Decomposition, acetaldehyde methane

Decomposition. Acetaldehyde decomposes at temperatures above 400°C, forming principally methane and carbon monoxide [630-08-0]. The activation energy of the pyrolysis reaction is 97.7 kj/mol (408.8 kcal/mol) (27). There have been many investigations of the photolytic and radical-induced decomposition of acetaldehyde and deuterated acetaldehyde (28—30). 

At elevated temperatures, acetaldehyde (CH3CHO, A) undergoes gas-phase decomposition into methane and carbon monoxide. The reaction is second-order with respect to acetaldehyde, with kA = 22.2 L mol-1 min-1 at a certain T. Determine the fractional conversion of acetaldehyde that can be achieved in a 1500-L CSTR, given that the feed rate of acetaldehyde is 8.8 kg min-1, and the inlet volumetric flow rate is 2.5 m3 min-1. Assime T and P arc unchanged... 

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

The photochemical decomposition of methanal in a solid Xe matrix has been studied. Work has also been reported dealing with the photodissociation dynamics of methanal, and ab initio calculations have been carried out on the photochemical decomposition of acetaldehyde into methane and CO. The photocatalytic decomposition of acetaldehyde to yield carbon dioxide has also been reported. The threshold for CC bond fission in propanal and the release of the CHO fragment has been shown to be at a wavelength of 326.26 nm. Chowdhury has reported the dissociation of propynal using multiphoton irradiation. Gas-phase photolysis of butyraldehyde in the 280-330 nm range has shown that the CHO radical is produced. ... 

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 ... 

Acetaldehyde, CH3CHO, decomposes into methane gas and carbon monoxide gas. This is a second-order reaction. The rate of decomposition at 140°C is 0.10 mole/liter sec when the concentration of acetaldehyde is 0.010 mole/liter. What is the rate of the reaction when the concentration of acetaldehyde is 0.50 mole/liter ... 

In words, we describe the process as initiated by the decomposition of acetaldehyde to form the methyl radical CH3 and the formyl radical CHO. Then methyl attacks the parent molecule acetaldehyde and abstracts an H atom to form methane and leave the acetyl radical CH3CO, which dissociates to form another methyl radical and CO. Finally, two methyl radicals combine to form the stable molecule ethane. 

Our first example of a chain reaction, the decomposition of acetaldehyde to methane and CO, is endothermic so the reactor tends to cool as reaction proceeds. However, the oxidation of H2 is exothermic by 57 kcal/molc of H2, and the oxidation of CH4 to CO2 and H2O is exothermic by 192 kcal/mole of CH4. Thus, as these reactions proceed, heat is released and the temperature tends to increase (strongly ). Thus thermal ignition is very important in most combustion processes. 

As previous studies had suggested that the selective oxidation of ethane might occur through the formation and further reaction of ethoxide, it seemed useful to investigate the effects of these molybdate catalysts in the decomposition of ethanol. The decomposition of ethanol at 603 K yielded acetaldehyde (64-69%), ethane (25-26%), ethylene (3-5%) and small amounts of methane and CO. A decay in catalytic activity was observed for all catalysts. At the steady state, neither the activity nor the selectivity differed significantly for these molybdates. 

Action of Diethylamine on Decomposition of Ethyl tert-Butyl Peroxide. The rate of decomposition of ethyl ferf-butyl peroxide is decreased by adding diethylamine (Figure 7), and the yield of products is altered (Table II). Again, the yield of methane is increased at the expense of ethane and f erf-butyl alcohol is increased at the expense of acetone. Ethanol and acetaldehyde are formed in considerably greater amounts. The yields of carbon monoxide and methyl ethyl ketone are decreased. 

With respect to the untreated Reactor I, the hydrogen peroxide yield was very small, and that of methane, ethylene, carbon monoxide, and acetaldehyde was large. The small ratio of hydrogen peroxide to propylene is possibly caused by the successive decomposition of hydrogen peroxide once formed. With aged Reactor II, the yield of hydrogen peroxide and methanol increased, while that of methane, ethylene, and carbon monoxide decreased significantly. 

The thermal decomposition of 3,5-dihydroxy-3,5-dimethyl-l,2-dioxolan (28) in glacial acetic acid or water at 110° is very complex, and leads to the following products acetic acid (60%), lactic acid (13%), propionic acid (6%), carbon dioxide (6%), acetylacetone (10%), methane (5%), carbon monoxide (2%), formic acid (1%), 3,5-diacetylheptane-2,6-dione (0.4%), and a mixture of acetone, methyl ethyl ketone, acetaldehyde, and methylglyoxal.89,40... 

