Heteropoly acid and ZrO2 bifunctionalized organosilica hollow nanospheres for esterification and transesterification

Article abstract of DOI:10.1039/c4ta02257b

Single-micelle-templated preparation of heteropoly acid and ZrO₂ bifunctionalized organosilica hollow nanospheres (H₃PW ₁₂O₄₀/ZrO₂-Et-HNS) is developed by co-hydrolysis and -condensation of bissilylated organic precursor, 1,2-bis(trimethoxysilyl)ethane (BTMSE), with a zirconium source (Zr(OC ₄H₉)₄) in the presence of H₃PW ₁₂O₄₀, triblock copolymer surfactant F127 and 1,3,5-trimethylbenzene (TMB) followed by boiling ethanol washing. Through tuning the molar ratios of Si/Zr in the initial gel mixture, the morphology transformation from the 3D interconnected mesostructure to the hollow spherical nanostructure is realized. The inner diameter of the H₃PW ₁₂O₄₀/ZrO₂-Et-HNS materials is in the range of 6-12 nm, and their shell thickness is ca. 2 nm. As novel organic-inorganic hybrid catalysts, the catalytic activity of H₃PW₁₂O ₄₀/ZrO₂-Et-HNS is evaluated by the model reactions of esterification of levulinic acid (LA) with methanol to methyl levulinate and transesterification of yellow horn oil with methanol to biodiesel at refluxing temperature (65 °C) and atmospheric pressure. The obtained excellent heterogeneous acid catalytic activity of H₃PW₁₂O ₄₀/ZrO₂-Et-HNS is explained in terms of their strong Broensted and Lewis acidity, unique hollow nanospherical morphology and hydrophobic surface. Finally, the recyclability of the hybrid catalysts is tested through three consecutive catalytic runs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ta02257b

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DOI: 10.1039/C4TA02257B
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ARTICLE TYPE
Heteropoly acid and ZrO bifunctionalized organosilica hollow
2
nanospheres for esterification and transesterification
a
Fang Su, Sai An, Daiyu Song, Xianghuan Zhang, Bo Lu, Yihang Guo*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Singleꢀmicelleꢀtemplated preparation of heteropoly acid and ZrO bifunctionalized organosilica hollow
2
nanospheres (H PW O /ZrO ꢀEtꢀHNS) is developed by coꢀhydrolysis and ꢀcondensation of bissilylated
3
12 40
2
organic precursor, 1,2ꢀbis(trimethoxysilyl)ethane (BTMSE), with zirconium source (Zr(OC H ) ) in the
4
9 4
presence of H PW O , triblock copolymer surfactant F127 and 1,3,5ꢀtrimethylbenzene (TMB) followed
3
12 40
1
0
by boiling ethanol washing. Through tuning the molar ratios of Si/Zr in the initial gel mixture, the
morphology transformation from 3D interconnected mesostructure to the hollow spherical nanostructure
is realized. The inner diameter of the H PW O /ZrO ꢀEtꢀHNS materials is in the range of 6−12 nm, and
3
12 40
2
their shell thickness is ca. 2 nm. As the novel organicꢀinorganic hybrid catalysts, the catalytic activity of
H PW O /ZrO ꢀEtꢀHNS is evaluated by the model reactions of esterification of levulinic acid (LA) with
3
12 40
2
1
5
methanol to methyl levulinate and transesterification of yellow horn oil with methanol to biodiesel under
refluxing temperature (65 C) and atmospheric pressure. The obtained excellent heterogeneous acid
o
catalytic activity of H PW O /ZrO ꢀEtꢀHNS is explained in terms of their strong Brönsted and Lewis
3
12 40
2
acidity, unique hollow nanospherical morphology and hydrophobic surface. Finally, the recyclability of
the hybrid catalysts is tested through three consecutive catalytic runs.
14–16
topic.
Therefore, the preparation route should be designed
2
0
1 Introduction
carefully to ensure the structural integrity of the functionalities
and perfect hollow nanospherical morphology with controllable
particle size. The general approaches for the fabrication of hollow
nanospheres composites/hybrids include soft/hard template and
selfꢀtemplating methods depending on the nature of the
Rational design of catalysts with unique morphological
characteristics and perfect textural properties has been the long
term research focus in the field of catalysis. To a great extent, the
catalytic activity of catalysts is determined by their
morphological characteristics and textural properties in addition
to the chemical structure and composition, which can influence
the active site numbers, the accessibility of active sites on a given
catalyst surface and inside the pores to the substrates as well as
5
5
6
6
7
0
5
0
5
0
17
template. For example, the assembly of silicate or organosilicate
2
3
3
4
4
5
0
5
0
5
species and the active precursors around the template (e.g.
surfactant micelles and PS spheres) through a solꢀgel process and
the subsequent removal of templates results in the formation of
silicaꢀ
or
organosilicaꢀbased
hollow
nanospheres
1
–5
massꢀtransport of the reactant or product molecules. Recently,
fabrication of catalysts with various nanostructures, such as
nanospheres, nanowires, nanotubes, nanorods and nanopores has
composite/hybrid catalysts. A successful example has recently
been reported by Yang’s group. They prepared a series of Pdꢀ
doped propylsulfonic acidꢀfunctionalized hollow nanospheres
6–10
been reported.
Among them, hollow nanospheres catalysts
(Pd/SO HꢀEꢀHS)
bis(trimethoxysilyl)ethane
by
coꢀcondensation
(BTMSE)
of
with
1,2ꢀ
3ꢀ
3
with outstanding properties including hollow interiors, permeable
and thin shells, low density, thermal and mechanical stability as
well as bimodal mesoporosity have attracted particular interests,
and they can serve as the novel nanoreactors for various chemical
mercaptopropyltrimethoxysilane (MPTMS) in the presence of
single surfactant (F127, EO106PO EO106) and saltingꢀout
70
inorganic electrolyte (NaOAc) followed by H O postsynthesis
1
1–13
2
2
transformations.
Generally, various active species could be
oxidation, and subsequent impregnation of metallic Pd. Asꢀ
incorporated into both the hollow interior and the shell of silica,
organosilica or carbon hollow nanospheres, however, compared
with the preparation of pure silica, organosilica or carbon hollow
nanospheres, the fabrication of silicaꢀ, organosilicaꢀ or carbonꢀ
based hollow nanospheres composites/hybrids functionalized by
various catalytically active components via oneꢀpot route rather
than postꢀsynthesis grafting method is more difficult, and only a
limited number of research papers is concerned about this
prepared Pd/SO HꢀEꢀHS exhibited a higher activity than their
3
bulk mesoporous counterparts (e.g. Pd/SO HꢀSBAꢀ15 and
3
Pd/SO HꢀFDUꢀ12) in the oneꢀpot synthesis of methyl isobutyl
3
18
ketone (MIBK) from acetone and hydrogen in liquid phase.
Motivated by the aforementioned work, herein, both Kegginꢀ
type heteropoly acid and ZrO2 functionalized ethaneꢀbridged
organosilica
hollow
nanospheres
hybrid
materials
(
H PW O /ZrO ꢀEtꢀHNS) are successfully fabricated via the
3
12 40
2
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Journal of Materials Chemistry A
Page 2 of 12
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DOI: 10.1039/C4TA02257B
carefully designed single step solꢀgel route. The preparation
procedures include oneꢀstep coꢀcondensation of BTMSE and 60 HNS is also compared with the reference acid catalysts including
Zr(OC H ) around the F127 polymer micelles in the presence of previously reported 2D hexagonal p6mm H PW O /ZrO ꢀEtꢀ
be realized. The catalytic activity of the H PW O /ZrO ꢀEtꢀ
3 12 40 2
4
9 4
3
12 40
2
19
33
1
,3,5ꢀtrimethylbenzene (TMB) and H PW O and subsequent
2Dhex
,
bridging ethyl groupꢀfree H PW O /ZrO ,
3 12 40 2
3
12 40
5
0
5
0
5
0
5
0
5
0
5
removal of the polymer. By the combination of unique
morphological characters such as spheres with hollow interiors,
H PW O , Cs H PW O , ZrO and commercially available
sulfonic ionꢀexchange resin (Amberlystꢀ15) under the same
3
12 40
2.5 0.5
12 40
2
extremely small particle size and thin shell thickness with 65 conditions. Based on the catalytic testing results, the influence of
1
9–21
Brönsted and Lewis acidity,
asꢀprepared H PW O /ZrO ꢀ
Brönsted acidꢀsite density, morphological and textural properties
as well as surface hydrophobicity on the heterogeneous acid
catalytic activity of asꢀprepared hybrid catalysts is revealed.
