Bimetallic Ru:Ni/MCM-48 catalysts for the effective hydrogenation of D-glucose into sorbitol

Article abstract of DOI:10.1016/j.apcata.2016.10.018

Three different bimetallic Ru:Ni catalysts supported on a mesoporous silica MCM-48 were prepared by consecutive wet impregnations, with a total metal loading of ca. 3% (w w^?1). Ru:Ni ratios spanned in the range of 0.15–1.39 (w w^?1) and were compared with the corresponding monometallic Ni/MCM-48. The catalysts so prepared were characterized by X-Ray Diffraction, Transmission Electron Microscopy, adsorption/desorption of N₂, Temperature Programmed Reduction, NH₃ ? TPD and Atomic Absorption, and tested in the liquid phase hydrogenation of D-glucose into sorbitol in the temperature range 120–140 °C under 2.5 MPa of H₂ pressure. Bimetallic catalysts with Ru:Ni ratios higher than 0.45 enhanced the catalytic behavior of the monometallic Ni/MCM-48 in the reaction, increasing the reaction rate and showing complete selectivity to sorbitol by minimizing the production of mannitol. Ru:Ni/MCM-48 (0.45) was recovered from the reaction media and tested for three reaction cycles, showing good stability under the selected experimental conditions.

Full text of DOI:10.1016/j.apcata.2016.10.018

Accepted Manuscript
Title: BIMETALLIC Ru:Ni/MCM-48 CATALYSTS FOR
THE EFFECTIVE HYDROGENATION OF D-GLUCOSE
INTO SORBITOL
´
Author: Alberto Romero Antonio Nieto-Marquez Esther
Alonso
PII:
S0926-860X(16)30513-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.10.018
APCATA 16034
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
21-6-2016
13-10-2016
18-10-2016
´
Please cite this article as: Alberto Romero, Antonio Nieto-Marquez, Esther
Alonso, BIMETALLIC Ru:Ni/MCM-48 CATALYSTS FOR THE EFFECTIVE
HYDROGENATION OF D-GLUCOSE INTO SORBITOL, Applied Catalysis A,
General http://dx.doi.org/10.1016/j.apcata.2016.10.018
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BIMETALLIC Ru:Ni/MCM-48 CATALYSTS FOR THE EFFECTIVE
HYDROGENATION OF D-GLUCOSE INTO SORBITOL
Alberto Romeroa, Antonio Nieto-Márquezb, Esther Alonsoa*
aHigh Pressure Processes Group. - Chemical Engineering and Environmental Technology
Department – Dr. Mergelina s/n, 47011 Valladolid, Spain.
bMechanical, Chemical and Industrial Design Engineering Department. –ETSIDI-
Technical University of Madrid (UPM). Ronda de Valencia 3, 28012 Madrid, Spain
*ealonso@iq.uva.es
Graphical abstarct

SORBITOL (S = 86%)
MANNITOL (S = 14 %)
SORBITOL
(S = 100%)
D-GLUCOSE
D-GLUCOSE
+ H2
+ H2
NI - RU
NI
MCM-48
T = 140 ºC, P = 2.5 MPa H2 and t = 90 min
MCM-48

Highlights
i)
Notable influence of the addition of different amounts of Ru to Ni/MCM-48
ii) Ru:Ni ratios higher than 0.45 improved the catalytic behavior of Ni/MCM-48
iii) Good stability of Ru:Ni/MCM-48 (0.45) was observed after three reaction cycles
iv) Ru:Ni/MCM-48 enhanced selectivity of sorbitol in comparison with Ni/MCM-48
Abstract
Three different bimetallic Ru:Ni catalysts supported on a mesoporous silica MCM-48
were prepared by consecutive wet impregnations, with a total metal loading of ca. 3 %
(w·w-1). Ru:Ni ratios spanned in the range of 0.15 – 1.39 (w·w-1) and were compared with
the corresponding monometallic Ni/MCM-48. The catalysts so prepared were characterized
by X-Ray Diffraction, Transmission Electron Microscopy, adsorption / desorption of N2,
Temperature Programmed Reduction, NH3 - TPD and Atomic Absorption, and tested in the
liquid phase hydrogenation of D-Glucose into sorbitol in the temperature range 120 – 140
ºC under 2.5 MPa of H2 pressure. Bimetallic catalysts with Ru:Ni ratios higher than 0.45
enhanced the catalytic behavior of the monometallic Ni/MCM-48 in the reaction, increasing
the reaction rate and showing complete selectivity to sorbitol by minimizing the production
of mannitol. Ru:Ni/MCM-48 (0.45) was recovered from the reaction media and tested for
three reaction cycles, showing good stability under the selected experimental conditions.
Keywords
Hydrogenation of sugars, sorbitol, ruthenium-nickel bimetallic catalysts, MCM-48, D-
Glucose

1. Introduction
Nowadays, environmental issues such as the poor management of fossil fuels, the
depletion of crude-oil reserves and the global warming have promoted a major effort in the
valorization of biomass in order to produce fuels, energy and fine chemicals [1].
Lignocellulosic biomass is one of the most promising renewable sources of carbon and it is
the only one that can be converted into solid, liquid or gas fuels by thermochemical or
biological processes [2]. Essentially, lignocellulosic materials comprise three main
fractions, whose average composition is 34 – 50 % cellulose, 19 – 34 % hemicellulose and
11 – 30 % lignin [3, 4] and it is a relatively low-priced source of biomass with a high
availability all over the world. In this sense, hydrolytic hydrogenation of cellulose into
sugar alcohols has attracted a lot of research interest [5-8].
Catalytic hydrogenolysis of cellulose consists of two consecutive steps where firstly
cellulose is hydrolyzed into D-Glucose, which is subsequently hydrogenated into sugar
alcohols like sorbitol and mannitol. Sorbitol is a versatile compound which has been used
for many different applications, like building block for the synthesis of fine chemicals such
as ascorbic acid (intermediate in the synthesis of Vitamin C) [9, 10], as additive in food,
cosmetics and paper industries [2], and its annual production is about 700.000 tones / year
[11]. Sorbitol is also used as feedstock for hydrolysis – hydrogenation processes in order to
produce isosorbide and valuable polyols such as triols, tetrols, glycerol, ethylene glycol and
1,2-propanediol [12]. Most of the sorbitol processing at industrial scale is performed by
catalytic hydrogenation of D-Glucose, which is a cheap raw material produced from starch
and sucrose [13, 14], using Raney–nickel catalysts [15]. Both noble metals (Ru, Rh, Pd
and Pt) and non-noble metals (Fe, Ni, Cu or Co) have been used as active phases in

