Which catalyst is used to prepare benzene from acetylene at 70°C?

Benzene can be synthesized from acetylene (ethyne) by a process known as trimerization, where three molecules of acetylene combine to form one molecule of benzene. This reaction is a fundamental example of how unsaturated hydrocarbons can polymerize to yield aromatic compounds.

The key to this transformation lies in the catalyst and the reaction conditions. When acetylene is heated to around 70°C in the presence of an organo-nickel catalyst, it undergoes a smooth catalytic trimerization:
3 C₂H₂ → C₆H₆

The organo-nickel catalyst facilitates the combination of acetylene molecules by forming intermediate nickel–acetylene complexes. These intermediates align the carbon–carbon triple bonds in such a way that cyclization can occur, leading to the formation of the aromatic benzene ring. The mild temperature (70°C) prevents side reactions and polymer formation, resulting in a high yield of benzene.

Other catalysts and conditions give different results:

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  Cr₂O₃ + Al₂O₃ + SiO₂ is used for the dehydrogenation of cyclohexane, not for trimerization.

  
  


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  Raney nickel generally catalyzes hydrogenation reactions, converting unsaturated hydrocarbons to saturated ones.

  
  


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  Nickel at 250–300°C can also convert acetylene to benzene, but at a higher temperature and through a different catalytic mechanism.

  
  

The organo-nickel catalyst is particularly efficient because it stabilizes the acetylene intermediates, promoting ring closure at relatively low temperatures. This method is widely studied in organic synthesis and industrial chemistry for its selectivity and efficiency.

Therefore, the correct answer is option C — Organo-nickel, the specific catalyst that allows benzene formation from acetylene at 70°C.