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A rare oxidation of camphene to acid and aldehyde in the presence of Lacunar Keggin heteropoly salts
Molecular Catalysis 478 (2019) 110589
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A rare oxidation of camphene to acid and aldehyde in the presence of Lacunar Keggin heteropoly salts
T
⁎
Castelo Bandane Vilanculoa, Márcio José da Silvab, , Sukarno Olavo Ferreirab, Milena Galdino Teixeirab a b
Chemistry Department, Pedagogical University of Mozambique, FCNM, Campus of Lhanguene, Av. de Moçambique, Km 1, Maputo, 4040, Mozambique Chemistry Department, Federal University of Viçosa, Viçosa, Minas Gerais State, 36590-000, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Camphene Lacunar Keggin heteropolyacid salts Hydrogen peroxide Catalytic oxidation
In this work, monolacunary Keggin heteropolyacid salts containing sodium as the countercation were synthesized and evaluated as catalysts on the camphene oxidation with hydrogen peroxide. Under reaction conditions (i.e., 333 K, air atmosphere, CH3CN solution), camphene (1) was selectively oxidized by H2O2, generating two rare products; camphene aldehyde (1a) and camphenylic acid (1b), which have attractive olfactive properties. Notably, these uncommon compounds were obtained with the camphene preserving their carbon skeletal. Among the lacunar Keggin heteropolyacid salts, the Na7PW11O39 lacunar salt was the most active and selective toward oxidation products, completely converting the camphene to (1a) and (1b) products. The catalyst load, the oxidant: camphene molar ratio and the reaction temperature allowed to control the (1a)/ (1b) proportion.
1. Introduction Monoterpenes are renewable raw material used to synthesize flavors, agrochemicals, and pharmacies intermediates [1]. Among them, camphene deserves highlight because their oxygenated derivatives have attractive olfactive properties and are widely used as an ingredient for perfumery and fragrance industries [2]. Camphene oxidation becomes still more interesting with the hydrogen peroxide, an environmentally friendly oxidant, that is too noninflammable, easily handling, inexpensive, and atom-efficient [2,3]. The greatest challenge to oxidize the camphene is that it has only one allylic hydrogen, which is in the bridgehead position, being thus hardly removable. In addition, the double bond of camphene has a large hindrance hysteric, an aspect that hampers its epoxidation [4]. Consequently, there are scarce cases where the metal catalysts commonly used in olefin oxidation reactions had success in the camphene oxidation, either with oxygen or hydrogen peroxide [5,6]. Heteropolyacids (HPAs) are considered environmentally benign catalysts and a good alternative to the common metal catalysts [7]. Keggin HPAs have tungsten and molybdenum as addenda metal atoms in their anions, being thus potentially active catalysts in oxidation reactions [8–10]. However, HPAs are strong Brønsted acids, a feature that promotes acid-catalyzed concurrent reactions, such as carbon skeletal
rearrangement and nucleophilic addition, which reduce the selectivity of the oxidation products [11,12]. To circumvent this drawback, an approach that is still few explored is to convert Keggin HPAs to neutral salts, exchanging their acidic protons by metal cations [13,14]. Moreover, the removal of one MO unit from the heteropolyanions (i.e., WO or MoO) results in a lacunar salt catalyst, which may be more active than a precursor with the saturated heteropolyanion [15–18]. In this work, we synthesized three lacunar Keggin HPAs sodium salts and assessing their catalytic activity on the oxidation of the camphene by hydrogen peroxide. All the lacunar salts were characterized by FT-IR, XRD, TG/DSC, SEM-EDS techniques, and their acidic strength were measured by n-butylamine titration. Effects of the main reaction variables were investigated. Special attention was devoted to linking the structural properties of the lacunar salts with their catalytic activity. 2. Experimental section 2.1. Chemicals All the chemicals were purchased from commercial sources and used as received. Camphene and acetonitrile were Sigma-Aldrich (ca. 99 wt. %). Sodium hydrogen carbonate was Vetec (ca. 99 wt. %).
⁎ Corresponding author at: Chemistry Department, Federal University of Viçosa, Campus Universitário, Avenue P.H. Rolfs, s/n, Viçosa, Minas Gerais State, CEP: 36570-000, Brazil. E-mail address: [email protected] (M.J. da Silva).
https://doi.org/10.1016/j.mcat.2019.110589 Received 23 July 2019; Received in revised form 19 August 2019; Accepted 23 August 2019 Available online 29 August 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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catalyst (ca. 1.34 mol %), the reaction was started and monitored during 8 h, periodically collecting aliquots that were analyzed in a GC equipment (Shimadzu 2010, FID, capillary column). All the characterization data of the products are in the Supplemental material.
Hydrated heteropolyacids (i.e., H3PW12O40, H3PMo12O40 and H4SiW12O40; ca. 99 wt. %) were acquired from Sigma-Aldrich. Aqueous hydrogen peroxide was from Alphatec (ca. 35 wt. %). 2.2. Synthesis of the lacunar sodium salts
2.7. Recovery and reuse of the catalyst The lacunar salt catalysts were synthesized as described in the literature [19]. Firstly, H3PW12O40.nH2O (ca. 1.0 g) was dissolved in water (ca. 30 mL), magnetically stirred and heated to 333 K. The pH was adjusted to 4.8 adding aqueous NaHCO3, and the solution was stirred and heated at 333 K/ 3 h keeping constant the pH value. The Na7PW11O39 salt was obtained after the vaporization of the solvent and recrystallization from water, being subsequently dried at 373 K/ 5 h. A similar procedure was used to synthesize the lacunar sodium salts of the phosphomolybdic and silicotungstic acids, adequately adjusting the pH of the solutions.
