Sulfonated graphene as highly efficient and reusable acid carbocatalyst for the synthesis of ester plasticizers

Article abstract of DOI:10.1039/c4ra11205a

Plasticizers are well known for their effectiveness in producing flexible plastics. The automotive, plastic and pharmaceutical industries, essential to a healthy economy, rely heavily on plasticizers to produce everything from construction materials to medical devices, cosmetics, children toys, food wraps, adhesives, paints, and 'wonder drugs'. Although H₂SO₄ is commonly used as commodity catalyst for plasticizer synthesis it is energy-inefficient, non-recyclable, and requires tedious separation from the homogeneous reaction mixture resulting in abundant non-recyclable acid waste. In this study, for the first time, we report an efficient synthesis of ester plasticizers (>90% yields) using sulfonated graphene (GSO₃H) as an energy-efficient, water tolerant, reusable and highly active solid acid carbocatalyst. The hydrothermal sulfonation of reduced graphene oxide with fuming H₂SO₄ at 120°C for 3 days afforded GSO₃H with remarkable acid activity as demonstrated by ³¹P magic-angle spinning (MAS) NMR spectroscopy. The superior catalytic performance of GSO₃H over traditional homogeneous acids, Amberlyst-15, and acidic ionic liquids has been attributed to the presence of highly acidic and stable sulfonic acid groups within the two dimensional graphene domain, which synergistically work for high mass transfer in the reaction. Furthermore, the preliminary experimental results indicate that GSO₃H is quite effective as a catalyst in the esterification of oleic and salicylic acid and thus may pave the way for its broad industrial applications in the near future.

Full text of DOI:10.1039/c4ra11205a

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Sulfonated graphene as highly efficient and
reusable acid carbocatalyst for the synthesis of
Cite this: RSC Adv., 2014, 4, 57297
ester plasticizers†
a
b
a
*
Bhaskar Garg, Tanuja Bisht and Yong-Chien Ling
Plasticizers are well known for their effectiveness in producing flexible plastics. The automotive, plastic and
pharmaceutical industries, essential to a healthy economy, rely heavily on plasticizers to produce everything
from construction materials to medical devices, cosmetics, children toys, food wraps, adhesives, paints, and
‘wonder drugs’. Although H2SO4 is commonly used as commodity catalyst for plasticizer synthesis it is
energy-inefficient, non-recyclable, and requires tedious separation from the homogeneous reaction
mixture resulting in abundant non-recyclable acid waste. In this study, for the first time, we report an
efficient synthesis of ester plasticizers (>90% yields) using sulfonated graphene (GSO3H) as an energy-
efficient, water tolerant, reusable and highly active solid acid carbocatalyst. The hydrothermal sulfonation
of reduced graphene oxide with fuming H2SO4 at 120 ꢀC for 3 days afforded GSO3H with remarkable
acid activity as demonstrated by 31P magic-angle spinning (MAS) NMR spectroscopy. The superior
catalytic performance of GSO3H over traditional homogeneous acids, Amberlyst™-15, and acidic ionic
liquids has been attributed to the presence of highly acidic and stable sulfonic acid groups within the
two dimensional graphene domain, which synergistically work for high mass transfer in the reaction.
Furthermore, the preliminary experimental results indicate that GSO3H is quite effective as a catalyst in
the esterification of oleic and salicylic acid and thus may pave the way for its broad industrial
applications in the near future.
Received 25th September 2014
Accepted 23rd October 2014
DOI: 10.1039/c4ra11205a
www.rsc.org/advances
demand for plasticizers will increase to more than 7.6 million
tonnes per year until 2018.5 The search for alternative
Introduction
approaches employing highly efficient and eco-friendly catalysts
in the synthesis of plasticizers is, therefore, acute.
Plasticizers or dispersants are one of the most critical accessory
ingredients which play a vital role in the elaboration of plasti-
cization efficiency and process performance1 of a macromolec-
ular material, especially, polyvinyl chloride (PVC),2 one of the
cheapest of all synthetic plastics currently used in daily life.
Among plasticizers of industrial interest, esters of poly-
carboxylic acids such as citric acid have received signicant
attention owing to their low toxicity, compatibility with the host
material, non-volatility as well as competitive price.3 Further-
more, the growing interest in biodegradable citric acid esters is
mainly due to their ubiquity and importance in cosmetic,
plastic, food, automobile, and pharmaceutical industries.4
Given the broad range of possible applications, it is predicted by
the market research institute, Ceresana, that the worldwide
In industry, liquid-acid catalysts such as H2SO4 and meth-
anesulfonic acid are predominantly used as crucial bulk
commodity catalysts for the synthesis of ester plasticizers.
However, these catalysts require challenging processes for their
separation from the homogeneous reaction mixture, thereby
resulting in abundant non-recyclable acid waste.6 To address
these issues, endeavours have been made using alternative acid
catalysts such as titanate,7 polyoxometalates (POMs),8 Zr-MCM-
41,9 and acid functional ionic liquids (ILs)10 to complement
liquid-acid catalysts in the synthesis of ester plasticizers. The as-
mentioned catalysts certainly overcome some shortcomings but
still are inappropriate due to the operational loss, high mass
transfer resistance, deactivation, dissolution, and high cost. In
this context, it is critical to search for a catalyst that combines
the toxological benets of metal-free and cost-effective
synthesis with the convenience of heterogeneous workup,
whilst retaining the high activity in the synthesis of plasticizers.
