Facile synthesis of mesoporous sulfated Ce/TiO2 nanofiber solid superacid with nanocrystalline frameworks by using collagen fibers as a biotemplate and its application in esterification

Article abstract of DOI:10.1039/c3ra44083d

Collagen fibers, one of the most abundant biomass in the natural world, were used as the biomass template to synthesize cerium-doped mesoporous TiO ₂ nanofiber (SO₄^2--Ce_x/TiO ₂) solid acid catalysts. The physiochemical properties of the as-prepared catalysts were well characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS). The fibrous morphology of native collagen fibers were found to be well preserved in SO₄^2--Ce _x/TiO₂, and the acid strength and textural property of the SO₄^2--Ce_x/TiO₂ were adjustable by changing the doping amount of Ce and the curing temperature. The catalytic properties of as-prepared catalysts were evaluated in the esterification reactions. It was found that the SO₄^2--Ce _x/TiO₂ exhibited high activity under mild reaction conditions. When reacted at 90 °C for 45 min with a catalyst amount of 2 wt%, the conversion yields of ethyl acetate, butyl acetate, hexyl acetate and ethyl caproate were 99.9%, 97.06%, 94.29% and 86.75%, respectively. Additionally, the SO₄^2--Ce_x/TiO₂ solid acid catalyst could be reused at least 6 times without significant loss of activity, exhibiting much better reusability as compared with the SO ₄^2--TiO₂* catalyst prepared by conventional approach.

Full text of DOI:10.1039/c3ra44083d

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Facile synthesis of mesoporous sulfated Ce/TiO2
nanofiber solid superacid with nanocrystalline
frameworks by using collagen fibers as a
Cite this: RSC Adv., 2014, 4, 4010
biotemplate and its application in esterification†
Gao Xiao, Jianfei Zhou, Xin Huang, Xuepin Liao*ab and Bi Shi*bc
ab
ab
ab
Collagen fibers, one of the most abundant biomass in the natural world, were used as the biomass template
2ꢀ
to synthesize cerium-doped mesoporous TiO
2
nanofiber (SO
4
x 2
–Ce /TiO ) solid acid catalysts. The
physiochemical properties of the as-prepared catalysts were well characterized by field emission
scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD),
2
N adsorption–desorption isotherms, X-ray photoelectron spectroscopy (XPS). The fibrous morphology
2ꢀ
of native collagen fibers were found to be well preserved in SO
4
–Ce
were adjustable by changing the doping amount of Ce and the
curing temperature. The catalytic properties of as-prepared catalysts were evaluated in the esterification
x 2
/TiO , and the acid strength and
2ꢀ
textural property of the SO
4
x 2
–Ce /TiO
2ꢀ
reactions. It was found that the SO
4
x 2
–Ce /TiO exhibited high activity under mild reaction conditions.
ꢁ
When reacted at 90 C for 45 min with a catalyst amount of 2 wt%, the conversion yields of ethyl
acetate, butyl acetate, hexyl acetate and ethyl caproate were 99.9%, 97.06%, 94.29% and 86.75%,
Received 1st August 2013
Accepted 5th December 2013
2ꢀ
respectively. Additionally, the SO
4
–Ce
x
/TiO
2
solid acid catalyst could be reused at least 6 times
DOI: 10.1039/c3ra44083d
42ꢀ
2
without significant loss of activity, exhibiting much better reusability as compared with the SO –TiO *
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catalyst prepared by conventional approach.
high catalytic activity as solid superacids in esterication.
Amongst these sulfated metal oxides, sulfated titania holds
Introduction
Esterication reactions of carboxylic acids with alcohols have great promise in the esterication of various carboxylic acids
been intensively investigated due to their importance in organic due to its superacidic property, good mechanical stability and
1
8
synthesis, pharmaceuticals, food preservatives, cosmetics, low-cost. More recently, the doping of rare earth in sulfated
2
biofuels, etc. Conventionally, strong acids, such as H
2
SO
4
are titania solid acid catalyst has drawn great attentions. As a kind
employed as homogenous catalysts in esterication reactions. of rare earth, cerium could be added into the solid acid catalysts
However, the homogenous acid catalysts oen suffer from the to improve their performances, owing to its redox and versatile
9
problem of difficult recovery. On the other hand, the corrosion acid–base properties. Furthermore, the unlike phase trans-
of reaction equipment caused by strong acid is a persistent formation from anatase phase to rutile phase can be substan-
3
10
problem. To overcome these disadvantages, great efforts have tially restrainted by Ce doping. All these advantages are
been devoted to exploring heterogeneous solid acid catalysts, benecial for improving the catalytic performances of the
which offer signicant advantages of easy recovery, little sulfated titania catalysts. Currently, it is highly desirable to
4
corrosion, low toxicity and environmental safety.
develop a simple and economical synthetic approach to
2
ꢀ
2ꢀ
Sulfated metal oxides (SO –M O ) such as sulfated synthesize cerium-doped SO
zirconia, sulfated titania and sulfated tin oxide have exhibited
4
2
–TiO .
