Pd Nanoparticles Stabilized on the Cross-Linked Melamine-Based SBA-15 as a Catalyst for the Mizoroki–Heck Reaction

Article abstract of DOI:10.1007/s10562-021-03691-9

Mesoporous SBA-15 silicate with a high surface area was prepared by a hydrothermal method, successively modified by organic melamine ligands and then used for deposition of Pd nanoparticles onto it. The synthesized materials were characterized with infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), nitrogen physisorption, scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR) and inductively coupled plasma (ICP-OES). The catalyst was effectively used in the Mizoroki–Heck coupling reaction of various reactants in the presence of an organic base giving the desired products in a short reaction time and with small catalysts loadings. The reaction parameters such as the base type, amounts of catalyst, solvents, and the temperature were optimized. The catalyst was easily recovered and reused at least seven times without significant activity losses. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03691-9

Catalysis Letters
https://doi.org/10.1007/s10562-021-03691-9
Pd Nanoparticles Stabilized on the Cross‑Linked Melamine‑Based
SBA‑15 as a Catalyst for the Mizoroki–Heck Reaction
Ayat Nuri1,2 · Abolfazl Bezaatpour1 · Mandana Amiri1 · Nemanja Vucetic2 · Jyri‑Pekka Mikkola2,3 · Dmitry Yu. Murzin2
Received: 31 March 2021 / Accepted: 29 May 2021
© The Author(s) 2021
Abstract
Mesoporous SBA-15 silicate with a high surface area was prepared by a hydrothermal method, successively modifed by
organic melamine ligands and then used for deposition of Pd nanoparticles onto it. The synthesized materials were charac-
terized with infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), nitrogen physisorption, scanning electron
microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), nuclear
magnetic resonance (NMR) and inductively coupled plasma (ICP-OES). The catalyst was efectively used in the Mizoroki–
Heck coupling reaction of various reactants in the presence of an organic base giving the desired products in a short reaction
time and with small catalysts loadings. The reaction parameters such as the base type, amounts of catalyst, solvents, and the
temperature were optimized. The catalyst was easily recovered and reused at least seven times without signifcant activity
losses.
* Dmitry Yu. Murzin
dmurzin@abo.f
1
Department of Applied Chemistry, Faculty of Science,
University of Mohaghegh Ardabili, Ardabil, Iran
2
Johan Gadolin Process Chemistry Centre, Laboratory
of Industrial Chemistry and Reaction Engineering, Åbo
Akademi University, Biskopsgatan 8, 20500 Turku-Åbo,
Finland
3
Technical Chemistry, Department of Chemistry,
Chemical-Biological Center, Umeå University, 90187 Umeå,
Sweden
Vol.:(0123456789)
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A. Nuri et al.
Graphic Abstract
Keywords Mesoporous SBA-15 · Mizoroki–Heck reaction · Pd nanoparticles
diferent functional groups. In most of the cases the reac-
tion is catalyzed by palladium, nonetheless other more
abundant metals can be catalytically active under specifc
conditions making it much more environmental friendly
alternative [3, 4]. Diferent reagents cane be utilized in
Mizoroki–Heck reactions from the less reactive bromo- and
chloro-compounds on one side to non-activated alkenes on
the other, thus becoming attractive for industrial productions
[5]. Reaction can yield E and Z types of products, but usu-
ally the E product type is predominant [6–9]. It can be used
for creating the bonds within the molecule (intra-molecular)
or between diferent reactants (inter-molecularly). Initially
the inter-molecular form of the reaction was extensively
studied in the past, [10] while intra-molecular reaction was
described more lately [11].
1 Introduction
In the synthetic organic chemistry and production of fne
chemicals C–C cross-coupling reactions are highly ranked.
In generally they present a robust way for C–C bonds for-
mation and production of the diverse molecules with high
selectivity toward R–R′ compounds. Among them the Mizo-
roki–Heck reaction is considered as one of the most impor-
tant tools for coupling of aryl and vinyl halides or trifates
with the molecules possessing sp2 CH bonds such as olefns
and acrylates (Scheme 1) [1, 2]. Its popularity is connected
to its versatility toward many reactants and resistance to
There was a long debate in the literature whether homo-
geneous or heterogeneous species are catalyzing the Mizo-
roki–Heck reaction. The most commonly agreed mechanism
of this reaction comprises leaching of palladium from the
catalyst surface, its participation in the catalytic cycle and
re-deposition back after the reaction completion [12–15].
