Tris(hydroxymethyl)aminomethane modified layered double hydroxides greatly facilitate polyoxometalate intercalation

Article abstract of DOI:10.1039/c4dt01827c

Polyoxometalate (POM) intercalation to layered double hydroxides (LDHs) has been greatly restricted by the geometry, charge and size of POMs. We report herein, for the first time, the intercalation of Na₃[PW₁₂O₄₀]·15H₂O into tris(hydroxymethyl)-aminomethane (Tris) modified layered double hydroxides (Tris-LDH-CO₃) using an ion exchange method, resulting in the formation of novel intercalated Tris-LDH-PW₁₂ under ambient conditions without the necessity of degassing CO₂. Theoretical calculations show the decreased energy and the slightly distorted LDH layer after Tris modification, indicating that the Tris-modified LDH layers greatly facilitate the intercalation of PW₁₂. Further application of Tris-LDH-PW₁₂ for oxygenation of sulfides shows highly efficient and selective catalytic activities under mild conditions. The Tris-LDH-PW₁₂ can be easily recovered and reused for more than 10 times without any obvious decrease of reactivity. This opens a completely new pathway for engineering POM-LDH advanced functional materials. This journal is

Full text of DOI:10.1039/c4dt01827c

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Tris(hydroxymethyl)aminomethane modified
layered double hydroxides greatly facilitate
Cite this: DOI: 10.1039/c4dt01827c
polyoxometalate intercalation†
Yang Chen,‡ Dongpeng Yan‡ and Yu-Fei Song*
Polyoxometalate (POM) intercalation to layered double hydroxides (LDHs) has been greatly restricted by
the geometry, charge and size of POMs. We report herein, for the first time, the intercalation of
Na3[PW12O40]·15H2O into tris(hydroxymethyl)-aminomethane (Tris) modified layered double hydroxides
(Tris-LDH-CO3) using an ion exchange method, resulting in the formation of novel intercalated Tris-
LDH-PW12 under ambient conditions without the necessity of degassing CO2. Theoretical calculations
show the decreased energy and the slightly distorted LDH layer after Tris modification, indicating that the
Tris-modified LDH layers greatly facilitate the intercalation of PW12. Further application of Tris-LDH-PW12
for oxygenation of sulfides shows highly efficient and selective catalytic activities under mild conditions.
The Tris-LDH-PW12 can be easily recovered and reused for more than 10 times without any obvious
decrease of reactivity. This opens a completely new pathway for engineering POM-LDH advanced
functional materials.
Received 19th June 2014,
Accepted 19th August 2014
DOI: 10.1039/c4dt01827c
www.rsc.org/dalton
conventional synthetic methods; (3) LDHs exhibit high affinity
for CO3 anions, and ion exchange reactions for preparation
Introduction
2−
Polyoxometalates (POMs) are a class of discrete anionic metal– of POMs intercalated into LDHs do not work in air in most
oxide clusters with unrivalled structural types1,2 as well as a cases.16
wide range of unique physical properties and applications in
Since the strong affinity between POM layers and guest
areas as diverse as catalysis,3,4 medicine5 and nano-science.6 CO32− anions originates from the electrostatic interactions and
Layered double hydroxides (LDHs) comprise an unusual class hydrogen bonding, it is highly interesting whether it is possi-
of layered materials with positively charged nanosheets/layers ble to decrease such host–guest interactions, and thereby
and the balancing anions located in the interlayer region.7–10 promote the ion exchange. Bearing this in mind, and inspired
The POM/LDH nanocomposites have attracted wide interest as by the recent work on the tris(hydroxymethyl)aminomethane
they have shown great advantages over both traditional LDH
and POM compounds.11–15 Although POM intercalated LDHs
possess superior application to POMs or LDHs, the relevant
intercalation process faces serious challenges: (1) it is almost
impossible to obtain POM/LDH nanocomposites with no
impurity using traditional synthetic methods such as co-
precipitation and reconstitution pathways; (2) the POM intercala-
tion is closely related to the geometry, charge and size of
POMs.12 As such, a number of POM anions such as the classi-
cal Keggin cluster of [PW12O40]3− with the negative charge
below 4 are very unlikely to intercalate into LDHs using the
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, P. R. China. E-mail: songyufei@hotmail.com,
songyf@mail.buct.edu.cn
†Electronic supplementary information (ESI) available: Sample characterization,
calculation details and analysis of Tris-LDH-CO3 and Tris-LDH-PW12. See DOI:
3−
10.1039/c4dt01827c
Scheme 1 Schematic representation of the synthesis of [PW12O40]
‡These authors have the same contribution.
intercalated into Tris-stabilized LDHs.
