Two novel organic phosphorous-based MOFs: synthesis, characterization and photocatalytic properties

Article abstract of DOI:10.1039/C8DT04106G

Two novel metal-organic framework (MOF) photocatalysts with different structures [(Cu(H₂L)(4,4′-bipy)_0.5(H₂O)] (1) and [Co(C₁₄H₁₄O_6.5P₂)(4,4′-bipy)_0.5(H₂O)₂]·H₂O (2) were synthesized using a hydrothermal method using a phosphonate ligand [H₄L = 1,1′-biphenylene-4,4′-bis(methylene)-bis(phosphonic acid)] and 4,4′-bipyridine ligand. All the samples were characterized by elemental analysis, thermal analysis, and single crystal X-ray diffraction. As novel porous materials, the two complexes showed active performance for the reduction of Cr(vi) to Cr(iii) and the photodegradation of methylene blue (MB) dye in aqueous solution under UV light. Control experiments showed that the pH value was vital for Cr(vi) reduction, meanwhile, the use of a hole scavenger of methanol promoted the photocatalytic reduction significantly. It was also demonstrated that complexes 1 and 2 were efficient for the degradation of MB. Moreover, the possible reaction mechanism of the reaction was also investigated in detail. Finally, the cyclic experiments indicated the two photocatalysts were stable and reusable, enabling them to be potential candidates for use in environment governance.

Full text of DOI:10.1039/C8DT04106G

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Two novel organic phosphorous-based MOFs:
synthesis, characterization and photocatalytic
properties†
Cite this: Dalton Trans., 2019, 48,
523
Tianyu Zeng, Liwen Wang, Lu Feng, Hailong Xu, Qingrong Cheng * and
Zhiquan Pan*
Two novel metal–organic framework (MOF) photocatalysts with different structures [(Cu(H2L)(4,4’-
bipy)0.5(H2O)] (1) and [Co(C14H14O6.5P2)(4,4’-bipy)0.5(H2O)2]·H2O (2) were synthesized using a hydro-
thermal method using a phosphonate ligand [H4L = 1,1’-biphenylene-4,4’-bis(methylene)-bis(phosphonic
acid)] and 4,4’-bipyridine ligand. All the samples were characterized by elemental analysis, thermal ana-
lysis, and single crystal X-ray diffraction. As novel porous materials, the two complexes showed active per-
formance for the reduction of Cr(VI) to Cr(III) and the photodegradation of methylene blue (MB) dye in
aqueous solution under UV light. Control experiments showed that the pH value was vital for Cr(VI)
reduction, meanwhile, the use of a hole scavenger of methanol promoted the photocatalytic reduction
significantly. It was also demonstrated that complexes 1 and 2 were efficient for the degradation of MB.
Moreover, the possible reaction mechanism of the reaction was also investigated in detail. Finally, the
cyclic experiments indicated the two photocatalysts were stable and reusable, enabling them to be
potential candidates for use in environment governance.
Received 14th October 2018,
Accepted 21st November 2018
DOI: 10.1039/c8dt04106g
rsc.li/dalton
etrations, high toxicity, and high chemical oxygen demand,
which make them toxic not only to human beings but also to
Introduction
Nowadays, a large amount of waste water containing inorganic plants in photosynthesis.7 Therefore, it is of great importance
heavy metal ions and organic compounds is produced with the to develop effective and economical technologies to reduce
rapid development of modern industry.1,2 The wastewater has Cr(VI) to Cr(III) and to degrade organic dyes in the treatment of
low biodegradability, which means the pollutants are easy to wastewater.
be concentrated in the environment and can enter living
Various methods, such as physical, chemical, and biologi-
organisms through the food chain. Among the inorganic pollu- cal techniques, have been reported to deal with wastewater.
tants, Cr(VI) pollution is particularly critical. The toxicity of Among these, photocatalytic technology has been broadly used
Cr(VI) is 100 times higher than that of Cr(III), which can cause in green wastewater treatment due to it being mild and envir-
serious health problems as certain classes of Cr(VI) are terato- onmentally friendly.8–10 Since the semiconductor TiO2 was
genic and carcinogenic and can cause mutations.3,4 In contrast initially discovered to be useful for pollutants degradation in
to Cr(VI), trivalent chromium Cr(III) is slightly toxic and can be the photocatalytic system, many semiconductors, including
easily precipitated or adsorbed in the solid phase.5,6 Therefore, ZnS, SnO2, ZnO, and CdS, have been investigated as potential
the reduction of Cr(VI) to Cr(III) is considered an effective photocatalysts
for
eliminating
contaminants
in
method for the remediation of Cr(VI)-contaminated waters. wastewater.11–15 However, these semiconductor photocatalysts
Moreover, organic dyes are notorious due to their low pen- have several disadvantages, such as low surface area, easy
photoelectron–hole recombination, and rapid deactivation.16
Consequently, it has become urgent to find new efficient
photocatalysts for the reduction of Cr(VI) and for the degra-
dation of organic pollutants.
Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute
of Technology, Wuhan 430073, P. R. China. E-mail: chengqr383121@sina.com,
zhiqpan@163.com
Metal–organic frameworks (MOFs), an extensive class of
multi-functional crystalline materials with novel structures, are
made up of metal ions or metal clusters and organic ligands
by self-assembly.17 Due to their ultrahigh porosity, enormous
internal surface areas, diverse structural topologies, and rich
†Electronic supplementary information (ESI) available: More detailed infor-
mation about the characterization of MOFs, as well as that for photocatalytic
study as noted in the text (PDF). CCDC complex 1 1872673 and complex 2
1872661. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8dt04106g
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active site, MOFs have attracted considerable attention for ongoing studies on the photocatalysis, the photocatalytic reac-
their potential applications in gas storage and separation, tions of the new phosphonate-based MOFs were investigated
luminescent, catalysis, and proton conductivity.18–23 Moreover, under different experimental conditions, and the results
some MOFs have exhibited semiconducting properties under showed that these new MOFs are highly efficient photocatalysts
illumination, implying that they can be potentially used as for aqueous Cr(VI) reduction and MB degradation under ultra-
photocatalysts.24,25 Compared with conventional inorganic violet light. The possible mechanism of the reaction was also
semiconductors, the high porosity and desirable topology of investigated by measuring the major active species in the cata-
MOFs would shorten the transport distance of charge carriers lytic process. Finally, the photocatalytic activity of these two
to the pore surface throughout the structure in reactions, photocatalysts showed high stability, and the activity did not
which would extremely inhibit the recombination of electrons decrease after five consecutive cycles.
and holes. Garcia’s team demonstrated that MOF-5 shows
photocatalytic activity for phenol degradation, in which the
lifetime of electrons and holes reached the microsecond
Experimental
Preparation of the ligand (H4L)
level.16,26 Furthermore, some bridging ligands with conjugate
structures provide channels for the transport of electrons,
which provide the possibility to enhance the separation of
Sinopharm Chemical Reagent Co., Ltd and used as received
photogenerated electrons and holes. In addition, the band gap
in MOF systems is closely related to the HOMO–LUMO gap,
which allows for the fine-tuning and rational design of these
Chemical reagents were purchased commercially from
without further purification. The ligand 1,1′-biphenylene-4,4′-
bis(methylene)bis(phosphonic acid) (H4L, Scheme S1†) was
prepared according to the literature method.35
photocatalysts at the molecular level.27 Recently, Gascon
reported two Zn(II) MOFs, assembled by terephthalic acid or
1,4-naphthalene diacid, with energy band gaps (Eg) of 3.4 eV
and 3.3 eV, respectively.28 Their results demonstrated that the
Eg of MOFs can be adjusted by changing the length of ligands,
without altering the topological structure, which make MOFs
particularly suitable for photocatalysis. Up to now, many car-
boxylate-based MOFs and N-heterocycle-based MOFs have
been reported as catalysts for photocatalysis. For example,
Wang et al. disclosed Cr(VI) reduction by NH2-MIL-125(Ti) as
the photocatalyst, which showed a relatively high efficiency.29
Cheng et al. reported [Cu(4,4′-bipy)Cl]n and [Co(4,4′-
bipy)·(HCOO)2]n as photocatalysts using H2O2 on MB degra-
dation to achieve considerable efficiency under visible light
irradiation.30 Zhao et al. demonstrated that NNU-36
([Zn2(BPEA)(BPDC)2]·2DMF) could achieve the excellent photo-
catalytic reduction of Cr(VI) and the degradation of dyes (RhB,
MB, R6G).31 It is well known that the stability and recyclability
of a photocatalyst are crucial for its catalytic application. Wang
and co-workers used carboxylate-based MOF (named BUC-21)
in photocatalytic Cr(VI) reduction, and the results showed that
the photocatalytic activity under the condition of pH = 1 was
significantly lower than that under pH = 2 or 3, possibly
because of destruction of the framework of BUC-21.32 In con-
trast to metal carboxylates, metal phosphonates show higher
thermal stability and are harder to be soluble even in highly
acidic media,33,34 which indicated that MOFs based on phos-
phonate might be more stable for the photocatalytic degra-
dation of organic dyes and the reduction of Cr(VI) to Cr(III) in
acidic environments. However, investigations on the photo-
catalytic activities of phosphonate-based MOFs are limited.
Preparation of complexes
[Cu(H2L)(4,4′-bipy)0.5(H2O)] (1). The complex was hydrother-
mally synthesized by the reaction of Cu(CH3COO)2·H2O
(0.130 g, 0.650 mmol), 4,4′-bipyridine (0.101 g, 0.650 mmol),
and H4L (0.222 g, 0.650 mmol) dissolved in H2O (40.0 g,
2.22 mol), then HCl (0.350 mL, 6.00 mol L−1) was added. After
stirring under air atmosphere for 30 min, the solution was
transferred into a 150 mL Teflon-lined stainless-steel autoclave,
sealed, and heated up to 140 °C for 72 h. By slow cooling of
the reaction mixture to room temperature over 24 h, blue crys-
tals of complex 1 were obtained and were washed with H2O
and ethanol several times for further use. Yield: 66.62% (based
on Cu). Calcd for C19H20CuNO7P2 (499.85): C, 45.65; H, 4.03;
N, 2.80. Found: C, 45.37; H, 4.24; N, 2.42. Selected IR data (KBr
pellet cm−1): 3442(s), 3033(w), 2909(w), 2370(w), 1607(m),
1494(m), 1417(m), 1251(w), 1132(s), 1067(s), 932(s), 807(m),
724(w), 642(w), 594(w), 558(m), 464(w).
[Co(C14H14O6.5P2)(4,4′-bipy)0.5(H2O)2]·H2O (2). The same syn-
thetic procedure was used to synthesize 2 as that for 1 except
Co(CH3COO)2·4H2O was used instead of Cu(CH3COO)2·H2O
and the ratio was 1 : 1 : 1 : 3415 [Co(CH3COO)2·4H2O : 4,4′-
bipy : H4L : H2O]. Yield: 71.23% (based on Co). Calcd for
C19H24CoNO9.5P2 (539.28): C, 42.32; H, 4.49; N, 2.60. Found:
C, 42.45; H, 4.28; N, 2.72. Selected IR data (KBr pellet cm−1):
3428(m), 3030(w), 2910(w), 2369(w), 1604(m), 1534(w), 1497(m),
1410(m), 1265(m), 1161(m), 1126(s), 1099(s), 1041(s), 958(s),
838(m), 806(m), 757(w), 714(w), 615(w), 592(w), 544(s), 498(w).
