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Template-free synthesis of AlPO4-H1, -H2, and -H3 by microwave heating
Microporous and Mesoporous Materials 52 (2002) 159–167 www.elsevier.com/locate/micromeso
Template-free synthesis of AlPO4-H1, -H2, and -H3 by microwave heating Katsuyuki Kunii b
a,b
, Kazuhiro Narahara a, Shoji Yamanaka
a,c,*
a Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Takamatsu Technical Research Institute of Shikoku Instrumentation Co., Ltd., 2217-7 Hayashi-cho, Takamatsu, Kagawa 761-0301, Japan c CREST, Japan Science and Technology Corporation (JST), Kawaguchi 322-0012, Japan
Received 12 October 2001; received in revised form 16 January 2002; accepted 16 January 2002
Abstract Aluminophosphate molecular sieves (AlPO4 ’s) were prepared by microwave heating without using organic template reagents. The microwave heating enhanced the crystallization of aluminophosphate gels, and AlPO4 -H1, -H2(AHT), and -H3(APC) were successfully obtained as single phases in a relatively short reaction time. The reaction conditions used for the crystallization of the three kinds of AlPO4 ’s were very similar to each other, but the use of different aluminum sources resulted in different structures; amorphous aluminum hydroxide, gibbsite, and boehmite were used for the preparation of AlPO4 -H1, -H2, and -H3, respectively. The crystallization temperatures were 125, 118, and 90 °C, respectively. The addition of hydrochloric acid was needed for the crystallization of AlPO4 -H1 and -H2. The addition of seed crystals and stirring the gels were found to be essential for the crystallization of AlPO4 -H3. Nitrogen and water adsorption properties were also measured. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Microwave heating; Template-free synthesis; AlPO4 -H1; AlPO4 -H2(AHT); AlPO4 -H3(APC)
1. Introduction Aluminophosphate molecular sieves (AlPO4 ’s) adapt a variety of zeolite-like microporous structures. Their preparation methods have been extensively studied using organic molecules as
* Corresponding author. Address: Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. Tel./fax: +81-82424-7740. E-mail address: [email protected] (S. Yamanaka).
structure-directing agents [1,2]. One of the most striking discoveries in such efforts was the synthesis of VPI-5(VFI) aluminophosphate with 18membered tetrahedral (18-T) rings by Davis et al., [3,4], which had the largest pores ever found in crystalline microporous compounds. VPI-5 was prepared by a hydrothermal treatment of aluminophosphate gel solutions containing organic template reagents such as di-n-propylamine under a specific condition. Recently, it was pointed out that VPI-5 aluminophosphate was isostructural with AlPO4 -H1 [5], which had been prepared about 40 years ago by d’Yvoire [6] without using organic template. In the present study, AlPO4 -H1
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 3 1 4 - 1
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is prepared without using organic template more efficiently. We use the microwave heating instead of conventional heating in a furnace. Recently, much attention has been paid to the microwave heating in hydrothermal synthesis [7–20,11,22–26]. The advantages of microwave heating are as follows: microwave can raise the temperature of the reactant to a desired range quickly in a few minutes. The heat is supposedly induced by the friction of molecular rotation enhanced by microwave irradiation, and thus, it is possible to heat the reactants selectively and homogeneously from the inside. Some reports [12–14] showed that the reaction rate was 1 or 2 orders faster than that of a conventional heating. The first study of microwave synthesis of AlPO4 was made of the cobalt substituted derivative CoAlPO4 -5 using triethylamine as a template agent [10]. The time required for the crystallization of aluminophosphate hydrous gels was found to be accelerated from 1–2 days to 20 min using microwave heating [12–14]. The microwave effects for the synthesis of AlPO4 -5(AFI) were found in the rapid crystallization [14–20], narrow particle size distribution [12,13], and the large particle sizes of the obtained crystals [10,11]. Kodaira et al. [21] reported the dependence of the morphology of AlPO4 -5 on the pH of the starting gels in the synthesis using microwave heating. In this study, the microwave heating is applied to the synthesis of AlPO4 -H1 to accelerate the crystallization of the gels without using templates. Recently we have reported the template-free synthesis of AlPO4 -H3(APC) by a conventional heating [27]. The microwave heating will also be applied to the template-free synthesis of AlPO4 -H3 and -H2(AHT), which have 8-T and 10-T rings, respectively [28,29].
2. Experimental 2.1. Synthesis Aluminophosphate hydrous gels were prepared using three different kinds of aluminum sources, amorphous aluminum hydroxide (Aldrich, 54% Al2 O3 ), gibbsite (Wako, 65% Al2 O3 ), and boeh-
mite (DISPAL from Vista, 80% Al2 O3 ), and subjected to microwave heating. The microwave solvothermal equipment used was made by Shikoku Instrumentation Co. Ltd. The maximum power and the frequency of the microwave were 1.45 kW and 2.45 GHz, respectively. The reactor was made of Pyrex glass (36 mm in inner diameter and 250 mm in length) and the temperature was monitored and controlled by a fiber thermometer (FX8000, ANRITSU) placed in the reactor. AlPO4 -H1 was prepared using amorphous aluminum hydroxide, which was dispersed in deionized water, and then orthophosphoric acid and hydrochloric acid were added to make hydrous gels with various molar compositions (Al2 O3 /P2 O5 / HCl/H2 O). The gels were hydrothermally heated by microwave irradiation. After the reaction, the products were separated by centrifugation, washed with water, and dried at room temperature. AlPO4 H2 was also prepared by a similar manner except that gibbsite was used as an aluminum source instead of amorphous aluminum hydroxide. AlPO4 -H3 was prepared using boehmite as an aluminum source. The AlPO4 hydrous gels were prepared by mixing a 1 M boehmite dispersion in water and a 1 M orthophosphoric acid solution in various ratios; ion-exchanged water was added to the mixture to adjust the final concentrations. About 0.02 wt.% of AlPO4 -H3 seed crystals were added to the gel, and the mixture was stirred for 3 h at 25 °C. The mixed gels were then crystallized by microwave heating at 90 °C for 1 h, which were then washed with water and dried at room temperature in air. The dried AlPO4 -H3 was converted into AlPO4 -D by the thermal treatment at 300 °C for 3 h in air. 2.2. Analyses X-ray powder diffraction (XRD) data were measured using a diffractometer of MAC SCIENCE M18XHF-SRA with graphite monochromated Cu Ka radiation. Nitrogen adsorption isotherms were measured at liquid nitrogen temperature by a homemade computer controlled volumetric apparatus. AlPO4 -H1 and -D were degassed at 200 °C for 3 h in vacuum; AlPO4 -H2 was degassed at 60 °C for 5 h in vacuum prior to the measurement. Adsorption
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3.1. Synthesis of AlPO4 -H1
isotherms of water vapor were measured at 25 °C on a gravimetric apparatus constructed with a Cahn electric balance (model 1000). Chemical analysis was performed using an inductively coupled plasma spectrometer (Seiko Instruments SPS1500VR) on the samples dissolved in a mixture of hydrochloric acid and nitric acid solutions. The scanning electron micrographs were taken with JEOL-6640F SEM.
