Microwave-assisted silylation of graphite oxide and iron(III) porphyrin intercalation

Article abstract of DOI:10.1016/j.poly.2014.07.003

The hybrid material graphite oxide (GO) intercalated with an iron(III) porphyrin was obtained upon a silylation reaction of GO with 3-bromotrimethoxypropylsilane (BrTMS) followed by metalloporphyrin immobilization. Diverse reaction conditions and microwav

Full text of DOI:10.1016/j.poly.2014.07.003

Polyhedron 81 (2014) 475–484
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Microwave-assisted silylation of graphite oxide and iron(III) porphyrin
intercalation
a
a
a,
⇑
c
Monika E. Lipi n´ ska , João P. Novais , Susana L.H. Rebelo , Belén Bachiller-Baeza ,
c
b
a,
⇑
Inmaculada Rodríguez-Ramos , Antonio Guerrero-Ruiz , Cristina Freire
a
REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua Campo Alegre s/n, 4169-007 Porto, Portugal
Dpto. de Química Inorgánica y Química Técnica, Facultad de Ciencias, UNED, Senda del Rey 9, 28040 Madrid, Spain
Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, Cantoblanco, 28049 Madrid, Spain
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The hybrid material graphite oxide (GO) intercalated with an iron(III) porphyrin was obtained upon a
silylation reaction of GO with 3-bromotrimethoxypropylsilane (BrTMS) followed by metalloporphyrin
immobilization. Diverse reaction conditions and microwave versus conventional heating were tested.
The materials were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy
Received 28 March 2014
Accepted 2 July 2014
Available online 10 July 2014
(
XPS), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry (TGA) and temperature pro-
Keywords:
grammed desorption (TPD). Microwave-assisted synthesis allowed functionalization reactions of 1 h
instead of 24 h (conventional heating) with equivalent or improved yields on both silylation and metal-
loporphyrin immobilization reactions. The immobilizations performed in anhydrous solvent and absence
of other exfoliation agents led to an increase on the GO interlayer distance of 0.14 nm, in a total space of
Graphite oxide
Iron(III) porphyrin
Microwave-assisted synthesis
Silylation
7
Å that match the metalloporphyrin thickness.
Catalysis
Ó 2014 Elsevier Ltd. All rights reserved.
1
. Introduction
Graphite oxide (GO) is a layered carbon material with high
enable the fine tuning of surface properties, which is important
to obtain selective adsorption and dispersibility in different
solvents [15]. In this context, the grafting of silicon-containing
molecules on GO, such as polymers [16] or organosilanes [17,18],
provides straightforward covalent functionalization strategies.
Most of the up to date studies have been focused on the anchoring
of chloro- and alkoxysilanes carrying an extended aliphatic chain
or with a second functionality, in order to increase the interlayer
distances or allow the anchoring of other moieties. These studies
revealed that the organosilanes mainly react with the hydroxyl
groups on the GO surface to establish Si–O-GO bonds [19,20].
The presence of exfoliating or stabilizing agents was also
considered [21,22], but its excess showed to prevent the attack
of organosilane on the GO surface, thus reducing the extension of
the reaction [21].
When aminoalkylethoxysilanes were used, the organosilane
moieties or coupling dimmers can bind to two different superim-
posed GO sheets, through nucleophilic attack on the silane ethoxy
groups by OH-GO functionalities and by amidation reaction on the
HOOC-GO groups or nucleophilic attack on epoxy-GO groups [23].
This can led to structures with stable interlayer distance, assuring a
good dispersion, avoiding aggregation and enabling the formation
of the matrices for molecules intercalation.
hydrophilic character, owing to the presence of a significant
amount of oxygen groups, mainly epoxy and hydroxy groups in
the basal layer surfaces or carboxylic groups in the edges of the
graphene sheets [1]. These functional groups assure good dispersi-
bility, easy processability and compatibility with other materials,
being particularly prone to obtain further functionalizations [2,3].
Moreover, the swelling and exfoliation capabilities ensure the
formation of solid matrices and host materials, allowing the
intercalation of different molecules and the construction of
supramolecular systems [4]. Together with the good mechanical
properties and the large surface area of GO, these inherent charac-
teristics made of this material one of the most promising carbon
materials for different applications [5], like photochemical reac-
tions [6], drug delivery [7] and catalysis [8–11].
Derived materials can be originated from chemical and temper-
ature treatments of GO as reduced GO [12], GO framework materi-
als [13] or pillared GO [14]. Moreover, the functionalizations
⇑
Corresponding authors. Tel.: +351 220402590; fax: +351 220402695.
E-mail addresses: susana.rebelo@fc.up.pt (S.L.H. Rebelo), acfreire@fc.up.pt
(
C. Freire).
http://dx.doi.org/10.1016/j.poly.2014.07.003
277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
0

4
76
M.E. Lipi n´ ska et al. / Polyhedron 81 (2014) 475–484
Metalloporphyrins have been extensively investigated in differ-
The error in determination of electron binding energies and line
widths did not exceed 0.2 eV. The raw XPS spectra were deconvo-
luted by curve fitting of the peak components using the software
XPSPEAK 4.1 with no preliminary smoothing. Symmetric
Gaussian–Lorentzian product functions were used to approximate
the line shapes of the fitting components after a Shirley-type
background subtraction. Atomic ratios were calculated from exper-
imental intensity ratios and normalized by atomic sensitivity
factors.
ent fields, such as in artificial photosynthesis, in medicine and
catalysis [24]. For all these applications, the development of suit-
able methodologies for their immobilization on appropriate sup-
ports is presently
a goal of huge relevance. Hybrids of
porphyrinic chromophores and GO have been prepared as donor–
acceptor structures for solar light energy conversion, non-linear
optical absorption [25–27] and biosensing [28]. Thus, the applica-
tion of GO as a versatile, easily available and inexpensive support
for metalloporphyrin catalysts or as carrier for metalloporphyrins
with biological activity or sensing capabilities is very promising
29–33], although, in practice, these have been quite unexplored
fields.
