Arabinogalactan hydrolysis and hydrolytic hydrogenation using functionalized carbon materials

Article abstract of DOI:10.1016/j.cattod.2014.07.019

Hydrolysis of the hemicellulose arabinogalactan was studied in this work over several functionalized carbon materials, which were specifically treated to increase their acidities. Hydrolytic hydrogenation of arabinogalactan was investigated using the same materials in a mechanical mixture with ruthenium supported on active carbon. Application of these mixtures resulted in formation of polyols, suppressing simultaneously the generation of side products hydroxymethylfurfural (HMF) and furfural. Formation of high molecular weight compounds (aggregates of sugars and humins) was still quite substantial with a mechanical mixture of Ru/C and a carbon material prepared from sucrose by activation with zinc chloride to increase porosity. Post-treatment of this carbonaceous material with sulphuric acid significantly influenced kinetics of high molecular weight products formation resulting also in elevation of sugar alcohols yields.

Full text of DOI:10.1016/j.cattod.2014.07.019

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Arabinogalactan hydrolysis and hydrolytic hydrogenation using
functionalized carbon materials
D.Yu. Murzina,∗, E.V. Murzinaa, A. Tokareva, N.D. Shcherbanb, J. Wärnåa,c, T. Salmia
a Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Department of Chemical Engineering,
Åbo Akademi University, FI-20500 Åbo/Turku, Finland
b L.V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences, 03028, Kiev, Ukraine
c Umeå University, SE-901 87 Umeå, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrolysis of the hemicellulose arabinogalactan was studied in this work over several functionalized
carbon materials, which were specifically treated to increase their acidities. Hydrolytic hydrogenation
of arabinogalactan was investigated using the same materials in a mechanical mixture with ruthenium
supported on active carbon.
Received 14 February 2014
Received in revised form 25 June 2014
Accepted 16 July 2014
Available online xxx
Application of these mixtures resulted in formation of polyols, suppressing simultaneously the gener-
ation of side products hydroxymethylfurfural (HMF) and furfural. Formation of high molecular weight
compounds (aggregates of sugars and humins) was still quite substantial with a mechanical mixture of
Ru/C and a carbon material prepared from sucrose by activation with zinc chloride to increase porosity.
Post-treatment of this carbonaceous material with sulphuric acid significantly influenced kinetics of high
molecular weight products formation resulting also in elevation of sugar alcohols yields.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Arabinogalactan
Bifunctional catalysis
Biomass upgrading
Mesoporous carbon
Ruthenium
1. Introduction
industrially [1,4] using for example mineral acids such as H2SO4 as
catalysts [9]. However, several disadvantages (corrosion, treatment
Catalytic conversion of lignocellulosic biomass gained a recent
revival in the interest due to a general interest in catalytic valo-
rization of biomass [1–3], especially because lignocellulose does
not compete with food resources. The main chemical components
of lignocellulosic biomass are cellulose (40–50%), hemicelluloses
(25–35%) and lignin (23–33%), as well as minor amounts (<5%) of
extractives and mineral matter [1–3].
Among various options for transformation of biomass, mild
hydrolysis of cellulose and hemicellulose is one of the most promis-
ing since it leads to depolymerisation yielding a pool of molecules
which could be used as platform chemicals. A review on hydroly-
sis of cellulose as an entry point to biorefinery schemes has been
published recently [4]. During the acid hydrolysis of cellulose and
hemicelluloses, the glycosidic bonds between the sugar units are
cleaved to form partially hydrolyzed oligomers and, if the hydrol-
ysis is complete, to produce the respective monosaccharides.
