Acrolein, oxidation

Both fixed and fluid-bed reactors are used to produce acrylonitrile, but most modern processes use fluid-bed systems. The Montedison-UOP process (Figure 8-2) uses a highly active catalyst that gives 95.6% propylene conversion and a selectivity above 80% for acrylonitrile. The catalysts used in ammoxidation are similar to those used in propylene oxidation to acrolein. Oxidation of propylene occurs readily at... 

The newest and most commercially successful process involves vapor phase oxidation of propylene to AA followed by esterification to the acrylate of your choice. Chemical grade propylene (90—95% purity) is premixed with steam and oxygen and then reacted at 650—700°F and 60—70 psi over a molybdate-cobait or nickel metal oxide catalyst on a silica support to give acrolein (CH2=CH-CHO), an intermediate oxidation product on the way to AA. Other catalysts based on cobalt-molybdenum vanadium oxides are sometimes used for the acrolein oxidation step. 

The acrolein coordinated with catalyst then gives an acyl radical by abstracting its aldehyde hydrogen. In the general oxidation of aldehydes, the acyl radical is considered to be discontinuing coordination with catalyst, as was described by Bawn (3) and Hoare and Waters (17). However, in the acrolein oxidation, the acyl radical formed by hydrogen abstraction may not conform to this proposal, as described below. 

Kuznetsova etal19 20 show that in the V-Mo-0 system the active phase in acrolein oxidation is VMo3On in which nearly all the vanadium is present as non-e.s.r. active V4+ ions. In several other papers, e.s.r. is also applied to determine the number and type of V4+ions present,21-32 in most cases without closely correlating this with the catalytic properties. 

Ovsitser et al. (2002) TEM, SEM (MoVW)s014 Activation, defect formation, structure-function correlations + + Acrolein oxidation... 

Mo is the essential element of effective catalysts for propene oxidation to acrolein and acrolein oxidation to acrylic acid, while V is an essential element for effective catalysis of acrolein oxidation to acrylic acid. Mo-V-Nb oxide catalysts are capable of activating propane even at 573 K, but yields products of acetic acid, acetaldehyde, and carbon oxides. The addition of Te or Sb to Mo-V-Nb oxides induces certain structural changes leading to the formation of acrylic acid. ... 

A typical catalyst for the acrolein oxidation composed mainly of Mo-, V-, Cu-oxides was prepared according to the patent specification EP 17000. In order to avoid the influence of mass transfer processes on the rate of conversion pellets of egg shell type with a thin active layer of about 200 pm thickness were used for the kinetic measurements. 

The oxidation of acrylic acid also follows the reaction mechanism of Mars-van Krevelen, figure 8. In contrast to the acrolein oxidation no hindrance of reoxidation is observed when the partial pressure of acrylic acid is raised Addition of water vapor causes only a small increase of the rate of acrylic acid oxidation also in contrast to the considerable effect in acrolein oxidation, figure 9. 

Finally the selectivity towards acrylic acid of acrolein oxidation is given for constant concentration of acrylic acid, figure 10. The selectivity is defined as ... 

The kinetics of acrolein oxidation is of first order with respect to acrolein, which can be easily understood by the competition of strongly adsorbed acrylic acid and less strongly adsorbed acrolein. The reduced rate of acrolein oxidation at elevated acrolein concentration is interpreted by adsorption of acrolein on reduced sites and consequently described by a negative reaction order with respect to acrolein in the reoxidation term r(02). In this context the independence of the acrolein oxidation rate from acrylic acid concentration is surprising. This result may be understood by the reaction mechanism proposed by T.V. Andrushkevich in which formed acrylate anions are shifted to sites on vanadium cations and after protonation are desorbed as acrylic acid. Parallel to the protonation the reduced sites are reoxidized. 

The experiments of acrolein oxidation in absence of molecular oxygen confirm a dense adsorption layer of oxidized Cj-compounds, acrylic anion + acrylic acid, figure 11. It is remarkable that despite of the increasing adsorption of the oxidized Cj-compounds the rate of acrolein consumption is hardly reduced. IR analysis of surface intermediates have lead to the assumption that deprotonation of chemisorbed acrolein and oxidation of the intermediate to acrylic anion are fast reactions. Therefore, the protonation of the acrylic anion and the following desorption of acrylic acid are regarded as rate determining steps leading to accumulation of the acrylic anion on the surface of the catalyst [12]. 

