Oxidative catalysis of Keggin anion [XW11O39Z(H2O)]n− pillared clays in the reaction of acetaldehyde with H2O2

In aqueous solution, the layered double hydroxides (LDHs) pillared by heteropolyanions, Co2Al–XW11Z and Ni2Al–XW11Z (X=B3+, Si4+, Ge4+ and As5+; Z=Cu2+, Co2+ and Ni2+), have been synthesized by the direct ion exchange reaction of the monosubstituted Keggin-type heteropolyanion containing transitional element with the Co2Al–NO3 and Ni2Al–CO3 precursors. The results of XRD and FTIR measurements indicate that they are the pillared layered compounds with gallery height of 0.99±0.01 nm and that Keggin-type framework of the heteropolyanion still remain inside the layers. The composition and properties of these compounds have been evidenced by TG and ICP analyses. The catalytic feature of these pillared layered clays for the oxidative reaction of acetaldehyde with H2O2 has been studied. The formation rate of the product acetic acid was between 20–53 mmol (g-cat.)−1 h−1 and the selectivity for acetic acid was nearly 100%, showing that they were excellent oxidative catalysts in liquid–solid system. In addition, if the catalytic activities are expressed as per mmole of heteropolyanion instead of per gram of the catalyst, the catalytic activities of Co2Al–XW11Z and Ni2Al–XW11Z were between three times and nine times that of XW11Z as potassium salts in homogeneous system. This may be because the clay layer has a significant effect on the heteropolyanions against decomposition resulting from H2O2. Furthermore, the composition of the layer and heteropolyanion also influences on the catalytic activity. The experimental results of catalytic activity for the two kinds of clay catalysts of Co2Al–SiW11Cu and Co2Al–NO3 calcined at different temperatures show that not only the pillared layered structure makes a contribution to the catalytic activity but also the thermo-decomposition product of them, a complex oxide with spinel structure, is a fine catalyst for the oxidation of acetaldehyde into acetic acid.