- Email: [email protected]
Structure of mixed matrix membranes made with SAPO-5 zeolite in polyurethane matrix
Available online at www.sciencedirect.com
Microporous and Mesoporous Materials 115 (2008) 61–66 www.elsevier.com/locate/micromeso
Structure of mixed matrix membranes made with SAPO-5 zeolite in polyurethane matrix Gabriela Ciobanu a,*, Gabriela Carja a, Octavian Ciobanu b b
a Gh. Asachi Technical University of Iasi, Faculty of Chemical Engineering, D. Mangeron Building, No. 71A, 700050, Iasi, Romania Gr. T. Popa Medicine and Pharmacy University of Iasi, Faculty of Medical Bioengineering, Universitatii Street, No. 16, 700115, Iasi, Romania
Received 5 July 2007; accepted 22 January 2008 Available online 19 February 2008
Abstract SAPO-5 zeolite has been used to modify the structure of polyurethane membranes and to improve their properties. Scanning electron microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR), Bubble-point test, water permeability, swelling in water and several alcohols (methanol, ethanol, 1-propanol and 1-butanol) and density measurements have been used to study the incorporation of SAPO-5 zeolite in polyurethane membranes, the changes of supramolecular structure of polyurethane in the membrane network and the structural uniformity of the obtained membranes. As a function of the preparation method, anisotropic membranes can be obtained. The incorporation of SAPO-5 zeolite in polyurethane matrix also induces changes in the membrane morphology. The zeolite plays a cross-linking function in the membrane structure. All the tested membranes show a tendency to swell with ethanol. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Mixed matrix membranes; SAPO-5 zeolite; Polyurethane matrix; SEM; FTIR
1. Introduction Mixed matrix membranes for gas or liquid separation processes have been the subject of growing interest in the past 20 years [1]. These membranes are a new class of materials that offer the potential of significantly advancing the current technology. Mixed matrix membranes that are fabricated by encapsulating the molecular sieves into the polymer matrix have been recognized as a promising alternative to the conventional membranes. These hybrid materials combine the separation properties of zeolites or molecular sieves with the low cost and processability of polymers into one membrane. Zeolite-filled membranes are frequently mentioned in literature where are related to zeolite membranes with silicalite, ZSM-5, ZSM-35, L, Y, NaA, etc. [1–10]. Remarkable improvements on membrane performances due to incorpo*
Corresponding author. Present address: Str. A. Panu 32-2A, Iasi, 700020, Romania. Tel.: +4 0332 436344; fax: +4 0232 271311. E-mail address: [email protected] (G. Ciobanu). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.01.049
ration of zeolites have been described. Several specific properties of zeolites have already been brought to expression in composite membranes, such as their ion-exchange capacity, adsorption capacity, molecular sieving effect and hydrophobic/hydrophilic character [11–14]. In this study, SAPO-5 zeolite has been used to modifying the structure of polyurethane membranes and to improving their properties. As a function of the preparation method anisotropic membranes can be obtained. The incorporation of SAPO-5 zeolite in polyurethane matrix also induces changes in the membrane morphology. The zeolite plays a cross-linking function in the membrane structure. The SAPO-5 molecular sieves are microporous solids which contain non-intersecting tubular channels circumscribed by 12-membered rings with a free diameter of ˚ . Their surface selectivity varies from moderate 7.3–8.0 A up to high hydrophilic; consequently, SAPO-5 molecular sieves adsorb both polar and nonpolar molecules. SEM, FTIR, water permeability, swelling in water and several alcohols (methanol, ethanol, 1-propanol and 1-butanol) and density measurements have been used to study the
62
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66
incorporation of SAPO-5 zeolite in polyurethane membranes, the changes of supramolecular structure of polyurethane in the membrane network and the structural uniformity of the obtained membranes. 2. Experimental The structure of used polyurethane polymer is shown in Fig. 1. Polyurethane membranes were prepared using the method and materials described elsewhere [15,16]. The membranes were prepared by casting a polyurethane (PU) + cellulose acetate (CA) + N,N-dimethylformamide (DMF) solution containing calculated amounts of PU and CA polymers on a glass plate and allowing the solvent to evaporate under caloric radiation. We have prepared some pure membranes, the most performing one was denoted PM. The mixed matrix membranes were obtained by adding a calculated amount of zeolite SAPO-5 into the polymer solution and by mixing thoroughly before casting. The casting and curing of the mixed matrix membranes are identical with those of the pure PU membranes. Series of membranes with loading varying between 10 and 70 wt% SAPO-5 were made and denoted as: PMZ-1, PMZ-2 and PMZ-3. These membranes contain 20 wt% SAPO-5 zeolite (PMZ-1 sample), 35 wt% SAPO-5 zeolite (PMZ-2 sample) and 50 wt% SAPO-5 zeolite (PMZ-3 sample). SAPO-5 zeolite synthesis were carried out using silica ROMSIL (82.62% SiO2, Reactivul Bucuresti, Romania), Al2(SO4)3 18H2O (>98%, Merck) and H3PO4 (89%, Merck) as silicon, aluminum and phosphorus sources, respectively, triethylamine (TEA) (>99%, Merck) as templating agent. The silicoaluminophosphate SAPO-5 was synthesized by hydrothermal crystallization according to the procedure described elsewhere [17].
