Syntheses, Crystal Structure, Characterization, and Properties of Two Novel Superamolecular Phosphotungstate-Pyridine Complexes

Article abstract of DOI:10.1080/15533174.2014.988248

Two new inorganic-organic compounds constructed from phosphotungstate and pyridines have been synthesized under hydrothermal conditions. The formulas of the two compounds are (C₅H₇N₂)₃(C₅H₆N₂)₃(PW1₂O₄₀)·2H₂O (1) and (C₆H₈N)₃(C₆H₇N)(PW1₂O₄₀)·9H₂O (2). They have been characterized by IR, UV-vis, TG-DTA, FL, CV, and single-crystal X-ray diffraction. The two compounds both show three-dimensional networks. These compounds exhibit good catalytic activity for catalytic oxidation of methanol over a continuous-flow fixed-bed microreactor, when the initial concentration of methanol is 5.37 gsm^-3, and flow rate is 4.51 mLsmin^-1. 0.2 g compound 1 (or compound 2) as the catalyst can eliminate methanol completely at 150°C (175°C for compound 2).

Full text of DOI:10.1080/15533174.2014.988248

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Syntheses, Crystal Structure, Characterization
and Properties of Two Novel Superamolecular
Phosphotungstate-Pyriding Complexes
Yu-Zhe Han, Chuan-Lei Zhang, Qing Cai, Yang Cheng, Si-Xia Ai & Qian Deng
To cite this article: Yu-Zhe Han, Chuan-Lei Zhang, Qing Cai, Yang Cheng, Si-Xia Ai & Qian
Deng (2015): Syntheses, Crystal Structure, Characterization and Properties of Two Novel
Superamolecular Phosphotungstate-Pyriding Complexes, Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry, DOI: 10.1080/15533174.2014.988248
To link to this article: http://dx.doi.org/10.1080/15533174.2014.988248
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Syntheses, Crystal Structure, Characterization and Properties of Two Novel
Superamolecular Phosphotungstate-Pyriding Complexes
Yu-Zhe Hana, Chuan-Lei Zhanga, Qing Caib, Yang Chenga, Si-Xia Aia, Qian Deng a*
a. Hunan University of Science and Technology, College of Chemistry and Chemical
Engineering, Xiangtan, 411201, P. R. China.
b. Chemistry Department, City College of New York, City University of New York, 10016,
USA
*Corresponding author: E-mail: dengqian10@126.com
Abstract: Two new inorganic-organic compounds constructed from phosphotungstate and
pyridines have been synthesized under hydrothermal conditions. The formula of two
compounds are (C5H7N2)3(C5H6N2)3(PW12O40)·2H2O (1), (C6H8N)3(C6H7N)(PW12O40)·9H2O
(2). They have been characterized by IR, UV-vis, TG-DTA, FL, CV and single-crystal X-ray
diffraction. The two compounds are both show three-dimensional network. These compounds
exhibit good catalytic activity for catalytic oxidation of methanol over a continuous-flow
fixed-bed micro-reactor, when the initial concentration of methanol is 5.37 gm-3, and flow
rate is 4.51 mLmin-1. 0.2 g compound 1 (or compound 2) as the catalyst can eliminates
methanol completely at 150 °C (175 °C for compound 2).
Key words: Crystal Structure; Phosphotungstate; Methanol; Catalytic elimination; Pyridine;
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INTRODUCTION
Polyoxometalates (POMs) as a good building block and excellent performance [1,2]
have been attracted an increasing attention for their versatile topological structure [3],
excellent chemical, physical, biologic properties [4-5] and widely applications in catalysis
[6], medicine, biology, electrochemistry and material science et al. Due to the strong acidity
and high oxidizability, polyoxometalates can easily become new multiple structures of
inorganic-organic compounds [7] with transition metals and rare earth metals, which have
strong coordination ability and specific coordination sites.
Yang Guoyu et al.[8] synthetized two novel polyoxometalate frameworks
{[Ni(dap)2(H2O)]2[Ni4(Hdap)2(B--HSiW9O34)2]}·7H2O
and
{[Ni(dap)2(H2O)]2
[Ni(dap)2]2[Ni4(Hdap)2(B--HGeW9O34)2]}·7H2O based on the [Zn2Sb2(B-ZnW9O34)2]14-
building block by hydrothermal. A series of heteropolyacid compounds that conducted on
Keggin [SiMnW12-nO40]m- (M=Mo, V, Nb) and Well-Dawson [P2MnW18-nO62]m- (M=W, Mo, V)
two types synthetized by Xavier Lo´pez [9], which displays good redox abilities due to those
different metals in different frameworks.
