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Synthesis of mesostructured lamellar aluminophosphates in the presence of alkylpyridinium cationic surfactant
Materials Chemistry and Physics 68 (2001) 110–118
Synthesis of mesostructured lamellar aluminophosphates in the presence of alkylpyridinium cationic surfactant Zhong-Yong Yuan a,b,∗ , Tie-Hong Chen a , Jing-Zhong Wang a , He-Xuan Li a a
b
Department of Chemistry, Nankai University, Tianjin 300071, China Beijing Laboratory of Electron Microscopy, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Science, P.O. Box 2724, Beijing 100080, China Received 8 February 2000; received in revised form 18 April 2000; accepted 26 April 2000
Abstract A series of mesostructured lamellar aluminophosphates and silicoaluminophosphates have been synthesized hydrothermally using alkylpyridinium bromide surfactant templating or mixed surfactants of alkylpyridinium and cetyltrimethylammonium bromide. The synthesis of present aluminophosphate-based mesostructured materials can be performed within the following molar gel composition: Al2 O3 :(0.8–1.9) P2 O5 :(0.3–1.0) CPBr:(0.8–4.2) TMAOH:(100–350) H2 O:(0–1.0) SiO2 at various temperatures in the range of 80–150◦ C for 0.5–4 days. The effect of various synthetic factors, such as P/Al ratio, TMAOH/Al ratio, water content, surfactant concentration, synthesis temperature, and reaction time, has been discussed. The addition of Si in the mixture gel requires more base content. The interlayer distance of materials decreases with either increasing water content in the reaction mixture or decreasing alkyl chain length of the templating surfactant alkylpyridinium cation. The mesostructured aluminophosphate-based materials were also prepared with templates mixtures of cetylpyridinium bromide and cetyltrimethylammonium bromide, and their interlayer distance of materials can be finely controlled by changing the CPBr/CTABr molar ratio. 27 Al and 31 P nuclear magnetic resonance spectra have been carried out to understand the local atomic arrangements and coordination environments of these solid products. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Alkylpyridinium bromide; Aluminophosphates; Mesostructured material; NMR spectroscopy
1. Introduction Since the discovery of the M41S silica-based crystalline mesoporous molecular sieves and their preparation in the presence of surfactant [1,2], these materials have attracted increasing attention. Recently, there has been much interest in synthesizing mesoporous aluminophosphates owing to the previous success in the synthesis of microporous aluminophosphates with framework topology identical or similar to aluminosilicate zeolites [3,4]. It has been demonstrated that the liquid-crystal templating approach for the synthesis of mesostructured materials can be extended to aluminophosphates since mesostructured organo-aluminophosphates are expected to be useful for the formation of mesoporous aluminophosphates to enlarge the variety and possible applications of mesoporous materials [5].
∗ Corresponding author. Tel.: +86-10-8264-9453; fax: +86-10-6256-1422. E-mail address: [email protected] (Z.-Y. Yuan).
Lamellar mesostructures of aluminophosphate have been synthesized by using an amphiphilic alkylamine (C10 H21 NH2 ) in tetraethylene glycol [6] and by using dimethylamines with C8 –C16 alkyl groups in water [7], but these materials are thermally unstable. Feng et al. [8] demonstrated the synthesis of the hexagonal mesostructured aluminophosphate in the presence of F− ions and using EtOH–H2 O as a solvent and its phase transition from the hexagonal phase into the lamellar phase was reported. Zhao et al. [9] and Chakraborty et al. [10] also reported the syntheses of thermally stable mesoporous aluminophosphates and silicoaluminophosphates, respectively. The preparations of lamellar and hexagonal mesostructured aluminophosphates (AlPOs) were also reported by Kimura et al. [11,12]. We have synthesized mesoporous silicon and cobaltsubstituted aluminophosphate (SAPO, CoAPO, CoAPSO) molecular sieves using self-assembled ordered organic supramolecular templates [13]. Here, we describe a series of mesostructured lamellar aluminophosphates and with substituted silicon synthesized hydrothermally, using alkylpyridinium bromide surfactant templating or mixed surfactants with cetyltrimethylammonium bromide.
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2. Experimental 2.1. Synthesis The synthesis of aluminophosphate-based mesostructured materials can be performed within the following molar gel composition: Al2 O3 :(0.8–1.9) P2 O5 :(0.3–1.0) CPBr:(0.8–4.2) TMAOH:(100–350) H2 O:(0–0.2) SiO2 . In a typical synthetic procedure, aluminium triisopropoxide was mixed with water under vigorous stirring, followed by the addition of an 85% solution of phosphoric acid, and the mixture was homogenized by stirring for 1 h. Then, cetylpyridinium bromide (CPBr) was added under stirring. Finally, a 10% solution of tetramethylammonium hydroxide (TMAOH) was added dropwise to the mixture, which was then stirred for another 1 h. For the synthesis of silicoaluminophosphate samples, a solution of tetraethyl orthosilicate (TEOS) was added dropwise to the above mixture and stirred for an additional 30 min. The reaction mixture was heated in a stainless-steel autoclave for 24–48 h at various temperatures in the range of 80–150◦ C. Subsequently, the product was filtered, washed with deionized water, and air-dried. 2.2. Characterization X-ray diffraction (XRD) measurements were made on a Rigaku D/max 2500 powder diffractometer with CuK␣ radiation (50 kV, 200 mA), 0.01◦ step size and 4◦ min−1 scanning rate. The morphology of the products were obtained using a Hitachi X-650 scanning electron microscope. Transmission electron microscopy (TEM) was carried out with a Jeol 2010 electron microscope operating at 200 kV. The specimens for TEM were dispersed in alcohol by ultrasonic treatment, then dropped onto a wholly carbon–copper grid. 27 Al and 31 P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were obtained on a Bruker MSL-400 spectrometer at 104.26 and 161.98 MHz, respectively. Magic angle spinning speeds of 10 kHz were used to 27 Al nuclei and 9 kHz to 31 P nuclei. Each 27 Al and 31 P spectra were acquired with 3000 and 16 scans, respectively. A 1 mol l−1 solution of aluminium nitrate and 85% H3 PO4 were used as external references. Thermal analyses were performed on a Rigaku standard type TG–DTA instrument under air atmosphere with a rate of 10◦ C min−1 . Framework infrared spectra were collected on a Perkin-Elmer 684 spectrometer using KBr wafer technique. Elemental analyses were made by conventional chemical analysis methods.
