Liquid-Phase Catalytic Oxidation of Limonene to Carvone Over ZIF-67(Co)

catalysts

Article Liquid-Phase Catalytic Oxidation of Limonene to Carvone over ZIF-67(Co)

Yizhou Li, Yepeng Yang, Daomei Chen, Zhifang Luo, Wei Wang, Yali Ao, Lin Zhang, Zhiying Yan * and Jiaqiang Wang *

National Center for International Research on Photoelectric and Energy Materials, Yunnan Provincial Collaborative Innovation Center of Green Chemistry for Lignite Energy, Yunnan Province Engineering Research Center of Photocatalytic Treatment of Industrial Wastewater, The Universities’ Center for Photocatalytic Treatment of Pollutants in Yunnan Province, School of Chemical Sciences & Technology, Yunnan University, Kunming 650091, China; [email protected] (Y.L.); [email protected] (Y.Y.); [email protected] (D.C.); [email protected] (Z.L.); [email protected] (W.W.); [email protected] (Y.A.); [email protected] (L.Z.) * Correspondence: [email protected] (Z.Y.); [email protected] (J.W.); Tel.: +86-871-6503-1567 (Z.Y.); +86-871-6503-1567 (J.W.) Received: 15 March 2019; Accepted: 16 April 2019; Published: 21 April 2019

Abstract: Liquid-phase catalytic oxidation of limonene was carried out under mild conditions, and carvone was produced in the presence of ZIF-67(Co), cobalt based zeolitic imidazolate framework, as catalyst, using t-butyl hydroperoxide (t-BHP) as oxidant and benzene as solvent. As a heterogeneous catalyst, the zeolitic imidazolate framework ZIF-67(Co) exhibited reasonable substrate–product selectivity (55.4%) and conversion (29.8%). Finally, the X-ray diﬀraction patterns of the catalyst before and after proved that ZIF-67(Co) acted as a heterogeneous catalyst, and can be reused without losing its activity to a great extent.

Keywords: liquid-phase catalytic oxidation; limonene; carvone; zeolitic imidazolate frameworks

1. Introduction As we know, the reaction about the allylic oxidation plays a critical role in developing fine chemicals with high additional value from biomass, and has great value in the synthesis of unsaturated aldehydes and ketones [1,2]. Carvone is a main ingredient, derived from plant essential oils, used for cosmetics and food flavors, and is also used in the preservation of meat, fruits, and vegetables because it has good antioxidant activity, analgesic effects, and antibacterial effects [3,4]. In addition, carvone is an important intermediate in industrial chemistry, where it can further be used to synthesize carvone thioether and cyanoacetone [5]. The applicability of traditional methods of transforming limonene to carvone through catalytic oxidation, such as epoxidation and nitrosochlorination, is limited; largely because environmentally unfriendly reagents are often used and poisonous secondary products are produced [6]. Compared with these methods, the use of heterogeneous catalysts has broader applicability prospects in the allylic oxidation of cycloolefins, due to their remarkable advantages in catalyst recovery and stability. However, current research results show that the selectivity of the catalytic oxidation of limonene to carvone by heterogeneous reaction has not been satisfactory. For instance, 36% substrate conversion and 25% selectivity for carvone were obtained by using chromium-containing mesoporous molecular sieves MCM-41 [7]. Lower than 5% selectivity for carvone was obtained by using the III III [Fe (BPMP)Cl(µ-O)Fe Cl3] complex as catalyst [8]. A 21% conversion and 20% selectivity for carvone were obtained in a reaction using Fe/EuroPh catalysts [9]. In another study, high conversion up to 93%

Catalysts 2019, 9, 374; doi:10.3390/catal9040374 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 374 2 of 13 of limonene was attained, while the selectivity for carvone was less than 7% [10]. In our previous study, cobalt-doped mesoporous silica templated by reed leaves exhibited high substrate conversion (100%) and relatively good product (carvone) selectivity (40.2%) for the allylic oxidation of limonene to carvone [11]. Metal–organic frameworks (MOFs) have attracted considerable attention in catalysis and adsorption in recent years, owing to their advantages, such as high surface areas, well-defined structures, special metal centers, their ease of processing, and their structural diversity [12–16]. Thus, the performance of MOF materials in this reaction has also been reported [17,18], but their application is limited by their thermal and chemical stability [19]. However we are sure that it is valuable to use some MOFs materials in this reaction, and this research has aroused our great interest. Zeolitic imidazolate frameworks (ZIFs), which are constructed from tetrahedrally coordinated divalent cations (Zn2+ or Co2+) linked by the uninegative imidazolate ligands, are a new class of porous metal–organic framework (MOF) [20]. They are widely used in organic synthesis [21–25], CO2 capture of [26], and olefin/paraffin separation [27] because of their excellent thermal and chemical stability. They benefit from strong interactions between imidazolium salts and metal ions, which make the frameworks of zeolitic imidazolates maintain their structural integrity even in water, and this is difficult for other MOFs [28]. Additionally, ZIF-67 has a high-porosity zeolite structure, which is conducive to catalytic reactions. According to our surveys, ZIFs have been widely used to catalyze different types of organic synthesis reactions in recent years, such as cyclohexene hydrogenation [29], glycerol esterification [30], and transesterification reactions [31].To this end, the zeolitic imidazole framework ZIF-67(Co) was selected because ZIF-67 is easy to prepare at large scale in water under ambient conditions without using toxic solvents, and the preparation of ZIF-67(Co) has good reproducibility. In this continuation of our work, we successfully synthesized ZIF-67(Co) by hydrothermal synthesis, and it displayed good catalytic properties in the catalytic oxidation of limonene. Furthermore, the application of tert-butyl hydroperoxide (t-BHP) to the allylic oxidation of cycloolefins offers an alternative to traditional unfriendly oxidants. In this work, the effects of solvents, oxidants, and reaction temperature on the productivity and reaction time were investigated. Under the optimal conditions, although the conversion of limonene was only 29.8%, we increased the selectivity for carvone to 55.4%, which is more than that in all the literature we have been able to refer to. For comparison, the catalytic activities of more MOFs were also investigated. This is the first report about the oxidation of limonene to carvone over ZIF-67(Co).

