Effect of Ni Metal Content on Emulsifying Properties of Ni/CNTox Catalysts for Catalytic Conversion of Furfural in Pickering Emulsions

Article abstract of DOI:10.1002/cctc.202001045

Ni/CNTox catalysts with variable metal content have been prepared to investigate their emulsifying and catalytic properties for the liquid-phase conversion of furfural. The solid catalysts and emulsions were analyzed by several characterization techniques. The catalytic activity linearly increased with increasing Ni content (up to 10 wt.%) before dropping down again for a Ni content of 15 wt.%. The loss of catalytic activity was attributed to larger emulsion droplets formed by the inhibition of hydrophilic sites. All Ni/CNTox catalysts were highly selective to cyclopentanone as a main product, while several changes regarding secondary products were observed. Ni/CNTox catalysts with a Ni content up to 10 wt.% favor the formation of levulinic acid, while catalysts with a Ni content of 15 wt.% were selective to tetrahydrofurfuryl alcohol. This was attributed to an inhibition of the acid sites thus favoring the catalyst's hydrogenation capacity.

Full text of DOI:10.1002/cctc.202001045

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Effect of Ni Metal Content on Emulsifying Properties of Ni/
CNTox Catalysts for Catalytic Conversion of Furfural in Pickering
Emulsions
Authors: C. Herrera, J. Pinto-Neira, D. Fuentealba, C. Sepúlveda, A.
Rosenkranz, J. L. García-Fierro, M. González, and Nestor
Escalona
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ChemCatChem
10.1002/cctc.202001045
FULL PAPER
Effect of Ni Metal Content on Emulsifying Properties of Ni/CNTox
Catalysts for Catalytic Conversion of Furfural in Pickering
Emulsions
C. Herrera, [a, b] J. Pinto-Neira, [a, b] D. Fuentealba, [a] C. Sepúlveda, [b, c] A. Rosenkranz, [d] J. L. García-
Fierro, [e] M. González, [f] and N. Escalona *[a, b, g, h]
[
a]
C. Herrera, J. Pinto-Neira, D. Fuentealba, N. Escalona.
Departamento de Química física, Facultad de Química y Farmacia
Pontificia Universidad Católica de Chile
Av. Vicuña Mackenna 4860, Macul, Santiago, Chile.
E-mail: cpherrera3@uc.cl
neescalona@ing.puc.cl
[
[
b]
c]
C. Herrera, J. Pinto-Neira, C. Sepúlveda, N. Escalona.
ANID – Millennium Science Initiative Program- Millennium Nuclei on Catalytic Process towards Sustainable Chemistry (CSC), Chile.
C. Sepúlveda.
Facultad de Ciencias Químicas
Universidad de Concepción, Chile.
Casilla 160C, Universidad de Concepción, Chile.
[
d]
A. Rosenkranz.
Departamento de Ingeniería Química, Biotecnología y Materiales, Facultad de Ciencias Físicas y Matemáticas
Universidad de Chile.
[
[
e]
f]
J. L. García-Fierro.
Instituto de Catálisis y Petroleoquímica CSIC, Cantoblanco, Madrid, España.
M. González.
Departamento de Ingeniería y gestión de la construcción
Pontificia Universidad Católica de Chile.
N. Escalona.
Departamento de Ingeniería Química y Bioprocesos, Escuela de Ingeniería.
Pontificia Universidad Católica de Chile.
N. Escalona.
[
[
g]
h]
Unidad de Desarrollo Tecnológico
Universidad de Concepción.
Supporting information for this article is given via a link at the end of the document.
Abstract: Ni/CNTox catalysts with variable metal content have been
prepared to investigate their emulsifying and catalytic properties for
the liquid-phase conversion of furfural. The solid catalysts and
emulsions were analyzed by several characterization techniques. The
catalytic activity linearly increased with increasing Ni content (up to 10
wt.%) before dropping down again for a Ni content of 15 wt.%. The
loss of catalytic activity was attributed to larger emulsion droplets
formed by the inhibition of hydrophilic sites. All Ni/CNTox catalysts
were highly selective to cyclopentanone as a main product, while
several changes regarding secondary product were observed.
Ni/CNTox catalysts with a Ni content up to 10 wt.% favor the formation
of levulinic acid, while catalysts with a Ni content of 15 wt.% were
selective to tetrahydrofurfuryl alcohol. This was attributed to an
inhibition of the acid sites thus favoring the catalyst’s hydrogenating
capacity.
proportion of cellulose is suitable as a source for making
components for electronic and energy devices.[ Biomass has
gained notable importance since it can be considered as a
renewable, abundant, and low-cost raw material for the
production of biofuels and add-value chemicals.[5,6] An important
4]
product obtained from fast pyrolysis of biomass is the so-called
bio-oil.[
7,8]
However, due to its complex composition, bio-oil
displays several undesirable characteristics such as chemical and
thermal instability, corrosivity and low calorific values.[9–11]
Consequently, catalytic process such as hydrogenation (HY) and
hydrodeoxygenation (HDO) [
12,13]
are the most widely studied for
improve the fuel properties of bio-oil and to produce value-added
chemicals.[11]
It has been reported that several lignocellulosic biomass
derivates including levulinic acid, sorbitol, glucose, guaiacol, 5-
hydroxymethylfurfural and furfural can be converted to obtain
add-value chemicals and renewables plastics over different eco-
friendly catalysts.[
14–18]
Recently, Liao et al.
[19]
reported on the
Introduction
selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-
furandicarboxylic acid over engineered catalysts, which consisted
of Au-Pd alloy nanoparticles (NPs) and cobalt oxide supports
using a metal-organic framework (MOF) as a precursor prepared
via de novo synthesis.
The catalytic production of fuels, fuel additives and chemicals
from biomass, which is known as biorefinery, has received
considerable attention regarding the replacement of petroleum-
based production and the reduction of greenhouse emissions.[
Moreover, it has been reported that biomass with a high
1–3]
1
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ChemCatChem
10.1002/cctc.202001045
FULL PAPER
According to the US Department of Energy, furfural (FAL) has
been selected as one of the top 30 building blocks obtained from
biomass,[20] which can be converted directly or indirectly to more
post-synthesis treatment. For instance, carbon materials such as
graphene, fullerene, carbon onions, carbon nanotubes among
others, are mostly hydrophobic. However, it is possible
tailor/decrease their hydrophobic character by chemical
[21,22]
than 80 chemicals due to its high chemical reactivity.
