Selective Oxidation of Ethane to Acetic Acid Catalyzed by a C-Scorpionate

molecules

Article ArticleSelective Oxidation of Ethane to Acetic Acid Selective Oxidation of Ethane to Acetic Acid CatalyzedCatalyzed byby aa C-ScorpionateC-scorpionate Iron(II) Complex: A AHomogeneous Homogeneous vs. vs. Heterogeneous Heterogeneous Comparison Comparison

Inês A. S. Matias, Ana P. C. Ribeiro * and Luísa M. D. R. S. Martins * Inês A. S. Matias , Ana P. C. Ribeiro * and Luísa M. D. R. S. Martins * CentroCentro dede QuQuímicaímica Estrutural,Estrutural and Departamento Departamento de Engenharia de Engenharia Química, Química, Instituto Instituto Superior Superior Técnico, Técnico, UniversidadeUniversidade dede Lisboa,Lisboa, 1049-0011049-001 Lisboa,Lisboa, Portugal;Portugal; [email protected]@tecnico.ulisboa.pt ** Correspondence: Correspondence: [email protected]@tecnico.ulisboa.pt (A.P.C.R.);(A.P.C.R.); [email protected] [email protected] (L.M.D.R.S.M.); (L.M.D.R.S.M.); Tel.:Tel.:+ +351-218-419-389351-218-419-389 (L.M.D.R.S.M.)(L.M.D.R.S.M.) Academic Editor: Noel Nebra Academic Editor: Noel Nebra Received:Received: 3 November 2020; Accepted: 27 November 2020; Published: 30 November 2020

Abstract:Abstract: TheThe direct, direct, one-pot one-pot oxidation oxidation of ethane of ethane to acetic to acid acetic was, acid for was,the first for time, the performed ﬁrst time, performedusing a C-scorpionate using a C-scorpionate complex complexanchored anchoredonto a ontomagnetic a magnetic core-shell core-shell support, support, the 3 3 theFe3O Fe4/TiO3O4/2TiO/[FeCl2/[FeCl2{κ -HC(pz)2{κ -HC(pz)3}] composite.3}] composite. This This catalytic catalytic system system,, where where the the magnetic magnetic catalyst catalyst isis easilyeasily recoveredrecovered andand reused,reused, isis highlyhighly selectiveselective toto thethe aceticacetic acidacid synthesis.synthesis. TheThe performedperformed greengreen metricsmetrics calculationscalculations highlighthighlight thethe “greeness”“greeness” ofof thethe newnew ethaneethane oxidationoxidation procedure.procedure.

Keywords:Keywords: ethane oxidation; acetic acid; magnetic catalyst;catalyst; core-shell;core-shell; C-scorpionate; iron; heterogeneousheterogeneous catalysis;catalysis; greengreen metricsmetrics

1.1. Introduction High-value,High-value, sustainablesustainable commoditiescommodities obtainedobtained throughthrough novelnovel catalytic processes are the answer towardstowards developingdeveloping aa cleaner,cleaner, moremore eefficient,ﬃcient, modernmodern andand futurefuture orientedoriented industry.industry. AA signiﬁcantsignificant exampleexample isis thethe productionproduction of of acetic acetic acid, acid, a a large large tonnage tonnage commodity commodity of of high high importance importance in viewin view of itsof wideits wide industrial industrial use anduse wordand word demand demand (theglobal (the global acetic acetic acid market acid market reached reached a volume a volume of 17.28 of million 17.28 tonsmillion in 2019tons [in1]), 2019 which [1]), is which expected is expected [2] to continue [2] to continue increasing increasing in the next in the few next years few (Figure years1 (Figure). 1).

Figure 1. Worldwide acetic acid market forecasts. Reproduced with the permission of [2]. Figure 1. Worldwide acetic acid market forecasts. Reproduced with the permission of [2]. The current preferred manufacturing of acetic acid is through carbonylation of methanol. This process uses methanol and carbon monoxide as raw materials, which are produced from natural gas, coal and heavy residual oil, among other materials [3] (Scheme1). Molecules 2020, 25, x; doi: FOR PEER REVIEW www.mdpi.com/journal/molecules Molecules 2020, 25, 5642; doi:10.3390/molecules25235642 www.mdpi.com/journal/molecules MoleculesMolecules 20202020,, 2525,, xx FORFOR PEERPEER REVIEWREVIEW 22 ofof 1313

TheThe currentcurrent preferredpreferred manufacturingmanufacturing ofof aceticacetic acidacid isis throughthrough carbonylationcarbonylation ofof methanol.methanol. ThisThis process uses methanol and carbon monoxide as raw materials, which are produced from natural gas, Moleculesprocess 2020uses, 25 methanol, 5642 and carbon monoxide as raw materials, which are produced from natural2 ofgas, 13 coalcoal andand heavyheavy residualresidual oil,oil, amongamong otherother materialsmaterials [3][3] (Scheme(Scheme 1).1).

SchemeScheme 1. 1. AceticAcetic acidacid productionproduction from from the the raw raw materials, materials, involving involving the the carbonylation carbonylation of of methanol methanol in in thethe third third step. step.

InIn 1960,1960,1960, BASF BASFBASF (using (using(using cobalt cobaltcobalt catalysts) catalysts)catalysts) industrialized industrializedindustrialized the aforementioned thethe aforementionedaforementioned acetic acid manufacturing aceticacetic acidacid manufacturingprocess,manufacturing followed process,process, by Monsanto followedfollowed (with byby MonsantoMonsanto rhodium (with catalysts)(with rhodiumrhodium in 1970, catalysts)catalysts) and in in 1996in 1970,1970, a new andand process inin 19961996 for aa newnew the processcarbonylationprocess forfor thethe ofcarbonylationcarbonylation methanol to ofof acetic memethanolthanol acid toto was aceticacetic announced acidacid waswas by announannoun BP Chemicals,cedced byby BPBP Chemicals, basedChemicals, on a basedbased promoted onon aa iridium catalyst package named CativaTM. Although the most recent process presents benefits such promotedpromoted iridiumiridium catalystcatalyst packagepackage namednamed CativaCativaTMTM.. AlthoughAlthough thethe mostmost recentrecent processprocess presentspresents benefitsasbenefits the high suchsuch yield asas thethe of acetichighhigh yieldyield acid, ofof the aceticacetic disadvantages acid,acid, thethe disadvantagesdisadvantages include environmental includeinclude enen onesvironmentalvironmental and the risk onesones of andand severe thethe riskcorrosionrisk ofof severesevere of the corrosioncorrosion equipment ofof thethe (resulting equipmentequipment from (resulti(resulti the requiredngng fromfrom iodide thethe requiredrequired cocatalyst). iodideiodide The cocatalyst).cocatalyst). needed alternative TheThe neededneeded of alternativesynthesizingalternative ofof acetic synthesizingsynthesizing acid directly aceticacetic through acidacid directlydirectly oxidation throughthrough of a gaseous oxidationoxidation alkane ofof [4aa, 5gaseousgaseous] that appears alkanealkane to [4,5][4,5] be much thatthat wiser,appearsappears clean toto bebe and muchmuch cheaper, wiser,wiser, is cleanclean also challenging. andand cheaper,cheaper, isis alsoalso challenging.challenging. EthaneEthane is is the the most most suitable suitable alkane alkane for for such such aa processprocess byby choice,choice, althoughalthough somesome advancesadvances on on obtainingobtaining aceticaceticacetic acid acidacid from fromfrom methane methanmethan (viaee (via(via carboxylation carboxylationcarboxylation with withwith CO) CO)CO) have havehave also alsoalso been beenbeen reported reportedreported [6–9]. [6–9].[6–9]. In 2005, InIn 2005,SABIC2005, SABICSABIC of Saudi ofof ArabiaSaudiSaudi ArabiaArabia commercialized commercializedcommercialized an acetic anan acid aceticacetic plant acidacid [10 plantplant] based [10][10] on basedbased a low-cost onon aa ethanelow-costlow-cost oxidation ethaneethane oxidationprocessoxidation where processprocess the catalystwherewhere thethe is a catalystcatalyst calcined isis mixture aa calcinedcalcined of oxides mixturemixture of Mo,ofof oxidesoxides V, Nb ofof and Mo,Mo, Pd, V,V, but NbNb with andand selectivities Pd,Pd, butbut withwith to selectivitiesaceticselectivities acid of toto just aceticacetic 80%. acidacid ofof justjust 80%.80%. AA widewide rangerange range ofof of catalystscatalysts catalysts havehave have beenbeen been testedtested tested forfor forthethe partial thepartial partial oxidationoxidation oxidation ofof ethaneethane of ethane toto aceticacetic to acid,aceticacid, eithereither acid, ineitherin homogeneoushomogeneous in homogeneous oror heterogeneousheterogeneous or heterogeneous conditionsconditions conditions [11][11] [.. 11 However,However,]. However, soso sofar,far, far, nonenone none hashas has demonstrateddemonstrated demonstrated the the performanceperformance requiredrequired forfor industrialindustrial commercialization.commercialization. InIn pursuitpursuit ofof of ourour our interestinterest interest onon on oxidationoxidation oxidation catalysiscatalysis catalysis ofof ofindustrialindustrial industrial significancesignificance significance [5,12,13],[5,12,13], [5,12, 13 herein,herein,], herein, wewe we successfully explored the catalytic activity of the C-scorpionate iron(II) complex [FeCl {κ3-HC(pz) }] successfullysuccessfully exploredexplored thethe catalyticcatalytic activityactivity ofof thethe C-scorpionateC-scorpionate iron(II)iron(II) complexcomplex [FeCl[FeCl222{{κκ33-HC(pz)-HC(pz)333}]}] (pz(pz === pyrazol-1yl, pyrazol-1yl,pyrazol-1yl, Figure FigureFigure2)[ 2)2)14 ][14][14] for theforfor selective thethe selectiveselective oxidation oxidationoxidation of ethane ofof toethaneethane acetic to acid.to aceticacetic This acid.acid. iron compound ThisThis ironiron compoundpreviouslycompound provedpreviouslypreviously its e provedffiprovedciency itsits in efficiencyefficiency the catalytic inin thethe oxidation catalyticcatalytic of oxidationoxidation cyclohexane ofof cyclohexanecyclohexane [15–24] but was[15–24][15–24] not butbut applied waswas notfornot otherappliedapplied (more forfor other challenging)other (more(more challenging)challenging) alkane substrates. alkanealkane Moreover, substrates.substrates. to Moreover,Moreover, our knowledge, toto ourour no knowledge,knowledge, iron(II) coordination nono iron(II)iron(II) coordinationcompoundcoordination was, compoundcompound to date, reportedwas,was, toto date,date, as a reported catalystreported for asas theaa caca oxidationtalysttalyst forfor thethe of ethane oxidationoxidation to acetic ofof ethaneethane acid. toto aceticacetic acid.acid.

