Ruthenium on phosphorous-modified alumina as an effective and stable catalyst for catalytic transfer hydrogenation of furfural

Article abstract of DOI:10.1039/d0ra00415d

Supported ruthenium was used in the liquid phase catalytic transfer hydrogenation of furfural. To improve the stability of Ru against leaching, phosphorous was introduced on a Ru/Al₂O₃ based catalyst upon impregnation with ammonium hypophosphite followed by either reduction or calcination to study the effect of phosphorous on the physico-chemical properties of the active phase. Characterization using X-ray diffraction, solid state ³¹P nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy, temperature programmed reduction with H₂, infrared spectroscopy of pyridine adsorption from the liquid phase and transmission electron microscopy indicated that phosphorous induces a high dispersion of Ru, promotes Ru reducibility and is responsible for the formation of acid species of Br?nsted character. As a result, the phosphorous-based catalyst obtained after reduction was more active for catalytic transfer hydrogenation of furfural and more stable against Ru leaching under these conditions than a benchmark Ru catalyst supported on activated carbon.

Full text of DOI:10.1039/d0ra00415d

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Ruthenium on phosphorous-modified alumina as
an effective and stable catalyst for catalytic transfer
hydrogenation of furfural†
Cite this: RSC Adv., 2020, 10, 11507
ab
c
*
Thibault Fovanna,
‡
Sebastiano Campisi, ‡c Alberto Villa,
Anastasios Kambolis, §a Gael Peng,{ Daniel Rentsch,
Oliver Krocher,
¨
a
d
ab
a
a
*
Maarten Nachtegaal
and Davide Ferri
Supported ruthenium was used in the liquid phase catalytic transfer hydrogenation of furfural. To improve
the stability of Ru against leaching, phosphorous was introduced on a Ru/Al2O3 based catalyst upon
impregnation with ammonium hypophosphite followed by either reduction or calcination to study the
effect of phosphorous on the physico-chemical properties of the active phase. Characterization using X-
ray diffraction, solid state 31P nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy,
temperature programmed reduction with H2, infrared spectroscopy of pyridine adsorption from the
liquid phase and transmission electron microscopy indicated that phosphorous induces a high dispersion
of Ru, promotes Ru reducibility and is responsible for the formation of acid species of Brønsted
character. As a result, the phosphorous-based catalyst obtained after reduction was more active for
catalytic transfer hydrogenation of furfural and more stable against Ru leaching under these conditions
than a benchmark Ru catalyst supported on activated carbon.
Received 14th January 2020
Accepted 10th March 2020
DOI: 10.1039/d0ra00415d
rsc.li/rsc-advances
2-methylfuran and furfuryl alcohol (Scheme 1).5 The latter is the
result of a selective single-step reduction of the carbonyl group of
1 Introduction
furfural and is a key intermediate in the production of lysine,
ascorbic acid (vitamin C), polymers, synthetic bers, rubbers,
resins and lubricants.6 The conventional catalytic hydrogenation
process currently used for producing furfuryl alcohol requires high
Biomass can be converted into a pool of highly functionalized,
platform molecules1,2 to enable the synthesis of ne chemicals,
pharmaceuticals, intermediates, polymers and additives. The
production of value-added compounds from biomass involves
catalytic transformations, where the catalyst design represents
a key factor for achieving the desired efficiency and selectivity.3
Furfural (furan-2-carbaldehyde) is an example of a building
block from hemicellulose-derived pentoses, which can be trans-
formed into various compounds of industrial interest.4 The
hydrogenation of furfural can proceed along several pathways
leading to various useful chemicals such as tetrahydrofuran, furan,
ꢀ
temperature (130–200 C) and pressure (up to 30 bar) and toxic
elements, i.e. copper chromite.7 Therefore, the development of
heterogeneous catalytic processes operating under mild condi-
tions and with less-toxic metal catalysts remains a priority to
achieve a more sustainable industrial process. In this view, the
catalytic transfer hydrogenation (CTH), consisting in the use of
organic molecules as sources of hydrogen, represents a valid
alternative, which minimizes safety hazards deriving from
handling high pressure hydrogen gas.8
aPaul Scherrer Institut, CH-5232 Villigen PSI, Switzerland. E-mail: davide.ferri@psi.