The
relevance of FAL catalytic conversion comes from the large
number of chemicals and biofuels that can be obtained from its
transformation.[23–27] FAL can be converted to long-chain products
3 2 4
functionalization using acids (such as HNO and H SO ), which
allows for the incorporation of hydrophilic sites on their surface.
This procedure favors the formation of solid particles with
[
28,29]
[59,60]
by C-C coupling condensation reaction,
valuable fuel
amphiphilic character.
[
30]
additives such as 2-methyltetrahydrofuran (2-MTHF)
and
and to value-added compounds such as
Regarding the final emulsion properties, it has been reported
that parameters such as particle’s wettability, organic/aqueous
solvent ratio and sonication time can modify the emulsion type,
droplet size and emulsion fraction formed.[ All these studies
point to the fact that the wettability of the catalysts plays a key role
in the HDO of biomass-derived molecules and it should be
carefully designed to obtain the desired catalytic performance.
Despite of the encouraging results reported, the use of theses
[
31,32]
furfuryl ether (FFE),
furan by decarbonylation, furfuryl alcohol by hydrogenation[34,35]
[33]
61]
and by hydrogenation/ring rearrangement to cyclopentanone
[36–40]
(CPO).
Particular focus has been put on the transformation of furfural
to aliphatic cyclic molecules such as cyclopentanone and
cyclopentanol because they are important intermediate in fine
chemical industry with a broad range of application in the perfume, nanohybrid materials has been limited to the hydrogenation of
[
40,41]
[62,63]
medicine, and agriculture field.
Several catalytic studies
vanillin (biomass-derived polyol compounds)
coupling reactions over supported-Pd catalysts.[
and some C-C
Only a few
54,64]
(
Table S1, supplementary material) have been focusing on the
[
42–44]
[34]
effect of the active phase,
catalytic support [
In this regard, Jia et al.
metal content,
and the reaction media, among others.
nature of the
reports in the literature can be found addressing the application
of these nanohybrid materials, which make use of cheaper and
more abundant element such as Ni regarding the conversion of
furfural in emulsion systems. Recently, Herrera et al.[65] modified
CNT with different acidic treatments to study the effect on the
amphiphilic-catalytic properties of Ni/CNTox catalysts for furfural
upgrading. They verified that a more severe acid treatment
notably increased the hydrophilic character of the solid particles,
thus decreasing the catalytic activity. It is worth to emphasize that
this study has been performed with a constant Ni content. In
single-phase catalytic conversion, the metal loading is a crucial
parameter that significantly changes the activity and the products’
selectivity.[ A detailed look at the existing research literature
reveals that there is no study published addressing the effect of
the metal content on the wettability of the solid particles and their
catalytic activity in emulsions system. Therefore, this study aims
at investigating the effect of the Ni content (2, 7, 10 and 15 wt.%)
on the amphiphilic properties of solid catalysts and their
performance for catalytic conversion of furfural at the interface of
Pickering emulsions.
34,45–47]
[48]
[49]
studied the aqueous phase
hydrogenation of furfural to cyclopentanone over NiFe-supported
catalysts. The author found that the addition of Fe to Ni inhibited
further hydrogenation of the furan ring to tetrahydrofurfuryl alcohol
and enhanced CPO selectivity.
Currently, single liquid-phase conversion of furfural often
hinders the separation of the key products. Moreover, this does
not represent the real conditions, in which this platform molecule
[50]
would be found in the biphasic bio-oil.
Therefore, the most
appropriate approach for the conversion of biomass-derived
molecules is over a new amphiphilic catalysts capable to disperse
at the liquid-liquid interface thus forming stable emulsion
66]
[51]
droplets.
In phase-transfer catalysis, an emulsifier is added to the
biphasic mixture of two immiscible solvents with the purpose to
enhance the interfacial surface area.
disadvantage of using homogeneous emulsifiers is their difficult
separation from the reaction media.[53] This drawback can be
overcome by replacing the molecular surfactants with solid
[
52]
However, the main
[54,55]
particles that possess amphipathic character.
The catalytic
properties of Pickering emulsions has gained tremendous
attention due to many advantages of using emulsions, such as
enhancement of the rate of mass transfer between phases based
upon an increased interfacial area. However, the concept of
solid stabilized emulsions has not been widely used in the
catalytic upgrading of biomass derivatives.
Results and Discussion
Characterization of the catalysts
[56]
The results of the XRD analysis for all xNi/CNTox nanohybrid
catalysts (x = 2, 7, 10 and 15 wt.%) are depicted in Fig. 1. All the
catalysts displayed diffraction peaks at 25.9 and 42.7° (2θ)
corresponding to the (002) and (100) diffraction planes of graphite,
respectively (ICDD N°01-075-0444).
The formation of amphiphilic catalysts can be realized by the
fusion of two solid particles with opposite properties. For instance,
the fusion of carbon materials and oxide nanoparticles[
generates a material with adjustable amphiphilic properties.
57]
[67]
Additionally, the
xNi/CNTox catalysts showed diffraction peaks at 44.2 and 51.7°
[58]
Crossley et al.
deposited Pd nanoparticles onto carbon
(
2θ), which correspond to the characteristic peak of (111) and
nanotube-silica hybrid catalyst for water-oil HDO of vanillin. They
demonstrated that the vanillin hydrogenation over the nanohybrid
catalyst took place at the aqueous interface of the emulsion
droplets forming vanillin alcohol. Then, as the reaction proceeded,
reaction products such as 2-methoxy-4-methylphenol and 2-
methoxyphenol were obtained, which migrated to the organic
phase of the emulsion due to their relative solubility. Another
approach of tuning amphiphilic properties of a solid particle is to
modify their surface chemistry during synthesis or by a simple
0
(200) crystalline planes of Ni (JCPDS 04-0850). Moreover, the
diffraction peak located at 43.1°, which partially overlaps with the
100) reflection of graphite, can be assigned to NiO. It can be
(
observed that the peak intensity of the diffractions observed at
0
44.2 and 51.7° (corresponding to Ni ) gradually increased with
increasing Ni content, which is consistent with Ni loading in the
catalysts. The presence of NiO in the catalysts could be attributed
to some nickel oxide species that interact strongly with the support
or to the passivation layer.
2
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ChemCatChem
10.1002/cctc.202001045
FULL PAPER
particles for CNT-supported nickel catalysts.[69] Therefore, the
reduction peak observed at 376 °C can be assigned to the
reduction of NiO nanoparticles located in the exterior wall of the
CNTs.[
70,71]
Regarding to the reduction peak observed at 290 °C,
it can be attributed to the reduction of NiO nanoparticles with
different strengths of interaction with the support or to the
reduction of NiO particles inside of the CNT channels.[72]
Table 1. Textural properties of CNTox supports and calcined xNi/CNTox
catalysts quantified by N
2
physisorption.