hydrogenation reactions. Nickel–based catalysts have achieved a good piece of attention
according to their low cost and moderate to good catalytic activity [16]. Nevertheless,
nickel–based catalysts are susceptible to show deactivation after its recycling [2, 17, 18]
due to leaching of the active nickel into the reaction media [19], sintering of the active
metal [18, 20] and poisoning of metallic nickel surface attributed to organic byproducts of
the reaction [21]. The current trend consists on the preparation of ruthenium–based
catalysts, which show catalytic activities per mass of active metal 20 – 50 times higher in
comparison with nickel [13]. However, the high price of noble metals is the main
drawback. Thus, the development of novel bimetallic nickel-based catalysts with
comparable high activity to noble metal catalysts still remains a technological challenge.
Noteworthy efforts were carried out to enhance catalytic activity of nickel-based catalysts
in the catalytic conversion of D-Glucose into sorbitol. Hoffer et al. determined that the
addition of Mo and Cr had a positive effect promoting Raney - Nickel catalysts activity and
stability in the hydrolytic hydrogenation of D-Glucose [17]. Bizhanov et al. studied the
influence of noble metals such as Pt, Ru, Rh and Pd on Raney nickel catalysts and they
observed that Ni/Ru was the most promising option [22]. In that case the catalytic material
was an unsupported catalyst; however, to the best of our knowledge, supported Ni-Ru-
based catalysts have never been tested in the hydrogenation of D-Glucose. With this aim,
we present the hydrogenation of D-Glucose over bimetallic Ru:Ni catalysts, using MCM-
48 as porous support, which has shown an excellent catalytic behavior in previous works
[23, 24].
In the present work, we report the catalytic behavior of Ru:Ni-based bimetallic MCM-
48 catalysts in comparison with monometallic Ni/MCM-48 for the selective hydrogenation

of D-Glucose into sorbitol. The influence of the addition of small amounts of ruthenium
over Ni/MCM-48 in the catalytic activity is reported in this work.
2. Materials and methods
2.1. MCM-48 preparation
MCM-48 has been prepared using a conventional hydrothermal synthesis, according to
the procedure described by Schumacher et al. [25]. 2 g of n-Hexadecyltrimethylammonium
bromide template (CH3(CH2)15N(Br)(CH3)3 ≥ 98%, Sigma – Aldrich) was dissolved in 42
cm3 of deionized water, 13 cm3 of ammonium hydroxide (20% as NH3, Panreac), and 18
cm3 of absolute ethanol (partially denaturated QP, Panreac). The resulting solution was
stirred for 15 min and 4 cm3 of tetraethyl orthosilicate (TEOS, purity ≥ 99% GC, Sigma –
Aldrich), were added dropwise during 1 minute approximately. The solution was further
stirred for 18 h in a water bath at 30 ºC; the white precipitate was then collected by
filtration, washed with distilled water and dried at 60 ºC overnight. Template was removed
from dried samples by calcination with a heating rate of 2 ºC·min-1 from 80 ºC to 550 ºC
and maintained at 550 ºC overnight.
2.2. Catalyst preparation
Monometallic Ni/MCM-48 with a metal loading close to 3 % by weight was prepared
by the conventional wet impregnation (WI) method using the so prepared MCM-48 as
carrier. For this synthesis, nickel (II) nitrate hexahydrate (Ni(NO3)2.6·H2O, 99.999% trace
metal basis Sigma Aldrich) and MCM-48 were sonicated in water previously to their
mixture during ten minutes. Then, nickel nitrate solution and the dispersion containing
MCM-48 were mixed and heated with a rate of 1 ºC·min-1 from room temperature to 105 ºC

using a Stuart model SD162 heating plate. The impregnation finished when the solvent was
completely evaporated. Then, it was dried overnight at 105 ºC. Bimetallic Ru:Ni/MCM-48
with a total metal loading around 3 %, were prepared by consecutive wet impregnations
over MCM-48. Ruthenium (III) chloride anhydrous (RuCl3.anhydrous, Strem Chemicals
Inc.) and nickel nitrate were used as ruthenium and nickel precursors, respectively. In this
case, the metal with the highest loading in the final catalyst was deposited first. An example
of the nomenclature of bimetallic catalysts is presented: M1:M2/S (M1/M2), where M1 is
ruthenium, M2 is nickel, S is MCM-48 support and M1/M2 is the mass ratio of ruthenium to
nickel. Finally, both monometallic and bimetallic catalytic systems were reduced under H2
atmosphere at 250 ºC according to TPR-H2 conditions.
2.3. Support and catalyst characterization
Small Angle X-Ray Scattering (SAXS) and X-Ray Diffraction (XRD) were performed
in a Bruker Discover D8 diffractometer using the Cu Kα radiation (λ = 0.15406 nm). The
diffraction intensities were measured, for XRD, over an angular range of 5° < 2θ < 80°
with a step size of 0.03° and a count time of 2 s per step. In case of SAXS, 2° < 2θ < 6º was
selected as angular range with a step size of 0.02° and a count time of 1 s per step. Nitrogen
adsorption / desorption isotherms were performed with ASAP 2020 (Micromeritics, USA)
to obtain surface and pore properties of the support and the catalyst. Prior to analysis, the
samples were outgassed overnight at 350 ºC. Total specific surface areas were determined
by the multipoint BET method at P/P0 ≤ 0.3, total specific pore volumes were evaluated by
single point adsorption from N2 uptake at P/P0 ≥ 0.99. Pore diameter was obtained by BJH
(adsorption average, 4·V·A-1). Pore size distribution was derived from the adsorption
branch of the isotherm by BJH (dV/dD) Halsey:Faas correction. Temperature Programmed

