Gold-catalyzed oxidation of substituted phenols by hydrogen peroxide

Article abstract of DOI:10.1016/j.apcata.2010.08.018

Gold nanoparticles deposited on inorganic supports are efficient catalysts for the oxidation of various substituted phenols (2,6-di-tert-butyl phenol and 2,3,6-trimethyl phenol) with aqueous hydrogen peroxide. By contrast to more conventional catalysts such as Ti-containing mesoporous silicas, which convert phenols to the corresponding benzoquinones, gold nanoparticles are very selective to biaryl compounds (3,3′,5,5′-tetra-tert-butyl diphenoquinone and 2,2′,3,3′,5,5′-hexamethyl-4,4′- biphenol, respectively). Products yields and selectivities depend on the solvent used, the best results being obtained in methanol with yields >98%. Au offers the possibility to completely change the selectivity in the oxidation of substituted phenols and opens interesting perspectives in the clean synthesis of biaryl compounds for pharmaceutical applications.

Full text of DOI:10.1016/j.apcata.2010.08.018

Applied Catalysis A: General 387 (2010) 129–134
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Gold-catalyzed oxidation of substituted phenols by hydrogen peroxide
Yohan Cheneviere, Valérie Caps1, Alain Tuel∗
IRCELYON, Institut de recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS-Université de Lyon 1, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 June 2010
Received in revised form 30 July 2010
Accepted 10 August 2010
Available online 18 August 2010
Gold nanoparticles deposited on inorganic supports are efficient catalysts for the oxidation of vari-
ous substituted phenols (2,6-di-tert-butyl phenol and 2,3,6-trimethyl phenol) with aqueous hydrogen
peroxide. By contrast to more conventional catalysts such as Ti-containing mesoporous silicas,
which convert phenols to the corresponding benzoquinones, gold nanoparticles are very selective to
biaryl compounds (3,3ꢀ,5,5ꢀ-tetra-tert-butyl diphenoquinone and 2,2ꢀ,3,3ꢀ,5,5ꢀ-hexamethyl-4,4ꢀ-biphenol,
respectively). Products yields and selectivities depend on the solvent used, the best results being obtained
in methanol with yields >98%. Au offers the possibility to completely change the selectivity in the oxida-
tion of substituted phenols and opens interesting perspectives in the clean synthesis of biaryl compounds
for pharmaceutical applications.
Keywords:
Gold nanoparticles
Catalytic oxidation
Hydrogen peroxide
2,6-Di-tert-butyl phenol
2,3,6-Trimethyl phenol
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
in the preparation of vitamin E. In 1996, Pinnavaia and co-workers
already mentioned the activity of Ti-HMS samples in the oxidation
Oxygen-containing molecules like alcohols, aldehydes, ketones,
epoxides and carboxylic acids are important molecules in the
production of polymers and fine chemicals. They are generally
obtained by oxidation of organic substrates with inorganic oxi-
dizing agents like chromium and manganese oxides or nitric acid.
As a result, in addition to corrosion problems, the reaction is not
very selective and leads to a large amount of undesirable and
polluting compounds. Nowadays, environmentally unacceptable
oxidation methods are being progressively replaced by cleaner
methods, in particular heterogeneous catalytic methods. Therefore,
the discovery of new efficient catalysts for the selective oxidation
of hydrocarbons with clean oxidizing agents has become extremely
important in chemistry research.
Quinones represent a very important class of molecules used
to produce drugs, fragrances and vitamins [1]. Recently, hetero-
geneous catalytic routes have been reported to produce quinones
from the corresponding phenols in the liquid phase, using tert-butyl
hydroperoxide (TBHP), hydrogen peroxide (H2O2) or molecu-
lar oxygen as oxidants [2]. The most efficient catalysts consist
in heteropolyacids [3], immobilized metal complexes like iron
phthalocyanines [4–8] and transition metal containing molec-
ular sieves [9–15]. These catalysts give excellent yields in the
oxidation of 2,3,6-trimethyl phenol (TMP) to the corresponding
trimethyl-1,4-benzoquinone (TMBQ), an important intermediate
of di-tert-butyl phenol (DTBP) with hydrogen peroxide to the cor-
responding 2,6-di-tert-butyl benzoquinone (DTBQ) [16]. Later, the
excellent performance of Ti-MCM-41 samples was confirmed in the
oxidation of TMP and various functionalized phenols by H2O2. High
substrate conversions are obtained when the reaction is performed
in acetonitrile or acetone, with high quinone selectivity and very
low amount of by-products [15].