Herein, an iron(III) porphyrin carrying tetrafluorodimethylami-
Fourier transform infrared spectra (FTIR) of the materials were
collected in a Jasco FT/IR-460 Plus spectrophotometer in the
ꢁ1
ꢁ1
[
400–4000 cm range with a resolution of 4 cm and 32 scans.
The spectra were obtained in KBr pellets containing 0.2% weight
of GO materials.
no groups was immobilized onto GO (Fig. 1). Initially, the silylation
of GO with 3-bromopropyltrimethoxysilane (BrTMS) was devel-
oped followed by metalloporphyrin intercalation; both reactions
were optimized screening reaction conditions and using micro-
wave versus conventional heating [34,35]. The epoxidation of
cis-cyclooctene was used as a proof of concept for the iron(III)
porphyrin immobilization on its catalytic-active form.
Thermogravimetric analysis (TGA) was performed on a CI Elec-
tronics microbalance (MK2-MC5). All samples (typically 10 mg)
were heated under helium flow to 850 °C at a rate 5 °C/min.
Temperature programmed desorption – mass spectroscopy
(TPD–MS) experiments were carried out under vacuum in a
conventional volumetric apparatus connected to a SRS RGA-200
mass spectrometer. The sample was evacuated for 30 min at room
temperature and then ramped to 700 °C at a rate 5 °C/min.
GC-FID chromatograms were obtained with a Varian CP-3380
gas chromatograph equipped with a FID detector, using hydrogen
as carrier gas and a fused silica Varian Chrompack capillary column
2
. Experimental
2.1. Materials
CP-Sil 8 CB Low Bleed/MS (30 m ꢂ 0.25 mm id; 0.25
lm film thick-
All reagents and solvents were used as received without further
ness). The epoxidation reactions were followed with temperature
ꢁ1
purification. In the synthesis reactions and catalytic studies were
used: natural graphite powder (99.999% stated purity, 200 mesh,
Alfar Aesar), fuming nitric acid (>90%, Panreac), potassium chlorate
program: 70 °C (1 min), 20 °C min , 200 °C (8 min); injector
temperature: 200 °C; detector temperature: 250 °C.
(
(
ACS Reagent + 99%, Sigma–Aldrich), pentafluorobenzaldehyde
P98%, Alfa Aesar), pyrrole (98%, Sigma–Aldrich), diethyl
2.3. Microwave synthesis
ether (P98%, Merck), dichloromethane (98%, Fischer), methanol
>99.8%, Chem-Lab NV), iron(II) chloride (FeCl O, Merck),
ꢀ4H
chloroform (98%, Fischer), 3-bromopropyltrimethoxysilane
BrTMS, ABCR), anhydrous toluene (P99.8%, Sigma–Aldrich), cis-
cyclooctene (95%, Sigma–Aldrich), decane (P99%, Sigma–Aldrich),
hydrogen peroxide (H 2(aq) 30% (w/w), Sigma–Aldrich), N,N-
dimethylformamide (DMF, 98%, Fischer), N-methyl-2-pyrrolidone
NMP, P99%, Sigma–Aldrich), ethanol absolute (98%, Fischer),
o-dichlorobenzene anhydrous (>99%, Aldrich). In all filtration
Microwave synthesis was carried out using CEM Discover
LabMate reactor with pressure and temperature control system
in 10 mL reaction vessel. The reaction mixture was constantly irra-
diated in closed system using 150 W maximum power at the
desired temperature.
(
2
2
(
2
O
2.4. Preparation of the materials
(
2.4.1. Preparation of graphite oxide (GO)
procedures were used 0.2
Whatman).
lm polyamide membrane filters (NL16
Graphite oxide was synthesized from natural graphite powder
using a modified Brodie’s method [36]. Typically, 10 g of graphite
3
was added to 200 mL of fuming HNO , which was previously
2
.2. Physicochemical characterization
cooled to 0 °C in an iced bath. Then 80 g of potassium chlorate
was slowly added. The reaction mixture was kept under stirring
for 21 h, then filtered. Obtained material was extensively washed
with deionized water until neutral pH and dried under vacuum
at 60 °C overnight.
Powder X-ray diffractograms (XRD) were recorded on a Polycri-
stal XPert Pro PANalytical apparatus using Ni-filtered Cu K radia-
tion (k = 0.15406 nm) and a graphite monochromator. For each
sample, Bragg’s angles between 4° and 90° were scanned at a rate
of 0.04°/s.
a
2.4.2. Synthesis of iron(III) porphyrin (FeP)
X-ray photoelectron spectroscopy (XPS) was performed on
2
The base porphyrin H TPFPP and its iron(III) complex were syn-
ESCAPROBE
EA-125 hemispherical multichannel Electronics analyzer. The
excitation source was the Mg K line (h = 1253.6 eV, 300 W).
P
spectrometer from OMICRON equipped with
thesized through previously reported methodologies [37]. How-
ever, in the pyrrole and pentafluorobenzaldehyde condensation
reaction, nitrobenzene as hazardous solvent was eliminated and
a
m
Fig. 1. Schematic representation of the GO silylation with bromosilane followed by iron(III) porphyrin immobilization. Both reactions were carried out in toluene at 110 °C
using conventional and microwave heating.

