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Homogeneous and heterogeneous processes in the gas-phase oxidation of isobutane and isobutene

1965; Elsevier BV; Volume: 10; Issue: 1 Linguagem: Inglês

10.1016/s0082-0784(65)80180-1

1878-027X

J. N. Hay, John H. Knox, James M.C. Turner,

Advanced Chemical Physics Studies

According to most theories of the slow gas-phase oxidation of hydrocarbons, the major product producing reactions are homogeneous and it is supposed that the main effect of surface is on kinetically important reactions which produce relatively little final product. According to one theory of hydrocarbon oxidation all primary products arise from the homogeneous unimolecular decomposition of peroxy radicals or from reactions of radicals produced from such decompositions. If this is so, the distribution of the products formed directly from the peroxy radicals should be unaffected by reactant pressures (in particular, oxygen pressure) and should change gradually with temperature provided that conditions are chosen to avoid secondary oxidation. By analyzing the reaction products from the oxidation of isobutane and isobutene between 250° and 350°C when the extent of oxidation is sufficiently small that secondary oxidation is negligible we have found that, while the percentage yields of the major products (about 80% isobutene from isobutane, and about 75% acetone+formaldehyde from isobutene) are little affected, reactant pressure and temperature have such profound effects on the distribution of the minor products that the simple hypothesis outlined above is untenable. Furthermore, investigations of the effect of change of surface have revealed that the initial distribution of minor products is highly sensitive to the nature of the reaction vessel wall: the surfaces investigated were clean Pyrex; HF washed Pyrex; KCl-, B2O3−, and NaOH-coated Pyrex; and silverplated Pyrex. It is concluded that, contrary to general belief, a considerable part, if not the whole, of the minor products arise in heterogeneous processes which by consequence play a very important part in the reactions between 250° and 350°C. However, general considerations demand that the heterogeneous processes are not independent but are consequent on primary homogeneous processes involving free radicals. It is proposed that three primary homogeneous reactions occur in the early stages of the oxidation of isobutane and three in the oxidation of isobutene: Isobutane C4H10+X=C4H9+HX C4H9+O2=C4H8+HO2 C4H9+O2=C4H9OO Isobutene C4H8+HO2=Me2CO+CH2OH C4H8+HO2=Me2COH+CH2O C4H8+HO2=C4H8OOH and that these homogeneous reactions are followed by the heterogeneous decompositions of the peroxy radicals into complex surface sensitive mixtures of products, C4H9OO+wall=complex mixture A C4H8OOH+wall=complex mixture B With isobutene there will necessarily be secondary homogeneous processes producing formaldehyde or CO by oxidation of CH2OH and acetone by oxidation of Me2COH. This mechanism explains why the ratio of major to minor products is little affected by composition, total pressure, temperature, or surface while the distribution of minor products is highly sensitive to these parameters.