Tuning the Adsorption Properties of Isoreticular Pyrazolate-Based Metal–Organic Frameworks through Ligand Modification

Two isoreticular series of pyrazolate-based 3D open metal–organic
frameworks, MBDP_X, adopting the NiBDP and ZnBDP structure types [H2BDP = 1,4-bis­(1H-pyrazol-4-yl)­benzene], were synthesized with the new tagged
organic linkers H2BDP_X (X = −NO2, −NH2, −OH).
All of the MBDP_X materials have been characterized through
a combination of techniques. IR spectroscopy proved the effective
presence of tags, while X-ray powder diffraction (XRPD) witnessed
their isoreticular nature. Simultaneous TG/DSC analyses (STA) demonstrated
their remarkable thermal stability, while variable-temperature XRPD
experiments highlighted their high degree of flexibility related to
guest-induced fit processes of the solvent molecules included in the
channels. A structural isomer of the parent NiBDP was
obtained with a sulfonate tagged ligand, H2BDP_SO3H. Structure solution from powder diffraction data collected at three
different temperatures (room temperature, 90, and 250 °C) allowed
the determination of its structure and the comprehension of its solvent-related
flexible behavior. Finally, the potential application of the tagged
MOFs in selective adsorption processes for gas separation and purification
purposes was investigated by conventional single component adsorption
isotherms, as well as by advanced experiments of pulse gas chromatography
and breakthrough curve measurements. Noteworthy, the results show
that functionalization does not improve the adsorption selectivity
(partition coefficients) for the resolution of gas mixtures characterized
by similar high quadrupole moments (e.g., CO2/C2H2); however, the resolution of gas mixtures containing
molecules with highly differentiated polarities (i.e., N2/CO2 or CH4/CO2) is highly improved.