catalysts

Article Highly Active Trifloaluminate Ionic as Recyclable Catalysts for Green Oxidation of 2,3,6-Trimethylphenol to Trimethyl-1,4-Benzoquinone

Piotr Latos 1 , Agnieszka Siewniak 1 , Natalia Barteczko 1 , Sebastian Jurczyk 2, Sławomir Boncel 3 and Anna Chrobok 1,*

1 Department of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland; [email protected] (P.L.); [email protected] (A.S.); [email protected] (N.B.) 2 Łukasiewicz Research Network—Institute for Engineering of Materials and Dyes, Skłodowskiej-Curie 55, 87-100 Toru´n,Poland; [email protected] 3 Department of Organic Chemistry, Bioorganic Chemistry and Biotechnology, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland; [email protected] * Correspondence: [email protected]

 Received: 9 November 2020; Accepted: 15 December 2020; Published: 16 December 2020 

Abstract: An effective method for the synthesis of 2,3,6-trimethyl-1,4-benzoquinone via the oxidation of 2,3,6-trimethylphenol as the key step in the in the preparation of vitamin E was presented. An aqueous solution of H2O2 was used as the oxidant and Lewis acidic trifloaluminate ionic liquids [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 or 0.15 as catalysts. Trifloaluminate ionic liquids were synthesised by the simple reaction between 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (triflate) [emim][OTf] and aluminium triflate used in sub-stoichiometric quantities. The influence of the reaction parameters on the reaction course, such as the amount and concentration of the oxidant, the amount of catalyst, the amount and the type of organic , temperature, and the reaction time was investigated. Finally, 2,3,6-trimethyl-1,4-benzoquinone was obtained in high selectivity (99%) and high 2,3,6-trimethylphenol conversion (84%) at 70 ◦C after 2 h of oxidation using a 4-fold excess of 60% aqueous H2O2 and acetic as the solvent. The catalytic performance of trifloaluminate ionic liquids supported on multiwalled carbon nanotubes (loading of active phase: 9.1 wt.%) was also demonstrated. The heterogeneous ionic liquids not only retained their activity compared to the homogenous counterparts, but also proved to be a highly recyclable catalysts.

Keywords: ionic liquids; supported ionic phase; carbon nanotubes; trifloaluminate ionic liquids; Lewis ; oxidation of 2,3,6-trimethylphenol; 2,3,6-trimethylhydroquinone

1. Introduction Functionalised benzoquinones are attractive building blocks for a wide range of bioactive compounds, fine chemicals, and pharmaceuticals [1,2]. Trimethyl-1,4-benzoquinone (TMBQ), obtained by the oxidation of 2,3,6-trimethylphenol (TMP), is an intermediate in the synthesis of vitamin E, the most important biological antioxidant soluble in lipids (Figure1) [ 3]. As the first stage of this synthesis, the oxidation of TMP to TMBQ (Figure1a) takes place. In the next step, the of TMBQ to 2,3,6-trimethylhydroquinone (Figure1b) proceeds with the subsequent cycloaddition of 2,3,6-trimethylhydroquinone to isophytol leading to the formation of vitamin E (Figure1c) [4].

Catalysts 2020, 10, 1469; doi:10.3390/catal10121469 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1469 2 of 10 Catalysts 2020, 10, x FOR PEER REVIEW 2 of 10

