Noble Metal Supported on Activated Carbon for “Hydrogen Free” HDO Reactions: Exploring Economically Advantageous Routes for Biomass Valorisation

Article abstract of DOI:10.1002/cctc.201900841

An innovative route for bio-compounds upgrading via “hydrogen-free” hydrodeoxygenation (HDO) is proposed and evaluated using guaiacol as a model compound in a high-pressure batch reactor. Experimental results showed that noble metal supported on activated carbon catalysts are able to conduct tandem multiple steps including water splitting and subsequent HDO. The activity of Ru/C catalyst is superior to other studied catalysts (i. e. Au/C, Pd/C and Rh/C) in our water-only HDO reaction system. The greater dispersion and smaller metal particle size confirmed by the TEM micrographs accounts for the better performance of Ru/C. This material also presents excellent levels of stability as demonstrated in multiple recyclability runs. Overall, the proposed novel approach confirmed the viability of oxygenated bio-compounds upgrading in a water-only reaction system suppressing the need of external H₂ supply and can be rendered as a fundamental finding for the economical biomass valorisation to produce added value bio-fuels.

Full text of DOI:10.1002/cctc.201900841

Accepted Article
Title: Noble metal supported on activated carbon for “hydrogen free”
HDO reactions: exploring economically advantageous routes for
biomass valorisation
Authors: Wei Jin, Jose Luis Santos, Laura Pastor-Perez, Sai Gu,
Miguel Angel Centeno, and Tomas Ramirez Reina
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201900841
Link to VoR: http://dx.doi.org/10.1002/cctc.201900841
A Journal of
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10.1002/cctc.201900841
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Noble metal supported on activated carbon for “hydrogen free”
HDO reactions: exploring economically advantageous routes for
biomass valorisation
Wei Jin,[a] José Luis Santos,[b] Laura Pastor-Perez,[a] Sai Gu,[a] Miguel Angel Centeno *[b] and Tomas
Ramirez Reina *[a]
Abstract: An innovative route for bio-compounds upgrading via
“hydrogen-free” hydrodeoxygenation (HDO) is proposed and
evaluated using guaiacol as a model compound in a high-pressure
batch reactor. Experimental results showed that noble metal
supported on activated carbon catalysts are able to conduct tandem
multiple steps including water splitting and subsequent HDO. The
potential safety hazard in transportation and operation processes
due to its flammability. More importantly, the production of
aromatic hydrocarbon was limited by successive hydrogenation
of the aromatic rings or direct hydrogenolysis of C-C bonds in the
condition of high pressure H2.[6] All these drawbacks have
encouraged the bio-oil upgrading research towards seeking
activity of Ru/C catalyst is superior to other studied catalysts (i.e. Au/C, alternative approaches, which could realize the oxygen removal
Pd/C and Rh/C) in our water-only HDO reaction system. The greater
dispersion and smaller metal particle size confirmed by the TEM
micrographs accounts for the better performance of Ru/C. This
material also presents excellent levels of stability as demonstrated in
multiple recyclability runs. Overall, the proposed novel approach
confirmed the viability of oxygenated bio-compounds upgrading in a
water-only reaction system suppressing the need of external H2
supply and can be rendered as a fundamental finding for the
economical biomass valorisation to produce added value bio-fuels.
without the supply of high-pressure H2. In this sense,
hydrodeoxygenation using H donors (e.g. alcohols, formic acid)
has been considered as a promising method to suppress the
supply of external H2, in which the production of high valued
aromatic hydrocarbons, i.e. BXT[7] can be enhanced.
Guaiacol, with two different oxygen-containing function groups
(including -OH and -OCH3) is the representative of phenolic
derivatives and widely employed as lignin model compounds to
discern mechanistic information during the bio-oil upgrading
process. Numerous studies have report excellent activities of
noble metal such as palladium (Pd), rhodium (Rh), platinum (Pt),
ruthenium (Ru), etc. on various supports in HDO reaction of
Introduction
guaiacol.[8] For example, for the guaiacol HDO over Ru/TiO2-
[8e]
ZrO2[8a], Pd/TiO2[8b], Pt/H-MFI-90[8c] and Rh/ZrO2
catalysts,
Hydrodeoxygenation (HDO) is a fundamental technology to
upgrade bio-oils through catalytic processes to produce
deoxygenated fuels or high-value chemicals.[1] Lignin, as one of
the three main components of lignocellulose, possesses a great
potential for producing aromatics and transportation fuels [2] since
it is composed with a variety of phenylpropane-units (C9 units).
