Catalytic wet hydrogen peroxide oxidation of isoeugenol to vanillin using microwave-assisted synthesized metal loaded catalysts

Article abstract of DOI:10.1016/j.mcat.2021.111537

Supported metal nanoparticles (Cu, W, Mo among others) have been stabilized on a silica matrix following an efficient and sustainable one-pot microwave-assisted protocol. The materials synthesized were characterized, mainly, by nitrogen adsorption desorption measurements, X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDS). The materials exhibited large surface area and pore size in the mesopore range. The incorporation of metal particles in the silica matrix was also confirmed with particle size in a wide range between 2 and 52 nm. The catalytic activity of the resulting materials was evaluated in the catalytic wet oxidation of isoeugenol into vanillin using hydrogen peroxide as oxidant. The catalysts shown moderate conversions, up to 50–60 % for Cu-MINT with acceptable selectivity towards vanillin production (22–45 %), obtaining dimers as main side products. The stability of the catalysts was investigated as well, obtaining an imperceptible decrease after the fourth use.

Full text of DOI:10.1016/j.mcat.2021.111537

Molecular Catalysis 506 (2021) 111537
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Catalytic wet hydrogen peroxide oxidation of isoeugenol to vanillin using
microwave-assisted synthesized metal loaded catalysts
a
a
b
a
Paloma García-Albar , Noelia L a´ zaro , Zeid A. ALOthman , Antonio A. Romero ,
Rafael Luque
a,
b
,
c
,
a,
*, Antonio Pineda *
a
Departamento de Química Org a´ nica, Universidad de C ´o rdoba, Edificio Marie Curie (C 3), Campus de Rabanales, Ctra Nnal IV-A, Km 396, E14014, Cordoba, Spain
Peoples Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya str., 117198, Moscow, Russia
b
c
Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh, 11451, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Supported metal nanoparticles (Cu, W, Mo among others) have been stabilized on a silica matrix following an
efficient and sustainable one-pot microwave-assisted protocol. The materials synthesized were characterized,
mainly, by nitrogen adsorption desorption measurements, X-ray diffraction (XRD), scanning electron micro-
scopy/energy dispersive X-ray analysis (SEM/EDS). The materials exhibited large surface area and pore size in
the mesopore range. The incorporation of metal particles in the silica matrix was also confirmed with particle
size in a wide range between 2 and 52 nm. The catalytic activity of the resulting materials was evaluated in the
catalytic wet oxidation of isoeugenol into vanillin using hydrogen peroxide as oxidant. The catalysts shown
moderate conversions, up to 50–60 % for Cu-MINT with acceptable selectivity towards vanillin production
Wet oxidation
Heterogeneous catalysis
Microwave-assisted reactions
Metal nanoparticles
(
22–45 %), obtaining dimers as main side products. The stability of the catalysts was investigated as well,
obtaining an imperceptible decrease after the fourth use.
1
. Introduction
chemicals, energy and materials [8]. In addition, the sizes, shapes and
properties of the surface can be designed with greater precision to
achieve more specific functions [9]. This field has been the focus of
many researchers due to the importance of nanotechnology and the
immense potential for handling of materials at an ultra-small nanometer
scale (between 1ꢀ 10 nm) [9]. These research efforts have resulted in a
high number of applications for nanoparticles across many different
areas including biomass valorisation [10–13].
Although nanoparticle catalysts have high catalytic activities due to
their high surface-volume ratio, they suffer from some disadvantages
such as reuse and performance on large-scale reactors [14]. For these
reasons, nanoparticle stabilisation on supports is crucial for improved
catalytic efficiencies [14]. The use of mesoporous materials including
porous silicates has been extensively reported [15–17]. Mesoporous
silica is one of the most researched and well-known mesoporous mate-
rials, ideal to control the structure, texture and shape of the final ma-
terials without causing any undesirable interactions [18].
Catalysis is considered to be a key component on the path towards
sustainability [1]. While it has traditionally been in the best interest of
companies to employ catalysts for process optimization from an eco-
nomic standpoint, the push for greener processes has generated the
demand for other aspects of catalytic optimization including waste
elimination and avoiding the use of toxic or hazardous materials [1–3].
In addition to the need for improved catalyst design to take these aspects
into consideration for existing processes, new developments in biomass
valorisation and advanced processes still require significant progress
with yields and reaction conditions before being practical at industrial
scale [3–5].
