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Preparation of magnetic nickel hollow fibers with a trilobe structure using cellulose acetate fibers as templates
Applied Surface Science 266 (2013) 214–218
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of magnetic nickel hollow fibers with a trilobe structure using cellulose acetate fibers as templates Changfeng Zeng a , Ping Li b , Lixiong Zhang b,∗ a
College of Mechanic and Power Engineering, Nanjing University of Technology, No 5 Xin Mofan Rd., Nanjing 210009, China State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, No 5 Xin Mofan Rd., Nanjing 210009, PR China b
a r t i c l e
i n f o
Article history: Received 2 August 2012 Received in revised form 29 October 2012 Accepted 29 November 2012 Available online 7 December 2012 Keywords: Cellulose acetate fiber Electroless plating Nickel hollow fiber Trilobe structure Magnetization
a b s t r a c t Nickel hollow fibers with trilobe shape in cross section and monolithic nickel structures composed of trilobe shaped nickel hollow fibrous networks were prepared by using cellulose acetate fibers from cigarette filters as the template. Magnetic ZSM-5/Ni hollow fibers were then fabricated by using the nickel-based hollow fibers as the support. The samples were characterized by scanning electron microscopy, energy dispersive X-ray spectrometer, and X-ray diffraction. The results indicate that nickel hollow fibers and ZSM-5/Ni hollow fibers retain the morphology of the cellulose acetate fibers, and the monolithic nickel structures can be prepared by pre-shaping the cellulose acetate fibers. The thickness of the nickel layer can be regulated by controlling the electroless plating times. The saturation magnetization and coercivity of the trilobe shaped nickel hollow fibers and ZSM-5/Ni hollow fibers are 27.78 and 21.59 emu/g and 78 and 61 Oe, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Catalysts with various external morphological characteristics are commonly used in industrial heterogeneous catalytic processes [1]. For example, beads, rings, pellets, extrudates and flakes are used in a fixed-bed reactor, while fine spherical powers are applied in a fluidized reactor. These shaped catalysts used in fixed-bed reactors encounter obvious problems, such as maldistributions, high pressure drop, and sensitivity to fouling by dust [2]. Special shaped catalysts (e.g. trilobe form) or structured catalysts (e.g. monolith) are reported to solve at least partially these problems, and the trilobe shape is well-proven in refinery hydrotreating applications [3]. Trilobe shaped catalysts are generally manufactured by the extrusion process. Cellulose acetate fibers from cigarette filters exhibit trilobe shape in cross section [4], making them as potential templates to synthesize hollow fibers with a trilobe structure. Furthermore, they can be easily shaped to designed structures, such as monolithic fibrous networks, which make the preparation of various monolith structures be viable. The template method has been widely applied to create inorganic or organic hollow fibers by depositing precursors on surfaces of the templates, followed by removing the templates to create the hollow structures. Thus, the structures of resulting fibers are easily controllable. Various templates, such as carbon fibers [5], cotton
∗ Corresponding author. Tel.: +86 25 8317 2265; fax: +86 25 8317 2261. E-mail address: [email protected] (L. Zhang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.151
[6,7], poly(vinylidene fluoride) (PVDF) hollow fiber [8], and even virus [9] have been used. The resulting hollow fibers follow the original morphology of the template. Therefore, by using trilobe shaped cellulose acetate fibers as the template, hollow fibers with the trilobe structure can be fabricated. In addition, the discarded cigarette filters can thus be recycled, which is benefit for the environment protection. Electroless plating, as an easy and simple preparation method [10–13], is applied by using trilobe shaped cellulose fibers as the template for the preparation of nickel hollow fibers and monolith structure composed of hollow fibrous networks. Nickel hollow fibers are suitable catalysts or catalyst carriers for magnet fluidized bed (MFB) reactors, which have the advantage of combining the low pressure drop of a fluidized bed with the bubble-free operation of a fixed bed and improves the mass transfer rate of the reactants [14]. Monolith nickel hollow fibrous networks can provide large void volume, entirely open structure and high permeability, and are ready to entrap small particulates within the fibrous networks to form catalytic composites [15,16]. They can also be potentially used as structured catalysts or catalyst carriers for fixed-bed reactors. Zeolites, as quite important catalysts in the petrochemical and refining industries, can be grown on various substrates by in situ synthesis or secondary growth. In this work, we prepared trilobe shaped nickel hollow fibers and monolith structure composed of compact trilobe shaped nickel hollow fibrous networks using cellulose acetate fibers from cigarette filters as the template. Zeolite ZSM-5 crystals were grown on nickel hollow fibers to demonstrate, as an example, that trilobe shaped nickel
C. Zeng et al. / Applied Surface Science 266 (2013) 214–218
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Fig. 1. SEM images of the cellulose acetate fibers: overview (a) and the cross-section (b).