OH O OH

oxidant H2

catalyst catalyst

abO OH

isophytol catalyst HO c

O 2

Figure 1. FigureThree-step 1. Three-step synthesis synthesis of vitamin of vitamin E: E:(a ()a oxidation) oxidation ofof 2,3,6-trimethylphenol; 2,3,6-trimethylphenol; (b) hydrogenation (b) hydrogenation of of c 2,3,6-trimethyl-1,4-benzoquinone;2,3,6-trimethyl-1,4-benzoquinone; and and (c () )cycloaddition cycloaddition of, of, 2,3,6-trimethylhydroquinone 2,3,6-trimethylhydroquinone to isophytol to [ 5isophytol]. [5]. The constantly growing number of environmental regulations have forced the development of sustainable catalytic methods for the oxidation processes. For ecological and economic reasons, the use The ofconstantly inexpensive growing and commercially number available of environmenta oxidants togetherl regulations with an active have catalyst forced is the crucial development for the of sustainablesynthesis catalytic of the methods key intermediate for the oxidation TMP. The current processes. industrial For methodecological for the and production economic of TMPreasons, the is the oxidation of TMBQ with oxygen using copper chloride in stoichiometric amounts [5,6]. As a use of inexpensive and commercially available oxidants together with an active catalyst is crucial for result of the presence of a chlorine atom in the catalyst , chlorinated by-products are formed. the synthesisThis issue of the generates key intermediate problems with TMP. corrosion. The Tocurr tackleent thisindustrial problem, method to reduce for the the metal production chloride to of TMP is the oxidationcatalytic amounts,of TMBQ ionic with liquid oxygen 1-butyl-3-methylimidazolium using copper chloride chloride in stoichiometric and n-butanol asamounts a co-solvent [5,6]. As a result of werethe presence used, affording of a chlorine a 86% yield atom of TMP in the [7]. catalyst Another approachmolecule, is tochlorinated use copper(II) by-products nitrate catalyst are in formed. This issuewater generates generating problems halogen-free with conditions corrosion. [8]. To tackle this problem, to reduce the metal chloride to Simultaneously, the studies on the active catalysts for the oxidation of TMP with the use of H O catalytic amounts, ionic liquid 1-butyl-3-methylimidazolium chloride and n-butanol as a2 co-solvent2 as a green oxidant were carried out. However, due to the fact that is a weak were used, affording a 86% yield of TMP [7]. Another approach is to use copper(II) nitrate catalyst in oxidant, the -stable catalysts which can activate H2O2 are still in demand. According to this water generatingtrend, the usehalogen-free of heteropolyacid conditions catalyst [8]. allowed to obtain TMBQ in an 85% yield, after 4 h at 70 ◦C, Simultaneously,using a 60% aqueous the studies solution on of the H2 Oactive2 [9]. catalysts The use of for a more the oxidation concentrated of 83% TMP solution with the of H use2O2 of H2O2 as a greenwith oxidant methyltrioxorhenium(VII) were carried out. as aHowever, catalyst and due acetic to acid the in fact the oxidationthat hydrogen of TMP yieldedperoxide in 70% is a weak of TMBQ after 2 h at 60 ◦C[10]. The ruthenium compounds were also used as a catalyst for this oxidant, the water-stable catalysts which can activate H2O2 are still in demand. According to this process. Under the optimised conditions (30% aqueous solution of H2O2, as a solvent, 2 h trend, the use of heteropolyacid catalyst allowed to obtain TMBQ in an 85% yield, after 4 h at 70 °C, at 80 ◦C), TMBQ was obtained with 89% of yield [11]. The use of iron(III) compounds as catalysts using a 60%allowed aqueous to achieve solution a high of yield H2O of2 TMBQ[9]. The (79%) use after of a 2 more h at 80 conc◦C[12entrated]. In all cases, 83% practicallysolution of 100% H2O2 with methyltrioxorhenium(VII)conversion of the substrate as a wascatalyst achieved, and nevertheless, acetic acid the in lower the yieldsoxidation were causedof TMP by theyielded presence in 70% of TMBQ afterof oxidation 2 h at 60 side-products. °C [10]. The ruthenium compounds were also used as a catalyst for this process. A series of heterogeneous silica-supported titania, Ti- and V-containing mesoporous mesophase Under the optimised conditions (30% aqueous solution of H2O2, acetic acid as a solvent, 2 h at 80 °C), catalysts as well as Ti-monosubstituted polyoxometalate catalysts, were developed by Kholdeeva’s TMBQ wasgroup obtained [13–18]. Towith date, 89% the mostof yield active [11]. catalyst The for us thee of oxidation iron(III) of TMPcompounds with hydrogen as catalysts peroxide isallowed a to achieve aheteropolyacid high yield of with TMBQ vanadium (79%) and after tungsten 2 h atoms at 80 in °C its structure,[12]. In basedall cases, on the practically tetra n-butylammonium 100% conversion of the substratecation TBA was4H[ achieved,γ-PW10V2O nevertheless,40]. The use of a the 35% lower aqueous yields solution were of Hcaused2O2 and by acetonitrile the presence as a solvent of oxidation side-products.led to a 99% conversion of TMP with 99% selectivity to TMBQ after only 5 min at 80 ◦C[18]. A series Searchingof heterogeneous for halogen-free silica-supported and water-stable tita Lewisnia, Ti- acidic and catalysts V-containing for organic mesoporous synthesis, in mesophase our previous work, new acidic trifloaluminate ionic liquids were presented [19,20]. Trifloaluminate ionic catalysts liquidsas well were as Ti-monosubstituted synthesised in analogy polyoxometalate to chlorometallate catalysts, systems [21were] with developed the difference by Kholdeeva’s that group [13–18].instead To of date, the chlorido the most active on catalyst Al atoms, for trifluoromethanesulfonate the oxidation of TMP (triflate,with hydrogen OTf) ligands peroxide is a heteropolyacidwere present. with The vanadium synthesis isand the simpletungsten reaction atoms between in its triflate structure, ionic liquids based [cation][OTf] on the tetra n- + + butylammonium(1--3-methylimidazolium cation TBA4H[γ-PW cation—[emim]10V2O40]. Theand use [omim] of a )35% and aluminiumaqueous solution triflate used of H in2O2 and acetonitrilesub-stoichiometric as a solvent led quantities to a 99% (molar conversion ratio Al(OTf) of3 TMP: χAl(OTf)3 with= 0.1599% and selectivity 0.25). As ato result, TMBQ viscous after only 5 ionic liquids with higher melting points than the chloroaluminate counterparts and lower acidity min at 80 °C [18]. Searching for halogen-free and water-stable Lewis acidic catalysts for organic synthesis, in our previous work, new acidic trifloaluminate ionic liquids were presented [19,20]. Trifloaluminate ionic liquids were synthesised in analogy to chlorometallate systems [21] with the difference that instead of the chlorido ligands on Al atoms, trifluoromethanesulfonate (, OTf) ligands were present. The synthesis is the simple reaction between triflate ionic liquids [cation][OTf] (1-alkyl-3- methylimidazolium cation—[emim]+ and [omim]+) and aluminium triflate used in sub-stoichiometric quantities (molar ratio Al(OTf)3: χAl(OTf)3 = 0.15 and 0.25). As a result, viscous ionic liquids with higher melting points than the chloroaluminate counterparts and lower acidity (Acceptor Number AN = ca. 68 versus 96), however, with higher stability towards hydrolysis in the presence of atmospheric moisture, were obtained. The oligonuclear anionic complexes having various triflate bridging modes

