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.

Influence of the Compensating Cation Nature on the Water Adsorption Properties of Zeolites

“…As expected, no N 2 adsorption is observed. This phenomenon was already mentioned in the literature [39,[54][55][56] and is due to the position of Na + cations near to the pore opening which obstruct the accessibility of N 2 to the microporosity.…”

“…The N 2 adsorption isotherm of NaX-0 zeolite (Figure 9b) show a nitrogen adsorption capacity around 170 cm 3 g −1 (microporous volume of 0.27 cm 3 g −1 ) which is lower than the one expected for FAU-type zeolite (around 0.32 to 0.34 cm 3 g −1 ) [56,58]. The LTA impurities contained in the raw material (see Figure 2b) could explain this lower value.…”

Permselectivity improvement in membranes for CO2/N2 separation

“…Zeolites were synthesized at the Instituto de Tecnología Química in Valencia, according to procedures reported in literature [32,33,34], with a Si/Al molar ratio of 5 and the properties that are summarized in Table 1. parameter.…”

Insights into CO2 and CH4 Adsorption by Pristine and Aromatic Amine-Modified Periodic Mesoporous Phenylene-Silicas

“…Thus, the ideal adsorbent is still undiscovered and needs to present all optimal features as, for instance, high capacity, perfect selectivity, long-term durability, stability, fast uptake processes and easy regeneration consistent with negligible capacity loss on multiple adsorption/desorption cycles. 30 In an attempt to achieve this, we are exploring the possibility of applying periodic mesoporous phenylene-silicas (PMOs) [31][32][33] as adsorbents for CO2/CH4 gas separation. The interest to use PMOs as adsorbents in this field relies in the assumption that these materials possess high specific surface areas, high pore volumes and are easy to be modified.…”