CATALYTIC WET AIR OXIDATION OF ACETIC-ACID ON CARBON-SUPPORTED RUTHENIUM CATALYSTS

Ruthenium catalysts prepared by ion exchange of active carbons and hig h-surface-area graphites are active for the wet air oxidation of aqueo us solutions of acetic acid (5-20 g/liter). A total conversion into CO 2 can be achieved between 448 and 473 K using air as oxidizing agent. No leaching of ruthenium can be detected which indicates that the reac tion proceeds on the heterogeneous catalysts. For the same particle si ze (1 nm), graphite-supported ruthenium catalysts are much more active (up to 0.4 mol h(-1) g(Ru)(-1) at 473 K in a stirred batch reactor pr essurized with air at 10 MPa) than active carbon-supported catalysts. The lower activities of the latters could be due to internal diffusion limitation since the I-nm Ru particles are located inside the micropo res. However, graphite-supported catalysts might be intrinsically more active because of an electron transfer from graphite to metal particl es which would increase the resistance of ruthenium to oxygen poisonin g. It was also shown that the activity of ruthenium is particle size d ependent: the smaller the sizes, the lower the activities. This effect could be interpreted by the higher adsorption energy of oxygen on the small particles which produces a poisoning of the metal surface. From measurements of the reaction rates on the Ru/HSAG graphite catalyst a t different temperatures, pressures, and acetic acid concentrations, i t was established that the reaction orders were zero and 0.65 with res pect to the concentration and oxygen pressure, respectively; the activ ation energy of the reaction was 100.5 kJ mol(-1). An equation describ ing the reaction kinetics was proposed. (C) 1997 Academic Press.