Catalysts 2020, 10, 1469 3 of 10

(Acceptor Number AN = ca. 68 versus 96), however, with higher stability towards hydrolysis in the Catalysts 2020, 10, x FOR PEER REVIEW 3 of 10 presenceCatalysts 2020 of,atmospheric 10, x FOR PEER moisture, REVIEW were obtained. The oligonuclear anionic complexes having various3 of 10 triflate bridging modes were postulated to create anions. The catalytic performance of these systems as were postulated to create anions. The catalytic performance of these systems as acidic catalysts was acidicwere postulated catalysts was to create presented anions. in the The synthesis catalytic of pe chromanesrformance [of19 ]these and alkylsystems levulinates as acidic [ 20catalysts]. was presented in the synthesis of chromanes [19] and alkyl levulinates [20]. presentedIn this in work, the synthesis the use of of trifloaluminate chromanes [19] ionic and liquidsalkyl levulinates as catalysts [20]. in the homogenous oxidation of In this work, the use of trifloaluminate ionic liquids as catalysts in the homogenous oxidation of TMPIn with this an work, aqueous the use solution of trifloaluminate of H2O2 to TMBQ ionic liquids was proposed. as catalysts To facilitatein the homogenous the catalyst oxidation separation of TMP with an aqueous solution of H2O2 to TMBQ was proposed. To facilitate the catalyst separation andTMP its with recycling, an aqueous the ionic solution liquid of was H2O also2 to immobilised TMBQ was onproposed. multiwalled To facilitate carbon nanotubesthe catalyst (MWCNTs), separation and its recycling, the ionic liquid was also immobilised on multiwalled carbon nanotubes presentingand its recycling, the superactive, the ionic selective liquid and was stable also heterogeneous immobilised catalyst on multiwalled crucial for thecarbon development nanotubes of (MWCNTs), presenting the superactive, selective and stable heterogeneous catalyst crucial for the sustainable(MWCNTs), synthetic presenting processes. the superactive, selective and stable heterogeneous catalyst crucial for the development of sustainable synthetic processes. development of sustainable synthetic processes. 2. Results and Discussion 2. Results and Discussion 2. ResultsTrifloaluminate and Discussion ionic liquids based on 1-ethyl-3-methylimidazolium cation [emim]+ with various Trifloaluminate ionic liquids based on 1-ethyl-3-methylimidazolium cation [emim]+ with molarTrifloaluminate ratio of Al(OTf) ionic3 to [emim][OTf]: liquids basedχAl(OTf)3 on 1-ethyl-3-methylimidazolium= 0.25; χAl(OTf)3 = 0.15 were selectedcation [emim] as the+ Lewis with various molar ratio of Al(OTf)3 to [emim][OTf]: χAl(OTf)3 = 0.25; χAl(OTf)3 = 0.15 were selected as the Lewis acidicvarious catalysts molar ratio (Figure of Al(OTf)2). The3 to synthesis [emim][OTf]: was performed χAl(OTf)3 = 0.25; in theχAl(OTf)3 glove-box = 0.15 were to avoid selected the as hydrolysis the Lewis acidic catalysts (Figure 2). The synthesis was performed in the glove-box to avoid the hydrolysis underacidic thecatalysts solventless (Figure conditions. 2). The synthesis Ionic liquids was perf wereormed obtained in the in glove-box 99% yield to as avoid yellow the liquids hydrolysis and under the solventless conditions. Ionic liquids were obtained in 99% yield as yellow liquids and characterisedunder the solventless using the conditions. NMR spectroscopy Ionic liquids for thewere estimation obtained ofin activity99% yield coe asffi cientsyellow of liquids the target and characterised using the NMR spectroscopy for the estimation of activity coefficients of the target componentscharacterised (NEAT-NMR). using the NMR spectroscopy for the estimation of activity coefficients of the target components (NEAT-NMR). components (NEAT-NMR).

Figure 2. The synthesis of ionic liquids. FigureFigure 2.2. TheThe synthesissynthesis ofof ionicionic liquids.liquids.

