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Crystal morphology control of AlPO4-11 molecular sieves by microwave irradiation
Materials Chemistry and Physics 113 (2009) 899–904
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Crystal morphology control of AlPO4 -11 molecular sieves by microwave irradiation Yashao Chen a,∗ , Xiaolin Luo a,b , Pengmei Chang a , Shouhua Geng a a Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), Ministry of Education, School of Chemistry and Materials Science, Xi’an 710062, PR China b Chemistry & Chemical Engineering Department, Baoji University of Art Sciences, Baoji 721013, PR China
a r t i c l e
i n f o
Article history: Received 8 July 2007 Received in revised form 4 August 2008 Accepted 9 August 2008 Keywords: AlPO4 -11 molecular sieves Microwave irradiation Morphology HF
a b s t r a c t AlPO4 -11 molecular sieves with morphologies of aggregated sphere, clump, faggot and rod were successfully synthesized by microwave irradiation using diisopropylamine as template. By adjusting the crystallization time, the morphology of the AlPO4 -11 molecular sieves changed from sphere to clump and then to faggot. With the increasing concentration of HF, the morphology of AlPO4 -11 molecular sieves changed from sphere to rod gradually, which exactly consisted with the variation of the XRD patterns. Slow nucleation rate caused by the chelated function of Al–F may be the main reason to direct the crystal growth on the special direction. Meanwhile, some control experiments including use of HCl and increasing the concentration of water also showed the same preferential growth of the crystals. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Porous materials with pore sizes near molecular dimensions, such as zeolites and aluminophosphate molecular sieves (AlPO), have been widely used in catalysis and separation, and development for new applications in membranes, sensors, optics, etc., is in progress [1]. In the early 1980s, the discovery of a new class of molecular sieves was reported by researchers from Union Carbide [2,3]. These new materials comprise a series of crystalline, microporous aluminophosphates (AlPOs) hydrothermally prepared from reaction mixtures containing inorganic sources of Al and P and an organic template (such as amine or a quaternary ammonium salt). And these materials present strict alternation of AlO4 − and PO4 + tetrahedral with a neutral framework [4–7]. As being one member of the AlPO-n family, AlPO4 -11 molecular sieve has unidirectional, non-intersecting, 10-membered ring channels with elliptical pore apertures of 0.63 nm × 0.39 nm. The topology consists of sheets of six-ring–six-ring–four-ring (S6R–S6R–S4R) units. Recently, this subject has raised a lot of attention due to its potential application in heterogeneous catalysis [8–12]. At the same time, microwave irradiation is more efficient for transferring thermal energy to a volume of material than conventional thermal processing which transfers heat to the material by convection, conduction and radiation. The microwave is a kind of
∗ Corresponding author. E-mail address: [email protected] (Y. Chen). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.08.038
electromagnetic radiation with a high frequency between 0.3 and 300 GHz. Applications of microwave energy in the synthesis of inorganic materials have been exploited since the mid-1980s [13,14]. Until now, microwave technique has been widely applied in the synthesis of the zeolite and molecular sieves because of the reduced reaction time and improved crystal quality. Microwave technique offers more rapid crystallization than conventional hydrothermal method and it is believed more nuclei are generated simultaneously, thus the growth of the crystals is homogeneous [15,16]. Recent years, some research groups have reported the morphology control of porous materials such as VSB-5 [17] and SBA-16 [18] under microwave irradiation and have showed the microwave is a very efficient tool to control the morphology of porous materials. Because many emerging applications of porous materials require precise control of crystal morphology, strategies to control the crystal shape and size are necessary for special applications [3,4]. Large single crystals with a well-defined orientation and structure are necessary for the development of novel optical, electronic and magnetic materials [19,20]. Several studies have been conducted to synthesize homogeneous and well-defined AFI crystals having various morphologies. One promising route to control crystal size and shape is through confined synthesis from reagents dissolved in water-in-oil microemulsions. Lin et al. [21] successfully synthesized the AlPO4 -5 molecular sieve fibers directed by microemulsion. But the final crystal size is larger than the microemulsion droplets, so continued growth must occur through solution transport outside of the microemulsion. And Jhung et al. [22] also successfully used microwave techniques to synthesize AFI-
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Table 1 Reaction conditions and properties of the samples Sample no.