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(Tris) modified LDHs (Tris-LDH-CO3),17 we report, for the first Oxygenation of sulfides
time, ion exchange of the Tris-LDH-CO3 with the classical
1 mmol substrate, 30% H2O2 aqueous solution, 0.25 mol%
catalyst (Tris-LDH-PW12 contain polyoxometalate anions
2.5 μmol) and 0.2 ml n-propanol were placed in a 20 ml glass
bottle at room temperature and the reaction mixture was kept
stirring vigorously. The reaction was effectively quenched after
5 h. The resulting oily products were extracted with diethyl
ether, analyzed by gas chromatography with a flame ionization
detector (GC-FID). The conditions were as follows: injection
port temperature 340 °C; detector temperature 250 °C; oven
temperature 70 °C; carrier gas: ultrapure nitrogen; sample
injection volume 1 mL.
Keggin cluster of Na3PW12O40·15H2O (Na-PW12
)
under
ambient conditions without the necessity of degassing CO2
(Scheme 1). As a result, Tris-LDH-PW12 can be prepared for the
first time using the ion exchange method.
Experimental section
Chemicals
All the used chemicals and solvents were purchased from Alfa
Aesar and used directly without further purification.
Preparation
Characterization
Na3[PW12O40]·15H2O18 (Na–PW12) and the tripodal ligand-
stabilized layered double hydroxide (Tris-LDH-CO3)17 were syn-
thesized according to the literature procedures, respectively.
The POMs were intercalated into Tris-LDH-CO3 using the
anion-exchange method under CO2-existing conditions. Tris-
Powder X-ray diffraction (XRD) patterns were recorded on a
Rigaku XRD-6000 diffractometer under the following con-
ditions: 40 kV, 30 mA, Cu Kα radiation (λ = 0.154 nm). FT-IR
spectra were recorded on a Bruker Vector 22 infrared spectro-
meter using KBr pellets. The solid state NMR experiments
were carried out at 75.6 MHz for 13C and at 121.0 MHz for 31P
on a Bruker Avance 300M solid-state spectrometer equipped
with a commercial 5 mm MAS NMR probe. The N2 adsorp-
tion–desorption isotherms were measured using a Quanta-
chrome Autosorb-1 system at the liquid nitrogen temperature.
Scanning electron microscopy (SEM) images and energy dis-
persive X-ray (EDX) analytical data were obtained using a Zeiss
Supra 55 SEM equipped with an EDX detector. Transmission
electron microscopy (TEM) micrographs were recorded using a
Hitachi H-800 instrument. HRTEM images were conducted on
a JEOL JEM-2010 electron microscope operating at 200 kV.
Thermogravimetric and differential thermal analyses (TG-DTA)
were performed on a TGA/DSC 1/1100 SF from Mettler Toledo
in flowing N2 with a heating rate of 10 °C min−1 from 25 °C to
1000 °C. X-ray photoelectron spectroscopy (XPS) measure-
ments were performed with monochromatized Al Kα exciting
X-radiation (PHI Quantera SXM). An inductively coupled
plasma emission spectroscope (ICP-ES, Shimadzu ICPS-7500)
was used to measure the concentration of W in the catalysts.
GC analyses were performed with an Agilent 7820A GC system
using a 30 m 5% phenylmethyl silicone capillary column with
an ID of 0.32 mm and 0.25 mm coating (HP-5).
LDH-CO3 (2 mg mL−1) was re-dispersed in the Na3[PW12O40
]
solution (0.1 M) then stirred 2 h at room temperature. The pre-
cipitate was then filtered, washed with water and acetone, and
dried in an oven to obtain the Tris-LDH-PW12.