Scheme 1 Scheme of the Mizoroki–Heck reaction
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Pd Nanoparticles Stabilized on the Cross-Linked Melamine-Based SBA-15 as a Catalyst for the…
Such a mechanism requires a careful choice of the catalyst
and the reactor. Evidently, in batch processes homogene-
ous palladium catalysts can be used, however, such type
of catalysis comes along with an important issue of cata-
lyst separation and reusability, whereas recovery of noble
metals is very important in terms of costs and environment
impact. On the other hand, easy recycling of heterogeneous
catalysts is an important advantage, with a clear disadvan-
tage related to insufcient selectivity and catalyst leaching.
Development of an easily separable heterogeneous catalyst
was in the focus of many research groups and brought ver-
satile solutions [16–18]. For example, many eforts were
put on designing a heterogenized ligand system capable of
stabilizing Pd giving thereby a highly active and reusable
catalyst [19]. Besides high activity, a goal is to achieve a
maximum catalyst recovery level. This could be done by
taking an advantage of magnetic features embedded into a
palladium complex deposited on silica coatings [20].
Parallel with development of catalytically active com-
ponents various types of supports were explored. Highly
ordered mesoporous silica materials SBA-15 have a poten-
tial to be utilized in many diferent applications, catalysis
being one of the most important. The reasons for it are their
properties, such as high surface areas (700–1200 m2 g−1),
non-toxicity, chemical and physical inertness, abundance
of silanol groups capable for functionalization, along with
tunable physical properties such as the pore diameter and
walls thickness. SBA-15 possesses hydrothermal stability
due to thick and uniform silica walls (3.1 to 6.4 nm) with a
two-dimensional (2D) hexagonal (p6mm) meso-structure of
the channels and a large pore size (4.6–10.0 nm). As a conse-
quence, SBA-15 was considered as a material appropriate for
simple functionalization and tuned selectivity in catalysis,
as well as for deposition and release of pharmaceutically
active compounds in drug delivery applications [21–24]. A
typical example is design of palladium supported DHBP@
SBA-15 catalyst obtained by adding 2,2-dihydroxybenzo-
pheneone ligand to amine functionalized mesoporous silica
followed by metalation with Pd(II). This yielded a highly
active catalyst in Mizoroki–Heck (arylation) and hydrogena-
tion reactions without a signifcant loss of activity in several
reaction cycles [25].
2 Experimental
2.1 Materials
Pluronic P123, tetraethyl orthosilicate (TEOS), 3-aminopro-
pyltriethoxysilane (APTS), melamine, paraformaldehyde,
PdCl2, NaBH4, iodobenzene, ethyl acrylate and organic/
inorganic bases were obtained from Sigma Aldrich and used
without further purifcation. Other chemicals were of ana-
lytical purity grade obtained from Alfa Aesar.
2.1.1 Synthesis of SBA‑15
SBA-15 was synthesized according to the following pro-
cedure found in the literature [26]. In 30 mL of water 4.0 g
of P123 was dissolved, followed by adding the mixture to
120 mL of 2 M HCl solution. After the solution became
homogeneous, 9.0 g of tetraethyl orthosilicate was added,
and the mixture was stirred for 8 h at 40 °C. The uniform
reaction mixture was transferred into a Tefon-lined stain-
less-steel autoclave and aged at 100 °C for 24 h. After cool-
ing down to the room temperature, the product was fltered,
washed with water and EtOH, then dried overnight at 100
°C. The white solid was calcined at 560–600 °C for 6 h to
remove the template.
2.1.2 Synthesis of SBA‑15/Pr‑NH2
In order to functionalize mesoporous silica SBA-15 1.00 g of
this material reacted under refux conditions with 1.5 ml of
3-aminopropyltrimethoxysilane (6.5 mmol) in 50 ml dry tol-
uene under N2 for 24 h. The product was fltered and washed
three times with 40 mL diethylether and 40 mL toluene to
remove unreacted reagents, followed by evaporation of the
solvent giving a white powder (SBA-15/Pr-NH2) [27].
