Design of a highly ordered mesoporous H3PW12O 40/ZrO2-Si(Ph)Si hybrid catalyst for methyl levulinate synthesis

Article abstract of DOI:10.1039/c3gc36912a

A highly ordered mesoporous ZrO₂-based material functionalized by benzene-bridged organosilica groups and the Keggin type heteropolyacid, prepared in a single co-condensation-hydrothermal treatment step, exhibited much higher catalytic activity towards methyl levulinate synthesis compared to alkyl-free H₃PW₁₂O₄₀/ZrO₂.

Full text of DOI:10.1039/c3gc36912a

Green Chemistry
View Article Online
View Journal | View Issue
COMMUNICATION
Design of a highly ordered mesoporous
H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst for
methyl levulinate synthesis†
Cite this: Green Chem., 2013, 15, 885
Received 29th November 2012,
Accepted 31st January 2013
DOI: 10.1039/c3gc36912a
Fang Su, Ling Ma, Daiyu Song, Xianghuan Zhang and Yihang Guo*
www.rsc.org/greenchem
A highly ordered mesoporous ZrO2-based material functionalized surface hydrophobicity, and stability of the catalysts need to be
by benzene-bridged organosilica groups and the Keggin type considered. Bearing this in mind, herein, ZrO2 materials func-
heteropolyacid, prepared in a single co-condensation–hydrothermal tionalized by both Keggin type HPA and benzene-bridged
treatment step, exhibited much higher catalytic activity towards organosilica moieties with ordered mesostructure in hexagonal
methyl levulinate synthesis compared to alkyl-free H3PW12O40/ZrO2. p6mm symmetry (H3PW12O40/ZrO2–Si(Ph)Si) were successfully
developed by careful design of a one-step co-condensation–
hydrothermal route. HPAs are an interesting class of well-
defined super strong Brönsted acids, exhibiting excellent cata-
lytic behaviors in a wide variety of acid-catalyzed reactions.
1. Introduction
The disadvantages of HPAs including small specific surface
area and high solubility in polar media can be overcome by
dispersing them within porous supports. Recently,
H6P2W18O62/SiO2 and H3PW12O40/K10 have been reported to
exhibit catalytic activity towards ethyl and n-butyl levulinate
production. However, their catalytic activity is unsatisfactory
since only H6P2W18O62 or H3PW12O40 is the active site. For
example, over a period of 10 h the ethyl levulinate yield
reached 78% under the conditions of the H6P2W18O62/SiO2
amount of 1.6 wt%, 78 °C and a LA to ethanol molar ratio of
1 : 3.5 For the synthesis of n-butyl levulinate catalyzed by
H3PW12O40/K10 (H3PW12O4 loading of 20 wt%), 97% LA con-
version and 100% selectivity towards n-butyl levulinate were
achieved under the conditions of the catalyst amount of 10 wt%,
120 °C, a LA to n-buthanol molar ratio of 1 : 6 and 4 h.6 Com-
pared with the above supported HPA catalysts, the as-designed
multifunctionalized H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst
possessed the following advantages: (i) ZrO2 itself displays
Brönsted acid sites capable of protonating pyridine,7 after
introduction of HPA in the ZrO2 framework, the Brönsted
acidity is enhanced compared with their individual com-
ponents due to the interaction between the Keggin unit and
the ZrO2 support, which can promote the release of protons
more easily.8,9 Moreover, this strong interaction is expected to
inhibit the leaching of the Keggin unit in polar solvents, which
often occurred in the supported HPA systems prepared by the
post-synthesis route; (ii) incorporation of benzene-bridged
organosilica groups can ameliorate the textural properties and
enhance the surface hydrophobicity of the as-prepared hybrid
catalyst simultaneously. In comparison with disordered
Levulinate esters including methyl, ethyl, and n-butyl levu-
linate are the esterification products of levulinic acid (LA), and
they are versatile chemical feedstocks with numerous potential
applications in the flavoring and fragrance industry or as a