2. Results and Discussion

2.1. Characterization of ZIF-67(Co) Powder XRD patterns of the ZIF-67(Co) are shown in Figure1. The di ﬀraction peaks of the 2θ values at 7.48◦, 10.42◦, 12.74◦, and 18.06◦, were assigned to the (011), (002), (112), and (222) planes of ZIF-67, which has been reported by other researchers [20,21,29]. Figure1 also reﬂects the fact that synthetic ZIF-67(Co) is a phase-pure product. The N2 sorption experiments for the ZIF-67(Co) yielded the typical type I isotherms in Figure2. 2 The Brunauer–Emmett–Teller (SBET) and Langmuir surface areas of ZIF-67 were 1378 and 1805 m /g, and the pore volume was 0.62 cm3/g, which are near to the previously reported values [32,33]. At lower relative pressures, the micropore in the material makes the adsorption capacity increase rapidly, while at higher relative pressures, due to the existence of spin/large porosity nanoparticles structure, the enhancement of adsorption capacity is lower. The pore size distribution curve was calculated by the Horvath–Kawazoe (HK) method, which clearly showed that there were two kinds of micropores in ZIF-67(Co). This result is similar to an earlier work [22]. Catalysts 2019, 9, x FOR PEER REVIEW 3 of 12

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84 Figure 1. XRD patterns of the zeolitic imidazolate framework ZIF-67(Co).

85 The N2 sorption experiments for the ZIF-67(Co) yielded the typical type I isotherms in Figure 2. 86 The Brunauer–Emmett–Teller (SBET) and Langmuir surface areas of ZIF-67 were 1378 and 1805 m2/g, 87 and the pore volume was 0.62 cm3/g, which are near to the previously reported values [32,33]. At 88 lower relative pressures, the micropore in the material makes the adsorption capacity increase 89 rapidly, while at higher relative pressures, due to the existence of spin / large porosity nanoparticles 90 structure, the enhancement of adsorption capacity is lower. The pore size distribution curve was 91 83 calculated by the Horvath–Kawazoe (HK) method, which clearly showed that there were two kinds 92 of micropores in ZIF-67(Co).Figure 1. XRD This patterns result of is the similar zeolitic to imidazolate an earlier frameworkwork [22]. ZIF-67(Co). 84 Figure 1. XRD patterns of the zeolitic imidazolate framework ZIF-67(Co).

85 The N2 sorption experiments for the ZIF-67(Co) yielded the typical type I isotherms in Figure 2. 86 The Brunauer–Emmett–Teller (SBET) and Langmuir surface areas of ZIF-67 were 1378 and 1805 m2/g, 87 and the pore volume was 0.62 cm3/g, which are near to the previously reported values [32,33]. At 88 lower relative pressures, the micropore in the material makes the adsorption capacity increase 89 rapidly, while at higher relative pressures, due to the existence of spin / large porosity nanoparticles 90 structure, the enhancement of adsorption capacity is lower. The pore size distribution curve was 91 calculated by the Horvath–Kawazoe (HK) method, which clearly showed that there were two kinds 92 of micropores in ZIF-67(Co). This result is similar to an earlier work [22].

Figure 2. N2 adsorption/desorption isotherms of ZIF-67(Co) samples at 77 K and Horvath–Kawazoe 94 Figure(HK) pore 2. N size2 adsorption/desorption distribution curve. isotherms of ZIF-67(Co) samples at 77 K and Horvath–Kawazoe 95 (HK) pore size distribution curve. The morphology and particle size of the samples revealed by scanning electron microscopy 96 (SEM)The are morphology shown in Figureand particle3. The size SEM of picturethe samples reveals revealed that the by particles scanning were electron submicroscopic microscopy 97 (SEM)crystals are with shown polyhedral in Figure shapes, 3. eachThe surfaceSEM picture was a homogeneousreveals that the quadrilateral, particles were and thesubmicroscopic particle sizes 98 crystalsranged fromwith 200polyhedral to 500 nm. shapes, This each also provessurface thatwas the a homogeneous as-synthesized quadrilateral, ZIF-67(Co) was and composed the particle of 99 sizessubmicroscopic ranged from crystals. 200 to 500 nm. This also proves that the as-synthesized ZIF-67(Co) was composed 100 of submicroscopicFT-IR and TGA crystals. were employed to obtain further structural information about the ZIF-67(Co). 1 1 93 Figure4 presents FT-IR spectra of ZIF-67(Co) between 400 and 4000 cm − . The peak at 3420 cm− is a contribution of the asymmetric characteristic absorption peaks of O–H bonds in hydroxyl groups 1 94 adsorbedFigure on 2. theN2 adsorption/desorption surface of materials. Thereisotherms were of alsoZIF-67(Co) some peakssamples between at 77 K and 500 andHorvath–Kawazoe 1500 cm− caused 95 by plane-bending(HK) pore size distribution vibrations curve. and stretching vibrations of the imidazole ring. Meanwhile, the two 1 small peaks at 758 and 534 cm− were attributed to the stretching vibration peak of the Co–N bond. 96 The morphology and particle size of the samples revealed1 by scanning electron microscopy In addition, there were two bands at 3134 and 2931 cm− caused by the asymmetric absorption 97 vibrations(SEM) are ofshown the C–H in bondFigure in 3. the The methyl SEM groups. picture reveals that the particles were submicroscopic 98 crystals with polyhedral shapes, each surface was a homogeneous quadrilateral, and the particle 99 sizes ranged from 200 to 500 nm. This also proves that the as-synthesized ZIF-67(Co) was composed 100 of submicroscopic crystals.