Molar composition
Time (h)
Synthesized method
Morphology
Surface area (m2 g−1 )
A B C D E F G H I J K L M N
Al2 O3 :1.25P2 O5 :DIPA:100H2 O Al2 O3 :1.25P2 O5 :DIPA:100H2 O Al2 O3 :1.25P2 O5 :DIPA:100H2 O:0.005CTAC Al2 O3 :1.25P2 O5 :DIPA:100H2 O:0.025CTAC Al2 O3 :1.25P2 O5 :DIPA:100H2 O:0.025CTAC Al2 O3 :1.25P2 O5 :DIPA:100H2 O:0.025CTAC Al2 O3 :1.25P2 O5 :HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :0.4HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :1.5HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :DIPA:300H2 O Al2 O3 :1.25P2 O5 :HCl:DIPA:100H2 O
12 6 6 6 8 10 4 8 12 24 4 4 6 4
C M M M M M M M C C M M M M
Sphere Sphere Sphere Sphere Clump Faggot Rod Rod Rod Rod Elongated sphere Rod Layer Cube
– 194.8 – – – 213.1 193.3 – – – – – 199.6 –
C: conventional hydrothermal method; M: microwave irradiation.
type molecular sieves with various morphologies under varying reaction conditions. To the best of our knowledge, the synthesis of AEL molecular sieves with different morphologies by microwave technique has not been reported so far. Herein, by utilizing microwave techniques, we present the synthesis of AEL molecular sieves with various morphologies such as aggregated sphere, plate, rod and aggregated faggot. The effects of fluoride on the synthesis are investigated, and the morphology control may be explained by the combined result of XRD and SEM. 2. Experimental 2.1. Synthesis In a typical procedure, pseudo-boehmite (Al2 O3 , 70 wt.%, CP), phosphoric acid (H3 PO4 , 85 wt.%, AR) and hydrofluoric acid (HF, 40 wt.%, AR) were dispersed in deionized water and stirred for 1.5 h at room temperature. After that, diisopropylamine (DIPA, 98 wt.%, AR) was added dropwise into the reaction mixture under 0.5 h vigorously stirring. The suspension was transferred into Teflon autoclave, which was sealed and placed in a microwave oven (WR-E, Meicheng, China). The precursor mixture was heated at 400 kPa (about 393 K) with the rate of 10 kPa min−1 for various periods of time [23]. There is a difference of controlling way for microwave irradiation which uses pressure as reference to finish the established process. For conventional hydrothermal method, the gel was loaded in a Teflon lined autoclave and put in a preheated electric oven at 443 K for a fixed time without agitation. After the synthesis, the autoclave reactor was cooled to room temperature. All products were filtrated, washed with distilled water several times and dried in an oven at 383 K for 5 h. Finally, the organic template in the synthesized AlPO4 -11 molecular sieve was burn out in the muffle at 823 K for 4 h.
results (Fig. 2a and b), though both of two crystals with plate-like shapes arrange to spherical aggregates with the uniform diameter about 5 m, the plate-like crystals synthesized by microwave technique are much thinner and more uniform than those synthesized by conventional hydrothermal method. Because the nucleation rate increases, the crystal size decreases under microwave irradiation [15,22]. In order to improve the morphology of the crystalline aggregates, cetylpyridinium chloride (CTAC) was added to the initial mixture under stirring. Fig. 2c and d also display the AlPO4 -11 aggregates synthesized with different concentration of CTAC using microwave technique. If only the concentration of CTAC was beyond the critical micelle concentration (CMC, 9.04 × 10−4 mol L−1 ), the shape of spherical aggregates (Fig. 2c) was improved gradually. Once the concentration (5 CMC) of surfactant is suitable for the system, it is possible to get more perfect spherical aggregates of AlPO4 -11 molecular sieve (Fig. 2d) with the diameter of about 20 m. Because the microenvironment of synthesis was improved by CTAC, the surface of the synthesized crystal spheres became smoother. Besides, the morphology changes of AEL crystals were also observed when the time of microwave irradiation was prolonged from 6 h to 10 h after adding 5 CMC CTAC. The morphologies of the crystals transferred from aggregated spheres (Fig. 2d) to
2.2. Characterization The crystal structure and crystallinity of the samples were analyzed by powder X-ray diffraction (XRD, D/Max-3c, Rigalcu, Japan) using Cu K␣ radiation, with the diffraction angle (2) at a range of 5–50◦ . The degree of crystallinity was estimated by summing the areas of the five major diffraction peaks. The morphology of the products was examined with scanning electron microscopy (SEM, Quanta 200, Philips-FEI, Holland) after coating with Au. The surface areas according to the Brunauer–Emmett–Teller (BET) method were determined with a volumetric adsorption apparatus (ZXF-6, NWRICI, China) at 77 K after evacuation at 573 K.
3. Result and discussion 3.1. Transformation of AEL crystal morphologies The synthesized conditions are summarized in Table 1. The crystal phase of products with AEL crystal structure was determined by powder XRD as shown in Fig. 1. The XRD patterns of the two crystals correspond well with AEL molecular sieve and do not show any noticeable impurity phase. But the crystallinity of the sample (B) is higher than the sample (A), which obviously exhibits advantage on fast crystallization of microwave technique. From the SEM
Fig. 1. XRD patterns of the synthesized AlPO4 -11 molecular sieves: (a) sample A and (b) sample B of Table 1.