The investigations of polyoxometalates, which considered as the acid and oxidized
catalysts have captured the intriguing attention for years [10] ascribing to the discrete
structures of the polyanion. Hisao Hori et al.[11] obtained H2 from decomposing glycerol
solution
by
using
Na21K4[{P2W15Ti3O57.5(OH)3}4Cl]·104H2O
and
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Na18H15[{P2W15Ti3O59(OH)3}4{μ3-Ti-(OH)3}4Cl]·105H2O as the photocatalysts. This is the
first example that the macromolecular polyoxometalates complex applied in the
photochemistry.
Using cesium dodecyl sulfate (CDS) and Keggin-type polyoxometalates, such as
H3PMo12O40, Galia Maayan et al. [12-14] successfully composed a new type of
polyoxometalate nanoparticles. Their catalytic performance of the catalytic oxidation of
sulfur compound into sulfoxide and sulfone has been studied. It is explored that how the size,
shape and structure of polyoxometalate nanoparticles to impact their catalytic properties. The
study showed that the clustered heteropoly compounds have higher catalytic activity than
nonclustered
After that, Itsik Bar-Nahum et al [15] also reported
a
bipyrimidinyl
phosphovanadomolybdate hybrid complex which used as a catalyst supported on silica for
oxidation from methane to methanol and partially further to formaldehyde at the mild,
aerobic (50-60 °C, 1-2 bar O2) in aqueous.
With the increasing awareness of environmental protection more and more people pay
their attention to disposing the environmental pollutants. The catalyst [16, 17] became the
focus of degrading, eliminating and transiting environmental pollutants. Reactivity and
selectivity of catalysts and the problem of secondary pollution spurred the researchers to
explore the environment-friendly catalyst.
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Shi Yingdong et al. [18] synthesized ten organic-inorganic hybrids based on rare earth
metals and [-AsW11O39]9- anion. All the compounds showed some performance of the
photodegradation of RhB, which degradation rate can reach up to 80%.
To reduce pollution, the research of catalytic automotive exhaust gas desulfurization has
become increasingly. Ying An et al [19] prepared a novel molybdophosphate-based ionic
liquid [hmim]3PMo12O40 using as a catalyst to catalytic oxidation desulphurization reactions .
The rate of desulfurization was 100%, when the mole ratio of the hydrogen peroxide and
sulfur is 10 at 50℃.
Methanol is the typical small molecule organic pollutant. The Chinese industrial
standard GB16297-1996 states that the maximum allowable concentration of methanol is
0.22 g·m-3 in workshop atmosphere. Therefore, the elimination of methanol is still significant
in environment protection field.
In this paper, two new types of organic-inorganic hybrid compounds utilizing
2-aminopyridine and 3-methylpyridine as organic ligands were synthesized by the
hydrothermal method. Their crystal structures were resolved. And their thermal stability,
oxidation-reduction characteristics and fluorescence phenomena were also researched.
Methanol, as a representative VOCs was catalytic eliminated over the hybrid compounds to
evaluate the catalytic activity of two compounds. Their application in catalytic elimination of
gas phase organic pollutants has rarely been reported.
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Results and discussion
Description of crystal structures
Crystal structure of Compound 1
Compound 1 consists of one typical Keggin polyoxoanion [PW12O40]3-, three protonated
2-aminopyridine [C5H7N2]+ cations, three [C5H6N2] molecules and two water molecules (Fig
1). [PW12O40]3- anion contains a center PO4 tetrahedron and twelve WO6 octagonal. Three
WO6 octagonal are linked by the edge to form a W3O13 group, while four W3O13 groups and
one PO4 center tetrahedron are linked by angles. There are four types of oxygen atoms in
H3PW12O40; Oa (the central oxygen atoms in PO4 tetrahedron), Ob (the corner-sharing oxygen
between different W3O13 sets), Oc (the edge-sharing oxygen-bridge within W3O13 sets), and
Ot (the terminal oxygen atoms).
The O-P-O bond angles are in the range 109.23(19) °-109.7(18) °, P-O distance ranging
from 1.526(12) Å to 1.543(13) Å. The bond distances of W-Oa, W-Ob/c and W-Ot are from
2.417(3) Å to 2.456(3) Å, 1.835(3) Å to 1.996(3) Å and 1.670(4) Å to 1.684(4) Å
respectively.
O-W-O bond angles are between 71.88(12) ° and 171.63(16) °, which suggest that the
[PW12O40]3- anion is the typical Keggin-type with Td symmetry. The center PO4 presents
tetrahedron coordination, WO6 presents octagonal coordination. The above data imply that
the WO6 octagonal are the distorted octagonal. The bond distances and angles are slightly
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ranging, which suggest that the structure of the compound is relatively stable. The unit cell
consists of four crystallographically independent molecules (Fig 2).