Single phase AlPO4 -H1 was obtained when amorphous aluminum hydroxide was used as an aluminum source, and the molar ratio of the starting aluminophosphate gels (Al2 O3 :P2 O5 :HCl: H2 O) was in the range of 1:0.6–0.8:0.8–1.2:50. The optimum molar ratio of the gel was 1:0.8:1:50 (Table 1, #1). If the molar ratio P2 O5 /Al2 O3 > 0:8 and HCl/Al2 O3 < 0:8, AlPO4 -H1 was accompanied by AlPO4 -H4 and AlPO4 -H2, respectively (Table 1, #3 and #4). The temperature range for the crystallization of the single phase of AlPO4 -H1 was very narrow. If the reaction temperature was above 125 °C, tridymite type AlPO4 was formed with AlPO4 -H1, and the amount of the tridymite
3. Results and discussion Table 1 summarizes the molar ratios of starting aluminophoshate gels, crystallization conditions and the products obtained.
Table 1 Microwave synthesis of AlPO4 -H1, -H2, and -H3 in various gel compositions with (þ) and without () stirring or seed Al sourcesa
Seed
Stirring
Temp. (°C)
Time (h)
Products
50 50 50 50 50 50
A A A A A A
þ
125 125 125 125 130 115
1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0
50 50
G B
125 125
1.0 1.0
H1 H1, H3 H1, H4 H1, H2 H1, tridymite H1, amorphous Al(OH)3 H1, gibbsite H4
0.95 0.80 0.95 1.00 0.95 0.95 0.95 0.95 0.95
0.6 0.6 1.0 0.6 0.6 0.6 0.6 0.6 0.6
50 50 50 50 50 50 50 50 50
G G G G G G G A B
þ
118 118 118 118 118 125 114 118 118
0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67
H2 H1, H1, H2, H3 H2, H2, H1, H4
1.1 1.1 1.1 1.5 0.5
1.0 1.0 1.0 1.0 1.0
0 0 0 0 0
222 222 222 222 222
B B B B B
þ þ þ þ
þ þ þ þ
90 90 90 90 90
1.0 1.0 1.0 1.0 1.0
#23
1.1
1.0
0
222
A
þ
90
1.0
#24
1.1
1.0
0
222
G
þ
90
1.0
H3 Boehmite Boehmite H3 Variscite, metavariscite Amorphous Al(OH)3 Gibbsite
Reaction
Gel compositions (mol) Al2 O3
P2 O5
HCl
#1 #2 #3 #4 #5 #6
1.0 1.0 1.0 1.0 1.0 1.0
0.80 0.80 1.00 0.80 0.80 0.80
1.0 1.0 1.0 0.6 1.0 1.0
#7 #8
1.0 1.0
0.80 0.80
#9 #10 #11 #12 #13 #14 #15 #16 #17
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
#18 #19 #20 #21 #22
a
H2 O
A, amorphous Al(OH)3 ; G, gibbsite; B, boehmite.
H2 H2 H4 tridymite gibbsite H2
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increased with the increase of the temperature (Table 1, #5). If the crystallization was carried out at temperatures below 115 °C, the reactant amorphous aluminum hydroxide remained partly unreacted (Table 1, #6). The gel should not be stirred during crystallization. The agitation of the gel under microwave heating produced AlPO4 -H1 associated with AlPO4 -H3 as a minor phase (Table 1, #2). The selection of the aluminum source is important; AlPO4 -H1 was obtained as single phase only when amorphous aluminum hydroxide was used. The use of gibbsite gave AlPO4 -H1 with gibbsite remaining unreacted (Table 1, #7), while boehmite yielded AlPO4 -H4 (Table 1, #8). AlPO4 H4 was prepared by d’Yvoire [6] for the first time, and the crystal structure was recently determined by Poojary et al. [30], which was found to be a condensed aluminum phosphate phase with a composition of AlPO4 H2 O. The product #8 of Table 1 was assigned on the basis of the XRD data given by d’Yvoire [6]. The XRD pattern of AlPO4 -H1 obtained in this study is shown in Fig. 1(a). The diffraction peaks were indexed on the basis of a hexagonal cell with , in agreement a ¼ 18:995ð9Þ and c ¼ 8:124ð8Þ A with the XRD data reported by d’Yvoire [6] and the data for VPI-5 by Davis et al. [3]. Duncan et al. [5] reported a template-free synthesis of AlPO4 -H1 using amorphous aluminum hydroxide and conventional heating at a higher temperature of
Fig. 1. X-ray diffraction patterns of AlPO4 -H1 (a), -H2 (b), and -H3 (c) obtained by microwave heating.
