This is the peer reviewed version of the following article:
Stability improvement of Cu3(BTC)2 metal–organic frameworks under steaming
conditions by encapsulation of a Keggin polyoxometalate.
Danilo Mustafa,a Eric Breynaert,*a Sneha R. Bajpe,a Johan A.
Martensa and Christine E. A. Kirschhocka Chem. Commun., 2011,47, 8037-8039.
which has been published in final form at
ARTICLE TYPEARTICLE TYPE
Stability improvement of Cu3(BTC)2 metal-organic framework under
steaming conditions by encapsulation of Keggin polyoxometalate
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1
Danilo Mustafa,a Eric Breynaert,*a Sneha R Bajpe, a Johan A Martensa and Christine E A Kirschhock a
Accepted 31st May 2014
Cu3(BTC)2 with incorporated Keggin polyoxometalate was
demonstrated to be stable under steaming conditions up to
483K, while the isostructural HKUST-1 degrades and
transforms into [Cu2OH(BTC)(H2O)]n.2nH2O from 343K
Porous hybrid materials such as Cu3(BTC)2 metal-organic
frameworks (MOFs) have been intensively studied in the last
decade1, 2 because of their interesting properties such as a large
surface area, well-defined micro- and mesoporosity in
combination with attractive sorption properties, easily accessible
transition metal centers and the ability to arrange them in tailored
meso-structures such as thin films3, patterned single crystals4, etc.
More recently HKUST-1 MOF was identified as candidate for
energy storage5, 6, CO2 capture7-11 and safe storage and separation
of propane and propylene12. Exploiting its hydrophilic properties,
HKUST-1 has been applied for reversible adsorption of water at
room temperature and in this context used in humidity sensors13.
At present Cu3(BTC)2 is one of the few MOF that is produced by
the chemical industry (BasoliteTM C300 - BASF).
Despite its interesting applications, one of the major concerns
impairing industrial application of this material is its low stability
in steam conditions. Schlesinger et al.14 showed that the stability
of hydrated Cu3(BTC)2(H2O)3 deteriorates from 343K onwards.
The anhydrous material is stable until 473K, implying that water
molecules coordinated to the paddle-wheel units15 have to be
removed carefully before crossing the 343K stability boundary.
Catalyst decomposition at 353K also prevented Schlichte et al.16
from optimizing the turnover frequency in HKUST-1 based liquid
phase cyanosilylation of benzaldehyde.
Keggin type heteropolyacids (HPAs) have been successfully
applied as molecular level template for the formation of
Cu3(BTC)2 MOFs in water and ethanol solvent mixture at room
temperature17, 18. Here we report that the resulting Cu3(BTC)2,
incorporating Keggin type PW12O403- [HPA@Cu3(BTC)2] is more
robust in steam conditions compared to HKUST-1 which is very
important in view of applications.
a Center for Surface Chemistry and Catalysis, Department of Microbial
and Molecular Systems , KU Leuven, Belgium 3000. Fax: +32 16
321998; Tel: +32 16 321598; E-mail: Eric.Breynaert@biw.kuleuven.be
Supplementary information for this article is available on the www
As described by Bajpe et al.18 HPA@Cu3(BTC)2 was prepared
by dissolution of Cu(NO3)2.3H2O (Fluka) in a 10-3M H3PW12O40
(Fluka) solution prepared using an 0.1M NaNO3 in 50% vol.
ethanol (VWR)-water solution. Upon complete dissolution, a
1.259x10-2M 1,3,5-H3BTC (Acros organics) solution in 0.1M
NaNO3 in 50% vol. ethanol-water was added and aged statically
for 24h at room temperature. The HKUST-1 sample was prepared
according to the modified recipe of Rowsell et al.19. Millipore®
water was used throughout. After synthesis, the solids were
collected, washed twice with absolute ethanol and dried at 343K
to remove physisorbed solvents, while retaining the chemisorbed
water in the paddle-wheels of the structure. Sealed 0.5mm glass
capillaries containing the compounds were subjected to a thermal
treatment at 363, 393, 423, 453 and 483K for 24 h.
PXRD patterns were obtained for all samples from 3 to 90
degrees 2θ using a Stadi P (CuKα1, STOE& Cie GmbH) in θ-2θ
geometry and capillary mode. The data was processed using the
STOE Software WinXPOW.
The HKUST-1 structure consists of three types of pores1, 20,
small (octahedral) pores in the center of Cu12BTC8 units with a
diameter of only 6Å, and two larger pores around 11-15Å formed
by the organization of these units in the HKUST-1 structure. Due
to the hydrophobic character of the benzyl core and the lack of
unoccupied metal coordination sites, the small pore exhibits
hydrophobic character21. In one of the big pores, water molecules
are bound to the available copper coordination sites and as
consequence this pore shows a highly hydrophilic character.
