The enhancement of xylose monomer and xylotriose degradation by inorganic salts in aqueous solutions at 180 °C

Article abstract of DOI:10.1016/j.carres.2006.07.017

The inorganic salts KCl, NaCl, CaCl₂, MgCl₂, and FeCl₃, and especially the latter, significantly increased xylose monomer and xylotriose degradation in water heated to 180 °C with unaccountable losses of xylose amounting to as high as 65% and 78% for xylose and xylotriose, respectively, after 20 min incubation with 0.8% FeCl₃. Furthermore, losses of both xylose and xylotriose were well described by first order homogeneous kinetics, and the rate constants for xylose and xylotriose disappearance increased 6- and 49-fold, respectively, when treated with 0.8% FeCl₃ solution compared to treatment with just pressurized hot water at the same temperature. Although the addition of these inorganic salts produced a significant drop in pH, the degradation rates with salts were much faster than could be accounted for by a pH change. For example, the rate constants for the disappearance of xylose and xylotriose with 0.8% FeCl₃ were 3-fold and 7-fold greater, respectively, than for treatment with very dilute sulfuric acid at the same pH. In addition, xylose losses were greater than could be accounted for by just furfural production, suggesting that other degradation products were also formed, and xylose losses to unidentified compounds increased significantly with the addition of FeCl₃. The unidentified compounds could be formed through aqueous furfural resinification and condensation reactions that are accelerated by FeCl₃, but the actual mechanisms are still not clear.

Full text of DOI:10.1016/j.carres.2006.07.017

Carbohydrate Research 341 (2006) 2550–2556
The enhancement of xylose monomer and xylotriose degradation
by inorganic salts in aqueous solutions at 180 ꢀC
Chaogang Liu and Charles E. Wyman*
Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, USA
Received 10 March 2006; received in revised form 21 July 2006; accepted 30 July 2006
Available online 22 August 2006
Abstract—The inorganic salts KCl, NaCl, CaCl2, MgCl2, and FeCl3, and especially the latter, significantly increased xylose mono-
mer and xylotriose degradation in water heated to 180 ꢀC with unaccountable losses of xylose amounting to as high as 65% and 78%
for xylose and xylotriose, respectively, after 20 min incubation with 0.8% FeCl3. Furthermore, losses of both xylose and xylotriose
were well described by first order homogeneous kinetics, and the rate constants for xylose and xylotriose disappearance increased 6-
and 49-fold, respectively, when treated with 0.8% FeCl3 solution compared to treatment with just pressurized hot water at the same
temperature. Although the addition of these inorganic salts produced a significant drop in pH, the degradation rates with salts were
much faster than could be accounted for by a pH change. For example, the rate constants for the disappearance of xylose and xylo-
triose with 0.8% FeCl3 were 3-fold and 7-fold greater, respectively, than for treatment with very dilute sulfuric acid at the same pH.
In addition, xylose losses were greater than could be accounted for by just furfural production, suggesting that other degradation
products were also formed, and xylose losses to unidentified compounds increased significantly with the addition of FeCl3. The
unidentified compounds could be formed through aqueous furfural resinification and condensation reactions that are accelerated
by FeCl3, but the actual mechanisms are still not clear.
ꢁ 2006 Published by Elsevier Ltd.