2.1.3 Cross‑Linked Melamine Functionalization of SBA‑15
To synthesize the cross-linked melamine functionalized
SBA-15 in a 4-neck round bottom fask all amino-function-
alized SBA-15 from the previous step together with mela-
mine (2.8 mmol, 0.35 g) and paraformaldehyde (22.4 mmol,
0.68 g) was added and sonicated in 20 ml of DMF under
nitrogen for 30 min. After sonication, the reaction mixture
was refuxed upon stirring for 24 h at 90 ºC under nitrogen
atmosphere. The mixture was cooled to room temperature
and then washed with DMF, distillated water and acetone.
Finally, the solid cross-linked melamine functionalized
SBA-15 was dried overnight at 60 ºC [28].
The current work discusses design and characterization
of a new catalyst derived from melamine modifed SBA-
15 loaded with Pd nanoparticles. The catalyst performance
was optimized for the Mizoroki–Heck reaction of iodoben-
zene and methyl acrylate in which it displayed high activity
and reusability. The scope was extended to diferent iodo-,
bromo- and chloro-aromatic compounds.
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A. Nuri et al.
2.1.4 Preparation of SBA‑15/Pr‑NH‑CH2‑Melamine‑Pd(0)
The reaction yield was followed by HP 6890 Series gas
chromatograph (GC) equipped with the fame ionization
detector and HP-5, 5% phenyl methyl siloxane capillary col-
umn (30.0 m, 320 µm, 0.25 µm). The injection temperature
was 280 °C while the gas fow rate was 9.5 ml min−1. The
aliquots were diluted and mixed with an internal standard
solution of hexadecane.
To deposit the metal 1.00 g of synthesized SBA-15/Pr-
NH-CH2-Melamine was frst dispersed in 10 ml EtOH at
room temperature. Into SBA-15 suspension 0.10 g of PdCl2
dissolved in 20 ml EtOH was added, followed by stirring for
24 h at room temperature. Limited solubility of palladium
chloride in the solvent required several deposition steps until
the metal precursor was completely deposited. Finally, the
solid catalyst was separated by centrifugation and washed
three times with EtOH and twice with dicholorometane to
remove the unreacted reagents, then kept overnight at 70 ºC.
Subsequently, the solid product was reduced by 25 mL of
0.1 M sodium borohydride aqueous solution to give a dark
brown mixture, separated by centrifugation, washed with
distilled water and EtOH [29]. Reduction with a chemical
reducing agent -sodium borohydride is a well-established
method for generating Pd(0) particles [30]. After drying a
grey solid was obtained containing 6.0%wt Pd according to
ICP-OES.
2.3 Mizoroki–Heck Coupling
In general Mizoroki–Heck reaction was performed by
using 1.0 mmol of iodobenzene, 1.5 mmol methyl acrylate,
1.5 mmol triethylamine (Et3N), 0.001 g SBA-15/Pr-
NH-CH2-Melamine-Pd(0) as a catalyst (0.28 mol % Pd to
aryl halide) and 1.0 ml of dimethylformamide (DMF) as a
solvent at 120 °C for 15 min. The reaction was carried out in
glass vial tubes supplied with a magnetic stirrer and heated
by an oil bath, while the reaction progress and the halide
consumption was monitored by TLC. Upon completion, the
aliquots were analyzed with GC and the catalyst was sepa-
rated from the slurry by fltration. The reaction mixture was
washed with diethyl ether and ethyl acetate. The organic
phase was separated after evaporation of the solvent under
reduced pressure and the pure product was obtained by the
plate chromatography on silica.
2.2 Characterization
Modification of SBA-15 by organics was monitored by
infrared spectroscopy (Mattson ATI Genesis FTIR Spec-
trometer). The specifc surface area and the pore size were
studied by N2 physisorption performed on Sorptometer
1900 (Carlo-Erba Instruments). The structure of SBA-15
derived material coupled with elemental analysis was inves-
tigated by scanning electron microscopy (Zeiss Leo—1530
Gemini) with a Thermo-NORAN vantage X-ray detector.
