Enhanced Furfural Yields from Xylose Dehydration in the Γ-Valerolactone/Water Solvent System at Elevated Temperatures

Article abstract of DOI:10.1002/cssc.201800730

High yields of furfural (>90 %) were achieved from xylose dehydration in a sustainable solvent system composed of γ-valerolactone (GVL), a biomass derived solvent, and water. It is identified that high reaction temperatures (e.g., 498 K) are required to achieve high furfural yield. Additionally, it is shown that the furfural yield at these temperatures is independent of the initial xylose concentration, and high furfural yield is obtained for industrially relevant xylose concentrations (10 wt %). A reaction kinetics model is developed to describe the experimental data obtained with solvent system composed of 80 wt % GVL and 20 wt % water across the range of reaction conditions studied (473–523 K, 1–10 mm acid catalyst, 66–660 mm xylose concentration). The kinetic model demonstrates that furfural loss owing to bimolecular condensation of xylose and furfural is minimized at elevated temperature, whereas carbon loss owing to xylose degradation increases with increasing temperature. Accordingly, the optimal temperature range for xylose dehydration to furfural in the GVL/H₂O solvent system is identified to be from 480 to 500 K. Under these reaction conditions, furfural yield of 93 % is achieved at 97 % xylan conversion from lignocellulosic biomass (maple wood).

Full text of DOI:10.1002/cssc.201800730

Accepted Article
Title: Enhanced Furfural Yields from Xylose Dehydration in the
gamma-Valerolactone/Water Solvent System at Elevated
Temperatures
Authors: Canan Sener, Ali Hussain Motagamwala, David Martin
Alonso, and James Dumesic
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201800730
Link to VoR: http://dx.doi.org/10.1002/cssc.201800730
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10.1002/cssc.201800730
ChemSusChem
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Enhanced Furfural Yields from Xylose Dehydration in the γ-
Valerolactone/Water Solvent System at Elevated Temperatures
Canan Sener,[a,b] Ali Hussain Motagamwala,[a,b] David Martin Alonso,[a] and James A. Dumesic*[a,b]
Abstract: High yields of furfural (>90%) were achieved from xylose
dehydration in sustainable solvent system composed of γ-
additives.[1,9] Furfural is currently used in refining lubrication oils,
as a fungicide, and mostly in the production of resins via the
production of furfuryl alcohol, which accounts for more than 75%
of the global furfural consumption. Producing furfural more
effectively and at lower cost may enable other promising
technologies, such as the production of furan through
decarbonylation, and in production of tetrahydrofuran (THF)
through hydrogenation of furan, providing a biomass-based route
for the production of industrial solvents. Furfural may also serve
as a building block for potential transportation fuels like 2-
methylfuran and 2-methyltetrahydrofuran.[10–12] As such, furfural
can be used to bridge the gap between carbohydrate chemistry
and petroleum-based industrial chemistry.[13] The global furfural
solvent market is expected to reach USD 153.4 million by 2022.
Industrially, furfural is mainly produced from xylose obtained by
acid-catalyzed hydrolysis of biomass rich in pentosan, such as
corn cobs, oat hulls and bagasse. Many commercial process have
been developed, including the Quaker Oats process,
AGRIFURANE process, and the ROSENLEW process; however,
the highest furfural yield achieved in these processes is
a
valerolactone (GVL), a biomass derived solvent, and water. It is
identified that high reaction temperatures (e.g., 498 K) are required to
achieve high furfural yield. Additionally, it is shown that the furfural
yield at these temperatures is independent of the initial xylose
concentration, and high furfural yield is obtained for industrially
relevant xylose concentrations (10 wt%). A reaction kinetics model is
developed to describe the experimental data obtained with solvent
system composed of 80 wt% GVL and 20 wt% water across the range
of reaction conditions studied (473 – 523 K, 1–10 mM acid catalyst,
66 – 660 mM xylose concentration). The kinetic model demonstrates
that furfural loss due to bimolecular condensation of xylose and
furfural is minimized at elevated temperature, whereas carbon loss
due to xylose degradation increases with increasing temperature.
Accordingly, the optimal temperature range for xylose dehydration to
furfural in the GVL/H2O solvent system is identified to be from 480 to
500 K. Under these reaction conditions, furfural yield of 93% is
achieved at 97% xylan conversion from lignocellulosic biomass
(maple wood).
