Synthesis and crystal morphology control of AlPO4-5 molecular sieves by microwave irradiation

Solid State Sciences 29 (2014) 41e47

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Synthesis and crystal morphology control of AlPO4-5 molecular sieves by microwave irradiation Wei Yang a, Yu Song a, *, Ying Mu b, Shangru Zhai a, Yinghuan Fu a, Qingda An a, Bin Zhai a, Xiaowei Song b a b

Dalian Polytechnic University, Dalian 116034, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 June 2013 Received in revised form 7 January 2014 Accepted 12 January 2014 Available online 22 January 2014

AlPO4-5 molecular sieves have been synthesized by the hydrothermal and solvothermal reactions using triethylamine as a template, aluminum isopropoxide and orthophosphate as the aluminum and phosphorus resource under microwave irradiation. The influences of various experimental parameters, such as reaction time, reaction temperature and reaction power, have been systematically investigated. The morphology control of AlPO4-5 molecular sieves was achieved by changing the dosage of solvent and HF to control the solvent polarity and control the nucleation respectively. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and so on. The results show that the aspect ratio of the AlPO4-5 molecular sieves increases with the increase of the solvent polarity and with the increasing concentration of HF, the morphology of AlPO4-5 molecular sieves changes from hexagonal plate to hexagonal rod. Ó 2014 Elsevier Masson SAS. All rights reserved.

Keywords: Microwave irradiation Molecular sieves Morphology Solvent polarity

1. Introduction Microporous materials with pore sizes near molecular dimensions, such as zeolites and aluminophosphate molecular sieves (AlPO4-n), are widely used in separation and catalysis, and are still being developed for new applications in membranes, sensors, optics, etc [1]. In the early 1980s, the discovery of a new class of molecular sieves was reported by Flanigen E. M. and Wilson S. T. from Union Carbide Corporation [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. As one member of the AlPO-n family, AlPO4-5 molecular sieve has onedimensional channel of 0.73 nm
0.73 nm (delineated by 12 MR) and excellent thermal stability [4,5]. Because many emerging applications of microporous materials require precise control of crystal morphology, strategies to control the crystal shape and size are necessary for special applications [6]. Several studies have been conducted to synthesize homogeneous and well-defined AFI crystals having various morphologies [7e9]. To date microporous materials have been synthesized mainly with conventional hydrothermal heating [10]. One

* Corresponding author. Tel.: þ86 411 86323726; fax: þ86 411 86323736. E-mail address: [email protected] (Y. Song). 1293-2558/$ e see front matter Ó 2014 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2014.01.004

promising route to control crystal size and shape is through microwave synthesis, which is a new synthetic method in the field of molecular sieve materials preparation and has many advantages, such as fast crystallization [11,12], high product purity and selectivity [13,14], narrow particle size distribution and easy morphology control [15e20] in comparison with conventional heating mode. Applications of microwave energy in the synthesis of inorganic materials have been explored since the mid-1980s [21]. 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 [22]. Some research groups have reported the morphology control of porous materials such as VSB-5 [23] and SBA-16 under microwave irradiation and have showed the microwave is a very efficient tool to control the morphology of porous materials. Jhung et al. [7] successfully used microwave techniques to synthesize AFI-type molecular sieves with various morphologies under varying reaction conditions. Tian et al. [8,9,24] also successfully synthesized the AlPO4-5 molecular sieves with various morphologies by microwave irradiation in mixed-solvents system. Thus far, microwave heating method as a significant way in promoting chemical reactions has been developed into an independent chemical research area, and can also be a very efficient tool to control the morphology of microporous materials. In this work, by utilizing microwave techniques, we present the synthesis of AlPO4-5 molecular sieves with various morphologies.

