Synthesis of hybrid interfacial silica-based nanospheres composite as a support for ultra-small palladium nanoparticle and application of PdNPs/HSN in Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.jpcs.2017.07.012

The silica based hollow nanosphere (silica-HNS) containing polymer of polyaniline was synthesized and chosen as a promising support for Pd_NPs. Then it was applied as a green catalyst in the reaction of Heck coupling with high yield. TEM and SEM-EDX/mapping images were used to study the structure and morphology. FT-IR spectroscopy, Thermal gravimetry analysis (TGA), and BET were used to characterize and investigate the catalyst. Also, the amounts of Pd loading were characterized by ICP-AES technique. Catalyst recyclability showed 5 successful runs for the reaction.

Full text of DOI:10.1016/j.jpcs.2017.07.012

Accepted Manuscript
Synthesis of hybrid interfacial silica-based nanospheres composite as a support for
ultra-small palladium nanoparticle and application of Pd
reaction
/HSN in Mizoroki-Heck
NPs
Sadegh Rostamnia, Saba Kholdi
PII:
S0022-3697(17)30939-3
10.1016/j.jpcs.2017.07.012
PCS 8130
DOI:
Reference:
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 29 May 2017
Revised Date: 9 July 2017
Accepted Date: 15 July 2017
Please cite this article as: S. Rostamnia, S. Kholdi, Synthesis of hybrid interfacial silica-based
nanospheres composite as a support for ultra-small palladium nanoparticle and application of Pd
HSN in Mizoroki-Heck reaction, Journal of Physics and Chemistry of Solids (2017), doi: 10.1016/
j.jpcs.2017.07.012.
/
NPs
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis of hybrid interfacial silica-based nanospheres composite as a support for ultra-
small palladium nanoparticle and application of PdNPs/HSN in Mizoroki-Heck reaction
Sadegh Rostamnia, * Saba Kholdi
Organic and Nano Group (ONG), Department of Chemistry, Faculty of Science, University of
Maragheh, P.O. Box. 55181-83111, Maragheh, Iran. Fax: +98(421)2276066; Tel:
+98(421)2278001(108); E.mail: rostamnia@maragheh.ac.ir; srostamnia@gmail.com
Abstract:
The silica based hollow nanosphere (silica-HNS) containing polymer of polyaniline was
synthesized and chosen as a promising support for PdNPs. Then it was applied as a green catalyst
in the reaction of Heck coupling with high yield. TEM and SEM-EDX/mapping images were
used to study the structure and morphology. FT-IR spectroscopy, Thermal gravimetry analysis
(TGA), and BET were used to characterize and investigate the catalyst. Also, the amounts of Pd
loading were characterized by ICP-AES technique. Catalyst recyclability showed 5 successful
runs for the reaction.
Keyword: Mesoporous; PdNPs/HSN; Mizoroki-Heck reaction; Green chemistry.
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1. Introduction
Palladium-based catalysts are extensively studying due to their inherent ability in transition metal
catalyzed transformations such as insertion, oxidative addition and reductive elimination which
leads to many kinds of cross-coupling reactions such as Mizoroki-Heck reaction of organohalides
and activated alkenes [1-3]. However, the traditional homogeneous catalysis in this area has
reinforced the problems of product purification, palladium isolation and recyclability, and
aggregation of Pd(0) and therefore its deactivation which in practice, they totally lead to an
inconvenient catalyst specifically in pharmaceutical industry and from point view of green
chemistry. Over the past few decades, since the global concerns for environmental issues have
been increased, demands and tendencies for the green or greener protocols and systems to
develop recoverable and reusable catalysts and detoxified metals in the reaction medium have
also been raised. Efficient palladium supported catalysts have had a major impact on this area.
Various inorganic and organic supports such as porous silica [4], carbon [5] and organic
polymers [6] based materials have been explored to design and identify the eco-friendly protocols
for cross-coupling reactions. Specifically, low palladium loadings [7], phosphine-free [8] and
recyclable catalyst systems have all been demonstrated as the suitable conditions reaction
catalysis. Among various supports, nanoporous materials have been utilized in numerous diverse
designs and systems which can be attributed to sort of mesopores which are distinguished by
their structures, grafted ligands and metals [9]. However, concerns and efforts on providing a
highly efficient and greener catalytic system for these reactions are still under consideration.