Photocatalytic experiments
Herein, an organic phosphorous ligand H4L [1,1′-bipheny- The photocatalytic experiments for aqueous Cr(VI) to Cr(III)
lene-4,4′-bis(methylene)bis(phosphonic acid)] containing π–π were carried out under 125 W UV irradiation with a 380 nm
conjugate structure was designed and synthesized, and two cut-off filter at ambient conditions in a 50.0 mL quartz reactor
new MOF photocatalysts, [(Cu(H2L)(4,4′-bipy)0.5(H2O)] (1) and containing 4.00 mg MOFs-based photocatalysts which were
[Co(C14H14O6.5P2)(4,4′-bipy)0.5(H2O)2]·H2O (2), with different dispersed into 40.0 mL Cr(VI) aqueous solution (30.0 ppm).
band gaps and band positions were synthesized. As part of The distance between the light source and the quartz reactor
524 | Dalton Trans., 2019, 48, 523–534
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containing the reaction mixture was fixed at 10.0 cm. Prior to with graphite-monochromatic Mo/Kα radiation (λ = 0.71073 Å)
irradiation, the suspension was magnetically stirred for 60 min in ω-scan mode. The collected data were reduced using the
in the dark to ensure an adsorption–desorption equilibrium software package SAINT,37 and semi-empirical absorption cor-
and then 0.200 M H2SO4 was added to adjust the acidity in the rection was applied to the intensity data using SADABS
reaction, with the pH values determined using a pH detector. program.38 The structures of 1 and 2 were solved using direct
During the reduction of aqueous Cr(VI), stirring was maintained methods, and all nonhydrogen atoms were refined anisotropi-
to keep the mixture in suspension, 2.00 mL of suspension was cally by least squares on F2 using the SHELXTL-2014
withdrawn at regular intervals and the photocatalyst was separ- program.39 Hydrogen atoms were placed in calculated posi-
ated by centrifugation. Then, the Cr(VI) content in the solution tions and refined isotropically using the riding model. Details
was determined colorimetrically by the diphenylcarbazide of the crystallographic data of 1 and 2 are summarized in
method (DPC) with a detection limit of 0.005 mg L−1 36
The Table S1.† Selected bond lengths (Å) and angles (°) for 1 and 2
.
Cr(VI) content in the solution at different illumination times was are listed in Table S2.†
monitored by measuring its maximum absorbance (centered at
540 nm) using a UV-visible spectrophotometer. The reduction
ratio of Cr(VI) was calculated using the following expression:
reduction ratio of Cr(VI) = (C0 − Ct)/C0 × 100%.
Structure description of [Cu(H2L)(4,4′-bipy)0.5(H2O)] (1)
Single-crystal structure analysis revealed that complex 1 crystal-
ˉ
lizes in a space group P1. The asymmetric unit in 1 contains
The photocatalytic experiments for the degradation of dyes
of MB were carried out under 125 W UV irradiation with a
380 nm cut-off filter at ambient conditions in a 50 mL quartz
reactor containing 5.00 mg MOF-based photocatalysts, which
were dispersed in 40.0 mL MB aqueous solution (10.0 ppm).
Prior to irradiation, the suspension was magnetically stirred
for 60 min in the dark to ensure adsorption–desorption equili-
brium. During the photodegradation reaction, 2.00 mL of sus-
pension was taken out each time from the reactor at scheduled
intervals and the photocatalyst was separated by centrifu-
gation. The MB concentration changes at different illumina-
tion times were monitored by measuring the absorption inten-
sity at its maximum absorbance wavelength of λ = 664 nm
using a UV-visible spectrophotometer.
one Cu(II) ion, one [H2L]2− anion, 1/2 4,4′-bipy molecule and
one coordination water molecule (Fig. 1a). Each Cu ion coordi-
nates with an half 4,4′-bipy(N1), one water molecule (O7), and
three oxygen atoms from the phosphonate ligand (O1, O2, O4),
forming a slightly distorted square pyramidal type of geometry
(Fig. 1b). The distances of Cu–O and Cu–N are in the range of
1.894(4)–2.323(5) Å and 2.072(5) Å, respectively. In the asym-
metric unit, the H2L anion acts as a tridentate ligand to
chelate three Cu atoms, with each {CP(1)O3} group chelating
two Cu(II) sites through O1 and O2, respectively, while each
{CP(2)O3} group is monodentate for O4. So, the coordination
mode can be adopted as with μ3–η1η1η0η1η0η0 (Fig. S1†). As
shown in Fig. 1c, two equivalent Cu2+ ions are bridged by two
[H2L]2− anion with the connection of (P1–O2–Cu1–O4–P2)2,
forming a 30-membered ring, then two Cu ions are connected
with two nitrogen atoms in the 4,4′-bipy molecule, respectively,
which separates the ring into the same two parts. The rings
are further bridged by P1–O1–Cu1 through the coordination
Results and discussion
Crystal structure
Single-crystal X-ray diffraction data for complexes 1 and 2 were interactions, leading to a two-dimensional plane along the
collected on a Bruker Smart CCD area-detector diffractometer c-axis with Cu1⋯Cu1 separation of 4.9493 (Fig. 1d).
Å
Fig. 1 (a) The asymmetric unit of complex 1; (b) the coordination environment of Cu(II) in complex 1; (c) two-membered Cu-dimer ring formed in
complex 1; (d) 2D plane structure in complex 1; (e) hydrogen-bond interactions between the inorganic planes along the a-axis; (f) hydrogen-bond
interactions between the inorganic planes along the b-axis.
