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Synthesis and characterization of a new microporous aluminophosphate [Al2P2O8][OCH2CH2NH3] with an open-framework analogous to AlPO4-D
Microporous and Mesoporous Materials 39 (2000) 281±289
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Synthesis and characterization of a new microporous aluminophosphate [Al2P2O8][OCH2CH2NH3] with an open-framework analogous to AlPO4-D Kaixue Wang, Jihong Yu, Guangshan Zhu, Yongchun Zou, Ruren Xu * Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, People's Republic of China Received 15 December 1999; accepted 21 March 2000
Abstract A new microporous aluminophosphate [Al2 P2 O8 ][OCH2 CH2 NH3 ], denoted APO-CJ3 (CJ3: China, Jilin University, number 3), has been prepared from aqueous as well as non-aqueous systems. The as-synthesized product is characterized by scanning electron microscopy, X-ray powder diraction, 27 Al and 31 P magic-angle-spinning NMR, thermogravimetric and dierential thermal analyses. The structure has been solved by single-crystal X-ray diraction analysis. b 8:583(3) A, c 19:705(4) A and APO-CJ3 crystallizes in the orthorhombic space group Pbca, with a 9:993(2) A, Z 8. The structure resembles microporous aluminophosphate AlPO4 -D, and is featured by chains of four-membered rings that are connected by UDUD linkages. The compound contains two kinds of Al atoms, one is four-coordinated and the other is six-coordinated linking another six-coordinated Al atom by the linkages of bridging O atom of ethanolamine template. Upon removal of the template above 450°C, the structure of APO-CJ3 transforms to AlPO4 D. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Aluminophosphate; APO-CJ3; Hydrothermal; Microporous; AlPO4-D
1. Introduction The ®rst discovery of microporous aluminophosphates by Wilson et al. [1,2] in 1982 provided opportunities for the synthesis of novel compounds with special structures. So far, a great number of aluminophosphates, including microporous AlPO4 -n with an Al/P ratio equal to 1, and AlPO4 with an Al/P ratio lower than unity [3±5], have been developed in aqueous and non-aqueous systems. These materials show vast structural diversities with *
Corresponding author. Fax: +86-431-567-1975. E-mail address: [email protected] (R. Xu).
1-D chains [6±8], 2-D layers [9±14] and 3-D open frameworks containing various pore structures including 8- [15], 10- [16], 12- [17], 14- [18], 18- [19] and 20-membered rings (MRs) [20]. These materials have potential use in the ®eld of gas absorption, catalysis, ion exchange and molecule recognition, like aluminosilicate zeolites. Typically, the primary building units for these aluminophosphates are tetrahedral AlO4 and PO4 units which share a common oxygen vertex; however, in some cases, Al atoms are ®ve- or six-coordinated, sharing oxygen with extra-framework OH or H2 O species [21±23]. These materials are synthesized with organic amine templates that act as space ®ller, structural
1387-1811/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 0 ) 0 0 2 0 7 - 9
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directing agent or charge balancer for some anionic frameworks. In this work, using ethanolamine (HOCH2 CH2 NH2 ) as a template, we have synthesized a new open-framework aluminophosphate (APO-CJ3) both in aqueous and non-aqueous systems. In the structure, ethanolamine not only plays the above roles but also acts as a ligand coordinated to two Al atoms through oxygen atom. The framework of APO-CJ3 resembles that of AlPO4 -D which can be obtained by the calcination of AlPO4 -C at 250°C [24]. On calcination, APOCJ3 transforms to AlPO4 -D [25] above 450°C. 2. Experimental 2.1. Synthesis APO-CJ3 was typically synthesized from an aqueous system using ethanolamine (ETA) as template. Aluminum triisopropoxide [(i-PrO)3 Al] and phosphoric acid (H3 PO4 85%, in water) were used as the aluminum and phosphorous sources, respectively. The typical synthesis procedure was as follows: 1.25 g (i-PrO)3 Al was ®rst dispersed into 10 ml distilled water with stirring, 0.58 ml phosphoric acid was then added dropwise, the mixture was stirred until homogenous. Finally, 2.0 ml ETA was added, and a gel was formed. After stirring for about 1 h, the gel was loaded into te¯on-lined stainless steel autoclaves, and heated in an oven for ®ve days. The resulting crystals were ®ltered, washed with distilled water thoroughly and dried at
ambient temperature. The yield was about 89%. The synthesis procedure preformed in non-aqueous system was the same as above with the organic solvents instead of distilled water. Table 1 presents the synthesis conditions and results. 2.2. Characterization Powder X-ray diraction (XRD) pattern were recorded on a Siemens D5005 diractometer with Scanning elecCu-Ka radiation (k 1:5418 A). tron micrograph (SEM) was taken on a Hitachi X-650B scanning electron microscope. Thermogravimetric (TG) and dierential thermal analysis (DTA) were performed on a Perkin±Elmer TGA7 thermogravimetric analyzer and a DTA-1700 differential thermal analyzer, respectively, in air at a heating rate of 10 K minÿ1 . Inductively coupled plasma (ICP) analysis was performed on a Perkin± Elmer Optima 3300 DV ICP instrument, which gave a P/Al ratio of the product of 1:1. Elemental analysis was carried out on a Perkin±Elmer 2400 element analyzer, and the following results, N:4.12, C:7.58 and H:2.36% were obtained. The magic-angle-spinning (MAS) NMR spectra were recorded on a Varian Unity-400 Spectrometer. The 27 Al NMR spectra were obtained at a resonance frequency of 104.2 MHz with a spinning rate of 4 kHz. The 31 P NMR spectra were taken at 161.9 MHz with a spinning rate of 3 kHz. Chemical shifts were referenced to 1 M AlCl3 /H2 O for 27 Al and 85% H3 PO4 for 31 P. For the pulse width
Table 1 Summary of synthesis conditions and results for APO-CJ3 Runsa
P2 O5
ETA
Solvent
pH
Result
1 2 3 4 5 6 7 8 9 10
1.4 1.4 1.4 1.2±5.0 1.4 1.4 1.4 1.4 1.4 1.4
1.0±2.0 3.0±6.0 7.0±12.0 11.0 6.0 6.0 6.0 6.0 6.0 8.0
180H2 O 180H2 O 180H2 O 145H2 O 53 EG 22 tEG 17 TEG 32s-BuOH 32i-BuOH 37 EGME
3±6 6±10 10±12 12±6 ± ± ± ± ± ±
AlPO4 -21 AlPO4- 21 APO-CJ3 APO-CJ3 crystal APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase
ETA: Ethanolamine, tEG: Triethyleneglycol. TEG: Tetraethyleneglycol and EGME: Ethylene glycol monomethyl ether. a Al2 O3 1.0; reaction temperature 180°C; reaction time 5 days.
