Light-Induced Efficient Hydroxylation of Benzene to Phenol by Quinolinium and Polyoxovanadate-Based Supramolecular Catalysts

Article abstract of DOI:10.1002/anie.202011164

Direct Hydroxylation of benzene to phenol with high yield and selectivity has been the goal of phenol industrial production. Photocatalysis can serve as a competitive method to realize the hydroxylation of benzene to phenol owing to its cost-effective and environmental friendliness, however it is still a forbidding challenge to obtain good yield, high selectivity and high atom availability meanwhile. Here we show a series of supramolecular catalysts based on alkoxohexavanadate anions and quinolinium ions for the photocatalytic hydroxylation of benzene to phenol under UV irradiation. We demonstrate that polyoxoalkoxovanadates can serve as efficient catalysts which can not only stabilize quinolinium radicals but also reuse H₂O₂ produced by quinolinium ions under light irradiation to obtain excellent synergistic effect, including competitive good yield (50.1 %), high selectivity (>99 %) and high atom availability.

Full text of DOI:10.1002/anie.202011164

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Light-Induced Efficient Hydroxylation of Benzene to Phenol
by Quinolinium and Polyoxovanadate-based Supramolecular
Catalysts
Authors: Yongge Wei, Yaqi Gu, Qi Li, Dejin Zang, Yichao Huang, and
Han Yu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202011164
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10.1002/anie.202011164
Angewandte Chemie International Edition
RESEARCH ARTICLE
Light-Induced Efficient Hydroxylation of Benzene to Phenol by Quinolinium
and Polyoxovanadate-based Supramolecular Catalysts
Yaqi Gu, [a] Qi Li, [a] Dejin Zang, [a] Yichao Huang, [a] Han Yu[a],* and Yongge Wei [a,b],
*
Abstract: Direct Hydroxylation of benzene to phenol with high yield
and selectivity has been the goal of phenol industrial production.
Photocatalysis can serve as a competitive method to realize the
hydroxylation of benzene to phenol owing to its cost-effective and
environmental friendliness, however it is still a forbidding challenge
to obtain good yield, high selectivity and high atom availability
meanwhile. Here we show a series of supramolecular catalysts
based on alkoxohexavanadate anions and quinolinium ions for the
photocatalytic hydroxylation of benzene to phenol under UV
irradiation. We demonstrate that polyoxoalkoxovanadates can serve
as efficient catalysts which can not only stabilize quinolinium radicals
but also reuse H2O2 produced by quinolinium ions under light
irradiation to obtain excellent synergistic effect, including competitive
good yield (50.1%), high selectivity (> 99%) and high atom availability.
with O2 as the oxidant might be the most promising method for
hydroxylation of benzene.
Some important photocatalysts for the photocatalytic oxidation
of benzene to phenol with O2 as the oxidant have been
reported.13-18 Among them, DDQ-O2-TBN/HNO3 photocatalytic
systems were reported with the highest yield (93 %) and
selectivity (98 %) of phenol to date, however, as mentioned
above, additional oxidants TBN/HNO3 were essential to oxidize
DDQH2 to DDQ under aerobic conditions in the catalytic
cycle.13,14 There are also several systems need no additional
redox agents, besides noble metal contained catalysts,15 the
reported TiO2/silicate (with 77.6 % benzene conversion and
22.7 % phenol selectivity) and quinolinium ion catalysts also
show appreciable activity of benzene oxidation to phenol.2, 16-18
Introduction
Quinoline itself has photocatalytic activity for the oxidation of
benzene to phenol.[16-18] Under light conditions, it can catalyse
the formation of phenol and hydrogen peroxide from benzene,
water and oxygen gas.[16] But quinolinium ion catalysts are less
reported till now. Some efforts have been made to analyse the
effects of substituents (-CH3/-H/-CN) on quinolinium rings,[16] the
Phenol, as an important chemical raw material, still has many
limitations in its industrial production, such as cumbersome
synthesis steps, high energy consumption, low yield and low
selectivity.[1] One-step production of phenol with low energy
consumption, high yield and high selectivity has always been the
goal of researchers. Many progresses have been made in direct
hydroxylation of benzene to phenol.[2-6] In the field of chemical
catalysis, a series of vanadium silicalite zeolites were reported
which can catalyze the hydroxylation of benzene to phenol in
30.8% yield and high selectivity of 99 % with H2O2 as the sole
oxidant.[4] H2O2 is the most commonly used green oxidizer of
benzene oxidation to phenol to date.[3-6] Compared with H2O2, O2
is more attractive because it is cheaper and much more safety.
