Nature-mimic fabricated polydopamine/MIL-53(Fe): Efficient visible-light responsive photocatalysts for the selective oxidation of alcohols

Article abstract of DOI:10.1039/c9nj04929k

Polydopamine/MIL-53(Fe) (PDA/MIL-53(Fe)) nanocomposite photocatalysts were synthesized with PDA (PDA = polydopamine) and MIL-53(Fe) using a nature-mimicking method. The structures, morphologies, optical properties and thermal stabilities of all the synthesized materials were characterized using a series of methods. In particular, the separation efficiency of photogenerated charge significantly increased after the incorporation of PDA into MIL-53(Fe), which resulted in an elevated photocatalytic activity of PDA/MIL-53(Fe) compared with the control groups. The PDA/MIL-53(Fe) nanocomposite could accelerate the conversion of primary or secondary alcohols into the corresponding aldehydes or ketones with a high specificity by direct hole-involving oxidation under visible-light irradiation and room temperature. The catalysts could be cycled at least three times without a significant decrease in the catalytic activity and this result showed the excellent recyclability and stability of the catalysts.

Full text of DOI:10.1039/c9nj04929k

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ISSN 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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Nature-Mimic Fabricated Polydopamine/MIL-53(Fe): Efficient
Visible-light Responsive Photocatalysts for the
Selective Oxidation of Alcohols
Shuangyan Menga, Wei Zenga, Mingming Wanga, Litong Niua, Shaoping Hua, Bitao Sua, Yaoxia
Yanga, Zhiwang Yanga,*, Qunji Xueb,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Polydopamine/MIL-53(Fe) (PDA/MIL-53(Fe)) nanocomposite photocatalysts
were synthesized with PDA (PDA = polydopamine) and MIL-53(Fe) by a nature-
mimicking method. The structures, morphologies, optical properties and thermal
stabilities of all the synthesized materials were characterized by a series of
methods. In particular, the separation efficiency of photogenerated charge
significantly increased after the incorporation of PDA into MIL-53(Fe), which
resulted in an elevated photocatalytic activity of PDA/MIL-53(Fe) compared with
the control groups. PDA/MIL-53(Fe) nanocomposite could accelerate the
converting of the primary or secondary alcohols into the corresponding aldehydes
or ketones with a high specificity by direct hole-involving oxidation under visible-
light irradiation and room temperature. The catalysts could be cycled at least three
times without significant catalytic activity decrease and this result showed
excellent recyclability and stability of the catalysts.
assembly strategy4 and regarded as potential
photocatalysts11. MOFs have unique micro
1. Introduction
Recently, the rapid increase of consumption of
structure, various adsorption properties, diverse
catalyst sites12, open metal sites13, 14, high surface
area15, easily functionalized structure16, well-
defined pore structures15, 17 and tunable chemical
energy caused by growing population and
industrialization worldwide and the shortage of
clean and renewable energy is a major conflict of
our world. Efficient use of solar energy is one of
properties18
compared
with
inorganic
the promising ways to deal with the conflict1, 2
.
semiconductor. Varieties of photoactive MOFs,
such as ZIFs, MILs and UiOs series, are developed
by researchers as promising photocatalysts19.
Among them, Fe-based MOFs attracted more and
more attention for the earth-abundant20,
inexpensive, nontoxic properties and visible light
responsible of Fe element21.
Solar energy being directly converted into the form
of heat or other energy forms is a problem that
many researchers are trying to solve with different
technologies3. Among many technologies, the use
of photocatalysis is regarded as one of the most
advantageous strategies.
Visible-light-responsive photocatalysts are
pivotal in conversion of solar energy. Metal-
organic frameworks (MOFs), a class of crystalline
nanoporous materials, which are widely used in H2
storage5, carbon dioxide capture6, catalysis7, gas
storage8, gas separation9 and drug delivery10, are a
new class of hybrid solids prepared by self-
MIL-53(Fe),
a
typical
metal-organic
frameworks, comprising 1,4-bezenedicarboxylate
(H2BDC) linker and FeO4(OH)2 octahedra, is a
typical three-dimensional porous material22. MIL-
53(Fe) has unique features, such as small size of
Fe(III)-oxide cluster, better visible light absorption
and limited recombination of the photogenerated
charge. The application of MIL-53(Fe) in the
decolorization of organic dyes23, reduction of
Cr(VI)24, degradation of pollutant25 and selective
oxidation of alcohols is mediated by its excellent
photocatalytic activity26.