Results and discussion
Synthesis and characterization of Tris-LDH-PW12
Tris-modified layered double hydroxides (LDHs) have been pre-
pared successfully by mixing MgCl2, AlCl3 and Tris in aqueous
solution, leading to the formation of Tris-LDH-CO3.17 Ion
exchange of the classical Na-PW12 with Tris-LDH-CO3 under
ambient conditions without necessity of degassing CO2 results
in the formation of new intercalated assembly of Tris-
LDH-PW12. The XRD patterns of the Tris-LDH-CO3 (Fig. 1)
show the characteristic (003), (006), (110) and (113) at 2θ =
2−
11.5°, 23.4°, 60.9° and 62.2°. After the ion exchange of CO3
with [PW12O40]
3−, the XRD patterns of Tris-LDH-PW12 show
the (003) and (006) at 2θ = 8.4° and 18.1°, corresponding to d
values of 1.1 and 0.5 nm, respectively. Compared with the XRD
pattern of Tris-LDH-CO3, the basal (003) and (006) reflections
of Tris-LDH-PW12 shift to lower 2θ, indicating the successful
intercalation of [PW12O40]3− into the Tris-modified layer
double hydroxides. It is noted that 1.1 nm ≈ 2 × 0.5 nm, which
Desulfurization experiments
In a typical experiment, a solution of dibenzothiophene (DBT), suggests a typical layered structure of Tris-LDH-PW12.19 Based
benzothiophene (BT), and 4,6-dimethyl-dibenzothiophene on the elemental analysis [Mg = 4.13%, Al = 2.29%, W =
(4,6-DMDBT) in n-octane was used as model oil with an S 62.53% and N = 0.35%], elemental composition of the product
content of 1000 ppm. The catalytic oxidative desulfurization can be expressed as Mg0.66Al0.33(C4H8NO3)0.097(OH)1.71
experiments were performed in a 50 mL two-necked flask, to (PW12O40 0.11·0.68H2O (Table S1†).
which 0.08 mL of 30 wt% H2O2, 5 mL of model oil, 1 mL of As Pinnavaia et al. pointed out that it is very unlikely to
-
)
[bmim]BF4, and Tris-LDH-PW12 (H2O2/DBT/Cat = 100 : 20 : 1) intercalate the classical Keggin cluster of [PW12O40]3− into the
were added. The reaction mixture was stirred at 75 °C. During LDHs because of their negative charge below 4.12 In contrast,
the reaction, the upper layer of the model-oil phase was period- the mono-lacunary Keggin cluster of [PW11O39]7− (PW11) has
ically withdrawn and analyzed by gas chromatography with a been intercalated into LDHs as contrast (Fig. S1†). The result-
flame ionization detector (GC-FID). DBT, BT, and 4,6-DMDBT ing LDH-PW11 exhibits the reflections of the layered structure
were identified using reference standards.
with (003) and (006) at 2θ = 6.1° and 12.1°, corresponding to
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the interlayer CO3 species.17 In the case of Tris-LDH-PW12,
no signal at ∼170 ppm can be observed (Fig. 1c), which is due
to the successful exchange of [PW12O40]3− with the inter-
layered CO32−. One set of signals centred at δ = −4 ppm for the
2−
[PW12O40]3− anions in the 31P NMR spectrum of Tris-
3−
LDH-PW12 (Fig. 1d), suggesting that the [PW12O40
]
anions
have been intercalated into the Tris-modified LDHs. The
energy-dispersive X-ray (EDX) result of Tris-LDH-PW12
(Fig. S3†) reveals the presence of Mg, Al, C, N, P and W
elements, and the P/W molar ratio close to the expected 12/1,
indicating the completeness of [PW12O40]3− anions in the
interlayer of Tris-modified LDHs.
XPS analysis has been employed to characterize the oxi-
dation state of W species in the Tris-LDH-PW12 (Fig. 1e). The
XPS curve fitting procedure is according to the theory of
Doniach and Sunjic.23 The W4f spectrum can be deconvoluted
into doublets, as indicated in Fig. 1e. This doublet consists of
W4f7/2 line at 35.5 eV and W4f5/2 line at 37.5 eV, which are
assigned to the W in the W–O bond configuration and typically
observed for the W6+.24 All of these results reveal that the
Keggin structure is retained after being intercalated into Tris-
modified LDHs. This has been further proved by the theore-
tical calculation below.
