A hydrophobic titanium doped zirconium-based metal organic framework for photocatalytic hydrogen peroxide production in a two-phase system

Article abstract of DOI:10.1039/c9ta11120d

Photocatalytic synthesis of hydrogen peroxide (H₂O₂) from O₂ coupled with the synthesis of valuable products from benzyl alcohol (BA) by a hydrophobic metal organic framework (MOF) is attractive. Herein, a hydrophobic titanium doped zirconium-based MOF (OPA/Zr_100-xTi_x-MOF), whose metal Zr clusters are alkylated by octadecylphosphonic acid (OPA), is synthesized and used for photocatalytic H₂O₂ production in a two-phase system (water/BA). This allows the formation of H₂O₂ in the water phase and benzaldehyde, the oxidation product of BA, in the organic phase, respectively. The hydrophobic OPA/Zr_92.5Ti_7.5-MOF exhibited a remarkable H₂O₂ production rate of 9.7 mmol L^-1 h^-1 under the irradiation of visible light (λ > 420 nm), which is about 4.5 times higher than that of the parent zirconium-based MOF (Zr₁₀₀-MOF). The enhanced activity is attributed to the effective Ti-doping and the unique hydrophobic nature of the catalyst. The Ti species played a role in effectively promoting electron transfer from photoexcited linkers of the MOF to Ti, inhibiting the recombination of photogenerated electron-hole pairs in the hydrophobic MOF matrix. The unique hydrophobic nature, exemplified by a 139° water contact angle, prevented overreduction of H₂O₂ by spatial separation of the MOF in BA and H₂O₂ in water. The recycling test of H₂O₂ production over the catalysts demonstrated that OPA/Zr_92.5Ti_7.5-MOF displayed good stability. This study has greatly enriched the applications of hydrophobic MOFs in the field of photocatalytic energy production.

Full text of DOI:10.1039/c9ta11120d

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Page 1 of 7
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DOI: 10.1039/C9TA11120D
ARTICLE
Hydrophobic Titanium Doped Zirconium-based Metal Organic
Framework for Photocatalytic Hydrogen Peroxide Production in
Two-phase System
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaolang Chen,a Yasutaka Kuwahara,abc Kohsuke Mori,ab Catherine Louis,d and Hiromi Yamashita*ab
DOI: 10.1039/x0xx00000x
Photocatalytic synthesis of hydrogen peroxide (H2O2) from O2 coupled with the synthesis of valuable products from benzyl
alcohol (BA) by hydrophobic metal organic framework (MOF) is attractive. Herein, a hydrophobic titanium doped zirconium-
based MOF (OPA/Zr100-xTix-MOF), whose metal Zr clusters were alkylated by octadecylphosphonic acid (OPA), was
synthesized and used for photocatalytic H2O2 production in two-phase system (water/BA). This allows the formation of H2O2
in water phase and benzaldehyde, the oxidation product of BA, in the organic phase, respectively. The hydrophobic
OPA/Zr92.5Ti7.5-MOF exhibited a remarkable H2O2 production rate of 9.7 mmol•L-1•h-1 under the irradiation of visible light (λ>
420 nm), which is about 4.5 times higher than that of the parent zirconium-based MOF (Zr100-MOF). The enhanced activity
is attributed to the effective Ti-doping and the unique hydrophobic nature of the catalyst. Ti species played the role of
effectively promoting the electron transfer from photoexcited linkers of MOF to Ti, inhibiting the recombination of
photogenerated electron-hole pairs in the hydrophobic MOF matrix. The unique hydrophobic nature, exemplified by a 139°
of water contact angle ，prevented overreduction of H2O2 by spatial separation of the MOF in BA and H2O2 in water.
Recycling test of H2O2 production over the catalysts demonstrated that OPA/Zr92.5Ti7.5-MOF displayed a good stability. This
study has greatly enriched the new application of hydrophobic MOFs in the field of photocatalytic energy production.
disadvantages such as high energy consumption or potentially
explosive danger.11-13 Therefore, it is necessary to explore a safer,
more efficient, environmental-friendly and low-cost technology for
Introduction
Hydrogen peroxide (H2O2) can act as both multifunctional oxidant
and reductant,1 and is widely used in various chemical industries (
like organic synthesis and paper-bleaching) and environmental
H2O2 production.
