Acrolein production

The oxidation of propene to acrolein has received much attention for several reasons. Firstly, the process is of industrial importance in itself, and it is also a suitable model reaction for the even more important, but at the same time more complicated, ammoxidation. Secondly, propene oxidation is, in many aspects, representative of that of a class of olefins which possesses allylic methyl groups. Last, but not least, the allylic oxidation is a very successful example of selective catalysis, for which several effective metal oxide systems have been discovered. The subject has therefore attracted much interest from the fundamental point of view. 

The oxidation process is carried out in the temperature range 300— 450°C, and generally studied at atmospheric pressure. Excess air is usually applied (with some exceptions) and substantial amounts of water vapour may be added to the feed. High initial selectivities ( 95%) are feasible, and, although some further oxidation (combustion) of the product is unavoidable, yields of 70—90% are reported in the patent literature. The main by-products are carbon oxides, in addition to minor amounts of acrylic acid, acetaldehyde and formaldehyde. Acrylic acid may be a main product depending on specific catalyst properties and reaction conditions, as described in more detail in Sect. 2.2.3. 

The allylic oxidation of propene is catalyzed by (compound) metal oxides, which essentially contain metal ions of variable valency. It is commonly accepted that a redox mechanism is operative in such a way that the catalyst acts as the oxidizer and that lattice oxygen is incorporated in the oxidation products. The assumptions have been proved for several catalysts by the analysis of cation valency changes and by experiments with labelled oxygen. 

The reaction between propene and the catalyst is, in general, rate-determining, as catalyst reoxidation is a relatively fast reaction. This implies that the degree of catalyst reduction under steady state reaction conditions is fairly low (i.e. less than 10% with respect to the total amount of oxygen that can be removed with propene). Thus the observed kinetics 

For many cases, the kinetics are adequately represented by a simple first-order model, viz. 

---

The significance of industrial acrolein production may be clearer if one considers the two major uses of acrolein—direct oxidation to acryUc acid and reaction to produce methionine via 3-methyhnercaptopropionaldehyde. In acryUc acid production, acrolein is not isolated from the intermediate production stream. The 1990 acryUc acid production demand in the United States alone accounted for more than 450,000 t/yr (28), with worldwide capacity approaching 1,470,000 t/yr (29). Approximately 0.75 kg of acrolein is required to produce one kilogram of acryUc acid. The methionine production process involves the reaction of acrolein with methyl mercaptan. Worldwide methionine production was estimated at about 170,000 t/yr in 1990 (30). (See Acrylic ACID AND DERIVATIVES AmINO ACIDS, SURVEY.)... 

Again, points on the curve were the measured acrolein production rates, and the line is the predicted production rate based on the current and the stoichiometry according to eq 9. At higher conversions, we observed significant amounts of CO2 and water, sufficient to explain the difference between the acrolein production and the current. It should be noted that others have also observed the electrochemical production of acrolein in a membrane reactor with molybdena in the anode. The selective oxidation of propylene to acrolein with the Cu—molybdena— YSZ anode can only be explained if molybdena is undergoing a redox reaction, presumably being oxidized by the electrolyte and reduced by the fuel. By inference, ceria is also likely acting as a catalyst, but for total oxidation. 

Acrolein Production. Adams et al. [/. Catalysis, 3,379 (1964)] studied the catalytic oxidation of propylene on bismuth molybdate catalyst to form acrolein. With a feed of propylene and oxygen and reaction at 460°C, the following three reactions occur. 

At lower temperatures the Mars-van Krevelen mechanism no longer applies. Sancier et al. (440) studied propylene oxidation in the presence of 1802 over bismuth molybdate and found that the acrolein product contained 180 and not exclusively leO from the oxide lattice in contrast with results obtained by Keulks and co-workers (441, 442) at higher temperatures. This lower-temperature oxidation must involve adsorbed oxygen in some form but the nature is not clear. It is now accepted that not all these oxidation reactions do involve lattice oxygen (442,443). 

Kinetics. The kinetics of the oxidation of propene over bismuth molybdate follow the general lines described above [Sect. 2.2.2(a)]. As acrylic acid, acetic acid and formaldehyde are minor by-products, a simple scheme well suited to describe the acrolein production is... 

Many authors assume that the initial reaction step in the dimerization is identical with that in the acrolein production, namely hydrogen abstraction and formation of an allylic intermediate. Dimerization is then supposed to occur because the ability to oxidize the allyl radical to acrolein is absent. 

Ott, L., Bicker, M. and Vogel, H. 2006. Catalytic Dehydration of Glycerol in Sub- and Supercritical Water A New Chemical Process for Acrolein Production. Green Chem., 8, 214-220. 

Acrolein Zirconium and niobium mixed oxides have been shown to catalyze the dehydration of glycerol to acrolein, at 300°C in the presence of water with high selectivity (72%) at nearly total glycerol conversion [50]. Silica-supported niobia catalysts can also be used with similar catalytic performance [51]. Catalytic results for small-sized H-ZSM 5 zeolites showed that the high density of Bronsted acid sites favors acrolein production [52]. Acrolein production from glycerol has also been carried out in subcritical water at 360°C and 34 MPa with catalytic quantities of ZnS04 (791 ppm [g/g]) [52],... 

Methacrylic acid is produced by a number of different processes, one of which is based on the oxidation of isobutene (or of t-butyl alcohol) via the intermediate formation of methacrolein (Equation 32). The general features and the catalyst for the first-stage process are not dissimilar to those for acrolein production, whereas the oxidation of methacrolein to MMA differs in that it is catalyzed by... 

This work has been repeated by many workers using a variety of batch and flow reactors and with binary and multicomponent catalysts, and in all circumstances the essential result has been confirmed, namely the oxygen which appears in the acrolein product is inserted via the lattice and not directly from the gas phase. 

A similar lack of clarity pervades other areas concerning the relationship between the catalytic performance and fundamental properties of the catalysts. Wakabayashl et al. (10) reported that the optimized conversion of propylene to acrolein (>7%) over alumina-supported tin-antimony oxide (3 1) was dependent on the sintering temperature of the catalyst and was maximized after heating at 10(X)°C for 3 hr. Further work (22) showed that both electrical conductivity and surface area were maximized in the material containing 3% antimony and a close association between acrolein production and solid solution formation was suggested. 

TABLE E3.2 Acrolein Production Versus Carbon Deposition (mol h )... 

FIGURE E3.2Sa Carbon Deposition Versus Acrolein Production... 

Solution 3.2a. The first step in examining this problem is to plot carbon deposition (the response variable) as a function of acrolein production rate, which is shown in Figure E3.2Sa. 

The graph shows an approximately linear relationship between acrolein production and carbon deposition (as measured by the CO c evolution during the regeneration step). Note that the data does not pass through the origin, although it should evidently, when no acrolein is produced, no carbon is be deposited on the surface (unless all of the glycerol forms coke). 

FIGURE E3.2Sb Carbon Deposition versus Acrolein Production with Regression Line... 

---