Size selectivity of a copper metal-organic framework and origin of catalytic activity in epoxide alcoholysis

Article abstract of DOI:10.1002/chem.200901510

{(Cu(bpy)(H₂O)₂(BF₄)₂(bpy)} (Cu-MOF; MOF = metal-organic framework; bpy = 4,4'-bipyridine), with a 3D-interpenetrated structure and saturated Cu coordination sites in the framework, possesses unexpectedly high a

Full text of DOI:10.1002/chem.200901510

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FULL PAPER
DOI: 10.1002/chem.200901510
Size Selectivity of a Copper Metal–Organic Framework and Origin of
Catalytic Activity in Epoxide Alcoholysis
Dongmei Jiang,[a] Atsushi Urakawa,[a] Maxim Yulikov,[b] Tamas Mallat,[a]
Gunnar Jeschke,[b] and Alfons Baiker*[a]
Abstract: {Cu
(Cu-MOF;
G
E
E
(bpy)}
and UV/Vis spectroscopies. Cu-MOF
has highly dynamic structural proper-
ties that respond to MeOH; its frame-
work dimensions change from 3D to
2D by restructuring to a symmetric co-
ordination of four bpy units to Cu. This
interaction is accompanied by the par-
tial dissolution of Cu-MOF as multi-Cu
clusters, in which Cu2+ ions are con-
nected with bpy ligands. Although both
molecular and surface catalysis contrib-
ute to the high rate of alcoholysis, the
soluble oligomeric species (Cumbpyn)
are far more active. Finally, addition of
diethyl ether to the reaction mixture
induces the reconstruction of dissolved
and solid Cu-MOF to the original
framework structure, thereby allowing
excellent recyclability of Cu-MOF as
an apparent heterogeneous catalyst. In
contrast, the original Cu-MOF struc-
ture is maintained upon contact with
larger alcohols, such as iPrOH and
tBuOH, thus leading to poor activity in
epoxide ring opening.
framework; bpy=4,4’-bipyridine), with
a 3D-interpenetrated structure and sa-
turated Cu coordination sites in the
framework, possesses unexpectedly
high activity in the ring-opening reac-
tion of epoxides with MeOH, although
the reaction rate drops remarkably
with more bulky alcohols. This (appar-
ent) size selection and the single Cu2+
sites in an identical environment of the
crystalline matrix resemble zeolites.
The real nature of active sites was in-
vestigated by attenuated total reflec-
tion infrared (ATR-IR), Raman, EPR,
Keywords: alcoholysis
epoxides EPR spectroscopy
metal–organic frameworks
·
copper
·
·
·
Introduction
sible to the substrate.[4,6–10] In contrast to such “convention-
al” structures, less attention has been paid to MOFs that
have a dynamic and flexible framework, although they have
shown promising potential application in adsorption and
separation.[11–21] They could provide new opportunities in de-
signing solid catalysts due to the dynamic structural change
in response to guest molecules in a manner reminiscent of
enzymes.[22–25]
An open-pore structure within the solid catalyst, that is,
the accessibility of the active sites for the substrate, is crucial
to achieve high catalytic activity. In contrast to expectations,
excellent catalytic performance of some nonporous MOFs
has also been observed. Blake et al. developed a novel hy-
Metal–organic framework (MOF) materials are promising
candidates as new heterogeneous catalysts due to their high
surface area inside the pores and their tunable structure.[1–5]
An attractive feature of these solids is the presence of
single-site active species in an identical environment of the
crystalline matrix. The research on catalytic applications has
been focused on rigid MOFs with permanent porosity and
coordinatively unsaturated metal active sites that are acces-
[a] Dr. D. Jiang, Dr. A. Urakawa, Dr. T. Mallat, Prof. Dr. A. Baiker
Institute for Chemical and Bioengineering
Department of Chemistry and Applied Biosciences
ETH Zurich, Hçnggerberg, HCI, 8093 Zurich (Switzerland)
Fax : (+41)44-632-1163
drogen-bond-regulated, dynamic, Cu-based MOF, {Cu
(H2O)2A(BF4)2A
ACHTUNGTRENNUNG
G
N
CHTUNGTRENNUNG
consists of two-dimensional sheets (Scheme 1).[26] The octa-
hedral Cu2+ sites are linked directly through bpy units and
also through intermediate hydrogen-bonded water mole-
cules. The sheets are connected with hydrogen bonds be-
E-mail: baiker@chem.ethz.ch
[b] Dr. M. Yulikov, Prof. Dr. G. Jeschke
Laboratorium fꢀr Physikalische Chemie, ETH Zurich
Department of Chemistry and Applied Biosciences
ETH Zurich, Hçnggerberg, HCI, 8093 Zurich (Switzerland)
À
tween an F atom of a BF4 anion and an H atom of a water
molecule from the neighboring sheet. In the structure of Cu-
MOF, the Cu sites are coordinatively saturated and there
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901510.