Hence, we embedded polyaniline (PANI) within the walls of silica hollowed nanospheres (HNS)
with assembly method to obtain a highly efficient and hybrid polymer-inorganic HNS. Our
investigations disclosed PANI/HNS act as a support for Pd2+ ions which produces uniform and
available Pd nanoparticles supported onto the surface of PANI/silica-HNS. Currently, PANI-
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silica gel composites are developed and engaged for analytical applications [10]. There are also
some reports for PANI-based mesoporous nanocomposites [11] with advanced applications.
However, these materials suffer from lower surface area, mechanical properties, and decreasing
pore sizes. Embedding PANI inside the walls of silica can be more useful to solve these
problems. Therefore, we developed the synthesis of PANI/silica based HNS (PANI/silica-HNS)
by a one-pot method. And its application and catalytic activity in the Mizoroki-Heck coupling
reaction. In continues of our recent reports in mesoporous field [12], we report synthesis of
PdNPs@PANI/silica-HNSs and its catalytic application in Mizoroki-Heck reaction.
2. Experimental
Materials were of analytical grade and used as received without any further purification. Triblock
copolymer EO106PO70EO106 (F127), tetramethyl orthosilicate (TMOS) and polyaniline
(emeraldine base) average Mw ~5,000 were purchased from Sigma-Aldrich Company Ltd. Other
reagents were obtained from Merck (Germany) and Fluka (Switzerland) and were used without
further purification. All organic product yields were obtained as isolated method or by gas
chromatography (GC). GC analyses was recorded on GC Chrom (Teif Gostar Faraz Co., Iran).
Scanning electron microscopy (SEM/EDX/mapping) images was recorded on Zeiss-DSM 960A
microscope microscope. Transmission electron microscopy (TEM) images were recorded on
Zeiss EM 900 electron microscope. IR spectra were recorded on Shimadzu IR-460 spectrometer,
and absorbents are reported in cm-1.
Synthesis of PANI/silica-HNS
PANI/silica-HNS was synthesized by dissolving F127 (1 g) and K2SO4 (3.49 g), in 60 mL H2O
o
and 1 g mesitylene which were vigorously mixed together for 3 h at 13.5 C. Then, in another
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glass, 0.106 g of PANI was added to 4 mL of DMF and allowed to stir for 30 min and the then,
3.04 g TMOS was added to stir for next 30 min under the same conditions. Finally, the latter was
to former glass and allowed to stir for subsequent 24 h at 13.5 oC. The obtained solid was sealed
in autoclave and allowed to age for 24 h at 100 oC. Afterwards, reaction mixture was filtered and
washed with methanol and water. The template was extracted by ethanol for 24 h in soxhlet.
Synthesis of Pd(OAc)2@PANI/silica-HNS and PdNPs@PANI/silica-HNS
1 g of PANI/silica-HNS was dispersed in CH3CN and 0.02 g of Pd(OAc)2 in CH3CN at room
temperature was added to PANI/silica-HNS and allowed to stir for 2 h time. Finally, the reaction
mixture was filtered the obtained was washed with methanol for three time to gain pure
Pd(OAc)2@PANI/silica-HNS. Then, 5 mL of 0.2 mol/L of freshly prepared NaBH4 (in methanol)
was added to mixture of prepared Pd(OAc)2@PANI/silica-HNS in 50 mL methanol during 15
min. Finally, PdNPs@PANI/silica-HNS collected by simple filtration. The amounts of Pd loading
was 4.7 w% characterized by ICP-AES technique.
