Well-dispersed sulfated zirconia nanoparticles as high-efficiency catalysts for the synthesis of bis(indolyl)methanes and biodiesel

Article abstract of DOI:10.1016/j.catcom.2013.07.006

Well-dispersed sulfated zirconia nanoparticles were synthesized with poly(N-vinylpyrrolidone) as a surfactant. The resultant sulfated zirconia nanoparticles are characterized by SEM, XRD, FT-IR and XPS. These nanoparticles were directly used as catalysts for the synthesis of bis(indolyl)methanes and biodiesel via electrophilic substitution reaction of indole with various aldehydes and the esterification of long-chain free fatty acids and exhibited excellent catalytic activity. The mechanism of the formation of the synthesized zirconia nanoparticles was also proposed.

Full text of DOI:10.1016/j.catcom.2013.07.006

Catalysis Communications 41 (2013) 70–74
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Well-dispersed sulfated zirconia nanoparticles as high-efficiency
catalysts for the synthesis of bis(indolyl)methanes and biodiesel
b
a,
a
a
a
Guochang Chen a, , Cun-Yue Guo , Hongbin Qiao , Mingfu Ye , Xiaoning Qiu , Caibo Yue
⁎
⁎
a
College of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan, 243032, China
Polymer Chemistry and Physics School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Well-dispersed sulfated zirconia nanoparticles were synthesized with poly(N-vinylpyrrolidone) as a surfac-
tant. The resultant sulfated zirconia nanoparticles are characterized by SEM, XRD, FT-IR and XPS. These
nanoparticles were directly used as catalysts for the synthesis of bis(indolyl)methanes and biodiesel via elec-
trophilic substitution reaction of indole with various aldehydes and the esterification of long-chain free fatty
acids and exhibited excellent catalytic activity. The mechanism of the formation of the synthesized zirconia
nanoparticles was also proposed.
Received 22 May 2013
Received in revised form 5 July 2013
Accepted 5 July 2013
Available online 13 July 2013
Keywords:
© 2013 Elsevier B.V. All rights reserved.
Solid superacid
Nano-sulfated zirconia
bis(indolyl)methane
Biodiesel
1. Introduction
drying, the desired sulfated zirconia aerogel was obtained [7]. This
approach is a very sensitive one that requires considerable effort to
As environmentally benign catalysts, inorganic solid acids are
gaining much attention in recent years due to recognized advantages
of heterogeneous catalysts, like simplified product isolation, mild re-
action conditions, high selectivities, ease in recovery and reuse of
the catalysts and reduction in the generation of wasteful by-products
[1]. Among these solid acids, sulfated zirconia interestingly exhibits
excellent activity not only for alkylation, isomerization and cracking
reactions [2] but also for a wide range of organic syntheses and trans-
formation reactions [3].
Several different methods have been attempted for the synthesis
of sulfated zirconia catalyst. For example, mesoporous sulfated zirco-
nia has been synthesized from various surfactants [4], both monoclin-
ic and tetragonal sulfated zirconia with high surface area have been
prepared by a one-step crystallization method [5], and sulfated zirco-
nia has been supported into the pores of ordered mesostructured sil-
ica [6]. These preparation methods of sulfated zirconia could be
mainly classified into two types. The first one is one-step sol-gel
method. Most of the sol-gel methods are based on the hydrolysis of
water or organic solvent-soluble zirconia precursor, like zirconium
alkoxide, and subsequent condensation to the inorganic framework.
Sulfuric acid solution is added during the sol-gel formation of the
alcogel in such a way that the sulfate is incorporated into the alcogel
network, leading to a zirconium-sulfate cogel. After supercritical
develop convenient size- and shape-controllable processes.
