Ruthenium nanoparticles supported on zeolite y as an efficient catalyst for selective hydrogenation of xylose to xylitol

Article abstract of DOI:10.1016/j.molcata.2013.04.011

Zeolite Y (HYZ) supported ruthenium (Ru) nanoparticles catalysts are prepared by simple impregnation method and characterized by using different techniques such as TEM, TEM-EDX, SEM, XRD, FT-IR, surface area analysis and CO chemisorption. HYZ (different ratio of Si/Al) supported ruthenium catalysts are evaluated in hydrogenation of xylose to xylitol under green aqueous phase system. The reaction conditions are optimized by varying the stirring rate, ruthenium percent loading, xylose concentration, hydrogen partial pressure, reaction temperature and amount of catalyst to achieve the maximum conversion of xylose and selectivity to hydrogenated product xylitol. The activity of Ru/HYZ is also compared with that of conventional Ru/C catalyst at optimum reaction condition (120 C and 5.5 MPa pressure of H₂ in 2 h). The reusability test of catalyst is carried out four times by recovering the catalyst from product solution.

Full text of DOI:10.1016/j.molcata.2013.04.011

Journal of Molecular Catalysis A: Chemical 376 (2013) 63–70
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ruthenium nanoparticles supported on zeolite Y as an efficient catalyst for
ଝ
selective hydrogenation of xylose to xylitol
∗
Dinesh Kumar Mishra, Aasif Asharaf Dabbawala, Jin-Soo Hwang
Bio-refinery Research Center, Korea Research Institute of Chemical Technology (KRICT), Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Zeolite Y (HYZ) supported ruthenium (Ru) nanoparticles catalysts are prepared by simple impregnation
method and characterized by using different techniques such as TEM, TEM-EDX, SEM, XRD, FT-IR, surface
area analysis and CO chemisorption. HYZ (different ratio of Si/Al) supported ruthenium catalysts are eval-
uated in hydrogenation of xylose to xylitol under green aqueous phase system. The reaction conditions
are optimized by varying the stirring rate, ruthenium percent loading, xylose concentration, hydrogen
partial pressure, reaction temperature and amount of catalyst to achieve the maximum conversion of
xylose and selectivity to hydrogenated product xylitol. The activity of Ru/HYZ is also compared with
that of conventional Ru/C catalyst at optimum reaction condition (120 C and 5.5 MPa pressure of H2
in 2 h). The reusability test of catalyst is carried out four times by recovering the catalyst from product
solution.
Received 20 December 2012
Received in revised form 9 April 2013
Accepted 9 April 2013
Available online 17 April 2013
Keywords:
Hydrogenation
Xylose
Xylitol
Ruthenium nanoparticles
Zeolite Y
◦
© 2013 The Authors. Published by Elsevier B.V. All rights reserved.
1
. Introduction
Nowadays, considerable interests of researchers are increas-
view. The hydrogenation of xylose to xylitol is conventionally car-
ried out in a three-phase slurry batch reactor using Raney nickel as
catalyst [13]. Some of the advantages for the utilization of nickel
catalysts in the hydrogenation of carbohydrates is related to its
lower catalyst prices, ease of use as a suspended slurry in typical
batch reactions and good activity [12–14]. The formation of by-
products and accumulation of organic impurities on the catalyst
surface easily deactivated nickel catalyst due to the poisoning of the
active sites and also led to metal leaching-off by the morphologi-
cal changes take place on the metal surface [15–18]. Consequently,
the catalyst activity and selectivity deteriorate considerably. More-
over, additional purification of xylitol usually is required by means
of expensive ion-exchange, filtering and crystallization steps and
these are very crucial for the success of the hydrogenation per-
formed in the presence of nickel catalysts [17]. In this field, research
is ensuing in search for the proper metal and support material com-
bination catalyst systems with less sensitive to the deactivation
that can lead xylitol production. In terms of activity and selectivity,
ruthenium has been pointed out as more effective than other metal
catalysts such as nickel, palladium and rhodium for the hydrogena-
tion of sugar to sugar alcohol [19–21]. On other hand, activated
carbon has mainly been used as support material with ruthenium
and its catalyst is extensively studied to replace conventional nickel
catalysts for the hydrogenation of xylose into xylitol [22,23].
