13193
J. Phys. Chem. 1995,99, 13 193- 13197
Influence of Zeolites in PDMS Membranes: Pervaporation of Water/Alcohol Mixtures Ivo F. J. Vankelecom,* Dirk DeprC, Stijn De Beukelaer, and Jan B. Uytterhoeven Centrum voor Oppervlaktechemie en Katalyse, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium Received: October 27, 1994; In Final Fonn: March 7, 1 9 9 9
A strong influence of the zeolite was found on the pervaporation of alcohoVwater mixtures using filled PDMS membranes. In all cases, the zeolites reduced the swelling of the PDMS. Incorporation of the hydrophilic zeolite Y increased water fluxes, while ZSM-5 reduced both water and alcohol fluxes due to a partial retention of these molecules in the zeolite. For branched alcohols, the extreme importance of diffusion led to a strong dependency of selectivity and flux on temperature. Compared with commercially available PDMS membranes, better results were realized with the self-prepared membranes.
Introduction Since 1988,' zeolite-filled PDMS membranes have gained increasing attention in the literature.2-'0 For the separation of watedethanol mixtures, an exclusion and tortuosity effect together with an altered concentration profile in the membrane' was proposed to explain the increased fluxes and selectivities caused by incorporation of silicalite. Long and branched alcohols showed a limited diffusion through the composite membrane. For the pure zeolite, the order for total sorption of alcohols' from aqueous solutions (expressed as mg/g) was as follows: butanol > propanol > ethanol > tert-butyl alcohol > methanol, while diffusion coefficients were in direct relation to the alcohol chain length.I2 For PDMS, higher sorptions were found for higher alcohol^,^^ and the diffusion coefficient of water was twice that of ethanol.I4 In a previous report,15the incorporation of zeolites in PDMS was studied from the viewpoint of membrane preparation, leading to the concept of a zeolite-dependent cross-linking action on the PDMS. This knowledge concerning zeolite-polymer interactions will now be applied to explain pervaporationresults using these zeolite-filled membranes. Some selected alcohol/ water mixtures are chosen with varying chain length and kinetic diameter. Both sorption and pervaporation aspects are investigated, first on the pure polymer and then on the composite membrane to study the influence of the different zeolite types incorporated. Indirectly [(P(permeability) = D (diffusion) x S (solubility)]and on a merely qualitative basis, these numbers are used to provide information on diffusion. Activation energies for pervaporation are given, and the self-prepared membranes are finally compared with commercially available GFI' membranes in the pervaporation of ethanol from an aqueous solution.
Materials and Methods Membrane Preparation. Membranes were prepared using the method and materials described elsewhere,15 but with an improved solvent evaporation and polymer curing. Zeolite samples were weighed after equilibration at constant humidity, and dry weight was calculated by subtracting the water content. Water was removed in a vacuum oven for a minimum of 1 h at 180 "C. It was proven experimentally that this drying of the zeolites was indeed sufficient: after several periods of drying, @
Abstract published in Advance ACS Abstracts, August 1, 1995.
Pump
Cold rrap
LJ-
Figure 1. Schematic representation of the pervaporation unit.
