Development of mesoscopically assembled sulfated zirconia nanoparticles as promising heterogeneous and recyclable biodiesel catalysts

Article abstract of DOI:10.1002/cctc.201300192

The nanoassembly of nearly monodisperse nanoparticles (NPs) as uniform building blocks to engineer zirconia (ZrO₂) nanostructures with mesoscopic ordering by using a template as a fastening agent was explored. The mesophase of the materials was investigated through powder X-ray diffraction and TEM analysis (TEM) and N₂ sorption studies. The TEM results revealed that the mesopores were created by the arrangement of ZrO₂ NPs with sizes of 7.0-9.0nm and with broad interparticle pores. Moreover, the N₂ sorption study confirmed the results. The surface chemical analysis was performed to estimate the distribution of Zr, O, and S in the sulfated ZrO₂ matrices. The materials in this study displayed excellent catalytic activity in the biodiesel reaction for effective conversion of long-chain fatty acids to their methyl esters, and the maximum biodiesel yield was approximately 100%. The excellent heterogeneous catalytic activity could be attributed to the open framework, large surface area, presence of ample acidic sites located at the surface of the matrix, and high structural stability of the materials. The catalysts revealed a negligible loss of activity in the catalytic recycles.

Full text of DOI:10.1002/cctc.201300192

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Fasten your nanoparticles! Monodis-
perse nanoparticles are used as building
blocks for a mesoscopic ZrO2 nanoarchi-
tecture by using the template as a fas-
tening agent. Successive integration of
the sulfate functionality into the porous
framework makes the material a hetero-
geneous and recyclable catalyst for bio-
diesel production with a maximum bio-
diesel yield of 100%.
S. K. Das, S. A. El-Safty*
&& – &&
Development of Mesoscopically
Assembled Sulfated Zirconia
Nanoparticles as Promising
Heterogeneous and Recyclable
Biodiesel Catalysts
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DOI: 10.1002/cctc.201300192
Development of Mesoscopically Assembled Sulfated
Zirconia Nanoparticles as Promising Heterogeneous and
Recyclable Biodiesel Catalysts
Swapan K. Das[a] and Sherif A. El-Safty*[a, b]
The nanoassembly of nearly monodisperse nanoparticles (NPs)
as uniform building blocks to engineer zirconia (ZrO2) nano-
structures with mesoscopic ordering by using a template as
a fastening agent was explored. The mesophase of the materi-
als was investigated through powder X-ray diffraction and TEM
analysis (TEM) and N2 sorption studies. The TEM results re-
vealed that the mesopores were created by the arrangement
of ZrO2 NPs with sizes of 7.0–9.0 nm and with broad interparti-
cle pores. Moreover, the N2 sorption study confirmed the re-
sults. The surface chemical analysis was performed to estimate
the distribution of Zr, O, and S in the sulfated ZrO2 matrices.
The materials in this study displayed excellent catalytic activity
in the biodiesel reaction for effective conversion of long-chain
fatty acids to their methyl esters, and the maximum biodiesel
yield was approximately 100%. The excellent heterogeneous
catalytic activity could be attributed to the open framework,
large surface area, presence of ample acidic sites located at
the surface of the matrix, and high structural stability of the
materials. The catalysts revealed a negligible loss of activity in
the catalytic recycles.
Introduction
Design and synthesis of porous crystalline metal oxides have
been receiving great interest from the scientific community be-
cause of their versatile applications, such as solar cells, sensors,
filters, and catalysts.[1–7] Metal oxides fabricated through con-
ventional sol–gel procedures are usually amorphous in nature,
and their crystallization require high-temperature treatment,
which often results in the collapse of mesostructures. Thus,
their applications are limited.[8] This limitation has motivated
researchers in studying the formation of uniform crystalline-
building nanounits that can be utilized as precursor artificial
atoms for the fabrication of crystalline mesoporous structures
in mild reaction conditions.[9] Different routes, such as sol–gel,
hydrothermal, and solvothermal crystallization, for the synthe-
sis of metal oxide nanocrystals of various sizes and shapes and
several nanoparticle (NP) assemblies with porous structures,
such as TiO2,[10] CeO2,[4,11] CeO2–Al(OH)3,[11] and SnO2,[12] have
been reported. Chen et al. reported the hydrothermal fabrica-
tion of nanocrystalline zirconia (ZrO2) by using a mixed-surfac-
tant route with mixed tetragonal and monoclinic phases that
exhibit both macropores and small mesopores.[13] We recently
prepared self-assembled mesoporous ZrO2 NPs through evapo-
ration-induced self-assembly method through a nonaqueous
route in the presence of Pluronic F127.[14] However, the synthe-
sis of crystalline porous self-assembled ZrO2 NPs with controlla-
ble pore structures by using a template as a fastening agent is
still a big challenge.