K. Wang et al. / Microporous and Mesoporous Materials 39 (2000) 281±289
of 20.0 and 10.0 ls a total of 5000 and 4000 scans were taken for 27 Al and 31 P, respectively. The recycle delay times for both 27 Al and 31 P were 0.5 s. 2.3. Structure determination A suitable single crystal with dimensions 0:096 0:072 0:064 mm3 was selected for X-ray diraction analysis. The intensity data were collected on a Siemens SMART diractometer equipped with a CCD bidimensional detector using graphitemonochromatic MoKa radiation (k(MoKa) at a temperature of 20°C 2°C. Data 0.71073 A) processing was accomplished with SAINT processing program [26]. The structure was solved in the space group Pbca by direct methods and re®ned on F 2 by full-matrix least squares using the SHELXTL crystallographic software package [27]. The heaviest atoms were easily located and all the non-hydrogen atoms were re®ned anisotropically. The hydrogen atoms of the amine were re®ned with restraints applied to maintain the C±H and N±H geometries. Details of the data collection and structure re®nement are listed in Table 2.
3. Results and discussion The P2 O5 /Al2 O3 and ETA/Al2 O3 ratios in the starting mixture have a great in¯uence on the resulting products. In aqueous system without the change in the ratio of phosphoric acid, highly pure APO-CJ3 crystals are crystallized when the ETA/Al2 O3 ratio is higher than 7.0. When the ETA/Al2 O3 ratio is less than 2.0, pure AlPO4 -21 crystals are obtained. With the ETA/Al2 O3 ratio ranging from 2.0 to 6.0, it is noted that the APOCJ3 phase gradually predominates with the reduction of the AlPO4 -21 phase. The pH value was aected by the ETA/Al2 O3 and P2 O5 /Al2 O3 ratios. It is known that microporous aluminophosphates are mainly prepared in weakly acidic or neutral conditions, but seldom in the basic conditions. APO-CJ3 is an exception, which can only be prepared with a pH value higher than 10. In addition, APO-CJ3 phase not only can be synthesized in the aqueous system, but also can be crystallized in various non-aqueous systems using EG, tEG, TEG, s-BuOH, etc. as solvents instead of water.
Table 2 Crystal data and structure re®nement for APO-CJ3 Identi®cation code Empirical formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions Volume Z, calculated density Absorption coecient F(0 0 0) Crystal size (mm3 ) Theta range for data collection Limiting indices Re¯ections collected/unique Completeness to theta 23:36 Re®nement method Data/restraints/parameters Goodness of ®t on F 2 Final R indices I > 2r I R indices (all data) Largest di. peak and hole
283
APO-CJ3 Al2 P2 O9 C2 H7 N 305.99 293(2) K 0.71073 A Orthorhombic, Pbca a 90° a 9:9926(19) A, b 90° b 8:583(3) A, c 90° c 19:705(4) A, 3 1690.0(7) A
8, 2.397 mg/m3 0.766 mmÿ1 1232 0:096 0:072 0:064 2.07±23.36° ÿ10 6 h 6 10, ÿ9 6 k 6 4, ÿ21 6 l 6 18 3207/1199 [R int 0:0673] 97.3% Full-matrix least-squares on F 2 1199/0/145 0.838 R1 0:0375, wR2 0:0728 R1 0:0716, wR2 0:0785 ÿ3 0.413 and ÿ0.399e A
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Fig. 1. Scanning electron micrograph of as-synthesized APOCJ3.
The SEM photograph of APO-CJ3 is shown in Fig. 1. The crystals appear to have deformed hexagonal prism shape. The experimental powder XRD pattern of the as-synthesized crystals and simulated XRD patterns based on single-crystal X-ray diraction analysis are given on Fig. 2. Their peak positions are consistent with each other. This suggests the purity of as-synthesized
Fig. 2. Experimental and simulated XRD patterns of as-synthesized APO-CJ3.
product. The dierences in intensity may be due to some preferred orientation eects on the experimental XRD pattern. Single-crystal X-ray diraction analysis indicates that APO-CJ3 crystallizes in the ortho rhombic space group Pbca, with a 9:993(2) A, b 8:583(3) A, c 19:705(4) A and Z 8. Tables 3 and 4 list the atomic coordinates, and selected
Table 3 2 ´ 103 ) for APO-CJ3a Atomic coordinates (´104 ) and equivalent isotropic displacement parameters (A P(1) P(2) Al(1) Al(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) N(1) C(1) C(2) a
x
y
z
U(eq)
6483(2) 9332(2) 9337(2) 6503(2) 6533(4) 8160(3) 6124(3) 10523(3) 8935(4) 7820(3) 9074(4) 5390(3) 9718(4) 6344(5) 8583(6) 7314(6)
2262(2) 2611(2) 3873(2) 3989(2) 592(4) 3637(4) 3341(4) 2946(4) 923(4) 2724(4) 4926(4) 2603(4) 2877(4) 5085(5) 4203(6) 5001(7)
4410(1) 3026(1) 4571(1) 2997(1) 4165(2) 2801(2) 3804(2) 2571(2) 2919(2) 4712(2) 5433(2) 4939(2) 3754(2) 5700(2) 6024(3) 6265(3)
15(1) 14(1) 15(1) 13(1) 17(1) 16(1) 15(1) 16(1) 20(1) 14(1) 15(1) 16(1) 16(1) 21(1) 11(1) 23(1)
U(eq) is de®ned as one third of the trace of the orthogonalized Uij tensor.
