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.