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101

102 Figure 3. SEM pictures of ZIF-67(Co).

103 FT-IR and TGA were employed to obtain further structural information about the ZIF-67(Co). 104 Figure 4 presents FT-IR spectra of ZIF-67(Co) between 400 and 4000 cm−1. The peak at 3420 cm−1 is a 105 contribution of the asymmetric characteristic absorption peaks of O–H bonds in hydroxyl groups 106 adsorbed on the surface of materials. There were also some peaks between 500 and 1500 cm−1 caused 107 by plane-bending vibrations and stretching vibrations of the imidazole ring. Meanwhile, the two 108 small peaks at 758 and 534 cm−1 were attributed to the stretching vibration peak of the Co–N bond. In 101 109 addition, there were two bands at 3134 and 2931 cm−1 caused by the asymmetric absorption Figure 3. SEM pictures of ZIF-67(Co). 102110 vibrations of the C–H bond in theFigure methyl 3. SEMgroups. pictures of ZIF-67(Co).

103 FT-IR and TGA were employed to obtain further structural information about the ZIF-67(Co). 104 Figure 4 presents FT-IR spectra of ZIF-67(Co) between 400 and 4000 cm−1. The peak at 3420 cm−1 is a 105 contribution of the asymmetric characteristic absorption peaks of O–H bonds in hydroxyl groups 106 adsorbed on the surface of materials. There were also some peaks between 500 and 1500 cm−1 caused 107 by plane-bending vibrations and stretching vibrations of the imidazole ring. Meanwhile, the two 108 small peaks at 758 and 534 cm−1 were attributed to the stretching vibration peak of the Co–N bond. In 109 addition, there were two bands at 3134 and 2931 cm−1 caused by the asymmetric absorption 110 vibrations of the C–H bond in the methyl groups.

111 Figure 4. FT-IR spectra of ZIF-67(Co). 112 Figure 4. FT-IR spectra of ZIF-67(Co). The synthesized ZIF-67(Co) submicroscopic crystals had high thermostability, which was proved 113 by TGAThe andsynthesized diﬀerential ZIF-67(Co) scanning submicroscopic calorimetry (DSC) crystals as shown had high in Figure thermostability,5. The test which increased was 114 temperatureproved by TGA to 873 and K differential from room temperaturescanning calorimetry at 5 K/min. (DSC) When as calcinedshown in below Figure 373 5. K,The the test total increased weight 115 losstemperature of the sample to 873 was K veryfrom small room (about temperature 8%), relating at 5 toK/min. the removal When ofcalcined guest molecules below 373 such K, asthe water. total 116 Betweenweight loss 373 of and the 510 sample K, the was curve very remained small (about smooth 8%), overall, relating indicating to the removal that the of skeleton guest molecules of ZIF-67(Co) such 117 hasas water. good thermalBetween stability 373 and below 510 K, 510 the K. curve When remained the temperature smooth overall, was greater indicating than 520 that K, the the skeleton mass loss of 118 beganZIF-67(Co) at the has stage good of skeletal thermal decomposition. stability below This 510 wasK. When attributed the temperature to the collapse was of greater the ligands, than which520 K, 111 119 wasthe mass caused loss by began the decomposition at the stage of ofskeletal 2-methylimidazole. decomposition. The This weight-loss was attributed process to the was collapse complete of the at 120 ligands, which was caused by the decomposition of 2-methylimidazole. The weight-loss process was 112 620 K, and the residue was Co3O4Figure. 4. FT-IR spectra of ZIF-67(Co). 121 complete at 620 K, and the residue was Co3O4. 113 The synthesized ZIF-67(Co) submicroscopic crystals had high thermostability, which was 114 proved by TGA and differential scanning calorimetry (DSC) as shown in Figure 5. The test increased 115 temperature to 873 K from room temperature at 5 K/min. When calcined below 373 K, the total 116 weight loss of the sample was very small (about 8%), relating to the removal of guest molecules such 117 as water. Between 373 and 510 K, the curve remained smooth overall, indicating that the skeleton of 118 ZIF-67(Co) has good thermal stability below 510 K. When the temperature was greater than 520 K, 119 the mass loss began at the stage of skeletal decomposition. This was attributed to the collapse of the 120 ligands, which was caused by the decomposition of 2-methylimidazole. The weight-loss process was 121 complete at 620 K, and the residue was Co3O4.

