Enhancing the Kinetic Separation of Nitrogen and Methane on Alkaline Earth Metal-Modified Carbon Molecular Sieves

The commercial carbon molecular sieves (CMSs) were modified with alkaline earth metals (Mg, Ca, Sr, and Ba) by an incipient wetness impregnation method. The metal species were well dispersed on pristine CMSs and existed in the stable form of divalent cations. The modification showed greater influence on specific surface area rather than total pore volume and average diameter, which was mostly related to the location of metal species at the pore entrance, leading to the enhancement of diffusion resistance. The Ca-, Mg-, Sr-, and Ba-modified adsorbents weakened the adsorption of methane by more than 32% at 0.7 MPa without affecting the adsorption of nitrogen. These types of metals affected strongly the separation selectivity, tuning the methane selectivity between 3.7 and 5.5, and they also have significant influence on adsorption kinetics especially for methane diffusion. Compared to CMSs, the sample with Mg modification hardly affects the diffusion of nitrogen. The alkaline metal-modified adsorbents exhibited good stability, and the adsorption capacity can be recovered by thermal regeneration.