hollow fibers were good catalyst carriers. ZSM-5 is an aluminosilicate zeolite with MFI structure and a SiO2 /Al2 O3 ratio ranging from 15 to 100. This structure contains a three-dimensional well-defined and narrow pore system, resulting in a unique shape selectivity in reactions. In addition, it exhibits adjustable physical and chemical properties, making it as one of the most important zeolitic catalysts. The magnetic properties of trilobe shaped nickel hollow fibers and the resulting zeolite-nickel hollow fibers were also examined. 2. Experimental 2.1. Preparation of nickel hollow fiber and monolithic nickel hollow fibers The nickel hollow fiber and monolithic nickel hollow fibers were obtained by calcination of Ni/cellulose acetate hollow fibers, which were prepared by using our reported method [17]. First, the cellulose acetate fibers from cigarette filters were sensitized in a solution prepared by mixing 1 g of SnCl2 , 2 g of HCl (35 wt.%) and 97 g of deionized H2 O for 1 min, followed by washing with sufficient deionized water. Then, the sensitized fibers were activated in a solution prepared by mixing 0.025 g of PdCl2 , 0.25 g of HCl (35 wt.%) and 97.5 g of deionized water for 1 min and washed with sufficient deionized water afterwards. The process of sensitization and activation was repeated for 4–6 cycles until the fibers became brown in color. This color change indicates reduction of Pd2+ absorbed on the fibers in the activation step to Pd0 by Sn2+ absorbed on the fibers in the sensitization step. Then, the brown cellulose acetate fibers were submerged into the electroless plating solution prepared by mixing 4 g of nickel sulfate, 5.9 g of sodium citrate, 3.2 g of ammonium chloride and 2.7 g of sodium hypophosphite with 84 g of deionized water at 80 ◦ C for 30, 60 and 90 min. After the electroless plating reaction, the fibers were washed, dried at 80 ◦ C overnight and calcined in air at 500 ◦ C for 3 h with a heat rate of 0.5 ◦ C/min. For the preparation of monolithic nickel hollow fibers, the cellulose acetate fibers were pre-shaped to monolith structure in Raschig rings, followed by the same procedures mentioned above. 2.2. Preparation of ZSM-5/Ni hollow fibers Preparation of ZSM-5/Ni hollow fibers was conducted by a secondary growth method [18]. First, ZSM-5 nanocrystals were coated on Ni hollow fibers by an electrostatic adsorption process to promote the growth of ZSM-5 crystals on the fibers. The ZSM-5 nanocrystals were prepared by mixing appropriate amounts of tetrapropylammonium hydroxide (TPAOH), sodium hydroxide (NaOH), aluminum sulfate (Al2 (SO4 )3 ·18H2 O), tetraethyl orthosilicate (TEOS) and deionized water. The synthesis solution with
a molar composition of 9TPAOH: 25SiO2 : 0.208Al2 O3 : 0.5NaOH: 495H2 O experienced the hydrothermal synthesis at 80 ◦ C for 80 h. The resulting ZSM-5 nanocrystals were recovered by repeated centrifugating and washing with deionized water and drying at 70 ◦ C overnight. Then, the Ni hollow fibers were immersed in a 1 wt.% cationic poly(diallyldimethyl ammonium chloride) (PDDA) solution for 10 min to modify their surface. After rinsed with water to remove the excess PDDA, they were immersed in a 2 wt.% ZSM5 nanocrystals suspension for 10 min to facilitate electrostatically adsorption of the nanoparticles [18]. Finally, the seeded sample was mixed with ZSM-5 synthesis solution with a molar composition of 3TPAOH: 25SiO2 : 0.25Al2 O3 : 2NaOH: 1600H2 O and experienced the hydrothermal synthesis at 80 ◦ C for 48 h. The resulting ZSM5/Ni hollow fibers were collected by centrifugating, washing with deionized water, drying at 80 ◦ C overnight. 2.3. Characterization Scanning electron microscope (SEM) images were taken with a Philips Quanta-200 microscope. The elemental analysis was conducted by an energy dispersive X-ray spectroscopy (EDXS) attached to the Philips Quanta-200 microscope. The phase structure of the samples was examined by X-ray diffraction (XRD, Bruker D8 Advance) with Cu K␣ radiation at 40 kV and 30 mA. The magnetization curves of Ni/cellulose acetate fibers, nickel-based hollow fibers and ZSM-5/Ni hollow fibers were measured with a vibrating sample magnetometer (VSM, Lake Shore 7307). 3. Results and discussion 3.1. Preparation of nickel hollow fiber Fig. 1 shows SEM images of cellulose acetate fibers. The cross section of cellulose acetate fibers is about 33 m in width with a trilobe structure, and the length of the cellulose acetate fibers is about 2 cm. In our research, the cellulose acetate fibers were cut to about 0.5–2 mm to experience the electroless plating. Fig. 2a shows the XRD pattern of the Ni/cellulose acetate fibers prepared with an electroless plating time of 30 min. A broad diffraction peak at 2 of about 44.6◦ indicates the presence of amorphous nickelphosphorus alloy. The nickel and phosphorus contents determined by EDXS are about 92.9% and 7.1% (Fig. 2b). Fig. 3 shows SEM images of the Ni/cellulose acetate fibers. The nickel layer homogenously coats on the surface of cellulose acetate fibers and forms trilobe Ni/cellulose acetate composite fibers with smooth surfaces. The nickel layer on cellulose acetate fibers is composed of nickel particles with different sizes, which is similar to those on PVDF fibers [19]. In the electroless plating process, the nickel layer on cellulose
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Ni2+ + 2Had → Ni + 2H+
(3)
