Towards a bio-based industry: Benign catalytic esterifications of succinic acid in the presence of water

Article abstract of DOI:10.1002/chem.200700037

The biorefinery of the future will need to integrate bioconversion and appropriate low environmental impact chemical technologies (Green Chemistry) so as to produce a wide range of products from biomass in an economically effective and environmentally acceptable manner. The challenge for chemists is to develop chemistry that works with fermentation-derived dilute, aqueous mixtures of oxygenated chemicals (platform molecules) rather than the petroleum-derived non-aqueous, non-oxygenated feedstocks we have been working with for 50+ years and to avoid energy intensive and wasteful concentration and purification steps. Here we show that a new family of tuneable mesoporous carbonaceous catalysts derived from starch can be used to accomplish efficient chemistry in aqueous solution. Our new aqueous catalytic chemistry relies on the ability to adjust the surface properties including the hydrophobicity-hydrophilicity balance of mesoporous Starbons by carbonisation at different temperatures (250-750°C). Simple treatment of these materials with sulfuric acid then provides a series of porous solid acids that can function under a range of conditions including dilute aqueous solution. The reactions of succinic acid (platform molecule) in aqueous alcohol demonstrate the outstanding activities of these new catalysts.

Full text of DOI:10.1002/chem.200700037

DOI: 10.1002/chem.200700037
Towards a Bio-Based Industry: Benign Catalytic Esterifications of Succinic
Acid in the Presence of Water
Vitaly Budarin, Rafael Luque, Duncan J. Macquarrie, and James H. Clark*[a]
Abstract: The biorefinery of the future
will need to integrate bioconversion
and appropriate low environmental
impact chemical technologies (Green
Chemistry) so as to produce a wide
range of products from biomass in an
economically effective and environ-
mentally acceptable manner. The chal-
lenge for chemists is to develop
chemistry that works with fermenta-
tion-derived dilute, aqueous mixtures
of oxygenated chemicals (platform
molecules) rather than the petroleum-
derived non-aqueous, non-oxygenated
feedstocks we have been working with
for 50+ years and to avoid energy in-
tensive and wasteful concentration and
purification steps. Here we show that a
new family of tuneable mesoporous
carbonaceous catalysts derived from
starch can be used to accomplish effi-
cient chemistry in aqueous solution.
Our new aqueous catalytic chemistry
relies on the ability to adjust the sur-
face properties including the hydropho-
bicity–hydrophilicity balance of meso-
porous Starbons by carbonisation at
different temperatures (250–7508C).
Simple treatment of these materials
with sulfuric acid then provides a series
of porous solid acids that can function
under a range of conditions including
dilute aqueous solution. The reactions
of succinic acid (platform molecule) in
aqueous alcohol demonstrate the out-
standing activities of these new cata-
lysts.
Keywords: biorefinery · esterifica-
tion
· heterogeneous catalysis ·
mesoporous materials
Introduction
Here we report the application of tuneable carbonaceous
materials prepared from carbonisation of expanded starch
(Starbon),[6] after sulphonation, as acid catalysts for chemis-
try in dilute aqueous environments. Changes in preparation
temperature (250–7508C) provide the possibility to produce
a wide range of carbonaceous materials with an unique con-
tinuous trend in surface chemistries from more starch-like
(hydrophilic) to more graphitic-like (hydrophobic) together
with a highly stable mesoporous region. The feasibility and
simplicity of achieving an optimum catalytic activity by
tuning the hydrophilicity/hydrophobicity surface properties
of these mesoporous carbons provides us with outstanding
potential for optimizing reactions such as the esterification
of organic acids in water. In this study, we have tested the
activity of our new solid acids in the esterification of succin-
ic acid in aqueous ethanol (Scheme 1).
Succinic acid is one of the top (bio)platform molecules
and is available from the bioconversion of glucose at con-
centrations as high as approximately 6 wt%.[7] Esterification
reactions are one of the most useful transformations for or-
ganic acids,[8] especially for a dicarboxylic acid as the diester
can be used as an intermediate in the manufacture of poly-
mers, fine chemicals, perfumes, plasticizers and solvents.[8–11]
Many acid catalysts have been reported in these reactions,[12]
although only a few authors have dealt with the esterifica-
The biorefinery concept is based on the selective conversion
of biomass to value-added products. Biomass is a fully re-
newable resource that will increasingly be used for the pro-
duction of heat and power, chemicals, materials and other
products with substantial environmental, economic and se-
curity benefits.[1,2] The biorefinery of the future is likely to
integrate both bioconversion and appropriate chemical tech-
nologies. It is, however, essential that for truly sustainable
production, the technologies that are used have a low envi-
ronmental impact. In the case of chemical technologies and
chemical production, this means the use of green chemistry
methods, such as heterogeneous catalysis,[3] and the applica-
tion of green chemistry principles.[4,5] The main problem is
that chemical catalytic technologies have been developed
for the transformation of hydrophobic molecules in hydro-
phobic environments whereas fermentation processes pro-
duce hydrophilic platform molecules in aqueous media.