blending component in biodiesel.1 The synthesis of levulinate
esters from LA has been attracting special attention because
LA is one of the top biomass-derived platform molecules that
can be made by the treatment of 6-carbon sugar carbohydrates
from renewable biomass such as starch or lignocellulosics
with an acid.2 Traditional catalysts for the synthesis of levu-
linate esters are strong Brönsted mineral acids. More recently,
a “green” approach to levulinate ester synthesis has stimulated
the application of recyclable strong solid acids as replacements
for such liquid acid catalysts so that the use of harmful sub-
stances and generation of toxic wastes are avoided. For this
purpose, environmentally benign solid acid catalysts including
sulfated metal oxides (i.e., SO42−/TiO2 and SO42−/ZrO2), ion-
exchange resins (i.e., Amberlyst 70), SBA-15 silica-supported
sulfonic acid (SO3H-SBA-15), and silica-supported or acid-
treated clay montmorillonite-supported heteropolyacids (i.e.,
HPA/SiO2 and HPA/K10) were used in this process.3–6
Nevertheless, design of robust and stable solid acid cata-
lysts that can improve the efficiency of levulinate ester pro-
duction dramatically is still an important challenge, and a
number of factors such as acid strength, textural properties,
School of Chemistry, Northeast Normal University, Changchun, 130024, P. R. China.
E-mail: guoyh@nenu.edu.cn; Tel: +86 431 85098705
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3gc36912a
This journal is © The Royal Society of Chemistry 2013
Green Chem., 2013, 15, 885–890 | 885

View Article Online
Communication
Green Chemistry
mesoporous catalysts that possess broad pore size distri- samples were outgassed under vacuum at 363 K for 1 h and
butions and small surface areas, the catalysts with ordered 373 K for 4 h. The surface areas were calculated using the Bru-
mesostructure exhibit obvious advantages including well-dis- nauer–Emmett–Teller (BET) equation, while pore size distri-
tributed pore sizes, large surface areas and high pore volumes, bution curves were calculated using the Barrett–Joyner–
which in turn can provide more active sites and speed up the Halenda (BJH) desorption branch of the isotherms, and the
mass-transport of reactants and products inside the pores.10,11 pore volume was accumulated up to P/P0 = 0.99. 31P MAS NMR
Therefore, the development of catalysts with ordered meso- and 13C CP-MAS NMR spectra were recorded on a Bruker
structure has generated increasing interest in heterogeneous AVANCE III 400 WB spectrometer equipped with a 4 mm stan-
catalysis; and (iii) a one-step preparation route can ensure dard bore CP MAS probehead. The dried and finely powdered
homogeneous dispersion of the Keggin units throughout the samples were packed in the ZrO2 rotor closed with a Ke–F cap,
catalysts. By the combination of the above advantages the cata- and were spun at a 12 kHz rate. Chemical shifts for all
lytic activity and stability of the H3PW12O40/ZrO2–Si(Ph)Si 31P MAS NMR and 13C CP-MAS NMR spectra were referenced
hybrid catalyst towards the esterification of LA with methanol to the signal of NH4H2PO4 standard (δ = 0.00) and C10H16 stan-
to produce methyl levulinate is expected to be further dard (δCH2 = 38.5), respectively.
improved.
Esterification of LA was carried out in a three-necked round
bottomed glass flask fitted with a water cooled condenser. For
each reaction, 88 mg (2 wt%) of air-exposed catalyst was
added, and the reaction was performed at 65 °C with different
LA to methanol molar ratios (1 : 1 to 1 : 9) and constant volume
(5 mL). The produced methyl levulinate was determined by a
Shimadzu 2014C gas chromatograph equipped with an HP-
INNOWAX capillary column (film thickness 0.5 μm; i.d.
0.32 mm; length 30 m) and a flame ionization detector. The
catalytic activity of the catalysts was evaluated quantitatively by
the yield of methyl levulinate.