M.E. Lipi n´ ska et al. / Polyhedron 81 (2014) 475–484
477
the reaction was carried out in glacial acetic acid. The complexa-
tion of the free porphyrin with iron(II) chloride in refluxing DMF
procedure, the material FeP-sGOmw was extensively washed with
the reaction solvent, ethanol and diethyl ether and finally dried
under vacuum at 120 °C overnight.
[
38] led to the meso-tetrakis-(4-N,N-dimethylamine-2,3,5,6-tetra-
fluoro)porphyrinate iron(III) chloride denoted as FeP.
2.4.5. Direct reaction of FeP and GO
About 250 mg of GO were dispersed in 200 mL of NaOH 0.015 M
2.4.3. Silylation of graphite oxide (sGO)
and stirred for 1 h at 100 °C. The obtained material was washed
with water until pH 8, filtered and dried in the oven. To 200 mg
of this material and 40 mg of FeP were added 8 mL of o-dichloro-
benzene and the mixture was sonicated for 30 min. The vessel
was introduced in the microwave apparatus and the reaction mix-
ture was heated at 180 °C and irradiated at the pressure of 200 psi.
After that time the solid was filtered, washed, dispersed again in
In order to obtain optimized conditions, different heating
modes (conventional or microwave), solvents (toluene, DMF,
NMP), reaction times and temperatures were tested as described
in Table 1.
Method 1 (conventional heating – ch): In a typical reaction,
00 mg of GO were sonicated for 30 min in 200 mL of the chosen
solvent and then 0.15 mL of BrTMS were added. The reaction mix-
ture was refluxed under argon atmosphere during 24 h. Then, the
solid was filtered, washed, dispersed again in a new portion of
8
6
1
0 mL of the solvent, sonicated for 10 min and refluxed during
h. After the filtration by the previous procedure, the material
FeP-GOmw was extensively washed with the clean o-dichloroben-
zene, ethanol and diethyl ether and finally dried under vacuum at
2
00 mL of the reaction solvent, sonicated for 10 min and refluxed
during 1 h. After the filtration, the material sGOch was extensively
washed with the reaction solvent and diethyl ether and finally
dried under vacuum at 120 °C overnight.
1
2
2
0
20 °C overnight.
.5. Catalytic studies
Method 2 (microwave-assisted synthesis – mw): In a typical
reaction, 200 mg of GO were sonicated for 30 min in 8 mL of the
chosen solvent and then 0.04 mL of BrTMS were added to the reac-
tion vessel. The vessel was introduced in the microwave apparatus
and reaction was heated and irradiated, following the conditions
presented in Table 1, at a pressure of 200 psi. Then, the solid was
filtered, washed, transferred to a round bottom flask, again dis-
persed in 200 mL of the same solvent, sonicated during 10 min
and refluxed during 1 h. After the filtration, the material sGOmw
was extensively washed with the reaction solvent and diethyl
ether and finally dried under vacuum at 120 °C overnight.
.5.1. Epoxidation of cis-cyclooctene
Typically the reaction mixture was prepared by dissolving
.5 mmol of cis-cyclooctene and 0.2 mmol of decane as internal
standard, in 1.5 mL of ethanol. To this mixture 25 mg of the mate-
rial to be tested as catalyst were added followed by sonication dur-
ing 15 min. Then, the mixture was kept under magnetic stirring,
protected from light and at controlled temperature between 20
2 2
and 23 °C. The oxidant [H O 30% w/w (aq.)], diluted 1:10 in etha-
nol, was progressively added in aliquots of 0.5 or 0.25 mmol, at 1 h
intervals. After 55 min of each oxidant addition the reaction was
sonicated during 5 min in order to disperse the material, an aliquot
of this mixture was removed for GC analysis and a novel oxidant
aliquot was added. To perform GC analysis, 0.1 mL of reaction mix-
ture was sampled, centrifuged (15 min, 6000 rpm) to separate the
solid and the supernatant was directly injected on the GC.
2
.4.4. Preparation of FeP-sGO materials
In order to obtain optimized conditions, different heating
modes (conventional or microwave), solvents (toluene, NMP),
reaction times and temperatures were tested as described in
Table 5.
Method 1 (conventional heating – ch): In a typical reaction,
0 mg of FeP were added to 400 mg of sGOch1 and sonicated dur-
ing 30 min in 120 mL of the chosen solvent. The reaction mixture
was refluxed under argon atmosphere for 24 h. Then, the solid
was filtered, rinsed and dispersed in a new portion of 120 mL of
the reaction solvent, sonicated during 10 min and refluxed during
h. After the filtration, the material FeP-sGOch was extensively
washed with the reaction solvent, ethanol and diethyl ether and
finally dried under vacuum at 120 °C overnight.
Method 2 (microwave-assisted synthesis – mw): In a typical
reaction, 40 mg of FeP and 200 mg of sGOmw were put in the reac-
tion vessel and dispersed in 8 mL of the chosen solvent by sonica-
tion during 30 min. The vessel was introduced in the microwave
apparatus and the reaction mixture was heated and irradiated, at
the pressure of 200 psi. After that time the solid was filtered,
washed, transferred to a round bottom flask, dispersed again in
2
.5.2. Catalytic parameters
Conversion (C%) was determined both by calculation of reacted
8
substrate using the internal standard method (Ai.s. = area of
internal standard) and based on the chromatographic peak areas
of substrate (Asub.) and products (Aprod.) for each chromatogram.
