Aerobic Catalytic Oxidation of Cyclohexene Over Tizrco Catalysts

catalysts

Article Aerobic Catalytic Oxidation of Cyclohexene over TiZrCo Catalysts

Tong Liu 1,2,3, Haiyang Cheng 1,2,*, Weiwei Lin 1,2, Chao Zhang 1,2, Yancun Yu 1,2 and Fengyu Zhao 1,2,*

1 State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China; [email protected] (T.L.); [email protected] (W.L.); [email protected] (C.Z.); [email protected] (Y.Y.) 2 Laboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China 3 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected] (H.C.); [email protected] (F.Z.); Tel.: +86-431-8526-2454 (H.C.); Tel./Fax: +86-431-8526-2410 (F.Z.)

Academic Editor: Michalis Konsolakis Received: 8 December 2015; Accepted: 12 January 2016; Published: 29 January 2016

Abstract: The aerobic oxidation of hydrocarbon is of great signiﬁcance from the viewpoints of both fundamental and industry studies as it can transfer the petrochemical feedstock into valuable chemicals. In this work, we investigated the aerobic oxidation of cyclohexene over TiZrCo catalysts, in which 2-cyclohexen-1-one was produced with a high selectivity of 57.6% at a conversion of 92.2%, which are comparable to the best results reported for the aerobic oxidation of cyclohexene over heterogeneous catalysts. The inﬂuences of kinds of solvent, substrate concentration and reaction temperature were evaluated. Moreover, the catalytic performance of the TiZrCo catalyst and the main catalytic active species were also discussed. The results of SEM, XRD and XPS suggested that the surface CoO and Co3O4 species are the catalytic active species and contribute to the high activity and selectivity in the present cyclohexene oxidation. The present catalytic system should have wide applications in the aerobic oxidation of hydrocarbons.

Keywords: aerobic oxidation; heterogeneous catalyst; cyclohexene; TiZrCo

1. Introduction The selective oxidation of hydrocarbon is of great importance in the chemical industry, and the oxidation of alkenes to value-added chemicals has been paid more attention [1–4]. For example, 2-cyclohexen-1-one, which can be produced from oxidation of the C–H bond at the allylic site of cyclohexene, is one of the important ﬁne chemicals because it is widely used in the manufacture of perfumes, pharmaceuticals, dyestuff and agrochemicals [5–7]. However, the yield of 2-cyclohexen-1-one produced from cyclohexene oxidation is quite low due to the existence of two active sites (C–H and C=C bond) on the cyclohexene molecule [8]. Thus, it is still a great challenge to enhance the selectivity of 2-cyclohexen-1-one by designing an efﬁcient catalyst. For oxidation of cyclohexene, the traditional oxidants are iodosylbenzene, sodium hypochlorite, chromium trioxide, t-butyl hydroperoxide, H2O2, etc. These strong oxidants can improve the oxidation rate and increase the yield of product, but they are expensive and harmful to the environment. Therefore, developing a green oxidation process is of importance in view of the academic research and industrial application. Recently, the oxidation with molecular oxygen as an oxidant has received much attention as it is a cheap, abundant and environmentally benign process and some achievements have been obtained [9–13]. For example, the ionic liquids with metal chelate anion [C10mim][Co(F6-acac)3] exhibited high catalytic

