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Heat treatment for improving performance of inner-side zeolite NaA membranes on composite hollow fibers
Author's Accepted Manuscript
Heat treatment for improving performance of inner-side zeolite NaA membranes on composite hollow fibers Zhiying Zhan, Nanke Ma, Hui Yan, Yong Peng, Zhengbao Wang, Yushan Yan
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PII: DOI: Reference:
S0376-7388(15)00200-8 http://dx.doi.org/10.1016/j.memsci.2015.03.024 MEMSCI13533
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Journal of Membrane Science
Received date: 31 December 2014 Revised date: 1 March 2015 Accepted date: 2 March 2015 Cite this article as: Zhiying Zhan, Nanke Ma, Hui Yan, Yong Peng, Zhengbao Wang, Yushan Yan, Heat treatment for improving performance of inner-side zeolite NaA membranes on composite hollow fibers, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.03.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heat treatment for improving performance of inner-side zeolite NaA membranes on composite hollow fibers
Zhiying Zhan a, Nanke Maa, Hui Yana, Yong Peng a, Zhengbao Wang a,*, Yushan Yan a,b a
College of Chemical and Biological Engineering, and MOE Engineering Research Center of
Membrane and Water Treatment Technology, Zhejiang University, Hangzhou 310027, PR China b
Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE
19716, USA Corresponding author
* Fax/Tel.: +86-571-8795-2391; E-mail: [email protected]
Abstract Zeolite NaA membranes are prepared in the lumen side of zeolite/polymer blend (PES-PI) composite hollow fibers. It is demonstrated for the first time that the post-synthesis heat treatment can be used to improve their separation performances. Effects of the heat treatment temperature and time are investigated. Properties of hollow fibers and zeolite membranes before and after heat treatment are characterized by scanning electron microscopy, mechanical test, mercury intrusion method and pervaporation of 90 wt% ethanol aqueous solution at 75 °C. It is found that the continuous zeolite layers keep their original morphologies without any cracks after heat treatment. On the other hand, the compressive strength and separation performance of inner-side zeolite NaA membranes are significantly improved by post-synthesis heat 1
treatment at 240 °C for 6 h. The flux and separation factor of the heat-treated zeolite membranes can be higher than 11.1 kg m-2 h-1 and up to 10,000, respectively.
Keywords: zeolite membranes, pervaporation, post-synthesis heat treatment, composite hollow fibers
1. Introduction Zeolite membranes have shown their potential for industrial applications, especially since the first PV plant of NaA zeolite membranes for the dehydration of organic solvents was built by Mitsui Engineering and Shipbuilding Co. Ltd. in Japan in 1998 [1]. NaA zeolite is hydrophilic and has uniform pore size, thus its membrane is applicable to separate alcohol (esp. ethanol)/water mixtures in the view of industrial applications. However, this technology has not been broadly applied in industry. One major obstacle has been its high cost [2-5]. Since the membrane cost is dominated by the support rather than the zeolite layer, the replacement of expensive ceramic tubular supports with cheap candidates is challenging. To decrease the support cost, we have developed alumina hollow fibers [6] and zeolite/polymer composite hollow fibers [7] as supports of zeolite membranes. The zeolite/polymer composite hollow fibers (CHF) are cheaper than the alumina hollow fibers because high temperature (1500-1600 oC) sintering is eliminated from the preparation of the CHF supports [7-9]. Moreover, the CHF supports have uniform distribution of zeolite NaA crystals on their surface, thus the seeding step can be omitted. Therefore, the composite hollow fibers are a good candidate for supports of 2
zeolite NaA membrane synthesis. Our group has already reported that high-performance zeolite NaA zeolite membranes can be obtained on zeolite/polymer composite hollow fibers (CHF) with zeolite NaA crystals of 85 wt% inside [7-9]. However, their mechanical strength is relatively low [8]. The mechanical strength of zeolite membranes on supports is mainly determined by the supports. The mechanical strength of zeolite membranes on inner sides of zeolite/polymer composite hollow fibers was slightly improved by incorporating polyimide (PI) into a polyethersulfone (PES) matrix to form a polymer blend [9]. We reported that the bending strength of zeolite/polymer CHF supports with 75 wt% zeolite crystals was twice as high as that of CHF supports with 85 wt% zeolite crystals [8]. This indicates that the mechanical properties can be improved by decreasing the zeolite weight percentage (e.g. 75 wt%) in the CHF supports. However, the mechanical strength of the CHF support with 75 wt% zeolite crystals is increased at the expense of its porosity [7]. The influencing factors of the separation performance of zeolite membranes have been extensively investigated [7,9-19]. It is found that the pervaporation flux of zeolite membranes increases with the porosity of the supports [6]. Therefore, we speculate that the separation flux of zeolite membranes on the zeolite/polymer CHF supports with low zeolite weight percentage will be decreased significantly. There is yet another problem with zeolite/polymer CHF supports with low zeolite weight percentage of larger crystals, that is, it is difficult to obtain high-performance zeolite membranes on them [8]. On the other hand, it has been reported that the separation properties of mixed matrix 3
membranes can be improved by annealing at certain temperatures [20-22]. For example, O2/N2 selectivity increased from less than 1 to more than 3 by annealing the mixed matrix membranes above glass transition temperature [22]. It is said that polymer chains will be more flexible and mobile while annealing and thus the inorganic particles closely adhere to the polymer removing the voids between inorganic particles and polymer matrix, as a result, mixed matrix membranes with fewer defects can be obtained and the gas separation selectivity can be improved [23,24]. As the zeolite/polymer composite hollow fibers contain polymer, its structure and property can be altered by annealing. Therefore, it is speculated that the separation performance of zeolite membranes on zeolite/polymer composite hollow fibers can be changed by heat treatment. In this paper, two kinds of zeolite/polymer blend composite hollow fibers (BCHF) with different zeolite crystal sizes are prepared and zeolite NaA membranes are synthesized on their inner surfaces. The effect of particle size is briefly discussed, and the effects of heat treatment on the properties of supports and zeolite membranes are investigated in detail. We demonstrate here that the separation performance of zeolite membranes on composite hollow fibers in dehydration of ethanol-water mixtures can be improved by post-synthesis heat treatment. In brief, the work provides an insight into the post-synthesis heat treatment of zeolite membranes on composite hollow fibers.
2. Experimental 2.1. Materials 4
NaA zeolites (Z0, 4.0 µm, Luoyang Jianlong Chemical Industrial Co. Ltd.; Z1, 1.5µm, Mizusawa Industrial Chemicals Co. Ltd.), Gafone-3000p Polyethersulfone (PES, Solvay Advanced Polymers), Polyimide (PI, Hangzhou Surmount Science & Technology Co. Ltd.), N-methyl-2-pyrrolidone (NMP, Lingfeng Chemical Reagent Co. Ltd.), Sodium metasilicate nonahydrate (Si source, Sinopharm Chemical Reagent Co. Ltd.), Sodium Aluminate (Al source, Sinopharm Chemical Reagent Co. Ltd.), Sodium hydroxide (>96%, Sinopharm Chemical Reagent Co. Ltd.) and Ethanol (>99.7%, Sinopharm Chemical Reagent Co. Ltd.) were purchased as chemical reagents and used as received, while deionized water was self-produced in the laboratory.
2.2. Fabrication of zeolite/polymer blend composite hollow fibers The detailed spinning procedure has been reported previously [8]. Briefly here a certain amount of PES (90%) and PI (10%) was first dissolved in NMP and then mixed at least 6 h in a three neck flask to form a homogeneous solution. Then dry NaA zeolites (75 wt% in the support) were slowly added into the solution under agitation to form a slurry, and the slurry was then stirred for more than 18 h to guarantee that the NaA zeolite crystals were well dispersed. Air bubbles in the slurry were removed by evacuation before spinning, and then transferred to the spinning slurry tank. Here we applied the dry-wet phase inversion method [25] to spin the zeolite/polymer blend composite hollow fibers (BCHF) with specific equipment shown previously [8]. After the spinning slurry was pushed out of the spinneret by N2 extrusion pressure (0.09 MPa), the nascent fibers pass a 2 cm air gap distance before entering the coagulant bath 5
(tap water). The bore fluid (deionized water) was simultaneously pumped at a volumetric flow rate of 1200 ml h-1 to exchange with solvent (NMP) in the spinning slurry. The nascent fibers were kept in the coagulation bath for more than 24 h to remove residual NMP and thoroughly set the geometry. These fibers were then dried at 60 °C for more than 4 h before use. Two types of zeolite/polymer blend composite hollow fibers (BCHFs) were prepared and they were designated as BCHF-Z0 and BCHF-Z1, where Z0 and Z1 refer to NaA zeolites with crystal sizes of 4.0 and 1.5 µm, respectively. The inner and outer diameters of the BCHFs are 1.2 mm and 2.5 mm, respectively.