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Table 4 and angles (°) for APO-CJ3 Selected bond lengths (A) P(1)±O(1) P(1)±O(8) P(2)±O(9) P(2)±O(4) Al(1)±O(6) Al(1)±O(1)#1 Al(1)±O(8)#3 Al(1)±Al(1)#2 Al(2)±O(2) Al(2)±O(4)#4 O(4)±Al(2)#6 O(7)±C(1) O(8)±Al(1)#7 C(1)±C(2) O(1)±P(1)±O(6) O(6)±P(1)±O(8) O(6)±P(1)±O(3) O(9)±P(2)±O(5) O(5)±P(2)±O(4) O(5)±P(2)±O(2) O(6)±Al(1)±O(9) O(9)±Al(1)±O(1)#1 O(9)±Al(1)±O(7)#2 O(6)±Al(1)±O(8)#3 O(1)#1±Al(1)±O(8)#3 O(6)±Al(1)±O(7) O(1)#1±Al(1)±O(7) O(8)#3±Al(1)±O(7) O(5)#1±Al(2)±O(3) O(5)#1±Al(2)±O(4)#4 O(3)±Al(2)±O(4)#4 P(2)±O(2)±Al(2) P(2)±O(4)±Al(2)#6 P(1)±O(6)±Al(1) C(1)±O(7)±Al(1) P(1)±O(8)±Al(1)#7 O(7)±C(1)±C(2)
1.513(4) 1.538(4) 1.503(4) 1.518(4) 1.829(4) 1.890(4) 1.909(4) 2.892(3) 1.727(4) 1.735(4) 1.735(4) 1.409(6) 1.909(4) 1.517(7) 110.1(2) 108.0(2) 110.4(2) 110.2(2) 107.7(2) 108.0(2) 93.0(2) 95.0(2) 94.3(2) 91.3(2) 170.2(2) 90.4(2) 86.8(2) 86.7(2) 109.7(2) 107.3(2) 107.7(2) 140.3(2) 157.1(2) 144.3(2) 124.4(3) 127.4(2) 110.5(5)
P(1)±O(6) P(1)±O(3) P(2)±O(5) P(2)±O(2) Al(1)±O(9) Al(1)±O(7)#2 Al(1)±O(7) Al(2)±O(5)#1 Al(2)±O(3) O(1)±Al(1)#5 O(5)±Al(2)#5 O(7)±Al(1)#2 N(1)±C(2)
1.516(4) 1.553(4) 1.517(4) 1.531(4) 1.862(4) 1.893(4) 1.942(4) 1.723(4) 1.727(4) 1.890(4) 1.723(4) 1.893(4) 1.477(8)
O(1)±P(1)±O(8) O(1)±P(1)±O(3) O(8)±P(1)±O(3) O(9)±P(2)±O(4) O(9)±P(2)±O(2) O(4)±P(2)±O(2) O(6)±Al(1)±O(1)#1 O(6)±Al(1)±O(7)#2 O(1)#1±Al(1)±O(7)#2 O(9)±Al(1)±O(8)#3 O(7)#2±Al(1)±O(8)#3 O(9)±Al(1)±O(7) O(7)#2±Al(1)±O(7) O(5)#1±Al(2)±O(2) O(2)±Al(2)±O(3) O(2)±Al(2)±O(4)#4 P(1)±O(1)±Al(1)#5 P(1)±O(3)±Al(2) P(2)±O(5)±Al(2)#5 C(1)±O(7)±Al(1)#2 Al(1)#2±O(7)±Al(1) P(2)±O(9)±Al(1) N(1)±C(2)±C(1)
114.9(2) 109.1(2) 104.17(19) 109.5(2) 112.7(2) 108.6(2) 96.0(2) 171.5(2) 87.6(2) 91.1(2) 84.3(2) 176.0(2) 82.1(2) 113.1(2) 111.1(2) 107.8(2) 126.1(2) 148.2(2) 166.9(3) 122.3(3) 97.9(2) 147.4(2) 109.6(5)
Symmetry transformations used to generate equivalent atoms: #1 ÿ x 3=2, y 1=2, z; #2 ÿ x 2, ÿy 1, ÿz 1; #3 x 1=2, ÿy 1=2, ÿz 1; #4 x ÿ 1=2, y, ÿz 1=2 and #5 ÿ x 3=2, y ÿ 1=2, z; #6 x 1=2, y, ÿz 1=2; #7 x ÿ 1=2, ÿy 1=2, ÿz 1.
bond lengths and angles. APO-CJ3 has the formula [Al2 P2 O8 ][OCH2 CH2 NH3 ]. Each asymmetric unit (Fig. 3) contains two crystallographically dierent Al atoms and two crystallographically dierent P atoms. Each P atom is tetrahedrally coordinated, and shares four oxygen atoms with adjacent Al atoms. Each Al(2) atom shares an oxygen atom with one P(1) and three oxygen atoms with three P(2) atoms, but each Al(1) atom is octahedrally coordinated to six O atoms, two of which are donated by two ethanolamine mole-
cules. In the distorted octahedral Al(1)O6 , the Al(1)±O distances range from 1.829 to 1.909 A except for the long distance of 1.942 A between Al(1) and O(7) atoms of ETA. It is noted that the O(7) atom donated by ETA bridges two Al(1) atoms, this causes a small O(7)±Al(1)±O(7A) angle of 82.1°. In the Al(2)O4 tetrahedron, the Al(2)±O and bond lengths are in the range 1.723±1.735 A the O±Al(2)±O angles are in the range 107.7± 113.1°, which are in good agreement with berlinite and the [28]. The P±O distances (1.503±1.553 A)
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Fig. 3. Asymmetric unit of APO-CJ3 showing the connection of ETA and octrahedral Al(1)O6 by the bridging O(7) atom.
