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Preparation of a MOF membrane with 3-aminopropyltriethoxysilane as covalent linker for xylene isomers separation
Inorganic Chemistry Communications 30 (2013) 74–78
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Preparation of a MOF membrane with 3-aminopropyltriethoxysilane as covalent linker for xylene isomers separation Zixi Kang a, Jinying Ding b, Lili Fan a, Ming Xue a,⁎, Daliang Zhang a, Lianxun Gao b, Shilun Qiu a,⁎ a b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, 130012, PR China State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China
a r t i c l e
i n f o
Article history: Received 15 December 2012 Accepted 29 January 2013 Available online 4 February 2013 Keywords: Metal organic framework SAMs modified supporter Nanocrystal Xylene isomer separation
a b s t r a c t In this study, a continuous-growth Zn2(BDC)2DABCO membrane was synthesized on the NH2-terminated SAMs modified porous SiO2 substrates by using the MOF nanocrystals as seed, which was prepared by an easily scalable method with acetic acid as capping reagent. The seed crystals and membranes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The thickness of the membranes can be varied by adjusting the repetitions of growing process. The xylene isomer mixture separation performance tests of MOF membrane were taken out. © 2013 Elsevier B.V. All rights reserved.
In the chemical industry, the largest fraction of production costs is related to the separation and purification of product streams. The separation of mixed xylene isomers is one of the most challenging issues since xylene isomers are important chemical intermediates. For example, p-xylene is exclusively used as raw material in the production of terephthalic acid (TPA) and dimethyl terephthalate (DMT), which then react with ethyleneglycol to form polyethylene terephthalate (PET). Distillation can be used to remove o-xylene, which fails for the other xylenes isomeric compounds because of similar boiling points (p-xylene: 138.37 °C and m-xylene: 139.12 °C). Nowadays, adsorption is widely used to separate xylene isomers, for example, in Parex or Ebex units in which simulated moving-bed processes are used for the recovery of p-xylene and ethylbenzene [1]. Zeolites X and Y exchanged with cations (Na+, K+, and Ba2+) discriminate very selectively between different xylene isomers and have been widely used as industrial adsorbents [2,3]. However, the process control of this method is complex. Zeolite membranes had been used to separate xylene isomers as well, which may require high permeance temperature and pressure [4–7]. Metal organic frameworks (MOFs) or porous coordination polymers (PCPs) are hybrid inorganic–organic materials made from an assembly of metal ions with organic linkers [8–11]. Their well-defined porosity and tunable chemical functionality make them extremely attractive for applications in gas storage, catalysis, and separation [12–17]. Apart from their use as bulk materials, MOFs are also potential candidates for membrane applications. Pioneering research on MOF films or membranes have already been reported in the literature [18–22]. For instance, a copper-net-supported ⁎ Corresponding authors. Tel./fax: +86 431 85168589. E-mail addresses: [email protected] (M. Xue), [email protected] (S. Qiu). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.01.027
HKUST-1 membrane with selectivity towards H2/N2 in gas separation has been reported by our group [23]. This material is anticipated to recycle hydrogen from other permanent gases. Besides, Tsapatsis and coworkers have prepared a MOF membrane with small pore size on the substrates of alumina, which showed high selectivity for H2/N2 [24]. Caro et al. describes the successful preparation of zeolite imidazolate framework (ZIFs) type membranes, which are applied in the separation of hydrogen from other gases [25,26]. Recently, several research as well as focused on the liquid mixtures separation properties of MOF membranes [27–29]. The guest-free Zn2(BDC)2DABCO has large, three dimensionally interconnected voids and high surface area [30]. The guest-accessible volume for this framework is estimated to be 62%, while the pore size of Zn2(BDC)2DABCO (7.5 Å) is too large for as separation. However, it fit well with the size of most liquid molecules (Fig. S1). Therefore, the Zn2(BDC)2DABCO membrane may have potential applications in liquid separation area. Due to the three-dimensional pore structure of Zn2(BDC)2DABCO with easily accessible BDC ligands as potential adsorption sites, we tend to prepare this kind of membrane for xylene isomers separation. Here, we fabricate the Zn2(BDC)2DABCO membranes by second-grown method using seed cystals which was prepared by an easily scalable method with acetic acid as capping reagent on the modified substrates (Scheme 1). To demonstrate the potential for general applicability of MOF membranes, the effect of membrane thickness and separation conditions on some aspects of permeation result has been investigated and is discussed. In order to obtain large area and alternate growth MOF membrane, the surface of porous SiO2 substrate was first modified with a linker 3-aminopropyl-triethoxysilane (APTES) to form the SAMs layer. We utilize the coordination of \NH2 groups in the ARTES and zinc sites in
Z. Kang et al. / Inorganic Chemistry Communications 30 (2013) 74–78
