Polyoxometalate-based metal-organic frameworks as catalysts for the selective oxidation of alcohols in micellar systems

Article abstract of DOI:10.1002/cplu.201400009

A series of nanosized metal-organic frameworks (MOFs) encapsulating different polyoxometalates (POMs) including H₃PW₄O ₁₂, H₅PMo₁₂O₄₀, H ₅PVMo₁₀O₄₀, H₅PV₂Mo ₁₀O₄₀, and H₅PV₃Mo ₁₀O₄₀ was synthesized and used in the selective oxidation of alcohols. The catalyst with a uniform size and morphology offered easy accessibility between substrates and catalyst. At the same time, the MOF ensured that the POM was encapsulated, which could dramatically prevent the assembly of the catalyst. Furthermore, the catalyst showed clear chemoselectivity, which was related to the size or accessibility of the substrates for surface pores. With cetyltrimethyl ammonium bromide aqueous solution as solvent, both improved reaction efficiency and simple recycling of the catalytic system were achieved to afford a green oxidation process. It's what's inside that counts: A series of polyoxometalate (POM)-based metal-organic frameworks (MOFs) was prepared and used in the selective oxidation of alcohols. Experiments with these MOFs were carried out in cetyltrimethyl ammonium bromide micellar solutions. With various alcohols as substrates, better to excellent conversion and selectivity were presented (see figure). Thus a green and efficient process was obtained. Copyright

Full text of DOI:10.1002/cplu.201400009

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DOI: 10.1002/cplu.201400009
Polyoxometalate-Based Metal–Organic Frameworks as
Catalysts for the Selective Oxidation of Alcohols in
Micellar Systems
Jie Zhu, Meng-nan Shen, Xue-jing Zhao, Peng-cheng Wang, and Ming Lu*[a]
A series of nanosized metal–organic frameworks (MOFs) encap-
sulating different polyoxometalates (POMs) including
H3PW4O12, H5PMo12O40, H5PVMo10O40, H5PV2Mo10O40, and
H5PV3Mo10O40 was synthesized and used in the selective oxida-
tion of alcohols. The catalyst with a uniform size and morphol-
ogy offered easy accessibility between substrates and catalyst.
At the same time, the MOF ensured that the POM was encap-
sulated, which could dramatically prevent the assembly of the
catalyst. Furthermore, the catalyst showed clear chemoselectiv-
ity, which was related to the size or accessibility of the sub-
strates for surface pores. With cetyltrimethyl ammonium bro-
mide aqueous solution as solvent, both improved reaction effi-
ciency and simple recycling of the catalytic system were ach-
ieved to afford a green oxidation process.
Introduction
Selective oxidation of primary and secondary alcohols into the
corresponding aldehydes or ketones is undoubtedly one of the
most important and challenging transformations in organic
chemistry.[1] Many catalysts have been developed to accom-
plish the reaction including (2,2,6,6-tetramethylpiperidin-1-
yl)oxyl (TEMPO),[2] metal oxides,[3] hypervalent iodine,[4] and so
on. Another common and efficient route uses polyoxometa-
lates (POMs) together with hydrogen peroxide (H2O2) as termi-
nal oxidant, which has commonly been considered a “green
oxidant” because of its high content of active oxygen species
and coproduction of only water. Research published since the
1980s has established the high potential of POMs for activation
of H2O2, thanks to the nanoarrays of d0 transition-metal ions
such as WVI, MoVI, and VV exposed on their surfaces.[5]
stable host HKUST-1,[10] as a well-known MOF, was reported to
be able to encapsulate various Keggin-type POMs. Wee and
co-authors reported on a microporous cubic material with
strictly repetitive 5 nm-wide mesopores and the stability was
enhanced by Keggin-type phosphotungstate (HPW) systemati-
cally occluded in the cavities constituting the walls between
the mesopores.[11] Although the approach successfully solved
the problem of POM aggregation in catalytic processes and
