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Hydrothermal pentose to furfural conversion and simultaneous extraction with
SC-CO2 – Kinetics and application to biomass hydrolysates
Krishan Gairola ⇑
, Irina Smirnova
Institute of Thermal Separation Processes, Hamburg University of Technology, Eissendorfer Strasse 38, 21073 Hamburg, Germany
h i g h l i g h t s
" Biomass converted to furfural with only water and CO2 as processing agents.
" Furfural yield significantly increased by simultaneous SC-CO2 extraction.
" Influence of temperature, time and pentose concentration modelled.
" Determination of L-arabinose dehydration kinetics under hydrothermal conditions.
a r t i c l e i n f o
Article history:
Received 3 May 2012
Received in revised form 9 July 2012
Accepted 11 July 2012
Available online 20 July 2012
Keywords:
CO2
Furfural
Hydrothermal
Biorefinery
Supercritical fluid extraction
a b s t r a c t
This work explores hydrothermal D-xylose and hemicellulose to furfural conversion coupled with simul-
taneous furfural extraction by SC-CO2 and the underlying reaction pathway. A maximum furfural yield of
68% was attained from D-xylose at 230 °C and 12 MPa. Additionally missing kinetic data for L-arabinose to
furfural conversion was provided, showing close similarity to D-xylose. Furfural yields from straw and
brewery waste hydrolysates were significantly lower than those obtained from model compounds, indi-
cating side reactions with other hydrolysate components. Simultaneous furfural extraction by SC-CO2 sig-
nificantly increased extraction yield in all cases. The results indicate that furfural reacts with
intermediates of pentose dehydration. The proposed processing route can be well integrated into existing
lignocellulose biorefinery concepts.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Vanishing fossil fuel resources and environmental damages due
to petroleum production and consumption highlight the impor-
tance of a shift to renewable sources for fuels and chemicals. The
carbohydrate derived chemical furfural is widely seen as a promis-
ing bio based platform chemical, which can be further converted to
a range of products including plastics, pharmaceuticals and agro-
chemicals (Lichtenthaler, 2005; Mamman et al., 2008).
Current production of furfural is mainly based on mineral acid
catalyzed hydrolysis of hemicellulose contained in agricultural res-
idues or wood to C5-sugars followed by acid catalyzed dehydration
to furfural. The furfural is further purified by azeotropic distillation
or solvent extraction (Zeitsch, 2000). As furfural easily reacts to
degradation products, usually around 50% of the furfural is lost
in side reactions mainly to organic acids, aldehydes and condensa-
tion products by isomerisation, fragmentation and condensation
reactions (Antal et al., 1991; Zeitsch, 2000). Another drawback is
the degradation of remaining cellulose in the biomass via acid cat-
alyzed hydrolysis and decomposition mainly to hydroxymethyl-
furfural, formic and levulinic acid and tar-like condensation
products (Hayes et al., 2005). Typical cellulose losses of 40–50%
render most established processes incompatible with biorefinery
approaches for complete utilization of all embodied valuable com-
ponents (Hoydonckx et al., 2002). However, some alternative pro-
cesses have emerged in the last decade, which aim to overcome
some of the mentioned shortcomings. Examples are the Vederni-
kovs process based on differential catalysis with strong acid and
salts (Gravitis et al., 2001) and the SuprayieldÒ
process developed
by Zeitsch utilizing elevated temperatures and a state of continu-
ous boiling (Zeitsch, 2000). Both claim to reach furfural yields of
around 70% of the theoretical maximum and Vedernikovs process
shall reduce cellulose losses significantly (Gravitis et al., 2001;
Zeitsch, 2000). Nevertheless, both processes still depend on
0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.biortech.2012.07.031
Abbrevations: SC-CO2, super critical carbon dioxide.
⇑ Corresponding author.
E-mail address: krishan.gairola@tu-harburg.de (K. Gairola).
Bioresource Technology 123 (2012) 592–598
Contents lists available at SciVerse ScienceDirect
Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech

mineral acids and approaches using solid catalysts are still under
development and not ready yet for commercialisation (Karinen
et al., 2011).