Photocatalytic H2O2 production has recently attracted much
attention due to its safety, eco-friendly consumption of low energy
and pollution-free.1, 10, 14 Much efforts have been devoted to the
photocatalytic H2O2 production by the reduction of O2. For example,
TiO2,15, 16 CdS17 and g-C3N4-based1, 10 photocatalysts were reported to
produce H2O2. However, the photocatalytic activity was still low, all
of these systems produced only micromolar levels of H2O2 because
treatment2 (such as disinfection and water treatment).3,
4
Furthermore, H2O2 is a promising green and environmentally friendly
energy alternative to hydrogen energy (H2),5-7 and can be used in fuel
cells.1 H2O2 is more convenient and safer to be stored and
transported than H2 because of its higher water solubility.8, 9 As a
potential energy carrier, the theoretical potential of aqueous H2O2 is
1.09 V, which is similar with that of a traditional H2/O2 fuel cell
(1.23V).1 The conventional anthraquinone method for H2O2
production meets some disadvantages such as high costs,
complicated industrial routes, and substantial toxic by-products.10
Moreover, both electrocatalytic oxygen reduction reaction (ORR)
route and the direct synthesis of H2O2 from H2 and O2 have
of the wide bandgaps or low catalytic efficiency of photocatalysts.16,
18
CH2OH
CHO
Cat.
hv
H2O2
+
(1)
O2
+
Metal organic frameworks (MOFs), consisting of organic linkers
connected with metal oxide clusters, have been widely utilized in
photocatalytic H2 evolution,19 CO2 reduction,20 dye degradation21 as
well as organic transformations22 due to theirs advantages, such as
large surface area, tunable bandgap and well-defined crystalline
structure, etc.23-25 Moreover, many MOFs have the property of
absorbing visible light through their amine-functionalized linkers,26
effectively improving the utilization rate of visible light. However,
there are still few reports on the photocatalytic H2O2 production by
MOFs. Recently, our group reported that photocatalytic H2O2
productioncoupled with oxidation of benzyl alcohol to benzaldehyde
in two-phase system (equation (1)) was achieved successfully via the
a. Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka
565-0871, Japan. E-mail: yamashita@mat.eng.osaka-u.ac.jp
b. Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University,
Katsura, Kyoto 615-8520, Japan
c. JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
d. Sorbonne Universités, UPMC Univ Paris 06, UMR CNRS 7197, Laboratoire de
Réactivité de Surface, 4 Place Jussieu, Tour 43-33, 3ème étage, Case 178, F-75252
Paris, France
† Electronic Supplementary Information (ESI) available: BET, FTIR, XPS, TG-DTA
data of the catalysts, the digital picture of samples in two-phase system, activity
of H2O2 production in single-phase system, benzaldehyde concentration and
characterization data of the used catalyst. See DOI: See DOI: 10.1039/x0xx00000x
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Journal of Materials Chemistry A
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ARTICLE
Journal Name
Synthesis of Zr100-MOF:
View Article Online
DOI: 10.1039/C9TA11120D
In a typical synthesis of NH2-UiO-66(Zr), ZrCl4 (233.2 mg, 1.0 mmol)
and 2-aminoterephthalic acid (181.2 g, 1.0 mmol) were dissolved in
DMF (50 mL), subsequently, the solution was transferred to a 100 mL
Teflon-lined stainless steel autoclave. The autoclave was sealed and
o
heated in an oven at 120 C for 48 h, after cooling naturally of the
oven in one hour, the samples were washed three time with DMF
and centrifuged, then washed three times with anhydrous methanol
and centrifuged, followed by dried under vacuum overnight at room
temperature, and activated in vacuum at 120 oC overnight to remove
the solvent molecular in the MOFs. The pristine NH2-UiO-66(Zr) is
denoted as Zr100-MOF.
Synthesis of Zr100-xTix-MOF:
The Ti doped NH2-UiO-66(Zr) were prepared according to the same
procedure as above but with addition of varying amounts of Ti4+
(titanium isopropoxide was used as the Ti4+ precursor). The total
amount of metals (Ti and Zr) was 1.0 mmol, and the selected molar
ratios of Ti/(Ti+Zr) were 5%, 7.5% and 10%. After synthesis, the
samples were dealt with the same way as for Zr100-MOF. The samples
are denoted as Zr95Ti5-MOF, Zr92.5Ti7.5-MOF and Zr90Ti10-MOF,
respectively.
Scheme 1 Synthesis of Zr100-MOF, OPA/Zr100-MOF, Zr100-xTix-MOF and OPA/Zr100-xTix-MOF.
hydrophobization of MOFs.27, 28
Zirconium-based MOF, due to its excellent structural stability,
mechanical pressure and chemical properties, has attracted much
attention in catalysis field.29-32 In addition, partial substitution of Zr
ions by doping with suitable metal ions as an electron mediator into
MOFs can promote charge separation and prevent the
recombination of photogenerated carriers in the MOF matrix.33 For
instance, Ti ions can effectively acts as active sites through a redox
process of Ti4+ into Ti3+ in the photocatalytic reaction.34, 35 Based on
this, we assumed that the combination of hydrophobic property of
MOF with partially substituted Zr ions by Ti ions in a zirconium-based
MOF (NH2-UiO-66(Zr)), which is relatively easy to be synthesized, will
generate new catalysts for photocatalytic H2O2 production. Thus, Ti
doped zirconium-based MOFs (Zr100-xTix-MOF) were first synthesized
with various of x. Then, octadecylphosphonic acid (OPA) with a long
alkyl chain was used to coordinate metal Zr ions of clusters36
(Scheme 1). Ultimately, hydrophobic Ti doped zirconium-based MOF
(OPA/Zr100-xTix-MOF) was prepared. One of the samples,
Synthesis of OPA/Zr100-xTix-MOF:
Unactivated MOFs (50 mg) were immersed in OPA ethanol solution
(50 mL, 50 mM) for 24 h at room temperature. The suspension was
stirred for 5 min every 2 h during the first 12 h. Then, it was
centrifuged and washed several times with ethanol to remove free
OPA. The washed samples were dried under vacuum at room
temperature and activated in vacuum at 120 oC overnight. The
corresponding samples are denoted as OPA/Zr100-MOF, OPA/Zr95Ti5-
MOF, OPA/Zr92.5Ti7.5-MOF and OPA/Zr90Ti10-MOF, respectively.