Fig. 4 shows SEM images of the calcined nickel-based hollow fibers with a trilobe structure. The nickel layer thicknesses are about 5, 8 and 10 m when the electroless times are of 30, 60 and 90 min, respectively. XRD pattern of the nickel-based hollow fibers (Fig. 2b) indicates that most amorphous Ni-P layers are transformed to Ni3 P crystals [22]. This result is the same as the finding that amorphous Ni-P alloy will undergo a self-crystalline transformation to Ni and Ni3 P at temperatures above 300 ◦ C, which occurs by two crystallization steps, i.e., transformation of amorphous alloy to crystalline Ni3 P alloys and the decomposition of Ni-P alloy into metallic Ni and free P [22]. This process is quite possibly ascribed to diffusion of component elements in the alloy [23]. Two small XRD diffraction peaks at 2 of 37.2 and 63.0◦ are ascribed to the presence of NiO. 3.2. Preparation of monolithic nickel structures Cellulose acetate fibers are flexible, and they can be pre-shaped to any structures. As an example, we packed the cellulose acetate fibers in a Raschig ring to pre-shape them to cylindrical and then conducted the electroless plating. Cylindrical monolithic nickel hollow fibers were consequently obtained by using the pre-shaped cellulose acetate fibers as the template. From the front and left views of this cylindrical monolith nickel hollow fibers shown in Fig. 5, we can see that the size of the monolith is ∅3.4 × 6.5 mm. The hollow fibers are compactly packed (Fig. 5a, b), and they are potential materials for catalyst entrapment [15,16]. The close observation of the monolithic structure in Fig. 5c indicates the trilobe structure of the hollow fibers. The mechanical strength of the monolithic nickel hollow fibers is strong enough for normal handing. 3.3. Preparation of ZSM-5/Ni hollow fibers
Fig. 2. XRD patterns (a) of Ni/cellulose acetate fibers prepared by electroless plating for 30 min (A), nickel hollow fibers (B) and ZSM-5/Ni hollow fibers (C) and EDX analysis of the Ni/cellulose acetate fibers (b).
acetate fibers also acts as a kind of catalyst to promote nickel layers grow continuously, and finally formed multilayer nickel layers [20]. The catalytic action of the nickel layer is taken by promoting the following reactions (Eqs. (1) and (2)), thus enhancing formation of Ni through reaction (Eq. (3)) [21]. H2 PO2 − + H2 O → HPO3 2− + 2Had + H+
(1)
H2 PO2 − → PO2 − + 2Had
(2)
The nickel hollow fibers prepared with an electroless time of 30 min are employed as the template to synthesize ZSM-5/Ni hollow fibers. XRD diffraction peaks shown in Fig. 2c at 2 of 7.9 and 8.8◦ indicate the presence of the ZSM-5 crystals. Fig. 6 shows SEM images of ZSM-5/Ni hollow fibers. It can be seen that the hollow structure of ZSM-5/Ni hollow fibers (Fig. 6a) is same as that of nickel hollow fibers (Fig. 4a). ZSM-5 crystals with sizes of 300–500 nm (Fig. 6b) are coated on nickel hollow fibers and formed a compact ZSM-5 layer with a thickness of about 900 nm (Fig. 6c). The magnetic properties of Ni/cellulose acetate fibers before and after calcination, and ZSM-5/Ni hollow fibers are measured and shown in Fig. 7. The saturation magnetization (Ms) and the coercivity value (Hc) of Ni/cellulose acetate fibers are 3.14 emu/g and 12 Oe, respectively. After calcination, Ms and Hc of the nickel-based hollow fibers greatly increase to 27.78 emu/g and 78 Oe, respectively. The increase of the magnetic property for the calcined sample relates to the structure change of the nickel-based layer from amorphous to
Fig. 3. SEM images of Ni/cellulose acetate fibers prepared by electroless plating for 30 min: overview (a), surface (b) and the cross-section (c).
C. Zeng et al. / Applied Surface Science 266 (2013) 214–218
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Fig. 4. SEM images of the cross-section view of nickel hollow fibers prepared by electroless plating for 30 min (a), 60 min (b) and 90 min (c).
Fig. 5. SEM images of radial (a), axial (b) and close (c) views of cylindrical monolithic nickel hollow fibers.
Magnetization (emu/g)
Fig. 6. SEM images of the overview of the ZSM-5/Ni hollow fiber (a), the outer surface (b) and the cross-section (c).
30
b
20
c
10
a
0 -10 -20 -30 -10000
-5000
0
5000
10000
Field (Oe) Fig. 7. Magnetization curves of the Ni/cellulose acetate fibers (a), nickel hollow fibers (b) and ZSM-5/Ni hollow fibers (c).
Ni and Ni3 P [18] and the removal of the cellulose acetate fibers. The structure change can be observed from the XRD patterns (Fig. 4). Previous research reveal that formation of a Ni crystalline phase should favor the ferromagnetism of Ni due to the presence of parallel spins for the vicinal Ni atoms within the Ni crystalline phase [24], while Ni3 P is Pauli paramagnetic [25]. Since Ni-P amorphorous alloy shows weak ferromagnetism [26], transformation of the amorphorous alloy to Ni3 P and Ni crystals after calcination of the as-prepared Ni/cellulose acetate fibers surely results in increase in the magnetic property. For ZSM-5/Ni hollow fibers, Ms and Hc decrease to 21.59 emu/g and 61 Oe, respectively, because the coating of the ZSM-5 layer on the nickel layer [18]. Nevertheless, the Ms values are higher than those of zeolite P/Ni/wollastonite fibers [27] and magnetic ZSM-5/Ni/fly-ash hollow microspheres [28]. 4. Conclusion Ni/cellulose acetate fibers were prepared by electroless plating using cigarette filter (cellulose acetate fibers) as the template.
