Preparation of crystal-like periodic mesoporous phenylene-silica derivatized with ferrocene and its use as a catalyst for the oxidation of styrene

Article abstract of DOI:10.1039/c3dt51931g

The surface silanol groups in crystal-like mesoporous phenylene-silica have been derivatized with trimethylsilyl, benzyldimethylsilyl and dimethylsilyl(ferrocene) groups by performing a post-synthetic grafting reaction with the corresponding chlorosilane precursors. The success of the grafting procedure was demonstrated by transmission FT-IR spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and ¹³C and ²⁹Si magic-angle spinning (MAS) NMR spectroscopy. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and N₂ adsorption data for the modified materials indicated preservation of the mesostructure as well as the molecular-scale periodicity in the pore walls. Ferrocene and the ferrocenyl-modified periodic mesoporous organosilica (PMO) were employed in the catalytic oxidation of styrene at 55 °C using either hydrogen peroxide or tert-butylhydroperoxide as an oxidant. The main reaction product was always benzaldehyde (BzCHO), and other products included styrene oxide, benzoic acid and 2-hydroxyacetophenone. Using a styrene:H₂O₂ molar ratio of 1:5, the highest BzCHO yields at 24 h were 65% (85% selectivity) for ferrocene (semibatch conditions involving stepwise addition of H₂O₂, 1 mol% Fe) and 34% (83% selectivity) for the modified PMO (batch conditions, 0.06 mol% Fe). The modified PMO could be recovered and reused, albeit with a drop in catalytic activity due to partial metal leaching during the first catalytic run.

Full text of DOI:10.1039/c3dt51931g

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PAPER
Preparation of crystal-like periodic mesoporous
phenylene-silica derivatized with ferrocene and its use
as a catalyst for the oxidation of styrene
Cite this: Dalton Trans., 2013, 42, 14612
Ana C. Gomes,a Maria J. Ferreira,b Sofia M. Bruno,a Nicolas Bion,a,c Paula Ferreira,b
Anabela A. Valente,a Martyn Pillinger,a João Rochaa and Isabel S. Gonçalves*a
The surface silanol groups in crystal-like mesoporous phenylene-silica have been derivatized with tri-
methylsilyl, benzyldimethylsilyl and dimethylsilyl(ferrocene) groups by performing a post-synthetic graft-
ing reaction with the corresponding chlorosilane precursors. The success of the grafting procedure was
demonstrated by transmission FT-IR spectroscopy and diffuse reflectance infrared Fourier transform spec-
troscopy (DRIFTS), and 13C and 29Si magic-angle spinning (MAS) NMR spectroscopy. Powder X-ray diffrac-
tion (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and N2
adsorption data for the modified materials indicated preservation of the mesostructure as well as the
molecular-scale periodicity in the pore walls. Ferrocene and the ferrocenyl-modified periodic mesoporous
organosilica (PMO) were employed in the catalytic oxidation of styrene at 55 °C using either hydrogen
peroxide or tert-butylhydroperoxide as an oxidant. The main reaction product was always benzaldehyde
(BzCHO), and other products included styrene oxide, benzoic acid and 2-hydroxyacetophenone. Using a
styrene : H2O2 molar ratio of 1 : 5, the highest BzCHO yields at 24 h were 65% (85% selectivity) for ferro-
cene (semibatch conditions involving stepwise addition of H2O2, 1 mol% Fe) and 34% (83% selectivity)
for the modified PMO (batch conditions, 0.06 mol% Fe). The modified PMO could be recovered and
reused, albeit with a drop in catalytic activity due to partial metal leaching during the first catalytic run.
Received 16th July 2013,
Accepted 7th August 2013
DOI: 10.1039/c3dt51931g
www.rsc.org/dalton
showed that the pore diameter can be varied from 32 to 39 Å
by changing the length of the hydrocarbon chain (C14 to C18)
Introduction
During the last two decades research into ordered mesoporous of the alkyltrimethylammonium surfactant used.8 The family
materials has undergone extraordinary growth,1 with some of of crystal-like PMOs has since been extended to include
the key developments being the discovery by the Mobil Oil materials containing 1,3-phenylene,9 biphenylene,10 ethy-
Company of the M41S family of surfactant-templated meso- lene,7,11 1,4-divinylbenzene,12 2,6-naphthylene13 and pyridine
porous silicas (1992),2,3 the synthesis of periodic mesoporous bridging units.14
organosilicas (PMOs) by three groups working independently
PMOs with molecular scale periodicity offer new possibili-
of one another (1999),4–6 and the synthesis by Inagaki and co- ties for various kinds of modifications by post-synthesis deriva-
workers of a phenylene-bridged PMO that showed not only a tization. In particular, grafted species for catalytic or
periodic arrangement of the mesopores but also molecular optoelectronic applications may be spatially organized along
scale periodicity within the pore walls (2002).7 The “crystal- the wall surface.15 Shylesh et al. found that oxodiperoxomolyb-
like” 1,4-phenylene-bridged PMO has a unique surface struc- denum complexes of the type (L–L)MoO(O2)2 tethered onto
ture with alternating hydrophobic phenylene and hydrophilic phenylene-bridged mesoporous organosilica showed up to a
silica layers with a periodicity of 7.6 Å.7 The work by Bion et al. 10-fold increase in catalytic activity and a high stability in
liquid phase olefin epoxidation reactions, with aqueous tert-
butylhydroperoxide (TBHP) or H2O2 as the oxidant, compared
aDepartment of Chemistry, CICECO, University of Aveiro, Campus Universitário de
Santiago, 3810-193 Aveiro, Portugal. E-mail: igoncalves@ua.pt;
to corresponding systems based on conventional ordered
mesoporous silica (MCM-41) supports.16 This effect was attri-
buted to the increased hydrophobicity of the framework walls
due to the presence of phenylene bridges in defined positions.
