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Epoxidation of Cyclooctene Using Water as the Oxygen Atom Source at Manganese Oxide Electrocatalysts Kyoungsuk Jin, Joseph H. Maalouf, Nikifar Lazouski, Nathan Corbin, Dengtao Yang, and Karthish Manthiram* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
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S Supporting Information *
ABSTRACT: Epoxides are useful intermediates for the manufacture of a diverse set of chemical products. Current routes of olefin epoxidation either involve hazardous reagents or generate stoichiometric side products, leading to challenges in separation and significant waste streams. Here, we demonstrate a sustainable and safe route to epoxidize olefin substrates using water as the oxygen atom source at room temperature and ambient pressure. Manganese oxide nanoparticles (NPs) are shown to catalyze cyclooctene epoxidation with Faradaic efficiencies above 30%. Isotopic studies and detailed product analysis reveal an overall reaction in which water and cyclooctene are converted to cyclooctene oxide and hydrogen. Electrokinetic studies provide insights into the mechanism of olefin epoxidation, including an approximate first-order dependence on the substrate and water and a rate-determining step which involves the first electron transfer. We demonstrate that this new route can also achieve a cyclooctene conversion of ∼50% over 4 h.
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INTRODUCTION The direct transfer of an oxygen atom into a target substrate is a fundamental reaction in organic chemistry. Over several decades, oxygen-atom transfer reactions have been investigated to make diverse chemicals,1−3 including epoxides. Epoxides are versatile intermediates for the manufacture of many chemical products including surfactants,4 epoxy resins,5 and pharmaceuticals.6 These epoxides are often prepared by oxidation of olefins. Ethylene is among a small number of olefin substrates that can be oxidized to the corresponding epoxide using molecular oxygen through heterogeneous, thermochemical routes; in the case of ethylene, the reaction is conducted at 270−290 °C and 20 bar with Ag-based catalysts. 7,8 Most alkene epoxidation reactions employ peroxide-based oxidants, such as tert-butyl hydroperoxide (TBHP) or m-chloroperoxybenzoic acid (mCPBA), or are conducted through the chlorohydrin process. Both of these routes involve difficult to handle reagents and generate undesirable stoichiometric byproducts.9,10 To avoid generation of side products, some processes use catalysts that generate hydrogen peroxide in situ as the oxidant.11,12 Hydrogen peroxide is produced in situ by either reducing oxygen or oxidizing water. In either case, this in situ approach to hydrogen peroxide generation has been proposed in both a chemical and an electrochemical fashion. Epoxidation catalysts explored in this context include bioinspired homogeneous complexes,2,13−15 transition-metal-based polyoxometalates (POM),16−18metal clusters,19 and nanoparticles.12 Sodium bromide has also been studied as an additive © XXXX American Chemical Society
in water and acetonitrile mixtures to catalyze electrochemical epoxidation of olefin substrates.20−22Hydrolysis of bromine in water generates HOBr, which transfers oxygen into olefin substrates. In addition, photochemical epoxidation has been studied using various transition-metal-based complexes with water as oxygen source.23−27For example, visible light irradiation with chemical oxidants such as K2PtCl6 can activate a ruthenium(II) porphyrin to make a RuO species from water, which can transfer oxygen into an olefin substrate to make an epoxide. Similarly, a combination of a photosensitizer, [RuII(bpy)3]2+ (bpy: 2,2′-bipyridine), and a one-electron oxidant, [CoIII(NH3)5Cl]2+, has been utilized to produce metal−oxo species, which can catalyze olefin epoxidation.26 [(bTAML)FeIII-OH (bTAML = biuret-modified tetraamido macrocyclic ligand),26 [(R,R-BQCN)MnII]2+ (R,R-BQCN = N,N′-dimethyl-N,N-bis(8-quinolyl)cyclohexanediamine),27 and [Ru (TMP)(CO)] (TMP = tetramesitylporphyrin) catalysts23,25 have shown high selectivity and yield for photochemical epoxidation. Although these catalysts exhibit high selectivity and yield, several limitations still remain: (i) required operating temperature (∼80−100 °C), (ii) difficulty of separating homogeneous catalysts following the reaction, (iii) required additional reagents such as chemical oxidants and photosensitizer, and (iv) risk of explosion due to the combination of oxidizing agents (oxygen and hydrogen peroxide) and fuels (flammable solvents and organics).28,29 Received: March 2, 2019
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DOI: 10.1021/jacs.9b02345 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
NPs were transformed into the Mn3O4 phase, as confirmed by powder X-ray diffraction (PXRD) measurement (Figure S4). Scanning electron microscope (SEM) analysis confirmed that the Mn3O4 nanoparticles were deposited on the carbon paper electrode (Figure S5). Electrochemical experiments were conducted using a one-compartment electrochemical cell. Acetonitrile with 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) was used as the solvent with varying concentrations of cis-cyclooctene and water. A platinum foil and manganese oxide-loaded carbon paper were used as the counter and working electrodes, respectively. A Ag/AgCl electrode (Innovative Instruments) was used as the reference, and aluminum foil was used as the current collector. All potentials were 85% IR compensated using electrochemical impedance spectroscopy (EIS) techniques and were calibrated by measuring the redox potentials of 5 mM ferrocene/ferrocenium (Figure S7).