140 °C for 4 h. For the preparation of VPI-5 in the presence of organic templates, boehmite was used as an aluminum source [4,31,32]; the preparation conditions should be determined specifically in terms of parameters such as time/temperature regimes, method and conditions of mixing. In the present method using microwave heating, AlPO4 H1 can be obtained at a relatively low temperature of 125 °C for a short crystallization period of 1 h by a simple microwave heating. 3.2. Synthesis of AlPO4 -H2 The single phase AlPO4 -H2 was obtained when gibbsite was used as an aluminum source, and the molar ratio of the starting aluminophosphate gels (Al2 O3 :P2 O5 :HCl:H2 O) was 1:0.95:0.6:50 (Table 1, #9). When the molar ratio HCl/Al2 O3 P 0:6 and Al2 O3 /P2 O5 < 0:95, AlPO4 -H1 was co-crystallized with AlPO4 -H2 (Table 1, #10 and #11). When the molar ratio Al2 O3 /P2 O5 > 0:95, AlPO4 -H4 was co-crystallized (Table 1, #12) with AlPO4 -H2. The agitation of the gel during the crystallization should be prohibited. If the gel was stirred under microwave heating, AlPO4 -H3 was obtained as a single phase (Table 1, #13). The crystallization temperature for the AlPO4 -H2 single phase synthesis was determined to be in a range of 115–120 °C. When the temperature rose above 120 ° C, the tridymite type AlPO4 was co-crystallized (Table 1, #14); when the crystallization was carried out at temperatures below 115 °C, the reactant gibbsite remained unreacted in part (Table 1, #15). The use of amorphous aluminum hydroxide as an aluminum source instead of gibbsite yielded a mixture of AlPO4 -H1 and -H2 as a minor phase (Table 1, #16). The use of boehmite gave AlPO4 -H4 (Table 1, #17). The XRD data of the obtained AlPO4 -H2 (Fig. 1(b)) were in agreement with that reported by d’Yvoire [6], and could be indexed on the basis of an orthorhombic cell with a ¼ 16:184ð5Þ, b ¼ . Duncan et al. [5] 9:914ð3Þ, c ¼ 8:134ð4Þ A attempted to prepare AlPO4 -H2 using amorphous aluminum hydroxide under template-free conditions by a conventional heating. However, an impure AlPO4 -H2 was obtained along with AlPO4 H3 and -H4. Single phase AlPO4 -H2 was prepared using dipentylamine as template reagent by Li and
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Davis [29]. In this study using microwave heating, the single phase AlPO4 -H2 was successfully obtained in the absence of organic templates in a very narrow crystallization temperature range of 115– 120 °C. 3.3. Synthesis of AlPO4 -H3 AlPO4 -H3 was obtained as a single phase when boehmite was used as an aluminum source, and the molar ratio of the starting aluminophosphate gels Al2 O3 /P2 O5 P 1:0 (Table 1, #18 and #21). A dilute gel was used compared with the gels used in the syntheses of AlPO4 -H1 and -H2. It should be noted that the crystallization temperature was as low as 90 °C, and crystallization completed within 1 h. When the molar ratio Al2 O3 /P2 O5 < 1:0, variscite and metavariscite were formed (Table 1, #22). For the synthesis of AlPO4 -H3, the agitation of the gels during crystallization was rather essential, and the addition of seed crystals was needed. Without the agitation of the gel during crystallization and without the addition of seed crystals, the reactant boehmite remained unreacted even with the molar ratio Al2 O3 /P2 O5 P 1:0 (Table 1, #19 and #20). The XRD pattern (Fig. 1(c)) can be indexed on the basis of an orthorhombic unit cell of a ¼ 19:348ð7Þ, , in good agreement b ¼ 9:731ð3Þ, c ¼ 9:759ð5Þ A with that reported by d’Yvoire. We have already succeeded in the preparation of AlPO4 -H3 under template-free and conventional heating conditions at 90 °C for 3 h [27]. Note that the crystallization time was much reduced to less than 1 h by using the microwave heating. The use of different kinds of aluminum sources gave different types of AlPO4 ’s; amorphous aluminum hydroxide, gibbsite, and boehmite gave AlPO4 -H1, -H2, and -H3, respectively. This can be explained in terms of the difference in the re-
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activity of the aluminum sources. The reactivity of the aluminum sources with phosphoric acid will decrease in the following order; boehmite > amorphous aluminum hydroxide > gibbsite. Only boehmite gave AlPO4 -H3 at a crystallization temperature as low as 90 °C, while the other sources needed higher reaction temperatures of 118 °C (Table 1, #13) for gibbsite; Duncan et al. [5] reported that AlPO4 -H3 was obtained at 140 °C using amorphous aluminum hydroxide. Gibbsite can give AlPO4 -H1, but partly remained unreacted if the same reaction conditions as that used for amorphous aluminum hydroxide was used (Table 1, #7). AlPO4 -H2 is prepared from gibbsite in a narrow temperature range between the temperatures suitable for the crystallization of AlPO4 -H1 and -H3. Gibbsite is not so reactive compared with amorphous aluminum hydroxide, but the crystallization temperature for AlPO4 -H2 is lower than that of AlPO4 -H1. This is the reason for the narrow crystallization temperature range, and gibbsite can be used for the synthesis of AlPO4 -H2, if the crystallization temperature is optimized for the single phase formation. 3.4. Chemical analysis and scanning electron microscopy Chemical analyses were performed on the AlPO4 samples calcined at 900 °C, and the results are listed in Table 2. As can be seen from the table, all the AlPO4 samples obtained in this study had the stoichiometric composition Al2 O3 /P2 O5 1. The SEM pictures of the samples are shown in Fig. 2. The AlPO4 -H1 and VPI-5 prepared by conventional heating have a needle-like morphology and are aggregated into bundles [5,33]. The AlPO4 -H1 crystals obtained in this study also showed a needle-like morphology, but the crystals
Table 2 Chemical analysis data for AlPO4 -H1, -H2, and -H3 AlPO4 H1 H2 H3
Ignition loss (wt.%) 24.4 16.7 18.7
Al2 O3 (wt.%) 42.8 42.4 43.2
P2 O5 (wt.%) 56.5 57.8 56.2
Total (wt.%) 99.3 100.2 99.4
Molar ratio Al2 O3 /P2 O5
H2 O/AlPO4
1.06 1.02 1.07
2.23 1.36 1.58
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Fig. 2. SEM photographs of AlPO4 -H1 (a,b), -H2 (c), and -H3 (d) obtained by microwave heating.