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Neighboring planes are further interlinked through hydrogen (P1′–O3′–Co1–O4–P2)2, forming a 30-membered ring, and then
bond interactions along the a-axis (such as O3–H3A⋯O12, O7– two nitrogen atoms in the 4,4′-bipy molecule connect with two
H7A⋯O8 and O14–H14C⋯O3 etc.) and b-axis (O9–H9A⋯O5 Co atoms, respectively, which separate the ring into approxi-
and O10–H6A⋯O6), as shown in Fig. 1e and f. The hydrogen mately the same two parts (Fig. 2b). The rings are further con-
bonds are formed between the non-coordinated phophonate nected by (Co1–O1–P1′–O3′)2 leading to a 3D frame structure
oxygen atoms and the coordinated water molecules, resulting along the b-axis with a Co1⋯Co1 separation of 5.7443 Å
in a 3D supramolecular assembly. To clearly understand the (Fig. 2c). As shown in Fig. 2e, neighboring frames are further
network, topological investigations were performed by using interlinked through hydrogen bond interactions along the
the software package TOPOS. The crystallographically indepen- a-axis (such as O8–H8A⋯O5, O5–H5A⋯O7 and O9–
dent metallic centers (Cu2+) with different coordination con- H9B⋯O10). Hydrogen bonds are formed between the non-co-
figurations were taken as network nodes, with the diphospho- ordinated phophonate oxygen atoms and the coordinated
nic acid and 4,4′-bipy being ensured by the bridging ligands, water molecules, resulting in a 3D supramolecular assembly
and the simplified structure is shown in Fig. S2.† The point (Fig. 2d). The simplified topological structure of 2 is shown in
symbol for the network of complex 1 is described as a 3,4-con- Fig. S5.† The point symbol for network of 2 can be described
nected network with the overall point symbol {3·4·5}{32·42·5.6}, as a 3-connected network with overall point symbol {42.6}, in
of which the Cu2+ ions are two-connected nodes.
which the Co(II) is a single node.
Structural description of [Co (C14H14O6.5P2) (4,4′-bipy) 0.5
Physical characterizations
(H2O) 2] ·H2O (2)
Thermal analyses and BET analysis of the samples. TGA
Complex 2 crystallizes in the space group P21/c. The asym- curves were made under flowing N2 for complexes 1 and 2 in
metric unit in 2 contains two Co(II) ions, two [C14H14O6.5P2]2− the temperature range 30–900 °C (Fig. 3a). Complex 1 showed
anions, one 4,4′-bipy molecule, four coordination water mole- thermal stability up to 80 °C with a weight loss of 3.2%, attrib-
cules, and two water molecules in the lattice (Fig. 2a). Each Co uted to the coordination water molecule (calcd. 3.6%). The
atom coordinates with two oxygen atoms from two water mole- second weight loss stage had a decomposition temperature
cules (O7, O8), three oxygen atoms from peroxyphosphonate range of 240–360 °C, with a weight loss of 21.9%, corres-
(O1, O3′, O10), one oxygen atom from phosphonate (O1), and a ponding to the loss of 4,4′-bipyridine (calcd. 19.4%), and the
nitrogen atom from a pyridine molecule (N1), forming a third weight loss stage had a decomposition temperature
slightly distorted monoctahedron coordination geometry range of 400–520 °C, with a weight loss of 55.9%, corres-
(Fig. S3†). The distances of Co–O and Co–N are in the range ponding to the combustion of phosphonate (calcd. 55.2%).
2.036(3)–2.208(4)
Å
and 2.277(4) Å, respectively. The Upon further heating, the final product obtained at 900 °C
[C14H14O6.5P2]2− anion acts as a tetradentate ligand to chelate could be attributed to Cu(PO3)2 (found 39.0%; calcd 44.4%).
three Co atoms, each peroxyphosphonate group chelates two The difference in the weight was due to loss of P2O5 from the
Co(II) sites through O1, O3′ and O10, respectively, while each residual mass.
phosphonate group chelates one Co(II) ion through O4. So the
For complex 2, the first step of weight loss in the range of
coordination mode can be adopted as with μ3–η1η0η1η1η0η0 40–105 °C is consistent with the removal of two coordination
(Fig. S4†). Similar to complex 1, two equivalent Co(II) ions are water molecules and one water molecule in the lattice (found
bridged by two [C14H14O6.5P2]2− anions with the connection of 10.2%; calcd 10.0%). Then it undergoes continuous weight
Fig. 2 (a) Asymmetric unit of complex 2; (b) two-membered Cu-dimer ring formed in complex 2; (c) 3D frame structure along the b-axis; (d) and (e)
hydrogen-bond interactions between the inorganic frames along the b-axis.
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Fig. 3 (a) TGA diagrams of complexes 1 and 2; (b) N2 adsorption–
desorption isotherms of complexes 1 and 2.
losses at the temperature from 290 °C to 560 °C due to the
combustion of organic groups, showing a constant residual
mass of 140.2 (26.0%) up to 900 °C. It is well known that
Co(PO3)2 can convert into Co2P2O7 by eliminating P2O5
(Co(PO3)2 → 1/2 Co2P2O7 + 1/2 P2O5) after heating at high
temperature for a long time.40 The final end product was sup-
posed to be 1/2 Co2P2O7 with a formula weight of 145.9 (27.0%).