[a] Dr. V. Budarin, Dr. R. Luque, Dr. D. J. Macquarrie, Prof. J. H. Clark
Chemistry Department, Green Chemistry Centre of Excellence
The University of York, Heslington, York YO10 5DD (UK)
Fax : (+44)190-443-2705
E-mail: jhc1@york.ac.uk
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FULL PAPER
a similar trend to those origi-
nally reported for Starbon ma-
terials,[6] both surface area and
microporous volume increasing
with increasing temperature of
preparation (Figure 1). SEM
micrographs show that the Star-
bon materials have a homoge-
neous spherical morphology
with macro-mesoporosity in the
materials that can be seen at
higher magnifications (Fig-
Scheme 1. Esterification of succinic acid in aqueous ethanol.
tion of either succinic acid[13] or its cyclic anhydride.[14] The
esterification reaction has been reported as being very sensi-
tive to water environments due to different equilibrium
steps in which water is involved (Scheme 1).[15] Water has
been proved to affect the reaction activity, in homogeneous
conditions,[14b,16a] resulting in a decrease of the rate of
esterification that has generally been attributed not only to
reverse hydrolysis but also to a competitive protonation step
involving the water and the alcohol (assuming the rate-de-
termining step is the interaction between the protonated al-
cohol and the carboxylic acid).[16] We can expect to increase
the reaction rates in comparison with the homogeneous re-
actions by altering the local water concentration in the
active centre of the solid acids through careful adjustment
of the local surface properties.
Figure 1. Effect of the temperature of preparation on the surface area
(SBET, m2 gÀ1) and SO3H loading (mmolgÀ1) of Starbon acids.
Results and Discussion
Starbon acids surface proper-
ties: The sulfuric acid treatment
did not significantly change the
materials surface properties, as
shown by N2 adsorption meas-
urements, SEM and DRIFT ex-
periments (Figures 1, 2 and 3).
Moreover, the acidity of Star-
bon acids (measured as SO3H
loading) was almost independ-
ent of the temperature of prep-
aration of the parent materials,
although
a decrease in the
SO3H loading was found at
high temperatures. This is con-
sistent with the unchanged mes-
oporous environment. Elemen-
tal analysis of the solid acids
confirms the %S
A
the trend in reduction at higher
temperatures (see the Experi-
mental Section).
Textural properties of Star-
bon acids were found to follow Figure 2. SEM micrographs of A) Starbon-500 and B) Starbon-500-SO3H at different magnifications.
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J. H. Clark et al.
experiments were in good agreement with the textural and
acid properties of the materials. There were no significant
changes in the peaks due to the main functionalities (C=O,
C=C, etc.) present in Starbon materials but new peaks do
appear in the 1200–600 cmÀ1 region consistent with sulfonic
acid/sulfonate species. Kinetic experiments using Starbon
acids in the esterification of succinic acid in aqueous ethanol
demonstrate their high activity. Reactions typically reach
close to quantitative conversion of the acid to the diester in
five hours, as shown in Figure 4. A useful 35% conversion
to monoester (<2 h) was also obtained.
Figure 4. Kinetic profiles (concentration versus time) of Starbon-400-
SO3H in the esterification of succinic acid in aqueous ethanol.
For comparison, the activity of some other established
solid acid catalysts (zeolites, sulphated zirconias and clays)
in the esterification reaction were screened. In general, all
of the reactions using known solid acids were very slow,
taking 24+ hours at 808C to obtain reasonable conversions
of concentrations of approximately 6 wt% succinic acid
(Table 1). Zeolite-beta was the most promising catalyst for
diester formation, whereas sulfated zirconia and montmoril-
Table 1. Catalytic activity comparison of different materials in the
esterification of succinic acid in aqueous ethanol.
Cat.
kconv
Conditions for maximum product yield
[mol%]
A
]
Products
Yieldmax [mol%] Tmax [min]
H2SO4
90
1440
1380
1380
840
ZrO2-SO3H
Figure 3. DRIFT spectra of Starbon materials prepared at different tem-
peratures A) 4008C and B) 6508C before (continuous line) and after sul-
phonation (discontinuous line).
b-25
diester
70[a]
1440[a]
600
DARCO-SO3H
6.5”10À5 monoester 52
diester
monoester 35
diester 90
90
1150
100
410
Starbon-400-SO3H 32”10À5
ure 2A3 and B3). Nitrogen adsorption measurements con-
firmed that Starbon acids were mostly mesoporous with an
average pore diameter of approximately 8–12 nm. DRIFT
[a] Reaction not completed.
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Esterifications of Succinic Acid
FULL PAPER
lonite KSF (not shown) gave the highest observed selectivi-
ties to the monoester.
