Selective oxidative dehydrogenation butenes

The oxidation of butane on these orthovanadates were tested at 500°C in a flow reactor using a butane oxygen helium ratio of 4 8 88. The observed products were isomers of butene, butadiene, CO, and CO2. The carbon balance in these experiments were within experimental errors, thus the amount of any undetected product if present should be small. The selectivity for dehydrogenation (butenes and butadiene) was found to depend on the butane conversion and be quite different for different orthovanadates. Fig. 4 shows the selectivity for dehydrogenation at 12.5% conversion of butane [15,18,19]. Its value ranged from a high of over 60% for Mg3(V04)2 to a low of less than 5% for... 

The possibilities for selective oxidation of butenes present a complex picture because of the various ways of introducing oxygen into the molecule and the possibilities of dehydrogenation and isomerization. Finally, there are catalysts with a capacity for producing dimerization and aro-matization. 

Selective Oxidative Dehydrogenation of Butenes on Ferrite Catalysts... 

Oxydehydrogenation of /i-Butenes. Normal butenes can be oxidatively dehydrogenated to butadiene in the presence of high concentration of steam with fairly high selectivity (234). The conversion is no longer limited by thermodynamics because of the oxidation of hydrogen to water. Reaction temperature is below about 600°C to minimise over oxidation. Pressure is about 34—103 kPa (5—15 psi). 

Feed enters the reactor at tube side, oxygen at shell side. Oxidative dehydrogenation of 1-butene to butadiene. W3Sb203 catalyst placed in the pores of the tube. T 462 C. Conversion 30%, Selectivity 92%. T = 505°C. Conversion 57%, Selectivity 88%. ... 

For the 8.2 V/nm sample, the products observed for the pulse reaction at 400°C consisted of only dehydrogenation products (butenes and butadiene) and carbon oxides. No oxygenates were observed, and the carbon balance for each pulse was satisfied within experimental error. The selectivity for dehydrogenation is shown in Fig. 3a as a function of 0. It shows that the selectivity was very low when the catalyst was in a nearly fully oxidized state, but increased rapidly when the catalyst was reduced beyond 0 = 0.15. It should be noted that the dependence of selectivity for dehydrogenation on 0 shown in the figure was not... 

Fig. 6. Dependence of selectivity for oxidative dehydrogenation of butane (to butenes and butadiene) on the reduction potential of the cations in orthovanadates of the formula M2 (V04)2 and M11V04. Reactions conditions 500°C, butane conversion = 12.5%, butane 02 He = 4 8 88 (from Ref. 39).

Many substances can be partially oxidized by oxygen if selective catalysts are used. In such a way, oxygen can be introduced in hydrocarbons such as olefins and aromatics to synthesize aldehydes (e.g. acrolein and benzaldehyde) and acids (e.g. acrylic acid, phthalic acid anhydride). A selective oxidation can also result in a dehydrogenation (butene - butadiene) or a dealkylation (toluene -> benzene). Other molecules can also be selectively attacked by oxygen. Methanol is oxidized to formaldehyde and ammonia to nitrogen oxides. Olefins and aromatics can be oxidized with oxygen together with ammonia to nitriles (ammoxidation). 

Oxidative dehydrogenation of butene 573-648 a-Fe203, a-Fe203/Si02 Selectivity for butadiene increases with decreasing Fe2Q3 crystallite size 54... 

VO)2P207 is superior for the selective dehydrogenation of n-butane to butene to other crystalline V-P oxides, while few differences exist between the oxidation of butene and butadiene, which are considered reaction intermediates. The abstraction of methylene hydrogen from n-butane is the slowest step. Hence this step determines the overall catalytic activity." 2) Selectivity in forming anhydride from C4 and C5 alkanes, but not in selective oxidation of lower (C2 and C3) or higher (Ce-Cg) alkanes." 3) The number of surface layers involving the catalytic reactions is limited to 2-3 in contrast to Bi-Mo-O catalysts. 

Molybdate based scheelites have been intensively studied in this respect, one reason being that they are found with molybdenum in both the penta- and hexavalent state. Bismuth molybdates in particular are useful catalysts for selective oxidation of propylene to acrolein, propylene ammoxidation to acrylonitrile and the oxidative dehydrogenation of butene to butadiene. 

On the basis mainly of results obtained in the oxidation of isobutene to methacrolein, the oxidative dehydrogenation of butene to butadiene and the oxygen-aided dehydration of formamide to nitriles, it was possible to show that oxides present in catalysts are located on a scale reflecting donor-acceptor properties (fig. 5). Some oxides are essentially acceptors (e.g., M0O3, some tellurates) they can potentidly cany active and selective sites, provided they receive spillover oxygen. Others are essentidly donors a-Sb204, in this respect, is typical it produces spillover oxygen but carries no sites active for oxidation. Other oxides have mixed properties. The acceptors are relatively covalent, the donors are more ionic [63,77]. 

Catalysts based on transition metal molybdates, typically bismuth, cobalt and nickel molybdates [2-6], have received recent attention. Of the transition metal molybdates, those based on nickel, and in particular the stoichiometric NiMo04, have attracted the greatest interest. NiMo04 presents two polymorphic phases at atmospheric pressure a low temperature a phase, and a high temperature P phase [2,7]. Both phases are monoclinic with space group dim. These phases differ primarily in the coordination of molybdenum which is distorted octahedral in the a phase and distorted tetrahedral in the P phase. The P phase has been shown to be almost twice more selective in propene formation than the a phase for comparable conversion at the same temp>erature [2]. A similar effect has been noted for oxidative dehydrogenation of butane, with the P phase being approximately three times more selective in butene formation than the a phase [8]. The reason for the difference in selectivities is unknown, but the properties of the phases are known to be dependent on the precursors from which they are derived. Typically, nickel molybdates are prepared by calcination of precipitated precursors. 

We applied our FT-IR technique also to selective oxidation catalysts [10]. We present here our results on an FT-IR study of the interaction and oxidative conversion of n-butenes over MgFe204, i.e. an active catalyst for butene oxy-dehydrogenation. The aim of the present work was to have information on the activation of olefins over allylic oxidation catalysts and to find information on the reasons why ferrites are good catalysts for allylic oxy-dehydrogenation of butenes but they are are not good catalysts for allylic oxidation of propene. 

Process studies on conversion of m-butenes to butadiene by oxidative dehydrogenation over unspecified catalysts were reported by Kolobikhin and co-workers (133) and by Alkazov and co-workers (134). These studies show high selectivities to butadiene, and present a certain amount of kinetic information. 

Oxidative dehydrogenation of w-butenes to butadiene is described by Kolobikhin and co-workers (133) using unspecified mixed Group V and VI metal oxides. An extension of this work by Kolobikhin and Emcl yanova (152), using Bi-Mo oxide catalysts on SiOg gel of 64 m /gm area, describes selectivities to butadiene of the order of 90%, and suggests the need of excess O2 to prevent catalyst reduction. 

Pt, Pd, Ir Au oxidative dehydrogenation of alkanes, n-butene to butadiene, methanol to formaldehyde improved selectivity... 

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