Polyoxometalate-based metal-organic coordination networks for heterogeneous catalytic desulfurization

Article abstract of DOI:10.1039/c5ce01533b

Three new polyoxometalate (POM)-based metal-organic coordination networks (MOCNs) with chemical formulae of [Co(BBTZ)_1.5(HBBTZ)(H₂O)₂][PW₁₂O₄₀]·H₂O (1), [Co_2.5(BBTZ)₄(H₂O)₂] [BW₁₂O₄₀]·4H₂O (2) and [Cu(BBTZ)₂]₅[BW₁₂O₄₀]₂·4H₂O (3) (BBTZ = 1,4-bis-(1,2,4-triazol-1-ylmethyl)benzene) were hydrothermally synthesized in a reaction system containing Keggin-type POMs, transition metal salts (cobalt salts and copper salts) and BBTZ ligands. All compounds were characterized by elemental analyses, IR, powder X-ray diffraction, TG analyses and single-crystal X-ray diffraction analyses. Compound 1 exhibits a POM-encapsulated 3-D supramolecular network, while compounds 2 and 3 display POM-supported 3-D coordination networks. Using the oxidative desulfurization of dibenzothiophene (DBT) as the model, the catalytic activities of compounds 1-3 are investigated. All three compounds show efficient catalytic activity for the oxidation of DBT with the order of 2 > 3 > 1. It is found that the POM species of compounds 1-3 play the main role in the catalytic oxidative desulfurization process, while the TM ions, the loading amounts of POMs, and the structural features of these POM-based MOCNs are also necessary factors that affect the catalytic activities. Furthermore, a surfactant-assisted hydrothermal synthesis method has been developed to prepare nanocrystal 2. SEM reveals that the as-synthesized nanocrystalline 2 is about 245 nm in diameter. The catalytic oxidative desulfurization experiments show that nanocrystal 2 possesses much higher catalytic activities than those of the large single-crystal products of 2.

Full text of DOI:10.1039/c5ce01533b

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ARTICLE
Polyoxometalate-Based Metal-Organic Coordination
Networks for Heterogeneous Catalytic
Desulfurization
Cite this: DOI: 10.1039/x0xx00000x
YuanꢀYuan Ma,a HuaꢀQiao Tan,*a YongꢀHui Wang,a XiuꢀLi Hao,*ab XiaoꢀJia Feng, ac
HongꢀYing Zang,a and YangꢀGuang Li*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Three new polyoxometalate (POM)ꢀbased metalꢀorganic coordination networks (MOCNs) with the
chemical formula of [Co(BBTZ)1.5(HBBTZ)(H2O)2][PW12O40]·H2O
[BW12O40]·4H2O and [Cu(BBTZ)2]5[BW12O40]2·4H2O (BBTZ
(
1
), [Co2.5(BBTZ)4(H2O)2]
www.rsc.org/
(
2
)
(
3
)
= 1,4ꢀbisꢀ(1,2,4ꢀtriazolꢀ1ꢀ
ylmethyl)benzene) were hydrothermally synthesized in the reaction system containing Kegginꢀtype POMs,
transition metal salts (cobalt salts and copper salts) and BBTZ ligands. All compounds were characterized
by elemental analyses, IR spectra, powder Xꢀray diffraction, TG analyses and singleꢀcrystal Xꢀray
diffraction analyses. Compound
1 exhibits a POMꢀencapsulated 3ꢀD supramolecular network, while
compound and display the POMꢀsupported 3ꢀD coordination networks. Using oxidative
2
3
desulfurization of dibenzothiophene (DBT) as the model, the catalytic activities of compounds 1-3 are
investigated. All three compounds show efficient catalytic activity for oxidation of DBT with the order of
2
> 3 > 1. It is found that the POM species of compounds 1-3 play the main role in the catalytic oxidation
desulfurization process, while the TM ions, the loading amount of POMs, and structural features of these
POMꢀbased MOCNs are also necessary factors to affect the catalytic activities. Furthermore, a surfactantꢀ
assisted hydrothermal synthesis method has been developed to prepare nanocrystal 2. SEM reveals that the
asꢀsynthesized nanocrystallines
2 are about 245 nm. The catalytic oxidation desulfurization experiments
show that nanocrystal
products of
2
possesses much higher catalytic activities than those of large single crystal
2
.
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Introduction
units is an effective way to maximize the loading amount
of POMs. Following this strategy, we have constructed
many POMꢀbased cationic MOCNs by the hydrothermal
reaction of polyoxoanions, transition metal (TM) cations
and various neutral Nꢀdonor bridging ligands.13 Although
The desulfurization of fossil fuels is a currently significant
task, which is directly associated with the living
environment of human beings.1 In this research field,
oxidative desulfurization has been considered as an
effective strategy to remove refractory organosulfur
substrates. To date, a key factor in this aspect is the
exploration of new oxidative catalyst systems.2
Polyoxomatalates (POMs), as a unique metalꢀoxo clusters
of early transition metals (such as Mo, W, V, Nb and Ta),
have been proved one kind of most promising catalysts for
desulfurization,3 because of their excellent controllability in
composition, structure, charge and redox properties at the
molecular level. However, POMs sometimes show good
solubility in the reaction system, and act as homogeneous
catalysts with a poor recovery.4 Furthermore, some POMs
may suffer from a hydrolyzation in different pH of solution,
which must to be surmounted in the homogeneous system.5
Moreover, the low specific surface area and the relatively
low loading rate of POMs as heterogeneous catalysts also
inhibit their catalytic applications.6 Therefore, developing a
new type of POMsꢀbased catalytic materials with good
stability, easyꢀrecycling, high efficient catalytic activity is
one of the currently challenging issues.