offers great potential for chemical and structural diversity, in-
vestigation of the catalytic properties of these kinds of materi-
als is rather limited.[12]
The use of water as a medium for promoting organic reac-
tions is very important and has received much attention in
recent years from the point of view of green and economical
chemistry. To perform oxidation reactions in water, the solubili-
ty problem of substrates or catalysts must be overcome, be-
cause of the limited mutual solubility between water and or-
ganic agents. Micelles, which are dynamic clusters of surfactant
molecules that possess both hydrophilic and hydrophobic
structures, may associate in aqueous media to circumvent the
problem. Micelles can concentrate the reactants within their
small volumes; stabilize substrates, intermediates or products;
and orient substrates. Thus, they can alter the reaction rate,
mechanism, and regio- and stereochemistry.[13]
No matter which POM is used, however, separation and re-
covery are usually key problems to be solved owing to the sol-
ubility of POMs. Much effort has been devoted to the field of
immobilizing POMs, with organic hybrids of POMs as an attrac-
tive possibility for various oxidation reactions. Examples in-
clude surfactant–POM combinations,[6] coordination poly-
mers,[7] polymer–POM conjugates,[8] and so on. However, the
difficulty in controlling the separation, the aggregation of
POMs in the preparation of the catalyst, and the limited varia-
tion in chemical formulation and functionality often limit their
applications.
In this study, selective oxidations were performed in micellar
systems with MOF-encapsulated POMs as catalyst. A serious of
nanosized metal–organic frameworks were synthesized encap-
sulating different POMs including H3PW4O12, H5PMo12O40,
H5PVMo10O40, H5PV2Mo10O40, and H5PV3Mo10O40 (noted as MOF-
HPW, MOF-HPMo, MOF-HPMoV, MOF-HPMoV2, and MOF-
HPMoV3, respectively), which all offered organized multiple
porosity and high surface area. Their catalytic performance for
the selective oxidation of various alcohols was tested and com-
pared in a micellar system. Both improved reaction efficiency
and simple recycling of the catalytic system were achieved in
Lately, the use of transition metals, organic ligands, and
POMs to assemble POM-based metal–organic frameworks has
emerged as one of the approaches to solve the problems.[9]
A
[a] J. Zhu, M.-n. Shen, X.-j. Zhao, P.-c. Wang, Prof. M. Lu
School of Chemical Engineering
Nanjing University of Science and Technology
Xiaolingwei 200, Nanjing, Jiangsu (P. R. China)
E-mail: luming@mail.njust.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201400009.
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cetyltrimethyl ammonium bromide (CTAB) micellar solutions.
Thus a green oxidation process was achieved.
Results and Discussion
The concept of the catalysts
The catalysts were easily prepared from a synthesis mixture
containing the molar composition of 18Cu/10BTC/POM/CTAB/
170EtOH/2000H2O (BTC=1,3,5-benzenetricarboxylate). Ther-
mogravimetric analysis (TGA) was applied to the catalysts as
shown in Figure S1 in the Supporting Information. From the
TGA data, weight contents of POMs were established to be
0.506, 0.429, and 0.330 for MOF-HPW, MOF-HPMo, and MOF-
HPMoV2, respectively, which matched well with their corre-
sponding theoretical values of 0.485, 0.374, and 0.362, respec-
tively.
Figure 1. IR spectra of a) MOF-HPW, b) HKUST-1, and c) H3PW4O12 raw materi-
al.
hibited, which clearly differs from that of bare HKUST-1 (curve
b). Also, when comparing the IR spectrum of the MOF with its
relative POM (curve a with c), curves a and b show detectable
changes around 1500 cmÀ1 that are characteristic of CÀH, CÀC,
and C=O vibrations, which suggests the formation of the
framework, and which are absent from the spectrum of the
POM alone. IR spectra of MOF-PMo, MOF-HPMoV1, MOF-
HPMoV2, and MOF-HPMoV3 are shown in the Supporting In-
formation.