The obtained ionic liquids were used for the oxidation of TMP with an aqueous solution of H2O2 TheThe obtained ionic ionic liquids liquids were were used used for forthe theoxidation oxidation of TMP of TMPwith an with aqueous an aqueous solution solution of H2O2 (Figure 3). It is worth underlining that the crucial factor in oxidation is selectivity. During the of(Figure H2O 23).(Figure It is 3worth). It underlining is worth underlining that the crucia thatl the factor crucial in oxidation factor in is oxidation selectivity. is selectivity.During the oxidation of TMP to TMBQ, the following side-products can be formed (Figure 3): 2,2′,3,3′,5,5′- Duringoxidation the of oxidation TMP to TMBQ, of TMP the to TMBQ,following the side-products following side-products can be formed can (Figure be formed 3): 2,2 (Figure′,3,3′,5,53):′- hexamethyl-4,4′-biphenol (BP), 2,3,6-trimethyl-1,4-benzoquinone epoxides (EQ) and 2-hydroxy-3,5,6- 2,2hexamethyl-4,40,3,30,5,50-hexamethyl-4,4′-biphenol (BP),0-biphenol 2,3,6-trimethyl-1,4-benzoquinone (BP), 2,3,6-trimethyl-1,4-benzoquinone epoxides (EQ) and epoxides 2-hydroxy-3,5,6- (EQ) and trimethyl-1,4-benzoquinone (HO-TMBQ). 2-hydroxy-3,5,6-trimethyl-1,4-benzoquinonetrimethyl-1,4-benzoquinone (HO-TMBQ). (HO-TMBQ).

Figure 3. Oxidation of 2,3,6-trimethylphenol (TMP) to 2,3,6-trimethyl-1,4-benzoquinone (TMBQ) and FigureFigure 3.3. Oxidation of 2,3,6-trimethylphenol (TMP)(TMP) toto 2,3,6-trimethyl-1,4-benzoquinone2,3,6-trimethyl-1,4-benzoquinone (TMBQ)(TMBQ) andand potential by-products. potentialpotential by-products.by-products. 2.1. Homogeneous System with Trifloaluminate Ionic Liquids 2.1.2.1. Homogeneous SystemSystem withwith TrifloaluminateTrifloaluminate IonicIonic LiquidsLiquids The preliminary tests were carried out using a 4-fold molar excess of 60% aqueous solution of TheThe preliminarypreliminary teststests werewere carriedcarried outout usingusing aa 4-fold4-fold molarmolar excessexcess ofof 60%60% aqueousaqueous solutionsolution ofof H2O2 in relation to TMP in solventless conditions at 70 °C (Table 1). HH22O2 inin relation relation to TMP in solventle solventlessss conditions at 70 ◦°CC (Table 1 1).).

Table 1. The influence of the structure and amount of trifloaluminate ILs (ionic liquids) on the course Table 1. The influence of the structure and amount of trifloaluminate ILs (ionic liquids) on the course of the oxidation of TMP to TMBQ under solventless conditions. Reaction conditions: TMP (5 mmol), of the oxidation of TMP to TMBQ under solventless conditions. Reaction conditions: TMP (5 mmol), 60% aq. H2O2 (20 mmol), 70 °C, 2 h. 60% aq. H2O2 (20 mmol), 70 °C, 2 h. TMP TMBQ Entry IL mol% of IL 1 TMP TMBQ Entry IL mol% of IL 1 Conversion (%) Selectivity (%) Conversion (%) Selectivity (%) 1 [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.15 3 41 73 1 [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.15 3 41 73 2 [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 3 77 79 2 [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 3 77 79 3 [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 2 83 72 3 [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 2 83 72 1 with respect to TMP, conversion and selectivity were measured using GC. 1 with respect to TMP, conversion and selectivity were measured using GC.

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Table 1. The influence of the structure and amount of trifloaluminate ILs (ionic liquids) on the course of the oxidation of TMP to TMBQ under solventless conditions. Reaction conditions: TMP (5 mmol),

60% aq. H2O2 (20 mmol), 70 ◦C, 2 h.

Entry IL mol% of IL 1 TMP Conversion (%) TMBQ Selectivity (%) [emim][OTf]-Al(OTf) , 1 3 3 41 73 χAl(OTf)3 = 0.15 [emim][OTf]-Al(OTf) , 2 3 3 77 79 χAl(OTf)3 = 0.25 [emim][OTf]-Al(OTf) , 3 3 2 83 72 χAl(OTf)3 = 0.25 1 with respect to TMP, conversion and selectivity were measured using GC.

In the absence of solvent, [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.15 showed very low catalytic activity, resulting in 41% TMP conversion and 73% selectivity to TMBQ (entry 1, Table1). The use of [emim][OTf]-Al(OTf)3,χAl(OTf)3 = 0.25 allowed to obtain higher conversion and selectivity values, i.e., 77 and 79%, respectively. As the amount of ionic liquid increased, the selectivity to TMBQ increased while the TMP conversion decreased. This is due to the formation of two phases during the reaction. At the start of the reaction, all the reagents and the catalyst were dissolved, but, after the reaction, the product along with the occluded side-products and the unreacted substrate precipitated as a red solid. Therefore, to improve the catalytic efficiency of the process, the addition of a solvent was used (Table2).