It is well known that metalꢀorganic coordination networks
(MOCNs) usually possess a good stability and relatively
low solubility in solutions.7 The introduction of POMs into
MOCNs usually can not only load and disperse POM units
into MOCNs at the molecular level, but also keep the redox
activity of POM species well. Based on such consideration,
a series of POMꢀbased MOCNs have been constructed as
the heterogeneous catalysts.8 In these compounds, a few of
them show good catalytic activities due to their porous
structural features, which allow for the contact between
reactants and the catalytic active sites (POMs).9 In
comparison with these limited porous compounds,
numerous nonꢀporous POMꢀbased MOCNs have still been
far unexplored as heterogeneous catalysts.10 Thus, it is
significant to develop new strategies to improve the
catalytic activities of these nonꢀporous hybrid compounds
so as to explore new catalysts for oxidative desulfurization.
9b,13d
the loading amount of POMs in our previous work
have been significantly improved (about 71 %), their
catalytic activities for oxidation desulfurization are still
relatively low. The factors that affect the catalytic activity
of these catalysts remain unexplored.
Herein, we report three new POMsꢀbased cationic MOCN
compounds:
[Co(BBTZ)1.5(HBBTZ)(H2O)2][PW12O40]·H2O
[Co2.5(BBTZ)4(H2O)2][BW12O40]·4H2O
[Cu(BBTZ)2]5 [BW12O40]2·4H2O ( ) (BBTZ = 1,4ꢀbisꢀ
(
1
),
(2)
and
3
(1,2,4ꢀtriazolꢀ1ꢀylmethyl) benzene). Using oxidative
desulfurization of dibenzothiophene (DBT) as the model,
we confirm that all three compounds exhibit effective
catalytic activity for oxidation of DBT with the order of
2 >
3
> 1
. The effects of POMs, transition metal cations, the
loading amount of POMs, and the structural features on the
catalytic activities of compounds 1-3 have been
investigated.
hydrothermal synthesis method is developed for the
preparation of nanocrystalline compound with high
specific surface area. SEM reveals that the asꢀsynthesized
nanocrystallines are about 245 nm, which exhibit much
higher catalytic activities than those of large single crystal
products of
Furthermore,
a
surfactantꢀassisted
2
2
2
.
Experimental section
Materials and methods All reagents for syntheses were
commercially purchased and used without further
purification.
The
ligand
1,4ꢀbisꢀ(triazoleꢀ1ꢀ
ylmethyl)benzene (BBTZ) was prepared according to the
reported procedure.14 H3PW12O40·n H2O and K5BW12O40·n
H2O were prepared based on the literature methods.15
Elemental analyses (C, H and N) were conducted on a
PerkinꢀElmer 2400 CHN elemental analyzer. W, Co and
Cu were determined by a Leaman inductively coupled
plasma (ICP) spectrometer. The IR spectra were recorded
in the range of 4000ꢀ400 cmꢀ1 on an Alpha Centaurt FT/IR
spectrophotometer with pressed KBr pellets. The TG
analyses were carried out under air flow by using a Pyris
Diamond TG instrument at a heating rate of 10 °C minꢀ1.
The powder Xꢀray diffraction (PXRD) patterns were
collected by using
a
Rigaku D/ maxꢀIIB Xꢀray
diffractometer with CuꢀKα (λ = 1.5418 Å) radiation in the
2θ range of 5° to 50° at a scanning rate of 1° per minute.
GCꢀMS spectra were obtained on an agilent 5875ꢀ6890N.
The HPLC analyses were performed on a shimadazu LCꢀ
15C instrument.
Scheme 1 Schematic view of POMꢀbased MOCNs in which POMs can act
as (a) nodes, (b) templates, and (c) pillars (Green balls represent POM units).
