Ketonisation of acetic acid on metal oxides catalysts

Abstract

The ketonisation of acetic acid to acetone was studied in the gas phase using γ-Al2O3, TiO2, ZrO2 and CeO2 as the catalysts in the temperature range of 180–350 °C and ambient pressure. Catalyst activity was found to increase in the order Al2O3 << TiO2 < ZrO2 < CeO2. Catalyst resistance to deactivation increased in the order CeO2 << ZrO2 < TiO2 in parallel with the amount of coke formed. TiO2 (Degussa P25) and ZrO2 were found to exhibit the best performance as represented by their activity and stability to deactivation. CeO2 and ZrO2 could be regenerated by air calcination to regain their activity. Given the very good ketonisation performance of the TiO2 and ZrO2 pure oxides, it was interesting to test the performance of mixed oxide Ti-Zr catalysts in this reaction. All the mixed oxides were characterised by BET measurements as well as powder XRD. It was shown that TiO2-ZrO2 mixed oxides, prepared by sol-gel synthesis, are active catalysts in the ketonisation reaction of acetic acid to acetone at 300 oC. The 1:2 Ti Zr catalyst showed the best acetic acid conversion, which exceeded the conversion over the pure TiO2 and ZrO2 oxides prepared by the sol-gel method. Although all of the catalysts showed varied performance in terms of conversion, all samples gave excellent selectivity to acetone of 98–100%. Overall, however, the sol-gel TiO2, ZrO2 and TiO2-ZrO2 catalysts had no advantages in activity and selectivity over the same catalysts prepared by the precipitation method. Moreover, mixed oxide TiO2-ZrO2 catalysts were considerably less active than the pure ZrO2 prepared by the precipitation method. Two mechanistically significant observations were made from our DRIFTS studies. First, facile exchange was found between the bidentate bridging acetate and co-adsorbed acetic acid on the oxide surfaces under mild conditions (130 oC) well below ketonisation temperatures. This demonstrates the lability of the surface bidentate bridging acetate species in the ketonisation system. This result shows that various types of surface acetate species are equilibrated in the ketonisation system, which makes them kinetically indistinguishable. Second, at higher temperatures typical for acid ketonisation, in the absence of gas-phase acetic acid, we found that the adsorbed bidentate bridging acetate-d3 species underwent H/D exchange with proton sites on oxide surfaces. This, for the first time, provides experimental evidence supporting the intermediacy of enolate species in the ketonisation of carboxylic acids.