Development of Methanol Permselective FAU-Type Zeolite Membranes and Their Permeation and Separation Performances
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membranes
Article
Development of Methanol Permselective FAU-Type Zeolite
Membranes and Their Permeation and Separation Performances
Ayumi Ikeda , Chie Abe, Wakako Matsuura and Yasuhisa Hasegawa *
Research Institute of Chemical Process Technology, National Institute of Advanced Industrial Science and
Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan; a-ikeda@aist.go.jp (A.I.);
abe-chie@aist.go.jp (C.A.); wakako.matsuura@aist.go.jp (W.M.)
* Correspondence: yasuhisa-hasegawa@aist.go.jp; Tel.: +81-22-237-3098
Abstract: The separation of non-aqueous mixtures is important for chemical production, and zeolite
membranes have great potential for energy-efficient separation. In this study, the influence of the
framework structure and composition of zeolites on the permeation and separation performance of
methanol through zeolite membranes were investigated to develop a methanol permselective zeolite
membrane. As a result, the FAU-type zeolite membrane prepared using a solution with a composition
of 10 SiO2 :1 Al2 O3 :17 Na2 O:1000 H2 O showed the highest permeation flux of 86,600 µmol m−2 s−1
and a separation factor of 6020 for a 10 wt% methanol/methyl hexanoate mixture at 353 K. The
membrane showed a molecular sieving effect, reducing the single permeation flux of alcohol with
molecular size for single-component alcohols. Moreover, the permeation flux of methanol and the
separation factor increased with an increase in the carbon number of the alcohols and methyl esters
containing 10 wt% methanol. In this study, the permeation behavior of FAU-type zeolite membranes
was also discussed based on permeation data. These results suggest that the FAU-type zeolite
Citation: Ikeda, A.; Abe, C.; membrane has the potential to separate organic solvent mixtures, such as solvent recycling and
Matsuura, W.; Hasegawa, Y. membrane reactors.
Development of Methanol
Permselective FAU-Type Zeolite Keywords: organic mixture separation; methanol removal; pervaporation
Membranes and Their Permeation
and Separation Performances.
Membranes 2021, 11, 627. https://
doi.org/10.3390/membranes11080627
1. Introduction
Academic Editor: Tomohisa Yoshioka The separation of organic mixtures is essential for the chemical and petrochemi-
cal industries to improve process efficiency. In particular, organic solvent recycling,
Received: 29 July 2021 isomer separation, and the integration of reaction and separation processes have been
Accepted: 13 August 2021 reported [1]. Membrane separation is an energy-efficient separation technology, and in-
Published: 15 August 2021 organic membranes have superior chemical stability and mechanical strength compared
to polymeric membranes. Among inorganic membranes, porous membranes, such as
Publisher’s Note: MDPI stays neutral zeolite, silica, and carbon membranes, can separate organic mixtures by molecular sieving
with regard to jurisdictional claims in and selective adsorption.
published maps and institutional affil- Recently, the separation of organic mixtures using membranes by the molecular sieving
iations. effect has been reported [2–4]. Tsuru et al. developed a methanol permselective organosilica
membrane using bis(triethoxysilyl)acetylene as a precursor [2]. The methanol flux was
24,300 µmol m−2 s−1 with a separation factor of 10 for a dimethyl carbonate mixture
containing 10 wt% methanol at 323 K. Lively et al. examined xylene isomer separation by
Copyright: © 2021 by the authors. reverse osmosis method using carbon molecular sieve hollow fiber membranes derived
Licensee MDPI, Basel, Switzerland. from cross-linked poly(vinylidene fluoride) [3]. The flux was 1100 µmol m−2 s−1 with a
This article is an open access article separation factor of 4 for a xylene mixture (o-/p-xylene = 50/50) at 395 K. When an MFI-type
distributed under the terms and zeolite membrane was used for xylene separation, the p-xylene flux was 2.3 µmol m−2 s−1
conditions of the Creative Commons with a separation factor of more than 100 at 373 K [4]. The high permeation and separation
Attribution (CC BY) license (https:// performances of the zeolite membrane were achieved because the membrane had few
creativecommons.org/licenses/by/ defects such as pinholes and cracks.
4.0/).
Membranes 2021, 11, 627. https://doi.org/10.3390/membranes11080627 https://www.mdpi.com/journal/membranes
Membranes 2021, 11, 627 2 of 13
Zeolite membranes can separate organic mixtures by selective adsorption, for exam-
ple, alcohol/alcohol, alcohol/ether, and aromatic/alkane mixtures [5,6]. Kanemoto et al.
evaluated the permeation performance of an MFI-type zeolite membrane for a mixture of
1-butanol, 2-propanol, and butyrate [5]. As a result, 1-butanol can selectively permeate
because it has a higher carbon number and more hydrophobicity than 2-propanol. Kita
et al. reported that an FAU-type zeolite membrane could separate benzene from cyclohex-
ane [6]. The benzene selectivity was attributed to the selective adsorption of benzene on
the zeolite by the interaction of the covalent electrons of benzene with Na+ in the zeolite.
The FAU-type zeolite membrane was also effective for the separation of methanol from
methyl tert-butyl ether.
Membrane reactors have attracted much attention for the development of energy- and
resource-saving chemical reactions. The application of zeolite membranes to transesterifi-
cation reactions has recently been investigated [7,8]. The selective removal of by-products
(such as alcohol) from reaction mixtures can shift the chemical equilibrium, which im-
proves the yield of the target ester. We investigated the influence of the reaction substrates
of transesterification reactions on the conversion increment in the membrane reactor [9].
As a result, the permeation of methanol through the zeolite membrane significantly im-
proved the yield of the target ester. However, it is unclear which zeolite frameworks and
compositions are suitable for alcohol separation from non-aqueous organic solutions.
In this study, four zeolite membranes were prepared, and their permeation and
separation performances of methanol from alcohols and methyl esters were evaluated
to determine the influence of the framework structure and composition of the zeolite
membrane. Since the FAU-type zeolite showed the highest permeation and separation
performance of methanol among the membranes, the permeation performances were inves-
tigated for single-component alcohols and several organic solvents containing methanol.
Furthermore, the permeation and separation behavior of the FAU-type zeolite membrane
are discussed based on these data in this manuscript.
