Supported sulfonic acids: Metal-free catalysts for the oxidation of hydroquinones to benzoquinones with hydrogen peroxide

Article abstract of DOI:10.1016/j.apcata.2011.10.032

Some silica- and polystyrene-supported sulfonic acids, prepared by tethering or sol-gel techniques, were employed as metal-free heterogeneous catalysts in the oxidation of hydroquinones to the corresponding 1,4-benzoquinones, carried out with 30% aqueous hydrogen peroxide. The activity of these catalysts was tested in the model oxidation of methylhydroquinone under batch conditions: high yield and selectivity at room temperature and under environmentally friendly conditions can be achieved in the presence of SiO ₂-(CH₂)₃-SO₃H. The heterogeneous catalyst, that can be easily recovered by simply filtration, can be used for at least three cycles affording the same good results (～80% yield, ～91% selectivity).

Full text of DOI:10.1016/j.apcata.2011.10.032

Applied Catalysis A: General 411–412 (2012) 146–152
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Supported sulfonic acids: Metal-free catalysts for the oxidation of hydroquinones
to benzoquinones with hydrogen peroxide
Raimondo Maggia,∗, Calogero G. Piscopoa, Giovanni Sartoria, Loretta Storarob, Elisa Morettib
a “Clean Synthetic Methodologies Group”, Dipartimento di Chimica Organica e Industriale dell’Università, Parco Area delle Scienze 17A, I-43124 Parma, Italy
b INSTM UdR di Venezia, Dipartimento di Scienze Molecolari e Nanosistemi, Università Cà Foscari di Venezia, Via Torino 155/b, I-30172 Venezia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 May 2011
Received in revised form 5 October 2011
Accepted 21 October 2011
Available online 28 October 2011
Some silica- and polystyrene-supported sulfonic acids, prepared by tethering or sol–gel techniques, were
employed as metal-free heterogeneous catalysts in the oxidation of hydroquinones to the corresponding
1,4-benzoquinones, carried out with 30% aqueous hydrogen peroxide. The activity of these catalysts was
tested in the model oxidation of methylhydroquinone under batch conditions: high yield and selectivity
at room temperature and under environmentally friendly conditions can be achieved in the presence of
SiO2–(CH2)3–SO3H. The heterogeneous catalyst, that can be easily recovered by simply filtration, can be
used for at least three cycles affording the same good results (∼80% yield, ∼91% selectivity).
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Oxidation
Hydrogen peroxide
Hydroquinones
Benzoquinones
Supported sulfonic acids
1. Introduction
Phenols and naphthols can be directly converted into quinones
via one-pot hydroxylation–oxidation process in the presence of
Eco-efficient oxidation of alcohols, phenols and their derivatives
can be carried out by using atmospheric oxygen, hydrogen perox-
ide or alkyl hydroperoxides in the presence of an appropriate solid
catalyst, that activates the oxidizing agent [1].
different oxidizing [14–19]. Particular interest shows the dye sen-
sitized photo-oxygenation of 1,5-dihydroxynaphthalene to Juglone
(5-hydroxy-1,4-naphthalenedione) with oxygen in the presence of
moderately concentrated sunlight [20].
In particular, oxidation of phenols and hydroquinones affords
quinones that are found as structural units in a great variety of
natural compounds showing antifungal, antibacterial, antiviral and
anticancer activity. These properties made quinones valuable inter-
mediates for the preparation of fine chemicals and pharmaceuticals
[2–4]. Furthermore, their use as dienophiles in Diels–Alder reac-
tions is well documented [5].
Moreover, some recent examples are reported showing the use
of heterogeneous catalysts for the efficient oxidation of hydro-
quinone to para-benzoquinone with hydrogen peroxide. Thus,
Cu(II) catalyst supported onto acrylic resin [21], mesoporous
titanium–silicate catalyst [22], and a catalyst based on platinum
nanoclusters incarcerated into polystyrene [23] allow the aerobic
oxidation of hydroquinones to quinones.