Table 2. The influence of the solvent on the course of the oxidation of TMP to TMBQ. Reaction

conditions: TMP (5 mmol), 60% aq. H2O2 (20 mmol), [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 (2 mol%), 3 solvent (3 cm ), 70 ◦C, 2 h.

Entry Solvent TMP Conversion (%) TMBQ Selectivity (%)

1 CH3COOH 88 92 1 2 CH3COOH 31 89 2 3 CH3COOH 79 79 4 CH3COOH + CH3CN 69 80 5 CH3CN 6 86 6 n-BuOH 10 73 7 MeOH 9 71 1 2 χ without IL; [emim][OTf]-Al(OTf)3, Al(OTf)3 = 0.25 (0.1 mol%), conversion and selectivity were measured using GC.

The use of polar (CH3CN, n-BuOH and MeOH) resulted in a lower reaction rate and a decrease in TMP conversion due to the dilution of the reaction system. On the other hand, the addition of acetic acid caused the increase in the catalytic activity (up to 88% conversion and up to 92% selectivity). Acetic acid emerged as the essential reagent in this free radical oxidation. It is known that it increases the speed of the termination step in the formation of the final product and reduces the contribution of the propagation step. Such a combination of events, in total, decreases the amount of side-products [22]. The use of acetic acid alone without the ionic liquid also allowed to obtain the product with high selectivity (89%), however, the TMP conversion was poor (31%). The lowering of the amount of IL did not cause a significant decrease in catalytic activity (entry 3, Table2). The amount of solvent and the duration of the process significantly influenced the reaction course (Table3). The addition of 1 mL of acetic acid provided homogeneous conditions and using only 0.1 mol% of IL led to a 77% conversion of TMP to TMBQ with 78% selectivity, after only 2 h. Extending the reaction time to 4 h allowed to achieve 84% selectivity with the nearly full conversion of TMP. Increasing the temperature to 100 ◦C increased the reaction rate resulting in 99% TMP conversion Catalysts 2020, 10, 1469 5 of 10 just after 2 h and 86% selectivity of product. However, due to the safety of experiments with the use of hydrogen peroxide, the process was carried out at 70 ◦C in the further tests.

Table 3. The influence of the amount of acetic acid and temperature on the course of the oxidation of

TMP to TMBQ. Reaction conditions: TMP (5 mmol), 60% aq. H2O2 (20 mmol), [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 (0.1 mol%), CH3COOH.

3 1 1 Entry CH3COOH (cm ) Temperature (◦C) Time (h) TMP Conversion (%) TMBQ Selectivity (%) 1 1 70 2 77 78 2 1 70 4 99 84 3 3 70 2 79 79 4 3 70 4 91 81 5 1 100 2 99 86 1 conversion and selectivity were measured using GC.

The concentration and the effect of the molar ratio of hydrogen peroxide to TMP were also studied (Table4). The lower concentration of hydrogen peroxide (35% aq) reduced the catalytic e fficiency due to the high water content and dilution of the reaction system. In turn, an increase in the molar ratio of H2O2 to TMP—with the optimal value 1:4—resulted in the increase in catalytic activity allowing 99% TMP conversion and 84% selectivity toward TMBQ.

Table 4. The influence of the concentration and amount of aqueous solution of H2O2 on the course of the oxidation of TMP to TMBQ. Reaction conditions: TMP (5 mmol), aq. H2O2, [emim][OTf]-Al(OTf)3, 3 χAl(OTf)3 = 0.25 (0.1 mol%), CH3COOH (1 cm ), 70 ◦C, 4 h.

1 1 Entry Concentration of H2O2 (%) Molar Ratio of TMP:H2O2 TMP Conversion (%) TMBQ Selectivity (%) 1 35 1:3 87 77 2 60 1:3 95 81 3 60 1:2 77 82 4 60 1:4 99 84 1 conversion and selectivity were measured using GC.

2.2. Heterogeneous System with Trifloaluminate Ionic Liquids The heterogenisation of the catalyst is crucial for handling, separation, and recycling. From the other point of view, the process of anchoring ionic liquid onto the solid support (SILP, supported ionic liquid phase) can significantly reduce the required amount of ionic liquid as the catalyst. The most important advantages of SILPs at the molecular level are: the accessibility of acidic centres and the ability to tune the structure of the solid carrier with the emphasis on the high specific surface area and well defined pore size [23]. Taking advantage of the heterogeneous system in this work, ionic liquid [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 was immobilised via the adsorption method on the surface of commercially available multiwalled carbon nanotubes (MWCNTs, CheapTubes). MWCNTs are very often used as versatile catalyst supports. Their properties, such as, e.g., large surface area, mechanical and thermal stability and small size, can significantly improve the activity and the selectivity of the catalysts supported on MWCNTs [24,25]. The catalyst was obtained by the simple mixing of the suspension of MWCNTs in n-hexane with [emim][OTf]-Al(OTf)3 catalyst, χAl(OTf)3 = 0.25 (mass ratio 2:1) in an ultrasonic bath for 2 h at room temperature. After the filtration and drying of the catalyst, the amount of the active phase supported on the surface of MWCNTs (9.1 wt.%) was determined using thermogravimetric analysis (TGA). The resulting MWCNT-[emim][OTf]-Al(OTf)3 catalyst, χAl(OTf)3 = 0.25, was used in the oxidation of TMP with 60% aq. H2O2 at 70 ◦C and compared to its soluble analogue using the same molar ratio of catalyst to TMP (0.1 mol% calculated per active phase) (Table5). Catalysts 2020, 10, 1469 6 of 10

