Accurate Understanding the Catalytic Role of MnO2 in the Oxidative-Coupling of 2-naphthols into 1,1′-bi-2-naphthols

Article abstract of DOI:10.1007/s10562-020-03353-2

Abstract: It has been reported that β-MnO₂ has photocatalytic activity for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols. Nevertheless, it is hard to exclude the possibility that the oxidative-coupling of 2-naphthols is initiated by β-MnO₂ catalysis in dark due to the insufficient investigations in the related reports. In the present work, the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols with different phases MnO₂ catalysis in dark and under visible-light irradiation were systematically investigated. The results revealed that the oxidative-coupling of 2-naphthols is jointly initiated by MnO₂ catalysis and O₂-oxidation, not by MnO₂ photocatalysis. Among the α-MnO₂, β-MnO₂, γ-MnO₂ and δ-MnO₂ catalysis, β-MnO₂ catalysis has the optimal performance, its selectivity for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols is close to 100%, and its catalytic capacity could be well retained after multiple using. Our findings provide comprehensive and accurate understanding the catalytic role of MnO₂ for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols. Graphic Abstract: In the present work, the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols was proved to be jointly initiated by MnO₂ catalysis and O₂-oxidation, not by MnO₂ photocatalysis. β-MnO₂ has the optimal catalytic activity for the oxidative-coupling of 2-naphthols relative to α-MnO₂, γ-MnO₂ and δ-MnO₂. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03353-2

Catalysis Letters
https://doi.org/10.1007/s10562-020-03353-2
Accurate Understanding the Catalytic Role of MnO2
in the Oxidative‑Coupling of 2‑naphthols into 1,1′‑bi‑2‑naphthols
Yuting Wu1 · Long Yang1 · Bo Wu1 · Jie Li1 · Binyao Liu1 · Gaili Ke1 · Faqin Dong1 · Yong Zhou2 · Huichao He1
Received: 22 May 2020 / Accepted: 5 August 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
It has been reported that β-MnO2 has photocatalytic activity for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naph-
thols. Nevertheless, it is hard to exclude the possibility that the oxidative-coupling of 2-naphthols is initiated by β-MnO2
catalysis in dark due to the insufcient investigations in the related reports. In the present work, the oxidative-coupling of
2-naphthols into 1,1′-bi-2-naphthols with diferent phases MnO2 catalysis in dark and under visible-light irradiation were
systematically investigated. The results revealed that the oxidative-coupling of 2-naphthols is jointly initiated by MnO2
catalysis and O2-oxidation, not by MnO2 photocatalysis. Among the α-MnO2, β-MnO2, γ-MnO2 and δ-MnO2 catalysis, β-
MnO2 catalysis has the optimal performance, its selectivity for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols
is close to 100%, and its catalytic capacity could be well retained after multiple using. Our fndings provide comprehensive
and accurate understanding the catalytic role of MnO2 for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols.
Graphic Abstract
In the present work, the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols was proved to be jointly initiated by MnO2
catalysis and O2-oxidation, not by MnO2 photocatalysis. β-MnO2 has the optimal catalytic activity for the oxidative-coupling
of 2-naphthols relative to α-MnO2, γ-MnO2 and δ-MnO2.
Keywords 2-Naphthols · Oxidative-coupling · MnO2 catalysis · 1,1′-Bi-2-naphthols · Mechanism
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article (https://doi.org/10.1007/s10562-020-03353-2) contains
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Y. Wu et al.
1,1′-bi-2-naphthols with diferent phases MnO2 catalysis
in dark and under visible-light irradiation. Our investiga-
tions revealed that the oxidative-coupling of 2-naphthols
is jointly initiated by MnO2 catalysis and O2-oxidation
regardless of light irradiation. Among the α-MnO2, β-
MnO2, γ-MnO2 and δ-MnO2 catalysis, β-MnO2 catalysis
has the optimal performance, its selectivity for the oxi-
dative-coupling of 2-naphthols into 1,1′-bi-2-naphthols is
close to 100%, and its catalytic capacity could be well
retained after multiple using. Our fndings in the present
work provide comprehensive and accurate understanding
the catalytic role of MnO2 for the oxidative-coupling of
2-naphthols into 1,1′-bi-2-naphthols.