ClCl NN NN FeFe CC HH NN NN ClCl NN NN

3 2 33 FigureFigure 2.2.2.Structure StructureStructure of theofof dichlorothethe dichlorodichloro hydrotris(pyrazol-1-yl)methane hydrotris(pyrazol-1-yl)methanehydrotris(pyrazol-1-yl)methane iron(II) complexiron(II)iron(II) [FeClcomplexcomplex2{κ -HC(pz)[FeCl[FeCl2{{κκ3}]-- HC(pz)(pzHC(pz)= pyrazol-1-yl).33}]}] (pz(pz == pyrazol-1-yl).pyrazol-1-yl).

C-homoscorpionatesC-homoscorpionates areareare tripodal tripodaltripodal nitrogen nitrogennitrogen tris(pyrazol-1-yl)-methane tristris(pyrazol-1-yl)-methane(pyrazol-1-yl)-methane ligands ligandsligands (introduced (introduced(introduced [25 [25]][25] by byTroﬁmenkoby TrofimenkoTrofimenko in the inin thethe late latelate 1960s), 1960s),1960s), the thethe designation designationdesignation of ofof which whichwhich arises arisesarises from fromfrom the thethepeculiar peculiarpeculiar coordination coordinationcoordination modemode ofof these these chelators chelators to to a a metalmetal likelike aa scorpionscorpion attack,attack, pictoriallypictorially compared compared to to an an embraceembrace of of thethe metalmetal center,center, like like the the pincerspincers andand and tailtail tail ofof of aa a scsc scorpionorpionorpion catchingcatching catching andand and stingingstinging stinging itsits its pray.pray. pray. TheirTheir Their easyeasy easy interchangeinterchange interchange ofof coordinationofcoordination coordination modesmodes modes betweenbetween between tri-tri- tri- andand and bi-dentatebi-dentate bi-dentate isis is believedbelieved believed toto to bebe be responsibleresponsible responsible forfor the the structuralstructural versatilityversatility andand chemicalchemical (catalytic)(catalytic) performanceperformance ofof theirtheir metalmetalmetal complexes.complexes.complexes. 3 Very recently, a hybrid material consisting of the aforementioned complex [FeCl2{κ -HC(pz)3}] 3 immobilized at magnetic Fe3O4/TiO2 core-shell particles, Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}], Molecules 2020, 25, x FOR PEER REVIEW 3 of 13

Very recently, a hybrid material consisting of the aforementioned complex [FeCl2{κ3-HC(pz)3}] immobilized at magnetic Fe3O4/TiO2 core-shell particles, Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}], was successfullyMolecules 2020, 25prepared, 5642 [26] and used as a catalyst for the selective oxidation of 1-phenylethanol3 of to 13 acetophenone. The magnetic, oxide covered, Fe3O4/TiO2 core-shell was revealed to be a convenient support for the iron complex, enabling the composite to be collected by an external magnetic field [27].was successfullyThis possibility prepared is a [26significant] and used operational as a catalyst advantage, for the selective as the oxidation usually ofrequired 1-phenylethanol separation to techniquesacetophenone. (e.g., The filtration magnetic, or centrifugation), oxide covered, leading Fe3O4/ TiOto high2 core-shell catalyst waslosses, revealed are avoided. to be a convenient supportDespite for the the iron current complex, significant enabling interest the composite towards the to be by collected far more by challenging an external oxidation magnetic of field ethane [27]. toThis acetic possibility acid, there is a significant are relatively operational few reports advantage, regarding as the the usually low requiredtemperature, separation heterogeneously techniques catalyzed(e.g., filtration selective or centrifugation), oxidation of ethane leading [11], to specially high catalyst using losses, non-precious are avoided. metals as catalysts. Fe/ZSM- 5 (30)Despite is the theso far current unique significant iron-based interest catalyst towards which the led by farto moreproductivities challenging of oxidationup to 65 mol of ethane ethane to acetic acid, there are relatively few reports regarding the low temperature, heterogeneously catalyzed converted·kgcat−1·h−1 at 56% ethane conversion, with acetic acid being the major product (70% selectivity,selective oxidation 39.1% yield) of ethane [11]. [11], specially using non-precious metals as catalysts. Fe/ZSM-5 (30) is the 1 1 so far unique iron-based catalyst which led to productivities of up to 65 mol ethane converted kgcat h In this work, the selective oxidation of ethane to acetic acid, successfully catalyzed· by− ·the− at 56% ethane conversion, with acetic acid being the major product (70% selectivity, 39.1% yield) [11]. reusable magnetic Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}] composite, is presented. To the best of our knowledge,In this work,this is the the selective first time oxidation that a of magnetic ethane to aceticcore-shell acid, iron successfully composite catalyzed [or even by thean reusableiron(II) 3 coordinationmagnetic Fe3 Ocompound]4/TiO2/[FeCl has2{κ been-HC(pz) used3}] as composite, a catalyst is presented.for the direct To theone-pot best of oxidation our knowledge, of ethane this to is aceticthe first acid. time that a magnetic core-shell iron composite [or even an iron(II) coordination compound] has been used as a catalyst for the direct one-pot oxidation of ethane to acetic acid. 2. Results and Discussion 2. Results and Discussion The freshly synthesized [14] dichloro hydrotris(pyrazol-1-yl)methane iron(II) complex [FeCl2{κ3- The freshly synthesized [14] dichloro hydrotris(pyrazol-1-yl)methane iron(II) complex HC(pz)3}] (pz = pyrazol-1-yl) was immobilized onto the above Fe3O4/TiO2 magnetic core-shells using [FeCl {κ3-HC(pz) }] (pz = pyrazol-1-yl) was immobilized onto the above Fe O /TiO magnetic our very2 recently 3reported procedure [26] (see materials and methods section)3 and4 the2 resulting core-shells using our very recently reported procedure [26] (see materials and methods section) and Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}] magnetic composite was analyzed by using the usual spectroscopic the resulting Fe O /TiO /[FeCl {κ3-HC(pz) }] magnetic composite was analyzed by using the usual techniques such3 as4 scanning2 2 electron micros3 copy and energy dispersive X-ray (SEM/EDX), spectroscopic techniques such as scanning electron microscopy and energy dispersive X-ray (SEM/EDX), thermogravimetry-differential scanning calorimetry (TG-DSC) and X-ray powder diffraction (XRD). thermogravimetry-differential scanning calorimetry (TG-DSC) and X-ray powder diffraction (XRD). The spectroscopic results fully matched the ones that were previously obtained [26]. For example, the The spectroscopic results fully matched the ones that3 were previously obtained [26]. For example, structure and morphology of Fe3O4/TiO2/[FeCl2{κ -HC(pz)3 3}] composite were assessed by using the structure and morphology of Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] composite were assessed by using SEM/EDX (Figure 3). The magnetite-rich powder coated by TiO2 (Fe3O4/TiO2) with a non-spherical SEM/EDX (Figure3). The magnetite-rich powder coated by TiO 2 (Fe3O4/TiO2) with a non-spherical morphology is clearly observed in Figure 3a, where TiO2, as expected, covers the entire surface. The morphology is clearly observed in Figure3a, where TiO 2, as expected, covers the entire surface. Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}] composite (Figure 3b) exhibits also a non-spherical morphology and The Fe O /TiO /[FeCl {κ3-HC(pz) }] composite (Figure3b) exhibits also a non-spherical morphology the correspondent3 4 2 EDX2 analysis detected3 the presence of the C-scorpionate iron(II) complex. An iron and the correspondent EDX analysis detected the presence of the C-scorpionate iron(II) complex. An iron content of 0.3%wt was determined by using inductively coupled plasma atomic emission content of 0.3%wt was determined by using inductively coupled plasma atomic emission spectroscopy. spectroscopy.

