Tailoring the Optical Absorption of Water-Stable ZrIV- and HfIV-Based Metal-Organic Framework Photocatalysts

Article abstract of DOI:10.1002/asia.201500641

New Zr^IV- and Hf^IV-based metal-organic framework photocatalysts, termed VNU-1 and VNU-2 (where VNU=Vietnam National University), were synthesized and their resulting structures fully characterized. By employing a highly π-conjugated

Full text of DOI:10.1002/asia.201500641

DOI: 10.1002/asia.201500641
Full Paper
Photocatalysis
IV
IV
Tailoring the Optical Absorption of Water-Stable Zr - and Hf -
Based Metal–Organic Framework Photocatalysts
[
a, b]
[b]
[a, b]
[a]
Tan L. H. Doan,
Ha L. Nguyen, Hung Q. Pham,
Nguyen-Nguyen Pham-Tran,
[a]
[b]
Thach N. Le,* and Kyle E. Cordova*
IV
IV
Abstract: New Zr - and Hf -based metal–organic framework
photocatalysts, termed VNU-1 and VNU-2 (where VNU=Viet-
nam National University), were synthesized and their result-
ing structures fully characterized. By employing a highly p-
conjugated linker, namely 1,4-bis(2-[4-carboxyphenyl]ethy-
nyl)benzene, the optical absorption properties were effec-
tively red-shifted into the visible light region. This strategy,
coupled with the high water stability of the materials, led to
enhanced MOF-driven photocatalytic degradation, under ul-
traviolet-visible light, of organic dye pollutants commonly
found in wastewater.
Introduction
ous solutions leading to inefficiency in recycling them for fur-
[
5]
ther use. Therefore, the development and exploration of new
heterogeneous photocatalytic systems that can extend their
function into the visible light region and can be effectively re-
cycled are highly desired.
The rapid advancement of the textile and dye industry, specifi-
cally in Vietnam, has led to serious concerns over the environ-
mental effects of the wastewater generated. This wastewater
most often contains toxic, environmentally unfriendly, and
chemically stable organic dyes, which are difficult to degrade
Metal–organic frameworks (MOFs) are a class of porous, crys-
talline materials constructed from inorganic clusters (termed
secondary building units or SBUs) that are connected via or-
[1]
by standard biological treatments. Furthermore, organic dye
waste poses significant challenges to the treatment of other
pollutants as these molecules absorb and reflect sunlight, thus
damaging the algal-bacterial growth needed for such treat-
[
6]
ganic linkers. These materials have attracted much interest
for a wide range of applications, including gas storage and
[
6,7]
[8]
separation,
carbon dioxide capture, heterogeneous cataly-
[
2]
[9]
[10]
ment. An increasingly attractive method for degrading organ-
ic dye pollution in wastewater is the use of heterogeneous
photocatalysts, which utilize sunlight to effectively eliminate
the hazardous nature of organic pollutants and transform
sis, and chemical sensing, among others. MOFs have also
received attention for their photocatalytic characteristics as
they typically exhibit semiconductor properties upon exposure
[
11]
to light. It has been reported that the band gap of MOFs is
closely associated to the HOMO–LUMO gap, which allows the
light-harvesting properties of MOFs to be tailored through var-
[
3]
them into biodegradable or even less toxic molecules. Com-
monly used heterogeneous photocatalysts include metal
oxides, such as TiO and ZnO; however, these materials have
iation of the electronic nature of the inorganic SBUs and or-
2
[
4]
[12,13]
drawbacks regarding their use for practical applications. First,
the band gaps of these materials are rather large (~3.2 eV),
which means that they primarily absorb light with wavelengths
ganic linkers comprising their structures.
Although there
have been a wide number of reported MOFs used for photoca-
[
11]
talytic applications, there remains significant impediments to
using these materials as photocatalysts in wastewater treat-
ment due to the instability of these structures in aqueous envi-
ronments.
[
4]
<
380 nm. These properties render them effective only in the
UV region for photocatalytic or photodegradation applications.
Second, these photocatalysts are generally suspended in aque-
In 2009, a water-stable and photoactive Ti-based MOF,
termed MIL-125(Ti), was reported, in which the material’s ability
[
a] T. L. H. Doan, H. Q. Pham, N.-N. Pham-Tran, Prof. T. N. Le
Faculty of Chemistry
University of Science, Vietnam National University
Ho Chi Minh City, 721337 (Vietnam)
E-mail: lenthach@yahoo.com
[14]
to photoinduce the oxidation of alcohols was demonstrated.
Furthermore, with the report of a series of highly chemical-
IV
and water-stable MOFs based on Zr clusters, termed UiO
[
b] T. L. H. Doan, H. L. Nguyen, H. Q. Pham, K. E. Cordova
Center for Molecular and NanoArchitecture (MANAR)
Vietnam National University
MOFs, attention has turned to exploring these materials in en-
vironments that were previously unsuitable for other MOF
[15]
photocatalysts.
Although these MOFs are stable in water,
Ho Chi Minh City, 721337 (Vietnam)
their photocatalytic properties are mainly applicable to the UV
region as the linkers absorb light primarily at wavelengths
E-mail: kcordova@manar.edu.vn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500641.
[
16]
<300 nm.
