Isobutyric acid, dehydrogenation

Dehydrogenation of Propionates. Oxidative dehydrogenation of propionates to acrylates employing vapor-phase reactions at high temperatures (400—700°C) and short contact times is possible. Although selective catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid have been developed in recent years (see Methacrylic ACID AND DERIVATIVES) and a route to methacrylic acid from propylene to isobutyric acid is under pilot-plant development in Europe, this route to acrylates is not presentiy of commercial interest because of the combination of low selectivity, high raw material costs, and purification difficulties. 

The contrast between the two types is also found for organic reactions. The reduction of H3PM012O40 by dehydrogenation of isobutyric acid or cyclohexene belongs to the bulk type II classification, and the reduction by oxygenation of methacrolein or acetaldehyde is a surface reaction. For example, when H3PM012O40 is reduced by repeated pulses of isobutyric acid and methacrolein... 

Bulk type 11 Oxidation of H2, oxidative dehydrogenation of cyclohexene, isobutyric acid. 

This reaction is another possible route for the production of methacrylic acid, since isobutyric acid can be obtained by an oxo process from propene and CO. Heteropoly compounds and iron phosphates are so far the most efficient catalysts for the reaction. The favorable role of the presence of an a-methyl group is remarkable for oxidative dehydrogenation, as the heteropoly compounds are not good catalysts for the dehydrogenation of propionic acid (338, 339). 

In the dehydrogenation of isobutyric acid, the by-products in addition to CO and C02 are propylene and acetone. Two reaction mechanisms were proposed (340, 341) and the latter is shown in Scheme 9 (340). The formation of methacrylic acid and acetone involves a common intermediate The El elimination of a proton from I yields the methacrylic acid while a nucleophilic SN1 attack of oxide ion produces C02 and acetone (344). On the other hand. 

For acid catalysis, the rates of bulk-type reactions show close correlations with the bulk acidity, while the catalytic activities for surface-type reactions are related to the surface acidity which is sensitive to the surface composition and often change randomly. Similarly, in the case of oxidation catalysis, good correlations exist between the oxidizing ability of catalyst and the catalytic activity for oxidation in both bulk-type and surface-type reactions. Acid and redox bifunctionality is another characteristic of HPAs. For example, the acidity and oxidizing ability work cooperatively for the oxidation of mcthacrolcin, whereas they function competitively for the oxidative dehydrogenation of isobutyric acid [5]. Interestingly, the former is of surface type and the latter of bulk type. 

Attempts have been made to relate spectroscopic quantities and catalytic behavior. Kim et al. (2007) correlated the performance of heteropoly acid catalysts in isobutyric acid oxidative dehydrogenation (ODH) to the position of the absorption edge after treatment at 603 K. 

Another route to methacryhc acid is via oxidative dehydrogenation of isobutyric acid (equation 13). This reaction is catalyzed by molybdovanadophosphoric acid (H3+ PMoi2- V 04o n = 0-3), whose redox potential and acidity are well-balanced for effecting this reaction. The acidity is necessary, although excess acidity accelerates the decomposition of isobutyric acid into CO and propene. 

The acidity and oxidizing ability work cooperatively for oxidation of methacrolein, while they function competitively for oxidative dehydrogenation of isobutyric acid. These two reactions also differ in that the former is a surface-type and the latter a bulk-type. From this standpoint, different considerations in effective catalyst design are necessary. 

Iron phosphates catalyze the oxidative dehydrogenation of isobutyric acid to methacrylic acid. The initial Fe or Fe phase is converted to the mixed valent orthophosphate Fe7(P04)6, but the catalyticaUy active phase may be a diphosphate Fe3(P2(>7)2. 

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. 

