CYCLOTRIMERIZATION OF ALKYNES ON REDUCED TIO2(001) SURFACES

The reactivity of simple alkynes (acetylene, propyne, and 2-butyne) on various reduced TiO2 (001) surfaces was investigated using temperatur e-programmed desorption(TPD). The predominant reaction pathway in each case was cyclotrimerization to form aromatic compounds (benzene from acetylene, trimethylbenzene from propyne, and hexamethylbenzene from 2 -butyne) with selectivities between 51% and 86%. Other less prevalent reactions included reductive dimerization to open chain dienes and hyd rogenation to olefins. The selectivity for cyclization increased with reactant carbon number for the series above, while those of the olefin and diene products decreased, as did the fraction of the alkyne adlay er converted. By analysis of the cracking fractions of the trimethylbe nzene produced during methylacetylene TPD, it was determined that the ratio of the 1,2,4- to 1,3,5-isomers was approximate to 3:1. This is t he isomeric ratio expected if cyclotrimerization proceeds through a me tallacyclopentadiene intermediate, with no preferential orientation of the methylacetylene molecules inserted. The yield of trimethylbenzene can be directly correlated with the population of Ti(2+) cations, qua ntified by XPS, on reduced TiO2 (001) surfaces. This surface site requ irement is analogous to that for formation of metallacyclopentadiene c omplexes in solution; the latter reaction with mononuclear complexes r equires transition metal cations capable of undergoing a two-electron oxidation. Direct precedents exist for formation of titanium(IV) cyclo pentadienyl complexes from Ti(II) species in solution. Although the cy clotrimerization of alkynes has been reported on several single crysta l metal surfaces, this is the first example of this reaction on a sing le-crystal metal oxide surface under ultrahigh-vacuum (UHV) conditions .