A large variety of metal-based catalysts have been proposed in
the literature as alternatives for CTH9 and include cobalt,10 nickel,11
copper,12 platinum,13 palladium14 and ruthenium.15 Ruthenium
emerged by its superior activity and efficiency under liquid phase
conditions and by its lower cost compared to other precious
metals.16 Ru/Al2O3 (ref. 17–20) and Ru on carbonaceous supports21
are well studied hydrogenation catalysts. Interest in Ru as an
effective metal for the hydrogenation of biomass-derived carbonyl
compounds in the aqueous phase22–25 has probably increased due to
its unique oxophilicity and ability to dissociatively adsorb water
molecules.26 However, Ru-based catalysts oen suffer from the lack
of stability, due to passivation, poisoning or leaching of Ru
species.27 Besides the possibility to alloy Ru with Au to improve the
ch; Tel: +41 56 310 2781
b
´
´ ´
Ecole polytechnique federale de Lausanne (EPFL), Institute of Chemical Sciences and
Engineering, CH-1015 Lausanne, Switzerland
c
`
Dipartimento di Chimica, Universita degli Studi di Milano, I-20133 Milano, Italy.
E-mail: alberto.villa@unimi.it; Tel: +39 02 503 14361
dSwiss Federal Laboratories for Materials Science and Technology (Empa),
¨
¨
Uberlandstrasse 129, CH-8600 Dubendorf, Switzerland
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra00415d
‡ These authors contributed equally to the work.
§ Present address: CheMa Laboratories, Xenofontos 2, GR-Korinthos 201 31,
Greece.
´
´ ´
{ Present address: Ecole polytechnique federale de Lausanne (EPFL), Institute of
Chemical Sciences and Engineering, CH-1015 Lausanne, Switzerland.
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Scheme 1 Reaction pathways for the catalytic hydrogenation of furfural.
stability for hydrogenation reactions,28 modiers and promoters
a
solution of ruthenium(III) nitrosyl nitrate (1.5 wt%
can be used to increase the efficiency and the stability of the cata- Ru(NO)(NO3)3 in diluted HNO3, Sigma-Aldrich) in order to
lysts.29 Organic linkers,30 metal oxides31 as well as heteroatoms achieve 5 wt% metal loading. Aer drying at 60 ꢀC/170 mbar in
introduced on the support32 can commonly act as promoters by a rotary evaporator, a portion of Ru_dry was calcined in air in
anchoring metal species, introducing defects or stabilizing partic- a muffle oven at 400 ꢀC for 3 h to obtain Ru_O2. A second aliquot
ular oxidation states of the active metal.33 P-containing functional- of Ru_dry was reduced at 500 ꢀC in 30 vol% H2 (Ru_H2). A third
ities (e.g. phosphates, phosphides) form a broad class of promoters, portion was further impregnated with a solution of ammonium
which are experiencing a growing interest in the eld of heteroge- hypophosphite (NH4H2PO2, Sigma-Aldrich) to a P/Ru ratio of
neous catalysis for biomass valorization.34–45 Besides acting as 1.4. Aer drying in a rotary evaporator at 60 C/170 mbar, an
ꢀ
ligands for the stabilization of metal species, phosphorous intro- aliquot of the material (RuP_dry) was reduced in 30 vol% H2 at
ꢀ
ꢀ
duces Brønsted acid sites on the support surface and promotes 500 C for 4 h, cooled to 180 C in the same atmosphere and
electron transfer phenomena resulting in enhanced activity.36,40
then under N2 to ambient temperature. Finally, the powder was
In this work, we investigate the effect of the modication of exposed to 1 vol% O2 at room temperature for 30 min (sample
alumina with phosphorous on the catalytic performance of Ru labelled as RuP_H2). For comparison, a portion of RuP_dry was
towards catalytic transfer hydrogenation of furfural in the calcined at 400 ꢀC (RuP_O2) for 3 h. An additional sample
presence of isopropanol as hydrogen donor. A suite of charac- (P_Al2O3) was prepared by impregnation of g-Al2O3 with
terization techniques (X-ray powder diffraction, X-ray absorp- NH4H2PO2 to achieve 5 wt% P loading. The material was dried
ꢀ
tion spectroscopy, attenuated total reectance infrared as described above then calcined at 400 C for 3 h. The overall
spectroscopy, temperature programmed reduction, solid-state sample preparation is summarized in Scheme 2.
nuclear magnetic resonance spectroscopy, transmission elec-
tron microscopy) was used to clarify the nature, structure,
speciation and coordination of Ru species as well as of P
functionalities on the alumina surface. This information
allowed rationalizing the catalytic results in the frame of
appropriate structure–activity relationships.
2.2 Characterization
X-ray powder diffraction – XRPD. XRPD patterns were
collected using a Bruker Advance D8 diffractometer (Cu-Ka
radiation) in the 2q range 15–90ꢀ with a step scan of 0.03ꢀ.
Specic surface area. N2 physisorption was performed with
a Quantachrome Autosorb-1 instrument at liquid N2 temperature.
Prior to N2 adsorption, the sample was outgassed at 250 ꢀC for 3 h.