were separated, forming small cotton-ball-like aggregates as shown in Fig. 2(a) and (b). It appears that the microwave heating gives rise to homogeneous nucleation, and leads to a different aggregation mode of AlPO4 -H1. AlPO4 -H2 has a needle-like, or ribbon-like morphology, and the crystals are aggregated into bundles as shown in Fig. 2(c). Duncan et al. [5] reported that AlPO4 -H2 prepared from amorphous aluminum hydroxide had a needle-like morphology similar to that of AlPO4 -H1. AlPO4 -H3 was obtained as spherical aggregates of platelet crystals of about 2 lm, as shown in Fig. 2(d). 3.5. Thermal stability and structural transformation AlPO4 -H1 is isotypic with VPI-5, and undergoes a topotactic transformation to AlPO4 -8(AET) with 14-T rings [34] under a mild thermal treatment. It was reported that AlPO4 -H1 was transformed to AlPO4 -8 at about 100 °C in air, whereas VPI-5 transformed at an elevated temperature of
about 550 °C [5,33]. The thermal stability of the AlPO4 -H1 obtained by microwave was very similar to that prepared by conventional heating. The transformed AlPO4 -8 was stable up to 1000 °C in air. It was found that the transformation of AlPO4 H1 to AlPO4 -8 depended on the atmosphere. The transformation of AlPO4 -H1 was suppressed up to 400 °C, if the sample was first dehydrated by evacuation and then heated as reported by Kenny et al. [35]. AlPO4 -H2 is much less stable against heating, and undergoes a topotactic transformation to the tridymite type AlPO4 at temperatures above 100 °C [29]. AlPO4 -H2 prepared by microwave heating was also transformed to the tridymite at 110 °C. The transformation proceeded similarly in air as well as in vacuum conditions. AlPO4 -H3 prepared by microwave heating was dehydrated and irreversibly converted into AlPO4 -D(APD) (orthorhombic; a ¼ 19:242ð9Þ, b ¼ ) at 200 °C, which revers8:579ð6Þ, c ¼ 9:786ð8Þ A ibly adsorbed water and changed into AlPO4 -H6.
K. Kunii et al. / Microporous and Mesoporous Materials 52 (2002) 159–167
These transformation behaviors are in agreement with those reported by d’Yvoire [6]. 3.6. Adsorption properties The nitrogen adsorption–desorption isotherms of AlPO4 -H1, -H2, and -D measured at liquid nitrogen temperature are shown in Fig. 3. The isotherm for AlPO4 -H1 was of typical Type I (Langmuir type) for microporous solids, and the Langmuir surface area was determined to be 580 m2 /g. Both of AlPO4 -H2 and -D showed a small specific surface area of <15 m2 /g. It appeared that the nitrogen adsorption occurred only on the external surface of the above two types of crystals. The adsorption–desorption isotherms for water measured at 25 °C are shown in Fig. 4. The adsorption isotherms had threshold vapor pressures for the steep increases in the amount of adsorption at relative pressures of P =P0 ¼ 0:020, 0.025, and 0.120 for AlPO4 -H1, -H2, and -D, respectively. AlPO4 -H1 adsorbed 0.32 ml/g of water by three consecutive adsorption steps, and the isotherm showed little hysteresis in the desorption. The three-step adsorption observed is consistent with the adsorption isotherm reported by Kenny et al. [35] for VPI-5. The first step at P =P0 ¼ 0:020 corresponds to the adsorption to strong adsorption sites, and the second step at P =P0 ¼ 0:055 is interpreted in terms of the adsorption to the
165
framework Al atoms to form the octahedral coordination. The gradual adsorption at the third step is attributed to the process of the rearrangement and densification of the hydrogen-bonded water structure. As mentioned in the forgoing paragraph, AlPO4 -H2 and -D derived from AlPO4 H3 by the transformation at 200 °C showed a very small specific surface area for nitrogen adsorption. On the contrary, they showed large adsorption capacities for water at 25 °C, 0.18 and 0.15 ml/g, respectively. Li and Davis [29] reported that the total adsorption capacities of water of VPI-5 and AlPO4 -H2 prepared by conventional heating are 0.31 and 0.19 ml/g, respectively, consistent with the values obtained in this study. These amounts of water are also in good agreement with the molar ratios H2 O/AlPO4 given in Table 1. The water adsorption isotherms of AlPO4 -H2 and -D showed large hysteresis. The pores of AlPO4 -H2 and -D are composed of elongated 10-T and 8-T rings, respectively, and the narrower diameters have dimensions comparable to that of water molecules [36]. The large hysteresis observed for AlPO4 -H2 and -D could be attributed to the narrow diameter of the elongated rings. In our previous study, we showed that the AlPO4 -D prepared by conventional heating had a large water adsorption capacity of 0.20 ml/g [27]. The reason of the difference in the total adsorption capacity of AlPO4 -D’s prepared by microwave heating and
Fig. 3. Nitrogen adsorption–desorption isotherms of AlPO4 -H1 (a), -H2 (b), and -D (c) prepared by microwave heating; () adsorption and ( ) desorption.
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Fig. 4. Water adsorption–desorption isotherms of AlPO4 -H1 (a), -H2 (b), and -D (c) at 25 °C; () adsorption and ( ) desorption.
