-
journal of
MEMBRANE
SCIENCE
Journal of Membrane Science 95 ( 1994) 22 l-228
Separation of ethanol/water mixture by silicalite membrane on
pervaporation
Tsuneji Sanoap*,Hiroshi Yanagishitab, Yoshimichi
Kenji Harayab
Kiyozumib, Fujio Mizukamib,
‘Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-12, Japan
bNationalInstitute ofMaterials and Chemical Research, Tsukuba, Ibaraki 305, Japan
Received 1 October 1993; accepted 17 May 1994
Abstract
Pure silicalite membranes were prepared on porous supports of sintered stainless steel or alumina discs. The
silicalite layer was characterized by X-ray diffraction, SEM and mercury porosimetry. Individual crystals were
intergrown in three dimensions into the polycrystalline phase. The membranes were not disintegrated by thermal
treatment in vacua or calcination for removing the organic amine occluded in the channels of silicalite, indicating
high thermomechanical
stability of the membrane. The liquid separation potential of the membrane was investigated by pervaporation of an aqueous ethanol solution. The high ethanol permselectivity with a separation factor
a( EtOH/H*O) of more than 60 was achieved for a 5 ~01% aqueous ethanol solution at 3O”C, indicating no cracks
and pores between the silicalite grains within the membrane. From adsorption experiments of ethanol and water
on silicalite, it was found that the high permselectivity is attributable to the selective sorption of ethanol into the
silicalite membrane.
Keywords: Separation; Ethanol; Silicalite membrane; Pervaporation
1. Introduction
Great interest has been focused on preparation of zeolite membranes from the standpoint
of application to gas or liquid separation. Most
of the zeolite membranes have been made by
embedding the zeolite crystals in polymer or ceramic matrixes, or by in situ crystallization on
porous substrates. Recently, several researchers
succeeded in preparing pure zeolite membranes
of silicalite, ZSM-5,ZSM-35, gmelinite, NaA and
SAP0 [l-9]. Although zeolite membranes are
polycrystalline films and very fragile, the gas
separation potential of zeolite membranes is indicated in Refs. [ 10-l 2 1. However, only few
studies have been dedicated to the separation of
liquid mixtures.
Te Hennepe et al. reported that the membrane
performance of the silicon rubber membrane,
which is effective for separation of ethanol/water
mixtures, is considerably improved by addition
of silicalite crystals to the silicon rubber matrix
[ 13 1. At 70 wt% silicalite content a high separation factor (Y( EtOH/H20) of N 20 was achieved.
This result suggests that if a pure silicalite mem-
* Corresponding author.
0376-7388 / 94/$07.00
0 1994 Elsevier Science B.V. All rights reserved
SSD10376-7388(94)00120-N
222
T. Sane et al. /Journal of Membrane Science 95 (1994) 221-228
brane were synthesized, a higher separation factor would be obtained.
From these standpoints, we have investigated
the liquid separation potential of the zeolite
membrane on pervaporation. It was already reported that the silicalite membrane is very effective for the separation of ethanol/water mixtures [ 141. In this paper, the pervaporation
performance of silicalite membrane for separation of ethanol/water mixtures was studied in
detail.
2. Experimental
The hydrothermal synthesis of silicalite membranes was performed as follows. Colloidal silica
(Cataloid SI-30 from Shokubai Kasei Co.; 30.4
wt% SiOZ,0.38 wt% NazO, 69.22 wt% water) was
added to a stirred solution of tetrapropylammonium bromide (TPABr ) and sodium hydroxide, to give a hydrogel with a composition of
0.1: 0.05 : 1: 80 TPABr-NazO-Si02-H20. Then
the hydrogel was transferred to a 300 ml stainless
steel autoclave. A porous support of sintered
stainless steel or alumina disc (5 cm diameter)
with an average pore diameter of 0.5-2 pm was
placed on the bottom of the autoclave. The autoclave was placed in an air-heated oven at 170’ C
for 48 h. After the completion of the crystallization under autogenous pressure without stirring,
the autoclave was cooled down, and the support
was recovered. The silicalite membrane on the
support was washed with deionized water and
dried at 100’ C. Then the silicalite membrane was
heated at 300-380’ C under vacuum, or calcined
at 500°C for 20 h in order to decompose the organic amine occluded in the zeolite framework.