The membranes were characterised by scanning electron microscopy (SEM) (VEGA//TESCAN SEM-EDX instrument), Fourier transform infrared spectroscopy (FTIR) (SPECTRUM 100 FTIR Perkin Elmer spectrometer), Bubble-point test (with a laboratory instrument ‘‘home-made”; details about its features were previously reported [15,16]) water permeability, swelling in water and several alcohols (methanol, ethanol, 1-propanol and 1-butanol) and density measurements. Density measurement is based on Archimedes’ law. First, a dried piece of membrane is weighed as such and then once more while immersed in a liquid of known density. From the difference in weight and the liquid density, the volume of the membrane is calculated. Dividing the membrane dry weight by this volume yields the density of the membrane. The membrane permeation tests were conducted by using a simple dead-end permeation cell. Circular membrane discs were cut and mounted in a cylindrical membrane test cell. Effective permeation area of each membrane was about 14.5 cm2. Feed pressure was controlled at about 5 bar, while the permeate side was opened to the atmosphere. Experiments were carried out at ambient temperature (25 °C). For the pure water, the flux J (m/s) is calculated by following equation: J¼
V tA
where V is volume of permeate collected (m3), t is the sampling time (s) and A is the membrane area (m2). The main characteristics of the membranes used in this study are summarised in Table 1. Swelling of pure liquids in the membranes is investigated on 1.5 5 cm2 membrane strips at room temperature (25 ± 2 °C). To desorb the water from the polymer, the membrane was treated at 150 °C under vacuum before measuring the swelling. A piece of membrane after drying to constant weight (wd) was immersed in the pure liquid (water or alcohol) for at least 1 h. When the sorption equilibrium was reached, the piece was weighed rapidly after blotting free surface liquid. The swelling coefficient (SC) is calculated using the following relationship: SC ¼
Fig. 1. The structure of the polyurethane polymer.
ð1Þ
ws wd 1 d wd
ð2Þ
Table 1 The characteristics of the polyurethane membranes Sample
Zeolite content
Thicknessa (lm)
b
PM PMZ-1 PMZ-2 PMZ-3 a b
0 20 35 50
By SEM method. By Bubble-point method.
245 220 215 95
J 104 (m/s)
Density (g/cm3)
Pore diameter (lm) a
In active layer
In substructure
Theoretical
Experimental
2.3–4.7 2.0–4.4 1.9–2.9 0.3–1.4
11–22 5–9 4–10 2–3
1.250 1.350 1.425 1.495
0.2831 0.3229 0.3962 0.9835
6.34 7.12 7.37 7.41
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66 Table 2 Swelling coefficients (SC) of polyurethane membranes at 25 °C Solvent
SC (cm3/g) PM
PMZ-1
PMZ-2
PMZ-3
Water Methanol Ethanol 1-Propanol 1-Butanol
3.52 3.84 3.86 3.64 3.38
2.98 3.74 3.88 3.65 3.20
2.94 3.31 3.33 3.16 3.13
0.61 0.56 0.62 0.59 0.57
63
litical) chemical cross-linking is possible involving a reaction of the surface zeolite hydroxyls with the polymer (Fig. 2b). The polymer containing terminal groups (– NCO and –OH) which may react with the surface hydroxyls on the outer surface of the zeolite particles, especially at the temperatures used for the membrane preparation. A covalent link may be formed between the polymer and the zeolite particles. 3.1. SEM investigations
where wd is the weight of dry membrane (g), ws is the weight of solvent swollen membrane (g) and d is density of solvent taken (g/cm3). The values of swelling coefficients (SC) for polyurethane membranes in different solvents are given in Table 2. 3. Results and discussion Polymers with low crystallinity, such as polyurethanes, are represented well by two-phase model for the coating process. In forming the membrane material three organisational levels for the polymer structure can be defined, namely: the chemical, the macromolecular and the supramolecular structure. The supramolecular structure presumes the spatial location of the polymer chains by crosslinking effect [15]. The surface of SAPO-5 zeolite contains hydroxyl groups, T-OH (T = Si, Al, P). Taking into account the reactivity of hydroxyl groups, the linking of these to –N– H groups (on polymer structure) is possible by hydrogen bonds, with partial destruction of the hydrogen bond associations within polymer chains. Fig. 2 shows the possible cross-linking in zeolite-filled polyurethane membranes. Apart from physical cross-linking (Fig. 2a) attributed to hydrogen bonds (between polymer chains and HO–Si zeo-
The SEM micrographs for unfilled- and zeolite-filled polyurethane membranes are presented in Fig. 3. The unfilled polyurethane membranes have an asymmetric structure consisting of the dense top layer (active layer) supported by the porous sub-layer (substructure) (Fig. 3a). The top layer is a thin and dense permselective skin. The thick substructure of the membrane consists of a band of intermediate density (the transition layer) consisting of widely open pores and a band of open-macro-voids. The estimated pore size of top layer for the pure polyurethane membranes is much smaller than of the supported layer (Table 1). Accordingly, the molecular sieve property of the pure polyurethane membranes appears to be determined by the pore size in the top layer of the polyurethane membrane. SEM pictures reveal an increase of anisotropy in zeolitefilled polyurethane membranes (Fig. 3b). These membranes have an asymmetric structure consisting of the top skin (active layer), the substructure and the bottom skin. The dense bottom skin layer is composed of some aggregates of zeolite crystals in a polymeric matrix. Sedimentation of the zeolite crystals in the mixture at the very beginning of the curing procedure explains this phenomenon. In SEM pictures of the cross-section of zeolite-filled polyurethane membranes were observed the voids, the void walls and the zeolite deposits. Zeolite crystals can be seen spread over the internal surface of the membrane as white spots. Zeolite spots are also visible within the supporting structure of the membrane (Fig. 3c). The cross-section confirms the good incorporation of the zeolite only in the top skin (active layer) and in the substructure of the membrane. The estimated pore size of layer for the zeolite-filled polyurethane membranes is smaller than of the pure polyurethane membranes (Table 1). 3.2. FTIR analysis
Fig. 2. Possible physical (a) and chemical (b) cross-linkings in zeolite-filled polyurethane membrane.