The authors proposed that the unique hydrophobic pores
could facilitate the adsorption of olefins close to the active
Fax: +351 234 370084; Tel: +351 234 378190
bDepartment of Materials and Ceramics Engineering, CICECO, University of Aveiro,
Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
cUniversity of Poitiers, CNRS, UMR 7285, Institut de Chimie des Milieux et
Matériaux de Poitiers (IC2MP), 4 rue Michel Brunet, 86022 Poitiers Cedex, France
14612 | Dalton Trans., 2013, 42, 14612–14620
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sites, and/or reduce the adsorption of the more polar epoxide spectra were collected using a Bruker RFS100/S FT instrument
and by-products (tert-butanol or H2O).
with 2 cm−1 resolution (Nd:YAG laser, 1064 nm excitation,
The post-synthetic modification of molecularly ordered InGaAs detector). Solid-state magic-angle-spinning (MAS) NMR
PMOs has mainly involved the derivatization of the organic spectra were recorded at 79.49 MHz for 29Si and at 100.62 MHz
groups by, for example, bromination,11b,12b sulfonation7,17 and for 13C using a Bruker Avance 400 spectrometer. 29Si MAS
amination,18 vapor or liquid phase treatment with M(CO)6 NMR spectra were recorded with 40° pulses, a spinning rate of
(M = Mo, Cr) to give arenetricarbonyl complexes,19 and treatment 5.0 kHz, 60 s recycle delays and 8 ms contact time. 29Si CP
−
with [(CH3CN)3RuCp]PF6 to give [C6H4RuCp]PF6 complexes.20 MAS NMR spectra were recorded with 4 μs 1H 90° pulses, 8 ms
Much less attention has been paid to the derivatization of contact time with a spinning rate of 5 kHz and 5 s recycle
1
residual silanol groups in crystal-like PMOs. This pathway was delays. 13C CP MAS NMR spectra were recorded with 4 μs H
used by Shylesh et al. in the study mentioned above,16 and by 90° pulses and 2 ms contact time with a spinning rate of 9.0 or
Sharifi et al. to anchor SO3H groups for enhanced proton con- 15.0 kHz and 5 s recycle delays. Chemical shifts are quoted in
ductivity.17b In the present work, we have anchored ferrocene parts per million from tetramethylsilane.
units onto phenylene-bridged PMO (PMO-Ph) by derivatisation
Preparation of PMO-Si(CH3)3 (1)
of silanol groups. The incorporation of ferrocene into meso-
porous materials has been a topic of considerable interest for A toluene solution (20 mL) of chlorotrimethylsilane (0.5 mL,
several years,21 especially for the preparation of electrochemi- 3.9 mmol) was added to PMO-Ph (0.4 g) and the suspension
cally-active22 and/or catalytically active materials.23 The ferro- was stirred at room temperature for 20 h. The white solid was
cene-PMO described herein has been characterised by various filtered, washed with dichloromethane (4 × 20 mL), and
techniques and examined as a catalyst for the oxidation of vacuum-dried. FT-IR (KBr, cm−1): ν = 3443 (s), 3062 (m) (ν(C–
styrene using hydroperoxides as oxidants.
H)), 2960 (m) (ν(CH3)), 1935 (w), 1829 (w), 1634 (m), 1388 (m),
1254 (m) (δ(Si–CH3)), 1160 (vs), 1101 (sh), 1065 (vs) (ν(Si–O–
Si)), 1020 (sh), 915 (m) (ν(Si–OH)), 861 (m), 848 (m) (γ(Si–
CH3)), 813 (m), 762 (m), 554 (vs), 519 (s), 430 (m), 388 (m).
Raman (cm−1): ν = 3186 (w), 3043 (m), 2961 (w), 2901 (m), 1596
(s), 1530 (w), 1412 (vw), 1307 (w), 1204 (w), 1103 (m), 998 (w),
Experimental
Materials and methods
All chemicals used were of reagent grade or better and used as 779 (m), 632 (w), 556 (w), 114 (w). 13C CP MAS NMR: δ = –0.4
received from commercial sources. PMO-Ph7,8 and CpFe- (CH3), 1.2 (CH3), 133.5 (PMO-Ph). 29Si MAS NMR: δ = 10.5
(C5H4SiMe2Cl)24 were prepared according to literature (Si(CH3)3), −61.5 (T1), −71.4 (T2), −81.1 (T3) [Tm = RSi(OSi)m-
procedures. Prior to use, PMO-Ph was vacuum-dried at 110 °C (OH)3−m]. 29Si CP MAS NMR: δ = 10.5 (Si(CH3)3), −61.3 (T1),
for 2 h.
