Alkali Metal Cations Influence the CO₂ Adsorption Capacity of Nanosized Chabazite: Modeling vs Experiment

Abstract

The CO 2 adsorption behavior of nanosized chabazite (CHA) containing different compositions of extra-framework alkali metal cations (Na + , K + , Cs + ) with a constant Si/Al ratio of 2.2 was investigated experimentally (adsorption isotherms and in situ Fourier transform infrared (FTIR) spectroscopy) and theoretically by Grand Canonical Monte Carlo (GCMC) modeling. The CO 2 adsorption isotherms were in good agreement with the GCMC modeling in the pressure range of 0−60 kPa for all samples except the Cs + -containing nanosized CHA. The GCMC modeling did not consider the kinetics of adsorption; thus, the CO 2 capacity of the Cs-CHA sample was overestimated; the partial blockage of the pores and inaccessibility of CO 2 to the free voids of the CHA structure was experimentally observed by recording adsorption isotherms and in situ FTIR study. The FTIR results revealed that CO 2 adsorbed mainly physically on all samples with the sorption capacity decreasing in the following order: Na-CHA ≃ K-CHA > Cs-CHA. The Na-CHA demonstrated significant micropore volume compared to the other samples as measured by N 2 physisorption due to the inability of Na + to effectively occupy the eight-membered rings (8MRs). This study implies that the different alkali metal cation forms of the nanosized CHA zeolite are of practical interest for flue gas purification, where a high CO 2 /N 2 selectivity is required.

Zeolites in Adsorption Processes: State of the Art and Future Prospects

“…These were first described by Shang et al in 2012, after observing remarkably high CO 2 /CH 4 selectivities on low silica exchanged chabazites . Since then, trapdoor phenomena in zeolite adsorption have received a great deal of attention and research effort, due to the large selectivities achievable. − These phenomena are usually susceptible to external stimuli, such as temperature, and are usually triggered by molecules that interact strongly with the extra framework cations present in the zeolite, such as water and carbon dioxide.…”

Altering the Atomic Order in Nanosized CHA Zeolites by Postsynthetic Silylation Treatment

“…All samples display five distinct resonances at −89, −94, −99, −104, and −109 ppm corresponding to the five crystallographic SiO 4 sites coordinated by either Al 3+ or hydroxyl groups assigned to Q 0–4 species. [ 13,20,40 ] The Si/Al ratio of all samples was determined using Engelhardt and Michel equation and the results are shown in Table 1. [ 41 ]…”

“…Six nanosized CHA samples (2.0 < Si/Al < 4.0) were prepared by hydrothermal post-synthetic treatment of the reference nanosized CHA zeolite sample (Si/Al = 2.0) synthesized according to the procedure developed by our group. [13,20] The structural details of the nanosized CHA samples are presented in Table 1 and Table S1 (Supporting Information). Based on the conditions of silylation, the samples were labeled as CHA-pHα-βSi, where α is the pH of the silylation solution and β is the TEOS/ zeolite ratio in mmol g −1 of zeolite.…”

“…All samples display five distinct resonances at −89, −94, −99, −104, and −109 ppm corresponding to the five crystallographic SiO 4 sites coordinated by either Al 3+ or hydroxyl groups assigned to Q 0-4 species. [13,20,40] The Si/Al ratio of all samples was determined using Engelhardt and Michel equation and the results are shown in Table 1. [41] Interestingly, the treatment of the CHA zeolite with 2 mmol g −1 of Si under basic condition (sample CHA-pH8.5-2Si), gives rise to a new peak at −113 ppm assigned to Q 4 species (Figure 2 and Figure S4a, Supporting Information).…”

“…Chabazite (CHA) zeolites with Si/Al < 3.0 are excellent CO 2 adsorbents used for separation of CO 2 from CH 4 or N 2 streams thanks to the interaction of CO 2 molecules with the framework Al tetrahedrons. [ 10–13 ] However, CHA zeolites with Si/Al > 3.0 are considered for various catalytic applications such as selective catalytic reduction (SCR) of NO x or methanol to olefins (MTO) reactions. [ 11 ]…”