Hydrocarbons oxidative dehydrogenation

Reactions. The most important commercial reaction of cyclohexane is its oxidation (ia Hquid phase) with air ia the presence of soluble cobalt catalyst or boric acid to produce cyclohexanol and cyclohexanone (see Hydrocarbon oxidation Cyclohexanoland cyclohexanone). Cyclohexanol is dehydrogenated with 2iac or copper catalysts to cyclohexanone which is used to manufacture caprolactam (qv). 

Beside their use in equilibrium-restricted reactions, CMRs have been also proposed for very different applications [6], like selective oxidation and oxidative dehydrogenation of hydrocarbons they may also act as active contactor in gas or gas-liquid reactions. 

Catalytic testings have been performed using the same rig and a conventional fixed-bed placed in the inner volume of the tubular membrane. The catalyst for isobutane dehydrogenation [9] was a Pt-based solid and sweep gas was used as indicated in Fig. 2. For propane oxidative dehydrogenation a V-Mg-0 mixed oxide [10] was used and the membrane separates oxygen and propane (the hydrocarbon being introduced in the inner part of the reactor). 

The activity of elemental carbon as a metal-free catalyst is well established for a couple of reactions, however, most literature still deals with the support properties of this material. The discovery of nanostructured carbons in most cases led to an increased performance for the abovementioned reasons, thus these systems attracted remarkable research interest within the last years. The most prominent reaction is the oxidative dehydrogenation (ODH) of ethylbenzene and other hydrocarbons in the gas phase, which will be introduced in a separate chapter. The conversion of alcohols as well as the catalytic properties of graphene oxide for liquid phase selective oxidations will also be discussed in more detail. The third section reviews individually reported catalytic effects of nanocarbons in organic reactions, as well as selected inorganic reactions. 

The ODH of ethylbenzene to styrene is a highly promising alternative to the industrial process of non-oxidative dehydrogenation (DH). The main advantages are lower reaction temperatures of only 300 500 °C and the absence of a thermodynamic equilibrium. Coke formation is effectively reduced by working in an oxidative atmosphere, thus the presence of excess steam, which is the most expensive factor in industrial styrene synthesis, can be avoided. However, this process is still not commercialized so far due to insufficient styrene yields on the cost of unwanted hydrocarbon combustion to CO and C02, as well as the formation of styrene oxide, which is difficult to remove from the raw product. 

Selective partial oxidation of hydrocarbons poses considerable challenges to contemporary research. While by no means all, most catalytic oxidations are based on transition-metal oxides as active intermediates, and the oxidative dehydrogenation of ethylbenzene to styrene over potassium-promoted iron oxides at a scale of about 20 Mt/year may serve as an example [1]. Despite this... 

A reaction of particular relevance with respect to applied catalysis is the oxidative dehydrogenation (ODH) of hydrocarbon by VmOn ions according to reaction 2, which involves a two-electron reduction of the cluster. By means of a systematic study of the reactions of various YmOn ions as well as the related oxo-vanadium hydroxides VmO H+ ions with a set of C4-hydrocarbons, it was demonstrated recently that the ODH activity of the cluster ions shows a clear correlation with the formal valence of vanadium in the cluster ions with a maximum reactivity for formal vanadium (V) (Fig. 3) [84]. In such a kind of reactivity screening, it is essential to include more than a single reagent as a probe for the reactivity of the different ions in order to reduce interferences by kinetic barriers of one particular combination of neutral and ionic reactants [85]. Accordingly, the sums of the relative rate constants for the ODH reactions of the four different butenes are considered and normalized to the most reactive ion studied, which turns out to be the formally pure vanadium (V) compoimd In addition to isomeric... 

Isopropyl alcohol production in 1950 exceeded 800,000,000 pounds, all made from petroleum. This alcohol is used mainly as a raw material for the production of acetone and also as a solvent. Acetone is made by the catalytic high temperature dehydrogenation or air oxidation of isopropyl alcohol. A much smaller part of the total acetone supply comes from fermentation and from hydrocarbon oxidation. Like isopropyl alcohol, part of the consumption is for solvent uses, but most serves as a raw material for other oxygenated compounds. 

Early studies with respect to the dehydrogenation of hydrocarbons to alkenes on oxide catalysts indicated that carbonaceous deposits formed in the early stages of the process on the surface of acidic catalysts act as the real active centers for the oxidative dehydrogenation. The hypothesis was later confirmed377 378 and verified by using carbon molecular sieves. With this catalyst 90% styrene selectivity could be obtained at 80% ethylbenzene conversion.379 Various coals for the synthesis of isobutylene380 and activated carbon in the synthesis of styrene381 were used in further studies. 