conventional heating is not clear. It might be caused by the difference in the morphology. The crystals obtained by microwave heating are spherical aggregates (Fig. 2(d)), while the crystals by conventional heating are separated platelets. The latter crystals may have more open pores accessible from the outer surface by water. 4. Conclusions Microporous crystals AlPO4 -H1, -H2, and -H3 have been prepared as single phases by microwave heating under template-free conditions in a relatively short crystallization time. The aluminophosphate hydrous gels prepared using different kinds of aluminum sources such as amorphous aluminum hydroxide, gibbsite, and boehmite, gave different types of AlPO4 ’s; AlPO4 -H1, -H2, and -H3, respectively. The crystallization temperature should be controlled in a narrow range of 115–125 and 115–120 °C for AlPO4 -H1 and -H2, respectively. The conditions with agitation, addition of HCl and seed crystals are also important factors to obtain single phases. Acknowledgements This study has been partially supported by grant-in-aid for Scientific Research (no. 13CE2002)
of the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
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Template-free synthesis of AlPO4-H1, -H2, and -H3 by microwave heating Katsuyuki Kunii b
a,b
, Kazuhiro Narahara a, Shoji Yamanaka
a,c,*
a Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Takamatsu Technical Research Institute of Shikoku Instrumentation Co., Ltd., 2217-7 Hayashi-cho, Takamatsu, Kagawa 761-0301, Japan c CREST, Japan Science and Technology Corporation (JST), Kawaguchi 322-0012, Japan
Received 12 October 2001; received in revised form 16 January 2002; accepted 16 January 2002
Abstract Aluminophosphate molecular sieves (AlPO4 ’s) were prepared by microwave heating without using organic template reagents. The microwave heating enhanced the crystallization of aluminophosphate gels, and AlPO4 -H1, -H2(AHT), and -H3(APC) were successfully obtained as single phases in a relatively short reaction time. The reaction conditions used for the crystallization of the three kinds of AlPO4 ’s were very similar to each other, but the use of different aluminum sources resulted in different structures; amorphous aluminum hydroxide, gibbsite, and boehmite were used for the preparation of AlPO4 -H1, -H2, and -H3, respectively. The crystallization temperatures were 125, 118, and 90 °C, respectively. The addition of hydrochloric acid was needed for the crystallization of AlPO4 -H1 and -H2. The addition of seed crystals and stirring the gels were found to be essential for the crystallization of AlPO4 -H3. Nitrogen and water adsorption properties were also measured. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Microwave heating; Template-free synthesis; AlPO4 -H1; AlPO4 -H2(AHT); AlPO4 -H3(APC)
1. Introduction Aluminophosphate molecular sieves (AlPO4 ’s) adapt a variety of zeolite-like microporous structures. Their preparation methods have been extensively studied using organic molecules as
* Corresponding author. Address: Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. Tel./fax: +81-82424-7740. E-mail address: [email protected] (S. Yamanaka).
structure-directing agents [1,2]. One of the most striking discoveries in such efforts was the synthesis of VPI-5(VFI) aluminophosphate with 18membered tetrahedral (18-T) rings by Davis et al., [3,4], which had the largest pores ever found in crystalline microporous compounds. VPI-5 was prepared by a hydrothermal treatment of aluminophosphate gel solutions containing organic template reagents such as di-n-propylamine under a specific condition. Recently, it was pointed out that VPI-5 aluminophosphate was isostructural with AlPO4 -H1 [5], which had been prepared about 40 years ago by d’Yvoire [6] without using organic template. In the present study, AlPO4 -H1
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 3 1 4 - 1
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is prepared without using organic template more efficiently. We use the microwave heating instead of conventional heating in a furnace. Recently, much attention has been paid to the microwave heating in hydrothermal synthesis [7–20,11,22–26]. The advantages of microwave heating are as follows: microwave can raise the temperature of the reactant to a desired range quickly in a few minutes. The heat is supposedly induced by the friction of molecular rotation enhanced by microwave irradiation, and thus, it is possible to heat the reactants selectively and homogeneously from the inside. Some reports [12–14] showed that the reaction rate was 1 or 2 orders faster than that of a conventional heating. The first study of microwave synthesis of AlPO4 was made of the cobalt substituted derivative CoAlPO4 -5 using triethylamine as a template agent [10]. The time required for the crystallization of aluminophosphate hydrous gels was found to be accelerated from 1–2 days to 20 min using microwave heating [12–14]. The microwave effects for the synthesis of AlPO4 -5(AFI) were found in the rapid crystallization [14–20], narrow particle size distribution [12,13], and the large particle sizes of the obtained crystals [10,11]. Kodaira et al. [21] reported the dependence of the morphology of AlPO4 -5 on the pH of the starting gels in the synthesis using microwave heating. In this study, the microwave heating is applied to the synthesis of AlPO4 -H1 to accelerate the crystallization of the gels without using templates. Recently we have reported the template-free synthesis of AlPO4 -H3(APC) by a conventional heating [27]. The microwave heating will also be applied to the template-free synthesis of AlPO4 -H3 and -H2(AHT), which have 8-T and 10-T rings, respectively [28,29].
2. Experimental 2.1. Synthesis Aluminophosphate hydrous gels were prepared using three different kinds of aluminum sources, amorphous aluminum hydroxide (Aldrich, 54% Al2 O3 ), gibbsite (Wako, 65% Al2 O3 ), and boeh-
mite (DISPAL from Vista, 80% Al2 O3 ), and subjected to microwave heating. The microwave solvothermal equipment used was made by Shikoku Instrumentation Co. Ltd. The maximum power and the frequency of the microwave were 1.45 kW and 2.45 GHz, respectively. The reactor was made of Pyrex glass (36 mm in inner diameter and 250 mm in length) and the temperature was monitored and controlled by a fiber thermometer (FX8000, ANRITSU) placed in the reactor. AlPO4 -H1 was prepared using amorphous aluminum hydroxide, which was dispersed in deionized water, and then orthophosphoric acid and hydrochloric acid were added to make hydrous gels with various molar compositions (Al2 O3 /P2 O5 / HCl/H2 O). The gels were hydrothermally heated by microwave irradiation. After the reaction, the products were separated by centrifugation, washed with water, and dried at room temperature. AlPO4 H2 was also prepared by a similar manner except that gibbsite was used as an aluminum source instead of amorphous aluminum hydroxide. AlPO4 -H3 was prepared using boehmite as an aluminum source. The AlPO4 hydrous gels were prepared by mixing a 1 M boehmite dispersion in water and a 1 M orthophosphoric acid solution in various ratios; ion-exchanged water was added to the mixture to adjust the final concentrations. About 0.02 wt.% of AlPO4 -H3 seed crystals were added to the gel, and the mixture was stirred for 3 h at 25 °C. The mixed gels were then crystallized by microwave heating at 90 °C for 1 h, which were then washed with water and dried at room temperature in air. The dried AlPO4 -H3 was converted into AlPO4 -D by the thermal treatment at 300 °C for 3 h in air. 2.2. Analyses X-ray powder diffraction (XRD) data were measured using a diffractometer of MAC SCIENCE M18XHF-SRA with graphite monochromated Cu Ka radiation. Nitrogen adsorption isotherms were measured at liquid nitrogen temperature by a homemade computer controlled volumetric apparatus. AlPO4 -H1 and -D were degassed at 200 °C for 3 h in vacuum; AlPO4 -H2 was degassed at 60 °C for 5 h in vacuum prior to the measurement. Adsorption
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161
3.1. Synthesis of AlPO4 -H1
isotherms of water vapor were measured at 25 °C on a gravimetric apparatus constructed with a Cahn electric balance (model 1000). Chemical analysis was performed using an inductively coupled plasma spectrometer (Seiko Instruments SPS1500VR) on the samples dissolved in a mixture of hydrochloric acid and nitric acid solutions. The scanning electron micrographs were taken with JEOL-6640F SEM.