3. Results and discussion 3.1. General characterization Mesostructured aluminophosphates were synthesized in the synthetic system of Al(OPri )3 –H3 PO4 –CPBr–TMAOH–
Fig. 1. XRD patterns of the synthesized samples with different P2 O5 :Al2 O3 ratios of (a) 1.4; (b) 1.6 and (c) 1.8.
H2 O within the molar composition described in Section 2. The powder X-ray diffraction patterns of these products indicate characteristic low-angle peaks typical of mesostructured materials, consisting of one set of equidistant reflections which may be normalized as 0 0 l (l=1, 2, 3) reflections of a single lamellar phase [2] (Fig. 1), as well as reported in mesostructured lamellar materials synthesized with other different methods [14,15]. In the range of diffraction angle 2θ =20–30◦ there are some small broadened reflections which are related to the structure of lamellar AlPO phase. These materials were thermally unstable above 200◦ C revealed by XRD. Fig. 2a shows a scanning electron micrograph (SEM) of AlPO samples synthesized using CPBr as a template at 100◦ C, indicating elementary particles of ≈1 m with a generally spherical morphology. Fig. 2b shows an AlPO sample synthesized using tetradecylpyridinium bromide (TDPBr) as a template at 150◦ C, exhibiting rhombic particles of the size up to 2.7×7.2 m. These results show that the particle’s morphology might be controlled by the synthesis temperature. The TEM photographs of the mesostructured AlPO samples synthesized with cetylpyridinium bromide indicate that clear striped patterns were observed and the repeat distance of the striped patterns was ca. 3.1 nm, being in good agreement with the d001 -spacing recorded by XRD. Solid-state MAS NMR was carried out in order to understand the local structure and environments of both aluminum and phosphorus in these samples. Fig. 3 is the solid-state MAS NMR spectra of 27 Al and 31 P nuclei of the as-synthesized samples with different P/Al contents, respectively, showing the existence of both four-coordinate Al and P in the materials. There are two kinds of signals with different chemical shifts presented in the 27 Al spectra of the aluminophosphate samples: ∼40 and 0 ppm. The signals around 40 ppm were assigned to tetrahedral aluminum bonded to phosphorus atoms via oxygen bridges, and those of around 0 ppm could be attributed to octahedral coordinated Al to water and/or PO4 groups. The chemical shifts of the peaks in the 31 P spectra of AlPO indicate a tetrahedral coordination of the phosphorus atoms in the structure: one
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Fig. 2. SEM photographs of mesostructured lamellar aluminophosphates synthesized with different templates at different temperature. (a) C16 PBr, 100◦ C; (b) C14 PBr, 150◦ C.
main peak at −20.4 ppm. Additionally, the broadened and overlapped resonance in the range of 0 to −19 ppm in the 31 P spectra might be related to the distribution of phosphorus among various sites with slightly different local coordination environments [13,16], or probably due to amorphous impurity [17]. Mortlock et al. [18,19] reported that boldface P atoms in Al(H2 O)5 (H2 PO4 ), Al(H2 O)4 (H2 PO4 )2 , and Al(H2 O)4 (H2 PO4 )(H3 PO4 ) are observed at −9.5 ppm, that P atoms in Al(H2 O)5 (H3 PO4 ), Al(H2 O)4 (H3 PO4 )2 , and Al(H2 O)4 (H3 PO4 )(H2 PO4 ) are observed at −12.6 ppm, and that P atoms in (HO)2 P{OAl(H2 O)5 }2 are observed at −16.5 ppm. We believe that the signals in the range of 0 to −19 ppm may be related to the incomplete condensation of PO4 units and the presence of six-coordinated Al coor-
Fig. 3. 27 Al and 31 P MAS NMR spectra of AlPO sample with different P2 O5 :Al2 O3 ratios: (a) 1.4; (b) 1.6 and (c) 1.8.
dinated to some water molecules, as mentioned by Kimura et al. [12]. The framework infrared spectra of the AlPO materials showed several peaks in the structural region of 1400–400 cm−1 , which were similar to those observed for mesostuctured silica-based materials [2]. On the basis of data for microporous AlPO4 -n materials [20], the peaks at 1225, 1160, 1030, 750, 640, 520, and 485 cm−1 could be due to the asymmetric stretching of Al–O–P, the symmetric stretching of Al–O–P, and the bending of O–T–O (T:Al or P), respectively. 3.2. Al2 O3 :P2 O5 :TMAOH ratio Under the current synthetic system, increasing P2 O5 :Al2 O3 ratio required the corresponding addition of organic base TMAOH in the precursor solution, in order to prepare the well-defined mesostructured materials at a certain region of pH value. Mesolamellar products with high degree of crystallinity could be synthesized via tuning the concentration of TMAOH when P2 O5 :Al2 O3 ratio was very high, for instance, P2 O5 :Al2 O3 ratio of 1.9 (correspondingly TMAOH:Al2 O3 ratio should be in the region of 3.6–4.5 when H2 O:Al2 O3 ratio was 335). The preferable pH value of mixture system was 8.2. The experimental results suggest that mesostructured lamellar aluminophosphate can be synthesized in the molar composition ranges of P2 O5 :Al2 O3 =1.2–1.9 and TMAOH:Al2 O3 =2.6–4.5 and H2 O:Al2 O3 =335. Fig. 4 shows the synthetic phase diagram of the Al(OPri )3 –H3 PO4 –CPBr–TMAOH–H2 O system at 100◦ C with CPBr:Al2 O3 =0.35–1.0 and H2 O:Al2 O3 =335 (the part in the range of continuous line), revealing narrow region of Al2 O3 :P2 O5 :TMAOH ratio suitable for the synthesis of mesostructured lamellar AlPO. Moreover, the presence of an organic base such as TMAOH is essential because replacement of TMAOH with a solution of NaOH
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ter or surfactant cations [15]. When the P2 O5 :Al2 O3 ratio in the mixture was low, one broad resonance band was observed in the range of 0 to −19 ppm, which decreased, even disappeared with increase in the P2 O5 :Al2 O3 ratio. 3.3. Silicon incorporation
Fig. 4. Part of the phase diagram of the Al(OPri )3 –H3 PO4 –CPBr– TMAOH–H2 O system at 100◦ C.
