DIVERGENCE OF ALLENE FROM METHYLACETYLENE OLIGOMERIZATION REACTIONS ON REDUCED TIO2 (001)SURFACES

Temperature-programmed desorption (TPD) studies of allene on reduced T iO2 (001) surfaces were undertaken to compare the chemistry of allene with simple alkynes (in particular, its isomer methylacetylene). The p rincipal product of the reaction of allene was the hydrogenation produ ct propylene; three dimerization products, dimethylene cyclobutane, be nzene, and an open-chain C6H10 dimer, were produced in significantly s maller 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 bas is. Adsorption of allene was greatest on the most reduced surfaces, an d decreased dramatically on less reduced surfaces prepared by prior an nealing at 650 K and above. All dimer products were extinguished on su rfaces annealed to over 650 K prior to adsorption, while smaller but n on-zero amounts of propylene continued to be produced on surfaces anne aled in this temperature range. All three dimeric products track the p opulation of Ti(+2) cations on the surface; this site requirement impl ies that these reactions involve a surface intermediate whose formatio n requires a two-electron oxidation of surface cations. A metallacyclo pentane intermediate is proposed to account for the formation of allen e dimerization products. This intermediate is similar to the metallacy clopentadiene involved in alkyne dimerization and cyclotrimerization o n reduced TiO2 surfaces. Although two of the products (propylene and t he C6H10 dimer) from allene TPD are common to methylacetylene also, no formation of trimethylbenzene (nor of any trimer product) was observe d from allene, in sharp contrast to the behavior of methylacetylene on reduced TiO2 surfaces. Except for the novel production of benzene fro m allene in this study, analogies may be found for the distinct reacti on pathways of both allene and methylacetylene in the chemistry of tra nsition metal complexes.