The Low Temperature Solvent-Free Aerobic Oxidation Of .
catalystsArticleThe Low Temperature Solvent-Free AerobicOxidation of Cyclohexene to Cyclohexane Diol overHighly Active Au/Graphite andAu/Graphene CatalystsOwen Rogers 1 , Samuel Pattisson 1 , Joseph Macginley 1Stuart H. Taylor 1, * ID and Graham J. Hutchings 1, * ID12*ID, Rebecca V. Engel 1 , Keith Whiston 2 ,Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place,Cardiff CF10 3AT, UK; RogersO@cardiff.ac.uk (O.R.); PattissonSD@cardiff.ac.uk (S.P.);macginleyjd@cardiff.ac.uk (J.M.); EngelR@cardiff.ac.uk (R.V.E.)INVISTA Performance Technologies, the Wilton Centre, Wilton, Redcar TS10 4RF, UK;Keith.Whiston-1@invista.comCorrespondence: TaylorSH@Cardiff.ac.uk (S.H.T.); Hutch@cardiff.ac.uk (G.J.H.);Tel.: 44-2920-874-062 (S.H.T.); 44-2920-874-059 (G.J.H.) Received: 2 July 2018; Accepted: 28 July 2018; Published: 31 July 2018Abstract: The selectivity and activity of gold-catalysts supported on graphite and graphene havebeen compared in the oxidation of cyclohexene. These catalysts were prepared via impregnation andsol immobilisation methods, and tested using solventless and radical initiator-free reaction conditions.The selectivity of these catalysts has been directed towards cyclohexene epoxide using WO3 as aco-catalyst and further to cyclohexane diol by the addition of water, achieving a maximum selectivityof 17% to the diol. The sol immobilisation catalysts were more reproducible and far more active,however, selectivity towards the diol was lower than for the impregnation catalyst. The resultssuggest that formation of cyclohexane diol through solventless oxidation of cyclohexene is limited bya number of factors, such as the formation of an allylic hydroperoxyl species as well as the amount ofin situ generated water.Keywords: cyclohexene; cyclohexane diol; adipic acid; catalytic oxidation; gold; graphene1. IntroductionIn recent decades, the impact of chemistry on society has largely been influenced by its capacityto make new molecules in an environmentally considerate and sustainable way. This has led to thedevelopment of catalysts with increasing efficiency and selectivity, which, when combined with theimprovements in reaction engineering and purification methods, leads to greater atom economiesand energy efficiencies [1]. The industrial production of adipic acid historically relies on the aerobicoxidation of cyclohexane at 125–165 C and 8–15 atm, usually in the presence of a homogenouscobalt-based catalyst. The resulting product mixture of cyclohexanol and cyclohexanone, known as KAoil, is then oxidised by nitric acid to adipic acid [2,3]. Alternative technology based on cyclohexene as araw material for cyclohexanol synthesis is also now practiced commercially, with the product still beingconverted to adipic acid using nitric acid oxidation. The process for producing adipic acid from KA oilvia nitric acid oxidation historically produced 400,000 metric tons of N2 O globally. N2 O has a globalwarming potential approximately 300 times that of CO2 [4]. However, recent catalytic and thermalabatement of these emissions within both adipic acid and nitric acid processes has resulted in verysignificant reductions in attendant N2 O emissions, and therefore of their environmental significance.Catalysts 2018, 8, 311; ysts