2. Materials and Methods
2.1. Materials
Sodium aluminate, sodium hydroxide, sodium silicate solution, tetrapropylammo-
nium bromide (TPABr), N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdOH) so-
lution, colloidal silica (LUDOX HS-40), NaY-type zeolite particles (HSZ-320NAA, Si/Al = 2.3)
and HY-type zeolite particles (HSZ-390HUA, Si/Al = 770) were used for the preparation of
zeolite membranes. Methanol, ethanol, 1-propanol, 1-butanol, 1-hexanol, methyl acetate,
methyl propionate, methyl butyrate, and methyl hexanoate were used in the pervaporation
experiments. All reagents were purchased from FUJIFILM Wako (Osaka, Japan) and used
without further purification.
2.2. Membrane Preparation
CHA-, LTA-, MFI-, and FAU-type zeolites were selected as the potential material
for methanol permselective membranes. CHA- and LTA-type zeolites have similar pore
diameters (0.38, 0.42 nm) to the molecular size of methanol (0.380 nm). Besides, MFI- and
FAU-type zeolite membranes can separate alcohol from alcohol/ester mixtures [5] and
methanol/organic solvent mixture separation [6], respectively. Therefore, we selected the
above four types of zeolite membranes.
Four types of zeolite membranes (CHA-, LTA-, MFI-, and FAU-type) were synthesized
on the outside of porous α-alumina support tubes by a secondary growth method. The prop-
erties of the support tube were as follows: Diameter = 3 mm, mean pore diameter = 0.3 µm,
and porosity = 50%. The membranes were prepared using the same method as previously
reported [10–12]. The detailed preparation procedures for zeolite membranes are described
as follows.
For the CHA-type zeolite membrane [10], a synthesis solution was prepared by mixing
sodium hydroxide, sodium aluminate, TMAdOH solution, and HY-type zeolite particles,
Membranes 2021, 11, 627 3 of 13
followed by stirring for 4 h at room temperature. The molar composition of the mixture
was 45 SiO2 :1 Al2 O3 :4.5 Na2 O:3.2 TMAdOH:4500 H2 O. The CHA-type zeolite particles
were manually implanted into the macropores of the support tube by rubbing with paper
wipers. The tube was added to a Teflon-lined stainless-steel autoclave filled with 30 g
of the synthesis solution. The autoclave was placed horizontally in an oven at 433 K for
24 h to form a polycrystalline layer on the support. After the autoclave was cooled to
room temperature, the tube was removed from the autoclave and washed with deionized
water several times. The tube was dried overnight at room temperature and then fired
at 773 K for 10 h to obtain the CHA-type zeolite membrane (hereafter referred to as the
CHA membrane).
For the LTA-type zeolite membrane [11], a synthesis solution was prepared by stirring
a mixture of sodium aluminate, sodium hydroxide, and sodium silicate solution for 1 h at
room temperature. The molar composition of the mixture was 2 SiO2 :1 Al2 O3 :2.3 Na2 O:300
H2 O. The outside of the support tube was rubbed with LTA-type zeolite particles, and the
tube was added to an autoclave filled with 30 g of the synthesis solution. The autoclave
was placed horizontally in an oven at 393 K for 5 h. After the reaction, the autoclave was
cooled to room temperature, and the tube was recovered and washed with deionized water
several times. Finally, the membrane was dried overnight under ambient conditions to
obtain the LTA-type zeolite membrane (hereafter referred to as the LTA membrane).
For the MFI-type zeolite membrane [12], a synthesis solution was prepared by mixing
sodium hydroxide, colloidal silica, deionized water, and TPABr as a structure-directing
agent, followed by stirring for 1 h at room temperature. The molar ratio of the synthesis
solution was 1 SiO2 :0.05 Na2 O:0.26 TPABr:80 H2 O. The outside of the support tube was
rubbed with MFI-type zeolite particles, and both ends of the tube were sealed with Teflon
tape to prevent crystal deposition on the inner surface of the tube. The tube was then
placed in a Teflon-lined stainless-steel autoclave filled with 30 g of the synthesis solution.
The autoclave was placed horizontally in an oven at 413 K for 24 h. After the autoclave
was cooled to room temperature, the tube was removed from the autoclave and washed
with deionized water several times. The tube was dried overnight at room temperature
and then fired at 673 K for 48 h to obtain the MFI-type zeolite membrane (hereafter referred
to as the MFI membrane).
For the FAU-type zeolite membrane [12], a synthesis solution was prepared by stir-
ring a mixture of sodium hydroxide, sodium aluminate, sodium silicate solution, and
deionized water for 4 h at room temperature. The molar composition of the mixture was
a SiO2 :1 Al2 O3 :17 Na2 O:1000 H2 O (a = 5, 10, 13, and 25). The NaY-type zeolite particles
were rubbed on the outside of the support tube, and the tube was added to an autoclave
filled with 30 g of the synthesis solution. The autoclave was set horizontally in an oven at
363 K for 16 h to grow the FAU-type zeolite crystals on the support tube. The tube was
then washed with deionized water several times after cooling the autoclave and dried
overnight at room temperature. Hereafter, the FAU-type zeolite membranes with molar
compositions of a = 5, 10, 13, and 25 are referred to as FAU(5), FAU(10), FAU(13), and
FAU(25) membranes, respectively.
The membrane morphology was observed using scanning electron microscopy (SEM,
JEOL, JCM-6000, Tokyo, Japan), and the composition was analyzed using an energy disper-
sive X-ray analyzer (EDX, JEOL, ED-2300, Tokyo, Japan) attached with the SEM. The crystal
structure was identified by X-ray diffraction (XRD, Rigaku, Smart-Lab, Tokyo, Japan).