The conversion values obtained with both procedures always
agreed within 5%.
1
ꢀ
ꢁ
ꢀ
ꢁ
A
A
sub:
i:s:
A
A
sub:
i:s:
ꢁ
t¼0
t¼xh
C% ¼
ꢀ
ꢁ
ꢂ 100
A
sub:
A
i:s:
t¼0
A
prod:
C% ¼ A
ꢂ 100
sub: þ Aprod:
The product selectivity was also calculated based on the chro-
matographic peak areas and calculated as follows:
6
1
0 mL of clean reaction solvent and followed by sonication during
0 min and reflux during 1 h. After the filtration by the previous
S% ¼ ½Aepoxideꢃ ꢂ 100=½Aprod:
ꢃ
The turnover number (TON) was calculated through the ratio
between the number of mols of the converted substrate (nconv.sub.
and the mol of the metalloporphyrin in the catalyst (nFeP) and the
turn over frequency (TOF) was determined by the calculation of
TON after the 1 h of the reaction (TONt=1h). The nFeP corresponds
to the iron loading obtained by XPS, in mmol/g (material).
Table 1
)
Experimental conditions used for GO silylation in the different synthesis.
Entry
Material
Heating
Solvent
T (°C)
t (h)
Other
1
2
3
4
5
6
7
sGOch1
sGOch2
conventional
conventional
microwave
microwave
microwave
microwave
microwave
toluene
toluene
toluene
toluene
toluene
DMF
110
60
24
5
1
2
1
sGOmw1
sGOmw2
sGOmw3
sGOmw4
sGOmw5
110
110
110
140
200
n
conv:sub:
TON ¼
Open vessel
n
FeP
1
1
NMP
TOF ¼ TONt¼1h

4
78
M.E. Lipi n´ ska et al. / Polyhedron 81 (2014) 475–484
Table 2
3
. Results and discussion
The X-ray diffraction angels and corresponding interlayer distances.
3.1. Organosilylation of graphene oxide
Material
2h (°)
d (nm)a
GO
15.8
0.564
The preparation of iron(III) porphyrin-GO hybrid was preceded
sGOch1
sGOch2
sGOmw1
FeP-sGOch1
13.4
15.4; 15.7
13.0
0.660
0.575; 0.564
0.685
by the development of a practical procedure for GO silylation
with 3-bromopropyltrimethoxysilane (BrTMS). Previous studies
revealed that the effectiveness of GO silylation depends on the
temperature and reaction time, as well as, on the presence of silane
polymerization inducers, such as water or exfoliating agents
12.6
0.702
a
d-Spacing corresponding to the main (100) XRD reflection: d100 = k/(2sinh).
[
16,21]. The silylation of GO with BrTMS was developed by testing
different conditions as summarized in Table 1.
The reaction was carried out using conventional heating at
The diffraction peak of silylated material sGOch1 (entry 1,
1
10 °C, during 24 h in anhydrous toluene in order to avoid poly-
Table 1), is observed at 2h = 13.4° and indicates an increase in
the interlayer distance to 0.660 nm caused by the introduction of
organosilane [14]. In the material sGOch2 synthesized at lower
temperatures and lower reaction time (60 °C during 5 h, entry 2),
two diffraction peaks at 2h = 15.4° and 15.7° are observed, what
evidences the co-existence of poorly silylated and non silylated
regions on GO [21] and confirms the inefficiency of the experimen-
tal conditions used.
merization and loss of bromide from the alkyl bromide functional
groups; from this synthesis resulted material sGOch1 (entry 1,
Table 1). Longer reaction times have shown to not improve
silylation efficiency [21]. Due to the labiality of the alkylbromine
moiety, milder silylation conditions were also tested. In accor-
dance, the silylation was performed in the same solvent but at
6
0 °C during 5 h (entry 2) to afford material sGOch2 [16].
The microwave irradiation was tested in reflux of anhydrous
On the other hand, using microwave assisted synthesis, upon
1 h of the reaction time (Table 1, entry 3) sGOmw1 exhibits the
diffraction peak at 2h = 13.0°, which corresponds to an interlayer
distance of 0.685 nm, slightly larger compared with the material
sGOch1 prepared by the conventional heating. The intensity of
the diffraction peaks of the sGO materials decreased relatively to
GO, also corroborating the introduction of the silane moieties in
the interlayers of the material.
toluene, during 1 h (entry 3) or 2 h (entry 4), leading to materials
sGOmw1 and sGOmw2, respectively. Using the same conditions
as in entry 3, the reaction was also performed in ‘‘open vessel’’
condition, due to the possibility of using bigger reaction volumes
and increasing the reaction scale (sGOmw3, entry 5). Higher
reaction temperatures were also tested using higher boiling point
solvents, such as DMF (sGOmw4, entry 6) and NMP (sGOmw5, entry
7
), which also could led to an increased exfoliation/dispersion of
Table 3 summarizes the XPS surface atomic percentages for the
synthesized materials. The original GO presents 75.2% of carbon
and 24.8% of oxygen atoms. After reaction with BrTMS using con-
ventional and microwave-assisted heating to afford materials
sGOch1, sGOmw1 and sGOmw2, the carbon percentage was practi-
cally maintained, although, the oxygen percentage decreased of ca.
GO [39].