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Fig. 2. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample A, (b) sample B, (c) sample C and (d) sample D of Table 1. White scale bars correspond to 5 m.
clump (Fig. 3a) and last to faggot (Fig. 3b). On the basis of various reports, Oliver et al. [24] proposed that the crystallization of most aluminophosphates presumably follows a chain-to-layer transformation process wherein the parent chain is initially hydrolysed in solution, leading to the formation of other chain structure types. But we think that the chain-to-layer transformation process of the basic crystal structure not only occurs in the very beginning of crystallization but does in the period after the crystal formed. In our experiment, this transformation combined with the morphology changes of crystals was observed after the spheres formed. 3.2. Function of HF for the control of AEL crystal morphology Fluoride has been widely applied in the synthesis of microporous zeolites and AlPO-n molecular sieves [25]. As shown in
Fig. 4, after irradiated for 4 h, sample (G) is in the form of welldefined rod-like crystals of size about 3.0 m × 0.5 m without any aggregation. When the synthesized time was prolonged to 8 h, the morphology of sample (H) became the mixture of the big rectangular crystals and the small rod-like crystals. Because some of AlPO4 -11 crystals were accelerated to dissolve into the solution due to the chelated function between fluorin and aluminum, and grow onto large AlPO4 -11 crystals via a process known as Ostwald ripening [26] by prolonging the crystallization period. As a result, the rod-like AlPO4 -11 crystals with asymmetric diameters were obtained just like Fig. 4b. And the same phenomenon was also observed when the crystallization time was prolonged from 12 h (Fig. 4c) to 24 h (Fig. 4d) using conventional hydrothermal method after adding HF. After adding HF, the gel dissolves and the nucleation and crystallization rates are limited, thus large crystals were
Fig. 3. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample E and (b) sample F of Table 1. White scale bars correspond to 5 m.
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Fig. 4. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample G, (b) sample H, (c) sample I and (d) sample J of Table 1. White scale bars correspond to 5 m.
obtained with the prolonged synthesized time. And the slow rate of nucleation and crystallization with fluoride may be related to the low hydrolysis (metal–F bond to metal–O bond) rate of metal fluorides. For example, the bond strengths of Al–F and Al–O are 663.6 and 515.1 kJ mol−1 , respectively [22]. In fact, in the previous stirring process, it was ease to form gel without any aging after adding HF. In order to make clear the function of HF in the AlPO4 -11 crystallization, the concentration of the HF was carefully investigated. In the XRD patterns of AEL molecular sieves, the great variation of diffraction peaks appeared at d = 5.50 and d = 4.06, which corresponded to the lattice planes (2 0 0) and (0 0 2). As shown in Fig. 5, the diffraction intensity of the lattice planes (0 0 2) is much higher than the lattice planes (2 0 0) for the sample without HF (Fig. 5a). With the same crystallization conditions except adding a little HF, the diffraction intensity tendency of the two lattice planes changes to contrary (Fig. 5b and c). The diffraction intensity of the lattice planes (2 0 0) increased gradually, whereas the diffraction intensity of (0 0 2) decreased remarkably. Because the two lattice planes are perpendicular to each other, this variation may consist of the transformation of the crystals morphologies. And from the SEM micrographies, without any HF, the products were in form of spherical aggregates consisted with plate-like crystals (Fig. 2b). Once 0.4 molar ratio HF was added into the synthesized gel, it was seem as beginning to elongate the spherical aggregates into rod-like aggregates along the c-direction (Fig. 6a). Then, when the use of HF was further increased to 1.0 molar ratio, the perfect AlPO4 -11 crystals with rod-like shapes were observed (Fig. 4a). The transformation of the crystal morphologies exactly consisted with the variation of the XRD patterns. Because of the chelated function between fluorin and aluminum, the crystals growth is directed primarily in one direction (2 0 0) to produce rod-like crystals. A similar dependence of the relative intensity of the (0 0 2) and (1 0 0) diffractions on the aspect
ratio of the crystals has also been observed in the synthesis of SAPO5 [22]. However, when the excessive HF was used, the overmuch fluorin ions accelerated the dissolution of little crystals and limited the nucleation and crystal growth rate to form inhomogeneous crystals (Fig. 6b). Similar crystals can be obtained by increasing water. With low concentration of reactants, the crystals can be grown preferentially along the c-axis due to the slow growth rate. From Fig. 7a, the crystals aggregates axially into rod, which was made up of lamel-
Fig. 5. XRD patterns of the synthesized AlPO4 -11 molecular sieves: (a) sample B, (b) sample K and (c) sample G of Table 1.
