Dehydrogenation high-temperature

Athene formation requires that X and Y be substituents on adjacent carbon atoms By mak mg X the reference atom and identifying the carbon attached to it as the a carbon we see that atom Y is a substituent on the p carbon Carbons succeedmgly more remote from the reference atom are designated 7 8 and so on Only p elimination reactions will be dis cussed m this chapter [Beta (p) elimination reactions are also known as i 2 eliminations ] You are already familiar with one type of p elimination having seen m Section 5 1 that ethylene and propene are prepared on an industrial scale by the high temperature dehydrogenation of ethane and propane Both reactions involve (3 elimination of H2... 

Dehydrogenation of Propionates. Oxidative dehydrogenation of propionates to acrylates employing vapor-phase reactions at high temperatures (400—700°C) and short contact times is possible. Although selective catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid have been developed in recent years (see Methacrylic ACID AND DERIVATIVES) and a route to methacrylic acid from propylene to isobutyric acid is under pilot-plant development in Europe, this route to acrylates is not presentiy of commercial interest because of the combination of low selectivity, high raw material costs, and purification difficulties. 

Styrene undergoes many reactions of an unsaturated compound, such as addition, and of an aromatic compound, such as substitution (2,8). It reacts with various oxidising agents to form styrene oxide, ben2aldehyde, benzoic acid, and other oxygenated compounds. It reacts with benzene on an acidic catalyst to form diphenylethane. Further dehydrogenation of styrene to phenylacetylene is unfavorable even at the high temperature of 600°C, but a concentration of about 50 ppm of phenylacetylene is usually seen in the commercial styrene product. 

Other Technologies. As important as dehydrogenation of ethylbenzene is in the production of styrene, it suffers from two theoretical disadvantages it is endothermic and is limited by thermodynamic equiHbrium. The endothermicity requites heat input at high temperature, which is difficult. The thermodynamic limitation necessitates the separation of the unreacted ethylbenzene from styrene, which are close-boiling compounds. The obvious solution is to effect the reaction oxidatively ... 

In the alkylcyclohexane (ACH) to aromatic equihbrium, ACH Ar, aromatics are favored by high temperatures and low pressures. Normal reforming conditions promote rapid ACH dehydrogenation and a high conversion to aromatics. 

Dehydrogenation of isobutane to isobutylene is highly endothermic and the reactions are conducted at high temperatures (535—650°C) so the fuel consumption is sizeable. Eor the catalytic processes, the product separation section requires a compressor to facHitate the separation of hydrogen, methane, and other light hydrocarbons from-the paraffinic raw material and the olefinic product. An exceHent overview of butylenes is avaHable (81). 

The imida2olines can be dehydrogenated at high temperatures over metal oxide catalysts to give the corresponding imida2oles (46). 

Membrane Reactor. Another area of current activity uses membranes in ethane dehydrogenation to shift the ethane to ethylene equiUbrium. The use of membranes is not new, and has been used in many separation processes. However, these membranes, which are mostly biomembranes, are not suitable for dehydrogenation reactions that require high temperatures. Technology has improved to produce ceramic and other inorganic (90) membranes that can be used at high temperatures (600°C and above). In addition, the suitable catalysts can be coated without blocking the pores of the membrane. Therefore, catalyst-coated membranes can be used for reaction and separation. 

Dehydrogenation. The dehydrogenation of paraffins is equihbrium-limited and hence requites high temperatures. Using this approach and conventional separation methods, both Houdry and UOP have commercialized the dehydrogenation of propane to propylene (92). A similar concept is possible for ethane dehydrogenation, but an economically attractive commercial reactor has not been built. 

Dehydrogenation of A -imidazolines (294 Z = NR) gives imidazoles, but requires quite high temperatures and a catalyst such as nickel or platinum. Alternatively, hydrogen acceptors such as sulfur or selenium can be used (70AHC(12)103). 

The term, metal dusting, was first used about this time to describe the phenomenon associated with hydrocarbon processing. Butane dehydrogenation plant personnel noted how iron oxide and coke radiated outward through catalyst particles from a metal contaminant which acted as a nucleating point. The metal had deteriorated and appeared to have turned to dust. The phenomenon has been called catastrophic carburization and metal deterioration in a high temperature carbonaceous environment, but the term most commonly used today is metal dusting. 

This is an endothermic reaction in which a volume increase accompanies dehydrogenation. The reaction is therefore favoured by operation at reduced pressure. In practice steam is passed through with the ethylbenzene in order to reduce the partial pressure of the latter rather than carrying out a high-temperature reaction under partial vacuum. By the use of selected catalysts such as magnesium oxide and iron oxide a conversion of 35-40% per pass with ultimate yields of 90-92% may be obtained. 

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

It will be clear from the results so far presented that both C5 and C dehydrocyclization products can be formed, with aromatization proceeding (one would expect) by further dehydrogenation of the initially formed C6 ring-closure species. There is another pathway for the production of aromatics based upon cyclization of a linear triene (133), but this is of relatively small importance, and is only significant at all at quite high temperatures and low hydrogen partial pressures. 

The mechanism of thermal decomposition (pyrolysis) of methane has been extensively studied [90,91]. Because C-H bonds in the methane molecule are significantly stronger than C-H and C-C bonds of the products, the secondary and tertiary reactions contribute at the very early stages of the reaction, which obscure the initial processes. According to Holmen et al. [92], the overall methane thermal decomposition reaction at high temperatures can be described as a stepwise dehydrogenation as follows ... 

To examine the catalyst deterioration under the present superheated liquid-film conditions, a long-term tetralin dehydrogenation over the carbon-supported platinum catalyst (Pt/C) was carried out under the superheated liquid-film conditions (heating temperature 240°C) at the amount ratio of 1.1 g/0.5 mL/min [13]. Figure 13.21 shows the time courses of reaction rate and conversion in the tetralin dehydrogenation. High conversion (around 50%) was maintained for longer than 5 h. 

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