The FTIR spectroscopy is used for study of the specific interactions in the unfilled- and zeolite-filled polyurethane membranes. The FTIR spectrum of the pure polyurethane membrane (PM sample), is presented in Fig. 4. The N–H stretching vibration of urethane occurs approximately at 3400 cm1 and the peak has split into two sharp bands. The primary intermolecular interactions occurring in urethanes is the hydrogen bonding between the urethane C@O group of
64
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66
Fig. 3. SEM images of membrane samples in the cross-section: (a) PM, (b) PMZ-2, (c) the void wall in PMZ-2 membrane.
Fig. 4. Results of the FTIR performed on polyurethane membrane (PM sample), zeolite-filled polyurethane membrane (PMZ-2 sample) and SAPO-5 zeolite.
one unit with the urethane N–H group of another. Hence chains of hydrogen-bonded urethane groups are formed and at the ends of these chains are two non-hydrogenbonded groups, one C@O and one N–H. This is the reason why the N–H stretching vibration has split into two bands
one corresponding to the hydrogen-bonded N–H and the other for non-bonded. Correspondingly there is the C@O stretching vibration observed approximately at 1660 cm1 and the peak has split into two sharp bands. This again indicates the presence of bonded and non-bonded carbonyl groups. The FTIR spectrum of the calcined SAPO-5 sample, is presented in Fig. 4 and a broad band around 3200– 3600 cm1 is assigned to T-OH (where T = Si, Al, P) groups. For the zeolite-filled polyurethane membranes, the FTIR spectra were taken in order to investigate the changes in the chemical environment between the polymer chains and zeolite crystals. In the zeolite-filled polyurethane membrane (PMZ-2 sample), FTIR spectrum (Fig. 4) indicate hydrogen bonding between the urethane N–H groups of the polyurethane polymer and T-OH groups (T = Si, Al, P) of the zeolite. The small peak at 3220 cm1 reflected the presence of the hydrogen bonding between the zeolite and polymer. The zeolite is shown to be involved in the cross-linking of polyurethane in the membrane structure.
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66
65
3.3. Swelling and water permeability measurements
4. Conclusions
From the date on degree of swelling (Table 2), all membranes showed a tendency to swell with ethanol, while swelling only a little with water. This is due to the fact that the distance between the solubility parameter of membrane and that of water is too far for water to swell the membrane. Consequently, the polyurethane membranes are indicated for ethanol separation on water–ethanol mixtures by pervaporation process. Comparing filled and unfilled membranes, incorporation of SAPO-5 zeolite in polyurethane matrix lead to a reduced swelling of the membrane, to be ascribed to the cross-linking action of the zeolite. The degree of cross-linking of these composites can be explained from their swelling ratios. The degree of cross-linking of composites increased with decreasing swelling ratio. To study the effects of zeolite addition on polyurethane membrane performance, the pure water permeability was carried out on both polyurethane and zeolite–polyurethane membranes. The results are tabulated in Table 1. It shows that the pure water fluxes of the membranes increased as the loading of zeolite increased. This is due to the fact that the compositional complexity of the SAPO-5 framework in the three T elements (Si, Al and P), by their difference in electronegativity, confers to the surface of the SAPO crystals from moderate up to a high hydrophilic character. PMZ-3 membranes have a smaller thickness of ca. 95 lm (when compared to about 200 lm for the others), thus that keeping constant the driving force (about 4 bar) a larger flux may be expected for a material having similar pore size. The pore size is considerably smaller for PMZ-3, what could justify the similar water flux even the thickness being quite different.
