catalysts

Review Epoxidation of Terpenes

Yacoub Mahamat Ahmat 1 , Sara Madadi 1, Luc Charbonneau 2 and Serge Kaliaguine 1,*

1 Department of Chemical Engineering, Université Laval, Québec, QC G1V 0A6, Canada; [email protected] (Y.M.A.); [email protected] (S.M.) 2 Department of Chemistry, The University of Helsinki, 00560 Helsinki, Finland; luc.charbonneau@helsinki.fi * Correspondence: [email protected]

Abstract: Terpene epoxides are considered as potential primary intermediates in the synthesis of numerous green polymers including epoxy resins, polycarbonates, nonisocyanate polyurethanes and even some polyamides. In this chapter we describe recent efforts from our group to develop catalytic and noncatalytic processes for terpene epoxidation using a variety of oxidizing agents and process intensification methods. Most experimental tests deal with limonene epoxidation with applicability to some other terpenes also demonstrated.

Keywords: limonene; pinenes; dioxiranes; aerobic epoxidation

1. Introduction



In the recent quest for green substituents to carbonaceous monomers of fossil origin,
 terpenes occupy a special position [1–3]. Monoterpenes are naturally occurring olefinic hydrocarbons comprising ten carbon atoms. Their chemical properties are therefore similar Citation: Mahamat Ahmat, Y.; to those of significant petroleum derived monomers systematically used today in the Madadi, S.; Charbonneau, L.; Kaliaguine, S. Epoxidation of production of high tonnage polymers [4]. Terpenes. Catalysts 2021, 11, 847. Monoterpenes are dimers of isoprene (2-methyl-1,3-butadiene) biosynthesized by https://doi.org/10.3390/catal11070847 numerous plants at a global scale estimated to around 1000 Mt/year. Isoprene being volatile is released by biomass at a global scale of about 600 Mt/year. Only a few of these Academic Editors: Sébastien monoterpenes are produced commercially at much more modest scale. These are α- and Leveneur, Vincenzo Russo and β-pinene, 3-carene, and limonene (as illustrated in Scheme1). Pasi Tolvanen All four are present in coniferous tree resins. The two pinenes are produced mostly from turpentine by the pulp and paper industry at 300 kt/year (in 2015), whereas limonene Received: 15 June 2021 is recovered by steam extraction from citrus fruit peels, a waste product of the orange juice Accepted: 2 July 2021 industry (90 kt/year) [5]. Published: 14 July 2021 Notably, the US Company Amyris developed a process to produce farnesene from wood hydrolysates through a proprietary biochemical process. Farnesene is designated as Publisher’s Note: MDPI stays neutral a sesquiterpene, which means a 15-carbon olefin. with regard to jurisdictional claims in Squalene, a 30-carbon triterpene originally extracted from shark liver, is now mostly published maps and institutional affil- produced from vegetable sources including olive oil and sugar cane [6]. iations. The biosynthesis of terpenes is well-described in literature [7]. Recent work succeeded in reproducing it in vitro [8], but this approach is still too complex and not liable to any commercial application. Similarly, the in vitro biosynthesis of isoprene was also achieved [9–11]. A hybrid process involving glucose fermentation to mesaconic acid, Copyright: © 2021 by the authors. hydrodeoxygenation to 3-methylTHF, and dehydration to isoprene seems closer to an Licensee MDPI, Basel, Switzerland. industrial green isoprene production [12,13]. To our knowledge, however, a process for This article is an open access article dimerization of isoprene is still to be proposed. distributed under the terms and Epoxidation results from the reaction of an alkene with an oxidizing agent. In indus- conditions of the Creative Commons trial epoxide production, the choice of the oxidant is determining feasibility, efficiency, and Attribution (CC BY) license (https:// therefore, profitability of the epoxidation process. creativecommons.org/licenses/by/ 4.0/).

Catalysts 2021, 11, 847. https://doi.org/10.3390/catal11070847 https://www.mdpi.com/journal/catalysts

Review Epoxidation of Terpenes

Yacoub Mahamat Ahmat 1, Sara Madadi 1, Luc Charbonneau 2 and Serge Kaliaguine 1,*

1 Department of Chemical Engineering, Université Laval, Québec, QC G1V 0A6, Canada; Citation: Ahmat, Y.M.; Madadi S.; [email protected] (Y.M.A.); [email protected] (S.M.) Charbonneau L.; Kaliaguine S. 2 Department of Chemistry, The University of Helsinki, 00560 Helsinki, Finland; [email protected] Epoxidation of Terpenes. * Correspondence: [email protected] Catalysts 2021, 11, x. https://doi.org/10.3390/xxxxx Abstract: Terpene epoxides are considered as potential primary intermediates in the synthesis of numerous green polymers including epoxy resins, polycarbonates, nonisocyanate polyurethanes Academic Editors: Sébastien and even some polyamides. In this chapter we describe recent efforts from our group to develop Leveneur, Vincenzo Russo catalytic and noncatalytic processes for terpene epoxidation using a variety of oxidizing agents and and Pasi Tolvanen process intensification methods. Most experimental tests deal with limonene epoxidation with ap-

Received: 15 June 2021 plicability to some other terpenes also demonstrated. Accepted: 2 July 2021 Published: 14 July 2021 Keywords: limonene; pinenes; dioxiranes; aerobic epoxidation

Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- 1. Introduction tional affiliations. In the recent quest for green substituents to carbonaceous monomers of fossil origin, terpenes occupy a special position [1–3]. Monoterpenes are naturally occurring olefinic

hydrocarbons comprising ten carbon atoms. Their chemical properties are therefore sim- Copyright: © 2021 by the authors. Li- ilar to those of significant petroleum derived monomers systematically used today in the censee MDPI, Basel, Switzerland. production of high tonnage polymers [4]. This article is an open access article Monoterpenes are dimers of isoprene (2-methyl-1,3-butadiene) biosynthesized by distributed under the terms and con- numerous plants at a global scale estimated to around 1000 Mt/year. Isoprene being vol-

Catalystsditions of the2021 Creative, 11, 847 Commons At- atile is released by biomass at a global scale of about 600 Mt/year. Only a few of 2these of 17 tribution (CC BY) license (http://crea- monoterpenes are produced commercially at much more modest scale. These are α- and tivecommons.org/licenses/by/4.0/). β-pinene, 3-carene, and limonene (as illustrated in Scheme 1).

Scheme 1. Most important monoterpenes.

AllActually, four are a variety present of in such coniferous oxidants tree were resins. used The industrially. two pinenes The are conventional produced chloro-mostly fromhydrin turpentine process inby which the pulp alkenes and paper (e.g., indust propylene)ry at 300 react kt/year with (in chlorine 2015), whereas in the presence limonene of issodium recovered hydroxide by steam tends extraction to be replaced from citrus by morefruit peels, environmentally a waste product friendly of the ones orange owing juice to industryits large waste(90 kt/year) production [5]. [14,15]. Peracids such as m-CPBA (meta-chloroperbenzoic acid) were also used occasionally. Their oxidizing power depends on the leaving group capacity of the corresponding car- boxylate [16]. They leave a stoichiometric amount of the corresponding acid as waste product. They do not require using a catalyst but the acidity makes selective epoxidation challenging [17]. Catalysts 2021, 11, x. https://doi.org/10.3390/xxxxxDMDO (dimethyl dioxirane) is a relatively new, highly reactivewww.mdpi.com oxidant/journal/catalysts generated by reaction of potassium peroxomonosulfate (KHSO5) with acetone. A stabilized form (2KHSO5 + KHSO4 + K2SO4) is commercially available from Dupont Chemicals under the brand name of Oxone. The aqueous solution of Oxone should be maintained at pH between 8 and 9 to avoid its decomposition and acid hydrolysis of the epoxide. The hydrodynamics of injecting this solution in the reaction medium is also a challenge for large-scale application. Alkylhydroperoxides and hydrogen peroxide are commonly employed using a het- erogeneous catalyst [18]. In the large-scale production of propylene oxide, the Halcon process uses ter-butylhydroperoxide (TBHP), the Halcon–Arco process uses ethylbenzene hydroperoxide, whereas cumene hydroperoxide is the oxidant in the Sumitomo process [19]. All these processes generate corresponding alcohols as a stoichioemetric byproduct. Hydrogen peroxide is especially attractive since its only byproduct is water, and its weight ratio of active oxygen (47%) is the highest after molecular oxygen. Owing to its high rate of decomposition, however, it must be used in large excess, which upsets the large weight ratio asset [20]. The radicals formed during the decomposition process convert the olefins to alcohol or ketones, decreasing epoxide selectivity. For safety reasons, H2O2 is only available commercially in aqueous solutions (30 wt%), while water is a real inhibitor of catalysis by metals and greatly slows down the reaction [21].The epoxide selectivity is often poor in the presence of water or protic solvents due to the ring opening secondary reaction leading to the formation of glycol compounds [22,23]. Oxygen is by far the most desirable oxidant; it has the highest active oxygen content, the obviously lowest cost, and produces essentially no waste product. It is directly em- ployed in ethylene epoxidation, the largest epoxidation process (26 Mt global production in 2018) which uses Ag catalysts. With any other alkene, the selectivity of epoxide forma- tion is not sufficient. Recent developments led to transition metal-based catalysts with relatively high activity and epoxide selectivity [20]. The search for metal catalysts effective in activating molecular oxygen is therefore a current goal of industrial research. In this text we intend to review the work performed in our laboratory over the last five years, exploring the use of various oxidants in terpenes epoxidation. Our work was primarily targeting limonene epoxidation. As shown in Scheme1, limonene has two double bonds of different substitution numbers. The 1,2 double bond is trisubstituted and is therefore more easily epoxidized than the 8,9 one, which is only disubstituted, making the study of limonene epoxidation more informative. Catalysts 2021, 11, 847 3 of 17

Our contributions will be classified according to the oxidizing agent used for epoxidation.

2. Hydroperoxides

Hydroperoxides are generally far superior to H2O2. Hydrogen peroxide dissolution is limited to polar solvents, whereas hydroperoxides are also soluble in hydrocarbons [21,24,25]. Chiker et al. demonstrated that tert-butyl hydroperoxide (TBHP) is far more selective to- wards epoxides than H2O2 when supported titanium is used as the catalysts [26]. In dealing with epoxidation of limonene using t-butyl hydroperoxide as the oxidizing agent, we took advantage in the longstanding experience of our laboratory for Ti/SiO2 oxidation catalysts. Titanium on silica was initially proposed by Sheldon in the 1970’s [21,24] and firstly used by Shell to produce propylene oxide in liquid phase. It was also implemented by the Enichen scientists in designing titanium silicalites (TS-1), a work which provided inspiration for numerous oxidation catalysts [25–28]. In alkene epoxidation over TS-1 the oxidizing agent is, however, limited to H2O2, owing to diffusional restriction in silicalite microchannels for larger ones such as TBHP [25]. Similar restrictions obviously apply to bulky reactants and products with kinetic diameter larger than 0.6 nm. In all Ti/SiO2 oxidation catalysts, the active site is a coordination unsaturated Ti substituted in the silica lattice. In TS-1 for example the substitutional limit is 1.9% of the T atoms. Thus, to lift these two kinds of limitations, Berubé et al. initially studied the incorporation of titanium in mesostructued SBA-15 silica by coprecipitation; they concluded that this direct method was not the most suitable technique [29,30]. For the epoxidation reaction of cyclohexene by TBHP, Berubé et al. then developed a liquid phase soft-templating titanium postgrafting impregnation of SBA-15 using in situ generated acetylacetonate as the stabilizing chelate [31,32]. This synthetic method allows a high content of low coordination titanium on the surface of silica, reaching up to 13.8% of T atoms [31,32]. Up to 5%, isolated four coordinated Ti atoms were obtained, whereas intermediate 4–6 coordinations Ti were formed at 5–13.8% content. Over this latter range the rate of cyclohexene epoxidation by aqueous TBHP stood essentially constant but the catalyst became stable and reusable. In a more recent contribution, we studied coordination unsaturated titanium catalysts supported on SBA-16 mesoporous silica using anhydrous TBHP as the oxidizing agent for the epoxidation of limonene. The postgrafting impregnation of titanium method developed by Berube et al. was first modified by using soluble acetylacetonate dissolved in toluene. This approach was simpler and yielded the same catalytic activity as the in situ synthesized acetylacetonate titanium [33]. The reaction conditions including temperature, TBHP concentration, and solvent polarity were optimized. The conversion of limonene reached 80% with a 1,2-limonene oxide selectivity of 79%. The only observed secondary product was 8.9-limonene oxide. When SBA-16 was used, the mass transfer was enhanced compared to SBA-15, leading to a higher conversion of limonene [33]. For a similar Ti/Si atomic ratio, the conversion was only 38% when SBA-15 was used, compared to 80% for that of SBA-16. An increase of the 1,2-limonene epoxide trans-isomers was observed as the Ti/Si atomic ratio was increased to 13.4%. Attempts at increasing conversion to limonene dioxide (LDO) over Ti/SBA-16 using TBHP met with very little success [33,34]. Interestingly, a similar result was obtained in the recent work by Cunningham et al. [35]. When trying to epoxide limonene, over a Venturella phase transfer catalyst (VPTC; PW4O24(PTC)3) using H2O2 as the oxidizing agent, a high yield of 1,2-limonene epoxide was obtained with 30% aqueous H2O2 solution, whereas a 50% H2O2 solution was needed to reach partial conversion to LDO.

3. Dimethyl Dioxirane (DMDO) DMDO, sometimes designated as Murray’s reagent, is usually generated in situ from potassium peroxomonosulfate (in oxone) and a ketone (typically acetone) [36]. In this case the reaction medium is multiphasic with an organic phase containing the organic substrate (limonene) and acetone, and an aqueous solution of oxone with sodium bicarbonate as a buffer. DMDO is formed at the interface (as illustrated in Scheme2). Catalysts 2021, 11, x FOR PEER REVIEW 4 of 19

3. Dimethyl Dioxirane (DMDO) DMDO, sometimes designated as Murray’s reagent, is usually generated in situ from potassium peroxomonosulfate (in oxone) and a ketone (typically acetone) [36]. In this case Catalysts 2021, 11, 847 the reaction medium is multiphasic with an organic phase containing the organic4 sub- of 17 strate (limonene) and acetone, and an aqueous solution of oxone with sodium bicarbonate as a buffer. DMDO is formed at the interface (as illustrated in Scheme 2)

Scheme 2. Mechanism of dioxirane formatio formationn from oxone and ketones [[37].37].

Epoxidation cancan alsoalso bebe performed performed using using externally externally generated generated DMDO. DMDO. The The asymmet- asym- metricric epoxidation epoxidation of alkenes of alkenes is known is known as the as Shithe epoxidationShi epoxidation reaction reaction [38]. [38]. DMDO and other cyclic peroxides are known to be especially efficient efficient in transferring oxygen toto aa widewide variety variety of of alkenes alkenes while while being being rather rather mild mild towards towards oxidizing oxidizing the substratethe sub- strateand reaction and reaction products products [39]. DMDO [39]. DMDO is active is inac bothtive in electron both electron poor and poor electron-rich and electron-rich alkenes alkenesin very highin very yield high [40 yield]. [40] 3.1. Epoxidation of Limonene in Semi-Batch Reactor 3.1. Epoxidation of Limonene in Semi-Batch Reactor The biphasic nature of the reaction medium is not the most demanding aspect in terms The biphasic nature of the reaction medium is not the most demanding aspect in of reactor design. The aqueous solutions of oxone are acidic (pH =∼ 1–2) and would lead terms of reactor design. The aqueous solutions of oxone are acidic (pH ≅ 1–2) and would to hydrolysis of any epoxide formed at their contact. Thus, the aqueous phase must be lead to hydrolysis of any epoxide formed at their contact. Thus, the aqueous phase must buffered at pH of 7–9, which is accomplished by introducing sodium bicarbonate. Too high be buffered at pH of 7–9, which is accomplished by introducing sodium bicarbonate. Too basic pH would lead to decomposition of potassium monoperoxysulfate [36,41]. This very high basic pH would lead to decomposition of potassium monoperoxysulfate [36,41]. This strict pH control is to be performed not only on the instantaneous average pH value of the very strict pH control is to be performed not only on the instantaneous average pH value aqueous phase, but also on local values at any reaction time. This imposes constraints on of the aqueous phase, but also on local values at any reaction time. This imposes con- the feed flow rate of oxone to the reactor. The results shown in Table1 were obtained in a straints on the feed flow rate of oxone to the reactor. The results shown in Table 1 were round bottom flask, initially loaded with 10 mmol limonene dissolved in 30 mL acetone obtained in a round bottom flask, initially loaded with 10 mmol limonene dissolved in 30 and 4 g sodium bicarbonate. Adjusted numbers of moles of oxone were dissolved in 50 mL mL acetone and 4 g sodium bicarbonate. Adjusted numbers of moles of oxone were dis- deionized water, and this solution was fed to the reactor at the flow rate of 1 mL/min. This solveddropwise in 50 feeding mL deionized allows the water, solution and droplet this solu totion be rapidly was fed dispersed to the reactor in the at aqueous the flow phase, rate ofthus 1 mL/min. avoiding This over dropwise concentration feeding of allows oxone afterthe solution a few minutes droplet of to reaction be rapidly time. dispersed in the aqueousThe results phase, in Table thus1 avoiding were obtained over concen after twotration hours of ofoxone reaction after at a roomfew minutes temperature. of re- Theyaction clearly time. indicate that a complete conversion of limonene to LDO is possible under these conditions. No products of allylic oxidation, nor any trace of 8,9-limonene epoxide are observed. The over stoichiometric consumption of oxone is likely related to some slow oxone decomposition. Catalysts 2021, 11, 847 5 of 17

Table 1. Effect of total amount of oxone fed to reactor at constant flowrate [34].

Limonene Oxone 1,2-LO Oxone®/Limonene pH of Oxone® Conversion a Dioxide Conversion to Yield a Ratio Solution (%) Yield a Epoxide (%) (%) (%) 0.65 1.78 41 16 22 92.6 1.30 1.55 65 22 38 75 2.00 1.43 98 23 75 86 2.60 1.30 100 0 100 77 10 mmol of limonene, 30 mL of acetone, 4 g of sodium bicarbonate, various amounts of Oxone® dissolve in 50 mL of water was added to the flask at constant flowrate of 1 mL min−1, room temperature, total reaction time 2 h; a: Limonene conversion and yields were determined by GC-FID after purification.

3.2. Epoxidation under Ultrasound Obviously, the epoxidation process described in Scheme2 is occurring in a zone localized close to the interface between the aqueous and organic phases. This makes the process rate directly proportional to the surface area of this interface. One of the beneficial effects of applying ultrasound to a reaction medium is precisely an efficient and rapid dispersion in a biphasic system. Thus, a large rate increase of limonene epoxidation by in Catalysts 2021, 11, x FOR PEER REVIEWsitu generated DMDO upon applying ultrasound may be expected. 2 of 20 The results shown in Figure1a,b were obtained in similar conditions. In Figure1a, 10 mmol limonene was dissolved in 30 mL acetone with 4 g sodium bicarbonate. 50 mL of a 0.52 M oxone solution was added at a flowrate of 1 mL min−1. In Figure1b, 10 mmol Notably, the US Company Amyris developed a process to produce farnesene from limonene was dissolved in 40 mL of acetone with 4 g sodium bicarbonate. 60 mL of a wood hydrolysates through a proprietary biochemical process. Farnesene is designated 0.52 M oxone solution was added at a flowrate of 20 mL min−1, under ultrasound (power as a sesquiterpene, which means a 15-carbon olefin. 50 W, frequency 26 kHz). a and b tests were run at room temperature.

Figure 1. Comparison of limonene epoxidation results: (a) in absence of ultrasound; (b) under ultrasound. Reprinted with permission from Ref. [42]. CopyrightSqualene, 2018 American a 30-carbon Chemical triterpene Society. originally extracted from shark liver, is now mostly produced from vegetable sources including olive oil and sugar cane [6]. ComparingThe biosynthesis a and bof highlights terpenes theis well-described very significant in increase literature in epoxidation [7]. Recent ratework brought suc- aboutceeded by in ultrasonication. reproducing it in Similar vitro [8], rate but enhancements this approach were is still demonstrated too complex with andsome not liable other terpenesto any commercial including theapplication. two pinenes, Similarly, carvone, the carveol, in vitro andbiosynthesis farnesol [of42 ].isoprene was also achieved [9–11]. A hybrid process involving glucose fermentation to mesaconic acid, hy- drodeoxygenation to 3-methylTHF, and dehydration to isoprene seems closer to an in- dustrial green isoprene production [12,13]. To our knowledge, however, a process for di- merization of isoprene is still to be proposed. Epoxidation results from the reaction of an alkene with an oxidizing agent. In indus- trial epoxide production, the choice of the oxidant is determining feasibility, efficiency, and therefore, profitability of the epoxidation process. Actually, a variety of such oxidants were used industrially. The conventional chloro- hydrin process in which alkenes (e.g., propylene) react with chlorine in the presence of sodium hydroxide tends to be replaced by more environmentally friendly ones owing to its large waste production [14,15]. Peracids such as m-CPBA (meta-chloroperbenzoic acid) were also used occasionally. Their oxidizing power depends on the leaving group capacity of the corresponding car- boxylate [16]. They leave a stoichiometric amount of the corresponding acid as waste product. They do not require using a catalyst but the acidity makes selective epoxidation challenging [17]. DMDO (dimethyl dioxirane) is a relatively new, highly reactive oxidant generated by reaction of potassium peroxomonosulfate (KHSO5) with acetone. A stabilized form (2KHSO5 + KHSO4 + K2SO4) is commercially available from Dupont Chemicals under the brand name of Oxone. The aqueous solution of Oxone should be maintained at pH be- Catalysts 2021, 11, 847 6 of 17

3.3. Heterogeneous Polydioxiranes In a yet unpublished preliminary work, we tested, for limonene epoxidation, a method proposed by Kazemnejadi et al. for epoxidation of alkenes. In this method, a polydioxirane (PDOX) was generated by reaction of polysalicylaldehyde with oxone [43]. After epoxida- tion, PDOX is back converted to the polyaldehyde, which can be recovered by filtration. In [43], the converted PDOX was reused for eight cycles without significant loss of activity. Formerly, heterogeneous solid catalysts such as α-fluorotropinone supported on SiO2 and Merrified resin was studied for stereoselective epoxidation of alkenes by oxone [44]. For this purpose, trifluoromethylketone supported on silica, as a solid dioxirane support, was a good catalyst for epoxidation by dioxirane generating alkene epoxides in the presence of oxone [45,46]. Our preliminary results indicate that solid PDOX generated in the presence of oxone can also be employed in the case of terpenes.

3.4. In Microemulsions Another way besides ultrasonication to generate uniform dispersion of an organic phase into an aqueous oxone solution is the production of a microemulsion. Introduc- ing minute amounts of an appropriate surfactant, a stable dispersion of organic phase droplets with diameters in the 5 to 50 nm range can be generated [47]. We established that cetyltrimethylammonium hydrogen sulfate (CTHAS) is especially effective in microemul- sion epoxidation of terpenes by in situ generated DMDO. Figure2 shows the respective positions of the quaternary ammonium ions of CTHAS in the terpene droplet and the − Catalysts 2021, 11, x FOR PEER REVIEWHSO 5 ions attracted to the interface by electrostatic interaction. This favors the local7 of 19

formation of DMDO and consequently a rapid terpene epoxidation.

FigureFigure 2. 2.Dispersion Dispersion of of reagents reagents and and epoxidation epoxidation reaction reaction within within the the micelle. micelle.

Table 2. TheEpoxidation data reported of limonene in Table in presence2 illustrate and this absence increase of CTAHS in limonene [48]. epoxidation rate, since that introduction of small amounts of CTHAS in the reaction system results systematically Yield (%) Carbon CTAHS Vacetone inn aoxone significant Conversion increase in epoxide yield and active oxygen utilization. InOxygen these tests, Bal- 1.2 LO Balance (g) Vwater acetone(mmol) as the only(%) organic1.2 solvent LO is used LDO in small quantities. Also, almost 100%ance yield(%) (%) LDO can be reached under realistic conditions. No allylic+ LDO oxidation products or 8,9-LO 0 71 16 56 72 100 33 1/11 was12.4 observed. 0.05 100 0 99 99 99 51 0 61 25 36 61 100 35 1/7 8.6 0.05 100 9 89 98 98 67 0 55 27 24 51 92 37 1/5 6.2 0.05 85 26 58 84 98 71 0 45 36 10 46 100 46 1/3 3.7 0.05 52 33 20 53 100 61 Reaction conditions: 6.2 mmol of limonene diluted in 3 mL acetone; water and sodium bicarbonate are varied proportion- ally to noxone keeping oxone initial concentration to 0.37 mol/L and bicarbonate/oxone close to 4; room temperature; 90 min reaction time; oxone feeding protocol described in [48].

The results shown in Table 3 also indicate that CTHAS in an appropriate surfactant for epoxidation of pinenes by DMDO.

Catalysts 2021, 11, 847 7 of 17

Table 2. Epoxidation of limonene in presence and absence of CTAHS [48].

Yield (%) Carbon Oxygen CTAHS Vacetone noxone Conversion 1.2 LO Balance Balance (g) Vwater (mmol) (%) 1.2 LO LDO + LDO (%) (%) 0 71 16 56 72 100 33 1/11 12.4 0.05 100 0 99 99 99 51 0 61 25 36 61 100 35 1/7 8.6 0.05 100 9 89 98 98 67 0 55 27 24 51 92 37 1/5 6.2 0.05 85 26 58 84 98 71 0 45 36 10 46 100 46 1/3 3.7 0.05 52 33 20 53 100 61

Reaction conditions: 6.2 mmol of limonene diluted in 3 mL acetone; water and sodium bicarbonate are varied proportionally to noxone keeping oxone initial concentration to 0.37 mol/L and bicarbonate/oxone close to 4; room temperature; 90 min reaction time; oxone feeding protocol described in [48].

The results shown in Table3 also indicate that CTHAS in an appropriate surfactant for epoxidation of pinenes by DMDO.

