Amine-functionalized zirconium metal-organic framework as efficient visible-light photocatalyst for aerobic organic transformations

Article abstract of DOI:10.1039/c2cc34620f

An amine-functionalized zirconium metal-organic framework (MOF) was used as a visible-light photocatalyst for selective aerobic oxygenation of various organic compounds including alcohols, olefins and cyclic alkanes, at high efficiency and high selectivity. This study shows the great potential for design and application of MOF-based photocatalysts.

Full text of DOI:10.1039/c2cc34620f

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 11656–11658
www.rsc.org/chemcomm
COMMUNICATION
Amine-functionalized zirconium metal–organic framework as efficient
visible-light photocatalyst for aerobic organic transformationsw
Jinlin Long,*ab Sibo Wang,a Zhengxin Ding,a Shuchao Wang,b Yangen Zhou,a
Ling Huang*bc and Xuxu Wanga
Received 28th June 2012, Accepted 8th October 2012
DOI: 10.1039/c2cc34620f
An amine-functionalized zirconium metal–organic framework (MOF)
was used as a visible-light photocatalyst for selective aerobic
oxygenation of various organic compounds including alcohols,
olefins and cyclic alkanes, at high efficiency and high selectivity.
This study shows the great potential for design and application
of MOF-based photocatalysts.
of a wide range of organic compounds including alcohols,
olefins and cyclic alkanes at high efficiency and high selectivity.
As depicted in Fig. 1A, UiO-66-NH2 is comprised of hexa-
meric Zr6O32 units and 2-aminoterephthalate (ATA) linkers,4
and has a large BET surface area of 832 m2 gꢀ1 (Fig. S1Aw).
The –NH2 groups protrude into the micropores, and do not
change the topology, which can be confirmed by the uniform
binding energies of Zr, C and O atoms in both UiO-66 and
UiO-66-NH2 (Fig. S2w). The 399.5 eV binding energy of the N
atom in the amine group is slightly higher than that of the
ligand ATA (399.1 eV) because of the static polarization in
the confined MOF cavities. The XRD pattern in Fig. 1B
gives identical diffraction peaks as that of pristine UiO-66
(Fig. S3w), indicating that the crystal phase structure is
retained after amine functionalization. The UV-vis diffuse
reflectance spectrum (UV-vis DRS) in Fig. 1C indicates an
absorption band-edge at 450 nm, while UiO-66 is transparent
Although intense interest has been focused on the photocatalytic
production of hydrogen fuel from water and/or biomass over the
past decades, photocatalysis-driven transformation of organic
compounds into fine chemicals has been attracting increasing
attention because it represents a ‘‘clean’’ future that chemical
industry desires, and possesses great economic and ecological
potential towards the orientation of solar energy-based ‘‘green’’
chemistry. Indeed, modified metal oxides such as TiO2, WO3,
CeO2, polyoxometalates and Cr-supported silica have been used
as photocatalysts for organic transformations.1
On the other hand, the application of MOFs in photocatalysis is
emerging as an interesting topic since their cavity dimensions sit
exactly between molecular bioenzymes and porous solid catalysts.
A MOF was first used as a photocatalyst to degrade organic
pollutants by Mahata et al. in 2006,2 and other types of MOFs
including MOF-5, isoreticular MOFs (IRMOF-1, IRMOF-2,
IRMOF-7, IRMOF-8, IRMOF-9), MIL-53 (M = Al, Cr, Fe),
UiO-66 and UiO-673 were subsequently reported. However, all of
the above photocatalysts need either the assistance of a sacrificial
agent, a co-photocatalyst such as Ir, Re or Ru complexes, or
UV-light as the energy source. Thus the application of MOFs in
photocatalysis is still underdeveloped compared to inorganic
oxide photocatalysts. Here, we report an amine-functionalized
zirconium MOF (UiO-66-NH2)4 that under Z 420 nm light
(hereafter, light) irradiation, catalyzes the aerobic oxygenation
a Research Institute of Photocatalysis, Fujian Provincial Key
Laboratory of Photocatalysis–State Key Laboratory Breeding Base,
Fuzhou University, Fuzhou 350002, China. E-mail: jllong@fzu.edu.cn
b School of Chemical and Biomedical Engineering, Nanyang
Technological University, 70 Nanyang Drive, Singapore 637457.
