Phosphotungstic anion-paired quinoline salt for heterogeneous photocatalytic hydroxylation of benzene to phenol with air

Article abstract of DOI:10.1016/j.mcat.2019.110397

Heterogeneous catalysts are preferred in production of chemicals due to the facile production separation and catalyst reusage. However, it is still one challenge to construct efficient heterogeneous catalyst for target reaction. Herein, functional polyoxo

Full text of DOI:10.1016/j.mcat.2019.110397

MolecularCatalysis473(2019)110397
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Phosphotungstic anion-paired quinoline salt for heterogeneous
photocatalytic hydroxylation of benzene to phenol with air
Lingyu Zhang1, Qian Hou1, Yu Zhou⁎, Jun Wang⁎
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing, 210009, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photocatalysis
Ionic liquids
Polyoxometalates
Heterogeneous catalysts
Benzene hydroxylation to phenol
Heterogeneous catalysts are preferred in production of chemicals due to the facile production separation and
catalyst reusage. However, it is still one challenge to construct efficient heterogeneous catalyst for target re-
action. Herein, functional polyoxometalates paired ionic salts (IL-POMs) were prepared through pairing qui-
noline cations with Keggin-type phosphotungstic (PW) anions. Long alkyl chain in the organic cations enabled
the solid nature of these hybrids and endowed them as the recyclable heterogeneous photocatalysts. They ex-
hibited high yield (20.9%) and selectivity (> 99%) in the benzene hydroxylation to phenol by using atmospheric
air as the sole oxidant. The activity of the IL-POM hybrid is even higher than the quinoline salt precursor, thanks
to the unique redox property of PW anions that promotes the generation, separation and transport of photo-
induced carriers. After reaction, the catalyst was facilely reused with stable activity. The catalyst was also active
and recoverable in the oxidation of benzyl alcohol to benzaldehyde with atmospheric air.
1. Introduction
The utilization of H2O2 as the terminal oxidant was extensively studied,
but the oxidation of benzene with the low cost and easily available O2 is
Environment and energy problems are two of the crucial bottlenecks
that restrict the sustainable development, urgently encouraging the
search of clean, atom economy and environmentally friendly route to
produce fine chemicals [1–3]. Unfortunately, many important bulk
chemicals are still produced through traditional energy-consuming and
polluted pathways [3,4]. It is exemplified by the production of phenol,
one of the most valuable intermediates with wide applications such as
the precursors of resins, dyes, pharmaceuticals, and agrochemicals
[5–7]. Currently, the majority of phenol is industrially produced via a
three-step cumene process with many deficiencies such as low yield,
high energy consumption, harsh reaction conditions and the formation
of equimolar amounts of acetone as the byproduct [8–11]. One at-
tractive alternative is the direct oxidation of benzene to phenol with
green oxidants such as hydrogen peroxides (H2O2) and molecular di-
oxygen (O2) [12–15]. However, product phenol is more active than the
inert substrate benzene, making this selective oxidation great chal-
lengeable [14,16,17]. As a result, great efforts have been devoted to the
photocatalytic benzene hydroxylation to phenol through driving the
chemical transformation with the aid of light energy [18].
still scarce [22], attributable to that both benzene and O2 are inert
reactants. Up to now, several supported noble metal nanoparticles and
organic molecules were applied to the photocatalytic conversion of
benzene to phenol with O2 either under UV or visible light irradiation,
with their recycling performance unknown [23–27]. Recently, using
Bi2WO6/CdWO4 hybrid as a photocatalyst and O2 as the oxidant af-
forded a phenol yield of 7.3% and no apparent deactivation was ob-
served in the recycling test [28]. Thereupon, it is exceedingly requir-
able but still challengeable to design highly efficient heterogenous
recyclable photocatalyst for the oxidation of benzene to phenol with O2.
Organic molecules with extended conjugated structure such as
aromatic ketones/quinones, heterocycles and dyes are efficient and
versatile photoactive sites via the formation of the highly reactive in-
termediate under mild conditions [26,27,29–31]. They have almost
unlimited structure and their functionality can be task-specifically de-
signed at molecular level [29]. However, this kind of photocatalysts is
normally soluble in common solvents, which is disadvantageous for
their recovery and recycling utilization. Ionic liquids (ILs) and their
derivatives are organic ionic salts and have also applied in the homo-
geneous and heterogeneous photocatalytic systems [26,32,33]. The
artificial structure of ILs endows them great designability and wide
Various homogeneous and heterogeneous photocatalysts have been
explored in the photocatalytic oxidation of benzene to phenol [19–22].
⁎ Corresponding authors.
E-mail addresses: njutzhouyu@njtech.edu.cn (Y. Zhou), junwang@njtech.edu.cn (J. Wang).
1 These two authors contributed equally: Lingyu Zhang and Qian Hou.
https://doi.org/10.1016/j.mcat.2019.110397
Received 13 March 2019; Received in revised form 25 April 2019; Accepted 8 May 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

L. Zhang, et al.
MolecularCatalysis473(2019)110397
potential applicability, as special organic molecular catalysts [34].
