Marco
Conte
*a,
Xi
Liu
a,
Damien M.
Murphy
a,
Keith
Whiston
b and
Graham J.
Hutchings
*a
aCardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff, CF10 3AT, UK. E-mail: ConteM1@cardiff.ac.uk; Hutch@cardiff.ac.uk
bINVISTA Textiles (UK) Limited, P.O. Box 2002, Wilton, Redcar, TS10 4XX, UK
First published on 29th October 2012
The liquid phase oxidation of cyclohexane was undertaken using Au/MgO and the reaction mechanism was investigated by means of continuous wave (CW) EPR spectroscopy employing the spin trapping technique. Activity tests aimed to determine the conversion and selectivity of Au/MgO catalyst showed that Au was capable of selectivity control to cyclohexanol formation up to 70%, but this was accompanied by a limited enhancement in conversion when compared with the reaction in the absence of catalyst. In contrast, when radical initiators were used, in combination with Au/MgO, an activity comparable to that observed in industrial processes at ca. 5% conversion was found, with retained high selectivity. By studying the free radical autoxidation of cyclohexane and the cyclohexyl hydroperoxide decomposition in the presence of spin traps, we show that Au nanoparticles are capable of an enhanced generation of cyclohexyl alkoxy radicals, and the role of Au is identified as a promoter of the catalytic autoxidation processes, therefore demonstrating that the reaction proceeds via a radical chain mechanism.
However, it is currently debated if gold-based materials are real catalytic systems or rather act as promoters of the autoxidation pathways for the cyclohexane oxidation reaction.18 SiO2 supported gold catalysts, modified by doping with TiO2, were reported to be capable of high conversion,19,20 relative to the industrial catalyst, of ca. 10% and selectivity to the alcohol and ketone (K/A oil) > 70%, which was not observed in the absence of supported gold nanoparticles; the conclusion reached was that this was a real catalyst for cyclohexane oxidation. On the other hand, investigation of the cyclohexane oxidation over Au/Al2O3, Au/TiO2 and Au/SBA-1521 showed that the reaction proceeds via a pure radical pathway with products typical of autoxidation and the reaction could be fully inhibited by means of radical scavengers. Finally, it has also been shown that gold nanoclusters supported on hydroxyapatite were capable of displaying high activity towards cyclohexane22 and that no reaction was taking place in the absence of gold, although the reaction required the presence of radical initiators, typically tert-butylhydroperoxide (TBHP).
This lack of unambiguous evidence on the true catalytic role of gold in the cyclohexane oxidation, prompted us to carry out a mechanistic study using Au/MgO because of its excellent selective oxidation properties.23–25 To test if the cyclohexane oxidation reaction proceeds via a radical mechanism, we employed X-band EPR spectroscopy combined with the spin trapping technique26–28 as well as radical scavengers. The principle of the spin-trapping methodology relies on the fast selective addition, i.e. trapping, of short-lived radicals to a diamagnetic spin trap, usually a nitrone or a nitroso compound, such as 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The product of this addition, known as the spin adduct, is a persistent free nitroxide radical with a sufficiently long lifetime to enable detection by conventional EPR spectroscopy (Scheme 1).29 Because of the hyperfine coupling between the unpaired electron in the spin adduct and the 1H in the beta position for the chosen spin trap, it is often possible to assign the structure of the original short-lived radicals due to changes in the 14N and 1H hyperfine coupling constants of the spin trap molecule.30 In this paper we present the results of this mechanistic study.
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Scheme 1 Spin trapping mechanism for DMPO with a free radical (R˙). |
The spin trapping experiments were performed using the following procedure: 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) (0.1 mL of 0.1 M solution in cyclohexane) was added to the substrate (0.1 mL of 2.5 molar% solution of cyclohexyl-hydroperoxide – hereafter abbreviated CHHP – in cyclohexane), in an EPR sample tube. The mixture was deoxygenated by bubbling N2 for 1 min prior to recording the EPR spectra in order to enhance the signal intensity.30 For the samples containing the Au/MgO catalyst, deoxygenation was carried out at room temperature, 5 min after the mixing of the catalyst with the reaction mixture.
