Partial Oxidation Of N-butane Research Articles

TPD study of labile oxygen on a (VO) 2P 2O 7 catalyst active in n-butane partial oxidation

The interaction of oxygen with the surface of vanadyl pyrophosphate has been studied by Temperature-Programmed Desorption (TPD) under vacuum between 300 and 1000 K. It is shown that the adsorption of oxygen onto the catalyst does not occur at temperatures in the range 300–570 K and that it is activated. The behaviour of the main TPD peak around 850 K agrees with second order desorption kinetics showing that adsorption is dissociative. The use of Intermittent TPD (ITPD), which is a particular threshold method, shows that the labile superficial oxygen desorbing up to 900 K is virtually homogeneous with an activation energy of desorption of 327 kJ/mol O 2. Adsorption sites are tentatively assigned to cus V 4+, acting as Lewis acid centers, located in the (100) plane of the (VO) 2P 2O 7 structure, in the prolongation of the vanadyl columns. The role of labile oxygen in the catalytic oxidation of butane into maleic anhydride is discussed.

Oxidation of butane and butadiene to furan over vanadia–silica mixed oxides

Vanadia–silica mixed oxides (aerogels and xerogels), derived from the sol–gel preparation route, were tested in the partial oxidation of n-butane and 1,3-butadiene to furan. The dependence of the furan selectivity on morphological properties and vanadium dispersion in the silica matrix has been studied. The catalysts were characterized by N 2 and Ar physisorption, AAS, XPS and FTIR spectroscopy. Surface acidic centers of the materials were assessed by means of DRIFT-measurements of adsorbed NH 3 probe molecules. The structural properties, mainly influenced by the drying procedure of the sol–gel samples, had a major influence on the catalytic performance of the sol–gel materials. Aerogels were the most selective catalyst, converting butane to furan with only 3% selectivity. The maximum furan selectivity in butadiene transformation was about 25%, but considerable coke formation was also observed. Microporosity, found in conventionally dried sol–gel materials seems to be unfavorable for furan selectivity. The presence of Brønsted acidic sites was proved to be necessary for furan formation. Studies involving V 2O 5, SiO 2, and vanadia–silica low-temperature aerogels with 5–20% vanadia contents revealed that the selectivity to furan was diminished by silica due to its intrinsic high activity for total oxidation and cracking. A comparison to literature data indicated that vanadia–silica mixed oxides and vanadia grafted on silica are markedly less selective towards furan than VPO catalysts.

In SituStudy of Redox and of p-Type Semiconducting Properties of Vanadyl Pyrophosphate and of V–P–O Catalysts during the Partial Oxidation ofn-Butane to Maleic Anhydride

In situelectrical conductivity measurements have been performed on vanadyl pyrophosphate samples as a function of their preparation temperature and on V–P–O catalysts as a function of their activation time. In agreement with X-ray diffraction results, electrical conductivity has shown that heat treatment of vanadyl pyrophosphates between 450 and 800°C results in a less conducting state corresponding to a very crystalline form. Such a stabilized form can already be reached at a temperature between 600 and 700°C. For V–P–O catalysts, electrical conductivity decreased as a function of the activation time,tact, under the reaction mixture, reaching a steady value fortact≥84 h. Both types of solids were found to be p-type semiconductors with positive holes as the main charge carriers. During subsequent exposures ton-butane and to oxygen at reaction temperature (400°C), it was confirmed that vanadyl pyrophosphate constitutes the main phase of efficient V–P–O catalysts working with some excess of surface oxygen probably in relation with the presence of V5+species. The recorded changes in conductivity upon exposure to reactants provide illustrative evidence of a redox (Mars and van Krevelen) mechanism. The long activation period necessary to produce a high yield of maleic anhydride corresponds to structural and chemical changes of the surface which accommodates the reaction mixture and gives rise to active and selective sites able to convertn-butane to maleic anhydride.