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122 Figure 5. TGA and diﬀerential scanning calorimetry (DSC) curves of ZIF-67(Co) submicroscopic crystals. 123 Figure 5. TGA and differential scanning calorimetry (DSC) curves of ZIF-67(Co) submicroscopic 124 2.2. Catalyticcrystals. Experiments

125 2.2.1.2.2. Catalytic The Eff ectExperiments of Various Catalysts on the Reaction The comparison of the catalytic activities of different catalysts for the oxidation of limonene is 126 2.2.1. The Effect of Various Catalysts on the Reaction summarized in Table1. Using t-BHP as oxidant yielded carvone as the main product, and the reaction 127 timeThe was comparison 8 h. of the catalytic activities of different catalysts for the oxidation of limonene is 128 summarized in Table 1. Using t-BHP as oxidant yielded carvone as the main product, and the 129 reactionTable time 1. Comparison was 8 h. of the catalytic activities of various metal–organic frameworks (MOFs) for the oxidation of limonene. 130 Table 1. Comparison of the catalytic activities of various metal–organic frameworks (MOFs) for the Catalysts Conversion (%) Selectivity (%) 131 oxidation of limonene. MIL-101(Fe) 50.8 20.3 ZIF-8(Zn)Catalysts Conversion 16.5 (%) Selectivity 9.2 (%) MIL-101(Fe)ZIF-67(Co) 50.8 35.7 20.3 32.6 MIL-101(Cr) 44.1 26.6 ZIF-8(Zn) 16.5 9.2 HKUST-1(Cu) 20.4 7.9 MIL-125(Ti)ZIF-67(Co) 35.7 18.2 32.6 10.1 MIL-101(Cr)No catalyst 44.1 26.7 26.6 0.05 Conditions: limonene: 2HKUST-1(Cu) mL, catalyst: 100 mg, solvent:20.4 15 mL acetic acid, 7.9 oxidant: 15 mL t-BHP (tert-butyl hydroperoxide), temperature:MIL-125(Ti) 85 ◦C, reaction time: 8 h.18.2 10.1 No catalyst 26.7 0.05 132 First,Conditions: the reactivity limonene: of several2 mL, catalyst: common 100 catalysts mg, solven wast: compared 15 mL acetic without acid, any oxidant: condition 15 mL optimization. t-BHP 133 The activity(tert-butyl of hydroperoxid MIL-101(Fe)(Materialse), temperature: of Institute 85 °C, reaction Lavoisier time: Frameworks) 8 h. was the most outstanding for the oxidation of limonene—the conversion rate was 50.8% after 8 h; followed by MIL-101(Cr), 134 First, the reactivity of several common catalysts was compared without any condition whose conversion rate was 44.1%. ZIF-8 showed the worst activity, with a conversion rate of only 135 optimization. The activity of MIL-101(Fe)(Materials of Institute Lavoisier Frameworks) was the most 16.5%. However, after the reaction, the filtrates from HKUST-1(Cu)([Cu (BTC) (H O) ] ), MIL-101(Fe), 136 outstanding for the oxidation of limonene—the conversion rate was3 50.8%2 after2 38 nh; followed by and MIL-101(Cr) were light blue, dark red, and yellow-green, respectively. This indicates that metal 137 MIL-101(Cr), whose conversion rate was 44.1%. ZIF-8 showed the worst activity, with a conversion ions, as metal centers, leaked during the reactions with HKUST-1(Cu), MIL-101(Fe), and MIL-101(Cr). 138 rate of only 16.5%. However, after the reaction, the filtrates from HKUST-1(Cu)([Cu3(BTC)2(H2O)3]n), On the other hand, ZIF-67 showed the highest selectivity, and the selectivity for carvone reached 36.6% 139 MIL-101(Fe), and MIL-101(Cr) were light blue, dark red, and yellow-green, respectively. This after 8 h of reaction. The high selectivity of ZIF-67(Co) may be attributed to its unique ion centers. 140 indicates that metal ions, as metal centers, leaked during the reactions with HKUST-1(Cu), Heterogeneous catalysis with MOF materials is one of their extensively investigated applications [21]. 141 MIL-101(Fe), and MIL-101(Cr). On the other hand, ZIF-67 showed the highest selectivity, and the 142 selectivity for carvone reached 36.6% after 8 h of reaction. The high selectivity of ZIF-67(Co) may be 143 attributed to its unique ion centers. Heterogeneous catalysis with MOF materials is one of their 144 extensively investigated applications [21].

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145 2.2.2.2.2.2. The The E ffEffectect of of the the Catalyst Catalyst Dosage Dosage on on the the Reaction Reaction 146 TheThe catalyst catalyst dosage dosage had had a significant a significant eff ecteffect on theon the oxidation oxidation of limonene. of limonene. Five Five diff erentdifferent dosages dosages of 147 ZIF-67(Co)—35,of ZIF-67(Co)—35, 60, 85, 60, 110, 85, and 110, 135 and mg—were 135 mg—wer used whilee used keeping while all keeping the other all reaction the other parameters reaction 148 fixed,parameters and the fixed, experimental and the resultsexperimental are shown results in Figure are shown6. Conversions in Figure of6. 4.75%,Conversions 13.2%, of 28.1%, 4.75%, 30.3%, 13.2%, 149 and28.1%, 33.5%, 30.3%, corresponding and 33.5%, to corresponding the 35, 60, 85, 110,to the and 35, 135 60, mg 85, catalyst 110, and loadings, 135 mg respectively, catalyst loadings, were 150 obtainedrespectively, in the were experiment. obtained With in the increasing experiment. ZIF-67(Co) With increasing dosage, the ZIF-67(Co) conversion do ofsage, limonene the conversion increased. of 151 Whenlimonene the amount increased. of catalystWhen the was amount 135 mg, of thecatalyst conversion was 135 of limonenemg, the conversion was 33.5%. of Interestingly,limonene was 152 the33.5%. selectivity Interestingly, for carvone the increased selectivity at low for catalyst carvon contentse increased and thenat low decreased, catalyst relative contents to increasing and then 153 contentdecreased, of the relative catalyst. to When increasing the dosage content of of the the catalyst catalyst. was When 85 mg, the the dosage selectivity of the for catalyst carvone was reached 85 mg, 154 thethe maximum selectivity value for carvone (36.7%). reached After that, the themaximum yield of va carvonelue (36.7%). decreased After slightly that, the as yield the amount of carvone of 155 catalystdecreased continued slightly to as increase, the amount but the of catalyst rate of change continued was to small. increase, but the rate of change was small.

