Acrolein and Acrylic Acid

Acrolein and Acrylic Acid. Acrolein and acrylic acid are manufactured by the direct catalytic air oxidation of propylene. In a related process called ammoxida-tion, heterogeneous oxidation of propylene by oxygen in the presence of ammonia yields acrylonitrile (see Section 9.5.3). Similar catalysts based mainly on metal oxides of Mo and Sb are used in all three transformations. A wide array of single-phase systems such as bismuth molybdate or uranyl antimonate and multicomponent catalysts, such as iron oxide-antimony oxide or bismuth oxide-molybdenum oxide with other metal ions (Ce, Co, Ni), may be employed.939 The first commercial process to produce acrolein through the oxidation of propylene, however, was developed by Shell applying cuprous oxide on Si-C catalyst in the presence of I2 promoter. 

When a mixture of propylene, air, and steam (1 8-10 2-6) is reacted in the vapor phase (300-400°C, 1-3 atm) acrolein is formed in about 85% yield in an exothermic process898,915 [Eq. (9.173)] 5-10% of acrylic acid may also be isolated  

Oxidation in the original Sohio process941,942 was carried out over a bismuth molybdate catalyst, which was later superseded by bismuth phosphomolybdate with various amounts of additional metal ions (Ce, Co, Ni), and multicomponent metal oxides based on Mo, Fe, and Bi supported on silica. 

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Degenerate Explosion it was a free radical autocatalytic process and control was difficult, but manageable. The main disadvantage was that it produced as much or more acrolein as propylene oxide. Because no market existed for acrolein at that time, the project was abandoned. Within two years, the acrylic market developed and a new project was initiated to make acrolein and acrylic acid by vapor-phase catalytic oxidation of propylene. 

Much like the oxidation of propylene, which produces acrolein and acrylic acid, the direct oxidation of isobutylene produces methacrolein and methacrylic acid. The catalyzed oxidation reaction occurs in two steps due to the different oxidation characteristics of isobutylene (an olefin) and methacrolein (an unsaturated aldehyde). In the first step, isobutylene is oxidized to methacrolein over a molybdenum oxide-based catalyst in a temperature range of 350-400°C. Pressures are a little above atmospheric ... 

The two bands at around 1700 cm may be reasonably attributed to Vc=o two different adsorbed species, probably acrolein and acrylic acid. In this compound, in fact, the vc=o band is found at a frequency about 20 cm higher than in acrolein (9). In these adsorbed compounds [for example, on V-Mo oxides (9)], the vc=c band is expected at a nearly the same value as in 7C-bonded propylene (around 1625 cm ), whereas other IR active bands are covered by the stronger bands due to physisorbed propane. A more clear identification of the above species, therefore, is not possible. The shoulder at about 1425 cm" may be attributed to VsCOO in adsorbed acrylate, but the VasCOO band expected at around 1550 cm is absent. A more reasonable interpretation is the formation of alkene oligomers. In fact, propene adsorbed on HNaY gives rise to the formation of a main band at about 1460 cm (9), apart from vch 5ch bands that, in our case, are covered by the band of physisorbed propane. However, all adsorbed species are removed by evacuation, indicating their weak interaction with the surface. 

Oxidation of the allylic carbon of alkenes may lead to allylic alcohols and derivatives or a, 3-unsaturated carbonyl compounds. Selenium dioxide is the reagent of choice to carry out the former transformation. In the latter process, which is more difficult to accomplish, Cr(VI) compounds are usually applied. In certain cases, mixture of products of both types of oxidation, as well as isomeric compounds resulting from allylic rearrangement, may be formed. Oxidation of 2-alkenes to the corresponding cc,p-unsaturated carboxylic acids, particularly the oxidation of propylene to acrolein and acrylic acid, as well as ammoxidation to acrylonitrile, has commercial importance (see Sections 9.5.2 and 9.5.3). 

