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.