Miguel A. Bañares

Supported metal oxide and other catalysts for ethane conversion: a review

Abstract The conversion of ethane to ethylene, aromatics, and oxygenates and combustion on supported metal oxide catalysts are discussed. Updated information on the structure and performance of supported oxide catalysts in reactions is offered, underlining the importance of in situ characterization under reaction conditions in order to fully understand the structure and reactivity of supported metal oxide catalysts at molecular level. Support effects, the stability of supported oxide catalysts, molecular structures and oxidation states under reaction conditions, and the effects of metal oxide and additive loading are discussed. The role of terminal and bridging oxygen species is also addressed. Emphasis is placed on the relevance of surface species vs. bulk structure and characterization. A section devoted to new trends in the selective oxidation of ethane to afford high yields and lower working temperatures is included.

Dynamic behavior of supported vanadia catalysts in the selective oxidation of ethane in situ Raman, UV-Vis DRS and reactivity studies

Abstract The coordination and oxidation states of surface vanadia species on different oxide supports were studied by in situ UV–Vis DRS and in situ Raman spectroscopy. Surface vanadia species remain essentially oxidized during the steady-state ethane oxidation reaction. Polymeric surface vanadia species are more reducible than isolated ones, but this has only a minor effect on the ethane oxidation reactions. It appears that only one surface V site is involved in the rate-determining step for ethane oxidation. The reducibility of supported vanadium oxide species corresponds with the TOF values, but not with the average oxidation state under steady-state reaction. Ceria- and niobia-supported vanadia catalysts do not follow this trend due to solid-state reaction between the surface vanadia species and the oxide support that decreases the number of exposed vanadia sites. This solid-state reaction does not appear to affect the nature of the active site, which is associated with the V–O–Support bond rather than with the terminal VO bond.

Activation of methane by oxygen and nitrogen oxides

III. A NEW APPROACH TO METHANE ACTIVATION USING NITROGEN OXIDES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 A. Experimental Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 B. Modeling of NOx Gas Phase Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 36 IV. NETWORK AND MECHANISM OF CH4–O2-NOx OXIDATIONS . . . . 42 A. Kinetic Modeling of CH4–O2 Gas Phase Reaction . . . . . . . . . . . . . . . . 42 B. Kinetic Modeling of CH4–O2–NOx Gas Phase Reaction . . . . . . . . . . 44

The nature of cobalt species in Co and PtCoZSM5 used for the SCR of NOx with CH4

Abstract A thorough characterization of CoZSM5 and PtCoZSM5 before and after catalytic use was carried out using a battery of techniques. The bimetallic solid was more selective for N2 production. The TPR profiles showed significant differences. No solid, either fresh or used, exhibited any of the characteristic cobalt oxide X-ray reflections. The XPS data provided information concerning cobalt dispersion. The Raman spectroscopy clearly indicated that Co3O4 species were present only in the monometallic zeolites while a form of highly dispersed CoxOy moieties became dominant in the PtCoZSM5. The diffuse reflectance spectroscopy showed that Co2+ species in the monometallic solids were preferentially located at the main channels while in PtCoZSM5 these cations moved to higher coordination lattice sites. Through the combination of these tools, a much better understanding of the synergetic effect of Pt incorporated to CoZSM5 has been achieved. In view of these findings, related work previously published is revisited.

New Reaction: Conversion of Glycerol into Acrylonitrile

Glycerol is a major by-product from methanolysis during theproduction of biodiesel. Thus, it is an increasingly importantmolecule in the context of renewable biomass resources toprovide energy and chemical intermediates. However, the de-velopment of selective glycerol-based catalytic processes is amajor challenge as a result of their low selectivity.

Raman spectroscopy during catalytic operations with on-line activity measurement (operando spectroscopy): a method for understanding the active centres of cations supported on porous materials

Raman spectroscopy with on-line activity measurement (operando Raman spectroscopy) is used to reach a molecular understanding of the structure–activity relationship of supported vanadium oxide catalysts during alkane selective oxidation and ammoxidation reactions. The advantage of the operando methodology is that the molecular structures are recorded during true catalytic operation, thus affording information about the structures relevant to the catalytic performance. It is shown that efficient propane ammoxidation requires both surface mono-oxo vanadium oxide species and SbVO4 phases.