The utilization of porous solid materials in biodiesel produc-
tion is very interesting because it correlates with environmen-
tal issues and fuel crisis.[15,16] Biodiesel is considered as a prom-
ising alternative fuel compared with conventional petroleum
diesel because the former is eco-friendly and promotes a posi-
tive life cycle–energy balance. Biodiesel has also a higher
octane number and a higher flash point than diesel fuel, there-
by providing better performance and safer use.[17,18] Moreover,
alkyl esters of fatty acids have been utilized as important ingre-
dients in different uses, such as perfumes, flavors, cosmetics,
food additives, lubricants for textiles, detergents, soaps, and
plasticizers. Biodiesel is made from renewable resources and
low-cost feedstocks, such as yellow greases, rendered animal
fats, and trap greases that contain a high concentration of free
fatty acids.[19,20] Biodiesel is prepared through esterification of
free fatty acids or the transesterification of triglycerides with
methanol or other alcohols in the presence of acid or base cat-
alysts.[21–23] Industrial biodiesel production involves homogene-
ous base catalysis, which is correlated with a high production
cost because this method encounters neutralization and sepa-
ration problems.[24] Base-catalyzed procedure suffers from sev-
eral limitations of feedstock. For example, the free fatty acid
content in the feedstock should be lower than 0.5 wt%, other-
wise, biodiesel production will be badly hampered because of
soap formation.[24] On the other hand, concentrated sulfuric
acid catalyzed esterification is basically a homogeneous pro-
cesses, which is corrosive and critical for waste separation.[25]
[a] Dr. S. K. Das, Prof. S. A. El-Safty
National Institute for Materials Science (NIMS)
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan)
Fax: (+81)29-859-2501
E-mail: sherif.elsafty@nims.go.jp
[b] Prof. S. A. El-Safty
Graduate School for Advanced Science and Engineering, Waseda University
3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555 (Japan)
E-mail: sherif@aoni.waseda.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300192.
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Solid acid catalysts are considered as suitable candidates for
heterogeneous catalysis because of the limitations in homoge-
neous catalysis. Heterogeneous catalysis that involves the use
of solid acid catalysts does not encounter corrosion issues and
offers the usual advantages such as easy product isolation and
catalyst reusability, thereby minimizing the loss of products
during catalyst separation.[26] To date, various solid acid cata-
lysts such as zeolites,[27,28] sulfonated carbonized sugar,[29,30] sul-
fated ZrO2,[31–33] sulfated silica–ZrO2,[34] Zr–PMOs,[35] ion ex-
change resins and ionic liquid,[36] zirconium phosphate/metal
oxides,[37] and organosulfonic acid functionalized mesoporous
silica[38,39] have been developed in biodiesel reaction. Zeolites
are microporous solids and are no suitable candidates in bio-
diesel reaction because of the diffusion limitations of long-
chain fatty acid molecules. Ion exchange resins are also not
considered as potential candidates because of their low ther-
mal stability. Solid acids usually catalyze the esterification reac-
the active sites in the chemical reactions. Large pore sizes
favor the diffusion of large-size fatty acid molecules. With the
presence of strong acid sites, the biodiesel reaction rate signifi-
cantly accelerates in optimal reaction conditions.[40,41] The sul-
fated MAsZrNPs (MAsSZrNPs) functioned as heterogeneous
and recyclable catalysts in the biodiesel reaction, and the maxi-
mum biodiesel yield was approximately 100%. To our knowl-
edge, the fabrication of such mesoporous nanoassemblies by
using premade monodisperse NPs with a high surface area
and sodium dodecyl sulfate (SDS) as a fastening agent in the
hydrothermal method and their utilization as heterogeneous
solid acid catalysts in biodiesel reactions were not explored
until now. The mesoporous nanoassemblies provided efficient
catalytic reusability in the biodiesel reaction with negligible
loss of activity.
tion of fatty acids with methanol or with other small-chain al- Results and Discussion
cohols at high temperatures ranging from 373 K to 453 K. Kiss
Fabrication of MAsZrNPs biodiesel catalysts
et al. have displayed various interesting biodiesel catalysts,
such as niobic acid, sulfated ZrO2, sulfated titania, and sulfated
tin oxide, and have revealed that sulfated ZrO2 is the most
active in this aspect.[17,28] Another sulfated-ZrO2-anchored mes-
oporous silica catalyst has also been reported by Chen et al.
for the heterogeneous catalysis of the esterification of long-
chain fatty acids.[34] Myristic acid has been esterified with short-
chain alcohols by using sulfated ZrO2.[31] Rebeca et al. showed
that Zr-loaded mesoporous organic–inorganic hybrid silica cat-
alyzed the biodiesel production through esterification/transes-
terification of free fatty acids in the feedstock.[35] Thus, we have
also developed zirconium oxophosphates for the heterogene-
ous catalysis of the esterification reaction of different long-
chain fatty acids with methanol.[40] Moreover, studies have
shown that researchers are extremely interested in developing
heterogeneous recyclable catalysts in biofuel preparation, and
that the invention of suitable green catalysts is a big challenge
today.
The development of new MAsZrNPs by using premade NPs as
a framework building block through the use of a template as
a fastening agent (Scheme 1) is highly challenging. The entire
synthetic process involved the following: 1) Preparation of
a sol of highly disperse and very small ZrO2 NPs and their uti-
lization as framework building units. 2) Fastening of ZrO2 NPs
with a template molecule, because the pH of the solution (pH
ꢀ1) was below the point of zero charge (4–6),[14] thus, positive-
ly charged ZrO2 NPs interacted with the negative head group
of the anionic structure-directing agent (SDS) through electro-
static interaction, and a mesoscopic assembly architecture was
generated. 3) Generation of porous frameworks through the
removal of the fastening template molecules and the internal
rearrangement of NPs by high-temperature calcination; the
highly crystalline nature of NPs effectively sustained local
strain, which was owing to the mesophase formation.