2.3. Pervaporation Experiment
One end of the membrane was connected to a stainless-steel tube using a resin (GL Sci-
ence, Torr seal, Tokyo, Japan), and the other end was capped. The effective membrane areas
were 1.0 cm2 for the CHA, LTA, and MFI membranes and 2.0 cm2 for the FAU membrane,
respectively. All membrane performances were evaluated by a pervaporation method
using the apparatus shown in Figure 1. Ethanol, 1-propanol, 1-butanol, 1-hexanol, methyl
acetate, methyl propionate, methyl butyrate, and methyl hexanoate were used as solventsOne end of the membrane was connected to a stainless-steel tube using a resin (GL
Science, Torr seal, Tokyo, Japan), and the other end was capped. The effective membrane
areas were 1.0 cm2 for the CHA, LTA, and MFI membranes and 2.0 cm2 for the FAU mem-
brane, respectively. All membrane performances were evaluated by a pervaporation
Membranes 2021, 11, 627 method using the apparatus shown in Figure 1. Ethanol, 1-propanol, 1-butanol, 1-hexanol, 4 of 13
methyl acetate, methyl propionate, methyl butyrate, and methyl hexanoate were used as
solvents in the test solution containing 10 wt% methanol for the pervaporation experi-
ment. These alcohols and methyl esters are used as reaction substrates of transesterifica-
in the test solution containing 10 wt% methanol for the pervaporation experiment. These
tion reaction in our previous report [9]. The test solution (150 g) was added to a separable
alcohols and methyl esters are used as reaction substrates of transesterification reaction in
flask and heated at 303–353 K with stirring at 600 rpm. The membrane was immersed in
our previous report [9]. The test solution (150 g) was added to a separable flask and heated
the solution,
at 303–353 K and
withthe insideatof600
stirring therpm.
membrane was evacuated
The membrane using a rotary
was immersed pump below
in the solution, and
1the
kPa.
inside of the membrane was evacuated using a rotary pump below 1 kPa.6.0
In addition, helium was introduced into the membrane at 1.0, 3.0, or InmL min−1
addition,
as the standard.
helium The gasinto
was introduced composition
the membrane in the
at evacuated stream
1.0, 3.0, or 6.0 was−1analyzed
mL min using mass
as the standard. The
spectrometry
gas composition (Pfeiffer
in thevacuum,
evacuated QGA, Asslar,
stream was Germany).
analyzed using The permeation flux, Ji, (Pfeiffer
mass spectrometry of com-
ponent
vacuum, i was
QGA, calculated as follows [13]:
Asslar, Germany). The permeation flux, J , of component i was calculated
i
as follows [13]: N y
Ji =NHeHe yi i , (1)
Ji = S yHe, (1)
S y He
where NHe, S, and yi are the molar flow rate of helium, the membrane area, and the mole
where NHe , S, and yi are the molar flow rate of helium, the membrane area, and the mole
fraction of component i in the evacuated stream, respectively. The permeance of single
fraction of component i in the evacuated stream, respectively. The permeance of single
alcohol, Qi, component, was calculated using the following equation:
alcohol, Qi , component, was calculated using the following equation:
J
Qi = J i , (2)
p f , i − pp, i ,
i
Qi = (2)
p f ,i − p p,i
where pf,i and pp,i represent the saturated vapor pressure of component i of the feed solu-
tion
where and
pf,ithe
andpartial vapor pressure
pp,i represent of component
the saturated vapor pressure i on theofpermeate
componentside,
i ofrespectively. The
the feed solution
saturated vapor pressure
and the partial of component
vapor pressure i was calculated
of component i on the using the Antoine
permeate constants listed
side, respectively. The
saturated
in Table 1.vapor pressure of factor
The separation component i was calculated
of methanol for each using thesolvent,
organic Antoineα, constants listed
is defined as
in Table 1. The separation factor of methanol for each organic solvent, α, is defined as
follows:
follows: y yO
α (M/O) = yMM /yO , (3)
α(M/O ) = x x ,
xM M /xO O
where xxii is
where is the
the mole
mole fraction
fraction of component ii in
of component in the
the solution,
solution, and
and the
the subscripts
subscripts M
M and
and O
indicate methanol
indicate methanol andand organic
organic solvent, respectively.
Figure 1. Schematic diagram of pervaporation apparatus.Membranes 2021, 11, 627 5 of 13
Table 1. Antoine constants of primary alcohols.
Antoine Constant
Solvent Reference
A B C
Methanol 8.07919 1581.34 239.65 [14]
Ethanol 8.04494 1554.3 222.65 [14]
1-Propanol 7.751113 1441.6293 198.8507 [14]
1-Butanol 4.54607 1351.555 −93.34 [15]
2.4. Seed Particles Preparation for Zeolite Membrane
For the CHA membrane, the seed particles were prepared by mixing sodium hydrox-
ide, sodium aluminate, N,N,N-trimethyl-1-adamantammonium hydroxide solution (SDA,
25%, Sachem Asia, Osaka, Japan), and ultra-stable Y-type zeolite particles (HSZ-390HUA).
The molar composition of the solution was 40 SiO2 :1Al2 O3 :4 Na2 O:8 SDA:800 H2 O. The
mixture was poured into a Teflon-lined stainless-steel autoclave, and a hydrothermal reac-
tion was carried out at 433 K for four days. Solids were recovered by filtration, washed
with distilled water, and dried overnight at 383 K to obtain seed particles.
For the MFI membrane, the seed particles were prepared by mixing sodium hydroxide,
tetrapropylammonium hydroxide (TPAOH), and tetraethyl orthosilicate. The molar com-
position of the solution was 25 SiO2 :0.1 Na2 O:9 TPAOH:490 H2 O:100 EtOH. The mixture
was poured into a Teflon-lined stainless-steel autoclave, and a hydrothermal reaction was
carried out at 373 K for 72 h. Solids were recovered centrifugally and washed with distilled
water. The average particle size was 110 nm.
Commercially available NaA (A-4) and NaY-type zeolite particles (HSZ-320NAA)
were used as the seed particles in this study. They can be purchased from FUJIFILM Wako
Corp (Osaka, Japan).