Finally, the recyclability of the hybrid catalysts is tested through
70 three consecutive catalytic runs.
3
12 40
2
EtꢀHNS organicꢀinorganic hybrid materials are expected to be
promising heterogeneous acid catalysts in various important acidꢀ
catalyzed reactions.
1
1
2
2
3
3
4
4
5
5
In the H PW O /ZrO ꢀEtꢀHNS hybrid catalysts, H PW O
12 40
3
12 40
2
3
is an important super strong Brönsted acid that has exhibited
excellent catalytic behaviors in a wide variety of acidꢀcatalyzed
reactions, and the disadvantages of H PW O including small
2 Experimental
3
12 40
2
.1 Materials
specific surface area and high solubility in polar media can be
2
2–25
overcome by dispersing it within various porous materials.
Additionally, selecting ZrO (rather than catalytically inert silica)
H PW O •xH O, Pluronic F127 (EO PO EO106, where EO =
3
12 40
2
106
70
–CH CH O–, PO = –CH (CH )CHO–, M = 12600), Pluronic
2
2
2
2
3
w
26,27
as the support is due to its both Brönsted and Lewis acidity,
75 P123
(EO PO EO ,
Mw
=
5800),
1,2ꢀ
2
0
70
20
and the Brönsted acidity of the Keggin units is further enhanced
bis(trimethoxysilyl)ethane (BTMSE, 97%), zirconium nꢀbutoxide
(Zr(OC H ) , 76–80 % in nꢀbutanol) and levulinic acid (LA, 98
due to strong W−O−Zr covalent bindings between H PW O
3
12 40
4
9 4
clusters and ZrO support, which can promote the release of the
%) are purchased from Sigma–Aldrich; methyl levulinate (> 99.0
%) was purchased from TCI. Yellow horn oil is commercially
2
28,29
protons.
Meanwhile, the strong interaction between
H PW O and ZrO2 can effectively inhibit the leakage of 80 available. The above reagents are used without further
3
12 40
H PW O that generally occurred in most of the supported
purification.
.2 Catalytic preparation
Preparation of H
investigate the effect of molar ratios of precursors (i.e.
and BTMSE) on the morphology of the products,
3
12 40
heteropolyacids, leading to H PW O /ZrO ꢀEtꢀHNS a genuine
3
12 40
2
2
heterogeneous acid catalyst that will find out important
applications. Therefore, ZrO can act both acidic site and support
PW12O40/ZrO -Et-HNS hybrid catalysts. To
3 2
2
in the asꢀprepared hybrid catalysts. Finally, with ethyl groups
bridged in the silica/carbon framework of the prepared 85 Zr(OC
H )
9 4
4
H PW O /ZrO ꢀEtꢀHNS, the porosity and surface
various initial Si/Zr molar ratios (i.e. 0.5, 1.0, 1.5 and 2.0) were
employed to synthesis the hybrid catalysts while keeping the total
molar number of precursors constant (5 mmol). The detailed
synthesis route is as follow. F127 (0.5 g) and TMB (2.3 mL) were
3
12 40
2
hydrophobicity/hydrophilicity balance of the hybrid catalyst can
be tuned dramatically by changing the initial molar ratios of Si to
Zr, which can also influence the catalytic behaviors of the
catalyst.
−1
90 dissolved in an HCl solution (2 mol L , 29 mL). The solution
o
To evaluate the heterogeneous acid catalytic performance of
asꢀprepared H PW O /ZrO ꢀEtꢀHNS hybrid catalyst, the
was stirred at 40 C for 3 h. Subsequently, to the above solution,
BTMSE (0.25−0.63 mL), Zr(OC
H PW12O40 (0.110−0.112 g in 1.0 mL water) was added dropwise
3
H ) (1.8−1.1 mL) and aqueous
9 4
3
12 40
2
4
following two model reactions are selected under refluxing
o
o
temperature (65 C) and atmospheric pressure: i) synthesis of
at hourly interval. The resultant mixture was stirred at 40 C for
o
methyl levulinate from biomassꢀderived platform molecule, 95 24 h and aged at 100 C for another 24 h. Finally, the surfactantꢀ
o
levulinic acid (LA). Methyl levulinate is a member of levulinate
ester family that is versatile chemical feedstocks with numerous
potential applications in the flavoring and fragrance industry or as
free white precipitate was harvested through airꢀdried at 100 C
overnight, washed three time with boiling ethanol and then airꢀ
o
dried at 100 C for another 12 h. The product is denoted as
3
0−32
a blending component in biodiesel;
and ii) transesterification
H
3
PW12
O40/ZrO
2
ꢀEtꢀHNSn, where n is the molar ratio of Si/Zr.
40ꢀEtꢀHNS was prepared via the above
of yellow horn (a kind of inedible plant) oil with methanol to 100 For comparison, H
PW12O
3
produce biodiesel. Biodiesel is a mixture of C −C fatty acid
process except that BTMSE (5 mmol, 1.27 mL) was used as the
only precursor.
1
2
22
monoalkyl esters (FAMEs), and it is a sustainable, sulfurꢀfree,
biodegradable and nonꢀtoxic fossil fuel substitute that is widely
used worldwide. Conventional catalysts for both of the reactions
Preparation of H
PW12O40/ZrO -Et-2Dhex2.0. P123 (0.275 g)
2
3
was dissolved in a mixture of water (8.5 mL) and HCl (12 mol
−1
are inorganic liquid acids like HCl and H SO . Although these 105 L , 0.6 mL) under vigorous stirring at room temperature.
2
4
liquid acids are efficient to the above processes, they suffer from
serious contamination and corrosion problems that make essential
the implementation of good separation and purification steps. A
Subsequently, BTMSE (0.42 mL), Zr(OC
H ) (0.75 mL) and
4 9 4
aqueous H 40 (0.074 g in 1.5 mL water) were added
PW12O
3
dropwise to the above solution at hourly intervals, successively.
o
“green” approach to levulinate esters and biodiesel synthesis has
The resulting mixture was stirred at 40 C for 24 h, and then it
stimulated the application of sustainable solid acid catalysts as 110 was transferred into the autoclave and put it into the oven. The
o
replacements for such liquid acid catalysts so that the use of
harmful substances and generation of toxic wastes are avoided;
meanwhile, the ease of catalyst separation after the reactions can
temperature of oven was heated up to 120 C at a heating rate of 2
o
–1
o
C min and then held for 24 h at 120 C. The resulting white
o
solid powder was airꢀdried at 100 C overnight, and then P123 in
the product was removed by boiling ethanol washing.
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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Journal of Materials Chemistry A
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DOI: 10.1039/C4TA02257B
2
.3 Catalyst characterization
atmospheric pressure. The reaction progress was monitored by
taking a small portion (0.1 mL) of the reaction mixture at a
specified time. The solid catalysts were removed by
centrifugalization, and the obtained liquid was diluted with
acetone to 5 mL in order to analyze by GC (Shimadzu 2014C,
coupled with a flame ion detector and an HPꢀINNOWAX
TEM observations were performed on a JEMꢀ2100F high
resolution transmission electron microscope at an accelerating
voltage of 200 kV. Nitrogen gas porosimetry measurement was
performed on a Micromeritics ASAP 2020M surface area and
porosity analyzer after the samples were outgassed under vacuum
at 363 K for 1 h and 373 K for 4 h. The surface areas were
calculated using the BrunauerꢀEmmettꢀTeller (BET) equation,
while pore size distribution curves were calculated using Barrettꢀ
JoynerꢀHalenda (BJH) adsorption branch of the isotherms, and
6
6
7
7
8
0
5
0
5
0
5
0
5
0
capillary column (film thickness, 0.5 ꢁm; i.d., 0.32 mm; length,
o
3
0 m), the operation temperature was 220 C and flow rate of
nitrogen gas was 1.0 mL/min).