4
x y
5
6
7
To ensure higher catalytic activity, the sulfated Ce-doped
titania should have large surface and the pore size is desirably
controlled in mesopores or macropores. However, the sulfated
titania prepared through sol–gel method using surfactants as
templates oen suffer from collapse of mesoporous structure
and deactivation caused by pore plugging during the removal of
a
Department of Biomass Chemistry and Engineering, Sichuan University, Chengdu,
6
10065, P. R. China. E-mail: xpliao@scu.edu.cn
b
Key Laboratory of Leather Chemistry and Engineering of Ministry of Education,
Sichuan University, Chengdu, 610065, P. R. China
11
c
templates. Recently, synthesis of porous sulfated titania using
National Engineering Laboratory for Clean Technology of Leather Manufacture,
Sichuan University, Chengdu, 610065, P. R. China. E-mail: shibi@scu.edu.cn; Fax: organic matrices with ordered structure as the templates has
+
†
1
86-28-85400356; Tel: +86-28-85400356
been emerged as promising strategy for the effective and
convenient fabrication. Collagen ber (CF), one of the most
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ra44083d
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2
ꢀ
abundant biomass in the natural world, is self-assembled by air atmosphere, and then the SO
4
–Ce
x
/TiO
2
–Y catalysts were
12
highly ordered arrangement of collagen brils. The collagen obtained (Y represents the calcination temperature aer the
ꢁ

brils are formed by ve collagen molecules composed of three Ce /TiO nanobers were impregnated with H SO , Y ¼ 100 C,
x
2
2
4
ꢁ
ꢁ
ꢁ
ꢁ
polypeptide chains with triple helical structure, and packed 200 C, 300 C, 400 C, 500 C, respectively).
together longitudinally in a ‘quarter stagger’ arrangement, For comparison, the catalyst denoted as SO
which further form the mesopore holes and overlap zones in prepared by conventional precipitation method followed by
2
ꢀ
4
2
–TiO * was
13
2ꢀ
collagen ber. Most importantly, there are abundant func- impregnation with sulfuric acid. For the preparation of SO
4
–
tional groups such as –OH, –COOH, ꢀCONH
2
and –NH
2
in the TiO
2
*, 1.0 g of Ti(SO
4
)
2
were suspended in 10 mL of ammonium
ꢀ1
ꢁ
collagen ber, which could chelate with many kinds of metal hydroxide (1.0 g L ) aqueous solution at 30 C, and kept under
4
+
4+
4+
3+ 14
ions such as Ti , Zr , Cr , Al . Thus in principle, collagen constant stirring for 24 h. The product was thoroughly washed
ꢁ

ber could be used as template to prepare sulfated titania solid with deionized water, dried at 100 C for 24 h. The intermediate
superacid with mesoporous structure. In the present investi- product was then sulfated by impregnation with 10 mL of 1 mol
ꢀ
1
ꢁ
gation, we prepared a series of cerium-doped mesoporous
L
H
2
SO
4
for 4 h, followed by drying for 24 h at 100 C and
2
ꢀ
ꢁ
SO
4
–TiO
2
nanober solid acid by using CF as the biomass calcination in air at 550 C for 4 h. Follow the same steps, just
template. The crystalline phase, pore structure and acidity of replace the Ti(SO
4
)
2
with a mixture of Ce(SO
4 2 4 2
) and Ti(SO ) , the
2
ꢀ
2ꢀ
4
the as-preapred SO
4
–Ce
x
/TiO
2
were controlled by adjusting SO
–Ce0.02/TiO * prepared by conventional precipitation
2
the experimental conditions, including precursor dosage, method was also obtained.
calcination temperature, doped concentration of cerium, etc.
The catalytic properties of the as-prepared catalysts were eval-
uated using esterication reaction of carboxylic acids and
Catalyst characterizations
2
ꢀ
alcohols as the probe reactions.
The surface morphology of the SO4 –Ce /TiO nanobers was
x 2
observed by eld emission scanning electron microscopy
(
FESEM, Hitachi 4700, Japan). Transmission Electron Micro-
Experiments
Materials
2
ꢀ
ꢁ
scope (TEM) observation of the SO
nanober was taken on Tecnai G F20 (TEM, FEI, Netherlands)
4
–Ce0.02/TiO
2
– 300 C
2
CF was prepared from bovine skin according to the procedures operating at 200 kV. The X-ray diffraction analysis (XRD,
1
5
0
in our previous work. Briey, the cattle hide was cleaned, Philips X Pert Pro-MPD, Netherlands) using Cu-Ka X-radiation
2
ꢀ
unhaired, eshed, degreased, limed, and delimed according to (l ¼ 0.154 nm) of the SO
–Ce /TiO nanobers was per-
x 2
4
the procedures of leather processing to remove proteoglycan formed to identify the crystalline structures. The specic
and other noncollagenous substances. The obtained pelt was surface area and the pore size distribution of the samples
dehydrated by absolute ethyl alcohol, dried in vacuum and were determined by N adsoprtion–desorption at 77 K on a
2
pulverized into powder. Ti(SO
other chemicals were all analytical grade.
