Pd nanoparticles confined in mesoporous N-doped carbon silica supports: A synergistic effect between catalyst and support

Article abstract of DOI:10.1039/c9cy01920k

Palladium nanoparticles of similar size were deposited on different supports, layers of carbon materials (with and without nitrogen doping) on the surface of a MCF (mesocellular foam) silica. For the generation of the N-doped carbon coatings, three different N sources were used to also investigate a possible influence of the N-doped carbon precursor and thus the structure of the N-doped carbons on their performance as catalyst support. These catalysts were tested for the Suzuki coupling and hydrogenation reactions. For the Suzuki reaction, the carbon coatings showed to increase dramatically the stability of the MCF material. Furthermore, when N-doped carbon coatings were applied, strong improvement of the stability of the catalysts was observed due to an enhanced interaction between metal nanoparticles and the support, preventing metal particle growth. In hydrogenation reactions, the presence of the N-doped carbon coating on the silica support increases the adsorption of aromatic compounds causing an enhancement of the catalytic activity of Pd NPs when compared to the non-doped supports.

Full text of DOI:10.1039/c9cy01920k

Catalysis
Science &
Technology
View Article Online
View Journal
PAPER
Pd nanoparticles confined in mesoporous
N-doped carbon silica supports: a synergistic
Cite this: DOI: 10.1039/c9cy01920k
effect between catalyst and support†
ab
Julius Kerstien,b
*
Rafael L. Oliveira,
a
Reinhard Schomäckerb and Arne Thomas
*
Palladium nanoparticles of similar size were deposited on different supports, layers of carbon materials
(with and without nitrogen doping) on the surface of a MCF (mesocellular foam) silica. For the generation
of the N-doped carbon coatings, three different N sources were used to also investigate a possible
influence of the N-doped carbon precursor and thus the structure of the N-doped carbons on their
performance as catalyst support. These catalysts were tested for the Suzuki coupling and hydrogenation
reactions. For the Suzuki reaction, the carbon coatings showed to increase dramatically the stability of the
MCF material. Furthermore, when N-doped carbon coatings were applied, strong improvement of the
stability of the catalysts was observed due to an enhanced interaction between metal nanoparticles and
the support, preventing metal particle growth. In hydrogenation reactions, the presence of the N-doped
carbon coating on the silica support increases the adsorption of aromatic compounds causing an
enhancement of the catalytic activity of Pd NPs when compared to the non-doped supports.
Received 23rd September 2019,
Accepted 17th January 2020
DOI: 10.1039/c9cy01920k
rsc.li/catalysis
blockage or constriction formation inside the SBA-15
mesopores.19,20
Introduction
Ordered mesoporous silicas (OMS) have been extensively
explored as catalyst support over the last three decades.1–11
However, these materials face some limitations e.g. their low
mechanical and hydrothermal stability. In this respect,
ordered mesoporous carbons (OMC) have an advantage as
catalyst support due to their higher hydrothermal stability and
their resistance over a broad pH range. However, the synthesis
of OMCs is rather tedious, mostly encompassing the
replication of an OMS template, which has to be removed after
carbonization, thus resulting in many synthetic steps.12–18
Recently, hybrid materials, which combine some of the
beneficial features of OMSs and OMCs, were reported. Pham
et al. described an approach that involves the formation of a
layer of carbon on the surface of SBA-15 and showed that this
carbon film improved considerably the hydrothermal and
mechanical stability of the mesoporous silica. However, N2
physisorption measurements of the modified SBA-15 showed
changes of the hysteresis shapes, suggesting partial pore
While the increased hydrothermal stability of porous silica
after coating with carbons can be beneficial for catalytic
applications, the newly formed carbon surface shows a weak
interaction with noble metal particles, resulting in catalysts that
are quickly deactivated due to metal particles mobility and
growth. The physicochemical properties of carbon materials can
be changed by functionalization or by doping them with
heteroatoms such as nitrogen, sulfur, boron and
phosphorous.21–24 In the last decade, nitrogen-doped carbon
and nitrogen-functionalized materials have become an exciting
topic in catalysis due to the possibility of explored new metal-
free catalytic processes and new catalyst supports.25–39
As example, it has been reported that nitrogen atoms in a
carbon matrix can increase the interaction between metal
nanoparticles and the support, change the electronic
properties of the support and finally modifying the adsorption
energies of the reactants and products.40–47 However, it is
important to note that the influence of nitrogen-doping on
carbon supports is not generally beneficial for catalysis. For
example, Schlögl et al. reported that nitrogen-doping in
carbon nanotubes decreased the activity of Pd nanoparticles
for the hydrogenation of phenyl acetylene.38 Dupont et al. also
observed a lower catalytic activity of nitrogen-doped carbon
materials when compared to carbon black for the
hydrogenation of cyclohexene.39 On the other hand, Xia et al.
reported a drastically increased catalytic activity of Pd NPs in
a Technische Universität Berlin, Fakultät II, Institut für Chemie:
Funktionsmaterialien, Sekretariat BA2, Hardenbergstraße 40, 10623 Berlin,
Germany. E-mail: r.delimaoliveira@campus.tu-berlin.de, arne.thomas@tu-berlin.de
b Technische Universität Berlin, Fakultät II, Institut für Chemie, Sekretariat TC 8,
Straße des 17. Juni 124, 10623 Berlin, Germany
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cy01920k
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
the hydrogenation of aromatic carboxylic acids,27 which they
attributed to favourable interaction between the N-dopants
and benzoic acid moieties. Other groups also reported
examples of a positive synergetic effect between N-doped
carbon materials and metal nanoparticles.28–31,43,44,48
of 5 K min−1 under air atmosphere. XRD analysis was
performed on a Bruker D8 Advance instrument using Cu Kα
radiation (k = 1.54 Å). Scanning electron microscope (SEM)
images were obtained from a Hitachi S-2700 microscope.