the same sample was weighed until constant weight was reached. This was always the case within less than 1 h. Methyl isobutyl ketone (MIBK), dried over zeolite A, was added as to the zeolite. A treatment of 1 h in an ultrasonic bath was applied then to break the crystal aggregates and improve dispersion. The crosslinker was added to the zeolite dispersion, and this mixture was further stirred for 2 h. Finally, the prepolymer was added and mixing was continued for another hour. To obtain mixtures with comparable viscosity for all zeolite contents, more solvent was used for higher zeolite loadings. Air bubbles were removed from the mixture by vacuum treatment. After casting the 30 wt % membranes with a thickness of 500 pm, the glass plate was placed in a vacuum oven for 1 h at room temperature and then at 40 "C for another hour. The final curing took place at 150 "C during 45 min. The membrane was removed from the glass plate by immersion in warm water and was then air-dried. The zeolite content is expressed in volume percent as (weight of zeolite)/d,,,lite
+
(weight of zeolite)/dzeOlite (weight of polymer)/dpolymer
in which d is the density. Sorption Measurements. Sorption of pure liquids in the membranes was investigated on 1.5 x 5 cm2 membrane strips at room temperature. To desorb the water from the polymer and the zeolite in particular, the membrane was treated at 150 "C under vacuum before measuring the sorption. The membranes were then immersed in the pure liquids for at least 1 h. The amount sorbed was determined by weight. The membrane surface was wiped dry, before weighing as quickly as possible, so as to minimize evaporation and keep the error on the measurements below 2%. Pervaporation. The apparatus (GFT Co., Germany) is shown in Figure 1. The membrane is brought on a sintered stainless steel support and sealed with an O-ring, made form Teflon-coated silicone rubber. The permeate is trapped with liquid nitrogen and the vacuum is kept constant at about 2 mbar.
0022-3654/95/2099-13 193$09.00/0 0 1995 American Chemical Society
Vankelecom et al.
13194 J. Phys. Chem., Vol. 99, No. 35, 1995
TABLE 1: Hildebrandt Values for the Compounds Used
and the Experimental Sorption in PDMS" 6d 6, dh APDMS Awater sorption water ethanol propanol butanol rerr-butyl alcohol a
19.50 15.80 16.00 16.00 14.85
17.80 8.80 6.80 5.70 2.65
17.60 22.18 19.40 17.08 17.40 14.36 15.80 12.43 14.56 10.25
0.00 9.90 11.55 12.72 16.14
0.02 0.07 0.13 0.19 0.35
Hildebrandt values are taken from ref 17 for the liquids and from
ref 16 for the PDMS. d values are in sorption in mL/g.
6, A in &,
0.35
0.3
3 0.25 .5
t-But
0.2
and
0.15
E.
g
0.1 0.05
The fluxes are normalized to a membrane thickness of 100 pm, and selectivity is expressed as
0
PDMS
Si1
ZSM-5
ZeoY
Fimrre 2. Influence of the zeolite on the somtion of the different
in which y refers to the concentration of the components in the permeate and x to the concentration in the feed. Analysis of the alcohol solutions was done on GC (Type HP 5890 coupled to a HP 3392A integrator). A WCOT fused silica column was used with a CP-WAX-52 CB packing (50 m length, 0.32 mm i.d., 0.45 mm o.d., and 1.2 pm film thickness). The injector was at a temperature of 270 "C and the FID at 280 "C with a column head pressure of 62 kPa. Nitrogen was used as carrier gas with a column flow of 1.6 mumin and a split flow of 200 mumin. The oxygen flow was 158 mumin, the hydrogen 29.5 mL/min, and the nitrogen makeup gas 19.5 mL/min. The analysis was performed with an isothermal column temperature of 70 "C for ethanol, 160 "C for propanol, 200 "C for butanol, and 100 "C for tert-butyl alcohol. 2-Propanol was used as an external standard. For the calculations of the activation energies, fluxes were corrected for driving force. Measurements at three different temperatures (35, 55, and 75 "C) were done in triplicate.