For esterification reaction, ethyl laurate was applied as the
internal standard, and the catalytic activity was evaluated
quantitatively by the yield of methyl levulinate (Y, %) and
1
1
2
3
1
the pore volume was accumulated up to P/P = 0.99. P MAS
0
1
3
29
NMR, C CPꢀMAS NMR and Si MAS NMR spectra were
recorded on a Bruker AVANCE III 400 WB spectrometer
equipped with a 4 mm standard bore CP MAS probe head. The
dried and finely powdered samples were packed in the ZrO rotor
closed with Ke–F cap which were spun at 12 KHz rate. Chemical
shifts for all P MAS NMR, C CPꢀMAS NMR and Si MAS
NMR spectra were referenced to the signal of NH H PO
4
standard (δ = 0.00), C H standard (δCH2 = 38.5) and 3ꢀ
−1
turnover frequency (TOF, h ). Herein, Y (%) = (M /M ) × 100,
D
T
where MD and M is the number of moles of methyl levulinate
T
−
1
produced and expected, respectively; TOF (h ) = [M /(A
m)] × t , where A
×
D
titration
2
−1
+
is the number of equivalents of H
titration
3
1
13
29
determined by acidꢀbase titration (see section 2.4), m (g) is the
mass of hybrid catalyst used in the esterification reaction, and t (h)
is the reaction time.
For transesterification reaction, ethyl laurate was applied as the
internal standard, and the catalytic activity was evaluated
quantitatively by the yield of each FAME (Y, %). Herein, Y (%)
4
2
1
0
16
(trimethylsilyl)ꢀ1ꢀpropanesulfonic acid sodium salt standard (δ =
.0), respectively. FTIR spectra were recorded on a Nicolet
Magna 560 IR spectrophotometer. Thermogravimetric analysis
TGA) was performed with a Shimadzu TG instrument at a
0
=
(M /M ) × 100, where M and M are the number of moles of
D T D T
(
o
−1
FAME produced and expected, respectively. The FAMEs
produced in the reaction system were identified by a mass
spectrometry coupled with gas chromatography (HP6890GCꢀ
heating rate of 10 C min in air.
2
5
2.4 Determination of Brönsted acid-site density
5
973MSD) analysis. The GCꢀMS was equipped with HPꢀ5MS
The Brönsted acidꢀsite density of asꢀprepared materials was
determined by acidꢀbase titration. Fresh catalyst powder (60 mg)
was placed in 15 mL deionized water, and the mixture was sealed
capillary column (film thickness, 0.25 µm; i.d., 0.2 mm; length,
−1
3
0 m) and helium as the carrier gas at 1 mL min . The
o
85 temperature program was as follows: 150 °C for 2 min, 5 °C/min
up to 250 °C, hold time of 10 min. The GC injector and MS ion
source temperatures were 250 °C and 230 °C, respectively. The
MS detector was operated in the EI mode at 70 eV with a
scanning range of m/z 20ꢀ500. The concentrations of FAMEs
and stirred at 30 C for 24 h. The obtained suspension was cooled
3
0
down to room temperature, and then it was titrated by sodium
−1
hydroxide solution (0.0045 mol L ) that has been titrated with a
−1
standard potassium hydrogen phthalate solution (0.005 mol L ).
The Brönsted acidꢀsite density could be calculated from the
consumed sodium hydroxide amount, and it is expressed by the
90
were determined by GC.
+
−1 34
3
5
number of equivalents of H (Atitration, ꢁeq g ).
3
Results and discussion
2
.5 Characterization of Brönsted and Lewis acidity
3
.1 Preparation and characterization of H PW O /ZrO -Et-
3 12 40 2
Brönsted and Lewis acidity of the catalysts was characterized by
in situ FTIR spectroscopy with chemical adsorption of pyridine.
The catalyst samples were mixed with KBr powder and preꢀ
HNS organic-inorganic hybrid catalysts
H PW O /ZrO ꢀEtꢀHNS hybrid catalysts are prepared by a
3
12 40
2
o
95 single step solꢀgel route, and the procedures include coꢀ
hydrolysis and ꢀcondensation of bissilylated organic precursor
(BTMSE) with zirconium source (Zr(OC H ) ) in the presence of
H PW O , triblock copolymer surfactant F127 and TMB. The
procedures of preparation of H PW O /ZrO ꢀEtꢀHNS are
00 complicated due to poor processability of heteropoly acid
clusters. Moreover, the circumstance is more difficult especially
including nonꢀsilica component (i.e. ZrO ) in the hybrid materials
because the hydrolysis rate of the zirconium precursors is faster
in comparison of silicon precursors. H PW O begins to
05 decompose at pH higher than 1.5 and loses all acidic protons at
4
0
treated at 100 C for 12 h in vacuum. The samples were then
o
exposed to pyridine vapour at 60 C for 12 h in vacuum, followed
o
4
9 4
by pumping out at 150 C for 1 h to remove the physisorbed
3
12 40
pyridine. Then, the FTIR spectra of adsorbed pyridine were
recorded.
3
12 40
2
1
1
1
4
5
2.6 Catalytic tests
o
The catalysts were dried for 2 h at 120 C in vacuum before the
2
catalytic tests. All the reactions were carried out in a threeꢀnecked
round bottomed glass flask fitted with a water cooled condenser.
For esterification of LA with methanol, a mixture that consisted
of LA (1.474 g or 12.7 mmol), methanol (3.7 mL or 91.5 mmol)
3
12 40
o
35,36
4
65 C.
Therefore, creating H PW O /ZrO ꢀEtꢀHNS should
3 12 40 2
5
0
o
avoid decomposition of the Keggin unit. As a consequence,
acidity of the preparation system should be controlled at pH
lower than 1.5 and boiling ethanol washing rather than high
10 temperature calcination is selected to remove various additives
(e.g. F127 and TMB). In addition, the heteropolyanions can
and catalyst (88 mg or 2 wt.%) was heated to 65 C for 2 h at
atmospheric pressure. For transesterification of yellow horn oil
with methanol, the mixture that was composed of yellow horn oil
(
1.023 g or 1.1 mmol), methanol (4 mL or 98.9 mmol) and
o
5
5
catalyst (209 mg or 5 wt%) was heated to 65 C for 24 h at
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

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Page 4 of 12
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DOI: 10.1039/C4TA02257B
hollow spheres are formed. It has been reported that the single
surfactant micelles formed by copolymer (e.g. F127 or P123)
assembly in an aqueous solution are usually in the mesoꢀscale (2–
6
6
7
7
8
0
5
0
5
0
17
5
0
nm).
Therefore, F127 single micellesꢀtemplated
5
0
5
0
5
H PW O /ZrO ꢀEt hollow spheres are expected to be in the
3
12 40
2
particle size less than 100 nm; additionally, in the presence of
TMB, the charge density on the surface of F127 micelles was
further decreased due to the penetration of hydrophobic TMB
into the micelles, which may lead to dispersed single micelles and
thus finally to the wellꢀdispersed hollow nanospheres. Based on
the above discussion, schematic illustration of the formation of
H PW O /ZrO ꢀEtꢀHNS hybrid materials is presented in
1
1
2
2
3
12 40
2
Scheme 1.