General procedure for synthesis of trans-alkenes
In a typical method, 1 mmol of bromobenzene, 1 mmol of methyl acrylate, 0.04 g of
PdNPs@PANI/silica-HNS, and 2 mmol of Et3N were added to 5 mL of DMF:H2O (1:1) and
allowed to stirrer at 130 ºC. The reaction completion was monitored by TLC (n-hexane and ethyl
acetate mixture as solvent-5:1). After completion of the reaction, the reaction was cooled to room
temperature, the catalyst was removed by filtration. The catalyst was then washed with Et2O (3 ×
5 mL). The organic layer was separated and dried over anhydrous Na2SO4. The solvent was
evaporated under reduced pressure to give the corresponding aryl-olefins. To optimize the
reaction conditions, GC was used to investigate the yields under each condition. The NMR
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spectroscopic data of known compounds were found to be identical with those reported in the
literature [12].
3. Results and discussion
Synthesis of PdNPs@PANI/silica-HNS
For the synthesis of PANI/silica-HNS, F127 was used as a soft template and TMOS as a silica
source. Co-assembly of PANI with silicate in the presence of F127 under the ambient conditions
were performed. PANI was entrapped into silica shell using dissolving and entrapping PANI
during the hydrolysis of TMOS to synthesize hybrid PANI/silica based HNS (PANII/silica-
HNS). Then the surfactant was extracted from synthesized HNS. Finally, Pd(OAc)2 was added to
dispersion of PANI/silica-HNS in CH3CN to support the Pd nanoparticles (NP) in which the Pd2+
ions were reduced to PdNPs by methanolic NaBH4. A schematic pathway to synthesize
PANI/silica-HNS is indicated in Scheme 1.
Scheme 1. General synthesis pathway of PdNps@PANI/silica-HNS.
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Characterization of PdNPs@PANI/silica-HNS
To demonstrate the final structure of PANI/silica-HNS, FTIR, TGA, BET/BJH, EDAX, and
transmission electron microscopy (TEM) were used. FT-IR spectra provided evidence of
interaction between silica-HNS, PANI and PdNPs in the composite as shown in Fig. 2. In the FT-
IR spectrum of PANI/silica-HNS, the bands at 1580, 1460, 1300 and 1130 cm-1 arise from the the
C=C stretching of the quinoid and benzenoid ring, C-N stretching of the benzenoid and quinoid
unit, respectively. A Si-OH stretching band can also be found in the region of 920-830 cm−1
[12c]. A peak at 1080 cm-1 can be attributed to stretching of Si-O-Si bonds. The spectrum of
PdNPs@PANI/silica-HNS hybrid exhibited almost the same vibrational bands as PANI/silica-
HNS, but some peaks of PANI shifted to a higher wave number (1580, 1460 and 1300 cm-1) that
this result indicated the interaction between component of PdNPs@PANI/silica-HNS
nanocomposite. In the all samples, a broad silanol band in the region of 3400-3600 cm−1 is
appeared.
Figure 2. FT-IR analysis of PANI/silica-HNS (black) and PdNPs@PANI/silica-HNS (red).
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Thermal gravimetric analysis (TGA) was also performed to study the thermal behavior and for
measuring the amount of entrapped PANI inside the walls of PdNPs@PANI/silica-HNS.
According to this study, three weight losses were observed in this analysis. The first one at
temperature below 100 C is assigned to removal of physically adsorbed water and methanol or
acetonitrile solvents. The weight loss between 100 to 250 C attributed to elimination of
remained F127 surfactant template and non-entrapped PANI. According to this study, more than
o
9% weight loss between 350-800 C can be attributed to organic species which proves the
presence of entrapped PANI (Fig. 2).
Figure 2. Thermal gravimetric analysis (TGA) of PdNPs@PANI/silica-HNS.
The energy dispersive X-ray (EDAX) analysis showed the signals of silicon, oxygen, carbon,
nitrogen and palladium for the PdNPs@PANI/silica-HNS material. These analyses successfully
confirm well incorporation of PANI, silica-HNS and palladium species into/onto material
composite. Further, the EDX-mapping images is conducted with the PdNPs@PANI/silica-HNS
sample and the results are shown in Figure 3. Figure clearly showed that the PdNPs@PANI/silica-
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HNS solid is possessed carbon, nitrogen (from PANI) indicated the sample is contained with
silica, and also mapping image for Pd (L) indicates that Pd has been successfully supported to the
PANI/HNS (Fig.3).