Fig.1 The crystal structure of compound 1
Fig.2 The crystal cell structure of compound 1
Between the compound molecules, there exist many C-H··O and N··H-O intermolecular
hydrogen bonds and intermolecular hydrogen bonds (Fig 3).
In addition to the widely spread hydrogen bonds, there are many intramolecular and
intermolecular π-π stacking, which contains of the offset face-to-face and edge-to-face two
methods in the whole molecules. In offset face-to-face of π-π stacking, the centroid distance
of two aromatic rings (that containing N1 and N10) is 4.019(9) Å. The dihedral angle
between two aromatic rings molecules plane (N1 and N10) is 1.304 °. The centroid distance
of two aromatic rings (that containing N4 and N6) is 3.916(10) Å. The dihedral angle
between two aromatic rings molecules plane (N4 and N6) is 1.451 °. At the same time, in
edge-to-face (such as C7-H7··N8 aromatic ring), the centroid distance is 3.025 Å.
The compounds exhibit an extended three-dimensional supramolecular network via
hydrogen bonging and π-π stacking interactions (Fig 4).
Fig.3 The two-dimensional structure by hydrogen bonds connect in compound 1
Fig.4 The three- dimensional structure by hydrogen bonds connect and π-π stacking
interaction along the a-axis in compound 1
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Crystal structure of Compound 2
The crystal structure of compound 2 consists of a typical [PW12O40]3- anion, three
protonated 3-methylpyridine [C6H8N]+ cations, a [C6H7N] molecule and nine water molecules
(Fig 5). The structure of [PW12O40]3- anion is same of compound 1. The O-P-O bond angle
are in the range of 107.18(19) ° to 110.8(18) °, P-O bond are in the range of 1.550(4) Å to
1.559(3) Å. The bond distances of W-Oa, W-Ob/c and W-Ot are from 2.394(3) Å to 2.447(3)
Å, 1.785(3) Å to 1.943(3) Å and 1.645(4) Å to 1.702(4) Å respectively. O-W-O bond angles
are between 73.96(15) ° and 177.88(15) °. The above data imply that the WO6 octagonals
have slight distorted. The bond distances and angles are slightly ranging 0.007-0.055 Å for
distance or 2.12-6.04 ° for angles, which suggest that the structure of the compound is
relatively stable. The unit cell consists of four crystallographically independent molecules
(Fig 6).
Fig.5 The crystal structure of compound 2
Fig.6 The crystal cell structure of compound 2
There exists intramolecular and intermolecular C-H··O and N··H-O bonds between the
complex’s molecules. Compound shows an extended two-dimensional planar structure via
hydrogen bonds (Fig 7).
The intermolecular and intramolecular π-π stacking also exists in the compound 2. The
centroid distance of two aromatic rings (that containing N2 and N4) is 3.847(5) Å. The
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dihedral angle between two aromatic rings molecules plane (N2 and N4) is 2.054 ° in offset
face-to-face of π-π stacking. The shortest centroid distance between two aromatic rings plane
(contain N3 and N4) is 3.796(5) Å, the dihedral angle between two aromatic rings plane (N3
and N4) is 1.975 °.
The interaction of those three aromatic rings makes a great contribution to the stable of
whole crystal structure. The extensively existence of supermolecule affect for example, the
shortest centroid distance of two rings molecules plane is 2.847 Å, and the existence of
edge-to-face intramolecular between O3-H3A··N1-armatic ring all contribute to an extended
three-dimensional supramolecular network (Fig 8).
Fig.7 The two-dimensional structure of compound 2
Fig.8 Three-dimensional structure of compound 2
IR Spectra
The IR spectra of organic ligands and compounds were carried out in order to obtain the
generated information of the compounds. 2-aminopyridine and compound 1 infrared
spectrum are shown in figure 9 and 10, respectively. In figure 10, the peaks at 3347-3216
cm-1 turned out to N-H band in pyridine and the O-H band in the absorbed planar water of
pyridine macular. The IR spectrums (Fig 9) exhibit a complex pattern of bands at 1627 cm-1,
1568 cm-1, 1488 cm-1 and 1443 cm-1 which are the stretching vibration of 2-aminopyridine
molecule framework. In the IR spectra (Fig 10), characteristic peak at 3324-3156 cm-1 is
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contributed to (N-H) stretching vibration frequency in aminopyridine and (O-H) stretching
vibration frequency in crystallographic water of compound 1 respectively. Bonds at 1652
cm-1, 1612 cm-1, 1551 cm-1, 1483 cm-1 in compound 1 are contributed to the characteristic
peaks of 2-aminopyridine, which suggested the existence of 2-aminopyridine in 1. Four
characteristic vibration sharp peaks resulting from the Keggin-type polyanions, namely,
Vas(P-Oa), Vas(W-Ot), Vas(W-Ob) and Vas(W-Oc), appear at 1004 cm-1, 912 cm-1, 840 cm-1 and
751 cm-1 for compound 1. In comparison with the IR spectra of the precursor phosphotungstic
acid, which are 1079 cm-1, 983 cm-1, 890 cm-1 and 800 cm-1, respectively, the vibration
frequency for compound 1 has an apparent red shift. It is may be that the polyanion involved
in the coordination so that increased the P-O and W-O bond length, reducing the P-O and
W-O bond force constant. These results suggest that the 2-aminopyridine ligands are
coordinated to the metal ion by means of the oxygen atoms.
