Triazole/Triazine-Functionalized Mesoporous Silica As a Hybrid Material Support for Palladium Nanocatalyst

Article abstract of DOI:10.1021/acs.langmuir.7b01247

Noble and precious metal catalysts are sought for their remarkable efficiency in catalyzing numerous reactions in heterogeneous phase. However, they are costly and require the development of high-surface-area supports that favor their strong immobilization, dispersion, and stability. Toward this end, mesoporous silica-based materials can be regarded as unique supports for nanometric-sized noble metal catalysts provided they are functionalized with appropriate ligands. In this work, mesoporous silica SBA-15 was prepared and modified with 3-azidopropyltriethoxysilane and then clicked with alkyne derivatives of 1,3,5-triazine complex ligand. The resulting hybrid material contains triazole and triazine moieties covalently bound to the mesoporous silica network. The triazole/triazine minidendron was immobilized through a 1,3-dipolar cycloaddition click reaction, which was monitored by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The heterocyclic ligand-functionalized SBA-15 material served as a hybrid reactive platform for in situ deposition of palladium nanoparticles whose size is 3.154 ± 0.49 nm as assessed by X-ray diffraction and confirmed by transmission electron microscopy. The catalytic performance of the final palladium-decorated hybrid triazole/triazine-functionalized SBA-15 support was evaluated in the model reduction of 4-nitrophenol to 4-aminophenol by catalytic hydrogenation and stoichiometric reduction. Excellent catalytic performances were achieved, with reduction rate constant (K_app) of 16.8 × 10^-3 s^-1 for this model reaction. Moreover, the hybrid catalyst can be produced in high yield and recycled.

Full text of DOI:10.1021/acs.langmuir.7b01247

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Article
Triazole/triazine-functionalized mesoporous silica as
a hybrid material support for palladium nanocatalyst
ALI SAAD, Christian Vard, Manef Abderrabba, and Mohamed Mehdi Chehimi
Langmuir, Just Accepted Manuscript • Publication Date (Web): 21 Jun 2017
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Triazole/triazine-functionalized mesoporous silica as a hybrid material support for
palladium nanocatalyst
a, b *
c
a,
c ,*
Ali Saad
, Christian Vard , Manef Abderrabba * Mohamed M. Chehimi
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a) Laboratory of Materials, Molecules and Applications, IPEST, University of Carthage, Sidi Bou
Said road, B.P. 51 2070, La Marsa, Tunisia.
b) Faculté des Sciences de Tunis, Université El Manar, PO Box 248, El Manar II, 2092, Tunis,
Tunisia.
c) Université Paris Est, ICMPE (UMR 7182), CNRS, UPEC, 2ꢀ8 rue Henri Dunant, 94320 Thiais,
France.
Abstract
Noble and precious metal catalysts are sought for their remarkable efficiency in catalyzing
numerous reactions in heterogeneous phase. However, they are costly and request the
development of high surface area supports that favor their strong immobilization, dispersion
and stability. Towards this end, mesoporous silicaꢀbased materials can be regarded as unique
supports for nanometricꢀsized noble metal catalysts provided they are functionalized with
appropriate ligands. In this work, mesoporous silica SBAꢀ15 was prepared and modified with
3ꢀazidopropyltriethoxysilane then clicked with alkyne derivatives of 1,3,5ꢀtriazines complex
ligand. The resulting hybrid material contains triazole and triazine moieties covalently bound
to the mesoporous silica network. The triazole/triazine miniꢀdendron was immobilized
through 1,3ꢀdipolar cycloaddition click reaction which was monitored using Fourier transform
infrared spectroscopy (FTIR), Xꢀray photoelectron spectroscopy (XPS). The heterocyclic
ligandꢀfunctionalized SBAꢀ15 material served as hybrid reactive platform for the in situ
deposition of palladium nanoparticles whose size is 3.154 ±0.49 nm as assessed by XRD and
confirmed by TEM. The catalytic performances of the final palladiumꢀdecorated hybrid
triazole/triazineꢀfunctionalized SBAꢀ15 support were evaluated in the model reduction of pꢀ
nitrophenol (pꢀNP) into pꢀaminophenol (pꢀAP) by catalytic hydrogenation and stoichiometric
reduction. Excellent catalytic performances were achieved, with reduction rate constant (Kapp
)
ꢀ3 ꢀ1
of 16.8×10 s for this model reaction. Moreover, the hybrid catalyst can be produced in high
yield and recycled.
Keywords: mesoporous silica, 3ꢀazidopropyltriethoxysilane, nitrogen heterocycles, click
chemistry, Pd nanocatalyst.
Corresponding authors:
Ali Saad: ali.saad8803@gmail.com
Manef Abderrabba: abderrabbamanef@gmail.com
Mohamed M. Chehimi: chehimi@icmpe.cnrs.fr
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1 Introduction
Palladium nanoparticles (PdNPs) have received considerable attention due to remarkable
1
,2,3
catalytic performances in the reduction of nitro compounds.
They were also recognized to
be efficient catalysts of various organic chemical reactions in the chemical and
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pharmaceutical industries, including Suzuki–Miyaura coupling, Oꢀ and Nꢀarylations, and
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oxidation of alcohols. Interestingly, PdNPs can be even be produced as mesoporous
,
8,9 10
structures with unprecedented electrocatalytic performances.
However, it is difficult to
separate and thus to recover noble metal catalysts from the reaction mixture, with a result of
adverse environmental pollution, which limits their applications. To address this concern and
in order to facilitate the catalyst recovery, PdNPs are commonly dispersed onto solid matrices
to prepare heterogeneous palladium catalysts. Indeed, in order to limit agglomeration of Pd
nanoparticles, it is possible to immobilize them on various solid supports such as amorphous
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silica, titania, carbon, core–shell magnetic fibers and hexagonally ordered mesoporous
1
5 , 16
silica.