Figure 3. EDX of PdNPs@PANI/silica-HNS solid (a) and its related mapping image(b).
TEM images of PANI/silica-HNS and PdNPs@PANI/silica-HNS were shown in Fig. 4 (a and b).
The results reveals that palladium nanoparticles were successfully incorporated and dispersed on
the PANI/silica-HNS matrix. PANI/silica-HNS composite indicated hollow sphere-shaped (HNS)
silica in nanoparticles format called as hollow nanospheres (HNS) with average particle size of
nearly 21 nm (Fig. 4a). Moreover, PdNPs@PANI/silica-HNS composite evidently showed
particles of palladium with spherical shape onto the PANI/HNS matrix with the average particle
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size of 3.5 to 6 nm (Fig. 4b, c). The particle size histogram of Pd nanoparticle) over the matrix
shows that particle size ranged from 3.5 nm to 6 nm (Fig. 4c). The estimated average particle size
of deposited Pd nanoparticles by TEM and its particle size histogram from the Binary-TEM is
around 4.5 nm (Fig. 4c,d).
Figure 4. TEM images of PANI/silica-HNS (a) and PdNPs@PANI/silica-HNS (b) and Binary-
TEM (d) and particle size histogram of PdNPs@PANI/silica-HNS (c).
Nitrogen adsorption/desorption isotherm (BET analysis) of each PANI/silica-HNS (a-red) and
PdNPs@PANI/silica-HNS (a-blue) are indicated in Fig 5. Accordingly, by increasing the amount
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of loaded Pd, the surface area decreases from 277 to 238 m2.g-1 for PANI/silica-HNS and
PdNPs@PANI/silica-HNS, respectively. BJH pore size distribution analysis of PANI/silica-HNS
shows a pore diameter 21 nm. Adsorption-desorption isotherm truly showed that the both
structures of PANI/silica-HNS and PdNPs@PANI/silica-HNS are mesoporous and the other BET
analysis exhibited the surface area of 277 and 238 m2.g-1 which is maintained after Pd
nanoparticle deposition.
Figure 5. N2-adsorption/desorption isotherm of PANI/silica-HNS (a-red) and
PdNPs@PANI/silica-HNS (a-blue). BJH pore size distribution of PANI/silica-HNS (c).
Catalytic performance of PdNPs@PANI/silica-HNS
Mizoroki-Heck cross-coupling reaction was selected in investigating the catalytic performance of
PdNPs@PANI/silica-HNS based on this reaction importance in chemistry, industrial and
pharmacy [13]. In the case of Heck reaction, many different parameters were studied by the
model reaction of bromobenzene and methyl acrylate to observe the catalytic behavior. Based on
this study, solvent has a major effect on catalytic activity. In this regard, polar solvents were more
effective in the catalytic efficiency. Hence, the catalytic activity was excellent in the presence of
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H2O:DMF solvent [H2O:DMF (5 mL, 1:1)] Therefore, we chose the mixture of water and DMF
medium as a green, optimal, and alternative solvent (Fig.6).
Figure 6. The solvent effect on the progress of Heck coupling reaction (reaction conditions:
bromobenzene (1 mmol), methyl acrylate (1 mmol), Et3N (2 mmol), and Catalyst (40 mg of Cat.)
and 5 mL solvent in 3 h).
On the other hand, various temperatures and base were monitored to observe the catalytic activity
of PdNPs@PANI/silica-HNS over the model reaction. This study showed that the maximum
catalytic activity is shown at 130 oC and higher than that, no significant change was observed in
the reaction progress. To obtain better results, KOH, Et3N, Na2CO3 and K2CO3 (2 eq) were tested
as the base in the model reaction of Heck using PdNPs@PANI/silica-HNS. This study showed that
the maximum catalytic activity is shown for KOH and Et3N, but in case of KOH the reaction had
the highest leaching and Pd-black was also observed in this catalyst. In finding an ideal duration
for reaction completion, the reaction progress was monitored in each half an hour and the kinetic
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of the reaction was also depicted in the Fig. 7b. According to this plot, the reaction, the reaction
rate is initially slow and then the rate increases along the progress and finally, the reaction
progress stops after 3 h. Therefore, no significant change was observable after 3 h (Fig. 7b).