A more environmentally friendly and less corrosive alternative
to the acid based process is the hydrolysis and dehydration of
hemicellulose to furfural in pure water at elevated temperature
and pressure. With rising temperature, water itself acts as an acid
and base catalyst due to increased dissociation of water molecules
(Brunner, 2009; Kruse and Dinjus, 2007). Catalysis is further inten-
sified by organic acids released from hemicellulose and as side
product of pentose dehydration (Carvalheiro et al., 2004). There-
fore, the hot water environment can be tuned by temperature in
order to facilitate a selective hydrolysis of hemicelluloses at mod-
erate conditions up to 200 °C (Carvalheiro et al., 2004) and – after
separation from the solid cellulose and lignin residue – fast xylose
dehydration at temperatures above 220 °C (Usuki et al., 2008). At
temperatures above 180 °C, the reaction rate in pure water does
not differ from dilute sulphuric acid solutions with concentrations
below 0.001 M (Oefner et al., 1992). Knowledge of the high tem-
perature hydrothermal conversion of hemicelluloses to furfural is
so far limited to the model compound xylose. Neither the conver-
sion of other important C5-sugars as L-arabinose nor of native
hemicellulose or xylan oligomers has been covered in literature
at conditions above 200 °C.
A hydrothermal process solves the problems of acid consump-
tion and cellulose/lignin degradation, but does not lower furfural
losses. A reduction of furfural losses has been achieved by fast re-
moval of furfural from the reaction, either by evaporation (Zeitsch,
2000), organic solvent extraction (Gamse et al., 1997), adsorption
(Weil et al., 2002) or extraction with supercritical CO2 (Gamse
et al., 1997; Sako et al., 1992; Sangarunlert et al., 2007).
Extraction with supercritical CO2 is particularly interesting for
combination with a hydrothermal approach, as it can be employed
at high temperatures, is easily recoverable, environmentally
friendly and has acidifying properties. These acidifying properties
enhanced the rate of biomass depolymerisation and furfural for-
mation for corn stover and the depolymerisation of several poly-
saccharides, but the effect is reduced with rising temperatures
and diminishes above 240 °C (Miyazawa and Funazukuri, 2005;
Rogalinski et al., 2008; van Walsum and Shi, 2004). However, the
effect was not observed when treating aspen wood under hydro-
thermal conditions in the presence of CO2 (McWilliams and van
Walsum 2002). Sako et al., 1992; Sako et al., 1995) studied the
phase behavior of CO2-water-furfural systems and kinetics of xy-
lose to furfural conversion in dilute sulphuric acid in the presence
of supercritical CO2 at a temperature of 150 °C. The maximum yield
obtained was 70% of the theoretical maximum. Sangarunlert et al.
(2007) optimized the production of furfural from rice husks by acid
hydrolysis accompanied by supercritical CO2 extraction and re-
ported a maximum yield of almost 90%.
A high yielding approach for furfural recovery shall be investi-
gated, which can be easily integrated into a broader biorefinery
concept and that – differing from current acid catalyzed
approaches – purely relies on water and CO2 as processing and
reaction media. Such a process would consist of a pre extraction
step of valuables, followed by a liquid hot water pretreatment
(LHW) to dissolve the hemicellulose and as a last step the depoly-
merisation and dehydration of the dissolved hemicellulose under
hydrothermal conditions with simultaneous furfural extraction
by SC-CO2. The solid cellulose rich residue is removed before the
dehydration step to minimize cellulose degradation and can be fur-
ther converted to monomeric sugars and fermentation products.
In this work, the reaction pathway and potential differences in
the kinetics of the hydrothermal conversion of xylose and arabi-
nose to furfural shall be further elucidated. Two common reaction
schemes will be compared in terms of their ability to predict the
influence of SC-CO2 extraction on furfural yield. Besides working
with the model compounds L-arabinose and D-xylose, the conver-
sion of native biomass extracts to furfural shall be optimized.
2. Methods
2.1. Materials
D-xylose and L-arabinose (purity >99%) were supplied by Carl
Roth GmbH (Germany). Wheat straw pellets were supplied by Cor-
des Grasberg (Grasberg, Germany). Brewery residue was kindly
provided by a university campus brewing group (Campusperle)
and dried immediately at 60 °C until the moisture content was re-
duced from 80 wt.% to 4–6 wt.%. CO2 with a purity of 99.95% was
supplied by Yara (Grimsby, UK). Table 1 shows the composition
of hydrolysates derived from the mentioned biomass resources.
2.2. L-Arabinose dehydration studies
L-Arabinose dehydration studies were performed in a fluidized
sand bath (SBL-2D, Techne, USA). Arabinose solutions of 0.2, 1
and 5 wt.% were injected into two tubular stainless steel (ANSI
316 L) reactors of 250 mm lengt and 3.15 mm inner diameter,
equipped with a thermoelement. The remaining air was removed
from the samples by pressurizing the reactors with nitrogen. The
reactors were completely dipped into the preheated sand bath
for the time specified and then removed immediately followed
by quenching in a water bath. The weight of the reactors was mea-
sured before and after the reaction to detect any leakages. The
samples were frozen until analysis by HPLC.
