WO2015162162A1

WO2015162162A1 - Process for desulphurisation of fuels

Info

Publication number: WO2015162162A1
Application number: PCT/EP2015/058686
Authority: WO
Inventors: Valeria CONTE; Pierluca GALLONI; Massimo Centazzo; Alessia COLETTI
Original assignee: Qid S.R.L.
Priority date: 2014-04-23
Filing date: 2015-04-22
Publication date: 2015-10-29

Abstract

A process for desulphurisation of fuels is described, based on oxidation, catalysed and activated using microwaves in ionic liquids, of sulphur-containing diaromatic compounds which form impurities in said fuels, and concurrent extraction of the oxidised products in said ionic liquids. The process is selective for the oxidation of the sulphur-containing diaromatic compounds with respect to the epoxidation of the alkenes.

Description

Title

Process for desulphurisation of fuels

Field of the invention

The present invention relates to a method for desulphurisation of hydrocarbon- based fuels based on oxidising diaromatic compounds containing sulphur catalysed by catalysts consisting of a vanadium (V) species V^vO and subsequent removal of the oxidised compounds obtained by extraction using an ionic liquid.

Prior art

The removal of sulphur from gas oil for diesel engines and petrol is of importance for environmental protection standards, which are becoming stricter and stricter. In fact, the combustion of fuels converts sulphur-containing compounds into sulphuroxides SOx, leading in the first instance to pollution of the environment, which causes acid rain, and indirectly to deactivation of the catalytic converters in cars, increasing CO and NOx emissions.

At present, the method of choice for removing thiols, sulphides and disulphides is hydrodesulphurisation (HDS), but using this hydrogenation process it is not possible to remove sulphur-containing aromatic compounds efficiently, such as dibenzothiophene and 4,6-dimethyldibenzothiophene. Meanwhile, the method requires high-temperature and high-pressure conditions, and is not selective. In fact, as a side-effect of the hydrogenation, the alkene content is reduced. To achieve very strong desulphurisation, the oxidation reaction appears to be a very promising procedure [Kulkarni P.S. et al., 2010]. The oxidative desulphurisation strategy (ODS) is based on the different chemico-physical properties of the oxidised compounds. Because of the higher polarity thereof, the sulphoxides and sulphones obtained by oxidising the sulphides can be removed from the fuels by absorption onto a solid matrix, such as AI2O3, or extracted using a solvent. This process may be carried out under milder conditions than HDS, but the disposal of the products, in particular solvents or solid adsorbents containing the products of the oxidation reaction, is another aspect which needs to be improved to avoid further environmental pollution problems. The ionic liquids (IL) have various advantages as extraction solvents: they are nonvolatile, heat-stable, compatible with oxidising agents, and they have a much greater dissolving power that conventional molecular solvents.

Application WO 03/040264, for example, describes a method for removing organosulphorate compounds based on one or more extractions of these compounds using ionic liquids in combination with partial oxidation thereof to sulphoxides and sulphones before or during the extraction. The oxidation reaction is implemented using oxidising agents selected from air, oxygen, ozone, peroxides and peroxyacids, and is catalysed by catalysts selected from the transition metals platinum, palladium, vanadium, nickel, and salts or oxides thereof.

However, the greatest limitations on the use of ionic liquids are the high viscosity of the most common ILs and the concurrent extraction of aromatic hydrocarbons. This leads to a change in the composition of the fuel and increases the costs of recycling the ionic liquids used.

Meanwhile, a further problem with the combined use of oxidation and extraction using ionic liquids is the stability of the catalysts. In fact, the catalyst has to be stable under the process conditions and the sulphur-containing impurities typical of the fuels (BT benzothiophene, DBT dibenzothiophene and DMDBT 4,6- dimethyldibenzothiophene) are usually oxidised under harsh conditions. When hydrogen peroxide is used as an oxidising agent, the catalyst also has to be stable in water, since this is the solvent in which the oxidant is dissolved, as well as the product formed after the oxidation reaction.

As well as being stable under the oxidation conditions, the catalyst also has to be selective, or to oxidise sulphur-containing compounds preferentially, or even better exclusively, without removing the unsaturated part of the fuels, and may have to be recyclable.

For this purpose, metal complexes are used, which are basically Lewis acid of metals such as molybdenum and/or tungsten [Ding Y. et al., 201 1 ]. Recently, the use of Mo and W polyoxometalates (POMs) has also attracted considerable attention because of the variability of composition, size, shape, acid-base properties and redox potential [Zhang M. et al., 2013]. Fe species have also been proposed because of the possibility of recovering the catalyst by applying external magnetic fields [Jiang W. et al., 2013].

Catalysts based on V^v species have also been studied in the epoxidation of cyclooctene and in the oxidation of thioanisol and methyl p-tolylthioether [Conte V. et al., 2009]. The oxidation reaction has been studied in acetonitrile (MeCN), trifluoroethanol (TFE) and various ionic liquids, using H2O2 as an oxidising agent, under various temperature conditions. The catalytic activity found was low in the epoxidation of cyclooctene, whilst the results for the oxidation of methyl p- tolylthioether, a model compounds selected for the more electrophilic nature thereof, appeared more promising for some of the catalysts examined, even with respect to thioanisol. Subsequent studies on the oxidation reaction using hydrogen peroxide on the model compound phenyl methyl sulphide (PhSMe) in MeCN and at room temperature have been carried our using vanadium catalysts containing salen or salophen ligands. The results obtained suggest that the catalytic activity correlates with the electronic nature of the metal centre, although steric factors play a decisive role in determining the reaction yield [Coletti A. et al., 2012].

However, the primary sulphur-containing contaminants in the hydrocarbon fuels are diaromatic compounds, typically dibenzothiophene and derivatives thereof, which are significantly less reactive than benzothiophene (BT) under the reducing conditions of HDS because of the steric hindrance thereof, which inhibits the interaction with the active sites of the most commonly used catalysts. These diaromatic compounds are also unreactive in oxidising conditions because of the low electrophilicity thereof. Therefore, for these diaromatic compounds satisfactory removal procedures are yet to be found.