Y. Chen et al. / Materials Chemistry and Physics 113 (2009) 899–904
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Fig. 6. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample K and (b) sample L of Table 1. White scale bars correspond to 5 m.
Fig. 7. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample M and (b) sample N of Table 1. White scale bars correspond to 5 m.
lar crystals sized about 4.0 m × 1.0 m with cuspate heads. And the diffraction intensities of the lattice planes (2 0 0) is somewhat higher than (0 0 2) (Fig. 8a), which is in accord with the morphology character observed by SEM. Meanwhile, the same concentration of HCl was utilized instead HF for the synthesis of crystal. Because the bond strength of Al–Cl (about 502.5 kJ mol−1 ) is much lower than Al–F, the hydrolysis of
Al–Cl becomes very easy so that the reaction of Al–Cl can hardly induce the growth of crystals for some special direction. From Fig. 7b, the synthesized crystals are also with spherical morphologies and XRD pattern of this sample has no much special character (Fig. 8b). Because fluorin ions have much stranger chelated function with aluminum ions than chlorine ions according to the bond strength of Al–Cl and Al–F. As a result, the rod-like AlPO4 -11 crystals were obtained after added HF, while spheric AlPO4 -11 crystals were obtained after added HCl. 3.3. Physicochemical properties of the samples The synthesized AEL molecular sieves are microporous with BET surface areas of about 200 m2 g−1 as shown in Table 1, which are suitable for use in catalysis, adsorption and new applications. In fact, the sample (B) with strong diffraction intensity for the lattice planes (0 0 2) should expose more pores on the crystal surface because the lattice plane (0 0 2) is perpendicular to the 10membered ring channels. But in this work, there was no obvious variation of BET surface areas with the different aggregation. Therefore, the development of a method to disperse the aggregated AEL crystal spheres is necessary. 4. Conclusion
Fig. 8. XRD patterns of the synthesized AlPO4 -11 molecular sieves: (a) sample M and (b) sample N of Table 1.
AlPO4 -11 molecular sieves with morphologies of aggregated sphere, clump, faggot and rod were successfully synthesized by microwave irradiation using diisopropylamine as template. Although similar trends in crystallization were observed using microwave synthesis and conventional synthesis, the microwave
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irradiation offered much faster crystallization time than conventional hydrothermal method. By adjusting the crystallization time, the morphology of the AlPO4 -11 molecular sieves changed from sphere to clump and then to faggot. With the increase of HF, the morphology of AlPO4 -11 molecular sieves changed from sphere to rod gradually, which exactly consisted with the variation of the XRD patterns. Slow nucleation rate caused by the chelated function of Al–F was seemed to direct the crystal growth in the c-direction, and some control experiments included the use of HCl and the increase of water also showed the same preferential growth of the crystals. Therefore, all of the presented work may provide a series of simply but very effective methods to potentially control crystal morphology of molecular sieves. Acknowledgement This research work was fully supported by Items of Tackling Key Problem for Science and Technology of Shaanxi Province (No. 2006k06-G9). References [1] M.E. Davis, Nature 417 (2002) 813. [2] R.R. Xu, W.Q. Pang, Chemistry-Zeolites and Porous Materials, Science Publisher, Beijing, 2004, 225 pp.
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F. Schüth, W. Schmidt, Adv. Mater. 14 (2002) 629. Z.X. Chang, R. Koodali, R.M. Krishna, L. Kevan, J. Phys. Chem. B 104 (2000) 5579. B. Paizs, E. Tajkhorshid, S. Suhai, J. Phys. Chem. B 103 (1999) 5338. L. Domokos, L. Lefferts, K. Seshan, J.A. Lerchery, J. Catal. 203 (2001) 351. M. Höchtl, A. Jentys, H. Vinek, Appl. Catal. A 207 (2001) 397. F. Romilda, V.G. Marcus, O.P. Heloise, C. Dilson, Micropor. Mesopor. Mater. 53 (2002) 135. J.M.F.B. Aquino, C.D.R. Souza, A.S. Araujo, Int. J. Inorg. Mater. 3 (2001) 467. A.K. Sinha, S. Sainkar, S. Sivasanker, Micropor. Mesopor. Mater. 31 (1999) 321. A.K. Sinha, S. Seelan, Appl. Catal. A 270 (2004) 245. F. Ligia, V.D.V. Pascal, F.V. Etienne, A.S. Robert, M.W. Bert, Micropor. Mesopor. Mater. 39 (2000) 493. H.J. Sung, J.S. Chang, S.K. Dae, E.P. Sang, Micropor. Mesopor. Mater. 71 (2004) 135. H.J. Sung, K.H. Young, J.S. Chang, E.P. Sang, Micropor. Mesopor. Mater. 67 (2004) 151. H.J. Sung, J.S. Chang, S.H. Jin, E.P. Sang, Micropor. Mesopor. Mater. 64 (2003) 33. M. Sathupunya, E. Gulari, S. Wongkasemjit, Mater. Chem. Phys. 83 (2004) 89. H.J. Sung, W.Y. Ji, K.H. Young, S.C. Jong, Micropor. Mesopor. Mater. 89 (2006) 9. Y.K. Hwang, J.S. Chang, Y.U. Kwon, S.E. Park, Stud. Surf. Sci. Catal. 146 (2003) 101. S. Qiu, J. Yu, G. Zhu, O. Terasaki, Y. Nozue, W. Pang, R. Xu, Micropor. Mesopor. Mater. 21 (1998) 245. A.D. Lethbridge, J.J. Williams, R.I. Walton, K.E. Evans, C.W. Smith, Micropor. Mesopor. Mater. 79 (2005) 339. J.C. Lin, J.T. Dipre, M.Z. Yates, Chem. Mater. 15 (2003) 2764. S.H. Jhung, J.S. Chang, Y.K. Hwang, S.E. Park, J. Mater. Chem. 14 (2004) 280. X.L. Luo, Y.S. Chen, P.M. Chang, Acta Pet. Chem. Technol. 36 (2007) 21. S. Oliver, A. Kuperman, G.A. Ozin, Angew. Chem. Int. Ed. 37 (1998) 46. H. Kessler, J. Patarin, C.S. Darie, Stud. Surf. Sci. Catal. 85 (1994) 75. A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Crystal morphology control of AlPO4 -11 molecular sieves by microwave irradiation Yashao Chen a,∗ , Xiaolin Luo a,b , Pengmei Chang a , Shouhua Geng a a Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), Ministry of Education, School of Chemistry and Materials Science, Xi’an 710062, PR China b Chemistry & Chemical Engineering Department, Baoji University of Art Sciences, Baoji 721013, PR China
