International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
2
4
R. M. Abhang
*1
, Dr. K. S. Wani
2
, Dr. V. S. Patil
3
*1
Associate Professor and Head of Department, Department of Chemical Engineering, S.V.I.T., Chincholi,
Nashik, affiliated to the Savitribai Phule Pune University, Pune, (M.S.), India – 422101, PH:+ 91-255- 1271278.
2
Principal and Professor, Department of Chemical Engineering, S. S. B. T’s, C.O.E.T., Bambhori, affiliated to the N.M. U., Jalgaon, (M.S.), India- 425001, PH: +91-257-2258393.
#3
Professor, Department of Chemical Technology, University Institute of Chemical Technology, affiliated to the N.M. U. Jalgaon,(M.S.) India- 425001, PH: +91-257-2239060.
Abstract — Mixed matrix membranes with moderate filler loading have been shown to improve the properties of pure polymers for many gas separations.
Nano-size fillers mixed uniformly into polymer matrix would overcome the challenges of developing MMM by carefully dispersion in polymer matrix, high interfacial contact of polymer-filler even at low filler loading. The synthesized ZIF-8 in the earlier work was used as filler for MMM preparation. Its XRD pattern revealed that it was successfully synthesized.
Asymmetric neat Polyethersulfone (PES) membrane and PES/Zeolitic Imidazolate Framework-8 (PES/ZIF-
8) based mixed matrix membrane was prepared using solvent-evaporation method. The prepared membrane was coated with 2 wt% of polydimethylsiloxane
(PDMS) in n-hexane with moderate heat treatment and used for permeation of CO
2
/CH
4
. The gas permabilities of CO
2 and CH
4
by the PES/ZIF-8 MMM were increasing with increasing ZIF-8 loading at 3 bars. The incorporation of ZIF-8 at low loadings (˂
30 w/w %) improved the performances of the membrane.
While the addition of 10 w/w % ZIF-8 into polymer increased the permeability approximately twice for CO
2 and CH
4 while the ideal selectivities for
CO
2
/CH
4
gas pair showed a slight loss (~14 to 17%).
The increase in the permabilities was about 3 to 4 times, while the loss in selectivities for CO
2
/CH
4
gas pair was about approximately 16 % compared to results of pure PES membrane. For the higher ZIF-8 loadings (≥ 30 w/w %), the permabilities are increasing, but the ideal selectivities started to reduce very rapidly.
Since the most realistic increase in permabilities was observed in CH
4 but the selectivity decreases.
As a result, using nano ZIF-8 as filler into
PES matrix has revealed competent membrane for
CO
2
/CH
4
separation at moderate loading.
Keywords — Mixed Matrix Membrane (MMM),
Polyether sulfone (PES), Zeolitic Imidazole
Framework (ZIF-8), Gas separation, Permeability,
Selectivity.
I.
I NTRODUCTION of air and natural gas separation such as carbon dioxide separation and dew point adjustment etc [1].
Due to low capital cost, modest energy requirement and ease to fabricate, research on polymeric membrane has expand much attention in the last two decades. Polymeric membranes provide many advantages and its performance is studied by
Robeson‟s trade-off curve shows relation between selectivity and permeability [1].
Inorganic fillers such as carbon molecular sieve, various zeolite, carbon nanotube, activated carbon etc. posses separation properties surpass Robeson‟s tradeoff limit, were initially embedded into polymer. Still inorganic fillers shows poor interaction with polymer matrix and often lead to defective membrane.
Developing defect-free mixed matrix membranes remains major challenge [2] [3]. Membrane can be defected through particle agglomeration, un-selective voids formation, filler pore blockages and sieve-incage morphology affect its effectiveness. The Metal organic frameworks (MOFs) as potential filler due to organic linkers present in the structure have good interaction with polymer. Besides, MOFs consist of large surface area, high adsorption capacity, ease of modifications and high affinity towards certain gas, gives good potential for MOFs to be used as filler [4]
[5].
Among MOFs, zeolitic imidazole framework-8
(ZIF-8) is one of the most investigated MOFs
[6],[7],[8] and it has porous crystalline structure with
M-Im-M angle (M= metal) near to 145°, coincident with the Si–O–Si angle found in many zeolites
[7],[8],[9]. ZIF-8 has sodalite (SOD) topology with a pore size of 0.34 nm [6], [10], [8]. It has large pores of
11.6 A
0
which is two times larger than SOD zeolites.