4
)
2
, Ce(SO
4
)
2
$4H
2
O, H
2
SO
4
and Micrometrics ASAP-2010 adsorption apparatus. X-ray photo-
electron spectra analysis (XPS, Kratos XSAM800, England) of
2
ꢀ
ꢁ
SO
4
–Ce0.02/TiO – 300 C was conducted by employing Al-Ka
2
X-radiation (hn ¼ 1486.6 eV) with a pass energy of 31.5 eV, and
peaks from all the high-resolution core spectra were tted with
XPSPEAK4.1 soware using mixed Gaussian–Lorentzian
x 2
Preparation of mesoporous Ce /TiO nanobers
1
2
5.0 g of CF was suspended in 400 mL of deionized water at
5 C for 1.0 h. The pH of the mixture was adjusted to 1.8–2.0 by
ꢁ
2
ꢀ
functions. The acid strength of the SO
4
x 2
–Ce /TiO catalysts
using H
2
SO
4
–HCOOH solution (H
2
SO
4
: HCOOH ¼ 10, v/v).
was determined by CHEMBET-Palsar TPR/TPD (Automated
Chemisorption Analyzer) instrument.
ꢁ
Aer the mixture was stirred at 25 C for 3 h, 24.0 g of Ti(SO )
4
2
and a desired amount of Ce(SO ) was added in above mixture,
4
2
and kept under constant stirring for another 4.0 h. Subse-
quently, a proper amount of NaHCO solution (15%, w/w) was
3
Catalytic activity evaluation
drop-wise added into the reaction system within 3.0 h to
increase its pH to 3.8–4.0, and reacted at 40 C for another
The esterication of carboxylic acid with ethanol was carried
ꢁ
out as a probe reaction to evaluate the catalytic activity of
12.0 h. When the reaction was completed, the product was
2
ꢀ
SO4 –Ce /TiO as solid acid catalyst. The reaction was per-
x
2
collected by ltration, washed with deionized water, and dried
formed in a 50.0 mL autoclave reactor equipped with a
temperature controller and stirrer speed controller. A certain
amount of the catalyst was added into the mixture of carboxylic
acid and ethanol. Then, a proper amount of dichloromethane
was added into the reaction system as an internal standard.
The reaction was conducted at certain temperature under
constant stirring. When the reaction was completed, the
resultant products were analyzed using gas chromatograph and
ꢁ
at 50 C for 6 h. These intermediates were further treated by
ꢁ
temperature-programmed calcination at 600 C for 4.0 h to
remove the collagen ber template. Finally, a series of Ce
x
/TiO
2
+
4
nanobers were obtained (x represents the molar ratio of Ce
4
+
to Ti , x ¼ 0.01, 0.02, 0.03, 0.04, 0.05).
2
4
ꢀ
Preparation of mesoporous SO
x 2
–Ce /TiO
0
.5 g of brous Ce /TiO was impregnated in 20.0 mL of 1.0 M the catalyst was recovered by ltration, followed by washing
x 2
H
2
SO
4
solution for 4 h, and then collected by ltration, followed with ethanol and drying, and then reused under the same
ꢁ
by calcination at different temperature (100–500 C) for 3 h in experimental conditions.
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Fig. 1 shows the morphology of native collagen ber and the
Results and discussion
Catalyst preparation and characterizations
2
ꢀ
ꢁ
mesoporous SO4 –Ce0.02/TiO – 300 C nanober. As seen in
2
Fig. 1a and b, it is obviously observed that the natural collagen
ber slightly closely arranged in bundles compared with the
2
ꢀ
The schematic illustration for the preparation of SO
4
x
–Ce /
2
ꢀ
ꢁ
TiO
were rst chelated by the abundant functional groups of the standard deviations for both the collagen bers and SO
collage triple helix (–OH, –COOH, –CONH and –NH
) via the Ce0.02/TiO bers were calculated statistically. As shown in ESI
formation of coordination complexes, and then assembled 1,† the average diameter of brous SO
into an inorganic oxide framework surrounding the collage approximately 40–50 nm, which is smaller than the average
brils as basic building blocks, which are combined, oriented, diameter of natural collagen ber (120–140 nm). This fact sug-
and laid up to form a higher ordered structure of Ce /TiO
gested that the thermal contraction was occurred during
nanobers, as shown in Scheme 1. Subsequently, the organic calcining process. During this process, the TiO crystal was
2
nanobers is shown in Scheme 1. The Ti/Ce precursors SO
4
–Ce0.02/TiO – 300 C nanobers. The ber diameters with
2
2
ꢀ
4
–
2
2
2
16
2ꢀ
ꢁ
2
–Ce0.02/TiO – 300 C is
4

x
2
2
template was completely removed by high temperature calci- formed and rearrangement along the direction of collagen ber,
nation, and meanwhile, the metal complexes were accordingly and completely duplicated the morphology of collagen ber. In
converted to the corresponding oxides. Additionally, the vacant addition, the EDS analysis in Fig. 1d further conrms that Ce
spaces in the hierarchical structure of CF would lead to the and S were successfully incorporated into the TiO
2
nanobers.
