Dalton Transactions
Communication
Fig. 6 left: CAU-14 as synthesised, right thermally activated CAU-14.
ponds to the three coordinated water molecules. Around p/p0 =
0.2 a steep uptake of ten water molecules per formula sum is
observed (23.1 wt%). The water sorption is fully reversible
and the large loading lift lies within a narrow pressure range
(0.18 ≤ p/p0 ≤ 0.20). In addition only a comparatively small
hysteresis is observed (Fig. 5).
These properties could make CAU-14 a promising candidate
for heat storage and transformation applications with water as
working fluid.14
Fig. 4 Space filling view of one pore of CAU-14 along the c-axis. A
sphere of 9.4 Å in diameter is added to demonstrate the pore size. Water
molecules in the pores are omitted for clarity.
Thermogravimetric measurements (Fig. S2†) showed a
weight loss up to 150 °C due to the loss of solvent molecules.
The framework of CAU-14 is stable up to 380 °C in air. Hence
it was activated prior to the sorption experiments at 150 °C for
18 h under reduced pressure (10−2 kPa). Evaluation of the N2
sorption isotherm (Fig. S3†), recorded at 77 K, lead to a
specific surface area of aBET = 647 m2 g−1 and a micropore
volume of Vmic = 0.27 cm3 g−1 (theoretical micropore volume
Vmic = 0.37 cm3 g−1). CAU-14 is also porous towards CO2
(uptake of 1.1 mmol g−1 (4.84 wt%) at 100 kPa) and H2 (uptake
of 3.3 mmol g−1 (0.67 wt%) at 100 kPa) (Fig. S4 and S5†). Stabi-
lity of CAU-14 was demonstrated by PXRD measurements. A
comparison of the PXRD patterns of as-synthesized CAU-14
and CAU-14 after the sorption measurements with a simulated
PXRD pattern is given in Fig. S6.†
The activation leads also to the removal of the coordinated
water molecules, leaving the framework with unsaturated open
metal sites. This was confirmed by the results of the water
sorption measurement (Fig. 5) and the colour change of the
sample from light to dark green (Fig. 6). The water vapour iso-
therm shows two distinct steps; first three water molecules per
formula sum are adsorbed up to p/p0 = 0.05, which corres-
Conclusions
The small number of porous metal phosphonates has been
expanded by the new permanently porous metal phosphonate
CAU-14, which is thermally stable up to 380 °C and shows
reversible water uptake at 25 °C. The trigonal planar shape of
the phosphonic acid prevents the formation of metal phospho-
nate layers and leads to a porous honeycomb network. Hence
the use of trigonal linkers seems to be a promising way to
obtain porous metal phosphonates. The structure of CAU-14
[Cu3(PPT)(H2O)3]·10H2O was determined by single crystal X-ray
diffraction using synchrotron radiation. Permanent porosity
was demonstrated by sorption of N2, CO2, H2 and H2O. The
measured porosity towards nitrogen (Vmic = 0.27 cm3 g−1) is
comparable to the well studied porous metal phosphonate
STA-12, [M2(H2O)2(O3PCH2NC4H8NCH2PO3)]·xH2O, M = Mg,
Mn, Fe, Co, Ni. Upon activation the water molecules in CAU-14
which are bonded to the copper can be removed. This leads to
a framework with unsaturated metal sites which could be
useful for catalytic transformations.
Acknowledgements
We thank Diamond Light Source for access to beamline I19
(proposal number 9216) that contributed to the results pres-
ented here.
Notes and references
1 Themed issue on Metal Organic Frameworks, Chem. Rev.,
2012, 112, 673; Themed issue on Metal Organic Frame-
works, Chem. Soc. Rev., 2014, 43, 5405.
2 A. Clearfield and K. Demandis, in Metal Phosphonate Chem-
istry: From Synthesis to Applications, The Royal Society of
Chemistry, Cambridge, UK, 2012.
Fig. 5 Water sorption isotherm of CAU-14, measured at 298 K.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans.
Hermer
