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Surfactant-templated mesoporous aluminophosphate-based materials and the recent progress
Microporous and Mesoporous Materials 77 (2005) 97–107 www.elsevier.com/locate/micromeso
Review
Surfactant-templated mesoporous aluminophosphate-based materials and the recent progress Tatsuo Kimura Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Shimoshidami, Moriyama-ku, Nagoya 463–8560, Japan Received 12 July 2004; received in revised form 20 August 2004; accepted 25 August 2004 Available online 11 November 2004
Abstract This review provides an overview of surfactant-templated mesoporous materials whose main frameworks are composed of aluminophosphates (AlPOs). Recent developments of the synthetic strategies are also summarized. Potential applications of mesoporous AlPO-based materials are also mentioned. The synthetic strategies to control the mesostructures and the relation to the structures of AlPO frameworks are discussed. Lamellar and hexagonal mesostructured AlPOs reported so far can be classified as crystalline and non-crystalline materials, respectively. Although hexagonal mesostructured AlPOs can be converted to mesoporous materials with amorphous walls, it is difficult to maintain the mesostructural orderings of hexagonal phases completely after the removal of surfactants. Due to strong requirements for obtaining ordered mesostructured AlPOs by further improvement of synthetic methods, the synthesis of highly ordered mesoporous AlPO-based materials has been realized through block copolymer templating, organic modification technique, and so on. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Aluminophosphate; Mesostructured material; Mesoporous material; Ordered mesopore; Surfactant-templated synthesis
Contents 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
2.
Mesostructural control of AlPO-based materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.
Morphological control of mesostructured AlPO-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.
Mesoporous AlPO-based materials and their properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.
Frameworks of mesostructured AlPO-based materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.
Unique synthetic methods of mesostructured and mesoporous AlPOs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.
Improvement of mesostructural orderings of mesoporous AlPO-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
E-mail address: [email protected] 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.08.023
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8.
Summary and future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Since the discovery of ordered mesoporous silica [1,2], surfactant assemblies have effectively been utilized to obtain several types of ordered mesoporous silicas [3–6]. Mesostructured precursors for ordered mesoporous silicas can be formed through cooperative organization of inorganic species and amphiphilic molecules such as surfactants [7] and block copolymers [8–10]. The selfassembling ability of amphiphilic molecules interacting with inorganic species leads to the formation of mesostructured materials [11,12]. The formed mesostructures have been directed by the geometrical packing of used surfactant molecules [5,6]. Non-silica-based mesoporous materials have also been prepared by a similar synthetic strategy [12]. Although almost all of the mesostructured materials reported at earlier stages were thermally unstable because of the lamellar structures, the syntheses of mesostructured materials with three-dimensional (3-D) inorganic networks and their derivative mesoporous materials were achieved through various synthetic routes [13–15]. Hydrogen-bonded and ligand-assisted synthetic pathways were proposed for the formation of various non-silica-based mesoporous materials and used surfactants were easily removed by extraction. A family of microporous crystalline AlPO4-n and SAPO-n molecular sieves is well known as isomorphous substituted materials of silicates (zeolites) [16,17]. AlPO4-n and SAPO-n including their transition metal doped materials have attracted much attention as heterogeneous catalysts for oxidation, methanol conversion, ethylene dimerization, and so on. [18]. However, the preparation of mesoporous AlPO-based materials was not successful so readily in the early stage of the development of mesoporous materials. One of the main difficulties came from the organization of two different units on the surface of surfactant assemblies. Aluminum species are able to behave as both four- (AlO 4 ) and six3þ coordinated species (AlðH2 OÞ6 ) depending on the pH values of aqueous solutions [19] though phosphate anions are always tetrahedral (PO3 4 ). Various complexes are present in aqueous solutions containing both aluminum and phosphorus species [20]. Accordingly, the advance in the fabrication of mesostructured and mesoporous AlPO-based materials is of practical use for overcoming the difficulty in the general synthesis of non-silica-based mesostructured materials by controlling the formation of inorganic frameworks. Only one short review focused on mesostructured and mesoporous AlPO-based materials was published in 2001 [21] though a large number of reviews on silica-based and
non-silica-based materials have been published so far [13–15,22–29]. However, new synthetic routes to obtain ordered mesoporous AlPO-based materials have been reported recently and their synthetic methods are widely applicable to the synthesis of ordered mesoporous materials composed of a wide variety of inorganic frameworks including inorganic-organic hybrid materials. Therefore, mesostructured and mesoporous AlPO-based materials are reviewed here for further understanding of surfactant-templated mesophase materials.
