Acetaldehyde decomposition

Figure 10-1 Sketch of the chain reaction for acetaldehyde decomposition. The chain cycles between CHs and CH3CO" radicals in the propagation steps and is fed by acetaldehyde and terminated by methyl recombination.

Students may have seen the acetaldehyde decomposition reaction system described as an example of the application of the pseudo steady state (PSS), which is usually covered in courses in chemical kinetics. We dealt with this assumption in Chapter 4 (along with the equilibrium step assumption) in the section on approximate methods for handling multiple reaction systems. In this approximation one tries to approximate a set of reactions by a simpler single reaction by invoking the pseudo steady state on suitable intermediate species. 

This generic chain reaction can be sketched similarly to the acetaldehyde decomposition reaction as shown in Figure 10-2. The circular chain propagates itself indefinitely with a rate rp once initiated by rate ri, but it is terminated by rate ri, and in steady state Ti and rt control how fast the cycle runs. The overall reaction rate is controlled by the concentration or the chain-propagating radical Cr because this controls how many molecules are participating in the chain. This is why r and rt are so important in deterniining the overall rate. 

This reaction sequence is similar to that described for acetaldehyde decomposition to methane and carbon monoxide Reactions that produce stable products actually occur only... 

Following the reports of the effect of nitrogen doping anatase, visible light photocatalysis has also been reported for SrTiOs (NO elimination), MOx-ZnO (where M = W, V or Fe for acetaldehyde decomposition), and TaON, 135,136 methanol oxidation ). 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

The fact that there is a significant increase in the rate of methane formation shows that the NO is providing some entirely different mechanism for CH4 production. In this connection, it is interesting that in the ethane pyrolysis these is no HCN, which is formed in significant amounts in the acetaldehyde decomposition. A likely source of HCN is... 

It was proved by a separate experiment that isotope mixing in a mixture of methane and methane-i proceeds very slowly even above 600 °C. Thus, it must be concluded that, in the pyrolysis, the formation of the partially deuterated methanes is a result of free radical reactions and not of the secondary exchange of the methanes. Consequently, these results support the free radical mechanism of the acetaldehyde decomposition. 

Any mechanism intended to describe the kinetics of acetaldehyde decomposition should be in accord with the experimentally well established f initial reaction order. It can be readily seen that the o = f requirement excludes certain possibil-ites as chain initiation and termination steps . 

In 1934 Rice and Herzfeld assumed second-order termination, whereas some years later Taylor and Burton suggested a third-order recombination of methyl radicals in the acetaldehyde decomposition. Conclusions derived from other systems are also contradictory. The results of Laidler and Wojciechowski on ethane pyrolysis favour third-order combination. However, more recent investigations by Quinn , as well as by Trenwith , on the kinetics of ethane pyrolysis at around 500 °C and 100 torr pressure, show that the methyl recombination is practically second order. From the work of Lin and Back it follows that, at 550 °C, the recombination rate coefficient becomes pressure-dependent below approximately 200 torr. 

At low pressures the products are those of the acetaldehyde decomposition. At high pressures yields of acetaldehyde (vibrationally stabilized) are increased. A biradical mechanistic interpretation and review of this reaction has been made by Benson . 

Termination reactions destroy free radicals. The major termination reaction postulated for the acetaldehyde decomposition is termination by combination ... 

The acetaldehyde decomposition discussed in Section 2.5.3 is observed to produce minor amounts of ethane as expected from the proposed termination mechanism. Minor amounts of hydrogen are also observed. A postulated mechanism adds two chain transfer reactions to account for the hydrogen ... 

Nonintegral Order. A nonintegral-order rate equation, such as that for acetaldehyde decomposition [the first reaction of Table 1.1], does not fit into the pattern expected for the rates of true elementary steps, but it is convenient to consider it here in succession with the other simple-order rate laws. Here we have, for example A B, where the reaction is -order with respect to A ... 

Whereas, at 800 K, small amounts of added O2 (up to 10 %) greatly accelerate acetaldehyde decomposition, at 1030 K little eflFect is observed. The changing influence of O2 may be explained by the relative importance of the two competing initiation reactions, (30) and (la). 

We were able to observe clear evidence for the chain-type mechanism in experiments, involving acetaldehyde decomposition in the gas-phase [98], similar to those already discussed for 2-propanol. With acetaldehyde, the values exceeded the maximum value obtained for a similar film for 2-propanol oxidation (0.28) (Fig. 6). As already discussed, the latter value may be considered to be an intrinsic maximum value for this particular film. Therefore, if < > exceeds the intrinsic maximum value, it indicates that radical chain reactions are important, that is, a single photon can cause more than one photodecomposition reaction. 

ER14.14 Acetaldehyde, ethane, and other hydrocarbons are mixed and undergo two PFR reactors in parallel. The acetaldehyde decomposition into methane and carbon monoxide occurs preferentially at 520°C, but at 800°C ethane decomposes into ethylene and hydrogen. One introduces the mixture containing 9% acetaldehyde, 8% ethane, and stream as diluent (molar %). The other components are negligible. The first reactor works at 520° C and 1 atm and the second one at 800°C and 1.4 atm. To achieve 60% conversion, what should be the ratio between the volumetric flow at the entrance of the reactors, assuming that they have the same volume The rate constants are given by ... 

The acetaldehyde decomposition occurs in the first reactor, since ethane decomposition is thermodynamically negligible at that operating condition. On the other hand, ethane decomposition takes place in the second reactor. These components should be included into the global balance. 

The kinetics of the reactions is known. The acetaldehyde decomposition is of second order at variable volume and the ethane decomposition is of first order according to the unit of the rate constants. 

Many organic decompositions in the gas phase occur by composite mechanisms involving the participation of atoms and free radicals, and in 1934 F. O. Rice (b. 1890) and K. F. Herzfeld (1892-1978) proposed several types of chain mechanisms to explain the kinetic behavior observed. The acetaldehyde decomposition appears to occur largely by a mechanism that can be simplified to... 

---