Other technologies, already commercially applied or under development, are summarized in Figure 2.63b. Alternative routes of synthesis include (i) ethene hydroformylation to propionaldehyde, which then forms methacrolein by condensation with formaldehyde methacrolein is then oxidized to methacrylic acid (BASF process) (ii) isobuthyraldehyde conversion into isobutyric acid and then oxidative dehydrogenation to methacrylic add (Mitsubishi Kasei/Asahi process) and (iii) oxidation of terf-butyl alcohol to methacrolein followed by oxidation to methacrylic acid and esterification. 

In the preceding studies [2-4], it was found that iron phosphate catalysts show a high selectivity in the oxidative dehydrogenation of compounds in which the carbon atom at the a-position of an electron-attracting group (X) such as -COOH, -CHO, or CN, is tertiary for example, isobutyric acid, isobutyraldehyde, and isobutyronitrile, but that they are inactive for oxygen insertion reactions. 

Oxydative dehydrogenation of isobutyric acid to methacrylic acid over heteropolysalts of composition Kx(NH4)3-xPMoi204o effect of catalyst pretreatment and composition on the activity and selectivity. 

Acid-catalyzed addition of carbon monoxide to propylene gives isobutyric acid, which is dehydrogenated to methacrylic acid [466,467]. 

Desquilles, C., Bartoli, M.J., Bordes, E., Courtine, P., and Hecquet, G. Oxidative dehydrogenation of isobutyric acid by heteropoly compounds — effect of alkah containing supports. JnProceedings ofthe DGMK-Conference on Selective Oxidations in Petrochemistry, Erdoel Erdgas Kohle, Vol. 9204, 1992, 69—79. 

Millet, J.M.M. and Vedrine, J. Role of cesium in iron phosphates used in isobutyric acid oxidative dehydrogenation. Appl Catal. 1991, 76, 209-219. 

Isobutyric acid can be converted to methacrylic acid by oxidative dehydrogenation using heteropoly compounds. In this reaction, suppression of the catalyst acidity which accelerates the decomposition of isobutyric acid to propylene and CO is necessary to improve the selectivity for methacrylic acid. 

Methyl-3-hydroxybutyryl-CoA is oxidized to 2-methylacetoacetyl-CoA, which is then metabolized by a )8-ketothiolase to acetyl-CoA and propionyl-CoA. 3-Hydroxyisobutyryl-CoA is deacylated and the 3-hydroxy-isobutyric acid is dehydrogenated to methylmalonic semialdehyde by a NAD-dependent reaction (Robinson et al., 1957 Robinson and Coon, 1957 Tanaka, 1975). The methylmalonic semialdehyde is believed to be decarboxylated to form propionaldehyde prior to oxidation to propionyl-CoA via an aldehyde oxidase (Tanaka etal, 1975 Baretz and Tanaka, 1978). 

This transformation has been used widely to prepare optically active p-hydroxycarboxylic acids. This process takes place in two stages in initial dehydrogenation to the a,p-unsat-urated carboxylic acid and subsequent hydration. These steps utilize the enzymes of the P-oxidation pathway of lipid cataboUsm, and so the P-hydroxy acids produced are generally of the natural S) form. Both saturated carboxylic acids and their a,P unsaturated counterparts have been used as raw materials. For example, P-hydroxypropionic acid (HPA) has been prepared from acrylic acid through a process mediated by Fusarium [17], and Pseduomonas putida has been used to prepare (5)-P-hydroxyisobutyric acid from isobutyric acid [18]. The preparation of C -C (5)-P-hydroxycarboxylic acids from the corresponding tran -a,p-unsaturated carboxylic acids by microbial hydration catalyzed by resting cells of Mucor sp. has also been reported [19] (Scheme 7). 

Dehydrogenation to methyl methacrylate of the isobotyrate produced by the carbonyiation of propylene in the presence of methanol and a complex of rhodium, cobalt or palladium. The isobutyrate is converted with catalysts based on molybdenum, tungsten, vanadium, phosphomoiybdic acids and possibly heteropolyacids. It has been developed chiefly by Asahi, BASF, Eastman, Mitsubishi, Mobil, foray, etc. 

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