Pore morphologies and textural properties. TEM technique
is an effective measurement method to reveal the morphology
change of the nanomaterials. The representative TEM images of
the H PW O /ZrO ꢀEt organicꢀinorganic hybrid materials
3
12 40
2
prepared at different initial Si/Zr molar ratios (e.g. 0.5, 1.0, 1.5
and 2.0) are collected in Fig. 1. At Si/Zr molar ratio of 0.5, the
material (e.g. H PW O /ZrO ꢀEtꢀ3D ) mainly exhibits 3D
3
12 40
2
int
interconnected mesostructure, and only a small quantity of
sphereꢀlike hollow particles is observed (highlighted with arrow,
Fig. 1a). As for the materials prepared at Si/Zr molar ratio of 1.0,
1
.5 and 2.0 (e.g. H PW O /ZrO ꢀEtꢀHNS1.0, H PW O /ZrO ꢀ
3 12 40 2 3 12 40 2
EtꢀHNS1.5 and H PW O /ZrO ꢀEtꢀHNS2.0), they mainly
3
12 40
2
exhibit hollow spherical morphologies with the inner diameter in
Scheme
1
The representative procedure for the preparation of 85 the range of 6−12 nm (or particle size of 10−16 nm) and shell
H
3
PW12
O
40/ZrO
2
ꢀEt hybrid catalysts.
thickness of ca. 2 nm (Fig. 1bꢀd). In the case of ZrO ꢀfree
2
H PW O ꢀEtꢀHNS obtained by the same route, its hollow
3
12 40
perturb the selfꢀassembly of BTMSE, Zr(OC H ) and F127A
4
9 4
spherical nanostructre becomes perfect with the inner diameter of
ca. 12 nm (or particle size of 18 nm) and shell thickness of ca. 3
nm (Fig. 1e). As expected, 2D hexagonal p6mm mesostructure of
H PW O /ZrO ꢀEtꢀ2Dhex2.0 (obtained by one step P123ꢀ
30
35
40
45
50
55
micelles due to their strong complexation with nonꢀionic
37
surfactants. Therefore, the preparation conditions should be
controlled carefully to create a suitable environment for selfꢀ
9
0
3
12 40
2
assembling BTMSE and Zr(OC H9)4 around F127 micelles,
4
directed solꢀgel−hydrothermal process) can be clearly observed in
Fig. 1f. The above results suggest that initial Si/Zr molar ratios in
the preparation systems influence the morphology formation of
H PW O /ZrO ꢀEt organicꢀinorganic hybrid materials obviously.
which is the key step to fabrication of H PW O /ZrO ꢀEtꢀHNS.
3
12 40
2
At the beginning of preparation of organicꢀinorganic hybrid
hollow nanospheres materials, the single surfactant (F127)
micelles are formed by copolymer assembly via hydrogen
bonding and hydrophobic/hydrophilic interactions in a dilute HCl
95
3
12 40
2
On the one hand, the morphology of the hybrid materials is
related to the hydrophobicity/hydrophilicity of the precursors (e.g.
BTMSE and Zr(OC H ) ). Generally, BTMSE is a hydrophobic
−1
solution (2mol L ) with the assistance of TMB. That is,
hydrophobic PPO blocks form a core of the micelle, while
hydrophilic PEO blocks form a hydrated corona around the core.
These F127 micelles are served as soft templates for constructing
H PW O /ZrO ꢀEtꢀHNS. Subsequently, coꢀhydrolysis and ꢀ
4
9 4
silicate precursor due to the existence of organic groups (i.e.
00 ethyl), while Zr(OC H ) is a hydrophilic precursor compared
1
1
1
1
4
9 4
with BTMSE. Formation of F127 single spherical micelles can be
encouraged by the hydrophobic unhydrolyzed BTMSE since the
charge density on the surface of F127 micelles is decreased
owing to the penetration of the hydrophobic unhydrolyzed
05 BTMSE into the micelles. The hydrophobicity/hydrophilicity of
the preparation system tends to more hydrophobic with the
increased initial Si/Zr molar ratio, as a result, the morphology
transformation from 3D interconnected mesostructure to the
3
12 40
2
condensation of BTMSE with Zr(OC H ) in the presence of
4
9 4
+
H PW O
leads
Si(OSi) (OH) −C H −Si(OSi) (OH) (x = 1−4, n = 1−3)
3−n
to
H PW O /Zr(OC H ) (OH ) −
3
12 40
3 12 40 4 9 4ꢀx 2 x
n
3−n
2
4
n
species. It should be mentioned that the hydrolysis rate of
BTMSE is obviously slower than that of Zr(OC H ) . In order to
4
9 4
ensure the matching of hydrolysis rate between BTMSE and
o
Zr(OC H ) , prehydrolysis of BTMSE at 40 C for 1 h is
4
9 4
38
hollow spherical nanostructure occurs. On the other hand,
proceeded before the addition of Zr(OC H ) and H PW O .
4
9 4
3
12 40
10 hydrolysis/condensation rate of BTMSE and Zr(OC H )
4
9 4
The assembly of the hydrolyzed species around the corona of
individual F127 micelles results in the formation of spherical
influences the morphology of the resulting hybrid materials. The
precursor with faster hydrolysis/condensation rate (e.g.
Zr(OC H ) ) can produce more hydrolyzed species (e.g.
+
supramolecular complexes H PW O /Zr(OC H ) (OH ) −
3
12 40
4
9 4ꢀx
2
x
4
9 4
Si(OSi) (OH) −C H −Si(OSi) (OH)3−n−EO PO EO106. After
n
3−n
2
4
n
106
70
+
Zr(OC H ) (OH ) ) to condense around the single micelles,
4
9 4ꢀx
2
x
further aging and subsequent boiling ethanol washing for F127
removing, H PW O /ZrO −Si(OSi) −C H −Si(OSi)
3
15 which in turn results in the aggregation of F127 single micelles.
Accordingly, the hybrid materials with the bulk mesostructure are
3
12 40
2
3
2
4
(
abbreviated as H PW O /ZrO ꢀEt) organicꢀinorganic hybrid
3 12 40 2
4
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Journal of Materials Chemistry A
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DOI: 10.1039/C4TA02257B
5
1
1
2
0
5
0
Fig. 1 The representative TEM images of asꢀprepared hybrid catalysts. H
3
PW12
O40/ZrO
2
3 2 3 2
ꢀEtꢀ3Dint (a); H PW12O40/ZrO ꢀEtꢀHSN1.0 (b); H PW12O40/ZrO ꢀ
EtꢀHSN1.5 (c); H 40/ZrO ꢀEtꢀHSN2.0 (d); H 40ꢀEt-HSN (e) and H
3
PW12
O
2
3
PW12
O
3
PW12 40/ZrO
O
2
ꢀEtꢀ2Dhex2.0 (f).
2
3
3
4
5
0
5
0
Fig. 2 Nitrogen gas adsorptionꢀdesorption isotherm (a) and pore size distribution curves (b) of various hybrid catalysts.
obtained. Otherwise, the hydrolysis/condensation rate of BTMSE
is much slower, and increasing initial Si/Zr molar ratio can slow
materials as well as H PW O ꢀEtꢀHNS are present in the Fig. 2.
3 12 40
From Fig. 2a it is found that all tested hybrid materials exhibit
down the hydrolysis/condensation rate of the precursors. 60 type IV isotherm, implying their mesoporosity. In addition,
4
5
5
5
0
5
Therefore, the aggregation of F127 single micelles is inhibited
rapidly increased nitrogen gas adsorbed amount at low relative
effectively, leading to H PW O /ZrO ꢀEt hybrid materials with
pressure regions (P/P < 0.1) suggests that asꢀprepared hybrid
3
12 40
2
0
hollow nanospheres structure. Therefore, it is concluded that
changing the initial Si/Zr molar ratios in the preparation systems
materials also have microporosity. The microporosity originates
from the more hydrophilic EO chains of the surfactant, which
3
8
can
adjust
the
hydrophobicity/hydrophilicity
and 65 penetrate the wall during synthesis and lead to microporosity.
