Automated diffraction tomography for the structure elucidation of twinned, sub-micrometer crystals of a highly porous, catalytically active bismuth metal-organic framework

Article abstract of DOI:10.1002/anie.201204963

A combined approach: A permanent highly porous bismuth-containing metal-organic framework (CAU-7) has been synthesized and its structure determined by a combination of electron diffraction, Rietveld refinement, and DFT calculations. The compound is catalytically active in the hydroxymethylation of furan (see picture). Copyright

Full text of DOI:10.1002/anie.201204963

Angewandte
Chemie
DOI: 10.1002/anie.201204963
Metal–Organic Frameworks
Automated Diffraction Tomography for the Structure Elucidation of
Twinned, Sub-micrometer Crystals of a Highly Porous, Catalytically
Active Bismuth–Metal–Organic Framework**
Mark Feyand, Enrico Mugnaioli, Frederik Vermoortele, Bart Bueken, Johannes M. Dieterich,
Tim Reimer, Ute Kolb,* Dirk de Vos, and Norbert Stock*
[
7]
The number of metal–organic framework (MOF) compounds
has increased almost exponentially over the last decade as
a consequence of their fascinating structures and potential
DFT calculations have been applied. Recently automated
diffraction tomography (ADT) has been introduced as a new
method for collecting three-dimensional electron diffraction
[
1]
[8]
applications. They are composed of inorganic building units,
such as metal ions or clusters, which are connected through
organic linker molecules to form a porous three-dimensional
network. Most of the MOFs are based on rigid polycarbox-
data from single nanosized crystals, thus allowing single-
crystal analysis even for porous and organic sub-microcrystal-
line samples.
A trivalent metal that exhibits interesting catalytic
properties is bismuth. It is nontoxic, noncarcinogenic, and
for a rare metal relatively inexpensive, and thus bismuth
compounds are used as green catalysts. Despite these
characteristics, the number of bismuth-based MOFs is
rather limited and only a few compounds with limited
[
2]
ylate linker molecules, but a large variety of metal ions,
[
2c,3]
mainly transition-metal ions, have also been incorporated.
[9]
The chemical and thermal stability of metal carboxylate based
MOFs is crucial for potential applications and depends on the
[4]
metal ions incorporated. In general, metal ions in higher
oxidation states lead to more stable structures.
[10]
porosity have been described. This is in contrast to the
many known bismuth-oxo clusters, which could possibly be
used for the construction of new MOFs.
While the use of divalent metal ions often results in the
formation of single crystals, whose structures can be routinely
[
11]
[
2c,3c]
determined by single-crystal X-ray diffraction,
tri- and
Here, we present the synthesis of the first highly
crystalline, porous, and catalytically active bismuth-based
MOF Bi(BTB) (BTB = 1,3,5-benzenetrisbenzoate), whose
structure was elucidated by a combination of electron
diffraction, Rietveld refinement, and DFT calculations.
Bi(BTB), denoted as CAU-7 (CAU = Christian-
Albrechts-Universitꢀt) was synthesized by using conventional
as well as microwave (MW) assisted heating. The reaction of
Bi(NO ) ·5H O with H BTB in methanol at 1208C led to
tetravalent metal carboxylates are mostly obtained as micro-
crystalline powders and the determination of their structures
[
2a,3a,4c,5]
poses immense challenges.
Direct methods have been
successfully employed, but complicated structures with large
unit cells necessitate the use of nonstandard approaches.
Thus, computational assisted structure determination,
[
4c]
namely, the AASBU approach (assembling of secondary
[
2a,6]
building units), the ligand-replacement strategy,
and
3
3
2
3
phase-pure CAU-7 (for a detailed synthesis procedure see the
Supporting Information). The reaction time can be reduced
from 12 h to 20 min by using MW-assisted instead of conven-
tional heating, but this leads to the formation of 10–20 mm
large agglomerates of strongly intergrown elongated crystals
of about 100 nm (see Figures S2–S4 in the Supporting
Information). The addition of DMF in the conventional
synthesis results in the formation of larger rodlike crystals
ranging from 200 to 300 nm in length. Transmission electron
microscopy confirmed that isolated CAU-7 crystals have
a typical rodlike shape with different length/diameter ratios
(see Figure S5 in the Supporting Information). Such isolated
rods were used to collect electron diffraction data by
automated diffraction tomography (ATD).
[
*] M. Feyand, T. Reimer, Prof. Dr. N. Stock
Institut fꢀr Anorganische Chemie
Christian Albrechts Universitꢁt zu Kiel
Max-Eyth Strasse 2, 24118 Kiel (Germany)
E-mail: stock@ac.uni-kiel.de
Dr. E. Mugnaioli, Dr. U. Kolb
Institut fꢀr Physikalische Chemie
Johannes Gutenberg-Universitꢁt Mainz
Welderweg 11, 55099 Mainz (Germany)
Dr. J. M. Dieterich
Institut fꢀr Physikalische Chemie
Georg-August-Universitꢁt Gçttingen
Tammannstrasse 6, 37077 Gçttingen (Germany)
F. Vermoortele, B. Bueken, Prof. Dr. D. de Vos
Centre for Surface Chemistry and Catalysis
Kasteelpark Arenberg 23, 3001 Heverlee (Belgium)
Single-crystal ADT electron diffraction datasets were
collected using a cryo holder cooled to 120 K and mild
illumination conditions. To prevent beam damage and
improve the signal intensity, the diffraction data were
[
**] We thank Steffen Schmidt (LMU Mꢀnchen) for the SEM micro-
graphs, Michael T. Wharmby for his assistance in preparing the
graphics, and the DFG for financial support (SPP 1362, grant
agreement no. STO 643/5-2). This work has also been supported by
the Stiftung Rheinland-Pfalz fꢀr Innovation and the state Schleswig
Holstein.
[
12]
acquired in the precession mode. The three-dimensional
