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group functionality such as methyl acrylate and acrylic acid,
cycles indicating that even these Pd particles on MgF were
2
1
00% conversions with almost complete selectivity to methyl
propanoate and propanoic acid, respectively, were obtained
entries 9–10). The high conversions indicated stability of the
equally active for hydrogenation.
Kantam et al. recently reported the use of palladium sup-
ported on lanthanum-modified magnesium oxide nanoparti-
cles, Pd/La-MgO, as an efficient catalyst for the chemoselective
hydrogenation of olefinic double bonds at room temperature
and atmospheric pressure. However, 10 wt% Pd loading had
been used for this transformation. As the surface area of the
(
catalysts in the presence of carboxylic and electron-withdraw-
ing groups, which activated the olefinic C=C for catalytic hy-
drogenation. The effect of bulkier groups and electron-with-
drawing reducible carbonyl groups was studied by using ben-
zalacetophenone as substrate. The conversion decreased to
2
À1
support was low (ꢀ35 m g ) the palladium particles obtained
[49]
6
5% without affecting much the selectivity to 1,5-diphenylpen-
were larger, for example, in the range of 28–32 nm. In anoth-
er report, Asefa et al. used Pd encapsulated in poly(amido a-
mine) dendrimers supported on silica microspheres. The re-
ported activity was very high for room temperature hydroge-
nation of olefins, nitro aromatics, and carbonyl compounds
even with very low palladium loading and very high turnover
tanone. This lowered activity indicated a negative effect of
steric hindrance owing to the presence of phenyl rings
(entry 11). The geometrical isomers, cis and trans stilbene were
hydrogenated. The hydrogenation needed a slightly elevated
temperature (808C) owing to higher steric hindrance of the
bulkier phenyl group. trans-Stilbene showed lower activity
than the cis isomer. This may have owed to the higher relative
stability of the trans isomer than the cis isomer (entries 12–13).
Furthermore, the effect of the substituent on the phenyl ring
of styrene was examined by catalytic hydrogenation of 4-vinyl-
anisole. The reaction showed 95% conversion with 99% selec-
tivity for olefin hydrogenation (entry 14). If 2-vinylpyridine was
used as substrate to study olefinic hydrogenation in a heterocy-
clic system, 78% conversion with 99% selectivity was achieved
at 808C after 12 h (entry 15). This substrate screening study
showed sterically hindered olefins to be less reactive for cata-
lytic hydrogenation. The catalyst showed selectivity for olefinic
hydrogenation in the presence of other reducible functional
groups such as carbonyl and carboxylic groups. In addition,
sterically hindered olefins could be hydrogenated at elevated
reaction temperatures.
À1
frequencies were reported (24000 h ). However, the hydroge-
nation was effective only at higher pressures and the catalyst
[50]
preparation involved multistep preparation.
The results obtained in the present study are superior to
previous reports, indicating the positive effect of magnesium
hydroxide fluorides as support on the catalytic activity of palla-
dium nanoparticles. The results obtained for PdÀMgF -100 can
2
be correlated with the acidity of the surface of the catalyst,
size of the Pd nanoparticles, hydrogen adsorption capacity of
the catalyst, partial positive charge on the Pd sites, and strong
metal–support interaction. On comparison of the PdÀMgF -100
2
catalyst with other catalysts, it showed distinct characteristics
of high specific surface acidity equivalent to 0.328 mmol of
NH3 and high hydrogen adsorption capacity equivalent to
À5
À1
2.22ꢁ10 mmolg of catalyst.
Owing to the high hydrogen adsorption capacity of the cat-
alyst, it exhibited high metal dispersion of up to 47% as
a result of smaller metal particle formation and higher metal
surface area. Also, XPS studies showed the presence of elec-
tron-deficient Pd with partial positive charge, which activated
olefins after adsorption and helped in catalytic hydrogenation
compared to other catalysts such as Pd on charcoal.
The recyclability of the catalyst was tested by using styrene
hydrogenation as the model reaction under the conditions
given in Table 3. After completion of each reaction the catalyst
was separated, washed, dried, and reused for the next run, the
results of which are summarized in Figure S7. The catalyst
could be recycled very efficiently over five cycles without ap-
preciable decrease in styrene conversion. Possible Pd leaching
was investigated by separating the catalyst from the reaction
mixture by filtration after 1.5 h (57% styrene conversion) and
continuing the reaction (in the filtrate) without the catalyst.
However, even after continuing the reaction for an additional
In situ FTIR study and reaction mechanism
To study the effect of support on palladium-catalyzed hydroge-
nation reactions, in situ FTIR studies were performed by pass-
ing hydrogen and styrene over the catalyst surface in diffused-
reflectance mode and monitoring the formation of species on
the catalyst surface (Figure 7). Initially, only styrene was passed
across the catalyst surface at room temperature with a flow of
nitrogen gas to study if activation of styrene took place (Fig-
ure 7c). However, the activation of styrene could not be ob-
served on the catalyst surface as no significant shifts in the ole-
3
h, there was no increase in styrene conversion (Figure S8),
which strongly evidenced the absence of Pd leaching from the
catalyst. Additionally, Pd leaching was also tested by using in-
ductively coupled plasma atomic emission spectroscopy, which
showed no indication of Pd leaching into the reaction medium
(
at a Pd metal detection limit of 5 ppm). Therefore, PdÀMgF -
2
1
00 acted as true heterogeneous catalytic system without
À1
leaching. The TEM image of a recycled PdÀMgF -100 catalyst
finic C=C band (1600 cm ) and CÀH stretching frequency (in
2
À1
(after five recycles) revealed only a slight agglomeration of Pd
the range 2873–3085 cm ) were observed relative to those of
on the catalyst surface (Figure S9). In the fresh catalyst
Figure 6), formation of Pd sheets was observed whereas in the
styrene from the FTIR database (Figure 7a). This indicated that
only physisorption of styrene on the catalyst surface occurred.
Then, hydrogen gas was passed over the catalyst surface al-
ready saturated by physisorbed styrene and instantaneous for-
mation of ethyl benzene was observed on the catalyst surface
with a corresponding decrease in intensity of styrene peaks
(
recycled catalyst, Pd particles approximately 6 nm in diameter
were observed indicating partial agglomeration. However,
there was no decrease in the catalytic activity in successive
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 3182 – 3191 3188