Chui et al. revealed that the polymer framework of HKUST-
1 is composed of dimeric cupric tetracarboxylate units. Twelve
carboxylate oxygen atoms from the two BTC ligands bind to
four coordination sites for each of the three Cu2+ ions of the
formula unit. For most POM-bearing MOFs, the POM is added
to the hydrothermal synthesis and found in the cavities after-
wards. In the case of the HKUST-1, however, strong interaction
between CuII and Keggin-type ions is discovered, which leads
to spontaneous self-assembly of microporous MOF-POM.[14]
The Keggin ion acts as a templat-
To further confirm the structure of the MOF-POMs, X-ray dif-
fraction (XRD) studies were also performed. Figure 2 shows the
ing species during synthesis and
stabilizes the microporous struc-
ture by means of the synergism
between the metal and Keggin
ions.[15] A Cu2+/POM interaction
directs Cu2+ ions to preferential-
ly reside close to the positions
needed to build the MOF frame-
work. In such a scenario, the role
of the organic linker is simply to
connect the already structured
Cu2+/POM units through com-
plexation. Namely, the MOF as-
sembly results in strictly system-
Figure 2. I) XRD patterns of a) HKUST-1, b) MOF-HPW, and c) MOF-HPMo; II) XRD patterns in the low-angle region
atic encapsulation of POM mole-
cules. In addition, the nanosize
and high surface area of Cu-MOF
of d) MOF-HPMo and e) MOF-HPW.
make it an excellent choice for POM anchoring. MOFs and zeo-
lites are very similar in terms of their porous structures, but
quite different in the situation of immobilizing POMs. There-
fore it would be expected that firstly, the MOF could be used
as a catalyst for reactions and the accessibility between sub-
strates and the catalyst could be maintained; and secondly, dif-
ferent from a normal support in which the active site is anch-
ored on its surface, the MOFs ensured that the POM was en-
capsulated, which could dramatically prevent the assembly of
the catalyst.
XRD pattern of MOF-HPW and MOF-HPMo, and the results for
MOF-HPMoV, MOF-HPMoV2, MOF-HPMoV3, and POM raw ma-
terials are listed in the Supporting Information. From Figure 2,
a high degree of mesoscopic ordering is confirmed by the
sharp reflections appearing at low angles.
TEM and SEM images of MOF-HPW and MOF-HPMo
(Figure 3; images of MOF-HPMoV are shown in the Supporting
Information) indicate that the catalyst was a framework struc-
ture with uniform size and morphology. Viewed from the
bright field, highly ordered mesostructures with long-range or-
dering could be observed, which testified that Keggin ions
were contained in the microporous wall structures, next to the
less dense mesopores in the structure. Parallel lines with a dis-
Figure 1 shows the IR spectra of MOF-HPW, HKUST-1, and
H3PW4O12 raw material. It can be seen that some characteristic
peaks belonging to WÀO vibrations around 1000 cmÀ1 are ex-
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including cationic surfactant ce-
tyltrimethyl ammonium bromide
(CTAB),
anionic
surfactant
sodium dodecyl sulfate (SDS),
and neutral surfactant TritonX-
100 (TX-10). For all the surfac-
tants, experiments were per-
formed at their tenfold critical
micellar concentration (10cmc)
in the presence of MOF-HPW. In
addition, for comparison with
micellar systems, acetonitrile,
which is usually used in POM-
catalyzed oxidations, was chosen
as an organic solvent. As can be
seen in Figure 4, the CTAB micel-
lar system gave the best result,
followed by acetonitrile, TX-10,
and SDS, respectively. The yield
of benzaldehyde was able to
reach 98% after 3 h in CTAB
aqueous solution, which was
even faster than that in an or-
ganic solvent (acetonitrile solu-
tion). So using water as the reac-
tion media was a good choice in
view of the reaction efficiency as
well as green and economical
chemistry. The fact that the reac-
tion could be facilitated by use
of a cationic surfactant also testi-
fied to the nature of micellar re-
actions. The reactant benzyl al-
cohol accumulated not in the
surrounding water phase, but in
the micelle through interactions
with the micelle surface or
through insertion into the mi-
celle itself, as was the catalyst.
