Traditionally, in the early 1990s, sulfur used to be removed from petroleum-derived feedstocks by a hydrodesulphurization (HDS) process using γ-Al2O3-supported Mo or W sulfide catalysts promoted by Co or Ni (Topsøe et al., 1996;Ho, 2004 and 2008). However, because the thermodynamic limitations of bimetallic Co(Ni)Mo(W)/Al2O3 catalyst formulation to perform deep HDS reactions at high temperature, it was impossible to satisfy stringent legislative requirements for ultra-low sulfur transport fuels using those catalysts. After many efforts in the design novel catalyst formulations, supports and new preparation procedures, the recent catalyst developments and technologies allow production of ultra-low sulfur diesel (ULSD) (Soled et al., 2000; Topsøe et al., 1996). However, the determination that catalyst could be used depends on the specific refinery configuration, feedstock blends employed, and it might be a simple matter of economics (Topsøe et al., 2005).
The S-containing refractory compounds, which should be removed from the feedstock’s, are dibenzothiophene (DBT) and alkyl-substituted DBT. For those molecules, the HDS reaction occurs mainly via hydrogenation (HYD) and direct desulfurization (DDS) pathways. For the HDS of DBT reaction (see Box 1), direct desulfurization pathway (DDS) leads to formation of biphenyl (BP), whereas hydrogenation pathway (HYD) leads to formation of tetrahydrodibenzothiophene (THDBT), cyclohexylbenzene (CHB) and dicyclohexyl (DCH) (Ho, 2004 and 2008). It is well established that DDS route of this reaction is favored for the Co-promoted catalysts whereas Ni-promoted ones exhibit enhanced HYD route of DBT transformation, as it was confirmed for the all catalysts studied in this chapter. The presence of alkyl groups in the highly refractory 4,6-dimethyldibenzothiophene (4,6-DMDBT) hardly affects the hydrogenation (HYD) pathway whereas the DDS pathway is severely inhibited. The hindrance of the C-S bond cleavage in the THDBT was claimed to be the most probably reason for the low reactivity of 4,6-DMDBT (Ho, 2004 and 2008). To overcome this problem, novel HDS catalysts exhibit enhanced hydrogenation function.