Integration of auxochromic and bathochromic
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functionalities, such as ÀNH or ÀNO groups, into the linkers
2
2
[
14,17]
[13c,16]
of MIL-125(Ti)
and UiO-66 and -67
has proven success-
ful for shifting optical absorption from the UV to the visible-
light region and, thus, enhancing practical photocatalytic prop-
erties. Alternative methods for increasing optical absorption in
water-stable MOFs, proposed mainly through computational
work, include increasing the p-conjugation of the linker back-
bone through introduction of additional aromatic rings or
[
12,18,19]
alkyne units.
Indeed, this was recently realized by an iso-
reticular (having the same topology) UiO-66 structure con-
[
20]
structed from a chromophoric anthracene-derived linker.
In this study, we demonstrate a strategy for further tailoring
IV
the optical absorption properties of a water-stable, Zr -based
MOF through increasing the p-conjugation of the linker. Spe-
cifically, we report the synthesis and structural characterization
of a new Zr-MOF, termed VNU-1 (VNU=Vietnam National Uni-
versity), that was constructed from the ditopic linker 1,4-bis(2-
Scheme 1. Synthesis of the highly p-conjugated ditopic linker, H
2
CPEB, used
[
4-carboxyphenyl]ethynyl)benzene (H CPEB). Furthermore, the
2
to construct VNU-1 and VNU-2, was accomplished by a Sonagashira-cou-
pling strategy (see section S1 in the Supporting Information).
[21]
successful preparation of an isoreticular structure based on Hf-
clusters, termed VNU-2, is presented. Due to the increase in
conjugation of the linker, the absorption edge of VNU-1 was
effectively red-shifted into the visible-light region. This finding
led to the discovery of enhanced photocatalytic degradation
under UV/Vis light of the environmentally malignant methyl-
ene blue (MB) and methyl orange (MO) dyes. It is noted that
the photocatalytic activity of VNU-1 was significantly higher in
Table 1. Summary of crystallographic data for VNU-1-SC and VNU-2-SC.
VNU-1-SC VNU-2-SC
Empirical formula
Formula weight [gmol
T [K]
C
72
H
36Zr
3
O
16
72 3 16
C H36Hf O
À1
]
1430.67
298
1692.48
100
comparison to the widely used Degussa P-25 TiO photocata-
Crystal system
Space group
a [Š]
Cubic
Fd-3m
39.8961(7)
90
63503(3)
16
0.599
Cubic
Fd-3m
39.7901(8)
90
62998(4)
16
0.714
2
lyst.
a [8]
Volume [Š ]
3
Results and Discussion
Z
À3
Calculated density, 1calc [gcm
Absorption coefficient, m [mm
]
Synthesis and structural characterization
À1
]
1.821
3.793
R
R
wR2
int
0.0501
0.0362
0.1072
0.1295
0.0283
0.0726
To obtain a MOF with optical absorption properties extending
1
[I>2s(I)]
to the visible-light region, we prepared a highly p-conjugated
linker, termed H CPEB, based on a Sonogashira-coupling strat-
2
egy that was adopted accordingly from a previous report
[
21]
(
Scheme 1). With this in hand, VNU-1 and VNU-2 were solvo-
ideally capped alternatively by m -O and m -OH groups to form
3 3
thermally synthesized through a reaction containing H CPEB
the triangular faces (Figure 1). These SBUs are connected to-
gether by twelve carboxylate groups from CPEB linker units to
produce both tetrahedral and octahedral cages (Figure 1a, b).
With respect to the two-fold interpenetration, the SBU of one
fcu net occupies the center of the tetrahedral cages that
belong to the second net (Figure 1c). This results in one large
tetrahedral cage (~25 Š) and one smaller octahedral cage (9 Š)
(Figure 1b). The total solvent-accessible volumes of VNU-1 and
2
and ZrOCl ·8H O or HfCl , respectively, in N,N-dimethylforma-
2
2
4
mide (DMF) at 1208C with acetic acid added as a modulator.
The crystallinity of VNU-1 and À2 was regulated by varying the
amount of acetic acid modulator added as well as the length
[
22]
of the reaction time. Depending on the degree of modulator
added, single crystals of VNU-1-SC and VNU-2-SC (where SC=
single crystal) as well as a microcrystalline powder (VNU-1-P
and VNU-2-P, respectively; where P=powder) were obtained.
Single-crystal X-ray diffraction analysis revealed that VNU-1-
SC and VNU-2-SC crystallized in the space group, Fd-3m, with
a=39.8961 and 39.7901 Š, respectively (Table 1, Tables S1 and
S2 in the Supporting Information). These materials are isostruc-
tural to the PIZOF series, with the exception that the CPEB
linker is not functionalized and VNU-2-SC is constructed from
[
24]
VNU-2, as determined by PLATON, are 68% for both of these
structures.
As a result of the low yield obtained for reactions that pro-
duced single crystals, the amount of acetic acid modulator was
reduced to obtain bulk microcrystalline powder phases for
both VNU-1 and VNU-2 in higher yields. The purity of these
bulk phases were assessed by powder X-ray diffraction (PXRD)
measurements, in which the experimental as-synthesized PXRD
patterns for both VNU-1-P and VNU-2-P were in satisfactory
agreement to those patterns calculated for their respective
single-crystal structures (Figure 2a, Figure S5). Furthermore,
IV
IV [23]
Hf as opposed to Zr .
doubly interpenetrated
M O (OH) (CO ) ] (M=Zr or Hf for VNU-1 or VNU-2, respec-
Specifically, both materials adopt
a
fcu-c net based on
IV
IV
[
6
4
4
2 12
tively) SBUs, in which the inner M O (OH) (CO ) SBU cores are
6
4
4
2 12
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Architectural robustness and
porosity
Prior to further characterization,
as-synthesized samples of VNU-
1
-P and VNU-2-P were thorough-
ly washed with DMF to remove
unreacted species and then sol-
vent-exchanged with chloroform
over the course of three days.
