"> $&#q` gz bjbjqPqP *::FFFFFFFZbbb8 Z $= h, F, FFA > > > dFF > > > FF> v Ab* d> DW 0 > 3 X3> 3F> > , , X ZZZ DZZZ ZZZFFFFFF Many countries are mandating substantial sulfur reduction in transportation fuels. For example, the U. S. Environmental Protection Agency ruled that on-road diesel sulfur level should be reduced to 15 ppm by June 2006, from the current average of 350-500 ppm. This is why there has been a massive, worldwide effort in developing new or much improved hydrodesulfurization technologies in recent years. The problem of central concern is how to remove sulfur from 4-substituted and 4,6-disubstituted dibenzothiophenes (DBTs). These species are the most refractory sulfur species due to steric hindrance around the sulfur atom. The present paper aims to identify limiting factors bearing on the chemistry and catalysis of removing sulfur from DBTs with steric hindrance. Among the topics discussed are: reactivity-property relationships, catalyst functionalities, inhibiting effects, poisoning dynamics, nature of active sites, and new catalytic materials.
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