b ..-
e 0
4 "C
v)
Eth
But
tRut
700
600
=% 3 M
500 400
300
Results and Discussion
200
Sorption of Pure Liquids in PDMS. For pure PDMS, a clear relation is observed in Table 1 between the amount of alcohol sorbed and the Hildebrandt parameter APDMS, obtained from NijhuisI6 for PDMS and Watson and Payne" for the alcohols and water
100
in which p stands for polymer and s for solute. This is in agreement with Mulder.I3 The high tert-butyl alcohol sorption indicates a very strong interaction of this compound with the PDMS matrix. Sorption of Pure Compounds in Zeolite-Filled PDMS Membranes. In Figure 2, the trend found for pure PDMS is recognized when zeolites are incorporated. For all membranes, the same order is found: tert-butyl alcohol sorbs best, followed by the alcohols in the order of decreasing chain length and finally water. Only the zeolite Y-containing membrane is an exception. In this membrane, more water is sorbed than ethanol. The highly hydrophilic nature of this zeolite and its high zeolite sorption capacity induce this effect. Comparing filled and unfilled membranes, incorporation of all zeolites leads to a reduced swelling of the membrane, to be ascribed to the crosslinking action of the ze01ite.I~ However, a significant difference can be observed when the different kinds of zeolite are compared. The more hydrophobic character of silicalite is revealed upon comparison with ZSM-5. The former zeolite results in a higher membrane sorption capacity for the more
Prop
'otal ihol
0
Eth Prop But tBut
Eth Prop But tBut
Figure 3. Selectivities (a, top) and fluxes (b, bottom) for the pervaporation of 6 wt % alcohol solutions at 35 and 74 "C. hydrophobic alcohols and a reduced sorption for water. The fact that the silicalite-filled membrane sorbs more water than the unfilled PDMS indicates a more hydrophobic character for the polymer than for the zeolite. The sorption in the zeolite Y-containing membranes is always higher than in the other membranes. This is in contrast to the observations in which xylene s~rptions'~ were found to be lowest for the zeolite Y-filled membrane, due to the very strong physical cross-linking of this wide pore zeolite. Here, the opposite is observed, ascribed to the fact that the alcohols-in contrast to xylenes-have a much higher percentage sorbed in the zeolite than in the PDMS phase. This means xylene sorption is a good tool to observe changes in PDMS properties, but for alcohols a more important influence is exerted by the zeolite. Pervaporationwith Pure PDMS Membranes. Flues and Selectivity as a Function of Temperature. Figure 3 shows the selectivities (a) and fluxes (b) for the pervaporation of the different alcohols from aqueous solutions at two different temperatures, using unfilled PDMS. In agreement with what was found by te Hennepe,' higher fluxes and increased selectivitiesare observed for the linear alcohols as chain length increases. Water fluxes are almost constant in all pervaporations, irrespective of the alcohol permeating. This means that
J. Phys. Chem., Vol. 99, No. 35, 1995 13195
Zeolites in PDMS Membranes
45
200
40
175
35
150
30
125
'$
100
,i 20
75
50 25
Figure 4. Water, alcohol, and total fluxes as a function of the alcohol and the membrane used in the pervaporation of aqueous solutions containing 6 wt % alcohol at 35 "C.
the increased swelling of the PDMS with the higher alcohols is compensated by the reduced interaction in the membrane of the water with the sorbed alcohol molecules (reflected in the high Awaterin Table 1). In spite of the high sorption in PDMS, tertiary butanol fluxes are rather low. This reveals an important influence of diffusion on the pervaporation of this branched molecule. The very large differences found between the low- and high-temperature alcohol flux (9 times higher) and selectivity (2.5 times higher) confirm this. Indeed, in general a diffusion limited process is more influenced by temperature. For the linear alcohols, a smaller difference is found between both temperatures. The moderately high increase in butanol selectivity with increasing temperature (factor of 1.24) explains a rather important influence of diffusion on the pervaporation of this alcohol. Being a larger molecule, the temperature dependency is indeed expected to be strongest for propan01.l~ In contrast to Slater et a1.,18 a more detailed investigation of the influence of temperature did not reveal an optimum in ethanol selectivity, but a steady increase with rising temperature. Pervaporation Using Zeolite-Filled PDMS. Ethanol. Considering the total flux (Figure 4) in the pervaporation of ethanol-water mixtures, the very high flux for the zeolite Y membrane is striking. This flux can be fully assigned to the high water flux, which is undesirable for the pervaporation studied. Considering the high water sorption of zeolite Y (reflected in Figure 2) and the large pores allowing fast diffusion, this is not surprising. All other membranes-except the silicalite-filled membrane-show reduced total fluxes compared with PDMS, undoubtedly due to the cross-linking action of the zeolite on the p01ymer.l~ The high total flux for the silicalitefilled membrane is mainly a consequence of a very high ethanol flux. Simultaneously, water flux is slightly increased, due to the higher water sorption in the silicalite filled membrane in comparison to the pure PDMS (Figure 2). These observations are better expressed in Figure 5 showing selectivities. The silicalite-filledmembrane obviously performs best for ethanol, while the zeolite Y membrane shows the lowest selectivity. ZSM-5-PDMS has a higher selectivity than the pure PDMS, but this is merely due to the cross-linking effect of the zeolite. This effect leads not only to a decreased ethanol and water flux but also to a reduced swelling of the membrane, excluding the water molecules more. l9 Milestone and Bibby' reported that-apart from an increased sorption for ethanol with increasing A1 content-a stronger retention of the molecules occurred. The latter effect seems to be the strongest here, as the ethanol flux through the silicalite-filled membrane is higher than through the ZSM-5-filled PDMS. It also provides an explanation for the reduced water flux of the ZSM-5-filled membrane compared
Ifl
25 15 10
5
n
Figure 5. Selectivities obtained from the pervaporation of aqueous solutions containing 6 wt % alcohol at 35 "C.
