Low-cost hierarchical micro/macroporous carbon foams as efficient sorbents for CO2 capture

- Hierarchical coal-based carbon foams have been tested as adsorbents for CO2.
- Foams show good values of CO2 uptake as compared to active carbons and other foams.
- Some foams show a very good selectivity for CO2 and a good recyclability.

Hierarchical coal-based carbon foams with a low production cost and a high carbon yield have been synthesised using ZnCl2 and KOH as activation agents and tested as adsorbents for CO2, and a relation has been established between CO2 uptake and their textural and chemical properties. The foams showed a porous structure consisting of macropores in the range of 100-10 μm, a surface area of 534-866 m2 g− 1 and a total micropore volume of 0.21-0.25 cm3 g− 1. The foams activated with ZnCl2 at 500 °C, which displayed a homogeneous microporosity in the range of 0.8-1.2 nm, showed moderate values of CO2 uptake, ranging from 1.6 to 2.1 mmol g− 1. The CO2 adsorption capacity of the foams prepared by KOH activation was as high as 2.8 mmol g− 1, as a result of their higher proportion of narrow micropores (< 0.7 nm). This value is superior to those attained by commercial activated carbons and the highest ever reported for carbon foams. Regarding the chemical surface, the presence of acidic functionalities reduced the uptake of CO2. The foam prepared with a low coal/KOH ratio (0.5) showed a very good selectivity for CO2 adsorption over N2, a low regeneration energy expenditure and a good recyclability.