formation of mesoporous structures during the calcination Therefore, it could be concluded that the well-dened brous
1
7
2ꢀ
process. Finally, the SO4 –Ce /TiO nanobers was synthe- morphology of natural collagen ber was faithfully replicated in
x
2
2
ꢀ
sized by conventional impregnation treatment using the SO
calcined mesoporous Ce /TiO
nanobers as the support. It has exhibit the distinct advantages of high geometrical exibility,
been proposed that the strong acid property is related to sulfate and low mass transfer resistance when used in liquid phase
–Ce0.02/TiO catalyst. As we know, brous catalysts oen
4
2
x
2
21
2ꢀ
4
ions, and that high electron negativity of sulfur can induce reactions. Hence, the as-prepared SO
–Ce0.02/TiO
coordinates should possess high catalytic activity when applied in esteri-
to Ti or Ce metal ions through two of its oxygen atoms, a cation reactions.
ꢁ
bridging bidentate complex is formed on the surface of
2
TEM and HRTEM images of the SO –Ce0.02/TiO – 300 C
2
catalyst
18
2ꢀ
polarization of neighboring OH groups. If SO
4
2
ꢀ
4
19
hydrated titania as shown in Scheme 1. Depending on the nanober were given in Fig. 2. As can be seen, the marked white
nature of surface M–O bond characteristics, the sulfate groups parallel lines in the TEM pattern indicated that the outer
being hooked on as a bidentate conguration can possibly be diameter of SO
distorted or deformed with varying conguration and electronic mately 50 nm and aligned parallel to each other. Additionally,
2
ꢀ
ꢁ
–Ce0.02/TiO – 300 C nanober was approxi-
2
4
4
+
2ꢀ
4
properties. Ce ion as a Lewis acid is apparently superior to the the porous structure of SO
x 2
–Ce /TiO was also observed as
oxygen molecule in the capability of trapping electrons. Doping marked in the white ellipse region of TEM image in Fig. 2b. The
Ce into TiO crystal can result in the changes of ratio of strong crystal sizes of titania in the TEM images were consistent with
2
20
to weak acid sites and the increase of total acidity. This aspect the average crystal size of 10 nm estimated from the below XRD
is also reected in the results of XPS and NH -TPD studies.
results. In Fig. 2c, the partially enlarged HRTEM image of the
3
2ꢀ
Scheme 1 A schematic illustration for the preparation of the SO
4
–Ce
x
/TiO
2
nanofibers and a proposed mechanism on the generation of
strong acidity on the surface of the solid superacid catalysts.
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2ꢀ
ꢁ
42ꢀ
2 2
–Ce0.02/TiO – 300 C nanofiber (b and c) and the EDS analysis of the SO –Ce0.02/TiO –
Fig. 1 FESEM image of collagen fiber (a), the SO
4
ꢁ
3
00 C nanofiber (d).
have a polycrystalline structure. The highly magnied images of
the selected area exhibit lattice fringes of the small crystalline
having an interplanar spacing of 0.35 nm, corresponding to
2
2
TiO anatase {101} plane (shown in Fig. 2d).
2
2
ꢀ
2 x
42ꢀ
–TiO and SO –Ce /
The XRD patterns of the TiO
TiO calcined at various temperatures are shown in Fig. 3. The
XRD pattern shows that both anatase and rutile phases are
2
, SO
4
2
2
ꢀ
presented in TiO
phase is presented in the case of SO
anatase phase of TiO is benecial for the esterication reac-
2
and SO
4
2
–TiO , whereas only the anatase
2
ꢀ
4
x 2
–Ce /TiO . In general, the
2
23
tion. Compared with the pure TiO , sulfated samples display
2
smaller crystal size and the diffraction peaks corresponding to
the rutile phase of TiO
2
become broader and weaker, which
suggest that the presence of sulfate ions retarded the phase
24
transformation from anatase to rutile phase. On the other
2
ꢀ
hand, when SO
4
2
–TiO catalyst was doped with Ce, the titania
displays broader diffraction peak, indicating that the crystal
size of TiO became further smaller because of the doped Ce.
