Hydrodeoxygenation of guaiacol over Ni/carbon catalysts: Effect of the support and Ni loading

Article abstract of DOI:10.1039/c5ra22540j

Commercial carbon nanotubes (CNT), oxidized CNT (CNTox) and activated carbon (AC) were used as supports to prepare Ni/C catalysts with a nominal loading of 15 wt%. In addition, xNi/CNT catalysts with loadings of x: 10, 12, 15, 17, 20 wt% were prepared. The catalysts were characterized by N₂ physisorption, temperature-programmed decomposition (TPD), potentiometric titration, H₂-temperature programmed reduction (TPR), CO chemisorption and X-ray photoelectron spectroscopy (XPS). The catalysts were evaluated for the conversion of guaiacol at 573 K and 5 MPa H₂ pressure for 4 h in a batch reactor. The activity of the xNi/CNT catalysts was related to Ni dispersion, while their selectivity was similar and favored the formation of hydrogenation products. The activity of 15Ni/CNT catalyst was higher than those of 15Ni/CNTox and 15Ni/AC catalysts. The higher activity of the 15Ni/CNT catalyst compared to 15Ni/AC catalyst was possibly due to their different morphologies, while the lower activity of the 15Ni/CNTox catalyst was attributed to the limiting effect of surface oxygen groups on the support. In addition, the higher acidity of the 15Ni/CNTox catalyst enhanced its hydrogenolysis and deoxygenation capacities.

Full text of DOI:10.1039/c5ra22540j

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Page 1 of 41
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Hydrodeoxygenation of guaiacol over Ni/Carbon catalysts: effect of the support
and Ni loading.
AB. Dongil a, I. T. Ghampson a, R. García a, J. L. G. Fierro b, N. Escalonac,d*
,
aUniversidad de Concepción, Facultad de Ciencias Químicas, Casilla 160C,
Concepción, Chile.
b Instituto de Catálisis y Petroquímica, CSIC, Cantoblanco, 28049 Madrid, Spain.
c Departamento de Ingeniería Química y Bioprocesos, Escuela de Ingeniería, Pontificia
Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, Macul, Santiago,
Chile.
d Facultad de Químicas, Pontificia Universidad Católica de Chile.
Abstract
Commercial carbon nanotubes (CNT), oxidized CNT (CNTox) and activated carbon
(AC) were used as supports to prepare Ni/C catalysts with a nominal loading of 15
wt.%. In addition, xNi/CNT catalysts with loadings of x: 10, 12, 15, 17, 20 wt.% were
prepared. The catalysts were characterized by N2 physisorption, temperatureꢀ
programmed decomposition (TPD), potentiometric titration, H2ꢀtemperature
programmed reduction (TPR), CO chemisorption and Xꢀray photoelectron spectra
(XPS). The catalysts were evaluated for the conversion of guaiacol at 573 K and 5 MPa
H2 pressure for 4 h in a batch reactor. The activity of the xNi/CNT catalysts was related
to Ni dispersion, while their selectivity was similar and favored the formation of
hydrogenation products. The activity of 15Ni/CNT catalyst was higher than that of
15Ni/CNTox and 15Ni/AC catalysts. The higher activity of the 15Ni/CNT catalyst
compared to 15Ni/AC catalyst was possibly due to their different morphologies, while
the lower activity of the 15Ni/CNTox catalyst was attributed to the limiting effect of
surface oxygen groups on the support. In addition, the higher acidity of the
15Ni/CNTox catalyst enhanced its hydrogenolysis and deoxygenation capacities.
Keywords: Hydrodeoxygenation, Nickel, Carbon nanotubes, guaiacol.
To whom correspondence should be addressed
*Dr. Néstor Escalona B., e-mail: neescalona@ing.puc.cl
Tel: (56-2) 2354 4927
1

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Introduction
Interest in exploring and developing alternative sources of energy has spiked over the
last decade due to diminishing crude oil supplies, environmental and energy security
concerns across the globe. Lignocellulosic biomass has gained considerable interest as a
source of transportation fuel and aromatic chemicals, particularly through the fast
pyrolysis technology to produce bioꢀoil. However, the bioꢀoil produced is limited by its
high viscosity, low heating value, corrosivity, incomplete volatility and thermal
instability.1 These deleterious properties arise from the presence of oxygenꢀcontaining
compounds, and for this reason it is necessary to either lower (to be used as a chemical
feedstock) or completely eliminate the oxygen composition (to be used as a fuel
component).2 Hydrodeoxygenation (HDO) is the most widely studied upgrading
method, and it is typically performed at high temperature and high hydrogen pressure
with the help of a solid catalyst.3 One of the biggest challenges of advancing this
technology is the development of effective catalysts.
Phenolic model compounds such as guaiacol, anisole and phenol, obtained from the
depolymerization of the lignin component of woody biomass, have typically been used
to assess the reactivity of several catalytic systems for HDO reactions.2, 4ꢀ6 The two
most widely studied classes of catalysts are the conventional hydrotreating sulfide
catalysts and noble metal catalysts.4 The former was found to be capable of removing
the oxygen while minimizing hydrogen consumption, although it was hampered by
2, 4
instability associated with the active sulfide phase and the support.
On the other
hand, noble metal catalysts demonstrated excellent stability and activity; their main
disadvantage was the high cost of the catalyst and the excessive hydrogen consumption
stemming from their high hydrogenation activity. For these reasons, several researchers
have been studying alternatives to these catalysts, including transition metal phosphides
2

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17ꢀ20
7ꢀ10, carbides 11, 12, nitrides 13ꢀ16, rheniumꢀbased catalysts, etc.
These studies have
reported excellent activity and selectivity results and sustained efforts are ongoing to
optimize these catalytic systems to decrease the cost of producing biomassꢀderived fuels
and chemicals. Recently, there has been a particular emphasis on exploring classes of
catalysts that uses less expensive and earthꢀabundant elements.