The second approach includes the preparation of zirconium hy-
droxide in the first step followed by sulfate impregnation in the sec-
ond step. The type of precursors used for the preparation of
zirconium hydroxide play a vital role in the final texture and hence
on the performance of the final catalyst. Various zirconium com-
pounds, such as zirconium nitrate, zirconium chloride, zirconium
oxychloride and zirconium isopropoxide, are employed to prepare
the zirconium hydroxide, and the precipitation is mostly carried out
by using the agents, like ammonium hydroxide and urea. In the sec-
ond step, also the sulfate impregnation is carried out by using various
sulfating agents; the most commonly used are H2SO4 and (NH4)2SO4
[8]. Some sulfur compounds, like H2S and SO2, have also been used.
However, particle agglomeration usually occurs in the second step
for this method.
In this study,
a novel two-step precipitation method with
poly(N-vinylpyrrolidone) (PVP) as a surfactant to synthesize well-
dispersed sulfated zirconia nanoparticles was developed. In the first
step, zirconium oxychloride is employed to prepare the zirconium hy-
droxide, and the precipitation is carried out by using ammonium hy-
droxide. In the second step, the sulfate impregnation is carried out by
using the sulfating agent H2SO4. To prevent particle agglomeration,
the surfactant PVP was added in this step. Subsequent glycothermal
processing and calcination induce the formation of nanoparticles.
With the help of this method, the products can be synthesized at
gram scale. The synthesized sulfated zirconia nanoparticles can be di-
rectly used as catalysts to synthesize bis(indolyl)methanes and biodie-
sel, and exhibited excellent catalytic performances.
⁎
Corresponding authors at: College of Chemistry and Chemical Engineering, Anhui
University of Technology, Maanshan, 243000, China. Tel.: +86 555 2311552; fax: +86
555 2311644.
E-mail addresses: chengc@ahut.edu.cn (G. Chen), qiaohb@ahut.edu.cn (H. Qiao).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.07.006

G. Chen et al. / Catalysis Communications 41 (2013) 70–74
71
2. Experiment
2.1. Synthesis of well-dispersed sulfated zirconia nanoparticles
Improved procedures were adopted to synthesize sulfated zirco-
nia. Typically, 5 g of ZrOCl2 · 8H2O was dissolved in 100 mL distilled
water. To this clear solution, dilute aqueous ammonia was added
dropwise from a burette with vigorous stirring until the pH of the so-
lution reached 8. The obtained precipitate was washed with distilled
water until free from chloride ions and dried at 80 °C for 24 h. The
obtained zirconium hydroxide sample was ground to fine powder.
To a two-necked flask, 40 mL of ethylene glycol and 2 g of PVP
were transfered, and the mixture was stirred at 60 °C until the solu-
tion was clear. Thereafter, zirconium hydroxide powder (1 g) was
added and stirred for 2 h before 3 mL of H2SO4 solution (0.2 M)
was added. The suspension was continuously stirred for 12 h before
loaded into a poly(tetrafluoroethylene) (PTFE)-lined stainless steel
autoclave. The autoclave was sealed and maintained at 180 °C for
12 h and then cooled down to room temperature. The final products
were filtered and washed with distilled water and ethanol several
times to remove any impurities. The as-prepared precursors were fi-
nally calcined at 650 °C for 5 h.
Fig. 2. The XRD patterns of the synthesized sulfated zirconia. (●) Characteristic lines
due to tetragonal zirconia; (○) characteristic lines due to monoclinic zirconia.
Detailed catalytic reaction procedure, characterization, and NMR
data are provided as supporting information.
the XPS survey spectrum of the sample calcined at 650 °C for 1 h.
Four elements can be discriminated: zirconium (Zr3d, 183.8 ev), sulfur
(S2s, 234.4 ev), oxygen (O1s, 531.7 ev), and carbon (C1s,284.8 ev). The
carbon detected by XPS is adventitious carbon, which is typically intro-
duced from carbon compounds existing in the enviroment under test-
ing conditions. The surface atomic mole concentration of sulfur is
about 0.32%. Parameters such as the amount of zirconium hydroxide
and PVP, H2SO4 concentration, glycothermal temperature, etc., signifi-
cantly influence sulfated zirconia particle size and morphology.
