Asphaltenes Conversion by S-Alkylation and C-Alkylation Reactions

Abstract

Asphaltenes, defined as the fraction of crude oil soluble in toluene and insoluble in C5 to C7 paraffinic solvents, constitute the lowest value fraction of heavy oil. Sulfur is the most abundant heteroatom in asphaltenes. The present work explores two approaches for upgrading asphaltenes using chemical reactions focused on aliphatic C-S bonds, which are approximately 30 % of the C-S bonds in asphaltenes. The first approach in the present work investigated the sulfur alkylation enabled scission of C-S bonds using methyl iodide at 75 °C under nitrogen environment. Reaction of selected organosulfur compounds (acyclic thioethers, cyclic thioethers and thiophene) with methyl iodide was conducted to confirm the chemistry of C-S bond cleavage by S-alkylation. Successful C-S bond cleavage in aliphatic thioethers motivated an attempt to use the same reaction with n-pentane precipitated asphaltenes from Athabasca bitumen. Solid product from the reaction was analyzed to contain above 10 wt% iodine. This solid product was further analyzed using thermogravimetric analysis. The reaction affected the thermal behavior of the product in the temperature range 250â350 °C, which was mainly due to iodine-related loss in this temperature range. Reaction of methyl iodide did not increase the yield of paraffin soluble products from asphaltenes. The second approach explored C-S bond scission concomitant with C-alkylation in organosulfur compounds by 1-hexene in presence of an amorphous silica-alumina catalyst, SIRAL®40. The presence of an acid catalyst facilitated C-S bond scissions in some thioether bonds. Tetrahydrothiophene and diphenyl sulfide was found to be fairly stable at the reaction conditions of 325 °C temperature and self-generated pressure. Benzyl phenyl sulfide and dibenzyl sulfide were completely converted to products at the reaction conditions employed in this study. C-S bond scission in dibenzyl sulfide led to the formation of sulfur free molecules when the acid catalyst was present. In case of benzyl phenyl sulfide existing C-S bonds were broken and some new C-S bonds were formed, which included the formation of diphenyl sulfide. Detection of H2S in gas product indicated the C-S bond scission and sulfur elimination in case of alkyl sulfide, dibenzyl sulfide and benzyl phenyl sulfide.