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Figure 1. Ring-opening of styrene oxide with alcohols with Cu-MOF
(black) and the homogeneous catalyst Cu(BF4)2·H2O (gray). Reaction
AHCTUNGTRENNUNG
conditions: 0.11 mmol Cu, 5 mL alcohol, 1.25 mmol styrene oxide, RT,
2 h.[33]
Results and Discussion
Scheme 1. Representation of the ab plane of the Cu-MOF crystal (ac-
cording to ref. [26]).
Structural changes of Cu-MOF induced by alcohols: The
ATR-IR spectrum of Cu-MOF showed several bands char-
À
acteristic of BF4 , bpy, and structural water (Figure 2a). The
À
are no open pores due to sheet stacking. Nevertheless, it ad-
sorbs CO2 after partial removal of water molecules,[27–29] and
adsorption of small amounts of methanol and acetonitrile in-
duces structural changes that are well observable with magic
angle spinning (MAS) NMR
broad, intensive bands of the BF4 anion were observed in
the range of 950–1100 cmÀ1. The asymmetric stretching band
of the BF4À anion, which is symmetric in a free environment,
is considerably distorted, showing several distinct sharp
spectroscopy combined with
EPR spectroscopy.[30] In addi-
tion, it is a highly effective cata-
lyst in the oxidation of trime-
thylsilyl enolates to a-hydroxy
ketones using molecular oxygen
as oxidant.[31,32]
We have developed an alter-
native and more facile synthesis
method for Cu-MOF and ap-
plied it as a Lewis acid catalyst
in the ring-opening reaction of
epoxides with alcohols to the Figure 2. ATR-IR spectra of a) Cu-MOF, b) Cu-MOF wetted with MeOH, c) CD3OD, d) iPrOH, and
e) tBuOH. ꢂ marks the bands originating from the alcohols.
corresponding 1-alkoxy-2-phe-
nylethanols with excellent recy-
clability.[33] In particular, among
the investigated alcohols only
methanol showed high reactivity, comparable to that of the
homogeneous Cu catalyst (Figure 1).[34–36] The strong de-
pendence of the reaction rate on the steric bulkiness of the
reactant was interpreted as an indication to the location of
active sites inside the micropores of the catalyst. A further,
combined catalytic and spectroscopic analysis, however, re-
vealed that the origin of this change in reactivity is not the
hindered diffusion of the bulky alcohols inside the pores,
rather the substrate-induced structural changes of Cu-MOF.
We present here a detailed analysis of the nature of the cat-
alytically active sites in Cu-MOF using attenuated total re-
flection infrared (ATR-IR), Raman, EPR, and UV/Vis spec-
troscopic techniques that allowed us to rationalize the excel-
lent catalytic activity in the methanolysis of epoxides.
bands.[37,38] This implies a highly nonsymmetric surrounding
environment of the BF4 anion within the framework. The
spectrum clearly shows the doublet features at around 1610,
1415, 1220, and 820 cmÀ1 that originate from bpy. The first
two bands represent the ring stretching modes, and the
À
À
latter two are due to in-plane and out-of-plane C H bend-
ing modes, respectively. According to DFT calculations (see
the Supporting Information), the higher frequency bands of
these doublet bands can be assigned to the bpy molecule di-
rectly coordinating to Cu, whereas the lower frequency
bands are due to the bpy molecule bridged with H2O
through hydrogen bonding (Scheme 1). On the other hand,
the region of 1800–3700 cmÀ1 (Figure 2a) shows mainly the
bands due to structural H2O in Cu-MOF. An OH stretching
band is clearly seen at 3489 cmÀ1. The two broad bands in
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Copper Metal–Organic Framework
FULL PAPER
the ranges 2000–3000 cmÀ1 and 1700–2000 cmÀ1 are derived
from the hydrogen-bonding network of the framework, as
revealed by comparing the diffuse reflectance IR spectra of
Cu-MOF and dehydrated Cu-MOF.
