Graphene oxide: An efficient acid catalyst for alcoholysis and esterification reactions

Article abstract of DOI:10.1002/cctc.201402574

Evidence is presented for graphene oxide (GO), prepared by modified Hummers method, as a highly active, selective and reusable solid-acid catalyst for the production of alkyl levulinates via alcoholysis or esterification. 95.5% yield of ethyl levulinate was achieved by GO in furfuryl alcohol alcoholysis. Moreover, the surface SO₃H groups were identified as the primary active sites, while the surface carboxyl groups worked synergistically to adsorb furfuryl alcohol.

Full text of DOI:10.1002/cctc.201402574

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DOI: 10.1002/cctc.201402574
Graphene Oxide: An Efficient Acid Catalyst for Alcoholysis
and Esterification Reactions
Shanhui Zhu,*[a] Chengmeng Chen,[a, b] Yanfeng Xue,[a, c] Jianbing Wu,[a, c] Jianguo Wang,[a]
and Weibin Fan*[a]
Evidence is presented for graphene oxide (GO), prepared by
modified Hummers method, as a highly active, selective and
reusable solid-acid catalyst for the production of alkyl levuli-
nates via alcoholysis or esterification. 95.5% yield of ethyl levu-
linate was achieved by GO in furfuryl alcohol alcoholysis. More-
over, the surface SO3H groups were identified as the primary
active sites, while the surface carboxyl groups worked synergis-
tically to adsorb furfuryl alcohol.
ed into alkyl levulinates via alcoholysis or esterification, cata-
lyzed by an acid catalyst.
Alkyl levulinates are commonly used as flavoring agents, fuel
additives, solvents, and important precursors for producing g-
valerolactone.[3] A great number of catalysts including mineral
acid, zeolites, ion-exchanged resins, commercial sulfonic resin,
organic-inorganic hybrid solid acid, solid superacid, and SO3H-
functionalized ionic liquids have been explored for the synthe-
sis of alkyl levulinates in recent works.[4] Despite the relative ef-
ficiency of these catalysts in FAL alcoholysis and LA esterifica-
tion, most have low acid site densities, are expensive, involve
tedious preparation protocols, and suffer from rapid loss of
catalytic activity. Consequently, it is desirable to develop
a highly active, inexpensive, and reusable heterogeneous acid
catalyst.
With declining fossil resources, significant effort has been de-
voted to convert renewable biomass into fuels and valuable
chemicals.[1] Hemicellulose and cellulose can be converted into
abundant chemicals and biofuels, such as alkyl levulinates via
furanic platform chemicals furfural and 5-hydroxymethylfurfural
(HMF) (Scheme 1). Furfural can be hydrogenated to form fur-
furyl alcohol (FAL) over copper-based catalysts, and has been
established as an industrial process for decades.[2] HMF is an in-
termediate in the production of levulinic acid (LA) from cellu-
lose degradation. Both FAL and LA can be effectively convert-
Graphene and graphene oxide (GO) have gained tremen-
dous attention and research interest, owing to their unique
physical, chemical, and electrical properties, which find a wide
range of applications, such as electronic and photonic devices,
energy-storage materials, sensors, and catalysis.[5] Graphene
and GO are commonly used as
supports to immobilize active
species in catalytic reactions,
owing to their high surface area
and abundant deficient sites.[6]
Upon
exhaustive
oxidation
during preparation, GO contains
a variety of oxygen-containing
functionalities (e.g., alcohols, ep-
oxides, carboxylates, sulfate
groups), which provide its
medium acidic and oxidizing
properties. Dreyer et al.[7] pio-
neered the use of GO as carbo-
Scheme 1. The pathway for the production of alkyl levulinates via hemicellulose and cellulose conversion.
catalyst to catalyze the oxidation of various alcohols and hy-
dration of alkynes. Subsequently, GO has been employed as
viable solid-acid catalyst for Friedel–Crafts reaction, Aza-Mi-
chael addition, condensation, and hydrolysis.[8] Owing to the
non-stoichiometric and inhomogeneous nature of GO, the in-
trinsic active sites are still subject to debate.