After the end of the reaction, the excess of solvent (CH3CN) was removed under reduced pressure. After to add water (ca. 20 mL), the resulting mixture was extracted four times with diethyl ether. The aqueous phase was evaporated in a heating plate to near dryness and then dried at room conditions. The solid catalyst was weighted and reused in another catalytic run. 3. Results and discussion 3.1. Catalyst characterization
2.3. Synthesis of the saturated sodium phosphotungstate salt The FT-IR study was done to verify if the Keggin anion structure (i.e., primary structure) was retained after the synthesis of lacunar salt. It was done comparing the FT-IR spectra of the lacunar salt, with spectra of the saturated salt and parent HPA (Fig. SM1). The main absorption bands of H3PW12O40 were noticed at wavenumbers 1080, 980, 920, and 790 cm−1, which agree with the literature [21]. These bands were assigned to the stretching of PeOa, WeOd, WeObeW and WeOceW bonds. The typical stretching bands of the saturated sodium phosphotungstate salt were like the pattern H3PW12O4, meaning that the salt was successfully synthesized. The subscripts distinguish the oxygen atoms in relation to the position occupied in the heteropolyanion [22]. The absorption band at wavenumber 1080 cm−1, which was assigned to the vibration of PeOa bond, underwent a splitting that resulted in two new bands (ca. 1020 and 1060 cm-1). It is a guarantee that the lacunar heteropolyanion was formed [23–25]. This splitting was attributed to the decreasing of the symmetry of group PO4, resultant from the removal of the WO unit [26]. The same splitting occurred when lacunar salt of phosphomolybdic acid was synthesized (see Supplemental material, Fig. SM2) [27]. However, the same it was not observed when the lacunar silicotungstic salt was synthesized; the stretching of SieOa bond resulted in only an absorption band (see Supplemental material, Fig. SM3). XRD spectra give information about the secondary structure of the sodium salts. A comparison of the diffractograms obtained from the sodium salts and their acid parents allowed observing that they presented a lower level of crystallinity (Fig. SM4). This different crystallinity may be assigned to the exchange of the protons by sodium cations, and the alteration of the number of hydration water molecules. The other two lacunar sodium salts also presented a lower level of crystallinity than acid precursors (see Supplemental material, Figs. SM5 and SM6). TG curves obtained from lacunar sodium salts shows two regions of loss weight; the first one before 473 K, which was assigned to loss of all water molecules. The second one, assigned to the decomposition of POa-W framework, followed by the noticeable peak in the DSC curves around 793 K. The final products were a mixture of oxides. Similar behavior was noticed for other salts (Figs. SM7–SM9). The acidity strength of the sodium salts was measured by potentiometric titration with n-butylamine (Figs. SM10–SM12). Regardless of the Keggin anion, the titration curves of the lacunar salts presented a similar pattern; a quick decrease on electrode potential after the addition of a minimum volume of base. It is suggestive that only a residual Brønsted acidity remained after the protons exchange by the sodium cations, indicating that all of them were virtually removed. The value of the initial electrode potential allows classifying the acidy sites [27]. While all the precursor HPAs presented very strong acid sites (i.e., Ei > 100 mV), their lacunar sodium salts displayed strong acid
The saturated sodium salt (i.e., Na3PW12O40) was synthesized as described in the literature [22]. Typically, Na2WO4·2H2O (30 mmol, 10 g) was slowly added to 20 ml of distilled water and the mixture was warmed to 333 K and magnetically stirred. Then, H3PO4 85% (15 mmol, 1 mL) and HCl (100 mmol, 8 mL) were added and the resulting mixture was stirred 1 h. After solvent evaporation, it was twice recrystallized in hot water, and dried at 373 K/ 5 h. 2.4. Characterization of the catalysts The infrared spectra were recorded on Varian 660-IR spectrometer at wavenumber range of 500–1700 cm−1, that is the fingerprint region of the main absorption bands of Keggin heteropolyanions. Crystalline structure of the salts and HPAs were analyzed by X-rays diffraction (XRD) spectroscopy using a XRD-ray Diffraction System model D8-Discover Bruker using Ni filtered Cu-kα radiation (λ =1.5418 Å), working at 40 kV and 40 mA, with a counting time 1.0 s in the diffraction angle (2θ) ranging from 5 to 80°. The acidity strength of the catalyst was estimated by potentiometric titration, as described by Pizzio et al [20]. The electrode potential variation was measured with a potentiometer (i.e. Bel, model W3B). Typically, 50 mg of HPA salt was dissolved in CH3CN, and titrated with n-butylamine solution in toluene (ca. 0.05 mol L−1). The characterization of the surface of the solid lacunar salt, thin sections were selected and metalized with carbon for analysis in a scanning electron microscopy equipment (SEM) and dispersive energy x-ray spectrometer (EDS) using a JEOL JSM 6010LA SEM. 2.5. Identification of the main reaction products The main products were identified in a Shimadzu GC-2010 gas chromatography coupled with a MS-QP 2010 mass spectrometer (i.e., electronic impact 70 eV, scanning range of m/z 50–450). Afterward, they were separated by column chromatography (silica 60 G) using mixtures of hexane, ethyl acetate, dichloromethane and methanol as eluents. After this step, they were characterized by 1H and 13C NMR and IV (Varian FT-IR 660) spectroscopy analysis. NMRs spectra were taken in CDCl3 solutions, using a Varian 300 spectrometer at 300.13 and 75.47 MHz, respectively. Chemical shifts were expressed as δ (ppm) relative to the tetramethylsilane (TMS) as an internal reference. 2.6. Catalytic runs Catalytic tests were carried out in a 25 ml three-necked glass flask, equipped with a sampling system and a reflux condenser. Typically, camphene (2.75 mmol) was solved in CH3CN solution (ca. 10 mL), which was magnetically stirred and heated to 333 K. After to add the 2
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Table 1 Lacunar sodium HPA salt-catalyzed reactions of camphene oxidation by hydrogen peroxide.a Exp.
1 2 3
Catalyst
Na7PW11O39 Na7PMo11O39 Na8SiW11O39
Conversion (%)
99 53 68
Selectivity (%) 1a
1b
Oligomers
17 – –
73 – –
10 100 100
a Reaction conditions: camphene (2.75 mmol); hydrogen peroxide (11.0 mmol); catalyst load (1.34 mol %); temperature (333 K); CH3CN (10 mL).
(Table 1). The straight obtention of the carboxylic acids from camphene is an uncommon reaction; for this reason, to synthesize camphene esters, other reaction pathways have been explored [28]. The camphene double bond has a high steric hindrance, an aspect that makes difficult their oxidation. On the other hand, the synthesis of an aldehyde containing a five-membered ring was achieved starting from α-pinene under oxidative conditions, and in an acidic medium [29]. Therefore, all these aspects contribute to valorize this oxidative process of the camphene. The surprising activity of the lacunar sodium phosphotungstate salt motivated us to assess the efficiency of other lacunar salts of phosphotungstic acid containing different cations (Fig. 2). The presence of a vacancy in the phosphotungstic anion looks has a pivotal role in the camphene oxidation reaction (Fig. 2). The phosphotungstic acid converted the camphene exclusively to oligomers; no oxidation products were detected. Conversely, excepted the cesium salt, all the lacunar catalysts provided the camphene oxidation derivates (i.e., aldehyde and the acid), even though in different proportions. The lowest efficiency of the cesium salt may be a consequence of the poor solubility; all the other lacunar catalysts were completely soluble. Therefore, the Na7PW11O39 was the most active and selective catalyst and was selected to investigate the effects of main reaction parameters (i.e., reactants stoichiometry, temperature, and catalyst load).
Fig. 1. Kinetic curves of lacunar sodium Keggin HPA salt-catalyzed reactions of camphene oxidation by hydrogen peroxidea. a Reaction conditions: camphene (2.75 mmol); hydrogen peroxide (11.0 mmol); catalyst load (1.34 mol %); temperature (333 K); CH3CN (10 mL).
sites (0 < Ei < 100 mV), which may be assigned to the presence of residual protons. Saturated sodium phosphotungstate (i.e., Na3PW12O40) also showed a very strong acidity strength, probably due to the presence of remaining Bronsted acidity. EDS spectra of the lacunar sodium phosphotungstate salt is shown in Fig. SM13. The percentual composition determined by EDS agrees established theoretical formula of lacunar sodium salt. Saturated phosphotungstate salt and the other lacunar salts also had the theoretical composition confirmed by EDS analyses, although traces of Cl− ions had been detected.