Indubitably, the electronic “boom” that the world has expe-
rienced during the last decade is only due to one specic
material, ‘Graphene’. Graphene can be dened as a two
dimensional sp2 hybridized single-layer carbon sheet with
aDepartment of Chemistry, National Tsing Hua University, 101, Section 2, Kuang-Fu
Road, Hsinchu, 30013, Taiwan. E-mail: ycling@mx.nthu.edu.tw; Fax: +886
35727774; Tel: +886 35715131 ext. 33394
bDepartment of Chemistry, Government Degree College, Champawat, 262523,
Uttarakhand, India
† Electronic supplementary information (ESI) available: The characterization data
and copies of selected 1H and 13C NMR spectra of ester plasticizers. See DOI:
10.1039/c4ra11205a
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hexagonal packed lattice (honeycomb) structure.11 Owing to its further extended and examined in the esterication reactions of
exceptional properties,12 graphene has been increasingly used oleic and salicylic acid.
for wide technological applications13 including photocatalysis.14
On another front, carbocatalysis by graphene is relatively new
but rapidly emerging research area at the interface of organic,
green, and material chemistry.15 The discovery that graphene
Results and discussion
Preparation and characterization of GSO3H
oxide (GO) has an intrinsic acid activity16 has provided a handle
Considering the practical applications of GSO3H, the bulk
for the development of graphene-based materials for a variety of
synthesis of GO was the prerequisite in this study. However, it is
acid catalysed reactions.17 Certainly, however, this growing
well known that during washing/purication steps, GO disper-
interest in graphene-based acid catalysis has resulted ‘an
sions tend to undergo severe gelation rendering ltration more
unsweet smell’ of conceptual vagueness in many aspects.17 For
complicated and tedious.22 Taking this into account, an acid–
instance, the surface modication of GO with sulfonated and
acetone washing procedure was adopted for preventing gelation
sulfated groups is oen confused while the structural differ-
and speeding-up the purication step in the synthesis of GO.23
ences between sulfonation and sulfation reactions are quite
Furthermore, it is also identied that LiAlH4 is one of the
signicant as shown in Scheme 1. Despite this, the fabrication
strongest reducing agents in hydride family and serves as an
of graphene with –SO3H groups affording sulfonated graphene
attractive option, where most reducing agents exhibit moderate
is of crucial importance and increasingly used in catalysis,18a
reactivity toward carboxyl functions. Accordingly, LiAlH4 was
micro-solid-phase extraction,18b graphene-based composites,18c
applied for reduction of GO into reduced graphene oxide (rGO).
biomimetics,18d and other technological applications.18e–g
Fuming H2SO4 or oleum-assisted sulfonation is one of the
Among different strategies for graing –SO3H groups in
simple and efficient industrial strategies, especially, for versa-
graphene domain, oleum-assisted sulfonation has appeared as
tility in feedstock selection to build materials that are truly
an attractive strategy.19 Nevertheless, alternative reaction
application specic. Nevertheless, relatively high temperature
conditions, an upscale synthesis at relatively low cost, and the
conditions used in sulfonation reaction of rGO may introduce
simplicity in processing and handling are some of the impor-
some defects within graphene domain resulting in an insuffi-
tant tenets of ‘green chemistry’ and can be the best selections in
cient graing of –SO3H groups, which, in turn, will lower the
the catalysis research. Equally important is to investigate and
acid density of sulfonated material resulting in limited catal-
expanding the scope of existing catalysts for possible industrial
ysis. Consequently, the oleum-assisted sulfonation of rGO was
applications. With these realizations combined with a knowl-
performed at relatively low temperature for several days. The
edge of products-switching in tetrapyrrolic systems using acidic
present protocol not only avoids the use of expensive instru-
ionic liquid catalysts,20 we decided to investigate an energy-
ment handling but also permits the bulk synthesis of GSO3H
efficient, upscale, and alternative synthesis methodology for
with ease.
oleum-assisted sulfonation of graphene and expanding the
The Fourier transform infrared spectroscopy (FT-IR) spec-
scope of as-prepared sulfonated graphene (GSO3H) to novel
trum of GO as shown in Fig. 1 presented substantial bands at
applications such as in the synthesis of (biodegradable)ester
1054 and 1400 cmꢁ1 corresponding to C–OH vibrations. The
plasticizers. To the best of our knowledge, there is still no report
signal at 1230 cmꢁ1 was generated by stretching vibrations of
of using graphene-based solid acid catalysts in the aforemen-
C–O–C groups, and nally, two intense bands at 1623 and 1729
tioned synthesis.
cmꢁ1 were generated by C]O and –COOH vibrations, respec-
Intrigued by the signicant role of polycarboxylic acid esters
tively.24 Compared with GO, GSO3H exhibited one prominent
in automotive, food, plastic and polymer industries combined
with our ongoing interest in homo- and heterogeneous catal-
ysis,21,22 herein, we report the synthesis of highly active GSO3H,
a solid acid catalyst and its usefulness in the effective synthesis
of ester plasticizers. In line with this, the scope of GSO3H is
Scheme
1 Sulfonation vs. Sulfation reactions. Reproduced from
ref. 17.
