preparation and characterization of aga zeolite/polysulfone membranes
TRANSCRIPT
Preparation and characterization of AgA zeolite/polysulfone membranes
ZHANG Tian-Minga, HUANG Zhenb, ZHANG Xiao-Hongc and GUO Li-Yingd
Department of Packaging Engineering, Tianjin University of Commerce, Tianjin 300134, PR China
Keywords: Polysulfone; zeolite AgA; composite membrane; characterizations.
Abstract. In present study, a few polysulfone composite membranes with the introduction of silver ion-exchange treated zeolite were prepared and evaluated by several characterization methods. Regularly-ordered zeolite particles were generally finely dispersed in the continuous PSF phase with appreciated organic-inorganic interfacial interactions as reflected by SEM and FTIR results. Gas permeation test shows that after incorporating zeolite the polysulfone membrane exhibits significantly decreased gas permeability for H2, N2, and CO2 whereas they show increased permselectivity for CO2/N2, H2/CO2 and H2/N2 gas pairs as compared to neat polysulfone membrane.
Introduction
Membrane technology has gained ever-increasing attention in gas separation applications as an energy-saving and cost-reducing alternative. Among various membrane materials exploited, polymers seem to be superior to the others because they are rich in materials reservoir and can provide desired mechanical properties and the processing flexibility required for a variety of practical fabrications and applications. However, the polymeric materials for gas separation are criticized by an upper bound trade-off limitation between the productivity and the selectivity [1,2]. Thus modification in the physical and chemical structures of polymer membranes is of importance to achieve appreciated separation characteristics [3].
In this work, we have fabricated a series of polysulfone-based composite membranes for gas separations by incorporating zeolite 4A. Glassy polysulfone was used as the matrix for the zeolite incorporation as its permeability-selectivity combination in natural gas separations falls close to Robeson’s “upper bound” region and then it is promising for developing inorganic-organic composite membranes for gas separation applications [1, 4-8]. Here, the zeolite chosen is zeolite LTA but treated with sliver ion exchange as it has been reported to be apparently favorable for separating gases with small molecular sizes [9]. A solution casting protocol was used to fabricate zeolite filled polysulfone composite membranes and the resultant zeolite-integrated membranes were examined by several characterizations like XRD, SEM, TGA and FTIR analysis. The gas permeation measurement was performed on H2, N2 and CO2 at 35oC with a variable-pressure constant-volume method.
Experimental A commercial Udel P-1700 polysulfone (PSF, with a Tg of 190oC, Union Carbide, USA) was
selected as the continuous phase in composite membranes. The dope solvent used for casting membranes was N-methyl-2-pyrrolidone (NMP, Merck, Germany, > 99%). The zeolite 4A (i.e., NaA, sodium form of zeolite LTA) was purchased from the Sigma-Aldrich Co., USA. Its particle size varied from 1µm to 2 µm as examined by SEM. The silver nitrate solid was purchased from Merck and used without further purification. Zeolite AgA was prepared by using ion-exchange treatment. The treatment was performed by dispersing 1 g zeolite NaA in 100 ml of 1 mol/L aqueous AgNO3 solution. The resultant mixture was stirred for 1 h at 60-70oC in a temperature-controlled water bath. After treatment, the zeolite filtered was washed twice with 500 ml deionized water to completely remove the unreacted cations and then dried at 90oC overnight in an oven. The treated zeolite with silver ion was designated zeolite AgA. Prior to use, the zeolite powder was heated to remove the adsorbed water vapor or other organic vapors in an oven at heating rate of 2.5oC/min, maintained at 250oC for 2 h, cooled down naturally to room temperature, and then kept in a desiccator immediately.
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In present work, the PSF composite membranes filled with silver-type zeolite were fabricated through the solution casting method. The procedure used to prepare composites can be referred to our previous work [4]. The membranes at a zeolite loading of 20 and 30 wt% are named as PSF-20 and PSF-30, respectively. The neat PSF membrane was also made following the same method.
