Application of metal–organic frameworks with coordinatively unsaturated metal sites in storage and separation of methane and carbon dioxide

The metal–organic frameworks M2(dhtp)(H2O)2·8H2O (CPO-27-M, M = Ni, Mg) can be activated to give the empty framework compounds M2(dhtp) with a honeycomb analogous structure containing large micropores of 11–12 Å diameter and a high concentration of open metal sites. These sites play a major role in the adsorption of methane and carbon dioxide, which was studied at pressures up to 100 bar and 50 bar, respectively, and various temperatures in the range of 179 to 473 K. Both gases are taken up by the material in significant amounts. The maximum excess adsorption of CO2 observed at 298 K was 51 wt.% for Ni2(dhtp) and 63 wt.% for Mg2(dhtp). A surprisingly large amount of CO2, in the range 25–30 wt.%, was still adsorbed at 473 K. Up to 18 and 22 wt.% methane were adsorbed at 179 K in the nickel and the magnesium compound, respectively. Congruent with this result is the high isosteric heat of adsorption observed, which was found to be in the range 38–43 kJ mol−1 for CO2 and 20–22 kJ mol−1 for CH4, initially. The heat of adsorption decreases significantly after the open metal sites have been occupied, which also is reflected in the shape of the adsorption isotherms. The vacant coordination site at the metal atom also imparts favorable properties in respect to gas separation onto the material. Breakthrough experiments using Ni2(dhtp) and gas mixtures of CO2–N2 and CO2–CH4 demonstrate the ability of the material to separate these gases. It is shown that carbon dioxide is preferentially adsorbed over methane or nitrogen. In the case of CO2–N2, the retention is quantitative within the precision of the detection system.