Acetaldehyde, formation

H2 production technologies based on natural gas. Operating the reaction at relatively lower temperature, between 300 and 450 °C could minimize the CO formation because the equilibria for WGS and CO oxidation reactions are thermodynamically more favorable at lower temperatures. In order to achieve this goal, highly selective catalysts that are specific for reforming via acetaldehyde formation rather than ethanol decomposition to CH4 and/or ethylene are required. The success in the development of ethanol-based H2 production technology therefore relies on the development of a highly active, selective and stable catalyst. 

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. 

Concurrent with acetic anhydride formation is the reduction of the metal-acyl species selectively to acetaldehyde. Unlike many other soluble metal catalysts (e.g. Co, Ru), no further reduction of the aldehyde to ethanol occurs. The mechanism of acetaldehyde formation in this process is likely identical to the conversion of alkyl halides to aldehydes with one additional carbon catalyzed by palladium (equation 14) (18). This reaction occurs with CO/H2 utilizing Pd(PPh )2Cl2 as a catalyst precursor. The suggested catalytic species is (PPh3)2 Pd(CO) (18). This reaction is likely occurring in the reductive carbonylation of methyl acetate, with methyl iodide (i.e. RX) being continuously generated. 

Scheme III. Bimolecular Reaction Scheme for Acetaldehyde Formation...

Nakajima, S., Y. Hagiwara, H. Hagiwara, and T. Shibamoto. Effect of the antioxidant 2 -0-glycosylisovi-texin from young green barley leaves on acetaldehyde formation in beer stored at 50C for 90 days. J Agr Food Chem 1998 46(4) 1529-1531. 

Miyake, T., and T. Shibamoto. Inhibition of malonaldehyde and acetaldehyde formation from blood plasma HV092 oxidation by naturally occurring anti-... 

Figure 3. Effects of temperature on the selectivities of ethylene and acetaldehyde formation in the C2H6 + N2O reaction on silica supported metal molybdates.

Rb2Mo04 This catalyst was found to be much more active and selective than the divalent metal molybdates. As shown in Figure 2, very little decay was obsei ved in the conversion of ethane. In contrast with the previous catalysts, acetaldehyde was the main product of partial oxidation at 823 K it was fornied with a selectivity of 23-24%. The selectivity for ethylene was 10-13%. As it appears from Figure 2, the yield of acetaldehyde formation was about 5 times higher than on Mo03/Si02... 

With decrease of the reaction temperature, no change was experienced in the selectivity for ethylene, but there was an increase in the selectivity of acetaldehyde formation. The oxidation of ethane has been also investigated on Mo03/Si02. Under these conditions, this catalyst was found to be very active for the total oxidation of ethane. At 510 K, the conversion of ethane was 21%, the products of partial oxidation were formed only in trace amount. 

Rb2Mo04 This catalyst was found to be much more active than the divalent metal molybdates. Although the catalyst underwent a significant activity loss, even in the steady state the conversion was about 9.0%. The selectivities for ethylene and acetaldehyde formation in this state was 46% and 7.1%. Some data for the oxidation of ethane on molybdates with O2 are also listed in Table II. 

A somewhat higher conversion was measured for Rb2Mo04 and the selectivity of acetaldehyde formation was also higher (76.8%). Interestingly, no ethylene or methane formation was detected. 

In a separate investigation MargeHs and Roginekii1107 carried nut catalytic oxidation of ethylene at 350° over vanadium pentoxidc. reportedly similar to metallic silver in catalytic properties. TVv asoertainod that carbon dioxide was formed faster from, ethylene oxide, or from acetaldehyde under comparable conditions, than from ethylene itself. Further, they noted the formation of carbon monoxide, and determined that its rate of formation was considerably greater than that of carbon dioxide, increasing still more in the presence of adtk-d ethylene oxide. The addition of ethylene oxide also appeared to depro both ethylene oxide and acetaldehyde formation. They concluded that reactions leading to carbon dioxide and water did not proceed by wav of ethylene oxide, but by way of some other intermediates, and tlmt-this process could occur either on the catalyst surface or in the gas phase. 

Fig. 8.3 Profiles of acetaldehyde formation for photochemical reaction of C3H6-N02-diy air in the presence of various metal oxides at ambient temperature.(a) blank, CoO, NiO, Cr203, (b) Fe203, Sn02, (c)Zr02, W03, (d)ZnO, (e)Ti02, Ce02. Initial concentrations ot C3H6 and N02 are 200 and 100 ppm,respectively.

Some acetaldehyde formation is observed even under rigorously anhydrous conditions.535 537-539 Indeed, separate experiments show that Pd(II) salts catalyze the reaction of vinyl acetate with acetic acid to produce acetaldehyde and acetic anhydride, that is,... 

Figure 9.13. The acetaldehyde formation mechanism, where A and B are Lewis acid sites and Bronsted basic sites, respectively. Dehydration requires the combination of an acid and strong base site with an adjacent strong basic site. After Di Cosimo et al. [184].

The Wacker process was a major landmark and a great push towards the development of homogeneous catalysis. The mechanism of acetaldehyde formation differs fundamentally from the other oxidation processes as O2 itself is not directly involved. As is clear from Figure 28 the actual oxidant is Pd(II) which is reduced to Pd(0). The intimate pathway of the reaction involves nucleophilic attack and was the subject of much debate. 

After biochemical conversion of glucose to pyruvic acid intermediate, the next step in ethanol synthesis is nonoxidative decarboxylation and acetaldehyde formation catalyzed by a native decarboxylase, and then acetaldehyde reduction to ethanol catalyzed by a native dehydrogenase. 

Olefin complexes can often be prepared in solution by adding the olefin to a soluble Pd(II) salt. Thus NaaPdCl, in HOAc, will absorb ethylene reversibly to give solutions of a different color than the original solution of the Pd(II) salt (106). In some cases the intermediate tt complex can be detected in catalytic systems. Thus, in the oxidation of ethylene to acetaldehyde, formation of the intermediate tt complex, according to the following equilibrium... 

Glycol derivatives, e.g., 2-chloroethanol (eq. (31)) are to a small extent byproducts in the technical olefin oxidation. With a very high concentration (ca. 5 mol/L) of cupric chloride and high pressure, 2-chloroethanol is the main product of ethylene oxidations, besides some acetaldehyde [33]. Cupric chloride is essential. In its absence, in spite of a high chloride ion concentration absolutely no 2-chloroethanol is obtained. It is assumed that analogously to acetaldehyde formation a y9-hydroxyethyl species bonded to a bi- or oligo-Pd-Cu cluster is an intermediate from which 2-chloroethanol is liberated by reductive elimination. 

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