Development of carbonaceous nanomaterial based heterogeneous catalytic systems for organic transformations

Abstract

Within the broad area of nanoscience, catalysis plays an important role in academic scientific research as well as industrial production. Traditional homogeneous catalytic systems are highly efficient because the catalytic activity can be defined on a molecular level and the catalysts as well as the reactants are in sufficient contact in the reaction medium. However, removing them from the reaction medium without contamination of the target products requires tedious purification procedure. With the ecological and economical demands towards sustainable chemical methods, the recovery and reuse of catalysts is an important factor. With this motivation, various heterogeneous catalytic systems including mesoporous materials, metal-organic frameworks, metal oxides/sulphides, noblemetal nanoparticles etc. have been developed for photochemical and electrochemical catalysis, environmental remediation as well as catalyst for important organic transformations.As one of the most abundant elements on the earth, carbon is very attractive for catalytic applications. Several polytypes of carbon which include fullerenes, nanotubes, graphene, nanodimonds and amorphous porous carbon represent a rich class of solid carbonaceous materials with environmental acceptability and reusability. Their excellent thermal and chemical stability and adjustable surface functionality make them suitable for many applications ranging from catalysis, electrochemistry and adsorption to separation. In heterogeneous catalytic processes carbon materials are predominantly being used as support for other active catalysts. However, the recent developments involving various carbonaceous materials as efficient metal-free catalysts for several organic reactions has generated a new area of research broadly known as carbocatalysis. The catalytic activities of carbon materials are proportionately related to their defects, structure and surface chemistry. The unparallel flexibility in tailoring their physical (surface area and porosity) and chemical (surface functional groups) properties and their role in enhancing catalytic activity have generated a great interest in the scientific community.In this thesis, we explored the intrinsic catalytic activity of two important carbonaceous nanomaterials namely carbon dots and graphene oxide for variousimportant organic transformations. Further their composite with noble-metal nanoparticles and iron oxide nanoparticles have been explored for different oxidation reactions. The thesis is divided into the following chapters.Chapter 1: Introduction In this chapter, a general discussion and literature survey of homogeneous and heterogeneous catalysis including carbonanceous nanomaterials, their synthesis and application in organic reactions have been inscribed. Chapter 2: Graphene oxide as metal-free catalyst in oxidative dehydrogenative C−N coupling leading to α‑ketoamides and importance of dual catalytic activity In this chapter, we have shown graphene oxide (GO) as a heterogeneous, inexpensive and environmentally friendly carbocatalyst that enables the formation of α-ketoamides from activated aldehydes and amines through a crossdehydrogenative coupling pathway. Several controlled experiments and spectroscopic investigation revealed formation of hemiaminal as the intermediate. The dual catalytic activity of graphene oxide towards the C-N coupling reaction was established where the acidic charcter catalyzed the initial formation of hemiaminal intermediate and the oxidizing character catalyzed the oxidation of hemiaminal to α-ketoamide. Mechanistic studies by different experimental evidence proved that it was the carboxylic acid group that was only responsible for the observed catalytic activity of graphene oxide. Chapter 3: Probing carbocatalytic activity of carbon nanodots for the synthesis of biologically active dihydro/spiro/glyco quinazolinones and aza- Michael adducts In this chapter, we have shown carbon nanodots (CNDs) as an effective and recyclable carbocatalyst for the generation of carbon-hetero atom bond leading to quinazolinone derivatives and aza-Michael adducts under mild reaction conditions. The mild acidity imparted by the surface CO2H groups of this nanoscale form of carbon could act as an alternative carbocatalyst to severalraditional acid catalysts for important acid catalyzed organic transformations. We choose 􀈕-carotene as the carbon source for the synthesis of CNDs. The main motive of using 􀈕-carotene as the carbon source was that unlike other carbon sources used to make CNDs, 􀈕-carotene does not have any oxygen functionality in it. The catalytic activity is driven only by the surface CO2H group generated during the carbonization of 􀈕-carotene. The catalyst showed excellent activity towards the synthesis of variety of dihydro/spiro/glyco quinazolinones with structurally perplexing substituent. The mild acidic behaviour of CNDs could also be extended towards the synthesis of aza-Michael adducts at room temperature. Chapter 4: Au nanoparticle-polydopamine-reduced graphene oxide ternary nanocomposite as efficient catalyst for selective oxidation of benzylic C(sp3)- H bonds under mild conditions In this chapter, we have shown the excellent catalytic activity of a ternary nanocomposite comprising of Au nanoparticles (NPs), polydopamine and reduced graphene oxide towards oxidation of C-H bond in benzylic hydrocarbons under mild conditions in presence of N-hydroxyphthalimide (NHPI). The composite was synthesized by modifying the surface of graphene oxide by polydopamine followed by immobilization of Au nanoparticles. The composite was characterized by several spectroscopic and microscopic techniques. The nanocomposite could be used towards C-H oxidation in wide range of compounds with high activity and selectivity. All the components in the nanocomposite played important role in the effectiveness of the catalyst. Sufficient electron transfer from polydopamine/reduced graphene oxide to Au NPs made the nanoparticle surface more negatively charged favourable for molecular oxygen activation leading to C-H bond oxidation. The reaction followed a free-radical pathway as evidenced by detailed mechanistic studies. Further, easy separation and excellent reusability without significant loss in activity over several iterations fortify the ternary nanocomposite as excellent heterogeneous catalyst for C-H oxidation reactions.Chapter 5: One-pot magnetic iron oxide carbon nanodots composite catalyzed cyclooxidative aqueous tandem synthesis of quinazolinones in presence of tert-butyl hydroperoxide In this chapter, we have shown carbon nanodots stabilized iron oxide nanoparticles (Fe3O4-CNDs) as effective magnetically recoverable heterogeneous catalyst for the one-pot synthesis of quinazolinones using tert-butyl hydroperoxide (TBHP) as the principal oxidant in aqueous medium. Controlled experiments showed involvement of benzaldehyde and dihydroquinazolinone as the intermediates. The reaction followed a free radical pathway as evident from the experiment using radical scavenger. The catalyst can be recovered easily from the reaction mixture by using a simple magnet and reused for multiple cycles without significant loss in catalytic activity. The stability of the reactive oxygen species derived from tert-butylhydroperoxide bound to iron oxide surface may be the driving force for the exceptional activity of the catalyst. Chapter 6: Conclusion and Future Outlook This chapter summarizes the works described in the thesis. Further, the relevance and future prospects of the works have been discussed.