The main advantages of these frameworks are as follows:
1) individual NPs compose the pore walls of the matrix and
provide a high structural stability at high temperatures and in
harsh chemical reaction conditions. 2) Premade NPs minimize
the possibility of NP size increase throughout the porous
framework construction. 3) The special arrangement of NPs
generates a porous framework with a higher surface area than
that of bulk NPs. Hence, these frameworks are useful catalysts
because they have plenty of accessible active sites. Moreover,
large pores and an open framework structure facilitate the dif-
fusion of large fatty acid molecules. NP morphology and crys-
tallinity were confirmed by high-resolution TEM (HRTEM), se-
lected-area electron diffraction (SAED) analysis (Figure 1), and
wide-angle powder X-ray diffraction (PXRD) analysis (Fig-
ure 2B). The small-angle PXRD results of calcined sulfated ma-
terials suggested that the structure was retained and the mes-
oscale was porous (Figure 2A and Figure S1A in the Support-
ing Information). The electron diffraction spectroscopy results
for chemical surface analysis revealed the successful integra-
tion of the sulfate functionality (Figure S2). The main advantag-
es of the materials for catalysis were as follows: 1) They exhib-
In this context, we herein present the development of new
mesoscopically
assembled
zirconium
nanostructures
(MAsZrNPs) by using premade NPs as a framework building
block through the use of a template as a fastening agent in an
acidic aqueous medium. The porous frameworks were generat-
ed by removing the fastening template molecules and internal
rearrangement of NPs at high-temperature calcination. The
highly crystalline characteristic of the NPs effectively sustained
the local strain during mesophase formation. The pore walls of
the materials were composed of individual NPs, which provid-
ed high structural stability at high temperatures and in harsh
chemical reactions. The utilization of premade NPs minimized
the possibility of increased NP size during the entire porous-
structure creation, and provided a higher surface area than if
bulk NPs were used. The sulfated matrix was used as an effi-
cient heterogeneous solid acid catalyst in the biodiesel reac-
tion, such as in the conversion of long-chain fatty acids to
their corresponding esters in mild reaction conditions. A high
surface area facilitates the integration of the sulfate functionali-
ty and an open framework structure provides easy access to
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3) The heterogeneous catalytic
activity of MAsSZrNPs was ex-
cellent, which could be attribut-
ed to the large surface area, the
presence of ample acidic sites
located at the surface of the
matrix, and its stability towards
harsh chemical reaction condi-
tions. Two NH3 desorption
peaks were assigned at 278.9
and 639 K for MAsSZrNPs (Fig-
ure S3). Compared to the de-
sorption peak of recently pre-
pared sulfated zirconia at
635 K,[14] the peak of MAsSZrNPs
appeared at much higher tem-
perature. These findings re-
vealed that the fabrication strat-
egy of sulfonated nanoassem-
bled zirconia led to a strong
binding of ammonia to highly
acidic sites. 4) The catalyst ex-
hibited a negligible loss of ac-
tivity in the catalytic recycles.
Scheme 1. The formation of nanoassembly MAsZrNPs through the hydrothermal method by using an anionic
structure-directing agent (SDS) as a fastening agent.
ited a high surface area that integrated the sulfate functionali-
ty and an open-framework structure that provided easy access
to the active sites during the chemical reaction. 2) The sulfated
MAsSZrNPs functioned as heterogeneous and recyclable cata-
lysts in the biodiesel reaction during the transformation of
long-chain free fatty acids into their corresponding esters. The
obtained maximum biodiesel yield was approximately 100%.
This concept can, therefore, be utilized to design and synthe-
size other mesoscopic-assembly materials. These materials may
also be of great importance to other acid-catalyzed reactions.
Figure 2. A) Small-angle PXRD patterns of sulfated samples: a) MAsSZrNPs-1,
b) MAsSZrNPs-2, c) MAsSZrNPs-3, and d) MAsSZrNPs-4. B) Wide-angle PXRD
patterns of sulfated samples: a) MAsSZrNPs-1, b) MAsSZrNPs-2,
*
Figure 1. TEM images of calcined samples a) MAsZrNPs-1 and b) MAsZrNPs-2
seen through the direction perpendicular to the pore axis. The correspond-
ing SAED patterns are shown in the insets, respectively. HRTEM images of
the calcined materials: c) MAsZrNPs-1 and d) MAsZrNPs-2.
c) MAsSZrNPs-3, and d) MAsSZrNPs-4; ( ) monoclinic phase, ( ) tetragonal
*
phase.
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High-resolution TEM analysis
Representative TEM images of the calcined MAsZrNPs are
shown in Figure 1a,b. In these images, low-electron-density
spots (pores) were observed throughout the respective speci-
mens, and particles of sizes of approximately 7.0–9.0 nm were
arranged in a mesoscopic order. Interparticle pores, as seen in
these images, varied from 4.0 to 6.0 nm in length. The SAED
analysis results (insets in Figure 1a and b) of the mesoporous
MAsZrNPs confirmed that the pore walls of our studied materi-
als were made up of nanocrystalline oxides, which exhibited
characteristic diffuse electron diffraction rings. These well-re-
solved diffraction rings were attributed to the polycrystalline
nature of NPs.[13,14] HRTEM images revealed the orientation of
the NPs within the pore walls and several nanocrystallites with
well resolved lattice planes, as shown in Figure 1c and d. Here,
the marked white portion within the dashed circles were the
pores, whereas the marked black portion within the solid cir-
cles were the observed NPs. The average pore diameters mea-
sured from this micrograph were consistent with the pore size
distribution derived by using the nonlocal density functional
theory (NLDFT) method from the N2 adsorption/desorption iso-
therms, as shown in Figures 3 and 4.
Figure 4. NLDFT pore size distribution of A) calcined samples: a) MAsZrNPs-
1, b) MAsZrNPs-2, c) MAsZrNPs-3, and d) MAsZrNPs-4; B) calcined sulfated
samples a) MAsSZrNPs-1, b) MAsSZrNPs-2, and c) MAsSZrNPs-3. Vp =pore
volume, dp =pore diameter.