Catalysts 2018, 8, 3112 of 10However, in principle, processes for adipic acid synthesis that avoid the use of nitric acid as an oxidanteliminate concerns with N2 O emissions [5–7] and offer a higher atom economy. Routes to adipicacid based on aerobic oxidation with air as the terminal oxidant are therefore of continuing researchinterest. The possibility of using cyclohexene as a feedstock has lately become an option because ofrecent improvements in the partial hydrogenation of benzene to cyclohexene in the Asahi process [8].The higher reactivity of cyclohexene allows it to be oxidised at lower temperatures by oxidants suchas O2 and hydrogen peroxide, and therefore avoids N2 O formation when using nitric acid. The firststep in forming adipic acid from cyclohexene is epoxidation of the alkene, which can subsequentlyundergo hydrolysis to form cyclohexane diol. The diol can then be converted into adipic acid throughthe oxidative cleavage of the C–C bond, as demonstrated by Obara et al. [9] and Rozhko et al. [10]However, the prospect of achieving the one-step conversion of cyclohexene to adipic acid is hinderedby the low miscibility of cyclohexane diol in cyclohexene, therefore, a second step would be requiredto form adipic acid.Current epoxidations of cyclohexene require the use of stoichiometric oxygen donors such aschromates, permanganates, and periodates, which could result in complex heat management needsand by-products that are harmful to the environment [11]. Peroxides are also commonly used to act asradical initiators [12] or stoichiometric oxidants, however, these can increase the cost of the process andproduce unwanted waste [13]. High yields of cyclohexene oxide are also mostly achieved using specificsolvents, which can become uneconomical and contribute to the high levels of waste produced [14].Therefore, the most sustainable alternative would be to run the reaction under solventless conditionsusing air [15] or oxygen [16] as the oxidant. However, this leads to lower selectivity where mainlyproducts of allylic oxidation are observed [17,18].In previous work, Sato et al. achieved the one-pot solventless oxidation of cyclohexene to adipicacid in the presence of an Na2 WO4 ·2H2 O catalyst, a phase transfer catalyst, and 30% hydrogenperoxide [19]. This produced a 90% yield of adipic acid after 8 h at 90 C in air. However, thisreaction required 4.0 to 4.4 molar equivalents of H2 O2 to cyclohexene. This ratio renders the processuneconomical on an industrial scale because of the expense of using hydrogen peroxide in these relativeamounts. The presence of a toxic phase transfer catalyst is also a major drawback of this approach.Gold nanoparticles have been shown to be active in a number of reactions, despite being originallyconsidered chemically inactive. Since the pioneering work on gold nanoparticles by Haruta andHutchings [20,21], gold has been shown to be active in several reactions, namely, low-temperature COoxidation [22,23], C–C bond coupling [24,25], selective hydrogenation [26–28], and oxidations [29,30].Ovoshchnikov et al. assessed the reactivity of Au-based catalysts for cyclohexene oxidations usingsolvent-free and initiator-free conditions [31]. It was found that the selectivity of the oxidationscould be altered from allylic products to the epoxide via the choice of co-catalyst. A WO3 co-catalystwas found to shift the selectivity towards cyclohexene oxide, while MIL-101 catalysed conversion ofcyclohexenyl hydroperoxide to the allylic products. Using a WO3 and Au/SiO2 system, they couldachieve 50% conversion and 26% selectivity towards the epoxide as the main product at 65 C and1 atm O2 . WO3 achieves this by promoting the reaction between cyclohexenyl hydroperoxide andcyclohexene to create cyclohexene oxide and 2-cyclohexen-1-ol, as shown in Scheme 1. As a result ofthese findings, herein we present an investigation into the selective oxidation of cyclohexene undersolventless and radical initiator free conditions to target cyclohexanediol with Au-based catalysts.
