2
60
F. Liang et al. / Journal of Catalysis 378 (2019) 256–269
large amounts of solvents (Table S1). An analogous beneficial effect
is also observed in terms of both cyclohexane conversion and AA
selectivity (compare between Table S1 and Table 1), when operat-
ing with the hydrogen-bonded solvents. Notably, however,
oxidations carried out in the presence of small amounts of non-
hydrogen-bonded solvents in solution, did not show any further
increase in cyclohexane conversion, confirming again the activa-
tion role of hydrogen-bonded solvents to fully promote the HAT
process.
with the limit of mass transfer. The NHPI amounts on the promo-
tion effects of the solvent towards the cyclohexane conversion
are shown in Fig. 1B. The results reveal that the conversion and
3
AA selectivity increase in the CH CN solution with an increase in
the amount of NHPI. However, the conversion and selectivity for
AA are lowered to 24.5% and 73.5%, respectively, when the NHPI
amount is increased to 1240 mg. Maybe partial NHPI catalyst (at
the high concentration) is easy to form dimeric adducts (or higher
aggregates) by a self-coordination of hydrogen-bond acceptor
(C@O) and donor (NAOH) groups of NHPI, which leads to NHPI
deactivation [32]. The aerobic oxidation results at different tem-
peratures, pressures and time are also shown in Fig. 1. With the
reaction temperature rising from 80 to 120 °C, the cyclohexane
conversion increases with the concomitant gradual increase of
AA and the decrease of KA-oil selectivity (Fig. 1C). A similar reac-
tion trend is also observed with the reaction pressures rising from
0.6 to 1.0 MPa (Fig. 1D). These results clearly indicate that cyclo-
hexane is first converted into KA-oil intermediates and then fur-
ther converted into AA, and that reaction energy and oxygen
concentration have a significant effect on the oxidative cleavage
of KA-oil at a higher temperature and pressure. On increasing reac-
tion pressure to 1.2 MPa or raising the temperature to 130 °C, the
In order to further address these questions, we introduce three
analogs of CH
an ethyl or phenyl group, respectively. The different functional
groups in propionitrile have a similar -H atom from the alkyl
3
CN as solvent by substituting the methyl group with
a
group, but electronic effects may be different for various H-
abstraction elementary reactions in the comparison of the bridging
of ACH for CH CN. Benzonitrile, without an a-H atom, can be a
3 3
control used to probe the environmental hydrogen-bond interac-
tion, which may be bound by reaction intermediates. In Table 1,
it is seen that as the chain length of the primary nitriles increased
(
entries 7 and 8), the rate of reaction and AA selectivity became
significantly lower under the same conditions. In contrast, ben-
zonitrile leads to a pronounced decrease in the catalytic activity
for cyclohexane conversion (from 27.4% to 7.3%). Meanwhile, AA
3
catalytic efficiency of CH CN-NHPI doesn’t change significantly,
selectivity clearly displayed opposite trends as compared to CH
CN, where interestingly 9.4% selectivity of AA along with KA-oil
ketone/alcohol = 3.2) at 88.7% selectivity is observed (entry 6).
As a benchmark, the catalytic reaction was carried out in pyridine
as a solvent, and low conversions of below 0.1% were obtained,
3
-
while degradation rate of AA increased rapidly. It is well observed
that the cyclohexane conversion increases with prolonging time
(Fig. 1E) but the selectivity of AA first increases and then decreases
due to occurrence of AA degradation by deep oxidation at the too
much elongated reaction time.