The SAPO-5 zeolite-filled polyurethane membranes with perselectivity to the gases or liquids were prepared and morphological characterised. Polyurethane solutions were selected as organic network and a series of membranes, with content of zeolite varying between 10 and 70 wt%, were prepared and characterised by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Bubble-point test and density measurements. The preparation method used results in anisotropic membranes. The incorporation of SAPO-5 zeolite in polyurethane matrix induces some changes in the membrane morphology. A good dispersion of the zeolite is crucial but extremely difficult to obtain especially when small particles are involved. The morphology of the membranes was recognized by SEM studies and verified the absence of voids around the zeolites. This suggested that the zeolite and polymer had good contact at the interface. The cross-section confirms the good incorporation of the zeolite only in the top skin (active layer) and in the substructure of the membrane. Sedimentation of the zeolite crystals in the mixture at the very beginning of the curing procedure explains the dense bottom skin layer formation. The zeolite shows some interactions with the polymer. In the zeolite-filled polyurethane membranes, FTIR spectra indicate hydrogen bonding between the urethane N–H groups of the polyurethane polymer and T-OH groups (T = Si, Al, P) of the zeolite. The SAPO-5 zeolite acted as a cross-linker on the polyurethane polymer and, the final result being in reinforced membrane. The zeolite addition into polyurethane membranes induces an increase in the water flux. Swelling studies of several industrial solvents are investigated through polyurethane membranes, unfilled and filled with SAPO-5 zeolite. All the membranes show a tendency to swell with ethanol. Consequently, the polyurethane membranes are indicated for ethanol separation on water–ethanol mixtures by pervaporation process.
3.4. Density measurements Table 1 shows that membrane density change as zeolite is incorporated. Out of membrane and zeolite densities, a theoretical density of the composite membranes was calculated, assuming a perfect adhesion between the zeolite and the polymer (Table 1). SAPO-5 containing polyurethane membranes could be prepared for loadings up to 70 wt%. Unfortunately, these high loadings resulted in membrane pieces that were too brittle to perform characterisation on. The theoretical calculations for zeolite SAPO-5 containing polyurethane membranes are somewhat different since zeolite SAPO-5 has a greater density than the polyurethanes. By consequence, the theoretical density increases as zeolite content increases (Table 1). In all cases, the experimental value is much lower than the theoretical one, and the difference increases as the zeolite content of the membrane increases. This can be explained by the creation of greater voids in the substructure of the asymmetric membrane.
Acknowledgments The authors gratefully acknowledge the financial assistance from the Ministery of Education and Research, Romania (Grant CEEX, SINAPS, No. 69/2006 and PNCDI II Contract, NATOEPA, Nr. 71-020/2007). References [1] H.J.C. te Hennepe, D. Bargeman, M.H.V. Mulder, C.A. Smolders, J. Membr. Sci. 35 (1987) 39. [2] T. Sano, Y. Kiyozumi, M. Kawamura, F. Mizukami, H. Takaya, T. Mouri, W. Inaoka, Y. Toida, M. Watanabe, K. Toyoda, Zeolites 11 (1991) 842. [3] J.G. Tsikoyiannis, W.O. Haag, Zeolites 12 (1992) 126. [4] B.G. Karle, C.J. Brinker, M.L.F. Phillips, Micropor. Mesopor. Mater. 431 (1996) 237.
66
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66
[5] T.C. Bowen, R.G. Meier, L.M. Vane, J. Membr. Sci. 298 (2007) 117. [6] T.W. Pechar, S. Kim, B. Vaughan, E. Marand, M. Tsapatsis, H.K. Jeong, C.J. Cornelius, J. Membr. Sci. 277 (2006) 195. [7] Y. Li, H.M. Guan, T.S. Chung, S. Kulprathipanja, J. Membr. Sci. 275 (2006) 17. [8] Y. Li, T.S. Chung, C. Cao, Kulprathipanja, J. Membr. Sci. 260 (2005) 45. [9] H.B. Park, C.H. Jung, Y.K. Kim, S.Y. Nam, S.Y. Lee, Y.M. Lee, J. Membr. Sci. 235 (2004) 87. [10] S. Kulprathipanja, Ann. NY Acad. Sci. 984 (2003) 361. [11] R. Kiyono, G.H. Koops, M. Wessling, H. Strathmann, J. Membr. Sci. 231 (2004) 109.
[12] C. Dotremont, S. Goethaert, C. Vandecasteele, Desalination 91 (1993) 177. [13] M.D. Jia, K.V. Peinemann, R.D. Behling, J. Membr. Sci. 57 (1991) 289. [14] C. Bartels-Caspers, E. Tusel-Langer, R.N. Lichtenthaler, J. Membr. Sci. 70 (1992) 75. [15] G. Ciobanu, G. Carja, O. Ciobanu, Mat. Sci. Eng. C 27 (2007) 1138. [16] G. Ciobanu, M. Bezdadea, Rev. Chim.-Bucharest 55 (2004) 140. [17] N. Balba, G. Ciobanu, Analele Stiintifice ale Universitatii ‘‘Al.I. Cuza”, Iasi, sectia Chimie, Tomul I, 1993, 89.