S*BET[a]
V
p
[b]
d
p
[c]
Catalyst
S
BET
2
-1
2
-1
3
-1
(
m g )
(m g )
(cm g )
(nm)
38
CNTox
224
187
198
184
144
-
1.27
2
7
1
1
%Ni/CNTox
%Ni/CNTox
0%Ni/CNTox
5%Ni/CNTox
219
208
202
190
1.09
38
1.01
34
0.88
32
0.65
32
[
(
a] Estimated specific surface area calculated based on support contribution
BET value of CNTs). [b] Recorded at a relative pressure of 0.96. [c] Estimated
S
by 4V/A from BET. Where V is referred to total pore volume and A to surface
area.
Figure 1. XRD pattern of oxidized CNTs and the xNi/CNTox catalysts (x = 2, 7,
1
0 and 15 wt.%).
2
Fig. S1 shows the N physisorption isotherms recorded at
-196 °C of the oxidized (CNTox) support and the calcined
xNi/CNTox catalysts (x = 2, 7, 10 and 15 wt.%). It can be observed
that both the CNTox support and the impregnated Ni/CNTox
catalysts exhibit
macroporous solids according to the IUPAC classification.
a type II isotherm, which is typical for
[68]
Additionally, their BJH plots are shown in Fig. S1 (inserts in the
graphs). All xNi/CNTox catalysts displayed an average pore size
in the range of mesoporosity with minor macroporous
contributions. The surface area (SBET), estimated surface area
p p
(S*BET), total pore volume (V ) and pore diameter (d ) of the
support and the catalysts are summarized in Table 1. The
comparison of the SBET and S*BET values of the xNi/CNTox
catalysts indicates that these values did not significantly change
with the Ni content (considering an experimental error in the SBET
results of 10 %). This statement holds true for Ni contents up to
10 wt.%. In contrast, the 15%Ni/CNTox catalyst displayed a
significant decrease of SBET value, which may suggest a partial
pore blockage. This observation agrees well with the results
determined for the total pore volume. As can be seen Table 1, all
catalysts displayed a similar averaged pore diameter, which
implies that this parameter was not significantly modified by the
Ni content.
2
Figure 2. H -TPR profile of the xNi/CNTox (x = 2, 7, 10 and 15 wt.%) catalysts.
H
2
-TPR profiles of the xNi/CNTox nanohybrid catalysts (x = 2,
, 10 and 15 wt.%) are shown in Fig. 2. It can be observed that
7
all xNi/CNTox catalysts show three main reduction peaks at
around 290, 376 and 482 °C. In published research work, Suhong
These results suggest that the metal nanoparticles located inside
of the tubes can be reduced more easily than those deposited on
et al. have observed different H
2
-TPR profiles depending on the
the external surface [
66,70]
since the inner surface is less reactive
surface chemistry, particle size and/or location of the metal
than the defective external surface. It is important to mention that
3
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ChemCatChem
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FULL PAPER
previous chemical functionalization of CNTs with nitric acid tends
to increase the number of surface oxygen groups (defects).[
Furthermore, the following oxidation step may have partially
opened the ends of the nanotubes, which can be a possible
In order to estimate the Ni metal dispersion in the xNi/CNTox
catalysts (x = 2, 7, 10 and 15 wt.%), CO chemisorption was
performed (Table 3). It can be observed that the CO uptake
increased with increasing Ni content. Furthermore, the dispersion
65,73]
explanation of the peak observed at lower reduction temperatures. expressed as CO/Ni atomic ratio is given in Table 3. The CO/Ni
This agrees well with results published by Iglesias et al.[
74]
.
ratio demonstrated that the calculated dispersion did not
significantly change with an increasing Ni content. This, in turn,
implies that Ni nanoparticles are homogeneously dispersed over
the CNTox support. The average Ni particle size of 10.2 nm
matches fairly well with the results obtained by TEM.
Moreover, it can be observed that an increased Ni content
produces a shift of the reduction peak of NiO located at 290 °C to
lower reduction temperature. This result suggests a lower
interaction of metal particles with the support at high Ni contents
induced by the location of the Ni particles in the less reactive inner
channel of the CNTs. It has been reported that the H
2
Table 3. Measurement of the CO chemisorption over the xNi/CNTox catalysts
(x = 2, 7, 10 and 15 wt.%).
consumption peak observed at higher temperatures around
75]
4
87 °C can be associated to the reduction of the CNT support.[
Total and partial H
catalysts are summarized in Table 2. These results were
determined by deconvoluting the respective H -TPR profiles (Fig.
S2). Based upon these data, a significant increase of the total H
uptake with increasing Ni content can be observed. Moreover, the
uptake associated to the reduction of Ni nanoparticles at
2
uptake and reducibility of the xNi/CNTox
Catalyst
CO uptake
CO/Ni
Particle size
(nm)
3
-1
(cm g )
2
2
2
7
1
1
%CNTox
0.49
0.09
0.10
0.10
0.11
11.5
H
2
%Ni/CNTox
0%Ni/CNTox
5%Ni/CNTox
1.87
9.6
2
3
80 °C is greatly enhanced for all catalysts than that observed at
70 °C. This behavior is associated to a preferential deposition of
2.44
10.6
Ni nanoparticles inside of the nanotubes due to capillary forces of
4.19
9.2
[76]
the tubes. Moreover, the reducibility of all catalysts was similar
85-90%), which suggests a strong interaction of some NiO
(
species with the support, which is in concordance with the XRD
results.
Fig. S3a-d show the spectra of the Ni 2p3/2 region of the
xNi/CNTox catalysts (x = 2, 7, 10 and 15 wt.%) measured by XPS.
Curve fitting of the spectra revealed two partially overlapping
contributions and a satellite peak for all catalysts. Table 4
summarizes the binding energies (BE) of the most intense Ni 2p3/2
component, their relative proportion (shown in parenthesis) and
the Ni/C atomic surface ratio. It can be observed that all catalysts
exhibited a peak located at 852.8 ± 0.1 eV, which can be assigned
-1
Table 2. Total and partial hydrogen uptake (mmol g ) and reducibility (%) of
xNi/CNTox (x = 2, 7, 10 and 15 wt.%) obtained by Gaussian deconvolution of
2
the H -TPR profiles.
