V. A. Rogov

Understanding Methane Aromatization on a Zn-Modified High-Silica Zeolite†

Methane is the principle constituent of natural gas and also the most inert of the saturated hydrocarbons. Its conversion into more commercially useful chemicals and liquid fuels represents one of the most important challenges in modern catalysis. Coaromatization of methane and light hydrocarbons (paraffins and olefins) at 700–800 K is one of the alternative methods for the conversion of methane. It has been reported recently that the conversion of methane during coaromatization with higher alkanes or alkenes (C2–C6) at 670–870 K in the presence of bifunctional catalysts (mainly, high-silica ZSM-5 or ZSM-11 zeolites, modified with gallium or zinc) may reach 20–40%. However, previous experiments in which C-labeled methane was used did not confirm the presence of the C-labeled atoms from the methane in the aromatization products. This result gave rise to scepticism as to whether methane-involved aromatization occurred at all. Herein we report that transfer of isotopically C-labeled atoms from methane into the aromatic products does occur to a high degree during the co-conversion of methane and propane on the Zn-modified high-silica zeolite BEA. We have identified the nature of the intermediates formed during the activation of methane and established how the conversion of methane into aromatic compounds occurs. Figure 1 shows the C CP/MAS NMR spectra of the products (in their adsorbed state on the zeolite catalyst) which are formed from methane and propane at 823–873 K. The spectrum of the products formed from unlabeled CH4 and C3H8 exhibits only a weak signal at d = 8.5 ppm from methane (Figure 1a). When unlabeled CH4 was replaced with CH4, the spectrum of the reaction products showed two new signals, which undoubtedly belong to hydrocarbons containing the C labels from the CH4 (Figure 1b). The carbon atoms of the C-labeled methane molecules are incorporated into both methyl groups (signal at d = 20 ppm) and aromatic rings (d = 130 ppm) of the methyl-substituted aromatic compounds (Figure 1b,c). According to GC-MS analysis of the products extracted from the zeolite, a mixture of benzene and toluene, as well as mand p-xylenes (BTX) with C enrichment is formed from CH4 and unlabeled propane at 773–823 K (Figure 2). The presence of singly (C1), doubly ( C2), and triply ( C3) labeled molecules of BTX (Figure 2b) provides proof for the incorporation of C-labeled methane into both the methyl groups and the carbon atoms of the aromatic rings of BTX. Neat propane converts on Zn/H-BEA into a mixture of aromatic products and methane at lower temperature (573– 723 K; Figure 1d). According to the H MAS NMR spectra, approximately 1.6–1.7 methane molecules are produced per reacted propane molecule. The possible overall reaction which describe the aromatization of propane can be described by Equation (1). Figure 1. C CP/MAS NMR spectra of products in the adsorbed state formed from methane and propane on zeolite Zn/H-BEA: a) from CH4 and C3H8 at 823 K for 15 min; b,c) from CH4 and C3H8 at 823 K for 15 min (b) and at 873 K for 15 min (c); d) from [1-C]C3H8 at 723 K for 15 min. Asterisks (*) in Figures 1, 3, and 4 denote the spinning side bands.

Honeycomb-supported perovskite catalysts for high-temperature processes

Pechini route [US Patent No. 3,330,697 (1967)] was used for supporting perovskite-like systems on thin-wall corundum honeycomb support to prepare catalysts for high-temperature processes of methane combustion and selective oxidation into syngas. In this preparation, the surface of corundum monoliths walls was shown to be covered by strongly adhering porous perovskite layer formed by rounded crystals. At high temperatures when pore diffusion is expected to affect catalysts performance in fast reactions, this spatial distribution of the active component could be attractive. In the kinetically controlled region of methane oxidation, samples prepared via Pechini route possess activity comparable with that of samples made via support wet impregnation with mixed nitrate solutions, when an active component is uniformly distributed across the wall thickness. Corundum-supported lanthanum manganite and ferrite are the most active in the reaction of methane combustion, while its selective oxidation into syngas effectively proceeds on supported lanthanum cobaltite and nickelates. Corundum-supported perovskites are more thermally stable as compared with those on γ-alumina support.

n-Butane conversion on sulfated zirconia: the mechanism of isomerization and 13C-label scrambling as studied by in situ 13C MAS NMR and ex situ GC-MS

