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A novel method to synthesize high performance silicalite-1 membrane
Separation and Purification Technology 67 (2009) 58–63
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
A novel method to synthesize high performance silicalite-1 membrane Wei Xiao, Jianhua Yang, Jinming Lu, Jinqu Wang ∗ State Key Laboratory of Fine Chemicals of Dalian University of Technology, 116012 Dalian, PR China
a r t i c l e
i n f o
Article history: Received 9 November 2008 Received in revised form 4 March 2009 Accepted 4 March 2009 Keywords: Silicalite-1 membrane Precursor solution Counter-diffusion Hydrogen permeance
a b s t r a c t Silicalite-1 membranes supported on macroporous alumina tubes for hydrogen separation were successfully prepared by a specific synthesis protocol called counter-diffusion hydrothermal crystallization (CDHC). This method introduced silica source (tetraethyl orthosilicate, TEOS) and template agent (tetrapropyl ammonium bromide, TPABr) from opposite directions of the support wall, overwhelming the conventional in situ hydrothermal crystallization (ISHC). After an optimization process, the optimum molar composition of silicon precursor was 1SiO2 :0.135Na2 O:100H2 O:2EtOH and that of template precursor was 1.5TPABr:100H2 O. XRD and SEM analysis showed the as-prepared membranes were typical silicalite-1 membranes with high internal crystalline order as well as dense and continuous surface morphology. Single-gas permeation tests indicated the membrane prepared with optimized recipe combined high H2 permeance (1.24 × 10−6 mol m−2 s−1 Pa−1 ) and good H2 selectivity (˛H2 /SF6 = 155) at room temperature, 0.1 MPa pressure drop. Moreover, the consumption of template in this novel method was about one third of that used in conventional hydrothermal synthesis. This concept can also be extended to other many types of zeolite membranes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Membrane separation is a promising technology in comparison to conventional separation processes [1], which has potential to reduce operating costs, minimize unit operations, and lower energy consumption. Inorganic membranes for gas separation applications have attracted considerable attention in recent years due to their good separation performance and intrinsic thermal stability compared with polymeric separation membranes [2], especially for the zeolite-based selective membranes for hydrogen separation [3]. Silicalite-1 membrane, as a member of zeolite membranes, is particularly attractive due to their good thermal, chemical and structural stabilities and well-defined pores (about 0.5 nm) of molecular dimensions as well as opportunities for applications in gas, vapor, and liquid separations in the petrochemical industry [4–10]. However, the development of such commercial applications must depend on the availability of high quality membranes synthesized through simple and cost-effective methods. Silicalite-1 membranes have been widely synthesized by two typical techniques: (1) in situ crystallization method [11–16]; (2) secondary growth method [17–20]. In the first method, the bare porous support is put in contact with the zeolite precursors and submitted to hydrothermal synthesis conditions. Nucleation and
∗ Corresponding author. Tel.: +86 411 8899 3632; fax: +86 411 8365 3220. E-mail address: [email protected] (J. Wang). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.03.007
growth of zeolite material take place in presence of the support at the same time. The second one is a two-step process during which small zeolite crystals are deposited on top of the support and then these seeds are grown under hydrothermal synthesis conditions in order to form a continuous zeolite layer. The main drawbacks of these methods are: (1) difficulty in ensuring the uniformity of the crystallization conditions and limited reproducibility of high-quality membranes; (2) significant consumption of valuable chemicals and supports, resulting in the generation of a large volume of waste and high processing costs. Recently, much work has been done to modify these methods. Some researchers introduced counter-diffusion chemical vapor deposition and pore-plugging methods [21–23] to prepare silica membranes and ZSM-5 membranes. Alsyouri et al. [23] used acid catalyzed counter-diffusion self-assembly method to synthesize ordered mesoporous silica membranes within porous ceramic supports and summarized that good quality silica membrane was fabricated within the supports with hydrophobic internal pore facilitating transfer of silicon precursor. Xomeritakis et al. [24] investigated the variation of pore size of the ceramic membranes during the synthesis process. It seems that plugging pores of a substrate with well inter-grown zeolite crystals is easier to achieve than growing a continuous defect-free zeolite layer on top of a substrate. But the above methods which require expensive apparatus and strict process conditions are incompetent for large-scale production. Considering the practical problems, a novel technique to prepare high performance zeolite membranes is urgently needed.
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Fig. 1. Schemes of the different impregnation architectures of the precursors. (a) The ISHC method and (b) the CDHC method.
Herein, a simple method called counter-diffusion hydrothermal crystallization (CDHC) was presented and optimized to prepare high performance silicalite-1 membrane. Silicon source and template precursors diffused along the support channels from opposite directions, reacted when they encountered with each other and formed a dense membrane. The as-prepared membranes were characterized by XRD, SEM and single-gas permeation tests. 2. Experimental 2.1. Materials ␣-Al2 O3 tubes (OD 13 mm, ID 8 mm, length 60 mm, Foshan Ceramics Research Institute (FCRI)), were used as supports in this experiment. The average pore size and porosity of the tubes were 2–3 m and 30–40%, respectively. Other materials used in this study, such as tetra-propyl ammonium bromide (TPABr), tetrapropyl ammonium hydroxide (TPAOH), tetraethyl orthosilicate (TEOS), hydrochloric acid, ethanol and sodium hydroxide (NaOH) were purchased from Aldrich. 2.2. Preparation of precursors Silicon source precursor and template precursor were used in this technique, which were marked as A and B, respectively. Precursor A was prepared by mixing TEOS, NaOH, EtOH and deionized H2 O with a final molar composition of 1SiO2 :0.135Na2 O:x(80, 100, 120, 140)H2 O:2EtOH under stirring for 24 h. Precursor B was prepared by adding TPABr into deionized H2 O with a molar composition of y(0.5, 1.0, 1.5, 2.0)TPABr:100H2 O under stirring for 4 h. Thus, clear solutions of A and B were obtained.