Reduction (TPR) profiles were recorded using the commercial Micromeritics TPD/TPR
2900 unit. The samples were loaded into a U-shaped quartz cell, ramped (10 ºC·min−1)
from room temperature to 800 ºC under a flow of H2/N2 (5% v/v; 50 cm3·min−1, Air
Liquide) and kept at the final temperature until the signal returned to the baseline.
Hydrogen consumption was monitored by a thermal conductivity detector (TCD) with data
acquisition/manipulation using the ChemiSoft TPX V1.03™ software. TPD-NH3
experiments were performed in the same analyzer. In this case, the samples were activated
under TPR-H2 conditions (250 ºC) for 60 min. Prior to the analysis, the samples were
outgassed at 105 ºC using pure He during 60 min. Then they were saturated with ammonia
at 100 ºC during 30 min. NH3 was purged using pure He during 60 min and then samples
were heated from 100 ºC to 600 ºC (ramped 15 ºC·min-1) and kept at the final temperature
until the signal returned to the baseline. The amount of chemisorbed ammonia was
calculated according to calibrated volumes of this compound. Transmission electron
microscopy (TEM) analyses used a JEOL 2100 unit with an accelerating voltage of 200 kV.
Samples were prepared by ultrasonic dispersion in acetone with a drop of the resultant
suspension evaporated onto a holey carbon-supported grid. A counting of nickel
nanoparticles were carried out from TEM images of the different catalysts. At least 100
nickel nanoparticles were counted in each case and the mean Ni particle sizes were
̅
̅
calculated as number average diameter (푑푛), surface area-weighted diameter (푑푠), volume-
̅
weighted diameter (푑푉), according to equations 1, 2 and 3, respectively [26],
∑
푛푖 · 푑푖
푖
̅
푑푛 =
(1)
∑
푛푖
푖

푛푖 · 푑푖3
푛푖 · 푑푖2
∑
∑
푖
푖
̅
푑푠 =
(2)
(3)
푛푖 · 푑푖4
푛푖 · 푑푖3
∑
∑
푖
푖
̅
푑푣 =
where ni is the number of nickel particles with a diameter di. Since chemical reactions occur
on catalyst surface, surface-area weighted diameter is selected as the most meaningful
parameter to obtain mean Ni particles sizes for catalysis purposes. In order to determine
how closely the observed distribution approaches the true population, the standard
deviation of the number diameter (σm) must be calculated as described in Eq. 4.
2
2
̅
∑
푑푖 − 푛 · 푑푛
푖
ꢀ푚 =
(4)
푛2
EDS microanalysis (Oxford Instruments Inca X-Ray microanalysis system TEM 250)
provided elemental and chemical identification of nickel and ruthenium in all cases. In
addition, energy dispersive X-Ray spectroscopy mapping was performed in STEM mode
for Ni/MCM-48. Metal loadings of ruthenium and nickel were determined by atomic
absorption (AA) using a VARIAN SPECTRA 220FS analyzer. Digestion of the samples
was performed with HCl, H2O2 and HF using microwave at 250 ºC.
2.4. Catalytic hydrogenation of D-Glucose
Catalytic tests were performed in a stainless-steel high pressure reactor with an internal
volume of 25 cm3 (Berghof BR-25), agitated with a magnetic stirring bar and PID
controlled. Hydrogenation experiments were carried out in the temperature range 120–140
ºC at 2.5 MPa H2 using a stirring rate of 1400 rpm. Prior to reaction catalysts were
activated in situ by reducing under H2 atmosphere at 250ºC during 60 minutes. Mole of

carbon in feedstock to mole of total metal ratio (C:Ru) was kept at 142 in all the
experiments. The reactor was flushed with N2 for 10 minutes and subsequently fed with H2,
and then the reactor was heated up to the desired reaction temperature. Once the set point
was reached, 5 cm3 of a solution of 7.35 g·dm-3 D-Glucose was pumped (intelligent HPLC
pump, Jasco PU-2080 Plus) and the reactor was pressurized up to 2.5 MPa of H2. At the
end of the experiments, recovery of the catalyst was made by filtering the product solution
using a vacuum pump. Hydrogenation products were analyzed by HPLC. The HPLC
column used was a SUGAR SC-1011 from Shodex at 80 °C and a flow of 0.8 cm3·min-1
using water Milli-Q as the mobile phase. A Waters IR detector 2414 was used to identify
sugars, polyols and their derivatives. The error related to the concentrations so obtained by
HPLC was lower than 0.08 %. D-Glucose conversion, yields and selectivities to sorbitol
and manitol were calculated using equations 5, 6 and 7.
푚표푙푒 (퐷-퐺푙푢푐표푠푒0) − 푚표푙푒 (퐷-퐺푙푢푐표푠푒푓)
( )
푋퐷−퐺푙푢푐표푠푒 % =
· 100 (5)
푚표푙푒 (퐷-퐺푙푢푐표푠푒0)
푚표푙푒 (푝푟표푑푢푐푡)
푚표푙푒 (퐷-퐺푙푢푐표푠푒0) − 푚표푙푒 (퐷-퐺푙푢푐표푠푒푓)
( )
푆푝푟표푑푢푐푡 % =
( )
6
· 100
푚표푙푒 (푝푟표푑푢푐푡)
푚표푙푒 (퐷-퐺푙푢푐표푠푒0)
푋 · 푆
100
( )
% =
푌
· 100 =
(7)
푝푟표푑푢푐푡
푚푎푠푠 (푆표푟푏푖푡표푙)
푅푒푎푐푡푖표푛 푟푎푡푒 (푔푠표푟푏푖푡표푙 · 푔푚−1푒푡푎푙 · 푚푖푛−1) =
(8)
(
)
푚푎푠푠 푚푒푡푎푙 · 푡푖푚푒
푚표푙푆표푟푏푖푡표푙
푆푝푒푐푖푓푖푐 푟푒푎푐푡푖표푛 푟푎푡푒 =
(9)
푆푁푖 · 푡