The specific surface areas and pore structures of complexes
1 and 2 were studied using nitrogen adsorption–desorption
isotherms. As shown in Fig. 3b, both complexes 1 and 2
display type III isotherms with H3 hysteresis, indicating the
nitrogen has a weak interaction with the materials and the
materials are non-porous.41 The Brunauer–Emmett–Teller
(BET) specific surface areas, pore volumes and adsorption/de-
sorption average pore diameters of complexes 1 and 2 are
listed in Table S3.†
Fig. 4 UV-vis spectrum of complexes 1 (black) and 2 (red); the band
gap of complexes 1 (b) and 2 (c); the valence band XPS pattern of com-
plexes 1 (d) and 2 (e).
lysts had a strong ultraviolet light harvesting capacity for photo-
catalytic reactions. In addition, the band gaps of complexes 1
and 2 were 2.57 eV (Fig. 4b) and 3.23 eV (Fig. 4c), respectively.
Apart from the low band gap, the proper matching of the
valence band (Ev) (vs. NHE) and conduction band (Ec) (vs.
NHE) is also important for evaluating photocatalysis. The Ev
positions of the complexes were determined by linear extrapol-
ation of the leasing edges of VB-XPS spectra to the base lines.
As shown in Fig. 4d and e, the valence band (Ev) positions of 1
and 2 were located at ∼3.15 eV and ∼1.94 eV, respectively. By
combining the band gap result with the valence band (Ev) of
the samples, we can get the conduction band levels (0.58 eV
for 1, −1.29 eV for 2) from (Ev − Eg).
UV-Vis diffuse reflectance spectroscopy (DRS) and X-ray
photoelectron spectra (XPS). The optical properties of the
MOF-based photocatalysts were investigated by DRS and XPS.
The optical band gap energy of the samples were calculated
according to the energy dependence relationship of αhν =
Photocatalytic activities
Photocatalytic reduction of Cr(VI). The photocatalytic
(hν − Eg)1/2, where α, ν, and Eg are the absorption coefficient, reduction of aqueous Cr(VI) over the MOF-based photocatalysts
photon frequency, and energy gap of the semiconductor, was performed under ultraviolet light since they showed
respectively.42 The band gap of the samples could be calcu- broad-range UV light absorption. Fig. 5 shows the photo-
lated from the plot of (αhν)2 versus the energy of the absorbed catalytic activities of complexes 1 and 2 in the reduction of
light. The Eg value is determined by measuring the x-axis inter- aqueous Cr(VI) under different experimental conditions.
cept of an extrapolated tangential line from the linear regime of Taking a view of the overall activities from Fig. 5a and b, it can
the curve. As shown in Fig. 4a, both MOF-based photocatalysts be found that there was no significant reduction of Cr(VI) in
showed strong ultraviolet light absorption in the range of the absence of complexes 1 or 2 in the dark. On the contrary,
200–400 nm (Fig. 4a), which indicated the MOF-based photocata- the Cr(VI) concentration decreased obviously in the presence of
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Thus, methanol was used as a hole scavenger. The temporal
concentration variation of Cr(VI) reduction over different
amounts of methanol is shown in Fig. 5e and f, and the reac-
tions were all performed in acid conditions at the same pH
value of 3.21. The experiments showed that the reductive rate
of Cr(VI) was increased along with the amount of methanol.
For complex 2, Cr(VI) in the photocatalytic reaction could be
fully reduced after 30 min when 150 μL of methanol was
added (Fig. 5f), while complex 1 need about 50 min to com-
plete the reaction under the same conditions (Fig. 5e).
To gain a better understanding of the reaction kinetics of
the reduction of aqueous Cr(VI), the experimental data were
fitted by
a first-order model as expressed by eqn (2)
(Langmuir–Hinshelwood model), due to the fact that the value
of the rate constant k commonly gives an indication of the
activity of a photocatalyst,
ꢀInðC=C0Þ ¼ k ꢁ t
ð2Þ
where C0 and C are the initial and apparent concentration of
the Cr(VI), respectively, and k is the kinetic rate constant. The
values of k were obtained from the slope and the intercept of
the linear plot. Fig. 6a and b show a linear relationship
between ln(C/C0) and the irradiation time for the reduction of
aqueous Cr(VI) on complexes 1 and 2, respectively. As shown in
Fig. 6c, the kinetic rate constants for the photocatalytic
Fig. 5 Control experiments for the reduction of Cr(VI) over complexes 1
(a) and 2 (b) under different conditions; photocatalytic reduction of Cr
(VI) over complexes 1 (c) and 2 (d) at different pH values; photocatalytic
reduction of Cr(VI) complexes 1 (e) and 2 (f) with different methanol
addition.
reduction of Cr(VI) follow the order: complex 2 (0.0801 min−1
,
pH = 3.15) > complex 1 (0.0470 min−1, pH = 3.21). Compared
with other photocatalysts, complexes 1 and 2 have high photo-
catalytic efficiency with small dosage and need only a short
time. A comparison of photoreduction capacity of complexes 1
and 2 with other reported photocatalysts is shown in Table 1.
Furthermore, in the photocatalytic Cr(VI) reduction reaction,
photogenerated electrons in the CB of the MOF photocatalyst
are believed to have a strong reductive ability to attack stable
1 and 2 under ultraviolet light, and the reduction ratio of Cr(VI)
was rapidly increased to 46.7% and 49.4% after 50 min,
respectively. The effect of pH value on the photocatalytic
reduction of Cr(VI) over 1 and 2 was also investigated under
different H2SO4 concentrations. As shown in Fig. 5c and d, the
reduction rate of Cr(VI) was remarkably pH-dependent in all
the photocatalytic systems with 1 and 2, and the reduction pro-
cesses were greatly accelerated by decreasing the pH values
(for 1–2, 66.2%, 100% at about pH = 2.30 after 50 min, respect-
ively). This phenomenon was consistent with other reports
that the photocatalytic reduction of Cr(VI) was an acid-catalyzed
behavior.43,44 Under acid conditions, the predominating
species of chromium is Cr2O72−, and the protons participated
in the reduction reaction may be as follows (eqn (1)):
2−
Cr2O7 under acidic conditions. To investigate the percentage
of effective electrons at the surface of complexes 1 and 2,
AgNO3 was added as an electron scavenger in the system,
where the reactions were carried out under the conditions of
pH = 3.21 and the addition of 150 μL methanol (Fig. 7a). In
contrast to the reaction without AgNO3, the photocatalytic
Cr(VI) reduction efficiencies were decreased by 35.0% (1) and
43.6% (2) within 50 min, and the apparent reaction rate con-
stants of complexes 1 and 2 declined to 0.0135 min−1 and
0.00777 min−1, respectively (Fig. 7b).