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Table 1 Reaction conditions for synthesis of AlPO4-5 molecular sieves. Sample no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Molar composition Al2O3

P2O5

TEA

HF

H2O

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 0.125 0.25 1.0

225 225 225 225 225 225 225 225 225 225 225 225 225 225 225 225

Temp./ C

Time/h

Power/W

180 180 180 180 180 180 160 170 190 180 180 180 180 180 180 180

0.25 0.5 0.75 1 1.5 2 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

800 800 800 800 800 800 800 800 800 400 600 1000 800 800 800 800

The influences of experimental parameters, such as reaction time, reaction temperature and reaction power, have been systematically investigated. And the effects of HF and ethylene glycol on the synthesis were investigated in detail. The as-prepared samples

were further characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and elemental analysis. The results show that various morphologies are modified by controlling the factors of nucleation. 2. Experimental 2.1. Synthesis In a typical procedure, 1.0 g of aluminum isopropoxide (Al2O3, 24 wt.%, CP) was added to 10 mL of deionized water under vigorous stirring for 0.5 h at room temperature, followed by the addition of 0.33 mL of phosphoric acid (H3PO4, 85 wt.%, AR) to the above mixture. After stirring for 0.5 h, 0.24 mL of triethylamine (TEA, 99 wt.%, AR) was added dropwise and stirred for 0.5 h. After that, an amount of hydrofluoric acid (HF, 40 wt.%, AR) was added into the reaction mixture and stirred for 2 h. The synthesis solution with an initial molar composition of 1.0 Al2O3:1.0 P2O5:0.7 TEA:(0e1.5) HF:225 H2O was charged into a 70 mL Teflon-lined autoclave directly. The crystallization was conducted in a microwave oven (MDS-6) with pre-programmed heating profiles under autogenous pressure. The gel heating proceeded in two stages. At the first stage, the gels were heated quickly from room temperature to 180  C with a microwave power of 800 W and kept 180  C for 3 min. At the

Fig. 1. The SEM images of the products synthesized under different crystallization time. (a) 0.25 h, (b) 0.5 h, (c) 0.75 h, (d) 1.0 h, (e) 1.5 h (f) 2 h.

W. Yang et al. / Solid State Sciences 29 (2014) 41e47

43

scanning electron microscopy (SEM) was taken on a JSM-6460LV electron microscope operating at 25 kV. Thermogravimetric analysis (TGA) was performed on a PerkineElmer TGA7 unit in the air with a heating rate of 20 K min1. Inductively coupled plasma (ICP) analysis was performed on a PerkineElemer Optima 3300DV spectrometer. Elemental analysis was conducted on a PerkineElemer 2400 elemental analyzer.

(f)2h

(e)1.5h

(d)1h

3. Results and discussion

(c)0.75h

3.1. Effect of crystallization time (b)0.5h

(a)0.25h 5

10

15

20

25

30

35

40

Fig. 2. XRD patterns of the products synthesized under different crystallization time. (a) 0.25 h, (b) 0.5 h, (c) 0.75 h, (d) 1.0 h, (e) 1.5 h (f) 2 h.

second stage, the power were firstly changed to the power of 400 W, 600 W and 1000 W respectively and the gels remained at these conditions for 0.25e2 h which is much shorter than 1 days by using traditional heating methods. The powders were cooled to room temperature. The resulting product consisting of large single crystals was centrifugation, washed with distilled water, and finally dried overnight at room temperature. The reaction conditions are summarized in Table 1. 2.2. Characterization X-ray powder diffraction (XRD) data were collected on a XRD6100 diffractometer with CuKa radiation (l ¼ 1.5406  A). The