Transmission electron microscopy – TEM. Transmission
electron micrographs were recorded using a JEOL 2010 at an
2 Experimental
2.1 Catalyst synthesis
The catalyst precursor (Ru_dry) was obtained by impregnation acceleration of 200 keV and equipped with a LaB6 lament,
of g-Al2O3 (PURALOX NWa155, Sasol; 132 m2 gꢁ1) with using the INCA soware and a CCD camera (Orius, GATAN). The
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Scheme 2 Synthesis approach for the preparation of the various materials.
powders were suspended in isopropanol, and a 5 mL drop of this mrad, monochromatized by a continuous scanning Si(111)
suspension was deposited on a holey carbon lm supported on channel cut monochromator and subsequently focused using
3 mm copper grid.
a Pt coated toroidal mirror to a spot size of 100 ꢂ 100 mm at the
Temperature programmed reduction – H2-TPR. H2-TPR sample position. XAS spectra were background corrected,
experiments were carried out in a dedicated instrument normalized and energy corrected using the Demeter soware
(TPDRO 1100, ThermoScientic) equipped with a thermal package.46 Spectra of metallic Ru (mesh) and RuO2 were
conductivity detector (TCD). The sample (100 mg) was loaded at collected for reference purposes.
the bottom of a quartz reactor. H2-TPR proles were collected in
Operando experiments were carried out in a ow cell adapted
the temperature range of 30–500 ꢀC at a heating rate of for liquid phase experiments.47 The catalyst (35 mg) was loaded
5 C minꢁ1 in a gas ow of 10 vol% H2/Ar (20 mL minꢁ1).
ꢀ
between two quartz wool plugs. The cell was connected to a HPLC
Nuclear magnetic resonance spectroscopy – NMR. Single pump (Azura P 4.1S 10 mL minꢁ1 head; Knauer) and set to a ow
pulse magic angle nuclear magnetic resonance spectroscopy (MAS of 0.2 mL minꢁ1. At the outlet of the cell a backpressure regulator
NMR) experiments were performed at room temperature on (KCB series, Swagelok) maintained the reactive environment in
a Bruker Avance 400 MHz spectrometer using a 2.5 mm CP/MAS liquid phase at 16 bar. The catalyst bed was recorded dry at room
probe at spinning rates of 25 kHz. The 27Al and 31P NMR chem- temperature before allowing the Ar-saturated 2-propanol solution
ical shis were referenced to external samples of a 1.1 M solution of furfural (5 mM; Sigma-Aldrich, 99.9%) to enter the cell. Quick
of Al(NO3)3 in D2O and of solid NH4H2PO4 at 0.0 ppm, respectively. EXAFS spectra were ꢀrecorded for 1 min every 30 ꢀC until the
X-ray absorption spectroscopy – XAS. Transmission X-ray reactor reached 180 C. Once the desired reaction temperature
absorption spectra were obtained at the Ru K-edge (22 117 eV) was attained, the catalyst bed was le under reaction conditions
at the Swiss Norwegian beamline (SNBL) of the European for 30 min while collecting samples for analysis by gas chroma-
Synchrotron Radiation Facility (ESRF, Grenoble, France) and tography (see below) every 15 min. In order to improve the signal
the SuperXAS beamline of the Swiss Light Source (SLS, Paul to noise ratio, the time-resolved spectra were averaged over the
Scherrer Institute, Switzerland) using 15 cm long ionization whole acquisition period (1 min, 60 spectra). Fourier transforꢁm1
ꢁ1
˚
˚
chambers lled with 1 bar N2 and 1 bar Ar. A reference spectrum was performed in the k-range of 3–15 A for Ru_H2 and 3–11 A
of a Ru metal mesh was measured simultaneously with the for RuP_H2. The Fourier transformed spectra were not phase-
sample between the second and the third ionization chamber shi corrected. A curve tting analysis was carried out on the
˚
for energy calibration. At SNBL, the white beam of a 0.8 T ESRF data in the R-range of 1.9–3.0 A. Only the rst coordination shell
bending magnet was monochromatized by a Si(111) double was considered and metallic Ru was used as reference. The
crystal monochromator. Harmonic suppression was obtained amplitude reduction factor was set to 0.78 aer tting the EXAFS
by detuning the second crystal to 65% throughput. The beam spectra of the Ru mesh.