14Hþ þ Cr2O72ꢀ þ 6eꢀ ! 2Cr3þ þ 7H2O
ð1Þ
Therefore, the reduction efficiency of Cr(VI) is relatively high
To characterize the main mechanism of Cr(VI) removal in
at low pH because of the existence of abundant protons. In our experiment, the valence state of chromium bound to the
particular, complexes 1 and 2 could retain their highly catalytic MOF-based photocatalysts after catalytic reaction was con-
activities at the pH value of 1.24, which indicated that these firmed by X-ray photoelectron spectroscopy analysis. As shown
two catalysts have good stability in strong acid conditions.
in Fig. 7c and d, there was a significant contribution of the Cr
To further investigate the charge transfer during the photo- bound on the MOF-based photocatalysts. Significant bands of
catalytic reaction, a hole scavenger was introduced in the standard Cr(III) compounds appeared at binding energies of
experiments. As methanol could capture the photoinduced 577–579 and 586–588 eV, with the former corresponding to the
holes in the VB of MOF-based photocatalysts, it is able to Cr 2p3/2 orbital, while the latter corresponded to the Cr 2p1/2
suspend the hole–electron pair recombination, and more elec- orbital. Meanwhile, those of the standard Cr(VI) compound,
trons are thus available to participate in the surface reaction. K2Cr2O7, appeared at binding energies of 579–581 and
528 | Dalton Trans., 2019, 48, 523–534
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Fig. 7 (a) Photocatalytic reduction of Cr(VI) over complexes 1 and 2
with or without electron scavenger of AgNO3 (0.1 mmol); (b) compari-
son of the apparent reaction rate constants for reactions; Cr 2p spec-
trum bound to complexes 1 (c) and 2 (d) after photocatalytic Cr(VI)
reduction.
588–590 eV, respectively.48,49 The XPS result indicated Cr(VI) is
indeed reduced to Cr(III).
The recyclability and stability of complexes 1 and 2 were
tested by performing repeated usage cycles under identical
reaction conditions (pH = 1.24). After each cycling experiment,
the photocatalyst was recycled by filtration, followed by
washing with deionized water and ethanol several times,
respectively, to completely remove the absorbed Cr(III), and
then drying under room temperature. As shown in Fig. 8a and
b, complexes 1 and 2 retained their catalytic activities during
the whole reaction process even after five cycles, with very little
decrease in activity observed. In addition, the structures of the
complexes were recorded by PXRD pattern before and after
cycle reaction. The PXRD patterns revealed that the structures
of the complexes showed no significant changes during the
reaction process, which indicated that complexes 1 (Fig. 8c)
and 2 (Fig. 8d) possessed good structural stability, with no
structural transformation in MOFs occurring during the
photocatalytic Cr(VI) reduction under an extremely acidic
environment.
Fig. 6 Pseudo-first-order kinetics curves of photocatalytic Cr(VI)
reduction reactions over complexes 1 (a) and 2 (b) under different con-
ditions; (c) comparison of the apparent reaction rate constants of Cr(VI)
reduction reactions over complexes 1 and 2.
Photocatalytic degradation of dyes. MB was chosen as a
representative dye to evaluate the capability of the MOF-based
Table 1 Comparison of the Cr(VI) reduction capacity of complexes 1 and 2 with other photocatalysts
Catalyst/mg
Pollutants/mg L−1
Irrigation
Time (min)
Efficiency (%)
Ref.
Complex 1/4
Complex 2/4
Zn(bpy)L/15
AgSCN/200
TiO2 nanocrystals/50
Ni@graphene–ZnO/10
Cr(VI)/30
Cr(VI)/30
Cr(VI)/10
Cr(VI)/10
Cr(VI)/10
Cr(VI)/10
UV
UV
UV
UV
UV
UV
50
30
30
120
120
40
100
100
86a
100
100
99
This work
This work
32
45
46
47
a L = cis-1,3-dibenzyl-2-imidazolidone-4,5-dicarboxylic acid, bpy = 4,4′-bipyridine.
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was observed after 90 min under ultraviolet irradiation,
respectively. Complex 2 had a comparative predominance for
photocatalysis over complex 1, suggesting that the adsorption
capacities for MB of the MOF-based photocatalysts should play
a predominant role in their photocatalytic activities. Because
the photocatalytic reactions are commonly believed to occur
on the surface of the photocatalyst, the larger adsorption
amounts of MB onto the photocatalysts may contribute to the
faster degradation rate of MB.50,51 The values of the BET
specific surface areas and pore volumes of complexes 1 and 2
exhibited the order of 2 > 1 (Table S3†). The rate constants for
MB photodegradation over 1 and 2 under UV light were
0.00778 min−1 and 0.0174 min−1 (Fig. 9b). The satisfactory
photocatalytic efficiencies might result from the relatively slow
recombination of photogenerated electrons and holes, which
is consistent with the studies of Cr(VI) reduction. Compared
with other MOF photocatalysts, complexes 1 and 2 have high
photocatalytic efficiency with small dosage and short time. A
comparison of the photocatalytic performance with reported
photocatalysts in recent years for the degradation of MB is
listed in Table 2.