O±P±O angles (104.2±114.9°) are all typical for aluminophosphate materials. The framework structure of APO-CJ3 is shown in Fig. 4a. It is noted that the structure of APOCJ3 resembles that of AlPO4 -D (Fig. 4b), which is constructed from alternation of tetrahedral AlO4 and PO4 , respectively. The framework of APOCJ3 is featured by chains of 4-MRs which are connected by UDUD linkages (Fig. 4c). However, the 8-MRs of APO-CJ3 are more regular than that of AlPO4 -D, which are elongated in one direction. In the structure, the ethanolamine molecules projecting into the 8-MR channels parallel to the [0 1 0] direction uphold the 8-MR channels to be regular and ®ll the void space inside the framework. The organic template also acts as a ligand that coordinates to two Al(1) atoms from two adjacent 4.8.8 nets parallel to the ac plane. Ignoring the ethanolamines, the structure of APOCJ3 can be regarded as a 3-D framework based on up±down linkage among 4.8.8 nets that are parallel to the ac plane. Within the 4.8.8 net, each 4MR contains one tetrahedral and one octahedral A1, alternating with two tetrahedral P atoms, and the 8-MR have the sequence ±AlVI ±PIV ±AlVI ±PIV ± AlIV ±PIV ±AlIV ±PIV ± (VI: octahedral coordination; IV: tetrahedral coordination). It is noted that the 8-MRs are crinkled, and the 4-MRs are tilted out of the ac plane. The 3-D framework can also be considered built up from up-down linkages of alternation of the 6.6.6 nets and 4.8.8 nets parallel to the ab plane. The nets are crinkled to allow for bonding to the O atoms of ETA, but may become
Fig. 4. (a) Crystal structure of APO-CJ3 showing the 8-MR channels along the b direction; (b) the open-framework of AlPO4 -D viewing along the b direction; and (c) the chain formed by the up±down linkages of 4-MRs.
much regular when the template molecules are removed. To investigate the thermal properties of APOCJ3, TG-DTA analyses were carried out between 30°C and 712°C. The results are shown in Fig. 5. The TG curve shows a major weight loss occurring at 400°C. The total weight loss is 20.6%, which is consistent with the calculated value (20%) of one ETA per formula of the compound. The DTA curve exhibits one exothermic eect at 480°C and one endothermic eect at 650°C. The exothermic peak corresponds to the decomposition of the
K. Wang et al. / Microporous and Mesoporous Materials 39 (2000) 281±289
287
Fig. 5. TG and DTA curves of APO-CJ3.
template ETA, and the endothermic peak may be due to a phase transformation. Powder XRD analyses of the samples calcined at dierent temperatures are shown in Fig. 6. It is noted that APO-CJ3 transforms to AlPO4 -D at 450°C. Useful informations on the framework structure and structural changes during the removal of
Fig. 6. XRD patterns of samples calcined at (a) 250, (b) 350, (c) 450, (d) 550, (e) 650, and (f) 750°C for 2 h.
the template are obtained by (MAS) NMR analysis. The 27 Al and 31 P (MAS) NMR spectra are given in Fig. 7. The 27 Al spectrum of as-synthesized APO-CJ3 shows two signals at 27.5 and ÿ10.7 ppm, corresponding to the four- and sixcoordinated Al atoms, respectively. The signal at 27.5 ppm due to the four-coordinated Al atoms shifts to the high ®eld compared with that of AlPO4 -5 (35.3 ppm), AlPO4 -11 (33.3 ppm) and AlPO4 -17 (31.2 ppm) [29]. The high®eld chemical shift may be due to the organic template that coordinates to the framework. Owing to the sixcoordinated Al atoms, there are two crystallographically dierent P sites in the structure. This is also con®rmed by 31 P (MAS) NMR spectrum, which shows two main signals at ÿ16.0 and ÿ32.7 ppm. During the process of heating, calcination at about 450°C results in the transformation of all aluminum and phosphorous atoms into tetrahedral coordination. So, the 27 Al and 31 P (MAS) NMR spectra of the calcined samples exhibit only one signal at 33.0 and ÿ32.3 ppm, respectively. On comparing the 31 P (MAS) NMR spectra, it is found that the peak at ÿ16.0 ppm disappears for the calcined sample. Thus, the chemical shift at ÿ16.0 ppm can be assigned to the P(1) atom which is linked to three six coordinated Al atoms and one four coordinated Al atom in APO-CJ3. The shift at ÿ32.7 ppm is due to the P(2) atom linked to one
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Fig. 7. 27 Al (left) and 31 P (right) (MAS) NMR spectra of APO-CJ3: (a) as-synthesized and (b) calcined samples. The asterisks denote spinning sidebands.
six-coordinated Al atom and three four coordinated Al atoms, which has little change with the removal of the template. It is noted that the unit cell dimensions of APOCJ3, AlPO4 -C and AlPO4 -D are very similar showing the close relationship between them. APO-CJ3 becomes AlPO4 -D when the template is removed at about 450°C, and AlPO4 -C can transform to AlPO4 -D by calcination at around 250°C. The nature of structural dierence between AlPO4 -C and AlPO4 -D is the up±down linkages of two 4-MRs.
synthesized in both aqueous and non-aqueous systems. Highly pure crystals can be obtained only in the aqueous system with a pH value higher than 10, which is unusual for the preparation of microporous aluminophosphate materials. In the synthesis of APO-CJ3, the organic template not only acts as a space ®ller, structure director and charge balancer, but also acts as a ligand coordinating to two octahedral Al atoms. The framework of APOCJ3 is featured by the chains of four-membered rings that are connected by UDUD linkages. Upon calcination the structure of APO-CJ3 transforms to AlPO4 -D with the removal of the templates.
4. Conclusions
Acknowledgements
A new open-framework aluminophosphate [Al2 P2 O8 ][OCH2 CH2 NH3 ] (APO-CJ3) has been
We are grateful to the Pan Deng Foundation Project of China for the ®nancial support.