Material Characterization
Powder X-ray diffraction (XRD) was collected with a Rigaku Ultima
IV diffractometer with Cu Kα radiation (λ = 1.54056 Å). Ultraviolet
visible diffuse reflectance spectra (UV-Vis DRS) was recorded by a
Shimadzu UV-2450 spectrophotometer. BaSO4 was used as the
reference, and absorption spectra were obtained using the Kubelka-
Munk function. Nitrogen adsorption measurements were performed
using BELSORP-max system (Microtrac BEL Corp., Inc.) at -196 oC.
Samples were degassed under vacuum at 120 oC for at least 6 h
before data collection. TG-DTA was performed under air flow using a
Rigaku thermogravimetry unit, Rigaku Thermo Plus EVO II series
high-temperature differential thermal balance TG/DTA, with
OPA/Zr92.5Ti7.5-MOF showed
a 139° of water contact angle
(exceeding 90°). Moreover, this hydrophobic catalyst was used into
the two-phase system and brought about a remarkable H2O2
production rate of 9.7 mmol•L-1•h-1, which was about 4.5 times
higher than that of the parent MOF (Zr100-MOF).
Experiment section
Chemicals
2-Aminoterephthalic
acid
(98%),
Oxo[5,10,15,20-tetra(4-
pyridyl)porphinato]titanium(IV) ([TiO(tpypH4)]4+) (> 90%), and
octadecylphosphonic acid (OPA) were supplied by Tokyo Chemical
Industry Co., Ltd. (TCI). Hydrogen peroxide (30%), hydrochloric acid
(5 mol•L-1), perchloric acid (60%), acetonitrile (99%), zirconium
tetrachloride (ZrCl4, > 97%), N, N-dimethylformaldehyde (DMF, >
99.5%), ethanol (99.5%), methanol (> 99%) and benzyl alcohol (>
97%) were purchased from Nacalai Tesque. All chemicals were
obtained commercially and utilized without further purification.
temperature ramp of 1.0 ℃ min-1 up to 800 ℃. Fourier transform
infrared spectroscopy (FT-IR) was carried out with a JASCO FTIR-6100
by measuring the plates of MOFs added into KBr. X-ray
photoelectron spectra (XPS) was performed with a Shimadzu XPS
system, ESCA-3400, using the Mg K line (1253.6 eV). Binding energy
was calibrated using C 1s photoelectronic peak at 285 eV.
Photoluminescence measurements were performed using
fluorolog-3 spectrofluorometer (Horiba).
a
Catalysts preparation
2 | J. Name., 2012, 00, 1-3
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Page 3 of 7
Journal of Materials Chemistry A
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Journal Name
ARTICLE
Photocatalytic reaction
Results and discussion
View Article Online
DOI: 10.1039/C9TA11120D
In a typical photocatalytic reaction setup for two-phase system:
the photocatalysts (5.0 mg), water (2.0 mL) and benzyl alcohol (BA)
(5.0 mL) were added to a Pyrex® reaction vessel (30 mL), which was
sealed with a rubber septum. The resulting mixture was sonicated for
about 5 min to disperse the photocatalyst, then bubbled with oxygen
for 15 min with a rate of 20 mL•min-1 under magnetic stirring in the
dark. Subsequently, the reaction vessel was irradiated from the side
with a Xe lamp (500 W; SANEI ELECTRIC XEF-501S) through a glass
filter (λ> 420 nm) under magnetic stirring at ambient pressure and
temperature. The distance between the light source and reaction
vessel was approximately 2 cm. An aliquot of solution was collected
from the reaction mixture every one hour and analysed.
X-ray diffraction (XRD) patterns of the synthesized samples were
shown in Fig. 1a and b. They only display the characteristic diffraction
peaks of pristine Zr100-MOF but with weakened peak intensities due
to Ti-doping and addition of OPA. No other peaks associated with Ti4+
oxides or Ti-related impurities can be observed. This preliminary
indicates that the MOFs was doped by Ti ions.
