S. Seok et al. / Ultrasonics Sonochemistry 28 (2016) 178–184
183
Fig. 5. Cinnamylalcohol oxidation yield of (a) PdO@SNP for five cycles and (b) with catalyst (red) or without catalyst after about 50% yields (black). Reaction condition:
cinnamylalcohol (0.2 mmol), PdO@SNP (40 mg, Pd = 3.3 mol% of substrate), toluene (2 mL), 90 °C, O (1 atm). (For interpretation of the references to color in this figure legend,
2
the reader is referred to the web version of this article.)
3.2. Selective alcohol oxidation reaction
the PdO@SNP catalyst is essential to carry out the oxidation reac-
tion (Table 1: entry 14).
PdO@SNP prepared by sonochemical method was applied for
The scope of the PdO@SNP for primary and secondary benzylic
and aliphatic alcohols was explored under the pre-defined reaction
conditions. Table 2 shows the catalytic activity and selectivity for
the aerobic oxidation reactions of variety of alcohols to aldehydes
and ketones. The benzyl alcohol tolerated the PdO@SNP comfort-
ably to give benzaldehyde with high yield and selectivity
(Table 2: entry 2). A small amount of naphthalene and benzene
emerged in the reaction due to the decarbonylation process. This
phenomenon occurs frequently for Pd based catalysts [27].
Benzylalcohol bearing electron donating and electron with draw-
ing groups was converted into aldehydes with very high selectivity
(Table 2: entries 3 and 4). Secondary benzyl alcohols (Table 2:
entries 5, 12 and 13) were also selectively catalyzed by the
PdO@SNP. The electron with drawing group strongly affected the
reaction rate and reaction time was longer for 4-chlorobenzyl alco-
hol and 4-chlorobenzhydrol (Table 2: entries 4 and 13). The
2-thiopehenmethanol containing the heteroatom and aliphatic
2-octanol afforded the products in good yield and selectivity
(Table 2: entries 5–7). Although the conversion for geraniol was
a little low, the cinnamyl alcohol and trans-crotonyl alcohol pro-
duced aldehydes in high selectivity without double bond iso-
merism (Table 2: entries 8–10). PdO@SNP comfortably catalyzed
the selective aerobic oxidation of alcohols. The oxidation reaction
of 2-naphthylmethanol was carried out as model reaction as
shown in Scheme 1. The Initial reaction was carried out at room
temperature in the toluene under atmospheric pressure of dioxy-
gen without any refluxing agent as shown in Table 1.
In this reaction a small amount of 2-naphthylmethanol con-
verted to 2-naphthaldehyde with the naphthalene (Table 1: entry
). The reaction rate and selectivity of the 2-naphthaldehyde
increased as we increased the temperature from 25 °C to 50 °C
and then to 90 °C (Table 1: entry 2, 3). A nearly complete conver-
sion of 2-naphthylmethanol was obtained when the reaction was
carried out over the period of 10 h at 90 °C with PdO@SNP
1
(
40 mg, Pd content = 3.3 mol% of substrate) catalyst under an oxy-
gen atmosphere (Table 1: entry 3). The conversion and selectivity
almost equalized when a similar reaction was carried out in a fairly
large amount of the catalyst (100 mg, Pd content = 8.3 mol% of sub-
strate), (Table 1: entry 5). However reaction did not proceed well in
a small amount of the catalyst (20 mg, Pd content = 1.6 mol% of
substrate) under the similar reaction condition (Table 1: entry 4).
Therefore, the further experiments were carried out with
PdO@SNP catalyst (40 mg, Pd content = 3.3 mol% of substrate). At
the time this reaction was carried out in different solvents; conver-
sion and selectivity were not as good as using of toluene (Table 1:
entries 6–9). The reaction proceeded quickly and a complete con-
version was obtained with biphasic solvent of water and toluene,
but the selectivity for 2-naphthaldehyde proved to be quite low
the
sterically
hindered
cyclic
2-adamantanol
into
2-adamantanone without giving any by product (Table 2: entry
11).
The catalyst is totally heterogeneous and can be separated from
the reaction mixture by centrifugation process. Upon being iso-
lated, the catalyst could endure five turns for cinnamylalcohol oxi-
dation, while the catalytic reactivity was not changed (Fig. 5a).
Catalyst is robust enough that no leached product was found in
the mixture by ICP-AES analysis, when catalyst was removed after
about 50% conversion of cinnamylalcohol. The reaction ceased
completely, once the catalyst removed from the mixture, preclud-
ing the further progress of the reaction (Fig. 5b).
(
i.e. ꢀ72%, Table 1: entry 10). To show the synergetic effect of
PdO with SNP support, we performed three control reactions using
Pd(NO O, SNP and PdO catalysts. The reaction hardly pro-
ÁxH
ceeded with precursor Pd(NO O or support SNP particles
ÁxH
Table 1: entries 11 and 12). However, an appreciable amount of
3
)
2
2
)
3 2
2
(
2
2
-naphthaldehyde was obtained upon the catalytic reaction of
-naphthylmthanol with PdO under the similar reaction conditions
(
Table 1: entry 13). These results ensured that the PdO is the active
site in this catalytic reaction. The conversion and selectivity of
-naphthylmethanol oxidation was obviously higher for
4
. Conclusions
2
PdO@SNP than PdO, which explained the significance that SNP
served as a significant support material and thus exhibited the
capacity to that provided PdO with a large dispersion area.
Hence, SNP enhanced the availability of active sites of the PdO
NPs for catalytic reaction. Oxidation of 2-naphthylmethanol did
not take place in the absence of the catalyst, which shows that
We successfully employed a sonochemical method for the syn-
thesis of PdO-doped SNP nanocomposite. The ultrasonic irradiation
method provided advantages of fast reaction time and mild reac-
tion condition without any surface modification. By adopting silica
as support material, we prevented PdO NP aggregation and gener-
ated well-dispersed PdO NP on surface of SNP. Fabricated