Fe was determined by ICP-OES at C.A.C.T.I., University of
Vigo, Spain. Powder X-ray diffraction (XRD) data were collected
−71.1 (T2), −81.1 (T3).
Preparation of PMO-Si(CH3)2(CH2C6H5) (2)
using a Philips X’pert MPD diffractometer equipped with an Benzylchlorodimethylsilane (0.75 mL, 4.13 mmol) and toluene
X’Celerator detector, a graphite monochromator (Cu-Kα radi- (20 mL) were added to PMO-Ph (0.63 g) and the suspension
ation filtered by Ni (λ = 1.5418 Å)) and a flat-plate sample was stirred at 80 °C for 48 h. The solid was filtered, washed
holder, in a Bragg–Brentano para-focusing optics configuration extensively with dichloromethane (4 × 20 mL), and vacuum-
(45 kV, 40 mA). Samples were step-scanned in 0.02° 2θ steps dried at 50 °C. FT-IR (KBr, cm−1): ν = 3449 (s), 3066 (m) (ν(C–
with a counting time of 10 s per step. Nitrogen adsorption– H)), 3011 (w), 2935 (w), 1938 (w), 1832 (w), 1641 (m), 1497 (w),
desorption isotherms were recorded at −196 °C using a Micro- 1458 (w), 1388 (m), 1254 (sh) (δ(Si–CH3)), 1161 (vs), 1101 (sh),
meritics Gemini V 2380 surface area analyzer. Functionalized 1065 (vs) (ν(Si–O–Si)), 1020 (sh), 920 (m) (ν(Si–OH)), 845 (w)
PMO materials were heated overnight at 150 °C under flowing (γ(Si–CH3)), 813 (m), 762 (m), 558 (vs), 520 (s), 430 (m), 392
N2 and then placed under reduced pressure (<4 Pa) at ambient (m). Raman (cm−1): ν = 3186 (w), 3043 (m), 2986 (w), 2901 (w),
temperature prior to starting the analysis. The microstructures 1596 (s), 1530 (vw), 1205 (w), 1104 (m), 1001 (w), 779 (m), 632
were analyzed by scanning electron microscopy (SEM) using a (w), 556 (w). 13C CP MAS NMR: δ = −4.2 (CH3), 27.0 (CH2),
HITACHI SU-70 high resolution microscope equipped with a 127.4 (CH2Ph), 133.1 (PMO-Ph). 29Si MAS NMR: δ = 7.6
Bruker EDS detector, and by transmission electron microscopy (PhCH2Si(CH3)2–), −70.6 (T2), −80.9 (T3). 29Si CP MAS NMR:
(TEM) using a 300 eV HITACHI H9000-NA microscope. FT-IR δ = 8.1 (PhCH2Si(CH3)2–), −61.6 (T1), −70.8 (T2), −80.5 (T3).
transmission spectra were measured with a Unican Mattson
Preparation of PMO-Si(CH3)2(C5H4)Fe(C5H5) (3)
Mod 7000 FTIR spectrophotometer using KBr pellets. Diffuse
reflectance infrared Fourier transform spectroscopy (DRIFTS) (C5H5)Fe(C5H4)Si(CH3)2Cl (0.52 g, 1.87 mmol) and dry dichloro-
was performed using a NICOLET 6700 FTIR spectrometer methane (20 mL) were added to PMO-Ph (0.27 g) and the sus-
equipped with a Praying Mantis™ High Temperature Reaction pension was stirred at room temperature for 48 h. The pale
Chamber. The spectra were collected using an MCT detector yellow solid was filtered, washed extensively with dichloro-
with a resolution of 2 cm−1 and 64 scans at room temperature methane (4 × 20 mL), and vacuum-dried. FT-IR (KBr, cm−1):
and 50 °C under secondary vacuum (10−5 mbar). Raman ν = 3443 (s), 3062 (m) (ν(C–H)), 2965 (m) (ν(CH3)), 1938 (w),
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1829 (w), 1641 (m), 1385 (m), 1265 (m) (δ(Si–CH3)), 1160 (vs),
1101 (sh), 1065 (vs) (ν(Si–O–Si)), 1020 (sh), 910 (m) (ν(Si–OH)),
848 (m) (γ(Si–CH3)), 808 (m), 759 (m), 554 (vs), 519 (s), 428 (m),
388 (m). Raman (cm−1): ν = 3043 (m), 2966 (w), 2907 (w), 1596
(s), 1530 (vw), 1307 (w), 1205 (w), 1104 (m), 1001 (w), 779 (m),
632 (w), 556 (w). 13C CP MAS NMR: δ = −15.4 (CH3), 71.1
(C5H4), 67.8 (C5H5), 133.6 (PMO-Ph). 29Si MAS NMR: δ = −19.4
(Si(CH3)2), −71.1 (T2), −80.7 (T3). 29Si CP MAS NMR: δ = −20.5
(Si(CH3)2), −71.0 (T2), −80.7 (T3).
Catalysis
The catalytic reactions were carried out under air (autogenous
pressure) and stirred magnetically (1000 rpm) in a closed boro-
silicate reactor (10 mL capacity) equipped with a valve for
sampling, and immersed in an oil bath thermostated at 55 °C.
