Conjugated dehydrogenation

In the study of conjugated dehydrogenation, the authors [25] suggested a new elementary reaction (refer to subsequent chapters for details) ... 

The solution to such a task may be illustrated using the example of conjugated dehydrogenation of an organic substance by hydrogen peroxide [6], For this purpose, let us present the reaction in a traditional shape similar to schemes (3.4) and (3.5). 

The primary reaction of conjugated dehydrogenation by hydrogen peroxide in the gas phase at 400-650 °C represents peroxide dissociation which generates active sites in the system ... 

Similarly, conjugated dehydrogenation of ethylbenzene is performed [6], At low temperature this is an endergonic reaction requiring high temperatures (800-900 °C), at which equilibrium output is high ... 

From this point of view, it is desirable to consider two competing elementary stages of ethylbenzene conjugated dehydrogenation with participation of H202 in the example of H02 radical (refer to the following chapters for more detailed consideration of this mechanism) ... 

In the second series of experiments (Table 4.2, experiments 3-6) on ethylbenzene dehydrogenation by molecular oxygen, styrene yields were much lower than in the first series. On the quartz reactor walls condensation product precipitation in amounts up to 1.7% was observed. Moreover, if molecular hydrogen was absent in the products of conjugated dehydrogenation, the total amount of hydrogen equals 78-82% of the whole volume of gaseous products (2.2% in total products). 

Comparison of the data of the third experimental series with conjugated dehydrogenation results (experiments 1 and 2) indicates a quantitative difference in the composition of the reaction products. Styrene is the main product of the conjugated dehydrogenation (90-91%... 

For this purpose, conjugated dehydrogenation of some alkylbenzenes was executed in reactors treated by various inorganic compounds [48], The experimental results indicate that in all cases the reaction proceeding is highly sensitive to the reactor surface type. For example, target product yields decreased in the following sequence with respect to salt selected for treatment of the reactor ... 

Comparison of the sequences indicates that the surfaces most active in HO radical recombination are the worst for the implementation of hydrocarbon conjugated dehydrogenation on them. 

Figure 4.2 Dependence of conjugated dehydrogenation selectivity (1), isoprene yield (2) and total side products yield (3) on isoamylenes and nitrogen mixture rate. T = 553 °C volume ratio C5H1() N2 = 1 10 20% aqueous H202 rate is 0.185 ml/h.

Thus, it may be concluded that conjugated dehydrogenation of isoamylenes similar in isomeric composition to industrial fractions gives a high isoprene yield. Despite relatively... 

Figure 4.3 Temperature dependencies of propylene conversion (1), allene (2) and methyl acetylene (3) yields in conjugated dehydrogenation of propylene. Time of contact t = 0.045 s.

The efficiency of conjugated dehydrogenation of olefins with hydrogen peroxide was also confirmed in studies of propylene dehydrogenation to allene [60], The well-known process of propylene oxidative dehydrogenation to allene in the presence of iodine [61] at 625-700 °C gives low yields of the target product (3.6% allene) at low selectivity of the process (6%). 

In the case of conjugated dehydrogenation of propylene under optimal conditions (700 °C), allene and methyl acetylene yields equal 20% and 10%, respectively (Figure 4.3) [62], Methyl acetylene yield increases with temperature, whereas allene yield decreases. 

Figure 4.4 Kinetic curves for conjugated dehydrogenation of cyclopentane at (a) 450 °C and (b) 600 °C (1 unreacted cyclopentane 2 cyclopentene yield (per involved cyclopentane) and 3 cyclopen-tadiene yield (per involved cyclopentane)).

These studies determined the range of parameters of cyclopentane conjugated dehydrogenation with hydrogen peroxide [75],... 

Figure 4.5 Kinetic curves of cyclohexane conjugated dehydrogenation (1 cyclohexane 2 cyclohexene 3 benzene and 4 cyclohexadiene).

Experimental data [90] that give an opportunity of analyzing kinetics of the above processes were also obtained for conjugated dehydrogenation of vinylethylbenzenes. 

Conjugated dehydrogenation of isopropylbenzene was planned [87] by the method of experiments with the minimal number of tests. Total yields of styrene and a-methylstyrene per injected isopropylbenzene were taken for optimized parameters, because these monomers are equally valuable. At the gradient motion (T = 640 °C) the highest yield of the target products (styrene + a-methylstyrene) equaled 56.8% with about 90% selectivity. Further gradient motion was of no practical interest due to a process selectivity decrease down to 80%. 

The reaction of piperidine conjugated dehydrogenation with hydrogen peroxide [99] opens the way to dehydrogenation of natural compounds, which include piperidine fragments. As shown by experimental data, pyridine yield increases from 45% to 65.2% with hydrogen peroxide concentration from 20% to 25%, respectively, with reaction selectivity above 98%. A further increase of H202 concentration reduces pyridine yield to 59%. It follows from these... 

Figure 4.8 Temperature dependence of EP conjugated dehydrogenation product yield. Molar ratio 4-EP 30% H202 = 1 3, 4-EP volume rate is 0.045 IT1 (1 4-VP 2 4-VP N-oxide and 3 total...

Of interest is the application of the conjugated dehydrogenation technique to transformations of more complex objects, for example, A-alkylanilines, for /V-cthylaniline dehydrogenation, in particular. Besides illustrating new possibilities of the technique mentioned, all the above would help to solve several theoretical questions associated with A-alkylaniline behavior under reaction conditions. 

The suggested mechanism of iV-ethylaniline conversion to benzonitrile is proved by conjugated dehydrogenation of iV-methylaniline and benzylaniline in the presence of H202 [102], Hence, data from the literature were taken into account, which confirmed the mechanism... 

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