Sulphur-doped graphene as metal-free carbocatalysts for the solventless aerobic oxidation of styrenes

Article abstract of DOI:10.1016/j.catcom.2015.02.018

Pyrolysis of λ-carrageenan at 1000 °C and subsequent exfoliation lead to a S-doped graphene [(S)G]. Increasing pyrolysis temperature to 1200°C or the use of other carrageenans with lower sulphate content results in the failure of S doping. XPS confirms the presence of S atom (4.5%) with environments alike sulphide and sulphoxide. (S)G promotes the aerobic oxidation of styrenes to their corresponding benzaldehydes accompanied by lesser amounts of styrene oxide. In contrast, under the same experimental conditions reduced graphene oxide exhibits a negligible activity. (S)G was reused with almost coincident catalytic performance.

Full text of DOI:10.1016/j.catcom.2015.02.018

Sulfur-doped graphene as metal-free carbocatalysts for the solventless aerobic
oxidation of styrenes
Amarajothi Dhakshinamoorthy, Marcos Latorre-Sanchez, Abdullah M.
Asiri, Ana Primo, Hermenegildo Garcia
PII:
DOI:
Reference:
S1566-7367(15)00077-1
doi: 10.1016/j.catcom.2015.02.018
CATCOM 4235
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
2 January 2015
10 February 2015
12 February 2015
Please cite this article as: Amarajothi Dhakshinamoorthy, Marcos Latorre-Sanchez, Ab-
dullah M. Asiri, Ana Primo, Hermenegildo Garcia, Sulfur-doped graphene as metal-free
carbocatalysts for the solventless aerobic oxidation of styrenes, Catalysis Communications
(2015), doi: 10.1016/j.catcom.2015.02.018
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ACCEPTED MANUSCRIPT
Sulfur-doped graphene as metal-free carbocatalysts for the solventless
aerobic oxidation of styrenes
Amarajothi Dhakshinamoorthy*a, Marcos Latorre-Sanchez,b Abdullah M. Asiri,c Ana Primob
and Hermenegildo Garcia*b,c
aSchool of Chemistry, Madurai Kamaraj University, Tamil Nadu, India 625 021.
bInstituto de Tecnología Química CSIV-UPV, Av. De los Naranjos s/n, 46022, Valencia,
Spain. E-mail: hgarcia@qim.upv.es
cCentre of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah,
Saudi Arabia.
Abstract
o
Pyrolysis of carrageenan at 1000 C and subsequent exfoliation leads to a S-doped
o
graphene [(S)G]. Increasing pyrolysis temperature to 1200 C or the use of other
carrageenans with lower sulfate content result in the failure of S doping. XPS confirms the
presence of S atom (4.5 %) with environments alike to sulfide and sulfoxide. (S)G promotes
the aerobic oxidation of styrenes to their corresponding benzaldehydes accompanied by lesser
amounts of styrene oxide. In contrast, under the same experimental conditions reduced
graphene oxide exhibits a negligible activity. (S)G was reused with almost coincident
catalytic performance.
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Introduction
For the sake of sustainability, availability and affordability, there is a considerable
interest in developing metal-free catalysts.1-8 Graphene (G) and related materials hold
considerable promise as metal-free catalysts,9-15 due to the combination of the unique features
of single-layer sp2 carbons with the possibility to design active sites on it.16-19 Doping of G
with heteroatoms has been found to be a general way to introduce catalytic activity for a
range of reactions,16 including aerobic oxidation of benzylic hydrocarbons and of electron
rich alkenes.20 Recently, we have reported that N- and B-doped Gs can promote the aerobic
oxidation of styrene to a mixture of benzaldehyde and styrene oxide.20 Continuing in this line
of research it would be of interest to determine if doped Gs having other heteroatoms also
exhibit activity as catalysts of aerobic oxidation of hydrocarbons.
Specifically in the present work we report a convenient procedure for the preparation
of S-doped G [(S)G] and it will be shown that this doped G can promote the aerobic
oxidation of styrene. In an earlier precedent we have disclosed that pyrolysis of chitosan in
the absence of oxygen and subsequent exfoliation of the resulting graphitic carbon residue
renders N-doped G [(N)G].21 The advantages of this process is that chitosan, a polysaccharide
of glucosamine, acts as single source of C and N in the process and N-doping occurs
simultaneously with the formation of the G sheet. During the pyrolysis, a fraction of the N
atoms of the chitosan amino groups become incorporated into the G layer in three different
types of N dopant and in a percentage that decreases as the pyrolysis temperature increases
in the range from 900 to 1200 oC.