Catalysts 2016, 6, 24; doi:10.3390/catal6020024 www.mdpi.com/journal/catalysts Catalysts 2016, 6, 24 2 of 9 Catalysts 2016, 6, 24 2 of 9 activitycatalytic foractivity the allylic for the oxidation allylic ofoxidation cyclohexene of cyclohexene in the absence in the of solventabsence with of solvent a high selectivitywith a high of ´ 81%selectivity to 2-cyclohexen-1-one of 81% to 2-cyclohexen at a conversion-1-one of at 100% a conversion [10], and the of Cu-[cationic 100% [10], salphen][Br and the Cu]2-[cationiccomplex presentedsalphen][Br a− selectivity]2 complex of presented 64.1% to 2-cyclohexen-1-onea selectivity of 64.1% at to a 100%2-cyclohexen conversion-1-one for at the a aerobic100% conversion oxidation offor cyclohexene the aerobic inoxidation acetonitrile of cyclohexene [11]. Although in acetonitrile these homogeneous [11]. Although catalysts these are efficienthomogeneous for the catalysts selective allylicare efficient oxidation for the of cyclohexene,selective allylic their oxidation industrial of applicationcyclohexene, is limitedtheir industrial due to the application difficult separation, is limited anddue theto the residual difficult metal separation, ions will and affect the the residual quality metal of the ions product. will affect Hence, the quality heterogeneous of the product. catalysts Hence, have beenheterogeneous developed catalysts by immobilizing have been metal developed complexes by immobilizing on solid supports, metal suchcomplexes as resinate-immobilized on solid supports, Co(II),such as which resinate exhibited-immobilized high catalytic Co(II), activitywhich exhibited with a 44.4% high selectivity catalytic activity to 2-cyclohexen-1-one with a 44.4% selectivity at a 94.5% conversionto 2-cyclohexen of cyclohexene-1-one at a [ 1294.5%]. Core-shell conversion type of Fe 3cyclohexeneO4@chitosan-Schiff [12]. Core base-immobilized-shell type Fe3O Co(II),4@chitosan Cu(II)- andSchiff Mn(II) base- complexesimmobilized were Co(II), also reportedCu(II) and active Mn(II) for thecomplexes cyclohexene were oxidation,also reported and aactive selectivity for the of 77.2%cyclohexene to 2-cyclohexen-1-one oxidation, and a selectivity was obtained of 77.2% at a to conversion 2-cyclohexen of 46.8%-1-one [ 13was]. obtained On the other at a conversion hand, the supportedof 46.8% [13] transition. On the metalother orhand, oxides the weresupported also employed transition as metal heterogeneous or oxides were catalysts also foremployed the allylic as oxidationheterogeneous of cyclohexene catalysts for [6 the,14 –allylic20]. Recently,oxidation Au of cyclohexene nanoparticles [6,14 supported–20]. Recently, on modified Au nanoparticles bentonite andsupported silica gave on modified a high conversion bentonite and (92%) silica and ga anve excellenta high conversion selectivity (92%) (97%) and to an 2-cyclohexen-1-one excellent selectivity in the(97%) aerobic to 2-cyclohexen oxidation- of1-one cyclohexene in the aerobic without oxidation solvent of [cyclohexene21]. It was alsowithout reported solvent that [21] PdO/SBA-15. It was also wasreported an active that PdO/SBA catalyst for-15 the was oxidation an active of catalyst cyclohexene for the inoxidation acetonitrile, of cyclohexene and a conversion in acetonitrile, of 56% and a selectivityconversion ofof 82%56% toand 2-cyclohexen-1-one a selectivity of 82% were to 2 obtained-cyclohexen [6].-1 In-one addition, were obtained nitrogen-doped [6]. In addition, carbon nanotubes,nitrogen-doped and graphiticcarbon nanotubes, carbon nitride-supported and graphitic carbon FeO and nitride CoO-supported were also effectiveFeO andcatalysts CoO were for also the oxidationeffective catalysts of cyclohexene for the oxidation [14,22]. Until of cyclohexene now, it was [14,22] still a hot. Until topic now, to design it was astill heterogeneous a hot topic to catalyst design fora heterogeneous the aerobic allylic catalyst oxidation for the of aerobic cyclohexene. allylic oxidation of cyclohexene. In our previous previous work, work, we we found found that that Ti Ti-Zr-Co-Zr-Co metallic metallic catal catalystyst was was effective effective for for the the oxidation oxidation of ofcyclohexane cyclohexane and and ethylbenzene ethylbenzene [23 [23–25–25], ],in in which which cyclohexanol cyclohexanol and and cyclohexanone cyclohexanone were produced with a high selectivity of of 90% 90% at at a a conversion conversion around around 7% 7%,, and and acetophenone acetophenone was was produced produced with with a a69.2% 69.2% selectivity selectivity at ata high a high co conversionnversion of of61.9%. 61.9%. The The Ti- Ti-Zr-CoZr-Co metallic metallic catalyst catalyst is simple is simple and and cheap cheap in inproduction production and and sturdy sturdy to towearing wearing in inthe the utilization, utilization, as ascompared compared to tothose those reported reported catalysts catalysts such such as asmetal metal-organic-organic complex, complex, metal metal nanoparticles nanoparticles and and nanocarbon nanocarbon materials. materials. Therefore, Therefore, it inspires it inspires us usto tostudy study itsits efficiency efficiency in in catalyzing catalyzing other other hydrocarbon hydrocarbon oxidations oxidations.. In In the the present present work, work, the catalytic performancesperformances of of the the Ti Ti-Zr-Co-Zr-Co cata catalystslysts were were discussed discussed for for the the aerobic aerobic oxidation oxidation of cyclohexene. of cyclohexene. We Wefound found that that2-cyclohexen 2-cyclohexen-1-one-1-one was produced was produced with a withhigh aselectivity high selectivity of 57.6% of at 57.6% a conversion at a conversion of 92.2%, ofwhich 92.2%, are which comparable are comparable to the best to theresults best resultsreported reported for the foraerobic the aerobic oxidation oxidation of cyclohexene of cyclohexene over overheterogeneous heterogeneous catalysts catalysts.. It was It was confirmed confirmed the the surface surface CoO CoO and and Co Co3O3O4 4actedacted as as the the catalytic catalytic active sites and contributedcontributed toto thethe excellentexcellent conversionconversion andand selectivity.selectivity.

