Using combination of Mn–Co transition metal species with
N-hydroxyphthalimide as a catalyst for one-step oxidation of cyclohexane with molecular oxygen in acetic acid at 353 K can give more than 95% selectivity towards oxygenated products with
adipic acid as a major product at a high conversion (ca. 78%).
A turnover number of 74 for this partial oxidation are also recorded.
KEY WORDS: adipic acid; one-step oxidation; catalyst; cyclohexane oxidation; N-hydroxyphthalimide; radical; selectivity.
1. Introduction
A considerable research effort has been made to
develop efficient methods for oxidation of unactivated
C–H bonds of alkanes because of the importance of the
substrates in view of both industrial and synthetic
aspects [1–7]. Catalytic oxidation of alkanes has been
explored using different oxidants [8], and those reactions
with molecular oxygen under mild conditions [9] are
especially rewarding due to its cheap availability and
production of H2O as the main harmless by-product.
However, the oxidation of cyclohexane turns out to be
the least efficient of all major industrial processes [10].
At present, the industrial oxidation of cyclohexane to
adipic acid undergoes a two-step process. The first step
involves oxidation of cyclohexane to cyclohexanone and
cyclohexanol at around 423 K and 1–2 MPa pressure
using either a soluble cobalt catalyst or no catalyst, in
the liquid phase, in which the total cyclohexane conversion
is less than 4%. Under such low conversion the
selectivity towards cyclohexanol and cyclohexanone
(KA oil) can be optimized at 70–85% without much
over-oxidation of the products [11–13]. The second step
involves oxidation of the KA oil to adipic acid via the
use of nitric acid as a mild oxidant. The side product
produced from this step includes nitrous oxide (N2O),
which shows a global warming effect of 300 times higher
than carbon dioxide. Research is on going to improve
reaction selectivity of this two-step process. For example,
addition of boric acid to the oxidation mixture
proved to be effective, which allowed approximately
10% conversion of cyclohexane with 90% selectivity for
KA oil [14]. Murahashi et al. reported the oxidation of
cyclohexane over iron powder and observed 11% conversion
with 95% selectivity to KA oil [15]. However,
finding a one-step method for manufacturing adipic acid
from cyclohexane without producing NOx is the most
urgent issue from the viewpoint of green chemistry.
There have been some interesting work led by Thomas
and co-workers in using molecular sieve catalysts with
active site isolation for the direct oxidation of cyclohexane
to adipic acid in gas phase (solvent free) [16,17].
In a similar line of research, Moden et al. have claimed
the selective nature of MnAPO-5 catalysts for the
cyclohexane oxidation [18].
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