Powder X-ray diffraction
The small-angle PXRD patterns for the calcined MAsSZrNPs are
shown in Figure 2A and Figure S1A, respectively. Both types of
materials exhibited a single broad peak in their respective
small-angle PXRD patterns. This result suggests that the NPs
were arranged more or less disorderly in these materials, as
shown in the TEM images (Figure 1).[14] The calcined MAsZrNPs
exhibited a diffraction peak at ca. 2.008 (2q), which indicates
a d spacing of ca. 4.41 nm, whereas in MAsSZrNPs, this diffrac-
tion peak slightly shifted to approximately 1.958, and the corre-
sponding d spacing was approximately 4.52 nm. The XRD re-
sults were consistent with the TEM micrographs and the N2
sorption analysis results. The small-angle PRXD results demon-
strate the existence of mesoporosity in both types of materials,
and during high-temperature calcinations, individual NPs rear-
ranged to create mesoporosity. The highly crystalline nature of
the materials effectively sustained the local strain during meso-
phase formation.[4,11]
The wide-angle PXRD patterns of the calcined MAsSZrNPs
are shown in Figure 2B and Figure S1B, respectively. The XRD
results for both types of materials exhibited a mixture of well-
resolved characteristic of monoclinic and tetragonal phases of
individual ZrO2 NPs.[13,42–44] Calcined MAsZrNPs possessed a tet-
ragonal phase, which is its major characteristic (Figure 2B).
After sulfate integration, the monoclinic phase became more
prominent, as observed in the Figure S1B. Thus, integrated sul-
fate ions had a strong influence on phase modification. Sulfate
ions converted the metastable tetragonal phase to its more
thermodynamically stable monoclinic phase.[42] In the current
research, the particle sizes of NPs were calculated by using the
Scherrer equation. The estimated particle sizes varied from 7.0
to 9.0 nm (see the Supporting Information). These results were
in agreement with the TEM image analysis results (Figure 1).
Figure 3. N2 adsorption/desorption isotherms of A) calcined samples
a) MAsZrNPs-1, b) MAsZrNPs-2, c) MAsZrNPs-3, and d) MAsZrNPs-4; B) cal-
cined mesoporous sulfated samples a) MAsSZrNPs-1, b) MAsSZrNPs-2, and
c) MAsSZrNPs-3 measured at 77 K. Adsorption points are marked by filled
circles and desorption points by empty circles.
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N2 sorption studies
N2 sorption studies are important to determine the porous
nature of the materials. N2 sorption measurements were con-
ducted on the calcined and sulfated mesoporous MAsZrNPs at
77 K, and the type-IV adsorption-desorption isotherm exhibited
a small hysteresis loop, as shown in Figure 3A and B, respec-
tively. Type-IV isotherms are characteristic of mesoporous ma-
terials, and desorption hysteresis results suggested the exis-
tence of large mesopores in the sample.[45] This hysteresis was
an intermediate between typical H2- and H4-type hysteresis
loops in a relative pressure range P/P0 from 0.40 to 0.9. This
result suggested that large mesopores with cage-like pore
structures were connected by windows with small sizes.[10,46]
This characteristic is typical of capillary condensation within
uniform pores. In the current research, the BET surface area,
average pore diameter and pore volume for the calcined and
sulfated mesoporous MAsZrNPs are shown in Table 1 (en-
Figure 5. UV/Vis reflectance spectra of calcined samples: a) MAsZrNPs-1,
b) MAsZrNPs-2, c) MAsZrNPs-3, and d) MAsZrNPs-4.
identical. The absorption band at approximately 208 nm ap-
peared because of the ligand-to-metal charge-transfer transi-
tion (O2ꢁ!Zr4+). Furthermore, a weak broad band was ob-
served in the region from 224 to 230 nm. This result can be at-
tributed to the presence of ZrꢁOꢁZr linkages in the frame-
work.[34] The UV absorption edge wavelength is very sensitive
to the particle size of semiconductor nanocrystals.[47,48] For NPs
(<10 nm), the band gap energy increases with decreasing
crystal size, and the absorption edge of the interband transi-
tion is blueshifted. Such blueshifts of the interband transition
energy (band gap) were clearly observed in the UV region of
the diffuse reflectance spectra for very small ZrO2 nanocrystals
because of the appearance of an additional peak in the region
from 270 to 290 nm.[14] This spectroscopic result suggested
that ZrO2 nanocrystals compose the pore walls of the mesopo-
rous MAsZrNPs structure. However, unlike conventional meso-
porous materials with a continuous pore wall, the pore walls
of MAsZrNP materials can be considered as composed of dis-
crete nanodomains separated by voids.
Table 1. Physico-chemical properties of mesoscopic-assembly zirconia
and sulfated zirconia nanoparticles.
Entry Sample type Surface area Pore width Pore volume Particle size
[m2 gꢁ1
]
[nm]
[cm3 gꢁ1
]
[nm]
1
2
3
4
5
6
7
MAsZrNPs-1 183
MAsZrNPs-2 142
MAsZrNPs-3 139
6.26
4.26
4.24
4.02
0.272
0.265
0.207
0.082
7.63
8.13
8.34
8.69
7.94
7.21
8.21
MAsZrNPs-4
67
MAsSZrNPs-1 93
MAsSZrNPs-2 103
MAsSZrNPs-3 98
6.87/14.20 0.160
4.59
4.87
0.114
0.272
tries 1–4 for the calcined MAsZrNPs and entries 4–7 for the sul-
fated MAsZrNPs). The BET surface area of the calcined
MAsZrNPs-1, MAsZrNPs-2, MAsZrNPs-3, and MAsZrNPs-4 were
183, 142, 139, and 67 m2 gꢁ1, respectively. The surface area of
the sulfated MAsZrNPs-1, MAsZrNPs-2, and MAsZrNPs-3 were
93, 103, and 98 m2 gꢁ1, respectively. The pore volumes of the
corresponding calcined materials decreased after the incorpo-
ration of the sulfate group, as shown in Table 1 (entries 4–7).