156 Figure 6. The effect of the catalyst dosage on the reaction. Conditions: limonene: 2 mL, catalyst: 157 Figure 6. The effect of the catalyst dosage on the reaction. Conditions: limonene: 2 mL, catalyst: ZIF-67(Co), solvent: 15 mL acetic acid, oxidant: 15 mL t-BHP, temperature: 85 ◦C, reaction time: 8 h. 158 ZIF-67(Co), solvent: 15 mL acetic acid, oxidant: 15 mL t-BHP, temperature: 85 °C, reaction time: 8 h. 2.2.3. The Effects of Various Oxidizing Agents on the Reaction 159 2.2.3. The Effects of Various Oxidizing Agents on the Reaction Choosing a suitable oxidant is very important for improving the conversion and selectivity of 160 catalyticChoosing oxidation, a suitable because oxidant oxidants is arevery one important of the key for factors improving in catalytic the conversion oxidation. and We triedselectivity to get of 161 aircatalytic in the reaction oxidation, to usebecause oxygen oxidants as a reagent. are one In of addition, the key factors hydrogen in catalytic peroxide ox andidation. t-BHP We were tried used to get 162 asair oxidants. in the reaction The results to use of theseoxygen experiments as a reagent. are In recorded addition, in hydrogen Table2. When peroxide t-BHP and (70%) t-BHP was were added used 163 toas the oxidants. system, The the results conversion of these of limoneneexperiments increased are recorded to 28.1%, in Table and the2. When selectivity t-BHP for (70%) carvone was added was 164 36.7%.to the However, system, the if hydrogen conversion peroxide of limonene (30%) was increa usedsed as to oxidant, 28.1%, theand conversion the selectivity of limonene for carvone and thewas 165 selectivity36.7%. However, decreased if significantly.hydrogen peroxide (30%) was used as oxidant, the conversion of limonene and 166 the selectivity decreased significantly. Table 2. The effect of various oxidizing agents on the reaction. 167 Table 2. The effect of various oxidizing agents on the reaction. Oxidant Conversion (%) Selectivity (%) OxidantAir Conversion 11.3 (%) Selectivity 30.9 (%) 30% HAir2O2 6.811.3 30.9 9.9 70%30% t-BHP H2O2 28.16.8 36.7 9.9 Conditions: limonene: 2 mL, catalyst:70% t-BHP 85 mg of ZIF-67(Co),28.1 solvent: 15 mL acetic36.7 acid, oxidant: 5 mL H2O2 (30%) or 5 mL t-BHP, temperature: 85 ◦C, reaction time: 8 h. 168 Conditions: limonene: 2 mL, catalyst: 85 mg of ZIF-67(Co), solvent: 15 mL acetic acid, oxidant: 5 mL 169 H2O2 (30%) or 5 mL t-BHP, temperature: 85 °C, reaction time: 8 h.

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170 2.2.4.2.2.4. The The Eﬀ ectEffect of theof the Dosage Dosage of of the the Oxidant Oxidant on on the the Reaction Reaction 171 ExperimentalExperimental results results on on the the eeffectﬀect ofof oxidant dosa dosagege on on the the reaction reaction were were shown shown in Figure in Figure 7. We7 . 172 Wenoticed that that the conversion of of limonene limonene increase increasedd with with increasing increasing amounts amounts of of t-BHP. t-BHP. When When the the 173 amountamount of t-BHPof t-BHP was was 7 mL,7 mL, the the conversion conversion of of limonene limonene was was 33.1%.33.1%. Meanwhile, the the selectivity selectivity for for 174 carvonecarvone increased increased and and then then decreased decreased with with the the increase increase in in t-BHP. t-BHP. We We also also found found that that the the selectivity selectivity for 175 for carvone reached its maximum (43.5%) when the dosage of oxidant was 3 mL. After that, carvone reached its maximum (43.5%) when the dosage of oxidant was 3 mL. After that, increasing the 176 increasing the amount of oxidant increased the degree of oxidation, resulting in a decrease of amount of oxidant increased the degree of oxidation, resulting in a decrease of selectivity and yield. 177 selectivity and yield.

178 Figure 7. The effect of the oxidant dosage on the reaction. Conditions: limonene: 2 mL, catalyst: 179 Figure 7. The effect of the oxidant dosage on the reaction. Conditions: limonene: 2 mL, catalyst: 180 ZIF-67(Co),ZIF-67(Co), solvent: solvent: 15 15 mL mL acetic acetic acid, acid, oxidant: oxidant: t-BHP, t-BHP, temperature: temperature: 85 ◦°C,C, reaction time: time: 8 8h. h. 2.2.5. The Effects of Different Solvents on the Reaction 181 2.2.5. The Effects of Different Solvents on the Reaction The solvent is an important factor affecting heterogeneous catalytic reaction systems. The polarity, 182 The solvent is an important factor affecting heterogeneous catalytic reaction systems. The acidity, viscosity, and volatility of solvents have a great influence on the performance and reactivity 183 polarity, acidity, viscosity, and volatility of solvents have a great influence on the performance and 184 of thereactivity catalyst. of Somethe catalyst. experiments Some wereexperiments carried outwere to carried study theout etoffects study of dithefferent effects solvents of different on the 185 catalyticsolvents oxidation on the catalytic of limonene oxidation by ZIF-67(Co), of limonene and by ZIF-67(Co), the results areand shownthe results in Table are shown3. We in found Table that 3. 186 limoneneWe found conversion that limonene and carvone conversion selectivity and carvone were higher selectivity when were benzene higher was when used benzene as solvent was than used with as 187 aceticsolvent acid, than acetic with anhydride, acetic acid, or acet ethylic acetate.anhydride, or ethyl acetate.