The main focus in the literature is on hydrolysis of cellulose [5],
even if hemicelluloses are easier to hydrolyze than cellulose, due to
their branched and non-crystalline structures [6–8]. Hydrolysis of
lignocellulose using homogeneous acid catalysts was implemented
of generated waste, etc.) call for the introduction of heterogeneous
catalysts to conduct this reaction in an aqueous environment with-
out the above mentioned drawbacks [4,10]. A range of solid acids
has been used for hydrolysis of cellulose, including metal oxides,
polymer based acids, heteropolyacids, proton forms of zeolites
and sulfonated carbonaceous based acids as discussed recently in
a comprehensive review [10]. Due to cellulose crystallinity, ball-
milling is typically applied prior to hydrolysis. Among the materials
tested sulfonated carbons or silica/carbon nanocomposites dis-
played high activity and good recyclability [10,11]. Hemicelluloses
can be even easier to hydrolyze using heterogeneous catalysts, as
demonstrated for arabinogalactan [12] and xylan [13] with sulfonic
acid functionalized polymers and for xylan with proton form of zeo-
lites [13]. For xylan it was concluded that leaching of sulfonic groups
is rather strong, while proton form zeolites were much more stable
and active.
One of the attractive options to increase the selectivity in hydrol-
ysis of either cellulose or hemicellulose is to combine hydrolysis
and hydrogenation to a single step using only water as a solvent and
molecular hydrogen [14–29]. This bifunctional catalysis leading to
sugar alcohols helps to avoid generation of furan compounds, which
are formed on the acid sites favouring dehydration of the sugars.
Such strategy was pioneered by Balandin et al. [14] and reintro-
duced by Fukuoka and Dhepe [15–17]. The first step in the one-pot
∗
Corresponding author. Tel.: +358 22154985.
E-mail address: dmurzin@abo.fi (D.Yu. Murzin).
http://dx.doi.org/10.1016/j.cattod.2014.07.019
0920-5861/© 2014 Elsevier B.V. All rights reserved.
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process is hydrolysis of cellulose or hemicellulose into free sugars
in the liquid phase (in the presence of a diluted mineral acid) and/or
over an acidic support material. Hydrogenation of the free sugars
affording polyols takes place on the active metal sites deposited on
the support material. Typically transition metals (Pt or Ru) are used
as the active phase for hydrogenation [5]. There are quite a number
of studies on hydrolytic hydrogenation of cellulose and hemicel-
lulose using a variety of solid acids, such as acid-functionalized
mesoporous silica [18], zeolites and mesoporous materials [19–25].
Various carbon supported metal catalysts have been applied for
hydrolytic hydrogenation of cellulose [26–32].
Changes of the acidic properties of the parent support after
deposition of a metal are well documented resulting in loss of
strong Bronsted and Lewis sites in case of metal modified zeolites
[33,34]. An interesting option for hydrolytic hydrogenation could
be thus to apply in hydrolytic hydrogenation a mechanical mixture
of a solid acid catalyst and the most effective catalyst for hydro-
genation of sugars, namely Ru/C. This has been done for hydrolytic
hydrogenation of cellulose using Ru/C and sulfonic acid functional-
ized mesoporous silica MCM-41 [35] or heteropoly acids [36]. In the
former case, leaching of sulfonic groups was very rapid, which also
implies that contribution of homogeneous catalysis to the hydrol-
ysis becomes significant.
Cellulose consists of polymerized glucose units, whereas hemi-
celluloses are hetero-polymers made up of different units of
hexoses and pentoses. Arabinogalactan appears in large quanti-
ties in larch species such as Larix sibirica. The structural basis
of arabinogalactan is a backbone of ␤-d-galactopyranose with d-
galactopyranose and l-arabinofuranose side chains (Scheme 1). The
average molar ratio of galactose to arabinose in arabinogalactan is
about 6:1 and the molar mass is 20 000–100 000 g/mol [4]. Arabino-
galactan can easily be extracted from wood chips with water under
moderate conditions in industrial scale [37].
