Modified graphene oxide as a support for rhodium nanoparticles active in olefin hydroformylation

Article abstract of DOI:10.1007/s11172-014-0729-x

A new two-step procedure for modifying graphene oxide by the methylation of its surface with trimethyl-o-formate followed by the treatment with methyl iodide was developed. The methylation product was used as a support for the formation of rhodium nanoparticles. The obtained samples of graphene oxide containing rhodium nanoparticles were characterized by IR spectroscopy, X-ray fluorescence spectroscopy, and transmission electron microscopy. The Rh-containing nanocomposites were used as atalysts for the hydroformylation of olefins in upercritical carbon dioxide and organic solvents.

Full text of DOI:10.1007/s11172-014-0729-x

Russian Chemical Bulletin, International Edition, Vol. 63, No. 10, pp. 2243—2249, October, 2014
2243
Modified graphene oxide as a support for rhodium nanoparticles
active in olefin hydroformylation*
Yu. V. Ioni,a S. E. Lyubimov,b V. A. Davankov,b and S. P. Gubina
aN. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (495) 954 1279. Eꢀmail: Acidladj@mail.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 6549
A new twoꢀstep procedure for modifying graphene oxide by the methylation of its surface
with trimethylꢀoꢀformate followed by the treatment with methyl iodide was developed.
The methylation product was used as a support for the formation of rhodium nanoparticles. The
obtained samples of graphene oxide containing rhodium nanoparticles were characterized by
IR spectroscopy, Xꢀray fluorescence spectroscopy, and transmission electron microscopy. The
Rhꢀcontaining nanocomposites were used as catalysts for the hydroformylation of olefins in
supercritical carbon dioxide and organic solvents.
Key words: graphene oxide, rhodium nanoparticles, hydroformylation.
Graphene and related nanomaterials with a well develꢀ
oped specific surface have interesting mechanical, electriꢀ
cal, thermal, and optical properties. Therefore, they atꢀ
tract attention of researchers.1—4 There are many methods
for synthesis of graphene, but the most promising is the
chemical or thermal reduction of graphene, since with
these simple methods scaling techniques can be used.2,3
Graphene oxide is a graphene layer with diverse surface
functional oxygenꢀcontaining groups.5,6 These groups can
participate in various reactions to form a new product, soꢀ
called modified or functionalized graphene oxide. The
introduction of surface functional groups can enhance disꢀ
persion and increase compatibility of graphene oxide or
graphene with various objects. In addition, covalently funcꢀ
tionalized graphene oxide can further be used for the prepꢀ
aration of new substances. For example, the possibility of
functionalization of the graphene surface by amide groups
was reported.7 In several works modification of the
graphene oxide surface by diverse organic substances was
described.8—10 To change the optical properties, graphene
oxide was modified by porphyrinꢀcontaining primary
amines and fullereneꢀcontaining secondary amines. In
these reactions, porphyrin and fullerene molecules were
grafted to the graphene oxide surface via the formation of
amide bonds. The degree of functionalization of graphene
and graphene oxide significantly affects these properties
and, hence, new approaches to the chemical modification
of graphene and its derivatives are aimed at searching for
mild functionalization methods.
One of interesting examples for the modification of
graphene structures is preparation of supported metal
nanoparticles. Adsorption of Cu2+ ions on the graphene
oxide aerogel used for the preparation of a new sorption
material was described.11 Graphene oxide can act as an
efficient support for catalytically active surface nanoꢀ
particles on the surface. In some cases, metal nanopartiꢀ
cles immobilized on solid supports can manifest unique
catalytic properties.12—14 Catalytic activity of particles sigꢀ
nificantly depends on the surface area of metal accessible
for reacting molecules and on the support nature. Earliꢀ
er,15 platinum and palladium nanoparticles immobilized
on the graphene oxide surface were used in the Suzuki and
Heck crossꢀcouplings.