approximately 50%.[14] To improve the process, various catalysts
[15,16]
and solvent systems have been developed;
however, the
yields are still limited by the formation of undesired by-products,
humins,[17,18] which are proposed to be produced by condensation
of furfural with xylose or an intermediate formed during xylose
dehydration to furfural.[19] Several strategies have been
developed to minimize the formation of humins during xylose
dehydration to furfural to increase furfural yield. One promising
strategy is to use a bi-phasic system wherein furfural produced in
the aqueous phase during the reaction is continuously extracted
in the organic phase.[20,21] The organic phase is chosen such that
both xylose and the acid catalyst do not partition into the organic
phase, and the furfural extracted into the organic phase is
protected from condensation as well as catalytic degradation.[22–
Introduction
The primary challenge for the sustainable production of fuels and
chemical intermediates (e.g. alcohols, aldehydes, acids) from
renewable biomass resources is to develop cost-effective
processes to transform highly functionalized feedstock into value-
added chemicals.[1–8] In this regard, furfural can be produced by
acid-catalyzed dehydration of the pentoses obtained from
renewable biomass resources. Furfural has been identified by the
US-DOE as one of the top 30 building block chemicals that can
be produced from lignocellulosic biomass, and it is considered as
a platform molecule which can be upgraded to various platform
compounds that are suitable for replacing or supplementing
petroleum-derived commodity chemicals, fuels and fuel
24]
Accordingly, increased furfural yields of ∼60-70% have been
reported with the use of biphasic systems; however, most organic
solvents have shown poor partitioning of furfural into the organic
phase, necessitating large amount of organic solvent for
continuous extraction.[20] Solvents like 2-sec-butylphenol, 4-n-
hexylphenol and 2-methoxy-4-propylphenol have been employed
for furfural production due to their high partition coefficient for
furfural,[25] but these solvents are toxic and are not ideal for
industrial application. Another strategy employed to improve the
production of furfural is the addition of salts to form of a bi-phasic
system.[26] For example, NaCl has been utilized to improve the
partitioning of furfural to the organic phase while using more
environmentally benign organic solvents;[27,28] however, salt leads
to corrosion of the reactor and also creates additional waste
stream leading to a non-sustainable process. In addition to the
[a]
[b]
Dr. C. Sener, A. H. Motagamwala, Dr. D. M. Alonso, Prof. J. A.
Dumesic
Department of Chemical and Biological engineering
University of Wisconsin-Madison
Madison, Wisconsin 53706 (USA)
E-mail: jdumesic@wisc.edu
Dr. C. Sener, A. H. Motagamwala, Prof. J. A. Dumesic
DOE Great Lakes Bioenergy Research Center, Wisconsin Energy
Institute
1552University Ave.
Madison, Wisconsin 53726 (USA)
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10.1002/cssc.201800730
ChemSusChem
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use of organic solvents, ionic liquids have also been used Results and Discussion
successfully to produce furfural in biphasic as well as monophasic
systems.[29,30] Another alternative to remove furfural from the
Temperature Effect
reaction medium is the use of gas or steam stripping. Yields of
90% have been achieved in these systems, but the high cost of
steam or compressed gas is still an impediment for commercial
applications.[31–33] While all of these technologies have
demonstrated that it is possible to produce furfural at high yield,
they are normally limited to low initial xylose concentration. In
addition, the scale up of processes based on the removal of
furfural is challenging, as typically mass transfer issues must be
addressed. It is thus desirable to have a process which leads to
high furfural yield at high initial xylose concentrations and utilizes
environmentally friendly solvent systems.
The effect of reaction temperature on dehydration of xylose was
studied with 2 wt% (133 mM) xylose over a temperature range
from 443 to 523 K and at acid concentrations of 1 mM HCl (open
symbols) and 5mM HCl (solid symbols) in a solvent system
containing 80 wt% GVL, as shown in Figure 1. The initial rate of
xylose consumption and furfural yield increased with increasing
temperature, and high furfural yield (>80%) was achieved at 498
K and 523 K with a residence time of 120 seconds and 60 seconds,
respectively. The effect of temperature on furfural selectivity is
shown in Figure 2.
We have recently shown that using a monophasic solvent system
a)
composed of
a mixture of γ-valerolactone and water is
advantageous to produce furfural from xylose[34,35]. Recently,
several novel catalysts have been used with this solvent system
to improve furfural yields. Lin et al. used a SO3H-functionalized
ionic liquid as catalyst to produce furfural in GVL at low
temperature (413 K) obtaining 78% furfural yield.[36] Similar yields
were obtained by Zhu et al. using a functionalized porous carbon
catalyst.[37] Li et al. employed sulfonated catalysts to achieve up
to 93% furfural yield from corn-stover;[38] however, high catalyst
loading (ca. 30 wt%), irreversible deactivation of the sulfonated
catalyst and low initial biomass loading (ca. 2wt%) still limit the
applicability of this process at commercial scale.
With the goal to maximize the utilization of lignocellulosic biomass,
we have shown that the GVL system can be used to efficiently
fractionate biomass into three separate streams: a stream
containing xylose and furfural, which accounts for >95% of hemi-
cellulose present in the biomass; a high purity cellulose stream,
accounting for >90% cellulose present in the initial biomass, and
a stream consisting of lignin that retains the functionalities present
in the original biomass.[39] The carbohydrate stream containing
the xylose can be dehydrated to produce furfural from the hemi-
cellulose fraction of biomass. In the present work, we have
studied and optimized process parameters for dehydration of
xylose to furfural, including reaction temperature, acid type, acid
concentration, initial xylose concentration, and solvent
composition for a monophasic solvent system composed of γ-
valerolactone, a biomass derived solvent, and water. We show
that under optimal reaction conditions, xylose degradation is
minimized leading to high furfural yield. We develop a reaction
kinetics model to describe xylose dehydration across a range of
experimental conditions. We show that an environmentally benign
monophasic solvent system can be used to efficiently dehydrate
xylose, at high concentrations, to furfural. Moreover, the reactor
set-up presented is simple, as it does not involve phase
separations, purification and extractions and eliminates the need
for salt and other promoters. Our findings provide directions for
reactor design and operational parameter determination for
maximizing furfural yields.
b)
Figure 1. Effect of temperature on (a) xylose conversion and (b) furfural yield.