For H PW O /ZrO ꢀEtꢀ3D material, it has one capillary
hydrolysis/condensation rate of the precursors, which in turn
leads to the morphology transformation of the H PW O /ZrO ꢀ
3
12 40
2
int
condensation step at the relative pressure P/P = 0.40−0.80. In the
3
12 40
2
0
Et hybrid materials from the bulk mesostructure to hollow
spherical nanosturctue
The porosity and textural properties of asꢀsynthesized 70 they possess two capillary condensation steps at a relative
cases of H PW O /ZrO ꢀEtꢀHNS1.0, H PW O /ZrO ꢀEtꢀ
3
12 40
2
3
12 40
2
.
HNS1.5, H PW O /ZrO ꢀEtꢀHNS2.0 and H PW O ꢀEtꢀHNS,
3 12 40 2 3 12 40
materials are characterized by nitrogen gas physisorption analysis.
Nitrogen gas adsorptionꢀdesorption isotherms and pore size
distribution curves of various H PW O /ZrO ꢀEt hybrid
pressure P/P = 0.40−0.80 and P/P = 0.80−0.99 (close to the
0
0
saturation vapour pressure), respectively. Combination of the
textural properties with morphological characteristics of
3
12 40
2
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Journal Name, [year], [vol], 00–00 | 5

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DOI: 10.1039/C4TA02257B
a
Table 1 Textural parameters and Brönsted acidꢀsite density of various hybrid catalysts
2
−1
3
−1
A
titration/ꢁeq g−1
Sample
S
BET/cm g
V
p
/cm g
D
p
/nm
H
3
H
3
H
3
H
3
H
3
H
3
H
3
PW12
PW12
PW12
PW12
PW12
PW12
PW12
O
O
O
O
O
O
O
40/ZrO
40/ZrO
40/ZrO
40/ZrO
2
2
2
2
ꢀEtꢀ3Dint
151
256
265
339
603
313
353
0.32
0.43
0.57
0.71
1.34
0.64
0.44
7.5/22.6
7.6/17.0
9.0/17.7
9.1/18.0
11.0/43.4
9.1/18.0
6.4
2891
1902
1833
1500
333
ꢀEtꢀHSN1.0
ꢀEtꢀHSN1.5
ꢀEtꢀHSN2.0 1st
b
40ꢀEtꢀHSN
c
40/ZrO
2
2
ꢀEt-HSN2.0 3rd
ꢀEtꢀ2Dhex2.0
1200
1327
40/ZrO
a
Pore diameter (D
p
) was estimated from BJH adsorption determination; Pore volume (V
p
) was estimated from the pore volume determination using the
adsorption branch of the N
2
isotherm curve at P/P = 0.99 single point; Acid capacity (Atitration) of the tested catalysts was measured by titration with NaOH
0
−1
+
b
c
5
(0.0045 mol L ) and expressed by the number of equivalents of H in the catalysts. The first use of the catalyst in the reaction. The third time of the
catalyst was used in the reaction.
H PW O /ZrO ꢀEtꢀHNS hybrid materials it is speculated that
3
12 40
2
4
5
5
6
6
7
7
5
0
5
0
5
0
5
the hysteresis loop appeared at a lower relative pressure can be
assigned to the hollow interior of the spherical particles; and that
the other hysteresis loop shown up at a higher relative pressure
originates from void space between the loosely packed spherical
particles. Therefore, asꢀprepared H PW O ꢀEtꢀHNS hybrid
1
1
2
2
3
3
4
0
3
12 40
materials possess an interesting hierarchical porous structure.
The above results are consistent with BJH poreꢀsize
distribution curves displayed in Fig. 2b as well as the textual
parameters summarized in Table 1. From Table 1 it is found that:
i) the diameter of the primary and secondary pore calculated from
BJH adsorption determination is in the range of 7.6−11.0 nm and
17.0−43.4 nm, respectively, for H PW O /ZrO ꢀEt and
5
0
5
0
5
0
3
12 40
2
H PW O ꢀEtꢀHNS hybrid materials with various morphologies;
3
12 40
ii) with the increase of the initial Si/Zr molar ratio, the BET
2
−1
surface area (from 151 to 339 cm g ), pore diameter (from
3
7.5/22.6 to 9.1/18.0 nm) and pore volume (from 0.32 to 0.71 cm
−1
g ) of the hybrid materials increase gradually; and iii) among all
tested hybrid materials, ZrO ꢀfree H PW O ꢀEtꢀHNS exhibits
2
3
12 40
2
−1
the largest BET surface area (603 cm g ), the largest pore
diameter (11.0 nm) and highest pore volume (1.34 cm g ).
3
−1
Composition and structure. The loading of H PW O in all
3
12 40
products is 10.0−11.0 wt% except for H PW O ꢀEtꢀHNS (5.7
3
12 40
wt%), determined by a Leeman Prodigy Spec ICPꢀAES.
The structural integrity of the Keggin units and ethaneꢀbridged
organosilica fragments as well as the interaction between the
ZrO2 with organic or inorganic functionalities in the hybrid
organicꢀinorganic hollow nanospheres materials are provided by
31
13
29
P MAS NMR (Fig. 3a), C CPꢀMAS NMR (Fig. 3b) and Si
MAS NMR (Fig. 3c) spectra, and H PW O /ZrO ꢀEtꢀHNS2.0 is
3
12 40
2
selected as the representative.
31
The P MAS NMR spectrum of H PW O /ZrO ꢀEtꢀHNS2.0
3
12 40
2
shows that the strong resonance at −16.3 ppm, which is assigned
to the PO4 units within the bulk H PW O environment,
suggesting the structural integrity of the Keggin units after
formation of the organicꢀinorganic hybrid hollow nanoshperes.
3
12 40
3
1
13
29
Fig. 3 P MAS (a), C CPꢀMAS (b) and Si MAS (c) NMR spectra of
the asꢀprepared H 40/ZrO ꢀEtꢀHNS2.0 hybrid catalyst.
3
PW12
O
2
6
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Journal of Materials Chemistry A
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DOI: 10.1039/C4TA02257B
o
As for the weak signal at −11.0 ppm, it is due to the new
loss (ca. 6%) in the range of 365−600 C is due to partial
+
n−
41
(
≡ZrOH ) [H PW O ] species generated at the interface of 60 decomposition of H PW O . The TGA analysis indicates asꢀ
2 n 3ꢀn 12 40 3 12 40
3
9
the Keggin units and ZrO2. In this species, the terminal W=O
bonds of Keggin units can coordinate with the surface ≡ZrOH
prepared H PW O /ZrO ꢀEtꢀHNS is thermal stable at
3
12 40
2
o
temperature lower than 170 C.
5
0
5
0
5
0
5
0
5
groups of ZrO via Zr−O−W covalent bonds. The conclusion is
reasonable because of the wellꢀmatched electronegativity and
Based on the above chemical structure information, the
illustration of the shell structure and composition of the
2
4
+
6+
ionic radius of Zr ion (1.33, 0.072 nm) and W (1.70, 0.060 65 H PW O /ZrO ꢀEtꢀHNS is proposed and illustrated in Scheme 2.
3
12 40
2
3
1
nm). The interactions ensure strong combination of the Keggin
units and ZrO2 in the organicꢀinorganic hybrid hollow
nanospheres.
1
1
2
2
3
3
4
4
1
3
In the C CPꢀMAS NMR spectrum of H PW O /ZrO ꢀEtꢀ
3
12 40
2
HNS2.0, a predominant peak at 5.2 ppm originates from the
carbon species of ethaneꢀbridged organosilica unit
−Si−CH −CH −Si−), suggesting that ethaneꢀbridged organosilica
70
(
2
2
units are successfully incorporated into the framework of
H PW O /ZrO ꢀEtꢀHNS material through
Zr−O−Si−CH −CH −Si−O− linkages. In the cases of three weak
3
12 40
2
−
75
2
2
signals at 71.0, 57.8 and 16.4 ppm, they are due to residual F127
and the carbon species of the ethoxy group that are formed during
1
9
boiling ethanol washing process.