In addition, the UV-Vis DRS were shown in Fig. 1c and d. Both Zr100-
xTix-MOF and OPA/Zr100-xTix-MOF generally inherited the optical
absorption characteristics of the parent Zr100-MOF. The absorption
bands in the high energy region (λ < 300 nm) can be assigned to the
 →* transition of the linker, while the absorption band extending
to the visible region from 320 to 450 nm is originated from the ligand-
to-cluster charge transfer (LCCT) of amino functional group (-NH2) on
the organic linkers.38, 39 It indicates that all of the samples have the
visible-light-responsive characteristic, which is suitable for the
photocatalytic reaction induced by visible light. After doping with Ti
ions, the absorption edge position of MOFs slightly shifted to lower
wavelength compared with the sample without Ti doping because of
the generation of several energy bands between the lowest
unoccupied molecular orbital (LUMO) and highest occupied
molecular orbital (HOMO) states, confirming the successful
substitution of Ti ions.26
The N2 adsorption/desorption isotherms and Brunauer-Emmett-
Teller (BET) surface areas were shown in Fig. S1 and Table S1. All
samples exhibited a type I sorption isotherm, indicating a typical
microporous structure. The BET surface areas of Zr100-xTix-MOF
continue to decrease as the amount of Ti-doping increases,
respectively, compared with that of Zr100-MOF. OPA/Zr100-xTix-MOF
has the similar trend, compared with that of OPA/Zr100-MOF. This
could be resulted from the loss of the well-ordered structure due to
Ti-doping, as confirmed by the XRD (Fig. 1a and b). Additionally, the
BET surface areas of Zr100-xTix-MOF also decreases after modification
In a photocatalytic reaction setup for single-phase system: the
photocatalysts (5.0 mg), acetonitrile (5.0 mL) and BA (1.0 mL) were
added to a Pyrex® reaction vessel (30 mL), then the next procedures
were same as above two-phase system.
Recycling test
The MOFs (10 mg) were added to the two-phase system composed
of water (4.0 mL) and benzyl alcohol (10 mL). The mixture was
bubbled with oxygen for 15 min with a rate of 20 mL•min-1 in the
dark. Then, the system was irradiated with visible light (λ > 420 nm).
After 3 h, the quantification of H2O2 was detected and the reaction
suspension was transferred to the centrifuge tube from reaction
vessel, washed three times with ethanol and centrifuged to collect
the MOFs. The collected MOFs were dried in vacuum at room
temperature overnight. After that, benzyl alcohol (10 mL) was added
into the centrifuge tube, then the mixture was sonicated to get the
suspension, which was transferred to the reaction vessel for next
cycle.
Quantification of produced H2O2
The amount of produced H2O2 was determined by spectroscopic
titration with an acidic solution of [TiO(tpypH4)]4+ complex (Ti-TPyP
reagent) according to the literature.37 The Ti-TPyP reagent (34 mg)
was dissolved in 1.0 L of 50 mM hydrochloric acid. An aliquot (e.g.,
10 μL) from original reaction solution was diluted in a given volume
of distilled water and utilized as a sample solution. 0.25 mL of Ti-TPyP
reagent and 0.25 mL of 4.8 M perchloric acid were added into 0.25
mL of the sample solution. After waiting 5 min at room temperature,
the mixture was diluted to 2.5 mL with distilled water and used for
the spectroscopic measurement. Absorbance at λ= 434 nm was
measured using the Shimadzu UV-2600 UV-Vis spectrophotometer
(AS). A similar way, a blank solution, whose absorbance designated
as AB, was prepared by using distilled water instead of the sample
solution. The difference in absorbance was calculated by following
the equation: ∆A434 = AB – AS. Based on ∆A434 and the volume of
solution, the amount of H2O2 was deduced.
The amount of produced benzaldehyde was analysed by gas
chromatography (Shimadzu, GC-14B with Phenomenex ZB-FFAP
columns).
Fig. 1 XRD patterns of (a) Zr100-xTix-MOF and (b) OPA/Zr100-xTix-MOF before and after used.
UV-vis DRS of (c) Zr100-xTix-MOF and (d) OPA/Zr100-xTix-MOF.
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
indicating their hydrophilic property. By contrast, theVwieawtAerrticcleonOtnalincet
DOI: 10.1039/C9TA11120D
angles for OPA/Zr100-MOF and OPA/Zr92.5Ti7.5-MOF are 127° and 139°
(Fig. 3b and d). The higher water contact angles exceeding 90°
indicate that OPA has transformed MOFs into hydrophobic materials.