A logical extension of the preparation of (N)G will be to test if other natural
polysaccharides containing heteroatoms can similarly act as precursors of doped G.
Carrageenans are linear, high-molecular weight polysaccharide of galactose having sulfate
groups. Therefore, they are obvious candidates to become precursors of (S)G that
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subsequently could be tested as oxidation catalyst. The convenient preparation of (S)G shown
below constitutes another example of the methodology that uses pyrolysis of natural
biopolymers for the preparation of Gs.
Experimental Section
Preparation of (S)-G
There are three main types of carrageenans commercially available, but it was found
that -carrageenan with the highest S content is the only one that according to XPS renders
after pyrolysis G materials doped with S. -Carrageenan (Sigma Aldrich ref. 22049) was
o
o
pyrolized in argon atmosphere heating at 200 C for 2 h and then heating at 10 C/min up to
o
1000 C for 6 h. The resulting graphitic powder was sonicated at 700 W for 1 h in water to
obtain dispersed (S)G suspensions. This method of preparing doped Gs from pyrolysis of
natural polysaccharides was reported previously by our group to obtain N- and B-doped Gs.21
Two different temperatures were tested (1000 and 1200 ºC) and only the G sample pyrolyzed
at 1000 ºC was doped. It is a general observation that the percentage of dopant heteroatoms in
doped G prepared by pyrolysis decreases as the pyrolysis temperature increases.21
Characterization techniques
FT-Raman spectra were recorded at ambient temperature with 514 nm laser excitation
at 2 mW power by using a Renishaw In Via Raman spectrophotometer equipped with a CCD
detector. TEM images were recorded in a Philips CM300 FEG system with an operating
voltage of 100 kV. XP spectra were recorded on a SPECS spectrometer equipped with a
Phoibos 150 9MCD detector using a non monochromatic X-ray source (Al and Mg) operating
at 200 W. The samples were evacuated in the pre-chamber of the spectrometer at 1 × 10-9
mbar. The samples have been activated in situ in nitrogen flow at 450 °C for 3 h followed by
evacuation at 10−8 mbar. Deconvolution and fitting of the experimental peaks were carried
out after nonlinear Shirley-type background subtraction and peak correction by the
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transmission function of the spectrometer. Combustion elemental analyses were measured
using a Perkin-Elmer CHNOS analyzer. The detection limit of the technique was 10 ppm.
Experimental procedure for aerobic oxidation
A 3 mL microreactor was charged with the catalyst (10 mg), followed by the addition
of substrate (1 mL). The resulting slurry was sonicated for 30 min (700 W, horn sonicator).
o
The reaction was carried out under magnetic stirring by heating the suspension at 100 C in
most of the experiments or the indicated temperature using a preheated thermostated oil bath.
This reaction mixture was maintained with oxygen atmosphere. The course of the reaction
was periodically monitored by analyzing the sample with GC. An aliquot of 1 µL of the
reaction mixture was taken at different time intervals with a syringe, diluted with 0.1 mL of
acetonitrile and filtered throughout a 0.2 m Nylon filter prior to GC injection and analysed
immediately without delay to minimise possible on-going room temperature oxidations. The
mass balances of the reaction mixture accounted for more than 96 % of the initial substrate,
as estimated by GC using nitrobenzene as the external standard. The yields of the product
were determined by using nitrobenzene as the external standard considering the response
factors unity. The reaction products were further confirmed by GC-MS and co-injecting with
authentic samples.
Results and discussion
Scheme 1 summarizes the pyrolytic procedure followed in the preparation of (S)G.
Combustion chemical analysis of the three commercial K,  and  carrageenan samples
differing in the number of sulfate groups per disaccharide unit, one two or three, respectively,
shows that only the residue from -carrageenan having the maximum sulfate content
pyrolysed at 1000 oC contains S. The elemental analysis of (S)G was 80.8 % C and 4.5 % S.
The presence of some residual percentage of N (0.2 %) and H (0.3 %) was also detected.