In this regard, we propose that water would be attractive as a sustainable, abundant, and safe oxygen atom source for electrochemical epoxidation reactions, especially if a hydrogen peroxide intermediate can be avoided (Scheme 1). Although Scheme 1. Electrochemical Epoxidation Using Water as the Oxygen Source
olefin substrates are normally inert to water, we demonstrate in this study that the equilibrium can be shifted toward epoxide through the application of an anodic potential in an electrochemical reactor at ambient conditions, allowing for catalytic epoxidation of olefins with water as the sole oxygen atom source. Among the many candidate catalysts that could be explored for this new reaction, we were interested in manganese oxide nanoparticles. Previously, manganese oxide nanoparticles have been investigated as efficient water oxidation catalysts. According to recent studies, reactive high-valent Mn(IV)O intermediate species are generated during water oxidation catalysis,30 which we hypothesized may act as an oxygen atom donor to the target olefin substrate.
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RESULTS AND DISCUSSION Cyclic voltammetry (CV) analysis was conducted to investigate the effect of water, cyclooctene, and manganese oxide NPs on the epoxidation reaction (Figure 1B). The distinct redox features of manganese oxide NPs appeared at approximately 0.7 V vs Fc/Fc+ with 5 M H2O and 200 mM cyclooctene. The redox behavior only appeared in the presence of water and manganese oxide catalysts, which indicated that manganese redox originates from the added water. According to previous electrochemical studies,30 the observed redox peaks may correspond to the Mn(III)/Mn(IV) redox couple (Figure S8). To confirm the identity of the products, we performed chronoamperometry at various potentials by passing 20 C of charge, which is equivalent to a maximum conversion of ∼13% of the substrate (Figure 1C). All the products were collected and analyzed by nuclear magnetic resonance (NMR) and quantified using gas chromatograph−mass spectrometry (GCMS) (Figure S9; see the Supporting Information for full details). We were able to detect cyclooctene oxide (epoxide) as the major product starting from 0.8 V vs Fc/Fc+, which is well matched with the Mn redox features. The only detectable side product was cyclooctanone (ketone). Interestingly, at lower potentials, we only detected the epoxide without the formation of any ketone, while at higher potentials, both products were detected. The product ratio of epoxide and ketone at 1.45 V vs Fc/Fc+ was approximately 3.91:1. In the absence of Mn3O4 NP catalysts, only about 8% of Faradaic efficiency toward epoxide was obtained at 1.45 V vs Fc/Fc+, which corresponds to 0.34 mA/cm2 of iepoxide (Figure S10), whereas ∼30% of FE with 2.5 mA/cm2 was obtained in the presence of Mn3O4 NP with 5 M H2O and 200 mM cyclooctene (Figure S11). Having water serve as the only oxygen atom source for the epoxidation reaction is crucial. More detailed insights into the nature of the epoxide/ketone formation step are provided from measurements of the isotopic distributions of epoxide/ketone product obtained from the electrolysis with H218O-enriched electrolyte. We conducted GC-MS of synthesized epoxide (Figure 1D). The prominent ion peaks at a mass-to-charge ratio (m/z) of 125 and 126 were shifted to 127 and 128, respectively, when the electrolysis was conducted with a H218O oxygen source. We also excluded the involvement of free hydrogen peroxide in the epoxidation reaction by assay tests. We were not able to detect any hydrogen peroxide species during the anodic reaction with or without cyclooctene substrate (Figure S12). Thus, the spectroscopic data clearly
EXPERIMENTAL SECTION