3.2. Catalytic tests 3.2.1. Screening of lacunar Keggin HPA salt Initially, we have assessed the catalytic activity of three lacunar sodium salt catalysts on the camphene oxidation with hydrogen peroxide (Fig. 1). The reaction conditions were choice based on the literature [16]. a Reaction conditions: camphene (2.75 mmol); hydrogen peroxide (11.0 mmol); catalyst load (1.34 mol %); temperature (333 K); CH3CN (10 mL) Fig. 1 shows that lacunar sodium phosphotungstate salt (Na7PW11O39) was the most active catalyst. The highest conversion (ca. 99%) was achieved within two first hour reaction. Noticeably, two rare reaction products were obtained only on the Na7PW11O39-catalyzed reaction: camphene aldehyde (1a) and camphenylic acid (1b) (Scheme 1). Remarkably, these two products were obtained only when the Na7PW11O39 was the catalyst. When lacunar sodium silicotungstate or phosphomolybdate salts were the catalysts, almost no product was significantly formed. Through the mass balance of reaction (via GC analyses), we verified that undetected products were oligomers
3.2.2. Effect of the oxidant load on the Na7PW11O39-catalyzed oxidation of the camphene by hydrogen peroxide The oxidant load is a key aspect in oxidation reactions of olefins. An excess of hydrogen peroxide may favor the formation of oligomers (i.e., due to the radicalar reactions), or the hydration products (i.e., aqueous solution), compromising thus the oxidation selectivity. To assess this effect, we carried out reactions varying the ratio oxidant to camphene (Fig. 3). In the absence of the catalyst, an excess of oxidant was unable to improve the reaction conversions; only ca. 10% was achieved. Contrariwise, in the presence of the lacunar sodium phosphotungstate catalyst, both conversion and selectivity of the reactions were dependent on the proportion oxidant: camphene. When the camphene
Scheme 1. Oxidation of camphene by H2O2 in the presence of Na7PW11O39 catalyst in CH3CN solutions. 3
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Fig. 2. Effect of cation nature on the lacunar phosphotungstate catalyst in camphene oxidation reactions by hydrogen peroxidea. a Reaction conditions: camphene (2.75 mmol); hydrogen peroxide (11.0 mmol); catalyst load (1.34 mol %); temperature (333 K); CH3CN (10 mL).
oxidation was carried out at an equimolar amount of oxidant, only a poor conversion (ca. 30%), and a low selectivity for 1a and 1b was obtained (Fig. 4). Conversely, an increase of the oxidant load increased either the initial rate of reaction as well as the final conversion. It was found that the proportion of 1:4 was enough to virtually convert all the camphene to aldehyde and camphenylic acid, with a minimum formation of oligomers. The reaction selectivity was impacted by the oxidant load; using 1:2 and 1:3 M ratios, aldehyde was the major product; on the other hand, higher amounts of peroxide favored the formation of the camphenylic acid. We suppose that the requirement of peroxide excess may be explained if we consider that the sodium catalyst should be firstly peroxidized, generating the intermediate 1 (Scheme 2) [16,30]. Posteriorly, the intermediate 1 may transfer the oxygen atom to the camphene, generating the camphene aldehyde and, afterward, the camphenylic acid. The consecutive character of the camphene oxidations (i.e., firstly, aldehyde and posteriorly to camphenylic acid), could be confirmed monitoring the reaction selectivity in the presence of lacunar sodium salt during 8 h (Fig. SM14). To verify the hypothesis of Na7PW11O39 catalyst has been peroxidized during the reaction and generate the intermediate 1, we recorded FT-IR spectra of the samples of the catalyst in the presence of aqueous H2O2 (Fig. 5). The changes in the absorption bands present in the fingerprint region of the lacunar Na7PW11O39 suggests that the catalyst was peroxidized [30]. The literature has described that di-or tetra-nuclear peroxo tungstate fragments were formed in H3PW12O40-catalyzed reactions of terminal olefins with hydrogen peroxide [31]. However, herein the saturated heteropolyanion catalysts (i.e., Na3PW12O40) were inactive on the reactions of oxidation. It is suggestive that the action mechanism of the lacunar anion catalyst should be different from saturated phosphotungstate anion (i.e., which may involve di-peroxidized fragments). Therefore, we suppose that as described in previous work [16], a mono-peroxidized intermediate could be an intermediate on this reaction. Moreover, our experimental results (Fig. 2) demonstrated that the vacancy plays an essential role in the catalyst activity; only lacunar phosphotungstic catalysts were efficient to promote the oxidation of camphene with H2O2.
Fig. 3. Effect of oxidant load on kinetic curves of Na7PW11O39-catalyzed oxidation reactions of camphene with H2O2. a Reaction conditions: Camphene (2.75 mmol); reaction time (8 h); Na7PW11O39 (1.34 mol %); temperature (333 K); CH3CN (10 mL).
Fig. 4. Effect of the oxidant load on kinetic curves of Na7PW11O39-catalyzed oxidation reactions of camphene with H2O2. a Reaction conditions: camphene (2.75 mmol); reaction time (8 h); Na7PW11O39 (1.34 mol %); temperature (333 K); CH3CN (10 mL).
4
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Scheme 2. Reaction pathway of Na7PW11O39-catalyzed oxidation reactions of camphene with H2O2 (adapted from ref. [16]).
Fig. 5. FT-IR spectra of the samples: H2O2(aq), Na7PW11O39, Na7PW11O39 + H2O, and Na7PW11O39 + H2O2.
Fig. 7. Effect of catalyst load on kinetic curves of Na7PW11O39-catalyzed oxidation reactions of camphene with H2O2. a Reaction conditions: camphene (2.75 mmol); reaction time (8 h); H2O2 (11.0 mmol); temperature (333 K); CH3CN (10 mL).
3.2.3. Effect of the Na7PW11O39 catalyst load on the oxidation of camphene by H2O2 The kinetic curves in Fig. 6 shows that within period investigated the reactions of the camphene oxidation did not achieve the equilibrium; an increase of the catalyst load resulted in higher conversions. The reaction selectivity was also impacted by an increase of catalyst load; with a high Na7PW11O39 concentration the formation of camphenylic acid was remarkably favored, concomitantly reducing the selectivity of aldehyde (Fig. 7). It is reinforcing the importance of the lacunar sodium phosphotungstate catalyst on the formation of oxidation products.
Fig. 8. Effect of temperature on the conversion and selectivity of Na7PW11O39catalyzed oxidation reactions of camphene with H2O2. a Reaction conditions: camphene (2.75 mmol); reaction time (8 h); H2O2 (11.0 mmol); Na7PW11O39 catalyst concentration (134 mol %); CH3CN (10 mL).
3.2.4. Effect of temperature on the Na7PW11O39-catalyzed oxidation of camphene by H2O2 An increase of the reaction temperature enhanced the initial rate of the reaction (omitted by simplification) as well as the final conversion (Fig. 8). In addition, the conversion of the camphene to aldehyde (1a), and the oxidation of aldehyde to carboxylic acid (1b), were also notably improved by an increase of temperature. The results are evidence of the endothermal character of the oxidation reactions (i.e., camphene and camphene aldehyde). For this reason, the camphene conversion and the selectivity for the oxidation products was improved when the reaction temperature was increased.