The oxidation of substituted hydroquinones to the corre-
sponding quinones can be achieved by a variety of methods
based on the use of noxious oxidants and metal-based catalysts.
For example, the Fremy’s salt (potassium nitrodisulfonate) [6],
sodium dichromate/sulfuric acid mixtures [7], benzyltrimethy-
lammonium tribromide [8], iodine or hydroiodic acid/hydrogen
peroxide mixtures [9], diphenyl diselenide/hydrogen peroxide
mixtures [10], ammonium ceric nitrate/Montmorillonite K10 mix-
tures [11], phenyl iodosoacetate on alumina under microwave
irradiation [12], and a variety of transition metal derivatives [13]
have been exploited.
Recently we have demonstrated that a well known and estab-
lished reagent such as silver oxide can also be utilized as catalyst
with excellent results to perform these reactions [24]. Furthermore,
solid polyaniline has been utilized as non metal heterogeneous cat-
alyst to perform oxidation of hydroquinone to para-benzoquinone
with hydrogen peroxide [25].
New green legislations call for the lowering of waste production
and the use of more friendly alternative reagents and catalysts.
This goal can be achieved through the application of the princi-
ples of green chemistry that provide a framework for designing
more eco-compatible routes to fine chemicals production [26].
In addition, the presence of metals as impurities in products for
pharmaceutical or food applications is a great problem, even if they
are present only in traces. For these reasons, in the last decades
the use of non metal catalysts has emerged as one of the most
∗
Corresponding author. Tel.: +39 0521 905411; fax: +39 0521 905472.
E-mail address: raimondo.maggi@unipr.it (R. Maggi).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.10.032

R. Maggi et al. / Applied Catalysis A: General 411–412 (2012) 146–152
147
appealing instruments to perform regio- and enantio-selective
reactions including oxidations [27].
(1.50 ml; 6.3 mmol) in toluene (120 ml); the so functionalized silica
has been then sulfonated at the phenyl ring by refluxing in 1,2-
dichloroethane (60 ml) with chlorosulfonic acid (10 ml; 150 mmol).
SG–(CH2)3–SO3H: in a beaker containing EtOH (20 ml) and H2O
(25 ml), MPTS (1.9 ml; 10.1 mmol) and tetraethoxysilane (TEOS)
(8.9 ml; 40 mmol) were added together with dodecylamine (2.54 g;
13.1 mmol) as porogenic compound [28]. When the mixture begun
to become opalescent, the stirrer was stopped and the formed gel
aged for 24 h at rt in the same container covered with a filter paper.
The so obtained white monolith was then finely ground in a mortar,
washed in a Soxhlet apparatus with EtOH at reflux for 24 h, and the
resulting supported propylmercaptane has been oxidized to propy-
lsulfonic acid by treatment with 30% aq. H2O2 (100 ml; 1 mol) for
24 h under stirring at rt, adding few drops of concentrated sulfuric
acid after 12 h.
Catalysts based on polystyrene supported sulfonic acid (Nafion-
NR50, Amberlyst-15) have shown successful results in various
acid-catalyzed reactions [28–34], but they have rarely been
employed in oxidation reactions, although examples of efficient
activation of hydrogen peroxide through these catalysts have been
reported [35–38]. For this reason, in continuation of our investi-
gations on the preparation, characterization and use of supported
organic and organometallic catalysts [39–41], we have evaluated
the activity of silica-supported sulfonic acids as heterogeneous,
metal-free catalysts for the oxidation of hydroquinones to ben-
zoquinones with 30% aqueous hydrogen peroxide under batch
conditions. Sulfonic groups anchored onto solid materials have
been recently utilized as solid Brönsted acid catalysts [42].
All the catalysts were carefully washed and dried for 24 h at
100 ◦C before use.
2. Experimental
2.1. Reagents and catalysts
2.3. Catalyst characterizations
All the reagents were used as received without further purifica-
tion. Solvents were distilled under nitrogen and dried before use.
Before functionalization the siliceous supports were dried at 300 ◦C
for 5 h.