to the silicalite-filled one. 'Indeed, this more hydrophilic ZSM-5 sorbs more water (increased 5') and should consequently increase the water fluxes (increased P as P = D x S) of the composite membrane. When substituting a ZSM-5 for a silicalite, diffusion is expected not to change tremendously: the pathway for diffusion through the zeolite crystal is hardly changed. Therefore, the only explanation left for the reduced water flux is a stronger retention of ethanol and water in the ZSM-5 pores. The differences with the commercially available GFT membranes will be explained in the overview. Propanol. The same tendencies shown for ethanol can be observed for propanol. The zeolite Y membrane shows the highest fluxes due to the very high water flux. Silicalite-PDMS has the highest selectivity,even though the alcohol flux is lower than for pure PDMS. As shown in Table 1, propanol has a higher affinity for PDMS than ethanol. This means that a relatively large part of the transport will occur through the PDMS. Consequently, the propanol transport will be more affected by the reduced swelling of the PDMS when silicalite is added than in the case of ethanol. Furthermore, the alcohol transport through the silicalite will be less fast, since diffusion decreases with increasing chain length.l 2 For ZSM-5-filled membranes, a more pronounced reduction in alcohol flux compared to the silicalite-filled membrane can be observed than in the case of ethanol. It confirms the stronger retention of the longer alcohols in ZSM-5 zeolites, as reported by Milestone and Bibby." Butanol. For butanol, the same explanations hold as for propanol. The effects are even stronger, due to a more pronounced preference for transport through the PDMS. tert-Butyl Alcohol. The very high sorption values found for tert-butyl alcohol in Figure 2 are in contrast with the relatively low alcohol permeation fluxes. Again, this proves the importance of diffusion in the pervaporation process of this large molecule. Activation Energies. The activation energy for pervaporation is just a means to express the temperature dependency of this process. It cannot be interpreted as an activation energy in the strict sense of the word. It is more like an apparant activation energy of a process consisting of several steps, each of them showing a different temperature dependency. The calculation of the overall activation energies for flux confirms the explanations given above. The order of activation energy for the linear alcohols is ethanol > propanol > butanol (Figure 6). This is the opposite order as the one found for sorption. Indeed, higher sorption implies higher swelling of the membrane and consequently an easier permeation through the membrane. The branched alcohol forms an exception on this rule. As said before, tert-butyl alcohol is strongly hindered during the permeation through the membrane, which is reflected in high activation energies for this alcohol. The value found
Vankelecom et al.