Microporous and Mesoporous Materials 115 (2008) 61–66 www.elsevier.com/locate/micromeso
Structure of mixed matrix membranes made with SAPO-5 zeolite in polyurethane matrix Gabriela Ciobanu a,*, Gabriela Carja a, Octavian Ciobanu b b
a Gh. Asachi Technical University of Iasi, Faculty of Chemical Engineering, D. Mangeron Building, No. 71A, 700050, Iasi, Romania Gr. T. Popa Medicine and Pharmacy University of Iasi, Faculty of Medical Bioengineering, Universitatii Street, No. 16, 700115, Iasi, Romania
Received 5 July 2007; accepted 22 January 2008 Available online 19 February 2008
Abstract SAPO-5 zeolite has been used to modify the structure of polyurethane membranes and to improve their properties. Scanning electron microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR), Bubble-point test, water permeability, swelling in water and several alcohols (methanol, ethanol, 1-propanol and 1-butanol) and density measurements have been used to study the incorporation of SAPO-5 zeolite in polyurethane membranes, the changes of supramolecular structure of polyurethane in the membrane network and the structural uniformity of the obtained membranes. As a function of the preparation method, anisotropic membranes can be obtained. The incorporation of SAPO-5 zeolite in polyurethane matrix also induces changes in the membrane morphology. The zeolite plays a cross-linking function in the membrane structure. All the tested membranes show a tendency to swell with ethanol. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Mixed matrix membranes; SAPO-5 zeolite; Polyurethane matrix; SEM; FTIR
1. Introduction Mixed matrix membranes for gas or liquid separation processes have been the subject of growing interest in the past 20 years [1]. These membranes are a new class of materials that offer the potential of significantly advancing the current technology. Mixed matrix membranes that are fabricated by encapsulating the molecular sieves into the polymer matrix have been recognized as a promising alternative to the conventional membranes. These hybrid materials combine the separation properties of zeolites or molecular sieves with the low cost and processability of polymers into one membrane. Zeolite-filled membranes are frequently mentioned in literature where are related to zeolite membranes with silicalite, ZSM-5, ZSM-35, L, Y, NaA, etc. [1–10]. Remarkable improvements on membrane performances due to incorpo*
Corresponding author. Present address: Str. A. Panu 32-2A, Iasi, 700020, Romania. Tel.: +4 0332 436344; fax: +4 0232 271311. E-mail address: [email protected] (G. Ciobanu). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.01.049
ration of zeolites have been described. Several specific properties of zeolites have already been brought to expression in composite membranes, such as their ion-exchange capacity, adsorption capacity, molecular sieving effect and hydrophobic/hydrophilic character [11–14]. In this study, SAPO-5 zeolite has been used to modifying the structure of polyurethane membranes and to improving their properties. As a function of the preparation method anisotropic membranes can be obtained. The incorporation of SAPO-5 zeolite in polyurethane matrix also induces changes in the membrane morphology. The zeolite plays a cross-linking function in the membrane structure. The SAPO-5 molecular sieves are microporous solids which contain non-intersecting tubular channels circumscribed by 12-membered rings with a free diameter of ˚ . Their surface selectivity varies from moderate 7.3–8.0 A up to high hydrophilic; consequently, SAPO-5 molecular sieves adsorb both polar and nonpolar molecules. SEM, FTIR, water permeability, swelling in water and several alcohols (methanol, ethanol, 1-propanol and 1-butanol) and density measurements have been used to study the
62
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66
incorporation of SAPO-5 zeolite in polyurethane membranes, the changes of supramolecular structure of polyurethane in the membrane network and the structural uniformity of the obtained membranes. 2. Experimental The structure of used polyurethane polymer is shown in Fig. 1. Polyurethane membranes were prepared using the method and materials described elsewhere [15,16]. The membranes were prepared by casting a polyurethane (PU) + cellulose acetate (CA) + N,N-dimethylformamide (DMF) solution containing calculated amounts of PU and CA polymers on a glass plate and allowing the solvent to evaporate under caloric radiation. We have prepared some pure membranes, the most performing one was denoted PM. The mixed matrix membranes were obtained by adding a calculated amount of zeolite SAPO-5 into the polymer solution and by mixing thoroughly before casting. The casting and curing of the mixed matrix membranes are identical with those of the pure PU membranes. Series of membranes with loading varying between 10 and 70 wt% SAPO-5 were made and denoted as: PMZ-1, PMZ-2 and PMZ-3. These membranes contain 20 wt% SAPO-5 zeolite (PMZ-1 sample), 35 wt% SAPO-5 zeolite (PMZ-2 sample) and 50 wt% SAPO-5 zeolite (PMZ-3 sample). SAPO-5 zeolite synthesis were carried out using silica ROMSIL (82.62% SiO2, Reactivul Bucuresti, Romania), Al2(SO4)3 18H2O (>98%, Merck) and H3PO4 (89%, Merck) as silicon, aluminum and phosphorus sources, respectively, triethylamine (TEA) (>99%, Merck) as templating agent. The silicoaluminophosphate SAPO-5 was synthesized by hydrothermal crystallization according to the procedure described elsewhere [17].