In this work, a novel method for graphene oxide modꢀ
ification by the twoꢀstep methylation of its surface is deꢀ
scribed. Modified graphene oxide was used as a support
for rhodium surface nanoparticles, and the obtained nanoꢀ
composite was applied as a catalyst for alkene hydroformyꢀ
lation. Hydroformylation is widely used in industry to obꢀ
tain solvents, detergents, biodegradable surfactants, lubriꢀ
cants, and plasticizers.16—18 The development of an effiꢀ
cient heterogeneous catalyst would significantly improve
commercial processes, which are currently in use.
* Dedicated to Academician of the Russian Academy of Sciences
Yu. N. Bubnov on the occasion of his 80 birthday and to the 60th
anniversary of the foundation A. N. Nesmeyanov Institute of
Organoelement Compounds of the Russian Academy of Sciences.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2243—2249, October, 2014.
1066ꢀ5285/14/6310ꢀ2243 © 2014 Springer Science+Business Media, Inc.

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Ioni et al.
Scheme 1
Results and Discussion
Graphene oxide was prepared in several steps from
natural graphite using the modified Hummers method19
(see Experimental). The oxidation of natural graphite leads
to a nonꢀstoichiometric graphite oxide with surface oxygenꢀ
containing groups. Asꢀsynthesized graphite oxide was
dispersed in water using a powerful ultrasound treatment
to obtain a dispersion of graphene oxide (Fig. 1). Graphite
oxide is hydrophilic, since after ultrasonication it forms stable
dispersions in water and in other polar solvents. These
dispersions undergo no agglomeration within a long time.
The Xꢀray diffraction analysis (XRD) (Fig. 2) shows
that the structure of synthesized graphene oxide includes
phases of graphite oxide (2 = 10—12) and graphite (peaks
at 2 = 26.5 and 54.5).20
It is known2 that graphene oxide is a polyfunctional
ligand having a number of different oxygenꢀcontaining
groups (ОН, С=О, СООН) on its surface. Upon the adꢀ
dition of metal salts to graphene oxide, metal ions form
bonds with oxygenꢀcontaining surface groups. This interꢀ
action is visualized when the solution is bleached after the
addition of salts to a dispersion of graphene oxide.21 This
property was used for the introduction of Rh3+ ions onto
the graphene oxide surface. After the system was reduced
by NaBH4, rhodium nanoparticles stabilized by functionꢀ
al groups on the graphene oxide surface are formed
(Scheme 1).
The obtained sample was studied using transmission
electron microscopy (TEM) (Fig. 3). The TEM image
shows that rhodium nanoparticles on the graphene oxide
surface have a narrow particleꢀsize distribution (the averꢀ
age diameter of the nanoparticles is 2.3 nm). The Xꢀray
diffraction analysis (Fig. 4) showed that the maxima in
the diffraction patterns corresponded to the phase of meꢀ
tallic Rh with the cubic face centered lattice (JCPDS cards
is Rh nanoparticle, GO is graphene oxide.
i. Reducing agent.
# 05—0685) and graphene oxide. The reflections correꢀ
sponding to rhodium are broadened. This characterizes
a small size of the nanoparticles, which correlates well
I (arb. units)
3
GO
2
G
1
G
10
20
30
40
50
60
70
2/deg
100 nm
Fig. 2. Diffraction pattern of graphene oxide: GO is graphene
Fig. 1. TEM image of graphene oxide.
oxide, and G is graphite.

Modified graphene oxide
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 10, October, 2014
2245
at 50 С under a pressure of 25 atm of syngas is shown
in Scheme 2.
Scheme 2
i. Rh nanoparticles/GOmod (GOmod is modified GO), H2/CO
(1 : 1), 25 atm, solvent, 4 h.
100 nm
However, the primary experiments show that the use
of rhodium nanoparticles on the unmodified graphene oxꢀ
ide surface as a catalyst did not reveal any conversion
using toluene or scCO2 as solvents (Table 1).
Fig. 3. TEM image of the nanocomposite of Rh/GO nanoꢀ
particles.
It was assumed that the surface polar hydrophilic
groups С=О and OH inhibit hydroformylation. It is most
likely that the approach to the nanoparticle surface is hinꢀ
dered for hydrophobic styrene molecules. The twoꢀstep
methylation of graphene oxide was carried out according
to Scheme 3 in order to change polarity.
