Heterogeneous iron containing carbon catalyst (Fe-N/C) for epoxidation with molecular oxygen

Daniel Malko, Yanjun Guo, Pip Jones, George Britovsek, and Anthony Kucernak†

Department of Chemistry, Imperial College London, London SW7 2AZ, united Kingdom

Abstract

Pyrolized transition metal and nitrogen containing carbon materials (M-N/C) have shown

promising activities as electrocatalysts for oxygen reduction reactions (ORR) in fuel cell cathodes.

Similar materials have recently gained interest as heterogeneous catalysts. We report that ORR-

active heterogeneous M-N/C materials can catalyze the chemical epoxidation of olefins with

molecular oxygen and two equivalents of aldehyde at room temperature and ambient pressure.

The observed yield and selectivity is higher than that for homogeneous analogues and the

catalysts achieve TOF>2700 h-1 and TON>16000. The ability to recycle the catalyst several times is

also demonstrated.

Keywords

Heterogeneous catalysis • Epoxidation • Oxygen Activation • Mild conditions

1. Introduction

The activation of dioxygen to perform controlled oxidations is a desirable and useful aim for

electrochemical and chemical processes[1-4]. On the electrochemical side, the oxygen reduction

reaction (ORR) is crucial for green energy technologies such as fuel cells and metal-air batteries. For

chemical processes, the oxidation of organic compounds under mild conditions has the potential to

make many industrial processes more sustainable.[1, 4, 5] For instance, the epoxidation of olefins,

conducted on an annual multi-million ton scale, offers access to a variety of important building

blocks for the chemical industry as well as to fine chemicals and pharmaceuticals.[5] Often harsh

reaction conditions or the use of synthetic oxidants are necessary to conduct this reaction. Ideally

oxygen or air are used as the oxidant and several industrial oxidation processes have been

developed where peroxides are formed as intermediates, for example in the Sumitomo process for

† Corresponding author: anthony@imperial.ac.uk

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

1

propylene epoxidation or the Hock process for dual production of phenol and acetone from cumene.

[6]

Nature has mastered the task of oxygen activation with enzymes such as monooxygenases and

dioxygenases, using either heme- or non-heme metal complexes as the active sites. Heme-based

enzymes feature an iron-porphyrin structure,[7] and it has been shown that synthetic iron

complexes containing porphyrin ligands such as tetraphenylporphyrin (FeTPP, see Figure 1a) can

facilitate the epoxidation of olefins under mild conditions using a sacrificial reductant.[8-14]

However, their large scale application is prohibited by i) lack of stability, ii) high synthesis cost and iii)

the homogeneous nature of the reaction, which complicates product separation.

A heterogeneous catalyst which is more stable and has a lower cost of production could overcome

these issues. Heat treated iron-nitrogen/carbon materials (Fe-N/C) have emerged as promising

catalysts for oxygen reduction reactions in low temperature fuel cells and might replace Pt based

materials at the cathode.[1, 15-17] Extensive research effort has provided insight into the nature of

the catalytically active sites in these materials and the results indicate that a metal center,

coordinated by N-donors, similar to a metal porphyrin acts as the active site (see an example of such

a structure in Figure 1b).[16, 18, 19] Nanocarbon materials are also well known to act as

heterogeneous catalysts and have gained some interest recently.[20, 21] These materials are

especially interesting for oxidation reactions.[8, 11, 22] Selective oxidation of alcohols to aldehydes

or oxidative dehydrogenation has been demonstrated for metal-free nitrogen containing

nanocarbons, as well as FeO and CoO containing materials.[22, 23] Fe-N/C and Co-N/C catalysts have

also shown catalytic activity towards reductive transformations under mild conditions[22, 24, 25]

Most relevant to this work, Banerjee et al. showed that Co-N/C materials can facilitate the

epoxidation of olefins, utilizing tert-butyl hydroperoxide as the oxidant at 80 oC.[22]

2

Figure 1. (a) Iron tetraphenylporphyrin (FeTPP) (b) suggested structure of the active site in Fe-N/C catalyst (c) HAADF-STEM image of Fe-N/C catalyst.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

Here we demonstrate that heterogeneous Fe-N/C catalysts are highly active for the epoxidation of

olefins coupled to the concurrent oxidation of aldehydes to carboxylic acids, utilizing molecular

oxygen as the oxidant at room temperature and ambient pressure.[26, 27] These catalysts achieve

good to excellent conversions and selectivities, high turnover frequencies and good recyclability. The

yields and selectivities surpass those of homogeneous catalysts such as iron porphyrin or iron

chloride. The epoxidation/oxidation activity might be related to the ability of the catalyst to perform

the electrochemical ORR in acidic conditions, suggesting that the active site in both reactions is the

same. Although there are some reports of epoxidation of linear alkenes and unsaturated fatty

acids[28-30], it is surprising that the catalyst we describe shows such universal activity under

ambient conditions. To the best of our knowledge this is the first report of the use of such catalysts

for aerobic heterogeneous epoxidations of alkenes under the mild conditions of room temperature

and atmospheric pressure.

