Chemical Engineering Science 62 (2007) 4864 – 4868

www.elsevier.com/locate/ces

Manufacture of mesoporous alumina of boehmite type via subcritical drying

and application to purify liquid crystal
Tz-Bang Du a,∗ , Shyue-Ming Jang a,b , Bor-Wen Chen a
a Material and Chemical Research Laboratory of Industrial Technology Research Institute, Bldg. 23, 321 Kuang Fu Road, Sec. 2, Hsinchu, Taiwan 300, ROC
b Nano Technology Research Center of Industrial Technology Research Institute, Bldg. 67, 195 Chung Hsing Road, Sec. 4, Chutung, Hsinchu, Taiwan 310, ROC

Received 30 June 2006; received in revised form 25 December 2006; accepted 27 December 2006
Available online 17 January 2007

Abstract
Mesoporous alumina is prepared by using the sodium aluminate as the starting material in this study. The pore diameter of mesoporous
alumina is controlled between 2 and 20 nm. The factors of pH value, ageing, water washing and drying conditions on synthesising mesoporous
alumina have been discussed. We found that more amount of product can be obtained when the pH of sol is controlled around 7 in the
neutralisation process. Ageing temperature of 90 ◦ C and ageing time of 3 h is sufficient to form mesoporous alumina in this process. The
mesoporous alumina product with specific surface area of 367 m2 /g and pore volume of 0.98 cm3 /g can be obtained. Besides, when subcritical
drying of 250 ◦ C and 6.2 MPa is applied to obtained boehmite type mesoporous alumina, larger pore volume of 1.42 cm3 /g is observed
comparing with 0.55 cm3 /g pore volume of the comparison sample drying under normal pressure. Further, 89.5% sodium release of sample
can be reduced by water washing process. The synthesised mesoporous alumina product has been applied to liquid crystal purification test.
The superior purification efficiency is found to raise the specific resistance of liquid crystal from 1.8 × 1012 to 2.9 × 1013  cm.
䉷 2007 Elsevier Ltd. All rights reserved.

Keywords: Adsorption; Nanostructure; Porous media; Supercritical fluid; Sol–gel; Alumina

1. Introduction alkoxide precursor (Dumeignil et al., 2003) or aluminium salt

(Yao et al., 2001) in sol–gel method. The process will be sim-
Alumina (Al2 O3 ) is one of the most common ceramics ple if using alkoxide chemicals as precursor and leave little ion
widely used in the industries, experiments and our daily life, impurity in the final product. However, the cost is higher due to
for example, the packing of distillation tower, the catalyst of the expensive alkoxide precursor. When using the aluminium
reactions and the additive of paint and pigment. The sale price salt as the precursor, the main drawback is introducing water
of alumina is low in these applications, and mass produc- into the process. Water is a good solvent for dissolving the alu-
tion should be taken to reap tiny benefits. On the other hand, minium salt in this process. However, pores are easily cracked
mesoporous alumina is synthesised for fine chemical purifica- when water leaves the pore in the drying process due to the
tion usage. In this way, the unit price of mesoporous alumina high surface tension of water (7.28 × 10−2 N/m). One way to
largely increases. For example, the mesoporous alumina ab- overcome the shortage is replacing the water with organic sol-
sorbent for purifying liquid crystal (LC) is about 100 times vent (Cejka, 2003), like methanol (2.26 × 10−2 N/m), before
higher than the unit price of alumina. drying. Still, the surface tension of organic solvent will lead
Generally, fabricating mesoporous alumina in sol–gel the cracking of the pores of mesoporous alumina. Further, the
method includes the steps: dissolving to obtain sol, ageing superior pore properties of mesoporous alumina are obtained
to obtain gel, drying and calcination to obtain the final prod- by supercritical drying (Poco et al., 2001; Walendziewski et al.,
uct. Mesoporous alumina can be synthesised by aluminium 1999; Elaloui et al., 1997) due to the lower surface tension at
the condition near the supercritical point (about 0 N/m). How-
∗ Corresponding author. Tel.: +886 3 5732408; fax: +886 3 5732361. ever, most of the studies use the aluminium alkoxide as the
E-mail address: DuTzBang@itri.org.tw (T.-B. Du). precursor.

