D. Jiang et al. / Journal of Catalysis 257 (2008) 390–395
391
The solvent diffusion method, commonly used for the synthe-
Table 1
Ring-opening of styrene oxide with methanol
sis of Cu-MOF [21–23,26], provides well-developed single crystals
that are ideal for characterization; however, the procedure is time-
consuming, and the product yield is low. We could well repro-
duce the fundamental framework structure of Cu-MOF by apply-
ing a simplified, convenient synthesis method. This Cu-MOF is a
highly active, recyclable heterogeneous catalyst for the alcoholysis
of epoxides.
Catalyst
Conv.a (%)
Sel.a (%)
Without catalyst
Cu-MOF
0
93
98
22
0
–
95.5
99
91
–
Cu(BF4)2·H2O
2. Experimental
Cu(BF4)2·H2O + 4 equiv. Py
Cu(BF4)2·H2O + 40 equiv. Py
2.1. Chemicals
Note. Reaction conditions: catalyst (0.11 mmol Cu); CH3OH (5 mL); styrene oxide
(1.25 mmol); room temperature; 2 h; Py—pyridine.
The conversion and selectivity values represent the average of three parallel
reactions.
a
ꢀ
Hydrated Cu(II)-tetrafluoroborate (Cu: 21–22 mass%) and 4,4 -
bipyridine (98%) were purchased from Aldrich. Methanol, iso-
propanol, and tert-butanol were distilled from Mg before use.
Benzyl alcohol (99%) was purchased from Aldrich and used with-
out distillation. Aniline (99.5%), styrene oxide (98%), cyclohexene
oxide (98%), cyclopentene oxide (98%), and 2-hexene oxide (97%)
were purchased from Fluka; trans-stilbene oxide (99%) was pur-
chased from Acros; and cis-stilbene oxide (98%) was purchased
from Aldrich. The solvents (analytical grade) were used without
further purification.
For the aminolysis reaction, 1.25 mmol of cyclohexene oxide
and the catalyst Cu-MOF (0.11 mmol Cu) were stirred in 2 mL of
aniline at room temperature. After 4 h, the catalyst was filtered
off and the filtrate was concentrated. The crude product was pu-
rified by flash column chromatography on silica gel using petrol
ether/ethyl acetate (5/1) as eluent. The products were analyzed by
GC and GC-MS.
Reaction conditions for the homogeneous copper salt were the
same as described above, but using 0.11 mmol of copper salts in-
stead of Cu-MOF.
2.2. Synthesis of Cu(bpy)(H2O)2(BF4)2(bpy) (Cu-MOF)
ꢀ
First, 4,4 -bipyridine (0.312 g; 2 mmol) in 2 mL of ethanol
3. Results and discussion
was slowly added to an 8-mL aqueous solution of Cu(BF4)2·H2O
(0.309 g; 1 mmol) at room temperature. The blue precipitate was
formed gradually. The mixture was stirred for 4 h at room tem-
perature, after which the solid was filtered off, washed with water
and ethanol, dried in air at room temperature and then in vacuum
Here we first discuss the application of Cu-MOF as a Lewis acid
catalyst for epoxide ring-opening, then summarize the characteri-
zation of the catalyst before and after the reaction.
◦
at 100 C (2 h), and stored under Ar.
3.1. Ring-opening reactions of epoxides
2.3. Catalyst characterization
Cu-MOF is a highly active and selective heterogeneous catalyst
in the alcoholysis of epoxides. The facile transformation of styrene
oxide with methanol to 2-methoxy-2-phenylethanol is shown in
Table 1. Conversion and selectivity >90% were achieved at room
temperature in only 2 h, and no reaction occurred without the cat-
alyst under the mild conditions. For comparison, the homogeneous
catalyst Cu(BF4)2·H2O was only slightly more active and selective
than Cu-MOF. To mimic the effect of bipyridine ligands in Cu-MOF,
we repeated the reaction with Cu(BF4)2·H2O in the presence of
four equivalent pyridine (Py). Clearly, the N-heteroaromatic com-
pound deactivated the catalyst, and a further increase of the Py/Cu
ratio up to 40 led to a complete loss of activity. This effect is
due mainly to the coordination of Cu with pyridine, which hinders
the access of styrene oxide to the CuII active sites. In the light of
the poor activity of Cu(BF4)2·H2O in the presence of pyridine, the
excellent performance of Cu-MOF is even more impressive. Note
that Cu(BF4)2·H2O is an outstanding Lewis acid catalyst for vari-
ous transformations, including ring-opening reactions of epoxides
under very mild, solvent-free conditions [27–29]. In contrast, other
Cu-containing MOFs tested in the alcoholysis of epoxides were ei-
ther inactive [12] or required several days to complete the reaction
at room temperature [30,31].
X-ray diffractograms were recorded on a Siemens D5000 pow-
der diffraction system using CuKα radiation (45 kV and 35 mA)
and Cu as the reference. A Fa Leco CHN-900 instrument was used
for C, H, and N elemental analysis. Nitrogen sorption isotherms
were measured at 77 K on a Micromeritics ASAP 2000 system. The
sample was outgassed at 373 K for 2 h under 10
measurement.
Scanning electron microscopy (SEM) images were obtained on
a Gemini 1530 (Zeiss) at low voltage (U = 1 keV). For transmis-
sion electron microscopy (TEM), the material was deposited onto
a holey carbon foil supported on a copper grid. TEM and electron
diffraction investigations were performed with a CM30ST micro-
scope (FEI; LaB6 cathode, operated at 300 kV, point resolution
∼2 Å).
−3
Pa before the
2.4. Catalytic reaction
The ring-opening reactions of epoxides were carried out in
oven-dried glassware under air atmosphere. For the alcoholysis,
1.25 mmol of epoxide and the catalyst (Cu-MOF, 0.11 mmol Cu)
were stirred in 5 mL of alcohol at room temperature for 2 h.
The products were analyzed on an Agilent gas chromatograph
equipped with a flame ionization detector and a HP-5 capillary col-
umn, with nonane used as the internal standard. The isolated yield
was obtained by flash column chromatography on silica gel with
petrol ether/ethyl acetate = 3/1 as the eluent (petrol ether/diethyl
ether = 7:1 for stilbene oxide). The product was identified by GC-
MS analysis. The anti/syn ratio was determined by 1H NMR. The
initial TOF was calculated from the conversion achieved in 2 h.
The reaction could be extended to aliphatic and cyclo-aliphatic
epoxides (Table 2). The reaction rates, characterized by the turn-
over frequencies (TOF) calculated for the first 2 h, were lower than
those for styrene oxide, but the selectivities to
α-methoxy alco-
hols were always excellent, at least 95%. The structural effect on
the rate of methanolysis was similar to that observed with the ho-
mogeneous catalyst Cu(BF4)2·H2O (last column, values in brackets).
Methanolysis of trans- and cis-stilbene oxide was slow and incom-
plete even after 3 and 6 days, respectively. The dramatic difference