Grajciar et al.15 showed that the Cu-Cu distance in the paddle-
wheels is elongated due to the interaction with these water
molecules. This phenomenon leads to a square pyramidal
coordination of the cations as compared to a square planar
coordination in the anhydrous HKUST-1 structure. Upon careful
heating, the water molecules can be detached from the
paddlewheel without any damage to the original structure. In
steam conditions, however, Cu hydrolysis is enhanced22. The
hydroxylation of Cu2+ in the paddle-wheel changes the local
structure and consequently the compound. As shown in figure 1a,
the PXRD patterns for heat-treated hydrated HKUST-1 confirm
the occurrence of structural changes upon heating from room
temperature to 453K. Crossing the 343K stability boundary, the
diffraction lines broaden, hence indicating a phase transition. At
393K, an intermediate, currently unidentified phase with low
crystallinity is formed which in turn transforms into the green
information). At 423K the formation of this phase is complete
Figure 1a: PXRD patterns of hydrated HKUST-1 as function of
Figure 1b: PXRD patterns of the HPA@Cu3(BTC)2 as function of
The cubic unit cell of HKUST-1 consists of 2D layers
constructed from hexagonally organized BTC molecules
interconnected by copper pairs (figure 2). These (111) layers are
oriented perpendicular to the diagonal in space of the unit cell.
Viewed from the side, one set of these 2D layers is connected by
the two other sets of these planes intersecting under an angle of
70.53º, creating the cavity system between two consequent
parallel layers. Comparison of the PXRD pattern of the material
obtained at 393K with that of the intact structure revealed some
reflections persisting and remaining sharp during the initial
transformation. Some of these persistent reflections are associated
with indices h=k=l, and hence relate to the distance between
parallel 2D layers. In addition, reflections associated with indices
h=k, l=0 and h=k, l=2h are observed, which both relate to
distances in plane of the 2D layers. This indicates that in addition
to the distance between at least one set of 111 planes also their
dimensions are preserved during the transformation of the
structure between 298 and 393K. Further heating the material in
steam conditions up to 423K results in the formation of the green
[Cu2OH(BTC)(H2O)]n.2nH2O phase, which is a densely packed
layered material with very small interlayer distance (±3.5Å) and
altered intra-layer structure as compared to the layers present in
HKUST-1 (supplementary information).
Figure 1b shows the PXRD patterns for HPA@Cu3(BTC)2
samples after heat-treatment up to 483K in steam conditions.
Refinement of this series of identical patterns resulted in
unchanged unit cell parameters, confirming the absence of any
structural changes. As expected from the diffraction patterns, no
color-changes were observed. In HPA@Cu3(BTC)2, one type of
the big pores, empty in HKUST-1, is occupied by Keggin ions 17.
The presence of these guest species obviously prevents any
structural changes observed for empty HKUST-1 under steam
Figure 2: Cubic unit cell of HKUST-1. The parallel (111) layers are
composed by hexagonally organized BTC molecules (triangles)
interconnected by copper pairs (orange cylinders). The spheres inside the
pores represent the HPA.
The structure of the green [Cu2OH(BTC)(H2O)]n.2nH2O phase
obtained after steam treating unstabilised HKUST-1, cannot serve
as a host for large species like PW12O403- ( 11Å). The
transformation of HPA@Cu3(BTC)2 would hence require a
complete disassembly of the original structure before the densely
packed, layered material could be formed. The structural stability
of anhydrous HKUST-1 (up to 473K)16, demonstrates the
stability of the paddle-wheel geometry in absence of strongly
complexing ligands like OH-, so a full disintegration of HKUST-
1 and reassembly of its components into the dense layered phase
would be highly unlikely. Considering that the PXRD patterns
obtained at intermediate stages of the transformation indicate the
preservation of at least one set of the original 2D layers, it can be
concluded that the presence of Keggin ions between parallel
layers in the stabilised material prevent any lateral movement and
consequently lock the connecting BTC molecules in place. The
transformation of the paddle-wheel geometry in HKUST-1 into
the dinuclear µ-hydroxy
[Cu2OH(BTC)(H2O)]n.2nH2O would require significant structural
flexibility. In HPA@Cu3BTC2, the guest species prevents both
this structural flexibility and the re-orientation of BTC linking
parallel layers. Although the detailed transformation mechanism
for the conversion
[Cu2OH(BTC)(H2O)]n.2nH2O is currently not yet elucidated, the
results of this stability study indicate that the transformation only
occurs in presence of strong bridging ligands like hydroxyl and
that an initial delamination step is followed by a re-arrangement
of the BTC linkers.
In summary, we report a study investigating the stability of
Cu3(BTC)2 MOF in steam conditions. HPA@Cu3(BTC)2 is stable
until 483K in steam conditions, while the conversion of HKUST-
1 to Cu2OH(BTC) (H2O).nH2O starts at 343K. The superior
stability of HPA@Cu3(BTC)2 compared to the related,
isostructural HKUST-1 phase, increases its suitability for
practical applications in hydrous, heat conditions with
temperatures up to 483K.
J. Maes, S. Usé and W. Wouters are gratefully acknowledged
for their continuous technical assistance. We thank the Flemish
Government for long-term structural funding (Methusalem). E.B.
acknowledges the Flemish FWO for a postdoctoral fellowship.
C.E.A.K. and J.A.M. acknowledge support from FWO-
linked copper centres in
of HKUST-1 into
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