The sihcalite membrane was not disintegrated by
these processes.
The characterization of the membrane was
achieved by X-ray diffraction (Mac Sci.
MXP 18 ) with the incident beam at a glancing
angle of 1.OO".The surface and the cross section
of membrane were characterized by scanning
electron microscopy (SEM, Hitachi H-800) with
an energy-dispersive X-ray analysis system
(EDX). The pore size distribution of the membrane was measured by a mercury porosimeter
(Micromeritics AutoPore 9200).
The pervaporation measurements using an
aqueous ethanol solution as a feed were performed on a standard pervaporation apparatus
as shown in Fig. 1. Liquid nitrogen was used as a
cooling agent for the cold trap. The compositions of the feed and the permeate were determined by gas or liquid chromatography. Flux and
Thermometer
Pirani gauge
A
To
3.5
L-
Stirrer
Stirrer
Silicdite membrane
Pervaporation cell
Fig. 1. Schematic diagram of pervaporation
apparatus.
223
T. Sano et al. /Journal of Membrane Science 95 (1994) 221-228
3 shows the scanning electron micrographs of the
surface and the cross section of the membranes.
The surface was formed by an aggregate of crysFlux( kg/m2 h)
tals of 1O-25 pm. The growth on the porous sup(weight of permeate, kg)
port led to a randomly grown crystalline layer.
= (membrane area, m2) x (permeation time, h)
The average thickness of the silicalite layer (400500 pm) was confirmed by Si line analysis with
and
EDX. These results indicate that the surface of
Separation factor (Y( EtOH/H2 0)
the support was covered with densely packed silicalite crystals.
= [G,O”/G,O
IPermeate
If there are cracks or pores between the silicaltGtOHICH20lFeed
ite grains within the membrane, separation performance cannot be expected. Therefore, the pore
where C,,,, and CH20are the volume fractions
size distribution of the membrane was measured
of ethanol and water, respectively.
by mercury porosimetry. Fig. 4 shows the pore
To evaluate the adsorption performance of the
size distribution of the pure silicalite membrane
silicalite membrane, vapor-sorption experidetached from the stainless steel support by a
ments of water and ethanol on silicalite were carmechanical method. One peak was observed at
ried out with a Belsorp 18 adsorption apparatus.
N 500 nm. To clarify where the peak originated
from, a cross section of the membrane was measured by SEM at higher magnification (Fig. 5 ).
3. Results and discussion
The support side of the membrane consists of
small crystals 2-4 pm in size. Pores or interstices
3.1. Characterization of silicalite membrane
between the silicalite grains were visible. The
crystal sizes in the middle part of the membrane
Fig. 2 shows X-ray diffraction diagrams of silwere larger than those on the support side. Pores
icalite membranes supported on porous stainless
or interstices were also observed. On the other
steel and alumina discs. The reflection peaks
hand, the solution side (the top layer) of the
corresponding to silicalite were observed in the
membrane was composed of a layer of highly inX-ray diffraction diagrams. The peaks derived
tergrown zeolite crystals. Although the shape of
from the porous supports were not observed. Fig.
separation factor cy( EtOH/H20)
lated from:
were calcu-
(4
I(B)
silicalite/sintered stainless steel support
silicalitekalumina support
sintered stainless steel support
5
10
20
30
28 (degrees)
40
50
20
30
28 (degrees)
40
Fig. 2. X-ray diffraction diagrams of silicalite membranes on porous supports.