The membranes were characterised by scanning electron microscopy (SEM) (VEGA//TESCAN SEM-EDX instrument), Fourier transform infrared spectroscopy (FTIR) (SPECTRUM 100 FTIR Perkin Elmer spectrometer), Bubble-point test (with a laboratory instrument ‘‘home-made”; details about its features were previously reported [15,16]) water permeability, swelling in water and several alcohols (methanol, ethanol, 1-propanol and 1-butanol) and density measurements. Density measurement is based on Archimedes’ law. First, a dried piece of membrane is weighed as such and then once more while immersed in a liquid of known density. From the difference in weight and the liquid density, the volume of the membrane is calculated. Dividing the membrane dry weight by this volume yields the density of the membrane. The membrane permeation tests were conducted by using a simple dead-end permeation cell. Circular membrane discs were cut and mounted in a cylindrical membrane test cell. Effective permeation area of each membrane was about 14.5 cm2. Feed pressure was controlled at about 5 bar, while the permeate side was opened to the atmosphere. Experiments were carried out at ambient temperature (25 °C). For the pure water, the flux J (m/s) is calculated by following equation: J¼
V tA
where V is volume of permeate collected (m3), t is the sampling time (s) and A is the membrane area (m2). The main characteristics of the membranes used in this study are summarised in Table 1. Swelling of pure liquids in the membranes is investigated on 1.5 5 cm2 membrane strips at room temperature (25 ± 2 °C). To desorb the water from the polymer, the membrane was treated at 150 °C under vacuum before measuring the swelling. A piece of membrane after drying to constant weight (wd) was immersed in the pure liquid (water or alcohol) for at least 1 h. When the sorption equilibrium was reached, the piece was weighed rapidly after blotting free surface liquid. The swelling coefficient (SC) is calculated using the following relationship: SC ¼
Fig. 1. The structure of the polyurethane polymer.
ð1Þ
ws wd 1 d wd
ð2Þ
Table 1 The characteristics of the polyurethane membranes Sample
Zeolite content
Thicknessa (lm)
b
PM PMZ-1 PMZ-2 PMZ-3 a b
0 20 35 50
By SEM method. By Bubble-point method.
245 220 215 95
J 104 (m/s)
Density (g/cm3)
Pore diameter (lm) a
In active layer
In substructure
Theoretical
Experimental
2.3–4.7 2.0–4.4 1.9–2.9 0.3–1.4
11–22 5–9 4–10 2–3
1.250 1.350 1.425 1.495
0.2831 0.3229 0.3962 0.9835
6.34 7.12 7.37 7.41
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66 Table 2 Swelling coefficients (SC) of polyurethane membranes at 25 °C Solvent
SC (cm3/g) PM
PMZ-1
PMZ-2
PMZ-3
Water Methanol Ethanol 1-Propanol 1-Butanol
3.52 3.84 3.86 3.64 3.38
2.98 3.74 3.88 3.65 3.20
2.94 3.31 3.33 3.16 3.13
0.61 0.56 0.62 0.59 0.57
63
litical) chemical cross-linking is possible involving a reaction of the surface zeolite hydroxyls with the polymer (Fig. 2b). The polymer containing terminal groups (– NCO and –OH) which may react with the surface hydroxyls on the outer surface of the zeolite particles, especially at the temperatures used for the membrane preparation. A covalent link may be formed between the polymer and the zeolite particles. 3.1. SEM investigations
where wd is the weight of dry membrane (g), ws is the weight of solvent swollen membrane (g) and d is density of solvent taken (g/cm3). The values of swelling coefficients (SC) for polyurethane membranes in different solvents are given in Table 2. 3. Results and discussion Polymers with low crystallinity, such as polyurethanes, are represented well by two-phase model for the coating process. In forming the membrane material three organisational levels for the polymer structure can be defined, namely: the chemical, the macromolecular and the supramolecular structure. The supramolecular structure presumes the spatial location of the polymer chains by crosslinking effect [15]. The surface of SAPO-5 zeolite contains hydroxyl groups, T-OH (T = Si, Al, P). Taking into account the reactivity of hydroxyl groups, the linking of these to –N– H groups (on polymer structure) is possible by hydrogen bonds, with partial destruction of the hydrogen bond associations within polymer chains. Fig. 2 shows the possible cross-linking in zeolite-filled polyurethane membranes. Apart from physical cross-linking (Fig. 2a) attributed to hydrogen bonds (between polymer chains and HO–Si zeo-
The SEM micrographs for unfilled- and zeolite-filled polyurethane membranes are presented in Fig. 3. The unfilled polyurethane membranes have an asymmetric structure consisting of the dense top layer (active layer) supported by the porous sub-layer (substructure) (Fig. 3a). The top layer is a thin and dense permselective skin. The thick substructure of the membrane consists of a band of intermediate density (the transition layer) consisting of widely open pores and a band of open-macro-voids. The estimated pore size of top layer for the pure polyurethane membranes is much smaller than of the supported layer (Table 1). Accordingly, the molecular sieve property of the pure polyurethane membranes appears to be determined by the pore size in the top layer of the polyurethane membrane. SEM pictures reveal an increase of anisotropy in zeolitefilled polyurethane membranes (Fig. 3b). These membranes have an asymmetric structure consisting of the top skin (active layer), the substructure and the bottom skin. The dense bottom skin layer is composed of some aggregates of zeolite crystals in a polymeric matrix. Sedimentation of the zeolite crystals in the mixture at the very beginning of the curing procedure explains this phenomenon. In SEM pictures of the cross-section of zeolite-filled polyurethane membranes were observed the voids, the void walls and the zeolite deposits. Zeolite crystals can be seen spread over the internal surface of the membrane as white spots. Zeolite spots are also visible within the supporting structure of the membrane (Fig. 3c). The cross-section confirms the good incorporation of the zeolite only in the top skin (active layer) and in the substructure of the membrane. The estimated pore size of layer for the zeolite-filled polyurethane membranes is smaller than of the pure polyurethane membranes (Table 1). 3.2. FTIR analysis
Fig. 2. Possible physical (a) and chemical (b) cross-linkings in zeolite-filled polyurethane membrane.