In the field of catalysis, the use of heterogeneous catalysts has offered
environmental improvements when compared to homogenous catalysts,
along with easier handling and better separation [1,6,7]. Similarly, the
employment of nanomaterials in catalyst development offers several
advantages. The ability to directly work and control systems on the same
scale as nature provides a unique approach for the production of
Nanocatalysts can be prepared with these useful supports using a
wide variety of conventional approaches such as impregnation/
*
Corresponding authors at: Departamento de Química Org ´a nica, Universidad de C o´ rdoba, Edificio Marie Curie (C 3), Campus de Rabanales, Ctra Nnal IV-A, Km
3
96, E14014, Cordoba, Spain.
E-mail addresses: q62alsor@uco.es (R. Luque), q82pipia@uco.es (A. Pineda).
https://doi.org/10.1016/j.mcat.2021.111537
Received 31 July 2020; Received in revised form 17 March 2021; Accepted 17 March 2021
Available online 1 April 2021
2
468-8231/© 2021 Published by Elsevier B.V.

P. García-Albar et al.
Molecular Catalysis 506 (2021) 111537
reduction, ionexchange and chemical vapor deposition (CVD) [19].
Alternatively, there are novel procedures such as microwave-assisted
deposition and mechanochemistry [20,21] rather promising for the
preparation of supported metal nanoparticles. The use of microwave
irradiation, in particular, has been shown to induce the formation
nanostructures with the ability of controlling particle sizes and shapes
self-assembled nanotubular structures. According with such preparation
method, in atypical synthesis, 0.6 mmol of n-dodecylamine as surfac-
tant, 2 mmol of tetraethyl orthosilicate (TEOS), 0.2 g of the metal salt
precursor, 2 mL of water and 2 mL of acetonitrile were mixed together
and microwaved for times ranging between 5 and 15 min at 200 W,
◦
reaching a maximum temperature between 80 and 100 C, depending
[
22,23].
Luque et al. developed a microwave-assisted procedure for the syn-
on the material. Subsequently, the materials were washed with aceto-
nitrile, distilled water and acetone. Then, the surfactant was extracted
with ethanol for 6 and 24 h, respectively. Eventually, the material was
thesis of metal nanoparticles supported in the structure of hexagonal
mesoporous silica (HMS) materials [24]. These materials are prepared
by self-assembly from mixtures of n-dodecylamine, tetraethyl orthosi-
licate (TEOS), water and acetonitrile, giving rise to nanotubular struc-
tures, instead of the typical morphology of HMS materials [9,24]. The
denoted Microwave Induced Nanotubes (MINT) could offer improve-
ments to many different types of reactions, particularly in biomass val-
orisation and oxidation reactions to bring these sustainable technologies
closer to industrial applications.
◦
obtained after drying at 100 C. The materials were named as M-MINT,
where “M” stands for the metal incorporated and “MINT” is the
acronym for Microwave Induced NanoTubes. For those materials pre-
pared using heteropolyacid “HP” was added at the beginning of the
name of the material.
2.3. Materials characterization
A process of interest in biomass valorisation is the production of
vanillin ((4-hydroxy-3-methoxybenzaldehyde), a compound widely
used in many different industries including food, perfumes and phar-
maceuticals [25]. The annual production of vanillin is roughly 20,000
metric tons per year, and it is expected to continue increasing due to the
continuously growing demand [25]. At present, only 1% of total vanillin
production comes from the extraction of natural material, as it is an
expensive process, and the remaining 99 % from chemical and
biochemical routes [26]. Biochemical routes utilize lignin, one of the
main constituents of lignocellulosic biomass, as a starting material [26].
Eugenol, isoeugenol and ferulic acid derived from lignin are employed
as substrates for the production of vanillin via simple oxidation routes
Textural properties of as-synthesised materials were analysed using
nitrogen adsorption/desorption measurements at liquid nitrogen tem-
perature (77 K) though the use of a Micromeritics ASAP 2000 poros-
imeter (Micromeritics Instrument Corp., Norcross, GA, USA). The
◦
samples, prior to the analysis, were evacuated at 130 C for 24 h. The
surface area (SBET) of the synthesized nanostructures was calculated
using the linear part (0.05 < Po < 0.22) of Brunauer, Emmett and Teller
(BET) equation. Pore size distribution as well as pore volume were
estimated in the adsorption branch by the Barrett, Joyner, and Halenda
(BJH) equation (Barret-Joyner-Halenda).