Single phase AlPO4 -H1 was obtained when amorphous aluminum hydroxide was used as an aluminum source, and the molar ratio of the starting aluminophosphate gels (Al2 O3 :P2 O5 :HCl: H2 O) was in the range of 1:0.6–0.8:0.8–1.2:50. The optimum molar ratio of the gel was 1:0.8:1:50 (Table 1, #1). If the molar ratio P2 O5 /Al2 O3 > 0:8 and HCl/Al2 O3 < 0:8, AlPO4 -H1 was accompanied by AlPO4 -H4 and AlPO4 -H2, respectively (Table 1, #3 and #4). The temperature range for the crystallization of the single phase of AlPO4 -H1 was very narrow. If the reaction temperature was above 125 °C, tridymite type AlPO4 was formed with AlPO4 -H1, and the amount of the tridymite
3. Results and discussion Table 1 summarizes the molar ratios of starting aluminophoshate gels, crystallization conditions and the products obtained.
Table 1 Microwave synthesis of AlPO4 -H1, -H2, and -H3 in various gel compositions with (þ) and without () stirring or seed Al sourcesa
Seed
Stirring
Temp. (°C)
Time (h)
Products
50 50 50 50 50 50
A A A A A A
þ
125 125 125 125 130 115
1.0 1.0 1.0 1.0 1.0 1.0
1.0 1.0
50 50
G B
125 125
1.0 1.0
H1 H1, H3 H1, H4 H1, H2 H1, tridymite H1, amorphous Al(OH)3 H1, gibbsite H4
0.95 0.80 0.95 1.00 0.95 0.95 0.95 0.95 0.95
0.6 0.6 1.0 0.6 0.6 0.6 0.6 0.6 0.6
50 50 50 50 50 50 50 50 50
G G G G G G G A B
þ
118 118 118 118 118 125 114 118 118
0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67
H2 H1, H1, H2, H3 H2, H2, H1, H4
1.1 1.1 1.1 1.5 0.5
1.0 1.0 1.0 1.0 1.0
0 0 0 0 0
222 222 222 222 222
B B B B B
þ þ þ þ
þ þ þ þ
90 90 90 90 90
1.0 1.0 1.0 1.0 1.0
#23
1.1
1.0
0
222
A
þ
90
1.0
#24
1.1
1.0
0
222
G
þ
90
1.0
H3 Boehmite Boehmite H3 Variscite, metavariscite Amorphous Al(OH)3 Gibbsite
Reaction
Gel compositions (mol) Al2 O3
P2 O5
HCl
#1 #2 #3 #4 #5 #6
1.0 1.0 1.0 1.0 1.0 1.0
0.80 0.80 1.00 0.80 0.80 0.80
1.0 1.0 1.0 0.6 1.0 1.0
#7 #8
1.0 1.0
0.80 0.80
#9 #10 #11 #12 #13 #14 #15 #16 #17
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
#18 #19 #20 #21 #22
a
H2 O
A, amorphous Al(OH)3 ; G, gibbsite; B, boehmite.
H2 H2 H4 tridymite gibbsite H2
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increased with the increase of the temperature (Table 1, #5). If the crystallization was carried out at temperatures below 115 °C, the reactant amorphous aluminum hydroxide remained partly unreacted (Table 1, #6). The gel should not be stirred during crystallization. The agitation of the gel under microwave heating produced AlPO4 -H1 associated with AlPO4 -H3 as a minor phase (Table 1, #2). The selection of the aluminum source is important; AlPO4 -H1 was obtained as single phase only when amorphous aluminum hydroxide was used. The use of gibbsite gave AlPO4 -H1 with gibbsite remaining unreacted (Table 1, #7), while boehmite yielded AlPO4 -H4 (Table 1, #8). AlPO4 H4 was prepared by d’Yvoire [6] for the first time, and the crystal structure was recently determined by Poojary et al. [30], which was found to be a condensed aluminum phosphate phase with a composition of AlPO4 H2 O. The product #8 of Table 1 was assigned on the basis of the XRD data given by d’Yvoire [6]. The XRD pattern of AlPO4 -H1 obtained in this study is shown in Fig. 1(a). The diffraction peaks were indexed on the basis of a hexagonal cell with , in agreement a ¼ 18:995ð9Þ and c ¼ 8:124ð8Þ A with the XRD data reported by d’Yvoire [6] and the data for VPI-5 by Davis et al. [3]. Duncan et al. [5] reported a template-free synthesis of AlPO4 -H1 using amorphous aluminum hydroxide and conventional heating at a higher temperature of
Fig. 1. X-ray diffraction patterns of AlPO4 -H1 (a), -H2 (b), and -H3 (c) obtained by microwave heating.