1 Introduction
Due to their obvious surface asymmetry and easy reso-
lution into high purity enantiomers, 1,1′-bi-2-naphthols
are typical chiral compounds, which are widely used as
important intermediates in organic synthesis, pharmaceu-
tical, pesticide and dye industries [1, 2]. For the synthesis
of 1,1′-bi-2-naphthols, the oxidative-coupling of 2-naph-
thols is the fundamental and key reaction. Accordingly,
the researches on catalysts for the oxidative-coupling of
2-naphthols have been intensively conducted in the past
decades to achieve the low-cost and environment-friendly
production of 1,1′-bi-2-naphthols [3]. In recent years,
transition-metal-based complexes (such as, vanadium
complex, copper complex, iron complex) have been dem-
onstrated to be homogeneous catalysts for the oxidative-
coupling of 2-naphthols [3–6]. However, these transition-
metal-based complexes have several disadvantages in
preparation, recovery, activity retention due to their com-
plicated structure and homogeneous catalysis property.
Therefore, it is highly desirable to develop heterogeneous
catalyst for the oxidative-coupling of 2-naphthols.
2 Experimental Section
2.1 Preparation and Characterization of MnO2
Four kinds of MnO2 with diferent crystalline phases were
prepared by hydrothermal method. The preparation details of
α-MnO2 are as follows: 0.525 g MnSO4·H2O (Aladdin, AR)
and 1.25 g KMnO4 (Aladdin, AR) were frstly dissolved in
160 mL deionized water by magnetic stirring. Then, the pre-
cursor solution was transferred into a Tefon-lined autoclave
for hydrothermal reaction at 160 °C. After 12 h of hydrother-
mal reaction, the product was collected by centrifugation and
washed by deionized water. Finally, the product was dried
at 80 °C for 5 h, and then annealed at 300 °C in air for 3 h
in a mufe furnace.
Manganese (IV) dioxide (MnO2) is a typical transition-
metal oxide with multi-catalytic activity in dry-cell [7],
lithium ion batteries [8], and super capacitor [9]. In essence,
the various crystal of MnO2 determines the polytropy of
its atomic position, and thus provides abundant sites for
catalytic applications. Signifcantly, Pal et al. and Jain et al.
have reported that β-MnO2 has photocatalytic activity for the
oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols
[10, 11]. Nevertheless, in both of the reported works, the
contrastive investigation of β-MnO2 initiating 2-naphthols
oxidative-coupling in dark condition is insufcient. Based on
their investigations, it is hard to exclude the possibility that
the oxidative-coupling of 2-naphthols is initiated by β-MnO2
catalysis in dark condition, and the light irradiation may not
be the necessary condition. Undeniably, β-MnO2 is a semi-
conductor, which may have potential photocatalytic activity
for 2-naphthols oxidative-coupling. As is well known, the
organic synthesis through semiconductor photocatalysis is
fundamentally initiated by specifc active species (such as,
photogenerated holes, photogenerated electrons, radicals)
[12]. It is noteworthy that the photocatalytic activity of
MnO2 is very weak relative to the well-known photocata-
lysts (such as, TiO2, WO3, Fe2O3, C3N4, BiVO4). In addition,
2-naphthols and 1,1′-bi-2-naphthols have optical property,
and they are not stable enough under long time irradiation
[13]. Accordingly, it is necessary to check the mechanism
of MnO2 catalysis for the oxidative-coupling of 2-naphthols
into 1,1′-bi-2-naphthols carefully.
For the preparation of β-MnO2, γ-MnO2 and δ-MnO2,
their main diferences from the preparation of α-MnO2 were
the ingredient of precursor solution as well as the hydrother-
mal temperature and time. For the preparation of β-MnO2,
1.69 g MnSO4·H2O and 2.28 g (NH4)2S2O8 (Aladdin, AR)
were dissolved in 160 mL deionized water as the precur-
sor solution, and the hydrothermal reaction was conducted
at 140 °C for 12 h. For the γ-MnO2, its preparation pre-
cursor solution consisted of 3.375 g MnSO4·H2O, 4.575 g
(NH4)2S2O8 and 160 mL deionized water, and its hydro-
thermal temperature was 90 °C and reaction time was 24 h.