Fig. 6. Effect of Na7PW11O39 catalyst load on the oxidation by H2O2. a Reaction conditions: camphene (2.75 mmol); H2O2 (11 mmol); temperature (333 K); CH3CN (10 mL). 5
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exchange by sodium ions as well as the removal of an WO unit from the Keggin heteropolyanions were successfully accomplished. In almost of Na7PW11O39-catalyzed reactions, the camphene was majority converted to rare oxidation products; camphene aldehyde (1a) and camphenylic acid (1b), preserving the camphene carbon skeletal, which is rare due to easy Wagner-Meerwein rearrangement commonly observed in these oxidation reactions. The maximum selectivity combined toward these products was close 90%, with a total conversion of camphene. The catalyst load, molar ratio of hydrogen peroxide to camphene, and the reaction temperature were key aspects toward formation of the goalproducts. The effects of the countercation, and of the Keggin heteropolyanion were investigated. Remarkably, the camphene oxidation products were formed only in the presence of the lacunar phosphotungstic acid salt catalysts. This catalytic procedure demonstrated to be an efficient route to synthesize these rare camphene oxidation derivatives. The reusability of the catalyst was assessed. The low stability of catalyst along the recycle process resulted in the decomposition of lacunar anion.
Fig. 9. Conversion after the reuse of Na7PW11O39-catalyzed oxidation reaction of camphene with H2O2. a Reaction conditions: camphene (2.75 mmol); reaction time (8 h); H2O2 (11.0 mmol); Na7PW11O39 catalyst concentration (134 mol %); CH3CN (10 mL).
Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgements The authors are grateful for the financial support from CNPq and FAPEMIG (Brasil). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) Finance Code 001. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110589. References [1] P. Gallezoti, Conversion of biomass to selected chemical products, Chem. Soc. Rev. 41 (4) (2012) 1538–1558. [2] A.A. de Oliveira, M.L. da Silva, M.J. da Silva, Palladium-catalysed oxidation of bicycle monoterpenes by hydrogen peroxide in acetonitrile solutions: a metal, reoxidant-free and environmentally benign oxidative process, Catal. Lett. 130 (2009) 424–431. [3] M.J. da Silva, J.A. Gonçalves, R.B. Alves, O.W. Howarth, E.V. Gusevskaya, Palladium catalyzed transformations of monoterpenes: stereoselective deuteriation and oxidative dimerization of camphene, J. Organomet. Chem. 689 (2004) 302–308. [4] T. Michel, M. Cokoja, F.E. Kuhn, Catalytic epoxidation of camphene using methyltrioxorhenium(VII) as catalyst, J. Mol. Catal. A 368–369 (2013) 145–151. [5] M.J. da Silva, J.A. Gonçalves, R.B. Alves, O.W. Howarth, E.V. Gusevskaya, Palladium-catalyzed oxidation of monoterpenes: novel tandem oxidative couplingoxidation of camphene by dioxygen, J. Mol. Catal. A 176 (2001) 23–27. [6] N.V. Maksimchuk, M.S. Melgunov, J. Mrowiec-Białoń, A.B. Jarzębski, O.A. Kholdeeva, H2O2-based allylic oxidation of α-pinene over different single site catalysts, J. Catal. 235 (2005) 175–183. [7] S. Omwoma, C.T. Gore, Y. Ji, C. Hu, Y.-F. Song, Environmentally benign polyoxometalate materials, Coord. Chem. Rev. 286 (2015) 17–29. [8] F. Cavani, Heteropolycompound-based catalysts: a blend of acid and oxidizing properties, Catal. Today 41 (1998) 73–86. [9] S.G. Casuscelli, M.E. Crivello, C.F. Perez, G. Ghione, E.R. Herrero, L.R. Pizzio, P.G. Vázquez, C.V. Cáceres, M.N. Blanco, Effect of reaction conditions on limonene epoxidation with H2O2 catalyzed by supported Keggin heteropolycompounds, Appl. Catal. A 274 (2004) 115–122. [10] D. Amitouche, M. Haouas, T. Mazari, S. Mouanni, R. Canioni, C. Rabia, E. Cadot, C. Marchal-Roch, The primary stages of polyoxomolybdate catalyzed cyclohexanone oxidation by hydrogen peroxide as investigated by in situ NMR. Substrate activation and evolution of the working catalyst, Appl. Catal. A 561 (2018) 104–116. [11] H. Zheng, Z. Yan, S. Chu, J. Chen, Continuous synthesis of isobornyl acetate catalyzed by a strong acid cation exchange resin in an oscillatory flow reactor, J. Chen, Chem. Eng. Process. Process Intens. 134 (2018) 1–8. [12] A.L.P. de Meireles, K.A. da Silva Rocha, I.V. Kozhevnikov, E.V. Gusevskaya, Esterification of camphene over heterogeneous heteropoly acid catalysts: synthesis of isobornyl carboxylates, Appl. Catal. A 409–410 (2011) 82–86.
Fig. 10. FT-IR spectra of sample fresh Na7PW11O39 and after the second reuse.
3.3. Catalyst reuse The Na7PW11O39 is a solid catalyst, however, it is soluble in the reaction medium (i.e., CH3CN). However, we developed a simple procedure to recover the catalyst by a liquid-liquid extraction procedure (see Section 3.2). We achieved a rate recovery of 90–95% for the Na7PW11O39 catalyst (Fig. 9). This high recovery suggests that procedure used was very efficient. Therefore, it was possible to reuse the catalyst in another runs. We have observed a trend of decreasing on the conversion when the catalyst was reused. The selectivity for the oxidations products was being gradually reduced, increasing the formation of the oligomers. To assess the catalyst deactivation, we compared the infrared spectra before (i.e., fresh catalyst), and after the second reuse. (Fig. 10). Noticeably, the two bands corresponding to the splitting of PeOa absorption bond absorption (ca. 1020 and 1060 cm−1) were absent in the FT-IR spectrum of reused catalyst [23–25]. We suppose that during the recovery procedure, the pH, which is key aspect to synthesize the lacunar catalyst, should has been drastically modified, consequently, the catalyst was deactivated. 4. Conclusions In this work, the activity of the lacunar sodium Keggin HPA salts was assessed on the oxidation of the camphene with hydrogen peroxide. All the catalysts were spectroscopically characterized. The protons 6
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[13] R. Karcz, P. Niemiec, K. Pamin, J. Połtowicz, J. Kryściak-Czerwenka, B.D. Napruszewska, A. Michalik-Zym, M. Witko, R. Tokarz-Sobieraj, E.M. Serwicka, Effect of cobalt location in Keggin-type heteropoly catalysts on aerobic oxidation of cyclooctane: experimental and theoretical study, Appl. Catal. A 542 (2017) 317–326. [14] M.J. da Silva, C.M. de Oliveira, Catalysis by Keggin heteropolyacid salts, Curr. Catal. 7 (2018) 26–34. [15] L. Li, B. Liu, Z. Wu, X. Yuan, H. Luo, Preparation of Keggin-type mono-lacunar phosphotungstic-ammonium salt and its catalytic performance in ammoxidation of cyclohexanone, Chem. Eng. J. 280 (2015) 670–676. [16] M.J. da Silva, P.H.S. Andrade, S.O. Ferreira, C.B. Vilanculo, C.M. Oliveira, Monolacunary K8SiW11O39-catalyzed terpenic alcohols oxidation with hydrogen peroxide, Catal. Let. 148 (2018) 2516–2527. [17] K. Kamata, M. Kotani, K. Yamaguchi, S. Hikichi, N. Mizuno, N Olefin epoxidation with hydrogen peroxide catalyzed by lacunar polyoxometalate [γ-SiW10O34(H2O) 2]4−, Chem. Eur. J. 13 (2) (2007) 639–648. [18] N.C. Coronel, M.J. da Silva, Lacunar Keggin heteropolyacid salts: soluble, solid, and solid-supported catalysts, J. Clust. Sci. 29 (2018) 195–205. [19] S. Rana, K.M. Parida, A simple and efficient protocol using palladium based lacunary phosphotungstate supported mesoporous silica towards hydrogenation of pnitrophenol to p-aminophenol at room temperature, Catal. Sci. Technol. 2 (2012) 979–986. [20] L.R. Pizzio, P.G. Vásquez, C.V. Cáceres, M.N. Blanco, Supported Keggin type heteropoly compounds for ecofriendly reactions, Appl. Catal. A 256 (2003) 125–139. [21] O.A. Kholdeeva, M.P. Vanina, M.N. Timofeeva, R.I. Maksimovskaya, T.A. Trubitsina, M.S. Melgunov, E.B. Burgina, J. Mrowiec-Bialon, A.B. Jarzebski, C.L. Hill, J. Catal. 226 (2004) 363–371. [22] J.B. Moffat, Metal-Oxygen Clusters: The Surface and Catalytic Properties of Heteropoly Oxometalates, Plenum, New York, 2001.