However, oxygen-participated benzene oxidation generally
requires additional redox agents (for example, CO and H2),[7,8]
redox agents-free catalytic systems were less reported and most
of them usually need noble metal catalysts or non-noble metal
catalysts under harsh reaction conditions or accomplished in the
low selectivity.[2, 9-11, 15] Besides thermochemical catalysis, the
method of electrocatalysis has also been proposed for the direct
hydroxylation of benzene to phenol.[12] Yet, due to its cost-
effective and environmental friendliness, photocatalysis has
attracted more and more attention recently in the synthesis of
-
kind of counter anions (SO42-, Cl-, NO3 ) and co-catalyst on the
catalytic performance of hydroxylation of benzene,[17,18] while the
reuse of generated hydrogen peroxide has been totally
neglected.
Polyoxometalates (POMs) form a unique class of metal-oxide
clusters of the early transition metals (usually Mo, W, V)
generally in their highest oxidation states with diversity of
structures and properties. POMs are widely used in the catalytic
systems, especial for the catalytic oxidation due to their electron
reservoir abilities and the activity of their diverse reduced
forms.[19] The catalytic activity of POMs in the hydroxylation of
benzene to phenol has also been confirmed these years,[20,21]
however the yield of phenol is rather low and the kinds of POMs
are very limited. Developing POM–based catalytic systems with
high phenol yield and selectivity is still a formidable challenge.
H2O2 is the most commonly used oxidant for POM-based
catalytic systems,[19] and here POMs might be suitable
candidates for the reuse of hydrogen peroxide generated in a
photocatalytic system of quinolinium ions. Supramolecular
photocatalytic systems constructed from POM anions and
quinolinium ions may behave efficiently synergistic catalytic
activity of phenol preparation via the hydroxylation of benzene.
Recently, one example of phosphotungstic anion-countered
quinolinium salt (named as C16Qu-PW) was reported with
relatively low phenol yield (20.9 %),[22] furthermore, even though
the C16Qu-PW showed a higher yield of phenol (20.9 %) than
C16Qu and PW alone (14.1 % for C16Qu and <0.1 % for PW,
respectively), the synergistic effect between C16Qu and PW was
not very obvious, and the reported mechanism explanation is
phenol
compared with thermochemical catalysis and
electrocatalysis.[13-18] Developing novel photocatalytic systems
[a]
[b]
Dr. Y. Gu, Mr. Q. Li, Dr D. Zang, Dr Y. Huang, Prof. Dr. H. Yu,* Prof.
Dr. Y. Wei*
Key Lab of Organic Optoelectronics & Molecular Engineering of
Ministry of Education, Department of Chemistry
Tsinghua University
Haidian District, Beijing, 100084, P. R. China
E-mail: hanyu0220@tsinghua.edu.cn; yonggewei@tsinghua.edu.cn
Prof. Dr. Y. Wei*
State Key Laboratory of Natural and Biomimetic Drugs, Peking
University, Beijing 100191, P.R. China.
E-mail: ygwei@pku.edu.cn
This article is protected by copyright. All rights reserved.