size on the sample was set to 5 to 0.5 mm (l ꢂ h). At the
Pyridine adsorption by ATR-IR. The acidity of the materials
SuperXAS beamline, the polychromatic beam from the 2.9 T was studied by adsorption of pyridine using attenuated total
bending magnet was collimated by a Pt coated mirror at 2.5 reection infrared spectroscopy (ATR-IR). An aqueous slurry of
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the catalyst sample (10 mg in 1.5 mL) was allowed to dry over- a Thermo Scientic Trace ISQ QD Single Quadrupole GC-MS
night on the ZnSe internal reection element (IRE, 45ꢀ, 52 ꢂ 20 equipped with a capillary column (HP-5 30 m ꢂ 0.32 mm,
ꢂ 2 mm; Crystran) under the fume-hood. The coated IRE was 0.25 mm lm; Agilent Technologies).
mounted in a home-made cell and installed on a commercial
For catalyst recycling tests, each run was carried out under
four-mirror vertical ATR-IR assembly (Specac) within the sample the same conditions (furfural ¼ 0.3 M; F/Ru mol ratio, 100;
ꢀ
compartment of the IR spectrometer (Vertex 70, Bruker). 180 C, 5 bar N2). The catalyst was recycled to be used in the
Spectra were recorded at 20 kHz scanner velocity by averaging subsequent run (reaction times of 3 h each) aer ltration
60 scans at a spectral resolution of 4 cmꢁ1 using a liquid N2 without any further treatment.
cooled MCT detector. The catalyst layer was equilibrated in
cyclohexane at room temperature for 1 h prior to collection of
the background spectrum. Then, a 5 mM pyridine solution in
cyclohexane was allowed to enter the ATR-IR cell and adsorption
3 Results and discussion
3.1 Catalyst preparation and characterization
was monitored for 30 min followed by desorption in cyclo- The various catalyst materials that are compared in this study
hexane for 30 min. For comparison purposes, pyridine were prepared according to Scheme 2. Ruthenium was depos-
adsorption was carried out also on H-ZSM5, a material con- ited on g-Al2O3 by wet impregnation from ruthenium(III) nitro-
taining only Brønsted acid sites allowing for clear identication syl nitrate solution to attain a nal metal loading of 5 wt%. Aer
of pyridine adsorption on these sꢀites (Si : Al mol ratio 11.3, drying, Ru_dry underwent nitrosyl nitrate precursor decompo-
ABCR, 379 m2 gꢁ1, calcined at 550 C in air prior to use).
sition either by calcination in air (Ru_O2) or by reduction in H2
(Ru_H2). For both samples, specic surface areas of 169 and 163
m2 gꢁ1 were determined, respectively.
2.3 Catalytic activity
The P-promoted catalysts were prepared similarly by sequential
wet impregnation with ammonium hypophosphite. Aer
impregnation with the ruthenium precursor, Ru_dry was con-
tacted with the phosphite containing solution. The material con-
taining 7.5 wt% P (RuP_dry) was subsequently calcined (RuP_O2;
131 m2 gꢁ1) or reduced (RuP_H2; 126 m2 gꢁ1). The specic surface
area of the resulting samples decreased to approximately the value
obtained for the support (Al2O3, 132 m2 gꢁ1).
Furfural hydrogenation was performed at 180 ꢀC, using
a stainless steel reactor (30 mL capacity), equipped with heater,
mechanical stirrer, gas supply system and thermometer. The
furfural solution (15 mL; 0.3 M in 2-propanol) was added into
the reactor at room temperature and the desired amount of
catalyst (F/Ru ratio ¼ 100 mol molꢁ1) was suspended in the
solution. The N2 pressure was 5 bar. The mixture was heated to
the reaction temperature and was then mechanically stirred
(1250 rpm). At the end of the reaction, the autoclave was cooled
to room temperature, N2 ow stopped and the autoclave purged
with owing N2. Samples were removed periodically (0.2 mL)
using a micro-syringe through a withdrawal valve and were
analyzed by gas chromatography (HP 7820A gas chromatograph
equipped with a capillary column HP-5 30 m ꢂ 0.32 mm, 0.25
mm lm; Agilent Technologies). Standard solutions of reactants
and products were analyzed to determine separation times.
Quantitative analysis was performed using an external standard
(n-octanol). Identication of the products was performed using
The structural analysis by X-ray powder diffraction (XRPD,
Fig. 1) helped to identify the composition and structure of Ru
containing crystallites. Calcination of the Ru precursors on P-
free Al2O3 (Ru_O2) and aer impregnation with the
P
precursor (RuP_O2) produced large crystallites of RuO2 as sug-
gested by the presence of sharp reections of this phase.
