Dehydrogenation in the vapor phase

Zinc oxide-alumina-calcium oxide Dehydrogenation in the vapor phase... 

Palladium-asbestos Dehydrogenation in the vapor phase Vinyl from ethyl derivatives... 

Styrene. Commercial manufacture of this commodity monomer depends on ethylbenzene, which is converted by several means to a low purity styrene, subsequendy distilled to the pure form. A small percentage of styrene is made from the oxidative process, whereby ethylbenzene is oxidized to a hydroperoxide or alcohol and then dehydrated to styrene. A popular commercial route has been the alkylation of benzene to ethylbenzene, with ethylene, after which the cmde ethylbenzene is distilled to give high purity ethylbenzene. The ethylbenzene is direcdy dehydrogenated to styrene monomer in the vapor phase with steam and appropriate catalysts. Most styrene is manufactured by variations of this process. A variety of catalyst systems are used, based on ferric oxide with other components, including potassium salts, which improve the catalytic activity (10). 

In the petroleum (qv) industry hydrogen bromide can serve as an alkylation catalyst. It is claimed as a catalyst in the controlled oxidation of aHphatic and ahcycHc hydrocarbons to ketones, acids, and peroxides (7,8). AppHcations of HBr with NH Br (9) or with H2S and HCl (10) as promoters for the dehydrogenation of butene to butadiene have been described, and either HBr or HCl can be used in the vapor-phase ortho methylation of phenol with methanol over alumina (11). Various patents dealing with catalytic activity of HCl also cover the use of HBr. An important reaction of HBr in organic syntheses is the replacement of aHphatic chlorine by bromine in the presence of an aluminum catalyst (12). Small quantities of hydrobromic acid are employed in analytical chemistry. 

Synthesis from Citronellol. Citronellol is hydrated to 3,7-dimethyloctan-l,7-diol, for example, by reaction with 60% sulfuric acid. The diol is dehydrogenated catalytically in the vapor phase at low pressure to highly pure hydroxydihydrocitronellal in excellent yield. The process is carried out in the presence of, for example, a copper-zinc catalyst [68] at atmospheric pressure noble metal catalysts can also be used [69]. 

Dehydrogenation of/>-menthadienes and a-pinene in the vapor phase over catalysts such as chromia—alumina produces />-cymene (70). -Menthadienes can be disproportionated over a Cu—Ni catalyst to give a mixture of -menthane andy>-cymene (71). 

The catalysts were tested in the dehydrogenation of tetrahydrothiophene (DHN of THT), the hydrodesulphurization of thiophene (HDS of thiophene) and the hydrogenation of biphenyl (HN of BP). The reactions were carried out in the vapor phase using dynamic flow microreactors equipped with an automatic online analysis. Reaction conditions are given in Table 1. 

There are two ways to produce acetaldehyde from ethanol oxidation and dehydrogenation. Oxidation of ethanol to acetaldehyde is carried out in the vapor phase over a silver or copper catalyst (305). Conversion is slighdy over 80% per pass at reaction temperatures of 450—500°C with air as an oxidant. Chloroplatinic acid selectively catalyzes the liquid-phase oxidation of ethanol to acetaldehyde giving yields exceeding 95%. The reaction takes place in the absence of free oxygen at 80°C and at atmospheric pressure (306). The kinetics of the vapor and liquid-phase oxidation of ethanol have been described in the literature (307,308). 

Alkylation. Ethylbenzene [100-41-4], the precursor of styrene, is produced from benzene and ethylene. The ethylation of benzene is conducted either in the liquid phase in the presence of a Friedel-Crafts catalyst (A1C13, BF3, FeCl3) or in the vapor phase with a suitable catalyst. The Monsanto/Lummus process uses an aluminum chloride catalyst that yields more than 99% ethylbenzene (13). More recently, Lummus and Union Oil commercialized a zeolite catalyst process for liquid-phase alkylation (14). Badger and Mobil also have a vapor-phase alkylation process using zeolite catalysts (15). Almost all ethylbenzene produced is used for the manufacture of styrene [100-42-3], which is obtained by dehydrogenation in the presence of a suitable catalyst at 550—640°C and relatively low pressure, <0.1 MPa (<1 atm). 