Next, the structural changes induced by wetting Cu-MOF
with drops of alcohols (MeOH, CD3OD, iPrOH, and
tBuOH) were investigated by ATR-IR spectroscopy. For the
Cu-MOF wetted with iPrOH and tBuOH, the spectral fea-
À
tures of bpy, BF4 , and framework H2O did not change
(compare the spectra in Figure 2a, d, and e). It follows that
iPrOH and tBuOH have virtually no influence on the struc-
ture of Cu-MOF. In contrast, a remarkable restructuring of
Cu-MOF was detected upon addition of MeOH (Figure 2b),
as evidenced by the merging of all doublet bands into singlet
bands and by the disappearance of the two broad bands due
to H2O in 1700–3000 cmÀ1. The latter change implies the re-
lease of structural H2O from the Cu-MOF. Furthermore, the
Figure 3. Raman spectra of a) Cu-MOF, b) Cu-MOF wetted with MeOH,
c) iPrOH, and d) tBuOH. ꢂ marks the bands originating from the alco-
hols.
This vibration is the counterpart of the band that appeared
as a doublet in the IR spectrum at 1227 and 1219 cmÀ1 (Fig-
ure 2a). However, formation of the singlet in the Raman
spectrum may only be apparent and caused by the signifi-
cantly lower resolution of the Raman spectrum (4–8 cmÀ1)
compared to the resolution of the IR spectrum (1 cmÀ1).
Similar to the observations by IR spectroscopy (Figure 2),
there was neither band shift nor intensity change after addi-
tion of iPrOH and tBuOH (Figure 3c, d). This is a confirma-
tion that the size of the alcohol substrate plays a key role in
the restructuring of Cu-MOF.
À
band features of BF4 anion at 950–1100 cmÀ1 were altered
considerably; the broader and less distinct bands indicate a
more free and flexible environment of the anion after meth-
anol addition. At 700–1700 cmÀ1 the band features of the
samples treated with MeOH and CD3OD were identical
except some differences due to the alcohols themselves. In
the higher frequency region, new OH stretching bands at
3568 and 3351 cmÀ1 emerged by wetting with MeOH (Fig-
ure 2b). The assignment of the OH stretching vibrations was
helped by the wetting experiment using CD3OD that re-
vealed the corresponding bands at 2658 and 2530 cmÀ1 (Fig-
ure 2c). It should be noted that
An important feature of the Cu-MOF is that the structur-
al change induced by MeOH is reversible. Figures 4 and 5
a completely wet Cu-MOF im-
mersed into MeOH (shown
later in Figure 7a) or the Cu-
MOF wetted with drops of
MeOH gave identical results.
The OH stretching vibrations at
3568 cmÀ1 and its broad side
band at 3351 cmÀ1, which corre-
sponds to the OD stretching at
2658 and 2530 cmÀ1, likely have
a
similar origin: the former
being isolated and the latter in-
volved in hydrogen bonding.
This implies that there are
more than two kinds of MeOH
species present in the wetted
Cu-MOF.
Figure 4. ATR-IR spectra of Cu-MOF and CD3OD-wetted Cu-MOF in air with elapsed exposure time from a)
to d).
Raman spectroscopy was used to gain further insight into
the nature of the interaction of Cu-MOF with alcohols
(Figure 3). Due to different selection rules of IR and
Raman spectroscopies, the band positions of similar vibra-
tional motions are shifted and the band intensities appear
differently. It is clear from the Raman investigation that
after wetting the Cu-MOF with MeOH some doublet bands
merged into singlets, as observed in IR, and are shifted to
higher frequencies. The most prominent band at 1288 cmÀ1
À
of Cu-MOF, which was assigned to in-plane symmetric C H
stretching vibration of bpy, appeared as a singlet (Figure 3a).
Figure 5. Raman spectra of Cu-MOF and MeOH-wetted Cu-MOF in air
with elapsed exposure time from a) to c).
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A. Baiker et al.
show the evolution of the ATR-IR and Raman spectra
during air exposure of Cu-MOF wetted with CD3OD and
MeOH, respectively. The use of CD3OD was necessary to
avoid the signal overlapping in the OH stretching region.
