Oxidative Degradation of Multi-Carbon Substrates by an Oxidic Cobalt

Communication Cite This: Organometallics XXXX, XXX, XXX−XXX

Oxidative Degradation of Multi-Carbon Substrates by an Oxidic Cobalt Phosphate Catalyst Thomas P. Keane,† Casey N. Brodsky,† and Daniel G. Nocera* Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States

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S Supporting Information *

ABSTRACT: The development of heterogeneous catalysts to affect the activation of recalcitrant biomolecules has applications for biomass processing, biomass fuel cells, and wastewater remediation. We demonstrate that a cobalt oxygen evolution catalyst (Co-OEC) can catalyze the oxidation of carbon feedstocks completely to CO2. A quantitative analysis of the product distribution from the oxidative degradation of the C2 compound, ethylene glycol, is elaborated and a reaction sequence is proposed. The Co-OEC is also found to be competent for oxidatively degrading C2+ compounds, including glucose and lignin, to carbon dioxide at consequential Faradaic efficiencies.

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To probe the ability of CoPi to oxidize C−C bonds, the C2 molecule ethylene glycol (EG) was selected as an initial substrate, which permits the catalyst functionality to be examined in detail by avoiding the intractably complex product analysis expected for larger substrates. We also show that CoPi is able to oxidize glucose and kraft lignin and that CO2 is produced with Faradaic efficiencies of ∼10%, establishing that metal−oxyls are promising catalysts for the oxidative degradation of biomass and more generally presage watersplitting catalysts as potential anodic catalysts in biomass fuel cells. Differential electrochemical mass spectrometry (DEMS) was used to probe the gaseous products produced upon oxidation of an aqueous solution of ethylene glycol (EG) by CoPi catalyst. Catalyst films were deposited onto a glassy-carbon (GC) electrode from solutions of 1 mM Co2+ in 100 mM KPi at pH 7 by performing a controlled-potential electrolysis at 0.9 V vs Ag/AgCl (all potentials are referenced to Ag/AgCl unless noted otherwise) and passing a total charge of 35 mC/cm2. This catalyst film was then inserted into the electrochemical flow cell of the DEMS instrument (instrument details are provided in Supporting Information), while electrolyte (100 mM EG, 100 mM KPi, 200 mM KNO3 at pH 7) was flowed through the cell at a rate of 60 mL/h. Cyclic voltammograms (CVs) were performed from 0.9 to 1.0 V at a scan rate of 2 mV/s. This potential range is high enough for CoPi to perform water oxidation in addition to EG oxidation. Accordingly, mass spectrometry (MS) signals were detected for species of m/z

omplex biomolecules such as lignin, cellulose, and hemicellulose constitute the majority of plant biomass1 and as such represent a large untapped resource for the production of both energy and feedstock chemicals.2−4 As an energy supply, optimal power density requires that these compounds be completely oxidized to CO2 in order to fully exploit the energy stored in their chemical bonds. Nonetheless, oxidations of C2+ substrates are typically incomplete and undesired, partially oxidized products are obtained. A common method to recover the stored energy in biomass is via fuel cells, the most common of which are solid oxide and microbial fuel cells. The former requires high operating temperatures, which limit both their efficiency and scope of implementation,5,6 whereas the latter are able to oxidize multicarbon biomolecules at ambient temperatures but at low power densities with the further complication that the microbes are fragile and their operational environment must be highly regulated.7,8 For these reasons, molecular catalysts have been sought for biomass conversion. Reactive oxygen species (ROS) such as hydrogen peroxide9 or organic oxyl radicals such as TEMPO10 have been explored for the oxidation of lignin and other biomolecules under mild conditions. These studies point to the importance of highly reactive oxygen-based radicals to drive oxidation of C−C bonds. In this regard, we recently reported that a cobalt oxygen evolution catalyst (CoPi)11−13 derives its function from reactive cobalt-bound oxyl radical centers that are present in an aqueous and benign environment.14,15 We posited that MOECs, with their reactive metal−oxyl active sites, may offer a path to complete oxidation of organic molecules under aqueous and benign conditions, using the cobalt−oxyl as a reactive warhead to oxidatively sever C−C and C−H bonds. In this work, we demonstrate that CoPi is capable of fully oxidizing organic compounds containing C−C bonds. © XXXX American Chemical Society