2.3. Synthesis of inner-side zeolite NaA membranes The synthetic mixture was prepared by mixing an aluminate solution with a silicate solution. The aluminate solution was made by dissolving 1.55 g of NaAlO2 in 28.0 g of deionized water under stirring, and the silicate solution was made by dissolving 3.98 g of Na2SiO3·9H2O and 2.0 g of NaOH in 45.33 g of deionized water. After adding the aluminate solution into the silicate solution under vigorous stirring, the mixture was aged for 3 h at room temperature producing a final synthetic mixture with a molar composition of Na2O:SiO2:Al2O3:H2O=7.5:2:1:600. After that, the synthetic mixture was transferred into an autoclave where the supports stood vertically in a holder. All supports were wrapped with Teflon tape on their outer surfaces before being put into the autoclave to avoid any zeolitic deposition on the outer surfaces. The hydrothermal synthesis was carried out at 100 °C for 5 h in an oven and the autoclaves were rotated 6
inside at 30 rpm. After crystallization, the membranes were washed several times with deionized water and then dried at 100 °C for 6 h.
2.4. Heat treatment Heat treatment of the supports and the zeolite membranes was carried out in a muffle furnace. The furnace was heated to a certain temperature with a rate of 2 °C min-1 from room temperature and then kept at this temperature for a certain time (e.g. 6 h) until letting it cool down at a rate of 2 °C min-1 to 30 °C. The membranes treated here were all synthesized on the inner surfaces of the supports.
2.5. Characterization The morphologies of zeolite/polymer blend composite hollow fibers and synthesized zeolite membranes were observed by scanning electron microscopy (SEM, HITACHI TM-1000). The mechanical properties of the BCHF-Z1 supports before membrane synthesis and after membrane synthesis by heat treatment at different temperatures were measured by mechanical test according to our previous report [9]. The pore structure of the supports was measured by the mercury porosimeter (AutoPore IV 9510, Micrometrics Inc.). The zeolite membranes were further characterized by pervaporation of 90 wt% ethanol/water mixture at 75 °C. The setup and operation process were the same as previously reported [26]. The zeolite membrane synthesized on the BCHF support was pasted into a module and then connected to the vacuum system. The feed solution was 7
circulated through the lumen of the support with a peristaltic pump at a flow rate of 100 ml min-1. A cold trap in liquid N2 was used to collect the permeate after the ethanol solution flowed in the module for 10 min. The time for collecting the permeate was 15 min. The compositions of the feed and the permeate were analyzed by gas chromatography (GC-1690, Kexiao Co. in Hangzhou). The two most important variables were flux (J) and separation factor (α), which were defined as J=W/(A×t)
(1)
α=(YH2O/YEtOH)/(XH2O/XEtOH)
(2)
Where W is the total weight of the permeation (kg), A is the efficient separation area of the membrane (m2), t is the collection time (h), YH2O/YEtOH is the weight ratio of water to ethanol in the permeate and XH2O/XEtOH is the weight ratio of water to ethanol in the feed.
3. Results and Discussion 3.1 Effects of crystal size on the performance of zeolite membranes Previously, our group illustrated that compact and continuous zeolite NaA membranes could be synthesized on the outer surfaces of the zeolite (75 wt%) /polymer composite hollow fiber (CHF) supports with small crystals [8]. The effect of crystal size on properties of zeolite membranes synthesized on the lumen sides of BCHF supports with 75 wt% zeolite crystals was investigated in this study. Due to the space limitation of lumen side (inner diameter: 1.2 mm) in the hollow fiber, the synthesis composition is very important for obtaining continuous zeolite layer on the inner surface. Our previous 8
work
has
presented
an
optimal
synthesis
composition
of
Na2O:SiO2:Al2O3:H2O=7.5:2:1:600 [26]. Therefore, this synthesis composition was used in this study. The morphologies of the supports (BCHF-Z0 and BCHF-Z1) and zeolite NaA membranes synthesized on their inner surfaces are shown in Fig. 1. The number of uniformly distributed zeolite crystals on the inner surface of BCHF-Z1 (Fig. 1b1) for membrane growth was more than that of BCHF-Z0 (Fig. 1a1). Therefore, it is easier to obtain dense membranes on hollow fibers with small-sized seeds. Thus, the membrane in Fig. 1b2 was compact and defect-free with a thickness of 4-5 µm (Fig. 1b3), while the membrane in Fig. 1a2 was less intergrown and had large defects. Moreover, the two membranes had different behaviors in dehydrating ethanol/water mixtures (Table 1). The membrane M0 had a poor separation factor of 24 far lower than that of M1 (>10,000), but the flux of M0 (6.6 kg m-2 h-1) was a little higher than that of M1 (5.1 kg m-2 h-1). It is believed that the defects in M0 decrease the separation selectivity but increase the separation flux during pervaporation.
9
Fig. 1. SEM images of inner surfaces of supports [(a1) BCHF-Z0 and (b1) BCHF-Z1] and zeolite NaA membranes supported on the corresponding supports [(a2, b2) top view and (a3, b3) cross section].
The bending strength and the compressive strength of BCHF-Z1 with 75 wt% zeolite crystals were greatly increased to 22.56 MPa and 3.78 MPa, respectively, compared to that of BCHF supports with 85 wt% zeolite crystals (10.11 MPa and 1.67 MPa,
，
respectively) [9]. However the flux of zeolite membranes on BCHF-Z1 supports (5.1 kg m-2 h-1) were much lower than that of zeolite membranes on BCHF supports with 85 10
wt% zeolite crystals (10.9 kg m-2 h-1) [9]. Here, we demonstrate that post-synthesis heat treatment can improve the separation performance of zeolite membranes on BCHF-Z1 supports. Therefore, effects of heat treatment on properties of BCHF-Z1 before and after membrane synthesis were investigated. Table 1 Separation performance of zeolite membranes supported on the inner sides of BCHF-Z0 and BCHF-Z1 supports a
a
Membrane
Zeolite
M0
Z0
M1
Z1
J ( kg m-2 h-1)
α (dimensionless)
4
6.6
24
1.5
5.1
﹥10,000
Particle size (µm)
Feed: 90 wt% ethanol aqueous solution at 75 oC
3.2 Effects of heat treatment temperature Mixed matrix membranes have been reported to be less defective when annealed to achieve good gas separation properties [20,22-24,27]. Therefore, it is hypothesized that zeolite NaA membranes on the BCHF-Z1 supports may achieve better performance by heat treatment at a certain temperature. Effects of the heat treatment temperature were then investigated, and SEM images of the inner and outer surface of the BCHF-Z1 support heat-treated for 6 h are shown in Fig. 2. No significant changes happened to the BCHF-Z1 in terms of the inner or the outer surface after heat treatment at 100 ºC and 220 ºC. The inner and outer surfaces after heat treatment at 240 ºC (Fig. 2c1 and 2c2) showed slight coalescence of polymer blends with some big pores formed, while the polymer blends were totally densified when treated at 270 ºC leaving many big pores in both inner and outer surfaces of BCHF-Z1 (Fig. 2d1 and 2d2). 11
Fig. 2. SEM images of the (a1-d1) inner and (a2-d2) outer surface of the BCHF-Z1 supports heat-treated at (a) 100 °C, (b) 220 °C, (c) 240 °C, and (d) 270 °C for 6 h.
12
Fig. 3. Cross-sectional SEM images of the BCHF-Z1 supports heat-treated at (a) 100 °C, (b) 220 °C, (c) 240 °C, and (d) 270 °C for 6 h.