Transmission electron microscopy (TEM) images were
obtained from a FEI Tecnai G2 20 STWIN microscope at an
operating voltage of 200 kV. The histograms of particle size
distribution were done from the measurement of
approximately 400 particles found using representative
images. Nitrogen sorption measurements were carried out on
a Quantachrome Quadrasorb SI porosimeter, silica samples
were degassed at 200 °C for 12 h before measurement and the
samples with carbon layer and Pd were degassed at 120 °C for
12 h. The surface area was calculated by using Brunauer–
Emmett–Teller (BET) calculations, and the pore-sized
distribution plot was obtained from the adsorption branch
of the isotherms based on the NLDFT method. X-ray
photoelectron spectroscopy (XPS) was performed on a Thermo
Fisher Scientific ESCALAB 250Xi. The size of the X-ray spot on
the sample is 400 μm, 20 scans for survey, and 50 scans for
regions. For each sample, a survey and high resolution C 1s,
O 1s, N 1s and Pd 3d regions were measured. The avantage
software with pseudo-Voight Gaussian–Lorentzian product
functions and smart background was used for peak
deconvolution. The binding energies were assigned based in
previous reported.21,38 Elemental analysis (EA) was measured
on a Thermo Flash EA 1112 organic elemental analyzer as a
dynamic flash combustion analysis. Gas chromatography was
performed by using an Agilent GC system 7890A equipped
with a 30 m capillary column containing (5% phenyl)-
methylpolysiloxane as stationary phase using the following
parameters: initial temperature 50 °C, temperature ramp 30
°C min−1, final temperature 300 °C and injection volume 1
μL. Pd contents in the liquid from the catalytic test were
determined by an ICP-ES Varian 720. CO pulse chemisorption
was measured on BelCAT II. Hydrogen reduction was done at
100 °C for 1 h. The samples were cooled at 50° and pulse Co
chemisorption were carried out.
Pd immobilized on different supports have been used as
catalysts in many processes such as hydrogenation, oxidation,
and cross-coupling reactions among others.49–53 In the case of
C–C coupling reactions, the Suzuki reaction is one of the most
explored, but Pd nanoparticles immobilized on inorganic
supports often suffer from metal leaching and particle growth
resulting in
a
limited recyclability in most cases.54–57
Chemoselective hydrogenations over heterogeneous catalysts
have also been widely studied due to economic reasons
(production of chemicals and reduction in the number of
purification steps). For example, α,β-unsaturated alcohols are
extensively used as intermediate for pharmaceutical,
agrochemicals and fragrances products.58 N-Doped carbon
materials were explored as catalyst support on selective
hydrogenation of α,β-unsaturated aldehyde and α,β-
unsaturated ketones. In many of these studies, Pd
nanoparticles with different size were synthesized but the role
of nature and amount of nitrogen dopants on metal particles
stability was rarely addressed.34,48,59–61
In this study, layers of carbon materials (with and without
nitrogen-doping) where synthesized on the surface of a MCF
(mesocellular foam) silica. For the generation of the N-doped
carbon coatings three different N sources (cyanamide, 1-ethyl-
3-methylimidazolium dicyanamide and N-acetylglycine) were
used to also investigate a possible influence of the N-doped
carbon precursor and thus the structure of the N-doped
carbons on their performance as catalyst support. The carbon
coated silicas were used as supports for Pd nanoparticles, and
applied as catalyst for Suzuki couplings and hydrogenation
reactions. As the Pd NPs deposited on the different supports
initially have almost the same particle size distribution, the
influence of nitrogen doping of the supports on the catalytic
performance could be investigated.
Experimental
MCF synthesis
Materials and methods
MCF was synthesized by following a modified procedure
previously reported.62 Typically, 4 g of copolymer Pluronic
P123 was dissolved in an aqueous acidic solution (10 mL of
HCl and 65 mL of water) in a 250 mL polypropylene bottle at
room temperature overnight. Then, 3 grams of mesitylene
(TMB) was added to the reaction mixture at 35 °C dropwise
and stirred vigorously during 2 h. After this period, 8.5 grams
of TEOS was slowly added to the mixture (1.5 mL min−1) and
stirred vigorously during 5 minutes. The solution was aged at
40 °C for 20 hours under static condition. 46 mg of NH4F
was dissolved in 5 mL of water and added to the solution.
Then, the mixture was kept under a static condition for 24
hours at 80 °C. The solid product was collected by filtration,
washed with distilled water, dried at 60 °C during 24 h and
calcined at 550 °C in static air during 6 hours.
All reagents used in this work were of analytical grade and
were obtained from a commercial source. Triblock copolymer
polyIJethylene oxide)–polyIJpropylene oxide)–polyIJethylene oxide)
(P123), tetraethyl orthosilicate (98%), cyanamide (99%),
1-ethyl-3-methylimidazolium dicyanamide (98%), mesitylene
(99%), ammonium fluoride (98%), 4-bromoacetophenone
(98%), phenylboronic acid (97%), palladium acetate (98%)
were purchased from Sigma-Aldrich. Furfuryl alcohol (99%)
and allylbenzene (98%) were purchased from ABCR,
cinnamaldehyde (98%) was purchased from TCI (Tokyo
Chemical Industry) and chloride acid (37%) was purchased
from Carl Roth Chemicals.
Thermogravimetric analysis (TGA) experiments were
performed on a Netzsch TG209-F1 apparatus at a heating rate
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Carbon and N-doped carbon layer formation
(1/1, 2 × 40 mL), ethanol (30 mL) and dried at 60 °C
overnight. Then, a new solution mixture as described above
was added to the solid and allows reacting during the same
period. As just one product was observed, the conversions
were obtained by external calibration.
The carbon coating of MCF was formed using furfuryl alcohol
as a precursor. 115 mg of furfuryl alcohol was dispersed in 1
mL in methanol and subsequently added to 500 mg of MCF,
after methanol was slowly evaporated under vacuum.
For the N-doped carbon materials, 70 mg of furfuryl alcohol
and 75 mg of an ionic liquid (1-ethyl-3-methylimidazolium
dicyanamide) or 105 mg of furfuryl alcohol and 45 mg of
cyanamide or 50 mg of furfuryl alcohol and 150 mg of
N-acetylglycine were dissolved in 1 mL of methanol. The
prepared solution was added to different vials containing 500
mg of MCF each. Methanol was slowly evaporated under
vacuum. After this procedure, the obtained solids were placed
in an oven and pyrolyzed at 400 °C under argon during 2 hours
with a heat ramp of 3.1 °C min−1. The obtained materials were
called MCF-C when only furfuryl alcohol was used, MCF-N-IL,
MCF-N-Ac or MCF-N-Cy when ionic liquid, N-acetylglycine or
cyanamide was used, respectively.
For comparison, a N-doped carbon was synthesized without
using MCF as a template (CN-IL). In summary, furfuryl alcohol
(700 mg) was added to a round flash containing 750 mg of
ionic liquid (1-ethyl-3-methylimidazolium dicyanamide). These
two chemicals were mixed vigorous during 15 min. This liquid
was transferred to a porcelain crucible, then, placed into an
oven and pyrolyzed at 400 °C under argon during 2 hours with
a heat ramp of 3.1 °C min−1.