2. Experimental
The ordered 2D hexagonal H3PW12O40/ZrO2–Si(Ph)Si hybrid
material was prepared as follows. Pluronic P123 (0.275 g) was
dissolved in a mixture of water (8.5 mL) and HCl (12 mol L−1
,
0.6 mL) under vigorous stirring at room temperature. Sub-
sequently, 1,4-bis-(triethoxysilyl)benzene (abbreviated BTESB,
96%, 0.32 mL), Zr(OC4H9)4 (0.75 mL) and aqueous H3PW12O40
(0.1010 g in 1.5 mL water) were added dropwise to the above
solution at hourly intervals, successively. The resulting mixture
was stirred at 40 °C for 24 h, and then it was subjected to
hydrothermal treatment at 120 °C for another 24 h at a heating
rate of 2 °C min−1. The resulting white solid powder was air-
dried at 100 °C overnight, and then P123 in the product was
removed by washing in boiling ethanol. The final product is
denoted as H3PW12O40/ZrO2–Si(Ph)Si, and H3PW12O40 loading
(7.6 wt%) in the product was determined by a Leeman Prodigy
Spec ICP-AES. The above procedure has been illustrated in
Scheme 1. For comparison, H3PW12O40/ZrO2 (H3PW12O40
loading of 7.0 wt%) was also prepared via the above route in
the absence of BTESB. Similarly, ZrO2 was prepared by follow-
ing the above procedure in the absence of both BTESB and
H3PW12O40.
3. Results and discussion
It is a challenge to obtain ordered mesoporous HPA-based
catalysts by a one-step route, originating from the complicated
preparation process and special structure characteristics of
HPA clusters. In the present work, the above problems were
solved by carefully designing and controlling the co-conden-
sation–hydrothermal treatment procedure, and an ordered
mesoporous H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst was pre-
pared by using nonionic surfactant P123 as a structure
directing agent.
Successful introduction of both inorganic and organic func-
tionalities into the ZrO2 framework is confirmed by 31P MAS
NMR and 13C CP-MAS NMR measurements. Fig. 1a shows a
31P MAS NMR spectrum of the H3PW12O40/ZrO2–Si(Ph)Si
material, and two signals at −16.2 ppm (intense) and
−11.6 ppm (very weak) were found. The intense peak is
assigned to the resonance of the PO4 unit within the bulk
H3PW12O40 environment, indicating that the primary Keggin
structure remains intact after immobilization. As for the very
weak peak, it originates from the new species formed at the
interface between the Keggin unit and the ZrO2 support.12
This new species slightly perturbs the chemical environment
of P atoms in the PO4 units. During the hybrid catalyst pre-
paration procedure, the surface uZrOH groups of the ZrO2 fra-
mework were protonated in the presence of Keggin units,
XRD patterns were obtained on a D/max-2200 VPC diffract-
ometer using Cu Kα radiation. TEM observations were per-
formed on a JEM-2100F high resolution transmission electron
microscope at an accelerating voltage of 200 kV. A nitrogen
porosimetry measurement was performed on a Micromeritics
ASAP 2020M surface area and porosity analyzer after the
giving (uZrOH2 )n[H3−nPW12O40]n− species by an acid–base
+
reaction.8 Additionally, considering the well-matched electro-
negativity and ionic radius of the Zr4+ ion (1.33, 0.072 nm) and
Scheme 1 Route for the preparation of the H3PW12O40/ZrO2–Si(Ph)Si hybrid
catalyst.
886 | Green Chem., 2013, 15, 885–890
This journal is © The Royal Society of Chemistry 2013

View Article Online
Green Chemistry
Communication
homogeneous dispersion of Keggin units throughout the
material.