13
Fig. 3 shows the cross-sectional morphologies of the supports treated from 100 ºC to 270 ºC. No obvious differences can be observed at low magnification in Fig. 3a1-d1 and at high magnification in Fig. 3a2-c2. However, many large pores can be seen in Fig. 3d2 due to the strong densification of the polymer blend at elevated temperature. It is possible that increased number of large pores might be favorable to the decrease of the support resistance, but unfavorable to the increase of the mechanical strength of supports. The porosity and pure water flux of the hollow fibers heat-treated at different temperatures are shown in Table 2. The porosities of the supports heat-treated at 100, 220 and 240 ºC were more than 50%, while the porosity of the support heat-treated at 270 ºC was about 40%. The pure water flux of the support heat-treated at 220 ºC (187 kg m-2 h-1) slightly increased compared with the support treated at 100 ºC (138 kg m-2 h-1). It sharply increased to 1,273 kg m-2 h-1 when heat-treated at 240 ºC, while it reached more than 19,000 kg m-2 h-1 at 270 ºC. The increasing pure water flux here indicates that the inter-connectivity of pores in the hollow fiber could be increased (i.e. provision of larger pores at the expense of small pores or increase of interphase between zeolite crystals and polymer) by heat treatment. Table 2 Effects of heat treatment temperature on the porosity and pure water flux of BCHF-Z1 supports a
a
T b (°C)
Porosity (%)
Pure water flux (kg m-2 h-1)
100
54.58
138
220
53.85
187
240
52.88
1,273
270
40.92
19,089
The heat treatment time was 6 h; b heat treatment temperature. 14
It can also be observed in Fig. 4 that cumulative intrusion of mercury decreased, and it becomes easier for mercury to enter the supports as the heat treatment temperature increases. In other words, the porosity decreased and the inter-connectivity increased as described above. As also shown in Fig. 5, the pore size of the supports increased when the heat treatment temperature was above 220 ºC, while there were no significant differences at 100 ºC and 220 ºC. The average pore size at 270 ºC was about 1.6 µm. The pore size distribution of the supports became narrow as the heat treatment temperature increased. This effect is particularly obvious at 270 ºC. This is consistent with the SEM results, indicating that large pores may be formed due to the mergence of small pores.
Fig. 4. Cumulative intrusion of mercury in BCHF-Z1 supports heat-treated at different temperatures for 6 h.
15
Fig. 5. Pore size distribution of BCHF-Z1 supports heat-treated at different temperatures for 6 h.
The above results can be explained by the glass transition temperature of the support. The heat deflection temperature of PES ranges from 200 ºC to 220 ºC, and its glass transition temperature is from 220 ºC to 230 ºC. The glass transition temperature (Tg=223 ºC) of BCHF-Z1 can be calculated by the Fox equation [22]. 1/Tg=w1/Tg1+w2/Tg2
(3)
Where w1 (0.9) and w2 (0.1) are the mass fraction of PES and PI, Tg1 (220 ºC) and Tg2 (254 ºC) are the glass transition temperature of PES and PI, respectively. On the other hand, a high zeolite crystal weight percentage (75 wt%) might slightly increase the glass transition temperature. Therefore, the glass transition temperature of the BCHF supports might be higher than 223 ºC. The heat treatment temperatures of 100 and 220 ºC are lower than the glass transition temperature. Therefore, there is no significant change in the support structure. The heat treatment temperatures of 240 and 270 ºC are higher than the glass transition temperature, and therefore, the support structure changes. 16
Table 3 Mechanical properties of BCHF-Z1 heat-treated at different temperatures for 6 h Bending strength (MPa)
Compressive strength (MPa)
Temperature (°C) BS
AS
BS
AS
100
22.56
21.00
3.78
5.85
220
22.99
21.70
3.71
5.68
240
24.36
22.19
3.74
6.10
270
23.01
21.20
3.32
4.28
Notice: BS--before synthesis and AS--after synthesis of membrane.
Mechanical properties of supports and supported membranes after heat treatment at different temperatures are shown in Table 3. The data revealed that the bending strength of BCHF supports after membrane synthesis was slightly lower than that before membrane synthesis, which is probably due to the formation of a brittle zeolite membrane layer synthesized on the surface of the hollow fiber [8,9]. The compressive strength of supports after membrane synthesis was higher than that before membrane synthesis. It is possible that the intergrown zeolite NaA layer in the lumen side acts as an annular upholder to increase the compressive strength and the seeded growth of the crystals in the matrix of the polymer blend may also help stiffen the BCHF [9]. The bending strength and compressive strength after membrane synthesis by heat treatment at 240 ºC were 22.19 MPa and 6.10 MPa, respectively. Its compressive strength is higher than others, which is probably due to the increased stiffness of the polymer chain, better adherence between zeolite particles and the polymer matrix, and the densification
17
of the polymer blend at above Tg. The compressive strength after heat treatment at 270 ºC declined, probably due to the formation of large pores. Table 4 Separation performance of the inner-side zeolite NaA membranes by heat post-synthesis treatment at different temperatures a Membrane
T b(°C)
J (kg m-2 h-1)
M1
100
5.1
M2
220
6.6
M3
240
11.1
M4
270
10.5
α (dimensionless)
﹥10,000 ﹥10,000 ﹥10,000 ﹥10,000
a
Feed: 90 wt% ethanol aqueous solution at 75 oC, and the heat treatment time was 6 h; b heat treatment temperature.
Due to the thermal stability of zeolite NaA, the structure of the inner-side zeolite NaA membranes after heat treatment at different temperatures (lower than 270 ºC) for 6 h were still the same with Fig. 1b2 and b3. The membranes were thin and well intergrown without any cracks (Data not shown). The separation performances in pervaporation of 90 wt% ethanol/water mixtures are listed in Table 4. The flux increased from 5.1 to 11.1 kg m-2 h-1 (M3) when the temperature increased from 100 ºC to 240 ºC. When the heat treatment temperature increased to 270 ºC, the flux of M4 was 10.5 kg m-2 h-1, which is slightly lower than that of M3. It has been reported that the membrane flux increases with the porosity of the support [7]. As the porosities of the supports heat-treated at different temperatures (100-240 ºC) are close to each other (Table 2), the increasing permeate flux indicates that the increased inter-connectivity (i.e. provision of larger pores at the expense of small pores or increase of interphase between zeolite crystals and polymer, or reduction in the blockage of zeolitic pores by polymer) likely improved 18
the separation flux. The flux of M4 decreased, whereas the pure water flux of the heat-treated support increased significantly. The reason for this phenomenon is not very clear at present. The relatively low porosity of the support heat-treated at 270 ºC might be the main reason for the low flux of M4. There are also some other reasons: (1) high heat treatment temperature (much higher than Tg) leads to additional support resistance, causing the polymer to gather around the zeolite layer or block zeolitic pores of crystals in the polymer matrix; (2) capillary condensation of the permeate in the large support pores [28]; (3) some different structural changes in the supports take place between the heat treatment of BCHF supports with and without the zeolite membrane. Further investigation needs to be performed to understand all changes in the membrane by heat treatment. It is worthy to be mentioned that a dense zeolite membrane can be hardly obtained on a heat-treated BCHF support. Therefore, separation performance of zeolite membranes on heat-treated BCHF supports was not presented in this study. That is, only the post-synthesis heat treatment is an effective way to improve the separation flux. From the results above, we can conclude that the post-synthesis heat treatment at 240 ºC is viable for improving separation performance and even mechanical strength of zeolite membranes supported on the BCHF-Z1 supports.
19
Fig. 6. SEM images of the BCHF-Z1 supports heat-treated at 240 °C for (a1-a3) 12 h and (b1-b3) 24 h; (a1, b1) inner surface, (a2, b2) cross section and (a3, b3) magnification of black square in a2 and b2.
3.3 Effects of heat treatment time Effects of post-synthesis heat treatment time at 240 ºC were also investigated. Fig. 6 shows the morphologies of hollow fibers heat-treated at 240 ºC for 12 h and 24 h, respectively. There are relatively large pores on the inner surfaces and cross-sections, which are similar to supports treated at 270 ºC for 6 h (Fig. 2d1 and Fig. 3d2). The porosities of the supports treated at 240 ºC for 12 h and 24 h were above 50%, almost 20
the same as those treated for 6 h and their pure water flux surpassed 8,000 kg m-2 h-1 (Table 5), but less than those treated at 270 ºC for 6 h. It is also found that the cumulative intrusion of mercury and pore size distribution of hollow fibers at 240 ºC for 12 h and 24 h (data not shown) were almost identical with those treated for 6 h. Moreover, the zeolite membranes heat-treated at 240 oC for 12 h and 24 h were still dense and defect free, as shown in Fig. 7. We initially thought that the separation flux would be higher after heat treatment for 12 h and 24 h due to the high porosity and excellent interconnectivity. However, in fact, the flux decreased as the heat treatment time was increased over 6 h (Table 6). The reason for this phenomenon is not very clear at present. There are two possible mechanisms: (1) longer heat treatment time leads to additional support resistance, causing the polymer to gather around the zeolite layer or block zeolitic pores of crystals in the polymer matrix; (2) capillary condensation of the permeate in the support pores [28]. Table 5 Effects of heat treatment time on the porosity and pure water flux of BCHF-Z1a
a
Time (h)
Porosity (%)
Pure water flux (kg m-2 h-1)
0
54.58
138
6
52.88
1,273
12
53.85
8,222
24
52.88
8,580
The heat treatment temperature is 240 °C.
