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the benzyl alcohol and ethyl cyanoacetate to react for 2 h, we
removed the catalyst from the reaction vessel by filtration
while the conversion of benzyl alcohol and the yield of the
condensation product at this point reached 72% and 58%, re-
spectively (Figure 8A). The reaction mixture was kept under
XRD, FT-IR, 31P NMR, and XPS analyses demonstrate the reten-
tion of its structural integrity (Figure S4).
Conclusions
A series of bifunctional catalysts of the general formula Tris–
LDH–X4(PW9)2 (X=Mn, Fe, Co, Ni, Cu, and Zn; Tris=tris(hydrox-
ymethyl)aminomethane; LDH=layered double hydroxides)
have been fabricated by intercalating the polyoxymetalate
(POM) anions into Tris-modified LDH following a facile ion ex-
change method. The experimental results show for the first
time the efficient utilization of POM–LDH composite materials
as bifunctional heterogeneous catalysts for the promotion of
cascade reactions. The combinatorial effect of this family of
composite materials was demonstrated by the fine-tuning of
the oxidation catalyst (POM) and the basicity of the LDH layers
(Knoevenagel condensation). More specifically, the Tris–LDH–
Zn4(PW9)2 composite exhibited excellent activity and selectivity
under mild and soluble-base-free conditions in the cascade ox-
idation–Knoevenagel condensation reaction of various substi-
tuted benzyl alcohol and active methylene substrates. This
work demonstrates the POM-intercalated LDHs as very attrac-
tive low cost, energy-efficient and noble-metal-free alternative
catalysts. Moreover, the reported heterogeneous catalysts can
be easily recovered upon completion of the cascade reaction
and recycled for at least ten times without measurable deterio-
ration of their catalytic activity or structural integrity. This new
approach opens up endless possibilities for the engineering of
low-cost, environmentally friendly and modular multifunctional
POM-LDH catalysts, tailored for the promotion of specific cas-
cade reactions. Our current research effort is focused on ex-
ploring the potential of the family of POM–LDHs-based multi-
functional catalysts.
Figure 8. A) Leaching test for the cascade reaction of benzyl alcohol with
ethyl cyanoacetate over Tris–LDH–Zn4(PW9)2. a) conversion of benzyl alcohol,
b) yield of benzylidene ethyl cyanoacetate. Reaction conditions: benzyl alco-
hol (1 mmol), ethyl cyanoacetate (1.5 mmol), H2O2 (1 mL), solvent (3 mL), cat-
alyst (0.01 mmol), T=353 K. B) Recycling test of the Tris–LDH–Zn4(PW9)2 cata-
lyst for 10 times.
stirring for another 4 h under the same experimental condi-
tions while we kept monitoring any concentration changes of
the species. The obtained data (Figure 8A) demonstrate that
the removal of the solid catalyst, Tris–LDH–Zn4(PW9)2 from the
reaction mixture, brings the cascade reaction, and as a conse-
quence the conversion of benzyl alcohol and the yield of the
target product, to a halt. In addition, inductively coupled
plasma atomic emission spectroscopy (ICP–AES) measurements
of the filtrate after removing the catalyst indicate no evidence
of W content in the filtrate. These two experimental results
provide unambiguous proof that there is no leaching of POM
clusters from the Tris–LDH–Zn4(PW9)2 composite into the solu-
tion during the course of the cascade reaction. This can be at-
tributed to the multiple electrostatic and H-bonding interac-
tions between Tris-modified LDH and POM anions in the two-
dimensionally restricted region.
Experimental Section
Chemicals
MgCl2·6H2O, AlCl3·6H2O, Zn(NO3)2·6H2O, MnCl2·2H2O, FeCl3·6H2O,
Co(NO3)2·6H2O, Ni(NO3)3·6H2O, CuCl2·2H2O, Al(NO3)3·9H2O, Na2CO3,
NaWO4·2H2O, H3PO4, CH3COOH, KCl, tris(hydroxymethyl)aminome-
thane (Tris), benzyl alcohol, 4-methylbenzyl alcohol, 4-methoxyben-
zyl alcohol, 4-fluorobenzyl alcohol, 4-chlorobenzyl alcohol, 4-bro-
mobenzylalcohol, 4-aminobenzyl alcohol, 4-nitrobenzyl alcohol,
2,6-difluorobenzyl alcohol, ethyl cyanoacetate, malononitrile, dieth-
yl malonate, and phenylacetonitrile were purchased from Sigma–
Aldrich and used without any further purification. All solvents were
of analytical grade, purchased from Alfa Aesar and used without
further purification.
Preparation of the catalysts
Most importantly, the heterogeneous catalyst can be suc-
cessfully recovered upon completion of the cascade reaction
by filtration. As shown in Figure 8B, the Tris–LDH–Zn4(PW9)2
composite used for the promotion of the cascade reaction can
be easily recovered by filtration and recycled for at least 10
times without obvious deterioration of its catalytic activity. In
addition, the characterization of the recycled catalyst using
Tris–LDH–Zn4(PW9)2: K10[Zn4(H2O)2(PW9O34)2]·20H2O[34] (K–Zn4(PW9)2)
and the tripodal-ligand-stabilized layered double hydroxide (Tris–
LDH–CO3)[35] were synthesized according to previously reported
methodologies. The POMs were intercalated into Tris–LDH–CO3
using an anion-exchange method[36] under CO2-existing conditions.
Tris–LDH–CO3 (2 mgmLÀ1) was then redispersed in a solution of
K10[Zn4(H2O)2(PW9O34)2] (0.1m) and stirred for 2 h at RT. The precipi-
ChemCatChem 2016, 8, 929 – 937
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