Structure sensitivity in catalytic hydrogenation of glucose over ruthenium

Article abstract of DOI:10.1016/j.cattod.2013.12.031

The structure sensitivity was studied in the hydrogenation of glucose to sorbitol over supported ruthenium catalysts in a semi-batch reactor. Ruthenium on carbon supports with different ruthenium particle sizes was prepared and evaluated in the hydrogenation experiments. The highest turnover frequency was obtained with a catalyst bearing average ruthenium particle size of ca. 3 nm.

Full text of DOI:10.1016/j.cattod.2013.12.031

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Catalysis Today
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Structure sensitivity in catalytic hydrogenation of glucose
over ruthenium
Atte Ahoa, Stefan Rogganb, Olga A. Simakovaa,c, Tapio Salmia, Dmitry Yu. Murzina,∗
a Åbo Akademi University, Process Chemistry Centre, Laboratory of Industrial Chemistry, Biskopsgatan 8, FI-20500 Turku, Finland
b Bayer Technology Services GmbH, 51368 Leverkusen, Germany
c Georgia Institute of Technology, School of Chemical & Biomolecular Engineering, 311 Ferst Drive NW, Atlanta, 30332-0100 GA, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure sensitivity was studied in the hydrogenation of glucose to sorbitol over supported ruthe-
nium catalysts in a semi-batch reactor. Ruthenium on carbon supports with different ruthenium particle
sizes was prepared and evaluated in the hydrogenation experiments. The highest turnover frequency
was obtained with a catalyst bearing average ruthenium particle size of ca. 3 nm.
© 2014 Elsevier B.V. All rights reserved.
Received 21 October 2013
Received in revised form
20 December 2013
Accepted 28 December 2013
Available online xxx
Keywords:
Glucose
Hydrogenation
Ruthenium on carbon
Structure sensitivity
1. Introduction
particle size. The TOF can be declining as in oxidation of glycerol [8],
increasing until reaching a plateau as in Fischer–Tropsch synthesis
Hydrogenation of glucose to sorbitol is an important industrial
process. Sorbitol can be used as an alternative sweetener in food,
intermediate in pharmaceutical production and as a humectant in
cosmetics. Hydrogenation of glucose or other sugars to correspond-
ing sugar alcohols can be performed over nickel and ruthenium
catalysts as well as other metals belonging to the platinum group.
Typically Raney-type Ni catalysts are used in the industrial produc-
tion of sorbitol from glucose [1–3]. The main reason for using nickel
in industry is the low costs. However, there are some disadvan-
tages in using nickel as a hydrogenation catalyst. It has been found
that nickel deactivates through a loss of active surface (sintering),
leaching of nickel into the reaction mixture and poisoning [1–3].
Leaching of nickel into the sorbitol solution increases the purifica-
tion costs since nickel is toxic. These drawbacks can be overcome
by using a different catalyst, for instance ruthenium supported on
carbon. Glucose hydrogenation to sorbitol over ruthenium cata-
lysts has been extensively investigated [3–7]. Despite the large
number of publications on hydrogenation over ruthenium cata-
lysts, no reports on the effect of ruthenium particle size could be
found in the open literature. The structure sensitivity can be inves-
tigated by plotting the turnover frequency (TOF) as a function of the
[9] or constant. However, interesting reports on reactions where
the TOF goes through a maximum can be found in the open lit-
erature. These reactions include hydrogenation of crotonaldehyde
[10] and ethene [11], oxidation of methane [12], deoxygenation of
palmitic and stearic acids [13] on different metals other than ruthe-
nium. The selectivity to cinnamyl alcohol in the hydrogenation of
cinnamaldehyde over ruthenium increased over larger ruthenium
particles supported on carbon, while the specific activity expressed
per exposed ruthenium atoms remained constant [14].
In this work, the influence of ruthenium nanoparticle size on the
catalyst activity for glucose hydrogenation is investigated. For this
purpose, ruthenium on carbon catalysts with ruthenium particle
sizes ranging from 1 to 10 nm were synthesized and tested.