diffraction space reconstruction leads to lattice parameters
a = 32 ꢁ, b = 28 ꢁ, c = 4 ꢁ, a = b = g = 908, and extinction
group Pb-a. The reconstructed reciprocal space is shown in
Figure 1.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204963.
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Figure 1. Reconstructed three-dimensional diffraction space of CAU-7
projected along the main directions (top), a rod tilted to expose the
triangular base (bottom left), and a sketch of the trilling arrangement
(
bottom right).
The ADT data also showed that each rod is in fact a twin
aggregate composed of three individuals that grow following
the pseudohexagonal symmetry of the structure (Figure 1),
and thus a trilling is formed (see Figures S6 and S7 in the
Supporting Information). The three individuals share the
same [001] direction, which is also the main growth direction
of the rod. The [010] directions are rotated by 1208, and are
parallel to the “almost” equivalent [2À10] and [À2À10]
directions of the other domains.
Figure 2. Crystal structure of CAU-7. The connection of 1,3,5-benzene-
3
À
3+
trisbenzoate (BTB ) and Bi ions (top left) leads to face-sharing BiO9
polyhedra which form chains along the [001] direction (top right).
These chains are interconnected into a honeycomb network (bottom).
The structure of CAU-7 was determined on the basis of
electron diffraction data collected on two single domains.
Simulated annealing (SA) routines were used as implemented
[
13]
in SIR2011. The structure was finally solved in the space
group Pb2 a with one independent bismuth atom and one
1
3
À
[16]
independent BTB molecule in the unit cell. All six torsion
bond lengths that range from 2.250(6) to 3.110(6) ꢁ. The
3
À
3À
angles of the BTB molecule were kept free to rotate during
the solving process. The crystal structure was refined from X-
BTB linkers connect the chains into a slightly distorted
honeycomb network with approximately 1 nm wide, one-
[14]
[15b,17]
ray powder diffraction data using Topas Academics 4.1.
More details are given in the Supporting Information.
dimensional channels, as determined with PLATON.
Each carboxylate group of the linker molecule contains one
m- and one end-on-linked oxygen atom.
The thermal stability of CAU-7 was investigated by
thermogravimetric (TG) analysis and temperature-dependent
X-ray powder diffraction (TD-XRPD) measurements. The
as-synthesized product, which contains noncoordinating
DFT-based calculations on the CAU-7 crystal structure
were carried out to further support the experimental obser-
vations. A plane-wave approach, as implemented in the
CPMD program package was employed, using the PBE
functional together with Goedecker–Teter–Hutter pseudopo-
tentials and a final wavefunction cutoff of 100 Rydberg
H BTB molecules (see Figure S17 in the Supporting Infor-
3
(
density cutoff: 400 Ry) and the experimental lattice param-
mation for the IR spectra), was treated with DMF and
methanol. This was followed by ultrasonication in methanol
for 30 min. The TG curve (see Figure S15 in the Supporting
Information) shows a weight loss of 14.2% as a result of the
loss of solvent molecules from the pores between 25 and
808C. Above 3508C, the loss of one BTB molecule is
observed, which results at 10008C in Bi O (found: 52.1%,
calcd: 51.1%). The TD-XRPD measurements (see Figure S16
in the Supporting Information) confirm the stability of the
crystalline framework up to 3808C. Above this temperature,
decomposition takes place and results in an X-ray amorphous
phase at 4008C.
[
15]
eters. The calculated Bi-O distances are between 2.336 and
.221 ꢁ for the optimized structure, which is in good agree-
ment with the experimentally obtained values (2.16(1)–
.38(1) ꢁ). More details of the DFT calculations are given
in the Supporting Information.
In the crystal structure of CAU-7, the Bi ions are
3
3
3
+
2
3
3
À
ninefold coordinated by oxygen atoms of the BTB ions,
thereby forming threefold capped trigonal prisms. Face
sharing of the BiO polyhedra leads to chains along the c-
axis (Figure 2). Such polyhedra are already known for the
bismuth benzoate Bi(O C H ) , which shows very similar
9
2
6
5 3
2
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Prior to the sorption experiments CAU-7 was thermally
activated at 1608C for 12 h under vacuum. The elemental
analysis of the activated sample confirms the complete
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À1
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volume of V = 0.43 cm g
(V (calcd) = 0.43 cm g
).
m
m
These values are comparable to other BTB-based MOFs
with honeycomb networks, such as CAU-4 (Al(BTB):
2
À1
3
À1 [18]
A(BET) = 1520 m g , V = 0.61 cm g
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condensations and polymerization result; the optimum cata-
lyst should give high yields at high conversions. CAU-7 was
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Keywords: bismuth · heterogeneous catalysis · metal–
organic frameworks · structure elucidation
.
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Communications
Metal–Organic Frameworks
A combined approach: A permanent
highly porous bismuth-containing metal–
organic framework (CAU-7) has been
synthesized and its structure determined
by a combination of electron diffraction,
Rietveld refinement, and DFT calcula-
tions. The compound is catalytically
active in the hydroxymethylation of furan
(see picture).
M. Feyand, E. Mugnaioli, F. Vermoortele,
B. Bueken, J. M. Dieterich, T. Reimer,
U. Kolb,* D. de Vos,
N. Stock*
&&&& — &&&&
Automated Diffraction Tomography for
the Structure Elucidation of Twinned,
Sub-micrometer Crystals of a Highly
Porous, Catalytically Active Bismuth–
Metal–Organic Framework
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!