Fe Doped MIL-101/Graphene Nanohybrid for Photocatalytic Oxidation of Alcohols Under Visible-Light Irradiation

Article abstract of DOI:10.1007/s10562-020-03472-w

A novel photoactive porous material of GR/FeMIL-101 based on FeMIL-101 metal organic frameworks (MOFs) was successfully synthesized via a simple hydrothermal method. The structural and photoelectric properties of the GR/FeMIL-101 was analyzed by XRD, SEM, TEM, TGA, XPS, UV–vis DRS, FT-IR, PL and EIS methods. The photocatalytic performance for the selective oxidation of benzyl alcohol with GR/FeMIL-101 as catalysts was evaluated under visible light irradiation. The results showed that the GR/FeMIL-101 nanohybrid had better photocatalytic performance than both of FeMIL-101 and the pristine MIL-101. It was further found that the incorporation of Fe and MIL-101 caused valence fluctuations of Fe³⁺/Fe²⁺ which improved the absorption of visible-light and increased the separation efficiency of photogenerated charges. In addition, the combination of FeMIL-101 and GR could further promote the transfer rate of the photoelectrons. The mechanism of the reaction revealed that ·O₂^? was the dominating active specie in this reaction through active species trapping experiments. Graphic Abstract: [Figure not available: see fulltext.]Fe doped MIL-101/GR nanohybrid was successfully synthesized as an efficient photocatalyst for selective oxidation of alcohols under visible-light and shown a best conversion of 50%. Analyses revealed that Fe was successfully doped into the MIL-101, valence fluctuation of Fe2+/Fe3+ not only improved the visible-light absorption but also increased the separation rate of photoexcited carriers. Graphene further improved the transportation rate of electron (e-). Subsequently, the possible photocatalytic mechanism for the selective oxidation of alcohols was proposed. It was proved that superoxide radicals (·O2-) was the main active species when the reaction was performed under Oxygen atmosphere.

Full text of DOI:10.1007/s10562-020-03472-w

Catalysis Letters
https://doi.org/10.1007/s10562-020-03472-w
Fe Doped MIL‑101/Graphene Nanohybrid for Photocatalytic Oxidation
of Alcohols Under Visible‑Light Irradiation
Mingming Wang1 · Yali Ma1 · Bolin Lv1 · Fenglin Hua1 · Shuangyan Meng1 · Xuedi Lei1 · Qingtao Wang1 · Bitao Su1 ·
Ziqiang Lei1 · Zhiwang Yang1
Received: 24 September 2020 / Accepted: 16 November 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
A novel photoactive porous material of GR/FeMIL-101 based on FeMIL-101 metal organic frameworks (MOFs) was suc-
cessfully synthesized via a simple hydrothermal method. The structural and photoelectric properties of the GR/FeMIL-101
was analyzed by XRD, SEM, TEM, TGA, XPS, UV–vis DRS, FT-IR, PL and EIS methods. The photocatalytic performance
for the selective oxidation of benzyl alcohol with GR/FeMIL-101 as catalysts was evaluated under visible light irradiation.
The results showed that the GR/FeMIL-101 nanohybrid had better photocatalytic performance than both of FeMIL-101 and
the pristine MIL-101. It was further found that the incorporation of Fe and MIL-101 caused valence fuctuations of Fe3+/Fe2+
which improved the absorption of visible-light and increased the separation efciency of photogenerated charges. In addition,
the combination of FeMIL-101 and GR could further promote the transfer rate of the photoelectrons. The mechanism of the
reaction revealed that ·O2− was the dominating active specie in this reaction through active species trapping experiments.
Graphic Abstract
Fe doped MIL-101/GR nanohybrid was successfully synthesized as an efcient photocatalyst for selective oxidation of
alcohols under visible-light and shown a best conversion of 50%. Analyses revealed that Fe was successfully doped into the
MIL-101, valence fuctuation of Fe2+/Fe3+ not only improved the visible-light absorption but also increased the separation
rate of photoexcited carriers. Graphene further improved the transportation rate of electron (e-). Subsequently, the possible
photocatalytic mechanism for the selective oxidation of alcohols was proposed. It was proved that superoxide radicals (·O2-)
was the main active species when the reaction was performed under Oxygen atmosphere.