a r t i c l e
i n f o
Article history: Received 8 July 2007 Received in revised form 4 August 2008 Accepted 9 August 2008 Keywords: AlPO4 -11 molecular sieves Microwave irradiation Morphology HF
a b s t r a c t AlPO4 -11 molecular sieves with morphologies of aggregated sphere, clump, faggot and rod were successfully synthesized by microwave irradiation using diisopropylamine as template. By adjusting the crystallization time, the morphology of the AlPO4 -11 molecular sieves changed from sphere to clump and then to faggot. With the increasing concentration of HF, the morphology of AlPO4 -11 molecular sieves changed from sphere to rod gradually, which exactly consisted with the variation of the XRD patterns. Slow nucleation rate caused by the chelated function of Al–F may be the main reason to direct the crystal growth on the special direction. Meanwhile, some control experiments including use of HCl and increasing the concentration of water also showed the same preferential growth of the crystals. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Porous materials with pore sizes near molecular dimensions, such as zeolites and aluminophosphate molecular sieves (AlPO), have been widely used in catalysis and separation, and development for new applications in membranes, sensors, optics, etc., is in progress [1]. In the early 1980s, the discovery of a new class of molecular sieves was reported by researchers from Union Carbide [2,3]. These new materials comprise a series of crystalline, microporous aluminophosphates (AlPOs) hydrothermally prepared from reaction mixtures containing inorganic sources of Al and P and an organic template (such as amine or a quaternary ammonium salt). And these materials present strict alternation of AlO4 − and PO4 + tetrahedral with a neutral framework [4–7]. As being one member of the AlPO-n family, AlPO4 -11 molecular sieve has unidirectional, non-intersecting, 10-membered ring channels with elliptical pore apertures of 0.63 nm × 0.39 nm. The topology consists of sheets of six-ring–six-ring–four-ring (S6R–S6R–S4R) units. Recently, this subject has raised a lot of attention due to its potential application in heterogeneous catalysis [8–12]. At the same time, microwave irradiation is more efficient for transferring thermal energy to a volume of material than conventional thermal processing which transfers heat to the material by convection, conduction and radiation. The microwave is a kind of
∗ Corresponding author. E-mail address: [email protected] (Y. Chen). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.08.038
electromagnetic radiation with a high frequency between 0.3 and 300 GHz. Applications of microwave energy in the synthesis of inorganic materials have been exploited since the mid-1980s [13,14]. Until now, microwave technique has been widely applied in the synthesis of the zeolite and molecular sieves because of the reduced reaction time and improved crystal quality. Microwave technique offers more rapid crystallization than conventional hydrothermal method and it is believed more nuclei are generated simultaneously, thus the growth of the crystals is homogeneous [15,16]. Recent years, some research groups have reported the morphology control of porous materials such as VSB-5 [17] and SBA-16 [18] under microwave irradiation and have showed the microwave is a very efficient tool to control the morphology of porous materials. Because many emerging applications of porous materials require precise control of crystal morphology, strategies to control the crystal shape and size are necessary for special applications [3,4]. Large single crystals with a well-defined orientation and structure are necessary for the development of novel optical, electronic and magnetic materials [19,20]. Several studies have been conducted to synthesize homogeneous and well-defined AFI crystals having various morphologies. One promising route to control crystal size and shape is through confined synthesis from reagents dissolved in water-in-oil microemulsions. Lin et al. [21] successfully synthesized the AlPO4 -5 molecular sieve fibers directed by microemulsion. But the final crystal size is larger than the microemulsion droplets, so continued growth must occur through solution transport outside of the microemulsion. And Jhung et al. [22] also successfully used microwave techniques to synthesize AFI-
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Y. Chen et al. / Materials Chemistry and Physics 113 (2009) 899–904
Table 1 Reaction conditions and properties of the samples Sample no.