2
These results are in agreement with that the existence of CeO2
can effectively stabilize the TiO anatase phase and retard the
transformation of anatase phase to rutile phase. Additionally,
we did not nd the diffraction peak of cerium oxide, which
2
ꢀ
ꢁ
2
Fig. 2 (a) TEM image of SO
4
–Ce0.02/TiO
2
– 300 C nanofiber. (b)
24
2
ꢀ
ꢁ
High magnification TEM images of SO
nanofiber. (c) HRTEM images of partially enlarged SO
4
–Ce0.02/TiO
2
– 300
C
2ꢀ
4
–Ce0.02/TiO
2
ꢁ
–
300 C nanofiber. (d) Partially enlarged image of the white quadrate suggests that no sole crystalline phase caused by cerium oxide
25
area. The inset images show the corresponding fast Fourier transform and Ce is dispersed among the interfaces of catalyst.
(
FFT) patterns of the selected area.
Cerium can partially restrain the crystalline phase trans-
3
+
4+
˚
3+
formation. Since the ionic radii of Ce /Ce (1.03/1.02 A) are
2
ꢀ
ꢁ
4+
˚
single SO4 –Ce0.02/TiO – 300 C nanober revealed that the bigger than that of Ti (0.68 A), it is thus difficult for Ce and
mesoporous TiO nanobers have high crystallization. The fast Ce to replace Ti in the crystal lattice. These relatively larger
fourier transform (FFT) pattern of nanocrystal particles (inser- Ce /Ce cations at the grain boundaries and grain junctions
2
4
+
4+
2
3
+
4+
ted in Fig. 2c) revealed that the mesoporous TiO
2
nanobers inhibit crystallite growth of titania due to the formation of
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2ꢀ
2 x 2
42ꢀ
–TiO and SO –Ce /TiO nanofibers (A, anatase; R, rutile).
Fig. 3 XRD patterns of the TiO
2
, SO
4
26
Ce–O–Ti bonds. Facts proved that, the peak intensity of rutile
Furthermore, the ESI 2† also indicates that the XRD
phase of TiO is gradually decreased along with the increase of patterns are very different at different Ce doping and
2
Ce content, and the rutile phase was disappeared entirely as the calcining temperature. In ESI 2a,† it can be observed that the
Ce doped dosage increased to 0.03 (seen in the ESI 2a†). These peak intensity of rutile phase of TiO
facts suggested that the phase transition inhibition effect of Ce along with the increase of Ce content, and the rutile phase
to TiO is dosage dependence. Therefore, the rutile phase is not was disappeared entirely as the Ce doped dosage increased to
completely retard in the Ce0.02/TiO , probably due to the dosage 0.03. In addition, the crystal size of TiO was also reduced as
2
is gradually decreased
2
2
2
of Ce is not enough. However, excessive doping of Ce can lead to increasing amount of Ce doped. However, excessive doping
the reduction of the quantity of Brønsted acid sites of the of Ce can lead to the reduction of the quantity of Brønsted
2
ꢀ
2ꢀ
SO4 –Ce /TiO solid acid. So, the suitable doping amount of Ce acid sites of the SO4 –Ce /TiO solid acid. Therefore, the
x
2
x
2
2
ꢀ
is essential important for the preparation of SO
solid acid with higher activity. On the other hand, the presence calcining temperature, as shown in ESI 2b,† it was found that
of sulfate ions also can retard the phase transformation from the rutile phase of TiO become obvious at higher calcining
anatase to rutile phase to a certain extent. As shown in temperature, and the crystal size of TiO
4
–Ce
x
/TiO
2
dosage of Ce doped should be in a suitable range. In case of
2
2
is also enlarged,
2
ꢀ
2ꢀ
2ꢀ
SO
4
–Ce0.02/TiO
2
in Fig. 3, the rutile phase of SO
4
–Ce0.02
/
which will greatly reduce the catalytic activity of SO
4
x
–Ce /
TiO is completely disappeared.
TiO solid acid.
2
2
2ꢀ
42ꢀ
Fig. 4 (a) Nitrogen adsorption–desorption isotherms of the TiO
2
, SO
4
2 2
–TiO and SO –Ce0.02/TiO nanofibers cured at different temper-
ature. (b) BJH pore-size distributions of the samples.
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2
7
The N
2
adsorption–desorption isotherms and the pore size increase the positive polarity on the titanium cation. In Fig. 5d,
2
ꢀ
2ꢀ
distribution of TiO , SO –TiO2 and SO4 –Ce0.02/TiO2 are the multiple peaks around 900–910 eV indicate the co-existence
shown in Fig. 4. The type IV isotherm and an H1 hysteresis loop of Ce and Ce in SO4 –Ce0.02/TiO – 300 C nanober.