Conversely, reduction (Ru_H2) produced metallic Ru crystallites
of ca. 11 nm in diameter as estimated using the Scherrer
equation (Table S1†). The XRPD pattern of RuP_H2 displayed
only the reections of the Al2O3 support and no specic
reection of RuO2 nor metallic Ru could be detected. Trans-
mission electron micrographs of Ru_H2 and RuP_H2 (Fig. S1†)
conrmed the absence of well-dened particles in RuP_H2. This
observation suggests that the presence of phosphorous allowed
Ru to disperse uniformly on Al2O3 and prevented the growth of
Ru or RuO2 particles. It also provides a possible explanation for
the observed lower increase of surface area aer Ru wet
impregnation compared to the corresponding P-free materials.
On the other hand, phosphorous did not prevent the formation
of large RuO2 crystallites when RuP_dry was subjected to
calcination at 400 ꢀC (sample RuP_O2). Hence, reduction is
required to disperse Ru in the presence of phosphorous.
Information on the local environment of Ru was determined
by analysis of the Ru K-edge X-ray absorption near edge structure
(XANES, Fig. S2†) and extended X-ray absorption ne structure
(EXAFS, Fig. 2) spectra. The XANES spectra of Ru_O2 and RuP_O2
exhibited the typical white line of RuO2 (Fig. S2†) conrming the
presence of RuO2 already identied by XRPD. The edge energy
position of Ru_H2 was close to that of metallic Ru while the edge
Fig. 1 XRD patterns of Ru catalysts and of the aluminum oxide
reference material.
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of RuP_H2 was shied to higher energy suggesting the presence
of partially oxidized Ru species. The attenuation of the features of
the white line intensity of RuP_H2 compared to the intensity of
the Ru reference suggests the presence of small Ru domains. The
FT-EXAFS spectra of the two reduced samples (Fig. 2) are very
˚
different. Ru_H2 presented coordination shells up to 10 A, sug-
gesting the presence of large Ru metal particles, while RuP_H2
did not exhibit further coordination shells past the rst Ru–O
˚
coordination shell (at ca. 1.6 A). Hence, RuP_H2 possessed
smaller particles or clusters, thus justifying the absence of a Ru-
containing phase in XRPD and of evident particles in TEM.
The differences in the extent of aggregation, the oxidation state
and the coordination environment of Ru are expected to inuence
also the reactivity of Ru. Hence, the reducibility of the materials was
investigated by temperature programmed reduction (H2-TPR). The
H2-TPR proles of Ru_H2 and RuP_H2 shown in Fig. 3 conrmed
that the different Ru dispersion and aggregation also affected the
reducibility of Ru. The P-free sample presented a reduction peak at
Fig. 3 H2-TPR of Ru_H2 and RuP_H2.
pyridinium ion coordinated to Brønsted acid sites (BAS) were
ꢀ
negligible. Despite the similarities, signicant information was
195 C, while highly dispersed Ru in sample RuP_H2 was already
obtained from the inspection of the band at ca. 1490 cmꢁ1
,
reduced at 98 ꢀC, in agreement with the increased difficulty to
reduce small oxide particles than large particles. The opposite
behavior displayed by the small Ru domains present on RuP_H2
reveals that phosphorous decreases the activation barrier for
hydrogen dissociation on small Ru clusters.
which is attributed to the interaction of pyridine with both BAS
and LAS. The intensity of this signal is typically higher than that
of the signal at 1540 cmꢁ1 (BAS). It was centered at 1491 cmꢁ1 in
the case of pyridine adsorbed on RuP_H2, while the position
changed to 1485 cmꢁ1 in the case of Ru_H2 (Fig. 4a). While the
signal was asymmetric in both samples, the spectra obtained
aer pyridine desorption clearly conrmed the existence of two
signals close in energy. Pyridine adsorption on H-ZSM-5, which
Based on the characterization focused on Ru species, it
appears clear that phosphorus induces an improvement of the
dispersion of Ru on alumina. Phosphorus groups stabilize Ru
species of low nuclearity, which are characterized by a more
pronounced tendency to split hydrogen compared to larger Ru
species immobilized on P-free alumina.
contains mainly BAS (Fig. S3†), shows the characteristic signals
of BAS at 1540 and 1490 cmꢁ1
. In the absence of the former
49
signal, which is likely too weak to be detected in the spectrum of
pyridine adsorbed on RuP_H2, the signal at 1490 cmꢁ1 can be
taken as evidence of a contribution from pyridine coordinated
to BAS. Thus, phosphorous modied the acidity of alumina by
increasing the tendency of the surface to donate protons.