Both the rate of acrolein oxidation and the rate of catalyst reoxidation depend on the concentration of acrolein so that also steps towards the formation of acrylic anion must be taken into account as rate determining. In the whole the acrolein oxidation appears as the result of a network of coordinated reaction steps. 

The influence of water on the rate of acrolein respectively oxygen consumption is still included in the reaction rate constant of acrolein oxidation k, and in the constant of catalyst reoxidation klOj),. These constants are listed in the table for different water vapor contents of the reaction gas. 

While at a temperature of 280 °C and above the constant of acrolein oxidation k, is hearly independent of the water content, the reaction rate constant of the reoxidation k(02)i strongly increases with increasing water content. 

It has been shown earlier that the kinetics of selective acrolein oxidation can be described by using a simple network of chemical reactions [7]. CO2, CO and acetic acid generated in small amounts can be lumped together into one pseudo-species ( by-products ). The network consists of the main reaction from acrolein to acrylic acid, a parallel reaction of acrolein to byproducts and the consecutive reaction of acrylic acid to by-products. 

Figure 4 Product distribution and the natural logarithnm of (aoVpo2jtef) as a function of the modified residence time during selective acrolein oxidation. Symbols experimental data, lines calculated after curve fitting by use of rate constants given...

In the case of acrolein oxidation in the presence of cobalt acetyl-acetonate, Co(Acac)3, Table 5 gives the results obtained with different solvents [37]. The influence of solvents on both rate and selectivity may occur in a complex manner. Free acid selectivity depends in particular on the stability of this acid, because the oxidation of acrolein primarily produces acid almost quantitatively. Consequently, in a benzene—nitrobenzene mixture, acid is obtained with an 80% selectivity with conversions of 40% [39,40]. 

We are particularly interested in the Wacker oxidation of cyclohexene as the product, cyclohexanone, is a starting material in the synthesis of caprolactam, which is an intermediate in nylon production. Furthermore, we have strong interest in oxidation of acrolein in particular and acryhc compounds in general. Acrolein oxidation leads to a convenient route to 1,3-propanediol, while methyl acrylate oxidation leads to a starting material for adhesives. 

This strategy could also be applied to acrolein oxidation. Via the sequence of protection, oxidation, hydration, and hydrogenation, acrolein could be converted in high yield into the industrially important 1,3-propanediol. 

Acrylic acid, CH2=CHCOOH, can be produced by a series of processes direct oxidation of acrolein oxidation of ethylene to ethylene oxide, with further reaction with hydrogen cyanide to ethylene cyanhydrin, which is then saponified and dehydrated addition of carbon monoxide and water to acetylene and from acetone by pyrolysis to ketene and addition of formaldehyde to the ketene to produce jS-propiolactone. jS-Propiolactone polymerizes to the corresponding polyester, which depolymerizes at 150 C to acrylic acid ... 

Acrylic acid has traditionally been used as the raw material for acrylic estos, polyacrylates, cross-linked polyacrylates, and copolymers. The global acrylic acid capacity was ca. 4.7 million tons in 2006, with an estimated average growth of 4%. Nowadays, acrylic acid is industrially obtained from a physically separated two-step process with propylene as the starting raw material. Firstly, propylene is selectively oxidized to acrolein at 300-350 C employing multicomponent catalysts based on metallic mixed oxides, i.e. MoBiO, FeSbO, or SnSbO. Then, the acrylic acid is obtained in a second step from acrolein oxidation at 200-260°C using multicomponent catalysts based on Mo-V-W mixed oxides. Thus, an overall acrylic acid yield of 85-90% is reached. 

Tichy, J., Svachula, J., Farbotko, J., etal. (1991). Active Componentof Molybdenum Vanadium Oxide Catalysts in Acrolein Oxidation, Zesz. Nauk. Politech. LodzJca, Chem., 616, pp. 95-106. 

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