2
4
4
+
3+
2ꢀ
ꢁ
2
are clearly observed, which clearly indicate the mesoporous
2ꢀ
2ꢀ
properties of TiO
bers by using collagen ber as template. The pore size distri-
bution of all the as-prepared nanobers are mainly in the range
2
, SO
4
–TiO
2
and SO
4
2
–Ce0.02/TiO nano-
2
ꢀ
Determination of acid strength of SO
4
x 2
–Ce /TiO and its

catalytic activity in esterication reactions
of 2–10 nm (shown in Fig. 4b), implying its narrow mesoporous NH -TPD was employed as a qualitative probe for determining
3
size distribution. However, as shown in ESI 3a,† there is no the strength of acid site. The NH -TPD proles of the Ce0.02/TiO2
3
2
ꢀ
obvious hysteresis loop in N adsorption–desorption curves of and SO4 –Ce0.02/TiO2 calcined at various temperatures are
the SO4 –TiO * prepared by the conventional method, sug- shown in Fig. 6a. It can be observed that the acid sites of most
2
2
ꢀ
2
gesting almost no mesopores existed. For the pore diameter of samples are located in two temperature regions. The rst broad
2
ꢀ
ꢁ
2
–TiO *, as shown in ESI 3b,† there is no peak at meso- ammonia desorption prole in the range of 450–700 C can be
SO
4
porous range. The specic surface area and pore volume of considered as the presence of high concentration acid sites with
2
ꢀ
2
ꢀ1
3
ꢀ1
SO
much smaller than that of SO
using collagen ber as template is essential important to However, the Ce0.02/TiO and SO –Ce0.02/TiO – 500 C have
4
2
–TiO * were 16.09 m g and 0.046 cm g , respectively, moderate strength. The second desorption prole in the range
2
ꢀ
ꢁ
ꢁ
2
–TiO – 300 C. Therefore, of 700–850 C indicates the acid sites with super acidic nature.
4
2
ꢀ
ꢁ
2
4
2
prepare TiO based solid acids with mesoporous structure.
no obvious desorption peak, indicating that they almost have no
2
ꢁ
Pore volume (single point total pore volume of pores at P/P0 acid sites. At relatively low calcination temperature (#300 C),
2
ꢀ
¼
0.99), pore size (determined from BJH desorption isotherm), the bonding strength between SO4
and Ce0.02/TiO2 was
2
–TiO and improved and the strong acid sites were increased as the
2
ꢀ
particle size, and BET surface area of TiO
2
, SO
4
2
ꢀ
SO
4
2
–Ce0.02/TiO nanobers calcined at various temperatures increase of calcination temperature. The peak area presents
are summarized in the Table 1. It can be observed that the amount of NH desorbed from the catalyst can be considered as
3
ꢁ
impregnation of sulfuric acid and doping of cerium lead to the number of acid sites on the catalyst. The peak at 450–600 C
ꢁ
signicant increase of specic surface area, pore size and pore and 700–800 C emerges, which is denoted as peak L and H,
2
ꢀ
2ꢀ
volume of SO4 –Ce0.02/TiO nanober as compare with pure respectively. As shown in ESI 4,† for the SO4 –Ce0.02/TiO2
2
TiO and Ce /TiO . In general, the large surface area and pore catalysts, the areas of both peak L and peak H synchronously
2
0.02
2
size are benecial for the improvement of mass transfer during increases rst, and then decreases with the rising of heat
catalytic esterication reactions. Therefore, it is inferred that treatment temperature. This clearly indicates that impregnated
2
ꢀ
the SO
4
–Ce0.02/TiO
2
would exhibit signicantly improved sulfate groups strongly inuence the acidic properties of Ce0.02
/
2
ꢀ
catalytic activity compared with SO
tion is conrmed in the following experimental results.
The XPS analyses in Fig. 5a reveal that the SO
00 C nanober is composed of Ti, O, S and Ce elements. The and the SO4 –Ce0.02/TiO – 300 C owns the most strong acid
4
–/TiO
2
, and this specula- TiO nanobers. Sulfate groups can interact with the doped Ce
2
28
species and generate the super strong acid sites. The results
2ꢀ
ꢁ
4
–Ce0.02/TiO
2
–
indicate that the optimum heat treatment temperature is 300 C
ꢁ
2ꢀ
ꢁ
3
2
ꢁ
distinct peak of S 2p3/2 at 168.682 eV can be assigned to the sites. When calcination temperature was beyond 300 C, the
2ꢀ
4+
SO4 bidentately coordinated with Ti . In comparison, the decomposition of S]O bond happened and the total acidity
unsulfated Ce0.02/TiO exhibits no peak at this location as shown was rapidly decreased.