Nomenclature
EAi activation energy of component i [kJ mol-1
]
f fraction of furfural in reactor [mol mol-1
]
ki rate constant of i-th reaction [min-1
], [mol-1
min-1
],
[mol-2
min-1
]
k0i frequency factor [min-1
], [h-1
]
ni number of moles of component i [mol]
t reaction time [min]
Yi yield of component i [mol.%]
DPi degradation product i [mol]
Subscripts
0 at initial reaction time
A arabinose
DP degradation product
F furfural
X xylose
Table 1
Biomass hydrolysate composition after analytical digestion.
Component Unit Wheat straw 1 Wheat straw 2 Brewery residue
Arabinose g/l 0.5 3.6 3.9
Xylose g/l 3.0 14.4 9.6
Glucose g/l 0.6 5.5 4.1
Furfural g/l 0.3 2.9 2.2
K. Gairola, I. Smirnova / Bioresource Technology 123 (2012) 592–598 593

2.3. Biomass hydrolysis in hot compressed water
Biomass hydrolysis was performed according to Ingram et al.
(2009) in a 50 ml fixed bed reactor (Applied Separations, Allentown,
USA) in flow through mode. The hydrolysis temperature was 200 °C
and hydrolysis time 40 min. Dry biomass was filled into the reactor
and held in position by two sintered plates. The amount of biomass
was 7 g in case of wheat straw 1 and 14 g in all other cases. The PTFE
sealed reactor was placed in a preheated oven and after six minutes
water was pumped through the reactor either with a constant flow
rate of 6 ml/min (wheat straw 1 and brewery residue) or with inter-
vals without flow for enhanced oligosaccharide concentrations
(5 min break after 5 and 10 min break after 20 min, wheat straw
2). The hydrolysate was cooled down in a water bath and subse-
quently depressurized into a sample vessel. The pressure was main-
tained at 3 MPa by an adjustable spring valve.
2.4. Furfural production
A setup of six serial batch reactors (Berghof, Eningen, Germany)
with a volume of 45 ml was used for dehydration under hydrother-
mal conditions with simultaneous furfural extraction by SC-CO2.
The reactors were equipped with heating jackets, magnetic stirrers
and thermo elements connected to a PID temperature controller.
One or more reactors were filled with 20 ml of sugar solution or
hydrolysate and pressurized to 30 bar before heating up to the de-
sired temperature (approx. 15 min heat up time). The CO2 flow was
started shortly before reaching the operating temperature. Lique-
fied CO2 was sparged into the stirred solution (300 rpm) and was
then released into a vessel cooled by dry ice (cooling trap). The
CO2 stream was subsequently sparged into a vessel filled with
water to absorb remaining furfural. After the selected extraction
time had been elapsed, each reactor was cooled down rapidly in
a water bath and the remaining gaseous phase was slowly released
into the sample vessels. The pressure was maintained constant
throughout the process by a backpressure regulator. The CO2 flow
was adjusted via the reactor outlet valve and measured under
ambient conditions.
The collected samples and the remaining liquid in the reactor
were immediately frozen and later analyzed for furfural and xylose
concentration using HPLC.
Following terms are used to characterize the results: The term
‘‘Xylose in reactor’’ is defined as the moles of xylose left in the reac-
tor at the end of the experiment, divided by the initial amount of
xylose in the reactor in moles. ‘‘Furfural yield’’ is defined as the
sum of furfural left in the reactor at the end of the experiment
and the furfural extracted in moles, divided by the initial amount
of xylose in the reactor in moles. ‘‘Furfural extracted’’ is defined
as the moles of furfural extracted, divided by the initial amount
of xylose in the reactor in moles. ‘‘Furfural in reactor’’ is defined
as the moles of furfural left in the reactor at the end of the exper-
iment, divided by the initial amount of xylose in the reactor in mo-
les. ‘‘Furfural concentration in extract’’ is defined as the mass of
furfural in the extract divided by the total mass of the liquid ex-
tract (CO2 free). ‘‘Furfural per initial biomass’’ is defined as the
mass of furfural produced from biomass hydrolysate, divided by
the total dry mass of the biomass used for producing the biomass
hydrolysate.
2.5. Analytics
2.5.1. HPLC analysis
The samples were centrifuged at 16000g for five minutes
(repeatedly if necessary) and the supernatant was directly injected
(20 lL) in the HPLC-System (Agilent 1200). The Analysis was per-
formed on a Sugar Na Column (Macherey–Nagel) of
300 Â 7.8 mm with pure water as eluent. The flow was adjusted
to 0.3 ml/min and was maintained at a temperature of 85 °C using
a column oven (ERC, Germany). The Detector was an Agilent 1100
refractive index (RI) Detector. The mechanism of the separation is a
mixture of size exclusion (SEC) and ligand exchange (LEC)
chromatography.