Nickel catalysts, in particular, have been studied by several groups for HDO
21
reactions.21ꢀ28 Mortensen et al.
screened several catalytic systems for the HDO of
phenol at 10 MPa H2 and 548 K in a batch reactor, and reported that Ni was the only
active nonꢀnoble metal catalyst. In fact, Ni/ZrO2 was the best performing HDO catalyst
from a library which also included Ru/C and Pt/C noble metal catalysts. Bykova et al.
23, 25
developed Niꢀcontaining catalysts and tested them in the HDO of guaiacol in an
autoclave at 593 K and a H2 pressure of 17 MPa: conversion reaching 97% and
deoxygenation activity exceeding 97% were obtained for some of the catalysts. Jin et al.
26
studied the effect of support (using activated carbon [AC], SBAꢀ15, SiO2 and γꢀ
Al2O3) on the reactivity of Ni catalysts in the HDO of anisole in a batch reactor at fairly
moderate conditions. The authors reported a high selectivity to deoxygenated product
for the Ni/SiO2 catalyst, while the Ni/AC catalyst was more adept at saturating the
aromatic ring. Yang et al. 27 reported a study on the same topic but with a different type
of reactor and reaction conditions. They investigated the effect of metalꢀsupport
interaction (using SBAꢀ15, AlꢀSBAꢀ15, γꢀAl2O3, microporous carbon, TiO2 and CeO2
as support) in a continuous flow reactor under 0.3 MPa and 563ꢀ583 K. The authors
reported very high HDO activity, often reaching 100%. In contrast to the results from
26
Jin et al. discussed above, the activated carbonꢀsupported Ni catalyst exhibited the
highest selectivity to benzene (64%), a valuable feedstock as a fuel additive and as a
28
commodity chemical. In another recent study, Song et al.
evaluated bifunctional
3

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Ni/HZSMꢀ5 catalyst in the HDO of phenol, catechol and guaiacol in the aqueous phase.
They reported that the combined effect of the Ni catalyst and the acid sites of the
29
HZSMꢀ5 support significantly increased the HDO activity. Zhao et al. detailed the
kinetics and performance of Ni/HZSMꢀ5 and Ni/Al2O3ꢀHZSMꢀ5 catalysts for the
aqueous phase HDO of phenol, and reported a reaction sequence whose rateꢀ
determining step was phenol hydrogenation. The study also found the Ni/Al2O3ꢀHZSMꢀ
5 catalyst was five times more active than the Ni/HZSMꢀ5 catalyst.
The exceptional activity of these catalysts reported by these authors provides an
incentive to delve into the parameters controlling the reactivity. The reactivities of the
catalysts from the studies discussed above are due to interplay between support
properties and the intrinsic activity of Ni. Recently, carbon nanotubes (CNT) and
carbon nanofiber (CNF) have been making headway in the field of heterogeneous
catalysis due to their particularly high thermal conductivity, the high accessibility of the
active phase, good chemical stability in aggressive media, and the absence of any
microporosity.30 Due to their inert surface they are especially suited to study the effect
of particle size and reaction mechanisms. However, in HDO catalysis, CNT and CNF
have received little attention. Jongerius et al. 12 reported that Mo2C/CNF and W2N/CNF
displayed high conversions and high selectivities towards phenolics in the HDO of
guaiacol at 573ꢀ648 K and 5.5 MPa of H2 pressure. Ohta et al. 31 reported no dropꢀoff in
activity in the HDO of 4ꢀpropylphenol when comparing Pt catalyst supported on multiꢀ
walled carbon nanotube to those supported on other different types of carbon materials
such as activated carbon, CMKꢀ3 mesoporous carbon and carbon black. These results
provide an additional impetus to further the applicability of CNT in the HDO of ligninꢀ
derived phenolics.
4

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In this study, a series of carbon nanotubeꢀsupported Ni catalysts with different loadings
were prepared, characterized and evaluated for the HDO of guaiacol. Guaiacol was
chosen because it is the most abundant phenolic monomer from lignin
depolymerization.32 Additionally, the catalyst with the optimum loading for CNT was
then compared to a Ni/activated carbon and a Ni/CNTox catalysts (carbon nanotube was
oxidized by nitric acid prior to Ni impregnation) in order to ascertain the effect of the
carbon support properties on the activity and selectivity.
2. Experimental
2.1 Catalysts
Two different commercial carbon materials were used as supports: carbon nanotubes
Nanocyl 3100 (CNT, > 95% purity, Nanocyl) and activated carbon (AC, Norit SX Plus)
previously preꢀtreated at 1073 K under He flow for 4 h to remove the oxygen surface
groups. A portion of the parent CNT was oxidized in 10 ml HNO3 (65%) per gram of
support at 403 K for 24 h and labeled as CNTox. The catalysts were prepared by wet
impregnation in acetone (10 mL of solution per gram of CNT) using the corresponding
amount of Ni (NO3)2·6H2O (99.9%, Sigma–Aldrich) to obtain 10, 12, 15, 17 and 20
wt.% Ni. In addition, 15% wt. Ni supported on CNTox and AC supports were prepared
following the same procedure. After stirring for 12 h, the solvent was removed under
vacuum. Finally, the materials were treated at 623 K for 5 h under flowing He (50 mL
minꢀ1). In total, seven catalysts were prepared: (1) the CNT supported catalysts with
different loadings were denoted as xNi/CNT where x is the Ni loading (i.e. 10Ni/CNT,
12Ni/CNT, 15Ni/CNT, 17Ni/CNT, 20Ni/CNT); (2) 15Ni/CNTox and 15Ni/AC denoted
the CNTox and AC supported 15wt% Ni catalysts, respectively.