Molar composition
Time (h)
Synthesized method
Morphology
Surface area (m2 g−1 )
A B C D E F G H I J K L M N
Al2 O3 :1.25P2 O5 :DIPA:100H2 O Al2 O3 :1.25P2 O5 :DIPA:100H2 O Al2 O3 :1.25P2 O5 :DIPA:100H2 O:0.005CTAC Al2 O3 :1.25P2 O5 :DIPA:100H2 O:0.025CTAC Al2 O3 :1.25P2 O5 :DIPA:100H2 O:0.025CTAC Al2 O3 :1.25P2 O5 :DIPA:100H2 O:0.025CTAC Al2 O3 :1.25P2 O5 :HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :0.4HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :1.5HF:DIPA:100H2 O Al2 O3 :1.25P2 O5 :DIPA:300H2 O Al2 O3 :1.25P2 O5 :HCl:DIPA:100H2 O
12 6 6 6 8 10 4 8 12 24 4 4 6 4
C M M M M M M M C C M M M M
Sphere Sphere Sphere Sphere Clump Faggot Rod Rod Rod Rod Elongated sphere Rod Layer Cube
– 194.8 – – – 213.1 193.3 – – – – – 199.6 –
C: conventional hydrothermal method; M: microwave irradiation.
type molecular sieves with various morphologies under varying reaction conditions. To the best of our knowledge, the synthesis of AEL molecular sieves with different morphologies by microwave technique has not been reported so far. Herein, by utilizing microwave techniques, we present the synthesis of AEL molecular sieves with various morphologies such as aggregated sphere, plate, rod and aggregated faggot. The effects of fluoride on the synthesis are investigated, and the morphology control may be explained by the combined result of XRD and SEM. 2. Experimental 2.1. Synthesis In a typical procedure, pseudo-boehmite (Al2 O3 , 70 wt.%, CP), phosphoric acid (H3 PO4 , 85 wt.%, AR) and hydrofluoric acid (HF, 40 wt.%, AR) were dispersed in deionized water and stirred for 1.5 h at room temperature. After that, diisopropylamine (DIPA, 98 wt.%, AR) was added dropwise into the reaction mixture under 0.5 h vigorously stirring. The suspension was transferred into Teflon autoclave, which was sealed and placed in a microwave oven (WR-E, Meicheng, China). The precursor mixture was heated at 400 kPa (about 393 K) with the rate of 10 kPa min−1 for various periods of time [23]. There is a difference of controlling way for microwave irradiation which uses pressure as reference to finish the established process. For conventional hydrothermal method, the gel was loaded in a Teflon lined autoclave and put in a preheated electric oven at 443 K for a fixed time without agitation. After the synthesis, the autoclave reactor was cooled to room temperature. All products were filtrated, washed with distilled water several times and dried in an oven at 383 K for 5 h. Finally, the organic template in the synthesized AlPO4 -11 molecular sieve was burn out in the muffle at 823 K for 4 h.