(a) Fe3O4/TiO2. (b) Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}]

Figure 3.3. ScanningScanning electron electron microscopy microscopy images images of of core-shell core-shell Fe 33OO44/TiO/TiO2 2 (a(a) )and and of of 3 3 Fe3OO44/TiO/TiO22/[FeCl/[FeCl2{2κ{κ-HC(pz)-HC(pz)3}]3 }]composite composite (b ().b ).

Molecules 2020, 25, 5642 4 of 13

3 2.1. Oxidation of Ethane Catalysed by [FeCl2{κ -HC(pz)3}] 3 In order to assess the catalytic performance of the C-scorpionate Fe(II) complex [FeCl2{κ -HC(pz)3}] for the direct one-pot oxidation of ethane to acetic acid, the reaction was performed using potassium peroxodisulfate as an oxidant and the iron complex (ethane/Fe ratio: 67) as a catalyst. The catalytic experiments were initiated by choosing the traditional solvent for the oxidation of ethane, i.e., triﬂuoroacetic acid (TFA) [28–32]. A maximum acetic acid yield of 13.6% was attained after 20 h at 80 ◦C under a C2H6 pressure of 3 atm (entry 2, Table1) with a remarkable selectivity (acetic acid was the only product detected by using chromatographic analysis).

3 a Table 1. Catalytic performance of [FeCl2{κ -HC(pz)3] in the one-pot oxidation of ethane to acetic acid .

b c Entry Cat/mmol T/◦CC2H6/atm Time/h Solvent Yield /% TON 1 0.02 80 3 20 TFA 6.8 6 2 0.04 80 3 20 TFA 13.6 6 3 0.02 80 3 20 ACN/H2O 4.7 4 4 0.04 80 3 6 ACN/H2O 4.9 2 5 0.04 80 3 20 ACN/H2O 10.1 4 6 0.04 80 3 72 ACN/H2O 12.3 5 7 0.04 80 3 20 H2O 4.6 2 8 0.04 80 3 20 ACN 0.0 0 9 0.04 80 5 20 ACN/H2O 10.3 7 10 0.04 50 3 20 ACN/H2O 5.2 2 d 11 0.04 80 3 20 ACN/H2O 0.0 0 12 - 80 3 6 ACN/H2O 3.9 0 e 13 0.04 80 3 20 ACN/H2O 4.2 2 a b Reaction conditions: 4.33 mmol K2S2O8, 6 mL of solvent. Selectivity for acetic acid 99% for all assays. Yield (%) = (moles of acetic acid/100 moles of ethane). c Moles of acetic acid/mole of catalyst. d≥in the presence of 0.008 mol of e pyrazine carboxylic acid (HPCA). in the presence of 0.04 mmol iron chloride (FeCl2.4H2O).

Then, a mixture of water/acetonitrile (1:1) was chosen to replace TFA as a solvent, due to the hazardous character of the later, and the reactions were conducted exactly under the same reaction conditions. In aqueous acetonitrile at 80 ◦C for 20 h, 10.1% of ethane was converted into acetic acid (entry 5, Table1) as the only reaction product. An increase in the reaction time (until 72 h) led to only a slight increase of acetic acid yield to 12.3% (entry 6, Table1), as depicted in Figure4. Molecules 2020, 25, x FOR PEER REVIEW 5 of 13

Figure 4. AceticAcetic acid acid yield yield evolution evolution over over reaction reaction time in a water/acetonitrile water/acetonitrile mixture.

The type of solvent had a crucial effect (Figure 5) on the conversion of ethane under the tested reaction conditions (compare entries 2, 5, 7 and 8 of Table 1). The inhibiting effect of acetonitrile on the formation of acetic acid was minimized by the addition of water, while TFA allowed us to achieve the highest oxidation yield. The observed effect could be explained by the participation of water in the active site turnover, namely by filling surface oxygen vacancies to oxidative dehydrogenation of surface hydroxy groups. It is still not clear such effects are due to a direct participation of water in the main reaction pathway [33] or to a specific interaction of H2O with certain catalyst. The knowledge that Shul’pin and co-workers previously undertook [34] the partial oxidation of ethane over NaVO3 + H2SO4 and H2O2 in acetonitrile achieving the reaction selectivity trend: ethanol (51% selectivity), acetaldehyde (32%) and acetic acid (17%), prompted us to test the acid effect on our catalytic system. It was also found that the presence of an acidic medium resulted in an inhibiting effect on the oxidation of ethane (entry 11, Table 1).

13.6

10.1

4.6 Acetic acidyield /%

ACN H2O ACN/H2O TFA Solvent

Figure 5. Acetic acid yield evolution in different solvents (0.04 mmol of [FeCl2{κ3-HC(pz)3]; 20 h at 80 °C).

Our C-scorpionate Fe(II) complex [FeCl2{κ3-HC(pz)3}] exhibited much higher selectivity to the formation of acetic acid than the previously tested iron(III) complexes. In fact, the partial oxidation of ethane, using [FeIII(Me3tacn)(Cl-acac)Cl]+ type catalysts and oxone (KHSO5) as oxidants, reported by Tse et al. [35] led to a C2 oxygenate selectivity of 80% of acetic acid and 20% of ethanol. The authors have also studied ligand effects by using different bidentate and tridentate ligands to stabilize the active site. The most active catalyst was the B-scorpionate [FeIII(Tp)2][ClO4] [Tp = hydrotris(1-

Molecules 2020, 25, x FOR PEER REVIEW 5 of 13

Molecules 2020, 25, 5642 5 of 13

Figure 4. Acetic acid yield evolution over reaction time in a water/acetonitrile mixture. The use of FeCl2.4H2O as catalyst produced approximately 4% of yield, after 20 h. This is an indicationThe type that of iron solvent species had are a crucial active buteffect the (Figure scorpionate 5) on complex the conversion is much of more ethane effi cientunder that the the tested iron reactionsalt (compare conditions entries (compare 5 and 13 ofentries Table 2,1). 5, This 7 and suggests 8 of Table the favorable 1). The inhibiting involvement effect of theof acetonitrile C-scorpionate on theligand formation in the of iron acetic assisted acid was steps minimized of this catalytic by the oxidationaddition of reaction. water, while Moreover, TFA allowed the easy us interchange to achieve thebetween highest tri- oxidation and bi-dentate yield. The coordination observed effect modes co ofuld the be C-scorpionate explained by ligandthe participation provides theof water required in thestructural active site and turnover, chemical namely versatility by tofilling the ironsurface complex oxygen as vacancies a catalyst forto oxidative the oxidation dehydrogenation of ethane. of surfaceThe hydroxy type of groups. solvent It had is still a crucial not clear effect such (Figure effects5) on are the due conversion to a direct of participation ethane under of the water tested in thereaction main conditionsreaction pathway (compare [33] entries or to 2,a specific 5, 7 and interaction 8 of Table1 ).of The H2O inhibiting with certain e ffect catalyst. of acetonitrile on the formationThe knowledge of acetic acidthat wasShul’pin minimized and co-workers by the addition previously of water, undertook while [34] TFA the allowed partial usoxidation to achieve of ethanethe highest over NaVO oxidation3 + H yield.2SO4 and The H observed2O2 in acetonitrile effect could achieving be explained the reaction by the selectivity participation trend: of waterethanol in (51%the active selectivity), site turnover, acetaldehyde namely (32%) by filling and acetic surface acid oxygen (17%), vacancies prompted to us oxidative to test the dehydrogenation acid effect on our of catalyticsurface hydroxysystem. groups.It was also It is found still not that clear the such presence effects of are an due acidic to a medium direct participation resulted in ofan water inhibiting in the effectmain on reaction the oxidation pathway of [ 33ethane] or to (entry a specific 11, Table interaction 1). of H2O with certain catalyst.