led to only amorphous products, even one half replacement of TMAOH by NaOH results in amorphous products. As shown in Fig. 1, as well as evaluated by 27 Al and 31 P MAS NMR spectroscopy (Fig. 3), the crystallinity degree of the synthesized products of mesostructured lamellar AlPO increased with the P2 O5 :Al2 O3 ratio, suggesting a high P2 O5 :Al2 O3 ratio is proper to preparing well-defined mesolamellar AlPO materials. Based on Figs. 1 and 3, with increasing P2 O5 :Al2 O3 ratio, the crystallinity degree of the products increases, and the intensity of the resonance signal around 0 ppm in the 27 Al spectra of mesolamellar AlPO decreases. This suggests that the tetrahedral:octahedral ratio of Al is related to the crystallinity of the products and their P/Al ratios. The six-coordinate Al might be associated with amorphous aluminum phosphate or alumina [17]. In the 31 P spectra, one main peak is present at −20.4 ppm, corresponding to four-coordinate phosphorus coordinated to three AlO4 tetrahedra with the fourth oxygen bound to hydrogen, wa-
Fig. 5.
27 Al
and
31 P
The introduction of Si during synthesis could result in the formation of mesostructured lamellar silicoaluminophosphate (SAPO) materials. TEOS was used as a source of silicon to be SiO2 :Al2 O3 =0.2 in the mixture. Thus, optimum synthetic condition changed to P2 O5 :Al2 O3 ratio of 0.8–1.6 with corresponding TMAOH:Al2 O3 ratio of 2.1–4.0. The range of dotted line in Fig. 4 shows the phase diagram of Al2 O3 –P2 O5 –TMAOH system with SiO2 :Al2 O3 =0.2, H2 O:Al2 O3 =335, and CPBr:Al2 O3 =0.35–0.80 at 100◦ C. As to the synthesized SAPO products, their crystallinity increases with P2 O5 :Al2 O3 ratio. Fig. 5 shows the 27 Al and 31 P MAS NMR spectra of SAPO samples with different P2 O5 :Al2 O3 ratios. All SAPO samples show four-coordinated and six-coordinated Al, and four-coordinated P. The intensity of 31 P NMR signals increases with P2 O5 :Al2 O3 ratios. 3.4. The effect of surfactant The degree of crystallinity of the products is related with the surfactant concentration in the synthetic gel, as revealed by the XRD data. The intensity of the diffraction peaks (0 0 l) increases significantly with the concentration of CPBr. The increased crystallinity is also reflected in the 27 Al and 31 P MAS NMR spectra (Fig. 6). As shown in Fig. 6, with the increase of CPBr:Al2 O3 ratio, the resonance signal of six-coordinated Al in the 27 Al NMR spectra of synthesized samples decreases, and the ratio of tetrahe-
MAS NMR spectra of SAPO samples with different P2 O5 :Al2 O3 ratios: (a) 1.0 and (b) 1.6.
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Fig. 6.
27 Al
and
31 P
MAS NMR spectra of lamellar aluminophosphates with different CPBr:Al2 O3 ratios: (a) 0.35 and (b) 0.80.
dral to octahedral aluminium increases. Besides one peak at −20.6 ppm corresponding to tetrahedral phosphorus in the 31 P NMR spectra of the products synthesized with low CPBr:Al2 O3 ratio, the broad peaks at −12.4 and −0.5 ppm from amorphous material was observed. The intensity of the signal around −12.4 and −0.5 ppm decreases with the increase of CPBr:Al2 O3 ratio, even disappear. The highest quality products can be obtained at a high CPBr:Al2 O3
ratio, i.e. CPBr:Al2 O3 =1.0. A higher concentration of the surfactant in the synthetic mixture is beneficial for the crystallization of mesostructured aluminophosphate materials. The XRD patterns of silicoaluminophosphate materials synthesized with different concentrations of surfactant CPBr also show the same case that the intensity of the diffraction peak increases with the concentration of surfactant.
Fig. 7. XRD patterns of lamellar aluminophosphates synthesized from the composition Al2 O3 :1.8 P2 O5 :x C16 PBr:(1−x) CTABr:4.0 TMAOH:335 H2 O. (a) x=0; (b) x=0.2; (c) x=0.4; (d) x=0.6 and (e) x=0.8.
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The repeat distance between lamella and lamella in the present material can change with surfactant alkyl-chain length variation. Correspondingly, the d-spacing recorded by XRD generally increases with surfactant chain length under comparable reaction conditions, as well as the similar results for the silica-based materials such as MCM-41 [2] and HMS [21]. For example, when TDPBr or dodecylpyridinium bromide (DDPBr) was used as template in synthesis instead of CPBr, the d-spacing of the first diffraction line (0 0 l) of the products decreased from 3.1 to 2.9 or 2.7 nm, respectively. Ozin et al. [22] have reported the synthesis of MCM-41 type mesoporous silica using self-assembling micellar aggregates of a mixture of cetylpyridinium chloride and cetyltrimethylammonium chloride, in which the d100 -spacing of the mesoporous silica could be fine-tuned between 4.1 and 4.3 nm by variation of the molar ratio of these two surfactants in the mixture. Introduced this synthetic route to the synthesis of mesostructured aluminophosphate based materials, similar results could be obtained. Fig. 7 shows the XRD patterns of the synthesized products using a mixture of CPBr and cetyltrimethylammonium bromide (CTABr) with different molar ratio as a template under the reaction composition of Al2 O3 :1.8 P2 O5 :x CPBr:(1−x) CTABr:4.0 TMAOH:335 H2 O at 100◦ C for 48 h. As shown in Fig. 7, with the increase of the content of CPBr in the mixed CPBr/CTABr surfactant system, the d001 -spacing gradually decreased from 3.15 to 3.04 nm. Such a relationship of the d-spacing variation with the change of the CPBr/CTABr ratio was also shown in Fig. 8. Additionally,
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Fig. 8. d001 -spacing as a function of the C16 PBr/CTABr ratio as determined by XRD.