-
1
[a]
NiO[b]
Catalyst
H
2
uptake (mmol g )
Reducibility
(%)
-1
to Ni0 and a contribution of BE at 855.9 ± 0.1 eV, which
Peak 1
Peak 2
Total
(mmol g )
correspond to NiO species.[
72,77]
Additionally, all catalysts display
(
280 °C)
(370 °C)
0.029
0.123
0.288
0.219
a satellite peak with a BE of 860.0 ± 0.1 eV, thus confirming the
2
+ [77]
presence of Ni .
2
7
1
1
%Ni/CNTox
%Ni/CNTox
0%Ni/CNTox
5%Ni/CNTox
0.212
0.672
0.891
1.569
0.241
0.795
1.178
1.788
0.268
0.937
1.339
2.008
90
85
88
89
Table 4. Binding energies (eV) and atomic surface ratio of the xNi/CNTox
catalysts (x = 2, 7, 10 and 15 wt.%).
Catalyst
C 1s (eV)
O 1s (eV)
Ni 2p3/2 (eV)
Ni/C
[
a] Estimated by Gaussian deconvolution H
2
-TPR profiles. [b] Estimated by the
amount of NiO impregnated in the catalyst.
(atom/ atom)
0.0011
2
7
1
1
%Ni/CNTox
%Ni/CNTox
0%Ni/CNTox
5%Ni/CNTox
284.8 (79)
531.3 (24)
532.6 (35)
533.8 (41)
852.8 (87)
855.9 (13)
TEM micrographs of the amphiphilic xNi/CNTox catalysts (x = 2,
, 10 and 15 wt.%) and their corresponding histograms of the size
2
2
86.1 (17)
88.0 (4)
7
distribution of the Ni particles are depicted in Fig. 3. It can be seen
in this figure that the CNTox support consists of long graphitic
walls with defects created during the acid functionalization.
Moreover, it can be observed that most of the ends of the oxidized
CNTs are opened (as indicated by red arrows), which enabled the
deposition of Ni nanoparticles in the interior channels of the CNTs,
which agrees well with the results of H
histograms of the size distribution of the Ni particles of the 2, 7,
1
±
284.8 (81)
531.3 (20)
532.6 (36)
533.8 (44)
852.8 (86)
856.0 (14)
0.0026
0.0037
0.0050
2
2
86.1 (16)
87.9 (3)
284.8 (78)
531.3 (27)
532.6 (34)
533.8 (39)
852.7 (85)
856.0 (15)
2
-TPR. Moreover, the
2
2
86.2 (18)
88.0 (4)
0 and 15%Ni/CNTox catalysts show mean values at around 9.2
0.3 nm, 6.7 ± 0.1 nm, 7.4± 0.1 nm and 7.2 ± 0.2 nm, respectively.
284.8 (80)
531.3 (23)
532.6 (35)
533.8 (42)
852.8 (88)
856.0 (12)
Based upon these results, there is not significant change in the
size of the Ni nanoparticles with increasing Ni content, which
suggests a homogeneous distribution of Ni nanoparticles over the
support.
2
2
86.2 (15)
88.0 (5)
4
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FULL PAPER
Figure 3. TEM micrographs and corresponding particle size distribution of nickel metal of the different xNi/CNTox (x = 2, 7, 10 and 15 wt.%) catalysts.
The XPS data also demonstrate that the relative proportion
0
2+
(
shown in parenthesis) of Ni is much higher than that of Ni . This
holds true for all catalysts, which suggests that some Ni species
strongly interact with the support. This in turn, goes hand in hand
with the observed degree of the reducibility degree (85-90 %)
2
obtained by H -TPR and XRD. Additionally, Table 4 also
summarizes the observed peaks regarding the C 1s and O 1s
regions of the catalysts (spectra not shown). For all catalysts, it
can be seen that the C 1s peak consisted of three contributions
located at 284.8, 286.2 and 288.0 ± 0.1 eV. The O 1s peak had
also three contributions located at 531.3, 532.6 and 533.8 ± 0.1
eV. Concerning the C 1s region, the contribution with BE of 284.8
eV represents graphitic carbon.[71,78] The C 1s contribution located
at 286.2 eV and the O 1s signal at 532.6 eV correspond to the C–
[
66,79]
Figure 4. Relationship between the Ni content of the xNi/CNTox catalysts and
the Ni/C atomic surface ratio obtained by XPS.
O bonds in phenolic and/or ether groups.
groups can be correlated with the contribution located at 287.8 eV
C 1s signal) and 531.4 eV (O 1s signal).[80] The contribution
Carboxyl/carbonyl
(
Although ammonia desorption does not allow to discriminate
between Brønsted and Lewis acid sites, the amount of desorbed
NH is an indirect measure of the material’s overall acidity.
3
located at 533.8 eV of the O 1s signal can be assigned to C-O
bonds in carboxylic anhydride groups. Additionally, the relative
ratios (C 1s region, shown in parentheses) indicate the
predominant existence of graphitic carbon on the surface of all
samples. Moreover, it can be deduced from this table that the
relative proportion (O 1s region) did not change with increasing Ni
content and confirm the generation of different surface oxygen
groups during the chemical functionalization using nitric acid. The
Ni/C atomic surface ratio is a direct measure of the Ni
nanoparticles distribution over surface of the support. Fig. 4
shows the relationship between Ni/C atomic surface ratio and Ni
content over the xNi/CNTox catalysts (x = 2, 7, 10 and 15 wt.%).
It can be observed that the Ni/C atomic surface ratio linearly
increases with the Ni content. This result indicates that the Ni
nanoparticles are homogeneously dispersed over the
functionalized surface of the CNTox support, which is in good
agreement with the observed tendencies in TEM and CO.
According to previous studies, the acid strength can be classified
as weak acid sites (T < 300 ºC), medium acid sites (300 ºC < T <
81]
5
00 ºC) and strong acid sites (T > 500 ºC).[ In terms of acid
strength, it is observed that most of acid sites presented in the
catalysts are weak (under 300 °C) with minor contributions of
medium acid sites (300 - 500 °C). These acid sites present in the
Ni/CNTox nanohybrid catalysts arises from different surface
oxygen groups formed during the chemical functionalization.
Furthermore, as can be seen in Fig. 5, the ammonia amounts
desorbed decrease in the following order 2%Ni/CNTox
>
7%Ni/CNTox > 10%Ni/CNTox > 15%Ni/CNTox. It has been
reported that surface functional groups played a key role over the
final properties of catalysts. Bowden et al.[ proved that surface
functional groups act as anchoring centers for metal nanoparticles.