Table 5. The influence of the type of catalyst based on trifloaluminate IL during the course of the oxidation

of TMP to TMBQ. Reaction conditions: TMP (5 mmol), 60% aq. H2O2 (20 mmol), catalyst (0.1 mol%), 3 CH3COOH (1 cm ), 70 ◦C, 4 h.

1 1 2 3 3 1 Entry Catalyst TMP Conversion (%) TMBQ Selectivity (%) TON (10− ) TOF (min− ) [emim][OTf]-Al(OTf) , 1 3 99 84 83.2 347 χAl(OTf)3 = 0.25 MWCNT-[emim][OTf]-Al(OTf) , 2 3 98 84 79.8 333 χAl(OTf)3 = 0.25 1 conversion and selectivity were measured using GC; 2 turnover number; 3 turnover frequency. Catalysts 2020, 10, x FOR PEER REVIEW 6 of 10

The obtained SILP catalyst exhibited high activity, similar to the catalytic activity of the homogenous ionic liquid. The The turnover turnover number number (TON) (TON) and turnover frequency (TOF) values for −11 MWCNT-[emim][OTf]-Al(OTf)33,,χ χAl(OTf)3Al(OTf)3 == 0.250.25 were were comparable comparable (TON (TON 79,800 and TOFTOF 333333 minmin− ) to −1 1 those obtained for [emim][OTf]-Al(OTf)33, χAl(OTf)3 = 0.25= 0.25 (TON (TON 83,200 83,200 and and TOF TOF 347 347 min min).− ). To investigate thethe possibility of recycling the SILP catalyst, the catalyst was filtered filtered ooffff after each reaction cycle,cycle, washedwashed with withn n-hexane,-hexane, and and dried dried under under vacuum vacuum (0.1 (0.1 mbar) mbar) for for 2 h. 2 Theh. The recovery recovery of the of catalystthe catalyst was was very very high high and and remained remained at the at levelthe level of 95–99% of 95–99% in five in five reaction reaction cycles. cycles. Conversion and selectivity remained high in fivefive cyclescycles (Figure(Figure4 ).4). AA slightslight decreasedecrease inin thethe activity waswas observedobserved in in the the fourth fourth reaction reaction cycle, cycle, yielding yielding 96% 96% TMP TMP conversion conversion accompanied accompanied by 83% by selectivity.83% selectivity. The shapeThe shape of the of DTG the DTG (derivative (derivative thermogravimetry) thermogravimetry) curve curve after the after fifth the cycle fifth incycle the in range the ofrange 100–250 of 100–250◦C (Figure °C (Figure5b) suggested 5b) suggested the co-adsorption the co-adsorption of other of compounds other compounds of the reaction of the mixturereaction onmixture the surface on the ofsurface the MWCNTs, of the MWCNTs limiting, limiting the access the access to the to active the active centres centres of the of catalyst, the catalyst, mainly mainly due todue clogging to clogging the catalyst the catalyst pores. pores. Therefore, Therefore, to calculate to calculate the amount the amount of active of active phase phase after theafter fifth the cycle, fifth thecycle, range the fromrange 100 from to 250100 ◦toC was250 °C not was considered. not consid Theered. ionic The liquid ionic content liquid decreasedcontent decreased slightly afterslightly the fifthafter cyclethe fifth to 8.7 cycle wt.% to 8.7 compared wt.% compared to the fresh to the catalyst fresh (9.1 catalyst wt.%). (9.1 wt.%).

100 conversion selectivity

80

60 (%) 40

20

0 12345 cycle

Figure 4. Recycling ofof multiwalledmultiwalled carboncarbon nanotube nanotube (MWCNT)-[emim][OTf]-Al(OTf) (MWCNT)-[emim][OTf]-Al(OTf)3, 3χ, Al(OTf)3χAl(OTf)3 = 0.25.0.25. Reaction conditions: TMP (25 mmol),mmol), 60%60% aq.aq. HH22O22 (100(100 mmol),mmol), MWCNT-[emim][OTf]-Al(OTf)MWCNT-[emim][OTf]-Al(OTf)3,, 3 3 χAl(OTf)3 = 0.25= 0.25 (0.1(0.1 mol%), mol%), CH CH3COOH3COOH (5 cm (5), cm 70 ),°C, 70 4◦ h,C, conversion 4 h, conversion and selectivity and selectivity were measured were measured using usingGC. GC.