K. Wang et al. / Microporous and Mesoporous Materials 39 (2000) 281±289
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[14] J. Yu, I.D. Williams, S. Qiu, O. Terasaki, R. Xu, Supramol. Sci. 5 (1998) 297. [15] J.J. Pluth, J.V. Smith, Nature 318 (1985) 165. [16] J.M. Bennett, J.V. Smith, Z. Kristallogr. 171 (1985) 65. [17] J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth, J.V. Smith, Am. Chem. Soc. Symp. Ser. 218 (1983) 109. [18] E.T.C. Vogt, J.W. Richardson Jr., J. Solid State Chem. 87 (1990) 469. [19] M.E. Davis, C. Saldarriaga, C. Montes, J. Garces, C. Crowder, Nature 331 (1988) 698. [20] Q.S. Huo, R.R. Xu, S.G. Li, Z.G. Ma, J.M. Thomas, R.H. Jones, A.M. Chippindale, J. Chem. Soc. Chem. Commun. (1992) 875. [21] J.J. Pluth, J.V. Smith, J.M. Bennett, J.P. Cohen, Acta Cryst. C40 (1984) 2008. [22] J.M. Bennett, J.M. Cohen, G. Artioli, J.J. Pluth, J.V. Smith, Inorg. Chem. 24 (1985) 188. [23] J.J. Pluth, J.V. Smith, J.M. Bennett, Acta Cryst. C42 (1986) 283. [24] E.B. Keller, W.M. Meier, R.M. Kirchner, Solid State Ionics 43 (1990) 93. [25] M. Ryoo, H. Chon, J. Chem. Soc. Faraday Trans. 93 (1997) 3259. [26] SMART and SAINT (software package), Siemens Analytical X-ray Instruments, Inc., Madison, WI, 1996. [27] SHELXTL version 5.1, Siemens Industrial Automation, Inc., 1997. [28] D. Schwarzenbach, Z. Kristallogr. 123 (1966) 161. [29] C.S. Blackwell, R.L. Patton, J. Phys. Chem. 88 (1984) 6135.
www.elsevier.nl/locate/micromeso
Synthesis and characterization of a new microporous aluminophosphate [Al2P2O8][OCH2CH2NH3] with an open-framework analogous to AlPO4-D Kaixue Wang, Jihong Yu, Guangshan Zhu, Yongchun Zou, Ruren Xu * Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, People's Republic of China Received 15 December 1999; accepted 21 March 2000
Abstract A new microporous aluminophosphate [Al2 P2 O8 ][OCH2 CH2 NH3 ], denoted APO-CJ3 (CJ3: China, Jilin University, number 3), has been prepared from aqueous as well as non-aqueous systems. The as-synthesized product is characterized by scanning electron microscopy, X-ray powder diraction, 27 Al and 31 P magic-angle-spinning NMR, thermogravimetric and dierential thermal analyses. The structure has been solved by single-crystal X-ray diraction analysis. b 8:583(3) A, c 19:705(4) A and APO-CJ3 crystallizes in the orthorhombic space group Pbca, with a 9:993(2) A, Z 8. The structure resembles microporous aluminophosphate AlPO4 -D, and is featured by chains of four-membered rings that are connected by UDUD linkages. The compound contains two kinds of Al atoms, one is four-coordinated and the other is six-coordinated linking another six-coordinated Al atom by the linkages of bridging O atom of ethanolamine template. Upon removal of the template above 450°C, the structure of APO-CJ3 transforms to AlPO4 D. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Aluminophosphate; APO-CJ3; Hydrothermal; Microporous; AlPO4-D
1. Introduction The ®rst discovery of microporous aluminophosphates by Wilson et al. [1,2] in 1982 provided opportunities for the synthesis of novel compounds with special structures. So far, a great number of aluminophosphates, including microporous AlPO4 -n with an Al/P ratio equal to 1, and AlPO4 with an Al/P ratio lower than unity [3±5], have been developed in aqueous and non-aqueous systems. These materials show vast structural diversities with *
Corresponding author. Fax: +86-431-567-1975. E-mail address: [email protected] (R. Xu).
1-D chains [6±8], 2-D layers [9±14] and 3-D open frameworks containing various pore structures including 8- [15], 10- [16], 12- [17], 14- [18], 18- [19] and 20-membered rings (MRs) [20]. These materials have potential use in the ®eld of gas absorption, catalysis, ion exchange and molecule recognition, like aluminosilicate zeolites. Typically, the primary building units for these aluminophosphates are tetrahedral AlO4 and PO4 units which share a common oxygen vertex; however, in some cases, Al atoms are ®ve- or six-coordinated, sharing oxygen with extra-framework OH or H2 O species [21±23]. These materials are synthesized with organic amine templates that act as space ®ller, structural
1387-1811/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 0 ) 0 0 2 0 7 - 9
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directing agent or charge balancer for some anionic frameworks. In this work, using ethanolamine (HOCH2 CH2 NH2 ) as a template, we have synthesized a new open-framework aluminophosphate (APO-CJ3) both in aqueous and non-aqueous systems. In the structure, ethanolamine not only plays the above roles but also acts as a ligand coordinated to two Al atoms through oxygen atom. The framework of APO-CJ3 resembles that of AlPO4 -D which can be obtained by the calcination of AlPO4 -C at 250°C [24]. On calcination, APOCJ3 transforms to AlPO4 -D [25] above 450°C. 2. Experimental 2.1. Synthesis APO-CJ3 was typically synthesized from an aqueous system using ethanolamine (ETA) as template. Aluminum triisopropoxide [(i-PrO)3 Al] and phosphoric acid (H3 PO4 85%, in water) were used as the aluminum and phosphorous sources, respectively. The typical synthesis procedure was as follows: 1.25 g (i-PrO)3 Al was ®rst dispersed into 10 ml distilled water with stirring, 0.58 ml phosphoric acid was then added dropwise, the mixture was stirred until homogenous. Finally, 2.0 ml ETA was added, and a gel was formed. After stirring for about 1 h, the gel was loaded into te¯on-lined stainless steel autoclaves, and heated in an oven for ®ve days. The resulting crystals were ®ltered, washed with distilled water thoroughly and dried at
ambient temperature. The yield was about 89%. The synthesis procedure preformed in non-aqueous system was the same as above with the organic solvents instead of distilled water. Table 1 presents the synthesis conditions and results. 2.2. Characterization Powder X-ray diraction (XRD) pattern were recorded on a Siemens D5005 diractometer with Scanning elecCu-Ka radiation (k 1:5418 A). tron micrograph (SEM) was taken on a Hitachi X-650B scanning electron microscope. Thermogravimetric (TG) and dierential thermal analysis (DTA) were performed on a Perkin±Elmer TGA7 thermogravimetric analyzer and a DTA-1700 differential thermal analyzer, respectively, in air at a heating rate of 10 K minÿ1 . Inductively coupled plasma (ICP) analysis was performed on a Perkin± Elmer Optima 3300 DV ICP instrument, which gave a P/Al ratio of the product of 1:1. Elemental analysis was carried out on a Perkin±Elmer 2400 element analyzer, and the following results, N:4.12, C:7.58 and H:2.36% were obtained. The magic-angle-spinning (MAS) NMR spectra were recorded on a Varian Unity-400 Spectrometer. The 27 Al NMR spectra were obtained at a resonance frequency of 104.2 MHz with a spinning rate of 4 kHz. The 31 P NMR spectra were taken at 161.9 MHz with a spinning rate of 3 kHz. Chemical shifts were referenced to 1 M AlCl3 /H2 O for 27 Al and 85% H3 PO4 for 31 P. For the pulse width
Table 1 Summary of synthesis conditions and results for APO-CJ3 Runsa
P2 O5
ETA
Solvent
pH
Result
1 2 3 4 5 6 7 8 9 10
1.4 1.4 1.4 1.2±5.0 1.4 1.4 1.4 1.4 1.4 1.4
1.0±2.0 3.0±6.0 7.0±12.0 11.0 6.0 6.0 6.0 6.0 6.0 8.0
180H2 O 180H2 O 180H2 O 145H2 O 53 EG 22 tEG 17 TEG 32s-BuOH 32i-BuOH 37 EGME
3±6 6±10 10±12 12±6 ± ± ± ± ± ±
AlPO4 -21 AlPO4- 21 APO-CJ3 APO-CJ3 crystal APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase APO-CJ3 phase
ETA: Ethanolamine, tEG: Triethyleneglycol. TEG: Tetraethyleneglycol and EGME: Ethylene glycol monomethyl ether. a Al2 O3 1.0; reaction temperature 180°C; reaction time 5 days.