Monodisperse manganese oxide nanoparticles (NPs) were prepared using a hot injection method (Figure 1A).30 A ligand exchange procedure was performed on the NPs to replace hydrophobic myristic acid ligands on the manganese oxide NPs with NOBF4 ligands.31 Manganese oxide NPs dispersed in ethanol were then drop-cast onto hydrophilic carbon paper and subsequently annealed in a muffle furnace at 400 °C for 5 h (see the Supporting Information for a detailed procedure). During the annealing process, synthesized MnO
Figure 1. Mn3O4 nanoparticles for cyclooctene epoxidation. (A) TEM image of as-synthesized 10 nm sized manganese oxide nanoparticles and (B) cyclic voltammetry curves of Mn3O4 nanoparticles varying substrate and water condition (scan rate: 50 mV/s; 5 M water and 200 mM cyclooctene were used). (C) Faradaic efficiency for epoxide at various potentials and (D) GC-MS spectrum of the epoxide product generated from the epoxidation reaction by Mn3O4 NPs. Inset figures show the isotopic distribution patterns obtained from H216O (left) and H218O (right) (see Figure S6 for identification of the fragments). B
DOI: 10.1021/jacs.9b02345 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society
ment allows for theinterrogation of the reaction order of reactant species.32,33 The cyclooctene order dependence was obtained by plotting the partial current density for epoxide iepoxide versus the concentration of cyclooctene with 5 M H2O (Figure 3A). While no clear dependence on total current density exists (Figure S13), a roughly first-order dependence is obtained from the log(iepoxide) vs log(Ccyclooctene) plot. The water order dependence of catalytic current at Mn3O4 NPs was then examined by varying the concentration of water from 0.5 to 15 M at 200 mM cyclooctene. Similar to the substrate dependence, a roughly first-order dependence on water concentration was observed from 0.5 to 5 M water, whereas a negative-order water dependence is observed beyond 5 M H2O due to the reduced solubility of cyclooctene. The current−potential (Tafel) behavior of the Mn3O4 NPs in the region of olefin oxidation was measured from 1.25 to 1.45 V vs Fc/Fc+ in 50 mV increments. The chronoamperometry analysis was conducted at each potential in 5 M H2O and 200 mM cyclooctene condition until total passed charge reached 20 C. The Tafel slope of the Mn3O4 NPs is 150 ± 12 mV/dec (Figure 3C), which suggests that the rate-determining step (RDS) contains an electron transfer and there are no prior electron transfers that affect the RDS for the overall epoxidation reaction.34 Taken together, the electrokinetic study and isotopic labeling experiments lead us to suggest the following electrochemical rate law:
indicate the oxygen in cyclooctene oxide comes from the added water without involving a soluble peroxide intermediate. Because water and cyclooctene are the species that are being oxidized into epoxide or ketone products, we sought to understand the impact of concentration of those species on the Faradaic efficiency toward epoxidation. As the concentration of either cyclooctene or water increased, the total current density at 1.45 V vs Fc/Fc+ remained unchanged (Figure S13), which suggests the current is limited by a competition for the same manganese active sites. In terms of selectivity, the FEepoxide increased linearly with the cyclooctene concentration (Figure 2A). The higher cyclooctene concentration likely enables it to
Figure 2. Variation in the Faradaic efficiency toward cyclooctene oxide as a function of (A) cyclooctene (with 5 M H2O) and (B) water (with 200 mM cyclooctene) concentrations at 1.45 V versus Fc/Fc+. The FE increased monotonically as cyclooctene concentration increased while the FE slightly decreased when the water concentration exceeded 10 M. Each error bar denotes the standard deviation of data from three experiments.