by filling precursor A into the autoclave along the wall, paying attention to that liquid level of precursor A should not exceed the upper nozzle of the support. Immediately, precursor B about 5 mL was filled into the ␣-Al2 O3 tube from the upper nozzle, followed by sealing the upper nozzle and reinforcing precursor A again to exceed the support by about 1 cm. Fig. 1 shows the schemes of the impregnation process. After sealing the autoclave, it was placed in a preheated oven at 448 K for 24 h. As a contrast, the silicalite-1 membrane by ISHC method was also prepared, using a synthesis precursor of 1SiO2 :0.135Na2 O:0.27TPABr:100H2 O:2EtOH, which was marked as Ma . After the synthesis, the tubes were washed thoroughly with deionized water and dried at 393 K overnight. The synthesis was repeated until an uncalcined membrane had a N2 permeance of below 1 × 10−10 mol m−2 s−1 Pa−1 (298 K, 0.1 MPa pressure drop). The calcination process was carried out at 773 K for 30 h with heating and cooling rates of 0.5 K/min and 1.0 K/min, respectively. Table 1 shows the molar composition of synthesis precursors (A and B). 2.4. Characterization of silicalite-1 membranes The surface and cross-section morphologies of the as-prepared membranes were characterized by scanning electron microscopy (SEM) using a KYKY2800B at an acceleration voltage of 20 kV or 30 kV and a working distance of 10 mm after gold coating. X-ray diffraction (XRD) with a Philips Analytical X-ray diffractometer using Cu K␣ radiation analyzed the crystalline microstructure of the membranes. Gas permeation tests using H2 , N2 , n-C4 H10 and SF6 were performed at the temperature range of 298–523 K. The heating and cooling rates of the cell were maintained at 1 K/min so as to prevent thermal collapse of the zeolite membrane. The feed stream was pressurized, while downstream pressure was maintained at
2.3. Preparation of silicalite-1 membranes The ␣-Al2 O3 supports were treated as Ref. [25]. Then the pretreated support with the lower nozzle sealed with Teflon cap was put vertically into a Teflon lined stainless steel autoclave, followed Table 1 Molar composition of the synthesis precursors (A and B). Membrane No.
M1 M2 M3 M4 M5 M6 M7
Precursor A
Precursor B
SiO2
Na2 O
H2 O
EtOH
H2 O
TPABr
1 1 1 1 1 1 1
0.135 0.135 0.135 0.135 0.135 0.135 0.135
100 100 100 100 80 120 140
2 2 2 2 2 2 2
100 100 100 100 100 100 100
0.5 1.0 1.5 2.0 1.5 1.5 1.5
Fig. 2. XRD patterns of silicalite-1 membranes prepared with different B precursors: M1 y = 0.5; M2 y = 1.0; M3 y = 1.5; M4 y = 2.0.
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Fig. 3. SEM images of the as-prepared membranes. M1 (a and b); M3 (c and d); M4 (e and f); Ma (g and h).
atmospheric pressure; no sweep gas was used. Permeate flow rate was determined by a soap-film meter. During the experiments, the residual of the previously measured gas inside the membrane might have some effects on the permeation of the next. To deduce this influence, after the completion of a gas permeation measurement, the membrane was softly regenerated before the introduction of the next permeate by being flushed with helium at 423 K until the permeance of helium was restored to its original value.