PVPy poisoning test
The catalyst poison was added to the reactor before the
addition of the reaction solution. PVPy was used at 100 equiv.
of pyridine sites total of Pd.
Hydrogenation reactions
In a typical hydrogenation reaction, 35 mg of catalyst, 1 gram
of allylbenzene and 99 ml of methanol were added to a
reactor and stirred at 200 rpm under nitrogen at 35 °C. The
N2 was replaced by H2 (p = 1.2 atm) without stirring and the
reaction was started by turning the stirrer to 1250 rpm. The
cumulative hydrogen consumption and the pressure during
the reaction were recorded using a Bronkhorst flow meter
and pressure controller (Bronkhorst Mättig GmbH, Kamen,
Germany), respectively. The sensitivity of measured hydrogen
consumption was 0.4 mL.
After the reaction, the solution was analyzed by GC-MS to
confirm the conversion and selectivity. For recycling
experiments, the catalyst was centrifuged, washed with
ethanol, dried and reused in several runs by adding new
substrate and solvent.
MCF-N-ILs with different N-content were also synthesized.
The supports with
a higher content of nitrogen were
The same procedure was used to run the hydrogenation of
cinnamaldehyde, however, only 0.63 grams of the substrate
was added into the reactor and the reaction was run at 60 °C.
In this case, GC and GC-MS were used to obtain the
conversion values and product determination.
produced using 70 mg of furfuryl alcohol and 150 mg of ionic
liquid (1-ethyl-3-methylimidazolium dicyanamide) or pure
ionic liquid (230 mg). These precursors were mixed in 1 mL
of methanol. The solution was then added to different vials
containing 500 mg of MCF each. MCF-N-IL having lower
N-content was synthesized mixing 100 mg of furfuryl alcohol
and 50 mg of ionic liquid, followed by the same procedure Results and discussion
described before.
Synthesis of catalysts
To prepare carbon or N-doped carbon coatings, the pores of
Pd impregnation
the mesocellular foam (MCF) silica were infiltrated with
dilute solutions of either pure furfuryl alcohol (MCF-C) or a
mixture of furfuryl alcohol with nitrogen-rich molecules,
namely 1-ethyl-3-methylimidazolium dicyanamide (MCF-N-IL)
or cyanamide (MCF-N-Cy) or N-acetylglycine (MCF-N-Ac). After
evaporation of the solvent, the materials were carbonized at
400 °C under argon. N2 physisorption isotherms of pristine
MCF and modified MCF were measured and compared
(Fig. 1). The isotherms are type IV following the IUPAC
classification since they show one-step capillary condensation
in the adsorption branch, corresponding to the filling of
uniform mesopores by N2 molecules. No change in the
hysteresis loops (H1 type) is seen when carbon layers were
formed in MCF, just a slight decrease in surface area, pore
volume and pore size can be observed as expected (Table 1
and ESI† Fig. S1).
500 mg of a selected support was loaded by incipient wetness
impregnation with 0.1 M solution of palladium acetate in
dichloromethane, resulting in a solid with 2 wt% of Pd. The
materials were dried under vacuum, then, the solid was
transferred to a tubular oven and heated at 200 °C at a
heating rate of 10 °C min−1, during this period the solid was
exposed to an air flow for 30 minutes.
Suzuki reaction
In
a
typical
reaction,
a
mixed
solution
of
4-bromoacetophenone (0.5 mmol), phenylboronic acid (0.7
mmol), base (0.5 mmol,) and ethanol (4 mL) was added to a
Schlenk tube, followed by addition of 20 mg of catalyst (0.75
mol% of palladium relative to 4-bromoacetophenone). Then,
the mixture was reacted at the desired temperature and time.
For a recycle experiment, the catalyst was recovered by
centrifugation and the solid was washed with ethanol/water
Elemental analyses showed that MCF-N-IL, MCF-N-Cy and
MCF-N-Ac have almost the same nitrogen content of 3.1 wt%,
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
In addition, thermogravimetric (TGA) analyses were
applied to address the thermal stability of the carbon layer
synthesized on the silica surface. The carbon layers showed
to be stable until 350 °C under air atmosphere. At
temperatures higher than 350 °C, a considerable mass loss
was observed (ESI,† Fig. S4). The weight loss observed from
TGA fits well to the amount of carbon layers determined by
CHN elemental analysis, namely MCF-C (9.5% from TGA and
9% from CHN), MCF-N-Cy (13.5% TGA and 12.4% CHN),
MCF-N-IL (13.18% TGA and 11.9% CNH) and MCF-N-Ac
(15.5% TGA and 14.4% CNH). Comparing the weight loss
below 150 °C, which can be attributed to the removal of
adsorbed water, for pure and carbon-coated MCFs it is also
observed that after introduction of the carbon layers the
materials loose considerably their affinity to water (Fig. S4†).
TEM images and histograms of Pd particle size
distribution are presented in Fig. 4, showing mainly the
porous structure of the MCFs, while also the formation of
very small Pd nanoparticles inside the pores can be spotted.
STEM-HAADF combined with EDX chemical mapping was
used to image more clearly these small Pd nanoparticles,
confirming the formation of small Pd nanoparticles with
diameters around 0.5–11 nm (Fig. 5). In all cases, small metal
particles with a narrow size distribution were observed. MCF-
Pd, MCF-C-Pd, MCF-N-Cy-Pd, MCF-N-IL-Pd and MCF-N-Ac
have an average metal particle size of 2.4, 2.6, 2.6, 2.2 and 2.6
nm, respectively. This is also seen by XRD measurements of
the fresh catalysts (Fig. S6†), showing no diffraction peaks
related to Pd species confirming that no larger Pd particles
were formed. CO chemisorption was also used to study the
metal particle size and the dispersion of Pd nanoparticles
(ESI,† Table S11), the results show particle sizes slightly larger
than the ones determined by TEM measurement (Fig. 4).
N-Doped carbon was synthesized without the MCF support
(CN-IL), however, this procedure resulted in a material with a
low surface area (1 m2 g−1) (Fig. S9 and Table S1†). On this
support, small Pd nanoparticles could not be stabilized. XRD
analysis (ESI,† Fig. S10) showed sharp peaks related to
palladium oxide metal particles, revealing that the MCF is
essential for building stable and active catalysts.