I (arb. units)
GO
2
G
Rh
111
At the first step, the С=О groups on the graphene
oxide surface (see Scheme 3, А) interact with trimethylꢀoꢀ
formate (in the presence of catalytic amounts of pꢀtolueꢀ
nesulfonic acid) to form acetal groups (see Scheme 3, B).
At the second step, the obtained intermediate reacted with
MeI to methylate the hydroxo groups (see Scheme 3, C).
To confirm the efficiency of the proposed reaction scheme,
IR studies of graphene oxide graphene oxide surface (a),
intermediate product (b), and completely methylated
graphene oxide (c) have been performed (Fig. 5).
1
Rh
200
50
Rh
220
G
10
20
30
40
60
70
2/deg
Fig. 4. Diffraction pattern of the nanocomposite of Rh/GO nanoꢀ
particles.
According to the obtained data, the reaction of the
initial graphene oxide (see Fig. 5, a) with trimethylꢀoꢀ
formate (see Fig. 5, b) results in the appearance of new
bands in the spectrum. Among these are an intense band
at 2900 cm–1 corresponding to stretching vibrations of the
С—Н bonds in в the Me groups, a band at 2850 cm–1
corresponding to stretching vibrations of the С—Н bonds
in the Me groups attached to the oxygen heteroatoms, and
a mediumꢀintensity band at 1458 cm–1 corresponding to
antisymmetric bending vibrations of the С—Н bonds
in the Me groups. The spectrum also exhibits bands at
with the TEM data. Calculation of the average size of
rhodium nanoparticles by the Scherrer equation gives
a value of 3.7 nm.
The obtained rhodium nanoparticles on the graphene
oxide surface (Rh/GO) were used as catalysts for the
hydroformylation of styrene and other olefins (according
to the Xꢀray fluorescence analysis data, the rhodium conꢀ
tent was 4.1%). Hydroformylation is usually carried out in
organic solvents in combination with fire hazardous and
explosive agents. Therefore, it is interesting to use alternaꢀ
tive media for the process, in particular, supercritical
carbon dioxide (scСО2) since carbon dioxide is accessiꢀ
ble and environmentally friedly and fireꢀsafe.22 The use
of scСО2 makes it possible to operate with lowly boiling
reagents and products, isolation of which from organic
solvents is difficult because it requires fractional distillaꢀ
tion. Unlike this, carbon dioxide can readily be reꢀ
moved from the synthesis products. Hydroformylation of
styrene and other olefins in organic solvents and in scCO2
Table 1. Parameters of styrene hydroformylation using the
Rh/GO nanoparticle as a catalyst
Solvent
Pressure/atm
/h
Conversion (%)
Toluene
scСО2
25
250
5
5
0
0

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Russ.Chem.Bull., Int.Ed., Vol. 63, No. 10, October, 2014
Ioni et al.
Scheme 3
i. 36 h.
I
OH
a
C=O
3000
Me
2000
1000
/cm–1
I
b
C—O
OH
C=O
3000
Me
2000
1000
/cm–1
I
c
C—O
OH
C=O
3000
2000
1000
/cm–1
Fig. 5. IR spectra of the starting graphene oxide (a), intermediate methylation product (b), and modified graphene oxide (c).

Modified graphene oxide
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 10, October, 2014
2247
1023—1155 cm–1 assigned to stretching vibrations of the
С—О—С bonds in the epoxy groups. The IR spectrum of
the final methylation product is almost identical to the
spectrum of the intermediate. However, a comparison of
this spectrum with the spectra of the initial graphene oxide
and the product of the first methylation step shows that
the shape of the peak corresponding to stretching vibraꢀ
tions of the OH groups changes. A weak peak at 3550 cm–1
can be attributed to free OH groups, and a stronger peak at
3300 cm–1 can be assigned to the residual bound hydroxy
groups. A decrease in the band intensity indicates a nearly
complete methylation of graphene oxide. According to the
elemental analysis data (Table 2), the carbon content inꢀ
creases by 7.7% and the oxygen content decreases
by 6.5% in the sample, being the intermediate methylation
product. In the final product, the amount of carbon inꢀ
creases to 70% (an increase by 4.3% compared to the prodꢀ
uct of the first methylation step) and the amount of oxyꢀ
gen decreases to 26.5% (a decrease by 6% compared to the
product of the first methylation step). These data provide
the evidence of a chemical reaction between the graphene
oxide and methylation agents.
