Hydrogenolysis of Organic Sulfur Compounds

Hydrogenolysis of carbon-sulfur bonds is a widely utilized reaction for removing sulfur from sulfur-containing organic compounds, and is known as desulfurization or hydrodesulfurization. Bougault et al. used Raney Ni for the first time for the desulfurization of aliphatic thioalcohols and disulfides in neutral and alkaline solution.126 Since then the reaction has been widely applied, for example, for organic syntheses, purification of solvents and substrates, structural studies, and determination of sulfur contents.127 

The hydrogenolysis of sulfur-containing compounds is also an important industrial process, known as hydrodesulfurization, in the field of petroleum refinery to reduce the sulfur content of petroleum fractions. The most commonly used catalyst is a mixture of either cobalt or nickel and molybdenum oxides supported on alumina, which is sulfided before use and usually employed at about 300-400°C and 1-7 MPa H2.128 The basic reactions involved in the hydrodesulfurization process are represented in eq. 13.60. 

The reaction of a sulfide with Raney Ni may follow two simultaneous courses as shown in eq. 13.61.129,130 The source of hydrogen may be that associated with Raney Ni or the hydrogen produced by dehydrogenation of a solvent such as ethanol.131 According to Bonner, however, dehydrogenation of ethanol to acetaldehyde and hydrogen is merely a concurrent reaction.132 

Hauptmann et al. found that aromatic disulfides, thioesters, and thiols were transformed into thioethers when refluxed in xylene (140°C) in the presence of Raney Ni degassed at 200°C.130,134,135 However, when these compounds were heated at 220°C with degassed Raney Ni or reduced Ni, biphenyls were obtained in good yields. The yields of the products with Raney Ni degassed at 500°C were never lower and were sometimes higher than those obtained with Raney Ni degassed at 200°C, as seen from an example shown in eq. 13.64. On the basis of these results, it was concluded that the presence of hydrogen was not essential for these desulfurizations.136 

The desulfurization of organic sulfur compounds with Raney Ni using far greater amounts than a catalytic is not a catalytic hydrogenolysis in the strict sense but involves a stoichiometric chemical reaction, since the catalyst itself is converted into nickel sulfide. However, since it has found many useful applications with Raney Ni (a hydrogenation catalyst), this reaction has been treated in this section. 

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Impurities in natural gas are usually simple sulfur compoimds such as hydrogen sulfide and mercaptans and are easily removed. Where the gas supply is also used for domestic purposes a stenching agent such as a thioether or thiophene may have been added. These less-reactive compoimds have to be subjected to a hydrogenolysis treatment before they can be removed. Naphthas contain several hundred parts per million of more complex organic sulfur compounds and may be pre-treated in a hydrofiner before use in ammonia or hydrogen production. Variation in the sulfur content of feed does not affect the performance... 

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