Cu2(pyrazine-2,3-dicarboxylate)2(4,4′-bipyridine) Porous Coordination Sorbents: Activation Temperature, Textural Properties, and CO2 Adsorption at Low Pressure Range

The effect of activation temperature on the textural properties and low-pressure adsorption performance of the porous coordination polymer Cu-2(pzdc)(2)(bpy) [pzdc = pyrazine-2,3-dicarboxylate, bpy = 4,4'-bipyridine], better known as CPL-2, was considered to elucidate the material potential for separations. The effective activation temperature range was estimated via Coupled thermal gravimetric and Fourier transforms infrared spectroscopy analysis. A textural property analysis via the alpha(s)-plot, Dubinin-Radushkevich and Horvath-Kawazoe methods show that a significant reduction in effective surface area and micropore volume occurs when the activation temperature is increased from 373 to 423 K. Cooling of the sample in a moisture-free environment revealed that such reduction is nonreversible, as evidenced by single-component CO2 equilibrium adsorption tests. Although CO2 equilibrium adsorption isotherms exhibit a linear behavior in the ambient pressure range, an increase in activation temperature eventually decreases the pore size of the structure resulting in a considerable decrease in loading amounts. This was also corroborated by means of in situ high-temperature X-ray diffraction, which was used to monitor the lattice semiquantitative changes of CPL-2 during the thermal activation sequence. In addition, adsorption uptake data was gathered to estimate a diffusion time constant and elucidate preliminary information about the kinetics involved during the transport of CO2 through the micropores of CPL-2. After inspection of the adsorbent particle morphology via scanning electron microscopy, it became ostensible that the transport phenomenological model suitable to fit the uptake data was that of a slab-shape particle. For the sample pretreated at 373 K the analysis yields an average diffusion time constant of ca. 0.5 s(-1) at 298 K.