More importantly, compared with the neutral organic molecules, the
solubility of ILs is facilely modulated by varying the cations and anions
[35,36]. Solidification of ILs have been reached by pairing organic ionic
cations with polyoxometalate (POM) anions, the transition-metal oxo-
anion clusters with versatile adjustable structure plus versatile acid/
base and redox catalytic properties [35]. The unique properties above
promote the development of various POMs as efficient homogeneous
catalysts in numerous acids, bases and oxidation reactions
[35–38,40,41]. However, the dissolution of POMs in many polar sol-
vents increases the difficulties in the recovery, separation, and recycling
of the catalysts [35], just like the situation of organic molecules. Up to
now, both quinolinium ions, a kind organic molecule [26], and POM
species [37,41] have been applied in the photocatalytic benzene hy-
droxylation to phenol, yet, no organic molecule or POM related het-
erogeneous catalysts have been constructed for this process. Combina-
tion of organic cations and POM anions offered a series of POM-based
ionic salts (shorten as IL-POMs) as efficient heterogeneous catalysts in
many organic reactions [37–40]. It is proposed that IL-POMs would
include the character and catalytic properties of organic molecules and
POM species in one catalyst [35,36,39,40], and more importantly, en-
able a facile strategy towards heterogeneous catalysts. Besides, owing
to the existence of strong electrostatic interaction between the cations
and anions, the photochemistry properties of IL-POMs would be
modulated through task-specially designing the organic cations and
POM anions. However, to the best of our knowledge, the application of
IL-POMs in heterogeneous photocatalysis is to be explored.
In this work, a series of IL-POM hybrids were constructed by pairing
quinoline cations with phosphotungstic (PW) anions and their photo-
catalytic performance in benzene hydroxylation to phenol with air was
investigated. We demonstrate that the length of the carbon chain in the
alkyl groups of quinoline cations is crucial for the solubility of these IL-
POMs and long carbon chain led to the formation of heterogeneous
photocatalysts. The obtained IL-POM exhibited high photocatalytic
activity in the benzene hydroxylation to phenol by using air as the sole
oxidant. After reaction, the catalyst was facilely recovered and reused
with excellent recyclability. Based on the full characterization and
control experiments, a synergistic effect between IL cations and POM
anions is proposed to understand the reaction. Further, their applica-
tion in heterogeneous photocatalytic oxidation of benzyl alcohol to
benzaldehyde with air was also explored to reveal the rationality of the
structural design of this photocatalyst.
samples were degassed at 100 °C for 3 h. X-ray photoelectron spectro-
scopy (XPS) was performed on a PHI 5000 Versa Probe X-ray photo-
electron spectrometer equipped with Al Kα radiation (1486.6 eV).
Photoluminescence (PL) spectra were collected on a Varian Cary Eclipse
spectrophotometer. Attenuated total reflection-Fourier transform in-
frared spectra (ATR-FTIR) were measured on an Agilent Cary 660 in-
strument in the region 4000-400 cm−1. Solid UV–vis spectra were re-
corded on a SHI-MADZU UV-2600 spectrometer with barium sulfate
(BaSO4) as an internal standard. Electron spin-resonance (ESR) spectra
were measured with a Bruker EMX-10/12 spectrometer at the X-band.
Liquid chromatography-tandem mass spectrometry (LCeMS/MS) was
recorded by using an Agilent 1260 LC system equipped with an EC-C18
column and equilibrated with 90% solvent A (water) and 10% solvent B
(methanol) for the chromatographic separation. An Agilent
Technologies 6540 UHD quadrupole time-of-flight (Q-TOF) mass
spectrometer was used for the MS/MS analysis and operated in positive
electrospray ionization (ESI) mode. Photoelectrochemical properties
were analyzed by using a CHI 760E electrochemical station (Shanghai
Chen-hua, China) by using a conventional three electrode configura-
tions with Pt foil and Ag/AgCl (saturated KCl) as the counter electrode
and reference electrode, respectively. The working electrodes were
prepared by depositing the photocatalyst ink on glassy carbon. The
catalyst (2.0 mg) was dispersed in a mixture solution of ethanol (1 mL)
and nafion (100 μL) and then sonicated for 30 min. The resulting cat-
alyst ink (20 μL) was dropwise deposited onto glassy carbon electrode
(φ=6 mm). Photocurrent measurement and electrochemical im-
pedance spectroscopy (EIS) were measured at open circuit potential by
using 0.1 M Na2SO4 aqueous solution as the electrolyte.
2.2. Synthesis of IL-POM
IL-POM hybrids were prepared with a two-step synthetic method.
The following is the detailed synthesis of the typical sample C16Qu-PW.
Cetylquinolinium bromide (C16Qu-Br) was synthesized by referring to
our previous report [34]. Quinoline (20 mmol) and 1-bromohexadecane
(10 mmol) were mixed in a 25 mL round-bottomed flask and then
stirred at 80 °C for 24 h. After reaction, the precipitate was isolated by
filtration, washed with ethyl acetate, and then dried to afford C16Qu-Br
as a red solid. Elemental analysis (Table S1) calcd (wt%): C 69.11, H
3.22, N 9.28; found (wt %): C 69.16, H 3.13, N 9.17. 1H NMR
(500 MHz, D6-DMSO, TMS) (Fig. S1) δ (ppm) = 0.86 (t, 3H, −CH3),
1.28 (m, 24H, −CH2), 1.41 (t, 2H, −CH2), 1.98 (t, 2H, −CH2), 5.10 (t,
2H, −CH2), 8.08 (t, 1H, −CH), 8.23 (t, 1H, −CH), 8.31 (t, 1H, −CH),
8.54 (d, 1H, −CH), 8.67 (d, 1H, −CH), 9.34 (d, 1H, −CH), 9.64 (d, 1H,
−CH). The catalyst C16Qu-PW was synthesized through the reaction of
2. Experimental section
2.1. Materials and methods
C16Qu-Br and phosphotungstic acid (H3PW). A solution of H3PW
(0.25 mmol H3PW in 30 mL ethanol) was dropwise added into the so-
lution of C16Qu-Br (0.75 mmol C16Qu-Br in 30 mL ethanol). The mix-
ture was stirred at room temperature for 24 h. The formed pink pre-
cipitate C16Qu-PW was isolated by filtration, washed with ethanol and
then dried under the vacuum. Elemental analysis (Table S1) calcd (wt
%): C 22.86, H 3.07, N 1.07; found (wt %) C 23.03, H 3.16, N 1.03.