The decomposition of acetaldehyde (CH3CHO) to methane and carbon monoxide is an example of a free radical chain reaction. The overall reaction is believed to occur through the following sequence of steps ... 

The formation of moderate amounts of methane throughout the flame probably indicates the presence of methyl radicals, but these do not appear to be the precursor of methanol (which is present in highest concentration along the boundary between the blue and smoky parts of the flame) since it is not a product in the diffusion flames of acetaldehyde or acetone, both of which should give rise to high concentrations of methyl radicals. One suggested source of methanol is through decomposition of the peroxidized ether radical, viz. 

This ester resembles its methyl homologue in possessing three modes of decomposition [131]. It also supports a self-decomposition flame, the multiple reaction zones of which are clearly separated at low pressures [122, 123, 125]. Temperature and composition profiles in the low-pressure decomposition flame have been measured [133]. The products include formaldehyde, acetaldehyde and ethanol with smaller amounts of methane and nitromethane. The activation energy derived from the variation of flame speed with final flame temperature was 38 kcal. mole", close to the dissociation energy of the RO—NO2 bond. The controlling reaction is believed to be unimolecular in its low pressure regime, and the rate coefficient calculated from the heat-release profile is... 

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... 

Morris analysed the methanes formed in the pyrolysis of acetaldehyde and acetaldehyde-d by infrared spectrophotometry. If the decomposition were an intramolecular reaction then only CH4 and CD4 could be expected, while CH3D, CH2D2 and CHD3 could not. However, in a chain reaction partially deuterated methanes should be formed. With equimolar mixtures of acetaldehyde and acet-aldehyde-d4 at temperatures 480 and 535 °C, respectively, only CH4 and CD4 were formed. On the basis of these findings it was concluded that in the thermal decomposition of pure acetaldehyde mainly a unimolecular mechanism is operative the contribution of the chain decomposition was regarded to be only 10-20 % at the most. 

Later these experiments were repeated -with the conclusion that Morris findings were dubious. Zemany and Burton used equimolar mixtures of acetaldehyde and acetaldehyde- /4, at temperatures 510 and 465 °C, and found that partially deuterated methanes were formed in appreciable amounts. The ratio CHD3/ CD4 was found to be 1.2 and 1.0 at 510 and 465 °C, respectively (compared to the value of 1.6 obtained in the photolysis at 140 °C). These results clearly indicate the free radical origin of the methane. However, the fact that the CHD3/CD4 ratio is lower than the one found in the photolysis made the authors conclude that there IS some contribution from the molecular mechanism. An upper limit for the latter was estimated to be approximately 15 and 25 % of the total reaction at temperatures 510and465 °C, respectively. Zemany and Burton estimated the values for the ratios methane-rf3/ethane-d6 and methane-t /ethane-rfe, from which a chain length of 1000 can be derived, at 465 °C, for the Rice-Herzfeld type decomposition. 

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. 

O Neal and Benson S studied the photolysis of acetone at 3130 A in the presence of hydrogen iodide. The main products of the reaction were methane and acetaldehyde CO could be detected only at high temperatures. The dependence of the CH3CHO/CO ratio on pressure was considered to be a result of the pressure-dependent decomposition of CH3CO. The experimental results concerning the radical decomposition obeyed the relation predicted by the Hinshelwood-Linde-mann theory. The values reported for the limiting high-, and low-pressure rate coefficients were... 

As an example of the use of concentration measures other than that of moles per unit volume consider the gas reaction A —> B + C, say the decomposition of acetaldehyde vapor into methane and carbon monoxide. If the reaction proceeds at constant volume and temperature, it may be followed by the increase in pressure. Suppose TVo moles of A are introduced into a volume V at temperature T so that the initial pressure is Pq = MRTyr. When the gross extent of reaction is X there will be (M + X) total moles present and the pressure will be P = (TVo + X)Rr/ V, Hence y = (P — Pq) V/ RP. If the reaction is second order and so proportional to the square of the concentration (Aq — X)/ K, we have... 

Under certain conditions, such as exposure/to particular catalytic materials, each of these reactions may give yields asjiigh as SO per cent or more of theoretical. Each of these reactions are Reversible, practically completely so, under certain conditions where side reactions and decompositions are largely eliminated. Secondary decomposition of acetaldehyde to methane and carbon monoxide, reduction of the ethylene by hydrogen to ethane, break down of ether to lower molecular weight compounds, polymerizations, etc., so involve any equilibrium relations that the relative rates of the different reactions as well as the equilibria are difficult to obtain experimentally. Even where specific and directive catalysts are used, side reactions are present and complicate any precise analysis of the decomposition mechanism. 

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