2.5.2. Analytical digestion of oligomeric sugars
The hydrolysis of dissolved oligomers was performed by diges-
tion with sulphuric acid based on the procedure described by NREL
(Sluiter et al. 2006). One and a half millilitre of the sample is thor-
oughly mixed with the same volume of 8% sulphuric acid and incu-
bated in a drying oven at 121 °C for 1 h. After neutralization with
calcium carbonate and centrifugation as above, the sample is ana-
lyzed by HPLC.
2.6. Reaction analysis
2.6.1. Hemicellulose depolymerisation
Hydrothermal decomposition of hemicellulose can be modelled
by a sequential pseudo homogenous first order approach, taking
into account the dissolution of hemicellulose to oligosaccharides,
their depolymerisation and the degradation of the resulting mono-
meric sugars (Carvalheiro et al., 2005; Garrote et al., 2001). The
hydrolysis of hemicellulose from brewers spent grain under hydro-
thermal conditions (150–190 °C) to monosaccharides and degrada-
tion products has been thoroughly studied by Carvalheiro et al.
(2005). Of special interest for this work are the kinetics of the oli-
gosaccharides reaction to monomeric xylose and arabinose. Carv-
alheiro et al. (2005) determined an activation energy of 130 kJ/
mol and a frequency factor of 5.29 Â 1013
minÀ1
for xylose oligo-
saccharides and 117 kJ/mol and 2.63 Â 1012
minÀ1
for arabinose
oligosaccharides based on a first order reaction scheme. At temper-
atures above 220 °C the depolymerisation reaction proceeds very
fast - at 230 °C a conversion of 95% is reached after 1.6 (arabinose)
to 1.8 min (xylose) according to Eq. (1) and the above mentioned
kinetics. Eq. (1) is based on the first order kinetic approach for
the susceptible xylan fraction described in Carvalheiro et al.
(2005), solved for reaction time.
t ¼ À
lnð1 À conversionÞ
k
ð1Þ
2.6.2. Pentose dehydration
Several kinetic models for pentose dehydration to furfural and
the associated loss reactions in acidic or hydrothermal environ-
ment have been proposed. Two of the evolved reaction models
which have been applied to high temperature hydrothermal pen-
tose dehydration are shown in Eqs. (4) and (5). More complex
models including several intermediates and further degradation
products have been developed (Antal et al., 1991; Oefner et al.,
1992), but are beyond the scope of this work.
Pentose Furfural DP2
k3
k2
k1
DP1
ð2Þ
Pentose Furfural DP2
k3
k2a, k2b
k1
DP1
ð3Þ
Model 1: Jing and Xiuyang (2007) proposed among others the
simple first order reaction scheme depicted in Eq. (4), where a part
of the pentose reacts to a degradation product (DP1) and another
part to furfural. The furfural further reacts to a second degradation
product (DP2) (Jing and Xiuyang, 2007). The resulting differential
594 K. Gairola, I. Smirnova / Bioresource Technology 123 (2012) 592–598

equations (Eqs. (6) and (7)) can be solved analytically for the
amount of pentose and furfural (Eqs. (8) and (9)), where nx and
nF are the amount of xylose and furfural in mol and k is the overall
xylose degradation rate constant. This reaction scheme was so far
only tested with D-xylose. There is no literature data available for
L-arabinose conversion kinetics, but own experiments have shown
a similar behavior for L-arabinose as for D-xylose (see Table 2).
dnX
dt
¼ Àk1nX À k2nX ¼ Àknx ð4Þ
dnF
dt
¼ k1nX À k3nF ð5Þ
nX ¼ nX0expðÀktÞ ð6Þ
nF ¼
k1nX0
k3 À k
½expðÀktÞ À expðÀk3tÞŠ ð7Þ
The reaction constants are derived by fitting experimental data
of pentose and furfural concentrations to Eq. (7). The resulting
kinetic parameters based on an Arrhenius approach are given in
Jing and Xiuyang (2007). Furfural yield can now be calculated
when dividing Eq. (7) by nx0.
Model 2: Zeitsch (2000) describes in his comprehensive book on
furfural production a more complex model based on findings of
Root et al. (1959) as shown in Eq. (5). It considers the reaction of
furfural with two intermediates of xylose dehydration (Root
et al., 1959; Zeitsch, 2000). The resulting differential equations
for formation of DP1 and furfural are given in Eqs. (10) and (11).