13.6

10.1

4.6 Acetic acidyield /%

ACN H2O ACN/H2O TFA Solvent

3 Figure 5. Acetic acid yield evolution in different solvents (0.04 mmol of [FeCl2{κ -HC(pz)3]; Figure 5. Acetic acid yield evolution in different solvents (0.04 mmol of [FeCl2{κ3-HC(pz)3]; 20 h at 80 20 h at 80 C). °C). ◦ The knowledge that Shul’pin and co-workers previously undertook [34] the partial oxidation of Our C-scorpionate Fe(II) complex [FeCl2{κ3-HC(pz)3}] exhibited much higher selectivity to the ethane over NaVO + H SO and H O in acetonitrile achieving the reaction selectivity trend: ethanol formation of acetic 3acid 2than4 the previously2 2 tested iron(III) complexes. In fact, the partial oxidation (51% selectivity), acetaldehyde (32%) and acetic acid (17%), prompted us to test the acid effect on our of ethane, using [FeIII(Me3tacn)(Cl-acac)Cl]+ type catalysts and oxone (KHSO5) as oxidants, reported catalytic system. It was also found that the presence of an acidic medium resulted in an inhibiting by Tse et al. [35] led to a C2 oxygenate selectivity of 80% of acetic acid and 20% of ethanol. The authors effect on the oxidation of ethane (entry 11, Table1). have also studied ligand effects by using different3 bidentate and tridentate ligands to stabilize the Our C-scorpionate Fe(II) complex [FeCl2{κ -HC(pz)3}] exhibited much higher selectivity to the active site. The most active catalyst was the B-scorpionate [FeIII(Tp)2][ClO4] [Tp = hydrotris(1- formation of acetic acid than the previously tested iron(III) complexes. In fact, the partial oxidation of III + ethane, using [Fe (Me3tacn)(Cl-acac)Cl] type catalysts and oxone (KHSO5) as oxidants, reported by

Tse et al. [35] led to a C2 oxygenate selectivity of 80% of acetic acid and 20% of ethanol. The authors have also studied ligand eﬀects by using diﬀerent bidentate and tridentate ligands to stabilize the active site. III The most active catalyst was the B-scorpionate [Fe (Tp)2][ClO4] [Tp = hydrotris(1-pyrazolyl1)-borate] that showed a TOF of 12.0 mol ethane converted mol catalyst 1 h 1 at room temperature. − · − In spite of the herein revealed excellent selectivity to acetic acid formation (although in modest 3 yields), one main disadvantage of the [FeCl2{κ -HC(pz)3}] catalyst was its lack of reusability. In fact, its high solubility in aqueous acetonitrile impaired its easy recovering and possible reusing. Thus, our next goal was to try to stabilize the C-scorpionate Fe(II) complex by anchoring it at an inert (and easy to recover) supporting material as the magnetic Fe3O4/TiO2 core-shells. 3 The Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] composite was successfully achieved (see above) and its catalytic activity for the oxidation of ethane to acetic acid was studied, as described in the following section. Molecules 2020, 25, 5642 6 of 13

3 2.2. Oxidation of Ethane Catalysed by the Magnetic Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] Composite The oxidation of ethane was undertaken in the presence of the prepared magnetic 3 Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] composite. A signiﬁcant enhancement of the catalytic activity of the C-scorpionate iron(II) complex was achieved (Figure6) by its immobilization at the magnetic Fe3O4/TiO2 core-shell (which did not exhibit catalytic activity for this reaction per se; see entry 17, Table2). A maximum yield of 29.1% (entry 4, Table2) of acetic acid was reached under the same 3 conditions as the conditions previously used for [FeCl2{κ -HC(pz)3}] (which led to a maximum yield Moleculesof 10.1%, 2020 Table, 25, x1 FOR), while PEER maintainingREVIEW the selectivity of the homogeneous system. 7 of 13

Figure 6. Acetic acid yield obtained by homogeneous (left) and heterogeneous (right) oxidation of Figure 6. Acetic acid yield obtained by homogeneous (left) and heterogeneous (right) oxidation of ethane at 80 C in a water/acetonitrile mixture. ethane at 80 °C◦ in a water/acetonitrile mixture. 3 a Table2. Catalytic performance of Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] composite in the oxidation of ethane . The previously observed (Section 2.1) inhibiting effect of an acidic medium on the homogeneous Aditive oxidationEntry of ethaneCat/mmol was Talso/ CC observedH /atm for Time the /hoxidationSolvent catalyzed by theYield Fe3Ob4//TiO% 2/[FeClTON c2{κ3- ◦ 2 6 mmol HC(pz)3}] composite (entry 7, Table 2). Moreover, the presence of a base (NaOH) as additive also resulted1 in the inhibition 0.02 of 80 the iron 3composite 20catalytic activity TFA (entry - 8, Table 2). 3.0 3 2 0.04 80 3 20 TFA - 3.3 1 Interestingly, the solvent effect on the heterogeneous system differed from the effect found 3 0.04 80 3 6 ACN/H2O - 2.5 1 under homogeneous conditions (compare Figures 5 and 7). The Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}] 4 0.04 80 3 20 ACN/H2O - 29.1 12 composite5 exhibited 0.02 a significantly 80 superior 3 performance 20 ACN in/H the2O (1:1) -water/acetonitrile 21.3 mixture 18 (up to 21.3%6 yield of 0.02acetic acid) 80 than in 3the hazardous 72 TFAACN (3%/H acetic2O acidyield, entry 25.6 1, Table 212). This d 7 0.02 80 3 20 ACN/H2O 8 2.72 2 decreasee is probably due to the fact that TFA can be chemisorbed on the surface of the TiO films as 8 0.02 80 3 20 ACN/H2O 8 2.2 2 a trifluoroacetate complex. This TFA complex bound on the surface of TiO2 can act as an electron 9 0.02 50 3 20 ACN/H2O - 18.3 15 scavenger and, thus, reduces the recombination of electrons and holes. The TFA eventually 10 0.02 100 3 20 ACN/H2O - 21.5 18 decomposes11 under 0.02 the strong 80 oxidizing 5 environment 20 ofACN the/ Hcatalytic2O process.- 16.5 23 12 0.02 80 20 20 ACN/H2O - 13.7 75 13 0.02 80 3 20 ACN21.3 - 0.0 0 14 0.02 80 3 20 H2O - 5.9 5 15 0.00 80 3 20 ACN/H2O - 0.0 0 f 16 0.08 80 3 20 ACN/H2O - 6.7 6 f 17 0.02 80 3 20 ACN/H2O - 0.6 0 g 18 0.02 80 3 20 ACN/H2O - 0.0 0 a 3 Reaction conditions: 4.33 mmol K2S2O8, 6 mL of solvent. Cat is the quantity in mmol of [FeCl2{κ -HC(pz)3}] present. Selectivity for acetic acid 99% for all5.9 assays. b Yield (%) = (moles of acetic acid/100 moles of ethane). c ≥ d e Moles of aceticAcetic acid yield /% acid/mole of catalyst. 0.008 mmol of pyrazine carboxylic acid (HPCA); 0.008 mmol of sodium f g 3 hydroxide; Fe3O4/TiO2/FeCl02.2H2O. Fe3O4/TiO2 core-shell.

It is worth highlightingACN that high yields H2O (up to 21.3%, ACN/H2O entry 5, Table2) were TFA also obtained with 3 half of the [FeCl2{κ -HC(pz)3}] amount used in the homogeneous experiments. Solvent

Figure 7. Acetic acid yield evolution in different solvents (0.02 mmol Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}], 20 h at 80 °C).