Generally, POMs can act as nodes, templates and pillars
(Scheme 1) in the POMꢀbased MOCNs.11,12 In any cases, a
main concern is that how to introduce more POM units into
the MOCN skeletons, and let them expose as much as
possible in the reaction system. In this aspect, the
combination of cationic MOCNs and various polyoxoanion
Synthesis
[Co(BBTZ)1.5(HBBTZ)(H2O)2][PW12O40]·H2O (1)
A
mixture of H3PW12O40·nH2O (0.6g, 0.2mmol),
Co(OAc)2·4H2O (0.102g, 0.4mmol), and BBTZ (0.15g,
0.625mmol) was dissolved in 10 mL of distilled water at
2 | J. Name., 2012, 00, 1-3
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room temperature and stirred for 0.5 h. The pH of the
reaction mixture was adjusted to about 4.0 with 1.0 M
NaOH. The suspension was sealed in a 23ꢀmL Teflonꢀlined
autoclave and heated at 140 °C for 3 days. After slow
cooling to room temperature, brown block crystals were
filtered and washed with distilled water (65 % yield based
on W). Anal. Calcd for C30H37N15O43PW12Co: C 10.02, H
1.03, N 5.85, W 61.45, Co 1.65; Found: C 10.22, H 1.09, N
5.64, W 61.39, Co 1.67. Selected IR (solid KBr pellet, cmꢀ
1): 3421(m), 3129(w), 1634(m), 1522(s), 1436(m),
1378(w), 1280(s), 1221(w), 1124(m), 1078(s), 975(s),
898(s), 804(s). TG analysis suggests that the first weight
loss in the temperature range of 50 ~ 200 °C corresponds to
the loss of three lattice and/or coordinated water molecules.
[Co2.5(BBTZ)4(H2O)2][BW12O40]·4H2O (2)
A mixture of K5BW12O40·n H2O (0.4 g, 0.13 mmol),
Co(OAc)2·4H2O (0.102 g, 0.4 mmol), and BBTZ (0.15 g,
0.625 mmol) was dissolved in 10 mL of distilled water at
room temperature and stirred for 0.5 h. The pH of the
reaction mixture was adjusted to about 3.0 with 1.0 M HCl.
The suspension was sealed in a 23ꢀmL Teflonꢀlined
autoclave and heated at 140 °C for 3 days. After slow
cooling to room temperature, redꢀbrown block crystals
were filtered and washed with distilled water (78 % yield
based on W). Anal. Calcd for C48H60N24O46BW12Co2.5: C
14.14, H 1.47, N 8.25, W 54.19, Co 3.62; Found: C 14.16,
H 1.40, N 8.21, W 54.21, Co 3.48. Selected IR (solid KBr
pellet, cmꢀ1): 3413(s), 3124(w), 1617(m), 1527(s),
1426(m), 1352(w), 1278(s), 1216(w), 1135(s), 1017(m),
979(s), 924(s), 799(s). TG analysis suggests that the first
weight loss in the temperature range of 50 ~ 210 °C
corresponds to the loss of six lattice and/or coordinated
water molecules.
and ethanol alternately, and dried in an oven at 60 °C for
24 h (70 % yield based on W). The nanocrystals with an
average diameter of 245 nm were obtained based on the
SEM images (Fig 8). The nanocrystals were characterized
by FT/IR spectroscopy (Fig. S20†), PXRD (Fig. 11 and
S19†), and EDX (Fig S23†).
The procedure for catalytic oxidative desulfurization
The catalytic oxidation desulfurization was performed with
dibenzothiophene (DBT). In a typical case, the DBT
(0.0735g, 0.4mmol) and the catalyst (0.075mmol) were
added to 5 mL CH2Cl2 and stirred for 10 min. Then, TBHP
(2 mmol) were added to above mixture and heated at 50 °C
for 3ꢀ7 h. An aliquot of the mixture was periodically
removed and put into an ice chamber to stop the reaction.
The identity of the product was ascertained by FT/IR and
GCꢀMS, while the yield was obtained by HPLC analysis
with a UVꢀVis detector at λ = 254 nm using an Inertsil SILꢀ
100A C18 column. All analyses were performed with the
mobile phase: CH3CN:H2O = 90:10 at an operating flow
rate of 1 mL minꢀ1. After the reaction, the catalyst was
recovered by centrifugation, washed with distilled water
and anhydrous ethanol alternately, and then dried in an
oven at 60 °C for 24 h.
Model oil was obtained by dissolving desired amount of
dibenzothiophene (DBT) in nꢀoctane with a correspounding
Sꢀcontent of 500ppm, respectively. oxidation of
dibenzothiophene in fuels were carried out by mixing
0.075mmol of nanocrystalline compound
2 and 5ml model
oil in a twoꢀnecked kettle equipped with a magnetic stirrer
and a condenser. After then, a desired amount of TBHP
was rapidly added into the above mixture under vigorous
stirring at 50°C. After the reaction, the residual sulfur
content was removed by a polar extractant (for example,
acetonitrile or 1ꢀmethylꢀ2ꢀpyrrolidone), and monitored via
HPLC analysis.
[Cu(BBTZ)2]5[BW12O40]2·4H2O (3)
A mixture of K5BW12O40·n H2O (0.4 g, 0.13 mmol),
Cu(OAc)2·H2O (0.105g, 0.4 mmol), and BBTZ (0.15 g,
0.625 mmol) was dissolved in 10 mL of distilled water at
room temperature and stirred for 0.5 h. The pH of the
reaction mixture was adjusted to about 5.0 with 1.0 M
NaOH. The suspension was sealed in a 23ꢀmL Teflonꢀlined
autoclave and heated at 140 °C for 3 days. After slow
cooling to room temperature, blue block crystals were
filtered and washed with distilled water (55 % yield based
on W). Anal. Calcd for C120H128N60O84B2W24Cu5: C 16.93,
H 1.51, N 9.87, W 51.91, Cu 3.74; Found: C 16.96, H 1.53,
N 9.82, W 51.95, Cu 3.78. Selected IR (solid KBr pellet,
cmꢀ1): 3436(s), 3108(s), 1621(w), 1525(s), 1424(m),
1352(w), 1280(s), 1129(s), 999(w), 950(s), 902(s), 821(s).