The composite membranes were examined by a few characterization techniques. Powder X-ray diffraction (XRD) analysis, performed on a Shimadzu XRD-6000 spectrometer using Cu Kα radiation (1.5406 Å) with the samples scanned from 5 to 50° in steps of 0.02° at each point, was taken to investigate the composite microstructure and confirm the presence of zeolite structure. The membrane morphology and thickness were examined with a scanning electron microscope (SEM, JEOL JSM-6700F). Before the SEM examinations, the samples were chilled in liquid nitrogen for at least 30 second and then folded with a forceps to make a sharply broken interfacial cross-section, followed by gold coated under vacuum by a sputter coater. The thickness of the dried composite membranes was determined to vary from 30 to 50µm and the thickness variation was noted to be about 5µm for each membrane sheet. Thermal gravimetric analysis (TGA) was performed on a Shimadzu DTG-60 analyzer. The samples of about 5–10 mg were heated from 30 to 800oC at a ramp of 5oC min−1 under nitrogen atmosphere with a nitrogen flow rate of 30 mL min−1. TGA analysis was performed to determine thermal stability and the amount of solvent remained in membrane samples. The chemical interactions between polymer and zeolite were assessed using Fourier transform infrared spectroscopy (FTIR, α-ALPHA-T, Bruker Optics). The FTIR absorption spectra were obtained at room temperature in a range from 4000 to 400 cm−1 with a spectral resolution of 8 cm−1 and averaged over 32 scans. The single gas permeation properties were determined by means of a variable-pressure constant-volume method [4].
Results and discussion
Membrane characterization
To determine whether the silver ion-exchange treatment of zeolite is actually successful,
measurement of the elemental composition of the zeolite surface before and after the silver ion-exchange treatment was performed by EDX instrument. EDX data shown in Table 1 demonstrate that sodium ion in zeolite NaA has been replaced completely by silver ion in the aqueous AgNO3 solution after the ion-exchange treatment. The crystalline properties of newly developed zeolite AgA were studied through XRD spectra. Figure 1 shows X-ray diffraction patterns of zeolite NaA before and after the silver nitrate solution treatment, neat PSF membrane and composite membranes. As evidenced by the position and height of the characteristic peaks, the ion-exchanged zeolite NaA, i.e., zeolite AgA, has exactly the same XRD patterns with zeolite NaA. These characteristic peaks are typical of the zeolite LTA topology and identical to those reported in the literature [10]. Therefore, the silver ion-exchange treatment used in current study is confirmed to have not affected the crystallinity of zeolite NaA and, zeolite AgA obtained still maintains pure beta zeolite structure with high crystallinity as reflected by the very high peak intensity. Likewise, after dispersed into the PSF dope
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and processed by solvent evaporation and thermal treatment, no effects are observed on the zeolite crystal structure. However, the intensity of these peaks are much lower for composites and tend to go up with increase in the zeolite content. On the other hand, the incorporation of zeolite particles has not been found to obviously affect the microscopic packing structure of polysulfone matrices.
Figure 2 Cross-sectional SEM views of PSF composite membrane filled with 30% zeolite at different magnifications.
Fig. 2 displays cross-sectional SEM images for the zeolite AgA-filled polysulfone composite membrane, from which the particle distribution and morphologies can be examined. As we can see from Fig. 2a, zeolite AgA particles are generally homogeneously distributed throughout the whole polymer matrix, indicative of the feasibility of the membrane fabrication technique employed. The zeolite loading in the range of up to 30 wt% has not obviously affected the dispersion of rigid entitles in the continuous polymer phase, perhaps related to their hydrophilic nature of the two phases. At high magnifications, SEM images reflect that zeolite particles used are generally uniform and exhibit an average particle size of around 2 µm with a very narrow particle size distribution roughly ranging from 1 to 3 µm. It is clearly observed that the zeolite particles are closely packed in the PSF phases, indicative of very nice contact formed between dispersed phase and continuous phase. However, some clearly-cut gaps are found around polymer zeolite interfacial regions. These gaps observed may have arisen from the SEM pre-treatment where the samples were chilled in liquid N2, leading to the frozen polymer chains and the membrane brittleness, subsequently producing voids around zeolite particles in broken membranes.