Styrene. Styrene is the largest benzene derivative with annual consumption about 11.5 billion lb in the United States. It is produced mainly by catalytic dehydrogenation of high-purity ethylbenzene (EB) in the vapor phase. The manufacture process for EB is based on ethylene alkylation with excess benzene. This can be done in a homogeneous system with aluminum chloride catalyst or a heterogeneous solid acid catalyst in either gas or liquid-phase reaction. In the past decade, the liquid-phase alkylation with zeolite catalyst has won acceptance. Those processes have advantages of easier product separation, reducing waste stream, and less corrosion. In addition, it produces less xylene due to lower... 

Catalytic dehydrogenation of volatile alcohols is carried out by passing their vapors over copper at 300 °C [344] or over copper chromite at 275-325 °C for 1.7-4 h, with yields ranging from 20 to 80% [554]. Dehydrogenation in the liquid phase is accomplished by refluxing the alcohol with copper chromite in xylene for 4 h [556] or by heating the alcohol in paraffin oil with Raney nickel and a catalytic amount of potassium hydroxide at 150-180 °C for 6 h. Thus endo-norborneol is transformed into norcamphor in 95% yield [928]. 

Heteropoly acids can also act as oxidation catalysts both in the vapor and liquid phase. In the vapor phase they are effective dehydrogenation catalysts for saturated carboxylic acids and aldehydes readily converting isobutyric acid to methacrylic acid (Eqn. 10.19). Methacrylic acid is produced in 70% selectivity at 72% conversion over (NH4)3PMoi204q at 260°C. This reaction takes place only when there is a substituent a to the carbonyl group of the reactant. 

The presence of solution can dramatically affect dissociative chemisorption. In the vapor phase, most metal-catalyzed reactions are homolyticlike, whereby the intermediates that form are stabilized by interactions with the surface. Protic solvents, on the other hand, can more effectively stabilize charge-separated states and therefore aid in heterolytic activation routes. Heterolytic paths can lead to the formation of surface anions and cations that migrate into solution. This is directly relevant to methanol oxidation over PtRu in the methanol fuel cell. The metal-catalyzed route in the vapor phase would involve the dissociation of methanol into methoxy or hydroxy methyl and hydrogen surface intermediates. Subsequent dehydrogenation eventually leads to formation of CO and hydrogen. In the presence of an aqueous media, however, methanol will more likely decompose heterolytically into hydroxy methyl (—1) and intermediates. 

One mechanism that has been proposed for the conversion of ethyl alcohol to butadiene in the vapor phase consists of three steps (a) dehydration of the ethyl alcohol b) dehydrogenation of the ethyl alcohol (c) condensation of the ethylene and acetaldehyde in (a) and (b) to give butadiene, C Hg. At 400 . the following information is available for the three steps ... 

As in the classical dehydration of alcohol in the presence of sulfuric acid,181 to form ether, the same transformation may be accomplished under certain conditions in the vapor phase by passing ethanol vapors over dehydrating catalysts as has already been shown. Although this particular decomposition does not occur in the oxidation of ethanol over dehydrogenating catalysts, yet the presence of formaldehyde in the products obtained when certain border line catalysts have been used might indicate that the initial dehydration which occurs had gone partly to formation of ether which subsequently oxidized or decomposed. 

Acetaldehyde synthesis by dehydrogenation or partial oxidation of ethanol in the vapor phase (Fig. 8.1)... 

Dehydrogenation of the cyclohexanol/n clohexanone mixture to phenol. The alcohol forms with the phenol an azeotrope with a high phenol content (for example, 75 molar per cent at 12 kPa absolute). In order to employ distillation as a means of purification, this means that the reactor output must have a phenol content higher than that of the azeotrope. In practice, conversion takes place in the vapor phase with platinum base... 

Dehydrogenation itself takes place in the vapor phase, around 400 to 450°C, with catalysts consisting of mixed oxides of molybdenum, phosphorus, vanadium, iron. etc. It has been developed chiefly by Eastman, Hitachi. Mitsubishi, etc. 

The highest pyridine-yd-picoline yields with MFI were found with Si02/Al203 ratios in the 150-400 range [10,11,22]. At such low aluminium concentrations it is likely that Brpnsted acid sites, rather than Lewis sites, assist in the Aldol condensation, cyclization, and hydrogen transfer (or dehydrogenation) reaction steps. Discussion of the mechanism of formation of pyridine bases in the vapor phase and the nature of the acid site dates back to the time of Chichibabin and remains a topic for debate [5,27]. 

Catalytic dehydrogenation of alcohols is best effected in the vapor phase since the equilibrium ... 

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