1,721,005 research outputs found
Oxidation of 1-Butene and Butadiene to Maleic Anhydride. 2. Kinetics and Mechanism
No full textThe kinetics of the oxidation of 1-butene and butadiene over a vanadium-phosphorus mixed oxide catalyst prepared
by reduction of V,O, in an organic medium were investigated. Increased 1-butene concentration in the feed strongly
inhibited the rate of maleic anhydride formation but did not affect the rate of formation of the other products (methyl
vinyl ketone, crotonaldehyde, acetaldehyde, and carbon oxides). The inhibition was interpreted by a Langmuir-
Hinshelwood type model where the ratedetermining step is the reaction between adsorbed oxygen and the
intermediate butadiene adsorbed on two adjacent sites. The rates of formation of the other byproducts are
interpreted by expressions which correspond to the reaction between 1-butene or butadiene adsorbed on one site
and adsorbed or gaseous oxygen. Satisfactory fit of the derived rate expression with the experimental data was
obtaine
Key aspects of catalyst design for the selective oxidation of paraffins
No full textThis review examines some aspects in the development of heterogeneous catalysts for the oxyfunctionalization of light paraffins. Particular attention is devoted to the reaction of paraffin oxydehydrogenation to olefins and of -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 light of the stability of the product of partial oxidation towards consecutive unselective oxidation reactions, and with respect to the mechanism of paraffin activation
Vanadium/phosphorus mixed oxide from the precursor to the active phase:Catalyst for the oxidation of n-butane to maleic anhydride
No full textThis review examines the recent scientific and patent literature dealing with V/P/O-basedcatalysts for the synthesis of maleic anhydride by n-butane oxidation. Attention is focused on the different methods of preparation claimed by each company, as well as on the main parameters in precursor preparation and thermal treatment affecting the final catalytic performance. The role of the various promoters reported in the literature is also discussed. © 1995 Elsevier B.V
Enhancement of the catalytic performance of V2O5/Al2O3 catalysts in the oxidehydrogenation of propane to propylene by the use of monolith reactor
No full textA new catalyst for propane ammoxidation: The Sn/V/Sb mixed oxide
No full textThe effect of the preparation and calcination method of a Sn/V/Sb mixed oxide on its catalytic performance are analyzed with respect to ammoxidation of propane. Different samples have been prepared with coprecipitation technique (by dissolving the starting materials with ethanol, iso-butanol or water), or by solid state reaction between oxide and tin hydroxide. The sample prepared by ethanol shows the best catalytic performance: this solvent makes the partial alkoxides quite stable in solution and brings about a better coprecipitation with a more intimate mixture of tin, antimony and vanadium. The thermal transformation of the precursor of this sample has been followed during the calcination, both in air and in nitrogen with several characterization techniques. The thermal treatment in air at 700°C leads to the best catalytic performance, that is, good activity and high selectivity. This calcination procedure leads to a homogenous mixed oxide, with numerous well-crystallized microfields of tin oxide, incorporating antimony and vanadium ions in limited concentrations and an excess of poorly crystallized antimony oxide
Comparison of the Catalytic Performance of V2O5/g-Al2O3 in the Oxidehydrogenation of Propane to Propylene in Different Reactor Configuration: i) Packed-bed Reactor, ii) Monolith-.
No full textNature of vanadium species in SnO2 - V2O5-based catalysts. Chemistry of preparation, characterization, thermal stability and reactivity in ethane oxidative dehydrogenation over V-Sn mixed oxides
No full textTin-vanadium mixed oxides have been prepared either from V4+-Sn4+ solutions by coprecipitation, or by the solid-state reaction between SnO(OH)2 and V2O5, and characterized by means of chemical analysis, FTIR spectroscopy, EPR, X-ray diffraction and surface area measurements. Interaction between the hydroxy groups of the tin oxohydrate and the vanadium ions, reduction of V5+ ions to V4+ and stabilization inside the rutile structure led to the formation of a VxSn1-xO2 solid solution after calcination at 700 °C. A maximum amount of 10 atom% of vanadium entered the SnO2 lattice; at values of up to x = 0.02, V4+ was likely to be homogeneously dispersed, while higher amounts probably formed V4+ oxide clusters inside the rutile matrix. In addition, amorphous V5+ oxide was formed over the rutile surface, and at an overall vanadium content greater than 20-25 atom% crystalline V2O5 was also formed. In samples where x ≥ 0.02-0.03, the solid solution was not stable at temperatures greater than 700 °C, and some of the V4+ was released from the structure forming segregated amorphous V5+ oxide, while for x < 0.02 the solution was stable. The V-Sn mixed oxides were tested as catalysts for ethane oxidative dehydrogenation. The catalysts initially exhibited an unstable behaviour due to a reduction of the V5+ oxide in the reaction environment. Tin oxide activity was enhanced by the addition of V4+; for x = 0.018, also the selectivity to ethene at temperatures higher than 480 °C was significantly greater. In contrast, selectivity to ethene at low temperatures was lower for x > 0.018
Surface structure and reactivity of VTiO catalysts prepared by solid-state reaction 1. Formation of a VIV interacting layer
No full textThe solid-state interaction between V2O5 and TiO2 in the 700-800 K range of temperatures gives rise to the formation of VIV sites even in the absence of reducing agents. A VIV interacting layer covering the entire surface of TiO2 anatase may be created in the absence of any indication of partial transformation to the rutile phase. The nature, amount, and distribution of these VIV sites are characterized by means of titration combined with selective extraction, reactivity measurements in o-xylene oxidation, evaluation of redox properties, and by XRD, XPS, and ESR analyses. The amount of VIV depends on the crystallographic nature (anatase or rutile) and surface area of the TiO2 and on the conditions (temperature, time, and type of atmosphere) of the heat treatment. In the anatase sample the VIV sites can be reduced to VIII, but not oxidized to VV due to the strong interaction with the titania surface. In rutile samples part of the VIV may be reduced to VIII, but also oxidized to VV. The remaining VIV sites are present in solid solution in the rutile matrix and are not accessible to redox reagents. The model of a VIV-modified TiO2 (anatase) surface is discussed with reference to the problem of surface diffusion of vanadium species on the anatase surface. In TiO2 (rutile)-based samples, due to the competition of the migration of vanadium ions toward the bulk of the rutile with respect to surface diffusion, V2O4-like islands form that are coherently intergrown into the main rutile TiO2 matrix. © 1991
On the antimony-stabilized cubic structure of potassium/ammonium salts of 12-molybdophosphoric acid and its catalytic performance in the oxidehydrogenation of ethane
No full textPotassium/ammonium salts of 12-molybdophosphoric acid were modified by the addition of an Sb5+ salt. The addition of antimony led to a remarkable increase in the thermal structural stability of the compounds obtained. The incipient destruction of the Keggin unit in the mixed potassium/ammonium salt of 12-molybdophosphoric acid was shifted from 400-420°C to 500°C. The potassium salt modified by the addition of one Sb5+ atom per KU decomposed at temperatures higher than 600°C. This property allowed the samples to be used as catalysts for high temperature, gas-phase oxidation reactions, such as the oxidehydrogenation of ethane. The compounds did not undergo structural decomposition at temperatures as high as 540°C under reaction conditions, but were poorly active and selective in ethylene formation. Therefore, antimony-stabilized compounds were further modified by the addition of transition metal ions in order to improve the catalytic performance. The addition of small amounts of iron, chromium and cerium ions led to an improvement of the catalytic performance; the compound was apparently monophasic, and characterized by the cubic crystalline structure typical of the salts of 12-molybdophosphoric acid. © 1995 J.C. Baltzer AG, Science Publishers
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