Liquid phase oxidation of cyclopentanone over metal-free carbon catalysts

Abstract

Oxidation of cyclopentanone (CPO) was studied over metal-free carbon catalysts in solvent-free conditions. Two pristine carbons were used: activated carbon Norit SX plus (total BET surface area 900 m2 g-1, external surface area about 350 m2 g-1) and carbon black Vulcan XC72 (total BET surface area 250 m2 g-1). Catalysts were prepared by activation in nitrogen and by treatment with urea and calcined in nitrogen at 500 °C and 950 °C. Catalysts were characterised in terms of textural properties (size of particles, X-ray powder diffraction, specific surface, microporosity, distribution of pores), acid-base properties (the average pH of the suspension, Boehm titration), surface functional groups (FTIR), and surface composition (XPS). A small content of nitrogen was found on the surface of all the catalysts, whereas the highest value was for the Vulcan catalyst activated with urea and calcined at 500 °C. Oxidation tests, carried out with pure CPO at 0.45 MPa in temperature range 80 – 110 °C allowed to choose the 90 °C and the catalyst activated with urea as the best one for recycling. At about 25 % conversion, the selectivity of ca. 30 mol % to glutaric acid and ca. 3 mol % to succinic acid were achieved. An interesting finding was that Norit based catalysts despite their higher specific surface area than Vulcan based catalysts were less active, provided lower selectivity and were much prone to deactivation. According to our hypothesis a higher concentration of active sites immediately after the start of the oxidation reaction accelerates the reaction rate. Due to exothermicity more reaction heat is evolved and the surface temperature of catalyst is increased. A higher temperature on the surface enhances side reactions, which yield in heavier products sticking on the surface and deactivates the catalyst.