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The nickel layer coated on cellulose acetate fibers is continuous and compact, and the layer thickness is adjustable between 5 and 10 m when the electroless plating times are of 30–90 min. The nickel hollow fiber and monolithic nickel structures exhibit the trilobe structure, which inherits from the template of cellulose acetate fibers. ZSM-5/Ni hollow fibers were prepared by hydrothermal synthesis of ZSM-5 on nickel hollow fibers. ZSM-5 crystals are homogeneously coated on the nickel layer. Nickel hollow fibers and ZSM-5/Ni hollow fibers exhibit the magnetic property, and their saturation magnetization values are 27.78 and 21.59 emu/g, respectively. Acknowledgment We are grateful for financial support from the National Natural Science Foundation of China (No. 20676059, 21076107), Specialized Research Fund for the Doctoral Program of Higher Education (20093221110002) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] J.F. LePage, in: G. Ertl, H. Knozinger, J. Weitkamp (Eds.), Preparation of Solid Catalysts, Wiley-VCH, Weinheim, 1999, p. 4. [2] A. Cybulski, J.A. Moulijn, in: A. Cybulski, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, Marcel Dekker, New York, 1998, p. 1. [3] P. Broadhurst, Poisons, purification and performance, Hydrocarbon Engine 11 (2006) 71–75. [4] S. Polarz, B. Smarsly, J.H. Schattka, Hierachical porous carbon structures from cellulose acetate fibers, Chemistry of Materials 14 (2002) 2940–2945. [5] V. Valtchev, B.J. Schoeman, J. Hedlund, S. Mintova, J. Sterte, Preparation and characterization of hollow fibers of silicalite-1, Zeolites 17 (1996) 408–415. [6] H. Imai, Y. Iwaya, K. Shimizu, H. Hirashima, Preparation of hollow fibers of tin oxide with and without antimony doping, Chemistry Letters 19 (2000) 906–907. [7] W.W. Liu, C.F. Zeng, L.X. Zhang, H.T. Wang, N.P. Xu, Facile and versatile preparation of silicalite-1 hollow structures using cotton threads as templates, Materials Chemistry and Physics 103 (2007) 508–514. [8] H.Q. Lu, L.X. Zhang, W.H. Xing, H.T. Wang, N.P. Xu, Preparation of TiO2 hollow fibers using poly(vinylidenefluoride) hollow fiber microfiltration membrane as a template, Materials Chemistry and Physics 94 (2005) 322–327. [9] W. Shenton, T. Douglas, M. Young, Inorganic–organic nanotube composites from template mineralization of tobacco mosaic virus, Advanced Materials 11 (1999) 253–256.
[10] L.J. Wang, J. Li, H.B. Liu, A simple process for electroless plating nickelphosphorus film on the wood veneer, Wood Science and Technology 45 (2010) 161–167. [11] J.L. Jiang, H.Q. Lu, L.X. Zhang, N.P. Xu, Preparation of monodisperse Ni/PS spheres and hollow nickel spheres by ultrasonic electroless plating, Surface and Coatings Technology 201 (2007) 7174–7179. [12] Y.J. Li, R. Wang, F.M. Qi, C.M. Wang, Preparation, characterization and microwave absorption properties of electroless Ni Co P coated SiC powder, Applied Surface Science 254 (2008) 4708–4715. [13] H.J. Zhang, Y. Liu, Preparation and microwave properties of Ni hollow fiber by electroless plating-template method, Journal of Alloys and Compounds 458 (2008) 588–594. [14] Y.A. Liu, R.K. Hamby, R.D. Colberg, Fundamental and practical developments of magnetofluidized beds: a review, Powder Technology 64 (1991) 3–41. [15] Y. Liu, H. Wang, J.F. Li, Y. Lu, H.H. Wu, Q.S. Xue, L. Chen, Monolithic microfibrous nickel catalyst co-modified with ceria and alumina for miniature hydrogen production via ammonia decomposition, Applied Catalysis A 328 (2007) 77–82. [16] B.K. Chang, Y. Lu, B.J. Tatarchuk, Microfibrous entrapment of small catalyst or sorbent particulates for high contacting-efficiency removal of trace contaminants including CO and H2 S from practical reformates for PEM H2 O2 fuel cells, Chemical Engineering Journal 115 (2006) 195–202. [17] H.Q. Lu, J.F. Yao, L.X. Zhang, Y.Y. Wang, J.P. Tian, N.P. Xu, Preparation of Ni/TiO2 composite hollow fibers by electroless plating, Materials Science and Engineering A 466 (2007) 218–222. [18] J.L. Jiang, J.F. Yao, C.F. Zeng, L.X. Zhang, N.P. Xu, Preparation of magnetic hollow ZSM-5/Ni composite spheres, Microporous and Mesoporous Materials 112 (2008) 450–457. [19] F. Ochanda, W.E. Jone Jr., Sub-micrometer-sized metal tubes from electrospun fiber templates, Langmuir 21 (2005) 10791–10796. [20] Y. Chen, L. Liu, Y. Wang, H. Kim, Preparation of porous PVDF-NiB capsules as catalytic adsorbents for hydrogen generation from sodium borohydride, Fuel Processing Technology 92 (2011). [21] Q. Zhang, M. Wu, W. Zhao, Electroless nickel plating on hollow glass microspheres, Surface and Coatings Technology 192 (2005) 213–219. [22] C.S. Chen, X.H. Chen, Z. Yang, W.H. Li, L.S. Xu, B. Yi, Effect of multi-walled carbon nanotubes as reinforced fibres on tribological behaviour of Ni-P electroless coatings, Diamond and Related Materials 15 (2006) 151–156. [23] H. Li, H. Chen, S. Dong, J. Yang, J.