2. Mesostructural control of AlPO-based materials Preparation of lamellar mesostructured AlPO-based materials (SCS-22) was firstly reported in 1993 [30]. Diaminoalkanes (H2N(CH2)nNH2) were used as surfactants and SCS-22 materials were composed of crystalline AlPO frameworks. Surprisingly, the X-ray diffraction (XRD) peaks in low scattering angles remained slightly after calcination in spite of the lamellar structures. Later, Fyfe et al. succeeded in preparing lamellar mesostructured AlPOs by using alkyltrimethylammonium bromides (CnH2n+1(CH3)3NBr, CnTMABr) in 1995 [31]. The lamellar mesostructured AlPOs can be obtained when the alkyl chain length in CnTMABr was longer than 12. The pH values of the starting mixtures were adjusted by adding tetramethylammonium hydroxide (TMAOH). This finding is the beginning of the researches on the mesostructural control of AlPO-based materials because the CnTMA surfactants have often been used for the synthesis of ordered mesoporous silicas [1–4]. In the AlPO–C16TMACl–H2O system, the synthetic conditions of mesostructured AlPOs were investigated in detail [32]. Lamellar and hexagonal mesostructured AlPOs (named as APW-1 and APW-2, respectively) can be obtained without impurities and then their structures were characterized adequately. First successful preparation of hexagonal APW-2 was reported in 1996 [33]. The XRD pattern of lamellar APW-1 showed a (0 0 1) peak at the d-spacing of 3.1 nm and the higher order reflections. In addition to those peaks, some XRD peaks related to the crystalline framework were also observed in higher scattering angles. The 27Al and 31 P MAS NMR measurements of APW-1 showed that the crystalline AlPO framework is constructed by alternating AlO4 and PO4 units (Al/P = 0.75). In contrast, the XRD pattern of APW-2 showed only peaks due to the 2-D hexagonal mesostructural ordering (d100 = 4.0 nm). The 27Al and 31P MAS NMR spectra of APW-2 indicated that both AlO4 and AlO6 units are present in
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the hexagonal APW-2 and the units are bonded with various PO4 units (Al/P = 1.5) [32–34]. Accordingly, it is reasonable to describe that APW-2 is composed of amorphous and less condensed AlPO frameworks. Additionally, APW-2 was formed by transformation of a layered intermediate composed of AlPO oligomers and CnTMA cations. As described later, the removal of CnTMA surfactants from APW-2 is possible by calcination, yielding mesoporous AlPO-based materials [32,35,36]. Zhao and coworkers [37,38] synthesized not only mesoporous AlPO (UHM-1) but also mesoporous silicoaluminophosphate (SAPO) (UHM-3). The TEM images of UHM-1 and -3 showed disordered arrangements even before the removal of surfactants. The synthesis of UHM-1 and -3 was successful only at room temperature in the presence of TMAOH (pH > 8.5) under a wide gel composition such as Al2O3:0.6–3.4P2O5: 0.0–1.0SiO 2 :0.24–0.50C 16 TMACl:8.5–47.0TMAOH: 200–642H2O. TMA cations play an important role in modifying the strength of the electrostatic interaction between AlPO species and cationic surfactants. No mesostructured AlPOs are obtained in the presence of smaller Na+ cations because of the stronger ion-pair interaction of Na+ with the AlPO species. Khimyak et al. [39–41] also investigated the synthetic conditions of mesostructured AlPOs using C16TMACl by XRD and the local structures in the AlPO frameworks by solid-state NMR. Then, the presence of hexagonal mesostructured AlPOs with lesser condensed frameworks and lamellar mesostructured materials with highly condensed AlPO frameworks were found [42,43]. The above-mentioned studies have showed the relation between the formation of mesostructured AlPOs and the synthetic conditions. The synthetic conditions can be summarized simply through the investigation on the synthesis of mesostructured AlPOs at higher temperatures [32,43]. As shown in Scheme 1, the mesostructures of AlPO-based materials are changed systematically depending on the synthetic conditions. Several synthetic conditions are associated with one another. The pH value of the starting mixtures is changed according to the Al/P ratio and the amount of water in the starting mixtures. High temperature synthesis leads to the formation of lamellar mesostructured AlPOs because AlPO species are likely to be crystallized. In addition, the presence of four-coordinated species such as AlO4 species bonded to PO4 units in the starting mixtures also promotes the formation of lamellar mesostructured AlPOs. Such AlO4 species are likely to form at low Al/P molar ratios [20]. In the same manner, the addition of SiO4 species in the starting mixtures leads to the formation of lamellar phases. Thus, hexagonal mesostructured AlPOs are formed under restricted conditions. 2-D hexagonal mesostructured AlPOs are formed at room temperature under wider synthetic conditions
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Scheme 1. Effects of the synthetic conditions on the mesostructural control of AlPO-based materials. Reproduced with permission from [32].