The low-angle XRD pattern shown in Fig. S1b† indicates
that the H3PW12O40/ZrO2–Si(Ph)Si exhibits an intense diffrac-
tion peak at 0.86° and two weak diffraction peaks at 1.64° and
1.92°, which are indexed as the 100, 110, and 200 Bragg reflec-
tions of 2D hexagonal p6mm structure. As for the benzene-free
H3PW12O40/ZrO2, the above reflections were not found. The
result suggests that the incorporation of benzene-bridged
organosilica moieties plays a key role in creating ZrO2-based
hybrid material with ordered mesostructure. This structural
ordering is further confirmed by TEM observation, revealing
the 2D hexagonal arrangement of pore channels (Fig. 1c). The
result is consistent with that of the nitrogen porosimetry analy-
sis (Fig. 1d). H3PW12O40/ZrO2–Si(Ph)Si exhibits a type IV iso-
therm with an H1 hysteresis loop, characteristic of the
material with regular and even mesopores without intercon-
necting channels. The narrow BJH pore-size distribution curve
also indicates uniform pore sizes (mainly centering at 5.5 nm)
of the hybrid catalyst. As for the benzene-free H3PW12O40/
ZrO2, it has a type III isotherm with an H3 hysteresis loop,
together with a broad pore-size distribution curve (Fig. S2 of
ESI†), implying poor porosity of the material. Through analyz-
ing the textural parameters summarized in Table 1 it was
found that the BET surface area and pore volume of
H3PW12O40/ZrO2 were improved significantly after the intro-
duction of benzene-bridged organic silica groups.
Fig. 1 (a) The 31P MAS NMR spectrum, (b) 13C CP-MAS NMR spectrum, (c) TEM
image, and (d) nitrogen gas adsorption–desorption isotherms and pore size dis-
tribution profile of the H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst.
the W6+ ion (1.70, 0.060 nm), it can be reasonably inferred that
the terminal WvO groups within the Keggin units coordinate
to the surface uZrOH groups within ZrO2 via Zr–O–W bonds
in this species.13 Both of the interactions ensure strong combi-
nation of the two components. Fig. 1b displays the 13C
CP-MAS NMR spectrum of H3PW12O40/ZrO2–Si(Ph)Si material,
and two resonance signals at 133.4 ppm (intense) and
15.9 ppm (weak) were detected. The intense peak is due to the
carbon atoms from benzene-bridged organosilica groups, con-
firming successful introduction of the organic functional
groups into the ZrO2 framework through –Zr–O–Si–C6H4–Si–O–
linkages (Scheme 2). The weak peak is assigned to ethoxy
groups that were formed during the procedure of P123
removal.14
As-prepared H3PW12O40/ZrO2–Si(Ph)Si was successfully
tested in the esterification of LA with methanol to produce
methyl levulinate under atmosphere refluxing (65 °C)
(Scheme 2).
At first, the influence of the molar ratio of LA to methanol
was studied because it is one of the most important experi-
mental parameters of the esterification reaction. From the
result shown in Fig. 2 it is found that the yield of methyl levu-
linate continuously increases with the molar ratio of LA to
methanol from 1 : 1 to 1 : 7, and with further increasing the
molar ratio to 1 : 9, the yield begins to decrease. For example,
over the period of 3 h, methyl levulinate yield reaches 45.6%,
99.9% and 75.0%, respectively, at the LA to methanol molar
ratios of 1 : 1, 1 : 7 and 1 : 9. This is due to the fact that esterifi-
cation is a reversible reaction, and excessive methanol is favor-
able to ester formation. However, much more methanol may
dilute the reactant, thereby resulting in lower methyl levulinate
yield. Additionally, excessive methanol may also lead to side
reactions such as ether formation or methanol dehydration in
Wide-angle XRD patterns of the starting H3PW12O40 and
H3PW12O40/ZrO2–Si(Ph)Si hybrid catalysts are shown in
Fig. S1a.† The absence of diffraction peaks corresponding to
the starting H3PW12O40 for the hybrid catalyst suggests the
Table 1 Textural parameters and acid property of ZrO2-based materials
a
SBET
Dp
Vp
Atitration
Catalyst
(m2 g−1
)
(nm) (cm3 g−1
)
(μeq g−1
)
H3PW12O40/ZrO2
18
250
n.d.
