This	document	is	the	Accepted	Manuscript	version	of	a	Published	Work	that	appeared	in	final	form	in	ChemSoc.Rev,	copyright	©	The	Royal	
Society	 of	 Chemistry	 2017	 aGer	 peer	 review	 and	 technical	 ediIng	 by	 the	 publisher.	 To	 access	 the	 final	 edited	 and	 published	 work	 see:	
hKps://pubs.rsc.org/en/content/arIclelanding/2017/gc/c6gc02586b#!divAbstract

Green Chemistry
View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

PAPER

Cite this: Green Chem., 2017, 19,
2361

View Journal | View Issue

Interfacial acidity in ligand-modified ruthenium
nanoparticles boosts the hydrogenation of
levulinic acid to gamma-valerolactone†
Davide Albani,‡a Qiang Li,‡b Gianvito Vilé,a Sharon Mitchell,a Neyvis Almora-Barrios,b
Peter T. Witte,c Núria López*b and Javier Pérez-Ramírez*a
Gamma-valerolactone (GVL), a versatile renewable compound listed among the top 10 most promising
platform chemicals by the US Department of Energy, is produced via hydrogenation of levulinic acid (LA).
The traditional high-loading ruthenium-on-carbon catalyst (5 wt% Ru) employed for this transformation
suffers from low metal utilisation and poor resistance to deactivation due to the formation of RuOx
species. Aiming at an improved catalyst design, we have prepared ruthenium nanoparticles modified with
the water-soluble hexadecyl(2-hydroxyethyl)dimethylammonium dihydrogen phosphate (HHDMA) ligand
and supported on TiSi2O6. The hybrid catalyst has been characterised by ICP-OES, elemental analysis,
TGA, DRIFTS, H2-TPR, STEM, EDX, 31P and 13C MAS-NMR, and XPS. When evaluated in the continuousflow hydrogenation of LA, the Ru-HHDMA/TiSi2O6 catalyst (0.24 wt% Ru) displays a fourfold higher reaction rate than the state-of-the-art Ru/C catalyst, while maintaining 100% selectivity to GVL and no sign of
deactivation after 15 hours on stream. An in-depth molecular analysis by Density Functional Theory
demonstrates that the intrinsic acidic properties at the ligand–metal interface under reaction conditions
ensure that the less energy demanding path is followed. The reaction does not obey the expected

Received 15th September 2016,
Accepted 3rd October 2016

cascade mechanism and intercalates hydrogenation steps, hydroxyl/water eliminations, and ring closings
to ensure high selectivity. Moreover, the interfacial acidity increases the robustness of the material against

DOI: 10.1039/c6gc02586b

ruthenium oxide formation. These results provide valuable improvements for the sustainable production
of GLV and insights for the rationalisation of the exceptional selectivity of Ru-based catalysts.

rsc.li/greenchem

Introduction
The necessity of more sustainable products and processes has
led to the use of new methodologies which result in a low
carbon footprint. This implies, in line with the 12 principles of
green chemistry,1,2 the utilisation of biomass-derived reactive
substrates, non-toxic solvents (e.g., water), sustainable heterogeneous catalysts based on low precious metal loadings, and
continuous-flow processes.3 Gamma-valerolactone (GVL), for
example, has been recognised by the US Department of Energy

a
Institute for Chemical and Bioengineering, Department of Chemistry and Applied
Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland.
E-mail: jpr@chem.ethz.ch
b
Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16,
43007 Tarragona, Spain. E-mail: nlopez@iciq.es
c
BASF Nederland BV, Strijkviertel 67, 3454PK De Meern, The Netherlands
† Electronic supplementary information (ESI) available: Additional characterization of the catalysts, reaction mechanism, and calculated properties of
Ru(0001) and Ru(0001) + HHDMA. See DOI: 10.1039/c6gc02586b and DOI:
10.19061/iochem-bd-1-15
‡ These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2017

as an important bio-derived platform chemical.4–8 This compound can be used for the manufacture of a number of important commodities such as transportation fuels, polymer intermediates, pharmaceuticals, food ingredients, or it can be
directly applied as a renewable organic solvent. Gamma-valerolactone is synthesised from levulinic acid via a two-step
process involving selective hydrogenation and intramolecular
dehydration.8,9 Ruthenium-based catalysts are widely applied
for this reaction,10 due to the intrinsic ability of this metal to
selectively hydrogenate the CvO bond without reducing other
unsaturated functionalities.11 Nonetheless, conventional catalysts, such as 5 wt% Ru/C, have some drawbacks, including the
high metal content, possible metal leaching in water-based
solvents, and oxidation to RuO2 that can lead to severe activity
losses.12,13 In order to increase stability, some authors have
proposed the utilisation of RuRe, RuSn, and RuPd alloys.14–16
Similarly, it has been postulated that acidic moieties surrounding the catalytically active surface could be beneficial to reduce
the extent of deactivation.17–20
Alternative active phases to Ru, such as Cu,21 Pt,22 Ni,23,
Pd,24 Ir,24 and Au,25 are much less selective, yielding 2-methyl-