Thermogravimetry (TG) and differential thermal analysis
(DTA) curves for Tris-LDH-PW12 are shown in Fig. 1f. Two
weight-loss stages can be observed with the increase of temp-
erature from 75 to 700 °C. The first weight loss of 8.39% at
75–220 °C corresponds to the loss of surface and interlayer
adsorbed water. The second weight loss step of 10.49% at
220–660 °C can be attributed to the decomposition of the Tris
on LDHs and the collapse of the layered structure. Two
endothermic peaks in the DTA curve can be observed at 120 °C
and 347 °C.
Fig. 1 (a) The XRD patterns of the Tris-LDH-CO3 and Tris-LDH-PW12
;
(b) FT-IR spectra of Tris-LDH-CO3, Tris-LDH-PW12 and Na3PW12O40
;
(c) 13C and (d) 31P CP/MAS NMR spectra of Tris-LDH-PW12; (e) XPS spectra
for the W4f core level spectrum of the Tris-LDH-PW12; (f) TG-DTA of
Tris-LDH-PW12
.
d values of 1.5, and 0.7 nm, respectively. Moreover, a broad
diffraction following (003) can be observed, which is rather
common in various POM-intercalated LDH materials using the
conventional synthetic methods.20 Therefore, the Tris-modi-
fied layer double hydroxides show a much improved intercala-
tion ability in comparison to that of classical LDHs. Moreover,
no impurity can be observed in the XRD patterns of Tris-
LDH-PW12, which is in good contrast to those of the conven-
tional POMs intercalated LDHs.
The TEM image (Fig. 2a) of Tris-LDH-CO3 shows the
uniform nanoparticles with a rectangular shape and average
FT-IR spectra of Tris-LDH-CO3, Tris-LDH-PW12 and
NaPW12O40 are shown in Fig. 1b. The absorption bands at
1088 and 1135 cm−1 in the spectrum of Tris-LDH-CO3 are due
to the characteristic stretching vibration of M–O–C and C–C–O,
respectively.17 The asymmetric vibration peak 1072 cm−1 for
the P–O chemical bond still exists in the Tris-LDH-PW12. The
asymmetric vibration of the W–O bond also changes signifi-
cantly and the shift is from 986 cm−1 in Na3PW12O40 to
955 cm−1, the characteristic absorption bands shift to lower
frequency indicates the presence of strong electrostatic inter-
actions and hydrogen bondings between the host layers and
the guest anions.21 The peaks at 894 and 811 cm−1 in Tris-
LDH-PW12 are assigned to the vibration of W–O–W chemical
bonds, which are originally at 894 and 799 cm−1, respectively.
These results reveal that [PW12O40]3− anions still hold the Keggin
structure upon intercalation. The absorption band in the range
of 400–800 cm−1 in Tris-LDH-CO3 or Tris-LDH-PW12 can be
ascribed to O–M–O vibrations in the brucite-like layers of LDH.22
The 13C CP/MAS NMR spectrum of Tris-LDH-CO3 (Fig. S2†)
shows a strong signal at 171 ppm, which can be assigned to
Fig. 2 (a) TEM image and the corresponding Tyndall effects (inset) of
Tris-LDH-CO3, (b) HRTEM image and the corresponding Tyndall effects
(inset) of Tris-LDH-PW12; SEM images of the conventional LDH:
(c) LDH-CO3 and (d) LDH-PW11
.
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size ∼20 nm, which is very different from the conventional Theoretical calculations
LDH-CO3 with the plate shape and the average size of several
hundred nanometres (Fig. 2c). The HRTEM image of Tris-
To further understand how the Tris-modified LDHs can
tailor the host–guest interactions within the interlayer region,
periodic density functional theoretical (DFT) calculations
have been employed for the idealized models of LDH-CO3,
LDH-PW12, Tris-LDH-CO3 and Tris-LDH-PW12 structures (Fig. S4†).
The typical optimized structures for LDHs before and after
Tris-modification are shown in Fig. 4.
It can be observed that the Tris units can accommodate at
both the inner and the outer of the LDH layers, which do not
affect the change in the basal spacing relative to the pristine
form. This is consistent with the fact that the modification by
Tris has the same possibility at two sides of the LDH layers.