Figure 7. The effect of base (a) and time effect (b) on the progress of Heck reaction.
Various amounts of PdNPs@PANI/silica-HNS in terms of Pd mol% was examined in the model
reaction to reach an optimal amount of catalyst. According to this study, the ideal amount of
catalyst was 40 mg per each mmol of substrate which equals to 1.76 mol% Pd (Fig. 8).
Figure 8. The effect of catalyst amount on Heck reaction.
After a careful and successful catalytic study on the Heck coupling of bromobenzene with methyl
acrylate, and finding out that the catalyst can proceed the reaction. Under the optimized
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conditions, the reaction was carried out under basic conditions (by Et3N) in the presence of 40 mg
of the catalyst at 130 oC and 5 mL DMF/H2O (1:1) as solvent.in 3h, we turned to test its catalytic
power on different haloaryls and active alkenes. In this part, we found our proposed catalyst
active in every single derivatives of aryl bromides and aryl iodides. Because every derivative had
high to excellent yield, there was no distinguishable difference among derivatives. However, aryl
iodides had better results in yields under the similar conditions (Table 1).
Table 1. Synthesis of alkene derivatives under the optimized conditions. a
Time/Conv.
Entry Ar-
R
-Hal
Yield (%)
(h/%)
2.5/97
3/96
3/97
3/98
3/94
3/93
3/90
Ph-
1
CO2Me
CO2Et
CO2Et
CN
CO2Me
CN
-I
-I
-I
-I
-I
95
95
96
96
Ph-
2
3-NO2-Ph-
3-NO2-Ph-
4-Me-Ph-
4-MeO-Ph-
Ph-
3
4
5
6
91
-I
-I
85/7 (trans/cis)
Ph
7
88
Ph-
Ph-
Ph-
8
9
10
11
CO2Me
CO2Et
Ph
-Br
-Br
-Br
-Br
2.5/92
3/91
4/88
90
90
73/11 (trans/cis)
91
4-MeCO-ph CO2Me
3/94
Reaction conditions: haloaryl 1 mmol, 1 mmol of alkene, 40 mg of Pd, and 2 mmol of Et3N at
130 ºC in 5 mL of H2O:DMF (1:1).
For Heck reactions, we recovered the catalyst and reused it in the same reaction with the same
reaction conditions. In this study, all the reaction conditions were under the optimized parameters
except the catalyst in which was recovering and reusing at the end after each reaction cycle.
Accordingly, the kinetics of reaction in each cycle was similar to first run. However, small
decrease on the product yields happened after five consecutive cycles which is natural due to
small loss of catalyst in each recovery (Fig. 9). The stability of the catalyst was also investigated
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by the BET study of the reused catalyst (after 5th reaction cycle) in comparison of initial
mesoporous PANI/silica-HNS. These studies showed that the porosity of solid destroyed after 5th
cycle (Fig. 9b).
Figure 9. Recyclability of PdNPs@PANI/silica-HNS for model reaction of Heck (a) and BET of
recovered catalyst after 5 run (b-red).
Conclusion
In conclusion, we herein synthesized a new polymer-inorganic hollow nanospheres (PANI-silica-
HNS) for supporting and stabilizing of nanoparticles of Pd and developed it as a new efficient
and promising green catalyst catalytic system in the synthesis of trans-alkene derivatives which
were produced from Mizoroki-Heck cross-coupling reaction. In addition, low Pd leaching and
therefore low toxicity, ease of separation and work-up, and high yields and conversions were the
other scopes of this catalyst.
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Research highlights
► Synthesis of hybrid interfacial silica-based nanospheres.
► Hollow nanospheres as advanced support for Pd.
► PdNPs/HSN as green and interfacial nanocatalyst for Mizoroki-Heck reaction.