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2.2 Catalyst characterization
The specific surface area (SBET) and pore volume (Vp) of the supports and catalysts were
determined from nitrogen isotherms at 77 K using a Micromeritics ASAP 2010
equipment. Prior to the measurements, the samples were degassed at 473 K for 2 h.
The surface acidity of calcined samples was measured by potentiometric titration of a
suspension of the catalyst in acetonitrile with nꢀbutylamine, using an Ag/AgCl
electrode.33ꢀ35
Temperatureꢀprogrammed decomposition (TPD) analyses of the supports were carried
out under an inert atmosphere at a heating rate of 10 K min−1 up to 1373 K and a helium
flow of 50 mL min−1.
Xꢀray photoelectron spectra (XPS) were recorded using a VG Escalab 200R electron
spectrometer equipped with a hemispherical analyzer operating in a constant pass
energy mode, and a nonꢀmonochromatic MgꢀKα (hí=1253.6 eV, 1 eV=1.603 × 10−19 J)
xꢀray source operated at 10 mA and 12 kV. Prior to the analysis, the catalyst samples
were reduced in situ under the same conditions employed before the catalytic tests. The
binding energies (BE) were referenced to the carbon support at 284.6 eV. An estimated
error of ±0.1 eV can be assumed for all measurements. Intensities of the peaks were
calculated from the respective peak areas after background subtraction and spectrum
fitting by a combination of Gaussian/Lorentzian functions. The relative surface atomic
ratios were determined from the corresponding peak intensities, corrected with tabulated
sensitivity factors, with a precision of ±7%.
Metal dispersion was estimated from CO chemisorption using a Micromeritics ASAP
2020 apparatus. The catalysts (0.25 g) were reduced in situ under H2 flow using the
same conditions employed before the catalytic tests, evacuated under flowing He and
cooled down to 303 K. Then, CO chemisorption was measured at 303 K and 0.003ꢀ0.07
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MPa. Ni dispersion was calculated by assuming an average CO:Ni stoichiometry of
1:1.36
H2ꢀTemperature programmed reduction (TPR) studies were carried out in a quartz cell
on a conventional system equipped with a thermal conductivity detector. In each
experiment, 25 mg of the sample was heated under 5% H2/Ar with a flow of 50
cm3min−1. The sample was heated at a rate of 10 °C min−1 from 298 to 1173 K.
2.3 Catalytic test
Conversion of guaiacol was carried out in a 250 ml batch reactor equipped with a
hollowꢀshaft 6ꢀbladed mechanically driven turbine with four baffles on the wall of the
reactor to prevent vortex formation (model Parr 4841). The liquid reactant feed,
consisting of guaiacol (0.232 mol lꢀ1) in nꢀdodecane (80 ml) with hexadecane (0.0341
mol lꢀ1) as an internal standard, was introduced into the reactor. Then, approximately
200 mg of catalyst previously reduced under H2 (50 ml minꢀ1) at 723 K for 4 h were
transferred rapidly to the reactor limiting the exposition to air. The system was closed
and, to avoid any air contamination, N2 was bubbled through the solution for 10 min.
Still under N2 the reactor was heated to the reaction temperature of 573 K under stirring.
The pressure was adjusted to 5 MPa by H2 introduction into the reactor and kept
constant during the course of the experiment. Aliquots of around 0.5 ml were taken
periodically during the reaction. The reaction products were identified by Gas
Chromatography (Perkin Elmer ꢀ Clarus 680) coupled to GCMSꢀSQ8T and quantified
by gas chromatograph (PerkinꢀElmer ꢀ Clarus 400) using a Flame Ionization Detector
(FID) and a CPꢀSil 5 column (Agilent, 30 m x 0.53 mm x 1.0 ꢁm film thickness). The
specific rate for the total conversion of guaiacol was deduced from the initial slope of
the conversion as a function of time plot according to the following equation:
[
b × n
m
]
Eq. (1)
rs =
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where rs is the specific rate expressed in mol g−1s−1, in which
b represents the initial
slope of conversion of guaiacol (τ/t),
n is the initial number of moles of guaiacol, and m
the mass of catalyst (g). The hydrodeoxygenation (HDO) rate was calculated from
guaiacol conversion to Oꢀfree compounds (benzene, toluene, xylene: BTX; cyclohexane;
cyclohexene and hexane). On the other hand, the selectivities (%) were determined at
20% of guaiacol conversion, according to Eq. (2).
Xi
S% =
×100
Eq. (2)
XT
where Xi is the percentage of product formation
3. Results and discussion
i, and XT is the guaiacol conversion.
3.1 Support characterization
The CNT, CNTox and AC supports were characterized to assess the differences in their
morphology and surface chemistry. The nitrogen adsorption/desorption isotherms at 77
K are shown in Supplementary Information, Fig. S1. It can be observed that CNT
support showed very little nitrogen uptake at low relative pressure, characteristic of a
Type II isotherm corresponding to nonꢀmicroporous carbon materials. However, at a
relative pressure of 0.8–0.9 a noticeable increase in the amount of N2 adsorbed can be
observed, along with a wellꢀdefined hysteresis loop due to the presence of mesopores.