Green Chem., 2017, 19, 2361–2370 | 2361

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Paper

Green Chemistry

tetrahydrofuran and 1,4-pentandiol, and/or require harsher
reaction conditions to reach the same conversion level.
Applications of nanostructured hybrid materials in catalysis
are continuously expanding due to the unique properties that
can arise from the interaction of the organic and inorganic
components.26–28 In fact, their chemical versatility makes
these materials attractive for the possibility to combine the
advantages of heterogeneous catalysts (e.g., easy separation of
the catalyst from the reaction mixture) with the intrinsic
selectivity encountered over homogeneous catalysts.29 In the
last few years, a key development in the application of hybrid
materials for selective hydrogenations was the identification
of hexadecyl(2-hydroxyethyl)dimethylammonium dihydrogen
phosphate (HHDMA) as a suitable ligand for the synthesis of
metal nanoparticles.30,31 This molecule integrates both stabilising and reducing properties and can be applied in aqueous
medium, eliminating many hurdles which have long hampered the industrial exploitation of other routes to prepare
hybrid nanoparticles (e.g., the need to use low-boiling point
solvents, the fast addition of expensive or toxic reductants).
This enabled the production of hybrid Pd-HHDMA and PtHHDMA catalysts for alkyne and nitro-group hydrogenation,
respectively, commercialised by BASF under the trademark
NanoSelect.32–35 Due to the presence of a dihydrogen phosphate anion, the HHDMA ligand acts as a pH buffer, controlling the local acidity. This can be utilised for attaining beneficial effects in terms of reaction rate, while retaining high
selectivity levels. Since the selective hydrogenation of levulinic
acid requires catalyst acidity to improve the performance, we
decided to prepare and catalytically explore for the first time
HHDMA-modified Ru nanoparticles.
From a process viewpoint, the hydrogenation of levulinic
acid to GVL has been traditionally conducted in batch
reactors,21–25,36 mainly due to the simplicity of the equipment.
However, it is now widely recognised that continuous-flow
reactors offer important benefits in terms of process intensification, safety, evaluation of deactivation phenomena, and
kinetic analysis in steady state.37 For these reasons, in recent
times, several authors have explored the selective hydrogenation of levulinic acid in continuous mode.13,25,38–41
Herein, we report the synthesis of Ru-HHDMA colloids displaying a narrow particle size distribution centred at 1.5 nm,
and the subsequent deposition onto a TiSi2O6 carrier.
Characterisation by inductively coupled plasma optical emis-

Fig. 1

sion spectroscopy, elemental analysis, scanning transmission
electron microscopy, energy dispersive X-ray mapping, 31P and
13
C magic angle spinning nuclear magnetic resonance, X-ray
photoelectron spectroscopy, diffuse reflectance infrared spectroscopy, and temperature-programmed reduction with H2
confirms the desired interaction of the ligand and the metal
nanoparticle in the resulting catalyst. In order to explore the
advantages of ligand modification in comparison with the
state-of-the-art Ru/C material, the catalyst performance has
been evaluated in the continuous-flow three-phase hydrogenation of levulinic acid to γ-valerolactone in water at different
temperatures, pressures, and flow rates. Density Functional
Theory has been employed to derive mechanistic understanding of the high selectivity to GVL achieved over Ru-based catalysts and the enhanced activity of the ligand-modified
nanoparticles.

Experimental
Catalyst preparation
As depicted in Fig. 1, an aqueous solution of HHDMA (7 cm3,
30%) was diluted in deionised water (200 cm3) and mixed with
an aqueous solution of RuCl3 (2 cm3, 0.5 M) and HCl (0.5 cm3,
33%). The resulting mixture, containing 0.5 mg Ru per cm3, was
heated to 368 K and stirred at this temperature for 2 h to form
colloidal metal nanoparticles of approximately 1.5 nm. The colloidal solution was added to a suspension consisting of TiSi2O6
(Aldrich, 30 g) and deionised water (300 cm3). The suspension
was stirred for 1 h, filtered extensively with deionised water, and
dried overnight at 333 K. The Ru/C catalyst (Sigma-Aldrich, ref.:
24888169) was commercially available. Prior to use, this
material was reduced at 473 K for 1 h under flowing 10 vol%
H2/N2 (20 cm3 min−1) using a heating rate of 10 K min−1.
Catalyst characterisation
The ruthenium content in the catalysts was determined by
inductively coupled plasma-optical emission spectrometry
(ICP-OES) using a Horiba Ultra 2 instrument equipped with a
photomultiplier tube detector. The C, H, N, and P contents
were determined by infrared spectroscopy using a LECO
CHN-900 combustion furnace. Nitrogen isotherms were
obtained at 77 K using a Micrometrics ASAP 2020, after
evacuation of the samples at 393 K for 3 h. High-angle

Synthetic steps involved in the preparation of the Ru-HHDMA/TiSi2O6 catalyst.

2362 | Green Chem., 2017, 19, 2361–2370

This journal is © The Royal Society of Chemistry 2017

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Green Chemistry

annular dark field scanning transmission electron microscopy
(HAADF-STEM) images and elemental maps by energy dispersive X-ray (EDX) spectroscopy were acquired using an FEI Talos
S200 microscope operated at 200 kV, with a 70 mm C2 aperture
and 0.4 nA beam current. The catalysts were dispersed as dry
powders on lacey carbon coated copper grids. The particle size
distribution was assessed by counting more than 150 individual Ru nanoparticles. H2 chemisorption was performed using
a Thermo TPDRO 1100 unit. The samples were treated in He
(20 cm3 min−1) at 393 K for 60 min and reduced to 5 vol% H2/
N2 (20 cm3 min−1) at 423 K for 30 min. Then, pulses of 5 vol%
H2/N2 (0.344 cm3) were dosed to the catalyst at 308 K every
4 min. The ruthenium dispersion was calculated from the
amount of chemisorbed H2, considering an atomic surface
density of 1.57 × 1019 atoms per m2 and an adsorption stoichiometry Ru/H2 = 2.42 Temperature-programmed reduction
with hydrogen (H2-TPR) was performed using a Micromeritics
Autochem II 2920 unit connected to a MKS Cirrus 2 quadrupole mass spectrometer. The samples (0.1 g) were treated
under a He flow (20 cm3 min−1) at 393 K for 1 h, cooled down
to 308 K, and finally heated to 1273 K (10 K min−1) while
monitoring the consumption of H2. X-ray photoelectron spectroscopy (XPS) was conducted with a Physical Electronics
Instruments Quantum 2000 spectrometer using monochromatic Al Kα radiation generated from an electron beam operated at 15 kV and 32.3 W. The spectra were recorded under
ultra-high vacuum conditions (residual pressure = 5 × 10−8 Pa)
at a pass energy of 50 eV. In order to compensate for charging
effects, all binding energies were referenced to the C 1s at
288.2 eV. Thermogravimetric analysis was performed using a
Mettler Toledo TGA/DSC 1 Star system. Prior to measurement,
the samples were dried in N2 (40 cm3 min−1) at 393 K for 1 h.
The analysis was performed in air (40 cm3 min−1), heating the
sample from 298 K to 1173 K at a rate of 10 K min−1. 13C and
31
P magic-angle spinning nuclear magnetic resonance (MAS
NMR) spectra were recorded on a Bruker AVANCE III HD NMR
spectrometer at a magnetic field of 16.4 T corresponding to a
1
H Larmor frequency of 700.13 MHz. A 4 mm double resonance probe head at a spinning speed of 10 kHz was used for
all experiments. The 13C spectra were acquired using the cross
polarization mode with a contact time of 2.00 ms and a recycle
delay of 1 s. A total of 64 × 103 scans were added for each
sample. The 31P experiments used a single pulse excitation
sequence with a recycle delay of 1 s. Between 39 × 103 and 96 ×
103 scans were acquired depending on the sample. Both 13C
and 31P experiments used high-power 1H decoupling during
acquisition with the SPINAL-64 sequence. Fourier transform
infrared (FTIR) spectroscopy was performed using a Bruker
Optics Vertex 70 spectrometer equipped with a high temperature cell, ZnSe windows, and a mercury–cadmium–telluride
(MCT) detector. The cell was filled with a powdered catalyst
diluted with KBr in a weight ratio of 1 : 10 and carefully
levelled to minimise reflection from the sample surface. The
spectra were recorded in He (20 cm3 min−1) at 473 K, accumulating 64 scans in the 4000–1000 cm−1 range with a resolution
of 4 cm−1.