X-ray diffractograms were acquired using a Bruker D8D Discover
(40 kV, 40 mA) diffractometer (Bruker AXS, Karlsruhe, Germany) using
[
27,28]. However, low selectivity values have been a significant issue, as
the Cu K
α radiation (λ =1.54 Å). For the wide angle runs the scan speed
◦
◦
the purification and isolation steps are rather difficult due to the similar
structures of the other phenolic by-products [29].
employed was 0.05 s per step in the interval 10 < 2θ < 80 , while for
◦
◦
the low angle measurements the interval was 0.5 < 2θ < 5 for the
wide-angle acquisitions. The XRD instrument was coupled with a com-
puter fitted with the “Diffract.Suite EVA” software version 3.1 (Bruker
AXS, Karlsruhe, Germany) which allows particle size determination and
metal phase identification.
In this work, we report the microwave-assisted synthesis of metal
(
Cu, Nb, Mo, W) containing MINT like materials. The materials syn-
thesized were characterized in order evaluated the protocol for different
metals. The catalytic activity of as-synthesised materials was tested in
the wet oxidation of isoeugenol to vanillin using hydrogen peroxide as
oxidant under conventional heating.
Elemental analysis of the synthesized materials was obtained using a
JEOL JSM 7800 F scanning electron microscope (JEOL Ltd., Akishima,
Tokio, Jap o´ n) fitted with a X-max150 microanalysis system, window
type detector SiLi, detection range: from boron to uranium, 127 eV
resolution at 5.9 KeV.
2
. Materials and methods
2
.1. Materials
SEM images were obtained using a Phenom ProX SEM microscope
with an acceleration voltage of 15 kV.
All chemicals used for the preparation of materials synthesised in this
TEM micrographs were obtained employing a high-resolution
work were used as-received, without further purification. The silica
precursor was tetraethyl orthosilicate (TEOS), acquired from Sigma-
Aldrich, with 99 % purity. The surfactant employed was an amine, n-
dodecylamine, purchased from Sigma-Aldrich with a purity degree of 98
transmission electron microscope JEOL JEM 1400 with an accelera-
tion voltage of 80 kV and a resolution of 0.38 nm between points.
XPS measurements were carried out in an ultra-high vacuum (UHV)
multipurpose surface analysis system Specs™, equipped with the Phoi-
bos 150-MCD energy detector (Berlin, Germany). Previously, the sam-
%
and the solvent used was acetonitrile obtained from Panreac with a
purity of 99.9 %. The metal salt precursors employed were: ammonium
niobate oxalate hydrate, C NNbO .xH O, (Sigma Aldrich, purity: 99
), copper chloride (II) dihydrate, CuCl .2H O (Merck, purity: 99.5 %),
ammonium metatungstate hydrate, (NH 40.xH2O (Fluka, pu-
rity: 99 %), ammonium molybdate tetrahydrate, (NH MoO (Sigma
Aldrich, purity: 99,98 %), phosphomolybdic acid hydrate,H Mo12 40P.
xH₂O (Sigma Aldrich, purity: ≥99.99 %), phosphotungstic acid hydrate,
PW12 40.xH2O (Sigma Aldrich, purity: 99,995 %).
The reagents employed to assess the catalytic activity of the as-
synthesised materials were isoeugenol, C10 , (Sigma-Aldrich, pu-
rity: 98 %) and hydrogen peroxide as oxidant (Sigma-Aldrich, purity: 35
).
ꢀ 6
ples were outgassed under vacuum (<10 torr) for 12 h.
4
H
4
9
2
ICP-MS measurements were carried out to determine the possible
catalysts leaching in the reaction solution. The equipment used for the
analysis were an ElanDRC-e spectrometer (PerkinElmer NexionX)
equipped with a sample introduction, argon plasma ionization and
quadrupole ion detection systems; in addition, the device is equipped
with collision/reaction cells to eliminate some polyatomic interferences.