Palladium nanoparticles size distributions and SBA-15
channels dimensions were investigated by transmission elec-
tron microscopy (TEM) performed on LEO 912 OMEGA
(EFTEM) with acceleration voltage 120 kV and LaB6
electron source. Thermogravimetric analysis (TGA) was
conducted with a Linseis STA PT 1000 instrument using a
scanning rate of 10 °C min−1. The palladium content in solid
samples was determined by inductively coupled plasma opti-
cal emission spectroscopy (ICP-OES), with Optima 4300
DV optical atomic emission spectrometer (Perkin Elmer).
CP MAS 13C NMR spectrum of the catalyst was made with
a Bruker AVANCE-III HD 400 MHz spectrometer. The sam-
ple was spun at 9 kHz spin rate in a Bruker 1H broadband
double-resonance 4 mm CP MAS probe with the proton 90°
high-power pulse of 2.9 µs and the contact time of 2 ms.
The recovery delay time was 2 s and 30,000 scans were
accumulated.
3 Results and Discussion
3.1 Catalyst Synthesis and Characterization
SBA-15 was synthesized via the hydrothermal method from
TEOS and pluronic P123 polymer. After calcination, SBA-
15 was functionalized by amine groups and modifed by
melamine and paraformaldehyde under sonication. After
grafting the surface of SBA-15 by organic groups, PdCl2
was deposited on the surface and then reduced to Pd(0) by
NaBH4 giving the fnal catalyst SBA-15/Pr-NH-CH2-Mela-
mine-Pd(0) as illustrated in Scheme 2.
The FT-IR spectra for SBA-15, SBA-15/Pr-NH2, SBA-
15/Pr-NH-CH2-Melamine, SBA-15/Pr-NH-CH2-Mela-
mine-Pd(0) are shown in Fig. 1a. The peaks at 467, 812
and 1088 cm−1 in all samples are related to Si–O-Si bonds
in SBA-15. Vibrations of the silanol groups are present
at 3420 cm−1. Vibrations corresponding to NH2 bending
and hydrogen-bonding of water molecules on the surface
overlap at 1610 to 1640 cm−1. Asymmetric and symmet-
ric C–H stretching of propyl chain are present at 2882 and
2936 cm−1. After functionalization of SBA-15/(CH2)3-NH2
with melamine, three new peaks were revealed at the 1361,
1515 and 1572 cm−1 showing presence of melamine on the
surface of SBA-15 [31]. The spent catalyst exhibited essen-
tially the same vibrations as the fresh one (Fig. 1b).
Low‐angle XRD in the 2θ range 0.7–10° was conducted
with Philips X‐Pro (scanning rate 1° min−1) instrument oper-
ating at room temperature and equipped with an X‐ray radia-
tion source using Cu Kα (λ=0.15406 nm).
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Pd Nanoparticles Stabilized on the Cross-Linked Melamine-Based SBA-15 as a Catalyst for the…
Scheme 2 Synthesis of supported SBA-15/Pr-NH-CH2-Melamine-Pd(0)
Fig. 2 Solid-state 13C NMR spectrum of SBA-15/Pr-NH-CH2-Mela-
mine-Pd(0)
Fig. 1 FT-IR spectra for a SBA-15, SBA-15/Pr-NH2, SBA-15/Pr-
NH-CH2-Melamine, fresh SBA-15/Pr -NH-CH2-Melamine-Pd(0), b
reused SBA-15/Pr -NH-CH2-Melamine-Pd(0)
To confrm the structure of the organic moieties on the
surface, the solid state 13C-NMR analysis was performed, as
illustrated in Fig. 2. The sharp peak at ca. 167 ppm can be
Fig. 3 a SBA-15/Pr -NH2, b SBA-15/Pr -NH-CH2-Melamine-Pd(0)
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A. Nuri et al.