The gas permeance is defined as PM =
Q A · t · (pf − pp )
where Q is the moles of gas permeate through the membrane in a time period of t (s); A is the active membrane area (m2 ); and pf and pp are the permeating gas partial pressures (Pa) in the feed side and permeate side, respectively. The permselectivity of ˛A/B is defined
W. Xiao et al. / Separation and Purification Technology 67 (2009) 58–63
as the permeance ratio of their permeance values to evaluate the quality of the zeolite membrane. 3. Results and discussions 3.1. Precursor impregnation of different methods As an innovation, the model used for the precursor impregnation is called counter-diffusion with architecture different from the conventional methods, in which two surfaces (external surface and internal surface) of the support were in contact with different precursors (A and B). Because the hydrophilic surface of the support pores reduces the penetration of the hydrophobic silicon precursor and facilitates the transport of the hydrophilic template precursor through the pores [23], in this experiment impregnation precursor A before B is beneficial for the diffusion process as shown in Fig. 1b. While due to the large pore size (2–3 m) distribution of the cost-effective support, precursor A diffusion into the support is unavoidable before the impregnation of precursor B. To minimize this problem, the impregnation interval between precursor A and B is as short as possible. One can imagine that they diffuse along opposite directions of the support channels, meet in the surface region of the support and react to form a dense zeolite layer under proper conditions. After calculation, it was concluded that in this method the template consumption is only about one third of that in ISHC method due to the high utilization of the template in the new synthesis model. Fig. 1 shows the comparison of schemes of the different impregnation architectures. 3.2. Structures and morphologies of silicalite-1 membranes XRD analysis of membranes prepared with different B precursors is shown in Fig. 2. It can be seen that M1 –M4 exhibit typical diffraction peaks of MFI-type membrane at 2 in the ranges of 7–9 and 23–25◦ , respectively, with no evidence of other crystalline phase. While for M1 (y = 0.5), the intensity of the support peaks is faintish compared with that of other samples. This may be interpreted as follows: (1) in the first synthesis step, the template was insufficient (y = 0.5), so fewer nuclei were formed on the surface of the support. In the subsequent growth of the nuclei, much nutrition were absorbed by limited nuclei, resulting in larger zeolite crystals and a thick and loose membrane layer; (2) the silicalite-1 top layer covers the support, based on its thickness, diffraction angle and absorption coefficient of the silicalite-1 film, part of the signal from the support should be absorbed by the thick silicalite-1 layer (Fig. 3b). In Fig. 3, the morphologies of M1 , M3 , M4 and Ma can be seen clearly. The surface of M1 is not dense enough, and some intercrystalline pores can be seen, though they may not be run-through pores. Moreover, it is very thick (Fig. 3b), which can be roughly divided into three parts: the macroporous support (A), ca. 10 m porous layer (B) and ca. 15 m silicalite-1 layer (C). Different from M1 , as shown in Fig. 3c, it is clearly seen that silicalite-1 crystals shows a good inter-grown behavior with less inter-zeolitic spaces and a dense and continuous top layer was obtained. Moreover, the silicalite-1 top layer with 12–15 m in thickness displays a good coupling with the support as shown in Fig. 3d. Compared with M1 , the surface of M4 is relative continuous and dense, moreover, the crystal grains are smaller than those of M3 . While compared with M3 , the crystals on M4 surface are obviously discrete. On the other hand, some of the support pores were filled with colloidal substance from Fig. 3f. It can be presumed that in some large pore region, silicon and template precursors contacted with each other in the channels of the support, leading to the formation of zeolite crystals in the channels. Ma prepared by the ISHC method also displays
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Table 2 Permeance (×108 mol m−2 s−1 Pa−1 ) and permselectivity of the membranes (298 K, 0.1 MPa). Sample
H2
N2
n-C4 H10
SF6
˛1
˛2
˛3
M1 M2 M3 M4 M5 M6 M7 Ma
26.4 46.6 124 120 34.4 69.6 39.0 32.0
14.4 18.5 41.0 81.9 8.70 27.9 22.5 22.7
0.76 3.35 1.30 9.76 1.20 1.60 10.1 2.50
0.71 1.11 0.80 4.37 0.60 1.30 8.60 2.20
1.83 2.52 3.00 1.50 3.84 2.49 1.70 1.40
34.9 13.9 95.0 12.3 28.0 43.5 3.90 12.8
37.3 41.9 155 27.4 36.0 53.5 4.50 14.5
˛1 , ˛2 , and ˛3 denote the ideal separation factors of H2 /N2 , H2 /n-C4 H10 and H2 /SF6 , respectively.
sparse surface and irregular cross-section morphology with a large amount of defects compared with other samples. The structure of Ma foretells its poor permeation behavior. The above discussions show that in this method befittingly high concentration of template precursor is beneficial for the formation of dense, continuous and inter-grown membrane surface. When the concentration exceeds a proper value, however, the performance of the membranes begins to decrease, including the microstructures and the permeation property as shown in Table 2. As shown in Fig. 4a, when x = 80, the surface of M5 consists of some columelliform zeolite crystals with c-axis partially vertical to the support surface. But, it is not dense almost the same as M1 . And the membrane is thick without high internal crystalline order. This suggests the silicon precursor with a high concentration is disadvantageous for the formation of high performance membrane, which is consistent with M1 for which low concentration template results in a poor morphology and structure. When x = 140 (the lowest TEOS concentration investigated), from the SEM characterization, it can be seen that the morphology of M7 is different from those of other samples, which possessed unique platelet crystal structure as shown in Fig. 4c. Moreover, some silicalite-1 crystals behind this layer can be seen indistinctly. It should be noted most samples experienced two synthesis steps, but for M7 , three steps were required to make M7 impermeable to N2 before calcination. This unique crystal morphology is consistent with Lee and Shantz [26] who observed that platelets of silicalite-1 can be formed in a much narrower composition space as compared to the familiar coffin-like zeolite morphology. Although our synthesis process is different from that of Lee, it can be assumed after the initial crystallization of the silicalite-1 layer on the support by CDHC method, the further diffusion of precursor is difficult, so the special contact model between TEOS and TPABr and proper precursor concentration may lead to the same result as [26]. All these XRD and SEM characterizations suggest that silicalite-1 membranes, which possess high quality morphology and structure, can be prepared under optimized compositions of precursors by CDHC method. 3.3. Permeability and permselectivity XRD and SEM analysis can only show the structures and morphologies of the as-prepared membranes, which cannot evaluate the practical separation property well. Membrane quality is usually determined using the ideal selectivity for several gas pairs such as N2 /SF6 , H2 /SF6 , and n-C4 H10 /i-C4 H10 [27], for example Noack et al. used H2 /SF6 ideal selectivity higher than 45 as quality criterion [28]. Table 2 shows the permeation behavior of the membranes. It can be seen M1 has poor H2 permeation at room temperature, which is interrelated with its thickness and interconnectivity shown in Fig. 3b. M3 has very excellent gas permeation property. At 298 K, it reaches a H2 permeance and ˛H2 /SF6 up to 1.24 × 10−6 mol m−2 s−1 Pa−1 and 155, respectively, much higher
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Fig. 4. SEM images of the as-prepared membranes. M5 (a and b); M7 (c and d).
