Solvent-Free Esterification of Carboxylic Acids Using Supported Iron Oxide Nanoparticles As an Efficient and Recoverable Catalys

materials

Article Solvent-Free Esteriﬁcation of Carboxylic Acids Using Supported Iron Oxide Nanoparticles as an Efﬁcient and Recoverable Catalyst

Fatemeh Rajabi 1,*, Mohammad Abdollahi 1 and Rafael Luque 2

1 Department of Science, Payame Noor University, P.O. Box 19395-4697, Tehran 19569, Iran; [email protected] 2 Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Ediﬁcio Marie Curie (C-3), Ctra Nnal IV-A, km 396, Cordoba 14014, Spain; [email protected] * Correspondence: [email protected]; Tel.: +98-281-333-6366; Fax: +98-281-334-4081

Academic Editor: Eduardo J. García-Suárez Received: 9 May 2016; Accepted: 23 June 2016; Published: 12 July 2016

Abstract: Supported iron oxide nanoparticles on mesoporous materials ([email protected]) have been successfully utilized in the esteriﬁcation of a variety carboxylic acids including aromatic, aliphatic, and long-chain carboxylic acids under convenient reaction conditions. The supported catalyst could be easily recovered after reaction completion and reused several times without any loss in activity after up to 10 runs.

Keywords: supported iron oxide nanoparticles; esteriﬁcation; carboxylic acid

1. Introduction Esters play a significant role in daily living and the chemical industry. The reaction of carboxylic acids with alcohols to form esters is among the mildest and most efficient of organic transformations, largely a consequence of the high accessibility and stability of reactants. High ester usage in the synthesis of drugs, fine chemicals, pharmaceuticals, solvents and plasticizers as intermediates makes these substrates one of the most important types of compounds in organic chemistry [1]. In this context, some protocols for the synthesis of esters are well-known, including Fisher esterifications [2] and methylation reactions [3] of carboxylic acids. Due to the wide synthetic and biological applications of esters, a number of reagents such as ortho esters [4], N,N-dimethylformamide dialkyl acetals [5], triazene derivatives [6] and O-dialkyl isoureas [7] have been reported for the esterification of various aromatic/aliphatic carboxylic acids. The reaction of carboxylic acids and alcohols in the absence of catalysts is very slow and requires a long time for the reaction to reach equilibrium. To accelerate reaction rates, a number of catalysts have been reported inthe literature. These include classical solid acids such as ion exchange resins [8–10], zeolites [11,12], super acids [13,14], heteropolyacids [15–18] and supported chlorides [19]. Metal oxides such as CaO and MgO [20], metal-layered hydroxides [21,22], and efficient enzymatic catalysts [23,24] have also been employed in esterification reactions. However, many of these methods suffer from inherent drawbacks such as the need for expensive or harmful materials as reagents and catalysts, the formation of undesired side products, highly acidic conditions, the use of hazardous and toxic solvents, high reaction temperatures, low yield of products and prolonged reaction times. A need to develop an improved catalytic system for the synthesis of esters in terms of operational simplicity and economic viability is of utmost importance. Herein, a nanomaterial based on supported iron oxide nanoparticles on SBA-15 ([email protected]) has been utilized as an efficient catalyst for a mild esterification of various carboxylic acids to their corresponding esters (Scheme1). The combination of iron nanoparticles and the mesoporous structure of the material showed excellent synergistic effects on the enhancement of

Materials 2016, 9, 557; doi:10.3390/ma9070557 www.mdpi.com/journal/materials Materials 2016, 9, 557 2 of 9 Materials 2016, 9, 557 2 of 9

synergistic effects on the enhancement of activity and stability of the catalyst. Apart from a high Materials 2016, 9, 557 2 of 9 activityactivity, and stability the successful of the recycling catalyst. of Apart this catalytic from a system high activity, allows athe more successful economicrecycling and environmentally of this catalytic Materials 2016, 9, 557 2 of 9 systemfriendly allows process a more which economic is of and special environmentally advantage for friendly large-scale process preparations which is of and special industrial advantage for large-scalesynergistic preparationseffects on the andenhancement industrial of applications. activity and stability of the catalyst. Apart from a high activity,applications.synergistic the effectssuccessful on therecycling enhancement of this catalytic of activity system and allows stability a more of the economic catalyst. and Apart environmentally from a high friendlyactivity, theprocess successful which recycling is of ofspecial this catalytic advantage system for allows large-scale a more economicpreparations and environmentallyand industrial COOH COOMe applications.friendly process which is of [email protected] advantage for large-scale preparations and industrial applications. MeOH, Reflux COOH [email protected] COOMe COOH COOMe Scheme 1. Esterification [email protected] benzoic acid with FeNP supported on SBA-15. Scheme 1. Esterification ofMeOH, benzoic acidReflux with FeNP supported on SBA-15. MeOH, Reflux 2. Results and Discussion 2. Results and DiscussionScheme 1. Esterification of benzoic acid with FeNP supported on SBA-15. The catalyticScheme performance 1. Esterification of suppo ofrted benzoic iron acid oxide with nanoparticles FeNP supported has on been SBA-15. previously reported The2.by Results our catalytic research and performanceDiscussion group [25–27]. of supported In continuation iron oxide of nanoparticlesour previous study has been on previouslythe application reported of by our research2. Results group and Discussion [25–27]. In continuation of our previous study on the application of [email protected] [email protected] catalytic as aperformance recoverable catalystof suppo [26],rted we iron found oxide that nanoparticles the esterification has been of carboxylic previously acids reported in the as a recoverablepresence of [email protected] catalyst [26], as we an found effective that catalyst the esterification has not been investigated of carboxylic yet acids. Hence, in thewe decided presence of by ourThe research catalytic groupperformance [25–27]. of suppoIn continuationrted iron oxide of our nanoparticles previous studyhas been on previouslythe application reported of [email protected]@SBA-15toby investigateour research as anthe as effectiveacatalyticgroup recoverable [25–27]. effect catalyst catalyst of [email protected] has continuation [26], not we been found as investigated of a that promoterour the previous esterification yet.system Hence,study on theof on wecarboxylic rate the decided and application efficiency acids to investigate in the of the [email protected] effectof [email protected] of as of carboxylic a [email protected] recoverable as acids. an catalyst effective The as a material[26], promoter catalyst we [email protected] has system notthat been the on esterificationthe investigatedhas rate been and previously of yet efficiency carboxylic. Hence, described ofwe acids esterification decided in and the of carboxylictocharacterizedpresence investigate acids.of [email protected] theby The catalytica series material ofeffect as techniques an [email protected] ofeffective [email protected] includ catalysting has ashas Inductively beena notpromoter previouslybeen investigatedcoupled system described onplasma/Mass the yet rate. Hence, and and spectrometry characterizedefficiency we decided of by a seriesesterification(ICP/MS),to investigate of techniques X-ray ofthe carboxylic diffractioncatalytic including effect acids. (XRD), Inductively of The [email protected] Scanning material coupled [email protected] as a plasma/Masspromoter microscopy has system been (SEM), spectrometry on previously the Transmission rate and described (ICP/MS), efficiency electron and of X-ray diffractioncharacterizedmicroscopyesterification (XRD), (TEM) ofby Scanning carboxylica andseries X-ray electronof acids. techniques photoe microscopyThelectron material includ spectroscopying (SEM),[email protected] Inductively Transmission (XPS) coupled[27].has been electron plasma/Mass previously microscopy described spectrometry (TEM) and and X-ray(ICP/MS),characterized photoelectronTEM imagesX-ray by spectroscopy diffractionaof series the catalyst of (XRD),techniques (XPS)indicated Scanning [27 includ]. that ingtheelectron ironInductively oxidemicroscopy nanoparticle coupled (SEM), plasma/Mass sizes Transmission were inspectrometry the electron 5–7 nm range, with an excellent homogeneous dispersion of the iron oxide nanoparticles on the support TEMmicroscopy(ICP/MS), images X-ray (TEM) of thediffraction and catalyst X-ray (XRD),photoe indicated lectronScanning that spectroscopy theelectron iron oxidemicroscopy (XPS) nanoparticle[27]. (SEM), Transmission sizes were in electron the 5–7 nm (Figure 1). Fe species in the synthesized materials as measured by ICP/MS were found to be around range,microscopy withTEM an images excellent(TEM) of and the homogeneous X-ray catalyst photoe indicatedlectron dispersion that spectroscopy the iron of theoxide (XPS) iron nanoparticle [27]. oxide nanoparticles sizes were in onthe the5–7 supportnm 0.5–0.6 wt. %, with average iron oxide nanoparticle sizes in the 5–8 nm range. XRD of the materials (Figurerange,1).TEM Fe with species images an excellent inof thethe synthesizedcatalysthomogeneous indicated materials dispersion that the as ironof measured the oxide iron nanoparticle oxide by ICP/MS nanoparticles sizes were were found on in the the to support 5–7 be aroundnm (Figureconfirmedrange, with1). theFe an species presence excellent in ofthe homogeneous the synthesized hematite phase materialsdispersion (Fe2 Oas 3of, measuredJCPDS the iron card byoxide 39-0664) ICP/MS nanoparticles forwere [email protected] found on to the be support around(Figure 0.5–0.6 wt. %, with average iron oxide nanoparticle sizes in the3+ 5–8 nm range. XRD of the materials 0.5–0.62),(Figure which wt. 1). was %,Fe withspeciesalso confirmedaverage in the ironsynthesized by oxideXPS measurements nanoparticle materials as sizes (typicalmeasured in theFe by5–8 bands ICP/MS nm atrange. BE were 714 XRD foundeV(Fe2p of theto 3/2be materials) andaround 725 confirmedeV (Fe2p the1/2 presence), Figure 3), of thewith hematite only a very phase minor (Fe contribution2O3, JCPDS (<2%) card 39-0664)of zerovalent for [email protected] Fe. (Figure2), confirmed0.5–0.6 wt. the%, withpresence average of the iron hematite oxide nanoparticlephase (Fe2O3 ,sizes JCPDS in3+ thecard 5–8 39-0664) nm range. for [email protected] XRD of the materials (Figure which was also confirmed by XPS measurements (typical Fe bands at BE 714 eV(Fe2p3/2) and 725 eV 2),confirmed which was the alsopresence confirmed of the by hematite XPS measurements phase (Fe2O3 ,(typical JCPDS Fecard3+ bands 39-0664) at BE for 714 [email protected] eV(Fe2p3/2) and (Figure 725 (Fe2p1/2), Figure3), with only a very minor contribution (<2%)3+ of zerovalent Fe. eV2), which(Fe2p1/2 was), Figure also confirmed 3), with only by XPSa very measurements minor contribution (typical (<2%) Fe bands of zerovalent at BE 714 Fe. eV(Fe2p 3/2) and 725 eV (Fe2p1/2), Figure 3), with only a very minor contribution (<2%) of zerovalent Fe.