in Fig. 5b. In Fig. 5c, the binding energies of Ti 2p3/2 and Ti 2p1/
The TPD proles of SO4 –Ce /TiO – 300 C with different
in Ce0.02/TiO (Ti 2p3/2 at 458.238 eV and Ti 2p1/2 at 463.806 content of doped Ce are shown in Fig. 6b. All the samples except
eV) are shied to higher values (458.762 eV and 464.323 eV, SO4 –TiO – 300 C show two peaks in the temperature regions
2
2
ꢀ
ꢁ
x
2
2
2
2
ꢀ
ꢁ
2
ꢁ
ꢁ
respectively) aer sulfated (Fig. 5c), which suggest that the strong of 450–700 C and 700–850 C, respectively, which reveal that
interactions between the sulfate anion and titanium cation the samples contain acid sites with a wide distribution of acid
2ꢀ
42ꢀ
Table 1 Effect of doped Ce dosage and calcination temperature on the textural properties of TiO
2
, Ce0.02/TiO
2
, SO
4
–TiO and SO –Ce0.02/
2
a
TiO
2
Crystal size
(101) nm
Specic surface
area BET (m g
Specic surface
area BJH (m g
Pore diameter
(nm)
Pore volume
2
ꢀ1
2
ꢀ1
3
ꢀ1
Sample
Phase content
)
)
(cm g
)
Pure TiO
2
A (51.8%), R (48.2%)
A (86.7%), R (13.3%)
A (100%)
A (100%)
A (100%)
A (100%)
A (100%)
A (100%)
20.5
13.9
12.6
10.7
8.4
6.3
9.5
11.2
30.82
40.85
44.97
56.23
81.94
100.90
53.75
47.41
44.37
54.02
67.81
74.71
109.74
116.48
72.63
70.55
7.16
14.17
22.88
10.60
18.30
15.00
14.92
12.32
0.06
0.15
0.26
0.15
0.38
0.38
0.20
0.15
2
ꢀ
ꢁ
SO
4
–TiO
2
– 300 C
Ce0.02–TiO
2
2
2
2
2
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢁ
ꢁ
ꢁ
ꢁ
SO
4
SO
4
SO
4
SO
4
SO
4
–Ce0.02/TiO
–Ce0.02/TiO
–Ce0.02/TiO
–Ce0.02/TiO
–Ce0.02/TiO
2
2
2
2
2
– 100 C
– 200 C
– 300 C
– 400 C
ꢁ
– 500 C
a
2
A and R denote anatase and rutile phase of TiO , respectively.
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2
ꢀ
ꢁ
Fig. 5 X-ray photoelectron spectra of unsulfated Ce0.02/TiO
2
and anatase SO
4
–Ce0.02/TiO
2
– 300 C: (a) the whole survey of Ce0.02/TiO
2
and
and
2
ꢀ
ꢀ
ꢁ
2ꢀ
ꢁ
SO
SO
4
–Ce0.02/TiO
–Ce0.02/TiO
2
– 300 C; (b) S 2p XP spectrum of Ce0.02/TiO
– 300 C; and (d) Ce 3d XP spectrum of SO
2
and SO
4
–Ce0.02/TiO
–Ce0.02/TiO
2
– 300 C; (c) Ti 2p XP spectrum of Ce0.02/TiO
2
2
ꢁ
2ꢀ
ꢁ
4
2
4
2
– 300 C.
2
ꢀ
42ꢀ
calcined at various temperatures; (b) ammonia TPD spectra of SO –
Fig. 6 (a) Ammonia TPD spectra of Ce0.02/TiO
Ce /TiO
2
and SO
4
–Ce0.02/TiO
2
ꢁ
x
2
(x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) calcined at 300 C.
2
ꢀ
ꢁ
2ꢀ
ꢁ
strength. The plateau area of SO –Ce /TiO – 300 C above SO4 –Ce /TiO – 300 C samples. Also, from the ESI Table 1,† it
4
x
2
x
2
ꢁ
7
00 C corresponding to the super-acidic sites are much larger can be found that with increase Ce doping, the Brønsted acid
2
ꢀ
ꢁ
2
–TiO – 300 C. These results conrm that the sites increases rst, and then decreases, and the Ce doping
than the SO
4
doping of Ce strongly inuences the acidic properties of threshold is 0.02.
