Ethylbenzene Dehydrogenation Styrene Catalysts

Early catalysts were produced from calcined ferric oxide, potassium carbonate, a binder when required, and usually chromium oxide. Subsequently a wide range of other oxides replaced the chromium oxide typical compositions are shown in Table 7.5. The paste was extruded or granulated to produce a suitable shape and then calcined at a high temperature in the range 900°-950°C. Solid solutions of a-hematite and chromium oxide (the active catalyst precursors) were formed and these also contained potassium carbonate to inhibit coke formation. Catalyst surface area and pore volume were controlled by calcination conditions. It has been confirmed by X-ray diffraction studies that a-hematite is reduced to magnetite and that there is some combination of potash and the chromium oxide stabilizer. There is little change in the physical properties of the catalyst during reduction and subsequent operation. 

Catalyst activity and selectivity, at high or low steam ratios, can be controlled by selection of both the stabilizers and the calcination temperature. 

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Dehydrogenation of ethylbenzene to styrene occurs over a wide variety of metal oxide catalysts. Oxides of Ee, Cr, Si, Co, Zn, or their mixtures can be used for the dehydrogenation reaction. Typical reaction... 

An efficient oxidation catalyst, OMS-1 (octahedral mol. sieve), was prepared by microwave heating of a family of layered and tunnel-structured manganese oxide materials. These materials are known to interact strongly with microwave radiation, and thus pronounced effects on the microstructure were expected. Their catalytic activity was tested in the oxidative dehydrogenation of ethylbenzene to styrene [25]. 

Figure 7.24 Photoelectron emission microscopy images of two Fe304 surfaces that were used as model catalyst in the dehydrogenation of ethylbenzene to styrene at 870 K, showing carbonaceous deposits (bright). These graphitic deposits grow in dots and streaks on a surface of low defect density, but form dendritic structures on surfaces rich in point and step detects (from Weiss et al. f731).

Dehydrogenation of ethylbenzene to styrene. FejOj/AljOj catalyst pellets (occasionally potasium doped) packed on tube side of the reactor. 

Balandin and co-workers 143) have shown that the apparent activation energies for the dehydrogenation of ethylbenzenes to styrenes, as well as their similar previous data 144), can be correlated by the Hammett equation (series 116 and 117, three reactants in each, probably an Fe203 catalyst, negative slopes). 

Table 1 summarizes the experimental results obtained in our laboratory on the kinetics of the normal dehydrogenation of hydrocarbons (hexahydro-aromatics to aromatics, the open chain compounds butylene to butadiene, and ethylbenzene to styrene), of amines to ketimines, and of alcohols to aldehydes or to ketones, respectively, in the presence of metallic or oxide catalysts. Equation (1) was found to apply in all cases. Ko and h are given by... 

In particular, the dehydrogenation of ethylbenzene to styrene, a large-scale process, is performed with iron oxide-containing catalysts in the... 

Figure 5. The temperature dependences of benzene (curve 1) and styrene (curve 2) yields at cyclohexane and ethylbenzene dehydrogenation, respectively, on the catalyst, obtained from copolymer carbonisate, containing 7 mass per cent of Re.

Kito and Hattori et al. have described INCAP (IN-tegration of Catalyst Activity Patterns [21-23]), an expert system which rates the applicability of catalyst components for the desired reaction based on known activity patterns for different catalyst properties. The system was successfully applied for the selection of promoter components for the oxidative dehydrogenation of ethylbenzene to styrene. An improved version INCAP-MUSE (INCAP for MUlti-Componcnt catalyst SElcction [24-26]) selects as many catalyst components until all required catalyst properties are present. Although the system was successfully applied to oxidation reactions, more recently better results have been obtained by neural network methods (Section 2.6.2.2). 

A fixed-bed reactor often suffers from a substantially small effectiveness factor (e.g., 10 to 10 for a fixed-bed steam reformer according to Soliman et al. [1988]) due to severe diffusional limitations unless very small particles are used. The associated high pressure drop with the use of small particles can be prohibitive. A feasible alternative is to employ a fluidized bed of catalyst powders. The effectiveness factor in the fluidized bed configuration approaches unity. The fluidization system also provides a thermally stable operation without localized hot spots. The large solid (catalyst) surface area for gas contact promotes effective catalytic reactions. For certain reactions such as ethylbenzene dehydrogenation, however, a fluidized bed operation may not be superior to a fixed bed operation. To further improve the efficiency and compactness of a fluidized-bed reactor, a permselective membrane has been introduced by Adris et al. [1991] for steam reforming of methane and Abdalla and Elnashaie [1995] for catalytic dehydrogenation of ethylbenzene to styrene. 

It is often desirable to operate the reactor and the catalyst under isothermal conditions to achieve high reactor performance. Heat requirement of an endothermic reaction in a membrane reactor to maintain an isothermal condition can be challenging as in most of the dehydrogenation reactions such as conversions of ethylbenzene to styrene and prc pane to propylene. Maintaining an isothermal condition implies that some means must be provided to make the adequate heat input (e.g., from a burner) that is longitudinally dependent It is not trivial to make the temperature independent of the longitudinal position because the permeate flow also varies with the location in the axial direction. 

A classical example of the dehydrogenative formation of a carbon-carbon double bond conjugated with an aromatic ring is the dehydrogenation of ethylbenzene to styrene at 500-600 °C over a complex catalyst containing oxides of zinc and chromium [1090] or at 625 °C over vanadium pentoxide on alumina [478]. 

Large amounts of styrene are commercially produced by dehydrogenation of ethylbenzene (EB) in the presence of steam using iron oxide-based catalysts. Carbon dioxide, small amounts of which are formed as a by-product in the ethylbenzene dehydrogenation, was known to depress the catalytic activity of commercial catalyst [7,8]. However, it has been recently reported that several examples show the positive effect of carbon dioxide in this catalytic reaction [5,9,10]. In this study, we investigated the effect of carbon dioxide in dehydrogenation of ethylbenzene over ZSM-5 zeolite-supported iron oxide catalyst. 

The dehydrogenation reaction proceeds over an iron or an iron-chromium catalyst that usually also contains potassium in the form of potassium carbonate, so that at elevated temperatures various complex mixed carbonates and oxides are formed, e.g., KFe02. Temperatures are elevated, in the order of 630° C, and pressures are usually subatmospheric for improved per-pass conversions. Steam dilution is performed to further lower the partial pressure of the reactants. Because the reaction is strongly endothermic, various reaction stages with interheat (and interstage addition of superheated steam) are normally employed. Fig. 18 illustrates a typical process scheme for the dehydrogenation of ethylbenzene to styrene. 

Table 1 presents the properties of vanadium-magnesium oxide catalysts subjected to the heat treatment. The temperature of the heat treatment determines both the textural and the catalytic properties of the catalyst. Similar to the dehydrogenation of ethylbenzene into styrene [10,11], the most active catalysts occurred to be those... 

Recently, the remarkable properties of carbon nanotubes (CNTs) and related structures, such as carbon nanofibers (CNFs) and onionlike carbons, have attracted an increasing interest from the catalysis community [66], Although these materials are most often used as supports for active phases, some applications as catalysts have been reported, the oxidative dehydrogenation of ethylbenzene to styrene being the most frequently cited example [67-73], These reports basically confirm the mechanism proposed previously, based on a redox cycle involving the quinone surface groups. 

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