Experiments were conducted with 2 wt% xylose concentration in a solvent
system containing 80 wt% GVL. Black open squares represent 1 mM HCl and
473 K; red open circles represent 1 mM HCl and 498 K; blue open triangles
represent 1 mM HCl and 523 K; pink closed inverted triangles represent 5 mM
HCl and 498 K; green closed diamonds represent 5 mM HCl and 523 K. Solids
lines are prediction from kinetic model based on Scheme 1.
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a)
b)
Figure 2. Effect of reaction temperature on furfural selectivity. Experiments
were conducted with 2 wt% xylose and 5 mM HCl in a solvent system containing
80 wt% GVL. Black open squares represent 498 K; red open circles represent
523 K.
It is observed that high furfural selectivity is obtained for both
reaction temperatures studied. Importantly, it is observed that the
selectivity towards furfural remains relatively constant with
increasing xylose conversion. This result indicates that minimal
furfural degradation occurs due to the condensation of furfural
with xylose and/or intermediates produced during xylose
dehydration. This behavior is contrary to results obtained in water,
where furfural selectivity decreases with increasing xylose
conversion.[19] This fact has important implications for
a
commercial process where high xylose conversion and high initial
xylose concentrations (discussed later) are desired.
Figure 3. Effect of acid concentration on (a) xylose conversion and (b) furfural
yield. Experiments were conducted with 2 wt% xylose concentration in a solvent
system containing 80 wt% GVL. Black open squares represent 498 K and 1 mM
HCl; red open circles represent 498 K and 5 mM HCl; blue open triangles
represent 498 K and 10 mM HCl; pink closed squares represent 523 K and 1
mM HCl; green closed circles represent 523 K and 5 mM HCl; orange closed
triangles represent 523 K and 10 mM HCl. Solids lines are prediction from
kinetic model based on Scheme 1.
Effect of Acid Concentration
The effect of acid concentration on xylose dehydration was
studied at acid concentrations of 1, 5 and 10 mM, and at
temperatures of 498 and 523 K, as shown in Figure 3. The rate of
xylose dehydration increases with increased acid concentration;
however, furfural selectivity is independent of acid concentration,
as shown in Figure 4. It is observed from Figure 3 that xylose
dehydration is first order with respect to the acid concentration, in
agreement with previous reports.[19,35,40] It is observed that the rate
of xylose dehydration increases with increasing acid
concentration; however, furfural selectivity remains constant at
any given xylose conversion (Figure 4). Both the rate and acid
concentration have high impact on reactor design, as higher acid
concentration leads to higher reaction rate which allows shorter
residence time to achieve a given conversion. On the other hand,
higher acid concentration leads to faster corrosion and will lower
the reactor life. Thus, acid concentration must be judiciously
selected to maximize the output from a given reactor setup.
Effect of Xylose Concentration
The effect of xylose concentration on the dehydration of xylose
was studied with 5 mM acid concentration at temperatures of 473
and 498 K, as shown in Figure 5. Xylose conversion as well as
furfural yield were found to be independent of xylose
concentration, indicating that furfural selectivity is independent of
the initial xylose concentration, as shown in Figure 6. This
behavior is contrary to results obtained in previous studies
conducted at lower temperatures, wherein the furfural selectivity
decreased with increased xylose (and furfural) concentration.[35,40]
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Figure 4. Effect of acid concentration on furfural selectivity. Experiments were
conducted with 2 wt% xylose concentration in a solvent system containing 80
wt% GVL. Black open squares represent 498 K and 1 mM HCl; red open circles
represent 498 K and 5 mM HCl; blue open triangles represent 498 K and 10 mM
HCl; pink closed squares represent 523 K and 1 mM HCl; green closed circles
represent 523 K and 5 mM HCl; orange closed triangles represent 523 K and
10 mM HCl. Dashed black line is a visual guide.
Figure 6. Effect of initial xylose concentration on furfural selectivity.
Experiments were conducted with 5 mM HCl in a solvent system containing 80
wt% GVL. Black open squares represent 473 K and 1 wt% xylose; red open
circles represent 473 K and 4 wt% xylose; blue closed triangles represent 498
K and 1 wt% xylose; pink inverted triangles represent 498 K and 2 wt% xylose;
green closed diamonds represent 498 K and 4 wt% xylose. Dashed black line
is a visual guide.
a)
b)
Figure 7. Comparison of furfural yield and selectivity at low and high initial
xylose concentrations. Black represents xylose conversion, blue represents
furfural yield and red represents furfural selectivity. Experiments were
conducted in a solvent system containing 80 wt% GVL at 498 K with 5 mM HCl
and a residence time of 96 seconds.