100 nm
Fig. 6. TEM image of modified graphene oxide.
a
The dispersion of modified graphene oxide in hexane
was examined by TEM. The TEM image (Fig. 6) shows
that the sample is characterized by a layered uniform strucꢀ
ture, similarly to the initial graphene oxide, it is transꢀ
parent and has a few layers.
20 nm
Modified graphene oxide was also used as a support for
the formation of surface rhodium nanoparticles (see Experꢀ
imental). The sample of Rh/GOmod nanocomposite was
studied by TEM (Fig. 7). The obtained TEM image is
shown in Fig. 7. The TEM image of the sample of
Rh/GOmod nanoparticles shows that spheroid rhodium
nanoparticles tend to form "starꢀlike" agglomerates. Indiꢀ
vidual rhodium nanoparticles have a narrow particleꢀsize
distribution with an average particle size of 2.4 nm.
The catalytic activity of the nanocomposite of Rh/GOmod
nanoparticles was also studied in the hydroformylation of
unsaturated hydrocarbons (see Scheme 2).
b
N (%)
30
25
20
15
10
5
The use of the Rh/GOmod nanocomposite in toluene
as a catalyst results in a high conversion; however, the
regioselectivity of the obtained branched product with reꢀ
spect to unbranched aldehyde is 65% (Table 3, entry 1).
1.6 2.0 2.4 2.8 3.2 3.6
4.0 d/nm
Fig. 7. TEM image of the Rh/GOmod nanocomposite (a) and the
rhodium nanoparticle distribution in the nanocomposite (b).
Table 2. Results of C,Н,N analysis of the starting
graphene oxide (A) and intermediate (B) and final
(C) methylation products
The replacement of the solvent by benzene gives an insigꢀ
nificant increase in conversion, but the regioselectivity deꢀ
creases (see Table 3, entry 2). Among the solvents used,
THF is characterized by the lowest activity (see Table 3,
entry 3).
The replacement of organic solvents by scCO2 made it
possible to attain the complete conversion in the styrene
hydroformylation and to increase regioselectivity (see
Table 3, entry 4). The hydroformylation of 4ꢀmethylꢀ
styrene and 4ꢀbromostyrene in scCO2 also proceeds quanꢀ
Sample
Content of element (%)
Н
С
О
A
B
C
58.0
65.7
70.0
1.5
1.5
1.8
39.0
32.5
26.5

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Russ.Chem.Bull., Int.Ed., Vol. 63, No. 10, October, 2014
Ioni et al.
Table 3. Parameters of the hydroformylation of olefins
H2C=CH—R in the presence of Rh/GOmod nanoparticles as
a catalyst (65 C)
(6000 rpm, 12 min) to separate the solid product from the liquid
phase, and the final product was dried for 6 h at 60 С in vacuo
and in air.
Methylation of graphene oxide. Graphene oxide (500 mg)
was continuously stirred for 36 h in 6 mL of trimethylꢀoꢀformate
with the addition of a catalytic amount (4 mg) of pꢀtolueneꢀ
sulfonic acid. An excess of trimethylꢀoꢀformate was removed by
heating in vacuo. Methyl iodide (4 mL), K2CO3 (1 g), and acetoꢀ
nitrile (4 mL) was added to the dry residue after the first step of
the reaction, and the mixture was continuously stirred for 36 h at
~20 C. After the reaction, the solid residue was separated
from the liquid phase by centrifugation (4000 rpm, 5 min),
washed with water (5 mL) and acetone (2×5 mL), and dried
in vacuo for 1 h.
