Graphene oxide as a facile solid acid catalyst for the production of bioadditives from glycerol esterification

Article abstract of DOI:10.1016/j.catcom.2015.01.007

Graphene oxide (GO) has proved to be a highly active and reusable solid acid catalyst for glycerol esterification with acetic acid in the synthesis of bioadditives diacylglycerol (DAG) and triacylglycerol (TAG). The effects of reaction temperature, molar ratio of acetic acid to glycerol, catalyst amount and reaction time were investigated. A 90.2% combined selectivity of DAG and TAG with complete glycerol conversion was achieved at 120 °C for 6 h over GO. Final characterization shows that the active site of GO is the remaining SO₃H group.

Full text of DOI:10.1016/j.catcom.2015.01.007

Catalysis Communications 62 (2015) 48–51
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Graphene oxide as a facile solid acid catalyst for the production of
bioadditives from glycerol esterification
a,b
Xiaoqing Gao a,b, Shanhui Zhu a, , Yongwang Li
⁎
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
b
Synfuels China Co. Ltd., Taiyuan 030032, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Graphene oxide (GO) has proved to be a highly active and reusable solid acid catalyst for glycerol esterification
with acetic acid in the synthesis of bioadditives diacylglycerol (DAG) and triacylglycerol (TAG). The effects of
reaction temperature, molar ratio of acetic acid to glycerol, catalyst amount and reaction time were investigated.
A 90.2% combined selectivity of DAG and TAG with complete glycerol conversion was achieved at 120 °C for 6 h
over GO. Final characterization shows that the active site of GO is the remaining SO3H group.
© 2015 Elsevier B.V. All rights reserved.
Received 20 September 2014
Received in revised form 26 December 2014
Accepted 6 January 2015
Available online 8 January 2015
Keywords:
Glycerol
Esterification
Graphene oxide
Bioadditive
Diacylglycerol
Triacylglycerol
1. Introduction
Significant acid catalysts have been used for glycerol esterification,
including sulfated based superacids [9–11], heteropolyacid-based cata-
The limitation of fossil resources and global concerns regarding en-
vironmental issues stimulate the increasing attention for sustainable
production of fuels from renewable biomass. Among them, biodiesel
has attracted great interest because it possesses nontoxic, biocompati-
ble as well as biodegradable features, and substantially reduces CO2
emission. With the rapid development of biodiesel industry, abundant
byproduct glycerol has been produced via the transesterification of veg-
etable oil with low alcohol [1]. Therefore, it is urgent to convert glycerol
into valuable chemicals or biofuels to promote the benign development
of biodiesel industry. Several innovative strategies have been designed
to utilize glycerol involving hydrogenolysis [2], oxidation [3], esterifica-
tion [4–6], etherification [7], polymerization [1] and so on. In this con-
text, one of the most desirable processes is to perform glycerol
esterification with acetic acid to produce monoacylglycerol (MAG), di-
acylglycerol (DAG) and triacylglycerol (TAG). These products are widely
utilized in cryogenics, biodegradable polyester and cosmetics. Com-
pared with MAG, DAG and TAG own much higher economic value be-
cause of their potential applications as valuable liquid fuel additives in
improving octane number, cold and viscosity properties [8].
lysts [12–14], Amberlyst-15 [15–18], tin chloride [19], zeolite [18], Y/
SBA-3 [20], and ZrO2 based solid acids [21]. Regardless of their great ad-
vances, most have the drawbacks of rapid deactivation, complex prepa-
ration procedures, low reactivity, and expensive costs. To overcome
these disadvantages, it is imperative to develop a highly active, inexpen-
sive, robust and sustainable solid acid catalyst for glycerol esterification.
Recently, graphene and graphene oxide (GO) have gained consider-
able interest due to their unique physical, chemical and electrical prop-
erties [22]. Prepared by Hummer's method, GO undergoes exhaustive
oxidation and thus possesses rich oxygen-containing functionalities,
such as SO3H, carboxyl, hydroxyl and epoxide groups, which endows
it with moderate acidic and oxidizing properties [23]. GO has been
employed as an efficient carbocatalyst in hydration [23], oxidation
[23], Aza-Michael addition [24] and Friedel–Crafts addition [25]. Recent-
ly, GO has demonstrated to be a highly active and reusable solid acid
carbocatalyst for furfuryl alcohol alcoholysis in our previous work [22].
Consequently, it is envisaged that GO may be a potential and facile
solid acid catalyst for glycerol esterification with acetic acid. For the
first time, GO is reported as an acid catalyst for glycerol esterification.
Our results indeed show that GO is capable of achieving complete glyc-
erol conversion with ~90% combined selectivity to preferred DAG and
TAG. The unprecedented catalytic performance is related to the
oxygen-containing groups, particularly the SO3H groups.
⁎
Corresponding author at: State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China.
E-mail address: zhushanhui@sxicc.ac.cn (S. Zhu).
http://dx.doi.org/10.1016/j.catcom.2015.01.007
1566-7367/© 2015 Elsevier B.V. All rights reserved.