Fig. 4 SEM images of a SBA-15 and b SBA-15/Pr-NH-CH2-Melamine-Pd(0)
Table 1 The weight % and atomic % of SBA-15/Pr-NH-CH2-Mela-
mine-Pd(0)
Element
Wt (%)
Atom (%)
Carbon
41.2
13.6
38.0
5.8
0.2
1.0
48.9
13.9
33.9
2.95
0.1
0.1
0.1
Nitrogen
Oxygen
Silicon
Chlorine
Palladium
Sodium
Fig. 5 EDX analysis of SBA-15/Pr-NH-CH2-Melamine-Pd(0)
0.15
0.05
Aluminum
0.05
assigned to carbons in the aromatic chain [32]. The peak at
ca. 77 ppm is related to carbon between two nitrogen atoms
(NH-C-NH) [28]. Three signals at 46.6, 18.5 and 7.5 ppm
showed presence of the propyl chain on the silicate. The
non-hydrolyzed ethoxy group or unevaporated ethanol used
for washing after functionalization with amino and mela-
mine groups were also observed at ca. 15.5 (overlapping
with carbon of the propyl chain) and at 59.1 ppm [33].
In order to study thermal stability of the materials ther-
mogravimetric analysis of SBA-15/Pr -NH2 and the fnal
catalyst was performed (Fig. 3). Removal of physically and
chemically adsorbed water molecules is associated with a
mass loss of approx. 3.2% for both samples from ambient
temperature to 230 ºC. In total, the SBA-15/Pr-NH2 exhib-
ited 10% weight losses, whereas the diference between the
initial 3.2% and overall mass loss is approx. 6.6% and can be
attributed to the propylamine loaded onto the SBA-15, with
decomposition starting to happen above 310 ºC. In a case
of SBA-15/Pr -NH-CH2-Melamine-Pd(0) sample exhibited
26.1% of weight losses, which can be attributed to the Pro-
pyl-NH-CH2-Melamine loading on the SBA-15 in addition
to the initial water loss. It can be seen that the fnal catalyst
is thermally stable to ca. 250 ºC, well above the common
reaction temperatures.
Morphology of the samples was elucidated by SEM
analysis of SBA-15 and the fnal catalyst. Figure 4 displays
the SEM images of SBA-15 and SBA-15/Pr-NH-CH2-Mel-
amine-Pd(0) materials. As follows from SEM images that
the fresh SBA-15 has the rod shape which did not change
after functionalization and stabilization of Pd(0) on SBA-15,
indicating stability of the supporting structure during the
catalyst preparation.
A qualitative overview of the elemental composition of
SBA-15/Pr-NH-CH2-Melamine-Pd(0) was evaluated by
EDX at random points on the surface of the catalyst and is
shown in Fig. 5. The elemental measurements confrmed
the presence of all expected elements in the supported cata-
lyst. The weight % and atomic % of the elements are shown
in Table 1. They should be taken, however, with precau-
tion since the carbon content is overestimated due to the
analytical approach and a necessity to use a carbon tape to
deposit the powder sample prior analysis. The real loading
1 3

Pd Nanoparticles Stabilized on the Cross-Linked Melamine-Based SBA-15 as a Catalyst for the…
Fig. 6 TEM images of a SBA-15 and b SBA-15/Pr-NH-CH2-Melamine-Pd(0)
Fig. 8 Low-angle XRD patterns of the pristine SBA-15 and SBA-15/
Pr-NH-CH2-Melamine-Pd(0) catalyst
Fig. 7 Nitrogen physisorption of SBA-15
nanoparticles on the channels. After selecting random spots
on the TEM image of the catalyst, the average size of the
particles is found to be ca. 20 nm.
Table 2 Textural properties of the applied materials
The nitrogen adsorption–desorption isotherm (Fig. 7) was
analyzed with the Brunauer–Emmett–Teller (BET) method
for calculating the surface area of SBA-15. The BET surface
area for the non-functionalized SBA-15 was calculated to be
840 m2/g. After functionalization with organic groups and
deposition of palladium nanoparticles on mesoporous silica,
the surface area was diminished to 575 m2/g, while the pore
volume decreased from 1.10 to 0.14 cm3 g−1 confrming
that the organic ligands and Pd nanoparticles are located in
the pores of SBA-15. The Brunauer–Emmett–Teller (BET)
surface areas and pore volumes of the SBA-15 and the fnal
catalyst are summarized in Table 2.
Sample
Pore volume (cm3 g−1
)
SBET (m2 g−1
)
SBA-15
Catalyst
1.10
0.14
840
575
of palladium in the catalyst was determined by ICP-OES dis-
playing a much higher value, than obtained by EDX analysis.
