Carbon dioxide (CO₂) capture is increasingly important for mitigating greenhouse gas emissions and reducing environmental impacts associated with fossil fuel utilization. Although palm oil mill boiler ash has been extensively studied for heavy metal adsorption, its application as a CO₂ adsorbent remains limited. Meanwhile, the accumulation of boiler ash waste presents a major environmental challenge for the palm oil industry due to the increasing volume of solid waste generated annually. Therefore, this study investigates the potential utilization of palm oil mill boiler ash as a sustainable silica-based adsorbent for CO₂ capture. The study involved silica synthesis, KOH impregnation, and CO₂ adsorption evaluation. The effects of citric acid concentration (1–5 wt.%), KOH concentration (1–5 M), and impregnation temperature (60–90 °C) on the physicochemical properties and adsorption performance of the adsorbent were systematically investigated. The synthesized materials were characterized using SEM–EDX, FTIR, BET, and XRD analyses to evaluate their morphology, elemental composition, pore structure, and crystallinity. The results showed that silica synthesized using 4 % citric acid achieved the highest silica purity of 95.6%, with crystalline phases dominated by cristobalite and tridymite structures. Subsequent impregnation with 5 M KOH significantly enhanced the adsorption performance, resulting in a maximum CO₂ adsorption capacity of 144.8 mg/g at an impregnation temperature of 80 °C. Kinetic analysis revealed that the adsorption process followed a pseudo-second-order model, indicating that chemisorption was the dominant adsorption mechanism. Palm oil mill boiler ash-derived silica adsorbents possess promising potential as sustainable and low-cost materials for CO₂ capture applications.