Use this URL to cite or link to this record in EThOS:
Title: Development of phosphazene-polyoxometalate catalyst systems for multiphase oxidations with hydrogen peroxide
Author: Craven, M.
ISNI:       0000 0004 7428 6670
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
Availability of Full Text:
Access from EThOS:
Access from Institution:
Olefin epoxidation and oxidative desulfurization are two important reactions in both industry and academia. Olefin epoxidation produces epoxides which are important precursors and intermediates in materials and organic syntheses, respectively. Oxidative desulfurization is an emerging technology for the removal of heavy aromatic organosulfur compounds from vehicular diesel fuels which produce harmful SOx gases from their combustion and are difficult to remove using the conventional hydrodesulfurization technology. Hydrogen peroxide is an attractive oxidising agent for these reactions as it has a high oxygen atom efficiency in oxidation reactions and its only by-product is typically water. Reactions with H2O2 are commonly conducted in liquid-liquid biphasic systems due to immiscibility of the aqueous H2O2 and the fuel/olefin-containing organic solvent. Keggin-type polyoxometalates (POM, [XM12O40] 8-n , where M = MoVI, WVI and X = PV (n = 5), SiIV (n = 4)) are often used as catalyst precursors which transform in the presence of H2O2 to form active peroxo polyoxometalate (peroxo-POM) species. A suitable phase transfer catalyst (PTC) is required to transfer the peroxo POM to the organic layer where oxidation of the target substrate takes place. Many PTCs have been reported for these reactions over the years - typically quaternary ammonium cations - which are often cumbersome to prepare. In this work, organoaminocyclotriphosphazenes (P3N3(NHR)6, RPN, R = Bz, benzyl; iBu, iso-butyl; iPr, iso - propyl; n-Bu; n-butyl; Hex, hexyl; and Cy, cyclohexyl) are presented as alternative PTCs for these reactions, which are easily prepared in one-step reaction of commercially-available P3N3Cl6 with a range of primary amines. RPN-POM catalysts were formed by simply combining RPN and POM species together in suitable solvents or in situ. Three systems have been developed for olefin epoxidation and oxidative desulfurization with H2O2 using RPNPOM catalysts: homogeneous RPN-POM salt catalysts in aqueous biphasic solvent systems (System One); heterogeneous POM/RPN-SiO2 solid catalysts for aqueous biphasic solvent systems (System Two); and eutectic RPN-POM "ionic liquid" multifunctional solvent-catalyst systems for reactions in the absence of organic solvents (System Three). Reactions were conducted under mild conditions (atmospheric pressure and reaction temperature = 60 oC). With BzPN-PMo based catalysts, up to 100% dibenzothiophene (DBT) conversion could be achieved in 0.5 and 3h using Systems one and Two, respectively. Cyclooctene oxide yields of up to 99% were obtained in 0.5h using System One and in 4h using System Two with BzPN-PMo and HexCyPN-PMo as catalysts, respectively. These results compare well with some of the best results reported in the literature, which used similar catalyst-types and reaction conditions. A 76% cyclooctene oxide yield was obtained after 5h with PMo/BzPN-SiO2 in system two, which was a modest performance compared with other results reported for heterogenous catalysts under similar conditions. In all three systems, the final yield of cyclooctene oxide or conversion of DBT typically increased with decreasing aqueous stability and increasing oxidative potential of POM in the order: PMo > PW > SiW. With respect to benzothiophene in oxidative desulfurization, activity decreased in the order: DBT > (4,6-dimethyldibenzothiophene) DMDBT > (benzothiophene) BT due to the electrondonating and steric effects of the aromatic rings and methyl groups of benzothiophenes. Activity with respect to RPN was relative to the catalyst type, reactant and the solvent system that was used. The results show that RPN-POMs are promising catalysts for biphasic oxidations with H2O2 that can be modified to give homogeneous, heterogeneous and solventless catalyst systems. Mechanisms have been proposed for all three systems.
Supervisor: Kozhevnikov, Ivan ; Steiner, Alexander Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral