Gold Nanotriangles As Selective Catalysts for Cyclohexanol and Cyclohexanone Production

applied sciences

Article Gold Nanotriangles as Selective Catalysts for Cyclohexanol and Cyclohexanone Production

Inês A. S. Matias 1 , A. P. C. Ribeiro 1 , Rui P. Oliveira-Silva 2, Duarte M. F. Prazeres 2 and Luísa M. D. R. S. Martins 1,*

1 Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal; [email protected] (I.A.S.M.); [email protected] (A.P.C.R.) 2 IBB–Institute for Bioengineering and Biosciences, Department Bioengineering, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal; [email protected] (R.P.O.-S.); [email protected] (D.M.F.P.) * Correspondence: [email protected]; Tel.: +351-218-419-389

Received: 24 October 2018; Accepted: 14 December 2018; Published: 17 December 2018

Featured Application: The sustainable synthesis of cyclohexanol and cyclohexanone mixture (ketone and alcohol (KA) oil) is of primordial importance for the synthesis of nylon 6,6. The present industrial process uses a cobalt catalyst, dioxygen as oxidant, requires high temperatures (150 ◦C), being only able to reach 5–12% yields in order to assure a selectivity of approximately 85%. Thus, more efﬁcient catalytic systems and milder reaction conditions are needed. Herein, an alternative catalytic system based on gold nanotriangles is presented.

Abstract: The search for sustainable catalytic oxidation processes remains a challenge. One process of utmost industrial and economic importance is the selective oxidation of cyclohexane, in the route of nylon-6,6 production, which requires urgent improvement. Herein, Au nanotriangles (Au NTs) were prepared following a three-step (seed preparation, growth and shaping) procedure and applied, for the ﬁrst time, as catalysts for the selective oxidation of neat cyclohexane to ketone and alcohol (KA) oil (cyclohexanol and cyclohexanone mixture). The Au NTs successfully yield KA oil (up to 14%) under mild conditions (50 ◦C), using an alternative energy source (microwave irradiation) as reaction promotor.

Keywords: gold nanoparticles; gold nanotriangles; cyclohexane; selective oxidation; KA oil; cyclohexanol; cyclohexanone; microwave

1. Introduction Studies on the catalytic properties of gold nanostructures in oxidation reactions showed that such nanoparticles stabilized and well dispersed on various supports (metal oxides [1–6], carbon materials [7–12], metal–organic frameworks [13], zeolites [14], modiﬁed aluminums [15,16], ionic liquids [17], etc.) are able to successfully catalyze several oxidation reactions, as well as the selective partial oxidation of cyclohexane. The preference for cyclohexane as a model substrate arises from the signiﬁcance of the oxidized products (cyclohexanol and cyclohexanone) in the production of adipic acid, a building block for different processes in the pharmaceutical, chemical, and food industries. In fact, adipic acid is an important chemical commodity produced globally (over 3.5 million metric tons/year, growing ca. 5%/year) [18,19] mainly in the synthesis of Nylon-6,6 polyamide (primary use), but can also be used in the production of polyurethane resins and polyester [18].

Appl. Sci. 2018, 8, 2655; doi:10.3390/app8122655 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2655 2 of 11

Despite the efforts made to prepare and investigate Au nanocatalysts without any support to assess the activity of pure Au without support effects, the application of gold nanoparticles (Au NPs) in solution as catalysts for oxidation reactions is scarce, mainly due to aggregation issues that induce low reproducibility in the results. So far, Kulikova et al. [20] has disclosed the use of Au NPs stabilized by a 1-dodecanethiol monolayer to catalyze the aerobic oxidation of cyclohexene to produce cyclohexene oxide (11%), alkyl hydroperoxide (84%), and allyl ketone (5%). It is known that catalytic properties of metal NPs are size- and shape-dependent [21,22]; therefore, the ability to control sizes and morphologies of the Au NPs is an important issue in nanomaterial science. In this work, cetyltrimethylammonium chloride (CTAC)-stabilized Au nanotriangles (Au NTs) were synthetized by a seed-mediated approach and “face-blocking” growth mechanism [23]. The above CTAC-stabilized Au NTs were tested for the microwave (MW)-assisted oxidation of neat cyclohexane to ketone and alcohol (KA) oil under mild conditions using a “green” oxidant (aqueous tert-butylhydroperoxide, TBHP), in contrast to the current inefﬁcient industrial harsher conditions [18,24]. To our knowledge, this is the ﬁrst time that the successful use of Au NTs for the oxidation of cyclohexane is disclosed.

2. Materials and Methods All solvents and reagents were obtained from the commercial sources and were used without further purification. Cetyltrimethylammonium chloride-stabilized gold nanotriangles (CTAC-stabilized Au NTs) were synthetized following a three-step (seed preparation, growth and shaping) procedure previously described in the literature [23]. Briefly, first Au seeds were synthesized by the standard CTAC/NaBH4 procedure; then a “growing solution” (CTAC, HAuCl4 and NaI onto Milli-Q water) and a “shaping solution” (CTAC, HAuCl4 and NaI) were prepared. Ascorbic acid was added to both solutions and were manually stirred until they become colorless (AuIII → AuI). Consequently, 250 µL of seeds were added to solution 1 and manually stirred. Right after, 3.2 mL of solution 1 were added to solution and left to react in the dark during 1 h at room temperature. Finally, to induce flocculation of Au NTs, cetyltrimethylammonium bromide (CTAB) was added until the desired concentration of CTAC+CTAB was reached. The solution was left overnight to flocculate the gold nanotriangles. Scanning electron microscope (SEM) JEOL 7001F with Oxford light elements, energy dispersive (EDX) detector and electron backscatter diffraction (EBSD) detector (JEOL, Tokyo, Japan) was used to characterize the size distribution and morphology of Au triangles. Transmission Electron Microscopy (TEM) measurements were carried out on a Transmission Electron Microscope Hitachi 8100 with ThermoNoran light elements EDX detector and digital image acquisition (Hitachi, Tokyo, Japan). UV-visible spectroscopic measurements of CTAC-stabilized Au NTs aqueous solutions were performed on a PerkinElmer Lambda 750 UV-visible spectrophotometer (PerkinElmer, Inc., Waltham, MA, USA), using a 1 cm path length quartz cuvette. The catalytic oxidation reactions of cyclohexane were performed in an Anton Paar Microwave 300 reactor (Anton Paar GmbH, Graz, Austria) fitted with a rotational system and an infra-red (IR) temperature detector, in G10 borosilicate glass tubes. In a typical experiment, were introduced in a 10 mL Pyrex tube a desired volume of Au NTs solution, 5.00 mmol of cyclohexane and 50 µL of nitromethane (as internal standard). The additive (if any) was added and the reaction started by adding TBHP (70% aqueous solution, 10.0 mmol) in one portion. The mixture was irradiated with 10 W and stirred (650 rpm) at the desired temperature (30–70 ◦C) and reaction time (1–6 h). After the reaction, the obtained mixture was cooled to room temperature and centrifuged. A sample (4 µL) was taken from the organic phase and analyzed by gas chromatography (GC) upon the addition of an excess of PPh3 to reduce cyclohexyl hydroperoxide to cyclohexanol, following a method developed by Shul’pin [25,26]. For the experiments using air Appl. Sci. 2018, 8, 2655 3 of 11 as oxidant,Appl. Sci. 2018 atmospheric, 8, x FOR PEER pressure REVIEW was used, and the other reaction conditions are the same3 asof 12 the stated before. experiments using air as oxidant, atmospheric pressure was used, and the other reaction conditions GC measurements were performed in a FISONS Instruments GC 8000 series gas chromatograph are the same as the stated before. with a flame ionization (FID) detector and a capillary column (DB-WAX, column length: 30 m; internal GC measurements were performed in a FISONS Instruments GC 8000 series gas diameter: 0.32 mm) and the Jasco-Borwin v. 1.50 software (Jasco, Tokyo, Japan) using helium as the chromatograph with a flame ionization (FID) detector and a capillary column (DB-WAX, column carrier gas. The temperature of injection was 240 ◦C. The initial temperature was kept at 100 ◦C length: 30 m; internal diameter:◦ 0.32 mm) and the◦ Jasco-Borwin v. 1.50 software (Jasco, Tokyo, Japan) forusing 1 min, helium then as increased the carrier 10 gas.C/min The temperature until 180 ofC injection and maintained was 240 °C. at The this initial temperature temperature for 1was min. GC-MSkept at analyses 100 °C for were 1 min, executed then increased in a Perkin 10 °C/min Elmer Clarusuntil 180 600 °C C and instrument maintained (He at asthis the temperature carrier gas). Thefor ionization 1 min. GC-MS voltage analyses was 70 eV.were GC executed was conducted in a Perkin in the Elmer temperature-programming Clarus 600 C instrument mode, (He usingas the an SGEcarrier BPX5 gas). column The (30ionization m × 0.25 voltage mm ×was0.25 70 µeV.m). GC All was products conducted formed in the were temperature-programming identified by comparison of theirmode, retention using an timesSGE BPX5 confirmed column with (30 m×0.25 those of mm×0.25 commercially µm). All available products samples. formed were Blank identified experiments by werecomparison also carried of their out, retention and it was times confirmed confirmed that with in thethose absence of commercially of the Au available NTs almost samples. no oxidation Blank productsexperiments (only upwere to also 1.2%, carried see Table out,1 and) were it was obtained. confirmed Reaction that in products the absence mass of spectrathe Au NTs were almost compared no to fragmentationoxidation products patterns (only obtained up to 1.2%, from see the Table NIST 1) (National were obtained. Institute Reaction of Standards products and mass Technology) spectra spectralwere compared library stored to fragmentation in the computer patterns software obtained of the from mass the spectrometer. NIST (National Institute of Standards and Technology) spectral library stored in the computer software of the mass spectrometer. 3. Results 3. Results Cetyltrimethylammonium chloride-stabilized gold nanotriangles (CTAC-stabilized Au NTs) were synthetizedCetyltrimethylammonium by a 3 steps procedure chloride-stabilized [23]: seed preparation, gold nanotriangles growth and (CTAC-stabilized shaping (see Scheme Au NTs)1 and Materialswere synthetized and Methods by a section). 3 steps procedure [23]: seed preparation, growth and shaping (see Scheme 1 and“Face-blocking” Materials and Methods growth, section). i.e., directed growth upon selective adsorption of surfactants to most-favorable“Face-blocking” facets (capping), growth, wasi.e., directed chosen asgrowth growth upon mechanism. selective Byadsorption this “surfactant-templating”, of surfactants to an initialmost-favorable shape yielded facets above(capping), 50% was can bechosen achieved as growth by firstly mechanism. generating By this CTAC “surfactant-templating”, capped single-twinned an initial shape yielded above 50% can be achieved by firstly generating CTAC capped seeds (<10 nm) and then fast seed adding to a growth solution using ascorbic acid, yielding single-twinned seeds (<10 nm) and then fast seed adding to a growth solution using ascorbic acid, nanotriangles of 60–150 nm upon depletion-induced purification. The synthesis process yielded yielding nanotriangles of 60–150 nm upon depletion-induced purification. The synthesis process mostly triangles, although hexagons and small polyhedral particles are scarcely detected. yielded mostly triangles, although hexagons and small polyhedral particles are scarcely detected.

SchemeScheme 1. 1.Preparation Preparation ofof goldgold nanotriangles. Appl. Sci. 2018, 8, 2655 4 of 11 Appl.Appl. Sci.Sci. 20182018,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 44 ofof 1212 Characterization by TEM and SEM conﬁrm a regular triangular shape of the synthetized CharacterizationCharacterization byby TEMTEM andand SEMSEM confirmconfirm aa regularregular triangulartriangular shapeshape ofof thethe synthetizedsynthetized CTAC-stabilized Au NTs (Figure1). High-resolution TEM show nearly 60% of the total nanoparticle CTAC-stabilizedCTAC-stabilized AuAu NTsNTs (Figure(Figure 1).1). High-resolutionHigh-resolution TEMTEM showshow nearlynearly 60%60% ofof thethe totaltotal nanoparticlenanoparticle population are gold triangles and conﬁrmed the {111} nature of the planar facets of the nanotriangles populationpopulation areare goldgold trianglestriangles andand confirmedconfirmed thethe {111}{111} naturenature ofof thethe planarplanar facetsfacets ofof thethe nanotrianglesnanotriangles (Figure1a) which are very uniform in size distribution and exhibit an edge length of 71 ± 3 nm (Figure(Figure 1a)1a) whichwhich areare veryvery uniformuniform inin sizesize distributiondistribution andand exhibitexhibit anan edgeedge lengthlength ofof 7171±±33 nmnm (Figure1b) and heights up to 40 nm. Additional TEM images and histogram of Au NTs are depicted in (Figure(Figure 1b)1b) andand heightsheights upup toto 4040 nm.nm. AdditionalAdditional TEMTEM imagesimages andand histogramhistogram ofof AuAu NTsNTs areare depicteddepicted Figure S1 of Supplementary Information. inin FigureFigure S1S1 ofof SupplementarySupplementary Information.Information.

((aa)) ((bb))

FigureFigure 1. 1. TEMTEM (( aa)) andand SEMSEM (( bb)) imagesimages of of the the obtained obtained CTAC-stabilized CTAC-stabilized Au Au NTs. NTs.

TheThe catalytic catalytic performance performance of of the the CTAC-stabilized CTAC-stabilized gold gold nanotriangles nanotriangles was, was, forfor thethe firstfirst ﬁrst time, time, evaluatedevaluated forfor thethe cyclohexanecyclohexanecyclohexane oxidation, oxidation,oxidation, in inin terms termsterms of ofof products productsproducts yield, yield,yield, turnover turnoverturnover number numbernumber (TON, (TON,(TON, moles molesmoles of −1 ofKAof KAKA oil oiloil per perper mole molemole of catalyst)ofof catacatalyst)lyst) or frequencyoror frequencyfrequency (TOF, (TOF,(TOF, h hh,− TON/reaction−11,, TON/reactionTON/reaction time), time),time), catalyst catalystcatalyst amount, amount,amount, and andand type typetype of ofoxidantof oxidantoxidant used usedused (see (see(see Table TableTable1 and 11 andand Figures FiguresFigures2–5 ). 2–5).2–5). TheThe Au Au NTs NTs showshow anan effectiveeffective performanceperformance as as catalysts catalysts for for the the MW-assisted MW-assisted oxidation oxidation of of cyclohexane,cyclohexane, with with aqueous aqueous tertterttert-butylhydroperoxide-butylhydroperoxide (TBHP),(TBHP), to to KA KA oil, oil,oil, through throughthrough the thethe formation formation of of cyclohexylcyclohexyl hydroperoxide hydroperoxide (primary(primary product),product), underunder mildmildmild conditionsconditionsconditions (Scheme(Scheme(Scheme2 2).).2).

SchemeScheme 2. 2. OxidationOxidation of of cyclohexane cyclohexane with with peroxide, peroxide, catalyzed catalyzed by by Au Au NTs. NTs.

Importantly,Importantly,Importantly, the the cyclic cyclic KA KA oil oil are are the the only only prod productsproductsucts detected detected byby gas gas chromatographic chromatographic analysis, analysis, conﬁrming the high selectivity of the catalytic oxidative system (please see Table1). This achieved high confirmingconfirming thethe highhigh selectivityselectivity ofof thethe catalyticcatalytic oxoxidativeidative systemsystem (please(please seesee TableTable 1).1). ThisThis achievedachieved selectivity (100% selectivity for the conditions of entry 4, Table1) is of utmost economic importance high selectivity (100% selectivity for the conditions of entry 4, Table 1) is of utmost economic highsince itselectivity avoids costly (100% separation selectivity steps for in the KAconditions oil production of entry process. 4, Table In addition, 1) is of theutmost mild conditionseconomic importanceusedimportance also contribute sincesince itit avoidsavoids for a sustainability costlycostly separationseparation enhancement stepssteps inin thethe of theKAKA catalytic oiloil prodproduction process.uction process.process. InIn addition,addition, thethe mildmild conditionsconditions usedused alsoalso contributecontribute forfor aa sustainabilitysustainability enhancementenhancement ofof thethe catalyticcatalytic process.process.

TableTable 1.1. SelectedSelected datadataaa forfor thethe MW-assistedMW-assisted oxidationoxidation ofof cyclohexanecyclohexane toto KAKA oiloil catalyzedcatalyzed byby AuAu NTs.NTs.

CatalystCatalyst Yield/%Yield/% cc TT TimeTime ConversionConversion SelectivitySelectivity TotalTotal TotalTotal EntryEntry amountamount AdditiveAdditive //°°CC /h/h /%/%bb CyOCyO (K)(K) CyOH CyOH (A)(A) Total Total /%/% dd TONTON ee TOF/hTOF/h−−11 ff /µmol/µmol Appl. Sci. 2018, 8, 2655 5 of 11

Table 1. Selected data a for the MW-assisted oxidation of cyclohexane to KA oil catalyzed by Au NTs.

Catalyst Yield/% c ◦ b d e Total Entry Amount/ T/ C Time/h Additive Conversion/% CyO CyOH Selectivity/% Total TON −1 f Total TOF/h µmol (K) (A) 1 5 50 3 — 3.9 3.5 — 3.5 90 35 12 2 10 50 3 — 6.3 5.7 — 5.7 90 29 10 3 20 50 3 — 13.9 13.1 — 13.1 94 33 11 4 30 50 3 — 14.2 14.2 — 14.2 100 24 8 5 5 50 3 HNO3 1.9 1.7 — 1.7 89 17 6 6 10 50 3 HNO3 3.4 3.1 — 3.1 91 16 5 7 20 50 3 HNO3 12.8 10.3 1.9 12.2 95 31 10 8 20 50 3 H2SO4 3.5 3.3 — 3.3 94 8 3 9 20 30 3 — 5.4 4.7 — 4.7 87 12 4 10 20 70 3 — 14.2 12.3 1.6 13.9 98 35 12 11 20 50 1 — 2.6 2.5 — 2.5 96 6 6 12 20 50 6 — 14.9 11.3 3.1 14.4 97 36 6 13 — 50 3 — 1.5 1.2 — 1.2 80 — — 14 g 20 50 3 — 6.9 6.7 — 6.7 97 17 6 g 15 20 50 3 HNO3 6.2 5.9 — 5.9 95 15 5 16 h 20 50 3 — — — — — — — — a Reaction conditions, unless stated otherwise: MW (10 W), CyH (5.0 mmol), TBHP (70% aq. solution, 10.0 mmol), b c n(additive)/n(substrate) = 4. Based on gas chromatography (GC) analysis, after treatment with PPh3. Molar yield (%) based on substrate, that is, moles of products (cyclohexanol and cyclohexanone) per 100 moles of cyclohexane. d Moles of cyclohexanol and cyclohexanone per mole of converted cyclohexane. e TON = Total turnover number f −1 g (moles of cyclohexanol + cyclohexanone per mole of catalyst). TOF (h ) = TON/time. H2O2 (30% aq. solution, 10.0 mmol). h Using air as oxidant.

The best catalytic conditions consisted of using 20 µmol of catalyst and TBHP (70% aq. solution) as oxidant under MW irradiation at 50 ◦C for 3 h (Table1 and Figures2–5). In fact, the amount of catalyst plays a signiﬁcant role, as shown in Figure2. The increase from e.g., 10 to 20 µmol of Au NTs in the reaction medium increases the yield from 5.7 to 13.1%. The ratio of Au NTs surface to volume per mole of catalyst was determined for the different amounts of catalyst used and is presented in Table2. Calculations were performed using the Equations S1 and S2 (see Supplementary Information) for the area and volume of an equilateral triangle, respectively.

Table 2. Au NTs surface/volume ratio per mole of catalyst used for the MW-assisted cyclohexane oxidation to KA oil.

Catalyst Amount/µmol Au NTs Surface/Volume Ratio/×10−4 nm−1 µmol−1 5 50 10 25 20 13 30 8

The inﬂuence of the TON affected by the surface to volume ratio, on the total yield of products was determined (see Figure S2 of Supplementary Information), and it was found to exhibit a considerable dependence of the catalyst amount. Moreover, the presence of nitric acid (a commonly used cyclohexane oxidation promotor [27,28]) has a slight inhibitory effect on the catalytic activity of CTAC-stabilized Au NTs as depicted in Figure2 and entries 5–7 or 15 of Table1. However, sulfuric acid has a much stronger inhibitory effect than nitric acid (compare entries 8 and 7 of Table1) reducing the obtained KA oil yield to 3.3% and 12.2%, respectively. Possibly, the inhibiting effect observed is due to the cationic surfactant cetyltrimethylammonium chloride (CTAC) when combined with H2SO4. The complete dissociation of the acids occurs and the pKa of the formed species is higher for sulfuric acid (HSO4; pKa = 1.92) than − for nitric acid (weak conjugated base NO3 ). This higher value of pH of the reaction mixture probably promotes the decomposition of TBHP. Appl. Sci. 2018, 8, x 2655 FOR PEER REVIEW 66 of of 12 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 12

Figure 2. Effect of the Au NTs amount on the total yield of cyclohexanol and cyclohexanone obtained Figure 2. Effect of the Au NTs amount on the total yi yieldeld of cyclohexanol and cyclohexanone obtained by MW-assisted oxidation of cyclohexane with TBHP during 3 h at 50 °C◦ using (---) or without (⎯) by MW-assisted oxidation of cycl cyclohexaneohexane with TBHP during 3 h at 50 °CC using (---) (—) or without ( (—)⎯) nitric acid. nitric acid. The catalytic activity of the Au NTs is also sensitive to the reaction temperature and time of MW The catalytic activity of the Au NTs is also sens sensitiveitive to the reaction temperature and time of MW irradiation, as depicted in Figures 3 and 4, respectively. irradiation, as depicted in Figures3 3 and and4 ,4, respectively. respectively.

ºC º C Figure 3.3. InﬂuenceInfluence of of the the reaction reaction temperature temperature on theon yieldthe yield of KA of oil KA achieved oil achieved by the Au by NTs the catalyzedAu NTs Figure 3. Influence of the reaction temperature on the yield of KA oil achieved by the Au NTs catalyzedcyclohexane cyclohexane oxidation withoxidation TBHP with under TBHP MW under irradiation MW irradiation for 3 h. for 3 h. catalyzed cyclohexane oxidation with TBHP under MW irradiation for 3 h. The attempts to perform the aerobic catalytic oxidation using the above optimized conditions (20 µmol of Au NTs, 3 h of MW irradiation at 50 ◦C) failed (see entry 16 of Table1 and Figure5). As expected, molecular oxygen is harder to activate than a peroxide compound. Moreover, as presented in Table3, the pressure when using air as oxidant is much lower than that generated by hydrogen peroxide or TBHP. In addition, the use of the greener hydrogen peroxide (30% aq. solution) oxidant leads to a signiﬁcant reduction of the KA oil produced (Figure5). The auto-oxidation of hydrogen peroxide Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 12

Appl. Sci. 2018, 8, 2655 7 of 11

under the optimized conditions for TBHP is known, leading to decrease in the conversion of substrates. ◦ Decomposition of H2O2 also prohibits its use at temperatures above 50 C. TheAppl. Sci. maximum 2018, 8, x FOR pressure PEER REVIEW inside the reactor for the different oxidants is shown in Table37 .of 12

Figure 4. Dependence on the reaction time of the yield of KA oil attained by the Au NTs catalyzed oxidation of cyclohexane with TBHP under MW irradiation at 50 °C.

The attempts to perform the aerobic catalytic oxidation using the above optimized conditions (20 µmol of Au NTs, 3 h of MW irradiation at 50 °C) failed (see entry 16 of Table 1 and Figure 5). As expected, molecular oxygen is harder to activate than a peroxide compound. Moreover, as presented in Table 3, the pressure when using air as oxidant is much lower than that generated by hydrogen peroxide or TBHP. In addition, the use of the greener hydrogen peroxide (30% aq. solution) oxidant leads to a significant reduction of the KA oil produced (Figure 5). The auto-oxidation of hydrogen peroxide under the optimized conditions for TBHP is known, leading to decrease in the conversion of substrates. Decomposition of H2O2 also prohibits its use at temperatures above 50 °C. TheFigure maximumFigure 4. Dependence 4. Dependence pressure on oninside the the reaction reaction the reactortime time ofof for the thyiel yielde ddifferent of of KA KA oil oil oxidantsattained attained by is bythe shown theAu NTs Au in NTscatalyzed Table catalyzed 3. ◦ oxidationoxidation of cyclohexane of cyclohexane with with TBHP TBHP under under MWMW irradiation at at 50 50 °C.C. Table 3. MW reactor maximum pressure for the oxidation of cyclohexane to cyclohexanol and Table 3. MW reactor maximum pressure for the oxidation of cyclohexane to cyclohexanol and cyclohexanoneThe attempts catalyzed to perform by Au the NTs. aerobic catalytic oxidation using the above optimized conditions cyclohexanone catalyzed by Au NTs. (20 µmol of Au NTs, 3 h of MW irradiation at 50 °C) failed (see entry 16 of Table 1 and Figure 5). As MW-reaction conditions Pressure/atm expected, molecular oxygen MW-Reactionis harder to activate Conditions than a peroxide Pressure/atm compound. Moreover, as presented in Table 3, the pressure whenControl using (without air as oxidan catalyst)t is much lower 1.3 than that generated by hydrogen peroxide or TBHP. Control (withoutTBHP catalyst)5.3 1.3 TBHP 5.3 In addition, the use of the greenerH2O hydrogen2 peroxide (30%5.6 aq. solution) oxidant leads to a H2O2 5.6 significant reduction of the KA oil producedairair (Figure 5). The auto-oxidation1.0 1.0 of hydrogen peroxide under the optimized conditions for TBHP is known, leading to decrease in the conversion of substrates. Decomposition of H2O2 also prohibits its use at temperatures above 50 °C. The maximum pressure inside14 the reactor for the different oxidants is shown in Table 3. No additive Table 3. MW reactor maximum pressure for the oxidation of cyclohexane to cyclohexanol and 12 Nitric acid cyclohexanone catalyzed by Au NTs. 10 MW-reaction conditions Pressure/atm Control8 (without catalyst) 1.3 TBHP 5.3 6 H2O2 5.6 air 1.0

KA oil yield / / % KA oil yield 4

2 14 No additive 0 12 Nitric acid H2O2H2O2 TBHP O2O2 10 Figure 5. Inﬂuence of the oxidant type on the yield of cyclohexanol and cyclohexanone achieved by 8 the Au NTs catalyzed oxidation of cyclohexane under MW irradiation for 3 h at 50 ◦C. 6 Even using the very low MW irradiation power of 10 W, a favorable effect was observed for the

KA oil yield / / % KA oil yield 4 present catalytic system, as reported for other oxidative systems [28–32]. For example, only 2% of product, under the same conditions2 of those presented in entry 4 of Table1 but using an oil-bath as heating source (entry 4, Table4), was obtained. The accelerating effect of microwave-assisted reactions 0 results mainly from (i) the selective absorptionH2O2H2O2 ofTBHP the radiation O2O2 by polar compounds, (ii) the local

Appl. Sci. 2018, 8, 2655 8 of 11 overheating (“hot-spot” effect), and (iii) the non-thermal effects such as inﬂuence on the pre-exponential factor A in the Arrhenius equation [33].

Table 4. Selected data a for the oxidation of cyclohexane to cyclohexanol and cyclohexanone during 3 h at 50 ◦C using conventional heating.

Catalyst Yield/% b Entry Additive Total TON c Conversion/% Amount/µmol CyO (K) CyOH (A) Total 1 5 — 0.2 — 0.2 2 0.5 2 10 — 0.3 — 0.3 2 0.6 3 20 — 0.5 — 0.5 1 0.9 4 30 — 2.0 — 2.0 3 2 5 5 HNO3 0.6 — 0.6 6 0.8 6 d 20 — 0.2 — 0.2 1 0.3 d 7 20 HNO3 0.3 — 0.3 1 0.3 8 e 20 — — — — — — a Reaction conditions, unless stated otherwise: CyH (5.0 mmol), TBHP (70% aq. solution, 10.0 mmol), b n(additive)/n(substrate) = 4. Based on gas chromatography (GC) analysis, after treatment with PPh3; molar yield (%) based on substrate, that is, moles of products (cyclohexanol and cyclohexanone) per 100 mol of cyclohexane. c d TON = Total turnover number (moles of cyclohexanol + cyclohexanone per mole of catalyst). H2O2 (30% aq. solution, 10.0 mmol). e Using air as oxidant.

It is also worth to notice that the attained maximum yield value in the present study is also higher than the reported [34] for the currently industrial (commercial) process with the advantage of being much more selective (100% selectivity was obtained, see entry 4 of Table1) and obtained under mild conditions. The stability of the gold nanotriangles in the oxidative reaction medium was assessed by UV-vis spectrophotometry, through the localized surface plasmonic resonance (LSPR) of the AuNTs before and after the reaction. As depicted in Figure6, after the catalytic reaction, only a very small λmax shift from 533 nm (before the reaction) to 536 nm (after the reaction) of the LSPR was detected. As a common rule, bigger nanoparticles, as well as more complex morphologies, lead to longer red-shifted absorptions in the LSPR and therefore no signiﬁcant aggregation of the Au NTs appears to occur duringAppl. the Sci. catalytic 2018, 8, x reaction.FOR PEER REVIEW 9 of 12

0.50

0.45

0.40 Initial 0.35 After reaction

0.30 Abs

0.25

0.20

0.15

0.10

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0.00 400 500 600 700

λ /nm

Figure 6. Absorption spectra (400 – 700 nm) of the Au NTs before and after the catalytic Figure 6. Absorption spectra (400 – 700 nm) of the Au NTs before and after the catalytic oxidation oxidation reaction. reaction.

By analogy with reported mechanisms for different types of Mn/n-1 metal systems [6,7,35], the proposed mechanism for the present Au NTs/TBHP system follows (reactions (1)–(9), Scheme 3).

Scheme 3. Suggested mechanism for cyclohexane oxidation catalyzed by Au NTs.

Firstly, Au-catalyzed decomposition of TBHP occurs, leading to ROO and RO radicals upon oxidation by Aun or reduction by An-1 according to reactions (1) and (2), respectively. Then, H-abstraction from CyH by RO leads to the formation of cyclohexyl radical (Cy) (reaction (3)). CyOO is formed by the reaction of Cy with dioxygen (reaction (4)), and CyOOH can then be formed upon H-abstraction from TBHP by CyOO (reaction (5)). Au-assisted decomposition of CyOOH to CyOO and CyO (reactions (6) and (7)) leads to the formation of the desired products: cyclohexanol (CyOH) and cyclohexanone (Cy−H=O), according to reactions (8) and (9)).

This work created the possibility of using non-supported gold nanoparticles towards cyclohexane oxidation under mild conditions using peroxide as oxidant. The combination of suitable Au NPs, such as CTAC-stabilized Au NTs, and well-adjusted reaction parameters (in Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 12

0.50

0.45

0.40 Initial 0.35 After reaction

0.30 Abs

0.25

0.20

0.15

0.10

0.05

0.00 400 500 600 700

λ /nm

Appl. Sci.Figure2018 ,6.8, Absorption 2655 spectra (400 – 700 nm) of the Au NTs before and after the catalytic oxidation9 of 11 reaction.