The FTIR spectroscopy is used for study of the specific interactions in the unfilled- and zeolite-filled polyurethane membranes. The FTIR spectrum of the pure polyurethane membrane (PM sample), is presented in Fig. 4. The N–H stretching vibration of urethane occurs approximately at 3400 cm1 and the peak has split into two sharp bands. The primary intermolecular interactions occurring in urethanes is the hydrogen bonding between the urethane C@O group of
64
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66
Fig. 3. SEM images of membrane samples in the cross-section: (a) PM, (b) PMZ-2, (c) the void wall in PMZ-2 membrane.
Fig. 4. Results of the FTIR performed on polyurethane membrane (PM sample), zeolite-filled polyurethane membrane (PMZ-2 sample) and SAPO-5 zeolite.
one unit with the urethane N–H group of another. Hence chains of hydrogen-bonded urethane groups are formed and at the ends of these chains are two non-hydrogenbonded groups, one C@O and one N–H. This is the reason why the N–H stretching vibration has split into two bands
one corresponding to the hydrogen-bonded N–H and the other for non-bonded. Correspondingly there is the C@O stretching vibration observed approximately at 1660 cm1 and the peak has split into two sharp bands. This again indicates the presence of bonded and non-bonded carbonyl groups. The FTIR spectrum of the calcined SAPO-5 sample, is presented in Fig. 4 and a broad band around 3200– 3600 cm1 is assigned to T-OH (where T = Si, Al, P) groups. For the zeolite-filled polyurethane membranes, the FTIR spectra were taken in order to investigate the changes in the chemical environment between the polymer chains and zeolite crystals. In the zeolite-filled polyurethane membrane (PMZ-2 sample), FTIR spectrum (Fig. 4) indicate hydrogen bonding between the urethane N–H groups of the polyurethane polymer and T-OH groups (T = Si, Al, P) of the zeolite. The small peak at 3220 cm1 reflected the presence of the hydrogen bonding between the zeolite and polymer. The zeolite is shown to be involved in the cross-linking of polyurethane in the membrane structure.
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66
65
3.3. Swelling and water permeability measurements
4. Conclusions
From the date on degree of swelling (Table 2), all membranes showed a tendency to swell with ethanol, while swelling only a little with water. This is due to the fact that the distance between the solubility parameter of membrane and that of water is too far for water to swell the membrane. Consequently, the polyurethane membranes are indicated for ethanol separation on water–ethanol mixtures by pervaporation process. Comparing filled and unfilled membranes, incorporation of SAPO-5 zeolite in polyurethane matrix lead to a reduced swelling of the membrane, to be ascribed to the cross-linking action of the zeolite. The degree of cross-linking of these composites can be explained from their swelling ratios. The degree of cross-linking of composites increased with decreasing swelling ratio. To study the effects of zeolite addition on polyurethane membrane performance, the pure water permeability was carried out on both polyurethane and zeolite–polyurethane membranes. The results are tabulated in Table 1. It shows that the pure water fluxes of the membranes increased as the loading of zeolite increased. This is due to the fact that the compositional complexity of the SAPO-5 framework in the three T elements (Si, Al and P), by their difference in electronegativity, confers to the surface of the SAPO crystals from moderate up to a high hydrophilic character. PMZ-3 membranes have a smaller thickness of ca. 95 lm (when compared to about 200 lm for the others), thus that keeping constant the driving force (about 4 bar) a larger flux may be expected for a material having similar pore size. The pore size is considerably smaller for PMZ-3, what could justify the similar water flux even the thickness being quite different.
