F. Trifirò

Relationship between structure and activity of antimony mixed oxides in 1-butene oxidation☆

Abstract The mixed oxides of antimony with Sn, Fe, Co, Ni, Mn, Zn, U, Ce, and Cd were prepared and characterized by means of spectroscopic techniques. Fe, Co, Ni, Mn, Zn, Cd and, partially, U were found to form well-defined compounds with Sb oxide. The mixed oxides were tested as catalysts in the 1-butene oxidation. They showed a very different activity but about the same selectivity to butadiene suggesting that the active sites are the same for all compounds. It is proposed that the oxidative dehydrogenation properties of the catalysts are due to two “gem” Sb 5+ ue5fbO groups and that the role of the second metal is to adsorb the gaseous oxygen to reoxidize the reduced antimony ions. This reoxidation mechanism can be presented only by mixed oxides showing a particular structure.

Oxidation of butenes to maleic anhydride on MnMoO4 based catalyst

The oxidation of n-butene-1 was carried out in a stirred tank reactor and in a pulse reactor using MnMoO4 as a catalyst. This catalyst exhibits a fairly good selectivity to maleic anhydride. MnMoO4 shows a polyfunctional nature; it is possible to distinguish the properties of isomerization, dehydrogenation, oxidation with oxygen insertion, and complete oxidation by varying parameters such as temperature, oxygen concentration and contact time. n nThe compositions of the products in the oxidation of n-butene-1 carried out in a pulse reactor are completely different in the presence and in the absence of oxygen, respectively. In the absence of oxygen, MnMoO4 is a very selective catalyst in the dehydrogenation of n-butene-1 to butadiene. In the presence of oxygen, CO and CO2 are the main products together with small amounts of maleic anhydride. n nThe selectivity of MnMoO4 to butadiene formation has been attributed to the presence of Moue5fbO bonds which are responsible for dehydrogenation reactions. n nA monocenter oxidation mechanism, accounting for the formation of CO, CO2, and maleic anhydride, has been proposed in which the gaseous oxygen is considered to be adsorbed on the same center of the hydrocarbon.

The effect of sulfiding on the structure of a cobalt-molybdenum-alumina hydrodesulfurization catalyst

Abstract Ultraviolet, visible, and infrared spectra and magnetic properties of a fresh and sulfided cobalt-molybdenum-alumina hydrodesulfurization catalyst are reported. The fresh and sulfided catalyst both contain oxomolybdenum(VI) species. Properties of the fresh catalyst are consistent with the presence of linked [MoO4] tetrahedra and [CoO6] octahedra. Spectroscopic changes on sulfiding indicate that sulfide adds to [MoO4] and that no more than one or two oxide ions, probably bridging between molybdenum and cobalt, are replaced by sulfide. There is no evidence for discrete sulfides, e.g., MoS2 and Co9S8, in the sulfided catalyst. Aerial reoxidation of the sulfided catalyst gives a product identical with the fresh catalyst.

Chemical, structural and catalytic modifications of pure and doped iron(III) molybdate☆

In this paper various factors influencing the precipitation of Fe-Mo-O systems have been investigated. Stoichiometric Fe(III) molybdate, Mo-rich Fe(III) molybdate promoted with bismuth and an Fe-Mo-O system with excess of Mo obtained by precipitation at room temperature were studied and the changes in the catalyst properties with respect to the temperature of calcination were examined. The following techniques were employed: chemical analysis, X-ray diffraction, scanning electron microscopy, surface area measurements, differential thermal analysis and an abrasion resistance test. The oxidation of methanol to formaldehyde was also investigated using a fixed bed microreactor. Stoichiometric Fe(III) molybdate was confirmed as the active component. Excess of Mo had no influence on the specific activity of the investigated catalysts, but it was found to be responsible for the high coherence of the sample. The addition of Bi to this system improved mechanical properties by preserving the coherence at temperatures above 450 °C where a collapse of the structure was obtained in the absence of bismuth. Bi-promoted catalyst showed the formation of Bi molybdate 23 phase. Stoichiometric Fe(III) molybdate gave the highest conversions of methanol after prolonged treatment at high temperature.

The nature of active components in Fe2O3MoO3 System: IV. Bi-doped Fe(III)-molybdate: Characterization and activity

Abstract An investigation on the Feue5f8Moue5f8Bi oxides system in the Fe-rich region was carried out by adding various amounts of Bi to Fe(III)-molybdate. The principal catalytic modifications which already occurred at very low Bi levels (0.5% Bi 2 O 3 by wt), were: (i) strong decreasing in isomerizing power at low temperature; (ii) strong increase in the selectivity to and yield of butadiene; iii) decrease in 1-butene conversion; (iv) strong decrease in CO formation. The characterization of these catalysts with Mossbauer, ir, ATR, diffuse reflectance, X-rays seems to indicate that no new phase is present at these concentrations of Bi. Only at much higher levels (3.5% Bi 2 O 3 by wt) where the catalytic activity is already constant, does a new phase form in increasing amounts. This new phase is Bi-molybdate 2:1 modified probably by Fe 3+ . It is proposed that the active and selective phase of these ternary systems is Fe(III)-molybdate with Bi as dopant in interstitial or substitutional sites, which might give rise to Bi(V)ue5f8Mo(VI) continuous groups. Other particular features of these catalysts seem to be: 1. i. A higher activity than that of the typical values of Bi-molybdates. 2. ii. An activity constant also in very reducing conditions where Bi-molybdates become inactive. 3. iii. A strong influence of oxygen partial pressure: at very low oxygen level the selectivity increases considerably.