The experimental trend indicates that an increase of Ni content
reduces the density of superficial acid sites. This implies that Ni
nanoparticles are preferably anchored and dispersed over the
82]
NH
content on the acidity strength of the xNi/CNTox catalysts (x = 2,
, 10 and 15 wt.%). The corresponding profiles are depicted in
Fig. 5.
3
-TPD/MS were conducted to evaluate the effect of the Ni
7
5
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ChemCatChem
10.1002/cctc.202001045
FULL PAPER
surface oxygen groups thus producing a decrease in the acid site
density.
groups) on the surface of solid materials through oxidative
functionalization.[
60,84]
Fig. S4 shows that, after oxidative
treatment, the CNTox support presents a reduced contact angle
of 117.9 ° in comparison with the pristine CNT with a contact angle
of 138.1 °. As can be seen in Fig. 6, the Ni-based catalysts with
a Ni content of 2, 7, 10 and 15 wt.% display a contact angle of
119, 123, 127 and 135 °, respectively. These results indicate that
an increase of the Ni content tends to increase the hydrophobic
character of the catalysts, thus reducing the amphiphilic character
of the solid particles. The preferentially anchored of the Ni
nanoparticles at the created surface oxygen groups (hydrophilic
sites) produce a partial blocking of theses hydrophilic centers.
Therefore, these results together with the results obtained by
3
NH -TPD/MS points towards the fact that high Ni content reduce
the acid and amphiphilic character of Ni-based catalysts.
In Pickering emulsions, fine amphiphilic solid particles adsorbed
at the water/oil interface sterically hinder the coalescence of
droplets and effectively stabilize the emulsion.[ In Fig. 7a-d,
optical micrographs of the emulsion droplets stabilized over the
xNi/CNTox nanohybrid catalysts (x = 2, 7, 10 and 15 wt.%) are
shown. When the water-dodecane mixture was sonicated in
presence of the Ni/CNTox catalysts, amphiphilic particles were
dispersed at the liquid-liquid interfaces and an effective formation
of emulsion droplets was observed. However, several differences
can be observed with respect to the size of the resulting droplets.
Based upon Fig. 7a-d, it can be seen that the Ni-based catalysts
with a Ni content of 2, 7, 10 and 15 wt. % display average droplet
sizes average of 20 ± 6 µm, 28 ± 8 µm, 29 ± 7 µm and 44 ± 2 µm,
respectively. Ruiz et al.[ demonstrated that an increase of the
concentration of the solid particles may lead to an increase of
stabilized emulsion fraction and a decrease of the size of the
emulsion droplets, thus producing stable emulsions. The
amphiphilic character of the nanohybrids makes them segregate
naturally to the water/oil interface, thus becoming suitable for the
stabilization of emulsions with small droplet sizes and remarkable
stability.[ Therefore, for Ni contents below to 10 wt.% emulsions
with small droplet size can be formed, whereas the emulsion
droplets are bigger and, therefore, less stable for a Ni content of
56]
3
Figure 5. NH -TPD/MS profile of the xNi/CNTox (x = 2, 7, 10 and 15 wt.%)
catalysts.
To evaluate the effect of the Ni content on the wettability of the
xNi/CNTox nanohybrid catalysts (x = 2, 7, 10 and 15 wt.%), the
static contact angle (θ) was measured on the catalysts’ surface
85]
(Fig. 6 and Fig. S4). For particles stabilizing Pickering emulsions,
θ is the equivalent of the HLB (hydrophilic-lipophilic balance) of
surfactants.[83] It is well accepted that when θ < 90 °, particles are
considered to be hydrophilic and can stabilize oil-in-water
emulsions (o/w). In contrast, in case of θ > 90 °, particles are
mainly hydrophobic and favor the stabilization of water-in-oil
emulsion droplets (w/o).[54] Concerning carbon materials, they are
54]
well-known to be mainly hydrophobic. However, it is possible to
incorporate hydrophilic sites (represented by surface oxygen
15 wt.%.
Figure 6. Static water contact angle measurements for the xNi/CNTox (x = 2, 7, 10 and 15 wt.%) catalysts. The photos were captured after the deposition of the
water droplet on the surface of the self-supporting pressed sample disc.
6
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10.1002/cctc.202001045
FULL PAPER
Figure 7. Optic/fluorescent microscope images of w/o emulsion stabilized over a) 2%Ni/CNTox, b) 7%Ni/CNTox, c) 10%Ni/CNTox and d) 15%Ni/CNTox
catalysts.
Claire et al. verified that the wettability of the particles is essential
to control the emulsion type (o/w or w/o, simple or multiple) and
the droplet’s size.[ After the addition of a fluorescent and water-
soluble dye, it became possible to identify the type of emulsions
formed over the xNi/CNTox catalysts (x = 2, 7, 10 and 15 wt.%).
In Fig. 8, it can be observed that the internal area of the emulsion
droplets presented fluorescence, which proved that the
nanohybrid catalysts were capable to effective form w/o emulsion
droplets. These results are in good agreement with the measured
contact angle.
furfural (FAL) in water-dodecane emulsions stabilized over
xNi/CNTox catalysts (x = 2, 7, 10 and 15 wt.%) were investigated.
Fig. 9 shows the specific catalytic activities based upon the initial
55]
-6
-1 -1
reaction rates (x 10 molFAL gcat s ) obtained from Fig. S5. It can
be observed that the initial rate linearly increases with a Ni content
up to 10 wt.% before following a more curved trend (dotted line in
Fig. 9) for higher contents (15 wt.% of Ni). The increase in the
catalytic activity can be generally related with an increased
number of active sites on the surface (dispersion) and, therefore,
a loss of linearity at high metal contents could be associated to
the formation of agglomeration.[
66,86]
The increase of the catalytic
activity up to Ni content of 10 wt.% can be well related to the
surface accessibility of Ni sites as suggested by TEM, CO
chemisorption and XPS results. However, for higher Ni contents
(15 wt.%), the observed results cannot be connected with a loss
of dispersion since the characterization results of this catalysts
show the opposite. This implies that there are other parameters
affecting the catalytic activity in emulsion system, such as the
wettability of the solid nanoparticles and the size of the emulsion
droplets. Large emulsion droplets generate less stable drops and
are prone to coalescence process.[85,87] The optical micrographs
confirmed that the small droplet size did not change significantly
with an increasing Ni content up to 10 wt.%. In contrast, when a
15 wt.% of Ni were incorporated over the support surface, the
droplet average size increased notably (factor of 2). This reduced
the accessibility of hydrophilic sites (surface oxygen groups) and
changed the wettability of solid particles, thus forming larger
emulsion droplets, which tend to be less stable. Therefore, the
loss of the catalytic activity for catalysts containing more than 10
wt.% of Ni could be attributed to the increased droplet’s size and
the reduced stability of larger droplets. Fig. S5 shows the
conversion of FAL and the yield of products as a function of time
over the xNi/CNTox catalysts (x = 2, 7, 10 and 15 wt.%). The main
product obtained over all catalysts was cyclopentanone (CPO).