TG analysis suggests that the first weight loss in the
temperature range of 45 ~ 220 °C corresponds to the loss of
four lattice water molecules.
X-ray crystallography
Singleꢀcrystal Xꢀray diffraction data for compounds 1-3
were recorded by using a Bruker Smart Apex CCD
diffractometer with MoKα radiation (λ = 0.71073 Å) at the
temperature of 298(2) K. All structures were solved by the
direct method and refined by a fullꢀmatrix leastꢀsquares
method on F2 using the SHELXTLꢀ97 crystallographic
software package.16 All nonꢀhydrogen atoms were refined
anisotropically, except for the lattice water molecules. The
hydrogen atoms on organic carbon atoms were fixed in
calculated positions. Hydrogen atoms on water molecules
cannot be assigned from the weak reflection peaks but were
directly included into the final molecular formula. During
the refinement of compound 3, a number of O atoms on the
polyoxoanion are disordered in two positions with 50%
occupancy for each. Furthermore, the restraint command
“ISOR” was used to restrain nonꢀH atoms with ADP and
NDP problems in compounds 1-3. Moreover, the triazole
and benzene rings of the BBTZ ligands in all three
compounds were also restrained with the commands “AFIX
69”, “AFIX 59”, “DELU”, and “DFIX” so as to restrain the
Preparation of nanocrystal of compound 2
The preparation of nanocrystalline compound
2 was similar
to that of single crystal compound , except that the
2
surfactant sodium docecyl sulfate (SDS) (10 mg, 0.035
mmol) was added in the reaction system. The redꢀbrown
crystalline solid was filtered, washed with distilled water
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bond distance and angles of organic ligands with a
chemically reasonable structural features. All above
restrained refinements led to relatively high restraint values
of 229, 270 and 1212 for compounds 1-3, respectively. In
the elemental analysis, TG analysis and the SQUEEZE
calculation result, another one water molecule is directly
included in the final molecular formula of compound
2.
Crystal data and structural refinement for 1- 3 are listed in
Table 1. Selected bond lengths and angles of 1- 3 are listed
in Table S1ꢀ S3†. Crystallographic data for this paper has
been deposited at the Cambridge Crystallographic Data
the final refinement, compound
2 exhibits solvent
accessible voids but only three lattice water molecules can
be clearly assigned from the residual peaks. Thus, the
SQUEEZE program was further used to remove the
contributions of disordered moieties from all of the
observed structure factors.17 The new generated hkl file
was further used to refine the final crystal data. Based on
Center: CCDC 1063971 for
1
, 1063960 for
2, 1401904 for
3
. These data can be obtained free of charge from the
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam. Ac.uk/data_request/cif.
Table 1. Crystal data and structure refinement for 1-3.
Compounds
Formula
1
2
3
C30H37N15O43PW12Co
3591.85
298(2)
C48H60N24O46BW12Co2.5
4073.53
298(2)
C120H128N60O84B2W24Cu5
8506.54
298(2)
M
r
T / K
Cryst. Syst.
Space group
Monoclinic
C2/c
Triclinic
Pꢀ1
Monoclinic
C2/c
a
/Å
/Å
39.245(8)
15.242(3)
23.514(5)
90
11.7046(9)
14.5353(11)
28.585(2)
85.5140(10)
78.9940(10)
66.8170(10)
4388.3(6)
2
51.260(4)
15.4783(10)
24.3771(16)
90
b
c
/Å
/°
/°
/°
α
β
115.77(3)
90
103.921(2)
90
γ
V
Z
µ
F
/Å3
12666(4)
8
18773(2)
4
/
mmꢀ1
22.086
12768
16.215
15.292
(000)
3689
15484
Refls
Rint
10932
15268
16535
0.0868
1.022
0.0322
0.0560
GOF
1.026
1.024
R1
wR2 (all data)b
Note: a
1 = Σ||
[I>2σ(I
)]a
0.0459
0.0420
0.0866
0.0986
0.1011
0.1822
2
R
F
o| – |
F
c||/Σ|
F
o|; bwR2 = Σ[
w
(
Fo
–
Fc2)2]/Σ[
w
(
Fo2)2]1/2
.