This journal is © The Royal Society of Chemistry 2017

Paper

Catalyst testing
The hydrogenation of levulinic acid (ABCR, 98%) was carried
out in a fully-automated flooded-bed reactor (ThalesNano
H-Cube Pro™), in which the gaseous hydrogen produced
in situ via electrolysis of Millipore water and the liquid feed
(containing 5 wt% of the substrate and deionised water as the
solvent) flowed concurrently upward through a cylindrical cartridge of an approximately 3.5 mm internal diameter. The
latter contained a fixed bed composed of the catalyst (0.1 g of
Ru-HHDMA/TiSi2O6 or 0.05 g of Ru/C) well mixed with inert
titanium silicate as diluent (0.12 g, Sigma-Aldrich, 99.8%),
both with a particle size of 0.2–0.4 mm. The reactions were
conducted at various temperatures (373–423 K), total pressures
(1–60 bar), and liquid flow rates (0.3–1.2 cm3 min−1, resulting
in contact times (τ) between 4 and 16 s), keeping the H2 flow
rate constant (56 cm3 min−1). For every experimental condition, three data points were averaged. In particular, the products were collected after 20 min under each experimental
condition; this was appropriate, considering that, in our continuous-flow microreactor, steady-state operation is reached in
5–10 min. The product composition was analysed offline by
high-performance liquid chromatography (HPLC), using a
Merck LaChrome system equipped with a HPX-87H column,
refractive index and UV-vis detectors. A 0.005 M aqueous
H2SO4 solution flowing at 0.600 cm3 min−1 was used as the
eluent. The conversion (X) of a given substrate was determined
as the amount of the reacted levulinic acid divided by the
amount of the substrate at the reactor inlet. The reaction rate
(r) was expressed as the mole of LA reacted per hour and mole
of Ru in the catalyst. The selectivity (S) to each product was
quantified as the amount of the particular product divided by
the amount of reacted levulinic acid.
Computational details
Density Functional Theory (DFT) calculations were performed
using the Vienna ab initio Simulation Package (VASP),43,44
employing the generalised gradient approximation with the
Perdew–Burke–Ernzerhof exchange–correlation functional.45
Since van der Waals (vdW) contributions are required to effectively model large molecules such as LA and HHDMA, the
Grimme DFT-D2 method46,47 modified with C6 parameters
developed in our group has been applied.48 The interaction
between valence and core electrons was described by the projector augmented wave (PAW) pseudopotentials49,50 with a cutoff energy of 450 eV. The calculated lattice parameters for
Ru (2.712 Å with c/a = 1.582) agree well with the experimental
one (2.706 Å with c/a = 1.584).51 Gas-phase molecules such as
LA and GVL were optimised in a simulation box of 20 × 20 ×
20 Å3. The Ru/C catalyst (Ru(0001)) was modelled using periodic slabs with a thickness of four layers and in a p(4 × 4)
supercell, while for the Ru-HHDMA four dihydrogen phosphates and the corresponding cationic head groups of the surfactant were included.33 The slabs were separated from the
neighbouring ones through a vacuum gap of 20 Å to avoid
lateral interactions. In particular, the two upmost layers were

Green Chem., 2017, 19, 2361–2370 | 2363

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Paper

Green Chemistry

completely relaxed, whereas the atoms in the remaining layers
were fixed to mimic the bulk. The calculation of the adsorption and transition state energies was performed with a 3 ×
3 × 1 k-point. Dipole corrections were introduced to eliminate
the spurious contributions arising from the asymmetry of the
system. Solvation contributions for the molecules in the liquid
phase were also considered using the MGCM methodology.52
The climbing image nudged elastic band (CI-NEB)53,54 and
improved dimer method (IDM)55 were employed to locate the
transition states in the potential surfaces. The transition states
were confirmed by having only one imaginary value from frequency analysis.

Results and discussion
Catalyst properties
Table 1 shows the bulk composition and morphological
characteristics of the materials prepared. ICP-OES analysis of
the Ru/C and Ru-HHDMA/TiSi2O6 catalysts reveals a total
ruthenium content of 5 and 0.24 wt%, respectively. In particular, for the latter material, the presence of a ligand is confirmed by C, H, N, and P analysis. Considering the stoichiometry of the surfactant (C20H46NO5P), it is possible to estimate
a total ligand concentration of ca. 10 wt%. The inspection of
the scanning transmission electron micrographs reveals that
the ruthenium nanoparticles possess a uniform spherical
shape, with an average particle diameter of around 1.5 nm in
both cases (Fig. 2). As reported by Philippot et al.,56 this is
unsurprising since small Ru particles readily form due to the
strong interaction of this metal with a variety of C, Ti, and
Si-based supports. The nanoparticles are well distributed
and no evidence of particle agglomeration is observed.
Characterisation by N2 sorption confirms the distinct textural
properties of the two catalysts (Fig. S1†). In particular, the RuHHDMA catalyst exhibits the typical total surface area (200
m2 g−1) of titanium silicate carriers, and a high (meso)porosity.
The corresponding pore size distribution derived by the
Barrett–Joyner–Halenda model evidences a discrete range of
pore sizes centred around 10 nm. In contrast, the Ru/C catalyst
displays the surface area as 755 m2 g−1, owing to the high
microporosity evidenced by the high uptake in the N2 adsorption isotherm at low relative pressures. To confirm the microporous nature of the carbon support, an NLDFT model assuming slit-pore geometry has been employed, which evidenced a
pore size distribution centred at 0.8 nm. In order to characterise the ligand distribution over the catalyst, EDX mapping was

