Tuning the Adsorption Properties of Zeolites as Adsorbents for CO₂ Separation: Best Compromise between the Working Capacity and Selectivity

Abstract

The choice of an appropriate adsorbent for CO 2 separation by pressure-swing adsorption remains a field of intense research. In this work, several FAU and LTA zeolites with different Na contents (Si/Al ratios) are studied for the separation of CO 2 from mixtures of CO 2, CO, and CH 4 by means of breakthrough experiments. The breakthrough experiments were carried out between 1 and 5 bar at 303 K using two feed mixtures: 50/50 (v/v) CO 2 /CH 4 and 75/15/15 (v/v/v) CO 2 /CH 4 /CO. The most polar zeolites, i.e., those with high Na content, exhibit the highest adsorption capacity and selectivity for CO 2, but their regeneration is difficult; hence, their working capacity is low. The opposite is true for the least polar zeolites, i.e., those with low Na content. In order to quantify the trade-off between the selectivity and working capacity, the Ruthven statistical model (RSM) was used. It satisfactorily reproduced the experimental trends. We, therefore, used the RSM to identify the properties of the adsorbent that lead to an optimal compromise between the working capacity and separation factor. The critical parameter is the concentration of extraframework cations, which, in turn, depends on the framework charge of the zeolites FAU and LTA. The optimal trade-off zone is defined in terms of the Henry constant of CO 2 ( K CO 2 ). It is found that this zone is placed between K CO 2 = 5 × 10 –3 and 50 × 10 –3 molecules·bar –1 ·Å –3 . This interval corresponds to a heat of adsorption of CO 2 at zero coverage between 27 and 32 kJ·mol –1 . In our study, this optimal range of Henry constants was achieved for the zeolites Na-USY, SAPO-37, LTA (Si/Al = 5), and EMC-1.