188 TableTable 3. 3.The The eﬀ effectect of of di differentﬀerent solvents solvents on on thethe reaction.reaction.

SolventSolvent ConversionConversion (%) (%) SelectivitySelectivity (%) (%) AceticAcetic acid acid 25.9 25.9 33.3 33.3 AceticAcetic anhydride 38.1 15.9 38.1 15.9 anhydrideEthyl acetate 42.6 23.1 EthylBenzene acetate 42.620.3 23.1 49.4 Benzene 20.3 49.4 189 Conditions: limonene: 2 mL, catalyst: 85 mg of ZIF-67(Co), solvent: 15 mL, oxidant: 3 mL t-BHP, 190 Conditions:temperature: limonene: 85 °C, 2 reaction mL, catalyst: time: 85 8 mgh. of ZIF-67(Co), solvent: 15 mL, oxidant: 3 mL t-BHP, temperature: 85 ◦C, reaction time: 8 h. 191 2.2.6. The Effect of the Temperature on the Reaction 2.2.6. The Eﬀect of the Temperature on the Reaction 192 The oxidation of limonene was examined over the temperature range of 55–95 °C, and the 193 resultsThe oxidation are shown of in limonene Figure 8. was The examined conversion over of limonene the temperature increased range quickly of 55–95 from ◦6.5%C, and to 30.5% the results for are shown in Figure8. The conversion of limonene increased quickly from 6.5% to 30.5% for a reaction

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Catalysts 2019, 9, x FOR PEER REVIEW 8 of 12 Catalysts 2019, 9, x FOR PEER REVIEW 8 of 12 time of 8 h, when the temperature was increased from 55 to 65 ◦C. Thereafter, the trend of conversion 194 a reaction time of 8 h, when the temperature was increased from 55 to 65 °C. Thereafter, the trend of 194 hada reaction a slow decline. time of 8 Interestingly h, when the temperature enough, the was selectivity increased for from carvone 55 to reached 65 °C. Thereafter, its maximum the trend (48.7%) of at 195 conversion had a slow decline. Interestingly enough, the selectivity for carvone reached its 195 85conversionC. This might had bea becauseslow decline. the activity Interestingly of the catalystenough, deﬁnitely the selectivity increased for atcarvone the beginning reached ofits the 196 maximum◦ (48.7%) at 85 °C. This might be because the activity of the catalyst definitely increased at 196 temperaturemaximum (48.7%) rise, so at the 85 conversion °C. This might of limonene be because increased the activity rapidly. of the However,catalyst definitely after the increased temperature at 197 the beginning of the temperature rise, so the conversion of limonene increased rapidly. However, 197 the beginning of the temperature rise, so the conversion of limonene increased rapidly. However, 198 reachedafter the a certaintemperature value, reached the catalytic a certain activity value, ofthe the catalytic material activity did of not the increase material and did thenot byproductincrease 198 after the temperature reached a certain value, the catalytic activity of the material did not increase 199 formationand the byproduct did increase, formation so the selectivitydid increase, for so carvone the selectivity decreased. for carvone decreased. 199 and the byproduct formation did increase, so the selectivity for carvone decreased.

200 200 201 Figure 8. The effect of temperature on the reaction. Conditions: limonene: 2 mL, catalyst: 85 mg 201 FigureFigure 8. 8.The The e ffeffectect of of temperature temperature on on the the reaction. reaction. Conditions:Conditions: limonene: limonene: 2 2mL, mL, catalyst: catalyst: 85 85mg mg 202 ZIF-67(Co), solvent: 12 mL benzene, oxidant: 3 mL t-BHP, reaction time: 8 h. 202 ZIF-67(Co),ZIF-67(Co), solvent: solvent: 12 12 mL mL benzene, benzene, oxidant:oxidant: 3 mL t-BHP, reaction reaction time: time: 8 8h. h. 203 2.2.7.2.2.7. The The E ffEffectect of of the the Reaction Reaction Time Time on on the the ReactionReaction 203 2.2.7. The Effect of the Reaction Time on the Reaction 204 The catalytic oxidation of limonene, using the above conditions for different reaction times, was 204 TheThe catalytic catalytic oxidation oxidation of of limonene, limonene, usingusing the abov abovee conditions conditions for for different different reaction reaction times, times, was was 205 studied. As can be seen in Figure 9, the conversion of limonene increased with increasing reaction 205 studied.studied. As As can can be be seen seen in in Figure Figure9 ,9, the the conversion conversion ofof limonenelimonene increased with with increasing increasing reaction reaction 206 time, and the conversion was 37.8% at 12 h. Meanwhile, the selectivity for carvone increased first, 206 time,time, and and the the conversion conversion was was 37.8% 37.8% atat 1212 h.h. Meanwhile, the the selectivity selectivity for for carvone carvone increased increased first, first, 207 reached a maximum at 6 h (57.7%), and then decreased. Owing to the fact that t-BHP is not 207 reachedreached a maximum a maximum at 6at h (57.7%),6 h (57.7%), and thenand decreased.then decreased. Owing Owing to the factto the that fact t-BHP that is t-BHP not completely is not 208 completely decomposed over short times, the oxidation efficiency was undesirable, and with 208 decomposedcompletely overdecomposed short times, over the short oxidation times, e ffitheciency oxidation was undesirable, efficiency was and undesirable, with increasing and reactionwith 209 increasing reaction time, there were corresponding increases in the oxidants’ oxidation capacity as 209 time,increasing there were reaction corresponding time, there increases were corresponding in the oxidants’ increases oxidation in the capacity oxidants’ as oxidation well as the capacity conversion as 210 well as the conversion rate. However, selectivity of carvone decreased with reaction time increased 210 rate.well However, as the conversion selectivity rate. of However, carvone decreased selectivity withof carvone reaction decreased time increased with reaction because time of increased the depth 211 because of the depth oxidation of carvone and by-product increased. 211 oxidationbecause ofof carvonethe depth and oxidation by-product of carvone increased. and by-product increased. 212 212

