Pervaporation through TS-1 membrane

Pervaporation through TS-1 membrane

Available online at www.sciencedirect.com Microporous and Mesoporous Materials 115 (2008) 164–169 www.elsevier.com/locate/micromeso Pervaporation th...

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Available online at www.sciencedirect.com

Microporous and Mesoporous Materials 115 (2008) 164–169 www.elsevier.com/locate/micromeso

Pervaporation through TS-1 membrane Xiangshu Chen a, Pei Chen b, Hidetoshi Kita b,* b

a College of Chemistry and Chemical Engineering, Jingxi Normal University, Nanchang 330027, PR China Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan

Received 19 July 2007; accepted 12 December 2007 Available online 19 February 2008

Abstract Titanium-substituted silicalite (TS-1) membranes were synthesized onto the surface of a tubular mullite support by in situ crystallization using tetraethyl orthosilicate (TEOS), titanium tetrabutoxide (TBOT) and tetrapropylammonium hydroxide (TPAOH) as silica, titanium source and organic structure directing agent. The typical molar composition was 1SiO2:0.02TiO2:0.17TPAOH:120H2O. X-ray diffraction (XRD) patterns and Fourier transformed infrared (FT-IR) spectra of zeolite powders and membranes confirmed that the titanium was isomorphously incorporated into the MFI framework. The outer surface of the porous support was completely covered with randomly oriented, intergrown TS-1 crystals and the thickness of the membrane was about 10–20 lm, judging from the scanning electron microscopy observation. The membrane prepared at 200 °C for 16 h showed high ethanol selectivity for an ethanol/water mixture. For example, the separation factor and total flux through this TS-1 membrane were 127 and 0.77 kg/m2 h for 5 wt.% ethanol feed concentration at 60 °C, respectively. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Zeolite membrane; Silicalite membrane; TS-1; In situ crystallization; Pervaporation

1. Introduction It is well known that zeolite membranes have strong potential applications in gas separation, pervaporation (PV), and catalytic membrane reactors since zeolite membranes have high thermal, chemical and structural stability compared with polymer membranes. More than a decade ago, the first large-scale plant using zeolite A membranes for dehydration of organic liquids were commercialized by Mitsui Engineering and Shipbuilding Co. Ltd. in cooperation with Yamaguchi University [1–3]. In contrast to zeolite A membranes, hydrophobic silicalite membranes exhibit preferentially selective permeation of organic components from organic/water mixtures. In recent years, we have reported that silicalite membranes synthesized on porous mullite tubes showed the high PV performance [4–6].

*

Corresponding author. Tel.: +81 836 85 9661; fax: +81 836 85 9601. E-mail address: [email protected] (H. Kita).

1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.12.033

In 1983, Taramasso et al. [7] first reported the synthesis of titanium-substituted zeolite silicalite, and called it TS-1. Since then several patents and papers have been reported [8–11]. Zeolite silicalite framework incorporated with titanium shows very interesting properties. Now TS-1 is known to be an efficient catalyst for selective oxidation of alcohols, alkanes, and hydroxylation of aromatics, epoxidation of alkenes. On the other hand, TS-1 zeolite crystals also have a hydrophobic property so that TS-1 membranes are expected to exhibit preferentially selective permeation of organic components from organic/water mixtures. Titanium ions are not stable at high alkaline media normally used in zeolite synthesis because they form a very stable TiO2 phase, which does not dissolve into Ti(OH)4 species, as silicon, aluminum and gallium would do. On the other hand, TiO2 crystals are hydrophilic. When the stable TiO2 phase is dispersed in products and membranes, it not only decreases the catalytic activity, but also counteracts the hydrophobic property of TS-1 membranes. Up to now, there have two papers reported for the preparation of TS-1 membranes on porous stainless steel supports [12,13].