Nature of the active component in a Fe2O3MoO3 catalyst: I. Study on the catalyst reduction and oxidation

The reduction of a catalyst based on Fe2O3ue5f8MoO3 with N2H2 and N2NH3 as reducing mixtures has been studied by a thermogravimetric method. At the same time the products formed have been analyzed by IR and X-ray powder-diffraction techniques. The initial catalyst contains ferric molybdate, which is the first that is reduced (to ferrous molybdate), and molybdic anhydride, which is reduced to MoO2. Thermal treatments of the catalyst at a temperature higher than 600 °C in air flow can lead to irreversible phenomena, such as sintering. Oxidations, following a reduction, must be carried out at T > 450 °C to ensure a total reoxidation of ferrous molybdate, and with an oxidizing mixture containing a low percentage of O2 (e.g., 5%) in nitrogen, to avoid catalyst decomposition due to local overheating. The reoxidation rate is higher than the rate of reduction. n nThe behavior of ferric molybdate during reduction suggests that the combined iron oxide of the catalyst plays a determining role in the oxidation processes of organic molecules carried out in the presence of catalysts based on Fe2O3ue5f8MoO3.

Oxidation of 1-butene to maleic anhydride and carbon oxides on molybdate-based catalysts

Abstract The catalytic activity of MnMoO4, β-FeMoO4, a-CoMoO4 and CdMoO4 in the oxidation of butenes has been investigated in a wide range of experimental conditions both in a stirred-tank gradientless reactor and in a pulse microreactor. Some of the catalysts investigated showed good selectivity to maleic anhydride; the other products were mainly CO and CO2. The oxidation runs carried out in a pulse microreactor showed that the catalytic behaviour of the catalysts investigated strongly depended on whether oxygen was or was not present in the gas phase. At a low oxygen percentage, butadiene is the main product for all the catalysts. Therefore the partial pressure of oxygen is a key to modify the selectivity. In the absence of oxygen, CdMoO4 was not active, whereas CO and CO2 were the main products. It was therefore proposed that gaseous oxygen must take a direct part in the formation of oxidised products. As the second metal (Mn(II), Fe(II), Co(II), Cd(II)) of the catalysts investigated is unable to oxidise the reduced form of molybdenum, a direct interaction of reduced molybdenum with oxygen must be hypothesised. In addition, the tetrahedral coordination of Mo (a common structural feature of all the catalysts investigated) can play an important role in giving a reactive form of adsorbed molecular oxygen which is responsible for the formation of maleic anhydride, CO and CO2. Bismuth molybdates, which present an octahedral coordination of molybdenum and a second metal (Bi(III)) that can oxidise Mo(V), show a catalytic behaviour quite different from that of the investigated catalysts.

Liquid phase epoxidation of cyclohexene by tert-butyl hydroperoxide on a Mo-based catalyst

Abstract A kinetic and spectroscopic study has been carried out on the epoxidation of cyclohexene by tert-butyl-hydroperoxide in the presence of MoO2(oxine)2. This catalyst gives quantitative yields of cyclohexene oxide at 80 °C, and is quite stable in t-BuO2H itself. The kinetic investigation of the reaction leads to the following rate law rate = k[t-BuO2H] [cyclohexene] [MoO2(oxine)2]. On the basis of a spectroscopic study, the presence of a catalyst—hydroperoxide reversible complex as the active species in the epoxidation is advanced. The spectroscopic study also indicates that the formation of this active species comes from the opening of a Mo—chelate bond. An oxidative degradation product of the catalyst, which was less soluble and less active than MoO2(oxine)2 and had been obtained by prolonged heating, was isolated and tested as an epoxidation catalyst. The presence of molybdenum—peroxy groups in this compound is indicated by permanganate titration and its infrared spectrum. The influence of the nature of the ligand on activity and selectivity were also investigated. Very strong ligands lead to inactive catalysts, while very labile ligands lead to catalysts with low selectivity.

Preparation and activity of bismuth tungstates in oxidation and ammoxidation of olefins

Investigations have been carried out on the catalytic behavior of Bi tungstates at the different ratios BiW: 23, 11, 1.81, 21, 61 in the oxidation and ammoxidation of propylene and 1-butene. n nBi2WO6 is the only active and selective compound. WO3 is active but the main products of oxidation of both 1-butene and propylene are CO and CO2. At low temperatures WO3 shows a high isomerizing power towards 1-butene while Bi2WO6 is inactive. n nAmmonia present in the reagent mixture modifies the catalytic behavior of WO3, keeping the distribution of products of ammoxidation of olefines nearer to the distribution given by Bi2WO6. n nThe surface acidic sites and the formation of CO and CO2 were attributed to the pressence of bonds of covalent type. These types of bonds are not present in Bi2WO6 which has a rather polar structure.

Study of the interaction between CoMoO4 and 341-1341-1341-1-Al2O3 by Raman spectroscopy

Raman spectroscopy has been used to study the solid state reaction of MoO3 and CoMoO4 with γ-Al2O3 and the compounds formed upon the adsorption of Co(II) an Mo (VI) on γ-Al2O3. Results show that γ-Al2O3 endows CoMoO4 and MoO3 with a high reactivity, giving Raman-inactive compounds.AbstractСпектроскопия комбинационного рассеивания была использованиа для изучения твердофазной реакции MoO3 и CoMoO4 с γ-Al2O3 и соединениями, образующимися за счет адсорбции Co(II) и Mo(VI) на γ-Al2O6. Результаты указывают на то, что γ-Al2O3 придает высокую реактивность CoMoO4 и MoO3, давая при этом Раман-неактивные соединения.