Polydopamine (PDA) which is a catechol
derivative and contains many catechol groups27, a
synthetic
analogue
of
natural
melanin
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(eumelanin)28, is produced by oxidizing and self-
MIL-53(Fe) was prepared by means in the
View Article Online
DOI: 10.1039/C9NJ04929K
32
polymerizing under alkaline conditions (pH = 8.5) literature . 1 mmol of FeCl3·6H2O (0.5406 g) and
of dopamine29, Excellent biocompatibility and low 1 mmol of H2BDC (0.3323 g) were added to 0.56
cytotoxicity30 make it applied in chemistry, biology, mmol of DMF (43.14 mL) and stirred at room
medicine, materials sciences and photocatalytic temperature for 60 min in a beaker, then the
degradation of methylene blue (MB)27. Lee and co- mixture was transferred into a 100 mL Teflon-lined
o
workers described the current state of the art in autoclave and was sealed and heated to 170 C
polydopamine coating methods31. PDA which keeping for 24 h. After the reaction, the autoclave
possesses good light responsiveness and promising was allowed to cool naturally to room temperature.
photoconductivity under visible-light irradiation, The resultant suspension was washed with DMF
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showed
high
separation
efficiency
for and C2H5OH for three times, respectively.
photogenerated charge32,33. Due to its excellent Sequentially, the raw product was soaked in
photocatalytic properties, many researchers have methanol for three days to remove the guest
applied polydopamine in the filed of photocatalysis, molecules absorbed in the pores of the structure of
for example, Au-TiO2 and Polydopamine for MIL-53(Fe). The powder was then collected by
degradation of tetracycline under visible light34, centrifuging and dried under vacuum at 100 C
o
polydopamine-coated porous substrates as a overnight.
platform for the wastewater treatment under
2.2.2 Preparation of PDA
sunlight35 and Ag-Fe3O4 nanoparticles supported
on polydopamine-functionalized porous cellulose
PDA was prepared according to the literature
acetate microspheres for catalytic and antibacterial 27. Dopamine hydrochloride was added in water
applications36.
and the solution was stirred at room temperature for
In our previous study, we synthesized a series 60 min. After the pH value of the mixture was
of MOFs/conjugated polymer nanocomposite adjusted to 8.5 with 1 M NaOH solution, the
photocatalysts and applied those materials to mixture was magnetically stirred vigorously at 60
selective organic oxidation and reduction reactions oC for 24 h in an oil bath. The resulted black
26-27,
29-30. Following previous researches, we suspension was fully washed by water until the
expected to synthesize materials with better light filtrate was neutral. The residual was then dried
response properties, in addition, with higher under vacuum at 60 oC overnight.
conversion of selective oxidation products.
2.2.3 Preparation of PDA/MIL-53(Fe)
Therefore, polydopamine/MIL-53(Fe) (PDA/MIL-
53(Fe)) nanocomposites were generated in our lab
A
variety of polydopamine/MIL-53(Fe)
concerned with the excellent light-involving (PDA/MIL-53(Fe)) with different mass ratios of
properties of MIL-53(Fe) and polydopamine in this polydopamine (PDA) was prepared by a nature-
article. The properties and the application of the mimicking method 27. In detail, 500 mg of MIL-
materials in the selective oxidation of alcohols 53(Fe) was added to 100 mL of water, the
were investigated.
suspension was dispersed evenly after half an hour
of sonication treatment. Then different mass of
dopamine hydrochloride were added and stirred for
1 h. After the pH value of the mixture was adjusted
2 Experimental
2.1 Materials
to 8.5 with 1 M NaOH, the solution was stirred in
o
FeCl3·6H2O, CCl4 were supplied by Beijing
an oil bath at 60 C for 24 h and then cooled and
Chemical
hydrochloride was obtained from Shanghai Yuheng
Medical Technology Center. 1,4-
Reagent
Co.,
Ltd.
Dopamine
centrifuged. In order to remove the unreacted
dopamine, the collected solid samples were
dispersed and rinsed with deionized water for
several times. The final product was then dried
under vacuum at 60 oC overnight.
Benzenedicarboxylic acid (H2BDC), benzalcohol
and its derivatives were obtained from Aladdin
Reagent Co., Ltd. and Beijing J&K Scientific Co.,
Ltd. Absolute ethanol, anhydrous methanol, N,N-
dimethylformamide (DMF) were obtained from
Tianjin Chemical Reagent Co., Ltd. NaOH was
obtained from Sinopharm Chemical Reagent Co.,
Ltd.
2. 3 Characterization
The Fourier transform infrared (FT-IR,
Nicolet NEXUS 670 spectrometer) spectroscopy
was used to characterize the structure of samples.
X-ray photoelectron spectroscopy (XPS, Thermo
2.2 Preparation of photocatalysts
2.2.1 Preparation of MIL-53(Fe)
radiation) measurements characterization was used
for investigate the chemical states of the catalysts.