the seed crystals to enhance the binding force between MOF membrane and support [25]. The modified substrates were characterized by XPS, and the results were shown in Fig. 1. The hydrocarbon C1s signal at 284.8 eV was used as a reference for surface charging. XPS was performed on a sample monolayer to confirm the monolayer formation and the introduction of each different component. According to theoretical relative concentration of the APTES layer, the relative concentration of C1s: N1s was 69.4%: 30.6%, in agreement with the theoretical values of 71.4%: 28.6%, which approve that a NH2-terminated SAMs was successfully grafted on the porous SiO2 substrates. As a capping reagent, acetic acid was used to compose Zn2(BDC)2DABCO nanocrystals, which were used as seeds for membrane second growth on porous SiO2 wafers. The role of acetic acid in reducing the particle size is attributed to its modulating effect on the coordinating interactions between the metal ions and organic linkers [31,32]. In the initial stage of synthesis, Zn cations coordinate to carboxylic groups, which are not only from organic linker BDC but also from acetic acid added. The crystal growth is, therefore, impeded in a very early stage allowing more nuclei to be formed. Moreover, the competitive coordination of the capping reagent is speculated to regulate the rate of crystal growth. Fig. 2 shows the SEM pictures of the Zn2(BDC)2DABCO seed crystals synthesized at different conditions. When the reaction solution was acetic acid free, the crystals become larger and have no fixed shape (Fig. 2c). Similarly, the crystals grow much bigger with higher concentration (Fig. 2a) and longer reaction time (Fig. 2b). However, adding acetic acid to reaction solution significantly decelerated the rate of crystal growth and facilitated the formation of nanorods with high aspect ratios (Fig. 2d). The average lengths of the major and minor axis of seed crystals synthesized under this condition are about 700 nm and 300 nm. Fig. S3 shown the PXRD patterns of Zn2(BDC)2DABCO simulated and seed crystals. As shown in Fig. S2, the XRD pattern illustrate that the synthesized seed crystals are composed of cubic phase of Zn2(BDC)2DABCO and a small amount of other phase, because the metal center and ligands may assemble to another hexagon structure in a short period of time (Fig. S3c) [33], which is not stable. Increasing growth time, this structure will be transformed into a relatively stable cubic structure, while the crystal size will increase. Fortunately this hexagon phase does not affect the secondary growth of MOF membrane. Morphology details of the seed layer and the synthesized Zn2(BDC)2DABCO membrane is shown in SEM images as well (Fig. 3). Seed layer with good continuity were uniformly spread on the top of the support and the support surface were fully covered (Fig. 3a). The membrane grown on this basis is proved to be well intergrown (Fig. 3b). From the cross-sectional view, it can be seen that the crystal layer is about 100 μm thickness and there are no pinholes inside the membrane. The crystals connect with the support compactly (Fig. 3c). EDS map indicated that the Zn2(BDC)2DABCO membrane only grew on the support surface (Fig. 3d). SEM pictures of membranes with varied thickness are shown in Fig. S4, which illustrate the thickness of these membranes with 2–5 cycle growth are about 40 μm, 80 μm, 100 μm and 150 μm respectively.
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In order to evaluate the crystallinity, as synthesized membrane were characterized by X-ray diffraction (XRD). As it shows in Fig. 4, the XRD patterns are consistent well with that of the simulated Zn2(BDC)2DABCO, indicating that the membranes grown on the substrate are in one pure phase and exhibit high crystallinity. It is worth noting that XRD pattern peaks of the guest-free membranes shift to lower 2θ values, which reveals Zn2(BDC)2DABCO structure expand after the activated process because of guest-dependent dynamic behavior [30]. In addition, XRD patterns of different thickness membranes are recorded in Fig. S5. As the the times of growth increase, the intensity of diffraction peaks becomes higher. The powder samples were collected by scraping from actvated membranes to take the TGA and N2 sorption isotherms measurements, and the TGA curves indicate that all guest molecules had been removed after the activation process (Fig. S6). As shown in Fig. S7, the nitrogen sorption isotherms of samples is a typical type I curves, which indicates uniform microporous structures of Zn2(BDC)2DABCO materials. The Brunauer–Emmett–Teller (BET) specific surface areas are calculated from the adsorption curve to be as high as 1589 m 2 g−1. Some groups have reported the application of MOF crystals in the simulate and adsorption separation of xylene isomers, which relate to the behavior of xylene isomers in the microporous metal–organic framework and breakthrough experiments of equimolar quaternary mixtures [34–37]. In this work, liquid separation studies of xylene isomer mixtures on the Zn2(BDC)2DABCO membranes with varied thickness and different temperatures were carried out (Fig S2). The separation performances of the membranes are displayed in Fig. 5. On the contrary to the results of the zeolite membrane, o-xylene and m-xylene molecules with larger kinetic diameter have greater permeability on this MOF membrane, which was consistent with the selective adsoption results on this MOF [34]. This phenomenon can be explained by an adsorption–diffusion model. The molecules which are more easy to adsorped by framework materials take precedence inside the channels and as well as hinder other molecules to get through the membrane. Selective factors of membranes prepared by different cycles of growth were obtained by Gas Chromatography are shown in Fig. 5b, it is clear that the addition of grown cycle in membranes synthesis increased the selectivity and decreased the xylene permeability at the same time. It is most likely due to the reduction of defect pores and increase in membrane thickness. The poor selectivity on the 5 cycles growth membrane is caused by the large shape of crystals, which are easily resulted in cracks and defects. The permeance test results of xylene isomers under different temperature (25 °C, 100 °C, 150 °C, 175 °C and 200 °C) were revealed in Table S1. High transmittance is obtained in membranes at high temperature, because the molecular kinetic energy increases when the temperature gets higher. As the operating temperature increasing from 25 to 150 °C for m-x/p-x binary system, the permeance of m-x increase from 826 g/m2h to 2094 g/m2h, while the p-x only slightly increases from 406 g/m2h to 533 g/m2h. Thus the resulting selective factor rises from 1.095% to 1.934%. However, at 175 °C and 200 °C, the membrane gradually became unstable, which
Scheme 1. Preparation of the Zn2(BDC)2DABCO membranes by using APTES as covalent linker between the Zn2(BDC)2DABCO membrane and the porous SiO2 support.
