Thiophene nickel catalyst poisoning

In the case of 3,3-dimethyl-1-butanol and 1-methyl-1-hydroxymethylcyclohexane, the reductive dehydroxylation in the presence of thiophene was accompanied by a skeletal isomerization 2,2- and 2,3-dimethylbutane were formed from the former alcohol and a mixture of ethylcyclohexane, methylcycloheptane, and 1,2-dimethylcy-clohexane from the latter alcohol. It has been suggested that the sulfirr compounds accentuate the acid properties of a nickel catalyst through their ability to poison the catalyst. 

Benzene and aikyibenzenes are quantitatively converted to cyclohexanes by catalytic hydrogenation. Modem procedures employ liquid-phase hydrogenation over nickel catalysts at 100-200° or over platinum catalysts at room temperature. Nickel catalysts are poisoned by traces of thiophene and water. Small quantities of hydrogen halide increase the effectiveness of platinum catalysts. Isomerization occurs during the reduction of benzene over nickel at 170° the cyclohexane formed is probably contaminated with methylcyclopentane, Partial reduction of benzene to 1,4-dihydrobenzene is accomplished by sodium in liquid ammonia at —45°. ... 

In order to confirm their proposed mechanism (surface reaction between chemisorbed benzene and chemisorbed hydrogen), poisoning experiments were employed using thiophene as a poison. This showed that the deactivation rate decreases with increasing hydrogen and benzene partial pressures, and hence they concluded from their data that hydrogen and benzene are both chemisorbed on the catalyst surface and their proposed dual-site adsorption kinetic model is suitable to describe the hydrogenation of benzene on nickel. 

By ESR study of nickel catalysts supported on K-clinoptilolite containing iron, a correlation was fotmd between the Ni-Fe interaction and the resistance of the catalyst to thiophene poisoning [93 A2]. 

Catalytic hydrogenation of thiophene poses a problem since noble metal catalysts are poisoned, and Raney nickel causes desulfurization. Best catalysts proved to be cobalt polysulfide [425], dicobalt octacarbonyl [426], rhenium heptasulfide [5i] and rhenium heptaselenide [54]. The last two require high temperatures (230-260°, 250°) and high pressures (140, 322 atm) and give 70% and 100% of tetrahydrothiophene (thiophane, thiolene), respectively. 

The kinetic experiments, activity tests, and poisoning experiments were carried out in a gas-flow isothermal fixed bed reactor [6) at the benzene partial pressure of 7.55 kPa hydro gen partial pressure 99.82 kPa thiophene partial pressure 0.032 kPa and the reaction temperatures 403, 427 and 448 K. The size of the commercial cylindrical catalyst pellet was 5x5mm (21% Ni on alumina, supplied by BASF). The nickel oxide containing precursor was activated by reduction with hydrogen at 743 K for 10 hr. 

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 corresponding change with thiophene does not take place because the catalysts are poisoned by the sulfur. In order to hydrogenate thiophene, therefore, sulfur-resisting catalysts must be sought. These have been found in the sulfides of molybdenum and nickel, described by Moldavskii and Kumari (29) and by Cawley and Hall (30). In both these series of experiments, static systems under pressure were used, so that no sulfur was carried away from the reaction zone and at least part of the catalyst remained in the form in which it was originally added. The structures of M0S2 and NiS are therefore to be considered. 

Catalytic reductions of the thiophene ring, or of substituents attached to it, are complicated by two factors poisoning of the catalyst, and the possibility of competing hydrogenolysis - reductive removal of sulfur, particularly with Raney nickel - indeed the use of thiophenes as templates on which to elaborate a structure, followed finally by desulfurisation, is an important synthetic strategy... 

The alloy hydride exhibits different reactivities in comparison with classical hydrogenation catalysts (Raney nickel, Pd, Ph). The alloy is not poisoned by thiophene ring, there is no reduction of oximes nor hy-drogenolysis of benzylic C-0 bonds. The compound SmMg reacts with anthracene dissolved in THF (13). The complex absorbs hydrogen and then catalyzes the hydrogenation of ethylene. 

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