Fig. 1 FT-IR spectra of (a) GO and (b) GSO3H.
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average size of sp2 hybridized graphene domains due to the
incorporation of abundant –SO3H groups.25
Fig. 3 shows the X-ray diffraction (XRD) patterns of GO, rGO,
and GSO3H. GO displayed a sharp peak at 10.7ꢀ at the expense
of a diffraction peak at 26.2ꢀ, associated with graphite. The
appearance of a peak at 10.7ꢀ clearly indicates the presence of
oxygen functionalities in graphene domains. Upon reduction,
the peak at 10.7ꢀ completely disappeared and rGO exhibited a
broad diffraction peak, centred at 24.2ꢀ. The GSO3H showed
almost similar XRD patterns (24.2ꢀ)19 as rGO which suggests
their similar graphene domains.
Fig. 2 Raman spectra of (a) GO, (b) rGO, and (c) GSO3H.
band at 1099 cmꢁ1 and relatively a weak band at 1192 cmꢁ1
,
which are associated with S]O stretching vibrations.19,25
However, similar to that of rGO (not shown here), the bands at
1729, 1623, 1400, 1054 and 1230 cmꢁ1 were severely attenuated
in the spectrum of GSO3H. These results combined with the 31
P
magic-angle spinning (MAS) NMR spectroscopy, as will be dis-
cussed below, conrmed the successful graing of –SO3H
groups in the as-prepared GSO3H.
In order to gain the structural information about GO, rGO
and GSO3H, Raman spectroscopy was used. As shown in Fig. 2,
the rst-order Raman spectra of GO exhibited two characteristic
D (indicative of the defects and ordered/disordered structure of
graphitic carbon) and G bands (indicative of pristine graphene
sheet) at 1354 and 1599 cmꢁ1, respectively. The obtained ID/IG
ratio of GO was 1.04 indicating an extensive disorder in the
graphitic structure due to the presence of a complex cocktail of
oxygen functionalities. Upon reduction, the rGO exhibited quite
similar Raman spectra to that of GO, however, the ID/IG ratio
decreased to 0.85. A reduced ID/IG ratio can be attributed to the
removal of oxygen-containing groups and the reintroduction of
large aromatic domains.26 Compared with rGO, GSO3H gave
relatively a higher ID/IG ratio (0.98) suggesting a decrease in the
Fig. 4 XPS spectra of the GSO3H; (a) Survey spectrum. (b) High
Fig. 3 XRD patterns of (a) GO, (b) rGO, and (c) GSO3H.
resolution C1s. (c) S2p spectrum.
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To investigate the surface composition and chemical state of presumably, due to the presence of O-acid sites, in particular,
the elements, GSO3H was further characterized by X-ray hydroxyl functions, other than –SO3H group on the surface of
photoelectron spectroscopy (XPS) analysis. As shown in GSO3H. Considering the higher O/S atom ratio (3.4 : 1) than the
Fig. 4a, the XPS spectrum of as-prepared GSO3H displayed a theoretical one (3 : 1), the presence of residual oxygen-con-
predominant C1s peak, a weaker O1s and much weaker S2p taining functional groups on GSO3H surface is further
peak. In detail, the high resolution C1s XPS spectrum (Fig. 4b) conrmed. Aside from the GSO3H, the neutralization titrations
can be deconvoluted into four peaks at 283.8, 286.2, 287.5 and of GO indicated an acid exchange capacity of 0.89 mmol H+ gꢁ1
,
289.1 eV which are respectively attributed to C–C, C–O–C, C]O, which may be attributed to the presence of –O–SO3H groups on
and O]C–OH species.19 Notably, the intensities for the peaks the surface of GO.
corresponding to the carbonyl and the carboxylate functions
In order to further shed light on the acid strength of as-
exhibited much smaller relative contents suggesting that most prepared GSO3H, 31P MAS NMR spectroscopy was executed
of the oxygen-containing functional groups are successfully using triethylphosphine oxide (TEPO) as an adsorbed base
removed in GSO3H.27 The S2p spectrum of GSO3H showed a probe molecule. Indeed, this method has been shown to be
peak at 168.7 eV (Fig. 1c), which is slightly lower than those of sensitive and reliable technique capable of providing exclusive
–SO3H functionalized ordered mesoporous carbon (OMC– information about relative acidities of various solid acids. In
SO3H; 168.8 eV), Amberlyst™-15 (168.9 eV), and sulfonated particular, the larger 31P NMR chemical shis (d31P) correlates
hollow sphere carbon (HSC–SO3H, 169.1 eV).19,28 In this milieu, the higher acidic strength of respective solid acid.30 Fig. 5 shows
it is important to stress that binding energy of S2p is a direct the 31P NMR spectrum of TEPO adsorbed on different extracted
measure and quite sensitive to the acidic strength.29
materials: GO, rGO, GSO3H, and a sulfonic resin, Amberlyst™-15.