Traditionally, zeolites are used as ion exchange, adsorbents and
separation agents. Zeolites have been extensively used as solid cat-
alyst as such in various organic transformations [24–29] and are
considered as suitable host to support the metal nanoparticles
ing on the conversion of biomass-derived carbohydrates into
molecules or building block molecules [1–5], because these
products are known as important intermediates that can be fur-
ther transformed into a variety of valuable chemicals [5,6]. The
molecules as prior mentioned are formed either by catalytic
hydrogenation or dehydration of carbohydrates including sugar
aldehydes [7,8]. According to US Department of Energy, 2004, one
of top ten platform molecules [9] is xylitol and its interesting
properties such as anti-carcinogenic, sugar substitute and higher
sweetening capacity than sucrose make it valuable contender for
wide range of applications in food, pharmaceutical, cosmetic, and
synthetic resin industries [10–12]. Since xylitol is an important
molecule for the synthesis of other chemicals and it has tremen-
dous applications, therefore the production of xylitol should be
increased. Even though xylitol is produced from biomass-derived
carbohydrates, xylitol production by catalytic hydrogenation of
xylose (a five carbon sugar aldehyde) that is obtained from
hemicellulose is essential for large scale and commercial point of
ଝ
This is an open-access article distributed under the terms of the Creative
Commons Attribution License, which permits unrestricted use, distribution and
reproduction in any medium, provided the original author and source are credited.
∗
Corresponding author. Tel.: +82 42 860 7382; fax: +82 42 860 7676.
E-mail address: jshwang@krict.re.kr (J.-S. Hwang).
1
381-1169/$ – see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.04.011

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D.K. Mishra et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 63–70
exhibiting high activity in hydrogenation of various compounds
under mild conditions [30,31]. Because of ordered porous
structures and confined void spaces in zeolites, it also restricts
the growth of nanoclusters and lead to an increase in the cata-
lyst performance [30–32]. Prompted by the various application and
its properties such as thermal stability, crystalline in nature, and
shape selective, HYZ is selected as support material. The applica-
tion of HYZ as support with variable acidic sites for Ru catalyzed
xylose hydrogenation was not explored so far. Here in, we report
Ru/HYZ catalyzed selective hydrogenation of xylose to xylitol in
liquid phase using water as green solvent with added advantages
over conventional catalysts.
Emmett, Teller (BET) method. CO chemisorption was carried out
by using an instrument model ASAP 2020C V1.09 G. Before adsorp-
tion of the CO, the catalysts (weighed approximately 0.12 g) were
pre-treated in He for 35 min, and in O2 for 15 min, and were then
reduced for 30 min in a (5.0%) H /Ar gas flow of 50 mL/min, and
2
◦
in He gas flow for 15 min at 400 C in a reaction chamber. After
◦
this pre-treatment, the samples were cooled down to 50 C under
He gas flow and CO pulse measurements were carried out using
(5.0%) CO/He gas flow of 50 mL/min. Finally, the surface concen-
tration and dispersion of metallic Ru were obtained from the CO
pulse analysis data. The metal contents (amount of Ru loading) in
the catalysts were determined by using EDX, Quantax 200 Energy
Dispersive X-ray Spectrometer, Bruker. Fourier transform infrared
(FT-IR) spectra were recorded using KBr pellet on a Nicolet Magna-
2
. Experimental
5
60 IR spectrophotometer. Scanning electron microscopy (SEM)
2.1. Materials
images of support and catalyst were measured on SEM, JEOL JSM-
8
40 A.
Ruthenium trichloride (RuCl ·3H O) was provided by Strem
3
2
Chemicals, USA. The zeolites (Zeolites Y hydrogen form with differ-
ent Si/Al from 5.1 to 80) used as support materials were procured
from ZEOLYST International Company, USA. Xylose, and NaBH4
were purchased from M/s Sigma–Aldrich Chemicals, USA and used
as received. Ru(5.0%)/C purchased from M/s Sigma–Aldrich Chem-
icals, USA is used after reduction. De-ionized water was used as
a solvent for hydrogenation experiments. All the chemicals were
used as such without further purification. Hydrogen and nitrogen
gases (99.9%) used were purchased from Deokyang Co. Ltd.