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o
When -carrageenan was pyrolysed at 1200 C, no S was detected. Pyrolysis under inert
atmosphere leading to G are strongly reducing conditions.22 In earlier precedent using the
phosphate ester of alginate, it has been observed the formation of elemental phosphorous that
partly sublimes as the red allotrope in the pyrolysis process and partly becomes incorporated
in the G sheet.23 Also, alginate borate ester as precursor of (B)G should require a chemical
reduction step of B atoms in the process of G doping.20 Similarly, it is proposed here that the
sulfate groups present in natural carrageenan would be reduced to elemental sulfur that partly
would become incorporated during the formation of carbon residue on the G sheets and partly
will sublime. In earlier precedents it was found that an excess of the precursor containing the
heteroatom is necessary to produce G doping. For this reason, under our conditions, other
carrageenans have been found to lead to carbon residues in which no sulfur is detected by
combustion chemical analysis.
Scheme 1. Preparation procedure of (S)G using -carrageenan as precursor that is pyrolysed
at 1000 oC (i) and exfoliated by sonication (ii).
FT-Raman spectroscopy is a suitable characterization technique for graphitic
materials. The Raman spectrum recorded for (S)G (Figure 1) showed three expected peaks
that correspond to the 2D band (2890 cm-1), the G and D bands at 1592 and 1355 cm-1,
respectively.
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Figure 1. The Raman spectrum of (S)G.
The presence of S atoms and the fact that they are incorporated into G sheets was
ascertained by XPS measurements. Deconvolution of the C1s peak (Figure 2) leads to two
components at binding energies of 284.5 eV and 285.7 eV that correspond to graphitic carbon
atoms and C atoms bonded to S. The S2p peak was deconvoluted into two components at
binding energies of 164.1 eV and 168.2 eV (Figure 2), that according to the literature 24 can
be attributed to S atoms bonded to C and S atoms bonded to C and O, respectively.
S2p
C1s
Figure 2. XPS C1s and S2p peaks recorded for (S)G and the best deconvolution peaks.
The typical layered G morphology of (S)G was observed by transmission electron
microscopy (TEM). The images showed layers of around 600 nm in size with (S)G.
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Figure 3. TEM images of (S)-G with different magnifications.
Catalytic studies on aerobic oxidation of styrenes
Aerobic oxidation of olefins to epoxides with high selectivity is a key reaction in
industry due to the commercial importance of epoxides. However, this transformation is often
accompanied by the formation of many secondary products. In the present work, the catalytic
activity of (S)G is evaluated in the aerobic oxidation of styrene under solvent-free conditions.
Styrene can undergo oxidative C=C double-bond cleavage to form benzaldehyde (BA) and
benzoic acid, but can also can undergo epoxidation to give styrene oxide (SO). The direct
conversion of styrene to SO by molecular oxygen as the oxidant is particularly challenging
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because this reagent typically produces undesirable side reactions and degradation. The
importance of this reaction comes from the fact that the aerobic oxidation of alkenes is a
reaction of considerable economic importance that is performed using noble or transition
metals as catalysts, and, therefore, it would be especially advantageous to develop metal-free
catalysts for this process.
Control experiments in the absence of catalyst resulted in 4 % conversion of styrene
after 7 h probably due to the auto-oxidation process. On the other hand, when the aerobic
oxidation was performed at 100 oC with (S)G as catalyst, the conversion of styrene increased
gradually as a function of time. After 7 h, 13 % conversion was observed with the selectivity
of BA and SO being 70 and 20 %, respectively. In addition, the reaction mixture also showed
some dimeric and oligomeric derivatives of styrene. The formation of styrene oligomers is in
agreement with the recent literature report by Bielawski and co-workers that have shown the
remarkable catalytic activity of graphene oxide (GO) to promote styrene oligo- and
polymerization.25 In contrast to these results, an analogous sample of GO obtained by the
same procedure had been found previously to act as a carbocatalyst for the aerobic oxidation
of benzyl alcohol, although about half the weight of GO with respect to the organic substrate
was employed than in this reaction.17 This catalyst to substrate weight ratio is considerably
much higher than the one employed here (1 wt.%).
For the sake of comparison, the catalytic activity of reduced graphene (rGO) was
tested under the same experimental conditions in the aerobic oxidation of styrene, observing a
negligible conversion compared to (S)G as catalyst. Furthermore, no conversion of styrene
o
was also observed in the absence of oxygen or at lower reaction temperature (80 C). These
control experiments clearly suggest the active role played by molecular oxygen as oxidizing
reagent and the importance of the temperature on the reaction rate. Furthermore, these data
explain the role of S doping in the promotion of aerobic oxidation by tuning the properties of
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native G. Figure 4 shows the time-conversion plot for the oxidation of styrene using (S)G and
rGO as catalysts.
