Heterogeneous Selective Oxidation of Sulfides with H₂O₂ Catalyzed by Ionic Liquid-Based Polyoxometalate Salts

Abstract

A group of ionic liquid (IL)-based polyoxometalate (POM) salts were prepared via anion-exchange of imidazolium IL precursors tethered by different carbon chain alkyls with various Keggin POMs. The resultant imidazolium POM salts were tested as the catalysts for oxidation of sulfides with aqueous H 2 O 2 . For understanding the POM salt-catalyzed oxidation of sulfides, catalyst compositions, substrates, and reaction conditions were changed. The results show that the imidazolium POM salts are very active and selective heterogeneous catalysts for oxidations of sulfides, adding the advantages of convenient recovery, steady reuse, simple preparation, and flexible composition. Characterizations by Fourier transform infrared (FT-IR), ultraviolet− visible (UV−vis), X-ray diffraction (XRD), and elemental analysis further reveal that the heterogeneous nature of the oxidation processes associates with the good crystallinities of the butyl-and hexyl-functionalized imidazolium POM salts and the hydrogen bonding networks among cations and anions. The POM salts' excellent performances arise from the promoted redox property of the POM-anions by the intramolecular charge transfer from the IL-cations.

Efficient and selective oxidation of sulfides in batch and continuous flow using styrene-based polymer immobilised ionic liquid phase supported peroxotungstates

“…28 Ionic liquids are an intriguing class of solvent that has been widely utilized for immobilisation of catalysts under homogeneous, liquid-liquid biphasic and liquid-solid (SILP) biphasic conditions, in some cases with remarkable success. 29 Recent endeavours in this area include highly selective sulfoxidations catalysed by a SILP system based on imidazolium modified SBA-15 and [MoO(O2)2(H2O)n], 30 a magnetically recoverable sulfoxidation catalyst based on magnetic nanoparticles entrapped in a tungstate-functionalised polyionic liquid, 31 an eco-friendly protocol for the oxidation of sulphides to sulfones catalysed by V2O5 in [C12mim][HSO4], 32 efficient and selective sulfoxidation catalysed by peroxotungstates immobilised on multilayer ionic liquid brushes-modified silica 33 Other recent developments include selective oxidation of sulfides with H2O2 catalysed by heterogeneous ionic liquidbased polyoxometalates, 34 selective oxidation of sulphides with a sulfoacid-hexafluorotitanate(IV) bifunctional ionic liquid, 35 ionic liquid-mediated oxidation of sulphides to sulfoxides, 36 efficient eco-friendly selective oxidation of sulphides to sulfoxides with molecular oxygen catalysed by Mn(OAc) 2 in [C12mim][NO3], 37 rapid oxidation of sulfides by themoregulated polyoxometalate based ionic liquids, 20b,38 selective and efficient desulfurization by amphiphilic polyoxometalate-based ionic liquid supported silica, 39 and heterogeneous selective sulfoxidation with polymeric ionic liquid nanogel-immobilised tungstate anions. 40 We have recently applied the concept of SILP-based technology to develop peroxotungstate-based Polymer Immobilised Ionic Liquid Phase (PIILP) oxidation catalysts in order to combine the favourable and tuneable properties of ionic liquids with the advantages of a solid porous support.…”

Efficient and selective hydrogen peroxide-mediated oxidation of sulfides in batch and segmented and continuous flow using a peroxometalate-based polymer immobilised ionic liquid phase catalyst

“…Our initial catalyst evaluation, comparison and optimization focused on thioanisole as the benchmark substrate as this oxidation has recently been catalysed by peroxometalate-based systems immobilized on the surface of ionic liquid modified silica, 23b,c ionic liquid-based polyoxometalates salts, 28 a temperature responsive phase transfer catalyst based on a Keggin-type lacunary anions 20b as well as tungstate ions embedded in mesochannels of SBA-15 23a and poly(ionic) liquid entrapped magnetic nanoparticles. 29 A series of batch reactions were first conducted to explore the effect of the H 2 O 2 :substrate mole ratio on conversion and selectivity in acetonitrile and methanol, details of which are presented in Table 1.…”

“…23 Ionic liquids (ILs) are a fascinating class of solvent that have been widely embraced by the catalysis community as a method for immobilization of catalysts under homogeneous, liquid-liquid biphasic and liquid-solid (SILP) biphasic conditions. 24 Recent relevant examples include an efficient protocol for the oxidation of sulfides to sulfones catalyzed by V 2 O 5 in [C 12 mim] [HSO 4 ], 25 highly selective sulfoxidation of sulfides to sulfoxides catalysed by the bifunctional IL combination [SO 3 HC 3 py][TiF 6 ]/[C 4 py] [BF 4 ], 26 a selective catalyst-free oxidation of sulfides to sulfoxides that is faster in ionic liquid than conventional solvents, 27 and, most recently, heterogeneous selective oxidation of sulfides catalysed by ionic liquid-based polyoxometalates 28 or tungstate-based poly(ionic)liquid entrapped magnetic nanoparticles. 29 While good conversions and high selectivities have been obtained In IL media, surprisingly few recycle studies have been reported.…”

Epoxidation of Olefins Catalyzed by Polyoxomolybdates Formed in-situ in Ionic Liquids

“…In 2012 Wang et al synthesized several imidazolium polyoxometalate salts using Keggin-type polyoxometalate ions as counter-ions while varying the length of the carbon chains at the imidazolium cations. The salts provided good yields in the oxidation of sulfides with aqueous H 2 O 2 [25].…”