Abstract Using 13 C MAS NMR, conversion of selectively 13 C-labeled n -butane on sulfated zirconia catalyst has been demonstrated to proceed initially via two parallel routes: scrambling of the selective 13 C label in the n -butane molecule and selective formation of isobutane. The combination of the results obtained by both in situ 13 C MAS NMR and ex situ GC-MS analysis provides evidence for the monomolecular mechanism of the 13 C-label scrambling, whereas isomerization into isobutane proceeds through a pure bimolecular mechanism. Further, the intermolecular mechanism of n -butane isomerization is complicated and turns into conjunct polymerization. Besides isobutane, conjunct polymerization gives also the products of butane disproportionation, propane and pentanes, as well as the stable cyclopentenyl cations; the latter may be in charge of catalyst deactivation.

Forms of oxygen in La1 − xCaxMnO3 + δ (x = 0–1) perovskites and their reactivities in oxidation reactions

The effects of substitution in the cationic sublattice and of the synthesis procedure on the reactivity of different forms of oxygen in La1 − xCaxMnO3 + δ perovskites synthesized by mechanochemical and ceramic processing was studied by temperature-programmed reduction (TPR) with hydrogen. As the calcium content of the perovskite is raised, the maxima of the TPR peaks shift to lower temperatures and the extent of reduction of the perovskite increase, implying an increase in the reactivity of the system. Conversely, raising the calcination temperature or extending the calcination time shifts the maxima of the peaks to higher temperatures and diminishes the extent of reduction of the sample. TPR data for the intermediate-composition samples can be explained in terms of the dependence of microstructure on the synthesis procedure (near-surface calcium segregation in the mechanochemically synthesized samples and the microheterogeneous structure of the ceramic samples). The reduction process Mn4+ → Mn2+ takes place in the low- and medium-temperature regions. According to the literature, the bulk reduction process Mn3+ → Mn2+ occurs at high temperatures. The activity of the system in CO oxidation is correlated with the amount of the most reactive surface oxygen, which is eliminated in hydrogen TPR runs below 250–300°C.

Synthesis and properties of nanocomposites with mixed ionic-electronic conductivity on the basis of oxide phases with perovskite and fluorite structures

Nanocomposites consisting of phases with fluorite (doped CeO2) and perovskite (LaMnO3, GdMnO3) structures are synthesized using the method of ester polymeric precursors (the Pechini method) and two sources of rare-earth elements (Ln), such as pure cerium and gadolinium salts or a commercial mixture of rare-earth carbonates containing La, Ce, Pr, Nd, and Sm cations. The genesis of the nanocomposite structure as a function of the sintering temperature is investigated using X-ray diffraction and electron microscopy. It is revealed that the genesis of the nanocomposite structure is governed, in many respects, by the fact that the decomposition of the ester polymeric precursor leads to the formation of a metastable phase, namely, a fluoritelike solid solution based on ceria with an excess concentration of the cations Ln3+ (Ln3+ = La3+, Pr3+, Nd3+, Sm3+) as compared to the equilibrium concentration. As a result, the perovskite phase (identified by X-ray diffraction analysis) is formed only after the subsequent annealing at temperatures higher than 800°C, when Ln3+ cations escape from particles of the solid solution. It is demonstrated that, at annealing temperatures of up to 1100°C, particles of both phases have nanometer sizes and are characterized by a uniform spatial distribution necessary for percolation. The nanocomposites possess a high total electrical conductivity and a high mobility of lattice oxygen. The reduction rate of the nanocomposites with hydrogen or methane is higher than the reduction rate of the individual phases. The characteristics of the nanocomposites prepared from the commercial mixture of rare-earth carbonates are better than those of the samples synthesized from the pure salts.

Modified Ceria-Zirconia Fluorite-Like Catalysts for the Combustion of Methane

Abstract For dispersed ceria-zirconia-based solid solutions prepared via the polymerized complex method and annealed at 700 °, effects of bulk doping by Ca, Mn, Co, Bi or Nb cations and surface modification by Mn and Pt on their structural features, surface/bulk oxygen reactivity and catalytic activity in methane combustion are considered. With up to 20 mol% doping, a structural type of homogeneous solid solutions of anion-deficient fluorite with disordered anion vacancies is formed. Doping by transition metal cations or Pt increases the mobility and reactivity of the surface/bulk oxygen. A broad variation in specific rates of methane combustion for the studied systems was observed, suggesting structural sensitivity of this reaction. In general, there is no universal relationship between the oxygen mobility, the reactivity and the catalytic activity in methane combustion, which is explained by the factor of specific methane activation on surface active sites. For the Pt-promoted samples, Pt efficiency in methane activation depends on the Pt-support interaction, and the most favorable ones being mixed Pt/MnOx and Pt/NbOx clusters on the surface of the supports that exhibit high lattice oxygen mobilities.