Entry | Catalyst | Au loading (wt%) | Initiator | Conversiona (%) | Selectivityb (%) | K/A ratio | ||||
---|---|---|---|---|---|---|---|---|---|---|
K | A | CHHP | AA | Total | ||||||
a We detected a closed carbon mass balance within an experimental error of 5%. b The missing components are carboxylic acids as ring opening products. | ||||||||||
1 | Au/MgO-Imp | 1 | — | 1.9 | 30 | 51 | 7 | 0 | 88 | 0.58 |
2 | Au/MgO-Imp | 0.1 | — | 1.7 | 35 | 44 | 15 | 0 | 94 | 0.8 |
3 | Au/MgO-Imp | 0.01 | — | 1.3 | 28 | 36 | 27 | 0 | 91 | 0.78 |
4 | Autoxidation | — | — | 1.1 | 19 | 22 | 57 | 0 | 98 | 0.86 |
5 | MgO | — | — | 1.4 | 36 | 25 | 4 | 0 | 65 | 1.44 |
6 | Au/MgO-Imp | 1 | AIBN | 4.5 | 34 | 51 | 6 | 0 | 91 | 0.67 |
7 | Autoxidation | — | AIBN | 6.7 | 32 | 57 | 2 | 0 | 91 | 0.56 |
8 | Au/MgO-Imp | 1 | TBHP | 5.0 | 29 | 50 | 0 | 19 | 98 | 0.58 |
9 | Autoxidation | — | TBHP | 6.6 | 33 | 52 | 0 | 15 | 100 | 0.63 |
However, because the conversion values are just above those obtained by autoxidation, these data suggest that the reaction still proceeds via a radical chain mechanism. A control experiment using MgO in the absence of gold (entry 5) showed that the conversion is higher than the one observed for autoxidation, but lower than that observed for the gold containing catalyst, where the selectivity is shifted to the ketone.
In view of this, the effect of initiators such as azo-bis-isobutyronitrile (AIBN) and tert-butylhydroperoxy radical (TBHP) were evaluated (Table 1). When AIBN was used (entries 6 and 7), the activity of Au/MgO catalyst increased to ca. 4% and with selectivity values quite similar to those obtained for the catalyst in absence of initiators (entry 1). However, despite this apparent increase in conversion, this is less than the value obtained in the presence of AIBN only (ca. 7%). This is consistent with the role of Au as an inhibitor for the reaction. In contrast, if TBHP was used (entries 8 and 9), the activity of Au/MgO increased, (up to ca. 5%) but, also in this case, it was lower than that for the reaction carried out in presence of TBHP only (ca. 6.6%). Moreover, the selectivity control to cyclohexanol and cyclohexanone was limited, and a significant amount of adipic acid was detected, an effect that should be considered a direct consequence of the large amount of peroxides in solution.38
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Fig. 1 XRPD patterns of: (a) MgO starting material, (b) Au/MgO catalyst 1 wt%, (c) Au/MgO catalyst 0.1 wt%, (d) Au/MgO catalyst 0.01 wt% and (e) MgO support impregnated with water. The symbols used indicate: (○) MgO – periclase (□) Mg(OH)2 – brucite, and (△) Au. |
From the XRPD pattern it was also possible to estimate the particle size of gold, using the Au(111) reflection at 38.2° 2θ.42 These were estimated to be ca. 17 and 8 nm for the materials containing 1 and 0.1 wt% Au respectively. In contrast, no gold reflection for the 0.01 wt% Au sample was detected, indicating a particle size below the detection limit of the XRD method, which is ca. 4 nm. Gold nanoparticles were also well identified by means of plasmon resonance43via diffuse reflectance UV-Vis spectroscopy (Fig. 2), and the band intensity of the spectra is consistent with the particles size estimation obtained by XRPD; i.e., with gold particles less than 4 nm diameter for the 0.01 wt% Au sample.
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Fig. 2 Diffuse reflectance UV-Vis spectra of: (a) MgO starting material, (b) MgO support impregnated with water, (c) Au/MgO catalyst 0.01 wt%, (d) Au/MgO catalyst 0.1 wt% and (e) Au/MgO catalyst 1 wt%. |
A detailed analysis of the diffuse reflectance spectra shows that MgO, even for the untreated sample, presents absorption bands in the UV range at ca. 210 and 280–300 nm. While MgO is commonly regarded as a white standard for DR-UV in the visible region,44 absorption bands at 213 nm and 282 nm are associated with the excitation of four-fold and three-fold coordinated surface O2− anions at edge and corner positions of MgO crystals respectively.45,46 This is a consequence of the use of a commercial purity grade MgO as catalyst support, rather than the use of an optically purity grade MgO.
