Fuel precursors 2,5-bis(2-furylmethylidene)cyclopentanone (F₂Cp) and 2-(2-furylmethylidene)cyclopentanone (FCp) were obtained through aldol condensation of cyclopentanone and furfural over solid acid catalysts under solvent-free conditions. Nafion exhibited the best performance among the investigated catalysts (Nafion, Amberlyst-15, Amberlyst-36, H-USY, ZSM-5, H-β, and SiO₂/Al₂O₃). Under optimized conditions, yields of F₂Cp and FCp were 37.48% and 23.77%, respectively. Selectivities of F₂Cp and FCp were 50.74% and 32.18%, respectively. Nafion showed good stability and did not deactivate during 4 runs. Meanwhile, a certain amount of humin was also generated during the reaction. The reaction mechanism for the aldol condensation of cyclopentanone and furfural and the formation mechanism of humin were also proposed.