Oxyfunctionalization of n-pentane and n-hexane by oxovanadium complexes supported on carbamated modified silica gel

The bis(maltolato)oxovanadium complexes cis-[VO(OCH3)(ma)2] 1, cis-[VO(OC2H5)(ma)2] 2, [VO(ma)2] 3 and [VO(py)(ma)2] 4 have been anchored to chemically modified silica gel and are shown to act as supported catalysts for the oxidation of n-pentane and n-hexane, with molecular oxygen under relatively mild reaction conditions, to the corresponding ketones (with good selectivities) and alcohols (in smaller amounts). cis-[VO(OCH3)(ma)2] 1 provides the best results in both cases and the effects of various factors (temperature, time, O2 pressure, catalyst amount) were also investigated towards the optimization of the reaction conditions. 2-pentanone (84% selectivity) and a small amount of 2-pentanol are formed from n-pentane at 150 °C, p(O2) = 15 atm, while 2-hexanone (58% selectivity), 1-hexanol and acetic acid are obtained from n-hexane at 160 °C, p(O2) = 10 atm. The involvement of a carbon- and oxygen-centred radical mechanism is suggested by experiments with free radical traps. Heteroaromatic acids were tested as possible co-catalysts and 2-pyrazinecarboxylic acid was the most effective one. Thermogravimetric analysis (TGA) indicates the catalyst is stable up to ca. 220 °C and inductive couple plasma (ICP) shows a notable extensive vanadium loss from the supported catalyst after the reaction, allowing further use of the catalyst even without reactivation.