Minor amounts of 2-methyltetrahydrofuran (2-MTHF) and
cyclopentanol (CPOL) were observed. Concerning the detected
Figure 8. Optic/Fluorescent microscope image of w/o emulsion droplets
stabilized by 2%Ni/CNTox catalyst. A water-soluble and fluorescent dye (green
color, fluorescent Na salt) was added to identify the emulsion’s type.
Catalytic activity
Since bio-oil derived from biomass is a complex liquid that is
partially soluble in either water or hydrocarbon solvents, the
hydrogenation reaction in water-oil systems are more desirable
from a practical point of view. Therefore, catalytic conversion of
7
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ChemCatChem
10.1002/cctc.202001045
FULL PAPER
metal sites.[ The formation of CPO takes place via furanic ring
rearrangement of FUR-OH through Piancatelli reaction,[ under
reduced atmosphere in aqueous medium, while CPOL is obtained
by consecutive hydrogenation of CPO. Moreover, THF-OH can be
obtained via direct hydrogenation of FUR-OH. Hydrogenolysis of
FUR-OH allows for the formation of 2-methylfuran (2-MF),[
which is further hydrogenated to 2-MTHF. Finally, LA is obtained
via direct ring opening through acid hydrolysis reaction.[ It has
42]
48]
88]
89]
been reported that several parameters can affect the FAL
rearrangement towards CPO.[
48]
Hronec et al. studied the
aqueous FAL conversion towards CPO over Pd-Cu/C catalysts.[
40]
They found that the higher catalytic activity displayed for these
0
+
catalysts was related to the proper distribution of Pd and Cu
species. Furthermore, CPO formation was studied by Bradley et
al.,[90] who demonstrated that the strong FAL adsorption over
groups 8-10 metals is connected to the interaction between the
orbital π and the orbitals d of the metals. This interaction weakens
2
the C-O bond, thus helping to stabilize the ƞ -(C-O) aldehyde
Figure 9. Initial rate of furfural conversion as a function of the Ni content over
the different xNi/CNTox catalysts. The dotted line represents the theorical linear
trend.
complex.[90] The same behavior could potentially explain the ring
opening and the FAL rearrangement to CPO over xNi/CNTox (x
=
2, 7, 10 and 15 wt.%). In Scheme 2 the reported catalytic
[47]
rearrangement of FAL to CPO is depicted.
Initially, FAL is
secondary products, several changes were observed as a
function of the metal content. When FAL conversion was
performed over the Ni-based catalysts with a Ni content up to 10
wt.%, levulinic acid (LA) was mainly formed. In case of
hydrogenated towards FUR-OH over the active sites of the metal.
Afterwards, the water molecules attack via the fifth position of the
furan ring to form the oxycation.[ Then, under acidic conditions,
the rearrangement of the formed oxycation to 4-hydroxy-
cyclopentenone (4HCPTO) happens through Piancatelli
rearrangement. Finally, further dehydration of 4HCPTO,
catalyzed by acid sites, give rise to very unstable intermediate 2-
cyclopentanone (2CPTO) before being further hydrogenated to
CPO.
38]
15%Ni/CNTox catalyst, tetrahydrofurfuryl alcohol (THF-OH) was
the second mayor product. These results indicate that at high Ni
content (15 wt.%) the dehydrating ability of acid sites is minimized,
while the hydrogenating capacity was increased, according to the
Scheme 1. This could be explained by a modification of the active
sites as inferred by the characterization results. Scheme 1 shows
the main products obtained in aqueous hydrogenation of furfural
in presence of hydrogen and metal/acid-functionalized
Fig. 10 shows the products’ selectivity calculated at a furfural
conversion of 10 %. It can be observed that all catalysts display a
fairly similar selectivity to CPO (about 63% aprox.) at lower FAL
conversion.
[42,48]
catalysts.
It can be seen that FAL can be initially converted
to FUR-OH through a direct hydrogenation of C=O bond over
Scheme 1. Main products during the hydrogenation of furfural in water under hydrogen atmosphere and metal-acid catalysts.[40,48]
8
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10.1002/cctc.202001045
FULL PAPER
Scheme 2. Catalytic rearrangement of furfural (FAL) to cyclopentanone (CPO) showing hydrogenation, rearrangement, and dehydration reactions over amphiphilic
[47]
Ni/CNTox catalysts.
Moreover, it can be observed that the selectivity of secondary
products is strongly modified for higher Ni contents. The Ni-based
catalysts with a Ni content up to 10 wt. % showed the highest
selectivity to LA (> 20%) with a lower selectivity to THF-OH (about
This suggests that catalysts with strong Brønsted acid sites were
essential to obtain LA as a primary product. According to
experimental results, Ni-based catalysts with a Ni content below
10 wt.% display a higher concentration of acid sites (with weak
acidity) compared to the catalyst with 15%Ni/CNTox (Fig. 5),
which favors the formation of LA as a secondary product. The
preferential THF-OH formation was influenced by the deposition
of the Ni. The metal nanoparticles would preferably deposit over
the surface functional groups. However, in case of higher Ni
content (15 wt.%), an inhibition of the surface groups can be
achieved, thus minimizing the acid function of the catalyst.
Consequently, changes in the selectivity were observed.
Moreover, the modification of the products distribution due to a
change in the environment of Ni nanoparticles, produced by large
emulsion droplets cannot be ruled out.