2. Results Results and and Discussion Discussion For the oxidation of cyclohexene, it is very difﬁcultdifficult to control product selectivity due to the existence of the the two two active active groups groups of of the the C C-H-H bond bond atat the the allylic allylic site site and and the the C=C C=C bond, bond, as when as when the theC–H C–H bond bond is oxidized, is oxidized, 2-cyclohexene 2-cyclohexene-1-ol,-1-ol, 2-cyclohexene 2-cyclohexene-1-one-1-one or cyclohexene or cyclohexene hydroperoxide hydroperoxide will willbe generated be generated;; as the as theC=C C=C bond bond is isoxidized, oxidized, cyclohexene cyclohexene oxide, oxide, cyclohexanol, cyclohexanone,cyclohexanone, cyclohexanediol and dialdehydedialdehyde willwill bebe producedproduced (Scheme(Scheme1 1))[ 26[26]]. .

Scheme 1. ReactiReactionon route for the aerobic oxidation of cyclohexene.cyclohexene.

Firstly, we examined several organic solvents for the oxidation of cyclohexene and the results Firstly, we examined several organic solvents for the oxidation of cyclohexene and the results are listed in Table 1. Among the examined solvents, acetone and acetonitrile are more effective, and are listed in Table1. Among the examined solvents, acetone and acetonitrile are more effective, the higher conversion and selectivity of 2-cyclohexen-1-one were obtained; in contrast, ethanol and cyclohexane are less efficient. However, it was found that acetone could be oxidized (to 2,2-

Catalysts 2016, 6, 24 3 of 9 and the higher conversion and selectivity of 2-cyclohexen-1-one were obtained; in contrast, ethanol and cyclohexane are less efﬁcient. However, it was found that acetone could be oxidized (to 2,2-diethoxypropane) during the reaction. As a result, acetonitrile is a suitable solvent which led to a conversion of cyclohexene of 38.0% and a selectivity to 2-cyclohexen-1-one of 60.6%. Therefore, the acetonitrile was selected as solvent in the following studies.

Table 1. Effect of solvent on the oxidation of cyclohexene over Ti60Zr10Co30 catalyst.

Selectivity (%) a Solvent Conversion (%) (1) (2) (3) (4) (5) Others b Acetone 43.9 7.3 0 1.5 13.3 71.8 6.1 Acetonitrile 38.0 3.0 0.4 1.4 13.6 60.6 21.0 Ethanol 22.0 1.4 0 0.3 4.7 25.0 68.6 c Cyclohexane 6.2 - 0.2 7.4 9.9 37.8 44.7 d

Reaction conditions: cyclohexene 1 mL (at a concentration of 4.8%), solvent 20 mL, Ti60Zr10Co30 20 mg, O2 2 MPa, 100 ˝C, 12 h. a (1) cyclohexene oxide; (2) cyclohexanol; (3) cyclohexanone; (4) 2-cyclohexen-1-ol; (5) 2-cyclohexen-1-one; b Others may consist of reaction intermediate such as cyclohexene hydrogen peroxide, deeply oxidized products such as some ring-opening acids or the byproduct from solvent reacting with substrate. c The ethanol was oxidized into acetic acid and ethyl acetate. d A certain amount of cyclohexane was found to be oxidized to cyclohexanol and cyclohexanone. The result could not exact calculated.

Next, the reaction conditions were evaluated for the oxidation of cyclohexene in acetonitrile. The results for the effect of the concentration of cyclohexene are shown in Table2. The conversion of cyclohexene increased signiﬁcantly with the concentration of cyclohexene; it increased from 33.8% to 97.5% when the cyclohexene concentration was raised from 4.8% to 13.0%. However, the selectivity to 2-cyclohexen-1-one decreased linearly due to deep oxidation of 2-cyclohexen-1-one to undesired byproducts at a higher conversion. The aerobic oxidation of cyclohexene is a radical reaction; it contains the chain-initiation, -propagation and -termination steps [1]. The produced radical in the initial step could promote the following steps and was more efﬁcient at higher cyclohexene concentrations as it enhanced the impact probability of radicals, resulting in an increase of the conversion of cyclohexene. It is notable that when the concentration of cyclohexene increased to 16.7%, the reaction became very violent with a sharp increase of pressure (up to 10 MPa at 3.3 h); for safety, the concentration of cyclohexene was controlled below 16.7% under the present reaction conditions.