A recently proposed removal method, for example, provides desulphurisation by DBT-catalysed oxidation using a system based on an ionic liquid, an amphiphilic catalyst consisting of a vanadium polyoxymetalate V^v and H2O2 [Ge J. et al., 2012]. These catalysts have been found to be effective in the catalytic oxidation of DBT, but not of BT. One problem with this technology is the need for high oxidant/DBT ratios and long reaction times in order to achieve high conversions of the sulphurised substrate. Meanwhile, another method describes a procedure based on oxidation catalysed by vanadium complexes V^lvO(acac)2 variously substituted with H2O2 and ionic liquids. However, this procedure is not equally effective, in particular as regards the amount of catalyst, for all of the diaromatic sulphurised derivatives examined [Mota A. et al., 2012].

Summary

The purpose of the present invention is, thus, to develop an effective and advantageous method for selectively removing the sulphur-containing diaromatic compounds of fuels, and all the same without depleting them of alkenes, such as cyclooctene, which are components necessary for the quality of the fuels.

The inventors have now found that species of vanadium V^vO with tetradentate salen and salophen ligands can expediently be used for exhaustive catalysed oxidation of the model compound dibenzothiophene. The catalysed oxidation can be carried out in a three-layer system consisting of a hydrophobic ionic liquid, hydrogen peroxide and hydrocarbon-based fuel to be treated (typically petrol), and that the use of microwave activation (MW) considerably decreases the reaction times, making the process even more beneficial and innovative with respect to the prior art.

Therefore, in one aspect the object of the invention is a method for removing sulphur-containing diaromatic compounds contained in a hydrocarbon-based material, wherein the removal is carried out in a three-layer (or three-phase) system consisting of:

- a first organic layer formed by a hydrophobic ionic liquid comprising a catalyst consisting of complexes of the V^vO species with tetradentate ligands selected from salen or salophen ligands, derivatives and salts thereof;

- a second aqueous layer comprising an oxidising agent consisting of an aqueous solution of hydrogen peroxide;

- a third organic layer consisting of the hydrocarbon-based material to be treated comprising the sulphur-containing diaromatic compounds to be removed and olefin, which do not have to be oxidised. In the above-described three-phase system, the removal of the sulphur-containing diaromatic compounds by the method according to the invention comprises at least the following steps:

- a catalysed oxidation of the sulphur-containing diaromatic compounds comprised in the hydrocarbon-based material and an extraction of oxidised compounds thereof by means of a hydrophobic ionic liquid;

- a separation of the three phases consisting of: the hydrophobic ionic liquid comprising the catalyst and the oxidised sulphur-containing diaromatic compounds; water; and treated hydrocarbon-based material; and - an extraction of the oxidised sulphur-containing diaromatic compounds from the hydrophobic ionic liquid with recovery of the same.

The catalysed oxidation reaction is carried out under stirring and is preferably activated by a microwave treatment.

Therefore, the object of the present invention is a method for removing sulphur- containing diaromatic compounds comprised in a hydrocarbon-based material, wherein the removal is carried out in a three-phase system consisting of:

an organic phase formed by a hydrophobic ionic liquid comprising a catalyst consisting of complexes of the V^vO species with tetradentate ligands selected from salen or salophen ligands, derivatives and salts thereof;

an aqueous phase comprising an oxidizing agent consisting of an aqueous solution of hydrogen peroxide;

a further organic phase consisting of the hydrocarbon-based material to be treated,

comprising at least the steps of:

performing under stirring an oxidation of the sulphur-containing diaromatic compounds, wherein the oxidation is catalyzed by means the said catalyst in an amount of at least 0.5 mol% and the oxidizing agent is at least in an amount of 2 eq., and an extraction of oxidized compounds thereof by means of the hydrophobic ionic liquid; a separation of the phases consisting of: the hydrophobic ionic liquid comprising the catalyst and the oxidized sulphur-containing compounds; water; and treated hydrocarbon-based material; and

- an extraction of the oxidized sulphur-containing compounds from the hydrophobic ionic liquid with recovery of the same.

The method and possible embodiments thereof, as well as the advantages thereof, will be clearer from the following detailed description of the invention, along with the provided illustrative, non-limiting examples of the invention.

Brief description of the drawings

Figure 1 : The drawing schematically shows the three-phase system developed for desulphurisation of hydrocarbon-based fuels based on catalysed oxidation and extraction in a petrol model containing dibenzothiophene and cyclooctene.

Detailed description of the invention

As stated previously above, the inventors studied a series of catalysts based on the V^vO species with salen or salophen ligands in the epoxidation of cyclooctene and the oxidation of thioanisol and methyl p-tolylthioether [Conte V. et al., 2009, Coletti A. et al., 2012].

The catalysts described and considered, including for the purpose of the desulphurisation method for catalysed oxidation of sulphur-containing diaromatic compounds, according to the invention, are represented, for the complexes of the V^vO species with salen ligands, by general formula (I)

(I)

and, for the complexes of the V^vO species with salophen ligands, by general formula (II)

where:

- X and Y, equal or different from each other, are: H, halogen, linear or branched Ci-C₄ alkyl or alkoxy thereof.

The preferred meanings for halogen is CI, for linear or branched Ci-C₄ alkyl is f-butyl and for alkoxy is MeO.

In particular embodiments for these vanadium complexes (V^v), according to the meanings of X and Y the salen or salophen ligands can be:

ligands

f.q. (I) f.q. (ID

X =Y = H; salen salophen

- x ₌Y ₌ ci; 3,3',5,5'-CI₄ salen 3,3',5,5'-CI₄ salophen

- X = CI, Y = H; 5,5'-CI₂ salen 5,5'-CI₂ salophen

- X = iBut, Y = H; 5,5'-(f-but)₂ salen 5, 5'-(f-but)₂ salophen

- X =Y = iBut; 3,3',5,5'-(f-but)₄ salen 3,3',5,5'-(f-but)₄ salophen

- X = H, Y = MeO; 5,5'-(MeO)₂ salen 5,5'-(MeO)₂ salophen

- X = MeO, Y = H 3,3'-(MeO)₂ salen 3,3'-(MeO)₂ salophen.

These complexes are preferably in the form of salts and the counter-anions are selected from Br, CI^", CIO₄ ^", BF₄ ^" and CF3SO3 and preferably the counter-anion is

Therefore, the preferred catalysts for the complexes of the V^vO species with tetradentate salen ligands are selected from [salenV^vO]CF3SO3, [5,5'- CI₂salenV^vO]CF₃SO₃, [5,5'-(f-Bu)₂salenV^vO]CF₃SO₃, [3,3'- (OMe)₂salenV^vO]CF₃SO₃, [5,5'-(OMe)₂salenV^vO]CF₃SO₃, [3,3',5,5'-

CI₄salenV^vO]CF₃SO₃, [3,3',5,5'-(f-Bu)₄salenV^vO]CF₃SO3, whilst the preferred catalysts among the complexes of the V^vO species with tetradentate salophen ligands are selected from [salophenV^vO]CF₃SO₃, [5,5'-CI₂salophenV^vO]CF₃SO3, [5,5'-(i-Bu)2salophenV^vO]CF3S03, [3,3'-(OMe)2salophenV^vO]CF₃S0₃, [5,5'- (OMe)2salophenV^vO]CF₃S0₃, [3,3',5,5'-CI₄salophenV^vO]CF₃S03, [3,3',5,5'-(f- Bu)₄salophenV^vO]CF₃S0₃.

