epoxidation along with O₂ in the presence of a small amount of H₂O with C₃H₆ conversion of 0.88% and PO selectivity of 52%. Interestingly, it can be observed that, CO₂ as the major product that was produced with the selectivity of exceeding 80%, however, the PO cannot be able to generate due to the absence of H₂O from the feed gas [9].

Figure 1. Propylene epoxidation with H₂ and O₂ on trimethylsilylated Au/mesoporous TiSiO₂ [8].

Corma and co-workers have investigated the AuNPs with new unconventional supports [10]. For the past two decades, Graphene (GN), is found to be one of the most interesting material with a two-dimensional (2D) material that comprised of carbon layer atoms packed with a honeycomb network structure.GN is widely used as the support for the heterogeneous catalysis is concerned, because of its high thermal and chemical stability with an excellent dispersion in the organic solvents [11]. It was reported that Au/Graphene catalyst with a highly selective epoxidation of propene was used with H₂/H₂O/O₂ mixtures. Further, they have concluded that the propene combustion was energetically hampered on this material (Au1/G1V) with the dissociation of O₂ into two oxygen atoms.

Thomson et al. have supported the presence of H₂ endorsed by the formation of gold hydroperoxide (Au-OOH) species which can afford the PO with more selectively [12]. Au atoms can generate gold hydroperoxide H-Au-OOH intermediates from H₂ and O₂ and the fact of isolation is being very strongly bound to the support which renders for the subsequent chemical reaction with propene, to give the yield of PO and H₂O. For preserving the morphology, electronic properties, and the ability to activate the O₂, the Au clusters can be strongly bonded with the defective graphene, and the DFT calculations were also indicates the Au atoms and sub nanometer clusters. Inspite, the coordinating capability of an isolated Au atom is been limited, due to the fact of H₂ dissociation which is energetically feasible with the non-interaction of propene along with H atoms that can be appended to Au, and therefore, suppressing the propene hydrogenation. However, over the larger clusters, the propene adsorption are found to be close with H atoms which would be possible for the formation of propane [10].

Our SNB team have emphasize this research article to enrich our viewer’s knowledge about the ecofriendly epoxidation of olefins supported by AuNPs catalyst. Because of the instability of the epoxide ring, especially with allyl acetate, the production of epoxides is found to be a challenging one. A brief overview from the promising results of the supported nanocatalysts materials based on Au, Co, Ag and Mo NPs were provided here describing with some unique properties that have exploited brilliantly for a wide range of olefin oxidations [1]. Various support materials like graphene, dendrimer, polymer resins, HAP, LDH, MOF, POM, polyoxometalates, metal oxides, zeolites, CNT, carbon materials, etc. were been successfully applied. Outstandingly, the supported nanocatalysts will provide a load sized amounts of active sites exceptionally with high accessibility on comparison to the conventional catalysts with multifold increase of its catalytic activity. Nanocatalysis is not just a field of academic curiosity. Undoubtedly, it would be an emerging field to develop a greener and sustainable processes in the near future which is an evitable one.

References

1. S. A. Sharma, Green Chemistry, DOI: 10.1039/D0GC01927E (2020).
2. Nexant Process Evaluation Research Planning (PERP) Report-Propylene oxide, (2018).
3. F. Cavani, et al., ChemSus Chem., 2, 508 (2009).
4. S.T.Oyama, In mechanism in Homogeneous and Heterogeneous Epoxidation catalysis, ed., Elsevier:Amsterdam, 2008, chapter 13, pp 355-371.
5. Metal Nanoparticles for catalysis Advances and Applications, ed. F. Tao, The Royal Society of Chemistry, 2014.
6. V. Polshettiwar, et al.,Nanocatalysis: Synthesis and applications, ed. John Wiley & Sons Publication, 2013.
7. Green Photoactive Nanomaterials: Sustainable Enery and Environmental Remediation, ed. N. Nuraje, et al., Royal Society of Chemistry, London, Chapter 8, pp 168-201 (2016).
8. B. Chowdhury, J.J.B. Suaarez, M. Date, S. Tsubota, M. Haruta, Angew. Chem., Int.Ed. 45, 412 (2006).
9. J.H. Huang, T. Takei, H. Ohashi, M. Haruta, Appl. Catal. A., 435-436, 115 (2012).
10. A. Pulido, M. Boronat, A. Corma, J. Phys. Chem. C., 116, 19355 (2012).
11. U.C. Rajesh, J. Wang, S. Prescott, T. Tsuzuki, D.S. Rawat, ACS Sustainable Chem.Eng., 3, 9 (2015).
12. A.M. Joshi, et al., J. Phys. Chem. C., 111, 7841 (2007).

 

Dr. S. Thirumurugan

                                                       Assistant Professor 

               Department of Chemistry 

National College (SF), 

Thiruchirappalli

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