2. Materials and methods
2.1. Ruthenium on carbon catalysts
Several different ruthenium on carbon catalysts were studied in
this work. Both commercial and in-house prepared catalysts were
tested and compared.
The commercial samples were a 4.6% ruthenium on activated
carbon (AC) catalyst and a 0.7% ruthenium on carbon extrudates
catalyst, denoted as Catalysts A and B. Prior to the experiments
the Catalyst B was crushed and sieved to a fraction smaller than
∗
Corresponding author. Tel.: +358 22154985.
E-mail address: dmurzin@abo.fi (D.Yu. Murzin).
0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.12.031
Please cite this article in press as: A. Aho, et al., Structure sensitivity in catalytic hydrogenation of glucose over ruthenium, Catal. Today (2014),
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A. Aho et al. / Catalysis Today xxx (2014) xxx–xxx
125 ␮m. The in-house prepared catalysts were made by deposition
of ruthenium on activated carbon, carbon nanotubes (CNT), and
on nitrogen doped carbon nanotubes (NCNT). The catalyst samples
were not separately reduced under hydrogen before the hydro-
genation experiments. However, heating of the reactor and the
catalyst was done in hydrogen atmosphere.
80 ml/min) was applied. The temperature was raised to 450 ◦C at a
3 ◦C/min heating ramp and was kept during 8 h.
2.2. Catalyst characterization
Ruthenium content in the solid catalysts was measured by
inductively coupled plasma optical emission spectroscopy (ICP-
OES) using a Spectro ICP spectrometer (Model: Arcos) at a mean
wavelength of Ru, which was determined from the following three
wavelengths of Ru: 240.272 nm, 267.876 nm, 245.553 nm.
The size and size distributions of the ruthenium nanoparti-
cles were measured by transmission electron microscopy (TEM).
TEM measurements were performed with a FEI TECNAI 20 elec-
tron microscope operated at an acceleration voltage of 200 kV.
LaB6 was used as the electron source. Digital images were taken
with a side mounted CCD camera (Olympus, MegaView III). For
TEM preparation a few milligrams of each sample were ultrasoni-
cated in 2 ml of ethanol during 15 min and a drop of this dispersion
was placed on a 300 mesh holey carbon coated copper TEM-grid
(plano S147-1). The mean sizes of Ru particles were estimated from
TEM micrographs by single particle measurement of at least 140
particles. The dispersion of the ruthenium nanoparticles was also
determined by CO chemisorption with a Micromeritics AutoChem
2910. A ratio of CO:Ru = 1:1 was used in the calculations. Prior to
the measurement the sample was reduced under hydrogen flow at
300 ◦C.
2.1.1. Catalyst C
The deposition of ruthenium on the activated carbon was per-
formed by precipitation of [RuCl3·HCl] with NH4OH, followed by
reduction of ruthenium hydroxychloro-complexes by NaBH4.
2.1.2. Catalyst D
CNTs were used as catalyst support materials in the preparation
of Catalyst D. Before deposition of ruthenium the CNTs were oxi-
dized by treatment with concentrated HNO3 (70%, Aldrich) in order
to increase the hydrophilicity by introduction of surface carboxylic
groups [15]. Ruthenium(IV) oxide was precipitated on the oxidized
CNT according to the method described by Fu et al. [16]: 30% H2O2
(Aldrich) aqueous solution was added dropwise to a stirred mixture
of RuCl3·H2O (40.9%, Haereus) and oxidized CNT in water. The rate
of addition was controlled to keep the temperature of the reaction
mixture ≤60 ◦C. After completed addition the mixture was stirred
at 80 ◦C for additional 3 h. After cooling to room temperature the
Ru/CNT material was filtered and repeatedly washed with water
and dried at 100 ◦C in an oven overnight.
2.3. Hydrogenation reactor set-up
2.1.3. Catalyst E
NCNT support material was prepared as described elsewhere
[17]. Ruthenium was deposited on NCNT adapting the method of
Fu et al. [16] outlined above.