results (Fig. 2a and b), though both of two crystals with plate-like shapes arrange to spherical aggregates with the uniform diameter about 5 m, the plate-like crystals synthesized by microwave technique are much thinner and more uniform than those synthesized by conventional hydrothermal method. Because the nucleation rate increases, the crystal size decreases under microwave irradiation [15,22]. In order to improve the morphology of the crystalline aggregates, cetylpyridinium chloride (CTAC) was added to the initial mixture under stirring. Fig. 2c and d also display the AlPO4 -11 aggregates synthesized with different concentration of CTAC using microwave technique. If only the concentration of CTAC was beyond the critical micelle concentration (CMC, 9.04 × 10−4 mol L−1 ), the shape of spherical aggregates (Fig. 2c) was improved gradually. Once the concentration (5 CMC) of surfactant is suitable for the system, it is possible to get more perfect spherical aggregates of AlPO4 -11 molecular sieve (Fig. 2d) with the diameter of about 20 m. Because the microenvironment of synthesis was improved by CTAC, the surface of the synthesized crystal spheres became smoother. Besides, the morphology changes of AEL crystals were also observed when the time of microwave irradiation was prolonged from 6 h to 10 h after adding 5 CMC CTAC. The morphologies of the crystals transferred from aggregated spheres (Fig. 2d) to
2.2. Characterization The crystal structure and crystallinity of the samples were analyzed by powder X-ray diffraction (XRD, D/Max-3c, Rigalcu, Japan) using Cu K␣ radiation, with the diffraction angle (2) at a range of 5–50◦ . The degree of crystallinity was estimated by summing the areas of the five major diffraction peaks. The morphology of the products was examined with scanning electron microscopy (SEM, Quanta 200, Philips-FEI, Holland) after coating with Au. The surface areas according to the Brunauer–Emmett–Teller (BET) method were determined with a volumetric adsorption apparatus (ZXF-6, NWRICI, China) at 77 K after evacuation at 573 K.
3. Result and discussion 3.1. Transformation of AEL crystal morphologies The synthesized conditions are summarized in Table 1. The crystal phase of products with AEL crystal structure was determined by powder XRD as shown in Fig. 1. The XRD patterns of the two crystals correspond well with AEL molecular sieve and do not show any noticeable impurity phase. But the crystallinity of the sample (B) is higher than the sample (A), which obviously exhibits advantage on fast crystallization of microwave technique. From the SEM
Fig. 1. XRD patterns of the synthesized AlPO4 -11 molecular sieves: (a) sample A and (b) sample B of Table 1.
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Fig. 2. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample A, (b) sample B, (c) sample C and (d) sample D of Table 1. White scale bars correspond to 5 m.
clump (Fig. 3a) and last to faggot (Fig. 3b). On the basis of various reports, Oliver et al. [24] proposed that the crystallization of most aluminophosphates presumably follows a chain-to-layer transformation process wherein the parent chain is initially hydrolysed in solution, leading to the formation of other chain structure types. But we think that the chain-to-layer transformation process of the basic crystal structure not only occurs in the very beginning of crystallization but does in the period after the crystal formed. In our experiment, this transformation combined with the morphology changes of crystals was observed after the spheres formed. 3.2. Function of HF for the control of AEL crystal morphology Fluoride has been widely applied in the synthesis of microporous zeolites and AlPO-n molecular sieves [25]. As shown in
Fig. 4, after irradiated for 4 h, sample (G) is in the form of welldefined rod-like crystals of size about 3.0 m × 0.5 m without any aggregation. When the synthesized time was prolonged to 8 h, the morphology of sample (H) became the mixture of the big rectangular crystals and the small rod-like crystals. Because some of AlPO4 -11 crystals were accelerated to dissolve into the solution due to the chelated function between fluorin and aluminum, and grow onto large AlPO4 -11 crystals via a process known as Ostwald ripening [26] by prolonging the crystallization period. As a result, the rod-like AlPO4 -11 crystals with asymmetric diameters were obtained just like Fig. 4b. And the same phenomenon was also observed when the crystallization time was prolonged from 12 h (Fig. 4c) to 24 h (Fig. 4d) using conventional hydrothermal method after adding HF. After adding HF, the gel dissolves and the nucleation and crystallization rates are limited, thus large crystals were
Fig. 3. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample E and (b) sample F of Table 1. White scale bars correspond to 5 m.
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Fig. 4. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample G, (b) sample H, (c) sample I and (d) sample J of Table 1. White scale bars correspond to 5 m.
