Selective extraction of neutral nitrogen compounds found in diesel feed by 1-butyl-3-methyl-imidazolium chloride

Abstract

To produce ultra low-sulfur gasoline and meet the new regulations, the improvement of the HDS reaction by the removal of inhibitors, like nitrogen-containing compounds (N-compounds) commonly found in middle distillates used for diesel feed, could be worth considering. Liquid–liquid extraction using 1-butyl-3-methyl-imidazolium chloride (BMIm⁺Cl⁻) was found to be a very promising approach for the removal of neutral N-compounds with high selectivity towards sulfur. BMIm⁺Cl⁻ was first evaluated using synthetic solution with dibenzothiophene and carbazole as model compounds, and a high selectivity for N-compounds was found. In order to explain the observed selectivity, the extraction of heterocyclic aromatic compounds from a hydrocarbon mixture by BMIm⁺Cl⁻ was studied as the function of the toluene/n-dodecane ratio. The extraction of sulfur-containing compounds and basic N-compounds decreased when the mass fraction of toluene increased, whereas the extraction of proton donor N-compounds was found to be almost independent of the mass fraction of toluene. We thus assumed that the selective extraction of indole and carbazole by BMIm⁺Cl⁻ could be explained both by electron pair acceptor properties of the imidazolium ring and by the binding of the hydrogen-donor group with chloride anion of the RTIL. BMIm⁺Cl⁻ was then evaluated using straight-run diesel feed, containing 13400 ppm S and 105 ppm N. An extraction of up to 50% of the N-compounds was obtained in one step, whereas the sulfur concentration reduction was only 5%. The selectivity of the extraction process towards heterocyclic aromatic compounds and polyaromatic compounds using BMIm⁺Cl⁻ was further emphasized by two-dimensional GC-MS analysis of the extracted compounds.