[a] Dr. S. Zhu, Dr. C. Chen, Y. Xue, J. Wu, Prof. J. Wang, Prof. W. Fan
State Key Laboratory of Coal Conversion
Institute of Coal Chemistry, Chinese Academy of Sciences
Taiyuan 030001 (P.R. China)
E-mail: zhushanhui@sxicc.ac.cn
fanwb@sxicc.ac.cn
[b] Dr. C. Chen
Herein, for the first time, the inherent reactivity of GO was
explored in FAL alcoholysis and LA esterification, which has
presented fascinating catalytic performance. The enrichment of
-SO3H groups was identified as the primary active sites, while
surface carboxyl functionality worked synergistically to form
hydrogen bonding sites, thus facilitating acid-catalyzed reac-
tions.
Key Laboratory of Carbon Materials
Institute of Coal Chemistry, Chinese Academy of Sciences
Taiyuan 030001 (P.R. China)
[c] Y. Xue, J. Wu
University of Chinese Academy of Sciences
Beijing 100039 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402574.
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Table 1. Catalytic performance of FAL alcoholysis to EL over different
catalysts.[a]
Entry
Catalyst
Amount
[g]
Yield [%]
EMF
EL
3.1
TEP
1[b]
2[c]
3
4
5
6
7
8
9
10
11[d]
12
13
14
15
16
17
18
19[e]
GO
GO
GO
GO
ZSM-5
ZSM-48
Hb
WO3/ZrO2
MoO3/ZrO2
HPW/ZrO2
GO
0.02
0.05
0.05
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0
0.1
0.1
0.1
0.1
0.01
0.1
0.1
22.2
32.0
23.1
0.3
8.5
54.9
8.4
73.7
59.2
40.0
0.7
1.0
0.6
7.4
0.9
1.5
1.7
0.6
38.8
12.3
12.8
6.1
0.1
2.3
10.4
3.8
5.9
15.7
1.8
0.2
0.2
0.1
0.8
0.1
0.1
0.1
0.5
11.7
47.9
64.7
95.5
85.8
19.0
82.6
16.4
12.8
48.0
93.8
0.1
blank
AC
0.1
89.6
0.3
SO3H/AC
acetic acid
benzoic acid
H2SO4
p-TSA
GO-HT
0.4
92.5
94.6
39.7
Figure 1. A) SEM image (scale bar=100 nm), B) EDS, C) FTIR, and D) C1s XPS
spectrum of GO.
[a] Conditions: FAL 0.4 g, ethanol 9.6 g, 1208C, 6 h. [b] 808C. [c] 1008C.
[d] Reusability test for the fourth time. [e] GO-HT was obtained by hydro-
thermal treating GO for 12 h at 1508C and S content was 0.39%.
GO was synthesized by a modified Hummers method,[9] in
which graphite was exhaustively oxidized under harsh oxidiz-
ing conditions followed by exfoliation of graphite oxide. Pre-
sented in Figure 1A is a SEM of the crumpled and layered
structure of GO. The interlayer spacing was calculated to be
0.74 nm, as calculated from the (002) peak centered at 11.68 in
the XRD pattern (Figure S1). Analysis of the EDS spectra (Fig-
ure 1B) revealed that GO contained 88.35% carbon, 10.25%
oxygen and 1.40% remaining sulfur. Analysis of the FTIR (Fig-
ure 1C, Figure S2) and XPS (Figure 1D) confirmed that this GO
possessed numerous oxygen-containing groups including
SO3H, COOH, hydroxyl, and epoxy species.