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leads to the reduction of separating capability of the membranes. The selectivity data indicate that best separation effect (o-x/p-x=1.934; m-x/p-x=1.617) can be obtained at the temperature close to the boiling point of xylene for both o-x/p-x and m-x/p-x binary system (Fig. 5a). However, this MOF membrane has very little separation effect for the o-x/m-x mixture. As we all known, Zn2(BDC)2DABCO is a dynamic structure, which may cause the decrease of membrame quaulity after the
activated process and result in the poor selectivity and relatively high flux of xylene isomers on this membrane. In summary, a large scale continuous MOF (Zn2(BDC)2DABCO) membrane has been successfully synthesized by second growth approach on the modified porous SiO2 substrate surface. The seed crystals were prepared using acetic acid as tapped agent. The thickness of the membrane can be varied by adjusting the repetitions of growing process. The separation performance tests of xylene isomer mixtures were taken out. Comparing with the conventional zeolite type membranes, separation properties of Zn2(BDC)2DABCO membrane for xylene isomers are derived from the selective adsorption effect of the framework. As MOF materials are self-assembled from metal centers and a wide range of organic ligands, the pore structure and pore size can easily changed, which is unprecedented in other types of materials. It is expected that MOF membrane with different structures and property will be developed for the targeted application of xylene isomers separation in the future. Acknowledgment This work was supported by the National Basic Research Program of China (2011CB808703, 2012CB821700), National Natural Science Foundation of China (Grant nos. 91022030, 21101072), and “111” project (B07016). References
Fig. 1. XPS spectra for the surface of membrane (a), N 1s (b) and C 1s (c) regions of APTES monolayers deposited for 3 h.
[1] D.B. Broughton, R.W. Neuzil, J.M. Pharis, C.S. Brearley, Chem. Eng. Prog. 66 (1970) 70–75. [2] V. Cottier, J.P. Bellat, M.H. Simonot-Grange, A. Methivier, J. Phys. Chem. B 101 (1997) 4798–4802. [3] M. Goddard, D.M. Ruthven, Zeolites 6 (1986) 275–282. [4] W.H. Yuan, Y.S. Lin, W.S. Yang, J. Am. Chem. Soc. 126 (2004) 4776–4777. [5] J. Hedlund, J. Sterte, M. Anthonis, A.-J. Bons, B. Carstensen, N. Corcoran, D. Cox, H. Deckman, W.D. Gijnst, P.-P. Moor, F. Lai, J. McHenry, W. Mortier, J. Reinoso, J. Peters, Microporous Mesoporous Mater. 52 (2002) 179–189. [6] C.J. Gump, V.A. Tuan, R.D. Noble, J.L. Falconer, Ind. Eng. Chem. Res. 40 (2001) 565–577. [7] Z.P. Lai, G. Bonilla, I. Diaz, J.G. Nery, K. Sujaoti, M.A. Amat, E. Kokkoli, O. Terasaki, R.W. Thompson, M. Tsapatsis, D.G. Vlachos, Science 300 (2003) 456–460. [8] D.J. Tranchemontagne, J.L. Mendoza-Cortés, M. O'Keeffe, O.M. Yaghi, Chem. Soc. Rev. 38 (2009) 1257–1283. [9] S. Kitagawa, R. Matsuda, Coord. Chem. Rev. 251 (2007) 2490–2509. [10] B.L. Chen, S.C. Xiang, G.D. Qian, Acc. Chem. Res. 43 (2010) 1115–1124. [11] S.L. Qiu, G.S. Zhu, Coord. Chem. Rev. 253 (2009) 2891–2911. [12] L.J. Murray, M. Dinca, J.R. Long, Chem. Soc. Rev. 38 (2009) 1294–1314. [13] R.J. Kuppler, D.J. Timmons, Q.R. Fang, J.R. Li, T.A. Makal, M.D. Young, D.Q. Yuan, D. Zhao, W.J. Zhuang, H.C. Zhou, Coord. Chem. Rev. 253 (2009) 3042–3066. [14] J.Y. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc. Rev. 38 (2009) 1450–1459. [15] Y.L. Liu, J.F. Eubank, A.J. Cairns, J. Eckert, V.C. Kravtsov, R. Luebke, M. Eddaoudi, Angew. Chem. Int. Ed. 46 (2007) 3278–3283. [16] Y.K. Hwang, D.Y. Hong, J.S. Chang, S.H. Jhung, Y.K. Seo, J. Kim, A. Vimont, M. Daturi, C. Serre, G. Férey, Angew. Chem. Int. Ed. 47 (2008) 4144–4148. [17] L.Q. Ma, C. Abney, W.B. Lin, Chem. Soc. Rev. 38 (2009) 1248–1256. [18] D. Zacher, O. Shekhah, C. Wöll, R.A. Fischer, Chem. Soc. Rev. 38 (2009) 1418–1429. [19] O. Shekhah, J. Liu, R.A. Fischer, Ch. Wöll, Chem. Soc. Rev. 40 (2011) 1081–1106. [20] A. Bétard, R.A. Fischer, Chem. Rev. 112 (2012) 1055–1083. [21] Y. Yoo, H.K. Jeong, Chem. Commun. 21 (2008) 2441–2443. [22] Y. Liu, Z. Ng, E.A. Khan, H.K. Jeong, C. Ching, Z.P. Lai, Microporous Mesoporous Mater. 118 (2009) 296–301. [23] H.L. Guo, G.S. Zhu, I.J. Hewitt, S.L. Qiu, J. Am. Chem. Soc. 131 (2009) 1646–1647. [24] R. Ranjan, M. Tsapatsis, Chem. Mater. 21 (2009) 4920–4924. [25] Y.S. Li, F.Y. Liang, H. Bux, A. Feldhoff, W.S. Yang, J. Caro, Angew. Chem. Int. Ed. 49 (2010) 558–561. [26] H. Bux, F.Y. Liang, Y.S. Li, J. Cravillon, M. Wiebcke, J. Caro, J. Am. Chem. Soc. 131 (2009) 16000–16001. [27] Y.X. Hu, X.L. Dong, J.P. Nan, W.Q. Jin, X.M. Ren, N.P. Xu, Y.M. Lee, Chem. Commun. 47 (2011) 737–739. [28] Z.X. Zhao, X.L. Ma, Z. Li, Y.S. Lin, J. Membr. Sci. 382 (2011) 82–90. [29] W.J. Wang, X.L. Dong, J.P. Nan, W.Q. Jin, Z.Q. Hu, Y.F. Chen, J.W. Jiang, Chem. Commun. 48 (2012) 7022–7024. [30] D.N. Dybtsev, H. Chun, K. Kim, Angew. Chem. Int. Ed. 43 (2004) 5033–5036. [31] T. Tsuruoka, S. Furukawa, Y. Takashima, K. Yoshida, S. Isoda, S. Kitagawa, Angew. Chem. Int. Ed. 48 (2009) 4739–4743. [32] H.L. Guo, Y.Z. Zhu, S.L. Qiu, J.A. Lercher, H.J. Zhang, Adv. Mater. 22 (2010) 4190–4192. [33] H. Chun, J. Moon, Inorg. Chem. 46 (2007) 4371–4373.
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Fig. 2. SEM images of the Zn2(BDC)2DABCO nanorods and microrods. (a): higher concentration (0.06 M); (b):longer reaction time (40 min); (c): without acetic acid; and (d): seed prepared for membrane second-growth.
Fig. 3. Top view SEM pictures of (a): seed layer on porous SiO2 substrate and (b): four cycles grown Zn2(BDC)2DABCO membrane. Cross-section SEM pictures (c) and EDS map (d) of four cycles grown Zn2(BDC)2DABCO membrane.
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Z. Kang et al. / Inorganic Chemistry Communications 30 (2013) 74–78 [34] M.M. Nicolau, P.S. Ba'rcia, J.M. Gallegos, J.C. Silva, A.E. Rodrigues, B.L. Chen, J. Phys. Chem. C 113 (2009) 13173–13179. [35] L. Alaerts, C.A. Kirschhock, M. Maes, M.A. van der Veen, V. Finsy, A. Depla, J.A. Martens, G.V. Baron, P.A. Jacobs, J.M. Denayer, D. De Vos, Angew. Chem. Int. Ed. 46 (2007) 4293–4297. [36] J.R. Li, J. Sculley, H.C. Zhou, Chem. Soc. Rev. 38 (2009) 1477–1504. [37] Z.Y. Gu, D.Q. Jiang, H.F. Wang, X.Y. Cui, X.P. Yan, J. Phys. Chem. C 114 (2010) 311–316.
Fig. 4. X-ray diffraction patterns of (a): simulated Zn2 (BDC)2 DABCO material; (b): as-synthesized four cycles grown Zn2(BDC)2DABCO membranes; and (c): activated Zn2 (BDC)2DABCO membranes.