The elemental analysis was performed with GO, rGO and
Spectrum for rGO (as prepared by LiAlH4 triggered reduction
GSO3H. Specically, GO and rGO resulted in C/O ratios of 1.09 of GO) exhibited a broad 31P peak centred at 55.5 ppm that may
(C, 49.01%; O, 44.56%) and 13.5 (C, 89.2%; O, 6.6%), respec- correspond to TEPO adsorbed on residual –OH and –COOH
tively. An increase in the C/O ratio in rGO certainly signify the groups on the surface of rGO. The as-prepared GSO3H shared a
efficacy of LiAlH4. Alongside, the elemental analysis revealed resonance at nearly same chemical shi (58.3 ppm) as rGO and
that the mass ratios of carbon, oxygen and sulphur in GSO3H is in accordance with results reported by other authors.31
are 86.48, 8.56, and 4.96%, respectively. That is, the density of However, relatively a high intensity of this peak than that of rGO
–SO3H groups in GSO3H is calculated to be 1.55 mmol gꢁ1
.
may be attributed to the introduction of additional oxygen-
However, the acid–base titrations revealed that GSO3H has an containing groups during sulfonation reaction of rGO. In
actual acid exchange capacity of 1.69 mmol H+ gꢁ1. This is, addition, GSO3H exhibited two overlapping but strong 31P NMR
Fig. 5 31P NMR spectra of TEPO chemically adsorbed onto GO, rGO, GSO3H, and Amberlyst™-15. The asterisks mark spinning sidebands.
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signals at 74.5 and 84.8 ppm, respectively, authenticating the can also be seen in GSO3H that may have the potential advan-
presence of highly acidic –SO3H groups on the surface of gra- tage in heterogeneous acid catalysis.
phene domains. Interestingly, the signal at 84.8 ppm is in good
agreement with Amberlyst™-15, which displays a very high
acidity (86.6 ppm;32 single, sharp and strong resonance) due to
the presence of a macro reticular sulfonic polystyrene network.
These results clearly demonstrate that just by simple switching
in experimental conditions, highly acidic sulfonated graphene
can be obtained with ease.
Catalytic activity of GSO3H: optimization of the reaction
conditions
Having established the essential nature of the GSO3H, the
conversion of citric acid (1) and n-butanol (2a) into tributyl
citrate (3a) was chosen as a model reaction in order to explore
the catalytic activity of GSO3H under different reaction condi-
tions. (Table 1). In a preliminary experiment, 1 (5.2 mmol) in 2a
(25 mL) was reuxed for 4 h in the presence of GSO3H (200 mg).
To our delight, aer ltration of catalyst and removal of the
volatile solvent, 3a could be obtained in an excellent yield of ca.
94% (Table 1, entry 3).
Aer observing quantitative conversion of 1 into 3a, subse-
quent efforts were directed toward optimizing reaction condi-
tions such as GSO3H loadings and the reaction time. In
particular, minimal changes in the isolated yields were
observed upon increasing the reaction time and GSO3H load-
ings (Table 1, entries 4–6). Nevertheless, it was found that
variations in the GSO3H loadings (ranging from 50–100 mg) had
a signicant effect on the isolated yields of the plasticizer
products (Table 1, entries 1, 2). While 50 mg of GSO3H afforded
35% of the target product (Table 1, entry 1), increasing the
loading to 200 mg of GSO3H was found to be sufficient to drive
the esterication reaction in the forward direction (Table 1,
entry 3). When 2a was employed in stoichiometric amount (3.2
Aside from these observations, the 31P NMR spectrum of GO
displayed a single strong resonance at 64.2 ppm that may be
connected to the TEPO adsorbed on mixed acidic centers
arising from the presence of hydrosulphates (–O–SO3H) in
conjunction with abundant –OH groups. It would be worthwhile
to mention here that the acidic strength of GO is nearly
comparable to aluminium-substituted mesoporous silica (Al-
MCM-41; 66–69 ppm) or even signicantly higher than that of
conventional SBA-15 mesoporous silica (ꢂ60 ppm) as evaluated
31
by TEPO adsorbed P NMR chemical shis.32
Fig. 6 depicts the eld-emission scanning electron micros-
copy (FESEM) images of GO, rGO, and GSO3H. The FESEM
image clearly indicates the layered structure of GO (Fig. 6a).
Compared with GO, the rGO (Fig. 6b) exhibited crumpling
features in exfoliated graphene domains. The sulfonation of
rGO led to further exfoliation (Fig. 6c) of crumpled layers into
sheet-like structure as evidenced by high resolution FESEM
image (Fig. 6d). Indeed, to some extent, the crumpling features
Fig. 6 The FESEM images of (a) GO, (b) rGO, and (c and d) GSO3H.