Results and discussion
Crystal Structure of 1
Singleꢀcrystal Xꢀray diffraction analysis revealed
compound crystallizes in the monoclinic space group
1
C2/c, and the crystallographically asymmetric unit consists
of one polyoxoanion [PW12O40]3ꢀ, one Co2+ (Co1) ion, one
BBTZ bridging ligand and one monoprotonated HBBTZ
ligand in general positions, another half BBTZ ligand lying
about a twofold axis, two coordinated water molecules
connected to Co1 center, and one lattice water molecule
(Fig. 1, 2 and Fig. S1†). The cationic metalꢀorganic
fragment [Co(BBTZ)1.5(HBBTZ)(H2O)2]3+ contains one
crystallographically independent Co2+ center, which adopts
a sixꢀcoordinated mode with four nitrogen atoms derived
from four BBTZ ligands and two coordinated water
molecules (Fig. S1†). The bond lengths of Co–N range
from 2.087(2) to 2.232(2) Å and the NꢀCoꢀN bond angles
vary from 91.0(7) to 169.0(6)°. The bond distances of Coꢀ
O are in the range of 2.095(1) ꢀ 2.097(1) Å. It is noteworthy
Fig. 1
unit in
(
1
a) Ballꢀandꢀstick and (b) schematic views of 1ꢀD ladderꢀlike chain
formed by one Co2+ center and BBTZ ligands with three different
kinds of structural configurations.
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that the BBTZ ligands in this metalꢀorganic cationic moiety
can be divided into three groups which are labeled with La,
Lb and Lc (Fig. 1a and Fig. S2†). The first group (La)
adopts transꢀconfiguration and links adjacent two Co atoms
with the Co center. In the packing arrangement, the
adjacent 1ꢀD chains are hydrogenꢀbonded by the lattice
water molecules (O3w), forming
a
porous 2ꢀD
supramolecular network on the bc plane (Fig. 2a and 2b
and Fig. S3†). The guest Kegginꢀtype POM anions and
lattice water molecules are encapsulated in these voids
(Fig. 2). Furthermore, the adjacent POMꢀencapsulated 2ꢀD
supramolecular networks are parallel with each other on the
bc plane, and stack into 3ꢀD supramolecular frameworks
(Fig. 2c and 2d).
into 1ꢀD chains; The second group (Lb) possesses cis
ꢀ
configuration and can be viewed as the “middle rail” that
connects the adjacent 1ꢀD chains to form a ladderꢀlike
chain; The third group (Lc) displays cisꢀconfiguration and
acts as “dangling arm”, protruding upper and lower sides of
the ladder. It is noteworthy that the third group (Lc) is
monoprotonated and only one triazole group coordinate
Fig. 2
(
a) POMꢀencapsulated 2ꢀD supramolecular network in
1
viewed along
a
axis. 1ꢀD ladderꢀlike chains are shown with yellow, blue and purple color for clarity.
The lattice water molecules are shown with red ball. POM units are shown with green polyhedra. (b) Schematic view of the 2ꢀD supramolecular network topology in
viewed along axis; (c) Polyhedral and ballꢀandꢀstick and (d) Schematic view of the 3ꢀD POMꢀencapsulated supramolecular network of viewed along axis.
1
a
1
b
coordinated with three N atoms derived from three BBTZ
ligands, two O atoms derived from two polyoxoanions, and
one coordinated water molecule (Fig. 3a, 3b and Fig. S4†).
The bond distances of CoꢀN and CoꢀO are in the range of
Crystal Structure of 2
Compound crystallizes in the triclinic space group
2
Pꢀ1
and the crystallographically asymmetric unit contains one
Kegginꢀtype polyoxoanion [BW12O40]5ꢀ, two and half Co2+
(Co1, Co2, Co3) ions connected to the polyoxoanion, four
BBTZ bridging ligands, two coordinated water molecules
and four lattice water molecules (Fig. 3, Fig. 4 and Fig.
2.094(6)
ꢀ 2.136(6) Å and 2.067(8) ꢀ 2.173(9) Å,
respectively. The bond angles of N(O)–Co–N(O) are in the
range of 82.5(4)° ꢀ 175.5(3)°. In the 1ꢀD chain of
compound
BBTZ bridging ligands (
chain (Fig. 3a). The adjacent Co(2) ion are linked by the
transꢀBBTZ ligands ( b) to form a 1ꢀD zigꢀzag Co(2)ꢀ
BBTZ chain (Fig. 3a). Meanwhile, another transꢀBBTZ
ligands ( c) bridge two Co(1)ꢀBBTZ chains and one zigꢀ
2
, adjacent Co(1) ions are connected by the cis
ꢀ
S4†). In
BBTZ ligands lies in general positions while the other two
lie about independent inversion centers. In , there are two
2, the Co3 atom lies on an inversion center, two
L
a) to form a 1ꢀD Co(1)ꢀBBTZ
2
L
types of metalꢀorganic coordination polymer fragments,
that is, the 1ꢀD chain and 2ꢀD network (Fig. 3). The metalꢀ
organic 1ꢀD chain possesses two crystallographically
independent Co centers (Co1, Co2), which are hexaꢀ
L
zag Co(2)ꢀBBTZ chain together to generate an unusual 1ꢀD
metalꢀorganic chains with the “N”ꢀtype crossꢀsection
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viewed along
a axis (Fig. S5†). The metalꢀorganic 2ꢀD
network possesses one crystallographically independent Co
center (Co3), which is hexaꢀcoordinated with four N atoms
derived from four BBTZ ligands and two O atoms derived
from the two POM units (Fig. 4c and Fig. S4†). The bond
lengths of Co(3)ꢀN are in the range of 2.109(6) ꢀ 2.117(6)
Å and the bond length of Co(3)ꢀO is 2.148(8) Å. The bond
angles of N(O)–Co–N(O) are in the range of 87.4(3)° ꢀ
180°. In such 2ꢀD network, all BBTZ ligands possess
similar structural transꢀconfigurations and link the Co(3)
centers into 2ꢀD network with 44 sqp topology (Fig. 3c and
3d). Each mesh is large with the size of 11.70(5) × 26.72(5)
Å (Fig. S6†). In compound
D chains and 2ꢀD networks are connected by the Kegginꢀ
type polyoxoanions to form POMꢀsupported 3ꢀD
2, the cationic metalꢀorganic 1ꢀ
a
framework (Fig. 4 and Fig. S8†). From the topological
viewpoint, POM units can be viewed as a fiveꢀconnected
node, the Co(1) and Co(2) centers can be reduced to fiveꢀ
connected nodes, and Co(3) can be regarded as a sixꢀ
connected node (Fig. S7†). Thus, the whole framework
adopts the 4ꢀnodal 5,5,5,6ꢀc net with stoichiometry (5ꢀ
c)2(5ꢀc)2(5ꢀc)2(6ꢀc), and the point symbol for the net is
{3.43.52.64}2{32.4.54.63}2{44.54.62}2{44.610.8} (Fig. 4b).