Upon increasing the Tris units anchored on the LDH layer, the
values of lattice energy for both LDH-CO3 and LDH-PW12 have
a decreasing trend (ca. 247 Hartree per Tris unit, Table S2†),
which shows that the LDH lattice can be highly stabilized
upon formation of the Tris-modified LDHs structures. The
decreased energy and the slightly distorted LDH layer observed
after surface modification by Tris further suggests that the flexi-
ble Tris-modified LDH layers may facilitate the intercalation
of PW12 as observed in the experiment. Moreover, the configur-
ation of the PW12 anions is nearly unchanged upon inter-
calation, which is in agreement well with the FT-IR and XPS
results.
In addition, to detect the influences of the different models
on the calculation results, a 4 × 3 × 1 Mg-Al-LDH supercell
(Fig. S5†, denoted as Model 2) has been further selected to
accommodate the pure PW12 anions in the absence of inter-
calated CO32−. It can be observed that the calculated lattice
energy for the LDH-PW12 shows a similar decreasing trend
(Table S3†) upon increasing the Tris units as those of the cal-
culation with the 6 × 4 × 1 LDH layer in the presence of inter-
calated CO32−. Moreover, it is also observed that the electron
density of the HOMOs and the LUMOs are mainly populated
on the W and O atoms of the interlayer PW12 (Fig. S6†). These
results confirm that the selected models (such as different
LDH-PW12 exhibits irregular morphology with uniform dis-
3−
persion of [PW12O40
]
as small black dots (Fig. 2b). In con-
trast, the hexagonal plate morphology of LDH-PW11 can be
clearly observed in the TEM images (Fig. 2d). The colloidal dis-
persions of Tris-LDH-CO3 and Tris-LDH-PW12 are transparent
and show the typical Tyndall effect (inset in Fig. 2a and 2b),
indicating the presence of a highly dispersed colloidal solution.
BET measurements have been carried out on the Tris-
LDH-CO3 and Tris-LDH-PW12. The specific surface area, pore
volume, and average pore diameter (estimated from N2 adsorp-
tion–desorption isotherms) are presented in Table 1. BET data
show that the surface area of Tris-LDH-CO3 and Tris-
LDH-PW12 of 58.8 and 15.9 m2 g−1, pore volume of 0.096 and
0.018 cm3 g−1, average pore diameter of 4.2 and 3.8 nm,
respectively. Furthermore, Tris-LDH-CO3 and Tris-LDH-PW12
display H4 type hysteresis loops (Fig. 3), indicating that the
pores are produced by the aggregation of slit-shaped pores
with some microporosity,25 which is similar to the convention-
al Mg3Al–PW11, but different from the conventional Mg3Al–
CO3 with H3 type hysteresis loops.26 It is worth noting that
despite the fact that the surface area and pore volume of Tris-
LDH-PW12 are smaller than that of Tris-LDH-CO3, the nano-
scale particles of Tris-LDH-PW12 guarantee the good dispersion
and thereby facilitate the access of the substrates to the
active species. As such, Tris-LDH-PW12 shows excellent catalytic
activity.
Table 1 Comparison of physio-chemical properties of Tris-LDH-CO3
and Tris-LDH-PW12
Surface area
Pore volume
Average pore
diameter (nm)
Materials
(m2 g−1
)
(cm3 g−1
)
Tris-LDH-CO3
Tris-LDH-PW12
58.8
15.9
0.096
0.018
4.2
3.8
Fig. 3 Adsorption–desorption isotherms of Tris-LDH-CO3, Tris-LDH-PW12
Fig. 4 The optimized geometric structures of the (a1) LDH-CO3; (a2)
and the conventional LDH including LDH-CO3, LDH-PW11
.
and (a3) Tris-LDH-CO3; (b1) LDH-PW12; (b2) and (b3) Tris-LDH-PW12.