(b)
10
8
6
4
2
(a)
0
0
1
2
3
4
5
6
7
8
Time (h)
Figure 4. Aerobic oxidation of styrene using rGO (a) and (S)G (b) as catalysts.
Table 1. Aerobic oxidation of styrene and its derivatives using (S)G catalyst.a
Run Catalyst
No
T
Substrate
Time Conv. Sel. (%)b
(oC)
100
100
(h)
7
(%)b
BA SO
1
-
styrene
styrene
4
44
100
82
74
72
70
71
67
-
18
2
(S)G
1
0.8
3
3
2
18
20
19
20
18
19
-
4
3
4.8
5.5
13
12
12
-
5
4
6
7
7c
8d
9
(S)G
(S)G
rGO
100
100
100
styrene
styrene
styrene
7
7
7
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10
11
12
13
14
15
(S)G
(S)Ge
(S)G
80
styrene
7
7
1
5
7
7
7
-
-
-
100
100
styrene
< 1
3
98
87f
85
76
12
-
-
α-methylstyrene
10
13
20
81g
-
9
13
6
(S)G
(S)G
100
100
4-chlorostyrene
16
4-methoxystyrene
23
a
o
Reaction conditions: substrate (1 mL), catalyst (10 mg), oxygen atmosphere, 100 C;
c
e
b Determined by GC; First reuse; dSecond reuse; Reaction performed under inert
atmosphere; fThe product is acetophenone; gThe selectivity corresponds to the mixture of the
epoxide and carbonyl compounds.
The initial results achieved with (S)G on the aerobic oxidation of styrene prompted us
to extend this catalytic system to other styrene derivatives. Under identical conditions, α-
methylstyrene exhibited 13 % conversion with 76 % of acetophenone and 20 % of the
corresponding epoxide at 7 h. On the other hand, 4-chlorostyrene gave 6 % conversion after 7
h with 12 % selectivity to benzaldehyde which comes from the dechlorination of 4-
chlorobenzaldehyde. This reaction also afforded 4-chlrobenzaldehyde and the corresponding
epoxide in 81 % selectivity. In contrast, 4-methoxystyrene resulted in higher conversion of 23
% but mainly undefined oligo- and polymeric product were formed and neither aldehyde nor
the corresponding epoxide was detected. These experiments clearly indicate that the rate of
the reactions mainly depend on the nature of the substituent present on the aromatic ring.
Also, achieving high selectivity to epoxide is challenging due to the fact that the initially
formed epoxide may undergo further secondary reactions like ring opening or epoxide
rearrangement, thus, leading to other secondary products with lower selectivity of epoxide.
These side reactions are generally catalysed by adventitious acid or basic sites.13 Therefore, it
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may be that as-synthesised (S)G contains some defects and very low acidic sites that promote
these secondary reactions. The key point is to develop a highly efficient and selective catalyst
for the aerobic oxidation of olefin to increase the density of the active sites which can
promote epoxidation, avoiding the presence of those sites which can act as acid or basic sites
activating secondary reactions.
Reusability of (S)G was tested for the aerobic oxidation of styrene, performing two
consecutive runs after separation of the (S)G catalyst under identical conditions. No
significant changes in styrene conversion compared with the fresh catalyst were observed. On
the other hand, the selectivity of the products also remains the same with slight changes.
These results indicate that (S)G is a stable catalyst to promote the aerobic oxidation of
styrene under the present experimental conditions.
Conclusions
(S)G has been synthesized from naturally occurring -carrageenan having high sulfate
content upon optimization of the pyrolysis temperature. The G structure of the resulting (S)G
was determined by combination of Raman, XPS and TEM measurements. XPS clearly
indicated the presence of two types of sulphur atoms on (S)G, alike to sulfide and sulfoxide,
respectively. The typical layered structure expected for G based materials was clearly
observed by TEM. The catalytic activity of as-synthesized (S)G was tested in the aerobic
oxidation of styrenes to their corresponding benzaldehydes accompanied by styrene oxide. In
contrast to the catalytic activity of (S)G, rGO exhibited no activity under identical conditions.
The (S)G catalyst can be reused two times with no significant changes in conversion and
selectivity.
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Acknowledgements
A.D.M. thanks University Grants Commission, New Delhi for the award of Assistant
Professorship under its Faculty Recharge Programme. Financial support by the Spanish
Ministry of Economy and Competitiveness (CTQ-2012-32315 and Severo Ochoa) and
Generalidad Valenciana (Prometeo 2012-014) is gratefully acknowledged. The research
leading to these results has received partial funding from the European Community’s Seventh
Framework Programme (FP7/2007–2013) under grant agreement no. 228862.