Fig.9 IR spectrum of 2-aminopyridine
Fig.10 IR spectrum of compound 1
The IR spectrum exhibits some vibration bands of the 3-methypyridyl (Fig 11). The
strong peak at 3438-3306 cm-1 can be ascribed the N-H vibration of the pyridine and the O-H
stretching vibration of 3-methylpyridine ring. The peak at 3032.14 cm-1 can be ascribed the
C-H vibration of the pyridine and the O-H stretching vibration of 3-methylpyridine molecule.
The peak at 1634.68, 1578.64, 1480.18, 1414.71 cm-1 are the vibrations of 3-methylpyridine
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aromatic ring skeleton. The infrared spectra of compound 2 exhibit characteristic peaks of the
3-methylpyridine (Fig 12). The peak at 3540-3222 cm-1 ascribed to the N-H vibration of the
pyridine ring and the stretching vibration of O-H in crystallographic water. Peak at 2945.36
cm-1 contributed to the C-H stretching vibration of the 3-methypyridyl molecular. Bands at
1628.34 cm-1, 1544.12 cm-1, 1466.45 cm-1, 1368.88 cm-1 in compound 2 are contributed to
3-methylpyridine molecular skeleton vibrations. In comparison with IR spectra of the free
3-methylpyridine, the C-C vibration frequency for 2 has a few shifts. All these results suggest
that the polyanions have certain impact to 3-methylpyridine. Bands at 1058.44cm-1, 945.76
cm-1, 857.62 cm-1, 776.38 cm-1 in compound 2 belong to the [PW12O40]3- anion vibrations to
consistent with typical Keggin-type polyanion characteristic vibrations, which declare that
the complex remains stay Keggin structure. These results are consistent with the crystal
structure analysis.
Fig.11 IR spectrum of 3-methylpyridine
Fig.12 IR spectrum of compound 2
Thermogravimetric analysis
In order to cognize the thermal stability of compounds, the TG-DTA analysis has been
tested. The TG-DTA analysis of compounds 1 and 2 were performed in the temperature range
of room temperature to 700 °C (Fig. 13, 14). There are exothermic peaks for both two
compounds accompanied weightlessness.
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There are three obvious exothermic peaks for 1 (309, 394 and 560 °C), three exothermic
peaks for 2 (297, 323, and 569 °C). TG analysis of 1 shows a 18.21% weight loss before
469.9 °C (17.65% for 2 before 536.9 °C) which correspond to the release of crystal water and
organic ligand molecules in accordance with the calculated value (17.33% for 1 and 15.73%
for 2). The peak at 560.4 °C accompanied 5.84% weight loss for 1 and at 569.8 °C
accompanied 6.34% weight loss for 2 may be due to the thermal decomposition of the
polyoxoanions framework and the sublimation of P2O5. Above 560 °C , the Keggin structure
of the polyoxoanions is completely destroyed.
The final residue is WO3 proved by the IR spectra. The whole process shows an overall
25.15% weight loss for 1 (26.55% for 2), which agrees with the calculated value of 23.22%
(25.35% for 2), which consistent with the theoretical weight loss rate.
Fig.13 TG-DTA curve of compound 1
Fig.14 TG-DTA curve of compound 2
TG-DTA analysis showed the two compounds both have the higher decomposition
temperature, which can meet the need of the catalyst. This result also provides a gist for
setting temperature of the catalytic reaction to ensure that the highest temperature of the
catalytic reaction is lower than the compounds decompose temperature.