E-mail: lhuang@ntu.edu.sg; Tel: +65 6316 8825
c Institute of Advanced Materials, Nanjing University of Technology,
Nanjing 210009, China. E-mail: iamlhuang@njut.edu.cn
w Electronic supplementary information (ESI) available: XRD, TEM,
SEM, XPS, UV-vis DRS, EPR data, photocatalytic activity as a
function of irradiation time, and GC-MS results of 18O-labeling
experiments. See DOI: 10.1039/c2cc34620f
Fig. 1 Structural scheme (A), XRD pattern (B), UV-vis DRS spectrum
(C), PL spectrum (D), Mott–Schottky plot (E) and visible-light
photocurrent response (F) of the UiO-66-NH2 catalyst.
c
11656 Chem. Commun., 2012, 48, 11656–11658
This journal is The Royal Society of Chemistry 2012

View Online
between 350–800 nm (Fig. S4w). The visible-light absorption is
due to the conjugated p electron transition from the amine-
containing chromophores to the Zr centers. Based on the
relation Eg = 1240/l, the calculated optical bandgap of
UiO-66-NH2 is 2.75 eV (inset in Fig. 1C). Under 400 nm laser
irradiation at room temperature, UiO-66-NH2 shows a broad
photoluminescence (PL) peak centered at 477 nm (Fig. 1D),
suggesting that electron–hole pairs can be efficiently generated
and separated in UiO-66-NH2 upon photon absorption, which
satisfies the prerequisite for visible-light photocatalysis.
Table 1 Photocatalytic activity of UiO-66-NH2 for organic
transformations
Selectivity (%)
Conversion (%) Aldehyde/
Substratea
Solvent (TOF/hꢀ1
)
ketone
Epoxide
b-Methylstyrene
CH3CN 14.0 (1.234)
TFT
79.1
76.9
80.1
81.0
57.0
83.4
81.0
83.9
91.6
70.1
35.0
73.1
0.0
20.9
23.1
19.9
19.0
43.0
16.5
19.0
16.2
8.4
29.9
65.0
26.9
100
100
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
10.3 (0.908)
Acetone 7.8 (0.687)
CHCl3 9.1 (0.802)
DMF
CH3CN 7.7 (0.766)
TFT 3.8 (0.378)
13.4 (1.181)
Styrene
Electrochemical analysis of UiO-66-NH2 shows a typical
Mott–Schottky plot in the dark in Fig. 1E, and the positive
slope suggests an n-type semiconductor. The flat-band
potential is ꢀ0.60 V vs. NHE, which is more negative than
Acetone 6.7 (0.666)
CHCl3 5.3 (0.527)
2.2 (0.219)
0.5 (0.031)
CH3CN 0.7 (0.043)
CH3CN 0.8 (0.070)
DMF
1,2-Diphenylethylene TFT
the redox potential of O2 ꢁ/O2 (ꢀ0.28 eV),5 and benefits
ꢀ
Cyclooctene
greatly the photogenerated electron transfer from catalyst to
adsorbed oxygen molecules. Impedance spectroscopy of UiO-
TFT
CH3CN 11.1 (1.227)
TFT 13.1 (1.448)
0.5 (0.044)
0.0
Benzyl alcoholb
100
100
100
100
100
100
100
100
100
100
100
66-NH2 shows a relatively large resistance of 1.34 ꢂ 106
O
(Fig. S5w), and it drops to 9.73 ꢂ 105 O upon light irradiation
due to the formation of electron–hole pairs. Furthermore,
light irradiation of a UiO-66-NH2 film coated on indium tin
oxide glass shows a strong short circuit photocurrent response
(Fig. 1F), indicating that these photogenerated carriers can
be efficiently separated upon contacting with a high work-
function conductor, which is consistent with the PL result
reported above.