The alcohol evaporated with time and the spectral features,
such as doublets, were gradually recovered. The regenera-
tion of the original Cu-MOF structure is clearly proven by
these two series of experiments. Evolution of the spectra in
the OH stretching region (Figure 4a–d) indicates that the
structural H2O, removed by wetting with MeOH, gradually
returned to the original positions and the specific structural
hydrogen bonds between H2O and bpy were re-formed. The
tion appears at around 1614 cmÀ1, whereas the band corre-
sponding to bpy dissolved in MeOH appears at a significant-
ly lower frequency, at 1599 cmÀ1, at which bpy presumably
interacts with methanol through hydrogen bonding as shown
later (Figure 7c). Moreover, the release of bpy from the CuII
coordination sites would destroy the Cu-MOF framework
completely and lead to significant dissolution of Cu-MOF.
Upon wetting with methanol, the color of Cu-MOF changed
from blue to purple without any sign of dissolution of the
solid material, thus ruling out the possibility of the first sce-
nario.
In contrast, the second assumption explains the experi-
mental observations and spectral changes well. The four bpy
coordination at the equatorial CuII sites can form a stable
2D network, as illustrated in Figure 6. In this structure the
ligand arrangement at the CuII center is highly symmetric
and the bpy molecules would appear in the spectrum as
single bands. The absence of free bpy dissolved in methanol
(as shown later in Figure 7c), also supports the symmetric
coordination without altering the Cu/bpy ratio. It leads us to
conclude that upon wetting, methanol occupies the sites that
À
band characteristics of BF4 in the original Cu-MOF were
also recovered with time as methanol evaporated.
Probable structure of Cu-MOF wetted by methanol: An in-
triguing question is the structure of the methanol-wetted
Cu-MOF. There are two types of bpy ligands present in the
original framework structure. They appear as distinct double
bands in the IR and Raman spectra; one ligand is directly
coordinated in an octahedral CuII site and the other one is
connected to CuII through hydrogen bonding to water
(Scheme 1). Wetting the Cu-MOF with methanol resulted in
the merging of the doublet bpy bands to singlets in both the
IR and Raman spectra, thereby indicating that all bpy units
become structurally identical. Other important elements of
the restructuring are the release of structural water, a less-
À
are available in the original framework for water and BF4 .
À
We assume that a mixture of BF4 , water, and methanol is
À
present between the 2D sheets and the BF4 anions are lo-
cated near the CuII centers due to the Coulomb interactions.
The mixture likely behaves as a “solution” between the 2D
À
sheets, which can interpret the environment of BF4 without
À
specific free environment of BF4 , and at least two different
specific intermolecular interactions upon addition of MeOH
as observed by IR spectroscopy. In this reconstructed frame-
work, methanol is present as a ligand coordinating to the
CuII center and also in the “solution” between the 2D
sheets. In the IR spectra, the various methanol molecules
that are located in the “solution” phase and interact with
kinds of methanol species in the wetted Cu-MOF. Two pos-
sible structures may be envisaged based on the singlet
À
nature of the bpy bands: 1) Complete breakage of Cu N-
A
(bpy) bonds, coordination of methanol to the CuII center,
and stabilization of the released bpy by methanol, and 2) re-
À
À
moval of H2O and BF4 by their strong interaction with
BF4 , bpy, or another methanol molecule may be further
methanol and coordination of bpy to Cu, thereby yielding
four identically coordinated bpy units at the equatorial Cu
sites (Figure 6).
The first scenario explains the merging of the bpy bands
but fails to account for the band positions of bpy upon wet-
ting with methanol. The resulting bpy-ring stretching vibra-
distinguished.
The nature of active sites in the alcoholysis of epoxides: The
IR and Raman spectroscopy investigations revealed a dy-
namic and reversible structural change of Cu-MOF upon in-
teraction with liquid methanol. In contrast, Cu-MOF pre-
served its original structure in
the presence of the more bulky
alcohols, iPrOH and tBuOH.
Since the BET surface area of
Cu-MOF used in the present
and former[33] studies is relative-
ly low (16.7 m2 gÀ1), the majori-
ty of the CuII active sites are
hidden in the bulk of the parti-
cles. The low fraction of surface
CuII sites exposed to the reac-
tants and the absence of the
more active, dissolved oligo-
meric species may explain the
low catalytic activity of Cu-
MOF in epoxide ring-opening
Figure 6. Diagram of the MeOH-induced reconstruction of Cu-MOF.
with iPrOH and tBuOH
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