The isotherm of CNTox differed from the isotherm of the parent CNT support: the
nitrogen uptake at low relative pressures increased and the hysteresis loop was extended
to medium relative pressures (0.4ꢀ0.6). The oxidation treatment of the CNT support
with nitric acid may have opened the carbon nanotubes to some extent, which will lead
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to an increase of the support’s mesoporosity.37 On the other hand, the isotherm obtained
for AC displayed higher adsorption volume at low partial pressures (P/P0 ≤ 0.1) which
is consistent with a Type I isotherm, indicative of microporous structure. The hysteresis
loop due to capillary condensations at higher partial pressures matches that observed for
a Type IV isotherm and is associated with mesoporosity. The estimated BET surface
areas (SBET), micropore (Vmicro) and mesopore (Vmeso) pore volumes are summarized in
Table 1: the AC support displayed the highest SBET, in agreement with the presence of
the largest amounts of micropores; the CNT and CNTox supports presented similar
textural properties and only a slight increase of the SBET could be observed suggesting
that the oxidative preꢀtreatment with HNO3 may have open a small fraction of the
nanotubes. In relation to the porosity, the CNT and CNTox supports possess a higher
pore volume compared to the AC support as a consequence of the spatial arrangement
of the nanotubes as bundles. This arrangement leaves an interstitial space between the
nanotubes, which is responsible for their porosity in contrast to the small pores of the
AC structure.
The acid strength and total acidity of the supports were evaluated and the results are
summarized in Table 2. The acid strength can be determined according to the criterion
proposed by Cid and Pecchi33: E0 > 100 mV, very strong sites; 0 < E0 < 100 mV, strong
sites; −100 < E0 < 0 mV, weak sites; E0 < −100 mV, very weak sites. Accordingly, these
results show that the CNT and AC supports have very weak acid sites, while the CNTox
support has very strong acid sites. The highest acid strength and total acidity displayed
for the CNTox support can be attributed to the creation of surface oxygen groups by the
wetꢀoxidation treatment.
The surface chemistry of the supports was studied by TPD analyses shown in Fig.S2. In
summary, the TPD profiles for CNT and AC barely showed peaks, suggesting that the
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amount of oxygen groups on the surface of the parent CNT was very low and that the
high temperature treatment applied to the activated carbon effectively removed most of
the surface groups. On the contrary, the CNTox support exhibited the typical peaks
associated with oxidized carbon materials38: it presented three maxima at 434, 750 and
1130 K depicting the decomposition of carboxylic, lactonic, phenolic, carbonyl,
anhydride and quinone groups.38 The presence of these functionalities agrees well with
the acidity results from potentiometric titration, shown in Table 2, and confirms that the
oxidation of the parent carbon nanotubes was effective in creating oxygen groups on the
surface.
XPS analyses were performed to identify and quantify the surface species present on the
carbon supports and catalysts. Curve fitting of the spectra revealed five C 1s peaks for
the asꢀreceived CNT support, while the CNTox and AC supports showed four and three
C 1s peaks, respectively. The binding energies (BEs) of the C 1s and O 1s regions as
well as the O/C atomic ratio of all the supports are summarized in Table 3. All the
supports displayed a main contribution at 284.6 eV, which is assigned to carbon in
Csp2.39, 40 Also, these supports displayed a peak at 286.2 ± 0.2 eV attributed to carbon
in C–O bonds 41 in phenolic or ether groups 42 and a signal at 287.7 ± 0.2 eV assigned to
carbon in C=O bonds in carbonyl groups.41 The CNT and CNTox supports also
exhibited a peak at 289.1 ± 0.2 eV ascribed to C–O–OR bonds in carboxyl or ester
groups.41 On the other hand, only the CNT support displayed a peak with BE at 290.4
eV attributed to π → π* transition 43 which is absent when the support was subjected to
the wetꢀoxidation treatment.
In regards to the O1s region, Table 3 shows that all the supports displayed BEs at 531.7
± 0.4 eV and 533.2 ± 0.4 eV, attributed to oxygen in C=O bond of carbonyl groups 39, 41
and to oxygen in C–O bonds in phenolic or ether groups, respectively.41 Moreover,
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CNT and CNTox displayed a peak at 534.4 ± 0.2 eV ascribed to oxygen in carboxyl or
ester groups. The relative intensity of this latter peak increases after the oxidation
treatment at the expense of the other two contributions. Finally, Table 3 shows that the
O/C atomic ratio followed the order of CNTox > AC > CNT. The highest value
obtained for CNTox was attributed to the creation of surface oxygen groups by the wetꢀ
oxidation treatment of the CNT support. This trend is in good agreement with the
interpretation from the TPD profiles.
3.2 Catalysts characterization
The nitrogen adsorption/desorption isotherms at 77 K of the catalysts were also
performed to study their textural properties and the results are shown in Table 1. For the
xNi/CNT catalysts, a continuous decrease of the BET surface area can be observed with
increasing Ni loading. The same trend can be observed for both the total pore and
mesoporous volumes. On the other hand, the incorporation of Ni to the AC support
significantly decreased the surface area and the pore volume. This effect has been
previously reported and is attributed to pore blockage.44 It should also be pointed out
that the textural properties of the 15Ni/CNTox catalyst were similar to the 15Ni/CNT
catalyst, in agreement with the similarities in the textural properties of their respective
supports.
The acid strength and acid sites density of the 15 wt.% Ni catalysts supported on
different supports were evaluated and the results are compared to the parent supports in
Table 2. It can be observed that while the acidity values of 15Ni/CNT and 15Ni/AC
catalysts did not vary significantly compared to their supports, the same was not true for
the 15Ni/CNTox catalyst and its corresponding support and a decrease in the acid
strength and total acidity was observed. The observed decrease for 15Ni/CNTox catalyst
is due to the reduction treatment under H2 performed prior to the reaction and
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characterization in order to obtain metallic Ni nanoparticles. Nonetheless, the value was
still within the range ascribed to strong acid sites according to the Cid and Pecchi
criterion.33
CO chemisorption was performed to assess the number of the accessible surface Ni0
atoms and the results are summarized in Table 4. A continuous increase of the CO
uptake with Ni loading can be observed. The calculated dispersion in Table 4, as CO/Ni
atomic ratio, was fairly identical and independent of the Ni loading up to 15 wt.% and
then it decreased slightly at higher loadings, which may be related to the formation of
Ni agglomerates. Moreover, the results obtained for the catalysts with 15 wt.% Ni
loading supported on CNT and CNTox were very similar, consistent with their textural
properties, which indicates that nitric acid treatment did not influence the accessibility
of CO molecules to the metal active sites. On the contrary, the CO/Ni ratio was higher
for the catalyst 15Ni/AC, possibly due to its higher surface area.