For the preparation of δ-MnO2, 0.275 g MnSO4·H2O and
1.5 g KMnO4 were dissolved in 160 mL deionized water as
the precursor solution, and the hydrothermal reaction was
conducted at 240 °C for 24 h. For mechanism confrmation,
monoclinic BiVO4 as a contrast to β-MnO2 was prepared by
hydrothermal method according to a reported reference [14].
To investigate the photocatalytic activity of β-MnO2,
P25(TiO2, Aladdin) and monoclinic BiVO4, their powder
was coated on FTO glass substrate to form flm electrode
by the following procedures. First of all, 10 mg of pow-
der sample was dispersed in 1250 μL of deionized water
and 1250 μL of ethanol by sonication for 20 min. Then,
Based on above analyses, we systematically inves-
tigated the oxidative-coupling of 2-naphthols into
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Accurate Understanding the Catalytic Role of MnO2 in the Oxidative-Coupling of 2-naphthols…
50 μL of 5 wt% Nafon solution was added into the dispersed
solution and further sonicated for 30 min. Finally, 50 μL of
the dispersion was drop-cast onto an FTO glass substrate
(1 cm×2 cm) and then dried at room temperature for 60 min
in air. The photocurrent measurement for the flms was con-
ducted on a CH Instruments 760E workstation with a three-
electrode quartz cell. 300 W xenon lamp (Beijing China
Education Au-light Co., Ltd) was used as light source, and
the illumination on the flms was adjusted to 100 mW/cm2.
The as-prepared flms were the working electrode, a Pt wire
(0.5 mm diameter) was used as the counter electrode, and the
reference electrode was a saturated calomel electrode (SCE,
Shanghai INESA Scientifc Instrument). The electrolyte was
0.1 M Na2SO4 aqueous solution.
3 Results and Discussion
3.1 Characterizations of MnO2
To check the catalytic activity of MnO2 for the oxidative-
coupling of 2-naphthols into 1,1′-bi-2-naphthols, MnO2
with four crystalline phases were prepared and studied. As
shown in Fig. 1a, the difraction peaks of the as-prepared
MnO2 samples well correspond to the α-MnO2 (JCPDS
44-0141), β-MnO2 (JCPDS 24-0735), γ-MnO2 (JCPDS
14-0644) and δ-MnO2 (JCPDS 80-1098), respectively.
In addition, the XRD pattern of the monoclinic BiVO4
and P25 (TiO2) was exhibited in Fig. S1 (please see the
Electronic supplementary information), BiVO4 and P25
were used as contrasts to β-MnO2 for the photocatalytic
oxidative-coupling of 2-naphthol in the present work. The
XPS investigations indicated that the Mn 2p and O 1 s
peaks of the β-MnO2 sample are consistent with the typi-
cal peaks of β-MnO2. The morphology of the as-prepared
MnO2 samples was shown in Fig. 1e, f, in which obvious
morphology diference between the samples was observed
due to their diferent phases. However, four kinds of MnO2
samples all have nanoscale morphology and high specifc
surface area for catalytic reaction.
The crystalline phase of the as-prepared MnO2 samples
before and after reacting with 2-naphthol was checked by
X-ray difractometer (XRD-6000, Shimadzu). X-ray photo-
electron spectroscopy (XPS, ESCALAB 250, Thermo Sci-
entifc) with monochromated Al-Kα radiation was used to
analyze the chemical states of the MnO2 samples. In addi-
tion, the morphology of the MnO2 samples was observed by
scanning electron microscopy (SEM, Zeiss SUPRA 55 VP).
2.2 The Synthesis of 1,1′‑bi‑2‑naphthols
For the synthesis of 1,1′-bi-2-naphthols, 0.1 g MnO2 powder
was added into 40 mL of 25 mm 2-naphthols acetonitrile or
toluene solution. Due to the toxicity of toluene solvent and
same synthetic efect (see Fig. S2), our investigations were
mainly conducted in 2-naphthols acetonitrile solution. The
synthesis reaction was performed at atmospheric pressure
and room temperature, and the MnO2 powder was continu-
ously stirred. For the sake of comparison, the 2-naphthols
solution/MnO2 powder reaction system was put in dark and
visible-light irradiation condition, respectively. In addition,
2-naphthol solution/P25(TiO2) powder reaction system and
2-naphthol solution/BiVO4 powder reaction system were
similarly treated. As the reaction time increased, the 2-naph-
thol acetonitrile solution gradually changed from colorless
into brown. To detect the formation of 1,1′-bi-2-naphthols,
the resultant solution was analyzed by liquid chromatogra-
phy (LC-16, Shimadzu) and liquid chromatography-mass
spectrometry (LCMS-8040, Shimadzu), respectively. The
detection wavelength of liquid chromatography was 280 nm,
and the volume ratio of mobile phase was 70% methanol-
H2O. Before the detection of 1H and 13C NMR spectroscopy
(JEOL Eclipse 400), the 1,1′-bi-2-naphthols product was
purifed by SiO2 gel thin plate using ethyl acetate/hexane
(4:6) as an unfolding agent. In addition, for the 2-naphthol/β-
MnO2 powder system, the dissolved Mn in the reaction solu-
tion after 12 h was detected by inductively coupled plasma
mass spectrometry (ICP-MS, Thermo ICAP6300).