The interaction between phosphorus and alumina and thus
the nature of phosphorous was studied by magic angle spinning
nuclear magnetic resonance (MAS-NMR). The broad resonance
at ꢁ10 ppm observed in the 31P MAS-NMR spectra of RuP_H2
(Fig. 4b) can be assigned to surface phosphate species.50,51 The
same line shape was noticed in the spectrum of RuP_O2, indi-
cating that the same phosphate species must be present in both
cases irrespective of the synthesis conditions. The shi from the
value of 0 ppm of the NH4H2PO2 precursor provides an addi-
tional proof of the interaction of phosphorous with alumina by
the formation of an amorphous aluminum phosphate species
that is not visible by XRPD because it is not yet a well crystallized
AlPO4 phase. The similarity of the spectrum of P_Al2O3 to those
of RuP_H2 and RuP_O2 conrmed that no specic interaction
with Ru could be detected. No additional information was ob-
tained from the 27Al MAS-NMR data (Fig. S4†).
Understanding the role of phosphorous includes also anal-
ysis of possible changes in the acidity of the samples. The type
of acidity (Brønsted or Lewis) of Ru_H2 and RuP_H2 was
investigated by adsorption of pyridine in cyclohexane solvent
using ATR-IR (Fig. 4a). The spectra of both samples (Fig. S3†)
exhibited signals of pyridine coordinated to Lewis acid sites
(LAS) at 1448 cmꢁ1 (n19b) and 1610 cmꢁ1 (n8a).48 The vibrations at
1540 cmꢁ1 (n19b mode) and 1637 cmꢁ1 (n8a) corresponding to the
In summary, the characterization data indicate that phospho-
rous is present in the form of phosphate groups on the surface of
alumina, likely anchoring sites for Ru species, thus ensuring the
observed high metal dispersion and nally, it changes the acid
properties of the surface by providing more BAS.
Fig. 2 Ru K-edge FT-EXAFS of Ru catalysts and of the Ru0 and RuO2
references.
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Fig. 4 (a) Spectral region of interest of ATR-IR spectra of pyridine adsorption (solid) and desorption (dashed) on Ru_H2 and RuP_H2. Enlarged
region of the spectra are provided in Fig. S3.† (b) 31P MAS NMR spectra of RuP_H2, RuP_O2 and of reference materials.
ltered solution evidenced 4% of Ru leaching in the case of
RuP_H2 between the rst and the second run and no further
leaching in the subsequent runs. On the contrary, Ru_H2 leached
10% of Ru between the rst and the second run, and another 5%
in the subsequent catalytic test. For comparison, 7% of Ru leached
into the solution in the case of Ru/AC in the same sequence.
These results indicate that besides increasing the dispersion
and the reducibility of Ru, phosphorus strengthens the inter-
action between Ru and alumina. We consider this behavior
a result of the increased dispersion of the active phase and the
presence of phosphate groups.
The effect of reduction of oxidized Ru species on the activity
was investigated by reducing the catalysts (7 bar H2, 150 ꢀC, 1 h)
in situ prior to reaction. The catalytic activity and product
selectivity are summarized in Table 2. The benecial effect of
the pre-reduction is evident from comparison with the data of
Table 1. RuP_O2, in particular, reproduced the same conversion
prole of the reduced counterpart RuP_H2 (Fig. S6†), thus
conrming the hypothesis that metallic ruthenium favors the
hydrogenation reaction. In situ reduction produced a slight
decrease in the activity of RuP_H2, likely in agreement with the
observation that the co-existence of RuOx species and Ru metal
has a positive impact on the performance of Ru-based cata-
lysts.53 Therefore, the lower activity of pre-reduced RuP_H2 is
ascribable to the complete reduction of the dispersed RuOx
species (most likely Ru–OH species as indicated by EXAFS)
present in the pristine catalyst. Whether remainders of oxidized
Ru were present on Ru_O2 and RuP_O2 aer reduction was not
ascertained.
3.2 Catalytic activity
Table 1 shows the selectivity and activity data of the prepared
catalysts compared to a commercial 5 wt% Ru supported on
activated carbon (Ru/AC) as benchmark catalyst for the liquid
phase catalytic transfer hydrogenation of furfural in batch
conditions. We identied furfuryl alcohol as the major product
for all materials.
Ru_O2 and RuP_O2 demonstrated the lowest activity, likely
due to the fact that reduction of RuO2 needs to take place rst
(Fig. 5a). The hydrogen-treated sample (RuP_H2) exhibited the
highest activity, 93% selectivity to furfuryl alcohol aer 15 min
of reaction and 58 (molF –molF ) molRuꢁ1 hꢁ1 compared to 84%
0
i
for the benchmark catalyst (38 (molF –molF ) molRuꢁ1 hꢁ1). This
0
i
improved performance can be attributed to the superior
reducibility and the presence of Ru metal. Ru/Al2O3-based
catalysts did not produce 2-methylfuran, which was observed in
the case of Ru/AC.