Activation of chloroform-washed
samples of VNU-1-P and VNU-2-
P was performed under reduced
pressure at 1208C for 24 h. To
assess the activation procedure
as well as the architectural ro-
bustness of these two structures,
thermal gravimetric analysis
IV
Figure 1. Crystal structure and topology of VNU-1 and -2: (a) Cuboctahedron [M
6
O
4
(OH)
4 2
(CO )12] (where M=Zr or
IV
Hf for VNU-1 or -2, respectively) secondary building unit (SBU) and H
2
CPEB linear linker; (b) Crystal structure of
VNU-1, as a representative example, showing the large 25 Š tetrahedral cages and smaller cages that result from
the interpenetration of a second net; (c) Interpenetrated fcu-c net of VNU-1 and VNU-2 in which the cuboctahe-
dron SBU of one net is located at the center of a tetrahedral cage belonging to the second net. It is noted that
the SBU is replaced with cuboctahedron shapes, which are connected by linear-shaped linkers. Atom colors: Zr,
blue and orange polyhedra; O, red; C, black; H atoms are omitted for clarity. The yellow and salmon colored balls
represent the free space in the framework.
(
TGA) was performed under air-
flow. VNU-1-P and VNU-2-P ex-
hibit a small decrease in weight
percent up to the decomposi-
tion temperatures of 430 and
4
608C, respectively (Figures S8
and S9). After decomposition,
the calculated weight percent of
the residue was found to be
2
7.4 and 38.5% for VNU-1-P and
VNU-2-P, respectively, which is in
line with the theoretical values
calculated from the crystal struc-
tures (25.3 and 36.9% for VNU-1-
P and VNU-2-P, respectively). Fur-
thermore, the calculated weight
percent of the linker, as deter-
mined from the TGA curve, was
found to be 72.6 and 63.2% for
VNU-1-P and VNU-2-P, respec-
tively. This weight percent is also
consistent with those calculated
from elemental microanalysis
(
73.5 and 63.8% for VNU-1-P and
VNU-2-P, respectively). The po-
rosity of both samples was es-
tablished by N2 isotherms at
Figure 2. (a) PXRD analysis of VNU-1. The calculated pattern from the single-crystal data (black) is compared to
the experimental patterns from the as-synthesized powder sample (blue) and activated powder sample (red).
(
2
b) N isotherm at 77 K for activated VNU-1-P. Closed and open circles represent the adsorption and desorption
branches, respectively. (c) PXRD analysis of VNU-1-P after immersing in a basic solution (pH 11) for 6 h (green), an
acidic solution (pH 1) for 24 h (blue), and boiling water for 24 h (red). (d) UV-DRS spectrum of VNU-1-P. Inset: Opti-
cal image highlighting the yellow color of the material.
7
7 K, which afforded type-IV be-
haviors due to the presence of
mesopores within the structures
(
Figure 2b, Figure S10). The cal-
culated Brunauer–Emmett–Teller
(Langmuir) surface areas of VNU-1-P and VNU-2-P were 2100
optical microscopy images taken for VNU-1-P and VNU-2-P dis-
played homogeneous crystal morphology, lending further sup-
port to the purity of the bulk phases (Figures S6 and S7). It is
noted that microcrystalline powder samples, VNU-1-P and
VNU-2-P, were used for all further characterizations and meas-
urements as described below.
2
À1
(2600) and 1700 (2100) m g , respectively. Following these
findings, we sought to demonstrate the chemical and water
stability of VNU-1-P, which functions as a representative exam-
ple. As shown in Figure 2c, VNU-1-P remarkably maintains its
crystallinity after being immersed in boiling water for 24 h, in
Chem. Asian J. 2015, 10, 2660 – 2668
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an acidic solution (pH 1) for 24 h, as well as in a basic solution
pH 11) for 6 h.
(
Photoabsorption analysis
UV/Vis diffuse reflectance spectroscopy (UV-DRS) measure-
ments were performed on activated samples of VNU-1-P and
VNU-2-P (Figure 2d, Figure S11). As depicted in Figure 2d, the
spectrum of VNU-1-P exhibits an intense absorption band at
about 380 nm, whose edge extends to approximately
[
25]
5
40 nm.
This absorption in the visible-light region corre-
sponds well with the yellow color of the powder sample (Fig-
ure 2dinset). The UV-DRS spectra of VNU-2-P and the original
H CPEB linker display absorption band-edges of 369 nm and
2
3
20 nm, respectively (Figures S11 and S12). It is noted that the
absorbance band-edge of VNU-1-P is significantly red-shifted
in comparison to most other water-stable and photoactive
[
16,19]
[19]
[13c]
Figure 3. Calculated band structure in the first Brillouin zone and projected
density of states of the non-interpenetrated VNU-1. The Fermi level is shifted
at zero.