Recently, supported gold catalysts have been found to be par-
ticularly effective in the oxidation of a large variety of substrates
like alcohols, alkenes, amines, . . ., using molecular oxygen [17–23]
or hydrogen peroxide [24–30]. Up to date, reports on the use of Au
for the oxidation of substituted phenols are scarce. Kholdeeva et al.
have shown that 2-methyl-1-naphthol was oxidized to 2-methyl-
1,4-naphthaquinone with molecular oxygen over Au/C and Au/TiO2
catalysts, but yields were not as good as those obtained on Ti-MCM-
41 and iron phthalocyanines. Actually, the authors suggested that
gold could favor over-oxidation of the substrate, thus decreasing
the selectivity in the target product [8].
The present work examines the performance of gold nanopar-
ticles in the oxidation of substituted phenols using hydrogen
peroxideasoxygen donor. TheactivityofAucatalystsin thehydrox-
ylation of 2,6-di-tert-butyl phenol (DTBP) and 2,3,6-trimethyl
phenol (TMP) was systematically compared with that of more con-
ventional Ti-containing mesoporous silica.
2. Experimental
∗
Corresponding author. Tel.: +33 472 445 395, fax: +33 472 445 399.
E-mail address: alain.tuel@ircelyon.univ-lyon1.fr (A. Tuel).
Present address: KAUST Catalysis Center (KCC), 4700 King Abdullah University
1
The Au/TiO2 catalyst (1.4 wt.% Au) was a gold reference cata-
lyst (Sample No 84A) supplied by the World Gold Council (the
of Science and Technology, Thuwal 23955-6900, Saudi Arabia.
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.08.018

130
Y. Cheneviere et al. / Applied Catalysis A: General 387 (2010) 129–134
Table 1
Oxidation of di-tert-butyl phenol (DTBP) over Ti-HMS and Au/TiO2 catalysts.
Catalyst
Oxidant
Solventa
6 h
24 h
DTBP conv. (%)
Product yield (%)
DTBQ
DTBP conv. (%)
Product yield (%)
DTBQ
DPQ
DPQ
TBHP
H2O2
TBHP
AC
MeCN
MeOH
AC
MeCN
MeOH
O
3
1
0
51
40
59
3
2
1
0
45
33
53
3
1
0
0
6
7
9
3
5
69
58
87
9
6
3
4
70
62
71
4
3
0
1
9
6
Ti-
HMS
O
6
0
36
5
AC
O
Au/TiO2
MeCN
MeOH
MCH
5
7
2
3
7
0
2
0
2
11
11
5
6
8
2
5
3
3
H2O2
AC
MeCN
MeOH
O
24
13
90
20
10
0
4
3
100
27
21
91
22
11
0
5
10
100
a
AC O: acetone, MeCN: acetonitrile, MeOH: methanol, MCH: methylcyclohexane.
mean diameter of the gold particles, determined by TEM, was
3.7 1.5 nm; the TiO2 support is P25).
3. Results
Au/Al2O3 was prepared by direct anionic exchange [31] using
HAuCl4·3H2O as gold precursor. Typically, 0.0808 g HAuCl4·3H2O
is dissolved in 1 L H2O and 2 g ␥-Al2O3 are added to the solution
under stirring. The temperature is increased to 70 ◦C and stirring
is maintained at this temperature for 1 h. The solid is then filtered,
washed with a NH4OH solution (7 × 10−2 mol L−1), dried overnight
at 100 ◦C and calcined in air at 300 ◦C for 4 h. Characterization of
the sample gave a SBET of 189 m2/g, Au content of 1.38 wt.% and a
mean particle size of ca. 2.3 0.8 nm [32].
Supported gold nanoparticles are traditionally used to oxidize
organic molecules with molecular oxygen [20–23]. Even in the case
of epoxidation reactions performed with TBHP, Lignier et al. clearly
showed that the latter is a radical initiator, oxygen from the air
being the primary oxidant [36,37]; yields of epoxide one order of
magnitude higher than the initial amount of TBHP present could
be obtained and increasing TBHP initial concentration essentially
increased the epoxide production rate. Moreover, the nature of the
solvent was of prime importance, the best results being achieved
with alkylbenzenes [38] or methyl-substituted cyclohexanes [39].