X. Gao et al. / Catalysis Communications 62 (2015) 48–51
49
2. Experimental
2.1. Catalyst preparation
GO was prepared by a modification to the Hummer's method de-
scribed elsewhere [22]. Briefly, graphite (5 g) and NaNO3 (2.5 g) were
introduced into 115 mL H2SO4 in an ice bath under vigorous stirring.
The reaction system was heated to 35 °C and stirred for an additional
30 min after adding 15 g KMnO4. This system was diluted by adding
230 mL deionized water, heated to 98 °C, and followed by adding
700 mL water. The mixture was filtered, washed and dried at 50 °C
after introducing 50 mL H2O2 (30 wt.%). The dispersed graphite oxide
in water was kept in sonication for 60 min, centrifugated and dried at
ambient temperature.
The preparation methods of other catalysts (ZSM-48, ZSM-5,
H-mordenite, WO3/ZrO2, MoO3/ZrO2, HPW/ZrO2, and Cs2.5PW) were
depicted in supporting information.
cps/eV
2.2. Catalyst characterization
16
14
12
10
8
Scanning Electron Microscopy (SEM) was conducted on a JSM 7001-
F microscope. EDX spectra were carried out using 20 kV primary elec-
tron voltages. The IR spectra were measured by Vertex 70 (Bruker)
FT-IR spectrophotometer in the range of 400–4000 cm−1 with a resolu-
tion of 4 cm−1. The samples were mixed with KBr and pressed into
translucent disks at room temperature. The S content was measured
by elemental microanalysis (EA) (vario MICRO cube, Elemental).
S
C
O
S
6
2.3. Catalytic reaction
4
The catalytic tests for glycerol esterification with acetic acid were
carried out in a stainless steel autoclave with a magnetic stirring. Typi-
cally, 2.0 g glycerol and 13.0 g acetic acid with 0.1 g GO were added
into the reactor and then the mixture was heated to the desired temper-
ature for a specified time. Reaction conditions were changed to in-
vestigate the dependence of main variables such as time, reaction
temperature, catalyst loading and molar ratio of acetic acid/glycerol.
When the reaction was terminated, the reactor was cooled in an ice-
water bath. After the mixture was separated by centrifugation, the ob-
tained products were diluted with ethanol and analyzed by GC with a
FID using a DB-1 capillary column. Corrected area normalization meth-
od was used to quantify the products. The assignments of these prod-
ucts were also determined by GC–MS. The conversion of glycerol and
selectivity of products were determined by using the following equa-
tions.
2
0
1
2
3
4
5
6
7
8
9
10
keV
Fig. 1. Typical SEM–EDS image of GO.
0.378 mmol/g SO3H groups in GO. As displayed in Fig. 2, an intense
and broad peak located at ca. 3400 cm−1, corresponding to the stretching
mode of OH bond. Additional hydroxyl peaks were located at 1390 cm−1
and 1085 cm−1. The strong bands centered at 1640 cm−1 and 1730 cm−1
(νC_O) were ascribed to carboxylic acid and carbonyl groups. Addition-
ally, the bands at 1260 cm−1 and 800 cm−1 reflected the appearance of
C–O–C (epoxy) groups [22].
molesof glycerolðinÞ−molesof glycerolðoutÞ
Conversationð%Þ ¼
molesof allglycerolðinÞ
ꢀ 100
molesof oneproduct
molesof allproduct
Selectivityð%Þ ¼
ꢀ 100
3. Results and discussion
3.1. Catalyst characterization
The synthesized GO was characterized by SEM–EDX and FTIR. As
indicated by SEM in Fig. 1, GO presented a crumpled and layered
structure. EDX spectroscopy showed that GO primarily contained carbon
and oxygen, as well as small amounts of sulfur. The large amount of oxy-
gen was an indication of rich oxygen-bearing functionalities. The remain-
ing sulfur confirmed the presence of SO3H group, which can be further
corroborated by FTIR (at around 1160 cm−1) [26]. The EA results also
showed that the S amount was as high as 1.21 wt.%, corresponding to
Fig. 2. FTIR spectra of fresh and spent GO.