The intermediates are substituted by xylose concentration.
dnDP1
dt
¼ k2anXnF þ k2bnXn2
F ð8Þ
dnf
dt
¼ knX À k2anXnF À k2bnXn2
F À k3nF ð9Þ
An analytical solution of Eq. (9) is not known and the related ki-
netic parameters have so far not been calculated.
One of the major differences of model 2 to model 1 is the depen-
dence of the first loss reaction on furfural concentration. This
should lead to declining furfural yield with increasing initial xylose
concentration and strongly increasing yield when the furfural con-
centration in the reactor is minimized.
Influence of furfural extraction: According to model 1, xylose deg-
radation is much more pronounced than furfural degradation, thus
limiting the impact of furfural extraction. The maximum possible
yield with immediate and complete furfural extraction can be
obtained from Eq. (7), when setting k3 to zero and approaching
infinite reaction time:
Ymax ¼
nF;max
nX0
¼ limt!1 À
k1
k
ðeÀkt
À 1Þ

¼
k1
k
ð10Þ
This shows that according to model 1, the maximum possible
yield is limited depending on temperature to 58% at 200 °C and
41% at 260 °C.
According to model 2, xylose degradation is a function of furfu-
ral concentration and therefore a strong positive influence of furfu-
ral extraction on yield can be expected. Sako et al. (1992) studied
the effect of SC-CO2 extraction of furfural during acid catalyzed xy-
lose conversion. They could show that furfural concentration in the
reactor is maintained at a low and almost constant level during the
course of extraction. With the assumption of constant furfural con-
centration, which only depends on initial xylose concentration, Eq.
(9) can be solved, giving Eq. (11):
nF ¼ nX0ðf nX0k2a þ ðf nX0Þ2
k2b À 1Þ eÀkt
À 1
À Á
À nX0fk3t ð11Þ
where nF is the amount of furfural and f is the fraction of furfural
present in the reactor throughout conversion, in terms of initial
amount of xylose. Furfural yield can now be calculated when divid-
ing Eq. (11) by nx0.
3. Results and discussion
3.1. Kinetics of L-arabinose dehydration
D-Xylose is the main constituent of most plant hemicellulose
and is therefore generally used as model compound for hemicellu-
lose to furfural conversion. Nevertheless L-arabinose can make up a
significant fraction of hemicellulose and hemicellulose hydroly-
sates, as shown in Table 1. The kinetics of L-arabinose dehydration
based on model 1 have been therefore studied and are presented in
this section. The experiments were conducted in a sand bath for
rapid heating and cooling.
L-Arabinose disappearance was measured for several tempera-
tures and time steps to determine the aggregated kinetic parame-
ter k = k1 + k2 and the associated Arrhenius parameters (Table 2).
The experimental and calculated values are presented in Fig. 1.
The results show a good correlation with the experimental values
over a wide temperature range. The results in Fig. 1 further show
that a temperature of more than 220 °C is necessary to reach an al-
most complete disappearance of arabinose within less than
60 min. The Activation energy and frequency factor given in Table 2
are close to the values obtained for L-arabinose degradation by
Carvalheiro et al. and are similar to the values obtained for D-xylose
by several authors (Carvalheiro et al., 2005; Jing and Xiuyang,
2007; Oefner et al., 1992). However, they strongly differ from
kinetic parameters obtained by hydrothermal conversion of
D-arabinose (Usuki et al., 2008).
Table 2
Kinetic parameters of L-arabinose dehydration to furfural. The letter ‘‘a’’ denotes
results obtained with k3 taken from literature Jing and Xiuyang (2007), while ‘‘b’’
denotes results obtained with own k3.
i EAi [kJ/mol] ln(k0i) [minÀ1
]
– 117 ± 4.1 25.8 ± 1.04
1a 109 ± 3.2 23.2 ± 0.80
1b 112 ± 3.2 23.8 ± 0.80
2a 123 ± 4.6 26.7 ± 1.21
2b 121 ± 4.7 26.2 ± 1.23
3a⁄ 59 7.63
3b 110 ± 15.3 21.0 ± 3.49
⁄, taken from Jing and Xiuyang (2007).
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60
Arabinosedisappearance[mol-%]
Time [min]
Fig. 1. Arabinose dissapearance as function of time for temperatures of 180 °C (j),
200 °C (Â), 220 °C (Ç), 240 °C (N) and 260 °C (d). The dashed lines are calculated
based on model 1.

The remaining rate constants (k1, k2 and k3) can now be derived
by fitting model 1 to the measured furfural concentrations.