Indeed, it has been postulated that activation of molecular oxy-
gen was carried out by a substituted cyclohexyl radical, produced
by abstraction of one the tertiary atoms of the solvent molecule
by tert-butyl hydroperoxy radicals. As shown in Table 1, attempts
to convert 2,6-di-tert-butyl phenol with TBHP were not successful,
even when the reaction was performed in methylcyclohexane. For
a TBHP/DTBP ratio of one, the DTBP conversion does not exceed
11% in methanol and acetonitrile and is even lower in methylcy-
clohexane. Hydrogen peroxide is by far a better oxidant than TBHP
in the reaction. However, conversions and selectivities strongly
depend on the nature of the solvent and are very different from
a catalyst to another. As already reported, Ti-HMS converts DTBP
to the corresponding benzoquinone (DTBQ) [9,16]. The selectivity
in DTBQ is good but a second product, identified by NMR as 3,3ꢀ-
5,5ꢀ-tetra-tert-butyl-4ꢀ4ꢀ-diphenoquinone (DPQ) was also obtained
as secondary product (Scheme 1). A higher conversion is obtained
in methanol, but this solvent favors the formation of the biaryl
compound.
P25, purchased from Degussa, is a mixed-phase titanium oxide
containing 80% anatase and 20% rutile with a BET surface area close
to 50 m2/g.
A sample of Ti-containing mesoporous silica Ti-HMS was pre-
pared according to a published procedure [33]. Primary amines
were removed from the mesopores by ethanol extraction, followed
by calcination in air at 550 ◦C. The BET surface area and mean pore
diameter of the solid were 879 m2/g and 3.6 nm, respectively. More-
over, the Ti content in the dry solid was 1.87 wt.%. The UV–vis
spectrum of the calcined dried sample showed a maximum around
230 nm without any signal above 320 nm, suggesting highly dis-
persed Ti species in the silica network [34,35].
Phenol hydroxylation was performed in a round-bottomed flask
equipped with condenser and magnetic stirrer. In a typical exper-
iment, 5 × 10−3 mol phenol was dispersed in 20 mL solvent. Then
an amount of catalyst corresponding to 2 × 10−6 mol Au was added
and the mixture was heated at 60 ◦C under stirring. For Ti-HMS,
experimental conditions were adapted from literature data and
reactions were performed with 200 mg catalyst [16]. After stabi-
lization of the temperature, an amount of H2O2 (35 wt.% solution
in water, Aldrich) corresponding to an oxidant/phenol molar ratio
of 1 was added in one lot. For comparison, experiments were also
carried out using tert-butyl hydroperoxide (TBHP, 70 wt.% in water,
Alfa Aesar). Then, samples were periodically taken and analyzed
by gas chromatography using a 30 m × 0.32 mm × 0.25 ␮m Varian
capillary column filled with CP-WAX 52 CB.
In all three solvents, the use of Au nanoparticles as catalyst
increases the selectivity to DPQ, the C–C coupling product of
phenoxy radicals. Methanol appears to be the best solvent, with
conversions far beyond those obtained in acetone or acetonitrile.
Indeed, after 6 h, the conversion is almost complete (90%) in MeOH
whilst it is only 24 and 13% in acetone and acetonitrile, respec-
For reactions performed under argon atmosphere, the solvent,
phenol and catalyst were preliminary mixed and stirred overnight
at 60 ◦C under Ar bubbling. Then, hydrogen peroxide was added in
one lot and the reaction was continuously performed under Ar.
Biaryl compounds (3,3ꢀ-5,5ꢀ-tetra-tert-butyl-4ꢀ4ꢀ-diphenoqui-
none and 2,2ꢀ,3,3ꢀ,5,5ꢀ-hexamethyl-4,4ꢀ-biphenol) were identified
by comparing their NMR spectra with those of pure compounds.
These solids were separated from catalytic batches by filtration,
dissolved in methyl alcohol-d (99.5+ atom % D, Aldrich) and spectra
were recorded on a Bruker Avance 250 spectrometer.
Scheme 1. Main products of the oxidation of 2,6-di-tert-butyl phenol by hydrogen
peroxide.

Y. Cheneviere et al. / Applied Catalysis A: General 387 (2010) 129–134
131
Scheme 3. Main products of the oxidation of 2,3,6-trimethylphenol by hydrogen
peroxide.