2
.4. Catalytic hydrogenation of xylose
The xylose hydrogenation experiments catalyzed by Ru/HYZ
were carried out batch wise in 300 mL of three phase slurry reactor
in the temperature range from 100 to 140 C at hydrogen pressure
◦
(2.0–5.5 MPa) by various stirring rate (400–1200 rpm). In typical
hydrogenation experiment, required amount of catalyst (Ru/HYZ)
and 100 mL of xylose solution were charged into stainless steel
autoclave reactor. The reactor was fitted air tight and flushed with
nitrogen gas three times at room temperature. Then, reactor was
brought to desired temperature and pressurized with hydrogen
which was considered as the zero reaction time. Hydrogenation
reaction was initiated by stirring the entire reaction mass. Con-
stant hydrogen pressure was maintained by supplying hydrogen
gas manually through gas inlet valve during the reaction. During
hydrogenation at different time intervals, the product components
were analyzed using a HPLC (Younglin Instrument, Acme 9000)
equipped with refractive index (RI) detector and Sugar-Pak column.
De-ionized water was used as an eluent for the analysis at a flow
rate of 0.4 mL/min at 70 C. After a stipulated period, the stirring
was stopped and the reactor was abruptly cooled down, depressur-
ized, flushed with N , opened and decanted the reaction mixture
from the catalyst to collect sample for final analysis. Xylose (XLS)
conversion, selectivity to main xylitol (XTL) and arabitol (ARB) are
calculated using following expressions.
2.2. Preparation of HYZ supported ruthenium (Ru/HYZ) catalyst
The HYZ supported Ru catalysts were prepared by using conven-
tional impregnation-reduction method as reported in the literature
31]. In procedure to incorporate Ru (1.0% by weight) on HYZ-
[
8
0 (Si/Al = 80) support, 1.0 g of HYZ-80 and RuCl ·3H O (26 mg)
3
2
were placed together with 10 mL ethanol in a two neck 50 mL
round bottom flask equipped with a mechanical stirrer and a
nitrogen inlet. The resulting mixture was stirred at room temper-
ature under an N2 atmosphere for a period of 24 h. Then, 0.2 M
◦
solution of NaBH in ethanol was added drop wise to reaction mix-
4
ture with constant stirring; and entire reaction mass was stirred
2
(
500 rpm) under N2 atmosphere for a day at room temperature.
Ru(III) was reduced and the Ru(0) nanoclusters were formed which
ware stabilized by HYZ framework. Finally, catalyst was separated
by filtration; washed with ethanol and dried to give dark gray
HYZ-80 supported ruthenium catalyst, 1.0Ru/HYZ-80 (Scheme 1).
The catalysts having different Ru contents such as 0.5Ru/HYZ-80,
ꢀ
ꢁ
mole of xylose at particular time
initial mole of xylose
XLSConv. (%) = 1 −
× 100
2.0Ru/HYZ-80 and 3.0Ru/HYZ-80 and different Si/Al ratio such as
1.0Ru/HYZ-5.1, 1.0Ru/HYZ-5.2, 1.0Ru/HYZ-30, 1.0Ru/HYZ-60 and
1.0Ru/HYZ-80 were also prepared as described above by vary-
ꢀ
ꢁ
mole of XTL
ing RuCl ·3H O amount and Si/Al of HYZ. In the entire paper,
XTLSelc. (%) =
× 100
3
2
mole of all products formed
0.5Ru/HYZ-80, 1.0Ru/HYZ-80, 2.0Ru/HYZ-80, 3.0Ru/HYZ-80 corre-
spond to 0.5, 1.0, 2.0 and 3.0 wt% of ruthenium supported on HYZ
and where as 5.1, 5.2, 30, 60 and 80 represent Si/Al of HYZ, respec-
tively.
ꢀ
ꢁ
mole of ARB
mole of all products formed
ARBSelc. (%) =
× 100
2.3. Instrumentation
3. Results and discussion
Powder X-ray diffraction (XRD) patterns of all samples were
obtained by a Rigaku diffractometer (D/MAX IIIB, 2 kW) using Ni-
filtered Cu K␣-radiation (40 kV, 30 mA, ꢀ = 1.5406 A˚ ) and a graphite
The HYZ supported Ru catalysts were characterized by different
methods and evaluated in liquid phase hydrogenation of xylose.
