The present study investigates synthesis of waste-derived heterogeneous acid catalyst from rubber de-oiled cake using sulfuric and chlorosulfonic acids. The synthesized catalyst was characterized using XRD, XPS, FE-SEM, EDX, BET, and acid density titration. Maximum biodiesel yields of 91.2 +/- 1.1% and 93.7 +/- 1.3% were obtained at molar ratio = 12.8:1, catalyst loading = 8.18% (w/w), and temperature = 63 degrees C in single-step process (duration 3 h) and two-step process (duration 1 h), respectively. Two-step process resulted in lower activation energy of 55.98 kJ/mol, as compared to 69.62 kJ/mol in single-step process. The predominant role of ultrasound in process intensification was observed in two-step process, where sum of activation energies (27.47 kJ/mol) of three individual reaction steps was lower as compared to mechanically agitated system (49.07 kJ/mol). The catalyst retained activity until three cycles of reuse.