[37,38] because the room temperature synthesis suppresses the crystallization of AlPO-based frameworks. Even though the Al/P molar ratios in starting mixtures are less than 1.0, it is not difficult to prepare 2-D hexagonal mesostructured AlPOs at room temperature [39]. The formation diagrams of mesostructured AlPOs are shown in Fig. 1. The presence of TMAOH is indispensable for obtaining mesostructured AlPOs and 2-D hexagonal mesophases are formed at higher pH. 2-D hexagonal mesostructured AlPOs with lesser condensed frameworks can be obtained at much higher TMAOH contents. The condensation of the AlPO frameworks can be controlled during the drying process of as-synthesized materials at different temperatures [44]. Therefore, 2-D hexagonal mesostructured AlPOs described in different papers should be compared carefully. Feng et al. [45] prepared lamellar and hexagonal mesostructured AlPOs in the presence of fluoride anions. The organic fractions have not been removed yet to obtain mesoporous materials. Perez et al. [46] also investigated the synthesis of mesoporous AlPOs through a fluoride route. The formed 2-D hexagonal phase changed to a dense phase with a cristobalite-type structure by calcination. A lamellar mesostructured AlPO was obtained when the synthesis was conducted at 130 °C in a mixed solvent of water and methanol [47]. Yuan et al. [48] synthesized lamellar mesostructured AlPOs by using hexadecylpyridinium bromide (C16PBr) and the d001spacing was finely controlled in the mixed surfactant (C16PBr/C16TMABr or C16PBr/C14PBr) systems.
3. Morphological control of mesostructured AlPO-based materials In general, lamellar mesostructured AlPOs show plate-like morphologies (Fig. 2), reflecting the layered nature [44]. Nevertheless, Khimyak and Klinowski [42]
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condensed AlPO framework [42,43]. Several research groups have reported the preparation of lamellar mesostructured AlPOs by using neutral alkylamine (CnH2n+1NH2) surfactants under aqueous and nonaqueous conditions [49–58]. Oliver and coworkers [49– 51] reported that lamellar mesostructured AlPOs with various surface patterns can be prepared in the presence of tetraethyleneglycol (TEG) and the representative SEM pictures are shown in Fig. 3. The formation mechanism of the lamellar phase was proposed that the materials are formed via layered alkylammonium-phosphates [52,53]. The curvature of the surfactant assemblies increases by the presence of the TEG molecules that are penetrating into around the hydrophilic groups in the surfactant assemblies. Very recently, Shen also reported the synthesis of lamellar mesostructured AlPOs by using CnH2n+1NH2 [59,60]. When the alkyl chain length was 10, shell-like products were formed by the long-chain amine-stabilized oil-in-water emulsification methodology [59]. The use of a short-chain amine (C7H15NH2) provided the formation of cellular-like products through biomineralization reaction in interconnected water
Fig. 1. Formation diagrams of mesostructured AlPOs prepared at room temperature under (a) medium (1.00Al2O3:0.70–0.83C16TMACl: 325H2O) and (b) low water contents (1.00Al2O3:0.70–0.83C16TMACl: 200H2O). Reproduced with permission from [39].
Fig. 2. SEM image of the lamellar mesostructured AlPO (APW-2) with a plate-like morphology prepared using C16TMACl [44].
found the formation of a lamellar mesostructured AlPO prepared by using C16TMACl with a spherical aggregated morphology. The spherical aggregated morphology was observed in the lamellar phase with highly
Fig. 3. SEM images of lamellar mesostructured AlPOs with various macroscopic morphologies and surface patterning. Reproduced with permission from [49,51].