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UV-Vis spectra
The UV spectra (Fig 15) of compound 1, 2-aminopyridine and phosphotungstic acid
were measured in the aqueous solution at room temperature in the region of 360-200 nm. The
spectra of 2-aminopyridine appear absorption peaks in 229 nm and 290 nm, which
corresponds to annular conjugate system π-π* transition absorption. The spectra of
phosphotungstic acid appear absorption peaks in 207 nm and 257 nm, which corresponding to
the Ot→W LMCT (ligand-to-metal charge-transfer) and Ob/c→W LMCT, respectively. The
spectra of compound 1 show an absorption band at 204 nm, 228 nm, 250 nm and 294 nm. The
peak at 204 nm corresponds to Ot→W charge transfer transition absorption [20]. The peak at
250 nm is the absorption of Ob,c→W transfer transition. The bands at 228 nm and 294 nm in
compound 1 belong to π-π* transition absorption of 2-aminopyridine molecular [21].
Fig.15 UV spectroscopy of compound 1
Fig.16 UV spectroscopy of compound 2
The UV spectra of compound 2, 2-aminopyridine and phosphotungstic acid were
measured (Fig 16). The spectra of 3-methylpyridine appear absorption peaks in 202 nm and
263 nm, which corresponds to annular conjugate system π-π* transition absorption. The
UV-Vis spectrum of compound 2 shows Ot→W and Ob/Oc→W charge-transfer peaks at 210
nm and 267 nm respectively which are characteristic absorption of Keggin-type polyanion.
The absorption peak for compound 2 is red-shifted which attributed to the annular conjugate
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system of 3-methypyridine. These indicate the target compounds have generated.
Fluorescence properties
The fluorescent spectra of compounds and phosphotungstic acid solid are depicted upon
excitation at 229 nm or 220 nm respectively at room temperature in Figure 17. The maximum
emission peak of 1 is located at 509 nm (2 at 551 nm, and phosphotungstic acid at 467 nm).
3
The broad-band emission induced by T1u→1A1g transition derived from the excitation into
O→W LMCT band of phosphotungstate anion. Compound 1 has a strong and wide peak,
which means the fluorescence intensity increases greatly. The maximum emission band of 2
is red-shifted in comparison with phosphotungstic acid.
Fig.17 Emission spectrum for compounds in solid state
Cyclic Voltammetry
The Cyclic Voltammetry experiments of phosphotungstic acid, compounds 1 and 2 were
tested to explore the redox properties of compounds (Fig 18). The Cyclic Voltammetry
measured in 1.0 M sulfuric acid aqueous solution using methyl alcohol to dissolve
compounds at room temperature. The experiment uses a three-electrode system. One glassy
carbon electrode was used as the working electrode and one platinum electrode was used as
the indicator electrode. The calomel electrode was used as the reference electrode. The scan
rate was 20 mV s-1and potential range was from -0.2 to +1.2 V [22].
There is one pair of redox peak at 0.482/0.582 V for compound 1. The mean peak
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potentials E1/2=((Epa+Epc)/2) was 0.532 V, which corresponding to one single-electron redox
process of tungsten atom. The ΔEp=Epa-Epc=57/n-67/n mV was 100 mV(>57/n mV), which
indicate this process is irreversible. Compared with the cyclic voltammograms of
phosphotungstic acid the peak potential of compound 1 significantly increased, which
indicate that the oxidability of compound 1 enhanced.
Fig.18 CV cures of H3PW12O40 and compounds
There was one pair redox peak at 0.264/0.467 V for compound 2. The potentials
E1/2=((Epa+Epc)/2) was 0.36V. The ΔEp= Epa-Epc=57/n-67/n mV was 203 mV(>57/n mV)
which indicate an irreversible process.
Comparing with the cyclic voltammograms of phosphotungstic acid, it was found that
oxidation peaks value of two compounds increased, explaining oxidation of compounds
enhanced. The data showed that the compounds had a good redox properties, which provided
theoretical basis for the complexes used as catalysts.
Catalytic properties
Blank experiments showed that the methanol was hardly eliminated without compounds
as the catalyst. From the results of the effluent gases after catalytic reaction, there is no new
peak in the GC diagram, indicating no new organic compounds formed; lime saturated water
formed precipitate and the color of the 0.2% PdCl2 solution did not change, indicating that
there was CO2 but no CO. Products of the methanol elimination were CO2 and H2O, without
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CO or other by-products, which proved that methanol has been thoroughly mineralized.
Effect of the reaction temperature on the elimination rate of methanol showed in figure
16. With the rise of temperature from 25 to 150 °C the catalytic activity of compounds 1 and
2 improved successively.
The elimination rate of methanol is 100% (for compound 1) and 94% (for compound 2)
when the reaction temperature is 150 °C, flow velocity of 4.51 mL·min-1, and the
concentration of methanol of 5.37 g·m-3. For compound 2, the elimination rate of methanol
reaches 100% when the reaction temperature is 175 °C, the catalytic abilities exhibit tiny
differences at different reaction temperature. Compound 2 shows weaker catalytic ability than
compound 1, which may contribute to that the compound 1 have higher oxidation than
compound 2, according the potentials E1/2=0.40V for compound 1 and 0.36V for compound 2.