Catalysts 2018, 8, 3113 of 10Catalysts 2018, 8, x FOR PEER REVIEW3 of 10SchemeScheme 1.1. ProposedProposed mechanismmechanism byby OvoshchnikovOvoshchnikov etet al.al. forfor cyclohexenecyclohexene oxidationoxidation [31].[31].2. ResultsThe aim of this work was to modify conditions used by Ovoshchnikov et al. in order to achieveFor allall catalysts,catalysts, thethe Au content wasfurther oxidation of the epoxide to yield cyclohexanecyclohexane diol. Formaintained atat intainedsupport.Thecatalystswerepreparedviaviaanan impregnationmethoda henthenanalysedanalysed byby TEMTEM (transmissionimpregnationmethodor ora solelectron microscopy) and STEM (scanning transmission electron microscopy) techniques to quantifynanoparticle size and establish a relationship between nanoparticle size and selectivity to the desired C for 24 h, withUnless otherwiseotherwise stated,stated, allall reactionsreactions werewere runrun atat 3 bar O22 atat 6060 Cproducts. Unlessfor 24 h, with 0.1 g of6 Au mol %). The results are presented in Table 1.catalyst (5.08 1010 6 Aumol %). The results are presented in Table 1.aTable 1.Table1. PerformancePerformance ofof catalystscatalysts inin cyclohexenecyclohexene oxidationoxidation a.CatalystCatalystConversion (%)Conversion e Au/graphene WO1%WO33 .43.9 3.915.655.315.655.329.829.831.122.231.122.20.4 0.416.437.016.437.010.110.118.253.318.253.30.0 0.018.656.018.656.07.1 7.121.346.421.346.40.0 .25.29.09.0aReaction conditions:cyclohexene(10 (10mL),mL),n-decane(1 mL)(1asmL)an internalstandard,catalyst (0.1g), O2 (0.1(3 atm),Reactionconditions:cyclohexenen-decaneas an internalstandard,catalystg),60 C, 24 h, glass reactor. b All conditionsbare the same as a with additional WO(0.1g).I Impregnationmethod3a2 (3 atm), 60 C, 24 h, glass reactor. All conditions are the same as with additional WO3 (0.1 g). I OS Sol immobilization; Cy–oxide cyclohexene oxide; Cy–ol 2-Cylohexen-1-ol; Cy–one 2-Cyclohexen-1-one;Cy–diol Cyclohexanediol.Impregnationmethod S Sol immobilization; Cy–oxide cyclohexene oxide; Cy–ol 2-Cylohexen-1aol; Cy–one 2-Cyclohexen-1-one; Cy–diol Cyclohexanediol.It was found in this study that the highest activity for a catalyst without any co-catalyst wasobserved1% Au/graphenevia sol immobilisation,however,catalyst gaveIt wasforfoundin this study (73.2%)that thepreparedhighest activityfor a catalyst eparedsolobserved for 1% Au/graphene (73.2%) prepared via sol immobilisation, however, this h selectivity to the allylic products. Similarly, the 1% Au/graphite catalyst prepared via solThe catalysts preparedimpregnationare alsosignificantlyless active,withonly 8.5%and 25.9%immobilisationalso gaveviahighconversion, butsuffered fromthe robservationcatalysts prepared via impregnation are significantly less active, with only 8.5% and 25.9%was that theforimpregnationcatalystsshowedhigh batch tobatch variabilityin theobservedcatalyticconversionthe 1% Au/graphiteand1% onactivity.this is dueto a largevariationthe Auparticlesize bly,the impregnationcatalystsshowedhigh inbatchto batchvariabilityin the observedactivity. Presumably, this is due to a large variation in the Au particle size distribution obtainedbetween samples. Contrastingly, the sol immobilisation catalysts consistently gave high conversions
Catalysts 2018, 8, 3114 of 10Catalysts 2018, 8, x FOR PEER REVIEW4 of 10samples.Contrastingly,the soloverimmobilisationcatalystsconsistentlyhigh conversionsbetweenbetween batches,with 61.4%the graphitesupportedcatalyst gaveand 73.2%over the atalystand73.2%overthegraphenesupportedsupported analogue. The type of support appears to have a slight impact on the total conversionanalogue.The istypeof supportappearshave a grapheneslight impactonabsencethe totalofconversionobserved.observed. Thisevidencedby theresulttoutilisingin thegold, whichgave bsenceofgold,whichgaveaconversionofconversion of 4.3%, higher than the graphite, which showed only trace amounts of more, the