Oxidative dehydrogenation reactions

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

Figure 3.37 Selectivity-conversion diagram of the oxidative dehydrogenation reaction [1].

There are certainly quite significant advantages that membrane reactor processes provide as compared to conventional reaction processes. The reactor can be divided by the membrane into two individual compartments. The bulk phases of the various components or process streams are separated. This is of importance for partial oxidation or oxidative dehydrogenation reactions, where undesirable consecutive gas phase reactions leading to total oxidation occur very often. By separating the process stream and the oxidant. 

Oxidation and oxidative dehydrogenation reactions over oxide catalysts have been widely studied in recent years. The precise role of oxygen in these reactions remains elusive, but slowly a more detailed picture is emerging which suggests that both oxide ions of the lattice and oxygen species on the surface can play an important role (/, 2). 

Oxidative dehydrogenation reactions of alcohols and amines are widespread in enzymatic biochemistry, and are of potential importance with regard to the operation of fuel cells based on simple alcohols such as methanol. The nature of products, and their rates of formation, may vary depending on the reaction conditions, and a role of metal ions has been recognized. The oxidation of amines may lead to a variety of products (nitriles, nitro species, etc.) although dehydrogenated diimine products are obtained quantitatively when the oxidation of the amine occurs via coordination to metal centers. A review is available on the mechanisms of oxidative dehydrogenations of coordinated amines and alcohols (93). 

The data presented above showed that the oxidative dehydrogenation reactions of the various alkanes share many common features. Thus it is tempting to discuss selectivity for alkane oxidative dehydrogenation with a common scheme. The reaction scheme for ethane oxidation [Eqs. (5)-(7)] provides a useful basis for such a discussion. It shows that the primary reaction of alkane oxidation can take on three different pathways depending on the reaction temperature (Scheme I). The first step in all three pathways is breaking a C—H bond, which is the rate-limiting step. The three pathways are described below. 

Reaction 7 means that hydrogen peroxide can be a radical source by decomposition—i.e., one of the reaction products is the origin of radicals. Accordingly this oxidative dehydrogenation reaction has the feature of the degenerative branching. Naturally one may also assume that other peroxides or aldehydes could be origins of radicals. However, Reaction 7 is considered to be the sole source of radicals from the reaction products. 

Oxidative dehydrogenation reactions can produce macrocycles with extensive delocalization, e.g. Schemes 9 and 12. In Scheme 12 the 15-7i-electron radical complex (36) can be further oxidized to the 14-7i-electron complex (37), or reduced to the 16-Ti-electron complex (38) by reversible one-electron redox steps. 

Although oxidative dehydrogenation reactions are particularly well characterised with macrocyclic complexes, even very simple amine ligands such as 1,2-diaminoethane may be oxidised to the corresponding imines (Fig. 9-23). 

Symyx entered this competition in 1997 in collaboration with Hoechst with the goal of creating and validating primary and secondary synthesis and screening technologies and the use of this workflow to broadly explore mixed metal oxide compositions so as to discover and optimize new hits . The initial goal was a 10-fold increase in the space-time yield relative to the state-of-the-art MoVNb system for the ethane oxidative dehydrogenation reaction to ethylene. 

This polymer is red in its oxidized form and yellow in its reduced form, and lends itself to spectroscopic study. Its activity was compared with that of molybdate catalysts, which are well known to be active for oxidative dehydrogenation and many parallels between the organic and inorganic materials were found. The color changes of this catalyst during reaction made it clear that we are not concerned with a surface reaction, but that at least a part of the bulk of the material participates in the oxidative dehydrogenation reaction, a phenomenon we have mentioned several times in these pages. 

Su DS, et al. Nanocarbons in selective oxidative dehydrogenation reaction. Catal Today. 2005 102 110—4. 

The main drawbacks of the non-oxidative dehydrogenation reaction can be summarized as, the thermodynamic limitation, the low conversion rate, the need for recovery of unreacted ethylbenzene, the high energy consumption, and deactivation of the catalyst. Thus in recent years several alternatives to overcome those problems have been investigated. 

Khodakov, A., Olthof, B., BeU, A.T. and Iglesia, E. (1999) Structure and catalytic properties of supported vanadium oxides support effects on oxidative dehydrogenation reactions. Journal of Catalysis,... 

It follows that one may conclude that basic properties as well as atomic arrangements at molecular level play an important role in alkane oxidative dehydrogenation reactions. Such a conclusion could also be reached from the study of VMgO catalysts [42] for oxidative dehydrogenation of several alkanes as ethane, propane and butane under similar conditions (see e g. fig. 5 in ref 42). 

The oxidative dehydrogenation reactions over these catalysts are similar to the gas phase result of shock tube experiments determined by Skinner et al. (ref. 6). This observation supports the fact that the recombination reactions of methyl radicals in the gaseous phase are an important source of ethane and that the ethene is a secondary product derived from ethane. This secondary reaction proceeds in the gaseous phase as well as the catalyst surface. The major role of the MgO surface is to produce the methyl radical efficiently. The active sites for cleaving the H-CH3 bond should be moderated by Li to enforce C2 selectivity. In addition to gas phase oxidation, the direct surface oxidation of the hydrocarbon adsorbate is very significant especially for acidic materials. 

Previously we have studied such catalysts in hydrocarbon dehydrogenation and oxidative dehydrogenation reactions [6,7]. Instrumental methods such as XRD, X-ray, photoelectron spectroscopy, DTA, UV-spectroscopy, EM were used. It has been found that activity of the Zn-Cr catalysts is determined by the stoichiometric spinel ZnCr204 [8]. In the case of the vanadium-magnesium system the activity and selectivity depend upon the presence of ions V and V grouped on the catalyst surface into clusters of 2-3 vanadium ions [9]. This was taken as a principal for the purposeful synthesis of the catalytic systems mentioned. In this work an attempt was made to spread the obtained experience on the dehydrogenation of alcohol groups. 

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