Special Issue: Organometallic Electrochemistry: Redox Catalysis Going the Smart Way Received: May 18, 2018

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DOI: 10.1021/acs.organomet.8b00337 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 32, 28, and 44, corresponding to O2, CO, and CO2, respectively (Figure S1). Although the flow solution was sparged with Ar gas for 30 min prior to each DEMS experiment, it was impossible to fully exclude N2 gas from the solution; thus the m/z 28 CO signal was convoluted with spikes in N2 due to gas bubbles. To isolate a clear signal for each product, the same experiment was repeated using 13C-EG. Figure 1a shows the resulting MS signals for m/z 32, 29, and

fractionates to CO in the MS detector, which does not account for the high CO signals observed in Figure 1a. In addition, little to no CO2 was detected (Figure S4) when a CO-saturated solution was passed over CoPi poised at an anodic applied potential. These experiments establish that the CO detected in the CV experiments is a product of EG oxidation. Although DEMS experiments identify CO2, CO, and O2 as gaseous products, they do not furnish quantitative yields. Gas chromatographic analysis was employed to quantitate gasphase product distributions. Additionally, nonvolatile solutionphase products are not observed in the DEMS experiment and, consequently, they were identified and quantified by 1H NMR spectra of electrolyzed solutions. The GC and NMR quantification experiments were performed using a 15 mC/ cm2 CoPi catalyst deposited onto FTO-coated glass and immersed in solutions consisting of 500 mM EG and 100 mM KPi at pH 7; the CoPi catalyst was operated at 1.0 V. Table 1 Table 1. Products and Their Distribution from the Electrooxidation of Ethylene Glycol by CoPi at 1.0 V vs Ag/ AgCl product 1,1,2-ethanetriol glycoaldehyde glycolate formate CO2 CO O2

Figure 1. DEMS experimental data for CoPi operated in electrolyte containing (a) 100 mM 13C-EG and (b) 250 mM EG. In (a), five consecutive CVs are scanned from 0.9 to 1.0 V vs Ag/AgCl at 2 mV/s, while MS ion current is detected for mass channels m/z 32, 29, 45 (bottom to top). In (b), a linear sweep voltammogram from 0.6 to 1.1 V vs Ag/AgCl, taken at 5 mV/s, is plotted versus potential, for m/z 32 (black), 28 (blue), and 44 (magenta).

Faradaic efficiency (%)a 42.1 2.33 4.6 7.0 17.5 3.2 5.1

± ± ± ± ± ± ±

10 0.4 2 1 2 1.6 3.0

a

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Conditions: [EG] = 500 mM, 100 mM KPi solutions (pH 7), CoPi electrodeposited by passing 15 mC/cm2.

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45, corresponding to O2, CO, and CO2, respectively. The O2 signal is stable over five CV cycles, while CO and CO2 decrease slightly with cycling. In order to explore whether the onset of carbon-based oxidation occurs at a lower potential in comparison to that in oxygen evolution, the DEMS experiment was repeated with a higher concentration of EG (250 mM). Figure 1b shows a linear sweep voltammogram for the production of O2, CO, and CO2. The onset of the CO2 is 40 mV less positive than that of the O2; this 40 mV represents a window in which the CoPi valence is capable of activating EG but is not oxidizing enough to effect water oxidation. In addition to 13C isotope labeling, 18O-labeled CoPi DEMS experiments were also performed. Catalyst was deposited from solutions of 1 mM Co2+ in 100 mM KPi at pH 7 in 97% H218O, and the flow solution (100 mM EG, 100 mM KPi, 200 mM KNO3 at pH 7) was of natural isotopic abundance. As shown in Figure S5, gas products were detected corresponding to labeled 12C18O, singly labeled 12C16O18O, and doubly labeled 12 18 18 C O O. This result confirms that the oxygens in the detected products originate from CoPi oxo/hydroxo groups and importantly that two O atoms can be incorporated from the CoPi catalyst into the substrate, to yield the doubly labeled 12 18 18 C O O. A series of control experiments was performed to ensure that oxidized products were due to CoPi-catalyzed EG oxidation. Performing the CV experiments of Figure 1 with a blank GC electrode yields only a small amount of O2 and no carbon products (Figure S2), confirming that CoPi is necessary for EG oxidation. Next, CO2-saturated electrolyte solution was passed through the DEMS in the absence of CoPi and with no applied potential (Figure S3). Approximately 10% of the CO 2