6
푆푁푖 =
(10)
̅
ꢁ푁푖 · 푑푠
Catalytic activity was expressed as reaction rate (Eq. 8) in Table 2 to be compared with
those reported previously by other authors. In addition, catalytic activity was also stated as
specific reaction rate (Eq. 9) in Table 2. In order to test catalyst reusability, after its
recovery from reaction media, the solid catalyst was washed several times with deionized
water and dried at 105 ºC overnight. Then, the catalyst was reactivated under TPR
conditions and tested again in D-Glucose hydrogenation.
3. Results and discussion
3.1. Support characterization
Figure S1(A) shows Small Angle X-Ray Scattering (SAXS) pattern of MCM-48.
Calcined MCM-48 exhibits three main Bragg diffraction peaks in the 2θ range from 2-5 º,
that can be assigned to (211), (220) and (332) planes. These results are in good agreement
with the high quality of mesoporous MCM-48, where the cubic phase belongs to a Ia3d
space group symmetry [25, 27].
To study adsorption properties of calcined MCM-48 material, typical adsorption /
desorption isotherms of N2 at -196 ºC were determined and results are illustrated in Figure
S1(B). This isotherm shows the typical features of a mesoporous silica material, and it can
be classified as a type IV according to the IUPAC [28]. First, a sharp nitrogen uptake at
P/P0 in the range of 0 – 0.02 due to a monolayer adsorption on the walls of MCM-48 is
observed. This step is followed by an abrupt increase in the volume of nitrogen adsorbed at
P/P0 in the range of 0.2 – 0.3 associated to capillary condensation of N2 in the channels of
MCM-48, suggesting uniformity of the channels and a narrow pore size distribution [29].

No hysteresis was observed between adsorption and desorption branches, therefore MCM-
48 shows a reversible type IV isotherm, comparable to those reported by Morey et al. [29].
Absence of hysteresis is attributed to the presence of small-sized mesopores. Materials such
as MCM-48 usually shows type H1 hysteresis, where the width of the hysteresis loop
slightly increases with increasing the pore size [30]. Figure S1(C) illustrates the pore
volume (cm3·g-1·Å-1) as a function of pore diameter (Å) for MCM-48. A unimodal pore
size distribution with a well-defined peak centered at 20.3 Å was observed. This small
mesopore size, in the limit between meso and micropores, is in good agreement with the
shape of the isotherm. Textural properties for MCM-48 are described in Table 1, where it
can be observed that BET surface area and pore volume values were 1289 m2·g-1 and 0.87
cm3·g-1, respectively, characteristic of these materials.
3.2. Characterization of Ni and Ru-Ni-based catalysts
A monometallic Ni/MCM-48 and three bimetallic Ru:Ni/MCM-48 catalysts were
prepared with a metal loading around 3 %, according to atomic absorption results (Table 1).
The bimetallic catalysts presented Ru:Ni ratios in the range of 0.15-1.39
Figure 1 shows the temperature programmed reduction (TPR-H2) profiles of the
monometallic Ni/MCM-48 and bimetallic Ru:Ni/MCM-48 catalysts between 25 ºC and 400
ºC, since at temperatures higher than 400 ºC other reduction peaks were not recorded. For
Ni/MCM-48, one reduction peak centered at 255 ºC is observed. This profile can be
attributed to reduction of nickel nitrate species with two displaced ligands of water by
terminal silanol groups (Ni(NO3)2·4H2O·2(SiOH)) from MCM-48. A smaller and broader
peak was also observed from 275 ºC to 350 ºC corresponding to smaller amounts of
Ni(NO3)2·5H2O·(SiOH) with higher interaction with the silica MCM-48 framework [31].

Ru/MCM-48 was characterized in terms of H2-TPR in previous works showing a unique
reduction peak at ca. 125 ºC corresponding to the reduction of Ru3+ to Ru0 [23, 24]. The
reduction patterns of Ru:Ni/MCM-48 bimetallic catalysts are clearly different from those
obtained from the monometallic Ni/MCM-48 and Ru/MCM-48 catalysts, showing complex
H2-TPR profiles consisting on three overlapped peaks where reduction starts at 100 ºC and
finishes at 260 ºC, approximately. In the case of Ru:Ni/MCM-48 (0.15), which presented
the highest nickel loading in comparison with the other bimetallic systems, a low-intense
peak centered at 122 ºC was observed attributed to the reduction of RuCl3. Then, a broader
peak was detected at 182 ºC suggesting the presence of Ru/Ni alloys formed during the
impregnation process, as it was reported by other authors [32-34] and a narrow peak at 258
ºC corresponding to the reduction of Ni(NO3)2.4H2O.2(SiOH) to Ni0 was observed. Similar
behavior was detected for Ru:Ni/MCM-48 (0.45) and Ru:Ni/MCM-48 (1.39), though a
decrease in the reduction temperature of ruthenium was observed when Ru:Ni ratio
increased suggesting the presence of bigger ruthenium nanoparticles, with lower interaction
with the MCM-48 framework. Also, the addition of different amounts of ruthenium over
Ni/MCM-48 catalyst enhanced the reducibility of nickel species due to chemisorption of H2
molecules on Ru0 and subsequent spillover. According to the above results, 250 ºC was
selected as an adequate common reduction temperature.
XRD patterns for Ni/MCM-48 and Ru:Ni/MCM-48 catalysts, after reduction under H2-
TPR conditions, are shown in Figure 2(A). Monometallic Ni/MCM-48 showed a broad
characteristic metallic diffraction peak at 2θ = 44.5º (JCPDS card No. 4-850),
corresponding to (111) crystallographic plane of Face-Centered Cubic (FCC) nickel. The
position, shape and size of this peak suggests the presence of very small Ni0 nanoparticles