obtained with the prolonged synthesized time. And the slow rate of nucleation and crystallization with fluoride may be related to the low hydrolysis (metal–F bond to metal–O bond) rate of metal fluorides. For example, the bond strengths of Al–F and Al–O are 663.6 and 515.1 kJ mol−1 , respectively [22]. In fact, in the previous stirring process, it was ease to form gel without any aging after adding HF. In order to make clear the function of HF in the AlPO4 -11 crystallization, the concentration of the HF was carefully investigated. In the XRD patterns of AEL molecular sieves, the great variation of diffraction peaks appeared at d = 5.50 and d = 4.06, which corresponded to the lattice planes (2 0 0) and (0 0 2). As shown in Fig. 5, the diffraction intensity of the lattice planes (0 0 2) is much higher than the lattice planes (2 0 0) for the sample without HF (Fig. 5a). With the same crystallization conditions except adding a little HF, the diffraction intensity tendency of the two lattice planes changes to contrary (Fig. 5b and c). The diffraction intensity of the lattice planes (2 0 0) increased gradually, whereas the diffraction intensity of (0 0 2) decreased remarkably. Because the two lattice planes are perpendicular to each other, this variation may consist of the transformation of the crystals morphologies. And from the SEM micrographies, without any HF, the products were in form of spherical aggregates consisted with plate-like crystals (Fig. 2b). Once 0.4 molar ratio HF was added into the synthesized gel, it was seem as beginning to elongate the spherical aggregates into rod-like aggregates along the c-direction (Fig. 6a). Then, when the use of HF was further increased to 1.0 molar ratio, the perfect AlPO4 -11 crystals with rod-like shapes were observed (Fig. 4a). The transformation of the crystal morphologies exactly consisted with the variation of the XRD patterns. Because of the chelated function between fluorin and aluminum, the crystals growth is directed primarily in one direction (2 0 0) to produce rod-like crystals. A similar dependence of the relative intensity of the (0 0 2) and (1 0 0) diffractions on the aspect
ratio of the crystals has also been observed in the synthesis of SAPO5 [22]. However, when the excessive HF was used, the overmuch fluorin ions accelerated the dissolution of little crystals and limited the nucleation and crystal growth rate to form inhomogeneous crystals (Fig. 6b). Similar crystals can be obtained by increasing water. With low concentration of reactants, the crystals can be grown preferentially along the c-axis due to the slow growth rate. From Fig. 7a, the crystals aggregates axially into rod, which was made up of lamel-
Fig. 5. XRD patterns of the synthesized AlPO4 -11 molecular sieves: (a) sample B, (b) sample K and (c) sample G of Table 1.
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Fig. 6. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample K and (b) sample L of Table 1. White scale bars correspond to 5 m.
Fig. 7. SEM micrographies of the synthesized AlPO4 -11 molecular sieves: (a) sample M and (b) sample N of Table 1. White scale bars correspond to 5 m.
lar crystals sized about 4.0 m × 1.0 m with cuspate heads. And the diffraction intensities of the lattice planes (2 0 0) is somewhat higher than (0 0 2) (Fig. 8a), which is in accord with the morphology character observed by SEM. Meanwhile, the same concentration of HCl was utilized instead HF for the synthesis of crystal. Because the bond strength of Al–Cl (about 502.5 kJ mol−1 ) is much lower than Al–F, the hydrolysis of
Al–Cl becomes very easy so that the reaction of Al–Cl can hardly induce the growth of crystals for some special direction. From Fig. 7b, the synthesized crystals are also with spherical morphologies and XRD pattern of this sample has no much special character (Fig. 8b). Because fluorin ions have much stranger chelated function with aluminum ions than chlorine ions according to the bond strength of Al–Cl and Al–F. As a result, the rod-like AlPO4 -11 crystals were obtained after added HF, while spheric AlPO4 -11 crystals were obtained after added HCl. 3.3. Physicochemical properties of the samples The synthesized AEL molecular sieves are microporous with BET surface areas of about 200 m2 g−1 as shown in Table 1, which are suitable for use in catalysis, adsorption and new applications. In fact, the sample (B) with strong diffraction intensity for the lattice planes (0 0 2) should expose more pores on the crystal surface because the lattice plane (0 0 2) is perpendicular to the 10membered ring channels. But in this work, there was no obvious variation of BET surface areas with the different aggregation. Therefore, the development of a method to disperse the aggregated AEL crystal spheres is necessary. 4. Conclusion
Fig. 8. XRD patterns of the synthesized AlPO4 -11 molecular sieves: (a) sample M and (b) sample N of Table 1.
AlPO4 -11 molecular sieves with morphologies of aggregated sphere, clump, faggot and rod were successfully synthesized by microwave irradiation using diisopropylamine as template. Although similar trends in crystallization were observed using microwave synthesis and conventional synthesis, the microwave
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irradiation offered much faster crystallization time than conventional hydrothermal method. By adjusting the crystallization time, the morphology of the AlPO4 -11 molecular sieves changed from sphere to clump and then to faggot. With the increase of HF, the morphology of AlPO4 -11 molecular sieves changed from sphere to rod gradually, which exactly consisted with the variation of the XRD patterns. Slow nucleation rate caused by the chelated function of Al–F was seemed to direct the crystal growth in the c-direction, and some control experiments included the use of HCl and the increase of water also showed the same preferential growth of the crystals. Therefore, all of the presented work may provide a series of simply but very effective methods to potentially control crystal morphology of molecular sieves. Acknowledgement This research work was fully supported by Items of Tackling Key Problem for Science and Technology of Shaanxi Province (No. 2006k06-G9). References [1] M.E. Davis, Nature 417 (2002) 813. [2] R.R. Xu, W.Q. Pang, Chemistry-Zeolites and Porous Materials, Science Publisher, Beijing, 2004, 225 pp.
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