3.2 MnO2 Catalysis for the Oxidative‑Coupling
of 2‑naphthol
Based on the detections of 2-naphthol and 1,1′-bi-2-naph-
thol (shown in Figs. S2, S3 and S4), the catalytic activ-
ity of β-MnO2 in dark and under visible-light irradiation
for the oxidative-coupling of 2-naphthol into 1,1′-bi-
2-naphthol was displayed in Fig. 2a. It is apparent that
2-naphthol was gradually turned into 1,1′-bi-2-naphthol
with β-MnO2 catalysis both in dark and under visible-light
irradiation, and the selectivity of β-MnO2 catalysis for the
oxidative-coupling of 2-naphthol was close 100%. For the
2-naphthol/β-MnO2 reaction in dark and under visible-
light irradiation, the slight diferences in the conversion
rate of 2-naphthol and selectivity indicated that irradiation
is not the necessary condition for the initiation of β-MnO2
catalysis. Further investigations found that the infuence
of reaction temperature on β-MnO2 catalysis in dark for
the oxidative-coupling of 2-naphthol is not signifcant
(see Fig. 2b), suggestting that the synthesis of 1,1′-bi-
2-naphthol through β-MnO2 catalysis can be achieved at
mild temperature. In addition, in the 2-naphthol/α-MnO2,
2-naphthol/γ-MnO2 and 2-naphthol/δ-MnO2 reaction sys-
tem, their conversion of 2-naphthol into 1,1′-bi-2-naphthol
was lower than the 2-naphthol/β-MnO2 reaction system
at same reaction time and dark condition (Fig. S5 and
1 3

Y. Wu et al.
Fig. 1 XRD pattern, Mn 2p and O 1s XPS peaks, SEM images of the as-prepared MnO2 samples
Fig. 2c). Since the contrast investigation was conducted
using same amount of MnO2 sample, this fnding suggests
that the catalytic activity of MnO2 for 2-naphthol oxida-
tive-coupling is closely related to its crystalline phase.
It should be noted that our observations on MnO2 catal-
ysis for the oxidative-coupling of 2-naphthol into 1,1′-bi-
2-naphthol is remarkably diferent from the previous reports
by [10, 11]. They reported that the oxidative-coupling of
2-naphthol into 1,1′-bi-2-naphthol was initiated by β-MnO2
photocatalysis, but not provided rigorous contrasts conduct-
ing in dark condition. To further confrm our fndings, we
investigated the possibility of synthesizing 1,1′-bi-2-naph-
thol by P25 (TiO2) photocatalysis and monoclinic BiVO4
photocatalysis. Herein, P25 and BiVO4 were selected as
contrasts to β-MnO2 because they are well-known transi-
tion-metal oxide photocatalysts [15, 16]. Additionally, the
Ti atom of TiO2 and the V atom of BiVO4 have close posi-
tion to the Mn atom of MnO2 in the periodic table, thery
1 3

Accurate Understanding the Catalytic Role of MnO2 in the Oxidative-Coupling of 2-naphthols…
Fig. 2 a Under diferent time of β-MnO2 catalysis and β-MnO2 pho-
tocatalysis, the conversion rate of 2-naphthol into 1,1′-bi-2-naphthol
and the reaction selectivity. b Under diferent temperature of β-MnO2
catalysis (12 h), the conversion rate of 2-naphthol into 1,1′-bi-2-naph-
thol and the reaction selectivity. c The conversion rate of 2-naphthol
into 1,1′-bi-2-naphthol with α-MnO2, β-MnO2, γ-MnO2 and δ-MnO2
catalysis (12 h). The photocurrent response of d β-MnO2, e P25 and f
BiVO4 in 0.1 M Na2SO4 aqueous solution at 0 V vs SCE
