Catalysis of photooxidation reactions through transformation between Cu2+ and Cu+ in TiO2-Cu-MOF composites

Article abstract of DOI:10.1039/c8cc03505a

Novel porous TiO₂@Cu₃(BTC)₂ composites, which were synthesized using ionic liquids (ILs) as solvents, exhibited excellent activity for photooxidation of styrene to 4-aryl tetralones and promoting the Glaser coupling reacti

Full text of DOI:10.1039/c8cc03505a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Catalysis of photooxidation reactions through transformation
2
+
+
between Cu and Cu in TiO -Cu-MOF composites
2
Received 00th January 20xx,
Accepted 00th January 20xx
a, b
a
a, b
a
a, b
a, b
Chunjun Chen, Tianbin Wu,* Dexin Yang, Pei Zhang, Huizhen Liu, Youdi Yang, Guanying
a
a, b
Yang and Buxing Han*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Novel porous TiO
2
@Cu
3
(BTC)
2
composites, which were tetralones was only 15%, mainly due to the fast recombination
synthesized using ionic liquids (ILs) as solvent, exhibited excellent of photogenerated electron-hole pairs and the lack of catalytic
activity for photooxidation of styrene to 4-aryl tetralones and sites. Therefore, the preparation of heterogeneous catalysts
2
promoting Glaser coupling reaction with O under light irradiation. for highly effective cyclization of styrene is very interesting.
2
+
+
It was discovered that the transformation between Cu and Cu
was crucial for enhancing photocatalytic performance.
Ionic liquids (ILs), which have some unique properties,
which provide many opportunities to design and prepare
various advanced materials. Especially, ILs can be used as a
template for the preparation of porous materials. Herein we
Metal-organic frameworks (MOFs) are a class of crystalline
micro-mesoporous hybrid materials with an extended 3D
proposed
TiO @Cu
a
new strategy to synthesize novel porous
composites in ILs by one step method. It was
@Cu (BTC) catalysts, in which
dispersed uniformly in the porous TiO framework,
network, which have shown
1
a
variety of potential
2
3
(BTC)
2
-5
applications. Especially, MOFs are used for the development
of heterogeneous catalysts, because inorganic nodes in MOFs
6, 7
found that heterogeneous TiO
Cu (BTC)
2
3
2
3
2
2
can be intrinsic active sites for catalysis.
However, the
exhibited very high activity and stability for photooxidation of
styrene to 4-aryl tetralones and benzaldehyl under simulated
solar light irradiation. It was discovered that electrons
transferred from TiO to Cu (BTC)2 and transformation
photocatalytic performance of MOFs alone as photocatalysts is
8
still not comparable to that of inorganic semiconductors,
mainly due to very low efficiency of exciton generation and
9
2
3
, 10
2
+
+
charge separation in MOFs.
An effective way to overcome
between Cu and Cu played important roles for the reactions.
To the best of our knowledge, this is the first report on
photooxidation reaction promoted by the transformation
these problems is fabricating inorganic semiconductors-MOFs
composites to boost the photocatalytic efficiency by
combining the merits of inorganic semiconductors and MOFs.
Tetralone and its derivatives with prominent biological
activity have attracted much attention due to their application
2
+
+
between Cu and Cu in MOFs.
1
1-13
in medicinal chemistry.
The traditional methods for
synthesizing 4-aryl tetralone involve Friedel-Crafts cyclization
using complicated starting materials, and usually require acidic
1
4, 15
media, multiple steps and complex operation.
Although a
few articles have reported the construction of 4-aryl tetralones
via light induced cyclization of styrenes with O2 using
homogeneous reaction, it is difficult for separation of products Figure 1. The route for the synthesis of the TiO @Cu (BTC) composites.
2
3
2
1
and catalysts. TiO as common photocatalyst has also been
6
2
used to construction of 4-aryl tetralones via light induced
The proposed method to synthesize porous TiO @Cu (BTC)
2
2
3
1
cyclization of styrenes with O2. However, the yields of 4-aryl
7
composites is shown in Figure 1. Tetrabutyl titanate (TBOT),
,3,5-benzenetricarboxylic acid (H BTC) and Cu(NO ) ·3H O
1
3
3 2
2
were charged into the mixed solvent consisting of IL 1-decyl-3-
methylimidazolium tetrafluoroborate (Figure S1), acetic acid,
ethanol, and water (Figure 1a). The Cu (BTC) and TiO were
3
2
2
generated gradually by the coordination reaction and
hydrolysis reaction, respectively, and the Cu (BTC) crystals
3
2
formed were trapped in the TiO framework together with
2
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2
.
