Post-synthetic modification of crystal-like periodic mesoporous phenylene-silica with ferrocenyl groups

Article abstract of DOI:10.1016/j.jorganchem.2013.10.022

Amination of crystal-like 1,4-phenylene-bridged periodic mesoporous organosilica (Ph-PMO) was achieved with about 35% conversion of phenylene groups. Ferrocenylimine groups were subsequently anchored onto this material by condensation of acetylferrocene w

Full text of DOI:10.1016/j.jorganchem.2013.10.022

Journal of Organometallic Chemistry 751 (2014) 501e507
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Post-synthetic modification of crystal-like periodic mesoporous
phenylene-silica with ferrocenyl groups
a
b
a
b
Ana C. Gomes , Mirtha A.O. Lourenço , Sofia M. Bruno , Paula Ferreira ,
a
a
a,
*
Anabela A. Valente , Martyn Pillinger , Isabel S. Gonçalves
Department of Chemistry, CICECO, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
a
b
Department of Materials and Ceramics Engineering, CICECO, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 May 2013
Received in revised form
Amination of crystal-like 1,4-phenylene-bridged periodic mesoporous organosilica (Ph-PMO) was ach-
ieved with about 35% conversion of phenylene groups. Ferrocenylimine groups were subsequently
anchored onto this material by condensation of acetylferrocene with amino groups. Elemental analysis
2
October 2013
indicated that about 15% of amino groups in PMO-NH
2
were derivatized, resulting in an iron loading of
Accepted 3 October 2013
ꢀ1
0
.21 mmol g . Evidence for the presence of ferrocenylimine groups in the derivatized material (PMO-Fc)
13
was obtained from C cross-polarization (CP) magic-angle spinning (MAS) NMR and FT-IR spectros-
Keywords:
Periodic mesoporous organosilica
Ferrocene
Ferrocenylimine
Condensation reaction
Styrene oxidation
29
copies. PMO-Fc was further characterized by Si MAS NMR spectroscopy, powder X-ray diffraction
(
XRD), N
edesorption data for PMO-NH
scale periodicity in PMO-NH
2
adsorptionedesorption, and thermogravimetric analysis (TGA). Powder XRD and N
2
adsorption
2
and PMO-Fc indicated that the mesoporous structure and molecular-
2
were largely retained upon treatment with acetylferrocene. The material
ꢁ
PMO-Fc was examined as a catalyst for the oxidation of styrene at 55 C using hydroperoxides as oxi-
dants. The reaction products were benzaldehyde (major) and styrene oxide (minor), with the aldehyde
being formed in yields of 25e27% at 24 h. Recycling experiments indicated that the material was sus-
ceptible to leaching of catalytically active species into the liquid phase due to the pronounced water
sensitivity of the azomethine linkage.
Ó 2013 Elsevier B.V. All rights reserved.
ꢀ
1. Introduction
diameter can be varied from 32 to 39 A by changing the length of
the hydrocarbon chain (C14eC18) of the alkyltrimethylammonium
In 1999, a new family of nanoporous organic-inorganic hybrid
materials termed periodic mesoporous organosilicas (PMOs) were
reported [1e3]. These materials are synthesized by the hydrolysis
and polycondensation of bridged organosilanes of the type
surfactant used [11]. The family of crystal-like PMOs has since been
extended to include materials containing 1,3-phenylene [12],
biphenylene [13,14], ethylene [9,15,16], 1,4-divinylbenzene [17,18],
2,6-naphthylene [19] and pyridine bridging units [20].
(
ZO)
3
SieReSi(OZ)
3
in the presence of a micellar surfactant tem-
PMOs with crystal-like pore walls offer new opportunities for
various kinds of modification by post-synthetic derivatization. In
particular, grafted guest species for catalytic or optoelectronic ap-
plications may be spatially organized along the wall surface [21e
23]. To date, only a few studies have addressed the post-synthetic
chemical modification of the organic groups in these materials.
