Liquid-phase air oxidation of hydrocarbons, notably p-xylene, cumene, ethylbenzene/isobutane, cyclohexane, and n-butane, is of great scientific, technological, and commercial importance. This state-of-the-art paper covers the chemistry and engineering science aspects of these reactions. The role of uncatalyzed reactions and metal ion and mixed metal ion catalysts with bromide activation is discussed. An analysis is presented for the role of mass transfer in influencing the rate of reaction and selectivity for the desired product. Different types of reactors that are used, notably bubble-column reactors and mechanically agitated reactors, are analyzed, and a simple basis is provided for selection of reactors. Some emerging oxidation systems, notably oxidation of cycloalkenes (cyclohexene/cyclooctene/cyclododecene) and oxidation of isobutane under supercritical conditions, are presented. New strategies for conducting air oxidations, such as in biphasic systems (including fluorous biphasic systems), biocatalysis, photocatalysis, etc., are emerging and illustrate the considerable tailoring of the reaction microenvironment that is becoming possible. In some cases, it may be possible to manipulate chemo-, regio-, and enantioselectivity in these reactions.

Engineering Aspects of Industrial Liquid-Phase Air Oxidation of Hydrocarbons

Industrial production and use of adipic acid

Despite their industrial importance, the detailed reaction mechanism of autoxidation reactions is still insufficiently known. In this work, complementary experimental and theoretical techniques are employed to address the radical-chain initiation in the autoxidation of cyclohexane with a particular focus on the "lighting-off" of the oxidation by (added) cyclohexanone. We used a newly developed method to quantify the intrinsic rate of chain initiation as well as the rate enhancement by cyclohexanone and several other (oxygenated) molecules. On the basis of first principles, the hitherto assumed perhemiketale mechanism was found to be many orders of magnitude too slow to account for the observed initiation enhancement by the ketone. Instead, it is shown that the pronounced chain-initiation enhancement by the ketone is attributable to a newly proposed concerted reaction between cyclohexyl hydroperoxide and cyclohexanone, in which the (.)OH radical breaking away from the hydroperoxide abstracts an alphaH atom from the ketone, thereby energetically assisting in the cleavage of the RO--OH bond. This reaction is highly efficient in generating radicals as it quasi-excludes geminate in-cage recombination. As a result, the ketone oxidation product at a level of 1 mol % increases the initiation rate by one order of magnitude, and so acts as a highly efficient "autocatalyst" in autoxidation reactions. An analogous reaction with cyclohexanol, although estimated to be even faster, has only a marginal effect on the overall kinetics, owing to the fast subsequent formation of HO(2) (.) radicals that very rapidly terminate with other ROO(.) radicals. Finally, solid evidence is presented that, also in absence of oxygenates, ROOH initiation is actually a bimolecular reaction, involving concerted H abstraction from the alkane substrate by the nascent (.)OH.

To the core of autocatalysis in cyclohexane autoxidation.

Mechanisms of physical and reaction enhancement of mass transfer in a gas inducing stirred slurry reactor

The gas-liquid oxidation of cyclohexane is performed at high temperature (>200 degrees C) and pressure (up to 25 bar) using pure oxygen in a Pyrex capped silicon etched microreactor which allows convenient screen reaction conditions well above the flammability limit.

Gas–liquid selective oxidations with oxygen under explosive conditions in a micro-structured reactor

Mass transfer coefficients for the solution, without reaction, of oxygen in cyclohexane over a wide range of temperature and pressure have been measured in a flat interface reactor (FIR) and a gas sparged stirred-tank reactor (STR). Solubilities have also been measured and it is suggested that a modified Henry's law coefficient of 1.09 x lo-' kmol/m3 . bar can be regarded as constant over the temperature range 293-435 K. The physical mass transfer coefficient in the STR shows very little variation over this temperature range and a value of 3.5 x m/s can be assumed for 423-435 K. However, coefficients in the FIR increase with agitation rate as expected. The difference is ascribed to interference by neighboring bubbles in the transfer process, when diffusivities are high, and also to the lowering of surface tension at high temperatures. A model proposed by Gal-Or and Resnick is reviewed in its application to the present system.

Mass Transfer and Solubility in Autocatalytic Oxidation of Cyclohexane

Liquid phase oxidation of hydrocarbons is a major source of organic chemicals. This paper concerns itself with the oxidation of cyclohexane. The process is one in which gas absorption is accompanied by chemical reaction, but little information has been reported. A major factor has been safety considerations, the Flixborough explosion being a constant reminder. In this paper attention is given to the kinetics and to the interaction of kinetics and mass transfer, showing that in this autocatalytic system the dissolved oxygen level rises to saturation and falls as the rate of reaction increases. Initially zero order in oxygen, the reaction becomes first order at low oxygen levels. Oscillations and enhancement due to reaction within the boundary layer are noted. A kinetic model is developed based on simplification of the accepted free radical scheme and it is reasonably successful in accounting for the results reported. Experiments at temperatures up to 170°C and 70 bar using uncatalyzed reaction are reported from three different reactors: a homogenous batch reactor, a flat interface reactor, and a sparged agitated bubbling reactor.

Autocatalytic oxidation of cyclohexane—modeling reaction kinetics

In an industrial gas-liquid reactor both mass transfer and kinetics have, in principle, a role to play This paper describes studies on the oxidation of cyclohexane in two-phase, gas-liquid reactors Regimes of absorption have been clarified by measurements of dissolved oxygen concentrations The behavior is found to be complex, arising from the fact that the reaction is autocatalytic, and zero order in oxygen over almost the entire absorption range As reaction velocity increases, due to the autocatalytic kinetics, enhanced physical mass transfer rates arise Bubble swarms have properties that prevent the simple application of mass transfer and enhancement from flat-surface systems A theory is developed here that accounts qualitatively for the enhancement found in bubble swarms in the present case The theory is also shown to predict quantitatively the appropriate physical mass transfer coefficients

A. K. Suresh

Papers

Mass Transfer and Solubility in Autocatalytic Oxidation of Cyclohexane

Autocatalytic oxidation of cyclohexane—modeling reaction kinetics

Autocatalytic oxidation of cyclohexane—mass transfer and chemical reaction

Temperature-dependent physical properties in physical gas absorption

Mass transfer during reaction - cyclohexane oxidation