Fig. 4 (a) Polyhedral and ballꢀandꢀstick and (b) schematic views of the
POMꢀsupported 3ꢀD network of
2 viewed along a axis.
Crystal Structure of 3
Compound
3 crystallizes in the monoclinic space group
C
2/c and the crystallographically asymmetric unit consists
of one Kegginꢀtype polyoxoanion [BW12O40]5ꢀ, two and
half Cu2+ ions (Cu1, Cu2, Cu3) connected to the
polyoxoanion, five BBTZ bridging ligands and two lattice
water molecules (Fig. S9†). It is also worth noting that the
Cu2 center lies on a twofold axis, three BBTZ ligands lie in
general positions and two half BBTZ ligands lie about
Fig. 3 (a) Ballꢀandꢀstick and (b) schematic views of 1ꢀD chainlike unit in
c) Ballꢀandꢀstick and (d) schematic views of 2ꢀD network moiety in
2;
(
2.
independent
inversion
centers.
In
3,
three
crystallographically independent Cu centers are all hexaꢀ
coordinated with four N atoms derived from four BBTZ
ligands and two O atoms derived from two different POM
units (Fig. 5, Fig. S9ꢀS10†). The bond lengths of CuꢀN and
CuꢀO are range from 2.004(1) – 2.08(2) Å and 2.211(1) –
2.791(1) Å, respectively. The five BBTZ ligands exhibit
two types of structural configurations (Fig. S9†), that is,
two cisꢀBBTZ ligands connect the Cu1 and Cu3 centers,
while three transꢀBBTZ ligands connect the Cu1ꢀCu3
centers, Cu1ꢀCu2 centers, Cu2ꢀCu2 centers and Cu3ꢀ
Cu3(generated by symmetry operations) centers,
respectively. From the topological viewpoint, all three Cu
centers can be reduced to a fourꢀconnection node. Thus, the
whole cationic metalꢀorganic framework adopt the 3ꢀnodal
4,4,4ꢀc net with stoichiometry (4ꢀc)2(4ꢀc)(4ꢀc)2, and the
point symbol for the net is {63.72.8}2{66}2{74.82} (Fig. 5b).
In such a cationic MOF framework, each guest Kegginꢀ
6 | J. Name., 2012, 00, 1-3
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Catalytic Oxidative Desulfurization
type POM unit also connect with five Cu centers via the
CuꢀO bonds, further stabilizing the whole framework. Thus,
if the POM unit can be reduced to a fiveꢀconnectin node,
the whole framework shall adopt the 4ꢀnodal 5,6,6,6ꢀc net
with stoichiometry (5ꢀc)2(6ꢀc)2(6ꢀc)(6ꢀc)2, and the point
symbol for the net is {32.43.54.66}2{32.44.54}2
{32.45.55.62.7}2{42.54.68.7} (Fig. 6).
The desulfurization of fossil fuels is one of currently
significant tasks, since their combustion waste can lead to
serious atmospheric pollution, equipment corrosion and
respiratory problems. In this research field, oxidative
desulfurization has been developed an effective approach to
remove the refractory organosulfur compounds by the
catalytic oxidation into corresponding sulfones under mild
reactions.17ꢀ22
During
the
research
process,
dibenzothiophene (DBT) is usually employed as a typical
refractory sulfur compound in fuel to evaluate the catalytic
activities of test catalysts. In this case, the catalytic
oxidation experiments of DBT with compounds 1-3 and a
group of contrast catalysts were performed in CH2Cl2
medium at 50 °C by using tertꢀbutyl hydroperoxide
(TBHP) as the oxidant (Scheme 2 and Table 2). The
corresponding oxidized products were confirmed by FT/IR
spectroscopy and GCꢀMS (Fig. S13 to S15†).
Scheme 2 Oxidation reaction of sulfide to corresponding sulfoxides and
sulfones.