3. Results and Discussions
3.1. Membrane Characterization
Figure 2 shows the SEM images of the top surface and cross-section and elemental
mapping by EDX of the CHA, LTA, MFI, and FAU(10) membranes. The outer surface of
the support was covered with well-intergrown polycrystalline layers with sizes of 2–5 µm
for CHA, 3–4 µm for LTA, 2–3 µm for MFI, and 1–4 µm for FAU. The thicknesses of the
polycrystalline layers were in the range of 1.2–4.5 µm. The Si/Al ratios of CHA, LTA, MFI,
and FAU(10) membranes were 18, 1.0, 75, and 1.3. by EDX. The values of CHA, LTA, and
FAU(10) were similar to theoretical Si/Al ratios, while that of the MFI membrane was
different from the theoretical Si/Al ratio (=∞). This proposes the zeolite layer contains
aluminum derived from the α-alumina support. As shown in Figure 3, the XRD patterns
of the membranes contain both the α-alumina support and desired zeolite. Table 2 shows
major XRD peaks of four zeolites and the relative intensities. For CHA, the peaks due to
(1,0,0), (1,0,−1), and (2,−1,0) were observed at 2θ = 9.5, 12.9, and 20.6◦ , respectively. LTA,
MFI, and FAU membranes also give peaks due to corresponding zeolites. These results
indicate that CHA, LTA, MFI, and FAU(10) membranes can be prepared on the supports.
The peak intensity ratios due to zeolites were almost the same as those of the particles.
This means that the zeolite crystals forming the polycrystalline layer are not oriented in a
unique direction.
Figure 4 shows the effect of the molar composition of the synthesis solution on the
morphology of the FAU membranes. When the molar composition of the synthesis solution
was 25 SiO2 :1 Al2 O3 :17 Na2 O:1000 H2 O (SiO2 /Al2 O3 = 25), many particles were deposited
on the outer surface. However, few particles were observed at SiO2 /Al2 O3 ratios of 5, 10,
and 13. The thicknesses of the polycrystalline layers were almost the same as 1.7–2.5 µm
for all SiO2 /Al2 O3 .Membranes 2021, 11, x FOR PEER REVIEW 6 of 13
Membranes 2021,
Membranes 2021, 11,
11, 627
x FOR PEER REVIEW 6 6of
of 13
13
Figure 2. SEM images and cross-section elemental mapping of CHA, LTA, MFI, and FAU(10) membranes (Green: Si, Red:
Figure
Figure 2.
Al). 2. SEM
SEM images and cross-section
images and cross-sectionelemental
elementalmapping
mappingofof CHA,
CHA, LTA,
LTA, MFI,
MFI, andand FAU(10)
FAU(10) membranes
membranes (Green:
(Green: Si, Red:
Si, Red: Al).
Al).
Figure
Figure 3. XRD
3. XRD patterns
patterns of CHA,
of CHA, LTA,
LTA, MFI,
MFI, andand FAU(10)
FAU(10) membranes.
membranes. Symbols
Symbols denote
denote thethe diffraction
diffraction
Figure
peaks
peaks 3.
of of XRD
CHACHA patterns
(diamond),of CHA,
(diamond),LTA LTA, MFI,triangle),
(inverted
LTA (invertedand FAU(10)
MFI
triangle), membranes.
(triangle),
MFI Symbols
FAU
(triangle), denote
(circle),
FAU and
(circle), the
and diffraction
α-alumina
α-alumina
peaks
(square). of CHA (diamond), LTA (inverted triangle), MFI (triangle), FAU (circle), and α-alumina
(square).
(square).Table 2. XRD peaks and relative intensity, Irel, for CHA, LTA, MFI, and FAU(10) particles.
CHA Table 2. XRD peaks and relative intensity, Irel, for CHA, MFI
LTA LTA, MFI, and FAU(10) particles. FAU(10)
2θ (°) (h,k,l)
CHA I rel (%) 2θ (°) (h,k,l)
LTA I rel (%) 2θ (°) (h,k,l)
MFI I rel (%) 2θ (°) (h,k,l)
FAU(10)Irel (%)
9.5 (1,0,0)
2θ (°)2021,
Membranes (h,k,l)
11, 627 100Irel (%) 7.2
2θ (°) (2,0,0)
(h,k,l) 100 Irel (%) 7.9 2θ (°) (0,1,1)
(h,k,l) 100 Irel (%) 6.2
2θ (°) (1,1,1)
(h,k,l) 100Irel
7 of(%)
13
12.99.5 (2,−1,0)(1,0,0) 47.2 100 10.27.2 (2,2,0)
(2,0,0) 76.9 100 8.97.9 (2,0,0)
(0,1,1) 59.7 100 15.66.2 (3,3,1)
(1,1,1) 33.8 100
20.612.9 (1,0,−1)
(2,−1,0) 50.4 47.2 16.1 10.2 (4,2,0)
(2,2,0) 36.276.9 23.18.9 (0,5,1) (2,0,0) 11059.7 23.615.6 (5,3,3) (3,3,1) 30.733.8
20.6 (1,0,−1) 50.4 16.1 (4,2,0) 36.2 23.1 (0,5,1) 110 23.6 (5,3,3) 30.7
Table 2. XRD peaks and relative intensity, Irel , for CHA, LTA, MFI, and FAU(10) particles.
Figure 4 shows the effect of the molar composition of the synthesis solution on the
CHA morphology FigureofLTA
4the FAUthe
shows membranes.
effect of theWhen
molarthecomposition
MFI molar composition of the
of the synthesis synthesis
FAU(10)solutionsolu-
on the
tionmorphology
was 25 SiO 2 :1
of Al
the 2 O3
FAU:17 Na 2 O:1000
membranes. H 2O (SiO
When 2
the/Al 2 O
molar 3 = 25), many
composition particles
of the were depos-
synthesis solu-
2θ (◦ ) (h,k,l) Irel (%) ◦
2θ ( ) (h,k,l) Irel (%) ◦
2θ ( ) (h,k,l) Irel (%) ◦
2θ ( ) (h,k,l) Irel (%)
itedtion
on was
the outer
25 SiOsurface. However, few particles were observed at SiO2/Al2O3 ratios
2:1 Al2O3:17 Na2O:1000 H2O (SiO2/Al2O3 = 25), many particles were depos-
of 5,
9.5 (1,0,0) 100 10, and 7.2 13. The
(2,0,0)
thicknesses 100 of the 7.9 (0,1,1) layers
polycrystalline 100were almost
6.2 the(1,1,1)
same 100
ited 2/Al2Oas 1.7–2.5
12.9 (2,−1,0) 47.2 10.2on the(2,2,0)
outer surface.
