Sulfur and nitrogen removal from transportation fuels by π-complexation

The sulfur and nitrogen compounds in transportation fuels (gasoline, diesel and jet fuels) can be selectively removed by a new class of adsorbents, referred to as π-complexation sorbents. The sorbents contain d-block metal cations (e.g., Cu⁺, Ag⁺, Ni²⁺, Pd²⁺) that form π-complexation bonds with the thiophene rings of the sulfur molecules or the pyridine and pyrrole rings of the organo-nitrogen molecules in the fuel. A summary of the development of these sorbents is given. High separation factors for sulfur and nitrogen containing compounds over the fuel aromatics were first predicted from molecular orbital calculations. Desulfurization and denitrogenation of a commercial diesel by using a fixed-bed of Cu(I)Y zeolite were demonstrated. Likewise, NiX and NiY zeolites also showed high selectivity for sulfur removal from a commercial diesel. The sulfur capacities of the sorbents were further increased by using a thin "guard" bed of a commercial sorbent (activated alumina or activated carbon) that removed the heavy aromatics from competing with the sulfur molecules for the cation sites. The sorbents could be effectively regenerated by heating in air at 350°C, followed by reduction of Cu²⁺ to Cu⁺. Furthermore, it was shown that ultrasound-assisted desorption could be a promising technique for regeneration at ambient temperature. For desulfurization, the detrimental effects of organo-nitrogen, moisture, heavy (polynuclear) aromatics, oxygenates (ethanol and MTBE) and possibly other fuel additives are discussed.