13196 J. Phys. Chem., Vol. 99, No. 35, 1995 8o h
5
O'
60
.
r
1
ZeoY
PDMS .Si1
;20
,GFTsiI GFT m
ZSM-5 0 ' 0
5
IO
15
20
25
Ethanol flux (g/m'.h) Alcohol
Figure 6. Activation energy for fluxes of water and alcohols through the composite membranes.
for ethanol through PDMS is in agreement with the 39.3 kJ/ mol found by Slater et a1.18 Within the experimental error, the permeation of water through the membranes is always less activated than the permeation of the copermeating alcohol. It indicates the fact that water is entrained by the alcohol through the membrane. The shorter the alcohol chain, the stronger this effect. Indeed, the water-alcohol interactions are stonger for the more hydrophilic compounds as shown in Table 1 by Awater.Again, other explanations hold for tert-butyl alcohol. Even though this molecule does not show strong interactions with the water, it interacts with the membrane polymer so strongly that the increased swelling enables a*lot of water to be copermeated, in spite of the low interactions. Comparison with Literature. The absolute values reported in te Hennepe' are difficult to compare with the values found here as they were taken under different experimental circumstances. However, it is remarkable that in te Hennepe' all alcohol fluxes increase with increasing silicalite content, while in our case the permeation of only one compound, namely ethanol, is favored by zeolite incorporation. The only differences appear in the membrane synthesis. Probably, our procedure leads to a better dispersion and an improved interaction between the zeolite and the polymer, inducing a higher degree of cross-linking in the membrane. The explanation given by te Hennepe concerning the exclusion of water molecules from the zeolite pores does not hold for our systems or, anyway, is not the most important factor. As shown in Figure 3b, the water flux is almost independent of the copermeating alcohol. For methanol, ethanol, and propanol, te Hennepe found a decreasing selectivity for increasing temperature. In Figure 3a, this is shown not to be the ease here. Again, it indicates a stronger influence of alcohol diffusion through the self-prepared membranes, caused by the improved zeolite-polymer interaction. Overview and Comparison with Commercial Membranes. Figure 7 summarizes the performance of the six membranes tested in the pervaporation of ethanol. To reduce the energy for condensation of the permeate in potential applications, it is required to have water fluxes as low as possible. From that viewpoint, the best membranes are situated in the right bottom corner of the graph. It is clear, then, that the incorporation of silicalite in the PDMS membrane is highly recommended for the removal of ethanol from aqueous solutions. Compared with the pure PDMS membrane, organic flux is increased to 167% of its original value, while water flux is reduced to 80%. As discussed previously,l5 the self-prepared PDMS films show much higher fluxes than the commercial GFT membranes. This can be ascribed to the attachment of the GFT top layer on a support, reducing swelling of the membrane and consequently the fluxes. The incorporation of silicalite improves membrane performance far more for the self-prepared membranes, even
Figure 7. Schematic overview of the performance of the different membranes in the pervaporation of ethanol from aqueous solutions at 35 "C.
though only 20 vol % of zeolite is added here in comparison with the 50 vol % for the GFT-filled membrane. The chemical nature of the GFT-PDMS can provide an explanation for this phenomenon, since the GFT-PDMS chains carry terminal acrylate groups that cross-link after electron beam irradiation. This excludes the PDMS chains from reaction with the zeolite surface. It means that no drastic changes happen in this polymer material when zeolite is added. Meanwhile, the attachment of the active layer to the support keeps swelling equally limited in both the filled and the unfilled membrane. The silicalitefilled GFT membrane (GFTsil) has the highest flux, due to the increase in both water and alcohol flux. The increased water flux is probably caused by a poor adhesion between the acrylate groups and the zeolite particles. The self-prepared zeolite-filled membranes, on the other hand, undergo a cross-linking from the silicalite,15 reducing fluxes and especially the swelling of the membrane, leading to higher selectivities than for the pure PDMS. The introduced zeolite pore system in the polymer provides a pathway for fast diffusion of the small ethanol molecules, while water is almost completely excluded from the pores in the case of silicalite.