The SAPO-5 zeolite-filled polyurethane membranes with perselectivity to the gases or liquids were prepared and morphological characterised. Polyurethane solutions were selected as organic network and a series of membranes, with content of zeolite varying between 10 and 70 wt%, were prepared and characterised by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Bubble-point test and density measurements. The preparation method used results in anisotropic membranes. The incorporation of SAPO-5 zeolite in polyurethane matrix induces some changes in the membrane morphology. A good dispersion of the zeolite is crucial but extremely difficult to obtain especially when small particles are involved. The morphology of the membranes was recognized by SEM studies and verified the absence of voids around the zeolites. This suggested that the zeolite and polymer had good contact at the interface. The cross-section confirms the good incorporation of the zeolite only in the top skin (active layer) and in the substructure of the membrane. Sedimentation of the zeolite crystals in the mixture at the very beginning of the curing procedure explains the dense bottom skin layer formation. The zeolite shows some interactions with the polymer. In the zeolite-filled polyurethane membranes, FTIR spectra indicate hydrogen bonding between the urethane N–H groups of the polyurethane polymer and T-OH groups (T = Si, Al, P) of the zeolite. The SAPO-5 zeolite acted as a cross-linker on the polyurethane polymer and, the final result being in reinforced membrane. The zeolite addition into polyurethane membranes induces an increase in the water flux. Swelling studies of several industrial solvents are investigated through polyurethane membranes, unfilled and filled with SAPO-5 zeolite. All the membranes show a tendency to swell with ethanol. Consequently, the polyurethane membranes are indicated for ethanol separation on water–ethanol mixtures by pervaporation process.
3.4. Density measurements Table 1 shows that membrane density change as zeolite is incorporated. Out of membrane and zeolite densities, a theoretical density of the composite membranes was calculated, assuming a perfect adhesion between the zeolite and the polymer (Table 1). SAPO-5 containing polyurethane membranes could be prepared for loadings up to 70 wt%. Unfortunately, these high loadings resulted in membrane pieces that were too brittle to perform characterisation on. The theoretical calculations for zeolite SAPO-5 containing polyurethane membranes are somewhat different since zeolite SAPO-5 has a greater density than the polyurethanes. By consequence, the theoretical density increases as zeolite content increases (Table 1). In all cases, the experimental value is much lower than the theoretical one, and the difference increases as the zeolite content of the membrane increases. This can be explained by the creation of greater voids in the substructure of the asymmetric membrane.
Acknowledgments The authors gratefully acknowledge the financial assistance from the Ministery of Education and Research, Romania (Grant CEEX, SINAPS, No. 69/2006 and PNCDI II Contract, NATOEPA, Nr. 71-020/2007). References [1] H.J.C. te Hennepe, D. Bargeman, M.H.V. Mulder, C.A. Smolders, J. Membr. Sci. 35 (1987) 39. [2] T. Sano, Y. Kiyozumi, M. Kawamura, F. Mizukami, H. Takaya, T. Mouri, W. Inaoka, Y. Toida, M. Watanabe, K. Toyoda, Zeolites 11 (1991) 842. [3] J.G. Tsikoyiannis, W.O. Haag, Zeolites 12 (1992) 126. [4] B.G. Karle, C.J. Brinker, M.L.F. Phillips, Micropor. Mesopor. Mater. 431 (1996) 237.
66
G. Ciobanu et al. / Microporous and Mesoporous Materials 115 (2008) 61–66
[5] T.C. Bowen, R.G. Meier, L.M. Vane, J. Membr. Sci. 298 (2007) 117. [6] T.W. Pechar, S. Kim, B. Vaughan, E. Marand, M. Tsapatsis, H.K. Jeong, C.J. Cornelius, J. Membr. Sci. 277 (2006) 195. [7] Y. Li, H.M. Guan, T.S. Chung, S. Kulprathipanja, J. Membr. Sci. 275 (2006) 17. [8] Y. Li, T.S. Chung, C. Cao, Kulprathipanja, J. Membr. Sci. 260 (2005) 45. [9] H.B. Park, C.H. Jung, Y.K. Kim, S.Y. Nam, S.Y. Lee, Y.M. Lee, J. Membr. Sci. 235 (2004) 87. [10] S. Kulprathipanja, Ann. NY Acad. Sci. 984 (2003) 361. [11] R. Kiyono, G.H. Koops, M. Wessling, H. Strathmann, J. Membr. Sci. 231 (2004) 109.
[12] C. Dotremont, S. Goethaert, C. Vandecasteele, Desalination 91 (1993) 177. [13] M.D. Jia, K.V. Peinemann, R.D. Behling, J. Membr. Sci. 57 (1991) 289. [14] C. Bartels-Caspers, E. Tusel-Langer, R.N. Lichtenthaler, J. Membr. Sci. 70 (1992) 75. [15] G. Ciobanu, G. Carja, O. Ciobanu, Mat. Sci. Eng. C 27 (2007) 1138. [16] G. Ciobanu, M. Bezdadea, Rev. Chim.-Bucharest 55 (2004) 140. [17] N. Balba, G. Ciobanu, Analele Stiintifice ale Universitatii ‘‘Al.I. Cuza”, Iasi, sectia Chimie, Tomul I, 1993, 89.