Deep desulfurization of model fuels by metal-free activated carbons: The impact of surface oxidation and antagonistic effects by mono- and polyaromatics

The main novel objective of this work is the evaluation of the adsorptive deep desulfurization efficiency under ambient conditions and studying in detail the effects arisen from the presence of competitors as in real fuels. Towards this direction, several activated carbons (commercially available and chemically modified/oxidized counterparts) were examined using a (diesel) model fuel containing initially 20 ppmwS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) and 20 ppmwS of dibenzothiophene (DBT), as well as mono- and di-aromatic competitors, namely benzene and naphthalene, in hexadecane. To gain better insights regarding the adsorption sites, the involved mechanisms, and the impact/antagonistic effect arisen by the presence of the aromatics in high concentration, simpler model fuels were also utilized while emphasis was also given on exploring which physicochemical features, textural (N-2 adsorption) or surface chemistry (Boehm titration and surface pH measurements), play a key role. In all cases, the oxidation treatment enhanced the adsorption capacity of the carbons, despite a decrease on the porosity, by introducing additional surface functional groups, indicating that surface chemistry plays a vital role in adsorptive desulfurization and that difference in chemical nature adsorption sites exists. The best performing oxidized carbon (SX PLUS-ox) presented a 92.1% (5.2 mgS/g capacity) removal of DBT in hexane and a 67.7% 4,6-DMDBT removal (4.0 mgS/g) in pure hexadecane. In the case of 4,6-DMDBT in hexadecane, the addition/co-presence of the aromatics in low concentration led to slightly lower desulfurization of 57.8% (3.6 mgS/g), revealing the limited antagonistic effect due to the aromatics. When DBT was also present, the 4,6-DMDBT removal was 51.2% (3.5 mgS/g) while the DBT removal 50.9 % (3.6 mgS/g), leading to a high total thiophenic adsorption capacity of 7.1 mgS/g. Even with the addition of the mono- and di-aromatics with concentrations as in real diesel fuel, the best performing carbons after oxidation demonstrated a superior desulfurization performance, since >10 ppmwS can still be removed from the model fuel. The results designate that the oxidation of porous activated carbon can be considered as an effective materials design strategy in adsorptive deep desulfurization, since different oxygen containing surface functional groups that can act as adsorption sites are created and have a crucial selectivity towards thiophenes compared to the aromatics. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

The main objective of this work is to evaluate the efficiency of adsorptive deep desulfurization under ambient conditions and study the effects of competitors in real fuels. By examining various activated carbons and model fuels, the study found that oxidation treatment enhances the adsorption capacity of carbons and can effectively remove sulfur compounds from the model fuel.