K. Wang et al. / Microporous and Mesoporous Materials 39 (2000) 281±289
of 20.0 and 10.0 ls a total of 5000 and 4000 scans were taken for 27 Al and 31 P, respectively. The recycle delay times for both 27 Al and 31 P were 0.5 s. 2.3. Structure determination A suitable single crystal with dimensions 0:096 0:072 0:064 mm3 was selected for X-ray diraction analysis. The intensity data were collected on a Siemens SMART diractometer equipped with a CCD bidimensional detector using graphitemonochromatic MoKa radiation (k(MoKa) at a temperature of 20°C 2°C. Data 0.71073 A) processing was accomplished with SAINT processing program [26]. The structure was solved in the space group Pbca by direct methods and re®ned on F 2 by full-matrix least squares using the SHELXTL crystallographic software package [27]. The heaviest atoms were easily located and all the non-hydrogen atoms were re®ned anisotropically. The hydrogen atoms of the amine were re®ned with restraints applied to maintain the C±H and N±H geometries. Details of the data collection and structure re®nement are listed in Table 2.
3. Results and discussion The P2 O5 /Al2 O3 and ETA/Al2 O3 ratios in the starting mixture have a great in¯uence on the resulting products. In aqueous system without the change in the ratio of phosphoric acid, highly pure APO-CJ3 crystals are crystallized when the ETA/Al2 O3 ratio is higher than 7.0. When the ETA/Al2 O3 ratio is less than 2.0, pure AlPO4 -21 crystals are obtained. With the ETA/Al2 O3 ratio ranging from 2.0 to 6.0, it is noted that the APOCJ3 phase gradually predominates with the reduction of the AlPO4 -21 phase. The pH value was aected by the ETA/Al2 O3 and P2 O5 /Al2 O3 ratios. It is known that microporous aluminophosphates are mainly prepared in weakly acidic or neutral conditions, but seldom in the basic conditions. APO-CJ3 is an exception, which can only be prepared with a pH value higher than 10. In addition, APO-CJ3 phase not only can be synthesized in the aqueous system, but also can be crystallized in various non-aqueous systems using EG, tEG, TEG, s-BuOH, etc. as solvents instead of water.
Table 2 Crystal data and structure re®nement for APO-CJ3 Identi®cation code Empirical formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions Volume Z, calculated density Absorption coecient F(0 0 0) Crystal size (mm3 ) Theta range for data collection Limiting indices Re¯ections collected/unique Completeness to theta 23:36 Re®nement method Data/restraints/parameters Goodness of ®t on F 2 Final R indices I > 2r I R indices (all data) Largest di. peak and hole
283
APO-CJ3 Al2 P2 O9 C2 H7 N 305.99 293(2) K 0.71073 A Orthorhombic, Pbca a 90° a 9:9926(19) A, b 90° b 8:583(3) A, c 90° c 19:705(4) A, 3 1690.0(7) A
8, 2.397 mg/m3 0.766 mmÿ1 1232 0:096 0:072 0:064 2.07±23.36° ÿ10 6 h 6 10, ÿ9 6 k 6 4, ÿ21 6 l 6 18 3207/1199 [R int 0:0673] 97.3% Full-matrix least-squares on F 2 1199/0/145 0.838 R1 0:0375, wR2 0:0728 R1 0:0716, wR2 0:0785 ÿ3 0.413 and ÿ0.399e A
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Fig. 1. Scanning electron micrograph of as-synthesized APOCJ3.
The SEM photograph of APO-CJ3 is shown in Fig. 1. The crystals appear to have deformed hexagonal prism shape. The experimental powder XRD pattern of the as-synthesized crystals and simulated XRD patterns based on single-crystal X-ray diraction analysis are given on Fig. 2. Their peak positions are consistent with each other. This suggests the purity of as-synthesized
Fig. 2. Experimental and simulated XRD patterns of as-synthesized APO-CJ3.
product. The dierences in intensity may be due to some preferred orientation eects on the experimental XRD pattern. Single-crystal X-ray diraction analysis indicates that APO-CJ3 crystallizes in the ortho rhombic space group Pbca, with a 9:993(2) A, b 8:583(3) A, c 19:705(4) A and Z 8. Tables 3 and 4 list the atomic coordinates, and selected
Table 3 2 ´ 103 ) for APO-CJ3a Atomic coordinates (´104 ) and equivalent isotropic displacement parameters (A P(1) P(2) Al(1) Al(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) N(1) C(1) C(2) a
x
y
z
U(eq)
6483(2) 9332(2) 9337(2) 6503(2) 6533(4) 8160(3) 6124(3) 10523(3) 8935(4) 7820(3) 9074(4) 5390(3) 9718(4) 6344(5) 8583(6) 7314(6)
2262(2) 2611(2) 3873(2) 3989(2) 592(4) 3637(4) 3341(4) 2946(4) 923(4) 2724(4) 4926(4) 2603(4) 2877(4) 5085(5) 4203(6) 5001(7)
4410(1) 3026(1) 4571(1) 2997(1) 4165(2) 2801(2) 3804(2) 2571(2) 2919(2) 4712(2) 5433(2) 4939(2) 3754(2) 5700(2) 6024(3) 6265(3)
15(1) 14(1) 15(1) 13(1) 17(1) 16(1) 15(1) 16(1) 20(1) 14(1) 15(1) 16(1) 16(1) 21(1) 11(1) 23(1)
U(eq) is de®ned as one third of the trace of the orthogonalized Uij tensor.