In order to know the influence of crystallization time on the crystallization of AlPO4-5 molecular sieves, we examined the products taken from 0.25 h, 0.5 h, 0.75 h, 1 h, 1.5 h and 2 h, shown in Table 1(1e6). Fig. 1 shows the SEM images of the products synthesized under different crystallization time. For 0.25 h with microwave irradiation (Fig. 1(a)), the products are mainly polycrystalline and irregular crystals. For 0.5 h, the products are mainly prismatic crystals with good uniformity, and the aspect ratio is consistent (Fig. 1(b)). For 0.75 h, the products are given priority to prismatic crystals with different aspect ratio, and the uniformity is not good (Fig. 1(c)). For 1 h, products are obviously observed in hexagon crystallites in addition to the prismatic crystals (Fig. 1(d)). For 1.5 h, prismatic crystals and hexagon crystallites grow up, and some of the crystals become amorphous phases (Fig. 1(e)). For 2 h, there are bigger prismatic crystals of larger aspect ratio with amorphous phase increasing, but with the constantly consumption of reactants, the uniformity is not good (Fig. 1(f)). Results show that with the increase of crystallization time of microwave irradiation, crystal purity and particle size distribution have been changed, and 0.5 h can synthesize the AlPO4-5 molecular sieves with high purity and uniform particle size distribution. After that, extending the time of microwave irradiation, the main influence is to promote the crystal growth and accumulate the nucleation. The crystallization time can influence the aspect ratio

Fig. 3. The SEM images of the products synthesized under different crystallization temperature. (a) 160  C, (b) 170  C, (c) 180  C, (d) 190  C.

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W. Yang et al. / Solid State Sciences 29 (2014) 41e47

(d)1000W o

(d)190 C

(c)800W o

(c)180 C

(b)600W

o

(b)170 C

o

(a)160 C 5

10

(a)400W 15

20

25

30

35

40

Fig. 4. XRD patterns of the products synthesized under different crystallization temperature. (a) 160  C, (b) 170  C, (c) 180  C, (d) 190  C.

and uniformity. The XRD patterns for the products are shown in Fig. 2, and are in agreement with the standard pattern [2]. 3.2. Effect of crystallization temperature Figs. 3 and 4 show the SEM images and XRD patterns of AlPO4-5 synthesized under different temperature, respectively. When the temperature is below 160  C, there is no aim product. When the temperature is 160  C, the crystallization is incomplete (Fig. 3(a)). For 170  C (Fig. 3(b)) and 180  C (Fig. 3(c)), the products are mainly prismatic crystals. As high temperature helps nucleation, when the temperature is 180  C, the homogeneity of products is better. When the temperature is 190  C (Fig. 3(d)), prismatic crystals reduce. The

5

10

15

20

25

30

35

40

Fig. 6. XRD patterns of the products synthesized under different power. (a) 400 W, (b) 600 W, (c) 800 W, (d) 1000 W.

results show that the microwave irradiation temperature has a great influence on the synthesis of AlPO4-5 molecular sieve. When the temperature is high, the crystal nucleation and growth are accelerated, but excessive temperature is not conducive to the perfect single crystals. 3.3. Effect of power The products synthesized under different power are characterized by XRD and SEM analysis (Figs. 5 and 6). Within 800 W microwave irradiation powers, the products are mainly prismatic crystals. At 400 W, products in addition to the prismatic crystals include a small amount of amorphous phases. At 600 W, the

Fig. 5. The SEM images of the products synthesized under different power. (a) 400 W, (b) 600 W, (c) 800 W, (d) 1000 W.

W. Yang et al. / Solid State Sciences 29 (2014) 41e47

amorphous phases reduce. Under the condition of 800 W, prismatic crystals are uniform. When the microwave power is 1000 W, the products get together, and have irregular crystal formation. The results show that microwave power can influence the AlPO4-5 molecular sieve synthesis, and the power should be controlled in 800 W. When the power is 1000 W, the size of crystals is small and easy to aggregate. The reason is that the high power and the fast heating rate can promote crystal nucleation largely and rapidly. Meanwhile, the crystal surrounding “mother liquid concentration” fall rapidly, which hinder further growth of the crystals. 3.4. Effect of hydrofluoric acid In order to investigate the effect of HF on the morphologies of the products, some experiments have been done according to Table 1(13e16). Fig. 7 is the SEM images of the AlPO4-5 molecular sieves synthesized adding different amount of HF. The products are mostly irregular spheres with none of HF, shown in Fig. 7(a). After joining a small amount of HF (0.0125 mL), the products are spherical crystals, shown in Fig. 7(b). With the increase of HF volume to 0.025 mL, the products are polycrystalline. It can be seen that globular crystals are formed by prismatic crystals crossing, shown in Fig. 7(c). When the HF is 0.05 mL, rule prismatic crystals are dispersed, shown in Fig. 7(d). When the amount of HF is two times and three times than the original, the products are prismatic crystals, and shape has not been changed. By adding fluorine ions, F, 6 Al and P form AlF3 which release Al and P by hydrolysis. 6 and PF The appropriate concentration of HF can promote the formation of the AlPO4-5 crystals. However, when the concentration of HF is very small, crystals formed globular polycrystalline. 3.5. Effect of the solvent ratio The solvent ratio of the reaction mixture is a crucial factor for the synthesis of the AlPO4-5 (Table 2). Fig. 8 is the SEM images of