Table 2. Effect of the cyclohexene concentration on the oxidation reaction.

Selectivity (%) Entry Cyclohexene (%) a Time (h) Conversion (%) (1) (2) (3) (4) (5) Others b 1 4.8 5 33.8 3.1 - 1.7 19.1 71.3 4.9 2 9.1 5 67.0 5.5 - 0.7 11.6 62.5 19.8 3 13.0 5 97.5 - - 0.6 3.7 49.5 46.1 4 c 16.7 3.3 92.8 1.9 0.9 1.4 6.2 30.2 59.4 ˝ a Reaction conditions: acetonitrile 20 mL, Ti60Zr10Co30 20 mg, O2 2 MPa, 120 C. Concentration of cyclohexene b = Vcyclohexene/(Vcyclohexene + Vacetonitrile) ˆ 100%; Others may consist of reaction intermediate such as cyclohexene hydrogen peroxide, deeply oxidized products such as some ring-opening acids; c When the concentration of cyclohexene reached 16.7%, the reaction proceeded very quickly with a suddenly pressure rising, and the reaction was stopped at 3.3 h for safety.

It is well known that temperature is one of the most important factors for oxidation reactions. Generally, high temperature is in favor for the oxidation of hydrocarbons. As the results show in Figure1, the conversion increased from 17.8% to 98.2% while the selectivity of 2-cyclohexen-1-one decreased from 70.1% to 43.7% with the temperature rising from 80 to 140 ˝C. Deep oxidation is serious at higher temperatures and undesirable byproducts such as ring-opening acids are produced. The selectivity of cyclohexene oxide, cyclohexanol and cyclohexanone changed slightly, within 5%. Catalysts 2016, 6, 24 4 of 9

Catalysts 2016, 6, 24 4 of 9 The optimal temperature was 120 ˝C, at which a 57.6% selectivity of 2-cyclohexene-1-one was obtained The optimal temperature was 120 °C, at which a 57.6% selectivity of 2-cyclohexene-1-one was Catalysts 2016, 6, 24 4 of 9 at a highobtained conversion at a high of conversion 92.2%. of 92.2%. The optimal temperature was 120 °C, at which a 57.6% selectivity of 2-cyclohexene-1-one was obtained at a high conversion of 92.2%.

Figure 1. Effect of temperature on the oxidation of cyclohexene. Reaction conditions: acetonitrile 20 mL, Figure 1. Effect of temperature on the oxidation of cyclohexene. Reaction conditions: acetonitrile cyclohexene 1 mL (at a concentration of 4.8%), Ti60Zr10Co30 20 mg, 2 MPa O2, 12 h. 20 mL, cyclohexene 1 mL (at a concentration of 4.8%), Ti60Zr10Co30 20 mg, 2 MPa O2, 12 h. Figure 1. Effect of temperature on the oxidation of cyclohexene. Reaction conditions: acetonitrile 20 mL, In addition, the influence of reaction time on the conversion and product selectivity were cyclohexene 1 mL (at a concentration of 4.8%), Ti60Zr10Co30 20 mg, 2 MPa O2, 12 h. Inexamined addition, at conditions the inﬂuence of 120 of°C reaction, a cyclohexene time onconcentration the conversion of 9.1% andand an product oxygen selectivity pressure of were 2 MPa. As shown in Figure 2, ˝the conversion increased to 96.1% with extending the reaction time to examinedIn ataddition, conditions the influence of 120 C,of areaction cyclohexene time onconcentration the conversion ofand 9.1% product and anselectivity oxygen were pressure 8 h. The selectivity of 2-cyclohexen-1-ol and 2-cyclohexen-1-one decreased from 20.3% to 4.0% and of 2 MPa.examined As shownat conditions in Figure of 1202, the °C, conversion a cyclohexene increased concentration to 96.1% of 9.1% with and extending an oxygen the reactionpressure timeof to 68.8% to 55.3% due to the formation and accumulation of deep oxidation products with the reaction 8 h. The2 MPa. selectivity As shown of in 2-cyclohexen-1-ol Figure 2, the conversion and 2-cyclohexen-1-oneincreased to 96.1% with decreased extending from the reaction 20.3% time to 4.0% to and proceeding. The other products derived from deep oxidation such as the ring-opening acids 8 h. The selectivity of 2-cyclohexen-1-ol and 2-cyclohexen-1-one decreased from 20.3% to 4.0% and 68.8%increased to 55.3% with due a toselectivity the formation up to 40% and (8 accumulationh) from 6% (1 h), of and deep the oxidation intermediates products such as with cyclohexene the reaction 68.8% to 55.3% due to the formation and accumulation of deep oxidation products with the reaction proceeding.oxide, cyclohexanol The other products and cyclohexanone derived from changed deep very oxidation little (<5.5%). suchas the ring-opening acids increased proceeding. The other products derived from deep oxidation such as the ring-opening acids with a selectivity up to 40% (8 h) from 6% (1 h), and the intermediates such as cyclohexene oxide, increased with a selectivity up to 40% (8 h) from 6% (1 h), and the intermediates such as cyclohexene cyclohexanol and cyclohexanone changed very little (<5.5%). oxide, cyclohexanol and cyclohexanone changed very little (<5.5%).