MOFs, notably UiO-66,
UiO-67,
(Table 2). The optical band gaps of
VNU-1-P and VNU-2-P were calculated to be 2.88 eV and
.36 eV, respectively, based on the relationship E =1240/l (Fig-
UiO-66-NO2,
UiO-66-
[13c]
[13b,14]
Br,
and MIL-125(Ti)
3
g
ure S13). At present, it is not clear as to why the CPEB linker
did not positively influence the optical absorption properties
of VNU-2-P, especially when considering that Hf and Zr have
error is within the limitation of the DFT calculation in terms of
band gap prediction. Additionally, the interpenetration ob-
served in the real crystal structure can also play a contributing
role to this difference. According to the DOS of VNU-1, both
the valence band and conduction band are primarily com-
posed of deprotonated CPEB linkers with little contribution
arising from the Zr O (OH) (CO ) SBUs. This implicates the or-
[18]
the same valence electron configurations.
To gain further understanding about the origins of the pho-
toabsorption property in VNU-1-P, density functional theory
calculations were employed to determine the electronic band
structure and the density of states (DOS). These results are de-
picted in Figure 3. As is shown, the calculated theoretical band
gap is 3.23 eV, which is slightly higher than the experimentally
determined value. This discrepancy is acceptable since the
6
4
4
2 12
ganic CPEB linker as playing an important role in effectively
[
16b,c,26]
tuning the band gap of the resulting MOF material.
Our
findings are consistent with the combined theoretical and ex-
[
26]
perimental observations made for UiO-66 and MOF-5. It is
noted that the conduction band of VNU-1 is very narrow
whereas the valence band is wider. These findings are indica-
Table 2. Summary of the photoabsorption properties and pertinent
structural information of VNU-1, VNU-2, and other related MOF structures.
tive of the fact that VNU-1 is a n-type semiconductor, similar
[16b,26]
to UiO-66 and UiO-66-NH2.
Nevertheless, the significant
MOF
SBU
Linker
Band Edge Band Gap Ref.
extension of the absorption band of VNU-1-P into the visible-
light region, in conjunction with the material’s high chemical
and water-stability, led us to investigate the UV/Vis driven pho-
tocatalytic performance of VNU-1-P for wastewater treatment.
[nm]
[eV]
[
a]
UiO-66
UiO-67
MIL-
Zr
Zr
6
O
O
4
(OH)
(OH)
4
4
(CO
(CO
2
)
)
12 BDC
305
337
345
4.07
3.68
3.60
[19]
[19]
[13b]
[
b]
6
4
2
12 BPDC
BDC
Ti
8
O
8
(OH)
(OH)
(OH)
4
(CO
(CO
(CO
2
)
6
1
25(Ti)
UiO-66- Zr
Br
VNU-2-P Hf
6
O
4
4
2
)
12 BDC(Br) 360
3.44
3.36
[13c]
Photocatalytic degradation of organic dye pollutants
6
O
4
4
2
)
12 CPEB
369
This
work
[18]
In order to investigate the photocatalytic ability of VNU-1-P
and VNU-2-P, two commonly used dyes, methylene blue (MB)
and methyl orange (MO), were chosen as model dye pollutants
in aqueous media. The photocatalytic performance of VNU-1-P
and VNU-2-P in the UV/Vis light region was highlighted by the
color variation (dark to light color) in the reaction system after
as little as 30 min. The performances of the two photocatalysts
were quantified by monitoring the disappearance in the ab-
sorbance bands of MB and MO at lmax =661 and 464 nm, re-
spectively, which is related to structural changes of the chro-
mophoric unit of the dye molecules (Figure 4, Figure S14). The
degradation rate profiles for the VNU-1-P photocatalytic
system are shown in Figure 5, in which VNU-1-P is clearly ob-
[
c]
[d]
UiO-68
UiO-66- Zr
NO
Zr
6
O
O
4
(OH)
(OH)
4
4
(CO
(CO
2
)
)
12 TPDC
n.d.
) 400
3.23
3.10
6
4
2
2
2
2
12 BDC(NO
12 CPEB
2
[13c]
2
VNU-1-P Zr
6
O
6
O
6
O
4
(OH)
4
(OH)
4
(OH)
4
4
4
(CO
(CO
(CO
)
)
)
430
2.88
2.75
2.47
This
work
[16b]
UiO-66- Zr
12 BDC(NH
2
) 450
502
NH
UiO-
6(AN)
2
[
e]
Zr
12 ANDC
[20]
6
[
a] BDC=1,4-benzenedicarboxylate; [b] BPDC=biphenyl-4,4’-dicarboxy-
late; [c] TPDC=[1,1’:4’,1“-terphenyl]-4,4”-dicarboxylate; [d] Based on den-
sity functional theory calculation; [e] ANDC=anthracene-9,10-dicarboxy-
late; n.d.=no experimental data available.
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the photocatalytic VNU-1-P. It is noted that these conversions
are significantly higher than those observed for the compari-
son catalyst, Degussa P-25 TiO . These results are expected, as
2
the absorbance band-edge of VNU-1-P is 430 nm and extends
to about 540 nm, which allows VNU-1-P to absorb more visible
light than the Degussa P-25 TiO . Moreover, we presume an
2
additional reason for the higher activity of VNU-1-P is due to
a larger accessible surface area, providing more photocatalytic
[
13c]
active sites and promoting the transport of charge carriers.
