2Glycerol is oxidized by hydrogen peroxide to formic acid with excellent selectivity in the presence of iron salts. The oxidation takes place at room temperature inwater; at the end of the reaction the catalytic system is still active and available to restart the oxidation.partially_openopenFarnetti, Erica; Crotti, CorradoFarnetti, Erica; Crotti, Corrad

C. Crotti

E. Farnetti

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Catalysis Communications

Selective oxidation of glycerol to formic acid catalyzed by iron salts

Glycerol is oxidized by hydrogen peroxide to formic acid with excellent selectivity in the presence of iron salts. The oxidation takes place at room temperature inwater; at the end of the reaction the catalytic system is still active and available to restart the oxidation

FARNETTI, ERICA

Crotti, Corrado

Archivio istituzionale della ricerca - Università di Trieste

1Selective oxidation of glycerol to formic acid catalyzed by iron saltsE. Farnetti a, C. Crotti b,⁎a Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste, Via L. Giorgieri 1, 34127 Trieste, Italyb CNR – Istituto Struttura della Materia, Unità Operativa di Supporto di Trieste, S.R.14, Km163.5, 34149, Basovizza, Trieste, Italy⁎ Corresponding author.E-mail address: crotti@ism.cnr.it (C. Crotti).a b s t r a c ta r t i c l e i n f o Accepted 20 May 2016 Glycerol is oxidized by hydrogen peroxide to formic acid with excellent selectivity in the presence of iron salts.The oxidation takes place at room temperature in water; at the end of the reaction the catalytic system is still ac-tive and available to restart the oxidation.Keywords:Iron catalystsGlycerol valorizationSelective oxidationHydrogen peroxideFormic acid1. IntroductionIn recent years the blooming business of biodiesel fuels has led to anincreasing surplus of glycerol as the main by-product oftransesterification of biomass-derived triglycerides with methanol. Asa consequence, studies regarding the development of feasible and con-venient ways to transform glycerol in higher value products have pro-portionally increased in number. Such valorization would make thebiofuels production significantly cheaper and therefore affordable incomparison to traditional fossil fuels.From several points of view glycerol is a versatilemolecule, howeverits high number of functional groups - which is a valuable characteristicfor its use as chemical building block - causes at the same time thehardest difficulties in the set-up of selective reactions. Among the possi-ble glycerol valorization routes oxidation is one of the most studied,since the potentially obtainable products are all commercially relevant(Scheme 1) [1,2]. However, some of the most appealing products ob-tainable by glycerol oxidation, e.g. dihydroxyacetone (DHA), are them-selves rather reactive compounds: therefore, selective oxidation ofglycerol often leads to poor yields due to subsequent decomposition ofthe oxidation products [3].The difficulties experienced in developing selective routes for a par-tial oxidation of glycerol would be overcome by pushing the oxidationprocess to its end, for example to formic acid (FA) as final product.Few studies report glycerol oxidation to FA in the past literature [4],but their number has been growing in very recent years [5–8]. Most ofthe reported systems are based on heterogeneous catalysts and the ox-idant agent can be hydrogen peroxide as well as molecular oxygen.The rising interest in efficient FA production lies in its use as hydro-gen carrier. With its 4.4% hydrogen content and several well-knownsystems able todecompose it inmild conditions to hydrogen and carbondioxide [9–12], FA might provide a supply for the hydrogen fuel cells[13], as well as for the hydrogenation process of biomasses [14] andeven directly as automotive alternative fuel [15] which would perfectlyfit with the biofuel production above cited. On the other hand, FA is alsoa convenient source of C1 raw material for the chemical industry, as itcan be easily transformed into carbon monoxide [16].The