A new nitrogen-enriched biochar modified by ZIF-8 grafting and annealing for enhancing CO2 adsorption

To increase the CO2 adsorption capacity of biochar, a facile modification method of biochar is proposed. Through in-situ synthesis and annealing (< 500 °C), ZIF-8 (zeolitic imidazolate frameworks) was successfully grafted onto biochar to form nitrogen-enriched modified biochar. The results show that the in-situ synthesis method improves the dispersion of ZIF-8 nanoparticles and the modified biochar inherits the parental characteristics of high nitrogen content of ZIF-8 (>10 at.%). Compared with raw biochar, the pore structure of modified biochar was significantly improved: the maximum surface area was increased from 10 to 882 m2/g. The biochar modified by ZIF-8 grafting and annealing of 400 °C (MNBC3–400) shows the highest CO2 dynamic adsorption capacity of 1.8 mmol/g at 30 °C and 1 bar and it is regenerable and stable during 10 cycles of CO2 adsorption-desorption. Moreover, MNBC3–400 also exhibits the maximum CO2 uptake of nearly 2 mmol/g from CO2 adsorption isotherms and the highest CO2/N2 selectivity of 16.0 at 25 °C and 1 bar. The density functional theory (DFT) results show that the modified biochar has a higher CO2 adsorption energy than the parent materials and the van der Waals effect is dominant in the CO2 adsorption system.