Revisiting the Bronsted acid catalysed hydrolysis kinetics of polymeric carbohydrates in ionic liquids by in situ ATR-FTIR spectroscopy

Article abstract of DOI:10.1039/c3gc41174e

A new versatile method to measure rates and determine activation energies for the Bronsted acid catalysed hydrolysis of cellulose and cellobiose (and other polymeric carbohydrates) in ionic liquids is demonstrated by following the C-O stretching band of the glycoside bond with in situ ATR-FTIR. An activation energy in excellent agreement with the literature was determined for cellulose hydrolysis, whereas a distinctly lower activation energy was determined for cellobiose hydrolysis. The methodology also allowed to independently determine activation energies for the formation of 5-hydroxymethylfurfural in the systems.

Full text of DOI:10.1039/c3gc41174e

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Revisiting the Brønsted acid catalysed hydrolysis
kinetics of polymeric carbohydrates in ionic liquids by
Cite this: Green Chem., 2013, 15, 2843
in situ ATR-FTIR spectroscopy†
Andreas J. Kunov-Kruse,*a Anders Riisager,*a Shunmugavel Saravanamurugan,a
Rolf W. Berg,b Steffen B. Kristensena and Rasmus Fehrmanna
A new versatile method to measure rates and determine activation energies for the Brønsted acid cata-
lysed hydrolysis of cellulose and cellobiose (and other polymeric carbohydrates) in ionic liquids is demon-
strated by following the C–O stretching band of the glycoside bond with in situ ATR-FTIR. An activation
energy in excellent agreement with the literature was determined for cellulose hydrolysis, whereas a dis-
tinctly lower activation energy was determined for cellobiose hydrolysis. The methodology also allowed
to independently determine activation energies for the formation of 5-hydroxymethylfurfural in the
systems.
Received 18th June 2013,
Accepted 22nd July 2013
DOI: 10.1039/c3gc41174e
www.rsc.org/greenchem
than if the crystalline fibres are just suspended in a hetero-
geneous mixture. Most studies concerned with the hydrolysis
Introduction
The demand for substituting the petroleum products used of cellulose in ionic liquids in the presence of homogeneous
today with biomass derived chemicals and fuels is continu- or heterogeneous acid catalysts have focused on the product
ously increasing.1,2 Extensive effort has in particular been put distribution after hydrolysis8–10 and paid less attention to the
into developing systems based on the furanic platform, which hydrolysis kinetics. However, an interesting effect of dissolving
are able to provide a diverse range of products.3–5 However, the cellulose in ionic liquids is that the mechanism of hydrolysis
initial production of 5-hydroxymethylfurfural (HMF) is in most seems to be altered as studies of the hydrolysis kinetics in
cases focused on applying sugars such as fructose and ionic liquids report lower activation energies than normally
glucose, which are derived from corn, starch sugar canes or observed in water.11–13 Generally, the apparent activation
other edible resources.6 These resources are far from un- energy observed for acid cellulose hydrolysis and its oligomers
limited, and in a longer term perspective such processes must in water is relatively high around 120–130 kJ mol−1 14
The high
.
apply lignocellulosic materials instead, which are available in activation energy is induced by an unfavourable oxocarbenium
surplus from various agricultural processes and waste streams. ion which has considerably higher energy than observed for
However, there still needs to be a breakthrough in the conver- oxocarbenium ions in general, because the pyranose ring
sion technologies of cellulose into sugars to make this structure prevents the preferred planar geometry around the
approach competitive to food crops.
CvO+ group (activation energies of simple acetals are only of
The acid catalysed hydrolysis of cellulose in ionic liquids the order of 50–60 kJ mol−1).15,16 In ionic liquids the strongly
has been attracting immense interest in the last decade since ionic environment may stabilise the positive charge of the oxo-
Rogers and co-workers7 discovered that some ionic liquids are carbenium ion and thus lower its energy.13 This is supported
able to dissolve cellulose. The dissolution of cellulose makes by the finding that the hydrolysis rate of the glucoside bond in
the glycoside bond more susceptible to attack by a catalyst water–1,4-dioxane mixtures is significantly enhanced in the
presence of chloride and bromide ions.17
Cellulose and short chain cellulose oligomers are insoluble
in conventional solvents. This makes it very challenging to esti-
aCentre for Catalysis and Sustainable Chemistry, Department of Chemistry, Building
207, DK-2800 Kgs. Lyngby, Technical University of Denmark, Denmark.