impregnation and sol immobilisation catalysts supported on graphene both appearimpregnationsol immobilisationgrapheneThisbothmayappeargiveto give higherandconversionsthan the catalystsgraphite supportedsupported onanalogues.be todueto abilising effect of the graphene sheets on the cyclohexenyl radical as observed by Yang et al. ingraphenesheets oncyclohexenylradicalas observedby Yanget autocatalytical. in the aerobicoxidationofaerobic oxidationofthecyclohexane,therebyincreasingthe rateof thereaction[32]. on, the Au/graphene catalysts consistently provided higher selectivities for the allyliccatalystshigherfor thecatalysts.allylic oducts inprovidedcomparisonwithselectivitiesthe Au/graphiteThis addsfurther inweightto thewiththe Au/graphitecatalysts.Thistheaddsfurtherweightto thehypothesisthat graphenehypothesisthat graphenepromotesallylicoxidationroute,which,as suggestedbefore, promotesmay be atheallylicoxidation route,which,as suggestedradical,before, whichmay bea resultof a stabilisingon theresultof a stabilisingeffect onthe cyclohexenylleadsto allylicoxidation, effectas ation,asshowninScheme2.Scheme 2.OOH[Au]OOOHO O2Scheme 2. ProposedProposed mechanism for the allylic oxidation route adapted from the literature [32].The effect of adding WO33 to the reaction is seen to significantly increase the selectivity towardsepoxide thwiththe /graphite(impregnation)catalyst,the conversionincreasingfrom from8.5% to17.7%.This increasein conversionoccurs dueto theWOco-catalystshowingalso increasing8.5%to 17.7%.This increasein conversionoccursdueto3 theWO3 co-catalystslightactivityconvertingcyclohexeneto cyclohexeneoxide in theabsenceAu. TheeffectTheofshowingslightinactivityin convertingcyclohexeneto cyclohexeneoxidein the ofabsenceof tbyincreasingconversionandselectivityeffect ffectbyincreasingconversionand3tothe diol,tohowever,is observeda lesser toextent.BrightfieldTEMfieldand STEMimagesofimagesthe 1%selectivitythe diol,thishowever,this istoobserveda lesserextent.BrightTEM andSTEMAucatalystsgraphenegraphitearegraphiteshown inareFigure1. Themean diametersof Aunanoparticlesof the1% Auoncatalystsonandgrapheneandshownin Figure1. The meandiametersof AuweredeterminedbydeterminedTEM for thecatalysts and BSE(back-scatteredelectrons) fornanoparticleswerebysol-immobilisedTEM for the sol-immobilisedcatalystsand BSE (back-scatteredthecatalystsvia impregnation.were unable toWedeterminethe AutonanoparticlesizesAuofelectrons)forpreparedthe catalystsprepared viaWeimpregnation.were unabledetermine theAu/graphitesynthesisedby impregnationusingbya TEMapproach. usingThis waspredictedto be thenanoparticle assizesof Au/graphiteas synthesisedimpregnationa TEMapproach.Thisresultwasofa wide dispersionof nanoparticlesthe catalystsurface andacrossthe possibilityof ansurfaceagglomerationpredictedto be the resultof a wide acrossdispersionof nanoparticlesthe catalystand theoflarge, highlyparticlescouldbe moredifficultto find.However,particlescouldpossibilityof andispersedagglomerationof thatlarge,highlydispersedparticlesthatcould bethemoredifficulttothenlocated afterswitchingto BSE.TEM imagesof the sol-immobilisedcatalystshowofanequalfind. beHowever,the particlescouldthenThebe locatedafter switchingto BSE. The rossthedispersiongraphite andgraphene surface,wouldbe expectedof thisimmobilisedshow anequalof nanoparticlesacrossasthegraphiteand graphenetechnique.Au begraphitecatalysthastechnique.a mean particlesizediameterof 4.2 nmthe Augraphenesurface, as Thewouldexpectedof thisThe Augraphitecatalysthasanda meanparticlesizehasa similarmeansize4.5 nm. Thedeviationsare alsosimilar1.1 andnm fordiameterof 4.2nmparticleand theAuofgraphenehasstandarda similarmean particlesizeof erespectively,suggestingthat the supportsdeviationssimilarsupportedat 1.1 andcatalysts,1.2 nm forthe graphiteand graphenesupportedthemselvescatalysts,haveno impacton the particleor dispersion.The particlealsoparticlemuch narrower,respectively,suggestingthat