3.2. Physicochemical characterization of silylated graphite oxide
The XRD diffractograms of GO and silylated GO materials –
1
.2% at the expense of the appearance of silicon and bromine
sGOch1, sGOch2 and sGOmw1 – are collected in Fig. 2. The diffrac-
tion peak of GO is observed at 2h = 15.8° that corresponds to lattice
parameters (001) and a d-spacing of 0.564 nm (Table 2). This
indicates a graphite oxidation in a median extension relatively to
other functionalization protocols described in the literature [40],
aiming to provide enough functional groups to obtain further
derivatizations, without a strong disruption of the graphene layers
planar structure. The diffraction peak characteristic for graphite
at 2h = 26° is absent in GO material, and this indicates the
homogeneous functionalization/oxidation of graphite that is being
observed in one single phase.
ꢁ1
elements. The material sGOch1 shows 1.1% of Si (830
and 0.2% of Br (150
l
molꢀg
)
ꢁ1
l
molꢀg ). The low atomic ratio Br/S = 0.2, rel-
atively to the expected Br/S = 1, is the consequence of the labiality
of the alkylbromine moiety and its probable reaction with nucleo-
philic groups. It can be hypothesized that some of the organosilane
units are bonded to both superior and inferior sheets (resulting
from reaction of the methoxy groups in one side and by reaction
of the alkylbromine moieties with the hydroxyl groups in the other
side); this can maintain a stable interlayer distance and form a
more appropriate host material.
Fig. 2. XRD diffractograms for (a) original GO and (b) functionalized materials.

M.E. Lipi n´ ska et al. / Polyhedron 81 (2014) 475–484
479
ꢁ
1
Table 3
1060 cm
(m
C–O) are endorsed to hydroxyl, carboxyl and epoxy
a
Surface atomic percentages of materials obtained by XPS.
groups. Some of these bands are overlapped by the vibrations from
the carbon structure, at 1620 cm
ꢁ1
ꢁ1
At%
(m C@C) and at 1060 cm (C–C
skeletal tangential motions) [42]. The spectrum also confirms the
presence of higher amounts of hydroxyl and epoxy groups than
carboxylic functionalities.
Sample
C1s
O1s Si2p3/2 Br3d5/2 N1s F1s Fe2p3/2 Cl2p3/2
GO
sGOch1
75.2 24.8
75.1 23.6 1.1
75.1 23.8 0.9
75.1 23.4 1.3
75.2 24.7 0.1
74.4 20.5 0.8
0.2
0.2
0.2
0.02
–
sGOmw1
sGOmw2
sGOmw3
FeP-sGOch1
FeP-sGOmw1 72.9 21.7 0.4
FeP-sGOmw2 73.3 21.1 0.5
After the silylation, the most notorious changes in the spectral
features are related with the introduction of three bands, centered
ꢁ1
at 2920 cm
1
(m C–H) together with the shoulder at near
2.0
1.8
1.8
2.0 0.1
2.7 0.2
2.8 0.2
0.2
0.3
0.3
ꢁ1
480 cm (d C–H) in accordance with the introduction of alkyl
–
–
and alkoxy chains from grafted organosilane [42]. The series of
ꢁ1
the bands between 1200 and 1000 cm are also assigned to asym-
metric stretching vibrations of Si-O-R groups [19].
a
Determined by the areas of the respective bands in the high-resolution XPS
spectra.
The TGA curve of GO in an inert atmosphere (Fig. 5) shows that
GO is thermally stable up to ca. 250 °C, where occurs an intense
mass decrease associated with the pyrolysis of the label oxygen
groups and release of different gases (see below the TPD analyzes).
After this temperature, GO shows a progressive, slow rate mass
loss up to a total mass loss of 34% (Table 4).
The silylated materials show thermogravimetric profiles similar
to original GO (Fig. 5 and Fig. 2, SI). For material sGOch1 the strong
mass loss occurred at a slightly lower temperature relatively to GO
(Fig. 5) and from Table 4, it can be seen that the final total mass loss
increased to 37% relatively to the 34% observed for original GO.
Material sGOch2 does not show higher mass loss relatively to GO,
corroborating the very low functionalization indicated previously.
Material sGOmw2 showed the highest mass loss among the silylat-
ed materials, namely 44%, in the region 200–300 °C, and is
observed at slightly lower temperature compared with GO. Other
materials silylated with BrTMS also showed a decomposition step
in the range 200–300 °C [35].
Materials sGOmw1 and sGOmw2 show Si% in the range 0.9 –
.3% although the Br% = 0.2 is identical for the three materials.
1
Thus, the results evidence identical functionalization degree for
the three materials and attest the advantage of the microwave-
assisted synthesis relatively to conventional heating, which allows
lower consumption of time and energy. Furthermore, these results
allow concluding that there is no advantage in extending the reac-
tion time for more than 1 h in the microwave protocol.
The corresponding high resolution spectra of the relevant
elements of GO and sGOmw1 materials are collected in Fig. 3. Very
similar spectral patterns are observed for sGOch1; the correspond-
ing spectra are presented in Fig. 1a, SI.
The XPS high resolution spectrum of GO in the C1s region was
deconvoluted into four bands (Fig. 3a). The small peak at the low-
est binding energy (B.E.), at 283.7 eV is ascribed to carbon in the
2
graphitic structure and aromatic rings (sp ); the peak at 285.1 eV
Temperature-programmed desorption (TPD) was performed
monitoring the release of possible degradation fragments, namely
3
to carbon–carbon single bond in the carbon structure (sp ). The
other two peaks are associated with the presence of the oxygen-
containing groups: the peak at 286.9 eV with the hydroxyl and
epoxy/ether groups (C–O) and the smaller peak at 288.6 eV with
the carbonyl/carboxyl groups (C@O). In the O1s region spectrum
two peaks are present: a small peak at 529.3 eV is assigned to
O@C bond and a much more intense at 531.2 eV to O–C bond, sug-
gesting that hydroxyl or epoxide groups are more abundant than
the carboxylic acid groups [41].
m/z 44 (CO
42 (CH CH
lyzed temperature range the most significant fragments in GO
were CO , CO, H O, and H , while in silylated materials were CO
CO, H O and the fragment with m/z 43 from the CH CH CH moi-
ety. The results in the temperature range of 150–450 °C are shown
Fig. 3, SI, since in the temperature range of 450–750 °C no signifi-
cant gas release was observed.