Acetone 7.5 (0.829)
CHCl3 8.0 (0.884)
DMF
CH3CN 2.8 (0.308)
TFT 2.7 (0.297)
5.4 (0.597)
Cyclohexanolc
CHCl3 1.6 (0.176)
Acetone 2.1 (0.231)
CH3CN 0.9 (0.082)
CH3CN 0.1 (0.009)
Hexyl alcohold
Cyclohexanee
0.0
0.0
a
Standard reaction conditions: 1200 rpm stirring speed, 12 h reaction time,
light intensity ca. 270 mW cmꢀ2, light irradiation area of ca. 1.5 cm2. Main
products are either benzaldehyde or epoxide. Products are b benzaldehyde,
c cyclohexanone, d hexanal and e cyclohexanone.
The photocatalytic results of UiO-66-NH2 for the aerobic
oxygenation of various organic compounds are listed in
Table 1. None of the reactions occur in the dark, but progress
as a function of the light irradiation time as shown in Fig. S6A,
C, E and Gw. Moreover, control experiments show that UiO-66
and ZrO2 are catalytically inactive for all the reactions under
the same conditions, further confirming the unique action of the
amine groups. The photocatalytic conversion of all the selected
alkenes increases monotonously with the irradiation time,
which proves that the organic transformation is a photocata-
lysis-driven process. b-Methylstyrene and 1,2-diphenylethylene
show a constant selectivity (Fig. S6B and Fw), suggesting two
reaction pathways: i.e. a competition between the photo-
catalytic oxidation and epoxidation, which are isolated and
parallel. The photo-oxidation proceeds via a two-step manner,
epoxide due to its stable geometrical conformation. These
results suggest that the selectivity to epoxide is closely related
to the chemical stability of the intermediates formed during
the photocatalytic process. Comparison of the reaction pro-
ducts in different solvents shows that polar organic media play
a crucial role in stabilizing the intermediate epoxides and
ꢀ
ꢁ
1
active oxygen species such as O2 and O2. Thus acetonitrile
gives higher conversion rate than reactions in benzotrifluoride
(TFT), N,N-dimethylformamide (DMF), CHCl3 and acetone,
while TFT and DMF are more favorable for improving the
selectivity to aromatic alkene epoxides.
hv;O2
hv;O2
i.e., alkenes
epoxides
oxides, where epoxides are
ƒƒ!
ƒƒ!
Benzyl alcohol, cyclohexanol, hexyl alcohol and cyclo-
hexane were chosen to prove the wide applicability of UiO-
66-NH2 (Table 1). 100% selectivity is achieved for the four
reactions, but the conversion rate, which depends mainly on
the activation energy of a-C–H bonds, decreases in the order
of benzyl alcohol 4 cyclohexanol 4 hexyl alcohol 4 cyclo-
hexane. The highest light quantum efficiency of 0.41% is
attained for TFT-mediated photocatalytic oxidation of benzyl
alcohol. An 18O isotope labeling experiment using the photo-
catalytic epoxidation of cyclooctene as a model reaction
confirms that oxygen in the product comes from exterior
molecular oxygen (Fig. S7w), suggesting that the photoactiva-
tion of reacting substrates and molecular oxygen is one of
the crucial steps determining the photocatalytic efficiency of
UiO-66-NH2. The conversion may be mainly contributed from
the kinetic diameter and C–H activation energy of the reacting
substrates.
the only intermediates formed, and can be further converted
to benzaldehyde by deep oxidation. The photo-oxidation
product of styrene proves the occurrence of the second
reaction pathway for aromatic alkenes. Since styrene epoxide
can be converted to benzaldehyde, an increase in selectivity
to benzaldehyde and a corresponding decrease in selectivity
to styrene epoxide with prolonged reaction time is observed
(Fig. S6C and Dw).