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However, either no separation data was presented or the resulted membranes cracked upon calcination. Based on our successfully synthetic technology for the preparation of silicalite membranes, a completely different approach from those reported in early patents and papers was adopted to prepare defect-free TS-1 membranes with the high PV performance in this paper. 2. Experimental 2.1. TS-1 membrane preparation TS-1 membranes were prepared by in situ crystallization on the outer surface of porous mullite tubes (12 mm outer diameter, 1.5 mm thickness, 1.0 lm average pore size and 10 cm length). The surface of the mullite tubular support was very rough so that the outer surface was polished with SiC paper. Then it was cleaned with distilled water three times by ultrasonic cleaner. A mullite porous tube was chosen as a support because it is much cheaper than pure aAl2O3, especially at pore size of less than 1 lm [2]. The sol molar composition studied in this work is: SiO2:TiO2:TPAOH:H2O = 1:x:0.17:120, where x is less than 0.03. A gel for the synthesis of TS-1 membranes was prepared in the following procedure. First, tetrapropylammonium hydroxide (TPAOH, 20–25 wt.% in water, Tokyo Kasei) was mixed with water under stirring. After stirring for a short time, tetraethyl orthosilicate (TEOS, 98 wt.% Aldrich) was added to the TPAOH solution. Then a yellow solution was prepared by mixing titanium tetrabutoxide (TBOT, 97 wt.%, Aldrich), hydrogen peroxide (30 wt.%, Wako) and water under stirring. After a suitable time the obtained TBOT solution was added to the above sol under vigorous stirring. Subsequently 250 g of the synthesis gel obtained was poured into a Teflon-lined autoclave. Finally the support tube was vertically placed in the autoclave. The autoclave was transferred in a convection oven preheated at a given temperature. After a given period of reaction time, the autoclave was removed from the oven and cooled to room temperature. The sample was then recovered, washed with distilled water and dried at room temperature and 100 °C each for several hours. Subsequently, it was calcined at 500 °C in air for 10 h at heating and cooling rates of 0.15 °C/min and 0.25 °C/min, respectively.

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4000 cm1. The membrane morphology was observed with field emission scanning electron microscopy (FE-SEM, JEOL JSM 6335F). PV experiments were carried out with a batch system as described elsewhere [4–6]. The inside of the membrane tube was evacuated through a vacuum line. The permeate vapor was collected by a condensed trap cooled with liquid nitrogen. The downstream pressure was maintained below 0.1 torr. The effective membrane areas were about 28 cm2. The amount of the feed solution was about 2500 g. During PV, a proper amount of organic component (ethanol) was added to the feed solution at intervals of 60 min to keep the constant feed concentration due to high flux of the silicalite membranes. The compositions of the feed and permeate were analyzed by a gas chromatograph (GC, SHIMADZU GC-8A) equipped with 3 m column packed with Polarpack Q poly (ethylene glycol)-1000. The flux was calculated by weighing the condensed permeate. The separation factor was determined as aA=B ¼ ðY A =Y B Þ=ðX A =X B Þ where XA, XB, YA, and YB denote the mass fractions of components A (ethanol) and B (water) at the feed and permeate sides. 3. Results and discussion 3.1. Structural and morphological characterization To confirm titanium isomorphously incorporated into the silicalite framework, the powders scraped off from the surface of the membrane and the membrane prepared with the molar composition of 1SiO2:0.02TiO2:0.17TPAOH: 120H2O by in situ crystallization at 200 °C for 20 h of hydrothermal treatment were characterized by FT-IR and XRD. For comparison, the silicalite powders and the silicalite membrane was also characterized by FT-IR and XRD. Fig. 1 shows FT-IR spectra of the powders scraped off from

2.2. Characterization and permeation experiments The powders collected from the bottom of the autoclave were characterized by X-ray diffraction (XRD, SHIMADZU XRD-6100) with Cu Ka radiation. The spectra were scanned in the 5–45 2h range at a scanning rate of 4° per min. The powders were also analyzed by Fourier transformed infrared (FT-IR, JASCO FT/IR-610) spectroscopy using KBr pellets technique. For FT-IR, an amount of 1 mg of TS-1 powders was mixed with 250 mg of KBr and a pellet of 200 mg was used for this characterization. The IR spectra were recorded between 400 and

Fig. 1. FT-IR spectra of (a) TS-1 powders, and (b) silicalite powders.

