Role of Al Distribution in CO2 Adsorption Capacity in RHO Zeolites

Tailoring the CO2 adsorption performance of high-aluminum containing zeolites is typically considered from the perspective of controlling the type and location of extra-framework cations. In this work, using solid-state 29Si nuclear magnetic resonance (NMR), we show that local order, i.e., the aluminum distribution within the framework of Na,Cs-RHO type zeolites with different Al contents, plays a fundamental role in governing the CO2 adsorption capacity and structural flexibility. From this analysis, the cation type and location within the RHO structure as a consequence of the framework Al distribution are not the only parameters that deserve consideration. This is despite their paramount importance in optimizing the adsorption capacity of samples with a fixed Al content. In addition, we observe strong correlations between the 29Si NMR barycenter and ellipticity of the eight-ring pore apertures and the nearest neighbor and next-nearest neighbor framework atom distances. From this analysis, we rationalize that the zeolite framework flexibility can be viewed as a consequence of the distribution of Si species rather than being exclusively a consequence of the type of cation loading only.

Automated summary

Using solid-state 29Si nuclear magnetic resonance (NMR), this study demonstrates that the distribution of aluminum within the framework of Na,Cs-RHO type zeolites plays a crucial role in determining CO2 adsorption capacity and structural flexibility. The flexibility of the zeolite framework is not solely dependent on the type of cation loading, but also on the distribution of Si species and pore aperture shape.