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
2 3
*
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.) Laboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China 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 Co3 O4 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, H2 O2 , 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 [C10 mim][Co(F6 -acac)3 ] exhibited high catalytic Catalysts 2016, 6, 24; doi:10.3390/catal6020024
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activity the allylic oxidation cyclohexene in the absence of solvent a high selectivity of catalyticfor activity for the allylic of oxidation of cyclohexene in the absencewith of solvent with a high ´ ] complex 81% to 2-cyclohexen-1-one at a conversion of 100% [10], and the Cu-[cationic salphen][Br 2 selectivity of 81% to 2-cyclohexen-1-one at a conversion of 100% [10], and the Cu-[cationic presented a −selectivity 64.1% to 2-cyclohexen-1-one at to a 100% conversion for at thea aerobic oxidation salphen][Br ]2 complexofpresented a selectivity of 64.1% 2-cyclohexen-1-one 100% conversion of cyclohexene in acetonitrile [11]. Although these homogeneous catalysts are efficient for the selective for the aerobic oxidation of cyclohexene in acetonitrile [11]. Although these homogeneous catalysts allylic oxidation of cyclohexene, their industrial application is limited due to the difficult separation, are efficient for the selective allylic oxidation of cyclohexene, their industrial application is limited and residual metal ions willand affect quality of the product. Hence, heterogeneous catalystsHence, have due the to the difficult separation, thethe residual metal ions will affect the quality of the product. been developed by immobilizing metal complexes on solid supports, such as resinate-immobilized heterogeneous catalysts have been developed by immobilizing metal complexes on solid supports, Co(II), exhibited high catalytic with a 44.4% to 2-cyclohexen-1-one at a 94.5% such aswhich resinate-immobilized Co(II), activity which exhibited highselectivity catalytic activity with a 44.4% selectivity conversion of cyclohexene Core-shell type of Fe3cyclohexene O4 @chitosan-Schiff base-immobilized Co(II), Cu(II) to 2-cyclohexen-1-one at a[12]. 94.5% conversion [12]. Core-shell type Fe3O 4@chitosanand Mn(II) complexes were also reported active for the cyclohexene oxidation, and a selectivity of Schiff base-immobilized Co(II), Cu(II) and Mn(II) complexes were also reported active for the 77.2% to 2-cyclohexen-1-one was obtained at a to conversion of 46.8% [13]. On the other hand, the cyclohexene oxidation, and a selectivity of 77.2% 2-cyclohexen-1-one was obtained at a conversion supported transition metal or oxides were also employed as heterogeneous catalysts for the allylic of 46.8% [13]. On the other hand, the supported transition metal or oxides were also employed as oxidation of cyclohexene [6,14–20]. Recently, Au nanoparticles supported on modified bentonite heterogeneous catalysts for the allylic oxidation of cyclohexene [6,14–20]. Recently, Au nanoparticles and silica gave a high conversion (92%) andgave an excellent selectivity(92%) (97%) to an 2-cyclohexen-1-one in supported on modified bentonite and silica a high conversion and excellent selectivity the aerobic oxidation of cyclohexene without solvent [21]. It was also reported that PdO/SBA-15 (97%) to 2-cyclohexen-1-one in the aerobic oxidation of cyclohexene without solvent [21]. It was also was an active catalyst for the oxidation cyclohexene acetonitrile, and a conversion of 56% and reported that PdO/SBA-15 was an activeof catalyst for thein oxidation of cyclohexene in acetonitrile, a selectivity 2-cyclohexen-1-one were obtained [6]. In addition, nitrogen-doped carbon conversionof of82% 56%to and a selectivity of 82% to 2-cyclohexen-1-one were obtained [6]. In addition, nanotubes, and graphitic carbon nitride-supported FeO and CoO were also effective catalysts foralso the nitrogen-doped carbon nanotubes, and graphitic carbon nitride-supported FeO and CoO were oxidation of cyclohexene [14,22]. Until now, it was [14,22]. still a hot topic to design heterogeneous effective catalysts for the oxidation of cyclohexene Until now, it was astill a hot topic tocatalyst design for the aerobic allylic oxidation of cyclohexene. a heterogeneous catalyst for the aerobic allylic oxidation of cyclohexene. In our previous previous work, work,we wefound foundthat thatTi-Zr-Co Ti-Zr-Cometallic metalliccatalyst catalystwas was effective oxidation effective forfor thethe oxidation of of cyclohexane and ethylbenzene [23–25],ininwhich whichcyclohexanol cyclohexanoland and cyclohexanone cyclohexanone were produced cyclohexane and ethylbenzene [23–25], with a high selectivity of of 90% 90% at at aa conversion conversionaround around7%, 7%,and andacetophenone acetophenonewas wasproduced producedwith witha a69.2% 69.2%selectivity selectivity a high conversion 61.9%. The Ti-Zr-Co metallic catalyst is simple and cheap atat a high conversion ofof 61.9%. The Ti-Zr-Co metallic catalyst is simple and cheap in in production and sturdy wearing utilization, compared those reported catalysts such production and sturdy to to wearing in in thethe utilization, as as compared to to those reported catalysts such as as metal-organic complex, metal nanoparticles and nanocarbon materials. Therefore, it inspires metal-organic complex, metal nanoparticles