Strategical design of bridged vanillic Schiff-bases as jet fuel antioxidants

Two vanillic Schiff-base antioxidants, 2-methoxy-4-(((4-(phenylamino)phenyl)imino)methyl)phenol (PPIMP) and 2-methoxy-4-(((4-(phenylamino)phenyl)amino)methyl)phenol (PPAMP), have been synthesized through the Schiff-base condensation reaction and hydrogenation to improve the thermal-oxidative stability of hydrocarbon fuels. Both novel antioxidants exhibited remarkable performance in decalin and JP-10 fuel. For example, pressurized differential scanning calorimetry (P-DSC) tests showed that adding 100 ppm of PPIMP or PPAMP to JP-10 improved the oxidation induction time from 9.5 min to 19.2 and 27.8 min, respectively. JFTOT tests showed that adding 100 ppm of either antioxidant helped alleviate JP-10 deposition. Theoretical calculations on electron properties and orbital conformation revealed that the C = N bridge in PPIMP had an electron donation effect and conjugative stabilization effect, which can stabilize intermediate antioxidant radicals and improve the antioxidants’ reactivity. The hydrogenation of C = N further enhanced the performance of PPAMP by introducing an additional antioxidant group. Therefore, both C = N and C-N in the synthesized antioxidants were found to be essential. The chemical combination of small molecular antioxidants achieves 1 + 1 > 2 synergism with the assistance of the azomethine group.