The lower furfural yields in these studies were due to the
condensation reaction between xylose and furfural. Therefore, it
was suggested to operate dehydration at lower xylose
concentration to obtain high furfural selectivity.[14,19] However, for
industrial applications, it is desirable to operate dehydration at
high xylose concentrations to reduce separation and overall
capital costs. Figure 7 shows a comparison of furfural yield
obtained at initial xylose concentrations of 2 and 10 wt% in the
Figure 5. Effect of initial xylose concentration on (a) xylose conversion and (b)
furfural yield. Experiments were conducted with 5 mM HCl in a solvent system
containing 80 wt% GVL. Black open squares represent 473 K and 1 wt% xylose;
red open circles represent 473 K and 4 wt% xylose; blue closed triangles
represent 498 K and 1 wt% xylose; pink inverted triangles represent 498 K and
2 wt% xylose; green closed diamonds represent 498 K and 4 wt% xylose.
Dashed lines are prediction from kinetic model based on Scheme 1.
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GVL/water (80 wt% GVL) solvent system containing 5mM HCl at
498 K. It should be noted that the furfural selectivity is high (>90%),
even at high xylose concentration of 10 wt%.
Effect of acid type
Two of the most studied mineral acids for xylose dehydration are
hydrochloric acid and sulfuric acid. Accordingly, studies were
performed to compare the efficiency of xylose dehydration with
these mineral acids, and Figure 8 shows a comparison between
the xylose conversion for the these two acids. The concentrations
of HCl and H2SO4 were kept constant at 10 mM even though
sulfuric acid is diprotic. This concentration was chosen because
the second dissociation constant of sulfuric acid decreases with
increasing temperature[41,42] and sulfuric acid behaves essentially
as a mono-protic acid at the temperature used in this study. It can
be observed from Figure 8 that the rate of xylose dehydration is
higher using hydrochloric acid as the catalyst. However, we
observe that the effect of the chloride ion decreases with an
increase in temperature (see Figure 8). More importantly, the
selectivity to furfural at high xylose conversion is similar for both
of these acid catalysts studied, as shown in Figure 9.
Figure 9. Effect of acid type on Furfural selectivity. Experiments were conducted
with 2 wt% Xylose in a solvent system containing 80 wt% GVL. Black open
squares represent 498 K and 10 mM H2SO4; red open circles represent 498 K
and 10 mM HCl; blue open triangles represent 523 K and 10 mM H2SO4; pink
solid inverted triangles represent 523 K and 10 mM HCl; open green diamonds
represent 498 K 5mM H2SO4; open black stars represent 498 K and 5mM HCl.
dehydration increases with addition of NaCl, showing the positive
effect of the presence of chloride ion on dehydration of xylose. It
should be noted that the solvent system is monophasic at these
salt concentration (∼30 mM) and the improvement observed in
furfural yield is not due to the partitioning of furfural to the organic
phase. Additionally, it has been shown that the identity of the
cation has little effect on the rate of dehydration[43]. We also show
that the effect of chloride ion saturates as the chloride ion
concentration increases from 15 to 30 mM (Figure. 10).
a)
Figure 8. Effect of acid type on xylose dehydration. Experiments were
conducted with 2 wt% Xylose in a solvent system containing 80 wt% GVL. Black
open squares represent 498 K and 10 mM H2SO4; red open circles represent
498 K and 10 mM HCl; blue closed triangles represent 523 K and 10 mM H2SO4;
pink inverted triangles represent 523 K and 10 mM HCl. Solids lines are visual
guides.
It is known that chloride ions promote xylose dehydration
reactions,[43] and the difference observed here between the
reactivity of hydrochloric acid and sulfuric acid at 498 K (Figure 8)
is an indication of this chloride ion effect. However, the difference
in reactivity can also be caused by a difference in dissociation
constants of the mineral acids in the GVL/H2O solvent at elevated
temperature. To elucidate the effect of chloride ions on the
dehydration of xylose in GVL/H2O solvent system, NaCl was
added to GVL/H2O solvent containing sulfuric acid, and xylose
dehydration was conducted at 443 and 498 K. The results are
shown in Figure 10, and it can be observed that the rate of xylose
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b)
b)
Figure 10. Figure Caption Effect of chloride ion on (a) xylose conversion and
(b) on furfural selectivity. Experiments were conducted with 2 wt% Xylose in a
solvent system containing 80 wt% GVL. Pink open inverted triangles represent
443 K and 5 mM H2SO4; green open diamonds represent 443 K, 5 mM H2SO4
and 15 mM NaCl; black solid squares represent 498 K and 5 mM H2SO4; red
solid circles represent 498 K, 5 mM H2SO4 and 15 mM NaCl; blue solid triangles
represent 498 K, 5 mM H2SO4 and 30 mM NaCl.
Figure 11. Effect of GVL concentration on (a) xylose conversion and (b) furfural
yield. Experiments were conducted with 2 wt% xylose at 498 K with 5 mM HCl.