sizesthe supportsthemselveshavesizenodistributionimpact on isthesizes oraswould be Theexpectedof thetechnique,whichgoodcontrol overnanoparticlesize. Thedispersion.particlesizedistributionis canalsoallowmuchnarrower,as wouldbe expectedof BSEtheimagesof theimpregnationcatalystshowovera muchwider dispersionof particles,as istechnique,whichcan allow goodcontrolnanoparticlesize. TheandBSEdistributionimages of theimpregnationcommonin thistechnique.particlesa mean diameterof 19.3and a standarddeviationcatalyst showa muchwiderThesedispersionandhaddistributionof particles,as nmis commonin this technique.of6.6 nm.The particlesalsoranged offromtoa55.3nm. Thislarge rangein particlesize couldTheseparticleshad a meandiameter19.38.7nmnmandstandarddeviationof 6.6 nm.The particlesalsoaccountfor its8.7loweractivityrelativethe rangesol-immobiliseddueto the forgeneralinactivityofranged fromnm to55.3 nm.This tolargein particle catalystssize couldaccountits loweractivitylargeparticlescatalytic epoxidation[33].thisinactivityparticle sizebe inaccuraterelativeto thetowardssol-immobilisedcatalysts duetoHowever,the generalof rangelarge mayparticlestowardsdueto theepoxidationdifficulty in[33].measuringparticles,which sizemayrangebe toomaysmallresolve. TheimagessuggestcatalyticHowever,this particlebetoinaccuratedueBSEto thedifficultyinthatthe of resolve.the graphiticrather suggestthan at thesheetmeasuringparticles,beintooThe sheets,BSE imagesthatthe edgeslargeaswith thesol catalysts.explainsthey werenot observedduringinitialstudies.particlesmainlyreside inThisthe centreof whythe graphiticsheets,rather thanat thesheetTEMedgesas with thesol catalysts. This explains why they were not observed during initial TEM studies.
Catalysts 2018, 8, 311Catalysts 2018, 8, x FOR PEER REVIEW5 of 105 of 10Figure 1.1. (a,b)(a,b) BrightBright fieldfield transmissiontransmission electronelectron microscopymicroscopy (TEM)(TEM) imagesimagesofof1%1%Au/grapheneAu/graphene viasol immobilisation show mean particle diameter of 4.5 nm, standard deviation of 1.2 nm shown inin (c).(c).field graphite viavia solsol immobilisationimmobilisation showshow meanmean particleparticle diameterBright fieldof 4.2 nm, standard deviationdeviation ofof 1.11.1nmnmshownshown inin(f).(f). Back-scatteredBack-scattered electronselectrons (BSE)(BSE) images (g,h) of esizeof19.3nmanda standard deviationAu/graphite prepared by impregnationof 6.6 nm shown in (i).Despite their high activity,activity, thethe solsol immobilisationimmobilisation catalysts do not display promising selectivityto the epoxideepoxide eveneven withwith thethe additionaddition ofof aa co-catalyst.co-catalyst. In addition to this,this, thethe graphenegraphene supportedsupportedmaterials seemed to favour allylic products. Therefore,Therefore, furtherfurther studiesstudies were conductedconducted on the 1%Au/graphite,prepared viavia impregnation,impregnation, toto furtherfurther oxidiseoxidise thethe epoxideepoxide toto thethe diol due to its highAu/graphite, preparedselectivity to the epoxide (29.8%)(29.8%) inin thethe presencepresence ofof WOWO33. Small amounts of diol were observed in allthe previous reactions, through hydrolysis of the epoxide. However, these may have been limited bythe amount of available water in the reaction. Water is formed as a by-product in the dehydration ofthe hydroperoxyl intermediate to the allylic ketone. We thus decided to add additional water at thestart of the reaction to assess this theory and attempt to hydrolysehydrolyse all epoxideepoxide throughthrough toto thethe diol.diol. Onadditiontotothediol,achievinga totalselectivityofaddition ofof ga iceabledropinconversionto11.8%,whichsuggeststhatof 17.0% (Table 2). However, there was a noticeable drop in conversion to 11.8%, which suggestswater has a detrimentaldetrimental effect on overall reaction and the amount may need to be further tuned oradded in a stepwise manner.manner.