0009-2509/$ - see front matter 䉷 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ces.2006.12.073
 T.-B. Du et al. / Chemical Engineering Science 62 (2007) 4864 – 4868 4865

To decrease raw material cost of fabricating porous boehmite 70

type alumina, one of the industrial route of synthesis is start-
ing from sodium aluminate (NaAlO2 ) (Schuth et al., 2002). 60
Boehmite type mesoporous alumina with specific surface area

Wight of wet cake (g)

50
ranging from 200 to 300 m2 /g has been obtained. In this study,
we have developed a process to prepare the aluminium gel from 40
sodium aluminate, and a process to washing the gel to remove
ion impurity. When subcritical drying is applied, the boehmite 30
type mesoporous alumina product equips large specific surface 20
area of 393.3 m2 /g.
10
2. Experimental
0
-1 1 3 5 7 9 11 13
GR grade hydrogen chloride and methanol supplied by
pH of solution
ALDRICH and sodium aluminate supplied by SHOWA were
used as received. De-ionised (DI) water was used in preparing Fig. 1. Weight of wet cake obtained versus pH of aluminate solution.
aqueous solution in these experiments. The sol–gel process in
this study primary consisted of steps: to prepare sodium alu-
minate aqueous solution in controlled concentration as sol, to alumina with different ageing conditions are measured by BET
neutralise the sol by adding hydrogen chloride until the con- and shown in Table 1. For short ageing time period of 3 h,
trolled pH value was reached to obtain gel, to age the gel in hot lower temperature of 60 ◦ C offers insufficient energy to gel
water bath with controlled temperature, to wash the aged gel the sol and is not able to form mesoporous structure. As a
with water, to replace the water with methanol in the gel and to result, non-porous alumina with pore properties of low specific
dry and calcine at elevated temperature to obtain mesoporous surface area as 9 m2 /g and small pore volume as 0.03 cm3 /g
alumina product. The crystalline property was measured by is obtained. In contrary, at higher ageing temperature of 90 ◦ C,
X-ray diffraction analysis (XRD, Mac Science MXP3). The the mesoporous alumina with specific surface area of 367 m2 /g
pore properties were measured by Brunauer–Emmett–Teller and pore volume of 0.98 cm3 /g can be obtained.
analysis (BET, Micromeritics ASAP 2010). The possible error For long ageing time period of 16 h, smaller specific sur-
produced in repetition experiments is observed to be 5–8%. face area of 81.9 m2 /g and pore volume of 0.32 cm3 /g of the
The release of metal was quantified by graphite atomic ab- mesoporous alumina has been obtained as shown in Table 1.
sorption instrument (AA, 220Z Varian). With longer ageing time period, more dry gel cake has been
observed, which indicates that more water of the gel has been
3. Results and discussion evaporated. It is known that the leaving of solvent from the
pores at the mesoporous alumina will lead to crack of the pores
3.1. The pH value factor due to surface tension of the solvent (Cejka, 2003). To verify
the affection of solvent surface tension in this study, the gel
Ten grams of sodium aluminate is mixed with DI water to without replacing water with methanol is calcining to obtain
prepare aqueous solution sol of 10 wt%. The sol is transparent mesoporous alumina as shown in Table 1. Much smaller spe-
and the pH value of the sol is 13.2. Opaque dispersive solution cific surface area of 35.1 m2 /g and pore volume of 0.1. cm3 /g
is obtained after adding hydrogen chloride. Wet cake is obtained of the mesoporous alumina has been obtained. It is known that
by filtering the solution and is weighted to determine the pH the surface tension of water is 72.8 × 10−3 N/m and that of
value with maximum solid product. As shown in Fig. 1, the methanol is 22.6 × 10−3 N/m. The higher surface tension of
solid content of the solution will vary with the controlled pH water will result in larger capillary shrinking force in drying
value of the sol, the weight of wet cake obtained is 30 g at step and lead to more serious crack of pores. As a result, meso-
pH = 11 and increases with decreased pH value to a maximum porous alumina with poor pore properties is produced in sample
amount of 57 g at pH=7. When the pH value is lower than 7, the without methanol replacing step.
weight decreases with decreased pH value and the precipitate
is totally dissolved at pH −0.8. As a result, pH=7 is controlled 3.3. The drying conditions factor
at neutralisation step in this study.
In this study, subcritical drying of gel to obtain mesoporous
3.2. The ageing conditions factor alumina has been examined by adding three parts of methanol
by one part of wet cake into high-pressure vessel after methanol
The obtained opaque dispersive solution transforms into gel replacing step. The temperature of the vessel is then raised to
after ageing in 90 ◦ C hot water bath. Differing from transparent 250 ◦ C (Tc of methanol is 239.4 ◦ C). After 1 h equilibrium, the
silicate gel, the aluminate gel is observed to be opaque. The gels pressure is observed as 6.2 MPa and the vent valve is opened.
of different ageing conditions are calcined at 800 ◦ C for 4 h to The methanol gas of the vessel has been released and the pres-
obtain mesoporous alumina. The pore properties of mesoporous sure of the vessel drops to normal pressure of 0.1 MPa within
4866 T.-B. Du et al. / Chemical Engineering Science 62 (2007) 4864 – 4868