10.1002/anie.202011164
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 1. Molecular structures of the supramolecular catalysts: (C4-Quin)2V6 (a), (C8-Quin)2V6 (b), (C16-Quin)2V6 (c) and (C8-Quin-NH2)2V6 (d). The solvent
molecules are omitted for clarity. Color code: V - orange, O - red, N - blue and C - grey.
unsuitable for the reuse of hydrogen peroxide generated during
the photocatalytic process.
confirmed
by
1H
NMR
(Figs.
S1-
S6).
[(C4H9)4N]2[V6O13{(OCH2)3CCH2OH}2] ( abbreviated as (TBA)2V6)
was prepared according to a previously reported procedure.[23]
The crystals of these four supramolecular catalysts were
obtained by diffusion method and used for single crystal X-ray
diffraction analysis subsequently. Their complete molecular
structures and crystal data were provided in Fig. S22 and Tab.
S3, respectively. The molecular structures of supramolecular
catalysts without solvents were given in Figure 1, each molecule
includes two quinolinium ions and one alkoxohexavanadate
anion, with or without co-crystallized solvents. In particular,
hydrogen bond interactions between anions in (C4-Quin)2V6 was
found (shown in Fig. S21 and Tab. S2). Besides, 1H NMR spectra
(Figs. S7-S10) and FT-IR (Figs. S11- S14) spectra of these
supramolecular catalysts after dried were also provided.
In
[C9H7N(C4H9)]2[V6O13{(OCH2)3CCH2OH}2]
(abbreviated as (C4-Quin)2V6,
[C9H7N(C8H17)]2[V6O13{(OCH2)3CCH2OH}2]
(abbreviated as (C8-Quin)2V6,
this
article,
four
supramolecular
catalysts:
3CH3CN
1957103),
(4/3)CH3CN
1957107),
•
CCDC:
•
CCDC:
[C9H7N(C16H32)]2[V6O13{(OCH2)3CCH2OH}2] • (5/4)CH3CN
•
C4H10O (abbreviated as (C16-Quin)2V6, CCDC: 1057109) and
[NH2C9H6N (C8H17)]2[V6O13{(OCH2)3CCH2OH}2] (abbreviated as
(C8-Quin-NH2)2V6, CCDC: 1057110) were constructed from the
perspectives of improving atomic availability by a strategy of
molecular self-assembly based on alkoxohexavanadate anion
[V6O13{(OCH2)3CCH2OH}]2- (abbreviated as V6) and a series of
quinolinium bromide, including C9H7N(C4H9)Br, C9H7N(C8H17)Br,
C9H7N(C16H33)Br and NH2C9H6N(C8H17)Br. These four catalysts
Photocatalytic hydroxylation of benzene under conditions
without H2SO4 and the reaction mechanism. Considering the
supramolecular catalysts all have absorption at 365 nm, 365 nm
Led lamps were chosen as light source to perform photocatalytic
experiments (Fig. S23). The photocatalytic experiments were
performed under UV-light (25 °C, 10 W led lamp) irradiation of
various catalysts in water-acetonitrile mixtures containing
benzene and dodecane (internal standard) (Table 1). GC-MS
were used for the detection and quantification of phenol. No
phenol was detected without catalysts (Table 1, Entry 11) or
without light irradiation (Table 1, Entry 9-10). No phenol was
detected by (TBA)2V6 with O2 or H2O2 as the oxidant (Table 1,
Entry 12, 13), it reflected that the alkoxohexavanadate V6 alone
could not catalyze O2 or H2O2 to oxidize benzene to phenol here.