Fig. 5. Comparison of the selective factors for xylene mixtures at different temperatures and by different cycles of growth at 150 °C.
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Preparation of a MOF membrane with 3-aminopropyltriethoxysilane as covalent linker for xylene isomers separation Zixi Kang a, Jinying Ding b, Lili Fan a, Ming Xue a,⁎, Daliang Zhang a, Lianxun Gao b, Shilun Qiu a,⁎ a b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, 130012, PR China State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China
a r t i c l e
i n f o
Article history: Received 15 December 2012 Accepted 29 January 2013 Available online 4 February 2013 Keywords: Metal organic framework SAMs modified supporter Nanocrystal Xylene isomer separation
a b s t r a c t In this study, a continuous-growth Zn2(BDC)2DABCO membrane was synthesized on the NH2-terminated SAMs modified porous SiO2 substrates by using the MOF nanocrystals as seed, which was prepared by an easily scalable method with acetic acid as capping reagent. The seed crystals and membranes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The thickness of the membranes can be varied by adjusting the repetitions of growing process. The xylene isomer mixture separation performance tests of MOF membrane were taken out. © 2013 Elsevier B.V. All rights reserved.
In the chemical industry, the largest fraction of production costs is related to the separation and purification of product streams. The separation of mixed xylene isomers is one of the most challenging issues since xylene isomers are important chemical intermediates. For example, p-xylene is exclusively used as raw material in the production of terephthalic acid (TPA) and dimethyl terephthalate (DMT), which then react with ethyleneglycol to form polyethylene terephthalate (PET). Distillation can be used to remove o-xylene, which fails for the other xylenes isomeric compounds because of similar boiling points (p-xylene: 138.37 °C and m-xylene: 139.12 °C). Nowadays, adsorption is widely used to separate xylene isomers, for example, in Parex or Ebex units in which simulated moving-bed processes are used for the recovery of p-xylene and ethylbenzene [1]. Zeolites X and Y exchanged with cations (Na+, K+, and Ba2+) discriminate very selectively between different xylene isomers and have been widely used as industrial adsorbents [2,3]. However, the process control of this method is complex. Zeolite membranes had been used to separate xylene isomers as well, which may require high permeance temperature and pressure [4–7]. Metal organic frameworks (MOFs) or porous coordination polymers (PCPs) are hybrid inorganic–organic materials made from an assembly of metal ions with organic linkers [8–11]. Their well-defined porosity and tunable chemical functionality make them extremely attractive for applications in gas storage, catalysis, and separation [12–17]. Apart from their use as bulk materials, MOFs are also potential candidates for membrane applications. Pioneering research on MOF films or membranes have already been reported in the literature [18–22]. For instance, a copper-net-supported ⁎ Corresponding authors. Tel./fax: +86 431 85168589. E-mail addresses: [email protected] (M. Xue), [email protected] (S. Qiu). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.01.027
HKUST-1 membrane with selectivity towards H2/N2 in gas separation has been reported by our group [23]. This material is anticipated to recycle hydrogen from other permanent gases. Besides, Tsapatsis and coworkers have prepared a MOF membrane with small pore size on the substrates of alumina, which showed high selectivity for H2/N2 [24]. Caro et al. describes the successful preparation of zeolite imidazolate framework (ZIFs) type membranes, which are applied in the separation of hydrogen from other gases [25,26]. Recently, several research as well as focused on the liquid mixtures separation properties of MOF membranes [27–29]. The guest-free Zn2(BDC)2DABCO has large, three dimensionally interconnected voids and high surface area [30]. The guest-accessible volume for this framework is estimated to be 62%, while the pore size of Zn2(BDC)2DABCO (7.5 Å) is too large for as separation. However, it fit well with the size of most liquid molecules (Fig. S1). Therefore, the Zn2(BDC)2DABCO membrane may have potential applications in liquid separation area. Due to the three-dimensional pore structure of Zn2(BDC)2DABCO with easily accessible BDC ligands as potential adsorption sites, we tend to prepare this kind of membrane for xylene isomers separation. Here, we fabricate the Zn2(BDC)2DABCO membranes by second-grown method using seed cystals which was prepared by an easily scalable method with acetic acid as capping reagent on the modified substrates (Scheme 1). To demonstrate the potential for general applicability of MOF membranes, the effect of membrane thickness and separation conditions on some aspects of permeation result has been investigated and is discussed. In order to obtain large area and alternate growth MOF membrane, the surface of porous SiO2 substrate was first modified with a linker 3-aminopropyl-triethoxysilane (APTES) to form the SAMs layer. We utilize the coordination of \NH2 groups in the ARTES and zinc sites in
Z. Kang et al. / Inorganic Chemistry Communications 30 (2013) 74–78