2. Experimental

2.1 Catalyst preparation

In short, 1,5-diaminonaphthalene was oxidatively polymerized with (NH4)2S2O8 in an ethanolic

solution. The metal was introduced as FeCl2 or CoCl2. The solvent is removed under reduced pressure

and the remaining black powder is subjected to heat treatment in a tube furnace to 950 °C at a

heating rate of 20 °C/min for 2h while supplying a constant stream of inert nitrogen gas. The

resulting black powder is refluxed for 8h in 0.5M H2SO4 to remove residual metal. After filtering and

drying in an oven at 60 °C overnight, the catalyst is subjected to a second heat treatment at 950 °C at

a heating rate of 20 °C/min for 2h while supplying a constant stream of inert nitrogen gas. After

cooling down, the catalyst is ready to use. Further details for the synthesis process is discussed in the

ESI[16].

2.2 Catalyst characterisation

The catalyst materials post-treatment were assessed to have loadings of 1±0.2 wt% Co (Co-N/C) and

1.5±0.2 wt% Fe (Fe-N/C). The reference material with no addition of a metal (N/C) contained 6020

ppm residual Fe‡ (see TXRF - Total Reflection X-ray Fluorescence results in ESI). Extensive

microscopic, X-Ray diffraction, and Mössbauer spectroscopy has shown no evidence of secondary

phase carbide or nitride material[16, 31]. Figure 1c shows a HAADF-STEM image of the Fe-N/C

catalyst. We interpret the bright spots as the atomic iron sites. Further detailed material

‡‡ Note that although many reports in the literature discuss “metal free” catalysts, even catalysts

prepared without added metal (N/C) contain small amounts of residual metal due to adventitious

metal in the precursors.

3

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

1

2

3

characterization (XPS, TEM, HR-TEM, Lattice spacing analysis, X-Ray Fluorescence, BET) are provided

in the ESI. A further discussion of the material properties, especially with respect to the

electrochemical properties, are reported elsewhere[16, 31]. This work focuses on the catalytic

activity of these materials.

2.3 Electrochemical oxygen reduction reaction

For the electrochemical experiments we utilised a Rotating Ring Disk Electrode (Pine Instruments,

model AFE6R1AU having a mirror polished glassy carbon as disk and rotator model AFMSRCE), the

catalyst was deposited on the glassy carbon disk following a procedure described in the literature. 2 A

three compartment electrochemical glass cell was used. 0.5M H2SO4 was prepared by dilution from

the concentrated acid (H2SO4 97% Aristar from VWR). The RHE reference electrode (Gaskatel

HydroFlex) was ionically connected to the main compartment of the electrochemical glass cell via a

Luggin-Haber-Capillary. A glassy carbon rod was used as counter electrode and ionically connected

to the main compartment of the glass cell through a porous frit. Glassy carbon was used instead of

Pt in order to avoid contamination with catalytically active precious metals. A potentiostat (Autolab,

model PGSTAT20) was used for potential or current control during the electrochemical

measurements. Steady state oxygen reduction reaction polarization curves, performed in O 2-

saturated electrolyte solutions were obtained via step potentials of 30mV with waiting time of 30

seconds. Ultrapure gases utilized in this study were, Nitrogen and Oxygen (BIP plus-X47S, Air

products).

2.4 Epoxidation reaction

Detailed descriptions of all the epoxidation reactions are given in the SI. All experiments were

performed at 25 1 oC and under a pure oxygen atmosphere at atmospheric pressure. For all

experiments we used 2:1 mol ratio aldehyde:alkene, and a mol ratio of alkene to iron of ~1400:1

(assuming all the iron in the catalyst is active), apart from the specific cases noted. We determined

the target alkene:iron ratio from tests on how the reaction rate varied with the amount of catalyst

added, and found that the catalyst was saturated at a ratio of ~1400:1 (i.e. we were in the linear

regime in which doubling the amount of catalyst doubles the rate of reaction). Note the reason for

the slight variation in amount of catalyst added is because of the difficulty in adding a small amount

of catalyst to these reaction mixtures – we report the amount of catalyst added (not the target

weight to add). We included the reaction time in all of the results. Reaction times were determined

by following the reaction to completion by monitoring the flow of oxygen into the reaction flask

utilising a mass flow meter. The reactions were terminated when the flow reached zero. Hence the

reaction times allow assessment of times required for practical completion of the reaction. Further

4

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

details are in the ESI. Apart from the initial experiments performed to assess the oxygen reduction

activity of the catalysts, all epoxidation reactions have been with the materials operating purely as a

chemical catalyst – i.e. we do not perform any of the epoxidation reactions under electrochemical

control and have not tested our catalyst for the electrochemical epoxidation reaction[32] – however

this may prove a fruitful future avenue.

