Cation Control of Molecular Sieving by Flexible Li-Containing Zeolite Rho

Abstract

The adsorption of CO 2 on zeolite Li-Rho (unit cell composition Li 9.8 Al 9.8 Si 38.2 O 96 ) has been investigated by the measurement of adsorption isotherms (273 -300 K), breakthrough curves with a CO 2 /CH 4 /He mixture (308 K) and in situ synchrotron X-ray powder diffraction in CO 2 (298 K). The Rho framework distorts when in the Li-form to give a shape selective adsorbent for CO 2 over CH 4 , although breakthrough curves reveal diffusional limitations. In situ synchrotron powder XRD follows the expansion of the Li-Rho unit cell upon adsorption, which remains single phase to a CO 2 pressure of ca. 0.6 bar. Partial cation exchange of Li-2 Rho by Na + or Cs + gives two series of M,Li-Rho zeolites (M = Na, Cs). Where the occupancy of window sites (8R, D8R) between lta cages is less than 50%, hysteresis is not observed in CO 2 isotherms at 298 K. For Cs 1.8 Li 8 -Rho, which has a larger unit cell and a wider window than zeolite Li-Rho due to the presence of large Cs + cations in double 8-ring sites, breakthrough curves indicate faster CO 2 diffusion without significant loss of selectivity. We propose this control of adsorption kinetics of the flexible zeolite Rho via modification of cation content as a mechanism for cation controlled molecular sieving.

The Origin of Selective Adsorption of CO2 on Merlinoite Zeolites

“…This adsorption mechanism is substantially different from the cation gating mechanism found in rigid chabazites, where the gating cations temporarily move away from the 8MR center for the passage of guest molecules, without significant changes in the 8MR window dimension [6b] . In addition, we can distinguish between the CO 2 adsorption mechanism in Na‐MER‐2.3 here and the breathing mechanism in flexible zeolites Li‐rho‐3.9 (RHO) and K‐MER‐3.8 [7a,b] . It is noted that structural changes in the former zeolite occur during CO 2 adsorption due to the rearrangement of extraframework cations which are not window‐keepers.…”

Selective Adsorption of CO2 on Zeolites NaK-ZK-4 with Si/Al of 1.8–2.8

“…There is an ongoing discussion on the structural details of 8-ring zeolites and how those relate to the trapdoor, swing door, potential gating of the CO 2 adsorption, and framework flexibility. 14 , 15 , 15 , 16 , 42 , 43 We recently deduced from in situ XRD experiments that the cations of zeolite NaK-A did not redistribute upon CO 2 adsorption but instead were slightly displaced toward the center of the α-cage. Here, we performed a similar in situ X-ray diffraction study to investigate the cation position in zeolite K-ZK-4 upon CO 2 adsorption and the positioning of the CO 2 molecules upon adsorption.…”

Analysis and control of acid sites in zeolites

“…The widespread application of zeolites is a direct consequence of their properties, and by tuning their features, zeolites become accessible to exhibit the desired performance in a specific application. The possibilities of zeolite usage in catalysis and gas separation with novel substrates are intensively investigated, and every several years a new process is reported [14][15][16]. Hence, zeolites are undoubtedly going to continue to be broadly employed and, accordingly, improving their properties to enhance their performance is imperative.…”