%
2
2
4 6 2 12
) H W O
4
)
2
4
3
O
H
3
O
2.4. Catalytic activity
12 2
H O
The catalytic activity of the synthesized materials was investigated in
the wet peroxide oxidation of isoeugenol to vanillin using hydrogen
peroxide as oxidant. The reaction was carried out in a parallel reaction
station Carrousel Reaction Station TM (Radleys Discovery Technolo-
gies). The reaction conditions were: 0.8 mL of isoeugenol, 1.2 mL of
hydrogen peroxide, 8 mL of acetonitrile as solvent and 0.1 g of catalyst
%
2
.2. Materials synthesis
The nanostructures were synthesized according to the protocol re-
◦
ported by Gonzalez-Arellano et al. [30,31] for the preparation
at 80 C for 6 h. Moreover, the reaction was carried out adding the
2

P. García-Albar et al.
Molecular Catalysis 506 (2021) 111537
hydrogen peroxide by two steps (0 min and 30 min reaction-time) to
study the possible improve of catalytic rate. The samples were analysed
in a gas chromatograph Agilent Technologies 7890 A using a Petrocol™
Table 1
Textural properties for the metal loaded synthesized MINT materials from ni-
trogen adsorption/desorption measurements.
2
ꢀ 1
3
ꢀ 1
3
ꢀ 1
g )
DH (100 m×0,25 mm×0,50
μ
m) capillary column and a flame ionisa-
Materials
SBET (m
g
)
DBJH (nm)
Vmeso (cm
g
)
VBJH (cm
tion detector (FID).
Cu-MINT
675
87
5.2
0.70
0.20
0.47
0.23
0.10
0.79
0.77
0.32
0.63
0.32
0.20
1.04
W-MINT
15.0
12.0
11.6
6.4
3
. Results and discussion
The protocol followed for the preparation of the materials investi-
HPW-MINT
Mo-MINT
HPMo-MINT
Nb-MINT
206
156
127
397
10.7
gated in this work was developed by Gonzalez-Arellano et al. [30,31].
According to this protocol nanotubular structures could be obtained,
influenced, mainly by the nature of the metal salt precursor. This pro-
tocol seems to be a promising alternative for the immobilization of
heteropolyacids using a siliceous support.
corresponding, mostly, to the observed mesoporosity.
The morphology of the materials could be clearly observed in SEM
images, being similar to previous reported tubular shape morphology for
related MINT materials [30] (Fig. 2.a). Interestingly, TEM images for
this material also revealed a good dispersion of tungsten oxide nano-
particles with an average particle size of 2 nm, much smaller than the
particle size determined for other materials, measured using the
Scherrer equation.
◦
◦
Low angle X-ray diffractograms (0,5 <2θ<5 ) were recorded as
depicted in Fig. 1. CuMINT did not show any distinctive diffraction lines
as consequence of not having any degree of ordering. In contrast, other
materials such as HPMo-MINT did show a not well resolved single signal
◦
at 2θ values around 3 similar to that present in Hexagonal Mesoporous
Silica materials (HMS) that may confirm the presence of this type of
mesoporous arrangement in these materials [32]. The incorporation of
metals on the materials was confirmed by XRD. As expected, the pres-
ence of reduced metal species was confirmed for most of the cases with
the exception of the material W-MINT, where no distinctive diffraction
lines could be identified due either to a low incorporation of the metal
into the material or a good dispersion on the silica support (Fig. S1). XPS
spectra in the Cu 2p region for Cu-MINT are displayed in Fig. S2. For
both samples, fresh (a) and used (b) catalyst, the presence of Cu2 was
confirmed by the presence of two peaks at 934 eV and 954 eV corre-
sponding to Cu 2p3/2 and Cu 2p1/2, respectively. In addition, a satellite
of Cu 2p3/2 in the region between 940 and 943 eV. Noticeably, satellite
peak-shape is different for both fresh and used catalyst, while for the
fresh Cu-MINT sample the shape of the satellite peak is typical of Cu
The elemental analysis of selected samples is displayed on Table 2
was determined by EDS technique. These results confirm the effective
incorporation of the different metals in the catalysts synthesised by
microwave irradiation. Interestingly, the metals were incorporated in a
surprisingly high loading, especially in the case of molybdenum, indis-
tinctively of the salt precursor employed (either ammonium molybdate
or in the form of heteropolyacid). In contrast, tungsten loading was
small in the material HPW-MINT as compared to the metal loading
found in the other analysed samples. These results were in good agree-
ment with previous characterisation data (e.g. X-ray diffractograms
without any distinctive line corresponding to the metal phase, Fig. S1)
and TEM micrographs (Fig. 2.b) where tungsten oxide nanoparticles
could be visualised. In addition, chlorine was detected in Cu-MINT as
indicative of the incomplete decomposition of the salt precursor during
the material preparation. With regards to particle size, it is important to
mention that the particle size measured for the materials investigated in
this work was unexpectedly high, above 20 nm, with already mentioned
exception of HPW-MINT.