Whether the additional acidity determined by pyridine
adsorption impacts catalytic activity and selectivity cannot be
completely excluded from the present data. Brønsted acidity
was shown to promote etherication reactions52 and Table 1
shows that RuP_H2 promoted the production of ethers at 80%
conversion of furfural (14% selectivity towards ethers)
compared to the P-free catalysts but at the expense of selectivity
to furfuryl alcohol. This may indicate that the presence of
Brønsted acidity at the surface of the solid catalyst may be
a strategy to direct the reaction towards etherication (Fig. S5†).
The stability of the catalysts to the reaction conditions and the
liquid phase environment was evaluated by performing six
consecutive recycling tests (Fig. 5b). Both Ru_H2 and RuP_H2
experienced a decrease in furfural conversion between the rst and
the second run. However, the extent of activity loss was signi-
cantly higher for the P-free catalyst. Elemental analysis of the
Operando XAS experiments were performed on the most
active of the pristine catalysts, i.e. Ru_H2 and RuP_H2 to
monitor the Ru speciation under reaction in ow. Fig. 6 shows
the FT-EXAFS spectra obtained while heating an Ar-saturated 2-
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Fig. 5 Catalytic transfer hydrogenation of furfural: (a) furfural conversion without catalyst pre-reduction and (b) consecutive leaching tests.
Table 1 Catalytic transfer hydrogenation of furfural on the indicated catalysts without pre-reduction
Selectivityf (%)
Tetrahydro furfuryl
alcohol
Samplea
Ru/AC
Activityb
Furfuryl alcohol
2-Methyl furan
Ethers
38
40
58
84c
80d
98c
96e
93c
81e
86c
70c
—
2d
2c
12c
16d
—
—
—
2e
3c
2d
—
3e
Ru_H2
1e
2c
RuP_H2
5c
3e
3c
14e
10c
9c
Ru_O2
RuP_O2
14
18
1c
18c
1c
a
Reaction conditions: F0 ¼ 0.3 M; F/Ru ratio ¼ 100 mol molꢁ1, 180 ꢀC, 5 bar N2. F0, initial concentration of furfural. b (molF –molF ) (molRu
) ;
ꢁ1 hꢁ1
0
i
molF , initial mol of F; molF , mol of F at time i (i ¼ 15 min). c At 30% conversion. d At 50% conversion. e At 80% conversion. f Selectivity is calculated
0
i
as Sj ¼ 100 molj (molF –molF )ꢁ1; molF , initial mol of F; molF , mol of F at conversion value k (k ¼ 30 and 80%).
0
k
0
k
Table 2 Catalytic transfer hydrogenation of furfural on the indicated catalysts after in situ pre-reduction
Selectivitye (%)
Tetrahydro-furfuryl
alcohol
Samplea
Ru/AC
Activityb
42
Furfuryl alcohol
2-Methyl-furan
Ethers
80c
76d
84c
88c
70c
62c
—
2d
18c
20d
—
1c
1d
Ru_H2
RuP_H2
Ru_O2
38
48
30
42
4c
10c
11c
9c
1c
—
18c
17c
1c
RuP_O2
2c
17c
a
Reaction conditions: F0 ¼ 0.3 M; F/Ru ratio ¼ 100 mol molꢁ1, 180 ꢀC, 5 bar N2. F0, initial concentration of furfural. b (molF –molF ) (molRu
) ;
ꢁ1 hꢁ1
0
i
molF , initial mol of F; molF , mol of F at time i (i ¼ 15 min). c At 30% conversion. d At 80% conversion. e Selectivity is calculated as Sj ¼ 100 molj
(molF0 –molF )ꢁ1; molF , initial mol of F; molF , mol of F at conversion value k (k ¼ 30 and 80%).
i
0
k
0
k
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Fig. 6 Operando non-phase shift-corrected FT-EXAFS spectra at the Ru K-edge under reaction conditions of Ru_H2 and RuP_H2 determined
from 30 ꢀC to 180 ꢀC. Conditions: 5 mM furfural in Ar-saturated 2-propanol at a flow rate of 0.2 mL minꢁ1, under 16 bar and with 35 mg of
material.
Table 3 EXAFS fitting results of operando spectra during continuous flow catalytic transfer hydrogenation of furfural
b
c
2
CNa
R (A)
DW (A )
DEd (eV)
R factore
Average sizef (nm)
˚
˚
Sample
Ru_H2
RuP_H2
8.12 ꢃ 1.1
4.92 ꢃ 1.0
2.67 ꢃ 0.01
2.67 ꢃ 0.01
0.006 ꢃ 0.001
0.009 ꢃ 0.002
3.7 ꢃ 1.0
3.6 ꢃ 1.4
0.024
0.019
1.2
0.7
a
b
c
d
e
Coordination
number.
