The catalytic condensation of furfural and 2-methylfuran was performed to produce C15 diesel precursors using sulfonated acidic carbon catalysts, including multi-walled carbon nanotubes, graphene oxide, carbon aerogel, activated carbon, and carbon black. Sulfonated multi-walled carbon nanotubes were prepared with mild sulfonate grafting procedure, and displayed negligible modification of their pore structures. They exhibited the highest turnover frequency, indicating that the acid strength, depicted by the pKa, determined the catalytic condensation activity. The pKa values of the catalysts needed to be less than 4 for enabling the condensation of furfural and 2-methylfuran, and catalysts with pKa values greater than 4 exhibited poor condensation. The pKa, or acid strength, was determined by the environment of the surface sulfonates, which were electronically modified to adjust the stability of the sulfonate anions to release protons.