i EF zy zz iepoxide = 2Fk 0θ[cyclooctene]0.70 [water]0.77 expjjj k 2.54RT { where θ is coverage of surface Mn active sites, k0 is a potentialindependent rate constant, F is Faraday’s constant, T is the absolute temperature, and R is the ideal gas constant. On the basis of the equation, we can propose an electrochemical epoxidation reaction mechanism (Scheme 2 and section E in
compete better for oxygen-atom intermediates at Mn active sites. On the other hand, FEepoxide initially increased with the water concentration, reaching a maximum FEepoxide at 10 M H2O and then monotonically decreased when the water concentration was over 10 M (Figure 2B). This can be rationalized by the fact that at water concentrations where the solubility of cyclooctene drops below 200 mM (>10 M H2O) the electrolyte becomes slightly turbid, and the concentration of cyclooctene is reduced, resulting in attenuated Faradaic efficiency toward epoxide. Electrokinetic measurements were conducted to understand the electrochemical epoxidation mechanism. The dependence of the partial current density on the concentration of cyclooctene and water was ascertained from chronoamperometry measurements at 1.45 V vs Fc/Fc+ (see the Supporting Information for full details). Chronoamperometry measure-
Scheme 2. Proposed Mechanism for Electrochemical Epoxidation by Manganese Oxide Nanoparticles
Figure 3. Electrokinetic studies. (A) Cyclooctene concentration (at 5 M H2O) and (B) water concentration (at 200 mM cyclooctene) dependences of epoxide partial current at 1.45 V vs Fc/Fc+. (C) Tafel plot for cyclooctene oxide formation with 5 M H2O and 200 mM cyclooctene. Each error bar denotes the standard deviation of data from three experiments. C
DOI: 10.1021/jacs.9b02345 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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remaining FE is largely attributable to ketone formation (∼7.7%) as well as solvent oxidation. To fully understand the overall reaction in the electrochemical cell, we checked the products of the cathode using gas chromatography and confirmed that only hydrogen gas was produced with 94 ± 5% Faradaic efficiency (Figure 4B). Taken together with the data of the product analysis, isotopic labeling experiments, and electrokinetic studies, the overall electrochemical epoxidation reaction is proposed to involve the reaction of cyclooctene and water to generate cyclooctene oxide and hydrogen gas (Figure 4C). Hence, compared to a conventional water electrolyzer which produces valuable hydrogen and vents oxygen gas, we can make use of the anodically produced oxygen atoms to conduct an epoxidation reaction. Lastly, we have attempted to apply our method to other olefin substrates. Investigation of a wider range of substrates is imperative to show the broader applicability of our concept. Hence, a series of linear, cyclic, and other aliphatic substrates have been examined (Scheme 3). The concentrations of water
the Supporting Information). We hypothesize a Mn(IV)O species as the resting state, which was reported in previous studies on manganese oxide catalysts for water oxidation.30,35 In addition, Mn(IV)O has also been regarded as the resting state for homogeneous epoxidation reactions.36,37 Addition of cyclooctene to the Mn(IV)O will make cyclooctene oxide, leaving Mn(II)-vacant sites. We consider a reversible oxygen atom transfer step that exists as a pre-equilibrium step, which was previously observed in other oxygen atom transfer reactions.38,39 To probe the nature of the pre-equilibrium step, we added 1,2-epoxyoctane to the electrolyte during an electrochemical epoxidation reaction of cyclooctene. Generation of 1-octene was confirmed by GC-MS analysis (Figure S14), demonstrating the existence of reversible oxo-transfer steps during the epoxidation reaction. While the quantitative order dependence on added product is of smaller magnitude than expected (∼−0.3), this may be due to further oxidation of the added product. We found that the H/D kinetic isotope effect (KIE) value was 1.54, which does not provide evidence for the involement of a proton in the rate-determining step (Figure S15). As shown in Scheme 2, nucleophilic attack of water to vacant Mn(II) sites is proposed as the rate-determining step, accompanied by a one-electron transfer, resulting in the formation of Mn(III)−OH2. Indeed, for water oxidation catalysis by manganese oxide, one-electron oxidation of Mn(II) to Mn(III)−OH2 has been considered as the rate-determining step. 40−42 Strong Jahn−Teller