Facile assembly for fast construction of intercalation hybrids of layered double hydroxides with anionic metalloporphyrin

Article abstract of DOI:10.1039/c4dt00967c

Anionic manganese tetrasulfonatophenyl porphyrin (MnTSPP) has been intercalated into the interlamellar space of Mg-Al and Ni-Al layered double hydroxides (LDHs) through the exfoliation/restacking approach by using exfoliated LDH nanosheets and guest molecules as building blocks. The obtained hybrids were characterized by a variety of analytical techniques such as CHN analysis, XRD, FTIR, SEM, HRTEM, UV-vis spectroscopy and thermal analysis. Interlayer spacings determined from XRD patterns reveal a perpendicular orientation of the MnTSPP anions between the hyroxylated layers of both LDHs. The results of zeta potential measurements give information about the surface charge change of LDH nanoparticles associated with the spontaneous coassembly process. The catalytic performance of the heterogeneous catalysts MnTSPP/Mg-Al LDH_2.0 and MnTSPP/Ni-Al LDH_1.0 for the epoxidation of cyclohexene was investigated using molecular oxygen as an oxidant and isobutylaldehyde as a co-reductant. The intercalated hybrids appear to be promising catalysts owing to their good catalytic activity and selectivity.

Full text of DOI:10.1039/c4dt00967c

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Facile assembly for fast construction of
intercalation hybrids of layered double hydroxides
with anionic metalloporphyrin†
Cite this: Dalton Trans., 2014, 43,
9909
Juanjuan Ma,a Lin Liu,a Shanzhong Li,a Yonghao Chen,a Meng Zhuo,a Feng Shao,a,b
Junyan Gonga and Zhiwei Tong*a,c
Anionic manganese tetrasulfonatophenyl porphyrin (MnTSPP) has been intercalated into the interlamellar
space of Mg–Al and Ni–Al layered double hydroxides (LDHs) through the exfoliation/restacking approach
by using exfoliated LDH nanosheets and guest molecules as building blocks. The obtained hybrids were
characterized by a variety of analytical techniques such as CHN analysis, XRD, FTIR, SEM, HRTEM, UV-vis
spectroscopy and thermal analysis. Interlayer spacings determined from XRD patterns reveal a per-
pendicular orientation of the MnTSPP anions between the hyroxylated layers of both LDHs. The results of
zeta potential measurements give information about the surface charge change of LDH nanoparticles
associated with the spontaneous coassembly process. The catalytic performance of the heterogeneous
catalysts MnTSPP/Mg–Al LDH2.0 and MnTSPP/Ni–Al LDH1.0 for the epoxidation of cyclohexene was
investigated using molecular oxygen as an oxidant and isobutylaldehyde as a co-reductant. The inter-
calated hybrids appear to be promising catalysts owing to their good catalytic activity and selectivity.
Received 2nd April 2014,
Accepted 30th April 2014
DOI: 10.1039/c4dt00967c
www.rsc.org/dalton
III
formula of LDHs is [MII
M )
x(OH)2]x+(An− x/n·mH2O, with MII
1−x
Introduction
and MIII representing the metal cations octahedrally co-
Porphyrins are a well-known class of naturally occurring com- ordinated by hydroxyl groups and An− representing a hydrated
pounds that represent many important enzymes including counterion situated between the layers.11,12 The highly tunable
hemes, chlorophylls and horseradish peroxidase.1,2 The com- intralayer composition coupled with the widest possible choice
bination of porphyrins and appropriate host materials pro- of anionic organic moieties makes LDHs a promising building
duces the regulated structure and excellent functionality in block for hybrid materials. There has been growing interest in
biological systems.3 For example, in enzymes, the structures of the intercalation compounds LDH-porphyrin and LDH-
the proteins surrounding the porphyrin derivatives are crucial phthalocyanine due to their technological importance in cata-
for their functions. As a result, functional hybrids have been lysis, sensors and optics development.13–16 Several approaches
prepared by immobilization of porphyrin derivatives in various (coprecipitation, ion-exchange reaction and reconstruction
organic and inorganic host materials (polymers,4 clays,5,6 methods) have been applied to introduce porphyrin molecules
layered semiconductors,7–10 etc.) with potential applications in between the hydroxide layers.17–19 However, these methods
many fields such as photochemistry, electrochemistry and suffer from one or more drawbacks such as long reaction
catalysis.
times, low yield of the products and tedious work-up. Thus,
Layered double hydroxides (LDHs), also known as hydro- the development of a simple, efficient and rapid procedure is
talcite-like compounds, form a class of anionic clay. The general in demand.
