Oxidation of alkenes to 1,2-diols: FT-IR and UV studies of silica-supported sulfonic acid catalysts and their interaction with H2O and H 2O2

Article abstract of DOI:10.1016/j.jcat.2012.06.014

Supported sulfonic acids can be employed as efficient catalysts in the dihydroxylation of 1-methylcyclohexene with aqueous hydrogen peroxide, without the use of additional solvents, under mild condition. The reaction was deeply studied in terms of catalys

Full text of DOI:10.1016/j.jcat.2012.06.014

Journal of Catalysis 294 (2012) 19–28
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Oxidation of alkenes to 1,2-diols: FT-IR and UV studies of silica-supported
sulfonic acid catalysts and their interaction with H O and H O
2
2 2
a,
⇑
b
a
b
Raimondo Maggi , Gianmario Martra , Calogero Giancarlo Piscopo , Gabriele Alberto ,
a
Giovanni Sartori
a
‘
‘Clean Synthetic Methodology Group’’, Dipartimento di Chimica Organica e Industriale dell’Università, and Consorzio Interuniversitario ‘La Chimica per l’Ambiente’ (INCA),
UdR PR2, Parco Area delle Scienze 17A, I-43124 Parma, Italy
b
Dipartimento di Chimica & Centro Interdipartimentale di Eccellenza ‘‘Nanostructured Interfaces and Surfaces – NIS’’, Università degli Studi di Torino, Via P. Giuria 7, I-10125
Torino, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Supported sulfonic acids can be employed as efficient catalysts in the dihydroxylation of 1-methylcyclo-
hexene with aqueous hydrogen peroxide, without the use of additional solvents, under mild condition.
The reaction was deeply studied in terms of catalyst, H O /1-methylcyclohexene molar ratio, and per-
2 2
centage of aqueous hydrogen peroxide to achieve the best reaction conditions.
Experimental results, catalytic efficiency, and spectroscopy data allow to advance some hypothesis on
the reaction mechanism, based on the interaction, likely via H-bond, of hydrogen peroxide with the sul-
fate groups resulting from the deprotonation of supported sulfonic acids.
Ó 2012 Elsevier Inc. All rights reserved.
Received 26 March 2012
Revised 19 June 2012
Accepted 20 June 2012
Available online 9 August 2012
Keywords:
Alkene dihydroxylation
Hydrogen peroxide
Supported catalysts
Sulfonic acids
In situ IR spectroscopy
DR UV–Vis spectroscopy
1
. Introduction
Hydrogen peroxide is an ideal oxidant for many reasons: it is
[18–21] and Baeyer–Villiger (BV) oxidation of ketones to esters
or lactones [22–26]. In all these cases, the reaction involves an
electrophilic activation of hydrogen peroxide; the same mecha-
nism is reported with fluorinated solvents, as it was clearly shown
by Sheldon and Berkessel [27,28].
With regard to the crucial role of peracids added to the reaction
mixture or produced in situ from carboxylic acids, many studies
are reported in the literature [29]; however, the role of different
acids such as sulfonic, arsonic, and phosphonic acids showing a
similar behavior is not as much studied and needs further deepen-
ing. For example, supported sulfonic acids were reported to be
good heterogeneous and reusable catalysts for the BV oxidation
of cyclopentanone [30], and more recently, Sato reported a very
efficient procedure for the solvent-free dihydroxylation of alkenes
with hydrogen peroxide in the presence of Nafion resin [31].
1,2-Diols are important intermediates for the pinacol rearrange-
ment [32] and for the preparation of more sophisticated synthons
cheap, its efficiency is excellent, and water is theoretically the sole
by-product [1–3]. The activation of aqueous hydrogen peroxide is a
challenge that has attracted many research groups in order to per-
form oxidation reactions, under environmentally friendly condi-
tions [4,5].
From a mechanistic point of view, the activation of hydrogen
peroxide can take place in three ways [6]. First, in the presence
of metal catalysts, according to the well-known Haber–Weiss reac-
tion, involving the hydroxyl radical, that frequently leads to unse-
lective processes in organic synthesis [7]; second, in the presence
ꢀ
of a base, through the formation of the strong HOO nucleophile,
that can oxidize, for example, electrophilic alkenes [1]; third,
according to the heterolytic oxidation process, that is probably
the most important reaction from a synthetic point of view [8].
In the presence of catalysts containing different transition met-
als such as Ti, W, Mn, Re and Sn [9–13], hydrogen peroxide under-
goes efficient oxidation reactions, for example, alkene epoxidation
such as
a-hydroxyketones and dioxanes [33,34] and are also pre-
pared to be utilized in perfumery and fragrance industry, for the
manufacture of cosmetics and for the synthesis of commercial
products such as photographic materials and lubricants and in
drugs and foods [31].
[
14–17], conversion of sulfides into sulfoxides and sulfones
Usually, 1,2-diols are synthesized through pinacol coupling
reaction [35] or better from direct alkene oxidation [36,37] or from
epoxides opening [38].
⇑
Corresponding author. Fax: +39 0521 905472.
E-mail address: raimondo.maggi@unipr.it (R. Maggi).
0
021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.06.014

2
0
R. Maggi et al. / Journal of Catalysis 294 (2012) 19–28
2
N adsorption–desorption isotherms, obtained at 77 K on a
Micromeritics PulseChemiSorb 2705, were used to determine spe-
cific surface areas (SABET), while pore volume and size were calcu-
lated by using the BJH model. Before measurements, all samples
ꢀ4
were outgassed at 383 K until a residual pressure 1.0 ꢁ 10 Torr
(
1 Torr: 133.33 Pa).
The surface acidity and surface area of all catalysts were deter-
mined by a reported titration method [44] and by the BET method
45], respectively (Table 1).
[
Scheme 1. Possible products derived from 1-methylcyclohexene oxidation.
