FAU zeolites from coal/rice husk co-combustion ash (co-ash) for CO2 adsorption: Synthesis, characterization, adsorption and regeneration performance

Synthesis of zeolite from industrial wastes has become an important environmental protection strategy. This study explored the synthesis of nano-sized NaX and NaY zeolites from coal/rice husk co-combustion ash (co-ash) using alkali-melting-hydrothermal synthesis and seed guiding methods, followed by modification with Li + and K + exchange. NaY zeolite, synthesized at a SiO 2 /Al 2 O 3 ratio of 5 and crystallization temperature of 100 °C, exhibited the most complete crystal structure and optimal CO 2 adsorption capacity. The specific surface area (SSA) of NaX and NaY zeolites (583.0 m 2 /g and 790.1 m 2 /g, respectively) are comparable to those of commercial zeolites, emphasizing their structural parity, and demonstrating their potential as effective CO 2 adsorbents. NaY demonstrated a CO 2 adsorption capacity of 5.31 mmol/g at 298.15 K and 1 bar, competitive with other conventionally synthesized and commercial zeolites. Cation exchange had a minimal impact on CO 2 adsorption of NaY, with LiX improving capacity by 11.65 % compared to NaX, while LiY showed only a 2.94 % increase. K + exchange negatively affected both zeolites. Kinetic and thermodynamic analyses, using the proposed first-order (PFO) and Langmuir-Freundlich models, indicated that CO 2 adsorption is a rapid physisorption process with thermal effects ranging from 22 to 41 kJ/mol. Both zeolites exhibited excellent CO 2 /N 2 selectivity and regeneration stability, with the loss of adsorption capacity retaining only 10.57 % and 9.63 % for NaX and NaY after fifteen cycles. This highlights their potential as efficient CO 2 adsorbents.