and it indicates the successful reduction of nickel species into Ni0. Calculations based on
the Scherrer equation and Ni (111) diffraction, determined a nickel crystallite diameter
around 2.7 nm. Comparing monometallic Ni/MCM-48 and bimetallic Ru:Ni/MCM-48
XRD patterns, significant differences were observed as a result of the addition of small
amounts of ruthenium over nickel catalysts. In the case of bimetallic Ru:Ni/MCM-48
catalysts, characteristic diffraction peaks corresponding to FCC Ni0 reflections were
observed at 2θ = 44.5º, 51.7º and 76.1º (JCPDS card No. 4-850) related to (111), (200), and
(220) crystallographic planes, respectively. Bimetallic catalysts showed Ni0 diffraction
peaks with higher intensity than in the case of the monometallic catalyst, which is
indicative of the presence of bigger metallic nickel nanoparticles. This fact can be attributed
to the additional drying and coprecipitation steps employed for bimetallic materials
compared to the monometallic catalyst. In addition, Ru0 reflections were detected at 2θ
=38.8º, 42.2º, 43.8º, 58.2º, 69.6º and 78.4º (JCPDS No. 06–0663), indicating the presence
of Hexagonal Close Packing (HCP) Ru0 nanoparticles. Thus, the most intense diffraction
peaks of Ru0 at 2θ =43.8º and Ni0 at 2θ =44.5º overlapped. A expanded region of XRD
analysis for Ru:Ni/MCM-48 is shown in Figure 2(B). A shift of the resulting peak was
observed towards lower angles as the atomic percentage of Ru was increased. Similar
behavior has been reported by different authors in the literature [35, 36]. In the case of
Ru:Ni/MCM-48 (0.15), overlapped peak is close to 2Ɵ = 44.5º since this catalysts had the
highest nickel metal loading. However, this peak moved around 2Ɵ = 43.8º for
Ru:Ni/MCM-48 (1.39) because in this case Ru:Ni ratio was the highest one in comparison
with the other bimetallic catalysts. This fact makes more inaccurate the application of
Scherer equation for crystallite size determination. Therefore, shape and particle sizes of
metallic species were evaluated by TEM.

Textural properties of the catalysts are summarized in Table 1. A sharp decrease of
BET surface area for MCM-48 from 1289 to 572 m2·g-1 upon nickel introduction was
observed, and pore volume was reduced from 0.87 to 0.44 cm3·g-1, suggesting a high pore
blockage for Ni/MCM-48 due to the deposition of metallic particles into the mesoporous
network. This is consistent with TEM image for Ni/MCM-48 (Figure 3(A)), where Ni
nanoparticles where located in the mesoporous network of MCM-48. It must be noted that
nickel nanoparticle size was comparable to the average pore diameter of MCM-48.
Therefore, the location of Ni nanoparticles in the porous network can lead to a forced
increase of pore diameter, indicating a slight alteration of the structure after nickel
deposition. According to these facts, some differences in the adsorption / desorption
isotherms of Ni/MCM-48 (Figure S2(A)) and pore size distribution (Figure S2B) were
observed in comparison with that from MCM-48. After the deposition of nickel into the
pores of the support, the shape of the adsorption / desorption isotherm changes and a
hysteresis loop appears between P/P0 0.4 – 0.9. The presence of the hysteresis loop in the
case of Ni/MCM-48 is due to the observed modification in pore diameter. It is in good
agreement with the results reported by other authors, where the increase of pore diameter
promoted small increases in the width of the hysteresis loop [30]. The pore size distribution
(dV/dD) did not show a clear maximum, exhibiting a wider diameter distribution and lower
associated pore volumes than the corresponding MCM-48. In the case of bimetallic
catalysts, the decrease of BET surface and pore volume after the introduction of ruthenium
and nickel was smaller, being this decrease sharper at higher nickel loadings. Significant
differences in adsorption / desorption isotherms and pore size distribution of bimetallic
samples (not shown) were not observed in comparison with that obtained for MCM-48.
TEM images, EDS analysis and Ni particle size distributions for all the samples are

presented in Figure 3, 4, 5 and 6. TEM micrograph of the monometallic Ni/MCM-48
(Figure 3(A)) shows pseudospherical MCM-48 particles where small nickel nanoparticles
with low contrast are detected, which could be distributed into the mesoporous network of
MCM-48. X-Ray maps coupled to TEM images for Ni/MCM-48 demonstrated the
homogeneous distribution of nickel nanoparticles into pore structure of MCM-48 (Figure
3(C), left side for Ni and right side for Si). A counting of nickel nanoparticles (> 100
nanoparticles) from TEM images was carried out for Ni/MCM-48 (Figure 3(B)). The
histogram shows a nickel nanoparticle size distribution in the range 0.91-3.20 nm.
Calculations based on equation 2 determined a surface-area weighted diameter of 2.3 nm,
in the range of that obtained from XRD analysis. The small size of nickel nanoparticles can
be related to the small size of MCM-48 channels where metallic nanoparticles were
deposited during the synthesis [32]. Comparing TEM images from bimetallic Ru:Ni/MCM-
48 with Ni/MCM-48, very different results were observed (Figures 4-6). In the case of
Ru:Ni/MCM-48 (0.15) which presented the lower amount of ruthenium, two types of
images were observed. Most of the pictures are similar to that presented in Figure 4(A),
where EDS only detected the presence of nickel. Other images presented irregular
agglomerates of ruthenium particles (Figure 4(B)). Due to the irregular geometry of the
areas where ruthenium was identified and the impossibility of visually difference each
metal, counting of nanoparticles (> 100) was only done for nickel. The distinction of Ni and
Ru areas was carried out by EDS analysis. In this case, a broader nickel nanoparticle size
distribution was obtained with a surface-area weighted diameter of 20.6 nm. Increasing
Ru:Ni ratio up to 0.45, a heterogeneous distribution of the metallic particles was observed,
where three different type of regions were identified: a) irregular agglomerates, where EDS
analysis determined the main presence of ruthenium (Figure 5(A)), b) better dispersed