50
X. Gao et al. / Catalysis Communications 62 (2015) 48–51
Table 1
Catalytic performance of glycerol esterification with acetic acid over different catalysts.a
Catalyst
Conversion (%)
Selectivity (%)
MAG
DAG
TAG
Blank
GO
ZSM-48
ZSM-5
H-mordenite
WO3/ZrO2
MoO3/ZrO2
HPW/ZrO2
Cs2.5PW
48.9
98.5
68.0
76.3
75.0
74.2
72.7
73.2
73.0
31.8
15.5
35.7
30.8
35.3
36.9
33.2
38.7
34.9
63.3
60.0
58.0
62.2
58.3
56.7
59.4
55.4
59.2
4.9
24.5
6.3
7.0
6.4
6.4
7.4
5.9
5.9
a
Reaction conditions: reaction temperature = 120 °C, molar ratio of glycerol with acetic
acid = 1:10, catalyst amount = 0.1 g, reaction time = 1 h.
3.2. Catalytic esterification of glycerol
Fig. 4. Recycling results of glycerol esterification with acetic acid over GO catalysts. Reac-
tion conditions: reaction temperature = 120 °C, molar ratio of glycerol with acetic acid =
1:10, catalyst amount = 0.1 g, reaction time = 6 h.
Table 1 displays the results from the reaction of glycerol esteri-
fication with acetic acid over GO and a series of representative solid
acid catalysts. Even without the catalyst, 48.9% glycerol was converted
to MAG, DAG and TAG, implying that glycerol esterification is a self-
catalysis reaction wherein the acidic protons from acetic acid can cata-
lyze this reaction itself. Compared to the blank experiment, ZSM-48
only presented minor promotion for this reaction due to lower acidity.
Medium activities were obtained for strong acidic zeolites ZSM-5 and
H-mordenite. Other catalysts, such as WO3/ZrO2, MoO3/ZrO2, HPW/
ZrO2 and Cs2.5PW, showed ca. 70% conversion with low TAG selectivity.
Surprisingly, excellent reaction performance was achieved for GO, 98.5%
conversion and 24.5% TAG selectivity. The combined results clearly
demonstrate the high efficiency of GO in glycerol esterification. GO con-
tains 0.378 mmol/g SO3H groups, which provides an abundance of
acidic sites. As glycerol esterification is a typical acid-catalyzed reaction,
the superior performance of GO should be mainly attributed to the large
amounts of strong acidic SO3H groups. Additionally, the unique layered
structure facilitates to adsorb reactants and promote the reaction.
3.3. Influence of reaction conditions
Reaction temperature has significant impact on the catalytic activity
and product distribution. As shown in Fig. 3A, glycerol conversion
improved remarkably with the increasing reaction temperature and
Fig. 3. Effect of reaction temperature (A), molar ratio of acetic acid to glycerol (B), catalyst loading (C) and reaction time (D) on the catalytic performance of glycerol esterification with
acetic acid over GO catalysts. Reaction conditions: reaction temperature = 120 °C, molar ratio of glycerol with acetic acid = 1:10, catalyst amount = 0.1 g, reaction time = 6 h (except
the variables).