T. Kimura / Microporous and Mesoporous Materials 77 (2005) 97–107
droplet channels [60]. Sayari et al. [54] succeeded in preparing a vesicular mesostructured AlPO by using C12H25NH2 under the aqueous condition. Gao et al. [57,58] reported the synthesis of lamellar mesostructured AlPOs with a plate-like morphology by using CnH2n+1NH2 in the presence of alkyl alcohols and ethylene glycol (EG) was used as the solvent. Alkyl alcohols act as co-surfactants and then the basal spacing of the lamellar mesostructured AlPOs is changed according to the alkyl chain length of the alkyl alcohols. The AlPO framework was built from hydrated AlO4 units connected with PO4 units. Because of the presence of the hydrated AlO4 units, the material is useful as a good adsorbent for H2O molecules. Particles reflecting periodic mesostructures have not been obtained in the case of 2-D hexagonal mesostructured AlPOs. This is because hexagonal mesostructured AlPOs, whose frameworks are less condensed, are formed via layered intermediates [32]. Morphological variations of mesostructured AlPO-based materials have been presented only in the papers on lamellar mesostructured AlPOs with crystalline frameworks. Because particle morphologies of materials reflect the crystal structures, plate-like materials are likely to form whenever lamellar mesostructured materials are prepared. Exceptionally, morphologies of the particles are designed according to the presence of co-solvents [49–51,59,60], the formation of defects in the frameworks, and so on.
4. Mesoporous AlPO-based materials and their properties Hexagonal mesostructured AlPOs (APW-2) can be converted to mesoporous AlPOs with tunable disordered mesopores (1.6–3.9 nm) upon calcination [32, 35,36]. Although the structure of APW-2 became disordered by substantial shrinkage during calcination, the mesoporous structure was confirmed by the N2 adsorption–desorption measurement (Fig. 4). Depending on the presence of uniform mesopore, no hysteresis loops were observed and the specific surface areas were higher than 700 m2 g1. In the case of UHM-1 with high specific surface area, a hysteresis loop was observed in the N2 isotherm [37,38]. Cabrera et al. [61] showed an interesting result for the synthesis of mesoporous AlPOs (ICMUV-3). The pore size (1.3–3.7 nm) was changed by different Al/P ratios in the starting mixtures without varying the chain length of C16TMA surfactants. Tiemann et al. [62] succeeded in preparing mesoporous AlPOs when alkylamine surfactants were used as the structure-directing agents. The stability and structural order were improved by postsynthetic thermal treatment of the as-synthesized materials and the surfactants were quickly removed by acidic solvent extraction. Kriesel et al. [63] reported that meso-
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Fig. 4. N2 adsorption–desorption isotherms of calcined APW-2 prepared by using (a) C16TMACl, (b) C22TMACl, and (c) C22TMACl with triisopropylbenzene. Reproduced with permission from [36].