gives the products detected, along with their respective Faradaic yields (experimental details and NMR spectra are given in the Supporting Information). All product quantification experiments were performed in triplicate with little variability among runs. In agreement with DEMS data, the primary gaseous products as detected by GC-MS are CO2 (17.5% ± 2%), CO (3.2% ± 1.6%), and O2 (5.1% ± 3.0%). The low O2 Faradaic efficiency is not simply the result of an increase in total current due to EG oxidation. As shown in Figure S6, the addition of EG to the electrolysis solution actually suppresses absolute OER activity. This result suggests that EG is capable of reacting with (and thus quenching) an individual cobalt− oxyl species before the formation of an adjacent cobalt−oxyl and subsequent intermolecular radical coupling to form oxygen. Thus, the low Faradaic efficiency for O2 production demonstrates that EG oxidation is able to significantly outcompete the simultaneous oxygen evolution reaction (OER). Observing that gaseous products only constitute ∼25% of the charge passed in this system, 1H NMR spectra of postelectrolysis solutions were taken following the passage of 10−20 C. The products detected were 1,1,2-ethanetriol (42.1% ± 10%), glycoaldehyde (2.3% ± 0.4%), formate (7.0% ± 1%), and glycolate (4.6% ± 2%) with the majority of current directed toward the production of 1,1,2-ethanetriol (glycoaldehyde exists in chemical equilibrium with 1,1,2ethanetriol in aqueous solution). The product yields were quantified by integration and referenced to spectra of the B

DOI: 10.1021/acs.organomet.8b00337 Organometallics XXXX, XXX, XXX−XXX

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Organometallics internal standards (DMSO and phenol) of known concentration (Figure S7). In order to confirm that the observed liquid-phase species were in fact EG oxidation products, an electrolysis was performed using 13C-EG. A 13C NMR (Figure S8) of the postelectrolysis solution revealed 13C-labeled versions of the products described above, confirming them to be direct products of EG oxidation. Figure S9 shows control NMR spectra of each intermediate used for 1H and 13C assignments. The organic intermediates given in Table 1 are successive oxidative products beginning with EG. Scheme 1 shows a

Figure 2. Liquid phase product distribution for 500 mM EG, 100 mM KPi solutions (pH 7) following electrolysis at 1.0 V vs Ag/AgCl with CoPi films deposited by passing 3, 15, or 60 mC/cm2: (a) Faradaic efficiencies for each carbon product; (b) percentage of total carbon Faradaic efficiency for each product. Detected products were 1,1,2ethanetriol/glycoaldehyde (green), glycolate (blue), formate (purple), CO2 (red), and CO (black).

Scheme 1. Proposed Reaction Sequence for CoPi-Mediated EG Oxidation

Figure 2b, the distribution of EG oxidation products in thicker films is shifted toward more fully oxidized products (CO2), and away from less oxidized intermediates (1,1,2-ethanetriol). This may be attributed to longer residence times for intermediate species within thicker films, thus favoring the production of more oxidized products. The CoPi is capable of oxidizing higher content carbon substrates of interest to biomass fuel cell conversions. Parts a and b of Figure 3 show that CoPi is able to oxidize glucose and