76.9 However,8.9 few (2,0,0)
particles were 59.7 observed
15.6 at SiO(3,3,1) 3 ratios
33.8 of 5,
μm10, forand
all SiO /Al2O
13. 2The 3.
thicknesses of the polycrystalline layers were almost the same as 1.7–2.5
20.6 (1,0,−1) 50.4 16.1 (4,2,0) 36.2 23.1 (0,5,1) 110 23.6 (5,3,3) 30.7
μm for all SiO2/Al2O3.
Figure 4. SEM images of FAU membranes prepared using the synthesis solutions with SiO2/Al2O3 ratios of 5, 13, and 25.
Figure 4. SEM images of FAU membranes prepared using the synthesis solutions with SiO2 /Al2 O3 ratios of 5, 13, and 25.
Figure 4. SEM images of FAU membranes prepared using the synthesis solutions with SiO2/Al2O3 ratios of 5, 13, and 25.
Figure
Figure55showsshowsthe theinfluence
influence ofof the
the SiO
SiO22/Al
/Al2O3 ratio of the synthetic solution on the
2 O3 ratio of the synthetic solution on the
Si/Al
Si/Alratios
Figureofofthe
5the FAU
shows zeolite
FAUthe membrane
influence of theand
SiOthe
2/Alrecovered
2O3 ratio of particles. At SiO
the syntheticSiO 2/Al2O3 = 5,the
solution
ratios zeolite membrane and the recovered particles. At 2 /Al2 O3on
= 5,
the
theSi/Al
Si/Al
Si/Al ratios
ratios of the
of the
ratios FAU
of the particles and
zeolite and
particles membrane
membrane
membrane were
and were 1.35 and
the recovered 1.31, respectively.
1.35 and particles. At SiO2/Al
1.31, respectively. As the
2Othe
As 3 = 5,
SiO 2/Al
the 2O3 ratio
Si/Al of of
thethe
solution increased, the
theSi/Al ratios ofofthe
theparticles and
andmembrane
SiO 2 /Al 2 O3ratios
ratio the particles
solution and membrane
increased, were
Si/Al 1.35
ratios and 1.31, respectively.
particles As the
membrane
also
alsoincreased.
SiO 2/Al2O3 ratio of the solution increased, the Si/Al ratios of the particles and membrane
increased.
also increased.
InfluenceofofSiO
Figure5.5.Influence
Figure SiO2/Al
2 /Al
2O23Oratio
3 ratio
ofof synthesis
synthesis solutionononthe
solution theSi/Al
Si/Al ratioofofFAU
ratio FAUmembranes
membranes
and
andparticles.
particles.
Figure 5. Influence of SiO2/Al2O3 ratio of synthesis solution on the Si/Al ratio of FAU membranes
and particles.
Lechert et al. investigated the influence of the molar composition of the synthesis
solutions on the Si/Al ratio of FAU-type zeolite crystallites [16]. Assuming that the molar
composition of the zeolite synthesis solution is a SiO2 :1 Al2 O3 :c Na2 O:d H2 O, the theoretical
Si/Al ratio of FAU-type zeolite crystallites can be described as follows:
a+c−1
Si/Al = . (4)
c+1Membranes 2021, 11, 627 8 of 13
The calculation result is represented by the dashed line in Figure 5. The good agree-
ment between the experimental data and theoretical values indicates that the membrane
composition can be controlled by the composition of the synthesis solution.
3.2. Methanol Separation Performance of CHA, LTA, MFI, and FAU Membranes
Table 3 shows the results of pervaporation experiments for a 10 wt% methanol/methyl
hexanoate mixture at 353 K. For all the zeolite membranes, methanol permeated selectively,
and the permeation fluxes of methyl hexanoate were less than 32 µmol m−2 s−1 . The lower
fluxes indicate that there are few large pinholes in the zeolite membranes.
Table 3. Permeation and separation performances of methanol through zeolite membranes for 10 wt%
methanol/methyl hexanoate mixture at 353 K.
Flux (µmol m−2 s−1 ) Separation Factor
Zeolite
Methanol Methyl Hexanoate (-)
CHA 13,800 24.0 1270
LTA 1070 10.5 225
MFI 1300 17.2 168
FAU(10) 86,600 31.9 6020
The FAU(10) membrane with a large pore size showed an excellent methanol per-
meation flux of 86,600 µmol m−2 s−1 (=9.9 kg m−2 h−1 ) and a high separation factor of
6020. The reason was considered that FAU-type zeolite has the largest pore size (0.74 nm)
among the tested zeolite membranes. The methanol selectively permeates through the
FAU(10) membrane by selective adsorption and molecular sieving as discussed later
(Figures 7 and 9). Because the pore diameter of the CHA-type zeolite (0.38 nm) is close to
the molecular size of methanol (0.380 nm), the separation factor of the CHA membrane
was as high as 1270. However, the permeation fluxes of the LTA and MFI membranes
were lower than 1300 µmol m−2 s−1 . The lower permeation fluxes could be attributed to
the lower adsorption capacity of the LTA-type zeolite [17] and the inhibition of methanol
diffusion by methyl hexanoate for MFI-type zeolite [18].
3.3. Effect of Si/Al Ratio of FAU Membrane
Next, the effect of the composition of the synthesis solution on the separation and per-
meation performance of methanol through the FAU membrane was investigated. Table 4
shows the effect of the SiO2 /Al2 O3 ratio on the permeation performance of the FAU mem-
branes for the 10 wt% methanol/methyl hexanoate mixture at 353 K. When SiO2 /Al2 O3 = 5,
the methanol permeation flux and separation factor were 51,900 µmol m−2 s−1 and 3890,
respectively. As the SiO2 /Al2 O3 ratio of the synthetic solution increased, the permeation
flux increased, reaching 184,000 µmol m−2 s−1 at SiO2 /Al2 O3 = 25. In contrast, a maximum
separation factor of 6020 was obtained for SiO2 /Al2 O3 = 10. The FAU(13) and (25) mem-
branes were supposed to exist pinholes since the permeation fluxes of methyl hexanoate
were two orders of magnitude higher. Especially, the FAU(25) membrane observed a
lot of particles on the surface (Figure 4), so it indicated the formation of zeolite particles
was superior to the membrane formation at SiO2 /Al2 O3 = 25, whereas the FAU(5) and
(10) membranes had few pinholes, since the permeation fluxes of methyl hexanoate were
low. By increasing the SiO2 /Al2 O3 ratio from 5 to 10, the hydrophobicity of the FAU(10)
membrane could be increased. As the result, the permeation flux of methanol through the
FAU(10) membrane was obtained 1.7 times higher than that of the FAU(5) membrane.Membranes 2021, 11, x FOR PEER REVIEW 9 of 13
FAU(10) membrane could be increased. As the result, the permeation flux of methanol
Membranes 2021, 11, 627 9 of 13
through the FAU(10) membrane was obtained 1.7 times higher than that of the FAU(5)
membrane.