21
Fig. 7. Morphologies of inner-side zeolite NaA membranes after heat-treated at 240 oC for (a, b) 12 h and (c, d) 24 h; (a, c) inner surface and (b, d) cross section.
Moreover, the heat treatment at 240 ºC for more than 12 h decreased the mechanical strength of zeolite membranes. For example, the bending and compressive strengths of zeolite membranes heat-treated for 12 h were 19.16 MPa and 4.74 MPa, respectively. Thus, longer heat treatment times than 6 h at 240 ºC do not improve the properties of the zeolite membranes. Table 6 Effects of post-synthesis heat treatment time on pervaporation properties of zeolite membranes supported on BCHF-Z1 a Time (h)
a
J (kg m-2 h-1)
0
5.1
6
11.1
12
9.6
24
7.9
The heat treatment temperature is 240 °C. 22
α (dimensionless)
﹥10,000 ﹥10,000 ﹥10,000 ﹥10,000
4. Conclusions Compact and defect-free zeolite NaA membranes were successfully synthesized on the inner-side surfaces of zeolite/polymer blend composite hollow fibers with 75 wt% zeolite small crystals (e.g. BCHF-Z1) due to more seed crystal numbers on the surface of the supports. Heat treatment had significant influence on the structure of the supports and the separation performances of zeolite membranes in dehydration of ethanol/water mixtures. The post-synthesis heat treatment at 240 ºC for 6 h greatly enhanced the flux to 11.1 kg m-2 h-1 from 5.1 kg m-2 h-1 with a separation factor of up to 10,000. Although the post-synthesis heat treatment at 270 ºC also improved the flux of zeolite membranes, the pores in the support became too large and the mechanical strength of the membrane was decreased. Heat treatment longer than 12 h at 240 ºC increased the pure water flux of supports, but, decreased the flux of zeolite membranes in pervaporation. It is suggested that both the porosity and interconnectivity of the hollow fibers have effects on the separation flux of zeolite membranes. This work opens a new path to the improvement of separation performances of zeolite membranes after synthesis.
Acknowledgement The authors would like to thank the National Natural Science Foundation of China (21236006), the National Basic Research Program (2013CB228104), Science and Technology Department of Zhejiang Province (2009R50020), and open research fund of Top Key Discipline of Chemistry in Zhejiang Provincial Colleges and Key Laboratory 23
of the Ministry of Education for Advanced Catalysis Materials (Zhejiang Normal University) (ZJHX201302) for financial support.
References
[1] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif .Technol. 25 (2001) 251-260. [2] C.L. Yu, C. Zhong, Y.M. Liu, X.H. Gu, G. Yang, W.H. Xing, N.P. Xu, Pervaporation dehydration of ethylene glycol by NaA zeolite membranes, Chem. Eng. Res. Des. 90 (2012) 1372-1380. [3] S.L. Wee, C.T. Tye, S. Bhatia, Membrane separation process-pervaporation through zeolite membrane, Sep. Purif. Technol. 63 (2008) 500-516. [4] A. Urtiaga, E.D. Gorri, C. Casado, I. Ortiz, Pervaporative dehydration of industrial solvents using a zeolite NaA commercial membrane, Sep. Purif. Technol. 32 (2003) 207-213. [5] J. Caro, M. Noack, P. Kolsch, Zeolite membranes: from the laboratory scale to technical applications, Adsorption. 11 (2005) 215-227. [6] Z.B. Wang, Q.Q. Ge, J. Shao, Y.S. Yan, High performance zeolite LTA pervaporation membranes on ceramic hollow fibers by dipcoating-wiping seed deposition, J. Am. Chem. Soc. 131 (2009) 6910-6911. [7] Q.Q. Ge, Z.B. Wang, Y.S. Yan, High-performance zeolite NaA membranes on polymer-zeolite composite hollow fiber supports, J. Am. Chem. Soc. 131 (2009) 17056-17057. [8] J.G. Li, J. Shao, Q.Q. Ge, G.L. Wang, Z.B. Wang, Y.S. Yan, Influences of the zeolite loading and particle size in composite hollow fiber supports on properties of zeolite NaA membranes, Micropor. Mesopor. Mater. 160 (2012) 10-17. [9] Z.Y. Zhan, Y. Peng, Z.B. Wang, Y.S. Yan, High performance zeolite NaA membranes synthesized on the inner surface of zeolite/PES-PI blend composite hollow fibers, J. Membr. Sci. 471 (2014) 299-307. [10] S.Y. Guo, C.L. Yu, X.H. Gu, W.Q. Jin, J. Zhong, C.L. Chen, Simulation of adsorption, diffusion, and permeability of water and ethanol in NaA zeolite membranes, J. Membr. Sci. 376 (2011) 40-49. [11] M. Nomura, T. Yamaguchi, S. Nakao, Ethanol/water transport through silicalite membranes, J. Membr. Sci. 144 (1998) 161-171. [12] I.F.J. Vankelecom, B. Moermans, G. Verschueren, P.A. Jacobs, Intrusion of PDMS top layers in porous supports, J. Membr. Sci. 158 (1999) 289-297. [13] X.B. Chen, W.S. Yang, J. Liu, X.C. Xu, A.S. Huang, L.W. Lin, Synthesis of NaA zeolite membrane with high performance, J. Mater. Sci. Lett. 21 (2002) 1023-1025. [14] F.T. de Bruijn, L. Sun, Z. Olujic, P.J. Jansens, F. Kapteijn, Influence of the support layer on the flux limitation in pervaporation, J. Membr. Sci. 223 (2003) 141-156. [15] Y. Hasegawa, C. Abe, M. Nishioka, K. Sato, T. Nagase, T. Hanaoka, Formation of high flux CHA-type zeolite membranes and their application to the dehydration of alcohol solutions, J. Membr. Sci. 364 (2010) 318-324. [16] X.J. Shu, X.R. Wang, Q.Q. Kong, X.H. Gu, N.P. Xu, High-flux MFI zeolite membrane supported on 24
YSZ hollow fiber for separation of ethanol/water, Ind. Eng. Chem. Res. 51 (2012) 12073-12080. [17] J. Shao, Z.Y. Zhan, J.G. Li, Z.B. Wang, K. Li, Y.S. Yan, Zeolite NaA membranes supported on alumina hollow fibers: Effect of support resistances on pervaporation performance, J. Membr. Sci. 451 (2014) 10-17. [18] A. Thongsukmak, K.K. Sirkar, Extractive pervaporation to separate ethanol from its dilute aqueous solutions characteristic of ethanol-producing fermentation processes, J. Membr. Sci. 329 (2009) 119-129. [19] J.R. McCutcheon, M. Elimelech, Influence of membrane support layer hydrophobicity on water flux in osmotically driven membrane processes, J. Membr. Sci. 318 (2008) 458-466. [20] G.C. Kapantaidakis, S.P. Kaldis, G.P. Sakellaropoulos, E. Chira, B. Loppinet, G. Floudas, Interrelation between phase state and gas permeation in polysulfone/polyimide blend membranes, J. Polym. Sci. Pol. Phys. 37 (1999) 2788-2798. [21] G.C. Kapantaidakis, G.H. Koops, M. Wessling, Preparation and characterization of gas separation hollow fiber membranes based on polyethersulfone-polyimide miscible blends, Desalination. 145 (2002) 353-357. [22] A.F. Ismail, R.A. Rahim, W.A.W.A. Rahman, Characterization of polyethersulfone/Matrimid (R) 5218 miscible blend mixed matrix membranes for O2/N2 gas separation, Sep. Purif. Technol. 63 (2008) 200-206. [23] K. Liang, J. Grebowicz, E. Valles, F.E. Karasz, W.J. Macknight, Thermal and rheological properties of miscible polyethersulfone polyimide blends, J. Polym. Sci. Pol. Phys. 30 (1992) 465-476. [24] R. Mahajan, W.J. Koros, Mixed matrix membrane materials with glassy polymers. Part 2, Polym. Eng. Sci. 42 (2002) 1432-1441. [25] T.S. Chung, Z.L. Xu, W.H. Lin, Fundamental understanding of the effect of air-gap distance on the fabrication of hollow fiber membranes, J. Appl. Polym. Sci. 72 (1999) 379-395. [26] L.L. Lai, J. Shao, Q.Q. Ge, Z.B. Wang, Y.S. Yan, The preparation of zeolite NaA membranes on the inner surface of hollow fiber supports, J. Membr. Sci. 409 (2012) 318-328. [27] R. Mahajan, W.J. Koros, Mixed matrix membrane materials with glassy polymers. Part 1, Polym. Eng. Sci. 42 (2002) 1420-1431. [28] O. Trifunovic, G. Tragardh, The influence of support layer on mass transport of homologous series of alcohols and esters through composite pervaporation membranes, J. Membr. Sci. 259 (2005) 122-134.