In order to compare the performance of VNU-1-P with isoretic-
ular structures that lack responsivity to UV/Vis light, control re-
actions were performed using UiO-66 and UiO-67 as the pho-
tocatalysts. From these reactions, it was clearly evident that
UiO-66 and À67 displayed significantly less photocatalytic ac-
tivity for dye degradation, thus highlighting the advantage of
the visible-light-responsive VNU-1-P (Figure S15). Furthermore,
an additional control experiment, without any catalyst present,
demonstrated that both MB and MO were not able to be effec-
tively photodegraded by UV/Vis light alone. In contrast to the
highly active VNU-1-P photocatalyst, the photocatalytic activity
of VNU-2-P was observed to be less effective than that of De-
gussa P-25 TiO (Figure S16). The final concentration of MB and
2
MO by VNU-2-P after irradiation for 3 h was calculated to be
5
3 and 72% of the initial concentration, respectively, which
was expected due to the larger measured band gap for this
material.
We reason that the photocatalytic degradation of MB and
MO by VNU-1-P and VNU-2-P occurs via generation of elec-
tron–hole pairs under UV/Vis irradiation. Photoluminescence
(
PL) spectroscopy measurements, under 365 nm laser irradia-
tion, display broad PL peaks for H CPEB, VNU-1-P, and VNU-2-P,
2
which were centered at 432, 450, and 410 nm, respectively
(
Figure S17). The redshift and lower intensities observed for
Figure 4. UV/Vis absorption spectra of the (a) MB and (b) MO solutions over
the course of irradiation with UV/Vis light in the presence of VNU-1-P.
VNU-1-P in comparison to H CPEB indicate that electron–hole
2
[
16b,c]
pairs were efficiently generated and separated.
Through
this mechanism, which is
common amongst many photo-
catalytic MOF systems, hydroxyl
radicals are produced as a result
of a photoinduced energy trans-
fer to adsorbed oxygen and
[
11,27]
water molecules.
The result-
ing hydroxyl radicals effectively
react and decompose MB and
[
11,27]
MO accordingly.
After photodegradation of the
MB and MO dyes was completed
(
3 h), VNU-1-P and VNU-2-P were
collected by centrifugation and
washed three times with 20 mL
Figure 5. Degradation profiles of methylene blue (a) and methyl orange (b) under UV/Vis light by VNU-1-P (trian-
gles), Degussa P-25 TiO (squares), and no catalyst (diamonds, control experiment). The lines connecting the
2
of ethanol for a day. Following
shapes are guides for the eyes.
this, VNU-1-P and VNU-2-P were
immersed in 20 mL of chloro-
form for a day, then filtered, and
served to have a higher activity than the commercially avail-
subsequently regenerated under reduced pressure. PXRD anal-
ysis of the regenerated photocatalysts indicated that the crys-
tallinity of these materials were retained (Figure 6, Figure S18).
able Degussa P-25 TiO under identical conditions. After 3 h,
2
MB and MO were 100% and 83% degraded, respectively, by
Chem. Asian J. 2015, 10, 2660 – 2668
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VNU-1 was capable of being recycled, without appreciable loss
of activity, for further use.
Experimental Section
Materials and general methods
Methyl 4-iodobenzoate, 1,4-diethynylbenzene, bis(triphenylphos-
phine)palladium dichloride, copper(I) iodide, potassium hydroxide,
ammonium chloride (NH Cl), sodium chloride (NaCl), methylene
4
blue (MB), methyl orange (MO), triethylamine (TEA), toluene, meth-
anol (MeOH), acetic acid, chloroform, and N,N-dimethylformamide
(
DMF) were purchased from Aldrich Chemical Company. Zirconyl
Figure 6. PXRD analysis of VNU-1-P before and after the photocatalytic deg-
radation reaction in comparison to the calculated pattern from single-crystal
data.
chloride octahydrate (ZrOCl ·8H O), hafnium chloride (HfCl ), potas-
sium hydroxide, and tetrahydrofuran (THF) were obtained from
Acros Organics. P-25 TiO was purchased from Degussa. All chemi-
2
2
4
2
cals were used without further purification. Water used
was double distilled and filtered through a millipore
membrane. All experiments were performed in air.
1
13
H and C NMR spectra were acquired on a Bruker AVB-
00 MHz spectrometer. High-resolution electrospray ioni-
5
zation mass spectra were obtained on a Bruker micrO-
TOF-Q. Elemental microanalyses (EA) were performed in
the Microanalytical Laboratory of the College of Chemis-
try at UC Berkeley, using a PerkinElmer 2400 Series II
CHNS elemental analyzer. FT-IR spectra were analyzed
from KBr pellets by using a Bruker Vertex 70 instrument.
