L.H. Wee et al. / Catalysis Today 171 (2011) 275–280
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Fig. 7. Esterification reaction of acetic acid with 1-propanol catalyzed by differ-
ent catalysts: (a) without catalyst, (b) Cu3(BTC)2 (HKUST-1), (c) hydrothermally
synthesized HPW/Cu3(BTC)2, (d) ultrastable Y zeolite (CBV-720), (e) 65 nm-sized
HPW/Cu3(BTC)2 and (f) 50 nm-sized HPW/Cu3(BTC)2 catalysts. Reaction conditions:
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10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
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2 Theta (degree)
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T = 60 C, mole ratio of acetic acid to 1-propanol at 1:40 and 2.3 wt% of catalyst based
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Relaꢀve pressure (P/Po)
Keggin HPW is necessary for catalytic activity, and the cupper nodes
of the Cu (BTC) (HKUST-1) at most play a role for propanol activa-
Fig. 8. N2 adsorption isotherms of as-synthesized 65 nm-sized HPW/Cu3(BTC)2
samples washed with (a) water and ethanol mixture [1:1 (v/v)] and (b) water only.
The insert shows the corresponding XRD patterns.
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tion. The more limited dissolution of HPW/Cu (BTC)2 showed that
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the immobilization of the Keggin anions within the HKUST-1 matrix
enhanced the framework stability even at high acetic acid concen-
tration. This is in agreement with facile formation of the structure
at room temperature and the remarkable synergy between Keggin
and the pore structure of this MOF [15].
a rather shorter intracrystalline diffusional path length, leading to
an improved catalytic efficiency.
The porosity of 65 nm-sized HPW/Cu (BTC) nanomaterial after
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two catalytic reactions and recycling was investigated using N2
adsorption. There was a decline in BET surface area from 392 to
By increasing the mole ratio of acetic acid to 1-propanol from 1:2
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−1
to 1:20, the leaching of Cu2+ into the reaction mixture was reduced.
305 m g after 2 cycles of esterification reaction. This was inter-
pretated as partial pore blocking by products and side products, as
well as some limited sintering. This moderate loss of surface area
might be responsible for the minor loss of catalytic activity in con-
secutive runs. A similar observation of a gradual loss of surface area
has been reported for HPW/MIL-101 catalyst after use in cyclohex-
Visually a trend of decreasing copper leaching into solution was
observed depending on the concentration of acetic acid from blue
to pale blue as shown in Fig. 4 (insert). The degree of HPW leaching
probed by residual catalytic activity in homogeneous phase after
solid catalyst removal following the same trend (Fig. 4). At a mole
ratio of acetic acid to 1-propanol of 1:40, colorless supernatant was
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ene oxidation performed at 50 C [16]. It shall be mentioned that the
obtained after centrifugation indicating no leaching of Cu2+. Also,
no further conversion was noted after removal of the solid catalyst
nano catalyst is stable in common organic solvents such as ethanol,
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DMF, acetonitrile, dioxane and toluene at 65 C with stirring up to
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0 h. Continuous washing with water at room temperature causes
structure deterioration as evidenced with XRD and N2 adsorption
Fig. 8).
(
Fig. 4). Under such conditions the esterification reaction catalyzed
over the nanocatalyst is truly heterogeneous without leaching of
copper or HPW. The structure, crystal size and morphology of the
HPW/Cu (BTC) nanomaterials were preserved after catalytic test-
(
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ing under conditions preventing leaching as evidenced by XRD
4. Conclusions
(
Fig. 6c) and SEM (Fig. 6b), respectively.
Having identified reaction conditions under which
A facile though highly reproducible method was demonstrated
for the synthesis of nanocrystalline HPW/Cu3(BTC)2. The crystal-
lization at room temperature is extremely fast and is achieved
within minutes. The crystal growth can be interrupted via freezing
of the suspension in liquid nitrogen. Lyophilization is convenient
for recovering of the MOF product. The HPW/Cu3(BTC)2 nanoma-
terials with particle size of ca. 50 nm is an exceptional catalyst that
outperforms a reference zeolite in the model esterification reaction
HPW/Cu (BTC) is a truly heterogeneous catalyst, the catalytic
performance of the nanosized HPW/Cu (BTC) was compared with
reference catalysts including both micron-sized HPW/Cu (BTC)2
and ultrastable Y zeolite. The mole ratio of acetic acid to 1-propanol
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was set at 1:40. The catalytic activity at 60 C was estimated from
the conversion of acetic acid after 7 h. The only detected product
of the reaction was propyl acetate with a selectivity of 100%. The
conversion of acetic acid catalyzed by different catalysts is shown
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of acetic acid with 1-propanol at 60 C. Provided a high ratio of 1-
in Fig. 7. The 50 nm and 65 nm-sized HPW/Cu (BTC)2 samples
showed higher catalytic activity with 45.4 and 37.7% conversion,
respectively, as compared to the reference catalysts ultrastable Y
propanol to acetic acid of 40:1 is used, HPW/Cu3(BTC)2 behaves
as a true heterogeneous catalyst. Some decrease of BET surface
area was noted after catalyst reuses even under optimized reac-
tion conditions and resulted in a gradual loss of catalytic activity
after been reused for several cycles, although the crystallinity and
structure were well preserved. The catalyst is unstable, however,
at high concentrations of acetic acid.
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zeolite (25%) and micron-sized HPW/Cu (BTC)2 (12.5%). A control
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experiment without the introduction of catalyst showed only
ca. 3% conversion. Cu (BTC)2 (HKUST-1) catalyst under similar
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reaction conditions showed negligible catalytic activity. Thus, the
results clearly indicated that Keggin HPW species occluded in
Cu (BTC) pores are significantly active in esterification catalysis.
Acknowledgements
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Although the origin of the observed difference in catalytic activity
between nano and micron-sized HPW/Cu (BTC) catalysts remains
elusive, one possible explanation is that the nanomaterials possess
The authors gratefully acknowledge financial support of the
Flemish Government (Methusalem Funding) and the Belgian Gov-
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