Since the oxidant H2O2 was in
the aqueous phase,[16] the oxida-
Figure 3. a) TEM image of MOF-HPW; b) SEM image of MOF-HPW; c) TEM image of MOF-HPMo; d) SEM image of
MOF-HPMo; e) TEM image of MOF-HPMoV2; f) SEM image MOF-HPMoV2.
tance of 3.6 nm could be observed in Figure 3c, similar to
those previously reported in reference [11], which means that
the catalyst we prepared follows an analogous structure. For
MOF-HPMo and MOF-HPMoV, the size of the framework
changed greatly, but the mesostructures were still clearly visi-
ble, which also confirmed that the Keggin ion acted as the
templating species during the synthesis of MOF.
Exploration of the reaction conditions
First, MOF-HPW was taken as a model catalyst to show the im-
portant effect of micellar media upon the reactions. Oxidations
of benzyl alcohol were studied in different systems and the
Figure 4. Oxidation of benzyl alcohol with MOF-HPW in different systems:
a) CTAB micellar solution; b) CH3CN; c) TX-10 micellar solution; and d) SDS
micellar solution.
comparative results are demonstrated in Figure 4. Three
common surfactants were chosen to generate micellar systems
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tion process took place at the surface of the micelles. As
a result of the attraction of the POM anion to the cationic sur-
factant, the reaction rate was accelerated greatly in CTAB mi-
cellar solution.
To further optimize the reaction conditions, preliminary ex-
periments were performed with benzyl alcohol as the sub-
strate and H2O2 as the oxidant in CTAB micellar solution. The
comparable results are summarized in Table 1. It was found
Table 1. Oxidation of benzyl alcohol with MOF-HPW under various condi-
tions.[a]
Figure 5. Oxidation of benzyl alcohol by MOFs with different POMs (selectivi-
ty was for benzaldehyde). Reaction conditions: benzyl alcohol (2 mmol),
CTAB micellar solution (10 mL), catalyst (50 mg), H2O2 (4 mmol), 808C, 2 h.
Entry Catalyst
Catalyst
amount [mg] [mmol]
30% H2O2
T
t
Conv. Yield
[8C] [h] [%] [%]
1
2
3
4
5
6
7
8
–
–
50
50
50
100
25
50
50
50
50
50
50
6
6
4
4
4
4
2
1
4
4
4
4
80 12 trace trace
80 12 trace trace
HKUST-1
MOF-HPW
MOF-HPW[b]
MOF-HPW
MOF-HPW
MOF-HPW
MOF-HPW
MOF-HPW
MOF-HPW
MOF-HPW
H3PW4O12
80
80
80
80
80
80
3
3
2
5
3
5
98
43
96
95
83
79
98
43
96
94
82
78
water. A combined influence of framework Mo and V was sup-
posed to be responsible for the activity difference between
single POM-functionalized MOFs and vanadium-containing
ones.[17]
The distribution of products was further defined with MOF-
HPW and MOF-HPMoV3 as catalyst. As shown in Figure 6, ben-
zaldehyde was almost the only product in the MOF-HPW-cata-
9
25 12 trace trace
10
11
12
40
60
80
6
4
3
88
92
99
87
91
99
[a] Reaction conditions: benzyl alcohol (2 mmol), CTAB micellar solution
(10 mL). [b] Reaction conditions: benzyl alcohol (2 mmol), H2O (10 mL).
that almost no product was obtained in aqueous solution
without any catalyst (entry 1) or with HKUST-1 without POMs
(entry 2). Also, if no surfactant was added, the reaction turned
out to be less efficient in two phases (entry 4). For both MOF-
HPW and MOF-HPMo, a catalytic amount of catalyst was
enough for the reaction to proceed and reaction termination
was observed on its removal. A further increase of catalyst
amount would be meaningless. The reaction was initiated at
about 408C and high temperature was clearly beneficial for
the reaction. But at the same time, a high temperature acceler-
ates the decomposition of H2O2 so more H2O2 was required to
complete the reaction. The reaction conditions were finally op-
timized to be 808C with 4 mmol H2O2.