Table 1
Pore properties of mesoporous alumina sample with various ageing conditions and methanol replacing conditions

Ageing temperature (◦ C) Ageing time (h) Specific surface area (m2 /g) Pore volume (cm3 /g) Pore diameter (nm) Methanol replacing

60 3 9.0 0.03 12.9 Y

90 3 367.0 0.98 10.6 Y
90 16 81.9 0.32 15.0 Y
90 16 35.1 0.10 10.9 N

found in the pore diameter distribution as shown in Fig. 4. The

pore diameter distribution of normal pressure drying sample is
divided into two main groups, while that of subcritical drying
sample shows only one peak.

3.4. The washing conditions factor

CPS

To reduce the metal impurity release of the mesoporous alu-

a mina product, there is a water wash step following the ageing
step. One part of the aged cake is washed with 30 parts of DI
water for three times. The sodium release of final product is
b
tested by adding 1 g of sample into 10 ml ultra-pure water in
bottle and shaking for 1 min to mix the solution in the bottle.
0 10 20 30 40 50 60 70 80 90
The up layer water of the solution is sampled after 24 h im-
2
mersion. The sodium content in water sample is measured by
Fig. 2. The XRD pattern of (a) subcritical drying sample and (b) normal pres-
graphite AA. It is found that the unwashed mesoporous alu-
sure drying sample (o indicates the characteristic peak of boehmite alumina). mina product releases 731.3 ppm sodium ion, while the washed
product releases 76.5 ppm. As a result, water wash step can re-
move about 89.5% released sodium ion of mesoporous alumina
15 min. Dry powder of mesoporous alumina is obtained and the synthesised in this study.
crystalline and pore properties of the product are measured with
XRD and BET analysis. For comparison, a parallel controlled 3.5. The test of mesoporous alumina product in purifying LC
experiment with the same batch and amount of precursor has
been proceeded by putting into the same high-pressure vessel. Mesoporous alumina is known to be able to adsorb ion im-
Also, the temperature of the vessel is raised to 250 ◦ C and the purity from the matrix. To test the purification ability of the
sample is heated in the same time period of subcritical drying product, the mesoporous alumina synthesised in this study is
process. Contrarily, the valve keeps opening during whole dry- used to purify thin film transistor type LC. One gram of meso-
ing time period. The obtained dry powder of mesoporous alu- porous alumina product is added into 10 g LC in polypropylene
mina is also examined with XRD and BET analysis. The XRD (PP) cup and agitated by PP rod for 3 min. The up layer LC is
results are shown in Fig. 2. It is known that mesoporous alu- filtered and the specific resistance (SR) is measured. The SR
mina with boehmite (refer to Yao et al., 2001) crystal structure of LC sample is raised from 1.8 × 1012 to 2.9 × 1013  cm. It
is obtained in these processes. The subcritical drying sample is known that the SR of LC indicates the ion impurity content
exhibits sharper peak than the normal pressure drying sample. level of LC. The LC with higher SR contains fewer ion impuri-
As shown in Table 2, the BET results indicate that the sample ties. On the contrary, the LC with lower SR contains more ion
of subcritical drying equips larger pore volume up to 1.42 than impurities. As a result, the increase from lower SR to higher
0.55 cm3 /g of normal pressure drying sample. It is supposed SR of LC indicates that ion impurity has been removed dur-
that the lower surface tension of methanol during subcritical ing the purification process. It is supposed that the mesoporous
drying helps to maintain the pore of mesoporous alumina. Be- alumina synthesised in this study is able to purify LCs.
sides, as shown in Fig. 3, the BET adsorption and desorption
curves indicate that the pore shape of subcritical drying sam- 4. Conclusions
ple is homogeneous cylinder type (refer to Dumeignil et al.,
2003). On the other hand, that of normal pressure drying sam- Mesoporous alumina is prepared by using sodium aluminate
ple exhibits a mixing type of cylinder type and inkbottle type. as starting material in this study. With respect to using alu-
It is supposed that parts of pores of mesoporous alumina are minium alkoxide as starting material of preparing mesoporous
cracked into smaller pores during normal pressure drying due alumina, the aluminate salt is much cheaper. Although water has
to the surface tension of methanol. The evidence can also be to be used to dissolve salts in such process, solvent replacing
 T.-B. Du et al. / Chemical Engineering Science 62 (2007) 4864 – 4868 4867