In the previous work of the authors [22], Ru–beta zeolites with
different Si/Al ratio were used in hydrolytic hydrogenation of ara-
binogalactan because they contain Brønsted acid sites similar to
mineral acids such as HCl and H2SO4, i.e. well-known homogeneous
catalysts for hydrolysis of cellulose and hemicelluloses. Incorpora-
tion of ruthenium by impregnation resulted in the decrease of the
overall strength of the acid sites. Hydrolysis of AG was ascribed [22]
to the acid sites on the zeolite support material as well as homo-
geneous catalysis by H3O+. The latter is becoming more prominent
with the temperature increase being related to a decrease of pKw
value of water (14 at 298 K compared to 11.4 at 458 K). Ru sites
are needed for hydrogenation of the sugars and the introduction of
Ru metal influences the selectivity by suppressing the formation of
dehydration and degradation products.
0,15
0,10
0,05
0,00
-0,05
-0,10
-0,15
200
300
400
500
0
600
7
Temperature,С
Fig. 1. NH3-TPD profile of the sample CIII.
from sucrose [38]. The structure of the carbon is composed of a
hexagonal arrangement of 1-D carbon rods. CI material being con-
ventional mesoporous carbon is not acidic. Since it was proposed in
the literature that mesoporous carbons such as CMK-3 can catalyze
the hydrolysis of cellulose to glucose even in the absence of strong
acidity [39], it was interesting to test performance of this material
also for hydrolytic hydrogenation of hemicelluloses.
Mesoporous carbon (CII) was obtained via activation of sucrose
with zinc chloride. A weighted amount of sucrose was stirred in 60%
solution of ZnCl2 (mass ratio ZnCl2/sucrose was 7) for 30 min, then
it was evaporated at 85 ◦C for 1 h. Thereafter the obtained mixture
was heated at 110 ◦C for 1 day and subjected to pyrolysis in an inert
atmosphere (argon) for 3 h at 700 ◦C (heating rate 3 ◦C/min). After
cooling, the obtained samples were washed with distilled water,
then with 1.2 M hydrochloric acid and with hot (85–90 ◦C) distilled
water until the pH of the washing water was ∼6–7.
The treatment with zinc chloride as an activating agent is
applied in bulk carbonization of some organic compounds in order
to promote the development of porosity of carbon framework.
Among the substances that are easily polymerized besides cellulose
or lignin, also sucrose is often used, while such activating agents as
phosphoric acid or zinc chloride were found to be efficient [40].
Such carbon materials with well developed surface and high meso-
porous volume were described in details in our previous work [40].
During the material preparation HCl was used for dissolving of the
products of hydrolysis and subsequent thermal transformation of
zinc chloride. The obtained carbon CII is similarly to CI not acidic.
A mesoporous carbon catalyst (CIII) was prepared by sulfona-
tion of mesoporous carbon (CII) similar to a conventionally used
approach for treatment of for example CMK3 [39]. For sulfonation,
1 g of the carbon sample was added to 15 ml of concentrated sul-
phuric acid (98%). Then, the mixture was placed in a Teflon-sealed
autoclave and maintained at 180 ◦C for 24 h. The obtained black
product was filtered and washed with water up to neutral pH in
the wash water, and then dried at 100 ◦C overnight. The structure
and sorption properties of the sample CIII were almost the same
as for the starting material (the reduction of the sorption parame-
ters is not more than 10%). The concentration of acid sites in this
sample is about 2.9 mmol/g, which was determined through back
titration. In a typical procedure, a weighted amount of the sam-
ple (ca. about 0.5 g) was added into 20 ml of 0.1 M NaOH aqueous
solution. The resulting suspension was stirred at room tempera-
ture for 24 h until an equilibrium was reached and titrated by drop
wise addition of 0.1 M HCl. The acid strength of the sample was
determined by thermo-programmed desorption of ammonia (NH3-
TPD). The results (Fig. 1) showed that CIII had strong acid sites, as
ammonia was only desorbed at 400–600 ◦C.