porous AlPOs (UCB1-AlP) are successfully prepared by using triblock copolymers. The pore diameter (6.6–7.8 nm) and pore volumes (1.1 cm3 g1) of UCB1-AlP were quite larger than those reported for other mesoporous AlPOs. Ordered mesoporous silicas prepared by the surfactant-templated method have hydrophobic surfaces [3,4,64,65] that can be slightly weakened by incorporating AlO4 units [66]. Mesoporous aluminas also show hydrophobic nature because the inorganic frameworks are mainly composed of AlO6 units; water molecules are adsorbed on the surface physically [67]. In contrast, the water adsorption–desorption measurement for mesoporous APW-2 showed a type IV behavior, indicating that the surface has a hydrophilic nature [36]. Because the framework of mesoporous APW-2 consists of both AlO4 and PO4 units after calcination, water molecules are coordinated to the AlO4 units. Incorporation of metals into mesoporous AlPO frameworks has not been studied so extensively. Zhao and coworkers [68–70] tried to incorporate transition metals (Mn, V, Cu, Ni, Cr) into the frameworks of UHM-1 and UHM-3. Other research groups also reported the preparation of B-, Si-, Mg-, Ti-, Co-, and Fe-containing mesoporous AlPOs [71–77]. Chakraborty et al. [78,79] reported the synthesis of mesoporous SAPOs. Because of the lower stability of mesoporous AlPOs, there have been few reports on acidities and ion-exchange properties of metal-containing mesoporous AlPOs. Holland and coworkers [80,81] investigated
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anion-exchange properties of mesoporous AlPO formed through a layered intermediate composed of Keggin-like ions and the mesoporous AlPO showed the effective exchange property for several organic dyes. However, the mesoporous structure almost collapsed after the experiments. Recently, a novel strategy to improve the thermal stability of mesostructured AlPO-based materials by surface modification was reported [82]. Deformation of mesoporous AlPO-frameworks is suppressed according to enhanced hydrophobic nature due to the presence surface organic groups. Bae et al. studied the photoionization of N-alkylphenothiazines in UHM-3 and transition metal substituted UHM-3 [70,83]. Kapoor and Raj showed that the higher catalytic activity of Ti-containing mesoporous AlPO for epoxidation of some olefins using H2O2 in the presence of acetonitrile than that observed for Ti-MCM-41 [75]. Karthik et al. [77] showed the catalytic performance of Mg and Co substituted mesoporous AlPOs. Their mild acidities are useful for tert-butylation of m-cresol with tert-butyl alcohol. Selvam and co-workers have investigated the catalytic properties of metal species (Me=Fe, Co, Cr, and Ti) in the frameworks of mesoporous AlPOs (Me-AlPOs) [84–89]. Mesoporous Fe-AlPO exhibits a high catalytic activity for the oxidation of cyclohexene under mild conditions and the catalyst was repeatedly used without deactivation [84]. Mesoporous Fe-AlPO is a versatile heterogeneous catalyst for hydrogenation of nitro and carbonyl compounds and reductive cleavage of azo functions (Scheme 2) [85,86]. Other functional groups such as –CN, –CHO, –Cl, –CH3, –OCH3, and –NH2 groups in the reactants are not reacted during the reactions. Mesoporous Co-AlPO was also useful for selective hydrogenation of nitro and carbonyl compounds [87]. Cr- and Ti-AlPOs showed remarkable activity for the oxidation of cyclohexane [88] and alkyl substituted phen-
Scheme 2. (a) Hydrogenation of aromatic nitro and (b) carbonyl compounds and (c) reductive cleavage of azo functional groups [85,86].
ols [89] under mild conditions, respectively. As described above, several reports clearly show the catalytic properties of metal-doped mesoporous AlPOs. However, we are still in the very early stage in the studies on the catalytic performances of this class of materials. Comparative studies with those on other mesoporous materials are also a next step to be disclosed.
