Application Note SC-XRD 51
Complete characterization of a series of
MOF structures using a X8 PROSPECTOR
Introductions
Metal Organic Framework (MOF) structures are well
known for their many interesting applications, such as gas
storage and separation, chemical sensing, and catalysis [1].
The same structural features that allow for such excellent
chemical properties [2] cause difficulty in characterization
by crystallographic methods. Owing to the highly porous
nature of MOFs, crystalline samples tend to lose solvent
rapidly, degrading the quality of diffraction. Additionally, a
high degree of disorder over long ranges frustrates single
crystal structure determination efforts.
In assessing the suitability of a certain MOF system for
various applications, it is necessary not only to elucidate
the structural features via single-crystal diffraction, but
also to confirm the purity of the bulk sample by comparative PXRD techniques. In order to completely characterize
difficult MOF samples, our lab has investigated the
complementary techniques of single crystal and powder
diffraction utilizing only a CuKα IμS Kappa geometry
diffractometer, equipped with QUAZAR MX optics and an
APEX II detector, the X8 PROSPECTOR.
X8 PROSPECTOR diffraction system
Single Crystal Structure
Determination
Single crystals of two new, highly porous MOF
structures (NU-108-Cu and NU-108-Zn, (Figure
1) were grown via literature methods [3].
Individual crystals were mounted in oil in the
cold stream (225K) of the CuKα IμS equipped
X8 PROSPECTOR diffractometer. Crystals
were on the order of 200-300 μm in their
largest dimensions. The highly porous nature
of these materials resulted in very weak
diffraction intensities, which could only be
observed to approximately 1.5-1.0 Å resolution,
necessitating the use of our QUAZAR MX
optics equipped microfocus source.
Each structure exhibits a non-catenated
framework: a 3,24 network rht-topology for
NU-108-Cu (Figure 2), and a new 3,3,6 network
for NU-108-Zn (Figure 3). The rht-topology of
NU-108-Cu forms three porous cages ranging
in size from 1.9-2.8nm, resulting in a total void
space of over 87% of the structure volume
(Figure 4). The new topology exhibited in the
Zn structure yields three distinct channels, the
largest of which with a size of 1.0 × 1.1nm2.
This results in a total void space of nearly 80%
of the structure volume (Figure 5).
Figure 1: Single crystals of NU-108-Cu (A) and NU-108-Zn (B), and the organic strut
ligand (center). Figure adapted from [3].
Figure 2: Packing diagrams of NU-108-Cu showing A) the cage structure around one
ligand and B) the long-range packing of the MOF. Figure adapted from [3].
Figure 4: The various pores (large
purple spheres) in NU-108-Cu. Figure
adapted from [3].
Figure 3: Structural diagram of NU-108-Zn showing A) the atoms of the metal nodes, B) a
polyhedral view of the node, and C) the arrangement around on ligand.
Figure adapted from [3].
Figure 5: Packing diagram perpendicular
(A) and parallel (B) to the channels in
NU-108-Zn. Figure adapted from [3].
Powder Diffraction on a X8 PROSPECTOR
To determine the purity of the bulk syntheses of NU-108-Cu,
and –Zn, it is necessary to employ Powder X-Ray Diffraction
techniques. Single crystals of the NU-108 MOFs will
decompose rapidly when pulled from the mother liquor.
Consequently, it is imperative to perform the PXRD data
collections in capillaries with crystals suspended in solvent.
The QUAZAR MX optics equipped on the X8 PROSPECTOR
produce a small high-flux X-Ray beam which in turn yields
narrow peaks in the PXRD scan which can be resolved and
matched to known unit cells and predicted patterns, or indexed
using TOPAS.
Samples are loaded into borosilicate glass capillaries with a
Figure 6: Some MOF samples loaded in borosilicate
capillaries under solvent.
Figure 7: A representative powder pattern taken on the X8 PROSPECTOR.
PXRD pattern from NU-108-Cu (A) and -Zn (B)
minimal amount of mother liquor. Capillaries are sealed
with wax and mounted on the X8 PROSPECTOR at room
temperature. Data are collected as rotation frames over
180° in φ at 2θ values of 12°, 24°, 36°, and 48° and
exposed for 10 minutes for each frame. At a distance of
150 mm, the APEX II detector area covers 24° in 2θ.
Overlapping sections of data are matched and the
resulting pattern integrated using the Bruker APEX2
Phase ID program. Powder pattern data were treated for
Figure 8: PXRD patterns for NU-108-Cu (A) and –Zn (B) generated from
amorphous background scatter using EVA for visual
the X8 PROSPECTOR (red) compared to calculated patterns based on
comparison to simulated patterns. Unit cell indexing can
the single crystal XRD structures (black). Figure adapted from [3].
be carried out with TOPAS by
allowing for a background
PXRD pattern from NU-108-Zn
correction during the calculation.
Figure 9: An indexed PXRD pattern from NU-108-Zn (solid blue trace) showing the indexed peak
positions (blue lines) and their deviation from refined values (red lines).
Conclusion
The X8 PROSPECTOR diffraction system provides an excellent
combination of a high intensity CuKα IμS microfocus source
with the sensitive APEX II detector for the efficient
characterization of Metal Organic Framework (MOF)
structures. Structures that could previously only be elucidated
at synchrotron facilities or using high powered rotating anode
X-ray generators can now be reliably determined in the home
lab using low maintenance diffraction systems. The X8
PROSPECTORS experimental flexibility and the APEX2
software also allow for powder diffraction experiments on the
same diffractometer for confirming the purity of the bulk
sample.
[1] (a)Murray, L. J.; Dinca, M.; Long, J. R. Chem. Soc. Rev.
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Sarjeant, Senior Research Associate, X-Ray Crystallography,
Department of Chemistry, Northwestern University.
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References