2.2. Catalyst preparation
Despite the numerous examples reported in the literature con-
The catalysts were synthesized by tethering procedure as re-
cerning the catalytic dihydroxylation of olefins with hydrogen per-
oxide [39–41], the selectivity toward 1,2-diols is usually around
ported in the literature [46] (Fig. 1). All the catalysts were carefully
washed with acetone (3 ꢁ 20 mL) and water (3 ꢁ 50 mL) and dried
for 24 h at 100 °C before use.
9
0%, due to the formation of epoxides, alcohols, ketones, and/or
ethers as by-products (Scheme 1).
Moreover, in most of the reported synthetic methods, the use of
chlorohydrocarbons or other organic solvents is required. For these
reasons, in continuation of our investigation on the preparation,
characterization and use of organic and organometallic catalysts
supported onto different materials [42,43,26], we have evaluated
the activity of some silica-supported sulfonic acids as heteroge-
neous catalysts for the dihydroxylation of 1-methylcyclohexene
with hydrogen peroxide. In addition, molecular features of the sup-
2
.2.1. Supported arylsulfonic acids [46]
Silica (8.0 g) was refluxed under stirring for 24 h with phenyltri-
ethoxysilane (2.0 mL, 8.3 mmol) or phenylethyltrimethoxysilane
1.8 mL, 8.3 mmol) in toluene (120 mL), filtered, and washed with
(
toluene. The supported phenyl groups were then sulfonated by
refluxing the functionalized material with chlorosulfonic acid
(
(
10 mL, 150 mmol) under stirring for 4 h in 1,2-dichloroethane
60 mL). The solid material was then recovered by filtration and
ported acids and of their interaction with H
gated by IR spectroscopy in controlled atmosphere and DR UV–Vis
spectroscopy of the catalysts in contact with H /H O solutions.
2 2
O have been investi-
carefully washed with 1,2-dichloroethane (3 ꢁ 20 mL), acetone
(
3 ꢁ 20 mL), and water (3 ꢁ 50 mL). The catalysts derived from sul-
2
O
2
2
fonation of the materials obtained from tethering to silica phenyl-
triethoxysilane and phenylethyltrimethoxysilane will be hereafter
referred to as catalyst A and C, respectively (Fig. 1).
2
. Experimental
.1. Materials
Toluene, ethanol, acetonitrile, acetone, 1,2-dichloroethane,
A new method was developed for the preparation of the silica-
supported 4-propoxybenzensulfonic acid (hereafter referred to as
catalyst B). A mixture of amorphous silica gel (2.0 g) and BPTS
2
(
0.76 mL, 4.0 mmol) in toluene (80 mL) was refluxed, under stir-
ring, for 24 h. The resulting silica-supported 3-bromopropane
was recovered by filtration and washed with toluene (3 ꢁ 50 mL).
A mixture of this material (2.00 g) and sodium phenoxide (0.60 g,
DMF, 1-methylcyclohexene, bromobenzene, phenyltriethoxysi-
lane, phenylethyltrimethoxysilane, 3-bromopropyltrimethoxysi-
lane (BPTS), chlorosulfonic acid, hydrogen peroxide 30% and 50%
and manganese(II) oxide were purchased in the highest purity
available and used without further purification. Methanol was dis-
tilled before use.
H O (bidistilled) and D O (Euriso-top; 99.90%D) were purified in
2 2
vacuo and rendered gas-free by several ‘‘freeze–pump-thaw’’ cy-
cles to be employed in the IR spectroscopic measurements. The
same was done for a H O /H O solution (Sigma–Aldrich; P30%;
2 2 2
6
.0 mmol) in DMF (100 mL) was heated to 100 °C under stirring
for 24 h. Afterward, the material was filtered off and washed with
DMF (3 ꢁ 20 mL) and acetone (3 ꢁ 20 mL). Finally, a mixture of this
material (2.0 g) and chlorosulfonic acid (4 mL, 60 mmol) in 1,2-
dichloroethane (60 mL) was stirred for 4 h at reflux. The catalyst
was recovered by filtration and washed with 1,2-dichloroethane
(
3 ꢁ 20 mL), acetone (3 ꢁ 20 mL), and water (3 ꢁ 50 mL).
Trace SELECT) that was also used to prepare the slurries for DR
UV–Vis spectroscopic investigations.
2.3. Catalyst characterization
Gas-chromatographic analyses were accomplished on a Trace-
GC ThermoFinnigan instrument with a fused silica capillary col-
umn SPB-20 from Supelco (30 m ꢁ 0.25 mm).
Infrared spectra of the catalysts were performed in transmission
mode on powders pressed in self-supporting pellets (‘‘optical
Table 1
Codes, textural features (SSABET, porosity) of the materials, and catalytic efficiency of the supported sulfonic acids in the dihydroxylation of 1-methylcyclohexene.a
2
Total pore volumeb (cm /g)
3
Average pore widthb (nm)
+
Yield (Sel.)c (%)
Entry
Material
SiO
Catalyst A
Catalyst B
Catalyst C
Amberlyst-15
Specific surface area (m /g)
Surf. acidity (mmol H /g)
1
2
3
4
5
2
450
440
415
300
53
0.88
0.87
0.80
0.43
–
5.8
5.8
5.3
4.2
–
–
–
80 [94]
55 [93]
90 [96]
55 [93]
0.65
0.70
1.20
4.70
a
b
c
Reaction conditions: 1-methylcyclohexene (0.5 mL, 4.2 mmol), 30% aq H
Measured on the desorption branch.
Determined by GC analysis.
2 2
O (0.86 mL, 8.4 mmol), 70 °C, 22 h.

R. Maggi et al. / Journal of Catalysis 294 (2012) 19–28
21
lows: a 10-mL round-bottomed flask equipped with a magnetic
stirring bar and reflux condenser was charged with the selected so-
lid catalyst (2% mol with respect to 1-methylcyclohexene) and 30%
aqueous H O (0.86 mL, 8.4 mmol). The mixture was stirred at rt
2 2
for 10 min, after which 1-methylcyclohexene (0.50 mL, 4.2 mmol)
was added. The mixture was heated at 70 °C under stirring for
2
2 h and then cooled to rt. The catalyst was removed by filtration
and washed with methanol (3 ꢁ 5 mL); MnO
2
(ca. 100 mg) was
added to the solution to destroy any hydrogen peroxide excess.