Bromination experiments performed for ethylene- and
divinylbenzene-bridged PMOs showed that up to 35% of the organic
groups were accessible for functionalization [16,18]. Crystal-like
mesoporous phenylene-silica can be sulfonated in one step (by
treatment with fuming sulfuric acid or chlorosulfonic acid)
plate [4e8]. Important features of PMOs include well-ordered
mesostructures with narrow pore size distributions, homoge-
neous distribution of organic groups within the pore walls, high
specific surface areas and high pore volumes. In 2002, Inagaki and
co-workers reported a phenylene-bridged PMO that showed not
only a periodic arrangement of the mesopores but also molecular
scale periodicity within the pore walls [9,10]. The “crystal-like” 1,4-
phenylene-bridged PMO has a unique surface structure with
alternating hydrophobic phenylene and hydrophilic silica layers
ꢀ
with a periodicity of 7.6 A. Work by Bion et al. showed that the pore
[9,24,25] or aminated in two steps (by nitration with HNO
3 2 4
eH SO
followed by treatment with SnCl eHCl) [26]. N-Alkylation of the
2
amino-functionalized material could be achieved with about 87%
conversion by using a potassium iodide catalyzed method and
*
Corresponding author. Fax: þ351 234 401470.
E-mail address: igoncalves@ua.pt (I.S. Gonçalves).
0
022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.10.022

502
A.C. Gomes et al. / Journal of Organometallic Chemistry 751 (2014) 501e507
ꢁ
microwave-assisted heating [27]. Phenylene-bridged PMO has been
successfully modified with organometallic groups by treatment
isotherms were recorded at ꢀ196 C using a Micromeritics Gemini
V 2380 surface area analyzer. Functionalized PMO materials were
dehydrated overnight at 150 C prior to analysis. TGA was carried
out using a Shimadzu TGA-50 system with a heating rate of
5 C min under a static air atmosphere. FT-IR (ATR) spectra were
measured with a Bruker Tensor 27 spectrophotometer in the range
of 4000 to 350 cm with 256 scans and 4 cm resolution. Solid-
state MAS NMR spectra were recorded at 79.49 MHz for Si and
100.62 MHz for C on a Bruker Avance III 400 spectrometer
operating at 9.4 T. Si MAS NMR spectra were recorded with 40
flip angle pulses, a spinning rate of 5.0 kHz, and 60 s recycle delays.
ꢁ
with M(CO)
arenetricarbonyl (epHM(CO)
1], and with [(CH CN) RuCp]PF
ruthenium (e[pHRuCp]PF
e; Cp ¼ cyclopentadienyl) complexes
32].
To the best of our knowledge there are only a few papers that
6
(M ¼ Mo, Cr; vapor or liquid-phase) to give supported
e; pH ¼ eC e) complexes [28e
to give supported organo-
3
H
6 4
ꢁ
ꢀ1
3
3
3
6
6
ꢀ1
ꢀ1
[
2
9
13
describe the incorporation of ferrocene into PMOs [33e35]. Such
materials may have interesting catalytic properties. In particular,
ferrocene-containing PMO (prepared by co-condensation) [34] and
mesoporous SBA-15 (prepared by post-synthesis grafting) [36]
were found to be active catalysts for the hydroxylation of ben-
zene (giving, in the former case, phenol with 65% selectivity at ca.
2
9
ꢁ
2
9
1
ꢁ
Si CP MAS NMR spectra were recorded with 4
contact time of 8 ms, a spinning rate of 5 kHz, and 5 s recycle delays.
C CP MAS NMR spectra were recorded with 4 ms H 90 pulses, a
ms H 90 pulses, a
13
1
ꢁ
3
1% conversion).
contact time of 1 ms, a spinning rate of 9 or 15.0 kHz, and 4 s recycle
delays. Chemical shifts are quoted in parts per million from
tetramethylsilane.