3. Results and discussion

Our initial motivation for this study was to ascertain whether these heterogeneous catalysts, which

are active for oxygen reduction and have been utilized as such in fuel cells, are also active towards

activating oxygen for chemical reactions.

3.1 Comparison between electrochemical oxygen reduction and epoxidation activity

Displayed in Figure 2(a) is the performance towards the electrochemical oxygen reduction reaction

as assessed utilising the rotating disk electrode technique in 0.5 M H 2SO4. We find that for our

catalysts, the iron containing analogue is the most active followed by the cobalt containing analogue

and lastly the nominally metal-free analogue. These results are similar to others reported in the

literature.[33, 34] Taking the same catalysts and assessing their activity towards the chemical

epoxidation reaction utilising cyclohexene as a substrate provides the time dependent results shown

in Figure 2(b). In these experiments the dispersed catalyst was combined with the substrate

cyclohexene and isobutyraldehyde (IBA) and oxygen (1 bar). Aliquots were removed from the

reaction mixture and assessed using gas chromatography (details in SI). In these experiments we find

a similar ordering of catalysts as for the oxygen reduction reaction with the Fe-N/C>Co-N/C>N/C.

(a) (b) (c)

0.76 0.78 0.80 0.82 0.84 0.86 0.88 0.900

20

40

60

80

100

yield conv. catalystFe-NC

Co-NC NC%

yiel

d, %

conv

ersi

on

ORR onset potential / V

Figure 2. Comparison between electrochemical and chemical catalytic performance of Fe-N/C, Co-N/C and N/C catalysts. (a) Electrochemical steady-state rotating disk electrode (RDE) measurement in 0.5 M H2SO4, rotating speed: 1600 rpm, 30 s hold, 30 mV step potential, catalyst loading: 750 μg cm-2. (b) Time dependent chemical epoxidation of cyclohexene with (M)-N/C catalysts. 2 eq of isobutyraldehyde and 0.2 wt% catalyst to cyclohexene, 1 atm O2. Values determined by GC. (c) Comparison between the yield of epoxide and conversion of alkene after 18hr reaction with the electrochemical onset potential for the oxygen reduction reaction for all three catalysts.

5

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24252627282930

A frequently used activity descriptor for catalysts used for electrochemical oxygen reduction is the

onset potential for the reduction process – a higher value for this onset potential represents a more

active catalyst. Hence we plotted the yield and conversion of the cyclohexene to its corresponding

epoxide as a function of the ORR onset potential, Figure 2(c) (see SI for more details). A strong and

near linear correlation is seen for both parameters†. A complete analysis of the performance of

these catalysts towards the epoxidation of cyclohexene is displayed in Table 1. In these experiments

we limited the reaction time to six hours. The yield and therefore selectivity follows the trend Fe-N/C

> Co-N/C > N/C, the same as seen above for the onset for the electrochemical oxygen reduction

reaction.

Table 1. Chemical epoxidation of cyclohexene with different M-N/C catalysts.

0.2 wt% catalyst2 eq. IBA, 1 atm O2

MeCN, RT, 6hO

Entry Catalyst Conversion[a] [%] Yield[a] [%] Selectivity[a] [%]

1 Fe-N/C 80 70 87

2 Co-N/C 79 59 74

3 N/C 58 40 69

4 no catalyst 16 4.8 30

5 no aldehyde[b] 0 0 0

6 no oxygen[b] 0 0 0

†† We might expect an asymptotic response as the oxygen reduction onset potential is limited to a thermodynamic maximum of 1.23V.

6

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

12

Conditions: 2 eq isobutyraldehyde (IBA), 0.2 wt% catalyst, 1 atm O2, 6 h at room temperature in acetonitrile (MeCN). [a]

determined by GC-FID, which was calibrated with the respective standards, using the relative response factor;

[b] Fe-N/C catalyst

For the best performing catalyst, Fe-N/C, an excellent selectivity of 87% is achieved. For the N/C

material, some activity is seen and we attribute this to the small amount of residual iron present in

that catalyst as previously discussed. Although the reaction proceeds also without catalyst, the

conversion and selectivity are significantly lower, further demonstrating the beneficial effect of the

catalyst. We did not detect any significant amount of a specific side product in our GC traces (see SI).

