In this work, natural rubber vulcanizate (VR) was chemically modified to the heterogeneous acid catalyst as sulfonic acid-containing rubber via a controlled oxidation with hydrogen peroxide (H₂O₂) and formic acid (HCOOH). The starting VR were prepared by varying mass ratio of N-cyclohexyl-2-benzothiazole sulfonamide (CBS), as an accelerator, to sulfur (S/S). The influences of oxidation conditions, including concentration of H₂O₂, mass ratio of H₂O₂ to HCOOH and the mass ratio of rubber to oxidizing solution were studied. The original and oxidized natural rubber vulcanizate (OVR) were measured for the sulfur content by bomb calorimetry, the swelling degree according to ASTM D471 and the sol/get ratio according to ASTM D471. Moreover, the physicochemical properties of rubbers were characterized by Fourier-transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Thermogravimetric/differential thermal analysis (TG/DTA) and acid-base titration. The FTIR results indicated the presence of the polar acid functions, especially sulfonic acid and carboxylic acid groups in the oxidized vulcanizate. Because of the high amount of sulfur (1.47 mmol/g), acidity contains (0.96 mmol/g) and high stability in toluene/ethanol solvents, OVR-4 prepared from the VR-4 oxidation (S = 5 phr, A/S = 7.25) was suitable for apply as the heterogeneous acid catalyst in the esterification of octanoic acid with 1- octanol. Finally, the octanoic acid conversion and the catalyst recovery were analyzed by the gas chromatography and the filtration, respectively. The OVR-4 was successfully applied as the catalyst in the esterification of octanoic acid with 1-octhanol at 120 ℃ for 8 h in which the octanoic acid conversion was 86.4%. The results showed that its efficiency was very similar to that of the commercial catalyst Amberlyst15. However, the catalyst recovery was low due to the dissolution of catalyst in the reaction mixture.
