In the next step, a multicomponent gas mixture containing CO₂/SO₂/NO/N₂ was used to characterize the adsorption behavior of the 13X zeolite monoliths and beads under both dry and humid conditions. Figure 6 displays the concentration fronts for the three adsorbents collected during dry-contaminated and humid-contaminated breakthrough runs. It should also be noted that SO₂ adsorption did not break through the bed in any experiment. On the other hand, He broke through almost immediately, while N₂ and NO₂ broke through at ~10 min for all samples and exhibited sharp breakthrough wavefronts under both dry and humidified conditions. This was attributed to the low affinity of 13X towards He, N₂and NO, but high affinity towards SO₂. Namely, Deng et al.¹³ reported that zeolites have a high affinity towards SO₂ compared to N₂, NO, and CO₂ because SO₂ is the only molecule which contains a permanent dipole moment. In turn, this increases the Henry’s Law constant for SO₂ compared to the other compounds and promotes greater adsorption on zeolites. In comparison, the Henry’s Law constants for NO and N₂ are much lower and, because of this, these species do not adsorb in high quantities on zeolite 13X, as further demonstrated by their rapid breakthrough times. On the other hand, because the SO₂ concentration in the stream was so low, and the zeolite affinity towards the gas was so high, SO₂ saturation was never achieved and the gas was not detected. It is also worth noting here that, similar to CO₂, the pre-humidification likely reduced the overall SO₂ adsorption capacity, as water also exhibits a permanent dipole moment and should compete with SO₂ on zeolite.²⁴ However, given the low SO₂concentration relative to H₂O, it is impossible to say for certain whether or not its capacity was reduced under humidified conditions. This being the case, because the goal of the study was to examine CO₂ adsorption behavior in the presence of SO₂, N₂, and NO, and not focusing on the adsorption behavior for the contaminant species, the experiments were terminated after CO₂ saturation was observed. For reference, the SO₂ desorption profiles are displayed inFigure S3, Supporting Information .

As evident from Figure 6 , introduction of the acidic contaminants produced significantly longer CO₂breakthrough times for all three adsorbents under both dry and wet conditions. This caused the CO₂ adsorption capacity to increase by 0-20% in the dry experiments and by ~100% in the wet experiments. This also corresponded to an increase in CO₂ selectivity of 5-30% across the dry experiments for the various samples, whereas the selectivity increased by 50% or more between the humidified-clean and contaminated runs. These increases in adsorption capacity were to be expected from established literature for multicomponent adsorption, which illustrates the promotional effect of SO₂ for CO₂ capture.²⁵This may be attributed to the promotional effect of the acidic contaminants towards CO₂, which becomes more pronounced in the presence of water, on account of the formation of carbonate species. Specifically, under dry operation, the acidic SO₂ increase the sorbents basicity by reacting with the slightly basic Na₂O species, to produce oxygen atoms which share free radicals. In turn, this causes the contaminants to act as chemisorbents and increases the overall affinity towards CO₂,¹⁴ but also degrades the zeolitic alumina. Under humid mode, however, the Na₂O readily reacts with water to form NaOH, which then bonds to the oxygen free radicals in the SO₂, to produce species which do not interact with the Al₂O₃ centers. These free electrons can then react with CO₂ to form carbonate, similar to the promotional mechanism of water on CO₂ for amine-modified solids⁴ or the promotional effect which occurs between SO_x, NO_x, CO₂, in other solid adsorbent systems.^25,26

Comparison of CO₂ concentration profiles obtained under each mode of breakthrough experiments are presented in Figure 7 . It can be observed that the breakthrough time was shortened significantly in the humid bed as a result of competitive adsorption of CO₂ and water, however, when SO₂ and NO were present, a reverse trend was observed and total dynamic adsorption amount was enhanced. Such cooperative adsorption occurred across all three samples examined in this study and became further exacerbated under humid-contaminated conditions. Considering the shape of wavefronts, the zeolite 13X beads (Figure 7a ) exhibited the sharpest breakthrough front under dry-clean conditions, however, exhibited nearly double the breakthrough width under both humidified modes, on account of the competitive CO₂/water adsorption behavior. In comparison, the dry-contaminated mode did not increase the breakthrough width nearly as much as the humidified modes of operation. However, the additional chemisorption still broadened the profile by 65% from the dry-clean run. Both the 600 cpsi (Figure 7b ) and 800 cpsi (Figure 7c ) monoliths experienced similar changes in wavefront broadness across the different modes, however, the increases in front width were much less compared to the beads. This was attributed to the monoliths’ higher rate of film mass transfer compared to the beads, which is a well-established benefit of using monolithic contactors.²⁷ Between the two monolith samples, the 800 cpsi sample experienced greater increases in front width from the dry-clean to the other modes compared to the 600 cpsi monolith. As discussed previously, this was attributed to the increased kinetic dependence on molecular mass transfer which occurs at elevated cpsi monoliths, leading to more bead-like behavior.^12,16

After breakthrough tests, TGA runs were conducted over the used samples to investigate the degree of CO₂ capacity loss after exposure to impurities. Figure 8 compares the CO₂ uptake capacities of the three adsorbents after each mode of breakthrough runs with that of fresh materials. As can be seen, the differences in CO₂ adsorption between the fresh, dry-clean, and humid-clean samples were, overall, marginal. Notably, the dry-clean samples all exhibited slightly lesser CO₂adsorption from the fresh and humid-clean samples, however, these losses were only ~8% and could have been caused by slight changes in intraparticle bonding from the repeated heating and cooling of the samples. On the other hand, the losses in adsorption capacity for the dry-contaminated runs could not be discounted, as the beads, 600, and 800 cpsi monoliths exhibited 38%, 58%, and 14% losses in CO₂ adsorption capacity, respectively. As previously discussed, these sizable losses could likely be attributed to decomposition of the sodium centers after reaction with the acid gases. Following the humidified-contaminated runs, however, the losses in CO₂ capacity were only marginal, as the reductions in CO₂ adsorption capacity were only 25%, 14%, and 11% for the beads, 600 cpsi monolith, and 800 cpsi monolith, respectively. On the basis of these results, it was concluded that water can act as a protective barrier during CO₂ adsorption on zeolite when SO_x/NO_x are present, as it allows the acid gases to react with hydroxide clusters instead of decomposing the alumina centers.