The amorphous silica used was the commercial Kieselgel 60 (KG-
60) purchased from Merck.
MCM-41 mesoporous silica was prepared, according to the
procedure reported in the literature [43], by mixing, at room
temperature and under vigorous stirring, 25% sodium silicate
solution, tetramethylammonium hydroxide, 25% cetyltrimethy-
lammonium chloride solution and water, and aging at 110 ◦C for
4 days the resulting gel (composition: SiO2·0.39Na2O·0.25CTMACl·
0.24TMAOH·50.7H2O).
Nitrogen adsorption–desorption measurements were per-
formed at liquid nitrogen temperature (−196 ◦C) with an ASAP 2010
apparatus of Micromeritics. Before each measurement, the sam-
ples (0.1 g) were outgassed first at 100 ◦C for 12 h at 5 × 10−3 Torr
and then at room temperature for 2 h at 0.75 × 10−6 Torr. The
N2 isotherms were used to determine the specific surface areas
through the BET equation (S.A.BET), the specific pore volume (Vs)
was calculated at p/p◦ = 0.98, and pore size distributions were calcu-
lated from the BJH method calculated from the adsorption branch.
Thermogravimetric and Differential Thermal Analyses
(TGA–DTG) and differential scanning calorimetry (DSC) were
carried out with a Netzsch STA 409 instrument. Typically, 10 mg of
sample were heated in a crucible made of alumina, at constant rate
(10 ◦C/min) in a stream of air from 30 to 1000 ◦C. The temperature
of maximum weight loss was obtained by derivative TGA (DTG)
plots.
2.2. Catalysts preparation (Scheme 1)
SiO2–(CH2)3–SO3H and MCM-41–(CH2)3–SO3H: the siliceous
support (amorphous KG-60 silica or mesoporous MCM-41 sil-
ica) (8 g) has been refluxed under stirring for 24 h with
(3-mercaptopropyl)trimethoxysilane (MPTS) (1.15 ml; 6.1 mmol)
in toluene (120 ml) and the resulting supported propylmercaptane
has been oxidized to propylsulfonic acid by treatment with 30% aq.
H2O2 (100 ml; 1 mol) for 24 h under stirring at rt, adding few drops
of concentrated sulfuric acid after 12 h.
The surface acidity was determined by a reported titration
method [44].
Elemental analyses were performed by using a Carlo Erba CHNS-
0 EA1108 Elemental Analyzer instrument.
2.4. Reaction procedure
2.4.1. Oxidation of hydroquinones
SiO2–(C6H4)–SO3H: the amorphous KG-60 silica (8 g) has been
refluxed under stirring for 24 h with phenyltrimethoxysilane
To a mixture of the selected hydroquinone (1.0 mmol),
SiO2–(CH2)3–SO3H (0.1 g, 0.02 mmol) and MeOH (2 ml), a solution
H2O2 aq
Toluene
120 °C
MeO
MeO
SiO2
Si
SH
MPTS
+
+
SiO2
SiO2
SiO2-(CH2)3-SO3H
SO3H
SH
SH
H2SO4
OMe
H2O2 aq
H2SO4
MeO
MeO
Toluene
120 °C
Si
SH
MCM
MCM
SO3H
MCM-41-(CH2)3-SO3H
MCM
OMe
MPTS
TMPS
Toluene
120 °C
HSO3Cl
MeO
MeO
SO3H
SiO2-C6H4-SO3H
SiO2
+
SiO2
SiO2
SG
Si
OMe
EtO
MeO
H2O2 aq
H2SO4
H2O/EtOH
OEt
Si
SH
SH
SO3H
SG-(CH2)3-SO3H
Si
+
SG
MeO
EtO
TEOS
OMe
dodecylamine
OEt
MPTS
Scheme 1. Preparation of the supported sulfonic acids.