particles, where EDS only could identified nickel (Figure 5(B)) and c) the combination of
the two previous morphologies, where both ruthenium and nickel were observed (Figure
5(C)). The counting of nickel nanoparticles resulted in a narrower nickel particle size
distribution than the previous one. Surface-area weighted diameter of 19.2 nm was
determined for Ru:Ni/MCM-48 (0.45). Finally, Ru:Ni/MCM-48 (1.39) showed a similar
morphology to Ru:Ni/MCM-48 (0.45). This sample showed nickel regions (Figure 6(A)),
ruthenium regions (Figure 6(B)) and other areas with presence of both (Figure 6(C)). The
narrowest nickel nanoparticle size distribution was obtained for this catalyst, with a surface
area-weighted diameter of 10.9 nm. Nickel nanoparticles were not detected into the pores
of MCM-48 in the case of bimetallic catalysts by TEM; however, this fact cannot be
discarded according to the reduction peaks observed at temperatures higher than 200 ºC,
which can be attributed to small nickel nanoparticles deposited into the pores, as in the case
of the monometallic catalyst. Derived number, surface and volume weighted diameters, as
well as the standard deviation related to number diameter are given in Table 1. Standard
deviations in number diameter were in the range 18 – 34 %. According to the additional
drying and coprecipitation steps in the case of bimetallic catalysts, wider particle size
distributions were obtained compared to Ni/MCM-48, in line with greater values of
standard deviation.
Acidic properties are critical for the use of these catalysts in one-pot applications, which
is the ultimate goal of our research [23, 24]. Acidic features from NH3-TPD of the support
and the reduced catalysts are presented in Table 1 and Figure 7. In general terms, all the
samples showed two ammonia desorption peaks in the temperature range of 170 – 250 ºC
and 520 – 590 ºC, which are related to weak and strong acid sites, respectively. Total

amount of acid sites (mmolNH3·g-1catalyst) is consistent with the following sequence of
increasing acidity: MCM-48 < Ni/MCM-48 < Ru:Ni/MCM-48. Compared to MCM-48
sample, an increase of acid sites was observed after the deposition of nickel into the
framework of the support in the case of Ni/MCM-48 catalyst. In the case of bimetallic
Ru:Ni/MCM-48 catalysts, the presence of ruthenium resulted in a slight increase of acidity
in comparison with the monometallic catalyst. This fact is attributed to the higher trend of
ruthenium atoms to adsorb ammonia molecules [37], while the influence of chlorine atoms
cannot be discarded.
3.3.Activity test: D-Glucose hydrogenation
Hydrogenation of model compounds into sorbitol such as D-Glucose seems a very easy
reaction pathway, but experimentally it is not as simple, since a decrease in the selectivity
into sorbitol can be observed because of D-Glucose conversion into different byproducts by
different ways. D-Glucose transformation into D-Mannose and D-Fructose can be obtained
by Lobry de Bruyn–Alberda van Ekenstein rearrangements [14]. Subsequent hydrogenation
of D-Mannose and D-Fructose produces mannitol and mixtures of sorbitol/mannitol,
respectively. In addition, 5-HMF can be detected from dehydration of D-Glucose, as well
as other derivatives, such as aldehydes [38]. In general terms, sorbitol is the major product
in the catalytic hydrogenation of D-Glucose in hot compressed water, though sorbitol
isomerizes into mannitol, which is the most important by-product of this reaction as well.
Both the features of the carrier as the selected active metal play a very important role in D-
Glucose hydrogenation reactions. In this sense, the monometallic Ni/MCM-48 and
bimetallic Ru:Ni/MCM-48 catalysts were tested in the hydrogenation of D-Glucose at 120
ºC and 2.5 MPa H2 at different reaction times. Then, Ni/MCM-48 and Ru:Ni/MCM-48

(0.45) were used at 130 ºC and 140 ºC in order to check the influence of temperature in the
reaction. Preliminary experiments were carried out in the high pressure reactor, in order to
confirm that the hydrogenation of D-Glucose was not mass transfer limited when stirring
rate was 1400 rpm and catalyst particles presented a particle size smaller than 70 µm,
neither limited by H2 diffusion when pressure was adjusted to 2.5 MPa. According to the
experimental data presented in Figure 8(A) and plotting – Ln (1 - X) versus reaction time in
Figure 8(B), where X is conversion of D-Glucose, it was observed a linear fitting in all the
cases. Given the excess of H2 employed, a pseudo-first order dependence respect to D-
Glucose was detected, which is consistent with previous data reported by other authors.
Wisniak and Simon [39] found a first order dependency respect to D-Glucose concentration
using Raney-Nickel catalysts. Mishra et al. proved that the hydrogenation of D-Glucose
over Ru/HYZ followed a first order dependence respect to D-Glucose as well [38]. In this
sense, a pseudo-first order dependence respect to D-Glucose concentration was observed
during the hydrolytic hydrogenation (Figure 8(B)). Figure 8(A) shows the catalytic
behavior of the different catalysts during the hydrogenation process. A clear difference was
observed between monometallic and bimetallic catalysts, since D-glucose was efficiently
hydrogenated into sorbitol over bimetallic catalysts (100 % selective to sorbitol), while
Ni/MCM-48 showed selectivities to sorbitol in the range of 93-95%. The slight decrease in
sorbitol selectivity was attributed to the isomerization of sorbitol into mannitol, thus the
addition of small amounts of ruthenium improved sorbitol selectivity being 100 % in all
cases. The highest conversion of D- Glucose, around 70 %, was achieved over
Ru:Ni/MCM-48 (1.39) after 90 min at 120 ºC and 2.5 MPa H2. Experimental data presented
in Table 2 provides further information about the behavior of the catalysts. Monometallic
Ni/MCM-48 showed a kinetic constant of 9.7 dm3·g-1·min-1, which corresponds to a slower