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the surface of the membrane and the silicalite powders. It can be seen that the scraped powders showed an absorption peak at about 960 cm1 (Fig. 1a), which was not observed in the silicalite powders (Fig. 1b). This band was attributed to an asymmetric stretching mode of SiO2 entities boned to Ti4+ ions in a tetrahedral zeolite site, and has been used as the fingerprint to characterize the presence of framework Ti in the MFI structure [14]. XRD spectra of the TS-1 membrane and the powders are shown in Fig. 2. From Fig. 2, it can be clearly seen that the XRD spectra of both the powders that formed at the same time as the membrane and the membrane prepared with the molar composition of SiO2:0.01TiO2:0.17TPAOH:120H2O by in situ crystallization at 200 °C for 20 h were essentially similar to that of the standard TS-1 sample [6,9]. Therefore, it is evidenced that titanium was isomorphously incorporated into the zeolite silicalite framework. Moreover, the strong intensity of TS-1 peaks with the approximate absence of the support reflection peaks for the TS-1 membrane (Fig. 2a) reveals that the support was completely covered by the TS-1 crystals. The absence of any other hetero-phase of titanium oxide is proved by the UV–vis spectrum [15]. The dominant peak at 220–240 nm indicates the presence of the tetrahedral titanium in the zeolite framework. A well defined absorption band around 330 nm that is attributed to anatase was not observed. We can thus conclude that the zeolites do not contain anatase-like oxide species inside the channels. Fig. 3 shows the surface and cross-sectional SEM views of TS-1 membranes prepared on unseeded mullite tubes with the molar composition of 1SiO2:0.01TiO2:0.17TPAOH:120H2O at 200 °C for crystallization time from 16 to 24 h. From the surface views as shown in Fig. 3a, after 16 h of hydrothermal treatment, the unseeded mullite tube was completely covered with randomly oriented TS-1 crystals. A similar morphology was also observed in the other

Fig. 2. XRD patterns of (a) mullite support, (b) silicalite membrane, (c) TS-1 membrane synthesized for 20 h at 200 °C (SiO2:0.01TiO2:0.17TPAOH:120H2O), and (d) TS-1 powders.

membranes synthesized for 20 and 24 h of hydrothermal treatment, although the length of the TS-1 crystals increased from about 15 lm to 30 lm. From the SEM cross-sectional views shown in Fig. 3e–g, the thickness of the dense crystal layer increased from about 10 lm to 20 lm with increasing hydrothermal treatment time from 16 h to 24 h. Furthermore it also reveals a random orientation of the crystals on the incompact top layer. A similar morphology shown in Fig. 3d and h was observed for the TS-1 membrane prepared with the molar composition of 1SiO2:0.02TiO2:0.17TPAOH:120H2O. In addition, it was very difficult to determine the interface between the mullite support and the TS-1 crystal layer because of not only the very rough surface of the support, but also formation of the TS-1 crystals inside the support. This indicated that a strong interaction is present between the top surface layer and the support. 3.2. PV performance The membrane separation performance (permeability and selectivity) is related to the quality of a zeolite layer formed on a porous support. Under the certain gel composition for synthesis of zeolite membranes, a crystal growth behavior such as growth rate, morphology, and orientation of crystals is strongly related to the crystallization temperature, time, and alkalinity etc. Consequently, the membrane separation properties are affected by these factors. In this section, the PV performance of TS-1 membranes corresponding to the synthesis conditions was presented. The PV performance of the TS-1 membranes prepared on unseeded mullite tubes with the starting molar composition of 1SiO2:0.01TiO2:0.17TPAOH:120H2O at 200 °C as a function of different treatment time is shown in Table 1. After 12 h of hydrothermal treatment, all the pores of the support could not be fully covered with TS-1 crystals formed; as a consequence of a defective membrane. After 16 h of treatment, the TS-1 membrane had a separation factor for ethanol over water and a total flux were 66 and 1.17 kg/m2 h, respectively. For further treatment, the separation factors first increased, combining with decrease in fluxes. The highest separation factor of 104 was obtained for treatment time of 20 h. However, for a prolonged treatment time of up to 40 h, the membrane had a lower separation factor of 41 with a higher flux of 0.585 kg/m2 h than that for 24 h. That might result from the formation of defects during calcination or other unclear reasons. As confirmed by the SEM observations, the membrane thickness increased with increasing in hydrothermal treatment time, which resulted in decreasing fluxes. Table 2 shows the PV performance of the TS-1 membranes prepared on unseeded mullite tubes at different synthesis temperatures from 170 to 210 °C. It can be seen that the separation factors increased with increasing synthesis temperature until 200 °C. At 170 °C, the crystal growth rate was so low that even after 48 h of hydrothermal treatment, the compact crystal layer could not be formed on the

