CO₂ capture by ZSM-5 with varied Si/Al molar ratios: Isothermal adsorption capacity of CO₂ vs dynamic CO₂ adsorption capacity

ZSM-5 zeolites with varying Si/Al molar ratios were synthesized to investigate their CO₂ adsorption capabilities. It was found that the isothermal adsorption capacity of CO₂ showed a volcano curve with respect to Si/Al ratio, maximum at Si/Al = 300, which was in relation with surface area and proportion of mesopores. Specifically, ZSM-5-300 displayed an impressive isothermal adsorption capacity of 2.76 mmol/g at 1 atm. Isosteric heats of adsorption analysis indicated that CO₂ adsorption predominantly involved physical adsorption on silica. In contrast, dynamic CO₂ capacities for all ZSM-5 samples remained relatively low, ranging from 0.006 to 0.007 mmol/g, representing less than one percent of the isothermal adsorption capacity. These suggests a strong relationship between CO₂ capacity and CO₂ concentration, emphasizing the potential significance of dynamic CO₂ capacity as a more relevant indicator for direct air capture applications, given the typical CO₂ concentration in air of approximately 400 ppm. Furthermore, while acid treatment and amine grafting were observed to enhance dynamic CO₂ capacity, including ZMS-5, ZSM-5-AT, and NH2-ZSM-5-AT samples, the resulting capacities remained quite modest, all falling below 0.1 mmol/g. These findings indicated that isothermal CO₂ capacity was primarily influenced by physisorption and chemisorption, which, in turn, was influenced by the texture of the adsorbents. Conversely, dynamic CO₂ capacity was closely associated with chemisorption, reflecting the interaction between CO₂ and the adsorbents.