Understanding capacity loss of activated carbons in the adsorption and regeneration process for denitrogenation and desulfurization of diesel fuels

• Loss of micropores and surface functional groups reduces adsorption capacity.
• Trace amount of N/S compounds was left on carbon sorbent after regeneration.
• Coke formed due to the pyrolysis of N and/or S residues blocked the micropores.
• Surface functional groups decreased during adsorption and regeneration.

Activated carbon (AC), after successive adsorption–regeneration processes, is found to lose its capacity in the adsorptive removal of nitrogen (N) and sulfur (S) containing compounds from diesel fuels. This work is the first attempt to identify the root causes of reduction in adsorption capacity of diesel spent AC due to solvent regeneration. The diffusion resistance from micropores prevents the adsorbed N- and S-containing compounds from being removed via solvent extraction. Meanwhile, coke is formed on the inner walls of the pores in AC during carbon re-activation. The residues and the coke result in a decrease in the specific surface area and porosity of the regenerated carbons, which thus lowers their performance. The loss of oxygen functional groups such as carboxylic acid groups, pyrone type groups, lactone groups, carbonyl groups, carboxylic anhydride groups and phenolic groups might also cause the reduction in the re-adsorption of N and S compounds.