Black squares represent 70 wt% GVL; red circles represent 80 wt% GVL and
blue triangles represent 90 wt% GVL.
Reaction Kinetics Analysis
Kinetics of furfural degradation:
Effect of GVL Concentration
We observe high furfural yields under our reaction conditions,
indicating that there is minimal furfural degradation. Hence, we
first studied the degradation of furfural by self-condensation in the
absence of xylose. Figure 12 shows results for furfural
degradation at 473, 498 and 523 K with 100 mM HCl and 2 wt%
furfural. Note that high acid concentration of 100 mM (20 times of
what is required for xylose dehydration) and high residence time
were required to achieve appreciable furfural conversion for
kinetic analysis. We obtained the rate constant, ꢀ3, and the
activation energy, ꢁꢂ,3, for furfural degradation by fitting the data
in Figure 12 to the following equation (see Scheme 1, reaction 3)
We studied the effect of GVL concentration in solvent system on
xylose dehydration at temperatures of 498 and 523 K. These
studies were performed with solvent systems composed of 70, 80
and 90 wt% GVL. The rate of xylose dehydration increases with
increasing GVL concentration, which is in agreement with the
literature reports.[34,35] The furfural yields at high xylose
conversion were similar for all studied GVL concentrations (see
Figure 11). This behavior is an important advantage when
working at high xylose concentrations, as the dehydration of
xylose to furfural produces three moles of water for each mole of
furfural produced, thereby changing the composition of the
solvent as the reaction proceeds.
ꢃ[ꢄꢅꢆꢇꢅꢆꢂꢈ]
+
[
][
]
= −ꢀ3 ꢊꢋꢌꢍꢋꢌꢎꢏ ꢐ3ꢑ
[Eq. (1)]
ꢃꢉ
a)
k3 at the reaction temperature of 498 K, ꢀ3498, was estimated to be
2.85x10-2 M-1s-1 and the activation energy, ꢁꢂ,3, was estimated to
be 76 ± 8.4 kJ/mol. These values of ꢀ3498and ꢁꢂ,3 were used for
modeling furfural degradation throughout this study.
Bimolecular condensation of furfural and xylose:
The high furfural yield obtained in this study indicate that the rate
of condensation between xylose and furfural is negligible under
our reaction conditions. This behavior is in contrast to the previous
reaction kinetics studies of xylose dehydration conducted in
GVL/H2O wherein the loss of xylose and furfural due to
condensation reaction was significant.[35] We also observe that
furfural selectivity is independent of initial xylose concentration at
the high temperatures of this study (Figure 6 and Figure 7),
whereas furfural selectivity would decrease with increasing initial
xylose concentration if bimolecular condensation were significant.
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Figure 12. Furfural degradation in presence of 100 mM HCl. Experiments were
conducted with 2 wt% Furfural. Furfural concentration is shown with open
symbols and furfural conversion is shown with solid symbols. Circles represent
experiments conducted at 473 K; squares represent experiments conducted at
498 K, triangles represent experiments conducted at 523 K. Solid lines are
predictions from the kinetic model described in equation 3.
It was also shown by Root et al.,[19] that higher furfural yield of
∼60% is obtained in water at high temperature (553 K), whereas
low furfural yields (∼30-40%) are obtained at lower temperature
(453 K). Root et al. proposed that the improved furfural yields at
higher temperatures were due to minimal furfural loss by
condensation reactions.[19] Additionally, condensation reactions
are analogous to polymerization reactions, and like the
polymerization reaction, the condensation reaction becomes
insignificant at elevated temperatures.[44] We also performed
experiments to study the conversion of xylose in the presence of
furfural in GVL/H2O using hydrochloric acid as catalyst at 498 K.
Figure 13(a) shows the concentration of xylose and furfural as a
function of the residence time, and Figure 13(b) shows the xylose
conversion and the furfural yield as a function of the residence
time. We note here that furfural yield is calculated by subtracting
the initial furfural concentration from the furfural concentration
obtained after reaction. It can be seen from Figure 13 that high
concentrations of initial furfural do not affect xylose dehydration to
furfural, indicating that condensation reactions are diminished
under the reaction conditions studied. In view of these findings,
we can neglect any losses from condensation reactions.
Figure 13. Xylose dehydration in presence of furfural with 5 mM HCl and at 498
K. Experiments were conducted with 2 wt% xylose and 2 wt% Furfural in a
solvent system containing 80 wt% GVL. (a) Xylose and furfural concentration
as a function of residence time. (b) Xylose conversion and furfural yield as a
function of residence time. Black squares represent xylose and red circles
represent furfural.
hydrogen ion concentration in aqueous media using mineral acids
as catalysts. The reaction rates for both xylose and furfural
disappearance were verified to be linear with respect to the acid
concentration. Under these conditions, the rate equation for
xylose consumption and furfural production can be written as:
Kinetic study of xylose dehydration:
Reaction kinetics analyses of xylose dehydration to furfural were
based on reaction Scheme 1, which includes dehydration of
xylose to furfural as a single step, direct xylose degradation, and
direct furfural degradation.