Results in Table 1 also indicate that the final selectivity of
products is dependent on the reaction medium and reacting
substrates. For example, epoxides obtained from b-methyl-
styrene, styrene and 1,2-diphenylethylene were at a broad
selectivity ranging from 15 to 65%. For cyclooctene, conver-
sion yield is low as a result of its larger kinetic diameter
compared to the pore size (5.1 A)6 of UiO-66-NH2, styrene
and b-methylstyrene, but gives 100% selectivity to cyclooctene
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11656–11658 11657

View Online
transformations (Scheme 1). Upon light irradiation, electrons on
the HOMO composed of O, C and N 2p orbitals jump to the
LUMO, and transfer to O2 molecules adsorbed on the Zr3+
ꢀ
sites to form O2 ꢁ, while the photogenerated holes oxidize the
organic reactive substrates adsorbed on the amine sites to
carbonium ions. The formed superoxide radicals further react
with carbocations, which leads to the final products. The
nanometer-sized cavities in UiO-66-NH2 are excellent micro-
photoreactors where the formed superoxide radical and reactive
intermediates may also be stabilized by polar organic solvents
due to the confinement effect.
In summary, UiO-66-NH2 was used for the first time as an
efficient visible-light photocatalyst for aerobic oxidation of
organic compounds. Considering its unique role of bridging the
gap between organometallic molecular and semiconducting
photocatalysts, this study proves great potential in developing
novel MOF-based photocatalysts.
Scheme 1 Photocatalytic mechanism of UiO-66-NH2.
This work was supported by the NSFC (21003021 and
21173044), the Natural Science Foundation of Fujian Province
(2010J05024), and National Basic Research Program of China
(973 Program, 2012CB722607 and 2010CB234604). L. H. thanks
the financial support from NTU Start-Up Grant and Tier 1 Grant
(RG20/09) from Ministry of Education, Singapore.
Fig. 2 (A) EPR spectra of UiO-66-NH2 dispersed in (a) methanol,
(b) trifluorotoluene, (c) acetonitrile and (d) benzyl alcohol without
light irradiation. (B) The intensity of the EPR line at g = 2.009 as a
function of irradiation time and the decay upon light off.
Notes and references
1 (a) H. Yamashita, M. Ariyuki, K. Yoshizawa, K. Kida, S. Ohshiro
and M. Anpo, Res. Chem. Intermed., 2004, 30(2), 235–245;
(b) M. Zhang, C. Chen, W. Ma and J. Zhao, Angew. Chem., 2008,
120, 9876–9879; (c) S. Higashimoto, N. Kitao, N. Yoshida,
T. Sakura, M. Azuma, H. Ohue and Y. Sakata, J. Catal., 2009,
266, 279–285; (d) S. Higashimoto, N. Suetsugu, M. Azuma, H. Ohue
and Y. Sakata, J. Catal., 2010, 274, 76–83; (e) N. Zhang, Z. Xian
and Y. Xu, J. Mater. Chem., 2011, 21, 8152–8158; (f) N. Wu,
J. Wang, D. N. Tafen, H. Wang, J.-G. Zheng, J. P. Lewis, X. Liu,
S. S. Leonard and A. Manivannan, J. Am. Chem. Soc., 2010, 132,
6679–6685.