Well-dispersed yorlk-like sulfated zirconia nanoparticles with the size
of 700–900 nm were obtained under synthesis conditions of 1.0 g of
zirconium hydroxide, 2.0 g of PVP, and 1.2 mL of H2SO4 (0.2 M) as
shown in Fig. 5(b). Interestingly, when the amount of zirconium hy-
droxide was decreased to 0.5 g, near-spherical sulfated zirconia
nanoparticles were obtained, as shown in Fig. 5(a). The diameters of
the nanoparticles range between 40 and 70 nm. These in all manifest
that well-dispersed sulfated zirconia nanoparticles were obtained.
3. Results and discussion
3.1. Characterization of the synthesized sulfated zirconia nanoparticles
Scanning electron microscopy (SEM) images show that sulfated zir-
conia nanoparticles are in a high dispersed state (Fig. 1). Ellipse
nanoparticles with the size of 500 to 800 nm were presented with the
improved method. Powder X-ray diffraction (XRD) analysis discloses
structural information about the phase and the size of the crystalline
domains of the sulfated zirconia. The XRD pattern of the synthesized
nanoparticles (Fig. 2) exhibits that the tetragonal phase is predominant,
which is represented by reflections at 2θ = 30.18° as well as at 31.56°,
34.62°, 35.28°, 50.21°, 59.29°, 60.19°, and 62.72° [9]. The width of the re-
flections varies slightly, indicating that there is not a substantial change
in the crystalline domain size [10]. Fourier transform infrared spectra
(FT-IR) exhibits obvious bands at 997, 1045, and 1142 cm−1, which
are assigned to typical bands for chelating bidentate sulfate ion coordi-
nated to zirconium cation [11], as showed in Fig. 3. Better resolved
bands appeared clearly at 1640 cm−1, characteristic of Brønsted acidic
sites [12]. X-ray photoelectron spectroscopy (XPS) was applied to de-
tect the surface chemical component of the sulfated zirconia. Fig. 4 is
3.2. Formation mechanism of sulfated zirconia nanoparticles
The combination of glycols and hydrothermal treatment proves
itself a successful method for preparing well-dispersed sulfated
Fig. 1. SEM image of sulfated zirconia nonoparticles.
Fig. 3. XPS survey spectrum of the synthesized sulfated zirconia calcined at 650 °C.

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G. Chen et al. / Catalysis Communications 41 (2013) 70–74
mechanism, the sulfated zirconia particles were also synthesized
without PVP under the same conditions. In contrast, only irregular
sulfated zirconia particles were obtained, and the diameters of these
irregular particles vary greatly from 5 to 20 μm. In fact, highly dis-
persed sulfated zirconia nanoparticles can only be obtained within a
narrow window of the concentration of sulfuric acid. The mechanism
of the formation of the synthesized zirconia nanoparticles is proposed
(Fig. S1). PVP plays key roles in preventing hard agglomerate forma-
tion during glycothermal process. Nanometer-sized particles of
supermicrostructured surfactant-zirconia composite with surface hy-
droxyl groups could form as soon as the Zr precursor is added to the
surfactant solution under given condition. The unconsumed surfac-
tant molecules cover the nanoparticles of the supermicrostructured
surfactant-zirconia composite due to the affinity between the hy-
droxyl groups of the nanoparticles and the carbonyl of the surfactant
molecules. The presence of surfactant PVP molecules surrounding the
surface of the composite improves the size uniformity and promotes
the formation of well-dispersed sulfated zirconia nanoparticles. The
presence of S in sulfated zirconia detected by XPS (Fig. 4) also pro-
motes the formation of the PVP-nanoparticle composite. In the
study, glycothermal process was conducted at 180 °C. Hence, in
order to obtain high dispersion of the sulfated zirconia nanoparticles,
an apropriate temperature, autoclave pressure, the presence of sulfur
and the addition of PVP are indispensable in glycothermal reaction.