5. Frameworks of mesostructured AlPO-based materials Mesostructured and mesoporous AlPO-based materials have been developed by surfactant-templated synthetic strategies and those synthetic conditions are listed in Table 1. Low Al/P ratios in starting mixtures and higher reaction temperatures, which can allow AlPO frameworks to crystallize, are required for the synthesis of lamellar mesostructured AlPOs. Hexagonal mesostructured AlPOs are prepared at lower reaction temperatures under conditions with higher pH values adjusted by the addition of TMAOH. Mesostructural orders of lamellar and hexagonal mesostructured AlPOs are usually characterized by XRD in low scattering angles and observed by TEM directly. Electron diffraction data are also important for defining the mesostructures. The crystallinity of AlPO frameworks is confirmed by XRD and solid-state NMR measurements. However, such results are not discussed adequately in almost all of the papers. This is because the synthesis of mesoporous AlPOs has been a main topic in this research field and the mesostructured precursors are always composed of amorphous AlPO frameworks. In contrast, only lamellar mesostructured AlPOs can be composed of crystalline inorganic frameworks. On the basis of the compositions, structural features and properties, KraushaarCzarnetzki et al. [30] proposed that the lamellar SCS-22 prepared by using H2N(CH2)nNH2 in an aqueous system takes an intermediate position between 3-D AlPOs and 2-D zirconium phosphates. Feng et al. [90] identified the crystal structures of lamellar mesostructured materials prepared by using H2N(CH2)nNH2 (n = 9–12) in a non-aqueous system (UCSB-50, 51, 52, 53). The formula and the space group were [H2N(CH2)nNH2]7(H2O)8[Al13(PO4)18H] and R3, respectively. The inorganic framework consists of both fourand six-membered rings and the periodicity of the hexagonal c axis is unusually long and varies at around 5.1–6.2 nm. Adjacent crystalline AlPO sheets, which are separated one another by the presence of layered H2N(CH2)nNH2 assemblies, can be arranged periodically according to the interaction between the AlPO sheets and the amino groups at the both ends of the hydrocarbon chain. On the basis of the XRD and NMR results, other lamellar mesostructured AlPOs are also thought to be composed of crystalline AlPO
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Table 1 Synthetic conditions of mesostructured AlPO-based materials Surfactant (S)
Typical composition of starting mixture
Temperature, time, pH
Lamellar Kraushaar-Czarnetzki et al. [30] Fyfe et al. [31] Fyfe et al. [31] Kimura et al. [32] Oliver and coworkers [49–51] Sayari and coworkers [54–56] Gao et al. [57,58]
H2N(CH2)nNH2 (n = 8–12) H2N(CH2)nNH2 (n = 8) CnTMABr (n = 12–18) CnTMACl (n = 16) CnH2n+1NH2 (n = 6–18) CnH2n+1NH2 (n = 8–12) CnH2n+1NH2 (n = 6–12)
120 180 150 150 180 100 180
Khimyak and Klinowski [42,43]
CnTMACl (n = 16)
Yuan et al. [48]
CnPBrc/CnTMACl (n = 14, 16) H2N(CH2)nNH2 (n = 9–12)
0.5Al2O3:1.0P2O5:1.0S:80H2O 1.0Al2O3:1.4P2O5:0.7S:50H2O 0.3Al2O3:1.0P2O5:1.0S:1.6TMAOH:15H2O 0.5Al2O3:1.0P2O5:1.0S:2.0TMAOH:65H2O 0.9Al2O3:1.8P2O5:3.0S:2.5H2O:14TEGa 1.0Al2O3:1.0P2O5:1.0S:60H2O 0.5Al2O3:0.9P2O5:3.4S: 1.7H2O:3.4n-CmH2m+1OH:13.8EGb 0.83Al2O3:1.0P2O5:0.8S: 1.10TMAOH:190H2O(L1) 0.83Al2O3:1.0P2O5:0.6S: 0.75TMAOH:110H2O(L2) 1.0Al2O3:1.8P2O5:1.0S:4.0TMAOH:335H2O
100 °C, 2 d
0.5Al2O3:1.0P2O5:0.32 S:0.95H2O:13.5 EG
180 °C, 4 d (pH 7.0)
1.0Al2O3:1.0P2O5:1.0S:2.0TMAOH:65H2O 0.38Al2O3:1.0P2O5:0.014SiO2:0.44S:85.4H2O 1.0Al2O3:1.3P2O5:0.26S:1.7TMAOH: 0.8HF:117H2O:8.2EtOH 1.24Al2O3:1.0P2O5:1.0S:1.3TMAOH:196H2O 1.0Al2O3:1.0P2O5:1.0S:64 EtOH:6H2O 1.0Al2O3:1.0P2O5:0.5S:32EtOH Using a single source precursor such as [Al(PO4)(HCl)(C2H5OH)4]4 1.0Al2O3:1.0P2O5:0.35S:2.0–4.0TMAOH:335H2O 0.5Al2O3:0.75P2O5:0.2SiO2:1.0S: 1.4TMAOH:8.0EtOH:138H2O Using a molecular precursor such as [Al(OiPr)2O2P(OtBu2)]4 Using inorganic acid-base pair sources such as AlCl3 and H3PO4
<130 °C (pH 10) 110 °C, 12 h (pH 2.5) <70 °C (pH 8.3)
Feng et al. [90] 2-D Hexagonal (or disordered) Kimura et al. [43–47] Chakraborty et al. [78,79] Feng et al. [45]
CnTMACl (n = 12–22) CnTMABr (n = 16) CnTMABr (n = 16)
Zhao and coworkers [37,38] Tiemann et al. [62] Tiemann and Fro¨ba [96]
CnTMACl (n = 16) CnH2n+1NH2 (n = 12–16) CnH2n+1NH2 (n = 12–16)
Yuan et al. [72] Zhao and Lu [74]
CnTMABr (n = 16) CnTMABr (n = 14)
Kriesel et al. [63]
EO106PO70EO106 EO20PO70EO20 EO106PO70EO106 EO20PO70EO20
Wang and coworkers [105,106] a b c
°C, °C, °C, °C, °C, °C, °C,
24 h 16 h 5d 5 d (pH 8.5) 3d 24 h (pH 2.5–3.5) 8d
130 °C, 2 d
RT (pH 9.5) 90 °C, 2 d 90 °C, 2 d
RT, 24 h 70 °C, 2 d (pH 8.5) 135 °C, 12 h RT
TEG: tetraethylene glycol. EG: ethylene glycol. C16PBr: cetylpyridinium bromide.
