catalysts Article Highly Active Trifloaluminate Ionic Liquids 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 Polymer 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 solvent, 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 acid 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 liquid phase; carbon nanotubes; trifloaluminate ionic liquids; Lewis acids; 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 hydrogenation 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 molecule, 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 hydrogen peroxide is a weak were used, affording a 86% yield of TMP [7]. Another approach is to use copper(II) nitrate catalyst in oxidant, the water-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, acetic acid 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 ligands 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-alkyl-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].