5.5
5.3
0.05
0.38
0.36
1331
933
915
H3PW12O40/ZrO2−Si(Ph)Si
H3PW12O40/ZrO2−Si(Ph)Si3rd 235
Scheme 2 Wall components of H3PW12O40/ZrO2–Si(Ph)Si and esterification of
levulinic acid with methanol to produce methyl levulinate catalyzed over the
H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst.
a Acid capacity (Atitration) of the tested catalysts was measured by
titration with NaOH (0.002 mol L−1).15
This journal is © The Royal Society of Chemistry 2013
Green Chem., 2013, 15, 885–890 | 887

View Article Online
Communication
Green Chemistry
Fig. 2 Influence of the molar ratio of LA to methanol on the yield of methyl
levulinate. H3PW12O40/ZrO2–Si(Ph)Si (2 wt%); 65 °C; atmosphere refluxing.
the presence of an acid catalyst. However, in the current cata-
lytic system, 100% selectivity towards methyl levulinate is
achieved, and neither dehydration products nor ether is
detected.
Subsequently, the influence of the H3PW12O40/ZrO2–Si(Ph)Si
amount on the yield of methyl levulinate was studied at a
molar ratio of LA to methanol of 1 : 7. From the result shown
in Fig. S3† it can be seen that the catalytic activity of
H3PW12O40/ZrO2–Si(Ph)Si is significantly enhanced after
increasing the catalyst amount from 1 to 2 wt%, however, the
yield of methyl levulinate remains unchanged when the cata-
lyst amount was further increased to 3 or 5 wt%.
Finally, at a LA to methanol molar ratio of 1 : 7 and a cata-
lyst amount of 2 wt%, the catalytic activity of H3PW12O40/ZrO2–
Si(Ph)Si was further compared with as-prepared ZrO2-based
catalysts (i.e., H3PW12O40/ZrO2 and ZrO2) and the reference
catalysts (i.e., H3PW12O40 and H0.5Cs2.5PW12O40) under identi-
cal conditions. As can be seen in Fig. 3a, the catalytic activity
of H3PW12O40/ZrO2–Si(Ph)Si outperforms H3PW12O40/ZrO2
and ZrO2 as well as H3PW12O40 and H0.5Cs2.5PW12O40. For
example, over a period of 3 h, methyl levulinate yield reaches
99.9% (H3PW12O40/ZrO2–Si(Ph)Si), 69% (H3PW12O40/ZrO2),
61.8% (ZrO2), 49.9% (H0.5Cs2.5PW12O40) and 69% H3PW12O40,
respectively.
Fig. 3 (a) Comparison of the catalytic activity of the H3PW12O40/ZrO2–Si(Ph)Si
hybrid catalyst with other ZrO2-based materials, homogeneous H3PW12O40 and
H0.5Cs2.5PW12O40 towards the esterification of LA. LA : MeOH = 1 : 7; catalyst
amount 2 wt%; 3 h; 65 °C; atmosphere refluxing, and (b) recyclability of
H3PW12O40/ZrO2–Si(Ph)Si for the synthesis of methyl levulinate. LA : MeOH =
1 : 7; catalyst amount 2 wt%; 3 h; 65 °C; atmosphere refluxing.
polyanions. Larger polyanions can effectively delocalize the
negative charge required for the formation of Brönsted acid
centers, ultimately resulting in strong Brönsted acidity of the
H3PW12O40/ZrO2.9,13,16 The decreased acid capacity of
The excellent heterogeneous acid catalytic performance of
the H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst is explained in
terms of its strong Brönsted acid property, well-defined meso-
structure and surface hydrophobicity. Among these factors, the
strong Brönsted acid property of the hybrid catalyst plays a
dominant role in its catalytic activity. From the results sum-
marized in Table 1 it can be seen that H3PW12O40/ZrO2 pos-
sesses a strong Brönsted acid property with an acid capacity of
1331 μeq g−1, mainly due to the contribution from both
H3PW12O40/ZrO2–Si(Ph)Si (933 μeq g−1
) compared with
H3PW12O40/ZrO2 is due to the introduction of non-acidic
benzene-bridged organic silica groups into the hybrid catalyst.