mate the initial reaction rate of hydrolysis by conventional
analytical methods. Furthermore, analytical techniques
dealing with the analysis of all the hydrolysis products, includ-
ing long chain oligomers, are very laborious.11 Analysis is
further complicated by the formation of secondary products,
as acid catalysts are active in glucose dehydration to, e.g., HMF
E-mail: ar@kemi.dtu.dk, ajoku@kemi.dtu.dk; Fax: +45 45883136;
Tel: +45 45252390, +45 45252233
bDepartment of Chemistry, Building 207, DK-2800 Kgs. Lyngby, Technical University
of Denmark, Denmark
†Electronic supplementary information (ESI) available: Experimental details,
FT-IR spectra and interpretation of data, and HPLC analytic results. See DOI:
10.1039/c3gc41174e
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and levulinic acid.13 Such reactions occur simultaneously with
hydrolysis and at considerable rates, making the produced
glucose an unsuitable measure of the hydrolysis rate. Since the
activation energies for cellulose hydrolysis and glucose dehy-
dration by Brønsted acids are similar,18,19 methods solely taking
the produced glucose into account, and not the products pro-
duced hereof, are thus likely to underestimate the actual hydro-
lysis rate and thus overestimate the activation energy.
In this work we introduce a new approach to measure the
hydrolysis kinetics of cellulose. Hence, instead of monitoring
the formation of hydrolysis products, the disappearance of gly-
coside bonds is followed during hydrolysis by in situ ATR-FTIR
in a specially designed stirred micro reactor.20
Results and discussion
Fig. 1 Corrected ATR-FTIR difference spectra recorded during the first 30 min
of sulfuric acid catalysed hydrolysis of Avicel cellulose in [BDMIm]Cl at 120 °C.
The symmetric and anti-symmetric glucoside C–O–C stretching bands are high-
lighted with boxes. The insert shows the 1800–1600 cm−1 range (full region
spectra can be found in ESI, Fig. S20–S22†).
Initially, we performed a thorough investigation of vibrational
modes in cellulose using DFT model calculations (B3LYP/
6-311+G(d,p)) (ESI, Fig. S10†) to identify quantifiable bands.
Among several new IR-active modes that are characteristic of
the glucose moieties in cellulose, we were able to locate the
position of the two group vibrations comprising C–O–C
stretching vibrations at around 1165–1155 and 965–975 cm−1
(anti-symmetric and symmetric modes, respectively; see ESI,
Table S2†) in very good agreement with interpretations found
in the literature.21–30 The modes are schematically shown in
Scheme 1.
When using the popular ionic liquids 1-butyl-3-methylimi-
dazolium chloride and 1-ethyl-3-methylimidazolium chloride
([BMIm]Cl and [EMIm]Cl) as solvents for the hydrolysis, it was
only possible to follow the time evolution of the very weak sym-
metric stretching mode at 965 cm−1. Moreover, the 1160 cm−1
band was here unsuited for accurate quantitative measure-
ments due to overlap with a very strong mode comprising the
bending of the C2–H proton on the [BMIM]+ cation. However,
this C–H bending mode disappeared when the hydrogen in
the C2 position was substituted with a methyl group, thus
allowing quantitative analysis via the intense band due to anti-
symmetric stretching of the glycoside bond (ESI, Fig. S15†).
The ionic liquid 1-butyl-2,3-dimethylimidazolium chloride
([BDMIm]Cl) was accordingly chosen for the kinetic
experiments.
Time-resolved difference spectra measured for the sulfuric
acid catalysed hydrolysis of Avicel cellulose in [BDMIm]Cl are
shown in Fig. 1. Notably, the spectra reflect the total change in
the composition during hydrolysis and show thus a mixture of
disappearing bands for both cellulose and glucose as the reac-
tion proceeds. Therefore, care must be taken when a quantitat-
ive interpretation is done because the pyranose backbone of
the cellulose chains show many similarities with glucose and
several of the types of vibrational bands are found in the
spectra of both structures (see ESI, Table S2 and Fig. S19†).