Conclusions Using zeolite-filled PDMS membranes in the pervaporation of alcohols from aqueous solutions, sorption was found to play an important role and to be predictable by the Hildebrandt parameters. On the level of sorption, the influence of the zeolite is especially an effect of increased membrane cross-linking. The pervaporation of aqueous alcohol solutions showed a strong influence of the zeolite, even though only 20 vol % was added. In all cases, the incorporation of the hydrophilic zeolite Y caused an increase in water flux. The incorporation of ZSM-5 reduced-fluxes compared with the silicalite-filled membrane, due to a partial retention of both alcohols and water in the zeolite pore system. In only one case, the use of a zeolite-filled membrane increased the organic flux compared with PDMS: silicalite-filledPDMS in the pervaporation of ethanol. For the higher alcohols, transport through the polymer phase became more important, and diffusion through the zeolite pores was too slow. Selectivities on the other hand increased for all pervaporations using silicalite-filled membranes, as a consequence of reduced swelling and the hydrophobic character of the zeolite. The extreme importance of diffusion for the branched alcohol led to a very strong temperature dependency of selectivity and flux. The two commercial membranes showed low fluxes, basically because of the reduced possibility for swelling and low selectivities due to the chemical nature of the membrane. A very small difference between the silicalite-filledGFT-PDMS and the unfilled GFT-PDMS was observed, attributed to a poor adhesion between the polymer and the zeolite.
Zeolites in PDMS Membranes
Acknowledgment. We are grateful to the Belgian Govemment for support in the form of a IUAP-PA1 grant on Supramolecular Catalysis. I.F.J.V. acknowledges a fellowship as Research Assistant from the Catholic University of Leuven, Belgium. References and Notes (1) te Hennepe, J. PhD Thesis, University of Twente, 1988. (2) Bartels-Caspers, C.; Tusel-Langer, E.; Lichtenthaler, R. N. J. Membr. Sci. 1992, 70, 75-83. (3) Dotremont, C.; Goethaert, S . ; Vandecasteele, C. Desalination 1993, 91, 177-186. (4) Duval, J.-M.; Folkers, B.; Mulder, M. H. V.; Desgrandchamps, G.; Smolders, C. A. J. Membr. Sci. 1993, 80, 189-198. (5) Duval, J.-M. PhD Thesis, University of Twente, 1993. (6) Goethaert, S.; Dotremont, C.; Kuijpers, M.; Michiels, M.; Vandecasteele, C. J . Membr. Sci. 1993, 80, 189-198. (7) Goldman, M.; Fraenkel, D.; Levin, G. J. Appl. Polym. Sci. 1989, 37, 1791-1800. (8) Jia, M.-D.; Peinemann, K.-V.; Behling, R.-D. J. Membr. Sci. 1991, 57, 289-296.
J. Phys. Chem., Vol. 99, No. 35, 1995 13197 (9) Jia, M.-D.; Peinemann, K.-V.; Behling, R.-D. J. Membr. Sci. 1992, 73, 119-128. (10) Lamer, T.; Voilley, A. Proc. Int. Con$ Peruaporation Processes Chem. Ind. 5th 1991, 110-122. (1 1) Milestone, N. B.; Bibby, D. M. J. Chem. Technol. Biorechnol. 1983 , 34A, 73-79. (12) Nayak, V. S.; Moffat, J. B. J. Phys. Chem. 1988, 92,7079-7102. (13) Mulder, M. Peruaporation Membrane Processes; Huang, R. Y. M., Ed.; Elsevier: Amsterdam, 1991; Chapter 3. (14) Okamoto, K.; Butsuen, A,; Tsuru, S . ; Tanaka, K.; Kita, H.; Asakawa, S . Polym. J. 1987, 19, 747-756. (15) Vankelecom, I. F. J.; Scheppers, E.; Heus, R.; Uytterhoeven, J. B. J. Phys. Chem. 1994, 98, 12390-12396. (16) Nijhuis, H. H. PhD Thesis, University of Twente, 1990. (17) Watson, J. M.; Payne, P. A. J. Membr. Sci. 1990, 49, 171-205. (18) Slater, C. S . ; Hickey, P. J.; Juricic, F. P. Sep. Sci. Technol. 1990, 25, 1063. (19) Feng, X.; Huang, R. Y. M. J. Membr. Sci. 1992, 74. 171-181. JP942899A