Table 1

Fig. 2 Scanning transmission electron micrographs and particle size
distributions of Ru-HHDMA/TiSi2O6 (a) and Ru/C (b).

performed over Ru-HHDMA/TiSi2O6 (Fig. 3). Interestingly,
phosphorus appears to be mainly localised in the areas where
Ru is also present, corroborating previous hypotheses that the
ligand is bound to the metallic nanoparticle through the phosphate and to the carrier through N–OH.31 This clear partition
between the type of ligand adsorbed on the nanoparticles and
that on the support enables, for the first time, the direct estimation of the ligand content per nanoparticle. Assuming an
hcp unit cell with an atomic packing of 0.76,57 and considering the dispersion of Ru, we have first calculated the moles of
surface Ru atoms. The ratio between the moles of P obtained
by elemental analysis and those of surface Ru provided the
number of HHDMA molecules per nanoparticle (ca. 250
HHDMA molecules per nanoparticle). To determine the
thermal stability of the ligand shell, thermogravimetric analysis in air was conducted (Fig. 4a). In line with the previous
results,33–35 the ligand decomposition started at 500–550 K,
confirming that the metal–ligand interaction is independent
of the metal identity. To further elucidate the chemical identity
of the ligand molecule, DRIFT spectra were acquired on both
the Ru-HHDMA/TiSi2O6 catalyst and the TiSi2O6 support
(Fig. 4b). The spectrum of the catalyst shows, between 3000
and 2800 cm−1, the C–H vibrational mode of the aliphatic

Characterisation data of the catalysts

Catalyst

Rua/wt%

Cb/wt%

Hb/wt%

Nb/wt%

Pb/wt%

Weight lossc/%

SBET d/m2 g−1

Vpore e/cm3 g−1

dRu f/nm

Ru-HHDMA/TiSi2O6
Ru/C

0.24
5.0

4.5
95

1.8
—

0.3
—

0.9
—

12
—

281
755

0.21
0.65

1.3
1.5

a

Inductively coupled plasma optical emission spectroscopy. b Elemental analysis. c Thermogravimetric analysis. d BET method. e Volume of N2 at
p/p0 = 0.95. f H2 chemisorption.

2364 | Green Chem., 2017, 19, 2361–2370

This journal is © The Royal Society of Chemistry 2017

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Green Chemistry

Fig. 3 Energy dispersive X-ray maps of Ru and P in Ru-HHDMA/
TiSi2O6. The insets show a zoom of 4 times.

Fig. 4 Thermogravimetry of Ru-HHDMA/TiSi2O6 in air (a). DRIFT
spectra of the Ru-HHDMA/TiSi2O6 catalyst (blue) and the TiSi2O6 carrier
(orange) (b).

This journal is © The Royal Society of Chemistry 2017

Paper

chain of the ligand and, between 1200 and 1100 cm−1, the
stretching mode of the phosphate group. In addition, the
broad band at 3500 cm−1, which is present in both the support
and catalysts, is attributed to the stretching of O–H groups
associated with the surface of the TiSi2O6 support.
The sample was further characterised by solid-state 13C and
31
P MAS NMR. The 31P MAS NMR spectra (Fig. 5a) display a
single, broad isotropic signal at around 0 ppm, which is consistent with the presence of an orthophosphate anion. This
peak is flanked on either side by a spinning sideband giving a
rough estimate of the chemical shift anisotropy of about
50 ppm. Contrarily, the peaks in the 13C MAS NMR spectra
(Fig. 5b) can be assigned in analogy with the signals observed
in the solution state NMR spectra. The signals above 40 ppm
all originate from carbon positions in close proximity to the
nitrogen. These resonances display a markedly increased linewidth when compared to many of the peaks below 40 ppm,
which mainly correspond to positions in the aliphatic tail of
the molecule. These observations are consistent with previous
findings,35 indicating a reduced mobility (on the NMR timescale) of the head-group with respect to the tail, for which the
signals are dynamically equilibrated. In fact, the 31P peak with
a full width at the half height of approximately 2 kHz seems to
be inhomogeneous in nature and this is indicative of a
statically disordered environment.
Notwithstanding the determination of the atomic state of
Ru-based materials is not straightforward for carbon supported materials, due to the overlapping of the Ru 3d core
level signal with C 1s one, a feature centred at 281.3 eV indicative of the presence of RuOx species for the Ru/C sample was

Fig. 5 31P (a) and 13C (b) magic-angle spinning nuclear magnetic resonance spectroscopy of Ru-HHDMA/TiSi2O6.

Green Chem., 2017, 19, 2361–2370 | 2365

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Paper

Green Chemistry

Fig. 6 Ru 3d core level X-ray photoelectron spectra of the Ru-HHDMA/
TiSi2O6 (blue) and Ru/C (green).

observed.58 In the case of Ru-HHDMA/TiSi2O6, a single Ru 3d5/2
core level at 280.2 eV was displayed (Fig. 6), confirming that
the ruthenium surface is fully metallic.58,59 An additional tool
to further prove the oxidation state of the Ru catalysts is H2TPR (Fig. S2†). Apart from the decreased signal intensity
observed at room temperature due to the stabilisation of the
MS filament, Ru-HHDMA/TiSi2O6 did not show any hydrogen
consumption peak at around 400 K, the expected temperature
of reduction of RuOx species to Ru0.60 In contrast, the Ru/C
sample shows two peaks at 400 and 560 K. The first is attributed to the reduction of oxidic Ru species to metallic Ru,
whereas the second represents the H2 consumption due to the
formation of methane catalysed by Ru.

Fig. 7 Reaction rate (in 103 h−1, left) and selectivity to γ-valerolactone
(in %, right) as a function of temperature and pressure in the hydrogenation of levulinic acid over Ru-HHDMA/TiSi2O6 (a) and Ru/C (b).
Conditions: FL(levulinic acid + water) = 0.3 cm3 min−1, and FG(H2) =
54 cm3 min−1. The contour maps were obtained through spline interpolation of the experimental points indicated by black dots.