For the polymeric glucose species the introduction of acetal
groups with the glycoside bonding creates several modes invol-
ving deformation around the oxygen atom inside the pyranose.
Consequently, new strong bands from C–O–C stretching in the
pyranose ring and new C–C stretching and C–H bending
modes were found in the cellulose spectrum around
1070–1060 cm−1. The intensities of these bands decreased
markedly during the hydrolysis of cellulose, and indeed these
bands initially showed a linear decrease as a function of time
that possibly could be used to determine the initial rates (ESI,
Fig. S8†). However, glucose also had intense bands in the
same region. Therefore, these bands were unsuitable for quan-
titative analysis, as they expressed an unknown fraction of the
hydrolysis rate of both cellulose and glucose (an elaborate dis-
cussion follows later). The anti-symmetric stretching of the gly-
coside bond has a relatively strong absorption band at around
1155–1160 cm−1, which is found in cellulose and in cellulose
oligomers including the dimer cellobiose (ESI, Fig. S18†). The
spectra of glucose showed an intense band at around
1140 cm−1 probably due to C1–O1 bond stretching and
bending of C1–H and O1–H bonds, whereas cellulose and the
Scheme 1 Schematic representation of the two characteristic anti-symmetric
(a) and symmetric (b) vibrations of the glycoside bond.
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cellulose oligomers showed only a weak absorption band.
These bands can thus easily be distinguished and quantified
by deconvolution of the ATR-corrected difference spectra. No
general linear time dependence of the area of the 1140 cm−1
band was observed during the experiments. The difference
between these bands was further confirmed by experimental
spectra of cellulose oligomers, which all showed that the C–O–C
stretching band was only present in the polymeric species
(ESI, Fig. S18†).
Knowledge about the kinetics of cellulose hydrolysis in
ionic liquids is quite limited due to the earlier mentioned
analytical difficulties, although some activation energies of cel-
lulose hydrolysis in ionic liquids catalysed by strong mineral
acids or similar systems have been reported.12,13 Here, we firstly
investigated the hydrolysis using sulfuric acid as a catalyst to
allow direct comparison of the kinetics obtained by the in situ
FTIR method to kinetics derived from studies using more con-
ventional analytical methods. The cellulose hydrolysis was
investigated in situ in [BDMIm]Cl from 90 to 140 °C in steps of
Fig. 3 Corrected ATR-FTIR difference spectra recorded during the first 15 min
of sulfuric acid catalysed hydrolysis of cellobiose in [BDMIm]Cl at 120 °C. The
symmetric and anti-symmetric glucoside C–O–C stretching bands are high-
10 °C using 1.7 wt% sulfuric acid catalyst. The small volume of lighted with boxes. The insert shows the 1800–1600 cm−1 range (full region
spectra can be found in ESI, Fig. S23–S25†).
the stirred detachable micro reactor ensured fast heating and
equilibration of the cooled premixed reaction mixture.
In the IR spectra a slight increase of the intensity of the
1157 cm−1 band was initially observed at the set temperatures,
determined rate constants for the hydrolysis are plotted in an
Arrhenius plot as ln(−k) against 1/T. From the rate constants
(first order with respect to catalyst concentration) the acti-
vation energy for the hydrolysis of cellulose with sulfuric acid
was determined to be 96.4 4.1 kJ mol−1, which is in excellent
agreement with recent values of 92–96 kJ mol−1 reported for
analogous cellulose hydrolysis in [EMIm]Cl.12,13
probably due to incomplete dissolution of the cellulose (see
the development in ESI, Fig. S1†). However, within a short
time period a steady decrease of the band was measured for all
the temperatures allowing first the initial rates to be deter-
mined by integration of the 1157 cm−1 band of the deconvo-
luted spectra and, secondly, the initial rates (expressed as
absorbance per second) to be established with high accuracy
from the in situ experiments (ESI, Fig. S1 and Table S1†). A
plot of ln(abs) as a function of time confirmed that the reac-
tion was first order in concentration of glucoside bonds, as
Cellobiose is often used as a model for cellulose when mod-
elling cellulose hydrolysis, due to simplicity and the compara-
tive higher solubility of cellobiose in many solvents.12
Accordingly, we also investigated cellobiose hydrolysis under
the same conditions as described for cellulose above (Fig. 3
and ESI Fig. S2 and S3†). Notably, we found here the apparent acti-
expected (ESI, Fig. S3†). In Fig.