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R. Maggi et al. / Applied Catalysis A: General 411–412 (2012) 146–152
0.10
500
450
400
350
300
250
200
150
100
50
0.08
0.06
0.04
0.02
0.00
2
4
6
8
10
12
14
16
18
20
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p/p°)
Pore Diameter (nm)
Fig. 1. N2 adsorption (full symbol)–desorption (empty symbol) isotherms of the
functionalized silica materials MCM-41–(CH2)3–SO3H (ꢀ), SiO2–(CH2)3–SO3H (᭹),
SiO2–(C6H4)–SO3H (ꢁ) and SG–(CH2)3–SO3H (ꢂ).
Fig. 2. Pore size distribution (nm) of the functionalized mesoporous silica materials
SiO2–(CH2)3–SO3H (᭹) and SiO2–(C6H4)–SO3H (ꢁ).
of 30% aq. H2O2 (0.3 ml, 3.0 mmol) in MeOH (5 ml) was added
dropwise under stirring during 30 min at rt. The mixture was con-
tinuously stirred for additional 5.5 h. The reaction mixture was then
diluted with H2O (10 ml) and extracted with Et2O (2× 15 ml). The
combined organic layers were washed with H2O (2× 10 ml) and
dried over Na2SO4. The solvents were evaporated under reduced
pressure affording the corresponding pure product after separation
on a TLC plate using a mixture of hexane/EtOAc 10:1.
with a BET surface area and total pore volume about half of the
other samples.
The thermal stability of these materials and the percentage
of anchoring of the organic group were determined by means
of TGA. Fig. 3 shows the TGA–DTG profile of SiO2–(CH2)3–SO3H,
MCM-41–(CH2)3–SO3H and SG–(CH2)3–SO3H samples. Desorption
of physisorbed solvent was observed below 100 ◦C accompanied
by an endothermic DSC peak (not shown) in all the synthesized
organofunctionalized samples. The MCM-41–(CH2)3–SO3H DTG
profile shows a large shoulder centered at about 360 ◦C probably
due to the decomposition of the supported propylmercaptane not
oxidized to propylsulfonic acid by treatment with H2O2 [51]. The
decomposition of the propylsulfonic acid groups seems to occur in
a large range of temperature as evidenced by the broad DTG peak,
which coincides with an exothermic DSC peak, centered at about
510 ◦C. Two-step weight loss, at 320 ◦C and 512 ◦C, both exother-
mic, was observed in the siliceous material prepared via sol–gel,
SG–(CH2)3–SO3H, the former due the decomposition of both the
organic compounds that are part of the silica network and the not
oxidized propylmercaptane, and the latter to the decomposition of
the propylsulfonic acid groups.
All the products gave melting points and spectral data consistent
with values previously reported in the literature [45–48].
3. Results and discussion
The catalysts were synthesized by tethering procedure on both
amorphous and mesoporous MCM-41 silica or by sol–gel technique
as reported in the literature [28,49] (Scheme 1).
The N2 adsorption–desorption isotherms of the functionalized
mesoporous silica materials are shown in Fig. 1. The isotherms of
SiO2–(CH2)3–SO3H and SiO2–(C6H4)–SO3H materials are classified
as type IV characteristic of the mesoporous materials [50]. Pore
size distribution of these samples (Fig. 2, Table 1), estimated by
employing the BJH method, indicates the existence of uniform large
mesopores. The MCM-41–(CH2)3–SO3H exhibits a reversible type
IV isotherm with a narrow hysteresis loop at p/p◦ > 0.4 and a sharp
pore filling step at ca.0.2 < p/p◦ < 0.3, characteristic of uniform pores.
The pore size distribution is very sharp and centered at 2.2 nm.
The total pore volume Vs of these materials (Table 1) calculated
from the N2 amount adsorbed at p/p◦ close to the saturation vapour
pressure (p/p◦ = 0.98) is almost the same value (about 0.6 cm3 g−1).
Very different is the N2 adsorption–desorption isotherm of the
SG–(CH2)3–SO3H sample. The material has a disordered porosity
From the TGA curve of the siliceous samples containing the
organic alkylsulfonic group –(CH2)3–SO3H (Fig. 3) was calculated
a weight loss, relative to the thermal decomposition of the teth-
ered groups, always higher than that obtainable from the titration
of the materials. The different data were ascribed to the incom-
plete oxidation of supported propylmercaptane to propylsulfonic
acid [52].