Therefore, we infer that the amount of substrate which was not converted to the epoxide might

have been either oxidised to CO2 by the organic peroxide or might have polymerised through radical

polymerisation. Interestingly, compared to the method described by Banerjee et al., where an

organic peroxide was used as oxidant, it can be seen that the reaction proceeds significantly faster (6

h vs 36 h) and at lower temperature (25 vs. 80 oC).[22]

Another striking feature of the here presented catalysts is the high turnover number. In this case,

only 0.2 wt% of catalyst (compared to the weight of olefin substrate) were sufficient to facilitate the

reaction. Assuming a metal content of ~1.5 wt% (as determined by TXRF) for the Fe-N/C catalyst (see

ESI) and further assuming that all the metal is utilised in single metal-atom active sites, a TON of

~16100 per iron site would be present (i.e. average TOF ~2700 h -1). However, these numbers could

be even higher, as it has been shown that many active sites in such materials are not at the surface

but buried within the bulk of the carbon[16]. Experiments conducted without oxygen do not show

product formation, confirming that oxygen is indeed the terminal oxidant. Furthermore, no reaction

was observed without the aldehyde and only a small conversion was observed in the absence of the

catalyst. This confirms that the catalyst has a major influence on the reaction rate and outcome.

3.2 Comparison of epoxidation activity to other catalysts

In order to assess the relative efficacy of the Fe-N/C catalyst to other commonly used catalysts in the

literature we examined the performance of homogeneous catalysts previously reported for this and

related reactions.[14, 35] Iron porphyrin was chosen as it has been reported, together with other

metal macrocycles, to catalyze this epoxidation reaction with a high efficiency and resembles the

proposed active site in our catalyst. Iron(III) chloride was chosen, as it might be possible that this

reaction is catalyzed simply by iron salts, which have been shown to be an efficient catalyst for

Fenton-type oxidations. The catalyst was dispersed (Fe-N/C) or dissolved (FeCl3 and FeTPP) in a

7

1

2

345

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

mixture of CH2Cl2 and MeOH (9:1) as solvent, which was chosen to enable solubilisation of FeCl 3 and

the iron-porphyrin catalysts. The dissolved/dispersed catalyst was then combined with the substrate

cyclohexene and isobutyraldehyde (IBA) and oxygen (1 bar). In order to ensure comparability of the

results, the relative amounts of iron species, which is presumably the catalytically active site, was

kept constant, and in line with what was used in the literature, namely at a Substrate:Fe molar ratio

of 3000:1. This was achieved for the Fe-N/C catalyst by assuming that all the 1.5wt% iron within the

material, as determined by TXRF (see ESI) is accessible and active. All catalysts facilitated the

epoxidation of cyclohexene, but the heterogeneous catalyst afforded the highest conversion, yield

and selectivity for cyclohexene oxide, as shown in Figure 3. This was confirmed by gas

chromatography analysis (GC, see ESI for examples).

This result is striking, as it is often for homogeneous catalysts to have a higher activity and selectivity

as compared to heterogeneous catalysts. Homogeneous catalysts are usually able to perform under

milder reaction conditions and provide higher selectivity as the homogeneous nature makes the

catalyst more accessible to the reactants and sophisticated catalytic cycles are possible supported by

ligand exchange and steric effects.[36] This indicates however, in the case of this epoxidation

reaction, that the active site within the heterogeneous catalyst might be far more active than the

homogeneous analogues and points towards a distinct chemical moiety, such as a Fe-N4 site (see

Figure 1b) which is commonly suggested for these materials in the electrochemical ORR.

Interestingly for the electrochemical ORR a similar behaviour is observed, meaning the heat-treated

Fe-N/C moiety is far more active than the iron porphyrin analogue deposited onto a carbon

substrate. The higher heterogeneous activity might also indicate a possible participation of the

matrix carbonaceous material in the catalytic cycle, rather than initiating a radical chain reaction, as

suggested for similar materials.[14]

8

Conversion Yield Selectivity0

20

40

60

80

100

%

Fe-N/C FeCl3 FeTPP

Figure 3. Conversion, yield and selectivity of cyclohexene to cyclohexene oxide, comparing the Fe-N/C catalyst to FeCl3 and FeTPP. Conditions: cyclohexene:Fe molar ratio 3000:1, assuming all 1.5 wt% of Fe in the Fe-N/C catalyst is active, 5 eq of isobutyraldehyde to alkene, 1 atm O 2, 298 K in CH2Cl2:MeOH 9:1, 24h. Values determined by GC.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

3.3 Assessment of activity as a function of substrate and aldehyde

To gain further insight, the scope of the reaction was extended. Various olefins and aldehydes were

tested under similar conditions. The olefins were chosen to assess whether general groups of olefins,

i.e. linear, aromatic and sterically hindered ones would react under the present conditions. The

results are presented in Table 2.

We found that even alkenes such as 1-hexene, which are notoriously difficult to epoxidise, can be

transformed at room temperature and ambient pressure with molecular oxygen as terminal oxidant.

[5]

Table 2. Chemical epoxidation of different alkenes with Fe-N/C catalyst.