Hydrogenation of levulinic acid
The performance of the Ru-based catalysts has been assessed
in the continuous hydrogenation of levulinic acid in water
(Fig. 7 and 8). The contour maps in Fig. 7 show the effect of
temperature and pressure on levulinic acid conversion and on
the selectivity to γ-valerolactone for Ru-HHDMA/TiSi2O6 and
Ru/C. An excellent selectivity degree (>95%) was obtained at all
temperatures and pressures investigated for both catalysts,
highlighting the intrinsic selectivity of Ru-based materials for
these types of chemical transformations (vide infra).
1,4-Pentanediol was also formed in the reaction, but with a
selectivity lower than 5%. In terms of reaction rate, the ligandmodified catalyst exhibits, at 403 K and 20 bar, a fourfold
higher activity compared to the state-of-the-art catalyst, indicating that the combination of inorganic and organic counterparts boosts the catalytic activity (this important aspect is elaborated below). We have also studied the effect of the contact
time (τ) on the reaction rate (r) for both catalysts. A linear
increase of r with the contact time is observed in both cases
(Fig. 8a). Note that, even with four times longer residence
times, high selectivity levels were retained. In order to verify
the stability against leaching of the hybrid catalyst, a long-term
test was conducted. The catalyst was stable for 15 h on stream,
displaying no signs of deactivation (Fig. 8b). To further prove
that both the ligand and the metal were retained after the reaction, elemental analysis of the used catalyst was conducted,

2366 | Green Chem., 2017, 19, 2361–2370

Fig. 8 Reaction rate (in 103 h−1) as a function of the contact time (τ) in
the hydrogenation of levulinic acid over Ru-HHDMA/TiSi2O6 (blue) and
Ru/C (green) (a). Stability of Ru-HHDMA/TiSi2O6 in the hydrogenation of
levulinic acid (b). Conditions: T = 403 K, P = 10 bar, FL(levulinic acid +
water) = 0.3–1.2 cm3 min−1, and FG(H2) = 54 cm3 min−1 (a); T = 393 K,
P = 20 bar, FL(levulinic acid + water) = 0.3 cm3 min−1, and FG(H2) =
54 cm3 min−1 (b).

This journal is © The Royal Society of Chemistry 2017

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Green Chemistry

showing negligible differences in the loading of both Ru and
ligand with respect to the fresh catalyst. Additionally, STEM
micrographs of the used catalysts have been acquired to verify
if the ligand can hinder the sintering of the Ru nanoparticles.
Interestingly, as depicted in Fig. S3,† the average particle size
was retained upon hydrogenation. Contrarily, as reported elsewhere,61 extensive particle sintering is typically observed for
Ru/C, confirming that the Ru nanoparticles are susceptible to
clustering if not effectively shielded in the presence of water.
Mechanism over bare Ru nanoparticles
To obtain molecular insights into the mechanism of the reaction over Ru and Ru-HHDMA surfaces, DFT calculations were
conducted. Fig. 9 depicts the generally accepted cascade mechanism for the conversion of LA to GVL.6,62 LA would either
undergo hydrogenation to produce 4-oxopentanal and 4-hydroxypentanoic acid, or intramolecular esterification to produce
angelica lactone. It is important to highlight that this pathway
only provides a macroscopic framework for the reaction. When
considering the reaction from the microscopic viewpoint, the
mechanism for LA and levulinate to GVL consists of numerous
and much more complex elementary steps, including C–O
bond cleavage, hydrogenation ( protonation) of the oxygen
atoms, hydrogenation of carbon in the ketone group, and ring
closing steps. To construct a solid theoretical model, we have
considered all 34 intermediates generated from the 68 corresponding elementary steps (see the ESI, Fig. S4 and S5†). The
sequence of these reactions could affect the result, determining the most preferable pathway. Due to the complexity of the
reaction network, all aforementioned steps have been first
computed for a bare Ru surface representing the naked nanoparticle, and then extrapolated these results to understand the
Ru-HHDMA case (vide infra). To maximise the relevance of the
theoretical calculations, DFT calculations using dispersion
interactions have been applied. Besides, to determine the
nature of the interaction between LA and the surface, we have
also considered that the reactions take place in water,
suggesting that LA, which is an acid with a pKa of 4.5, is

Fig. 9

Paper

prevalent in the anionic state in aqueous solution. The first
step of the reaction is hydrogen dissociation on the Ru metal
surface, which is barrierless and exothermic (0.68 eV per
atom). This step is followed by the adsorption of levulinate on
the surface (black box in Fig. 10 and ESI†). As shown in
Fig. 10, once the levulinate is in the adsorbed state, several
paths can follow: C–O cleavage, followed by ring closure, or
hydrogenation. From the analysis of the reaction energies and
energy barriers of all possible steps (see the ESI†), it emerges
that mainly the hydrogenation of the ketonic group leading to
17-c is likely to occur. In fact this step needs a relatively low
energy barrier (0.57 eV) compared to the competitive protonation of O1 and O2 (1.34 and 1.33 eV, respectively). Besides, the

Fig. 10 Reaction network for levulinic acid (blue)/levulinate to
γ-valerolactone. Blue arrows describe the path for levulinic acid on RuHHDMA. Contrarily, all the steps are possible in the levulinate transformation on the bare Ru(0001) surface.

Cascade process for the hydrogenation of LA to GVL, 1,4-pentanediol, and valeric acid.