Keywords: Sugar degradation; Xylose; Xylotriose; Inorganic salts; Ferric chloride
1. Introduction
hemicellulose is in the form of oligomers.6,10 However,
the total yield of xylose monomer and oligomers is only
about 65% for batch pretreatment of corn stover,2 and
xylan losses of about 15–20% were reported for the pre-
treatment of poplar and agricultural residues by uncata-
lyzed steam explosion.11 Furthermore, for biomass
pretreatment with liquid hot water, the total xylose yield
decreased with increasing solid concentration.12 These
low hemicellulose sugar yields are apparently due to
the heterogeneous nature of the hemicellulose hydrolysis
reactions and the rapid decomposition of sugars derived
from hemicellulose.2,11 Temperature, acid concentra-
tion, and residence time all contribute to the degrada-
tion of monomeric sugars including xylose, glucose,
galactose, arabinose, and mannose.13 Degradation of
monomeric sugars in dilute acids at a low pH (<1) has
been extensively studied, and these reactions are
normally considered to be general acid–base catalyzed
reactions with monosaccharide degradation following
Biomass pretreatment is one of the most important but
expensive unit operations in the processing of cellulosic
biomass to ethanol via biological methods.1 Biomass
pretreatment with just hot water or steam (180–
230 ꢀC), or autohydrolysis, is a relatively straightfor-
ward process that has been widely applied in biomass
fractionation, pulp and paper making, fiberboard pro-
duction, and the animal feed industry.2,3 Autohydrolysis
can solubilize most of the hemicellulose,4 remove a large
fraction of lignin,5,6 and produce a very digestible
fiber.7–9 For biomass pretreatment with just hot water
or very dilute acid, a large fraction of the solubilized
*
Corresponding author at present address: Bourns College of Engi-
neering, Center for Environmental Research and Technology,
University of California, 1084 Columbia Avenue, Riverside, CA
92507, USA. Tel.: +1 951 781 5703; e-mail: cewyman@engr.ucr.edu
0008-6215/$ - see front matter ꢁ 2006 Published by Elsevier Ltd.
doi:10.1016/j.carres.2006.07.017

C. Liu, C. E. Wyman / Carbohydrate Research 341 (2006) 2550–2556
2551
1
first-order kinetics over a pH range of 1–4 and temper-
atures of 170–230 ꢀC.14 Reaction rates are a minimum
over a pH range of 2–2.5, and above pH 2.5, monosac-
charide degradation becomes more rapid due to the
base-catalyzed reaction.15 However, few papers have re-
ported degradation reactions in aqueous solution at a
high temperature and a near neutral pH, although there
is evidence that sugar oligomers directly degrade in just
hot water at neutral pH.16
Sugar loss is also observed in sucrose processing, and
inorganic salts have been shown to increase its thermal
degradation, even at moderate conditions (pH 7.0,
690 ꢀC).17,18 Many forms of lignocellulosic biomass,
including agricultural residues, have a high ash content
(ꢀ6%), with the major cations being calcium, potassium,
magnesium, sodium, alumina, and iron, and most of the
cations are considered to be bound to inorganic anions
(SO42À, PO43À, Cl1À, OH1À). A number of researchers
have reported the impact of inorganic salts on thermal
degradation of cellulose and hemicelluose for biomass
gasification.19,20 It has been shown that minerals in lig-
nocellulosic biomass neutralize a portion of the added
acid during acid-catalyzed biomass pretreatment, thus
impacting hemicellulose hydrolysis and enzymatic diges-
tion of pretreated substrates.21–23 However, little has
been reported on how such inorganic salts affect the deg-
radation of hemicellulose sugars released during bio-
mass pretreatment.
0.1
0.01
water only
0.8% KCI
0.8% NaCl
0.8% CaCI22
0.001
0.0001
0.8% MgCI2
2
0.8% FeCl33
0
10
20
30
40
50
60
Time, min
Figure 1. The effect of addition of inorganic salts on degradation of
xylose in hot water at 180 ꢀC.
same temperature and same reaction time (see Fig. 5).
Overall, the impact of the different salts increased in
the following order: NaCl, KCl, CaCl2, MgCl2, and
FeCl3.
A similar trend was observed for treatment of xylotri-
ose with hot water at 180 ꢀC, as illustrated in Figure 2.
However, although the order of impact was the same
as for xylose, all the salts had a greater impact on xylo-
triose than on xylose. Furthermore, addition of 0.8%
FeCl3 at 180 ꢀC resulted in an almost 100% reaction of
xylotriose after 1 min while it took more than 20 min
to react 60% of the xylotriose when only water was
added (see Fig. 6).
Because xylose monomer and oligomers are the pri-
mary products in hemicellulose hydrolysis and biomass
pretreatment with just hot water, the fates of xylose
and xylotriose were followed at various concentrations
of inorganic salts (KCl, NaCl, CaCl2, MgCl2, and
FeCl3) in this study to shed light on how the latter affect
reactions of sugar oligomers and monomers that are
derived during hemicellulose hydrolysis in hot water at
180 ꢀC. In addition, degradation rate constants were
determined from the xylose and xylotriose data to better
understand how salts alter reaction pathways.