However, the results will be more accurate when the furfural deg-
radation rate constant k3 is determined analytically. The degrada-
tion kinetics of furfural (k3) cannot be determined directly, as
there is no method known to quantify the resulting degradation
products. If furfural degradation is independent from the presence
of sugar and sugar degradation products, the kinetics can be deter-
mined by measuring furfural disappearance in heated furfural
solutions as done by Jing and Xiuyang (2007). However this leads
to a rather low rate of furfural degradation, which is not backed
by experimental results at high temperatures, as shown in Fig. 2:
There is a marked decline in furfural concentration after reaching
the maximum furfural yield, which is not explained by furfural
degradation under isolated conditions. Therefore, a new rate con-
stant (k3b) was calculated from the declining furfural concentra-
tions at 240 and 260 °C. However, only a few data points were
available and only the higher end of the temperature range was
covered. The results nonetheless fit the experimental data well
(Fig. 2). A change of k3 also affects k1 and k2. The Arrhenius param-
eters for all rate constants are given in Table 2. The fact that the
presence of arabinose and arabinose degradation products influ-
ence furfural degradation indicates that model 2 might be more
appropriate to explain the pathway of furfural formation than
model 1.
The resulting kinetic paramters for conversion of L-arabinose to
furfural are are close to values determined for D-xylose by Jing and
Xiuyang (2007). It might be therefore justified to solely use D-xy-
lose as a model compound for hemicellulose to furfural conversion.
3.2. Furfural production from D-xylose
D-xylose was selected as a model compound for hemicellulose,
as it is the major constituent in most hemicelluloses (Ebringerová
and Heinze, 2000) and its kinetic parameters are similar to L-arab-
inose as shown in the previous section, another anbundent constit-
uent of typical arabino-xylan hemicelluloses. The aim of the model
compound based experiments was to identify optimal operating
conditions for xylose to furfural conversion and simultaneous fur-
fural extraction with SC-CO2 and to clarify the reaction pathway,
before using complex biomass hydrolsysates as substrate.
Reactions in the SC-CO2 phase were not considered as xylose was
found to be insoluble in this phase and furfural does not react with
the water vapor fraction (Zeitsch, 2000).
3.2.1. Optimal operation conditions
Temperature, pressure, reaction time, CO2 flow rate and xylose
concentration were identified as relevant variables for furfural
extraction and total furfural yield (furfural extracted plus furfural
remaining in the reactor). The time–temperature relationship
observed in the previous chapter for L-arabinose dehydration indi-
cates a reasonable reaction time of 13–45 min in the temperature
range of 220–240 °C. Preliminary experiments have shown that
furfural yield increased with rising CO2 flow (not shown), which
was technically constrained to a maximum of 3.6 g/min. Higher
flows led to flooding of the reactor.
To confirm the optimum of furfural yield in the temperature
range of 220–240 °C, experiments were conducted along the line
of expected 95% xylose conversion. The resulting molar composi-
tion in terms of initial amount of xylose in the range of 220–
240 °C is shown in Table 3. Actual xylose conversion is in all cases
97%. At a CO2 pressure of 8 MPa, the highest overall yield of 51%
and extract yield of 48% is achieved at 230 °C. Furfural recovered
as extract at 240 °C is comparably low (36%). The difference be-
tween observed and predicted conversion might be attributed to
a rate enhancement due to a catalytic effect of carbonic acid
formed by dissolved CO2 and water. Such a rate enhancement
was observed by several authors for the decomposition of polysac-
charides under hydrothermal conditions in the presence of dis-
solved CO2 (Miyazawa and Funazukuri, 2005; Rogalinski et al.,
2008).
An increase in furfural yield from 51% to 68% was observed
when increasing pressure from 8 to 12 MPa. A further increase to
16 MPa did not improve furfural yield significantly. A pressure of
12 MPa was therefore selected as optimal setting.
Experiments at varying initial xylose concentrations were
undertaken to clarify, whether there is an influence of initial xylose
concentration (model 2) or not (model 1). A high xylose concentra-
tion should further lead to a higher furfural concentration in the
extract, reducing downstream processing efforts.
Table 4 shows the resulting furfural yield, the furfural residue in
the reactor and the furfural concentration in the extracts. While
there is no significant difference in yield in the range of 2-4 wt.%
initial xylose concentration, an almost linear decline in yield from
68% to 37% was observed above 4 wt.%. The furfural concentration
in the extract increased up to an initial xylose concentration of
10 wt.%. No further increase in furfural concentration was observed
when doubling xylose concentration. The fraction of furfural re-
tained in the reactor was decreasing slightly with initial xylose
concentration and ranges from 4-7 mol.%.