The catalysts based on the V^vO species with tetradentate salophen ligands are preferred.

The ox s:

To check whether the oxidation reaction for the purposes of the ODS of hydrocarbon fuels was also applicable to sulphur-containing diaromatic compounds, which are the typical contaminants thereof, dibenzothiophene was selected as a model substrate compound, whilst the catalysts used were [salophenV^vO]CF3S03 and [salenV^vO]CF₃S0₃, together with the derivative [5,5'-(f-Bu)2salenV^vO]CF₃S03 thereof.

The oxidation of dibenzothiophene (DBT) by comparison with that of aryl thioethers, reported in the aforementioned publications [Conte et al., 2009; Coletti et al., 2012], has completely different features as regards the reactivity of DBT towards the peroxide oxidants and the working conditions. It is therefore not possible to predict the progression of the oxidation, in particular in the presence of MW activation, on the basis of the previous published works, mainly because the low reactivity of sulphur-containing diaromatic compounds respect to the aryl thioethers reactivity. As shown in the following by the examples, the oxidation reaction was studied in acetonitrile, under various conditions, such as temperature, equivalents of oxidant (H2O2), amount of catalyst. It was thus possible to check that when the reaction temperature is increased from 25 °C to 60 °C the reaction times decrease, whilst the conversion of DBT increases in particular with the [salophenV^vO]CF3S03 complex, but the selectivity towards the formation of dibenzosulphone does not improve. However, it is clear that only an amount between 0.5 and 1 % is required for the catalyst at the temperature of 60 °C. For the purpose of improving the selectivity of the DBT conversion in the corresponding sulphone, it is necessary to increase the amount of oxidising agent from 2 equivalents to 4-6 equivalents. The use of 4 equivalents of H2O2 increases the conversation of the substrate. Increasing the temperature from 60 to 70 °C reduces the reaction times, but the substrate conversion is lower, suggesting that at these temperatures the decomposition of the H2O2 is competing with the oxidation reaction. The concentration of the catalyst is also important for minimising the decomposition of the oxidant. From the results shown in the examples, it is in fact apparent that the selectively towards the sulphone increases when less catalyst is used in the presence of more H2O2 oxidant in solution.

It is also observed that the cyclooctene added as a substrate, in an equimolar amount with the DBT, does not compete with the oxidation reaction of the sulphurised compound, confirming a higher reactivity of DBT by comparison with the alkenes.

On the basis of the best reaction conditions found for the oxidation of DBT in acetonitrile, i.e. the conditions with the highest conversion of the substrate, the reaction was carried out using ionic liquids as solvents for the catalyst. The aim was to obtain a catalytic system where the oxidation of the sulphur-containing compounds and the extraction of the oxidant products thereof are combined together, and thus obtaining a new oxidative/extractive desulphurisation process (OEDSP) for fuels.

Both hydrophilic (i.e. BMImCFsSOs) and hydrophobic (i.e. BMImPFe) ionic liquids were used. In BMImCFsSOs, no oxidation reaction was observed even when more equivalents of oxidant were used. In this case, the CF3SO3 counter-ion, also provided by the solvent and thus greatly in excess, is competing with the substrate or with the oxidant in the interaction with the catalyst, causing this catalyst to be deactivated. In BMImPF6, when four equivalents of oxidant are used, the conversion of DBT is almost quantitative and the reaction is selective in the formation of DBTO2. The addition of two more equivalents of H2O2 increases the reaction time.

Since the ionic liquid extracts the sulphone better from the sulphoxide, the advantage of this oxidation catalysed by these catalysts under these conditions is clear. This selectivity of the reaction towards the sulphone is not substantially impaired by the addition of cyclooctene or cyclohexane, notwithstanding the fact that they both give the reaction medium a different polarity.

It has further been observed that heating the reaction mixture at temperatures from 50 or 60 °C to 100 °C with microwaves significantly and unexpectedly increases the reaction speed and the conversion (%) in comparison with a standard heating at the same temperatures.

It is, thus, apparent from the results obtained that these catalysts are advantageous for having selectivity for the conversion of dibenzothiophene into the oxidised products thereof with respect to the epoxidation reaction of the cyclooctene and the decomposition reaction of the H2O2, these reactions competing with the conversion of the dibenzothiophene.

Therefore, for the purposes of the method for removing sulphurised compounds from hydrocarbon fuels according to the invention, the first oxidation phase is implemented using vanadium catalysts (V^v) consisting of complexes of the V^vO species with salen ligands of general formula (I) or salophen ligands of general formula (II). The complexes of the V^vO species with salophen ligands of general formula (II) are preferred. The most preferred catalysts are:

a) for the complexes with the salen ligands: [salenV^vO]CF₃S0₃; [5,5'- Cl2salenV^vO]CF₃S0₃; [5,5'-(f-Bu)₂salenV^vO]CF₃S0₃; [3,3'- (OMe)₂salenV^vO]CFsS0₃; [5,5'-(OMe)2salenV^vO]CF₃S0₃; [3,3',5,5'-

CI salenV^vO]CF₃S0₃; [3,3',5,5'-(f-Bu) salenV^vO]CF₃S0₃;

b) for the complexes with the salophen ligands: [salophenV^vO]CF₃S0₃;

[5,5'-Cl2salophenV^vO]CF₃S0₃; [5,5'-(f-Bu)₂salophenV^vO]CF₃S0₃; [3,3'- (OMe)2salophenV^vO]CF₃S0₃; [5,5'-(OMe)₂salophenV^vO]CF₃S0₃; [3,3',5,5'- CUsalophenV^vO]CF₃S0₃; [3,3',5,5'-(f-Bu)₄salophenV^vO]CF₃SOs.