The pores are accessible through small channels (3.4
A
0
). It exhibits thermal stability up 400
0
C and it has a
BET surface area around 1300 to 1600 m
2
/g or even more [6], [10], [11], [8]. ZIF-8 shows good chemical stability against polar and
Gas separation membranes have found many applications such as hydrogen, oxygen–nitrogen separation, vapor–vapor separation, and dehydration
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
T EXTURAL P
TABLE 1
ROPERTIES OF THE ZIF-8
MOF
Type
Pore topology
Pore diameter
(nm)
BET
Surface area m 2 /gm
Approximate
Particle size (nm)
ZIF-8
Cage/
Window
1.16 /0.34 1214 170
TABLE 2
G ENERAL PROPERTIES OF GASES CO
2
, N
2
AND CH
4
Gas
Molecular
Mass
(g/mol)
Critical
Temperature
(
0
K)
Kinetic
Diameter
(nm)
0.33 CO
2
N
2
CH
4
44
28
16
304
126
190
0.36
0.38 non-polar solvents [9], reorientation of its structure at high pressure and mechanical strength. Above table-1 shows the textural properties of the ZIF-8 [11],
[12],[8].
Another important concern about MMM is the amount of filler loading. The particle size of filler material is also an important factor in determining the gas transport properties of the mixed matrix membrane. High filler loading would provide higher penetrant-filler interaction with increase separation properties, but leads to particles agglomeration, directly reflect on membrane production cost and deteriorating its performance. In contrast, incorporating small amount of filler give irrelevant improvement on membrane separation properties, but highly unlikely for particles to agglomerate. Lowest filler loading with considerable improvement of membrane performance would be the ideal MMM [13]
[5].
CO
2
has a smaller kinetic diameter 0.33 nm compared to CH
4 gas, and much upper critical temperature compared to N
2
and CH
4
as shown in above table-2.
The lesser kinetic diameter and prominent critical temperature (higher condensability) of CO
2
support in higher diffusion rate and solubility coefficients and hence higher permeability compared to N
2
and CH
4
.
Polyethersulfone (PES) was used for membrane preparation which is a commercial polymer provided by Solvay. It is commercially attractive due to its high chemical resistance, thermal degradation and stability to oxygen. The glass transition temperature (Tg) and weight average molecular weight of the polymer (PES) are 220
0
C and 53,000, respectively [14].
Its gas transport properties lie near the upper bound line on the middle region of Robeson‟s plot for attractive gas pairs like CO
2
/CH
4
, H
2
/CH
4
.
Dimethylformamide (DMF) was used as solvent due to its strong dissolving power for many components. It has the chemical formula of C point of 153
0
C [15].
3
H
3
ON, and boiling
In order to develop high performance MMM at low to moderate filler loading, nano filler with good polymer-filler interaction is necessary. Thus, this study intend to utilize synthesized nano sized ZIF-8 as a filler to prepare mixed matrix membrane and study the influence of ZIF-8 loading on performance of gas separation. Typically the a lot of research focused on development of ZIF-8-MMM dense membrane, but the purpose of this study was to preparation of asymmetric MMM using solvent evaporation method at varied filler percentage loading of ZIF-8 to analyse the performance of CO
2
/CH
4
separation.
II.
EXPERIMENTAL
A. Materials: Zinc nitrate hexahydrate
[Zn(NO
3
)
2
.6H
2
O], methanol were obtained from
Fisher Scientific and 2-methylimidazole [C
4
H
6
N
2
] was obtained from Sigma-Aldrich (India).
Polyethersulfone (PES) [Radel A-100 grade] provided by Solvay. Polydimethylsiloxane (PDMS) was obtained from Sigma Aldrich and n-hexane and
Dimethyl formamide (DMF) were purchased from
Merk. Deionized water was obtained from Thomas
Baker. All chemicals were used as received without any further purification.
