Figure 1 PXRD patterns of MOF-74 and py-MOF-74.
PXRD of MOF-74 show typical
dominated diffraction peaks at 2θ of 7° and 12°, which is consistent
with the simulated pattern obtained from the single-crystal structure.
Pyrazine-modified
MOF-74, namely py-MOF-74a, py-MOF-74b and py-MOF-74c show featured
diffraction peaks analogous with that of MOF-74, indicative of no loss
of crystallinity after pyrazine modification (Figure 1).
The pristine MOF-74 crystal maintains stability with a tiny weight loss
before ~420°C. An incipient structural decomposition
appears at the higher temperature. Modified MOF-74 crystals show
negligible weight loss below 250°C
(Figure 2a), which has been beyond the vaporization temperature of
pyrazine (115oC at 1 atm). It suggests that pyrazine
molecules are embodied into the framework of MOF-74 by chemical bonds.
It is worth mentioning that the introduction of pyrazine molecules
improves the thermal stability of the MOF-74 framework; a significant
weight loss appears after 500°C. In the light of above analysis, the
weight loss of py-MOF-74 before this temperature, corresponding to the
pyrazine removal, was utilized to estimate the pyrazine loading ratio.
The weight loss for py-MOF-74a, py-MOF-74b and py-MOF-74c is 16.4,
26.0% and 33.9%, respectively, which can be translated into py-MOF-74
with the formula of Co2(dhtp)(py)0.765,
Co2(dhtp)(py)1.37 and
Co2(dhtp)(py)2.00. Py-MOF-74c permits
one pyrazine molecule insert to one open metal site, giving the highest
pyrazine loading content. A vibration peak at 440 cm-1in FTIR spectra of pyrazine modified MOF-74 can be assignable to the
formation of Co-N bonds (Figure 2b). And the peak becomes strong with
the increase of pyrazine content. It further indicates that
pyrazine molecules are bonded to
the open metal sites [Co (II)] of MOF-74, forming
pyrazine-interior-embodied composites. N2 adsorption
isotherms were measured at 77K (Figure 3a). The parent framework and
modified MOF-74, except for py-MOF-74c, exhibit typical Type-I
adsorption isotherms: a steep adsorption behavior at the relative
pressure P/P0 < 0.01 with the saturation
capacity reaching 316, 138 and 68 cm3g-1 for MOF-74,
py-MOF-74a and py-MOF-74b,
respectively. Owing to the blocking effect, N2 can
hardly access to the py-MOF-74c with the highest pyrazine loading.
Accordingly, the Brunauer-Emmett-Teller (BET) surface area is 605, 300
and 42 m2 g-1for
py-MOF-74a,
py-MOF-74b and py-MOF-74c, which is inferior to parent MOF-74 (1358
m2 g-1). The block effect arising
from pyrazine molecules also leads to a significant shift of pore size
distribution (PSD) from 9.85 nm (MOF-74) to 9.28 nm (py-MOF-74a).
Further increasing the loading of pyrazine, the pore volume of
py-MOF-74b and py-MOF-74c sharply shrinks, along with the PSD almost
undetectable by the current gas probe. We suppose the existence of
extremely narrow pores in py-MOF-74, which are inaccessible to
N2 molecule.