This journal is © The Royal Society of Chemistry 2017

Green Chem., 2017, 19, 2361–2370 | 2367

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Paper

protonation of O1 and O2 is highly endothermic (0.69 and 1.13
eV). C–O cleavage of LA is also unlikely to occur. In fact, the
direct C–O bond scission from the carboxylate group of levulinate (structure 02-c) and 17-c needs large energy barriers (1.56
and 1.39 eV, respectively). These high barriers arise from the
strong interaction of the metal surface with the bidentate O
atoms from COO− which is η2-O,O coordinated to the surface.
Since the reaction is carried out in water, we have investigated
the possibility that the solvent assists the C–O cleavage,63
resulting in a barrier decrease to 1.08 and 1.17 eV, respectively.
Since these barriers remain pretty large, only moderate contributions from ring closing reactions are expected from R–COO−
species, due to the strong binding of the O of the carboxyl
group, inhibiting the internal C–O bond formation. Thus, after
the formation of 17-c, the water-assisted C–O cleavage takes
place, leading to the formation of 33-c that gives the final
product, GVL (33-r) via ring closing. The C–O bond breaking is
the rate determining step of the whole process. Note, however,
that the reaction network can encompass other species, like
those shown in Fig. 9, and that a complete reaction rate analysis with the contribution of the different paths under
different conditions would require a full microkinetic analysis
including coverage effects.
The thermodynamic selectivity concept, which refers to the
energy difference between the adsorption of the reactants and
that of the products, is the simplest descriptor that can be
used to rationalise activity–selectivity patterns of challenging
hydrogenation reactions.64–66 High thermodynamic selectivity
requires that the reactants are much more strongly adsorbed
to the catalyst surface than the products. This concept can be
applied here to rationalise the high product selectivity
obtained in the reaction. Specifically, GVL over-hydrogenation
cannot occur since the calculated barrier for this step is
approximately 1.36 eV. This is higher than the desorption
barrier for GVL (1.22 eV, which reduces to 0.80 eV once solvation is taken into account). Thus, once GVL is formed, it
easily desorbs from the surface.
Mechanism over Ru-HHDMA
Since the reaction network is very complex, the role of the RuHHDMA can be inferred from the Ru(0001) network, and complemented with a few calculations on the modelled RuHHDMA interface. The preferred path on Ru-HHDMA is highlighted by the blue arrows in Fig. 10. The major molecular
difference in the mechanism of reaction over the Ru-HHDMA
surface is in the activation of molecular H2. Due to the presence of HHDMA ligands, hydrogen dissociation is barrierless
and exothermic on the Ru-HHDMA interface (0.45 eV per
atom). This facilitated hydrogen activation produces a buffer
of hydrogen atoms that at high coverage can be stabilised
between the phosphate anions and co-adsorbed water.
Particularly, part of these hydrogen atoms can be trapped in
the form of hydronium (H3O+) species. These protons being
more acidic than LA reverse the equilibrium of the reactant
towards its acidic form (LA), instead of the anionic intermediate described above. This trapping is almost thermoneutral

2368 | Green Chem., 2017, 19, 2361–2370

Green Chemistry

(+0.12 eV). Thus, when adsorbing the levulinate to the RuHHDMA, the proton buffered at the interface is transferred,
converting the levulinate adsorbate into LA (01-c, see the blue
box in Fig. 10). Due to the absence of hapticity (that is, the
coordination of a ligand to a metal via a contiguous series of
atoms), LA has a weaker interaction with the surface than
RCOO− (02-c), and therefore the C–OH breaking from
R–COOH (01-c to 26-c) features a lower barrier (0.46 eV) than
any other reaction (ca. 1.0 eV). This enhanced C–OH cleavage
results in a fourfold increase in the reaction rate observed
experimentally (Table 2). After that, the ring closing takes
place with a small barrier (0.44 eV), followed by the hydrogenation of 26-r to 33-r (0.87 eV).
To further characterise the intrinsic acidity of the surface,
we have employed a set of small compounds: formic acid,
acetic acid, chloro, dichloro, and trichloro acetic acid. In
Fig. 11, the adsorption energy of the corresponding anions
(red) and the energy required to protonate them (black) have
been calculated with respect to the anions in solution. The
difference of these gives the adsorption energy of the acid
(blue line). This energy is equal to zero when the acid/anion
are identical, enabling the determination of the intrinsic pKa
of the Ru-HHDMA interface. From our calculations, the interface is buffered around pKa = 1, indicating that reactions featuring molecules of a lower pKa would not benefit in terms of
reaction rate. Similarly, Ru nanoparticles stabilised on reduced
graphene oxide (GO) have also shown a fourfold increase in

Table 2 Main elementary steps in the mechanisms for LA hydrogenation to GVL on Ru(0001) and Ru-HHDMA surface

Ru(0001)
Step 1
Step 2
Step 3
Step 4
Step 5

Ru-HHDMA

Adsorption of levulinate
C2 hydrogenation
C–O breaking from R–COO
Ring closing to GVL
GVL desorption

Adsorption of levulinate
Protonation of levulinate to LA
C–OH breaking from R–COOH
Ring closing
C2 hydrogenation to GVL
GVL desorption

Fig. 11 Relationship between pKa and adsorption energy of the corresponding carboxylate (Eads, black), protonation energies (ΔE, red), as well
as the sum of them (Eads + ΔE, blue). Linear fitting: Eads = (−0.23 ± 0.02)
pKa + (0.44 ± 0.06), ΔE = (−0.09 ± 0.01)pKa + (0.80 ± 0.02), and Eads +
ΔE = (−0.32 ± 0.03)pKa + (0.35 ± 0.08) with r2 values of 0.97, 0.98, and
0.97, respectively. Estimated pKa of the interface is 1.09.

This journal is © The Royal Society of Chemistry 2017

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Green Chemistry

the activity when compared with standard Ru formulations.12
Since in GO supports some acidic centres are retained even
upon reduction, we believe that these centres can act as promoters in the same way as the interface does in Ru-HHDMA.
This concept can be further transposed to any reaction catalysed by ionic liquids with sulfonate terminations.67 The pH
control at the interface adds to recent electrochemical effects
found for other NanoSelect catalysts.68
In order to rationalise the outstanding selectivity of the Rubased catalyst in breaking C–O bonds, a comparison with
other metals could also be crucial. For instance, C–OH bond
breaking on less oxophilic metals, such as Pd(111) and Pt(111)
surfaces, presents high barriers (1.28 and 0.84 eV, respectively),
while the competitive hydrogenations exhibit barriers of ca.
1 eV. Therefore, on these metals, the selectivity levels achieved are
much lower since the activation energy window is very narrow.
Finally, considering that the main issue of Ru catalysts for
biomass conversion is the relative instability against the formation of oxidic phases (RuOx),12 the ligand-modified catalyst
was tested in a stability test (Fig. 8b) and complemented with
the analysis of oxygen adsorption. We have found that the
adsorption of large quantities of oxygen on the Ru-HHDMA
nanoparticles is inhibited due to the repulsion of the orthophosphate group (see the ESI, Table S3 and Fig. S6†). Under a
H2 atmosphere, the interface acidity modulates the oxidative
effect of the oxygens and converts them into hydroxyls that are
less prone to oxide formation. These computational results
give the key for the outstanding selectivity of the Ru-based
catalyst in the conversion of LA to GVL, indicating that acid
properties at the interface improve the activity and the stability
of the Ru-HHDMA nanoparticles.