Thus, the surface areas as well as the pore volumes of the cal-
cined matrices decreased upon sulfate integration, which can
be attributed to the dispersion of sulfate groups on the surface
of the porous framework. Moreover, a kind of pore blocking
can also occur.[26] The pore sizes of MAsZrNPs, as shown in Fig-
ure 4A and estimated by employing the NLDFT method, were
in agreement with the pore widths obtained from TEM images
(Figure 1) and XRD analysis results (Figure 2A and Figure S1B).
FTIR spectroscopy
The FTIR spectra of the calcined and sulfated MAsSZrNPs are
shown in Figure 6. The absence of bands at approximately n=
2854 and approximately 2925 cmꢁ1 in these samples, which
are ascribed to the symmetric and asymmetric vibrations of
the CꢁH groups, indicated the complete removal of surfactant
molecules after calcination. The broad bands at approximately
n=3000 to 3600 and 1620 cmꢁ1 were attributed to the asym-
metric OH stretching and vibration bending of the adsorbed
water molecule, respectively.[48] The spectral feature ranging
from n=1400 to 900 cmꢁ1 was very important in characteriz-
ing the presence of sulfate moieties in MAsSZrNPs, and all
MAsSZrNP materials exhibited almost similar spectral features.
The observed band at n=960 cmꢁ1 indicated SꢁO symmetric
stretching, whereas other bands at higher frequencies, such as
UV/Vis diffuse reflectance spectra
UV/Vis spectroscopy was performed to characterize the optical
properties of the ZrO2 nanocrystals, which contained pore
walls of the mesoporous self-assembly architecture. In Figure 5
and Figure S3, UV/Vis diffuse reflectance spectra of the cal-
cined and sulfated mesoporous MAsZrNPs are shown, respec-
tively. The spectral features of these materials were almost
n=1033, 1069, 1128, and 1236 cmꢁ1 were attributed to SꢁO
,
asymmetric stretching.[49] Furthermore, the presence of another
band at n=1380 cmꢁ1 indicated the asymmetric stretching of
S=O, which was bonded to the ZrO2 NPs.[50] From the spectral
data, MAsSZrNPs-2 had numerous intense bands in the sulfate
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with a turnover number (TON) of 18.53 at 323 K after 8 h of re-
action time (entry 13). If the fatty acid (oleic acid) to alcohol
(methanol) molar ratio was changed to 10, the yield and TON
were not significantly affected (entry 15). TON calculation was
performed based on chemical analysis, as shown in the Sup-
porting Information (Figure S2). A decrease in Zr/S molar ratio
to 15.13 (MAsSZrNPs-1), which corresponded to an increase in
the integrated sulfate group amount in the matrix, lowered
the yield to 77% and reduced the TON to 14.26 (Table 2,
entry 14). MAsSZrNPs-1 at 3 wt% resulted in a lauric acid
methyl ester yield of 89% with a TON of 17.11 (entry 8), where-
as 6 wt% of the respective catalyst resulted in a yield of 91%.
The increase was small, but the TON decreased to 8.75
(entry 9) without changing other reaction parameters.
Figure 6. FTIR spectra of the sulfated samples: a) MAsSZrNPs-1,
b) MAsSZrNPs-2, and c) MAsSZrNPs-3. In the inset, the corresponding FTIR
spectra ranging from n=1800 to 800 cmꢁ1 are shown.
To study temperature effects, lauric acid was used as a refer-
ence. For example, the temperature was lowered to 283 K in
the case of MAsSZrNPs-2, which resulted in a decrease of the
yield to 38% and TON to 7.04 (Table 2, entry 17), whereas in-
creasing the temperature to 303 K resulted in an increase of
the yield to 86% and the TON to 15.93 (entry 6). Increasing the
temperature of the reaction to 323 K increased the yield to
96% and the TON to 17.78 (entry 10). If the system was cata-
lyzed by a nonsulfated MAsZrNPs-1 catalyst, the yield de-
region of asymmetric stretching, and this result may explain
the higher catalytic efficiency of MAsSZrNPs-2 in biodiesel reac-
tion (acid-catalyzed reaction) than that of the other two sam-
ples. Thus, this spectral investigation described the integration
of sulfate moieties into the ZrO2 NPs. The partial ionic nature
of the SꢁO bonds was responsible for the strong Brønsted
acidity of the sulfate-modified
ZrO2 NPs.[51] The high surface
Table 2. Biodiesel reaction of different long chain fatty acids with methanol catalyzed by mesoscopic assembly
area
of
the
mesoporous
zirconia and sulfated zirconia nanoparticles.[a]
MAsZrNPs facilitated the inte-
gration of the sulfate functional-
ity to a suitable extent within
its framework; hence, acid-cata-
lyzed reactions were accelerat-
ed.
