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Sorption and diffusion of VOCs in DAY zeolite and silicalite-filled PDMS membranes
iournalor MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 133 (1997) 231-243
Sorption and diffusion of VOCs in DAY zeolite and silicalite-filled PDMS membranes •
a
M . V . C h a n d a k a, Y.S. L l n ' a
•
, W . Ji b, R . J . H i g g i n s b
Department of Chemical Engineering, University of Cincinnati, ML 171, CincinnatL OH 45221-171, USA b CeraMem Corporation, 12 Clematis Avenue, Waltham, MA 02154, USA Received 31 January 1997; received in revised form 31 March 1997; accepted 2 April 1997
Abstract The sorption and diffusion properties of ethanol, 1,1,l-trichloroethane (TCA) and trichloroethylene (TCE) were determined in silicalite-filled and dealuminized-Y-zeolite (DAY)-filled poly[dimethylsiloxane] (PDMS) membranes at 25, 100 and 150°C. Zeolite filling results in increased solubility coefficients (S) for polar solvents like ethanol over pure PDMS. No significant increase in S is observed in case of TCA and TCE which act as good solvents for PDMS. However, at higher temperatures, the sorption is higher in zeolite-filled membranes even for the good solvents. The VOC diffusivity decreases with increasing degree of zeolite filling because of higher characteristic diffusion time in zeolites (for ethanol) and increasing tortuosity of the diffusion path (for TCA). Due to the presence of carbon=carbon double bond, TCE exhibits marginal diffusivity drop in zeolite-filled membranes. The specific zeolite-polymer interactions, that is, tendency of zeolite pore blocking by polymer chains or the formation of voids on zeolite-polymer interface are influenced by the zeolite pore size and type of VOC permeating through the composite membrane. The variation in experimentally observed ethanol permeability due to zeolite filling could be qualitatively estimated from the sorption-diffusion data.
Keywords: DAY zeolite; Diffusion; Silicalite; Sorption; VOC control; Zeolite-filled polymeric membranes
1. Introduction Certain types of dense polymeric membranes offer good potential for selective VOC removal using vapor permeation or pervaporation processes due to their high permeability. However, their current limitations arise from their limited selectivities for organics over air or water. Addition of an adsorptive filler to the
*Corresponding author. Fax: +1 513 556 3473; e-mail: jlin @alpha.che.uc.edu. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0376-73 88(97)00082-3
polymeric membrane has been demonstrated to be an effective way to improve membrane performance by enhancing membrane sorption capacity for one of the compounds to be separated. Silicalite-filled poly[dimethylsiloxane] (PDMS) membranes were first applied for combined pervaporation and fermentation of alcohol-water mixtures in a membrane bioreactor by Hennepe et al. [1]. Both selectivity and permeability of silicone rubber membranes were enhanced by the incorporation of silicalite during pervaporation of ethanol/water mixtures [2-4]. This was due to the lower water sorption capacity of
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M.V. Chandak et al. /Journal of Membrane Science 133 (1997) 231-243
silicalite. Also, the alcohol could diffuse through both zeolite and polymer phases, while water had to follow a more tortuous path due to the hydrophobicity of silicalite. These membranes have also been applied for gas separation applications involving N2, 02, He, H2, CH4, CO2, n-C4Hlo, i-C4Hlo, 1-Call 8 and Ar [5-8]. Apart from PDMS, various other types of membranes have been investigated for filling, including polyetherblock-polyamides (PEBA)/poly[urethane] (PUR)/ silicone-poly[carbonate] copolymer (SPC) [9], PDMS-acrylate, poly[vinyl alcohol] (PVA) [4,8,10] ethylene-propylene rubber (EPDM)/poly[chloroprene] (PCP)/nitrile-butadiene rubber (NBR) [6] and poly[ethersulfone] (PES) [7]. These studies investigated microporous adsorbent fillers such as zeolites, carbon molecular sieves, activated carbons and carbonaceous adsorbents. The aim was to choose fillers with selectivity toward the same molecules as the unfilled membrane. Most of the studies in literature are limited to alcohol/water pervaporation or removal of liquid phase chlorinated organics from aqueous solutions. Very few studies have dealt with vapor permeation properties of organics in zeolite-filled polymer membranes. The vapor phase investigations of sorption and permeation of chlorinated solvents in PDMS were restricted to highlight the effect of number of chlorine atoms, degree of branching and molecular weight on the sorption and diffusion behavior of these chlorinated organics in silicalite-filled PDMS membranes [111. The application of zeolite-filled membranes for vapor permeation VOC control processes has recently gained attention. In view of the solution-diffusion mechanism for permeant transport, ideal fillers for preparation of VOC-selective polymer membranes should have hydrophobicity and high sorption capacity for organic compounds and fast diffusion of the organic molecules within the filler. Zeolites have been used extensively in large scale adsorption and catalytic processes. They can be used as adsorptive fillers for improving membrane properties because of their unique crystalline microporous pore structure, surface chemistry and mechanical strength. Among many synthetic zeolites, only two zeolites are hydrophobic, silicalite and DAY, both of which are pure crystalline silica molecular sieves. Silicalite is a dealuminized
form of ZSM-5 zeolite. DAY zeolite is a DeAluminized form of Y zeolite. These zeolites were found to be thermally stable and acid resistant up to a temperature of 1000°C [12]. The adsorption and diffusion properties of VOCs in these two zeolites have been compared and reported elsewhere [13]. It was shown that DAY zeolite was a better adsorbent for VOCs compared to silicalite. DAY zeolite exhibited higher equilibrium uptake capacity, and faster kinetics compared to silicalite. This was attributed to the larger pore volume and pore size of DAY zeolite. However, silicalite was the only hydrophobic zeolite investigated as a filler in all the studies reported in literature. There has been no reported study on DAY zeolite-filled polymeric membranes. Hence, in view of its superior adsorptive and diffusion properties, the application of DAY zeolite as a filler in PDMS membrane merits investigation. Also, none of the studies on zeolite-filled membranes have investigated VOC sorption and diffusion in zeolites and pure polymers independently. In a recent study reported elsewhere, the sorption and diffusion properties of VOCs were investigated in pure PDMS membranes [14]. PDMS had good sorption capacity for trichloroethane (TCA) and trichloroethylene (TCE), which act as good solvents. However, much lower sorption capacity was observed for ethanol due to its polar character and clustering tendency. Also, DAY zeolite and silicalite were compared for their VOC adsorption and diffusion properties [13]. The present work is a continuation of these two investigations and utilizes the experimental findings reported therein. In the present study, the properties of sorption and diffusion of VOCs in zeolite-filled PDMS membranes were measured at 25, 100 and 150°C. Silicalite and DAY zeolite are compared as fillers. The three VOCs chosen are ethanol, 1,1,1-trichloroethane (TCA) and trichloroethylene (TCE). The effect of VOC type, zeolite type and degree of zeolite filling on the VOC sorption and diffusion properties are determined. The deviations between the actual and predicted sorption are used to quantitatively determine the polymer-zeolite interactions. The effect of zeolite pore size and VOC type was considered on these interactions. The variation in ethanol permeability with degree of zeolite filling is estimated using
233
M. V Chandak et al./Journal of Membrane Science 133 (1997) 231-243
sorption-diffusion data and compared to experimental permeation data.
3. Results and discussion 3.1. Sorption uptake
2. Experimental 2.1. Zeolites and polymer
The zeolites, DAY zeolite and silicalite, were procured from Degussa Corporation and UOP, respectively. The zeolite crystal sizes were determined to be 1 lam [12]. Silicalite has a pore size of 5.4 ~, [15] whereas DAY zeolite has a pore size of 7.4 ~, [16]. DAY zeolite has larger pore volume (0.30 ml/g) and surface area (800m2/g) than silicalite (0.19 ml/g, 450 m2/g, respectively). Two-part silicone rubber from General Electric was selected as the polymer phase in the zeolite-filled membranes. Rubber A (10 parts), cross-linker B (1 part) and the zeolite were mixed thoroughly. The membranes were cast on a glass plate from the resulting homogeneous paste consisting of silicone rubber and the zeolite. After curing at room temperature overnight, the membranes were placed in an oven at 80°C for 5-6 h to assure complete cross-linking. Filled PDMS membranes containing 20 and 40 wt% silicalite and DAY zeolite were synthesized. Typical thickness of the zeolite-filled membrane is 100 tam.
The ethanol isotherms on pure PDMS, 20 wt% silicalite/PDMS and 40 wt% silicalite/PDMS membranes are given in Fig. 1 at 25°C. The molar uptake (Mp) is plotted against the VOC activity al defined as PIPs. P is the VOC partial pressure and Ps is the saturated vapor pressure. The effect of the zeolite filler is immediately seen in the form of increased ethanol molar uptake by the membrane. This increasing molar uptake (Mp) automatically translates into increasing solubility, that is, VOC volume fraction (~b 1 = MpPmV1)and solubility coefficient (S = c/p). Pm is the membrane density, VI the VOC liquid molar volume and c the VOC uptake (cm 3 (STP)) per unit volume of membrane. The relative increase in sorption is greater at lower partial pressures of ethanol, as this corresponds to the region where the zeolite is not saturated. At higher activities, the zeolite tends to saturate. Hence, the fractional improvement in sorption over pure PDMS decreases. Similar isotherms were observed for DAY zeolite-filled PDMS membranes. This is consistent with the observations of earlier investigators that
5 2.2. Experimental procedure
The VOCs, l,l,l-trichloroethane (Fisher, certified grade), trichloroethylene (Fisher, certified ACS grade) and ethanol (Aaper, absolute - 200 proof) were procured and used without further treatment. The sorption and diffusion studies were carried out gravimetrically in a modified CAHN C-1000 micro-balance. A membrane film of 10-20 mg was loaded in one ann of the balance and activated by heating to 175°C. The membrane was exposed subsequently to increasing partial pressures of pure phase VOC vapor. At each VOC activity, the transient and equilibrium weight changes of the membrane samples were measured and recorded for analysis. The entire range of VOC partial pressures was covered. The experiments were conducted at 25, 100 and 150°C. The details of the experimental setup and procedure have been reported elsewhere [14].
I
ethanol@25 °C
~4 ~3
PDMS
/
, 20%sil
E 1
0.0
0.2
0.4
0.6
0.8
1.0
activity(a~) Fig. 1. Ethanol isotherms on silicalite-filledPDMS membranesat 25°C.