2 the experimentally
vation energy for cellobiose hydrolysis to be 69.6 3.0 kJ mol−1
,
which is significantly lower than for cellulose. In the earlier
work using [EMIm]Cl the activation energy for cellobiose
hydrolysis was not reported, but it was observed that the
hydrolysis rate of cellobiose was first order with respect to the
concentration of glucoside bonds and indeed significantly
faster than for cellulose.12,13 This directly supports our finding
of a lower activation energy, even though the nature of the
glucoside bonds of cellobiose and cellulose are rather similar,
as also indicated by our DFT calculations.
A recent theoretical study on cellobiose hydrolysis suggests
the high energy transition state to be associated with distor-
tion of the stable chair conformation, when the protonated
glucoside bond is split into glucose and the oxocarbenium
ion.31 In the transition state the two glucose units are orien-
tated in a 90 degree angle, which is possible due to flexibility
of both glucose units in the cellobiose dimer around the glyco-
side bond.32 This conformation allows the structure to
rearrange so the reaction barrier for hydrolysis is lowered. In
contrast, the polymeric cellulose chain will in general not
Fig. 2 Arrhenius plot showing the temperature dependence of the initial rates
for cellulose and cellobiose hydrolysis with the sulfuric acid catalyst.
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bend 90 degrees around each glucoside bond, and thus
behave more rigidly with no possibility to undergo an analo-
gous conformation change that can stabilise the transition
state. The reason why the activation energies for hydrolysis of
cellulose and cellobiose in water are still found to be similar
seems to be an effect of poor stabilisation of the positive
charge on the oxocarbenium ion. This effect can, however, be
altered in a strongly ionic environment to lower the reaction
barrier significantly.11–13,17 Hence our results suggest that the
difference in magnitude of the reaction barrier for the hydro-
lysis of cellobiose and cellulose in ionic liquids is mainly
caused by conformational effects.
A dehydration experiment with 10 wt% glucose in [BDMIm]-
Cl and 1.7 wt% sulfuric acid was performed to exclude that the
observed decrease in intensity of the 1157 cm−1 band from
cellobiose was due to simultaneous conversion of the formed
glucose into HMF. Such a misinterpretation would namely
increase the apparent rate and thus decrease the calculated
activation energy. In addition, the experiment could corrobo-
rate that the observed intensity loss of the 1142 cm−1 band of
glucose was distinguishable from the glycoside 1157 cm−1
band. From the results of the experiment it was clear that
glucose was converted mainly into HMF and to a lower degree
into carboxylic acids (increase of νCvO bands at 1669 and
1705 cm−1, respectively; see the discussion below), and that
the 1157 and 1142 cm−1 bands could easily be distinguished
by deconvolution. Moreover, no absorption bands at
1157 cm−1 were observed in the difference spectra as otherwise
seen in the difference spectra of cellulose and cellobiose (ESI,
Fig. S9†). The experiment further showed that the strong
vibration bands overlapping with the cellulose backbone at
1070–1060 cm−1 cannot be used for the kinetic analysis, since
almost identical activation energies for the acid catalysed cellu-
lose hydrolysis and the glucose dehydration to HMF18,19 result
in an overall deceptive linear decrease. Examination of the
spectra recorded during cellobiose hydrolysis at lower tempera-
tures of 90 and 100 °C revealed that the contribution from the
simultaneous dehydration to the glucose backbone vibration
bands at 1140–970 cm−1 was significantly less pronounced.
This strongly suggests that the cellobiose hydrolysis was faster
than the subsequent glucose dehydration.