In a preliminary experiment of FAL alcoholysis with ethanol
(Table 1, entry 1). 39.7% FAL was converted to ethyl levulinate
(EL), ethoxymethylfuran (EMF), 4,5,5-triethoxypentan-2-one
(TEP) over GO at 808C, indicating that GO can catalyze this re-
action smoothly. When the temperature and catalyst loading
were increased to 1008C and 0.05 g respectively, FAL was com-
pletely converted with a sharp increase in the EL yield to
47.9% (entry 2). A further increase in the EL yield to 64.7% was
achieved by increasing the temperature to 1208C (entry 3).
When the amount of GO increased to 0.1 g, GO achieved the
superior performance, up to 95.5% yield of EL, which was
among the best reported results for FAL alcoholysis (entry 4).[4]
To explore the kinetic behavior of FAL alcoholysis, the prod-
uct yield as a function of reaction time over GO is displayed in
Figure 2. The FAL converted quickly in the initial stage and was
consumed completely after only 30 min. The EL yield monot-
onically increased to 95.5% with the reaction time, while those
of EMF and TEP exhibited a volcano trend. This implied that
FAL alcoholysis to EL occurred via two consecutive reaction
pathways with EMF and TEP as intermediates, and the reaction
routes are illustrated in Scheme S1, consistent with previous
Figure 2. Reaction kinetics of FAL alcoholysis over GO.
reports.[4e,h] Notably, the rapid consumption of FAL to EMF and
TEP, but further slow transformation revealed that the conver-
sion of EMF and TEP to EL should be the rate-determining
step.
For comparison, the catalytic performance of a series of typi-
cal solid-acid catalysts, including ZSM-5, ZSM-48, Hb, WO3/ZrO2,
MoO3/ZrO2, and H3PW12O40/ZrO2 (HPW/ZrO2), were performed
under the same conditions as those employed for GO (en-
tries 5–10). Surprisingly, all of these catalysts exhibited lower
EL yield than that of GO, particularly for ZSM-48, WO3/ZrO2,
and MoO3/ZrO2, likely due to their lower acidity. Although
HPW/ZrO2, as a strong acid catalyst, presented excellent per-
formance in dehydration and esterification,[10] to give moderate
EL yield. Moreover, a leaching test was performed to exclude
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the contribution of leached S and O species (see the Support-
ing Information). The activity of GO did not decline obviously
upon four consecutive reaction cycles (entry 11). The above re-
sults unequivocally demonstrated that GO was an outstanding
acidic catalyst in terms of reactivity and reusability.
Table 2. Alcoholysis of FAL with various alcohols to alkyl levulinates over
GO catalyst.[a]
Entry
Alcohol
Product
Yield [%]
1
2
3
4
5
6
7
8
9
methanol
ethanol
methyl levulinate
ethyl levulinate
95.4
95.5
92.3
91.7
91.4
90.0
78.2
87.9
87.0
It is interesting to know why GO is so highly active for FAL
alcoholysis. To explore the intrinsic active sites of GO, control
experiments were exhibited over catalysts containing SO3H or
carboxyl groups. For completeness, a blank experiment was
run (Table 1, entry 12). Activated carbon (AC) was found to be
unable to catalyze FAL alcoholysis at all (Table 1, entry 13).
After grafting SO3H into AC, 89.6% yield of EL was achieved,
which was attributed to the increase in strong acidity
(entry 14). Moreover, both H2SO4 and p-toluenesulfonic acid (p-
TSA) presented excellent EL yield (entries 17–18). When remov-
ing approximately 70% of the SO3H groups by hydrothermal
treatment, GO gave only 39.7% EL yield (entry 19). The above
results unambiguously revealed that SO3H groups were the in-
trinsic active sites. Conversely, the catalysts containing only
carboxyl groups did not exhibit obvious reactivity towards FAL
alcoholysis, such as acetic acid or benzoic acid (entries 15–16),
demonstrating that weakly acidic carboxyl groups cannot ef-
fectively catalyze this reaction.
1-propanol
2-propanol
1-butanol
iso-butanol
tert-butanol
1-pentanol
1-hexanol
1-propyl levulinate
2-propyl levulinate
1-butyl levulinate
iso-butyl levulinate
tert-butyl levulinate
1-pentyl levulinate
1-hexyl levulinate
[a] Conditions: FAL 0.4 g, alcohol 9.6 g, 1208C, 6 h.