-F. Deng, Study on the crystallization process of Ni-P amorphous alloy, Applied Surface Science 125 (1998) 115–119. [24] AllenBai, Chi-ChangHu, Effects of annealing temperatures on the physicochemical properties of nickel-phosphorus deposits, Materials Chemistry and Physics 79 (2003) 49–57. [25] K. Zeppenfeld, W. Jeitschko, Magnetic behaviour of Ni3 P, Ni2 P, NiP3 and the series Ln2 Ni12 P7 (Ln = Pr, Nd, Sm, Gd·Lu), Journal of Physics and Chemistry of Solids 54 (1993) 1527–1531. [26] A. Berrada, M.F. Lapierre, B. Loegel, P. Panissod, C. Robert, J. Beille, Magnetic properties of amorphous NiP alloys, Physica B+C 86–88 (1977) 790–792. [27] J.L. Jiang, X. Lu, C. Duanmu, S. Zhou, X. Gu, J. Chen, Preparation of magnetic Ni/wollastonite and zeolite P/Ni/wollastonite composite fibers, Applied Surface Science 258 (2012) 8283–8288. [28] H.Q. Tao, J.F. Yao, L.X. Zhang, N.P. Xu, Preparation of magnetic ZSM-5/Ni/flyash hollow microspheres using fly-ash cenospheres as the template, Materials Letters 63 (2009) 203–205.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of magnetic nickel hollow fibers with a trilobe structure using cellulose acetate fibers as templates Changfeng Zeng a , Ping Li b , Lixiong Zhang b,∗ a
College of Mechanic and Power Engineering, Nanjing University of Technology, No 5 Xin Mofan Rd., Nanjing 210009, China State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, No 5 Xin Mofan Rd., Nanjing 210009, PR China b
a r t i c l e
i n f o
Article history: Received 2 August 2012 Received in revised form 29 October 2012 Accepted 29 November 2012 Available online 7 December 2012 Keywords: Cellulose acetate fiber Electroless plating Nickel hollow fiber Trilobe structure Magnetization
a b s t r a c t Nickel hollow fibers with trilobe shape in cross section and monolithic nickel structures composed of trilobe shaped nickel hollow fibrous networks were prepared by using cellulose acetate fibers from cigarette filters as the template. Magnetic ZSM-5/Ni hollow fibers were then fabricated by using the nickel-based hollow fibers as the support. The samples were characterized by scanning electron microscopy, energy dispersive X-ray spectrometer, and X-ray diffraction. The results indicate that nickel hollow fibers and ZSM-5/Ni hollow fibers retain the morphology of the cellulose acetate fibers, and the monolithic nickel structures can be prepared by pre-shaping the cellulose acetate fibers. The thickness of the nickel layer can be regulated by controlling the electroless plating times. The saturation magnetization and coercivity of the trilobe shaped nickel hollow fibers and ZSM-5/Ni hollow fibers are 27.78 and 21.59 emu/g and 78 and 61 Oe, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Catalysts with various external morphological characteristics are commonly used in industrial heterogeneous catalytic processes [1]. For example, beads, rings, pellets, extrudates and flakes are used in a fixed-bed reactor, while fine spherical powers are applied in a fluidized reactor. These shaped catalysts used in fixed-bed reactors encounter obvious problems, such as maldistributions, high pressure drop, and sensitivity to fouling by dust [2]. Special shaped catalysts (e.g. trilobe form) or structured catalysts (e.g. monolith) are reported to solve at least partially these problems, and the trilobe shape is well-proven in refinery hydrotreating applications [3]. Trilobe shaped catalysts are generally manufactured by the extrusion process. Cellulose acetate fibers from cigarette filters exhibit trilobe shape in cross section [4], making them as potential templates to synthesize hollow fibers with a trilobe structure. Furthermore, they can be easily shaped to designed structures, such as monolithic fibrous networks, which make the preparation of various monolith structures be viable. The template method has been widely applied to create inorganic or organic hollow fibers by depositing precursors on surfaces of the templates, followed by removing the templates to create the hollow structures. Thus, the structures of resulting fibers are easily controllable. Various templates, such as carbon fibers [5], cotton
∗ Corresponding author. Tel.: +86 25 8317 2265; fax: +86 25 8317 2261. E-mail address: [email protected] (L. Zhang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.151
[6,7], poly(vinylidene fluoride) (PVDF) hollow fiber [8], and even virus [9] have been used. The resulting hollow fibers follow the original morphology of the template. Therefore, by using trilobe shaped cellulose acetate fibers as the template, hollow fibers with the trilobe structure can be fabricated. In addition, the discarded cigarette filters can thus be recycled, which is benefit for the environment protection. Electroless plating, as an easy and simple preparation method [10–13], is applied by using trilobe shaped cellulose fibers as the template for the preparation of nickel hollow fibers and monolith structure composed of hollow fibrous networks. Nickel hollow fibers are suitable catalysts or catalyst carriers for magnet fluidized bed (MFB) reactors, which have the advantage of combining the low pressure drop of a fluidized bed with the bubble-free operation of a fixed bed and improves the mass