Divergence of allene from methylacetylene oligomerization reactions on reduced TiO2 (001) surfaces

Temperature-programmed desorption (TPD) studies of allene on reduced TiO₂ (001) surfaces were undertaken to compare the chemistry of allene with simple alkynes (in particular, its isomer methylacetylene). The principal product of the reaction of allene was the hydrogenation product propylene; three dimerization products, dimethylene cyclobutane, benzene, and an open-chain C₆H₁₀ dimer, were produced in significantly smaller amounts. Conversion of allene was around 70% on the most active (most highly reduced) surfaces, with propylene production accounting for about two-thirds of the reactant converted on a carbon-content basis. Adsorption of allene was greatest on the most reduced surfaces, and decreased dramatically on less reduced surfaces prepared by prior annealing at 650 K and above. All dimer products were extinguished on surfaces annealed to over 650 K prior to adsorption, while smaller but non-zero amounts of propylene continued to be produced on surfaces annealed in this temperature range. All three dimeric products track the population of Ti( + 2) cations on the surface; this site requirement implies that these reactions involve a surface intermediate whose formation requires a two-electron oxidation of surface cations. A metallacyclopentane intermediate is proposed to account for the formation of allene dimerization products. This intermediate is similar to the metallacyclopentadiene involved in alkyne dimerization and cyclotrimerization on reduced TiO₂ surfaces. Although two of the products (propylene and the C₆H₁₀ dimer) from allene TPD are common to methylacetylene also, no formation of trimethylbenzene (nor of any trimer product) was observed from allene, in sharp contrast to the behavior of methylacetylene on reduced TiO₂ surfaces. Except for the novel production of benzene from allene in this study, analogies may be found for the distinct reaction pathways of both allene and methylacetylene in the chemistry of transition metal complexes.