The activity of the HYZ supported Ru catalysts, effect of stirring
rate, metal loading, hydrogen partial pressure, reaction tempera-
ture, catalyst amount and reusability of the catalysts were studied
systematically in details. The results obtained from these studies
are presented and discussed below.
crystal monochromator. Surface area measurements were carried
out using Micromeritics, Tristar II analyzer. The samples were
◦
−2
activated at 250 C for 4 h under vacuum (5 × 10 mmHg) prior
to N adsorption measurements. The specific surface areas, pore
2
diameters, and pore volumes of the samples were calculated from
nitrogen adsorption isotherms measured at 77 K as per Brunauer,

D.K. Mishra et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 63–70
65
Scheme 1. A schematic approach for the synthesis of HYZ-80 supported ruthenium cataysts.
3
.1. Characterization of catalysts
of 0.5Ru/HYZ-80, 1.0Ru/HYZ-80, 2.0Ru/HYZ-80, 3.0Ru/HYZ-80 are
identical to HYZ-80 support. Only the intensity of the peaks are
attenuated with increasing the Ru loading. It indicated that the
crystallinity of the HYZ-80 supported ruthenium catalysts is not
significantly affected by the method pursued for the preparation
of Ru(0) nanoparticles and insertion. Even though Ru nanopar-
ticles are highly dispersed on the HYZ-80 support, there is no
additional peak of Ru nanoparticles in XRD patterns of HYZ-80
supported ruthenium catalysts due to low percent loading of ruthe-
nium nanoparticles [33].
Zeolite incorporated ruthenium catalysts are generally prepared
in two steps: first impregnation or ion exchange of Ru(III) followed
by reduction using reducing agents or H2 at higher temperatures
[
30,31]. In the case of reduction by H2 at higher temperature
may lead to affect the distribution of metal and zeolite frame-
work due to the formation of an unstable acid form at high
temperature treatment. Considering above, HYZ supported ruthe-
nium(0) nanoparticles were prepared by the reduction with sodium
borohydride (NaBH ) after impregnation following reported pro-
The BET surface areas of the support HYZ-80 and catalyst
4
2
2
cedure with modification [31]. In order to confirm formation of Ru
nanoparticles and alteration in the HYZ-80 framework after load-
ing of Ru nanoparticles, the HYZ-80 supported Ru catalysts were
thoroughly characterized by various instrumental techniques.
XRD patterns of HYZ-80 support and HYZ-80 supported Ru cat-
alysts are shown in Fig. 1. It can be seen that the XRD patterns
(1.0/HYZ-80) were 780 m /g and 768 m /g, respectively. No signif-
icant change in surface area of the catalyst was observed might be
due to low percent loading of ruthenium on HYZ-80 support. The
obtained results are in very agreement with the results reported by
other research group [34].
In order to get further information on the morphology of ruthe-
nium nanoparticles, TEM characterization has been performed.
The transmission electron microscopy (TEM) images of catalyst
1.0Ru/HYZ-80 showed the formation of ruthenium nanoclusters
and well dispersion of Ru(0) nanoparticles (∼2.0 nm) throughout
the crystalline zeolite framework (Fig. 2). This is in agreement
with the results obtained from CO chemisorption. The dispersion
of metallic ruthenium nanoparticles was 23.6%. TEM-EDX spec-
trum of 1.0Ru/HYZ-80 further confirmed the presence of Ru in the
3
.0Ru/HYZ-80
2
.0Ru/HYZ-80
1
0
.0Ru/HYZ-80
.5Ru/HYZ-80
HH YY ZZ -- 88 00
1
.0Ru/HYZ-80 catalyst (Fig. 2).
The ruthenium particles size distribution has been also esti-
mated from the measurement of ruthenium particles from the
given area of TEM image and is presented in Fig. 3. It is worth to
add that the measurement is only related to the size of ruthenium
nanoparticles. In fact, TEM analysis provides the direct information
on the metal particles size and maximum in the range of 2.0–2.5 nm.
3
.2. Catalytic activity of the HYZ supported ruthenium (Ru/HYZ)
10
20
30
40
50
60
70
80
catalysts
2
theta/degree
Liquid phase catalytic hydrogenation of xylose to xylitol, in
principle, seems to be simple but the formation of small amount
Fig. 1. XRD patterns of support HYZ-80 and HYZ-80 supported ruthenium catalysts.

