This is the peer reviewed version of the following article Green Chem., 2017,19,
2361-2370 which has been published in final form
http://pubs.rsc.org/en/content/articlelanding/2017/gc/c6gc02586b#!divAbstract
Interfacial acidity in ligand-modified ruthenium nanoparticles boosts the
performance for the continuous hydrogenation of levulinic acid to gammavalerolactone
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-Ramí a
rez*
Gamma-valerolactone (GVL), a versatile renewable compound listed among the top 10 most promising platform chemical 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
hydroxyethyl-dimethyl ammonium 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
continuous-flow hydrogenation of LA, the Ru-HHDMA/TiSi2O6 catalyst (0.24 wt.% Ru) displays a fourfold higher reaction rate than the stateof-the-art Ru/C catalyst, while maintaining a 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 of the ligand-metal interface under
reaction conditions ensure that the less energy demanding path is followed. The reaction does not obey the expected 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 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.

Introduction
The necessity for 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 Gammavalerolactone (GVL), for example, has been recognised by the US Department of Energy 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 a selective hydrogenation and an 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 C=O
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 Au25, are much less selective, yielding 2-methyltetrahydrofuran and
1,4-pentandiol, and/or require harsher reaction condition 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 years, a key development in the application of hybrid materials for selective hydrogenations was the
identification of hydroxyethyl-dimethyl ammonium 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 head, 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 have 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 under 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 emission spectroscopy, elemental
analysis, scanning transmission electron microscopy, energy dispersive X-ray mapping, 31P and 13C 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 continuousflow three-phase hydrogenation of levulinic acid to -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 Figure 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 cm-3, 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 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 photomultiplier tube detection. The C, H, N, and P contents were
determined by infrared spectroscopy using a LECO CHN-900 combustion furnace. Nitrogen isotherms were measured at 77 K in a
Micrometrics ASAP 2020, after evacuation of the samples at 393 K for 3 h. High-angle 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 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 in 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

Fig. 1 Synthetic steps involved in the preparation of the Ru-HHDMA/TiSi2O6 catalyst.

Table 1. Characterisation data of the catalysts.
Catalyst

Rua /
wt.%

Ca /
wt.%

Ha /
wt.%

Na /
wt.%

Pa /
wt.%

Weight
lossb / %

SBETc /
m2 g-1

Vpored /
cm3 g-1

dRue/
nm

Ru-HHDMA/TiSi2O6

0.24

4.5

1.8

0.3

0.9

12

281

0.21

1.3

Ru/C

5.0

95

-

-

-

-

0.65

1.5

aInductively

coupled plasma optical emission spectroscopy;
transmission electron microscopy.

bElemental

analysis;

cThermogravimetric

analysis;

755
dBET

method;

eV

ads

at p/p0 = 0.95; fScanning

dispersion was calculated from the amount of chemisorbed H2, considering an atomic surface density of 1.57×1019 atoms m-2 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 in 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 in a Physical Electronics Instruments Quantum 2000 spectrometer using
monochromatic Al K radiation generated from an electron beam operated at 15 kV and 32.3 W. The spectra were collected under ultrahigh 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 in 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 31P 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 1H 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 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 SPINAL-64
sequence. Fourier transform infrared (FTIR) spectroscopy was performed in a Bruker Optics Vertex 70 spectrometer equipped with high
temperature cell, ZnSe windows, and a mercury-cadmium-telluride (MCT) detector. The cell was filled with powdered catalyst diluted with
KBr in a weight ratio (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.
Catalyst testing. The hydrogenation of levulinic acid (ABCR, 98%) was carried out in a fully-automated flooded-bed reactor (ThalesNano HCube ProTM), in which the gaseous hydrogen produced in situ via electrolysis of Millipore water and the liquid feed (containing 5 wt.% of
substrate and deionised water as solvent) flow concurrently upward through a cylindrical cartridge of 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 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 rate (0.3-1.2 cm3 min-1, resulting in contact times () between 4-16 s),
keeping constant the H2 flow rate (56 cm3 min-1). For every experimental condition, three data points were averaged. In particular, the
products were collected after 20 min at 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, refraction index and UV-Vis detectors. A
0.005 M aqueous H2SO4 solution flowing at 0.600 cm3 min−1 was used as eluent. The conversion (X) of a given substrate was determined as
the amount of reacted levulinic acid divided by the amount of substrate at the reactor inlet. The reaction rate (r) was expressed as the
number of moles 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 revised 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) pseudopotencials49,50 with cut-off energy of 450 eV. The calculated lattice
parameters for Ru (2.712 Å with c/a = 1.582) agrees 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 Å 3. The Ru/C catalyst (Ru(0001)) was modelled using periodic slabs with a
thickness of five layer and in a p(4×4) supercell, while for the Ru-HHDMA four dihydrogenophophates and the cationic head 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 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

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

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.