(
demonstrating that the
role in a hydrogen-bonding interaction for enhancing catalytic
activity. Interesting, transition metals salts (Mn(OAc) and Co
OAc) ) or organic additives (diacetylmonoxime) are known to pull
a-H atom of solvents may play a positive
3.2. Oxidation of various hydrocarbons by CH
3
CN-NHPI system
2
(
2
The metal-free aerobic oxidation by the NHPI-CH
3
CN catalytic
hydrogen out from organic compounds forming radicals and/or to
promote the decomposition of peroxide species in the NHPI-based
catalytic system [37]. However, these additives have negative
influences on the catalytic activity and AA selectivity under our
experimental conditions (entries 13–16). In addition, in order to
confirm whether solvent polarity has a key role on the b-scission
in formed alkoxyl radicals for the formation of AA with the reaction
being strongly accelerated by polar solvents in catalytic system,
system provides a sustainable one-step route to AA, although the
catalytic oxidation of cyclohexane by transition-metal/NHPI leads
to an AA as a main product along with a mixture of KA-oil. With
the optimized reaction conditions, this metal-free catalytic system
can be extended to catalyze the oxygenation of various hydrocar-
bons (Table S2). Firstly, we test cycloalkanes to the corresponding
dicarboxylic acids. Cyclopentane is oxidized with 18.6% conversion
and 70% selectivity of glutaric acid under the same conditions
(entry 1). 66.5% of cyclooctane is also compatible, affording the
desired acid products with 53.2% selectivity (entry 2). In addition,
we have also explored the oxidation of linear alkanes. N-Pentane
can give rise to the pent-2-one with 43.6% selectivity, along with
pentan-3-one (34.6%, entry 3). 37.6% N-hexane, 31.3% N-heptane
and 40.3% N-octane can also be transformed into the correspond-
ing oxidated products efficiently, respectively. However, about
17% propionic acid or valeric acid are formed as the carbon-
carbon bond cleaved products by the cleavage of hexan-3-one or
heptan-3-one and octan-4-one, respectively. Meanwhile, the
mixed products such as 2-one, 3-one and 4-one are obtained with
50% total selectivity (entries 4, 5 and 6). The above results indicate
that these cleaved products such as valeric acid, propionic acid and
acetic acid seem to be formed via b-scission of the corresponding
alkoxy radical derived from the alkyl hydroperoxide decomposi-
tion by solvent participation, which is similar to the previous
transition-metal/NHPI catalytic system by metal ions as co-
catalyst for the decomposition of alkyl hydroperoxide [43,44]. Usu-
ally, the extent of the b-scission is thought to be related to the
alkoxy radicals stability. Therefore, the b-scission of a 2-oxy radical
to form organic acid occurs more easily than that of a 3-oxy or 4-
oxy radical. It is interesting that the higher catalytic efficiencies of
several commonly used solvents such as
trifluoroethanol were added (Table 1, entries 11 and 12) [41].
The reaction with H O solvent did not proceed well, which give
2
H O and 2,2,2-
2
trace results. This suggested that the activation role of the polar
solvent should be likely involved in the HAT process by stabilisa-
tion of the transition state with the favorable intermolecular
hydrogen-bonding interactions (with certain solvents), which is
consistent with previous results in this area [22,42].
3
The CH CN-NHPI reaction system presents the highest catalytic
efficiency for the one-step synthesis of AA, so we investigated the
effect of reaction parameters (Fig. 1). The solvent effect of changing
the cyclohexane concentration is studied and the results are shown
in Fig. 1A. It is observed that with an increase in the cyclohexane
concentration, initially the cyclohexane conversion increases con-
tinuously (till 2.0 mol/L), but after 2.0 mol/L the conversion of
cyclohexane decreases. The product selectivity of cyclohexane oxi-
dation is also dependent on the cyclohexane concentration (sol-
vent content); the AA selectivity gradually increases with
increasing cyclohexane content and reaches a maximum in the
cyclohexane concentration of 4.7 mol/L, whereas the selectivity
to KA-oil intermediate rapidly decreases. This is easily understand-
able, as the ‘cage’ effect of the highly-concentrated solvent would
reasonably be regarded as the inhibiting factor for further oxida-
tion of KA-oil to AA [43]. The use of cyclohexane in large excess
for increasing cyclohexane oxidation catalysis may, however, not
be a proper strategy due to the formation of a two-phase system
3
the branched alkane such as 3-methyl pentane in CH CN solvent
are found with 63.2% conversion, a b-scission of a 3-methyl-3-
oxypentan radical to pentan-3-one as a major product (58.4%)
along with 3-methylpentan-3-ol (19.0%) occurs (entry 7). One