2.2. Determination of reaction rate constants
As shown in Figures 1 and 2, a plot of ln(X1/X1,0) versus
time and ln(X3/X3,0) versus time, where X1,0 and X3,0 are
the initial xylose and xylotriose concentrations, respec-
tively, and X1, X3 represent xylose and xylotriose con-
centrations at any time, respectively, followed a linear
relationship. Thus, xylose and xylotriose degradation
in hot water with and without added inorganic salts
can be described as first-order homogeneous reactions.
Accordingly, reaction rate constants were determined
by minimizing the sum of the squares of the differences
between predictions of first-order homogeneous kinetic
models and the experimental data using the Solver rou-
tine in Microsoft Excel. As shown in Table 1, the rate
constant for treatment of xylose with just hot water at
180 ꢀC was 0.0200 minÀ1. However, addition of 0.8%
KCl, NaCl, CaCl2, MgCl2, and FeCl3 increased the rate
constants by factors of 1.3, 1.2, 1.4, 1.8, and 6.1, respec-
tively. A similar trend was observed for the addition of
these salts to xylotriose, with the enhancement ratios
now being even greater at 1.5, 1.4, 2.2, 2.4, and 48.7,
respectively. Thus, we see that all of these salts pro-
moted the reaction of xylose and xylotriose and that
2. Results and discussion
2.1. Impact of inorganic salts on xylose and xylotriose
disappearance
Figure 1 summarizes some of our data on the treatment
of xylose with hot water at 180 ꢀC with and without
addition of KCl, NaCl, CaCl2, MgCl2, or FeCl3. As ex-
pected, the xylose concentration decreased with increas-
ing reaction times because of the decomposition of
xylose at high temperatures, but all of the inorganic salts
increased the rate of xylose decomposition. Further-
more, FeCl3 had a particularly strong effect, with 90%
of the initial xylose disappearing after 20 min versus
only about 40% for addition of just hot water at the

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C. Liu, C. E. Wyman / Carbohydrate Research 341 (2006) 2550–2556
1
0.1
water only
0.8% KCI
0.8% NaCl
0.01
0.8% CaCI22
0.8% MgCI22
0.8% FeCl33
0.001
0
10
20
30
40
50
60
Time, min
Figure 2. The effect of addition of inorganic salts on degradation of xylotriose in hot water at 180 ꢀC.
Table 1. Rate constants for the disappearance of xylose and xylotriose when reacted in hot water containing inorganic salts at 180 ꢀC
Solvent
pH
Xylose
Rate constant (minÀ1
Xylotriose
Rate constant (minÀ1
)
Ratio to control
)
Ratio to control
Water only (control)
0.8% KCl
0.8% NaCl
0.8% CaCl2
0.8% MgCl2
0.8% FeCl3
6.98
5.68
5.72
5.59
5.87
1.86
0.0200
0.0251
0.0239
0.0281
0.0360
0.1229
—
0.0458
0.0690
0.0650
0.1012
0.1082
2.2304
—
1.3
1.2
1.4
1.8
6.1
1.5
1.4
2.2
2.4
48.7
addition of FeCl3 significantly increased the rate of dis-
appearance of xylotriose.
sulfuric acid, and the FeCl3 solution shown in Figure
4 also revealed that FeCl3 has a larger influence on reac-
tion than does the use of low pH water alone. However,
the addition of sulfuric acid had a greater relative effect
on the disappearance of xylotriose compared to xylose,
and the rate constants for xylotriose disappearance were
0.0458, 0.3159, and 2.2304 minÀ1 for treatment with just
water, pH-adjusted water, and 0.8% FeCl3, respectively,
at 180 ꢀC.
The large differences in the effects of 0.8% FeCl3 and
water adjusted to the same pH suggested that a different
mechanism may exist for the reaction in the presence of
FeCl3 versus reaction at a low pH without adding this
salt. Two mechanisms seemed possible: (1) FeCl3 im-
pacts the structure of water17,24 and (2) FeCl3 catalyzes
the dehydration of carbohydrates. However, the fact
that xylotriose disappears much faster than xylose under
the same conditions and that the relative effects of acid
are different for xylose and xylotriose suggests that the
mechanisms for FeCl3 action could be different for the
two carbohydrates. We plan a more in depth investiga-
tion to try to explain these differences in the future.