Particularly, mesoporous silica offers great advantages due to its outstanding
features such as high surface area, regular tunable and accessible pores and control over their
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sizes, high thermal stability, and mechanical properties.
One can take advantage of
the nature of mesoporous silica pore structure to functionalize it with alkoxysilyl compounds
2
1,22
in order to facilitate the incorporation of metal complexes in the said pores.
An attractive method that allows for fineꢀtuning of catalytic material at the molecular level
consists in the use of organic molecules for covalent functionalization. Another commonly
followed route is the functionalization of mesoporous silica via addition reactions, namely
cycloaddition, nucleophilic and freeꢀradical additions, and reductive alkylation onto/inside the
2
3
silica pores in order to design hybrid materials. Among the most investigated click reactions
Huisgen 1,3ꢀcycloaddition, DielsꢀAlder, thiolꢀyne, thiolꢀene, Michael addition), Huisgen
(
reaction has raised much interest owing to its simplicity and efficiency either in bulk solutions
or in suspensions where it is employed to functionalize or bridge compounds, and plays a very
24
important role in the design of novel hybrid materials. Huisgen reaction is actually one of
the most efficient routes for binding appropriately functionalized molecules to solid supports
through triazole linkages. Therefore, azidoꢀfunctionalized SBAꢀ15 materials have been widely
employed to covalently attach various moieties to the inorganic support. For example,
mesoporous silicaꢀbased hybrid materials were designed and evaluated in biosensing
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5
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applications, removal of antibiotics, immobilization of enzymes and as catalyst
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supports, to name but a few.
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It is well known that Nꢀ2 and Nꢀ3 nitrogen atoms present in the triazole ring are good Hꢀbond
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acceptors owing to the 1,2,3ꢀtriazole large dipole moment. Moreover, triazole groups are
excellent nitrogenꢀdonor ligands and thus are ideal for coordinating a variety of transition
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metals.
As far as triazine derivatives are concerned, they belong to the compounds with
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2
nitrogen–carbon double bonds and which exhibit excellent ability to immobilize proteins
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and catalysts on the underlying materials surfaces. One can take advantage of the chemical
and electronic structure of triazine to implement this functional group in the inorganic/organic
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hybrid systems in order to enhance the coordination of metal ions and therefore to design a
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support for anchoring metal NPs evenly dispersed on the active sites.
Whilst triazine and triazole were used separately to functionalize mesoporous silicas for the in
situ deposition of metallic nanocatalysts, we have reasoned that we can take advantage of
their remarkable and synergetic properties in view of making one single dendronꢀlike
structure attached to silicas in view of immobilizing the said metallic nanocatalysts. With this
objective in sight, we relied on the efficiency of click chemistry to prepare a clickable
dendronꢀlike nitrogen heterocyclic structure to react with azidoꢀfunctionalized mesoporous
SBAꢀ15 and end up with a unique organic/inorganic hybrid platform for dispersing and
immobilizing nanocatalysts. To the very best of our knowledge this strategy has not been
described previously; this is what has motivated us to construct the unique hybrid materials
for the attachment of metallic nanocatalysts.
Herein, we have prepared palladium NPs supported on 1,2,3ꢀtriazoles from alkyne derivatives
of 1,3,5ꢀtriazine complexes, designed via ‘‘click’’ chemistry. The rationale for using a high
surface area support (mesoporous silica) is to achieve excellent dispersion of the nanocatalyst
and to enhance its catalytic activity. Metal complex speciation and catalytic reaction were
shown to be influenced by surface loading. In addition, it is important to chemically tailor an
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environment within the support to achieve metal affinity and ligand stability.
The Pdꢀ
catalyzed reduction of 4ꢀnitrophenol into aminophenol using NaBH as reducing agent was
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performed in order to assess the catalytic performances of the hybrid silicaꢀimmobilized noble
metal nanocatalyst.
The synthesis of 1,2,3ꢀtriazoles from alkyne derivatives of 1,3,5ꢀtriazines was obtained by
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,3ꢀdipolar cycloaddition reactions between organic azides and terminal alkynes of
trispropargylmelamine, using a threeꢀstep methodology (Figure 1): (i) a clickable group (N −)
3
was introduced onto mesoporous silica by covalently grafting the azide using
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azidopropylsilane ; (ii) synthesis of trispropargylmelamine; and (iii) the click chemistry
reaction inside the mesoporous silica formed organic compounds triazoleꢀtriazine complexes
grafted into the pores. The nitrogenꢀrich triazole/triazine group is expected to act as bidendate
or even multidentate ligand towards Pd(II) ions. This chelation mechanism leads to the
dispersion and stabilization of the Pd NPs on the hybrid mesoporous support.
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O
H
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EtO
EtO Si
EtO
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(a)
SBA-15
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Triethylamine/THF/Reflux
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3Tr-M-SBA-15
H
(a + b)
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Si
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(iii)
N N
Si
O
O
O
Figure 1. Schematic representation of the preparation 1,2,3ꢀtriazoles from alkyne derivatives
of 1,3,5ꢀtriazines complex supported mesoporous silica palladium complex.
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Experimental
2.1 Materials
Poly(ethyleneoxide)ꢀbꢀpoly(propyleneoxide)ꢀbꢀpoly(ethyleneꢀoxide)
Pluronic P123, 5800], tetraethylorthosilicate
[EO20PO70EO20,
M
w
=
(TEOS,
98%),
3ꢀ
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chloropropyltriethoxysilane, triethylamine, tetrahexylammonium bromide, sodium ascorbate,
sodium azide (NaN
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), 1,3,5ꢀtriaminoꢀtriazine (melamine), propargyl bromide (80%), copper
·5H O), toluene, pentane, acetonitrile, propanꢀ2ꢀol,
(II) sulfate pentahydrate (CuSO
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tetrahydrofurane (THF), HCl and ethanol (AldrichꢀSigma products) were of high purity grade
and used as received. Water was deionized using the MilliꢀQ system of Millipore.