2
), 28 (CO); 18 (H
2
O), 32 (O
2
), 2 (H
2
), 43 (CH
3 2 2
CH CH ),
2
2
CH ), 80 (HBr), 27 (CH
2
3
CH
2
), 94 (CH
3
Br). In the ana-
2
2
2
2
,
2
3
2
2
The C1s spectrum of sGOmw1 (Fig. 3b) shows a small increase in
the intensity of the band at 283.9 eV, which is related with the con-
tribution of C–Si band due to the grafted organosilane [16]. In the
O1s region of this material there are some shifts in the peak posi-
tions and the band at 529.4 eV is relatively more intense than the
corresponding band in GO O1s spectrum, what is a result of the
contribution of O–Si band from the grafted organosilane [19].
The Si2p spectrum shows a broad band at 103.1 eV, being a slightly
higher B.E. than the typical values observed for Si–OH and Si–O–C
bonds and was previously observed when these peaks are over-
lapped by peaks due to the presence of Si–O–Si bonds, the latter
being a consequence of polymerization or intermolecular silane
coupling [19]. Even if an anhydrous solvent was used, the highly
polar and protonic GO surface can catalyze the hydrolysis of the
methoxy groups from the organosilane and consequently, these
Si–OH groups can undergo nucleophilic attack on the silicon atom
of one other silane molecule [40]. So, it can also be considered that
some organosilane dimmers or polymers are present.
The oxygenated surface groups are thermally decomposed
releasing CO and/or CO and the decomposition temperature and
2
the type of the gas released during the analysis give information
about the nature of the groups [19]. Water molecules are present
in the interlayer space and maintain the GO structure by hydrogen
bridging. The detection of the fragment m/z 43 further confirmed
2
the occurrence of the silylation reaction. The CO release at lower
temperature (near 300 °C) is associated with the decomposition
of carboxylic acids groups, while the CO release near that temper-
ature results from the decomposition of the epoxy or non-aromatic
hydroxyl groups, since phenolic hydroxyl or quinone groups
decompose releasing CO at higher temperatures, near 800 °C, and
phenolic groups positioned in the deep basal planes are expected
to decompose at even higher temperatures. In our case these last
decompositions are above the monitoring range [19].
In Fig. 6 and in Fig. 3, SI, the TPD profiles of the materials show
that CO is the gas released in the highest amount, followed by
2
water and carbon monoxide. This observation, although commonly
observed in the TPD of GO materials [43,14], apparently contrast
with the higher number of hydroxyl groups relatively to carboxylic
groups observed by the previously discussed techniques, namely
XPS and FTIR. This can be explained considering that only non-aro-
matic hydroxyl groups and epoxy moieties are being monitored in
the studied temperature range and not the hydroxyl groups from
phenolic moieties, which are expected at higher temperatures [44].
In the Br3d3 spectrum it can be observed a band at B.E. 76.8 eV
ascribed to the presence of Br–C bond in the alkyl chain of alkyl-
bromosilane [35].
The FTIR spectrum of GO is depicted in Fig. 4 and shows peaks
related with the oxygen-containing groups present on its surface
ꢁ
ꢁ1
1
and with the carbon structure. The bands at 3460 cm
(
m
O–H),
ꢁ
1
ꢁ1
1
740 cm
(m
C@O), 1620 cm
(d O–H), 1390 cm
(d C–OH),

4
80
M.E. Lipi n´ ska et al. / Polyhedron 81 (2014) 475–484
Fig. 3. XPS high resolution spectra of relevant elements present on materials: (a) GO; (b) sGOmw1; (c) FeP-sGOmw1.
It can also be observed that in material sGOch1 the amount of
is lower than for original GO and for material sGOmw2
Fig. 6a); this can be related with the longer reaction times used
in the conventional heating protocol, with increased decomposi-
tion of the material, namely of carboxylic groups. On the other
hand, CO is released in lower amount from material sGOmw2 than
from original GO (Fig. 6b); this can confirm the silylation reaction
of the OH groups from GO. However, in material sGOch1 the CO
peak approach the one of GO, which is probably due to increased
hydrolysis of the organosilane methoxy groups to hydroxylated
CO
(
2

M.E. Lipi n´ ska et al. / Polyhedron 81 (2014) 475–484
481
Table 5
Experimental conditions for the immobilization of the Fe(III)porphyrin.
Entry
Material
Heating
Solvent
T (°C)
t (h)
1
2
3
4
5
FeP-sGOch1
FeP-sGOmw1
FeP-sGOmw2
FeP-sGOmw3
conventional
microwave
microwave
microwave
microwave
toluene
toluene
toluene
NMP
110
110
110
200
180
24
1
2
1
1
a
FeP-GOmw
o-dichlorobenzene
a
Reaction of metalloporphyrin directly on GO (non-silylated).
for this material confirms this hypothesis (Fig. 6c). The results give
evidence for the better maintenance of materials integrity upon
microwave functionalization relatively to the procedure under
conventional heating. The temperatures of the CO , CO and H O
2 2
decomposition peaks match the steps of mass loss observed on
thermogravimetry analyzes.
