ABSTRACT
Abstract Aerobic oxidation of formaldehyde in water under mild conditions (20-40 ˚C, 1 atm of air or O2) in the presence of Ce-substituted POMs affords formic acid with high selectivity. Introduction The development of catalysts for the oxidation of organic compounds by air under ambient conditions is of both academic and practical importance (1). Formaldehyde is an important intermediate in synthetic chemistry as well as one of the major pollutants in the human environment (2). While high temperature (> 120 ˚C) catalytic oxidations are well known (3), low temperature aerobic oxidations under mild conditions have yet to be reported. Polyoxometalates (POMs) are attractive oxidation catalysts because these extensively modifiable metal oxide-like structures have high thermal and hydrolytic stability, tunable acid and redox properties, solubility in various media, etc. (4). Moreover, they can be deposited on fabrics and porous materials to render these materials catalytically decontaminating (5). Here we report the aerobic oxidation of formaldehyde in water under mild conditions (20-40 ˚C, 1 atm of air or O2) in the presence of Ce-substituted POMs (Ce-POMs). Results and Discussion Various Ce-POMs have been synthesized and evaluated as catalysts for the title reaction at ambient conditions along with simple Ce-compounds (Figure 1). The most active catalyst was the monosodium acid salt of the Ce(IV)- monosubstituted silicotungstate, NaH3SiW11CeO39·7H2O (1), especially when combined with a 4-fold molar excess of AgNO3 (Figure 1). Surprisingly, no precipitation of the silver salt of the Ce-POM occurred, contrary to many other POMs, including PMo10V2O405-(6). The accelerating effect of AgNO3, compound known to co-catalyze oxidations but be minimally active itself, was considerably less pronounced when the reaction temperature was increased.