As evident from the literature overview above, compared to cel-
lulose, there is almost no effort reported in the literature devoted
to hydrolytic hydrogenation of hemicelluloses using sulfonic acid
functionalized carbons containing Ru or applying a combination of
Ru/C and such functionalized carbons. The current work is aimed to
fill this gap by performing hydrolytic hydrogenation of one of the
hexose hemicellulose – arabinogalactan applying a one pot tandem
approach, i.e. combining a commercial Ru/C catalyst with several
functionalized nanoporous carbons. Hydrolysis of hemicellulose
was also carried out with the functionalized carbons alone in the
absence of a hydrogenation catalyst albeit in hydrogen atmosphere.
2. Experimental
2.1. Catalyst preparation
The CMK-3 material (CI), which is a carbon replica of meso-
porous molecular sieve SBA-15, was obtained by matrix synthesis
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3
Scheme 1. Structure of arabinogalactan.
hydrogen partial pressure of 20 bar at 185 ◦C. The stirring rate was
1000 rpm to minimize external diffusion affecting activity mea-
surements. When the desired temperature was reached, stirring
was applied and the reactant solution from the pre-reactor was fed
into the reactor. This was considered as the initial reaction time.
These conditions were chosen in order to make easier the direct
comparison of these results to the previously reported by our group
[20,22]. Liquid samples from the reaction mixture were periodi-
cally withdrawn for analysis. The amounts of withdrawn samples
were considered in the calculations of the reactant and product
concentrations.
A commercial Ru/C catalyst (4.6 wt% Ru, surface area 700 m2/g,
metal cluster size according to TEM 2–3 nm) was applied in the
mechanical mixtures with the carbon materials.
2.2. Catalyst characterization
Nitrogen and hydrogen adsorption isotherms with a purity of
99.99% were measured by volumetric method at 77 K up to atmo-
spheric pressure (р ∼ 1 × 10−3–760 Tоrr) on Sorptomatic 1990. The
specific surface area (SВET) was estimated by the BET equation.
The mesopore size was determined by Barrett–Joyner–Halenda and
Dollimore Heal methods. The mesopore size was determined by the
adsorption branch in the cases where the ad(de)sorption hysteresis
on the isotherms ended in the ultimate strength of the meniscus of
2.4. Product analysis
The liquid phase samples taken from the reaction mixture with
an Acrodisc LC 13 mm syringe through 0.45 ␮m PVDF membrane
filter were quantitatively analysed by HPLC without any further
pretreatment. Two different columns were used according to the
different groups of compounds that can be analysed by each one.
The Bio-Rad Aminex HPX-87C column connected to a refractive
index (RI) detector was used to analyse arabinogalactan, sugars,
sugar alcohols and furan compounds. A diluted calcium sulphate
CaSO4 solution was used as the mobile phase, with a concentration
of 1.2 mM. The eluent flow rate was 0.4 ml/min and the temper-
ature was set to 353 K. In order to analyse the acid compounds
and other degradation products, an Aminex cation H+ column was
used, with a 0.005 M sulphuric acid solution as mobile phase. The
eluent flow rate was 0.5 ml/min and the column temperature was
set to 338 K. The individual components were identified by GC–MS
analysis of the silylated components with a HP 6890-5973 GC-
quadrupole-MSD instrument (Hewlett-Packard, Palo Alto, CA, USA)
equipped with a HP-1 GC column (25 m × 0.20 mm i.d., 0.11 ␮m).
More details are given in [20,22]. The total organic carbon (TOC-V
CSN, Shimadzu) analysis was conducted to determine the amount
of total organic carbon dissolved in the liquid phase. The carbon
mass balance was calculated considering the concentration of ara-
binogalactan, sugars, polyols, furfurals and low molecular weight
compounds analysed by HPLC.
the liquid adsorbate (nitrogen) to break (at р/р ∼ 0.45–0.50). The
о
micropore size was calculated from Horvath–Kavazoe equation.