Table 2
Pore properties of mesoporous alumina sample with different drying conditions

Drying temperature (◦ C) Drying pressure (MPa) Drying time (h) Specific surface are (m2 /g) Pore volume (m3 /g) Pore diameter (nm)

250 0.1 4 341.6 0.55 6.3

250 6.2 4 393.3 1.42 13.8

a Isotherm Plot a BJH Desorption dv/dlog(D) Pore Volume

1000
3.0
900

800 2.5
Volume Absorbed g STP

700

Pore Volume, (cm g)

2.0
600

500
1.5
400

300 1.0

200

100 0.5

0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0
Relative Pressure (P/Po) 1e+0.2 1e+0.3
Pore Diameter, (A)
b Isotherm Plot
b BJH Desorption dv/dlog(D) Pore Volume

350
1.6

300
1.4
Volume Absorbed g STP

250
1.2
Pore Volume, (cm g)

200
1.0

150
0.8

100 0.6

50 0.4

0 0.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Relative Pressure (P/Po) 0.0
+ Adsorption Desorption 1e+0.2 1e+0.3
Pore Diameter, (A)
Fig. 3. The BET adsorption and desorption curves of (a) subcritical drying
Fig. 4. The pore diameter distribution of (a) subcritical drying sample and
sample and (b) normal pressure drying sample.
(b) normal pressure drying sample.

process and subcritical drying process can be applied to di- with respect to that prepared by using aluminium alkoxide, wa-
minish the possible cracks of pores due to drying and produce ter washing step can be introduced to the synthesis process and
mesoporous alumina product with superior pore properties. Al- reduce the ion impurity of the product to a lower level. It is
though the final product tends to contain more ion impurity supposed that the mesoporous alumina product synthesised in
4868 T.-B. Du et al. / Chemical Engineering Science 62 (2007) 4864 – 4868

this study would remove ion impurity in the LCs instead of Poco, J.F., Satcher, J.H., Hrubesh, L.W., 2001. Synthesis of high porosity,
releasing ion impurity and result in successful purifying LCs. monolithic alumina aerogels. Journal of Non-Crystalline Solids 285,
57–63.
Schuth, F., Sing, K.S.W., Weitkamp, J., 2002. Handbook of Porous Solids,
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