29
In the Si MAS NMR spectrum of H PW O /ZrO ꢀEtꢀ
Scheme
2
3 2
The shell composition of H PW12O40/ZrO ꢀEt hollow
3
12 40
2
HNS2.0, the characteristic resonances at −60.0 and −65.8 ppm 80 nanospheres as well as the processes of levulinic acid esterification and
3 2
yellow horn oil transesterification catalyzed over H PW12O40/ZrO ꢀEt
correspond
to
the
organosiloxane
species
of
−
hollow nanospheres hybrid catalysts.
2
3
CH CH −Si(OSi) (OH) (T ) and −CH CH −Si(OSi) (T ) within
2
2
2
2
2
3
the ethaneꢀbridged organosilica groups, respectively. The result
confirms full framework linkage of ethaneꢀbridged organosilica
7
groups in the shell of the hybrid material. Additionally, the
8
9
9
5
0
5
absence of any resonance signal assignable to SiO species such
4
3
4
as Q [Si(OSi) (OH), −90 ppm] and Q [Si(OSi) , −120 ppm]
3
4
confirms that cleavage of the Si–C bond of Si−CH CH −Si
2
2
moiety in the silica/carbon framework of H PW O /ZrO ꢀEtꢀ
3
12 40
2
HNS2.0 has been avoided.
Fig. 5 FTIR spectra of pyridine adsorbed on various asꢀprepared hybrid
catalysts.
Brönsted acid-site density. The Brönsted acidꢀsite density of
the H PW O /ZrO ꢀEt hybrid materials is measured through
3
12 40
2
−1
1
00 acidꢀbase titration with NaOH (0.0045 mol L ). From the acid
values summarized in Table 1 it is found that the acidꢀsite density
of the tested materials decreases gradually with increasing the
initial Si/Zr molar ratio from 0.5, 1.0, 1.5 to 2.0. The finding is
owing to the fact that the proportion of the nonꢀacidic silica
05 increases and goes parallel with the initial Si/Zr molar ratio from
1
0
.5 to 2.0. As for ZrO ꢀfree H PW O ꢀEtꢀHNS it has the lowest
,
2
3
12 40
Fig. 4 TGA profile of asꢀprepared H
3
PW12
O
40ꢀEtꢀHNS2.0 hybrid material.
−1
acid capacity (333 ꢁeq g ).
Brönsted and Lewis acidity. PyridineꢀFTIR spectra of
various H PW O /ZrO ꢀEt and H PW O ꢀEtꢀHNS materials
o
5
0
TGA is conducted in the range of 25–600 C, and the TGA
plot of H PW O /ZrO ꢀEtꢀHNS2.0 is depicted in Fig. 4. The
3
12 40
2
3
12 40
2
3
12 40
TGA profile shows three weight loss steps: the first weight loss 110 are provided to investigate the different types of acid sites in asꢀ
o
(
ca. 6%) that occurred below 100 C is related to the loss of
prepared hybrid materials (Fig. 5). Three characteristic peaks at
−1
physisorbed water at the hybrid material surface. A main weight
loss (ca. 11%) in the range of 170–365 C is assigned to the
decomposition of ethaneꢀbridged group. Although the residual
surfactants are also decomposition in the same range, it is very 115 bond to the unsaturated surface Zr . As for the peak at 1540 cm ,
1450, 1490 and 1540 cm are observed for four tested
o
−1
5
5
H PW O /ZrO ꢀEt materials. The peak at 1450 cm is assigned
3
12 40
2
4
0
to the Lewis acid sites, which is due to pyridine coordinatively
4+
−1
2
6
difficult to quantify the amount of surfactant. The third weight
it is related to pyridinium ions formed due to the protonation of
Journal Name, [year], [vol], 00–00 |
This journal is © The Royal Society of Chemistry [year]
7

Journal of Materials Chemistry A
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5
0
5
1
1
Fig. 6 Catalytic activity of asꢀprepared catalysts towards esterification of levulnic acid with methanol (a) and transesterification of yellow horn oil with
o
methanol (b). Conditions: molar ratio of LA: methanol = 1: 7, 2 wt% catalyst, 65 C, 1.5 h, atmospheric pressure (esterification); oil: 1.023 g (1.1 mmol),
o
methanol: 4 mL (98.9 mmol), 5 wt% catalyst, 65 C, 24 h, atmospheric pressure (transesterification).
−1
20
the Brönsted acid sites. These Brönsted acid sites are contributed
the catalytic activity in terms of per acid site, TOF (h ) of each
catalyst are given in the inset of Fig. 6a. This result indicates that
from the protons of H PW O and surface Zr−OH groups of
3
12 40
ZrO . The coꢀexistence of Brönsted and Lewis acid sites is 60 the catalytic activity of H PW O /ZrO ꢀEt increases gradually
2
3
12 40
2
−1
confirmed by the characteristic peak at 1490 cm . The above
with the increased initial Si/Zr molar ratio; additionally,
results indicate that the H PW O /ZrO ꢀEt hybrid materials
H PW O ꢀEtꢀHNS2.0 is the most active among the five tested
3
12 40
2
3
12 40
25
possess both Brönsted and Lewis acid sites, regardless of their
hybrid catalysts.
morphological characteristic. In the case of H PW O ꢀEtꢀHNS,
To expand the scope of their application, the heterogeneous
3
12 40
it only exhibits Brönsted acidity since no peak related to Lewis 65 acid catalytic activity of asꢀprepared H PW O /ZrO ꢀEt
3
12 40
2
acid sites or coꢀexistence of Brönsted and Lewis acid sites are
catalysts is further tested in transesterification of yellow horn oil
with methanol to produce biodiesel under the conditions of
methanol to oil molar ratio of 90: 1, 5 wt% catalyst, 65 C and
atmospheric pressure. Low price and high yield of this feedstock
lead to the biodieselꢀproduction process from inedible yellow
horn oil is practical and economical. Thereby, biodieselꢀderived
from yellow horn oil is expected to be competitive with
petroleumꢀbased diesel for the commercial use.
found in the H PW O ꢀEtꢀHNSꢀpyridineꢀFTIR spectrum.
3
12 40
o
30
3.2 Evaluation of heterogeneous acid catalytic performance of
H PW O /ZrO -Et organic-inorganic hybrid catalysts
3
12 40
2
7
7
8
8
9
0
5
0
5
0
Catalytic activity. H PW O /ZrO ꢀEtꢀHNS organicꢀinorganic
3
12 40
2
hybrid catalysts with advantages of strong Brönsted and Lewis
acidity, hollow nanosphere morphology as well as surface
hydrophobicity are expected to exhibit the excellent catalytic
performance in heterogeneous acidꢀcatalyzed reactions. In current
work, the heterogeneous acid catalytic performance of
35
40
45
50
55
In the H PW O /ZrO ꢀEtꢀcatalyzed transesterification
3
12 40
2
system, the transesterification products (i.e. FAMEs) are
identified by a GCꢀMS. The main FAMEs produced are methyl
palmitate (MP, C16:0), methyl stearate (MS, C18:0), methyl
oleate (MO, C18:1) and methyl lioleate (ML, C18:2);
additionally, a small quantity of methyl eicosenoate (ME, C20:1)
and methyl docosenate (MD, C22:0) are also found. Herein, the
transesterification acivity of asꢀprepared three H PW O /ZrO ꢀ
H PW O /ZrO ꢀEtꢀHNS is evaluated by esterification of LA
3
12 40
2
with methanol to synthesize methyl levulinate and
transesterification of yellow horn oil with methanol to produce
biodiesel (Scheme 2). Both of the reactions need Brönsted and
Lewis acid sites; moreover, Lewis acid sites are more active in
3
12 40
2
4
2
promoting transesterification reactions.