2.3. Enhancement of reactions by FeCl3
As reported in Table 1, all of the solutions containing
inorganic salts were acidic, but the pH of 0.8% FeCl3
solution was particularly low at 1.86. Thus, we might ex-
pect that the low pH could account for the enhanced
degradation of xylose and xylotriose. To test this
hypothesis, the pH of HPLC grade water (pH 6.98)
was first adjusted to a value of 1.86 by adding dilute sul-
furic acid and then used to treat xylose and xylotriose at
the same temperature as used previously. The rate con-
stants were determined for the reaction of xylose and
xylotriose in this water without the addition of any salts
and compared to the rate constants for just the water at
pH 6.98 and for the addition of 0.8% FeCl3 (Fig. 3). As
expected, xylose degraded more rapidly in water with its
pH adjusted to 1.86 by the addition of sulfuric acid, and
the rate constant of disappearance of initial xylose
increased to 0.0332 minÀ1 versus 0.0200 minÀ1 for treat-
ment of xylose with just HPLC grade water (pH 6.98)
at the same temperature. However, the rate constant
was even higher (0.1229 minÀ1) when xylose was treated
with 0.8% FeCl3 at the same temperature. The results
for treatment of xylotriose with stock HPLC grade
water, HPLC grade water adjusted to a low pH with
2.4. Xylose losses and degradation products
The xylose remaining, furfural recovered, and unac-
counted for xylose losses were determined as a percent
of the initial xylose added over time for treatment of xy-

C. Liu, C. E. Wyman / Carbohydrate Research 341 (2006) 2550–2556
2553
1
-1
Rate constant was 0.0200 min , for
0.9
treatment with just water (pH 6.98)
0.8
-1
Rate constant was 0.0332 min , for
0.7
treatment with water adjusted to
pH 1.86 with dilute acid
0.6
0.5
0.4
0.3
-1
0.2
Rate constant was 0.1229 min
for treatment with 0.8% FeCl
(pH 1.86)
,
3
0.1
0
0
10
20
30
40
50
60
Time, min
Figure 3. The enhancement of xylose disappearance by 0.8% FeCl3 in hot water at 180 ꢀC.
1
-1
0.9
Rate constant was 0.0458 min , for
treatment with just water (pH 6.98)
0.8
0.7
0.6
-1
Rate constant was 0.3159 min , for
treatment with water adjusted to
pH 1.86 with dilute acid
0.5
0.4
0.3
-1
0.2
Rate constant was 2.2304 min
,
for
treatment with 0.8% FeCl (pH 1.86)
3
0.1
0
0
10
20
30
Time, min
40
50
60
Figure 4. The enhancement of xylotriose disappearance by 0.8% FeCl3 in hot water at 180 ꢀC.
lose with hot water at 180 ꢀC, with and without added
salts, as shown in Figure 5. As expected, the yield of fur-
fural, a key degradation product, increased with reac-
tion time. However, a large fraction of xylose losses
was not accounted for by the increase in furfural con-
centration, and these unidentified xylose losses increased
significantly with FeCl3 addition. For example, when xy-
lose was treated with 180 ꢀC water containing 0.8%
FeCl3 for 20 min, the loss of xylose increased to 65%
versus about 32% for treatment of xylose with just hot
water at the same temperature and residence time
(Fig. 5b and a).
This enhancement in the unaccountable xylose loss
was also observed for xylotriose treatment with 0.8%
FeCl3, as shown in Figure 6, in terms of percent of the
xylotriose present initially. In this case, xylotriose degra-
dation at this temperature produces xylobiose, xylose,
and furfural, with the relative amounts depending on
the reaction time. For example, when xylotriose was
treated with hot water at 180 ꢀC for less than 5 min, pri-
marily xylotriose and xylobiose could be measured, but
almost 15% of the material added initially could not be
accounted for. After 5 min, xylose and furfural were also
produced, and both increased with an increase in the
reaction time after that (Fig. 6a). This result is consistent
with the traditional concept that a higher degree of poly-
merization (DP) oligomers first degrade to lower DP
oligomers and then to monomer and furfural.25 How-
ever when xylotriose was treated with hot water contain-
ing 0.8% FeCl3 at the same temperature, xylotriose
disappeared completely in only 1 min, and xylose, the
only degradation product measurable at that time, could
not nearly account for all of the xylotriose added ini-
tially (Fig. 6b). This result showed that FeCl3 addition
significantly increased direct degradation of xylotriose.