Only small amount of phenol was detected by (C4-Quin)Br, (C8-
Quin)Br and (C16-Quin)Br with 1 %, 1.3 %, 1.0 % phenol yield,
respectively, while the reaction can be catalyzed efficiently by
(C4-Quin)2V6, (C8-Quin)2V6 and (C16-Quin)2V6 with 14.0 %, 21.7 %
and 22.5 % phenol yield, respectively, much higher than V6 itself
and their corresponding quinolinium bromides. Considering the
produced H2O2 by quinolinium rings under photocatalytic
1
were characterized by single crystal X-ray diffraction, H NMR
and FT-IR. Their UV-visible absorption spectra were also
determined. The influence of assembly structure, substituents in
quinolinium ring and the acidity of reaction system on the
synergistic catalytic effect was focused on, the possible involved
mechanisms of synergistic catalysis were proposed.
Results and Discussion
Synthesis of supramolecular catalysts C9H7N(C4H9)Br
(abbreviated as (C4-Quin)Br), C9H7N(C8H17)Br (abbreviated as
(C8-Quin)Br), C9H7N(C16H33)Br (abbreviated as (C16-Quin)Br)
were synthesized by mixing quinoline with excess brominated
alkanes (bromobutane, n-octyl bromide, 1-Bromohexadecane)
at 80 °C for 24h. Because of the activity of amino group,
NH2C9H6N (C8H17)Br (abbreviated as (C8-Quin-NH2)Br) cannot
be obtained by the direct reaction of 6-aminoquinoline with n-
octyl bromide. It was obtained by more complex steps, as
showed in Scheme S1. Related compounds involved here were
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10.1002/anie.202011164
Angewandte Chemie International Edition
RESEARCH ARTICLE
conditions here cannot be exploited by V6, a mechanism of
hydrogen peroxide reuse is not involved. EPR measurements
were performed then to further understand the role of V6 in
photocatalytic system. Signals attributed to DMPO-OOH (DMPO:
5,5-dimethyl-1-pyrroline N-oxide, spin trap) derived from O2•- (Fig.
S15 (a), AN = 1.3 mT, AHβ = 1.0 mT, AHγ=0. 16 mT, g = 2.0042)
were captured on irradiation of a reaction system containing (C4-
Quin)Br, while signals mainly attributed to carbon-centered
radicals (Fig. S15 (b), AN = 1.5mT, AHβ= 2.1 mT, g= 2.0047) were
captured on irradiation of a reaction system containing (C4-
Quin)2V6. Consider no carbon-centered compounds other than
quinolinium ions were introduced into the EPR testing system,
the captured carbon-centered radicals most likely belong to the
quinolinium radical c (Scheme 1). A reported photocatalytic
mechanism of quinolinium ions is as follow[16]: quinolinium ions
(shorted as QuCN+) can transform into excited state (shorted as
QuCN+*) under photo irradiation, and then transforms into
radicals (shorted as QuCN • ) via seizing an electron from
chains in (C4-Quin)2V6 is too short to bring big channels or pores
and only pores with 3.2 Å maximum diameter are obtained
(Figure 2(a)). It is worth mentioning that the phenol selectivity of
(C16-Quin)2V6 ＞(C8-Quin)2V6 ＞(C4-Quin)2V6 is also related to
the assembly structures, the larger the pore/channel size of
supramolecular catalysts, the better the separation of phenol and
the prevention of excessive oxidation of phenol on the surface of
catalysts. The result is that the longer carbon chains of
quinolinium ions, the larger channels/pores formed in
supramolecular catalysts, the higher catalytic efficiency and
good phenol selectivity obtained.
Table 1. Photocatalytic hydroxylation of benzene to phenol without additive.
Entry
1ace
Catalyst
Phenol Yield (%)
1.0 %
Sel. (%)
＞99 %
＞99 %
(C4-Quin)Br
(C8-Quin)Br
2ace
1.3 %
benzene which forms the benzene cation correspondingly due to
3ace
4ace
(C16-Quin)Br
(C8-Quin-NH2)Br
(C4-Quin)2V6
(C8-Quin)2V6
(C16-Quin)2V6
(C8-Quin-NH2)2V6
(C16-Quin)Br
(C16-Quin)2V6
No catalyst
1.0 %
＞99 %
•
the loss of an electron, QuCN• can reduce O2 to produce O2
-
•
•
followed by protonation of O2 - to afford HO2 (as shown in Fig.
