Role of Al Distribution in CO₂ Adsorption Capacity in RHO Zeolites

Abstract

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.

Visualizing the Flexibility of RHO Nanozeolite: Experiment and Modeling

“…Recently, we described the organic structure-directing agent (OSDA)-free synthesis of Na,Cs-RHO nanozeolite with an improved Si/Al ratio, thermal stability, and well-defined discrete nanoparticle morphology . By increasing the Si/Al ratio from 1.5–1.7 to 2, the Na,Cs-RHO nanozeolite exhibited a single crystalline structure upon dehydration, distinct to that of more aluminous analogues (Si/Al ≤ 1.7). , However, unlike more siliceous equivalents, the as-prepared Na,Cs-RHO nanozeolite (Si/Al = 2) possesses the I -43 m space group at room temperature due to the occupation of the D8Rs by Cs + extra-framework cations. − This illustrates the potential complexity of the structural behavior of as-prepared Na,Cs-RHO zeolites as a consequence of simply tuning the Si/Al ratio and the consequences this has on extra-framework cation and guest-molecule spatial distribution . Understanding the adsorbate-temperature relationships and their structural influence on RHO zeolite is central to their optimization and implementation as physical adsorbents, because of the significant temperature variations that can occur in separation processes due to the exothermic nature of CO 2 adsorption.…”

“…29−31 This illustrates the potential complexity of the structural behavior of as-prepared Na,Cs-RHO zeolites as a consequence of simply tuning the Si/Al ratio and the consequences this has on extra-framework cation and guest-molecule spatial distribution. 32 Understanding the adsorbate-temperature relationships and their structural influence on RHO zeolite is central to their optimization and implementation as physical adsorbents, because of the significant temperature variations that can occur in separation processes due to the exothermic nature of CO 2 adsorption.…”

Flexibility in zeolites: origin, limits, and evaluation

“…119 In our group, we recently showed for a series of RHO zeolites with different Si/Al ratios and lattice parameters, that changes to the Al distribution within the nanosized RHO samples, represented by the shi in the 29 Si NMR barycenter, could be correlated with the CO 2 adsorption capacity. 120 Changing the Si/Al ratio of RHO from 1.5 to 1.7 resulted in an increase of the CO 2 capacity from 1.37 to 2.01 mmol g −1 . 120 This is due to changes in the framework charge distribution, which play a fundamental role in controlling the CO 2 adsorption capacity and should not be overlooked as the cation distribution will always be guided by the negative charge distribution within the framework.…”

“…120 This is due to changes in the framework charge distribution, which play a fundamental role in controlling the CO 2 adsorption capacity and should not be overlooked as the cation distribution will always be guided by the negative charge distribution within the framework. 120…”

RHO Zeolites with High Hydrothermal Stability

“…The RHO framework consists of a body-centered cubic arrangement of alpha cages linked via double eight-rings (Figure ). Synthesized with 18-crown-6 ether as OSDA, phase-pure RHO zeolite with a Si/Al ratio of up to 4.5–8.6 has been reported. , Syntheses in the presence of sodium and cesium cations result in RHO materials with Si/Al ratios in the range 1.3–3.0. ,, …”