Fig. 3 Energy dispersive X-ray maps of Ru and P in Ru-HHDMA/TiSi2O6. The insets
show a zoom of 2 times.

Results and Discussion
Catalysts 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 reveal a total ruthenium content of 5 and 0.24 wt.%, respectively. In particular, for the latter
material, the presence of ligand is confirmed by C, H, N, and P analysis. Considering the stoichiometry of the surfactant (C 20H46NO5P), it is
possible to estimate a total ligand concentration of ca. 10 wt.%. 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
(Figure 2). As reported by Philippot et al.,56 this is unsurprising since small Ru particles readily form due to the strong intrinsic 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 (Figure 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 is 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 performed over Ru-HHDMA/TiSi2O6 (Figure 3). Interestingly, phosphorous 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 phosphate heads and to the carrier through N-OH dispersion interactions.31 This clear partition between the type of
ligand adsorbed on the nanoparticles and that on the support enable, for the first time, the direct estimation of the ligand content per
nanoparticle. Assuming an hcp unit cell with 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 molecule per nanoparticle (ca. 250 HHDMA molecules/nanoparticle). To determine the thermal stability of the ligand shell,
thermogravimetric analysis in air was conducted (Figure 4a). In line with previous results,33-35 the ligand decomposition started at 500550 K, confirming that the metal-ligand interaction is independent from 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 (Figure 4b). The spectrum of
the catalyst shows, between 3000-2800 cm-1, the C-H vibrational mode of the aliphatic chain of the ligand and, between 1200-1100 cm-1,
the stretching mode of the phosphate group. In addition, the

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

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

broad band at 3500 cm-1, which is present in both the support and
catalysts, is attributed to stretching of O-H groups associated with
the surface of the TiSi2O6 support.
The sample was further characterised by solid-state 13C and 31P MAS
NMR. The 31P MAS NMR spectra (Figure 5a) display a single, broad
isotropic signal 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 (Figure 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
core level X-ray
when compared to many of the peaks below 40 ppm, which mainly Fig. 6 Ru 3d2O6 (blue) and Ru/Cphotoelectron spectroscopy spectra of the RuHHDMA/TiSi
(green).
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 is indicative of a statically disordered environment.
Notwithstanding the determination of the atomic state of Ru-based materials is not straightforward for carbon supported material, 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 observed.58 In the case of the Ru-HHDMA/TiSi2O6, a single Ru 3d5/2 core level at 280.2 eV was displayed (Figure 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 (Figure S2). Apart from the decreased signal intensity observed at room temperature due to the stabilisation of the MS filament, RuHHDMA/TiSi2O6 did not show any hydrogen consumption peak 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.
Hydrogenation of levulinic acid. The performance of the Ru-based catalysts has been assessed in the continuous hydrogenation of levulinic
acid in water (Figure 7, and 8). The contour maps in Figure 7 show the effect of temperature and pressure on levulinic acid conversion and
on the selectivity to -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 transformation (vide infra).

Fig. 7 Reaction rate (in 103 h-1, left) and selectivity to -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 20 experimental points indicated by
black dots.

Fig. 8 Reaction rate (in 103 h-1) as a function of the contact time ( ) 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).

1,4-pentanediol was also formed in the reaction, but with a selectivity lower than 5%. In term of reaction rate, the ligand-modified 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 () on the reaction rate (r) for both catalysts. A linear increase of r with the contact time is observed in both cases
(Figure 8a). Note that, even with four times longer residence times, high selectivity levels were retained, proving the intrinsic selectivity of
Ru-based catalysts. 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 (Figure 8b). To further prove that both the ligand and the metal were retained
after the reaction, elemental analysis of the used catalyst was conducted, showing negligible differences in 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 Figure S3, the average particle size was retained upon
hydrogenation. Contrarily, as reported elsewhere,61 extensive particle sintering is typically observed on Ru/C, confirming that the Ru
nanoparticles are susceptible of clustering if not effectively shielded in the presence of water.
Mechanism over bare Ru nanoparticles. To obtain molecular insights on the mechanism of reaction over Ru and Ru-HHDMA surfaces, DFT
calculations were conducted. Figure 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-hydroxyentanoic acid, or intramolecular esterification to produce
angelicalactone. 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 ESI, Figure 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 the aforementioned steps have been first computed for a bare Ru
nanoparticle and then extended to 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 pKa of 4.5, is prevalently in
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 atom-1). This step is followed by the adsorption of levulinate on the surface (black box in Figure 10 and ESI). As shown
in Figure 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 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