transfer rate of the reactants [14]. Monolith nickel hollow fibrous networks can provide large void volume, entirely open structure and high permeability, and are ready to entrap small particulates within the fibrous networks to form catalytic composites [15,16]. They can also be potentially used as structured catalysts or catalyst carriers for fixed-bed reactors. Zeolites, as quite important catalysts in the petrochemical and refining industries, can be grown on various substrates by in situ synthesis or secondary growth. In this work, we prepared trilobe shaped nickel hollow fibers and monolith structure composed of compact trilobe shaped nickel hollow fibrous networks using cellulose acetate fibers from cigarette filters as the template. Zeolite ZSM-5 crystals were grown on nickel hollow fibers to demonstrate, as an example, that trilobe shaped nickel
C. Zeng et al. / Applied Surface Science 266 (2013) 214–218
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Fig. 1. SEM images of the cellulose acetate fibers: overview (a) and the cross-section (b).
hollow fibers were good catalyst carriers. ZSM-5 is an aluminosilicate zeolite with MFI structure and a SiO2 /Al2 O3 ratio ranging from 15 to 100. This structure contains a three-dimensional well-defined and narrow pore system, resulting in a unique shape selectivity in reactions. In addition, it exhibits adjustable physical and chemical properties, making it as one of the most important zeolitic catalysts. The magnetic properties of trilobe shaped nickel hollow fibers and the resulting zeolite-nickel hollow fibers were also examined. 2. Experimental 2.1. Preparation of nickel hollow fiber and monolithic nickel hollow fibers The nickel hollow fiber and monolithic nickel hollow fibers were obtained by calcination of Ni/cellulose acetate hollow fibers, which were prepared by using our reported method [17]. First, the cellulose acetate fibers from cigarette filters were sensitized in a solution prepared by mixing 1 g of SnCl2 , 2 g of HCl (35 wt.%) and 97 g of deionized H2 O for 1 min, followed by washing with sufficient deionized water. Then, the sensitized fibers were activated in a solution prepared by mixing 0.025 g of PdCl2 , 0.25 g of HCl (35 wt.%) and 97.5 g of deionized water for 1 min and washed with sufficient deionized water afterwards. The process of sensitization and activation was repeated for 4–6 cycles until the fibers became brown in color. This color change indicates reduction of Pd2+ absorbed on the fibers in the activation step to Pd0 by Sn2+ absorbed on the fibers in the sensitization step. Then, the brown cellulose acetate fibers were submerged into the electroless plating solution prepared by mixing 4 g of nickel sulfate, 5.9 g of sodium citrate, 3.2 g of ammonium chloride and 2.7 g of sodium hypophosphite with 84 g of deionized water at 80 ◦ C for 30, 60 and 90 min. After the electroless plating reaction, the fibers were washed, dried at 80 ◦ C overnight and calcined in air at 500 ◦ C for 3 h with a heat rate of 0.5 ◦ C/min. For the preparation of monolithic nickel hollow fibers, the cellulose acetate fibers were pre-shaped to monolith structure in Raschig rings, followed by the same procedures mentioned above. 2.2. Preparation of ZSM-5/Ni hollow fibers Preparation of ZSM-5/Ni hollow fibers was conducted by a secondary growth method [18]. First, ZSM-5 nanocrystals were coated on Ni hollow fibers by an electrostatic adsorption process to promote the growth of ZSM-5 crystals on the fibers. The ZSM-5 nanocrystals were prepared by mixing appropriate amounts of tetrapropylammonium hydroxide (TPAOH), sodium hydroxide (NaOH), aluminum sulfate (Al2 (SO4 )3 ·18H2 O), tetraethyl orthosilicate (TEOS) and deionized water. The synthesis solution with
a molar composition of 9TPAOH: 25SiO2 : 0.208Al2 O3 : 0.5NaOH: 495H2 O experienced the hydrothermal synthesis at 80 ◦ C for 80 h. The resulting ZSM-5 nanocrystals were recovered by repeated centrifugating and washing with deionized water and drying at 70 ◦ C overnight. Then, the Ni hollow fibers were immersed in a 1 wt.% cationic poly(diallyldimethyl ammonium chloride) (PDDA) solution for 10 min to modify their surface. After rinsed with water to remove the excess PDDA, they were immersed in a 2 wt.% ZSM5 nanocrystals suspension for 10 min to facilitate electrostatically adsorption of the nanoparticles [18]. Finally, the seeded sample was mixed with ZSM-5 synthesis solution with a molar composition of 3TPAOH: 25SiO2 : 0.25Al2 O3 : 2NaOH: 1600H2 O and experienced the hydrothermal synthesis at 80 ◦ C for 48 h. The resulting ZSM5/Ni hollow fibers were collected by centrifugating, washing with deionized water, drying at 80 ◦ C overnight. 2.3. Characterization Scanning electron microscope (SEM) images were taken with a Philips Quanta-200 microscope. The elemental analysis was conducted by an energy dispersive X-ray spectroscopy (EDXS) attached to the Philips Quanta-200 microscope. The phase structure of the samples was examined by X-ray diffraction (XRD, Bruker D8 Advance) with Cu K␣ radiation at 40 kV and 30 mA. The magnetization curves of Ni/cellulose acetate fibers, nickel-based hollow fibers and ZSM-5/Ni hollow fibers were measured with a vibrating sample magnetometer (VSM, Lake Shore 7307). 