conversion of guaiacol all reach 100 % at optimized reaction
condition, with total deoxygenated compound, cyclohexane, as
the main product. High conversion (>95%) of guaiacol was also
obtained over Pt/C [9] and Ru/C [10] catalysts using H donor solvent
(2-propanol and decalin) without the supply of external H2, with
partial deoxygenated compound, cyclohexanol, as the main
product. All these results indicate the high activity of noble metal
catalysts in the HDO reaction. Nevertheless, the systematical
comparison of metal effect on the catalytic performance in in-situ
HDO has not been explored yet. Obviously, a promising but
challenging approach for in-situ HDO is using cheap and available
water as the hydrogen source. If successful this approach would
have a tremendous impact the process sustainability and
economic efficiency. Indeed, bio-oil has a high water content (15
wt.%-30 wt.% [11]). Water splitting on the surface of the metal
catalyst is speculated to provide hydrogen in hydrodeoxygenation
process. However, to the best of our knowledge, few studies
investigated the in-situ HDO by using water as the hydrogen
Hydrodeoxygenation of bio-oil from pyrolysis of lignin [3], and lignin
[4]
model compounds
in particular is currently gaining intensive
research attention. Typically, liquid-phase hydrodeoxygenation
involves treatment with high pressure (50-100 bar) at high
o
temperature (250-500 C) in a solvent (mainly polar protic and
non-polar solvent) with H2 supply.[5] The “Achilles Heel” of this
approach is the supply of H2―an expensive resource which
imposes a tremendous economic limitation to the implementation
of HDO in large scale production units. Moreover, H2 exhibits
[a]
Miss W. Jin, Dr L. Pastor-Perez, Prof. S. Gu, Dr. T.R. Reina
Department of Chemical and Process Engineering, Faculty of
Engineering and Physical Sciences
University of Surrey
source in bio-oil or model compounds upgrading process [12]
.
In our research, different noble metal (Ru, Rh, Pd and Au)-
based catalysts have been prepared, characterized and studied
in guaiacol in-situ HDO reaction using water as hydrogen source.
Activated carbon (AC) was selected as the support considering
the low affinity for carbon deposition, high selectivity for direct
oxygen removal and hydrophobic nature [13]. The catalysts were
comprehensively characterized by transmission electron
microscopy (TEM), high-angle annular dark field (HAADF) with
high resolution transmission electron microscope (HR-TEM),
Guildford, UK
E-mail: t.ramirezreina@surrey.ac.uk
J.L.Santos, Dr M.A.Centeno
Departamento de Química Inorgánica
Universidad de Sevilla, Instituto de Ciencias de Materiales de
Sevilla Centro mixto US-CSIC Avda.
Américo Vespucio 49, 41092 Seville, Spain.
E-mail: centeno@icmse.csic.es
[b]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900841
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inductively coupled plasma mass spectroscopy (ICP-MS), diffuse
reflectance infrared fourier transform spectroscopy (DRIFTS),
energy dispersive X-ray spectroscopy (EDX) mapping and
temperature programmed decomposition in helium (He-TPD).
The effect of metal species were investigated. The recycling test
over the most active catalysts (Ru/C) was also conducted. This
seminal work on “H2-free” HDO process opens a novel route with
potential impact on bio-oil and biomass upgrading technologies.
HAADF-STEM and HR-TEM. HAADF-STEM observations
were performed with a FEI Talos F200X CTEM/STEM,
operated at 200 kV. High-resolution images were formed by
scanning a probe across to the sample and by recording
scattering electrons synchronously with an annular detector
set from 50 to 110 mrad.
HR-TEM micrographs were operated at an acceleration
voltage of 200 kV, a spherical aberration coefficient of 0.6
mm, a beam convergence of 0.2 mrad and spread of defocus
of 4.9 nm. Digital images were taken with a side mounted
Ceta 16M camera.
Experimental
DRIFTS. The diffuse reflectance infrared Fourier
Transform Spectra (DRIFTS) were obtained on a JASCO
FTIR 6200 spectrometer with a DRIFTS accessory Pike
model EASI-DIFF, at room temperature and accumulating
100 scans with a resolution of 4 cm-1.
