VAPO as catalyst for liquid phase oxidation reactions. Part II: stability of VAPO-5 during catalytic operation

The stability of VAPO has been studied in detail. VAPO-5 was extensively re-used as catalyst for the epoxidation of 3-phenyl-2-propen-1-ol and the oxidation of 3-octanol by tert-butylhydroperoxide (TBHP). It appears that during the reaction the structure of VAPO stays largely intact, however the micropores become obstructed by organic compounds. Calcination is necessary to restore the activity completely. Removal of the VAPO from the reaction mixture after a short reaction time showed an ongoing reaction in the filtrate. In the solution a small amount of vanadium could be detected (between 0.9–12.5% loss of vanadium, depending on the VAPO and reaction conditions). The leaching process stops soon after the start of the reaction, which can be explained by protection due to strong absorption of organic molecules. Soxhlet extraction of VAPO with water did not give a stable system. Controll experiments with tiny amounts of homogeneous catalyst (VO(acac)2) show that the activity can be largely explained by these small amounts of vanadium in solution. For this reason it is concluded that VAPOs (and more generally MeAPOs) are not suitable as catalysts for liquid phase oxidation reactions.