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2.2. Synthesis of SBA-15 mesoporous silica
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The mesoporous SBAꢀ15 was synthesized following the procedure described by Zhao et al.
First, 3g of nonionic triblock copolymer Pluronic P123 was dissolved in16.5 ml of HCl (12
M) and 112 ml of water. The solution was vigorously stirred for 3 h at 40 °C. Second, after
complete dissolution of Pluronic P123, 7.427 g of TEOS was rapidly added to the acidic
solution under continuous stirring. A synthesized gel was obtained after resting for 2 h at 40
°C the mixture was transferred to an oven for further condensation at 90 °C under static
conditions for 24 h. The precipitate obtained after filtration was washed and dried. Finally, the
template removal from as synthesized SBAꢀ15 was removed by calcination (550 °C, 8 h).
2.3. Preparation of Pd(II)/1,2,3-triazole from alkyne derivatives of 1,3,5-triazine complex
supported mesoporous silica
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2.3.1 Synthesis of N -SBA-15.
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The 3ꢀazidopropyltriethoxysilane was prepared according to the literature.
To a solution
of tetrahexylammonium bromide (0.86 g, 2 mmol) and sodium azide (1.08 g, 16.6 mmol) in
dry acetonitrile (50 mL) was added 3ꢀchloropropyltriethoxysilane (2 g, 8.3 mmol). The
heterogeneous mixture was then heated to 82 °C under an inert atmosphere for 18 h. After
completion of the reaction, the solvent was removed under reduced pressure, and the crude
mixture was diluted with pentane. The suspension was filtered through celite and the solvent
was removed by vacuum distillation. The 3ꢀazidopropyltriethoxysilane (AzPTES) was
1
obtained as a colorless liquid. H NMR (CDCl ): δ (ppm) = 3.75 (q, 6 H; CH CH O), 3.20 (t,
3
3
2
2 3 2 2 3 2 2
2H; CH N ), 1.64 (m, 2H; SiCH CH ), 1.16 (t, 9H, CH CH O), 0.61 (m, 2H; SiCH ).
To a suspension of 1 g of SBAꢀ15 in 50 mL of toluene, 2 mL AzPTES were added, and the
mixture was stirred for 24 h at 80 °C under nitrogen atmosphere. After the completion of the
reaction, the products were cooled, filtered and washed with toluene until it became free from
AzPTES. The sample was then dried at 80 °C for 8 h.
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2.3.3. Synthesis of trispropargyl melamine “3Tr-M”
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.87 g (6.9 mmol) of 1,3,5ꢀtriaminoꢀtriazine was added to a mixture of 3.65 g of propargyl
bromide (80%) and triethylamine (0.68 g, 6.9 mmol) in THF (120 mL). The solution was then
heated to 80 °C for 24 h and filtered. The filtrate was dried under vacuum. Finally the product
was isolated as a white powder and was then dried at 70 °C.
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2.3.3. Synthesis of 3Tr-M-SBA-15
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TrꢀMꢀSBAꢀ15 was obtained by 1,3ꢀdipolar cycloaddition reactions as follows: the N
3
ꢀSBAꢀ
1
5 (1.22g), CuSO4 (0.43, 0.1M), and sodium ascorbate solution (0.77g, 0.1M) was added to a
solution of trispropargyl melamine (0.15, 1M) in 50% aqueous propanꢀ2ꢀol (20 ml) under
stirring for 24 h at 25 °C. The resulting product 3TrꢀMꢀSBAꢀ15 was filtered and washed
thoroughly with ethanol and acetone and dried under vacuum for 24 hours.
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.3.4. In-situ synthesis of palladium nanoparticles
3
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At first, palladium catalyst was prepared according to Chergui et al. Chelatant nanosupports
3
TrꢀMꢀSBAꢀ15 (50 mg) were ultrasonically dispersed in 10 mL of NaOH solution (1 M), and
ꢀ2
then 10 mL of an aqueous solution of PdCl (10 M) was added. After stirring for1h,
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subsequent reduction of the metal was performed by addition a fresh aqueous solution of
4
NaBH (0.1M), to the mixture. The solution was stirred for 1 h at room temperature. The
resulting palladium nanoparticleꢀdecorated mesoporous silica were filtered, then washed
several times with water, and dried under vacuum.
2.4. Catalytic reduction of 4-nitrophenol
Typically, in a standard quartz cuvette, 4ꢀnitrophenol (30 µL, 7.4 mM), NaBH
M), and deionized water (2 mL) were added. After adding Pd/3TrꢀMꢀSBAꢀ15 17 µL catalyst
1.75 g/L), the mixture was vigorously stirred using a small glass stirring rod. As the reaction
4
(30 µL, 0.82
(
progressed, the bright yellow color of the solution gradually faded. UV−vis spectra were
taken after set periods of time. After the catalytic reaction was completed, the catalyst was
collected and washed with ethanol prior to reuse for a next catalytic test.
2
.5. Characterization
A K Alpha (Thermo) machine equipped with a monochromatic, 400 µmꢀsized Al K Xꢀray
source was used for Xꢀray photoelectron spectroscopy (XPS) measurements. The pass energy
was set to 200 and 50 eV for the survey and the narrow regions, respectively. The CꢀC/CꢀH
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C1s component was set at 285 eV to calibrate the spectra. The composition was assessed
using the sensitivity factors provided by the manufacturer.