234
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
5=
trichloroethylene / @25C /
04
PDMS
"--
~ 20% DAY
|
/ /
0
E 1 0
~-'
0.0
' I ......
0.2
,
' ' ' ,' . . . . .
0.4
0.6
, ....
0.8
1.0
activity (at) Fig. 2. Trichloroethylene isotherms on DAY zeolite-filled PDMS membranes at 25°C.
the zeolite filler acts as an adsorptive reservoir and increases the solubility of alcohols in PDMS. In case o f TCE, however, the zeolite fails to increase the sorption capacity appreciably, as seen in Fig. 2 for
DAY z e o l i t e / P D M S . The increase in uptake over pure PDMS is very low and confined to low activities o f VOC. Similar trends were observed in case of T C A on both DAY and silicalite-filled PDMS and are not shown for avoiding repetition. T C A and TCE are good solvents for PDMS, hence, the zeolite filler does not increase their uptake to the same extent, as in case of ethanol. In fact after a critical VOC partial pressure, the uptake in zeolite-filled membranes drops below that o f pure PDMS. This observation will be discussed in more detail later. Typical isotherms measured at 25, 100 and 150°C are given in Fig. 3 for T C A sorption on 40 wt% silicalite/PDMS. The membrane sorption capacity drops with temperature, though less than that observed earlier in pure PDMS [14]. Solubility coefficients (S) were calculated from the VOC sorption isotherms and the average solubility coefficients at the three temperatures are reported in Table l(a). At each temperature, five values of the solubility coefficient were calculated at VOC activities, a t - 0 , 0.2, 0.4, 0.6, 0.8 and 1.0. The average of heat of adsorption (AHs) values corresponding to these activities are reported in Table 1. The average solubility coefficient at 25°C for ethanol in zeolite-filled P D M S increases from 2.69 in pure
/
trichloroethane on
._. 4 7
40 % silicalite/PDMS o 2 o0 , 100oc
E3
o
.¢
o 150 °C
1
0
-
0.0
0.2
0.4
0.6
activity (at)
0.8
1.0
Fig. 3. Effect of temperature on trichloroethane isotherms on 40% silicalite/PDMS membranes.
M. V. Chandak et al. /Journal of Membrane Science 133 (1997) 231-243
235
Table 1 Average VOC solubility coefficients and heat of sorption in zeolite-filled membranes a: Solubility coefficient (S) cm3(STP)/cm 3 cmHg VOC Ethanol
TCA
TCE
T (°C) 25 100 150 25 100 150 25 100 150
PDMS 2.69 1.77 1.02 4.15 0.29 0.08 7.33 0.38 0.07
20% DAY 6.77 3.76 3.48 5.10 0.50 0.30 7.20 0.31 0.10
40% DAY
20% Sil.
40% Sil.
24.70 23.90 16.10 3.03 0.52 0.31 5.71 0.90 0.20
10.23 4.47 3.83 4.15 0.92 0.54 10.21 2.54 1.17
14.11 4.43 3.00 4.63 1.03 0.70 9.30 4.36 1.31
b: Heat of adsorption (-zSd-/s)
Ethanol TCA TCE
PDMS
DAY
silicalite
20% DAY
40% DAY
20% Sil.
40% Sil.
6.62 32.58 35.37
28.87 26.78 31.38
18.41 9.20 23.85
5.82 25.39 38.69
3.10 24.82 30.03
8.51 17.24 17.98
13.14 16.20 15.29
PDMS to 6.77 in 20 wt% DAY and 24.7 in 40 wt% DAY-filled PDMS. Similar but slightly lower increase was found for silicalite-filled PDMS. The higher increase in S for DAY zeolite-filled membranes compared to silicalite can be attributed to the higher pore volume of the former. The zeolites act as sorptive fillers in increasing the ethanol solubility coefficient. However, the increase in S for TCA, a good solvent for PDMS, is marginal. The variation in average solubility coefficient is only from 4.15 for pure PDMS to 5.1 in 20 wt% DAY and 3.03 in 40 wt% DAY. In fact for both good solvents, TCA and TCE, the solubility tends to be lower for 4 0 w t % DAY/PDMS compared to 20 wt% DAY/PDMS. This phenomenon will be discussed later. At higher temperatures, however, the solubility coefficient in filled membranes tends to be higher than that in pure PDMS even for the good solvents. TCE solubility coefficient at 100°C increases from 0.38 in case of pure PDMS to 2.54 in 20 wt% silicaIite/PDMS to 4.36 in 40 wt% silicalite/PDMS. This was probably due to the better effectiveness of the zeolite filler at higher temperatures. Hence, the values of heat of adsorption for the filled membranes remain within those for pure polymer and pure zeolite or falls slightly lower than either of them as seen in Table l(b).
3.2. Cluster function analysis The Zimm and Lundberg clustering integral function (G) enables sorption characteristics to be determined in terms of the degree of clustering of the sorbed molecules [17]. The clustering tendency of only ethanol is considered in zeolitefilled membranes as the increase in ethanol sorption is maximum due to zeolite filling. As seen in Table 1, zeolite filling does not affect TCA and TCE sorption to the same extent. Also, these VOCs did not show appreciable clustering tendency in pure PDMS [14]. Fig. 4 compares the Zimm plots for ethanol sorption in pure PDMS and silicalite-filled PDMS membranes. Ethanol demonstrates significant clustering tendencies in pure PDMS (G >> - 1 in Fig. 4) [14]. Zeolites have a strong influence in ethanol sorption. For silicalite-filled membranes, the value of cluster function remains < - 1 for almost the entire range of ethanol activity, which suggests strong immobilization of ethanol in the filled membranes [17]. In this case, the immobilization is caused by the zeolites, which act as adsorptive fillers. Similar cluster function trends were observed in case of DAY zeolite-filled membranes. Thus incorporation of zeolites brings about a radical change in the sorption mechanism of polar
236
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243 400 •
300
ethanol @ 25 C
"
---•--- P D M S
*
A
ov m
.20% silicalite
---e-- 40% silicalite
200 0')
..E= 100
U
.
-*--.
..
t
~
i I I I I I I l l l l
i
b
''" - a -'~°°° l
l
l
[
l
l
~
~ . 8
l
~
-
10
-1 O0
activity (al)
Fig. 4. Zimm plot for ethanol adsorption in PDMS and silicalite-filled PDMS membranes.
molecules like ethanol which show a clustering tendency in PDMS.
3.3. Diffusivity analysis The results of the diffusivity calculations for filled membranes are reported in Table 2. The plane sheet model [18] was used for calculation of diffusivities in zeolite-filled PDMS as in case of pure PDMS [14]. This is because similar uptake curves were observed in the two cases. Hence, the reported values can be considered as effective diffusivities in the zeolitefilled membranes. The diffusivities of the VOCs in zeolites (Dz) were found to be ,,~10-i°cmZ/s for DAY zeolite and ,,~10-12 cm2/s for silicalite [13]. In pure PDMS membranes, the diffusivities (Dp) were ,-~10 - 6 c m 2 / s [ 14]. Hence, the diffusivities in pure PDMS membranes are ~ 1 0 4 times the diffusivities in zeolites. However, the important parameter to be considered is the characteristic diffusion time. ~'z = r2/Dz is in the range of about 102 and 10as for DAY zeolite and silicalite, respectively, rc is the nominal zeolite crystal size. For PDMS polymer, the characteristic diffusion time is *p = 412/Dp which is in the range of 50 and 125 s. 21 is the membrane thickness. The diffusion time in
zeolites, ~-z, is 2-80 times larger than ~-p. Hence, the VOC diffusivity in filled membranes decreases compared to pure PDMS as denoted by the values in parenthesis. The lower ethanol diffusivities in zeolite-filled membranes reported in Table 2 are due to immobilization of ethanol in the zeolite pores as suggested by the cluster integral function in Fig. 4. Hence, the diffusivity decreases sharply with increasing zeolite content of the filled membrane. The drop is greater for silicalite-filled PDMS membranes compared to DAY zeolite-filled PDMS membranes. This is due to the smaller pore size of silicalite compared to DAY zeolite. This highlights the importance of diffusion through the zeolite pores in the overall permeation of ethanol through zeolite-filled membranes. TCA and TCE are good solvents for PDMS itself. Filling zeolites does not appreciably increase their sorption in filled membranes. Hence for TCA, the drop in diffusivity is due to lower diffusivity in the zeolite phase and also due to the increasing tortuosity of the TCA diffusion path through the polymer network in the filled membrane. The drop in diffusivity is almost identical for 20% silicalite and 20% DAY membranes, implying that the transport through the polymer phase plays a more dominant role in the overall permeation.
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
237
Table 2 VOC diffusivities and diffusional activation energies in zeolite-filled membranes a: Diffusivity (D) x 108 cm2/s VOC Ethanol
TCA
TCE
T (°C) 25 100 150 25 100 150 25 100 150
PDMS 151.5 156.6 359.1 323.2 749.1 1292.8 143.6 610.5 725.4
20% DAY 50.8 (0.34) a 62.9 98.3 122.9 (0.38) 304.9 336.9 118.0 (0.82) 242.0 340.1
b: Diffusional activation energy (ED) kJ/mol PDMS DAY
silicalite
Ethanol TCA TCE
15.48 17.57 15.48
6.15 11.59 14.21
8.79 12.97 17.57
40% DAY
20% Sil.
40% Sil.
4.5 (0.03) 4.8 8.5 13.3 (0.04) 24.1 41.4 30.2 (0.21) 33.4 120.7
13.0 (0.09) 27.1 27.7 118.3 (0.37) 162.0 388.8 68.0 (0.47) 187.3 337.0
1.4 (0.04) 2.1 2.4 23.1 (0.07) 58.8 78.4 103.9 (0.72) 198.0 223.4
20% DAY
40% DAY
20% Sil.
40% Sil.
