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Aerobic Electrochemical Oxygenation of Light Hydrocarbons Catalyzed by an Iron-Tungsten Oxide Molecular Capsule Marco Bugnola, Raanan Carmieli, and Ronny Neumann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00477 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018
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Aerobic Electrochemical Oxygenation of Light Hydrocarbons Catalyzed by an Iron-Tungsten Oxide Molecular Capsule Marco Bugnola,1 Raanan Carmieli,2 and Ronny Neumann1* 1
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel
2
Department for Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel
Supporting Information Placeholder ABSTRACT: The selective oxidation of light hydrocarbons
and their valorization with only dioxygen (O2) are important transformations towards development of efficient chemical processes. Monooxygenase enzymes can catalyze selective aerobic reactions under reducing and protic conditions. The translation of such enzymatic pathways to the practical electro-catalytic oxidation of light, gaseous hydrocarbons, using O2 as sole oxidant is now reported. An iron-tungsten oxide inorganic molecular catalyst with a capsular structure {Fe30W72} stabilized inside by sulfate/bisulfate anions provides a protic environment where three iron atoms are located at each of the pores of the capsule leading to a unique and potent active site for the oxidation reactions. Under mild electrochemical conditions, 1.8 V, in water at room temperature, using O2 from air, we demonstrate the low-pressure (1-2 bar) hydroxylation of alkanes, notably ethane to acetic acid, and the ozone like cleavage of the carbon-carbon double bonds of alkenes. Typical turnover frequencies were 300-400 min-1. Initial mechanistic studies support a reaction through a very active iron-oxo species.
Keywords: Polyoxometalate • Electrocatalysis • Cathodic oxidation • C-H bond activation • Hydroxylation
INTRODUCTION
The activation of dioxygen (O2) from air under mild reaction conditions towards the oxidation of hydrocarbons, especially of alkanes and alkenes, is one of the holy grails of catalysis. The reaction between O2 and a hydrocarbon has a high activation barrier with the formation of CO2 and H2O as the thermodynamically preferred products. Such reactions proceed by a free radical autoxidation mechanism that can also be catalyzed.1 Although there are some high temperature industrial processes for the oxygenation of alkenes, for example, ethylene to ethylene oxide and propylene to acrylic acid, the selective and efficient aerobic oxygenation of light alkanes remains an unsolved problem. In this area the selective
oxygenation of the main components of natural gas (methane, ethane and propane) towards their valorization is of special interest. The lack of a viable aerobic oxygenation pathway has led to alternative approaches such as the use of SO3, 2-4 or KIO3 5 as oxidants along with proposals to use strong electrophiles.6 Oxygenation through in situ formation of hydroxyl radicals, typically from H2O2 has also been espoused.7 Nature has evolved copper and iron monooxygenase enzymes that utilize air, paradoxically under reducing conditions, where one atom of O2 is sacrificed to form H2O as co-product, Equation 1. For alkane oxidation, enzymes with multiple metal sites appear to be preferred.8,9 RH(aq) + 2 H+(aq) + 2 e–(aq) + O2(aq)
ROH(aq) + H2O(l)
(1)
The reaction mechanism of these enzymes has been widely investigated often also using complexes that mimic their active sites.10-14 In most of this research, the so-called “shunt” pathways have been studied using mono-oxygen donors or peroxides as oxidant. But the use of O2 in the context of application of monooxygenases and functional models has been elusive.15,16 For example, the reductive activation of O2 with ascorbate using a di-iron complex proceeded only substoichiometrically,17 and the electrochemical cathodic activation of O2 has not been very successful; manganese(III) heme mimics show little activity and require the presence of anhydrides to form the reactive oxygenating intermediate.18,19 Electrocatalytic alkane oxidation using cytochrome P-450 enzymes has also not been successful,20 although recently a combination of Alkane Hydroxylase (AlkB) and Rubredoxin2 (AlkG) enzymes were successfully used for hydroxylation of higher alkanes albeit with low faradaic efficiency.21 Since catalyst stability is an important issue for the development of a viable alkane oxidation reaction, there have been several recent reports on the use of intrinsically stable zeolites incorporating copper or iron sites as catalysts. In these cases, high temperature activation, typically at around 400 °C, of O2 is needed and in some cases nitrous oxide is required to yield intermediates competent for alkane oxidation.22-25 There have been no reports on purely inorganic
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compounds (no organic ligands) that show monooxygenase activity using O2 under reductive conditions. This is true also of polyoxometalates, a large sub-class of molecular inorganic compounds, although use of mono-oxygen donors, notably iodosobenzene, for mostly alkene epoxidation is long known.26 In the last decade, porous polyoxometalate capsules consisting of 132 molybdenum atoms have been studied in the context of host-guest chemistry.27,28 In addition, the guest cavities of these capsules have been used as “nanoreactors” showing very significant acceleration of acid catalyzed hydrolysis of ethers.29,30 Here, we present the electrochemical cathodic oxygenation of light alkanes and alkenes, notably ethane to acetic acid at 1.8 V under aerobic conditions in water at room temperature using an iron-tungsten porous spherical capsule, {FeIII30WVI72} in shorthand,31 as catalyst Figure 1.