Entry
Catalyst type
Fatty acid
Alcohol/acid
molar ratio
Catalyst
[wt%]
Temperature
[K]
Yield
[%]
TON[b]
1
2
3
4
5
6
7
8
9[c]
10
11[d]
12[e]
13
14
15
16
17[f]
18[g]
19[h]
MAsSZrNP-1
MAsSZrNP-2
MAsSZrNP-1
MAsSZrNP-2
MAsSZrNP-1
MAsSZrNP-2
MAsSZrNP-3
MAsSZrNP-1
MAsSZrNP-1
MAsSZrNP-2
MAsSZrNP-2
MAsSZrNP-2
MAsSZrNP-2
MAsSZrNP-1
MAsSZrNP-2
MAsSZrNP-3
MAsSZrNP-2
MAsSZrNP-1
–
decanoic acid
decanoic acid
decanoic acid
decanoic acid
lauric acid
lauric acid
lauric acid
lauric acid
lauric acid
lauric acid
lauric acid
lauric acid
oleic acid
oleic acid
oleic acid
oleic acid
lauric acid
lauric acid
lauric acid
10
10
10
10
10
10
10
10
10
10
20
20
20
10
10
10
10
10
10
3.48
3.48
3.48
3.48
3.00
3.00
3.00
3.00
6.00
3.00
3.00
3.00
2.14
2.14
2.14
2.14
3.00
3.00
–
303
303
323
323
303
303
303
323
323
323
303
323
323
323
323
323
283
323
323
57
86
81
97
76
86
42
89
91
96
93
95
100
77
99
72
38
21
4
10.96
15.93
15.58
17.96
14.61
15.93
7.78
Catalytic reaction
The catalytic performance of
sulfated MAsZrNPs in biodiesel
synthesis
was
investigated
through the esterification of dif-
ferent long-chain fatty acids
with methanol. In this study,
methanol was used as a reactant
as well as a solvent for the
liquid phase. In all cases, only
the methyl ester of the corre-
sponding fatty acid was formed
as a product with 100% selec-
tivity. The results of the catalytic
activities of MAsSZrNPs are
given in Table 2. In view of their
good catalytic activity, a sulfate-
loaded MAsSZrNPs-2 catalyst
17.11
8.75
17.78
17.22
17.59
18.53
14.26
18.34
13.33
7.04
4.32
–
[a] Conditions, unless stated otherwise: The esterification of long-chain fatty acids with methanol was per-
formed with a mole ratio of fatty acid/methanol of 1:10 and 1:20 (entries 11–13). The catalyst (ꢀ30 mg) and
fatty acid (5 mmol) were used for each set of the reaction. [b] TON=moles of product or yield/mole of active
sites (e.g., disperse Zr sites) of the catalyst. [c] Esterification of lauric acid with double amount of the catalyst
(60 mg) at 323 K. [d] Esterification of lauric acid with a mole ratio of fatty acid/methanol of 1:20 at 303 K.
[e] Esterification of lauric acid with a mole ratio of fatty acid/methanol of 1:20 at 323 K. [f] Esterification of
lauric acid at 283 K. [g] Esterification of lauric acid in the presence of unsulfated MAsZrNPs-1 catalyst.
[h] Esterification of the lauric acid in the absence of a catalyst. In each case, the product selectivity was 100%,
and the conversion of all the products was measured after 8 h.
was considered, at
a typical
loading of 2.14 wt% with re-
spect to fatty acid (oleic acid),
to obtain a maximum yield of
100% (Zr/S molar ratio of 16.54)
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creased to 21% and the TON to 4.32 (entry 18) during lauric
acid esterification at 323 K for 8 h. Thus, sulfate functionaliza-
tion is important for the efficient biodiesel reaction of different
fatty acids with methanol in mild reaction conditions. Blank ex-
periments confirmed that fatty acid esterification did not occur
considerably in the absence of the MAsSZrNPs catalysts
(entry 19). From the catalytic results, the MAsSZrNPs-2 catalyst
provided the highest efficiency in biodiesel reaction compared
with the other two materials, and also provided the maximum
yield of oleic acid.
charge (d+) on the carbonyl carbon of the reacting fatty acid
through the protonation of the adjacent oxygen atom. Conse-
quently, the nucleophilic methanol (MeOH) molecule attacks
the carbonyl carbon, because the latter has a very high electro-
philicity. After internal rearrangement, the methyl ester of the
corresponding fatty acid and water are formed in the reaction.
In the end, the final products are easily desorbed from the sur-
face of the catalyst to the bulk reaction medium because the
polarity of the esterified products is lower compared to that of
the reactant (free fatty acids). In general, the MAsZrNPs cata-
lysts exhibited a catalytic activity significantly higher than that
of other heterogeneous or homogeneous acid and base cata-
lysts,[17,25,28,34,40,52,53] which also require intensive design and ex-
perimental conditions in such biodiesel catalytic reactions
(Table 3).
Methanol was used in a large excess relative to the fatty
acids because it functions as a reactant as well as a solvent in
this type of reaction. The use of a small amount of methanol
(methanol/fatty acid molar ratio 1:1) relative to the fatty acid
in the respective biodiesel reaction provided a viscous mixture
of the reactant and product, which was difficult to separate.[23]
Excess methanol helps dissolve the fatty acid; thus, other sol-
vents are not needed. Methanol also facilitates the conversion
to the desired product. Excess methanol used in the reaction
can be reused in the next run. The boiling point of methanol is
relatively low; thus, the separation of excess methanol from
the product mixture is not difficult. Particularly, the isolation of
the biofuel product becomes much easier if methanol is used
in excess in the reaction mixture.
Table 3. Comparative study of reported biodiesel heterogeneous cata-
lysts with our studied materials.[a]
Entry Sample
Amount Fatty acid/ Temp Time Conv Ref.