B. ZIF-8 Synthesis: The ZIF-8 crystals were synthesized [16] [17] by the room temperature synthesis method [9] with some modifications with the help of methanol and de-ionized water. In this method a solution of 3 gm of [Zn(NO
3
)
2
.6H
2
O] added in 100 ml of methanol and other solution of 6.6 gm of 2methylimidazole added in 100 ml of methanol were prepared and then mixed with each other under vigorous stirring for one hour at room temperature.
After stirring, the resulting ZIF-8 crystals were separated by centrifugation at around 10000 rpm for
10-12 minutes, followed by washing with methanol two to three times and dried under vacuum at 45
0
C for four to five hours and stored dry for further analysis and use. Similarly, ZIF-8 synthesized by via deionized water with accurate proportion of Zinc Nitrate
Hexahydrate (1.17gm) and 2-methylimidazol (22.7gm) was dissolved in 88ml de-ionized water. Synthesis methodology applied for ZIF-8 crystals is previously studied in our work [13].
C. Membrane Preparation Methodology: The morphology and the transport properties of mixed matrix membranes are strongly related to the types of polymer, solvent, filler material and the additives used in fabrication [18]. Solvent-evaporation method was used for preparation of the membranes. PES and ZIF-8 were dried at 80
0
C and 180
0
C overnight before using in the membrane synthesis. Two different types of membranes were prepared in this study, pure PES,
PES/ZIF-8 with different ZIF-8. PES concentration in
DMF, 20 w/v %, was kept constant for all membranes.
Asymmetric flat sheet neat membrane was prepared by casting solution consisted of Polyethersulfone (PES)
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016) and DMF. Overnight dried PES was added into the solvent DMF step by step in order to prevent a sudden increase in the viscosity of solution and ease of stirring. Then, the solution was stirred for overnight by a magnetic stirrer. Casting process was performed by hand-casting at ambient atmosphere. Asymmetric flat sheet MMM was prepared by overnight dried ZIF-
8 was dispersed in the solvent DMF in three or four steps according to the amount of ZIF-8. Between each two steps, the solution was ultrasonicated for 20 to 30 min in order to ease the dispersion and minimize the agglomeration of ZIF-8 particles in the solution. After completing the ZIF-8 addition, PES was primed by adding 15 wt % of the total amount so as to increase the compatibility between ZIF-8 and PES and the solution was stirred for overnight by a magnetic stirrer.
Then, remaining amount of PES was added to the solution in three or four steps with 20 to 30 minute ultrasonication in between the steps and again the solution was stirred for overnight. While the PES concentration was kept constant, the ZIF-8 contents in the membranes were varied between 10- 30 w/w % and the compositions of prepared membranes [19] [20]
[21]. After that, membranes were dip-coated for 10 to
12 minutes in 2wt% PDMS in n-hexane to seal possible pinhole on membrane surface. Then, membrane undergoes “curing” at 50°C overnight.
Gas permeation tests were performed with a designed membrane permeation cell by using carbon dioxide (CO
2
), and methane (CH
4
). Circular membrane discs with an effective permeation area of
7.069 cm
2
were used. Feed pressure was maintained at
3 bar while permeate side was opened to atmosphere.
Permeability „P‟ of 1 barrer corresponding to 10 -10 related to cubic centimetres per second (volume at
STP) was calculated by using following equation:
Where „i‟ represent the gas component i, Vi is gas volume permeated through the membrane (cm
„A‟ the effective membrane area (cm 2 pressure drop (cmHg) [10]. The unit of permeability usually is Barrer, where,
III.
THEORY
Selectivity was obtained using Equation (2):
3
, STP),
), „t‟ is the time of permeation (s) and „Δp‟ is the transmembrane
Where, x i
and y i
are mole fractions of component „i‟ in the gas mixture in the feed and permeate sides respectively.
IV.
RESULTS AND DISCUSSION
The X -Ray Diffractometer (XRD) [9], [19] were used to confirm the phase purity of ZIF-8 and results found in our earlier literature [13]. XRD is a non destructive analysis to measure wavelength of sample and to identify structure. The XRD will release X-rays to the sample and the X-Rays diffracted at different angles and intensity by CuKa irradiation with a wavelength (λ) 1.54 A 0
at room temperature. The schematic graphical representation of synthesized ZIF-
8 crystalline powder by room temperature synthesis method and synthesis procedure of ZIF-8 crystals [15] by using de-ionized water and by using methanol has been studied in our earlier work [13].