using mixtures of two alkylpyridinium cationic surfactants with different alkyl chain lengths could also succeed the fine-tuning of the d-spacings on a subangstrom length scale. 3.5. Water content The single lamellar structure phase could be synthesized at high water content, such as H2 O:Al2 O3 =335 or higher. When the water content decreased to H2 O:Al2 O3 =230 and 133, the obtained products were mixed phases of two lamellar structures, and the d-spacings increased with the decrease of the water content in the synthetic gel, and the diffraction peaks shifted to lower 2θ angles. Fig. 9 shows the XRD patterns of the obtained products synthesized
Fig. 9. XRD patterns of samples synthesized from different compositions: (a) Al2 O3 :1.2 P2 O5 :0.9 C16 PBr:1.3 TMAOH:230 H2 O; (b) Al2 O3 :1.2 P2 O5 :0.9 C16 PBr: 0.9 TMAOH:133 H2 O.
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Fig. 10. XRD patterns of samples synthesized at different temperatures: (a) 100◦ C; (b) 130◦ C and (c) 150◦ C.
from different molar compositions of Al2 O3 :1.2 P2 O5 :0.9 CPBr:1.3 TMAOH:230 H2 O and Al2 O3 :1.2 P2 O5 :0.9 CPBr:0.9 TMAOH:133 H2 O at 130◦ C for 28 h. 3.6. Synthesis temperature A constant range of crystalline temperature is needed for the synthesis of mesolamellar aluminophosphates. The optimum temperature range is from 80 to 150◦ C. Crystallizing at ambient temperature results in the precipitation of amorphous solids. Fig. 10 shows the XRD patterns of the AlPO samples synthesized at different temperatures from the composition mixture of Al2 O3 :1.7 P2 O5 :0.8 TDPBr:3.6 TMAOH:335 H2 O. As shown in Fig. 10, with the increasing of the synthesis temperature, the intensity of diffraction lines increased, indicating that a relatively higher temperature is favorable to prepare mesolamellar aluminophosphates. It is interesting that the morphology of the product particle changed from spherical to rhombic with the increase of the synthesis temperature (see Fig. 2).
Fig. 12 presents the 27 Al and 31 P spectra of the intermediates in the synthesis of mesolamellar AlPO. It can be observed that the transferring from the original mixture to the crystalline lamellar AlPO needs only a short time. The 27 Al spectra of amorphous solids collected from the original mixture show two peaks around −43 and −8 ppm, corresponding to four-coordinate and six-coordinate aluminum, respectively. After treated 1.5 h, the population of six-coordinate Al decreased, and the ratio of four-coordinate
3.7. Reaction kinetics Fig. 11 shows the crystallinity curves of mesolamellar AlPO and SAPO from respective molar compositions of Al2 O3 :1.7 P2 O5 :0.8 CPBr:3.6 TMAOH:335 H2 O and Al2 O3 :1.6 P2 O5 : 0.8 CPBr:4.0 TMAOH:335 H2 O:0.2 SiO2 at 100◦ C. Lamellar aluminophosphate based materials with relatively highly crystallinity could be obtained after hydrothermal crystallization of 2 h. The hydrothermal reactions were practically complete after 5 h.
Fig. 11. Degree of crystallinity of the intermediates in the synthesis of (a) AlPO and (b) SAPO estimated on the basis of the relative intensity of the (0 0 1) reflections.
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broad peak around −12.5 ppm, corresponding to the resonance signal of amorphous material. After hydrothermally treated 1.5 h, this amorphous signal decreased much, and one narrow resonance peak was present around −20.5 ppm. The amorphous signal could gradually disappear with time, and the resonance at −20.5 ppm increased, reflecting the transformation of the coordinate state of 31 P during synthesis. Fig. 13 shows the 27 Al and 31 P spectra of the intermediates in the synthesis of mesolamellar SAPO, in which the similar results as AlPO samples could be observed. After 3 h, the hydrothermal reactions were almost complete for SAPO samples.
4. Conclusions
Fig. 12. 27 Al and 31 P MAS NMR spectra of the intermediates in the synthesis of mesolamellar AlPO samples. Asterisks denotes spinning sidebands.
to six-coordinate increased quickly. After 5 h, the ratio of four-coordinate to six-coordinate remained stable, suggesting the crystallization has finished at this moment. In the 31 P MAS NMR spectra, the original mixture showed one
A series of mesostructured lamellar aluminophosphate and silicoaluminophosphate materials have been synthesized by supramolecular templating of alkylpyridinium cation. Optimal synthesis conditions have been established. Higher P2 O5 :Al2 O3 ratio, higher surfactant concentration, and higher synthesis temperature are beneficial to crystallize the mesolamellar structure. The addition of Si in the mixture gel requires more base content of TMAOH. The interlayer distance of the materials and their particle morphology can be different by the variation of the alkyl chain length of the alkylpyridinum template, the ratio of two mixed surfactant with cetyltrimethylammonium bromide, the water content in the original mixture, or a change in the synthesis temperature. The results of MAS NMR spectroscopy reveals the coordination state of aluminum and phosphorus in the structure of the synthesized products, and provides strong evidence for the variation of the coordination state of framework elements during the synthesis.
Acknowledgements This research work was supported by the National Natural Science Foundation of China. We are grateful to the Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Belgium for its supporting of NMR experiments. References
Fig. 13. 27 Al and 31 P MAS NMR spectra of the intermediates in the synthesis of mesolamellar SAPO samples. Asterisks denotes spinning sidebands.