6
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D.K. Mishra et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 63–70
Fig. 2. TEM images and EDS spectrum of 1.0Ru/HYZ-80.
Fig. 3. Particles size distribution (measurement of ruthenium particles form provided TEM image).
◦
of by-product, e.g. xylulose makes the process complicated since
xylulose, an isomer of xylose, is hydrogenated further to other by-
product arabitol as well as xylitol (Scheme 2). In some cases under
severe conditions (high temperatures and high concentrations of
alkali), there is also possibility not only for the formation of fur-
fural, but also xylonic acid as another by products. It is known that
alkali-catalyzed Cannizzaro reaction is responsible for formation
of xylonic acid. In general, the reaction conditions are chosen in
such a way that the formation of xylonic acid and furfural is pro-
hibited. Thus these by-products, xylulose and arabinitol, and their
generation is suppressed by using low temperature, high hydrogen
pressure and ruthenium catalysts.
The catalytic performances of 1.0Ru/HYZ catalysts with vary-
ing Si/Al ratio are summarized in Table 1. The Si/Al ratio of HYZ
varied from 5.1 to 80. It was clear from the experimental results
that change in Si/Al ratio of HYZ has a significant effect on cat-
alytic behavior of 1.0Ru/HYZ. Interestingly, with an increase in Si/Al
ratio of 1.0Ru/HYZ from 5.1 to 80, the conversion of xylose and
selectivity to xylitol gradually increased. Under identical reaction
condition (120 C and 5.5 MPa hydrogen pressure), Ru supported
on HYZ of Si/Al ratio 80, 1.0Ru/HYZ-80 showed superior catalytic
performance in term of conversation and selectivity to xylitol. At
Si/Al ratio of 5.1, the conversion and selectivity to xylitol were 46
Table 1
Effect of Si/Al ratio of HYZ on xylose hydrogenation catalyzed by 1.0Ru/HYZ.a
Entry
Catalyst
Ratio
Si/Al
%Conv.
XLS
%Sel.
XTL
ARB
NI
1
2
3
4
5
6
HYZ-80
80
5.1
5.2
30
60
80
0
46
47
52
58
62
0
68
70
86
94
98
0
22
20
11
4
0
10
10
3
2
0
1.0Ru/HYZ-5.1
1.0Ru/HYZ-5.2
1.0Ru/HYZ-30
1.0Ru/HYZ-60
1.0Ru/HYZ-80
2
XLS, xylose; XTL, xylitol; ARB, arabitol; NI, not identified.
Reaction conditions: substrate (xylose) = 20 g, catalyst (1.0Ru/HYZ) = 0.5 g,
temp. = 120 C, pressure (H2) = 5.5 MPa, water = 100 mL, str. speed = 1200 rpm,
a
◦
time = 1 h.

D.K. Mishra et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 63–70
67
1
00
100
A
B
8
6
4
2
0
0
0
0
0
9
8
7
6
0
0
0
0
1
1
8
4
200 rpm
000 rpm
00 rpm
00 rpm
1200 rpm
1000 rpm
800 rpm
4
00 rpm
0
30
60
90
120
150
180
0
20
40
60
80
100
Hydrogenation time, min.
Xylose conversion, %
◦
Fig. 4. Effect of stirring rate on xylose conversion (A) and xylitol selectivity (B). Reaction conditions: substrate (xylose) = 20 g, catalyst (1.0Ru/HYZ-80) = 0.5 g, temp. = 120 C,
pressure (H2) = 5.5 MPa, water = 100 mL.
