W.-J. Hu et al.
Catalysis Communications 159 (2021) 106353
was not significantly affected by the amount of catalyst. But when the
amount of catalyst was increased continuously, the selectivity of benzoic
acid generated increased slightly (entry 7 in Table S1).
UV–vis spectra of cobalt porphyrin catalyst. From Fig. 3, as the reaction
proceeded, the Soret band of CoTPP (408 nm) gradually decreased, the
formation of a new intermediate with an electronic absorption band at
425 nm was observed. Such changes for operando UV–vis spectra of
cobalt porphyrin could be attributed to the generation of the cobalt
high-valent species during the oxidation. The results are consistent with
our previous works on toluene oxidation in acetonitrile solvent [25].
Based on above experiments and previous works [33], a plausible
mechanism of aerobic oxidation of toluene catalyzed by CoTPP and
NHPI in scCO2 was proposed as shown in Fig. 4. Firstly, cobalt high-
valence radical (PorCoIII-O2•) was generated from the reaction be-
tween CoIITPP and one oxygen molecule. The hydrogen atom in hy-
3.3. Effect of NHPI amount and reaction temperature on toluene
oxidation
Then the effect of NHPI amount on toluene oxidation catalyzed by
CoTPP was examined (Fig. 1a). A higher amount of NHPI improved
toluene conversion, which increased to 21.6% when NHPI amount was
6.0 mol% (based on toluene). However, no obvious improvement in the
activity was observed as increased the NHPI dosage consciously. In
addition, the selectivity of benzoic acid did not fluctuate significantly
with the increased amount of NHPI. It indicated that the role of NHPI
was to initiate free radical. The formation mechanism of products maybe
not involve the participation of NHPI.
droxyl of NHPI was abstracted by PorCoIII-O2 to produce PINO
•
(phthalimide N-oxyl radical). Followed, benzyl radical was generated
–
from the dehydrogenation of primary benzylic C H bond by PINO.
Benzyl radicals combined another oxygen molecule to form benzyl
hydroperoxyl radical. After the decomposition of benzyl hydroperoxyl
radical by Co(III) species, benzyl alcohol and benzaldehyde were pro-
duced. Then, benzoic acid product was generated from the over-
oxidation of benzyl alcohol and benzaldehyde. It was confirmed by the
independent experiments on the oxidation of benzyl alcohol and benz-
aldehyde under the reaction conditions (Fig. S4 and Fig. S5).
Toluene conversion and the selectivity of products are related to
reaction temperature. As presented in Fig. 1b, the conversion of toluene
was greatly influenced by the reaction temperature. Toluene conversion
increased as the temperature rose. The reaction rate increased sub-
stantially from 80 ◦C to 160 ◦C, where conversion improved remarkably
from 6.2% to 22.8%. Moreover, high temperature also promoted the
deep oxidation of benzyl alcohol and benzaldehyde.
4. Conclusions
3.4. Effect of oxygen prerssure on toluene oxidation
–
In summary, efficient aerobic oxidation of primary benzylic C
H
The effect of oxygen pressure on toluene oxidation was also tested
(Fig. S2). Toluene conversion generally increased with the oxygen
pressure. The conversion increased from 12.3% to 21.9% with the in-
crease of oxygen pressure from 0.5 MPa to 2.5 MPa. But the increase rate
was not significant when the oxygen pressure was higher 2.0 MPa. In
addition, the higher oxygen concentration is favorable for the genera-
tion of benzoic acid. Considering the slight increase in conversion of
toluene, oxygen pressure was suggested not higher than 2.0 MPa.
bond by cobalt porphyrin and NHPI in scCO2 was developed. The in-
fluence of various reaction parameters such as catalyst, catalyst dosage,
reaction temperature and pressure on the activity and selectivity were
evaluated. Under the optimized conditions, 21.6% toluene conversion
and 81.2% selectivity towards benzoic acid was obtained. The catalytic
efficiency in scCO2 was much higher than that of carried out in organic
solvent, which could be attributed to the enhanced transport and
peculiar solvating properties. Moreover,
a plausible mechanism
involving free radical and high-valence cobalt species was proposed.
3.5. Substrate scope
Declaration of Competing Interest
With the efficient protocol for toluene oxidation established, the
substrate scope was extended as shown in Table S2. Various toluene
derivatives with different electron property groups at the para-position
of phenyl ring were subjected to the reaction.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
As shown in Table S2, the result is indicative of good tolerance, in
–
which the primary benzylic C H bond of most substrates could be
Acknowledgments
oxidized to corresponding products. It seemed that the efficiency was
dependent on the electronic property of para substituents of toluene
derivatives. Compared to the substrate with electron-withdrawing
groups at para position, electron-donating groups exhibited better effi-
ciency for the oxidation. The electron-donating effect could accelerate
This work was financially supported by the National Key Research
and Development Program of China (2020YFA0210900), the National
Natural Science Foundation of China (No. 21938001 and 21878344),
Innovation Project of Guangdong Colleges (Natural Science)
(2019ktscx110), Natural Science Research Project of Guangdong Uni-
versity of Petrochemical Technology (2019rc047), Research and Inno-
vation Team Construction Project of Guangdong University of
Petrochemical Technology.
–
the dehydrogenation of C H bond. In addition, for the substrate with
electron-withdrawing groups, satisfactory efficiency could be obtained
by prolonging reaction time properly. For example, 14.9% conversion of
4-nitrotoluene was achieved when the oxidation was conducted for 5 h
(entry 8 in Table S2).
Appendix A. Supplementary data
3.6. Plausible mechanism of toluene oxidation in scCO2
Supplementary data to this article can be found online at https://doi.
The profiles of toluene oxidation catalyzed by CoTPP with NHPI and
molecular oxygen in scCO2 is shown in Fig. 2. In the first one hour,
toluene conversion slowly increased. Followed, the reaction rate accel-
erated rapidly. After 3 h, the reaction rate was no significant change.
There is an obvious induction period in the catalytic oxidation system.
To test the free-radical mechanism, free-radical inhibitor (2,6-di-tert-
butylphenol, BHT, 0.5 mmol) was added in the solution. It was observed
that the oxidation was subsequently quenched (Fig. S3).
References
The oxidation of toluene was conducted in a stainless-steel reactor,
which connected to an AvaSpec-2048 spectrometer to test operando
4