Unless otherwise stated, the reactor was loaded with 6.3 mg
(34 µmol) of ferrocene or 40 mg (equivalent to 2.15 µmol of
iron) of material 3, substrate (3.4 mmol) and oxidant
(5.1–17.2 mmol). The oxidants tested were aqueous hydrogen
peroxide (30 wt% H2O2, Sigma-Aldrich) or aqueous tert-butyl-
hydroperoxide solution (70 wt% in water, Sigma-Aldrich). The
total amount of oxidant was added initially or gradually
throughout the first 5 h of the reaction. The cosolvents tested
were 1,2-dichloroethane (1.0 mL, DCE, Aldrich, 99%), acetone
(1.5 mL, Fluka, 99.5%) or acetonitrile (3.0 mL, Aldrich, 99%),
added in sufficient amounts to obtain a single liquid phase.
The olefin and cosolvent were preheated in separate vessels
(10 min at the reaction temperature) and then the corres-
ponding desired amounts were transferred to the reactor (with
preheated walls) containing the catalyst. The instant the reac-
tion began was taken as the instant the preheated mixture of
the substrate, cosolvent and catalyst was put into contact with
the oxidant.
The course of the reactions was monitored using a Varian
3800 GC equipped with a BR-5 (Bruker) capillary column (30 m
× 0.25 mm; 0.25 μm) and a flame ionization detector, using H2
as the carrier gas and undecane as the internal standard. After
24 h of reaction, polystyrene (PS, identified by FT-IR) was
sometimes isolated by the following procedure: the reaction
mixture was centrifuged and the solid obtained was washed
with n-hexane (Sigma-Aldrich, PA), and dried at room tempera-
ture overnight. The isolated yields of PS were in general less
than 4%, but may be underestimated due to the partial solubi-
lity of PS in the reaction mixtures.
Fig. 1 Low angle powder XRD patterns of PMO-Ph (a), and the derivatized
materials 1 (b), 2 (c) and 3 (d).
56.0 Å (Fig. 1). The broad, weak shoulder on this peak toward
higher angles (3–4° 2θ) comprises the overlapping (110) and
(200) reflections. In addition to the low-angle peaks, a
medium-range reflection is observed at d = 7.6 Å, which arises
from the molecular-scale periodicity in the PMO-Ph pore walls
along the channel direction.7,8
As mentioned in the introduction, the post-synthetic modi-
fication of PMO-Ph may be performed by derivatization of the
phenylene groups and/or the residual silanol groups. The
DRIFTS spectrum of PMO-Ph reveals the presence of non-con-
densed isolated surface silanol groups via the SiO–H stretching
vibration at 3728 cm−1 (Fig. 2).25 29Si MAS NMR spectroscopy
shows the presence of organosilica species of the type [RSi-
(OSi)(OH)2] and [RSi(OSi)2(OH)] (in addition to fully con-
densed [RSi(OSi)3] sites), with the latter single silanol species
being much more abundant than the geminal silanol species.8
Treatment of PMO-Ph with chlorotrimethylsilane, benzyl-
chlorodimethylsilane and chlorodimethylsilyl-ferrocene in
either toluene or dichloromethane gave the derivatized
materials 1–3, respectively (Fig. 3). A decrease in the popu-
lation of isolated OH groups in the three materials (relative to
PMO-Ph) was revealed by a pronounced decrease in the relative
intensity of the sharp IR absorption band at 3728 cm−1 in the
DRIFTS spectra, especially noticeable for 1 and 3 (Fig. 2). The
presence of methylsilyl groups in all three materials was con-
Results and discussion
Synthesis
Phenylene-bridged PMO (PMO-Ph) was prepared by the hydro- firmed by the three new absorption bands at 845–850 (γ(Si–
thermal synthesis reported by Inagaki et al. using octadecyltri- CH3)), 1255–1265 (δ(Si–CH3)) and 2960–2965 cm−1 (ν(CH3)) in
methylammonium bromide and 1,4-bis(triethoxysilyl)benzene the IR spectra (Fig. 2 and 4), resonances between −15 and
as starting materials.7,8 The powder XRD pattern of PMO-Ph 2 ppm in the 13C CP MAS NMR spectra (Fig. 5), and resonances
shows one strong low-angle peak at a d-spacing of 48.5 Å, between −20 and 10 ppm in the 29Si MAS NMR spectra (Fig. 6).
which is assigned to the (100) reflection of a two-dimensional Additional 13C NMR signals at 27.0 and 127.4 ppm for 2 are
hexagonal symmetry (p6mm) lattice with a lattice constant a = assigned to the carbon atoms of the benzyl group. The
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Fig. 2 DRIFTS spectra in the range 2500–4000 cm−1 of PMO-Ph (a), and the
derivatized materials 2 (b), 1 (c) and 3 (d).
Fig. 4 FT-IR spectra (KBr) of (a) PMO-Ph and the modified materials 1 (c), 2 (d)
and 3 (e), and of the solids recovered from a 24 h catalytic run using an Fe : Sty :
oxidant molar ratio of 0.06 : 100 : 500, and semibatch conditions for (b) PMO-Ph
and (f ) 3. Spectrum (g) is of the solid recovered from a second run using 3. The
bands indicative of Si–CH3 groups are highlighted by the yellow bars.