T. Sano et al. /Journal ofMembrane Science 95 (1994) 221-228
224
(A)
silicalitekintered
stainless steel
support
(B) silicalite/alumina support
Surface
Cross section
Fig. 3. SEM images and Si line analysis for surface and cross section of silicalite membranes on porous supports.
T. Sano et al. /Journal ofMembrane Science 95 (1994) 221-228
225
size distribution as shown in Fig. 4 is attributable to the pores between the silicalite grains in
the middle and the support side of the membrane. These characteristics of the silicalite
membrane prepared are consistent with those reported in the literature [ lo- 12 1.
3.2. Pervaporation performance
0.0
----+-----
10
,./f
102
L..
103
RP
_
d-l.
104
___.__.
.-
105
(nm)
Fig. 4. Pore size distribution of silicalite membrane detached
from stainless steel support determined
by mercury
porosimetry.
the crystal is not clear due to intergrowth, the
crystal size appears to become larger. No pores
originated from silicalite grains were observed.
This indicates that the peak at 500 nm in the pore
As mentioned in the experimental section, the
organic amine (TPABr) is used in the silicalite
synthesis as a template. The organic amine remains in the channels of the silicalite crystals. In
order to use the membrane as a pervaporation
membrane, the amine must be removed from the
channels by certain procedures. As the silicalite
membrane experiences irregular stresses that
arise from a difference in the thermal expansion
between the support and the silicalite crystals and
from removal of volatile materials from the zeolite framework, cracks are easily formed within
the membrane during the treatment process.
Solution side
Fig. 5. SEM images of cross section of silicalite membrane detached from stainiess steel support at high magnification.
T. Sano et al. /Journal of Membrane Science 95 (1994) 221-228
226
Therefore, the influence of pretreatment conditions of the membrane on the pervaporation performance was studied. The pervaporation measurements were conducted at 60°C using an
aqueous ethanol solution of 5 ~01% as the feed.
The results obtained are summarized in Table 1.
The silicalite membrane after air-drying at 100’ C
(containing the template still) showed a very low
flux combined with a separation factor smaller
than 1, indicating no cracks and pores between
the silicalite grains within the membrane before
the pretreatment. However, in the case of the
membrane after thermal treatment under a vacuum, the flux and the separation factor increased
with an increase in treatment temperature and
time. The membrane calcined at 500°C to remove the template completely showed a high flux
combined with a high separation factor of N 60.
This separation factor is the most favorable one
among organic and inorganic membranes which
have been published so far. These results suggest
that the membrane changes from a water-selective membrane to an ethanol-selective one by
decreasing the amount of template occluded in
the zeolite framework, and that the separation of
ethanol/water takes place by transport through
the zeolite channels.
Fig. 6 shows the pervaporation performance of
several silicalite membranes, which were prepared by the same method. It is noted that each
membrane shows a slight different performance.
We have considered that the difference in membrane performance is attributable to a slight difference in membrane thickness or to very small
cracks formed during the thermal treatment of
the membrane. It is also noted that the ethanol
Table 1
Influence of pretreatment
conditions on pervaporation
performance
Treatment condition
air-drying
in vacua
in vacua
calcination
Feed temperature:
concentrations of the permeates for the alumina
supports are slightly lower than those for the
stainless steel ones. Taking into account that the
hydrophobicity of the ZSM-5 type zeolite decreases when the content of framework aluminum increases [ 15,161, this suggests that the top
layer of the membrane on the alumina support is
not composed of silicalite crystals but composed
of ZSM-5 ones. The difference between ZSM-5
and silicalite is the content of framework aluminum, which is nil for the latter. Although the synthesis mixture does not contain an aluminum
source, it is reasonable to consider that a part of
the surface of the alumina support is dissolved
in the high-pH solution. Actually, the surface
Si0JA120, ratio of the membrane on the alumina support measured by EDX was 392.