Figure 1. TEM micrograph of [email protected] material.

Figure 1. TEM micrograph of [email protected] material. FigureFigure 1. 1.TEM TEM micrograph micrograph of [email protected] material. material.

Figure 2. XRD pattern of [email protected] Bottom lines correspond to the JCPDS 39-0664 card of hematite phase Fe2O3.

Figure 2. XRD pattern of [email protected] Bottom lines correspond to the JCPDS 39-0664 card of Figure 2. XRD pattern of [email protected] Bottom lines correspond to the JCPDS 39-0664 card of Figurehematite 2. XRD phase pattern Fe2O3.of [email protected] Bottom lines correspond to the JCPDS 39-0664 card of hematite phase Fe2O3. hematite phase Fe O . 2 3 Materials 2016, 9, 557 3 of 9 Materials 2016, 9, 557 3 of 9

FigureFigure 3.3. XPS spectra ofof Fe2pFe2p ((leftleft panelpanel)) andand surveysurvey ((rightright panelpanel)) ofof [email protected]@SBA-15.

The influence of [email protected] in the esterification of benzoic acid with methanol was firstly The influence of [email protected] in the esterification of benzoic acid with methanol was firstly investigated (Table 1). Blank runs indicated that the esterification of benzoic acid with methanol in the investigated (Table1). Blank runs indicated that the esterification of benzoic acid with methanol absence of [email protected] did not take place at 60 °C (Table 1, Entry 1). A simple addition of 0.3 mol. % in the absence of [email protected] did not take place at 60 ˝C (Table1, Entry 1). A simple addition [email protected] 0.3 mol. % [email protected] as catalyst to as the catalyst mixture to theof benz mixtureoic ofacid benzoic (1 mmol) acid (1and mmol) methanol and methanol (2 mmol) (2 already mmol) alreadyprovided provided a low ayield low yieldof methyl of methylbenzoatebenzoate (10%) (10%) at room at room temperature temperature (Table (Table 1,1 ,Entry 2).2). TheThe optimizationoptimization ofof the the reaction reaction conditions conditions eventually eventually led led to quantitativeto quantitative yields yields of the of corresponding the corresponding ester (Tableester (Table1, Entry 1, 4).Entry Eventually, 4). Eventually the reaction, the reaction of benzoic of benzoic acid (1acid mmol) (1 mmol) with with methanol methanol (2 mmol) (2 mmol) in the in presencethe presence of 0.1 of mol. 0.1 %mol. [email protected] % [email protected] under methanolunder methanol reflux was reflux selected was asselected the optimum as the conditionoptimum forcondition the esterification for the esterification reaction due reaction to the highdue to efficiency the high and efficiency short reaction and short times reaction (typically, times 6 (typically, h, Table1, Entry6 h, Table 8). 1, Entry 8).

Table 1.1. ScreeningScreening ofof reactionreaction conditionsconditions inin thethe esteriﬁcationesterification ofofbenzoic benzoicacid acidwith withmethanol methanola a..

Entry FeNP (mol. %) Time (h) T (°C) Yield (%) b Entry FeNP (mol. %) Time (h) T (˝C) Yield (%) b 1 - 10 60 - 12 -0.3 1010 r.t. 60 10 - 2 0.3 10 r.t. 10 33 0.30.3 1010 40 40 22 22 44 0.30.3 1010 reflux reflux 99 99 55 0.20.2 1010 reflux reflux 99 99 66 0.10.1 1010 reflux reflux 99 99 7 0.07 10 reflux 48 87 0.10.07 610 reflux reflux 48 99 98 0.10.1 46 reflux reflux99 91 a All reactions9 were carried0.1 out with 1 mmol of benzoic4 acid andreflux 2 mmol MeOH; b Isolated91 yield. a All reactions were carried out with 1 mmol of benzoic acid and 2 mmol MeOH; b Isolated yield. The efficiency and scope of the present protocol was further extended to a broad range of The efficiency and scope of the present protocol was further extended to a broad range of aromatic and aliphatic carboxylic acids containing electron-donating or -withdrawing groups with methanolaromatic and and aliphatic ethanol ascarboxylic esterifying acids reagents containing under electron-donating optimized reaction or -withdrawing conditions. As groups shown with in Tablemethanol2, most and carboxylic ethanol as acids esterifying underwent reagents esterification under optimized to afford reaction the corresponding conditions. estersAs shown in excellent in Table yields2, most(88% carboxylic to 99%). acidsThe underwent introduction esterification of substituents to afford often the changes corresponding the activity esters of thein excellent aromatic yields ring but(88% changing to 99%). the The aromatic introduction substitution of substituents from an electron-donating often changes the group activity to anof electron-withdrawingthe aromatic ring but groupchanging did the not aromatic significantly substitution influence from yields an to elec productstron-donating as clearly group indicated to an in electron-withdrawing Table2. Our system exhibitedgroup did an not almost significantly analogous influence efficiency yields towards to prod bothucts activated as clearly and non-activatedindicated in Table aromatic 2. Our carboxylic system acidsexhibited (Table an2, Entriesalmost 2analogous and 6). The efficiencyα,β-unsaturated towards carboxylic both activated acids were and also non-activated efficiently esterified aromatic to thecarboxylic corresponding acids (Table esters 2, without Entries any2 and observable 6). The α,β reaction-unsaturated at the carboxylic double bond acids (Table were2, Entriesalso efficiently 16–18). Theesterified reaction to ofthe aliphatic corresponding carboxylic este acidsrs without with alcohols any observable in this work reaction did notat the show double obvious bond differences (Table 2, Entries 16–18). The reaction of aliphatic carboxylic acids with alcohols in this work did not show obvious differences and the corresponding esters were also obtained in high yields (Table 2, Entries

