Showing papers on "Partial oxidation published in 1996"

New catalytic routes for syngas and hydrogen production

Abstract: This review summarizes different catalytic options for the production of syngas and hydrogen starting from simple hydrogen-containing molecules. Particular attention is given to new direct catalytic alternatives in natural gas conversion. Improvements in syngas technology are discussed, including partial oxidation, autothermal reforming, combined reforming and carbon dioxide reforming, and the energy efficiencies of direct and indirect methane conversion are compared. Processes, issues and practical difficulties are discussed with academic and applied efforts presented in parallel. It is emphasized that most of the ongoing research related to the direct processes is at the exploratory stage while technology utilizing indirect approach has advanced to semi-works and initial commercialization plants. The new emerging processes based on partial oxidation features are unique for syngas generation. Further enhancement of such processes plus improvements in other second generation technologies and advances in direct processes are anticipated to provide additional, new attractive paths to the chemical conversion of natural gas. Similarly, on board generation of hydrogen-rich gaseous fuels either for spark ignition engines or for coupled-fuel-cells electric engines is discussed within the scope of both partial oxidation and catalytic decomposition of methanol. A concept based in the thermochemical water splitting, which provides a renewable portable fuel from water in the form of H2, is also presented. Above all, as the chemistry involved in most of these catalytic alternatives takes place under extreme conditions, highly stable catalysts and engineering concepts are being developed.

Key Aspects of Catalyst Design for the Selective Oxidation of Paraffins

Abstract: This review examines some aspects in the development of heterogenous catalysts for the oxyfunctionalization of light paraffins. Particular attention is devoted to the raction of paraffin oxydehydrogenation to olefins and of n-butane oxidation to maleic anhydride. Most catalyst compositions are based on vanadium oxide as the main component, and the peculiar properties of this element with respect to the catalytic performance are discussed. These properties are also examined in ligh of the stability of the product of partial oxidation towards consecutive unselective oxidation reactions, and with respect to the mechanism of paraffin activation.

On the partial oxidation of propane and propylene on mixed metal oxide catalysts

Abstract: The present review analyses the literature data reported on the partial oxidation of propane to organic compounds (acrolein, acrylic acid and acrylonitrile) over mixed metal oxides, mainly magnesium vanadates, vanadia bismuth molybdates and vanadia antimony. The data were compared to those reported on the partial oxidation of propylene over bismuth molybdate and antimony—tin multicomponent oxides and over cuprous simple oxide. For both reactions, we analyzed the involved reaction mechanisms, intermediate species, active phases and active sites. The role of water produced during the reaction and that of the Bronsted acid sites were shown to be important in the determination of the selectivity of the expected products. The main conclusion of our study is that a good mix of acid-base and redox properties of the oxide surface should permit a controlled orientation of the reaction towards selective products.

Microsecond Catalytic Partial Oxidation of Alkanes

Abstract: A single layer of woven platinum-10 percent rhodium gauze was used as a catalyst for the partial oxidation of ethane, propane, n -butane, and isobutane. This configuration produced oxygen conversions of equal to or greater than 90 percent and had high selectivity to olefins and oxygenated hydrocarbons at contact times as short as 10 microseconds at atmospheric pressure. This reactor operates by rapid chemical heating (∼5 microseconds) followed by rapid quenching to avoid homogeneous reactions, which decompose products. Mixing of the cold unreacted gas passing between the gauze wires with the hot product gas results in rapid quenching (∼200 microseconds) of the products from ∼800° to ∼400°C. The rapid quenching avoids successive homogeneous decomposition reactions of unstable products such as olefins and aldehydes but still allows homogeneous chain reactions that produce these species. The strong variations in products with the feed alkane showed that the reaction pathways operating at ∼10−5 seconds are quite different than those operating at longer residence times. These results could have a significant effect on the direct conversion of alkanes to oxygenates such as formaldehyde.