Hydrogenation of glucose over different ruthenium on carbon
catalysts was investigated in a Parr 4561 autoclave (300 ml). The
autoclave was equipped with a gas entrainment impeller, baffles,
heating jacket and a cooling coil, sampling line, pressure, tem-
perature and stirring rate controllers. The glucose solution was
pre-heated and saturated with hydrogen in a separate chamber.
The catalyst sample was put in the reactor which was flushed with
nitrogen and hydrogen before heating. When the temperature was
120 ◦C the glucose solution was fed to the reactor and the pressure
was increased to 19 bar of hydrogen. Samples (1–2 ml) were peri-
odically withdrawn through a 0.5 ␮m sinter during the semi-batch
experiments. A constant 19 bar hydrogen (5.0, AGA) was applied
by controlled addition and the reaction temperature was 120 ◦C. A
0.1 mol/L 120 ml glucose (Fluka, ≥98% purity) solution was used.
The stirring rate was 1000 rpm. The experiments were carried out
between 120 and 180 min and the amount of catalyst was between
0.1 and 0.2 g.
2.1.4. Catalyst F
The Catalyst F was prepared in a three step procedure. In the first
step NCNTs were suspended in a solution of polyvinylpyrrolidone
(PVP) (58 000 g/mol, ABCR) in ethanol (Aldrich) and the mixture
was sonicated for 2 h. Then, the solid material was filtered off,
repeatedly washed with ethanol and dried in an oven at 100 ◦C.
The pretreated support material was mixed with an aqueous solu-
tion of RuCl3, sonicated for 2 h and stirred 4 h at room temperature.
Finally, after complete evaporation of the water, the (pre-) cata-
lyst was reduced by heating the NCNT-PVP-RuCl3 solid mixture at
195 ◦C in excess of ethylene glycol (Aldrich) during 2 h.
2.1.5. Catalyst G
The pH of sugar and the catalyst slurry was measured for some
cases being for example equal to 6.7 for Catalyst A. It is well known
that isomerization of glucose to fructose can occur at much higher
alkaline pH (ca. 12–13) as demonstrated in [18]. Such isomerization
leads to subsequent hydrogenation of fructose forming mannitol
and sorbitol [19]. Much milder conditions in the present work did
not thus result in formation of fructose, mannitol and degradation
products.
A
NCNT-PVP-RuCl3 mixture was analogously prepared as
described for Catalyst F. After drying the impregnated support, it
was put into a tube furnace and a flow of Ar (5.0 Linde) (80 ml/min,
30 min) was passed over it at room temperature in order to remove
air from the tube. Reduction was performed applying a mixture of
Ar/H2 (5.0 Linde) (H2 20 ml/min; Ar 80 ml/min), while the temper-
ature was increased to 450 ◦C by applying a 3 ◦C/min heating rate.
After reaching 450 ◦C, the temperature was maintained for another
4 h.
2.4. Glucose and sorbitol analysis
2.1.6. Catalyst H
The concentrations of glucose and sorbitol were determined by
high-performance liquid chromatography (HPLC) (HITACHI Chro-
master HPLC) equipped with an RI detector. A Biorad HPX-87C
carbohydrate column was used, the mobile phase was 1.2 mM
CaSO4. The temperature of the column was 70 ◦C and the flow rate of
the mobile phase 0.5 ml/min, the detector was at 40 ◦C. Calibrations
were made for glucose and sorbitol. The by-products, mannitol and
glycerol, were also analyzed by HPLC.
Catalyst H was prepared by a similar method as applied for Cat-
alyst G, but a different procedure was applied for reduction in order
to obtain larger ruthenium nanoparticles. A NCNT-PVP-RuCl3 mix-
ture was analogously prepared as described for Catalyst F and the
catalyst precursor placed into a tube furnace. In the air atmosphere,
the temperature was raised by 3 ◦C/min to 300 ◦C, kept at 300 ◦C for
2 h. Then a flow of Ar (80 ml/min, 30 min) was used to remove air
from the tube. Subsequently, a mixture of Ar/H2 (H2, 20 ml/min; Ar,
Please cite this article in press as: A. Aho, et al., Structure sensitivity in catalytic hydrogenation of glucose over ruthenium, Catal. Today (2014),
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Table 1
Catalyst characterization results.