pushing requirements of the twelve Green Chemistry principles[17] urge to shift towardsmore sustainable reactions and the use of ironcatalysts in place of rarer and more expensive transition metals [18]meets such expectations, as demonstrated by the growing interest intheir applications [19]. Iron compounds have been known to promotealcohols and diols oxidation by hydrogen peroxide since nineteenthcentury and even glycerol oxidation was briefly described in 1899[20], but also in the last years both iron salts [21–24] and iron complexes[25–28]were reported to catalyze alcohol oxidation by eithermolecularoxygen or hydrogen peroxide.Our group has recently described glycerol oxidation by hydrogenperoxide catalyzed by iron complexes with the tridentate ligand bis(2-pyridinylmethyl)amine (BPA) [29]; these studies have shown that bycareful tuning of the experimental setup a remarkably selective forma-tion of DHA can be obtained: on the other hand, in some cases we alsodetected a significative amount of FA as by-product.Here we present our results in the oxidation of glycerol by hydrogenperoxide catalyzed by iron salts to give FA with high selectivities and invery mild conditions.Scheme 1. Possible products of glycerol oxidation.22. Results and discussionThe oxidation of glycerol by hydrogen peroxide catalyzed byFe(OTf)2 (Tf = trifluoromethanesulfonate) and BPA in acetonitrile/water 2:1 gave a 25% selectivity of FA besides the main product DHA(Table 1 entry 1). The reactionwas carried out at r.t. with a 2.8 hydrogenTable 1Oxidation of glycerol with H2O2 catalyzed by iron compoundsa.Entry Catalyst [BPA(tot)]/[Fe] Solvent [H2O2]/[gly1 Fe(OTf)2 + BPA 3 MeCN:H2Oc 2.82 [Fe(BPA)2(OTf)2] + BPA 3 MeCN:H2Oc 2.83 Fe(OTf)2 + BPA 3 MeCN:H2Oc 11.2d4 Fe(OTf)2 + BPA 3 MeCN:H2Oc 11.2e5 Fe(OTf)2 0 MeCN:H2Oc 11.2e6 Fe(OTf)2 0 MeCN:H2Oc 4.4d7 Fe(OTf)2 0 MeCN:H2Oc 4.2e8 Fe(OTf)2 0 H2O 4.2e9 Fe(OTf)2 + BPA 3 H2O 4.2e10f Fe(OTf)2 0 H2O 4.2e11g Fe(OTf)2 0 H2O 4.2e12i Fe(OTf)2 0 H2O 4.2e13j Fe(OTf)2 0 H2O 4.2e14k Fe(OTf)2 0 H2O 4.2l15 Fe(OTf)3 0 H2O 4.2e16 FeCl2 0 H2O 4.2e17 FeCl3 0 H2O 4.2da Experimental conditions: [Fe] = 1.0 × 10−2 M; [glycerol]/[Fe] = 35. Isolated yields couldtherefore all yields are based on GC analyses (see Experimental section in Supplementary datab Calculated as % of glycerol reacted.c MeCN:H2O ratio = 2:1.d Addition of H2O2 in portions of 40 μl (1.1 equivalents) each.e Addition of H2O2 in a single portion.f Reaction carried out in closed and thermostatted vessel.g A second load of fresh glycerol was added after the end of the first reaction and a second ah Overall results.i A third load of fresh glycerol was added after the end of the second reaction and a third adj [glycerol]/[Fe] = 100.k Reaction carried out with t-BuOOH as oxidant.l [t-BuOOH]/[glycerol] (t-BuOOH 70% w/w).peroxide/glycerol ratio and three equivalents of BPAwith respect to theiron salt. Similar results were obtained using the preformed complex[Fe(BPA)2(OTf)2] in the presence of added BPA as catalytic system(entry 2): all these findings are consistent with those reported in ourprevious paper [29].In order to push theoxidation further,we increased the total amountof H2O2 to 11 equivalents, whichwere added in 1.1 eq portions at inter-vals of 30 min (entry 3 and Fig. 1). GC analysis of the reaction mixtureafter each addition showed that the first two equivalents of hydrogenperoxide caused partial oxidation of glycerol to a mixture of DHA andglyceraldehyde; upon addition of further amounts of hydrogen perox-ide we detected the formation of FA which rapidly became the mainproduct of the reaction. In the course of the addition of H2O2, initiallyformed glyceraldehyde disappeared and besides FA only traces of DHAwere detected in solution, together withminor amounts of a new prod-uct which was identified as glycolic acid (GA) (see Scheme 2). After tenadditions (11 equivalents of hydrogen peroxide), the oxidation of glyc-erol was almost complete to produce FA and GA; no other productswere detected. Further addition of H2O2 caused no