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
A rare oxidation of camphene to acid and aldehyde in the presence of Lacunar Keggin heteropoly salts
T
⁎
Castelo Bandane Vilanculoa, Márcio José da Silvab, , Sukarno Olavo Ferreirab, Milena Galdino Teixeirab a b
Chemistry Department, Pedagogical University of Mozambique, FCNM, Campus of Lhanguene, Av. de Moçambique, Km 1, Maputo, 4040, Mozambique Chemistry Department, Federal University of Viçosa, Viçosa, Minas Gerais State, 36590-000, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Camphene Lacunar Keggin heteropolyacid salts Hydrogen peroxide Catalytic oxidation
In this work, monolacunary Keggin heteropolyacid salts containing sodium as the countercation were synthesized and evaluated as catalysts on the camphene oxidation with hydrogen peroxide. Under reaction conditions (i.e., 333 K, air atmosphere, CH3CN solution), camphene (1) was selectively oxidized by H2O2, generating two rare products; camphene aldehyde (1a) and camphenylic acid (1b), which have attractive olfactive properties. Notably, these uncommon compounds were obtained with the camphene preserving their carbon skeletal. Among the lacunar Keggin heteropolyacid salts, the Na7PW11O39 lacunar salt was the most active and selective toward oxidation products, completely converting the camphene to (1a) and (1b) products. The catalyst load, the oxidant: camphene molar ratio and the reaction temperature allowed to control the (1a)/ (1b) proportion.
1. Introduction Monoterpenes are renewable raw material used to synthesize flavors, agrochemicals, and pharmacies intermediates [1]. Among them, camphene deserves highlight because their oxygenated derivatives have attractive olfactive properties and are widely used as an ingredient for perfumery and fragrance industries [2]. Camphene oxidation becomes still more interesting with the hydrogen peroxide, an environmentally friendly oxidant, that is too noninflammable, easily handling, inexpensive, and atom-efficient [2,3]. The greatest challenge to oxidize the camphene is that it has only one allylic hydrogen, which is in the bridgehead position, being thus hardly removable. In addition, the double bond of camphene has a large hindrance hysteric, an aspect that hampers its epoxidation [4]. Consequently, there are scarce cases where the metal catalysts commonly used in olefin oxidation reactions had success in the camphene oxidation, either with oxygen or hydrogen peroxide [5,6]. Heteropolyacids (HPAs) are considered environmentally benign catalysts and a good alternative to the common metal catalysts [7]. Keggin HPAs have tungsten and molybdenum as addenda metal atoms in their anions, being thus potentially active catalysts in oxidation reactions [8–10]. However, HPAs are strong Brønsted acids, a feature that promotes acid-catalyzed concurrent reactions, such as carbon skeletal
rearrangement and nucleophilic addition, which reduce the selectivity of the oxidation products [11,12]. To circumvent this drawback, an approach that is still few explored is to convert Keggin HPAs to neutral salts, exchanging their acidic protons by metal cations [13,14]. Moreover, the removal of one MO unit from the heteropolyanions (i.e., WO or MoO) results in a lacunar salt catalyst, which may be more active than a precursor with the saturated heteropolyanion [15–18]. In this work, we synthesized three lacunar Keggin HPAs sodium salts and assessing their catalytic activity on the oxidation of the camphene by hydrogen peroxide. All the lacunar salts were characterized by FT-IR, XRD, TG/DSC, SEM-EDS techniques, and their acidic strength were measured by n-butylamine titration. Effects of the main reaction variables were investigated. Special attention was devoted to linking the structural properties of the lacunar salts with their catalytic activity. 2. Experimental section 2.1. Chemicals All the chemicals were purchased from commercial sources and used as received. Camphene and acetonitrile were Sigma-Aldrich (ca. 99 wt. %). Sodium hydrogen carbonate was Vetec (ca. 99 wt. %).
⁎ Corresponding author at: Chemistry Department, Federal University of Viçosa, Campus Universitário, Avenue P.H. Rolfs, s/n, Viçosa, Minas Gerais State, CEP: 36570-000, Brazil. E-mail address: [email protected] (M.J. da Silva).
https://doi.org/10.1016/j.mcat.2019.110589 Received 23 July 2019; Received in revised form 19 August 2019; Accepted 23 August 2019 Available online 29 August 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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catalyst (ca. 1.34 mol %), the reaction was started and monitored during 8 h, periodically collecting aliquots that were analyzed in a GC equipment (Shimadzu 2010, FID, capillary column). All the characterization data of the products are in the Supplemental material.
Hydrated heteropolyacids (i.e., H3PW12O40, H3PMo12O40 and H4SiW12O40; ca. 99 wt. %) were acquired from Sigma-Aldrich. Aqueous hydrogen peroxide was from Alphatec (ca. 35 wt. %). 2.2. Synthesis of the lacunar sodium salts
2.7. Recovery and reuse of the catalyst The lacunar salt catalysts were synthesized as described in the literature [19]. Firstly, H3PW12O40.nH2O (ca. 1.0 g) was dissolved in water (ca. 30 mL), magnetically stirred and heated to 333 K. The pH was adjusted to 4.8 adding aqueous NaHCO3, and the solution was stirred and heated at 333 K/ 3 h keeping constant the pH value. The Na7PW11O39 salt was obtained after the vaporization of the solvent and recrystallization from water, being subsequently dried at 373 K/ 5 h. A similar procedure was used to synthesize the lacunar sodium salts of the phosphomolybdic and silicotungstic acids, adequately adjusting the pH of the solutions.