Catalysts 2018, 8, x FOR PEER REVIEWCatalysts 2018, 8, 311Table 2. Effect of the addition of water to cyclohexene oxidation reaction a.6 of 106 of 10Selectivity (%)Table 2. Effect of the addition ofwaterCatalystCon(%)to cyclohexene oxidation reaction a .Cy–OxideCy–OlCy–OneCy–Diol1% Au/graphiteI8.513.717.338.41.4Selectivity (%)b1% Au/graphiteI29.831.122.25.1Catalyst .82.022.546.317.01% Au/graphite WO3 H2O ca Reaction1% Au/graphiteI mL), n-decane8.513.7as an internal17.3 standard,38.4catalyst (0.11.4g),conditions: cyclohexene (10(1 mL)bI b all conditions17.729.822.2 WO3 (0.15.1Au/graphiteWO3(3 atm),60 C, 24 h,glassreactor.are thesame as a 31.1with additionalg). cO21%c1%Au/graphite WO HOI11.82.022.546.317.0b32All conditions are the same as with additional H2O (1 mL). I Impregnation method; Cy–oxide aReaction conditions:mL), n-decane (1 mL)Cy–oneas an internalstandard, catalyst (0.1 g),Cy–diolO2 (3 atm),cyclohexeneoxide;cyclohexeneCy–ol (102-Cylohexen-1-ol; 2-Cyclohexen-1-one; 60 C, 24 h, glass reactor. b all conditions are the same as a with additional WO3 (0.1 g). c All conditions are the sameCyclohexanediol.as b with additional H2 O (1 mL). I Impregnation method; Cy–oxide cyclohexene oxide; Cy–ol 2-Cylohexen-1-ol;Cy–one 2-Cyclohexen-1-one; Cy–diol Cyclohexanediol.3. Discussion3. DiscussionThese results demonstrate the challenges in utilising cyclohexene as an alternative substrate forTheseresultsdemonstratethe challengesutilisingcyclohexenean ofalternativesubstratethe formationof adipicacid throughsolventless inandgreen conditions.Theasusediatomic oxygenasforformationof adipicacidthroughsolventlessconditions.The use However,of diatomicthe theoxidantis highlydesirablebothfrom enas here,the oxidanthighly desirablebothfrom an economicdescribedand inis previouswork byOvoshchnikovet al.,andthe environmentalformation of viewpoint.the allylicHowever,as describedhere, ultimatelyand in previousOvoshchnikoval., the toformationof the ofallylichydroperoxylintermediatelimitsworkthe bypossibleyield of etepoxide50% becausethehydroperoxylintermediateultimatelylimits alcohol.the possibleof epoxideto yield50% becauseof theformation of equimolaramountsof the allylicThis yieldthen limitspossibleto cyclohexaneformationequimolar amountsof the allylicalcohol.Thislimitsmechanismpossible yieldto thecyclohexanediol, a keyofintermediatein the formationof adipicacid.Anthenadaptedfrompaper bydiol,a key intermediatein theinformationacid. An ofadaptedmechanismfromthepaper byOvoshchnikovet al. is shownScheme 3offoradipicthe formationcyclohexanediol withtheadditionofOvoshchnikovet al.is shownin Scheme3 for the formationof cyclohexanediolinwithaddition ofwater [31]. Theresultsin Table2 demonstratethe importanceof waterthetheformationwater[31]. TheresultsTable 2 demonstrateof waterin the formationof cyclohexanecyclohexanediol,and inthereforeany process thethatimportanceachieves highselectivityto the epoxidewould bediol,andbecausethereforeanylackprocesshigh selectivityto the epoxidewould be limitedbecauselimitedof theof inthatsituachieveswater formedvia dehydrationof the hydroperoxylintermediate.ofthe lack of inanysitulargewaterimprovementsformed via dehydrationthe ade to ofyieldcyclohexane diolwould faceadditionalanylarge improvementsmade to yieldcyclohexane Synthesisdiol wouldoffaceadditionalproblemsto itsproblemsdue to its insolubilityin cyclohexene.adipicacid wouldthusduerequireinsolubilityin thecyclohexene.of adipicacid wouldthus requireoxidation,separationthe produceddiol,separation ofproduced Synthesisdiol, solvationin water,and subsequentasofshownby the worksolvationin al.water,subsequentof Obara etand andRozhkoet al. oxidation, as shown by the work of Obara et al. and Rozhko et al.Scheme 3.3. Adaptedscheme fromfrom OvoschnikovOvoschnikov etet al.al. forfor effecteffect ofof additionaladditional waterwater [31].