modifications tothe reaction mixture. On the other hand, no significative differenceswere observed when the same overall amount of hydrogen peroxidewas added in a single step (entry 4).When a catalytic test was performed replacing glycerol with FA assubstrate no reaction was observed, as opposed to the results reportedby Walton [4] on the iron catalyzed oxidation of FA to carbon dioxideand water.The use of Fe(OTf)2 as catalyst in the absence of BPA gave even betterresults, with an almost complete glycerol conversion to FA and onlytraces of DHA and glycolic acid as by-products (entry 5). We repeatedseveral times the catalytic cycle by adding new loads of fresh glyceroland hydrogen peroxide, obtaining comparable yields of FA: thus, atthe end of the reaction the catalytic systemwas still active and availableto restart the oxidation without significative loss of activity.We repeated the reaction reported in entry 5 by adding hydrogenperoxide in 1.1 eq portions at intervals of 30 min (entry 6 and Fig. 2)and we verified that the evolution of the reaction catalyzed bycerol] t (min) Conv.b (%) DHA Sel. (%) GA Sel. (%) FA Sel. (%)90 25 72 3 2590 18 74 4 22280 99 Traces 6 9336 96 2 6 9236 99 Traces Traces 99106 100 Traces 1 986 97 Traces Traces 966 99 Traces 4 9436 82 24 12 636 99 1 3 956 99h Tracesh 3h 95h6 98h 0h 5h 95h16 98 Traces 2 9760 0 0 0 06 99 Traces 1 996 99 Traces 4 956 99 Traces 3 97not be provided, since all efforts to separate FA from reaction mixture were unsuccessful;).ddition of H2O2 was repeated.dition of H2O2 was repeated.Fig. 1. Glycerol oxidation by H2O2 in MeCN:H2O (2:1) catalyzed by Fe(OTf)2 + BPA (1:3).Exp. conditions: [Fe]= 1.0 × 10–2M; [glycerol]/[Fe]= 35; (-●-): glycerol; (-◆-): formicacid; (-■-): DHA; (-+-): glyceraldehyde; (-✕-): glycolic acid.Fig. 2. Glycerol oxidation by H2O2 in MeCN:H2O (2:1) catalyzed by Fe(OTf)2. Exp.conditions: [Fe] = 1.0 × 10–2 M; [glycerol]/[Fe] = 35; (-●-): glycerol; (-◆-): formicacid; (-■-): DHA; (-✕-): glycolic acid.3Fe(OTf)2was the same either in the presence or in the absence of the co-catalyst BPA (compare entries 3 and 6); the main differences in the lat-ter case were a lower production of DHA and glyceraldehyde and theuse of fewer equivalents of hydrogen peroxide to obtain a completeglycerol oxidation.In several reports of heterogeneously catalyzed glycerol oxidation,the reaction was proposed to occur via formation of intermediatessuch as DHA, glyceraldehyde, glycolic acid, glyceric acid and oxalicacid [30–32]. In the present case, the results shown in Figs. 1 and 2 indi-cate that the initial glycerol oxidation products were DHA and glyceral-dehyde,which upon further addition of hydrogen peroxidewere in turnoxidized to FA; in contrast, formation of glycolic acid was not followedScheme 2. Glycerol oxidation to formic acid (FA) and glycolic acid (GA).by further oxidation. Notably, other possible oxidation products werenever detected in the reaction mixture.The results of entry 6 and Fig. 2 evidence that after addition of 4.4equivalents of H2O2 oxidation of glycerol to FA was complete underthese experimental conditions, in agreement with the stoichiometryof the oxidation from glycerol to formic acid. Therefore, we carried outa reaction by slow addition of 4.2 equivalents of H2O2 to the reactionmixture (one drop every 15 s, 6 min altogether): glycerol oxidationwas completed immediately after the last H2O2 addition, with no fur-ther reaction occurring afterwards (entry 7). A faster addition of hydro-gen peroxide gave rise to somewhat lower conversion, probably due topartial hydrogen peroxide decomposition which is known to occur inthe presence of iron salts [21,25].We tested water as solvent to improve the sustainability of the reac-tion; the catalytic reactions performed in water gave similar results tothose in acetonitrile/water, with only a slight decrease in FA selectivity(entry 8), thus confirming the full feasibility of the reaction in environ-mentally friendly setting. As already observed in acetonitrile/watermix-ture (see entries 4 and 5), also