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Table 1 Esterification of citric acid (1) with n-butanol (2a) into tributyl
citrate (3a) over GSO3H and other catalystsa
from the reaction mixture at ca. 50% formation of 3a. Aer
removal of the GSO3H, the reaction was progressed again at
reux. In the absence of GSO3H, no further product formation
was observed even aer 2 h indicating that catalysis occurs on
the surface of GSO3H and the process is truly heterogeneous.
These experiments clearly demonstrate the indispensable role
of GSO3H in facilitating the esterication reaction to quantita-
tive conversion.
In the reaction of 1 with 2a, GO exhibited good catalytic
activity (though poorer than GSO3H), when used at higher doses
under otherwise optimized conditions (Table 1, entry 10). The
catalytic activity of GO might be attributed to its inherent acidic
nature due to the presence of surface functional –O–SO3H
groups, which can be introduced during the synthesis of GO
under relatively harsh acidic conditions.33 Nevertheless,
minimal yields of the plasticizer product were obtained when
GSO3H was replaced by natural ake graphite or rGO (Table 1,
entries 11, 12). Likewise, no product formation was recognized
in the absence of a carbon promoter under otherwise identical
conditions (reux, 4 h) or even prolonging the reaction time
(Table 1, entry 8).
Entry
Catalyst
Loading
Time (h)
Yieldb (%)
1
2
3
4
GSO3H
GSO3H
GSO3H
GSO3H
GSO3H
GSO3H
GSO3H
—
50 mg
4
4
4
6, 8
4
4
4
4, 8, 10, 12
4
4
35
69
94
92, 90
100 mg
200 mg
200 mg
300 mg
400 mg
200 mg
—
200 mg
200 mg
400 mg
200 mg
200 mg
3 mol%
5 mol%
5 mol%
10 wt%
5
93
93
90
—
6
7c
8
9d
10
GSO3H
GO
56e, 64f, 92g
42
62
5
2.3
94
78
70
79
11
12
r-GO
Graphite
H2SO4
p-TSA
H3PW12O40
Amberlyst™-15
6–8
6–24
4
4
4
4
13
14h
15i
16j
Comparison of GSO3H activity with other acid catalysts
To evaluate the catalytic performance of GSO3H, various tradi-
tional acid catalysts were also examined and the results are
summarized in Table 1. Specically, homogeneous acids such
as H3PW12O40, p-toluenesulfonic acid (p-TSA), and H2SO4 were
examined in the reaction and afforded 3a in good (70%, 78%) to
excellent (94%) yields, respectively (Table 1, entries 13–15).
A relatively high yield of 3a in the presence of H2SO4,
comparable to that use of GSO3H, was not surprising as H2SO4
exhibits higher acidity than those of H3PW12O40 and p-TSA.
Furthermore, the unique spatial separation as well as the self-
similarity of structures between the active sites in H2SO4 allow
consistent energetic interactions between each of active site and
reaction substrate. However, the technical challenges, of either
the separation issue associated with H3PW12O40 or reuse
performances of these homogeneous acids were of high
concern.
Aside from homogeneous acids, Amberlyst™-15 was of
further interest due to its established activity in important acid-
catalyzed reactions such as esterication, and alkylation.34 The
reaction of 1 with 2a in the presence of Amberlyst™-15 afforded
3a in 79% yield (Table 1, entry 16). Considering the much higher
acidic content of Amberlyst™-15 (4.7 eq. per kg)34 than that of
GSO3H, the higher catalytic activity of GSO3H over Amberlyst™-
15 may be associated to its unique two dimensional sheet
structure, where most of the –SO3H groups are well dispersed
and exposed to the reactants resulting in the high mass transfer.
The another possible explanation for the stronger acidity of the
GSO3H is that some of the –SO3H groups in GSO3H may be
linked by strong hydrogen bonds ensuing in the higher acidity
due to mutual electron-withdrawal.35 On the other hand,
Amberlyst™-15 represents a porous structure resulting in rela-
tively slow mass transfer. As a consequence, some of the –SO3H
17k
6 mL
5 mL
4
4
94
18l
Dissolved
19m
5 mL
4
Dissolved
a
Reaction conditions: 1 (5.2 mmol), 2a (25 mL), and catalyst (type and
amount indicated) were combined in 50 mL round-bottomed ask and
reuxed at 120 ꢀC unless otherwise stated for the time indicated.
b
c
d
Isolated yield. Aer ve runs. 1 (5.2 mmol), 2a (16.6 mmol), and
e
f
g
solvent (25 mL). DCM, 45 ꢀC. THF, 75 ꢀC. Toluene, 112 ꢀC.
h
i
j
p-Toluene sulfonic acid. Phosphotungstic acid. Catalyst was
k
activated at 105 ꢀC for 4 h and dried prior to use. Dried under
vacuum for 6 h and stored in dry box prior to use; 1 (5.2 mmol).
l
Dried under vacuum for 6 h and stored in dry box prior to use; 2a
m
(16.6 mmol). Dried under vacuum for 6 h and stored in dry box
prior to use; 90 ꢀC.
equiv.) in the presence of different solvents, 3a was obtained in
moderate to excellent yields (Table 1, entry 9). The relatively
lower yield of 3a in DCM and THF may be attributed to the effect
of reuxing temperature and consequently lower reaction rate.