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Synthesis of mesostructured lamellar aluminophosphates in the presence of alkylpyridinium cationic surfactant Zhong-Yong Yuan a,b,∗ , Tie-Hong Chen a , Jing-Zhong Wang a , He-Xuan Li a a
b
Department of Chemistry, Nankai University, Tianjin 300071, China Beijing Laboratory of Electron Microscopy, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Science, P.O. Box 2724, Beijing 100080, China Received 8 February 2000; received in revised form 18 April 2000; accepted 26 April 2000
Abstract A series of mesostructured lamellar aluminophosphates and silicoaluminophosphates have been synthesized hydrothermally using alkylpyridinium bromide surfactant templating or mixed surfactants of alkylpyridinium and cetyltrimethylammonium bromide. The synthesis of present aluminophosphate-based mesostructured materials can be performed within the following molar gel composition: Al2 O3 :(0.8–1.9) P2 O5 :(0.3–1.0) CPBr:(0.8–4.2) TMAOH:(100–350) H2 O:(0–1.0) SiO2 at various temperatures in the range of 80–150◦ C for 0.5–4 days. The effect of various synthetic factors, such as P/Al ratio, TMAOH/Al ratio, water content, surfactant concentration, synthesis temperature, and reaction time, has been discussed. The addition of Si in the mixture gel requires more base content. The interlayer distance of materials decreases with either increasing water content in the reaction mixture or decreasing alkyl chain length of the templating surfactant alkylpyridinium cation. The mesostructured aluminophosphate-based materials were also prepared with templates mixtures of cetylpyridinium bromide and cetyltrimethylammonium bromide, and their interlayer distance of materials can be finely controlled by changing the CPBr/CTABr molar ratio. 27 Al and 31 P nuclear magnetic resonance spectra have been carried out to understand the local atomic arrangements and coordination environments of these solid products. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Alkylpyridinium bromide; Aluminophosphates; Mesostructured material; NMR spectroscopy
1. Introduction Since the discovery of the M41S silica-based crystalline mesoporous molecular sieves and their preparation in the presence of surfactant [1,2], these materials have attracted increasing attention. Recently, there has been much interest in synthesizing mesoporous aluminophosphates owing to the previous success in the synthesis of microporous aluminophosphates with framework topology identical or similar to aluminosilicate zeolites [3,4]. It has been demonstrated that the liquid-crystal templating approach for the synthesis of mesostructured materials can be extended to aluminophosphates since mesostructured organo-aluminophosphates are expected to be useful for the formation of mesoporous aluminophosphates to enlarge the variety and possible applications of mesoporous materials [5].
∗ Corresponding author. Tel.: +86-10-8264-9453; fax: +86-10-6256-1422. E-mail address: [email protected] (Z.-Y. Yuan).
Lamellar mesostructures of aluminophosphate have been synthesized by using an amphiphilic alkylamine (C10 H21 NH2 ) in tetraethylene glycol [6] and by using dimethylamines with C8 –C16 alkyl groups in water [7], but these materials are thermally unstable. Feng et al. [8] demonstrated the synthesis of the hexagonal mesostructured aluminophosphate in the presence of F− ions and using EtOH–H2 O as a solvent and its phase transition from the hexagonal phase into the lamellar phase was reported. Zhao et al. [9] and Chakraborty et al. [10] also reported the syntheses of thermally stable mesoporous aluminophosphates and silicoaluminophosphates, respectively. The preparations of lamellar and hexagonal mesostructured aluminophosphates (AlPOs) were also reported by Kimura et al. [11,12]. We have synthesized mesoporous silicon and cobaltsubstituted aluminophosphate (SAPO, CoAPO, CoAPSO) molecular sieves using self-assembled ordered organic supramolecular templates [13]. Here, we describe a series of mesostructured lamellar aluminophosphates and with substituted silicon synthesized hydrothermally, using alkylpyridinium bromide surfactant templating or mixed surfactants with cetyltrimethylammonium bromide.
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2. Experimental 2.1. Synthesis The synthesis of aluminophosphate-based mesostructured materials can be performed within the following molar gel composition: Al2 O3 :(0.8–1.9) P2 O5 :(0.3–1.0) CPBr:(0.8–4.2) TMAOH:(100–350) H2 O:(0–0.2) SiO2 . In a typical synthetic procedure, aluminium triisopropoxide was mixed with water under vigorous stirring, followed by the addition of an 85% solution of phosphoric acid, and the mixture was homogenized by stirring for 1 h. Then, cetylpyridinium bromide (CPBr) was added under stirring. Finally, a 10% solution of tetramethylammonium hydroxide (TMAOH) was added dropwise to the mixture, which was then stirred for another 1 h. For the synthesis of silicoaluminophosphate samples, a solution of tetraethyl orthosilicate (TEOS) was added dropwise to the above mixture and stirred for an additional 30 min. The reaction mixture was heated in a stainless-steel autoclave for 24–48 h at various temperatures in the range of 80–150◦ C. Subsequently, the product was filtered, washed with deionized water, and air-dried. 2.2. Characterization X-ray diffraction (XRD) measurements were made on a Rigaku D/max 2500 powder diffractometer with CuK␣ radiation (50 kV, 200 mA), 0.01◦ step size and 4◦ min−1 scanning rate. The morphology of the products were obtained using a Hitachi X-650 scanning electron microscope. Transmission electron microscopy (TEM) was carried out with a Jeol 2010 electron microscope operating at 200 kV. The specimens for TEM were dispersed in alcohol by ultrasonic treatment, then dropped onto a wholly carbon–copper grid. 27 Al and 31 P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were obtained on a Bruker MSL-400 spectrometer at 104.26 and 161.98 MHz, respectively. Magic angle spinning speeds of 10 kHz were used to 27 Al nuclei and 9 kHz to 31 P nuclei. Each 27 Al and 31 P spectra were acquired with 3000 and 16 scans, respectively. A 1 mol l−1 solution of aluminium nitrate and 85% H3 PO4 were used as external references. Thermal analyses were performed on a Rigaku standard type TG–DTA instrument under air atmosphere with a rate of 10◦ C min−1 . Framework infrared spectra were collected on a Perkin-Elmer 684 spectrometer using KBr wafer technique. Elemental analyses were made by conventional chemical analysis methods.