The values of the micropore volume were refined with the com-
parative t-plot method for porous materials which contain micro-
and mesopores in the one structure. The surface area, as well as
the pore volumes of the carbon materials are shown in Table 1. The
carbon materials are characterized by a high surface area and large
mesopore volume. More detailed characterization of CII sample is
provided in [40].
The metal loading of Ru/C catalyst was determined by ICP-
OES using a Spectro-CircoSCCD ICP-spectrometer. Approximately
50 mg of the sample was inserted into a teflon bomb; 4 ml of HF,
1 ml of HCl and 0.5 ml of HNO3 were added. The sample was dis-
solved in a microwave oven, and diluted with deionized water
and analysed in the spectrometer. The Ru particle morphology was
analysed by transmission electron microscopy (TEM) in a MET JEOL-
2000 EX II microscope. Analyzing the morphology of 100 particles
observed in these micrographs it was possible to determine the size
metal distributions.
2.3. Experimental setup
Hydrolysis in the presence of hydrogen over functionalized car-
bons and hydrolytic hydrogenation experiments over the same
materials and also 5%Ru/C were carried out in a 300 ml Parr auto-
clave reactor connected to a pre-reactor with a 200 ml volume. The
autoclave was equipped with a 0.5 ␮m filtered sampling outlet,
which prevented the small catalyst particles from passing through
it. The temperature was measured with a thermocouple and con-
trolled automatically (Brooks Instrument). At the beginning of each
experiment, 250 mg of the reactant, arabinogalactan, was dissolved
in 90 ml of deionized water, loaded to the pre-reactor and sub-
sequently bubbled with hydrogen. Thereafter, 200 mg of carbon
materials along with 200 mg of 5%Ru/C catalyst (when the later was
used) with a particle size below 63 ␮m to suppress internal diffu-
sion limitations were loaded into the reactor containing 10 ml of
deionized water. The total pressure was 31 bar, which implied the
3. Results and discussion
3.1. Hydrolysis and hydrolytic hydrogenation of arabinogalactan
Hydrolysis of arabinogalactan in the presence of hydrogen was
studied using different carbon containing materials. For the sake of
comparison, results in the absence of any catalysts as well as with
a beta zeolite (Si/Al ratio equal to 11) reported previously [22] are
also briefly discussed.
The acid nature of the solid catalysts is a parameter which can
play a role in the first step of hydrolysis of the hemicellulose.
The mechanism, which has been proposed for the acid catalyzed
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Table 1
Textural properties of carbon materials.
SBET, m2/g
Vt., cm3/g
Smeso, m2/g
Vmeso, cm3/g
Dmeso, nm
Vmicro, cm3/g
Dmicro, nm
SBA-15
CI
CII
600
1120
1557
1.00
1.22
1.59
540
865
1450
0.95
1.10
1.54
6.8
3.6
0.05
0.12
0.001
1.16
0.48
–
3.8 0.35.4 1.1
Vt. – total pore volume.
Smeso, Vmeso, Dmeso – mesopore specific surface area, volume and diameter.
Vmicro – micropore volume calculated by the method t-plot.
Dmicro – micropore diameter.
100
AG and humins
100
90
80
70
60
50
40
30
20
10
0
Oligomers
Galactose
Arabinose
75
AG and humins
Oligomers
Galactose
Arabinose
HMF
Furfural
50
25
0
0
50
100
150
200
250
0
50
100
150
200
250
time, min
time, min
Fig. 2. Non catalytic hydrolysis of arabinogalactan at 185 ◦C.
Fig. 3. Hydrolysis of AG at 185 ◦C in the presence of hydrogen with CI material.
hydrolysis of hemicelluloses in the aqueous phase, proceeds via
the protonation of the glycosic bonds between the sugar units in
the hemicellulose chain, resulting in the formation of t monosac-
charides [20].
the formed sugars are hydrogenated to corresponding alcohols and
that the formation of dehydration products is very minimal.
