Etsuro Echigoya

12-Heteropolymolybdates as catalysts for vapor-phase oxidative dehydrogenation of isobutyric acid: 3. Molybdotungstophosphoric and molybdovanadophosphoric acids

The effect of condensing metal atoms (tungsten and vanadium) on the catalytic properties of 12-heteropolyphosphoric acids, as heterogeneous oxidation catalysts, has been investigated. While both the reducibility by isobutyric acid and carbon monoxide and reoxidizability by gaseous oxygen of molybdovanadophosphoric acids (H3+n[PMo12−nVnO40], n = 0, 1, 2, and 3) paralleled their highest oxidation-reduction potential and showed the maximum at H4[PMo11V1O40], the reoxidizability of molybdotungstophosphoric acids (H3[PMO12−nWnO40], n = 0, 1, 2, … 6, and 12) increased with lowering their highest oxidation-reduction potential and their reducibility decreased markedly with increasing the content of tungsten atom irrespective of the change in their highest oxidation-reduction potential. The preferential formation of V4+ in the reduced molybdovanadophosphoric acids and that of Mo5+ in the reduced molybdotungstophosphoric acids, which were revealed by ESR, also suggested different mechanistic roles of condensing metal atoms between tungsten and vanadium atoms. These results are discussed as the effect of condensing metal atoms on the reducibility and reoxidizability of 12-heteropolyphosphoric acids, and it is shown that unlike the tungsten atoms as a MoO bond strength modifier the vanadium atoms play the role of an electron reservoir: their redox properties decisively affect the reducibility and reoxidizability of molyb-dovanadophosphoric acids. It is also shown that potassium, rubidium, and cesium salts of H4 [PMo11V1O40] are fairly selective catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid.

12-Heteropolymolybdates as catalysts for vapor-phase oxidative dehydrogenation of isobutyric acid: 2. Group Ib, IIb, IIIb, and VIII metal salts

Abstract The effect of cations on the catalytic properties of 12-molybdophosphates, as oxidation catalysts, has been investigated in the vapor-phase oxidative dehydrogenation of isobutyric acid at 300 °C. Both the reducibility by isobutyric acid and carbon monoxide and reoxidizability by gaseous oxygen of 12-molybdophosphates increased with increasing standard electrode potential (SEP) of cations (group Ib, IIb, IIIb, and VIII metal ions and Pb 2+ , Mn 2+ , Ce 3+ , and La 3+ ). On the other hand, while the oxidizing activity (i.e., an affinity for electron) of Mo 6+ in these 12-molybdophosphates paralleled the SEP of the cations, the electrons formed by reduction of these 12-molybdophosphates were preferentially captured by the cations when the cations had a high SEP. These results are discussed as the effect of cations on the electrochemical properties of the molybdenum atoms, and the catalytic results obtained are explained on the basis of the exclusive participation of the lattice oxygen, the delocalization of the electrons by the cations, and the acid-base properties of the catalyst. It is proposed that these cations play the role of an electron-reservoir: their electron-capturing nature and electron-donating nature, i.e., their redox properties decisively affect the reducibility and reoxidizability of 12-molybdophosphates. Mechanistic differences in the promotional effects of cations between the metal ions employed in this work and alkali metal and alkaline-earth metal ions are also shown.

Kinetic and adsorption studies on vapor-phase catalytic oxidation of olefins over silver