Fig. 8 The photocatalytic Cr(VI) reduction during five consecutive runs
over complexes 1 (a) and 2 (b); the PXRD patterns for complexes 1 (c)
and 2 (d).
photocatalysts for organic pollutions degradation. As shown in
Fig. 9a, the degradation of MB occurred merely up to 13.5%
after 90 min under UV irradiation in the absence of the MOF-
based photocatalysts. The variation of MB concentration was
8.90% and 20.5% when complexes 1 and 2 were added in the
dark after 60 min, respectively, suggesting that the MB mole-
cule could be adsorbed on the surface of the MOF photocata-
lysts. For 1 and 2, about 55.9% and 84.3% degradation of MB
Mechanism
Based on the experimental results of Cr(VI) reduction and MB
degradation, a plausible mechanism for complexes 1 and 2
mediated photocatalysis is proposed.31 As illustrated in
Fig. 10, upon light irradiation, the photogenerated electron–
hole pairs separate through the ligand to enable a metal
charge transfer (LMCT) process,56 leaving a photoinduced elec-
tron (e−) in the conduction band and a positive-charged hole
(h+) in the valence band of the MOF-based photocatalysts
(eqn (3)).
fMOFsg þ hv ! fMOFsgðhþ þ eꢀÞ
methanol þ hþ ! CO2 þ H2O þ others
ð3Þ
ð4Þ
Inorganic species with a reduction potential more positive
than that of the conduction band of the semiconductor can
consume the electrons and complete the redox reaction
cycle.57 The reduction potential of Cr(VI) to Cr(III) is pH depen-
dent, which is more positive than the position of conduction
band of complexes 1 and 2 (0.58 V and −1.29 V vs. NHE) in
acid conditions, indicating that the photogenerated electrons
in the CB possess a strong reduction ability and can reduce
the adsorbed Cr(VI) to Cr(III) (eqn (1)). Meanwhile, the thermo-
dynamic driving force for Cr(VI) reduction decreases with
increasing pH, showing that the photocatalytic reduction of
Cr(VI) is beneficial at low pH. The photogenerated holes (h+)
could easily react with organic species (methanol) to produce
CO2, H2O, and other minerals, thereby limiting the recombina-
tion between photoinduced electrons and holes to accelerate
the target reaction (eqn (4)). The introduction of a hole scaven-
ger, such as methanol, was proven to be effective for promot-
Fig. 9 (a) Control experiments for the photocatalytic degradation of
MB over complexes 1 and 2; (b) pseudo-first-order kinetics curves of the
photocatalytic degradation of MB over complexes 1 and 2 under UV
light.
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Table 2 Comparison of the MB degradation capacity of complexes 1 and 2 with other photocatalysts
Catalyst/mg
Pollutants/mg L−1
Irrigation
Time (min)
Efficiency (%)
Ref.
Complex 1/5
Complex 2/5
[Ni(bismip)(bpy)(H2O)]·H2O/40
[Cu3(4-bpah)4(1,3,5-btc)2]·8H2O/150
[Cu(3-dpye)(1,3-bdc)]·3H2O/150
ZIF-8/LDO/20
MB/10
MB/10
MB/10
MB/10
MB/10
MB/10
MB/10
UV
UV
UV
UV
UV
UV
UV
90
90
180
240
240
180
30
55.9
84.3
80a
50b
80c
This work
This work
52
53
54
55
32
58d
93e
Zn(bpy)L/15
a Bismip = 5-(1H-benzoimidazol-2-ylsulfanylmethyl)-isophthalic acid, bpy = 2,2′-bipyridine. b 4-Bpah = N,N′-bis(4-pyridinecarboxamide)-1,2-cyclo-
hexane, 1,3,5-H3BTC = 1,3,5-benzenetricarboxylic acid. c 3-Dpye = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane, 1,3-H2BDC = 1,3-benzenedicar-
boxylic acid. d LDO = Zn–Al layered double oxides. e L = cis-1,3-dibenzyl-2-imidazolidone-4,5-dicarboxylic acid, bpy = 4,4′-bipyridine.
•
and tert-butyl alcohol (t-BuOH) was chosen as an OH scaven-
•
ger because it reacts with OH radicals with a high rate con-
stant (k = 6 × 108 M−1 s−1).59,60 For complex 1, the degrading
rate of MB increased to about 95.3% when TEOA (10.0 mM)
was added into the reaction solution after 60 min under UV
light irradiation (Fig. 11a). Compared with the system without
a hole scavenger, the degrading rate of MB increased signifi-
cantly in the system with TEOA. We suspected that the MB was
•
–
not oxidized by the holes but by O2 radical anions, which
were generated by the reaction of e− and O2. The TEOA would
capture photoinduced holes in the VB of complex 1 and more
electrons could escape from the pair recombination, which
could then be captured by O2 to form superoxide radicals
–
(•O2 ) (eqn (6)). To confirm this conjecture, nitrotetrazolium
Fig. 10
A schematic illustration of the energy position and Cr(VI)
reduction over complexes 1 and 2.
blue chloride (NBT) was chosen as an effective superoxide
radical scavenger for the system.61 There was only about a
13.5% degradation of MB observed after 90 min when NBT
(10.0 mM) was added into the reaction solution. Compared
ing photocatalytic Cr(VI) reduction over complexes 1 and 2 in
our experiments.