25
Nomenclature W A t J C T YH2O/YEtOH XH2O/XEtOH Tg Tg1 Tg2 w1 w2
permeation water mass (kg) permeation area of membrane (m2) permeation time (h) permeation flux for water (kg m-2 h-1) concentration of ethanol-water mixture (wt %) heat treatment temperature (oC) weight ratio of water to ethanol in the permeation (%) weight ratio of water to ethanol in the feed (%) glass transition temperature of BCHF-Z1 (oC) glass transition temperature of PES (oC) glass transition temperature of PI (oC) mass fraction of PES mass fraction of PI
Greek symbols α pervaporation separation factor
26
Captions Fig. 1. SEM images of inner surfaces of supports [(a1) BCHF-Z0 and (b1) BCHF-Z1] and zeolite NaA membranes supported on the corresponding supports [(a2, b2) top view and (a3, b3) cross section].
Fig. 2. SEM images of the (a1-d1) inner and (a2-d2) outer surface of the BCHF-Z1 supports heat-treated at (a) 100 °C, (b) 220 °C, (c) 240 °C, and (d) 270 °C for 6 h.
Fig. 3. Cross-sectional SEM images of the BCHF-Z1 supports heat-treated at (a) 100 °C, (b) 220 °C, (c) 240 °C, and (d) 270 °C for 6 h.
Fig. 4. Cumulative intrusion of mercury in BCHF-Z1 supports heat-treated at different temperatures for 6 h.
Fig. 5. Pore size distribution of BCHF-Z1 supports heat-treated at different temperatures for 6 h.
Fig. 6. SEM images of the BCHF-Z1 supports heat-treated at 240 °C for (a1-a3) 12 h and (b1-b3) 24 h; (a1, b1) inner surface, (a2, b2) cross section and (a3, b3) magnification of black square in a2 and b2.
Fig. 7. Morphologies of inner-side zeolite NaA membranes heat-treated at 240 oC for (a, b) 12 h and (c, d) 24 h; (a, c) inner surface and (b, d) cross section.
Highlights •
Zeolite/polymer composite hollow fibers as supports of zeolite membranes.
•
The zeolite (<1.5 µm) content in hollow fibers is 75 wt%.
•
Flux can be greatly increased by post synthesis heat treatment.
27
•
No cracks occur in the membrane layer after post synthesis heat treatment.
28
Heat treatment for improving performance of inner-side zeolite NaA membranes on composite hollow fibers Zhiying Zhan, Nanke Ma, Hui Yan, Yong Peng, Zhengbao Wang, Yushan Yan
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PII: DOI: Reference:
S0376-7388(15)00200-8 http://dx.doi.org/10.1016/j.memsci.2015.03.024 MEMSCI13533
To appear in:
Journal of Membrane Science
Received date: 31 December 2014 Revised date: 1 March 2015 Accepted date: 2 March 2015 Cite this article as: Zhiying Zhan, Nanke Ma, Hui Yan, Yong Peng, Zhengbao Wang, Yushan Yan, Heat treatment for improving performance of inner-side zeolite NaA membranes on composite hollow fibers, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.03.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heat treatment for improving performance of inner-side zeolite NaA membranes on composite hollow fibers
Zhiying Zhan a, Nanke Maa, Hui Yana, Yong Peng a, Zhengbao Wang a,*, Yushan Yan a,b a
College of Chemical and Biological Engineering, and MOE Engineering Research Center of
Membrane and Water Treatment Technology, Zhejiang University, Hangzhou 310027, PR China b
Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE
19716, USA Corresponding author
* Fax/Tel.: +86-571-8795-2391; E-mail: [email protected]
Abstract Zeolite NaA membranes are prepared in the lumen side of zeolite/polymer blend (PES-PI) composite hollow fibers. It is demonstrated for the first time that the post-synthesis heat treatment can be used to improve their separation performances. Effects of the heat treatment temperature and time are investigated. Properties of hollow fibers and zeolite membranes before and after heat treatment are characterized by scanning electron microscopy, mechanical test, mercury intrusion method and pervaporation of 90 wt% ethanol aqueous solution at 75 °C. It is found that the continuous zeolite layers keep their original morphologies without any cracks after heat treatment. On the other hand, the compressive strength and separation performance of inner-side zeolite NaA membranes are significantly improved by post-synthesis heat 1
treatment at 240 °C for 6 h. The flux and separation factor of the heat-treated zeolite membranes can be higher than 11.1 kg m-2 h-1 and up to 10,000, respectively.
Keywords: zeolite membranes, pervaporation, post-synthesis heat treatment, composite hollow fibers
1. Introduction Zeolite membranes have shown their potential for industrial applications, especially since the first PV plant of NaA zeolite membranes for the dehydration of organic solvents was built by Mitsui Engineering and Shipbuilding Co. Ltd. in Japan in 1998 [1]. NaA zeolite is hydrophilic and has uniform pore size, thus its membrane is applicable to separate alcohol (esp. ethanol)/water mixtures in the view of industrial applications. However, this technology has not been broadly applied in industry. One major obstacle has been its high cost [2-5]. Since the membrane cost is dominated by the support rather than the zeolite layer, the replacement of expensive ceramic tubular supports with cheap candidates is challenging. To decrease the support cost, we have developed alumina hollow fibers [6] and zeolite/polymer composite hollow fibers [7] as supports of zeolite membranes. The zeolite/polymer composite hollow fibers (CHF) are cheaper than the alumina hollow fibers because high temperature (1500-1600 oC) sintering is eliminated from the preparation of the CHF supports [7-9]. Moreover, the CHF supports have uniform distribution of zeolite NaA crystals on their surface, thus the seeding step can be omitted. Therefore, the composite hollow fibers are a good candidate for supports of 2
zeolite NaA membrane synthesis. Our group has already reported that high-performance zeolite NaA zeolite membranes can be obtained on zeolite/polymer composite hollow fibers (CHF) with zeolite NaA crystals of 85 wt% inside [7-9]. However, their mechanical strength is relatively low [8]. The mechanical strength of zeolite membranes on supports is mainly determined by the supports. The mechanical strength of zeolite membranes on inner sides of zeolite/polymer composite hollow fibers was slightly improved by incorporating polyimide (PI) into a polyethersulfone (PES) matrix to form a polymer blend [9]. We reported that the bending strength of zeolite/polymer CHF supports with 75 wt% zeolite crystals was twice as high as that of CHF supports with 85 wt% zeolite crystals [8]. This indicates that the mechanical properties can be improved by decreasing the zeolite weight percentage (e.g. 75 wt%) in the CHF supports. However, the mechanical strength of the CHF support with 75 wt% zeolite crystals is increased at the expense of its porosity [7]. The influencing factors of the separation performance of zeolite membranes have been extensively investigated [7,9-19]. It is found that the pervaporation flux of zeolite membranes increases with the porosity of the supports [6]. Therefore, we speculate that the separation flux of zeolite membranes on the zeolite/polymer CHF supports with low zeolite weight percentage will be decreased significantly. There is yet another problem with zeolite/polymer CHF supports with low zeolite weight percentage of larger crystals, that is, it is difficult to obtain high-performance zeolite membranes on them [8]. On the other hand, it has been reported that the separation properties of mixed matrix 3
membranes can be improved by annealing at certain temperatures [20-22]. For example, O2/N2 selectivity increased from less than 1 to more than 3 by annealing the mixed matrix membranes above glass transition temperature [22]. It is said that polymer chains will be more flexible and mobile while annealing and thus the inorganic particles closely adhere to the polymer removing the voids between inorganic particles and polymer matrix, as a result, mixed matrix membranes with fewer defects can be obtained and the gas separation selectivity can be improved [23,24]. As the zeolite/polymer composite hollow fibers contain polymer, its structure and property can be altered by annealing. Therefore, it is speculated that the separation performance of zeolite membranes on zeolite/polymer composite hollow fibers can be changed by heat treatment. In this paper, two kinds of zeolite/polymer blend composite hollow fibers (BCHF) with different zeolite crystal sizes are prepared and zeolite NaA membranes are synthesized on their inner surfaces. The effect of particle size is briefly discussed, and the effects of heat treatment on the properties of supports and zeolite membranes are investigated in detail. We demonstrate here that the separation performance of zeolite membranes on composite hollow fibers in dehydration of ethanol-water mixtures can be improved by post-synthesis heat treatment. In brief, the work provides an insight into the post-synthesis heat treatment of zeolite membranes on composite hollow fibers.