Fig. 1 N2 physisorption of synthesized materials. The isotherms were
offset by 1200 (MFC-C), 2200 (MCF-N-IL), 3200 (MCF-N-Cy) and 4200
(MCF-N-Ac).
3.3 wt% and 2.9 wt% respectively. However, the N 1s XPS
spectra (Fig. 3) showed that the chemical nature of the
nitrogen atoms differ, depending on the used N-doped
carbon precursor. The N 1s areas of the respected spectra
were deconvoluted into three components related to different
nitrogen types. In the case of MCF-N-IL, the use of the ionic
liquid precursor yield
a high percentage of pyridinic
nitrogens (79%). When cyanamide was used, the amount of
pyridinic nitrogens decreased to 48.9%, while the amount of
pyrrolic nitrogen increased to 42.4% (ESI,† Table S2). On the
other hand, N-acetylglycine favoured the formation of pyrrolic
nitrogen-type (57%).
SEM analysis of MCF, MCF-C, MCF-N-IL and MCF-N-Cy
are shown in the ESI† Fig. S2. After carbon layer formation,
the materials retained the same morphology as pristine
MCF (spherical shape). The distribution of carbon, silicon
and oxygen on MCF-C was examined by energy-dispersive
X-ray spectroscopy combined with secondary electron
images; the maps show a homogenous distribution of
carbon throughout MCF-C (Fig. 2). Energy-filtered
transmission (EFTEM) image also shows the presence of
carbon around the porous structure (carbon K edge). The
distribution of N and C on MCF-N-IL and on MCF-N-Cy
surfaces were further evaluated by energy-dispersive X-ray
spectroscopy (SEM-EDS) and the obtained maps showed
uniform distribution of carbon and nitrogen throughout
the samples (Fig. S3†).
To elucidate if the support affect the electronic properties
of the Pd NPs, XPS analyses of the Pd 3d core peak of the
fresh catalysts were explored (Fig. 6). In all cases, a doublet
which corresponds to 3d3/2 and 3d5/2 was observed. The Pd
3d areas were deconvoluted into two components related to
different Pd types (Pd oxide at 337.5 eV and metallic Pd at
Table 1 Structure and composition of synthesized materials
Material
BET surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Pore sizea (nm)
Carbon contentb (wt%)
Nitrogen contentb (wt%)
MCF
MCF-C
MCF-N-IL
MCF-N-Cy
MCF-N-Ac
786
2.1
1.6
1.6
1.5
1.35
36
34
34
34
33
—
—
—
3.1
3.3
2.9
538.8
510.4
503.4
470.5
9.0
8.8
9.1
11.5
a
Determined by NLDFT. b Determined by CNH analysis.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Fig. 2 Elemental mapping of MCF-C.
335.3 eV), showing that the electronic states of Pd NPs were
considerably influenced by the nature of the support surface.
Thus, a higher percentage of divalent Pd (69.8%) was
observed for MCF-N-IL where almost 79% of the N atoms
were assigned as Npyridinic. In the case of MCF-N-Cy, which
present a lower quantity of Npyridinic and higher amount of
showed a further increased amount of Pd metallic (66.13%)
(ESI,† Table S3). For the supports MCF-C and pristine MCF,
the percentage of metallic Pd was 50.4% and 69.9%,
respectively.
Another notable fact is the significant decrease of free
Npyridinic species in the N-doped supports when Pd NPs are
deposited (Fig. S11, Table S4†), suggesting that Pd are
preferably positioned at Npyridinic sites. Schlögl et al.
described that pyridinic nitrogen species in N-doped carbons
interact strongly with Pd by charge transfer, yielding partial
positive charges on the metal, resulting in a Pd 3d core level
of divalent Pd. This observation is in line with our XPS
N
pyrrolic, a much lower degree of divalent Pd (58.2%) was
seen, yielding a much higher amount of metallic Pd (41.8%).
MCF-N-Ac which presents the highest percentage of Npyrrolic
results, showing an increase of divalent Pd species with
38
increasing amount of Npyridinic
.
Catalytic test
Suzuki
reaction.
The
reaction
between
4-bromoacetophenone and phenylboronic acid was used as a
model reaction to elucidate the activity and stability of the
catalysts. MCF-N-IL-Pd was first explored to find optimum
reaction conditions regarding the used base and solvents
(ESI,† Table S5). CN-IL-Pd was also tested, however this
catalyst presented
a much lower activity. The turnover
frequency (TOF) of MCF-N-IL-Pd was compared to other Pd
based catalysts reported previously in literature (ESI,† Table
S8), showing a higher or a similar activity.
All catalysts were applied under these optimum conditions
and recyclability test were carried out to examine their
stability. All fresh catalysts showed almost the same
conversion values after 1 hour, however, the performance of
the catalysts change considerably after the first reaction cycle.
Thus, Pd-MCF suffered a strong deactivation already after the
first run, while MCF-C showed an enhanced stability, but also
a severe deactivation after several runs. In contrast, the
N-doped supports (MCF-Cy, MCF-IL and MCF-Ac) showed no
sign of deactivation even after six consecutive catalytic
reactions (Fig. 7). 4-Bromobenzonitrile was also used as
substrate to extend the study, showing the same behaviour as
observed to 4-bromoacetophenone (Fig. S23†).
SEM analyses and N2 physisorption of the spent catalyst
were used to investigate the cause of the extensive
deactivation of MCF-Pd (ESI,† Fig. S12 and S14). The N2
physisorption analysis of spent catalyst suggested that the
pore structure of MCF collapse during the Suzuki reaction, as
Fig. 3 XPS of N 1s of MCF-N-IL (top) MCF-N-Cy (middle) and MCF-N-
Ac (bottom).
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Fig. 4 TEM images (A) MCF-Pd; (B) MCF-C-Pd; (C) MCF-N-Cy-Pd; (D) MCF-N-IL-Pd; (E) MCF-N-Ac-Pd.
Pd particles has not changed after four reaction cycles (ESI,†
Fig. S13).
