Phase Dehydrogenation of Ethanol

Estimate the overall rate of reaction when the bulk phase gas-phase composition is = 0.6 y = 0.2 y3 = 0.2 

ANALYSIS The flux of ethanol is determined by the rate of chemical reaction at the surface 

Furthermore, for every mole of ethanol that diffuses to the catalyst surface, 1 mol of acetaldehyde and 1 mol of hydrogen are produced and these components diffuse away from the catalyst surface. That is, 

The molar fluxes in the vapor phase will be calculated from 

We have identified the interface mole fractions with and the bulk vapor mole fractions with y.  

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The reaction kinetics for the dehydrogenation of ethanol are also weU documented (309—312). The vapor-phase dehydrogenation of ethanol ia the presence of a chromium-activated copper catalyst at 280—340°C produces acetaldehyde ia a yield of 89% and a conversion of 75% per pass (313). Other catalysts used iaclude neodymium oxide and samarium hydroxide (314). 

Consider the vapor-phase dehydrogenation of ethanol (1) to produce acetaldehyde (2) and hydrogen (3) ... 

Adapted from Kabel, R. L. and Johanson, L. N., Reaction kinetics and adsorption equilibrium in the vapor-phase dehydrogenation of ethanol, AJChE J 8 (5), 623 (2004). 

There are two ways to produce acetaldehyde from ethanol oxidation and dehydrogenation. Oxidation of ethanol to acetaldehyde is carried out ia the vapor phase over a silver or copper catalyst (305). Conversion is slightly over 80% per pass at reaction temperatures of 450—500°C with air as an oxidant. Chloroplatinic acid selectively cataly2es the Uquid-phase oxidation of ethanol to acetaldehyde giving yields exceeding 95%. The reaction takes place ia the absence of free oxygen at 80°C and at atmospheric pressure (306). The kinetics of the vapor and Uquid-phase oxidation of ethanol have been described ia the Uterature (307,308). 

Tamaru (110) also discusses examples from heterogeneous catalysis in which reaction rates of one step seem to be influenced by the adsorption of other components. For example, Nishimura et al 115) studied the dehydrogenation of ethanol to acetaldehyde and hydrogen over a specially prepared Nb/Si02 catalyst at 523 K. The studies were done in a recirculating closed (batch) reactor. The rate is about constant as time increases, and the IR spectrum of an adsorbed intermediate remains constant. A sudden evacuation of the gas-phase ethanol stops the reaction but does not affect... 

The vapor-phase catalytic dehydrogenation of ethanol to acetaldehyde involves the diffusion of ethanol to the catalyst surface where it reacts to produce acetaldehyde and hydrogen. Under typical reactor conditions (temperature = 548 K, pressure = 101.3 kPa) the binary diffusivities of the three binary pairs encountered are... 

Oxidation of Ethanol. Ethanol may be dehydrogenated or oxidized to acetaldehyde in the vapor phase with good yields. Shreve reports yields of 85-95 per cent by air oxidation of ethanol to acetaldehyde with a silver catalyst at 550°C and per-pass conversions of 50-55 per cent. Dehydrogenation of ethanol to acetaldehyde over an asbestos-supported copper catalyst activated by small additions of cobalt and chromium has been re-... 

CiambeUi P, Sannino D, Palma P, Vaiano V, Mazzei RS. (2011) Intensification of gas-phase photoxidative dehydrogenation of ethanol to acetaldehyde by using phosphors as light carriers. Photochem. Photobiol. Sci., 10 414. 

Acetic Acid. Acetic acid production in the United States has increased by large numbers in the last half century, since the monomer has many uses such as to make polymers for chewing gum, to use as a comonomer in industrial and trade coatings and paint, and so on. In the 1930s, a three-step synthesis process from ethylene through acid hydrolysis to ethanol followed by catalytic dehydrogenation of acetaldehyde and then a direct liquid-phase oxidation to acetic acid and acetic anhydride as co-products was used to produce acetic acid... 

Dehydrogenations of some 2,3-dihydropyrazines to the corresponding pyrazines have been effected as follows 2,3-dimethyI-5,6-dihydro [reflux with potassium hydroxide in ethanol (333) heat in ethylene glycol at 197° (653, 1558) reflux with potassium hydroxide and manganous oxide in ethanol (633a) vapor phase reaction in the presence of a catalyst, such as copper chromite, and clay (472) heat... 

The conversion of ethanol is carried out in the presence of gas-phase oxygen molecules - oxidative dehydrogenation [47,48] - and the oxygen vacancies created in step 3a of Scheme 7.1 are regenerated by gas-phase oxygen. The dehydration of ethanol to ethylene and water does not consume surface oxygen atoms (step 3b). 