Nitrogen adsorption and desorption studies were carried out at 100 °C and 0.1 MPa for 12 h
before measurements using a autosorb IQ, Quantachrome. The specific surface (SBET) values
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were obtained using the BrunauerꢀEmmettꢀTeller (BET) method whereas the mesopore size
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was assessed using the BarrettꢀJoynerꢀHalenda (BJH) method from the desorption branch of
the isotherm.
The thermal and decomposition characteristics of the materials were determined by thermal
gravimetric analyses, were performed using a SETSYS evolution 16 apparatus from
SETARAM, in the temperature range of 10ꢀ800 °C with a heating rate of 10C/min under a
3
ꢀ1
flow of air at (55 cm (STP)/min) with a heating rate of 10 °C min .
Fourier transform infrared (FTIR) spectra were recorded using an EQUINOX 55 (Bruker)
−1
−1
Spectrometer ranging from 4000 to 400 cm . Spectra were accumulated 64 times at 4 cm
resolution and processed using OPUS/IR software.
Transmission electron micrographs were obtained on a (TEMꢀTecnai F20 with a Field
Emission Gun 200 kV, punctual resolution 0.24 nm and Energy Filtering GIF). Samples were
prepared by dispersing in ethanol using an ultrasonic bath. Few drops from these suspensions
were then gently deposited onto a carbonꢀcovered copper grid and then ethanol was
evaporated.
1
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The H, and C NMR spectra were obtained with a Bruker AVANCE 300 wide bore
spectrometer using deuterated chloroform as the solvent and internal standard. The chemical
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shifts are reported in parts per million (ppm). The C CP MAS NMR spectra were recorded
on a Bruker AVANCE 400 wide bore spectrometer with a 12ꢀkHz MAS spinning rate (the
typical contact time was 3ms, and Typically 10000 to 25000 scans with a recycle delay of 3s
were collected depending on the sensitivity of the sample.
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3. Results and discussion
3.1. Physicochemical properties of materials
3.1.1. FT-IR spectroscopy analysis.
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The FTꢀIR spectra before and after inclusion of propargylamine are presented in Figure 2a.
The spectrum for pure 1,3,5ꢀtriaminoꢀtriazine exhibits the characteristic absorption bands in
ꢀ1
ꢀ1
the 478–1662 cm range i.e. 1624, 1532, 1428, 808 and 584 cm bands can be assigned to the
aromatic stretching modes in triazine ring, C=N (imine stretching) inꢀplane deformation, CꢀN
stretching and C–H outerꢀbending vibrations, respectively. These characteristic bands of the
triazine ring also appear after reaction with propargyl bromide. Also as can be seen
ꢀ
1
appearance of the new adsorption peaks at 2109, 2923, 2961, 3128 and 3244 cm are due to
(CꢀC≡CH) stretch, νsy CH in ꢀCH – group), νas CH (in ꢀCH – group), (NꢀH secondary
2
2
amine) and the alkyne CꢀH vibrations. All these bands confirmed the successful obtaining the
desired compound. Figure 2b shows spectra of SBAꢀ15 based materials. The (3ꢀazidopropyl)
ꢀ1
triethoxysilane grafted mesoporous silica yields a new peak observed at 2110 cm which is a
2
6
characteristic stretching vibration of organic azides. This band confirms the effective
incorporation of the azido functional group to the materials. The associated SiꢀOꢀSi bands are
ꢀ
1 27
observed at 1274, 1050, 910, 797, and 448 cm . After the click reaction the typical band for
the azido group was no longer present, indicating that the click reaction proceeded to
completion. Upon clicking, a new band at 1610, 1470 and 1402 can be assigned to the
35, 41
aromatic stretching modes in triazine and triazole rings.
1
1
0
0
,5
,2
,9
,6
(
b)
(a)
SBA-15
-SBA-15
trispropargyl melamine
1,2
N
3
3
Tr-M-SBA-15
2
123
2
985
2
938
melamine
2110
1647
0
,9
3
371
1
401
7
53
1
610
470
1
797
448
1
402
0,6
1
050
1000
500
1000
1500
2000
2500
3000
3500
4000
5
00
1500
2000
2500
3000
3500
4000
-
1
-1
Wavenumbers (cm )
Wavenumbers (cm )
Figure 2. FTIR spectra of: (a) 1,3,5ꢀtriaminoꢀtriazine (melamine) before and after
modification with propargyl bromide , (b) SBAꢀ15, N ꢀSBAꢀ15 and 3TrꢀMꢀSBAꢀ15.
3
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1
3
To confirm FTꢀIR studies we have employed solid state C crossꢀpolarized magic
angle spinning (CPꢀMAS) NMR. N ꢀSBAꢀ15 and 3ꢀTrꢀMꢀSBAꢀ15 spectra are displayed in
Figure S1 (see Supporting Information). For the N ꢀSBAꢀ15 (Figure S1a), three peaks at δ=
3
3
9.4, 22.5 and 54 ppm are ascribed to the C signals of azidopropyl. One can also note
additional peaks assigned to residual P123 template (71 and 17 ppm) and to EtO groups (59
10
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15
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28
and 17 ppm). After azideꢀalkyne 1,3ꢀdipolar cycloaddition (CuAAC) of N
3
–SBAꢀ15, along
with all three peaks of propyl attached SBAꢀ15, extra peaks are clearly visible at δ= 102,
25.5 and 132.9 ppm, which corresponds to the triazoleꢀtriazine group attached to the surface
Figure S1b).
