Oxidative conversion of alkyl naphthalenes to carboxylic acids in hot pressurised water

Special polymers on polyethylene-naphthalate (PEN) basis contain 2,6-naphthalene dicarboxylic acid (2,6-NDA) as building block. Besides various routes for the synthesis, oxidation of substituted dialkylnaphthalenes seems to be advantageous. Our research focused on the oxidative conversion of 2,6-diisopropylnaphthalene (2,6-DIPN) to 2,6-NDA using hot pressurised water. In general, oxidation runs were carried out in a sealed tube (volume 10 ml) in a temperature region of 300–420 °C at pressures of 150–250 bar. Co/Br or Mn/Br were used as catalyst systems and aqueous H2O2 served as oxidant. Some complementary experiments were realised with a stirred batch autoclave (volume 250 ml). Products were analysed by GC (FID) and GC/MS. The catalytic tests have shown that 2,6-DIPN can be converted to 2,6-NDA. At reaction times of 30–60 min at 400 °C and p = 250 bar (supercritical conditions) in the sealed tube a complete conversion was observed and 2,6-NDA was formed with a selectivity of 60 to nearly 80%. At 300 °C (subcritical conditions) conversion is less and formation of incompletely oxidised by-products increases.