Catalyst sample
Support
Ru loading (wt%)
Ru particle size (nm)
Catalyst A
Catalyst B
Catalyst C
Catalyst D
Catalyst E
Catalyst F
Catalyst G
Catalyst H
AC
AC
AC
CNT
NCNT
NCNT
NCNT
NCNT
4.6
0.7
2.0
3.3
3.6
4.1
3.6
3.4
2.5 (CO)
2.9 (CO)
10.0 (CO)
1.3 (TEM)
1.2 (TEM)
1.5 (TEM)
3.3 (TEM)
5.3 (TEM)
Fig. 4. TEM image of Catalyst E, ruthenium particle size 1.2 nm.
Fig. 1. TEM image of Catalyst A, ruthenium particle size 2.5 nm.
Fig. 5. TEM image of Catalyst F, ruthenium particle size 1.5 nm.
Fig. 2. TEM image of Catalyst B, ruthenium particle size 2.9 nm.
Fig. 6. TEM image of Catalyst G, ruthenium particle size 3.3 nm.
Fig. 3. TEM image of Catalyst D, ruthenium particle size 1.3 nm.
Fig. 7. TEM image of Catalyst H, ruthenium particle size 5.3 nm.
3. Results and discussion
3.2. Glucose hydrogenation results
3.1. Catalyst characterization results
The glucose conversion as a function of reaction time multi-
plied by the amount of ruthenium in the reactor is displayed in
Fig. 8. The screened catalysts showed different behavior in terms of
activity. Full conversion was achieved in most cases, although some
The ruthenium content and average particle size of the cata-
lysts are listed in Table 1. TEM images of the samples are shown in
Figs. 1–7.
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100
For such shape of nanoparticles the fraction of edges to the total
number of atoms on the surface, which besides edges also includes
square and triangular faces, is given by [20]
80
60
40
20
0
Nedges
Catalyst A
Catalyst B
Catalyst C
Catalyst D
Catalyst E
Catalyst F
Catalyst G
Catalyst H
fedges
=
(1)
Nedges + Nsquare faces + Ntriangular
faces
The fraction of edges can be described in a simplified way as
fedges ≈ 1/dcluster when d is given in nm [21].
It was previously shown [21] that the rate constants of adsorp-
tion and desorption are given respectively by
ꢀ
(ꢀG
ads,edges
−ꢀG
ads,terraces
)]/RT
kads = kadse[−˛f
;
edges
ꢀ
(ꢀG
ads,edges
−ꢀG
ads,terraces
)]/RT
kdes = kadse[(1−˛)f
(2)
edges
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
t*m_Ru (min*g_Ru)
where ˛ is the Polanyi parameter and ꢁGads,edges corresponds to
adsorption on edges. Eq. (2) in combination with (1) gives a possi-
bility to describe turnover frequency dependencies on cluster size
for different reaction mechanisms.
Fig. 8. Conversion vs. reaction time multiplied by the amount of ruthenium in the
reactor.
When hydrogenation kinetics follows Langmuir–Hinshelwood
type of mechanism with noncompetitive adsorption of hydrogen
and the organic substrate the reaction rate per exposed site is
expressed by
Table 2
Selectivity (%) to sorbitol over the screened catalysts.