than 24 in the literature [6]. M4 has almost the same H2 permeance (1.2 × 10−6 mol m−2 s−1 Pa−1 ) as that of M3 , however, N2 and SF6 are of high permeance. At room temperature the ˛H2 /N2 is only 1.5, which is only 40% of the Knudsen value of 3.74. All these suggest that M4 has more non-zeolite pores or cracks, although the surface morphology is relatively preferable. M5 , M7 and Ma have low H2 permeance and permselectivity in accordance with the poor structures as shown in Figs. 3 and 4. For M3 the temperature programmed permeation results of single gases are shown in Fig. 5. With the increasing temperature, H2 permeance increases from 1.24 × 10−6 mol m−2 s−1 Pa−1 to 1.5 × 10−6 mol m−2 s−1 Pa−1 at 523 K. As a notable difference, the influence of temperature on N2 permeance is almost negligible, which maintains at 4.0 × 10−7 mol m−2 s−1 Pa−1 . So one can conclude the dominant diffusion mechanism for H2 is activation diffusion, temperature having a positive effect on the permeance [8]. While for N2 activation diffusion and Knudsen diffusion are both the
dominant mechanisms which have opposite effects on N2 permeance, resulting in the changeless permeability. This is the indicator of the defect inexistence, since H2 is a small molecule which can permeate faster through inter-crystalline defects [6,8]. Compared with H2 , n-C4 H10 (0.364 nm) and SF6 (0.55 nm) have a certain adsorptive property in zeolite channels, and its diffusion process includes adsorption step and diffusion step. When increasing the temperature from 298–523 K an increasing mobility of SF6 , n-C4 H10 and H2 can be expected especially for the larger SF6 , n-C4 H10 molecules. Thus, ˛H2 /SF6 and ˛H2 /n-C4 H10 decrease dramatically from 103 to 10.2 and 95 to 9.25, respectively, however, they are still higher than the Knudsen values of 8.54 and 5.38. In order to check the presence of viscous flux for these gases, permeance was plotted versus the pressure difference through the membrane (Fig. 6). For M3 at 523 K, each permeance remains constant with the increase in the pressure, and the H2 ideal separation factors of H2 /N2 , H2 /n-C4 H10 , and H2 /SF6 also have no change in the pressure range. So, no viscous flux contributes to the total perme-
Fig. 5. Permeances of single gases for M3 as a function of temperature.
Fig. 6. Permeances of single gases for M3 as a function of pressure difference.
W. Xiao et al. / Separation and Purification Technology 67 (2009) 58–63
Fig. 7. The Arrhenius relation of H2, N2 and SF6 permeances with temperatures for M3 .
ance of single gases and a further conclusion is that this membrane almost has no large defects (such as pin-holes, cracks). Valuable information can also be gathered from the examination of the temperature programmed permeation results, which are presented in the case as Arrhenius plots (Fig. 7). The activation energy of H2, N2 , n-C4 H10 , and SF6 permeation through M3 appears to be 0.8163 kJ/mol, 0.1670 kJ/mol, 13.85 kJ/mol and 17.84 kJ/mol, respectively, which are in accordance well with the above discussions (Fig. 5). Takata [29] showed the activation energy of He (0.28 kJ/mol) and N2 (−2.45 kJ/mol) for a silicalite-1 membrane. Other reports showed the activation energy of H2 (3–10 kJ/mol), n-C4 H10 (42.6 kJ/mol) and SF6 (10.4 kJ/mol) [30]. One of the explanations for the differences in activation energies is the differences of inner microstructure and density of defects such as the intercrystalline and/or intracrystalline pores as well as the cracks formed during the calcination process, which needs to be studied further. 4. Conclusion Silicalite-1 membranes on macroporous ␣-Al2 O3 supports were successfully prepared by CDHC method. In this method, silicon source and template agent can encounter with each other during the diffusion process near the substrate surface, resulting in a good coupling between membranes and the supports. This method can control the amounts and time of the encountered precursors effectively, making the membrane dense, continuous and stable. The membrane prepared with optimized recipe has excellent H2 permeance and permselectivity. Moreover, the amount of template agent used in this method is about one third of that used in conventional methods. This method can also be extended to the preparation of other many types of zeolite membranes. It should be noted that the membranes prepared by this method are relatively thick, so next step how to obtain thin membranes with good performance by this method will be explored. Acknowledgements We acknowledge the Petrochemical Corporation and Shanghai Yiming Filtration Technology Co. Ltd. of China for supporting this work. References [1] T.S. Chung, L.Y. Jiang, Y. Li, S. Kulprathipanja, Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci. 32 (2007) 483–507.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
A novel method to synthesize high performance silicalite-1 membrane Wei Xiao, Jianhua Yang, Jinming Lu, Jinqu Wang ∗ State Key Laboratory of Fine Chemicals of Dalian University of Technology, 116012 Dalian, PR China