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Product12). werewere ions ions usinglessusingless (Este (Este (Tablereactive(Tablereactive supportedsupportedrr)) 2,2, (in Entry(inEntry ironiron comparisoncomparison 8). 8). oxideoxide Alcohol Alcohol Interestingly,Interestingly, nanoparticlesnanoparticles withwith unhindered)unhindered)YieldYield aa biomass- biomass- aa.. (%)(%) bb derivedderived platformplatform chemicalchemical suchsuch asas succinicsuccinic acidacid (a(a C4C4 diacid)diacid) couldcould bebe efficientlyefficiently esterifiedesterifiedaa toto chemical suchdimethylindimethylin thethe as catalyticcatalyticTable succinic succinatesuccinate 2. systemEsterificationEsterificationsystem acid inin high underhighunder (a yieldof C4yieldof optimum optimum carboxyliccarboxylic diacid) (Table(Table reaction reaction acid2,acid2, could EntryEntry derivativesderivatives condit condit 12). be12). efficientlyions ions usingusing (Table(Table supportedsupported esterified 2,2, EntryEntry ironiron 8). 8). oxideoxide Interestingly,Interestingly, to nanoparticlesnanoparticles dimethyl aa biomass- succinatebiomass- .. bb in high derivedderivedEntryEntry platformplatformCarboxylicCarboxylic chemicalchemical AcidAcid suchsuch asas succinicsuccinic acidacid Product Product (a(a C4C4 (Este (Este diacid)diacid)rr)) couldcould bebe Alcohol Alcohol efficientlyefficiently esterifiedesterifiedYieldYieldaa (%)(%) toto indimethylindimethyl thethe catalyticcatalyticTableTable succinatesuccinate 2.2. system EsterificationsystemEsterification inin high underhighunder yield ofyieldof optimumoptimum carboxyliccarboxylic (Table(Table reactionreaction acid2,acid2, EntryEntry derivativesderivatives conditcondit 12).12). ions ions usingusing (Table(Table supportedsupported 2,2, EntryEntry ironiron 8).8). oxideoxide Interestingly,Interestingly, nanoparticlesnanoparticles aa biomass- biomass- .. bb derivedderivedEntryEntry platformplatformCarboxylicCarboxylic chemicalchemicalCOOCOO AcidAcid suchsuchHH asas succinicsuccinic acidacid Product Product (a(a COOMeCOOMeC4C4 (Este (Este diacid)diacid)rr)) couldcould bebe Alcohol Alcohol efficientlyefficiently esterifiedesterifiedYieldYieldaa (%)(%) toto yield (TabledimethyldimethylEntry2, Entry TableTable succinatesuccinate 12). 2.2. EsterificationEsterificationCarboxylic inin highhigh Acid yieldofyieldof carboxyliccarboxylic (Table(Table acid 2,acid2, EntryEntry derivativesderivatives Product 12).12). usingusing (Este supportedrsupported) ironiron oxideoxide Alcohol nanoparticlesnanoparticles Yield .. (%) bb derivedderivedEntry platformplatformCarboxylic chemicalchemicalCOOCOO Acid suchsuchHH asas succinicsuccinic acidacid Product (a(a COOMeCOOMeC4C4 (Este diacid)diacid)r) couldcould bebe Alcohol efficientlyefficiently esterifiedesterifiedYieldaa (%) toto dimethyldimethylEntryEntry11 TableTable succinatesuccinate 2.2. EsterificationEsterificationCarboxylicCarboxylic inin highhigh AcidAcid yieldofyieldof carboxyliccarboxylic (Table(Table acid 2,acid2, EntryEntry derivativesderivatives Product12). Product12). usingusing (Este (Este supportedsupportedrr)) ironiron oxideoxide AlcoholCH AlcoholCH33OHOH nanoparticlesnanoparticles YieldYield 99.99. (%)(%) bb COOCOOHH COOMeCOOMe aa bb dimethyldimethylEntryEntry1 TableTable succinatesuccinate 2.2. EsterificationEsterificationCarboxylicCarboxylic inin highhigh AcidAcid yieldofyieldof carboxyliccarboxylic (Table(Table acid2,acid2, EntryEntry derivativesderivatives Product12). Product12). usingusing (Este (Este supportedsupportedrr)) ironiron oxideoxide AlcoholCH Alcohol33OH nanoparticlesnanoparticles YieldYield 99.. (%)(%) 1 COOH COOMe CH OH aa99 11 TableTable 2.2. EsterificationEsterification ofof carboxyliccarboxylic acidacid derivativesderivatives usingusing supportedsupported ironiron oxideoxideCHCH33OHOH nanoparticlesnanoparticles 99.99. bb a TableEntryEntry 2. Esterification CarboxylicCarboxylic ofCOO carboxylicCOO AcidAcidHH acid derivatives Product ProductCOOMe usingCOOMe (Este (Esterr)) supported iron Alcohol Alcohol oxide nanoparticlesYieldYieldaa (%)(%) . 11 TableTable 2.2. EsterificationEsterification ofof carboxyliccarboxylic acidacid derivativesderivatives usingusing supportedsupported ironiron oxideoxideCH33OH nanoparticlesnanoparticles 99.. bb EntryEntry CarboxylicCarboxylicCOOCOO AcidAcidHH Product ProductCOOMeCOOMe (Este (Esterr)) Alcohol Alcohol YieldYielda (%)(%) EntryEntry11 TableTable 2.2. EsterificationEsterificationCarboxylicCarboxylicCOOCOO AcidAcid COOHofCOOHof H Hcarboxyliccarboxylic acidacid derivativesderivatives Product Product COOMeusingusingCOOMe (Este (EsteCOOMeCOOMe supportedsupportedrr)) ironiron oxideoxide AlcoholCH AlcoholCH33OH OH nanoparticlesnanoparticles YieldYield a99.99. (%)(%) bb EntryEntry11 CarboxylicCarboxylicCOOCOO AcidAcidCOOHCOOHHH Product ProductCOOMeCOOMe (Este (EsteCOOMeCOOMerr)) AlcoholCH AlcoholCH33OHOH YieldYield9999 (%)(%) bb EntryEntryEntry1212 CarboxylicCarboxylicCarboxylic AcidAcid Acid Product Product Product (Este(Ester) (Esterr)) AlcoholCH AlcoholCH33OHOH YieldYield99989998 (%)Yield(%) bb (%) b COOCOOCOOHCOOHHH COOMeCOOMeCOOMeCOOMe 2112 COOCOOCOOHHH COOMeCOOMeCOOMe CHCH33OHOH 98999998 MeOMeO MeOMeO 33 2121 COOCOOCOOHCOOHHH COOMeCOOMeCOOMeCOOMe CHCH33OHOH 98999899 2121 MeOMeO COOHCOOH MeOMeO COOMeCOOMe CHCH333OHOH 989999