Thermal gravimetric analyses (TGA) were carried out on
a TA Q500 Thermal Analysis System under airflow. Nitro-
gen adsorption isotherms at 77 K were measured on
a Quantachrome Autosorb-iQ2, using helium of 99.999%
purity for the estimation of dead space. Ultra-high-purity
Figure 7. Degradation profiles of methylene blue (a) and methyl orange (b) by VNU-1-P
over three consecutive cycles. The lines connecting the shapes are guides for the eyes.
grade N (99.999% purity) was used in the adsorption
2
experiments. Optical microscopy images were acquired
This finding points to the remarkable stability of VNU-1-P and
VNU-2-P under the aqueous reaction conditions. In order to
demonstrate the recyclability of these heterogeneous photoca-
talysts, VNU-1-P and VNU-2-P were dispersed again in 100 ppm
solutions of MB and MO. As shown in Figures 7 and S19, VNU-
using a Nikon SMZ1000 microscope. UV-Vis diffuse reflectance
spectra were recorded by using a JASCO V-670 UV-Vis spectropho-
tometer equipped with a diffuse reflectance accessory and barium
sulfate as reference standard. UV-Vis spectra of photocatalytic dye
degradation samples were collected on a JASCO V-670 spectropho-
tometer. Photoluminescence spectra were acquired on a Horiba
Jobin Yvon spectrometer iHR320. Photocatalysis experiments were
performed on an Oriel Newport system equipped with 300 W
Xenon lamp.
1
-P and VNU-2-P exhibited efficient photodegradation of both
MO and MB after three consecutive cycles. Specifically, the final
relative concentrations of MO and MB were found to have de-
creased by only 10% over the course of three cycles.
Synthesis of 1,4-bis(2-[4-carboxyphenyl]ethynyl)benzene
(
H CPEB) linker
Conclusions
2
IV
Synthesis of 1,4-bis(4-carbomethoxyphenylethynyl)benzene,
precursor 1: Methyl 4-iodobenzoate (0.524 g, 2.00 mmol) and 1,4-
diethynylbenzene (0.126 g, 1.00 mmol) were dissolved in a 50 mL
Schlenk flask containing a 10 mL mixture of triethylamine and tolu-
ene (v/v=1:1). Once dissolved, bis(triphenylphosphine)palladium
dichloride (35.2 mg, 0.0500 mmol) and copper(I) iodide (2.0 mg,
10 mmol) were then added in the reaction mixture. The solution
was subsequently stirred at room temperature for 24 h. Next, the
solvent was evaporated under reduced pressure and the remaining
solid was washed consecutively with copious amounts of hexanes,
Two novel heterogeneous MOF photocatalysts based on Zr
IV
and Hf inorganic SBUs and a highly p-conjugated linker were
synthesized and their structures fully characterized. The struc-
tures of these MOFs, termed VNU-1 and VNU-2, respectively,
were determined by single-crystal X-ray diffraction analyses, in
which a doubly interpenetrated fcu-c net was found. By incor-
porating the highly p-conjugated linker, the optical absorption
properties of VNU-1 were tuned to the visible-light region. In
turn, this allowed VNU-1 to function as an effective heteroge-
neous photocatalyst in the UV/Vis light degradation of two
model dye pollutant molecules, especially in comparison to
a saturated solution of NH Cl, and a saturated solution of NaCl.
4
The final pink product was dried under vacuum (0.335 g,
1
0
.850 mmol, 85% yield). H NMR (500 MHz, CDCl , 258C, TMS): d=
3
the commercially available Degussa P-25 TiO photocatalyst. Fi-
2
3.94 (s, 6H), 7.54 (s, 4H), 7.59 (d, 4H, J=8.5 Hz), 8.02 ppm (d, 4H,
13
nally, VNU-1 was shown to remain crystalline after photocata-
lytic reactions were completed, and it was demonstrated that
J=8.5 Hz). C NMR (125 MHz, [D ]DMSO, 258C, TMS): d=165.92,
6
131.74, 131.55, 129.76, 129.57, 127.33, 122.97, 91.38, 52.26 ppm.
Chem. Asian J. 2015, 10, 2660 – 2668
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2665
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
+
MS (ESI) calculated for C H O : m/z=394.12 ([M+H] ); Found m/
Synthesis of single crystals of [Hf O (OH) (CPEB) ], VNU-2-SC:
HfCl (10.9 mg, 0.0340 mmol) and H CPEB (12.4 mg, 0.0340 mmol)
4 2
2
6
18
4
6
4
4
6
À1
z=395.12. FT-IR (KBr, 4000–400 cm ): n˜ =2958 (w), 2849 (br), 1721
(
(
1
s), 1652 (w), 1636 (w), 1604 (m), 1558 (w), 1541 (w), 1518 (w), 1488
w), 1435 (m), 1406 (m), 1375 (m), 1309 (m), 1281 (s), 1191 (m),
were dissolved in DMF (3.75 mL) in a 10 mL capped vial with acetic
acid (0.25 mL) added as a modulator. The solution was subsequent-
ly heated at 1208C for 3 days in an isothermal oven to yield a color-
less crystalline solid. After cooling the vial to room temperature,
the crystalline product was separated from the mother liquor via
176 (m), 1149 (m), 1107 (s), 955 (w), 857 (w), 838 (m), 810 (w), 768
À1
(
m), 697 (m), 549 (br), 524 (w), 490 (w), 419 cm (w).
Synthesis of 1,4-bis(2-[4-carboxyphenyl]ethynyl)benzene
(
centrifugation (35% yield based on the H CPEB linker). The single
H CPEB) linker: Potassium hydroxide (0.420 g, 7.50 mmol) and
2
2
crystals collected were then used for single-crystal X-ray diffraction
analysis.