3. Results and discussion 3.1. General characterization Mesostructured aluminophosphates were synthesized in the synthetic system of Al(OPri )3 –H3 PO4 –CPBr–TMAOH–
Fig. 1. XRD patterns of the synthesized samples with different P2 O5 :Al2 O3 ratios of (a) 1.4; (b) 1.6 and (c) 1.8.
H2 O within the molar composition described in Section 2. The powder X-ray diffraction patterns of these products indicate characteristic low-angle peaks typical of mesostructured materials, consisting of one set of equidistant reflections which may be normalized as 0 0 l (l=1, 2, 3) reflections of a single lamellar phase [2] (Fig. 1), as well as reported in mesostructured lamellar materials synthesized with other different methods [14,15]. In the range of diffraction angle 2θ =20–30◦ there are some small broadened reflections which are related to the structure of lamellar AlPO phase. These materials were thermally unstable above 200◦ C revealed by XRD. Fig. 2a shows a scanning electron micrograph (SEM) of AlPO samples synthesized using CPBr as a template at 100◦ C, indicating elementary particles of ≈1 m with a generally spherical morphology. Fig. 2b shows an AlPO sample synthesized using tetradecylpyridinium bromide (TDPBr) as a template at 150◦ C, exhibiting rhombic particles of the size up to 2.7×7.2 m. These results show that the particle’s morphology might be controlled by the synthesis temperature. The TEM photographs of the mesostructured AlPO samples synthesized with cetylpyridinium bromide indicate that clear striped patterns were observed and the repeat distance of the striped patterns was ca. 3.1 nm, being in good agreement with the d001 -spacing recorded by XRD. Solid-state MAS NMR was carried out in order to understand the local structure and environments of both aluminum and phosphorus in these samples. Fig. 3 is the solid-state MAS NMR spectra of 27 Al and 31 P nuclei of the as-synthesized samples with different P/Al contents, respectively, showing the existence of both four-coordinate Al and P in the materials. There are two kinds of signals with different chemical shifts presented in the 27 Al spectra of the aluminophosphate samples: ∼40 and 0 ppm. The signals around 40 ppm were assigned to tetrahedral aluminum bonded to phosphorus atoms via oxygen bridges, and those of around 0 ppm could be attributed to octahedral coordinated Al to water and/or PO4 groups. The chemical shifts of the peaks in the 31 P spectra of AlPO indicate a tetrahedral coordination of the phosphorus atoms in the structure: one
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Fig. 2. SEM photographs of mesostructured lamellar aluminophosphates synthesized with different templates at different temperature. (a) C16 PBr, 100◦ C; (b) C14 PBr, 150◦ C.
main peak at −20.4 ppm. Additionally, the broadened and overlapped resonance in the range of 0 to −19 ppm in the 31 P spectra might be related to the distribution of phosphorus among various sites with slightly different local coordination environments [13,16], or probably due to amorphous impurity [17]. Mortlock et al. [18,19] reported that boldface P atoms in Al(H2 O)5 (H2 PO4 ), Al(H2 O)4 (H2 PO4 )2 , and Al(H2 O)4 (H2 PO4 )(H3 PO4 ) are observed at −9.5 ppm, that P atoms in Al(H2 O)5 (H3 PO4 ), Al(H2 O)4 (H3 PO4 )2 , and Al(H2 O)4 (H3 PO4 )(H2 PO4 ) are observed at −12.6 ppm, and that P atoms in (HO)2 P{OAl(H2 O)5 }2 are observed at −16.5 ppm. We believe that the signals in the range of 0 to −19 ppm may be related to the incomplete condensation of PO4 units and the presence of six-coordinated Al coor-
Fig. 3. 27 Al and 31 P MAS NMR spectra of AlPO sample with different P2 O5 :Al2 O3 ratios: (a) 1.4; (b) 1.6 and (c) 1.8.
dinated to some water molecules, as mentioned by Kimura et al. [12]. The framework infrared spectra of the AlPO materials showed several peaks in the structural region of 1400–400 cm−1 , which were similar to those observed for mesostuctured silica-based materials [2]. On the basis of data for microporous AlPO4 -n materials [20], the peaks at 1225, 1160, 1030, 750, 640, 520, and 485 cm−1 could be due to the asymmetric stretching of Al–O–P, the symmetric stretching of Al–O–P, and the bending of O–T–O (T:Al or P), respectively. 3.2. Al2 O3 :P2 O5 :TMAOH ratio Under the current synthetic system, increasing P2 O5 :Al2 O3 ratio required the corresponding addition of organic base TMAOH in the precursor solution, in order to prepare the well-defined mesostructured materials at a certain region of pH value. Mesolamellar products with high degree of crystallinity could be synthesized via tuning the concentration of TMAOH when P2 O5 :Al2 O3 ratio was very high, for instance, P2 O5 :Al2 O3 ratio of 1.9 (correspondingly TMAOH:Al2 O3 ratio should be in the region of 3.6–4.5 when H2 O:Al2 O3 ratio was 335). The preferable pH value of mixture system was 8.2. The experimental results suggest that mesostructured lamellar aluminophosphate can be synthesized in the molar composition ranges of P2 O5 :Al2 O3 =1.2–1.9 and TMAOH:Al2 O3 =2.6–4.5 and H2 O:Al2 O3 =335. Fig. 4 shows the synthetic phase diagram of the Al(OPri )3 –H3 PO4 –CPBr–TMAOH–H2 O system at 100◦ C with CPBr:Al2 O3 =0.35–1.0 and H2 O:Al2 O3 =335 (the part in the range of continuous line), revealing narrow region of Al2 O3 :P2 O5 :TMAOH ratio suitable for the synthesis of mesostructured lamellar AlPO. Moreover, the presence of an organic base such as TMAOH is essential because replacement of TMAOH with a solution of NaOH
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ter or surfactant cations [15]. When the P2 O5 :Al2 O3 ratio in the mixture was low, one broad resonance band was observed in the range of 0 to −19 ppm, which decreased, even disappeared with increase in the P2 O5 :Al2 O3 ratio. 3.3. Silicon incorporation
Fig. 4. Part of the phase diagram of the Al(OPri )3 –H3 PO4 –CPBr– TMAOH–H2 O system at 100◦ C.