have similar electronic structure. For the 2-naphthol/P25
and 2-naphthol/BiVO4 photocatalysis system, the color of
2-naphthol acetonitrile solution was changed from colorless
to light-yellow after the 12 h irradiation due to the opti-
cal activity of 2-naphthol (Fig. S6). However, both of P25
photocatalysis and BiVO4 photocatalysis did not initiate the
oxidative-coupling of 2-naphthol into 1,1′-bi-2-naphthol (see
Figs. S7 and S8). In 0.1 M Na2SO4 aqueous solution, both
of P25 and BiVO4 showed obviously higher photocurrent
than β-MnO2, confrming their higher photocatalytic activ-
ity (Fig. 2d–f). In essence, the conversion of 2-naphthol into
1,1′-bi-2-naphthol is a oxidation reaction, which involves
2-naphthol dehydrogenating and coupling. If the oxidative-
coupling of 2-naphthol is initiated by β-MnO2 photocataly-
sis, the active species of coupling reaction must be origi-
nated from the photogenerated holes or electrons of β-MnO2.
As shown Fig. 2d–f, β-MnO2 has weaker photocatalytic
activity than P25 and BiVO4, but no 1,1′-bi-2-naphthol was
formed in the 2-naphthol/P25 and 2-naphthol/BiVO4 pho-
tocatalysis systems. Using the yield of 1,1′-bi-2-naphthol in
the normal 2-naphthol/β-MnO2 system as the reference, we
investigated the yield of 1,1′-bi-2-naphthol in N2-bubbling
and O2-bubbling 2-naphthol/β-MnO2 system in dark and
under visible-light irradiation, respectively. As summarized
1 3

Y. Wu et al.
in Table S1, the yield of 1,1′-bi-2-naphthol was obviously
reduced in the N2-bubbling 2-naphthol/β-MnO2 systems,
while in the O2-bubbling 2-naphthol/β-MnO2 systems was
increased, regardless of irradiation. These observations indi-
cated that O2 plays a oxidant role in the oxidative-coupling
of 2-naphthol into 1,1′-bi-2-naphthol, β-MnO2 is the cata-
lyst. Actually, O2 has been demonstrated to be the oxidant
in the vanadium complex-catalyzed, and iron complex-cat-
alyzed coupling of 2-naphthols [4, 5]. From above observa-
tions, we could confrm that the 2-naphthol coupling into
1,1′-bi-2-naphthol is jointly driven by MnO2 catalysis and
O2-oxidation, not by MnO2 photocatalysis.
MnO2 catalysis for the oxidative-coupling of 2-naphthols
into 1,1′-bi-2-naphthols, we comparatively analysed the β-
MnO2 catalysts before and after reacting with 2-naphthol.
Figure 3a shows the capacity retention of same β-MnO2
catalysts that used for the oxidative-coupling of 2-naph-
thol within six times. Evidently, the catalytic activity of
β-MnO2 was stably maintained even if reused six times,
and the conversion rate of 2-naphthol into 1,1′-bi-2-naph-
thol for the sixth repeating was close to 95%. The slight
activity attenuation of β-MnO2 was jointly caused by the
dissolution of MnO2 during the reaction and the formation
of amorphous MnOOH, which were confrmed by the ICP-
MS and XPS investigations. In the 2-naphthol/β-MnO2
system, 0. 21 μM of Mn ion was detected in the acetoni-
trile solution after 12 h reaction. For the β-MnO2 before
and after catalytic reaction, the comparative XRD char-
acterization shown that the crystalline phase of the after-
used catalysts is still β-MnO2, but its peaks difraction
intensity is weaker than the pristine β-MnO2 (Fig. S12).
