The effect of the zeolite pore size on the Lewis acid strength of extra-framework cations

Abstract

The catalytic activity and the adsorption properties of zeolites depend on their topology and composition. For a better understanding of the structure–activity relationship it is advantageous to focus just on one of these parameters. Zeolites synthesized recently by the ADOR protocol offer a new possibility to investigate the effect of the channel diameter on the adsorption and catalytic properties of zeolites: UTL, OKO, and PCR zeolites consist of the same dense 2D layers (IPC-1P) that are connected with different linkers (D4R, S4R, O-atom, respectively) resulting in the channel systems of different sizes (14R × 12R, 12R × 10R, 10R × 8R, respectively). Consequently, extra-framework cation sites compensating charge of framework Al located in these dense 2D layers (channel-wall sites) are the same in all three zeolites. Therefore, the effect of the zeolite channel size on the Lewis properties of the cationic sites can be investigated independent of other factors determining the quality of Lewis sites. UTL, OKO, and PCR and pillared 2D IPC-1PI materials were prepared in Li-form and their properties were studied by a combination of experimental and theoretical methods. Qualitatively different conclusions are drawn for Li⁺ located at the channel-wall sites and at the intersection sites (Li⁺ located at the intersection of two zeolite channels): the Lewis acid strength of Li⁺ at intersection sites is larger than that at channel-wall sites. The Lewis acid strength of Li⁺ at channel-wall sites increases with decreasing channel size. When intersecting channels are small (10R × 8R in PCR) the intersection Li⁺ sites are no longer stable and Li⁺ is preferentially located at the channel-wall sites. Last but not least, the increase in adsorption heats with the decreasing channel size (due to enlarged dispersion contribution) is clearly demonstrated.