Catalytic wet oxidation of p-coumaric acid: Partial oxidation intermediates, reaction pathways and catalyst leaching

Abstract: The catalytic oxidation of p -coumaric acid, a compound representative of the polyphenolic fraction typically found in olive processing and wine-distillery wastewaters, has been investigated using various homogeneous and heterogeneous catalysts. Experiments have been performed with homogeneous Fe 2+ , Cu 2+ , Zn 2+ and Co 2+ ions at pH = 1, and with metal oxide catalysts in suspension at pH 3.5, 7 and 12. Additional uncatalyzed experiments have been performed and the results are compared to those of the catalyzed runs. The temperature was 403 K and the oxygen partial pressure was 2.8 MPa in all runs. The distribution of the reaction intermediates was determined, using HPLC and GCMS as the main analytical techniques, and reaction pathways are speculated. It was found that the use of catalysts could increase the rate of destruction of p -coumaric acid compared to the uncatalyzed reaction, while the distribution of the intermediate compounds was strongly dependent on the pH of the solution. A CuO·ZnO Al 2 O 3 heterogeneous catalyst was found to be effective for the oxidation of p -coumaric acid although leaching of dissolved metals to the solution was found to occur. The stability of the heterogeneous catalysts was investigated by measuring the extent of metal leaching into the solution. The results are discussed with respect to the impact of various conditions (catalyst, pH) on the oxidation of p -coumaric acid and compared to those of the uncatalyzed reaction, studied in previous work.

Partial oxidation of methane to synthesis gas via the direct reaction scheme over Ru/TiO2 catalyst

Abstract: The partial oxidation of methane to synthesis gas has been investigated over various supported metal catalysts. The effects of operational variables on mass and heat transport resistances were investigated for defining the kinetic regime. It is observed that, in the absence of significant mass and heat transfer resistances, high selectivity (up to 65%) to synthesis gas is obtained over Ru/TiO2 catalysts in the low methane conversion range (\(X_{O_2 }< 100\% \)) whereas only negligibly small selectivity to synthesis gas is observed over all other catalysts investigated under similar conditions. This indicates that the Ru/TiO2 catalyst possesses unique properties, offering high selectivity to synthesis gas formation via the direct reaction scheme, whereas the other catalysts promote the sequence of total oxidation of methane to CO2 and H2O, followed by reforming reactions to synthesis gas. An increase of selectivity to synthesis gas, in the presence of oxygen, is achieved over the Ru/TiO2 catalyst by multi-feeding oxygen, which is attributed to suppression of deep oxidation of H2 and CO.

On the functional significance of substrate accessibility in the photosynthetic water oxidation mechanism

Abstract: Recent evidence suggests that after selective perturbation of the protein structure in photosystem II, hydrogen peroxide is formed at the water oxidation catalytic site instead of molecular oxygen. In this communication, we review the interpretation of these observations and elaborate on the hypothesis that an essential factor in determining the end-product of photosynthetic water oxidation is one of substrate accessibility. It is argued that normally the access of water to the catalytic site is controlled by a hydrophobic domain in the surrounding protein matrix and that the production of O 2 is optimized by an ordered binding of the two substrate water molecules. It is proposed that upon perturbation of the hydrophobic domain (for example, through the removal of various extrinsic proteins) the catalytic site becomes exposed to excess water from the external solvent phase. As a consequence, additional water binds at the catalytic site during intermediate oxidation steps and undergoes a partial oxidation reaction to form hydrogen peroxide. The importance of water accessibility to the structure/function relationships of photosystem II is discussed, particularly with respect to photoinhibitory damage through the formation of hydrogen peroxide.