•
with the system without an OH scavenger, the degrading rate
MB þ hþ ! CO2 þ H2O þ others
ð5Þ
ð6Þ
ð7Þ
ð8Þ
of MB was scarcely increased in the system with t-BuOH
(10.0 mM) (Fig. 11a). These results clearly suggest that the
–
main active component was superoxide radicals (•O2 ) for
•
h
þ þ H2O ! OH þ Hþ
complex 1 system. For complex 2, the addition of tert-butyl
alcohol and triethanolamine led to 69.1% after 90 min and
89.9% after 70 min quenching in the system, respectively, and
NBT could significantly inhibit the MB degradation (Fig. 11b).
•
ꢀ
O2 þ eꢀ ! O2
•
•
ꢀ
MB þ OH þ O2 ! CO2 þ H2O þ others
The photocatalytic mechanism of complexes 1 and 2 in dye All being said, the main reactive species in the system with 2
•
•
–
molecule degradation was similar to those of reported photo- were both OH radicals and O2 , which was consistent with
catalysts.58 The photoexcited holes have a strong oxidation the PL experiments62 (Fig. 11c).
capacity to oxidize MB molecules directly (eqn (5)) or to react
To further confirm the presence of active radicals in the MB
with water molecules to produce hydroxyl radicals (•OH) photocatalytic degradation over 1 and 2, electron spin reso-
(eqn (6)). Meanwhile, the photoexcited electrons in the con- nance (ESR) spin-trap technique was conducted by using 5,5-
duction band could be captured by O2 to form superoxide dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trap reagent
–
radicals (•O2 ) (eqn (7)). Both of the resulting hydroxyl radicals under UV light irradiation. Fig. 12 shows four ESR character-
(•OH) and superoxide radicals (•O2 ) have a strong oxidation istic signals during light irradiation, attributable to the
–
–
•
–
capacity to decompose MB molecules into CO2 and H2O and DMPO-•O2 adducts.63 Therefore, O2 radicals play a key role
others (eqn (8)). The process of photocatalytic reaction is as in the photocatalytic degradation of MB over 1 and 2.
shown in eqn (5)–(8):
In order to determine whether the radicals lead to
In order to explore the insights into the role of the reactive decomposition of the complexes, the removal of metal ions
species during the MB degradation, several trapping experi- from the MOFs after the photoreaction experiments was quan-
ments were performed. Triethanolamine (TEOA) was chosen tified by inductively coupled plasma optical emission spec-
as an effective hole scavenger for the photocatalysis system, trometry (ICP-OES). The concentrations of Cu(II) and Co(II) in
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–
Fig. 12 ESR spectra of complexes 1 (a) and 2 (b) for DMPO-•O2
.
Conclusions
In summary, two metal–organic frameworks based on phospho-
nate ligands were synthesized by a general hydrothermal method
and were then structurally characterized. Both complexes demon-
strated that they were highly efficient heterogeneous photocata-
lysts for aqueous Cr(VI) reduction and for the degradation of MB
under UV light irradiation. The experimental results indicated
that the photocatalytic reduction of Cr(VI) to Cr(III) was highly pH-
dependent, and the addition of a hole scavenger of methanol sig-
nificantly promoted the photocatalytic reaction. The high photo-
catalytic efficiency of the catalysts depends on the phosphorous
ligands with conjugate structure, which enhances the separation
of photogenerated electrons and holes. Moreover, for the degra-
dation of MB, the superoxide radicals and hydroxyl radicals to be
the main reactive species in the system with the complexes. The
results also indicated that the two photocatalysts were stable and
reusable, enabling them to be potential candidates for waste-
water purification. The metal–organic frameworks based on
phosphonate ligands were first applied for the photocatalytic
reduction of Cr(VI) and the degradation of dyes as new photocata-
lysts. It is hoped that our current work could widen the appli-
cation range of MOFs based on phosphonate ligands and
provide new inroads into the exploration and utilization of MOFs
for industrial wastewater treatment.
Fig. 11 Effect of photocatalytic reaction as different trapping agents
were added to the system, (a) and (b) are for 1 and 2, respectively; (c) the
photoluminescence spectra of TAOH formed by the reaction of TA with
•OH radicals generated from different photocatalytic systems under UV-
light for 60 min.
MB aqueous solution after photocatalysis were 0.460 mg L−1
and 1.98 mg L−1, respectively, and the results are listed in
Table S4.† Accordingly, the removal rate of metal ions from
complexes 1 and 2 was 0.360% and 1.80%, respectively. In
addition, the metals and P components of complexes 1 and 2
after photocatalysis were checked by XPS analysis (Fig. S6†). All
of the binding energies (BEs) in this XPS analysis were cor-
rected for specimen charging by referencing them to the C 1s
peak (set at 284.8 eV). For complex 1, the peaks of 2p3/2 (934.4
eV) and 2p1/2 (953.8 eV), and their shake-up satellite peaks at
943.1 and 962.7 eV, corresponding to Cu(II), were observed
(Fig. S7a†).64 For 2, in the Co 2p spectrum of Fig. S7b,† the two
Co 2p3/2 and Co 2p1/2 peaks located at 781.2 and 797.1 eV were
accompanied by two prominent shake-up satellite peaks (786.5
and 803.8 eV), which distinctly verified the dominating pres-
ence of the Co(II).65 All P2p spectra from complexes 1 and 2
revealed a single peak at 133.1 eV, corresponding to P–O-metal
(Fig. S8†).66 All of the test results indicated that the radicals
have little effect on the decomposition of the complexes.
Conflicts of interest
There are no conflicts to declare.
532 | Dalton Trans., 2019, 48, 523–534
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21 I. A. Ibarra, T. W. Hesterberg, B. J. Holliday, V. M. Lynch
Acknowledgements
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