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Table 2 Reaction conditions of AlPO4-5 molecular sieves synthesized in mixed-solvents. Sample no.

V(EG)/mL

V(H2O)/mL

17 18 19 20 21 22

9 4 3 2 1 1

1 1 2 3 4 9

AlPO4-5 with different volume ratio of EG/H2O in a mixture system of ethylene glycol and water. When the volume ratio of EG/H2O is 9:1, the products are sharp on both ends of the prismatic crystals, shown in Fig. 8(a). When the volume ratio of EG/H2O is reduced to 4:1, the products are prismatic crystals with both ends changing plane and aspect ratio being longer, shown in Fig. 8(b). However, both ends of the crystals are plane and aspect ratio is not changed under the condition of ratio of 3:2, shown in Fig. 8(c) (2:3 of volume ratio of EG/H2O is the same). In the ratio of 1:9, the aspect ratio of prismatic crystals is larger, shown in Fig. 8(d) (4:1 of volume ratio of EG/H2O is the same). The results show that with the decrease of the EG content in the system which makes the solvent polarity increase, the aspect ratio of the AlPO4-5 molecular sieves increases and the ends of them are changed from sharp tips to plane tips. When the EG content decreases to a certain extent, tips grow slowly, and aspect ratio increases. The addition of EG can actually inhibit the growth of the crystal tips. 4. Conclusions By using a MDS-6 automatic variable frequency microwave digestion/extraction instrument, the AlPO4-5 molecular sieves with different morphologies have been synthesized. At the microwave irradiation condition of 180  C, 30 min, 800 W, the AlPO4-5 molecular sieves with the highest purity and particle size uniformity

Fig. 7. The SEM images of the AlPO4-5 molecular sieves synthesized under different concentration of the HF. (a) n(HF)/n(Al2O3) ¼ 0, (b) n(HF)/n(Al2O3) ¼ 0.125:1, (c) n(HF)/ n(Al2O3) ¼ 0.25:1, (d) n(HF)/n(Al2O3) ¼ 0.5:1.

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W. Yang et al. / Solid State Sciences 29 (2014) 41e47

Fig. 8. The SEM images of the AlPO4-5 molecular sieves synthesized under different solvent proportion conditions. (a) V(EG)/V(H2O) ¼ 9:1, (b) V(EG)/V(H2O) ¼ 4:1, (c) V(EG)/ V(H2O) ¼ 3:2, (d) V(EG)/V(H2O) ¼ 2:3, (e) V(EG)/V(H2O) ¼ 1:4, (f) V(EG)/V(H2O) ¼ 1:9.

have been acquired. HF has a great influence on the morphologies of molecular sieves. The results show that when the concentration of HF is very small, the shape of the crystals changes from hexagonal plate to hexagonal rod. By changing the system in the ethylene glycol and water volume ratio, there is a great influence on the morphologies of the products. With the decrease of the glycol content in the system, the crystal tips grow faster. When the ethylene glycol content decreases to a certain extent, the growth of crystal tips reduces, and aspect ratio of the products increases. Acknowledgments Financial support from the Opening Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University (No. 2013-16), the Educational Commission of Liaoning Province of China (No. L2012192) and the Science & Technology Foundation of Liaoning of China (No. 201102111) are kindly appreciated. References [1] M.E. Davis, Nature 417 (2002) 813e819.

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