Mechanochemistry: the activation method of VPO catalysts for n-butane partial oxidation

The paper describes the influence of the introduction of alkali and alkaline-earth metals into the VPO precursor and the effect of bismuth additive introduced into the VPO precursor by the mechanochemical treatment. XRD, XPS, DTA, TPD of NH 3 and determination of catalytic activity and selectivity in n-butane oxidation have been carried out. The preparations studied after mechanochemical treatment in a planetary mill were: (a) VPO, composed of VOHPO 4 · 0.5H 2O, (b) VPBiO, containing the bismuth additive introduced during synthesis, (c) VPO/Bi 2O 3 and VPO/BiPO 4, mechanical mixtures of the VPO precursor and Bi 2O 3 or BiPO 4, respectively. Introduction of alkali and alkaline-earth metals additives increases the basicity of surface oxygen in vanadium pyrophosphate, which entails increase in the rate of n-butane oxidation. The mechanochemical treatment of the preparations changed their morphology, favouring the formation of the vanadyl (001) plane in the phosphate precursor. The XRD peaks of bismuth phosphate appeared instead of those of bismuth oxide and the +0.8 eV chemical shift in the XPS Bi4f band was observed. The change in the chemical state of VPO/Bi 2O 3 are an evidence of the occurrence of chemical reaction. Mechanochemical treatment of VPO/BiPO 4 and VPO/Bi 2O 3 catalysts increases their activity in n-butane oxidation and the selectivity to maleic anhydride. Thus, mechanochemistry may be a promising method for introduction of promoting additives into the basic VPO composition.

Mechanochemistry as activation method of the V [sbnd]P [sbnd]O catalysts for n-butane partial oxidation

The mechanochemical treatment of the V P O catalysts caused a substantial increase of both the catalytic activity in n-butane oxidation and the selectivity towards maleic anhydride. It was accompanied by a relative growth of the contribution of vanadyl plane (001) of VOHPO 4·0.5H 2O which transforms into the topologically similar (100) plane of (VO) 2P 2O 7. The mechanochemical treatment of the V P O catalysts containing bismuth changes the sample morphology, favouring the formation of the vanadyl plane and provoking surface chemical reaction. The data obtained for V P O/BiPO 4 and V P O/Bi 2O 3 samples demonstrate that mechanochemistry is a new promising method for the introduction of promoting additives into the basic V P O composition.

New Layered Vanadyl(IV) Phosphite as a Precursor to Vanadyl Pyrophosphate Catalysts for Partial Oxidation of n-Butane to Maleic Anhydride

The synthesis and characterization of layered vanadyl(IV) phosphonates, VOC{sub n}H{sub 2n+1}PO{sub 3}{center_dot}xH{sub 2}O containing the V{sub 2}O{sub 8} dimers have been recently reported. The authors present here the results of partial oxidation of n-butane on VPO catalysts derived from the simplest of these phosphonates, vanadyl phosphite. The results indicate that vanadyl(IV) phosphite is the precursor to vanadyl pyrophosphate catalysts with high surface area (ca. 45 m{sup 2}/g) and selectivity to maleic anhydride in n-butane partial oxidation comparable to the conventional unpromoted catalyst. 10 refs., 3 figs., 1 tab.

Electrical properties of doped vanadium phosphate phases and VPO catalysts used in the partial oxidation of n-butane to maleic anhydride

Electrical properties of doped vanadium phosphate phases and VPO catalysts used in the partial oxidation of n-butane to maleic anhydride

Infrared chemiluminescence of CO and CO 2 produced by molecular-beam surface reactions

Vibrationally excited molecules produced by molecular-beam surface reactions have been studied by using infrared emission spectroscopy. The product CO desorbed by partial oxidation of n-butane on Pt at 1500 K is substantially vibrationally excited, but rotationally very cool (rotational temperature; T R = 360 K), while the product CO 2 desorbed by the CO + O 2 reaction on Pt is both vibrationally and rotationally excited. The CO 2 molecules produced by CO oxidation on Pt using an atomic oxygen beam are much more highly excited (both vibrationally and rotationally) than those by the CO + O 2 reaction. These results showed that both reactant and product molecules do not necessarily thermally accommodate to the surface, and their energy distributions will give us information on the dynamics of surface-catalyzed reactions.