Firstly, the heterogeneous acid catalytic activity of the
H PW O /ZrO ꢀEt materials prepared at various initial Si/Zr
EtꢀHNS hybrid catalysts is evaluated by the yields of four main
FAMEs (i.e. MP, MS, MO and ML). For comparison,
H PW O /ZrO ꢀEtꢀ3D and ZrO ꢀfree H PW O ꢀEtꢀHNS are
3
12 40
2
3
12 40
2
int
2
3
12 40
molar ratios are evaluated in the esterification of LA with
methanol under the conditions of a 1: 7 molar ratio of LA to
also tested under the same conditions. From the result displayed
in Fig. 6b it is found that the transesterification reaction proceeds
more slowly in comparison of esterification reaction. This result
is explained by the following three reasons: i) transesterification
of triglycerides (TGs) in oily feedstocks is a more complex
process with multiple steps; ii) transesterification of TGs can
produce more intermediates; and iii) transesterification of TGs
o
methanol, 2 wt% catalyst, 65 C and atmospheric pressure. From
the result presented in Fig. 6a it is found that four tested
H PW O /ZrO ꢀEt materials show excellent esterification
3
12 40
2
activity, and their catalytic activity follows the order
H PW O /ZrO ꢀEtꢀHNS1.0 < H PW O /ZrO ꢀEtꢀHNS1.5 <
3
12 40
2
3
12 40
2
H PW O /ZrO ꢀEtꢀHNS2.0 ≈ H PW O /ZrO ꢀEtꢀ3D . For
21
3
12 40
2
3
12 40
2
int
needs higher activation energy. From Fig. 6b it is also found
example, the yield of methyl levulinate reaches 78, 85, 89 and
that the hybrid catalysts follow the activity order H PW O ꢀEtꢀ
3
12 40
92% after the reaction proceeded for 1 h. As for ZrO ꢀfree
2
HNS < H PW O /ZrO ꢀEtꢀ3D < H PW O /ZrO ꢀEtꢀHNS1.0
3 12 40 2 int 3 12 40 2
H PW O ꢀEtꢀHNS, it shows the lowest yield of methyl
3
12 40
95 < H PW O /ZrO ꢀEtꢀHNS1.5 < H PW O /ZrO ꢀEtꢀHNS2.0.
3 12 40 2 3 12 40 2
levulinate (3.6%) under the same conditions. In order to compare
For the most active H PW O /ZrO ꢀEtꢀHNS2.0ꢀcatalyzed
3
12 40
2
8
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transesterification reaction, the yields of MP, MS, MO and ML
activity is explained in terms of the strong Brönsted and Lewis
reach 75, 77, 76 and 63%, respectively, after the reaction 60 acidity, unique hollow nanospherical morphology and
proceeded for 24 h.
hydrophobic surface.
Compared with the results shown in Fig. 6a and b it is found
that the reaction activity of the tested hybrid catalysts (expressed
in terms of the yield) towards esterification and transesterification
First of all, the inherent strong Brönsted and Lewis acidity
plays the dominant role to the excellent catalytic activity of the
H PW O /ZrO ꢀEtꢀHNS hybrid catalysts. The Brönsted acidity
5
0
5
0
5
0
3
12 40
2
follow the different sequences. For transesterification reaction, 65 of the hybrid catalysts originates from H PW O and ZrO2;
3
12 40
mass transfer of TG molecules in yellow horn oil is the rate
controlling step because TGs are bulky and high viscosity
compounds. The disordered bulk mesostructure with poor
porosity (e.g. smaller BET surface area, lower pore volume and
moreover, the Brönsted acid strength can be enhanced owing to
+
n−
the formation of (≡ZrOH ) [H PW O ] species at the
2
n
3ꢀn
12 40
8,29,43
2
1
1
2
2
3
interface between H PW O and ZrO2.
With the aid of the
3
12 40
protons, carbonyl groups of LA or TG molecules are protonated
smaller pore diameter) of the H PW O /ZrO ꢀEtꢀ3D leads to 70 at the beginning of the esterification or transesterification
3
12 40
2
int
the increased mass transfer limit of bulky TG molecules and
inaccessible to the acid sites in transesterification process, and
reaction. This is the key step for both of the reactions, and strong
Brönsted acidity of the hybrid catalysts can facilitate the reactions
proceeding at a fast rate. On the other hand, the Lewis acid sites
thereby, H PW O /ZrO ꢀEtꢀ3D
exhibits obviously lower
transesterification activity compared with H PW O ꢀEtꢀ
3
12 40
2
int
4
+
of the hybrid catalysts come from coordinatively unsaturated Zr
3
12 40
4+
HNS2.0. In the case of H PW O /ZrO ꢀEtꢀ3D ꢀcatalyzed 75 sites of ZrO , which leave the exposed Zr ion to interact directly
3
12 40
2
int
2
esterification
process,
higher
catalytic
activity
of
with LA or TG molecules and act as the acceptor of the electronꢀ
H PW O /ZrO ꢀEtꢀ3D may due to its very high Brönsted acidꢀ
pair. In the subsequent steps, they follow the same procedures as
those of the Brönsted acidꢀcatalyzed esterification or
transesterification reaction.
3
12 40
2
int
−
1
site density (2891 ꢁeq g ), which plays a dominating role in its
esterificaiton activity.
8
8
9
9
0
5
0
5
Secondly, unique hollow nanospherical morphology of the
H PW O /ZrO ꢀEtꢀHNS hybrid catalysts with shell thickness in
3
12 40
2
several nanometers can serve as the nanoreactors where
esterification or transesterification reaction takes place. For the
H PW O /ZrO ꢀEtꢀHNSꢀcatalyzed reactions, the diffusion path
3
12 40
2
of the reactants and products is shortened; moreover, the
accessibility of acid sites to the reactants could be increased. Both
of the factors can speed up the mass transport of the reactants
and/or products, which results in the enhancement of the
17,18
esterification and transesterification activity.
Additionally,
unique hollow nanospherical structure of H PW O /ZrO ꢀEtꢀ
3
12 40
2
HNS led to them excellent textural properties including
hierarchical porous structure, larger BET surface area (256−339
2
−1
3
−1
3
5
m g ) and high pore volume (0.43−0.71 cm g ), which can
provide better dispersion of the acid sites throughout the hybrid
catalysts and increase the population of the acid sites.
Accordingly, the accessibility of LA or TGs molecules to the acid
sites also could be increased, giving a positive influence on the
catalytic activity. For the tested three hollow nanospherical
Fig. 7 Comparison of the catalytic activity of H
3
PW12
PW12 40/ZrO
and Amberlystꢀ15 towards
O
40/ZrO
2
ꢀEtꢀ
HNS2.0 with
H
3
PW12
pure
O40/ZrO
2
ꢀEtꢀ2Dhex2.0,
H
3
O
2
,
ZrO
2
,
4
0
C
2.5
H0.5PW12
O
40
,
H
3
PW12O
40
esterification of levulnic acid with methanol. Conditions: molar ratio of
o
hybrid catalysts, H
PW12O40/ZrO ꢀEtꢀHNS1.0, H PW12O40/ZrO ꢀ
3 2 3 2
LA: methanol = 1: 7, 2 wt% catalyst, 65 C, 1 h, atmospheric pressure.