The catalytic oxidation of olefins gives a variety of industrially
important products such as epoxides, carbonyl compounds, diols
and oxidative cleavage products of C]C [37]. For example, the
oxidation of styrene gives styrene oxide, benzaldehyde, styrene
glycol, benzoic acid, acetophenone and phenylacetaldehyde, which
are useful intermediates or final products, e.g. as reactive diluent
for epoxy resins, or for synthesizing fragrances, flavors and aromas
2
2.2. Preparation of PMO-NH (1)
The amination of the phenylene moieties in Ph-PMO was carried
out by using the two step approach reported by Inagaki and co-
[37,38]. Several iron-containing ordered mesoporous silicas (MCM-
workers [26]. First, a mixed acid solution of sulfuric acid
4
1 and SBA-15) have been applied as catalysts for the liquid-phase
(16.7 mL) and nitric acid (5.1 mL) was added slowly to Ph-PMO
(1 g). After stirring for 3 days at room temperature, the mixture
was poured into cold distilled water. The solid product was filtered
and washed several times with a large amount of distilled water.
oxidation of styrene [38e43]. For example, a catalyst comprising
ferric oxide nanoparticles supported on SBA-15 gave benzaldehyde
as the main product in high selectivity, under mild reaction con-
ꢁ
ditions (91e94% at 7e12% conversion, 50 C) [38]. The catalyst was
The nitrated Ph-PMO (PMO-NO
yellow powder. The aminated PMO (PMO-NH
reduction of the nitro group. A solution of tin chloride (3.33 g) in
hydrochloric acid (31.60 mL) was added slowly to PMO-NO (1 g).
2
, 1.07 g) was obtained as a pale
prepared by calcination of ferrocene-loaded SBA-15 (prepared by a
physical-vapor-infiltration method). No catalytic results were re-
ported for the uncalcined material.
2
) was obtained by
2
Considering the above points, we set out to prepare a ferrocenyl-
modified PMO as a possible catalyst for the oxidation of styrene. A
new approach for the incorporation of organometallic groups in
crystal-like mesoporous phenylene-silica is reported, based on the
further modification of the aminated material mentioned above.
Specifically, the amino groups were condensed with acetylferro-
cene to give anchored ferrocenylimine moieties. The resultant
material has been characterized by various techniques and exam-
ined as a catalyst for the oxidation of styrene using hydroperoxides
as oxidants.
The mixture was stirred for 3 days at room temperature and then
mixed with 300 mL of distilled water. After filtration, the powder
was washed with a large amount of distilled water followed by a
mixture of isopropylamine (20 mL) in ethanol (100 mL). The PMO-
2
NH was obtained as a pale pink powder (1.04 g) after drying at
ꢁ
13
60 C overnight. Anal. found: N, 2.09%. C CP MAS NMR:
(C1 (Ph-NH )), 133.5 (Ph-PMO, C3,5 (Ph-NH )), 123.9, 120.7 (C2,4,6
(Ph-NH
d
¼ 150.3
2
2
2
9
2
)) (see Fig. 1 for atom numbering scheme). Si MAS NMR:
¼ ꢀ70.7 (T ), ꢀ81.1 (T ) [T ¼ RSi(OSi)
2
3
m
29
d
m
(OH)3ꢀm]. Si CP MAS
2
3
NMR:
d
¼ ꢀ70.7 (T ), ꢀ80.8 (T ).
2
. Experimental
2.3. Preparation of PMO-Fc (2)
PMO-NH
ꢁ
2.1. Materials and methods
2
(0.84 g) was predried under vacuum at 110 C for 2 h.
A dry toluene solution (20 mL) of acetylferrocene (3.28 g,
14.4 mmol) was then added and the suspension was stirred for 48 h
at reflux. The cream colored solid was filtered, washed extensively
with toluene (3 ꢂ 20 mL), 1,2-dichloroethane (4 ꢂ 20 mL) and
dichloromethane (4 ꢂ 20 mL), and finally vacuum-dried. Anal.
