SURFACE-INDUCED HETEROCYCLE FORMATION - SULFUR ATOM ABSTRACTION DURING CYCLIZATION

We have studied the mechanism of a novel cyclization reaction in which acetylene abstracts sulfur from the Pd(111) surface in the form of thiophene. The proposed mechanism is one in which adsorbed acetylene first dimerizes to form a C4H4 intermediate which is then capable of abstracting atomic sulfur to form thiophene. The appearance of thiophene in the gas phase is then rate limited by thiophene desorption kinetics. This sequential mechanism has also been proposed for acetylene cyclotrimerization to benzene on the clean Pd(111) surface. Thiophene formation from acetylene is sensitive to both the surface sulfur coverage and the temperature at which the sulfur overlayer is produced. The reaction of a C4H4 species produced from cis-3,4-dichlorocyclobutene exhibits exactly the same sensitivity to surface characteristics and hence is identified as the intermediate produced by acetylene dimerization. We discuss the potential of this chemistry as a process for the regeneration of catalytic surfaces deactivated by sulfur contamination.