Conclusions
We have prepared and characterised a new type of hybrid Ru
nanoparticle using HHDMA as a modifier. ICP-OES, elemental
analysis, TGA, DRIFTS, H2-TPR, STEM, EDX, 31P and 13C
MAS-NMR, and XPS have been used for confirming the ligand–
metal interaction and determining the textural and spectroscopic features of the catalyst. The catalyst was evaluated in
the continuous-flow hydrogenation of LA in water. Under all
conditions screened (T = 373–423 K, P = 1–60 bar, and τ =
4–16 s), Ru-HHDMA/TiSi2O6 displays an outstanding fourfold
higher reaction rate than that of the benchmark 5 wt% Ru/C
catalyst, and full selectivity to GVL. Since the Ru/C catalyst is
known to deactivate easily in water, a stability test over the
ligand-modified catalyst has been carried out, showing no sign
of deactivation for 15 h on stream. Density Functional Theory
has unravelled the reasons behind this remarkable activity and
intrinsic selectivity of the Ru-HHDMA catalyst in the selective
hydrogenation of levulinic acid. In particular, the metal/surfactant/water interface causes a local increase in the pH that can
protonate the levulinate anion, resulting in a reaction network
featuring low barriers and a fourfold reaction rate increase.
Due to the structure of the nanocatalysts, this enhancement is

This journal is © The Royal Society of Chemistry 2017

Paper

maintained, demonstrating the robustness of the catalytic
system. The high selectivity is of thermodynamic nature,
meaning that once GVL is formed, it is easily desorbed from
the surface, avoiding further hydrogenation. Contrary to the
deactivation suffered by the Ru/C catalyst due to surface oxidation, the formation of RuOx compounds on the ligand-modified catalyst is impeded for two reasons: the phosphate anions
at the surface shield the nanoparticle, thus reducing the
oxygen uptake from by-products, and the high pH at the interface does not allow the formation of the oxide.

Acknowledgements
Financial support from ETH Zurich, ICIQ Foundation, and the
Spanish Ministerio de Economía y Competitividad
(CTQ2012–33826 and “Ayuda formación Posdoctoral”
Fellowship (N. A.-B.)) is acknowledged. Dr Roland Hauert
(EMPA, Dubendorf ) and Dr Réne Verel are acknowledged for
the XPS analyses, and 13C and 31P MAS-NMR, respectively.
ScopeM of the Swiss Federal Institute of Technology is
acknowledged for providing access to its facilities. BSC-RES is
thanked for generous computational resources. Dr. M. GarcíaRatés is acknowledged for the solvation values.

References
1 P. Anastas and N. Eghbali, Chem. Soc. Rev., 2010, 39, 301.
2 C. O. Tuck, E. Pérez, I. T. Horváth, R. A. Sheldon and
M. Poliakoff, Science, 2012, 337, 695.
3 R. A. Sheldon, Green Chem., 2014, 16, 950.
4 D. M. Alonso, S. G. Wettstein and J. A. Dumesic, Green
Chem., 2013, 15, 584.
5 I. T. Horváth, H. Mehdi, V. Fábos, L. Bode and L. T. Mika,
Green Chem., 2008, 10, 238.
6 M. Besson, P. Gallezot and C. Pinel, Chem. Rev., 2014, 114,
1827.
7 R. Palkovits, Angew. Chem., Int. Ed., 2010, 49, 4336.
8 F. D. Pileidis and M.-M. Titirici, ChemSusChem, 2016, 9,
562.
9 A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107,
2411.
10 W. R. H. Wright and R. Palkovits, ChemSusChem, 2012, 5,
1657.
11 C. Michel and P. Gallezot, ACS Catal., 2015, 5, 4130.
12 C. Xiao, T.-W. Goh, Z. Qi, S. Goes, K. Brashler, C. Perez and
W. Huang, ACS Catal., 2016, 6, 593.
13 O. A. Abdelrahman, A. Heyden and J. Q. Bond, ACS Catal.,
2014, 4, 1171.
14 S. G. Wettstein, J. Q. Bond, D. M. Alonso, H. N. Pham,
A. K. Datye and J. A. Dumesic, Appl. Catal., B, 2012, 117,
321.
15 D. J. Braden, C. A. Henao, J. Heltzel, C. C. Maravelias and
J. A. Dumesic, Green Chem., 2011, 13, 1755.
16 W. Luo, M. Sankar, A. M. Beale, Q. He, C. J. Kiely,
P. C. A. Bruijnincx and B. M. Weckhuysen, Nat. Commun.,
2016, 6, 6540, DOI: 10.1038/ncomms7540.

Green Chem., 2017, 19, 2361–2370 | 2369

View Article Online

Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili on 4/12/2019 2:14:54 PM.