Nevertheless, EDX is still a valuable verifcation tool.
To gain more information on the structure of the catalyst
and Pd size distribution TEM imaging was done. Figure 6
displays the images of SBA-15 and SBA-15/Pr-NH-CH2-
Melamine-Pd(0) catalyst. Both materials exhibit uniform
mesopores with a long-range order of a hexagonal array
characteristic for SBA-15 materials. The dark spots in the
image of SBA-15/Pr-NH-CH2-Melamin-Pd(0) with 20 to
50 nm diameter, can be attributed to the agglomerated Pd(0)
The low-angle XRD spectra for SBA-15 and SBA-15/Pr-
NH-CH2-Melamine-Pd(0) are shown in Fig. 8. For SBA-15,
the three well-determined difraction peaks are seen at 2θ of
ca. 0.9–1.9°. Both synthesized materials exhibited a single
strong peak (100) in XRD pattern followed by two additional
1 3

A. Nuri et al.
Table 3 Optimization of
conditions for the Mizoroki–
Heck reaction
Entry
Solvent
Pd (mol %)
Base
Temperature Time (min)
(°C)
Yield (%)c
1
2
3
4
5
6
7
8
DMAC
DMSO
DMF
NMP
H2O
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.56
0.84
1.12
Free
0.28
0.28
0.28
0.28
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
K2CO3
CsNO3
Cs2CO3
HMTAb
Et3N
DBU
NaOAc
Free
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
80
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
15
15
15
15
15
15
15
15
15
6.3
4.3
7.9
0.0
0.0
0.0
0.0
2.5
80
80
80
80
80
80
80
80
Dioxane
EtOH
CH3CN
Acetone
Toluene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
9
1.1
0.0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
80
100
100
100
100
100
100
100
100
100
100
100
100
100
60
trace
trace
trace
11.5
77.5
trace
6.8
trace
48.4
50.6
51.1
52.4
0.0
80
85
˃99
˃99
80
120
140
aReaction conditions: iodobenzene (1 mmol), methyl acrylate (1.5 mmol), solvent (1 ml), base (1.5 mmol)
and SBA-15/Pr-NH-CH2-Melamine-Pd(0) as catalyst
bHexamethylenediamine
cGC yield
peaks (110, 200) which can be ascribed to a hexagonal lat-
tice. Compared with SBA-15, the peak intensity of the fnal
catalyst is weaker, because of a poorly ordered mesophase
after functionalization of the Pr-Melamine groups inside the
channels.
iodobenzene, due to its oxidative potential [35]. In the frst
set of experiments performed at a relatively low reaction
temperature the optimal solvent was determined (entry
1–10). Polar aprotic solvents showed to be most efective in
the reaction, out of which dimethylformamide (DMF) gave
the best results and 7.9% yield. In general, the polar aprotic
solvents such as DMF could lead to catalyst leaching giving
thereby a higher activity [36]. With the optimal solvent and
at a slightly higher temperature diferent organic and inor-
ganic basses were tested (entry 11–18). Low solubility of
inorganic basses in DMF makes them unsuitable for systems
studied in the current work, while triethylamine led to a fully
homogenous reaction slurry and gave the highest yield of the
coupling product (entry 15, 77.5%) [37].
3.2 Optimization of reaction conditions
For optimizing the reaction conditions the Mizoroki–Heck
reaction of iodobenzene and methyl acrylate was used as a
model reaction. Variation of most important reaction param-
eters such as the choice of solvents, base type, the catalyst
amount and reaction temperature gave the results presented
in Table 3. The initial reactants ratio with an excess of
methyl acrylate was adopted in order to make up for the
loss of the reactant due to evaporation [34], also avoiding
palladium leaching caused by the presence of unreacted
In addition to an improvement in the reaction rates utili-
zation of triethylamine is benefcial for Pd re-precipitation
[35]. In order to estimate an adequate amount of the catalyst
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Pd Nanoparticles Stabilized on the Cross-Linked Melamine-Based SBA-15 as a Catalyst for the…
capable of catalyzing the reaction within a reasonable time
span, diferent catalyst loadings and reaction time were
elucidated (entry 19–23). A minor increase in conversions
was detected when the loading was increased, indicating
that even small amounts of Pd are sufcient. In fact, lower
loadings of Pd were claimed to be benefcial for improv-
ing the catalyst stability because the excess of Pd can lead
to catalyst deactivation through Pd nanoparticles growth
and agglomeration [38]. The catalyst loading of 0.28 mol
% of palladium (entry 19, 48.4% yield) gave satisfactory
results and was further used for studying the infuence of
diferent reaction temperatures (entry 24–27). As expected,
a temperature increase afected positively the reaction rate,
most probably because of the aril halide activation [39]. In
the current case at 120 °C the full conversion was achieved
within 15 min (entry 26, ˃99% yield).