A noncatalytic experiment conducted at 458 K and total pressure
21 bar was described in ref. [22] demonstrating (Fig. 2) that even
in the absence of any acid catalyst arabinose and galactose were
formed along with oligomers.
3.2. Influence of functionalized carbon on hydrolysis and
hydrolytic hydrogenation
Modification of the carbon properties by thermal treatment
of sucrose in the presence of zinc chloride and following with
hydrochloric acid for removing the salt (CII carbon) significantly
changed the hydrolysis of arabinogalactan (Fig. 5).
As can be seen from Fig. 5, the conversion of AG is substan-
tially higher that for CI. Moreover, one could see clearly formation
of degradation products. An unusual behaviour of AG and the con-
centration profiles of humins can be associated with the fact that
the curve represents a sum of their concentrations. While AG is
depolymerized humins are formed simultaneously.
In [22] a general reaction network for hydrolytic hydrogena-
tion of arabinogalactan was proposed which included formation
of humins from furfural and five HMF as well as a possible for-
mation of aggregates (not separable from humins in HPLC) from
sugars and sugar alcohols due to hydrogenation of the former. In
the case of arabinogalactan hydrolysis in the absence of hydro-
gen and the metal catalyst this general scheme could be modified
(Fig. 6).
A set of differential equations considering for simplicity first
order reactions with concentrations directly taken from Fig. 5 can
be easily written based on the reaction network in Fig. 6. Numerical
data fitting of this set for the experimental data in Fig. 5 was done
with the backward difference method by minimization of the sum
of residual squares, SRS, with non-linear regression analysis using
the Simplex and Levenberg–Marquardt optimization algorithms
implemented in the a parameter estimation software ModEst [42].
The reason for such behaviour is the presence of in-situ gen-
erated protons which enhance the hydrolysis step even without
any heterogeneous catalyst. In the hydrolysis of hemicelluloses
H3O+ from water acts as a homogeneous acid catalyst. Besides the
main sugars originating from the hemicellulose also dehydration
products of sugars, such as furfural and 5-hydroxymethyl furfural
were detected in inferior quantities. Their formation can be also
attributed to H3O+ generated in-situ at high temperature.
A rather strange behaviour of the AG–hunins curve in Fig. 2 is the
attributed to co-production of polymeric humins, which are formed
during acid catalyzed conversion of hexoses with HMF [41] or
through transformations of HMF first to 2,5-dioxo-6-hydrohexanal
[10]. AG and humins cannot be separated in applied HPLC analysis
procedure as these polymers elute at the same time. It was men-
tioned in ref. [22] that when arabinose is removed from the side
chain of the hemicellulose it can lead to aggregation of galactose.
It was interesting to compare the hydrolysis in the case of blank
experiments with the results for CI carbon, which is not acidic. The
results presented in Fig. 3 clearly demonstrate that even if there is
a difference between a noncatalytic experiment and the case when
CI carbon is utilized, namely lower humins formation, the over-
all reaction also completely stops although at a somewhat higher
conversion (ca. 50% for CI and 75% for a noncatalytic experiment).
Introduction of ruthenium on carbon catalyst (Fig. 4) does not
substantially change the overall picture. Fig. 4 demonstrates that
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a)
AG and humins
AGhumins
Oligomers
Galactose
Arabinose
Unknown
HMF
Oligomers
Galactose
Arabinose
Unknown
Arabitol
100
80
60
40
20
0
100
a)
80
60
40
20
0
Galactitol
HMF
Furfural
Furfural
0
50
100
0
50
100
150
200
250
time, min
time, min
b)
Galactose
Arabinose
HMF
Galactose
Arabinose
Unknown
Arabitol
Galactitol
HMF
Furfural
20
16
12
8
b)
15
10
5
Furfural
4
0
0
0
50
100
150
200
250
0
50
100
time, min
time, min
Fig. 4. Hydrogenation of AG at 185 ◦C with CI material and Ru/C: a) all concentra-
tions, b) concentration of low molecular mass products.