2. Experimental 2.1. Materials 4
NaA zeolites (Z0, 4.0 µm, Luoyang Jianlong Chemical Industrial Co. Ltd.; Z1, 1.5µm, Mizusawa Industrial Chemicals Co. Ltd.), Gafone-3000p Polyethersulfone (PES, Solvay Advanced Polymers), Polyimide (PI, Hangzhou Surmount Science & Technology Co. Ltd.), N-methyl-2-pyrrolidone (NMP, Lingfeng Chemical Reagent Co. Ltd.), Sodium metasilicate nonahydrate (Si source, Sinopharm Chemical Reagent Co. Ltd.), Sodium Aluminate (Al source, Sinopharm Chemical Reagent Co. Ltd.), Sodium hydroxide (>96%, Sinopharm Chemical Reagent Co. Ltd.) and Ethanol (>99.7%, Sinopharm Chemical Reagent Co. Ltd.) were purchased as chemical reagents and used as received, while deionized water was self-produced in the laboratory.
2.2. Fabrication of zeolite/polymer blend composite hollow fibers The detailed spinning procedure has been reported previously [8]. Briefly here a certain amount of PES (90%) and PI (10%) was first dissolved in NMP and then mixed at least 6 h in a three neck flask to form a homogeneous solution. Then dry NaA zeolites (75 wt% in the support) were slowly added into the solution under agitation to form a slurry, and the slurry was then stirred for more than 18 h to guarantee that the NaA zeolite crystals were well dispersed. Air bubbles in the slurry were removed by evacuation before spinning, and then transferred to the spinning slurry tank. Here we applied the dry-wet phase inversion method [25] to spin the zeolite/polymer blend composite hollow fibers (BCHF) with specific equipment shown previously [8]. After the spinning slurry was pushed out of the spinneret by N2 extrusion pressure (0.09 MPa), the nascent fibers pass a 2 cm air gap distance before entering the coagulant bath 5
(tap water). The bore fluid (deionized water) was simultaneously pumped at a volumetric flow rate of 1200 ml h-1 to exchange with solvent (NMP) in the spinning slurry. The nascent fibers were kept in the coagulation bath for more than 24 h to remove residual NMP and thoroughly set the geometry. These fibers were then dried at 60 °C for more than 4 h before use. Two types of zeolite/polymer blend composite hollow fibers (BCHFs) were prepared and they were designated as BCHF-Z0 and BCHF-Z1, where Z0 and Z1 refer to NaA zeolites with crystal sizes of 4.0 and 1.5 µm, respectively. The inner and outer diameters of the BCHFs are 1.2 mm and 2.5 mm, respectively.
2.3. Synthesis of inner-side zeolite NaA membranes The synthetic mixture was prepared by mixing an aluminate solution with a silicate solution. The aluminate solution was made by dissolving 1.55 g of NaAlO2 in 28.0 g of deionized water under stirring, and the silicate solution was made by dissolving 3.98 g of Na2SiO3·9H2O and 2.0 g of NaOH in 45.33 g of deionized water. After adding the aluminate solution into the silicate solution under vigorous stirring, the mixture was aged for 3 h at room temperature producing a final synthetic mixture with a molar composition of Na2O:SiO2:Al2O3:H2O=7.5:2:1:600. After that, the synthetic mixture was transferred into an autoclave where the supports stood vertically in a holder. All supports were wrapped with Teflon tape on their outer surfaces before being put into the autoclave to avoid any zeolitic deposition on the outer surfaces. The hydrothermal synthesis was carried out at 100 °C for 5 h in an oven and the autoclaves were rotated 6
inside at 30 rpm. After crystallization, the membranes were washed several times with deionized water and then dried at 100 °C for 6 h.
2.4. Heat treatment Heat treatment of the supports and the zeolite membranes was carried out in a muffle furnace. The furnace was heated to a certain temperature with a rate of 2 °C min-1 from room temperature and then kept at this temperature for a certain time (e.g. 6 h) until letting it cool down at a rate of 2 °C min-1 to 30 °C. The membranes treated here were all synthesized on the inner surfaces of the supports.
2.5. Characterization The morphologies of zeolite/polymer blend composite hollow fibers and synthesized zeolite membranes were observed by scanning electron microscopy (SEM, HITACHI TM-1000). The mechanical properties of the BCHF-Z1 supports before membrane synthesis and after membrane synthesis by heat treatment at different temperatures were measured by mechanical test according to our previous report [9]. The pore structure of the supports was measured by the mercury porosimeter (AutoPore IV 9510, Micrometrics Inc.). The zeolite membranes were further characterized by pervaporation of 90 wt% ethanol/water mixture at 75 °C. The setup and operation process were the same as previously reported [26]. The zeolite membrane synthesized on the BCHF support was pasted into a module and then connected to the vacuum system. The feed solution was 7
circulated through the lumen of the support with a peristaltic pump at a flow rate of 100 ml min-1. A cold trap in liquid N2 was used to collect the permeate after the ethanol solution flowed in the module for 10 min. The time for collecting the permeate was 15 min. The compositions of the feed and the permeate were analyzed by gas chromatography (GC-1690, Kexiao Co. in Hangzhou). The two most important variables were flux (J) and separation factor (α), which were defined as J=W/(A×t)
(1)
α=(YH2O/YEtOH)/(XH2O/XEtOH)
(2)
Where W is the total weight of the permeation (kg), A is the efficient separation area of the membrane (m2), t is the collection time (h), YH2O/YEtOH is the weight ratio of water to ethanol in the permeate and XH2O/XEtOH is the weight ratio of water to ethanol in the feed.
3. Results and Discussion 3.1 Effects of crystal size on the performance of zeolite membranes Previously, our group illustrated that compact and continuous zeolite NaA membranes could be synthesized on the outer surfaces of the zeolite (75 wt%) /polymer composite hollow fiber (CHF) supports with small crystals [8]. The effect of crystal size on properties of zeolite membranes synthesized on the lumen sides of BCHF supports with 75 wt% zeolite crystals was investigated in this study. Due to the space limitation of lumen side (inner diameter: 1.2 mm) in the hollow fiber, the synthesis composition is very important for obtaining continuous zeolite layer on the inner surface. Our previous 8
work
has
presented
an
optimal
synthesis
composition
of
Na2O:SiO2:Al2O3:H2O=7.5:2:1:600 [26]. Therefore, this synthesis composition was used in this study. The morphologies of the supports (BCHF-Z0 and BCHF-Z1) and zeolite NaA membranes synthesized on their inner surfaces are shown in Fig. 1. The number of uniformly distributed zeolite crystals on the inner surface of BCHF-Z1 (Fig. 1b1) for membrane growth was more than that of BCHF-Z0 (Fig. 1a1). Therefore, it is easier to obtain dense membranes on hollow fibers with small-sized seeds. Thus, the membrane in Fig. 1b2 was compact and defect-free with a thickness of 4-5 µm (Fig. 1b3), while the membrane in Fig. 1a2 was less intergrown and had large defects. Moreover, the two membranes had different behaviors in dehydrating ethanol/water mixtures (Table 1). The membrane M0 had a poor separation factor of 24 far lower than that of M1 (>10,000), but the flux of M0 (6.6 kg m-2 h-1) was a little higher than that of M1 (5.1 kg m-2 h-1). It is believed that the defects in M0 decrease the separation selectivity but increase the separation flux during pervaporation.