Figure 9. Cascade process for the hydrogenation of LA to GVL, 1,4-pentanediol, and valeric acid.

low energy barrier (0.57 eV) compared to the competitive
protonation of O1 and O2 (1.34 and 1.33 eV, respectively). Besides,
the protonation of O1 and O2 are highly endothermic (0.69 and
1.13 eV). C-O cleavage of LA is also unlike to occur. In fact, the direct
C-O bond scission from carboxylate group of levulinate (structure 02c) and 17-c need 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
2-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. Notice, however, that the
reaction network can encompass other species, like those shown in
Figure 9, and that a complete reaction rate analysis with the
Figure 10. Reaction network for levulinic acid (blue)/levulinate to - contribution of the different paths under different conditions would
valerolactone. Blue arrows describe the path for levulinic acid on Ru-HHDMA. require a full microkinetic analysis including coverage effects.
Contrarily, all the steps are possible in the levulinate transformation on bare Ru
The thermodynamic selectivity concept, which refers to the energy
nanoparticle.
difference between the adsorption of the reactants and that of the
products, is one 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 that the desorption barrier for GVL (1.24 eV, which reduces to 0.36 eV considering the
solvation phenomena). 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 Ru-HHDMA can be inferred from the Ru(0001)
network, and complemented with a few calculations on the modelled Ru-HHDMA interface. The preferred path on Ru-HHDMA is
highlighted by the blue arrows in Figure 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 RuHHDMA interface (0.45 eV atom-1). 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 (+0.12 eV). Thus, when adsorbing the levulinate
to the Ru-HHDMA, the proton buffered at the interface is transferred, converting the
levulinate adsorbate into LA

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

Ru-HHDMA

Step 1

Adsorption of levulinate

Adsorption of levulinate

Step 2

C2 hydrogenation

Protonation of levulinate to LA

Step 3

C—O breaking* from R-COO

C—OH breaking from R-COOH

Step 4

Ring closing to GVL

Ring closing

Step 5

GVL Desorption

C2 hydrogenation to GVL
GVL Desorption

Figure 11. Relationship between pKa and adsorption energy of the correspective
carboxylate (Eads, black), protonation energies (ΔE, red), as well as the sum of
them (Eads+ΔE , blue). Liner fitting are: 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 pK a of the
interface is 1.09.

(01-c, see the blue box in Figure 10). Due to the absence of hapticity
(that is, the coordination of a ligand to a metal via a contiguous
series of atoms), this does have 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 the 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 Figure 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 those gives the adsorption energy of
the acid (blue line). This energy equals to zero when the acid/anion are identical, enabling the determination of intrinsic pKa of the RuHHDMA interface. From our calculations, the interface is buffered at pKa = 1, indicating that reactions featuring molecules of 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 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
In order to rationalise the outstanding selectivity of Ru-based catalyst in breaking C-O bonds, a comparison with other metals could also be
crucial. For instance, C-OH bond breaking on less oxophylic 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 (vide supra) 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 ESI, Table S3 and Figure S6). Under H2 atmosphere, the interface acidity modulates the
oxidative effect of the oxygens as converts them into hydroxyls less prone to oxide formation. These computational results give the key for
the outstanding selectivity of 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 nanoparticles 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
textural and spectroscopic features of the catalyst. The catalyst was evaluated in the continuous-flow hydrogenation of LA in water. At all
conditions screened (T = 373-423 K, P = 1-60 bar, and  = 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 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 Ru-HHDMA catalyst in the selective hydrogenation of
levulinic acid. In particular, the metal/surfactant/water interface creates 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 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 from the deactivation
suffered by the Ru/C catalyst due to surface oxidation, the formation of RuOx compounds in 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 Economí y Competitividad (CTQ2012-33826 and
a
“Ayuda formación Posdoctoral” Fellowship (N.A-B.)) are 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.

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