Figure 1. The iron-tungsten capsule, {FeIII30WVI72}. Leftpolyhedral presentation, middle-stick model with an arrow pointing towards the pore, right-a closer view of the atoms lining the pore. In this capsule, there are 12 icosahedrally arranged pentagonal {(W)W5} units connected by 30 Fe(III) linkers. Iron-green, tungsten-black, oxygen-red. Not shown are the 25 disordered sulfate/bisulfate ligands coordinated to W and Fe within the capsule that supply a protic environment and NH4+ cations on the outside of the capsule; both stabilize the structure.
tion, as is common in such reductive activation reactions with O2, is the uncoupled reaction where the active oxidizing species, for example a high valent Fe-oxo species, is reduced to water instead of reacting with the substrate and thereby reducing the Faradaic efficiency of the hydrocarbon oxidation.15 Also notable is the observation that as opposed to the reported electrochemical oxidation of cyclooctene, no anhydride is need for O2 activation. 18,19
Figure 2. Electrocatalytic oxidation of ethane. (top) – description of cathodic reactions; (bottom) – product distribution at different temperature and pressure conditions. Reaction conditions: 1.9 µmol {FeIII30WVI72}, 2.5 mL D2O, 24 h, platinum mesh cathode, platinum wire anode, potential 1.8 V. Products, in µmol, were quantified by 1H NMR.
The results of the cathodic oxidation of additional light hydrocarbons at 20 °C with Pair = 1 bar and Phydrocarbon = 2 bar are presented in Scheme 1 and the turnover frequencies are summarized in Figure 3. One can see from Figure 3, that turnover frequencies, TOFs, are typically in the range of 300 – 400 min-1 depending on the substrate.
RESULTS AND DISCUSSION
800
The cyclic voltammogram of {FeIII30WVI72} shows a fast, reversible, diffusion limited reduction of Fe(III) to Fe(II) at 0.47 V vs. Ag/AgCl, Figure S1. Constant potential coulometry, Figure S2, shows that at 0 V vs. Ag/AgCl the capsule is reduced by 10 electrons yielding {FeII10FeIII20WVI72} and suggesting that one iron atom is reduced at each pore. Using this information, cathodic oxidation of ethane was first tested in an electrocatalytic reaction where 2.5 mL of a 0.74 mM aqueous solution of {FeIII30WVI72} was pressurized with a mixture of air and ethane. The potential between the platinum mesh cathode and the platinum wire anode was 1.8 V, Figure 2. From the results, the six-electron oxidation of ethane to acetic acid was the predominant reaction at 20 °C under 0.25 bar air and 0.75 bar ethane. Some further oxidation to formic acid was observed, but still the selectivity to acetic acid was >95%. Lowering the temperature or increasing the ethane pressure, thereby increasing the concentration of ethane in water increased the yield and Faradaic efficiency, and also allowed the observation of the hydroxylation reaction intermediates, ethanol and acetaldehyde. The side reac-
700 600
TOF, min-1
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500 400 300 200 100 0 Methane
Ethane
Propane
i-Butane
Ethene
Propene
Figure 3. The relative reactivity of light hydrocarbons. Reaction conditions: 1.9 µmol {FeIII30WVI72}, 2.5 mL D2O, air – 1 bar, hydrocarbon 2 bar, 20 °C, 24 h, platinum mesh cathode, platinum wire anode, potential 1.8 V. The turnover frequency (TOF) was calculated as follows: TOF = molproducts/(molcat on electrode * time) where molproducts is the total of all products as shown in Scheme 2. The surface of the Pt cathode was 2.5 cm2 = 2.5 *1014 nm2; the diameter of {FeIII30WVI72} was taken as 3.0 nm which can be translated to a coverage of 7.065 nm2 per molecule. At 50% coverage molcat on electrode = 2.5*1014*0.5/7.065 = 1.77*1013 molecules = 2.94*10-11 mol.
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These values are on par with the TOFs observed for the electrocatalytic oxidation of >C3 alkanes using a combination of recombinant alkane hydroxylase and rubredoxin-2, the only reported successful enzymatic electrocatalytic hydroxylation of alkanes reported to date.21 The oxidation of methane was sluggish and proceeded only with a very low,