Table 4. Effect of SiO2/Al2O3 ratio of synthesis solutions on the permeation performances of the FAU
Table 4. Effect of SiO /Al2 O3 ratio of synthesis solutions on the permeation performances of the
zeolite membranes for 210wt% methanol/methyl hexanoate mixture at 353 K.
FAU zeolite membranes for 10wt% methanol/methyl hexanoate mixture at 353 K.
SiO2/Al2O3 Flux (μmol m−2 s−1) Separation Factor
SiO2 /Al2 O3 Flux (µmol m−2 s−1 ) Separation Factor
Ratio (-) Methanol Methyl Hexanoate (-)
Ratio (-) Methanol Methyl29.5
Hexanoate (-)
5 51,900 3890
105 51,900
86,600 29.5
31.9 3890
6020
10
13 86,600
106,000 31.9
1520 6020
155
13 106,000 1520 155
25
25 184,000
184,000 2710
2710 150
150
3.4. Single Alcohol Permeation Performance
3.4. Single Alcohol Permeation Performance
Figure 6 shows the temperature dependence of the permeation fluxes of single-com-
Figure 6 shows the temperature dependence of the permeation fluxes of single-
ponent alcohols through the FAU(10) membrane. The methanol permeation flux was 5900
component alcohols through the FAU(10) membrane. The methanol permeation flux was
μmol m−2 s−1 at−2303 K. The permeation flux decreased as the carbon number increased, and
5900 µmol m s−1 at 303 K. The permeation flux decreased as the carbon number increased,
that of 1-butanol was 94.4 μmol m−2 s−1−at 303
− K. The permeation flux of methanol in-
and that of 1-butanol was 94.4 µmol m s 1 at 303 K. The permeation flux of methanol
2
creased with increasing solution temperature, reaching 41,400 μmol m−2 s−1−at 338 K. Eth-
increased with increasing solution temperature, reaching 41,400 µmol m 2 s−1 at 338 K.
anol and 1-propanol showed temperature dependencies similar to that of methanol. In
Ethanol and 1-propanol showed temperature dependencies similar to that of methanol. In
contrast,
contrast,the
theeffect
effectof
oftemperature
temperatureon
onthe
thepermeation
permeationflux
fluxof
of1-butanol
1-butanolwas
wassmaller
smallerthan
than
that of the other alcohols.
that of the other alcohols.
Temperaturedependency
Figure6.6.Temperature
Figure dependencyofofpermeation
permeationfluxes
fluxesofofsingle
singlealcohols
alcoholsthrough
throughthe
theFAU(10)
FAU(10)
membrane.
membrane.
Figure77shows
Figure showsthe theinfluence
influenceof ofthe
the molecular
moleculardiameter
diameteron onthe
the permeation
permeationflux flux of
of
single-component alcohols through the FAU(10) membrane at 323,
single-component alcohols through the FAU(10) membrane at 323, 333, and 348 K. The 333, and 348 K. The
permeationfluxfluxofofmethanol
methanolwas was 31,100 −2 s−1 at 333 K. The flux decreased
permeation 31,100 µmol
μmol m−2ms−1 at 333 K. The flux decreased with
with increasing
increasing molecular
molecular diameter,
diameter, and that and that of 1-butanol
of 1-butanol was 122 wasμmol122 m−2µmol m−significant
s−1. This
2 s−1 . This
significant
effect effect of
of molecular molecular
size size on theflux
on the permeation permeation fluxto
is attributed is the
attributed
molecularto the molecular
sieving func-
tion of the zeolite, even though the pore size of the FAU-type zeolite (0.74 nm) (0.74
sieving function of the zeolite, even though the pore size of the FAU-type zeolite nm)
is larger
is larger
than than the molecular
the molecular diameters diameters of the(ethanol:
of the alcohols alcohols0.430
(ethanol: 0.430 nm, 1-propanol:
nm, 1-propanol: 0.469 nm,
0.469 nm, and 1-butanol: 0.504 nm). These results also support
and 1-butanol: 0.504 nm). These results also support our claim that there our claimare
thatnothere
largeare no
pin-
large pinholes in the FAU membranes, as discussed
holes in the FAU membranes, as discussed in Table 3. in Table 3.
Because the permeation flux is described as the product of the permeance and the
partial pressure difference, the permeation flux in Figure 6 includes the temperature
dependence of the vapor pressure. To discuss only the membrane permeation phenomenon,
the permeation flux was converted to permeance. The temperature dependence of the
vapor pressure was calculated using the Antoine constant (Table 1).Membranes
Membranes2021,
2021,11,
11,x627
FOR PEER REVIEW 1010ofof13
13
Figure 7. Influence of kinetic diameter of alcohol on permeation flux of single alcohol through the
FAU(10) membrane at 323 K (circle), 333 K (square), and 348 K (triangle).
Because the permeation flux is described as the product of the permeance and the
partial pressure difference, the permeation flux in Figure 6 includes the temperature de-
pendence of the vapor pressure. To discuss only the membrane permeation phenomenon,
Figure
Figure
the Influence
7.7.Influence
permeation ofofkinetic
flux kinetic
was diameterof
diameter
converted ofalcohol
to alcoholon
permeance.onpermeation
permeation fluxofofsingle
flux
The temperature single alcoholthrough
alcohol
dependence through
of the the the
va-
FAU(10)
FAU(10)
por membrane
membrane
pressure atat323
323KK(circle),
was calculated (circle),
using 333
333 KKAntoine
the (square),and
(square), and348
348KK(Table
constant (triangle).
(triangle).
1).