He-TPD. Ru/C sample (50 mg) was decomposed in a
quartz U reactor under a 50 ml/min flow of an inert gas
(helium) from room temperature up to 500°C at a heating rate
of 10°C/min and the gaseous products emitted followed by
mass spectrometry (Pfeiffer vacuum). m/z signals from 2 to
100 were continuously monitored.
Catalyst preparation
A commercial charcoal powder DARCO® with 100 mesh
particle size purchased from Sigma-Aldrich, was employed as the
carbon support. Noble metal catalysts, including Pd/C, Rh/C and
Ru/C (loading 2 wt.% of reduced metal) were synthesized by
using wetness impregnation method. Acetone was used as the
solvent for preparing Ru/C and Pd/C while distilled water for that
Rh/C catalyst. Prior to the HDO reaction, all these catalysts were
reduced under a 300 ml/min gas flow (N2: H2=1: 1) at 300 oC for
2h. It is difficult to obtain well-dispersed gold nanoparticles
through wetness impregnation method.[14] Accordingly, the
synthesis of the Au/C (2 wt.% Au) catalyst with nano-sized metal
particles was realized by colloidal method assisted by polyvinyl
alcohol (PVA) where NaBH4 was used as reducing agent .[15]
Colloidal route is an appropriate method to obtain homogeneous
and reproducible gold nanoparticle size distribution, which allows
a more direct gold size/activity correlation investigation.[16] Further
details about the catalysts preparation can be found in our
previous work.[17]
Catalytic experiment
The HDO reactions were conducted in a high-pressure
batch reactor (Parr Series 5500 HPCL Reactor with a 4848
Reactor Controller). Approximately 0.5 g of guaiacol, 49.5 g
of distilled water and 0.2 g of catalyst were loaded in a glass-
lined stainless steel vessel of 300mL. To avoid any air
presence, N2 was bubbled through the loaded reactor for 10
mins under a stirring speed of 300 rpm before closing the
reaction system. Then, the reactor was heated to the reaction
temperature 250 oC and hold at this temperature of 4h under
a stirring speed of 300 rpm. Typically, the pressure of vessel
reaches 50 bar at the reaction condition. After the reaction
and the system cooling down to room temperature, the spent
catalyst was separated from the liquid by filtration and
followed by drying. The organic products were dissolved and
recovered with ethyl acetate. Identification of organic
products was conducted by a gas chromatograph-mass
spectrometry system (GC/MS). Quantitative analysis was
performed with a gas chromatograph-flame ionisation
detector (GC/FID). The GC injector temperature was 280 oC.
The GC separation was performed using a Carboxen Packed
Analytical Column (30m×320µm×0.25µm). A split ratio of
20:1 was held. The column was firstly held at 50 oC for 1 min,
Catalyst characterization
TEM. The size and size distributions of the metal
nanoparticles were measured by transmission electron
microscopy. TEM micrographs were acquired with a FEI
Talos electron microscope operated at an acceleration
voltage of 200 kV, equipped with a Field Emission filament.
Digital images were taken with a side mounted Ceta 16M
camera. For TEM preparation, a few milligrams of each
sample were deposited directly on a 300 mesh holey carbon
coated copper TEM-grid. The mean particles sizes were
estimated from TEM micrographs by single particle
measurements of at least 200 particles.
The mean metal particle diameter was considered on the
basis of its homogeneity, dispersion and number of particles.
The average particle size was estimated according to Eq. 1,
o
o
then increased to 240 C at a heating rate of 5 C/min and
held at 240 oC for 10 min.
3
∑
푛 푑
푖
푖
퐷푝 =
Eq. 1
2
푖
∑
푛 푑
푖
The conversion of guaiacol and selectivity (based on C
mol) of typical products can be calculated using Eq. 2 and
where di is the geometric diameter of the ith particle, and ni is
the number of particles with this diameter.
ICP-MS. Metal content in the catalysts was measured by
inductively coupled plasma mass spectroscopy (ICP-MS)
using a Spectro ICP spectrometer (Model: ULTIMA 2). The
samples were burned in air at 1073 K and and the residue
was disolved in aqua regia for the ICP analysis.