Paper

17 C. Moreno-Marrodan and P. Barbaro, Green Chem., 2014,
16, 3434.
18 V. V. Kumar, G. Naresh, M. Sudhakar, C. Anjaneyulu,
S. K. Bhargava, J. Tardio, V. K. Reddy, A. H. Padmasri and
A. Venugopal, RSC Adv., 2016, 6, 9872.
19 A. M. R. Galletti, C. Antonietti, V. De Luise and
M. Martinelli, Green Chem., 2012, 14, 688.
20 X. Zhang, P. Murria, Y. Jiang, W. Xiao, H. I. Kenttämaa,
M. M. Abu-Omar and N. S. Mosier, Green Chem., 2016, 18,
5219.
21 A. M. Hengne and C. V. Rode, Green Chem., 2012, 14, 1064.
22 M. Sudhakar, V. V. Kumar, G. Naresh, M. L. Kantam,
S. K. Bhargava and A. Venugopal, Appl. Catal., B, 2016, 180,
113.
23 Z.-p. Yan, L. Lin and S. Liu, Energy Fuels, 2009, 23, 3853.
24 L. E. Manzer, Appl. Catal., A, 2004, 272, 249.
25 X.-L. Du, Q.-Y. Bi, Y.-M. Liu, Y. Cao and K.-N. Fan,
ChemSusChem, 2011, 4, 1838.
26 G. E. Oosterom, J. N. H. Reek, P. C. J. Kamer and
P. W. N. M. van Leeuwen, Angew. Chem., Int. Ed., 2001, 40, 1828.
27 V. I. Pârvulescu and C. Hardacre, Chem. Rev., 2007, 107,
2615.
28 A. Kaushik, R. Kumar, S. K. Arya, M. Nair, B. D. Malhora
and S. Bhansali, Chem. Rev., 2015, 115, 4571.
29 M. A. Boles, D. Ling, T. Hyeon and D. V. Talapin, Nat.
Mater., 2016, 15, 141.
30 P. T. Witte, P. H. Berben, S. Boland, E. H. Boymans,
D. Vogt, J. W. Geus and J. G. Donkervoort, Top. Catal.,
2012, 55, 505.
31 P. T. Witte, S. Boland, F. Kirby, R. van Maanen,
B. F. Bleeker, D. A. M. de Winter, J. A. Post, J. W. Geus and
P. H. Berben, ChemCatChem, 2013, 5, 582.
32 E. H. Boymans, P. T. Witte and D. Vogt, Catal. Sci. Technol.,
2015, 5, 176.
33 G. Vilé, N. Almora-Barrios, S. Mitchell, N. López and
J. Pérez-Ramírez, Chem. – Eur. J., 2014, 20, 5926.
34 G. Vilé, N. Almora-Barrios, N. López and J. Pérez-Ramírez,
ACS Catal., 2015, 5, 3767.
35 D. Albani, G. Vilé, S. Mitchell, P. T. Witte, N. AlmoraBarrios, R. Verel, N. López and J. Pérez-Ramírez, Catal. Sci.
Technol., 2016, 6, 1621.
36 A. Yepez, S. De, M. S. Climent, A. A. Romero and R. Luque,
Appl. Sci., 2015, 5, 532.
37 M. Irfan, T. N. Glasnov and C. O. Kappe, ChemSusChem,
2011, 4, 300.
38 A. S. Piskun, J. E. de Haan, E. Wilbers, H. H. van de
Bovenkamp, Z. Tang and H. J. Heeres, ACS Sustainable
Chem. Eng., 2016, 4, 2939.
39 R. A. Bourne, J. G. Stevens, J. Ke and M. Poliakoff, Chem.
Commun., 2007, 4632.
40 J.-P. Lange, R. Price, P. M. Ayoub, J. Louis, L. Petrus,
L. Clarke and H. Gosselink, Angew. Chem., Int. Ed., 2010,
49, 4479.
41 J. M. Bermudez, J. A. Menéndez, A. A. Romero, E. Serrano,
J. Garcia-Martinez and R. Luque, Green Chem., 2013, 15,
2786.

2370 | Green Chem., 2017, 19, 2361–2370

Green Chemistry

42 J. Okal, M. Zawadzki, L. Kępiński, L. Krajczyk and W. Tylus,
Appl. Catal., A, 2007, 319, 202.
43 G. Kresse and J. Furthmüller, Comput. Mater. Sci., 1996,
6, 15.
44 G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter,
1996, 54, 11169.
45 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.,
1996, 77, 3865.
46 S. Grimme, J. Comput. Chem., 2006, 27, 1787.
47 T. Bučko, J. Hafner, S. Lebègue and J. G. Ángyán, J. Phys.
Chem. A, 2010, 114, 11814.
48 N. Almora-Barrios, G. Carchini, P. Błoński and N. López,
J. Chem. Theory Comput., 2014, 10, 5002.
49 P. E. Blöchl, Phys. Rev. B: Condens. Matter, 1994, 50, 17953.
50 G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter,
1999, 59, 1758.
51 D. Lide, CRC Handbook of Chemistry and Physics, CRC
Press, Boca Ranton, 84th edn, 2003–2004.
52 M. Garcia-Ratés and N. López, J. Chem. Theory Comput.,
2016, 12, 1331.
53 G. Henkelman and H. Jónsson, J. Chem. Phys., 2000, 113, 9978.
54 G. Henkelman, B. P. Uberuaga and H. Jónsson, J. Chem.
Phys., 2000, 113, 9901.
55 A. Heyden, A. T. Bell and F. J. Keil, J. Chem. Phys., 2005,
123, 224101.
56 L. M. Martínez-Prieto, A. Ferry, L. Rakers, C. Richter,
P. Lecante, K. Philippot, B. Chaudret and F. Glorius, Chem.
Commun., 2016, 52, 4768.
57 A. B. Ellis, M. J. Geselbracht, B. J. Johnson, G. C. Linsensky
and W. R. Robinson, Teaching General Chemistry:
A Material Science Companion, American Chemical Society
Publication, 1st edn, 1993, p. 103.
58 A. M. Ruppert, M. Jędrzejczyk, O. Sneka-Płatek, N. Keller,
A. S. Duman, C. Michel, P. Sautet and J. Grams, Green
Chem., 2016, 18, 2014.
59 D. J. Morgan, Surf. Interface Anal., 2015, 47, 1072.
60 I. Rossetti, N. Pernicone and L. Forni, Appl. Catal., A, 2003,
248, 97.
61 E. P. Maris, W. C. Ketchie, V. Oleshko and R. J. Davis,
J. Phys. Chem. B, 2006, 110, 7869.
62 M. J. Climent, A. Corma and S. Iborra, Green Chem., 2014,
16, 516.
63 C. Michel, J. Zaffran, A. M. Ruppert, J. Matras-Michalska,
M. Jędrzejczyk, J. Grams and P. Sautet, Chem. Commun.,
2014, 50, 12450.
64 Y. Segura, N. López and J. Pérez-Ramírez, J. Catal., 2007,
247, 383.
65 M. García-Mota, B. Bridier, J. Pérez-Ramírez and N. López,
J. Catal., 2010, 273, 92.
66 G. Vilé, D. Albani, N. Almora-Barrios, N. López and J. PérezRamírez, ChemCatChem, 2016, 8, 21.
67 J. Julis, M. Hölscher and W. Leitner, Green Chem., 2010, 12,
1634.
68 N. Almora-Barrios, G. Vilé, M. García-Ratés, J. PérezRamírez and N. López, ChemCatChem, 2016, DOI: 10.1002/
cctc.200161134.

This journal is © The Royal Society of Chemistry 2017