1
(
3.1.2. Nitrogen adsorption–desorption isotherms.
The surface area, diameter and pore volume of SBAꢀ15, N
3
ꢀSBAꢀ15 and 3TrꢀMꢀSBAꢀ15
samples were assessed by nitrogen adsorption/desorption isotherms (Figure 3). The textural
properties are gathered in Table 1. For all materials, it can be observed that, as defined by
4
2
Brunauer et al., the sorption resulted in a typical type IV isotherm hysteresis loop at high
relative pressure values (P/P
0
= 0.65). This behaviour is characteristic of materials possessing
2
regular cylindrical mesopores with a narrow pore size distribution.
12
800
10
8
SBA-15
-SBA-15
3Tr-M-SBA-15
N
3
6
4
600
2
0
2
4
6
8
10
12
Pore size (nm)
4
00
00
0
2
SBA-15
-SBA-15
N
3
3Tr-M-SBA-15
0,8 1,0
0
,0
0,2
0,4
0,6
Relative pressure p/p0
3
Figure 3. Nitrogen adsorption/desorption isotherms of (a) SBAꢀ15, (b) N ꢀSBAꢀ15,
(c) 3TrꢀMꢀSBAꢀ15.
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2
2
2
2
3
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4
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5
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5
5
5
5
5
5
5
6
The specific surface area, total pore volume, and pore size decreased with the increase of
azide content as shown in Table 1. The multiple points BET surface areas of SBAꢀ15,
2
N
3
ꢀSBAꢀ15 and 3TrꢀMꢀSBAꢀ15 were 734, 375 and 289 m /g, respectively. The pore size
distribution, determined for pristine and modified SBAꢀ15 samples by the BJH method, is
also shown in Figure 3. The pore diameter (DBJH) distributions of SBAꢀ15, N ꢀSBAꢀ15 and
TrꢀMꢀSBAꢀ15 are centered at ∼ 6.8, 5.8 and 5.4 nm, respectively. However, the
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
incorporation of organic groups to the mesoporous silica resulted in a decrease in pore volume
compared to pristine SBAꢀ15 (Table 1). This significant decrease in the porosity, surface area
and total pore volume of the functionalized materials is due to the partial blocking of the
4
3
pores by grafting of organic groups on the inner walls of SBAꢀ15.
Table 1. Specific surface areas, mean pore diameters and porous volumes for SBAꢀ15 and
ligandꢀmodified SBAꢀ15.
Materials
2 -1
SBET (m g )
BJH
Dp (nm)
a
3
V (cm g )
-1 b
p
SBA-15
733
6.81
1.15
N -SBA-15
3Tr-M-SBA-15
375
289
5.83
5.43
0.529
0.374
3
a
Mean pore diameter determined using the BJH method
Total pore volume determined at P/P = 0.97
b
0
3
.1.3. XPS analysis
Figure 4 displays highꢀresolution C 1s and N 1s spectra of 3TrꢀMꢀSBAꢀ15 and its precursor
ꢀSBAꢀ15. For the latter, C1s is fitted with three components centered at 285, 286.7 and
88.7 eV, which are attributed to CꢀC/CꢀH, CꢀN and OꢀC=O, respectively (Figure 4a). The
N
3
2
high binding energy component noted at 288.7 eV can be assigned to OꢀC=O from SBAꢀ15.
This feature is most of the time detected at the surface of the high energy materials such as
4
4
metals, metal oxides and ceramics. As far as the N 1s region is concerned, the spectrum is
ꢀ
+
fitted with three components assigned to NꢀH/NꢀC (399ꢀ400 eV), N /NꢀC (~401 eV), and N
+
(
~404.5 eV), respectively (Figure 4b). The N atom is the central nitrogen atom in the azido
+
ꢀ
ꢀ
+
45
group which coꢀexists in the following mesomeric forms: ꢀN=N =N ꢀ ꢀN ꢀN ≡N. The main
peak centred at ~401 is due to the two other nitrogen atoms which can be negatively charged.
The first peak at 399ꢀ400 eV is assigned to NꢀH/NꢀC bonds. We would like to insist at this
stage on the fact that azido groups are known to undergo Xꢀray photodegradation during XPS
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analysis as we and others have demonstrated previously. As far as we are concerned we
have given guidelines for analyzing material surfaceꢀbound azido groups. We have strictly
followed the procedures in order to be able to check by XPS the presence of surface
3
bound –N prior to any significant Xꢀray induced photodegradation. After reaction with
trispropargyl melamine, for 3TrꢀMꢀSBAꢀ15, the C=N/CN peak component is significantly
higher (Figure 4c), suggesting that more N species have been incorporated into the skeleton of
the support which is in the line with the surface elemental composition reported in Table S1.
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+
The azide is transformed into triazole and the N peak component has vanished indicating
complete reaction, a result that supports the FTIR findings. The N=N atoms have a single
3
component centred at 400.2 eV and the sp nitrogen atom has a peak centred at 402 eV. The
N1s region has additional component centred at 398.6 assigned to N=C (Figure 4d).
-
N
401
C-C/C-H
285
(a)
16k
14k
N -SBA-15
3
N3-SBA-15
(b)
12000
+
N
4
04.5
9000
6000
3000
C-N
1
1
2k
0k
2
86.7
N-H/N-C
99.7
3
O-C=O
88.7
2
8k
292
290
288
286
284
282
280
408
406
404
402
400
398
396
394
Binding Energy (eV)
Binding Energy (eV)
2
2
8k
4k
3Tr-M-SBA-15
C-H/C-C
284.9
19k 3Tr-M-SBA-15
(d)
(c)
N=C
1
1
8k
7k
398.6
N=N
400.2
20k
C-N
286.7
1
6k
C=C/C=N
N-H/N-C/N-N
402.4
288.8
16k
15k
12k
8
k
1
4k
408
2
92
290
288
286
284
282
280
406
404
402
400
398
396
394
Binding Energy (eV)
Binding Energy (eV)
palladium oxide
Pd 3d 5/2 (e)
335.6
36k
metallic palladium
Pd 3d 3/2
40.7
3
0k
4k
8k
2k
3
2
1
1
3
48
346
344
342
340
338
336
334
332
Binding Energy (eV)
3
Figure 4. (a) C 1s , (b) N1s XPS spectra of the N ꢀSBAꢀ15 before , (c) C 1s , (d) N1s after
click reaction with trispropargylmelamine and (e) Pd3d after immobilization of palladium
nanoparticles.