Acetylferrocene (SigmaeAldrich), anhydrous toluene (99.5%,
Panreac), 1,2-dichloroethane (99%, SigmaeAldrich), dichloro-
methane (>99%, SigmaeAldrich), sulfuric acid (95e97% v/v, Pan-
reac), nitric acid (65% v/v, Panreac), tin chloride (98%, Aldrich),
hydrochloric acid (37% v/v, Carlo Erba), isopropylamine (>99.5%,
Aldrich) and ethanol (PA, Panreac) were purchased from commer-
13
found: Fe, 1.15%. C CP MAS NMR:
d
¼ 150.8 (C1 (Ph-N]C(CH
3
)Fc)),
133.5 (Ph-PMO, C3,5 (Ph-N]C(CH
3
)Fc)), 122.1 (C2,4,6 (Ph-N]
ꢀ
29
2
3
29
cial sources. The anhydrous toluene was stored over activated 4 A
C(CH
3
)Fc)), 68.9 (Fc). Si MAS NMR:
d
¼ ꢀ71.8 (T ), ꢀ81.1 (T ). Si
3
2
molecular sieves prior to use. 1,2-Dichloroethane and dichloro-
CP MAS NMR:
d
¼ ꢀ71.0 (T ), ꢀ80.9 (T ).
2
methane were dried over CaH , distilled under inert atmosphere,
ꢀ
and stored over activated 3 A molecular sieves. The phenylene-
bridged PMO (Ph-PMO) was synthesized according to the litera-
ture procedure [9,11]. Subsequent derivatizations were performed
using standard Schlenk techniques.
2.4. 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 sam-
Microanalyses for N were performed at the Department of
Chemistry, University of Aveiro. Fe was determined by ICP-OES at
C.A.C.T.I., University of Vigo, Spain. Powder XRD data were acquired
ꢁ
pling, and immersed in an oil bath thermostated at 55 C.
Typically, the reactor was loaded with catalyst in an amount
with a Rigaku Geigerflex D Max-C Series diffractometer using Cu-
equivalent to 2.15
Aldrich) and oxidant. Aqueous hydrogen peroxide (30 wt.%
, SigmaeAldrich), urea hydrogen peroxide (97%, Aldrich),
mmol of iron, 3.4 mmol of styrene (Sty; 99%,
ꢁ
Ka
radiation. Samples were step-scanned in 0.02 2
q
steps with a
counting time of 60 s per step. Nitrogen adsorptionedesorption
2 2
H O

A.C. Gomes et al. / Journal of Organometallic Chemistry 751 (2014) 501e507
503
2
Fig. 1. Schematic illustration of Ph-PMO and its modification to give the aminated material PMO-NH and the derivatized material containing anchored ferrocenyl groups (PMO-Fc).
tert-butylhydroperoxide (TBHP) in decane (5e6 M, Sigmae
Evidence for the presence of ferrocenylimine groups,
ꢀ
13
Aldrich; dried over activated 4 A molecular sieves prior to use), or
aqueous TBHP (70 wt.% aqueous, SigmaeAldrich) were tested as
oxidants (6.9 mmol) under batch conditions. In a different
[Fe(C
5
4
H C(CH
3
)NeAr)(C
5
H
5
)], in PMO-Fc was obtained from the
C
13
CP MAS NMR and FT-IR (ATR) spectra. The C CP MAS NMR spec-
trum of Ph-PMO displays a single resonance at 134 ppm for the 1,4-
substituted phenylene carbon atoms. After the two-step function-
2 2
approach, aqueous H O was added gradually to the reactor
throughout the first 5 h of reaction (system referred to as semi-
2
alization to give PMO-NH , resonance peaks at 121, 124, 134 and
continuous). The cosolvents (3 mL) tested were acetonitrile
150 ppm are observed (Fig. 2), in agreement with that reported
previously [26,27]. The latter peak is assigned to the aromatic car-
bon atom directly bonded to NH groups, while the overlapping
2
peaks at 121 and 124 are assigned to the C2,4,6 carbons of amino-
substituted phenylene groups. The intense resonance at 134 is
assigned to unmodified phenylene moieties and C3,5 carbons of
amino-substituted phenylene groups. The subsequent treatment of
(
99%, Aldrich) and tetrahydrofuran (THF; 99.99%, Fischer Scien-
tific), which allowed a single liquid phase to be obtained. For the
reaction using TBHPdec as oxidant, CH CN was predried using
3
ꢀ
activated 4 A molecular sieves. The substrate was preheated in a
separate vessel for 10 min at 55 C, and then the desired amount
ꢁ
was transferred to the reactor (with preheated walls) containing
the catalyst and cosolvent. The instant t ¼ 0 was taken as the
instant the preheated mixture of the substrate, cosolvent and
catalyst was put into contact with the oxidant.