9
Fig. 1. SEM images of inner surfaces of supports [(a1) BCHF-Z0 and (b1) BCHF-Z1] and zeolite NaA membranes supported on the corresponding supports [(a2, b2) top view and (a3, b3) cross section].
The bending strength and the compressive strength of BCHF-Z1 with 75 wt% zeolite crystals were greatly increased to 22.56 MPa and 3.78 MPa, respectively, compared to that of BCHF supports with 85 wt% zeolite crystals (10.11 MPa and 1.67 MPa,
，
respectively) [9]. However the flux of zeolite membranes on BCHF-Z1 supports (5.1 kg m-2 h-1) were much lower than that of zeolite membranes on BCHF supports with 85 10
wt% zeolite crystals (10.9 kg m-2 h-1) [9]. Here, we demonstrate that post-synthesis heat treatment can improve the separation performance of zeolite membranes on BCHF-Z1 supports. Therefore, effects of heat treatment on properties of BCHF-Z1 before and after membrane synthesis were investigated. Table 1 Separation performance of zeolite membranes supported on the inner sides of BCHF-Z0 and BCHF-Z1 supports a
a
Membrane
Zeolite
M0
Z0
M1
Z1
J ( kg m-2 h-1)
α (dimensionless)
4
6.6
24
1.5
5.1
﹥10,000
Particle size (µm)
Feed: 90 wt% ethanol aqueous solution at 75 oC
3.2 Effects of heat treatment temperature Mixed matrix membranes have been reported to be less defective when annealed to achieve good gas separation properties [20,22-24,27]. Therefore, it is hypothesized that zeolite NaA membranes on the BCHF-Z1 supports may achieve better performance by heat treatment at a certain temperature. Effects of the heat treatment temperature were then investigated, and SEM images of the inner and outer surface of the BCHF-Z1 support heat-treated for 6 h are shown in Fig. 2. No significant changes happened to the BCHF-Z1 in terms of the inner or the outer surface after heat treatment at 100 ºC and 220 ºC. The inner and outer surfaces after heat treatment at 240 ºC (Fig. 2c1 and 2c2) showed slight coalescence of polymer blends with some big pores formed, while the polymer blends were totally densified when treated at 270 ºC leaving many big pores in both inner and outer surfaces of BCHF-Z1 (Fig. 2d1 and 2d2). 11
Fig. 2. SEM images of the (a1-d1) inner and (a2-d2) outer surface of the BCHF-Z1 supports heat-treated at (a) 100 °C, (b) 220 °C, (c) 240 °C, and (d) 270 °C for 6 h.
12
Fig. 3. Cross-sectional SEM images of the BCHF-Z1 supports heat-treated at (a) 100 °C, (b) 220 °C, (c) 240 °C, and (d) 270 °C for 6 h.
13
Fig. 3 shows the cross-sectional morphologies of the supports treated from 100 ºC to 270 ºC. No obvious differences can be observed at low magnification in Fig. 3a1-d1 and at high magnification in Fig. 3a2-c2. However, many large pores can be seen in Fig. 3d2 due to the strong densification of the polymer blend at elevated temperature. It is possible that increased number of large pores might be favorable to the decrease of the support resistance, but unfavorable to the increase of the mechanical strength of supports. The porosity and pure water flux of the hollow fibers heat-treated at different temperatures are shown in Table 2. The porosities of the supports heat-treated at 100, 220 and 240 ºC were more than 50%, while the porosity of the support heat-treated at 270 ºC was about 40%. The pure water flux of the support heat-treated at 220 ºC (187 kg m-2 h-1) slightly increased compared with the support treated at 100 ºC (138 kg m-2 h-1). It sharply increased to 1,273 kg m-2 h-1 when heat-treated at 240 ºC, while it reached more than 19,000 kg m-2 h-1 at 270 ºC. The increasing pure water flux here indicates that the inter-connectivity of pores in the hollow fiber could be increased (i.e. provision of larger pores at the expense of small pores or increase of interphase between zeolite crystals and polymer) by heat treatment. Table 2 Effects of heat treatment temperature on the porosity and pure water flux of BCHF-Z1 supports a
a
T b (°C)
Porosity (%)
Pure water flux (kg m-2 h-1)
100
54.58
138
220
53.85
187
240
52.88
1,273
270
40.92
19,089
The heat treatment time was 6 h; b heat treatment temperature. 14
It can also be observed in Fig. 4 that cumulative intrusion of mercury decreased, and it becomes easier for mercury to enter the supports as the heat treatment temperature increases. In other words, the porosity decreased and the inter-connectivity increased as described above. As also shown in Fig. 5, the pore size of the supports increased when the heat treatment temperature was above 220 ºC, while there were no significant differences at 100 ºC and 220 ºC. The average pore size at 270 ºC was about 1.6 µm. The pore size distribution of the supports became narrow as the heat treatment temperature increased. This effect is particularly obvious at 270 ºC. This is consistent with the SEM results, indicating that large pores may be formed due to the mergence of small pores.
Fig. 4. Cumulative intrusion of mercury in BCHF-Z1 supports heat-treated at different temperatures for 6 h.
15
Fig. 5. Pore size distribution of BCHF-Z1 supports heat-treated at different temperatures for 6 h.
The above results can be explained by the glass transition temperature of the support. The heat deflection temperature of PES ranges from 200 ºC to 220 ºC, and its glass transition temperature is from 220 ºC to 230 ºC. The glass transition temperature (Tg=223 ºC) of BCHF-Z1 can be calculated by the Fox equation [22]. 1/Tg=w1/Tg1+w2/Tg2
(3)
Where w1 (0.9) and w2 (0.1) are the mass fraction of PES and PI, Tg1 (220 ºC) and Tg2 (254 ºC) are the glass transition temperature of PES and PI, respectively. On the other hand, a high zeolite crystal weight percentage (75 wt%) might slightly increase the glass transition temperature. Therefore, the glass transition temperature of the BCHF supports might be higher than 223 ºC. The heat treatment temperatures of 100 and 220 ºC are lower than the glass transition temperature. Therefore, there is no significant change in the support structure. The heat treatment temperatures of 240 and 270 ºC are higher than the glass transition temperature, and therefore, the support structure changes. 16
Table 3 Mechanical properties of BCHF-Z1 heat-treated at different temperatures for 6 h Bending strength (MPa)
Compressive strength (MPa)
Temperature (°C) BS
AS
BS
AS
100
22.56
21.00
3.78
5.85
220
22.99
21.70
3.71
5.68
240
24.36
22.19
3.74
6.10
270
23.01
21.20
3.32
4.28
Notice: BS--before synthesis and AS--after synthesis of membrane.
Mechanical properties of supports and supported membranes after heat treatment at different temperatures are shown in Table 3. The data revealed that the bending strength of BCHF supports after membrane synthesis was slightly lower than that before membrane synthesis, which is probably due to the formation of a brittle zeolite membrane layer synthesized on the surface of the hollow fiber [8,9]. The compressive strength of supports after membrane synthesis was higher than that before membrane synthesis. It is possible that the intergrown zeolite NaA layer in the lumen side acts as an annular upholder to increase the compressive strength and the seeded growth of the crystals in the matrix of the polymer blend may also help stiffen the BCHF [9]. The bending strength and compressive strength after membrane synthesis by heat treatment at 240 ºC were 22.19 MPa and 6.10 MPa, respectively. Its compressive strength is higher than others, which is probably due to the increased stiffness of the polymer chain, better adherence between zeolite particles and the polymer matrix, and the densification
17
of the polymer blend at above Tg. The compressive strength after heat treatment at 270 ºC declined, probably due to the formation of large pores. Table 4 Separation performance of the inner-side zeolite NaA membranes by heat post-synthesis treatment at different temperatures a Membrane
T b(°C)
J (kg m-2 h-1)
M1
100
5.1
M2
220
6.6
M3
240
11.1
M4
270
10.5
α (dimensionless)
﹥10,000 ﹥10,000 ﹥10,000 ﹥10,000
a
Feed: 90 wt% ethanol aqueous solution at 75 oC, and the heat treatment time was 6 h; b heat treatment temperature.