3. Results and discussion 3.1. Preparation of nickel hollow fiber Fig. 1 shows SEM images of cellulose acetate fibers. The cross section of cellulose acetate fibers is about 33 m in width with a trilobe structure, and the length of the cellulose acetate fibers is about 2 cm. In our research, the cellulose acetate fibers were cut to about 0.5–2 mm to experience the electroless plating. Fig. 2a shows the XRD pattern of the Ni/cellulose acetate fibers prepared with an electroless plating time of 30 min. A broad diffraction peak at 2 of about 44.6◦ indicates the presence of amorphous nickelphosphorus alloy. The nickel and phosphorus contents determined by EDXS are about 92.9% and 7.1% (Fig. 2b). Fig. 3 shows SEM images of the Ni/cellulose acetate fibers. The nickel layer homogenously coats on the surface of cellulose acetate fibers and forms trilobe Ni/cellulose acetate composite fibers with smooth surfaces. The nickel layer on cellulose acetate fibers is composed of nickel particles with different sizes, which is similar to those on PVDF fibers [19]. In the electroless plating process, the nickel layer on cellulose
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Ni2+ + 2Had → Ni + 2H+
(3)
Fig. 4 shows SEM images of the calcined nickel-based hollow fibers with a trilobe structure. The nickel layer thicknesses are about 5, 8 and 10 m when the electroless times are of 30, 60 and 90 min, respectively. XRD pattern of the nickel-based hollow fibers (Fig. 2b) indicates that most amorphous Ni-P layers are transformed to Ni3 P crystals [22]. This result is the same as the finding that amorphous Ni-P alloy will undergo a self-crystalline transformation to Ni and Ni3 P at temperatures above 300 ◦ C, which occurs by two crystallization steps, i.e., transformation of amorphous alloy to crystalline Ni3 P alloys and the decomposition of Ni-P alloy into metallic Ni and free P [22]. This process is quite possibly ascribed to diffusion of component elements in the alloy [23]. Two small XRD diffraction peaks at 2 of 37.2 and 63.0◦ are ascribed to the presence of NiO. 3.2. Preparation of monolithic nickel structures Cellulose acetate fibers are flexible, and they can be pre-shaped to any structures. As an example, we packed the cellulose acetate fibers in a Raschig ring to pre-shape them to cylindrical and then conducted the electroless plating. Cylindrical monolithic nickel hollow fibers were consequently obtained by using the pre-shaped cellulose acetate fibers as the template. From the front and left views of this cylindrical monolith nickel hollow fibers shown in Fig. 5, we can see that the size of the monolith is ∅3.4 × 6.5 mm. The hollow fibers are compactly packed (Fig. 5a, b), and they are potential materials for catalyst entrapment [15,16]. The close observation of the monolithic structure in Fig. 5c indicates the trilobe structure of the hollow fibers. The mechanical strength of the monolithic nickel hollow fibers is strong enough for normal handing. 3.3. Preparation of ZSM-5/Ni hollow fibers
Fig. 2. XRD patterns (a) of Ni/cellulose acetate fibers prepared by electroless plating for 30 min (A), nickel hollow fibers (B) and ZSM-5/Ni hollow fibers (C) and EDX analysis of the Ni/cellulose acetate fibers (b).
acetate fibers also acts as a kind of catalyst to promote nickel layers grow continuously, and finally formed multilayer nickel layers [20]. The catalytic action of the nickel layer is taken by promoting the following reactions (Eqs. (1) and (2)), thus enhancing formation of Ni through reaction (Eq. (3)) [21]. H2 PO2 − + H2 O → HPO3 2− + 2Had + H+
(1)
H2 PO2 − → PO2 − + 2Had
(2)
The nickel hollow fibers prepared with an electroless time of 30 min are employed as the template to synthesize ZSM-5/Ni hollow fibers. XRD diffraction peaks shown in Fig. 2c at 2 of 7.9 and 8.8◦ indicate the presence of the ZSM-5 crystals. Fig. 6 shows SEM images of ZSM-5/Ni hollow fibers. It can be seen that the hollow structure of ZSM-5/Ni hollow fibers (Fig. 6a) is same as that of nickel hollow fibers (Fig. 4a). ZSM-5 crystals with sizes of 300–500 nm (Fig. 6b) are coated on nickel hollow fibers and formed a compact ZSM-5 layer with a thickness of about 900 nm (Fig. 6c). The magnetic properties of Ni/cellulose acetate fibers before and after calcination, and ZSM-5/Ni hollow fibers are measured and shown in Fig. 7. The saturation magnetization (Ms) and the coercivity value (Hc) of Ni/cellulose acetate fibers are 3.14 emu/g and 12 Oe, respectively. After calcination, Ms and Hc of the nickel-based hollow fibers greatly increase to 27.78 emu/g and 78 Oe, respectively. The increase of the magnetic property for the calcined sample relates to the structure change of the nickel-based layer from amorphous to
Fig. 3. SEM images of Ni/cellulose acetate fibers prepared by electroless plating for 30 min: overview (a), surface (b) and the cross-section (c).