The
TGA–DTG
profiles
of
the
SiO2–(C6H4)–SO3H
organic–inorganic hybrid material (Fig. 4) clearly show that
most of the arylsulfonic acid groups remained anchored to the
Table 1
Physico-chemical parameters of the tested catalysts.
S.A.BET (m2 g−1
)
Surface acidity (meq H+/g)
Vs (cm3 g−1
b
)
Dp (nm)
a
c
Entry
Catalyst
1
2
3
4
5
6
7
SiO2
MCM-41
540
1240
510
445
340
1100
53
–
–
0.80
1.13
0.67
0.60
0.31
0.68
0.40
4.0
2.5
4.7
5.3
–
SiO2–(CH2)3–SO3H
SiO2–(C6H4)–SO3H
SG–(CH2)3–SO3H
MCM-41–(CH2)3–SO3H
Amberlyst-15
0.20
0.65
0.32
0.42
4.70
2.2
30.0
a
Specific BET surface area.
Specific pore volume at p/p◦ = 0.98.
Diameter of pores according to the maximum of the BJH pore size distribution.
b
c

R. Maggi et al. / Applied Catalysis A: General 411–412 (2012) 146–152
149
100
90
80
70
60
50
200
400
600
800
1000
200
400
600
800
1000
Temperature (°C)
Temperature (°C)
Fig. 3. Thermogravimetric (left) and differential thermogravimetric (right) curves of SiO2–(CH2)3–SO3H (solid line), MCM-41–(CH2)3–SO3H (dash-dot line) and
SG–(CH2)3–SO3H (short dash line) samples.
silica matrix up to 610 ◦C which indicate the higher thermal
stability of the –(C6H4)–SO3H groups tethered to SiO2 respect
to the –(CH2)3–SO3H ones [34]. From the TGA curve of the
SiO2–(C6H4)–SO3H sample (Fig. 4) was calculated a 9% (w/w)
weight loss relative to the thermal decomposition of the aryl-
sulfonic acid groups accompanied by an exothermic DSC peak.
In this case TGA results are quite in agreement with the surface
acidity data obtained from titration method probably due to the
different preparation approach. These results are in agreement
with previous studies carried out on aminopropyl silicas [53],
and reflect the high thermal stability of the materials also in an
oxidative environment.
Methylhydroquinone, hereafter 1a, has been utilized as model
reagent in a series of experiments aimed at achieving the optimum
conditions for the oxidation to methylbenzoquinone, hereafter 3a,
with 30% aqueous hydrogen peroxide catalyzed by all the prepared
catalysts and the commercially available Amberlyst-15. The exper-
iments were carried out at room temperature in different solvents
by adding dropwise aqueous hydrogen peroxide diluted in the same
solvent during 30 min. The reaction mixture was stirred for a total
time of 6 h. Results are reported in Table 2.
In the reactions carried out in methanol, methylbenzoquinone
3a has been obtained in good yields. The propylsulfonic acid teth-
ered onto both amorphous and mesoporous MCM-41 silica showed
optimum activities [SiO2–(CH2)3–SO3H: 79% yield, 92% selectiv-
ity; MCM–41–(CH2)3–SO3H: 78% yield, 90% selectivity], better in
terms of yield and selectivity to that obtained under homogeneous
Table 2
Reaction of methylhydroquinone with aqueous H2O2 in the presence of different
solid catalysts and solvents.a
.