The photoluminescence (PL) measurements were further
performed to verify the intrinsic role of Ti dopant in MOFs during the
photocatalysis. The corresponding PL emission spectra of Ti doped
MOFs are shown in Fig. 4a and b. Zr92.5Ti7.5-MOF and OPA/Zr92.5Ti7.5
-
MOF both show weaker PL emission intensities than Zr100-MOF and
OPA/Zr100-MOF without Ti dopant. This result confirms that Ti dopant
effectively promotes electrons transfer, inhibiting the recombination
of electron-hole pairs in the photoexcited MOFs.40-42
H2O2 production in two-phase system was performed under visible
light irradiation (λ> 420 nm). The hydrophilic Zr100-xTix-MOF was
dispersed in aqueous phase, while hydrophobic OPA/Zr100-xTix-MOF
was dispersed in BA phase (Fig. S6). When H2O2 was produced, it was
concentrated in aqueous phase, while benzaldehyde, the oxidation
product of BA in the BA phase. The concentration of H2O2 on a certain
time are shown in Fig. 4c and d. Hydrophobic OPA/Zr100-xTix-MOF
showed higher activity of H2O2 production (Fig. 4d) than hydrophilic
Zr100-xTix-MOF (Fig. 4c), reflecting the better catalytic performance of
hydrophobic MOFs. Usually, overreduction of H2O2 to OH- and •OH
occur during photocatalysis, which results in lowering H2O2
production. Spatial separation of MOFs and produced H2O2
suppresses this phenomenon.28 To illustrate this point, H2O2
decomposition in two-phase system was measured and the results
are shown in Fig. 5a. OPA/Zr92.5Ti7.5-MOF showed higher H2O2
concentration than Zr92.5Ti7.5-MOF, demonstrating that the
hydrophobic MOF in organic phase does inhibit the overreduction of
H2O2. Additionally, OPA/Zr92.5Ti7.5-MOF have better activity than
other OPA modified MOFs (Fig. 4d). Similar trend was shown in Fig.
4c for Zr92.5Ti7.5-MOF. It illustrates that doping Ti with a molar rate of
7.5% is optimal. Hence, Ti species plays a crucial role in the
improvement of activity for H2O2 production. Finally, the
hydrophobic OPA/Zr92.5Ti7.5-MOF possesses a H2O2 production rate of
9.7 mmol•L-1•h-1, which is about 4.5 times higher than that of the
pristine Zr100-MOF. A further increase of doping amount of Ti leads
to a significant decrease of activity, which can be attributed to the
lower crystallinities, as confirmed by XRD (Fig. 1a and b) and
decreased surface areas (Table S1) of samples. Specially, the formed
Fig. 2 XPS spectra of the synthesized samples: Ti 2p of (a) Zr100-xTix-MOF and (b) OPA/Zr100-
xTix-MOF. (c) P 2p of Zr92.5Ti7.5-MOF and OPA/Zr92.5Ti7.5-MOF. (d) Zr 3d of Zr92.5Ti7.5-MOF
and OPA/Zr92.5Ti7.5-MOF.
by OPA, compared with that of OPA/Zr100-xTix-MOF. This is due to the
modification of OPA into the interior of MOFs, as verified by the
reduced pore volume.
The chemical states of Ti, P and Zr elements in Zr100-xTix-MOF and
OPA/Zr100-xTix-MOF were determined by XPS in Fig. 2a, the peaks at
around 459.0 eV and 465.0 eV attributed to the Ti 2p3/2 and Ti 2p1/2
,
respectively, further confirm the Ti4+-doping in Zr100-xTix-MOF and
OPA/Zr100-xTix-MOF. As the amount of Ti increases, the corresponding
peak signals are stronger. Furthermore, the corresponding peak
intensities of Ti4+ are more weakened in the hydrophobic OPA/Zr100-
xTix-MOF in Fig.2b. This could be due to the modification of OPA into
both surface and interior of MOFs, resulted in weaker signals.
Additionally, the XPS peaks of P 2p at around 134.0 eV for Zr100-MOF,
OPA/Zr100-MOF, Zr92.5Ti7.5-MOF and OPA/Zr92.5Ti7.5-MOF were also
shown in Fig. S2 and Fig. 2c. It further indicates that the MOFs are
modified by OPA successfully, which is consistent with the results of
FT-IR spectra in Fig. S3. Two peaks assigned to the symmetric and
asymmetric stretching vibration of CH2 in the alkyl –CH2– groups of
OPA are observed at around 2920 and 2850 cm-1 in the FT-IR spectra
of OPA/Zr100-xTix-MOF (Fig. S3b), which are not observed in the
spectra of Zr100-xTix-MOF (Fig. S3a). Moreover, the binding energies
of Zr 3d in OPA/Zr92.5Ti7.5-MOF both show positive shifts of 0.3 eV,
compared to that of Zr92.5Ti7.5-MOF in Fig. 2d. The same result for
OPA/Zr100-MOF and Zr100-MOF was shown in Fig. S4. This result
demonstrates an interaction between OPA and the Zr atoms of metal
-
•O2 inside MOFs will not be diffused well to the aqueous phase to
form H2O2 due to the smaller surface areas and smaller pore volume.
clusters, resulting from the formation of Zr-O-P36 in OPA/Zr92.5Ti7.5
-
MOF and OPA/Zr100-MOF, sustaining the alkylation of Zr clusters by
OPA. TG-DTA measurements in Fig. S5 revealed that 7.3 % and 8.7 %
of the Zr atoms in the clusters were alkylated by OPA for OPA/Zr100
-
MOF and OPA/Zr92.5Ti7.5-MOF, respectively.