Keywords GR/FeMIL-101 · Photocatalytic · Benzyl alcohol · Oxidation · Visible light · Mofs
* Zhiwang Yang
yangzw@nwnu.edu.cn
Extended author information available on the last page of the article
Vol.:(0123456789)
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M. Wang et al.
photocatalytic activity under visible-light for selectively
aerobic oxidation of aromatic alcohols [24] and the deg-
radation performance [25]. However, it is also important
to study the efect of Fe on the photocatalytic properties
of other MOFs materials.
1 Introduction
Nowadays, with the development and progress of human
society, the problems of energy shortage and environmen-
tal pollution have become two major challenges for people.
Rich and clean solar energy has been attracted the atten-
tions of many researchers [1–3]. The heterogeneous photo-
catalysis can efectively utilize solar energy to solve these
two major problems. Therefore, it ofers many possibili-
ties for the development of semiconductor photocatalytic
technology. It’s worth noting that the nanomaterials based
on metal organic frameworks (MOFs) have a great poten-
tial for visible light photocatalytic reactions [4–6]. For
instance, Wang et al. reported an aromatic heterocycle-
grafted photocatalytic material of NH2-MIL-125(Ti) for
selectively oxidizes alcohol under visible light. The results
indicated that the aromatic heterocycle-grafted MOFs
exhibited a signifcant activity for oxidation of alcohols
under visible light [7].
In this work, the Fe-doped MIL-101 catalyst was success-
fully synthesized by a simple hydrothermal method and the
photocatalytic performance was improved after the doping
of Fe species. Then, the GR/FeMIL-101 nanohybrid was
obtained by the combination of the prepared FeMIL-101 and
graphene (GR) to further improve the photocatalytic activity
of selective oxidation of benzyl alcohol under visible light.
The possible mechanism of GR/FeMIL-101 photocatalytic
process was also explored by trapping experiments and
Mott-Schottky plots measurements.
2 Experimental
2.1 Materials
As we know, MOFs are porous coordination materi-
als formed by transition-metal ions or metal clusters with
organic linkers [8–10]. In recent years, because of the
excellent structural characteristics such as large specifc
surface area, large active center, porosity and fexible
structure tunability, MOFs have been widely used in gas
storage and separation [11], drug carriers [12], sewage
treatment [13], photocatalytic conversion of organic com-
pounds [14, 15] electrocatalysis [16] and fuorescence
sensing [17]. Moreover, MIL-101 is one of the promis-
ing metal organic framework materials with a regular
octahedral structure and a larger specifc surface area
(2191 m2g−1) as well as a high stability in air or strong
acid solutions. It has been applied to gas separation
[18], degradation of dye pollutants [19], photocatalytic
hydrogen generation [20]. As a photocatalytic material,
MIL-101 can absorb visible-light due to its narrow band
gap (2.06 eV) [21]. However, the material with narrow
Eg is not absolutely good for the separation of electrons
and holes. Therefore, it is signifcant to explore how to
improve the photocatalysis of MIL-101. We found that the
partial suitable metal ions, as an electron mediator in the
nodes, can greatly improve the photocatalytic performance
via decreasing the recombination rate of charge efciently
by valence fuctuation. Yamashita et al. reported that the
Ce ions were successfully doped in the nodes of MIL-101
and promoted H2 production under visible light irradiation
[22]. According to the above researches, it can be seen that
doping redoxactive (multi) metallic nodes such as Fe, Mn,
Ni, Co, Cu and Ce is an efective method to improve the
photocatalytic performance of MOFs [23]. In the previ-
ous works of our group, a series of Fe ions doped UiO-66
nanoparticles were successfully synthesized to enhance
All the chemical reagents used in this research were analyti-
cally pure purchased from the commercial companies, and
directly employed without further purifcation.
2.2 Preparation of Photocatalysts
Graphene oxide (GO) nanostructure was obtained accord-
ing to the literature [26]. 0.5 g graphite was frstly dispersed
in 12 mL H2SO4 (95%) solution and was being stirred at
− 5 °C, then 1.5 g KMnO4 was added into the solution, the
mixture was ultrasonically treated for 12 h at room tem-
perature. The resulted suspension was quickly transferred to
15% H2O2 aqueous solution and continuously stirred for 2 h.
After the reaction, the solid was collected by centrifugation
and washed several times with H2O until pH 7, and then it
was freeze-dried for further using.