The H2ꢀTPR profiles of the catalysts are shown in Fig. 1. In general, the profiles showed
two main peaks with maximum at around 600 K and 800 K, except for the 15Ni/CNTox
and 15Ni/AC catalysts which displayed an additional contribution at 530 and 570 K,
respectively. The reduction of bulk NiO has been observed at 638 K.45, 46 It has been
reported that the nature of the support used and the synthesis conditions may affect the
reducibility of Ni species: for example, for nickel supported on carbon nanotubes,
different TPR profiles have been reported depending on the surface chemistry, particle
size and/or location of the particles.47, 48 The first reduction peak at 600 K in Fig.1
corresponds to the reduction of NiO and the temperature at which it appears is in the
50
range of previously reported values.49,
An additional reduction peak at lower
temperature is observed for 10Ni/CNT which could be due to the reduction of NiO
nanoparticles with different strength of interaction with the support or, as has been
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proposed in the literature, to the reduction of Ni (III) species formed by oxygen
chemisorption on highly dispersed NiO. 51
The estimated H2 consumption (in Table 4) within this range of temperature
corresponds quite well with the expected value corresponding to the reduction of NiO,
except for the sample 15Ni/CNTox. The larger experimental value displayed by
15Ni/CNTox compared to the theoretical value, can be ascribed to the decomposition
and desorption of the oxygen groups. Moreover, the 15Ni/CNTox sample displayed an
additional reduction peak at lower temperature, i.e. 523 K, which could be due to the
presence of particles interacting differently with the support. It has been suggested that
metal nanoparticles located inside the tubes are more easily reduced than those on the
external surface, and considering that the previous oxidation step may have opened the
tips of the nanotubes to a certain extent, this could explain the presence of the peak at
lower reduction temperature.45, 52 On the other hand, it has been previously reported that
H2 consumption peaks observed at higher temperature are due to Ni nanoparticles
catalyzing the methanation of the carbon support.45, 53 Nonetheless, the contribution of
the reduction of larger Ni nanoparticles or the reduction oxygen groups on the surface of
15Ni/CNTox which would normally take place in this temperature range cannot be
disregarded; in fact, this phenomenon might explain the greater difference between the
experimental and the theoretical value for the H2 consumption.54
Figure 2 shows the XPS of the Ni 2p region of the different catalysts with 15 wt.% Ni
loading. Curve fitting of the spectra of the Ni 2p region revealed two partially
overlapped Ni 2p doublets in addition to a satellite peak. Similar spectra were obtained
for the xNi/CNT catalysts as a function of Ni contents (not shown). Table 5 summarizes
the BEs of the most intense Ni 2p3/2 component of each catalyst, their relative
proportion in parenthesis, and the Ni/C atomic ratio. All the catalysts displayed a peak
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at BE of 853.0 ± 0.2 eV assigned to Ni0 and another peak at higher binding energy at
855.7 ± 0.2 eV corresponding to NiO species.55, 56 These results indicate that under the
reduction conditions employed a certain proportion of NiO was still present, which
might be reduced at higher temperatures in agreement with the second contribution of
the H2ꢀTPR profiles. The data also show that the relative proportion of NiO species is
relatively close in all the catalysts. These results suggest that the reduction of NiO was
not significantly modified by either the Ni loading or the wetꢀoxidation preꢀtreatment of
CNT support. The BEs in the C 1s and O 1s regions of the catalysts are also presented
in Table 5. The data reveals that all the catalysts displayed values similar to their
respective supports: a BE at 284.6 eV assigned to carbon in Csp2 57, 58, a BE at 286.2
eV attributed to carbon in C–O bonds 41 in phenolic or ether groups 42 and a BE at 287.7
eV assigned to carbon in C=O bonds in carbonyl groups.41 Unlike the C 1s XPS data for
the CNT and CNTox supports, contributions ascribed to carboxylic groups and to the π
→ π* transition were absent, suggesting that the reduction conditions employed
decreased the carboxyl or ester groups in the supports. This would be in agreement with
the different acidity displayed by 15Ni/CNTox catalyst compared to CNTox support.
Also, Table 5 shows that all the catalysts displayed two contributions in the O1s region
at 531.3 eV and 533.2 eV attributed to C=O bonds of carbonyl groups 39, 41 and to C–O
bonds in phenolic or ether groups, respectively.41 The O/C and Ni/C atomic ratios of the
catalysts are also presented in Table 5. The O/C atomic ratio values for the xNi/CNT
catalysts were similar and comparatively higher than the CNT support (~0.027 vs.
0.011). This is not surprising and could be explained by the presence of nonꢀreduced
NiO nanoparticles on the surface. On the other hand, a decrease in the O/C ratio was
observed for the 15Ni/CNTox catalyst in comparison to the CNTox support (0.067 vs.
0.096), possibly due to decomposition and desorption of surface oxygen groups on the
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support during the H2 reduction. Nonetheless, a significant amount of oxygen groups
was still present after the hydrogen treatment in agreement with the potentiometric
titration results. Therefore, the wetꢀoxidation treatment introduced oxygen groups on
the surface which are resistant to reduction. The Ni/C atomic ratio represents the
relative Ni dispersion on the surface of the carbon support. Figure 3 illustrates the
relationship between the Ni/C atomic ratio and the Ni loading for the CNT supported
catalysts where the dotted line represents the linear behavior with Ni content. The Ni/C
ratio increased linearly (within the margin of error) up to 15 wt.% of Ni loading and
then remained relatively constant afterwards. The deviation from linearity at higher
loadings is suggestive of the formation of agglomerates of Ni particles, in agreement
with CO chemisorption results. Also, Table 5 shows that the Ni/C atomic ratio for the
15Ni/AC is higher than for 15Ni/CNT and 15Ni/CNTox catalysts, suggesting a higher
dispersion of Ni nanoparticles on the 15Ni/AC catalyst which can be attributed to the
higher surface area of AC support.