The temperature plays an important role in the oxidation of ethane in the presence of Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}], conceivably due to the high energy involved in the activation of such a small reactive substrate such as ethane (Figure 8). Although this does not occur as significantly as under homogeneous conditions (see entries 10 and 5, respectively, of Table 1), the product yield increased under heterogeneous as the temperature rose from 50 to 80 °C (entries 9 and 5, respectively, Table 2), whereas for higher temperatures (100 °C), it remained almost unchanged (compare entries 5 and 10 of Table 2). Therefore, like under homogeneous conditions, the temperature of 80 °C was

Molecules 2020, 25, x FOR PEER REVIEW 7 of 13

Molecules 2020, 25, 5642 7 of 13 Figure 6. Acetic acid yield obtained by homogeneous (left) and heterogeneous (right) oxidation of ethane at 80 °C in a water/acetonitrile mixture. The previously observed (Section 2.1) inhibiting effect of an acidic medium on the homogeneous The previously observed (Section 2.1) inhibiting effect of an acidic medium on the homogeneous3 oxidation of ethane was also observed for the oxidation catalyzed by the Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] oxidation of ethane was also observed for the oxidation catalyzed by the Fe3O4/TiO2/[FeCl2{κ3- composite (entry 7, Table2). Moreover, the presence of a base (NaOH) as additive also resulted in the HC(pz)3}] composite (entry 7, Table 2). Moreover, the presence of a base (NaOH) as additive also inhibition of the iron composite catalytic activity (entry 8, Table2). resulted in the inhibition of the iron composite catalytic activity (entry 8, Table 2). Interestingly, the solvent effect on the heterogeneous system differed from the effect found under Interestingly, the solvent effect on the heterogeneous system differed from3 the effect found homogeneous conditions (compare Figures5 and7). The Fe 3O4/TiO2/[FeCl2{κ -HC(pz)3}] composite under homogeneous conditions (compare Figures 5 and 7). The Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}] exhibitedcomposite a significantlyexhibited a significantly superior performance superior performance in the (1:1) waterin the/ acetonitrile(1:1) water/acetonitrile mixture (up mixture to 21.3% (up yield ofto acetic 21.3%acid) yield thanof acetic in the acid) hazardous than in the TFA hazardous (3% acetic TFA acid (3% yield,acetic acid entry yield, 1, Table entry2). 1, ThisTable decrease 2). This is probablydecrease due is probably to the fact due that to TFAthe fact can bethat chemisorbed TFA can be onchemisorbed the surface on of the the surface TiO2 films of the as aTiO trifluoroacetate2 films as complex.a trifluoroacetate This TFA complex. complex This bound TFA on complex the surface bound of TiOon the2 can surface act as of an TiO electron2 can act scavenger as an electron and, thus, reducesscavenger the recombinationand, thus, reduces of electrons the recombination and holes. Theof TFAelectrons eventually and holes. decomposes The TFA under eventually the strong oxidizingdecomposes environment under the strong of the catalyticoxidizing process. environment of the catalytic process.

21.3

5.9 Acetic acid yield /% 0 3

ACN H2O ACN/H2O TFA

Solvent

Figure 7. Acetic acid yield evolution in different solvents (0.02 mmol Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3 3}], Figure 7. Acetic acid yield evolution in diﬀerent solvents (0.02 mmol Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}], 20 h at 80 °C). 20 h at 80 ◦C).

TheThe temperaturetemperature plays an an important important role role in in th thee oxidation oxidation of ofethane ethane in inthe thepresence presence of of 3 3 FeFe3O3O44//TiOTiO22/[FeCl/[FeCl2{κ2{-HC(pz)κ -HC(pz)3}], conceivably3}], conceivably due dueto the to high the high energy energy involved involved in the in activation the activation of such of such a a smallsmall reactive reactive substrate substrate such such as as ethane ethane (Figure (Figure8). 8). Although Although this this does does not not occur occur as signiﬁcantlyas significantly as underas homogeneousunder homogeneous conditions conditions (see entries (see entries 10 and 10 5, respectively,and 5, respectively, of Table of1 ),Table the product1), the product yield increased yield increased under heterogeneous as the temperature rose from 50 to 80 °C (entries 9 and 5, respectively, underMolecules heterogeneous 2020, 25, x FOR PEER as the REVIEW temperature rose from 50 to 80 ◦C (entries 9 and 5, respectively,8 Tableof 13 2), Table 2), whereas for higher temperatures (100 °C), it remained almost unchanged (compare entries whereas for higher temperatures (100 ◦C), it remained almost unchanged (compare entries 5 and 10 of 5 and 10 of Table 2). Therefore, like under homogeneous conditions, the temperature of 80 °C was Tableconsidered2). Therefore, to be the like most under adequate homogeneous for heterogeneous conditions, theconditions, temperature combining of 80 ◦yieldC was and considered energy to beefficiency. the most adequate for heterogeneous conditions, combining yield and energy eﬃciency.

FigureFigure 8. 8. EffectE ofﬀ ectthe temperature of the temperature on the acetic onacid theyield aceticevolution acid (0.02 yield mmol evolutionFe3O4/TiO2/[FeCl (0.022{κ mmol3- 3 FeHC(pz)3O4/TiO3}],2 20/[FeCl h at 280{κ °C).-HC(pz) 3}], 20 h at 80 ◦C).

Another source of iron (II), Fe3O4/TiO2/FeCl2.2H2O, was tested (entries 16 and 17, Table 2) and it is clear that, as observed under homogeneous conditions (see above), the [FeCl2{κ3-HC(pz)3}] scorpionate complex is more active as a catalyst for the oxidation of ethane than the iron in the tested salt (compare entries 5 and 17, Table 2). In addition, the Fe3O4/TiO2 core shell was tested as a catalyst under the same reaction conditions but no catalytic activity towards the oxidation products formation was detected. This is an indication that the role of the core shell as a support is efficient due to its high stability. The activity of Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}] composite in the successive oxidative reactions was evaluated through its potential recyclability for up to four consecutive catalytic cycles, as described in Section 3.2, using the optimized conditions (entry 4 of Table 2). As expected, the recovering of the magnetic catalyst was extremely easy and efficient, avoiding catalyst loss during the separation process. As presented in Figure 9, under the above-mentioned reaction conditions, there was a small initial decrease of 6.5% in the catalytic activity between 1st and the 2nd cycle. Then, from the 2nd to the 3rd cycle, the acetic acid production loss increased by up to 30% (indicating a higher loss of activity) and by the end of the 4th cycle, the loss was 89%. This consecutive decreasing activity was sufficient to prevent further catalytic cycles from being run.

Figure 9. Recycling tests for the oxidation of ethane catalyzed by Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}]. % of acetic acid loss = (yield of n cycle per initial yield) × 100.

Molecules 2020, 25, x FOR PEER REVIEW 8 of 13 considered to be the most adequate for heterogeneous conditions, combining yield and energy efficiency.

Figure 8. Effect of the temperature on the acetic acid yield evolution (0.02 mmol Fe3O4/TiO2/[FeCl2{κ3-

HC(pz)3}], 20 h at 80 °C). Molecules 2020, 25, 5642 8 of 13

Another source of iron (II), Fe3O4/TiO2/FeCl2.2H2O, was tested (entries 16 and 17, Table 2) and it 3 is clearAnother that, sourceas observed of iron (II),under Fe 3Ohomoge4/TiO2/neousFeCl2. 2Hconditions2O, was tested (see (entriesabove), 16the and [FeCl 17, Table2{κ -HC(pz)2) and it3}] is 3 scorpionateclear that, as complex observed is undermore active homogeneous as a catalyst conditions for the (seeoxidation above), of the ethane [FeCl than2{κ -HC(pz) the iron3 in}] scorpionatethe tested saltcomplex (compare is more entries active 5 and as a 17, catalyst Table for2). theIn addition, oxidation the of ethaneFe3O4/TiO than2 core theiron shell in was the tested saltas a (comparecatalyst underentries the 5 andsame 17, reaction Table2). conditions In addition, but the no Fecatalyti3O4/TiOc activity2 core shelltowards was the tested oxidation as a catalyst products under formation the same wasreaction detected. conditions This is butan noindication catalytic that activity the role towards of the the core oxidation shell as products a support formation is efficient was due detected. to its highThis stability. is an indication that the role of the core shell as a support is efficient due to its high stability. 3 3 TheThe activity activity of of Fe Fe33OO4/TiO4/TiO2/[FeCl2/[FeCl2{2κ{κ-HC(pz)-HC(pz)3}]3 }]composite composite in inthe the successive successive oxidative oxidative reactions reactions waswas evaluated evaluated throughthrough itsits potential potential recyclability recyclability for upfor toup four to consecutivefour consecutive catalytic catalytic cycles, ascycles, described as describedin Section in 3.2 Section, using 3.2, the optimizedusing the optimized conditions conditions (entry 4 of (entry Table2 ).4 Asof Table expected, 2). As the expected, recovering the of recoveringthe magnetic of the catalyst magnetic was extremelycatalyst was easy extremely and effi cient,easy and avoiding efficient, catalyst avoiding loss during catalyst the loss separation during theprocess. separation As presented process. inAs Figure presented9, under in Figure the above-mentioned 9, under the above-me reactionntioned conditions, reaction there conditions, was a small thereinitial was decrease a small of initial 6.5% indecrease the catalytic of 6.5% activity in the between catalytic 1st activity and the between 2nd cycle. 1st and Then, the from 2nd thecycle. 2nd Then, to the from3rd cycle,the 2nd the to acetic the 3rd acid cycle, production the acetic loss acid increased production by up loss to 30% increased (indicating by up a higherto 30% loss (indicating of activity) a higherand by loss the of end activity) of the 4thand cycle, by the the end loss of was the 89%.4th cycle, This the consecutive loss was decreasing89%. This consecutive activity was decreasing sufficient to activityprevent was further sufficient catalytic to prevent cycles from further being catalytic run. cycles from being run.