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285
Table 4 and angles (°) for APO-CJ3 Selected bond lengths (A) P(1)±O(1) P(1)±O(8) P(2)±O(9) P(2)±O(4) Al(1)±O(6) Al(1)±O(1)#1 Al(1)±O(8)#3 Al(1)±Al(1)#2 Al(2)±O(2) Al(2)±O(4)#4 O(4)±Al(2)#6 O(7)±C(1) O(8)±Al(1)#7 C(1)±C(2) O(1)±P(1)±O(6) O(6)±P(1)±O(8) O(6)±P(1)±O(3) O(9)±P(2)±O(5) O(5)±P(2)±O(4) O(5)±P(2)±O(2) O(6)±Al(1)±O(9) O(9)±Al(1)±O(1)#1 O(9)±Al(1)±O(7)#2 O(6)±Al(1)±O(8)#3 O(1)#1±Al(1)±O(8)#3 O(6)±Al(1)±O(7) O(1)#1±Al(1)±O(7) O(8)#3±Al(1)±O(7) O(5)#1±Al(2)±O(3) O(5)#1±Al(2)±O(4)#4 O(3)±Al(2)±O(4)#4 P(2)±O(2)±Al(2) P(2)±O(4)±Al(2)#6 P(1)±O(6)±Al(1) C(1)±O(7)±Al(1) P(1)±O(8)±Al(1)#7 O(7)±C(1)±C(2)
1.513(4) 1.538(4) 1.503(4) 1.518(4) 1.829(4) 1.890(4) 1.909(4) 2.892(3) 1.727(4) 1.735(4) 1.735(4) 1.409(6) 1.909(4) 1.517(7) 110.1(2) 108.0(2) 110.4(2) 110.2(2) 107.7(2) 108.0(2) 93.0(2) 95.0(2) 94.3(2) 91.3(2) 170.2(2) 90.4(2) 86.8(2) 86.7(2) 109.7(2) 107.3(2) 107.7(2) 140.3(2) 157.1(2) 144.3(2) 124.4(3) 127.4(2) 110.5(5)
P(1)±O(6) P(1)±O(3) P(2)±O(5) P(2)±O(2) Al(1)±O(9) Al(1)±O(7)#2 Al(1)±O(7) Al(2)±O(5)#1 Al(2)±O(3) O(1)±Al(1)#5 O(5)±Al(2)#5 O(7)±Al(1)#2 N(1)±C(2)
1.516(4) 1.553(4) 1.517(4) 1.531(4) 1.862(4) 1.893(4) 1.942(4) 1.723(4) 1.727(4) 1.890(4) 1.723(4) 1.893(4) 1.477(8)
O(1)±P(1)±O(8) O(1)±P(1)±O(3) O(8)±P(1)±O(3) O(9)±P(2)±O(4) O(9)±P(2)±O(2) O(4)±P(2)±O(2) O(6)±Al(1)±O(1)#1 O(6)±Al(1)±O(7)#2 O(1)#1±Al(1)±O(7)#2 O(9)±Al(1)±O(8)#3 O(7)#2±Al(1)±O(8)#3 O(9)±Al(1)±O(7) O(7)#2±Al(1)±O(7) O(5)#1±Al(2)±O(2) O(2)±Al(2)±O(3) O(2)±Al(2)±O(4)#4 P(1)±O(1)±Al(1)#5 P(1)±O(3)±Al(2) P(2)±O(5)±Al(2)#5 C(1)±O(7)±Al(1)#2 Al(1)#2±O(7)±Al(1) P(2)±O(9)±Al(1) N(1)±C(2)±C(1)
114.9(2) 109.1(2) 104.17(19) 109.5(2) 112.7(2) 108.6(2) 96.0(2) 171.5(2) 87.6(2) 91.1(2) 84.3(2) 176.0(2) 82.1(2) 113.1(2) 111.1(2) 107.8(2) 126.1(2) 148.2(2) 166.9(3) 122.3(3) 97.9(2) 147.4(2) 109.6(5)
Symmetry transformations used to generate equivalent atoms: #1 ÿ x 3=2, y 1=2, z; #2 ÿ x 2, ÿy 1, ÿz 1; #3 x 1=2, ÿy 1=2, ÿz 1; #4 x ÿ 1=2, y, ÿz 1=2 and #5 ÿ x 3=2, y ÿ 1=2, z; #6 x 1=2, y, ÿz 1=2; #7 x ÿ 1=2, ÿy 1=2, ÿz 1.
bond lengths and angles. APO-CJ3 has the formula [Al2 P2 O8 ][OCH2 CH2 NH3 ]. Each asymmetric unit (Fig. 3) contains two crystallographically dierent Al atoms and two crystallographically dierent P atoms. Each P atom is tetrahedrally coordinated, and shares four oxygen atoms with adjacent Al atoms. Each Al(2) atom shares an oxygen atom with one P(1) and three oxygen atoms with three P(2) atoms, but each Al(1) atom is octahedrally coordinated to six O atoms, two of which are donated by two ethanolamine mole-
cules. In the distorted octahedral Al(1)O6 , the Al(1)±O distances range from 1.829 to 1.909 A except for the long distance of 1.942 A between Al(1) and O(7) atoms of ETA. It is noted that the O(7) atom donated by ETA bridges two Al(1) atoms, this causes a small O(7)±Al(1)±O(7A) angle of 82.1°. In the Al(2)O4 tetrahedron, the Al(2)±O and bond lengths are in the range 1.723±1.735 A the O±Al(2)±O angles are in the range 107.7± 113.1°, which are in good agreement with berlinite and the [28]. The P±O distances (1.503±1.553 A)
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Fig. 3. Asymmetric unit of APO-CJ3 showing the connection of ETA and octrahedral Al(1)O6 by the bridging O(7) atom.