Delamination of LDHs, defined as their segregation into
single entities through soft chemistry methods,20–22 is an
aDepartment of Chemical Engineering, Huaihai Institute of Technology,
interesting route for producing positively charged thin plate-
Lianyungang 222005, P.R. China. E-mail: zhiweitong575@hotmail.com;
lets with the thickness of a few atomic layers. The development
Fax: +86-518-85892778; Tel: +86-518-85892778
bSchool of Chemical Engineering, China University of Mining and Technology,
of inorganic–inorganic or organic–inorganic multifunctional
nanocomposites based on exfoliated LDH nanosheets has
attracted increasing attention during the past few years.
Various techniques have been used to fabricate these hybrids
such as electrostatic sequential deposition, flocculation or
layer-by-layer (LBL) methods.23–27 Among them, flocculation is
Xuzhou, 221116, P.R. China
cSORST, Japan Science and Technology (JST), Kawaguchi Centre, Building 418,
Honcho Kawaguchi-shi, Saitama 332-0012, Japan
†Electronic supplementary information (ESI) available: Chemical composition,
FTIR spectra, SEM images and zeta potential measurement results discussed in
the manuscript. See DOI: 10.1039/c4dt00967c
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Al(NO3)3·9H2O and urea were dissolved in 80 mL of deionized
water to give the final concentrations of 0.1, 0.05 and 0.245 M,
respectively. The mixture was transferred to a Teflon-lined
autoclave and placed in a preheated oven at 140 °C for 24 h.
The resulting precipitate was collected by centrifugation and
washed with deionized water and anhydrous ethanol several
times. Anal. calc. for [Mg0.66Al0.34(OH)2](CO3)0.17·0.63H2O:
C, 2.53; H, 4.07; found: C, 2.58; H, 4.11. Anal. calc. for
[Ni0.68Al0.32(OH)2](CO3)0.16·0.57H2O: C, 1.88; H, 3.09; found:
C, 1.84; H, 3.06.
0.2 g of M(II)–Al–CO3 LDH was further treated with 200 mL
of an aqueous solution containing 1.5 M NaNO3 and 5 mM
HNO3 and stirred under a nitrogen atmosphere at ambient
temperature for 24 h. The exchanged product was recovered
using the same procedure as that described for the pristine
material.35,36
Fig. 1 Schematic illustration of the preparation process for metallo-
porphyrin intercalated LDH nanocomposites via the exfoliation/
restacking route.
To perform the delamination of M(II)–Al–NO3 layered struc-
ture and access to a colloidal suspension, 0.1 g of the LDH
sample was dispersed into 100 mL of formamide and stirred
vigorously for 2 days under a nitrogen atmosphere. The result-
ing translucent colloidal suspension was then centrifuged at
2000 rpm for 10 min to remove the unexfoliated particles.