3 3 FigureFigure 9. 9.RecyclingRecycling tests tests for forthe theoxidation oxidation of ethane of ethane catalyzed catalyzed by Fe by3O Fe4/TiO3O42/[FeCl/TiO2/2[FeCl{κ -HC(pz)2{κ -HC(pz)3}]. % of3}]. acetic% of aceticacid loss acid = loss(yield= of(yield n cycle of n per cycle initial per initialyield) yield)× 100. 100. ×

Although the % of acetic acid loss, which is the ratio between the yield of n cycle over the yield obtained in the first cycle shows a continuous increase (Figure9), the selectivity of 100% to acetic acid was always observed. This could mean that the catalyst suffered a deactivation and a partial leaching 3 process occurred. SEM/EDX analysis of Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] after the 4th consecutive cycle (Figure 10) revealed the occurrence of leaching of the C-scorpionate iron(II) complex from the core-shell surface, in accordance with the diminishing of the composite catalytic activity. Despite the current high interest towards the activation of lower alkanes, namely by its activation, there are still relatively few reports regarding heterogeneously catalyzed selective oxidation of ethane. Nevertheless, our magnetic catalytic system was benchmarked to other heterogeneous catalytic systems that were previously reported, and we can essentially highlight that: (i) Li et al. [36] used a non-aqueous environment system (methylnitrate and trifluoroacetic acid), where acetic acid and formic acid are formed after 12 h at 85 ◦C. The attained yield and acid selectivity values were lower than in our case; (ii) Shul’pin et al. used titanosilicalite TS-1 to catalyze the partial oxidation of ethane with H2O2 but did not produce acetic acid [37]; (iii) Rahman et al. showed the direct oxidation of ethane to acetic and formic acid using ZSM-5, aqueous H2O2, 30 bar of ethane, 120 ◦C, 2 h and PPh3 as the relevant variables. Under these conditions, 35.1% ethane conversion with major product selectivities of acetic acid (48.5%), formic acid (36.3%) and CO2 (11.9%) were reached [38], which again were lower acetic Molecules 2020, 25, x FOR PEER REVIEW 9 of 13

Although the % of acetic acid loss, which is the ratio between the yield of n cycle over the yield obtained in the first cycle shows a continuous increase (Figure 9), the selectivity of 100% to acetic acid was always observed. This could mean that the catalyst suffered a deactivation and a partial leaching process occurred. SEM/EDX analysis of Fe3O4/TiO2/[FeCl2{κ3-HC(pz)3}] after the 4th consecutive cycle (Figure 10) revealed the occurrence of leaching of the C-scorpionate iron(II) complex from the core- shell surface, in accordance with the diminishing of the composite catalytic activity. Despite the current high interest towards the activation of lower alkanes, namely by its activation, there are still relatively few reports regarding heterogeneously catalyzed selective oxidation of ethane. Nevertheless, our magnetic catalytic system was benchmarked to other heterogeneous catalytic systems that were previously reported, and we can essentially highlight that: (i) Li et al. [36] used a non-aqueous environment system (methylnitrate and trifluoroacetic acid), where acetic acid and formic acid are formed after 12 h at 85 °C. The attained yield and acid selectivity values were lower than in our case; (ii) Shul’pin et al. used titanosilicalite TS-1 to catalyze the partial oxidation of ethane with H2O2 but did not produce acetic acid [37]; (iii) Rahman et al. showed the Molecules 2020, 25, 5642 9 of 13 direct oxidation of ethane to acetic and formic acid using ZSM-5, aqueous H2O2, 30 bar of ethane, 120 °C, 2 h and PPh3 as the relevant variables. Under these conditions, 35.1% ethane conversion with major product selectivities of acetic acid (48.5%), formic acid (36.3%) and CO2 (11.9%) were reached acid yield values than the ones reported herein; and iv) iron phthalocyanine complexes [39] supported [38], which again were lower acetic acid yield values than the ones reported herein; and iv) iron at SiO2 ((FePc)2N/SiO2) led to higher reaction yields (34%) but lower selectivity (69%), under mild phthalocyanine complexes [39] supported at SiO2 ((FePc)2N/SiO2) led to higher reaction yields (34%) conditions (60 C). but lower selectivity◦ (69%), under mild conditions (60 °C).

3 FigureFigure 10. 10.A A scanning scanning electron electron microscopy microscopy (SEM) (SEM) image image ofof magneticmagnetic FeFe33O4/TiO/TiO22/[FeCl/[FeCl2{2κ{3κ-HC(pz)-HC(pz)3}]3 }] compositecomposite after after the the 4th 4th reaction reaction cycle. cycle.

2.3.2.3. Green Green Metrics Metrics ToTo assess assess the the quantitative quantitative improvement improvement brought brought about about by ourby ethaneour ethane oxidation oxidation procedure, procedure, several greenseveral analytical green analytical chemistry chemistry metrics [ 40metrics] were [40] calculated were calculated and are and presented are presented in Table in3 .Table 3.

TableTable 3. 3. GreenGreen chemistrychemistry metrics for the the oxidation oxidation of of ethane ethane to to acetic acetic acid acid catalyzed catalyzed by by 3 3 [FeCl[FeCl2{κ2{κ-HC(pz)3-HC(pz)3}]3}] or or FeFe33O4/TiO/TiO22/[FeCl/[FeCl2{2κ{κ3-HC(pz)-HC(pz)3}].3 }].

EntryEntry SolventSolvent Catalyst Catalyst E-Factor E-Factora a AEAE/%/% b b AU/%AU/% c c f df d 1 H2O/ACN 0.5 108.4 1 H O/ACN 3 0.5 108.4 2 [FeCl[FeCl{κ32{-HC(pz)κ -HC(pz)}] 3}] 20.020.0 100 100 22 TFATFA 2 3 2.7 2.7 702.3 702.3 3 H2O/ACN Fe3O4/TiO2/ 0.7 122.1 3 H O/ACN Fe3O4/TiO2/ 0.7 20.0 100 122.1 2 20.0 100 44 TFATFA [FeCl2{[FeCl2{κ3-HC(pz)3}]κ3-HC(pz)3}] 1.9 1.9 570.3 570.3 a Environmental factor (E-factor) [41] = total weight (in kg) of all waste generated in an industrial a Environmental factor (E-factor) [41] = total weight (in kg) of all waste generated in an industrial process per b kilogramprocess of per product; kilogramb Atom of product; Economy Atom (AE) =Economythe conversion (AE) = ethefficiency conversion of a chemical efficiency process of a in chemical terms of all atoms involved and the desired products produced [42] c Reaction Mass Efficiency (RME) [41] = measures the efficiency with which reactant mass ends up in the desired product; c Atom Utilization (AU) = is the ratio of the molecular weight of the desired product to the sum total of all the materials (excluding solvents) used; d Solvent and Catalyst Environmental Impact Parameter (f) = the ratio of the quantity of all materials used to the quantity of materials produced.

The use of the mixture water/acetonitrile led always to lower E-factor values than the ones obtained when using TFA as solvent (compare entry 1 and 2 or 3 and 4 of Table3), indicating a lesser amount of waste is generated and, therefore, that the process exhibits higher environmental sustainability. On the other hand, an atom economy (EA) of 20% (Table3) is a clear indication that one of the species is oﬀ balancing the atomic balance of the process: the oxidant, K2S2O8 (reagent to oxidant ratio of 1:6). AU values (Table3) also reﬂect the “green economy” of the process, as all components of the reaction were used to produce the desired acetic acid, minimizing waste production. The Solvent and Catalyst Environmental Impact Parameter should have a value as low as possible if only if all materials used in the process are recycled, recovered or eliminated; otherwise, it will measure the need for improvement in the process. It is clear that a simple change in the solvent can lead to a decrease of approximately 80% in the value of f (entry 3 and 4 of Table3) for the heterogeneous catalytic process. In the homogeneous process, the improvement in the metrics is 85% just by changing the solvent (entry 1 and 2 of Table3). Molecules 2020, 25, 5642 10 of 13

Therefore, it is of crucial importance to assess the improvement in the processes by using metrics and proving that a sustainable process can also be quantiﬁed.