6.74 9.00 13.23
4.25 10.40 6.64
5.06 8.86 8.83
4.59 7.31 9.95
a Values in parenthesis denote ratio of VOC diffusivity in filled membranes to pure PDMS~
This also suggests partial blockage of DAY zeolite pores by polymer, making its effective pore size smaller. This will be discussed in more detail later. In case of TCE, also a good solvent, the diffusivity does not drop much in the filled membranes. TCE has a C=C double bond, and there is a tendency of rotation about the double bond which increases the mobility of TCE molecules. Earlier, it was observed that unlike TCA and ethanol, TCE had identical diffusivities in DAY and silicalite, despite the smaller pore size of silicalite [13]. Hence, TCE diffusivity does not drop much in zeolite-filled membranes, due to the higher mobility of TCE and its flexible structure. The diffusional activation energies (ED), calculated from the diffusional data at different temperatures using the Arrhenius plots, are summarized in Table 2(b). The calculated VOC diffusional activation energies in PDMS [14] and zeolites [13] are also included for comparison. The diffusional activation energy in zeolite-filled membranes decreases compared to pure PDMS for all VOCs. Thus the diffusion process becomes less activated due to zeolite filling. For TCA and TCE, zeolites increase membrane solubility coefficients at higher temperatures as seen in Table 1. Hence, TCA/TCE diffusivity in zeolite-filled membranes is restricted at higher temperatures due to immobilization in zeolite pores, resulting in lower
activation energies. Ethanol has a lower ED value compared to other VOCs probably due to the effect of clustering, which is a weak function of temperature.
3.4. Prediction of filled membrane sorption isotherms In the filled membranes, the zeolite crystals of size rc = 1 ktm are embedded in the polymer network, which has a typical thickness o f 2 / = 100 ~tm. Assuming the polymer and the zeolite phase do not influence each other, the VOC molar uptake in zeolite-filled membranes can be estimated from that of pure PDMS and zeolite uptake at any particular VOC activity, al, by the following equation, Mf (mmol/g) = Mp × (100 - f ) ( m m o l / g ) + Mz x f ( m m o l / g )
(1)
w h e r e f i s the degree of filling by wt% of zeolite. Mp, Mz, and Mf are the molar uptakes of VOC per gram by the PDMS, zeolite and filled membrane, respectively. Mz and Mp values were taken from earlier studies [13,14]. These calculated adsorption capacities, Mr, were compared with the actual filled membrane sorption isotherms. If there is no discrepancy, it implies that the polymer and zeolite sorption capacities are
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
238
8
5
i
trichloroethane@ 25 C
6 E Q
m
A(:tua120 % sil
O
Actual 40% sil
1
3.4.1. TCA/TCE ..-
Cale 40% sl
. . . . . . .
3
oO.
'
0
__
e.Q.-
"~
.o " '°"
0
~'~-'
0.0
,
,
i
I
I
' I ....
0.2
,
,
i
i
,
I ....
Figs. 5 and 6 demonstrate the actual data for TCA and TCE adsorption in 20 and 40% silicalite/PDMS. For TCA sorption, the solid and dotted lines in Fig. 5 give the estimated adsorption amount calculated from pure PDMS [14] and zeolite data [13] using Eq. (1). At any given VOC activity al, the sorption data of the filled membrane can be estimated within experimental error by adding the pure zeolite and polymer adsorption amounts. This indicates the zeolite and the PDMS polymer adsorb the VOCs independently. The zeolite pores are not blocked by the polymer and the polymer swelling is not affected by the zeolite particles. Figs. 7 and 8 show the estimated and actual adsorption data for TCA and TCE adsorption in 20% DAY and 40% DAY/PDMS. For TCA sorption, the uptake by 20% DAY/PDMS can be accurately estimated. However, for 40% DAY, the actual uptake is lower than estimated. In case of TCE, however, both the 20 and 40% DAY/PDMS membranes give lower than expected sorption uptake as shown in Fig. 8. Thus, either the polymer phase or the zeolite phase does not adsorb as expected. It is proposed that the PDMS polymer chains enter and block the larger DAY zeolite
E
."
~n
¢'~
mmol/g of VOC, but it can be qualitatively extended to VOC solubility coefficient as well.
,
0.4
1 ~"
,
i
i
i
~ .....
i
0.6
i
i
,
,
, ....
i
,
0.8
0
.0
activity at Fig. 5. E s t i m a t e d a n d actual T C A adsorption in 20 a n d 40 w t % silicalite/PDMS.
additive and they do not influence each other due to zeolite filling. However, any discrepancy between the two quantities can be utilized to understand the nature of the interface and interaction between the zeolite particles and the polymer network. The following discussion is based on sorption uptake (Mr) in 5
4
trichloroethylene @ 25 C _~
4
.i .//
n Actua,2o%s,,
A
.¢
3 ~)
,'/O O
,,
Actual 40% sil
-~ " 2 •"~
.t'~//
c.,o , o ~ . ,
.......
m
.-7
.- a .,"
o
E
~"
2"~ " :~
"°'" 6 0.°-
0
'
0.0
'
'
1 . . . .
0.2
1 . . . .
0.4
I
. . . .
0.6
1 . . . .
0.8
0
1.0
activity al Fig. 6. Estimated a n d actual T C E adsorption in 20 a n d 40 w t % s i l i c a l i t e / P D M S .
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
5
5
trichloroethane @ 25 C
J
[3/
3 .......~~ c aDlAYc"' . ."~. . ."'I~ 4"""o/' '/~"° % °o 2 "~.~c~ ~
3 ~ ~2
Actual 40% DAY
0
e~l
/~"
i
......
1'~
..''" / n .°'/,~/_
0
239
0
13 _ ^
O
_ _ . ,
O
0
O
..............
0
0.0 0.2 0.4 0.6 0.8 1.0 activity al
Fig. 7. Estimated and actual TCA adsorption in 20 and 40 wt% DAY/PDMS.
6
trichloroethylene @25C
"~5 "6
[]
~4
--
Cale20% DAY
o
Actual 40% DAY
.......
"
"
Cal¢ 40 % DAY
"
I~i 1
~
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"~
3
."
. /
,=2
o
:' 4
Actual 20% DAY
.~¢
o
5
E
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oOO
[]
1
°..°-'"°
"f,°,~,
,. . . . . . . . , . . . . . . . . , . . . .
,
0
0.0 0.2 0.4 0.6 0.8 1.0 activity at
Fig. 8. Estimated and actual TCE adsorption in 20 and 40 wt% DAY/PDMS.
pores. Thus, the DAY zeolite crystals embedded in the polymer network cannot adsorb the same amount of VOC as it would otherwise, resulting in lower than expected solubility. TCA and TCE are good solvents for PDMS. With increasing VOC activity and sorption, the polymer
swells as it contributes more to the total sorption. The polymer chains reorient and thus enter and block the larger DAY zeolite pores. Thus the effective diameter of DAY zeolite pores is lowered. The almost identical drop in TCA and TCE diffusivities observed in DAY and silicalite-filled membranes as seen in Table 2
240
M.V. Chandak et al./Journal ¢~ Membrane Science 133 (1997) 231-243
validates this hypothesis. For TCA, the deviation between actual and estimated sorption is lower for 20% DAY, due to reduced tendency to block the pores because of fewer number of zeolite particles in the polymer network. Silicalite pores are smaller than DAY zeolite pores, and thus, the PDMS chains cannot enter and block the pores. Hence, for both TCA and TCE, very good agreement was found between estimated and actual silicalite-filled membrane sorption amounts. Vankelecom et al. investigated the effect of filling of ZSM-5 and Y zeolite on the physical properties of PDMS membranes [19]. They reported increase in tensile strength of PDMS membranes due to incorporation of Y zeolite. However ZSM-5 decreased the tensile strength of the PDMS membranes. This was explained on the basis of partial invasion of the polymer chains in to the Y zeolite framework. The authors suggested that Y zeolite acted as a physical 'cross-linker', thus increasing the tensile strength of the membrane. The pore structure of Y zeolite and DAY zeolite are similar. Hence, it is proposed that DAY zeolite similarly reinforces the PDMS network, due to blockage of the pores by the polymer chains. This reduces the sorption capacity of both zeolite and PDMS polymer for TCA and TCE, respectively. Thus, the results of the gravimetric sorption experiments in this study are compatible with the physical tensile strength measurement investigations reported in literature. Blume et al. reported that the toluene sorption in pure PDMS was inversely proportional to the degree of cross-linking [20]. The addition of DAY zeolite increases the amount of cross-linking and thus decreases polymer sorption capacity.
3.4.2. Ethanol The actual and estimated ethanol uptakes by 20 and 40 wt% silicalite/PDMS are shown in Fig. 9. At low VOC activities, the actual sorption by filled membranes is close to estimated. However, as the VOC activity increases, the uptake by the filled membrane is higher than estimated. This could be due to formation of voids on the zeolite-polymer interface. The presence of voids in case of ethanol sorption can be explained by considering the lack of affinity of
3
!
3
ethanol @ 25 C A
El
o=
A
O1
~3
A c t u a l 2 0 % Sil
i
--Calc
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2=0
20% sil A c t u a l 4 0 % sil
E
[] O
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o
. . - " -"
#....--~
Ill
--
|
"
. .aS~..-o"
.o"
e~ -I
/
o
17
o.oOo o-
.,° O 0.0
~
I 0.2
~
O
I
....
0.4
0.6
I .... 0.8
0 1.0
activity al
Fig. 9. Estimatedand actual ethanol adsorption in 20 and 40 wt% silicalite/PDMS.
hydrophobic PDMS to polar ethanol. From the sorption data for ethanol in pure PDMS, it can be seen that ethanol does not have high affinity for the PDMS polymer as evident from the low sorption (~l,max = 0.04) and high heat of mixing ( - - A H m - - - - 3 1 . 9 4 k J / m o l ) [14]. However, both the zeolites have a high saturation capacity for ethanol. Thus, maximum contribution to ethanol adsorption in zeolite-filled membranes occurs from the zeolite fillers (>80%). The presence of ethanol in the zeolite micropores prevents the blocking of the zeolite pores by the PDMS polymer chains. The PDMS polymer experiences a kind of a 'repulsive force' from the ethanol adsorbed in the zeolite particles, creating void spaces on the interface. The higher than estimated adsorption in zeolite-filled membranes is due to these void spaces. For DAY zeolite, the filled membrane adsorption amount is slightly lower than estimated at low ethanol activity as seen in Fig. 10. The zeolite is not yet saturated, hence, some PDMS polymer chains block the DAY zeolite pores, which results in lower than expected zeolite adsorption. However, at higher ethanol activities, the zeolite tends to saturate, driving out the PDMS chains from the zeolite pores, due to low affinity of the PDMS polymer for ethanol. This reorientation creates voids at the zeolite-polymer inter-
M. V. Chandak et al. /Journal of Membrane Science 133 (1997) 231-243
241
3.5. Permeability prediction ethanol @ 25 C A
..=4 o
[] --CaLc
0
E E 0
Actual 20% DAY
o
3
.......
i
20% DAY
O
E
Actual 40% DAY Cale
e
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Q.