After the end of the reaction, the excess of solvent (CH3CN) was removed under reduced pressure. After to add water (ca. 20 mL), the resulting mixture was extracted four times with diethyl ether. The aqueous phase was evaporated in a heating plate to near dryness and then dried at room conditions. The solid catalyst was weighted and reused in another catalytic run. 3. Results and discussion 3.1. Catalyst characterization
2.3. Synthesis of the saturated sodium phosphotungstate salt The FT-IR study was done to verify if the Keggin anion structure (i.e., primary structure) was retained after the synthesis of lacunar salt. It was done comparing the FT-IR spectra of the lacunar salt, with spectra of the saturated salt and parent HPA (Fig. SM1). The main absorption bands of H3PW12O40 were noticed at wavenumbers 1080, 980, 920, and 790 cm−1, which agree with the literature [21]. These bands were assigned to the stretching of PeOa, WeOd, WeObeW and WeOceW bonds. The typical stretching bands of the saturated sodium phosphotungstate salt were like the pattern H3PW12O4, meaning that the salt was successfully synthesized. The subscripts distinguish the oxygen atoms in relation to the position occupied in the heteropolyanion [22]. The absorption band at wavenumber 1080 cm−1, which was assigned to the vibration of PeOa bond, underwent a splitting that resulted in two new bands (ca. 1020 and 1060 cm-1). It is a guarantee that the lacunar heteropolyanion was formed [23–25]. This splitting was attributed to the decreasing of the symmetry of group PO4, resultant from the removal of the WO unit [26]. The same splitting occurred when lacunar salt of phosphomolybdic acid was synthesized (see Supplemental material, Fig. SM2) [27]. However, the same it was not observed when the lacunar silicotungstic salt was synthesized; the stretching of SieOa bond resulted in only an absorption band (see Supplemental material, Fig. SM3). XRD spectra give information about the secondary structure of the sodium salts. A comparison of the diffractograms obtained from the sodium salts and their acid parents allowed observing that they presented a lower level of crystallinity (Fig. SM4). This different crystallinity may be assigned to the exchange of the protons by sodium cations, and the alteration of the number of hydration water molecules. The other two lacunar sodium salts also presented a lower level of crystallinity than acid precursors (see Supplemental material, Figs. SM5 and SM6). TG curves obtained from lacunar sodium salts shows two regions of loss weight; the first one before 473 K, which was assigned to loss of all water molecules. The second one, assigned to the decomposition of POa-W framework, followed by the noticeable peak in the DSC curves around 793 K. The final products were a mixture of oxides. Similar behavior was noticed for other salts (Figs. SM7–SM9). The acidity strength of the sodium salts was measured by potentiometric titration with n-butylamine (Figs. SM10–SM12). Regardless of the Keggin anion, the titration curves of the lacunar salts presented a similar pattern; a quick decrease on electrode potential after the addition of a minimum volume of base. It is suggestive that only a residual Brønsted acidity remained after the protons exchange by the sodium cations, indicating that all of them were virtually removed. The value of the initial electrode potential allows classifying the acidy sites [27]. While all the precursor HPAs presented very strong acid sites (i.e., Ei > 100 mV), their lacunar sodium salts displayed strong acid
The saturated sodium salt (i.e., Na3PW12O40) was synthesized as described in the literature [22]. Typically, Na2WO4·2H2O (30 mmol, 10 g) was slowly added to 20 ml of distilled water and the mixture was warmed to 333 K and magnetically stirred. Then, H3PO4 85% (15 mmol, 1 mL) and HCl (100 mmol, 8 mL) were added and the resulting mixture was stirred 1 h. After solvent evaporation, it was twice recrystallized in hot water, and dried at 373 K/ 5 h. 2.4. Characterization of the catalysts The infrared spectra were recorded on Varian 660-IR spectrometer at wavenumber range of 500–1700 cm−1, that is the fingerprint region of the main absorption bands of Keggin heteropolyanions. Crystalline structure of the salts and HPAs were analyzed by X-rays diffraction (XRD) spectroscopy using a XRD-ray Diffraction System model D8-Discover Bruker using Ni filtered Cu-kα radiation (λ =1.5418 Å), working at 40 kV and 40 mA, with a counting time 1.0 s in the diffraction angle (2θ) ranging from 5 to 80°. The acidity strength of the catalyst was estimated by potentiometric titration, as described by Pizzio et al [20]. The electrode potential variation was measured with a potentiometer (i.e. Bel, model W3B). Typically, 50 mg of HPA salt was dissolved in CH3CN, and titrated with n-butylamine solution in toluene (ca. 0.05 mol L−1). The characterization of the surface of the solid lacunar salt, thin sections were selected and metalized with carbon for analysis in a scanning electron microscopy equipment (SEM) and dispersive energy x-ray spectrometer (EDS) using a JEOL JSM 6010LA SEM. 2.5. Identification of the main reaction products The main products were identified in a Shimadzu GC-2010 gas chromatography coupled with a MS-QP 2010 mass spectrometer (i.e., electronic impact 70 eV, scanning range of m/z 50–450). Afterward, they were separated by column chromatography (silica 60 G) using mixtures of hexane, ethyl acetate, dichloromethane and methanol as eluents. After this step, they were characterized by 1H and 13C NMR and IV (Varian FT-IR 660) spectroscopy analysis. NMRs spectra were taken in CDCl3 solutions, using a Varian 300 spectrometer at 300.13 and 75.47 MHz, respectively. Chemical shifts were expressed as δ (ppm) relative to the tetramethylsilane (TMS) as an internal reference. 2.6. Catalytic runs Catalytic tests were carried out in a 25 ml three-necked glass flask, equipped with a sampling system and a reflux condenser. Typically, camphene (2.75 mmol) was solved in CH3CN solution (ca. 10 mL), which was magnetically stirred and heated to 333 K. After to add the 2
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Table 1 Lacunar sodium HPA salt-catalyzed reactions of camphene oxidation by hydrogen peroxide.a Exp.
1 2 3
Catalyst
Na7PW11O39 Na7PMo11O39 Na8SiW11O39
Conversion (%)
99 53 68
Selectivity (%) 1a
1b
Oligomers
17 – –
73 – –
10 100 100
a Reaction conditions: camphene (2.75 mmol); hydrogen peroxide (11.0 mmol); catalyst load (1.34 mol %); temperature (333 K); CH3CN (10 mL).
(Table 1). The straight obtention of the carboxylic acids from camphene is an uncommon reaction; for this reason, to synthesize camphene esters, other reaction pathways have been explored [28]. The camphene double bond has a high steric hindrance, an aspect that makes difficult their oxidation. On the other hand, the synthesis of an aldehyde containing a five-membered ring was achieved starting from α-pinene under oxidative conditions, and in an acidic medium [29]. Therefore, all these aspects contribute to valorize this oxidative process of the camphene. The surprising activity of the lacunar sodium phosphotungstate salt motivated us to assess the efficiency of other lacunar salts of phosphotungstic acid containing different cations (Fig. 2). The presence of a vacancy in the phosphotungstic anion looks has a pivotal role in the camphene oxidation reaction (Fig. 2). The phosphotungstic acid converted the camphene exclusively to oligomers; no oxidation products were detected. Conversely, excepted the cesium salt, all the lacunar catalysts provided the camphene oxidation derivates (i.e., aldehyde and the acid), even though in different proportions. The lowest efficiency of the cesium salt may be a consequence of the poor solubility; all the other lacunar catalysts were completely soluble. Therefore, the Na7PW11O39 was the most active and selective catalyst and was selected to investigate the effects of main reaction parameters (i.e., reactants stoichiometry, temperature, and catalyst load).
