Structural Chemistry, Flexibility, and CO₂ Adsorption Performance of Alkali Metal Forms of Merlinoite with a Framework Si/Al Ratio of 4.2

Abstract

Small pore zeolites that show framework flexibility, such as merlinoite (topology type MER), possess a high potential for the selective adsorption of small gas molecules including CO 2 . The CO 2 adsorption properties of Na-, K-, and Cs-exchanged forms of a merlinoite zeolite with Si/Al = 4.2 have been measured at 298 K, and in situ PXRD was used to follow their structural response to dehydration and CO 2 uptake. The Na-and Cs-forms convert from a wide-pore to a narrow-pore form upon dehydration, while the K-form remains in the wide-pore form. The Na-and Cs-forms exhibit stepped CO 2 adsorption isotherms, consistent with breathing behavior and expansion from narrow-to wide-pore phases, while K 6.2 -MER remains in the wide-pore structure throughout. Synchrotron PXRD of the Kand Cs-forms reveals the effects of CO 2 adsorption on the cation site distributions and the framework configuration. All cation forms of MER (4.2) show enhanced adsorption kinetics for Ar compared to those with lower Si/Al, and the wide-pore structure of K 6.2 -MER (4.2) shows particularly rapid sorption for both Ar and CO 2 . Breakthrough curves over K 6.2 -MER (4.2) demonstrate good separation of CO 2 from CH 4 in flowing CO 2 /CH 4 mixtures, even in pelletized form with an alumina binder.

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

“…Similar but more severe trapdoor effects upon CO 2 adsorption are observed in low silica aluminosilicate CHA materials containing potassium and prepared in the absence of an OSDA. ,, Nonetheless, said CHA materials operate poorly under the presence of water, which is a major issue for industrial implementation . Merlinoite is another zeolite which presents guest-induced flexibility upon CO 2 adsorption and very promising CO 2 /CH 4 selectivities. ,, Additionally, Hong et al have reported its OSDA-free synthesis, making it a very promising and potentially cheap CO 2 adsorbent …”

“…515 g Breakthrough data of 10/30/50 (CO 2 /CH 4 /He), 1 bar total prressure. 517 h Breakthrough data of a CO 2 /CH 4 40:60 mixture 520 at 5 bar total pressure. i Breakthrough data of equimolar CO 2 /CH mixtures 262 the search for better adsorbents that can further improve the efficiency and economy of the process is still a very active research field.…”

“…516 Merlinoite is another zeolite which presents guest-induced flexibility upon CO 2 adsorption and very promising CO 2 /CH 4 selectivities. 264,517,518 Additionally, Hong et al have reported its OSDA-free synthesis, making it a very promising and potentially cheap CO 2 adsorbent. 264 Table 3 lists many of the mentioned adsorbents and includes relevant descriptors and parameters related to the adsorption of CO 2 and its separation from CH 4 .…”

“… f Breakthrough data of equimolar CO 2 /CH 4 mixture, 2 bar total pressure g Breakthrough data of 10/30/50 (CO 2 /CH 4 /He), 1 bar total prressure h Breakthrough data of a CO 2 /CH 4 40:60 mixture at 5 bar total pressure. i Breakthrough data of equimolar CO 2 /CH 4 mixtures at 1 bar total pressure. …”

A forgotten participant in pore deblocking of zeolites: dicarbonate in NaMeA zeolites, Me = Na, K, Rb, Cs

“…3 Cation gating appeared in different cationic form zeolites as cationic displacements in the presence of CO 2 or its mixtures. 3–6 The flexibility-driven effects are observed in the same systems in parallel. More precisely, the unusual growth of CH 4 adsorption of Me′MER zeolites (Me′ = Na, K, Cs with different Si/Al modulus) after the minor addition of CO 2 to the gas has been interpreted as a consequence of the transformation of the 8R windows.…”