and 68% respectively in 1 h, which were escalated up to 62 and
8% respectively at Si/Al ratio of 80 (Table 1, entry 6), indicat-
3.3. Effect of stirring rate and ruthenium percent loading
9
ing crucial role played by the strength and concentration of acid
sites of HYZ support. Indeed it has been established that low Si/Al
ratio has higher acidity that decreases gradually with an increase
in Si/Al ratio [35–38]. Additional information about formation of
side products is presented in Scheme 2. It may also provide indi-
rect information of product distributions over HYZ based catalysts
having different Si/Al ratio. As shown in Scheme 2, xylose may
undergo hydrogenation in addition to the isomerization reactions
over highly acidic zeolite having lower Si/Al ratio (Table 1). While
an increase in Si/Al ratio of HYZ from 5.1 to 80, the intensity of
both the Bronsted acid sites and the Lewis acid sites were grad-
ually decreased, that promote hydrogenation over isomerization
and increases overall xylitol selectivity. Based on the abovemen-
tioned results, it can be realized that the higher conversion and
selectivity of HYZ containing catalysts were connected primarily
with Si/Al ratio of HYZs. As comparative high Si/Al ratio with the
weak acid sites favors the hydrogenation over the isomerization as
the hydrogenation of xylose is competitive reaction. To ascertain
the role of Ruthenium present in the HYZ supported Ru catalysts,
In order to see the influence of gas liquid mass transfer resis-
tance, the effect of stirring rate on the conversion and selectivity of
hydrogenation of xylose was studied by varying the stirring speed
from 400 to 1200 rpm (400, 800, 1000 and 1200 rpm) keeping sub-
strate concentration (20 wt% in water) and catalyst amount (0.5 g,
◦
substrate/catalyst = 40) constant at 120 C temperature and 5.5 MP
hydrogen pressure. It can be seen from results (Fig. 4) that xylose
conversion (A) and xylitol selectivity (B) affected by changing the
stirring rates, particularly when the stirring rate is low. The hydro-
genation reaction proceeded slowly and formation of side products
is somewhat higher at low stirring rate due to the slow diffusion
of H2 gas to liquid. The hydrogenation experiments performed at
400–800 rpm had clearly a lower reaction rate and gave somewhat
lower xylitol selectivity indicating gas–liquid mass transfer limita-
tions in this range. An increase in stirring rate from 400 to 1000 rpm
gradually enhanced both conversion and selectivity (Fig. 4) due to
the rapid diffusion of H2 in the liquid. A further increase in stirring
rate from 1000 to 1200 rpm, no significant changes in conversion
and selectivity were observed. Thus, the stirring rate was fixed
at 1200 rpm in all of hydrogenation experiments to avoid the gas
liquid mass transfer limitation.
one hydrogenation experiment was carried out using pristine HYZ
◦
(
without Ru) as a catalyst at 120 C and 5.5 MPa partial pressure
of hydrogen. No hydrogenation of xylose was observed even after
0 h reaction time (Table 1, entry 1), which indicated that the Ru
present in the HYZ supported Ru catalysts is an active metal center
for hydrogenation of xylose.
In order to check the influence of Ru loading, the ruthenium
contents were varied from 0.5 to 3.0% in the Ru/HYZ-80 catalyst. As
expected, the conversion of xylose was found to increase with an
increase in Ru content. However, selectivity to xylitol was not much
1
Scheme 2. Hydrogenation of xylose with possible side reactions.

6
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D.K. Mishra et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 63–70
4
3
2
1
0
Ln k = αLn p(H ) + Ln ρ(k1)
-
-
2
4
2
2
1
0 wt% xylose
α = 1.0
R² = 0.9656
f
i
20 wt% xylose
-6
40 wt% xylose
-
8
0
15
30
45
60 75
2.5
3
3.5
4
4.5
Hydrogenation time, min.
Ln p(H2)
Fig. 5. Effect of xylose concentration.
Fig. 7. First order with respect to hydrogen pressure.
affected (Table 2). The conversion and xylitol selectivity were low at
low Ru content (0.5%) and improved with an increase in Ru content
catalyst (98%). The 3.0Ru/HYZ-80 catalyst even at lower Ru load-
ing than 5.0Ru/C displayed comparable catalytic activity and high
selectivity of xylitol (Table 2, entries 4 and 5). The effect of other
reaction parameters like catalyst amount, substrate concentration,
temperature and pressure of hydrogen on conversion and selectiv-
ity of xylitol in the hydrogenation experiments using 1.0Ru/HYZ-80
catalysts were then investigated in detail.