In the comparative XPS investigations, the Mn 2p and O
1s spectrum of the after-used β-MnO2 were observed to
be diferent from the pristine β-MnO2 (Fig. 3b, c). In par-
ticular, the Mn–O–H oxygen atoms accounted for a higher
proportion in after-used β-MnO2 relative to the pristine
3.3 MnO2 Catalysis for the Oxidative‑Coupling
of 2‑naphthols
The applicability of MnO2 catalysis for the oxidative-cou-
pling of 2-naphthol derivatives into corresponding 1,1′-bi-
2-naphthols was further investigated and the result was
summarized in Table 1 and Figs. S9–S11. Similar to the
reaction of 2-naphthol coupling into 1,1′-bi-2-naphthol,
MnO2 catalysis exhited satisfactory activty for the oxida-
tive-coupling of others 2-naphthol derivatives into corre-
sponding 1,1′-bi-2-naphthols. To understand and evaluate
Table 1 The yield of 2-naphthols oxidative-coupling into 1,1′-bi-2-naphthols through β-MnO2 catalysis
Reactant
Product
Yield (%)
99.3
88.6
85.5
82.8
1 3

Accurate Understanding the Catalytic Role of MnO2 in the Oxidative-Coupling of 2-naphthols…
Fig. 3 a The catalytic capacity of the same β-MnO2 catalysts for the
oxidative-coupling of 2-naphthol into 1,1′-bi-2-naphthol in six cycles.
b Mn 2p and c O 1s XPS peaks of the β-MnO2 catalysts before and
after the catalytic synthesis of 1,1′-bi-2-naphthol. d Schematic illus-
tration of the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naph-
thols with MnO2 catalysis
β-MnO2, suggesting the presence of manganese hydroxide
[17]. Similar fnding was also observed in the Mn 2p spec-
trum of the after-used β-MnO2, which can be deconvoluted
into Mn(III) and Mn(IV) [17, 18]. Considering that the
oxidative-coupling of 2-naphthol into 1,1′-bi-2-naphthol
involves eletrons losing and the pKa value of acetonitrile
solvent is 25 (at 25 °C), thus the formation of amorphous
manganese hydroxide (MnOOH) on β-MnO2 catalysts is
very probable in thermodynamics. On the basis of our
investigations, the mechanism of MnO2 catalysis for the
oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols
could be illustrated as Fig. 3d.
oxidative-coupling of 2-naphthols relative to the α-MnO2,
γ-MnO2 and δ-MnO2. Though the dissolution of β-MnO2 and
the formation of MnOOH were present, the catalytic selec-
tivity of β-MnO2 for the oxidative-coupling of 2-naphthols
into 1,1′-bi-2-naphthols was close to 100%, and its catalytic
capacity could be well retained after multiple using. Our
fndings provide comprehensive and accurate understand-
ing the catalytic role of MnO2 for the oxidative-coupling of
2-naphthols into 1,1′-bi-2-naphthols.
Acknowledgements This work was supported by NSF of China (Grant
No. 41702037) and Project of State Key Laboratory of Environment-
friendly Energy Materials, Southwest University of Science and Tech-
nology (Grant No. 18fksy0216).
Compliance with Ethical Standards
4 Conclusions
Conflict of interest No confict of interest between the authors.
In summary, the oxidative-coupling of 2-naphthols into
1,1′-bi-2-naphthols was proved to be jointly initiated by
MnO2 catalysis and O2-oxidation, not MnO2 photoca-
talysis. β-MnO2 has the optimal catalytic activity for the
1 3

Y. Wu et al.
10. Jana S, Pande S, Sinha A, Pal T (2008) Synthesis of superpara-
magnetic β-MnO2 organosol: a photocatalyst for the oxidative
phenol coupling reaction. Inorg Chem 47:5558–5560
11. Kumar A, Aathira M, Pal U, Jain S (2018) Photochemical oxida-
tive coupling of 2-naphthols using a hybrid reduced graphene
oxide/manganese dioxide nanocomposite under visible-light irra-
diation. ChemCatChem 10:1844–1852
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Afliations
Yuting Wu1 · Long Yang1 · Bo Wu1 · Jie Li1 · Binyao Liu1 · Gaili Ke1 · Faqin Dong1 · Yong Zhou2 · Huichao He1
2
* Huichao He
hehuichao@swust.edu.cn
Ecomaterials and Renewable Energy Research Center,
School of Physics, Nanjing University, Nanjing 211102,
China
1
State Key Laboratory of Environmental-Friendly Energy
Materials, Key Laboratory of Solid Waste Treatment
and Resource Recycle of Ministry of Education, School
of Materials Science and Engineering, Southwest University
of Science and Technology, Mianyang 621010, China
1 3