140 °C for 4 h. For the preparation of VPI-5 in the presence of organic templates, boehmite was used as an aluminum source [4,31,32]; the preparation conditions should be determined specifically in terms of parameters such as time/temperature regimes, method and conditions of mixing. In the present method using microwave heating, AlPO4 H1 can be obtained at a relatively low temperature of 125 °C for a short crystallization period of 1 h by a simple microwave heating. 3.2. Synthesis of AlPO4 -H2 The single phase AlPO4 -H2 was obtained when gibbsite was used as an aluminum source, and the molar ratio of the starting aluminophosphate gels (Al2 O3 :P2 O5 :HCl:H2 O) was 1:0.95:0.6:50 (Table 1, #9). When the molar ratio HCl/Al2 O3 P 0:6 and Al2 O3 /P2 O5 < 0:95, AlPO4 -H1 was co-crystallized with AlPO4 -H2 (Table 1, #10 and #11). When the molar ratio Al2 O3 /P2 O5 > 0:95, AlPO4 -H4 was co-crystallized (Table 1, #12) with AlPO4 -H2. The agitation of the gel during the crystallization should be prohibited. If the gel was stirred under microwave heating, AlPO4 -H3 was obtained as a single phase (Table 1, #13). The crystallization temperature for the AlPO4 -H2 single phase synthesis was determined to be in a range of 115–120 °C. When the temperature rose above 120 ° C, the tridymite type AlPO4 was co-crystallized (Table 1, #14); when the crystallization was carried out at temperatures below 115 °C, the reactant gibbsite remained unreacted in part (Table 1, #15). The use of amorphous aluminum hydroxide as an aluminum source instead of gibbsite yielded a mixture of AlPO4 -H1 and -H2 as a minor phase (Table 1, #16). The use of boehmite gave AlPO4 -H4 (Table 1, #17). The XRD data of the obtained AlPO4 -H2 (Fig. 1(b)) were in agreement with that reported by d’Yvoire [6], and could be indexed on the basis of an orthorhombic cell with a ¼ 16:184ð5Þ, b ¼ . Duncan et al. [5] 9:914ð3Þ, c ¼ 8:134ð4Þ A attempted to prepare AlPO4 -H2 using amorphous aluminum hydroxide under template-free conditions by a conventional heating. However, an impure AlPO4 -H2 was obtained along with AlPO4 H3 and -H4. Single phase AlPO4 -H2 was prepared using dipentylamine as template reagent by Li and
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Davis [29]. In this study using microwave heating, the single phase AlPO4 -H2 was successfully obtained in the absence of organic templates in a very narrow crystallization temperature range of 115– 120 °C. 3.3. Synthesis of AlPO4 -H3 AlPO4 -H3 was obtained as a single phase when boehmite was used as an aluminum source, and the molar ratio of the starting aluminophosphate gels Al2 O3 /P2 O5 P 1:0 (Table 1, #18 and #21). A dilute gel was used compared with the gels used in the syntheses of AlPO4 -H1 and -H2. It should be noted that the crystallization temperature was as low as 90 °C, and crystallization completed within 1 h. When the molar ratio Al2 O3 /P2 O5 < 1:0, variscite and metavariscite were formed (Table 1, #22). For the synthesis of AlPO4 -H3, the agitation of the gels during crystallization was rather essential, and the addition of seed crystals was needed. Without the agitation of the gel during crystallization and without the addition of seed crystals, the reactant boehmite remained unreacted even with the molar ratio Al2 O3 /P2 O5 P 1:0 (Table 1, #19 and #20). The XRD pattern (Fig. 1(c)) can be indexed on the basis of an orthorhombic unit cell of a ¼ 19:348ð7Þ, , in good agreement b ¼ 9:731ð3Þ, c ¼ 9:759ð5Þ A with that reported by d’Yvoire. We have already succeeded in the preparation of AlPO4 -H3 under template-free and conventional heating conditions at 90 °C for 3 h [27]. Note that the crystallization time was much reduced to less than 1 h by using the microwave heating. The use of different kinds of aluminum sources gave different types of AlPO4 ’s; amorphous aluminum hydroxide, gibbsite, and boehmite gave AlPO4 -H1, -H2, and -H3, respectively. This can be explained in terms of the difference in the re-
163
activity of the aluminum sources. The reactivity of the aluminum sources with phosphoric acid will decrease in the following order; boehmite > amorphous aluminum hydroxide > gibbsite. Only boehmite gave AlPO4 -H3 at a crystallization temperature as low as 90 °C, while the other sources needed higher reaction temperatures of 118 °C (Table 1, #13) for gibbsite; Duncan et al. [5] reported that AlPO4 -H3 was obtained at 140 °C using amorphous aluminum hydroxide. Gibbsite can give AlPO4 -H1, but partly remained unreacted if the same reaction conditions as that used for amorphous aluminum hydroxide was used (Table 1, #7). AlPO4 -H2 is prepared from gibbsite in a narrow temperature range between the temperatures suitable for the crystallization of AlPO4 -H1 and -H3. Gibbsite is not so reactive compared with amorphous aluminum hydroxide, but the crystallization temperature for AlPO4 -H2 is lower than that of AlPO4 -H1. This is the reason for the narrow crystallization temperature range, and gibbsite can be used for the synthesis of AlPO4 -H2, if the crystallization temperature is optimized for the single phase formation. 3.4. Chemical analysis and scanning electron microscopy Chemical analyses were performed on the AlPO4 samples calcined at 900 °C, and the results are listed in Table 2. As can be seen from the table, all the AlPO4 samples obtained in this study had the stoichiometric composition Al2 O3 /P2 O5 1. The SEM pictures of the samples are shown in Fig. 2. The AlPO4 -H1 and VPI-5 prepared by conventional heating have a needle-like morphology and are aggregated into bundles [5,33]. The AlPO4 -H1 crystals obtained in this study also showed a needle-like morphology, but the crystals
Table 2 Chemical analysis data for AlPO4 -H1, -H2, and -H3 AlPO4 H1 H2 H3
Ignition loss (wt.%) 24.4 16.7 18.7
Al2 O3 (wt.%) 42.8 42.4 43.2
P2 O5 (wt.%) 56.5 57.8 56.2
Total (wt.%) 99.3 100.2 99.4
Molar ratio Al2 O3 /P2 O5
H2 O/AlPO4
1.06 1.02 1.07
2.23 1.36 1.58
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Fig. 2. SEM photographs of AlPO4 -H1 (a,b), -H2 (c), and -H3 (d) obtained by microwave heating.