(
3.0%). These results further corroborate that Ru is the active metal
center as the conversion and selectivity varied with Ru content. It is
of interest to compare the catalytic performance of the Ru/HYZ-80
with 5.0Ru/C (5.0% Ru supported on carbon), the hydrogenation of
xylose was carried out using 5.0Ru/C catalyst under similar reac-
◦
tion conditions (temp. 120 C, 5.5 MPa hydrogen pressure). The
5
.0Ru/C catalyst yielded 78% conversion of xylose with 96% selec-
tivity to xylitol. The conversion was observed as compared to the
.0Ru/HYZ-80 catalyst (77%) due to the higher loading of Ru and
high surface area of the carbon support while selectivity to xylitol
was low for 5.0Ru/C (96%) than that observed for the 3.0Ru/HYZ-80
3
.4. Determination of order of reaction with respect to xylose and
3
hydrogen (H ) pressure
2
The hydrogenation experiments with 10, 20 and 40 wt% of
◦
xylose concentration were carried out at 120 C temperature and
5
.5 MPa hydrogen pressure keeping catalyst amount constant
Table 2
a
Effect of Ru metal loading on hydrogenation of xylose catalyzed by Ru/HYZ.
(1.0Ru/HYZ-80 catalyst = 0.5 g). The xylose hydrogenation results
with varying concentration are presented in Fig. 5. The variation in
xylose concentration showed influence on reaction rate slightly at
the experimental range. Plot of Ln(C /C ) versus time gave straight
Entry
Catalyst
Ru (%)
%Conv.
XLS
%Sel.
XTL
TON
ARB
NI
i
f
1
2
3
4
5
0.5Ru/HYZ-80
1.0Ru/HYZ-80
2.0Ru/HYZ-80
3.0Ru/HYZ-80
5.0Ru/C
0.5
1.0
2.0
3.0
5.0
45
62
69
77
78
96
98
98
98
96
4
2
2
2
2
0
0
0
0
2
2426
1671
929
692
420
lines for all these hydrogenation experiments carried out at differ-
ent concentrations, indicating the reaction is close to the first-order
with respect to xylose concentration.
The hydrogenation experiments were also carried out by chang-
◦
ing the hydrogen pressure from 2.0 to 5.5 MPa at 120 C using
XLS, xylose; XTL, xylitol; ARB, arabitol; NI, not identified; TON, turn over number.
a
1
.0Ru/HYZ-80 catalyst. An increased hydrogen pressure had pos-
Reaction conditions: substrate (xylose) = 20 g, catalyst (Ru/HYZ-80) = 0.5 g,
◦
itive effect on reaction rate (A) and xylitol selectivity (B) (Fig. 6). In
hydrogenation of xylose to xylitol, it is assumed that hydrogen was
temp. = 120 C, pressure (H2) = 5.5 MPa, water = 100 mL, str. speed = 1200 rpm,
time = 1 h.
1
00
1
00
A
B
9
6
8
6
4
2
0
0
0
0
0
92
88
84
80
5
4
2
5 bar
0 bar
0 bar
55 bar
40 bar
20 bar
0
30
60
90
120
150
180
20 30 40 50 60 70 80 90 100
Xylose conversion, %
Hydrogenation time, min.
Fig. 6. Effect of hydrogen pressure on xylose conversion (A) and xylitol selectivity (B). Reaction conditions: xylose concentration (Ci) = 20 wt% (1.3322 mol/L), catalyst
◦
Ru/HYZ-80 = 1.0 g, temp. = 120 C, str. speed = 1200 rpm, pressure (H2) = 5.5 Mpa.

D.K. Mishra et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 63–70
69
1
00
100
A
B
9
8
2
4
8
6
4
2
0
0
0
0
0
76
68
60
140oC
140oC
120oC
100oC
1
1
20oC
00oC
0
30
60
90
120 150 180
0
20
40
60
80
100
Hydrogenation time, min.
Xylose conversion, %
Fig. 8. Effect of temperature on xylose conversion (A) and xylitol selectivity (B). Reaction conditions: xylose concentration (Ci) = 20 wt% (1.3322 mol/L), catalyst Ru/HYZ-
◦
8
0 = 1.0 g, temp. = 120 C, str. speed = 1200 rpm, pressure (H2) = 5.5 Mpa.
spread first from air to the liquid membrane. Then, H2 dissolved
in the gas–solution interface and it spread from the liquid mem-
brane to xylose in the liquid phase. It is supposed that H2 did not
react with the carbonyl group of xylose, but instead was adsorbed
by the active centers of the catalyst, producing activated H on the
catalyst. Finally, xylose reacts with the activated H on the surface
of catalyst, which is an irreversible reaction, and then the prod-
uct desorbs from the catalyst and diffuses into the liquid phase.