Fig. 5. Hydrolysis of AG at 185 ◦C in hydrogen atmosphere with CII material: a) all
concentrations, b) concentration of low molecular mass products.
Fig. 6. Reaction network for hydrolysis of arabinogalactan.
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CI
CII
CIII
1,6
1,4
1,2
1,0
0,8
0,6
0
50
100
150
200
250
time, min
Fig. 8. Weight yields of arabinose and galactose for tested catalysts in hydrolysis of
AG and 185 ◦C.
negligible compared to other constants thus not being included in
the Table 2.
The galactose-to-arabinose ratio during hydrolysis is close to
unity (Fig. 8) and is much smaller than one would expect from
the stoichiometry (six in native AG). This implies that the release
a)
AG and humins
Oligomers
100
80
60
40
20
0
Fig. 7. Comparison between experimental data (Fig. 5) and calculations: a) higher
molecular weight compounds, b) oligomers and lower molecular weight com-
pounds.
0
50
100
ThesumofsquareswasminimizedstartingwithSimplexandthere-
after switching to Levenberg–Marquardt method.
time, min
Estimated parameters (numbering corresponds to reactions in
Fig. 6) and the standard errors are presented in Table 2, while the
results are displayed in Fig. 7, showing a good correspondence
between the experimental and calculated data with a degree of
explanation above 99%. The majority of parameters are rather well
identified, while few have large errors, which is understandable
taking into account a relatively limited set of experimental data. In
the parameter estimation the rate constants of steps 2 and 3 were
b)
Galactose
14
12
10
8
Arabinose
Arabitol
Galactitol
HMF
Furfural
Table 2
6
Values of parameters for Fig. 7.
Constant
Value
Error, %
4
k1
k4
k5
k6
k7
k8
1.77
11.0
>100
>100
>100
>100
7.3
0.99 × 10−7
2
1.48
9.05
7.06
2.65
0
0
50
100
k9
6.07
1.1
0.85
2.17
4.0
7.0
5.2
2.6
time, min
k10
k11
k12
k13
Fig. 9. Hydrogenation of AG at 185 ◦C with CII material and 5%Ru/C at 31 bar: a)
concentration of AG, humins and oligomers, b) concentration of low molecular mass
products.
0.38
>100
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a)
a)
AG and humins
Oligomers
100
100
80
60
40
20
0
AG and humins
Oligomers
80
60
40
20
0
0
50
100
150
200
250
0
50
100
150
200
250
time, min
time, min
18
16
14
12
10
8
b)
Galactose
Arabinose
HMF
b)
28
24
20
16
12
8
Furfural
Galactose
Arabinose
Arabitol
Galactitol
HMF
6
4
Furfural
2
4
0
0
50
100
150
200
250
0
time, min
0
50
100
150
200
250
time, min
Fig. 11. Hydrogenation of AG at 185 ◦C with CIII material and 5%Ru/C at 31 bar: a)
concentration of AG, humins and oligomers, b) concentration of low molecular mass
products.
Fig. 10. Hydrolysis of AG at 185 ◦C with CIII material: a) concentration of AG, humins
and oligomers, b) concentration of low molecular mass products.
with different carbon supports can be explained that not only H3O+
but also the acid sites of the support influence the hydrolysis.
From Fig. 9 it also follows that the ratio between arabitol and
galactitol is even higher than the ratio between corresponding
sugars, which in fact implies that arabinose is preferentially hydro-
genated under actual experimental conditions.
It was interesting to further investigate the catalytic properties
of carbon CII when sulfonic groups are introduced into this material
(Fig. 10).
