Catalytic wet-air oxidation of aqueous solutions of formic acid, acetic acid and phenol in a continuous-flow trickle-bed reactor over Ru/TiO2 catalysts

Catalytic wet-air oxidation of aqueous solutions of formic acid, acetic acid and phenol was carried out in a trickle-bed reactor at T = 328–523 K and total pressures up to 50 bar over various Ru/TiO2 catalysts. Complete oxidation of formic acid was obtained at mild operating conditions, and no catalyst deactivation occurred that could be attributed to the dissolution of active ingredient material. It was observed that besides oxidation route thermal decomposition contributes significantly to the removal of formic acid; Ru/TiO2 catalysts could be thus efficiently used for transformation of HCOOH to H2 and CO2 in an inert atmosphere. Liquid-phase oxidation of acetic acid was found to be structure sensitive; the highest catalyst activity was obtained, when Ru phase on the catalyst surface prevailed in zero-valent state. Due to simultaneous oxidation of metallic Ru to RuO2 during the reaction course, correspondingly lower conversion of acetic acid was measured in the reactor outlet. The employed Ru/TiO2 catalysts enable complete removal of phenol and TOC at temperatures above 483 K; at these conditions, no carbonaceous deposits were accumulated on the catalyst surface. Apparent catalyst deactivation observed at temperatures below 463 K is attributed to strong adsorption of partially oxidized intermediates on the catalyst surface, which can be avoided by conducting the CWAO process at sufficiently high temperatures. The acute toxicity to Daphnia magna and Vibrio fischeri of end-product solutions compared with that of the feed solutions was significantly reduced by oxidative treatment over Ru/TiO2 catalysts.