Reaction orders with respect to xylose and furfural were found to
follow apparent first-order kinetics for all reaction steps in the
reaction network. This behavior is in agreement with previous
studies showing apparent first-order rates for xylose and furfural
in aqueous media using mineral acids as catalyst.[40,43,45] Also, the
rates of xylose and furfural disappearance are proportional to the
ꢃ[ꢒꢓꢈꢔꢕꢖ]
+
+
[
][ ]
]
[
][
= −ꢀ1 ꢗꢘꢏꢙꢚꢛ ꢐ3ꢑ − ꢀ2 ꢗꢘꢏꢙꢚꢛ ꢐ3ꢑ
[Eq. (2)]
[Eq. (3)]
ꢃꢉ
ꢃ[ꢄꢅꢆꢇꢅꢆꢂꢈ]
+
+
[
][ ]
]
[
][
= ꢀ1 ꢗꢘꢏꢙꢚꢛ ꢐ3ꢑ − ꢀ3 ꢊꢋꢌꢍꢋꢌꢎꢏ ꢐ3ꢑ
ꢃꢉ
For the following initial conditions
[
]
[
]
[
]
@ꢜ = 0, ꢝꢘꢏꢙꢚꢛ = ꢝꢘꢏꢙꢚꢛ ꢔ ꢎꢞꢟ ꢊꢋꢌꢍꢋꢌꢎꢏ = 0,
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the rate equations can be solved to yield the following expression
for the xylose and furfural concentrations as a function of the
residence time.
[
]
[
]
(
(
)[
+] )
ꢝꢘꢏꢙꢚꢛ = ꢝꢘꢏꢙꢚꢛ ꢔ ꢛꢝꢠ − ꢀ1 + ꢀ2 ꢐ3ꢑ ꢜ
+
[
]
(
(
) [
]
)
= ꢝꢘꢏꢙꢚꢛ ꢔ ꢛꢝꢠ − 1 + ꢡ ꢐ3ꢑ ꢀ1 ꢜ [Eq. (4)]
+
[
[
]
]
(
[
+](
) )
ꢀ1 + ꢀ2 − ꢀ3 ꢜ �
ꢀ1ꢛꢝꢠ − ꢐ3ꢑ ꢀ3ꢜ �1 − ꢛꢝꢠ − ꢐ3ꢑ
[
]
[
]
ꢝꢘꢏꢙꢚꢛ
ꢊꢋꢌꢍꢋꢌꢎꢏ
=
=
ꢔ
ꢔ
ꢀ1 + ꢀ2 − ꢀ3
+
+
�
(
1+ꢨ−ꢦ
)
ꢖꢢꢣ�−�ꢤ
ꢥ
�ꢦ ꢧ ꢉ��1−ꢖꢢꢣ�−�ꢤ
ꢥ
ꢧ
ꢉ �
�
1
3
1
3
[
]
ꢝꢘꢏꢙꢚꢛ
[Eq. (5)]
1+ꢨ−ꢦ
ꢫ
1
where, ꢩ = ꢀ3/ꢀ1, ꢡ = ꢀ2/ꢀ1, and ꢀꢪ = ꢀꢆꢖꢇꢛꢝꢠ �− ꢬ,ꢭ �ꢯ1 − ꢯꢰꢱꢲ��.
ꢪ
ꢮ
ꢀꢪꢆꢖꢇ is the rate constant for the
reaction measured at the
ith
reference temperature ꢳꢆꢖꢇ of 498 K, ꢁꢂ,ꢪ is the activation energy
for the ꢴꢉℎ reaction, ꢳ is the reaction temperature and ꢵ is the gas
constant. The measured concentrations of xylose and furfural in
Equation [Eq. (4)] and Equation [Eq. (5)] were fitted to the
experimental data obtained at 473, 498 and 523 K to obtain the
rate constants at 498 K and the activation energies of the
individual reactions in Scheme 1. The values of the reaction
kinetics parameters are reported in Table 1. Figure 14 shows the
parity plot of xylose and furfural concentrations for the kinetic
model described in Scheme 1. It can be seen from Figure 14 that
the kinetic parameters reported in Table 1 are able to capture the
reaction trends for various reaction condition studied. It is
observed from Table 1 that the activation energies obtained in this
work are lower than previously reported values.[19,40,45] This result
can due to the stabilization of acidic protons by the polar aprotic
solvent (GVL) leading to a decrease in the activation energy for
acid catalyzed reactions, as described previously.[35]
Figure 14. Parity plot of (a) xylose conversion and (b) furfural yield. Model
predictions are from Scheme 1.
reaction temperature must be sufficiently high to suppress
condensation reactions. We estimate from the kinetic model that
high furfural yields (e.g., 90%) can be obtained for xylose
dehydration conducted in the GVL/H2O solvent system consisting
of 80 wt% GVL and 10 mM hydrochloric acid at reaction
temperatures between 480 and 500 K and at residence times near
100 sec. We note that there exists an optimal temperature range
which suppress furfural loss due to condensation reactions
leading to high furfural yield.