1HNMR (500 MHz, D6-DMSO, TMS) (Fig. S2) δ (ppm) = 0.85 (t, 3H,
−CH3), 1.23 (m, 24H, −CH2), 1.43 (t, 2H, −CH2), 2.01 (t, 2H, −CH2),
5.09 (t, 2H, −CH2), 8.08 (t, 1H, −CH), 8.21 (t, 1H, −CH), 8.31 (t, 1H,
−CH), 8.52 (d, 1H, −CH), 8.63 (d, 1H, −CH), 9.31 (d, 1H, −CH), 9.57
(d, 1H, −CH).
All the commercial chemicals were analytical grade and used as
received. The CHN elemental analysis was performed on an elemental
analyzer Vario EL cube. Thermo gravimetry (TG) analysis was carried
out on a STA409 instrument (dry air, 10 °C min−1). Scanning electron
microscopy (SEM) images and energy-dispersive X-ray spectrometry
(EDS) elemental mapping analysis was recorded on a HITACHI S-4800
field-emission scanning electron microscope. Transmission electron
microscopy (TEM) images were collected on a jeol 2100 plus field-
emission transmission electron microscope. 1H nuclear magnetic re-
sonance (NMR) spectra were collected on a Bruker DPX 500 spectro-
meter at ambient temperature by using dimethyl sulfoxide (DMSO) as
the solvent and tetramethylsilane (TMS) as the internal reference. Solid
state 13C and 31P magic angle spinning (MAS) NMR spectra were re-
corded on a Bruker AVANCE-III spectrometer (9.4 T, 100 MHz for 13C
nuclei and 161.996 MHz for 31P nuclei with a CP/MAS unit). X-ray
diffraction (XRD) patterns ranging 5-80° (0.2° s−1) were recorded using
a SmartLab diffractometer from Rigaku equipped with a 9 kW rotating
anode Cu source (40 kV, 20 mA). N2 sorption isotherms were measured
at 77 K using a BELSORP-MINI analyzer. Before measurement, the
2.3. Catalytic activity
Photocatalytic benzene hydroxylation to phenol was carried out
under ambient conditions (room temperature and atmospheric pres-
sure). Typically, benzene (1.28 mmol), water (1 mL), catalyst (25 μmol)
and acetonitrile (10 mL) were added into a 70 mL Pyrex glass bottle.
The suspension was stirred for 30 min and then irradiated by a high
pressure mercury lamp (500 W, λ ≤ 365 nm) using a photocatalysis
2

L. Zhang, et al.
MolecularCatalysis473(2019)110397
Scheme 1. Preparation of CnQu-PW hybrids.
instrument BL-GHX-V (Shanghai Bi Lang Co., LTD, China). After reac-
tion, the liquid phase was isolated by centrifugation and analyzed by
gas chromatography (GC, Agilent GC 7890B) equipped with a hydrogen
13C NMR spectrum (Fig. 1C), the peak at 138 ppm is ascribed to the C7
and C9 carbon atoms in the quinoline ring. The peak at 132 ppm is
assignable to the C4-6 carbon atoms. The peaks at 151, 149, 124, and
119 ppm are respectively attributable to the C3, C1, C2, and C8 carbon
atoms. The peak at 61 ppm comes from the methylene (C10) bonded
directly to the nitrogen atom. The peaks at 31 and 23 ppm respond
respectively to the other methylene (C11-23 and C24) in the hexadecyl
line. The peak at 15 ppm is corresponding to the terminal methyl (C25)
moiety of hexadecyl line. All of these 13C NMR
flame ionization detector and
a capillary column (HP-5, 30 m
×0.25 mm × 0.25 μm). The potential by-products such as benzoqui-
none and benzenediol were not detected by GC, giving the GC se-
lectivity above 99%. Yield of phenol = mmol (phenol) / mmol (initial
benzene) × 100%. Photocatalytic selective oxidation of benzyl alcohol
was performed similarly. Reusability was evaluated in a six-run re-
cycling test. After reaction, the catalyst was isolated by centrifugation,
washed with ethanol and then directly charged into the next run.
signals additionally confirm the structure of C16Qu-PW. 31P NMR
spectrum of C16Qu-PW (Fig. 1D) demonstrated a dissymmetry signal at
–15.9 ppm. Phosphotungstic acid itself has a sharp and symmetric re-
sonance peak at –15.2 ppm characteristic of phosphorous in the central
position of the Keggin unit [46]. The asymmetrical resonance signal
with slight shifting to up-field for C16Qu-PW is attributable to the strong
interaction between organic cations and Keggin type POM anions [47].