Eq. 3:
Conversion of guaiacol=(푚
× 100% Eq.2
× 100% Eq.3
퐺,푖푛−푚퐺,표푢푡)/푀퐺×푁퐺
푚퐺,푖푛/푀퐺×푁퐺
푚푥/푀푥×푁푥
Selectivity of product 푥=(푚
퐺,푖푛−푚퐺,표푢푡)/푀퐺×푁퐺
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푚퐺,푖푛: Initial mass of guaiacol; 푚퐺,표푢푡: Detected mass of guaiacol
in the organic phase; 푚푥: Mass of product 푥.
ascertain the chemical functionalities present in the prepared
solids. Figure 2 shows representative DRIFTS spectra of the
studied catalysts. The spectra are dominated by an adsorption
band at 1590 cm-1, that is due to the ν(C=C) vibrations belonging
to the sp2 rings, those around 1430 assigned to aliphatic C-C
bonds and/or carboxylate groups.[20] and a wide set of bands in
the 1000-1300 cm-1 region that includes mainly bands associated
with C-O stretching in ethers, lactones, phenols and carboxylic
anhydrides.[21] Additionally, the spectral features that appear in
the region 700-900 cm-1 can be attributed to the ν(C-H) vibrations
out of the plane of aromatic compounds and C-H stretching bands
of aliphatic species are observed in the 2800-3000 cm-1 range.[22]
푀퐺: Molar mass of guaiacol; 푀푥: Molar mass of product 푥.
푁퐺 : Number of Carbon in guaiacol; 푁푥 : Number of Carbon in
pRreodsuucltt푥s.and Discussion
Catalysis characterization
The samples were thoughtfully characterised in our
previous work.[17] Briefly, the samples are composed by
noble metals nanoparticles supported on commercial
activated char. The commercial charcoal powder (Cdarco
)
used in this study contained 75.7 wt.% C, 0.2 wt.% H, 0.4
wt.% N and 23.7 wt. % O on a dry ash free basis.[17] The XRD
diffraction patterns show an excellent metallic particles
dispersion and also indicate that the Cdarco bare support does
not suffer any remarkable structural modification upon
metals deposition.[17] In terms of textural properties, the
Au/Cdarco
Pd/Cdarco
Rh/Cdarco
samples are mesoporous materials with
a type IV
isotherm.[17] Overall there are not significant discrepancies in
terms of texture and structure (XRD) among the studied
series.
TEM. Particle size is key in the HDO process since the
activation of the reactive molecules typically takes place on
the metal-support interface.[18] Figure 1 shows representative
TEM images of the prepared catalysts (Au/C, Pd/C, Rh/C
and Ru/C), no large agglomerations being observed. The
distribution of metal particle size has been calculated based
on the TEM micrographs (Table 1). Results showed that all
catalysts, apart from Rh, presented a monomodal distribution
with the mean values of metal particle lower than 7 nm. In
particular, Ru/C exhibited extremely homogeneous
dispersion on the C support, with the smallest particle size,
2.6 nm (Table 1).
As previously reported, greater deoxygenation ability in
guaiacol HDO reaction was observed for Ru/C compared to
other noble metals such as Pd/C and Rh/C.[8d] The superior
catalytic performance of Ru/C can be also found in Jelle and
co-workers’ study. Ru/C is superior to catalysts with different
support, such as Ru/TiO2 and Ru/Al2O3 and other noble
metal-based catalysts, such as Pt/C and Pd/C, achieving
high deoxygenation ratio (up to 90wt.%) in bio-oil upgrading
process.[19]
Table 1 also shows the dispersion of the active phase,
which was calculated from TEM data, assuming
cuboctahedra particles and the active phase loading
measured by ICP analysis. Metallic dispersion are somehow
comparable with values ca. 20-30% except for the Ru based
catalysts that outstands with a calculated metallic dispersion
of 48%.
Ru/Cdarco
ICP-MS. The ICP-MS results are also summarized in
Table 1. For all the synthesized catalysts, the deposited
amount of noble metal is close to a nominal loading of 2 wt.%
indicating successful immobilization of the metal on the
carbon surface.
DRIFTS. Surface groups may influence the catalytic activity in
the HDO reaction and hence a DRIFTS study is relevant to
Figure 1. TEM micrographs of catalysts and metal particle size distribution.