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5
5
5
6
The electronic state of metallic palladium particles in the catalyst was further analyzed by the
XPS spectrum. The result (Figure. 4e) demonstrated that the Pd 3d5/2 and 3d3/2 doublet
binding energy positions are observed at 335.6 and 340.7 eV, which is consistent with the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
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8
9
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9
0
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range of values compiled by Powell and coꢀworkers for palladium in the metallic state.
Palladium is also detected in the oxidation state (doublet pair at 337.2 and 342.4 eV) due to
PdꢀO bond formation upon air exposure of the hybrid catalyst.
Table S1 reports the surface chemical composition of unmodified, azidoꢀsilanized and
palladium immobilized grafted mesoporous silicas Pd(0)/3TrꢀMꢀSBAꢀ15. The N/Si atomic
ratio is equal to ∼0.09 after azidoꢀsilanization but increases to ∼0.167 upon click chemistry
grafting. Indeed, the mesoporous silica gets wrapped by the trispropargyl melamine which
brings nitrogen atoms in melamine groups, hence the increase in the N/Si ratio. Incorporation
of nitrogenꢀcontaining groups alters the surface composition and induces surface hydrophilic
character which favors the dispersion in water and other polar solvents. The high extent of
2
+
nitrogen atoms brought by the ligand facilitates the complexation of the Pd ions and their
tight immobilization onto the support. As previously demonstrated, pyridinic and quaternized
N species facilitate the coordination of metal ions as well as the attachment of metal
35,48,49,50
NPs.
The XPS surface analysis of all materials permitted to track the subtle changes of the
composition following azidoꢀsilanization and click chemistry grafting of trispropargyl
melamine.
3.1.4. Thermal analysis
Thermal degradation of pristine and modified SBAꢀ15 samples was studied using TGA by
heating the samples from room temperature to 800 °C under a nitrogen atmosphere, at a rate
−1
of 10 °C min . Thermogravimetric (TG) analyses were carried out to confirm the effective
modification of silica with the functional groups resulting from silanization and subsequent
click reaction. As shown in Figure 5 the first weight loss below 170 °C is ascribed to the
volatilization of water adsorbed on all samples. A second significant weight loss of 12.7 and
3
23.4% can be observed between170 and 500 °C for N ꢀSBAꢀ15 and 3TrꢀMꢀSBAꢀ15. This
weight loss is attributed to the decomposition of both the organic groups and the bridging
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ethylene groups, respectively. Similar results were reported previously for other “click”ꢀ
3
5, 51
functionalized solid periodic mesoporous organosilica supports.
The weight loss
difference between azidopropyl functionalized mesoporous silica and triazoleꢀtriazine groups
attached to the SBAꢀ15 surface is 10.6%. This reflects the efficiency of alkyne derivatives of
1,3,5ꢀtriazine to react with azidoꢀfunctionalized mesoporous silica through Huisgen’s 1,3ꢀ
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cycloaddition resulting in triazole linkages.
0
2.77
(a)
(b)
5.18
-5
12.72
23.35
-
-
-
-
-
10
15
20
25
30
(
c)
-35
0
200
400
600
800
Temperature °C
Figure 5 Thermogravimetric analysis of pristine mesoporous silica SBAꢀ15 (a),
ꢀSBAꢀ15 (b), and 3TrꢀMꢀSBAꢀ15 (c).
N
3
3.1.5. Textural studies.
Highꢀresolution transmission electron microscopy (HRꢀTEM) has been used in order to
visually detect the presence of palladium nanoparticles, their dispersion and to verify their
size distribution. The TEM image of the SBAꢀ15 sample (Figure 6a) exhibits large domains of
highly ordered stripeꢀlike arranged structures, which accounts for the 2D hexagonal
arrangement of uniformely sized 1D channels.
TEM images Figure 6(b,c,d) of the front and side views of the nanocatalyst show that
palladium has been introduced within the channels of the structure. This suggests that the
mesoporous silica possesses sufficient structural stability required to retain morphological
integrity despite the loading of metal NPs. Black dots, observed in the TEM images of the
5
2
resulting nanocomposite materials, are assigned to Pd(0) NPs. Indeed, after the anchoring of
Pd(II) complex inside the mesoporous channels of SBAꢀ15, the image displays distinct deep
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2
2
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2
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6
5
3
contrasting meso parallel channels with respect to the light shaded surface (Figure 6b). This
might be interpreted as due to the presence of the 3TrꢀM–Pd complex mostly inside the
mesopores of the SBAꢀ15, yet some nanoparticles are loaded on the surface of the hybrid
support.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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8
9
0
1
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
a)
(b)
(d)
(c)
(
e)
(f)
1
500
200
900
4
0
2
4
6
8
0
1
3
2
1
6
00
00
0
3
2
0
40
60
80
100
1
2
3
4
5
Particle size (nm)
2
θ
(degree)
Figure 6. (a) Typical TEM images of SBAꢀ15 (a), and Pd(0)/3TrꢀMꢀSBAꢀ15 nanocatalyst
b,c,d). XRD pattern of nanocatalyst (e) and histogram of the Pd particle size distribution (f).
