Ultra-deep desulfurization of transportation fuelsvia charge-transfer complexes under ambient conditions

Abstract

Very stringent regulations on the maximum sulfur content of gas oil have led to intense research into all aspects of desulfurization. Deep desulfurization of diesel fuels is particularly challenging due to the difficulty of removing refractory sulfur compounds, particularly 4,6-dialkyldibenzothiophenes, using conventional hydrodesulfurization processes (HDS). A novel approach to the potential desulfurization of fuels such as diesel is proposed. It relies on the ability of 4,6-dialkyldibenzothiophene to form charge-transfer complexes (CTC) with π-acceptor molecules. We present the synthesis of a new π-acceptor molecule, 4,5-dicyano-2,7-dinitrofluorenone, solution redox behaviour and the crystal structure of the charge-transfer complex with the refractory 4,6-dimethyldibenzothiophene. This π-acceptor compound was then immobilized on a hydrophobic support (poly(styrene-co-divinylbenzene)). The selectivity of the CTC process was confirmed by the analysis of the sulfur compounds trapped from straight run Arabian Light (SR) containing 13600 ppm S. The functionalized polymer can be used in multiple cycles for the removal of refractory S-containing compounds from hydrotreated SR. It can also be regenerated with toluene. The high selectivity of this material permits diesel fuel to be desulfurized to a level that meets future regulatory requirements, i.e. less than 10 ppm S, at ambient temperature and without hydrogen consumption.