MIL-101 was synthesized through a simple hydro-
thermal method according to the literature [27]. Briefy,
0.48 g Cr(NO3)3·9H2O, 0.2 g 1,4-terephthalic acid and 22
µL hydrofuoric acid (40%) were added to 6 mL H2O and
stirred for 30 min at room temperature, then the solution was
transferred to a Tefon-lined stainless steel autoclave and
maintained at 220 °C for 8 h. After being cooled to room
temperature, the obtained mixture was centrifuged at 8000
r/min and washed several times with DMF and anhydrous
ethanol. Then, in order to activate MIL-101, the as-prepared
sample was dispersed in 50 mL ethanol and transferred to
65 mL autoclave which was then being kept at 90 °C for
20 h. Finally, after naturally cool down, the obtained MIL-
101 was washed by ethanol several times and dried at 160 °C
for 8 h to remove the surplus DMF. FeMIL-101 was synthe-
sized and activated by the same way just as the synthesis of
1 3

Fe Doped MIL-101/Graphene Nanohybrid for Photocatalytic Oxidation of Alcohols Under…
MIL-101 but adding 0.2 mmol FeCl3·6H2O to the aqueous
solution.
the selectivity of the alcohols were obtained by the follow
formulas:
GR/FeMIL-101 nanohybrid was obtained by thermal
treating as the follow process. 3.0 mg GO was ultrasoni-
cally dispersed in 2 mL H2O and added to 6 mL aqueous
solution including 0.48 g Cr(NO3)3·9H2O, 0.2 g 1,4-ben-
zene dicarboxylic acid, 22 µL hydrofuoric acid (40%) and
0.2 mmol FeCl3·6H2O. After being stirred for 30 min, the
mixture solution was transferred to a Tefon-lined stainless
steel autoclave and heated at 220 °C for 8 h. At last, after
being purifed and activated, GR/FeMIL-101 nanohybrid
was obtained.
[(
)
]
Conversion of the alcohol (%) = C0 − Cn ∕C0 × 100
[
(
)]
Selectivity of the aldehyde∕ketone (%) = Ca∕ C0 − Cn × 100
where C0 is the concentration of alcohol before irradiation,
Cn and Ca is the concentration of alcohol and aldehyde/
ketone after irradiation via gas chromatograph (Shimadzu
GC-2010) analysis.
3 Results and Discussions
2.3 Catalyst Characterization
3.1 Characterization
X-ray powder difraction (XRD) patterns of the samples
were recorded on a Rigaku D/Max-2400 with Cu Kα radia-
tion. Fourier transform infrared (FT-IR) spectroscopy was
carried on a Nicolet NEXUS 670 with the scanning range
from 400 to 4000 cm−1. The photoluminescence (PL)
spectra of the samples were conducted with the excita-
tion wavelength of 450 nm on a PELS-55 luminescence/
fuorescence spectrophotometer. UV–vis difuse refectance
spectra (UV–vis DRS) of the samples were characterized
by a Cary 500 UV–vis NIR spectrophotometer. X-ray pho-
toelectron spectroscopy (XPS) method was used to analyze
the elements of the catalysts by a Thermo VG Scientifc
Sigma Probe instrument with an Al Kα radiation. N2 adsorp-
tion desorption isotherms of the samples were carried on
an Autosorb-iQ2-MP instrument. The thermogravimetric
analysis (TGA) was conducted on a TA Instruments Dis-
covery Thermo gravimetric Analyzer from 25 to 800 °C at
the rate of 10 °C min−1 in N2 atmosphere. SEM images were
observed through a Carl Zeiss Ultra Plus scanning electron
microscope. TEM images were measured by a TECNAIG2
transmission electron microscope. Electrochemical imped-
ance spectroscopy (EIS) was performed on PGSTAT302N
workstation to observe chemical properties of the samples.
The X-ray powder difraction (XRD) was used to character-
ize the crystal and phase structure of the samples. The XRD
patterns of MIL-101, FeMIL-101, GO and GR/FeMIL-101
were shown in Fig. 1. The spectrum of pristine MIL-101 can
be great ftted with the simulated MIL-101, indicating the
successful synthesis of MIL-101 [29]. FeMIL-101 main-
tained the similar characteristic peaks as the pure MIL-101,
and no additional refection of Fe presented, indicating the
doping Fe into MIL-101 did not change the framework struc-
ture and no new phase of Fe compounds formed [22, 29].
It was worthy to pay attention that the XRD peaks of GR/
FeMIL-101 were similar to that of MIL-101 and FeMIL-
101, but the peaks intensity slightly decreased. It was ascribe
to the successful combining of GR and FeMIL-101.
