Synthesis and characterization of zeolitic imidazolate framework derived activated carbon materials for carbon dioxide adsorption

Abstract

The rising levels of CO2 emissions from industrial activities (especially combustion of fossil fuels in power plants) are spiralling out of control, resulting in devastating global warming effects. Carbon capture and storage is among the most efficient systems to curb CO2 levels in the atmosphere with post-combustion capture being a promising technology. In post-combustion capture, adsorption is attractive with research geared towards enhancing the efficacy of adsorbents by increasing their capacity and improving selectivity for CO2 while minimizing adsorption of impurities. The challenge is to find a stable and hydrophobic adsorbent without compromising the surface properties and structure. In this study, zeolitic imidazolate frameworks (ZIFs) (i.e. ZIF-8, ZIF-67, and their core-shells) were utilized together with cellulose acetate (CA) as an extra source of carbon for the synthesis of activated carbons (ACs). For determination of optimum conditions, variation of synthesis parameters including activation temperature, activation time and precursor ratio were conducted for the ZIF-8 derived ACs. All prepared samples were characterized and CO2 adsorption-desorption at 298 K and 1 bar were measured for selected ACs. The results showed the feasibility of producing highly porous AC adsorbents through the utilization of ZIFs with CA as an additional carbon source. It was found that even though ZIF-8 and ZIF-67 were synthesized utilizing the same organic ligand and solvent, they produced distinct levels of porosity in the derived ACs. The ZIF-8 demonstrated the capacity to generate increased microporosity (up to 1823 m2g-1; 0.80 cc/g) and specific surface area (up to 2190 m2g-1) for the ACs, while ZIF-67 yielded highly mesoporous and decreased surface area (601 m2g-1) ACs owing to their different metal ions. This was further reflected in the ACs derived from core-shell ZIFs, which had textural properties that were intermediate between those of the parent ACs. The variation in temperature from 550 up to 750 °C led to a reduction in surface area and microporosity compared to the initial findings at 850°C. This trend was similarly observed in the experiments involving precursor ratio and activation time variation. An increase in the ratio (from 1:10 up to 4:10) resulted in a higher presence of unreacted particles, and lower surface area and microporosity. Reducing the duration from 2 h to 1 h prevented the evaporation of metal ions, while prolonging the duration to 2.5 h led to the collapse of pores. Evaluation of CO2 adsorption capacity revealed a strong correlation with microporosity of the ACs. The highest adsorption capacity was 3.29 mmol g-1 obtained for ZIF-8 derived AC. All the ACs adsorbed CO2 and exhibited complete reversibility. Hence, the utilization of ZIF-derived ACs as adsorbents for CO2 capture in post-combustion processes is feasible.