SEM images indicated that the primary particles of C16Qu-PW are
irregular ones with the size of hundreds of nanometers (the inset image
of Fig. 2A). These primary particles are highly interconnected and fused
with each other to generate secondary particles with the size from
several to tens of micrometers. Elemental mapping images showed the
existence of C, N, O, P and W elements, which suggested that phos-
photungstic acid was exchanged successfully. TEM images further vi-
sualized the morphology information observed in the SEM images.
Nitrogen sorption experiment shown the low uptake in the whole
pressure range (Fig. S5), indicating that the obtained C16Qu-PW is of
nonporous structure with the surface area below 1 m2 g−1. TG curve in
Fig. S6 illustrated that C16Qu-PW possesses a thermally stable structure
with the starting decomposition temperature up to 330 °C. The drastic
weight loss ranging from 330 to 400 °C and the slight weight loss from
400 to 530 °C are assigned to the decomposition of hexadecyl group of
the organic cations. The weight loss from 530 to 640 °C is mainly at-
tributed to the decomposition of the quinoline group. The total weight
loss of 26.9% in the range of 330-640 °C is consistent with the theo-
retical one 27.0%, further verifying the rationality of the chemical
composition of this IL-POM hybrid.
3. Results and discussion
3.1. Structure characterization
IL-POM hybrids were synthesized through the reaction of phos-
photungstic acid (H3PW) and alkyl functional quinoline salts (Scheme
1). These quinoline-based ionic salts and IL-POMs are termed
CnQu-Br and CnQu-PW (n = 2, 8, 12, 16 or 18, denoting the length
of the carbon chain in the alkyl groups), respectively. Experimental
CHN elemental analyses (Table S1) were matching with the molecular
structure of CnQu-Br and indicated that each CnQu-PW molecule is
composed of three CnQu+ cations and one phosphotungstate anion
(PW12O403−, shorten as PW anion). Additional characterization was
performed on the typical ionic salt C16Qu-Br and corresponding IL-POM
hybrid C16Qu-PW. 1H NMR spectra (Fig. S1 and S2) and liquid chro-
matography-tandem quadrupole time-of-flight mass spectrometry (LC-
Q-TOF) analysis (Fig. S3) additionally proved the above-mentioned
molecular formula of C16Qu-Br and C16Qu-PW.
Fig. 1A shows the FT-IR spectra of C16Qu-PW and its two precursors
of H3PW and C16Qu-Br. The IR spectrum of H3PW displayed four
characteristic vibration bands for Keggin structure, in which the bands
at 1080, 983, 888 and 813 cm−1 are assignable to the stretching vi-
bration of the central oxygen ν(PeOa), terminal oxygen ν(W = Ot),
inter-octahedral oxygen ν(WeObeW) and intra-octahedral oxygen
ν(WeOceW), respectively [42]. The structure of Keggin-type phos-
photungstic (PW) anions is supplemented in Fig. S4. The IR spectrum of
The surface chemical compositions and bonding configurations of
C16Qu-Br and C16Qu-PW were investigated by X-ray photoelectron
16Qu-PW also exhibited the peaks at 1626−1363 cm−1 and
spectroscopy (XPS). The C 1s, N 1s and Br 3d signals at 284.14, 401.82,
67.24 eV were observed in the survey scan spectrum of C16Qu-Br (Fig.
S7). The high-resolution C 1 s signal was fitted with three peaks at
289.33, 286.26 and 285.19 eV for the C on the alkyl chain, aromatic
ring and the end of chain, respectively (Fig. S8) [48]. Only one N 1s
peak was identified at 401.82 eV for quinoline nitrogen (Fig. 3A) [49].
The Br 3d spectrum was deconvoluted into two peaks at 68.44 eV and
67.50 eV, which is belonged to Br anion (Fig. 3B) [50]. The survey scan
spectrum of C16Qu-PW exhibited the C 1s, N1s, P 2p and W 4f signals at
284.69, 401.82, 135.08 and 36.35 eV. The presence of P and W signals
and the absence of Br signal indicated that the Br anions has been ex-
changed to be PW anions (Fig. S7). Compared with C16Qu-Br, the C 1s
signals of C16Qu-PW shifted to the lower binding energy. One weak P
2p signal was found at 135.08 eV (Fig. 3C), while the W 4f signal was
deconvoluted into two sets of peaks (Fig. 3D). The ones at 36.66 eV (W
4f7/2) and 39.20 eV (W 4f5/2) were equal to that of W6+ species that
are same as the PW anions of neat H3PW. W 4f peaks at lower binding
energies at 35.82 and 37.99 eV were corresponded to the W5+ species,
the formation of which comes from the electron transfer from C16Qu
cations to PW anions due to the strong electronic cation-anion
C
2992−2850 cm−1 for the quinoline unit and alkyl chain, which were
found in the spectrum of C16Qu-Br [43]. Four peaks for the Keggin
structure of PW anions were still observable in the spectrum of C16Qu-
PW. The slight peak shifts suggest the strong ionic interaction between
organic cations and PW anions, which accounts for the solid nature of
C
16Qu-PW. This hybrid is modestly soluble in DMSO but is insoluble in
most common solvents, such as water, alcohols, acetonitrile, acetone,
and ethyl acetate. By contrast, samples C2-12Qu-PW obtained from the
quinoline salts tethered with short alkyl chains were soluble in these
solvents, whereas C18Qu-PW has the same solubility as C16Qu-PW. This
phenomenon implies that the solubility of CnQu-PW relies on the length
of the carbon chain in the organic cations. XRD patterns of H3PW,
C
16Qu-Br and C16Qu-PW are shown in Fig. 1B. The neat H3PW presents
a set of sharp Bragg peaks for the secondary crystal structure. C16Qu-Br
also exhibited crystal structure. All of these peaks didn’t appear in the
spectrum of C16Qu-PW and only a broad Bragg peak was observable at
2θ = 8.1° with a d spacing of 1.1 nm, close to the theoretical size of a
primary unit of IL-POM hybrid [44,45]. The chemical structure of
C
16Qu-PW was further investigated by solid state NMR analyses. In the
3

