Raman Spectroscopic Investigation of CH₄ and N₂ Adsorption in Metal−Organic Frameworks

The adsorption behavior of CH4 and N2 (298 K, 30 bar) in a series of isoreticular metal−organic frameworks (IRMOFs) was investigated by Raman spectroscopy. For CH4, the ν1 vibrational mode shifted to lower frequency by 7.6, 8.4, 11.0, 10.3, and 10.1 cm-1 from 2917 cm-1 when adsorbed to IRMOF-1, -6, -8, -11, and -18, respectively. Along this same series, the adsorbed N2 stretch exhibited smaller shifts of 2.7, 3.1, 4.2, 4.1, and 3.7 cm-1. These shifts arise because of interactions within the framework pores, and not with the outer crystal surface. In all cases, Raman spectra at pressures up to 30 bar showed that saturation of the sorption sites does not occur. The observed shifts of the vibrational modes for each gas indicate different chemical environments within different IRMOFs, pointing to the important role the linkers play in the adsorption of gases.