Acetaldehyde formation acetic acid production

For example, the reaction of methyl acetate and synthesis gas at 170 C and 5000 psig with a Co-Lil-NPh catalyst results in the formation of acetaldehyde and acetic acid. The rate of acetaldehyde formation is 4.5 M/hr, and the yield based on Equation 15 is nearly 100%. Methane (1-2%) and ethyl acetate (1-2%) are the only by-products. The product mixture does not contain water, methanol or 1,1-dimethoxyethane. The acetic acid can easily be recycled by esterification with methanol in a separate step. 

An alternative scheme to simultaneous formation of acetaldehyde and acetic anhydride could entail the carbonylation of methyl acetate to acetic anhydride which is subsequently reduced to acetaldehyde and acetic acid. The reaction of acetaldehyde with excess anhydride would form EDA. In fact, Fenton has described production of EDA by the reduction of acetic anhydride using both rhodium and palladium salts as catalysts when modified with triphenylphosphine (26). Two possible mechanisms for the reduction are postulated in equation 16. 

During the production of recovery flavours, apple wines or brandies, the interaction with ethanol, acetaldehyde and acetic acid represents the next level of interactions. The reaction products contain compounds which result from esterification and acetal formation reactions, which are summarised in Table 21.4. 

In-situ FTIR studies on the electrooxidation of ethanol on polycrystalline Pt [97-99] as well as on single-crystal Pt electrodes [lOO/lOl] have shown the formation of acetaldehyde and acetic acid in addition to cmbon dioxide as soluble products. Figure 29 shows the typical features for thesq/products, which were assigned according to Ihble 1. 

Methanol is widely available as a solvent and in paints and antifreezes, and may be consumed as a cheap substitute for ethanol. As little as 10 ml may cause permanent blindness and 30 ml may kill, through its toxic metabolites. Methanol, like ethanol, is metabolised by zero-order processes that involve the hepatic alcohol and aldehyde dehydrogenases, but whereas ethanol forms acetaldehyde and acetic acid which are partly responsible for the unpleasant effects of hangover, methanol forms formaldehyde and formic acid. Blindness may occur because aldehyde dehydrogenase present in the retina (for the interconversion of retinol and retinene) allows the local formation of formaldehyde. Acidosis is due to the formic acid, which itself enhances pH-dependent hepatic lactate production, so that lactic acidosis is added. 

Fig. 7. Branch point between fermentation and respiration. At low pyruvate flux, the low of the Pdh complex for pyruvate results in oxidative decarboxylation to form acetyl CoA and NADH. The acetyl CoA can then can go into energy generation (via respiration) or fatty acid synthesis. At high glycolytic flux, pyruvate accumulates, and the higher of Pdc favors acetaldehyde formation and ethanol production. Accumulation of acetate can interfere with mitochondrial function. Pyk Pyruvate kinase Pdh pyruvate dehydrogenase Pdc pyruvate decarboxylase Aid (Dha) aldehyde dehydrogenase Adh alcohol dehydrogenase Acs acetyl CoA synthetase. (Taken from Postma et al. [169])...

In general, acetic acid production via acetaldehyde oxidation takes place continuously in a bubble column at 50-80 °C with pressures of 1-10 bar. The construction material of choice for the reactor is austenitic Cr-Ni-steel. The acetic acid product serves as process solvent and the concentration of acetaldehyde is kept at 3%. It is necessary to keep the temperature over 50 °C to obtain a sufficient peroxide decomposition and oxidation rate. To remove the heat of the exothermic reaction, the reaction mixture is circulated through an external heat exchanger. Accurate temperature control is important to decrease oxidative degradation of acetic acid to formic acid, CO2, and water. The reaction mixture is separated by several distillation units. The process yields are typically in the range of 90-97% and the purity of acetic acid is higher than 99%. Typical by-products are CO2, formic acid, methyl acetate, methanol, methyl formate, and formaldehyde. 

The formation of acetaldehyde and acetic acid is due to the fact that rupture of the C-C bond in the original ethanol molecule would be required for CO2 formation. In the chemical oxidation of ethanol at high temperatures (combustion), this bond is readily ruptured in the hot flame, and the only reaction product (in addition to water) is CO2. In electrochemical oxidation occurring at temperatures below 2(X)°C, this bond is very difficult to rupture, and reactions involving the sole rupture of C-H bonds occur much more readily, thus leading to the side products noted. 

The complete electrooxidation of ethanol to CO2 releases 12 electrons and two molecules of CO2 per molecule of ethanol. Alas, in aqueous acid medium at room temperature, the partial oxidation of ethanol is the most favorable route, leading to the formation of acetaldehyde and acetic acid releasing of 2 and 4 electrons, respectively (see Figure 3.1). Whereas acetaldehyde can be further oxidized to acetic acid and CO2, acetic acid is a dead-end product of the electrooxidation of ethanol in acid medium. The formation of CO2 implies the scission of the C—C bond, a process which seems to be the bottleneck step for the complete oxidation of ethanol. Many aspects of the electrooxidation of ethanol still remain unclear in particular it not yet understood how the cleaving of the C—C bond proceeds. The nature of the ethanol adsorbate(s) and the intermediate adsorbed species leading to the cleavage of the C—C bond are also still under debate. Some authors propose that C—C scission can happen directly from ethanol whereas others claim that acetaldehyde (or acetyl) species are formed before C—C scission. The nature of the active site for the cleavage of the C—C scission is also under debate. 