O±P±O angles (104.2±114.9°) are all typical for aluminophosphate materials. The framework structure of APO-CJ3 is shown in Fig. 4a. It is noted that the structure of APOCJ3 resembles that of AlPO4 -D (Fig. 4b), which is constructed from alternation of tetrahedral AlO4 and PO4 , respectively. The framework of APOCJ3 is featured by chains of 4-MRs which are connected by UDUD linkages (Fig. 4c). However, the 8-MRs of APO-CJ3 are more regular than that of AlPO4 -D, which are elongated in one direction. In the structure, the ethanolamine molecules projecting into the 8-MR channels parallel to the [0 1 0] direction uphold the 8-MR channels to be regular and ®ll the void space inside the framework. The organic template also acts as a ligand that coordinates to two Al(1) atoms from two adjacent 4.8.8 nets parallel to the ac plane. Ignoring the ethanolamines, the structure of APOCJ3 can be regarded as a 3-D framework based on up±down linkage among 4.8.8 nets that are parallel to the ac plane. Within the 4.8.8 net, each 4MR contains one tetrahedral and one octahedral A1, alternating with two tetrahedral P atoms, and the 8-MR have the sequence ±AlVI ±PIV ±AlVI ±PIV ± AlIV ±PIV ±AlIV ±PIV ± (VI: octahedral coordination; IV: tetrahedral coordination). It is noted that the 8-MRs are crinkled, and the 4-MRs are tilted out of the ac plane. The 3-D framework can also be considered built up from up-down linkages of alternation of the 6.6.6 nets and 4.8.8 nets parallel to the ab plane. The nets are crinkled to allow for bonding to the O atoms of ETA, but may become
Fig. 4. (a) Crystal structure of APO-CJ3 showing the 8-MR channels along the b direction; (b) the open-framework of AlPO4 -D viewing along the b direction; and (c) the chain formed by the up±down linkages of 4-MRs.
much regular when the template molecules are removed. To investigate the thermal properties of APOCJ3, TG-DTA analyses were carried out between 30°C and 712°C. The results are shown in Fig. 5. The TG curve shows a major weight loss occurring at 400°C. The total weight loss is 20.6%, which is consistent with the calculated value (20%) of one ETA per formula of the compound. The DTA curve exhibits one exothermic eect at 480°C and one endothermic eect at 650°C. The exothermic peak corresponds to the decomposition of the
K. Wang et al. / Microporous and Mesoporous Materials 39 (2000) 281±289
287
Fig. 5. TG and DTA curves of APO-CJ3.
template ETA, and the endothermic peak may be due to a phase transformation. Powder XRD analyses of the samples calcined at dierent temperatures are shown in Fig. 6. It is noted that APO-CJ3 transforms to AlPO4 -D at 450°C. Useful informations on the framework structure and structural changes during the removal of
Fig. 6. XRD patterns of samples calcined at (a) 250, (b) 350, (c) 450, (d) 550, (e) 650, and (f) 750°C for 2 h.
the template are obtained by (MAS) NMR analysis. The 27 Al and 31 P (MAS) NMR spectra are given in Fig. 7. The 27 Al spectrum of as-synthesized APO-CJ3 shows two signals at 27.5 and ÿ10.7 ppm, corresponding to the four- and sixcoordinated Al atoms, respectively. The signal at 27.5 ppm due to the four-coordinated Al atoms shifts to the high ®eld compared with that of AlPO4 -5 (35.3 ppm), AlPO4 -11 (33.3 ppm) and AlPO4 -17 (31.2 ppm) [29]. The high®eld chemical shift may be due to the organic template that coordinates to the framework. Owing to the sixcoordinated Al atoms, there are two crystallographically dierent P sites in the structure. This is also con®rmed by 31 P (MAS) NMR spectrum, which shows two main signals at ÿ16.0 and ÿ32.7 ppm. During the process of heating, calcination at about 450°C results in the transformation of all aluminum and phosphorous atoms into tetrahedral coordination. So, the 27 Al and 31 P (MAS) NMR spectra of the calcined samples exhibit only one signal at 33.0 and ÿ32.3 ppm, respectively. On comparing the 31 P (MAS) NMR spectra, it is found that the peak at ÿ16.0 ppm disappears for the calcined sample. Thus, the chemical shift at ÿ16.0 ppm can be assigned to the P(1) atom which is linked to three six coordinated Al atoms and one four coordinated Al atom in APO-CJ3. The shift at ÿ32.7 ppm is due to the P(2) atom linked to one
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Fig. 7. 27 Al (left) and 31 P (right) (MAS) NMR spectra of APO-CJ3: (a) as-synthesized and (b) calcined samples. The asterisks denote spinning sidebands.
six-coordinated Al atom and three four coordinated Al atoms, which has little change with the removal of the template. It is noted that the unit cell dimensions of APOCJ3, AlPO4 -C and AlPO4 -D are very similar showing the close relationship between them. APO-CJ3 becomes AlPO4 -D when the template is removed at about 450°C, and AlPO4 -C can transform to AlPO4 -D by calcination at around 250°C. The nature of structural dierence between AlPO4 -C and AlPO4 -D is the up±down linkages of two 4-MRs.
synthesized in both aqueous and non-aqueous systems. Highly pure crystals can be obtained only in the aqueous system with a pH value higher than 10, which is unusual for the preparation of microporous aluminophosphate materials. In the synthesis of APO-CJ3, the organic template not only acts as a space ®ller, structure director and charge balancer, but also acts as a ligand coordinating to two octahedral Al atoms. The framework of APOCJ3 is featured by the chains of four-membered rings that are connected by UDUD linkages. Upon calcination the structure of APO-CJ3 transforms to AlPO4 -D with the removal of the templates.
4. Conclusions
Acknowledgements
A new open-framework aluminophosphate [Al2 P2 O8 ][OCH2 CH2 NH3 ] (APO-CJ3) has been
We are grateful to the Pan Deng Foundation Project of China for the ®nancial support.
K. Wang et al. / Microporous and Mesoporous Materials 39 (2000) 281±289
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