Abstract
4-Methyldibenzothiophene and 4, 6-dimethyldibenzothiophene, which are typical refractory sulfur compounds in diesel fuel, were synthesized, and their desulfurization reactivity was compared with that of dibenzothiophene in decalin.
There are two desulfurization routes for alkyldibenzothiophenes: one is desulfurization after hydrogenation of the neighboring phenyl groups; i. e., hydrodesulfurization route, and the other is desulfurization without apparent hydrogenation; i. e., direct desulfurization reaction. The ratio of direct desulfurization to hydrodesulf urization decreased markedly in the order of dibenzothiophene(0.94)>4-methyldibenzothiophene(0.37)> 4, 6-dimethyldibenzothiophene(0.12), according to the number of substituted methyl groups. The methyl groups substituted at neighboring sulfur atom in alkyldibenzothiophenes inhibit direct desulfurization so strongly that hydrodesulfurization pathway becomes dominant on both NiMo and CoMo catalysts. Higher reaction temperatures, higher hydrogen pressures, and longer reaction times enhanced the desulfurization through hydrogenation.
Kinetic analyses revealed that the rate constant for the desulfurization step of the hydrogenated intermediates was about 30 to 35 times larger than that of direct desulfurization for 4-methyldibenzothiophene, and about 100 times that of 4, 6-dimethyldibenzothiophene, reflecting the preference for the hydrodesulfurization route.
Although the rate constants at direct desulfurization step of alkyldibenzothiophenes were much smaller (k2=2.02×10-5 for 4, 6-dimethyldibenzothiophene and k10=6.9×10-5 for 4-methyldibenzothiophene) than the constant of dibenzothiophene (k13=1.82×10-4), the desulfurization rate constants of hydrogenated alkyldibenzothiophenes were larger than the rate constant of dibenzothiophene, indicating the loosened C-S bond and reduced steric hindrance by the methyl groups in their hydrogenated forms.
It appears reasonable to assume that NiMo possesses higher desulfurization activity for alkyldibenzothiophenes than CoMo because of its higher hydrogenation activity under the desulfurization conditions presently used.