The FT-IR spectra of MIL-101, FeMIL-101 and GR/
FeMIL-101 nanocomposites were showed in Fig. 2. For
MIL-101, the peaks at the range of 2825–3687 cm−1 were
attributed to the crystalline water and the OH groups in
the ligand of H2BDC. The peaks at 1405–1610 cm−1 were
ascribed to the stretching vibrations of C=C bonds. The
2.4 Evaluations of Photocatalytic Activity
A 500 W xenon lamp with a 400 nm cut of flter was used
to get the wave length (λ) greater than 400 nm visible light
to test the photocatalytic oxidation activity of catalysts at
room temperature. Firstly, 10 mg catalyst and 0.1 mmol
alcohols were added into a quartz vial. Then the reactor was
treated under ultrasound for 1 min and shaken to make sure
that the catalyst was evenly dispersed in the solvent. After-
wards, the reacted solution was kept under dark for 2 h in
order to insure that the construction of the alcohols’ adsorp-
tion and desorption equilibriums on the catalyst surface. In
the end, the samples were irradiated under visible light for
8 h. After the photocatalytic reaction, the conversion and
Fig. 1 XRD patterns of the samples
1 3

M. Wang et al.
was ascribed to the absorption signals of epoxide group [26].
The FT-IR spectra of FeMIL-101 and GR/FeMIL-101 were
in common with that of the pure MIL-101, which indicated
that the framework of FeMIL-101 and GR/FeMIL-101 were
not afected by the doping of Fe and combination of GR
[29–32].
Figure 3 showed the Raman spectra of the obtained sam-
ples. The GO sample exhibited two characteristic bands at
1337 cm−1 and 1587 cm−1 while MIL-101 had four charac-
teristic bands at 1611, 1453, 1142, and 866 cm−1. As for the
spectrum of FeMIL-101, besides the main bands of MIL-
101, the band at 629 cm−1 could be assigned to the oxidation
state of Fe [31]. The successful incorporation of Fe into
the MIL structure without preventing MIL crystal formation
was proved. In the case of GR/FeMIL-101, besides the main
bands of FeMIL-101, the band at 1336 cm−1 was ascribed
to the GO which indicated FeMIL-101 was coated by the
GR layers.
Fig. 2 FT-IR spectra of the samples
Figure 4a showed the UV–vis DRS absorption proper-
ties of the samples. The absorption edge around 300 nm
was ascribed to the π-π* electronic jumping of the aromatic
ring and the absorption in the visible region was ascribed
to the d-d spined transition of Cr3+ in MIL-101 [22]. Com-
pared with the pure MIL-101, there was a great red shift
of FeMIL-101 caused by the doping of Fe into the crystal
structure of MIL-101. Moreover, GR/FeMIL-101 had obvi-
ous enhanced absorption due to the good pristine visible
light absorption of carbon materials [10, 26, 28, 34].
The photoluminescence (PL) spectra can give the separa-
tion efciency of the charges in the photocatalysts [10, 26,
28]. As shown in Fig. 4b, we could fnd that the PL emis-
sion intensity of GR/FeMIL-101 was far lower than those
of MIL-101 and FeMIL-101, which clearly indicated that
the doping of Fe into MIL-101 could efciently promote
the separation of photo-generated carriers. And the excellent
charges separation efciency was somewhat due to not only
the own photoelectronic performance but also the excellent
conductivity of GR [28, 33].
Fig. 3 Raman spectra of the samples
peak at 1400 cm−1 was linked to the symmetric stretching of
O=C=O groups in the carboxylate group of the BDC ligand.