Fig. 3 Schematic illustration of PMO-Ph, showing the three grafting reactions
carried out to give materials 1–3.
presence of ferrocenyl species in 3 is supported by the weak
peak at 67.8 ppm and the broad shoulder at 71.1 ppm in the
13C CP MAS NMR spectrum, which are attributed to C5H5 and
C5H4Si, respectively.21d,26 The low intensity of these signals
is consistent with the low Fe content of 0.3 wt%. All three
materials exhibit the expected strong 13C NMR resonance at
133.5 ppm for the phenylene groups of the PMO support.
The powder XRD patterns for 1–3 are similar to those for
PMO-Ph, which indicates retention of the mesoporous struc-
ture as well as the molecular-scale periodicity (Fig. 1). SEM
studies for all three materials revealed very large aggregates of
irregular, thin and mostly elongated particles (Fig. 7). Material
3 was further characterized by TEM and N2 adsorption–deso-
rption isotherms measured at −196 °C. In accordance with the
XRD results, TEM shows a locally well-ordered 2D hexagonal
mesostructure (Fig. 8). Materials 1–3 exhibit a type IV nitrogen
adsorption–desorption isotherm. Condensation inside the
mesopores occurred at p/p0 in the range of 0.1–0.4, and multi-
layer adsorption was noticeable as p/p0 tended to unity,
Fig. 5 13C CP MAS NMR spectra of PMO-Ph (a), and the derivatized materials 1
(b), 2 (c) and 3 (d). Spinning sidebands are indicated by asterisks. The signals
marked with +(δ 15.9, 58.3) are due to residual ethoxy groups present in the
starting material PMO-Ph.
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Fig. 6 29Si MAS (a, c, e) and CP MAS (b, d, f) NMR spectra of the derivatized
materials 1 (a, b), 2 (c, d) and 3 (e, f).
indicating that the samples possessed significant external
specific surface area. Similar features were observed for the
starting material PMO-Ph, confirming that the mesostructure
was essentially preserved during the modification treatments.
The BET specific surface areas (calculated for p/p0 in the range
0.01–0.1) for 1–3 were 655, 627 and 738 m2 −1, respectively,
g
and the pore size distribution curves (BJH algorithm applied
to the adsorption branch) had maxima (Dp) for 1–3 at ca. 3.7,
3.5 and 3.6 nm of pore width, respectively. The SBET and Dp for
1–3 are lower than those for PMO-Ph (999 m2 g−1 and 3.9 nm,
respectively, in agreement with the literature data8), most
likely due to filling of the internal void spaces by the anchored
species.
Fig. 7 Representative SEM images of the derivatized materials 1 (a), 2 (b)
and 3 (c).
Catalytic oxidation of styrene
The catalytic performances of ferrocene and material 3 were
investigated in the oxidation of styrene. To the best of our
knowledge, there is only one very recent report on the appli-
cation of ferrocene in oxidation catalysis.27 Specifically,
Shul’pin et al. described the oxidation of (cyclo)alkanes into
alkyl hydroperoxides using a catalytic system consisting of a
mixture of ferrocene and pyrazine-2-carboxylic acid (PCA) or
2,2′-bipyridine, and H2O2 as an oxidant. These catalytic oxi-
dation systems were effective under relatively moderate reac-
tion conditions (50 °C). In turn, supported ferrocene catalysts
have been previously tested in oxidation reactions, specifically
the hydroxylation of phenol or benzene, but not the oxidation
of styrene.23a–g
in organic chemistry, allowing, for example, oxygen atoms to
be introduced into molecules (e.g. forming carbonyl com-
pounds) or to fragment large molecules.28 The classical
methods used for oxidative cleavage of olefins involve the use
of ozone and/or heavy metals, posing serious safety/environ-
mental concerns.29
The catalytic performance of ferrocene was investigated in
the reaction of styrene (Sty) using either DCE, CH3CN or
acetone as cosolvents, at 55 °C (Fe : Sty : oxidant molar ratio of
1 : 100 : 150). Under these conditions, benzaldehyde (BzCHO)
and polystyrene (PS, less than 4 mol%) were detected as pro-
ducts, although less than 14% conversion was reached at 24 h
The oxidation of styrene to benzaldehyde involves bond
cleavage. Oxidative cleavage of olefins is a useful synthetic tool
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A possible explanation for the poor catalytic results for
ferrocene is the “non-productive” catalytic decomposition of
the oxidant. Wang et al. reported significant decomposition of
H2O2 by iron-containing silica materials, tested as catalysts in
the reaction of Sty at 70 °C.30 In order to check this hypothesis,
ferrocene was mixed with each oxidant in separate experiments
(without Sty and cosolvent, using a Fe : oxidant molar ratio of
1 : 150) for 3 h at 55 °C, and subsequently the concentration of
the unreacted oxidant was measured by iodometric titration.
