Propionaldehyde from oxidation

BASF led the development of a route based on ethylene and synthesis gas. Its four step process begins with the production of propionaldehyde from ethylene, CO, and H2 using a proprietary catalyst mixture that they aren t telling anything about. Reaction with formaldehyde gives methacrolein. The last two steps are the same as above—oxidation with air yields the MAA subsequent reaction with methanol yields MMA. 

Chemical processes dominate the production of short-chain organic acids. The primary route of synthesis employs the "Oxo process (Billig and Bryant 1991). Propionic acid is made by oxo synthesis of propionaldehyde from ethylene, CO, and H2 with a rhodium catalyst. liquid-phase oxidation of the aldehyde yields propionic add. Butyric acid is made by air oxidation of butyraldehyde, which is synthesized by the 0x0 process fi-om propylene, CO, and H2. The triphenylphosphine-modified rhodium 0x0 process, termed the LP Oxo process, is the industry standard for the hydroformylation of ethylene and propylene (Billig and Bryant 1991). Also pure propionic acid can be obtained from propionitrile or by oxidation of propane gas. 

Although the selectivity of isopropyl alcohol to acetone via vapor-phase dehydrogenation is high, there are a number of by-products that must be removed from the acetone. The hot reactor effluent contains acetone, unconverted isopropyl alcohol, and hydrogen, and may also contain propylene, polypropylene, mesityl oxide, diisopropyl ether, acetaldehyde, propionaldehyde, and many other hydrocarbons and carbon oxides (25,28). 

Acrolein is produced according to the specifications in Table 3. Acetaldehyde and acetone are the principal carbonyl impurities in freshly distilled acrolein. Acrolein dimer accumulates at 0.50% in 30 days at 25°C. Analysis by two gas chromatographic methods with thermal conductivity detectors can determine all significant impurities in acrolein. The analysis with Porapak Q, 175—300 p.m (50—80 mesh), programmed from 60 to 250°C at 10°C/min, does not separate acetone, propionaldehyde, and propylene oxide from acrolein. These separations are made with 20% Tergitol E-35 on 250—350 p.m (45—60 mesh) Chromosorb W, kept at 40°C until acrolein elutes and then programmed rapidly to 190°C to elute the remaining components. 

Isomerization and Hydrogenolysis. lsomeri2ation of propylene oxide to propionaldehyde and acetone occurs over a variety of catalysts, eg, pumice, siUca gel, sodium or potassium alum, and 2eohtes (80,81). Stronger acid catalysts favor acetone over propionaldehyde (81). AHyl alcohol yields of 90% are obtained from use of a supported lithium phosphate catalyst (82). 

The most common procedure previously employed to effect the elimination of thiols from thioacetals has been heating in the presence of a protic acid. For example, propionaldehyde diethyl thioacetal is converted to 1-ethylthio-l-propene on heating at 175° in the presence of phosphoric acid. The relatively high temperature and acidic conditions of such procedures are, however, distinct disadvantages of this method. Another approach consists of oxidation of a thioacetal to the mono S-oxide and thermal elimination of a sulfenic acid at 140-150°. ... 

In comparable reaction conditions as Pd +Cu +Y, Pd + and Cu2+ exchanged pentasil and ferrierite zeolites show a different type of activity [31]. The main products formed by propylene oxidation on these catalysts are acrolein and propionaldehyde below 120°C and 2-propanol above 120 C. Above 150°C consecutive oxidation of 2-propano1 to acetone is observed. The catalytic role of Pd and Cu in the 2-propanol synthesis is proposed to follow the Wacker concept. It is striking that when Pd + and Cu2+ are exchanged in 10-membered ring zeolites, oxidation of a primary carbon atoms seems possible, as acrolein and propionaldehyde are obtained from propylene. 

Data for aliphatic aldehyde enolisation are very scarce, probably because the enolisation process is often complicated by oxidation and hydration. Nevertheless, the rate constants for base- and acid-catalysed iodination of R R2CHCHO were determined in aqueous chloroacetic acid-chloroacetate ion buffers (Talvik and Hiidmaa, 1968). The results in Table 4 show that alkyl groups R1 and R2 increase the acid-catalysed reactivity in agreement with hyperconjugative and/or inductive effects. This contrasts with aliphatic ketones for which steric interactions are important and even sometimes dominant. Data for base-catalysis are more difficult to interpret since a second a methyl group, from propionaldehyde to isobutyraldehyde, increases the chloroacetate-catalysed rate constant. This might result from a decrease of the a(C—H) bond-promoted hyperconjugative stabilisation of the carbonyl compound... 

Other routes to MMA start from ethylene, propylene or propyne and involve metal catalysis at some stage of multi-step transformations for example by the hydroformylation of ethylene to intermediate propionaldehyde, oxidation to propionic acid, followed by condensation with formaldehyde. The Pd-catalyzed carbonylation of propyne to MMA is a further method. However only the ethylene route has found some industrial application (see Chapter 4, Section 4.3.1). 

Table 12. Oxidation potentials, Ep, of pora-substituted 2-aryl-propionaldehydes 88 and their tautomeric enols 89 in acetonitrile/DMSO [171] compared with calculated , 2 (89)- The enol content was taken from Ref. [187]...

Fig. 6. The variation of initial percentage yields with surface at 270 and 300 °C. isobutene , propionaldehyde , acetone cp, propene 6, isobutyraldehyde , methacrolein >, acetaldehyde , isobutene oxide. (From ref. 44.)...

Fig. 25. The variation with initial pressure of the initial percentage yield of products from the oxidation of isobutane at 310 °C. Isobutane oxygen = 1 2 volume of reaction vessel = 500 cm. isobutene , acetaldehyde O, propionaldehyde propene t>, fcrf-butyl hydroperoxide , isobutene oxide o, acetone.

Thus there are compelling reasons for regarding the initiation step (la) as a low activation energy heterogeneous process at low temperatures. In any case from estimates of ki (homogeneous) for acetaldehyde and propionaldehyde made by Baldwin et al. [21, 22] a value for ja for 40 kcal. mole" was calculated. Although this may represent an upper limit (see also Sect. 4.4), it is obvious that the rate of (la) in the gas phase below 200 °C is far too low to allow the oxidation to get started. 

Fig. 28. Typical pressure—time curves for the propionaldehyde oxidation at 440 °C using boric acid coated vessels. O2, 30 torr C2H5CHO (torr) , 1 x, 2 o, 4 v, 6. O2,8 torr C2 H5 CHO (torr) A, 4. (From ref. 22 by permission.)...

Fig. 29. Variation of the maximum rate of propionaldehyde oxidation with temperature using boric acid coated reaction vessels. (From ref. 22 by permission.)...

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