The peak with medium strength at 580 cm−1 was due to the
vibrations of Cr–O bonds, which proved that MIL-101 had
been formed. For GO, the peak at 1720 cm−1 was assigned
to the C=O stretching vibration, and the peak at 692 cm−1
XPS data of GR/FeMIL-101 were shown in Fig. 5. The
XPS survey scanning indicated the atom content of the O,
Fig. 4 UV–vis DRS (a) and PL
spectra of the samples (b)
1 3

Fe Doped MIL-101/Graphene Nanohybrid for Photocatalytic Oxidation of Alcohols Under…
Fig. 5 XPS spectra of GR/MIL-101: survey (a); C1s (b); O1s (c); Cr2p (d) and Fe2p (e)
C, Cr and Fe on the surface of GR/FeMIL-101 were 73.98%,
21.35%, 4.48% and 0.19%, respectively (Fig. 5a). Inductively
couple plasma (ICP) was also carried out to study the con-
tent of Fe element. The result calculated was 0.21% which
was close to the result of XPS. In the XPS spectrum of C1s,
the ftted peaks at 284.9 eV, 288.4 eV and 286.2 eV can be
attributed to the C=C, C=O and C–C bond from H2BDC in
the framework of FeMIL-101, respectively (Fig. 5b). For
the spectrum of O1s, the peaks at 532.0 eV and 533.7 eV
was related to the signals of C=O and O–H groups, respec-
tively. And the peak at 533.1 eV represented the Cr–O and
Fe–O species (Fig. 5c). Figure 5d presented the XPS spectra
of Cr2p. It was found that the spectra of Cr2p showed two
peaks at 577.8 eV and 587.5 eV, which were corresponding
to the Cr2p1/2 and Cr2p3/2 signals, respectively. The Fe2p
core level spectrum consists of Fe2p3/2 and Fe2p1/2 excita-
tions. The XPS signals of GR/FeMIL-101 revealed the exist-
ence of the Fe2+. The peak at 711.0 eV was for Fe2p3/2, and
the peak at 723.7 eV was for Fe2p1/2, respectively The peak
.
at 715.5 eV also proved the existence of Fe–O species in
GR/FeMIL-101 nanocomposites (Fig. 5e). These data were
well in agreement with the reported results [24, 35].
N2 adsorption–desorption isotherms of the samples were
shown in Fig. 6. The N2 adsorption–desorption curves of
the samples were shown in Fig. 6a and the average pore
diameters of the samples were shown in Fig. 6b. The specifc
surface area of the pure MIL-101 was 4397 m2g−1, and the
average pore diameter was 2.23 nm. The specifc surface
Fig. 6 Nitrogen adsorption–des-
orption isotherms (a) and pore
size distribution curves (b) of
the samples
1 3

M. Wang et al.
area of FeMIL-101 was found to be 2554 m2g−1, and the
average pore diameter was 2.24 nm. It showed an obvious
change of the surface area of FeMIL-101 after the doping of
Fe into MIL-101, indicating that the introducing of Fe would
change the framework but a lot of micropores were still
existed in FeMIL-101. The specifc surface area decreased
again when GR was combined with FeMIL-101. It showed
that the specifc surface area of GR/FeMIL-101 decreased
to 1851 m2g−1 due to a small number of GR entered into the
aperture of FeMIL-101. However, the average pore diameter
maintained during the combination. The average pore diam-
eter of FeMIL-101 was 2.35 nm.
catalysts, during the initial stage of heating up to the tem-
perature of 100 °C, a rapid weight loss occurred due to the
removal of water from the larger cage of MOFs and GO. For
MIL-101, FeMIL-101 and GR/FeMIL-101, the second stage
which was observed in the temperature range of 100–300 °C
was related to the removal of DMF solvent from the crystal
structure of MIL-101. As for pure MIL-101, during the tem-
perature range of 300–480 °C, the observed weight loss was
corresponded to the collapse of crystal structure [36]. When
Fe was doped into MIL-101, the stability of FeMIL-101 was
signifcantly increased, and the collapse occurred at the tem-
perature range of 400–520 °C. The stability of GR/FeMIL-
101 was further improved when FeMIL-101 was combined
with GR due to the good compatibility as well as the strong
molecular interaction between FeMIL-101 and GR.
Thermogravimetric analyses (TGA) of MIL-101, FeMIL-
101, GR/FeMIL-101 and GO were shown in Fig. 7. For all
The SEM images of the samples at diferent magnifca-
tion times were shown in Fig. 8. Most of the samples were
basically octahedral pyramid shaped. It was noted that
FeMIL-101 presented a rough surface obviously compared
with pure MIL-101 after the doping of Fe into MIL-101. For
GR/FeMIL-101, it maintained the adhesive morphology due
to GR attaching on the surface of FeMIL-101.
