12026-66-3Relevant articles and documents
The Effect of Cation Type and H+ on the Catalytic Activity of the Keggin Anion [PMo12O40]3- in the Oxidative Dehydrogenation of Isobutyraldehyde
Hu, Ji,Burns, Robert C.
, p. 360 - 375 (2000)
The oxidative dehydrogenation of isobutyraldehyde to methacrolein over [PMo12O40]3--containing catalysts has been shown to proceed through bulk catalysis-type II, which depends on the rates of diffusion of the redox carriers (H+ and e-) into the catalyst bulk. Variations in catalyst behaviour have been shown to change with the countercation and appear to be related to the polarizing ability of the cation, which can be represented by the ionic potential (charge/ionic radius). This, in turn, may indicate that the active site at the [PMo12O40]3- ion is close to an attendant countercation. For the alkali metal ions Li+, Na+, K+, Rb+, and Cs+ as well as the (isoelectronic) ions of the series Cs+, Ba2+, La3+, and Ce4+, the studies have shown that conversion generally decreases with increasing ionic potential, while selectivity to methacrolein is less affected by changes in this property. However, the observed enhancement in selectivity to methacrolein and methacrylic acid in the case of Ce4+ is attributed to the ability of this cation to undergo reduction to Ce3+, with the Ce4+/Ce3+ couple likely acting to expedite the transfer of electrons between anions and hence into the bulk. Trends in the conversion and selectivities among the countercations Cs+, NH+4, and (CH3)4N+ show significant increases along this series, which is consistent with progressively increasing dissipation of the formal cationic charge. A mechanism for the oxidative dehydrogenation of isobutyraldehyde is proposed. The presence of H+ enhances the activity of salts of [PMo12O40]3-, most likely by protonation of the organic component, as little evidence from extended-Hueckel molecular orbital calculations could be found for any reduction in the HOMO-LUMO gap of the anion upon protonation, which would make it easier to reduce the anion as part of the catalytic process.
Albonetti, S.,Cavani, F.,Trifiro, F.,Gazzano, M.,Koutyrev, M.,et al.
, p. 491 - 502 (1994)
Effects of NH4/+, Cs+, and H+ counterions of the molybdophosphate anion in the oxidative dehydrogenation of isobutyric acid
Marchal-Roch,Laronze,Villanneau,Guillou,Teze,Herve
, p. 173 - 181 (2000)
Mixed NH3-Cs salts of the molybdophosphoric acid (Csx(NH4)3-x[PMo12O40]) were prepared by solid-state cationic exchange using the NH3 salt impregnated with the appropriate amount of Cs nitrate at incipient wetness. Structural characterization of the solids revealed that the exchange was quantitative and that NH3 and Cs cations were randomly distributed in the lattice. Catalytic activity of these salts in the oxidative dehydrogenation of isobutyric acid increased with increases in the amount of Cs ≤ 3 Cs per heteropolyanion. Between 3 and 3.1 Cs, the catalytic activity suddenly fell and, simultaneously, the major product switched from methacrylic acid (MAA) to acetone. This abrupt change in the activities for x ~ 3 corresponded with the strong decrease of the rates of formation of MAA and propylene, while the rate of formation of acetone remained almost unmodified. Departure of NH3 was total for x ≥ 2 and partial for x 3-Cs salts when x 2 and acid Cs salts when 2 x 3. Very few protons were needed to obtain an active and selective catalyst in MAA.
Ammoxidation of 2-methyl pyrazine on supported ammonium salt of 12-molybdophosphoric acid catalysts: The influence of nature of support
Narasimharao, Katabathini,Babu, B. Hari,Lingaiah,Prasad, P. S. Sai,Al-Thabaiti, Shaeel A.
, p. 487 - 498 (2014)
Influence of the nature of support on the formation of catalytically active species was investigated to clarify the key factor for the synthesis of supported ammonium salt of 12-molybdophosphoric acid (AMPA) catalyst which maintains the activity of ammoxidation during 2-methylpyrazine reaction. With this aim, different loadings of niobia-, silica- and alumina-, supported AMPA catalysts were prepared. The AMPA loading was varied in the range of 5-25 wt%. The synthesized solids were characterized by nitrogen adsorption for BET surface area, XRD and 31P MAS NMR techniques. All the AMPA-supported samples are poorly crystalline even after 25 wt% AMPA loading. Investigations using 31P MAS NMR spectroscopy of samples revealed that Keggin ion existed as at least five different species on the supports. The investigated properties were acidity of the support and amount of AMPA loading on the support. Active sites for the ammoxidation of MP on supported AMPA catalysts seem to be the interacted and/or the lacunary species. Maximum catalytic activity could be obtained at lower loadings with AMPA deposited on acidic supports whereas the less acidic supports require higher loading. It was found that in order to efficiently generate the active interactive species, the support must have an acidity which promotes the formation of support-AMPA interactive species. It is possible to enhance the catalytic activity of the supported AMPA catalyst for ammoxidation of 2-methylpyrazine by controlling the acidity of the support and AMPA loading on the support.