Figure 2. The variation of the conversion and selectivity with extending reaction time. Reaction conditions: cyclohexene 2 mL (at a concentration of 9.1%), acetonitrile 20 mL , Ti60Zr10Co30 20 mg, 2 MPa O2, 120 °C. Figure 2. The variation of the conversion and selectivity with extending reaction time. Reaction Figure 2. The variation of the conversion and selectivity with extending reaction time. conditions: cyclohexene 2 mL (at a concentration of 9.1%), acetonitrile 20 mL, Ti60Zr10Co30 20 mg, ReactionGenerally, conditions: the aerobic cyclohexene radical 2 oxidation mL (at a concentrationof hydrocarbons of 9.1%),can automatically acetonitrile 20occur mL, in Ti theZr absenceCo 2 MPa O2, 120 °C. 60 10 30 of catalyst [27], but it is˝ very slow and the selectivity of the desired product is poor. The oxidation of 20 mg, 2 MPa O2, 120 C. cyclohexene can also automatically occur somewhat at certain conditions without catalyst; however, Generally, the aerobic radical oxidation of hydrocarbons can automatically occur in the absence the conversion and selectivity to 2-cyclohexen-1-one are quite low [22]. Herein, we also found that of catalyst [27], but it is very slow and the selectivity of the desired product is poor. The oxidation of Generally,the cyclohexene the aerobic can convert radical with oxidation a conversion of hydrocarbons of 14.4% under can the automatically aerobic oxidation occur conditions in the absence cyclohexene can also automatically occur somewhat at certain conditions without catalyst; however, of catalystwithout [27 catalyst.], but it isAs very expected, slow andthe thepresence selectivity of TiZrCo of the catalysts desired can product promote is poor. the reaction The oxidation rate of the conversion and selectivity to 2-cyclohexen-1-one are quite low [22]. Herein, we also found that cyclohexenesignificantly can alsoas shown automatically in Figure 3. occur The catalytic somewhat performances at certain conditionsof TiZrCo catalysts without were catalyst; discussed however, the cyclohexene can convert with a conversion of 14.4% under the aerobic oxidation conditions according to the composition and the surface active species. It is clear that the catalytic activity the conversionwithout catalyst. and selectivity As expected, to 2-cyclohexen-1-one the presence of TiZrCo are quite catalysts low [22can]. promote Herein, wethe alsoreaction found rate that the depends on the Co content in the TiZrCo metallic catalysts, as the conversion of cyclohexene cyclohexenesignificantly can as convert shown within Figure a conversion 3. The catalytic of 14.4% performances under the of aerobic TiZrCo oxidation catalysts were conditions discussed without increased to 38.5% over Ti50Zr10Co40 from 32% over Ti70Zr10Co20, while the selectivity to 2-cyclohexene- catalyst.according As expected, to the composition the presence and of the TiZrCo surface catalysts active species. can promote It is clear the that reaction the catalytic rate signiﬁcantly activity as showndepends in Figure on the3. TheCo catalyticcontent in performances the TiZrCo metallic of TiZrCo catalysts, catalysts as the were conversion discussed of accordingcyclohexene to the compositionincreased andto 38.5% the over surface Ti50Zr active10Co40 species.from 32% Itover is clearTi70Zr10 thatCo20 the, while catalytic the selectivity activity to depends2-cyclohexene on the- Co content in the TiZrCo metallic catalysts, as the conversion of cyclohexene increased to 38.5% over