The optimal initial xylose concentration lays between 4 and
10 wt.%, depending whether the main focus is put on maximising
yield or furfural concentration.
Simultaneous SC-CO2 extraction increases furfural yield from
xylose by 65% from 41 mol.% (not shown) to 68 mol.%. The fraction
of furfural recovered as extract is 90%. These values are comparable
to the results of Sako et al. (1992) for the conversion of 2 wt.% D-xy-
lose at 150 °C in the presence of 0.05 mol/l sulphuric acid and
simultaneous SC-CO2 extraction of furfural. A maximum furfural
yield of 69.9% is reported, while 85.6% of furfural was recovered
as extract at these conditions. Furfural yield without extraction
did not exceed 40%.
3.2.2. Comparison of kinetic models
The two selected reaction pathways show both room for
improvement of furfural yield by simultaneous extraction during
formation. Nevertheless according to model 1 only the amount of
0
5
10
15
20
25
30
35
40
45
50
0 10 20 30 40 50 60
Furfuralyield[mol-%]
Time [min]
Fig. 2. Furfural yield as function of time for temperatures of 180 °C (j, solid line),
200 °C (Â, finely dashed line), 220 °C (Ç dashed/dotted line), 240 °C (N, coarsely
dashed line) and 260 °C (d, dotted line). The lines are based on model 1. Parameters
are calculated with own k3.

direct furfural degradation can be reduced, while model 2 suggests
also a potential reduction of pentose degradation.
To determine the kinetic parameters for model 2, samples were
taken during the course of extraction. Eq. (11) was fitted to the
experimental values to determine k2a and k2b using MATLAB 7.9
(The MathWorks Inc.), giving 883.0 minÀ1
molÀ1
for k2a and
32.0 min-1
mol-2
for k2b. It was assumed that the furfural concen-
tration in the reactor retained the final value of 6.8 mol.% through-
out the extraction process. The results are compared to the
prediction based on model 1 in Fig. 3. Model 2 outperforms model
1 in prediction accuracy, but this might be partially attributed to
the fact that it was directly fitted to the extraction yield data. A fi-
nal assessment can be made based on the influence of initial xylose
concentration on furfural yield, as model 1 predicts a concentration
independent furfural yield, whereas model 2 predicts a decreasing
furfural yield with rising xylose concentration.
As already shown above, an overall decline in furfural yield was
observed with rising xylose concentration. Model 1 predicts the
same yield of 46% independent of initial xylose concentration,
while Model 2 reproduces the falling trend, but overestimates
the influence of concentration. While the predicted yield of 66%
at an initial xylose concentration of 4is close to the experimental
value of 68%, the predicted yield of less than 10% for an initial xy-
lose concentration of 20 wt.% is visibly below the experimental va-
lue. This can be at least partially attributed to the uncertainty of
the amount of furfural in the reactor. Nevertheless the results
prove that model 2 is more suitable for the prediction of furfural
yield than model 1.
According to model 2, the negative influence of initial xylose
concentration on furfural yield can be strongly decreased with
increasing extraction efficiency. A reduction of f to 1% for example
would lead to a theoretical yield of 84% for a 10% xylose solution.
3.3. Furfural production from biomass
After validating the positive effect of SC-CO2 extraction on fur-
fural yield from D-xylose solutions, the method was extended to
the conversion of native biomass hydrolysates to furfural. The fast
depolymerisation of hemicelluloses predicted by the model of
Carvalheiro et al. (2005) was verified by experiments with biomass
hydrolysates. A complete conversion to furfural and degradation
products of close to 99% was already reached after 25 min, a longer
treatment time did not lead to a significant increase in conversion
(not shown). SC-CO2 extraction improved furfural yield signifi-
cantly (see Table 5), but furfural yield was in all cases far lower
than in experiments with pure xylose and large amounts of a char
like resin formed inside the reactor (this was also observed to a les-
ser extend in the model compound experiments).