1 1212 MeOMeO COOHCOOHCOOHCOOH MeOMeO COOMeCOOMeCOOMeCOOMe CHCH33OHOHOH 99989998 99 22 MeO COOHCOOHCOOHCOOH MeO COOMeCOOMeCOOMeCOOMe CHCH33OHOH 9898 2323 MeOMeO COOH MeOMeO COOMe CHCH33OHOH 98979897 MeOMeO COOHCOOHCOOH MeOMeO COOMeCOOMeCOOMe 3223 COOHCOOHCOOH COOMeCOOMeCOOMe CHCH33OHOH 97989897 MeOMeOII MeOMeOII 33 3232 COOHCOOHCOOHCOOH COOMeCOOMeCOOMeCOOMe CHCH33OHOH 97989798 2 3232 MeOMeOII MeOMeOII CHCH333OHOHOH 979898 98 MeOMeO COOHCOOH MeOMeO COOMeCOOMe 3 2323 II COOHCOOHCOOHCOOH II COOMeCOOMeCOOMeCOOMe CHCH33OHOH 98979897 33 MeOMeOII MeOMeOII CHCH33OHOH 9797 COOHCOOHCOOHCOOH COOMeCOOMeCOOMeCOOMe 33 MeOMeOII COOH MeOMeOII COOMe CHCH33OHOH 9797 44 II COOHCOOHCOOH II COOMeCOOMeCOOMe CHCH33OHOH 9696 33 COOHCOOHCOOH COOMeCOOMeCOOMe CHCH33OHOH 9797 44 CHCH33OHOH 9696 3 33 II COOHCOOHCOOH II COOMeCOOMeCOOMeCOOMe CHCH3OHOH 9797 97 4 COOH CH33OH 96 3 433 II COOHCOOH II COOMeCOOMe CHCH3OHOH 969797 33 4343 II II CHCH33OHOH 969797

COOHCOOH COOMeCOOMe 44 II II CHCH33OHOH 9696

COOHCOOH COOMeCOOMe 44 II II CHCH33OHOH 9696 COOHCOOH COOMeCOOMe 44 COOHCOOH COOMeCOOMe CHCH33OHOH 9696

55 COOHCOOH COOMeCOOMe CHCH OHOH 9090 44 CHCH33OHOH 9696 4 CH OH 96 COOHCOOH COOMeCOOMe 55 COOH COOMe CHCH33OHOH 9090 44 COOHCOOHCOOH COOMeCOOMeCOOMe CHCH33OHOH 9696 55 CHCH33OHOH 9090 44 COOH COOMe CHCH33OHOH 9696

55 CH33OH 90 44 COOHCOOH COOMeCOOMe CHCH33OHOH 9696

55 COOHCOOH COOMeCOOMe CHCH33OHOH 9090 COOHCOOH COOMeCOOMe

55 CHCH33OHOH 9090 66 COOHCOOH COOMeCOOMe CHCH33OHOH 9494

55 CHCH33OHOH 9090 5 COOHCOOH COOMeCOOMe CH3OH 90 656 CHCH33OHOH 949094 5 FF COOHCOOH FF COOMeCOOMe CH OH 90 6565 CHCH333OHOH 94909490 COOHCOOH COOMeCOOMe 6565 FF COOHCOOH FF COOMeCOOMe CHCH333OHOH 949090 COOHCOOH COOMeCOOMe 5656 FF COOHCOOHCOOHCOOH FF COOMeCOOMeCOOMCOOMe e CHCH33OHOH 90949094 66 F F CHCH33OHOH 9494 6 COOHCOOHCOOHCOOH COOMeCOOMeCOOMCOOMe e CH3OH 94 6767 FF FF CHCH33OHOH 94959495 FF COOHCOOHCOOH FF COOMeCOOMeCOOMCOOMe e 7667 COOHCOOHCOOH COOMeCOOMeCOOMee CHCH33OHOH 95949495 BBFrr BBFFrr 33 7676 F COOHCOOHCOOHCOOH COOMeCOOMeCOOMCOOMe e CHCH33OHOH 95949594 7676 BBFFrr COOHCOOH BBFFrr COOMCOOMe e CHCH333OHOH 959494 FF FF 7 6767 BBrr COOHCOOHCOOHCOOH BBrr COOMeCOOMeCOOMCOOMee CHCH33OHOHOH 94959495 95 FF FF 3 77 Brr COOHCOOHCOOHCOOH Brr COOMeCOOMeCOOMCOOMee CHCH3OHOH 9595 7878 BBFFrr BBFFrr CHCH33OHOH 95889588 BBrr COOHCOOHCOOHCOOH BBrr COOMeCOOMeCOOMCOOMee 8778 COOHCOOHCOOH COOMeCOOMCOOMee CHCH33OHOH 88959588 Br OOMMee BBrr OOMMee 33 8787 Br COOHCOOHCOOHCOOH COOMeCOOMeCOOMCOOMee CHCH33OHOH 88958895 8 8787 BBrr OCOOHOCOOHMMee BBrr OCOOMeOMMee CHCH333OHOHOH 889595 88 Br BBrr COOMe 3 7878 Br OCOOHCOOOCOOCOOHMMeeHH OCOOMeOCOOMeMMee CHCH33OHOH 95889588 BBrr BBrr 3 88 OCOOCOOHCOOHCOOMeeHH OCOOMeCOOMeMee CHCH3OHOH 8888 88 BBrr OOMMee BBrr OOMMee CHCH33OHOH 8888 OCOOCOOHOCOOCOOHMMeeHH OCOOMeOCOOMeMMee 8998 COOCOOHCOOHH COOMeCOOMe CHCH33OHOH 88909088 88 OOMMee OOMMee CHCH33OHOH 8888 99 COOCOOHCOOHCOOHH COOMeCOOMe CHCH33OHOH 9090 33 898 OOMMee OOMMee CHCH33OHOH 889088 9 9 COOOCOOMeHH COOMeOCOOMeMe CH 3OHOH 90 90 9898 OOMMee OMe OOMMee OMe CHCH33OHOH 908888 OCOOCOOOMMeeHH OCOOMeCOOMeOMMee 99 OOMMee COOOCOOMeHH OOMMee COOMeOCOOMeMe CHCH33OHOH 9090 99 OOMMeeOOOCOOCOOMeHH OOMMeeOOOCOOMeCOOMeMe CHCH33OHOH 9090 99 OMeeOOCOOCOOHH OMeeOOCOOMeCOOMe CHCH33OHOH 9090 101099 OOMMee COOCOOHH OOMMee COOMeCOOMe CHCH33OHOH 90939390 99 OOMMeeOO OHOH OOMMeeOO OMeOMe CHCH33OHOH 9090 10 1010 O O CHCH33OHOHOH 9393 93 99 OOMMe16e16 OOMMe16e16 CHCH33OHOH 9090 1010 OO OHOH OO OMeOMe CHCH33OHOH 9393 101099 OOMMe16e16O OHOH OOMMe16e16 OMeOMe CHCH33OHOH 939090 OMeO OMeOO 1010 OM16e16 OH OM16e16 OMe CHCH33OHOH 9393 OOMM16e16eOO OOMMe1616eOO MaterialsMaterials1010 20162016,, 99,, 557557 OO OHOH OO OMeOMe CHCH33OHOH 9393 55 ofof 99 MaterialsMaterials 20162016,, 99,, 557557 OOMMe16e16OO OHOH OOMMe16e16OO OMeOMe 55 ofof 99 11 10111011 1616OO 1616OO CHCH33OHOHOH 93959395 95 1010 OHOH OMeOMe CHCH33OHOH 9393 1111 OO OO CHCH33OHOH 9595 1010 16141614 OHOH 16141614 OMeOMe CHCH33OHOH 9393 1111 OO OHOOHO OO OMeOMeOO CHCH33OHOH 9595 1010 1616 OHOH 1616 OMeOMe CHCH33OHOH 9393 1111 141416OO OHOHOO 141416OO OMeOMeOO CH33OH 95 16 OHOH 16 OMeOMe 3 10111011 HOHO 1414 OH MeOMeO 1414 OMe CHCH33OHOH 93959395 1212 14161416OO OHOH 14161416OO OMeOMe CHCH33OHOH 9090 1111 HOHO OO OHOHOHOH MeOMeO OO OMeOMeOMeOMe CHCH33OHOH 9595 12 1212 16141614 OHOH 16141614 OMeOMe CHCH33OHOHOH 9090 90 1111 1414OO OHOH 1414OO OMeOMe CHCH33OHOH 9595 11 OHOH OMeOMe CH33OH 95 11 OO 1414OO OO 1414OO CH OH 95 1111 OO OO OHOH OO OO OMeOMe CHCH33OHOH 9595 1111 1414 OHOH 1414 OMeOMe CHCH33OHOH 9595 COOCOO1414 HH 1414 1111 OHOH COOCOOEOMeEOMett CHCH33OHOH 9595 COOCOO1414 HHOHOH COOCOO1414 OMeEOMeEtt 1414 1414