ICP-OES showed a very low but still detectable amount of
Pd leaching for Pd-MCF-C (ESI,† Table S6), while this is not
the case for the N-doped samples. To further elucidate the
reason for deactivation of MCF-C-Pd, TEM and XRD analyses
were carried out to investigate possible metal nanoparticles
growth during the Suzuki reaction. Fig. 8A and B shows TEM
and HAADF-STEM images of the spent catalysts MCF-C-Pd
after 4 cycles. While the cage pore structure of MCF is still
present, the Pd nanoparticle distribution changed drastically
at the MCF-C surface. Moreover, XRD analysis of spend MCF-
C-Pd shows a new peak arising, which can be assigned to Pd
oxide (PdO) confirming the formation of larger crystallites
(ESI,† Fig. S16). The particle size distributions and
complementary TEM images are shown in ESI† Fig. S18–S20,
the average Pd particle size of spent catalyst changes to 17.8
nm. CO chemisorption also showed a considerable increase
in Pd average particle size to 25 nm (Table S11†). In contrast,
formation of such large crystallites is not observed for MCF-
N-IL-Pd in XRD measurements. Indeed, TEM and HAADF-
STEM images (Fig. 8C and D and S17†) showed that the
majority of particles remain small (2–4 nm), but also some
larger particles around 10 nm were formed. The particle size
distribution (ESI,† Fig. S20) showed a bimodal distribution of
Pd nanoparticles size.
Fig. 5 HAADF-STEM image of MCF-N-IL-Pd and Pd mapping.
the type IV isotherm observed for the fresh material was not
seen for spent catalyst, indicating no presence of mesopores.
Moreover, a considerable decrease in surface area and pore
volume was observed after Suzuki reaction (Table S7†). SEM
analysis also confirms that the conditions applied for the
Suzuki reaction resulted in a change of the morphology of
MCF-Pd. Beside the collapse of the silica structure caused by
alkaline conditions applied for Suzuki reaction, many Pd
centers might be inaccessible. Moreover, severe Pd leaching
for this material was observed (ESI,† Table S6). MCF-C-Pd
did as well show a loss in surface area and pore volume
compared to the fresh catalyst, even though pore collapse
seem to be much less significant as for MCF-Pd. Moreover,
the N-doped materials also show higher stability than
MCF-Pd (ESI,† Fig. S15). Also the morphology of the MCF-C-
Previous reports have indicated that during the Suzuki
reaction, leached Pd species formed during catalysis are of
considerable importance for the conversion of reactants to
Fig. 6 XPS of Pd 3d of (A) MCF-Pd; (B) MCF-C-Pd; (C) MCF-N-Cy-Pd; (D) MCF-N-Ac-Pd; (E) MCF-N-IL-Pd.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
pyridine sites to total Pd). This indicates that Pd was
liberated from the surface of the support and that these
leached species were responsible for most of the catalytic
performance.
In the cases where leached species are formed, Pd species
in solution can redeposit on the supporting surface at the
end of the catalytic process, forming particles larger than the
ones present in the fresh catalyst as observed for MCF-C-Pd.
In the case of MCF-N-IL-Pd, Pd nanoparticles just grew
slightly and this metal particles growth took place inside
the pore mostly, suggesting the lower mobility of Pd species
compared to MCF-C-Pd. Thus, the importance of nitrogen on
carbon matrices relates to the control of leaching and
recapturing of Pd, thus affecting the stability or mobility of
Oswald ripening species, extending the lifetime of the
catalysts.
Based on the fact that Pd species strongly interact with
pyridinic nitrogen, this stronger interaction might cause
enhancement of the activation barrier for the metal
displacement across the support. In the case of MCF-C, the
oxygenated functional groups from furan rings (confirmed by
infra-red spectroscopy, ESI† Fig. S24) on the surface have a
lower affinity to palladium than the nitrogen species. This
might result in a weaker interaction between the metal
particles and the support causing higher mobility and
diffusion of the Pd species with their consequent growth.
The influence of the amount of nitrogen on the support
on the Suzuki reaction was also studied. MCF-N-IL supports
with different N-content were synthesized adjusting the
amount of furfuryl alcohol and ionic liquid. Beside MCF-N-IL
with a nitrogen content of 3.1 wt%, solids with 1.8 wt%, 6.4
wt% and 10.1 wt% of nitrogen were produced (determined by
elemental analysis). These solids were named MCF-N-IL-2,
MCF-N-IL-6 and MCF-N-IL-10 respectively. TGA, XRD and N2
physisorption analyses of these MCF-ILs are shown in the
ESI† (Fig. S5, S7 and S8, respectively).
Fig. 7 Recyclability of materials for the Suzuki reaction 20 mg of
catalyst (0.75 mol% of Pd), 0.5 mmol of 4-bromoacetophenone,
0.7 mmols of phenylboronic acid, 0.5 mmol of K2CO3, 4 mL EtOH,
40 °C, 1.0 hour.
products, thus, the actual catalytic species are in
solution.57,63–67 However, this is
a controversial subject
because other authors have reported that C–C coupling
reactions take place preferentially on the metal particle
surface, thus in a heterogeneous fashion.56,68–70 We used the
PVP test to investigate the leaching process.71 Insoluble PVPy
(2% cross-linked) can be used as a trap to confirm the
presence of leached Pd. In the current work, we applied
insoluble PVPy to complement the elemental analysis of the
solution and address the formation of active species in
solution. For all cases studied, the catalytic activity was
suppressed by the addition of PVPy (100 equivalents of
Applying the same conditions shown in Fig. 7, similar
conversion values were observed for MCF-N-IL-2-Pd and MCF-
N-IL-Pd. However, the catalysts with higher nitrogen content
showed lower catalytic activity. In the case of MCF-N-IL-6-Pd,
the conversion dropped slightly (80%) while for MCF-N-IL-10-
Pd a considerable decrease in the activity was observed (51%)
(Fig. S21†).
Hydrogenation reactions. The electronic structure of
transition metals has a strong influence on their catalytic
properties, such as modifying the adsorption strength of
alkenes and alkynes.72–76 To elucidate the influence of
these electronic properties and influence of N species, the
hydrogenation of allylbenzene was explored.
Fig. 9 displays the conversion of allylbenzene versus time
for the here described catalysts and a commercial catalyst
(Pd on carbon). Despite having the same metal particle size,
MCF-N-IL-Pd shows a far better activity compared to MCF-Pd
and MCF-C-Pd and also a slightly higher activity as MCF-N-
Cy. Turnover numbers (TON) of MCF-N-IL-Pd, MCF-N-Cy-Pd,
MCF-N-Ac-Pd, MCF-C-Pd and MCF-Pd were 1286, 990, 921,
Fig. 8 TEM and HAADF-STEM of spent catalysts after 4 cycles MCF-C
(A and B) and MCF-N-IL (C and D).