led to only amorphous products, even one half replacement of TMAOH by NaOH results in amorphous products. As shown in Fig. 1, as well as evaluated by 27 Al and 31 P MAS NMR spectroscopy (Fig. 3), the crystallinity degree of the synthesized products of mesostructured lamellar AlPO increased with the P2 O5 :Al2 O3 ratio, suggesting a high P2 O5 :Al2 O3 ratio is proper to preparing well-defined mesolamellar AlPO materials. Based on Figs. 1 and 3, with increasing P2 O5 :Al2 O3 ratio, the crystallinity degree of the products increases, and the intensity of the resonance signal around 0 ppm in the 27 Al spectra of mesolamellar AlPO decreases. This suggests that the tetrahedral:octahedral ratio of Al is related to the crystallinity of the products and their P/Al ratios. The six-coordinate Al might be associated with amorphous aluminum phosphate or alumina [17]. In the 31 P spectra, one main peak is present at −20.4 ppm, corresponding to four-coordinate phosphorus coordinated to three AlO4 tetrahedra with the fourth oxygen bound to hydrogen, wa-
Fig. 5.
27 Al
and
31 P
The introduction of Si during synthesis could result in the formation of mesostructured lamellar silicoaluminophosphate (SAPO) materials. TEOS was used as a source of silicon to be SiO2 :Al2 O3 =0.2 in the mixture. Thus, optimum synthetic condition changed to P2 O5 :Al2 O3 ratio of 0.8–1.6 with corresponding TMAOH:Al2 O3 ratio of 2.1–4.0. The range of dotted line in Fig. 4 shows the phase diagram of Al2 O3 –P2 O5 –TMAOH system with SiO2 :Al2 O3 =0.2, H2 O:Al2 O3 =335, and CPBr:Al2 O3 =0.35–0.80 at 100◦ C. As to the synthesized SAPO products, their crystallinity increases with P2 O5 :Al2 O3 ratio. Fig. 5 shows the 27 Al and 31 P MAS NMR spectra of SAPO samples with different P2 O5 :Al2 O3 ratios. All SAPO samples show four-coordinated and six-coordinated Al, and four-coordinated P. The intensity of 31 P NMR signals increases with P2 O5 :Al2 O3 ratios. 3.4. The effect of surfactant The degree of crystallinity of the products is related with the surfactant concentration in the synthetic gel, as revealed by the XRD data. The intensity of the diffraction peaks (0 0 l) increases significantly with the concentration of CPBr. The increased crystallinity is also reflected in the 27 Al and 31 P MAS NMR spectra (Fig. 6). As shown in Fig. 6, with the increase of CPBr:Al2 O3 ratio, the resonance signal of six-coordinated Al in the 27 Al NMR spectra of synthesized samples decreases, and the ratio of tetrahe-
MAS NMR spectra of SAPO samples with different P2 O5 :Al2 O3 ratios: (a) 1.0 and (b) 1.6.
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Fig. 6.
27 Al
and
31 P
MAS NMR spectra of lamellar aluminophosphates with different CPBr:Al2 O3 ratios: (a) 0.35 and (b) 0.80.
dral to octahedral aluminium increases. Besides one peak at −20.6 ppm corresponding to tetrahedral phosphorus in the 31 P NMR spectra of the products synthesized with low CPBr:Al2 O3 ratio, the broad peaks at −12.4 and −0.5 ppm from amorphous material was observed. The intensity of the signal around −12.4 and −0.5 ppm decreases with the increase of CPBr:Al2 O3 ratio, even disappear. The highest quality products can be obtained at a high CPBr:Al2 O3
ratio, i.e. CPBr:Al2 O3 =1.0. A higher concentration of the surfactant in the synthetic mixture is beneficial for the crystallization of mesostructured aluminophosphate materials. The XRD patterns of silicoaluminophosphate materials synthesized with different concentrations of surfactant CPBr also show the same case that the intensity of the diffraction peak increases with the concentration of surfactant.
Fig. 7. XRD patterns of lamellar aluminophosphates synthesized from the composition Al2 O3 :1.8 P2 O5 :x C16 PBr:(1−x) CTABr:4.0 TMAOH:335 H2 O. (a) x=0; (b) x=0.2; (c) x=0.4; (d) x=0.6 and (e) x=0.8.
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The repeat distance between lamella and lamella in the present material can change with surfactant alkyl-chain length variation. Correspondingly, the d-spacing recorded by XRD generally increases with surfactant chain length under comparable reaction conditions, as well as the similar results for the silica-based materials such as MCM-41 [2] and HMS [21]. For example, when TDPBr or dodecylpyridinium bromide (DDPBr) was used as template in synthesis instead of CPBr, the d-spacing of the first diffraction line (0 0 l) of the products decreased from 3.1 to 2.9 or 2.7 nm, respectively. Ozin et al. [22] have reported the synthesis of MCM-41 type mesoporous silica using self-assembling micellar aggregates of a mixture of cetylpyridinium chloride and cetyltrimethylammonium chloride, in which the d100 -spacing of the mesoporous silica could be fine-tuned between 4.1 and 4.3 nm by variation of the molar ratio of these two surfactants in the mixture. Introduced this synthetic route to the synthesis of mesostructured aluminophosphate based materials, similar results could be obtained. Fig. 7 shows the XRD patterns of the synthesized products using a mixture of CPBr and cetyltrimethylammonium bromide (CTABr) with different molar ratio as a template under the reaction composition of Al2 O3 :1.8 P2 O5 :x CPBr:(1−x) CTABr:4.0 TMAOH:335 H2 O at 100◦ C for 48 h. As shown in Fig. 7, with the increase of the content of CPBr in the mixed CPBr/CTABr surfactant system, the d001 -spacing gradually decreased from 3.15 to 3.04 nm. Such a relationship of the d-spacing variation with the change of the CPBr/CTABr ratio was also shown in Fig. 8. Additionally,
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Fig. 8. d001 -spacing as a function of the C16 PBr/CTABr ratio as determined by XRD.