L. Zhang, et al.
MolecularCatalysis473(2019)110397
Fig. 1. (A) FT-IR spectra, (B) XRD patterns of H3PW, C16Qu-Br and C16Qu-PW; (C) 13C and (D) 31P MAS NMR spectra of H3PW and C16Qu-PW.
Fig. 2. (A, B) SEM and corresponding EDX mapping images for C, N, P, W, and O elements, and (C, D) TEM images of C16Qu-PW.
4

L. Zhang, et al.
MolecularCatalysis473(2019)110397
Fig. 3. High-resolution (A) N 1s, (B) Br 3d, (C) P 2p, and (D) W 4f XPS spectra.
interaction [51].
strengthened photocurrent density of C16Qu-PW relative to that of
C16Qu-Br indicates that more excited electrons were generated and
UV-vis spectra were employed to detect the optical properties of
these IL-POM hybrids (Fig. 4A). H3PW presented two strong absorption
bands centered at 260 and 310 nm for the ligand to metal charge
transfer (O2–W6+) in the Keggin units [52]. The absorption band cen-
tered at 320 nm and a broad peak centered at 500 nm belong to π-π*
transition and n-π* transition of organic compound [53]. Similar
UV–vis spectra of C16Qu-Br and C16Qu-PW were observed, suggesting
that these adsorption bands mainly come from the organic cations
[54,55]. The enhanced absorption band from 200 to 400 nm of C16Qu-
PW is a synergistic effect of organic cations and PW anions, reaching an
enhanced absorption in the UV region. The inset of Fig. 4A shown en-
ergy gaps calculated from Tauc plots, which are 2.90, 2.72 and 2.80 eV
for H3PW, C16Qu-Br and C16Qu-PW respectively. The electron beha-
viors and photophysical properties of C16Qu-Br and C16Qu-PW were
investigated by photoluminescence (PL) emission spectra. PL emission
reflects the recombination of photogenerated charge carriers, in which
weaker PL intensity is generally corresponding to lower recombination
rate of photogenerated electrons and holes [56]. Fig. 4B compares the
PL emission spectra of C16Qu-Br and C16Qu-PW. It was found that
efficiently separated on the former [58,59]. Fig. 4D compares EIS
spectra of these two samples. Only one arc/semicircle in the frequency
ranges was observed in each spectrum, indicating the involvement of
surface charge-transfer in the photocatalytic reaction system. The
semicircle diameter of C16Qu-PW was much smaller than that of C16Qu-
Br, reflecting the reduced charge transfer resistance that would effi-
ciently promote the separation of photoinduced electron-hole pairs and
substantially accelerate the interfacial charge transfer [60]. All these
photoelectrochemical results reveal the improved efficient in the gen-
eration, separation and transport of photoinduced carriers on C16Qu-
PW relative to C16Qu-Br. Ultimately, more separated photogenerated
charge carriers give rise to the higher photocatalytic activity.
3.2. Photocatalytic hydroxylation of benzene to phenol
The photocatalytic performance of CnQu-PW series and various
control materials were assessed in hydroxylation of benzene to phenol
using atmospheric air as the sole oxidant (Table 1). No phenol was
detectable in the absent of a catalyst and only trace amount of phenol
formed by using quinoline (Qu), 1-bromohexadecane (C16Br) or Na3PW
as the homogeneous photocatalysts (Entries 1-4). The yield of phenol
was 14.1% over the organic salt C16Qu-Br, which is also soluble in the
reaction mediate of mixture of acetonitrile and water (Entry 5). C16Qu-
PW served as a heterogeneous catalyst and offered a high yield of
20.9% (Entry 6). No formation of phenol was observable when per-
forming the C16Qu-PW catalyzed reaction in the dark (Entry 7), illus-
trating the photocatalytic nature. The higher activity of C16Qu-PW than
C
16Qu-Br exhibited emission at 600 nm, indicating that electrons can be
excited by C16Qu-Br. Compared to C16Qu-Br, the intensity of PL emis-
sion spectrum for C16Qu-PW showed a distinct drop off at this band
[57]. This dramatic difference suggests that the recombination of
photoexcited carriers is efficiently restrained after pairing C16Qu+ ca-
tions with PW3- anions. In addition, photocurrent
measurement and electrochemical impedance spectroscopy (EIS)
were carried out on C16Qu-Br and C16Qu-PW. The photocurrent in-
creases sharply upon light irradiation and returns quickly to its dark-
current state when the light turns off (Fig. 4C). The dramatically
C
16Qu-Br is in line with the weaker peak intensity in the PL spectrum
5

L. Zhang, et al.
MolecularCatalysis473(2019)110397
Fig. 4. (A) UV–vis spectra (the inset shows the corresponding Tauc plots) of H3PW, C16Qu-Br and C16Qu-PW; (B) PL spectra, (C) transient photocurrent response and
(D) electrochemical impedance spectra of C16Qu-Br and C16Qu-PW.
