Mendeleev Commun., 2012, 22, 237–238
O
Ph
O
Pd/graphene
2 nm
2 nm
(a)
(b)
PhHal
+
DMF, Et3N,
120–125 °C
OEt
OEt
Hal = I, Br
Scheme 1
B(OH)2
Pd/graphene
PhBr
+
K2CO3, 55 °C
Me
Me
Scheme 2
Figure 2 Two representative high resolution TEM images of Pd particles
on the graphene shells that have been used for estimation of interplanar
spacing.
Table 1 Conversion of aryl halides in the Heck reaction with ethyl acrylate.
Entry
ArHal
Time/h (cycle)
Yield (%)
of Pd crystalline lattice (0.389 nm). Electroconductivity of the
samples did not change after adding palladium.
1
2
3
4
PhI
PhI
PhI
PhBr
2 (1)
2 (2)
4 (3)
100
100
80
The fact that the average size of Pd nanoparticles was about
2.5 nm seems to be very important. It was found earlier12 that
for Pd the maximum catalytic activity is observed for size within
3–4 nm. Note that size of Pd nanoparticles fixed on the oxidized
graphene with a different method was also 2 nm in average.13
The size effect in catalysis was observed for nanoparticles of
other metals as well. So, for Pt the maximum specific activity
was found at size 2.2 nm14, for Au ~2.0 nm.15 Interestingly, the
character of catalytic reactions may differ (oxidation, reduction,
electrosynthesis, etc.), and the nature of size effect is supposed to
be connected with maximum fraction of the catalytically active
crystallographic faces in the particles of the definite size.
As follows from the experiments, the activity of the novel Pd
catalyst supported on graphene shells is moderate in the Heck
reaction (Scheme 1) and relatively high in the Suzuki reaction
(Scheme 2).† Recycling the Pd/graphene catalyst in the Heck
reaction is shown in Table 1.
20 (1)
16
High activity was recently demonstrated16 for the palladium
supported on the oxidized graphene. On the whole, Pd immo-
bilized on carbon nanotubes7,8 is more active in the Heck and
Suzuki reactions. Catalytic activity of Pd/nanodiamond in these
reactions was not studied but in hydrogenation it was found to be
very high.10
This work was supported by the Division of Chemistry and
Materials Science of the RussianAcademy of Sciences (programme
no. 01) and the Russian Foundation for Basic Research (grant no.
11-03-00252).
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†
Starting graphene material, graphene shells, prepared as described,
contained according to combustion analysis 94.32% of carbon. Method
of carbon combustion for the determination of the percentage of non-
volatile admixtures in carbon materials was standard. It consisted in the
heating the samples in air at 800°C when carbon is burnt off followed
by weighing residue.
Preparation of Pd/graphene catalyst. Complex Pd2(dba)3 (43.5 mg) was
dissolved in toluene (20 ml). The red-wine coloured solution was filtered
through a paper filter to remove insoluble particles and was magnetically
stirred for 30 min under argon at 50°C in the presence of graphene (160 mg).
Solution turned to light yellow colour. Then the reaction mixture was
cooled to room temperature, the black precipitate was separated, washed
with toluene (4×20 ml), pentane (4×20 ml), and dried in vacuo to afford
169 mg of the catalyst. Found (%): C, 89.26; H, 2.04; Pd, 5.2.
Heck reaction. The mixture of aryl halide (3 mmol), ethyl acrylate
(7.5 mmol), triethylamine (7.5 mmol) and 10 mg of the Pd/graphene
catalyst was stirred in DMF (12 mmol) at 120–125°C. In the case of
iodobenzene reaction was complete in 2 h, whereas conversion of bromo-
benzene was only 16% in 20 h. The mixture was treated with water, the
product was extracted with ethyl acetate (2×50 ml), dried over Na2SO4
and evaporated. The product was identified by 1H NMR spectra.
1
Ethyl (2E)-3-phenylacrylate. H NMR (CDCl3) d: 1.35 (t, 3H, Me),
4.28 (q, 2H, CH2), 6.46 (d, 1H, CH=CH, J 16.0 Hz), 7.39 (m, 3H, Ph),
7.52 (m, 2H, Ph), 7.71 (d, 1H, CH=CH, J 16.0 Hz).
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Suzuki reaction. The mixture of bromobenzene (2 mmol), m-tolyl-
boronic acid (3 mmol), K2CO3 (4 mmol) was stirred in MeOH (12 ml)
and H2O (4 ml) for 5 min. Then catalyst Pd/graphene (7 mg) was added and
stirring was continued at 55°C for 30 min. The mixture was diluted with
water, the product was extracted with ethyl acetate (2×50 ml), dried over
Na2SO4, evaporated. Product was taken into hexane, passed through the thin
layer of SiO2, once more evaporated and identified by 1H NMR spectrum.
3-Methylbiphenyl: yield 90%. 1H NMR (CDCl3) d: 2.55 (s, 3H, Me),
7.29 (m, 1H, Ph), 7.48 (m, 2H, Ph), 7.55 (m, 4H, Ph), 7.70 (m, 2H, Ph).
16 S. Moussa, Ali R. Siamaki, B. F. Gupton and M. S. El-Shall, ACS Catal.,
2012, 2 (1), 145.
Received: 12th April 2012; Com. 12/3910
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