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sizes of the LDH layer and in the presence/absence of the The oxidation of DBT to DBTO2 can be completed in about
co-intercalated CO32−) do not influence the calculation con- 25 min. As such, the catalyst shows high catalytic efficiency for
clusions obviously.
the oxidation of sulfide to sulfone, and the catalytic reaction
Electronic densities of states (DOSs) and frontier orbital strictly obeys zero-order kinetics with almost 100% selectivity
analysis (Fig. S7–S8†) show that for the LDH-PW12 system, the for DBTO2.
electron density of the highest occupied molecular orbitals
(HOMOs) and the lowest unoccupied molecular orbitals
(LUMOs) are mainly contributed from W and O atoms of the
PW12 anions, and the LDH layer do not participate in the fron-
tier orbital distribution (Fig. S9†). These suggest that no elec-
tronic transfer occurs between the LDH and PW12, and the
dCt
dt
ꢀ
¼ k
ð1Þ
ð2Þ
CoꢀCt ¼ kt
In the case of 4,6-dimethyldibenzothiophene (4,6-DMDBT)
valence electrons localized in the PW12 are confined within the and benzothiophene (BT), which are difficult to remove owing
energy blocking by the LDH layers. Similar results were also to the relatively lower electron density of sulfur atoms on both
obtained for the Tris-LDH-PW12 system (Fig. S10–S12†), substrates,27 the catalytic system exhibits a high desulfuriza-
suggesting that there is no obvious change in the energy levels tion efficiency with almost 100% sulfur removal for 4,6-
and active sites of the Tris-LDH-PW12 system after the Tris DMDBT and BT in 55 and 60 min, respectively (Fig. 5b).
modification. This conclusion is also in agreement with the
XPS observation above.
To compare the catalytic oxidative desulfurization
3−
efficiency of Tris-LDH-PW12 with [PW12O40
]
or LDH-con-
taining catalysts reported in the literature, we have summar-
ized the results in Table 2. It can be seen that deep
desulfurization of DBT can be achieved using Tris-LDH-PW12
in 25 min at 75 °C with H2O2/DBT/Cat = 100 : 20 : 1 (Table 2,
entry 8), which is one of the highly efficient catalysts reported
so far. The main advantage of heterogeneous catalysts in a
liquid-phase reaction is the ease of separation and reuse of
the catalyst in catalytic cycles. As such, the recycled Tris-
LDH-PW12 catalyst has been applied for further oxidation of
DBT. After the desulfurization procedure, the upper layer can
be separated by decantation. Then, 50 mL of deionized water
is added to the water phase to decrease the viscosity of
[bmim]BF4. After that, the catalyst can be separated by
centrifugation and filtration. The diluted [bmim]BF4 in de-
ionized water is concentrated under vacuum to recycle the ionic
liquid. As such, the IL and the catalyst are both reusable. The
catalyst can be recycled and reused at least ten times without
any obvious decrease of the catalytic efficiency (Fig. 5c). The
recycled catalyst and the oxidative product have been charac-
terized (Fig. S13–S15†).
Application of Tris-LDH-PW12 for oxygenation
To further explore the potential application of Tris-LDH-PW12,
which has been applied for extraction and catalytic desulfuri-
zation (ECODS) of dibenzothiophene (DBT), experiments have
been carried out with H2O2/DBT/Cat = 100 : 20 : 1 at 75 °C. The
percentage sulfur removal and Ct are plotted against reaction
time in Fig. 5a, in which C0 and Ct are the initial DBT concen-
tration and DBT concentration at time t, respectively. The
linear fit of the data reveals that the catalytic reaction exhibits
zero-order kinetics for the desulfurization process with R2
=
0.9953. The rate constant k of the oxidation reaction was deter-
mined to be 41.5 mg L−1 min−1 on the basis of eqn (1) and (2).
Comparative experiments using Tris-LDH-CO3/H2O2/
[bmim]BF4 and/or H2O2/[bmim]BF4 as desulfurization
systems (Fig. 5d) show sulfur removal efficiency is less than
20% in 60 min, whereas the Tris-LDH-PW12/H2O2/[bmim]BF4
exhibits 99% desulfurization in only 25 min by applying the
experimental conditions of T = 75 °C, H2O2 = 0.080 mL,
model oil = 5 mL (S = 1000 ppm), [bmim]BF4 = 1 mL, H2O2/
DBT/Cat = 100 : 20 : 1. This result indicates the synergetic
interactions between the Tris-modified LDHs and the inter-
calated PW12 anions lead to the large enhancement of the
desulfurization efficiency in the presence of H2O2 and
[bmim]BF4. Besides being used for extraction and catalytic
Fig. 5 (a) Sulfur removal of DBT; Ct as a function of reaction time at oxidative desulfurization (ECODS), Tris-LDH-PW12 can also
75 °C; (b) the comparison of the reactivity of DBT, 4,6-DMDBT, BT;
(c) the recycling experiments for desulfurization of DBT using Tris-
LDH-PW12 at 75 °C; (d) comparison of sulfur removal efficiency of
be applied for oxidation of various sulfides. As shown in
Table 3, different sulfides are oxygenated to the corres-
ponding sulfones (ca. 100%) with >99% selectivity. These
different catalytic systems. Reaction conditions: T = 75 °C, H2O2
=
experiments suggest that sulfides could be effectively oxi-
dized by Tris-LDH-PW12
0.080 mL, model oil = 5 mL (S = 1000 ppm), [bmim]BF4 = 1 mL, H2O2/
DBT/Cat = 100 : 20 : 1.