ca. 5% of Pd leached into the reaction mixture. TEM analy-
sis of the spent catalyst after fve cycles revealed presence
of Pd nanoparticles and confrmed catalyst stability under
the studied conditions (Fig. 9b). In addition, agglomeration
of the nanoparticles was detected which potentially can be
another reason for a subsequent decrease in activity. After
selecting random spots on the TEM image of the reused
catalyst, the average size of the particles was found to be
approximately 27 nm.
Comparison of the results in Table 5 for diferent reported
catalysts used in the cross-coupling reaction of iodobenzene
and methyl acrylate illustrates high activity of the cata-
lyst developed in the current work. SBA-15/Pr-NH-CH2-
Melamine-Pd(0) catalyst exhibited a remarkable activity
with turnover frequency of 1429 h−1 which is a signifcant
improvement compared to the already reported melamine
based catalysts (entries 8–10) [45–47]. Benchmarking the
current study with the literature examples can be difcult
since the model reaction is often not the same. By consider-
ing analogous catalysts with melamine originating auxilia-
ries, such as SBA-15/CCMet/Pd(II) [48], Pd-MPMO [49],
Pd/G1,2-SBA-15 [50] it can be, nevertheless, stated that in
comparison with the literature reports, the current study is
much more comprehensive covering the most important var-
iables such as temperature, catalyst amounts, solvents and
bases. The synthesized catalyst enabled selective process-
ing of a vide scope of the reactants, including low reactive
compounds such as non-activated bromoarenes with the use
of low palladium loadings and a short reaction time, making
it much more economically and environmentally desirable
option.
After optimizing the reaction conditions, the scope of the
reaction was examined by applying the supported catalyst for
the Mizoroki–Heck coupling reaction with a wide range of
aryl halides (Cl, Br and I) and olefns. The results are listed
in Table 4 containing also as a reference the melting points
of obtained products and their comparison with the literature
data. As can be seen from Table 4, although the palladium
loading was as low as 0.28 mol %, excellent yields were
obtained for the examined aryl iodides and bromides (entries
1 to 11). Both, activated and non-activated halides were ef-
ciently utilized. Typically, aryl chlorides are less reactive
than other halides. The same behavior was also observed
in the current work under the optimal reaction conditions
(entries 12 and 13). Therefore, it was concluded that the
catalyst can efciently promote the coupling reaction of aryl
iodides and bromides with diferent olefns.
The reaction mechanism for SBA-15/Pr-NH-CH2-Mela-
mine-Pd(0) seems to follow the one recently proposed in the
literature when after coordination of the olefn to the sup-
ported Pd(0), the base (Et3N) attacks the alkene coordinated
complex to give an alkyl σ-complex. The aryl halide then
oxidizes the Pd(0) center, the attached base is released and
the alkene coordinates to the Pd(II) complex. Subsequent
hydride elimination and removal of hydrogen halide give
the desired product and regenerate the catalyst to Pd(0) [56].
One of the most important features of heterogeneous
catalysts is their recyclability. In order to estimate stabil-
ity of supported SBA-15/Pr-NH-CH2-Melamine-Pd(0) for
the Mizoroki–Heck reaction the performance of the recy-
cled catalyst under optimized conditions in the reaction of
iodobenzene and methyl acrylate was tested. Each reaction
cycle was performed for 15 min followed by quenching the
reaction by cooling to the room temperature, separating the
catalyst by fltering and fnally fushing several times the
residue with diethyl ether and ethyl acetate. Activity of the
catalyst was maintained for at least fve cycles without a drop
in conversion, thereafter a slight decrease was observed still
showing high activity for up to the seventh cycle (Fig. 9a).