Phthalocyanines grafted on silica generally give moderate yields
with TBHP as oxidant, but can be very selective in the biaryl
compound with molecular oxygen, in the presence of a sacrificial
aldehyde [5,41]. Catalytic data reported in Table 2 confirm that both
Ti-HMS and Au/TiO2 are poorly active with TBHP, whatever the
solvent used. By contrast, both catalysts are active with hydrogen
peroxide, and their behaviors are quite similar to those previ-
ously observed with DTBP (Table 1). Over Ti-HMS, the conversion is
almost complete after 6 h, which indicates that TMP is certainly eas-
ier to oxidize than DTBP. Moreover, reaction rates and final product
selectivities do not significantly depend on the nature of the sol-
vent. The major product formed is TMBQ, in agreement with the
reaction mechanism reported in the literature [13]. Quinone selec-
tivities are very high, most likely because fast reaction rates limit
the formation of coupling products.
Reactions performed over Au/TiO2 produce exclusively biaryl
compounds. Once more, a comparison of conversions at 6 h clearly
shows the particular properties of methanol as compared to ace-
tone and acetonitrile. Conversions at 6 h do not exceed 30 and 40%
in acetonitrile and acetone, respectively, but reach 92% in MeOH.
The reaction in methanol is very selective and leads to a unique
product, which precipitates as a gray powder and was identified as
2,2ꢀ,3,3ꢀ,5,5ꢀ-hexamethyl-4,4ꢀ-biphenol (HMBP) by NMR. It is inter-
esting to note that the diphenoquinone, which results from the
over-oxidation of the biphenol and was the only compound found
with DTBP, is not observed with TMP. The difference between DTBP
and TMP has been explained by a sterical hindrance of the 3 substi-
tuting groups to further oxidation [13]. In acetone and acetonitrile,
the reaction is less selective and a second product is detected by
GC. This product, which selectivity did not exceed 10%, could not
Scheme 2. Two different pathways for the oxidation of 2,6-di-tert-butyl phenol by
H2O2 over Ti-HMS and Au/TiO2.
tively. MeOH was already an excellent solvent when using Ti-HMS
but the difference with other solvents was not so clear, particularly
when compared with acetone. Furthermore, the gold catalyst is
100% selective in DPQ when the reaction is performed in methanol.
By contrast to acetone or acetonitrile, in which both DTBQ and DPQ
are formed, and by contrast with the titanium catalyst, quinone has
never been observed and the biaryl compound is the only product
formed all along the reaction.
Catalytic data in the hydroxylation of 2,6-di-tert-butyl phenol
strongly suggest that the main reaction mechanisms with H2O2 are
different over Ti-HMS and Au/TiO2. Whatever the catalyst used,
methanol as solvent enhances the production of the biaryl com-
pound, but this compound becomes the sole product of reaction
in the case of gold. In other words, gold nanoparticles inhibit the
formation of the benzoquinone (Scheme 2).
In order to confirm the difference, the two catalysts have
been tested in the hydroxylation of a second substrate, i.e. 2,3,6-
trimethyl phenol (TMP) (Scheme 3).
Previous results showed that the H2O2/TiO2–SiO2 system is very
active and also very selective in the corresponding quinone (2,3,6-
trimethyl benzoquinone or TMBQ) [40].
Table 2
Oxidation of 2,3,6-trimethyl phenol (TMP) over Ti-HMS and Au/TiO2 catalysts.
Catalyst
Oxidant
Solventa
TMP conv. (%) after 6 h
Product yield (%)
TMBQ
HMBP
Othersb
TBHP
H2O2
TBHP
H2O2
AC
MeCN
MeOH
AC
MeCN
MeOH
AC
MeCN
MeOH
O
2
–
3
86
98
97
–
2
–
2
74
85
87
–
0
–
1
–
–
–
6
3
–
–
–
1
4
3
–
–
–
–
–
Ti-
HMS
O
6
10
10
–
–
2
37
27
94
97
–
O
Au/TiO2
–
5
–
2
AC
O
41
30
96
97
3
MeCN
MeOH
MeOHc
MeOH
MeOH
MeOH
–
–
–
–
4
–
No
TiO2
Au/Al2O3
H2O2
H2O2
H2O2
6
92
2
92
a
AC O: acetone, MeCN: acetonitrile, MeOH: methanol.
This product was assumed to be a phenoxyphenol (PP). The yield was estimated by the difference between the conversion and the amount of known products
b
(TMBQ + HMBP).
c
Reaction performed under argon atmosphere.

132
Y. Cheneviere et al. / Applied Catalysis A: General 387 (2010) 129–134
Scheme 4. Reaction pathway for the oxidation of 2,3,6-trimethylphenol by H2O2 over Au/TiO2.
be isolated and completely identified. However, based on previ-
ous studies on the oxidation of TMP with H2O2, the unidentified
product could be tentatively assigned to a phenoxyphenol (PP).