..
o ....--
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2
_~ -" "-~i~
o
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,
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.°o
0
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I 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
0
0.0
i
i
i
i
0.2
04
0.6
0.8
1.0
activity at
Fig. 10. Estimatedand actual ethanol adsorptionin 20 and 40 wt% DAY/PDMS.
face, which results in higher than expected adsorption at higher activities. The pore blocking, however, does not occur for silicalite, for which the smaller pore size does not allow entry of the PDMS polymer chains. Hence, the actual sorption capacity for filled membranes stays close to expected at lower ethanol partial pressures and is higher than estimated at higher partial pressures. Caspers et al. investigated ethanol sorption in pure PDMS-acrylate and ZSM-5-filled PDMS-acrylate membranes [10]. They found little deviation between calculated sorption as per Eq. (1) and the experimentally observed ethanol sorption in ZSM-5-filled membranes. These acrylate end-groups undergo crosslinking which inhibits interaction of PDMS polymer with the zeolite crystals. Hence, the polymer properties do not change drastically due to zeolite filling. Ethanol is a polar molecule and the presence of acrylate end groups gives rise to significant polar character in the membrane itself. In this study, PDMS and TCA/TCE are both non-polar. This, along with the findings of this study, suggests that the sorption in filled membranes can be estimated from pure membrane and zeolite sorption data, if both the polymer and VOC are identically polar or non-polar.
The effect of zeolite filling was favorable only for ethanol. Hence, the discussion for this part is limited to ethanol sorption in DAY zeolite-filled membranes. The solubility coefficients (S) and diffusivity (D) values at 3000 ppm VOC concentration were used to estimate the increase in ethanol permeability due to zeolite filling using the solution-diffusion law. A quantity of 20 and 40 wt% DAY zeolite-filled membranes were considered. The ethanol permeability at 3000 ppm in pure PDMS and DAY zeolite-filled PDMS membranes were experimentally determined using a standard permeation setup at 25°C. The solution-diffusion model predicted a 5.5 factor increase in permeability for 20 wt% DAY/PDMS compared to pure PDMS. This estimated ratio was only 1.9 for 40 wt% DAY/PDMS. Experimentally the ratio of permeabilities were 3.6 for 20wt% DAY/PDMS and 1.1 for 40 wt% DAY/PDMS. The solution--diffusion model was able to qualitatively determine the variation in VOC permeability due to zeolite filling. The permeability in DAY zeolite-filled membranes increases for 20 wt% filling and decreases as the zeolite filling is increased to 40 wt%. This can be attributed to the much lower ethanol diffusivity in 40 wt% DAY/PDMS, which offsets the increase in solubility coefficient. Hence, the optimum DAY zeolite filling amount is close to 20 wt% for maximum ethanol permeability. However, more experiments have to be performed with other zeolite filling values for a more accurate estimate.
4. C o n c l u s i o n s
Zeolite filling increased the membrane solubility coefficient (S) for ethanol, a polar VOC. The zeolites act as sorptive fillers and the ethanol solubility coefficient increased with the degree of zeolite filling. The increase in S was higher in case of DAY zeolite-filled membranes due to larger pore volume of DAY zeolite. The Zimm and Lundberg clustering function indicated strong ethanol immobilization in the zeolite pores. The PDMS membrane itself had good sorption capacity for TCA and TCE. Hence, the incorporation of zeolites did not increase the solubility coefficient appreciably.
242
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
The VOC diffusivities in zeolite-filled membranes were lower than in pure PDMS due to higher characteristic diffusion times in zeolites. For ethanol, the drop in diffusivity was due to immobilization in the zeolite particles which contributed more than 80% to the sorption capacity. For TCA, the diffusivity drop was primarily due to the increasing tortuosity of the diffusion path through the polymer. The sorption of TCA and TCE in zeolite-filled PDMS membranes could be estimated accurately from the pure PDMS and zeolite sorption data with silicalite as filler. However, TCA and TCE caused considerable polymer swelling, resulting in reorientation of the polymer chains, which tended to enter and block the larger DAY zeolite pores. Hence some zeolite and polymer sorption capacities were lost, resulting in lower than estimated VOC sorption in DAY zeolitefilled membranes. Maximum contribution to ethanol sorption in zeolite-filled membranes was from the zeolites. The presence of ethanol in the zeolite micropores prevented the blocking of the zeolite pores by the PDMS polymer chains. The PDMS polymer did not have a high affinity for polar ethanol (high AHm). The polymer chains experienced a 'repulsive force' from the ethanol adsorbed in the zeolite particles, creating void spaces on the interface. The higher-than-estimated adsorption amounts in zeolite-filled membranes were due to these void spaces. Thus, the nature of the VOCpolymer interactions had a profound effect on the zeolite-polymer interface. The solution-diffusion model could qualitatively determine the variation in ethanol permeability for 20 and 40 wt% DAY zeolite-filled membranes with respect to pure PDMS for 3000 ppm ethanol concentration and at 25°C. The permeability increased for 20wt% DAY/PDMS and decreased for 40 wt% DAY/PDMS. Thus, the decrease in diffusivity more than offsets the increase in solubility coefficient for higher degrees of zeolite filling.
Dz
ED f AHs G l Mf Mp Mz p Ps rc S T V1
VOC diffusivity in zeolites (cm2/s) Diffusional activation energy (kJ/mol) Degree of zeolite filling in membrane (wt%) Heat of adsorption (kJ/mol) Cluster integral function Half thickness of membrane (m) Molar uptake of VOC by zeolite-filled membrane (mmol/g) Molar uptake of VOC by polymer (mmol/g) Molar uptake of VOC by zeolite (mmol/g) VOC partial pressure (cmHg) VOC saturation pressure (cmHg) Zeolite nominal crystal size (~tm) Solubility coefficient (cm3(STP)/cm 3 cmHg) Temperature (K) Liquid VOC molar volume (cm3/mol)
5.1. Greek Letters
Pm
VOC volume fraction in membrane Membrane density (g/cm 3)
5.2. Subscripts
1 f max p z
VOC zeolite-filled membrane maximum pure polymer zeolite
Acknowledgements We are grateful for financial support of the NSF, through an SBIR grant to CeraMem Co., on this project.
References 5. List of symbols at c D Dp
VOC VOC VOC VOC
activity ( - - p i p s ) uptake by polymer (cm 3 (STP)/cm 3) diffusivity in membrane (cm2/s) diffusivity in pure PDMS (cm2/s)
[1] H.J.C. Hennepe, D. Bargeman, M.H.V. Mulder and C.A. Smolders, Zeolite filled silicone rubber membranes:Part I, J. Membrane Sci., 35 (1987) 39. [2] H.J.C. Hennepe, C.A. Smolders, D. Bargeman and M.H.V. Mulder, Exclusion and tortuosity effects for alcohol/water separation by zeolite-filled PDMS membranes, Separation Sci. Tech., 26 (1991) 585.
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
[3] M.D. Jia, K.V. Pienermann and R.D. Behling, Preparation and characterization of thin-film zeolite PDMS composite membranes, J. Membrane Sci., 73 (1992) 119. [4] Z. Gao, Y. Yue and W. Li, Application of zeolite-filled pervaporation membrane, Zeolites, 16 (1996) 70. [5] M.D. Jia, K.V. Pienermann and R.D. Behling, Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas permeation, J. Membrane Sci., 57 (1991) 289. [6] J.M. Duval, B. Folkers, M.H.V. Mulder, G. Desgrandchamps and C.A. Smolders, Adsorbent filled membranes for gas separation. Part I. Improvement of gas separation properties of polymeric membranes by incorporation of microporous adsorbents, J. Membrane Sci., 80 (1993) 189. [7] M.G. Siier and Ba¢, N.L. Yilmaz, Gas permeation characteristics of polymer-zeolite mixed matrix membranes, J. Membrane Sci., 91 (1994) 77. [8] T.M. Gur, Permselectivity of zeolite filled polysulfone gas separation membranes, J. Membrane Sci., 93 (1994) 283. [9] W. Ji, S.K. Sikdar and S.T. Hwang, Sorption, diffusion and permeation of 1,1,1-trichloroethane through adsorbent-filled polymeric membranes, J. Membrane Sci., 103 (1995) 243. [10] C. Bartels-Casper, E. Tusel-Langer and R.N. Lichtenthaler, Sorption isotherms of alcohols in zeolite-filled silicone rubber and in PVA-composite membranes, J. Membrane Sci., 70 (1992) 75. [ll] C. Dotremont, B. Brabants, K. Geeroms, J. Mewis and C. Vandecasteele, Sorption and diffusion of chlorinated hydrocarbons in silicalite filled PDMS membranes, J. Membrane Sci., 104 (1995) 109.
243
[12] S.G. Deng and Y.S. Lin, Sulfur dioxide sorption properties and thermal stability of hydrophobic zeolites, Ind. Eng. Chem. Res., 34 (1995) 4063. [13] M.V. Chandak, Sorption and diffusion of volatile organic compounds in zeolites and zeolite filed polymer membranes, M.S. Thesis, University of Cincinnati, Ohio, 1996. [14] M.V. Chandak, Y.S. Lin, W. Ji and R.J. Higgins, Sorption and diffusion of VOCs in poly(dimethylsiloxane) membranes, J. Appl. Polym. Sci., accepted. [15] E.M. Flanigen, J.M. Bennett, R.N. Rgrose, J.R. Cohen, R.L. Patton, R.M. Kirchner and J.V. Smith, Silicalite, a new hydrophobic crystalline silica molecular sieve, Nature, 257 (1978) 512. [16] E. Gail, W. Otten and T. Frey, DeAluminized Y-zeolites Properties and applications, presented at AIChE 1993 annual meeting, Session No. 183, November 1993. [17] B.H. Zimm and J.L. Lundberg, J. Phys. Chem., 60 (1956) 425. [18] J. Crank, The Mathematics of Diffusion, Oxford University Press, London, 1975. [19] I.J.F. Vankelecom, E. Scheppers, R. Hues and J.B. Uytterhoeven, Parameters influencing zeolite incorporation in PDMS membranes, J. Phys. Chem., 98 (1994) 12390. [20] I. Blume, A. Bos, P.J.F. Schwering, M.H.V. Mulder and C.A. Smolders, Transport phenomena of vapor and liquid permeants in elastomeric membranes, J. Membrane Sci., 61 (1991) 85.