a r t i c l e
i n f o
Article history: Received 9 November 2008 Received in revised form 4 March 2009 Accepted 4 March 2009 Keywords: Silicalite-1 membrane Precursor solution Counter-diffusion Hydrogen permeance
a b s t r a c t Silicalite-1 membranes supported on macroporous alumina tubes for hydrogen separation were successfully prepared by a specific synthesis protocol called counter-diffusion hydrothermal crystallization (CDHC). This method introduced silica source (tetraethyl orthosilicate, TEOS) and template agent (tetrapropyl ammonium bromide, TPABr) from opposite directions of the support wall, overwhelming the conventional in situ hydrothermal crystallization (ISHC). After an optimization process, the optimum molar composition of silicon precursor was 1SiO2 :0.135Na2 O:100H2 O:2EtOH and that of template precursor was 1.5TPABr:100H2 O. XRD and SEM analysis showed the as-prepared membranes were typical silicalite-1 membranes with high internal crystalline order as well as dense and continuous surface morphology. Single-gas permeation tests indicated the membrane prepared with optimized recipe combined high H2 permeance (1.24 × 10−6 mol m−2 s−1 Pa−1 ) and good H2 selectivity (˛H2 /SF6 = 155) at room temperature, 0.1 MPa pressure drop. Moreover, the consumption of template in this novel method was about one third of that used in conventional hydrothermal synthesis. This concept can also be extended to other many types of zeolite membranes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Membrane separation is a promising technology in comparison to conventional separation processes [1], which has potential to reduce operating costs, minimize unit operations, and lower energy consumption. Inorganic membranes for gas separation applications have attracted considerable attention in recent years due to their good separation performance and intrinsic thermal stability compared with polymeric separation membranes [2], especially for the zeolite-based selective membranes for hydrogen separation [3]. Silicalite-1 membrane, as a member of zeolite membranes, is particularly attractive due to their good thermal, chemical and structural stabilities and well-defined pores (about 0.5 nm) of molecular dimensions as well as opportunities for applications in gas, vapor, and liquid separations in the petrochemical industry [4–10]. However, the development of such commercial applications must depend on the availability of high quality membranes synthesized through simple and cost-effective methods. Silicalite-1 membranes have been widely synthesized by two typical techniques: (1) in situ crystallization method [11–16]; (2) secondary growth method [17–20]. In the first method, the bare porous support is put in contact with the zeolite precursors and submitted to hydrothermal synthesis conditions. Nucleation and
∗ Corresponding author. Tel.: +86 411 8899 3632; fax: +86 411 8365 3220. E-mail address: [email protected] (J. Wang). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.03.007
growth of zeolite material take place in presence of the support at the same time. The second one is a two-step process during which small zeolite crystals are deposited on top of the support and then these seeds are grown under hydrothermal synthesis conditions in order to form a continuous zeolite layer. The main drawbacks of these methods are: (1) difficulty in ensuring the uniformity of the crystallization conditions and limited reproducibility of high-quality membranes; (2) significant consumption of valuable chemicals and supports, resulting in the generation of a large volume of waste and high processing costs. Recently, much work has been done to modify these methods. Some researchers introduced counter-diffusion chemical vapor deposition and pore-plugging methods [21–23] to prepare silica membranes and ZSM-5 membranes. Alsyouri et al. [23] used acid catalyzed counter-diffusion self-assembly method to synthesize ordered mesoporous silica membranes within porous ceramic supports and summarized that good quality silica membrane was fabricated within the supports with hydrophobic internal pore facilitating transfer of silicon precursor. Xomeritakis et al. [24] investigated the variation of pore size of the ceramic membranes during the synthesis process. It seems that plugging pores of a substrate with well inter-grown zeolite crystals is easier to achieve than growing a continuous defect-free zeolite layer on top of a substrate. But the above methods which require expensive apparatus and strict process conditions are incompetent for large-scale production. Considering the practical problems, a novel technique to prepare high performance zeolite membranes is urgently needed.
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Fig. 1. Schemes of the different impregnation architectures of the precursors. (a) The ISHC method and (b) the CDHC method.