The single gas permeability‟s of pure PES and
PES/ZIF-8 MMM are presented together in figure-1 and shows that, the single gas permeability of the
PES/ZIF-8 MMMs are increasing with increasing ZIF-
8 loadings.
Especially, with the addition of 30 w/w % and higher ZIF-8 nano-crystals, the raise in the permeability was very strong and the highest increase was in the permeability of CH
4
. In literature, there are similar increasing permeability trends with increasing loadings of nano-size filler materials. However, for the higher percentage of ZIF-8 nano-crystals, the gas permeability‟s were reduced. This trend was claimed as the result of reduction in the amount of polymer for gas transport, increase in the diffusion path length for the gas penetrants, and reducing free volume in the membrane due to increasing density. The increase observed in the permeability values with increasing
ZIF-8 loading may be due to the enhanced free volume and ZIF-8 –polymer interfaces that the gas molecules can cross through the membrane.
The selectivity‟s of the PES/ZIF-8 MMM for
CO
2
/CH
4
gas pair are represented in table-3. The incorporation of ZIF-8 at low loadings (˂ 30 w/w %) improved the performance of the membranes.
By the addition of 10 w/w % ZIF-8 into polymer, increased the permeability performance about two times for gases CO
2 and CH
4, while the ideal selectivity for
CO
2
/CH
4
gas pair showed a slight loss (~14 to 17%).
The increase in the permeability‟s was about 3 to 4 times while the loss in selectivity‟s for CO
2
/CH
4
, gas pairs was about approximately 16 % compared to results of pure PES membrane.
For the higher ZIF-8 loadings (≥ 30 w/w %), while the permeability‟s are increasing, the ideal selectivity‟s started to reduce very rapidly.
Since the most sensible increase in permeability‟s were observed in CH
4 but the selectivity decreased CO
2
/CH
4
for all membranes.
In general the ideal selectivity‟s are improved with the addition of filler into glassy polymers up to 25% and
30% loadings [22].
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
TABLE 3
P ERMEABILITY AND SELECTIVITY OF NEAT
PES AND PES/ZIF-8 AT 10, 20, 30 % LOADING
PERCENTAGE
Permeability Selectivity
CO
2
/ CH
4
Membrane
Type
Neat PES
10% ZIF-8
20% ZIF-8
30% ZIF-8
CO
2
4.4
8.6
14.7
23.8
CH
4
x10
2.4
3.2
6.6
16.8
35.6
30.8
29.6
23.3 types of membranes used, the feed pressure is the other important parameter appreciably affected the separation performances for gas pair. Thus, the MMM prepared with moderate nano-filler loading shows that there will be great potential to be further improvement and various applications in the gas and vapour separation.
A CKNOWLEDGMENT
25
20
15
10
5
0
0
Fig.1. Effect of percentage ZIF-8 loading on single gas
Permabilities of PES–ZIF-8 MMM at 3 bar 30
0
C
40
35
30
25
20
15
10
5
0
0
10
Percentage of ZIF-8 loading
Fig.2. Effect of percentage ZIF-8 loading on selectivity
CO
2
/CH
4
of PES–ZIF-8 MMM at 3 bar 30
0
C.
V.
Percentage of ZIF-8 Loading
CO2
CH4
10
20
20
CONCLUSION
30
30
40
40
The incorporation of synthesized ZIF-8 crystals into continuous PES matrix resulted in high performance gas separation membranes at good dispersion of fillers and high improvement of permabilities with considerable ideal selectivity. The permeability of gases increased with ZIF-8 loading, while the ideal selectivity showed a slight decrease compared to neat PES membrane. The addition of 20 to 25 w/w % ZIF-8 was selected as optimum filler loading for membrane formulation considering the permeation performances at 3 bar pressure. For all
The authors of this research paper would like to thanks the University Institute Chemical Technology,
N.M.U., Jalgaon, (M.S.), India and the Department of
Chemical Engineering, S.S.B.T.‟s, C.O.E.T., Jalgaon, affiliated N.M.U., Jalgaon, (M.S.), India and
Department of Chemical Engineering S.V.I.T.,
Chincholi, Nashik, affiliated to S.S.P.U. Pune, (M.S.),
India, for their technical and research facilities.
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