Zeolite 13X was synthesized from palm oil mill fly ash (POMFA) using an alkaline fusion method. A zeolite 13X yield of 85% was obtained at a mass ratio of NaOH/(SiO2 and Al2O3 precursors) of 1.6, a fusion temperature of 600 degrees C and a fusion time of 60 min. Acid activation was performed on the zeolite 13X product in order to enhance its CO2 adsorption capacity. Acid activation with 4-8 M HCl for 4 h produced a marked increase in the mesopore and total pore volumes of the zeolite 13X. This increased mesopore volume resulted in CO2 adsorption capacities of the acid-activated zeolite 13X at 6 M HCl for 4 h that were 22% higher than those of the unactivated zeolite 13X and 11% higher than those of the commercial zeolite 13X. The CO2 adsorption capacity obtained was 6.42 mol CO2/kg at a pressure of 403 kPa and a temperature of 32 degrees C. (C) 2016 Elsevier Ltd. All rights reserved.