The morphology of MIL-101 and GR/FeMIL-101 were
also observed by TEM as shown in Fig. 9. MIL-101 showed
regular octahedral morphology with a smooth surface. How-
ever, the images of GR/FeMIL-101 sample were very difer-
ent from the images of MIL-101. From the images of GR/
FeMIL-101 sample, after the doping Fe, a rough surface
could be seen, and it was clear that a number of GR were
well dispersed on the surface of FeMIL-101. These results
Fig. 7 TGA profles of the samples
Fig. 8 SEM images for MIL-101 (a, b, c), FeMIL-101 (d, e) and GR/FeMIL-101 (f)
1 3

Fe Doped MIL-101/Graphene Nanohybrid for Photocatalytic Oxidation of Alcohols Under…
Fig. 9 TEM images of MIL-101
(a, b) and GR/FeMIL-101 (c, d)
Fig. 10 The EDS pictures of
GR/FeMIL-101
suggested that the heterojunctions between GR and FeMIL-
101 were formed, which would improve the charge separa-
tion and transfer efciency of the photoinduced carriers.
The component of GR/FeMIL-101 nanocomposite was
further investigated by the TEM EDS characterization
shown in Fig. 10. It found that the materials were composed
of C, O, Cr, and Fe elements which were evenly distributed
over the surface of GR/FeMIL-101.
EIS Nyquist plots of MIL-101, FeMIL-101 and GR/
FeMIL-101 were shown in Fig. 11. The arcs refected the
charge-transfer resistance at the surface of the electrode. The
smaller arc radius indicated that a faster interfacial charge
transfer and an efective separation of the photo-generated
electron–hole pairs occurring in a photocatalyst [37, 38].
From the Fig. 11 we could find that FeMIL-101 had a
much smaller arc radius than the pure MIL-101, while GR/
Fig. 11 EIS Nyquist plots of samples
1 3

M. Wang et al.
Table 2 Photooxidation of benzyl alcohol in diferent solvents
Table 1 Photocatalytic oxidation of benzyl alcohol with diferent cat-
alysts
Entry
Solvent
Con. (%)
Sel. (%)
Entrya
Catalyst
Con. (%)b
Sel. (%)c
1
2
3
4
5
6
7
8
Carbon tetrachloride
Ethyl acetate
1, 2-Dichloroethane
Acetone
5
98
99
98
98
99
99
99
99
1
2
3
none
MIL-101
FeMIL-101
GR/FeMIL-101
GR/FeMIL-101
0
4
25
50
3
/
50
25
34
5
15
8
99
99
99
99
4
Ethanol
5d
Acetonitrile
Isopropanol
H2O
aConditions: 10 mg catalyst, 0.1 mmol benzyl alcohol, ethyl acetate
10 mL, light λ>400 nm, reaction time 8 h
14
b,cThe conversion (Con. %) and the selectivity (Sel. %) were calcu-
lated by GC analysis (similarly hereinafter)
Conditions: catalyst 10 mg, benzyl alcohol 0.1 mmol, light
λ>400 nm, solvent 10 mL, reaction time 8 h
dWithout visible light irradiation
conversion of benzyl alcohol was just 5% (Table 2, entry 1).
Meanwhile, the weaker trapping electron of 1, 2-dichloroeth-
ane caused the conversion of benzyl alcohol in 1, 2-dichlo-
roethane was lower than that in ethyl acetate but higher than
that in carbon tetrachloride (Table 2, entry 1, 2, 3).
FeMIL-101 had a smallest arc radius among all the samples,
suggesting the most excellent charge transferred efciency
over the surface of GR/FeMIL-101.
3.2 Photocatalytic Performance
The substrate expansion results with the catalysis of GR/
FeMIL-101 were summarized in Table 3. It showed that
all the photooxidations aforded high aldehyde selectivity
more than 98%, however, moderate conversions not high
than 60% were provided for all the reactions. The substitu-
ents on the aromatic rings and the molecular structure had
complex efect on all the photooxidations. Both the electron-
donating group and the electron-withdrawing group have
efects on the conversion of benzyl alcohol. According to the
mechanism, the free radical is the key species in the oxida-
tion. And only a single electron exists in the p orbit of the
center carbon in the free radical. When the benzene ring is
attached to an electron-donating group, the single electron in
the p orbital of the center carbon in the benzyl group will be
partially paired with the electron from the electron-donating
group, and which would make it more stable. On the other
hand, as for the benzene ring with the electron-withdrawing
group, the single electron in the p orbit will be transferred to
the electron-withdrawing group and the electron density of
the p orbit will be somewhat empty, so that the p orbit was
also relatively stable. The scopes for the application of GR/
FeMIL-101 were worthy to further investigations.