13 1313 CHCH33CHCH22OH2OHOH 9090 90 1313 CHCH33CHCH22OHOH 9090

NONO NONO 22 22 NO NO NONO 22 22 COOHCOOH COOCOOEEtt COOHCOOH COOCOOEEtt

1414 CHCH33CHCH22OHOH 9292 1414 CHCH33CHCH22OHOH 9292

FF FF FF FF COOHCOOH COOCOOEEtt COOHCOOH COOCOOEEtt 1515 CHCH33CHCH22OHOH 9494 1515 CHCH33CHCH22OHOH 9494

OO OO OO OO 1616 OHOH OO CHCH33CHCH22OHOH 9595 1616 OHOH OO CHCH33CHCH22OHOH 9595

OO OO OO OO 1717 OHOH OO (CH(CH33))22CHOHCHOH 9595 1717 OHOH OO (CH(CH33))22CHOHCHOH 9595

OO OO OO OO 1818 OHOH OO CHCH33OHOH 9898 1818 OHOH OO CHCH33OHOH 9898

aa AllAll reactionsreactions werewere carriedcarried outout withwith thethe molarmolar ratioratio ofof substrate/ROHsubstrate/ROH (1:2)(1:2) inin thethe presencepresence ofof aa AllAll reactionsreactions werewere carriedcarried outout withwith thethe molarmolar ratioratio ofof substrate/ROHsubstrate/ROH (1:2)(1:2) inin thethe presencepresence ofof supportedsupported ironiron oxideoxide nanoparticlesnanoparticles (0.1(0.1 mol.mol. %)%) atat refluxreflux conditionsconditions forfor 66 h;h; bb IsolatedIsolated yields.yields. supportedsupported ironiron oxideoxide nanoparticlesnanoparticles (0.1(0.1 mol.mol. %)%) atat refluxreflux conditionsconditions forfor 66 h;h; bb IsolatedIsolated yields.yields. TheThe solventlesssolventless reactionreaction waswas alsoalso performedperformed withwith solidsolid phasephase alcohols.alcohols. AsAs example,example, thethe reactionreaction TheThe solventlesssolventless reactionreaction waswas alsoalso performedperformed withwith solidsolid phasephase alcohols.alcohols. AsAs example,example, thethe reactionreaction betweenbetween benzoicbenzoic acidacid andand 4-chlorobenzyl4-chlorobenzyl alcoholalcohol (1:1)(1:1) underunder optimizedoptimized conditionsconditions forfor 1212 hh couldcould betweenbetween benzoicbenzoic acidacid andand 4-chlorobenzyl4-chlorobenzyl alcoholalcohol (1:1)(1:1) underunder optimizedoptimized conditionsconditions forfor 1212 hh couldcould provideprovide anan isolatedisolated esterester yieldyield ofof 55%.55%. TheThe presenpresencece ofof thethe organicorganic ligandligand graftedgrafted onon thethe SBA-15SBA-15 provideprovide anan isolatedisolated esterester yieldyield ofof 55%.55%. TheThe presenpresencece ofof thethe organicorganic ligandligand graftedgrafted onon thethe SBA-15SBA-15 surfacesurface diddid notnot seemseem toto havehave anyany eeffectffect onon thethe catalyticcatalytic activityactivity inin thethe systems,systems, withwith aa negligiblenegligible surfacesurface diddid notnot seemseem toto havehave anyany eeffectffect onon thethe catalyticcatalytic activityactivity inin thethe systems,systems, withwith aa negligiblenegligible esterificationesterification activityactivity observedobserved forfor aminopropyaminopropyl-functionalizedl-functionalized SBA-15SBA-15 (in(in thethe absenceabsence ofof FeFe22OO33 NPs).NPs). esterificationesterification activityactivity observedobserved forfor aminopropyaminopropyl-functionalizedl-functionalized SBA-15SBA-15 (in(in thethe absenceabsence ofof FeFe22OO33 NPs).NPs). TheThe esterificationesterification reactionreaction ofof benzoicbenzoic acidacid withwith methanolmethanol catalyzedcatalyzed byby severalseveral differentdifferent catalystscatalysts TheThe esterificationesterification reactionreaction ofof benzoicbenzoic acidacid withwith methanolmethanol catalyzedcatalyzed byby severalseveral differentdifferent catalystscatalysts reportedreported inin thethe literatureliterature hashas beenbeen summarizedsummarized inin TableTable 3.3. AsAs cancan bebe seen,seen, thethe catalyticcatalytic performanceperformance reportedreported inin thethe literatureliterature hashas beenbeen summarizedsummarized inin TableTable 3.3. AsAs cancan bebe seen,seen, thethe catalyticcatalytic performanceperformance ofof [email protected]@SBA-15 waswas remarkablyremarkably improvedimproved asas comparedcompared toto datadata reportedreported inin thethe literatureliterature inin termsterms ofof [email protected]@SBA-15 waswas remarkablyremarkably improvedimproved asas comparedcompared toto datadata reportedreported inin thethe literatureliterature inin termsterms ofof catalyticcatalytic activityactivity andand mol.mol. %% ofof usedused catalyst.catalyst. ofof catalyticcatalytic activityactivity andand mol.mol. %% ofof usedused catalyst.catalyst.

MaterialsMaterials 20162016,, 99,, 557557 55 ofof 99 MaterialsMaterials 20162016,, 99,, 557557 55 ofof 99 Materials 20162016,, 99,, 557557 55 ofof 99 MaterialsMaterials 20162016,, 99,, 557557 OO OO 55 ofof 99 MaterialsMaterials 20162016,, 99,, 557557 OO OO 55 ofof 99 HOHO MeOMeO 1212 O O CHCH33OHOH 9090 HOHO OO OHOH MeOMeO OO OMeOMe 1212 HO OO OHOH MeO OO OMeOMe CHCH33OHOH 9090 12 HOHO OO OH MeOMeO OO OMe CH33OH 90 1212 CHCH33OHOH 9090 HOHO OO OHOH MeOMeO OO OMeOMe 1212 CHCH33OHOH 9090 O OHOH O OMeOMe COOCOOHH COOCOOEEtt OO COOCOOHH OO OO OO COOCOOEEtt COOH COOEtt Materials 2016, 9, 557 COOCOOHH 33 22 5 of 9 1313 COOCOOHH COOCOOEEtt CHCHCHCHOHOH 9090 1313 COOCOOEEtt CHCH33CHCH22OHOH 9090 13 CH33CH22OH 90 NONO 1313 NONO 22 CHCH33CHCH22OHOH 9090 22 NO NO 1313 NO 22 CHCH33CHCH22OHOH 9090 NO 22 Table 2. ContNO . NO 22 2 NONO COOHNOCOOHNO COOCOO22 EEt t 22 NONO NOCOOHNOCOOH COOCOO22 EEt t Entry CarboxylicCOOH22 Acid ProductCOO (Ester)Ett Alcohol Yield (%) b COOHCOOH COOCOOEEtt 1414 COOHCOOH COOCOOEEtt CHCH33CHCH22OHOH 9292 1414 CHCH33CHCH22OHOH 9292 14 CH33CH22OH 92 1414 CHCH33CHCH22OHOH 9292 FF FF 14 1414 CHCH33CHCH22OHOHOH 9292 92 FF FF 3 2 F COOHCOOH F COOCOO EEtt FF COOHCOOH FF COOCOO EEtt FF COOH FF COO Ett 1515 COOHCOOH COOCOOEEtt CHCH33CHCH22OHOH 9494 1515 COOHCOOH COOCOOEEtt CHCH33CHCH22OHOH 9494 15 CH33CH22OH 94

