A new class of materials for carbon capture and storage (CCS): adsorption of CO2 and N2 on pure-silica zeolites—the rpa cavity

Abstract Pure-silica zeolites are expected to exhibit an excellent performance for CO 2 /N 2 separation due to the combination of hydrophobicity and favorable kinetic effects. Pure-silica chabazite separates CO 2 from N 2 in the presence of water, with good selectivity and much higher CO 2 adsorption capacity than the benchmark zeolite 13X. This performance encouraged us to develop a systematic theoretical study of these materials, aiming at a molecular-level understanding of the adsorption phenomena that could assist the experimentalist to synthesize new and more efficient porous materials for CCS. The present work reports the results for the rpa cavity, one of the simplest cavities to match the above-mentioned criterion, using a methodology previously developed and validated for the pure-silica chabazite. The calculated molar enthalpies of adsorption for CO 2 and N 2 are both higher on t-rpa than on t-cha . However, < ∆H > CO2 is increased by 12.9 kJ/mol, whereas < ∆H > N2 is increased by only 1.4 kJ/mol. As a result, the difference < ∆H > CO2 − < ∆H > N2 is higher in t-rpa than in t-cha . Considering that this quantity is an estimator of the relative affinity of CO 2 /N 2 for the adsorbent, and also that Si-CHA has already been experimentally proved to be an excellent adsorbent for separating CO 2 from N 2 in the presence of water, we conclude that pure-silica zeolites with rpa cavities would be very promising materials for this kind of separation. Moreover, in spite of the CO 2 adsorption energy increase, < ∆H > CO2 is still small enough to allow CO 2 to be easily desorbed.