and higher photocurrent density (Fig. 4B) of former relative to those of
latter, indicating that the present strategy not only reaches the het-
erogenization
Table 1
Photocatalytic selective oxidation of benzene to phenola.
of the organic salt but also improves the photocatalytic activity by
inhibiting the recombination of photoexcited carriers to promote their
generation, separation and transport. Moreover, inert activity of Na3PW
suggests that C16Qu+ cations mainly respond to the photocatalytic
activity and pairing with PW3− anions affords the heterogeneous cat-
alyst with improved activity. Parallel test of other CnQu-Br and CnQu-
PW samples (Table S2) demonstrated that the activity is closely related
to the length of the carbon chain in the organic cations and CnQu-PW
always exhibited the higher yield than the corresponding CnQu-Br
precursor. The yield of phenol was 14.4% by using C2Qu-PW and in-
creased continuously with increasing carbon chain, reaching the max-
imum over C16Qu-PW (Entries 6–9). Owing to the solubility of C2-18Qu-
Br and C2-12Qu-PW in the polar solvents, these samples are homo-
geneous catalysts under the present reaction conditions. The sample
Entry Catalyst
Catalyst
dosage
(μmol)
Phenomenon
Atmosphere Yield
(%)
Sel.
(%)
1
2
None
Qu
–
–
Air
Air
Air
Air
Air
< 0.1
0.3
–
75
75
25
75
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Heterogeneous Air
Heterogeneous Air
Heterogeneous O2
Heterogeneous N2
Heterogeneous Air
Heterogeneous N2
Heterogeneous O2
> 99
–
–
> 99
> 99
–
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
–
3
C16Br
< 0.1
< 0.1
14.1
20.9
< 0.1
21.0
20.5
14.8
4.7
4
Na3PW
C16Qu-Br
5b
6
C
16Qu-PW 25
C16Qu-PW 25
7c
8
9
10
11
12
13b
14d
15e
16e
C16Qu-PW 25
C16Qu-PW 25
C16Qu-PW 2.5
C16Qu-PW 2.5
C16Qu-PW 2.5
C18Qu-PW also has the heterogeneous nature and afforded similar ac-
tivity (yield: 20.5%) to that of C16Qu-PW (Entry 10). These results
suggest that the long carbon chain in the organic cations favors to
construct high performing heterogeneous catalyst. After reaction, the
15.2
1.3
C16Qu-Br
C16Qu-PW 25
C16Qu-PW 25
7.5
Homogeneous
Heterogeneous O2
Heterogeneous N2
N2
20.6
4.8
catalyst
Photocatalytic recyclability was measured in a six-run test (Fig. 5A).
16Qu-PW displayed the stable recycling activity, suggestive of the re-
markable reusability. Structural characterization of the recovered
16Qu-PW after 6th run indicated that the spent catalyst exhibited al-
C16Qu-PW was conveniently recovered by filtration.
C16Qu-Br
25
Homogeneous
N2
< 0.1
C
a
Reaction conditions: benzene 1.28 mmol, acetonitrile 10 mL, water 1 mL,
atmospheric air, RT, 10 h.
C
b
75 μmol of C16Qu-Br was used to guarantee that equimolar quinoline or-
most the same IR (Fig. S9) and UV–vis spectra (Fig. S10) as those of the
fresh one. The result of elemental analysis over this spent C16Qu-PW
(Table S1, Entry 11) is also identical to that of the fresh catalyst. These
preserved structural characters are responsive to the well recycling
performance.
ganic cations as that in 25 μmol of C16Qu-PW was charged into the reaction.
c
In dark.
d
A mixture of acetonitrile (10 mL) and D2O (1 mL) was used as the solvent.
Acetonitrile (10 mL) was used as the solvent.
e
6

L. Zhang, et al.
MolecularCatalysis473(2019)110397
Fig. 5. Catalytic reusability of C16Qu-PW in (A)
the selective oxidation of benzene (Reaction
conditions: benzene 1.28 mmol, acetonitrile
10 mL, water 1 mL, atmospheric air, RT, 10 h)
and (B) the selective oxidation of benzyl al-
cohol (Reaction conditions: benzyl alcohol
0.1 mmol, acetonitrile 10 mL, atmospheric air,
RT, 4 h).
Fig. 6. Influences of reaction conditions on the
photocatalytic hydroxylation of benzene to
phenol by using C16Qu-PW. (A) irradiation
time, (B) amount of catalyst, (C) solvent type
and (D) amount of water. Reaction conditions:
benzene 1.28 mmol, solvent 10 mL, atmo-
spheric air, RT. For each figure there is a spe-
cific parameter changed.
Activity of C16Qu-PW under different reaction conditions was in-
of acetonitrile and water provides better solubility of reactant benzene
than water. On the other hand, water served as the intermediate O
source in the reaction (Fig. S11, details seen below) [62], facilitating
the O transfer, and thus a mixture of acetonitrile and water endows
more intermediate O resource than that of acetonitrile. The catalyst
dosage was explored in a wide range from 1.25 to 35 μmol (Fig. 6D).
