CHEN Jiaqi et al. / Chinese Journal of Catalysis, 2011, 32: 1446–1451
Tableꢀ1 Comparative reaction of oxidation of benzene to phenol with O2
14
12
10
8
Catalyst
(mmol)
0.1
TEMPO
(mmol)
—
Ascorbic acid
Yphenol
/
Entry
(mmol)
5.0
%
1
2
3
4
5
trace
8.3
(4)
(2)
(3)
0.1
1.0
5.0
—
1.0
—
no reaction
no reaction
no reaction
0.1
1.0
—
6
—
1.0
5.0
Reaction conditions: benzene 0.78 g, acetonitrile 5.0 g, O2 2.0 MPa, 80
oC, 1 h.
4
(1)
2
The raw material was obtained after crystallization for 6 h at
180 °C. V-AlPO5 was synthesized by washing, drying, and
calcining in air at 600 °C.
0
1
2
3
4
5
6
Time (h)
For a typical reaction: 0.2 g catalyst [(CH3)4N]4-PMo11VO40
(or 0.172 g V-AlPO5), 5 g acetonitrile as solvent containing
0.78 g benzene, 0.15 g TEMPO, and 0.9 g ascorbic acid were
added into a 25 ml PTFE liner. The liner was put into a stainless
steel reactor and subsequently pressurized with O2 at 2.0 MPa.
After stirring for 1 h at 80 °C the reactor was cooled to 10 °C.
The 1,4-dioxane was added as an internal standard. Quantifi-
cation of the products was carried out using a GC-Agilent
4890 series instrument with a SE-30 capillary column (inter-
nal standard method).
Fig.ꢀ1. Benzene hydroxylation under different conditions. Reaction
conditions: catalyst [(CH3)4N]4PMo11VO40 0.21 g, benzene 0.78 g, ace-
o
tonitrile 5.0 g, TEMPO 0.156 g, ascorbic acid 0.9 g, O2 2.0 MPa, 80 C.
(1) TMA-PMoV and ascorbic acid; (2) TMA-PMoV, TEMPO, and ascor-
bic acid; (3) TMA-PMoV, TEMPO, and ascorbic acid upon adding an-
other 0.156 g TEMPO after 1 h; (4) TMA-PMoV, TEMPO, and ascorbic
acid upon adding 0.9 g ascorbic acid at 0 h, 2 h, and 4 h.
added after 4 h, which meant that the catalytic system could be
regenerated by supplementing with ascorbic acid.
A series of control experiments for benzene hydroxylation is
shown in Table 1. The reaction was first investigated over the
catalyst TMA-PMoV with ascorbic acid but phenol was only
obtained in trace amounts (Table 1, entry 1). Surprisingly,
when TEMPO was added the phenol yield increased to 8.3%
with a high selectivity of 95% in a relatively short reaction time
(Table 1, entry 2). We also found that the reaction activity was
poor when using TEMPO alone (Table 1, entry 3) and when
using both TEMPO and the catalyst (Table 1, entry 4) or
ascorbic acid (Table 1, entry 5). The efficient benzene hy-
droxylation catalytic system only worked when the three in-
gredients- TMA-PMoV, ascorbic acid and TEMPO were used.
Figure 1 shows that the phenol yield remains low even after
6 h in the absence of TEMPO. A higher phenol yield was ob-
tained 1 h after the addition of TEMPO but the yield remained
the same at 3 h. To determine whether TEMPO was consumed
in this reaction extra TEMPO was added 1 h after the reaction
beginning but the yield decreased to 7.6% in the subsequent
hour, indicating that TEMPO was not consumed. Ascorbic acid
is known to be spent during the hydroxylation of benzene with
oxygen, therefore, extra ascorbic acid was added after 2 h in
another reaction. The phenol yield increased to nearly 11.3% in
the subsequent hour and then remained constant. Interestingly,
the phenol yield improved if ascorbic acid was repeatedly
The H2O2 system with the vanadium-based catalyst has been
studied previously [22] and here TEMPO was added to the
H2O2 system to improve the reactivity but no phenol product
was obtained (Scheme 1). With the normal hydrogen peroxide
approach described in Scheme 2 (Route A) the catalyst
TMA-PMoV reacts with H2O2 to form a V(V) peroxo species,
which then interacts with the substrate benzene to produce a
metallo-peroxy-arene intermediate species. However, Route A
is interrupted and replaced by Route B upon adding TEMPO
where hydroxyl hydrogen transfer occurs and the vanadium
species loses the capability to attack the benzene ring. There-
fore, the reactivity of the H2O2 system with TEMPO is hardly
significant.
As described in Scheme 3 (Circle A), the OH radical can be
produced by the plausible Fenton-type process [24]. Both the
oxidant oxygen and the reductant ascorbic acid are necessary
for the production of phenol from benzene oxidation upon
catalysis by the vanadium-based catalyst. This shows that the
V(V) species is reduced to V(IV) by ascorbic acid. The re-
sulting V(IV) species causes the O2 reduction to form hydrogen
peroxide in the presence of H+. The OH radicals are generated
by the decomposition of the formed H2O2 in the presence of the
V(IV) species and a proton attacks benzene to form a hy-
droxycyclohexadienyl radical (the transition state A in Scheme
OH
OH
TMA-PMoV
TMA -PMoV
͖
TEMPO, H2O2
55 oC, 2 h
TEMPO, O2, Ascorbic acid
80 oC, 1 h
Scheme 1. Benzene hydroxylation with TEMPO.