CatalystsCatalysts2020 2020, ,10 10,, 1469 x FOR PEER REVIEW 77 of of 10 10

FigureFigure 5.5. TGA and DTG curves, recorded in nitrogen (pyrolytic conditions),conditions), forfor thethe freshfresh catalystcatalyst MWCNT-[emim][OTf]-Al(OTf) , χ . and after the fifth cycle. (a) TGA curves; (b) DTG curves. MWCNT-[emim][OTf]-Al(OTf)3 3,Al(OTf)3 χAl(OTf)3= = 0.25. and after the fifth cycle. (a) TGA curves; (b) DTG curves. 3. Materials and Methods 3. Materials and Methods 3.1. Materials 3.1. Materials 1-Ethyl-3-methylimidazolium triflate—[emim][OTf], Al(OTf)3 (Sigma-Aldrich, St. Louis, MO, USA).1-Ethyl-3-methylimidazolium 2,3,6-Trimethylphenol (98%) (Acros, triflate—[emim][OTf], Waltham, MA, USA) Al(OTf) and MWCNTs3 (Sigma-Aldrich, (>95 wt.%, St.CheapTubes Louis, MO, Inc.,USA). Grafton, 2,3,6-Trimethylphenol VT, USA) were used. (98%) (Acros, Waltham, MA, USA) and MWCNTs (>95 wt.%, CheapTubes Inc., Grafton, VT, USA) were used. 3.2. Experimental Procedures 3.2. Experimental Procedures 3.2.1. Homogenous Catalyst 3.2.1. Homogenous Catalyst Trifloaluminate ionic liquids were obtained according to the procedure described earlier [19]. Briefly,Trifloaluminate [emim][OTf] ionic (2.60 liquids g, 0.0100 were mol) obtained and according Al(OTf)3 into the various procedure molar described ratios (0.84–1.58 earlier [19]. g, 3 0.0018–0.0033Briefly, [emim][OTf] mol), were(2.60 introducedg, 0.0100 mol) into and a 25Al(OTf) cm 3round-bottom in various molar flask ratios sealed (0.84–1.58 with ag, septum. 0.0018– The0.0033 reactants mol), were were introduced stirred (300 into ppm) a 25 in cm the3 gloveround-bottom box at 85 flask◦C for sealed 2 h. Thewith product a septum. was The obtained reactants in 99%were yield stirred as yellow(300 ppm) liquid in andthe glove was further box at used85 °C without for 2 h. purification. The product was obtained in 99% yield as yellow liquid and was further used without purification. 3.2.2. Heterogeneous Catalyst 3.2.2. Heterogeneous Catalyst Ionic liquid [emim][OTf]-Al(OTf)3], χAl(OTf)3 = 0.25 (0.62 g, 0.02 mol), MWCNTs (1.24 g) and 3 n-hexaneIonic (10 liquid cm ) [emim][OTf]-Al(OTf) as a solvent were added3], χ toAl(OTf)3 the round-bottom = 0.25 (0.62 g, flask 0.02 sealed mol), withMWCNTs a septum, (1.24 and g) stirredand n- inhexane an ultrasonic (10 cm3) bathas a solvent (25 ◦C, were 2 h). added Then, theto the heterogeneous round-bottom catalyst flask sealed was filtered with a throughseptum, and a Büchner stirred funnel, washed with n-hexane (5 25 cm3). The catalyst was dried at 25 C on a Schlenk line (0.1 mbar, in an ultrasonic bath (25 °C, 2 h).× Then, the heterogeneous catalyst was◦ filtered through a Büchner 2funnel, h). washed with n-hexane (5 × 25 cm3). The catalyst was dried at 25 °C on a Schlenk line (0.1 mbar, 2 h). 3.2.3. Synthesis of 2,3,6-Trimethyl-1,4-Benzoquinone

3.2.3.General Synthesis procedure: of 2,3,6-Trimethyl-1,4-Benzoquinone [emim][OTf]-Al(OTf)3 or immobilised [emim][OTf]-Al(OTf)3 (0.1–3.0 mol% in the respect to TMP), TMP (0.68 g, 5 mmol) and solvent (1–3 cm3) were added into a 25 cm3 General procedure: [emim][OTf]-Al(OTf)3 or immobilised [emim][OTf]-Al(OTf)3 (0.1–3.0 mol% two-neck round bottom flask equipped with a reflux condenser. Then, 35 or 60% H O (0.57–1.46 g, in the respect to TMP), TMP (0.68 g, 5 mmol) and solvent (1–3 cm3) were addedaq into2 2a 25 cm3 two- 10–20 mmol) was added. The reactants were stirred (1000 ppm) at 70–100 ◦C for 2–4 h under reflux. neck round bottom flask equipped with a reflux condenser. Then, 35 or 60%aq H2O2 (0.57–1.46 g, 10– The reaction progress was monitored by GC. 20 mmol) was added. The reactants were stirred (1000 ppm) at 70–100 °C for 2–4 h under reflux. The reaction progress was monitored by GC.