213 213 214 FigureFigure 9. 9.The The eﬀ effectect of of the the reaction reaction time time on on the the reaction.reaction. Conditions: limonene: limonene: 2 2mL, mL, catalyst: catalyst: 85 85mg mg 214 Figure 9. The effect of the reaction time on the reaction. Conditions: limonene: 2 mL, catalyst: 85 mg 215 ZIF-67(Co),ZIF-67(Co), solvent: solvent: 12 12 mL mL benzene, benzene, oxidant:oxidant: 3 mL t-BHP, temperature: 75 75 °C.◦C. 215 ZIF-67(Co), solvent: 12 mL benzene, oxidant: 3 mL t-BHP, temperature: 75 °C.

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Catalysts 2019, 9, x FOR PEER REVIEW 9 of 12 2.2.8. Study of the Catalyst Stability 216 2.2.8. Study of the Catalyst Stability Stability is an important parameter of catalyst performance. In order to investigate the stability of 217 ZIF-67(Co),Stability XRD is an spectra important of ZIF-67(Co) parameter beforeof catalyst and performance. after the ﬁrst reactionIn order roundto investigate were obtained the stability and 218 areof ZIF-67(Co), presented here.XRD spectra As shown of ZIF- in Figure67(Co) 10 before, the characteristicand after the first peak reaction was still round present, were but obtained its strength and 219 decreasedare presented slightly. here. So,As weshown can bein Figure sure that 10, the the structure characteristic of ZIF-67 peak still was existed. still present, but its strength 220 decreased slightly. So, we can be sure that the structure of ZIF-67 still existed.

221 222 Figure 10. XRD spectra of ZIF-67(Co) as synthe synthesizedsized and after the first ﬁrst recycle.

Concurrently, the speciﬁc data about repetitive experiments are revealed in Table4. 223 Concurrently, the specific data about repetitive experiments are revealed in Table 4.

Table 4. The data from reuse experiments. 224 Table 4. The data from reuse experiments.

TimesTimes ConversionConversion (%) (%) SelectivitySelectivity (%) (%) TON FirstFirst round round 29.8 29.8 55.4 55.4 18.1 Second round 33.5 41.6 13 Second round 33.5 41.6 13 Third round 32.5 28.0 8.7 Third round 32.5 28.0 8.7 Conditions: limonene: 2 mL, catalyst: 85 mg of ZIF-67(Co), solvent: 12 mL benzene, oxidant: 3 mL t-BHP, 225 temperature:Conditions: 75limonene:◦C, reaction 2 mL, time: catalyst: 8 h. Turnover 85 mg number of ZIF- (TON)67(Co),= carvone solvent: (yield) 12 mL mmol benzene,/total Co siteoxidant: mmol. 3 mL 226 t-BHP, temperature: 75 °C, reaction time: 8 h. Turnover number (TON) = carvone (yield) mmol / total 227 Co site mmol. The data in Table4 reveal the repeatability of ZIF-67(Co) in the catalytic oxidation of limonene. 228 As canThe be data seen in from Table the 4 datareveal in the Table repeatability4, we found of that ZIF-67(Co) the selectivity in the forcatalytic carvone oxidation declined of slightlylimonene. in 229 theAs can second be seen round. from At the the data same in time, Table the 4, conversionwe found that of limonene the selectivity declined for ascarvone well. Wedeclined conclude slightly that 230 thisin the may second be related round. to At a partial the same collapse time, of the the conversion basic structure of limonene of ZIF-67(Co). declined as well. We conclude 231 that this may be related to a partial collapse of the basic structure of ZIF-67(Co). 3. Materials and Methods 232 3. Materials and Methods 3.1. Materials and Solvents 233 3.1. MaterialsCobaltous and nitrate Solvents hexahydrate was supplied by Xilong Chemical Co., Ltd. (Shantou, China) and 2-methylimidazole was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, 234 Cobaltous nitrate hexahydrate was supplied by Xilong Chemical Co., Ltd. (Shantou, China) and China). All other reactants used in the synthesis of the ZIF-67(Co) catalyst samples were supplied 235 2-methylimidazole was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All by Xilong Chemical Co., Ltd. Methanol and acetic acid were purchased from Xilong Chemical Co., 236 other reactants used in the synthesis of the ZIF-67(Co) catalyst samples were supplied by Xilong Ltd. The tert-butyl hydroperoxide and limonene were obtained from Adamas Reagent Co., Ltd. 237 Chemical Co., Ltd. Methanol and acetic acid were purchased from Xilong Chemical Co., Ltd. The (Shanghai, China). 238 tert-butyl hydroperoxide and limonene were obtained from Adamas Reagent Co., Ltd. (Shanghai, 239 China).

240 3.2. Catalyst Preparation 241 In a typical preparation process [33], 0.45 g cobaltous nitrate hexahydrate was dissolved in 3 mL 242 of distilled water, and then 5.5 g of 2-methylimidazole was dissolved in 100 mL of distilled water.

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3.2. Catalyst Preparation In a typical preparation process [33], 0.45 g cobaltous nitrate hexahydrate was dissolved in 3 mL of distilled water, and then 5.5 g of 2-methylimidazole was dissolved in 100 mL of distilled water. Then the two solutions were mixed evenly and stirred at room temperature for 6 h. The products obtained were washed with distilled water and methanol, and then dried in at 80 ◦C for 24 h.