Entry
Catalyst
1a/2 ratio
Solvent
3a yield
[Sel.]b (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
–
1/3
1/3
1/2c
1/3
1/2
1/3
1/2
1/3
1/2
1/3
1/2
1/3
1/3
1/3
1/3
1/3
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
Acetone
H2O
∼3
49 [50]
57 [93]
79 [92]d
55 [61]
79 [79]
32 [65]
53 [62]
58 [91]
78 [90]
51 [93]
60 [92]
71 [81]
10 [32]
80 [85]
37 [73]
100
98
96
94
92
90
88
86
CH3SO3H
SiO2–(CH2)3–SO3H
SiO2–(CH2)3–SO3H
SiO2–(C6H4)–SO3H
SiO2–(C6H4)–SO3H
SG–(CH2)3–SO3H
SG–(CH2)3–SO3H
MCM-41–(CH2)3–SO3H
MCM-41–(CH2)3–SO3H
Amberlyst-15
Amberlyst-15
SiO2–(CH2)3–SO3H
SiO2–(CH2)3–SO3H
SiO2–(CH2)3–SO3H
SiO2–(CH2)3–SO3H
MeCN
Dioxane
a
Catalyst: 0.02 mmol, 1a: 1 mmol, solvent: 2 ml, reaction time: 30 min addition
and 5.5 h reaction.
100 200 300 400 500 600 700 800 900 1000
Temperature(°C)
b
Determined by GC analysis.
When the reaction was performed with a stoichiometric amount of H2O2, prod-
c
uct 3a was obtained in a modest yield (41%).
d
When the reaction was performed on dark, product 3a was obtained with the
Fig. 4. Thermogravimetric (TGA – solid line) and differential thermogravimetric
same yield (78%).
(DTG – dotted line) curves of the SiO2–(C6H4)–SO3H sample.

150
R. Maggi et al. / Applied Catalysis A: General 411–412 (2012) 146–152
100
80
60
40
20
0
0.10
0.08
0.06
3a Yield
0.04
0.02
0.00
before
after
0
0.002
0.005
0.01
0.02
0.04
SiO2-(CH2)3-SO3H/1a molar ratio
Fig. 5. Effect of the catalyst amount in the model reaction performed in methanol
for 6 h at room temperature.
2
4
6
8
10
12
14
16
18
20
Pore Diameter (nm)
Fig. 6. Pore size distribution (nm) of the functionalized mesoporous silica materials
SiO2–(CH2)3–SO3H before and after catalytic cycle.
conditions performing the reaction with methanesulfonic acid [49%
yield, 50% selectivity]; also the phenylsulfonic acid tethered onto
silica [SiO2–(C6H4)–SO3H] gave product 3a in good yield, but the
selectivity was somewhat lower (79% yield, 79% selectivity).
SG–(CH2)3–SO3H, prepared by sol–gel method, gave product 3a
in a lower yield (53%) and selectivity (62%); these results could be
attributable to the method utilized in the catalyst preparation. In
fact, in the material prepared by sol–gel technique it is expected
that variable amounts of the sulfonic acid are incorporated within
the siliceous framework and it appears that the sulfonic acid con-
tent, measured by the titration method [44], does not necessarily
correspond to the same amount of catalytically active acid sites,
since the access to these sites could be reduced or even hampered
by the shape of the solid matrix.1 Moreover, the micropore struc-
ture of this material can hamper the reagent diffusion inside the
catalyst.
increasing SiO2–(CH2)3–SO3H amount the yield of product 3a did
not show significant variations.
The efficiency of hydrogen peroxide utilization in the model
reaction, determined by evaluating the oxygen evolution after addi-
tion to the reaction crude of a suspension of manganese(IV) oxide,
was found to be 52%.
The possible leaching of active species into solution and coexis-
tence of homogeneous and heterogeneous catalysis was excluded
by performing the Sheldon test [55]: the catalyst was removed by
filtration after 1 h when the methylbenzoquinone yield was 34%.
Any yield increase was observed by keeping at room temperature
the filtrate for further 5 h.
The reusability of SiO2–(CH2)3–SO3H has then been checked in
the model reaction. The catalyst, simply recovered by filtration and
washed with hot methanol, can be reused two times, without fur-
ther activation, giving product 3a with the following yields and
selectivities: 1st run 79% (92%); 2nd run 76% (91%); 3rd run 82%
(90%). It is important to underline that after the catalytic runs the
catalyst maintained its textural characteristics: the BET surface area
and the pore size distribution (Fig. 6) of the material substantially
did not change and no loss of specific pore volume was registered.