3.3 Reaction results
3.3.1 Effect of Ni metal loading
The influence of Ni metal loading on the activity, transformation and products
selectivity of the CNTꢀsupported catalysts was evaluated for the conversion of guaiacol.
Figure 4 shows the evolution of products and conversion of guaiacol over xNi/CNT
catalysts. The main products at 100% conversion were cyclohexanol, cyclohexane and
methoxycyclohexanol, while the minor products include cyclohexanone, phenol,
anisole, methylphenol, light compounds and other HDO products such as benzene and
xylene (BTX) and cyclohexene. In addition, Fig. 4 shows that cyclohexanol and
methoxycyclohexanol first increased and then decreased with time, suggesting that they
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are intermediate products. Similar behavior was found for phenol and cyclohexanone.
Based on the literature 59ꢀ62 and the observed products, the proposed reaction scheme for
catalytic transformation of guaiacol over xNi/CNT catalysts is depicted in Fig. 5.
Guaiacol can be initially transformed into phenol and anisole via demethoxylation
(DMO) and direct deoxygenation (DDO) pathways, respectively. The formation of
methoxycyclohexanone and methoxycyclohexanol took place via hydrogenation (HYD)
of the aromatic ring of guaiacol, and then the DMO pathway led to the formation of
cyclohexanone and cyclohexanol, respectively. On the other hand, phenol can undergo
consecutive deoxygenation and hydrogenation (HYD) reactions to form benzene,
cyclohexene and cyclohexane. Benzene could also be produced through
demethoxylation of anisole, in agreement with the mechanism proposed by
Nimmanwudipong et al.62 Finally, the formation of methylꢀcompounds
(methylcyclohexane, methylphenols, etc.) may be attributed to methyl transfer from the
methoxylgroups to the aromatic ring through acid catalyzed transalkylation reactions, as
suggested by Zhu et al.63 It can be summarized from Fig. 4 that the xNi/CNT catalysts
were highly active for hydrogenation reactions while their hydrogenolysis activity was
low. In summary, the HYD and DMO routes were the principal routes in the conversion
of guaiacol over these catalysts.
The products distribution of the xNi/CNT catalysts was calculated at 20% conversion of
guaiacol and it is shown in Fig. 6. It can be observed that all the catalysts displayed
similar trends in product distribution, suggesting that the changes in the Ni dispersion
by varying the Ni loading did not significantly modify the active sites on these catalysts.
This is confirmed by XPS relative proportion results which showed similar distribution
of Ni species (Table 5). Figure 6 also shows that cyclohexanol, methoxycyclohexanol
and phenol were the predominant products at low conversion. It is important to stress
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that the products distribution for this catalytic system was similar to that previously
obtained by Escalona et al.59 over La1−xCexNiO3 perovskite as support of Ni
nanoparticles. It thus illustrates the unique active sites available on Niꢀbased
nanocatalysts for the conversion of guaiacol and its independence on the type of support
used.
The effect of Ni loading on the initial rate is shown in Fig. 7: the initial rate increases
with the Ni loading up to 15 %, then decreased at higher loading. Similar trend was
obtained for the intrinsic rate shown in Table 6 (normalized by the number of Ni surface
atoms derived from COꢀchemisorption results). As earlier discussed, Ni/C atomic ratio
increased almost linearly with Ni loading up to 15 wt.%, suggestive of an increase in the
number of active sites due to the homogeneous distribution of Ni on the surface, and
consequently leading to an increase in the rate of guaiacol conversion. Conversely, there
is a decrease in the initial specific rate for catalysts with higher loadings above 15 wt.%
which can be attributed to the formation of Ni aggregates (inferred from their similar
XPS Ni/C atomic ratios and CO chemisorption results), suggestive of a decrease in the
number of active sites on the catalysts with Ni loading above 15 % wt.
3.3.2 Effect of the support.
The effect of support was investigated by comparing three carbon materials with
different surface properties: (1) the asꢀreceived CNT support; (2) oxidized CNT support
by wetꢀoxidation treatment with concentrated nitric acid (CNTox) and (3) a thermallyꢀ
treated commercial activated carbon with high surface area. Figure 8 shows the
conversion of guaiacol and yield of products as a function of time for the 15Ni/support
catalysts. It can be observed that the yield of products obtained were different for the
three catalysts. Firstly, with the 15Ni/CNT catalyst cyclohexanol and cyclohexane were
the main products obtained at 100% conversion. Methoxycyclohexanol was also
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detected and its profile indicated that it is an intermediate compound. The main HDO
product was cyclohexane and only small amounts of cyclohexene and benzene could be
detected. Possible secondary products such as methylphenols, toluene and
methylcyclohexene compounds were detected in trace amounts. The same products
were obtained with the 15Ni/AC catalyst; however, their evolution with time was
different. Methoxycyclohexanol and cyclohexanol were also the main products, but
methoxycyclohexanol was observed in a higher proportion. Other differences, i.e. the
lower amount of detected cyclohexane and the continuous increase of
methoxycyclohexanol during the reaction time, are probably due to the lower
conversion achieved with 15Ni/AC catalyst during the experiment time. On the other
hand, the yield of products obtained with the 15Ni/CNTox catalyst was different: the
main reaction products at 100% conversion were methoxycyclohexanol and
cyclohexane while cyclohexanol was barely detected. Moreover, anisole was detected as
an intermediate product.