Fig. 4. FT-IR spectra of the synthesized sulfated zirconia.
zirconia nanoparticles with narrow size distribution and low aggrega-
tion. However, the reaction mechanisms in nonaqueous solution are
complex, and currently there is very limited information regarding
the reaction thermodynamics, kinetics, and underlying crystallization
mechanisms. Only a few investigations have dealt with the use of
diols to synthesize crystalline zirconia powders under hydrothermal
conditions [13]. In order to get more information about the forming
3.3. Catalytic performances in synthesis of bis(indolyl)methane
derivatives and biodiesel
The catalytic activity of the as-synthesized nanocomposite was eval-
uated by synthesizing bis(indolyl)methanes via electrophilic substitu-
tion reaction of indole with various aldehydes and biodiesel via the
esterification of long-chain free fatty acids. Indoles and their derivatives
are important intermediates in organic synthesis and widely featured in
variety of pharmacologically active compounds [14]. Table 1 lists the
synthesis of bis(indolyl)methane derivatives with the synthesized sul-
fated zirconia nanoparticles as the catalyst. When the molecular frac-
tion of the catalyst was 30%, based on the mole number of indole, the
yield of diindolylphenylmethane was up to 97% in the reaction time of
24 h at room temperature (Table 1, entry 3). Entries 1 and 2 of
Table 1 show the yield of diindolylphenylmethanes under different re-
action conditions. When the amount of sulfated zirconia increased
from 15% to 30%, the yield of diindolylphenylmethane increased from
36% to 57% in the reaction time of 6 h. Under optimized conditions, var-
ious aldehydes smoothly reacted with indole and formed correspond-
ing bis(indolyl)methanes in excellent yields in desired reaction times.
Hence, the synthesized sulfated zirconia is an efficient catalyst for the
synthesis of bis(indolyl)methanes.
As a “green fuel,” biodiesel is gaining much attention in recent
years as a renewable fuel [15]. In the present work, biodiesel was syn-
thesized via the esterification of long-chain free fatty acids in the
presence of the synthesized sulfated zirconia nanoparticles as the cat-
alyst. Table 2 shows the esterification of palmitic acid and methanol
under different conditions. As the palmitic acid:methanol molar
ratio increases, the conversion of palmitic acid increases first then de-
creases. The optimal molar ratio of palmitic acid:methanol was 1:40.
Similar resluts can also be found in the literature [16]. When the alco-
hol concentration increases, the tendency of equilibrium, in principle,
would be to move in favor of products, increasing thereby the yield of
the ester formed. In addition, the increase in the concentration of al-
cohol may be promoting a reduction in the viscosity of the mixture,
leading to better mixing of reagents and catalyst and to an improve-
ment in the rate of mass transfer, resulting thus in a higher conver-
sion. When the molar ratio of palmitic acid:methanol is over 1:40,
there is a competition between methanol and palmitic acid for active
sites of the catalyst and lead to the decrease of catalytic activity. Like-
wise, for entry 8-12, as the catalyst loading increases, the conversion
Fig. 5. SEM images of sulfated zirconia particles synthesized under different synthesis
conditions (a: 0.5 g Zr(OH)4, 2.0 g PVP 1.2 mL H2SO4; b: 1.0 g Zr(OH)4, 2.0 g PVP
1.2 mL H2SO4).