The phenol yield increased rapidly at the low catalyst dosage
(1.25–5 μmol) and varied slightly at higher catalyst dosage
(5–35 μmol). It was found that the maximum yield was observed with
the catalyst dosage of 25 μmol. No obvious light scattering effect was
observed [63]. Based on these results, the suitable catalyst dosage was
fixed at 25 μmol (Fig. 6D).
vestigated by varying the reaction time, catalyst dosage and solvent. As
is shown in Fig. 6A, the yield of phenol increased continuously with the
reaction time, reaching the highest one at 10 h. Further increasing the
time caused no obvious variation of the yield. Fig. 6B shows the in-
fluence of catalyst dosage. Gradually increasing the dosage ranging
0–5 μmol led to a rapid rise of phenol yield and excessive catalyst only
slightly enhanced the activity. When the catalyst dosage was higher
than 25 μmol, the phenol yield kept constant. Interestingly, no by-
product was detectable at high catalyst dosage, visualizingthe high
selectivity. By fixing the reaction time at 10 h and catalyst dosage at
25 μmol, the solvent effect was explored (Fig. 6C). Among them, the
yield of phenol by using acetonitrile was 16.2%, higher than those by
using trifluorotoluene, ethanol, water, cyclohexane or acetone. Higher
yield was found by using a mixture of acetonitrile and water that those
by using a single solvent. The composition of this binary solvents sig-
nificantly affected the activity and the presence of small amount of
water afforded a high yield of 20.9%. There are two major reasons
contributing to the higher yield of phenol by using a mixture of acet-
onitrile and water than those by using a single solvent. On the one
hand, because benzene is soluble in organic acetonitrile but insoluble in
water [61], biphasic reaction medium composed of suitable proportion
C16Qu-PW catalyzed photocatalytic hydroxylation of benzene to
phenol was carried out under different conditions (Table 1). When
25 μmol catalyst was used, the yields were 21.0% and 20.5% under O2
and N2 atmosphere (Entries 8 and 9), respectively. These two values are
close to that under air atmosphere. The same yield of phenol under O2,
air and N2 atmosphere by using 25 μmol C16Qu-PW is assigned to that
the oxygen in the PW anions [64] and water [62] served as the inter-
mediate oxygen sources that afforded sufficient oxygen atom to be
transferred into phenol even under N2 atmosphere when large dosage of
catalyst was charged into the reaction. When the catalyst dosage of
7

L. Zhang, et al.
MolecularCatalysis473(2019)110397
C16Qu-PW was decreased to be 2.5 μmol, the phenol yields decreased to
15.2%, 14.8% and 4.7% under O2, air and N2 atmosphere (Entries
10–12), respectively. The dramatically declined yield under N2 atmo-
sphere is attributed to the decreased intermediate oxygen source from
the PW anions, the reduced state of which is not regenerated under N2
atmosphere. D-isotope experiment was performed by using a mixture of
acetonitrile and H2O into D2O (Fig. S11 and Table 1, Entry 14). The
formation of partial CH4DOH indicates the participation of water in the
reaction [62]. C16Qu-PW (25 μmol) catalyzed reaction under N2 at-
mosphere using acetonitrile as the solvent provided lower phenol yield
of 4.8% than that (20.5%) using a mixture of acetonitrile and water
(Table 1, Entry 15), additionally revealing the participation of water. In
addition, under N2 atmosphere, the phenol yield was 1.3% over C16Qu-
Br without PW anions by using a mixture of acetonitrile (Table 1, Entry
13) and water and then declined to < 0.1% by using acetonitrile
(Table 1, Entry 16). These results additionally validate the involvement
of PW and water in the reaction as the intermediate oxygen source.
Owing to this reason, similar phenol yield was observed by using the
large dosage of C16Qu-PW. These results under N2 atmosphere also
suggest that the molecular dioxygen in air or O2 atmosphere serves as
the terminal oxygen source. On the basis of the observations described
above, we deduce that photocatalytic hydroxylation of benzene to
phenol involves multipaths. Oxygen in the PW anions was pre-
ferentially transferred into the phenol, and water also involved in the
reaction to promote product formation.
To gain deep insight, the electronic state of C16Qu-PW during the
reaction was monitored by in-situ ESR spectra, as is shown in Fig. 7A.
Before illumination, no ESR signal was observed, suggestive of the high
valent for the W6+ species in the original C16Qu-PW. A signal at 3650 G
attributable to low-valent W5+ species emerged under irradiation and
the intensity of this peak increased gradually with elongating the ir-
radiation time. This phenomenon indicates that the initial W6+ species
were gradually transformed into W5+ species during this photocatalytic
reaction [65]. After N2-mediated reaction, the spent C16Qu-PW was
recovered and the corresponding time-resolved UV–vis spectra were
collected (Fig. 7B). Compared with the fresh catalyst, this spent one
exhibited an additional absorption band ranging 600–850 nm that is
assignable to the formation of heteropoly blue containing low-valenced
W5+ species, in line with the result of in-situ ESR spectra [66]. The
intensity of the above absorption band was rapidly attenuated to a low
level in about 15 min when the spent catalyst was exposed in the air,
indicating that the W5+ species were oxidized into W6+ species. These
results also indicate that oxygen in the PW anions acts as the inter-
mediate oxygen source and the one in the O2 serves as the terminal
oxygen source to regenerate the PW anions.
