Study on regeneration mechanism of composite adsorbent by Mg-MOF-74-based modified biochar.

ABSTRACT

In this paper, composite adsorbent was prepared from biochar and Mg-MOF-74 by in-situ growth method to investigate regeneration mechanism. The effects of O2 and temperature on regeneration characteristics were investigated by CO2 adsorption properties and characterization techniques, and the optimal regeneration conditions were determined. Regeneration mechanism of adsorbent was revealed by adsorption kinetics and elemental valence analysis. The related wave function parameters were calculated based on DFT to reveal the repair mechanism of the failed oxidation sites from the microscopic level. The mechanism of CO2 adsorption by the repaired oxidation sites was explored based on the regenerated adsorption configuration. It was found that the regeneration performance of the adsorbent exhibited a trend of increasing and then decreasing with the increase of O2 concentration and temperature, and the optimal regeneration conditions were determined to be 5 % O2 concentration and 200 °C. At optimal regeneration conditions, a synergistic interaction between O2 and poly-metals was generated to enhance the adsorbent polarity. O2 also reacted with the adsorbent in a redox reaction to produce new oxygen-containing functional groups and cause pore expansion, the mass transfer and diffusion was enhanced. The oxidation site adsorbed O2 to undergo electron rearrangement and release the adsorbed CO2. Due to the nature of common orbital hybridization between metals, the metals underwent conjugation and synergistic effects with O2 to form tetrahedral co-coordination structures with lower energies. The electron density and electric field effects of the system were enhanced. The former enhanced interaction with CO2 to form carbonate. The latter increased the activity of the neighboring N atom, which in turn generated a stable ring structure with carbonate, and CO2 adsorption was enhanced.