2
PMO-NH with acetylferrocene resulted in only slight changes to
13
the C CP MAS NMR spectrum in the chemical shift range of 120e
150 ppm. A new peak at 69 ppm is attributed to cyclopentadienyl
carbon atoms of ferrocenyl groups.
The course of the reactions were monitored using a Varian
3
800 GC equipped with
m) and a flame ionization detector, using
as the carrier gas and cyclododecane oxide (95%, Aldrich) as
a
BR-5 (Bruker) capillary column
Owing to the low iron content of 0.21 mmol gꢀ1, resonances due
to the imine (expected in the range 150e170 ppm) and methyl
group (expected at ca. 25 ppm) carbon atoms could not be observed
(
30 m ꢂ 0.25 mm; 0.25
m
H
2
13
internal standard. After a 24 h batch run, the solid phase was
separated from the reaction mixture by centrifugation, washed
with n-hexane (PA, SigmaeAldrich), and dried at room temperature
overnight to give the recovered solids referred to in the discussion.
in the C CP MAS NMR spectrum of PMO-Fc. The presence of
methyl groups was, however, supported by FT-IR spectroscopy
2
(Fig. 3). The ATR FT-IR spectra of PMO-NH and PMO-Fc are similar
3
. Results and discussion
3.1. Synthesis
The phenylene-bridged PMO (Ph-PMO) was prepared by the
hydrolysis and polycondensation of 1,4-bis(triethoxysilyl)benzene
in the presence of octadecyltrimethylammonium bromide [9,11].
After synthesis the template was extracted with an ethanol/HCl
solution. The amination of phenylene moieties in Ph-PMO was
achieved by a two-step procedure involving treatment with very
strong acid solutions of HNO
followed by treatment with SnCl
denoted PMO-NH . Elemental analysis of PMO-NH
group density of close to 1.5 mmol g , indicating that 35% of
phenylene moieties in the parent PMO were functionalized with
3
eH
2
SO
eHCl [26]. The final material is
gave an amino
4
to give a nitrated material,
2
2
2
ꢀ1
2
the amino group. Condensation of amino groups in PMO-NH with
acetylferrocene gave the modified material PMO-Fc containing
anchored ferrocenylimine moieties (Fig. 1). ICP-OES analysis gave
ꢀ1
an iron content of 0.21 mmol g . Hence, overall, about 5% of the
phenylene moieties in the PMO were derivatized with ferrocenyl
groups.
Fig. 2. 13C CP MAS NMR spectra of (a) PMO-NH
are denoted by asterisks.
2
and (b) PMO-Fc. Spinning sidebands

504
A.C. Gomes et al. / Journal of Organometallic Chemistry 751 (2014) 501e507
ꢀ1
in the range of 400e4000 cm except that the latter shows an
ꢀ
1
additional weak band at 1460 cm , which is assigned to an
asymmetric deformation mode of methyl groups. A band for the
azomethine (C]N) stretching vibration, generally found between
ꢀ
1
1
620 and 1650 cm , may be present but is obscured by the ab-
ꢀ1
sorption centered at 1625 cm , which has contributions from the
in-plane NeH bending vibration (“scissoring” mode) of aromatic
primary amine groups, and a water bending vibration.