in water in the presence of BPA thereaction was less selective, giving mixtures of DHA, glycolic acid andformic acid (compare entries 8 and 9). These results can be interpretedin terms of formation of two catalytically active species, i.e. a free cation-ic iron species which promotes glycerol oxidation to formic acid on onehand, and on the other a Fe/BPA complex which is responsible for theglycerol oxidation to DHA.In order to minimize the effects of temperature variations, we per-formed the reaction in a vessel closed by a serum cap and kept at con-stant temperature (21 °C) in a thermostatted oil bath: the results ofsuch experiment (entry 10) were comparable to those obtained in anopen vessel. At the end of the reaction a second catalytic cycle with anew glycerol load and subsequent oxidation by H2O2 addition was per-formed, followed by an analogous third cycle. The results, reported inentries 11 and 12 respectively, show for both catalytic cycles a remark-able selectivity in FA, with no apparent loss of catalytic activity. The onlydetectable side effect was the absence of DHA and a moderate increaseof glycolic acid yield. The high catalytic activity was confirmed by in-creasing the catalytic ratio [glycerol]/[Fe] to 100 (entry 13), without sig-nificative variation on both glycerol conversion and FA selectivity.The effect of choice of the oxidizing agent was tested by replacinghydrogen peroxide with another peroxide: use of tert-butyl hydroper-oxide, a frequently employed oxidant in iron promoted alcohol4oxidation [25,27] was totally ineffective in the catalytic reactions underinvestigation (entry 14).We also tested the effect on the catalytic activity of both catalystcounterion and iron oxidation state (entries 15–17): the results werenot affected by replacing either triflate with chloride, or Fe(II) withFe(III) salts. Our findings on the catalytic activity of iron(III) salts, al-though not in agreement with those reported by Fenton [20], are how-ever coherent with other literature reports of oxidation by hydrogenperoxide catalyzed by Fe(III) salts [21,22,24,26]. On the other hand, itis known that Fe2+ could be easily oxidized to Fe3+ in air or when ex-posed to oxidizing agents; that might account for the similar activityshowed by FeCl2, FeCl3, Fe(OTf)2 and Fe(OTf)3.In fact the underlying chemistry in Fenton- or Gif-type reactions, i.e.the oxidation systems based on Fen+ with H2O2, is very complex andstill subject of studies and discussions [24,33,34], one of themost debat-ed points being the iron oxidation states throughout the catalytic cycle.The two proposedmechanisms, i.e. a free radicalmechanism as opposedto H2O2 nucleophilic addition to the metal centre, have been supportedby mechanistic studies and it was shown that the actual reaction path-way depends on several experimental parameters, one of which is thepH of the reaction medium [24,34]. Although we do not have any posi-tive proof indicating the actualmechanismof the reaction here described,the high selectivity and the absence of tar polymers might be both evi-dences (see Supplementary data) in favour of a non-radical pathway[24,35]; at the same time, the neat changes of colours during the firststages of the H2O2 addition - from orange to deep purple to clear yellow- might indicate fast variations through different iron oxidation states.Further studies are presently in progress to shed light on the influ-ence of the various experimental parameters – including the presenceof added ligands – on the course of the catalytic reactions: all evidencesobtained by us aswell as by other groupsmay contribute to a better un-derstanding of Fenton-type chemistry.3. ConclusionsGlycerol was oxidized by hydrogen peroxide to formic acid with ex-cellent selectivity in the presence of iron salts; the only by-product pres-ent in the final reaction mixture was glycolic acid. 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Selective oxidation of glycerol to formic acid catalyzed by iron salts

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