Methacrolein and Methacrylic Acid. A two-stage technology, essentially the same as the propylene oxidation process for the manufacture of acrolein and acrylic acid, was developed to oxidize isobutylene to methacrolein and methacrylic acid 949-951 Two different molybdenum-based multicomponent catalysts are used. In a typical procedure949 isobutylene is reacted with excess steam and air (5 30 65) at about 350°C to produce a mixture of methacrolein and methacrylic acid with 80-85% selectivity at a conversion of 98%. In the second stage this reaction mixture is oxidized at slightly above 300°C to yield methacrylic acid (80% selectivity at >90% conversion). 

Heterogeneous oxidative processes operate at high temperatures (250-450 6C) and are useful for the synthesis of acrolein and acrylic acid from propylene over bismuth molybdate catalysts, the synthesis of maleic and phthalic anhydrides from the oxidation of benzene (or C4 compounds) and naphthalene (or o-xylene) respectively over vanadium oxide,101 arid the synthesis of ethylene oxide from ethylene over silver catalysts.102... 

Glyoerol3 gives both glyceric aldehyde and trioxymethylene in sulphuric acid with carbon and platinum anodes, but in alkaline solution acrolein and acrylic acid are formed. 

FIGURE 1 Manufacture of acrolein and acrylic acid by oxidation of propylene. 

Acrolein and Acrylic Acid from Propylene for Super-Absorbent Polymers, Paints, and Fibres... 

Acrolein and acrylic acid are both made by vapor phase oxidation of propylene. U.S. 6,281,384 (to E. I. du Pont Nemours and Atofina) describes a fluidized bed process, while U.S. 5,821,390 (to BASF) describes an isothermal reactor cooled by heat transfer to a molten salt. U.S. 6,858,754 and U.S. 6,781,017 (both to BASF) describe alternative processes based on a propane feed. Compare the economics of acrylic acid production from propane with production from propylene. Is the conclusion different if the process is stopped at acrolein ... 

Meanwhile, acetaldehyde is scarcely oxidized by Pd to acetic acid under these conditions. A more pronounced difference is observed for propylene in acidic aqueous solution of the Pd-561 cluster propylene is converted successively to allyl alcohol, acrolein, and acrylic acid. In contrast to the reaction mediated by Pd only traces of acetone are found. 

However, catalysis of Diels-Alder reactions is satisfactory only with specific dienophiles, for example, the two cited in the examples, acrolein, and acrylic acid. Another limitation is that some dienes polymerize in the presence of a Friedel-Crafts catalyst. 

Multitubular reactors are mainly used in gas-phase partial oxidation processes, such as the air oxidation of light olefins, paraffins, and aromatics. Examples of chemistries where these reactors are used include the partial oxidation of methanol to formaldehyde, ethylene to ethylene oxide, ethylene and acetic acid to vinyl acetate, propylene to acrolein and acrylic acid, butane to maleic anhydride, isobutylene to methacrolein and methacrylic acid, and o-xylene to phthalic anhydride. An overview of the multitubular reactor process for the partial oxidation of n-butane to maleic anhydride is given here. 

The Diels-Alder reaction (diene synthesis) is the addition of compounds containing double or triple bonds (dienophiles) to the 1,4 positions of conjugated dienes with the formation of six-membered hydroaromatic rings. Hydrocarbons most often used in the reaction are 1,3-butadiene, cyclopentadiene, and isoprene, and dienophiles used include maleic anhydride, acrolein, and acrylic acid. The literature on this process is thoroughly reviewed by Alder (1), Kloetzel (59), Holmes (48), and Norton (82). 

Partial oxidation of propane was investigated in the presence of molybdenum oxide based catalysts. We have shown the existence of a synergetic effect between the two phases aNiMo04 and aMoOs. Indeed activity and selectivity towards acetic acid and acrylic acid were maximal with a ratio aMo03 / (aNiMo04 + aMoOj) close to 0.25. These results could be explained by an interaction and a mutual covering of the two phases. The addition of bismuth to these mixed systems led to a total or a partial inhibition in the production of acetic acid and an increase in the formation of acrolein and acrylic acid. 