Catalysts 2021, 11, x FOR PEER REVIEW a 8 of 19 Catalysts Catalysts 2021 2021, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 88 of of 19 19 Table 3. Effect of CTAHS on terpene epoxidation .

a Table 3. Effect of CTAHS on terpene epoxidation a. TableTable 3. 3. Effect Effect of of CTAHS CTAHS on onReaction terpene terpene epoxidation epoxidation a . . Active c Terpene Reaction CTAHS (g) b Vacetone/Vwater Yield (%) Reaction b acetone water c Terpene ReactionTime CTAHS (min) (g) Active O/Terpene b V acetone/V water O/TerpeneYield (%) c TerpeneTerpene Time (min) CTAHSCTAHS (g) (g) ActiveActive O/Terpene O/Terpene b VVacetone/V/Vwater YieldYield (%) (%) c TimeTime (min) (min) 0 3.2 1/9 44 00 3.23.2 01/91/9 4444 3.2 1/9 44 30 3030 300.05 3.2 1/9 100 0.050.05 3.23.2 0.05 1/9 1/9 100100 3.2 1/9 100

0 1.8 1/5 54 00 1.81.8 01/51/5 5454 1.8 1/5 54 30 30 3030 0.05 1.8 1/5 98 0.050.05 1.81.8 0.05 1/5 1/5 9898 1.8 1/5 98

0 2 1/6 47 00 22 01/61/6 4747 2 1/6 47 45 45 4545 0.05 2 1/6 97 0.050.05 22 0.05 1/6 1/6 9797 2 1/6 97 a Reactiona conditions: 6.2 mmol terpene diluted in 3 mL acetone; oxone initial concentration 0.37 mol/L; a Reaction conditions: 6.2 mmol terpene diluted in 3 mL acetone; oxone initial concentration 0.37 a Reaction Reaction conditions: conditions: 6.2 6.2 mmol mmol terpene terpene diluted diluted in in 3 3 mL mL acetone; acetone; oxone oxone initial initial concentration concentration 0.37 0.37 b oxone c mol/L; bicarbonate/oxone 4; room temperature. b active Ob /terpene = 2 n oxone/terpene c yields of c bicarbonate/oxonemol/L;mol/L; bicarbonate/oxone bicarbonate/oxone 4; 4; room4; room room temperature. temperature. temperature. b active active O O /terpene /terpeneactive = = 2 2 On noxone /terpene/terpene/terpene c yields yields = of 2of n /terpene yields of LDO, α-pinene oxide, LDO, αα-pinene oxide, and ββ-pinene oxide. oxone LDO,LDO, α -pinene-pinene oxide, oxide, and and β -pinene-pinene oxide. oxide. and β-pinene oxide. 4. Aerobic Epoxidation 4.4. Aerobic Aerobic Epoxidation Epoxidation Among the various oxidizing agents proposed for alkene epoxidation, molecular ox- AmongAmong the the various various oxidizing oxidizing agents agents proposed proposed for for alkene alkene epoxidation, epoxidation, molecular molecular ox- ox- ygen (dioxygen) is by far the greenest and less expensive one [49]. Using molecular oxy- ygenygen (dioxygen) (dioxygen) is is by by far far the the greenest greenest and and le lessss expensive expensive one one [49]. [49]. Using Using molecular molecular oxy- oxy- 4. Aerobicgen is however Epoxidation limited by a spin forbidden process which makes it unreactive towards gengen isis howeverhowever limitedlimited byby aa spinspin forbiddenforbidden processprocess whichwhich makesmakes itit unreactiveunreactive towardstowards organic molecules at low temperature or suffer from lack of selectivity [50]. organicorganic molecules molecules at at low low temperature temperature or or suffer suffer from from lack lack of of selectivity selectivity [50]. [50]. AmongWorks in our the group various aimed at developing oxidizing heterogeneous agents catalysts proposedfor the two most for alkene epoxidation, molecular WorksWorks inin ourour groupgroup aimedaimed atat developingdeveloping heterogeneousheterogeneous catalystscatalysts forfor thethe twotwo mostmost relevant liquid phase aerobic epoxidation processes, as applied to limonene epoxidation. oxygenrelevantrelevant (dioxygen) liquid liquid phase phase aerobic aerobic is epoxidation epoxidation by far processes, processes, thegreenest as as applied applied to to limonene limonene and epoxidation. epoxidation. less expensive one [49]. Using molecular 4.1. Aerobic Epoxidation Using an Aldehyde as Sacrificial Reductant (Mukaiyama oxygen4.1.4.1. Aerobic Aerobic is however Epoxidation Epoxidation Using Using limited an an Aldehyde Aldehyde by as as a Sacrificial Sacrificial spinforbidden Reductant Reductant (Mukaiyama (Mukaiyama process which makes it unreactive towards Reaction) Reaction)Reaction) organicA moleculessolution of cobalt ac atetylacetonate low temperature (Co(acac)2) was used orto synthesize suffer Co/SBA-16 from by lack of selectivity [50]. AA solution solution of of cobalt cobalt ac acetylacetonateetylacetonate (Co(acac) (Co(acac)2)2) was was used used to to synthesize synthesize Co/SBA-16 Co/SBA-16 by by postgrafting impregnation method. The resultant catalyst was employed in the epoxida- postgraftingpostgrafting impregnation impregnation method. method. The The result resultantant catalyst catalyst was was employed employed in in the the epoxida- epoxida- tionWorks of limonene in with our oxygen group in the presence aimed of isobutyraldehyde at developing and ethylacetate heterogeneous as a catalysts for the two most tiontion ofof limonenelimonene withwith oxygenoxygen inin thethe presencepresence ofof isobutyraldehydeisobutyraldehyde andand ethylacetateethylacetate asas aa greengreen solventsolvent andand waswas foundfound toto bebe activeactive underunder veryvery mildmild conditionsconditions [51].[51]. TheThe naturenature ofof relevantgreen solvent liquid and was phase found to aerobic be active under epoxidation very mild+2 conditions processes, [51]. The nature of as applied to limonene epoxidation. the metal affects the epoxide selectivity of the catalyst. Co +2 has a high ability to catalyze thethe metal metal affects affects the the epoxide epoxide selectivity selectivity of of the the catalyst. catalyst. Co Co+2 has has a a high high ability ability to to catalyze catalyze chain branching and promote the chain initiation. Also, the activation of dioxygen and/or chainchain branching branching and and promote promote the the chain chain initiati initiation.on. Also, Also, the the activation activation of of dioxygen dioxygen and/or and/or aldehydealdehyde viavia coordinationcoordination onon thethe metalmetal centercenter werewere foundfound thethe reasonreason forfor higherhigher activityactivity aldehyde+2 via coordination on the metal center were found the reason for higher activity 4.1. Aerobicof Co +2 compounds. Epoxidation Therefore, it could Using justify the an alkene Aldehyde epoxidation as by O Sacrificial2 in the presence Reductant (Mukaiyama Reaction) of Co+2 compounds. Therefore, it could justify the alkene epoxidation by O22 in the presence of Co compounds. Therefore, it could justify2+ the alkene epoxidation by O in the presence of an aldehyde. In the case of limonene, Co 2+ was also confirmed more active than other of an aldehyde. In the case of limonene, Co2+ was also confirmed more active than other of an aldehyde. In2+ the 2+case of limonene,2+ Co was also confirmed more active than other ionsA including solution Fe 2+, Ni of2+, and cobalt Mn 2+ [51]. acetylacetonate Interaction of coordinating (Co(acac) ligands such as acety-) was used to synthesize Co/SBA-16 by ionsions including including Fe Fe2+, ,Ni Ni2+, ,and and Mn Mn2+ [51]. [51]. Interaction Interaction of of coordina coordinatingting ligands ligands such such as as2 acety- acety- lacetone (acac) with surface hydroxyls contributes in low coordination oxidic species lacetonelacetone (acac)(acac) withwith surfacesurface hydroxylshydroxyls contributescontributes inin lowlow coordinationcoordination oxidicoxidic speciesspecies postgraftinggrafting on theimpregnation surface of the support and method. provides high The dispersion resultant of the active catalystCo sites. was employed in the epoxidation graftinggrafting on on the the surface surface of of the the support support and and provid provideses high high dispersion dispersion of of the the active active Co Co sites. sites. The effect of cobalt loading on catalytic performances of Co/SBA-16 is shown in Table TheThe effect effect of of cobalt cobalt loading loading on on catalytic catalytic performances performances of of Co/SBA-16 Co/SBA-16 is is shown shown in in Table Table of limonene4. Very low activity with wasoxygen observed in the in absence the of presence catalyst. The selectivities of isobutyraldehyde of both 1,2- and ethylacetate as a green 4.4. Very Very low low activity activity was was observed observed in in the the absence absence of of catalyst. catalyst. The The selectivities selectivities of of both both 1,2- 1,2- limonene oxide and limonene dioxide increased with Co/Si ratio up to 4.5%, and further solventlimonenelimonene and oxide oxidewas and and limonene limonene found dioxide dioxide to increased increased be active with with Co/Si Co/Si under ratio ratio up up to to very4.5%, 4.5%, and and mild further further conditions [51]. The nature of the enhancing the cobalt loading resulted in a negative impact on epoxides selectivity. enhancingenhancing the the cobalt cobalt loading loading resulted resulted in in a a negative negative impact impact on on epoxides epoxides selectivity. selectivity. +2

metal affects the epoxide selectivity of the catalyst. Co has a high ability to catalyze chain branching and promote the chain initiation. Also, the activation of dioxygen and/or aldehyde via coordination on the metal center were found the reason for higher activity of +2 Co compounds. Therefore, it could justify the alkene epoxidation by O2 in the presence of an aldehyde. In the case of limonene, Co2+ was also confirmed more active than other Catalysts 2021, 11, 847 8 of 17

ions including Fe2+, Ni2+, and Mn2+ [51]. Interaction of coordinating ligands such as acetylacetone (acac) with surface hydroxyls contributes in low coordination oxidic species grafting on the surface of the support and provides high dispersion of the active Co sites. The effect of cobalt loading on catalytic performances of Co/SBA-16 is shown in Table4. Very low activity was observed in the absence of catalyst. The selectivities of both 1,2-limonene oxide and limonene dioxide increased with Co/Si ratio up to 4.5%, and further enhancing the cobalt loading resulted in a negative impact on epoxides selectivity.

