Effects of cations introduced into 12-molybdophosphoric acid on the catalyst properties

Ordered Array Formation and Negative Differential Resistance Behavior of Cation-Exchanged Heteropoly Acids Probed by Scanning Tunneling Microscopy

“…The reduction temperature of cation-exchanged 12-molybdophosphate HPAs decreases with increasing Tanaka electronegativity of the countercation. 3 In other words, the reduction potential of the HPA increases with increasing electronegativity of the countercation. The applied voltage at which NDR behavior was observed in the tunneling spectrum of these HPA salts also decreased in magnitude with increasing electronegativity of the countercation, with some scatter as shown in Figure 4.…”

“…28 Also, X-ray and IR observations 27 and STM images have all shown that these heteropoly acids still maintain their characteristic Keggin structure after changing the identity of the countercations. It has been shown that the activities for oxidation of methacrolein 3 and methyl ethyl ketone 27 on various cation-exchanged 12-molybdophosphoric acids (HPA salts) increase with increasing electronegativity of the countercation. When the reduction temperature (defined as the temperature at which the H 2 consumption by the HPA salt reached 0.3 mole per mol of the pure 12-molybdophosphoric acid in TPR experiments) was employed as the index of reducibility, it became apparent 3,27 that the increase in the oxidation activity was due to the increase in the reduction potential of the HPA salts with more electronegative countercations.…”

“…These results suggest a correlation of the NDR behavior in the tunneling spectra of different HPA salts not only with the electronegativity of the countercation of the HPA salt but with the reducibility or the "reduction temperature" of the salt as well. Figure 4 shows the reciprocal of the reduction temperature as measured by Ai 3 and the electronegativity of the countercations, both plotted with respect to the NDR peak voltages measured for arrays of cationexchanged 12-molybdophosphates. Reduction temperature data for magnesium and potassium salts were not available.…”

“…O 40 , are known to perform excellently in certain mild oxidation reactions, notably in "acid-formation-type" reactions. 3 One such reaction involves the commercial production of methacrylic acid by direct oxidation of methacrolein. 31 It has been found that when the identity of the countercation of the heteropoly anion (12-molybdophosphate) is fixed and the extent of exchange is varied, the oxidation activity for methacrolein varies significantly in a manner that is distinctly different from the acidity of the catalyst measured by means of the adsorption of NH 3 .…”

“…[1][2][3] Many are highly soluble in polar solvents such as water, alcohols, and amines but insoluble in some nonpolar solvents such as benzene and olefins. 4,5 The solubility of these compounds in turn is closely connected to their ability to adsorb various reactants.…”