0
some solvent (Figure 1b). TiO
2
@Cu
3
(BTC)
2
composites were TiO
2
The Cu2p3/2 core level spectrum revealed that a main
subsequently obtained after washing and drying to remove the peak at 934.57eV, and a series of shake-up satellites peaks
solvent (Figure 1c). (939.58 and 943.66eV, respectively) were observed,
In order to understand the crystal structure of the catalysts, demonstrating the existence of the Cu valence state in pure
2
+
2
1
+
the obtained TiO
by X-ray diffraction (XRD), as shown in Figure S2. For these of the Cu
samples, a group of strong diffraction peaks before 20° obviously higher (Figure S8b), probably due to the strong
2
@Cu
3
(BTC)
2
composites were characterized Cu
3
(BTC)
2
.
Interestingly, the strength of the Cu valence state
3
(BTC) in the TiO @Cu (BTC) composites was
2
2
3
2
1
8
matched well with the simulated Cu
the formation of Cu (BTC) . In addition, the crystal planes of absorption properties and the band gap of TiO
anatase TiO
were observed, suggesting the formation of the composites were investigated by UV−vis diffuse reflectance
TiO @Cu (BTC) composites. The Cu (BTC) contents were spectroscopy, as shown in Figure S9.
3
(BTC)
2
signals, showing interaction between the TiO
2
and Cu
3
(BTC)
2
. The optical
3
2
2
@Cu (BTC)
3
2
2
2
3
2
3
2
determined by ICP-AES (VISTA-MPX) and the results are shown
in Figure S3.
2 3 2 3 2
Figure 2. Characterization of TiO @Cu (BTC) (19% Cu (BTC) ) composites. a)
SEM image. b) TEM image. c) HR-TEM image. d) Elemental mappings. e) Ti
element. f) Cu element.
Figure 3. The effect on the catalytic performance for the photocatalytic
oxidation of styrene under simulated solar light irradiation with TiO @Cu (BTC)
19%) composites: the reaction time, the pressure of O was 2 MPa (a);the
oxygen pressure, the reaction time was 6h (b); the Cu (BTC) content, the
pressure of O was 2 MPa and the reaction time was 6h (c); the recycling test,
the pressure of O was 2 MPa and the reaction time was 6h (d)
2
3
2
(
2
3
2
The SEM, TEM and high-resolution TEM images of the
2
TiO
2
@Cu
3
(BTC)
2
composites are shown in Figure 2. As shown in
@Cu (BTC) composites had mesopores up to
2
Figure 2b, TiO
2
3
2
about 10 nm, resulted from the removal of the entrained
solvent in Figure 1. The lattice spacing of 0.35 nm was assigned
We investigated the photocatalytic performance of the
TiO @Cu (BTC) composites for the oxidation of styrene under
2
3
2
to the (101) lattice planes of the anatase TiO
However, the lattice spacing of 0.27 nm was assigned to the
110) lattice planes of the CuO, which was probably generated
from the destruction of Cu (BTC) crystal by the electron beam
irradiation of TEM during TEM characterization because it
was also observed for the pure Cu (BTC) , as shown in Figure
S4a and S4b. The Energy dispersive X-ray elemental mappings
of the TiO @Cu (BTC) composites are shown in Figures 2d, 2e
2
(Figure 2c).
simulated solar light irradiation. The experimental results
showed that both benzaldehyl and 4-aryl tetralones were main
oxidation products, as can be known form the NMR spectra of
the products in Figure S10. It is deduced from Figure 3a and
Figure 3b that the optimal reaction time and oxygen pressure
were 6 h and 2 MPa, respectively. As shown in Figure 3c, it is
(
3
2
1
9
3
2
found that Cu
affecting the photocatalytic performance for oxidation of
styrene. When Cu (BTC) content increased to 26%, the yield
3 2
(BTC) content was a very important factor
2
3
2
and 2f. It is clearly shown that Ti and Cu elements were
dispersed uniformly in composite materials. The images of the
3
2
of 4-aryl tetralone could reach up to 31%, which was much
17
higher than that reported (15%). The reason for the
pure TiO
2
and pure Cu
3
(BTC)
2
are shown in Figures S4c, S4d.
The BET surface areas, average pore size and pore volumes for
the obtained samples are presented in Table S1. As shown in
occurrence of the highest yield is discussed in detail based on
the UV-Vis spectra of TiO
related discussion). There was no decrease in yield after four
cycles, which shows that TiO @Cu (BTC) composites had
excellent stability, as shown in Figure 3d. The physical mixture
of TiO and Cu (BTC) and TiO /CuO (XRD pattern, pore
2 3 2
and Cu (BTC) (Figure S9 and the
Figure S5 and Figure S6, the TiO
2
@Cu
3
(BTC)
2
composites had
had only
micropores and mesopores. However, the pure TiO
mesopores.
