Cyclohexane oxidation has been studied in supercritical carbon dioxide medium for homogenizing the initial reaction mixture to produce cyclohexanone and cyclohexanol as the chief reaction products. The kinetic experiments have been performed at three temperatures 410, 423, and 433 K and two pressures 170 and 205 bar. The results have been interpreted in the light of transition state theory and cage effects. Conversions obtained are low compared to the liquid phase oxidation because of dilute concentrations of the reactants. Cyclohexanone is more selectively formed and favored by both pressure and temperature. A 20% increase in pressure results in (i) reduction of the induction period by 50%, (ii) a change in activation energy from 13.0 kcal/mol at 170 bar to 22.6 kcal/mol at 205 bar, (iii) an increase in the preexponential factor by 5 orders of magnitude, and (iv) an increase in the first-order rate constant at 433 K by about 70%. The variation in the observed activation volume from 36 cm3/mol at 410 K and 170 bar to -775 cm3/mol at 433 K and 205 bar suggests that the reaction in supercritical CO2 medium can be greatly manipulated.
Cyclohexane has been oxidized in supercritical carbon dioxide (SC
CO2) medium in the presence
of oxygen to give the main products, cyclohexanone and cyclohexanol.
Kinetic studies have been
performed to study the effects of the proximity to the plait point,
nature of the phase and initial
feed concentration on the product profiles, selectivities, and rates of
product formation. The
initial reaction conditions have been chosen to be at different regions
of space on the ternary-phase diagram of the initial reaction mixture comprising cyclohexane,
oxygen, and carbon dioxide.
These conditions encompass different thermodynamic phases such as
(i) the homogeneous
subcritical (mixture) phase rich in SC CO2, (ii) the
homogeneous supercritical (mixture) phase,
(iii) the SC CO2-rich vapor−liquid two phase, and (iv)
the CO2 dissolved liquid phase. It has
been observed that the density and the proximity to the plait point of
the reaction mixture
influence the reaction conversion, rates, and pathways. The
first-order rate constants are
observed to be dependent on the thermodynamics state of the
feed.
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