S16). Compared the EPR spectra of the reaction system of (C4-
Quin)Br with that of (C4-Quin)2V6, the disappearance of DMPO-
No detected
14.0 %
-
5ade
97.5 %
6ade
21.7 %
98.6 %
•
OOH derived from O2 -and the appearance of species c
7ade
22.5 %
99 %
(Scheme 1) in (C4-Quin)2V6 system’s EPR spectra suggests that
8ade
No detected
No detected
No detected
No detected
No detected
No detected
-
-
-
-
-
-
•
the introduction of V6 limits the formation of O2 -and helps to
9bce
stabilize species c. DFT simulations reveals that the absorption
energy between c (R=H, n=2) and V6 is -18.6309 Kcal/mol, while
the absorption energy between c and Br- is -3.9868 Kcal/mol
(The models for V6- c and Br-- c were shown in Fig. S17). The
DFT simulations further confirm V6 is beneficial for stabilizing
quinolinium radicals. The catalytic mechanism of supramolecular
catalysts, O2 and H2O under light irradiation conditions was
proposed at last (Scheme 1). The quinolinium ions (a) can
transform into excited state (b) under irradiation, and then
transforms into radicals (c) accompanied by the formation of
benzene cation. The hydrogen abstraction of c, which is
stabilized by V6, from the OH-adduct radical generated from
benzene cation and water molecule affords phenol and d. d can
be oxidized by V6 (e) and regenerates a. The reduced V6 (f) can
be oxidized back to e by O2. The higher catalytic efficiency of
supramolecular catalysts than their corresponding quinolinium
bromide may be associated with the simpler way of benzene
oxidation. Beside, V6 may also promote the generation,
separation and transport of the photo-induced carriers. [22,24,25]
10bde
11ae
12ade
13adf
(TBA)2V6
(TBA)2V6
All the experiments were carried out in a mixed solutions (H2O/CH3CN, 3:17 =
v:v) containing substrate (0.5 mmol) and internal standard (dodecane, 44 μmol)
for 12 h at 25℃. The other details are show as below: a, with UV-light
irradiation. b, in dark. c, the catalyst dosage is 0.025 mmol. d, the catalyst
dosage is 0.0125 mmol. e, with O2 as oxidant. f, with H2O2 as oxidant. Entry.
1-4, 9, 11-13 are homogeneous system and Entry. 5-8, 10 are heterogeneous
system.
No phenol was detected by (C8-Quin-NH2)Br and no enhanced
catalytic efficiency from (C8-Quin-NH2)2V6 to (C8-Quin-NH2)Br
was observed (Table 1, Entry 4, 8), much lower than the
obviously enhanced catalytic efficiency from (C8-Quin)2V6 to (C8-
Quin)Br. There are three main reasons behind this phenomenon.
The first is the difference between (C8-Quin-NH2)Br and (C8-
Quin)2V6 in assembly structure. Rectangular channels with 3.3 x
1.5 Å2 cross-sectional area was found in (C8-Quin-NH2)2V6
(Figure 2(d)), smaller than π x 2.3 x 2.3 Å2 of (C8-Quin)2V6. The
second reason maybe lies in strong electron donation properties
of amino groups in C8-Quin-NH2, similar to the reported work by
Prof. Shunichi Fukuzumi, catalytic efficiency of quinolinium ions
with different substituents behaves as QuCN+ ＞ QuMe+ ＞
QuH+.[16] Finally, the alkalescence of -NH2 group may also lead
to the low catalytic efficiency. Alkaline condition of reaction
solution caused by -NH2 limits the reoxidation of the reduced V6
anion and reduces the yield of phenol at last. From this viewpoint,
acidic environment with abundant H+ is more conducive to the
reaction. A new photocatalytic system was then constructed with
concentrated H2SO4 as additive to maintain acidic environment
(Table 2).