As shown in Fig. 7 and Table 2, compounds 1-3 all exhibit
good catalytic activity on the oxidation of DBT into
sulfoxide and sulfone. The maximum conversions can be
observed after 7 h with the values of 91.57% for
1, 99.63%
for and 96.68% for , respectively, which are much
2
3
higher than the conversion of 12.23 % in the contrast group
(without catalyst). As shown in table S4, † compared with
previous POMꢀbased MOCNs catalysts, the catalytic
activity of these three compounds have been improved
remarkably. In order to explore the catalytical active
moieties of compounds 1-3, the catalytic activities of the
precursors of compounds 1-3 were performed (see entry 5-
12 in Table 2). It is found that the POM precursors exhibit
obviously higher catalytic activity than those of TM
precursors (Fig. S16†). Thus, POM units should be the
catalytically active species in the composite coordination
networks. This result is consistent with the reported
catalytic mechanism of POM species. The Mo/W centers of
POMs may activate and/or “capture” the peroxo groups,
forming peroxoꢀPOM intermediates, which can oxidize
sulfide into sulfoxide and/or sulfone. During the oxidative
process, the peroxoꢀPOM species is transferred into POM
species again and enter into next catalytic cycle.[19,20]
Fig. 5 (a) Ballꢀandꢀstick and (b) schematic views of the 3ꢀD metalꢀorganic
network in 3.
It is also noteworthy that the catalytic activities of
compounds 1-3 are much higher than those of H3PW12O40
and K5BW12O40 POM salts (entry 5ꢀ6 in Table 2 and Fig.
7), indicating that the uniform dispersion of POM units into
the metalꢀorganic coordination networks in 1-3 at the
molecular level may expose more POMs active sites, and
thus remarkably improve the catalytic activities of the
catalysts.
Fig. 6 (a) the chemical and (b) schematic view of the POMꢀsupported 3ꢀD
open framework in
3
.
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(PXRD). Generally, the strong diffraction peaks of PXRD
exhibit the advantage facets orientation of the crystalline
compounds. In this case, the advantage facets {002} of
compound
facets {ꢀ111} of compound
2
expose more POM units than the advantage
(see Fig. S17†), suggesting
3
that the advantage facets containing more POM units may
be an important factor to effect the activity of catalysts.
Fig. 7 Removal of DBT verse reaction time with different catalysts.
Table 2 Catalytic oxidation of sulfide to sulfoxide and sulfone
with TBHP [a]
Entry
Catalyst
Time /h
Conversion%[c]
91.57
1
2
3
4
5
6
7
8
9
Compound
Compound
1
2
7
7
3
7
7
7
7
7
7
99.63
Fig. 8 (a) The images of single crystals
particle size distribution of single crystals
nanocrystals (d) the particle size distribution of nanocrystals 2.
2
with the scale bar of 1mm; (b) the
Nanocrystal compound
2
99.74
2; (
c) the SEM image of
Compound
H3PW12O40
3
96.68
2
;
·
n
H2O
H2O
48.94
Another possible route to improve the catalytic property of
these hybrid catalysts is reducing the size of such
crystalline catalysts, which can increase their specific
surface area and expose more POM active sites during the
catalytic process. In this case, the best efficient catalyst
K5BW12O40
·
n
50.61
Co(OAc)2·4H2O
Cu(OAc)2·H2O
25.26
23.03
K5BW12O40
Co(OAc)2·4H2O [b]
K5BW12O40 H2O,
Cu(OAc)2·H2O [b]
H3PW12O40 H2O,
·
n
H2O,
65.39
compound
2 is used as the representative sample to prepare
the nanoꢀcrystallines. In this research field, the surfactantꢀ
assisted method has already been explored as an effective
way to prepare nanoscale MOF materials, since the suitable
surfactants can strongly influence the size of crystals and
control the nucleation rate for the production highly
crystalline materials.23ꢀ29 Up to date, the nanocrystallines of
POMꢀbased MOCNs prepared by this surfactantꢀassisted
hydrothermal synthesis method have not been reported yet.
During the preparation, the key factor is to choose suitable
surfactants. Herein, we successfully prepared the
10
11
·
n
7
7
7
62.82
63.79
12.23
·
n
Co(OAc)2·4H2O [b]
12
None
[a]
Conditions: a mixture of catalyst (0.075 mmol), DBT (0.4 mmol), and
[b]
TBHP (2.0 mmol, 5 equiv.) in CH2Cl2 (2 mL) was stirred at 50°C.
Both
[c]
POM and TM salts catalysts are 0.075 mmol.
HPLC analysis.
The values are based on
nanocrystalline compound
2 by the use of anionic
As shown in Fig. 7, the catalytic activity of K5BW12O40 is
slight higher than that of H3PW12O40. Therefore, the
catalytic activities of compounds 1-3 are slightly different
surfactant sodium dodecyl sulfate (SDS).30ꢀ33 SEM images
show that uniform crystals are rectangle morphology with
the size of about 245 nm in diameter (Fig. 8). The powder
Xꢀray diffraction (PXRD) pattern and EDAX of
with the order of
These catalytic differences between compound
2
(99.63%) >
3
(96.68%) >
1
(91.57%).
and 2-3
1
nanocrystal
2
are same as those of the large singleꢀcrystal
may mainly derive from their difference of POM species.