Due to the thermal stability of zeolite NaA, the structure of the inner-side zeolite NaA membranes after heat treatment at different temperatures (lower than 270 ºC) for 6 h were still the same with Fig. 1b2 and b3. The membranes were thin and well intergrown without any cracks (Data not shown). The separation performances in pervaporation of 90 wt% ethanol/water mixtures are listed in Table 4. The flux increased from 5.1 to 11.1 kg m-2 h-1 (M3) when the temperature increased from 100 ºC to 240 ºC. When the heat treatment temperature increased to 270 ºC, the flux of M4 was 10.5 kg m-2 h-1, which is slightly lower than that of M3. It has been reported that the membrane flux increases with the porosity of the support [7]. As the porosities of the supports heat-treated at different temperatures (100-240 ºC) are close to each other (Table 2), the increasing permeate flux indicates that the increased inter-connectivity (i.e. provision of larger pores at the expense of small pores or increase of interphase between zeolite crystals and polymer, or reduction in the blockage of zeolitic pores by polymer) likely improved 18
the separation flux. The flux of M4 decreased, whereas the pure water flux of the heat-treated support increased significantly. The reason for this phenomenon is not very clear at present. The relatively low porosity of the support heat-treated at 270 ºC might be the main reason for the low flux of M4. There are also some other reasons: (1) high heat treatment temperature (much higher than Tg) leads to additional support resistance, causing the polymer to gather around the zeolite layer or block zeolitic pores of crystals in the polymer matrix; (2) capillary condensation of the permeate in the large support pores [28]; (3) some different structural changes in the supports take place between the heat treatment of BCHF supports with and without the zeolite membrane. Further investigation needs to be performed to understand all changes in the membrane by heat treatment. It is worthy to be mentioned that a dense zeolite membrane can be hardly obtained on a heat-treated BCHF support. Therefore, separation performance of zeolite membranes on heat-treated BCHF supports was not presented in this study. That is, only the post-synthesis heat treatment is an effective way to improve the separation flux. From the results above, we can conclude that the post-synthesis heat treatment at 240 ºC is viable for improving separation performance and even mechanical strength of zeolite membranes supported on the BCHF-Z1 supports.
19
Fig. 6. SEM images of the BCHF-Z1 supports heat-treated at 240 °C for (a1-a3) 12 h and (b1-b3) 24 h; (a1, b1) inner surface, (a2, b2) cross section and (a3, b3) magnification of black square in a2 and b2.
3.3 Effects of heat treatment time Effects of post-synthesis heat treatment time at 240 ºC were also investigated. Fig. 6 shows the morphologies of hollow fibers heat-treated at 240 ºC for 12 h and 24 h, respectively. There are relatively large pores on the inner surfaces and cross-sections, which are similar to supports treated at 270 ºC for 6 h (Fig. 2d1 and Fig. 3d2). The porosities of the supports treated at 240 ºC for 12 h and 24 h were above 50%, almost 20
the same as those treated for 6 h and their pure water flux surpassed 8,000 kg m-2 h-1 (Table 5), but less than those treated at 270 ºC for 6 h. It is also found that the cumulative intrusion of mercury and pore size distribution of hollow fibers at 240 ºC for 12 h and 24 h (data not shown) were almost identical with those treated for 6 h. Moreover, the zeolite membranes heat-treated at 240 oC for 12 h and 24 h were still dense and defect free, as shown in Fig. 7. We initially thought that the separation flux would be higher after heat treatment for 12 h and 24 h due to the high porosity and excellent interconnectivity. However, in fact, the flux decreased as the heat treatment time was increased over 6 h (Table 6). The reason for this phenomenon is not very clear at present. There are two possible mechanisms: (1) longer heat treatment time leads to additional support resistance, causing the polymer to gather around the zeolite layer or block zeolitic pores of crystals in the polymer matrix; (2) capillary condensation of the permeate in the support pores [28]. Table 5 Effects of heat treatment time on the porosity and pure water flux of BCHF-Z1a
a
Time (h)
Porosity (%)
Pure water flux (kg m-2 h-1)
0
54.58
138
6
52.88
1,273
12
53.85
8,222
24
52.88
8,580
The heat treatment temperature is 240 °C.
21
Fig. 7. Morphologies of inner-side zeolite NaA membranes after heat-treated at 240 oC for (a, b) 12 h and (c, d) 24 h; (a, c) inner surface and (b, d) cross section.
Moreover, the heat treatment at 240 ºC for more than 12 h decreased the mechanical strength of zeolite membranes. For example, the bending and compressive strengths of zeolite membranes heat-treated for 12 h were 19.16 MPa and 4.74 MPa, respectively. Thus, longer heat treatment times than 6 h at 240 ºC do not improve the properties of the zeolite membranes. Table 6 Effects of post-synthesis heat treatment time on pervaporation properties of zeolite membranes supported on BCHF-Z1 a Time (h)
a
J (kg m-2 h-1)
0
5.1
6
11.1
12
9.6
24
7.9
The heat treatment temperature is 240 °C. 22
α (dimensionless)
﹥10,000 ﹥10,000 ﹥10,000 ﹥10,000
4. Conclusions Compact and defect-free zeolite NaA membranes were successfully synthesized on the inner-side surfaces of zeolite/polymer blend composite hollow fibers with 75 wt% zeolite small crystals (e.g. BCHF-Z1) due to more seed crystal numbers on the surface of the supports. Heat treatment had significant influence on the structure of the supports and the separation performances of zeolite membranes in dehydration of ethanol/water mixtures. The post-synthesis heat treatment at 240 ºC for 6 h greatly enhanced the flux to 11.1 kg m-2 h-1 from 5.1 kg m-2 h-1 with a separation factor of up to 10,000. Although the post-synthesis heat treatment at 270 ºC also improved the flux of zeolite membranes, the pores in the support became too large and the mechanical strength of the membrane was decreased. Heat treatment longer than 12 h at 240 ºC increased the pure water flux of supports, but, decreased the flux of zeolite membranes in pervaporation. It is suggested that both the porosity and interconnectivity of the hollow fibers have effects on the separation flux of zeolite membranes. This work opens a new path to the improvement of separation performances of zeolite membranes after synthesis.
Acknowledgement The authors would like to thank the National Natural Science Foundation of China (21236006), the National Basic Research Program (2013CB228104), Science and Technology Department of Zhejiang Province (2009R50020), and open research fund of Top Key Discipline of Chemistry in Zhejiang Provincial Colleges and Key Laboratory 23
of the Ministry of Education for Advanced Catalysis Materials (Zhejiang Normal University) (ZJHX201302) for financial support.
References
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25
Nomenclature W A t J C T YH2O/YEtOH XH2O/XEtOH Tg Tg1 Tg2 w1 w2
permeation water mass (kg) permeation area of membrane (m2) permeation time (h) permeation flux for water (kg m-2 h-1) concentration of ethanol-water mixture (wt %) heat treatment temperature (oC) weight ratio of water to ethanol in the permeation (%) weight ratio of water to ethanol in the feed (%) glass transition temperature of BCHF-Z1 (oC) glass transition temperature of PES (oC) glass transition temperature of PI (oC) mass fraction of PES mass fraction of PI
Greek symbols α pervaporation separation factor
26
Captions Fig. 1. SEM images of inner surfaces of supports [(a1) BCHF-Z0 and (b1) BCHF-Z1] and zeolite NaA membranes supported on the corresponding supports [(a2, b2) top view and (a3, b3) cross section].
Fig. 2. SEM images of the (a1-d1) inner and (a2-d2) outer surface of the BCHF-Z1 supports heat-treated at (a) 100 °C, (b) 220 °C, (c) 240 °C, and (d) 270 °C for 6 h.
Fig. 3. Cross-sectional SEM images of the BCHF-Z1 supports heat-treated at (a) 100 °C, (b) 220 °C, (c) 240 °C, and (d) 270 °C for 6 h.
Fig. 4. Cumulative intrusion of mercury in BCHF-Z1 supports heat-treated at different temperatures for 6 h.
Fig. 5. Pore size distribution of BCHF-Z1 supports heat-treated at different temperatures for 6 h.
Fig. 6. SEM images of the BCHF-Z1 supports heat-treated at 240 °C for (a1-a3) 12 h and (b1-b3) 24 h; (a1, b1) inner surface, (a2, b2) cross section and (a3, b3) magnification of black square in a2 and b2.
Fig. 7. Morphologies of inner-side zeolite NaA membranes heat-treated at 240 oC for (a, b) 12 h and (c, d) 24 h; (a, c) inner surface and (b, d) cross section.
Highlights •
Zeolite/polymer composite hollow fibers as supports of zeolite membranes.
•
The zeolite (<1.5 µm) content in hollow fibers is 75 wt%.
•
Flux can be greatly increased by post synthesis heat treatment.
27
•
No cracks occur in the membrane layer after post synthesis heat treatment.
28