Fig. 3 The water contact angles of Zr100-MOF, OPA/Zr100-MOF, Zr92.5Ti7.5-MOF and
OPA/Zr92.5Ti7.5-MOF (20μL of water droplets on tablets of MOFs, the tablets were
produced by pressing corresponding ~20 mg of MOFs at 10 MPa for 5 min).
The hydrophobicity of MOFs was examined by measuring their
water contact angles (Fig. 3). The water contact angles of Zr100-MOF
and Zr92.5Ti7.5-MOF are 30° and 0° (Fig. 3a and c), respectively,
4 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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DOI: 10.1039/C9TA11120D
Fig. 6 The proposed mechanism for photocatalytic H2O2 production of (a) Zr100-MOF and
(b) OPA/Zr92.5Ti7.5-MOF.
reaction indicates that two peaks at around 2920 and 2850 cm-1 of
CH2 in the alkyl –CH2– groups in OPA are retained (Fig. S9b), implying
that its hydrophobic nature is maintained. These results adequately
demonstrate that OPA/Zr92.5Ti7.5-MOF is stable in the two-phase
system.
Based on the above results, a reaction mechanism was proposed
in Fig. 6. For hydrophilic Zr100-MOF without Ti-doping (Fig. 6a),
photoexcited electrons formed and gathered in the LUMO after the
absorption of visible light by the linker,43, 44 while photoexcited holes
generated in the HOMO.44 Photoexcited electrons in LUMO would
Fig. 4 The Photoluminescence (PL) emission spectra (λex = 400 nm) of (a) Zr100-MOF and
Zr92.5Ti7.5-MOF, (b) OPA/Zr100-MOF and OPA/Zr92.5Ti7.5-MOF. Time courses of H2O2
production of (c) hydrophilic Zr100-xTix-MOF and (d) hydrophobic OPA/Zr100-xTix-MOF
under photoirradiation (λ > 420 nm) of two-phase system composed of BA (5.0 mL) and
water (2.0 mL) catalyzed by 5.0 mg of photocatalysts.
-
reduce O2 to •O2 , while photogenerated holes in HOMO were
quenched by the oxidation of benzyl alcohol to benzaldehyde. The
This can be further verified in Fig. S7. H2O2 production utilizing Zr100
-
-
•O2 then formed H2O2 through disproportionation process28, 45 in
MOF and OPA/Zr100-MOF was examined in a single-phase system,
which was composed of an acetonitrile solution (5.0 mL) of BA (1.0
mL). OPA/Zr100-MOF displayed lower activity than Zr100-MOF due to
the much lower surface areas and smaller pore volume caused by
modification of OPA. Similarly, this may also influence the diffusion
of produced benzaldehyde, resulting in lower detectable
concentration of benzaldehyde (Fig. S8) than that of H2O2 production
after 3 h of reaction.
aqueous phase. However, the recombination of photoexcited
electrons and holes is prone to happen due to the lack of an effective
46
LCCT mechanism.44,
In addition, the hydrophilic MOF would
inevitably decompose H2O2 when both the catalyst and H2O2 are in
the same phase. Those factors cause an undesirable activity
decrease. In contrast, the LCCT mechanism is effectively constructed
after Ti-doping in hydrophobic OPA/Zr92.5Ti7.5-MOF. The doped Ti4+
ions in clusters were reduced to Ti3+ by the photoexcited electrons in
LUMO of hydrophobic OPA/Zr92.5Ti7.5-MOF (Fig. 6b). Then, the
Results of recycling tests were displayed in Fig. 5b. OPA/Zr92.5Ti7.5
-
MOF still remains high activity of H2O2 production after 7 cycles of
experiment, while Zr92.5Ti7.5-MOF shows a continuous decrease. This
is due to the more stable structure of hydrophobic MOF in organic
phase, while the structure of hydrophilic MOF in the aqueous phase
could be destroyed, as verified by XRD of the samples before and
after recycling tests (Fig. 1 and S9a). It reveals that the characteristic
diffraction peaks of OPA/Zr92.5Ti7.5-MOF are maintained
substantially, indicating its intact structure is remained, whereas the
peak intensity for Zr92.5Ti7.5-MOF is weakened, and an impurity peak
at 18° has appeared. The FTIR of OPA/Zr92.5Ti7.5-MOF before and after
-
reduction of O2 to •O2 by Ti3+ was achieved, while photogenerated
holes in HOMO were quenched by the oxidation of benzyl alcohol to
benzaldehyde. We have already reported that in two-phase reaction
-
system, •O2 would transfer to the aqueous phase before it formed
H2O2 in aqueous phase.28 It should be pointed out that the Ti4+
species in the MOF played
a vital role in promoting the
photogenerated charge transfer from MOF thanks to the redox
reaction of Ti4+/Ti3+, inhibiting the recombination of photogenerated
electron–hole pairs. Additionally, the spatial separation of the MOF
and H2O2 prevented overreduction of H2O2, also resulting in a higher
activity.
