Zinc-Decorated and Nitrogen-Functionalized Hierarchical Porous Carbons for Carbon Dioxide Capture

This study developed a highly facile method to synthesize Zn-decorated and nitrogen-doped hierarchical porous carbons for carbon dioxide (CO2) adsorption. Zeolitic imidazolate framework-8 (ZIF-8) was used as the raw material, which was subjected to a thermal treatment to obtain ZIF-8-derived carbons (ZDCs) in order to develop nanocarbons with a stable framework structure, a high CO2 adsorption capacity, and high selectivity under normal pressure. The crystallinity evolution of the samples changed from the typical ZIF-8 structure to having features of graphite carbons upon heating. The average particle sizes of the products were between 34 and 105 nm, and the specific surface areas ranged from 618 to 1862 m2/g. The nitrogen and zinc contents gradually decreased with increasing carbonization temperatures, but the changes in the distributions of the functional groups were different. The interactions between CO2 and the ZDCs were significantly enhanced, resulting in a higher isosteric heat of adsorption. The ZIF-8 carbonized at 1123 K exhibited the highest CO2 uptake, i.e., 3.57 mmol/g at 298 K and 101.3 kPa, while higher CO2 uptakes at 15 kPa occurred on the ZIF-8 carbonized at 923 and 1023 K due to their high isosteric heat of adsorption of CO2. The higher adsorption selectivity of Z8-650 for CO2 over N2 may be due to its higher V<0.7nm/Vmi ratio and nitrogen and zinc contents. Consequently, the micropore area ratio and surface functional groups primarily determined the CO2 adsorption capacity at 15 kPa. In addition, an appropriate metal Zn to Zn2+ ratio may have a positive effect on CO2 adsorption. On the other hand, the ultramicropore volume ratio, micropore volume ratio, micropore area, and SSA played more significant roles at 101.3 kPa of pressure.