Porous dipeptide crystals as selective CO₂adsorbents: experimental isotherms vs. grand canonical Monte Carlo simulations and MAS NMR spectroscopy

Abstract

Molecular crystals of dipeptides containing open channels can selectively absorb CO 2 over N 2 and CH 4 , as shown by experimental and simulated isotherms. The efficient CO 2 capture enabled separation from methane and allowed the direct observation by 2D MAS NMR of CO 2 sequestered in the peptide-based biozeolites.

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

“…Desorption isotherms were also determined in every case (not shown) and hysteresis was never detected. N2 adsorption isotherms have been determined before, for 25 ºC 4 , and are similar to the ones here reported. The adsorption isotherms were numerically fitted with the Volmer equation,…”

“…A previous report 4 showed that CO2 adsorption in VA-class dipeptides has a mixed localised/mobile character. However, the N2 and O2 molecules and the Ar atom are much shorter and more electronically homogeneous than the CO2 molecule.…”

“…Crystalline hydrophobic dipeptides of the VA-class 1, 2 are a type of ultramicroporous solids that has received considerable attention in recent years [3][4][5][6] . The structure of these molecular crystals is stabilised by H-bonds and hydrophobic interactions, being essentially the same for all VA-class dipeptides.…”

“…Different side-chain combinations create slightly different crystal structures, with different pore sizes 1 . The micropores are helical and have an approximately circular crosssection, somewhat uniform throughout the unit cell 4,7 . Nominal, average pore sizes of VA-class dipeptide crystals range from 0.37 nm to 0.50 nm 2,7 .…”

“…This observation is supported by vibration spectroscopy 15 and 129 Xe NMR 3 studies. For guest molecules smaller than the pore, there has yet to be demonstrated any clear effect resulting from framework flexibility 4 .…”