This carbon material is the most acidic one, thus it could be
expected that hydrolysis would be most prominent in the presence
of it. It should be noted that in a special leaching test (the heating
of the catalyst in distilled water at 185 ◦C for 4 h) it was shown
that the amount of sulfonic groups leaching from carbon in the
experimental conditions is about 1–2%, which is within the error
of the performed titrations. Thus the catalytic influence should be
predominantly ascribed to the carbon material. It should be also
noted that the ratio between galactose and arabinose is somewhat
higher for CIII being still much lower than the stoichiometric one
observed previously for mineral acids used in hydrolysis under
milder conditions (ca. 100 ◦C). This result might imply that some
minor hydrolysis happens also in the liquid phase due to presence
of a homogeneous catalyst because of some inferior leaching of sul-
fonic groups. As can be seen from Fig. 10 the disappearance of AG is
rate of arabinose is higher than for galactose reflecting preferential
cleavage of the side chain containing the former sugar.
This behaviour was reported previously for AG hydrolysis using
an ion-exchange resin with sulphonic groups at milder conditions
(e.g. ca. 90 ◦C) [43]. Utilization of mineral acids such as HCl leads to
the ratio of sugars consistent with stoichiometry [43].
It was previously shown [22] that when hydrolysis is carried out
in water but in the presence of a Ru/C catalyst, the galactose- to-
arabinose ratio is ca. 2.5 decreasing from this value to ca. 1.25 when
an acidic Ru/beta zeolite is applied. Interestingly enough, the ratio
of sugars is much smaller for the tested carbon materials being even
below unity for CII, clearly illustrating the preferential cleavage of
side arabinose groups.
The same material in the presence of a commercial 5%Ru/C cat-
alyst accelerated the reaction leading to a higher conversion of
AG (Fig. 9). Moreover, introduction of Ru in the reaction mixture
completely suppressed the formation of furanic compounds and
resulted in formation of sugar alcohols. A similar influence of the
metal has been described for metal-modified Ru zeolites [22] also
giving polyols with fewer amounts of degradation products. The
hemicellulose, AG, is hydrolyzed first to the corresponding sug-
ars, arabinose and galactose, which are subsequently hydrogenated
to arabitol and galactitol. A clear difference between experiments
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not accompanied with any increase in the concentration of humins.
Not surprisingly formation of furans due to dehydration reactions
was not negligible.
When CIII functionalized carbon was used together with 5%Ru/C
catalyst, the rate of AG disappearance was almost the same
(Fig. 11a), while the formation of sugar alcohols was visible as well
as suppression of furan formation (compare Fig. 11b with Fig. 10b).
The combined yield of sugar alcohols was also higher than for the
mechanical mixture of CII and 5%Ru/C (Fig. 9).
The final carbon balance closure in the liquid phase with the
products that could be reasonably well identified was checked to
be around 90% with the least acidic catalyst CI and close to 70% for
the most acidic ones. The latter results imply that besides sugars,
sugar alcohols and furans also degradation products are present
in the reaction milieu. Previously with H-␤-11 some C2–C5 degra-
dation products from the released monomers were detected, such
as ethylene glycol, propylene glycol, glycerol, dihydroxy-propanoic
and butanoic acids, butanediol (-triol), pentanediol (-triol) and
their derivatives [22] being formed through various degradation
reactions. These reactions of unstable monosaccharides take place
at high temperatures and especially with increasing acidity of
catalysts.
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The majority of studies in the literature on hydrolytic hydro-
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Finally the experimental data for arabinogalactan hydrolysis
were modelled based on the proposed reaction network cover-
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oligomers and heavy molecular weight products eluting at the same
time as the initial substrate.
Acknowledgements
This work is part of the activities at the Åbo Akademi University
Process Chemistry Centre. Academy of Finland is acknowledged for
financial support within the framework of Finland–India research
program.
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Please cite this article in press as: D.Yu. Murzin, et al., Arabinogalactan hydrolysis and hydrolytic hydrogenation using functionalized
carbon materials, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.07.019