Oxide-catalyzed conversion of acetic acid into acetone: an FTIR spectroscopic investigation

Adsorptive and catalytic interactions of gas phase acetic acid with surfaces of alumina, titania and ceria were observed by in situ Fourier-transform infrared (FTIR) spectroscopy on heating from room temperature up to 400degreesC. The results revealed that, on alumina the acid was irreversibly, non-dissociatively adsorbed in the form of hydrogen-bonded molecules, and dissociatively in the form of bidentate bound acetate species over the full range of temperature scanned. In the gas phase, no chemical change was observed. On titania, the gas phase remained unchanged on heating up to 300degreesC, but at 400degreesC the acetic acid was largely converted into acetone (as well as CO2 and H2O) and minor products of isobutene and methane. Similar changes to the gas phase were observed on ceria, however, at 300degreesC. The acetic acid conversion on ceria was almost complete. The appearance of acetone molecules in the gas phase was pertained by the emergence of IR absorptions implying the formation on the surface of unsaturated carbonyl species. Thus, the formation of these surface species, together with isobutene and CH4 molecules in the gas phase, was considered consequent to the occurrence of further surface reactions of acetone molecules. Observance of structural and chemical stability of the surface and bulk of TiO2 and CeO2 throughout the reaction was taken to emphasize the catalytic nature of the acetic acid conversion into acetone in the gas phase, a process that is evidently more simple and economic than the conventional pyrolytic synthesis of acetone from metal acetate compounds in the solid state. The catalytic sites were suggested to be Lewis acid-base pair sites, with the Lewis acid sites (Ti4+ or Ce4+) being reducible. Mechanistic pathways were proposed for the observed adsorptive and catalytic interactions of acetic acid molecules on the test oxides. (C) 2002 Elsevier Science B.V. All rights reserved.