3. Methods and Materials All reagents were used as received, purchased from Sigma-Aldrich (Munich, Germany). Solvents were purified using standard methods and freshly distilled under dinitrogen prior to use. 3 The chloro C-scorpionate iron(II) complex [FeCl2{κ -HC(pz)3}] (pz, pyrazolyl) was prepared and characterized according to a published method [14]. 3 The synthesized core-shell particles, before and after functionalization with [FeCl2{κ -HC(pz)3}], were characterized by using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Morphology of the core shells and the catalyst were characterized using a scanning electron microscope (JEOL 7001F with Oxford light elements EDS detector and EBSD detector, JEOL, Tokyo, Japan). The iron content of the core-shell supported catalyst was analyzed by using inductively coupled plasma atomic emission spectroscopy (ICP-AES) carried out in an ICP-AES model Ultima (Horiba Jobin-Yvon, Kyoto, Japan) apparatus. Gas chromatographic (GC) was made using a FISONS Instruments GC 8000 series gas chromatograph equipped with a flame ionization (FID) detector and a capillary column (DB-WAX, column length of 30 m; column internal diameter of 0.32 mm) and run by the software Jasco-Borwin v.1.50 (Jasco, Tokyo, Japan). Helium was the carrier gas and the temperature of injection was 240 ◦C. After injection, the temperature was maintained at 140 ◦C for 1 min, then raised by 10 ◦C/min until 220 ◦C was reached and conditions were held for 1 min at this temperature. All products were identified by comparison of their retention times with a known reference.

3 3.1. Synthesis of Magnetic Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] Composite 3 The synthesis of Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] composite was performed by following a very recently reported [26] procedure. Briefly, a mixture of two salts ((NH ) Fe(SO ) 6H O and FeCl ) was 4 2 4 2· 2 3 dissolved in distilled water at 50 ◦C and stirred while a solution of NaOH 1 M was added, until the pH 10 was achieved. The magnetic Fe3O4 was collected (with a magnet, after cooled down to room temperature), washed with distilled water and ethanol and dried overnight at 80 ◦C. In a second step, to ethanol, the previously synthetized Fe3O4, Ti(IV) isopropoxide and distilled water were added, and the mixture was left for two hours under reflux. As in the previous step, after the solution cooled to room temperature, the magnetic Fe3O4/TiO2 was separated, washed and dried under vacuum overnight. 3 Finally, the as-prepared Fe3O4/TiO2 and [FeCl2{κ -HC(pz)3}] were dissolved in distilled water and stirred for 24 h. The desired composite was obtained by using filtration and washed with distilled water. In order to control the presence of chlorides in the solution and understand when the compound 3 was washed enough, a silver nitrate solution was used. Finally, Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] was washed with methanol and dried under vacuum overnight.

3.2. Catalytic Oxidation of Ethane to Acetic Acid All the reactions were carried out in stainless steel autoclaves by reacting at a typical temperature 3 of 50–100 ◦C and in a selected medium (3 mL/3 mL), ethane (3–20 atm), [FeCl2{κ -HC(pz)3}], 3 Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] or FeCl2.H2O (0.02 to 0.08 mmol) and potassium peroxodisulfate (4.33 mmol) as an oxidant. The reaction mixture was stirred typically between 6 and 72 h using an oil bath. Afterwards, it was cooled, degassed, opened and the contents transferred to a Schlenk ﬂask. Diethyl ether (9.0 mL) and 90 µL of cycloheptanone (GC internal standard) were added. 3 The recyclability of catalyst Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] was investigated under the highest yield experimental reaction conditions. Each cycle was initiated after the preceding one by adding of new above-mentioned portions of all other reagents. After completion of each run, the products were separated for analysis (see above) and catalyst separated by a magnet, washed and dried overnight at 70 ◦C. Molecules 2020, 25, 5642 11 of 13

4. Conclusions This is the ﬁrst time that a C-scorpionate complex anchored onto a magnetic core-shell support, 3 Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}], was used as a catalyst for the challenging oxidation of ethane to 3 acetic acid. The use of the Fe3O4/TiO2/[FeCl2{κ -HC(pz)3}] composite led to a highly selective catalytic method for the acetic acid synthesis, where the magnetic catalyst is easily recovered and reused. In view of the promising results achieved, also attested to by the determined green metrics, further optimization studies will be attempted to help with the design of a sustainable catalytic process for this industrially important reaction.

Author Contributions: Conceptualization, A.P.C.R. and L.M.D.R.S.M.; investigation, I.A.S.M. and A.P.C.R.; writing—original draft preparation, I.A.S.M. and A.P.C.R.; writing—review and editing, L.M.D.R.S.M.; supervision, A.P.C.R. and L.M.D.R.S.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Fundação para a Ciência e Tecnologia through UIDB/00100/2020 project of Centro de Química Estrutural. APCR thanks Instituto Superior Técnico for the Scientific Employment contract IST-ID/119/2018 and IASM is thankful to FCT for her PhD fellowship (SFRH/BD/146426/2019). Conflicts of Interest: The authors declare no conflict of interest.

References

1. Global Acetic Acid Market to Reach 24.51 Million Tons by 2025. Available online: https://www. expertmarketresearch.com/pressrelease/global-acetic-acid-market (accessed on 8 October 2020). 2. Global Organic Acid Market by Region. Available online: https://www.maximizemarketresearch.com/wp- content/uploads/2019/07/Global-Organic-Acid-Market-By-Region.png (accessed on 8 October 2020). 3. Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Wiley-VCH: Weinheim, Germany, 2002. 4. Pombeiro, A.J.L.; Silva, M.F.C.G. (Eds.) Alkane Functionalization; Wiley & Sons: Hoboken, NJ, USA, 2019; Chapter 6; pp. 173–187. ISBN 978-1-119-37880-8. 5. Martins, L.M.D.R.S. C-scorpionate complexes: Ever young catalytic tools. Coord. Chem. Rev. 2019, 396, 89–102. [CrossRef] 6. Nishiguchi, T.; Nakata, K.; Takaki, K.; Fujiwara, Y. Transition metal catalyzed acetic acid synthesis from methane and CO. Chem. Lett. 1992, 21, 1141–1142. [CrossRef] 7. Silva, T.F.; Luzyanin, K.V.; Kirillova, M.V.; Silva, M.F.G.; Martins, L.M.D.R.S.; Pombeiro, A.J.L. Novel scorpionate and pyrazole dioxovanadium complexes, catalysts for carboxylation and peroxidative oxidation of alkanes. Adv. Synth. Catal. 2010, 352, 171–187. [CrossRef] 8. Kirillova, M.V.; Kirillov, A.M.; Kuznetsov, M.L.; Silva, J.A.; Silva, J.J.F.; Pombeiro, A.J. Alkanes to carboxylic acids in aqueous medium: Metal-free and metal-promoted highly eﬃcient and mild conversions. Chem. Commun. 2009, 17, 2353–2355. [CrossRef][PubMed] 9. Kirillova, M.V.; Silva, J.A.; Silva, J.J.F.; Palavra, A.; Pombeiro, A.J. Highly eﬃcient direct carboxylation of propane into butyric acids catalyzed by vanadium complexes. Adv. Syn. Cat. 2007, 349, 1765–1774. [CrossRef] 10. Available online: https://www.icis.com/explore/resources/news/2005/05/24/679925/saudi-arabia-s-sabic- starts-up-acetic-acid-plant-at-yanbu/ (accessed on 10 October 2020). 11. Armstrong, R.D.; Hutchings, G.J.; Taylor, S.H. An overview of recent advances of the catalytic selective oxidation of ethane to oxygenates. Catalysts 2016, 6, 71. [CrossRef]