Figure 8 shows the Arrhenius plot of the single-component alcohol permeance
Figure 8the
Because shows the Arrhenius
permeation flux isplot of the single-component
described as the product of alcohol
the2.7 permeanceand
permeance through
through the FAU(10) membrane. The permeance of methanol was × 10−7 −mol −2the
2 s−m1 Pas−1
−1
the FAU(10) membrane. The permeance of methanol was 2.7 × 10 − 7 mol m
partial
Pa pressure difference, the permeation flux in Figure 6 includes
−1 at 303 K, increased with temperature, and reached 4.0 × 10−7 mol m−2 s−1 Pa−1 at 338 K. the temperature de-
at 303
pendence K, ofincreased
theenergy
vapor with temperature,
pressure. and only
To discuss reachedthe 4.0 × 10−7 mol
membrane m−2 s−1phenomenon,
permeation Pa−1 at 338 K.
The activation
Thepermeation
activation flux
energy for
for permeation
permeation was 9.1 kJ
was 9.1 kJ mol mol −−1 by calculation
1 by calculation with
with the Arrhenius
the Arrhenius
the
equation. The was
permeance converted
of ethanol to permeance.
also showed The temperature
aa similar dependence
temperature of the va-
dependence to
equation.
por pressure The permeance
wasancalculated of ethanol
using theforalso showed
Antoine constant similar
(Table temperature dependence
1). −−11. However, 1-Butanol to
methanol,
methanol, with
with an activation
activation energy
energy for permeation
permeation of
of 14.1
14.1 kJ
kJ mol
mol . However, 1-Butanol
showed Figurethe 8 showstrend,the Arrhenius plot of energy
the single-component ofalcohol kJpermeance
thereverse with
withanan activation forforpermeation −44.2 mol −1. The−1
showedthe
through reverse trend,
FAU(10) membrane. The activation
permeance energy
of methanol permeation
was 2.7 of
× −−7
10 44.2
mol kJm mol
−2 s−1 .
effect
The of
effect temperature
of increased
temperature on the permeance of 1-propanol
on the permeance of 1-propanol was −7 was small (activation
small (activation energy for
Pa −1 at 303 K, with −1energy for
−1 −1temperature, and reached 4.0 × 10 mol m s Pa at 338 K.
−2 −1
permeation
permeation ==−0.9 − mol).
0.9kJkJmol ).
The activation energy for permeation was 9.1 kJ mol−1 by calculation with the Arrhenius
equation. The permeance of ethanol also showed a similar temperature dependence to
methanol, with an activation energy for permeation of 14.1 kJ mol−1. However, 1-Butanol
showed the reverse trend, with an activation energy for permeation of −44.2 kJ mol−1. The
effect of temperature on the permeance of 1-propanol was small (activation energy for
permeation = −0.9 kJ mol−1).
Arrheniusplot
Figure8.8.Arrhenius
Figure plotofof permeances
permeances of of single
single alcohols
alcohols through
through the the FAU(10)
FAU(10) membrane
membrane (the(the
ac-
tivation
activationenergy for for
energy permeation:
permeation: Methanol; 9.1 kJ
Methanol; 9.1mol −1 , ethanol;
, ethanol;
kJ−1mol 14.1 kJ14.1
molkJ mol−1 , 1-propanol;
−1, 1-propanol; −0.9 kJ
mol kJ mol−1 , 1-butanol; kJ mol−1 ).
−1, 1-butanol; −44.2 kJ mol −1).
−0.9 −44.2
The
The molecules
molecules adsorbed in the the pores
poresdiffuse
diffuseacross
acrossthe
thezeolite
zeolitemembrane
membrane according
according to
thethe
to concentration
concentrationgradient.
gradient.TheThe
adsorbed
adsorbedamountamountdecreased as theastemperature
decreased increased,
the temperature in-
while 8.the
Figure
creased, diffusion
Arrhenius
while coefficient
theplot
diffusion increased
of permeances inversely.
of single
coefficient alcohols
increased The activation
through
inversely. energymembrane
the FAU(10)
The activation for permeation
energy ac-is
(theper-
for
tivation
expressedenergy
as for
the permeation:
sum of the Methanol;
heat of 9.1 kJ
adsorption mol −1, ethanol; 14.1 kJ mol−1, 1-propanol; −0.9 kJ
and the activation energy
meation is expressed as the sum of the heat of adsorption and the activation energy for for diffusion. This
mol −1, 1-butanol; −44.2 kJ mol−1).
suggests that the permeation of methanol and ethanol with positive activation energies
was strongly influenced by diffusion, while adsorption had a significant effect on the
The molecules
permeation adsorbed
of 1-butanol. It in
is the pores diffuse
understood across the is
that 1-butanol zeolite membrane
difficult to adsorbaccording
into the
tozeolite
the concentration gradient. The adsorbed
pores, as considering the results in Figure 6. amount decreased as the temperature in-
creased, while the diffusion coefficient increased inversely. The activation energy for per-
meation is expressed as the sum of the heat of adsorption and the activation energy fordiffusion. This suggests that the permeation of methanol and ethanol with positive acti-
vation energies was strongly influenced by diffusion, while adsorption had a significant
effect on the permeation of 1-butanol. It is understood that 1-butanol is difficult to adsorb
into the zeolite pores, as considering the results in Figure 6.
Membranes 2021, 11, 627 11 of 13
3.5. Permeation and Separation Performance of Methanol from Methanol/Organic Solvent
Mixture
3.5. Table
Permeation and Separation
5 shows Performance
the influence of Methanol
of organic from Methanol/Organic
solvent species Solvent
on the permeation andMixture
sepa-
rationTable
performance of methanol
5 shows the influence through
of organicthe FAU(10)
solvent membrane.
species When methanol
on the permeation was
and separa-
mixed with ethanol,
tion performance of the methanol
methanol permeation
through flux was
the FAU(10) 14,700 μmol
membrane. Whenm −2 s−1 andwas
methanol the mixed
sepa-
ration factor was
with ethanol, the8methanol
at 348 K. permeation
These increased with14,700
flux was the carbon
µmol m −2 s−1 of
number andalcohols mixed
the separation
factor
with was 8 atthus,
methanol; 348 K.forThese increased
1-hexanol, with the carbon
the methanol number
permeation fluxofand
alcohols mixed
separation with
factor
methanol;
reached thus,μmol
51,200 for 1-hexanol,
m−2 s−1 andthe980,
methanol permeation
respectively. The flux
sameand separation
trend factor reached
was observed when
51,200 µmol −2 s−1 and 980, respectively. The same trend was observed when methanol
methanol wasmmixed with methyl esters (except in the case of methyl butyrate).
was mixed with methyl esters (except in the case of methyl butyrate).