Entry Alkene Epoxide Con. [%] Yield [%] Sel. [%] Time [h]

1 99 91 91 6

2

72 27 38 7

3 70 12 17 7

478 52 67 17

579 76* 96 17

Conditions: 2 eq of isobutyraldehyde and 2.0-4.5 wt% Fe-N/C catalyst (Substrate:Fe molar ratio ~1400:1, assuming all 1.5

wt% of Fe in the Fe-N/C catalyst is active), 1 atm O2, room temperature in acetonitrile. Values determined by

GC. Reaction times are the time required for oxygen uptake to fall to zero.

Although the selectivity is somewhat lower, the remarkable fact is that the product can be formed

under such mild conditions. As for styrene, the low yield in this case could be explained by

formation of side products, such as polystyrene. Strikingly, the epoxidation of the sterically hindered

trans and cis stilbenes proceeds with high selectivity. While trans-stilbene is exclusively transformed

9

O

O

O

O

OO

1 : ~9

1

2

3

4

5

6

7

8

9

101112

13

14

15

16

into trans-stilbene oxide, cis-stilbene is converted predominantly into the more stable trans-stilbene

oxide, in line with observations with homogeneous systems[37, 38]. Hence, it can be deduced that

the mechanism does not proceed via concerted oxygen addition across the double bond, but more

likely involves the formation of radical intermediates, where there is free rotation around the bond

between the reacting carbons. The presence of a radical intermediate was confirmed by the

addition of a radical scavenger (Butylated hydroxytoluene, Alkene:BHT 500:1), which inhibited the

reaction.

We note from Table 1 that there is a requirement for the presence of an aldehyde in this reaction.

This aspect has been commented upon by a number of authors with some mechanistic studies. [14,

28, 37]. Hence we decided to study the influence of the aldehyde and the results are summarised in

Table 3.

Table 3. Chemical epoxidation of cyclohexene with Fe-N/C catalyst using different aldehydes.

Entry Aldehyde Con.Alkene

[%]

Con.Aldehydes

[%]

Yield

[%]

Sel.

[%]

Time

[h]1

96.7 94.5 80.0 82.5 24

2

87.0 74.0 68.7 79.0 24

3

76.6 --* 53.9 70.4 24

4

33.0 30.2 9.829.6

24

5

-- -- 0 0 24

6

-- -- 0 0 24

Conditions: 2 eq of isobutyraldehyde to substrate cyclohexene, 3.6wt% Fe-N/C catalyst to alkene, 1 atm O2, room temperature in Acetonitrile (MeCN). Values determined by GC. * The conversion of propionaldehyde was unable to be detected due to the GC peak overlapping with the solvent acetonitrile.

The highest conversions are obtained with sterically hindered aliphatic aldehydes such as

butyraldehyde (run 1), in line with previous observations [34]. Aromatic aldehydes are ineffective

(runs 5-6), whereas some conversion was obtained with a sterically encumbered benzylic aldehyde

(run 4). The reason for the difference in reactivity is believed to be related to the stability of radical

intermediates formed from these aldehydes [34].

10

O

O

O

O

OCl

O

1

2

3

4

5

6

7

8

9

10

11

12

13

141516

17

18

19

20

21

22

Two main mechanistic pathways are suggested in the literature for metal-based catalysis. On the

one hand, a radical chain mechanism involving acylperoxy radicals as the oxidising species, where

the metal is merely involved in the initiation through oxidation of the aldehyde. [35] On the other

hand, a pathway that involves the formation of a peracid intermediate which is activated by the

catalyst itself.[14, 37] We followed the consumption of oxygen in the reaction over time and find

that under the conditions studied, oxygen is consumed in direct proportion to the isobutyraldehyde,

Figure 4 (details in SI). Fitting the oxygen consumption to a simple first order consumption model

(with two different time constants) gives a good fit to the data (line in Figure 4). The asymptotic

consumption of the oxygen is 9.550.25 mmol O2. GC analysis of the reaction mixture after the

reaction shows that 9.45 mmol of the IBA was consumed, very close to the amount of oxygen.

Analysis also shows that the selectivity to the epoxide is 82.5% with a yield of 80%. The high ratio of

oxygen consumed to aldehyde also suggests that the rate of reaction of percarboxylic acid with

aldehyde to give two equivalents of carboxylic acid is also low, as this would decrease the ratio

below 1.

As this amount of oxygen consumed exceeds the amount of oxygen required to convert all of the IBA

to isobutyric acid and all the cyclohexene to cyclohexene oxide, we interpret this result as meaning

that the net reactions occurring under our conditions are as shown in Scheme 1.

Scheme 1

11

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

0 200 400 600 800 1000 1200 1400 1600

0.10.20.30.40.50.60.70.80.91.0

n(O

2) / n

(IBA

)

t / min

Figure 4. Integrated oxygen consumption for the epoxidation of cyclohexene. Amount of oxygen is ratioed to the amount of IBA in the mixture. Points: experimental data; Line: Fit to 1 st order exponential. 15 mg catalyst, 25 ml acetonitrile, 5 mmol inhibitor free cyclohexene, 10 mmol isobutyraldehyde, 25oC, pure oxygen. Oxygen flow measured using a mass flow meter.