Preparation of rhodium nanoparticles on the surface of graphꢀ
ene oxide and methylated graphene oxide. A solution of RhCl3•
•4H2O (46.5 mg) in methanol (4 mL) was added to graphene
oxide (330 mg) with continuous stirring. After 30 min, an excess
of dry NaBH4 (77 mg) was added by portions. The solid precipiꢀ
tate was washed with hot water (4×5 mL) and acetone (3×5 mL)
and dried in vacuo for 6 h.
Entry
R
Solvent
/h
Converꢀ Selectivity*
sion (%)
(%)
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Toluene
Benzene
THF
scCO2
scCO2
scCO2
scCO2
scCO2
4
4
4
4
4
4
4
5
88
90
12
100
100
100
84
65/35
56/44
51/49
84/16
86/14
87/13
0/100
0/100
Ph
4ꢀMePh
4ꢀBrPh
But
But
100
* Ratio of the branched to linear reaction products.
titatively with high regioselectivity (see Table 3, entries 5
and 6). The use of 3,3ꢀdimethylbutꢀ1ꢀene as a substrate
resulted in the selective formation of 4,4ꢀdimethylpentaꢀ
nal with 84% within 4 h (see Table 3, entry 7). The prolonꢀ
gation of the reaction to 5 h resulted in the complete conꢀ
version of olefin (see Table 3, entry 8).
Catalytic hydroformylation of olefins. The catalyst 30 mg,
0.01 mmole of Rh) and the corresponding unsaturated substrate
(1 mmol) were placed in a 10ꢀmL stainless steel autoclave. The
autoclave was filled with syngas (25 atm, PH /PCO = 1 : 1) and
2
then with carbon dioxide to a pressure of 200 atm with a High
Pressure Equipment manual press. The reactor was heated to the
corresponding temperature for 5 min, and the experiments were
carried out with magnetic stirring. After a certain time, the autoꢀ
clave was cooled to 5 С for 20 min, reaction gases were slowly
fed, and the reaction mixture was diluted in 1.5 mL of CDCl3,
filtered through a thin silica gel layer to separate catalyst resiꢀ
dues, and analyzed by the 1H NMR method. The spectral charꢀ
acteristics of the hydroformylation products correspond to
the literature data: 2ꢀphenylpropanal and 3ꢀphenylpropanal
(R = Ph), 2ꢀ(4ꢀmethylphenyl)propanal and 3ꢀ(4ꢀmethylphenyl)ꢀ
propanal (R = 4ꢀMePh), 2ꢀ(4ꢀbromophenyl)propanal and
3ꢀ(4ꢀbromophenyl)propanal (R = 4ꢀBrPh), and 4,4ꢀdimethylꢀ
propanal. Experiments in organic media were carried out simiꢀ
larly using 2 mL of the corresponding solvent.
Thus, a twoꢀstep modification procedure for graphene
oxide was developed. The methylated graphene oxide was
used as a support for the formation of rhodium nanoꢀ
particles (2—3 nm) on the surface. The prepared catalyst
showed moderate and high substrate conversion as well as
regioselectivity in the hydroformylation of various olefins
in in the "green" medium of scCO2. The results of this
study can provide new prospects for the further investigaꢀ
tion of the chemical properties of graphene oxide and its
application as a support in other catalytic reactions.
Experimental
IR absorption spectra were recorded on a Specord Mꢀ82
spectrometer (VEB Carl Zeiss Jena, Germany) in the range
400—4000 cm–1 with a scan increment of 4 cm–1. The sample
was triturated in an agate mortar together with anhydrous KBr
(0.1 mg of the studied powder per 100 mg of КBr) and then
pressed in pellets on a hydraulic press (pressing effort 6 metric
tons). The Xꢀray diffraction measurements were carried out on
a Bruker D8 Advance spectrometer operating in the reflectance
mode with CuꢀK radiation (35 kV, 30 mA). Elemental analysis
(C, N, H) was carried out on a VarioEL instrument by burning
microweights of the samples of the determined substance. The
content of rhodium nanoparticles on the graphene oxide surface
was determined by Xꢀray fluorescence spectroscopy on a Zeiss
Jena VRAꢀ30 spectrometer. The size and morphology of Rh0
nanoparticles were studied with a JEMꢀ1011 transmission elecꢀ
tron microscope at an accelerating voltage of 100 kV.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 14ꢀ03ꢀ31813
mol_a).