distortion increases the instability of Mn(III) species, which leads to high energy barriers.41,43 We believe a similar phenomenon occurs for epoxidation reactions as well. This mechanism has a first-order dependence on cyclooctene and water concentrations and a theoretical 120 mV/dec Tafel slope, consistent with our experimental data. There are additional proposed mechanisms which may also be consistent with the experimental data (Figure S18 and see the Supporting Information section E). Long-term electrolysis was conducted to demonstrate sustainable chemical production. Epoxide products were analyzed at 25, 50, 75, and 100 C of passed charge (Figure 4A and Figure S16). Epoxide production was confirmed over a 4 h electrolysis with ∼50% total conversion. The drop in FE during electrolysis is due to substrate depletion, which is consistent with our data showing the dependence of FE on cyclooctene concentration (Figure 2A). We believe that the
Scheme 3. Epoxidation of Other Olefin Substrates
and substrates were varied depending on the solubility of substrates in the water/acetonitrile mixture (Tables S1 and S3). We were able to obtain >50% selectivity toward epoxide from most of the olefin substrates. Detailed reaction conditions and results are summarized in the Supporting Information section F. We found that selectivity toward epoxide decreased as chain length of linear aliphatic olefin substrates increased. For instance, selectivity among observed products for 1-hexene epoxidation was 92.6%, while 25.4% selectivity was obtained using 1-dodecene (Table S2). Ketone and aldehyde were generated as side products (Figure S21). On the other hand, more selective epoxidation was achieved as ring size increased. Ring contraction reaction occurred when the ring size was smaller than n = 8 (Figure S23). For instance, cyclopentyl aldehyde and cyclohexanone were the major products for cyclohexene oxidation reaction. Cyclic alkene substrates of smaller ring size (n < 8) have higher strain energy,44 which could cause them to preferentially go through ring contraction reactions. Only 8.0% selectivity was observed for cyclohexene, whereas 72.1% selectivity toward epoxide was achieved from cyclooctene oxidation. We believe higher Faradaic efficiencies and yields could be obtained through further engineering, which we are undertaking in future work.
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CONCLUSIONS In summary, we have developed a new electrochemical method to epoxidize olefin substrates at room temperature and ambient pressure using water as the sole oxygen atom source and monodisperse manganese oxide nanoparticles as the catalyst. From electrokinetic studies, a reaction mechanism was proposed where epoxide formation is a pre-equilibrium step to a rate-limiting electron transfer. In addition, >30%
Figure 4. Overall reaction. (A) Chronoamperometry curves and FEepoxide of Mn3O4 NPs at 1.45 V vs Fc/Fc+ and (B) FE for hydrogen at the cathode. (C) Complete reaction scheme of electrochemical cyclooctene epoxidation. Each error bar denotes the standard deviation of data from three experiments. D
DOI: 10.1021/jacs.9b02345 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Faradaic efficiency toward epoxidation was achieved at the anode, while hydrogen was coproduced at the cathode with >94 ± 5% Faradaic efficiency. We believe that these results will provide a new pathway to catalyze oxygen atom transfer reactions electrochemically.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b02345.
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Experimental methods, additional electrochemical data, and mechanistic analysis (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kyoungsuk Jin: 0000-0003-3009-6691 Dengtao Yang: 0000-0002-8315-5467 Karthish Manthiram: 0000-0001-9260-3391 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge Kindle Williams for insightful discussions and experimental assistance. We also thank the MIT Department of Chemistry Instrumentation Facility (DCIF) for the use of their NMR spectrometer. Funding for this research was provided by the Department of Chemical Engineering at MIT. J.H.M., N.L., and N.C. were supported by National Science Foundation Graduate Research Fellowships under Grant 1122374.
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DOI: 10.1021/jacs.9b02345 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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DOI: 10.1021/jacs.9b02345 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