[31].SchemeAdapted reactionreaction schemestudy, wewe havehave alsoalso demonstrateddemonstrated thethe variationvariation inin activitiesactivities andand selectiviesselectivies attainableattainableIn this study,through byOvoshchnikovfocusedon thesimple modification of gold catalysts. The previous study by Ovoshchnikov focusedonsynthesisof edonvariousoxidesupports.thesynthesisof tedonvariousoxide3This dation isachievablethrough theuse of simplesupports.workshowshighfor cyclohexeneoxidationis achievablethroughthe usesolofimmobilisationcatalysts. However,promisingdiatomic ofoxygenandoxygensubsequentsimplesol immobilisationcatalysts.despiteHowever,despite activationpromisingofactivationdiatomicandcleavage of cleavagethe ablefurther improvementsare requiredareinsubsequentof theresultingperoxyl intermediates,further oxideordiol.Itispossiblethatthereisastrongrequired in order to obtain high yields of cyclohexene epoxide or diol. It is possible that there is adependenceon nanoparticlesize eon nanoparticlesizetheof thegoldepoxidation,consideringthethelargelarge differencesbetween solsol immobilisedimmobilised andand impregnationimpregnation catalysts.catalysts. Theobserved here betweenThe highhigh activity of the solimmobilisation catalysts was such that only trace amounts of epoxide were observed even in thepresence of WO33, whichwhich is ableable to direct the breakdownbreakdown ofof thethe hydroperoxyhydroperoxy intermediateintermediate (Table(Table bleamountsofdiolformation,whichsuggestsHowever, with these catalysts, there were appreciable amounts of diol formation, which suggests thatthat epoxidewas formedsomeinstagein the andreactionandfurthertooxidisedto athediol as aepoxidewas formedat someatstagethe reactionfurtheroxidisedthe diol asconsequenceconsequenceof in situgeneratedwater. Thehighest toselectivityto theepoxidewas byachievedby theofin situ generatedwater.The highestselectivitythe epoxidewasachievedthe ithWO3.Thislargediscrepancybetweensolimpregnation catalyst in combination with WO3 . This large discrepancy between sol immobilisationimmobilisationandimpregnationis likelylinkedtheiractivity, asobservedthe smallandimpregnationselectivityis likelyselectivitylinked to theiractivity,as thetosmallnanoparticlesin
Catalysts 2018, 8, 3117 of 10the sol immobilisation catalyst rapidly catalyse the breakdown of the intermediate species to allylicproducts. This also suggests that optimisation of reaction temperature and the stoichiometry of catalystand co-catalyst used could yield higher amounts of either epoxide or diol. The latter may be limited atlower temperatures because of hydrolysis of the epoxide occurring at around 60 C [34]. Temperatureramping studies could allow for the higher formation of epoxide followed by ring opening to the diol.However, as per the previously mentioned limitations with respect to the reaction mechanism, the diolyield would likely peak at 50%.Taking into consideration the above points, the use of this system for production of cyclohexanediol would not be a suitable replacement of the current industrial process. Formation of adipic acidby further oxidation of this diol in a one-pot approach is further