the seed crystals to enhance the binding force between MOF membrane and support [25]. The modified substrates were characterized by XPS, and the results were shown in Fig. 1. The hydrocarbon C1s signal at 284.8 eV was used as a reference for surface charging. XPS was performed on a sample monolayer to confirm the monolayer formation and the introduction of each different component. According to theoretical relative concentration of the APTES layer, the relative concentration of C1s: N1s was 69.4%: 30.6%, in agreement with the theoretical values of 71.4%: 28.6%, which approve that a NH2-terminated SAMs was successfully grafted on the porous SiO2 substrates. As a capping reagent, acetic acid was used to compose Zn2(BDC)2DABCO nanocrystals, which were used as seeds for membrane second growth on porous SiO2 wafers. The role of acetic acid in reducing the particle size is attributed to its modulating effect on the coordinating interactions between the metal ions and organic linkers [31,32]. In the initial stage of synthesis, Zn cations coordinate to carboxylic groups, which are not only from organic linker BDC but also from acetic acid added. The crystal growth is, therefore, impeded in a very early stage allowing more nuclei to be formed. Moreover, the competitive coordination of the capping reagent is speculated to regulate the rate of crystal growth. Fig. 2 shows the SEM pictures of the Zn2(BDC)2DABCO seed crystals synthesized at different conditions. When the reaction solution was acetic acid free, the crystals become larger and have no fixed shape (Fig. 2c). Similarly, the crystals grow much bigger with higher concentration (Fig. 2a) and longer reaction time (Fig. 2b). However, adding acetic acid to reaction solution significantly decelerated the rate of crystal growth and facilitated the formation of nanorods with high aspect ratios (Fig. 2d). The average lengths of the major and minor axis of seed crystals synthesized under this condition are about 700 nm and 300 nm. Fig. S3 shown the PXRD patterns of Zn2(BDC)2DABCO simulated and seed crystals. As shown in Fig. S2, the XRD pattern illustrate that the synthesized seed crystals are composed of cubic phase of Zn2(BDC)2DABCO and a small amount of other phase, because the metal center and ligands may assemble to another hexagon structure in a short period of time (Fig. S3c) [33], which is not stable. Increasing growth time, this structure will be transformed into a relatively stable cubic structure, while the crystal size will increase. Fortunately this hexagon phase does not affect the secondary growth of MOF membrane. Morphology details of the seed layer and the synthesized Zn2(BDC)2DABCO membrane is shown in SEM images as well (Fig. 3). Seed layer with good continuity were uniformly spread on the top of the support and the support surface were fully covered (Fig. 3a). The membrane grown on this basis is proved to be well intergrown (Fig. 3b). From the cross-sectional view, it can be seen that the crystal layer is about 100 μm thickness and there are no pinholes inside the membrane. The crystals connect with the support compactly (Fig. 3c). EDS map indicated that the Zn2(BDC)2DABCO membrane only grew on the support surface (Fig. 3d). SEM pictures of membranes with varied thickness are shown in Fig. S4, which illustrate the thickness of these membranes with 2–5 cycle growth are about 40 μm, 80 μm, 100 μm and 150 μm respectively.
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In order to evaluate the crystallinity, as synthesized membrane were characterized by X-ray diffraction (XRD). As it shows in Fig. 4, the XRD patterns are consistent well with that of the simulated Zn2(BDC)2DABCO, indicating that the membranes grown on the substrate are in one pure phase and exhibit high crystallinity. It is worth noting that XRD pattern peaks of the guest-free membranes shift to lower 2θ values, which reveals Zn2(BDC)2DABCO structure expand after the activated process because of guest-dependent dynamic behavior [30]. In addition, XRD patterns of different thickness membranes are recorded in Fig. S5. As the the times of growth increase, the intensity of diffraction peaks becomes higher. The powder samples were collected by scraping from actvated membranes to take the TGA and N2 sorption isotherms measurements, and the TGA curves indicate that all guest molecules had been removed after the activation process (Fig. S6). As shown in Fig. S7, the nitrogen sorption isotherms of samples is a typical type I curves, which indicates uniform microporous structures of Zn2(BDC)2DABCO materials. The Brunauer–Emmett–Teller (BET) specific surface areas are calculated from the adsorption curve to be as high as 1589 m 2 g−1. Some groups have reported the application of MOF crystals in the simulate and adsorption separation of xylene isomers, which relate to the behavior of xylene isomers in the microporous metal–organic framework and breakthrough experiments of equimolar quaternary mixtures [34–37]. In this work, liquid separation studies of xylene isomer mixtures on the Zn2(BDC)2DABCO membranes with varied thickness and different temperatures were carried out (Fig S2). The separation performances of the membranes are displayed in Fig. 5. On the contrary to the results of the zeolite membrane, o-xylene and m-xylene molecules with larger kinetic diameter have greater permeability on this MOF membrane, which was consistent with the selective adsoption results on this MOF [34]. This phenomenon can be explained by an adsorption–diffusion model. The molecules which are more easy to adsorped by framework materials take precedence inside the channels and as well as hinder other molecules to get through the membrane. Selective factors of membranes prepared by different cycles of growth were obtained by Gas Chromatography are shown in Fig. 5b, it is clear that the addition of grown cycle in membranes synthesis increased the selectivity and decreased the xylene permeability at the same time. It is most likely due to the reduction of defect pores and increase in membrane thickness. The poor selectivity on the 5 cycles growth membrane is caused by the large shape of crystals, which are easily resulted in cracks and defects. The permeance test results of xylene isomers under different temperature (25 °C, 100 °C, 150 °C, 175 °C and 200 °C) were revealed in Table S1. High transmittance is obtained in membranes at high temperature, because the molecular kinetic energy increases when the temperature gets higher. As the operating temperature increasing from 25 to 150 °C for m-x/p-x binary system, the permeance of m-x increase from 826 g/m2h to 2094 g/m2h, while the p-x only slightly increases from 406 g/m2h to 533 g/m2h. Thus the resulting selective factor rises from 1.095% to 1.934%. However, at 175 °C and 200 °C, the membrane gradually became unstable, which
Scheme 1. Preparation of the Zn2(BDC)2DABCO membranes by using APTES as covalent linker between the Zn2(BDC)2DABCO membrane and the porous SiO2 support.