Figure 6. Distribution of products with MOF-HPW and MOF-HPMoV3 as cata-
lyst: a) conversion of benzyl alcohol with MOF-HPMoV3 as catalyst; b) yield
of benzaldehyde with MOF-HPMoV3 as catalyst; c) yield of benzoic acid with
MOF-HPMoV3 as catalyst; d) conversion of benzyl alcohol with MOF-HPW as
catalyst; e) yield of benzaldehyde with MOF-HPW as catalyst. Reaction condi-
tions: benzyl alcohol (2 mmol), CTAB micellar solution (10 mL), catalyst
(50 mg), H2O2 (4 mmol), 808C.
With the optimized reaction conditions, the catalytic per-
formance of MOFs with different kinds of POMs was tested
and compared. As shown in Figure 5, similar results could be
obtained with MOF-HPW and MOF-HPMo as catalysts. Howev-
er, when the MOFs of vanadium-containing POMs were used
as catalyst, notable improvement was observed in the oxida-
tion efficiency of benzyl alcohol. Also, with an increase of the
vanadium content in the POMs, the conversion of benzyl alco-
hol showed a rising trend. However, further oxidation reactions
generated benzoic acid in a proportion that could not be ne-
glected. The results showed excellent catalytic oxidizability of
these vanadium-containing POMs, but at the same time, selec-
tivity of the oxidation tended to be less controllable. The
mechanism of the reaction occurs in two steps including the
oxidation of organic substrate catalyzed by POMs and the re-
generation of reduced POMs by H2O2 with the formation of
lyzed system. Whereas for MOF-HPMoV3, the yield of benzalde-
hyde decreased after a maximum at 1 h. At the same time,
benzoic acid was generated gradually and showed an increas-
ing yield in the following reaction period. The total conversion
of benzyl alcohol followed a similar trend to that of MOF-HPW.
Overall, excellent selectivity could be obtained through con-
trolling the reaction time for MOF-HPW and MOF-HPMo,
whereas for vanadium-containing POMs, improvement in oxi-
dation efficiency could be obtained but the selectivity of the
oxidation tended to be less controllable.
All these facts indicated that it was still Keggin anions that
served as the active species for oxidation reactions. The encap-
sulation process did not change the reaction mechanism and
an activity decrease was not obvious either. The good catalytic
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performance of this system probably resulted from two as-
pects. First, the nanosized metal–organic framework containing
the active sites provided a large surface area for the ready ac-
cessibility of substrate molecules. Second, a pseudo-homoge-
neous process was obtained owing to the assistance of an
emulsion system, which also greatly increased the chances for
the contact between substrate and catalyst.
Table 3. Oxidation of aliphatic alcohols.[a]
Catalytic performance of MOF-HPW
To examine the utility and generality of this methodology for
oxidation of alcohols, we applied the present system to a varie-
ty of substrates with MOF-HPW as the model catalyst. As
shown in Table 2, the substrate scope could be extended to
benzylic, allylic, heterocyclic, alicyclic, and aliphatic alcohols. In
[a] Reaction conditions: benzyl alcohol (2 mmol), CTAB micellar solution
(10 mL), catalyst (100 mg), H2O2 (6 mmol), 808C. The conversion is shown
in parentheses and the GC yield is outside of the parentheses.
Table 2. Oxidation of aromatic alcohols.[a]
for allyl alcohol and cinnamyl alcohol. More importantly, it was
found that the oxidative efficiency was not affected by the ex-
istence of a double bond and the double bond remained
stable during the oxidation process. Isooctyl alcohol performed
somewhat better than 1-C8H17OH, but unfortunately both re-
sults were unsatisfactory even after lengthening the reaction
time.