Analysis of the spent catalyst after fve reaction cycles also
indicated the catalyst stability. FT-IR spectrum of the reused
catalyst shown in Fig. 1b confrms stability of the catalyst
structure exhibiting also a sharp peak at 2936 cm−1 related
to solvents trapped in the catalyst.
4 Conclusions
In summary, an efcient catalyst for the Mizoroki–Heck
reaction has been developed. The catalyst comprising Pd
nanoparticles deposited on melamine-grafted mesoporous
SBA-15 was synthesized and characterized by FT-IR, NMR,
TGA, ICP-OES, nitrogen physisorption, XRD, TEM and
SEM coupled with EDX. Its performance was studied in the
reaction of iodobenzene and methyl acrylate while optimiz-
ing the reaction conditions. The usability of the catalyst was
demonstrated for a broad scope of the reactants, displaying
A small decrease in conversion in the following cycle
presumably can be connected with Pd leaching. More pre-
cisely, in the frst cycle ca. 8% of Pd and in the second cycle
1 3

A. Nuri et al.
X
R'
Table 4 The Heck reaction
of aryl halides with olefns
catalyzed by the supported
catalyst
Cat., Et3N
+
R'
DMF, 120°C
R
R
m. p. (°C)
Found Ref.
Aryl
halide
O
Time Yield
(min) (%) b
Entry
1
Olefin
Product
O
O
O
O
Oil
Oil
Oil[41]
80
15
80
96
99
98
O
O
Br
O
O
2
3
Oil[42]
O
I
O
O
O
O
108-
110
105-
108[43]
O
Br
O
O
O
O
O
89-91 88-89[42] 15
99
99
99
O
4
5
6
O
I
O
O
145-
147
148-
85
25
150[44]
Br
O
I
135-
137
135-
137[43]
O
118-
120
118.5-
7
8
35
30
96
99
121.5[44]
I
O
O
Oil[42]
O
Oil
O
Br
Br
9
71-73 70-71[44] 75
37-39 36-38[42] 30
96
99
96
19
16
O
O
O
10
11
12
13
O
Br
Br
Cl
Cl
118-
120
118.5-
45
121.5[45]
O
O
37-39 Oil[42]
25c
25 c
O
O
118-
120
118.5-
121.5[45]
aReaction conditions: aryl halide (1.0 mmol), olefn (1.5 mmol), solvent (1 mL) and base (1.5 mmol),
0.28 mol % Pd, reaction temperature 120 °C
bGC yield
cTime (h)
1 3

Pd Nanoparticles Stabilized on the Cross-Linked Melamine-Based SBA-15 as a Catalyst for the…
Fig. 9 a Catalyst reusability study in the reaction of iodobenzene and methyl acrylate under the optimal conditions. b TEM image of the spent
catalyst used in fve reaction cycles
Table 5 Comparison of activity
of diferent catalysts in the
cross-coupling reactions of
iodobenzene with methyl
acrylate
Entry
Catalyst
Time (h)
T (ºC)
Yield (%)
TOF (h−1 b
)
1
2
3
4
5
6
7
8
Pd/C [34]
Pd(OAc)2 (ligand free) [51]
Palladacycle [52]
0.5
1.5
0.3
24
24
0.5
2
3
8
0.5
0.25
140
80
100
91
100
37
99
90
99
92
96
98
714
60
602
0.3
12
180
198
13
140
100
110
100
120
110
100
100
120
Pd/C particles [53]
Pd(nano)/MNP@IL-SiO2 [54]
Pd-DABCO-γ-Fe2O3 [55]
MNP@SiO2 @NHC–Pd(II) [56]
Pd@MFR1 [47]
9
10
11
MNPs-Mel-Pd [45]
3
MNPs-Mel-Pd [46]
158
1429
SBA-15/Pr-NH-CH2-Melamin-Pd(0)a
˃99
aThis study
bTOF=moles of reactant converted/(moles of palladium x time (h))
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
its efciency in coupling iodo- and bromo-aromatics with
diferent olefns, while its high activity enabled application
of small amounts of Pd and short reaction times. The mate-
rial was possible to re-use for at least seven consecutive
cycles preserving its high activity.
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