The materials PMO-NH
by 29Si MAS NMR spectroscopy (Fig. 4). The Si MAS and CP MAS
NMR spectra of PMO-NH are in agreement with those reported
previously [27], showing signals for single silanol organosilica
2
and PMO-Fc were further characterized
29
2
2
species of the type [RSi(OSi)
2
(OH)] (T ) and fully condensed
] sites (T ). No significant changes were observed upon
treatment of PMO-NH with acetylferrocene to give PMO-Fc. None
3
[RSi(OSi)
3
2
n
of the spectra display Q signals corresponding to silicon atoms
connected entirely to OSi and/or OH units, which indicates that all
silicon atoms are covalently bonded within the PMO network and
no SieC bond cleavage took place during the chemical
modifications.
Powder XRD and N
and PMO-Fc indicated that the mesoporous structure and the
molecular-scale periodicity in PMO-NH were largely retained
2 2
adsorptionedesorption data for PMO-NH
2
upon treatment with acetylferrocene. Thus, the powder XRD
Fig. 4. 29Si MAS (a, c) and CP MAS (b, d) NMR spectra of PMO-NH2 (a, b) and PMO-Fc (c,
d).
pattern of Ph-PMO shows one strong low-angle peak at a d-spacing
ꢀ
of 48.5 A, which is assigned to the (100) reflection of a two-
dimensional hexagonal symmetry (p6mm) lattice with a lattice
ꢀ
constant a ¼ 56.0 A (Fig. 5). The broad, weak shoulder on this peak
ꢁ
and organosilica framework) due to the functionalization of the
material (which results in a slight loss of surface area and pore
volume, as described below).
towards higher angles (3e4
2q) comprises the overlapping (110)
and (200) reflections. In addition to the low-angle peaks, a
ꢀ
medium-range reflection is observed at d ¼ 7.6 A, which arises from
The N
condensation inside mesopores occurred in the p/p
.4, and multilayer adsorption occurred as p/p tended to unity
Fig. 6). Hence, PMO-Fc possesses a mesoporous structure and
significant external specific surface area. Similar features were
observed for PMO-NH , indicating that the pore structure was
essentially preserved during the modification treatment. Material
2
adsorption isotherm for PMO-Fc indicated that capillary
the molecular-scale periodicity in the Ph-PMO pore walls along the
0
range of 0.2e
2
channel direction [9e11]. The patterns for PMO-NH and PMO-Fc
0
0
are similar except that the (110) and (200) reflections are no
longer clearly resolved, and the (100) reflection is relatively weaker
when compared with that for Ph-PMO. These changes may be due
to a reduction in the XRD scattering contrast (between the pores
(
2
2
Fig. 3. FT-IR (ATR) spectra of (a) acetylferrocene, (b) PMO-NH and (c) PMO-Fc. The
ꢀ
1
arrow marks the appearance of a band at 1460 cm , which is assigned to an asym-
metric deformation mode of methyl groups.
2
Fig. 5. Low angle powder XRD patterns of (a) Ph-PMO, (b) PMO-NH and (c) PMO-Fc.

A.C. Gomes et al. / Journal of Organometallic Chemistry 751 (2014) 501e507
505
Fig. 6. N
2
adsorption isotherms of PMO-NH2 (B) and PMO-Fc (ꢂ).
Fig. 8. TGA curves of acetylferrocene (e $ e $ e $), Ph-PMO ($ $ $ $ $ $), PMO-NH
2
(e e
e e) and PMO-Fc (dd).
0
PMO-Fc possessed a BET specific surface area (calculated for p/p in
2
ꢀ1
the range of 0.01e0.1) of 434 m g , and pore widths (calculated
from the BJH algorithm applied to the adsorption branch) in the
range of 2.8e3.4 nm. The SBET and pore sizes for PMO-Fc are lower
ꢁ
above 200 C may lead to the iron-promoted decomposition and
2
ꢀ1
release of phenylene groups at a temperature significantly lower
than that observed for PMO-NH . For PMO-NH , the mass loss (on a
2 2
dry basis, i.e., after correcting for the physisorbed water content) is
5.9% in the temperature range of 350e775 C, while the mass loss
for PMO-Fc in the range of 200e650 C is 38.6%. Assuming that in
g of PMO-NH
ferrocenyl groups as shown in Fig. 1 (to give a final Fe loading of
.21 mmol g ), the calculated increment in mass loss for the
than those for PMO-NH (580 m g and 3e3.5 nm, respectively),
2
most likely due to the filling of the internal void spaces by the
anchored organometallic species.
