Hydrophobic dipeptide crystals: a promising Ag-free class of ultramicroporous materials showing argon/oxygen adsorption selectivity

Abstract

The adsorption isotherms of nitrogen, oxygen and argon in four VA-class hydrophobic dipeptides are presented. Isotherms were determined at 5, 20 and 35 °C, for a pressure range of 0–6 bar. Under these conditions, adsorption is still in the Henry region. For all materials and temperatures, the sequence of preferential adsorption is Ar > O₂ > N₂, a highly abnormal result. At 5 °C, the dipeptide with the smallest pores, VI, has Ar/O₂ adsorption equilibrium selectivities up to 1.30, the highest ever measured in Ag-free adsorbents. Gas uptakes, at 1 bar and 20 °C, are ∼0.05 mol kg⁻¹, very low relative values that are partially explained by the low porosity of the solids (<10%). The significance of these results for the development of new materials for the process of O₂ generation by pressure swing adsorption (PSA) is discussed. The results indicate some of the structural and chemical properties that prospective Ag-free adsorbents should have in order to have Ar/O₂ selectivity, hydrophobic pores, less than 0.5 nm-wide, and porosity of, at least, 20%.