Table 2, the yield of alkyl levulinate yield declined gradually
with increasing alcohol chain length. Moreover, branched alco-
hols, such as tert-butanol, also gave low alkyl levulinate yield.
This can be explained by the steric hindrance effect of larger
alkyl chains of alcohols in FAL alcoholysis.[4h] As mentioned
above, alkyl levulinates can also be produced by esterification
of LA with alcohols. Table S4 shows that GO also presented ex-
cellent yield of alkyl levulinates in LA esterification with various
alcohols. The above results demonstrate that GO is a highly
active acid catalyst in alcoholysis and esterification.
However, SO3H/AC, H2SO4, and p-TSA possessed much more
SO3H groups than GO (Table S1), but displayed lower EL yield,
suggesting that other functional groups of GO may play an im-
portant part in promoting the conversion of FAL. Adsorption
experiments were performed to probe the affinity of GO and
SO3H/AC to the substrate and product.[11] As compiled in
Table S2, the adsorption capacity of FAL in GO was two times
higher than SO3H/AC, reflecting a higher affinity of GO towards
FAL. Additionally, both GO and SO3H/AC presented lower ad-
sorption ability toward EL, which can explain the high yield of
EL in FAL alcoholysis. Compared to SO3H/AC, the rich oxygen-
containing groups of GO can be capable of adsorbing hydroxyl
group of FAL, forming strong hydrogen bond and facilitating
to proceed the reaction.
In conclusion, GO prepared by Hummers method has been
shown to be a highly active, selective, and reusable acid cata-
lyst for the production of alkyl levulinates by alcoholysis and
esterification. The control experiments and characterization re-
sults revealed that SO3H groups were the dominant active
sites, while carboxyl groups facilitated the adsorption of FAL. It
can be reasonably envisaged that such an environmental
friendly and cheap carbocatalyst will hold great potential for
a wide range of acid-catalyzed reactions.
Experimental Section
Detailed procedures for the catalyst preparation, characterization,
and catalytic tests are provided in the Supporting Information. GO
was prepared by modified Hummers oxidation of graphite and
subsequent exfoliation. All the reactions were performed in
a Teflon-lined stainless steel autoclave with magnetic stirring. The
reaction products were identified by GC-MS and analyzed by GC.
It is well-established that heat treatment of GO can eliminate
partial oxygen-containing groups.[8d] Indeed, some hydroxyl
groups of GO were removed, while carboxyl and SO3H groups
were kept intact upon heat treatment at 100 and 1508C, as in-
dicated by XRD and FTIR in Figure S3 and S4. Consequent reac-
tivity tests revealed that the reactivity did not decline
(Table S3), suggesting that hydroxyl groups should not be the
active sites for FAL alcoholysis. When the heat treatment tem-
perature was increased to 2008C, large amounts of carboxyl
groups and partial SO3H groups were removed, leading to
a significant decrease in EL yield, 35.7%. Clearly, it was reason-
able to infer that the superior performance of GO could be as-
cribed to the synergistic effect of SO3H and carboxyl groups in
FAL alcoholysis. Specifically, SO3H groups were the primary
active sites, which acted as strong acid sites and favored the
protonation of FAL. Simultaneously, carboxyl groups facilitated
the adsorption of FAL and synergistically worked with SO3H
groups.
Acknowledgements
This work was financially supported by the Major State Basic Re-
search Development Program of China (973 Program)
(2011CB201403), the National Natural Science Foundation of
China (21403269, 21273264), and the Foundation of State Key
Laboratory of Coal Conversion (J13-14-606).
Keywords: alcoholysis · biofuels · esterification · graphene
oxide · heterogeneous catalysis
GO is highly active and selective for FAL alcoholysis with var-
ious alcohols, an indication of its great versatility. As shown in
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