Spinel oxides with a general formula AB2O4 (i.e. the so-called normal spinels) are important materials in industrial catalysis. They are thermally stable and maintain enhanced and sustained activities for a variety of industrially important reactions including decomposition of nitrous oxide [1], oxidation and dehydrogenation of hydrocarbons [2], low temperature methanol synthesis [3], oxidation of carbon monoxide and hydrocarbon [4], and oxidative dehydrogenation of butanes [5]. A major problem in the applications of this class of compound as catalyst, however, lies in their usually low specific surface area [6]. 

The multifunctionality is achieved through either the combination of two different compounds (phase-cooperation) or the presence of different elements inside a single crystalline structure. In antimonates-based systems, cooperation between the metal antimonate (having a rutile crystalline structure), employed for propane oxidative dehydrogenation and propene activation, and the dispersed antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over the former. In metal molybdates, one single crystalline structure contains both the element active in the oxidative dehydrogenation of the hydrocarbon (vanadium) and those active in the transformation of the olefin and in the allylic insertion of the N H2 species (tellurium and molybdenum). 

Selective oxidations, e.g., propane to acroleine, butane to maleic anhydride, ethylene to ethylene oxide Oxidative dehydrogenations of hydrocarbons Oxidative coupling of methane Methane oxidation to syngas... 

Let us now pass on to hydrocarbon oxidation mechanisms at high temperatures, when cracking and dehydrogenation become the main reactions in the system. 

The statements of the possible role of HO radicals in saturated hydrocarbon oxidation processes is proved by experimentally determined formation of sufficient amounts of hydrogen peroxide and HO radicals during oxidation of propane [27] and paraffin dehydrogenation products [28-30],... 

Let us discuss in general gas-phase processes of oxidative dehydrogenation of hydrocarbons involving as reagents substances that easily induce free radical transformations of substrates. Many such substances are known that dissociate to free radicals or induce free radical reactions. However, the most widespread in investigations are compounds that are able to shift dehydrogenation and cracking product ratios toward the first process. 

As follows from a brief consideration of the role of HO radicals in gas-phase oxidation reactions, they are the key active sites in the high temperature range, and studies of homogeneous oxidative dehydrogenation of hydrocarbons allocate the dominating role in unsaturated compound formation to them. 

Lanthanum oxide is well-known as an active isomerization (406) and hydrogenation catalyst (407), and it has attracted much attention recently (along with other basic oxides, such as MgO and CaO) as a catalyst for the oxidative dehydrogenation and coupling of methane to C2 hydrocarbons (408-410). Its activity for selective NO reduction of CH4 in excess oxygen has also been demonstrated (411, 412). 

Rozanska X, Sauer J. Oxidative dehydrogenation of hydrocarbons by V307+ compared to other vanadium oxide species. J Phys Chem A. 2009 113(43) 11586—94. 

Short-contact-time reactions, defined as reactions occurring on a timescale of milliseconds, offer potential for conversion of hydrocarbons in one step into valuable products. Examples are the selective oxidation of methane in s)mgas without the formation of b)q5roducts (CO2, H2O, and coke) and the oxidative dehydrogenation of alkanes to give olefins or oxygenates. 

The heterobimetallic complexes [N(n-Bu)4] [Os(N)R2(/u.-0)2Cr02] catalyze the selective oxidation of alcohols with molecular oxygen. A mechanism in which alcohol coordinates to the osmium center and is oxidized by B-hydrogen elimination (see -Hydride Elimination) is consistent with the data. The hydroxide adduct of OSO4, [0s(0H)204], with ferric cyanide and other co-oxidants catalyzes the oxidative dehydrogenation of primary aromatic and aliphatic amines to nitriles, the oxidation of primary alcohols to carboxylic acids, and of secondary alcohols to ketones. Osmium derivatives such as OsCb catalyze the effective oxidation of saturated hydrocarbons in acetonitrile through a radical mechanism. ... 

In its literal form, this reaction is only of academic interest because a molecule is unlikely to break up or isomerize irreversibly in two or more different ways. However, situations frequently encountered in practice are those of multistep parallel first-order decomposition reactions and of parallel reactions that involve coreactants but are pseudo-first order in the reactant A. An example of the first kind is dehydrogenation of paraffins, examples of the second kind include hydration, hydrochlorination, hydroformylation, and hydrocyanation of olefins and some hydrocarbon oxidation reactions. All these reactions are multistep, but the great majority are first order in the respective hydrocarbon, and pseudo-first order if any co-reactant concentration is kept constant. 

---