The oligomerization of acetylene in hydrogen over Ni/SiO₂ catalysts: Product distribution and pathways

The characteristics of nickel on silica catalysts for the reaction of acetylene in the presence of hydrogen in a flow system are investigated. Samples containing 2 wt% Ni are capable of complete acetylene conversion over a period of 5 h at 140 °C when using a 25% C₂H₂/75% H₂ feed. Ethylene and ethane account for approximately one-half of the acetylene converted with methane formation low. The remaining acetylene is converted to a multitude of oligomers with even-numbered ones typically three times as common as odd-numbered ones. The predominant C₄ and C₅ products are linear alkenes in a non-equilibrium distribution (1 > cis-2 > trans-2) followed by branched alkenes (isobutene and the three methylbutenes), alkanes and traces of dienes. A further major class in the C₆ fraction are 3-methyl branched products, such as cis- and trans-3-methyl-2-pentene and 3-methyl-1,3-pentadiene, which lack hydrogen atoms at the branch. The product distribution amongst higher oligomers appears to be similar based on the structures of the alkanes produced when Pt/SiO₂ is placed downstream of Ni/SiO₂. Aromatics are minor products-less than 3% of the acyclic products formed at each of carbon numbers 6, 7 and 8 and less than that of the corresponding cyclopentanes observed when the Pt/SiO₂ is present. Formation of all product classes, with the exception of those with a 2-methyl branch such as isobutene, can be rationalised in terms of chain growth initiated by addition of C₂ units to surface species established in surface science studies of acetylene adsorbed on nickel surfaces. Rearrangement is necessary for formation of the 3-methyl branched products. Crown