This compound, which results from the C–O coupling between phe-
noxy radicals, was previously observed in the oxidation of TMP with
H2O2 in acetonitrile [11]. TMBQ is never detected as a primary prod-
uct by chromatography, even at the level of traces, confirming the
difference between Ti and Au in the reaction.
4. Mechanism
As previously reported in the literature, gold nanoparticles cat-
alyze the decomposition of alkyl hydroperoxides to alkoxy radicals,
which can further react with the peroxide to form alkylperoxy rad-
icals [39]. These species can carry out hydrogen abstraction from a
tertiary C–H bond at 80 ◦C, triggering a low temperature oxidation
mechanism involving molecular oxygen [43]. Hydrogen peroxide
has proven unsuccessful in initiating such a reaction [39]. We can
thus assume that Au will not simply cleave the H2O2 peroxide bond,
but will form Au–H2O2 adducts (Scheme 4), as previously proposed
[44]. These Au–hydroperoxide species will then react with DTBP
or TMP to form phenoxy radicals, like those produced by reaction
with Ti–OOH species [45]. The subsequent formation of quinone
(Route 1) then relies on an intermediate complex involving Ti per-
oxo species and phenoxy radicals [13]. However, by contrast to
Ti–OOH species, which are very stable and can be easily charac-
terized in solution (deep yellow color), Au–H2O2 intermediates are
In order to confirm the role of catalysts in the conversion of
substituted phenols, additional experiments have been performed.
First, a reaction was carried out in methanol with TMP as substrate
in the absence of any catalyst. After 6 h, the conversion was only 3%
and it reached 8% after 24 h (Table 2). In a second experiment, 30 mg
of P25, which is the titania support for the commercial Au/TiO2 cat-
alyst, were added to the reaction mixture. Indeed, the activity of
Ti-HMS suggested that TiO2 could also be active in the reaction.
Even after 24 h, the conversion did not exceed 10%, far below that
obtained in the presence of Au nanoparticles, and both TMBQ and
HMBP are formed (Table 2). Third, the same reaction was performed
using Au/Al2O3 as catalyst. Comparison with Au/TiO2 is straightfor-
ward because the two catalysts possess very similar Au content and
particle sizes. This catalyst was as active as Au/TiO2 and HMBP was
once more the only product formed (Table 2). All these experiments
clearly demonstrate the primary role of gold nanoparticles in the
catalytic process, as recently shown in the radical oxidation of bulky
alkenes [42]. They also show that bulk TiO2 is inactive and that the
nature of the support has no influence on the intrinsic activity of
Au particles in the present reaction.
Despite high activity of Au-based catalysts, phenol conversions
are never complete, even after 48 h, when reactions are performed
with H2O2/phenol = 1. Actually, the conversion reaches a maximum
(80–90%) after approx. 10 h and then it remains constant. After
one day, the reaction immediately restarts upon addition of 0.5 mL
H2O2, and the phenol conversion is complete within 3 h. Clearly, the
reaction does not stop because of catalyst deactivation, but because
active oxygen species are not available anymore. By contrast to pre-
vious observations, molecular oxygen from the air does not seem to
take part in the reaction. A reaction performed under argon atmo-
sphere confirmed that neither the reaction rate nor the selectivity in
biaryl compounds was significantly affected by the absence of air.
This suggests that oxygen species involved in the process essen-
tially result from the decomposition of H2O2 over gold particles
(Table 2 and Fig. 1), as will be discussed later.
Fig. 1. . 2,3,6-Trimethylphenol conversion in methanol over Au/TiO2 under air (ꢁ)
and argon (᭹) atmospheres.

Y. Cheneviere et al. / Applied Catalysis A: General 387 (2010) 129–134
133
5. Conclusions
The H2O2–Au system has proven to be particularly effective in
the mild oxidation of substituted phenols in the liquid phase. How-
ever, the product distribution is different from that obtained using
Ti-containing silica catalysts: whilst Ti is very selective to the cor-
responding benzoquinones, Au directs the reaction to C–C coupling
products. This has been explained by the low stability of Au–H2O2
adducts as compared to Ti–OOH species, thus favoring the forma-
tion of phenoxy radicals and free-radical coupling reactions, and
inhibiting the catalytic formation of benzoquinone. The reaction
rate and product selectivities depend on the nature of the solvent
but yields in biaryl compounds higher than 90% have been obtained
in methanol. Gold is thus complementary to Ti-silicas and could be
a promising catalyst for the clean synthesis of biaryl compounds
(low temperature, no by-products, green oxidants).
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