Herein, a simple method called counter-diffusion hydrothermal crystallization (CDHC) was presented and optimized to prepare high performance silicalite-1 membrane. Silicon source and template precursors diffused along the support channels from opposite directions, reacted when they encountered with each other and formed a dense membrane. The as-prepared membranes were characterized by XRD, SEM and single-gas permeation tests. 2. Experimental 2.1. Materials ␣-Al2 O3 tubes (OD 13 mm, ID 8 mm, length 60 mm, Foshan Ceramics Research Institute (FCRI)), were used as supports in this experiment. The average pore size and porosity of the tubes were 2–3 m and 30–40%, respectively. Other materials used in this study, such as tetra-propyl ammonium bromide (TPABr), tetrapropyl ammonium hydroxide (TPAOH), tetraethyl orthosilicate (TEOS), hydrochloric acid, ethanol and sodium hydroxide (NaOH) were purchased from Aldrich. 2.2. Preparation of precursors Silicon source precursor and template precursor were used in this technique, which were marked as A and B, respectively. Precursor A was prepared by mixing TEOS, NaOH, EtOH and deionized H2 O with a final molar composition of 1SiO2 :0.135Na2 O:x(80, 100, 120, 140)H2 O:2EtOH under stirring for 24 h. Precursor B was prepared by adding TPABr into deionized H2 O with a molar composition of y(0.5, 1.0, 1.5, 2.0)TPABr:100H2 O under stirring for 4 h. Thus, clear solutions of A and B were obtained.
by filling precursor A into the autoclave along the wall, paying attention to that liquid level of precursor A should not exceed the upper nozzle of the support. Immediately, precursor B about 5 mL was filled into the ␣-Al2 O3 tube from the upper nozzle, followed by sealing the upper nozzle and reinforcing precursor A again to exceed the support by about 1 cm. Fig. 1 shows the schemes of the impregnation process. After sealing the autoclave, it was placed in a preheated oven at 448 K for 24 h. As a contrast, the silicalite-1 membrane by ISHC method was also prepared, using a synthesis precursor of 1SiO2 :0.135Na2 O:0.27TPABr:100H2 O:2EtOH, which was marked as Ma . After the synthesis, the tubes were washed thoroughly with deionized water and dried at 393 K overnight. The synthesis was repeated until an uncalcined membrane had a N2 permeance of below 1 × 10−10 mol m−2 s−1 Pa−1 (298 K, 0.1 MPa pressure drop). The calcination process was carried out at 773 K for 30 h with heating and cooling rates of 0.5 K/min and 1.0 K/min, respectively. Table 1 shows the molar composition of synthesis precursors (A and B). 2.4. Characterization of silicalite-1 membranes The surface and cross-section morphologies of the as-prepared membranes were characterized by scanning electron microscopy (SEM) using a KYKY2800B at an acceleration voltage of 20 kV or 30 kV and a working distance of 10 mm after gold coating. X-ray diffraction (XRD) with a Philips Analytical X-ray diffractometer using Cu K␣ radiation analyzed the crystalline microstructure of the membranes. Gas permeation tests using H2 , N2 , n-C4 H10 and SF6 were performed at the temperature range of 298–523 K. The heating and cooling rates of the cell were maintained at 1 K/min so as to prevent thermal collapse of the zeolite membrane. The feed stream was pressurized, while downstream pressure was maintained at
2.3. Preparation of silicalite-1 membranes The ␣-Al2 O3 supports were treated as Ref. [25]. Then the pretreated support with the lower nozzle sealed with Teflon cap was put vertically into a Teflon lined stainless steel autoclave, followed Table 1 Molar composition of the synthesis precursors (A and B). Membrane No.
M1 M2 M3 M4 M5 M6 M7
Precursor A
Precursor B
SiO2
Na2 O
H2 O
EtOH
H2 O
TPABr
1 1 1 1 1 1 1
0.135 0.135 0.135 0.135 0.135 0.135 0.135
100 100 100 100 80 120 140
2 2 2 2 2 2 2
100 100 100 100 100 100 100
0.5 1.0 1.5 2.0 1.5 1.5 1.5
Fig. 2. XRD patterns of silicalite-1 membranes prepared with different B precursors: M1 y = 0.5; M2 y = 1.0; M3 y = 1.5; M4 y = 2.0.
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Fig. 3. SEM images of the as-prepared membranes. M1 (a and b); M3 (c and d); M4 (e and f); Ma (g and h).
atmospheric pressure; no sweep gas was used. Permeate flow rate was determined by a soap-film meter. During the experiments, the residual of the previously measured gas inside the membrane might have some effects on the permeation of the next. To deduce this influence, after the completion of a gas permeation measurement, the membrane was softly regenerated before the introduction of the next permeate by being flushed with helium at 423 K until the permeance of helium was restored to its original value.