X. Gao et al. / Catalysis Communications 62 (2015) 48–51
51
reached 100% at 100 °C over GO catalyst. Simultaneously, the selectivity
for TAG increased drastically with the increasing temperature at the ex-
pense of MAG and DAG, indicating that the improvement of tempera-
ture is instrumental in promoting the further esterification of MAG
and DAG to produce TAG. Because glycerol consecutive esterification
is a highly endothermic reaction, higher temperature facilitates to im-
prove the extent of esterification [4].
towards glycerol esterification to valuable bioadditives. The excellent
performance of GO, complete glycerol conversion with 90.2% combined
selectivity to DAG and TAG, was mainly attributed to the rich SO3H groups
and unique lamellar structure. Moreover, GO presented consistent con-
version in consecutive catalytic tests. It can be envisioned that such a
cheap and robust solid acid catalyst holds great potential for a wide
range of acid-catalyzed reactions.
Comparing the effect of temperature, molar ratio of acetic acid to
glycerol had minor effect on glycerol conversion (Fig. 3B). GO presented
good conversion, 91.0% at low molar ratio of 4:1. Nevertheless, the se-
lectivity to TAG increased obviously with the increase in molar ratio of
acetic acid to glycerol. The increase in acetic acid concentration provides
more esterification agent which undergoes the formation of TAG
through further esterification from MAG and TAG.
Acknowledgments
The authors gratefully acknowledge the financial support of the
National Natural Science Foundation of China (no. 21403269).
As illustrated in Fig. 3C, complete conversion of glycerol was achieved
when using only 0.025 g GO, implying that GO is extremely active for
glycerol esterification. Regarding the selectivity, the enhancement of
catalyst loading slightly increased the selectivity to desired TAG.
The reaction time dependence of glycerol esterification over GO is
illustrated in Fig. 3D. Glycerol conversion of 93.8% was obtained within
just 30 min and reached 100% after 2 h. As the reaction proceeded, the
selectivity to MAG and DAG declined while TAG selectivity increased
correspondingly. These reactivity trends implicitly indicate the consec-
utive feature of glycerol esterification. The interaction of acetic acid
with proton sites from GO results in the protonation of carbonyl
group, which is attacked by glycerol and generated an acylium ion inter-
mediate [6]. This intermediate tends to approach other acetic acid mol-
ecule and further proceeds consecutive nucleophilic attack on acetic
acid. After reacting for 6 h, the combined selectivity of desired DAG
and TAG was up to 90.2% over GO, which is among the best results
(Table S1). Compared to the published reports, the combined yield to
DAG and TAG is higher than that of PrSO3H–SBA15 [9], sulfated zirconia
[10], HPW/AC [13], Y/SBA-3 [20], and MoOx/TiO2–ZrO2 [21].
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.01.007.
References
[1] C.H.C. Zhou, J.N. Beltramini, Y.X. Fan, G.Q.M. Lu, Chem. Soc. Rev. 37 (2008) 527–549.
[2] S. Zhu, X. Gao, Y. Zhu, Y. Zhu, H. Zheng, Y. Li, J. Catal. 303 (2013) 70–79.
[3] J.A. Sullivan, S. Burnham, Catal. Commun. 56 (2014) 72–75.
[4] M.S. Khayoon, B.H. Hameed, Bioresour. Technol. 102 (2011) 9229–9235.
[5] S. Zhu, Y. Zhu, X. Gao, T. Mo, Y. Zhu, Y. Li, Bioresour. Technol. 130 (2013) 45–51.
[6] S. Zhu, X. Gao, F. Dong, Y. Zhu, H. Zheng, Y. Li, J. Catal. 306 (2013) 155–163.
[7] M. Ayoub, M.S. Khayoon, A.Z. Abdullah, Bioresour. Technol. 112 (2012) 308–312.
[8] N. Rahmat, A.Z. Abdullah, A.R. Mohamed, Renew. Sustain. Energy Rev. 14 (2010)
987–1000.
[9] I. Kim, J. Kim, D. Lee, Appl. Catal. B Environ. 148–149 (2014) 295–303.
[10] I. Dosuna-Rodríguez, C. Adriany, E.M. Gaigneaux, Catal. Today 167 (2011) 56–63.
[11] P.S. Reddy, P. Sudarsanam, G. Raju, B.M. Reddy, J. Ind. Eng. Chem. 18 (2012)
648–654.
[12] A. Patel, S. Singh, Fuel 118 (2014) 358–364.
[13] P. Ferreira, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Catal. Commun. 12
(2011) 573–576.
3.4. Reusability test
[14] C.E. Gonçalves, L.O. Laier, A.L. Cardoso, M.J. da Silva, Fuel Process. Technol. 102
(2012) 46–52.
[15] L. Zhou, T.-H. Nguyen, A.A. Adesina, Fuel Process. Technol. 104 (2012) 310–318.
[16] L. Zhou, E. Al-Zaini, A.A. Adesina, Fuel 103 (2013) 617–625.
[17] L.N. Silva, V.L.C. Goncalves, C.J.A. Mota, Catal. Commun. 11 (2010) 1036–1039.
[18] V.L.C. Goncalves, B.P. Pinto, J.C. Silva, C.J.A. Mota, Catal. Today 133 (2008) 673–677.
[19] C. Gonçalves, L. Laier, M. Silva, Catal. Lett. 141 (2011) 1111–1117.
[20] M.S. Khayoon, S. Triwahyono, B.H. Hameed, A.A. Jalil, Chem. Eng. J. 243 (2014)
473–484.
[21] P.S. Reddy, P. Sudarsanam, G. Raju, B.M. Reddy, Catal. Commun. 11 (2010)
1224–1228.
[22] S. Zhu, C. Chen, Y. Xue, J. Wu, J. Wang, W. Fan, ChemCatChem 6 (2014) 3080–3083.
[23] D.R. Dreyer, H.-P. Jia, C.W. Bielawski, Angew. Chem. Int. Ed. 122 (2010) 6965–6968.
[24] S. Verma, H.P. Mungse, N. Kumar, S. Choudhary, S.L. Jain, B. Sain, O.P. Khatri, Chem.
Commun. 47 (2011) 12673–12675.
As shown in Fig. 4, the recovered GO was reused four times without
a decline in catalytic activity. Moreover, the product distribution did not
show appreciable change during the complete test. The spent catalyst
was characterized by FTIR; the results were compared with the fresh
one in Fig. 2. As can be seen, the spent catalyst displayed similar struc-
tures with fresh GO, revealing that the oxygen-containing groups
were mostly preserved during the reaction.
4. Conclusions
[25] A. Vijay Kumar, K. Rama Rao, Tetrahedron Lett. 52 (2011) 5188–5191.
[26] J. Wang, W. Xu, J. Ren, X. Liu, G. Lu, Y. Wang, Green Chem. 13 (2011) 2678–2681.
Compared to conventional solid acid catalyst, GO prepared by
Hummer's method has demonstrated to be a highly active acid catalyst