H. Garcia et al.
[17]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(BTC) ] behaves as a Lewis acid.
Furthermore, we ob-
Lewis acidic sites to promote styrene oxide ring opening.
2
served that [Al2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(BDC) ] exhibits much lower activity than
The remarkable difference between Fe
ACHTUNGTNERNU(GN NO ) as a homoge-
3 3
3
[Fe
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(BTC)] and [Cu
A
H
U
G
R
N
U
G
neous catalyst and [Fe(BTC)] as a heterogeneous catalyst
ACHTUNGTRENNUNG
3
2
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(BDC) ] can be easily understood when considering that the
must be explained in terms of the impeded accessibility of
Lewis acid sites within the micropores of MOFs when com-
pared with homogeneous catalysts and the fact that Fe ion
3
3
+
Al ion in this MOF is fully coordinated by BDC linkers
and does not have a free coordination position available to
act as a Lewis acid.
III
in [Fe
tion in comparison to dissolved Fe ACHTUNGTNERNU(GN NO ) , in which Fe
3 3
ACHTUNGTRENNU(GN BTC)] has only one position available for coordina-
III
The data obtained showed that [Fe
A
H
U
G
E
N
N
(BTC)] is a highly
active and selective heterogeneous catalyst in the ring open-
ing of styrene oxide with methanol and the selectivity was
always more than 95%. To provide evidence for the exis-
should have more freedom to coordinate with the epoxide.
Additionally, the carboxylate groups coordinating Fe must
III
also reduce the Lewis acidity of these metal sites by giving
III
tence of Lewis acidic sites in [Fe
A
H
U
G
R
N
U
G
electron density to the Fe ions. In any case, comparison
ment was performed for the ring opening of styrene oxide
with methanol in the presence of pyridine as described in
Table 1, entry 11. The percentage conversion was <2 after
with iron citrate clearly reveals the benefits of large micro-
porosity and the presence of free coordination positions in
[Fe ACHUTNGRNEGUN( BTC)] so as to bind with styrene oxide to promote the
1
h and this failure can be easily explained by the poisoning
ring opening.
of the Lewis acid sites by strong interactions with pyridine.
This is clear evidence to support the presence of Lewis acid
To verify whether the catalysis of [Fe AHCTUNGTERNNUNG( BTC)] is truly het-
erogeneous or is due to some leached iron species present in
the liquid phase, the reaction was carried out under the opti-
mised conditions described in Table 1 and the [Fe ACHTUNGTRENNUNG( BTC)]
solid catalyst was filtered from the reaction mixture at 38%
formation of 2-methoxy-2-phenylethanol. After removal of
sites in [Fe
In addition, a quantitative EPR spectroscopic study on
Fe(BTC)] was undertaken. Figure S1 in the Supporting In-
formation shows the EPR spectrum recorded for [Fe(BTC)],
which gives 2.01 as the g value, as well as that of the Cu-
(SO )·12H O standard used to quantify the intensity of the
ACHTUNGTRENNUNG( BTC)].
[
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
AHCTUNGTRENNUNG
the [Fe ACHUTNGRENNUG( BTC)] catalyst, the solution in the absence of solid
was again stirred at 408C. After 1 h, no further product for-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
4
2
signal. It was estimated that the solid contained about 28%
of Fe species, thus confirming the +III paramagnetic state
mation was observed in the absence of solid (see Figure 2).
III
of Fe in [Fe ACHTUNGTRENNUNG( BTC)], which is in agreement with the stoichi-
ometry of the solid and the elemental analyses results (see
the Supporting Information), which show the presence of
25% iron in the material.
To determine the productivity of [Fe ACHUTNGRNENUG( BTC)] for the con-
version of styrene oxide to 2-methoxy-2-phenylethanol, the
reaction was carried out in methanol with an eight-fold
excess of styrene oxide with respect to [Fe ACHUTNGRENNUG( BTC)] (see
Table 1, entries 7 and 8). At 24 h reaction time, 72% of the
styrene oxide was converted to 2-methoxy-2-phenylethanol
with very high selectivity. After a prolonged time, the per-
centage conversion of styrene oxide was increased to 88%
without much decrease in the product selectivity. As expect-
ed, an eight-fold increase of styrene oxide concentration de-
creases the rate of the reaction, but the fact that the reac-
tion progresses after 90 h and a high conversion is achieved
Figure 2. Leaching test and evidence for heterogeneity in the catalysis of
styrene oxide ring opening by methanol at 408C a) in the presence of
[
Fe AHCTUNTGERNNUNG( BTC)] or b) after filtration of the solid at 38% conversion.
indicates that [Fe
when a 40-fold excess of styrene oxide with respect to the
typical styrene oxide/[Fe(BTC)] ratio is used, is deactivation
of [Fe(BTC)] observed (see Table 1, entry 9).
ACHTUNGTRENNUNG( BTC)] does not become deactivated. Only
AHCTUNGTRENNUNG
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Hence, it can be concluded that catalysis occurs in the solid
phase and the process is truly heterogeneous.
The reusability of [Fe AHCTUNGTERUNNN(G BTC)] was investigated for the
For the sake of comparison, iron citrate and iron nitrate
III
nonahydrate were used as model catalysts for Fe as Lewis
acid sites for the same reaction. As shown Table 1, entry 16,
iron citrate showed very low conversion and this may be
due to poor solubility in the medium, the absence of micro-
pores in the solid and/or the lack of free coordination posi-
tions of the Fe ion. In contrast, Fe nitrate becomes com-
pletely dissolved in the medium and showed very high activ-
ity, reaching an almost complete styrene conversion toward
ring-opening reaction of styrene oxide with methanol under
identical conditions as described in Table 1. After the re-
quired time, the reaction mixture was allowed to settle
down and the supernatant liquid was decanted and the solid
used for another consecutive run without further treatment.
After three consecutive reuses, the catalyst did not show
any changes in its crystallinity. Further evidence in support
of the catalyst stability was obtained from powder XRD re-
sults. Comparison of the XRD patterns of the fresh and
III
III
the 2-methoxy-2-phenylethanol in only 10 min. The result of
III
Fe ACHTUNGTRENNUNG( NO ) clearly indicates the intrinsic activity of Fe as
3 3
8532
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8530 – 8536