Valorization of Olive Stone Biomass into Graphitic-Like Carbon for Cyclic and Selective CO 2 Adsorption Under Postcombustion Conditions

Abstract

This work focuses on investigating the conversion of olive stone waste into porous, graphitic carbon materials using a sequence of thermal and catalytic treatments. Three distinct processing strategies were investigated: (i) chemical activation with potassium hydroxide (AC-OS-KOH), (ii) thermal pyrolysis in an inert atmosphere yielding biochar (C-OS), and (iii) catalytic graphitization using transition metals (Ni or Fe) either in combination with KOH activation (AC-OS-KOH-Ni, AC-OS-KOH-Fe) or applied directly to biochar (C-OS-Ni). The structural, morphological, and textural properties of the resulting carbon materials were characterized using X-ray diffraction, scanning electron microscopy, and carbon dioxide (CO₂) physisorption at 0°C.

Among all synthesized materials, AC-OS-KOH and AC-OS-KOH-Fe displayed superior microporosity and well-developed pore architectures, leading to enhanced CO₂ adsorption capacities compared with nonactivated and nickel-catalyzed samples. Notably, the dual strategy of chemical activation and nickel catalysis facilitated the transformation of olive stone precursors into graphitic-like porous carbon with a crystallinity index reaching 61%, indicating successful partial graphitization. CO₂ adsorption–desorption experiments were conducted at 25°C and 50°C under two CO₂ concentrations (90% and 10%, balanced with N₂). The KOH-activated carbons, with or without metal doping, exhibited fast adsorption–desorption kinetics, in contrast to the sluggish performance of the C-OS-Ni sample. This behavior underscores the critical role of micropore size and volume in governing CO₂ molecular diffusion and access to active sites. At elevated CO₂ concentration (90%), AC-OS-KOH demonstrated the greatest adsorption capacity, achieving 13.64 wt.% at 25°C and 8.98 wt.% at 50°C. In contrast, under diluted CO₂ conditions (10%), the AC-OS-KOH-Fe sample showed superior performance, indicating a strong link between pore size distribution and selective gas adsorption. Furthermore, the KOH-activated carbons maintained consistent adsorption performance across six consecutive adsorption–desorption cycles, confirming their stability and regeneration potential for practical CO₂ capture applications.