Sequential carboxylic acid and ammonium hydroxide treatments for zeolites: Enhancing CO₂ adsorption under humid conditions

High-performance carbon dioxide (CO₂) capture in environments with high humidity remains a significant challenge. In this study, we proposed a sequential post-treatment method using carboxylic acids (oxalic acid, OA, and ethylenediaminetetraacetic acid, EDTA) followed by ammonium hydroxide (NH₄OH) treatment to enhance the CO₂/H₂O adsorption selectivity of zeolite adsorbents. This method was found effective for Y, Beta, and MOR zeolites, in which MOR zeolites with sequential OA and NH₄OH (OA-NH₄OH) treatment demonstrated the optimal CO₂/H₂O selectivity and the longest CO₂ breakthrough time in dynamic adsorption at 75 % relative humidity (RH). The outstanding performance can be attributed to its well-preserved porosity and improved surface silicon-to-aluminum ratio. Based on advanced characterizations such as CO₂ physisorption and solid state ²⁷Al NMR, the mechanism for OA-NH₄ treatment was proposed as follows: first, OA treatment increased the micropore volume by restoring partially coordinated Al (IV-2) species, subsequently, NH₄OH treatment removed Al (VI) (i.e., extra framework Al species) from the MOR zeolites. The sequential OA-NH₄OH treatment collectively boosted the zeolite's affinity for CO₂. Additionally, the modified zeolite adsorbent exhibited excellent recyclability, with a CO₂ adsorption capacity loss of less than 1 wt% over five cycles. This study offers new insights into developing “water-resistant” zeolite adsorbents through simple post-treatment approaches.