The photocatalytic activity of the prepared catalysts was
tested by the photocatalytic oxidation of benzyl alcohol with
visible light irradiation. The controlling experiment was also
carried out under the identical conditions. The results were
summarized in Table 1.
The results in Table 1 showed that poor catalytic activ-
ity with the only 4% conversion of benzyl alcohol could be
observed for MIL-101. The weak catalytic activity of single
MIL-101 could be due to the low charge separation ef-
ciency. However, doping Fe into MIL-101 greatly improved
the conversion of benzyl alcohol (Table 1, entry 3). Obvi-
ously, the conversion for the catalysis of GR/FeMIL-101
reached 50% under the same conditions, proving that the
combination of GR could efectively improve the photocata-
lytic activity of the catalyst.
The photooxidation of benzyl alcohol was carried out in
diferent solvent under identical reaction conditions, and the
corresponding results were listed in Table 2. It was obvious
that ethyl acetate was an ideal solvent in this photocatalytic
system, and the highest conversion of 50% could be reached
in the photooxidation.
The stability of GR/FeMIL-101 in the photooxidation of
benzyl alcohol was investigated and shown in Fig. 12. It
showed after three times of parallel experiments, the cata-
lyst still maintained good catalytic activity for the oxidation.
This meant that GR/FeMIL-101 had good stability in this
photocatalytic system.
The polarity of the solution was an important factor in the
catalytic reaction. As the polarity of the solution increased,
the conversion rate of benzyl alcohol decreased (Table 2,
entry 2, 4, 5, 7). The polarity of acetonitrile is greater than
that of ethanol but the conversion of benzyl alcohol in ace-
tonitrile was higher than in ethanol (Table 2, entry 5, 6).
This phenomenon was due to the nitrile group of acetonitrile
accelerating the oxidation of alcohol. Carbon tetrachloride,
as the electron capture agent, will trap electrons and block
In order to further confrm the stability of GR/FeMIL-101
structure, XRD and FT-IR of GR/FeMIL-101 before and
after the photocatalytic reaction were investigated and shown
in Fig. 13. The XRD patterns (Fig. 13a) and FT-IR spectra
(Fig. 13b) of GR/FeMIL-101 sample had quite little changes
−
the production of the superoxide radical (·O2 ), so that the
1 3

Fe Doped MIL-101/Graphene Nanohybrid for Photocatalytic Oxidation of Alcohols Under…
Table 3 Photooxidation of
various substituted benzyl
Entry
1
Substrate
Product
Con. (%)
50
Sel. (%)
98
alcohols over GR/FeMIL-101
under visible light irradiation
CHO
OH
CHO
OH
2
3
10
11
46
38
21
30
57
37
42
99
98
CHO
OH
O2N
O2N
CHO
OH
4
98
Cl
Cl
CHO
OH
5
98
H3CO
H3CO
H3CO
H3CO
CHO
OH
OCH3
6
99
OCH3
OH
O
7
>99
>99
>99
>99
Cl
Cl
O
OH
8
Cl
Cl
Cl
Cl
OH
O
9
H3CO
OCH3
O
OH
10
H3CO
OCH3
H3CO
OCH3
Conditions: GR/FeMIL-101 10 mg, substrate 0.1 mmol, ethyl acetate 10 mL, light λ>400 nm,
reaction time 8 h
after catalytic reaction. This implied that the structure of
GR/FeMIL-101 remained intact after at least three times of
parallel experiments and the stability of the composite was
further proved.
of GR/FeMIL-101, some controlled experiments were oper-
ated with diferent radical scavengers. To measure the efect
of ·OH radicals, isopropyl alcohol (IPA) was for removing
·OH. Ethylenediamine tetraacetic acid disodium (EDTA-
2Na), AgNO3 and 1, 4-benzoquinone (BQ) were also used
as the scavengers for the capturing of h+, e−, and superoxide
3.3 Reaction Mechanisms
−
radical (·O2 ), respectively [26, 39, 40], the results were
In order to explain the possible photocatalytic mechanism
of the photocatalytic oxidation of alcohol with the catalysis
shown in Fig. 14a. When BQ, AgNO3 and N2 were added,
the conversion of benzyl alcohol was sharply decreased.