Importantly, the conversion of 1 into 3a was quantitative in
toluene (Table 1, entry 9).
To verify whether or not the catalysis is truly heterogeneous
or due to some leached active species present in the liquid
phase, the reaction was carried out under the optimized
conditions as described in Table 1 and the GSO3H was ltered
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groups on this porous catalyst might not be accessible to reac- greater stability of the GSO3H even under harsh reaction
tants and thus limit the product yield to a reasonable extent.
Indubitably, ILs have been under the spotlight of the green
catalysis over the last decade.36 Consequently, the nal catalysts
screened were of acid functionalized Brønsted ILs which were of
conditions.
Catalytic scope of GSO3H
particular interest to us aer observing their unique catalytic The scope and limitations of GSO3H were examined by
potential in the organic synthesis20,21b as well as in the rapid screening a variety of structurally diverse alcohols 2b–2h in the
identication of tertiaryalkyl amines.37 Stirring 1 with 2a in the synthesis of biodegradable ester plasticizers such as trimethyl
presence of N-(4-sulfonic acid) butyl triethylammonium citrate (TMC), triethyl citrate (TEC), triisopropyl citrate (TIPC),
hydrogen sulfate [TEBSA][HSO4] at 90 ꢀC for 4 h afforded a clear triisobutyl citrate (TIBC), tri-n-hexyl citrate (THC), tri-n-active
biphasic reaction mixture. The upper phase was simply dec- amyl citrate (TAAC), and tri-n-octyl citrate (TOC).
anted and the desired 3a could be obtained in 94% yield (Table
As revealed in Table 2, the reaction of 1 with 2b–2h,
1, entry 17). The obtained yield was comparable to that of H2SO4 regardless of the presence of linear (Table 2, entries 1–4) or
and GSO3H. Nevertheless, during the reaction, both N-methyl-2- branched chain (Table 2, entries 5–7) functionalities could be
pyrrolidonium hydrogen sulfate [NMP][HSO4] and N-methyl-2- directly converted to their respective plasticizer products in
pyrrolidonium methane sulfonate [NMP][CH3SO3] dissolved in appreciable yields. Despite being all primary alcohols, the
the reaction mixture making it difficult in separation (Table 1, reactivity of alcohols having long alkyl chain appeared slightly
entries 18, 19).
lower (Table 2, entries 3, 4) than that of short or branched chain
alcohols (Table 2, entries 1, 2 and 5–7). Furthermore, 2g
exhibited the shortest reaction time, presumably, because of its
stronger nucleophilic activity over primary alcohols. Neverthe-
less, the formation of mono- and di-esterication products was
not recognized in any case under the experimental conditions.
In an effort to expand GSO3H activity within the family of
ester plasticizers, we turned next toward exploring the synthesis
of phthalic acid esters. The so called ‘phthalate plasticizers’
found applications for the rst time in 1920s and by far are the
most widely used plasticizers, primarily, to make so and ex-
ible PVC, in the 21st century.39 With phthalates, the polarisable
benzene nucleus is highly effective with respect to compatibility
with PVC, making the long polyvinyl molecules to slide against
one another. Considering the greater return upon charging and
less water to expel (in comparison to traditional acid catalysts)
at the end of esterication reaction, phthalic anhydride 4 is
used as a precursor rather than phthalic acid. As summarized in
Table 3, a broad range of alcohols 2 were successfully converted
to their corresponding plasticizers using GSO3H under opti-
mized conditions (7.0 mmol 4, 150 mg GSO3H, 20 mL solvent,
and reux). Excellent yields (>92%) of plasticizers were obtained
in all the cases including those that featured long or branched
O-alkyl substituents.
Considering its signicant biodegradability, lower carbon
dioxide/sulphur emission, and minor particulate pollutants,
biodiesel consisting of long-chain fatty acid alkyl esters (FAAE)
can be used both as an alternative fuel and as an additive for
petroleum diesel. On another front, methyl salicylate (MS) or oil
of wintergreen is an industrially important ne chemical.
Intrigued by the successful use of solid acid catalysts in the
production of biodiesel by esterication,40 a further attempt was
made to extend the feasibility of GSO3H in the esterication of
oleic acid to methyl oleate (MO) as well as in the synthesis of
MS.
Reusability of GSO3H
The reusability of GSO3H in conjunction with [TEBSA][HSO4],
and Amberlyst™-15 was investigated for the esterication
reaction of 1 with 2a under identical conditions as described in
Table 1 and Fig. 7. In general, aer the reaction, the catalysts
were separated from the reaction mixture and reused without
any further treatment except vacuum drying. Interestingly, the
yield was signicantly reduced in the next cycle (<50%) when
Amberlyst™-15 was used. This may be attributed to that of the
water-induced poisoning of the porous acidic sites in
Amberlyst™-15. On the other hand, [TEBSA][HSO4] was dis-
solved in the reaction mixture aer three consecutive cycles.