Table 4. Catalytic performances of limonene epoxidation using different cobalt loadings a, reproduced with permission from Ref. [51]. Copyright Elsevier, 2020.

Catalyst Co/Si (%) Conversion (%) 1,2-LO Selectivity (%) LDO Selectivity (%) None - 42 15 - Co/SBA16 1.1 99 45 25 Co/SBA16 2.7 99 49 30 Co/SBA16 4.5 99 50 33 Co/SBA16 6.9 99 48 31 Co/SBA16 9.7 99 35 27 a Reaction conditions: temperature 28 ◦C; oxygen flow rate 10 mL/min; catalyst mass 0.17 g; limonene 3.6 mmol; IBA 12 mmol; 200 min reaction time; solvent ethylacetate (1,2 LO: 1,2-limonene oxide; LDO: limonene dioxide).

The decrease in catalytic activity at higher cobalt loading (4.5 to 9.7%), is associated 3+ to the formation of octahedrally coordinated Co in the form of Co3O4, which leads to reducing the number of accessible active sites. The low coordination isolated cobalt (II) species, present on the surface of mesoporous SBA-16, are the highly active and selective catalytic sites in the epoxidation reaction. On the other hand, those additional cobalt species that are grafted on the materials are not catalytically selective for epoxidation reaction. A radical mechanism as exhibited in Scheme3 was proposed for aerobic epoxidation of limonene in the presence of an aldehyde catalyzed by SBA-16 supported Co2+. The acylperoxy radical is the main epoxidizing agent, generated in the reaction of isobutyralde- hyde (RCHO) with molecular oxygen and initiated over cobalt active species. This species could react either directly with limonene through pathway A to produce epoxide or with isolated Co2+ to generate Co-peroxy species (pathway B) and then transfer oxygen by the reaction of Co-peroxy with limonene through a concerted mechanism. In another study, a novel “pH-adjusting” method was used to graft cobalt in suit- able state onto mesoporous silica SBA-16 to be employed for aerobic epoxidation of limonene [52]. Using the optimum reaction conditions established in [51], the catalytic performances of Co/SBA-16 prepared by pH adjusting method at different cobalt load- ings were investigated and reported in Table5. Co/SBA-16 (1.1%) showed the highest optimized epoxide yield (85%), which has a lower cobalt loading compared to the optimal Co/SBA-16 (4.5%) synthesized by postgrafting of Co(acac)2 (as illustrated in Table4). The higher density of silanol groups on the surface of the catalyst in the pH adjustment method, resulting from no calcination of the support SBA-16 before introduction of the cobalt pre- cursor, accessible to the Co2+ ions, contributes to higher density of isolated tetrahedral Co2+ species. A reverse trend at epoxide yield was observed when cobalt loading increased (as illustrated in Table5). This suggests some parallel oxidation catalyzed by the higher coordination Co2+ ions, generated in the samples at higher cobalt loading. CatalystsCatalysts 20212021, 11, 11, ,x 847 FOR PEER REVIEW 9 of 17 10 of 19

Scheme 3. Proposed pathway for aerobic epoxidation of limonene, reproduced with permission from Scheme 3. Proposed pathway for aerobic epoxidation of limonene, reproduced with permission Ref. [51]. Copyright Elsevier, 2020. from Ref. [51]. Copyright Elsevier, 2020.

Table 5. Catalytic performances of aerobic epoxidation of limonene using Co/SBA16 catalyst, Table 5. Catalytic performances of aerobic epoxidation of limonene using Co/SBA16 catalyst, Re- Reproduced from Ref. [52] with permission from the Royal Society of Chemistry. produced from Ref. [52] with permission from the Royal Society of Chemistry. 1,2-LO LDO Conversion Epoxide CatalystCo/Si Co/Si Conversion (%) 1,2-LO Selectivity LDOSelectivity Selectivity Epoxide Yield Catalyst (%) Yield (%) (%) (%) (%)(%) (%)(%) (%) Co/SBA16Co/SBA16 1.1 1.1 99 99 44 44 41 41 85 85 Co/SBA16Co/SBA16 3 3 99 99 42 42 31 31 72 72 Co/SBA16 5.2 99 42 28 69 Co/SBA16Co/SBA16 5.2 7.5 99 99 42 41 25 28 65 69 Co/SBA16Co/SBA16 7.5 8.8 99 99 41 41 25 25 65 65 ReactionCo/SBA16 conditions: 8.8 temperature 28 99◦C; limonene 3.6 mmol; 41 catalyst mass 0.17 g; IBA 12 25 mmol; oxygen flow rate 65 10Reaction mL/min; conditions: 200 min reaction temperature time; solvent 28 ethylacetate °C; limonene (1,2 LO: 3.6 1,2-limonene mmol; cataly oxide;st LDO: mass limonene 0.17 g; dioxide). IBA 12 mmol; oxygen flow rate 10 mL/min; 200 min reaction time; solvent ethylacetate (1,2 LO: 1,2-limonene As illustrated in Figure3, the raise in oxygen pressure from 15 to 44 psi increased oxide; LDO: limonene dioxide). the formation of limonene epoxides to the highest combined yield of 95%, which then decreased with further elevation of oxygen pressure up to 58 psi. This can be explained by possibleAs secondaryillustrated oxidation in Figure or 3, ring the opening raise in of oxygen epoxide pressure at higher from pressure. 15 to 44 psi increased the formation of limonene epoxides to the highest combined yield of 95%, which then de- creased with further elevation of oxygen pressure up to 58 psi. This can be explained by possible secondary oxidation or ring opening of epoxide at higher pressure.

Catalysts 2021, 11, x FOR PEER REVIEW 11 of 19

Catalysts 2021, 11, 847x FOR PEER REVIEW 1110 of 19 17

Figure 3. Epoxidation of limonene using Co/SBA16 (Co/Si = 1.1%) catalyst at different oxygen Figure 3. Epoxidation of limonene using Co/SBA16 (Co/Si = 1.1%) catalyst at different oxygen pressuresFigure 3. (see Epoxidation Table 2 for of reaction limonene conditions). using Co/SBA16 Reproduced (Co/Si from = 1.1%) Ref. catalyst [52] with at differentpermission oxygen from pressures (see Table2 for reaction conditions). Reproduced from Ref. [ 52] with permission from Royalpressures Society (see of Chemistry.Table 2 for reaction conditions). Reproduced from Ref. [52] with permission from Royal Society of Chemistry. RoyalAn Society example of Chemistry. of time evolution of the reaction medium during limonene epoxidation in the presenceAnAn example of isobutyraldehyde of timetime evolutionevolution ofandof the the oxyg reaction reacention at medium44 medium psi oxygen during during pressure limonene limonene with epoxidation epoxidation Co/SBA- in 16inthe (1.1%) the presence presence as catalyst of of isobutyraldehyde isobutyraldehydeis shown in Figure and and 4. oxygen oxygen at 44at 44 psi psi oxygen oxygen pressure pressure with with Co/SBA-16 Co/SBA- 16(1.1%) (1.1%) as catalystas catalyst is shownis shown in Figurein Figure4. 4.