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Table 1. Mean metal content, particle size and dispersion
wt.% Metal
(ICP)]
Average metal particle
size (TEM, nm)
Dispersion (%)
±0.5
Sample
Ru/AC
Rh/AC
2.0
1.4
2.0
2.0
-
2.6
6.7
3.9
6.0
2.9
4.9
13.6
48
20
33
22
44
27
11
Pd/AC
Au/AC
Ru/AC PR [a]
Rh/AC PR [a]
Pd/AC PR [a]
-
-
[a] PR:Post reaction
Figure 3. Activity of noble catalysts in in-situ HDO Process
the conversion of guaiacol during in-situ HDO process. For
example, the conversion of guaiacol obtained over Au/C (23.46
wt.%) and Ru/C (25.07 wt.%) catalysts was more than twice
compared to that of blank result. The superior catalytic
performance of Ru/C could be ascribed to the higher dispersion
and small particle size of Ru particles on the carbon supported by
TEM analysis and also the higher intrinsic activity of Ru, since no
apparent different textural properties (SBET) exist among these
catalysts.[17] Very importantly, Ru/C preserve stable
physicochemical features after the HDO reaction (as supported
by the characterization of the spent catalysts).
Figure 2. DRIFTS spectra of C darco support and prepared catalysts
A simplified representation of the HDO pathways of guaiacol
based on the observed product distribution is proposed in Figure
4. Three typical mono-aromatic compounds, namely phenol,
cresol and catechol, were detected in the organic phase.
Unfortunatally, bezene was not produced during the experiment.
It is quite challenging to realize the entire deoxygenation since the
HDO reaction was conducted suppresing external hydrogen
supply. Guaiacol can undergo a transalkylation process in which
the methyl group on the methoxy group is transferred to the
aromatic ring at the ortho position to produce methyl-catechol.
Then, the two phenolic hydroxyls can be hydrogenolyzed to
produce m-cresol and o-cresol. The high selectivity of catechol in
all the product distribution indicates the presence of other HDO
pathways. The formation of catechol is the most preferred process,
because the C(sp3)-O bond is most likely to be broken since it
presents the lowest bond energy.[23] Guaiacol could undergo
demethylation (DME) process to produce catechol as well as CH4.
Catechol can further produce partially deoxygenated product,
phenol, through dehydroxylation (DHO) reaction during HDO
process with H2 supply.[23] In contrast, the dehydroxylation (DHO)
reaction hardly happen in the water-only reaction system, which
is the determinant for the low deoxygenation ratio in HDO process.
In this case, the further enhancement towards the production of
deoxygenated products lies in the dihydroxylation reaction for the
HDO using water as hydrogen donor. Another possible pathway
On the other hand, the bands observed in the 1950-1600 cm-1
range can be attributed to the C=O stretching vibration of carbonyl
groups in aldehydes, ketones, carboxylic acids, esters, and acid
anhydride.[20a] The wide band observed in the 3200-3700 cm-1
range indicates the presence of hydroxyl groups that are
interacting with the physisorbed water by hydrogen bonds,
although the univocal assignment in this region is complex since
it includes contributions from various groups.
The metal deposition does not affect substantially the shape
of the spectra, although a notorious loss of the surface groups is
produced. Also, it is interesting to note that those catalysts
prepared by impregnation in acetone (Pd and Ru) lead to
carbonaceous materials where the amount of surface hydroxyl
groups is lower than in the case of the catalysts prepared in water
media (Au and Rh).
Catalytic behaviour
Carbon support and four noble metal based catalysts, including
Pd/C, Rh/C, Au/C and Ru/C were tested in the HDO of guaiacol
without the supply of external H2, at 250 oC for 4h. A non-catalytic
experiment was performed at the same reaction condition for sake
of comparison. As shown in Figure 3, the conversion of guaiacol
without catalyst was incomplete, with only 9.87 wt.% of
conversion. The conversion of guaiacol remains at low level over
C support. In comparison, the present of noble catalyst improved
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487
500
412
58
325
200
5.0x10-12
4.0x10-12
3.0x10-12
2.0x10-12
1.0x10-12
m/z=28
450
400
350
300
250
200
150
100
50
m/z=2
m/z=32
m/z=18 (*0,1)
m/z=44
2000
3000
4000
5000
6000
Time (s)
Figure 5. He-TPD profile of Ru/C catalyst
Figure 4. Proposed reaction pathways of guaiacol HDO over noble metal
catalysts (DMO: Demethoxylation DME: Demethylation; DHO: Dehydroxylation).