G. Chen et al. / Catalysis Communications 41 (2013) 70–74
73
Table 1
Sulfated zirconia catalyzed synthesis of bis(indolyl)methane derivatives.a
Entry
1
Indole
Aldehyde
Time (h)
6
Catalyst amount (mole fraction)
15%
Yield (%)
36
57
97
2
3
6
30%
30%
24
4
5
24
24
30%
30%
84
95
6b
24
30%
87
7
24
6
30%
30%
90
96
8b
9b
6
30%
93
a
b
Reaction conditions: indole (1 mmol), aldehyde (4 mmol), and free-solvent, room temperature. Indole (1 mmol), aldehyde (1.5 mmol), and CH2Cl2 solvent (0.5 mL), room
temperature.
increases then decreases. Increasing the amount of catalyst in the re-
action environment consequently increased the number of active
sites necessary for the reaction to occur [17]. Thus, the catalytic activ-
ity increases accordingly. When the amount of catalyst was 15%,
efficienct active sites are close to saturation. To increase the catalyst
mass in the reaction medium would not bring significant variations
in the conversions (Table 2, entry 10,11). Too big usage amount of
catalyst in catalytic reaction would result in the catalyst aggregation
and lead to the decrease of catalytic activity. The conversion of
palmitic acid was 91% under the condition of molar ratio (palmitic
acid:ethanol) 1:40, reaction temperature 95 °C, reaction time 6 h,
and catalyst amount 5 wt% (Table 2, entry 2). It is better than that
of the sulfated zirconia prepared by conventional method [18] as
shown in Table 2 (entry 3). Over the conventional sulfated zirconia,
the conversion of palmitic acid was only 72% when the catalyst
amount is 5 wt% under same conditions. The conversion was up to
97% when the amount of the synthesized sulfated zirconia increases
to 15 wt% under similar conditions (Table 2, entry 10). Satisfactory
conversions were also obtained with different fatty acids and alcohols
as reactants under optimal reaction conditions (Table 3).
4. Conclusions
Table 2
Esterification of palmitic acid and methanol catalyzed by the synthesized sulfated zir-
conia under different conditions.
Well-dispersed sulfated zirconia nanoparticles were synthesized
with PVP as a surfactant. The presence of PVP molecules surrounding
Entry Molar ratio (palmitic Temperature Time Catalyst amount Conversion
acid:methanol)
(°C)
(h)
(wt%)
(%)
1
2
3
4
5
6
7
8
1: 20
1: 40
1: 40
1: 60
1: 100
1: 40
1: 40
1: 40
1: 40
1: 40
1: 40
1: 40
95
95
95
95
95
85
105
120
95
95
95
95
6
6
6
6
6
6
6
6
6
6
6
6
5
5
5
5
5
5
5
5
10
15
20
30
85
91
72a
86
86
81
89
87
93
97
96
91
Table 3
Esterification of different fatty acids and alcohols catalyzed by the synthesized sulfated
zirconia.a
Entry
Fatty acids
Alcohol
Conversion (%)
1
3
4
5
6
7
Palmitic acid
Palmitic acid
Palmitic acid
Palmitic acid
Oleic acid
Methanol
Ethanol
Propanol
Butanol
Ethanol
Ethanol
96
92
91
78
97
90
9
10
11
12
Lauric acid
(a)
(a)
Catalytic data for the conventional sulfated zirconia under the same reaction
conditions as in Entry 2.
Reaction conditions: molar ratio (fatty acids:alcohol) 1:40, reaction temperature
95 °C, reaction time 6 h, and catalyst amount 20 wt% based on fatty acids.

74
G. Chen et al. / Catalysis Communications 41 (2013) 70–74
the surface of the supermicrostructured surfactant-zirconia composite
improves the size uniformity and promotes the formation of sulfated
zirconia nanoparticles. The resultant sulfated zirconia nanoparticles
were directly used as catalysts to synthesize bis(indolyl)methanes via
electrophilic substitution reaction of indole with various aldehydes
and biodiesel via the esterification of long-chain free fatty acids. Satis-
factory yields and esterification ratio were obtained under optimal reac-
tion conditions.
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Acknowledgments
The authors thank the Key Project of Scientific Research Founda-
tion of Education Department of Anhui Province, China (grant nos.
KJ2013Z026 and KJ2013A064), the Jiangsu Planned Projects for Post-
doctoral Research Funds (grant no. 1202012C), and the Student
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.07.006.