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.

The reaction of acetylene–hydrogen mixtures over Ni/SiO2 catalysts is found to proceed by chain growth with complete conversion at 140 °C to ∼50% C2, ∼20% C4 and 30% C5+ products. The principal oligomers, exemplified for C6 products, are the terminal and cis-linear alkenes together with 3-methyl branched pentenes and pentadienes, with much lesser amounts of 2-methyl and cyclic products. Figure optionsDownload as PowerPoint slide