+
(
2
OH) , for the used material due to the oxidant environment two sat-
ellite peak arise as consequence of the complete transformation of the
hydroxide into CuO [33]
Textural properties of the synthesized materials are summarised in
Table 1. The isotherm plots for all the synthesized materials, according
to the IUPAC classification [34], correspond to type IV, as expected for
mesoporous materials. According with the results shown in Table 1, the
nanostructures prepared in this work exhibited high surface areas (SBET),
MINT like materials loaded with different metals synthesised were
employed as catalyst for the wet oxidation of isoeugenol to vanillin using
hydrogen peroxide as oxidizing agent under conventional heating
(Scheme 1). The wet peroxide oxidative conditions investigated in this
work may favour side reactions that lead to the formation of lignin-type
oligomers. According to the catalytic activity results are displayed on
Table 3, all investigated materials exhibited significantly improved
isoeugenol conversion as compared to blank runs (in the absence of
catalyst). Nb-loaded materials were able to convert a higher amount of
isoeugenol but in terms of selectivity towards vanillin it is not a signif-
icant catalyst as compared with other materials herein investigated. The
2
ꢀ 1
ranging between 87 to 675 m g . The highest surface area was found
for the material Cu-MINT, attributed to a higher microporosity observed
in this material as compared to other samples. In addition, materials
exhibiting lower surface areas presented a certain degree of macro-
porosity as observed in the isotherm plots (Fig. S3). Pore sizes (DBJH) of
studied materials in this work were in all cases in the range of meso-
pores. Remarkably, all samples showed high pore volume (VBJH) values,
Fig. 1. Representative low-angle XRD diffractograms corresponding to the materials: a) Cu-MINT and b) HPMo-MINT.
3

P. García-Albar et al.
Molecular Catalysis 506 (2021) 111537
Fig. 2. Micrographs images corresponding to the material HPW-MINT: a) SEM image, and b) TEM image.
Table 2
Elemental composition, obtained by EDS, of the nanomaterials synthesized, molar ratio Si/M (where M is Cu, Mo and W, respectively) and the particle size.
Elements measured (wt. %)
Materials
Molar ratio Si/M
Particle Size (nm)
Cu
Mo
W
Si
O
P
Cl
a
Cu-MINT
13.2
–
–
24.8
9.6
58.7
55.9
60.6
50.8
–
3.0
–
1.8
31
a
Mo-MINT
55.9
–
–
–
0.17
8.7
52
b
HPW-MINT
HPMo-MINT
–
–
3.78
32.9
6.3
3.3
2.4
–
2
a
40.4
–
–
0.16
22
a
Measured using the Scherrer equation.
Obtained from TEM images.
b
Scheme 1. Isoeugenol wet oxidation to vanillin using hydrogen peroxide as oxidant.
the last step of catalyst synthesis, the surfactant extraction with ethanol
was carried out for 24 h instead of 6 h (Cu-MINT-6H vs Cu-MINT-24 H)
to ensure complete template removal. This option has been preferred in
order to avoid the formation of agglomerates or further chemical mod-
ifications in as-synthesized catalysts. Materials were tested during 6 h-
reaction. At the same time, the experiments were also carried out
modifying the hydrogen peroxide addition (a) 1.2 mL time 0 min and b)
Table 3
Catalytic activity of the metal loaded MINT materials: isoeugenol conversion
(
T
X ), selective towards vanillin (Svanillin) and selectivity to undesired products
(
S
others).
