Catalytic Oxidative/Extractive Desulfurization of Model Oil using Transition Metal Substituted Phosphomolybdates-Based Ionic Liquids

Abstract

Polyoxometalates based ionic liquids (POM-ILs) exhibit a high catalytic activity in oxidative desulfurization. In this paper, four new POM-IL hybrids based on transition metal mono-substituted Keggin-type phosphomolybdates, [Bmim] 5 [PMo 11 M(H 2 O)O 39 ] (Bmim = 1-butyl 3-methyl imidazolium; M = Co 2+ , Ni 2+ , Zn 2+ , and Mn 2+ ), have been synthesized and used as catalysts for the oxidation/extractive desulfurization of model oil, in which ILs are used as the extraction solvent and H 2 O 2 as an oxidant under very mild conditions. The factors that affected the desulfurization efficiency were studied and the optimal reaction conditions were obtained. The results showed that the [Bmim] 5 [PMo 11 Co(H 2 O)O 39 ] catalyst demonstrated the best catalytic activity, with sulfur-removal of 99.8%, 85%, and 63% for dibenzothiophene (DBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and benzothiophene (BT), respectively, in the case of extraction combining with a oxidative desulfurization system under optimal reaction conditions (5 mL model oil (S content 500 ppm), n(catalyst) = 4 µmol, n(H 2 O 2 )/n(Substrate) = 5, T = 50 • C for 60 min with [Omim]BF 4 (1 mL) as the extractant). The catalyst can be recycled at least 8 times, and still has stability and high catalytic activity for consecutive desulfurization. Probable reaction mechanisms have been proposed for catalytic oxidative/extractive desulfurization.because it can proceed under mild reaction conditions, and oxidized products can be easily removed by extraction with organic extractants, owing to oxidized compounds being more polar than hydrocarbon molecules [20][21][22][23][24]. Although conventional organic extractants, including acetonitrile, methanol, dimethylsulfoxide (DMSO), dimethylformamide (DMF), sulfolane, and dichloromethane, reveal good extraction, their flammable and toxic properties limit their development and application in industries.Ionic liquids (ILs), as a class of green solvents, have numerous advantages over conventional organic solvents [25,26]. In 2001, Wasserscheid et al. first reported desulfurization of diesel fuel by extraction with ionic liquids [27]. The results demonstrated that ILs have the potential to play an important role in achieving clean fuel oil. Subsequently, significant literature has become available about desulfurization systems by extraction with various ILs, such as imidazolium-based ionic liquids [28][29][30][31], pyridiniuim-based ionic liquids [32], Lewis and Brösted acidic ionic liquids, and redox ionic liquids [7,33]. However, sulfur removal using only ILs as the extractant is insufficient and hardly meets stringent environmental regulations.Thus, researchers turned towards the addition of catalysts, combining peroxides with the ILs to achieve enhancement in efficiency. H 2 O 2 is often chosen as an oxidant in oxidative desulfurization, because it only produces water as a byproduct and is environmentally benign. Polyoxometalates (POMs) have received increasing attention as oxidative catalysts, due to their characterist...

Polyoxometalates as chemically and structurally versatile components in self-assembled materials

“…The Co II mono-substituted POM-IL exhibited the highest catalytic efficiency, removing 99.8% dibenzothiophene (500ppm) at 50°C after 1h. 29 In addition to their catalytic performance, the conductivity of POM-ILs has also been studied. Qingyin Wu and co-workers used tetraalkylammonium bromide and tetraalkylphosphonium chloride together with (P3) and (P4) -[PW 9 V 3 O 40 ] 6-, [P 2 W 16 V 2 O 62 ] 8-, [PW 10 V 2 O 40 ] 8-, to prepare a series of POM-ILs by exchange of the halide anions by the POMs.…”

“…They expanded their previous reported catalyst to a POM-IL system and synthesized 1-butyl 3-methyl imidazolium-based [PW 11 O 39 TM(H 2 O)] nÀ (TM ¼ Mn II , Co II , Ni II and Zn II ). The Co II mono-substituted POM-IL exhibited the highest catalytic efficiency, removing 99.8% dibenzothiophene (500 ppm) at 50 C aer 1 h. 29 In addition to their catalytic performance, the conductivity of POM-ILs has also been studied. Qingyin Wu and co-workers used tetraalkylammonium bromide and tetraalkylphosphonium chloride together with (P3) and (P4) 8À , to prepare a series of POM-ILs by exchange of the halide anions by the POMs.…”

Copper Phosphotungstate-Catalyzed Microwave-Assisted Synthesis of 5-Hydroxymethylfurfural In A Biphasic System

“…It is noteworthy that when metal-substituted polyoxometalates (POM) are synthesized removing an MO unit (M = W, Mo), a lacunar catalyst is generated, and when in this catalyst is introduced a metal cation into Keggin anion structure, metal-substituted POM salts are obtained, which are also very active photocatalysts (i.e., [PW 11 M 2+ O 39 ] 5− , M 2+ = Mn, Fe, Co, Ni, Cu, Zn) [52]. In the UV-Vis spectra of these types of doped-metal heteropoly salts, the presence of the transition metal ions in octahedral coordination leads to risen "d-d" transitions absorption bands, in the region from 450 to 780 nm [53]. However, comparatively to the charge transfer bands, "d-d" transitions lead to very weak absorption bands.…”

Polyoxometalates‐Based Ionic Liquids (POMs‐ILs) for Electrochemical Applications

“…combining [PW 12 O 40 ] 3− anion with a poly(ethylene glycol) (PEG)‐ containing quaternary ammonium cation that exhibited unusually high conductivity . Their potential has been explored in catalysis (namely in desulfurization, esterification, production of ketones, asymmetric di‐hydroxylation of olefins, in CO 2 conversion, in anti‐corrosion, antibacterial and antimicrobial coatings, and as photoluminescence materials . Concerning their electrochemical properties, POMs‐ILs have been prepared and studied as electrochromic materials for modification of carbon composite electrodes, for selective determination of ascorbic acid based on modified glassy carbon electrodes and towards the oxidation of the NADH coenzyme, for catalytic reduction of nitrite, bromate and hydrogen peroxide using a modified carbon paste electrodes, for electroluminescence as good electron‐ transport, as reversible phase transformation ionic‐liquid‐type gels, as new hybrid gel electrolytes, for the modification of polycrystalline gold electrode, as well as for high performance Li−S battery cathode …”