Tuning the adsorption potential. Separation of aromatic hydrocarbons by cobalt and zinc zeolitic imidazolate frameworks

The formation of solid solutions (alloys) is a well-established route for tuning the functional properties in materials science, e.g. steels, semiconductors, ceramics. As will be discussed below in this contribution, it appears as a convenient way to control the pore windows accessibility and surface properties. This study illustrates the tuning of the adsorption potential for aromatic hydrocarbons through the formation of solid solutions of Co(II) and Zn(II) imidazolates. The best separation capability and the highest adsorption heats were observed for the mixed compositions (alloys). The random substitution of Zn by Co atoms in the network of zinc methyl-imidazolate, and vice versa, create anisotropies in the metal coordination environment, which are responsible for the appearance of local electron field gradients. This is equivalent to an increase for the adsorption potential able to interact with molecules (adsorbates) with dipole and/or quadrupole moments. Such hypothesis was corroborated for the separation of benzene, toluene and p-xylene, three molecules with quadrupole moments. The highest adsorption heat values for these adsorbates correspond to the solid solution containing Zn and Co. The materials under study were characterized from chemical analysis, X-ray diffraction data, thermal analysis, and specific surface measurements.