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

• The properties of NaX and NaY zeolites synthesized from co-ash were initially reported. • Optimal preparation conditions for NaY zeolite were investigated. • Better CO 2 adsorption performance was achieved on NaY compared to NaX zeolite. • Cation exchange effect on CO 2 adsorption capacity of zeolites was verified. • Fast kinetics, low adsorption heat and superior regeneration were displayed in the synthesized zeolites. 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.