Internal Energy Distributions of Product Molecules Studied by Molecular-Beam/Infrared Emission Technique. Partial Oxidation of Butane and Decomposition and Oxidation of CH3OH on a Pt Surface.

Internal energy distributions in the CO and CO2 molecules formed by molecular-beam surface reactions on a polycrystalline Pt foil have been studied by using FT-IR emission spectroscopy. The CO molecules produced by partial oxidation of n-butane or iso-butane at a surface temperature of 1500 K are substantially vibrationally excited (vibrational temperature : Tv=2300 K), but rotationally very cool (rotational temperature : TR=360 K). The low TR value is not due to the rotational relaxation by the gas phase collision. In contrast, the CO molecules produced by CH3OH decomposition are not vibrationally excited at all. This impliee that the mechanism is different between the partial oxidation of butane and the decomposition of CH3OH. On the other hand, the CO2 molecules produced and desorbed by oxidation of CH3OH is both vibrationally and rotationally excited, but the internal energy distributions of the CO2 molecules are different from those formed by the, CO+O2 reaction or the CO+O reaction. These differences in internal energy distributions can provide us with some information on the dynamics (including the structure of the activated complex and the energy transfer processes) of surface catalyzed reactions.

Homogeneous oxidation of n-butane ill a self-stirred reactor

Partial oxidation of n-butane was carried out in a self-stirred reactor in the homogeneous gas phase under atmospheric pressure at 600–700 K. The influence of space time (0.8 to 5 s) and of the fraction of oxygen in the reactants was studied, either with premixed or with unmixed reactant feed. It was found that the selectivity in the required products (formaldehyde, methanol, acetic acid) with respect to water was essentially favoured by operation at low temperature. A kinetic ‘tendency model’ is shown to account for the dependence of selectivity upon various experimental parameters and for the ‘ignition’ of the reactor. This model relies on a limited number of stoichiometric equations but keeps the essential features of the very complex reaction system. With this model, we are able to simulate the reaction under new experimental conditions (reactor type, inlet temperature, feed composition, space time) and to optimize the yield in required products.

Influence of reductive pretreatments on the activity and selectivity of vanadium-phosphorus oxide catalysts for n-butane partial oxidation

The catalytic activities for n-butane conversion to maleic anhydride of two series of vanadium-phosphorus oxide catalysts with phosphorus:vanadium ratios in the range 0.94 to 1.10 and calcined at 773 or 923 K were compared before and after reduction by hydrogen. In almost all cases studied, reduction gave rise to increased conversion, and for catalysts with low phosphorus:vanadium ratios increased selectivity was also noted. When n-butane was contacted with these catalysts in the absence of gas phase oxygen, maleic anhydride and total oxidation products continued to be formed until changes in average oxidation state of vanadium of up to -1.5 were recorded. From considerations of the dynamic state of the catalysts during catalysis, it was concluded that a morphology consisting of oxidized surface layers on a reduced core favors high activities and selectivities for this process.

Influence of the phosphorus/vanadium ratio on the solid state chemistry and redox properties of vanadium phosphate-based catalysts

The influence of the P V atomic ratio on the redox properties of two series of V-P-O catalysts was investigated. The catalysts were calcined at 773 or 923 K and the P V ratio was varied in the range 0.94 to 1.07. The 4+ oxidation state of vanadium was highly stabilized during calcination at 773 K by a slight excess of phosphorus. This stabilization was less pronounced at the higher calcination temperature. The influence of the P V ratio on the reactivity toward hydrogen of the catalysts calcined at 773 K was difficult to establish because the phase composition of these samples varied over a large range. It was established for the series calcined at 923 K that excess phosphorus, far from activating reduction of vanadium, tended to inhibit this process. Excess phosphorus also inhibited reoxidation of the bulk of these catalysts but tended to have little effect on the reoxidation of near surface layers. The catalytic activity in n-butane partial oxidation to maleic anhydride was compared with the redox behavior and a model for selective oxidation is presented which envisages excess phosphorus as regulating the flow of oxygen anions from the bulk to the surface. The primary interaction is postulated as being between the hydrocarbon and oxygen anions associated with a V 5+ related phase.