ꢁ
3
2
A typical TEM image of PMO-NH along the pore channel axis is
ꢁ
shown in Fig. 7(a). The image shows a locally well-ordered 2D
hexagonal mesostructure. Accordingly, the corresponding FFT
shows bright diffraction spots in a hexagonal arrangement (inset of
1
2 2
0.22 mmol of NH groups were derivatized with
ꢀ
1
0
2
Fig. 7(a)). TEM images of PMO-NH perpendicular to the pore
decomposition step is 2.8% (assuming that Fe will be thermally
removed alongside organic moieties), which is in agreement with
the observed value of 2.7%.
channel axis showed large domains of ordered stripe-like ar-
rangements of the long parallel cylindrical channels. Similar images
were obtained for PMO-Fc (Fig. 7(c)).
The TGA curves for Ph-PMO, PMO-NH
weight loss below 100 C due to desorption of physisorbed water
2
and PMO-Fc exhibit a
ꢁ
3.2. Catalytic oxidation of styrene
2
(Fig. 8). For Ph-PMO [9,27], PMO-NH and PMO-Fc, the decompo-
sition and release of organic moieties from the framework takes
PMO-Fc was tested as a catalyst for the oxidation of styrene (Sty)
ꢁ
ꢁ
ꢁ
places in one step, starting at ca. 530 C for PMO (DTGmax ¼ 620 C),
with different types of oxidants, at 55 C, using CH CN as solvent
3
ꢁ
ꢁ
ꢁ
3
(
50 C for PMO-NH
2
(DTGmax ¼ 610 C) and 200 C for PMO-Fc
(Fe:Sty:oxidant molar ratios of 0.06:100:200). The oxidants used
were aqueous hydrogen peroxide (H O ), urea hydrogen peroxide,
ꢁ
DTGmax ¼ 505 C). Hence there is a progressive decrease in ther-
2
2
mal stability upon amination and subsequent anchoring of ferro-
cenyl groups. Decomposition of the ferrocenyl groups in PMO-Fc
and tert-butylhydroperoxide in decane or water. Of the tested ox-
idants, H O led to the highest Sty conversion at 24 h reaction (8%)
2 2
2
Fig. 7. TEM images of PMO-NH (a, b) and PMO-Fc (c). The inset in (a) is the corresponding fast Fourier transform (FFT).

506
A.C. Gomes et al. / Journal of Organometallic Chemistry 751 (2014) 501e507
with benzaldehyde (BzCHO) as the only reaction product; for the
remaining oxidants the reaction did not occur.
and 24 h reaction was comparable to that for the typical catalytic
test (without filtration), suggesting that active species were leached
from PMO-Fc into solution during the catalytic reaction. A likely
explanation is that in the presence of water the imine group hy-
drolyses back to the ketone and the amine, resulting in the release
of acetylferrocene into the liquid phase. The instability of the imine
group was noted previously for the ordered mesoporous silica
MCM-41 functionalized with a pyridylimine ligand, used for
complexation with molybdenum tetracarbonyl [47] and bischlor-
ocopper(II) [48] species.
The catalytic performance of PMO-Fc was further explored using
ꢁ
H
2
O
2
as oxidant, and CH
3
CN or THF as solvent, at 55 C (the oxidant
was added gradually throughout 5 h reaction, to give Fe:(initial
Sty):(total added oxidant) molar ratios of 0.06:100:500). The Sty
conversion at 24 h reaction was 16% and 19% for CH
solvents, respectively, and BzCHO was always the only reaction
product (Table 1). Using PMO-NH instead of PMO-Fc as catalyst
3
CN and THF as
2
under similar conditions led to 3% conversion at 24 h reaction.