Based on the results in Table 2, 4-methylbenzyl alcohol and
4-methoxybenzyl alcohol, which were thought to perform simi-
lar to or even better than benzyl alcohol, gave lower yields in-
stead. The MOF catalyst seemed to be chemoselective. So, to
probe the chemselectivity, substrates with increasing dimen-
sions were tested. The volume and dimension of the substrates
were calculated by using Gaussian 03 at the B3LYP/6-31G+ +
(d,p) level.[18] As illustrated in Figure 7, the conversion of
benzyl alcohol with a dimension of 89.35 cm3 molÀ1 reached
[a] Reaction conditions: benzyl alcohol (2 mmol), CTAB micellar solution
(10 mL), catalyst (50 mg), H2O2 (4 mmol), 808C. The conversion is shown
in parentheses and the GC yield is outside of the parentheses.
all of the cases tested, ketones or aldehydes were the only de-
tected products. Clearly, all primary benzylic alcohols were con-
verted into their corresponding aldehydes in high yields. Sec-
ondary alcohols such as benzhydrol and 2-phenylethanol also
gave moderate yields of 59 and 66%, respectively. Benzyl alco-
hol showed the highest yield among the substrates tested,
and the introduction of another group (electron-withdrawing
or electron-donating group) decreased the results more or
less. When the volume of substituted groups did not differ
much, substrates with electron-withdrawing groups performed
better than the those with electron-donating groups. Further-
more, oxidations of some aliphatic alcohols were also tested as
listed in Table 3. With double the amount of the catalyst, the
activities exhibited by secondary alcohols were also not so bad
in the system. Good to excellent conversions were observed
Figure 7. Chemoselectivity of MOF-HPW toward substrates with different di-
mensions (units in ꢁ).
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98% after 3 h. In contrast, the conversion of 4-methylbenzyl al-
cohol and 4-methoxybenzyl alcohol with dimensions of 101.27
and 107.57 cm3 molÀ1, respectively, decreased to 95 and 92%
under similar conditions. 4-tert-Butylbenzyl alcohol, a larger al-
cohol with molecular dimension of 142.72 cm3 molÀ1 gave
a conversion of about 63% after 12 h. Although the 4-tert-
butyl group activated the benzene ring the most (better than
the methyl and methoxy group), the conversion of 4-tert-butyl-
benzyl alcohol was the lowest. The catalytic activity and selec-
tivity of the MOF seemed to depend on the size of the sub-
strates and their accessibility to surface pores. It is thereby rea-
sonable that the reactants with smaller sizes such as benzyl al-
cohol could diffuse swiftly through the pores. In contrast, the
substrates with larger sizes were not readily diffused through
the pores, but they may adsorb onto the surface pores con-
taining Keggin complexes, and the polyanions catalyzed the
reaction at the surface of the pores. In many systems, the ali-
phatic alcohols with longer carbon chains performed better
than the shorter ones.[2,7] The same trend was observed in this
system when comparing the reaction efficiency of nC8H17OH
and nC12H25OH. However, nC12H25OH also showed a higher con-
version than nC16H31OH, which was assumed to be due to this
chemoselectivity.
Figure 8. The recycling study. Each column represents the conversion of
benzyl alcohol; the lines represent the selectivity of benzaldehyde.
ology, selectivity, and reusability of MOF-HPW were further
studied in detail. The catalytic activity and selectivity of the
catalyst seem to depend on the size of the substrates and
their accessibility to surface pores. The reactants with smaller
sizes such as benzyl alcohol could diffuse swiftly through the
pores thus a higher efficiency was obtained. Also, good recy-
clability was exhibited in the CTAB micellar system. After com-
pletion of the oxidation reaction, the mixture was allowed to
cool to room temperature and was extracted with diethyl
ether. The catalyst remained in the aqueous layer for use in
the next run. With benzyl alcohol as substrate, the procedure
was successfully repeated five times without any great loss of
catalytic activity.