Conclusions
Hydrophobic Ti doped NH2-UiO-66(Zr) was synthesized and used
to produce H2O2 in a two-phase system. Doping with Ti species
promotes the photogenerated electrons transfer and inhibits the
recombination of the photogenerated electron-hole pairs in the
MOFs, bringing about an improved activity of H2O2 production. The
hydrophobic nature of MOFs allows the spatial separation of the
photocatalyst while produced H2O2 is transferred to water phase.
Fig. 5 (a) Time courses of H2O2 (20 mM) decomposition in two-phase system at room
temperature in dark. (b) Recycling tests of Zr92.5Ti7.5-MOF and OPA/Zr92.5Ti7.5-MOF.
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ARTICLE
Journal Name
21 C.-C. Wang, J.-R. Li, X.-L. Lv, Y.-Q. Zhang and G. Guo, Energy
This is beneficial for inhibiting the overreduction of H2O2, and results
in an enhanced activity for H2O2 production. Furthermore, the
hydrophobic MOF sustains a good stability in the recycling tests. This
study greatly extended the application of hydrophobic MOFs in the
field of photocatalytic H2O2 production.
View Article Online
Environ. Sci., 2014, 7, 2831-2867.
22 X. Deng, Z. Li and H. García, Chem. EuDr.OJI.:,120.0101379,/2C39T, A1111112809D-
11209.
23 Z. L. Wu, C. H. Wang, B. Zhao, J. Dong, F. Lu, W. H. Wang, W.
C. Wang, G. J. Wu, J. Z. Cui and P. Cheng, Angew. Chem., Int.
Ed., 2016, 55, 4938-4942.
24 T. Zhang and W. Lin, Chem. Soc. Rev., 2014, 43, 5982-5993.
25 W. Wang, X. Xu, W. Zhou and Z. Shao, Adv. Sci., 2017, 4,
1600371.
26 M. Wen, K. Mori, Y. Kuwahara and H. Yamashita, ACS Energy
Lett., 2017, 2, 1-7.
Conflicts of interest
There are no conflicts to declare.
27 Y. Kawase, Y. Isaka, Y. Kuwahara, K. Mori and H. Yamashita,
Chem. Commun., 2019, 55, 6743-6746.
28 Y. Isaka, Y. Kawase, Y. Kuwahara, K. Mori and H. Yamashita,
Angew. Chem., Int. Ed., 2019, 58, 5402-5406.
29 Y. Han, M. Liu, K. Li, Q. Sun, W. Zhang, C. Song, G. Zhang, Z.
Conrad Zhang and X. Guo, Inorg. Chem. Front., 2017, 4, 1870-
1880.
30 J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S.
Bordiga and K. P. Lillerud, J. Am. Chem. Soc., 2008, 130, 13850-
13851.
Acknowledgements
The present work was supported by the Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT) of Japan (26220911). Part of this work was
performed under the management of “Elements Strategy Initiative
for Catalysts and Batteries (ESICB)” supported by the MEXT, Japan.
31 L. Valenzano, B. Civalleri, S. Chavan, S. Bordiga, M. H. Nilsen,
S. Jakobsen, K. P. Lillerud and C. Lamberti, Chem. Mater.,
2011, 23, 1700-1718.
32 Y. Kuwahara, H. Kango and H. Yamashita, ACS Sustain. Chem.
Eng. 2017, 5, 1141-1152.
33 M. A. Syzgantseva, C. P. Ireland, F. M. Ebrahim, B. Smit and O.
A. Syzgantseva, J. Am. Chem. Soc., 2019, 141, 6271-6278.
34 D. Sun, W. Liu, M. Qiu, Y. Zhang and Z. Li, Chem. Commun.,
2015, 51, 2056-2059.
35 A. Santiago Portillo, H. G. Baldoví, M. T. García Fernandez, S.
Navalón, P. Atienzar, B. Ferrer, M. Alvaro, H. Garcia and Z. Li,
J. Phys. Chem. C 2017, 121, 7015-7024.
36 Y. Sun, Q. Sun, H. Huang, B. Aguila, Z. Niu, J. A. Perman and S.
Ma, J. Mater. Chem. A, 2017, 5, 18770-18776.
37 C. Matsubara, N. Kawamoto and K. Takamura, Analyst, 1992,
117, 1781-1784.
38 D. Sun, Y. Fu, W. Liu, L. Ye, D. Wang, L. Yang, X. Fu and Z. Li,
Chem.–Eur. J., 2013, 19, 14279-14285.
39 C. Silva, I. Luz, X. Llabres, A. Corma and H. Garcia, Chem., 2010,
16, 11133-11138.
40 J. D. Xiao, Q. Shang, Y. Xiong, Q. Zhang, Y. Luo, S. H. Yu and H.
L. Jiang, Angew. Chem., Int. Ed., 2016, 55, 9389-9393.
41 M. D. L. Ruiz Peralta, U. Pal and R. S. Zeferino, ACS Appl.
Mater. Inter, 2012, 4, 4807-4816.