Subsequently, bromobenzene was added as standard, and the mix-
ture was diluted with methanol and analyzed by GC.
3
. Results and discussion
3.1. Catalyst characterization
Fig. 2 shows the IR spectra of the bare silica and of the three
SiO -supported catalysts recorded after outgassing at beam tem-
2
Fig. 1. The three supported benzenesulfonic acid catalysts prepared.
perature (ca. 50 °C) for 1 h.
Such treatment resulted in the complete desorption of water
thickness’’ in the 8–10 mg cm 2 range) and placed in a quartz IR
ꢀ
molecules initially adsorbed from air moisture, as witnessed by
cell equipped with KBr windows. The IR cell was connected to a
ꢀ1
the essential invariance of the signal at ca. 1650 cm after subse-
ꢀ5
conventional vacuum line (residual pressure p 6 10 Torr), allow-
ing adsorption/desorption experiments to be carried out in situ. A
Bruker Vector 22 spectrometer was employed for spectra collec-
tion in the Mid-IR region, and a Jasco 6100 one for selected mea-
surements extended to the Near-IR. In both cases, resolution was
cm , and a DTGS detector was employed.
All spectra were baseline corrected to remove the contribution
of light scattering and were normalized with respect to the inten-
sity of the signals at 1980 and 1870 cm due to the combinations
of bulk framework modes [47] in order to render differences in
intensity independent of differences in the thickness of the pellets.
Mid-IR spectra are reported down to 1250 cm , that is the on-
set of the cut-off due to the fundamental absorptions of the sili-
ceous lattice, that rendered opaque the samples at lower
frequency, except for a narrow region (950–850 cm , the so-
called ‘‘silica window’’). However, pellets of the catalysts appeared
too fragile to be produced so thin to exhibit a reasonable transpar-
ency in such range.
Diffuse reflectance (DR) UV–Vis spectroscopy measurements
were performed with a Cary 5000 spectrophotometer, equipped
d
quent exchange with D
2
O (dotted line s ). Actually, adsorbed water
molecules are expected to contribute to that IR band through their
deformation (d) mode, and this contribution should be depleted
after exchange with D
ring at ca. 1200 cm (below the cut-off of the sample).
2
O and substituted by the dD
2
O signal occur-
ꢀ1
ꢀ1
4
2
Focusing on the spectral pattern of the bare SiO support (curve
s), on the basis of well assessed literature data [47], the various
components can be assigned as follows
ꢀ1
ꢀ1
ꢂ narrow peak at 3740 cm : silanols in weak (van der Waals)
interaction with neighbors Si–OH;
ꢀ1
ꢀ1
ꢂ shoulder at 3710 cm : terminal dangling OH of chains of inter-
acting silanols;
ꢀ1
ꢂ subband at 3660 cm : outer/inner silanols involved in weak
ꢀ1
hydrogen bonds;
ꢀ1
ꢂ broad band at 3530 cm : silanols involved in stronger hydro-
gen bonds;
ꢀ1
ꢂ signals at 1980, 1870, and 1650 cm : combinations (the first
two) and overtone (the third one) of fundamental silica vibra-
tional modes occurring at lower frequency.
Ò
with an integrating sphere internally coated with Spectralon .
The same material was used as reference for the collection of the
background baseline. The samples, in the form of powder, were
put on a quartz optical window that was the bottom of a cylindrical
cell, closed by screwing a piston, pressed toward the powder by a
spring. The amount of sample was dosed in order to form a layer
ca. 3 mm thick, to extinguish the radiation path within the powder
[
48]. The samples were investigated both in air and in the form of
concentrated slurries, obtained by dropping H O or H /H O solu-
2
2
O
2
2
tion in proper amounts to obtain an incipient wetness of the pow-
ders. In all cases, the materials were diluted in a 1:30 ratio by
weight with highly pure amorphous silica (AOX50, Aerosil), in order
to attain the best compromise between the location above the 0% of
reflectance of the most intense signals and a satisfactory signal/
noise ratio for the weakest ones. The conversion with the Kub-
elka–Munk function was then attempted, but no longer considered,
as the most intense signals exhibited values of the function signif-
icantly higher than 1 (for the samples in the form of aqueous slur-
ries), then outside the limits of applicability of such algorithm [48].
2.4. Catalytic tests
Fig. 2. IR spectra of samples outgassed at beam temperature (ca. 50 °C) for 1 h:(s)
The catalytic tests were performed by procedures reported in
d
bare silica; (s ) bare silica after exchange with D
2
O; (a) catalyst A; (b) catalyst B; (c)
the notes of table and figures. The general procedure was as fol-
catalyst C.

2
2
R. Maggi et al. / Journal of Catalysis 294 (2012) 19–28
As expected, the last three signals remained unaffected by the
tethering of the organic moieties (curves a–c), whereas the peak
No specific signals due to Ar–Si, Ar–C, Ar–O bonds were ob-
served, because of the limit of transparency of the self-supported
pellet employed in this study [53].
ꢀ1
at 3740 cm decreased in intensity, until depletion, as the amount
of supported acids increases, and the series of absorptions at lower
frequency were overcame by progressively more intense, broad,
3.1.2. Bands due to the sulfonic group
ꢀ1
and complex pattern spread over the 3700–2000 cm range. It is
worthy to note that silanols present on the surface of the support
can be involved in two kinds of events as a consequence of the
functionalization with sulfonic acid molecules: (i) consumption
by reaction with the alcoxysilanes; (ii) H-bond interaction with
the sulfonic groups, with related frequency downshift and increase
in intensity of the „SiO–H stretching band.