Catalysts 2016, 6, 24 5 of 9

Ti50Zr10Co40 from 32% over Ti70Zr10Co20, while the selectivity to 2-cyclohexene-1-one was around Catalysts 2016, 6, 24 5 of 9 66%–69%, and changed very little. These results indicate that the ternary TiZrCo metallic catalysts are effective1-one was for around the present 66%–69% cyclohexene, and change oxidation,d very little and. These the results content indicate of Co affectsthat the theternary catalytic TiZrCo activity signiﬁcantly.metallic catalyst In orders are to effective check the for catalytic the present efﬁciency cyclohexene of Co oxidation species, inand the the present contentoxidation, of Co affect CoTis the 2 and Co3Ocatalytic4/TiOCatalysts2 activitywith 2016, 6, the 2 4significantly higher content. In order of Coto check species the were catalytic also efficien examined.cy of Co Unfortunately, species in the bothpresent5 of 9 of them gaveoxidation, lower activity CoTi compared2 and Co3O to4/TiO the2 ternarywith the TiZrCo higher metallic content catalysts, of Co species as seen were in Figure also 3examined.. These results 1-one was around 66%–69%, and changed very little. These results indicate that the ternary TiZrCo suggestedUnfortunately, not only both the of Co them species gave lower but the activity surface compared and bulk to the structure ternary TiZrCo of the TiZrCometallic catalystscatalysts, play metallic catalysts are effective for the present cyclohexene oxidation, and the content of Co affects the as seen in Figure 3. These results suggested not only the Co species but the surface and bulk structure an importantcatalytic role activity in the significantly present oxidation.. In order toIt check is assumed the catalytic that efficien the additioncy of Co ofspecies Zr may in the produce present surface of the TiZrCo catalysts play an important role in the present oxidation. It is assumed that the addition defectsoxidation, and induce CoTi the2 formationand Co3O4/TiO of many2 with morethe higher CoO andcontent Co 3ofO 4Coactive species species. were also examined. of Zr may produce surface defects and induce the formation of many more CoO and Co3O4 active species. Unfortunately, both of them gave lower activity compared to the ternary TiZrCo metallic catalysts, as seen in Figure 3. These results suggested not only the Co species but the surface and bulk structure of the TiZrCo catalysts play an important role in the present oxidation. It is assumed that the addition of Zr may produce surface defects and induce the formation of many more CoO and Co3O4 active species.

Figure 3. Comparison of the catalytic performances of different catalysts in the oxidation of Figure 3. cyclohexene.Comparison Reaction of theconditions: catalytic cyclohexene performances 2 mL of, differentacetonitrile catalysts 20 mL,in catalyst the oxidation 20 mg, O of2 cyclohexene.2 MPa, ˝ Reaction120 °C conditions:, 3 h. cyclohexene 2 mL, acetonitrile 20 mL, catalyst 20 mg, O2 2 MPa, 120 C, 3 h. Figure 3. Comparison of the catalytic performances of different catalysts in the oxidation of cyclohexene. Reaction conditions: cyclohexene 2 mL, acetonitrile 20 mL, catalyst 20 mg, O2 2 MPa, Therefore, the structure of the TiZrCo catalysts and their relationship to the catalytic Therefore,120 °C the, 3 h. structure of the TiZrCo catalysts and their relationship to the catalytic performances performances were discussed. The SEM image of a representative sample of Ti60Zr10Co30 is shown in were discussed. The SEM image of a representative sample of Ti Zr Co is shown in Figure4a. Figure 4a. The size of the particles is at a range of 50–100 μm, and the other60 10two samples30 with different The size ofTherefore the particles, the structure is at a rangeof the ofTiZrCo 50–100 catalystsµm, andand thetheir other relationship two samples to the catalytic with different compositionsperformances should were discussedhave a similar. The SEM morphology image of a representativeas all the samples sample wereof Ti60 crushedZr10Co30 isand shown screen in - compositions should have a similar morphology as all the samples were crushed and screen-separated separatedFigure 4aby. T he140 size meshes of the particles before isbeing at a range examined. of 50–100 T μmhe , andbulk the structure other two ofsamples these with samples different w as by 140characterized meshescompositions before with shouldbeing XRD ashave examined. shown a similar in Figure The morphology bulk 4b. structureFor Ti as60Zr all10 ofCo the these30 , sampleswith samples a molar were was ratio crushed characterized of Co/Ti and ofscreen 1/2, with the- XRD as showndiffractionseparated in Figure patterns by 4b.140 For aremeshes Tiin 60accord Zrbefore10Coance 30being, with examined. athe molar CoTi T ratio2 hephase bulk of Co/Tiwithoutstructure of other 1/2,of these crystal the diffractionsamples phases. w asF patterns or are inTi accordance70characterizedZr10Co20, an with I -withphase the XRD was CoTi as found shown2 phase beside in Figure without the 4CoTib. otherFor2 phaseTi60 crystalZr 10[28]Co3. phases.0 ,However, with a molar For for Ti ratio Ti7050ZrZr of1010 Co/TiCo4200, ,aof annew 1/2, I-phase theCoTi was diffraction patterns are in accordance with the CoTi2 phase without other crystal phases. For foundphase beside was the detected CoTi2 besidephase the [28 CoTi]. However,2 phase. for Ti50Zr10Co40, a new CoTi phase was detected beside Ti70Zr10Co20, an I-phase was found beside the CoTi2 phase [28]. However, for Ti50Zr10Co40, a new CoTi the CoTi2 phase. phase was detected beside the CoTi2 phase.