Catalyst
Selectivity
kKGCGKH CH
Catalyst A
Catalyst B
Catalyst C
Catalyst D
Catalyst E
Catalyst F
Catalyst G
Catalyst H
96.1
95.5
28.8
87.6
94.9
92.1
92.4
89.0
2
2
v =
(3)
(1 + KGCG)(1 + KH CH
)
2
2
where k is the rate constant, KG and KH are the adsorption
coefficients and CG and CH the concentrations of glucose and
hydrogen, respectively. Based on the previous considerations
[21] for Langmuir–Hinshelwood kinetics the rate expression for
turnover frequency can be described as
2
2
+ꢂ )/d
ke˛(ꢂ
cluster KGCGKH CH
G
H
2
2
2
TOF =
(4)
CG)(1 + KH2 eꢂ
cluster CH
)
2
/d
H
2
(1 + KGeꢂ
/d
G
cluster
catalysts displayed lower activity, i.e. Catalyst C. The duration of the
experiments was between 120 and 180 min.
The highest activities were observed for Catalysts A, B and G sup-
ported respectively on active carbon and NCNT bearing the metal
cluster size close to 3 nm.
where ˛ is the Polanyi parameter for the surface reaction between
adsorbed sugar and hydrogen and parameters x account for dif-
ferences in the Gibbs energy of adsorption on edges and terraces
respectively for hydrogen and glucose
Selectivity to sorbitol is listed in Table 2. High selectivities, up to
96.1%, at full conversion were achieved with most of the catalysts.
Inferior selectivity in terms of hydrogenation was obtained with
Catalyst C, which in fact was very active in hydrogenolysis, resulting
in glycerol as the main product.
(ꢁGads,edges,G − ꢁGads,terraces,G
)
ꢂG
=
;
RT
(ꢁGads,edges,H − ꢁGads,terraces,H
)
2
2
ꢂH2
=
(5)
RT
Leaching of ruthenium was investigated by ICP, the concentra-
tion of Ru was below the limit of quantification meaning than in
the worst case only 0.05 mg/L of ruthenium could be present in
the liquid after the reaction. This value corresponds to maximum
ruthenium leaching of 0.13%, in practice it can be even smaller.
Hydrogenation of sugars over ruthenium is often described by
zero order kinetics at high hydrogen pressures [22], thus allowing
to simplify Eq. (5)
/d
ke˛ꢂ
KGCG
G
cluster
TOF =
(6)
(1 + KGeꢂ
CG)
/d
G
cluster
3.3. Structure sensitivity
The structure sensitivity of glucose hydrogenation is illustrated
in Fig. 9 including also comparison of the experimental and cal-
culated values. A clear maximum in Fig. 9 indicates that glucose
hydrogenation is a structure sensitive reaction and that an optimal
catalyst should contain ruthenium particles around 3 nm in size.
Catalyst C was excluded from the data fitting since it gives mainly
other products. It should be also noted that even if only the cata-
lysts prepared on the carbon nanotubes are taken into account one
could still see the effect of structure sensitivity with a maximum
in the TOF plot at slightly larger particle size than what is reported
now when also some active carbon supported catalysts are con-
sidered. Preliminary calculations showed that the value of Polanyi
parameter is very close to 0.5, a typical value for a large num-
ber of heterogeneous catalytic reactions [23], therefore this value
was fixed. Direct experimental data for Gibbs adsorption energy for
sugars on different type of Ru sites (low and high coordinated) as
Utilization of different catalysts with different ruthenium
nanoparticle sizes made it possible to study the influence of the
average particle diameter on the catalytic activity. The different
carbon supports used should be inert in the hydrogenation of glu-
cose, therefore the catalytic activity should only depend on the
properties of the ruthenium.
The activity of the different catalysts was calculated based on
the initial glucose 50% conversion rates and divided by the amount
of exposed ruthenium. The amount of exposed ruthenium was cal-
culated from on the average particle size, which was determined
by TEM analysis or CO chemisorption.
Analysis of structure sensitivity is also founded on the average
ruthenium particle size, not considering particle morphologies or
size distribution. The model presented below assumes that ruthe-
nium particles are cubo-octahedral in symmetry.
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Acknowledgements
The research leading to these results has received funding
from the European Community’s Seventh Framework Program
[FP7/2007–2013] under Grant Agreement No. CP-IP 246095-2
(POLYCAT). The work is a part of Process Chemistry Centre (PCC), a
centre of excellence financed by Åbo Akademi.
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