Propane carbonylation on sulfated zirconia catalyst as studied by 13C MAS NMR and FTIR spectroscopy

Using in situ 13C MAS NMR, it has been demonstrated that propane is carbonylated with carbon monoxide to produce isobutyraldehyde and isobutyric acid at 100–150 °C on sulfated zirconia. Isobutyraldehyde represents the intermediate reaction product, which is almost completely converted into the acid at 150 °C. The selective formation of isobutyraldehyde at low temperatures on SZ indicates that principally the secondary CH bond of propane is activated, rather than its CC bond. This pathway of the alkane activation can be realized either on Lewis acid sites of SZ or by direct formylation by the formyl cation formed as equilibrated species from the formate. Infrared spectroscopy provides evidence that the sulfate groups of SZ are responsible for the aldehyde oxidation. The formation of surface dithionate species as a result of sulfur reduction is suggested. These dithionate species are readily reconverted into sulfate groups by an oxidizing treatment in an O2 atmosphere.