1
00 EtꢀHNS1.5 and H PW O /ZrO ꢀEtꢀHNS2.0, their BET surface
3
12 40
2
Subsequently, the heterogeneous acid catalytic activity of the
area, pore diameter and pore volume increase with Si/Zr molar
ratio. Accordingly, H PW O /ZrO ꢀEtꢀHNS1.0 and
H PW O /ZrO ꢀEtꢀHNS2.0 exhibit the lowest and highest
most active H PW O /ZrO ꢀEtꢀHNS2.0 is compared with six
3
12 40
2
3
12 40
2
4
5
5
5
0
5
reference catalysts including H PW O /ZrO ꢀEtꢀ2Dhex2.0,
3 12 40 2
3
12 40
2
H PW O /ZrO , H PW O , Cs2.5H PW O , ZrO2 and
Amberlystꢀ15 by selecting esterification of LA with methanol as 105 H PW O /ZrO ꢀEtꢀ2Dhex2.0, although it has the similar textural
esterification or transesterification activity. In the case of
3
12 40
2
3
12 40
0.5
12 40
3
12 40
2
the model reaction. From the result displayed in Fig. 7 it is found
that the heterogeneous acid catalytic activity of H PW O /ZrO ꢀ
properties to those of H PW O /ZrO ꢀEtꢀHNS2.0, its lower
3 12 40 2
catalytic activity with respect to its hollow sphereꢀlike
counterpart is due to the lengthened diffusion pathway (pore
channels of bulk mesoporous counterpart is longer). However, for
3
12 40
2
EtꢀHNS2.0 outperforms all of the tested reference catalysts. For
example, the yield of methyl levulinate reaches 89
(
H PW O /ZrO ꢀEtꢀHNS2.0),
64
(H PW O /ZrO ꢀEtꢀ 110 ZrO ꢀfree H PW O ꢀEtꢀHNS, although it has perfect hollow
3 2 2 3 12 40
3
12 40
2
12 40
2
Dhex2.0), 42 (H PW O ), 32 (H PW O /ZrO ), 29
nanospherical structure and excellent porosity characteristic (BET
3
12
4
3
12 40 2
2
−1
2
−1
(
Cs2.5H PW O ), 26 (ZrO2), and 22% (Amberlystꢀ15),
surface area 603 m g ; pore volume 1.34 cm g ), it exhibits the
lowest esterification and transesterification activity among all
tested hybrid catalysts. This is due to its low acidity because it
0.5
12
4
respectively, after the reaction performed for 1 h.
Discussion. Based on the physicochemical properties and
catalytic testing results of asꢀprepared H PW O /ZrO ꢀEtꢀHNS 115 only has the Brönsted acid sites. Therefore, it is inferred that the
3
12 40
2
organicꢀinorganic hybrid catalysts, their excellent catalytic
catalytic activity of asꢀprepared hybrid catalysts is closely related
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 9

Journal of Materials Chemistry A
Page 10 of 12
View Article Online
DOI: 10.1039/C4TA02257B
to their morphological properties in addition to the chemical
structure
and
composition.
5
10
15
Fig. 8 Reusability of H
3 2
PW12O40/ZrO ꢀEtꢀHNS2.0 hybrid catalyst towards the esterfication of levulnic acid with methanol (a) and transesterification of
o
20
yellow horn oil with methanol (b). Conditions: molar ratio of LA: methanol = 1: 7, 2 wt% catalyst, 65 C, 2 h, atmospheric pressure (esterification); oil:
o
1
.023 g (1.1 mmol), methanol: 4 mL (98.9 mmol), 5 wt% catalyst, 65 C, 12 h, atmospheric pressure (transesterification).
Finally, hydrophobic surface of the hybrid catalysts owing to
powder. The result indicates that H PW O /ZrO ꢀEtꢀHNS2.0
3 12 40 2
incorporation of bridging ethyl groups in the framework is also 60 hybrid catalyst is a recyclable and waterꢀtolerant solid catalyst for
25
responsible for the excellent esterification and transesterification
activity. The hydrophobic environment in the pore channels is in
favor of enrichment of hydrophobic reactants (i.e. LA or TG),
while the hydrophilic products like water or glycerol are expelled.
the LA esterification and yellow horn oil transesterification. The
high catalytic stability is attributed to: i) the strong chemical
interaction between the Keggin units and ZrO ; and ii) the
2
hydrophobic catalyst surface, which can inhibit the strong
Accordingly, the esterification of LA and transesterification of 65 adsorption of water or glycerol effectively.
yellow horn oil can proceed at a fast rate. At the same time, the
30
catalyst deactivation due to the strong interaction of the active
site with water or glycerol is dramatically reduced, which is
confirmed by the following recycling test (Fig. 8).
4 Conclusions
A
series of heteropoly acid and ZrO2 bifunctionalized
organosilica hollow nanospheres (H PW O /ZrO ꢀEtꢀHNS)
3
12 40
2
3
.3 Regeneration and reusability
hybrid catalysts are successfully fabricated by carefully designed
solꢀgel coꢀcondensation route. By the combination of
advantageous including strong Brönsted and Lewis acidity,
unique hollow nanospherical morphology and hydrophobic
surface, asꢀprepared organicꢀinorganic hybrid catalysts exhibit
excellent heterogeneous acid catalytic activity in the synthesis of
methyl levulinate from levulinic acid and biodiesel production
from yellow horn oil under mild conditions. The inherent
Brönsted and Lewis acidity of the hybrid catalysts facilitate both
of the esterifiation and transesterification reactions carrying out at
a fast rate; additionally, the unique hollow nanospherical
morphology of the hybrid catalysts with thin shell thickness and
unique textural properties including bimodal porous structure
with hollow interior and interparticle voids, larger BET surface
area and high pore volume can increase the accessibility of the
guest molecules to the acid sites, reduce the diffusion resistance,
shorten the diffusion pathway, provide better dispersion of the
acid sites throughout the hybrid catalysts and increase the
population of the acid sites; and finally, the hydrophobic surface
of the hybrid catalysts is in favor of hydrophobic reactants
enrichment as well as expels the hydrophilic products, which also
can speed up both of the reactions. The strong interaction
7
7
8
8
9
0
5
0
5
0
35
40
45
50
55
The stability of the active sites and the reusability of the
heterogeneous catalysts in the liquid reaction system have been of
concern for the hybrid/composite catalysts. Therefore, the most
active H PW O /ZrO ꢀEtꢀHNS2.0 catalyst is selected to study
3
12 40
2
the stability and reusability of asꢀprepared heterogeneous
catalysts. The processes are performed three times, and the results
are shown in Fig. 8. After the first use and before every
subsequent reuse, the catalyst is recovered by centrifugation,
o
washed three times with dichloromethane and dried at 100 C in
air. The new reaction begins with refilling new reactants and the
weighed reused catalyst under the optimum reaction conditions,
o
i.e. molar ratio of LA to methanol is 1: 7, 2 wt% catalyst, 65 C, 2
h and atmospheric pressure for LA esterification; molar ratio of
o
methanol to oil is 90 to 1, 5 wt% catalyst, 65 C, 12 h and
atmospheric pressure for yellow horn oil transesterification. From
the results displayed in Fig. 8 it is found that the catalytic activity
of H PW O /ZrO ꢀEtꢀHNS2.0 is almost retained after three
3
12 40
2
cycles for LA esterification (Fig. 8a) and yellow horn oil
transesterification reaction (Fig. 8b).
Additionally, P and W in the catalystꢀfree reaction solutions
are not detected by ICPꢀAES; meanwhile, changes of structural
properties and acidity of the reused hybrid catalyst are negligible
between the Keggin unit and ZrO effectively inhibit the leakage
2
of the acid sites, and thereby the hybrid catalysts can be reused at
least three times without significant activity loss.
(entry 6 in Table 1). Therefore, the loss of activity during the
recycling process may be owing to the loss of the catalyst
1
0
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Journal of Materials Chemistry A
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DOI: 10.1039/C4TA02257B
2
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DOI: 10.1039/C4TA02257B
Table of contents entry
Title: Heteropoly acid and ZrO
2
bifunctionalized oraganosilica hollow nanospheres
for esterification and transesterification
a
Authors: Fang Su, Sai An, Daiyu Song, Xianghuan Zhang, Bo Lu, Yihang Guo*
3 2
H PW12O40/ZrO -Et-HNS materials with strong Brönsted and Lewis acidity, unique
hollow nanospherical morphology and enhanced surface hydrophobicity exihibit
excellent catalytic activity.