XPS results are in support of the observations made from FT-IR,
XRD and Raman for fresh and used catalysts.
conditions the activity of this catalyst is comparatively low with
the present Ag STA catalyst.
3
The observed catalytic activity can be explained based on the
observed characteristics of the catalysts. The acidity of the cata-
lysts can be directly correlated to activity data. TPD of ammonia
Subsequently, the scope of the present catalyst with various
aromatic and aliphatic alkynes was examined. All the substrates
are efficiently converted into corresponding carbonyl compounds
in moderate to high yields. The results are summarized in
Table 3. All the products were identified by GC-MS and their
and FT-IR pyridine absorption techniques confirm that Ag STA
3
contains both Brønsted and Lewis acidic sites and these are
responsible for high catalytic activity. The importance of Lewis
acidic sites is supported by the less activity of bulk STA
1
identity was confirmed by H NMR. Substrates with various
electron donating or withdrawing groups at para or meta pos-
ition on aromatic ring underwent reaction smoothly to corre-
sponding carbonyl compounds in moderate to high yields
(Table 3, entries 2–8). Heteroaromatic alkyne 2-ethynylpyridine
is also reactive and gave 88% yield (Table 3, entry 9). In the
case of 1-ethynylcyclohexanol, it first underwent dehydration
followed by hydration of the alkyne to give the Rupe rearrange-
ment product (Table 3, entry 10). 4-Phenyl-butyne reacted
smoothly and gave corresponding carbonyl compound with
excellent yield (Table 3, entry 11). Aliphatic terminal alkynes are
also reactive but require prolonged reaction times to obtain
corresponding ketones with good yields (Table 3, entries 12–14).
Hydrations of internal alkynes are also studied. Diphenyl-
acetylene is unreactive under the present reaction conditions
whereas 1-phenyl-1-butyne gave a 20% yield (Table 3,
entry 16).
(without Ag) which showed only 52% conversion of phenyl
acetylene (Table 1). Whereas in the presence of AgNO (Table 2,
3
entry 1) alone the reaction proceeded very slow and obtained
low conversion (15%). On the other hand the hydration of
phenyl acetylene with AgNO in combination of mineral acid
3
H SO as co-catalyst improved the catalytic activity (Table 2,
2
4
entry 2). H SO itself is also active and gave 27% conversion
2
4
(Table 2, entry 3). The variation in hydration of phenyl acetylene
is related to the amount of both Brønsted and Lewis acidity,
which in turn depends on Ag substitution in STA. These results
clearly demonstrate that both Brønsted and Lewis acid sites
facilitate this reaction. The generation of Lewis acidity and the
increase in Brønsted acidity is related to the presence of the
intact Keggin ions of STA even after the exchange of Ag with its
protons. The XRD, IR, Raman and XPS results clearly suggest
the existence of the Keggin ion structure of the Ag salts of STA.
Further, the effect of metal ions in secondary structure of sili-
cotungstic acid was studied and the results are presented in
Table 2. Copper, iron and tin salts of STA were examined and
The recycling of catalyst was carried out by performing the
reaction with phenyl acetylene under the same experimental con-
ditions. After completion of the reaction the reaction mixture
was cooled to room temperature and then ethyl acetate was
added. The catalyst was separated by simple filtration and
washed three to four times with ethyl acetate. The retrieved cata-
lyst was dried in an oven at 100 °C for 2 h and was used for next
cycle. These recycling results are shown in Fig. 7. The catalytic
activity is consistent even after fourth cycle. A separate exper-
iment is carried to investigate the possible leaching of active
component of the catalyst during the reaction. The catalyst is
filtered in hot conditions, at the reaction temperature, to avoid
readsorption of any leached metal ions as proposed by Lempers
none of those are effective as that of Ag STA (Table 2, entries
3
4
–7). In addition to acetophenone a trace amount of 1,4-dipheyl-
buta-1,3-diyne a glacer coupling product was observed with the
copper exchanged STA catalyst.
The activity of the present Ag STA catalyst was compared
3
with various other heterogeneous solid acid catalysts. The results
are presented in Table 2. It is noticed that the catalytic activity of
Ag STA was much higher than the commonly exploited hetero-
3
geneous catalysts such as Amberlyst-15, SO /ZrO , WO /ZrO
2
4
2
3
36
and WO /TiO . Mizuno and co-workers reported tin–tungsten
and Sheldon. After removing the catalyst from the reaction
mixture, the reaction was continued further. It is not observed
any conversion thereafter. This result suggests that there is no
leaching of any active component during the reaction.
3
2
mixed oxide catalyzed hydration of alkynes which gave high
conversion and selectivity towards carbonyl compounds in the
1
2
presence of organic solvent. However, under solvent-free
The retention of catalytic activity of the reused catalyst might
be due to the stability of the Ag STA catalyst. The used catalyst
3
Table 2 Comparison of different catalysts with Ag
3
STA
was characterized by XRD, FT-IR, Raman spectroscopy and
XPS to see is there were any changes in its structural character-
istics. These characterization results are shown along with those
of the fresh catalyst in the respective figures. All these character-
a
a
Entry
Catalyst
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
AgNO
AgNO
3
3
15
51
27
85
71
83
98
53
66
59
86
100
100
100
87
100
100
100
100
100
100
100
b
ization techniques divulge that the Keggin structure of Ag STA
3
b
2
H SO
4
remains intact.
Cu1.5HSiW12
O
40
40
The plausible reaction mechanism for this catalyst is shown in
Scheme 1. The mechanism is assumed to proceed in the same
pathway as R. Casado et al. proposed for the gold(III) complex
Fe HSiW12
Sn1.5HSiW12
Ag STA
Amberlyst-15
O
O
40
3
11
catalyzed hydration of phenyl acetylene. First, phenyl acety-
WO
WO
3
/ZrO
/TiO
2
lene coordinates with the silver salt of STA to form intermediate
1
0
3
2
11
Sn–WO
3
1. Then intermediate 1 reacts with H O to give 2 by nucleophilic
2
attack of −OH. The nucleophile −OH attacked at the carbon
bearing phenyl group to generate the more stable carbocation.
Consequently intermediate 2 undergoes keto–enol tautomerism
to produce 3 that would generate the product, acetophenone 4,
2
Reaction conditions: Alkyne (2 mmol), H O (6 mmol), catalyst (50 mg),
b
a
1
00 °C, 6 h. Conversion and selectivity based on GC-MS. 0.25 mmol
of H SO
2
4
.
1512 | Green Chem., 2012, 14, 1507–1514
This journal is © The Royal Society of Chemistry 2012