15 1515 CHCH33CHCH22OH2OHOH 9494 94 1515 CHCH33CHCH22OHOH 9494 OO OO OO OO O O 1616 OO OHOH OO OO CHCH33CHCH22OHOH 9595 1616 OO OHOH OO OO CHCH33CHCH22OHOH 9595 16 CH33CH22OH 95 16 OH O CH3CH2OH 95 1616 CHCH33CHCH22OHOH 9595 OHOH OO 1616 OHOH OO CHCH33CHCH22OHOH 9595 OO OO OO OO

1717 O OHOH O OO (CH(CH33))22CHOHCHOH 9595 OO OO 17 1717 OHOH OO (CH(CH(CH33))22CHOHCHOHCHOH 9595 95 17 OO OO (CH(CH33))22CHOH 95 OH O 1717 (CH(CH33))22CHOHCHOH 9595

OHOH OO 1717 OHOH (CH(CH33))22CHOHCHOH 9595 O OO OOO OO OO O O 18 1818 OO OHOH OO OO CHCH33OHOHOH 9898 98 1818 OO OHOH OO OO CHCH33OHOH 9898 18 CH33OH 98 OH O 1818 OHOH OO CHCH33OHOH 9898 a 1818 CHCH33OHOH 9898 All reactionsaa AllAll werereactionsreactions carried werewere out carriedcarried withOHOH out theout molarwithwith thethe ratio molarmolar of substrate/ROH ratioratio ofof substrate/ROHsubstrate/ROHOO (1:2) in (1:2) the(1:2) presence inin thethe presencepresence of supported ofof iron aa b oxide nanoparticles AllAll reactionsreactions (0.1 were mol.were %)carriedcarried at reflux outout withwith conditions thethe molarmolar for 6ratioratio h; ofofIsolated substrate/ROHsubstrate/ROH yields. b(1:2)b(1:2) inin thethe presencepresence ofof supportedaasupported ironiron oxideoxide nanoparticlesnanoparticles (0.1(0.1 mol.mol. %)%) atat refluxreflux conditionsconditions forfor 66 h;h; IsolatedIsolated yields.yields. AllAll reactionsreactions werewere carriedcarried outout with the molar ratio of substrate/ROH b(1:2)b in the presence of asupportedasupported ironiron oxideoxide nanoparticlesnanoparticles (0.1(0.1 mol.mol. %)%) atat refluxreflux conditionsconditions forfor 66 h;h; IsolatedIsolated yields.yields. AllAll reactionsreactions werewere carriedcarried outout withwith thethe molarmolar ratioratio ofof substrate/ROHsubstrate/ROH b(1:2)b(1:2) inin thethe presencepresence ofof asupportedasupported AllAll reactionsreactions ironiron oxidewereoxidewere nanoparticlescarriednanoparticlescarried outout withwith (0.1(0.1 themol.mol.the molar molar %)%) atat refluxrefluxratioratio of conditionsofconditions substrate/ROHsubstrate/ROH forfor 66 h;h; (1:2)(1:2) IsolatedIsolated inin thethe yields.yields. presencepresence ofof supportedThesupportedThe solventlesssolventless ironiron oxideoxide reactionreaction nanoparticlesnanoparticles waswas alsoalso (0.1(0.1 performedperformed mol.mol. %)%) atat withwith refluxreflux solidsolid conditionsconditions phasephase foralcohols.foralcohols. 66 h;h; bb IsolatedIsolated AsAs example,example, yields.yields. thethe reactionreaction The solventlessbetweenbetweensupportedThesupportedThe solventlessbenzoicsolventlessbenzoic reaction ironiron acidacid oxideoxide reactionreaction andand was nanoparticlesnanoparticles 4-chlorobenzyl4-chlorobenzyl alsowaswas alsoalso performed (0.1(0.1 performedperformed mol.mol. alcoholalcohol %)%) atat with with with reflux reflux (1:1)(1:1) solidsolid conditionsunderconditionsunder phasephase phase optimizedoptimized for alcohols.foralcohols. 66 alcohols. h;h; bb IsolatedconditionsIsolatedconditions AsAs example,example, Asyields.yields. for example,for the the1212 reactionhreactionh couldcould the reaction betweenThe solventlessbenzoic acid reaction and 4-chlorobenzyl was also performed alcohol with (1:1) solid under phase optimized alcohols. conditions As example, for the12 reactionh could between benzoicprovideprovidebetweenTheThe an solventlessan acidsolventlessbenzoic isolatedisolated and acid reactionesterreactionester 4-chlorobenzyl and yieldyield 4-chlorobenzyl waswas ofof alsoalso 55%.55%. performedperformed TheThe alcohol alcohol presenpresen withwith (1:1)(1:1)cece solidsolidofof under underthethe phasephase organicorganic optimized optimizedalcohols.alcohols. ligandligand conditions AsAs graftedgrafted example,example, conditions ononfor thethe the12 reaction reactionSBA-15hSBA-15 forcould 12 h could providebetweenTheThe ansolventlesssolventlessbenzoic isolated acid reactionesterreaction and yield 4-chlorobenzyl waswas of alsoalso 55%. performedperformed The alcohol presen withwith (1:1)ce solidsolidof under the phasephase organic optimized alcohols.alcohols. ligand conditions AsAs grafted example,example, onfor the the12 reactionreactionhSBA-15 could provide anbetweensurfacebetweensurfaceprovide isolated did did anbenzoicbenzoic ester notisolatednot seemseem yieldacidacid ester toandtoand of havehave yield 55%.4-chlorobenzyl4-chlorobenzyl anyany of The e55%.effectffect presence The onon alcoholalcohol thethepresen catalyticcatalytic of (1:1)(1:1)ce the of underunder organictheactivityactivity organic optimizedoptimized inin ligand the theligand systems,systems, conditionsconditions grafted grafted withwith for onfor a a 12the 12 negligiblenegligible h hSBA-15 SBA-15 couldcould surface betweensurfaceprovidebetween did anbenzoicbenzoic isolatednot seem acidacid ester toandand have yield 4-chlorobenzyl4-chlorobenzyl any of 55%.effect The on alcoholalcohol thepresen catalytic (1:1)(1:1)ce of underunder theactivity organic optimizedoptimized in theligand systems, conditionsconditions graftedgrafted with onforonfor a thethe12 12negligible hhSBA-15SBA-15 couldcould esterificationesterificationsurface did not activityactivity seem observedobserved to have foranyfor aminopropyaminopropy effect on thel-functionalizedl-functionalized catalytic activity SBA-15SBA-15 in the (in(in systems, thethe absenceabsence with ofof FeaFe negligible22OO33 NPs).NPs). did not seemprovidesurfaceprovide to have did anan isolatedisolatednot any seem effect esterester to have yield onyield theany ofof catalytic 55%.55%.effectffect TheThe onon presenthethepresen activity catalyticcatalyticcece ofof in the theactivityactivity the organicorganic systems, inin thetheligandligand systems,systems, with graftedgrafted a withwith negligible onon aathethe negligiblenegligible SBA-15SBA-15 esterification provideesterificationprovide anan isolatedisolated activity ester esterobserved yieldyield for ofof aminopropy 55%.55%. TheThe presenpresenl-functionalizedcece ofof thethe organicorganic SBA-15 ligandligand (in the grafted graftedabsence onon of Fethethe22O SBA-15SBA-1533 NPs). surfacesurfaceesterificationTheThe diddid esterificationesterification notnot activity seemseem observed to reactiontoreaction havehave any ofanyforof benzoicbenzoic aminopropy eeffectffect on acidonacid thethe withwithl-functionalized catalyticcatalytic methanolmethanol activityactivity catalyzedcatalyzed SBA-15 inin thethe (in byby systems,systems, theseveralseveral absence differentdifferentwithwith of aa Fe negligiblenegligible catalystscatalystsO NPs). activity observedsurfaceesterificationsurfaceTheThe diddid esterificationforesterification notnot aminopropyl-functionalizedactivity seemseem observed to reactiontoreaction havehave any foranyofof benzoic benzoicaminopropy eeffectffect on acidonacid thethe withl-functionalizedwithl-functionalized catalyticcatalytic SBA-15 methanolmethanol activityactivity (in catalyzedcatalyzed SBA-15 theSBA-15 inin absence thethe (in (in byby systems,systems, thethe severalseveral absenceabsence ofFe differentwithdifferentwith2O ofof 3a aFeFe NPs).negligiblenegligible 22 catalystsOcatalysts33 NPs).NPs). reportedreported inin thethe literatureliterature hashas beenbeen summarizedsummarized inin TableTable 3.3. AsAs cancan bebe seen,seen, thethe catalyticcatalytic performanceperformance22 33 esterificationesterificationThe esterification activityactivity observedobserved reaction foroffor benzoic aminopropyaminopropy acid withl-functionalizedl-functionalized methanol catalyzed SBA-15SBA-15 (in (inby the theseveral absenceabsence different ofof FeFe OcatalystsO NPs).NPs). esterificationreportedesterification in the activityactivity literature observedobserved has been forfor aminopropyaminopropysummarizedl-functionalizedl-functionalized in Table 3. As can SBA-15SBA-15 be seen, (in(in thethethe absence absencecatalytic ofof performance FeFe22OO33 NPs).NPs). The esterificationofofreported [email protected]@SBA-15TheThe esterification esterificationin thereaction literature waswas remarkablyreactionremarkablyreaction has of benzoicbeen ofof benzoic benzoic improved improvedsummarized acid acidacid as withas withwith comparedcomparedin Table methanolmethanol 3. to toAs data datacatalyzedcatalyzed can catalyzed reportedreportedbe seen, byby several severalthe inin by the thecatalytic several literature differentliteraturedifferent performance different catalystsincatalystsin termsterms catalysts ofreportedreported [email protected] esterificationesterificationinin thethe literatureliterature was remarkablyreactionreaction hashas beenbeen ofof benzoicbenzoic improvedsummarizedsummarized acidacid as withwith comparedinin Table methanolmethanol 3. toAs data catalyzed catalyzedcan reportedbe seen, byby severalthe severalthein thecatalyticcatalytic literature differentdifferent performanceperformance catalystscatalystsin terms reported inreportedofreportedof the [email protected] literature inin activity theactivitythe literatureliterature haswas andand beenremarkably mol.mol. hashas % %summarizedbeenbeen ofof used used summarizedimprovedsummarized catalyst.catalyst. inas inincompared Table TableTable3 .3.3. As AstoAs data cancan bebereported be seen,seen, seen, thethe in the catalyticthecatalytic catalyticliterature performanceperformance in performance terms of reportedofreportedof [email protected] inin activity thethe literatureliterature was and remarkably mol. hashas % beenbeen of used summarizedimprovedsummarized catalyst. as incomparedin TableTable 3.3. toAsAs data cancan bereportedbe seen,seen, thethe in thecatalyticcatalytic literature performanceperformance in terms [email protected] [email protected]@SBA-15catalytic was activity remarkably waswas and remarkablyremarkably mol. improved % of usedimprovedimproved catalyst. as compared asas compared compared to toto data datadata reportedreported inin the inthe theliteratureliterature literature inin termsterms in terms of of of [email protected]@SBA-15 activity waswas and remarkablyremarkably mol. % of used improvedimproved catalyst. asas compared compared toto datadata reportedreported inin thethe literatureliterature inin termsterms catalytic activityof of catalyticcatalytic and activityactivity mol. andand % of mol.mol. used %% ofof catalyst. usedused catalyst.catalyst. of of catalyticcatalytic activityactivity andand mol.mol. %% ofof usedused catalyst.catalyst.