most convenient. Furthermore, since the interlayer of the LDH
is completely open, there is no steric restriction for the immo-
bilization of bulky guest moieties. As reported, inorganic
nano-colloids such as graphene oxide,28 Au nanoparticles,29
CdSe quantum dots30 and transition metal oxide nanosheets31–33
have been assembled with LDH nanosheets via spontaneous
flocculation, forming sandwich-structured hybrids. In contrast,
reports on the combination of LDH nanosheets and organic
molecules via flocculation are quite limited.34
Fabrication of MnTSPP/M(II)–Al LDH intercalation hybrids
The colloidal dispersion of LDH nanosheets has been purged
with nitrogen gas to eliminate interference from atmospheric
carbon dioxide. Then, 2.5, 5.0, or 10.0 mL of 1.26 mmol L−1
MnTSPP aqueous solution were added to the delaminated
LDH suspension, corresponding respectively to a MnTSPP/
LDH volume ratio of 0.5, 1.0, and 2.0, denoted hereafter as
MnTSPP/LDH0.5, MnTSPP/LDH1.0, and MnTSPP/LDH2.0. The
precipitation was centrifuged under 8000 rpm for 5 min, fol-
lowed by washing with deionized water and anhydrous ethanol
repeatedly to remove water and formamide completely. The
quantity of Mn porphyrins in the final hybrids has been listed
in ESI.†
Herein, we have reported the direct coassembly of LDH
nanosheets and Mn porphyrin anions to fabricate sandwich-
structured nanocomposites for the first time (Fig. 1). The gen-
erality of this protocol was examined by choosing LDHs with
different lateral sizes, Mg–Al LDH (lateral size in micrometres)
and Ni–Al LDH (lateral size in hundreds of nanometers). The
preparation method is as follows: synthesis of highly crystal-
line M(II)–Al–CO3 LDHs (M(II) = Mg, Ni); conversion of the
2−
−
interlayer CO3 to NO3 by means of salt–acid treatment; ex-
foliation of these M(II)–Al–NO3 LDHs in formamide; and finally
fabrication of metalloporphyrin/LDH lamellar nanocomposites
by adding metalloporphyrin aqueous solution into the col-
loidal dispersion of LDH nanosheets. The positively charged
LDH nanosheets have been employed as macrocationic tem-
plates to direct the self-assembly of metalloporphyrin anions
in solution resulting in the precipitation of MnTSPP/M(II)–
Al LDH hybrids. The assembly of metalloporphyrin anions into
the interlamellar space of the LDH host can be finished in
20 minutes at room temperature without complicated instru-
ments. This assembly system will pave the way for further appli-
cations of the diverse co-assemblies with novel functionalities.
Sample characterization
X-ray diffraction patterns were obtained with a RINT 2000
diffractometer (Rigaku), using Cu Kα radiation (λ = 0.154 nm)
with 2θ from 2° to 70°. The metal contents of LDH samples
were determined by inductively coupled plasma (ICP) emission
spectroscopy on a Shimadzu ICPS-7500 instrument. Carbon,
hydrogen and nitrogen analysis data were obtained with a
Perkin-Elmer 2400 CHN elemental analyzer. Infrared spectra
were recorded on a Shimadzu FTIR-8400S spectrometer with
the use of KBr pellets. Zeta potential of LDH colloids was
measured using a Malvern Zetasizer Nano instrument. The
morphology of the samples was investigated by a field-emis-
sion scanning electron microscope (FESEM; FEI Quanta 600)
and
a high-resolution transmission electron microscope
Experimental section
Preparation of exfoliated M(II)–Al LDH (M(II) = Mg, Ni)
(HRTEM; JEOL JEM-2100F). Samples for HRTEM measure-
ments were prepared by sonicating the products in ethanol for
30 min and evaporating a drop of the resulting suspension
nanosheets
Highly crystalline particles of M(II)–Al–CO3 LDH were syn- onto a copper grid. UV-Vis absorption spectra were recorded
thesized according to a previous report. Typically, M(II)(NO3)2, on a Thermo Fisher spectrometer (Evolution 220). Thermo-
9910 | Dalton Trans., 2014, 43, 9909–9915
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gravimetry (TG) and differential scanning calorimetry (DSC) of ∼11.7°, indicating a basal spacing of ∼0.76 nm. And the (110)
measurements were carried out simultaneously using diffraction peaks of Mg–Al and Ni–Al LDH are at about 60.8°
Netzsch STA449F3 apparatus at a heating rate of 10 °C min−1 and 61.1°, respectively. From the diffraction peaks of (110) and
a
under a nitrogen atmosphere.
(003), unit cell values of a and c can be calculated as 0.3048
and 2.2838 nm for Mg–Al LDH, and 0.3034 and 2.2760 nm for
Ni–Al LDH. The unit cell areas of Mg–Al and Ni–Al LDH
nanosheets are 8.046 and 7.972 Å2. Chemical analysis gives a
Mg/Al ratio of 1.94 and a Ni/Al ratio of 2.13 in the two carbo-
nated LDH samples, respectively. Accordingly, the layered
charge density of Mg–Al and Ni–Al LDHs can be obtained as
0.0423 and 0.0401 e+ per Å2, respectively (Table S1†).38
Catalytic oxidation reaction
The oxidation reactions were carried out at room temperature
under 1 atm of oxygen with constant stirring. Cyclohexene
(2 mmol), the solid catalyst (in a MnTSPP/substrate molar
ratio of 1 : 1000) and isobutylaldehyde (6 mmol) were sus-
pended in 4 mL of acetonitrile.37 The consumption of the sub-
strate and formation of oxidized products were monitored by
GC-MS (Shimadzu GCMS-QP2010) using naphthalene as an
internal standard substance. For the recycling experiments,
the catalyst was exhaustively washed with acetonitrile to
remove the occluded reactants and products. The recycled cata-
lysts were then dried at room temperature and reused using
the same experimental conditions as those described above.
