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