Table 3. Comparison of various systems in the esteriﬁcation of benzoic acid by methanol.

Entry Catalyst Mol (%) T (˝C) Acid/Methanol Molar Ratio Time (h) Yield(%) a Ref.

1 [email protected] 0.1 reﬂux 1:2 6 99 b 2 [email protected] 0.5 reﬂux 1:2 12 98 [28] 3 [C3SO3Hmim]HSO4 0.3 95 1:3 2 98 [29] Ionic liquids based on 4 5 120 1:4 8 97.9 [30] benzothiazoliumcations 5 Pd/C 0.5 gr 60 1:excess amount 4 90 [31] 6 HClO4-SiO2 1 100 1:1 3 96 [32] a Isolated yield; b Present work.

A proposed reaction mechanism for the esterification is shown in Scheme2 and Fe species are coordinated to the carbonyl oxygen, followed by intermediate generation and alcohol addition to generate the observed ester products in high yields. Materials 2016, 9, 557 6 of 9 Materials 2016, 9, 557 6 of 9 Table 3. Comparison of various systems in the esterification of benzoic acid by methanol. Table 3. Comparison of various systems in the esterification of benzoic acid by methanol. Acid/Methanol Time Yield Entry Catalyst Mol (%) T (°C) Ref. Acid/Methanol Time Yielda Entry Catalyst Mol (%) T (°C) Molar Ratio (h) (%) Ref. a 1 [email protected] 0.1 reflux Molar1:2 Ratio (h)6 (%)99 b 21 [email protected]@SBA-15 0.50.1 refluxreflux 1:21:2 126 9899 [28]b 32 [[email protected]]HSO 4 0.30.5 reflux95 1:31:2 122 9898 [29][28] 3 Ionic[C3SO liquids3Hmim]HSO based 4on 0.3 95 1:3 2 98 [29] 4 Ionic liquids based on 5 120 1:4 8 97.9 [30] 4 benzothiazoliumcations 5 120 1:4 8 97.9 [30] 5 Pd/Cbenzothiazoliumcations 0.5 gr 60 1:excess amount 4 90 [31] 65 HClOPd/C 4-SiO2 0.51 gr 10060 1:excess1:1 amount 34 9690 [32][31] 6 HClO4-SiO2 1 100 1:1 3 96 [32] a Isolated yield; b Present work. a Isolated yield; b Present work. A proposed reaction mechanism for the esterification is shown in Scheme 2 and Fe species are coordinatedA proposed to the reaction carbonyl mechanism oxygen, followed for the esterification by intermediate is shown generation in Scheme and 2alcohol and Fe addition species areto generateMaterialscoordinated2016 the, 9 observed,to 557 the carbonyl ester products oxygen, in followed high yields. by in termediate generation and alcohol addition6 of to 9 generate the observed ester products in high yields. 3+ O Fe 2+ 2+ O 3+ O Fe R'OH O Fe Fe3+ O Fe 2+ 2+ O 3+ O Fe R'OH O Fe Fe R OH R OH R OH R OH R OH OH R OH R OH R OR' HOR' H O Fe2+ 2+ O Fe2+ O O Fe 3+ -H2O O Fe2+ - Fe O 3+ -H2O - Fe R OH2 R OR' OH R OR' 2 R OR' R OR' OR' R OR' Scheme 2. Lewis acid-catalyzed esterfication mechanism. SchemeScheme 2.2. Lewis acid-catalyzed esterficationesterfication mechanism.mechanism. After reaction completion, the possibility of reusing supported FeNP catalyst was determined. After reaction completion, the possibility of reusing supported FeNP catalyst was determined. The catalystAfter reaction was easily completion, separated the from possibility the reacti of reusingon mixture supported through FeNP filtration, catalyst washed was determined. with ethyl The catalyst was easily separated from the reaction mixture through filtration, washed with ethyl acetateThe catalyst to remove was easilyresidual separated product fromand reused the reaction in a subsequent mixture through reaction. filtration, As an example, washed the with reaction ethyl acetate to remove residual product and reused in a subsequent reaction. As an example, the reaction ofacetate benzoic to remove acid residualand methanol product in and the reused presence in a subsequent of [email protected] reaction. Asafforded an example, methylbenzoate the reaction in of of benzoic acid and methanol in the presence of [email protected] afforded methylbenzoate in quantitativebenzoic acid andyields methanol even after in the 10 presence successive of [email protected] runs under optimized afforded methylbenzoate reaction conditions, in quantitative with an quantitative yields even after 10 successive runs under optimized reaction conditions, with an averageyields even yield after of 1097%, successive supporting runs underthe stability optimized and reaction reusability conditions, of the withcatalytic an average system yield (Figure of 97%, 4). average yield of 97%, supporting the stability and reusability of the catalytic system (Figure 4). Furthermore,supporting the XPS stability spectra and of reusabilitythe catalyst of reco therded catalytic after system the several (Figure runs4). show Furthermore, that the Fe XPS3+ species spectra are of 3+ mostlytheFurthermore, catalyst present recorded XPS in the spectra after catalyst. theof the several catalyst runs reco showrded that after the the Fe3+ severalspecies runs are mostlyshow that present the Fe in thespecies catalyst. are mostly present in the catalyst.