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Fig. 3. Surface and cross-sectional SEM views of TS-1 membranes for different crystallization times of 16 h (a, e), 20 h (b, f, d, h) and 24 h (c, g).

support yet. This membrane was leaking before calcination. With synthesis temperature rising in combination with a shortened hydrothermal treatment time, the separation factor initially increased, then decreased greatly to 54 in the case of the membrane M-11. The maximum separation factor of 127 together with a relatively high flux of 0.77 kg/m2 h was obtained for the membrane M-10 prepared at 200 °C for 20 h of hydrothermal treatment. These results suggested that the synthesis temperature is one of

the most important variables, which strongly affects the quality of the membrane. Table 3 shows the PV performance of the TS-1 membranes synthesized with different Ti/Si ratios at 200 °C for 20 h of hydrothermal treatment. From this Table, it can be clearly seen that with increasing Ti/Si ratio, the separation factor dramatically increased, while the flux kept almost constant. The membrane with the maximum PV separation factor was obtained at the Ti/Si ratio of 0.02.

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Table 1 PV performance of TS-1 membranes prepared for different hydrothermal treatment time at 200 °C (5 wt.% ethanol/water mixture at 60 °C) Membrane

Synthesis time (h)

Flux (kg/m2 h)

Separation factor

M-1 M-2 M-3 M-4 M-5

12 16 20 24 40

– 1.17 0.73 0.41 0.59

1 66 104 97 41

Note: SiO2:TiO2:TPAOH:H2O = 1:0.01:0.17:120.

Table 2 PV performance of TS-1 membranes prepared at different temperatures (5 wt.% ethanol/water mixture at 60 °C) Membrane

Synthesis temperature (°C)

Synthesis time (h)

Flux (kg/m2 h)

Separation factor

M-6 M-7 M-8 M-9 M-10 M-11

170 170 180 190 200 210

48 120 72 40 20 20

– 0.42 0.55 0.70 0.77 0.59

– 75 70 107 127 54

Table 4 PV performance of TS-1 membranes prepared with different H2O/SiO2 ratios at 200 °C for 20 h (5 wt.% ethanol/water mixture at 60 °C) Membrane

H2O/SiO2 (molar ratio)

Flux (kg/m2 h)

Separation factor

M-14 M-15 M-16 M-10 M-17

20 40 80 120 150

– 0.62 0.76 0.77 –

– 92 122 127 1

mal treatment, suggesting that under the present synthesis conditions the solution was too diluted. It is clear that the high H2O/SiO2 ratio would not result in a continuous crystal layer because not only fewer nuclei were directly formed on the surface or attached onto the support surface from the solution, but also the limited nutrients were provided for their crystal growth. On the other hand, with a more concentrated H2O/SiO2 ratio of 20, no crystal was found on the support.

Note: SiO2:TiO2:TPAOH:H2O = 1:0.02:0.17:120.

3.3. Comparison of separation performance among MFI membranes

Table 3 PV performance of TS-1 membranes prepared with different TiO2/SiO2 ratios at 200 °C for 20 h (5 wt.% ethanol/water mixture at 60 °C)

The pervaporation performances of typical MFI membranes in the literature are listed in Table 5. The membrane pervaporation performance (flux and selectivity) is related to both of the quality of the zeolite layer formed on a porous support and the physic property of zeolite crystals. The stronger hydrophobicity, the fewer defects would result in the higher separation factor for MFI zeolite membranes. In our previous works, the alkali-free and clear synthesis solutions were used to prepare the strong hydrophobic silicalite membranes and in situ crystallization was adopted to improve the quality of the crystal layer. As shown in Table 5, indeed the silicalite membranes showed the high flux and separation factor for ethanol/water mixtures. Table 5 also shows that the highest separation factor was obtained with the TS-1 membrane. The pore size of TS-1 is almost the same as that of silicalite-1. Compared with the silicalite membranes, the TS-1 membrane prepared by in situ crystallization was the more hydrophobic, confirmed by the contact angle measurements. The high separation factor for ethanol aqueous solution was also attributed to the stronger hydrophobicity of the TS-1 membrane prepared in the present work.