Journal of Membrane Science 133 (1997) 231-243
Sorption and diffusion of VOCs in DAY zeolite and silicalite-filled PDMS membranes •
a
M . V . C h a n d a k a, Y.S. L l n ' a
•
, W . Ji b, R . J . H i g g i n s b
Department of Chemical Engineering, University of Cincinnati, ML 171, CincinnatL OH 45221-171, USA b CeraMem Corporation, 12 Clematis Avenue, Waltham, MA 02154, USA Received 31 January 1997; received in revised form 31 March 1997; accepted 2 April 1997
Abstract The sorption and diffusion properties of ethanol, 1,1,l-trichloroethane (TCA) and trichloroethylene (TCE) were determined in silicalite-filled and dealuminized-Y-zeolite (DAY)-filled poly[dimethylsiloxane] (PDMS) membranes at 25, 100 and 150°C. Zeolite filling results in increased solubility coefficients (S) for polar solvents like ethanol over pure PDMS. No significant increase in S is observed in case of TCA and TCE which act as good solvents for PDMS. However, at higher temperatures, the sorption is higher in zeolite-filled membranes even for the good solvents. The VOC diffusivity decreases with increasing degree of zeolite filling because of higher characteristic diffusion time in zeolites (for ethanol) and increasing tortuosity of the diffusion path (for TCA). Due to the presence of carbon=carbon double bond, TCE exhibits marginal diffusivity drop in zeolite-filled membranes. The specific zeolite-polymer interactions, that is, tendency of zeolite pore blocking by polymer chains or the formation of voids on zeolite-polymer interface are influenced by the zeolite pore size and type of VOC permeating through the composite membrane. The variation in experimentally observed ethanol permeability due to zeolite filling could be qualitatively estimated from the sorption-diffusion data.
Keywords: DAY zeolite; Diffusion; Silicalite; Sorption; VOC control; Zeolite-filled polymeric membranes
1. Introduction Certain types of dense polymeric membranes offer good potential for selective VOC removal using vapor permeation or pervaporation processes due to their high permeability. However, their current limitations arise from their limited selectivities for organics over air or water. Addition of an adsorptive filler to the
*Corresponding author. Fax: +1 513 556 3473; e-mail: jlin @alpha.che.uc.edu. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0376-73 88(97)00082-3
polymeric membrane has been demonstrated to be an effective way to improve membrane performance by enhancing membrane sorption capacity for one of the compounds to be separated. Silicalite-filled poly[dimethylsiloxane] (PDMS) membranes were first applied for combined pervaporation and fermentation of alcohol-water mixtures in a membrane bioreactor by Hennepe et al. [1]. Both selectivity and permeability of silicone rubber membranes were enhanced by the incorporation of silicalite during pervaporation of ethanol/water mixtures [2-4]. This was due to the lower water sorption capacity of
232
M.V. Chandak et al. /Journal of Membrane Science 133 (1997) 231-243
silicalite. Also, the alcohol could diffuse through both zeolite and polymer phases, while water had to follow a more tortuous path due to the hydrophobicity of silicalite. These membranes have also been applied for gas separation applications involving N2, 02, He, H2, CH4, CO2, n-C4Hlo, i-C4Hlo, 1-Call 8 and Ar [5-8]. Apart from PDMS, various other types of membranes have been investigated for filling, including polyetherblock-polyamides (PEBA)/poly[urethane] (PUR)/ silicone-poly[carbonate] copolymer (SPC) [9], PDMS-acrylate, poly[vinyl alcohol] (PVA) [4,8,10] ethylene-propylene rubber (EPDM)/poly[chloroprene] (PCP)/nitrile-butadiene rubber (NBR) [6] and poly[ethersulfone] (PES) [7]. These studies investigated microporous adsorbent fillers such as zeolites, carbon molecular sieves, activated carbons and carbonaceous adsorbents. The aim was to choose fillers with selectivity toward the same molecules as the unfilled membrane. Most of the studies in literature are limited to alcohol/water pervaporation or removal of liquid phase chlorinated organics from aqueous solutions. Very few studies have dealt with vapor permeation properties of organics in zeolite-filled polymer membranes. The vapor phase investigations of sorption and permeation of chlorinated solvents in PDMS were restricted to highlight the effect of number of chlorine atoms, degree of branching and molecular weight on the sorption and diffusion behavior of these chlorinated organics in silicalite-filled PDMS membranes [111. The application of zeolite-filled membranes for vapor permeation VOC control processes has recently gained attention. In view of the solution-diffusion mechanism for permeant transport, ideal fillers for preparation of VOC-selective polymer membranes should have hydrophobicity and high sorption capacity for organic compounds and fast diffusion of the organic molecules within the filler. Zeolites have been used extensively in large scale adsorption and catalytic processes. They can be used as adsorptive fillers for improving membrane properties because of their unique crystalline microporous pore structure, surface chemistry and mechanical strength. Among many synthetic zeolites, only two zeolites are hydrophobic, silicalite and DAY, both of which are pure crystalline silica molecular sieves. Silicalite is a dealuminized
form of ZSM-5 zeolite. DAY zeolite is a DeAluminized form of Y zeolite. These zeolites were found to be thermally stable and acid resistant up to a temperature of 1000°C [12]. The adsorption and diffusion properties of VOCs in these two zeolites have been compared and reported elsewhere [13]. It was shown that DAY zeolite was a better adsorbent for VOCs compared to silicalite. DAY zeolite exhibited higher equilibrium uptake capacity, and faster kinetics compared to silicalite. This was attributed to the larger pore volume and pore size of DAY zeolite. However, silicalite was the only hydrophobic zeolite investigated as a filler in all the studies reported in literature. There has been no reported study on DAY zeolite-filled polymeric membranes. Hence, in view of its superior adsorptive and diffusion properties, the application of DAY zeolite as a filler in PDMS membrane merits investigation. Also, none of the studies on zeolite-filled membranes have investigated VOC sorption and diffusion in zeolites and pure polymers independently. In a recent study reported elsewhere, the sorption and diffusion properties of VOCs were investigated in pure PDMS membranes [14]. PDMS had good sorption capacity for trichloroethane (TCA) and trichloroethylene (TCE), which act as good solvents. However, much lower sorption capacity was observed for ethanol due to its polar character and clustering tendency. Also, DAY zeolite and silicalite were compared for their VOC adsorption and diffusion properties [13]. The present work is a continuation of these two investigations and utilizes the experimental findings reported therein. In the present study, the properties of sorption and diffusion of VOCs in zeolite-filled PDMS membranes were measured at 25, 100 and 150°C. Silicalite and DAY zeolite are compared as fillers. The three VOCs chosen are ethanol, 1,1,1-trichloroethane (TCA) and trichloroethylene (TCE). The effect of VOC type, zeolite type and degree of zeolite filling on the VOC sorption and diffusion properties are determined. The deviations between the actual and predicted sorption are used to quantitatively determine the polymer-zeolite interactions. The effect of zeolite pore size and VOC type was considered on these interactions. The variation in ethanol permeability with degree of zeolite filling is estimated using
233
M. V Chandak et al./Journal of Membrane Science 133 (1997) 231-243
sorption-diffusion data and compared to experimental permeation data.
3. Results and discussion 3.1. Sorption uptake
2. Experimental 2.1. Zeolites and polymer
The zeolites, DAY zeolite and silicalite, were procured from Degussa Corporation and UOP, respectively. The zeolite crystal sizes were determined to be 1 lam [12]. Silicalite has a pore size of 5.4 ~, [15] whereas DAY zeolite has a pore size of 7.4 ~, [16]. DAY zeolite has larger pore volume (0.30 ml/g) and surface area (800m2/g) than silicalite (0.19 ml/g, 450 m2/g, respectively). Two-part silicone rubber from General Electric was selected as the polymer phase in the zeolite-filled membranes. Rubber A (10 parts), cross-linker B (1 part) and the zeolite were mixed thoroughly. The membranes were cast on a glass plate from the resulting homogeneous paste consisting of silicone rubber and the zeolite. After curing at room temperature overnight, the membranes were placed in an oven at 80°C for 5-6 h to assure complete cross-linking. Filled PDMS membranes containing 20 and 40 wt% silicalite and DAY zeolite were synthesized. Typical thickness of the zeolite-filled membrane is 100 tam.
The ethanol isotherms on pure PDMS, 20 wt% silicalite/PDMS and 40 wt% silicalite/PDMS membranes are given in Fig. 1 at 25°C. The molar uptake (Mp) is plotted against the VOC activity al defined as PIPs. P is the VOC partial pressure and Ps is the saturated vapor pressure. The effect of the zeolite filler is immediately seen in the form of increased ethanol molar uptake by the membrane. This increasing molar uptake (Mp) automatically translates into increasing solubility, that is, VOC volume fraction (~b 1 = MpPmV1)and solubility coefficient (S = c/p). Pm is the membrane density, VI the VOC liquid molar volume and c the VOC uptake (cm 3 (STP)) per unit volume of membrane. The relative increase in sorption is greater at lower partial pressures of ethanol, as this corresponds to the region where the zeolite is not saturated. At higher activities, the zeolite tends to saturate. Hence, the fractional improvement in sorption over pure PDMS decreases. Similar isotherms were observed for DAY zeolite-filled PDMS membranes. This is consistent with the observations of earlier investigators that
5 2.2. Experimental procedure
The VOCs, l,l,l-trichloroethane (Fisher, certified grade), trichloroethylene (Fisher, certified ACS grade) and ethanol (Aaper, absolute - 200 proof) were procured and used without further treatment. The sorption and diffusion studies were carried out gravimetrically in a modified CAHN C-1000 micro-balance. A membrane film of 10-20 mg was loaded in one ann of the balance and activated by heating to 175°C. The membrane was exposed subsequently to increasing partial pressures of pure phase VOC vapor. At each VOC activity, the transient and equilibrium weight changes of the membrane samples were measured and recorded for analysis. The entire range of VOC partial pressures was covered. The experiments were conducted at 25, 100 and 150°C. The details of the experimental setup and procedure have been reported elsewhere [14].
I
ethanol@25 °C
~4 ~3
PDMS
/
, 20%sil
E 1
0.0
0.2
0.4
0.6
0.8
1.0
activity(a~) Fig. 1. Ethanol isotherms on silicalite-filledPDMS membranesat 25°C.
234
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
5=
trichloroethylene / @25C /
04
PDMS
"--
~ 20% DAY
|
/ /
0
E 1 0
~-'
0.0
' I ......
0.2
,
' ' ' ,' . . . . .
