Cation control of cooperative CO₂ adsorption in Li-containing mixed cation forms of the flexible zeolite merlinoite

The lithium-exchanged form of a merlinoite zeolite (MER) with Si/Al = 4.2 (unit cell composition Li_6.2Al_6.2Si_25.8O₆₄) possesses a strongly contracted framework when dehydrated (the unit cell volume decreases by 12.9% from the hydrated ‘wide-pore’ form to the dehydrated ‘narrow-pore’ form). It shows cooperative adsorption behaviour for CO₂, leading to two-step isotherms with the second step at elevated pressure (>2.5 bar at 298 K). Partially exchanging Na and K cations to give single phase Li,Na- and Li,K-MER materials reduces the pressure of this second adsorption step because the transition from narrow- to wide-pore forms upon CO₂ adsorption occurs at lower partial pressures compared to that in Li-MER: partial exchange with Cs does not reduce the pressure of this transition. Exsolution effects are also seen at K cation contents >2.2 per unit cell. The phase transitions proceed via intermediate structures, by
complex phase behaviour rarely seen for zeolitic materials. The strongly distorted narrow-pore structures adopted upon dehydration give one dimensional channel structures in which the percolation of CO₂ through the material requires cation migration from their locations in ste sites. This is slow in Li_3.4Cs_2.8-MER where Cs cations occupy these critical ste cavities in the
channels, causing very slow adsorption kinetics. As the partial pressure of CO₂ increases, a threshold pressure is reached where cooperative adsorption and Cs cation migration occur and the wide-pore form results, with a three dimensionally connected pore system, leading to a sharp increase in uptake. This is far in excess of the increase of unit cell volume because more of the pore space becomes accessible. Strong hysteretic effects occur upon desorption, leading to CO₂ encapsulation. CO₂ remaining within the material after repeated adsorption/desorption cycles without heated activation improves sorption kinetics and modifies the stepped isotherms.