Fig. 1. Kinetic curves of lacunar sodium Keggin HPA salt-catalyzed reactions of camphene oxidation by hydrogen peroxidea. a Reaction conditions: camphene (2.75 mmol); hydrogen peroxide (11.0 mmol); catalyst load (1.34 mol %); temperature (333 K); CH3CN (10 mL).
sites (0 < Ei < 100 mV), which may be assigned to the presence of residual protons. Saturated sodium phosphotungstate (i.e., Na3PW12O40) also showed a very strong acidity strength, probably due to the presence of remaining Bronsted acidity. EDS spectra of the lacunar sodium phosphotungstate salt is shown in Fig. SM13. The percentual composition determined by EDS agrees established theoretical formula of lacunar sodium salt. Saturated phosphotungstate salt and the other lacunar salts also had the theoretical composition confirmed by EDS analyses, although traces of Cl− ions had been detected.
3.2. Catalytic tests 3.2.1. Screening of lacunar Keggin HPA salt Initially, we have assessed the catalytic activity of three lacunar sodium salt catalysts on the camphene oxidation with hydrogen peroxide (Fig. 1). The reaction conditions were choice based on the literature [16]. a Reaction conditions: camphene (2.75 mmol); hydrogen peroxide (11.0 mmol); catalyst load (1.34 mol %); temperature (333 K); CH3CN (10 mL) Fig. 1 shows that lacunar sodium phosphotungstate salt (Na7PW11O39) was the most active catalyst. The highest conversion (ca. 99%) was achieved within two first hour reaction. Noticeably, two rare reaction products were obtained only on the Na7PW11O39-catalyzed reaction: camphene aldehyde (1a) and camphenylic acid (1b) (Scheme 1). Remarkably, these two products were obtained only when the Na7PW11O39 was the catalyst. When lacunar sodium silicotungstate or phosphomolybdate salts were the catalysts, almost no product was significantly formed. Through the mass balance of reaction (via GC analyses), we verified that undetected products were oligomers
3.2.2. Effect of the oxidant load on the Na7PW11O39-catalyzed oxidation of the camphene by hydrogen peroxide The oxidant load is a key aspect in oxidation reactions of olefins. An excess of hydrogen peroxide may favor the formation of oligomers (i.e., due to the radicalar reactions), or the hydration products (i.e., aqueous solution), compromising thus the oxidation selectivity. To assess this effect, we carried out reactions varying the ratio oxidant to camphene (Fig. 3). In the absence of the catalyst, an excess of oxidant was unable to improve the reaction conversions; only ca. 10% was achieved. Contrariwise, in the presence of the lacunar sodium phosphotungstate catalyst, both conversion and selectivity of the reactions were dependent on the proportion oxidant: camphene. When the camphene
Scheme 1. Oxidation of camphene by H2O2 in the presence of Na7PW11O39 catalyst in CH3CN solutions. 3
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Fig. 2. Effect of cation nature on the lacunar phosphotungstate catalyst in camphene oxidation reactions by hydrogen peroxidea. a Reaction conditions: camphene (2.75 mmol); hydrogen peroxide (11.0 mmol); catalyst load (1.34 mol %); temperature (333 K); CH3CN (10 mL).
oxidation was carried out at an equimolar amount of oxidant, only a poor conversion (ca. 30%), and a low selectivity for 1a and 1b was obtained (Fig. 4). Conversely, an increase of the oxidant load increased either the initial rate of reaction as well as the final conversion. It was found that the proportion of 1:4 was enough to virtually convert all the camphene to aldehyde and camphenylic acid, with a minimum formation of oligomers. The reaction selectivity was impacted by the oxidant load; using 1:2 and 1:3 M ratios, aldehyde was the major product; on the other hand, higher amounts of peroxide favored the formation of the camphenylic acid. We suppose that the requirement of peroxide excess may be explained if we consider that the sodium catalyst should be firstly peroxidized, generating the intermediate 1 (Scheme 2) [16,30]. Posteriorly, the intermediate 1 may transfer the oxygen atom to the camphene, generating the camphene aldehyde and, afterward, the camphenylic acid. The consecutive character of the camphene oxidations (i.e., firstly, aldehyde and posteriorly to camphenylic acid), could be confirmed monitoring the reaction selectivity in the presence of lacunar sodium salt during 8 h (Fig. SM14). To verify the hypothesis of Na7PW11O39 catalyst has been peroxidized during the reaction and generate the intermediate 1, we recorded FT-IR spectra of the samples of the catalyst in the presence of aqueous H2O2 (Fig. 5). The changes in the absorption bands present in the fingerprint region of the lacunar Na7PW11O39 suggests that the catalyst was peroxidized [30]. The literature has described that di-or tetra-nuclear peroxo tungstate fragments were formed in H3PW12O40-catalyzed reactions of terminal olefins with hydrogen peroxide [31]. However, herein the saturated heteropolyanion catalysts (i.e., Na3PW12O40) were inactive on the reactions of oxidation. It is suggestive that the action mechanism of the lacunar anion catalyst should be different from saturated phosphotungstate anion (i.e., which may involve di-peroxidized fragments). Therefore, we suppose that as described in previous work [16], a mono-peroxidized intermediate could be an intermediate on this reaction. Moreover, our experimental results (Fig. 2) demonstrated that the vacancy plays an essential role in the catalyst activity; only lacunar phosphotungstic catalysts were efficient to promote the oxidation of camphene with H2O2.
Fig. 3. Effect of oxidant load on kinetic curves of Na7PW11O39-catalyzed oxidation reactions of camphene with H2O2. a Reaction conditions: Camphene (2.75 mmol); reaction time (8 h); Na7PW11O39 (1.34 mol %); temperature (333 K); CH3CN (10 mL).
Fig. 4. Effect of the oxidant load on kinetic curves of Na7PW11O39-catalyzed oxidation reactions of camphene with H2O2. a Reaction conditions: camphene (2.75 mmol); reaction time (8 h); Na7PW11O39 (1.34 mol %); temperature (333 K); CH3CN (10 mL).
4
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Scheme 2. Reaction pathway of Na7PW11O39-catalyzed oxidation reactions of camphene with H2O2 (adapted from ref. [16]).
Fig. 5. FT-IR spectra of the samples: H2O2(aq), Na7PW11O39, Na7PW11O39 + H2O, and Na7PW11O39 + H2O2.
Fig. 7. Effect of catalyst load on kinetic curves of Na7PW11O39-catalyzed oxidation reactions of camphene with H2O2. a Reaction conditions: camphene (2.75 mmol); reaction time (8 h); H2O2 (11.0 mmol); temperature (333 K); CH3CN (10 mL).
3.2.3. Effect of the Na7PW11O39 catalyst load on the oxidation of camphene by H2O2 The kinetic curves in Fig. 6 shows that within period investigated the reactions of the camphene oxidation did not achieve the equilibrium; an increase of the catalyst load resulted in higher conversions. The reaction selectivity was also impacted by an increase of catalyst load; with a high Na7PW11O39 concentration the formation of camphenylic acid was remarkably favored, concomitantly reducing the selectivity of aldehyde (Fig. 7). It is reinforcing the importance of the lacunar sodium phosphotungstate catalyst on the formation of oxidation products.