After the decarbonation and anion exchange with NO3−, the
basal spacings of Mg–Al and Ni–Al LDH samples increased to
0.89 and 0.88 nm, respectively. The interlayer expansions are
consistent with that in previous work.39 Through treating the
as-prepared nitrated LDHs with formamide at room tempera-
ture for 2 days, colorless and light green transparent colloidal
suspensions were yielded for MgAl- and NiAl-LDH samples
respectively. Clear Tyndall light scattering was discerned for
both suspensions and no sediment was observed upon stand-
ing, demonstrating the collapse of layered structure and the
presence of abundant exfoliated nanosheets dispersed in for-
mamide. The delamination mechanism has been proposed by
Hibino40 and Li41 et al. The carbonyl group of formamide has
a strong interaction with the LDH hydroxyl slabs, whereas
the other end (–NH2) bonds weakly with NO3−. Therefore,
once the replacement of water by formamide takes place,
this weakens the interlayer attraction force through the
destruction of the strong hydrogen-bonding network and
promotes delamination.
Results and discussion
Preparation of LDH nanosheets
The XRD patterns (Fig. 2) of M(II)–Al–CO3 LDH samples
showed very intense basal reflection series, indicative of the
highly crystalline nature of these products. The (003) reflec-
tions of both LDHs were located on almost the same 2θ angle
Coassembly of LDH nanosheets and MnTSPP
Fig. 2a and b show the XRD patterns of the samples prepared
by mixing MnTSPP aqueous solution with a colloidal suspen-
sion of Mg–Al and Ni–Al LDHs in formamide. In comparison
with the corresponding precursors, the (00l) diffraction peaks
shift to lower angles, evidencing the stacking of LDHs with
large metalloporphyrin molecules in the interlayer. The
decrease in crystallinity after the restacking process is indi-
cated by not only the weaker intensity and broader profile of
the (00l) reflection lines, but also by the absence of many (10n)
and (01n) in-plane lines at high angle values. The structural
disorganization of the obtained composites probably arises
from the rapid precipitation and nature of the materials,
which is one of the main drawbacks of the exfoliation/restack-
ing method.24
Furthermore, it is interesting to note that the diffraction
peak shape of the resulting hybrids varies according to the
respective proportions of each component involved initially in
the restacking process. XRD patterns of MnTSPP/Mg–Al
LDH1.0, MnTSPP/Mg–Al LDH2.0 and MnTSPP/Ni–Al LDH1.0 give
a series of symmetric and well-defined peaks corresponding to
00l reflections, while in other cases, relatively broader and less
symmetric reflections were obtained, implying
number of stacking faults. The gallery heights of MnTSPP/Mg–
Al LDH2.0 and MnTSPP/Ni–Al LDH1.0 were estimated to be 2.32
a bigger
Fig. 2 XRD patterns of carbonated, nitrated Mg–Al and Ni–Al LDHs and
MnTSPP/LDH hybrids obtained at different volume ratios of MnTSPP
solution to colloidal suspension of LDH nanosheets.
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and 2.36 nm. By subtracting the thickness of the brucite-like
As observed in situ from Fig. 3, dark brown flocculent pre-
LDH layer (∼0.48 nm), the net interlayer height of 1.84 and cipitates were formed and they settled down to the bottom of
1.88 nm for metalloporphyrin intercalated MgAl- and NiAl- the container quickly after MnTSPP aqueous solution was
LDH could be obtained, respectively. Considering the mole- added to the M(II)–Al LDH nanosheet suspension. The reaction
cular dimension of the MnTSPP macrocycle (18.0 × 18.0 × is extremely fast, and is complete within 20 minutes. To get
4.3 Å, as obtained from steric energy minimization using the better insight into the restacking process, the surface charge
MM2 method), a perpendicular orientation of the macrocycles changes were traced through zeta potential measurements.
