Ultra-deep desulfurization and denitrogenation of diesel fuel by selective adsorption over three different adsorbents: A study on adsorptive selectivity and mechanism

Adsorptive desulfurization and denitrogenation were studied using a model diesel fuel, which contains sulfur, nitrogen and aromatic compounds, over three typical adsorbents (activated carbon, activated alumina and nickel-based adsorbent) in a fixed-bed adsorption system. The adsorptive capacity and selectivity for the various compounds were examined and compared on the basis of the breakthrough curves. The electronic properties of the adsorbates were calculated by a semi-empirical quantum chemical method and compared with their adsorption selectivity. Different adsorptive selectivities in correlation with the electronic properties of the compounds provided new insight into the fundamental understanding of the adsorption mechanism over different adsorbents. For the supported nickel adsorbent, the direct interaction between the heteroatom in the adsorbates and the surface nickel plays an important role. The adsorption selectivity on the activated alumina depends dominantly on the molecular electrostatic potential and the acidic–basic interaction. The activated carbon shows higher adsorptive capacity and selectivity for both sulfur and nitrogen compounds, especially for the sulfur compounds with methyl substituents, such as 4,6-methyldibenzothiophene. Hydrogen bond interaction might play an important role in adsorptive desulfurization and denitrogenation over the activated carbon. Different adsorbents may be suitable for separating different sulfur compounds from different hydrocarbon streams.