Table 1. Model-predicted reaction kinetics parameters (with associated 95 %
confidence intervals) for xylose dehydration in GVL/H2O solvent system.
Rate constant at reference temperature
Activation energy
−1
ꢳꢆꢖꢇ = 498ꢶ ꢷ ꢸꢙꢏ−1ꢚ−1
ꢀJ ꢸꢙꢏ
[
]
[
]
ꢀ1498
ꢀ2498
ꢀ3498
5.365±0.041
0.765±0.033
ꢁꢂ,1
ꢁꢂ,2
ꢁꢂ,3
74.7±2.9
118.6±11.8
76.0±8.4
0.0285±0.0049
The activation energy for xylose degradation is higher than that of
xylose dehydration (see Table 1), and it is predicted that higher
temperature leads to lower furfural selectivity. A comparison of
reaction time, reaction temperature and furfural yield is shown in
Figure 15. It is observed from Figure 15 that higher temperature
leads to lower furfural yield; however, as discussed above, the
Production of furfural from lignocellulosic biomass:
The optimal conditions identified using the kinetic model were
used for the production of furfural from maple wood. We used the
solvent system composed of 80 wt% GVL and 20 wt% water
containing 100 mM H2SO4 to depolymerize the hemi-cellulose
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10.1002/cssc.201800730
ChemSusChem
FULL PAPER
and the lignin fraction of biomass, while retaining the cellulose
Table 2. Composition of maple wood, sugar concentration and yield after
fractionation and after dehydration.
Composition of maple wood [wt%]
Xylan
19.3
Glucan
41.9
Klason lignin
24.9
Moisture
5
Hydrolysate after fractionation
Volume
[ml]
C5 sugar conc.
[wt%]
C5 sugar yield
[%]
Furfural
yield [%]
660
1.94
74.3
13.9
Hydrolysate after dehydration
C5 sugar
Furfural yield
conversion
[%]
Furfural
conc.
[wt%]
C5 sugars
[wt%]
[%]
0.046
96.9
93.2
1.21
Figure 15. Model-predicted furfural yield as a function of residence time and
reaction temperature. Acid concentration was fixed at 10 mM HCl.
Conclusion
A monophasic solvent system comprised of GVL (80 wt%) and
water (20 wt%) is used to produce high yields of furfural (>90%)
from xylose as well as from lignocellulosic biomass. It is identified
that the reaction temperature has the greatest influence on
furfural yield, whereas the acid type, acid concentration and initial
xylose concentration have minimal effect on furfural yield at
fraction as a solid. For fractionation of maple wood, we used a 2-
liter stirred Parr reactor fitted with a custom high-solids mixing
system (described in Klingenberg et al.[46] and Lutherbacher et
al.[47]) which led to effective mixing of biomass at high solid loading
(10 wt% biomass). After 30 min at 403 K with a stirring rate of 60
rpm, 74% of the xylan present in the wood was recovered as
soluble sugars in the hydrolysate (see Table 2). The soluble
sugars were separated from unreacted cellulose by filtration, and
the sugar solution was subjected to dehydration at 498 K for a
residence time of 135 seconds. Under these reaction conditions,
a furfural yield of 93% was achieved at 97% xylan conversion (see
Table 2) and the kinetic model predicts complete xylose
conversion and 85% furfural yield. The increased furfural yield
obtained with the hydrolysate is due to the presence of xylose
oligomers in the hydrolysate which are first hydrolyzed to xylose
in the presence of acid, and the xylose thus produced is
dehydrated to furfural leading to increased furfural yield. Also, it
should be noted that no additional sulfuric acid was added to the
hydrolysate before the dehydration reaction. Thus, our studies
have identified processing conditions for production of furfural at
high yields using high concentration of lignocellulosic biomass.
complete xylose conversion.
A reaction kinetic model is
developed for the dehydration of xylose which involves three
reactions: xylose dehydration to furfural, catalytic degradation of
furfural and catalytic degradation of xylose. Moreover, it is shown
that above a certain temperature (e.g., 473 K) the bimolecular
condensation of furfural and xylose can be neglected, and it is
possible to obtain high furfural yield (>90%). Through the kinetic
model we identify that the optimal temperature range is from 480
to 500 K for xylose dehydration to furfural in the GVL/H2O solvent
system. The optimal conditions determined through the kinetic
model are used to produce furfural from lignocellulosic biomass
and furfural yield of 93% is achieved.
Experimental Section
Materials and experimental methods
D-(+)-xylose (≥99%), γ-valerolactone (≥99%), hydrochloric acid, sulfuric
acid were purchased from Sigma-Aldrich and were used without further
purification. Furfural (99%) was purchased from Sigma-Aldrich. The
furfural was purified by distillation before use to remove polymers
produced by self-condensation during storage.[14] Furfural purity after
.