The low-frequency region of the spectra of the three catalysts is
ꢀ1
dominated by a broad band in the 1390–1325 cm range, assign-
able to the antisymmetric stretching mode of the O@S@O moiety
[57]. Actually, the maximum of this band appeared located at ca.
ꢀ
1
ꢀ1
1370 cm for catalyst A (curve a) and at 1355 cm for catalyst
C (curve c). This is the maximum position also in the case of cata-
ꢀ
1
lyst B, with a poorly resolved shoulder at ca. 1370 cm (curve b).
The overall trend suggests that the (O@S@O)asym mode monitors
Of course, the other consequence of the functionalization of the
m
ꢀ1
support was the appearance of a series of bands below 1650 cm
.
the interaction of the sulfonic group with at least two types of local
For the sake of clarity, the assignment of these signals will be re-
ported separately for the alkyl + aryl parts and for the sulfonic
group.
environment. As for sulfuryl symmetric stretching mode, it should
produce a band at ca. 1100 cm , well below the limit of transpar-
ency of the samples [57].
ꢀ1
However, the information on the sulfonic groups is extended by
the presence of the spectral features due to the SO–H stretching
mode, occurring in the high-frequency region. Focussing on the
comparison between the bare support (curve s) and catalyst A
(curve a), it can be noticed that as a consequence of the surface
functionalization, besides the decrease in intensity of the sharp
3
.1.1. Bands due to alkyl + aryl parts
All catalysts (curves a–c) exhibited bands at ca. 1600 and
ꢀ1
1
410 cm , which should be related to 8a and 19b modes (in the
Wilson notation [49–51]) respectively, of the aromatic ring (Ar,
in the following) [52,53a,54]. Their 8b and 19a partner modes, ex-
pected to produce significantly weaker absorptions [55,56], should
ꢀ1
peak at 3740 cm , the rest of the mOH pattern is constituted by
ꢀ1
be then responsible for the components at 1580 cm (8b mode) in
an asymmetric absorption with maximum similar in position
ꢀ1
ꢀ1
the spectrum of catalyst C (curve c) and at ca. 1490 cm (19a
mode) in the spectra of catalysts B and C [52,53a]. Noticeably, in
the case of the catalyst A, the 8a band appeared significantly weak-
er in intensity than expected with respect to the other two cata-
lysts on the basis of the relative amount of supported aromatic
species. It can be considered that this band arises from a vibration
in which the main dipole-moment change is produced by the
movements of the substituents on opposite sides of the ring acting
in opposition, and then in the presence of one polar substituent or
of two substituents with different polarity, they should produce
quite intense band [55].
(3510 cm ) and intensity of that present for the bare silica
ꢀ1
ꢀ1
(3530 cm ), and a tail extended down to 2800 cm . This absorp-
tion should result from the superposition of subbands due to O-H
stretching of sulfonic groups and silanols. The appearance of the
low-frequency tail suggests the occurrence of an H-bond interac-
tion between SO–H (expected to act as an H-bond donor) and SiOH
(expected to act as an H-bond acceptor). However, the supported
acid molecules synthesized starting from phenyltriethoxysilane
should be oriented with the sulfuric group pointing away from
the support. Thus, the supposed interaction might involve acid
molecular moieties formed in pores of the silica support, where sil-
anols properly located on the facing pore wall could be present. Of
course the presence of sulfonic groups involved in weaker H-bond
interaction cannot be excluded, and, if present, they should con-
tribute to the part of the asymmetric band close to the maximum
Actually, in catalyst A, the substituents on the aromatic ring are
sulfonic and –Si(OSi)
polarity. However, the –Si(OSi)
support by O–Si bonds. The consequent possible coupling along
the resulting (Ar)–Si–(OSi) –(support) oscillators could affect the
displacement of the Si atom bound to the aromatic ring, with a
consequent effect on the dipole-moment change. The same argu-
ment should hold for the 19a mode, in which the dipole-moment
change is produced by the movements of the substituents on oppo-
site sides of the ring, but moving in the same direction.
3
– groups, exhibiting a significant different
3
– is in turn directly linked to the
ꢀ1
3
at 3510 cm . Vice versa, significant amount of sulfonic groups not
involved in any interaction should be negligible, as, on the basis of
spectra of species entrapped in inert matrices [58] and of theoret-
ꢀ
1
ical calculations [59], they should absorb at ca. 3560 cm
.
ꢀ1
Focussing on catalyst B, the 3740 cm peak appeared slightly
less intense, whereas the band at lower frequency, with maximum
ꢀ1
Furthermore, in the case of catalyst B, a weak and broader com-
now located at 3460 cm , exhibited a significant increase in inten-
sity, accompanied by the appearance of a shoulder at ca.
ꢀ
1
ponent is present at 1630 cm , which can be due to the overtone
of the aromatic 10a CH mode [52], the fundamental producing the
strong band, expected around 815 cm , characteristic of para-
disubstituted benzene rings [53a]. As for the C–H stretching
modes, they are responsible for the weak features in the 3100–
ꢀ1
ꢀ1
c
3250 cm and a partly resolved subband at ca. 2400 cm , and a
further broadening toward lower frequency, tailing down to ca.
ꢀ1
ꢀ
1
2100 cm . Such behavior monitors the occurrence of stronger H-
bond interactions, and the pattern appears typical of the occur-
rence of a Fermi resonance between overtones of low-frequency
bending modes, with the continuous distribution of levels resulting
ꢀ
1
3
000 cm range.
The methylene groups of the alkyl links connecting the aro-
matic rings of catalysts B and C to the support produced the typical
series of bands expected for their stretching modes in the 3000–
from the coupling of
m
(A–Hꢃ ꢃ ꢃ) and (AHꢃ ꢃ ꢃB) modes (where A and
m
B represent H-donor and H-acceptor groups, respectively) [60,61].
In the present case, the role of H-donor is played by the sulfuric
groups, whereas the oxygen atoms of surface silanols can act as
H-acceptor.