Comparing Figures 5 and 6 reveals that less total xy-
lose including monomers and oligomers was lost for
treatment of xylotriose than for treatment of xylose
with just hot water at 180 ꢀC (24% vs 32% after
20 min). However, for treatment with water containing
0.8% FeCl3, total xylose losses were greater for the reac-
tion of xylotriose than of xylose. For example, xylose
losses were 78% when xylotriose was treated with hot
water containing 0.8% FeCl3 for 20 min versus losses
of 68% for treatment of xylose at otherwise identical
conditions.

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C. Liu, C. E. Wyman / Carbohydrate Research 341 (2006) 2550–2556
xylose remaining, % furfural, % xylose loss, %
xylose remaining, %
furfural, %
xylose loss, %
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
5 min
10 min 15 min
(a) hot water
20 min
5 min
10 min 15 min
(b) 0.8% FeCl3
20 min
Figure 5. The effect of 0.8% FeCl3 on xylose remaining, furfural formation, and unaccountable xylose losses for treatment of xylose with hot water at
180 ꢀC.
x3, % x2, % x1, % furfural,%% xxyylose lossss,,%
x3, % x2, % x1, % furfural, % xylose loss, %
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
1 min 5 min 10 min 15 min 20 min
(a) hot water
0.5 min 1 min 5 min 10 min 15 min 20 min
(b) 0.8% FeCl3
Figure 6. The effect of 0.8% FeCl3 on xylobiose (X3) remaining, xylobiose (X2) formation, xylose (X1) release, furfural production, and
unaccountable xylose losses for treatment of xylotriose with hot water at 180 ꢀC.
2.5. Mechanism for ‘xylose loss’
reactions that occur for furfural formation in aqueous
systems.26 In any event, it is clear that inorganic salts,
and particularly FeCl3, significantly increased produc-
tion of unidentified compounds (Fig. 7) suggesting that
inorganic salts not only catalyze furfural formation but
also accelerate furfural resinification and condensation.
It was reported that ash removal could reduce char for-
mation during biomass pyrolysis,27 indicating that
inorganic salts may also catalyze the formation of
chars at high temperatures. Furthermore, the chars
produced may also account for some of the xylose
disappearance. In the future, we plan to seek other
ways to characterize the unidentified degradation prod-
ucts and develop a mechanism that could explain why
inorganic salts and particularly FeCl3 accelerate sugar
losses.
It was hypothesized that intermediates or sugar–salt
complexes could be produced before the formation of
furfural that may account for the unaccounted xylose
losses. However, our HPLC detected no other products
before any furfural for short residence times. In addi-
tion, the peak heights were the same for sugars dis-
solved with salt addition as with just deionized water,
suggesting no sugar–salt complexes were formed. We
did observe at least two unidentified peaks before the
furfural peak for treatment of xylose or xylotriose in
hot water with or without the addition of organic salts,
for increased residence times (Fig. 7). The unidentified
compounds could be furfural resins formed by mecha-
nisms consistent with resinification and condensation

C. Liu, C. E. Wyman / Carbohydrate Research 341 (2006) 2550–2556
2555
9.00
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
X1 in DI water, 0 min
X1 in DI water, 40 min
X1 in 0.8% KCl solution, 40 min
X1 in 0.8% MgCl2 solution, 40 min
X1 in 0.8% FeCl3 solution, 40 min
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
55.00
60.00
65.00
70.00
Minutes
A
B
C
D
A: xylose (X1); B, C: unidentified compounds; D: furfural
a) xylose (X1) degradation
9.00
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
X3 in 0.8% FeCl3 solution, 2 min
X3 in 0.8% MgCl solution, 40 min
X3 in 0.8% KCl solution, 40 min
X3 in DI water, 40 min
X3 in DI water, 0 min
30.00
35.00
40.00
45.00
50.00
55.00
60.00
65.00
70.00
0.00
5.00
10.00
15.00
20.00
25.00
Minutes
1
2
3 4 5
6
1: xylotriose (X3); 2: xylobiose (X2); 3: xylose (X1); 4, 5: unidentified compounds; 6: furfural
b) xylotriose (X3) degradation
Figure 7. Chromatographs for degradation of xylose (X1) and xylotriose (X3) in hot water at 180 ꢀC with or without the addition of inorganic salts.
