Manganese-containing Catalysts Research Articles

OXIDATION OF n-OCTANE IN THE PRESENCE OF MANGANESE-POLYMER CATALYST AND SELECTION OF THE APPROPRIATE KINETIC MODEL

Oxidation of n-octane was carried out in a stationary reactor using immobilized manganese nanoparticles in various mass percentages (5 and 10 wt.%) on poly(4-vinylpyridine). It was found that manganese-containing catalysts immobilized in polymer are more active in the oxidation of n-octane than similar homogeneous catalysts. A kinetic model and an equation for calculating the oxidation of n-octane in the presence of a manganese-polymer catalyst were chosen for the calculation. Taking into account the kinetic model, the velocity constant of the specified process is calculated. A high conversion in the oxidation reaction of n-octane with molecular oxygen was obtained

Oxidation of Reactive Alcohols with Hydrogen Peroxide Catalyzed by Manganese Complexes

Two manganese-containing catalysts have been employed in the oxidation with hydrogen peroxide of two reactive alcohols (1-phenylethanol and glycerol): soluble catalyst [LMn(μ-O)3MnL](PF6)2 (1a) and heterogenized catalyst [LMn(μ-O)3MnL]2[SiW12O40] (1b) (L is 1,4,7-trimethyl-1,4,7-triazacyclononane, TMTACN). Oxidation of 1-phenylethanol catalyzed by 1a in acetonitrile solution proceeds at room temperature in the presence of a small amount of oxalic acid; the turnover number attains 15,000 after 3 h. It has been proposed on the basis of the kinetic study that an oxidizing species is a manganyl species containing fragment “Mn=O” rather that hydroxyl radical. This species reacts competitively with the alcohol, acetonitrile and hydrogen peroxide. In the case of 1b dependences of the initial rates of acetophenone accumulation on concentration of the alcohol and amount of 1b have plateau. Both homogeneous and heterogeneous catalysts are efficient in the oxidation of glycerol to produce dihydroxyacetone (DHA) as the main product. The oxidation catalyzed by 1a is one of the first examples of the glycerol oxidation by a catalytic homogeneous system. The yield of valuable products attained 45%. The oxidation of DHA in the absence of glycerol afforded mainly glycolic acid in yield 60% based on the starting DHA. The oxidation on 1b represents the first example of the glycerol transformation catalyzed by a heterogenized metal complex. Under certain conditions yields of products of deeper oxidation (glyceric, glycolic and hydroxypyruvic acids) are somewhat higher than the yield of dihydroxyacetone. Special experiments demonstrated that no leaching of active species occurs from catalyst 1b to the solution and that this catalyst can be re-used at least four times without substantial loss of activity. Manganese-containing complexes are very efficient catalysts in the oxidation of reactive alcohols (1-phenylethanol and glycerol) with H2O2: soluble [LMn(O)3MnL](PF6)2 and heterogenized [LMn(O)3MnL]2[SiW12O40] (L is 1,4,7-trimethyl-1,4,7-triazacyclononane).

Hydrogenation of carbon oxides over promoted Fe-Mn catalysts prepared by the microemulsion methodology

The microemulsion technology was used in order to improve the preparation of iron-based catalysts for the CO and CO 2 hydrogenation reactions. This technique led to a sample with a higher surface area when compared with a similar solid prepared by conventional precipitation. As a consequence, a higher catalytic activity was obtained. The hydrogenation of carbon oxides was also performed over promoted iron-manganese catalysts. The preparation of these samples by the microemulsion methodology yielded to homogeneous mixed oxides. Manganese-containing catalysts presented a higher activity towards the formation of hydrocarbons, at the same conversion levels, than the iron counterparts. These catalysts were promoted with copper, sodium and potassium. Carbon dioxide conversion was favoured by alkaline addition, especially by potassium, due to the promotion of the water-gas shift reaction. Furthermore, alkaline promotion enhanced selectivity towards long-chain products either in CO 2 and CO hydrogenation processes. Addition of copper caused a more facile reducibility of the solids, resulting in a significant increment in both carbon monoxide and carbon dioxide conversion without modifying products selectivity.

Oxidation of carbonaceous fragments by carbon dioxide over manganese-containing catalysts

Oxidation of surface carbonaceous fragments by carbon dioxide deposited over manganese-containing catalysts during conversion of n- and isobutane have been studied. It has been established that the reactivity of these fragments, deposited under steady-state conditions, depends on the nature of hydrocarbon interacting with carbon dioxide. Modification of manganese by vanadium increases the activity of coking fragments.

XPS characterization of the reduction and synthesis behaviour of Co/Mn oxide catalysts for Fischer-Tropsch synthesis

The surface composition of Co/Mn oxide catalysts after calcination, reduction and during CO hydrogenation experiments were investigated by XPS. The bulk changes during reduction were studied by Temperature Programmed Reduction (TPR). The calcined catalysts showed Co/Mn oxides of different compositions in their surfaces. The Co surface concentration, of the catalysts with high Co content, decreased after calcination compared to the bulk composition, but after reduction the bulk concentration was almost reached again. The catalysts with low Co content showed no decrease in the surface concentrations after calcination. Significant differences in surface concentrations for the catalysts Co20 and Co5 were observed by analysing the Co 2p and 3p levels, respectively; these can be explained by an internal reduction model. After reduction the sample Co100 was completely reduced to metallic cobalt. In the manganese-containing catalysts, after in-situ reduction, Co3+ and Co2+ were found; all manganese was reduced to Mn2+. A comparison of the results of the in-situ reduction and the TPR profiles led to the development of a so called “internal reduction” model. This model assumes migration of the Co2+ ions to the reduction front in inner layers of the catalysts, where they will be reduced to metallic cobalt; this is enriched in the bulk and cannot be found at the surface. The manganese matrix stabilizes the surface oxide layer so that all catalysts exhibited Co/Mn spinels in the surface. Synthesis experiments in the reaction chamber of the XPS apparatus did not lead to changes of the catalyst surfaces as a function of the reaction pressure, synthesis time or synthesis gas composition. The differences in the synthesis behaviour observed for the catalysts must be due to other effects, (i.e. a change in adsorption of hydrogen connected with a change in hydrogenation activity or the different cobalt concentrations).

Dehydrogenation of isobutane with carbon dioxide on manganese-containing catalysts

Dehydrogenation of isobutane with carbon dioxide on manganese-containing catalysts