It is may be that there are more hydrogen bonds and stronger π-π stacking in crystal texture of
the compound 2 than of compound 1, so compound 2 is more stable, more difficult to charge
transfer than compound 1.
Using 0.2 g the grinded phosphotungstic acid as catalyst, the control experiment was
done at the same test condition. The elimination rate of methanol is 50.87% when the reaction
temperature is 30 °C (Fig 19). But with the temperature increasing the elimination rate of
methanol drop rapidly to 0 at 100 °C, catalyst quickly inactivated. Moreover catalytic activity
of the complexes is greatly increased.
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Pure phosphotungstic acid as the catalyst has the low activity, poor stability and easy
inactivation characters. The catalytic activity of complexes is highly improved by
coordinating with 2-aminopyridine or 3-methylpyridine. The stability is also greatly
improved, which owe to the supramolecular network structure of the complexes. In the
meantime, organic ligands affinity for methanol also increased. It will be easier for methanol
molecular adsorbing on catalysts surface. So the elimination rate is increased.
Fig.19 Elimination rate of methanol for compounds
EXPERIMENTAL
Materials and methods
All chemicals were purchased from commercial sources and used without further
purification. Infrared spectra were recorded as KBr pellets on a Perkin-Elmer FTIR-2000
spectrometer. TG analyses were carried out on a WCT-1D simultaneous thermal analyzer in
air atmosphere between room temperature and 800 °C at a heating rate of 10 °C·min-1.
UV-Vis spectra were measured on a TU-1810 spectrophotometer. Emission/Excitation spectra
were obtained on a Hitachi F4500 fluorescence spectrophotometer. CV spectra were tested on
Electrochemical workstation CH1760C of Shanghai Chen-Hua Instrument with the scan rate
of 20 mV·s-1. The products of the catalytic reaction were analyzed on-line by a GC-9800 gas
chromatograph (GC) equipped with a column packed with Porapak-Q and using a hydrogen
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flame ionization detector.
Synthesis
Synthesis of Compound 1
A mixture of Na3PO412H2O (0.0760 g, 0.20 mmol), Na2WO4·2H2O (0.6600 g, 2.0 mmol)
and NaVO32H2O (0.0630 g, 0.40 mmol) was stirring dissolved in 15 mL distilled water at
80 °C. The pH of the mixture was adjusted to about 2.0 with 1:1 (v:v) H2SO4. The bright red
solution was obtained after stirring for 30min and cooled to the room temperature. The
Keggin-type phosphotungstic was formed. After added CuSO45H2O (0.0500 g, 0.20 mmol)
and 2-aminopyridine (0.0188 g, 0.20 mmol) to the above solution, this mixture was transferred
into a 25 mL Teflon-lined stainless autoclave and filled 68% of its total volume. The
autoclave was sealed and maintained at 160 °C for 120 h and then was cooled to room
temperature with cooling rate of 10 °Ch-1. Finally, the red-brown block crystals were
collected by filtration, washed with distilled water, and dried in air to give a yield of 53%
based on the initial Na2WO4·2H2O.
Synthesis of Compound 2
The synthesis method is similar to Compound 1 except using CoSO47H2O (0.0560 g,
0.20 mmol) and 3-methylpyridine (0.0190 g, 0.20 mmol) instead of CuSO45H2O and
2-aminopyridine respectively at 180 °C for 120 h. Brown block crystals were filtered and
washed with distilled water. Yield: 46% (based on Na2WO4·2H2O).
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X-ray crystallography
The crystal structures of the compounds 1 and 2 were tested. The size is 0.50 mm × 0.42
mm × 0.32 mm for compound 1 and 0.45 mm × 0.41 mm × 0.30 mm for compound 2. The
reflection intensities of the crystals were collected at 293(2) K using a Bruker Smart Apex II
CCD area detector single-crystal diffractometer with graphite-monochromatized M-K
(=0.71073 Å) radiation by the -scan method. The absorption corrections were based on
multiple and symmetry equivalent reflections in the data set using the SADABS program [23].
The structure was solved by direct methods using SHELXS-97 [24] and refined by
full-matrix least-squares on F2 using the Shelxtl-97 program package [23]. All non-hydrogen
atoms were finally refined with anisotropic displacement parameters. All hydrogen atoms of
the crystal water were added by different Fourier methods while other hydrogen atoms were
finally refined as a riding mode using the default Shelxtl parameters.