and nanocarbon materials. Therefore, it inspires us us to to study efficiency catalyzingother otherhydrocarbon hydrocarbonoxidations. oxidations.InInthe the present present work, work, the catalytic study itsits efficiency inincatalyzing performances of of the theTi-Zr-Co Ti-Zr-Cocatalysts catalystswere werediscussed discussed aerobic oxidation of cyclohexene. forfor thethe aerobic oxidation of cyclohexene. We We found 2-cyclohexen-1-one was produced high selectivity at a conversion found that that 2-cyclohexen-1-one was produced with awith high aselectivity of 57.6%ofat57.6% a conversion of 92.2%, of 92.2%, are comparable to the best results reported the aerobic oxidation of cyclohexene which arewhich comparable to the best results reported for thefor aerobic oxidation of cyclohexene over over heterogeneous catalysts. It was confirmed the surface CoO and Co O acted as the catalytic active heterogeneous catalysts. It was confirmed the surface CoO and Co3O 3 4 4acted as the catalytic active sites sites and and contributed contributed to to the the excellent excellent conversion conversion and and selectivity. selectivity. 2. Resultsand andDiscussion Discussion 2. Results For it is For the the oxidation oxidation of of cyclohexene, cyclohexene, it is very very difficult difficult to to control control product product selectivity selectivity due due to to the the existence of the two active groups of the C-H bond at the allylic site and the C=C bond, as when existence of the two active groups of the C-H bond at the allylic site and the C=C bond, as when the the bond is oxidized, 2-cyclohexene-1-ol, 2-cyclohexene-1-one or cyclohexene hydroperoxide C–HC–H bond is oxidized, 2-cyclohexene-1-ol, 2-cyclohexene-1-one or cyclohexene hydroperoxide will will be generated; as the C=C bond is oxidized, cyclohexene oxide, cyclohexanol, cyclohexanone, be generated; as the C=C bond is oxidized, cyclohexene oxide, cyclohexanol, cyclohexanone, cyclohexanediol cyclohexanediol and and dialdehyde dialdehyde will will be be produced produced (Scheme (Scheme 1) 1) [26]. [26].
Scheme 1. 1. Reaction for the the aerobic aerobic oxidation oxidation of of cyclohexene. cyclohexene. Scheme Reaction route route for
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 Table 1. 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-
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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 Ti60 Zr10 Co30 catalyst. Solvent Acetone Acetonitrile Ethanol Cyclohexane
Selectivity (%) a
Conversion (%) 43.9 38.0 22.0 6.2
(1)
(2)
(3)
(4)
(5)
Others b
7.3 3.0 1.4 -
0 0.4 0 0.2
1.5 1.4 0.3 7.4
13.3 13.6 4.7 9.9
71.8 60.6 25.0 37.8
6.1 21.0 68.6 c 44.7 d
Reaction conditions: cyclohexene 1 mL (at a concentration of 4.8%), solvent 20 mL, Ti60 Zr10 Co30 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 Table 2. 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. Entry 1 2 3 4c
Cyclohexene (%) a 4.8 9.1 13.0 16.7
Time (h) 5 5 5 3.3
Selectivity (%)
Conversion (%) 33.8 67.0 97.5 92.8
(1)
(2)
(3)
(4)
(5)
Others b
3.1 5.5 1.9
0.9
1.7 0.7 0.6 1.4
19.1 11.6 3.7 6.2
71.3 62.5 49.5 30.2
4.9 19.8 46.1 59.4
Reaction conditions: acetonitrile 20 mL, Ti60 Zr10 Co30 20 mg, O2 2 MPa, 120 ˝ C. a Concentration of cyclohexene = V cyclohexene /(V cyclohexene + V acetonitrile ) ˆ 100%; b 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 Figure 1, 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%.
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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 of 92.2%. at a high conversion obtained at a high conversion of 92.2%.
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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%), Ti60 Zr10 Co30 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.
examined at conditions of 120of °C,reaction a cyclohexene concentration of 9.1%and and an oxygenselectivity pressure ofwere In addition, the influence time on the conversion product ˝ 2 MPa. As shown in Figure 2, the conversion increased to 96.1% with extending the reaction time to examined conditions of 120 C, cyclohexene 9.1% and an oxygen pressure In at addition, the influence of a reaction time onconcentration the conversionofand product selectivity were 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. As shown in Figure 2, the increased to 96.1% withand extending the pressure reaction of time to examined at conditions of 120 °C, conversion a cyclohexene concentration of 9.1% an oxygen 68.8% to 55.3% due to the formation and accumulation of deep oxidation products with the reaction 2 MPa. As shown in Figure 2, the conversion increased to 96.1% with extending the reaction time to 8 h. The selectivity of 2-cyclohexen-1-ol and 2-cyclohexen-1-one decreased from 20.3% to 4.0% and proceeding. The other products derived from deep oxidation such as the ring-opening acids 8 h. selectivity 2-cyclohexen-1-one 20.3%with to 4.0% 68.8% toThe 55.3% due to of the2-cyclohexen-1-ol formation and and accumulation of deep decreased oxidationfrom products theand reaction increased with a selectivity up to 40% (8 h) from 6% (1 h), and the intermediates such as cyclohexene 68.8% to 55.3% due to the formation and accumulation of deep oxidation products with the reaction proceeding. The other products derived from deep oxidation such as the ring-opening acids increased oxide, cyclohexanol and cyclohexanone changed very little (