for the production of phenol is the direct demethoxylation (DMO)
of guaiacol. The presence of this pathway can be judged by the
detection methanol in the product distribution. Anisole can be
derived from the hydrogenation of C (sp2)-OH bonds. Anisole can
be further hydrodeoxygenated to phenol. The possibility of this
pathway can be further speculated from the product distribution
and the short HDO reaction time, since no anisole was detected
in our HDO products (4h’s reaction). The production of partial
deoxygenated products, phenol and cresol is greatly improved
over Rh/C and Ru/C catalysts, indicating the superior ability of
demethoxylation and/or dihydroxylation. The highest conversion
of guaiacol and selectivity of deoxygenated products indicate
Ru/C is the best performing catalysts compared to other noble
metal catalysts for the upgrading of guaiacol suppressing external
H2 supply.
Figure 6. DRIFS spectra of support, fresh and spent Ru/C catalysts
One of the main challenges in a “H2-fee” HDO process is
to incorporate the H2 from water splitting into the organic
molecules. Hence, in order to rule out the possible
contribution of hydrogen released from the support’s
functional groups whose presence is evident in the DRIFTS
analysis, a TPD analysis of the studied samples was
conducted. The He-TPD results of Ru/C catalyst can been
seen in Figure 5. H2O (m/z=18), CO2 (m/z=44), CO (m/z=28)
and H2 (m/z=2) released from the catalyst during the
temperature programmed process. H2 can only be found
when temperature is higher than 375 oC (the signal of m/z=2
observed at 58°C comes from H2O desorption). This result
confirms that our catalysts do not provide any hydrogen
during the in-situ HDO at the selected reaction temperature
DRIFTS. Representative DRIFTS spectra comparing fresh
and reacted Ru/C catalysts are shown in Figure 6. There is
no apparent difference between the fresh and spent Ru/C
catalysts, indicating the Ru/C is kept stable during the HDO
reaction and the functional groups remain unchanged
corroborating the TPD results.
TEM of spent catalysts. Considering metallic sintering as
the main danger for catalyst deactivation in HDO processes
(particularly taking into account the high pressures used in
the reaction) a detailed post reaction TEM study was
conducted. Figure 7 as well as Figures S1 to S6 in the
supporting info show the results of this study.
Clearly, the spent samples of Au and Pd based catalysts,
suffer for sintering of the active phase since their average
particle size increase from 6 nm to 28.5 nm and 3.9 to 13.6
respectively (Table 1). However, this effect is relaxed for the
Ru catalyst, where the sintering of the active phase is almost
negligible (Table 1) showing very similar particle size
o
(250 C) and hence the only source of H2 is water via water
splitting. Also carbon gasification (C+H2O
→ CO+ H2) is
discarded under the studied conditions since it requires
much higher temperatures as previously reported [23]
Characterization of spent catalysts
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Au/Cdarco PR
Pd/Cdarco PR
Ru/Cdarco PR
Rh/Cdarco PR
Figure 7. TEM and metal particle size distribution of spent catalysts
distributions. A particular case occurs for the rhodium
catalysts where a clear evidence of redispersion/breaking of
the metallic nanoparticles under reaction conditions was
found, leading to lower particle sizes in the spent catalyst.
This behavior has been extensively reported previously for
noble metal nanoparticles under reductive atmosphere (H2,
100
80
60
40
20
0
40
Conversion
Anisole
Phenol
Cresol
Catechol
30
CO, etc.) and high temperatures or pressure.[24]
.
Further structural information regarding the sintering
phenomena was gathered by HAADF HRTEM of the fresh
and spent samples (Fig. S1 and S3, supporting material)
EDX mapping analysis (Fig. S2 and S4) and HRTEM (Fig.
S5). In principle the sintering process may induce
modifications on the preferential exposition for certain family
of planes. In that case we could identify those planes and
obtain fundamental information about the metallic sintering
process for samples and potentially prevent it. Unfortunately,
the HR-TEM images reveal a simultaneous growing of the
gold and palladium particles affecting all the exposed family
of planes. Hence we cannot spot whether there is a particular
family of planes susceptible to sinter. Anyway, in practical
terms the HR-TEM back up the TEM experiments indicating
the superior stability of the Ru and Rh based materials
against Au and Pd which are certainly poorer choice for this
process in terms of stability.