oxidative reaction sequence that accounts for the observed products given in Table 1. In this oxidative transformation sequence, EG oxidation proceeds via multiple discrete oxygen insertion steps mediated by Co−O• radicals, followed by chemical dehydration and decomposition steps. We detect the stable intermediates 1,1,2-ethanetriol and glycolate, as well as the end product formate, by 1H NMR. The observation of doubly 18O labeled 12C18O18O via DEMS suggests that the mechanism proceeds through the dehydration of triol intermediates (1,1,1,2-ethanetetraol and 2,2,2-trihydroxyacetate). If this is a successive reaction sequence, then entering the sequence at any point should lead to successive downstream products and the generation of CO2. This is the case. Table S1 shows that the oxidation of each stable on-pathway intermediate by CoPi yields CO2 (Table S1). That the Faradaic efficiency increases for more oxidized intermediates (e.g., glyoxylate) is consistent with a successive reaction sequence. The selectivity of CoPi for EG oxidation vs OER is partially affected by the CoPi film’s thickness. As shown in Figure 2a, as the film thickness is increased, the Faradaic efficiency of EG oxidation products decreases. This is likely a result of the structure of the film, which is porous and is composed of cobalt clusters packed into nanoparticles to give rise to a film mesostructure.16,17 Whereas the catalyst films are permeable to water, they may be significantly less permeable to larger organic molecules such as EG and its oxidative products. As the film thickness increases, OER activity will increase proportionally to the nanoparticle volume, while EG oxidation may be expected to only scale with the nanoparticle surface area. Because EG molecules are unable to penetrate into nanoparticle domains, they are more likely to react with CoPi at the surface of the nanoparticles. Additionally, as shown in

Figure 3. DEMS experimental data for CoPi operated in (a) 100 mM glucose and (b) 50 mM kraft lignin. CVs were scanned at 2 mV/s from 0.59−0.84 and 0.44−0.79 V vs Ag/AgCl for glucose and lignin, respectively, while MS ion current was detected for mass channels m/ z 32, 28, 44 (bottom to top).

kraft lignin, respectively, to CO2. GC-MS quantification reveals Faradaic efficiencies for glucose degradation to CO2 to be 7.5 ± 0.8%, whereas the heterogeneity of kraft lignin precludes a molar determination of substrate and hence Faradaic efficiency. Thus for the kraft lignin, we can only report a ratio of 14 ± 4 mol e− passed/mol CO2 produced. Nonenzymatic, low-temperature catalysts known to oxidize glucose are composed of precious metals,18 are only operative under highly alkaline conditions,18−20 and typically are only able to mediate the first 2e− oxidation to gluconic acid, leaving the remaining 22e− unused.21 The ability to modulate selectivity for CO2 from EG by changing the film thickness (Figure 2) suggests that further optimization of the catalyst morphology may lead to improved glucose and lignin CO2 Faradaic efficiency. While we acknowledge that these CO2 Faradaic efficiencies are low relative to those of existing C

DOI: 10.1021/acs.organomet.8b00337 Organometallics XXXX, XXX, XXX−XXX

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Organometallics microbial22 (FECO2 = 80%) glucose oxidation fuel cells or solid oxide23 (FECO2 = 100%) fuel cells, the demonstrated ability of the CoPi system to produce non-negligible quantities of CO2 from these C2+ substrates is notable and suggests that watersplitting catalysts based on oxyl intermediates may be competent for C2+ oxidations and thus represent a fruitful line of future inquiry in the field of biomass conversion. The CoPi catalyst can be used to catalyze the oxidation of EG, glucose, and lignin in aqueous solution at neutral pH. We observe CO2 for all three substrates and additionally C1 (e.g., CO and formate) products for the oxidation of EG. These results demonstrate that the Co−oxyl radical of CoPi system is capable of breaking C−C bonds under mild aqueous conditions, a property which is essential for the development of catalysts that can access the full thermodynamic potential of large biomolecules. The ability to generate oxyl radicals in situ by the application of anodic potential allows oxidative degradation to be driven electrochemically, which is ideal for implementation in biomass fuel cells and wastewater remediation applications.

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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/acs.organomet.8b00337.

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Full experimental details and additional DEMS, NMR, and GC data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for D.G.N.: [email protected]. ORCID

Daniel G. Nocera: 0000-0001-5055-320X Author Contributions †

T.P.K. and C.N.B. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Evan Jones for help with 13C NMR, and to Cyrille Costentin and David Gygi for helpful discussions. T.P.K. and C.N.B. are supported by a Graduate Research Fellowship from the National Science Foundation. This material is based upon work supported under the Solar Photochemistry Program of the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, of the U.S. Department of Energy (DE-SC0017619).

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DOI: 10.1021/acs.organomet.8b00337 Organometallics XXXX, XXX, XXX−XXX