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The crystal morphology of the catalyst was concentrations of alcohol and aldehVyiedweA/rkticeletOonnlinee
DOI: 10.1039/C9NJ04929K
characterized by powder X-ray diffraction (XRD, measured at the time t, respectively.
Rigaku D/ max-2400, k = 1.5418 Å). The UV-vis
diffuse reflectance spectra (UV-vis DRS, Cary 500
2. 5 Electrochemical measurements
UV-vis-NIR
spectrophotometer)
and
the
The M-S curve and the EIS analysis were put
photoluminescence spectra (PL, FluoroSENS into effect with AC impedance-potential model in a
Luminescence Fluorescence spectrophotometer) standard three-electrode system with an electrolyte
were used to characterize the optical properties of solution of 0.2 M Na2SO4. That is to say, an
the samples. The morphology and structure of saturated calomel electrode acted as the reference
catalysts were observed with a scanning electron electrode, a Pt foil electrode acted as the counter
microscope (SEM, Ultra Plus, Carl Zeiss, Germany, electrode and a FTO glass electrodes acted as
the test voltage was 6.0kV) and transmission working electrodes, respectively. FTO glass is
electron microscope (TEM, JEOL model JEM 2010 pretreated with isopropanol, acetone and water (V1 :
EX microscope). The electrochemical analysis V2 : V3=1 : 1 : 1). Sequentially, the progress was
including the Mott-Schottky analysis (M-S), the operated at open circuit potential. The frequency
electrochemical impedance spectroscopy (EIS) was range of open circuit potential with an AC voltage
performed on an electrochemical workstation magnitude which increased 5 mV each time was
(CHI660E, Shanghaichenhua, Inc.). The thermo between 105 and 10-2 Hz. For the sake of preparing
gravimetric analysis (TGA, TA Instruments photocatalyst-based composite electrodes, the
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Discovery Thermo gravimetric analyzer) was prepared photocatalysts PDA/MIL-53(Fe) were
o
obtained from room temperature to 800 C at the dispersed in chitosan solution to form 3 mgmL-1
o
ramping of 10 C min-1 in N2 atmosphere. The solution and ultrasonicated for 30 min to form a
products of the selective oxidation of alcohols were colloidal solution, after then 60 uL of colloidal
recorded through gas chromatograph (GC, solution was dropped on the conductive side of the
Shimadzu GC-2010) analyses. The BET specific FTO glass and dry overnight at 100oC.
surface areas (SBET) were conducted on an
3 Results and discussion
were degassing at 250 C to remove moistures and 3.1 Studies on PDA/MIL-53(Fe)
Autosorb-iQ2-MP instrument (first, the samples
o
o
impurities, which were analyzed at -195 C (liquid
Fig.1 showed the typical XRD patterns of the
nitrogen) in the analysis room).
MIL-53(Fe) and a sequence of PDA/MIL-53(Fe)
nanocomposites. Distinctly, the well-defined
diffraction peaks disclosed the high crystallinity of
experimental synthesis MIL-53(Fe), which was in
good agreement with the simulated one 32. But
PDA/MIL-53(Fe) had not similar crystal form with
that of MIL-53(Fe). On the one hand, PDA might
wrap closely on the surface of MIL-53(Fe), which
blocking original MIL-53(Fe) diffraction peaks
from appearance. On the other hand, with the
increasing of the content of MIL-53(Fe), the
2. 4 Photocatalytic experimental details
The photooxidation ability of PDA/MIL-53(Fe)
nanocomposites was texted by oxidation of aryl
alcohol under visible light irradiation. In detail, 0.2
mmol of aryl alcohol and 5 mg of photocatalyst
sample were added in a solution of 6 mL of CCl4 in
an open quartz tube. In order to ensure the
adsorption-desorption
equilibrium
between
catalysts and substrates before the photocatalytic
reaction, so the suspension was stirred for 1 h in the
dark. Then the mixture solution was performed for
9 h under a visible-light illumination. The visible-
light source was a 500 W metal halogen lamp
(PLS-SXE 300, BiL on Co. Ltd. Shanghai China).
After reaction, the reaction results were recorded
on a gas chromatograph, the formulas (1) and (2)
were used to calculate the conversion of alcohol
and the selectivity of aldehyde or ketone in the
reaction process, respectively.
characteristic
diffractions
of
PDA
were
strengthened increasingly in the composites, and
the diffraction peaks moved to the big 2 degrees.