1
0
,4-bis(4-carbomethoxyphenylethynyl)benzene
.250 mmol) were stirred in a mixture of methanol (3 mL), THF
(0.985 g,
(
3 mL) and water (1.5 mL) at room temperature overnight. After
Synthesis of microcrystalline [Hf O (OH) (CPEB) ] powder, VNU-
6
4
4
6
the reaction, the organic solvents were removed under reduced
pressure, and the reaction solution was acidified by concentrated
HCl to afford a faintly yellow precipitate. The final product was fil-
tered once again, washed with copious amounts of water, and
2-P: HfCl4 (10.9 mg, 0.0340 mmol) and H CPEB (12.4 mg,
2
0.0340 mmol) were dissolved in
a solvent mixture of DMF
(3.80 mL), acetic acid (0.10 mL), and water (0.10 mL) in a 10 mL
capped vial. The solution was subsequently heated at 1208C for
3 days in an isothermal oven to yield light yellow microcrystalline
powder. After cooling the vial to room temperature, the light
yellow solid product, VNU-2-P, was separated from the mother
dried at 808C to yield 1,4-bis(2-[4-carboxyphenyl]ethynyl)benzene
1
(
0.0780 g, 0.213 mmol, 85% yield). H NMR (500 MHz, [D ]DMSO,
6
2
4
1
9
58C, TMS): d=7.66 (s, 4H), 7.68 (d, 4H, J=8.5 Hz), 7.98 ppm (d,
1
3
H, J=8.5 Hz). C NMR (125 MHz, [D ]DMSO, 258C, TMS): d=
liquor via centrifugation (59% yield based on the H CPEB linker).
6
2
66.61, 131.88, 131.59, 130.79, 129.55, 126.22, 122.37, 91.34,
The as-synthesized sample of VNU-2-P was washed with 10 mL of
DMF three times per day over the course of three days. Following
this, VNU-2-P was immersed in 10 mL chloroform, which was re-
placed three times per day for a total of three days. After the sol-
vent-exchange process was completed, VNU-2-P was activated
under reduced pressure at 1208C for 24 h. Elemental anal. calcd for
Hf6C144H O =[Hf O (OH) (C H O ) ]: C, 51.09; H, 2.27%; found: C,
À
0.73 ppm. MS (ESI) calculated for C H O : m/z=366.09 ([M-H] );
2
4
14
4
À1
found m/z=365.09 FT-IR (KBr, 4000–400 cm ): n˜ =3413 (br), 3076
(
(
br), 3039 (w), 3013(w), 2966 (w), 2842 (m, br), 2284 (w), 2214.34
w), 1945 (w), 1927 (w), 1720 (s), 1602 (m), 1555 (m), 1516 (w),
1
488 (w), 1433 (m), 1405 (m), 1375 (m), 1310 (m), 1281 (s), 1190
(
(
4
m), 1174 (m), 1148 (m), 1105 (s), 1015 (m), 952 (m), 859 (m), 837
76
32
6
4
4
24 12 4 6
m), 809 (m), 768 (m), 696 (m), 547 (m), 522 (w), 489 (m), 450 (w),
52.08; H, 2.67; N, 0.30%. Calcd. for Hf6C145.5H79.5N O
=
0.5
32.5
À1
23 cm (w).
[Hf O (OH) (C H O ) ]·0.5DMF: C, 51.01; H, 2.34; N, 0.29%. FT-IR
6
4
4
24 12 4 6
À1
(
KBr, 4000–400 cm ): n˜ =1656 (m), 1604 (m), 1544 (m), 1496 (m),
1
409 (s), 1391 (s), 1253 (w), 1163 (w), 1099 (w), 1065 (w), 920 (w)
À1
Synthesis of [Zr O (OH) (CPEB) ] (VNU-1) and
773 (w), 720 (w), 667 (m) 548 cm (w).
6
4
4
6
[
Hf O (OH) (CPEB) ] (VNU-2)
6 4 4 6
X-ray diffraction analysis
Synthesis of single crystals of [Zr O (OH) (CPEB) ], VNU-1-SC
6
4
4
6
(
where SC=single crystal): ZrOCl ·8H O (10.9 mg, 0.0340 mmol)
2 2
A single crystal of VNU-1-SC or VNU-2-SC was isolated from the
mother liquor of the reaction by a nylon loop and mounted. The
X-ray diffraction data for these materials were both collected on
a Bruker D8 Venture diffractometer outfitted with a PHOTON-100
CMOS detector using monochromatic microfocus CuKa radiation
and H CPEB (12.4 mg, 0.0340 mmol) were dissolved in DMF
2
(
3.80 mL) in a 10 mL capped vial with acetic acid (0.2 mL) added as
a modulator. The solution was subsequently heated at 1208C for 3
days in an isothermal oven to yield a colorless crystalline solid.
After cooling the vial to room temperature, the crystalline product
was separated from the mother liquor via centrifugation (40%
yield based on the H CPEB linker). The single crystals collected
were then used for single-crystal X-ray diffraction analysis.
(l=1.54178 Š) that was operated at 50 kW and 1.0 mA. The VNU-
1
-SC data was collected at room temperature whereas VNU-2-SC
2
was cooled down to 100 K by chilled nitrogen flow controlled by
a Kryoflex II system before data collection. Unit cell determination
was performed in the Bruker SMART APEX II software suite. The
data sets were reduced and a multi-scan spherical absorption cor-
Synthesis of microcrystalline [Zr O (OH) (CPEB) ] powder, VNU-
1
H CPEB (12.4 mg, 0.0340 mmol) were dissolved in a solvent mixture
of DMF (3.86 mL), acetic acid (0.042 mL), and water (0.10 mL) in
a 10 mL capped vial. The solution was subsequently heated at
6
4
4
6
-P (where P=powder): ZrOCl ·8H O (10.9 mg, 0.0340 mmol) and
2 2
[28]
rection was implemented in the SCALE interface. The structures
were solved with direct methods and refined by the full-matrix
2
[
29]
least-squares method in the SHELXL-97 program package. Once
the framework atoms were located in the difference Fourier maps,
the SQUEEZE routine in PLATON was performed to remove scatter-
1
208C for 1 day in an isothermal oven to yield a yellow precipitate.