to the layers with the four anionic groups interacting with the The zeta potentials of Mg–Al and Ni–Al LDH nanosheet disper-
hydroxylated sheets can be speculated. Thus, the surface area sions are 35.9 and 32.3 mV, respectively, which shows that
per unit charge (S0) for MnTSPP equals 19.4 Å2 per e−. The both dispersions are stable and well-dispersed. For the
comparison with S0 values of Mg–Al (23.6 Å2 per e+) and Ni–Al samples with addition of MnTSPP, supernatant liquid was col-
(24.9 Å2 per e+) LDH hosts indicates a good surface compatibil- lected and measured. As shown in Fig. 3c, a progressive
ity. Such a perpendicular arrangement has also been described increase of MnTSPP volume reduces the zeta potential of LDH
by Taviot-Guého et al.42 for ZnTSPP and FeTSPP and Abellán nanosheets gradually. When 2-fold volume of MnTSPP is intro-
et al.43,44 for CoTSPP.
duced, zeta potential absolute values for both nanoparticles
The IR spectra of carbonated LDHs, nitrated LDHs and the approach zero, suggesting extremely unstable suspensions. In
restacked materials are shown in Fig. S1.† MnTSPP/Mg–Al the case of MnTSPP/Ni–Al LDH2.0, coassembly of positively
LDH2.0 exhibits characteristic absorption bands associated charged LDH nanosheets with MnTSPP even converts the
with the intercalated manganese porphyrin: 856, 877, and surface properties of LDHs from positive to negative. The
1008 cm−1 (C–H pyrrole bending); 1349 and 1416 cm−1 (C–N larger the MnTSPP volume involved, the better the destabiliza-
stretching); 1491 and 1574 cm−1 (C–C phenyl and pyrrole tion of the colloidal solution is.
stretching, respectively); 1039, 1126, 1187 and 1221 cm−1 (sul-
Scanning electron microscopy (SEM) and high resolution
fonic groups).45 The bands in the range of 450–800 cm−1 arise transmission electron microscopy (HRTEM) images of the floc-
from the vibrations of the hydroxyl-based octahedral metallic culated MnTSPP/LDH nanocomposites are displayed in Fig. 4.
complexes belonging to the LDH slabs. Similar vibration For comparison, the morphologies of the pristine carbonated
bands are also observed in the IR spectrum of MnTSPP/Ni–Al LDHs are shown in Fig. S2.† The SEM image of Mg–Al–CO3
LDH1.0. These results further confirm the incorporation of the LDHs reveals hexagonal platelets with an average particle size
metalloporphyrin complex into the LDH interlamellar space. of about 2–4 μm. Ni–Al–CO3 LDHs are crystallized into irregu-
Additionally, the sharp, strong absorption peak at 1356 cm−1 lar plate crystals with the lateral dimension of about
in pristine carbonated LDHs belongs to the v3 stretching 200–400 nm, which is smaller than that of Mg–Al–CO3 LDHs.
mode of CO32−. Although less intense, this peak can also be For MnTSPP/LDH nanocomposites, the aggregation of plate-
observed in nitrated LDHs and the restacked materials, indi- like particles is formed as expected. This result clearly evi-
cating the residual presence of non-exchanged CO32− anions.
dences that after the coassembly of exfoliated M(II)–Al LDH
nanosheets with MnTSPP anions, a two-dimensional layered
structure is reconstructed. Additionally, this process also leads
Fig. 3 Digital photographs of colloidal suspensions of Mg–Al (a) and
Ni–Al (b) LDHs in formamide (left), MnTSPP aqueous solution (middle)
and the corresponding mixture of LDHs and MnTSPP (right). (c) The
relationship between zeta potential and MnTSPP/LDH volume ratio.
Fig. 4 SEM and HRTEM images of MnTSPP/Mg–Al LDH2.0 (a, b) and
MnTSPP/Ni–Al LDH1.0 (c, d).
