Activity of catalysts in thiophene synthesis from furan and hydrogen sulfide

Thiophene synthesis from furan and H₂S over acid catalysts is reported. Proton donor catalysts are low-active, nonselective, and prone to deactivation. Alumina-supported catalysts having Lewis acid sites, such as alumina-supported catalysts, are more efficient. With these catalysts, the thiophene formation rate per Lewis acid site increases with increasing site strength. It is assumed that the reaction proceeds via the formation of a surface intermediate consisting of an acid site bonded to an α carbon atom of the furan ring and an H₂S molecule nondissociatively adsorbed on a basic site. At atmospheric pressure, T = 250–450°C, initial furan concentrations of 1–20 vol %, and H₂S/furan = 0.4–20 (mol/mol), the thiophene formation reaction is first-order with respect to both reactants and its rate constant increases with increasing temperature. The thiophene formation rate depends on the H₂S/furan molar ratio. Under optimal conditions, the thiophene yield is 95–98 mol % and the thiophene formation rate is high.