were separated, forming small cotton-ball-like aggregates as shown in Fig. 2(a) and (b). It appears that the microwave heating gives rise to homogeneous nucleation, and leads to a different aggregation mode of AlPO4 -H1. AlPO4 -H2 has a needle-like, or ribbon-like morphology, and the crystals are aggregated into bundles as shown in Fig. 2(c). Duncan et al. [5] reported that AlPO4 -H2 prepared from amorphous aluminum hydroxide had a needle-like morphology similar to that of AlPO4 -H1. AlPO4 -H3 was obtained as spherical aggregates of platelet crystals of about 2 lm, as shown in Fig. 2(d). 3.5. Thermal stability and structural transformation AlPO4 -H1 is isotypic with VPI-5, and undergoes a topotactic transformation to AlPO4 -8(AET) with 14-T rings [34] under a mild thermal treatment. It was reported that AlPO4 -H1 was transformed to AlPO4 -8 at about 100 °C in air, whereas VPI-5 transformed at an elevated temperature of
about 550 °C [5,33]. The thermal stability of the AlPO4 -H1 obtained by microwave was very similar to that prepared by conventional heating. The transformed AlPO4 -8 was stable up to 1000 °C in air. It was found that the transformation of AlPO4 H1 to AlPO4 -8 depended on the atmosphere. The transformation of AlPO4 -H1 was suppressed up to 400 °C, if the sample was first dehydrated by evacuation and then heated as reported by Kenny et al. [35]. AlPO4 -H2 is much less stable against heating, and undergoes a topotactic transformation to the tridymite type AlPO4 at temperatures above 100 °C [29]. AlPO4 -H2 prepared by microwave heating was also transformed to the tridymite at 110 °C. The transformation proceeded similarly in air as well as in vacuum conditions. AlPO4 -H3 prepared by microwave heating was dehydrated and irreversibly converted into AlPO4 -D(APD) (orthorhombic; a ¼ 19:242ð9Þ, b ¼ ) at 200 °C, which revers8:579ð6Þ, c ¼ 9:786ð8Þ A ibly adsorbed water and changed into AlPO4 -H6.
K. Kunii et al. / Microporous and Mesoporous Materials 52 (2002) 159–167
These transformation behaviors are in agreement with those reported by d’Yvoire [6]. 3.6. Adsorption properties The nitrogen adsorption–desorption isotherms of AlPO4 -H1, -H2, and -D measured at liquid nitrogen temperature are shown in Fig. 3. The isotherm for AlPO4 -H1 was of typical Type I (Langmuir type) for microporous solids, and the Langmuir surface area was determined to be 580 m2 /g. Both of AlPO4 -H2 and -D showed a small specific surface area of <15 m2 /g. It appeared that the nitrogen adsorption occurred only on the external surface of the above two types of crystals. The adsorption–desorption isotherms for water measured at 25 °C are shown in Fig. 4. The adsorption isotherms had threshold vapor pressures for the steep increases in the amount of adsorption at relative pressures of P =P0 ¼ 0:020, 0.025, and 0.120 for AlPO4 -H1, -H2, and -D, respectively. AlPO4 -H1 adsorbed 0.32 ml/g of water by three consecutive adsorption steps, and the isotherm showed little hysteresis in the desorption. The three-step adsorption observed is consistent with the adsorption isotherm reported by Kenny et al. [35] for VPI-5. The first step at P =P0 ¼ 0:020 corresponds to the adsorption to strong adsorption sites, and the second step at P =P0 ¼ 0:055 is interpreted in terms of the adsorption to the
165
framework Al atoms to form the octahedral coordination. The gradual adsorption at the third step is attributed to the process of the rearrangement and densification of the hydrogen-bonded water structure. As mentioned in the forgoing paragraph, AlPO4 -H2 and -D derived from AlPO4 H3 by the transformation at 200 °C showed a very small specific surface area for nitrogen adsorption. On the contrary, they showed large adsorption capacities for water at 25 °C, 0.18 and 0.15 ml/g, respectively. Li and Davis [29] reported that the total adsorption capacities of water of VPI-5 and AlPO4 -H2 prepared by conventional heating are 0.31 and 0.19 ml/g, respectively, consistent with the values obtained in this study. These amounts of water are also in good agreement with the molar ratios H2 O/AlPO4 given in Table 1. The water adsorption isotherms of AlPO4 -H2 and -D showed large hysteresis. The pores of AlPO4 -H2 and -D are composed of elongated 10-T and 8-T rings, respectively, and the narrower diameters have dimensions comparable to that of water molecules [36]. The large hysteresis observed for AlPO4 -H2 and -D could be attributed to the narrow diameter of the elongated rings. In our previous study, we showed that the AlPO4 -D prepared by conventional heating had a large water adsorption capacity of 0.20 ml/g [27]. The reason of the difference in the total adsorption capacity of AlPO4 -D’s prepared by microwave heating and
Fig. 3. Nitrogen adsorption–desorption isotherms of AlPO4 -H1 (a), -H2 (b), and -D (c) prepared by microwave heating; () adsorption and ( ) desorption.
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Fig. 4. Water adsorption–desorption isotherms of AlPO4 -H1 (a), -H2 (b), and -D (c) at 25 °C; () adsorption and ( ) desorption.
conventional heating is not clear. It might be caused by the difference in the morphology. The crystals obtained by microwave heating are spherical aggregates (Fig. 2(d)), while the crystals by conventional heating are separated platelets. The latter crystals may have more open pores accessible from the outer surface by water. 4. Conclusions Microporous crystals AlPO4 -H1, -H2, and -H3 have been prepared as single phases by microwave heating under template-free conditions in a relatively short crystallization time. The aluminophosphate hydrous gels prepared using different kinds of aluminum sources such as amorphous aluminum hydroxide, gibbsite, and boehmite, gave different types of AlPO4 ’s; AlPO4 -H1, -H2, and -H3, respectively. The crystallization temperature should be controlled in a narrow range of 115–125 and 115–120 °C for AlPO4 -H1 and -H2, respectively. The conditions with agitation, addition of HCl and seed crystals are also important factors to obtain single phases. Acknowledgements This study has been partially supported by grant-in-aid for Scientific Research (no. 13CE2002)
of the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
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