Toluene formation via acetylene-propyne cyclotrimerization on Ag(111).

Selective C-C coupling between different alkynes on metal surfaces remains largely unexplored. Here, we show that coadsorbed acetylene and propyne on Ag(111) form toluene through a C2-C3 cross-cyclotrimerization pathway that competes with acetylene homotrimerization to benzene. Our study reveals how molecular-scale co-assemblies affect competing energy barriers which in turn govern reaction selectivity to benzene and toluene. These results extend efficient acetylene trimerization chemistry to hetero-alkyne coupling on metal surfaces. This work establishes a molecular-level framework for controlling hetero-alkyne cyclotrimerization and opens new opportunities for C2/C3 upgrading to substituted aromatics.