.
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Table 2 Comparison of catalytic oxidative desulfurization of DBTs using different catalysts containing [PW12O40
and Tris-LDH-PW12
]
3− or LDH reported in the literature
a
Entry
Catalysts
Oxidant
T [°C]
t [min]
S [ppm]
S removal [%]
Ref.
1
2
3
4
5
6
7
8
[(C18H37)2N(CH3)2][PW12O40
Na2HPW12O40
H3PW12O40
SBA15-PW12
[MIMPS]3PW12O40·2H2O
LDH-WO4
LDH-WO4
]
H2O2
H2O2
H2O2
t-BuOOH
H2O2
H2O2
H2O2
H2O2
60
60
60
70
30
70
40
75
40
150
60
120
60
60
180
25
500
5700
500
174
500
18 400
18 400
1000
96
95
99
97
100
29
28
29
30
31
32
33
34
90
99
Tris-LDH-PW12
This work
a SBA15-PW12 = SBA15-H3PW12O40, MIMPS = 1-(3-sulfonic group) propyl-3-methyl imidazolium phosphotungstate.
Science Foundation of China (21222104), and Fundamental
Research Funds for the Central Universities (RC1302) and the
Program for Changjiang Scholars and Innovative Research
Team in University.
Table 3 Oxygenation of sulfides by the Tris-LDH-PW12 in the presence
of H2O2
a
(Sulfoxide%/
sulfone%)
Entry
Substrate
Yield (%)
1
2
3
4
5
6
7
8
9
10
Methyl p-tolyl sulfide
Allyl phenyl sulphide
Tetrahydrothiapyran
4-Chlorothioanisole
Ethyl phenyl sulfide
Thioanisole
(0/100)
(0/100)
(0/100)
(0/100)
(0/100)
(0/100)
(0/100)
(0/100)
(0/100)
(0/100)
100
100
100
100
100
100
100
100
100
100
Notes and references
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Diethyl sulfide
4-Nitrophenylsulphide
4-Methoxythioanisole
4-Bromothioanisole
a Reaction conditions: Sulfide 1 mmol, Tris-LDH-PW12 2.5 µmol, H2O2
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Conclusions
In summary, we have prepared for the first time Na3[PW12O40]
intercalated Tris-modified LDHs without necessity of degas-
sing CO2, which results in the formation of a new intercalated
assembly of Tris-LDH-PW12 under mild conditions. This
cannot be realized using the traditional synthetic methods due
to the limitation of the [PW12O40]3− charge being below 4. The
XRD patterns of Tris-LDH-PW12 indicate clearly that there is
no impurity phase after (003) diffraction, which is in good con-
trast with the conventional POM intercalated LDHs. Further
application of Tris-LDH-PW12 for oxygenation of sulfides with
30% H2O2 exhibits highly efficient catalytic results under mild
conditions. The nanoscale particles of Tris-LDH-PW12 promote
the good dispersion of the catalysts and the access of the active
species to the substrates. As such, Tris-modified LDHs change
the host–guest interactions, and thereby greatly facilitate PW12
intercalation. Most importantly, they open a completely new
pathway for the development of POM-LDH advanced func-
tional materials.
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Acknowledgements
This research was supported by the National Basic Research 19 F. Cavani, F. Trifirò and A. Vaccari, Catal. Today, 1991, 11,
Program of China (973 program, 2014CB932104), the National
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