The amount of catalyst to be used in the oxidation reaction is at least 0.5 mol% and can be comprised from 0.5 to 5 mol%, and is preferably 1 mol%, with respect to the substrate, whilst the amount of oxidising agent H2O2 is at least 2 eq. and can be comprised from 2 to 6 equivalents, and is preferably 4 equivalents, with respect to the substrate. Hydrogen peroxide is the preferred oxidising agent both for the low cost and for the environmental sustainability thereof. The reaction temperature can be between 25 and 70 °C, and preferably the reaction temperature is between 50 and 60 °C, when a standard heating is envisaged. In the oxidation phase, the reaction mixture is subjected preferably to a microwave treatment which can be carried out with a power set from |2][LC1] to 40 W (preferably from 20 to 40 W) or a temperature set from 80 to 120 °C depending on the apparatus used and on the reaction conditions (type of ionic liquid and concentration of the reagents). The microwave treatment time is at least 15 minutes and can be repeated. The treatment of the reaction mixture with microwaves represents an activation of the catalysed oxidation to be preferred respect to the standard heating since this activation has a significant effect on the reaction time and efficiency of conversion.

The reaction medium for the oxidation phase is a hydrophobic ionic liquid selected from salts of imidazole variously substituted with linear or branched Ci-C₄ alkyls, equal or different from each other, containing anions such as hexafluorophosphate or bis(trifluoromethane)sulphonimide, and is preferably butylmethyl imidazole hexafluorophosphate (BMImPFe) or propylmethyl imidazole bis(trifluoromethane)sulphonimide (PMimTf2N). The ionic liquid has the dual purpose of a reaction medium and a medium for extracting the oxidised compounds obtained from the oxidation reaction.

For the method according to the invention, the ratio between ionic liquid and hydrocarbon material to be treated can be 1 to 1 by volume.

Since the starting reaction mixture is formed of three distinct, immiscible phases, the oxidation reaction is carried out with vigorous stirring.

At the end of the oxidation reaction and the concurrent extraction of the oxidised compounds, the extraction mixture is left to rest, breaking off the stirring, and the three phases reform, making them easy to separate. The separated ionic liquid, which contains the oxidised compounds and the catalyst, is subjected to a treatment to extract compounds containing oxidised sulphur, in particular dibenzosulphone and dibenzosulphoxide. The extraction can be achieved by means known to a person skilled in the art, for example by extraction using solvents. However, for reasons of environmental sustainability and efficiency, extraction using supercritical CO2 is to be preferred. The extraction using supercritical CO2 can be implemented by mixing it with the ionic liquid to be treated, and has various advantages in the industrial context. In fact, it is widely used in many processes (in particular for the extraction of compounds for pharmaceutical and food use from plant matter), since it makes it possible to recover a dry product without traces of toxic solvents. Further, the use of supercritical CO2 in combination with ionic liquids should be considered a well-established technology [Keskin S. et al., 2007]. In fact, it is found to be particularly advantageous with ionic liquids, since, although the supercritical CO2 is partially soluble in the ionic liquids, said ionic liquids are not soluble in the supercritical CO2 if they do not contain water. During this extraction, directed substantially to the recovery and the recycling of the ionic liquid, two phase can be formed, one consisting of the clean hydrophobic ionic liquid and one consisting of the supercritical CO2 containing the extracted oxidised products, these phases being easy to separate.

In a preferred embodiment, the process can be a three-phase oxidation/extraction as shown in Fig. 1 , wherein:

- the first phase (or layer) is an organic phase formed by a hydrophobic ionic liquid in which the catalyst is dissolved and in which the oxidised sulphur-containing compounds from the oxidation reaction are highly soluble;

- the second phase (or layer) is an aqueous phase comprising the oxidising agent, preferably hydrogen peroxide, immiscible with the ionic liquid; - the third phase (or layer) is an organic phase consisting of the fuel to be treated, immiscible with the ionic liquid.

The oxidation reaction can, thus, be implemented using the following method protocol:

- dissolving the catalyst based on the V^vO species with tetradentate ligands selected from salen or salophen ligands, the derivatives and salts thereof in a hydrophobic ionic liquid, so forming a first phase;

- adding an aqueous solution of oxidising agent to the phase consisting of hydrophobic ionic liquid with the catalyst, so forming a second phase;

- adding a hydrocarbon-based material to be treated, so forming a third phase;

- stirring the three-phase system prepared vigorously and preferably treating with microwaves. This three-phase system, which makes it possible to combine the oxidation and the extraction, has been found to be very efficient. In fact, as is shown in example 10, when a hydrophilic ionic liquid is used, such as butylmethyl imidazole tetrafluoroborate (BMIBF₄), so as to implement a two-phase system, no oxidation of the DBT partially extracted from the ionic liquid is observed, since this ionic liquid also solubilises the oxidising agent.

Therefore, in a most preferred embodiment, the method for removing sulphur- containing diaromatic compounds from hydrocarbon materials, like e.g. fuels such as petrol and gas oil, comprises the steps of:

- preparing a three-phase system consisting in: an organic phase formed by a hydrophobic ionic liquid in which the catalyst is dissolved; an aqueous phase comprising the oxidising agent; and a further organic phase consisting in the hydrocarbon-based material to be treated;

- stirring the three-phase system and performing an oxidation of the sulphur-containing diaromatic compounds catalysed by a catalyst of the

V^vO species with salen or salophen ligands which is comprised in a hydrophobic ionic liquid by means of the oxidising agent H2O2, preferably activated by a microwave treatment;

- extracting of the corresponding oxidised components of the sulphur- containing diaromatic compounds by means of the ionic liquid of the oxidation step;

- separating the phases by stopping the stirring;

- treating the phase consisting of the ionic liquid, containing the catalyst and the oxidised compounds, with supercritical CO2, with extraction of the oxidised compounds by means of said supercritical CO2; and

- recovering the ionic liquid.

EXAMPLES

Example 1 Synthesis of the tetradentate salen ligand [1,2-bis- (salicylideneamino)ethane] and derivatives thereof

The Schiff base used for synthesising the catalysts was prepared by the known reaction between salicylaldehyde and ethanediamine, with small changes from what is described in the literature [Horwitz CP. et al., 1993]. The method was applied with numerous substituted aldehydes.