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
All synthesized materials show to be stable under
hydrogenation of allylbenzene conditions, i.e. no pore
collapse was observed. However, when studying the
hydrogenation of furfural under fixed-bed continuous flow at
130 °C, Huo et al. observed the collapse of a SBA-15 covered
by carbon, most probably due to the much harsher reaction
conditions applied in this study.20 Moreover, the authors also
observed a much extensive metal particles growth under
applied conditions compared to the results herein reported.
Finally, to study the influence of nitrogen doping on
carbon in selective hydrogenation a α,β-unsaturated-aldehyde
(cinnamaldehyde) was investigated. The catalytic activity of
MCF-N-IL-Pd was compared to other catalysts reported for
this reaction previously (ESI,† Table S9).
Fig. 10 shows that the catalysts with nitrogen in their
structure present a higher activity than MCF-C-Pd and MCF-
Pd as expected. However, no strong influence in the
selectivity was observed. Controversially, some authors
reported a difference in the selectivity for hydrogenation of
cinnamaldehyde when N-doped materials were used.61,78
Fig. 9 Catalytic performance of synthesized materials for hydrogenation
of allybenzene Conditions: 8.46 mmols of allylbenzene, 99 mL of MeOH,
35 mg of catalysts (0.08% of Pd), 1.2 atm of H2 and 35 °C.
689, 604 respectively, suggesting the strong influence of the
electronic properties or N-species of Pd NPs on the activity of
the catalysts for the studied reaction. N-Doped carbon
synthesized without MCF (CN-IL-Pd) was also tested,
however, this catalyst presented a much lower activity as
shown in Fig. S22.†
To further investigate the influence of the initial electronic
properties of metal nanoparticles before the hydrogenation
reaction, MCF-C-Pd was reduced using NaBH4. After the
reduction, a higher percentage of metallic Pd was observed,
i.e. 73% of the total Pd is in the metallic state (ESI† Fig. S30
and Table S10). However, the reduction treatment of the
catalyst has almost no influence on catalytic activity (ESI,†
Fig. S31). The highest conversion observed for MCF-N-IL-Pd
might be related to the presence of Npyridinic. Studying the
hydrogenation of 1,5-cyclooctadiene, Tang et al. suggested
that the rings containing Npyridinic on the support surface
were contributing to increase the adsorption of 1,5-
cyclooctadiene considerably through π–π bond interactions,
based on DFT calculations and experimental data.77 Indeed
the activity of MCF-N-IL-Pd drops considerably when toluene
was used as solvent probably caused by competitive
adsorption (ESI,† Fig. S25).
The recyclability of the catalysts (ESI,† Fig. S26) showed
that all catalysts could be recycled 8 times without any
considerable loss in their activity. XRD analyses of the spent
catalysts after 8 cycles showed no sign of significant growth
of Pd particles, at least no peaks attributed to Pd or PdO can
be observed (ESI,† Fig. S28). However, TEM measurements on
the spent catalysts revealed a slight growth of metal particles
(ESI,† Fig. S27 and S29), that is from 2.4 to 3.1 nm for MCF-
Pd and 2.2 to 2.9 nm for MCF-N-IL-Pd. This is also supported
Fig. 10 Hydrogenation of cinnamaldehyde by the synthesized
catalysts 4.8 mmol of cinnamaldehyde, 99 mL of EtOH, 35 mg of
catalysts (0.14% of Pd), 1.2 atm of H2 and 60 °C.
by CO chemisorption measurements, showing
a slight
increase of Pd metal particle size (Table S11†).
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
However, in these studies, catalysts with metal particles of Notes and references
different sizes were applied, which are known to influence
strongly the selectivity of this specific reaction due to the
creation of new edges and corners with the increase of metal
particles size.79 Similar results were also observed for another
substrate, chalcone, as shown in Fig. S32.† MCF-N-IL-Pd
1 M. C. G. Albuquerque, I. Jiménez-Urbistondo, J. Santamaría-
González, J. M. Mérida-Robles, R. Moreno-Tost, E.
Rodríguez-Castellón, A. Jiménez-López, D. C. S. Azevedo,
C. L. Cavalcante Jr. and P. Maireles-Torres, Appl. Catal., A,
2008, 334, 35–43.
showed
a higher activity than MCF-Pd and MCF-C-Pd,
however, the selectivity were almost the same for all catalysts.
The influence of the amount of nitrogen on the catalyst
support (MCF-N-IL) was also explored for the conversion and
selectivity of cinnamaldehyde hydrogenation. An increase of
N-loading negatively influenced the catalytic activity of Pd
nanoparticles on these supports. MCF-N-IL10, which contains
10 wt% of nitrogen, showed a considerable decrease in the
catalytic activity (ESI† Fig. S33), however, the selectively
stayed constant. This observation is in line with results
reported by Dupont et al. who observed a considerable lower
catalytic activity of platinum nanoparticles supported on
N-doped carbons which a high content of nitrogen (above 10
wt%) compared to platinum supported on activated carbon.39
2 A. Taguchi and F. Schüth, Microporous Mesoporous Mater.,
2005, 77, 1–45.
3 M. Yildiz, Y. Aksu, U. Simon, K. Kailasam, O. Goerke, F.
Rosowski, R. Schomäcker, A. Thomas and S. Arndt, Chem.
Commun., 2014, 50, 14440–14442.
4 R. L. Oliveira, M. Shakeri, J. D. Meeldijk, K. P. de Jong and
P. E. de Jongh, Microporous Mesoporous Mater., 2015, 201,
234–239.
5 C. Li, H. Zhang, D. Jiang and Q. Yang, Chem. Commun.,
2007, 547–558.
6 H. Yang, L. Zhang, P. Wang, Q. Yang and C. Li, Green Chem.,
2009, 11, 257–264.
7 M. Shakeri, C. Tai, E. Göthelid, S. Oscarsson and J.-E.
Bäckvall, Chemistry, 2011, 17, 13269–13273.
8 S. H. Gage, J. Engelhardt, M. J. Menart, C. Ngo, G. J. Leong,
Y. Ji, B. G. Trewyn, S. Pylypenko and R. M. Richards, ACS
Omega, 2018, 3, 7681–7691.
9 C. H. Liu, C. Y. Lin, J. L. Chen, K. T. Lu, J. F. Lee and J. M.
Chen, J. Catal., 2017, 350, 21–29.
10 G. Mohammadi Ziarani, S. Rohani, A. Ziarati and A. Badiei,
RSC Adv., 2018, 8, 41048–41100.