In order to highlight the promising advantages of performing a
catalytic reaction in an emulsion system, the conversion of FAL
and the yield of products as a function of the reaction media over
the nanohybrid 10%Ni/CNTox catalyst after 1 h of reaction are
depicted in Fig. S6. It can be observed that the FAL conversion
in emulsion system is much higher than in the individual single
aqueous and organic phases. This result indicates that the
reactivity can be increased in emulsion systems due to a
generation of a higher interfacial areas. This behavior enhances
the rate of mass transfer between the involves phases, which
enables the diffusion of reactant to active sites. A similar behavior
was reported by Jimare et al.[95] for the HDO of vanillin over
10%). In contrast, the 15%Ni/CNTox catalyst displayed the
highest selectivity towards THF-OH (35%) with a lower selectivity
to LA (5%). These results clearly demonstrate that an increase of
Ni content to 15 wt.% decreased the dehydrating ability of acid
sites, thus leading to a modification of the reaction mechanism. It
has been reported that selectivity changes towards specific
products depends on the properties of the acid-base support (acid
[91–93]
strength and amount of acid sites).
Mellmer et al. reported on
the selective production of LA from FUR-OH over H-ZSM5
[94]
catalysts. In a first catalysts’ screening, they found that the solid
catalysts with weak-medium Brønsted acid sites, produced the
lowest yield to LA.
2
Pd/CNT-SiO nanohybrid in emulsion systems. These authors
demonstrated that the formation of stable emulsions remarkably
increased the value of the volumetric global mass transport
coefficients due to the growth of the interfacial area. Therefore,
the reaction rate can be greatly enhanced by increasing the mass
transport coefficient with solid-particle stabilized emulsions.
Additionally, it can be observed that the catalytic conversion of
FAL in non-polar solvents (dodecane) is eighteen times higher
than that in aqueous media. This result suggests that the
presence of non-polar solvents in either single or emulsion
Figure 10. Distribution of obtained products at a furfural conversion of 10% over
the xNi/CNTox catalysts (x = 2, 7, 10 and 15 wt.%) at 200 °C, 2.0 MPa of H
2
and water/dodecane ratio 1:1.
9
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ChemCatChem
10.1002/cctc.202001045
FULL PAPER
phases hinders the deactivation of active sites through the
formation of a hydrophobic film at the interface of the emulsion
droplets. This helps to avoid the irreversible adsorption of water
on the active sites.[ A clear dependence of the yield of reaction
products on the reaction media can be also observed in Fig. S6.
Furfuryl alcohol (FUR-OH) and 2-methyltetrahydrofuran (2-
MTHF) were the main products observed with dodecane only,
while tetrahydrofurfuryl alcohol (THF-OH) was a minor product.
This result suggests that furfural hydrogenation products were
obtained only in absence of water. In contrast, cyclopentanone
Prior to the synthesis of the Ni-based catalysts, 1.0 g of pristine CNT
was oxidized using 10.0 mL of a HNO solution at 65% during 3 h at 110 °C.
3
Then, the solid was filtered and washed with deionized water until reaching
a neutral pH. Finally, the functionalized support was dried at 110 °C for 12
h. The nanohybrid xNi/CNTox catalysts (x = 2, 7, 10 and 15 wt.%) were
96]
prepared by incipient impregnation of an aqueous solution of Ni(NO
O with different Ni contents. The catalysts were dried overnight at
110 °C, and then, calcined in air for 1 h at 300 °C. Finally, the catalysts
3 2
) x
6H
2
-
1
were reduced in H
passivated in 1% O
2
(flow rate of 60.0 mL min ) for 4 h at 400 °C and
-1
2
2
/N flow (60.0 mL min ) for 1 h with the reactor
immersed in a liquid nitrogen/isopropanol bath. Afterwards, the catalysts
were kept for 1.5 h at room temperature.
(CPO) was the main product observed in the aqueous medium,
while FUR-OH was detected in traces, which implies that the
presence of water inhibits to a certain extent the hydrogenation
capacity of the nanohybrid catalyst. In contrast, important CPO
and levulinic acid (LA) were produced in a high amount using an
equal proportion of solvent (emulsion), whereas FAL
hydrogenation products were only obtained in low concentrations.
The difference in the yield of CPO in w/o emulsion indicates that
the rearrangement reaction of FUR-OH is dominant in the CPO
formation, which is consistent with previous study. Therefore,
the results obtained suggest that equal amounts of organic and
aqueous phase favor the catalytic conversion of FAL and the
production of cyclopentanone. These results display a better
catalytic performance (greater activity and high selectivity to
cyclopentanone) in comparison to others catalytic systems
reported in the literature (Table S1) for the liquid conversion of
furfural, which is a clear indication of the potential of this catalytic
system.
Characterization of the catalysts
X-ray diffraction (XRD) was carried out using a Bruker D2 powder
diffractometer (radiation source Cu-Kα (λ = 1.5406 Å) operated at 30 kV/10
mA and a LinxEye solid-state detector. BET Surface area (SBET) and
textural properties of CNTox supports and xNi/CNTox catalysts (x= 2, 7,
1
0 and 15 wt.%) were determined from N
a Micromeritics 3Flex equipment. The BJH method was used to calculate
the distribution of the pore sizes. Temperature-programmed reduction (H
2
physisorption at -196 °C using
[62]
2
-
TPR) studies were obtained using a Micromeritics 3Flex equipment
equipped with a thermal conductivity detector. In each experiment, 0.035
2
g of the sample were heated under 5% H /Ar with a flow rate of 100 mL
-1
-1
min . The sample was heated at a rate of 10 °C min from 25 to 900 °C.
The degree of reducibility of the catalysts was calculated according to
Equation (1):
푚푚표ꢀ 퐻2
푚푚표ꢀ 푁ꢁ푂
푟푒푑푢푐푖푏푖푙푖푡푦 (%) =
× 100
(1)
where the mmol of H
2
was estimated by Gaussian deconvolution of the H
2
-
TPR profiles (Fig. S2, the H
2
uptake was calibrated previously) and the
moles of NiO was estimated from the percentage of NiO impregnated on
the catalysts. Transmission electron microscopy (TEM) images of
xNi/CNTox were acquired using a JEOL Model JEM-1200 EXII microscope.