9912 | Dalton Trans., 2014, 43, 9909–9915
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Fig. 5 UV-visible absorption spectra of (a) MnTSPP aqueous solution,
(b) MnTSPP/Mg–Al LDH and (c) MnTSPP/Ni–Al LDH hybrid.
to a reduction in the size of particles with respect to the pre-
cursors, indicating the breakage or fracture of sheets during
the delamination process. The lamellar fringe was not comple-
tely parallel but slightly twisted in some areas, which may be
due to the high flexibility of the nanosheets. Our research
demonstrates that both LDHs can be used to fabricate
MnTSPP/LDH intercalation hybrids whatever the lateral size of
LDH crystals is.
Fig. 5 shows UV-Vis absorption spectra of free MnTSPP in
aqueous solution and intercalation hybrids. For the MnTSPP
aqueous solution, the Soret band at 467 nm and two weaker Q
bands between 550 and 650 nm can be observed. The absorp-
tion spectra of the hybrids suggest that no demetallation
occurred (characterized by a blue shift of the Soret band
associated with
a
significant amount of free base
porphyrin).46–48 The insertion of MnTSPP anions into the Mg–
Al and Ni–Al interlayers led to a 6 nm and a 10 nm red shift in
the Soret band, respectively. A similar phenomenon has been
repeatedly observed by many groups who have worked with
this kind of immobilization reaction (metalloporphyrin
immobilized in inorganic supports).49–51 And the spectral
change of metalloporphyrin is mostly attributed to the steric
constraints caused by the support, which modifies the manga-
nese porphyrin structure substantially in the hybrid. Moreover,
a broadening of the Soret band is also observed for the inter-
calation hybrids, which is generally attributed to some degree of
aggregation and stacking of the metalloporphyrin molecules.15
Fig. 6 TG (solid line) and DSC (dashed line) curves for MnTSPP (a), Mg–
Al–CO3 LDH (b), MnTSPP/Mg–Al LDH2.0 (c), Ni–Al–CO3 LDH (d) and
MnTSPP/Ni–Al LDH1.0 (e).
interval can be attributed to the evaporation of physically
adsorbed and interlayer water. The water content for LDHs has
been extracted from this first step (see ESI, Table S1†). The fol-
lowing steps of weight loss, with two endothermic peaks at 290
and 429 °C for Mg–Al and at 273 and 337 °C for Ni–Al, are pro-
duced by the removal of hydroxyl groups from the layers and
the decarbonation (Fig. 6b and d).52
In contrast, metalloporphyrin-containing LDH samples
present more complicated thermal decomposition behaviors
(Fig. 6c and e). Besides the same decomposition steps as those
of the pristine LDHs discussed above, the weight loss above
320 °C could be related to the degradation of MnTSPP inter-
calated in the interlayer. The extent of intercalated MnTSPP in
the LDH host is also reflected in the residue obtained.
MnTSPP/Mg–Al LDH2.0 yields a lower amount of residue
(38.2%) at 800 °C compared with that for Mg–Al LDH (55.6%)
at the same temperature. Similarly, the residue of MnTSPP/
Thermal analysis
Thermogravimetry (TG) and differential scanning calorimetry
(DSC) analyses have been carried out for the samples (pure
metalloporphyrin, pristine LDHs and intercalation hybrids)
under a nitrogen atmosphere in the 25–800 °C range, as
shown in Fig. 6. The decomposition of pure metalloporphyrin
seems to occur in two steps. The first and sharp one at about
307 °C is attributed to the loss of chains of the porphyrin ring
and the second and broader one at about 566 °C corresponds
to collapse of the porphyrin skeleton (Fig. 6a). TG-DSC curves
of carbonated LDHs presented in Fig. 6b and d show two main
weight loss steps. The first step that occurs at the 25–210 °C
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Ni–Al LDH1.0 (49.5%) is also lower than that of Ni–Al LDH
The reuse of both heterogeneous catalysts gives a similar
(66.8%). This is another proof for the successful coassembly of selectivity (about 90%) and slightly lower yields with respect to
LDH nanosheets with metalloporphyrin molecules via the the first use. This can also be indicative of the resistance of
exfoliation/restacking route. Moreover, pure MnTSPP starts to manganese porphyrin to the harsh oxidative reaction con-
decompose at ca. 260 °C and the onset is shifted to about ditions and strong interaction between the manganese por-
325 °C and 340 °C after the direct assembly with Mg–Al and phyrin and LDH supports. LDHs seem to contribute to the
Ni–Al LDH nanosheets. This result indicates that host–guest stabilization of the metalloporphyrin complex, and also play a
interactions thermally stabilize the porphyrin molecules simi- role in constructing a two-dimensional nanoreactor. The oxi-
larly to other intercalated molecules.23,43
dation can proceed selectively in the molecule size of the sub-
strate by means of this reaction field.