1 3

M. Wang et al.
to UV–vis DRS characterization, the absorptions wave-
length edge of FeMIL-101 was 600 nm, and the correspond-
ing calculated Eg of FeMIL-101 was 2.07 eV. Hereby, the
estimated potential positions of CB and VB of FeMIL-101
were−0.46 V and 1.58 V vs NHE, respectively [41, 42].
According to the previous researches, the oxidation
potential of O2/O2− was−0.33 V vs NHE, therefor, the CB
potential (−0.46 V) of FeMIL-101 was capable to reduce
−
O2 to oxygen radicals (·O2 ). The VB of FeMIL-101 was
1.58 V, which was not enough to oxidize benzyl alcohol
completely, but to form active intermediate. Moreover, the
catalyst was not enough to generate ·OH radicals (2.4 V vs.
NHE, pH 7) under visible light [43, 44]. So we could fnd
that the concentration of ·OH did not show signifcant impact
on the photooxidation. Based on these fndings, a possible
mechanism was proposed and illustrated in Scheme 1.
When GR/FeMIL-101 was exposed to a visible light irra-
diation condition, e− and h+ generated in the catalyst. With
the reduction of Cr4+ and Fe3+ ions by the photoinduced
e−, ·O2− was produced subsequently. Simultaneously, benzyl
alcohol was oxidized by the photoinduced h+ to produce new
benzyl radicals. The newly produced benzyl radicals were
Fig. 12 The recycling property of GR/FeMIL-101 for the photooxid-
tion of benzyl alcohol
When EDTA-2Na and IPA were added into the reaction,
the conversion decreased slightly. It suggested that ·OH and
h+ was not vital in the reaction. As a result, it was obvious
that ·O2− was the main activity species in this photocatalytic
process.
−
Mott-Schottky plots were measured and shown in
Fig. 14b. It was clear that the fat band position of FeMIL-
101 is −0.56 V vs Ag/AgCl (−0.30 V vs NHE). According
then immediately oxidized by the ·O2 , and benzaldehyde
formed successfully. It showed that ·O2− was very important
for this photocatalytic process.
Fig. 13 XRD patterns (a)
and FT-IR spectra (b) of GR/
FeMIL-101 before and after the
catalytic reactions
Fig. 14 The species cap-
turing results (a) and the
Mott-Schottky plot (b) for
the photooxidation of benzyl
alcohol with the catalysis of
GR/FeMIL-101
1 3

Fe Doped MIL-101/Graphene Nanohybrid for Photocatalytic Oxidation of Alcohols Under…
7. Wu Z, Huang X, Zheng H, Wang P, Hai G, Dong W, Wang G
(2018) Appl Catal B 224:479–487
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Z (2016) Appl Catal B 198:112–123
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Scheme 1 Possible mechanism for the photocatalytic oxidation of
15. Meng S, Wang M, Lu B, Xue Q, Yang Z (2019) Acta Chim Sin
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alcohols over GR/FeMIL-101
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4 Conclusions
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In summary, the nanohybridized GR/FeMIL-101 was suc-
cessfully prepared by a simple hydrothermal method, which
showed the moderate catalytic activity for the phooxidation
of benzyl alcohol under visible light irradiation. It certifed
that the doping of Fe to MIL-101 and the combination of
GR with FeMIL-101 really could improve the charge sepa-
ration efciency of the catalyst. The mechanism revealed
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−
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tooxidation of benzyl alcohol. This photocatalytic system
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Acknowledgements The research was financially supported by
NSFC (52063026, 21563026), the Program for Changjiang Scholars
and Innovative Research Team in University (IRT15R56), the Inno-
vation Team Basic Scientifc Research Project of Gansu Province
(1606RJIA324), and the Science and Technology Program of Gansu
Province (19JR2RA020). We also thank the Key Laboratory of Eco-
functional Polymer Materials (Northwest Normal University), Ministry
of Education, and the Gansu International Scientifc and Technological
Cooperation Base of Water-Retention Chemical Functional Materials,
for fnancial support.
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Afliations
Mingming Wang1 · Yali Ma1 · Bolin Lv1 · Fenglin Hua1 · Shuangyan Meng1 · Xuedi Lei1 · Qingtao Wang1 · Bitao Su1 ·
Ziqiang Lei1 · Zhiwang Yang1
1
Key Laboratory of Polymer Materials of Gansu
Province, Key Laboratory of Eco-Functional Polymer
Materials, Ministry of Education, College of Chemistry
and Chemical Engineering, Northwest Normal University,
Lanzhou 730070, China
1 3