3.3. Immobilization of the metalloporphyrin
The preparation conditions of materials FeP-sGO (Fig. 1) are
summarized in Table 5 and also consider the use of the conven-
tional and microwave heating (entries 1–2), different reaction
times (entries 1–3) and different solvents. The use of NMP as sol-
vent was tested in order to improve the dispersibility of GO, as well
as, to allow the reaction to be performed at higher temperature
Fig. 4. Representative FTIR spectra of GO, sGOmw2, FeP-sGOch1, FeP-sGOmw2, FeP-
sGOmw3 and FeP-GOmw materials in comparison with FeP.
(
entry 4). Finally, a direct reaction of FeP with non-silylated GO
was performed (entry 5), in order to evaluate the relevance of
the pre-functionalization with organosilane in the immobilization
of the metalloporphyrin.
The most notorious change in the XRD diffractogram of material
FeP-sGOch1, relatively to the precursors, is the significant decrease
on the intensity of the (001) peak in accordance with the presence
of new metalloporphyrin moieties in the interlayer space (Fig. 2).
The (001) peak is observed at 2h = 12.6° (Table 2), corresponding
to a d-spacing of 0.702 nm, and indicates an increase inter-planar
spacing of 0.042 nm relatively to the precursor material sGOch1
and of 0.138 nm relatively to original GO. The decrease of the
(
001) peak intensity and slight shifts of its position to lower 2h
in the XRD analysis were previously observed for immobilization
of porphyrins on GO structures [25]. These evidences support the
intercalation of the FeP on the interlayers of GO. It is worth to high-
light that porphyrins are large planar structures with thickness of
about 6–10 Å, which are structurally prone to intercalate between
sheets of the obtained sGO materials.
The XPS atomic percentages indicated that after metalloporphy-
rin intercalation, the materials FeP-sGOch1, FeP-sGOmw1 and
FeP-sGOmw2, showed a decrease on C% and O% and that no
bromine was detected on FeP-sGO materials (Table 3), while the
presence of fluorine, nitrogen, iron and chlorine confirmed the
presence of FeP.
Fig. 5. Thermograms of GO, sGOch1, FeP-sGOch1 and FeP materials in an inert
atmosphere.
The metalloporphyrin loading is similar in all materials,
although a slightly higher immobilization was observed for nano-
hybrids obtained with microwave-assisted synthesis: the Fe%
was 0.1% in conditions ch1 (Table 5, entry1), corresponding to a
concentration within the material of metalloporphyrin of
Table 4
Total mass losses (%) obtained by TG analysis.
Material
% Total mass loss
GO
34
37
31
44
42
43
79
ꢁ1
8
0
l
molꢀg and 0.2% in conditions mw1 and mw2 (Table 5, entries
sGOch1
sGOch2
sGOmw2
FeP-sGOch1
FeP-sGOmw2
FeP
ꢁ1
2 and 3), corresponding to a concentration of 150
l
molꢀg
.
Concomitant variations in the atomic percentages of the other ele-
ments were observed. The absence of bromine and the presence of
the FeP in the same loading than the observed for bromine in the
ꢁ1
silylated precursor material (150
l
molꢀg ), could indicate the
reaction of bromoalkyl groups with the dimethylamino groups of
the metalloporphyrin through covalent immobilization, although
it can also indicate the reaction of the bromoalkyl moieties with
derivatives, due to the longer reaction times with exposition to
humidity and temperature. The higher amount of water released

4
82
M.E. Lipi n´ ska et al. / Polyhedron 81 (2014) 475–484
2 2
Fig. 6. Mass loss of the relevant molecular fragments in TPD analyzes for GO and silylated materials: (a) CO ; (b) CO; (c) H O.
hydroxyl groups from GO, leading to the bromide release from the
material.
materials, namely material FeP-sGOmw2 increased mass loss rela-
tively to the parent material is observed, as a confirmation of the
porphyrin loading [39].
Catalytic oxidation tests were performed using cis-cyclooctene
as a model substrate in order to obtain the proof of concept for
the immobilization of the metalloporphyrin catalyst in its active
form. The catalytic activity of the different hybrid materials was
accessed using eco-sustainable conditions, namely, hydrogen
peroxide as oxidant, in ethanol as reaction solvent and at room
temperature [31].
As in the case of the silylation reaction, no advantage was
observed in carrying out the metalloporphyrin immobilization
reaction in microwave for longer reaction times than 1 h, since
the XPS atomic percentages observed for conditions 2 and 3 were
almost identical.
