Evolution of naphthalene and its intermediates during oxidation in subcritical/supercritical water.

Abstract: Oxidation of naphthalene (Nap) in supercritical water (SCW) was carried out in a batch reactor (6-ml volume) up to 400°C and 300atm (maximum residence time of 1800s). Two systems were studied. System A consisted of pure naphthalene mixed with oxygen and water: 0.065M Nap+1.441M O₂ +8.549M H₂O. System B included, in addition to O₂ and H₂O, a heavy metal and an ash to simulate their roles in the destruction of Nap: 0.065M Nap+1.441M O₂ +0.008M Cr(NO₃)₃ +0.065M Al₂Si₂O₅·(OH)₄ +8.624M H₂O. It was shown that, in general, the oxidation process can be divided into three stages characterized by H₂O₂ decomposition to H₂O and O₂ (Stage 1), oxidation in saturated/subcritical water up to 360°C and 225atm (Stage 2), and fast oxidation in the supercritical region (Stage 3). The oxidation process revealed different reaction rates in different systems. Relatively low activation energy of 10–30kJ/mol was obtained below 360°C and 225atm in the System A and in the whole temperature range in the System B. High activation energy (139.5kJ/mol) was obtained for supercritical water oxidation of naphthalene in the System A. The catalytic effect of Cr³⁺/ash stimulated naphthalene oxidation. Time-resolved yields of major products and intermediates were obtained. The reaction pattern was proposed. It describes the evolution of naphthalene and intermediates as a sequence of reactions comprising transformations of the main compounds and their inter-conversion. This radical-involving reaction mechanism is mainly based on the quantification of the profiles of the intermediates. [ABSTRACT FROM AUTHOR]