In our laboratory we showed that Bi-Mo (W,V,Ti) Aurivillius or Sillen phases and bismuth-molybdenum oxides supported on titania were more selective for the P.O.D. to propene [8,9]. In this field our objective was to improve the activity of molybdenum based materials either for the P.O.D. to propene or the selective oxidation of propane to acrolein and acrylic acid. For these reasons we have focused on the catalytic performances of NiMo04 Mo03 phases doped with bismuth. 

The results of Table 6 showed that the addition of a very small amount of bismuth increases significantly the selectivity towards acrolein and acrylic acid with no change in the propane conversion. The propene formation and the acetic acid production decreased at the same time, which is quite an important result of the effect of bismuth on the reaction scheme. 

Like in the preceding example adding bismuth to the [Nio sMoO ] catalyst increases the selectivity towards acrolein and acrylic acid and decreases the formation of acetic acid and of propene (Table 7). 

Under the specific conditions used in our experiment, a selectivity of about 50% towards oxygenated compounds, containing mainly acrolein and acrylic acid, was obtained for a propane conversion of about 15% and for complete conversion of oxygen. 

Acetic acid formed via isopropanol and acetone could involve Mo " species. With the second way, acrolein and acrylic acid are obtained with the participation of Mo species. When the temperature increases the first way of the scheme is favored owing to a surface restructuration of the oxide Mo 03 which can contain pentacoordinated molybdenum species. The addition of bismuth to these phases decreases slightly the activity, the formation of acetic acid being supressed. When water is added to the reagent stream, the activity does not change, but the desorption of oxygenated compounds is favored and the selectivity towards acids enhanced. 

The catalyst is reduced by the reactants acrolein and acrylic acid and the reduced catalyst is reoxidized by oxygen from the gas phase. For the steady state of reoxidation and oxygen transfer the following set of kinetic equations results. 

The parameters cj), and cjij characterize the oxidation state of the catalyst. Because of the different reaction rates r, + rj and r, also different degrees of oxidation are assumed, which does not necessarily mean that specific sites for the oxidation of acrolein and acrylic acid exist. 

CsHjoOj Combustible liquid. Forms explosive mixture with air [explosion limits in air (vol %) 1.6 to uel unknown flash point 149°F/65°C Fire Rating 1]. Unless inhibited (200 ppm hydroquinone recommended), polymerization may occur avoid exposure to high temperatures, ultraviolet light, free-radical initiators. Reacts with water with release of heat may not be violent if not contained. Strong oxidizers may cause fne and explosions. Reacts violently with sodium peroxide, uranium fluoride. Incompatible with strong acids, nitrates. Incompatible with sulfuric acid, nitric acid, caustics, aliphatic amines, isocyanates, boranes. Thermal decomposition releases toxic acrid fumes of acrolein and acrylic acid. On small fires, use dry chemical powder (such as Purple-K-Powder), water spray, alcohol-resistant foam, or CO2 extinguishers. 

The effect of contact time was studied at 340 "C, by changing the contact time from 0.5 s to 2.5 s. Figure 3 shows the catalytic performance of CS2.5H1.5PV1M09W2O40, taken as an example. Conversion increased linearly as expected, while selectivity to propene decreased. Selectivities to acetic acid, CO and CO2 increased regularly while those to acrolein and acrylic acid reached a maximum, indicating that they were further oxidised to COv. 

The extrapolation of the curves at zero contact time shows that propene is the primary product. Acrolein and acrylic acid appear at non zero contact time which indicates that these compounds are secondary products. Their selectivities present a maximum with contact time, which shows that they are further transformed to COx. The variations of acetic acid formation show that it is a secondary product formed by another route than acrolein and acrylic acid. COx products show behaviour of typical end products, which are formed from the C-C cleavage of propane and the further total oxidation of acetic and acrylic acids. Thus, a reaction scheme can be proposed... 

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