Figure 4. RecyclingRecycling of the supported FeNP and the yield of isolated isolated methylbenzoate in 10 subsequent Figure 4. Recycling of the supported FeNP and the yield of isolated methylbenzoate in 10 subsequent runs. Reaction conditio conditions:ns: 2 2 mmol benzoic acid, 4 mmol MeOH, 7 mg [email protected] (0.2 mol. %) at runs. Reaction conditions: 2 mmol benzoic acid, 4 mmol MeOH, 7 mg [email protected] (0.2 mol. %) at refluxreﬂux conditions for 6 h. reflux conditions for 6 h.

3. Materials and Methods

3.1. General Information

Unless otherwise stated, all reagents and chemicals in this study were used as received and were not further puriﬁed (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany). Melting point was recorded on a RY-1 microscopic melting apparatus (Hangzhou Chincan Trading Co., Shanghai, China) and uncorrected. 1H-NMR and 13C-NMR spectra were respectively recorded on 500 MHz and 125 MHz by using a Bruker Avance 500 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany). Metal content in the materials was determined using inductively coupled plasma (ICP) in a Philips PU 70000 sequential spectrometer (Philips, Almelo, The Netherlands) equipped with an Echelle monochromator (0.0075 nm resolution). Samples were digested in HNO3 and subsequently analyzed by ICP. Nitrogen adsorption measurements were carried out at 77 K using an ASAP 2000 volumetric adsorption analyzer from Micromeritics (Micromeritics, Norcross, GA, USA). The samples were outgassed for 24 h at 100 ˝C under vacuum (10´2 Pa) and subsequently analyzed. Powder X-ray diffraction patterns were recorded on a Bruker-AXS diffractometer using a Cu Kα radiation (λ = 1.5409 Å). XPS measurements were performed in an ultra-high vacuum (UHV) multipurpose surface analysis system (Specs™) operating Materials 2016, 9, 557 7 of 9 at pressures <10´10 mbar using a conventional X-ray source (XR-50, Specs, Mg-Kα, 1253.6 eV) in a “stop-and-go” mode to reduce potential damage due to sample irradiation. The survey and detailed O and Si high-resolution spectra (pass energy 25 and 10 eV, step size 1 and 0.1 eV, respectively) were recorded at room temperature with a Phoibos 150-MCD energy analyzer (SPECS GmbH, Berlin, Germany). Powdered samples were deposited on a sample holder using double-sided adhesive tape and subsequently evacuated under vacuum (<10´6 Torr) overnight. Eventually, the sample holder containing the degassed sample was transferred to the analysis chamber for XPS studies.

3.2. Preparation of Aminopropyl-Functionalized SBA-15 Materials (SBA-15-NH2) Co-condensed amino-SBA-15 silicas were synthesized according to the procedure described by Wang et al. [33]. Aminopropyl-functionalized SBA-15 materials (denoted as SBA-15-NH2) were prepared by a one-pot synthesis method. Pluronic 123 (4 g) was dissolved in 125 g of 2.0 M HCl solution at room temperature. After TEOS was added, the resultant solution was equilibrated at 40 ˝C for prehydrolysis, and then APTES was slowly added into the solution. The molar composition of the mixture was 0.9 TEOS: 0.1 APTES: 6.1 HCl: 0.017 P123:165 H2O. The resulting mixture was stirred at 40 ˝C for 20 h and then reacted at 90 ˝C under static condition for 24 h. The solid product was recovered by filtration and dried at room temperature overnight. The template was removed from the material by refluxing in excess ethanol for 24 h. Finally, the material was filtered, washed several times with water and ethanol, and dried at 50 ˝C.

3.3. Preparation of Supported Iron Oxide Nanoparticles ([email protected]) Salicylaldehyde (2 mmol, 0.244 g) was added to excess absolute MeOH, to which Aminopropyl-functionalized SBA-15 materials (2.35 g, loading of NH2 group is 0.85 mmol/g) were then added. The solution became yellow due to imine formation. After 6 h, Fe(NO)3¨ 9H2O, (1 mmol), was added to the solution, and the mixture was stirred for a further 24 h to allow the new ligands to complex the iron and a red brown color was observed. The ﬁnal product was washed with MeOH and water until the washings were colorless. Further drying of the solid product was carried out in an oven at 80 ˝C for 8 h.

3.4. General Reaction Procedure In a typical reaction, 0.005 mmol of supported FeNP (0.5 mol. %) was added to a mixture of carboxylic acid precursor (5 mmol) and excess ROH (molar ratio 1:2) under reflux conditions for 6 h. The reaction progress was monitored by using thin-layer chromatography (TLC), after completion of the reaction; the catalyst was separated from the mixture through filtration and then washed with portions of 20 mL ethyl acetate and heated at 70 ˝C prior to its reuse in the next reaction. The combined filtrate and ethyl acetate washings were then washed with water and the organic layer separated and dried over magnesium sulfate. The product was obtained after removal of the solvent.

4. Conclusions A promising, efficient and green approach for the synthesis of various esters via reaction between carboxylic acids and alcohols in the presence of catalytic amounts of low-loaded iron oxide nanoparticles on SBA-15 materials under solvent-free conditions was successfully performed. The supported iron oxide nanocatalyst exhibited a remarkable stability under these conditions and could be easily removed from the reaction mixture by simple filtration and reused 10 times without any significant loss in activity. The versatility, convenient operation, and cost-effectiveness of this approach, in addition to the high yields, make it highly attractive both in laboratory research and potentially for scaling up.

Acknowledgments: F.R. is grateful to Payame Noor University for the support of this work. Author Contributions: M.A. conducted all experimental work and wrote the manuscript, F.R. and R.L. supervised, discussed and revised the manuscript. Materials 2016, 9, 557 8 of 9

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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