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Scheme 2 Radical chain pathway in the formation of cyclohexanol and cyclohexanone during the oxidation of cyclohexane. |
The first step of the oxidation is the initiation, which involves activation of the C–H bond via abstraction of an H atom. This can occur by a number of events, including: (i) cleavage by an unsaturated metal centre,49 (ii) H abstraction by a peroxide species (either peroxyl or alkoxy radicals present in solution),28 or (iii) H abstraction by a superoxide species (O2−) bound to metal centres or metal oxides.50 For each of these processes this results in the formation of a carbon centred parent radical (C6H11˙). It is well known that carbon-centred radicals are extremely reactive51 and they immediately react with O2 to give peroxyl radicals, in our case cyclohexyl peroxyl radical (C6H11–OO˙) according to (eqn (2)). In principle, the oxygen incorporated into the products can originate from oxygen dissolved in solution or from adsorbed oxygen species on the metal oxide surface.52 C6H11–OO˙ can react further with cyclohexane to give cyclohexyl hydroperoxide (CHHP) and another C6H11˙ radical, thus ensuring propagation of the reaction (eqn (3)). It should be stressed that, in this scheme C6H11–OO˙ is the main radical chain carrier, with CHHP reacting in a sequence that finally yields cyclohexanone (eqn (4) and (5)).
In contrast, cyclohexanol can be obtained by hydrogenation of cyclohexyl alkoxy radical C6H11–O˙ (eqn (6)), cleavage of CHHP (eqn (7)) or via recombination of two peroxyl radicals (eqn (8)).53 In principle, cyclohexanol can originate from insertion of lattice oxygen from the metal oxide to the C6H11˙ parent radical adsorbed over the catalyst surface.54 However, when MgO only was tested, no activity was detected which therefore rules out this latter possible route, and this is also not related to the purity level of MgO.
From this model, it is evident that the ketone is always obtained (eqn (5) and (8)) if autoxidation is operating. Therefore selectivity control, if any, can occur only in the decomposition step of CHHP (eqn (7)). More recently, solvent-cage models have also been proposed for the autoxidation pathways,55 to explain the alcohol and ketone formation.
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Scheme 3 AIBN decomposition pathway. |
In contrast TBHP can undergo homolytic cleavage of the O–O bond and from this to peroxyl condensation via the set of equations ((9)–(11)) reported in Scheme 4.56
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Scheme 4 TBHP decomposition pathway. |
Considering the mechanisms of action for these initiators and the results obtained in the current case (i.e., an enhanced oxidation when initiators are used, but a decrease when combined with Au/MgO) it would be possible to conclude that Au/MgO can actually quench some of the radicals generated by the initiators, such as cyanopropyl radicals,57,58 but at the same time acting as a promoter for the O–O cleavage in the TBHP decomposition;34–36 this could also explain the enhanced selectivity to cyclohexanol. It is possible that quenching comes at least in part from the support, with the activation role from the metal nanoparticle counterpart. Many metal oxides, such as MgO, present neutral oxygen vacancies59,60 that could possibly aid in the localisation of peroxyl species, and so partially inhibit the reaction when initiators are used. A similar quenching effect was observed for ZnO in the aldehyde oxidation by Au/ZnO catalysts.61 In fact, regardless of the insulator properties of MgO, compared to the semi-conducting properties of ZnO which may increase the tendency to radical quenching, the presence of defects in or on the MgO crystals may facilitate radical trapping and stabilisation.62 This effect has been experimentally observed for methyl radicals in gas phase.63
When the reaction was carried out at room temperature in the presence of DMPO as spin trap, CW EPR spectra were acquired in the presence (Fig. 3) and absence (Fig. 4) of Au/MgO. Simulation of the spectrum and comparison with literature values allowed the identification of all the radical intermediates which are expected in the autoxidation pathway of cyclohexane to cyclohexanone and cyclohexanol. In particular these species include: di-tert-butyl-nitroxide derivative,64 a DMPO–O–C6H1165 and a DMPO–OOC6H11 spin adduct,66 a DMPO–C6H11 carbon-centred adduct characteristic of the parent radical C6H11˙
67 and a carbon centred adduct which is possibly a DMPO–C(OH)R2 species.68 The spin Hamiltonian parameters of these spin adducts are reported in Table 2.