General procedure: Two equivalents of an appropriate salicylaldehyde were dissolved in a minimum volume of boiling methanol and added in drops with one equivalent of diamine (1 ,2-diaminoethane) to 5 ml of methanol. The solution was brought to and kept at the reflux temperature until the aldehyde disappeared completely. Subsequently, the mixture was brought to room temperature so as to bring about precipitation of the Schiff base as a yellow solid. The solid was filtered and washed with a small amount of methanol, then with diethyl ether, and subsequently dried. The following Schiff bases were prepared in this manner:

- Salen, [1 ,2-bis-(salicylideneamino)ethane]: yield 88%;

- 5,5'-Cl2 salen, [1 ,2-bis-(5-CI-salicylideneamino)ethane]: yield 67%;

- 5,5'-(M3u)2salen, [1 ,2-bis-(5-i-Bu-salicylideneamino)ethane]: yield 91 %;

- 3,3'-(OMe)2 salen, [1 ,2-bis-(3-methoxy-salicylideneamino)ethane]: yield 92%;

- 5,5'-(OMe)2 salen, [1 ,2-bis-(5-OMe-salicylideneamino)ethane]: yield 92%;

- 3,3',5,5'-CI₄ salen, [1 ,2-bis-(3,5-Cl2-salicylideneamino)ethane]: yield 70%;

- 3,3',5,5'-(M3u)₄ salen, [1 ,2-bis-(3,5-(i-Bu)2-salicylideneamino)ethane]: yield 72%.

All of these ligands exhibited a ¹H-NMR and a UV-vis spectrum consistent with the structure.

Example 2 Synthesis of the complexes of oxovanadium (V^lvO) with tetradentate salen ligands

The synthesis of these complexes was carried out by a procedure slightly different from what is described in the literature [Chang C.J. et al., 1997; Bonadies J. A. et al., 1986]. Numerous vanadium derivatives used in the literature were tested as precursors, i.e. VO(acetylacetonate)2 [Chang C.J. et al., 1997], vanadyl sulphate dihydrate [Salavati-Niasari M. et al., 201 1 ; Webereski M.P. Jr. et al., 201 1 ], and V(acetylacetonate)3 [Tsuchida E. et al., 1994]. The best results were achieved with V^m(acac)3 in terms of reproducibility of reaction of solubility of the complexes.

General procedure. The Schiff base, prepared according to example 1 , was dissolved in 100 ml of boiling methanol, or suspended if not very soluble. An equimolar amount of V^m(acac)3 was completely solubilised in a minimum volume of MeOH with the assistance of sonication and added in drops to the solution or suspension of the Schiff base, causing an immediate change of colour from yellow to green. After one night under stirring in an open reactor at room temperature, the reaction was stopped and the solid precipitate was collected, washed with diethyl ether and dried. No trace of the Schiff base was found. Any residue of V^m(acac)3 which had not reacted was washed away with hot acetone.

The following complexes of V^lvO were prepared by this method:

- SalenV^lvO, yield 89%, UV-vis in MeCN [Amax, nm (ε, Μ^"1οητ¹)] 242 (39000), 277 (18000) and 362 (7900);

- 5,5'-CI₂salenV^lvO, yield 67% UV-vis in MeCN [Amax, nm] 248 (49000), 280 sh, 370 (7400);

- 5,5'-(f-Bu)2salenV^lvO, yield 67%, UV-vis in MeCN [Amax, nm (ε, Μ^" οητ¹)] 246 (56000), 278 (27000), 370 (9200);

- 3,3'-(OMe)₂salenV^lvO, yield 93%, UV-vis in MeCN [Amax, nm (ε, Μ^" οητ¹)] 224 (16000), 296 (14000) and 381 (2800);

- 5,5'-(OMe)₂salenV^lvO, yield 86%, UV-vis in MeCN [Amax, nm (ε, Μ^"1οητ¹)] 251 (20000), 286 sh (8800) and 392 (9000);

- 3,3',5,5'-CI₄salenV^lvO, yield 70%, UV-vis in MeCN: Amax = 370 nm (it does not dissolve completely);

- 3,3',5,5'-(f-Bu)₄salenV^lvO, yield 72%, UV-vis in CH2CI2 [Amax, nm (ε, Μ^" οητ¹)] 252 (41000), 288 (27000) and 386 (3600).

Example 3 Synthesis of the complexes of oxovanadium (V^vO) with tetradentate salen ligands

These complexes were synthesised by a procedure slightly different from what is described in the literature [Tsuchida E. et al. 1994].

General procedure: 200 mg of the V^lvO complex, prepared according to example 2, were dissolved in 30 ml of CH2CI2 under stirring. The O2 is bubbled for 5 min into the solution kept at 0 °C. Using a latex reservoir, the presence of 1 atmosphere of O2 was subsequently provided. 1 .2 equivalents of trifluoromethanesulphonic acid were subsequently rapidly added, causing the solution to darken and a solid to precipitate. The reaction mixture was brought back to room temperature and kept stirring until the species of V^IV disappeared (5-20 h). The V^v complex is subsequently isolated after centrifuging the reaction mixture (6000 rpm) and decanting the supernatant solution. The following V^vO complexes were prepared:

- [salenV^vO]CF₃S0₃, yield 89%, UV-vis in MeCN [Amax, nm (ε, Μ^"1οητ¹)] 230 (30000), 296 (14000) and 347 (7400);

- [5,5'-Cl2salenV^vO]CF₃S03, yield 67%, UV-vis in MeCN [Amax, nm (ε, IV cnr 1)] 230 (30000), 248 (24000), and 285 (17000);

- [5,5'-(f-Bu)₂salenV^vO]CF₃S0₃, yield 5%, UV-vis in MeCN [Amax, nm (ε, Μ^"1οητ 1)] 230 (33400), 250 (27500) 290 (19000) and 348 (7500);

- [3,3'-(OMe)2salenV^vO]CF₃S0₃, yield 58%, UV-vis in MeCN [Amax, nm (ε, M^" cnr¹)] 281 (12000), 308 (10000) and 359 (4200);

- [5,5'-(OMe)₂SalenV^vO]CF₃S0₃, yield 89%, UV-vis in MeCN [Amax, nm (ε, M^" ¹cnr¹)] 288 (12000) and 391 sh (3000);

- [3,3',5,5'-CI₄salenV^vO]CF₃SOs, yield 66%, UV-vis in MeCN [Amax, nm (ε, M^" ¹cnr¹)] 293 sh (1300) and 345 sh (710);

- [3,3',5,5'-(f-Bu)₄salenV^vO]CF₃SOs, yield 80%. UV-vis in CH2CI2 [Amax, nm (ε, M^"1cm-¹)] 263 (8000), 307 (5600) and 367 sh (2900).