Nanocomposite Catalysts for Steam Reforming of Methane and Biofuels: Design and Performance

Vladislav Sadykov, Natalia Mezentseva, Galina Alikina, Rimma Bunina, Vladimir Pelipenko, Anton Lukashevich, Zakhar Vostrikov, Vladimir Rogov, Tamara Krieger, Arkady Ishchenko, Vladimir Zaikovsky, Lyudmila Bobrova, Julian Ross, Oleg Smorygo, Alevtina Smirnova, Bert Rietveld and Frans van Berkel, 1Boreskov Institute of Catalysis, Novosibirsk State University, 2University of Limerick, 3Powder Metallurgy Institute, 4Eastern Connecticut State University, 5Energy Research Center of the Netherlands, 1Russia 2Ireland 3Belarus 4USA 5Netherlands

Effect of chromium content on the properties of a microspherical alumina-chromium catalyst for isobutane dehydrogenation prepared with the use of a centrifugal thermal activation product of gibbsite

The states of chromium in both promoted and unpromoted alumina-chromium catalysts with various chromium contents prepared with the use of a centrifugal thermal activation product of gibbsite were studied in detail. The presence of five chromium species was found in the catalysts of this type: two Cr6+ and three Cr3+ species. The concentration of each particular chromium species depends on the total chromium content of the catalyst. Based on the experimental data, we hypothesized that highly disperse Cr3+ particles, the formation of which was completed at a chromium content of ∼7–9 wt %, are responsible for the catalytic activity of alumina-chromium samples in the reaction of isobutane dehydrogenation.

Interaction of platinum and molybdophosphoric heteropoly acid under conditions of catalyst preparation for benzene oxidation to phenol with an O2-H2 gas mixture

The transformations of platinum and a heteropoly acid (HPA) in binary systems prepared from H2PtCl6 or H2PtCl4 and H3PMo12O40 were studied using IR and UV-VIS spectroscopy, elemental analysis, XPS, EXAFS, TPR, and HREM. The calcination of platinum chloride with the HPA to 450°C resulted in the formation of a platinum salt of the HPA along with decomposition products (mixture I). The reduction of calcined samples containing Pt: HPA = 1: 1 with hydrogen at 300°C (mixture II) followed by exposure to air resulted in the regeneration of the HPA structure. The resulting solid samples of Pt1−n0PtnIIClmOxHy) (H3+pPMo12−pVIMopVO40) (III) contained platinum and molybdenum in both oxidized and reduced states. The following association species were isolated from mixtures I and II by dissolving in water: [PtnIIPMo12O40] (Is) (n = 0.3−0.8) and [Ptn0PMo12red O40] (IIs) (n ≈ 1). Under exposure to air, the solutions of Is were stable (pH ∼2), whereas Ptmet was released from IIs. After the drying of Is, the solid association species (PtnIIClmOxHy). (H3PMo12O40), where n = 0.3−0.8, m = 0.2−1, and x = 3−0, (Isolid) were obtained. The Isolid/SiO2 supported samples were prepared by impregnating SiO2 with a solution of Is and drying at 100°C. Platinum metal particles of size ∼20 Å and a mixed-valence association species of platinum with the HPA were observed after the reduction of Isolid/SiO2 with hydrogen at 100–250°C. These samples were active in the gas-phase oxidation of benzene to phenol at 180°C with the use of an O2-H2-N2 mixture.

Monolith composite catalysts based on ceramometals for partial oxidation of hydrocarbons to synthesis gas

Microchannel ceramometal monoliths of a high thermal and mechanical stability have been synthesized via hydrothermal treatment of powdered mixtures containing aluminum and additives (oxides or/and Ni-Cr(Co) alloy) followed by calcination. The phase composition and textural properties of monoliths depend on the additive nature affecting the aluminum reactivity toward oxidation and the interaction of remaining Al0 as well. Microchannel ceramometal monoliths and catalysts based on them ensure a high performance and stability in CPO of hydrocarbons at short contact times.