limited because of the formation ofcyclohexane dione rather than the hydroxyketone, as seen in aqueous systems with either oxygen orhydrogen peroxide as oxidant. The lack of water also provides significant barriers to the formation ofdiol and hydration of adipic anhydride to adipic acid.We suggest that these catalysts may be more interesting when utilised in a biphasic system foractivation of oxygen and subsequent oxidation of a co-catalyst for the highly selective oxidationof cyclohexene. For this system, an emulsion may need to be stabilised in work similar to thatdescribed by Zhou et al., who designed amphiphilic silica particles for the solvent-free acetalisationof long-chain fatty aldehydes with ethylene glycol [35]. In addition to this work, He et al. hassuggested that graphene oxide particles can also be used to stabilise emulsions of the o
oil, is then oxidised by nitric acid to adipic acid [2,3]. Alternative technology based on cyclohexene as a raw material for cyclohexanol synthesis is also now practiced commercially, with the product still being converted to adipic acid using nitric acid oxidation. The process for producing adipic acid from KA oil
May 02, 2018 · D. Program Evaluation ͟The organization has provided a description of the framework for how each program will be evaluated. The framework should include all the elements below: ͟The evaluation methods are cost-effective for the organization ͟Quantitative and qualitative data is being collected (at Basics tier, data collection must have begun)
Silat is a combative art of self-defense and survival rooted from Matay archipelago. It was traced at thé early of Langkasuka Kingdom (2nd century CE) till thé reign of Melaka (Malaysia) Sultanate era (13th century). Silat has now evolved to become part of social culture and tradition with thé appearance of a fine physical and spiritual .
VL R 20 MDI Polyisocyanate Solvent-free 31.5 132 VL R 21 MDI Polyisocyanate Solvent-free 31.5 132 VH 20 MDI Modified MDI Solvent-free 24.5 173 E 21 MDI Prepolymer Solvent-free 16 263 E 22 MDI Prepolymer Solvent-free 8.6 489 E 23 MDI Prepolymer Solvent-free 15.4 273 E 29 MDI Prepolymer Solvent-free 24 175 E 20100 MDI Prepolymer Solvent-free 15.9 268
On an exceptional basis, Member States may request UNESCO to provide thé candidates with access to thé platform so they can complète thé form by themselves. Thèse requests must be addressed to esd rize unesco. or by 15 A ril 2021 UNESCO will provide thé nomineewith accessto thé platform via their émail address.
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Dr. Sunita Bharatwal** Dr. Pawan Garga*** Abstract Customer satisfaction is derived from thè functionalities and values, a product or Service can provide. The current study aims to segregate thè dimensions of ordine Service quality and gather insights on its impact on web shopping. The trends of purchases have
Chính Văn.- Còn đức Thế tôn thì tuệ giác cực kỳ trong sạch 8: hiện hành bất nhị 9, đạt đến vô tướng 10, đứng vào chỗ đứng của các đức Thế tôn 11, thể hiện tính bình đẳng của các Ngài, đến chỗ không còn chướng ngại 12, giáo pháp không thể khuynh đảo, tâm thức không bị cản trở, cái được
tion using the AKTA Explorer 100 system (GE Health-care). Solvent A comprised 10 mM KH 2 PO 4 in 25% (v/v) ACN, whereas solvent B comprised solvent A with 500 mM KCl. Solvents A and B were applied at the follow-ing gradients: 0–10% solvent B for 2 min, 10–20% solvent B for 25 min, 20–45% solvent B for 5 min, and 50–100%