Example 4 Synthesis of the tetradentate salophen ligand [1,2-bis- (salicylideneamino)benzene] and derivatives thereof

These ligands were synthesised as described in example 1 , using salicylaldehyde and substituted derivatives thereof and 1 ,2-benzenediamine. The following Schiff bases were prepared:

- Salophen, [1 ,2-bis-(salicylideneamino)benzene]: yield 95.3%;

- 5,5'-Cl2 salophen, [1 ,2-bis-(5-CI-salicylideneamino)benzene]: yield >99%; - 5,5'-(M3u)2salophen, [1 ,2-bis-(5-f-Bu-salicylideneamino)benzene]: yield

75%;

- 3,3'-(OMe)2 salophen, [1 ,2-bis-(3-methoxy-salicylideneamino)benzene]: yield 77%;

- 5,5'-(OMe)2 salophen, [1 ,2-bis-(5-OMe-salicylideneamino)benzene]: yield 79%;

- 3,3',5,5'-CI₄ salophen, [1 ,2-bis-(3,5-Cl2-salicylideneamino)benzene]: yield 95%; - 3,3',5,5'-(M3u)₄ salophen, [1 ,2-bis-(3,5-(i-Bu)2-salicylideneamino)benzene]: yield 75%.

All of the ligands exhibited a ¹H-NMR and a UV-vis spectrum consistent with the structure.

Example 5 Synthesis of the complexes of oxovanadium (V^lvO) with tetradentate salophen ligands

The synthesis was carried out as described in example 2, and the following complexes of V^lvO were prepared:

- SalofenV^lvO, yield 73%, UV-vis in MeCN [Amax, nm (ε, Μ^"1οητ¹)] 242 (40000), 314 (22000) and 396 (18000);

- 5,5'-CI₂salofenV^lvO, yield 78%, UV-vis in MeCN [Amax, nm (ε, Μ^" οητ¹)] 305 (12000), 409 (10700);

- 5,5'-(f-Bu)2salofenV^lvO, yield 82%, UV-vis in MeCN [Amax, nm (ε, Μ^" οητ¹)] 246 (40900), 318 (22700), 409 (15400);

- 3,3'-(OMe)₂salofenV^lvO, yield 79%, UV-vis in MeCN [Amax, nm (ε, M^" cm^"1)] 221 (42000), 301 (24000), 313 (22000) and 335 (13000);

- 5,5'-(OMe)₂salofenV^lvO, yield 87%, UV-vis in MeCN [Amax, nm (ε, Μ^"1οητ¹)] 216 (70000), 243 (41000), 289 (30000), 300 (32000), 337 (26000) and 434 (9000);

- 3,3',5,5'-CI₄SalofenV^lvO: yield 78%, UV-vis in MeCN [Amax, nm (ε, M^" cm^"1)] 315 (9800), 412 (10000);

- 3,3',5,5'-(f-Bu)₄salofenV^lvO, yield 87%, UV-vis in MeCN [Amax, nm (ε, M^"1 cnr 1)] 250 (43300), 327 (25900), 416 (16100).

Example 6 Synthesis of the complexes of oxovanadium (V^vO) with tetradentate salophen ligands

The synthesis was carried out as described in example 3, and the following complexes were prepared:

- [salophenV^vO]CF₃S0₃, yield 95% UV-vis in MeCN [Amax, nm (ε, Μ^" οητ¹)] 242 (40000), 304 (26000) and 393 (10000);

- [5,5'-Cl2salophenV^vO]CF₃S03, yield 98%, UV-vis in MeCN [Amax, nm (ε, M^" ¹cnr¹)] 303 (21300), 408 (1 1200); - [5,5'-(i-Bu)2salophenV^vO]CF3S03, yield 35%, UV-vis in MeCN [Amax, nm (ε, M-¹ cm^"1)] 245 (42700), 320 (24300), 407 (14400);

- [3,3'-(OMe)2salophenV^vO]CF₃S03, yield 75%, UV-vis in MeCN [Amax, nm (ε, M^"1 cm^"1)] 220 (64000), 251 (37000), 302 (45000), 310 (42000), 340 (24000) and 437 (7200);

- [5,5'-(OMe)2salophenV^vO]CF₃S03, yield 82%, UV-vis in MeCN [Amax, nm (ε, M^" cm-¹)] 215 (68000), 242 (41000), 292 (46000), 300 (45000), 341 (29000) and 435 (4900);

- [3,3',5,5'-CI₄SalophenV^vO]CF₃S03, yield 35%, [Amax, nm (ε, Μ^"1οητ¹)] 313 (19200), 410 (13700);

- [3,3',5,5'-(i-Bu)₄salophenV^vO]CF₃S03, yield 87%, [Amax, nm (ε, Μ^"1οητ¹)] 244 (30700), 326 (35300), 416 sh (9300).

Example 7 Oxidation reaction of dibenzothiophene (DBT) with H2O2, catalysed by the complexes in acetonitrile

The reaction was carried out in a flask. The dibenzothiophene (DBT 0.16 M) was dissolved in 5 ml of acetonitrile along with an appropriate amount of catalysts selected from those which were synthesised, and subsequently 2 eq. of oxygenated water were added. The catalyst VO(acac)2 was used as a control. The reaction mixture was kept stirring at controlled temperatures of 25 °C and 60 °C until the H2O2 disappeared, monitored by means of iodised-starch paper iodine. 100 μΙ of the solution were diluted with acetonitrile in a 1 ml volumetric test tube and the solution obtained was analysed by GC or HPLC with naphthalene as an external standard or by ¹H-NMR. The conversion of the DBT is calculated by taking into account the amount of non-oxidised substrate, whilst the selectivity is given as a ratio between the sulphoxide and the sulphone which are produced.

Table 1 shows the results obtained. Table 1. Oxidation of DBT in acetonitrile with 2 equivalents of H2O2

T Catalysts Time Conversion Selectivity

[salens 0] CF3SO3 10 20 44 45 55

25 [salens 0] CF3SO3 5 20 62 50 50

[salens 0] CF3SO3 1 20 8 85 1 5

[salens 0] CF3SO3 5 2 57 55 45

[salens 0] CF3SO3 1 2 74 64 36

u)₂salenV^vO] 1 1 0 76 47 53

[salophenVVQ] CF3SO3 1 0.25 92 54 46

DBT 0. 16 M, H₂0₂ 2 eq.