11 H. V. Le, S. Parishan, A. Sagaltchik, H. Ahi, A. Trunschke, R.
Schomäcker and A. Thomas, Chem. – Eur. J., 2018, 24,
12592–12599.
12 R. Ryoo, S. H. Joo and S. Jun, J. Phys. Chem. B, 1999, 103,
7743–7746.
13 J. Zhang, F. Guo and X. Wang, Adv. Funct. Mater., 2013, 23,
3008–3014.
14 C. M. Yang, C. Weidenthaler, B. Spliethoff, M. Mayanna and
F. Schüth, Chem. Mater., 2005, 17, 355–358.
15 J. E. Hampsey, Q. Hu, Z. Wu, L. Rice, J. Pang and Y. Lu,
Carbon, 2005, 43, 2977–2982.
Conclusions
The deposition of carbon or N-doped carbon layers on MCF
surface was successfully developed. These carbon coatings
dramatically increase the stability of MCF material. For the
Suzuki reaction, Pd NPs on the pure mesoporous silica
produced an inefficient catalyst due fast collapse of the silica.
In contrast, MCFs with carbon coating were much more
stable catalyst support, however also this catalyst deactivated
due to metal particle growth probably caused by the weak
interaction between Pd NPs and the carbon layer. However,
when N-doped carbon coatings were applied, a further
improvement of the stability of the catalysts was observed
due to an enhanced interaction between metal nanoparticles
and support. In hydrogenation reactions, the presence of the
N-doped carbon coating on the silica support increases the
adsorption of aromatic compounds causing an enhancement
of the catalytic activity of Pd NPs deposited on MCF-N-IL,
MCF-N-Cy and MCF-N-Ac when compared to the non-doped
supports.
16 J. Lee, K. Sohn and T. Hyeon, Chem. Commun., 2002, 2674–2675.
17 P. F. Fulvio, M. Jaroniec, C. Liang and S. Dai, J. Phys. Chem.
C, 2008, 112, 13126–13133.
18 E. Ramasamy and J. Lee, Chem. Commun., 2010, 46, 2136–2138.
19 H. N. Pham, A. E. Anderson, R. L. Johnson, K. Schmidt-Rohr
and A. K. Datye, Angew. Chem., Int. Ed., 2012, 51, 13163–13167.
20 J. Huo, R. L. Johnson, P. Duan, H. N. Pham, D. Mendivelso-
Perez, E. A. Smith, A. K. Datye, K. Schmidt-Rohr and B. H.
Shanks, Catal. Sci. Technol., 2018, 8, 1151–1160.
21 R. L. Oliveira, C. S. Oliveira, R. Landers and C. R. D. Correia,
ChemistrySelect, 2018, 3, 535–543.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors would like to thank FAPESP for a grant 2015/
07773-0. CAPES, Humboldt foundation for the fellowship and
Deutsche Forschungsgemeinschaft (DFG, German Research
Foundation) under Germany's Excellence Strategy – EXC 2008/
1 – 390540038. We also thank LNNano (Brazil) and Fritz-
Haber Institute (Germany) for microscopy analyses.
22 L. Zhang, X. Liu, X. Zhou, S. Gao, N. Shang, C. Feng and C.
Wang, ACS Omega, 2018, 3, 10843–10850.
23 V. Campisciano, M. Gruttadauria and F. Giacalone,
ChemCatChem, 2019, 11, 90–133.
24 Y. Sun, J. Wu, J. Tian, C. Jin and R. Yang, Electrochim. Acta,
2015, 178, 806–812.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
25 W. Xiong, L. Wang, G. Cai, Y. Yang, F. Hao, P. Liu and H.
Luo, ChemistrySelect, 2017, 2, 11244–11249.
52 F. P. Da Silva, J. L. Fiorio and L. M. Rossi, ACS Omega,
2017, 2, 6014–6022.
26 Y. Z. Chen, G. Cai, Y. Wang, Q. Xu, S. H. Yu and H. L. Jiang,
Green Chem., 2016, 18, 1212–1217.
53 D. Elhamifar, A. Eram and R. Moshkelgosha, Microporous
Mesoporous Mater., 2017, 252, 173–178.
27 H. Jiang, X. Yu, R. Nie, X. Lu, D. Zhou and Q. Xia, Appl.
Catal., A, 2016, 520, 73–81.
28 P. Zhang, Y. Gong, H. Li, Z. Chen and Y. Wang, Nat.
Commun., 2013, 4, 1511–1593.
54 S. Macquarrie, B. Nohair, J. H. Horton, S. Kaliaguine and
C. M. Crudden, J. Phys. Chem. C, 2010, 114, 57–64.
55 B. Nohair, S. Macquarrie, C. M. Crudden and S. Kaliaguine,
J. Phys. Chem. C, 2008, 112, 6065–6072.
29 W. Gong, C. Chen, H. Zhang, G. Wang and H. Zhao, Catal.
Sci. Technol., 2018, 8, 5506–5514.
56 Y. Yang, C. E. Castano, B. F. Gupton, A. C. Reber and S. N.
Khanna, Nanoscale, 2016, 8, 19564–19572.
30 J. L. Fiorio, R. V. Gonçalves, E. Teixeira-Neto, M. A. Ortuño,
N. López and L. M. Rossi, ACS Catal., 2018, 8, 3516–3524.
31 J. Long, K. Shen and Y. Li, ACS Catal., 2017, 7, 275–284.
32 G. Hahn, J. K. Ewert, C. Denner, D. Tilgner and R. Kempe,
ChemCatChem, 2016, 8, 2461–2465.
57 K. Köhler, R. G. Heidenreich, J. G. E. Krauter and J. Pietsch,
Chemistry, 2002, 8, 622–631.
58 Y. Zhu, G. K. Chuah and S. Jaenicke, J. Catal., 2006, 241, 25–33.
59 Z. Wei, Y. Gong, T. Xiong, P. Zhang, H. Li and Y. Wang,
Catal. Sci. Technol., 2015, 5, 397–404.
33 Z. Li, J. Liu, C. Xia and F. Li, ACS Catal., 2013, 3, 2440–2448.
34 R. Nie, M. Miao, W. Du, J. Shi, Y. Liu and Z. Hou, Appl.
Catal., B, 2016, 180, 607–613.
60 A. Nagendiran, V. Pascanu, A. Bermejo Gómez, G. González
Miera, C. W. Tai, O. Verho, B. Martín-Matute and J. E.