Conversion of the product (%) = [(C0-Cn) / C0] × 100% (1)
Selectivity of the product (%) = [C a / (C0-Cn)] × 100%
(2)
Where C0 is the initial concentration of
alcohol prior to reaction, Cn and Ca are the
Fig. 1 XRD patterns of the samples
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For the sake of further studying the functional the PDA. The elemental mappingViewfAortricle Otnhlinee
DOI: 10.1039/C9NJ04929K
group of PDA/MIL-53(Fe) and analyzing its PDA/MIL-53(1 : 20) composites were presented in
molecular structure, the general features of FT-IR Fig.3f. It could be found that the surfaces of the
spectroscopy for MIL-53(Fe) and different composites were composed of carbon, oxygen,
PDA/MIL-53(Fe) composites were carried out, and nitrogen, and iron element (Fig.3g-j.). The
the results were shown in Fig.2. For the FT-IR elements contents of the composites were also
spectra of MIL-53(Fe), the absorption peak at 1597 presented in Fig.3k-l, and the corresponding
cm-1 can be attributed to the stretching vibration of contents of carbon, oxygen, nitrogen, and iron
the benzene ring skeleton. The asymmetric and elemental were 18.5, 25.01, 1.61, 54.88%,
symmetric vibrations of the carboxyl group respectively.
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produced two strong peaks at 1504 and 1392 cm-1,
respectively. The absorption peaks of C-H in
benzene in-plane bending vibrations could be
observed at 1018 cm-1. The peak at 749 cm-1
corresponded to the C-H stretching mode of the
benzene in the organic linkers. For PDA, the
indoline and indole structures of the stretching
vibration were observed at 1508 and 1620 cm-1 27
.
Besides, the characteristic peaks of MIL-53(Fe)
and PDA can be observed in the FT-IR of the
PDA/MIL-53(Fe) composites.
Fig. 3 SEM images of pure PDA (a); MIL-53(Fe) (b);
PDA/MIL-53(Fe) (1 : 20, c); TEM images (d, e), EDS
mapping (f-j) and the elements content (k-l) of
PDA/MIL-53(1 : 20)
Surface stoichiometry information of the
chemical composition and valence state of
PDA/MIL-53(1 : 20) composite was analyzed by
Fig. 2 FT-IR spectra of the samples
The morphology and microstructures of MIL-
53(Fe) and PDA/MIL-53(Fe) were observed using
SEM and TEM as showed in Fig.3. PDA presents a
microsphere structure as showed in Fig.3a. The
original MIL-53(Fe) showed the shuttle-like
structure with smooth surfaces in Fig.3b. From the
SEM images of PDA/MIL-53(Fe) in Fig.3c, it
could be seen that the nanocomposite
approximately maintained the shuttle-like, and
PDA was uniformly and tightly adhered on the
surface of MIL-53(Fe), resulting in a rough surface
of the nanocomposites, which might be due to the
directing growth of PDA microspheres on the
surface of MIL-53(Fe) during nature-mimicking
preparation. With the small changes of the structure
of the nanocomposites, the XRD analysis results
changed accordingly (Fig. 1). In order to further
confirm this conclusion, the TEM images of the
PDA/MIL-53(1: 20) were showed in Fig.3d and
Fig.3e. It can be seen that the surface of PDA/MIL-
53(1: 20) composite was fuzzy, and PDA appeared
at the edge of MIL-53(Fe). All results demonstrated
that the MIL-53(Fe) was successfully modified by
XPS. Fig.4a showed
a
survey spectra
demonstrating that Fe, N, O and C elements which
could be observed in the PDA/MIL-53 (1 : 20)
composites. The XPS spectra (Fig.4b) of C1s was
deconvoluted into five component peaks
corresponding to the binding energies at about
284.20, 285.45, 286.33 and 288.08 eV. The peak at
284.20 eV corresponded to the sp2 carbon in PDA
and benzoic rings. The peaks at 285.45 and 286.33
were in response to the C-C and the C-O bond in
PDA, respectively. The carboxylate groups (C=O)
33
in H2BDC linkers was emerged at 288.08 eV . In
addition, the N1s spectra (Fig.4c) of PDA/MIL-
53(Fe) was deconvoluted into two component
peaks with the binding energies at about 400.58
and 399.58 eV, due to the sp2-bonded aromatic N
of -NH2 and the tertiary N of -NH37,38, respectively.