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2ꢀ
a
Table 2 The influence of doping amount of Ce and calcination temperature on catalytic activity of SO
4
–Ce
x
/TiO
2
in esterification reaction
Conversion yield
of acetic acid (%)
Conversion yield
of acetic acid (%)
Catalysts
Catalysts
2
2
ꢀ
ꢀ
2ꢀ
2ꢀ
ꢁ
SO
SO
TiO
4
–TiO
–Ce0.02/TiO
2
*
60.08
75.76
9.83
52.17
71.32
93.89
97.06
SO
4
SO
4
SO
4
SO
4
SO
4
SO
4
SO
4
–Ce0.03/TiO
–Ce0.04/TiO
–Ce0.05/TiO
–Ce0.02/TiO
–Ce0.02/TiO
–Ce0.02/TiO
–Ce0.02/TiO
2
2
2
2
2
2
2
– 300 C
91.13
84.32
76.34
90.75
94.18
83.7
ꢁ
4
2
*
– 300 C
2
ꢀ
ꢀ
ꢁ
2
nanober
– 300 C
2
ꢁ
Ce0.02/TiO
2
nanober
nanober
– 100 C
2
2
2
ꢀ
ꢀ
ꢀ
2ꢀ
2ꢀ
2ꢀ
ꢁ
SO
4
SO
4
SO
4
–TiO
–Ce0.01/TiO
–Ce0.02/TiO
2
– 200 C
ꢁ
ꢁ
ꢁ
2
– 300 C
– 300 C
– 400 C
ꢁ
2
– 500 C
54.64
a
ꢁ
Reaction conditions: 90 C, catalyst amount: 2 wt%, alcohol : acid ¼ 3 : 1 (mole ratio), reaction for 45 min.
Table 2 shows the inuence of Ce doping and calcination
2
ꢀ
temperature on catalytic activity of SO4 –Ce /TiO in esteri-
x
2
cation reaction of n-butyl alcohol with acetic acid. Compared
2
ꢀ
with the SO
method, the SO
superior activity, which can be attributed to the fact that the
4
2
–TiO * prepared by conventional coprecipitation
2
ꢀ
2ꢀ
4
–TiO
2
and SO
4
x 2
–Ce /TiO both exhibit
2
ꢀ
2ꢀ
mesoporous structure of SO
have been reported, the mesoporous SO
bers would greatly improve the diffusion rate of reactants and
products in the reaction process, and greatly increase the
availability of the internal surface.
Ce content in SO
4
–TiO
2
and SO
4
ꢀ
–Ce
x
/TiO
2
. As
2
4
–Ce /TiO
x
2
nano-

29–31
Obviously, the increase of
2
ꢀ
4
–Ce /TiO can promote the activity of the
x 2
catalysts. Without the doping of Ce, the conversion yield of
acetic acid is 71.32%. When the mole ratio of doped Ce is 0.01,
the corresponding conversion yield is increased to 93.89%.
2
4
ꢀ
Once the mole ratio of doped Ce is 0.02, the SO
–Ce0.02/TiO
2
exhibits the highest catalytic activity, and the corresponding
conversion yield of acetic acid reaches 97. 06%. When the mole
ratio of Ce is higher than 0.02, the conversion yield is decreased
to 91.13%. It is likely that the amount of Brønsted acid sites
2
ꢀ
ꢁ
42ꢀ
* and SO –
Fig. 7 Recycle application investigation of SO
4
–TiO
2
ꢁ
Ce0.02/TiO
2
– 300 C. Reaction conditions: 90 C, catalyst amount:
.3 g, alcohol : acid: 3 : 1 (mole ratio), reaction for 90 min.
0
2ꢀ
ꢁ
a
Table 3 The esterification of various carboxylic acids with alcohols catalyzed by SO
4
–Ce0.02/TiO
2
– 300 C nanofiber
Entry
Substrates (A)
Substrates (B)
Product
Conversion (%)
99.9
Selectivity (%)
99
1
2
3
4
98.27
97.06
94.29
99
99
99
5
86.75
99
a
ꢁ
Reaction conditions: 90 C, catalyst amount: 0.3 g, alcohol : acid: 3 : 1 (mole ratio), reaction for 45 min.
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were reduced due to the excessive doping of Ce, as shown in
Acknowledgements
2
ꢀ
Fig. 6b. In addition, SO4 –Ce0.02/TiO * was prepared by the
2
conventional method and its catalytic activity was also tested. As The authors gratefully acknowledge the nancial supports
shown in Table 2, the corresponding conversion extent of acetic provided by the National Natural Science Foundation of China
2
ꢀ
acid is 75.76% in 45 min, which is higher than that of SO
TiO * (60.08%), indicating the benecial of Ce doped. However, China (863 Program, 2011AA06A108).
it is much lower than that of SO
4
–
(21176161) and the National High Technology R&D Program of
2
2
ꢀ
ꢁ
C
4
–Ce0.02/TiO
2
– 300
(
97.06%), which is prepared by using collagen ber as template.
References
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ꢀ
2ꢀ
4
Compared with powder SO
TiO – 300 C can be used 6 times without signicant reduction
of catalytic activity. These facts proved that the catalysts of
mesoporous SO4 –Ce /TiO ber prepared using collagen ber
as template have remarkable advantages on the activity and
reusability.
Furthermore, as presented in Table 2, the conversion yield of
acetic acid is rstly increased and then decreased with the
increase of calcination temperature. The maximum activity was
4
–TiO
2
* catalyst, SO
–Ce0.02/
ꢁ
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