reaction rate in comparison with Ru:Ni/MCM-48 (0.45) and Ru:Ni/MCM-48 (1.39), though
kinetic constant for Ni/MCM-48 is around 4.2 times higher than the obtained for
Ru:Ni/MCM-48 (0.15). This catalytic behavior is in good agreement with TEM results,
where smaller and better distributed nickel nanoparticles were observed in the
monometallic catalyst than in Ru:Ni/MCM-48 (0.15). Both this fact and the small amount
of ruthenium presented in Ru:Ni/MCM-48 (0.15) could be not enough to overcome the
catalytic behavior of Ni/MCM-48. In the case of Ru:Ni/MCM-48 (0.45) and (1.39),
reaction rates were around 1.9 and 6.8 times higher than monometallic nickel-based
catalyst, respectively, indicating that larger amounts of ruthenium conducted to more active
catalysts even with less disperse nickel. Monometallic Ni/MCM-48 demonstrated higher
catalytic activity in terms of reaction rate (1.2·10-3 gsorbitol·gmetal-1·s-1) than other nickel-
based catalyst reported in the literature under similar experimental conditions. Zhang et al.
used Alumel catalyst (Ni/Al, 47% nickel metal loading) (Aladdin Reagent Limited
Company. A. P. reagents) and nickel powder (Tianjin Kermel Chemical Reagents Limited
Company) for the hydrogenation of D-Glucose at 120ºC, 3 MPa H2 and 120 min obtaining
reaction rates of 1.2·10-4 gsorbitol·gmetal-1·s-1 and 9.8·10-5 gsorbitol·gmetal-1·s-1, respectively [40].
However, Schimpf et al. working at higher pressure, longer time-on-stream (120 ºC, 12
MPa and 5 h) and using a Ni/SiO2 achieved a reaction rate of 9.9·10-3 gsorbitol·gmetal-1·s-1.
Ni/MCM-48 presented a catalytic activity 8.3 times smaller than those obtained by Schimpf
et al [38]. There is not many information about the hydrogenation of D-Glucose over
Ru:Ni-based catalysts in the literature, but Bizhanov et al. reported the promoting effect of
small amounts of ruthenium (0.1 – 0.5 %) and Palladium (5 %) over unsupported Raney-
Nickel catalysts [22] which can raise its activity by as much as 30 %. In the case of
Ru:Ni/MCM-48 (0.45), the addition of a 0.76 % of ruthenium over Ni/MCM-48 catalyst

improved catalytic activity in a 100 %, approximately. In addition, catalytic activity of all
the samples was calculated as specific reaction rate based on nickel surface area, which was
calculated from the equation 9, where SNi is the specific Ni surface area calculated from
equation 10 and t the reaction time. In equation 10, 6 is a shape factor that assumes a
̅
spherical geometry, which its use is valid based on calculations from TEM analysis, 푑푠 is
surface-area weighted diameter of Ni from equation 2 and ꢁ푁푖 is nickel density (8.9 g·cm-3)
[41]. Specific reaction rate given in Table 2, are consistent with the following sequence of
increasing activity: Ni/MCM-48 < Ru:Ni/MCM-48 (0.15) < Ru:Ni/MCM-48 (0.45) <
Ru:Ni/MCM-48 (1.39). This sequence illustrates the impact of ruthenium addition per
specific surface of nickel. Moreover, Ru/MCM-48 with a ruthenium loading around 4%
reported in a previous work [23], showed a reaction rate of 3.2·10-2 gsorbitol·gmetal-1·s-1 for D-
Glucose hydrogenation at the same experimental conditions, which was 5.8 times higher
than that obtained by Ru:Ni/MCM-48 (1.39).
Ru:Ni/MCM-48 (0.45) was selected to study the influence of temperature in the
conversion of D-Glucose, yield and selectivity to sorbitol. The so obtained results were
compared with those acquired over Ni/MCM-48 (Figure 9) in the same range of
temperatures (120 – 140 ºC). Kinetic constants at each temperature for both catalyst are
given in the caption for Figure 9. An increase of the hydrogenation temperature produced a
slight improvement of the reaction rate using Ni/MCM-48. In addition, a clear decrease in
the selectivity to D-Sorbitol from 95 to 86 % was detected as a result of its isomerization
into mannitol by raising temperature. However, the influence of temperature had a major
effect in the reaction rate for Ru:Ni/MCM-48 (0.45). In this case, the conversion to sorbitol
was significantly increased with temperature from 31 to 59 %, while the selectivity to

sorbitol remained constant (Figure 9). Arrhenius plots for Ni/MCM-48 and Ru:Ni/MCM-48
(0.45) given in Figure 10(A). It can be observed that the activation energy value (Ea) for the
hydrogenation of D-Glucose over Ni/MCM-48 was 36 KJ·mol-1, which is comparable to
the values reported in the literature. Déchamp et al. carried out the hydrogenation of D-
Glucose in a trickle bed reactor in the temperature range 70 – 130 ºC and 8 MPa of H2
pressure over a commercial Silica-Alumina supported nickel catalyst (purchased from
Harshaw, ref. Ni-3266E 1/16 in.) and the authors reported an activation energy around 67
KJ·mol-1 [16]. Brahme et al. studied the hydrogenation of D-Glucose in the temperature
ranges of 77 – 100 ºC and 77 – 146 ºC over a Raney – Nickel catalyst and activation
energies of 6 KJ·mol-1 and 44 KJ·mol-1 were obtained, respectively [42]. Ru:Ni/MCM-48
(0.45) was more sensitive to temperatures changes in comparison with Ni/MCM-48 during
the hydrogenation of D-Glucose, showing a higher activation energy (70 KJ·mol-1). It is not
possible to make a relationship between reaction rates and activation energies for both
catalysts due to the observed differences in terms of pre-exponential factors (Figure 10(A)).
It should be noted that the bimetallic catalyst showed pre-exponential factor five orders of
magnitude higher than the monometallic, pointing out a compensation effect. Bizhanov et
al. reported activation energy values for bimetallic Ni-Pt (0.1 wt % Pt) (50 – 54 KJ·mol-1)
and Ni-Rh (38 – 42 KJ·mol-1 Rh) catalysts in the hydrogenation of D-Glucose at
temperatures between 80 – 130 ºC [22]. Activation energy values obtained for Ni/MCM-48
(36 KJ·mol-1) and Ru:Ni/MCM-48 (0.45) (70 KJ·mol-1) are larger than those obtained for
mass transfer limited processes (12 – 21 KJ·mol-1), which indicates that the reaction rate
was controlled by the kinetics on the metal surface.