Membrane

Ti/Si (molar ratio)

Flux (kg/m2 h)

Separation factor

M-12 M-7 M-10 M-13

0.005 0.01 0.02 0.03

0.71 0.73 0.77 1.03

88 104 127 17

For further increasing Ti/Si ratio to 0.03, the hydrophilic anatase (TiO2) was presented in the crystal layer indicated by DR UV–vis analysis, so that the membrane separation factor decreased greatly. Table 4 shows the PV performance of TS-1 membranes prepared on the unseeded mullite tubes with different H2O/ SiO2 ratios. All the sample preparation was carried out at 200 °C for 20 h of hydrothermal treatment. From Table 4, either the more diluted or the more concentrated synthesis solution was not suitable for preparation of the TS-1 membranes with high separation factors compared with the H2O/SiO2 ratio of 120. The sample prepared with H2O/SiO2 ratio of 150 was leaking after 20 of hydrother-

Table 5 Pervaporation performance of silicalite membranes for ethanol/water mixtures Support

Temperature (°C)

Time (h)

Feed concentration (EtOH wt.%)

PV temperature (°C)

Flux (kg/m2 h)

Separation factor

Ref.

Ml-tube SS-disk

175 170

16 144

200

20

60 30 60 60 30

0.93 0.29 0.97 0.77 0.17

106 120 84 127 134

[6] [16]

Ml-tube

5 4 4 5

Notes: Ml-tube: mullite tube; SS-disk: stainless steel disk.

This work

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4. Conclusions The titanium-substituted silicalite membranes were successfully synthesized onto mullite porous tubes by in situ crystallization. The randomly oriented, intergrown TS-1 crystals were formed on the porous tables. The typical zeolite TS-1 was the only crystalline phase presented in the membranes. The TS-1 membrane with the stronger hydrophobicity showed a high ethanol selectivity in pervaporation of a ethanol/water mixture. Acknowledgments This work was partly supported by a Grant-in-Aid for Scientific Research of Japan (18360377) and by the Ministry of Science and Technology of the People’s Republic of China (2006DFB53070). References [1] H. Kita, K. Horii, Y. Ohtoshi, K. Tanaka, K. Okamoto, J. Mater. Sci. Lett. 14 (1995) 206.

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[2] M. Kondo, M. Komori, H. Kita, K. Okamoto, J. Membrane Sci. 133 (1997) 133. [3] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, Sep. Purif. Technol. 25 (2001) 251. [4] X. Lin, H. Kita, K. Okamoto, J. Chem. Soc. Chem. Commun. 1889 (2000). [5] X. Lin, H. Kita, K. Okamoto, Ind. Eng. Chem. Res. 40 (2001) 4069. [6] X. Lin, X. Chen, H. Kita, K. Okamoto, AIChE J. 49 (2003) 237. [7] M. Taramasso, G. Perego, B. Notari, US Patent 4410501, 1983. [8] C. Neri, F. Buonomo, Eur. Patent 0100117 A1, 1984. [9] T. Tatsumi, M. Nakamura, S. Nagishi, H. Tominaga, J. Chem. Soc. Chem. Commun. 467 (1990). [10] M.G. Clerici, U. Romano, Eur. Patent 0230949 A2, 1987. [11] C. Neri, B. Anfossi, F. Buonomo, Eur. Patent 0100119 A1, 1983. [12] A. Esposito, C. Neri, F. Buonomo, US Patent 4480135, 1984. [13] K.T. Jung, J.H. Hyun, G.Y. Shul, K.K. Koo, AIChE J. 43 (1997) 2802. [14] M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti, G. Petrini, Stud. Surf. Sci. Catal. 48 (1989) 133. [15] P. Chen, X. Chen, K. Tanaka, H. Kita, Chem. Lett. 36 (2007) 1078. [16] H. Matsuda, H. Yanagishita, D. Kitamoto, T. Nakane, K. Haraya, N. Koura, T. Sano, Membrane 23 (1998) 259 (in Japanese).