[wt%]
alcohol
[K]
[h]
[%]
molar ratio
1
2
3
4
5
6
7
8
SZ
1
5
5
1
10
10
20
10
1
403
361
338 24
358
298 24
403
323
323
2
6
40
90
89
84
95
96
77
99
[17]
[34]
[40]
[39]
SiO2–SZ
porous ZrP
SBA-15–SO3H 10
HZnPS-1
H2SO4
MAsSZrNPs-1 2.14
MAsSZrNPs-2 2.14
The use of surface-sulfate-modified zirconia NPs was vital in
the esterification reaction of fatty acids, as shown in Scheme 2.
Studies have shown that sulfated ZrO2 has a strong Brønsted
acidity, and usually favors this type of reaction.[14,22,34,40,51,52] In
the reaction, the long-chain fatty acid is dissolved in methanol
in the presence of a catalyst (MAsSZrNPs), and the polar car-
boxylic group (ꢁCOOH) of the fatty acid is adsorbed to the cat-
alytic active sites on the surface of the material. Large pores fa-
cilitate the diffusion of bulky-sized fatty acid molecules
through the pore interior, and a high surface area provides
plenty of active sites for the reaction. The presence of sulfonic
acid on the surface of the catalyst leads to a slight positive
3
5
1
[23]
1
8
8
[25,28]
this work
this work
10
10
[a] Sulfated zircona=SZ.
The stability of the MAsSZrNPs catalysts as well as the heter-
ogeneous nature of the catalysis was tested by recycling the
catalyst. The hot filtration of a MAsSZrNPs-2 catalyst solution in
optimized reaction conditions allowed the separation of the
solid catalyst, which was then reused with fresh reagents in
the same reaction conditions. No loss of catalytic activity was
observed. Moreover, the filtered solution was immediately
used for catalysis upon the addition of methanol, and the cata-
lyst was not active in increasing fatty acid esterification. These
results indicated that during the reaction, no appreciable cata-
lyst leaching occurred, and the reaction was essentially hetero-
geneous. The activity of a regenerated catalyst was inspected
upon separation of the solid catalyst from a reaction mixture
through filtration, several times washing with methanol and
anhydrous acetone, and drying in an oven at 373 K overnight.
The catalyst was subsequently activated at 473 K for 4 h under
air flow, and was then utilized for the above reaction. The
same procedure was repeated five times with negligible loss of
activity, as shown in Figure 7. A marginal loss of catalytic activi-
ty of the sulfated catalyst in the biodiesel reaction was ob-
served after several reuses/cycles. This loss of catalytic activity
was possibly attributable to 1) formation of water molecules
during the esterification reaction, which assisted in the deacti-
vation of active sites. 2) Leaching of the sulfate sites to a negli-
gible extent from the catalyst in the polar alcohol medium
Scheme 2. The catalytic cycle during the biodiesel reaction of long-chain
fatty acids with methanol by using calcined sulfated MAsSZrNPs.
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composition of the sulfated mesoporous MAsZrNPs were estimated
through SEM (field-emission scanning electron microscope, JEOL
model 6500). A JEOL JEM 6500 field-emission scanning electron
microscope attached to an energy-dispersive spectroscope and op-
erated at 20 keV was used to determine the elemental composition
of the sulfated mesoporous MAsZrNPs. The nitrogen adsorption/
desorption isotherms of the samples were obtained by using a BEL-
SORP36 analyzer (JP. BEL Co., Ltd.) at 77 K. Prior to the gas adsorp-
tion/desorption measurements, all samples were degassed at 473 K
for 4 h in a high-powered vacuum. The BET specific surface area
was computed by using the adsorption data at a relative pressure
range of P/P0 from 0.05 to 0.30. The pore size distribution was de-
rived from the adsorption isotherms by using NLDFT. The total
pore volume was estimated from the amounts adsorbed at a P/
P0 =0.99. The NH3 temperature-programmed desorption (NH3-TPD)
measurements were conducted on an Autochem 2910 instrument,
and a thermal conductivity detector was used for continuous mon-
itoring of desorbed ammonia. Prior to TPD analysis, the sample
was pretreated at 4008C for 1 h in a flow of ultra-pure helium gas
(40 mLminꢁ1), and the sample was cooled to 1008C in the flow of
ultra-pure helium gas. The pretreated sample was then saturated
with 10% anhydrous ammonia gas (balance He, 60 mLminꢁ1) at
508C for 2 h and subsequently flushed with He (60 mLminꢁ1) at
1008C for 2 h to remove the physisorbed ammonia. The heating
rate of the TPD measurements, ranging from 1008C to 7008C, was
Figure 7. Product yields in various runs, upon catalyst recycling for biodiesel
reaction of long-chain fatty acid with methanol by using sulfated
MAsSZrNPs-2 catalyst. Oleic acid was used as the reference fatty acid.
during the course of reaction. Thus, the sulfated mesoporous
MAsZrNPs described herein have a great potential to be used
as a stable and highly active recyclable solid acid catalyst in
biofuel preparation.
Conclusions
108Cminꢁ1 1H NMR experiments were performed on a Bruker
.
Avance DRX 600 MHz (UltraShield Plus Magnet) NMR spectrometer
at ambient temperature. The FTIR spectra of these samples were
obtained by using an IR Prestige-21 spectrophotometer from Shi-
madzu. The UV/Vis diffuse reflectance spectra were obtained by
using a Shimadzu 3150 spectrophotometer.
