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

High-performance carbon dioxide (CO 2 ) 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 (NH 4 OH) treatment to enhance the CO 2 /H 2 O adsorption selectivity of zeolite adsorbents. This method was found effective for Y, Beta, and MOR zeolites, in which MOR zeolites with sequential OA and NH 4 OH (OA-NH 4 OH) treatment demonstrated the optimal CO 2 /H 2 O selectivity and the longest CO 2 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 CO 2 physisorption and solid state 27 Al NMR, the mechanism for OA-NH 4 treatment was proposed as follows: first, OA treatment increased the micropore volume by restoring partially coordinated Al (IV-2) species, subsequently, NH 4 OH treatment removed Al (VI) (i.e., extra framework Al species) from the MOR zeolites. The sequential OA-NH 4 OH treatment collectively boosted the zeolite's affinity for CO 2 . Additionally, the modified zeolite adsorbent exhibited excellent recyclability , with a CO 2 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. • CO 2 /H 2 O adsorption selectivity on zeolite was improved by carboxylic acid treatment. • Solid-state NMR demonstrated the changes in Al species during post-treatment. • Less than 1 % loss was found in CO 2 capacity after five cycles of regeneration.