GSO3H exhibited remarkable activity and could be reused
between the rst and h runs without any considerable loss in
the catalytic activity (Table 1, entry 7). It should be noted that
unlike –SO3H bearing resins, the reaction with the carbocatalyst
is not signicantly dependent on the amount of water present in
the reaction system38 resulting in the high activity of GSO3H in
esterication reactions. In addition, because GSO3H possess
–SO3H and few –COOH functions, it is expected that electron-
withdrawing nature of –COOH can increase the electron
density between the carbon and sulphur atoms resulting in the
As depicted in Table 4, the GSO3H catalysed esterication of
oleic and salicylic acid gave excellent conversions within 4 h.
Furthermore, the catalyst was equally effective during second
run demonstrating the superior catalytic performance of
GSO3H. The effect of variable reaction conditions on product
Fig. 7 The recycle activity of GSO3H. Bars denote as: blue bar
(Amberlyst™-15), green bar ([TEBSA][HSO4]), and red bar (GSO3H).
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Table 2 Synthesis of biodegradable plasticizers using GSO3Ha
Entry
Alcohol (2)
Molar ratio of 1 : 2
Product (3)
Method
Time (h)
4
Yieldb (%)
93
1c
1 : excess
Method A
2c
1 : excess
Method A
Method Bd
4
94
88
3
1 : 3.2
6e
4
1 : 3.2
Method Bd
6e
87
5
6
1 : 3.2
Method Bd
Method A
4
91
94
1 : excess
2.5
7
1 : 3.2
Method Bd
4e
93
a
All reactions were performed at 90 ꢀC without solvent using 5.2 mmol citric acid 1, 200 mg GSO3H for the indicated reaction time. GSO3H was kept
under high vacuum for 3 h prior to use. b Isolated yield. c HPLC grade solvents were used. The solvents were further dried and distilled by following
d
e
the standard procedures prior to their use. Toluene was the solvent of choice, 25 mL, 112 ꢀC. Aer 4 h, the progress of the reactions were
scrutinized in every 30 min by TLC. Aer ltration of catalyst and subsequent removal of solvent, the product was passed through a short silica
column and dried.
formation, however, must await a more detailed systematic (LAH) was purchased from Lancaster. KMnO4 was obtained
study in this regard.
from Fluka. Toluene (100% assays by GC) was purchased from
Mallinckrodt chemicals. All chemicals and reagents received
were of highest purity and used without further purication
unless otherwise mentioned.
Experimental
Chemicals and reagents
Natural graphite ake (7–10 micron, 99%), 1-hexanol,
1-butanol, triethylamine, 1,4-butane sultone, phthalic anhy-
Methods
dride were purchased from Alfa Aesar, UK. N-methyl- Fisher Scientic FS60 ultrasonic bath cleaner (150 W) was used
2-pyrrolidone, 2-methyl-1-propanol, 2-methyl-1-butanol, for performing sonication treatment of G-NMs. The XRD
Amberlyst™-15, p-TSA, glycerol, NaNO3, H2SO4, and Oleum patterns were recorded using a Shimadzu XRD-600 diffractom-
(30% SO3) were purchased from Sigma-Aldrich. Anhydrous cit- eter with Cuka radiation. The morphology of G-NMs were
ric acid, methanol, 1-octanol, 2-propanol, THF, and DCM were examined using FESEM (Hitachi, S-4800, 15 kV). The XPS
purchased from J. T. Baker (USA). Lithium aluminium hydride measurements were carried out using an ULVAC-PHI Quantera
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Table 3 Synthesis of phthalate plasticizers using GSO3Ha
Entry
1
Alcohol (2)
Molar ratio (4 : 2)
Product (5)
Method
Time (h)
4
Yieldb (%)
94
1 : excess
Method A
2
3
4
5
1 : excess
1 : excess
1 : 2.1
Method A
Method A
Method B
Method B
3
3
6
6
96
93
92
95
1 : 2.1
6
7
1 : 2.1
1 : 2.1
Method B
Method B
4
4
95
96
8
1 : 2.1
Method B
5
94
a
All reactions were performed at reux with (20 mL) or without solvent using 7.0 mmol phthalic anhydride 4, and 150 mg GSO3H for the indicated
b
reaction time. The other reaction conditions are virtually the same as described above in Table 2. Isolated yield.
Table 4 Esterification of oleic and salicylic acid with CH3OH in the
presence of GSO3Ha
Fremont, CA, USA). The confocal micro Raman spectroscopy
was performed using a Horiba Jobin-Yvon LAB RAM HR 800 UV
(Japan) spectrometer (laser source: 325 nm, He–Cd, 30 mW).
The ion-exchange capacities of the G-NMs were determined
by acid–base titrations. In a typical experiment, 0.05 g of solid
sample was added to aqueous solution of NaCl (0.1 M, 20 mL)
and the resulting suspension was allowed to equilibrate.
Thereaer, it was titrated by dropwise addition of aqueous
NaOH (0.01 M).