Example 8 Oxidation reaction of dibenzothiophene (DBT) with H2O2, catalysed by the Ψ complexes in acetonitrile in the presence of the competing substrate cyclooctene (COT)

The oxidation reaction was carried out as described in the preceding example 7, but introducing the following changes:

- temperatures = 50, 60 and 70 °C;

- amount of catalyst = 1 % or 0.5%;

- DBT = 0.1 6 M or 0.05 M;

- COT = 0.05 M.

Table 2 below shows the results obtained.

Table 2. Catalysed oxidation of DBT in acetonitrile in the presence of COT

Entry H₂O₂ Time Conversion Selectivity

(-C) (eq.) ^{a a yS S} (h) (%) DBTO DBTO₂

1 50 4 [salenV^vO] CF3SO3 7 97 37 63 2 4 [salophen^O] 2.5 99 14 86

3 60 4 [salenV^vO] CF3SO3 2.25 96 40 60

4 4 [salophenV^vO] 1 .5 98 23 77

5 4 [salenV^vO] CF3SO3 2 98 37 63

& 6 [salens 0] CF3SO3 3.5 99 13 87

7* 6 [salens Ό] CF₃S0₃ ^b 3 86 8 92

6 [salens 0] CF3SO3 4.5 86 16 84

9 6 [salenV^vO] CF3SO3 3 86 8 92

70

6 [salenV^vO] CF3SO3 2.5 86 19 81

Cat. 1%, DBT 0.16 M; ^a DBT 0.05 M; ^bcat. 0.5%; ^c addition of cyclooctene (COT)

0.05 M

Example 9 Oxidation reaction of dibenzothiophene (DBT) with H2O2 catalysed by the V^v complexes in ionic liquid.

The reaction was carried out in a 5 ml Schlenk reactor. The dibenzothiophene was dissolved in 3 ml of I L together with the catalyst ([salenV^vO]CF3S03) and the aqueous solution of hydrogen peroxide was added at the end. The mixture (homogeneous for BMImCF3S03^" and heterogeneous for BMImPFe) was kept stirring at a controlled temperature of 60 °C. 100 μΙ of the solution were diluted with dichloromethane in a 1 ml volumetric test tube and filtered over S1O2. The resulting solution was analysed by GC with naphthalene as the external standard. In this case too, the conversion of the DBT is calculated by taking into account the amount of non-oxidised substrate, whilst the selectivity is given as the ratio between the sulphoxide and the sulphone which are produced. Table 3 below shows the results obtained. Table 3. /V^vO]CF3S03-caia/ysec/ oxidation reaction of DBT with H2O2 in ionic liquids

H2O2 Time Conversion Selectivity

Ionic liquid

BMImCF₃S0₃ 5 3.5 0 0 0

BMImPFe 4 2 96 2 98

BMImPFe 6 12 100 0 100

BMImPFe 4 2 94 40 60

BMImPF^ 4 - 97 9 91

DBT 0. 16 M, cat. 0.5%, T=60 X); ^a addition of COT 0. 16 M; ^b addition of cyclohexane 0. 16 M

Example 10 Oxidation reaction of dibenzothiophene (DBT) with H2O2 catalysed by the complexes in ionic liquids in a three-phase system

The oxidation reaction in a three-phase system was carried out analogously to what was described above: 2 ml of ionic liquid were used and the organic substrates DBT and COT were dissolved in 2 ml of petrol. At the end, H2O2 10.54 M was added. To analyse the organic phase by GC, all of the petrol in solution was removed and diluted with dichloromethane in a 5 ml volumetric test tube, with a known amount of naphthalene as the external standard. 100 μΙ_ of ionic liquid were dissolved in a 5 ml volumetric test tube with dichloromethane containing naphthalene as the external standard. The solution was filtered with S1O2 and analysed by HPLC. In this case too, the conversion of the DBT is calculated by taking into account the amount of non-oxidised substrate, whilst the selectivity is given as the ratio between the sulphoxide and the sulphone which are produced. Table 4 below shows the results obtained. Table 4. Oxidation reaction in a three-phase system.

Time Conversion Selectivity

IL Catalyst

BMImPF₆ [salens 0]CF₃S0₃ 24 62 30 70

BMImPF₆ [salophenV^vO]CF₃S0₃ 24 98 - 100

BMImBF₄ [salophenV^vO]CF₃S0₃ 5 - -

BMImTf₂N [salophenV^vO]CF₃S0₃ 16 98 45 55

DBT = 0. 16 M, COT = 0. 16 M, cat:DBT=1 :200, H₂0₂ 4 eq., T 60 ^<€; ^a 60% of DBT extracted with IL

Example 1 1 Oxidation reaction of dibenzothiophene (DBT) with H2O2 catalysed by the V^v complexes in ionic liquids in a three-phase system with MW application The experiments were carried out using butyl-methyl-imidazole hexafluorophosphate (bmimPFe) hexyl-methyl-imidazole hexafluorophosphate (hmimPF6), butyl-methyl-imidazole bi-trifluoromethanesulphonylimide (bmimTf2N) and methyl-propyl-imidazole bis-trifluoromethanesulphonylimide (pmimTf2N).

In these experiments the catalyst, VO-salophen or VO-salen (in a ratio of 0.5% or 1 % to the DBT substrate) was solubilised in the ionic liquid (2 ml or 4 ml) and in the same reaction vial the model petrol (petrol ether 75-120 or ligroin, 2 or 4 ml), in which the cis-cyclooctene (COT) and the dibenzothiophene (DBT) are present at a concentration of 0.16 M, was added. The reaction was analysed on the gas chromatograph (GC) to detect the disappearance of the DBT. Before the start of the reaction, the integration of the peaks for COT and DBT in the chromatogram came out to approximately 50-50. After the aqueous solution of H2O2 10.35 M had been added (124 μΙ or 248 μΙ), the reaction mixtures were placed in the microwave reactor and heated with the microwaves to the temperature of 100 °C or 120 °C for 15 minutes.

After the first cycle of microwave irradiation, the reactions were subjected to the microwave treatment once again, both without any addition of oxygenated water. The petrol phases thus obtained were analysed on the GC again, where conversion, where greater than 98% conversion of the sulphurised substrate was observed for the reaction.