Catalysts 2020, 10, 1469 8 of 10

Product isolation: [emim][OTf]-Al(OTf)3, χAl(OTf)3 = 0.25 used at 0.1 mol% per TMP, TMP (0.68 g, 5 mmol) and acetic acid as solvent (1 cm3) were added into a 25 cm3 two-neck round bottom flask equipped with a reflux condenser. Then, 60%aq H2O2 (1.46 g, 20 mmol) was added. The reactants 3 were stirred (1000 ppm) at 70 ◦C for 4 h under reflux. After the addition of n-hexane (10 cm ), [emim][OTf]-Al(OTf)3 formed a second phase that was easy to separate. Then, the product was purified by column chromatography (silica; n-hexane:AcOEt 25:1, v/v). 2,3,6-trimethyl-1,4-benzoquinone was obtained as a yellow oil in 80% yield. 1 2,3,6-trimethyl-1,4-benzoquinone: H NMR (400 MHz, CDCl3) δ: 6.49 (q, J = 1,6 Hz, 1H), 1.97–1.96 (m, 13 3H), 1.96–1.95 (m, 3H), 1.95–1.93 (m, 3H). C NMR (150 MHz, CDCl3) δ: 187.86, 187.47, 145.35, 140.91, + 140.75, 133.09, 15.90, 12.38, 12.07. GC–MS: (EI) m/z (%) 150 (100, M ·), 122 (30), 107 (42), 96 (18), 79 (29), 68 (31), 54 (17), 39 (27). The heterogeneous catalyst MWCNT-[emim][OTf]-Al(OTf)3 was recycled, χAl(OTf)3 = 0.25. For the catalyst recycling tests, all reactants and reagents were used in 5-fold larger scale compared to screening tests. After the reaction, 15 cm3 of n-hexane was added to the reaction mixture. Then, the catalyst was filtered off and dried under vacuum on a Schlenk line (60 ◦C, 0.1 mbar, 2 h) and used in the next cycle of the reaction.

3.3. Analyses TMBQ and ILs were analysed by NMR (1H 400 MHz and 13C 150 MHz, tetramethylsilane was used as an internal standard. NMR spectra were recorded on Agilent 400-MR (Agilent Technologies, Santa Clara, CA, USA). Ils were analysed neat (capillaries with d6-DMSO were used as an external lock). Samples from the reaction mixtures were analysed on a gas chromatograph (SHIMADZU GC-2010 Plus, Shimadzu Corp., Kyoto, Japan) equipped with a Zebron GC column (ZB-5MSi, dimensions: 30 m 0.32 mm, a film thickness: 0.25 µm film, Phenomenex, Torrance, CA, USA) and on a gas × chromatograph coupled with a mass spectrometer (Agilent 7890C GC with a Agilent 5975C MS, Agilent Technologies, Santa Clara, CA, USA). A capillary column HP-5 MS (dimensions: 30 m × 0.25 mm, a film thickness: 0.25 µm film, Agilent Technologies, Santa Clara, CA, USA) was used as a GC column and an EI (70 eV) was used as an ionisation method. Thermogravimetric curves of the heterogeneous catalysts were recorded on a Mettler-Toledo thermobalance STAR851 (Mettler-Toledo, GmbH., Greifensee, Switzerland). The analyses were 1 performed in nitrogen (flow: 100 mL min− ), at temperatures ranging from 25 to 800 ◦C, at the heating rate 20 C min 1. Al O crucibles (70 µL) were used for samples (5 10 mg). ◦ − 2 3 − Turnover number (TON) and turnover frequency (TOF) were calculated according to formulae 1 and 2:

TON = yield of TMBQ (%) moles of TMP (mol) moles of the catalyst (mol) (1) × × TOF = TON/reaction time (min) (2)

4. Conclusions In conclusion, highly active catalytic systems based on trifloaluminate ionic liquid were developed for the oxidation of TMP to TMBQ using hydrogen peroxide as the green oxidant. Due to the formation of a separate IL-phase in the reaction system and the free radical mechanism of the oxidation process, the use of acetic acid as a solvent was necessary—it caused the homogenisation of the reaction mixture and increased the selectivity of the product. The immobilisation of the ionic liquid on MWCNTs allowed for the development of a highly effective catalyst that can be easily separated by filtration and used in five reaction cycles without a significant decrease in activity. The presented catalyst, both in the form of trifoaluminate IL and its heterogeneous counterpart, is characterised by the high activity in the oxidation reaction with hydrogen peroxide (99% conversion and 84% selectivity), with the ability to recycle the catalyst as the largest advantage for the heterogeneous system. Catalysts 2020, 10, 1469 9 of 10

Author Contributions: Conceptualisation, A.C.; methodology, P.L., N.B., S.B.; investigation, P.L., N.B.; thermogravimetric analysis, S.J.; data curation, P.L., N.B.; writing—original draft preparation, A.C., P.L. and A.S.; writing—review and editing, A.C., S.B., P.L. and A.S.; visualisation, A.S.; supervision, A.C. and S.B.; funding acquisition, N.B. All authors have read and agreed to the published version of the manuscript. Funding: The research was funded by Silesian University of Technology, Grant No. 04/050/BKM20/0104 BKM-547/RCH5/2020 and 04/050/BK_20/0097. Conflicts of Interest: The authors declare no conflict of interest.

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