Figure 4. Limonene conversion (1), 1,2-limonene oxide (2), 8,9-limonene oxide (3) and limonene

Figuredioxide 4. Limonene (4) selectivity conversion versus (1), time. 1,2-limonene Reproduced oxide from (2), Ref. 8,9-lim [52] withonene permission oxide (3) and from limonene Royal Society dioxideFigureof Chemistry. (4) 4. selectivityLimonene versusconversion time. (1), Reproduced 1,2-limonene from oxide Ref. (2),[52] 8,9-lim with permisonene sionoxide from (3) andRoyal limonene Society of dioxideChemistry. (4) selectivity versus time. Reproduced from Ref. [52] with permission from Royal Society of Chemistry.Diepoxide is generated by secondary epoxidation of monoepoxides. 58% selectivity of 1,2Diepoxide limonene is generated and 37% limoneneby secondary dioxide epoxidation were achieved of monoepoxides. for the total 58% use ofselectivity limonene of at1,2 250 limoneneDiepoxide min of and the is generated37% reaction. limonene by Different secondary dioxide conclusions we epoxidationre achieved were of for mademonoepoxides. the total about use the of 58%mechanism limonene selectivity at of 250ofaerobic min1,2 limonene of epoxidation the reaction. and 37% of Different limonene. limonene conclusions This dioxide was donewewerere throughachievedmade about investigating for the mechanismtotal use the of initial limoneneof aerobic reaction at epoxidation of limonene. This was done through investigating the initial reaction rates 250rates min using of the the reaction. optimized Different catalyst. conclusions Since the initial were ratesmade of about limonene the mechanism epoxidation of (R aerobice) and using the optimized catalyst. Since the initial rates of limonene epoxidation (Re) and iso- epoxidationisobutyraldehyde of limonene. oxidation This (R awas) offered done athrough systematic investigating difference, the mechanism initial reaction pathway rates B butyraldehyde oxidation (Ra) offered a systematic difference, mechanism pathway B in usingin Scheme the optimized3, which suggestscatalyst. RSincee = Rthea, couldinitial berates excluded. of limonene Therefore, epoxidation the other (Re) reactionand iso- 3+ − Schemebutyraldehydepathway 3, which C which oxidationsuggests uses the R (Reperoxy =a )R offereda, could intermediate a be systematic excluded. Co difference, OOTherefore,, was mechanism consideredthe other reaction aspathway proposed path- B in 3+ − waySchemeScheme C which 43,. However,which uses suggests the the peroxy strong Re = intermediateR dependencea, could be excluded. ofCo R eOOon,the Therefore,was aldehyde considered the concentration other as proposedreaction leads path- into Schemewayassumption C 4. which However, that uses RCHO the the strong isperoxy not dependence consumed intermediate in of the R Coe process on3+OO the− , butaldehyde was is neverthelessconsidered concentration as involved proposed leads in tothe in assumptionSchemeformation 4. thatHowever, of this RCHO intermediate. the is not strong consumed dependence in the ofprocess Re on butthe isaldehyde nevertheless concentration involved leadsin the to formationassumption of this that intermediate. RCHO is not consumed in the process but is nevertheless involved in the formation of this intermediate. Catalysts 2021, 11, x FOR PEER REVIEW 12 of 19

The observed unusual effect of a strong decrease in initial epoxidation rate at increas- Catalysts 2021, 11, 847 11 of 17 ing limonene concentration imposes that the reaction be only important at very low limo- nene concentration. This, of course, precludes large scale application of this process.

Scheme 4. Proposed pathway for aerobic epoxidation of limonene. Reproduced from Ref. [52] with Schemepermission 4. Proposed from Royal pathway Society for of aerobic Chemistry. epoxidation of limonene. Reproduced from Ref. [52] with permission from Royal Society of Chemistry. The observed unusual effect of a strong decrease in initial epoxidation rate at in- 4.2.creasing Solvent-Free limonene and concentration Initiator-Free imposes Epoxidation that the reaction be only important at very low limoneneDirect concentration.aerobic epoxidation This, of course,limonene precludes without largeusing scale a solvent, application sacrificial of this reductant process. or radical initiator was investigated using highly dispersed Ru supported on activated carbon4.2. Solvent-Free (AC) fabricated and Initiator-Free using various Epoxidation immobilization strategies [53]. Ruthenium/activated carbonDirect was found aerobic an epoxidation active catalyst of limonene for the environmentally without using a solvent,friendly sacrificial epoxidation reductant of lim- or oneneradical using initiator molecular was investigated oxygen as the using only highly oxidant dispersed under very Ru supported mild condition on activated without carbon any solvent(AC) fabricated and initiator. using The various selectivity immobilization of the catalyst strategies toward [53 ].epoxidation Ruthenium/activated was found carbonto be greatlywas found influenced an active by the catalyst preparation for the method environmentally of the catalyst. friendly The epoxidation catalytic activity of limonene of va- rietyusing Ru molecular supported oxygen catalysts as theusing only different oxidant ACs under with very various mild conditionmorphologies without and any different solvent preparationand initiator. methods The selectivity for the aerobic of the catalystepoxidation toward of limonene epoxidation under was solvent-free found to be condi- greatly tionsinfluenced and without by the any preparation radical init methodiator is presented of the catalyst. in Table The 6. catalytic activity of variety RuDespite supported higher catalysts conversion using obtained different over ACs th withe catalyst various prepared morphologies by the sol and immobili- different zationpreparation (SI) method methods (40.4%), for the the aerobic one prepared epoxidation by cation of limonene exchange under Ru/G60(CE) solvent-free showed conditions the highestand without selectivity any radicaland yield initiator toward is presentedepoxidation in products. Table6. The results indicated that de- spite the formation of small Ru nanoparticles on carbon materials in sample prepared by Table 6. Aerobic epoxidation of limonene over supported ruthenium catalysts on activated carbon prepared by different the SI method (particle size of 2.5 nm and dispersion of 53%), it suffered from high selec- methods [53]. tivity to the allylic oxidation products. All catalysts synthesized by incipient wetness im- Catalysts Conversionpregnation (%) (IWI) 1,2-LO yielded (%) 8,9-LOlower epoxide (%) LDO selectivity. (%) Carvone (%) Carveol (%) Others (%) The catalyst synthesized using a cation exchange method showed a good compro- Ru/AC G60 (IWI) 28 21 6.5 14.6 16.7 8 33.2 Ru/AC MRX (IWI)mise 22 between activity 21 and selectivity 6.7 to the 13.5 epoxides. A 17 higher oxygen 9.1 functional 32.7groups Ru/AC RB4C (IWI)surface 18 density 18generated by 6preoxidation 10 of high surface 15 area G60 7.5support, led 43.5to AC Ru/AC RX3 (IWI)supported 21 ruthenium 8.7 catalysts 2.6 with uniform 7.9 and isolated 10 Ru nanoparticles. 8 62.8 Ru/Graphite (IWI) 15 16 4 8.5 18 9.5 44 Ru/AC G60 (CE) 35 28 10 19 19 6.5 17.5 Ru/AC G60 (SI) 40.4 18.5 5.5 12.8 20.7 12.9 39.6 Reaction conditions: limonene 5 mL; catalyst mass 150 mg; temperature 80 ◦C; oxygen pressure 3 bar; time 6 h ((1,2-LO: 1,2-limonene oxide; 8,9-LO: 8,9-limonene oxide; LDO: limonene dioxide).

Despite higher conversion obtained over the catalyst prepared by the sol immobi- lization (SI) method (40.4%), the one prepared by cation exchange Ru/G60(CE) showed the highest selectivity and yield toward epoxidation products. The results indicated that Catalysts 2021, 11, x FOR PEER REVIEW 13 of 19

Table 6. Aerobic epoxidation of limonene over supported ruthenium catalysts on activated carbon prepared by different methods [53].

Conversion Catalysts 1,2- LO (%) 8,9 -LO (%) LDO (%) Carvone (%) Carveol(%) Others (%) Catalysts 2021, 11, 847 (%) 12 of 17 Ru/AC G60 (IWI) 28 21 6.5 14.6 16.7 8 33.2 Ru/AC MRX (IWI) 22 21 6.7 13.5 17 9.1 32.7 Ru/AC RB4C(IWI) 18despite the formation 18 of small 6 Ru nanoparticles 10 on carbon 15 materials in 7.5 sample prepared 43.5 Ru/AC RX3(IWI) 21by the SI method 8.7 (particle 2.6 size of 2.5 nm 7.9 and dispersion 10 of 53%), it suffered 8 from 62.8 high Ru/Graphite (IWI) 15selectivity to the 16 allylic oxidation 4 products. 8.5 All catalysts synthesized 18 by 9.5 incipient wetness 44 Ru/AC G60 (CE) 35impregnation 28 (IWI) yielded 10 lower epoxide 19 selectivity. 19 6.5 17.5 Ru/AC G60 (SI) 40.4 The catalyst 18.5 synthesized 5.5 using a cation 12.8 exchange method 20.7 showed 12.9 a good compromise 39.6 Reaction conditions: limonenebetween 5 mL; activitycatalyst mass and selectivity 150 mg; temperature to the epoxides. 80 °C; oxygen A higher pressure oxygen 3 functionalbar; time 6 groupsh ((1,2-LO: surface 1,2-limonene oxide; 8,9-LO:density 8,9-limonen generatede oxide; byLDO: preoxidation limonene dioxide). of high surface area G60 support, led to AC supported ruthenium catalysts with uniform and isolated Ru nanoparticles. Both epoxidation epoxidation and and allylic allylic oxidation oxidation products products may may occur occur through through oxidation oxidation of ole- of olefinsfins with with dioxygen. dioxygen. The Theallylic allylic oxidation oxidation to epoxide to epoxide products products ratio are ratio determined are determined by the bycompetition the competition between between the abstraction the abstraction of the ally oflic the hydrogen allylic hydrogen and the addition and the additionof the alkyl of theperoxy alkyl radical peroxy to radicalthe double to the bond. double In cata bond.lytic In epoxidation catalytic epoxidation of limonene of with limonene molecular with molecularoxygen generally, oxygen generally,electrophilic electrophilic attack on attackthe double on the bond double of limonene bond of limonene results in results the for- in themation formation of 1,2- ofand 1,2- 8,9- and epoxy 8,9- limonene epoxy limonene as the primary as the primary epoxidation epoxidation products products and 1,2,8,9- and 1,2,8,9-diepoxy diepoxy limonene limonene as the assecondary the secondary epoxidat epoxidationion product product while while allylic allylic H-abstraction H-abstraction re- resultssults in incarveol carveol and and carvone carvone formation. formation. The main The mainoxygenated oxygenated byproducts byproducts could be could gener- be generatedated by the by two the reaction two reaction pathways pathways shown shown in Scheme in Scheme 5. 5.

