Single Crystal X-ray Diffraction Studies of Carbon Dioxide and Fuel-Related Gases Adsorbed on the Small Pore Scandium Terephthalate Metal Organic Framework, Sc2(O2CC6H4CO2)3

Abstract

The adsorption behavior of the microporous scandium terephthalate Sc2(O2CC6H4CO2)3 for small fuel-related molecules has been measured. The structure shows an adsorption capacity for N2 and CO2 of 6.5 mmol g(-1) and is able to take up straight chain hydrocarbons. The mechanism of adsorption of CO2, CH4, and C2H6 has been determined by single crystal synchrotron X-ray diffraction at approximately 230 K. Adsorption of CO2 at 235 K and 1 bar pressure and H2 at 80 K and 0.25 bar results in each case in a symmetry change from orthorhombic Fddd to monoclinic C2/c through tilts in the terephthalate linkers. CO2 molecules take up different sites in the two symmetrically different channels that result from this symmetry change. The structure remains orthorhombic in 9 bar of CH4 and 5 bar of C2H6, and the adsorption sites are located. CH4 and C2H6 are observed to adopt sites within the channels, and C2H6 is also observed to adopt adsorption sites between phenyl groups in the channel walls, suggesting that the structure is sufficiently flexible to allow diffusion of small molecules between adjacent channels.