Catalytic performance of the intercalated manganese
porphyrin
Conclusions
Epoxidation of cyclohexene was studied in order to investigate
the efficiency and stability of manganese porphyrin as a cata-
lyst for alkene oxidation and also to have an idea of the acces-
sibility of the substrate and of the oxidant to the manganese(III)
sites in the intercalated catalysts. The results from cyclohexene
epoxidation by molecular oxygen catalyzed by the MnTSPP
either in solution or immobilized in matrixes are presented in
Table 1.
In the control reaction, only 20% cyclohexene is converted
after 5 h. As for MnTSPP, 95% cyclohexene is converted within
2 h, demonstrating that the metalloporphyrin catalyst is
crucial for the aerobic oxidation system. However unsatis-
factory selectivity for the epoxidation product is observed in the
homogeneous system. Substantially better yields are obtained
with the heterogeneous catalysts under the same reaction con-
ditions. Here we can recall that the net interlayer height of the
synthesized MnTSPP/Mg–Al LDH2.0 and MnTSPP/Ni–Al LDH1.0
were 1.84 and 1.88 nm, respectively. Thus, they seem to
provide sufficient space for cyclohexene of which the size is
about 0.45 nm to approach the central metal of porphyrin.6
The good catalytic capability of the intercalated porphyrin may
be attributed to the better distribution of the intercalated
manganese porphyrin than the aggregated form of porphyrin
in organic solution and the prevention of inactive µ-oxo
dimers.53
In summary, we have reported a simple and rapid exfoliation/
restacking synthetic route for preparing the intercalation
hybrids of layered double hydroxides with metalloporphyrin.
Based on XRD analysis, a perpendicular arrangement of the
porphyrin ring with respect to the LDH layers has been pro-
posed. The intercalation of MnTSPP into the LDH changes the
crystallinity and morphology of the resulting nanocomposites
substantially in relation to the LDH-CO3 precursor. Spectro-
scopic studies reveal that the intercalation hybrids display a
red-shift in the Soret-band of metalloporphyrin. Furthermore,
the thermal degradation temperature of MnTSPP in the nano-
composites is raised, implying that the host–guest interactions
thermally stabilize the porphyrin molecules between the LDH
nanosheets. Preliminary results demonstrate that the intercala-
tion hybrids present good catalytic properties in the cyclo-
hexene epoxidation. It can be expected that a wide variety of
functional bulky anions can be introduced into the interlayer
spacing of LDHs in a similar manner. By virtue of the highly
tunable compositions for both inorganic LDH hosts and
organic guest molecules, these hybrid composites can be tai-
lored with potential technological applications in numerous
fields, including catalysts, sensors and optoelectronic devices.
Acknowledgements
Table 1 Epoxidation of cyclohexene by molecular oxygen in the pres-
We are grateful for the support from the National Natural
Science Foundation of China (grant no. 21001048), the Natural
Science Fund of Jiangsu Province (BK2011399, SBK201220654),
the University Science Research Project of Jiangsu Province
(13KJB430005, 12KJD150001, 11KJA430008) and a project
funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
ence of various catalysts and isobutyraldehydea
Reaction
time (h)
Catalyst
Conv. (%)
Yield (%)
MnTSPP
2.0
2.0
2.0
2.0
2.0
2.0
2.0
5.0
5.0
5.0
95
>99
99
92
97
95
89
21
21
20
64
91
90
83
88
86
80
17
17
15
MnTSPP/Mg–Al LDH2.0
1st reutilization
2nd reutilization
MnTSPP/Ni–Al LDH1.0
1st reutilization
2nd reutilization
Mg–Al–CO3 LDHb
Ni–Al–CO3 LDHb
No catalystb
Notes and references
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a Conditions: cyclohexene (2 mmol), isobutyraldehyde (6 mmol),
catalyst (2
×
10−3 mmol), CH3CN (4 mL), O2 bubbling, room
temperature. b Control reaction performed under the same conditions
as the oxidation reaction described in (a).
9914 | Dalton Trans., 2014, 43, 9909–9915
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