The detailed crystallographic data and structure refinement parameters for Compounds 1
and 2 are summarized in Table 1. Selected bond angles and lengths of Compounds 1 and 2 are
listed in Table 2 and 3. The partial hydrogen-bonding data of two crystals are listed in Tab 4
and 5 respectively. Crystallographic data for the structures reported in this paper have been
deposited in the Cambridge Crystallographic Data Center. CCDC reference numbers are
999909 for 1 and 999908 for 2.
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Catalytic Reaction
The catalytic elimination of methanol was used as a model reaction to evaluate the
catalytic performances of compound 1 and 2. Catalytic reactions were carried out in a
continuous flow fixed-bed micro-reactor. 0.20 g of compound 1 (or compound 2) was loaded
into the catalytic reaction tube (φ=8 mm; L=200 mm) as catalyst.
The simulacrum of polluted air containing reaction substrate was prepared by bubbling
clean air into a container filled with methanol solution, and diluting the methanol gas with
clean dry air. The initial concentration of reaction substrate was controlled by adjusting the
flow of bubbling gas or dilution air. The feed gas flow through the reactor and catalytic
oxidation reaction occurred at different temperature, which controlled by a thermostat.
The concentration of feed gas was detected on-line by gas chromatographic (GC102-N,
FID detector) about every 10 minutes for a sample analysis until the concentration unchanged,
which means the reaction was in balance. The concentrations of organic compounds of the
effluent gases were analyzed on-line by a gas chromatograph. The inorganic products from
the effluent gas was monitored by a 0.2 (wt%) PdCl2 solution and a saturated lime-water
solution. Blank experiments were carried out without the compound 1 or 2.
In this research, the elimination rate X% of methanol was adopted to evaluate the
catalytic activity of the catalysts, which was calculated by the following function:
C0 C
X % 
100%
C0
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where C0 is the initial concentration of methanol, and C is the residual concentration of
methanol after reaction.
CONCLUSIONS
In this paper, two new organic-inorganic hybrids based on POM and aminopyridine or
methylpyridine ligands were successfully synthesized under hydrothermal conditions.
Compound 1 and 2 consists of one typical Keggin polyoxoanion [PW12O40]3-,
2-aminopyridine or 3- methylpyridine. Among these structures, there are many hydrogen
bonds and π-π stacking interactions forming the 3D supramolecular structure. Compound 1
shows cyan fluorescent, but 2 shows kelly fluorescent. Both compounds have strong redox to
catalytic eliminate methanol. When the reaction temperature is 150 °C for 1 (175 °C for 2),
flow velocity of 4.51 mL·min-1 and the concentration of methanol of 5.37 g·m-3, the
elimination rate of methanol is 100% by using only 0.2 g sample. Two compounds both have
a higher catalytic activity than maternal phosphotungstic acid, which may be attributed to
form the 3D supramolecular structure to make it structurally more stable.
ACKNOWLEDGEMENTS
The Project was supported by Special and Key Laboratory of Functional Materials and
Resource chemistry of Guizhou Provincial Education Department (No. GAFMRC201302)
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Key Laboratory of Theoretical Organic Chemistry and Functional Molecules of Ministry of
Education and the Innovation Foundation of Graduate Student of Hunan University of