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10.1002/cssc.201800730
ChemSusChem
FULL PAPER
distillation was determined to be >99%. Distilled furfural was stored under
Argon at 277 K and redistilled after 3 days in storage.
with a 0.2 μm PTFE filter before analysis. The temperature of the HPLC
column was maintained at 338 K, and the flow rate of the mobile phase
(pH=2 water, acidified by sulfuric acid) was kept constant at 0.6 ml/min.
External calibrations with known standards were used to quantify reactant
and product concentrations. Xylose concentration was measured with the
RI detector, while furfural concentration was measured with the PDA
detector (210 nm). Conversion of xylose and yield of furfural are calculated
as follows.
A schematic representation of the reactor system used for studying
dehydration of xylose to furfural is shown in Figure 16. The flow-
through reactor was comprised of a 20 cm glass lined stainless steel tube
(1.0 mm internal diameter) (Trajan Scientific) with corresponding stainless
steel valves and fittings (Swagelok). The heated zone of the reactor was
fitted between two aluminum blocks placed within an insulated furnace.
The volume of the heated zone of the reactor was 0.16 cm3. A type-K
thermocouple (Omega) was placed at the reactor wall and was used to
monitor and control the reactor temperature using a 16A series controller
(Love). The reactor was pressurized using Argon, and the pressure was
maintained at 550 psi for reaction at 473 and 498 K, 600 psi for 523 K
throughout the reaction using a back pressure regulator. A GVL/H2O
solvent system consisting of 80 wt% GVL and 20 wt% H2O, and containing
the desired amount of xylose and mineral acid, was introduced at a specific
flow rate using an HPLC pump to achieve the desired residence time. For
reaction with a high concentration of xylose (10 wt%), two HPLC pumps
were used to introduce the organic phase, containing GVL, and the
aqueous phase consisting of water, xylose and sulfuric acid into the reactor.
This approach was required because the solvent system separates into
two phases at high xylose concentration (10 wt%) at low temperature (e.g.,
303 K), but it becomes monophasic at reaction temperatures (>473 K).
Liquid hydrolysate at the reactor outlet was collected in a separator and
was periodically drained and analyzed for xylose and furfural
concentrations using HPLC analysis.
ꢻꢢꢔꢓꢈꢔꢕꢖ − ꢻꢢꢓꢈꢔꢕꢖ
(
)
ꢗꢘꢏꢙꢚꢛ ꢹꢙꢞꢺꢛꢌꢚꢴꢙꢞ % =
× 100
ꢻꢢꢔꢓꢈꢔꢕꢖ
ꢻꢄꢅꢆꢇꢅꢆꢂꢈ
ꢻꢢꢔꢓꢈꢔꢕꢖ
(
)
ꢊꢋꢌꢍꢋꢌꢎꢏ ꢘꢴꢛꢏꢟ % =
× 100
where ꢻꢢꢔꢓꢈꢔꢕꢖ, ꢻꢢꢓꢈꢔꢕꢖ ꢎꢞꢟ ꢻꢄꢅꢆꢇꢅꢆꢂꢈ are the concentration of xylose in the
feed, concentration of xylose in the product, and concentration of furfural
in the product, respectively.
Reaction Kinetics Analysis
Experimental data were used to estimate the kinetic parameters of the
proposed reaction kinetics models. For Scheme 1, the set of ordinary
differential equation was solved to obtain an analytical solution. The kinetic
parameters were estimated by minimizing the square of the sum of
absolute errors between estimated and observed values at experimental
sampling points using the non-linear optimization function in Matlab®.
Additionally, separate experiments with only furfural as feedstock were
performed to determine the kinetic parameters for furfural degradation
(Scheme 1, reaction 3).
3
1
Degradation
products
Furfural
Xylose
2
Degradation
products
Scheme 1. Reaction network for xylose dehydration.
Acknowledgements
Figure 15. Model-predicted furfural yield as a function of residence time and
reaction temperature. Acid concentration was fixed at 10 mM HCl.
This material is based upon work supported by the DOE Great
Lakes Bioenergy Research Center (DOE Office of Science BER
DE-FC02-07ER64494).
Analysis
The concentrations of the reactant and products in the liquid hydrolysate
were measured by High Performance Liquid Chromatography (HPLC)
using a Bio-Rad Aminex HPX-87H column on a Waters 2695 system
equipped with RI-2414 and PDA-2998 detectors. An aliquot of the product
mixture was diluted 10 times with Milli-Q water (18 MΩ.cm) and filtered
Keywords: Furfural • γ-valerolactone (GVL) • Homogeneous
catalysis • Sustainable Chemistry • Xylose
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Pushing the Limits on Furfural
Production: Furfural, a commodity
chemical, is produced from the
hemicellulose fraction of
Canan Sener, Ali Hussain
Motagamwala, David Martin Alonso,
and James A. Dumesic*
Page No. – Page No.
lignocellulosic biomass using a
solvent system composed of γ-
valerolactone and water. High
temperature leads to efficient furfural
production with minimal carbon loss.
Optimal reaction conditions are
identified to maximize furfural
production from xylose as well as
xylan.
Enhanced Furfural Yields from
Xylose Dehydration in the γ-
Valerolactone/Water Solvent System
at Elevated Temperatures
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