Next, the influence of the feed temperature on
the pervaporation performance was studied using the silicalite membrane supported on a sintered stainless steel disc (Fig. 7). The flux increased linearly with an increase in the feed
temperature due to the higher vapor pressure of
the components of the feed. The separation factor a(EtOH/H,O)
decreased slightly with the
temperature. A few introductory experiments
were carried out to investigate the influence of
the feed ethanol concentration on the pervaporation performance. Fig. 8 illustrates the relationship between flux and separation factor as function of the feed ethanol concentration. The flux
decreased rapidly by adding ethanol to water and
then increased slightly with the ethanol concentration. On the other hand, the separation factor
a!(EtOH/H20) reached a maximum value at a
feed ethanol concentration of w 3 ~01%and then
(kg/m’ h)
Separation factor
cu(EtOH/H,O)
0.00303
0.00840
0.0394
0.760
0.38
0.58
7.8
58
FlUX
Temp. (“C)
Time (h)
100
300
380
500
12
6
6
20
60°C; feed ethanol concentration:
5 ~01%.
T. Sano et al. /Journal ofMembrane Science 95 (1994) 221-228
2
T 80
iz
80
(A)
0
0
&
s 70
2
E
a
0
0
$ 60
B
c
0
'E 50
0
0
5
i=?
8 40
8
ii
227
/
30
0.4
0.2
0
0.6
0.8
20
1.2
1
40
60
80
100
EtOH concentration (~01%)
Flux (kg/m2h)
Fig. 6. Pervaporation performance of silicalite membranes
prepared. Feed ethanol concentration: 5 voll; feed temperature: 30°C. (0) Alumina support; ( l ) sintered stainless
steel support.
0
ch
1
--I
OS._.-* .
1
0.8
0
z
NE 0.6
&
z
5 0.4
ii
'0
/
I-+
-o----o
/
o-
_o_
7
0
0
7
0
40
Temperature
60
80
100
Fig. 8. Influence of feed ethanol concentration on pervaporation performance. Feed temperature: (A) 3O”C, (B) 60°C.
/
20
40
0,
EtOH concentration (~01%)
---I
0.2
20
io\r
60
80
(“C)
Fig. 7. Influence of feed temperature on pervaporation
formance. Feed ethanol concentration: 5 voll.
per-
Sorption property
It is well known that the transport mechanism
in pervaporation through membranes can be de-
scribed by the sorption-diffusion
model [ 171.
The membrane performance depends upon ( 1)
sorption of permeates in the feed side and (2)
diffusion through the membrane. In the case of
ethanol/water mixture, as the molecular sizes of
ethanol and water are smaller than 5 8, ( z the
pore size of silicalite), the sorption process determines the pervaporation performance mainly.
Therefore, in order to clarify the sorption properties of the silicalite membrane, adsorption isotherms of ethanol and water on silicalite powders were measured. Fig. 9 shows the adsorption
isotherms of ethanol and water at 30°C. It is
clearly shown that the amount of ethanol adsorbed on silicalite was N 3 times larger than that
of water. It is indicated that the high hydrophobic property of silicalite enhances the selective
T. Sano et al. /Journal of Membrane Science 95 (I 994) 221-228
228
8oI
EtOH
60
01
s
-0-
O----O
d
g40
>
::
8
_.0
0
0.2
0-e
I
0.4
H20
-o--.
I
0.6
I
0.8
1
P/PO
Fig. 9. Adsorption isotherms of ethanol and water on silicalite at 30°C.
sorption of ethanol into the membrane,
in the high separation factor.
resulting
4. Conclusions
( 1) The pure silicalite membranes exhibits
high ethanol permselectivity
with a separation
factor a(EtOH/HzO)
of more than 60 for a 5
~01% aqueous ethanol solution at 30’ C.
(2) Cracks or pores between the silicalite
grains, which affect the pervaporation performance, do not exist within the membrane.
(3) The high ethanol permselectivity
of the
membrane is attributable to the high hydrophilic
properties of silicalite.
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