Bäckvall, Chem. – Eur. J., 2016, 22, 7184–7189.
35 C. E. Chan-Thaw, A. Villa, G. M. Veith, K. Kailasam, L. A.
Adamczyk, R. R. Unocic, L. Prati and A. Thomas, Chem. -
Asian J., 2012, 7, 387–393.
61 A. S. Nagpure, L. Gurrala, P. Gogoi and S. V. Chilukuri, RSC
Adv., 2016, 6, 44333–44340.
62 Y. Han, S. S. Lee and J. Y. Ying, Chem. Mater., 2006, 18, 643–649.
63 R. L. Oliveira, J. B. F. Hooijmans, P. E. de Jongh, R. J. M. Klein
Gebbink and K. P. de Jong, ChemCatChem, 2014, 6, 3223–3230.
64 R. L. Oliveira, W. He, R. J. M. Klein Gebbink and K. P. de
Jong, Catal. Sci. Technol., 2015, 5, 1919–1928.
65 S. P. Andrews, A. F. Stepan, H. Tanaka, S. V. Ley and M. D.
Smith, Adv. Synth. Catal., 2005, 347, 647–654.
66 Y. Ji, S. Jain and R. J. Davis, J. Phys. Chem. B, 2005, 109,
17232–17238.
67 S. Reimann, J. St, R. Frahm, W. Kleist, J. Grunwaldt and A.
Baiker, J. Am. Chem. Soc., 2011, 133, 3921–3930.
68 N. J. S. Costa, P. K. Kiyohara, A. L. Monteiro, Y. Coppel, K.
Philippot and L. M. Rossi, J. Catal., 2010, 276, 382–389.
69 C. M. Crudden, M. Sateesh, R. Lewis and C. Kl, J. Am. Chem.
Soc., 2005, 127, 10045–10050.
70 H. Tsunoyama, H. Ito, M. Komori, R. Kobayashi, M. Shibuta,
T. Eguchi and A. Nakajima, Catal. Sci. Technol., 2018, 8,
5827–5834.
36 X. Tuaev, J. P. Paraknowitsch, R. Illgen, A. Thomas and P.
Strasser, Phys. Chem. Chem. Phys., 2012, 14, 6444.
37 N. Ranjbar Sahraie, J. P. Paraknowitsch, C. Göbel, A. Thomas
and P. Strasser, J. Am. Chem. Soc., 2014, 136, 14486–14497.
38 R. Arrigo, M. E. Schuster, Z. Xie, Y. Yi, G. Wowsnick, L. L.
Sun, K. E. Hermann, M. Friedrich, P. Kast, M. Hävecker, A.
Knop-Gericke and R. Schlögl, ACS Catal., 2015, 5, 2740–2753.
39 G. R. Bolzan, G. Abarca, W. D. G. Gonçalves, C. F. Matos, M. J. L.
Santos and J. Dupont, Chem. – Eur. J., 2018, 24, 1365–1372.
40 R. L. Oliveira, P. K. Kiyohara and L. M. Rossi, Green Chem.,
2009, 11, 1366.
41 R. L. Oliveira, D. Zanchet, P. K. Kiyohara and L. M. Rossi,
Chemistry, 2011, 17, 4626–4631.
42 R. Nie, H. Jiang, X. Lu, D. Zhou and Q. Xia, Catal. Sci.
Technol., 2016, 6, 1913–1920.
43 S. Li, C. Cheng, X. Zhao, J. Schmidt and A. Thomas, Angew.
Chem., Int. Ed., 2018, 57, 1856–1862.
44 M. Oschatz, J. P. Hofmann, T. W. van Deelen, W. S. Lamme,
N. A. Krans, E. J. M. Hensen and K. P. de Jong,
ChemCatChem, 2017, 9, 620–628.
71 J. Richardson and C. Jones, J. Catal., 2007, 251, 80–93.
72 Y.-S. Kim, X.-F. Guo and G.-J. Kim, Chem. Commun.,
2009, 4296–4298.
45 J. P. Paraknowitsch and A. Thomas, Energy Environ. Sci.,
2013, 6, 2839–2855.
73 T. Maschmeyer, F. Rey, G. Sankar and J. M. Thomas, Nature,
1995, 378, 159–162.
46 P. Xiao, Y. Zhao, T. Wang, Y. Zhan, H. Wang, J. Li, A.
Thomas and J. Zhu, Chem. – Eur. J., 2014, 20, 2872–2878.
47 T. Yuan, H. Gong, K. Kailasam, Y. Zhao, A. Thomas and J.
Zhu, J. Catal., 2015, 326, 38–42.
74 S.-Y. Chen, T. Yokoi, C.-Y. Tang, L.-Y. Jang, T. Tatsumi, J. C. C.
Chan and S. Cheng, Green Chem., 2011, 13, 2920–2930.
75 L. Liu, Q. Zhao, R. Liu and L. Zhu, Appl. Catal., B, 2019, 252,
198–204.
48 X. Xu, Y. Li, Y. Gong, P. Zhang, H. Li and Y. Wang, J. Am.
Chem. Soc., 2012, 134, 16987–16990.
76 R. Nie, H. Jiang, X. Lu, D. Zhou and Q. Xia, Catal. Sci.
Technol., 2016, 6, 1913–1920.
49 C. H. Liu, C. Y. Lin, J. L. Chen, K. T. Lu, J. F. Lee and J. M.
Chen, J. Catal., 2017, 350, 21–29.
77 D. Tang, X. Sun, D. Zhao, J. Zhu, W. Zhang, X. Xu and Z.
Zhao, ChemCatChem, 2018, 10, 1291–1299.
50 Y. Monguchi, T. Marumoto, T. Ichikawa, Y. Miyake, Y. Nagae,
M. Yoshida, Y. Oumi, Y. Sawama and H. Sajiki,
ChemCatChem, 2015, 7, 2155–2160.
78 K. Chizari, I. Janowska, M. Houllé, I. Florea, O. Ersen, T.
Romero, P. Bernhardt, M. J. Ledoux and C. Pham-Huu, Appl.
Catal., A, 2010, 380, 72–80.
51 P. Paramita Das, V. C. Prabhakaran, S. Nanda, D. Sen and B.
Chowdhury, ChemistrySelect, 2019, 4, 3823–3832.
79 A. Giroir-Fendler, D. Richard and P. Gallezot, Catal. Lett.,
1990, 5, 175–181.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020