References
1. A. K. Geim, K. S. Novoselov, Nat. Mater., 2007, 6, 183.
2. S. Park, R. S. Ruoff, Nat. Nanotechnol., 2009, 4, 217.
3. H. Bai, C. Li, C. Shi, Adv. Mater., 2011, 23, 1089.
4. C. Soldano, A. Mahmood, E. Dujardin, Carbon, 2010,
48, 2127.
5. D. R. Dreyer, S. Park, C. W. Bielawski, R. S. Ruoff, Chem.
Soc. Rev., 2010, 39, 228.
6. T. Szab, O. Berkesi, P. Forgo, K. Josepovits, Y. Sanakis,
D. Petridis, I. Dekany, Chem. Mater., 2006, 18, 2740.
7. S. Niyogi, E. Bekyarova, M. E. Itkis, J. L. McWilliams, J. Am.
Chem. Soc., 2006, 128, 7720.
Synthesis of graphene oxide. Graphite oxide obtained by the
modified Hummers method19 was placed in the solvent and subꢀ
jected to the powerful ultrasonication treatment for 30—60 min
(parameters: frequency 20.4 kHz, specific power 0.1—1 W cm–3).
The formed dispersion of graphene oxide was centrifuged
8. B. G. Choi, W. H. Hong, Y. M. Jung, H. S. Park, Chem.
Commun., 2011, 47, 10293.

Modified graphene oxide
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 10, October, 2014
2249
9. M. Quintana, K. Spyrou, M. Grzelczak, W. R. Browne,
P. Rudolf, M. Prato, ACS Nano, 2010, 4, 3527.
10. Z.ꢀB. Liu, Y.ꢀF. Xu, X.ꢀY. Zhang, X.ꢀL. Zhang, Y. S. Chen,
J. G. Tian, J. Phys. Chem. B, 2009, 113, 9681.
11. X. Mi, G. Huang, W. Xie, W. Wang, Y. Liu, J. Gao, Carbon,
2012, 50, 4856.
12. H. W. Ha, I. Kim, S. J. Hwang, R. S. Ruoff, Electrochem.
Solid State Lett., 2011, 14, B70.
13. G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth,
R. Mulhaupt, J. Am. Chem. Soc., 2009, 131, 8262.
14. B. Xiong, Y. Zhou, Y. Zhao, J. Wang, X. Chen, R. O´Hayre,
Z. Shao, Carbon, 2013, 52, 181.
17. J. Klosin, C. R. Landis, Acc. Chem. Res., 2007, 40, 1251.
18. F. Ungvary, Coord. Chem. Rev., 2007, 251, 2087.
19. W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc., 1958,
80, 1339.
20. S. V. Tkachev, E. Yu. Buslaeva, A. V. Naumkin, S. L. Kotoꢀ
va, I. V. Laure, S. P. Gubin, Inorg. Mater. (Engl. Transl.),
2012, 47, 796 [Neorg. Mater., 2012, 48, 909].
21. I. L. Laure, S. V. Tkachev, E. Yu. Buslaeva, E. V. Fatushina,
S. P. Gubin, Russ. J. Coord. Chem (Engl. Transl.), 2013, 39,
487 [Koord. Khim., 2013, 39, 387].
22. S. E. Lyubimov, E. A. Rastorguev, K. I. Lubentsova, A. A.
Korlyukov, V. A. Davankov, Tetrahedron Lett., 2013,
54, 1116.
15. Yu. V. Ioni, S. E. Lyubimov, A. A. Korlyukov, M. Yu.
Antipin, V. A. Davankov, S. P. Gubin, Russ. Chem. Bull.
(Int. Ed.), 2012, 61, 1825 [Izv. Akad. Nauk, Ser. Khim.,
2012, 1810].
16. V. Agabekov, W. Seichea, B. Breit, Chem. Sci., 2013, 4,
2418—2422.
Received June 18, 2014;
in revised form August 19, 2014