Catalyst
X
T
(mol %)
S
vanillin (%)
Sothers (%)
Blank
17
58
34
55
52
51
64
<10
88
–
33
34
36
22
92
12
>99
67
66
64
78
Cu-MINT
Mo-MINT
HPMo-MINT
W-MINT
0
.6 mL time 0 min and 0.6 mL at 30 min) to study potential improve-
ments in catalytic activity (Fig. S4). The catalytic results did not show
significant differences as compared with the material extracted for 6 h
and with complete hydrogen peroxide addition.
HPW-MINT
Nb-MINT
The stability and reusability of the synthesised materials (Cu-MINT-
6
H and Cu-MINT-24H, Table 4) was investigated by the recycling test of
the materials in the isoeugenol oxidation to vanillin. For such test, prior
to their reutilisation, the catalysts were washed with acetonitrile and
catalyst with a better performance in terms of selectivity to vanillin was
Cu-MINT (88 %) with an also important isoeugenol conversion of 58 mol
%
. As Fig. S4 shows, the isoeugenol oxidation rapidly achieves such
conversion value remaining the unaltered. The remaining materials
produced vanillin with a lower selectivity due the acid nature of these
materials that lead to the production of side products such as diphenyl
ether and calamenene. The differences in selectivity is especially inter-
esting between the materials Mo-MINT and HPMo-MINT, while the
material prepared using the heteropolyacid was able to produce vanillin
the analogous material synthesized from ammonium molybdate that
Table 4
Reusability studies of Cu-MINT materials extracted during 6 and 24 h, respec-
tively. Isoeugenol conversion (XT) and selective towards vanillin (Svanillin).
Cu-MINT-6H
(mol %)
Cu-MINT-24H
(mol %)
X
T
S
vanillin (%)
X
T
Svanillin (%)
Fresh
45
40
32
30
85
80
64
57
44
43
36
42
88
82
76
76
1
2
3
st reuse
nd reuse
rd reuse
3
may lead to the formation of MoO species, known for being a good
Br o¨ nsted catalysts and as consequence favour the undesired side re-
actions [35].
Reaction conditions: 0.8 mL isoeugenol, 1.2 mL hydrogen peroxide (35 %), 8 mL
Alternatively, Cu-MINT materials were synthesized with changes in
◦
acetonitrile (solvent) y 0.1 g catalyst, 80 C, 30 min.
4

P. García-Albar et al.
Molecular Catalysis 506 (2021) 111537
◦
dried overnight at 100 C. Results revealed that the materials herein
synthesised showed a high stability, keeping their catalytic performance
stable up to the 4th reuse.
Acknowledgments
Antonio Pineda gratefully acknowledges the support of “Plan Propio
de Investigaci o´ n” from Universidad de C o´ rdoba (Spain) and “Programa
Operativo” FEDER funds from Junta de Andalucía. Rafael Luque ac-
knowledges funding from MINECO under project CTQ2016-78289-P,
co-financed with FEDER funds. This publication has been supported
by RUDN University Strategic Leadership Program (R. Luque). This
work was supported by the Distinguished Scientist Fellowship Program
(DSFP) at King Saud University, Riyadh, Saudi Arabia.
The reusability of Cu-MINT-24H was also compared during 3 reac-
tion cycles (30 min reaction time, Table 4) with the material extracted
for 6 h (Supporting information, Figs. S4–S6, Table 1) to show subtle
differences between materials and a very minor deactivation (from 40 to
4
5 % to 30–32 %) was found for Cu-MINT-6H as compared to a stable
and highly reusable Cu-MINT-24H after three reuses both in terms of
activity and selectivity. These findings pointed to a better stability of Cu-
MINT-24H that could be related to a more efficient removal of the
template leading to better structured materials.
Appendix A. Supplementary data
The reaction filtrate was also analysed by means of ICP-MS. The
average amount of leached Cu found in the analysed samples for Cu-
MINT-6H after the fourth use supposes approximately a 11 % of the
initial amount loaded in the fresh catalyst which may also explain the
reduction in the catalytic activity found after the fourth use. Compa-
rably, Cu-MINT-24H exhibited a stable performance that correlates well
with the low Cu content in solution (ca. 4 % of the initial amount in the
fresh catalyst) determined by ICP-MS.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2021.111537.
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5 aluminosilicates as support for niobium, which provided isoeugenol
conversion of 69 % with a selectivity towards vanillin of 66 %.The re-
sults reported in this work, specially the data corresponding to Cu-MINT,
can be considered an improvement as compared other catalysts inves-
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