A variety of functionalized quinones were then prepared fol-
lowing the general optimized methodology. Synthetic results are
described in Table 3.
Amberlyst-15, utilized after water removal by heating at 100 ◦C
for 24 h, gave product 3a in good yield (60%), however lower than
that achieved with SiO2–(CH2)3–SO3H; this result could be ascribed
to both the modest surface area and the hydrophobic nature of
the polystyrenic support that could hamper the contact with the
hydrophilic reagents.
Results from Table 2 also suggest that methanol was the solvent
of choice in terms of yield, selectivity and solubility of both start-
ing materials and products. The modest results achieved when the
reaction was performed in acetone could be ascribed to the compet-
itive production of the less reactive dimethyl dioxirane by reaction
of hydrogen peroxide with acetone [54]. Similarly, the competitive
interaction of the acid catalyst with the oxygen atoms of the solvent
via H-bond could account for the surprisingly low activity observed
when the reaction was performed in 1,4-dioxane.
The process gave quinones with satisfactory to good yields
and selectivities starting from hydroquinones bearing both
Table 3
Reaction of various hydroquinones with aqueous H2O2.a
The successive studies were performed with the best cat-
alyst [SiO2–(CH2)3–SO3H] in methanol with
a methylhydro-
quinone/hydrogen peroxide molar ratio 1/3. The effect of the
catalyst amount was first evaluated in the model reaction per-
formed for 6 h at room temperature. In Fig. 5 the yield of product
3a is plotted versus the catalyst amount.
In the absence of catalyst, product 3a was obtained only in
traces (∼3%). However, the 3a yield was dramatically increased
to 45% when only 0.01 g of SiO2–(CH2)3–SO3H (corresponding
to 0.002 catalyst/1a molar ratio) was utilized. The maximum
performance was achieved when the SiO2–(CH2)3–SO3H/1a molar
ratio was 0.02, being product 3a obtained in 79% yield. By further
.
Entry
R1
R2
R3
Benzoquinone
Yield [Sel.]b (%)
1
2
3
4
5
6
Me
H
H
H
H
H
Me
OMe
H
H
H
H
Me
OMe
3a
3b
3c
3d
3e
3f
79 [92]
80 [90]
44 [99]
85 [99]
73 [78]
53 [67]
But
Cl
Me
H
a
Catalyst: 0.02 mmol, 1: 1 mmol, MeOH: 2 ml, 30% aq. H2O2: 3 mmol in MeOH
1
5 ml, reaction time: 30 min addition and 5.5 h reaction.
The acid base titration method requires a greatly prolonged exchange (∼24 h)
b
Determined by GC analysis.
with aq. NaCl to completely release the protic acidity from the solid.

R. Maggi et al. / Applied Catalysis A: General 411–412 (2012) 146–152
151
Scheme 2. Possible ionic reaction mechanism.
electronwithdrawing and electronreleasing groups. It is interesting
to note that very good selectivities are obtained with hydroquinone,
methylhydroquinone and chlorohydroquinone (Table 3, entries 1, 2
and 4); on the contrary, when the reaction is performed with hydro-
quinones bearing strong electronreleasing groups (entries 5 and 6),
a drop in both yield and selectivity is observed, due to high reac-
tivity of the starting materials that is responsible of the formation
of overoxidation products.
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line that a possible radical mechanism contribution [57] cannot
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observed (reaction carried out in the absence of BHT: 79% yield, 92%
selectivity; reaction carried out in the presence of BHT: 52% yield,
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In conclusion we have found that sulfonic acids, tethered onto
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Acknowledgments
The authors acknowledge the support of the Ministero
dell’Università e della Ricerca (MIUR), Italy, and the University of
Parma (National Project “Attivazione ossidativa catalitica e foto-
catalitica per la sintesi organica”). The Centro Interdipartimentale
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