Systematic evaluation of textural properties, activation temperature and gas uptake of Cu2(pzdc)2L [L = dipyridyl-based ligands] porous coordination pillared-layer networks

Abstract

In situ high temperature X-ray diffraction, nitrogen porosimetry and gas adsorption at room temperature were used to elucidate the effect of the degassing or activation temperature on the long-range and micropore textural properties of a series of coordination polymers with pillared-layer structures. Ramp-and-soak thermal gravimetric analysis performed at selected activation temperatures were used to verify the thermal stability of a CPL-n series [Cu₂(pzdc)₂L; pzdc = pyrazine-2,3-dicarboxylate; L = 4,4-azopyridine (apy) for CPL-4, 1,2-di-(4-pyridil)-ethylene (bpe) for CPL-5, N-(4-pyridyl)-isonicotinamide (pia) for CPL-6, and 1,2-di-(4-pyridyl)-glycol (dpyg) for CPL-7]. Although the activation temperatures were far below the decomposition point of the complexes, these resulted in significant and unique changes in micropore surface area and volume, even for CPL-4, -5 and -6, which contained pillar ligands with similar dimensions and similar structural long-range order. For the case of CPL-7, however, the framework appeared to be non-porous at any given activation temperature. Pure component equilibrium adsorption data gathered for CO₂, CH₄, and N₂ were used to elucidate the CPL-n materials potential for storage and separations at room temperature. All of the materials exhibited considerable selectivity toward CO₂, particularly at moderate pressures. Meanwhile, CO₂ isosteric heats of adsorption indicated that the pore functionalities arising from the pillar ligands provided similar interactions with the adsorbate in the cases of CPL-4 and -5. For CPL-6, the presence of the carbonyl (CO) group appeared to enhance interactions with CO₂ at low loadings.