Table 5 shows the results obtained. Table 5. Oxidation reaction with and without microwave treatment

IL Catalyst Temperature MW Time Conversion Extraction

(^<C) (s) of DBT of DBT-ox

% % bmimPF6 [salens 0] 60 no 86400 62 >85

bmimPF6 [salophenVvO] 60 no 86400 98 >90

bmimTf2N [salophenV^vO] 60 no 57600 98 >98

hmimPF6 [salens 0] 100 yes 870 64 >85

bmimPF6 [salophenVvO] 120 yes 860 97 >93

bmimTf2N [salophenVvO] 120 yes 780 96 >98

pmimTf2N [salens 0] 100 yes 820+820 85 >98

pmimTf2N [salophenVvO] 100 yes 820+820 98 >98

DBT = 0. 16 M, COT = 0. 16 M cat.:DBT=1 :200, H₂0₂4 eq., fuels:IL = 1:1

Example 12 Oxidation reaction of dibenzothiophene (DBT) with H2O2 catalysed by the [salophenV^vO] CF3S03 in ionic liquid pminTf2N in a three-phase system with standard heating and heating with MW application

The experiments were carried out using methyl-propyl-imidazole bis- trifluoromethanesulphonylimide (pmimTf2N). The catalyst [salophenV^vO] CF3SO3 (in a ratio 0.5% respect the substrate DBT) was dissolved in 4ml of IL and in the same reaction vial the model petrol (petrol ether 75-120 or ligroin, 4 ml), in which the cis-cyclooctene (COT) and the dibenzothiophene (DBT) are present at a concentration of 0.1 6 M, was added. The reaction was analysed on the gas chromatograph (GC) to detect the disappearance of the DBT. Before the start of the reaction, the integration of the peaks for COT and DBT in the chromatogram came out to approximately 50-50. After the aqueous solution of H2O2 1 0.35 M had been added (248 μΙ), the reaction mixtures were heated by immersion in a bath thermostated at temperatures from 50 to 1 00 °C (standard heating) for a time of 1000 sec.

Subsequently, fresh test samples, prepared as previously described, were subjected to heating by irradiation with microwaves (heating with MW) for a time of 1 000 sec with a power from 21 to 35 W (medium).

The results are shown in table 6.

Table 6. Oxidation reaction in a three-phase system with microwaves treatment in comparison with standard heating

T Time Power Conversion

heating

(°C) (sec) (medium) %

50 MW 1 000 21 49

50 Standard 1 000 - 6

70 MW 1 000 27 69

70 Standard 1 000 - 9

90 MW 1 000 32 88

90 Standard 1 000 - 1 5

1 00 MW 1 000 35 98

1 00 standard 1 000 1 8

DBT = 0. 16 M, COT = 0. 16 M, cat:DBT= 1 :200, H₂0₂ 4 eq., fuels:IL References

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Claims

A method for removing sulphur-containing diaromatic compounds comprised in a hydrocarbon-based material, wherein the removal is carried out in a three-phase system consisting of:

comprising at least the steps of:

performing under stirring an oxidation of the sulphur-containing diaromatic compounds, wherein the oxidation is catalyzed by means the said catalyst in an amount of at least 0.5 mol% and the oxidizing agent is at least in an amount of 2 eq., and an extraction of oxidized compounds thereof by means of the hydrophobic ionic liquid;

a separation of the phases consisting of: the hydrophobic ionic liquid comprising the catalyst and the oxidized sulphur-containing compounds; water; and treated hydrocarbon-based material; and

an extraction of the oxidized sulphur-containing compounds from the hydrophobic ionic liquid with recovery of the same.

The method according to claim 1 , wherein the catalyzed oxidation is activated by a microwave treatment.

The method according to claim 1 , wherein the catalyst consisting of complexes of the V^vO species with tetradentate ligands of the salen type is represented by the general formula (I)

(I) wherein:

X and Y, equal or different from each other, are: H, halogen, linear or branched Ci-C₄ alkyl or alkoxy thereof.

4. The method according to claim 1 , wherein the catalyst consisting of complexes of the V^vO species with tetradentate ligands of the salophen type is represented by the ge

wherein:

The method according to claims 3 or 4, wherein the catalyst consisting of complexes of the V^vO species with tetradentate ligands of the salen or salophen type is in the form of salt, and the counter-anions are selected from Br, CI^", CI0 -, BF₄ ^", CFsSOs^".

The method according to claims 3 and 5, wherein the catalyst consisting of complexes of the V^vO species with tetradentate ligands of the salen type is selected from [salenV^vO]CF₃S0₃, [5,5'-Cl2salenV^vO]CF₃S0₃, [5,5'-(f- Bu)2salenV^vO]CF₃SOs, [3,3'-(OMe)2salenV^vO]CF₃S0₃, [5,5'-

(OMe)₂SalenV^vO]CF₃S0₃, [3,3',5,5'-CI salenV^vO]CF₃S0₃, [3,3',5,5'-(f- Bu) salenV^vO]CF₃SOs.

The method according to claims 4 and 5, wherein the catalyst consisting of complexes of the V^vO species with tetradentate ligands of the salophen type is selected from [salophenV^vO]CF₃S0₃, [5,5'-CI₂salophenV^vO]CF₃S0₃, [5,5'- (f-Bu)2salophenV^vO]CF₃SOs, [3,3'-(OMe)2salophenV^vO]CF₃S0₃, [5,5'- (OMe)2salophenV^vO]CF₃SOs, [3,3',5,5'-CUsalophenV^vO]CF₃S0₃, [3,3',5,5'- (f-Bu) salophenV^vO]CF₃SOs.

The method according to claim 1 , wherein the catalyst is in an amount from 0.5 to 5 mol%.

9. The method according to claim 1 , wherein the oxidizing agent consisting of the aqueous solution of hydrogen peroxide is in an amount from 2 to 6 equivalents.

10. The method according to claim 1 , wherein the oxidation is carried out at a temperature between 25 and 70 °C.

1 1 . The method according to claim 2, wherein the microwave treatment is carried out with a power from 2 to 40 W or with a temperature between 80 and 120 °C.

12. The method according to claim 1 , wherein the hydrophobic ionic liquid is selected from imidazolium salts substituted with Ci-C₄ alkyls, equal or different from each other linear or branched, with anions selected from hexafluorophosphate or bis(trifluoromethane)sulfonamide.

13. The method according to claim 12, wherein the hydrophobic ionic liquid is selected from butyl-methylimidazolium-hexafluorophosphate (BMImPFe) or propyl-methylimidazolium-bis(trifluoromethane)sulfonamide (PMimTf2N).

14. The method according to claim 1 , wherein the extraction of the oxidized sulphur-containing diaromatic compounds from the ionic liquid is carried out with the supercritical CO2 technique.