Fig.4d showed the XPS spectra of O 1s. The
spectra could be deconvoluted into three peaks at
the binding energies of 532.33, 531.33 and 530.08
eV, which were matched to the oxygen atoms in the
carboxylate groups of the H2BDC linkers (531.57
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eV), the Fe-O bonds in MIL-53(Fe) (532.45 eV), oxygen to metal 4 1 . Moreover, tVhieweArtsiclme Oanlilnel
DOI: 10.1039/C9NJ04929K
and the O-C in H2BDC (530.20 eV), respectively. characteristic peak at 445 nm may be due to the
4
4
Fig.4e showed the Fe2p spectra of PDA/MIL- state transition (6A1g = > A1g + Eg (G)) in
53(Fe), which displayed two peaks with the binding
Fe (III) 42. Obviously, the onset of the main
energies at about 711.20 and 725.08 eV. The peak
characteristic absorption edge in the visible-light
at 711.20 eV was owing to the Fe2p3/2, and the
region with a wavelength of MIL-53(Fe) was
binding energy peak at 725.08 eV was assigned to
estimated to be 456 nm. Based on the relation of Eg
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the Fe2p1/2 39. The peak separation of the two peaks
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= 1240/λ , as a result, the calculated optical band
of Fe2p, as follows, Δ = 2p1/2 - 2p3/2 = 13.88 eV,
gap of MIL-53(Fe) was 2.72 eV. With more and
was on the verge of previously reported for α-
more PDA being wrapped on the surface of MIL-
Fe2O3 25. Besides, at the peak of the binding energy
53(Fe), the absorption spectra exhibited a slight red
of 713.33 eV, a shake up line of the lower Fe2p
3/2
shift to 710 nm and an enhanced absorption
to Fe2p1/2 transition in the satellite peak was also
intensity of light in the visible-light region, which
observed 40. These results were consistent with FT-
suggested that the PDA/MIL-53(Fe) composites
IR spectra, which further confirmed the
had a better ability to harvest more visible light
conformation of PDA/MIL-53(Fe) composites.
than the pure MIL-53(Fe).
Fig.5 The UV-vis DRS (a), PL spectra (b, c), EIS
Nyquist plot (d) and
Mott-Schottky plot (e) of the samples
Fig.4 XPS spectra of PDA/MIL-53(1 : 20): survey
(a); C 1s (b); N 1s (c); O 1s (d); Fe 2p (e)
As for photocatalysis, the lifetime of the
generated charge carriers: the hole (h+) as well as
the electron (e-), was a vital factor. Longer lifetime
would give higher photocatalytic activity. The
lifetime of the charge carriers can be revealed by
PL spectra of the catalyst 44. Fig.5b showed the PL
spectra of the as-prepared samples. Generally, a
lower PL intensity means a higher lifetime of
electron-hole pairs. Obviously, MIL-53(Fe) had
UV-vis diffuse reflectance spectra (UV-vis
DRS) revealed the optical absorption properties of
the samples. As showed in Fig.5a, all of the as-
prepared samples showed the strong absorption
bands in the wavelength range of 200-420 nm. The
spectra of MIL-53(Fe) characteristic absorption
signal was observed at about 250 nm, it was
assigned to the ligand-to-metal charge transfer
(LMCT), implying the bonding of carboxylate
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higher photoluminescence intensity in comparison
with that of the PDA/MIL-53(Fe) composites. As
shown in Fig.5c, it could also be observed that the
PL intensity of the as-prepared samples followed as
the order: PDA/MIL-53(1 : 5) > PDA/MIL-53(1 :
10) > PDA/MIL-53(1 : 100) > PDA/MIL-53 (1 :
50) > PDA/MIL-53(1 : 1) > PDA/MIL-53(1 : 2) >
PDA/MIL-53(1 : 20), the results showed that the
PDA/MIL-53(1 : 20) composites had the high
separation efficiency of electron-hole pairs. With
reference to the morphology structure of
PDA/MIL-53(1 : 20) composites (Fig. 3c-3e), it
could be concluded that the good interfacial contact
between MIL-53(Fe) and PDA could prolong the
lifetime of the photon-generated carrier. Namely,
PDA could be regarded as a good acceptor for the
photogenerated electrons, as a result, the separation
of the photoelectron-hole was promoted, thus the
long-lived hole (h+) produced, which would make
the PDA/MIL-53 (1 : 20) composites more widely
used in photocatalytic applications.
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Fig. 6 TGA profiles of the samples
The BET specific surface area test method
(SBET) was widely used in the adsorption
performance of catalyst surface. Fig.7 displayed the
N2 adsorption-desorption curve and the pores of the
photocatalyst. The pore volume was calculated
using BJH method. The SBET of MIL-53 was
67.534 m2 g-1 accompanied with an average pore
diameter of 4.2 nm. The SBET of PDA was found to
be 23.54 m2 g-1, and the average pore diameter of
PDA was 10.8 nm. However, the PDA/MIL-53(1 :
20) nanocomposite had a SBET value of 146.4 m2 g-1
higher than that of MIL-53(Fe) and PDA, it also
had an average pore diameter of 4.5 nm. Therefore,
larger SBET of PDA/MIL-53 (1 : 20) were favorable
for improving the photocatalytic activity.
EIS Nyquist plot could more closely explain
the separation efficiency of photogenerated
electrons and holes. Fig.5d showed the EIS
Nyquist plots of the MIL-53(Fe) and PDA/MIL-
53(1 : 20) composites. It could be found that
PDA/MIL-53(1 : 20) had a much smaller arc than
that of MIL-53(Fe), indicating that the transfer
efficiency of charge carriers was higher on the
surface of PDA/MIL-53(1 : 20) nanoparticles.