Furthermore, it must be considered that in the present case, the
H-acceptor groups (the silanols) possess an internal mode, the mOH
ꢀ1
2
(
850 cm range, with the
m
asym mode splitted in two components
at 2940 and 2900 cm in the spectrum of catalyst C, curve c) by
Fermi resonance with the overtone of the deformation mode, then
ꢀ1
ꢀ
1
expected around 1460 cm . Actually, weak bands are present at
ꢀ
1
1
470 and 1475 cm
for catalyst B (curve b) and C (curve c),
respectively. In particular, in the case of catalyst C, the weak com-
one, that absorbs in the same region, and can be involved in the
coupling. This effect should be responsible for the increase in
intensity of the high frequency part of the complex absorption,
near the maximum at 3510 cm , related to less perturbed oscilla-
tors. Indeed, such intensification cannot be ascribed to a higher
ꢀ1
2
ponent at 1475 cm should be due to the –CH – group linked to
the support, and the other, linked to the aromatic ring, should pro-
ꢀ1
ꢀ1
duce an absorption near 1420 cm , likely contributing to the on-
set of the 19b aromatic mode band.

R. Maggi et al. / Journal of Catalysis 294 (2012) 19–28
23
amount of silanols interacting each other, as, conversely, „Si–OH
should have been consumed in a slightly larger amount during
the synthesis of the supported sulfonic acid molecules (by assum-
ing the same Si–OH/alcoxide stoichiometry for both phenyltrieth-
oxy- and phenylethyltrimethoxysilane), slightly more numerous
in catalyst B than in catalyst A (see Table 1). The stronger interac-
tion between sulfonic groups and silanols should be related to pos-
sible orientations of the substituted aromatic ring different from
the normal to the surface, because the link to the support through
the alkyl moiety, with rotational degree of freedom around the C-C
bonds.
ꢀ1
Furthermore, the weak feature at ca. 2400 cm is similar in po-
sition to the low-frequency component, with a more intense part-
ꢀ1
ner at ca. 2900 cm , of the mOH pattern obtained for dehydrated
H-NAFION membranes [62], with sulfonic groups interacting each
other via H-bonding. Actually, the increase in intensity of this com-
ponent and the more clear presence of a broad subband at ca.
ꢀ
1
3
000 cm are the distinctive features of the mOH pattern of cata-
lyst C (curve c) that contains ca. 40% additional supported acid
moieties than catalyst B (see Table 1). Other consequences of such
increased content in supported molecules are the following: (i) the
Fig. 3. IR spectra of samples outgassed beam temperature (ca. 50 °C) for 1 h (curves
ꢀ1
0 0
2
x, the same as in Fig. 1) and in presence of 20 mbar of H O vapor (curves x ): (s,s )
depletion of the 3740 cm peak, indicating that all isolated sila-
nols have been chemically consumed and/or involved in interac-
tion with supported species and (ii) the appearance of sharp
0
0
0
bare silica; (a,a ) catalyst A; (b,b ) catalyst B; (c,c ) catalyst C. The maximum of the
main absorption in the high-frequency region was out of scale.
ꢀ1
minima at 3300 and 3200 cm , which might be narrow Evans
windows resulting from Fermi resonance between the levels
same occurred for the materials in contact with air (as some of
the UV–Vis measurements, see below), with the consequent for-
mation of liquid-like molecular layers by adsorption of air mois-
ture (Fig. SI-1 in the SI).
The deprotonation of sulfonic groups is quite relevant for the
proposal of the nature of catalytically active species. Indeed, as
indicated in the Introduction, supported monoperoxysulfate spe-
cies were hypothesized to be active in the selective oxidation of
cyclohexene to 1,2-cyclohexanediol [31]. However, the formation
of S–O–O–H groups by reaction between sulfuric/sulfonic groups
resulting from the coupling of
see above), and the overtones and combinations of internal modes
of the base B involved in the H-bond (in this case, the aromatic sul-
m
(A–Hꢃ ꢃ ꢃ) and
m
(AHꢃ ꢃ ꢃB) modes
(
ꢀ1
fate, exhibiting ring modes in the 1650–1550 cm ), as observed in
pyridine/acid zeolites systems [63].
Finally, it can be noticed that the difference in the interaction of
sulfonic groups with different neighbors (silanols and/or other sul-
fonic groups) should affect, other than the SO–H band, also the
S@O ones, and this could partly account for the difference in posi-
tion and shape of the antisymmetric O@S@O stretching band ob-
served for the three catalysts (see above).
2 2
and H O requires the presence of a S-O-H moiety, as for instance
in the case of sulfuric acid [66–68]:
O ! HSOꢀ4 þ H
þ
3.2. Interaction of catalysts with H
2
O
H
2
SO
4
þ H
2
3
O
ꢀ
To probe the acid behavior of the supported species, the cata-
lysts (and the bare support, for the sake of comparison), where
contacted with 20 mbar of H O vapor, with the consequent forma-
tion of adsorbed multilayers of molecular water [64] and possible
proton transfer from sulfonic groups to H O molecules (Fig. 3).
H O ! 2OH
2
2
ꢀ
ꢀ
ꢀ
ꢀ
2
HSO4 þ OH ! SO þ OH
4
ꢀ
ꢀ
2
SO þ OH ! SO
4
OH
4
The admission of water on the bare silica (Fig. 3, s,s’) simply re-
Conversely, IR data demonstrated the occurrence of a complete
deprotonation of supported sulfonic acids when in contact with
an aqueous environment.
sulted in the appearance of the bands due to the deformation (d, at
ꢀ
1
ꢀ1
1
630 cm ) and stretching modes (3730–2800 cm range, maxi-
mum out of scale), these latter overimposed to the silanols pattern.
As expected, the interaction with H O molecules heavily affected
2
ꢀ1
the peak at 3740 cm , as the related groups are now interacting
via H-bond with adsorbed species. Nevertheless, a small portion
of such signal with maximum at 3744 cm still remained, indicat-
ing that some of the surface silanols are so widely distributed to
form patches not hydrophilic enough for the adsorption of water
3.3. Interaction of catalysts with H
2 2 2
O /H O
ꢀ
1
Similar IR measurements were carried out by admitting the va-
por (20 mbar) of a H
samples.