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Fig. 4. SEM images of the cross-section view of nickel hollow fibers prepared by electroless plating for 30 min (a), 60 min (b) and 90 min (c).
Fig. 5. SEM images of radial (a), axial (b) and close (c) views of cylindrical monolithic nickel hollow fibers.
Magnetization (emu/g)
Fig. 6. SEM images of the overview of the ZSM-5/Ni hollow fiber (a), the outer surface (b) and the cross-section (c).
30
b
20
c
10
a
0 -10 -20 -30 -10000
-5000
0
5000
10000
Field (Oe) Fig. 7. Magnetization curves of the Ni/cellulose acetate fibers (a), nickel hollow fibers (b) and ZSM-5/Ni hollow fibers (c).
Ni and Ni3 P [18] and the removal of the cellulose acetate fibers. The structure change can be observed from the XRD patterns (Fig. 4). Previous research reveal that formation of a Ni crystalline phase should favor the ferromagnetism of Ni due to the presence of parallel spins for the vicinal Ni atoms within the Ni crystalline phase [24], while Ni3 P is Pauli paramagnetic [25]. Since Ni-P amorphorous alloy shows weak ferromagnetism [26], transformation of the amorphorous alloy to Ni3 P and Ni crystals after calcination of the as-prepared Ni/cellulose acetate fibers surely results in increase in the magnetic property. For ZSM-5/Ni hollow fibers, Ms and Hc decrease to 21.59 emu/g and 61 Oe, respectively, because the coating of the ZSM-5 layer on the nickel layer [18]. Nevertheless, the Ms values are higher than those of zeolite P/Ni/wollastonite fibers [27] and magnetic ZSM-5/Ni/fly-ash hollow microspheres [28]. 4. Conclusion Ni/cellulose acetate fibers were prepared by electroless plating using cigarette filter (cellulose acetate fibers) as the template.
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The nickel layer coated on cellulose acetate fibers is continuous and compact, and the layer thickness is adjustable between 5 and 10 m when the electroless plating times are of 30–90 min. The nickel hollow fiber and monolithic nickel structures exhibit the trilobe structure, which inherits from the template of cellulose acetate fibers. ZSM-5/Ni hollow fibers were prepared by hydrothermal synthesis of ZSM-5 on nickel hollow fibers. ZSM-5 crystals are homogeneously coated on the nickel layer. Nickel hollow fibers and ZSM-5/Ni hollow fibers exhibit the magnetic property, and their saturation magnetization values are 27.78 and 21.59 emu/g, respectively. Acknowledgment We are grateful for financial support from the National Natural Science Foundation of China (No. 20676059, 21076107), Specialized Research Fund for the Doctoral Program of Higher Education (20093221110002) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] J.F. LePage, in: G. Ertl, H. Knozinger, J. Weitkamp (Eds.), Preparation of Solid Catalysts, Wiley-VCH, Weinheim, 1999, p. 4. [2] A. Cybulski, J.A. Moulijn, in: A. Cybulski, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, Marcel Dekker, New York, 1998, p. 1. [3] P. Broadhurst, Poisons, purification and performance, Hydrocarbon Engine 11 (2006) 71–75. [4] S. Polarz, B. Smarsly, J.H. Schattka, Hierachical porous carbon structures from cellulose acetate fibers, Chemistry of Materials 14 (2002) 2940–2945. [5] V. Valtchev, B.J. Schoeman, J. Hedlund, S. Mintova, J. Sterte, Preparation and characterization of hollow fibers of silicalite-1, Zeolites 17 (1996) 408–415. [6] H. Imai, Y. Iwaya, K. Shimizu, H. Hirashima, Preparation of hollow fibers of tin oxide with and without antimony doping, Chemistry Letters 19 (2000) 906–907. [7] W.W. Liu, C.F. Zeng, L.X. Zhang, H.T. Wang, N.P. Xu, Facile and versatile preparation of silicalite-1 hollow structures using cotton threads as templates, Materials Chemistry and Physics 103 (2007) 508–514. [8] H.Q. Lu, L.X. Zhang, W.H. Xing, H.T. Wang, N.P. Xu, Preparation of TiO2 hollow fibers using poly(vinylidenefluoride) hollow fiber microfiltration membrane as a template, Materials Chemistry and Physics 94 (2005) 322–327. [9] W. Shenton, T. Douglas, M. Young, Inorganic–organic nanotube composites from template mineralization of tobacco mosaic virus, Advanced Materials 11 (1999) 253–256.
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