Thermal stability and composition of
materials were studied by TGA. As showed in
Fig.6, the as-prepared MIL-53(Fe) presents four
stages in weight loss profiles. The weight loss
below 120oC was attributed to the evaporation of
moisture on the surface of the material, and
between 150-420oC was the decomposition of
DMF, the structure collapse of MIL-53(Fe) (450-
550oC) and the completely decomposition (> 700oC)
of the material, respectively. As for PDA, an initial
weight loss was at about 100oC owing to the
evaporation of moisture absorbed on the surface of
materials . The weight loss in the second stage was
100-800oC, mainly because the dopant molecules
was removed from the polymer structure and the
polymer chain fractured. The PDA/MIL-53(1 : 20)
composites also had the similar weight loss trend as
that of PDA, but the weight loss was more slow
than PDA, especially when the temperature was
higher than 430oC. It meant that PDA/MIL-53(1 :
20) composites presented better thermal stability.
Fig. 7 Nitrogen adsorption-desorption isotherms (a)
and pore size distribution curves (b) of the samples
3.2 Photocatalytic oxidation of alcohols
In order to evaluate the photocatalytic activity
of PDA/MIL-53 (Fe) (1 : 20), we chose
photooxidation of aromatic alcohols under visible-
light irradiation. All experimental data were listed
in Table 1. Obviously, compared with MIL-53(Fe)
and PDA, PDA/MIL-53(1 : 20) composites
effectively enhanced the photocatalytic activity.
The conversion of benzalcohol was only 30% when
used MIL-53(Fe) as the catalyst under the same
conditions, but no oxidation reaction occurred
when PDA was used. When the PDA/MIL-53(1 :
20) composites was employed as photocatalyst, the
conversion of benzalcohol was enhanced from 30%
to 85% superior to that of MIL-53(Fe) at the same
conditions. Visible light without catalysts or
catalysts without visible light was used as two
controlled experiment. It showed that benzalcohol
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was hardly oxidized under the controlled
3
ethyl
acetate
acetonitrile
37
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DOI: 10.1039/C9NJ04929K
experimental conditions of PDA/MIL-53(1 : 20).
The results convinced that the oxidation of
benzalcohol to benzaldehyde was a photodriving
reaction. For different proportions PDA/MIL-53(Fe)
composites, they had different catalytic activities in
the oxidation reaction process, due to their different
sizes and different adsorption ability in visible light.
In the as-prepared PDA/MIL-53(Fe) series,
PDA/MIL-53 (1:20) achieved the optimal
conversion (85%) under the same conditions,
which was exactly the same as the previous optical
performance.
4
5
26
25
99
99
dichloromet
hane
1,2-
dichloroeth
ane
6
16
99
10
11
12
13
14
15
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60
7
8
THF
12
6
99
99
brominated
n-butane
methanol
9
5
4
2
1
1
99
99
99
99
99
Table 1 Blank control experiments of the photocatalytic
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toluene
ethanol
oxidation of benzalcohol
Entry
1a
2 b
3 c
Catalyst
Con./%
None
30
Sel./%
None
>99
0
None
isopropanol
DMF
MIL-53(Fe)
PDA
0
4 d
5 e
a Conditions: benzalcohol 0.2 mmol; PDA/MIL-53(1 :
20) 5 mg; visible light illumination; 9 h.
b and c: Calculated by GC analysis.
PDA/MIL-53
85
>99
0
PDA/MIL-53
0
a Benzalcohol 0.2 mmol, CCl4 6 mL, 9 h, visible light,
illumination under ambient conditions.
PDA/MIL-53(1:20)
was
applied
to
b Benzalcohol 0.2 mmol, MIL-53(Fe) 5 mg, CCl4 6 mL,
benzalcohol derivatives with different substituents
to investigate the catalytic range of the catalytic
system and to evaluated its photocatalytic activity.
Benzalcohol derivatives were oxidized into
corresponding aryl aldehydes or ketones exposure
to visible light for 9 h, and the results were
summarized in table 3. Due to the steric hindrance
near the hydroxyl group in 2-methylbenzalcohol,
the oxidation gave the lowest conversion in this
series. As for the secondary benzylic substrates, all
of the oxidation had relative lower conversions
compared with primary benzylic because of their
steric hindrance from themselves. It also could be
found that the electronic effect of the substituent
group on the benzene ring did not play an
important role in the oxidations.
9
h, visible light, illumination under ambient
conditions.
c Benzalcohol 0.2 mmol, PDA 5 mg, CCl4 6 mL, 9 h,
visible light, illumination under ambient conditions.
d
Benzalcohol 0.2 mmol, PDA/MIL-53(1 : 20) 5 mg,
CCl4 6 mL, 9 h, visible light, illumination under
ambient conditions.
e All the conditions was the same as d except for the
visible light illumination.