(
The catalyst Pd(0)/3TrꢀMꢀSBAꢀ15 is characterized by XRD, and their XRD patterns are
shown in Figure 6e. Three peaks at 2θ of 40.0°, 48.1° and 66.4° corresponding to the
diffraction of the (1 1 1), (2 0 0) and (2 2 0) lattice planes of the Pd nanoparticles planes in the
5
4
face centered cubic (FCC) crystal structure of Pd and the broad peak centred at 2θ of 23°
corresponding to the support of the mesoporous silica can be clearly observed.
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The histogram in Figure 6e exhibits a narrow size distribution of the Pd nanoparticles with a
mean value of 3.154 ±0.49 nm indicating that palladium nanoparticles did not aggregate upon
immobilization. Indeed, the formation of small palladium particles is ascribed to the use of the
38
4
reducing agent NaBH which permits their quick nucleation and growth.
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3
.1. Catalytic performance
Nitrophenol (4ꢀNP) is a water pollutant with high toxicity, which is of great environmental
55
concern. Its reduced form 4ꢀaminophenol is an important analogue of anilines, an
intermediate of several antipyretic and analgesic drugs such as paracetamol and acetanilide.
Hence, many processes have been developed for the NP removal such as adsorption,
microbial degradation, photocatalytic degradation of pꢀnitrophenol (pꢀNP) into
pꢀaminophenol (pꢀAP) by catalytic hydrogenation and stoichiometric reduction. In this regard,
metal NPs can be active catalysts for several electronꢀtransfer reactions. In this study, we
evaluated the catalytic activity of the Pd(0)/3TrꢀMꢀSBAꢀ15 in the model reaction of reduction
of pꢀnitrophenol to pꢀaminophenol with an excess amount of NaBH
The reduction process was monitored using UV−vis spectroscopy at different time intervals
Figure 7c). In the absence of any Pd(0)/3TrꢀMꢀSBAꢀ15 the reduction reaction does not
4
.
(
proceed even in a large excess of NaBH . Indeed, the absorption of 4ꢀnitrophenol in water
4
gives a characteristic peak at 317 nm which is simply shifted to 400 nm after addition of
4
NaBH without any significant decrease in peak intensity. This change from light to dark
yellow resulting in spectral shift to 400 nm (Figure 7a) is due to the formation of the
corresponding nitrophenolate anion.
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NO2
OH
O
2
2
1
1
0
0
,5
,0
,5
,0
,5
,0
(b)
H
B
a
4
2,5
(a)
OH
N
4
00
317
NO
2
2,0
NO 2
1
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,5
,0
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9
0
NH2
OH
0,5
0,0
200
300
400
50
250
300
350
400
450
500
wavelength (nm)
wavelength (nm)
-6
2,4
1,8
1,2
0,6
0,0
1
,0
,8
(c)
(d)
-5
-
3
Ln
(
C /C
)
= -16,810 t - 0,0805
R² = 0,9861
t
0
0
0 s
5 s
0 s
-4
-3
1
3
0,6
0,4
0,2
0,0
6
0 s
90 s
180 s
-2
3
00 s
-1
0
0
50
100
150
200
250
300
2
50
300
350
400
450
500
wavelength (nm)
time (s)
Figure 7. (a) UVꢀvis spectra of 4ꢀnitrophenol before and after adding NaBH solution, (b)
Reaction scheme for the reduction of pꢀnitrophenol to pꢀaminophenol, (c) Timeꢀdependent
UVꢀvis spectral changes in 4ꢀNP catalyzed by Pd(0)/3TrꢀMꢀSBAꢀ15 at 25 °C, (d) Linear
4
t 0
relationship of ln(C /C ) as a function of time.
The reduction of the nitroꢀgroup does not take place in the absence of any catalyst due to a
ꢀ
56
4
high kinetic barrier between the mutually repelling negative ions, 4ꢀNP and BH . In the
presence of SBAꢀ15, the concentration of pꢀnitrophenol decreased marginally (5ꢀ7 %), due to
adsorption on the surface. In contrast, when Pd(0)/3TrꢀMꢀSBAꢀ15 nanocatalysts were
introduced into the solution, the absorption band of the pꢀnitrophenolate ion at about 400 nm
gradually decrease and the color of the solution vanished thus indicating the reductive
transformation of 4ꢀnitrophenol into 4ꢀaminophenol (Figure 7b). The bright yellow solution
gradually turns colorless 600 s after adding the catalyst, a visual inspection that supports the
complete reduction of 4ꢀnitrophenol. The formation of 4ꢀaminophenol was confirmed by the
appearance of intense bands at 230 and 310 nm indicating that the –NO
2
group of pꢀ
nitrophenol has been reduced to the –NH group.
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Figure 8. The proposed mechanism of the conversion of 4ꢀNP to 4ꢀAP by
Pd(0)/3TrꢀMꢀSBAꢀ15.
On the basis of the catalytic reduction of 4ꢀNP to 4ꢀAP by Pd(0)/3TrꢀMꢀSBAꢀ15 schematized
in Figure 8, the catalytic mechanism of the reduction reaction can be described as follows:
ꢀ
both the donor BH
4
ions and the acceptor 4ꢀNP molecules adsorb onto the catalytic centers
with a result of electron transfer from the former to the latter. This process leads to the
reduction of 4ꢀNP into 4ꢀaminophenol.
The efficiency of the catalytic conversion of pꢀnitrophenol (4ꢀNP) into pꢀaminophenol (4ꢀAP)
can be calculated using equation (1):
ꢅꢆ ꢈꢆ ꢃ
ꢇ
ꢉ
(4 ꢀ ꢁꢂꢃ% ꢄ
ꢊꢋ100
(1)
ꢆꢇ
where C
t
is the concentration of 4ꢀNP measured at time t, and C
0
is the initial concentration of
4
ꢀNP at time zero.