0.4
0.6
, ....
0.8
1.0
activity (at) Fig. 2. Trichloroethylene isotherms on DAY zeolite-filled PDMS membranes at 25°C.
the zeolite filler acts as an adsorptive reservoir and increases the solubility of alcohols in PDMS. In case o f TCE, however, the zeolite fails to increase the sorption capacity appreciably, as seen in Fig. 2 for
DAY z e o l i t e / P D M S . The increase in uptake over pure PDMS is very low and confined to low activities o f VOC. Similar trends were observed in case of T C A on both DAY and silicalite-filled PDMS and are not shown for avoiding repetition. T C A and TCE are good solvents for PDMS, hence, the zeolite filler does not increase their uptake to the same extent, as in case of ethanol. In fact after a critical VOC partial pressure, the uptake in zeolite-filled membranes drops below that o f pure PDMS. This observation will be discussed in more detail later. Typical isotherms measured at 25, 100 and 150°C are given in Fig. 3 for T C A sorption on 40 wt% silicalite/PDMS. The membrane sorption capacity drops with temperature, though less than that observed earlier in pure PDMS [14]. Solubility coefficients (S) were calculated from the VOC sorption isotherms and the average solubility coefficients at the three temperatures are reported in Table l(a). At each temperature, five values of the solubility coefficient were calculated at VOC activities, a t - 0 , 0.2, 0.4, 0.6, 0.8 and 1.0. The average of heat of adsorption (AHs) values corresponding to these activities are reported in Table 1. The average solubility coefficient at 25°C for ethanol in zeolite-filled P D M S increases from 2.69 in pure
/
trichloroethane on
._. 4 7
40 % silicalite/PDMS o 2 o0 , 100oc
E3
o
.¢
o 150 °C
1
0
-
0.0
0.2
0.4
0.6
activity (at)
0.8
1.0
Fig. 3. Effect of temperature on trichloroethane isotherms on 40% silicalite/PDMS membranes.
M. V. Chandak et al. /Journal of Membrane Science 133 (1997) 231-243
235
Table 1 Average VOC solubility coefficients and heat of sorption in zeolite-filled membranes a: Solubility coefficient (S) cm3(STP)/cm 3 cmHg VOC Ethanol
TCA
TCE
T (°C) 25 100 150 25 100 150 25 100 150
PDMS 2.69 1.77 1.02 4.15 0.29 0.08 7.33 0.38 0.07
20% DAY 6.77 3.76 3.48 5.10 0.50 0.30 7.20 0.31 0.10
40% DAY
20% Sil.
40% Sil.
24.70 23.90 16.10 3.03 0.52 0.31 5.71 0.90 0.20
10.23 4.47 3.83 4.15 0.92 0.54 10.21 2.54 1.17
14.11 4.43 3.00 4.63 1.03 0.70 9.30 4.36 1.31
b: Heat of adsorption (-zSd-/s)
Ethanol TCA TCE
PDMS
DAY
silicalite
20% DAY
40% DAY
20% Sil.
40% Sil.
6.62 32.58 35.37
28.87 26.78 31.38
18.41 9.20 23.85
5.82 25.39 38.69
3.10 24.82 30.03
8.51 17.24 17.98
13.14 16.20 15.29
PDMS to 6.77 in 20 wt% DAY and 24.7 in 40 wt% DAY-filled PDMS. Similar but slightly lower increase was found for silicalite-filled PDMS. The higher increase in S for DAY zeolite-filled membranes compared to silicalite can be attributed to the higher pore volume of the former. The zeolites act as sorptive fillers in increasing the ethanol solubility coefficient. However, the increase in S for TCA, a good solvent for PDMS, is marginal. The variation in average solubility coefficient is only from 4.15 for pure PDMS to 5.1 in 20 wt% DAY and 3.03 in 40 wt% DAY. In fact for both good solvents, TCA and TCE, the solubility tends to be lower for 4 0 w t % DAY/PDMS compared to 20 wt% DAY/PDMS. This phenomenon will be discussed later. At higher temperatures, however, the solubility coefficient in filled membranes tends to be higher than that in pure PDMS even for the good solvents. TCE solubility coefficient at 100°C increases from 0.38 in case of pure PDMS to 2.54 in 20 wt% silicaIite/PDMS to 4.36 in 40 wt% silicalite/PDMS. This was probably due to the better effectiveness of the zeolite filler at higher temperatures. Hence, the values of heat of adsorption for the filled membranes remain within those for pure polymer and pure zeolite or falls slightly lower than either of them as seen in Table l(b).
3.2. Cluster function analysis The Zimm and Lundberg clustering integral function (G) enables sorption characteristics to be determined in terms of the degree of clustering of the sorbed molecules [17]. The clustering tendency of only ethanol is considered in zeolitefilled membranes as the increase in ethanol sorption is maximum due to zeolite filling. As seen in Table 1, zeolite filling does not affect TCA and TCE sorption to the same extent. Also, these VOCs did not show appreciable clustering tendency in pure PDMS [14]. Fig. 4 compares the Zimm plots for ethanol sorption in pure PDMS and silicalite-filled PDMS membranes. Ethanol demonstrates significant clustering tendencies in pure PDMS (G >> - 1 in Fig. 4) [14]. Zeolites have a strong influence in ethanol sorption. For silicalite-filled membranes, the value of cluster function remains < - 1 for almost the entire range of ethanol activity, which suggests strong immobilization of ethanol in the filled membranes [17]. In this case, the immobilization is caused by the zeolites, which act as adsorptive fillers. Similar cluster function trends were observed in case of DAY zeolite-filled membranes. Thus incorporation of zeolites brings about a radical change in the sorption mechanism of polar
236
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243 400 •
300
ethanol @ 25 C
"
---•--- P D M S
*
A
ov m
.20% silicalite
---e-- 40% silicalite
200 0')
..E= 100
U
.
-*--.
..
t
~
i I I I I I I l l l l
i
b
''" - a -'~°°° l
l
l
[
l
l
~
~ . 8
l
~
-
10
-1 O0
activity (al)
Fig. 4. Zimm plot for ethanol adsorption in PDMS and silicalite-filled PDMS membranes.
molecules like ethanol which show a clustering tendency in PDMS.
3.3. Diffusivity analysis The results of the diffusivity calculations for filled membranes are reported in Table 2. The plane sheet model [18] was used for calculation of diffusivities in zeolite-filled PDMS as in case of pure PDMS [14]. This is because similar uptake curves were observed in the two cases. Hence, the reported values can be considered as effective diffusivities in the zeolitefilled membranes. The diffusivities of the VOCs in zeolites (Dz) were found to be ,,~10-i°cmZ/s for DAY zeolite and ,,~10-12 cm2/s for silicalite [13]. In pure PDMS membranes, the diffusivities (Dp) were ,-~10 - 6 c m 2 / s [ 14]. Hence, the diffusivities in pure PDMS membranes are ~ 1 0 4 times the diffusivities in zeolites. However, the important parameter to be considered is the characteristic diffusion time. ~'z = r2/Dz is in the range of about 102 and 10as for DAY zeolite and silicalite, respectively, rc is the nominal zeolite crystal size. For PDMS polymer, the characteristic diffusion time is *p = 412/Dp which is in the range of 50 and 125 s. 21 is the membrane thickness. The diffusion time in
zeolites, ~-z, is 2-80 times larger than ~-p. Hence, the VOC diffusivity in filled membranes decreases compared to pure PDMS as denoted by the values in parenthesis. The lower ethanol diffusivities in zeolite-filled membranes reported in Table 2 are due to immobilization of ethanol in the zeolite pores as suggested by the cluster integral function in Fig. 4. Hence, the diffusivity decreases sharply with increasing zeolite content of the filled membrane. The drop is greater for silicalite-filled PDMS membranes compared to DAY zeolite-filled PDMS membranes. This is due to the smaller pore size of silicalite compared to DAY zeolite. This highlights the importance of diffusion through the zeolite pores in the overall permeation of ethanol through zeolite-filled membranes. TCA and TCE are good solvents for PDMS itself. Filling zeolites does not appreciably increase their sorption in filled membranes. Hence for TCA, the drop in diffusivity is due to lower diffusivity in the zeolite phase and also due to the increasing tortuosity of the TCA diffusion path through the polymer network in the filled membrane. The drop in diffusivity is almost identical for 20% silicalite and 20% DAY membranes, implying that the transport through the polymer phase plays a more dominant role in the overall permeation.
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
237
Table 2 VOC diffusivities and diffusional activation energies in zeolite-filled membranes a: Diffusivity (D) x 108 cm2/s VOC Ethanol
TCA
TCE
T (°C) 25 100 150 25 100 150 25 100 150
PDMS 151.5 156.6 359.1 323.2 749.1 1292.8 143.6 610.5 725.4
20% DAY 50.8 (0.34) a 62.9 98.3 122.9 (0.38) 304.9 336.9 118.0 (0.82) 242.0 340.1
b: Diffusional activation energy (ED) kJ/mol PDMS DAY
silicalite
Ethanol TCA TCE
15.48 17.57 15.48
6.15 11.59 14.21
8.79 12.97 17.57
40% DAY
20% Sil.
40% Sil.
4.5 (0.03) 4.8 8.5 13.3 (0.04) 24.1 41.4 30.2 (0.21) 33.4 120.7
13.0 (0.09) 27.1 27.7 118.3 (0.37) 162.0 388.8 68.0 (0.47) 187.3 337.0
1.4 (0.04) 2.1 2.4 23.1 (0.07) 58.8 78.4 103.9 (0.72) 198.0 223.4
20% DAY
40% DAY
20% Sil.
40% Sil.
6.74 9.00 13.23
4.25 10.40 6.64
5.06 8.86 8.83
4.59 7.31 9.95
a Values in parenthesis denote ratio of VOC diffusivity in filled membranes to pure PDMS~
This also suggests partial blockage of DAY zeolite pores by polymer, making its effective pore size smaller. This will be discussed in more detail later. In case of TCE, also a good solvent, the diffusivity does not drop much in the filled membranes. TCE has a C=C double bond, and there is a tendency of rotation about the double bond which increases the mobility of TCE molecules. Earlier, it was observed that unlike TCA and ethanol, TCE had identical diffusivities in DAY and silicalite, despite the smaller pore size of silicalite [13]. Hence, TCE diffusivity does not drop much in zeolite-filled membranes, due to the higher mobility of TCE and its flexible structure. The diffusional activation energies (ED), calculated from the diffusional data at different temperatures using the Arrhenius plots, are summarized in Table 2(b). The calculated VOC diffusional activation energies in PDMS [14] and zeolites [13] are also included for comparison. The diffusional activation energy in zeolite-filled membranes decreases compared to pure PDMS for all VOCs. Thus the diffusion process becomes less activated due to zeolite filling. For TCA and TCE, zeolites increase membrane solubility coefficients at higher temperatures as seen in Table 1. Hence, TCA/TCE diffusivity in zeolite-filled membranes is restricted at higher temperatures due to immobilization in zeolite pores, resulting in lower
activation energies. Ethanol has a lower ED value compared to other VOCs probably due to the effect of clustering, which is a weak function of temperature.