Therefore, xylose hydrogenation proceeds through H2 dissolution,
H2 diffusion, H2 adsorption on the active centers of the catalyst, to
produce activated H. Finally, the carbonyl group in xylose reacted
with the activated H on the surface of catalyst to produce xylitol.
The value of ˛ is obtained by plotting Lnk2 versus Ln (pH2)
From the Arrhenius plot of Ln(k ) versus 1/T, K (Fig. 9) at different
temperatures (100, 120 and 140 C), it was found that the activation
energy (Ea) for xylose hydrogenation was found 46.8 kJ/mol which
was lower than the reported values in the literatures [30,21]. The
low value of activation energy indicates that the Ru/HYZ-80 is very
active toward xylose hydrogenation to xylitol.
1
◦
3.6. Effect of catalyst amount and its reusability
The effect of catalyst amount on conversion and selectivity
◦
in hydrogenation of xylose was studied at 120 C and 5.5 MPa
hydrogen pressure by varying the amount of catalyst from 160 to
500 mg (xylose/catalyst ratio 40–125). The results are summarized
in Table 3. The reaction was very slow at a low amount of catalyst
and the conversion as well as selectivity to xylitol increased with an
increase in the catalyst amount. The 160 mg 1.0Ru/HYZ-80 catalyst
resulted in 37% conversion of xylose with 88% xylitol selectivity.
The conversion and selectivity to xylitol increased to 99% and 98%
respectively at 500 mg of catalyst in 2 h under the employed exper-
imental conditions. The high conversion and selectivity at higher
catalyst amount can be attributed to the higher number of active
sites present on the surface of the catalyst. Catalyst recycling tests
were also conducted using 1.0Ru/HYZ. For reusability of catalyst,
the catalyst was filtered after reaction from the reaction mixture,
washed with ethanol and dried before reuse. The recovered cata-
lyst was then used for next run by adding xylose and water under
similar reaction conditions as used for the fresh catalyst. No sig-
nificant changes in conversion as well as in xylitol selectivity were
observed up to four cycles suggesting that the Ru based catalyst is
reusable under the employed reaction conditions (Table 3).
(Fig. 7). The xylose hydrogenation was found to be of first order
with respect to hydrogen pressure.
3.5. Effect of temperature
The effect of the reaction temperature on the xylose hydro-
genation using 1.0Ru/HYZ-80 catalyst was examine by varying
◦
temperature 100 to 140 C at constant hydrogen pressure of
.5 MPa. The xylose hydrogenation results at different tempera-
5
ture are presented in Fig. 8A. The increase in reaction temperature
improved the reaction rate at the experiments between 100 and
◦
1
40 C where as the xylitol selectivity was decreased significantly
(Fig. 8B) due to the formation of by-products such as arabitol [39].
1
0
8
6
4
2
0
Table 3
Effect of catalyst amount on hydrogenation of xylose using 1.0Ru/HYZ-80 catalyst.a
Ea= 46.8 KJ/mol
Entry
Catalyst amount
(%Conv.)
XLS
%Sel.
XTL
ARB
NI
1
2
3
4
5
0.16
0.22
0.30
0.38
0.50
0.50
0.50
0.50
0.50
37
45
61
78
99
98
96
95
92
88
92
93
95
98
98
95
95
92
7
5
4
4
2
2
5
4
6
5
3
3
1
0
0
0
1
2
1
2
3
st recycle
nd recycle
rd recycle
2.4
2.5
1/T) x 10 , K
2.6
2.7
4th recycle
-
3
(
XLS, xylose; XTL, xylitol; ARB, arabitol; NI, not identified.
Reaction conditions: substrate (xylose) = 20 g, temp. = 120 C, pressure
(H2) = 5.5 MPa, water = 100 mL, str. speed = 1200 rpm, time = 2 h.
a
◦
Fig. 9. Arrhenius plots of the initial rates for the xylose hydrogenation.

7
0
D.K. Mishra et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 63–70
4
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