Commercial porous carbons, such as Darco, which were
sulphonated, had activities slightly higher than those of the
non-carbon solid acids we tested (Table 1). Activated car-
bons have been reported to adsorb large quantities of suc-
cinic acid at different temperatures[17] changing the initial
acid/water ratio in the active centres on the catalyst surface.
Figure 5 summarises the catalytic activity of the sulpho-
nated Starbons in the esterification reaction as a function of
the preparation temperature and the surface properties. At
4008C, we have shown that the Starbon has an unusual
blend of starch and carbon-like surface characteristics in-
cluding hydroxyl, aldehyde, and aromatic functions.[6] The
À
type of acid sites ( SO3H from reaction with aromatic or
À
C=C groups and OSO3H from reactions with hydroxyl
groups) will also vary with the preparation temperature for
the Starbon. Significantly, the activity of these materials
peaked at approximately 4008C with sharply reduced activi-
ties below or above this preparation temperature (Fig-
ure 5A). The rates of reaction for Starbon-400 acid were
found to be at least five times greater than any of the other
solid acid catalysts. The highly active sulphonated Starbon
catalyst was easily recovered from the aqueous reaction
mixture whereupon the reaction rates returned to the back-
ground values.
Figure 6. A) Catalytic activity (initial rate of succinic acid esterification)
comparison between mesoporous expanded Starbon and microporous
carbonaceous materials prepared from non-expanded starch. B) Starbon
acids recyclability (1st and 2nd use) in the succinic acid esterification re-
action.
The recovered Starbon acid could be added to fresh sub-
strate solutions giving almost identical behaviour to that ob-
served in the original reaction (Table 1, Figures 5B and 6B).
Thus the catalyst is stable as well as active under aqueous
conditions, which are most extraordinary characteristics.
The initial expansion process to generate the mesoporous
expanded starch that was then carbonised can be proven to
be critical to the catalysts high activity and reusability. We
prepared similar microporous non-expanded materials from
direct carbonisation at different temperatures of starch gran-
ules. The carbonaceous solids were subsequently functional-
ised with sulphonated groups in a similar way to that of the
Starbons. These non-expanded materials, essentially micro-
porous in nature, are reminiscent of the solid acids recently
reported for biodiesel production.[18] Sulphonated non-ex-
panded carbons exhibited a catalytic activity that was at
least five times lower than the optimized 4008C sulphonated
Starbons. Most interestingly, a maximum in activity was
found at approximately 5508C, which was shifted to 3508C
after the first use. Therefore, materials prepared at high
temperature (>5508C) with increased microporosity per-
formed better in the first use but the catalytic activity was
Figure 5. A) Catalytic activity (rate constant, sÀ1) of Starbon materials,
prepared at different temperatures, with different surface functionalities
in the esterification of succinic acid in aqueous ethanol. The grid on the
bottom of the figure represents distribution of functional groups on Star-
bons prepared at the different temperatures: gray scale to indicate rela-
tive amounts of different groups (black represents highest). B) Starbon-
400-SO3H reusability in the esterification reaction.
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J. H. Clark et al.
Catalytic test: A typical catalytic test was performed as follows: Succinic
acid (1 mmol , SA, 0.124 mg, 6 wt% in solution), EtOH (30 mmol,
2.4 mL) and water (50 mmol , 0.9 mL) were added to a round-bottomed
flask with sulphonated Starbon (0.1 g), increasing the temperature to
808C. Samples were withdrawn periodically from the reaction mixture
and the mixture was left reacting (5 h). Products were analysed by GC
analysis by using an Agilent 6890N GC model equipped with a 7683B
series autosampler. Response factors of the reaction products (succinic
anhydride, monoethyl succinate and diethyl succinate) were determined
with respect to succinic acid from GC analysis by using known com-
pounds in calibration mixtures of specified compositions.
dramatically reduced after reuse. This can be explained by
the involvement of physisorbed H2SO4 in the catalytic per-
formance (Figure 6).
Conclusion
The outstanding Starbon acid performance in aqueous
esterification is believed to be a consequence of an ideal
combination of properties including hydrophobicity encour-
aging partitioning of the organic acid into the pores and pro-
duction of the more hydrophobic esters, mesoporosity en-
hancing molecular diffusion rates, surface functionality al-
lowing the formation of (two different types of) acid sites
and surface energy providing strong site activity. We can
foresee our approach being successfully extended to many
of the “top” bioplatform molecules, together with other
chemistries including the production of other small mole-
cules (amides, imides, acylated and alkylated products) pro-
viding enormous potential applications in the Biorefinery
chemical production units of the future.
Acknowledgements
We thank the EPSRC for support and colleagues in the Green Chemistry
Centre, especially Mr. P. Elliott for technical support, Mr. R. White for
providing the expanded starch as starting material and Mr. P. Shuttle-
worth for his help and interesting discussions. We would also like to
thank Ms M. Stark for her help with the SEM measurements. We also
thank a referee for suggestions.
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Received: January 10, 2007
Published online: May 30, 2007
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