Although the epoxidation may be due to the Prilezhaev reaction (path a), it seems more likely that

the peroxycarboxylic acid undergoes a reaction to form a radical species which facilitates the

reaction (path b), as confirmed by the sensitivity to radical scavengers and the product distribution

when cis-stilbene is used.[14, 35, 37-39] Any excess peroxycarboxylic acid left in the solution will not

be detected in GC traces as it will decompose to the carboxylic acid during the heating phase,

however we have detected high concentrations of peroxycarboxylic in the reaction mixture utilising

a spectrophotometric assay based on reaction with ABTS (2.2'-Azino-Bis (3-Ethylbenzothiazoline-6-

Sulfonate)[40]. We are currently studying the mechanism in greater detail and will report that work

in a future paper.

3.4 Catalyst recycling

The benefit of a heterogeneous as compared to a homogeneous catalyst is the easy separation from

the product mixture and the ability to reuse the catalyst. To confirm that the catalyst can be reused,

it was recovered after the reaction via centrifugation and added to a new batch of precursors. Such

catalyst recycling studies need to be done with care, as has been previously pointed out, as excessive

amounts of catalyst and excessively long reaction times will not highlight any loss in activity[41, 42].

In order to guard against this possibility we have (a) limited the amount of catalyst used in the

reactions to an amount in which the catalyst is saturated (i.e. a mol ratio of alkene to iron of ~1400:1

assuming all the iron in the catalyst is active), and thus the rate of reaction is dependent on the

amount of catalyst present (see section 2.4 for discussion); (b) limited the length of experiment to

12

1

2345

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

that for which the fresh catalyst is active towards the epoxidation reaction as measured by oxygen

uptake (6 hr, Table 2, Entry 1). Under these conditions a reduction in catalyst activity would be

expected to reduce conversion. It can be seen in Figure 5(a) that the catalyst can be reused without

significant loss of conversion and selectivity over five runs.

TEM images of the catalyst material before and after four-fold reuse for epoxidation have shown

that the structure resembles that of other high surface area carbons and that the epoxidation

reaction does not seem to have a significant effect on the structure of the material, Fig 5(b).

4. Conclusions

We have shown that a heterogeneous Fe-N/C catalyst, which is typically investigated for fuel cell

applications, can be used as efficient and robust catalyst for alkene epoxidation under mild

conditions, using oxygen as the oxidant and isobutyraldehyde as coreductant. The general reactivity

and reaction conditions seem to be like homogenous metal porphyrins, implying a FeNx type active

site. However, surprisingly, the yield and selectivity surpasses that of these homogenous analogues,

implying either an active site with a significantly higher specific activity or a different mechanism due

to the heterogeneous nature of the material. We see that the activity of the catalyst is correlated

with the performance towards the electrochemical oxygen reduction reaction suggesting that there

are some similarities between the active site for the two reactions. We determine that the reaction

proceeds through the intermediate formation of a peroxycarboxylic acid. Investigations into the

mechanism of this reaction can reveal crucial information on the active site. This might lead to

improvements of this type of material not only for the here presented epoxidation reaction but also

for electrochemical oxygen activation in general. Optimization of the reaction conditions may

render this catalyst industrially useful.

13

1 2 3 4 50

20

40

60

80

100

%

Repeat

Conversion Yield Selectivity

(a) (b)

Figure 5. (a) Conversion, yield and selectivity of cyclohexene to cyclohexene oxide while recycling the Fe-N/C catalyst. Conditions: 4.5wt% Fe-N/C catalyst and. 2 eq of isobutyraldehyde to alkene, 1 atm O 2, room temperature, 6h. Values determined by GC. (b) TEM of catalyst before and after the repeated reaction.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

5. Acknowledgements and data statement

The authors would like to thank the U.K. Engineering and Physical Sciences research council under

project EP/J016454/1 for financial assistance. DM would like to thank the EPSRC Doctoral Prize

Fellowship scheme for financial support. The data used in the preparation of this figures in this paper

is available for download [DOI inserted at proofing stage]