Role of potassium on the structure and activity of alumina-supported vanadium oxide catalysts for propane oxidative dehydrogenation

Abstract The influence of potassium on the structure and properties of alumina-supported vanadium oxide catalysts has been studied by in situ Raman spectroscopy, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), a probe reaction of acid/base–redox sites (methanol chemisorption) and tested in oxidative dehydrogenation (ODH) of propane. Potassium coordinates to surface vanadium oxide species altering its structure but does not form bulk compounds, possibly because the total V+K coverage does not reach the monolayer coverage on alumina. The interaction of K with V weakens the terminal VO bond. K-doped alumina (KAl)-supported vanadia catalysts show lower acidity, a decrease of reducibility and a decrease of propane conversion values. These trends do not correspond with the changes in the terminal VO bond energy. Thus, it appears that the terminal VO bond of surface vanadium oxide species is not the active site for propane ODH, oxidation of methanol to formaldehyde and for the reduction of surface vanadium oxide species by hydrogen. Potassium also changes the acid–base characteristic of the system and decreases the acidic character of surface vanadia. This shift in the acid–base character to a more basic system must also account for the better selectivity in propane ODH due to a variation in the interaction between the intermediates and the surface.

Partial oxidation of methane to formaldehyde on silica-supported transition metal oxide catalysts

Abstract This work describes the partial oxidation of methane on high surface area silica-supported redox oxide catalysts (MO x /SiO 2 ; M = V, Mo, W and Re). Formaldehyde, C 2 H n , and CO 2 are primary products obtained in this reaction, while CO originates from further oxidation of formaldehyde and hydrocarbons. Supported vanadium oxide was found to be the most reactive due to its higher reducibility, thus providing additional sites for oxygen activation. Rhenium oxide exhibited high specific activity and selectivity, however it deactivates from sublimation of metal oxide under on-stream operation at high temperatures, typically above 773 K. Results suggest a reaction scheme where oxygenates and oxygen-free intermediates are present yielding HCHO, CO x and C 2 H n hydrocarbons.

Effect of alkali metal cations on the structure of Mo(VI)SiO2 catalysts and its relevance to the selective oxidation of methane and methanol

The effect of alkali metal additives on both the structure of MoO[sub 3]/SiO[sub 2] and its behavior as a catalyst for selective oxidation of methane and methanol to formaldehyde was investigated. The structure of the silica-supported molybdenum oxide catalysts was determined by in situ Raman spectroscopy and the amount of reducible oxygen in the catalyst was determined by temperature-programmed reduction. In the absence of alkali metal, only an isolated surface molybdenum oxide species was present on the silica support. Addition of alkali metals decreased the number of isolated surface molybdenum oxide species and formed new alkali-molybdate compounds due to the weak interaction of the Mo species with the silica surface. The oxygen associated with the alkali-molybdate compounds are generally not available for oxidation reactions. Consequently, the addition of alkali metal decreased the catalytic activity for the oxidation of both methane and methanol. The activity for methane oxidation was found to correlate with the number of remaining isolated Mo species and the activity for methanol oxidation was found to correlate with the amount of reducible oxygen present in the catalyst. 32 refs., 8 figs., 1 tab.

Bulk structure and catalytic properties of mixed Mo-V-Sb-Nb oxides for selective propane oxidation to acrylic acid

The bulk mixed Mo–V–Sb–Nb–O catalysts, which are a candidate catalytic system for the selective oxidation of propane to acrylic acid, were investigated to elucidate the bulk structure and catalytic behavior of these complex materials. These mixed oxides were prepared via a redox reaction between V 5+ and Sb 3+ in the presence of Mo 6+ and Nb 5+ and characterized by potentiometric titrations, XRD, Raman spectroscopy, electron microscopy (TEM), and bulk elemental analysis. A potentiometric titration method was used to determine concentrations of metal cations in various oxidation states. XRD and Raman spectroscopy identified Mo 6V9O40 ,M oO 3 ,S bVO 4 ,a nd a Nb-stabilized defect phase of a V-rich molybdate as the major phases present. Electron microscopy illustrated the heterogeneity of the bulk oxide phases present in the model Mo–V–Sb–Nb–O system on the submicron scale. MoO3 comprised the bulk of this mixed metal oxide system, while the surface region of these model catalysts contained mixed Mo–V–Sb–Nb oxides. The rutile SbVO4 phase was inefficient in propane oxidation to acrylic acid, while mixed Mo–V–Nb oxides were capable of producing acrylic acid at ∼ 20 mol% yield.  2003 Elsevier Science (USA). All rights reserved.