2
2
3
2
The X-ray photoelectron spectrum(XPS) measurement was
also employed to further study the surface chemical states of
2
3
2
2
properties and TEM images are given in Figure S11) have been
used for oxidation of styrene. Results showed that their
photocatalytic performances were lower than the
the two elements (Ti and Cu) in the TiO
2 3 2
@Cu (BTC)
composites, as shown in Figures S7 and S8. The peaks of Ti
2
2
p3/2 and Ti 2p1/2 for pure TiO were located at 458.58 and
TiO
2
@Cu
3
(BTC)
2
, as shown in Figure S12. In order to get the
4
64.27 eV, respectively, indicating the oxidation state of Ti in
+
4
2
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evidence for the generation of free radicals in the process of from Figure 4Bd that the XPS spectra of Cu were similar to the
-
·
reaction, p-benzoquinone (an O
2
radical scavenger) or fresh TiO
22, 23
2
@Cu
3
(BTC)
2
composites. So we can conclude that the
@Cu (BTC) composites after the
depends on the valence state of the Cu.
+
triethanolamine ( a h radical scavenger)
was added into change of the color of TiO
2
3
2
reaction system, respectively, and the photocatalytic oxidation illumination in the N
2
of styrene to 4-aryl tetralones was completely inhibited, It can be seen from Figures S2 that crystal structure of
•
−
+
indicating that O
formation of 4-aryl tetralones.
An interesting photochromic phenomenon was observed results show that the valence states of Cu in Cu
2
or h radicals were responsible for the TiO
2
@Cu
3
(BTC)
2
composites did not change after reaction, this
@Cu (BTC) composites were stable. These
(BTC) only
, while it can be
and styrene in MeCN were recovered to original valence states after exposed in the air.
irradiated under simulated solar light using N instead of O To understand the change of valence states of Cu in
The color of the reaction suspension changed from the original TiO @Cu (BTC) in N and O , we further explored the roles of
green to reddish brown. Then the color of TiO @Cu (BTC) N2 and O2 in photooxidation of styrene. The difference
composites gradually changed back to green after exposed 6 between O and N lies in the fact that the O can get electron
indicated that TiO
2
3
2
3
2
+
for TiO
2
@Cu
3
(BTC)
2
during the photocatalytic reaction. When changed to Cu after reaction using N
@Cu (BTC)
2
the suspension of TiO
2
3
2
2
2
.
2
3
2
2
2
2
3
2
2
2
2
•
−
+
2+
hours in the air, as shown in Figure 4Aa1-4Ad1. However, the to generate an anion radical O (O2 ) and oxidize Cu to Cu ,
2
+ 2+
color of TiO @Cu (BTC) composites did not change after the while it is difficult for N to get electron and oxidize Cu to Cu .
2
3
2
2
reaction in the O , as shown in Figure 4Aa2, which was in good So we cannot confirm that the Cu in Cu (BTC) can transform
2
2
3
2
agreement with the color of fresh TiO @Cu (BTC) (Figure from Cu to Cu in O . Thus another control experiment was
+
+
2
3
2
2
4
Aa3). We guessed that the different color of TiO @Cu (BTC)
2
carried out by photocatalytic hydrogen generation using
3
2
+
composites was due to the change of the valence states of the isopropyl alcohol as sacrificial agent in N . Although H can get
2
+ 2+
electrons to produce H , it cannot oxidize the Cu to Cu . The
Cu in Cu (BTC) .
2
3
2
control experiment showed that the yield of H2 can be
promoted by the combination of TiO and Cu (BTC) . Similar to
2
3
2
the photooxidation of styrene, the highest yield could be
reached when Cu (BTC) content increased to 26%, as shown
3
2
in Figure S13. So the photoelectron transfer path of
photocatalytic hydrogen generation was the same as
photooxidation of styrene. It was found that the
TiO @Cu (BTC) became reddish brown from green after
2
3
2
2
+
photocatalytic hydrogen generation, which indicated that Cu
+
in the Cu (BTC)2 was first reduced to Cu , and the H get
+
3
+
electrons to produce H on the Cu sites. So we can concluded
2
2
that the Cu in Cu (BTC)2 was first reduced to Cu in
+
+
3
+
Figure 4. A) 1 The change of the color of TiO
2
@Cu
3
(BTC)
2
composites with photooxidation of styrene, and then the Cu can be oxidated
different times exposed in the air after photooxidation of styrene in the N
h, b1 2h, c1 4h, d1 6h; 2 The color of TiO @Cu (BTC) compositeswith different
times exposed in the air after the reaction in the O : a2 0h, b2 2h, c2 4h, d2 6h; 3
The color of fresh TiO @Cu (BTC) composites. B) The XPS spectra of Cu with
different times exposed in the air after photooxidation of styrene in N : a 0h, b
h, c 4h, d 6 h.