As can be seen from Table 1, the yield of phenol increases
with the increase of carbon chain length of quinolinium ions in
supramolecular catalysts. (C16-Quin)2V6 and (C8-Quin)2V6 show
much enhanced catalytic efficiency than (C4-Quin)2V6. The
assembly structure of supramolecular catalysts in solid may be
associated with this phenomenon (Figure 2). The size of
channels/pores in these three supramolecular catalysts
influence the final catalytic effect seriously. The larger the pore
size, the easier the contact between catalyst and reactant, and
the higher the catalytic efficiency. The orderly accumulation of
carbon chains in (C16-Quin)2V6 leads to a one-dimensional
channel with 5.8 Å maximum diameter (Figure 2(c)). Carbon
chains in (C8-Quin)2V6 is long enough to bring big pores with 4.6
Å maximum diameter (Figure 2(b)). Conversely, the carbon
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10.1002/anie.202011164
Angewandte Chemie International Edition
RESEARCH ARTICLE
essential to produce catalytic active species for benzene
oxidation to phenol by (TBA)2V6. (C4-Quin)Br, (C8-Quin)Br, (C16-
Quin)Br and (C8-Quin-NH2)Br show improved phenol yield with
H2SO4 than without H2SO4(Table 2, Entry 14, 15, 16, 17), caused
2-
by the following two reasons. The first, the introduction of SO4
bringing more Mulliken charge population on [Cn-Quin-H]2+ or [C8-
Quin-NH3]2+ cation.[17] The second, abundant H+ provided by
•
H2SO4 makes the formation of HO2 become much easier. (C4-
Quin)2V6, (C8-Quin)2V6, and (C16-Quin)2V6 show obviously
enhanced catalytic activity with H2SO4 than without H2SO4 under
O2 conditions (Table 2, Entry 18, 19, 20), and the calculated
highest TON was 19.8 by (C8-Quin)2V6. The transition from
heterogeneous system to homogeneous system is one of the
reasons for the increase of catalytic efficiency. EPR
measurements were performed then to further understand
catalytic mechanism of H2SO4 photocatalytic system. Signals
attributed to DMPO-OOH derived from O2•- (Fig. S19, AN = 1.3
mT, AβH = 0.9 mT, g = 2.0049) were captured on irradiation of
(C4-Quin)2V6 with H2SO4, it means H2O2 can be produced by the
•
hydrogen abstraction of HO2 from the OH-adduct radical I
(Scheme 2) then. The related performance was exhibited with
that the yield of H2O2 and phenol were improved simultaneously
by quinolinium ions photocatalysis.[16] Therefore, the other
reason for the improvement of catalytic efficiency in H2SO4
system is that H2O2 produced by quinolinium ions can be used
further to oxidize benzene to phenol catalyzed by the
polyoxoalkoxovanadate anion. The slight difference in catalytic
effect of (C4-Quin)2V6, (C8-Quin)2V6, and (C16-Quin)2V6 resulted
from the difference in emulsification of the catalyst themselves.
With the increase of carbon chain length on quinolinium ions, the
emulsification increased first and then decreased.
In order to understand the photocatalytic mechanism more
comprehensively, the mechanism of thermochemical catalytic
hydroxylation of benzene by V6, H2O2, H2SO4 was also studied.
Radical scavenger test was first performed to verify whether the
hydroxylation of benzene by V6, H2O2 and H2SO4 without light
irradiation is a free radical reaction or not. No deactivation
happened in the presence of butylated hydroxytoluene (BHT,
scavenger for various radicals) reflects the process is non-radical
route (Table 2, Entry 25). In-situ EPR measurements were
performed at last. No signals were captured on V6 in CH3CN,
while V4+ signals were captured with the adding of H2SO4 (Fig.