As can be seen in Fig. 3, TGA results obtained from three samples of zeolite AgA/PSF composite membrane with 0, 20, 30 wt.% zeolite AgA loading, show that the thermal stability of membrane remarkably enhances with increasing the zeolite AgA loading in the polymer matrix. The neat PSF
(a) (b)
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membrane bears a good thermal stability till 450oC then loses mass. After loaded with the 20 and 30 wt.% zeolite AgA, the decomposition temperature of the membrane shift up to 490 and 540oC, respectively. From these TGA analysis results one can draw that by increasing the loading of inorganic zeolite AgA into the PSF matrix the degradation temperature of membrane rises and the thermal stability of membrane improve considerably.
To further understand the nature of the interaction between the zeolite AgA filler and the PSF matrix, attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR) were obtained for a pure PSF membrane, for zeolite AgA and for two zeolite AgA/PSF composite membranes, as shown in Fig. 4. One may see that for pure zeolite AgA there are four strong absorption bands at 462, 552, 998, 1633 and 3433 cm-1, respectively. The strong and broad peak at 3200-3690 cm−1 corresponds to surface hydroxyl groups in zeolite AgA whereas no visible absorptive peaks are observed for neat PSF. For the PSF-20 membrane two sharp peaks at 3440 and 3556 cm-1 can be seen and for the PSF-30 membrane three peaks at 3366, 3472 and 3617cm-1 are clearly observed. These respective hydroxyls can be assigned to isolated silanols and associated hydroxyls with different degrees of hydrogen bonding due to the presence of the PSF macromolecules. Further, peaks at 560, 1008 and1697 cm−1 related to the absorption of O=S=O group of polysulfone are seen to shift to 469, 1014 and 1685cm-1 for PSF-20, and to 470, 1014 and 1688 cm-1 for PSF-30, respectively. These variations in the IR absorptive peaks seem to suggest that there possibly occurs a hydrogen-bonding interaction between the surface hydroxyl groups of the zeolite AgA and the aryl ether groups or sulfonyl oxygen
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of PSF. Similarly, it has been reported [6] that the weak hydrogen-bonding interactions are observed between the aryl ether groups instead of sulfonyl oxygen of PSF and the surface hydroxyl groups of MCM-41 in the MCM-41/PSF composite membranes, which subsequently enhances contact between MCM-41 fillers and polymer matrix, leading to increase in permeability without a loss in selectivity.
Gas permeations
Table 2 presents single gas permeation results for three different PSF membranes. As can be seen, the gas permeabilities for the PSF membrane are significantly decreased after being incorporated with zeolite for three tested gases. Compared to the neat PSF counterpart, the single gas permeability is decreased after adding zeolite particles by 25 % for H2, by 45% CO2 and 55% for N2, respectively. Event though the magnitude of permeabilities are not the same for different gases, the general trend is similar, suggesting a similar permeation mechanism. Evidently, the added zeolite has taken effect in gas permeation through the membranes. As also shown in Table 2, the selectivities for CO2/N2, H2/CO2 and H2/N2 pairs are enhanced by around 15-20 %, 35-40 % and 60-65 % at zeolite loadings of 20, 30 wt%, respectively, as compared to the pristine PSF dense membrane counterparts. These observed selectivity increases for different gas pairs are mainly attributed to the distinct molecular sieving effect of zeolite AgA, which are consistent with our expectations as large difference in molecular size exists in each gas pair. Obviously, hydrogen enrichment could be improved very much after adding zeolite AgA particles, probably related to facilitated hydrogen transport.
Summary
In this work, a few zeolite AgA-embedded PSF composite membranes have been made with the solution casting method. The morphological, structural, thermal, infrared spectral and gas permeation properties are examined. Results show that rather good organic-inorganic interfacial compatibility could be obtained between dispersed and continuous phases. Permeation results evidence that added zeolite AgA has taken effect in gas permeation and improving separation performance.
Acknowledgements
The authors acknowledge Tianjin University of Commerce for partially supporting this research with the project SRT-2011037.
References
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Chemical Engineering and Material Properties II 10.4028/www.scientific.net/AMR.549 Preparation and Characterization of AgA Zeolite/Polysulfone Membranes 10.4028/www.scientific.net/AMR.549.401
DOI References
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