To elucidate the mechanism of the photocatalytic process,
EPR was used to determine the active intermediates formed
during light irradiation. Fig. 2A shows five EPR lines (2.033,
2.022, 2.009, 2.003, 1.984) of UiO-66-NH2 dispersed in differ-
ent solvents, and obtained without light irradiation which
are in good agreement with the solid-state EPR spectrum of
UiO-66-NH2 (Fig. S8Aw). It is obvious that these five EPR
lines do not belong to the parent ligand ATA, which does not
give any paramagnetic signals (Fig. S8Bw). According to
literature,7,8 the line at 1.984 is due to the defects in the
2 (a) P. Mahata, G. Madras and S. Natarajan, J. Phys. Chem. C,
2006, 110, 13759–13768; (b) M. Alvaro, E. Carbonell, B. Ferrer,
F. X. Liabre
5106–5112; (c) F. X. Liabre
J. Phys. Chem. C, 2007, 111, 80–85; (d) C. G. Silva, A. Corma and
H. Garcıa, J. Mater. Chem., 2010, 20, 3141–3156; (e) C. G. Silva,
L. Luz, F. X. Liabresi Xamena, A. Corma and H. Garcıa, Chem.–Eur. J.,
2010, 16, 11133–11138.
´
si Xamena and H. Garcı
´
a, Chem.–Eur. J., 2007, 13,
si Xamena, A. Corma and H. Garcia,
´
Zr6O32 core and the three EPR lines at 2.033, 2.009 and
–
´
2.003 are from O2 species adsorbed on Zr3+ sites. This
´
´
ꢁ
assignment can be fully confirmed by the EPR results of the
pristine UiO-66 with the same topologic structure upon UV
irradiation for 220 s (Fig. S9w). The signal at 2.022 is closely
related to the space-confined amine groups because it only
occurs in UiO-66-NH2, and is indiscernible in the pristine
UiO-66 either before or after UV irradiation. Interestingly, the
signals at 2.022, 2.009 and 2.003 increase in intensity with
prolonged irradiation time, and reach a plateau after 2.0 min
irradiation (Fig. 2B). Upon light off, a gradual decrease in
intensity is observed and slowly returns to the starting point,
indicating that the increase in intensity of these EPR lines is
3 (a) M. C. Das, H. Xu, Z. Wang, G. Srinivas, W. Zhou, Y.-F. Yue,
V. N. Nesterov, G. Qian and B. Chen, Chem. Commun., 2011, 47,
11715–11717; (b) L. Wen, J. Zhao, K. Lv, Y. Wu, K. Deng, X. Leng
and D. Li, Cryst. Growth Des., 2012, 12, 1603–1612; (c) C. Wang,
Z. Xie, K. E. deKrafft and W. Lin, J. Am. Chem. Soc., 2011, 133,
13445–13454; (d) Y. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu
and Z. Li, Angew. Chem., Int. Ed., 2012, 51, 3364–3367;
(e) J. R. Choi, T. Tachikawa, M. Fujitsuka and T. Majima,
Langmuir, 2010, 26, 10437–10443.
4 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.
5 A. Fujishima, T. N. Rao and D. A. Tryk, J. Photochem. Photobiol.,
C, 2000, 1, 1–21.
–
ꢁ
caused by the O2 formed from the photogenerated electrons.
–
6 Y. Huang, W. Qin, Z. Li and Y. Li, Dalton Trans., 2012, 41,
9283–9285.
The O2 ꢁ could be stabilized in the cavities of UiO-66-NH2 due
to its interaction with the amine groups and/or organic solvents,
which consequently benefits the photocatalytic oxygenations of
C–H and CQC bonds.
7 (a) C. Morterra, E. Giamello, L. Orio and M. Volante,
J. Phys. Chem., 1990, 94, 3111–3116; (b) M. Occhiuzzi,
D. Cordischi and R. Dragone, J. Phys. Chem. B, 2002, 106,
12464–12469.
8 H. Liu, L. Feng, X. Zhang and Q. Xue, J. Phys. Chem., 1995, 99,
332–334.
Thus we believe that UiO-66-NH2 follows a mechanism
initiated by photogenerated electron transfer during the organic
c
11658 Chem. Commun., 2012, 48, 11656–11658
This journal is The Royal Society of Chemistry 2012