These results indicate that the active species contain iron. Iodo-
metric titration of the mixture of PMO-Fc/H
substrate) obtained after stirring for 5.5 h at 55 C gave a “non-
2
O
2
/CH
3
CN (without
4. Conclusion
ꢁ
2 2
productive” decomposition of H O which lied within the range of
experimental error (5%). A four-fold increase in the catalyst loading
led to higher Sty conversion at 24 h: 28% and 30% conversion, for
In the present study we have demonstrated that crystal-like
mesoporous phenylene-silica can be derivatized by performing a
condensation reaction between phenylene-substituted amino
groups and acetylferrocene. A ferrocenylimine-modified PMO was
obtained and characterized in the solid state by various techniques.
Although the ferrocene-PMO hybrid is effective in the iron-
CH
and styrene oxide (StyO), with BzCHO formed in 25% and 27% yield
for CH CN and THF, respectively. It is noteworthy that the two
3
CN and THF, respectively. The reaction products were BzCHO
3
solvents tested led to similar catalytic results. The conversion of Sty
to BzCHO may involve StyO as intermediate (with subsequent hy-
drolysis) or the direct oxidative cleavage of the double bond of the
olefin (involving a radical mechanism) [40,42,44e46]. Using StyO
as substrate instead of Sty, and the same catalytic conditions, led to
catalyzed oxidation of styrene with H
2
O
2
under mild conditions
ꢁ
at 55 C, giving benzaldehyde as the major product with a selec-
tivity typically greater than 90%, active species are leached from the
modified PMO into solution during the catalytic reaction, probably
due to the pronounced water sensitivity of the azomethine linkage.
Other applications can be envisaged for the ferrocene-PMO hybrid,
such as in the field of optical and electro-optical materials. The
post-synthetic derivatization method reported here may be
generally applicable for incorporating a wide range of functional
species into crystal-like phenylene-bridged PMOs.
100% conversion at 24 h reaction, and BzCHO was formed in 55%
yield (additionally, 2-hydroxyacetophenone was formed in 33%
yield). Based on these results, the conversion of Sty to BzCHO in-
volves StyO as intermediate at least to a certain extent.
As mentioned in the Introduction, one related published work
on the oxidation of styrene is that of Zhang et al. [38], who used
mesoporous SBA-15-supported ferric oxide nanoparticles (6.5e
Acknowledgments
2
2 2
4 mol% Fe content; H O as oxidant; acetone as solvent), prepared
by calcination of a ferrocene-loaded precursor. With a catalyst
We are grateful to the Fundação para a Ciência e a Tecnologia
FCT), Fundo Europeu de Desenvolvimento Regional (FEDER),
QREN-COMPETE (QUI-QUI/113678/2009), the European Union, and
the Associate Laboratory CICECO (PEst-C/CTM/LA0011/2013) for
continued support and funding. The FCT and the European Union
are acknowledged for a post-doctoral grant to S.M.B (SFRH/BPD/
loading equivalent to 65e240 mol% of Fe relative to Sty, and a
(
ꢁ
Sty:H
2
O
2
molar ratio of 1:1, at 50 C, 7e12% conversion was reached
at 24 h reaction, and BzCHO was the main product (91e94%
selectivity). Based on these results, PMO-Fc seems to be a fairly
active and more selective catalyst than the Fe-containing SBA-15
material reported by Zhang et al.
4
6473/2008) cofunded by MCTES and the European Social Fund
3
The catalytic stability of PMO-Fc was investigated using CH CN
through the program POPH of QREN. The FCT is acknowledged for
financing the PhD grant SFRH/BD/80883/2011 (to M.A.O.L.). The
authors thank the National Network of electron microscopy e
University of Aveiro Project REDE/1509/RME/2005.
as solvent, by reusing the recovered solid for two consecutive 24 h
batch runs (Fe:Sty:oxidant molar ratios of 0.06:100:500). A
decrease in the catalytic activity from run 1 to run 2 was observed
(
16% and 10% conversion, respectively), and BzCHO was always the
only reaction product. A hot-filtration test (using a 0.2
m
m PTFE GVS
ꢁ
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