using mixtures of two alkylpyridinium cationic surfactants with different alkyl chain lengths could also succeed the fine-tuning of the d-spacings on a subangstrom length scale. 3.5. Water content The single lamellar structure phase could be synthesized at high water content, such as H2 O:Al2 O3 =335 or higher. When the water content decreased to H2 O:Al2 O3 =230 and 133, the obtained products were mixed phases of two lamellar structures, and the d-spacings increased with the decrease of the water content in the synthetic gel, and the diffraction peaks shifted to lower 2θ angles. Fig. 9 shows the XRD patterns of the obtained products synthesized
Fig. 9. XRD patterns of samples synthesized from different compositions: (a) Al2 O3 :1.2 P2 O5 :0.9 C16 PBr:1.3 TMAOH:230 H2 O; (b) Al2 O3 :1.2 P2 O5 :0.9 C16 PBr: 0.9 TMAOH:133 H2 O.
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Fig. 10. XRD patterns of samples synthesized at different temperatures: (a) 100◦ C; (b) 130◦ C and (c) 150◦ C.
from different molar compositions of Al2 O3 :1.2 P2 O5 :0.9 CPBr:1.3 TMAOH:230 H2 O and Al2 O3 :1.2 P2 O5 :0.9 CPBr:0.9 TMAOH:133 H2 O at 130◦ C for 28 h. 3.6. Synthesis temperature A constant range of crystalline temperature is needed for the synthesis of mesolamellar aluminophosphates. The optimum temperature range is from 80 to 150◦ C. Crystallizing at ambient temperature results in the precipitation of amorphous solids. Fig. 10 shows the XRD patterns of the AlPO samples synthesized at different temperatures from the composition mixture of Al2 O3 :1.7 P2 O5 :0.8 TDPBr:3.6 TMAOH:335 H2 O. As shown in Fig. 10, with the increasing of the synthesis temperature, the intensity of diffraction lines increased, indicating that a relatively higher temperature is favorable to prepare mesolamellar aluminophosphates. It is interesting that the morphology of the product particle changed from spherical to rhombic with the increase of the synthesis temperature (see Fig. 2).
Fig. 12 presents the 27 Al and 31 P spectra of the intermediates in the synthesis of mesolamellar AlPO. It can be observed that the transferring from the original mixture to the crystalline lamellar AlPO needs only a short time. The 27 Al spectra of amorphous solids collected from the original mixture show two peaks around −43 and −8 ppm, corresponding to four-coordinate and six-coordinate aluminum, respectively. After treated 1.5 h, the population of six-coordinate Al decreased, and the ratio of four-coordinate
3.7. Reaction kinetics Fig. 11 shows the crystallinity curves of mesolamellar AlPO and SAPO from respective molar compositions of Al2 O3 :1.7 P2 O5 :0.8 CPBr:3.6 TMAOH:335 H2 O and Al2 O3 :1.6 P2 O5 : 0.8 CPBr:4.0 TMAOH:335 H2 O:0.2 SiO2 at 100◦ C. Lamellar aluminophosphate based materials with relatively highly crystallinity could be obtained after hydrothermal crystallization of 2 h. The hydrothermal reactions were practically complete after 5 h.
Fig. 11. Degree of crystallinity of the intermediates in the synthesis of (a) AlPO and (b) SAPO estimated on the basis of the relative intensity of the (0 0 1) reflections.
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broad peak around −12.5 ppm, corresponding to the resonance signal of amorphous material. After hydrothermally treated 1.5 h, this amorphous signal decreased much, and one narrow resonance peak was present around −20.5 ppm. The amorphous signal could gradually disappear with time, and the resonance at −20.5 ppm increased, reflecting the transformation of the coordinate state of 31 P during synthesis. Fig. 13 shows the 27 Al and 31 P spectra of the intermediates in the synthesis of mesolamellar SAPO, in which the similar results as AlPO samples could be observed. After 3 h, the hydrothermal reactions were almost complete for SAPO samples.
4. Conclusions
Fig. 12. 27 Al and 31 P MAS NMR spectra of the intermediates in the synthesis of mesolamellar AlPO samples. Asterisks denotes spinning sidebands.
to six-coordinate increased quickly. After 5 h, the ratio of four-coordinate to six-coordinate remained stable, suggesting the crystallization has finished at this moment. In the 31 P MAS NMR spectra, the original mixture showed one
A series of mesostructured lamellar aluminophosphate and silicoaluminophosphate materials have been synthesized by supramolecular templating of alkylpyridinium cation. Optimal synthesis conditions have been established. Higher P2 O5 :Al2 O3 ratio, higher surfactant concentration, and higher synthesis temperature are beneficial to crystallize the mesolamellar structure. The addition of Si in the mixture gel requires more base content of TMAOH. The interlayer distance of the materials and their particle morphology can be different by the variation of the alkyl chain length of the alkylpyridinum template, the ratio of two mixed surfactant with cetyltrimethylammonium bromide, the water content in the original mixture, or a change in the synthesis temperature. The results of MAS NMR spectroscopy reveals the coordination state of aluminum and phosphorus in the structure of the synthesized products, and provides strong evidence for the variation of the coordination state of framework elements during the synthesis.
Acknowledgements This research work was supported by the National Natural Science Foundation of China. We are grateful to the Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Belgium for its supporting of NMR experiments. References
Fig. 13. 27 Al and 31 P MAS NMR spectra of the intermediates in the synthesis of mesolamellar SAPO samples. Asterisks denotes spinning sidebands.
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