(a) (b) (a) (b) Figure 4. The SEM image of Ti60Zr10Co30 (a); and the XRD patterns of Ti-Zr-Co catalysts and the JCPDS 07-0141Figure file 4. identifyingThe SEM image CoTi of2 (Tib60).Zr 10Co30 (a); and the XRD patterns of Ti-Zr-Co catalysts and the JCPDS Figure 4. a 07-0141The fi SEMle identifying image ofCoTi Ti260 (bZr). 10Co30 ( ); and the XRD patterns of Ti-Zr-Co catalysts and the JCPDS 07-0141 ﬁle identifying CoTi2 (b).

Catalysts 2016, 6, 24 6 of 9 Catalysts 2016, 6, 24 6 of 9

Therefore, thethe variationvariation of of Co Co content content will will impact impact the bulkthe bulk structure structure of TiZrCo of TiZrCo significantly, significantly which, wmayhich have may anhave effect an effect on the on catalytic the catalytic activity. activity By. By comparison, comparison, the the surface surface species species will will play play aa more important role in the catalysis due to the catalytic reaction always occurringoccurring on the surface of the heterogeneous catalyst.catalyst. TheThe surfacesurface compositioncomposition of of TiZrCo TiZrCo was was studied studied by by XPS. XPS As. A showns shown in in Figure Figure5, 5the, the Ti Ti and and Zr Zr existed existed on on the the surface surface with with oxidation oxidation states states of TiOof TiO2 and2 and ZrO ZrO2, and2, and Co Co mainly mainly existed existed on onthe the surface surface with with metallic metallic Co, oxidesCo, oxides of CoO of CoO and Co and3O 4Co, as3O confirmed4, as confirmed by the by peak the at peak the bindingat the binding energy energyof 777.6 of eV 777.6 and intenseeV and shake-upintense shake satellites-up satellites Co 2p XPS Co spectra 2p XPS around spectra 6 around eV above 6 eV the above primary the spin-orbit primary 3+ 2+ spinBes [-29orbit–31 ];Bes Co [293O4–31was]; Co confirmed3O4 was byconfirm Co /Coed by (2/1)Co3+/Co [322+]. (2/1) The surface[32]. The compositions surface compositions calculated calculatedaccording toaccording the results to the of XPSresults (Figure of XPS5) ( areFigure listed 5) inare Table listed3 .in To Table consider 3. To theconsider main the active main species active speciesof the Co of element,the Co element, it is clear it is that clear all thethatcatalysts all the catalysts contained contained the same the species same species of Co, CoOof Co, and CoO Co and3O4 Coon3 theirO4 on surfaces, their surface includings, including CoTi2 .CoTi The2. most The most active active Ti50Zr Ti1050CoZr1040Cocatalyst40 catalyst contains contain as higher a higher ratio ratio of ofCoO CoO on on the the surface surface and and different different bulk bulk phases, phases, which which suggested suggested CoO CoO may may be morebe more effective effective than than the theCo3 OCo4 3species,O4 species, and and the bulkthe bulk phase phase may bemay also be involved also involved in the presentin the present catalysis catalysis based on based the reaction on the reactionresults in results Figure in3. Figure In addition, 3. In addition the surface, the area surface of TiZrCo area of catalysts TiZrCo was catalysts also examined, was also examined as shown, inas Tableshown3; in it shouldTable 3 have; it should less effect have on less the effect present on the oxidation present compared oxidation tocompared the surface to the active surface species, active as speciesthe Ti70,Zr as10 theCo Ti2070withZr10Co the20 largestwith the surface largest area surface did showarea did a lower show conversion,a lower conversion, which contains which contains a lower aratio lower of CoOratio andof CoO Co3 andO4 on Co the3O4surface. on the surface.

Figure 5 The XPS spectra of (a) Co 2p; (b) Ti 2p and (c) Zr 3d for Ti-Zr-Co catalysts. Figure 5. The XPS spectra of (a) Co 2p; (b) Ti 2p and (c) Zr 3d for Ti-Zr-Co catalysts.