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leads to the reduction of separating capability of the membranes. The selectivity data indicate that best separation effect (o-x/p-x=1.934; m-x/p-x=1.617) can be obtained at the temperature close to the boiling point of xylene for both o-x/p-x and m-x/p-x binary system (Fig. 5a). However, this MOF membrane has very little separation effect for the o-x/m-x mixture. As we all known, Zn2(BDC)2DABCO is a dynamic structure, which may cause the decrease of membrame quaulity after the
activated process and result in the poor selectivity and relatively high flux of xylene isomers on this membrane. In summary, a large scale continuous MOF (Zn2(BDC)2DABCO) membrane has been successfully synthesized by second growth approach on the modified porous SiO2 substrate surface. The seed crystals were prepared using acetic acid as tapped agent. The thickness of the membrane can be varied by adjusting the repetitions of growing process. The separation performance tests of xylene isomer mixtures were taken out. Comparing with the conventional zeolite type membranes, separation properties of Zn2(BDC)2DABCO membrane for xylene isomers are derived from the selective adsorption effect of the framework. As MOF materials are self-assembled from metal centers and a wide range of organic ligands, the pore structure and pore size can easily changed, which is unprecedented in other types of materials. It is expected that MOF membrane with different structures and property will be developed for the targeted application of xylene isomers separation in the future. Acknowledgment This work was supported by the National Basic Research Program of China (2011CB808703, 2012CB821700), National Natural Science Foundation of China (Grant nos. 91022030, 21101072), and “111” project (B07016). References
Fig. 1. XPS spectra for the surface of membrane (a), N 1s (b) and C 1s (c) regions of APTES monolayers deposited for 3 h.
[1] D.B. Broughton, R.W. Neuzil, J.M. Pharis, C.S. Brearley, Chem. Eng. Prog. 66 (1970) 70–75. [2] V. Cottier, J.P. Bellat, M.H. Simonot-Grange, A. Methivier, J. Phys. Chem. B 101 (1997) 4798–4802. [3] M. Goddard, D.M. Ruthven, Zeolites 6 (1986) 275–282. [4] W.H. Yuan, Y.S. Lin, W.S. Yang, J. Am. Chem. Soc. 126 (2004) 4776–4777. [5] J. Hedlund, J. Sterte, M. Anthonis, A.-J. Bons, B. Carstensen, N. Corcoran, D. Cox, H. Deckman, W.D. Gijnst, P.-P. Moor, F. Lai, J. McHenry, W. Mortier, J. Reinoso, J. Peters, Microporous Mesoporous Mater. 52 (2002) 179–189. [6] C.J. Gump, V.A. Tuan, R.D. Noble, J.L. Falconer, Ind. Eng. Chem. Res. 40 (2001) 565–577. [7] Z.P. Lai, G. Bonilla, I. Diaz, J.G. Nery, K. Sujaoti, M.A. Amat, E. Kokkoli, O. Terasaki, R.W. Thompson, M. Tsapatsis, D.G. Vlachos, Science 300 (2003) 456–460. [8] D.J. Tranchemontagne, J.L. Mendoza-Cortés, M. O'Keeffe, O.M. Yaghi, Chem. Soc. Rev. 38 (2009) 1257–1283. [9] S. Kitagawa, R. Matsuda, Coord. Chem. Rev. 251 (2007) 2490–2509. [10] B.L. Chen, S.C. Xiang, G.D. Qian, Acc. Chem. Res. 43 (2010) 1115–1124. [11] S.L. Qiu, G.S. Zhu, Coord. Chem. Rev. 253 (2009) 2891–2911. [12] L.J. Murray, M. Dinca, J.R. Long, Chem. Soc. Rev. 38 (2009) 1294–1314. [13] R.J. Kuppler, D.J. Timmons, Q.R. Fang, J.R. Li, T.A. Makal, M.D. Young, D.Q. Yuan, D. Zhao, W.J. Zhuang, H.C. Zhou, Coord. Chem. Rev. 253 (2009) 3042–3066. [14] J.Y. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc. Rev. 38 (2009) 1450–1459. [15] Y.L. Liu, J.F. Eubank, A.J. Cairns, J. Eckert, V.C. Kravtsov, R. Luebke, M. Eddaoudi, Angew. Chem. Int. Ed. 46 (2007) 3278–3283. [16] Y.K. Hwang, D.Y. Hong, J.S. Chang, S.H. Jhung, Y.K. Seo, J. Kim, A. Vimont, M. Daturi, C. Serre, G. Férey, Angew. Chem. Int. Ed. 47 (2008) 4144–4148. [17] L.Q. Ma, C. Abney, W.B. Lin, Chem. Soc. Rev. 38 (2009) 1248–1256. [18] D. Zacher, O. Shekhah, C. Wöll, R.A. Fischer, Chem. Soc. Rev. 38 (2009) 1418–1429. [19] O. Shekhah, J. Liu, R.A. Fischer, Ch. Wöll, Chem. Soc. Rev. 40 (2011) 1081–1106. [20] A. Bétard, R.A. Fischer, Chem. Rev. 112 (2012) 1055–1083. [21] Y. Yoo, H.K. Jeong, Chem. Commun. 21 (2008) 2441–2443. [22] Y. Liu, Z. Ng, E.A. Khan, H.K. Jeong, C. Ching, Z.P. Lai, Microporous Mesoporous Mater. 118 (2009) 296–301. [23] H.L. Guo, G.S. Zhu, I.J. Hewitt, S.L. Qiu, J. Am. Chem. Soc. 131 (2009) 1646–1647. [24] R. Ranjan, M. Tsapatsis, Chem. Mater. 21 (2009) 4920–4924. [25] Y.S. Li, F.Y. Liang, H. Bux, A. Feldhoff, W.S. Yang, J. Caro, Angew. Chem. Int. Ed. 49 (2010) 558–561. [26] H. Bux, F.Y. Liang, Y.S. Li, J. Cravillon, M. Wiebcke, J. Caro, J. Am. Chem. Soc. 131 (2009) 16000–16001. [27] Y.X. Hu, X.L. Dong, J.P. Nan, W.Q. Jin, X.M. Ren, N.P. Xu, Y.M. Lee, Chem. Commun. 47 (2011) 737–739. [28] Z.X. Zhao, X.L. Ma, Z. Li, Y.S. Lin, J. Membr. Sci. 382 (2011) 82–90. [29] W.J. Wang, X.L. Dong, J.P. Nan, W.Q. Jin, Z.Q. Hu, Y.F. Chen, J.W. Jiang, Chem. Commun. 48 (2012) 7022–7024. [30] D.N. Dybtsev, H. Chun, K. Kim, Angew. Chem. Int. Ed. 43 (2004) 5033–5036. [31] T. Tsuruoka, S. Furukawa, Y. Takashima, K. Yoshida, S. Isoda, S. Kitagawa, Angew. Chem. Int. Ed. 48 (2009) 4739–4743. [32] H.L. Guo, Y.Z. Zhu, S.L. Qiu, J.A. Lercher, H.J. Zhang, Adv. Mater. 22 (2010) 4190–4192. [33] H. Chun, J. Moon, Inorg. Chem. 46 (2007) 4371–4373.
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Fig. 2. SEM images of the Zn2(BDC)2DABCO nanorods and microrods. (a): higher concentration (0.06 M); (b):longer reaction time (40 min); (c): without acetic acid; and (d): seed prepared for membrane second-growth.
Fig. 3. Top view SEM pictures of (a): seed layer on porous SiO2 substrate and (b): four cycles grown Zn2(BDC)2DABCO membrane. Cross-section SEM pictures (c) and EDS map (d) of four cycles grown Zn2(BDC)2DABCO membrane.
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Z. Kang et al. / Inorganic Chemistry Communications 30 (2013) 74–78 [34] M.M. Nicolau, P.S. Ba'rcia, J.M. Gallegos, J.C. Silva, A.E. Rodrigues, B.L. Chen, J. Phys. Chem. C 113 (2009) 13173–13179. [35] L. Alaerts, C.A. Kirschhock, M. Maes, M.A. van der Veen, V. Finsy, A. Depla, J.A. Martens, G.V. Baron, P.A. Jacobs, J.M. Denayer, D. De Vos, Angew. Chem. Int. Ed. 46 (2007) 4293–4297. [36] J.R. Li, J. Sculley, H.C. Zhou, Chem. Soc. Rev. 38 (2009) 1477–1504. [37] Z.Y. Gu, D.Q. Jiang, H.F. Wang, X.Y. Cui, X.P. Yan, J. Phys. Chem. C 114 (2010) 311–316.
Fig. 4. X-ray diffraction patterns of (a): simulated Zn2 (BDC)2 DABCO material; (b): as-synthesized four cycles grown Zn2(BDC)2DABCO membranes; and (c): activated Zn2 (BDC)2DABCO membranes.
Fig. 5. Comparison of the selective factors for xylene mixtures at different temperatures and by different cycles of growth at 150 °C.