Overall, the MOF-HPW catalyst demonstrated great catalytic
activity. For a series of alcohols including benzylic, allylic, heter-
ocyclic, and alicyclic alcohols as substrates, good to excellent
yields were obtained with ketones or aldehydes as the only de-
tected products. Furthermore, the catalyst showed clear che-
moselectivity, which was related to the size of the substrates
or their accessibility to surface pores.
The advantage of the catalytic system lies in not only the
high catalytic activity in aqueous solvent with H2O2 as oxidant,
but also the easy recovery of both catalyst and solvent. Since
both the catalyst and solvent were immiscible with diethyl
ether, the catalytic system could be recovered after extraction,
to the maximum amount. After completion of the oxidation re-
action, the mixture was allowed to cool to room temperature
and was extracted with diethyl ether. The aqueous layer, con-
taining the catalyst, was able to be separated and reused with
the addition of substrate without any treatment. The organic
layer, containing the products, was analyzed after drying with
anhydrous sodium sulfate. When benzyl alcohol was used as
a model substrate, the procedure was successfully repeated
five times without any great loss of catalytic activity (Figure 8).
Experimental Section
Materials and methods
All the solvents and reagents were purchased from Sinopharm
Chemical Reagent Co. Ltd and were used without further purifica-
tion. IR spectra were recorded on a NICOLET NEXUS870 instrument.
Products were identified by using a 6820 gas chromatograph (GC)
with an Agilent Technologies HP-Innowax (30 mꢂ0.32 mmꢂ
0.5 mm). XRD data were collected with CuKa radiation on Bruker C8
ADVANCE spectrometer. TEM images were recorded on a JEM-2100
instrument. TGA was performed on a TGA/SDTA851e instrument
under a N2 atmosphere from 50 to 7008C, with a heating rate of
108CminÀ1 and N2 flowing rate of 30 mLminÀ1. Calculation of the
volume was performed by using Gaussian 03 at the B3LYP/6-31G+
+(d,p) level.
Conclusion
Catalyst preparation
In conclusion, we have synthesized a series of POM-based
metal–organic frameworks in this study. The catalysts were
easily prepared and exhibited an ordered size and morphology.
The nanosized MOF-POMs, which offered organized multiple
porosity and high surface area, were proven to be an efficient
catalyst for various alcohol oxidations in CTAB micellar solu-
tion. With MOF-HPW as model catalyst, a wide set of aliphatic,
allylic, heterocyclic, and benzylic alcohols were oxidized into
their corresponding carbonyl compounds with good to excel-
lent yields. The reaction conditions, generality of the method-
Preparation of HKUST-1: HKUST-1 was prepared according to Ref-
erence [10b]. In a typical procedure, a slurry of Cu(OH)2 (0.98 g,
0.01 mol) in water (18 mL) was added to a solution containing tri-
mesic acid (TA, 1,3,5-benzenetricarboxylic acid; 2.1 g, 0.01 mol), di-
methyl formamide (DMF, 10 mL), and ethyl alcohol (46.6 mL) under
moderate stirring. The molar composition of the resulting mixture
was 1Cu/1TA/12.9DMF/80EtOH/100H2O. The crystallization was
carried out at room temperature under moderate stirring. After-
wards, the product was recovered by filtration and washed with
ethanol, and finally dried at 658C.
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Keywords: alcohols · metal–organic frameworks · micelles ·
oxidation · polyoxometalates
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Received: January 7, 2014
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FULL PAPERS
J. Zhu, M.-n. Shen, X.-j. Zhao, P.-c. Wang,
M. Lu*
It’s what’s inside that counts: A series
of polyoxometalate (POM)-based metal–
organic frameworks (MOFs) was pre-
pared and used in the selective oxida-
tion of alcohols. Experiments with these
MOFs were carried out in cetyltrimethyl
ammonium bromide micellar solutions.
With various alcohols as substrates,
better to excellent conversion and se-
lectivity were presented (see figure).
Thus a green and efficient process was
obtained.
&& – &&
Polyoxometalate-Based Metal–Organic
Frameworks as Catalysts for the
Selective Oxidation of Alcohols in
Micellar Systems
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