Scheme 1. The proposed mechanism of hydroxylation of benzene by
supramolecular catalysts, O2, H2O/CH3CN system under light irradiation
conditions (R=H, n= 2, 6, 14). The Color code in e and f: V - orange, O - red,
C - black and H – grey.
Photocatalytic hydroxylation of benzene under conditions
with H2SO4 as additive and the reaction mechanism. The
catalytic system of supramolecular catalysts becomes
homogeneous after the introduction of H2SO4. UV-vis spectra
were obtained first to confirm the supramolecular interaction
between alkoxohexavanadate anions and quinolinium ions (Fig.
S18), the different absorption intensity of (TBA)2V6 and (C4-
Quin)2V6 at 387 nm in the acidic catalytic system reflects the
existence of energy transfer phenomenon between
alkoxohexavanadate anions and quinolinium ions. (C8-Quin-
NH2)2V6 performed improved catalytic activity with H2SO4 than
without H2SO4 (Table 2, Entry 21). Interestingly, (TBA)2V6 show
much higher catalytic efficiency with H2SO4 than without H2SO4
in the presence of H2O2 (Table 2, Entry 23, 24), while still no
phenol was detected by (TBA)2V6 with H2SO4 under O2
conditions (Table 2, Entry 22). It means both acid and H2O2 are
S20). It reflects V5+ is turned into V4+ in the presence of H2SO4.
26]
As the previous works confirmed,[4,
the newly formed V4+
species (Scheme 2 (b), f) can react with H2O2 to form highly
active species (Scheme 2, g, h) and realize the hydroxylation of
benzene. A possible involved mechanism of hydroxylation of
benzene by supramolecular catalysts, O2, H2O and H2SO4 under
light irradiation conditions was proposed at last, as shown in
Scheme 2. In summation, the whole photocatalytic mechanism
of supramolecular catalysts includes: 1) phenol and hydrogen
peroxide are produced by quinolinium ions under photocatalytic
conditions first (Scheme 2, (a)) [16]. 2), the produced H2O2 is then
exploited
further
by
the
catalytic
activation
of
alkoxohexavanadate anion to hydroxylate benzene again
(Scheme 2, (b)). Besides, as mentioned before, V6 may also
promote generation, separation and transport of the photo-
induced carriers here. [22, 24, 25]
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10.1002/anie.202011164
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RESEARCH ARTICLE
Figure 2. Packing structures of (C4-Quin)2V6 (a), (C8-Quin)2V6 (b), (C16-Quin)2V6 (c), (C8-Quin-NH2)2V6 (d) in the crystals. The solvent molecules and H atoms are
omitted for clarity. Color code: V – orange, O – red, N – blue, C – gray and H – white.
Table 2. Photocatalytic hydroxylation of benzene to phenol under acidic conditions.
Entry
14ac
Catalyst
(C4-Quin)Br
(C8-Quin)Br
(C16-Quin)Br
(C8-Quin-NH2)Br
(C4-Quin)2V6
(C8-Quin)2V6
(C16-Quin)2V6
(C8-Quin-NH2)2V6
(TBA)2V6
Oxidant
O2
H2SO4
Light
Catalytic systems
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Phenol Yield (%)
1.7 %
Sel. (%)
＞99 %
＞99 %
＞99 %
＞99 %
＞99 %
＞99 %
＞99 %
＞99 %
—
√
√
15ac
16ac
O2
√
√
√
√
√
√
√
√
√
√
√
√
√
√
5.0 %
O2
1.9 %
17ac
18ad
19ad
20ad
21ad
22ad
23bd
24ad
25ade
1.0 %
O2
O2
45.0 %
50.1 %
47.2 %
4.0 %
O2
O2
O2
O2
√
√
√
√
No detected
17 %
H2O2
(TBA)2V6
＞99 %
＞99 %
＞99 %
H2O2
H2O2
√
√
×
×
(TBA)2V6
26.7 %
26.2 %
(TBA)2V6
All the experiments were carried out in a mixed solution (H2O/CH3CN, 3:17 = v: v) containing substrate (0.5 mmol), internal standard (dodecane, 44 μmol) and
H2SO4 (0.288 mmol) at 25 °C. The other details are show as below: a, the reaction time is 12 h. b, the reaction time is 4 h. c, the catalyst dosage is 0.025 mmol. d,
the catalyst dosage is 0.0125mmol. e, BHT (1 mmol) was added to the reaction system.