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Fig. 3 Deconvoluted EPR spectra of DMPO spin adducts obtained during the decomposition of CHHP in cyclohexane in the presence of Au/MgO: (a) experimental spectrum and (b) simulated spectrum; (c) di-tert-butyl-nitroxide derivative, (d) DMPO–O–C6H11 spin adduct, (e) a DMPO–OO–C6H11 adduct, (f) DMPO–C6H11 carbon centred adduct, and (g) carbon centred adduct, which is possibly a DMPO–C(OH)R2 species. |
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Fig. 4 Deconvoluted EPR spectra of the DMPO spin adducts obtained during cyclohexane autoxidation at room temperature in the presence of CHHP: (a) experimental spectrum and (b) simulated spectrum; (c) di-tert-butyl-nitroxide derivative, (d) DMPO–O–C6H11 spin adduct, (e) DMPO–OO–C6H11 adduct, and (f) carbon centred adduct which is possibly a DMPO–C(OH)R2 species. |
Radical | a N /G | a H(β)/G | a H(γ)/G |
---|---|---|---|
tert-Butyl nitroxide derivative | 14.2 (14.2) | — | — |
C6H11–O˙ | 13.4 (13.4) | 6.00 (6.00) | 1.80 (1.90) |
C6H11–OO˙ | 14.3 (14.2) | 10.8 (10.8) | |
C6H11˙ | 14.3 (n.d.) | 21.2 (n.d.) | |
R2(OH)C˙ | 15.6 (15.8) | 25.9 (25.7) |
It should be noted that the spin trapping technique only allows for semi-quantitative determination of the adducts detected. This is a consequence of the life-time of the spin adduct, the nature of the solvent, the temperature and the efficiency of the capture reaction which is different for each radical.28,30 With these limitation in mind, the simulation revealed the following semi-quantitative values: di-tert-butyl-nitroxide derivative (2%); DMPO–O–C6H11 (81%); DMPO–OO–C6H11 (8%); DMPO–C6H11 (4%); and possible DMPO–C(OH)R2 adduct (5%).
When the same experiment was carried out in the absence of Au/MgO, to assess the CHHP decomposition via the autoxidation pathway, the following species were obtained (Fig. 4): di-tert-butyl-nitroxide derivative, DMPO–O–C6H11 and DMPO–OO–C6H11 spin adducts and the possible DMPO–C(OH)R2 adduct. The hyperfine splitting constants of these spin adducts are reported in Table 2. These species were quantified as follow: di-tert-butyl-nitroxide derivative (2%); DMPO–O–C6H11 (50%); DMPO–OO–C6H11 (43%); and DMPO–C(OH)R2 (5%).
The species trapped in the presence and absence of Au/MgO are basically the same in both sets of experiments, but the parent radical is not detected in the autoxidation case. In contrast, the most remarkable difference between the two sets is an increased quantity of alkoxy (C6H11–O˙) species, from 50% to 80% when gold is used. This indicates that Au/MgO is capable of enhancing alcohol formation by cleavage of the O–O bond of CHHP.
However, as the spin trapping technique is prone to artifacts, control tests were necessary and we carefully carried out. In fact, spin adducts can be formed not just by the radical addition to a spin trap, but also by nucleophilic addition followed by oxidation of the spin adduct69,70 or by oxidation of the spin trap followed by nucleophilic addition, in our case by species such as −OH. Control tests in the presence of Au/MgO and the spin trap, but in the absence of substrate, did not reveal any trace of the DMPOX oxidation product.71 Moreover, the trace amount of di-tert-butyl-nitroxide derivative should be considered ubiquitous in these types of experiments and can be discounted.28,61
On the other hand, no DMPO–OH adduct was detected in the tests we carried out. If homolytic cleavage of a substrate is considered, it is not unprecedented, by using spin trapping, to detect just one of the two expected partners (in our case RO˙). This could be due to a failure of the spin trap molecule to capture the ˙OH species under the reaction conditions used, or by termination of ˙OH on the metal surface. In addition, it is known that ˙OH is among the most reactive known radical species,72 and therefore difficult to trap.
This still does not preclude a redox cycle mediated by the metal centre. At present, this has been accredited in the case of oxidation by means of Co(III) salts in agreement with the Haber–Weiss cycle,73 (Scheme 5, eqn (12)–(14)).
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Scheme 5 Haber–Weiss cycle for the oxidation of cyclohexane mediated by Co(III). |
No adduct from nucleophilic attack and oxidation was detected when Au was present. Moreover, systems like those in eqn (12)–(14) have a K/A ratio in the range of 1.5,2,21 while in our case the K/A ratio is in the range of 0.6 with clear selectivity to the alcohol, therefore supporting the conclusion that gold has to operate a homolytic cleavage of CHHP in agreement with eqn (7).
When CBrCl3 was used (Fig. 5) the reaction was completely inhibited at the initial stage (first 2 hours). Then as long as CBrCl3 was consumed in the reaction media, with consequent C6H11–Br formation which is detected after ca. 30 min, product formation is observed. In particular, when all CBrCl3 was consumed (after 2 hours), the oxidation reaction newly started as in the absence of inhibitor, thus demonstrating that Au/MgO promotes the decomposition of CHHP but within a free radical-chain reaction mechanism.
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Fig. 5 Product evolution in the liquid phase oxidation of cyclohexane over Au/MgO in presence of CBrCl3 as radical scavenger at 140 °C under 3 bar of O2: (▲) bromocyclohexane, (●) cyclohexanol and (●) cyclohexanone. |
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