The high resolution XPS spectra of the relevant elements
obtained for material FeP-sGOmw1 are shown in Fig. 3c. In C1s
and O1s regions no significant changes were observed relatively
to the precursor material sGOmw1. In the N1s region of the FeP-
sGOmw1 spectrum only one band at 400.0 eV was observed and
matches the contributions from the pyrrolic nitrogen in the por-
phyrin core and from the dimethylamine groups from the metallo-
porphyrin peripheral substituents [45]. The F1s spectrum shows an
intense peak at 687.4 eV that is attributed to the fluorophenyl
groups of the meso-position of the metalloporphyrin [46]. In the
Fe2p3/2 region only one band with a maximum at 711.3 eV can
be distinguished, due to the presence of Fe3 thus providing confir-
mation for the presence of iron inside the porphyrin core [35]. In
the Cl2p region of the spectrum, the band at 198.6 eV is attributed
to chlorine coordinated to iron(III) in the core of the porphyrin and
The performance of the catalysts was shown to be proportional
2 2
to the metalloporphyrin loading. When a rate of H O addition of
ꢁ1
0.5 mmolꢀh was used (1 mol equivalents of oxidant relatively
to the substrate per hour) the conversion after 2 h was 63% for
ꢁ
1
material FeP-sGOch1, showing a FeP loading of 80
lmolꢀg
(Table 6, entry 2) and 95% for material FeP-sGOmw2, showing a
ꢁ
1
FeP loading of 150
l
molꢀg (entry 8). The material FeP-sGOmw3
did not show any catalytic activity after 2 h of reaction time (entry
12), suggesting that the immobilized iron(III) porphyrin is inactive,
probably due to strong coordination of NMP (solvent used for its
immobilization) onto the axial positions of the metal centre and
+
2 2
thus avoiding the accessibility of H O to the metal centre and con-
compares with the B.E. of chlorine in FeCl
3
at 198.8 eV [35]. The
sequently, imposing its inactivation. Furthermore, the material
resulting from direct reaction of FeP with original GO (FeP-GOmw)
did not show any catalytic behavior. This corroborates the absence
of metalloporphyrin in this material as showed by FTIR. For all the
previous assays the selectivity was 100% for the 1,2-epoxycyclooc-
tane. A test control with GO material did not show any catalytic
activity in epoxidation reaction, only a conversion 65% affording
non identified products.
second peak can be ascribed to chloride ion. Similar deconvolution
patterns and peak positions were observed for FeP-sGOch1 and
FeP-sGOmw2 materials (Fig. S3b and c; SI).
FTIR spectra of materials FeP-sGO (Fig. 4) show the characteristic
bands of the organosilane, whereas in the spectra of materials FeP-
sGOmw2 and FeP-sGOmw3 it can be distinguished small features at
ꢁ
1
wavenumber below 1650 cm , which can be assigned to interca-
lated metalloporphyrin. These bands are hardly observed in mate-
rial FeP-sGOch due to the lower FeP loading in the material.
Furthermore, the spectrum of the material FeP-GO, resulting from
the direct reaction of the FeP with pristine GO, matches the spectral
pattern of original GO, revealing the inefficient functionalization.
The thermograms of FeP-sGO materials have similar mass loss
pattern to those observed for the precursor materials (Fig. 5, Fig
The test of material FeP-sGOch1 in the catalytic oxidation using
ꢁ
1
a slower rate of H
2
O
2
addition of 0.25 mmolꢀh (entries 4–6) led to
higher substrate conversations relatively to the observed with the
ꢁ
1
rate of 0.5 mmolꢀh (entries 1–3). The results can be rationalized
by the lower stability of the catalyst in the higher oxidizing med-
ium. The progress of conversion with time demonstrated that a
ꢁ1
2
rate of H O
2
addition at 0.25 mmolꢀh established the kinetic con-
2, SI). In Fig. 5, the thermogram of material FeP-sGOch1 is
trol of the reaction. The results evidence a molecular reactivity of
hydrogen peroxide-substrate in a stoichiometry close to 1:1 and
presented in comparison with the material sGOch1 and GO. The
pronounced mass loss in the range of 200–300 °C occurs at exactly
the same temperature as the precursor material sGOch1 ꢄ240 °C,
while in GO this mass loss occurs at ꢄ250 °C. In the range of
2 2
show the high efficiency of H O activation with these catalytic
systems, even at low oxidant concentrations.
In the overall, the results support the relevance of the silylation
reaction for further metalloporphyrin immobilization, confirming
that the increase in the interlayer distance obtained upon silylation
allowed the final metalloporphyrin entrapment. This may have
occurred by physical entrapment of the FeP molecules between
GO layers or by covalent bonding; this latter reaction was tested
with success on MWCNT decorated with hydroxyl groups [35],
2
80–800 °C, a progressive and lower rate of the mass loss occurs,
which achieves a maximum of 42%. An equivalent value of 43%
of the total mass loss was observed for the material prepared using
microwave heating, namely FeP-sGOmw2 (Table 5). In the studied
temperature range the decomposition of the metalloporphyrin is
not complete, achieving a maximum of 79%; however for hybrid

M.E. Lipi n´ ska et al. / Polyhedron 81 (2014) 475–484
483
Table 6
2 2
Catalytic epoxidation of cis-cyclooctene by H O
in ethanol.a
(mmolꢀhꢁ1)b
t (h)
C%c
TON
TOF (h )d
ꢁ1
Entry
Reaction
1
Catalyst
2 2
H O
1
2
3
4
5
6
7
8
9
FeP-sGOch1
0.5
1
2
3
1
2
3
1
2
5
2
2
1
60
63
64
47
77
78
90
93
95
0
150
158
160
118
192
195
118
122
125
0
77
2
3
0.25
0.5
60
FeP-sGOmw2
115
1
1
1
0
1
2
4
5
6
FeP-sGOmw3
FeP-GOmw
FeP
0.5
0.5
0.5
0
0
128
0
100
0
141
a
Reaction conditions: the substrate (0.5 mmol) and catalyst (25 mg of FeP-sGO or 4 mg of FeP, corresponding to the same mass loaded in 25 mg of material) were stirred in
.5 mL of ethanol at room temperature and H was progressively added to the reaction.
to the reaction mixture.
1
2 2
O
b
2 2
Rate of addition of H O
c
Substrate conversion was determined by GC analyses, as well as, the selectivity that in all cases was 100% since 1,2-epoxycyclooctane was the only product.
Calculated after the first hour of the reaction.
d
where efficient entrapment is not prone to occur. The
p–p
staking
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