Influence of p/v ratio on the phase composition and catalytic activity of vanadium phosphate based catalysts

The influence of the bulk P/V ratio of V-P-O catalysts on their phase composition, surface P/V ratio and catalytic activity for n-butane partial oxidation is reported. Catalysts were prepared by reduction of V 2O 5 in lactic acid followed by addition of O-H 3PO 4. Excess lactic acid was evaporated off and the solid precursor calcined in air at 773 K. The bulk P/V ratio was varied in the range 0.94 to 1.07. This range was sufficient to bring about large variations in the nature of the final catalyst obtained. Catalysts with tow P/V ratios formed as βVPO 5 with vanadium in the +5 oxidation state. Those with high P/V ratios formed in a phase which we label β in which vanadium was in the +4 oxidation state. Scanning electron microscopy indicated that the catalysts increased in crystallinity as the P/V ratio decreased and EPMA indicated that the P/V ratio was homogeneous throughout each catalyst. Overall conversion of n-butane was at a maximum for P/V close to unity whereas selectivity was highest for the highest P/V ratio studied. The XPS study indicated that the surface P/V ratio did not change in the range of bulk compositions in which selectivity increased from ca. 10 to 50%. Arguments are presented which suggest that it is the solid state properties of these catalysts which largely determine selectivity and that the primary interaction of the hydrocarbon occurs with the aid of a V +5 related surface phase.

Influence of cobalt on the textural, redox and catalytic properties of stoichiometric vanadium phosphate

The influence of coprecipitated cobalt on the catalytic, redox and textural properties of βVPO 5 is reported. Four catalysts were prepared in which the Co/V ratio was varied between 0 and 0.05 and the P/V ratio was held at unity. Cobalt strongly reduced the surface area of βVPO 5 and diminished reactivity toward hydrogen was observed as a monotonous feature as the cobalt content increased. By contrast this material showed complex kinetic behaviour during reoxidation from reduced states. When reoxidation was confined to surface layers a maximum in rate was observed for Co/V ratios of ca. 0.02. Bulk reoxidation was strongly sensitive to small amounts of cobalt but it was independent of cobalt content above Co/V = 0.01. Results of catalytic testing for n-butane partial oxidation to maleic anhydride indicate that conversion was at a maximum for the undoped material but selectivity showed an optimum value for Co/V of ca. 0.02. Above this value a sharp drop in selectivity was observed. A strong increase in areal reoxidisability was observed for Co/V above 0.02. X.P.S. data indicated that some surface segregation of cobalt had occured for Co/V = 0.05. Two domains of influence by cobalt on the catalytic activity of βVPO 5 were identified; one in which it influenced the solid state properties and increased selectivity, the other in which a surface enrichment in cobalt provoked total oxidation of n-butane.

THE VAPOR PHASE PARTIAL OXIDATION OF n-BUTANE—EFFECT OF PRESSURE, REACTION TIME, AND INLET GAS COMPOSITION

Data are presented on the product yields and the general course of the partial oxidation of n-butane at an ambient temperature of 725 °F., at pressures ranging from 50 to 175 p.s.i.a., reaction times of 1.0 to 4.5 sec., and inlet reactant compositions of 1.5 to 6.0 mole % n-butane and 1.5 to 6.0 mole % oxygen (the remainder being diluent nitrogen). Conditions were neither isothermal nor adiabatic and peak temperatures as much as 70 Fahrenheit degrees higher than the inlet temperatures were encountered. The reactor consisted of a 24-ft. length of 1/4-in. stainless steel pipe, immersed in a bath of boiling Aroclor 1254. Under optimum conditions, a 20% conversion of the butane to methanol, ethanol, acetaldehyde, acetone, and formaldehyde was obtained. In all the tests, an induction period of approximately 0.75 sec. was observed.