Additionally, to a great extent, the excellent acid catalytic
activity of H3PW12O40/ZrO2–Si(Ph)Si is also determined by its
unique textural properties and homogeneous dispersion of the
active sites throughout the hybrid catalyst. Herein, the textural
properties of H3PW12O40/ZrO2 are obviously improved owing to
the introduction of benzene-bridged organosilica moieties.
The highly ordered pore channel, larger pore diameter
(5.5 nm), and homogeneous dispersion of the active sites may
reduce the mass transfer limitations of the reactants and
+
H3PW12O40 and ZrO2. Additionally, new (uZrOH2 )n-
[H3−nPW12O40]n− species formed at the interface between
H3PW12O40 and ZrO2 can enhance the interaction between the
polyanions (PW12O403−) and hence the formation of larger
888 | Green Chem., 2013, 15, 885–890
This journal is © The Royal Society of Chemistry 2013

View Article Online
Green Chemistry
Communication
products to and from the active acid centers and increase the the catalyst due to the strong adsorption of H3PW12O40/ZrO2 to
accessibility to their acid sites. In order to support this point water was inhibited owing to the surface hydrophobicity of the
of view, the influence of the stirring rate on the yield of levu- hybrid catalyst.
linic acid was studied. From the result displayed in Fig. S4† it
In conclusion, the one step co-condensation–hydrothermal
is found that the catalytic activity of the H3PW12O40/ZrO2– treatment strategy was designed to create a 2D hexagonal
Si(Ph)Si is hardly affected by changing the stirring rate from mesostructured organic–inorganic hybrid catalyst, H3PW12O40/
300, 600, 900 to 1200 rpm, implying that the H3PW12O40/ZrO2– ZrO2–Si(Ph)Si. The catalyst exhibited excellent catalytic activity
Si(Ph)Si-catalyzed esterification reaction is free from the exter- towards esterification of LA to produce methyl levulinate
nal mass transfer limitations. Besides, the larger surface area under mild conditions; meanwhile, the catalyst could be
(250 m2 g−1) and the higher pore volume (0.38 cm3 g−1) of the reused three times without significant loss of the activity. The
hybrid catalyst may give rise to a higher population of available unique catalytic performances of the as-prepared hybrid cata-
acid sites. All of these factors ensure that the H3PW12O40/ lyst are due to the combination of strong Brönsted acidity,
ZrO2–Si(Ph)Si has significantly higher acid catalytic activity well-defined ordered mesostructure, homogeneous dispersion
compared with H3PW12O40/ZrO2, although the latter has a of the active sites and enhanced surface hydrophobicity,
higher Brönsted acid capacity.
making it a promising environmentally friendly solid acid cata-
Finally, the surface hydrophobicity of the hybrid catalyst is lyst for methyl levulinate synthesis.
increased due to the incorporation of phenyl groups, which
can tune the adsorption properties of the reactants and pro-
ducts, thereby enhancing the LA esterification rate. For
Acknowledgements
benzene-free H3PW12O40/ZrO2, its hydrophilic nature due to
the presence of surface hydroxyl groups is not ideal for the LA
esterification to produce methyl levulinate. H3PW12O40/ZrO2
favors H2O adsorption and thereby inhibits the adsorption of
LA molecules, which may slow down the formation rate of
methyl levulinate. On the other hand, esterification is a rever-
sible process, and the produced ester molecules can easily be
hydrolyzed by water formed during the esterification process,17
thereby the reaction is depressed to some extent. Enhance-
ment of the surface hydrophobicity by the incorporation of
phenyl groups can create a hydrophobic environment within
mesopores, which serves to exclude the yielded water from the
active sites, causing the catalyst to sustain a higher reaction
rate.
Catalyst recycling is an important step as it can reduce the
cost of the process. The reusability of as-prepared H3PW12O40/
ZrO2–Si(Ph)Si was evaluated through three catalytic recycles.
