CO2 adsorption on zeolite X/activated carbon composites

Two series of zeolite X/activated carbon composites with different ratios of zeolite and activated carbon were prepared through a combination process of CO₂ activation of the mixtures of elutrilithe and pitch and subsequent hydrothermal crystallization in alkaline solution. An additional surface modification was achieved in diluted NH₄Cl solution. CO₂ and N₂ uptakes on the composites before and after modification were determined for pressures up to 101?kPa at 273 and 298?K, respectively. Langmuir-Freundlich and Toth adsorption models were used to describe the adsorption isotherms of CO₂ and the corresponding heats of adsorption were estimated with the Clausius-Clapeyron equation. Both before and after modification, all composites exhibited a remarkable preferential adsorption of CO₂ compared to N₂, with the modified composites showing a higher adsorption selectivity to CO₂ over N₂ than the unmodified composites. With an increasing ratio of zeolite in the composites, adsorption capacity and adsorption heat of CO₂ on the composites increased simultaneously. Lower adsorption heat was observed both before and after modification especially at the low-loading region and when there was less energetic heterogeneity on the surface of the modified composites. The results may be attributed to the elimination of strong basic sites on the modified composites, which is favorable for desorption of CO₂ on the adsorbents and application in pressure swing adsorption processes.