Scheme 5. Proposed reaction pathway in solvent- and initiator-free aerobic epoxidation of Scheme 5. Proposed reaction pathway in solvent- and initiator-free aerobic epoxidation of limo- limonenenene [53]. [53]. One corresponds to the formation of metal–alkene adducts by chemisorption on the One corresponds to the formation of metal–alkene adducts by chemisorption on the catalyst surface and activation of limonene and oxygen using ruthenium with appropriate catalyst surface and activation of limonene and oxygen using ruthenium with appropriate catalyst design. The other pathway is the limonene autoxidation via a free radical chain catalyst design. The other pathway is the limonene autoxidation via a free radical chain mechanism in which the formation of limonene hydroperoxide by activation of allylic mechanism in which the formation of limonene hydroperoxide by activation of allylic C- C-H bonds of limonene leads to decomposition of limonene hydroperoxide and formation H bonds of limonene leads to decomposition of limonene hydroperoxide and formation of radicals capable of reacting with oxygen and yielding variously oxidized products, suchof radicals as epoxides capable andof reacting allylic products.with oxygen However, and yielding the resultsvariously in thisoxidized study products, suggests such the former pathway is favored over the latter due to the higher ratio of epoxide to allylic oxidation products. The performance shown in Table6 for the original catalyst Ru/G60(CE) is the best reported epoxide yield for aerobic epoxidation of limonene. This being obtained in absence of any solvent or initiator makes this process close to large scale applicability. Catalysts 2021, 11, x FOR PEER REVIEW 14 of 19

as epoxides and allylic products. However, the results in this study suggests the former pathway is favored over the latter due to the higher ratio of epoxide to allylic oxidation products. The performance shown in Table 6 for the original catalyst Ru/G60(CE) is the best Catalysts 2021, 11, 847 13 of 17 reported epoxide yield for aerobic epoxidation of limonene. This being obtained in ab- sence of any solvent or initiator makes this process close to large scale applicability.

5.5. Importance Importance and and Perspective Perspective of of Stereoselective Stereoselective Epoxidation Epoxidation LimoneneLimonene is is a chirala chiral molecule molecule with with the the (R)-enantiomer, (R)-enantiomer, often often designated designated as D-limonene as D-limo- ((+)-limonene),nene ((+)-limonene), being being the most the most abundant abundant in orange in orange peel peel essential essential oils. oils. As As shown shown in in SchemeScheme6 ,6,each each of of the the two two limonene limonene enantiomers enantiomers gives gives rise rise to to two two isomers isomers with with different different configurationsconfigurations of of the the epoxide epoxide bridges bridges in limonenein limonene diepoxide diepoxide (LDO). (LDO). These These various various isomers iso- showmers differentshow different reactivities reactivities in reactions in reactions of interest of interest for further for further polymerizations. polymerizations. In the above In the citedabove ref cited [35], ref Cunningham [35], Cunningham et al. et studied al. studied the epoxidationthe epoxidation of aof wide a wide range range of of terpenes terpenes havinghaving trisubstituted trisubstituted alkene alkene bonds bonds with with a a tungsten-based tungsten-based polyoxometalate polyoxometalate catalyst catalyst using using aqueousaqueous H H2O2O22 asas thethe oxidantoxidant andandwithout withoutsolvent. solvent. TheirTheir workworkindicates indicatesthat that in in all all cases, cases, twotwo diastereomers diastereomers (i.e.,(i.e., a racemic mixture mixture of of cis- cis- and and trans- trans- epoxides epoxides of ofeach each terpene) terpene) are areformed. formed. In Inthe the example example of of(R)- (R)-(+)-limonene,(+)-limonene, epoxidation epoxidation result resulteded in inthe the formation formation of ofan analmost almost 1:1 1:1 mixture mixture of ofcis- cis- and and trans- trans- 1,2-ep 1,2-epoxideoxide of limonene. of limonene. Aerobic Aerobic epoxidation epoxidation of lim- of limoneneonene over over cobalt cobalt substituted substituted mesoporous mesoporous SBA-16, SBA-16, was was studied studied in [51]. in [51 Epoxidation]. Epoxidation with withmolecular molecular oxygen oxygen resulted resulted in the in formation the formation of two of twocis- and cis- andtrans- trans- diastereomers diastereomers of limo- of limonenenene dioxide dioxide (LDO). (LDO). Epoxidized Epoxidized samples samples from from this this work work were quantifiedquantified inin [54[54].]. The The productsproducts consisted consisted of of 60:40 60:40 cis:trans cis:trans limonene limonene dioxide dioxide with with the the exact exact distribution distribution shown shown in Schemein Scheme6. 6.

SchemeScheme 6. 6.Different Different isomers isomers of of limonene limonene dioxidesynthesized dioxidesynthesized from from pure pure D-limonene D-limonene [54 [54].].

TheseThese various various terpene terpene isomersisomers (cis-(cis- andand trans-)trans-) areare notnot equallyequally reactivereactive inin polymer-polymeri- izationzation reactions.reactions. Poly(limonenePoly(limonene trans-R-carbonate)trans-R-carbonate) andand poly(limonenepoly(limonene trans-S-carbonate) trans-S-carbonate) werewere synthesized synthesized from from a a cis/trans cis/trans mixturemixture of of (R)- (R)- and and (S)-limonene (S)-limonene oxide, oxide, respectively, respectively, at at roomroom temperature temperature using using a a zinc zinc b-diminate b-diminate complex complex. [55 [55–58–58].]. The The catalyst catalyst readily readily allows allows polymerizingpolymerizing the the trans- trans- diastereomers diastereomers of of the the epoxide, epoxide, leaving leaving the the cis- cis- diastereomers diastereomers unre- un- acted,reacted, which which resulted resulted in ain highly a highly regio, regio, diastereoiso, diastereoiso, and and enantiomerically enantiomerically pure pure polymer. poly- Similarmer. Similar results results were observed were observed in recent in studies recent usingstudies tetrabutylammonium using tetrabutylammonium halides (TBAX) halides as(TBAX) catalysts; as catalysts; the formation the formation of polycarbonates of polycarbonates and cyclic and carbonates cyclic carbonates from a commercialfrom a com- mixturemercial ofmixture (40:60, of cis:trans (40:60, cis:trans LDO) exhibits LDO) exhibi a significantlyts a significantly higher conversionhigher conversion of the trans- of the isomertrans- isomer than the than cis- the isomer cis- isomer [58,59]. [58,59]. Very recently, Very recently, Schutz Schutz et al. comparedet al. compared the reactivity the reac- oftivity trans- of trans- and cis- and limonene cis- limonene dioxide dioxide (LDO) (LDO) by reactingby reacting them them separately separately to to form form epoxy epoxy resins upon crosslinking with branched polyethyleneimine (BPEI) [54]. The results from resins upon crosslinking with branched polyethyleneimine (BPEI) [54]. The results from Fourier transform infrared spectroscopy, DFT calculations, and kinetics of cross-linking Fourier transform infrared spectroscopy, DFT calculations, and kinetics of cross-linking studied by differential scanning calorimetry confirmed that the two (trans-LDO) are really studied by differential scanning calorimetry confirmed that the two (trans-LDO) are really functional, while the other two (cis-LDO) possess an epoxide group that is significantly less functional, while the other two (cis-LDO) possess an epoxide group that is significantly reactive. In the same vein, it was reported that the use of pure trans-limonene carbonate less reactive. In the same vein, it was reported that the use of pure trans-limonene car- to form nonisocyanate polyurethanes (NIPU) significantly improved such properties as bonate to form nonisocyanate polyurethanes (NIPU) significantly improved such proper- glass transition temperature, stiffness, and strength [60]. However, the low reactivity of diastereomeric cis- compared to that of trans- is related to its high energy of activation due to the “boat-like” structure of its transition state [61,62]. Evidently, the development of a strereoselective epoxidation technique to further synthesize the trans- configuration of terpenes is a necessity. Hauenstein et al. developed a large-scale stereoselective synthesis technique for the trans-isomer 1,2-oxide of limonene (>1 kg/batch) and suggested easy transfer to pilot plant scale [58]. The synthesis involves regio- and stereo-selective endocyclic bromohydrin of limonene in aqueous acetone with N-bromosuccinimide (NBS) as the bromine source to form a bromohydrate intermediate. This intermediate is readily converted to the corresponding limonene trans-1,2-oxide in Catalysts 2021, 11, x FOR PEER REVIEW 15 of 19