Interatomic
distance.
Debye–Waller
factor.
Shi
in
the
edge
energy.
Dened
as
P
P
2
2
2
2
f
R ¼ ½ReðceðRiÞ ꢁ ccðRiÞÞ þ ImðceðRiÞ ꢁ ccðRiÞÞ ꢄ= ½ReðceðRiÞÞ þ ImðceðRiÞÞ ꢄ: The average size was calculated using the hcp model as
described in ref. 55.
i
i
propanol solution of furfural to 180 ꢀC. While the XANES region
(Fig. S7†) demonstrates the reduction of both samples to Ru
metal by a weakening of the white line at higher temperature,
Fig. 6a shows the appearance of a Ru–Ru shell of the metal
4 Conclusion
In conclusion, the results from the characterization and from
catalytic activity data indicate that addition of phosphorous
changed signicantly the dispersion and reducibility of Ru.
Also, the type of acid site of the Al2O3 support was modied
from a Lewis-based material towards a more Brønsted-based
material. While it is difficult to disentangle which effect is
more critical for catalytic activity and selectivity because of
the vastly different Ru particle size in these catalysts, these
combined effects improved the activity of Ru/Al2O3 towards
catalytic transfer hydrogenation of furfural to furfuryl
alcohol. Further systematic study is needed to disentangle
the role of the various parameters involved, such as particle
size and P loading effects. Moreover, the nature of the
binding mode of P to Al2O3 and possibly to Ru should also be
addressed in future studies. Phosphorous improved also the
stability of Ru by reducing Ru leaching into the solution
signicantly.
˚
phase in RuP_H2 at ca. 2.5 A with a simultaneous gradual
˚
disappearance of the Ru–O shell (at ca. 1.5 A). This process also
occurs on Ru_H2, but the reduction of the oxidized phase
contributes to the increase in the fraction of Ru metal already
˚
present in the catalyst (at ca. 2.5 A, Fig. 6b). The less intense and
broader contribution of the rst Ru–Ru shell observed for
RuP_H2 is an indication that the Ru particles formed upon
reduction are smaller than in Ru_H2, which is conrmed by the
values of the Ru coordination number obtained from the t of
the rst Ru–Ru shell (Table 3) of both catalysts (EXAFS ts are
shown in Fig. S8†). Moreover, it is evident that the Ru–O species
do not completely disappear during reaction in both catalysts,
which can be related to the need of mixed RuOx/Ru interfaces.54
Aer 30 min at 180 ꢀC, furfural conversion was 98% on both
RuP_H2 and Ru_H2.
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18 C. Kellner, J. Catal., 2004, 75, 251–261.
Conflicts of interest
´
19 L. K˛epinski, L. Krajczyk, J. Okal, M. Zawadzki and W. Tylus,
Appl. Catal., A, 2006, 319, 202–209.
The authors declare no conicts of interest.
20 P. Betancourt, A. Rives, R. Hubaut, C. E. Scott and
J. Goldwasser, Appl. Catal., A, 1998, 170, 307–314.
21 M. Oubenali, G. Vanucci, B. Machado, M. Kacimi, M. Ziyad,
J. Faria, A. Raspolli-Galetti and P. Serp, ChemSusChem, 2011,
4, 950–956.
Acknowledgements
The authors are grateful to the Paul Scherrer Institut (CROSS
project) and the Competence Center for Energy and Mobility
(CCEM, project ARRMAT-PLUS) for nancial support. Beam-
time allocation at beamlines SNBL (ESRF, France) and Super-
XAS (SLS, Switzerland) is kindly acknowledged. The NMR
hardware was partially granted by the Swiss National Science
Foundation (SNSF, grant no. 206021_150638/1). D. F. thanks
22 P. Panagiotopoulou, N. Martin and D. G. Vlachos,
ChemSusChem, 2015, 8, 2046–2054.
´
23 J. M. Tukacs, M. Bohus, G. Dibo and L. T. Mika, RSC Adv.,
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24 J. Xu, F. Liu, A. Wang, B. Qiao, S. Miao, Y. Su, H. Sun, L. Li,
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Green Chem., 2018, 20, 1770–1776.
25 J. Jae, W. Zheng, R. F. Lobo and D. G. Vlachos,
ChemSusChem, 2013, 6, 1158–1162.
¨
Dr M. Schoneborn for providing the alumina sample and Dr C.
Proff for the TEM measurements. Dr D. Kuzmenko is thanked
for support with the EXFAS t.
26 C. Michel and P. Gallezot, ACS Catal., 2015, 5, 4130–4132.
27 S. G. Wettstein, J. Q. Bond, D. M. Alonso, H. N. Pham,
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