Table 5. Influence of solvent species on permeation and separation performances of methanol through the FAU(10) mem-
brane
Tablefor
5. 10 wt% methanol/solvent
Influence mixtures.
of solvent species on permeation and separation performances of methanol through the FAU(10)
membrane for 10 wt% methanol/solvent mixtures.
Temperature Flux (μmol m−2 s−1) Separation Factor
Solvent
(K)
Temperature MethanolFlux (µmol m−2 s−Solvent
1) (-) Factor
Separation
Solvent
Ethanol (K)
348 14,700
Methanol 11,000
Solvent 8(-)
1-Propanol
Ethanol 348
348 14,400
14,700 1740
11,000 408
l-Butanol
1-Propanol 348
348 27,900
14,400 695
1740 16040
l-Butanol
1-Hexanol 348
348 27,900
51,200 695
148 160
980
1-Hexanol
Methyl acetate 348
328 51,200
231 148
4.2 510980
Methyl acetate 328 231 4.2 510
Methyl propionate
Methyl propionate
328
328
24,400
24,400
62.0
62.0
1290
1290
Methyl
Methylbutyrate
butyrate 348
348 52,300
52,300 538
538 270
270
Methyl
Methylhexanoate
hexanoate 348
348 98,200
98,200 47.0
47.0 4630
4630
Figure
Figure9 9shows
shows the influence
the of the
influence carbon
of the number
carbon of the
number of organic solvent
the organic on theonper-
solvent the
meation
permeation fluxes of methanol and solvents for the FAU(10) membrane. As mentionedinin
fluxes of methanol and solvents for the FAU(10) membrane. As mentioned
Table
Table5,5,the
thepermeation
permeationfluxfluxof
ofmethanol
methanolincreased
increasedwith
withthe
thecarbon
carbonnumber
numberofofalcohols,
alcohols,
while
while that of mixed alcohols showed the reverse tendency. Even whenmethyl
that of mixed alcohols showed the reverse tendency. Even when methylesters
esterswere
were
mixed
mixedwith
withmethanol,
methanol,the
thepermeation
permeationflux
fluxofofmethanol
methanolshowed
showeda asimilar
similarincreasing
increasingtrend,
trend,
although
althoughthe themeasurement
measurementtemperature
temperaturewaswasdifferent.
different.
Influenceofofcarbon
Figure9.9.Influence
Figure carbonnumber
numberofof(a)
(a)alcohol
alcoholand
and(b)
(b)methyl
methylester
esteron
onpermeation
permeationfluxes
fluxesofofthe
the
FAU(10)membranes
FAU(10) membranesfor for10
10wt%
wt%methanol/solvent
methanol/solvent mixturesatat328
mixtures 328KK(unfilled)
(unfilled)and
and348
348KK(filled).
(filled).
Symbols
Symbolsdenote
denotemethanol
methanol(circle)
(circle)and
andsolvents
solvents(square).
(square).
As discussed in Figure 8, methanol and ethanol molecules adsorbed on the zeolite
diffuse into the pores and permeate through the zeolite membrane according to the concen-
tration gradient. Therefore, for a mixed solution of methanol and ethanol, the permeation
of methanol was inhibited by ethanol in the zeolite pores, which reduced the permeation
flux of methanol. In contrast, the adsorption of 1-butanol into zeolite pores is important
for the permeation of 1-butanol, as shown in Figure 8. For a mixture of methanol andMembranes 2021, 11, 627 12 of 13
1-butanol, the permeation of methanol is not inhibited by 1-butanol because it is difficult
for 1-butanol to adsorb on the zeolite pores. As a result, both the permeation flux and
separation factor improved for 1-hexanol and methyl hexanoate with more carbons than
1-butanol.
4. Conclusions
For developing a methanol permselective zeolite membrane, we investigated the influ-
ence of the framework structure, composition, and organic compounds on the permeation
and separation performance of methanol through zeolite membranes in this study. First,
CHA, LTA, MFI, and FAU membranes with few pinholes were successfully prepared on
porous support tubes. Then, the permeation and separation performances of methanol
were determined by pervaporation with a 10 wt% methanol/methyl hexanoate mixture.
As a result, the FAU membrane synthesized with a composition of 10 SiO2 :1Al2 O3 :17
Na2 O:1000 H2 O showed a high permeation flux of 86,600 µmol m−2 s−1 and a separation
factor of 6020 at 353 K. Next, FAU membranes were prepared using synthetic solutions
with different SiO2 /Al2 O3 ratios to investigate the influence of the zeolite composition on
the permeation and separation performances of methanol through the FAU membranes.
The highest separation factor was obtained at SiO2 /Al2 O3 = 10, although the permeation
flux of methanol increased with a higher SiO2 /Al2 O3 ratio. The FAU membrane showed a
molecular sieving effect that reduced the single permeation flux of alcohol with molecular
size. Moreover, diffusion affected the permeation of methanol and ethanol through the FAU
membrane, while adsorption significantly affected the permeation of 1-butanol. Finally,
pervaporation experiments using the FAU membrane were carried out for several alcohols
and methyl esters. As a result, the permeation flux of methanol increased with the carbon
number of organic solvents because of the reduction in the inhibition of methanol perme-
ation by these organic solvents. These results suggest that the FAU membrane has the
potential to separate organic mixtures, such as solvent recycling and membrane reactors.
Author Contributions: Conceptualization, Y.H. and A.I.; methodology, A.I.; validation, A.I. and
Y.H.; formal analysis, Y.H. and A.I.; investigation, A.I., W.M. and C.A.; resources, Y.H.; data curation,
A.I.; writing—original draft preparation, A.I.; writing—review and editing, Y.H.; visualization, A.I.;
supervision, Y.H. All authors have read and agreed to the published version of the manuscript.
Funding: Part of this paper is based on results obtained from a project commissioned by the New
Energy and Industrial Technology Development Organization (NEDO).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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