In order to investigate the influence of solvent
on photocatalytic oxidation, the corresponding
results were summarized in Table 2. Various
solvents such as trichloromethane, methanol,
tetrachloromethane (CCl4), 1,2-dichloroethane,
toluene, THF, dichloromethane, isopropanol,
acetonitrile, brominated n-butane, ethanol, DMF,
and ethyl acetate made a different results on the
oxidation reactions. From the results we could
found that CCl4 was the suitable solvent for the
selective oxidation of benzalcohol in this
photocatalytic system.
Table 3 Photooxidation of various substituted aromatic
alcohols over PDA/MIL-53(1 : 20)
a Entry
Substrate
Product
b Con./% c Sel./%
85
69
72
>99
>99
>99
1
2
3
Table 2 Photooxidation of benzalcohol in different
solvents
a Entry
1a
Solvent
CCl4
b Con./%
c Sel./%
84
99
35
>99
4
2
trichloromet
hane
53
94
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>99
>99
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>99
8
a Conditions: Substrate (0.2 mmol); PDA/MIL-53(1 : 20)
5 mg; CCl4 6 mL; visible light illumination; 9 h;
b and c: Calculated by GC analysis.
Fig. 8 The cycling result of PDA/MIL-53(1 : 20)
composites in the oxidation of the benzalcohol. The
conversion as well as selectivity (a); The XRD patterns
(b); FT-IR spectra (c) and the SEM image(d) of
PDA/MIL-53(1 : 20) before and after the catalytic
reactions.
3.3 Reusability of the photocatalyst
The stability of materials has a critical position
in photocatalysis. The stability and reusability of
PDA/MIL-53(1
:
20) nanocomposites was
3.4 Mechanism for the photooxidation of
benzalcohol over PDA/MIL-53(1 : 20)
appraised by carrying out the recycle experiments
under identical conditions. After the first
photocatalysis,
PDA/MIL-53(1
:
20)
We used a blank control experiment, the
active species trapping experiments and the M-S
curve to further investigate the possible
photooxidation mechanism of benzalcohol over
PDA/MIL-53(1 : 20).
nanocomposite, which was simply washed, filtered
o
and dried at 60 C under vacuum for 10 h, was
applied to another batch of oxidation in the same
conditions. After three times of photocatalytic
oxidation, the conversion efficiency of benzalcohol
into benzaldehyde decreased by 10% when
PDA/MIL-53(Fe) was as catalyst (Fig.8). It was
clear that this catalyst performed outstanding
circulation in the oxidation and excellent photo
stability.
The M-S curve was drawn by the Mott-
Schottky equation, while the Mott-Schottky
equation described the relationship between the
space charge layer differential capacitance Csc of a
semiconductor and the potential Δφ of the
semiconductor surface to the body, as shown in Fig.
5e. It could be observed that the slope of the M-S
curve was positive, indicating that the PDA/MIL-
To further evaluate the structural stability of
the catalyst, FT-IR, XRD and SEM were used to
characterize the crystalline structures and
morphology of fresh and the three-time reused
PDA/MIL-53(1 : 20) nanocomposites. The position
and intensity of the diffraction peaks in the XRD
pattern were basically the same (Fig. 8b), and no
new characteristic peaks appeared after the catalyst
being recycled, which fully proved that the crystal
structure had no obvious change in the
photocatalytic reaction process. Fig. 8c was the
infrared spectra of the samples, indicating that there
was no significant change before and after the
reaction. But the SEM image (Fig.8d) showed that
the crystal particle was somewhat destroyed after
the reaction. In short, it clearly indicated that the
PDA/MIL-53(1 : 20) nanocomposite had promising
stability in this reaction system.
53 (1
:
20) composites was an n-type
semiconductor 45. By calculation, the flat band
potential of PDA/MIL-53 (1 : 20) was 0.18 V vs
SCE at pH 7.0, and consistent with the -0.02 V vs
NHE. In general, for an n-type semiconductor, the
flat band potential is more positive than the
conduction band potential for about 0.2 eV46.