Samples were ground and dispersed in methanol and, then, transferred to
a copper grid (methanol dispersion method). To obtain the Ni particle size,
histogram plots for over 300 particles of the reduced-passivated
xNi/CNTox catalysts were depicted using the software Image Tool 3.0
software. The metal dispersion was estimated from CO chemisorption
using a Micromeritics 3Flex apparatus. The catalysts (0.05 g) were
Conclusion
Amphiphilic Ni/CNT particles with different metallic content have
been prepared in order to study their emulsifying properties for
the conversion of furfural (biomass-derived furan compound) at
the water-oil interface. It was found that the wettability of the
catalysts and the size of emulsion droplets are key parameter to
enhance the catalytic activity. The catalytic activity linearly
increased with increasing Ni content (up to 10 wt.%) before
dropping down again for a Ni content of 15 wt.%. The increase of
catalytic activity was related to the increase of active Ni sites. In
contrast, the loss of the catalytic activity observed for higher Ni
contents (15 wt.%) was attributed to the enlarged droplet’s size
and their resulting instability, which was produced by the inhibition
of hydrophilic sites in this catalyst. All catalysts were highly
selective towards the formation of cyclopentanone. However,
several changes were observed regarding to secondary products
formed. Ni-based catalysts with a Ni loading up to 10 wt.% were
selective to levulinic acid, while the 15%Ni/CNTox catalyst was
mainly selective to tetrahydrofurfuryl alcohol as a second major
product. Ni nanoparticles were anchored over the surface oxygen
groups, thus inhibiting the acid sites. The catalyst is less
dehydrating and more hydrogenating thus favoring the formation
of THF-OH.
2
reduced in-situ under H flow using the same reduction conditions as
described for the synthesis of catalysts. Ni dispersion was calculated by
assuming a CO:Ni stoichiometry of 1:1. X-ray photoelectron spectroscopy
(
XPS) of in-situ reduced catalysts was realized using a VG Escalab 200R
electron spectrometer with a MgKα (1253.6 eV) photon source. All binding
energies (BE) were referenced to the C 1s level of the carbon support at
284.8 eV. An estimated error of ± 0.1 eV can be assumed for all
measurements. The intensities of the peaks were calculated from the
respective peak areas after background subtraction and spectrum fitting
by the standard computer based statistical analysis. The acidity of
xNi/CNTox catalysts was determined by temperature-programmed
3
desorption of ammonia (NH -TPD/MS) using a 3Flex (Micromeritics)
equipment. The sample was pre-treated in flowing He (flow rate of 50 mL
min−1) at 40 °C for 0.5 h and, then, saturated with NH
3
using He (flow rate
of 50 mL min−1) as a carrier gas. The samples were purged with He for 0.5
h to remove physically adsorbed NH3 and, subsequently, cooled to
ambient temperature in He. Once the TCD baseline was restored, NH -
3
TPD/MS was performed with a heating rate of 10 °C min−1 up to 900 °C
and He flow (flow rate of 50 mL min−1). The wettability of the xNi/CNTox
catalysts were determined by measuring the static contact angles (θ) with
distilled water (droplet’s volume of 2 µL) under ambient conditions on a
self-supporting pressed sample using a drop shape analyzer DSA-25-E
Experimental Section
(
Krüss GmbH). The optical/fluorescent micrograph were obtained in an
Synthesis of catalysts
Axiostar plus Zeiss microscope equipped with a HBO 50 mercury vapor
lamp.
1
0
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ChemCatChem
10.1002/cctc.202001045
FULL PAPER
Preparation of the emulsions using the nanohybrid catalysts
[
[
[
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4]
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To prepare the water/oil emulsions with the different xNi/CNTox
nanohybrid catalysts (x = 2, 7, 10 and 15 wt.%), deionized water and
dodecane were used as aqueous and organic phases, respectively. In
each experiment, 50 mg of the catalyst was dispersed in 8 mL of water by
sonication with a horn sonicator (UP50H, Hielscher) at 25% of amplitude
for 15 minutes. Afterwards, 8 mL of dodecane was added (water:dodecane
ratio equal to 1:1) before sonicating the final mixture at an amplitude of
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reactor. In each experiment, 0.232 mol L-1 of furfural, 50 mg of the
xNi/CNTox catalyst (x = 2, 7, 10 and 15 wt.%) and 16 mL of the emulsion
(
water and dodecane mixture) were used. Before to reaction, the vessel
reactor was purged using a N flow for 15 minutes to remove the oxygen
content inside of the reactor. Afterwards, the reactor was heated up to the
reaction temperature (200 °C) and the H pressure was adjusted (2.0 MPa).
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flow was replaced by a
N
2
flow. Once the reactor reached room temperature, the emulsion was
[
[
11]
12]
broken by filtering the catalyst’s particles out. Both liquid phases were
separated and analyzed individually by GC using flame-ionization detector
(
thickness). The products were identified by their column retention time in
s
comparison with available standards. The specific rate r (mol gcat s )
was calculated from the initial slope of the plot of furfural conversion as a
function of time, according to Equation (2):
FID) and an Elite-1 column (Perkin Elmer, 30 m x 0.53 mm x 3.0 μm film
-1
-1
[
[
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[
ꢂ×푛]
ꢃ
푟푠 =
(2)
U. Thubsuang, S. Chotirut, K. Nuithitikul, A. Payaka, N.
Manmuanpom, T. Chaisuwan, S. Wongkasemjit, J. Colloid Interface
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-1
where b is the initial slope of conversion vs. time plot (s ), n is the initial
number of moles of furfural in the solution (mol), and m the amount of the
catalyst (g). The selectivity (%) for each product was calculated at a furfural
conversion of 10 %, according to Equation (3):
[
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푋ꢁ
푋푇
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ꢄ푒푙푒푐푡푖푣푖푡푦 (%) = × 100
(3)
ꢅ
[
[
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19]
A. H. Motagamwala, W. Won, C. Sener, D. M. Alonso, C. T.
Maravelias, J. A. Dumesic, Sci. Adv. 2018, 4, 1–8.
where X
converted (10 %).
i
is the percentage of product i and X
T
is the percentage of furfural
Y. Te Liao, V. C. Nguyen, N. Ishiguro, A. P. Young, C. K. Tsung, K.
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The authors are grateful to CONICYT-Chile for FONDECYT N°
180982, FONDEQUIP EQM 160070, PIA CCTE AFB 170007
[21]
1
project and CONICYT doctoral scholarship N° 21170881. This
work was funded by ANID – Millennium Science Initiative Program
[22]
–
NCN17_040. A. Rosenkranz gratefully acknowledges the
financial support given by CONICYT-Chile for FONDECYT N°
1180121 and the VID of the University of Chile in the framework
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Keywords: amphiphilic solid • nanotubes • emulsion • furfural •
surface chemistry • interface catalysis
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Graphical Abstract
Furfural conversion into add-value chemicals has been investigated at the interface of emulsion droplets. Special attention has been
focused on the effect of Ni content over the emulsifying properties. It was found that enlarged droplet’s size, which was at higher Ni
content, disfavor the activity and stability of catalytic system. Moreover, it was found that the products selectivity can be tuned in function
of metal content.
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4
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