Search for an efficient 4,6-DMDBT hydrodesulfurization catalyst: A review of recent studies

Almost complete removal of 4,6-dimethyl dibenzothiophene (4,6-DMDBT) will perhaps be inevitable for reducing the sulfur content of diesel to a level of 50 wppm and lower. The hydrodesulfurization (HDS) of 4,6-DMDBT does not tend to occur through direct desulfurization, a pathway typically followed by reactive sulfur compounds over conventional CoMo/Al2O3 catalysts. Its reactivity can be enhanced either by increasing the rate of direct desulfurization or by transforming it to a more activated molecule through hydrogenation, isomerization, demethylation, and C-C bond scission. Attempts have been made to develop better catalysts using these concepts. Different additives such as phosphorus, fluorine, and lanthanum have been added to the alumina support for developing the required catalytic properties. Various other supports such as zeolite, zirconia, titania, etc., by themselves or in admixtures with alumina have also been used to improve the HDS activities of the catalysts. This article reviews the results of recent studies conducted in this area and summarizes the advances that have taken place in this direction.