Fig. 8. Effect of temperature on the conversion and selectivity of Na7PW11O39catalyzed oxidation reactions of camphene with H2O2. a Reaction conditions: camphene (2.75 mmol); reaction time (8 h); H2O2 (11.0 mmol); Na7PW11O39 catalyst concentration (134 mol %); CH3CN (10 mL).
3.2.4. Effect of temperature on the Na7PW11O39-catalyzed oxidation of camphene by H2O2 An increase of the reaction temperature enhanced the initial rate of the reaction (omitted by simplification) as well as the final conversion (Fig. 8). In addition, the conversion of the camphene to aldehyde (1a), and the oxidation of aldehyde to carboxylic acid (1b), were also notably improved by an increase of temperature. The results are evidence of the endothermal character of the oxidation reactions (i.e., camphene and camphene aldehyde). For this reason, the camphene conversion and the selectivity for the oxidation products was improved when the reaction temperature was increased.
Fig. 6. Effect of Na7PW11O39 catalyst load on the oxidation by H2O2. a Reaction conditions: camphene (2.75 mmol); H2O2 (11 mmol); temperature (333 K); CH3CN (10 mL). 5
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exchange by sodium ions as well as the removal of an WO unit from the Keggin heteropolyanions were successfully accomplished. In almost of Na7PW11O39-catalyzed reactions, the camphene was majority converted to rare oxidation products; camphene aldehyde (1a) and camphenylic acid (1b), preserving the camphene carbon skeletal, which is rare due to easy Wagner-Meerwein rearrangement commonly observed in these oxidation reactions. The maximum selectivity combined toward these products was close 90%, with a total conversion of camphene. The catalyst load, molar ratio of hydrogen peroxide to camphene, and the reaction temperature were key aspects toward formation of the goalproducts. The effects of the countercation, and of the Keggin heteropolyanion were investigated. Remarkably, the camphene oxidation products were formed only in the presence of the lacunar phosphotungstic acid salt catalysts. This catalytic procedure demonstrated to be an efficient route to synthesize these rare camphene oxidation derivatives. The reusability of the catalyst was assessed. The low stability of catalyst along the recycle process resulted in the decomposition of lacunar anion.
Fig. 9. Conversion after the reuse of Na7PW11O39-catalyzed oxidation reaction of camphene with H2O2. a Reaction conditions: camphene (2.75 mmol); reaction time (8 h); H2O2 (11.0 mmol); Na7PW11O39 catalyst concentration (134 mol %); CH3CN (10 mL).
Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgements The authors are grateful for the financial support from CNPq and FAPEMIG (Brasil). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) Finance Code 001. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110589. References [1] P. Gallezoti, Conversion of biomass to selected chemical products, Chem. Soc. Rev. 41 (4) (2012) 1538–1558. [2] A.A. de Oliveira, M.L. da Silva, M.J. da Silva, Palladium-catalysed oxidation of bicycle monoterpenes by hydrogen peroxide in acetonitrile solutions: a metal, reoxidant-free and environmentally benign oxidative process, Catal. Lett. 130 (2009) 424–431. [3] M.J. da Silva, J.A. Gonçalves, R.B. Alves, O.W. Howarth, E.V. Gusevskaya, Palladium catalyzed transformations of monoterpenes: stereoselective deuteriation and oxidative dimerization of camphene, J. Organomet. Chem. 689 (2004) 302–308. [4] T. Michel, M. Cokoja, F.E. Kuhn, Catalytic epoxidation of camphene using methyltrioxorhenium(VII) as catalyst, J. Mol. Catal. A 368–369 (2013) 145–151. [5] M.J. da Silva, J.A. Gonçalves, R.B. Alves, O.W. Howarth, E.V. Gusevskaya, Palladium-catalyzed oxidation of monoterpenes: novel tandem oxidative couplingoxidation of camphene by dioxygen, J. Mol. Catal. A 176 (2001) 23–27. [6] N.V. Maksimchuk, M.S. Melgunov, J. Mrowiec-Białoń, A.B. Jarzębski, O.A. Kholdeeva, H2O2-based allylic oxidation of α-pinene over different single site catalysts, J. Catal. 235 (2005) 175–183. [7] S. Omwoma, C.T. Gore, Y. Ji, C. Hu, Y.-F. Song, Environmentally benign polyoxometalate materials, Coord. Chem. Rev. 286 (2015) 17–29. [8] F. Cavani, Heteropolycompound-based catalysts: a blend of acid and oxidizing properties, Catal. Today 41 (1998) 73–86. [9] S.G. Casuscelli, M.E. Crivello, C.F. Perez, G. Ghione, E.R. Herrero, L.R. Pizzio, P.G. Vázquez, C.V. Cáceres, M.N. Blanco, Effect of reaction conditions on limonene epoxidation with H2O2 catalyzed by supported Keggin heteropolycompounds, Appl. Catal. A 274 (2004) 115–122. [10] D. Amitouche, M. Haouas, T. Mazari, S. Mouanni, R. Canioni, C. Rabia, E. Cadot, C. Marchal-Roch, The primary stages of polyoxomolybdate catalyzed cyclohexanone oxidation by hydrogen peroxide as investigated by in situ NMR. Substrate activation and evolution of the working catalyst, Appl. Catal. A 561 (2018) 104–116. [11] H. Zheng, Z. Yan, S. Chu, J. Chen, Continuous synthesis of isobornyl acetate catalyzed by a strong acid cation exchange resin in an oscillatory flow reactor, J. Chen, Chem. Eng. Process. Process Intens. 134 (2018) 1–8. [12] A.L.P. de Meireles, K.A. da Silva Rocha, I.V. Kozhevnikov, E.V. Gusevskaya, Esterification of camphene over heterogeneous heteropoly acid catalysts: synthesis of isobornyl carboxylates, Appl. Catal. A 409–410 (2011) 82–86.
Fig. 10. FT-IR spectra of sample fresh Na7PW11O39 and after the second reuse.
3.3. Catalyst reuse The Na7PW11O39 is a solid catalyst, however, it is soluble in the reaction medium (i.e., CH3CN). However, we developed a simple procedure to recover the catalyst by a liquid-liquid extraction procedure (see Section 3.2). We achieved a rate recovery of 90–95% for the Na7PW11O39 catalyst (Fig. 9). This high recovery suggests that procedure used was very efficient. Therefore, it was possible to reuse the catalyst in another runs. We have observed a trend of decreasing on the conversion when the catalyst was reused. The selectivity for the oxidations products was being gradually reduced, increasing the formation of the oligomers. To assess the catalyst deactivation, we compared the infrared spectra before (i.e., fresh catalyst), and after the second reuse. (Fig. 10). Noticeably, the two bands corresponding to the splitting of PeOa absorption bond absorption (ca. 1020 and 1060 cm−1) were absent in the FT-IR spectrum of reused catalyst [23–25]. We suppose that during the recovery procedure, the pH, which is key aspect to synthesize the lacunar catalyst, should has been drastically modified, consequently, the catalyst was deactivated. 4. Conclusions In this work, the activity of the lacunar sodium Keggin HPA salts was assessed on the oxidation of the camphene with hydrogen peroxide. All the catalysts were spectroscopically characterized. The protons 6
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