3. Conclusions
4. Experimental
A number of inorganic salts significantly accelerated
degradation of xylose monomer and xylotriose in hot
water at 180 ꢀC. The addition of 0.8% FeCl3 had a par-
ticularly strong effect, with the rate constants for the dis-
appearance of xylose and xylotriose increasing 6.1- and
49-fold, respectively. In addition, FeCl3 addition re-
sulted in significantly greater losses of xylose to uniden-
tifiable compounds for treatment of xylose monomer
and xylotriose at 180 ꢀC. Furthermore, the addition of
FeCl3 increased losses of xylotriose to unknown com-
pounds even more than xylose. The unidentified com-
pounds could be furfural resins formed by aqueous
furfural resinification and condensation reactions that
are accelerated by salts. However, the mechanisms are
still not clear, and work is ongoing to define a pathway
that could account for the enhancement of xylose mono-
mer and xylotriose degradation by inorganic salts.
4.1. Materials
Pure monomeric xylose (purity P 99.9%) was obtained
from Sigma Chemical Co. (St. Louis, MO). Furfural
(purity > 99.9%) was obtained from Fisher Scientific
(Chicago, IL). Xylobiose and xylotriose (purity ꢀ 99.5%)
were purchased from Magazyme International Ireland,
Limited (Bray, County Wicklow, Ireland). All inorganic
salts (KCl, NaCl, CaCl2Æ2H2O, MgCl2Æ6H2O, FeCl3Æ
7H2O) were obtained from Sigma Chemical Co. (St.
Louis, MO). HPLC grade water was purchased from
Fisher Scientific Co. (Chicago, IL).
4.2. Experimental operation
HPLC grade water was added to pure xylose and xylo-
triose to make solutions with concentrations of 0.0266

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C. Liu, C. E. Wyman / Carbohydrate Research 341 (2006) 2550–2556
and 0.0228 mol/L, respectively. Then, additional HPLC
grade water containing 1.6 wt % KCl, NaCl, CaCl2,
MgCl2 or FeCl3 was added to each sugar solution to re-
duce the sugar concentration to half its initial value.
Additional solutions were prepared at the same dilutions
but using just HPLC grade water without added salts to
provide controls with the same xylose and xylobiose
concentrations.
Aliquots (600 lL) of the xylose or xylotriose solu-
tions described above were added to flat bottom clear
glass crimp vials and sealed with aluminum crimp tops
with TFE/silicone liners (7 mm · 40 mm, 800 lL, All-
tech Associates Inc.; Deerfield, IL). These tiny reactors
were then rapidly heated to the target temperature of
180 ꢀC by immersing them in a 22.8 cm ID · 35 cm
deep 4 kW model SBL-2D fluidized sand bath (Techne
Corporation, Princeton, NJ). After the appropriate
reaction time, the vials were immediately removed from
the sand bath and submerged in ice water to quench
the reaction. The contents of the vials were removed
with a syringe and transferred to a centrifuge vial fitted
with a 0.2 lm nylon filter. Enough calcium carbonate
was added to each vial to adjust the pH to a range
of 5–6. After centrifugation supernatants were trans-
ferred to 0.5 mL polyethylene HPLC vials for HPLC
analysis.
Thayer School of Engineering at Dartmouth College
for providing facilities and equipment for this study.
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Acknowledgements
The authors would like to thank the Japanese New En-
ergy and Industrial Technology Development Organiza-
tion (NEDO) International Joint Research Program for
supporting this research. We are also grateful to the
National Institute of Standards and Technology for
supporting the purchase of much of the equipment
employed through award 60NANB1D0064, and to the