Scheme 7.1 The proposed sequence of reaction steps for the concurrent dehydrogenation (steps 1, 2a, and 3a) and dehydration (steps 1, 2b, and 3b) of ethanol over Ti02 and UO2 single-crystal surfaces. V (s) surface O vacancy, (a) adsorbed phase, (g) gas phase, (s) surface species and M=TP " or Surface oxygen atoms are indicated in bold to highlight the distinction with molecular oxygen atoms...

Prior to 1916, acetaldehyde was manufactured by the oxidation of alcohol in the liquid phase with bichromate and sulfuric add.1 Since that time it has been ade quite largely by the hydration of acetylene in sulfuric acid solutions activated with mercury salts. However, the relatively low price of ethanol in America has made the formation of acetaldehyde by vapor phase dehydrogenation or limited oxidation of the alcohol attractive commercially. To this end several methods have been proposed for conducting the transformation industrially. Developments of processes employing vapor phase oxidation reactions have all been based largely on the prindples disclosed by the early work, a considerable portion of which had been undertaken purely for the purpose of research and not industrialization. 

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)... 

Acetaldehyde can be produced by the partial oxidation of ethanol and the direct oxidation of ethylene. The predominant commercial process, however, is the direct liquid phase oxidation of ethylene. As with many other ethylene-based petrochemicals, acetaldehyde was first produced commercially from acetylene. The acetylene process was developed in Germany more than 70 years ago and was still practiced until the mid-1970s when the high cost and scarcity of acetylene forced it into obsolescence. Another early route to acetaldehyde was based on ethanol. Ethyl alcohol can be either oxidized or alternatively dehydrogenated to acetaldehyde. Site-... 

The feedstock picture further diversified in 1920 with the commercialization of ethanol dehydrogenation to generate acetaldehyde. (The process was conducted in the vapor phase at 260-290°C using copper-chromite catalysts.) While the process was known as early as 1886 the development of adequate catalysts for the endothermic process would take nearly 35 years. Subsequent oxidation to acetic acid provided an additional source of acetic acid. These technologies would largely stay in place with only minor modification until the 1950 s. A summary of the chemical routes to the various acetyls in 1920 is shown in Figure 1. 

Acetaldehyde may be made (1) from ethylene by direct oxidation, with the Wacker-catalyst containing copper(II) and palladium(II) salts (2) from ethanol by vapor-phase oxidation or dehydrogenation or (3) from butane by vapor-phase oxidation. The direct oxidation of ethylene is the most commonly used process, accounting for 80% of acetaldehyde production. 

Acetaldehyde. Acetaldehyde has been made from ethanol by dehydrogenation and by catalytic hydration of acetylene. Today direct oxidation of ethylene in the liquid phase catalyzed by palladium and copper has replaced these earlier methods. Figure 10.14 shows an ethylene-to-acetaldehyde unit based on this last route. 

Acetaldehyde Oxidation. Ethanol [64-17-5] is easily dehydrogenated oxidatively to acetaldehyde (qv) using silver, brass, or bronze catalysts. Acetaldehyde can then be oxidized in the liquid phase in the presence of cobalt or manganese salts to yield acetic acid. Peracetic acid [79-21-0] formation is prevented by the transition metal catalysts (7). (Most transition metal salts decompose any peroxides that form, but manganese is uniquely effective.)... 

It has been observed that a series of 2,4-alkanedionato adducts of cobalt(III)(salen), salen = bis(salicylideneaminato) dianion, undergo a thermally induced, intramolecular one-electron transfer reaction to cobalt(II)bis(salicylideneaminato) . The concomitant formation in the gas phase of a mixture of the /9-diketone (not more than 50%), methanol, ethanol and acetone has been explained as follows the thermally induced, homolytic fission of the Co—Odik bond gives a /3-diketonato radical which abstracts a hydrogen atom from a second /3-diketonate to form the corresponding diketone, whereas the dehydrogenated /3-diketonato radical decomposes into compounds of lower molecular weight. 

The major reaction is oxidative dehydrogenation at the secondary hydroxyl site of lactic acid, but the product pyruvic acid in its free-acid form is unstable to decompose. Thus the substrate was supplied as ethyl ester to protect the carboxyl moiety. Esterification is also of benefit to vapor-phase flow operation in making acids more volatile. Hydrolysis of ethyl lactate gives free pyruvic acid with further decarboxylation to actaldehyde. Ethanol, which is another fragment of ester hydrolysis, eould be either oxidized to acetaldehyde or dehydrated to ethylene at higher temperature above 350°G. The reaction network is summerized in Scheme 1. 

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