The gas permeance is defined as PM =
Q A · t · (pf − pp )
where Q is the moles of gas permeate through the membrane in a time period of t (s); A is the active membrane area (m2 ); and pf and pp are the permeating gas partial pressures (Pa) in the feed side and permeate side, respectively. The permselectivity of ˛A/B is defined
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as the permeance ratio of their permeance values to evaluate the quality of the zeolite membrane. 3. Results and discussions 3.1. Precursor impregnation of different methods As an innovation, the model used for the precursor impregnation is called counter-diffusion with architecture different from the conventional methods, in which two surfaces (external surface and internal surface) of the support were in contact with different precursors (A and B). Because the hydrophilic surface of the support pores reduces the penetration of the hydrophobic silicon precursor and facilitates the transport of the hydrophilic template precursor through the pores [23], in this experiment impregnation precursor A before B is beneficial for the diffusion process as shown in Fig. 1b. While due to the large pore size (2–3 m) distribution of the cost-effective support, precursor A diffusion into the support is unavoidable before the impregnation of precursor B. To minimize this problem, the impregnation interval between precursor A and B is as short as possible. One can imagine that they diffuse along opposite directions of the support channels, meet in the surface region of the support and react to form a dense zeolite layer under proper conditions. After calculation, it was concluded that in this method the template consumption is only about one third of that in ISHC method due to the high utilization of the template in the new synthesis model. Fig. 1 shows the comparison of schemes of the different impregnation architectures. 3.2. Structures and morphologies of silicalite-1 membranes XRD analysis of membranes prepared with different B precursors is shown in Fig. 2. It can be seen that M1 –M4 exhibit typical diffraction peaks of MFI-type membrane at 2 in the ranges of 7–9 and 23–25◦ , respectively, with no evidence of other crystalline phase. While for M1 (y = 0.5), the intensity of the support peaks is faintish compared with that of other samples. This may be interpreted as follows: (1) in the first synthesis step, the template was insufficient (y = 0.5), so fewer nuclei were formed on the surface of the support. In the subsequent growth of the nuclei, much nutrition were absorbed by limited nuclei, resulting in larger zeolite crystals and a thick and loose membrane layer; (2) the silicalite-1 top layer covers the support, based on its thickness, diffraction angle and absorption coefficient of the silicalite-1 film, part of the signal from the support should be absorbed by the thick silicalite-1 layer (Fig. 3b). In Fig. 3, the morphologies of M1 , M3 , M4 and Ma can be seen clearly. The surface of M1 is not dense enough, and some intercrystalline pores can be seen, though they may not be run-through pores. Moreover, it is very thick (Fig. 3b), which can be roughly divided into three parts: the macroporous support (A), ca. 10 m porous layer (B) and ca. 15 m silicalite-1 layer (C). Different from M1 , as shown in Fig. 3c, it is clearly seen that silicalite-1 crystals shows a good inter-grown behavior with less inter-zeolitic spaces and a dense and continuous top layer was obtained. Moreover, the silicalite-1 top layer with 12–15 m in thickness displays a good coupling with the support as shown in Fig. 3d. Compared with M1 , the surface of M4 is relative continuous and dense, moreover, the crystal grains are smaller than those of M3 . While compared with M3 , the crystals on M4 surface are obviously discrete. On the other hand, some of the support pores were filled with colloidal substance from Fig. 3f. It can be presumed that in some large pore region, silicon and template precursors contacted with each other in the channels of the support, leading to the formation of zeolite crystals in the channels. Ma prepared by the ISHC method also displays
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Table 2 Permeance (×108 mol m−2 s−1 Pa−1 ) and permselectivity of the membranes (298 K, 0.1 MPa). Sample
H2
N2
n-C4 H10
SF6
˛1
˛2
˛3
M1 M2 M3 M4 M5 M6 M7 Ma
26.4 46.6 124 120 34.4 69.6 39.0 32.0
14.4 18.5 41.0 81.9 8.70 27.9 22.5 22.7
0.76 3.35 1.30 9.76 1.20 1.60 10.1 2.50
0.71 1.11 0.80 4.37 0.60 1.30 8.60 2.20
1.83 2.52 3.00 1.50 3.84 2.49 1.70 1.40
34.9 13.9 95.0 12.3 28.0 43.5 3.90 12.8
37.3 41.9 155 27.4 36.0 53.5 4.50 14.5
˛1 , ˛2 , and ˛3 denote the ideal separation factors of H2 /N2 , H2 /n-C4 H10 and H2 /SF6 , respectively.
sparse surface and irregular cross-section morphology with a large amount of defects compared with other samples. The structure of Ma foretells its poor permeation behavior. The above discussions show that in this method befittingly high concentration of template precursor is beneficial for the formation of dense, continuous and inter-grown membrane surface. When the concentration exceeds a proper value, however, the performance of the membranes begins to decrease, including the microstructures and the permeation property as shown in Table 2. As shown in Fig. 4a, when x = 80, the surface of M5 consists of some columelliform zeolite crystals with c-axis partially vertical to the support surface. But, it is not dense almost the same as M1 . And the membrane is thick without high internal crystalline order. This suggests the silicon precursor with a high concentration is disadvantageous for the formation of high performance membrane, which is consistent with M1 for which low concentration template results in a poor morphology and structure. When x = 140 (the lowest TEOS concentration investigated), from the SEM characterization, it can be seen that the morphology of M7 is different from those of other samples, which possessed unique platelet crystal structure as shown in Fig. 4c. Moreover, some silicalite-1 crystals behind this layer can be seen indistinctly. It should be noted most samples experienced two synthesis steps, but for M7 , three steps were required to make M7 impermeable to N2 before calcination. This unique crystal morphology is consistent with Lee and Shantz [26] who observed that platelets of silicalite-1 can be formed in a much narrower composition space as compared to the familiar coffin-like zeolite morphology. Although our synthesis process is different from that of Lee, it can be assumed after the initial crystallization of the silicalite-1 layer on the support by CDHC method, the further diffusion of precursor is difficult, so the special contact model between TEOS and TPABr and proper precursor concentration may lead to the same result as [26]. All these XRD and SEM characterizations suggest that silicalite-1 membranes, which possess high quality morphology and structure, can be prepared under optimized compositions of precursors by CDHC method. 3.3. Permeability and permselectivity XRD and SEM analysis can only show the structures and morphologies of the as-prepared membranes, which cannot evaluate the practical separation property well. Membrane quality is usually determined using the ideal selectivity for several gas pairs such as N2 /SF6 , H2 /SF6 , and n-C4 H10 /i-C4 H10 [27], for example Noack et al. used H2 /SF6 ideal selectivity higher than 45 as quality criterion [28]. Table 2 shows the permeation behavior of the membranes. It can be seen M1 has poor H2 permeation at room temperature, which is interrelated with its thickness and interconnectivity shown in Fig. 3b. M3 has very excellent gas permeation property. At 298 K, it reaches a H2 permeance and ˛H2 /SF6 up to 1.24 × 10−6 mol m−2 s−1 Pa−1 and 155, respectively, much higher
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Fig. 4. SEM images of the as-prepared membranes. M5 (a and b); M7 (c and d).