The influence of the substituents in benzene ring on catalytic
reactions was investigated by changing the kind of substrates.
Conclusion
As shown in Tab. S1, the introduction of Cl, Br, NO2 and OCH3
groups in benzene all decreases the reaction efficiency. The
alkoxohexavanadate anion and several quinolinium ions for
conversation of chlorobenzene to p-chlorophenol (20.8 %) is
higher than bromobenzene to p-bromophenol (4.9 %), and no
developed from the perspective of improving atomic availability
oxidation products were detected in the case of nitrobenzene
In conclusion, a series of supramolecular catalysts based on an
photocatalytic hydroxylation of benzene to phenol have been
by the strategy of molecular self-assembly. The influence of
and methoxybenzene as substrates. Because the electron-
assembly structure, substituents in quinolinium ring and the
withdrawing ability of substituents is NO2 >Cl >Br >OCH3, we
acidity of reaction system on the synergistic catalytic effect of
conclude that the electron-withdrawing ability of substituents is
alkoxohexavanadate anion and quinolinium ions have been
not the only factor affecting the reaction efficiency and more
details that influence the finally catalytic efficiency are still under
consideration in our laboratory.
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10.1002/anie.202011164
Angewandte Chemie International Edition
RESEARCH ARTICLE
level of photocatalytic synthesis of phenol from benzene using
O2 as the sole oxidant without extra redox agents.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 21971134, 21631007 and 21225103),
Doctoral Fund of Ministry of Education of China No.
20130002110042, Tsinghua University Initiative Foundation
Research Program No. 20131089204 and the State Key
Laboratory of Natural and Biomimetic Drugs K20160202. We
thank Ms Ye Bi, Mr. Han Wu, Dr. Bin Yuan, Mr. Xianjin Zhu and
Mr. Guoyong Dai for discussions and supports.
Conflict and interest
The authors declare no competing financial interests.
Keywords: supramolecular photocatalysis, polyoxovanadates,
quinolinium, hydroxylation of benzene, synergistic catalysis
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uncovered. In the heterogeneous catalytic case without H2SO4,
the longer the carbon chain of quinolinium ions, the larger the
holes/ channels of the assemblies, the more conducive to the
improvement of catalytic efficiency. The electron donor
group/alkaline substituent group on quinolinium ring is harmful to
the catalytic reaction. The acidity of the catalytic system helps to
improve catalytic efficiency. The introduction of H2SO4 not only
improves the solubility of supramolecular catalysts in the system,
but also contributes to the formation of H2O2 and V-peroxo
compound, improving the synergistic catalytic efficiency at last.
In the photocatalytic case with H2SO4 as an additive, (C8-
Quin)2V6 exhibits the highest phenol yield (50.1 %) and
selectivity (> 99 %) by using O2 as the sole oxidant, much higher
than (TBA)2V6(0) and C8-Quin (5.0 %) alone. The 50.1 % phenol
yield surpasses all the POM-based catalysts reported for the
hydroxylation of benzene to phenol to date and at the leading
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10.1002/anie.202011164
Angewandte Chemie International Edition
RESEARCH ARTICLE
TOC
O2, CH3CN/H2O, 25°C, 365 nm Led, 12h
（R = H / NH2, n=2, 6, 14）
The readily available series green V supramolecular catalysts can
catalyze the hydroxylation of benzene to phenol with high
conversion, selectivity and atom efficiency.
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