Sulfur poisoning thiophene

Theoretical equations developed in this paper are intended to model benzene hydrogenation in an industrial reactor and account for the loss of catalyst activity due to sulfur poisoning. The approach used to develop the equations results in an analytical solution for the partial differential equations describing hydrogenation, sulfur poisoning, and a catalyst active site balance. The solution, which accounts for thiophene... 

Two sulfur containing precursors were studied thiophene and butanethiol. Attempts to add the S containing precursor from HjS, and from a gas phase stream bubbled through a solution containing the S precursor did not result in reproducible results due to the low surface area of the catalysts. The procedure adopted was to add directly liquid droplets of a cyclohexane-thiophene or cyclohexane-1-butanethiol solution to the Pd films. Addition from the liquid phase yielded satisfactory reproducibility both in terms of the amount of S adsorbed by the films, as well as the activity after sulfur poisoning. Solutions with various concentrations of precursors were used to vary the amount of sulfur added to each sample. One cc of the solution was added via a syringe to the film catalysts (fresh and pre-reduced). The films were then placed on a hot plate at 60°C to evaporate the solvent. 

Sulfur poisoning curves were obtained by doping the feed with thiophene. Thiotolerance (TT), which is defined as the residual activity value in the poisoning curves (TT = a ), was used for determining the relative sensitivity of the catalysts to sulfur poisoning. The catalyst... 

Molybdenum disulfide catalysts, promoted by cobalt or nickel, are used to remove organosulfur compoimds from crude petroleum by hydrogenolysis— hydrodesulfurisation or HDS [124,125,126], These compounds are imdesirable because they poison motor vehicle autoexhaust catalysts and bum to sulfur dioxide, an environmental pollutant. The most difficult compounds to desulfurise are sulfur heterocyclics thiophene, benzothiophene, dibenzothiophene and their methyl substituted derivatives. A typical reaction is the removal of sulfiir from thiophene ... 

The hydrodesulfurization process is the removal of sulfur from organic sulfur compounds, such as thiols, sulfides, thiophene, and substituted thiophenes from petroleum feedstocks, to avoid or minimize sulfur poisoning of metallic or bimetallic reforming catalysts and to avoid introduction of S into the environment. 

Natural gas consists mainly of methane together with some higher hydrocarbons (Tab. 8.1). Sulfur, if present, must be removed to a level of about 0.2 ppm prior to the steam reforming process as it poisons the catalyst. This is typically done by cata-lytically converting the sulfur present as thiols, thiophenes or COS into H2S, which is then adsorbed stochiometrically by ZnO, at 400 °C, upstream of the reactor. 

Sulfur in cmde oil is mainly present in organic compounds such as mercaptans (R-SH), sulfides (R-S-R ) and disulfides (R-S-S-R ), which are all relatively easy to desulfurize, and thiophene and its derivatives (Fig. 9.2). The latter require more severe conditions for desulfurization, particularly the substituted dibenzothiophenes, such as that shown in Fig. 9.2. Sulfur cannot be tolerated because it produces sulfuric add upon combustion, and it also poisons reforming catalysts in the refinery and automotive exhaust converters (particularly those for diesel-fueled cars). Moreover, sulfur compounds in fuels cause corrosion and have an unpleasant smell. 

Hydrogenolysis of adsorbed thiophene (poison) is the rate determining step. Sulfur resistance was Pt > Pt-Ir > Pt-Re.65 66 ... 

Consistent with the fact that thiophene is a poison in metal-catalyzed hydrogenation reactions, thiophene is deuterated only slowly on the prereduced transition metals listed in Table XII, except for iridium. In both hydrogenation115 and exchange89 the poisoning has been attributed to the influence of the heteroatom in the adsorption process, presumably as species (30), or even of elemental sulfur as a consequence... 

With regard to the sulfur bound on the catalyst surface, differences exist between the various types of ammonia catalysts, especially between those that contain alkali and alkaline earth metals and those that are free of them. Nonpromoted iron and catalysts activated only with alumina chemisorb S2N2 and thiophene. When treated with concentrations that lie below the equilibrium for the FeS bond, a maximum of 0.5 mg of sulfur per m2 of inner surface or free iron surface is found this corresponds to monomolecular coverage [382], [383], The monolayer is also preserved on reduction with hydrogen at 620 °C, whereas FeS formed by treatment above 300 °C with high H2S concentrations is reducible as far as the monolayer. For total poisoning, 0.16-0.25 mg S/m2 is sufficient. Like oxygen, sulfur promotes recrystallization of the primary iron particle. 

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