Primary reaction steps and active surface sites in the rhodium-catalyzed partial oxidation of methane to CO and H2

Abstract: The nature of surface sites responsible for methane activation and COx formation on Rh catalysts for the partial oxidation of methane to syngas was investigated. The interaction of H4 with Rh-black after oxidative and reductive pretreatments was studied applying (a) pulse experiments at reduced total pressure (10−4 Pa) and 1013 K in the temporal-analysis-of-product (TAP) reactor and (b) in situ DRIFTS at 973 K. The saturation of the metal surface sites with oxygen was found to inhibit methane dissociation. Direct methane oxidation to CO2 on the oxidized surface sites proposed earlier was excluded. Methane is first dissociated on reduced surface sites; the carbon species formed, then, react with surface oxygen to CO2. Rh sites responsible for methane activation are neither related to the formation of the Rh2O3 nor Rh0. Probably the partially oxidized species (Rh+) or highly dispersed Rh3+ entities act as active surface centers for the dissociation of methane. For supported catalyst, such sites are stabilized by the support, which on the other side acts as a source of active oxygen involved in the oxidation of surface carbon and hydrogen.

Partial oxidation of methane to carbon monoxide and hydrogen over NiO/La2O3/γ-Al2O3 catalyst

Abstract: The production of H2 and CO by catalytic partial oxidation of CH4 at atmospheric pressure has been examined over NiO γ- Al 2 O 3 and NiO/La2O3/γ-Al2O3 catalysts. When NiO loading increased to 17.5 wt.-%, the syngas production over NiO γ- Al 2 O 3 is found to increase consistently. In that case, the addition of La2O3 can improve the catalytic activity significantly, with a NiO content of more than 17.5 wt.-%, a very small extent of change in the catalytic activity and the selectivity for the methane oxidation to syngas as well as a little effect of La2O3 on the activity and the selectivity are observed. Under the condition of high space velocity, the CO2 and H2O reforming of CH4 occurs to a very small extent over reduced 12.5 wt.-% NiO/0.5 wt.-% La 2 O 3 γ- Al 2 O 3 catalyst. This fact and the results of the effect of space velocity on the methane partial oxidation to syngas suggest that both H2, CO and CO2, H2O are the primary products of the methane oxidation. The results of characterizing 12.5 wt.-% NiO/0.5 wt.-% La 2 O 3 γ- Al 2 O 3 catalyst reveal that the active form of catalyst is metallic Ni and the promoter La2O3 facilitate the reduction of the oxidative nickel.

Selective catalytic oxidation of ammonia to nitrogen

Abstract: The activity of catalysts of different nature in ammonia partial oxidation has been studied. Vanadium-titanium, copper-titanium catalysts and copper-substituted zeolites are the most active in the reaction. These catalysts are promising if used as honeycomb monoliths.

Partial oxidation of ethane over alumina-boria catalysts

Abstract: Alumina-boria catalysts can be used in selective oxidation of ethane for ethylene production. The alumina-boria catalysts have been prepared from boric acid using a classical impregnation method on porous and non-porous alumina. To obtain more dispersed boron oxide on the surface of the support, some alumina-boria samples were prepared by chemical vapour deposition and a comparison with impregnated samples has been performed in the ethane selective oxidation. The contribution of gas-phase reactions to the formation of ethylene and the role of boron oxide at different percentages on alumina for ethane conversion are discussed. A maximum of activity (18.7% conversion) was observed with the sample with 30 wt.-% boron oxide on non-porous alumina. The main product was ethylene (16.7% yield) while CO and CH 4 were the secondary products, coming from the decomposition of ethylene. A correlation has been demonstrated between catalytic activity and the acidity of the alumina-boria catalysts. The boron oxide (with its specific acidity) was responsible for the activation of ethane and for the stabilisation of the ethylhydroxyperoxide radical formed during the reaction and favouring the partial oxidation reaction leading to ethylene formation while alumina was shown to result into deep oxidation due to its too strong acidic sites. It is concluded that the role of alumina consists mainly in dispersing boria, resulting in a much more selective catalyst for ethane oxidation to ethylene than the support itself.

Partial oxidation of methane to synthesis gas. Behaviour of different Ni supported catalysts

Abstract: The behaviour of Ni supported catalysts, obtained using Ni(NO3)2 and Ni-acetylacetonate as precursor compounds, is analyzed It is observed that initial activities and selectivities are similar for both systems, but the stability differs significantly The systems show different carbon structures and sintering rates, depending on the precursor compound employed