2
: a1
2+
to Cu by O2.
0
2
3
2
2
2
3
2
2
2
In order to verify our hypothesis, the valence states of the
Cu with different times exposed in the air after the
illumination of TiO @Cu (BTC) in styrene under N
2
2
3
2
atomshpare were characterized using the semi-in-situ X-ray
photoelectron spectroscopy (the specific operation is
described in the Supporting Information). The peak at 932.07
+
eV can be observed, indexed to the Cu 2p3/2 in its Cu valence
state, indicating that the Cu in TiO
2
@Cu
the valence state of Cu after the illumination in the N
shown in Figure 4Ba. When the TiO @Cu (BTC) composites
were exposed in the air, the peak at 934.37 eV and a series of
3
(BTC)
2
composites had
+
2
, as
2
3
2
shake-up satellites peaks with higher binding energy values Scheme 1. The possible mechanism for the photooxidation of styrene over the
2
+
2 3 2
TiO @Cu (BTC) composites
corresponding to Cu valence state gradually appeared and
became higher, as shown in Figures 4Bb and 4Bc. After
On the basis of the above experiments and knowledge in
exposed 6 hours in the air, the color of TiO
2 3 2
@Cu (BTC)
literature, we proposed a possible mechanism for the reaction
promoted by the composites, as shown in Scheme 1. Under
composites completely changed back into green due to
+ 2+
gradual oxidation of Cu to Cu in the air, and it can be found
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simulated solar light irradiation, the TiO
2
produced the There are no conflicts to declare.
-
+
electron (e ) and positive hole (h ) pairs after absorbing
photon. Then the electron transfer to Cu (BTC) , and the hole
transfer to the surface of TiO
3
2
. The hole can get an electron Acknowledgements
2
from 1a to return to the ground state and generate a cation
radical A. Then the formed intermediate A was added to
another 1a to yield intermediate B (Path 2), which further
transforms into intermediate C through an intermolecular
electrophilic addition and aromatization processes.
The work was supported by National Key Research and
Development Program of China (2017YFA0403103), National
Natural Science Foundation of China (21373230), and Chinese
Academy of Sciences (QYZDY-SSW-SLH013).
2
+
+
Meanwhile, the Cu in Cu
3
(BTC)
+
2
was reduced to Cu by the
2+
2
aggregated electrons. The Cu was oxidized to Cu by O , and
Notes and references
•−
•
the O
generated from O
intermediate D. Finally, the product 2a was generated from
the reductive elimination of H O of D. On the other hand,
reacted with 1a (Path 1), dioxetane
2
was reduced to O
2
. Then, the [O
+
2
H] , which was
•−
1
2
3
4
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2
with H , reacted with C to produce
2
•−
when the
O
2
intermediate E was formed, leading to the generation of
benzaldehyl as another product.
Encouraged by the above results, we employed
TiO
2
@Cu
3
(BTC)
2
to catalyze Glaser coupling reaction, which
5
6
7
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was a typical reaction for the formation of C-C bond. Cu-based
2
catalysts have been widely used for Glaser coupling reaction.
4
2
It is reported that the transformation of Cu and Cu can
+
+
2
promote the Glaser coupling reaction. However, the
5
2
+
traditional transform between Cu and Cu suffered from high
+
2
6
temperature. On the basis of the above experiments, we
verified that the Cu in TiO @Cu (BTC) can transform between
Cu and Cu under simulated solar light irradiation with O . So
we believe that the TiO @Cu (BTC) could also promote the
8
9
2
3
2
2
+
+
2
J. G. Santaclara, F. Kapteijn, J. Gascon, M. A. van der Veen,
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2
3
2
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the yield of 1, 4-Diphenylbut-1, 3-diyne could reach up to 90%
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In conclusion, we proposed a new strategy to synthesize novel
multiporous TiO @Cu (BTC) composites in IL by one step. The
Cu (BTC) dispersed uniformly in the TiO
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photooxidation of styrene to 4-aryl tetralones and Glaser
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Conflicts of interest
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