Fig. 4 Arrhenius plot showing the temperature dependence of the initial rates
for HMF formation during hydrolysis of cellulose and cellobiose with the sulfuric
acid catalyst.
CvO stretching band at 1669 cm−1 after deconvolution (ESI,
Fig. S12 and Table S3†). Initially, a low rate was found until a
steady glucose concentration was established from the hydro-
lysis reaction. Thereafter, the formation rate followed a pro-
nounced first order kinetics (see ESI, Fig. S14†). When the
rates were plotted in an Arrhenius plot (Fig. 4) the apparent
activation energy for the HMF formation was determined to be
94.7 3.7 kJ mol−1, which is remarkably close to the activation
energy of 96.4 kJ mol−1 determined for the cellulose hydro-
lysis. However, we interpret this result as an indirect reproduc-
tion of the cellulose hydrolysis kinetics, and not as the actual
activation energy for the acid catalysed dehydration of the
formed glucose.
The quantification method was also applied for the HMF
CvO stretching band at 1669 cm−1 during cellobiose hydro-
lysis (Fig. 4). As expected, the initial rate indicated instant
release of glucose from the hydrolysis of the glucoside bond in
cellobiose (ESI, Fig. S13†), and the activation energy was deter-
mined from the initial rates to be 85.0
1.8 kJ mol−1
.
This value is in perfect agreement with a recent value of
84.7 kJ mol−1 calculated from the reported data for glucose
degradation in [EMIm]Cl.12 However, it is higher than the acti-
vation energy of 69.6 kJ mol−1 determined for the hydrolysis of
the glucoside bond in cellobiose (vide supra), and resembles
therefore more likely the actual activation energy for acid cata-
lysed glucose dehydration in ionic liquids. This is further sup-
ported by the finding that the rate of HMF formation did not
follow first order kinetics at longer reaction times. Instead the
rate was here faster than predicted by a first order rate law
(ESI, Fig. S14†), thus suggesting the glucose concentration to
be regulated by a combination of fast first order kinetics of the
hydrolysis of the glucoside bonds and slower first order kine-
tics of glucose isomerisation to form fructose, which generally
is believed to be an essential step for HMF formation.5 The
activation energy for Brønsted acid catalysed fructose
During the hydrolysis of cellulose an intense and sharp
band at 1669 cm−1 appeared independently in the FT-IR
spectra along with two broad bands at 1705 and 1623 cm−1
,
respectively. As mentioned above, the former band can be
assigned to CvO stretching from HMF being formed from the
simultaneous glucose dehydration, while a related increase of
the band at 1623 cm−1 corresponded to O–H bending from the
water gradually being formed by the dehydration reaction.
Similarly, the broad shoulder band at 1705 cm−1 was assigned
to carboxylic CvO stretching from formic acid and levulinic
acid formed by the rehydration of HMF. Post-reaction analysis
by HPLC (high-performance liquid chromatography) con-
firmed that glucose, HMF and carboxylic acids were the main
products of the reaction (ESI, Fig. S11†). The rate of the HMF
formation was determined by quantification of the carbonyl
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dehydration in an analogous system with [BMIm]Cl has been
determined to be as low as 62.2 kJ mol−1 33
The large differ-
.
ence in activation for dehydration of fructose and glucose is
expected to facilitate dehydration of fructose to HMF in mixed
systems. Hence, the absence of fructose in reaction samples
analysed from ionic liquid glucose dehydration systems13 is
most likely related to kinetics and not necessarily indicative of
an alternative mechanism.