The mechanism of the catalytic oxidation of ethylene, propylene, butenes, and pentenes over silver has been investigated. For ethylene, the partial and complete oxidations at 230 °C had nearly the same kinetic dependence on olefin and oxygen concentrations. For the complete oxidation of propylene, the reaction order in olefin was smaller than that observed with ethylene whereas the reaction order in oxygen was greater. This change in these two kinds of reaction orders in reverse direction was also observed with butenes and pentenes, and the rate of complete oxidation of olefins (butenes and pentenes) normalized to one allylic hydrogen followed the relation in terms of the type of allylic hydrogens involved, primary < secondary < tertiary, irrespective of the kind of olefin oxidized. Relative rate measurement showed that the small but not negligible normal kinetic isotope effect of hydrogen existed in the complete oxidation of propylene and 2-methyl-1-propene. It is also shown that while ethylene is adsorbed only on the Agδ+ (0 < δ < 1) formed at the partially oxygenated silver surface, the olefins other than ethylene can be adsorbed on both the Agδ+ and the completely reduced silver surface. Pentenes inhibited ethylene oxidation over silver at 245 °C; the results obtained are readily understood on the basis of the mechanism for epoxide formation involving the reaction of the adsorbed diatomic oxygen with the adsorbed ethylene, not with gasphase ethylene. These findings are discussed in relation to previous suggestions regarding surface intermediates and the mechanism of these oxidations.

12-heteropolymolybdates as catalysts for vapor-phase oxidative dehydrogenation of isobutyric acid: I. Alkali and alkaline-earth metal salts

Abstract The effects of cations and heteroatoms on the catalytic activity and selectivity of 12-heteropolymolybdates, as catalysts, have been investigated in the vapor-phase oxidative dehydrogenation of isobutyric acid. The conversion of isobutyric acid at 300 °C increased with decreasing electronegativity of the cations (H + , alkali, and alkaline-earth metal ions) and the heteroatoms (P 5+ , As 5+ , and Si 4+ ), whereas it decreased with the change in the nature of the cations at 250 °C. The yield ratio (acetone/methacrylic acid) at 300 °C showed a behavior similar to the conversion, but the ratio was maximum over lithium and strontium salts. On the other hand, the electron affinity of Mo 6+ in these 12-heteropolymolybdate-type catalysts paralleled the electronegativity of the cations and the heteroatoms, and the reactivity of Mo 5+ to gaseous oxygen increased with decreasing electronegativity of these catalyst components (cations and heteroatoms). These results are discussed as the effects of these catalyst components on the electrochemical properties of the molybdenum atom, and the catalytic results obtained are explained on the basis of the exclusive participation of the lattice oxygen, the strength of molybdenum-oxygen bond, and the acid-base properties of the catalyst. It is shown that the molybdenum-oxygen bond is weakened as the catalyst components become less electronegative.

Dehydration of alcohols on/in heteropoly compounds

Abstract The dehydration of methanol, ethanol, 1-propanol, and 1-butanol were studied using 12-tung-stophosphates as acid catalysts, Even “neutral” salts displayed acid-catalytic properties. A good correlation between acidic strength of the salts and their catalytic activity was established. However, several abnormal acid-catalytic behaviors were observed, namely, change of activity with process time, conversion vs W/F relationship, and reactivity sequence of various alcohols. In particular, the alcohols reactivity under high partial pressures was found in the order: MeOH > EtOH > 1-PrOH > 1-BuOH, while a reverse sequence was obtained under low partial pressures of alcohols. These results were reasonably interpreted in view of a pseudo-liquid phase catalysis model.

Carrier effect in vapor-phase oxidation of butadiene over supported molybdena catalysts

Abstract The reactivity of Mo 5+ with adsorbed oxygen formed on various kinds of supported molybdena catalysts has been investigated by ESR. The results are discussed as a carrier effect with reference to the selectivity for maleic anhydride formation during vapor-phase oxidation of butadiene. Mo 5+ formed on silica and titania carrier can be easily oxidized and shows good selectivity, in contrast to Mo 5+ on magnesia and alumina. By ESR, there appeared only one adsorbed oxygen species O 2 − on MoO 3 MgO, MoO 3 Al 2 O 3 and MoO 3 SiO 2 , while two kinds of adsorbed oxygen species, O 2 − and O − , were observed on MoO 3 TiO 2 . It was also shown that O 2 − and O 2− , which was formed by the oxidation of Mo 5+ and proved to be Mo 6+ O 2− , were required for the production of maleic anhydride. Therefore, it was concluded that the higher selectivity of MoO 3 -SiO 2 and MoO 3 -TiO 2 catalyst compared with MoO 3 -MgO and MoO 3 -Al 2 O 3 can be attributed to the presence of the double bond oxygen.