Therefore, it was possible to obtain a conduction
band potential (ECB) of PDA/MIL-53 (1 : 20)
composites of 0.18 eV vs NHE. Then, with the
band gap energy (Eg) of the previous UV-vis DRS
spectral data, the optical Eg of PDA/MIL-53 (1 : 20)
could be calculated to be 1.75 eV. Accordingly to
the empirical formula of Eg = EVB - ECB, it could be
concluded that the final valence band position (EVB)
of the PDA/MIL-53 (1 : 20) nanocomposites was
determined to be 1.93 eV vs NHE, which was
between the potential of benzalcohol (1.88 eV vs
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NHE) and the potential of benzaldehyde (2.27 eV
View Article Online
DOI: 10.1039/C9NJ04929K
26
vs NHE). . As a result, in thermodynamically, the
conversion of benzalcohol to benzaldehyde was
satisfied and benzaldehyde could not be further
oxidized at the identical conditions. In addition, by
comparing ECB potential (0.18 eV vs NHE) and the
-
oxidation potential of O2/•O2 (-0.33 eV vs NHE), it
was believed that PDA/MIL-53(1 : 20) was
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-
incapable to generate •O2 by the reduction of O2
with the photo-induced electron. Therefore, the
-
oxidation of benzalcohol to benzaldehyde by •O2
Fig. 9 Effects of different reactive species scavengers
on the photocatalytic oxidation of benzalcohol by
PDA/MIL-53(1 : 20)
radicals could not occur. PDA had a more negative
potential of -0.08 eV (vs NHE) than that of ECB of
47
MIL-53(Fe) , but the outstanding ability of PDA 4 Conclusions
to accept electron made it beneficial for the transfer
In conclusion, a range of PDA/MIL-53(Fe)
of electrons from MIL-53(Fe) to PDA. Therefore,
the high photocatalytic activity of PDA/MIL-53(1 :
20) in this system could be ascribed to the excellent
light-harvesting ability as well as the fast
separation efficiency of photogenerated electrons
of PDA on the interface of PDA/MIL-53(Fe)
nanoparticles. In addition, CCl4 was a typical
electron canceller in the photocatalysis researches
26, the most of the photogenerated electrons could
be captured by CCl4, so CCl4 was selected as the
ideal solvent for this oxidation (Table 2).
nanocomposites was prepared via nature-mimic
method. It was certified that the as-prepared
PDA/MIL-53(Fe) nanocomposites showed more
outstanding photocatalytic activity than the MIL-
53(Fe) or PDA, the cause was that the existence of
the PDA would prolong the lifetime and maximize
the separation of photogenerated carrier. With the
catalysis of PDA/MIL-53(Fe) composites, a series
of aromatic alcohols were converted to the
corresponding aldehydes or ketones through an h+-
dominated oxidation process. Meanwhile, the
photocatalyst was reused at least three times, and
the catalytic performance did not change obviously.
It could be used as a potential and promising
visible-light-involving system for the selective
oxidation of alcohols.
As shown in Fig.9, investigating the
mechanism of the oxidation, a sequence of control
experiments with different kinds radical scavengers
were added to the reaction system. In all of these
reagents, p-benzoquinone (BQ) was applied to the
-
capture of superoxide radicals (•O2 ), AgNO3 for
the capture of electron (e-), isopropyl alcohol (IPA) Conflicts of interest
for the quenching of •OH, N2 flow for the
There are no conflicts to declar
.
excluding of O2 and ethylenediamine tetraacetic
acid dis-odium (EDTA-2Na) for the holes (h+) Acknowledgements
48
capturing . The experimental results showed that
The research was financially supported by
NSFC (21563026), the Program for Changjiang
Scholars and Innovative Research Team in
University (IRT15R56), and the Innovation Team
Basic Scientific Research Project of Gansu
Province (1606RJIA324). We also thank the Key
Laboratory of Eco-Environment-Related Polymer
Materials (Northwest Normal University), Ministry
of Education, and the Gansu International
Scientific and Technological Cooperation Base of
Water-Retention Chemical Functional Materials,
for financial support.
h+ was one of the core active substances for the
oxidation of benzalcohol, because only by adding
EDTA-2Na into the oxidation reaction system, the
conversion of benzalcohol would decrease rapidly.
When the other scavengers were added, the
conversion of benzalcohol was not decreased.
Although the elelctron capture agent of AgNO3 was
added, the conversion was largely unaffected,
which was ascribe to the electron transfer of PDA
as well as the canceller of electrons by CCl4. In a
word, it was clear that the selective oxidation of
benzalcohol to benzaldehyde was mainly triggered
by the positive photogerated holes (h+).
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Graphical Abstract
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7
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Nature-Mimic Fabricated Polydopamine/MIL-53(Fe):
Efficient Visible-light Responsive Photocatalysts for the
Selective Oxidation of Alcohols
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PDA/MIL-53(Fe) nanocomposites was prepared via nature-mimic method. It
showed that PDA/MIL-53(Fe) nanocomposites were efficient catalysts toward the
photocatalytic selectively oxidation of alcohols to aldehydes or ketones by a direct
hole-oxidation process under visible-light illuminated conditions. A possible
photocatalytic mechanism concerned to the direct photogenerated hole-oxidation
process was also proposed.