than 24 in the literature [6]. M4 has almost the same H2 permeance (1.2 × 10−6 mol m−2 s−1 Pa−1 ) as that of M3 , however, N2 and SF6 are of high permeance. At room temperature the ˛H2 /N2 is only 1.5, which is only 40% of the Knudsen value of 3.74. All these suggest that M4 has more non-zeolite pores or cracks, although the surface morphology is relatively preferable. M5 , M7 and Ma have low H2 permeance and permselectivity in accordance with the poor structures as shown in Figs. 3 and 4. For M3 the temperature programmed permeation results of single gases are shown in Fig. 5. With the increasing temperature, H2 permeance increases from 1.24 × 10−6 mol m−2 s−1 Pa−1 to 1.5 × 10−6 mol m−2 s−1 Pa−1 at 523 K. As a notable difference, the influence of temperature on N2 permeance is almost negligible, which maintains at 4.0 × 10−7 mol m−2 s−1 Pa−1 . So one can conclude the dominant diffusion mechanism for H2 is activation diffusion, temperature having a positive effect on the permeance [8]. While for N2 activation diffusion and Knudsen diffusion are both the
dominant mechanisms which have opposite effects on N2 permeance, resulting in the changeless permeability. This is the indicator of the defect inexistence, since H2 is a small molecule which can permeate faster through inter-crystalline defects [6,8]. Compared with H2 , n-C4 H10 (0.364 nm) and SF6 (0.55 nm) have a certain adsorptive property in zeolite channels, and its diffusion process includes adsorption step and diffusion step. When increasing the temperature from 298–523 K an increasing mobility of SF6 , n-C4 H10 and H2 can be expected especially for the larger SF6 , n-C4 H10 molecules. Thus, ˛H2 /SF6 and ˛H2 /n-C4 H10 decrease dramatically from 103 to 10.2 and 95 to 9.25, respectively, however, they are still higher than the Knudsen values of 8.54 and 5.38. In order to check the presence of viscous flux for these gases, permeance was plotted versus the pressure difference through the membrane (Fig. 6). For M3 at 523 K, each permeance remains constant with the increase in the pressure, and the H2 ideal separation factors of H2 /N2 , H2 /n-C4 H10 , and H2 /SF6 also have no change in the pressure range. So, no viscous flux contributes to the total perme-
Fig. 5. Permeances of single gases for M3 as a function of temperature.
Fig. 6. Permeances of single gases for M3 as a function of pressure difference.
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Fig. 7. The Arrhenius relation of H2, N2 and SF6 permeances with temperatures for M3 .
ance of single gases and a further conclusion is that this membrane almost has no large defects (such as pin-holes, cracks). Valuable information can also be gathered from the examination of the temperature programmed permeation results, which are presented in the case as Arrhenius plots (Fig. 7). The activation energy of H2, N2 , n-C4 H10 , and SF6 permeation through M3 appears to be 0.8163 kJ/mol, 0.1670 kJ/mol, 13.85 kJ/mol and 17.84 kJ/mol, respectively, which are in accordance well with the above discussions (Fig. 5). Takata [29] showed the activation energy of He (0.28 kJ/mol) and N2 (−2.45 kJ/mol) for a silicalite-1 membrane. Other reports showed the activation energy of H2 (3–10 kJ/mol), n-C4 H10 (42.6 kJ/mol) and SF6 (10.4 kJ/mol) [30]. One of the explanations for the differences in activation energies is the differences of inner microstructure and density of defects such as the intercrystalline and/or intracrystalline pores as well as the cracks formed during the calcination process, which needs to be studied further. 4. Conclusion Silicalite-1 membranes on macroporous ␣-Al2 O3 supports were successfully prepared by CDHC method. In this method, silicon source and template agent can encounter with each other during the diffusion process near the substrate surface, resulting in a good coupling between membranes and the supports. This method can control the amounts and time of the encountered precursors effectively, making the membrane dense, continuous and stable. The membrane prepared with optimized recipe has excellent H2 permeance and permselectivity. Moreover, the amount of template agent used in this method is about one third of that used in conventional methods. This method can also be extended to the preparation of other many types of zeolite membranes. It should be noted that the membranes prepared by this method are relatively thick, so next step how to obtain thin membranes with good performance by this method will be explored. Acknowledgements We acknowledge the Petrochemical Corporation and Shanghai Yiming Filtration Technology Co. Ltd. of China for supporting this work. References [1] T.S. Chung, L.Y. Jiang, Y. Li, S. Kulprathipanja, Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci. 32 (2007) 483–507.
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