3.3. Catalyst Characterization ZIF-67(Co) was characterized through a series of techniques. The diffraction patterns of the materials and the residual catalyst were obtained by powder X-ray diffraction (XRD, Rigaku Co., Tokyo, Japan) experiments using Cu Kα band radiation on a D/max-3B spectrometer. Scans were made over the 2θ range of 10–90◦ with a scan rate of 10◦/min (wide-angle diffraction). Pore size distributions, pore volumes, and specific surface area were measured by nitrogen adsorption/desorption using a Micromeritics ASAP 2460(Micromeritics, Norcross, GA, USA) at 77 K. The specific surface areas were calculated by the Brunauer–Emmett–Teller (SBET) and Langmuir methods, and the pore volumes and pore sizes were calculated by the BJH (Barrett–Joyner–Halenda) method. Thermogravimetric analysis (TGA) curves and differential scanning calorimetry (DSC) determinations were carried out on a NETZSCH STA 449 F3 synchronous TG-DSC thermal analyzer (Nuremberg, Germany) with a scanning rate of 5 K/min in a dry nitrogen atmosphere and heating from room temperature to 1073 K. The FT-IR(Fourier transform infrared spectroscopy) measurements were performed on a Thermo Nicolet 8700 instrument(Wilmington, MA, USA). Potassium bromide pellets were used in 1 FT-IR experiments at a spectral resolution of 4 cm− . Scanning electron microscopy (SEM) images were taken on a Quanta 200FEG microscope (FEI, Hillsboro, OR, USA) at an accelerating voltage of 15 kV with the pressure in the sample chamber set to 2.7 10 5 Pa. Double-sided adhesive tape was bonded × − on the carrier disc, then a small amount of powder sample was placed near the center of the carrier disc on the adhesive tape. Then, a rubber ball was used to blow the sample outward along the radial direction of the load plate, so that the powder was evenly distributed on the tape. The tape was then coated with conductive silver paste to connect the sample to the carrier plate. After the silver paste was dried, the final gold steaming process could be carried out.

3.4. Catalytic Performance In a typical reaction, 2 mL limonene with varying amounts of catalyst and solvents were used as received, without further purification, in a 25 mL three-neck flask. Each set of experiments was performed at a corresponding temperature, under atmospheric pressure and magnetic stirring, using an appropriate amount of solvent, e.g., acetic acid, acetic anhydride, ethyl acetate, benzene, etc. Afterwards, the catalyst was separated by filtration, and the sample solution was dried with anhydrous magnesium sulfate. A final filtration was carried out, and the samples were qualitatively and quantitatively analyzed by gas chromatography (GC-9560, HUAAI Chromatography, Shanghai, China), with a capillary column AE.OV-624, 30 m in length, 0.25 mm i.d., 0.5 µm film thickness, and an FID (Flame ionization detector). In order to assess the stability of the ZIF-67(Co), some sets of a repeatability experiment were performed. After the first reaction, the catalyst was centrifuged and separated from the reaction system, and after repeated washing with distilled water and anhydrous ethanol, the catalyst was reactivated at 90 ◦C in the vacuum drying chamber for 12 h. Thereafter, the reactor was supplied with the recovered dry catalyst, new reagents, and solvent for the second reaction. We repeated the above process three times.

4. Conclusions In conclusion, we proved that the zeolitic imidazolate framework ZIF-67(Co) was an efficient catalyst for the liquid-phase catalytic oxidation of limonene, under relatively mild reaction conditions, Catalysts 2019, 9, 374 11 of 13 to carvone. Based on current research, the catalytic oxidation of limonene comprises two parallel reactions, namely a synergistic reaction and a free-radical reaction. Carvone is the product of the free-radical reaction of limonene at the allyl group. The synergistic reaction produced limonene-1,2-oxides and limonene-1,2-diols. Meanwhile, under acidic conditions, the 1,2-oxide of limonene could be hydrolyzed to produce the 1,2-diol of limonene, and the 1,2-diol of limonene could be further rearranged and converted to carvone. This is the first report on catalyzing the oxidation of limonene using ZIF-67(Co) as catalyst. It was revealed that ZIF-67(Co) was an efficient catalyst, with selectivity of 55.4% for carvone in the catalytic oxidation of limonene, and could be reused with similar activity.

Author Contributions: Conceptualization, J.W. and Y.L.; Data curation, Z.Y. and Y.L.; Formal analysis, Y.L. and W.W.; Investigation, Y.L., Y.Y., D.C., and Z.L.; Methodology, J.W.; Project administration, J.W. and Z.Y.; Software, Z.L., Y.A., and L.Z.; Supervision, J.W. and Z.Y.; Writing—original draft, Y.L.; Writing—review and editing, J.W. and Z.Y. Funding: This research was funded by National Natural Science Foundation of China (Project 21573193 and 21464016). The authors also thank the Key Projects for Research and Development of Yunnan Province (2018BA065), the Scientific Research Fund of Department of Yunnan Education (Project 2016CYH04 and 2017ZZX223) for financial support. Acknowledgments: The work was supported by National Natural Science Foundation of China (Project 21573193 and 21464016). The authors also thank the Key Projects for Research and Development of Yunnan Province (2018BA065), the Scientific Research Fund of Department of Yunnan Education (Project 2016CYH04 and 2017ZZX223), and the Program for Innovation Team of Yunnan Province and Key Laboratory of Advanced Materials for Wastewater Treatment of Kunming for financial support. Conflicts of Interest: The authors declare no conflict of interest.

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