In conclusion, we presented a simple and convenient synthetic
method for the fabrication of MAsZrNPs by using very fine
monodisperse ZrO2 nanoparticles (NPs) through the template
pathway in an acidic aqueous medium. The self-assembly pro-
cess was explained by the constructive charge interactions
among NPs with micellar aggregates. The self-assembly meth-
odology, that is, NP/surfactant fastening interactions, is general
and could open up a new window to other types of meso-
scopic assembly of semiconductor NP systems, as well as oxo-
metalate cations and anions. High-resolution TEM and N2 sorp-
tion studies results revealed the formation of large mesopores
through the arrangement of NPs. The integration of sulfate
made this material an excellent heterogeneous biodiesel cata-
lyst for the effective conversion of long-chain fatty acids to
their methyl esters (yield ꢀ100%), as shown in Table 3. The ex-
cellent heterogeneous catalytic activity and stability could be
attributed to the high surface area and acidic sites located on
the surface of the MAsZrNPs catalysts. Moreover, this material
provided efficient reusability as a catalyst in the designed reac-
tion with negligible loss of activity, a feature of high impor-
tance in the heterogeneous catalysis in the face of fuel crisis
and environmental concerns. Catalytic significance also confers
its paramount importance to other acid-catalyzed reactions.
Chemicals
Anionic structure-directing agent CH3(CH2)11OSO3Na (SDS) and dif-
ferent fatty acids, such as decanoic acid, lauric acid, and oleic acid,
and methanol were purchased from Sigma–Aldrich. Zirconyl chlo-
ride (ZrOCl2·8H2O), ammonia (NH3, 28%, aqueous solution), and
nitric acid (HNO3, 60%) were obtained from Wako Chemicals.
Chloroform-d (CDCl3, 0.05% TMS (v/v)+99.8 atom% D) was re-
ceived from Isotec. All these chemicals were used without further
purification.
Synthesis
The synthesis involved two steps: 1) the synthesis of ZrO2 NPs and
2) the fabrication of a mesoscopic nanoassembly architecture.
Preparation of a sol of uniform monodisperse ZrO2 NPs: ZrO2 NPs
were prepared by using suitable modification of previous work
published elsewhere.[54] Briefly, in a typical synthesis, zirconyl chlo-
ride (ZrOCl2·8H2O, 3.22 g, 10 mmol) was dissolved in distilled water
(100 mL). The pH of the solution was rapidly adjusted to approxi-
mately 10 by using an NH3 solution to form hydroxide precipitates.
The precipitate was filtered and thoroughly washed with an excess
of distilled water to remove NH3 and chloride. The precipitate was
then transferred to an aqueous acidic (HNO3) solution and was so-
nicated until a transparent NPs sol was generated. The final pH of
the solution was <1, and the generated particles remained highly
dispersible without sedimentation for a prolonged period.
Experimental Section
Characterization techniques
PXRD analyses of the samples were performed by using a D8 Ad-
vance Bruker AXS diffractometer operated at 18 kW and calibrated
with a standard silicon sample. A Ni-filtered CuKa (l=0.15406 nm)
radiation was used. TEM images and SAED patterns were obtained
by using a JEOL JEM model 2100F microscope operated at 200 kV.
TEM images were obtained by using a CCD camera. SAED patterns
were obtained by using an image-plate magazine. The elemental
Preparation of MAsZrNPs: MAsZrNPs were constructed by using
premade ZrO2 NPs as building blocks. In the synthetic procedure,
premade ZrO2 NPs (1 mmol) were added to SDS solution (0.320 g,
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then stirred further in an oil bath at 353 K for 3 h and slowly
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The other three materials were prepared by varying the molar ratio
of the precursors, such as XZrO2/YSDS/ZH2O. In all these cases X=
1, and only Y and Z were varied. The four sets of variation were
Y=0.56, Z=2224; Y=0.28, Z=1112; and Y=0.14 and Z=556. The
sample abbreviations were NAsZrNPs-2, NAsZrNPs-3, and
NAsZrNPs-4, respectively.
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by calcination in air at 833 K (2 Kminꢁ1 ramping rate) for 2 h.
Catalytic reactions
The catalytic biodiesel reactions were performed in a round-
bottom flask fitted with a water condenser and placed in a temper-
ature-controlled oil bath with a magnetic stirrer. In the biodiesel re-
action, different long-chain fatty acids are esterified with a low-mo-
lecular weight alcohol, such as methanol. In a typical batch, fatty
acid (5 mmol) was dissolved in a suitable amount of methanol, and
an amount of catalyst was added. The solution was then placed in
an oil bath at a specified temperature and was stirred for 8 h. After
completion of the reaction, the product was collected by separat-
ing the solid catalyst through filtration. The methanol solvent was
then separated from the product mixture (filtrate) by vacuum
evaporation. We varied the catalyst amount from 3.5 wt% to
5.8 wt% depending on the substrate, and we varied the tempera-
ture from 283 K to 323 K. Moreover, we varied the molar ratios of
the fatty acids and methanol from 1:10 to 1:20. Methanol was used
in large excess because it was used as a reactant as well as a sol-
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Reuse of the catalyst
At the end of the reaction, the catalyst was separated from the re-
action mixture through filtration. The catalyst was thoroughly
washed with methanol and n-hexane or acetone for several times
to remove both nonpolar and polar compounds that were ad-
sorbed on the surface and in the interior pores of the catalysts.
After washing, the catalysts were activated by heating overnight at
373 K, followed by 473 K for 4 h, and then used again for the same
reaction in identical reaction conditions. The whole process was re-
peated five times for recycling experiments. Negligible loss of the
catalytic activity was observed. Catalyst reusability was tested by
using oleic acid and MAsSZrNPs-2 as the reference fatty acid and
catalyst, respectively (Figure 7). The product yields for various
cycles (Figure 7) were very consistent, suggesting high catalytic ef-
ficiency and stability of MAsSZrNPs in the biodiesel synthesis reac-
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Keywords: fatty acids · heterogeneous catalysis · mesoporous
materials · nanostructures · zirconium
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Received: March 16, 2013
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