Polymeric ionic liquid supported catalysts incorporating polyoxometalates and/or metal nanoparticles

Abstract

Metal nanoparticles have a significant interest because of their applications in catalysis and nanoscience, and their synthesis requires stabilisers to prevent nanoparticle aggregation. Metal nanoparticles can be stabilised by ionic liquids. Polymer ionic liquids have recently attracted attention. Doherty and Knight group has developed and utilized them as support to immobilize polyoxometalates, which were Keggin-type polyoxometalates [XM12O40] n- and Lindqvist type- polyoxometalates[M6O19] nthat Errington group designed and developed, and nanoparticles, and explored their applications. The stabilisation of metal nanoparticles such as Pd, Pt, Ag, Au, Ir, Rh, and Ru by polyoxometalates has been investigated by Weinstock and Papaconstintinou. This represents that polyoxometalate provides redox and Brønsted acid functionally that stabilised Metal nanoparticles, which are applied in chemical synthesis electrochemistry and photocatalysis. This project aimed to investigate the interface between the metal nanoparticles and polyoxometalates. A convenient reduction method by hydrazine was used to synthesize polyoxometalate-stabilised Auᵒ nanoparticles by forming electron-rich polyoxometalates by adding multiple electrons to the fully oxidized polyoxometalates, which increases the electron density. Reduced polyoxometalates can act as stabilisers and reducing agents to synthesize polyoxometalates-stabilised Au° nanoparticles. Chapter 1 briefly discusses the history of the development of polyoxometalates, the structure of polyoxometalates, and their application. Developing ionic liquids and utilizing polymer ionic liquids as support are introduced based on previous research and discussed. The second chapter describes the synthesis and characterization of polyionic liquid immobilised Au° nanoparticles and a hybrid catalyst incorporating Au° nanoparticles and polyoxometalates. The polystyrene-based immobilised ionic liquid was prepared via free radical polymerization and used to support Au° nanoparticles (PIIL@AuNP) and incorporate phosphotungstates H3PW12O40 (PIIL@AuNP-PW12). Various techniques, including thermogravimetric analysis (TGA), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM), have used to characterize all the prepared catalysts. The third chapter presents the results of the systematic evaluation of the efficacy of the newly prepared PIIL@AuNP and PIIL@AuNP-PW12 systems as catalysts for the selective reduction of nitrobenzene. PIIL@AuNP efficient catalysts in a 1:1 mixture of water and ethanol as the solvent for reduction of nitrobenzene to N-arylhydroxylamine, azoxybenzene, or aniline and incorporation of H3PW12O40 into PIIL@AuNP-PW12 switches the selectivity from Narylhydroxylamine to aniline. This might be ascribed to the synergic creation of Brønsted acid sites in the presence of polyoxometalates. Chapter 4 describes the multi-electron reduction of phosphomolybdate (H3PMo12O40 and (TBA)3[PMo12O40]) and phosphotungstates (H3PW12O40 and (TBA)3[PW12O40]) and the interactions between the reduced anions and metal nanoparticles in an aqueous, nonaqueous and solid state. Transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FT-IR), and 31P NMR spectroscopy were used to investigate the initial reduction of polyoxometalates with hydrazine and the synthesis of polyoxometalates-stabilised Au° nanoparticles (POM@AuNP) from reduced polyoxometalates/oxidized polyoxometalates, and metal precursors. In a solidstate system, polyoxometalates-stabilised metal nanoparticles (POM@MNP) (M=Au, Ag) are also prepared by using polyoxometalates as stabilisers in the presence of sodium borohydride using a ball mill. Chapter 5 describes the synthesis and characterization of a range of polyionic liquid immobilized Lindqvist-type polyoxometalates ([MW5]@PIIL), where [MW5]= (TBA)3[(MeO)4TiW5O18], (TBA)6[{MnW5O18H}2], (TBA)6[{FeW5O18H}2], and (TBA)6[{CoW5O18H}2]. [(MeO)TiW5O18] 3-@PIIL (TiW5@PIIL), [{MnW5O18}2] 3-@PIIL (MnW5@PIIL), [{FeW5O18}2] 3-@PIIL (FeW5@PIIL), and [{CoW5O18}2] 3-@PIIL (CoW5@PIIL) were prepared in methanol and acetonitrile solvent at room temperature. Various techniques, including solid-state NMR spectroscopy, thermogravimetric analysis (TGA), energy dispersive X-ray (EDX), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), Fourier-transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) have used to characterize all the prepared compounds.