As a result of crystal structures, the loading amount of
POMs in MOCNs of 2-3 is 70.13 % and 67.17 %,
respectively. So the tiny catalytic difference between
2
, confirming the structural consistence (Fig. S19 and
S23†).
compound
2 and 3 is probably due to the loading amount of
POM units, the different TM ions in the compounds (see
entry 7-11 in Table 2) and the different network structural
features of two compounds. Especially the last difference
can be found from the powder Xꢀray diffraction patterns
8 | J. Name., 2012, 00, 1-3
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nanocrystal
2
in
n
ꢀoctane also have been investigated and
the conversion of DBT still reached 96.3% after 5ꢀtimes
recycling without a significant decrease (Fig. S35).
Fig. 9 Removal of DBT versus Reaction time.
Fig. 11 PXRD patterns of nanocrystal
2
in 2θ range of 5 – 50o (a) simulated
pattern; (b) asꢀsynthesized sample of nanocrystal 2; (c) nanocrystal 2 after
catalytic reation.
Conclusions
In summary, we presented three new Kegginꢀtype POMꢀ
based metalꢀorganic coordination networks. Compound
exhibits a POMꢀencapsulated 3ꢀD supramolecular network,
while compound and display the POMꢀsupported 3ꢀD
1
2
3
coordination networks. All three compounds exhibited
good catalytic activities in the heterogeneous catalytic
oxidation desulfurization of DBT in contrast to their
precursor POM salts, indicating that the monoꢀdispersion
of POMs units in the metalꢀorganic coordination networks
at the molecular level can expose more active POM sites
and increase the catalytic activity of the POMꢀbased
crystalline catalysts. In such nonꢀporous POMsꢀbased
MOCN catalysts, a series of factors such as the POMs
species, transition metal cations, the loading amount of
POMs, and the structural features, can affect their catalytic
activities and POM species is the main affecting factor. In
order to further improve the catalytic activities of such type
of catalysts, a new surfactantꢀassisted hydrothermal method
for the preparation of nanocrystalline products has been
Fig. 10 The recycle experiments with nanocrystal
2
as catalyst.
The catalytic activity of nanocrystal
2
was studied in the
same condition. It is observed that the conversion of DBT
can almost reach 100 % after 3 h, indicating that the crystal
size of the catalyst has effect on the catalytic activity (Fig.
9) obviously. When the crystal size of the catalyst
2
decreases, the specific surface area of catalysts is
dramatically improved, thus more POM active sites were
exposed, leading to high catalytic activity. In such catalytic
reaction system, the nanocrystal
2 is insoluble and can be
easily recycled by simple centrifugal separation (Fig.
S18†). Recycling experiments reveal that the catalytic
activity of the catalyst shows just slight decrease after
reused for five cycles (Fig. 10), which might be related to
the slight loss of catalyst in the process of recycle. The
FT/IR spectra, UVꢀvis diffuse reflectance spectra and
PXRD patterns (Fig. 11) of nanocrystal
five catalytic cycles (Fig. 11, Fig. S19ꢀS21†) show
negligible change, suggesting that the nanocrystal is
developed. The uniform nanocrystals
prepared, which exhibits much better catalytic activity than
the one of the large single crystal . This result confirm that
2 were successfully
2
reducing the size of crystalline catalysts can increase the
specific surface area, expose more POM active sites, and
thus improve the catalytic activity of the crystalline
catalysts. This work confirms that the nonꢀporous POMꢀ
based MOCNs can also be used as the heterogeneous
catalysts for the oxidative desulfurization and the
preparation of uniform nanocrystalline POMꢀbased MOCN
products may pave new ways to explore such
heterogeneous POMꢀbased catalysts with high efficient
catalytic activity. This work is still ongoing in our research
team.
2 before and after
2
stable in such catalytic reaction system. In order to simulate
the oxidative desulfurization in fossil oil, the catalytic
activity of nanocrystal
studied. The conversion of DBT can reach 100% after 3 h
(Fig. S34), manifesting that the nanocrystal displayed
2 in the model oil nꢀoctane was also
2
excellent catalytic activity. The recycling experiments of
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Acknowledgements
The authors gratefully acknowledge the financial support
National Natural Science Foundation of China (grant nos.
21271039, 21201032, 21471028 and 21401131), Program
for New Century Excellent Talents in University (grant no.
NCET120813) and Fundamental Research Funds for the
Central Universities (grant no. 11SSXT140).
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a
Key Laboratory of Polyoxometalate Science of Ministry of
Education Faculty of Chemistry, Northeast Normal University,
Changchun, 130024, P.R. China.
Eꢀmail: tanhq870@nenu.edu.cn (H.ꢀQ. Tan) and
liyg658@nenu.edu.cn (Y.ꢀG. Li)
School of Chemical and Biological Engineering, Taiyuan
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Electronic Supplementary Information (ESI) available:
Summary of selected bond lengths and angles; catalytic experiment
data; IR, TG and PXRD and cif files of . CCDC reference
numbers: 871528 ), 871529 ), and 871981 ).. See
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