Ferrocene nearly completely decomposed H2O2 (98%), whereas
TBHP decomposition was negligible (2%, which is within the
range of experimental error). These results are consistent with
the release of gas bubbles observed from the reaction mixtures
using H2O2 (this decomposition gives water plus molecular
oxygen), which was not observed for TBHP. However, these
results do not correlate with the higher catalytic activity with
H2O2 as an oxidant in comparison to TBHP. Possibly, different
mechanisms are involved for the two oxidants. It has been
reported for the catalytic oxidation system ferrocene–PCA–
H2O2 that ferrocene–PCA adducts are formed and react with
H2O2 to give strong oxidizing hydroxyl radicals.27 This is some-
what consistent with a previous work reporting that protonated
ferrocene can react with H2O2 forming water and hydroxyl radi-
cals.31 On the other hand, a free radical reaction mechanism
has been proposed for iron-containing silicas tested as cata-
lysts in the reaction of Sty with H2O2 to give BzCHO and
StyO.30 For ferrocene (and 3, discussed below, using a
Sty : H2O2 molar ratio of 1 : 5) StyO was detected as a product
at 24 h of reaction (Table 1). The conversion of Sty to BzCHO
may involve StyO as an intermediate or the direct oxidative
cleavage of the double bond of the olefin. Using StyO as the
substrate instead of Sty, and ferrocene as the catalyst, led to
96% conversion at 24 h of reaction, and BzCHO was formed in
30% yield; the reaction products included benzoic acid (BzA)
and HAP formed in 25 and 15% yields, respectively. Based on
these results, it seems that for ferrocene the conversion of Sty
to BzCHO involves StyO as an intermediate at least to a certain
extent. Nevertheless, we cannot rule out that the direct oxi-
dative cleavage of the double bond of Sty takes place via a
parallel pathway. No PS was isolated for the reaction using
StyO as the substrate, suggesting that the polymer product is
formed via free radical polymerization of Sty possibly involving
hydroxyl radicals as initiators.32
Fig. 8 TEM micrograph of the derivatized material 3. The inset displays the
(001) projection, which proves the hexagonal arrangement of pores.
of reaction. Using TBHP as an oxidant instead of H2O2 did not
improve the catalytic results (<3% conversion at 24 h). PS was
only isolated when the oxidant was H2O2 (for TBHP, no PS was
isolated). Increasing the Sty : oxidant molar ratio from 1 : 1.5 to
1 : 5 (CH3CN as a cosolvent) led to an increase in the conver-
sion at 24 h from 7% to 37% for H2O2 (Table 1), whereas for
TBHP the reaction remained slow (<3% conversion at 24 h).
The main reaction product was always BzCHO (formed in 26%
yield and 72% selectivity at 24 h), and other products included
styrene oxide (StyO), 2-hydroxyacetophenone (HAP) and PS. A
similar trend was reported for the oxidation of cyclohexane in
the presence of ferrocene–PCA using CH3CN as a solvent at
50 °C, in that much better catalytic results were obtained for
H2O2 than for TBHP.27
Table 1 Reaction of styrene with H2O2 in the presence of ferrocene or material
3a
Selectivityd (%)
Operation Time Conv.c
Catalyst
modeb
(h)
(%)
BzCHO
StyO
BzA
HAP
Ferrocene Batch
5
24e
5
24
5
24
5
24
5
17
37
10
77
7
33
14
41
93
72
91
84
100
81
7
6
9
—
—
—
3
—
—
—
4
—
3
—
6
—
4
Ferrocene Semibatch
Ferrocene f Semibatch
6
In an attempt to improve the catalytic performance of ferro-
cene, the operation mode was changed from batch to semi-
batch in which the total amount of oxidant was added
gradually during the first 5 h of reaction (CH3CN as a solvent,
at 55 °C, and the total amount of added H2O2 corresponded to
a Sty : H2O2 molar ratio of 1 : 5). The conversion at 24 h was
much higher for the semibatch conditions (77%) than for the
batch ones (37%), Table 1. The main reaction product was
always BzCHO (formed in 65% yield and 84% selectivity at
24 h), and other products included StyO, BzA and HAP
(formed in 5%, 2% and 5% yield, respectively), and PS
(smaller amounts were isolated compared to the batch con-
ditions). Iodometric titration of the mixture of ferrocene–
—
13
—
9
3
3
Batch
100
81
—
4
Semibatch
8 (4) 100 (100) — (—) — (—) — (—)
40 (23) 71 (85) 10 (11) 7 (—) 8 (—)
24
a Reaction conditions: a Fe : Sty molar ratio of 1 : 100 for ferrocene and
1 : 1667 for 3, a Sty : H2O2 molar ratio of 1 : 5, 3 mL CH3CN as a solvent,
55 °C. b Batch mode when the total amount of oxidant was added at the
initial instant of the reaction, and semibatch mode when the total
amount of oxidant was added gradually during the first 5 h of reaction.
c Values in parentheses are for the second run. d Identified products:
BzCHO = benzaldehyde, StyO = styrene oxide, BzA = benzoic acid and
HAP
= 2-hydroxyacetophenone. Polystyrene was sometimes identified
as product at 24 h of reaction. e PS was detected with 19% selectivity.
f A ferrocene : Sty molar ratio of 1 : 1667 was used.
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Table 2 The literature data for iron-containing heterogeneous catalysts tested for the oxidation of styrene with H2O2
Catalysta
Reaction conditionsb
Fe : Sty (mol%)
Conv.c (%)
BzCHO sel.d (%)
Other productse
Ref.