After the first catalytic run, the catalyst was recovered by cen-
trifugation, and then it was washed with dichloromethane for
three times and dried in an oven at 100 °C, weighted and
returned to the reaction system again for the second and third
catalytic runs, respectively, under the same conditions and
regeneration method. As shown in Fig. 3b, the hybrid catalyst
showed good catalytic stability maintaining a similar level of
reactivity after three cycles. For the second and third catalytic
runs, the loss of activity after 1 h reaction time may be due to
the loss of catalyst powder during the recycling processes, but
further increasing the reaction time can restore the activity.
This work was supported by the Natural Science Fund Council
of China (21173036; 51278092) and the Science and Techno-
logy Project of Jilin Province (20086035).
References
1 J. Zhang, S. B. Wu, B. Li and H. D. Zhang, ChemCatChem,
2012, 4, 1230–1237.
2 D. W. Rackemann and W. O. S. Doherty, Biofuels, Bioprod.
Biorefin., 2011, 5, 198–214.
3 L. C. Peng, L. Lin, J. H. Zhang, J. B. Shi and S. J. Liu, Appl.
Catal. A: Gen., 2011, 397, 259–265.
4 J. P. Lange, W. D. van de Graaf and R. J. Haan,
ChemSusChem, 2009, 2, 437–441.
5 G. Pasquale, P. Vázquez, G. Romanelli and G. Baronetti,
Catal. Commun., 2012, 18, 115–120.
6 S. Dharne and V. V. Bokade, J. Nat. Chem., 2011, 20,
18–24.
7 A. M. Alsalme, P. V. Wiper, Y. Z. Khimyak,
E. F. Kozhevnikova and I. V. Kozhevnikov, J. Catal., 2010,
276, 181–189.
8 G. S. Armatas, G. Bilis and M. Louloudi, J. Mater. Chem.,
2011, 21, 2997–3005.
9 B. M. Devassy and S. B. Halligudi, J. Catal., 2005, 236,
313–323.
Additionally, P and W were not detected in the catalyst-free 10 Y. Wan, H. Y. Wang, Q. F. Zhao, M. Klingstedt, O. Terasaki
reaction solutions; meanwhile, changes of the textural property and D. Y. Zhao, J. Am. Chem. Soc., 2009, 131, 4541–4550.
and acidity of the reused hybrid catalyst were negligible 11 W. Schmidt, ChemCatChem, 2009, 1, 53–67.
(Table 1). The results revealed that the hybrid catalyst works 12 F. Su, L. Ma, Y. H. Guo and W. Li, Catal. Sci. Technol., 2012,
effectively as a recyclable and water-tolerant solid catalyst in
the esterification of LA to produce methyl levulinate. The 13 M. G. Kulkarni, R. Gopinath, L. C. Meher and A. K. Dalai,
high catalytic stability is due to the strong chemical Green Chem., 2006, 8, 1056–1062.
rather than physical interaction that existed between the 14 W. Li, F. Y. Ma, F. Su, L. Ma, S. Q. Zhang and Y. H. Guo,
2, 2367–2374.
Keggin unit and the ZrO2 support. Moreover, deactivation of
ChemSusChem, 2011, 4, 744–756.
This journal is © The Royal Society of Chemistry 2013
Green Chem., 2013, 15, 885–890 | 889

View Article Online
Communication
Green Chemistry
15 A. A. Kiss, A. C. Dimian and G. Rothenberg, Adv. Synth.
Catal., 2006, 348, 75–81.
T. Lopez, P. P. Lottici and D. Bersani, Appl. Catal. A: Gen.,
2000, 193, 215–225.
16 E. Lopez-Salinas, J. G. Hernandez-Cortez, I. Schifter, 17 M. Pramanik, M. Nandi, H. Uyama and A. Bhaumik, Green
E. Torres-Garcia, J. Navarrete, A. Gutierrez-Carrillo,
Chem., 2012, 14, 2273–2281.
890 | Green Chem., 2013, 15, 885–890
This journal is © The Royal Society of Chemistry 2013