20
10
0
Recycling 1
Recycling 2
Recycling 3
Figure 8. Recycling tests of Ru/C catalyst
catalyst remain at high level even though a slightly decrease
of guaiacol conversion can be seen after the third activity
test. The excellent stability results displayed by the Ru/C
catalysts is backed up by the TEM images of spent sample
after the multiple cycles (Figure S6 supporting info). As
shown in Figures S6 (supporting info) no sintering at all was
observed after 3 consecutive runs validating the robustness
of this catalyst for the H2-free HDO process. Leaching of the
Recycling test. The stability of the best performing
catalyst, Ru/C, was investigated in a recyclability test, in
which the activity of Ru/C catalyst was tested in three
consecutive runs. As shown in Figure 8, the activity of Ru/C
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There are no conflicts to declare.
active phase cannot be fully discarded and perhaps it
accounts for the subtle activity drop after 3 cycles. In any
case the obtained results with the Ru/C catalyst are very
encouraging.
Acknowledgements
Financial support in the UK for this work was provided by the
Department of Chemical and Process Engineering of the
University of Surrey and the EPSRC grants EP/J020184/2
and EP/R512904/1 as well as the Royal Society Research
Grant RSGR1180353. As for the Spanish team, financial
support was obtained from the Spanish Ministerio de
Economía y Competitividad (MINECO) (ENE2013-47880-
Conclusions
This work showcase an innovative route for biomass valorisation
via “H2-free” hydrodeoxygenation reactions. We propose the
production of hydrogen through water splitting and its further
implication in
a HDO reaction to achieve bio-compounds
deoxygenation. To verify the practicability of this route, HDO of
guaiacol was conducted over a series of noble metal catalysts in
C3-2-R) and Ministerio de Economía, Industria
y
Competitividad (ENE2017-82451-C3-3-R) co-financed by
FEDER funds from the European Union. J.L. Santos also
acknowledges the MINECO for his predoctoral fellowship
(BES-2014-068244). W.J. thanks the Chinese Scholarship
Council (CSC) and LPP also thanks Comunitat Valenciana
for her postdoctoral fellow (APOSTD2017).
a
high-pressure batch reactor. Results showed that the
conversion of guaiacol were all improved over noble metal based
catalysts, compared with a non-catalytic process. The conversion
of guaiacol in the best case is around 25% under the selected
conditions in catalytic HDO reactions.
Among the studied noble metal catalysts (i.e. Au/C, Pd/C,
Rh/C and Ru/C), Ru/C owns the highest activity in our “H2-
free” HDO reaction system. The high activity of Ru/C can be
attributed to the greater dispersion and small particle size of
metal as corroborated by TEM and HR-TEM. Moreover, the
Ru/C catalyst is a robust system with an excellent behaviour
during the recyclability test. Catalyst’s recyclability is indeed
a fundamental aspect when a real application is envisaged
and we foresee a great potential for Ru/C type systems.
Overall, this seminal work validates the viability of “H2-free”
bio-compounds upgrading reactions. We sympathise with
the not extremely high conversion values reported in this
work but still the fact the partial deoxygenation and
molecules upgrading can be achieved without external
hydrogen supply is a highly commendable result that could
represent tremendous advantages from operational cost and
process safety perspective. Building upon the results of this
paper we hope to inspire the catalysis community to develop
highly active materials for this process that may revolutionise
our current understanding of biomass upgrading
technologies.
Keywords: Hydrodeoxygenation • water splitting • noble metal
catalysts •activated carbon • guaiacol
Table of Contents
This work showcases an economically appealing route for
oxygenated bio-compounds upgrading via “H2-free” HDO
reactions. We have applied noble metal based catalysts in
the partial de-oxygenation of guaiacol as a model molecule
in a high-pressure batch reactor in which water is used as
hydrogen source. The activity of Ru/C catalyst is superior to
other studied catalysts (i.e. Au/C, Pd/C and Rh/C) which is
probably due to its greater dispersion and smaller metal
particle size and also to the intrinsic activity of Ru for this
process. Although still on an early stage we believe this route
constitutes a promising approach to develop cost-effective
processes for biomass valorisation
Conflicts of interest
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