So far we have shown that ethanol adsorption/oxidation on Pt is a complex process which leads to the formation of absorbed Ci species, (COad and CH c,ad) at relatively low potentials which by remaining adsorbed on the surface of the Pt electrode impede the ethanol electrooxidation reaction to proceed further. The main products of the electrooxidation of ethanol are acetaldehyde and acetic acid along with a minor amount of CO2. The selectivity to those products depends on the reaction conditions. The next sections describe the adsorption and oxidation of acetic acid and acetaldehyde on Pt. 

Acetic acid formation cannot be directly detected via DEMS due to its low vapor pressure, while ethyl acetate formation can be detected (through the m/z = 61 fragment) at high (> 0.5 M) ethanol concentrations. Therefore, acetic acid yields are determined indirectly by calculating the difference between the measured Faradaic current and the partial currents for ethanol oxidation to CO2 and acetaldehyde. This calculation is based on the assumption that only three reaction products, namely CO2, acetaldehyde, and acetic acid, are formed during ethanol oxidation. 

Fig. 6.4 compares the anode and cathode polarization between AEM and CEM. The cathode potential was increased about 200 mV with AEM compared with CEM, whereas the anode potential decreased to 80-300 mV depending on the current density. The advantage of AEM-type DEFCs is based on reduction of both anode and cathode over voltages [16]. Quantitative analyses of the product species during the operation of DEFCs were carried out to determine the stoichiometry of DEFCs. The formation of acetaldehyde or acetic acid as an oxidation product from ethanol was expected during the operation of AEM-type DEFCs, as described by Eqs. (6.4)... 

Carbonyl content is normally determined for polyethoxylated surfactants, such as EO/PO copolymers, that do not have a carbonyl group as part of their nominal structure. The presence of carbonyl compounds in polyethers indicates that some degradation of the product has taken place, or that lower molecular weight aldehydes were not stripped properly. Oxidation of ethoxylates results in the formation of formaldehyde, formic acid, acetaldehyde, and acetic acid, with the carbonyl functionality of primary interest being the aldehyde group (79,80). There is some interest in formate esters of the type R(OCH2CH2) OCHO, since they can be formed in photoinduced degradation (86,156). 

The process is similar to the catalytic liquid-phase oxidation of ethylene to acetaldehyde. The difference hetween the two processes is the presence of acetic acid. In practice, acetaldehyde is a major coproduct. The mole ratio of acetaldehyde to vinyl acetate can he varied from 0.3 1 to 2.5 1. The liquid-phase process is not used extensively due to corrosion problems and the formation of a fairly wide variety of by-products. 

In Figure 3 the merits of the two processes for p-xylene oxidation are compared. The main disadvantages of the Eastman Kodak/Toray cooxidation method are the need for a cosubstrate (acetaldehyde of methylethylketone) with concomitant formation of a coproduct (0.21 ton of acetic acid per ton product) and high catalyst concentration. The Amoco MC process, on the other hand, has no coproduct and much lower catalyst concentrations but has the disadvantage that the bromide-containing reaction mixture is highly corrosive, necessitating the use of a titanium-lined reactor. 

The catalyst is generally a palladium compound promoted with a trivalent amine or phosphine in the presence of methyl iodide as described earlier. Systems proven to bias acetaldehyde are utilized, of course (e.g. see Table I, run 12). A yield of 85% acetaldehyde from methyl acetate is typical by this method. It can be utilized in stoichiometric addition to easily prepared acetic anhydride resulting in EDA formation. When considering that the "boiling pot" reaction by-products are recyclable acetic acid, acetic anhydride and small amounts of EDA, the yield to vinyl acetate related products is 95%. 

The proposed reaction mechanism for the destruction of aqueous solutions of TCE or PCE predicts the formation of stable oxidized polar organic compounds. These compounds consist of acids, aldehydes, and possibly halo-acetic acids. Three possible mechanisms have been proposed for the formation of by-products due to the irradiation of aqueous solutions containing TCE and PCE. The first is for the formation of formaldehyde, acetaldehyde, and glyoxal, which are formed at a concentration of approximately two orders of magnitude less than the influent solute concentration. Second, the formation of formic acid decreased with increasing radiation dose. The formic acid concentration was found to be higher for PCE than TCE. These results are most probably due to the slower reaction rate constants of PCE with e and OH, compared to TCE. The third possible reaction is the formation of haloacetic acids when TCE and OH react. The mechanism of decomposition of PCE by OH is shown in Equation (12.30) to Equation... 

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