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

Abstract

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.