3
1 : 5, CH3CN, 55 °C, 24 h
1 : 1, Acetone, 50 °C, 24 h
1 : 1, Acetone, 50 °C, 24 h
1 : 3, CH3CN, 30 °C, 24 h
1 : 3, CH3CN, 30 °C, 24 h
1 : 13, CH3CN, rt, 24 h
1 : 1, water, 100 °C, 4 h
1 : 1, CH3CN, 80 °C, 4 h
1 : 1, CH3CN, 80 °C, 4 h
1 : 1, CH3CN, 80 °C, 4 h
1 : 1, DMF, 60 °C, 2 h
0.06
65
240
7.8
40
7
71
94
92
StyO, BzA, HAP
StyO
StyO
StyO, BzA, diol
StyO, BzA, diol
nm
—
StyO
This work
23h
23h
33a
33a
33b
35
36
36
36
34a
34a
34b
30
Fe-SBA-15
Fe-SBA-15
Fe(acac)-SBA-15
Fe(acac:salen)-SBA-15
Fe(salen)-clay
(FeOx)/SBA-15
Fe-SBA-1
Fe-SBA-1
Fe-SBA-1
Fe-MCM-41(s–g)
Fe-MCM-41(imp)
Fe-MCM-41
Fe-MCM-41
Fe-ZSM-5
11
22
45
nm
95
66
68
66
22
9
72
7.8
>99
nmg
100
73
74
72
35
45
80–96
37
63
nmf
3
0.18
0.36
0.54
0.18
0.18
0.06
0.32
0.21
StyO
StyO
StyO
1 : 1, DMF, 60 °C, 2 h
StyO, styrene glycol
nm
StyO, BzA, diol, MA
StyO
1 : 3, CH3CN, 50 °C, 2 h
1 : 0.98, DMF, 73 °C, 2 h
1 : 0.98, DMF, 73 °C, 2 h
14
14
5
30
a Salen = N,N′-ethylenebis(salicylideneaminato), acac = acetylacetonate, clay = K10-montmorillonite, s–g = catalyst synthesized via the sol–gel
technique, imp = catalyst synthesized via the impregnation technique. b Styrene : H2O2 molar ratio, solvent, reaction temperature, reaction
time (rt = room temperature). c Sty conversion. d BzCHO selectivity. e StyO = styrene oxide, BzA = benzoic acid, diol = 1-phenylethanediol, and
MA = mandelic acid. f nm = not mentioned. g BzCHO yield of 30%.
H2O2–CH3CN (without Sty) indicated that for the semibatch and BzCHO yields, the catalytic results for 3 seem to be either
conditions the decomposition of the oxidant after 330 min was comparable or superior to those reported for supported iron
not complete (50%), in contrast to that observed for the batch complexes bearing organic ligands23h,33 or iron-containing
operation mode. Based on these results it seems that the cata- silicas and zeolites (Table 2).30,34 However, much better cata-
lytic performance and the productive consumption of the lyst results have been reported for iron oxide nanoparticles
oxidant (or oxidant efficiency) may be improved by using the supported on mesoporous silica.35
semibatch conditions. These results are in agreement with
The catalyst stability of 3 was investigated by using the
those reported by Wang et al. for the oxidation of Sty with semibatch conditions (CH3CN–H2O2, 55 °C) and performing a
H2O2 in the presence of iron-containing silica catalysts, where second run. After the first run, the catalyst was separated by
the efficiency of the oxidant was improved by gradually adding centrifugation, washed with n-hexane and dried at room temp-
it to the reaction mixture.30
erature overnight. The reaction rate decreased from the first to
The catalytic performance of material 3 was investigated the second run; 40% and 23% conversion at 24 h for runs 1
using batch and semibatch conditions, with H2O2 as an and 2, respectively, and BzCHO was always the main product
oxidant and CH3CN as a cosolvent, at 55 °C. In contrast to that (Table 1). The FT-IR spectra of the recovered solids are similar
observed for ferrocene, with 3 the catalytic results for the semi- to those for the respective PMO-Ph and 3 (Fig. 4). In particular,
batch and batch conditions were comparable (40% and 41% for the derivatized material 3, the bands at about 850 and
conversion at 24 h, respectively) with BzCHO as the main reac- 1260 cm−1 due to methylsilyl groups are still present even after
tion product formed in 28% and 33% yield, respectively the second catalytic cycle.
(Table 1). Additional products were StyO, BzA and HAP (for
The drop in catalytic activity of 3 from run 1 to run 2 may
semibatch conditions phenylacetaldehyde was also formed). In be due to metal leaching. In order to check this hypothesis a
contrast to that observed for ferrocene, no measurable amount hot-filtration test was performed at 55 °C. The solid catalyst
of PS was isolated in the case of 3. The reaction using PMO-Ph was filtered through a 0.2 μm PTFE GVS membrane after
(without iron) instead of 3, under similar semibatch con- 330 min of reaction and the filtrate was left to stir at 55 °C
ditions, led to 3% conversion at 24 h, indicating that the iron until 24 h (counting from the initial instant of the hot-fil-
species play a determining catalytic role. A comparative study tration test). The increment in Sty conversion between 330 min
of ferrocene and 3 under similar reaction conditions (using a and 24 h of reaction was 25%, which is considerable in com-
Fe : Sty molar ratio of 1 : 1667, and semibatch conditions) parison to the increment of 32% observed for the typical cata-
showed that the latter led to somewhat higher conversion lytic test (without filtration). Hence, active species were
(33% and 40% for ferrocene and 3, respectively) and BzCHO leached from 3 into solution during the catalytic reaction.