Science and Technology (No. S130029).
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Table. 1 Crystal data and refinement details for compounds
Parameters
Compound 1
Compound 2
Empirical formula
Formula weight
Description
Temperature
Crystal system
Space group
a(Å)
C30H43N12O42PW12
3480.08
Red brown
293(2)
C24H49N4O49PW12
3414.82
Dark brown
293(2)
Monoclinic
P2(1)/n
13.3371(7)
22.5278(12)
21.8442(11)
90
Monoclinic
P21/c
12.0380(9)
20.1622(15)
26.2208(18)
90
b(Å)
c(Å)
 (°)
115.880(3)
90
110.421(3)
90
 (°)
 (°)
Volume/ Å3
5905.0(5)
4
5964.2(8)
4
Z
Dc/(gcm-3)
F(0 0 0)
3.824
3.787
5936
6008
Absorption coeff
 Range(°)
Crystal size/mm
Total data
Unique data
Parameters
Rint
23.405
23.177
1.70-25.00
0.50×0.42×0.32
33798
1.31-25.00
0.45×0.41×0.30
40319
10288
9916
773
425
0.1115
0.1308
R1[I>2(I)]a
wR2[I>2(I)]b
GOOF(S)
0.0648
0.0706
0.1682
0.1481
1.053
1.080
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Table. 2 Selected bond lengths [Å] and angles [°] for compound 1
P1-O8
P1-O31
W3-O1
W3-O14
1.526(15)
1.526(12)
1.932(15)
1.689(15)
2.434(12)
109.5(7)
109.1(7)
109.1(8)
86.4(6)
156.0(6)
86.0(6)
91.0(6)
154.9(6)
172.1(6)
85.1(5)
P1-O30
P1-O33
W3-O3
W3-O29
1.543(13)
1.535(13)
1.901(14)
1.878(15)
1.922(14)
109.7(8)
109.9(7)
109.5(7)
102.3(7)
71.3(5)
102.9(7)
72.5(5)
101.6(7)
102.0(7)
86.4(6)
W3-O31
W3-O32
O8-P1-O30
O8-P1-O33
O30-P1 - O33
O1-W3-O3
O1-W3-O29
O1-W3-O32
O3-W3- 29
O3-W3 - O32
O14-W3-O31
O29-W3-O31
O31-W3-O32
W3-O31-W7
W3-O31-P1
W7-O31-P1
W3-O3-W7
W3-O1-W8
O8-P1-O31
O30-P1-O31
O31-P1-O33
O1-W3-O14
O1-W3-O31
O3-W3-O14
O3-W3-O31
O14-W3-O29
O14-W3-O32
O29-W3-O32
82.4(5)
88.9(4)
124.9(7)
126.4(7)
125.1(8)
126.1(7)
W3-O31-W8
W7-O31-W8
W8-O31-P1
W7-O19-W8
89.4(4)
89.3(4)
126.2(8)
123.9(7)
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Table. 3 Selected bond lengths [Å] and angles [°] for compound 2
P1-O1
P1-O3
W3-O3
W3-O14
1.496(15) P1-O2
1.510(13) P1-O4
2.435(13) W3-O6
1.916(15) W3-O17
1.869(13) W3-O32
1.535(14)
1.489(15)
1.674(16)
1.867(14)
1.896(14)
109.9(8)
110.2(8)
109.8(8)
72.5(5)
W3-O27
O1-P1-O2
O1-P1-O4
O2-P1-O4
O3-W3-O6
O3-W3-O17
O3-W3-O32
O6-W3-O17
O6-W3-O32
O14-W3-O27
O17-W3-O27
O27-W3-O32
W3-O3-W8
W3-O3-P1
W8-O3-P1
W3-O32-W8
W5-O1-P1
110.2(8)
106.3(8)
110.3(8)
170.5(7)
82.4(5)
73.4(5)
104.1(7)
99.6(7)
O1-P1-O3
O2-P1-O3
O3-P1-O4
O3-W3-O14
O3-W3-O27
O6-W3-O14
O6-W3-O27
O14-W3-O17
O14-W3-O32
O17-W3-O32
84.9(5)
101.2(7)
102.5(7)
154.7(6)
87.5(6)
89.4(6)
84.7(6)
88.8(6)
158.0(6)
88.9(4)
124.7(8)
126.4(7)
124.9(7)
126.8(7)
W3-O3-W10
W8-O3-W10
W10 - O3 - P1
W8-O33-W10
W3-O14-W10
89.1(4)
89.0(4)
126.8(8)
125.4(7)
125.4(7)
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Table 4 Hydrogen-bonding geometry (Å deg.) for Compound 1
Donor - H··Acceptor Symmetry
operations
D-H
H··A
D··A
D-H··A
N(2)-H(3)··O(15)
N(2)-H(4)··O(28)
N(2)-H(4)··O(31)
N(1)-H(4D)··O(22)
N(3)-H(5)··O(19)
N(3)-H(6)··O(7)
N(3)-H(6)··O(16)
N(3)-H(6)··O(21)
C(4)-H(2B)··O(17)
C(3)-H(3A)··O(26)
C(5)-H(5A)··O(9)
C(6)-H(6B)··O(29)
4#
3#
2#
2#
0.90
0.90
0.90
0.86
0.90
0.90
0.90
0.90
0.96
0.96
0.96
0.96
1.96
2.47
2.13
2.52
2.08
2.26
2.52
2.59
2.57
2.58
2.57
2.53
2.856(7)
3.042(7)
2.964(6)
2.862(11)
2.882(10)
3.127(11)
3.018(10)
3.059(10)
3.271(9)
3.253(9)
3.062(13)
3.217(10)
169
125
146'
107
152
176
114'
114"
130
132
113
130
1#
1#
1#
4#
4#
2#
5#
Symmetry operation: 1# = x, 1/2 − y, z; 2# = − 1/2 + x, 1/2 − y, 3/2 − z; 3#= −1/2 + x, y, 3/2
− z; 4# = 1/2 − x, −y, 1/2 + z; 5# = 1/2 − x, − y, −1/2 + z.
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