Scheme 2 Schematic drawing of the stirred micro reactor used for the in situ
ATR-FTIR experiments.20
Conclusions
Time-resolved in situ ATR-FTIR spectroscopy has been intro-
duced as a viable analytical tool to determine reaction rates
and apparent activation energies for Brønsted acid catalysed
hydrolysis of polymeric carbohydrates and simultaneous HMF
formation from glucose in ionic liquids. The activation energy
for hydrolysis of cellulose with sulfuric acid was experimentally
found to be 96.4 4.1 kJ mol−1, which is in excellent agree-
ment with earlier reported activation energies. Notably, the
activation energy determined for the analogous hydrolysis of
cellobiose was significantly lower at 69.6 3.0 kJ mol−1. In con-
trast, the apparent activation energy for HMF formation from
GladiATR diamond cell (Pike Technologies). For each record-
ing 16–80 scans with 4 cm−1 resolution (approximately 15 s to
2 min recording time) were performed depending on the reac-
tion rate. All spectra were background corrected by subtracting
a constant under the assumption of zero absorbance at
4000 cm−1. Then the spectra were ATR corrected with a built-in
algorithm in OMNIC 8.2 assuming a sample refractive index of
1.5. Difference spectra were produced by subtracting a spec-
trum corresponding to the pseudo steady state from each spec-
trum before analysis (ESI, Fig. S4–S7, S22 and S25†), and
deconvoluted with PeakFit using the built-in Voigt area
approximation algorithm (ESI, Fig. S17†). The reaction rates of
the hydrolysis reactions were determined on the basis of the
area of the band at 1157 cm−1 in the deconvoluted spectra
(ESI, Fig. S1 and S2† for hydrolysis and Fig. S12 and S13† for
HMF formation).
glucose was determined to be 94.7
3.7 and 85.0
1.8 kJ
mol−1 with cellulose and cellobiose, respectively. The study
clearly emphasizes the limitations of using cellobiose as a
model substrate for cellulose in kinetic studies.
The work was performed within the framework of the Cata-
lysis for Sustainable Energy initiative funded by the Danish
Ministry of Science, Technology and Innovation.
Reference spectra of cellulose oligomers were recorded on
powder samples with cellotriose, cellotetraose and cello-
pentaose (Megazymes) (ESI, Fig. S18†).
Experimental and computational details
In situ ATR-FTIR spectroscopy
HPLC analysis
After hydrolysis of the cellulose with sulfuric acid, spot checks
(100 and 120 °C) were performed on the reaction mixtures to
confirm the presence of glucose and HMF. The reaction mix-
tures were analysed by HPLC with RI detection (Agilent 1200
series, 30 cm Aminex© HPX-87H column, 0.005 M H2SO4 in
MilliQ water as an eluent at a flow rate of 0.6 mL min−1). The
formed products (glucose, HMF, levulinic acid and formic
acid) were identified by comparison with retention times of
individual reference samples.
Cellulose (Avicel, Sigma-Aldrich) or 1,4-β-cellobiose (99%,
Sigma-Aldrich) was dissolved in 1-butyl-2,3-dimethylimida-
zolium (95%, Sigma-Aldrich) at 120 °C under reduced pressure
(10–15 mbar) by stirring for 2 h. One equivalent of water was
then added and the solutions were equilibrated for 2 h at
90 °C. About 2 g of the ionic liquids with water were sub-
sequently added to the appropriate catalyst at 60 °C, rapidly
mixed with a spatula for 1 min and then freeze quenched in
acetone/dry ice. The resulting solutions of cellulose and cello-
biose were 10.0 and 10.6 wt%, respectively, thus containing
almost identical concentrations of glucose monomers. A cata-
Calculation of theoretical IR spectra
lyst concentration of 1.7 wt% sulfuric acid (98%, Sigma- DFT calculations were performed with Gaussian0934 software
Aldrich) was used. The reaction mixtures were stored in a using a starting structure adopted from an experimentally
sealed glass container and cooled in dry ice prior to use.
determined crystal structure.35 Initially, the structure was opti-
For the FT-IR measurements approximately 15 mg of the mized with Hartree–Fock and the 6-31G(d) basis set and then
reaction mixture were loaded into a custom-made 25 μL glass refined with DFT using the B3LYP functional and the
reactor together with a small magnet rod. The reactor was 6-311+G(d,p) basis set (ESI, structures are shown in Fig. S10†
applied on top of a preheated diamond ATR-plate and magne- and the interpretation of the spectra in Table S2†).
tically stirred by a commissioned device (Scheme 2). The
The theoretical spectra that are found in ESI, Fig. S19† are
spectra were recorded isothermally using a Nicolet iS5 spectro- normalized with respect to the number of glucose units in the
meter equipped with a thermo regulated (up to 300 °C) structure.
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