Insights into the enhancement of CO2 adsorption on faujasite with a low Si/Al ratio: Understanding the formation sequence of adsorption complexes

Faujasites (FAUs) with low Si/Al ratios (SARs) show high CO2 adsorption capacities, making them popular adsorbents for CO2 capture. The present study aims to investigate the enhancement mechanisms from the aspect of the formation characteristics of CO2 adsorption complexes. First, we test the CO2 adsorption capacities of two FAU samples (SAR = 1.18 or 1.01). The intensity of the adsorption enhancement for the low-SAR FAU is found to increase, and then decrease, with the growth of CO2 coverage. Subsequently, we perform Monte Carlo simulations with a force field validated by adsorption experiments and molecular structures determined by X-ray diffraction. Various adsorption complexes are validated by the energy distributions and the lowest-energy configurations of the simulations. Five types of CO2 adsorption complexes are classified according to the interacting cations. We study their formation sequences and find that dual-site complexes, preferentially interacted with two Na+ cations in SIII sites (NaIII···CO2···NaIII), are formed earlier than single-site complexes. Multiple CO2 molecules (up to six) may share one single cation at high coverage. Energies calculated for complex NaIII···CO2···NaIII, NaIII···CO2···NaII, and NaII···CO2 are −55.5, −42.7, and −37.8 kJ/mol, respectively. We attribute the adsorption enhancement to a large number of high-energy complexes in the low-SAR FAU. The max numbers of NaIII···CO2···NaIII and NaIII···CO2···NaII are 31 and 46, respectively, for the low-SAR FAU. (16 and 33, for the other FAU). The experimental trend of adsorption enhancement is well explained by the average energies of complexes at different levels of CO2 coverages, in two FAUs.