59ꢀ62
According to the reaction scheme in Fig 5
and the reaction results, it appears that
the main reaction pathway followed by the 15Ni/CNT and 15Ni/AC was the
hydrogenation of the aromatic ring of guaiacol, followed by demethoxylation either to
form cyclohexanone or cyclohexanol. In addition, cyclohexanol could also be obtained
by the HYD of cyclohexanone and finally transformed into cyclohexene and
cyclohexane. The results corroborated previous literature reports indicating that Ni
favored the hydrogenation reaction.59 The significantly high cyclohexanol production
has also been reported for other catalytic systems based on noble metals.64 On the other
hand, for the 15Ni/CNTox catalyst the amount of cyclohexanol and
methoxycyclohexanol detected which was very low. Nonetheless, the appearance of
cyclohexane seems to indicate that HYD of cyclohexanol was somehow favored over
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this catalyst. Instead, the formation of deoxygenation compounds was appreciable
through the formation of anisole via the DDO pathway, which could then be further
deoxygenated to produce benzene and then hydrogenated to eventually produce
cyclohexane.
The products distribution calculated at 20% of guaiacol conversion over the
15Ni/support catalysts is shown in Fig. 9. The products distribution are different for the
catalysts as evidenced by the relative proportion of methoxycyclohexanol, cyclohexanol
and, more notably, the significant amount of anisole produced by the 15Ni/CNTox
catalyst. This suggests that the support slightly modified the active sites on the catalysts
and, as a consequence, the reaction path. Considering the similar particle size of Ni on
15Ni/CNT and 15Ni/CNTox that CO chemisorption and XPS results showed, the
differences in the products distribution over these catalysts might be due to the different
surface chemistry that the TPD and potentiometric titration analyses suggested. In this
sense, different behaviors have been reported depending on the catalytic system. For
17
65
example, C. Sepulveda et al. and Bui et al. found that acid sites strongly modifies
the selectivity on the conversion of guaiacol, favoring the demethylation (DME)
pathway for sulfide catalysts. It is also possible that defective carbon sites from
oxycarbide groups formed via the wetꢀoxidation treatment dissolved into Ni clusters and
66
32
modified the active sites on Ni. Song et al. reported that the presence of proximal
acid sites on HZSMꢀ5 increased the hydrogenolytic character of Ni by a synergistic
action in the conversion of guaiacol. These results are in contrast to those observed for
sulfide catalysts 17, 65, suggesting that Ni sites were instrumental in tuning the reaction
mechanism involved in the conversion of guaiacol. In our system, based on the
interpretation from acidity measurements, it is reasonable to propose that the acid sites
on the 15Ni/CNTox catalyst hindered the hydrogenation pathway while facilitating the
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DMO route. Hence, it can be concluded that the cooperative effect of Ni sites and strong
acid sites on carbon supports led the reaction through the DMO pathway. On the other
hand, despite the similarly weak acid strength displayed by the 15Ni/CNT and 15Ni/AC
catalysts they exhibited different products distribution, indicating that the acid sites on
carbon supports do not have an exclusive control on product selectivity. For example,
cyclohexanol was the dominant product over the 15Ni/CNT catalyst, while
methoxycyclohexanol was the main product over the 15Ni/AC catalyst, suggestive of
lower DMO activity of the latter catalyst. In this sense, the textural properties of the
support might play an indirect role on product selectivity of Ni sites on 15Ni/AC,
26
probably due to the different accessibility to certain active sites. Jin et al.
found
Ni/AC catalyst to be an HDO inefficient but excellent aromaticꢀring hydrogenation
catalyst in the conversion of anisole. It is clear in this study that 15Ni/CNT is an even
better hydrogenation catalyst; in addition, the higher proportion of phenol and light
products (methanol, etc.) over this catalyst suggests a higher degree of hydrogenolysis
as well. However, the minimal production of HDO products by 15Ni/CNT and
15Ni/AC catalysts compared with the 15Ni/CNTox catalyst suggests that although Ni
sites are critical in the first step of ring hydrogenation, the presence of acid sites is a
requisite to further CꢀO bond cleavage.67 In summary, the products distribution of
Ni/carbon support catalysts depends on the acid strength of the surface oxygen groups
on the support and on the textural properties of the support, consistent with data
obtained by Ghampson et al.15
The activities of the 15Ni/support catalysts were expressed by the initial and intrinsic
rates (normalized by number of surface Ni atoms obtained by COꢀchemisorption) are
shown in Table 6. The 15Ni/CNT catalyst displayed higher initial and intrinsic rates
than the 15Ni/CNTox and 15Ni/AC catalysts. It has been reported that the presence of
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68, 69
surface oxygen groups on carbon supports may improve
or hamper the catalytic
70, 71
performance.
The data obtained in this study reveals that the presence of oxygen
groups on 15Ni/CNTox may have reduced the catalytic activity of Ni sites, which might
be due to a limited adsorption and/or migration of guaiacol to the active sites of the
support. A similar effect of the organic surface groups on the conversion of guaiacol
over ReS2/activated carbon catalysts was previously observed by Sepulveda et al.17
However, deactivation of active sites by the formation of coke over the more acidic
surface of 15Ni/CNTox catalyst cannot be disregarded. On the other hand, as it has been
previously reported CNT support may offer improved activity compared to activated
carbon 70, 72 and this special behavior has been attributed to the different dynamic of the
molecules on the close nanotube surface.73
In summary, there appears to be a tradeꢀoff in functionalizing the surface of CNT
support: although acidic oxygen surface groups steers Ni catalysts through the
deoxygenation pathway, they significantly diminish the intrinsic activity of this catalyst.