3.4. Prediction of filled membrane sorption isotherms In the filled membranes, the zeolite crystals of size rc = 1 ktm are embedded in the polymer network, which has a typical thickness o f 2 / = 100 ~tm. Assuming the polymer and the zeolite phase do not influence each other, the VOC molar uptake in zeolite-filled membranes can be estimated from that of pure PDMS and zeolite uptake at any particular VOC activity, al, by the following equation, Mf (mmol/g) = Mp × (100 - f ) ( m m o l / g ) + Mz x f ( m m o l / g )
(1)
w h e r e f i s the degree of filling by wt% of zeolite. Mp, Mz, and Mf are the molar uptakes of VOC per gram by the PDMS, zeolite and filled membrane, respectively. Mz and Mp values were taken from earlier studies [13,14]. These calculated adsorption capacities, Mr, were compared with the actual filled membrane sorption isotherms. If there is no discrepancy, it implies that the polymer and zeolite sorption capacities are
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
238
8
5
i
trichloroethane@ 25 C
6 E Q
m
A(:tua120 % sil
O
Actual 40% sil
1
3.4.1. TCA/TCE ..-
Cale 40% sl
. . . . . . .
3
oO.
'
0
__
e.Q.-
"~
.o " '°"
0
~'~-'
0.0
,
,
i
I
I
' I ....
0.2
,
,
i
i
,
I ....
Figs. 5 and 6 demonstrate the actual data for TCA and TCE adsorption in 20 and 40% silicalite/PDMS. For TCA sorption, the solid and dotted lines in Fig. 5 give the estimated adsorption amount calculated from pure PDMS [14] and zeolite data [13] using Eq. (1). At any given VOC activity al, the sorption data of the filled membrane can be estimated within experimental error by adding the pure zeolite and polymer adsorption amounts. This indicates the zeolite and the PDMS polymer adsorb the VOCs independently. The zeolite pores are not blocked by the polymer and the polymer swelling is not affected by the zeolite particles. Figs. 7 and 8 show the estimated and actual adsorption data for TCA and TCE adsorption in 20% DAY and 40% DAY/PDMS. For TCA sorption, the uptake by 20% DAY/PDMS can be accurately estimated. However, for 40% DAY, the actual uptake is lower than estimated. In case of TCE, however, both the 20 and 40% DAY/PDMS membranes give lower than expected sorption uptake as shown in Fig. 8. Thus, either the polymer phase or the zeolite phase does not adsorb as expected. It is proposed that the PDMS polymer chains enter and block the larger DAY zeolite
E
."
~n
¢'~
mmol/g of VOC, but it can be qualitatively extended to VOC solubility coefficient as well.
,
0.4
1 ~"
,
i
i
i
~ .....
i
0.6
i
i
,
,
, ....
i
,
0.8
0
.0
activity at Fig. 5. E s t i m a t e d a n d actual T C A adsorption in 20 a n d 40 w t % silicalite/PDMS.
additive and they do not influence each other due to zeolite filling. However, any discrepancy between the two quantities can be utilized to understand the nature of the interface and interaction between the zeolite particles and the polymer network. The following discussion is based on sorption uptake (Mr) in 5
4
trichloroethylene @ 25 C _~
4
.i .//
n Actua,2o%s,,
A
.¢
3 ~)
,'/O O
,,
Actual 40% sil
-~ " 2 •"~
.t'~//
c.,o , o ~ . ,
.......
m
.-7
.- a .,"
o
E
~"
2"~ " :~
"°'" 6 0.°-
0
'
0.0
'
'
1 . . . .
0.2
1 . . . .
0.4
I
. . . .
0.6
1 . . . .
0.8
0
1.0
activity al Fig. 6. Estimated a n d actual T C E adsorption in 20 a n d 40 w t % s i l i c a l i t e / P D M S .
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
5
5
trichloroethane @ 25 C
J
[3/
3 .......~~ c aDlAYc"' . ."~. . ."'I~ 4"""o/' '/~"° % °o 2 "~.~c~ ~
3 ~ ~2
Actual 40% DAY
0
e~l
/~"
i
......
1'~
..''" / n .°'/,~/_
0
239
0
13 _ ^
O
_ _ . ,
O
0
O
..............
0
0.0 0.2 0.4 0.6 0.8 1.0 activity al
Fig. 7. Estimated and actual TCA adsorption in 20 and 40 wt% DAY/PDMS.
6
trichloroethylene @25C
"~5 "6
[]
~4
--
Cale20% DAY
o
Actual 40% DAY
.......
"
"
Cal¢ 40 % DAY
"
I~i 1
~
°~
"~
3
."
. /
,=2
o
:' 4
Actual 20% DAY
.~¢
o
5
E
I~,
oOO
[]
1
°..°-'"°
"f,°,~,
,. . . . . . . . , . . . . . . . . , . . . .
,
0
0.0 0.2 0.4 0.6 0.8 1.0 activity at
Fig. 8. Estimated and actual TCE adsorption in 20 and 40 wt% DAY/PDMS.
pores. Thus, the DAY zeolite crystals embedded in the polymer network cannot adsorb the same amount of VOC as it would otherwise, resulting in lower than expected solubility. TCA and TCE are good solvents for PDMS. With increasing VOC activity and sorption, the polymer
swells as it contributes more to the total sorption. The polymer chains reorient and thus enter and block the larger DAY zeolite pores. Thus the effective diameter of DAY zeolite pores is lowered. The almost identical drop in TCA and TCE diffusivities observed in DAY and silicalite-filled membranes as seen in Table 2
240
M.V. Chandak et al./Journal ¢~ Membrane Science 133 (1997) 231-243
validates this hypothesis. For TCA, the deviation between actual and estimated sorption is lower for 20% DAY, due to reduced tendency to block the pores because of fewer number of zeolite particles in the polymer network. Silicalite pores are smaller than DAY zeolite pores, and thus, the PDMS chains cannot enter and block the pores. Hence, for both TCA and TCE, very good agreement was found between estimated and actual silicalite-filled membrane sorption amounts. Vankelecom et al. investigated the effect of filling of ZSM-5 and Y zeolite on the physical properties of PDMS membranes [19]. They reported increase in tensile strength of PDMS membranes due to incorporation of Y zeolite. However ZSM-5 decreased the tensile strength of the PDMS membranes. This was explained on the basis of partial invasion of the polymer chains in to the Y zeolite framework. The authors suggested that Y zeolite acted as a physical 'cross-linker', thus increasing the tensile strength of the membrane. The pore structure of Y zeolite and DAY zeolite are similar. Hence, it is proposed that DAY zeolite similarly reinforces the PDMS network, due to blockage of the pores by the polymer chains. This reduces the sorption capacity of both zeolite and PDMS polymer for TCA and TCE, respectively. Thus, the results of the gravimetric sorption experiments in this study are compatible with the physical tensile strength measurement investigations reported in literature. Blume et al. reported that the toluene sorption in pure PDMS was inversely proportional to the degree of cross-linking [20]. The addition of DAY zeolite increases the amount of cross-linking and thus decreases polymer sorption capacity.
3.4.2. Ethanol The actual and estimated ethanol uptakes by 20 and 40 wt% silicalite/PDMS are shown in Fig. 9. At low VOC activities, the actual sorption by filled membranes is close to estimated. However, as the VOC activity increases, the uptake by the filled membrane is higher than estimated. This could be due to formation of voids on the zeolite-polymer interface. The presence of voids in case of ethanol sorption can be explained by considering the lack of affinity of
3
!
3
ethanol @ 25 C A
El
o=
A
O1
~3
A c t u a l 2 0 % Sil
i
--Calc
E v
O .......
2=0
20% sil A c t u a l 4 0 % sil
E
[] O
C.,alc 4 0 % sil
o
. . - " -"
#....--~
Ill
--
|
"
. .aS~..-o"
.o"
e~ -I
/
o
17
o.oOo o-
.,° O 0.0
~
I 0.2
~
O
I
....
0.4
0.6
I .... 0.8
0 1.0
activity al
Fig. 9. Estimatedand actual ethanol adsorption in 20 and 40 wt% silicalite/PDMS.
hydrophobic PDMS to polar ethanol. From the sorption data for ethanol in pure PDMS, it can be seen that ethanol does not have high affinity for the PDMS polymer as evident from the low sorption (~l,max = 0.04) and high heat of mixing ( - - A H m - - - - 3 1 . 9 4 k J / m o l ) [14]. However, both the zeolites have a high saturation capacity for ethanol. Thus, maximum contribution to ethanol adsorption in zeolite-filled membranes occurs from the zeolite fillers (>80%). The presence of ethanol in the zeolite micropores prevents the blocking of the zeolite pores by the PDMS polymer chains. The PDMS polymer experiences a kind of a 'repulsive force' from the ethanol adsorbed in the zeolite particles, creating void spaces on the interface. The higher than estimated adsorption in zeolite-filled membranes is due to these void spaces. For DAY zeolite, the filled membrane adsorption amount is slightly lower than estimated at low ethanol activity as seen in Fig. 10. The zeolite is not yet saturated, hence, some PDMS polymer chains block the DAY zeolite pores, which results in lower than expected zeolite adsorption. However, at higher ethanol activities, the zeolite tends to saturate, driving out the PDMS chains from the zeolite pores, due to low affinity of the PDMS polymer for ethanol. This reorientation creates voids at the zeolite-polymer inter-
M. V. Chandak et al. /Journal of Membrane Science 133 (1997) 231-243
241
3.5. Permeability prediction ethanol @ 25 C A
..=4 o
[] --CaLc
0
E E 0
Actual 20% DAY
o
3
.......
i
20% DAY
O
E
Actual 40% DAY Cale
e
0 0
40% DAY
O ~e
0 0
Q.
..
o ....--
.....