12. Ribeiro, A.P.C.; Martins, L.M.D.R.S.; Pombeiro, A.J.L. N2O-free single-pot conversion of cyclohexane to adipic acid catalysed by an iron(II) scorpionate complex. Green Chem. 2017, 19, 1499. [CrossRef] 13. Martins, L.M.D.R.S.; Pombeiro, A.J.L. Tris(pyrazol-1yl)methane metal complexes for catalytic mild oxidative functionalizations of alkanes, alkenes and ketones. Coord. Chem. Rev. 2014, 265, 74–88. [CrossRef] 14. Silva, T.F.S.; Alegria, E.C.B.A.; Martins, L.M.D.R.S.; Pombeiro, A.J.L. Scorpionate vanadium, iron and copper complexes as selective catalysts for the peroxidative oxidation of cyclohexane under mild conditions. Adv. Synth. Catal. 2008, 350, 706–716. [CrossRef] 15. Martins, L.M.D.R.S.; Martins, A.; Alegria, E.C.B.A.; Carvalho, A.P.; Pombeiro, A.J.L. Eﬃcient cyclohexane oxidation with hydrogen peroxide catalysed by a C-scorpionate iron (II) complex immobilized on desilicated MOR zeolite. Appl. Catal. A 2013, 464, 43–50. [CrossRef] Molecules 2020, 25, 5642 12 of 13

16. Martins, L.M.D.R.S.; Almeida, M.P.; Carabineiro, S.A.C.; Figueiredo, J.L.; Pombeiro, A.J.L. Heterogenisation of a C-scorpionate feii complex on carbon materials for cyclohexane oxidation with hydrogen peroxide. ChemCatChem 2013, 5, 3847–3856. [CrossRef] 17. Martins, L.M.D.R.S.; Ribeiro, A.P.C.; Carabineiro, S.A.C.; Figueiredo, J.L.; Pombeiro, A.J.L. Highly efficient and reusable CNT supported iron(II) catalyst for microwave assisted alcohol oxidation. Dalton Trans. 2016, 45, 6816. [CrossRef] 18. Ribeiro, A.P.C.; Martins, L.M.D.R.S.; Carabineiro, S.A.C.; Buijnsters, J.G.; Figueiredo, J.L.; Pombeiro, A.J.L. Heterogenised C-scorpionate iron(II) complex on nanostructured carbon materials as catalysts for microwave-assisted oxidation reactions. ChemCatChem 2018, 10, 1821–1828. [CrossRef] 19. Van-Dúnem, V.; Carvalho, A.P.; Martins, L.M.D.R.S.; Martins, A. Improved cyclohexane oxidation catalyzed by a heterogenised iron(II) complex on hierarchical Y zeolite through surfactant mediated technology. ChemCatChem 2018, 10, 4058–4066. [CrossRef] 20. Ribeiro, A.P.C.; Matias, I.A.S.; Alegria, E.C.B.A.; Ferraria, A.M.; Rego, A.M.B.; Pombeiro, A.J.L.; Martins, L.M.D.R.S. New trendy magnetic C-scorpionate iron catalyst and its performance towards cyclohexane oxidation. Catalysts 2018, 8, 69. [CrossRef] 21. Matias, I.A.S.; Ribeiro, A.P.C.; Alegria, E.C.B.A.; Pombeiro, A.J.L.; Martins, L.M.D.R.S. C-scorpionate iron(II) complexes as highly selective catalysts for the hydrocarboxylation of cyclohexane. Inorg. Chim. Acta 2019, 489, 269–274. [CrossRef] 22. Ribeiro, A.P.C.; Martins, L.M.D.R.S.; Kuznetsov, M.L.; Pombeiro, A.J.L. Tuning cyclohexane oxidation: Combination of microwave irradiation and ionic liquid with the C-scorpionate [FeCl2(Tpm)] catalyst. Organometallics 2016, 36, 192–198. [CrossRef] 23. Ribeiro, A.P.C.; Martins, L.M.D.R.S.; Pombeiro, A.J.L. Carbon dioxide-to-methanol single-pot conversion using a C-scorpionate iron(II) catalyst. Green Chem. 2017, 19, 4811–4815. [CrossRef] 24. Andrade, M.A.; Mestre, A.S.; Carvalho, A.P.; Pombeiro, A.J.L.; Martins, L.M.D.R.S. The role of nanoporous carbon materials in catalytic cyclohexane oxidation. Catal. Today 2019, 357, 46–55. [CrossRef] 25. Trofimenko, S. Scorpionates: The Coordination Chemistry of Polypyrazolylborate Ligands; Imperial College Press: London, UK, 1999. 26. Matias, I.A.S.; Ribeiro, A.P.C.; Ferraria, A.M.; Rego, A.M.B.; Martins, L.M.D.R.S. Catalytic performance of a magnetic core-shell Iron(II) C-scorpionate under unconventional oxidation conditions. Nanomaterials 2020, 10, 2111. [CrossRef] 27. Baig, R.B.N.; Varma, R.S. Organic synthesis via magnetic attraction: Benign and sustainable protocols using magnetic nanoferrites. Green Chem. 2013, 15, 398–417. [CrossRef] 28. Munz, M.; Strassner, T. Alkane C–H Functionalization and Oxidation with Molecular Oxygen. Inorg. Chem. 2015, 54, 5043–5052. [CrossRef] 29. Alegria, E.C.; Kirillova, M.V.; Martins, L.M.D.R.S.; Pombeiro, A.J.L. Pyrazole and trispyrazolylmethane rhenium complexes as catalysts for ethane and cyclohexane oxidations. Appl. Catal. A 2007, 317, 43–52. [CrossRef] 30. Kirillov, A.M.; Haukka, M.; Kirillova, M.V.; Pombeiro, A.J.L. Single-pot ethane carboxylation catalyzed by new oxorhenium(V) complexes with N,O ligands. Adv. Synth. Catal. 2005, 347, 1435–1446. [CrossRef] 31. Kirillova, M.V.; Kirillov, A.M.; Reis, P.M.; Silva, J.A.; Silva, J.J.F.; Pombeiro, A.J.L. Group 5–7 transition metal oxides as efficient catalysts for oxidative functionalization of alkanes under mild conditions. J. Catal. 2007, 248, 130–136. [CrossRef] 32. Kirillova, M.V.; Silva, J.A.; Silva, J.J.F.; Pombeiro, A.J.L. Direct and efficient transformation of gaseous alkanes into carboxylic acids catalyzed by vanadium containing heteropolyacids. Appl. Catal. A 2007, 332, 159–165. [CrossRef] 33. Takanabe, K.; Iglesia, E. Mechanistic aspects and reaction pathways for oxidative coupling of methane on

Mn/Na2WO4/SiO2 catalysts. J. Phys. Chem. C 2009, 113, 10131–10145. [CrossRef] 34. Shul’pina, L.S.; Kirillova, M.V.; Pombeiro, A.J.L.; Shul’pin, G.B. Alkane oxidation by the H2O2-NaVO3-H2SO4 system in acetonitrile and water. Tetrahedron 2009, 65, 2424–2429. [CrossRef] 35. Tse, C.-W.; Chow, T.W.-S.; Guo, Z.; Lee, H.K.; Huang, J.-S.; Che, C.-M. Nonheme iron mediated oxidation of light alkanes with oxone: Characterization of reactive oxoiron(IV) ligand cation radical intermediates by spectroscopic studies and dft calculations. Angew. Chem. Int. Ed. 2014, 53, 798–803. [CrossRef] Molecules 2020, 25, 5642 13 of 13

36. Lin, M.; Sen, A. A highly catalytic system for the direct oxidation of lower alkanes by dioxygen in aqueous medium. A formal heterogeneous analog of alkane monooxygenases. J. Am. Chem. Soc. 1992, 114, 7307–7308. [CrossRef] 37. Shul’pin, G.B.; Sooknoi, T.; Romakh, V.B.; Süss-Fink, G.; Shul’pina, L.S. Regioselective alkane oxygenation

with H2O2 catalyzed by titanosilicalite Ts-1. Tetrahedron Lett. 2006, 47, 3071–3075. [CrossRef] 38. Alvarez, L.X.; Sorokin, A.B. Mild oxidation of ethane to acetic acid by H2O2 catalyzed by supported µ-nitrido diiron phthalocyanines. J. Organomet. Chem. 2015, 793, 139–144. [CrossRef] 39. Rahman, A.K.M.L.; Indo, R.; Hagiwara, H.; Ishihara, T. Direct conversion of ethane to acetic acid over

H-ZSM-5 using H2O2 in aqueous phase. Appl. Catal. A Gen. 2013, 456, 82–87. [CrossRef] 40. Anastas, P.T. Handbook of Green Chemistry; Constable, D.J.C., Gonzales, C.J., Eds.; Wiley-VCH: Weinheim, Germany, 2018; Volume 11. 41. Sheldon, R.A. The E factor: Fifteen years on. Green Chem. 2007, 9, 1273–1283. [CrossRef] 42. Phan, T.V.T.; Gallardo, C.; Mane, J. GREEN MOTION: A new and easy to use green chemistry metric from laboratories to industry. Green Chem. 2015, 17, 2846–2852. [CrossRef]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).