References
(1)
(2)
Goettmann, F.; Fischer, A.; Antonietti, M.; Thomas, A. Chem. Commun. 2006, 4530-
Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228-
4532.
240.
(3)
(4)
Dreyer, D. R.; Bielawski, C. W. Chem. Sci. 2011, 2, 1233-1240.
Dreyer, D. R.; Ruoff, R. S.; Bielawski, C. W. Angew. Chem. Int. Ed. 2010, 49, 9336-
9344.
(5)
Su, D. S.; Zhang, J.; Frank, B.; Thomas, A.; Wang, X.; Paraknowitsch, J.; Schlögl, R.
ChemSusChem 2010, 3, 169-180.
(6)
Su, C.; Loh, K. P. Acc. Chem. Res. 2013, 46, 2275-2285.
Navalon, S.; Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. Chem. Rev. 2014, 114,
(7)
6179-6212.
(8)
Garcia, H. Adv. Chem. 2014, Article ID 906781.
(9)
Su, C.; Acik, M.; Takai, K.; Lu, J.; Hao, S.-J.; Zheng, Y.; Wu, P.; Bao, Q.; Enoki, T.;
Chabal, Y. J.; Loh, K. P. Nature Commun. 2012, 3, 1298.
(10)
(11)
(12)
Sengupta, A.; Su, C.; Bao, C.; Nai, C. T.; Loh, K. P. ChemCatChem 2014, 6, 2507-2511.
Qi, W.; Su, D. ACS Catal. 2014, 4, 3212-3218.
Kumar, B.; Asadi, M.; Pisasale, D.; Sinha-Ray, S.; Rosen, B. A.; Haasch, R.; Abiade, J.;
Yarin, A. L.; Salehi-Khojin, A. Nature Commun. 2014, 4, 2819.
(13)
Dhakshinamoorthy, A.; Alvaro, M.; Concepción, P.; Fornés, V.; Garcia, H. Chem.
Commun. 2012, 48, 5443-5445.
(14)
Dhakshinamoorthy, A.; Alvaro, M.; Puche, M.; Fornes, V.; Garcia, H. ChemCatChem
2012, 4, 2026-2030.
(15)
Primo, A.; Neatu, F.; Florea, M.; Parvulescu, V.; Garcia, H. Nature Commun. 2014, 5,
5291.
(16)
(17)
(18)
(19)
(20)
Kong, X.-K.; Chen, C.-L.; Chen, Q.-W. Chem. Soc. Rev. 2014, 43, 2841-2857.
Dreyer, D. R.; Jia, H.-P.; Bielawski, C. W. Angew. Chem. Int. Ed. 2010, 49, 6813-6816.
Lavorato, C.; Primo, A.; Molinari, R.; García, H. ACS Catal. 2014, 4, 497-504.
Lavorato, C.; Primo, A.; Molinari, R.; Garcia, H. Chem. A Eur. J. 2014, 20, 187-194.
Dhakshinamoorthy, A.; Primo, A.; Concepcion, P.; Alvaro, M.; Garcia, H. Chem. Eur. J.
2013, 19, 7547-7554.
(21)
Primo, A.; Atienzar, P.; Sanchez, E.; Delgado, J. M.; García, H. Chem. Commun. 2012,
48, 9254-9256
12

ACCEPTED MANUSCRIPT
(22)
Abellan, G.; Latorre-Sanchez, M.; Fornes, V.; Ribera, A.; Garcia, H. Chem. Commun.
(Cambridge, U. K.) 2012, 48, 11416-11418.
(23)
Latorre-Sanchez, M.; Primo, A.; Garcia, H. Angew. Chem., Int. Ed. 2013, 52, 11813-
11816.
(24)
(25)
Wang, H.; Bo, X.; Zhang, Y.; Guo, L. 2013; Vol. 108, p 404-411.
Dreyer, D. R.; Jarvis, K. A.; Ferreira, P. J.; Bielawski, C. W. Macromolecules 2011, 44,
7659-7667.
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Graphical abstract
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Highlights



Pyrolysis of -carrageeenan and exfoliation leads to S-doped graphene.
XP spectroscopy shows the presence of two types of sulfur atoms.
S-doped graphene is able to promote the aerobic oxidation of styrenes.
15