In Table 5 furfural yield from brewers spent grain hydrolysate is
compared to the yield obtained from wheat straw hydrolysate
(wheat straw I). Furfural yield from straw hydrolysate exceeded
the yield from brewery residue more than twice. The main differ-
ence in the hydrolysate composition is made up by the large
amount of proteins in the brewery waste hydrolysate (around
5.5 g/l). Proteins are known to react with furfural and other sugar
degradation products to form char and oil like compounds
(Minowa and Inoue, 2004; Peterson et al., 2010). In the case of
all hydrolysates, the main difference to the model compounds is
the additional presence of phenolic compounds and organic acids
Table 5
Effect of partial flow hot water hydrolysis on resulting furfural concentrations and
yield at 230 °C, CO2 flow rate of 3.6 g/min, 25–27 min reaction time and pressure of
8 MPa. No replicates for wheat straw 1.
Biomass Furfural
concentration
in extract
(wt.%)
Furfural yield
without
extraction
(mol.%)
Furfural yield
with
extraction
(mol.%)
Furfural per
initial
biomass
(wt.%)
Wheat
straw 1
0.2 20.3 28.9 2.4
Wheat
straw 2
(partial
flow)
0.9 ± 0.10 – 25.2 ± 0.27 3.2 ± 0.03
Brewery
waste
0.4 ± 0.02 4.8 ± 0.01 13.4 ± 0.79 –
0
20
40
60
80
100
0 5 10 15 20 25
Conversionandyield[%]
Time [min]
Xylose conversion (calc.)
Furfural yield (exp.)
Furfural yield (model 2)
Furfural yield (model 1)
Fig. 3. Furfural extraction vs. time, experimental and predicted values at at 230 °C,
12 MPa, CO2 flow rate of 3.6 g/min and initial xylose concentration of 4 wt.%.
Table 4
Effect of initial xylose concentration on resulting furfural concentrations and yield at 230 °C, CO2 flow rate of 3.6 g/min, 25 min reaction time and pressure of 12 MPa.
Initial xylose concentration (wt.%) Furfural in reactor (mol.%) Furfural conc. in extract (wt.%) Furfural yield (mol.%)
2 6 1.7 ± 0.38 64 ± 10.9
4 6.8 ± 1.33 3.1 ± 1.06 68 ± 9.0
10 5.7 ± 0.39 8.0 ± 0.58 46 ± 0.3
20 3.9 ± 1.08 8.0 ± 0.77 37 ± 9.2
Table 3
Molar product composition as function of temperature at 8 MPa, CO2 flow rate of 3.6 g/min, initial xylose concentration of 5 wt.% and reaction time aimed at 95% conversion.
Temperature (°C) Time (min) Xylose in reactor (mol.%) Furfural in reactor (mol.%) Furfural extracted (mol.%)
220 43 3.3 ± 0.2 0.9 ± 0.5 46.3 ± 9.0
230 24 2.8 ± 0.6 3.0 ± 1.4 48.1 ± 6.1
240 13 3.4 ± 0.2 10.1 ± 1.9 36.5 ± 2.2

(Ibbett et al., 2011). Furfural is known to crosslink lignin and phe-
nolic resins under hydrothermal conditions (Hamzeh et al., 2009;
Ryu et al., 2010). Pin´ kowska et al. (2011) observed extensive for-
mation of gases and char when treating xylan under hydrothermal
conditions in the range of 210–250 °C for 0–30 min and achieved
rather low furfural yields of 4.4%. This indicates that furfural for-
mation from native biomass or biomass hydrolysates deviates sig-
nificantly from the behavior of pure pentose solutions. Further
investigations are therefore necessary to clarify the role of phenolic
compounds, proteins and other biomass constituents on furfural
degradation, for example by using model mixtures of sugars, ami-
no acids, salts and phenolic compounds, accompanied by a thor-
ough analysis of the degradation products formed.
Besides furfural yield furfural concentration in the extract is an-
other important parameter to optimize. As shown with the model
compound, furfural concentration in the extract is a function of ini-
tial pentose concentration. Table 5 shows the furfural concentra-
tion in the extract of wheat straw hydrolysate. Due to the low
oligosaccharide concentration in wheat straw 1, experiments with
intermittent flow of water during straw hydrolysis were performed
to reach higher saccharide concentrations (wheat straw 2). This
leads to a considerable increase in furfural concentration, while
furfural yield from hydrolysate is slightly reduced as expected.
Intermittent water flow additionally leads to a higher recovery of
oligosaccharides from straw; consequently more furfural is re-
leased per initially utilized straw (32 g furfural per kg of straw).
4. Conclusions
Biomass was converted to furfural with water and CO2 as the
only processing agents. Simultaneous SC-CO2 extraction lead to
high furfural yields when xylose and furfural concentrations in
the reactor were kept low. This indicates that furfural degradation
is based on the reaction with pentose intermediates. 12 MPa and
230 °C were found to be optimal conditions for furfural production.
Missing information on the kinetics of hydrothermal L-arabinose
conversion was complemented, showing close similarities to the
behavior of D-xylose. Furfural yield from biomass hydrolysates
was considerably lower than from D-xylose.
Acknowledgements
This work was funded by BMBF (031-555-9A)
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