Catalytic activity of vanadates in oxidation of methanol

The investigations of catalytic activities of V/sub 2/O/sub 5/--NiO, V/sub 2/O/sub 5/--Fe/sub 2/O/sub 3/, and V/sub 2/O/sub 5/--Co/sub 3/O/sub 4/ systems in the reaction of methanol oxidation to formaldehyde show much higher selectivities for mixed than for pure oxides. The highest CH/sub 2/O yield was obtained for catalysts with atomic ratios V:Me = 1. Besides methanol oxidation by the conventional flow method at 310/sup 0/C on catalysts of different V;Me ratios, measurements for the three V/Me = 1 systems by the pulse method without oxygen were carried out, showing different reaction mechanisms, involving H/sub 2/ evolution. In case of the V--Ni catalyst, which is the most selective one, no H/sub 2/ was found in the reaction products. Infrared spectra taken of the 1:1 catalysts before and after 2 hr of methanol oxidation at 410/sup 0/C show different stabilities of the V = O bond, which is most constant in the V--Ni system. The resulting structure modifications are most significant in the Me--O bond region.

Participation of double bond type lattice oxygen in vapor-phase catalytic oxidation of olefins

The reactivity of double bond type lattice oxygen Mo6+O in supported molybdena catalyst has been investigated by means of ESR and ir spectroscopy. It has been shown that the double bond oxygen is similar to a free radical oxygen in nature, in contrast to σ-bond oxygen MoOMo. This character increases as a result of electron transfer from an adsorbed olefin species to a nonbonding atomic orbital of Mo6+ with some decrease in the strength of MoO bond during vapor-phase oxidation of lower olefins. The reactivity of the double bond oxygen as a radical increases inversely with the electronegativity of the molybdenum ion as modified with an oxide of a VA group element or of an alkali metal. This was supported by studies of the catalysis of the oxidation of propylene to acrolein, of butadiene to maleic anhydride and the oxidative dehydrogenation of trans-2-butene to butadiene. On the other hand, the activity for these reactions of catalyst modified with a high concentration of alkali metal oxide increases with the electronegativity of the alkali metal. This different effect due to the concentration of the alkali metal oxide is also discussed.

The mechanism of the reaction of nitrogen oxide with ammonia over Cr2O3Al2O3 and Cr2O3 catalysts: II. Isotope labeling studies☆

Abstract The reaction of NO with 15 NH 3 was studied over 9.3% Cr 2 O 3 Al 2 O 3 and Cr 2 O 3 as catalyst; relevant properties characterizing these oxides are described in Part I (preceding paper). The main isotope species in the products, nitrogen and nitrous oxide, were 15 NN and NNO when the reaction proceeds in a stationary state. A noncatalytic reaction of NO with prereduced surface to give NN was also significant, especially in the beginning. When the catalyst was preoxidized, the reaction was greatly enhanced and a large amount of 15 NNO was found. It is concluded that the reaction takes place on surface oxygens.

The mechanism of the reaction of nitrogen oxide with ammonia over Cr2O3Al2O3 and Cr2O3 catalysts: I. Characterization of the catalysts

Abstract Catalytic activities of various Cr 2 O 3 Al 2 O 3 catalysts for the reaction of nitrogen oxide (NO) with ammonia (NH 3 ) were determined. The activity, when plotted against Cr contents, showed two maxima, at 9.3% Cr and 100% Cr, respectively. X-Ray examination showed that γ-phase and α-phase solid solution occurred at low and high Cr contents, respectively. The BET area, reducibility and the amount of irreversibly adsorbed NO also showed their maxima at 9.3% Cr. Such properties explain, at least partially, the activity pattern in the low Crcontent region.