ties as glass transition temperature, stiffness, and strength [60]. However, the low reactiv- ity of diastereomeric cis- compared to that of trans- is related to its high energy of activa- tion due to the “ boat-like “ structure of its transition state [61,62]. Evidently, the development of a strereoselective epoxidation technique to further synthesize the trans- configuration of terpenes is a necessity. Hauenstein et al. developed a large-scale stereoselective synthesis technique for the trans-isomer 1,2-oxide of limonene Catalysts 2021, 11, 847 (>1 kg/batch) and suggested easy transfer to pilot plant scale [58]. The synthesis involves14 of 17 regio- and stereo-selective endocyclic bromohydrin of limonene in aqueous acetone with N-bromosuccinimide (NBS) as the bromine source to form a bromohydrate intermediate. This intermediate is readily converted to the corresponding limonene trans-1,2-oxide in thethe presence presence of of aqueous aqueous sodium sodium hydroxide. hydroxide. The The byproducts byproducts thus thus formed formed contain contain more more thanthan 85% 85% ofof thethe limonenelimonene trans-1,2-oxide. Very Very recently, recently, this this same same method method of of stereoselec- stereose- lectivetive synthesis synthesis was was employed employed to to synthesize limonenelimonene trans-dioxide.trans-dioxide. (as(as illustratedillustrated inin SchemeScheme7 )[7)63 [63].].

SchemeScheme 7. 7.Synthesis Synthesis ofof trans-LBEtrans-LBE viavia stereoselective di di-bromohydration-bromohydration of of both both double double bonds bonds of of (R)-(+)-limonene [58]. (R)-(+)-limonene [58].

TheThe technique technique consists consists in in a a first first step step to to form form a a di-bromohydrate di-bromohydrate of of the the endocyclic endocyclic and and exocyclicexocyclic double double bonds bonds of of limonene limonene to formto form the the di-bromohydrate di-bromohydrate intermediate intermediate of limonene. of limo- Thenene. latter The reacts latter withreacts an with aqueous an aqueous sodium sodi hydroxideum hydroxide solution solution to form to limonene form limonene trans- dioxide.trans-dioxide. The authors The authors investigated investigated such parameters such parameters as reaction as reaction temperature, temperature, limonene: limo- N- bromosuccinimidenene: N-bromosuccinimide molar ratio, molar and reactionratio, and time reac totion promote time to a promote very high a yieldvery ofhigh limonene yield of trans-dioxide.limonene trans-dioxide. A 97% yield A 97% of limonene yield of limon trans-dioxideene trans-dioxide was obtained was usingobtained a 1:2 using ratio a of1:2 limonene:ratio of limonene: N-bromosuccinimide N-bromosuccinimide at 60 ◦C afterat 60 5°C min after reaction 5 min time.reaction time. OtherOther techniques techniques for for the the stereoselective stereoselective epoxidation epoxidation of of chiral chiral alkenes alkenes in in the the presence presence ofof a a dioxyrane dioxyrane were were investigated investigated by by several several authors. authors. The The stereoselective stereoselective epoxidation epoxidation of of chiralchiral olefinsolefins overover Jacobsen’sJacobsen’s catalyst immobilized on on Al-MCM-41 Al-MCM-41 and and NH2-Si-MCM-41 NH2-Si-MCM- 41in inthe the presence presence of in of situ in situ generated generated DMDO DMDO as oxygen as oxygen source source was studied was studied [64]. The [64]. exper- The experimentalimental results results indicate indicate that selectivities that selectivities to cis- to and cis- trans- and trans- epoxides epoxides can be can obtained be obtained up to up86 toand 86 72%, and 72%,respectively. respectively. The developed The developed catalyst catalyst did not did undergo not undergo any significant any significant change changein its chemical in its chemical structure structure upon reaction. upon reaction. Therefore, Therefore, the reusability the reusability of the catalyst of the catalystcould be couldachieved be achieved when it whenwas immobilized it was immobilized by chemical by chemical bonding bonding of the ofsalt the ligand. salt ligand. In the In same the samevein, vein,a chiral a chiral dioxirane dioxirane was wasdeveloped developed for th fore thehighly highly stereoselective stereoselective epoxidation epoxidation of cis- of cis-olefinsolefins [65]. [65 ].The The authors authors were were able able to to produc producee cis-epoxides cis-epoxides of a widewide varietyvariety ofof olefins olefins withwith selectivityselectivity ofof 77–95%.77–95%. Other Other similar similar studies studies demonstrated demonstrated that that in in situ situ generated generated di- dioxiranesoxiranes from from chiral chiral ketones ketones are are highly stereoselectivestereoselective forfor thethe asymmetricasymmetric epoxidation epoxidation of of trans-olefinstrans-olefins and and trisubstituted trisubstituted olefins olefins (See (See Refs. Refs. [4–6] 4–6 in in [41]). [41]). Indeed,Indeed, the the above above cited cited technologies technologies for for alkene alkene stereoselective stereoselective epoxidation epoxidation need need to to bebe tested tested for for setereoselective setereoselective epoxidation epoxidation of of terpenes terpenes and, and, in in particular, particular, for for the the especially especially crucialcrucial formation formation of of trans-limonene trans-limonene diepoxides. diepoxides.

Enantioselective5.1. Enantioselective Separation Separation Steiner et al. developed an efficient separation process for a 1:1 mixture of cis and Steiner et al. developed an efficient separation process for a 1:1 mixture of cis and trans 1,2-limonene oxide using nucleophilic amines [4]. To isolate cis 1,2-limonene oxide, trans 1,2-limonene oxide using nucleophilic amines.[4] To isolate cis 1,2-limonene oxide, pyrrolidine and piperidine were used and reacted with only trans 1,2-limonene oxide by pyrrolidine and piperidine were used and reacted with only trans 1,2-limonene oxide by ring opening reaction. The unchanged cis 1,2-limonene oxide was isolated by liquid–liquid ring opening reaction. The unchanged cis 1,2-limonene oxide was isolated by liquid–liq- extraction and distillation. The recovery yield reached up to 88%. To obtain pure trans uid extraction and distillation. The recovery yield reached up to 88%. To obtain pure trans 1,2-limonene oxide, less nucleophilic amines such as triazole and pyrazole were used. The recovery of trans 1,2-limonene oxide using the same isolation procedure as for cis 1,2-limonene oxide, yielded up to 80% recovery. In both cases the isolated isomer had a purity higher than 98% [61]. Nucleophilic amines were not explored for the isolation of pure trans limonene dioxide. This approach may however be a potential alternative to stereoselective epoxidation.

6. Conclusions This minireview of our recent terpenes epoxidation works is a necessarily shortened presentation of some of the most significant results. The interested reader may like to access Catalysts 2021, 11, 847 15 of 17

to our cited references. Even more detailed presentations may be found in the already published theses [66–68] and in the ones in preparation. This kind of report is an opportunity to illustrate the philosophy that underlies our work. Firstly, we recognize that the chemical and parachemical industries make significant contributions to the wellbeing of mankind. For several reasons, this is not always acknowledged in the general public. This is not to say that mistakes were never made in these sectors and that significant changes are not necessary. Actually, these industries are increasingly conscious of the need to adapt to a changing world [69]. Our activities are attempts to help this coming adaptation. Epoxidation is an example of a process, currently used on a large scale to valorize compounds of fossil origin, that needs to be studied for application with natural products such as terpenes. In doing so, we considered several possible options, looking systematically at the technical feasibility and possibility of scaling up of the various processes, in addition to a scientific search for fundamental understanding. We believe both exercises should be conducted simultaneously. Note that at this stage, a strict economic assessment of profitability would be premature and probably counterproductive [69].

Funding: This series of works were funded through several grants from CRSNG and CRIBIQ with financial support of Soprema. Data Availability Statement: There is no supporting information to this minireview. Please refer to the cited references. Acknowledgments: We wish to thank Soprema (Drummondville, QC, Canada) for its continued sup- port and interest in this work, as well as Consortium de Recherche et d’Innovation en Biotechnologie Industrielle du Québec (CRIBIQ), and Centre de Recherche en Sciences Naturelles et en Génie du Canada (CRSNG) for joint financial support. Conflicts of Interest: The authors declare no conflict of interest.

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