Notes and references
1
Y. Kofuji, Y. Isobe, Y. Shiraishi, H. Sakamoto, S. Tanaka, S.
Ichikawa and T. Hirai, J. Am. Chem. Soc., 2016, 138, 10019-
10025.
2
3
4
J. M. Campos-Martin, G. Blanco-Brieva and J. L. G. Fierro,
Angew. Chem., Int. Ed., 2006, 45, 6962-6984.
Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K.
Nørskov and T. F. Jaramillo, Science, 2017, 355, eaad4998.
S. Siahrostami, A. Verdaguer-Casadevall, M. Karamad, D.
Deiana, P. Malacrida, B. Wickman, M. Escudero-Escribano, E.
A. Paoli, R. Frydendal, T. W. Hansen, I. Chorkendorff, I. E. L.
Stephens and J. Rossmeisl, Nat. Mater., 2013, 12, 1137.
Y. Zheng, Z. Yu, H. Ou, A. M. Asiri, Y. Chen and X. Wang, Adv.
Funct. Mater., 2018, 28, 1705407.
S. Zhao, T. Guo, X. Li, T. Xu, B. Yang and X. Zhao, Appl. Catal.,
B, 2018, 224, 725-732.
H. Yin, Y. Kuwahara, K. Mori and H. Yamashita, J. Mater. Chem.
A, 2018, 6, 10932-10938.
5
6
7
8
9
S. Zhao and X. Zhao, J. Catal., 2018, 366, 98-106.
S. Zhao and X. Zhao, Appl. Catal., B, 2019, 250, 408-418.
10 Z. Wei, M. Liu, Z. Zhang, W. Yao, H. Tan and Y. Zhu, Energy
Environ. Sci., 2018, 11, 2581-2589.
11 Z. Zheng, Y. H. Ng, D.-W. Wang and R. Amal, Adv. Mater.,
2016, 28, 9949-9955.
12 J. C. Pritchard, Q. He, E. N. Ntainjua, M. Piccinini, J. K. Edwards,
A. A. Herzing, A. F. Carley, J. A. Moulijn, C. J. Kiely and G. J.
Hutchings, Green Chem., 2010, 12, 915-921.
13 N. M. Wilson and D. W. Flaherty, J. Am. Chem. Soc., 2016, 138,
574-586.
14 H. i. Kim, O. S. Kwon, S. Kim, W. Choi and J. H. Kim, Energy
Environ. Sci., 2016, 9, 1063-1073.
15 G. h. Moon, W. Kim, A. D. Bokare, N. e. Sung and W. Choi,
Energy Environ. Sci., 2014, 7, 4023-4028.
16 Y. Shiraishi, S. Kanazawa, D. Tsukamoto, A. Shiro, Y. Sugano
and T. Hirai, ACS Catal., 2013, 3, 2222-2227.
17 S. Thakur, T. Kshetri, N. H. Kim and J. H. Lee, J. Catal., 2017,
345, 78-86.
42 J. D. Xiao, L. Han, J. Luo, S. H. Yu and H. L. Jiang, Angew. Chem.,
Int. Ed., 2018, 57, 1103-1107.
43 M. Wen, Y. Kuwahara, K. Mori, D. Zhang, H. Li and H.
Yamashita, J. Mater. Chem. A, 2015, 3, 14134-14141.
44 S. Yuan, J. S. Qin, H. Q. Xu, J. Su, D. Rossi, Y. Chen, L. Zhang, C.
Lollar, Q. Wang, H. L. Jiang, D. H. Son, H. Xu, Z. Huang, X. Zou
and H. C. Zhou, ACS Central Sci., 2018, 4, 105-111.
45 Y. Isaka, Y. Kondo, Y. Kawase, Y. Kuwahara, K. Mori and H.
Yamashita, Chem. Commun., 2018, 54, 9270-9273.
46 Y. Horiuchi, T. Toyao, M. Saito, K. Mochizuki, M. Iwata, H.
Higashimura, M. Anpo and M. Matsuoka, J. Phys. Chem. C,
2012, 116, 20848-20853.
18 R. Nakamura, A. Imanishi, K. Murakoshi and Y. Nakato, J. Am.
Chem. Soc., 2003, 125, 7443-7450.
19 K. Meyer, M. Ranocchiari and J. A. van Bokhoven, Energy
Environ. Sci., 2015, 8, 1923-1937.
20 Y. Chen, D. Wang, X. Deng and Z. Li, Catal. Sci. Technol., 2017,
7, 4893-4904.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9TA11120D
Table of contents
(graphic maximum size 8 cm x 4 cm and one sentence of text, maximum 20 words, highlighting the
novelty of the work)
A hydrophobic Ti doped Zr-based MOF was synthesized and used for photocatalytic H2O2 production
in two-phase system (aqueous/benzyl alcohol).