2 2 2 2 2
O/H O mixture (30% vol H O ) onto the
[
65].
In the case of catalysts (Fig. 3, a,a –c,c ), the additional conse-
quences of the interaction with water were the depletion of the
No specific features related to the interaction of hydrogen per-
oxide with the species supported on the surface of catalysts were
observed (Figure SI-2 in the SI and related comment) owing to
0
0
ꢀ1
sulfuryl signal in the 1390–1325 cm range, and the appearance
of a continuous absorption extended from 3500 to 1700 cm . This
latter is a kind of ‘‘fingerprint’’ of the formation of H (H O) clus-
2 n
the overwhelming contribution of H
of the H –surface species adducts.
Conversely, some information was obtained from the UV–Vis
2
O, and, likely, the weakness
ꢀ1
2 2
O
+
ters [63] and then probes the transfer of protons to the aqueous
spectra of the systems contacted with H
tion (Fig. 4).
2 2 2 2
O and the H O/H O solu-
ꢀ
layers from -SO
3
H groups, converted into -SO , that are expected
3
to absorb below the transparency limit of the samples [63].
Thus, it can be stated that the contact with an aqueous medium
results in a complete deprotonation of the sulfonic groups. The
As a first step, the electronic spectra of the materials in the form
of an aqueous slurry were collected (Fig. 4, panel A). The bare silica
exhibited a continuous absorption, progressively increasing in

2
4
R. Maggi et al. / Journal of Catalysis 294 (2012) 19–28
Fig. 4. DR UV–Vis spectra of slurries obtained by wetting the samples with different aqueous media. Panel A, samples in contact with H
2
O: (s) bare silica used as support; (s0)
0
highly pure silica used as diluent; (a) catalyst A; (b) catalyst B; (c) catalyst C. Panel A : samples in contact with H
2
O
2
/H
2
O solution: (s) bare silica; (a) catalyst A; (b) catalyst B;
/H O (the
same as in panel A) are compared with the spectrum of the bare silica support in contact with the same medium (curves s , the same as in panel A ) and with the spectra (a),
b) and (c) of catalyst the catalysts A, B, and C, respectively, in contact with H O (the same as in panel A).
0
0
0
(
c) catalyst C. Inset, zoomed view of the 350–500 nm region, where the spectra (a ), (b ), and (c ) of the catalysts A, B, and C, respectively, in contact with the H
2
O
2
2
0
0
(
2
intensity toward the shorter wavelengths, assignable to charge-
transfer transitions involving metal ion impurities (curve s). Con-
versely, the spectrum of the much more pure pyrogenic silica em-
ployed as diluent, appeared significantly less intense (curve s ).
Overimposed to the feature due to the support, the spectra of the
catalysts appeared characterized by a main band at ca. 220 nm
hydrogen peroxide (acting as an H-donor) and the sulfate groups
(acting as H-acceptor), resulting from the deprotonation of sup-
ported sulfonic acids. As for the competition with water molecules
0
for the interaction with sulfate groups, it can be recalled that H
in aqueous solution is a stronger H-bond donor (but a weaker
acceptor) than H O [72]. The result of the competition with hy-
O ions should be the subject of a future modeling, as
well as the effect of the sulfate–H interaction on the electronic
2 2
O
2
+
(
(
catalysts A and C, curves a and c, respectively) or at ca. 240 nm
catalyst B, curve b), with a weaker partner, exhibiting a partially
drated H
3
2 2
O
resolved fine structure, centered at ca. 260 nm (catalyst A and C,
curves a and c, respectively) or at ca. 280 nm (catalyst B, curve
b). These components can be identified as the primary (p) and sec-
states of hydrogen peroxide molecules.
The occurrence of such interaction can be relevant for the cata-
lytic process, as the results obtained by Neumann [8] on the selec-
ondary (
due to transitions involving the
aromatic ring [69]. Their displacement toward longer wavelengths
in the case of catalyst B can depend on the presence, as substituent
in para position with respect to the sulfonic group, of an oxygen
atom, capable of a larger electron contributing effect than the Si
a
) bands, this latter with the typical vibrational structure,
electron system of a benzylic
tivity attained in oxidation reactions by using
H
2
O
2
in
p
perfluorinated alcohol solvents suggested the possibility for an
electrophylic activation of hydrogen peroxide via H-bonding. Ten-
tative drawings are reported in Fig. 5, where no indication of the
2 2
H O dihedral angle has been reported, intentionally. In general,
it is known that hydrogen peroxide has a quite flexible molecular
structure, easily distorted by molecular interactions [73,74], as a
result of the only weakly hindered rotation of the OH groups
around the O–O bond [73,75].
2
atom and CH present on the aromatic ring in catalysts A and C,
respectively. The spectra of the samples in air appeared similar
to those of the aqueous slurries in terms of position of the absorp-
tion bands that exhibited a lower intensity (Fig. SI-3 in the SI). This
was likely due to the increased contribution of light scattering with
respect to absorption phenomena in the presence of only some lay-
ers of physisorbed water.
3.4. Reaction condition optimization
The activity of the supported sulfonic acid catalysts has been
evaluated in the 1,2-dihydroxylation of 1-methylcyclohexene (1)
with hydrogen peroxide (Table 1) [76].
Subsequently, the slurries obtained by mixing the samples with
the H O/H O solution were analyzed. All spectra exhibited only a
2 2 2
very intense and broad absorption band, with a deep reflectance
The experiments were carried out by adding 1-methylcyclohex-
ene to a suspension of the catalyst in 30% aqueous hydrogen perox-
ide and by stirring the reaction mixture for 22 h at 70 °C [77]. The
model reaction was first tested with phenylsulfonic acid supported
onto amorphous silica and with the commercially available
Amberlyst-15 [78]. In all cases, 1-methyl-1,2-cyclohexanediol (3)
was obtained as the major product. The silica-supported phenyl-
sulfonic acid (catalyst A, 2% molar ratio with respect to reagent
1) showed the best activity, affording product 3 in 80% yield and
94% selectivity (Table 1, entry 2). Lower efficiency was observed
with Amberlyst-15 (59% yield, 93% selectivity) (Table 1, entry 5).