After cooling the vial to room temperature, the yellow precipitate
product, VNU-1-P, was separated from the mother liquor via centri-
fugation (68% yield based on the H CPEB linker). The as-synthe-
sized sample of VNU-1-P was washed with 10 mL of DMF three
times per day over the course of three days. Following this, VNU-1-
P was immersed in 10 mL chloroform, which was replaced three
times per day for a total of three days. After the solvent-exchange
process was completed, VNU-1-P was activated under reduced
pressure at 1208C for 24 h. Elemental anal. calcd for Zr6C144H O =
[
24]
ing from disordered guest molecules residing in the pores. De-
tailed descriptions of structural refinement can be found in Tables
S1 and S2 in the Supporting Information. Crystallographic data for
VNU-1-SC and VNU-2-SC have been deposited in the Cambridge
Crystallographic Data Centre (deposition numbers: 1063477 and
1063478). This data can be obtained, free of charge, via the web
(www.ccdc.cam.ac.uk/data request/cif). PXRD patterns were col-
lected using a Bruker D8 Advance equipped with a Ni filtered CuKa
radiation (l=1.54178 Š) source. The diffractometer was also outfit-
ted with an anti-scattering shield that prevented incident diffuse
radiation from hitting the detector. Sample preparation included
placing samples of VNU-1-P and VNU-2-P on a zero background
holder and flattening them with a spatula. The 2q range was 3–308
with a step size of 0.028 and a fixed counting time of 0.3 s/step.
2
76
32
[
Zr O (OH) (C H O ) ]: C, 60.35; H, 2.67%; found: C, 56.94; H, 2.63;
6 4 4 24 12 4 6
N,
0.23%.
Calcd.
for
Zr6C143.3H95.6N O
=
0.5
40.8
[
Zr O (OH) (C H O ) (C H O ) ]·3(H O)·0.5DMF: C, 58.86; H, 2.92;
6 4 4 24 12 4 5.9 2 3 2 0.1 2
À1
N, 0.24%. FT-IR (KBr, 4000–400 cm ): n˜ =1657 (m), 1602 (m), 1544
(
(
m), 1413 (s), 1179 (w), 1102 (w), 1016 (w), 860 (w), 835 (w), 780
m), 697 (w), 663 (w), 478 cm (w).
À1
Chem. Asian J. 2015, 10, 2660 – 2668
www.chemasianj.org
2666
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Computational methods
Keywords: dye degradation
·
heterogeneous catalysis ·
Theoretical calculations have been carried out at the B3LYP level of
theory with the Gaussian-type functions (GTFs) CRYSTAL pro-
photocatalysis · porous materials · wastewater treatment
[30]
gram. All electron basis set was used for Zr and the triple-zeta
valence basis sets with polarization quality were employed for H,
[31]
[
[
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À10
ments were set to 10
a.u. and 0.00012 a.u., respectively. The
symmetry of the structure was maintained during the relaxation.
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4
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10 mg) was dispersed in a 20 mL quartz tube containing a 15 mL
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5
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[
[
2
1
8 cm from the light source and adequate stirring was maintained.
The reaction mixture was then irradiated at 258C under a 300 W
1
xenon lamp equipped with an IR cutting filter to cut off light with
À2
wavelengths>750 nm and
a
light intensity of 400 mWcm .
4
Sample aliquots were collected at regular intervals and were treat-
ed by filtration through a Millipore membrane, followed by centri-
fugation to remove the small amount of remaining VNU-1-P cata-
lyst particle contaminants. The concentration of MB or MO were
monitored by measuring the absorbance intensity at their maxi-
mum absorbance wavelengths of lmax =661 and 464 nm, respec-
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Acknowledgements
We acknowledge the financial support provided by Vietnam
National University, Ho Chi Minh City (Nos. B2014-50-01,
A2015-50-01-H“-KHCN, and MANAR-CS2014-02), and ONRG-
NICOP (No. N62909-15-1N056). We gratefully acknowledge the
use of computational resources of the HPCI system provided
by Cyberscience Center, Tohoku University through the HPCI
System Research (project ID:hp150084). We thank Ms. A. M.
Osborn, Dr. H. Furukawa, Prof. H. T. Nguyen, Mr. T. N. Tu, and
Ms. L. H. T. Nguyen at MANAR for their valuable discussions.
Furthermore, we appreciate Mr. C. Tischler and Prof. J. K. Kang
at Korea Advanced Institute of Science and Technology (KAIST)
for advice and instrument use during the early stages of this
work. Finally, we acknowledge Prof. O. M. Yaghi (UC Berkeley)
for starting and supporting the Center for Molecular and Nano-
Architecture.
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Manuscript received: June 21, 2015
Revised: July 31, 2015
1
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Accepted Article published: August 10, 2015
Final Article published: September 4, 2015
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