The acid properties of solid G-NMs and Amberlyst™-15 were
assessed by monitoring the 31P NMR chemical shi of chemi-
cally adsorbed TEPO onto the solid materials. In a typical
experiment, TEPO (0.015 g) was dissolved in anhydrous pentane
(5 mL), and this solution was mixed with dehydrated solid acids
(0.15 g). The resulting suspension was allowed to equilibrate
Entry
Conversionb (%)
Selectivityc (%)
MO
98.9
98.6
MS
95.7
95.6
MS
100
100
1
2d
a
Reaction conditions: ratio of CH3OH to oleic acid and salicylic acid ¼
5 : 1; GSO3H ¼ 200 mg; reuxed for 4 h at 90 ꢀC. b Based on GC analysis.
c
d
Selectivity for methyl salicylate (based on salicylic acid). Reuse
performance of GSO3H.
SXM spectrometer and data were recorded using a mono-
chromatic Al anode as the excitation source. The FT-IR was
carried out on a Perkin-Elmer system 2000 (Perkin-Elmer,
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under stirring for 30 min in an inert atmosphere and thereaer
dried at 50 C under vacuum.
Method B. A 50 mL round-bottomed ask tted with a reux
condenser and a magnetic stir bar was charged with 1, GSO3H
and an appropriate solvent. To this stirred mixture, 2 was added
in stoichiometric amount via a syringe and the reaction mixture
was stirred at reux for appropriate time. Aer completion of
the reaction, the GSO3H was ltered off and solvent was evap-
orated in a rotary evaporator. The 3 was occasionally passed
through a short silica-gel column and characterized by NMR
spectroscopy.
ꢀ
The 31P NMR measurements were performed on a Bruker
Avance III 400 spectrometer using a 4 mm double resonance
probe operating at a B0 eld of 9.4 T (400 MHz) with a 31P
Larmor frequency of 161.9 MHz. 31P {1H} MAS NMR spectra
were recorded by using a rotation speed of 12 kHz, a single
excitation pulse width of 1.9 ms, a radio-frequency eld
strength of 45 kHz, and 15 s recycle delay. A two-pulse phase
1
modulation scheme (TPPM-15) was used for H heteronuclear
decoupling.
Conclusions
The 1H NMR and 13C NMR were recorded on a nuclear
magnetic resonance spectrometer (Bruker Cryomagnet, Oxford)
operated under 600 MHz (1H) and 150 MHz (13C), respectively at
room temperature (with high concentration of plasticizers). The
chemical shis (d ppm) are referenced to the respective solvents
and splitting patterns are designed as s (singlet), d (doublet), t
(triplet), m (multiplet), br (broad) and bs (broad singlet). The
column chromatography was carried out using silica gel (100–
200 mesh). The TLC analysis was carried out on double coated
silica Merk plates. The ILs were prepared as reported previously
by us.20,37 The ILs were dried under vacuum for appropriate time
and kept in a dry box prior to use.
In conclusion, sulfonated graphene (GSO3H) bearing abundant
–SO3H functional groups have been synthesized in an alterna-
tive way at affordable cost. Relative to the traditional mineral
and solid acid catalysts, the as-prepared GSO3H has been
proven to be a highly efficient heterogeneous acid catalyst in the
synthesis of plasticizer esters, methyl oleate and methyl salicy-
late. The superior catalytic performance of GSO3H can be
attributed to the synergistic combination of the specic struc-
ture, water tolerant character, and the highly acidic–SO3H
functional groups on its surface. These features, in addition, are
quite favourable for the stability of catalyst and high mass
transfer in the reaction. The experimental results as-described
here is expected to contribute to GSO3H utilization as an alter-
native yet robust solid acid catalyst for the development of
Preparation of GSO3H catalyst
The graphene oxide (GO) was synthesized by using a modied industrially important O- and N-containing heterocycles.
Hummer's method.23 The rGO was synthesized by the chemical Furthermore, the investigations on the efficacy of GSO3H in
reduction of GO using LAH as reported previously.24 The biodiesel synthesis and making full use of its by-product, glyc-
synthesis of GSO3H was carried out in a standard fume erol, is currently underway in our laboratory.
cupboard. Specically, 1 L nitrogen ushed round-bottomed
ask equipped with a condenser and an efficient magnetic
stir bar was charged with rGO (5.0 g). Fuming H2SO4 (300 mL)
Acknowledgements
This work was supported by National Tsing Hua University
(102N1807E1) and the Ministry of Science and Technology
(NSC101-2113-M-007-006-MY3) of Taiwan.
was slowly added and the resulting suspension was stirred at
room temperature for 1 h. The ask was heated at 120 C with
ꢀ
vigorous stirring for 3 days under nitrogen atmosphere. Aer
completion of the reaction, the reaction mixture was cooled to
room temperature and added in portions to a well stirred
mixture of crushed ice and water (1 L) in order to quench the
reaction.‡ The resulting black precipitate was ltered and
washed repeatedly with water (2 L) followed by acetone (3 ꢃ 100
mL). The as-obtained GSO3H was dried at 60 ꢀC in vacuum
overnight for further use.
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4 (a) L. V. Labrecque, R. A. Kumar, V. Dave, R. A. Gross and
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