In order to check catalyst stability, RuNi/MCM-48 (0.45) was recovered after each
experiment and tested in D-Glucose hydrogenation at 120 ºC after three cycles. Results
shown in Figure 10(B) confirmed that catalytic activity of Ru:Ni/MCM-48 (0.45) was not
affected by the reusing of the catalyst. It was observed a slight decrease in the yield of
sorbitol from 31 % to 29 % after three reaction cycles, probably due to the formation of
impurities over the active surface of the catalyst, while the selectivity to sorbitol was 100 %
in all cases, demonstrating the good stability of Ru:Ni/MCM-48 (0.45) under the
experimental conditions.
4. Conclusions
As a result of the deposition of different amounts of Ru over Ni/MCM-48, significant
differences were observed related to catalyst properties and thus in their behavior during
hydrogenation of D-Glucose in comparison with monometallic Ni/MCM-48. According to
the results presented above, the following conclusions were obtained:
i)
The addition of different amounts of ruthenium over monometallic Ni/MCM-48
improved the reducibility of nickel and ruthenium species into their metallic
state, respectively.
ii)
Calculation from TEM images demonstrated that the preparation of bimetallic
Ru:Ni catalyst by consecutive wet impregnation caused an increase of the nickel
crystallite size in comparison with the monometallic catalyst. However, the
higher Ru:Ni ratio, the smaller the nanoparticle size of nickel.
iii)
The presence of Ru:Ni ratios higher than 0.45 in the materials improved the
catalytic behavior of the monometallic system in the catalytic hydrogenation of

D-Glucose, increasing the reaction rate and showing complete selectivities to
sorbitol.
iv)
Hydrogenation of D-Glucose into sorbitol was also carried out at different
temperatures over Ni/MCM-48 and Ru:Ni/MCM-48 and activation energies of
ca. 36 KJ·mol-1 and 70 KJ·mol-1 were obtained. Ru:Ni/MCM-48 (0.45) showed
a good catalytic behavior at higher temperatures than 120 ºC, enhancing the
reaction rate but maintaining a stable selectivity to sorbitol.
A good stability after three reaction cycles was observed for Ru:Ni/MCM-48
(0.45).
v)
vi)
In accordance with the results here presented, Ru:Ni/MCM-48 (0.45) stands as a
good option for the efficient hydrogenation of carbohydrate sugars into sugar
alcohols.
Acknowledgements
The authors gratefully acknowledge the Spanish Ministry, MINECO, and FEDER
founds for the financial support of this project CTQ2015-64892-R (MINECO/FEDER). A.
Romero thanks to the program of predoctoral scholarships from Junta de Castilla y León
Government for his grant (E-47-2015-0062773).
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Figure captions
Figure 1. H2-TPR profiles.
Figure 2. (A) XRD patterns of Ni/MCM-48, Ru:Ni/MCM-48 (0.15), Ru:Ni/MCM-48 (0.45) and
Ru:Ni/MCM-48 (1.39) and (B) expanded region for bimetallic catalysts.
Figure 3. (A) Transmission Electron Microscopy (TEM) micrographs, (B) Ni particle size distribution and
(C) Mapping images of Ni/MCM-48.
Figure 4. Transmission Electron Microscopy (TEM) micrographs and EDS of (A) nickel area and (B)
bimetallic area of Ru:Ni/MCM-48 (0.15), (C) Ni particle size distributions from TEM images of
Ru:Ni/MCM-48 (0.15).
Figure 5. (A,B,C) Transmission Electron Microscopy (TEM) micrographs and EDS of Ru:Ni/MCM-48
(0.45). (D) Ni particle size distributions from TEM images.
Figure 6. (A,B,C) Transmission Electron Microscopy (TEM) micrographs and EDS of Ru:Ni/MCM-48
(1.39). (D) Ni particle size distributions from TEM images.
Figure 7. NH3-TPD patterns for reduced catalysts.
Figure 8. (A) Evolution of D-Glucose conversion (line + symbol) and selectivity (open symbol) as a function
of reaction time and (B) pseudo-first fitting at C:Ru = 142, 120 ºC, 2.5 MPa H2 and 1400 rpm for ● Ni/MCM-
48, ■ Ru:Ni/MCM-48 (0.15), ▲ Ru:Ni/MCM-48 (0.45) and▼Ru:Ni/MCM-48 (1.39).
Figure 9. Effect of reaction temperature (120, 130, 140 ºC) in conversion of D-Glucose and selectivity to
sorbitol over ▲
Ru:Ni/MCM-48 (0.45) (k120ºC = 1.8·10-2, k130ºC = 3.4·10-2 and k140ºC = 5.1·10-2 dm3·g-1·min-1) at C:Ru
Ni/MCM-48 (k120ºC = 9.7·10-3, k130ºC = 1.3·10-2 and k140ºC = 1.7·10-2 dm3·g-1·min-1) and
■
= 142, 2.5 MPa H2, 1400 rpm and 90 min.
Figure 10. (A) Arrhenius plots of D-Glucose hydrogenation over ■ Ni/MCM-48 and ● Ru:Ni/MCM-48
(0.45) and (B) Stability test of Ru:Ni/MCM-48 (0.45) at C:Ru = 142, 120 ºC, 2.5 MPa H2, 1400 rpm and 90
min.

Figure 1
Ru:Ni/MCM-48 (1,39)
Ru:Ni/MCM-48 (0,45)
Ru:Ni/MCM-48 (0,15)
Ni/MCM-48
0
50
100
150
200
250
300
350
400
Temperature (ºC)