References

[1] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature, 486 (2012) 43-51.[2] R.V. Jagadeesh, H. Junge, M. Beller, “Nanorust”-catalyzed Benign Oxidation of Amines for Selective Synthesis of Nitriles, ChemSusChem, 8 (2015) 92-96.[3] B. Meunier, S.P. de Visser, S. Shaik, Mechanism of Oxidation Reactions Catalyzed by Cytochrome P450 Enzymes, Chemical Reviews, 104 (2004) 3947-3980.[4] J.H. Teles, I. Hermans, G. Franz, R.A. Sheldon, Oxidation, in: Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2000.[5] G. Sienel, R. Rieth, K.T. Rowbottom, Epoxides, in: Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2000.[6] H.-J. Arpe, in: Industrial Organic Chemistry, Wiley-VCH, 2010, pp. 274-282 and 366-370.[7] C. R. Hess, R. W. D. Welford, J. P. Klinman, T.P. Begley, Oxygen-Activating Enzymes, Chemistry of, in: Wiley Encyclopedia of Chemical Biology, John Wiley & Sons, Inc., 2007.[8] X. Cui, Y. Li, S. Bachmann, M. Scalone, A.-E. Surkus, K. Junge, C. Topf, M. Beller, Synthesis and Characterization of Iron–Nitrogen-Doped Graphene/Core–Shell Catalysts: Efficient Oxidative Dehydrogenation of N-Heterocycles, Journal of the American Chemical Society, 137 (2015) 10652-10658.[9] J.W. Brown, Q.T. Nguyen, T. Otto, N.N. Jarenwattananon, S. Glöggler, L.-S. Bouchard, Epoxidation of alkenes with molecular oxygen catalyzed by a manganese porphyrin-based metal–organic framework, Catalysis Communications, 59 (2015) 50-54.[10] J.T. Groves, T.E. Nemo, Epoxidation reactions catalyzed by iron porphyrins. Oxygen transfer from iodosylbenzene, Journal of the American Chemical Society, 105 (1983) 5786-5791.[11] L. Zhang, A. Wang, W. Wang, Y. Huang, X. Liu, S. Miao, J. Liu, T. Zhang, Co–N–C Catalyst for C–C Coupling Reactions: On the Catalytic Performance and Active Sites, ACS Catalysis, 5 (2015) 6563-6572.[12] A. Farokhi, H. Hosseini-Monfared, A recyclable Mn–porphyrin catalyst for enantioselective epoxidation of unfunctionalized olefins using molecular dioxygen, New Journal of Chemistry, (2016).[13] F. Ogliaro, S.P. de Visser, S. Cohen, P.K. Sharma, S. Shaik, Searching for the Second Oxidant in the Catalytic Cycle of Cytochrome P450:  A Theoretical Investigation of the Iron(III)-Hydroperoxo Species and Its Epoxidation Pathways, Journal of the American Chemical Society, 124 (2002) 2806-2817.[14] W. Nam, H.J. Kim, S.H. Kim, R.Y.N. Ho, J.S. Valentine, Metal Complex-Catalyzed Epoxidation of Olefins by Dioxygen with Co-Oxidation of Aldehydes. A Mechanistic Study, Inorganic Chemistry, 35 (1996) 1045-1049.[15] E. Proietti, F. Jaouen, M. Lefèvre, N. Larouche, J. Tian, J. Herranz, J.-P. Dodelet, Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells, Nature Communications, 2 (2011) 416.[16] D. Malko, T. Lopes, E. Symianakis, A.R. Kucernak, The intriguing poison tolerance of non-precious metal oxygen reduction reaction (ORR) catalysts, Journal of Materials Chemistry A, 4 (2015) 142-152.