The reaction kinetics can be described by equation (2):
ꢌꢍꢎꢈꢏꢐꢑ
ꢄꢋꢓꢔꢕꢕꢍ4 ꢀ ꢁꢂꢑꢖ
(2)
ꢌꢒ
where kapp is the apparent rate constant and n is the reaction order of [4ꢀNP].
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In this study, NaBH
4
was used in excess, and the catalytic reduction is assumed to be a
pseudoꢀfirstꢀorder process. The ln(C
t 0
/C )ꢀvsꢀtime plot is obtained by monitoring the
absorbance as a function of time (equation 3):
ꢆ
ꢉ
ꢀꢓꢔꢕꢕꢗ ꢄ ꢋln ꢋꢅ ꢃ
(3)
ꢆ
ꢇ
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0
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0
where C and C are defined as above, and kapp is the kinetic constant. Figure 7d exhibits an
t 0
excellent linear fit for the reduction reaction catalyzed by Pd(0)/3TrꢀMꢀSBAꢀ15. By
monitoring the decrease in the absorption band of the enolate ion at 400 nm, and assuming
pseudoꢀfirst order kinetics, it was possible to determine an apparent rate constant (see Figure
7c). The whole reaction is almost complete within 300 s in the presence of excess NaBH4. For
ꢀ
3
ꢀ1
the Pd(0)/3TrꢀMꢀSBAꢀ15 catalyst the kinetic rate constant (ka ) at RT equals to 16.8
×
10 s
pp
56,57
and is comparable to previously published data for other supports.
The high catalytic activity might be attributed to the structure of the hybrid platform for nano
palladium catalyst and the very small Pd nanoparticles on the substrate.
Pd(0)/3TrꢀMꢀSBAꢀ15 catalyst was prepared and served to catalyze conversion of 4ꢀNP to 4ꢀ
AP three times. After the first run, the catalyst was washed three times sequentially with
ꢀ1
ethanol, water and a solution of sodium carbonate (0.3 mol L ) in order to remove borates and
any other reaction byproducts remaining from the previous run. The washed catalyst was then
further used again to verify the stability of its catalytic performances.
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9
5
0
9
85
8
0
5
7
10
70
1
2
3
Recycle Number
Figure 9. Recycling test of Pd(0)/3TrꢀMꢀSBAꢀ15 catalyst in water medium catalytic
conversion of 4ꢀNP to 4ꢀAP.
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Figure 9 displays the recycling test of Pd(0)/3TrꢀMꢀSBAꢀ15; a marginal decrease of the
kinetic constant values kapp is noted. The kapp constant in the third cycle was found to be as
high as 13.1 indicating that the catalyst exhibits a good stability against poisoning by the
product of the reaction. The percentage of completion reduction (CR) was nearly the same in
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the first two cycles, decreasing only marginally (CR∼90%), possibly due to the leaching of a
small portion of Pd NPs from the hybrid support, to the oxidation of palladium upon exposure
58
to air or to the screening of the catalytic sites by the formed product. However, the latter
hypothesis could be ruled out as the hybrid catalysts were thoroughly washed after each
catalytic process.
4. Conclusions
In summary, novel 1,2,3ꢀtriazoles from alkyne derivatives of 1,3,5ꢀtriazines complex over the
surface of organoꢀfunctionalized N ꢀSBAꢀ15 and subsequent complexation with palladium
3
chloride have been fabricated by click reactions. The complexation of this ligand with PdCl
2
was performed to prepare heterogeneous catalytic systems. Modified SBAꢀ15 and Pd(0)/3Trꢀ
MꢀSBAꢀ15 were analyzed by XPS, XRD, N sorption measurement isotherm, TGA, FTꢀIR
2
and TEM. XPS and FTꢀIR spectroscopy results are consistent with the covalent attachment of
the heterocyclic organic moieties to the the surface of SBAꢀ15 as well as inside the silica
channels. TEM images showed that Pd NPs are remarkably well dispersed on the hybrid
SBAꢀ15 network the structure of which is hexagonal. The SBAꢀ15 based catalysts have high
surface area as judged by BET technique. The thermal stability of
Pd(0)/3TrꢀMꢀSBAꢀ15 was monitored by TGA. The catalytic activity of newly synthesized
heterogeneous catalyst was assessed in the reduction of pꢀnitrophenol the rate of which was
ꢀ3 ꢀ1
found to be as high as 16.8×10 s .
This work conclusively shows that the dualꢀfunctional clickꢀgenerated triazoleꢀtriazine/SBAꢀ
5 system acts as a unique hybrid platform for the in situ synthesis and immobilization of
1
palladium nanocatalysts. High surface concentration and robust attachment of the Pd NPs are
due to the central role of the dendronꢀlike structure provided by triazine and triazole
heterocycles. These miniꢀdendrons act as strong anchoring sites for the catalyst nanoparticles.
While the demonstration of proof of concept is provided herein with the catalyzed reduction
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of pꢀnitrophenol, the ternary SBAꢀ15/nitrogen heterocyle/Pd NP hybrid nanocatalyst has
certainly much to offer and could be envisaged for the catalysis of other reactions. Last but
not least, one can benefit from the strategy described above as the hybrid catalytic system can
be obtained in high yield, which anticipates the scalability of our protocol.
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Acknowledgements
AS is indebted to the Tunisian Ministry of Higher Education and Scientific Research for the
provision of travel grants to conduct research at ICMPE. All authors thank Ms Sena Hamadi
for her assistance with TGA analyses.
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
3
Surface chemical composition of the hybrid nanocatalyst and the SBAꢀ15 and N ꢀSBAꢀ15
1
3
precursors and solid state C crossꢀpolarized magic angle spinning (CPꢀMAS) NMR spectra
of hybrid materials.
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