4. Conclusion
We have prepared a series of Ni catalysts with different loadings supported on carbon
nanotubes, and studied their catalytic performance in the hydrodeoxygenation of
guaiacol, a lignin model compound. The effect of the support was also studied by
comparing the 15Ni/CNT catalyst with catalysts prepared over oxidized CNT
(15Ni/CNTox) and thermallyꢀtreated activated carbon (15Ni/AC).
The characterization performed showed that the Ni loading influenced the dispersion of
Ni nanoparticles, and the formation of agglomerates at Ni loadings higher than 15 wt.%
was suggested. The intrinsic activity of the differently loaded catalysts varied with the
relative Ni dispersion deduced from XPS. On the other hand, product selectivity did not
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significantly change with Ni loading, indicating that the nature of the active sites was
not modified by increasing the Ni content which is evident from the similar relative
proportion of Ni species present on the catalysts (deduced from XPS data). The main
products from guaiacol at 100% conversion over these catalysts were ring
hydrogenation products such as cyclohexanol, cyclohexane and methoxycyclohexanol.
In relation to the effect of the support, the 15Ni/AC catalyst displayed the highest
dispersion of all the 15Ni/support catalyst, likely due to the higher surface area of the
parent support, AC. In addition, in comparing 15Ni/CNT and 15Ni/CNTox, it appears
that nitric acid oxidation did not improve the Ni dispersion. The 15Ni/CNT catalyst
displayed higher initial and intrinsic rates than the 15Ni/CNTox and 15Ni/AC catalysts.
Based on XPS and acid strength results, these activity differences can be explained by
the different surface chemistry and textural properties of the support. It appears that the
presence of oxygen groups on 15Ni/CNTox catalyst may have reduced the activity of Ni
sites by hindering the adsorption and/or migration of guaiacol to the active sites. The
surface chemistry and, to some extent, the textural properties influenced the selectivity
of the reaction. The acid sites on the 15Ni/CNTox catalyst hindered the hydrogenation
pathway while facilitating the DMO route. It is also possible that defective carbon sites
from oxycarbide groups formed via the wetꢀoxidation treatment dissolved into Ni
clusters and modified the active sites on Ni. The difference in products distribution over
the 15Ni/AC and 15Ni/CNT catalyst was probably due to accessibility to active sites
which controlled the different hydrogenation capacities of the catalyst.
Acknowledgments
The authors acknowledge the financial support by CONICYTꢀFONDECYT grant
1140528, Fondecyt Postdoctoral grants 3130483 and 3150033, PFBꢀ27 grants and Red
Doctoral REDOC.CTA, MINEDUC project UCO1202 at Universidad de Concepción.
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RSC Advances
Page 26 of 41
View Article Online
DOI: 10.1039/C5RA22540J
Table 1. Composition and textural properties of the supports and the catalysts
S BET
(m2/g)
300
V micro
(cm3/g)
0.01
V meso
Sample
Ni
Ni Surface density
(atoms nmꢀ2)
Vt
loading (%)
(cm3/g)
(cm3/g)
CNT
CNTox
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
10
12
15
17
20
15
15
ꢀꢀꢀ
ꢀꢀꢀ
1.76
1.77
1.79
0.31
1.14
1.08
1.02
0.74
0.71
1.05
0.11
319
846
282
249
241
230
211
262
224
0.01
0.28
0.01
0.01
0.01
0.01
0.01
0.01
0.06
1.78
0.03
1.12
1.07
1.01
0.73
0.70
1.04
0.05
AC
ꢀꢀꢀ
10NiꢀCNT
12NiꢀCNT
15NiꢀCNT
17NiꢀCNT
20NiꢀCNT
15NiꢀCNTox
15NiꢀAC
3.8
4.7
6.0
7.0
8.5
5.7
2.1
26

Page 27 of 41
RSC Advances
View Article Online
DOI: 10.1039/C5RA22540J
Table 2. Acid strength of the supports and the reduced catalysts with 15% wt. Ni.
Acid strength
(mV)
Total acidity
(meq/g)
0.08
Samples
CNT
CNTox
ꢀ177
335
ꢀ105
ꢀ140
200
0.43
0.35
0.05
0.29
0.31
AC
15NiꢀCNT
15NiꢀCNTox
15NiꢀAC
ꢀ112
27

RSC Advances
Page 28 of 41
View Article Online
DOI: 10.1039/C5RA22540J
Table 3. Binding energies (eV) and atomic surface ratios of supports.
Supports
C1s
O1s
(O/C) at.
284.6 (79)
286.0 (14)
287.7 (3)
288.9 (2)
290.4 (2)
531.7 (30)
533.2 (65)
534.7 (5)
0.011
CNT
284.6 (67)
286.0 (18)
287.5 (8)
289.1 (6)
531.6 (26)
533.1 (56)
534.5 (18)
0.096
CNTox
284.6 (84)
286.2 (14)
287.8 (2)
531.3 (26)
532.8 (74)
AC
0.034
28

Page 29 of 41
RSC Advances
View Article Online
DOI: 10.1039/C5RA22540J
Table 4. Ni dispersion calculated from CO chemisorption and theoretical and experimental
hydrogen consumption from H2ꢀTemperature programmed reduction of the catalysts.
TPRꢀH2
mmol consumed/g
CO Uptake
Samples
CO/Ni
(ꢀmol gꢀ1)
Peak 1
Peak 1
theoretic
real.
10NiꢀCNT
12NiꢀCNT
15NiꢀCNT
17NiꢀCNT
20NiꢀCNT
15NiꢀCNTox
15NiꢀAC
39
45
56
59
63
59
76
0.023
0.022
0.022
0.020
0.018
0.023
0.029
1.72
2.07
2.59
2.93
3.45
2.59
2.59
2.02
2.33
2.36
2.56
3.35
3.90
2.89
29