Y. Liao et al. / Journal of Molecular Catalysis A: Chemical 347 (2011) 46–51
47
the porous materials with fibrous morphology could be obtained
if collagen fiber is used as the template. On the other hand, colla-
gen fiber has abundant functional groups, including –COOH, –OH
and –NH2 groups, which are highly active with various metal ions
X-radiation (ꢀ = 0.154 nm). The specific surface areas and the pore
size distribution of the SO42 /ZrO2 were determined by using
−
the N adsorption/desorption approach (Micromeritics TriStar Sur-
2
face Area and Pority Analyzer, nitrogen absorption apparatus). The
4
+
3+
3+
2−
such as Zr , Cr , and Al [22]. Consequently in principle, porous
acid strength of the SO4 /ZrO2 catalysts was determined in the
zirconia oxide (ZrO ) with fibrous morphology could be prepared
CHEMBET-Palsar TPR/TPD (Automated Chemisorption Analyzer)
instrument.
2
by reacting Zr precursor with CF template, followed by calcina-
tion to remove collagen fiber. Accordingly, sulfated porous ZrO2
fiber can be subsequently synthesized by conventional impregna-
tion treatment using the porous ZrO2 fiber as the support. In order
2.4. Catalytic reaction
to evaluate its catalytic activity, sulfated porous ZrO fiber was used
2
The esterification of acetic acid with n-butanol was carried out as
as the heterogeneous acid catalyst in catalytic esterification of n-
butyl alcohol with acetic acid, of which the resultant n-butyl acetate
is an important organic compound widely used in many fields such
as lacquer, artificial perfume and flavouring extraction [23].
a probe reaction to evaluate the catalytic activity of SO42 /ZrO2 as
solid acid catalyst. The reaction was performed in a 50.0 mL auto-
clave reactor equipped with a temperature controller and stirrer
speed controller. A certain amount of the catalyst was added into
the mixture of acetic acid and n-butanol. Then, a proper amount
of dichloromethane was also added into the above mixture as an
internal standard. The reaction system was conducted at certain
temperature under constant stirring. When the reaction was com-
pleted, the catalyst was recovered by filtration and the resultant
products were analyzed using gas chromatograph. The reusability
of the catalyst was also performed under the same experimental
conditions.
−
2
. Experimental
2.1. Materials
Collagen fiber (CF) was prepared from cattle skin according to
our previous work [24]. Zirconium sulfate was provided by a com-
mercial supplier (Sichuan Ting Jiang Fine Chemicals Co. Ltd., China).
Acetic acid, n-butanol, and other chemicals were all analytical grade
reagents.
3. Results and discussion
2.2. Catalyst preparation
3.1. Catalyst characterization
2.2.1. Preparation of Zr-immobilized collagen fiber and fibrous
zirconia oxide (ZrO2)
3.1.1. The surface morphology of ZrO2
1
5.0 g of collagen fiber was added into 400 mL of deion-
Fig. 1a–e shows the surface morphology of un-sulfated ZrO2
prepared using collagen fiber as the template and calcinated at
600 C. It was found that the surface morphology of ZrO2 was
◦
ized water at 25 C for 1.0 h. The pH of the resultant mixture
was adjusted to 1.8–2.0 by using H SO –HCOOH solution
◦
2
4
◦
(
4
H SO :HCOOH = 10, v/v). After the mixture was stirred at 25 C for
highly dependent on the loading amount of Zr on CF. The ZrO2 pre-
pared from low loading amount of Zr (22.5 mg/g) showed highly
packed ZrO2 crystals instead of fibrous morphology, as shown
in Fig. 1a. Possibly, low loading amount of Zr is not sufficiently
enough to react with the active sites of collagen fiber (–COOH,
–OH, –NH2, etc.), which causes the poor distribution of Zr on the
surface of collagen fiber. Under such conditions, majority of the
un-reacted collagen fiber was burned and released as CO2, which
further lead to the collapse of fibrous structure of collagen fiber.
Due to the structural collapse of collagen fiber, Zr precursor will
be gradually aggregated and transformed to ZrO2 crystal clusters,
just like no collagen fiber was used. These are possible explana-
tions that are responsible for the observed highly crystallized
ZrO2.
2
4
.0 h, a determined amount of Zr(SO4)2 was added, and kept under
constant stirring for another 4.0 h. Subsequently, a proper amount
of NaHCO solution (15%, w/w) was drop-wise added into the reac-
tion system within 3.0 h in order to increase its pH to 4.0–4.2.
When reached the desired pH, the temperature of the system was
heated to 40 C, and the reaction proceeded for 10.0 h. Finally, the
Zr-immobilized collagen fiber was collected by filtration, washing
and drying at 45 C.
The prepared Zr-immobilized collagen fiber was treated by
temperature-programmed calcination to remove the collagen fiber
template. The detailed calcination process was as follows. Zr-
immobilized collagen fiber was first kept at 100 C for 2.0 h in air,
and then heated (5 C/min) to 300 C and held at this temperature
3
◦
◦
◦
◦
◦
◦
for 2.0 h. After this, the temperature was further increased to 500 C
with a rate of 5 C/min and held at this temperature for 2.0 h. As a
When the loading amount of Zr was increased from 22.5 mg/g
to 30.3 mg/g, the prepared ZrO2 has well-defined fibrous morphol-
ogy, which appears as fiber bundles with a diameter of 2.5 ± 0.5 m
and a length of 0.25 ± 0.05 mm, as shown in Fig. 1b. Similar fibrous
morphology can still be observed in those ZrO2 samples with Zr
loading of 40.9 mg/g (Fig. 1c), and their corresponding FE-SEM
image is shown in Fig. 1e, in which nanoscaled ZrO2 fibers are
well arranged with defined fiber bundles. These observations sug-
gested that the morphology of the natural collagen fiber was well
preserved in ZrO2. More importantly, the fibrous morphology of
ZrO2 can still be preserved after the treatment of sulfuric acid
impregnation (Fig. 1f). According to the literature [25], fibrous cat-
alysts often exhibited the distinct advantages of high geometrical
flexibility and low mass transfer resistance when used in liquid-
◦
result, fibrous ZrO2 was obtained. Herein, a series of fibrous ZrO2
were prepared by varying the final calcination temperature from
◦
◦
5
2
2
00 C to 800 C.
.2.2. Preparation of fibrous SO 2 /ZrO2
−
4
◦
0.5 g of fibrous ZrO2 prepared at 600 C was impregnated into
0.0 mL of 1.0 M H SO4 solution for 4.0 h, and then collected by
filtration, followed by calcination at 300 C for 4.0 h. The resultant
product was sulfated ZrO , which was denoted as SO4 /ZrO2 in
the following text.
2
◦
2−
2
2
.3. Catalyst characterizations
phase or three-phase reactions. Therefore, the SO42 /ZrO2 catalyst
is expected to have high catalytic reactivity in the following probe
reaction.
−
The surface morphology of SO42−/ZrO2 was observed by scan-
ning electron microscopy (SEM, JEOL LTD JSM-5900LV). XRD
Philips X’Pert Pro-MPD) characterization was performed to iden-
(
However, when further increasing the loading amount of Zr to
48.7 mg/g, the obtained ZrO2 exhibited irregularly arranged ZrO2
2−
tify the crystalline structures of SO4 /ZrO2 by using Cu KR