− By analogy with reported reported mechanisms mechanisms for for different different types types of of M Mn/n-1n/n metal1 metal systems systems [6,7,35], [6,7, 35the], theproposed proposed mechanism mechanism for forthe thepresent present Au AuNTs/TBHP NTs/TBHP system system follows follows (reactions (reactions (1)–(9), (1)–(9), Scheme Scheme 3). 3).

Scheme 3. Suggested mechanism for cyclohexane oxidation catalyzed by Au NTs.

Firstly, Au-catalyzed Au-catalyzed decomposition decomposition of TBHP of TBHP occurs, occurs, leading leading to ROO to and ROO RO and radicals RO uponradicals oxidation upon − byoxidation Aun or reductionby Aun or by reduction An 1 according by A ton-1 reactionsaccording (1) to and reactions (2), respectively. (1) and Then,(2), respectively. H-abstraction Then, from CyHH-abstraction by RO leads from to CyH the formation by RO leads of cyclohexyl to the format radicalion (Cy) of cyclohexyl (reaction (3)). radical CyOO (Cy) is formed(reaction by (3)). the reactionCyOO is of formed Cy with by dioxygen the reaction (reaction of Cy (4))with, and dioxyg CyOOHen (reaction can then (4)), be and formed CyOOH upon can H-abstraction then be formed from TBHPupon H-abstraction by CyOO (reaction from (5)).TBHP Au-assisted by CyOO (reaction decomposition (5)). Au-assisted of CyOOH decomposition to CyOO and CyO of CyOOH (reactions to (6)CyOO and and (7)) leadsCyO (reactions to the formation (6) and of (7)) the leads desired to the products: formation cyclohexanol of the desired (CyOH) products: and cyclohexanone cyclohexanol (Cy(CyOH)−H=O), and according cyclohexanone to reactions (Cy−H=O), (8) and according (9)). to reactions (8) and (9)). This work created the possibility of using non-supported gold nanoparticles towards cyclohexane oxidationThis underwork mildcreated conditions the possibility using peroxide of using as oxidant.non-supported The combination gold nanoparticles of suitable Autowards NPs, suchcyclohexane as CTAC-stabilized oxidation under Au NTs, mild and conditions well-adjusted using reaction peroxide parameters as oxidant. (in particular,The combination the use ofof MWsuitable irradiation) Au NPs, can such lead as to CTAC-stabilized highly selective andAu efﬁcientNTs, and catalytic well-adjusted systems withreaction signiﬁcance parameters for the(in development of a sustainable process for cyclohexane oxidation to KA oil. In fact, the following advantages of using Au NTs instead of spherical Au NPs can be considered: (i) for the same diameter, triangular nanoplates present more gold atoms at the nanoparticle surface when compared with nanospheres and circular nanoplates; (ii) despite being stable, gold nanotriangles are less stable than small gold nanospheres and therefore easier to separate from solution by centrifugation (expensive and not an easy-to-scale process), which can be important for reusing these materials at an industrial level; and (iii) anisotropic materials such as nanotriangles present hot-spots that can concentrate the energy. This can be important to favor the reaction.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/8/12/2655/ s1, Figure S1: Additional Transmission Electron Microscopy (TEM) images (a) and (b) and histogram (c) of Au NTs, Figure S1: Dependence of turnover number (TON) affected by the surface to volume ratio, on the total yield of products. Author Contributions: Conceptualization, A.P.C.R. and R.P.O.-S.; methodology, I.A.S.M.; formal analysis, A.P.C.R. and R.P.O.-S.; investigation, A.P.C.R., I.A.S.M. and R.P.O.-S.; writing—original draft preparation, L.M.D.R.S.M.; writing—review and editing, L.M.D.R.S.M.; supervision, L.M.D.R.S.M. and D.M.F.P.; project administration, L.M.D.R.S.M.; funding acquisition, L.M.D.R.S.M. Funding: This research was funded by the Fundação para a Ciência e a Tecnologia (FCT), Portugal: projects UID/QUI/00100/2013, UID/BIO/04565/2013 and PTDC/QEQ-ERQ/1648/2014 and fellowships SFRH/BPD/90883/2012 to APCR and PD/BD/116850/2016 to RP Oliveira-Silva. Funding received by IBB-Institute for Bioengineering and Biosciences from Programa Operacional Regional de Lisboa 2020 (Project N. 007317) is also acknowledged. Conﬂicts of Interest: The authors declare no conﬂict of interest. Appl. Sci. 2018, 8, 2655 10 of 11

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