interaction of benzene radical cation with water yields the OH-adduct
radical. On the one hand, O2 can be reduced by C16Qu% to O2%−, pro-
tonation of which generates HO2%−. Phenol could be produced through
%−
the hydrogen abstraction of HO2 from the OH-adduct radical, asso-
ciating with formation of H2O2 (Fig. S12, Part A) [26]. In the presence
of PW anions (Fig. S12, Part B), intermolecular charge transfer from
C
16Qu% to PW anions caused the formation of PW% and regeneration of
organic cation to be C16Qu+. The interaction of PW• with benzene
could produce phenol and heteropoly blue species. This is responsive to
the phenol formation under N2 atmosphere when the reaction was
catalyzed by C16Qu-PW even under water-free condition (Table 1, Entry
15). Additional oxidation of W5+ species in the heteropoly blue re-
generated the PW anions. It means that the phenol formation catalyzed
by C16Qu-PW may undergo multi-pathways, in line with the higher
activity of this catalyst by using a mixture of acetonitrile and water than
that by using acetonitrile. Though the details are still to be explored,
the present results show that PW anions promote generation, separation
and transport of the photoinduced carriers and thus C16Qu-PW ex-
hibited higher activity than C16Qu-Br [67,68].
3.3. Photocatalytic selective oxidation of benzyl alcohol
Aromatic aldehydes are important industrial raw materials of fine
chemicals and have been widely applied in the fields of medicine, dyes,
perfumes and pesticides [69]. Among them, benzaldehyde is the sim-
plest aromatic aldehyde with an active carbonyl group and traditionally
produced through the oxidation of toluene or the hydrolysis of benzyl
chloride, associating with hazardous or corrosive reagents and low se-
lectivity inevitably [70]. Photocatalytic oxidation of benzyl alcohol
with easily available oxidant O2 is among the most promising atom
efficient and environmental way to synthesize benzaldehyde under
mild conditions. Although many photocatalysts, such as TiO2, BiOCl, g-
C3N4 and CdS, have been investigated in this selective oxidation reac-
tion [71], the design of highly efficient heterogeneous catalysts is still
one urgent affair. In this study, the catalyst C16Qu-PW was further
applied in the photocatalytic oxidation of benzyl alcohol to benzalde-
hyde with excellent activity and selectivity. Urgent affair is developing
more efficient photocatalysts to improve product selectivity. In this
study, the catalyst C16Qu-PW was further applied in the photocatalytic
oxidation of benzyl alcohol to benzaldehyde with excellent activity and
selectivity. Parallel test was also performed on other control samples.
As listed in Table 2, the yield of benzaldehyde was low by using Qu,
C
C
16Br or Na3PW as the homogeneous photocatalysts (Entries 1–3).
16Qu-Br afforded a yield of 82.5% and C16Qu-PW offered a higher
yield of 92.4% (Entries 4 and 5). By contrast, no benzaldehyde was
produced under a catalyst-free or irradiation-free condition (Entries 6
and 7). C16Qu-PW served as a heterogeneous catalyst in this reaction
mediate and exhibited stable reusability in a 6-run recycling test
(Fig. 5B). CnQu-Br and CnQu-PW samples were tested (Table S3) and the
Based on the results above and previous researches [26,27], a pos-
sible mechanism is proposed in Fig. S12. Under irradiation, C16Qu+
absorbed photon and transformed into excited state of C16Qu+*, which
reacted with benzene to form C16Qu% and benzene radical cation. The
Fig. 7. (A) In-situ ESR spectra of C16Qu-PW in reaction system (containing benzene 1.28 mmol, acetonitrile 10 mL, water 1 mL) and (B) UV–vis spectra of recovered
C16Qu-PW with different exposure time in the air.
8

L. Zhang, et al.
MolecularCatalysis473(2019)110397
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Photocatalytic selective oxidation of benzyl alcohol to benzaldehydea.
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Entry Catalyst
Catalyst dosage
(μmol)
Phenomenon
Yield (%) Sel. (%)
1
Qu
7.5
7.5
2.5
7.5
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Heterogeneous 92.4
Heterogeneous
3.4
92.3
–
–
98.6
97.4
–
2
C16Br
< 0.1
< 0.1
82.5
3
Na3PW
4
C16Qu-Br
5
C
16Qu-PW 2.5
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7
C16Qu-PW 2.5
None
< 0.1
< 0.1
–
–
–
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a
Reaction conditions: benzyl alcohol 0.1 mmol, acetonitrile 10 mL, atmo-
spheric air, RT, 4 h.
b
In dark.
formers were homogeneous catalysts with slightly inferior activity than
that of the latter ones. The solvent and reaction time of C16Qu-Br cat-
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benzyl alcohol to benzaldehyde. Excessive reaction time caused the
decline of the yield due to the over-oxidation of benzaldehyde (Fig.
S13). Acetonitrile was found to the best solvent for this reaction while a
mixture of acetonitrile and water offered a slightly inferior phenol yield
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involved in the oxidation of benzyl alcohol [72,73].
4. Conclusion
In summary, IL-POM hybrids composed of quinoline cations and
phosphotungstic (PW) anions were constructed as the efficient recycl-
able heterogeneous photocatalysts by modulating the carbon chain in
the organic cations. They exhibited the high yield and stable reusability
in the benzene hydroxylation to phenol and oxidation of benzyl alcohol
to benzaldehyde, in which the atmospheric air was used as the sole
terminal oxidant. The strong ionic interaction between the organic
cations and inorganic anions led to the heterogeneous nature of this
photocatalysis. The unique redox property of the POM anions benefits
to inhibit the recombination of photo-induced carriers and thus the
activity of heterogeneous IL-POM hybrid is superior to the homo-
geneous quinoline salt precursor. This present strategy provides an ef-
fective and versatile route towards highly efficient heterogeneous
photocatalyst.
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