The catalytic activity of two additional sulfonic acids supported
onto silica through arms with different length, namely 4-ethylphe-
nylsulfonic acid and 4-propoxyphenylsulfonic acid, has been
successively studied in the reaction performed under the best
conditions (2 mol% catalyst, 70 °C, 22 h, H O /1 molar ratio
minimum at ca. 250 nm and a tail toward longer wavelength
0
(
Fig. 4, panel A ), due to H
2
O
2
molecules in aqueous solution [70].
In order to render the spectral profiles independent on difference
in intensity due to the not perfect equivalent density of all slurries,
and better observe differences in shape, the data have been nor-
malized to the minimum of reflectance.
It can be then noticed that the onset of the absorption of cata-
lysts B and C appeared shifted toward longer wavelength with re-
spect to both the bare silica support and the catalysts simply in
0
contact with H
2
O (Fig. 4, panel A inset). Such feature can be rea-
2 2
sonably related to the interaction of H O molecules with sup-
ported organic species that apparently occurred in a lower extent
for catalyst A, the spectrum of which appeared not so different
with respect to that obtained in the presence only of water. Quan-
tum chemical calculations indicate the possibility of the formation
2
2
of complexes between H
2
O
2
and halide anions [71], and then, it is
= 2). The best result in terms of yield (90%) and selectivity
proposed that some H-bonding interaction can occur between
(96%) of 1-methyl-1,2-cyclohexanediol was obtained with the

R. Maggi et al. / Journal of Catalysis 294 (2012) 19–28
25
H
O
O
O
O
H
O
H
H
O
H
H
(
a)
(b)
(c)
2 2
Fig. 5. Possible interactions H O –catalyst.
silica-supported 4-ethylphenylsulfonic acid (Table 1, entry 4). The
lower activity (80% yield and 94% selectivity) observed for the sil-
ica-supported phenylsulfonic acid (Table 1, entry 2) could be as-
cribed to the fact that the direct anchoring of phenylsulfonic
group to the silica surface can partly hamper the availability of
the active acid sites with respect to the more accessible ethylphe-
nylsulfonic group. The modest activity of the silica-supported 4-
propoxyphenylsulfonic acid bearing the longest arm (Table 1, entry
strate molar ratio), a lowering in yield due to a non-controllable
acid-catalyzed side reactions of the hydroxyl groups was observed
(69%).
Concerning the optimum H
achieved with a H /1 molar ratio = 2, with lower or higher
amounts of H product 3 was obtained in lower yield due to
the competition of unproductive H decomposition in the first
2 2
O amount, the best result was
2 2
O
2 2
O
2 2
O
case and over oxidation reactions in the second one.
3
) was tentatively ascribed to the lower stability of the ethereal
The effect of the concentration of the aqueous hydrogen perox-
ide on the efficiency of the process has been successively evalu-
ated. In a series of experiments, the reaction was carried out in
the presence of 1.2%, 15%, 30%, and 50% hydrogen peroxide, by
using the same amount of the oxidizing agent. The optimum
hydrogen peroxide concentration was found to be 30% (80% yield,
chain in the reaction medium that is responsible for some catalyst
leaching. Indeed, the elemental analyses of all the utilized catalysts
showed that only catalyst B underwent a 45% loss of the total car-
bon after the first cycle.
The surface area of these catalytic materials seems to be irrele-
vant in the present reaction; similarly, it is impossible to gather a
linear correlation between surface acidity and catalytic activity
from experimental results reported in Table 1 (see, for example,
the poor activity showed by Amberlyst-15 that in water undergoes
2 2
94% selectivity), as the use of 50% H O afforded the diol in lower
yield (72%) and selectivity (88%), probably due to overoxidation
processes; on the contrary, when more diluted solutions of the oxi-
dizing agent were employed, lower yields and selectivities were
6
0–70% swelling).
detected for the competitive alkene water addition (15% H
2 2
O :
In order to evaluate the possible catalytic activity of the bare
49% yield, 80% selectivity; 1.2% H : 8% yield, 65% selectivity).
2 2
O
support, the reaction was performed in the presence of amorphous
silica: the siliceous support played no catalytic role. Similarly,
hydrogen peroxide itself is not able to oxidize 1-
methylcyclohexene.
The influence of catalyst A amount was then evaluated; the re-
sults are illustrated in Fig. 6.
The system was inert until the catalyst A/substrate 1 molar ratio
raised 0.002, and then, a sharp increase in product 3 yield was ob-
served: 1-methyl-1,2-cyclohexanediol was obtained in 55% yield
with a good level of selectivity (93%) by increasing the catalyst
A/substrate 1 molar ratio to ca. 0.05. A further yield increase was
observed by using a 0.02 catalyst/substrate molar ratio value
Concerning the efficiency of the hydrogen peroxide under the
best reaction conditions (catalyst/substrate molar ratio = 0.02,
30% aq H O /substrate molar ratio = 2, 70 °C, 22 h), a value of
2 2
65%, calculated as product (mol) per consumed H
was found.
2
O
2
(mol) ꢁ 100,
Addition of a solvent (2 mL) such as toluene, acetonitrile, meth-
anol or water resulted in a dramatic lowering (<10% yield) or com-
plete inhibition of compound 3 production, irrespective of the
polarity. Among the by-products isolated in these reactions, 1-
methylcyclohexan-1-ol, due to acid-catalyzed addition of water
to alkene, and 2-methylcyclohexanone, derived from the isomeri-
zation of epoxide intermediate or from the pinacol rearrangement
of 1-methyl-1,2-cyclohexanediol, were recognized in the final mix-
tures. When methanol was utilized, the expected 2-methoxy-1-
methylcyclohexan-1-ol, derived from the reaction of the methanol
with the intermediate epoxide, was also isolated in modest yield.
