H. Veisi et al. / Journal of Molecular Catalysis A: Chemical 382 (2014) 106–113
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2.4. Preparation of SWCNT-Met/Pd2+and SWCNT-Met/Pd0
nanocatalysts
the stability, recyclability, and catalytic activity of heterogeneous
Pd nanocatalysts. Owing to these interests and also as a part of our
ongoing research program on the application of catalysts for the
development of useful new synthetic methodologies [62], herein,
we report the synthesis of a heterogeneous palladium nanocatalyst
supported on metformine-grafted-SWCNTs (SWCNT-Met/Pd(II))
and its catalytic activity of the prepared catalyst was investigated
by employing Suzuki–Miyaura coupling reaction as a model reac-
tion. The result showed that the new catalyst retains the reactivity
characteristic of a homogeneous catalyst but at the same time it
was easy to separate off and reuse.
The SWCNT-Met (1 g) were dispersed in 30 mL acetonitrile by
sonication for 10 min and a aqueous solution of PdCl2 (10 mL,
0.6 mmol) was added to dispersion of SWCNT-Met. The mixture
subjected to centrifugation, washed with acetonitrile and DI water
and dried in vacuum at 40 ◦C for 12 h. The overall synthesis of
SWCNT-Met/Pd2+ nanocatalyst is schematically demonstrated in
Scheme 1.
The reduction of SWCNT-Met/Pd2+ by hydrazine hydrate was
performed as follows: 30 mg of SWCNT-Met/Pd2+ was dispersed in
added. The pH of the mixture was adjusted to 10 with 25% ammo-
nium hydroxide and the reaction was carried out at 95 ◦C for 2 h.
The final product SWCNT-Met/Pd0 was washed with water and
dried in vacuo at 50 ◦C. Scheme 1 depicted the synthetic proce-
dure of SWCNT-Met/Pd2+ and SWCNT-Met/Pd0. The concentration
of palladium in SWCNT-Met/Pd2+ and SWCNT-Met/Pd0 were 19
and 17 wt.%, respectively, which were determined by ICP-AES and
TGA.
2. Experimental
2.1. Materials
All the reagents were purchased from Aldrich and Merck used
without any purification. The pure SWCNTs without functional
groups were purchased from Petrol Co. (Tehran, Iran) (Fig. 1).
SOCl2, HCl, H2SO4, HNO3, H2O2 (30 wt.%, aq), deionized water, NaH
(80%), anhydrous dimethylformamide (DMF), CaH2, and metformin
hydrochloride were obtained from Sigma Aldrich and Merck.
2.2. Preparation of free metformin
2.5. Catalytic activity
1.65 g of metformin hydrochloride (10 mmol) and 0.40 g of
NaOH (10 mmol) were added to 100 ml of ethanol and the result-
ing suspension was stirred for 5 h. Then, the suspension was filtered
and ethanol was removed with rotary evaporation leading to free
metformin in 99% yield. The obtained free metformin was freshly
used in the next experiments.
2.5.1. Suzuki–Miyaura coupling reaction
In a typical reaction, 10 mg of the catalyst (1 mol%) was placed
in a 25 mL Schlenk tube, 1 mmol of the aryl halide in 5 mL
of water/ethanol (1:1) was added 0.134 g (1.1 mmol) of phenyl
boronic acid, 0.276 mg of K2CO3 (2 mmol). The mixture was then
stirred for the desired time at 50 ◦C. The reaction was monitored by
thin layer chromatography (TLC). After completion of reaction, the
reaction mixture was cooled to room temperature and the cata-
lyst was recovered by centrifuge and washed with ethyl acetate
and ethanol. The combined organic layer was dried over anhy-
drous sodium sulfate and evaporated in a rotary evaporator under
reduced pressure. The crude product was purified by column chro-
matography.
Pristine CNTs (p-CNTs) were refluxed under stirring in the
mixture of 1:3 (v/v) HNO3 and H2SO4 at 70 ◦C for 30 h, which
was followed by centrifugation and repeated washings with DI
water [63]. The carboxylated CNTs (CNTs-COOH) thus obtained
were dried at 60 ◦C for 1 day under reduced pressure and reacted
with excess of SOCl2 at room temperature for 24 h. The acylated
CNTs (CNTs-COCl) were separated by centrifugation, subsequently
washed with anhydrous THF to remove excess of SOCl2, and dried
in vacuum at 50 ◦C for 12 h. The final product was then subjected to
functionalization with metformine. Free metformine (metformine-
to-SWCNT weight ratio was 10:1) were mixed with 1 mL solution
of DMF and NaH (80%) and then stirred for 1 h. The obtained acyl
chloride SWCNTs in 20 mL DMF were then added to the suspension.
The reaction mixture was kept at 120 ◦C for 3 days. The solid was
then separated by filtration and washed with CH2Cl2 and deionized
water for several times and dried in vacuum.
3. Results and discussion
3.1. Catalyst characterization
The synthesis of metformine-functionalized SWCNT anchored
palladium catalyst consists in: (i) surface modification of carbon
nanotubes with nitric acid and sulfuric acid to introduce car-
boxyl group (ii) conversion of the surface carboxylic acid groups
into acid chloride groups by reaction with thionyl chloride, (iii)
attachment of metformine to the SWCNTs surface through reac-
tion with the acid chloride groups and (iv) complexation of the
Fig. 1. SEM images of SWCNT before (a) and after (b) functionalization.