yield at 24 h (27% and 28% for ferrocene and 3, respectively), Superior catalyst stabilities (based on the evaluation of the
Table 1.
catalytic activity in consecutive batch runs and metal leaching
A fair comparison of the catalytic performance of 3 with tests) have been reported for Fe(acac:salen) complexes and
those reported in the literature for other catalysts tested in the iron oxide nanoparticles supported on SBA-15,33a,35 and for
same reaction is not trivial due to the different ranges of reac- iron-containing MCM-41 with an iron content lower than
tion conditions used. Based solely on the reported conversions 0.8 wt%.30
14618 | Dalton Trans., 2013, 42, 14612–14620
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Paper
3 J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz,
C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson,
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O. Terasaki, J. Am. Chem. Soc., 1999, 121, 9611.
5 T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin,
Nature, 1999, 402, 867.
6 B. J. Melde, B. T. Holland, C. F. Blanford and A. Stein,
Chem. Mater., 1999, 11, 3302.
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2002, 416, 304.
Conclusion
In the present study we have demonstrated that crystal-like
mesoporous phenylene-silica can be derivatized by performing
a grafting reaction between the surface silanol groups and
chlorosilanes. A ferrocenyl-modified PMO was obtained and
shown to be effective in the iron-catalyzed oxidation of styrene
with H2O2 under mild conditions at 55 °C, giving benz-
aldehyde as the major product. However, active species are
leached from the modified PMO into solution during the cata-
lytic reaction. To the best of our knowledge, there is only one
very recent report on the application of ferrocene in oxidation
catalysis, namely oxidative alkane functionalization with H2O2
in the presence of pyrazine-2-carboxylic acid or 2,2′-bipyri-
dine.27 We report that catalytic amounts of ferrocene also
promote the oxidation of styrene to benzaldehyde with quite
8 N. Bion, P. Ferreira, A. Valente, I. S. Gonçalves and
J. Rocha, J. Mater. Chem., 2003, 13, 1910.
9 M. P. Kapoor, Q. Yang and S. Inagaki, Chem. Mater., 2004,
16, 1209.
high selectivity and, depending on the reaction conditions, 10 (a) M. P. Kapoor, Q. Yang and S. Inagaki, J. Am. Chem. Soc.,
moderate activity. Further studies are underway on the appli-
cation of ferrocene and ferrocenyl-modified mesoporous
materials in oxidation catalysis.
2002, 124, 15176; (b) Y. Yang and A. Sayari, Chem. Mater.,
2007, 19, 4117.
11 (a) Y. Xia, W. Wang and R. Mokaya, J. Am. Chem. Soc., 2005,
127, 790; (b) Y. Xia and R. Mokaya, J. Phys. Chem. B, 2006,
110, 3889.
12 (a) A. Sayari and W. Wang, J. Am. Chem. Soc., 2005, 127,
12194; (b) M. Cornelius, F. Hoffmann and M. Fröba, Chem.
Mater., 2005, 17, 6674.
Acknowledgements
We are grateful to the Fundação para a Ciência e a Tecnologia
(FCT), Fundo Europeu de Desenvolvimento Regional (FEDER), 13 N. Mizoshita, Y. Goto, M. P. Kapoor, T. Shimada, T. Tani
QREN-COMPETE (PTDC/QUI-QUI/113678/2009), the European and S. Inagaki, Chem.–Eur. J., 2009, 15, 219.
Union, and the Associate Laboratory CICECO (PEst-C/CTM/ 14 M. Waki, N. Mizoshita, T. Ohsuna, T. Tani and S. Inagaki,
LA0011/2013) for continued support and funding. The authors Chem. Commun., 2010, 46, 8163.
acknowledge the Portuguese network of electron microscopy, 15 (a) Q. Yang, M. P. Kapoor and S. Inagaki, J. Am. Chem. Soc.,
the RNME, FCT Project REDE/1509/RME/2005. The FCT and
the European Union are acknowledged for a post-doctoral
2002, 124, 9694; (b) B. Camarota, S. Mann, B. Onida and
E. Garrone, ChemPhysChem, 2007, 8, 2363.
grant to S.M.B. (SFRH/BPD/46473/2008) cofunded by MCTES 16 S. P. Shylesh, M. Jia, A. Seifert, S. Adappa, S. Ernst and
and the European Social Fund through the program POPH
of QREN.
W. R. Thiel, New J. Chem., 2009, 33, 717.
17 (a) S. Shylesh, A. Wagener, A. Seifert, S. Ernst and W. R. Thiel,
Angew. Chem., Int. Ed., 2010, 49, 184; (b) M. Sharifi, C. Köhler,
P. Tölle, T. Frauenheim and M. Wark, Small, 2011, 7, 1086.
18 M. Ohashi, M. P. Kapoor and S. Inagaki, Chem. Commun.,
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