To achieve further information on the behavior of these cata-
lysts, the reactions with silica-supported phenylsulfonic, ethylphe-
nylsulfonic, and propoxyphenylsulfonic acids were analyzed
during the first 10 h under the same reaction condition. Results
are reported in Fig. 7.
The three catalysts show completely different reaction profiles.
Propoxyphenylsulfonic acid (N) shows a slightly higher initial rate
with respect to the ethylphenylsulfonic (j) and phenylsulfonic
acids (ꢁ), without any induction period. Instead, variable induction
periods were observed with ethylphenylsulfonic and phenylsulfon-
ic acids. Spectroscopic studies indicated that these induction times
are not attributable to the production of supported peroxysulfonic
acid intermediates, and that the sulfonic groups in all the three cat-
alysts are completely deprotonated in the presence of water.
(
80%), whereas with larger amount of catalyst (0.04 catalyst/sub-
100
80
60
40
20
0
Yield
Selectivity
0
0.01
0.02
0.03
0.04
SiO -(C H )-SO H/1 molar ratio
2
6
4
3
Fig. 6. Influence of the catalyst amount on the yield and selectivity of product 3 in
the reaction carried out with catalyst A.

2
6
R. Maggi et al. / Journal of Catalysis 294 (2012) 19–28
as the stability and the swelling of the solid catalyst as well as
the nature of the solvent affect the efficiency of these catalytic
systems.
The higher initial rate is observed with catalyst B (arm: prop-
oxyphenyl). Nevertheless, the system reaches a plateau at the
modest 55% yield after about 10 h. As previously showed, we have
ascribed this drawback to the low stability of the catalyst in the
reaction medium. It is indeed expected that in the present acidic
medium, the phenyl-alkyl ether linkage could undergo breaking
with production of para-hydroxybenzene sulfonic acid in solution.
The production of an homogeneous arylsulfonic acid accounts for
the lack of any induction period and for the modest product 3 final
yield. Indeed, our blank experiments confirm that, under homoge-
neous conditions, para-toluensulfonic acid catalyses the reaction
with complete conversion of reagent 1 and modest yield (ꢄ60%)
of product 3 (see Supporting Information).
The induction periods showed with catalyst C (arm: ethylphe-
nyl) and, particularly with catalyst A (arm: phenyl), are observed
with the as prepared as well the recycled catalysts. We tentatively
ascribed this behavior to the nature of the reaction medium and to
its change when the reaction proceeds. Indeed, the reaction mix-
2 2
ture is initially triphasic, being constituted of water (H O ), 1-
methylcyclohexene, and he solid catalyst. This negative effect is
stronger with catalyst A where the phenylsulfonic acid is directly
anchored to the silica surface and consequently less available.
However, the initial production, even trace amount, of 1-methyl-
1
,2-cyclohexanediol (3) converts the liquid biphasic system into
an emulsion followed by the production of a single liquid phase
where the reaction is more and more easy.
Consistent with this, the reaction performed with catalyst A in
the presence of product 3 (5% mol with respect to reagent 1) affor-
ded the same product in 45% yield after 8 h without any induction
period (see Supporting Information).
Fig. 7. (a) Yield and selectivity versus time trend for the reactions carried out with
catalyst A (ꢁ), catalyst B (N) and catalyst C (j) catalysts; (b) zoomed view of the 0–
6
h region.
The catalyst efficiency is expected to be strongly affected by the
On the basis of previous results reported in the literature [31],
initially we were inclined to ascribe the hydrogen peroxide activa-
tion to the formation of a supported peroxysulfonic acid. But, from
our spectroscopic studies, this mechanism seems unlikely, and we
propose the catalytic mechanism depicted in Scheme 2.
In the present aqueous reaction medium, the supported ben-
zenesulfonic acid results completely deprotonated, affording the
accessibility of the supported sulfonic acid groups that, in turn,
mainly depends on the length of the carbon atom arm that binds
them to the support surface, according to the order propoxyphe-
nyl > ethylphenyl > phenyl, that reflects the different initial rates.
In addition, the higher concentration of the catalytic sites in the
surface catalytic islands and the lower concentration of silanols,
due to a major functionalization of the silica support, can also play
a crucial role in the process [79]. The spectroscopic studies confirm
that in catalyst C, the silanol number is lower and the concentra-
tion of the sulfonic acid catalytic sites is higher with respect to
the other two catalysts. However, some further parameters such
+
3
supported sulfonate anion in combination with H O cation. UV
and IR studies gave some information indicating the possible
occurrence of a selective H-bond interaction of H O (as H-donor)
2
2
and the sulfonate group (as H-acceptor) in the presence of water,
affording the three possible adducts depicted in Fig. 5. One of these
H2O2
+
H O
2
O
S
O
O
S
O
O
4
S
O
H
O
OH
O
O
H3O+
H
O
OH
OH
O
+ H O
2
Scheme 2. Possible reaction mechanism.

R. Maggi et al. / Journal of Catalysis 294 (2012) 19–28
27
The authors are grateful to Prof. Salvatore Coluccia (University
of Turin, Italy) for helpful suggestions and discussions.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.06.014.
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Acknowledgments
[
1
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The authors acknowledge the support of the Ministero dell’Uni-
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+
+
stirred at rt; the ion exchange between H and Na was allowed to proceed for
4 h. The catalyst was filtered off and washed with distilled water and then
(
National Project ‘‘Attivazione ossidativa catalitica e fotocatalitica
2
per la sintesi organica’’) and the University of Torino (Progetto di
Ricerca di Ateneo - Compagnia di San Paolo 2011 - Linea 1A prog-
etto ORTO11RRT5). The Centro Interdipartimentale Misure (CIM) is
acknowledged for the use of NMR instruments. G.A. is recipient of
an ASP post-doc fellowship.
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