Catalysts 2016, 6, 24 7 of 9

Table 3. Textural and surface properties.

b a Surface Content of Co (%) 2 c Entry Catalyst Bulk Phase SBET (m /g) Co CoO Co3O4

1 CoTi2 CoTi2, I-phase 31.5 37.8 30.7 - 2 Ti70Zr10Co20 CoTi2, I-phase 26.9 31.2 40.9 189 3 Ti60Zr10Co30 CoTi2 20.1 32.8 47.1 70 4 Ti50Zr10Co40 CoTi2, CoTi 11.5 70.3 18.2 76 a The bulk phase of the alloy catalysts was obtained based on the XRD patterns; b The surface composition of the alloy catalysts was calculated from XPS results; c The surface areas were calculated using the BET equation.

3. Experimental Section

3.1. Ti-Zr-Co Alloy Preparation The series of Ti-Zr-Co alloys, as reported in our previous works [25], were prepared by arc-melting of Ti (99 wt. %), Zr (97 wt. %) and Co (99 wt. %) metals with a certain mole ratio on a water-cooled cuprum hearth in a high-purity argon atmosphere at 250 A. To make the chemical compositions homogenous, the ingot of alloy was turned over and remelted at least three times. After that, the surface of the cast ingot was burnished in order to eliminate the oxide layer. Then the alloy ingot was crushed by repeated manual beating with a steel pestle and mortar, and the alloy powders were screen separated by 140 meshes.

3.2. Catalyst Characterization The phase composition and microstructure of the alloys were examined by X-ray diffraction (XRD) on a Bruker-AXS D8 ADVANCE (Bruker AXS, Karlsruhe, Germany) with Kα. The leaching of Ti, Zr or Co in the ﬁltrate was not detected by ICP-OES measurement (iCAP6300, Thermo Waltham, MA, USA). XPS measurements were performed by using a VG Microtech 3000 Multilab. The electronic states of Co 2p, Zr 3d and Ti 2p were determined. All XPS spectra were corrected to the C 1s peak at 284.6 eV. Scanning electron microscopy (SEM) image was performed on a Hitachi S-4800 ﬁeld emission scanning electron microscope (HITACHI, Tokyo, Japan) at an accelerating voltage of 10 kV, and the size of particles was in a range of 50–100 µm. Nitrogen porosimetry measurement was performed on a Micromeritics ASAP 2020M instrument (Micromeritics, Norcross, GA, USA). The surface areas were calculated using the BET equation.

3.3. Catalytic Tests The catalytic performance was tested in a stainless steel autoclave with a Teflon inner liner (50 mL). Typically, the Ti-Zr-Co catalyst (20 mg), cyclohexene (2 mL) and acetonitrile (20 mL) were ˝ added. The reactor was then sealed, and placed into an oil bath preset to 120 C for 5 min. O2 (2 MPa) was introduced and the reaction was started with a continuously stirring at 1200 rpm. When the reaction finished, the reactor was cooled to room temperature and then depressurized carefully. Then the reaction solution was diluted with ethanol to 50 mL, the composition of reaction products was confirmed by gas chromatography/mass spectrometry (Agilent 5890, Santa Clara, CA, USA) and analyzed with a gas chromatograph (Shimadzu, Kyoto, Japan, GC-2010) equipped with a capillary column (RTX-50, Bellefonte, PA, USA, 30 m ˆ 0.25 mm ˆ 0.25 µm, carrier: N2) and a flame ionization detector (FID). Safety warning: The use of compressed O2 in the presence of organic substrates requires appropriate safety precautions and must be carried out in suitable equipment. When the pressure decreased far more quickly, the reaction must be stopped immediately. Note, the reaction should be diluted by solvent, otherwise, the blast occur as the reaction is very violent. Catalysts 2016, 6, 24 8 of 9

4. Conclusions In summary, TiZrCo catalysts were studied for the ﬁrst time for the oxidation of cyclohexene. High selectivity (57.6%) to 2-cyclohexen-1-one was obtained at a high conversion (92.2%) of cyclohexene. CoO and Co3O4 on the surface are the main active species and contribute to the high activity and selectivity in the present cyclohexene oxidation. These results indicate that TiZrCo metallic catalysts are effective for the aerobic oxidation of cyclohexene. It is important and signiﬁcant to extend the studies of the metallic alloy catalyst in catalyzing the aerobic oxidation of hydrocarbons. It is expected that the TiZrCo catalysts may have a broad prospect in industrial applications.

Acknowledgments: The authors gratefully acknowledge the financial support from the One Hundred Talent Program of CAS and Youth Innovation Promotion Association CAS. Author Contributions: Tong Liu did the experiment and wrote the paper, Haiyang Cheng discussed the results and revised the paper, Chao Zhang, Weiwei Lin, Yancun Yu, Fengyu Zhao supervised this work. Conflicts of Interest: The authors declare no conflict of interest.

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