...
el.
°..--'"
_..
2
_~ -" "-~i~
o
~--
,
#
.°o
0
[]
I 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
0
0.0
i
i
i
i
0.2
04
0.6
0.8
1.0
activity at
Fig. 10. Estimatedand actual ethanol adsorptionin 20 and 40 wt% DAY/PDMS.
face, which results in higher than expected adsorption at higher activities. The pore blocking, however, does not occur for silicalite, for which the smaller pore size does not allow entry of the PDMS polymer chains. Hence, the actual sorption capacity for filled membranes stays close to expected at lower ethanol partial pressures and is higher than estimated at higher partial pressures. Caspers et al. investigated ethanol sorption in pure PDMS-acrylate and ZSM-5-filled PDMS-acrylate membranes [10]. They found little deviation between calculated sorption as per Eq. (1) and the experimentally observed ethanol sorption in ZSM-5-filled membranes. These acrylate end-groups undergo crosslinking which inhibits interaction of PDMS polymer with the zeolite crystals. Hence, the polymer properties do not change drastically due to zeolite filling. Ethanol is a polar molecule and the presence of acrylate end groups gives rise to significant polar character in the membrane itself. In this study, PDMS and TCA/TCE are both non-polar. This, along with the findings of this study, suggests that the sorption in filled membranes can be estimated from pure membrane and zeolite sorption data, if both the polymer and VOC are identically polar or non-polar.
The effect of zeolite filling was favorable only for ethanol. Hence, the discussion for this part is limited to ethanol sorption in DAY zeolite-filled membranes. The solubility coefficients (S) and diffusivity (D) values at 3000 ppm VOC concentration were used to estimate the increase in ethanol permeability due to zeolite filling using the solution-diffusion law. A quantity of 20 and 40 wt% DAY zeolite-filled membranes were considered. The ethanol permeability at 3000 ppm in pure PDMS and DAY zeolite-filled PDMS membranes were experimentally determined using a standard permeation setup at 25°C. The solution-diffusion model predicted a 5.5 factor increase in permeability for 20 wt% DAY/PDMS compared to pure PDMS. This estimated ratio was only 1.9 for 40 wt% DAY/PDMS. Experimentally the ratio of permeabilities were 3.6 for 20wt% DAY/PDMS and 1.1 for 40 wt% DAY/PDMS. The solution--diffusion model was able to qualitatively determine the variation in VOC permeability due to zeolite filling. The permeability in DAY zeolite-filled membranes increases for 20 wt% filling and decreases as the zeolite filling is increased to 40 wt%. This can be attributed to the much lower ethanol diffusivity in 40 wt% DAY/PDMS, which offsets the increase in solubility coefficient. Hence, the optimum DAY zeolite filling amount is close to 20 wt% for maximum ethanol permeability. However, more experiments have to be performed with other zeolite filling values for a more accurate estimate.
4. C o n c l u s i o n s
Zeolite filling increased the membrane solubility coefficient (S) for ethanol, a polar VOC. The zeolites act as sorptive fillers and the ethanol solubility coefficient increased with the degree of zeolite filling. The increase in S was higher in case of DAY zeolite-filled membranes due to larger pore volume of DAY zeolite. The Zimm and Lundberg clustering function indicated strong ethanol immobilization in the zeolite pores. The PDMS membrane itself had good sorption capacity for TCA and TCE. Hence, the incorporation of zeolites did not increase the solubility coefficient appreciably.
242
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
The VOC diffusivities in zeolite-filled membranes were lower than in pure PDMS due to higher characteristic diffusion times in zeolites. For ethanol, the drop in diffusivity was due to immobilization in the zeolite particles which contributed more than 80% to the sorption capacity. For TCA, the diffusivity drop was primarily due to the increasing tortuosity of the diffusion path through the polymer. The sorption of TCA and TCE in zeolite-filled PDMS membranes could be estimated accurately from the pure PDMS and zeolite sorption data with silicalite as filler. However, TCA and TCE caused considerable polymer swelling, resulting in reorientation of the polymer chains, which tended to enter and block the larger DAY zeolite pores. Hence some zeolite and polymer sorption capacities were lost, resulting in lower than estimated VOC sorption in DAY zeolitefilled membranes. Maximum contribution to ethanol sorption in zeolite-filled membranes was from the zeolites. The presence of ethanol in the zeolite micropores prevented the blocking of the zeolite pores by the PDMS polymer chains. The PDMS polymer did not have a high affinity for polar ethanol (high AHm). The polymer chains experienced a 'repulsive force' from the ethanol adsorbed in the zeolite particles, creating void spaces on the interface. The higher-than-estimated adsorption amounts in zeolite-filled membranes were due to these void spaces. Thus, the nature of the VOCpolymer interactions had a profound effect on the zeolite-polymer interface. The solution-diffusion model could qualitatively determine the variation in ethanol permeability for 20 and 40 wt% DAY zeolite-filled membranes with respect to pure PDMS for 3000 ppm ethanol concentration and at 25°C. The permeability increased for 20wt% DAY/PDMS and decreased for 40 wt% DAY/PDMS. Thus, the decrease in diffusivity more than offsets the increase in solubility coefficient for higher degrees of zeolite filling.
Dz
ED f AHs G l Mf Mp Mz p Ps rc S T V1
VOC diffusivity in zeolites (cm2/s) Diffusional activation energy (kJ/mol) Degree of zeolite filling in membrane (wt%) Heat of adsorption (kJ/mol) Cluster integral function Half thickness of membrane (m) Molar uptake of VOC by zeolite-filled membrane (mmol/g) Molar uptake of VOC by polymer (mmol/g) Molar uptake of VOC by zeolite (mmol/g) VOC partial pressure (cmHg) VOC saturation pressure (cmHg) Zeolite nominal crystal size (~tm) Solubility coefficient (cm3(STP)/cm 3 cmHg) Temperature (K) Liquid VOC molar volume (cm3/mol)
5.1. Greek Letters
Pm
VOC volume fraction in membrane Membrane density (g/cm 3)
5.2. Subscripts
1 f max p z
VOC zeolite-filled membrane maximum pure polymer zeolite
Acknowledgements We are grateful for financial support of the NSF, through an SBIR grant to CeraMem Co., on this project.
References 5. List of symbols at c D Dp
VOC VOC VOC VOC
activity ( - - p i p s ) uptake by polymer (cm 3 (STP)/cm 3) diffusivity in membrane (cm2/s) diffusivity in pure PDMS (cm2/s)
[1] H.J.C. Hennepe, D. Bargeman, M.H.V. Mulder and C.A. Smolders, Zeolite filled silicone rubber membranes:Part I, J. Membrane Sci., 35 (1987) 39. [2] H.J.C. Hennepe, C.A. Smolders, D. Bargeman and M.H.V. Mulder, Exclusion and tortuosity effects for alcohol/water separation by zeolite-filled PDMS membranes, Separation Sci. Tech., 26 (1991) 585.
M.V. Chandak et al./Journal of Membrane Science 133 (1997) 231-243
[3] M.D. Jia, K.V. Pienermann and R.D. Behling, Preparation and characterization of thin-film zeolite PDMS composite membranes, J. Membrane Sci., 73 (1992) 119. [4] Z. Gao, Y. Yue and W. Li, Application of zeolite-filled pervaporation membrane, Zeolites, 16 (1996) 70. [5] M.D. Jia, K.V. Pienermann and R.D. Behling, Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas permeation, J. Membrane Sci., 57 (1991) 289. [6] J.M. Duval, B. Folkers, M.H.V. Mulder, G. Desgrandchamps and C.A. Smolders, Adsorbent filled membranes for gas separation. Part I. Improvement of gas separation properties of polymeric membranes by incorporation of microporous adsorbents, J. Membrane Sci., 80 (1993) 189. [7] M.G. Siier and Ba¢, N.L. Yilmaz, Gas permeation characteristics of polymer-zeolite mixed matrix membranes, J. Membrane Sci., 91 (1994) 77. [8] T.M. Gur, Permselectivity of zeolite filled polysulfone gas separation membranes, J. Membrane Sci., 93 (1994) 283. [9] W. Ji, S.K. Sikdar and S.T. Hwang, Sorption, diffusion and permeation of 1,1,1-trichloroethane through adsorbent-filled polymeric membranes, J. Membrane Sci., 103 (1995) 243. [10] C. Bartels-Casper, E. Tusel-Langer and R.N. Lichtenthaler, Sorption isotherms of alcohols in zeolite-filled silicone rubber and in PVA-composite membranes, J. Membrane Sci., 70 (1992) 75. [ll] C. Dotremont, B. Brabants, K. Geeroms, J. Mewis and C. Vandecasteele, Sorption and diffusion of chlorinated hydrocarbons in silicalite filled PDMS membranes, J. Membrane Sci., 104 (1995) 109.
243
[12] S.G. Deng and Y.S. Lin, Sulfur dioxide sorption properties and thermal stability of hydrophobic zeolites, Ind. Eng. Chem. Res., 34 (1995) 4063. [13] M.V. Chandak, Sorption and diffusion of volatile organic compounds in zeolites and zeolite filed polymer membranes, M.S. Thesis, University of Cincinnati, Ohio, 1996. [14] M.V. Chandak, Y.S. Lin, W. Ji and R.J. Higgins, Sorption and diffusion of VOCs in poly(dimethylsiloxane) membranes, J. Appl. Polym. Sci., accepted. [15] E.M. Flanigen, J.M. Bennett, R.N. Rgrose, J.R. Cohen, R.L. Patton, R.M. Kirchner and J.V. Smith, Silicalite, a new hydrophobic crystalline silica molecular sieve, Nature, 257 (1978) 512. [16] E. Gail, W. Otten and T. Frey, DeAluminized Y-zeolites Properties and applications, presented at AIChE 1993 annual meeting, Session No. 183, November 1993. [17] B.H. Zimm and J.L. Lundberg, J. Phys. Chem., 60 (1956) 425. [18] J. Crank, The Mathematics of Diffusion, Oxford University Press, London, 1975. [19] I.J.F. Vankelecom, E. Scheppers, R. Hues and J.B. Uytterhoeven, Parameters influencing zeolite incorporation in PDMS membranes, J. Phys. Chem., 98 (1994) 12390. [20] I. Blume, A. Bos, P.J.F. Schwering, M.H.V. Mulder and C.A. Smolders, Transport phenomena of vapor and liquid permeants in elastomeric membranes, J. Membrane Sci., 61 (1991) 85.