14

1

2

3

4

5

6

789

10111213141516171819202122232425262728293031323334353637383940414243444546

[17] Y.Y. Liu, X.P. Yue, K.X. Li, J.L. Qiao, D.P. Wilkinson, J.J. Zhang, PEM fuel cell electrocatalysts based on transition metal macrocyclic compounds, Coordination Chemistry Reviews, 315 (2016) 153-177.[18] U.I. Kramm, M. Lefèvre, N. Larouche, D. Schmeisser, J.-P. Dodelet, Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell via 57Fe Mössbauer Spectroscopy and Other Techniques, Journal of the American Chemical Society, 136 (2014) 978-985.[19] J. Masa, W. Xia, M. Muhler, W. Schuhmann, On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction, Angewandte Chemie International Edition, 54 (2015) 10102-10120.[20] S. Navalon, A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Carbocatalysis by Graphene-Based Materials, Chemical Reviews, 114 (2014) 6179-6212.[21] C.K. Chua, M. Pumera, Carbocatalysis: The State of “Metal-Free” Catalysis, Chemistry – A European Journal, 21 (2015) 12550-12562.[22] D. Banerjee, R.V. Jagadeesh, K. Junge, M.-M. Pohl, J. Radnik, A. Brückner, M. Beller, Convenient and Mild Epoxidation of Alkenes Using Heterogeneous Cobalt Oxide Catalysts, Angewandte Chemie International Edition, 53 (2014) 4359-4363.[23] C. Bai, X. Yao, Y. Li, Easy Access to Amides through Aldehydic C–H Bond Functionalization Catalyzed by Heterogeneous Co-Based Catalysts, ACS Catalysis, 5 (2015) 884-891.[24] F.A. Westerhaus, R.V. Jagadeesh, G. Wienhöfer, M.-M. Pohl, J. Radnik, A.-E. Surkus, J. Rabeah, K. Junge, H. Junge, M. Nielsen, A. Brückner, M. Beller, Heterogenized cobalt oxide catalysts for nitroarene reduction by pyrolysis of molecularly defined complexes, Nature Chemistry, 5 (2013) 537-543.[25] F. Chen, A.-E. Surkus, L. He, M.-M. Pohl, J. Radnik, C. Topf, K. Junge, M. Beller, Selective Catalytic Hydrogenation of Heteroarenes with N-Graphene-Modified Cobalt Nanoparticles (Co3O4–Co/NGr@α-Al2O3), Journal of the American Chemical Society, 137 (2015) 11718-11724.[26] T. Mukaiyama, New Possibilities in Organic Synthesis, Aldrichimica Acta, 29 (1996) 59-76.[27] T. Nagata, K. Imagawa, T. Yamada, T. Mukaiyama, OPTICALLY-ACTIVE N,N'-BIS(3-OXOBUTYLIDENE)DIAMINATO-MANGANESE(III) COMPLEXES AS NOVEL AND EFFICIENT CATALYSTS FOR AEROBIC ENANTIOSELECTIVE EPOXIDATION OF SIMPLE OLEFINS, Bulletin of the Chemical Society of Japan, 68 (1995) 1455-1465.[28] T. Yamada, T. Takai, O. Rhode, T. Mukaiyama, Direct Epoxidation of Olefins Catalyzed by Nickel(II) Complexes with Molecular Oxygen and Aldehydes, Bulletin of the Chemical Society of Japan, 64 (1991) 2109-2117.[29] A. Köckritz, M. Blumenstein, A. Martin, Epoxidation of methyl oleate with molecular oxygen in the presence of aldehydes, European Journal of Lipid Science and Technology, 110 (2008) 581-586.[30] L. Vanoye, Z.E. Hamami, J. Wang, C. de Bellefon, P. Fongarland, A. Favre-Réguillon, Epoxidation of methyl oleate with molecular oxygen: Implementation of Mukaiyama reaction in flow, European Journal of Lipid Science and Technology, 119 (2017) 1600281.[31] D. Malko, A. Kucernak, T. Lopes, In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts, Nature Communications, 7 (2016) 13285.[32] K. Rossen, R.P. Volante, P.J. Reider, A highly diastereoselective electrochemical epoxidation, Tetrahedron Letters, 38 (1997) 777-778.[33] G. Wu, A. Santandreu, W. Kellogg, S. Gupta, O. Ogoke, H.G. Zhang, H.L. Wang, L.M. Dai, Carbon nanocomposite catalysts for oxygen reduction and evolution reactions: From nitrogen doping to transition-metal addition, Nano Energy, 29 (2016) 83-110.[34] G. Wu, Current challenge and perspective of PGM-free cathode catalysts for PEM fuel cells, Frontiers in Energy, 11 (2017) 286-298.[35] A.C. Serra, A.M.d.A. Rocha Gonsalves, Mild oxygen activation with isobutyraldehyde promoted by simple salts, Tetrahedron Letters, 52 (2011) 3489-3491.[36] J. Hagen, Industrial Catalysis: A Practical Approach, 3rd Edition edition ed., Wiley VCH, Weinheim, 2015.

15

123456789

101112131415161718192021222324252627282930313233343536373839404142434445464748495051

[37] B.B. Wentzel, P.L. Alsters, M.C. Feiters, R.J.M. Nolte, Mechanistic studies on the Mukaiyama epoxidation, Journal of Organic Chemistry, 69 (2004) 3453-3464.[38] B.B. Wentzel, P.A. Gosling, M.C. Feiters, R.J.M. Nolte, Mechanistic studies on the epoxidation of alkenes with molecular oxygen and aldehydes catalysed by transition metal-beta-diketonate complexes, Journal of the Chemical Society-Dalton Transactions, (1998) 2241-2246.[39] L. Vanoye, J. Wang, M. Pablos, C. de Bellefon, A. Favre-Reguillon, Epoxidation using molecular oxygen in flow: facts and questions on the mechanism of the Mukaiyama epoxidation, Catalysis Science & Technology, 6 (2016) 4724-4732.[40] U. Pinkernell, H.J. Luke, U. Karst, Selective photometric determination of peroxycarboxylic acids in the presence of hydrogen peroxide, Analyst, 122 (1997) 567-571.[41] S.L. Scott, A Matter of Life(time) and Death, ACS Catalysis, 8 (2018) 8597-8599.[42] C.W. Jones, On the Stability and Recyclability of Supported Metal–Ligand Complex Catalysts: Myths, Misconceptions and Critical Research Needs, Topics in Catalysis, 53 (2010) 942-952.

16

123456789

10111213

14

15