sheets [32,42,43]. However, the crystal structures of these materials have not been proved yet. The structural information (compositions and coordination numbers) of the inorganic frameworks in the mesostructured AlPOs is summarized in Table 2. The Al/P ratios observed for almost all lamellar mesostructured AlPOs are less than unity as well as those reported for layered crystalline AlPOs (Al/P = 0.75) [91–94]. Judging from the 27Al and 31P MAS NMR results including XRD data, the lamellar mesostructured AlPOs are composed of crystalline AlPO frameworks with alternating AlO4 and PO4 units. The reasonable explanation for the interactions of AlPO frameworks with positively charged surfactants is possible because of the formation of negatively charged sheets ([Al3(PO4)4]3). In the case of UCSB-50, 51, 52, and 53, aluminophosphate sheets ([Al13(PO4)18H]14) are also negatively charged and then can be interacted with protonated H2N(CH2)nNH2 molecules.
Hexagonal mesostructured AlPOs have unusual Al/P ratios higher than unity, being related to less condensed AlPO frameworks containing AlO6, AlO4, and PO4 units. However, all the aluminum species are converted to AlO4 species in mesoporous AlPOs after calcination at 600 °C [36], meaning that an Al2O3 impurity is not present in the mesoporous AlPOs. Acidic P–OH and Al–OH groups are exposed at the surface of mesopores [95]. In addition, the compositional control of mesoporous AlPOs is quite difficult. According to pH values and Al/P ratios, the composition and structure of AlPO oligomers are variable. Recently, the stoichiometrically controlled mesoporous AlPOs can be successfully obtained from single source precursors such as [Al(OiPr)2O2P(OtBu2)]4 through triblock copolymer templating [63] and ([Al(PO4)(HCl)(C2H5OH)4]4) through alkylamine templating [96]. These studies will open a new step for the formation of well designed AlPO frameworks retaining mesostructures.
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Table 2 Structural information on mesostructured AlPO-based materials Surfactant (S)
Lamellar Kraushaar-Czarnetzki et al. [30] Fyfe et al. [31] Kimura et al. [32] Oliver and coworkers [49–51] Sayari and coworkers [54–56] Gao et al. [57,58] Khimyak and Klinowski [42,43]
Yuan et al. [48] Feng et al. [90] 2-D Hexagonal (or disordered) Kimura et al. [32–36] Feng et al. [45] Zhao et al. [37,38] Tiemann et al. [62] Tiemann and Fro¨ba [96] Yuan et al. [72] Zhao and Lu [74] Kriesel et al. [63] Wang and coworkers [105,106]
Composition (molar ratio)
MAS NMR (ppm)
Al/P
S/(Al + P)
27
31
H2N(CH2)nNH2 H2N(CH2)nNH2 CnTMACl CnH2n+1NH2 CnH2n+1NH2 CnH2n+1NH2 CnTMACl(L1) CnTMACl(L2)
0.75 – 0.75 0.5 0.79 2.0 0.68 0.62
0.21 – 0.34 0.67 – 0.33 0.41 0.33
44.8 45.5 43.1 – 46.8, 10.4, 8.7 4.3 43.2 44.1, 36.5
CnPBrc/CnTMACl H2N(CH2)nNH2
– 0.72
– 0.23
40 –
8.4, 16.2 18.6 17.1, 20.3 – 13.0 7.8 17.5, 21.1 18.0, 20.7, 22.9, 30.8 20.4 –
CnTMACl CnTMABr CnTMACl CnH2n+1NH2 CnH2n+1NH2 CnTMABr CnTMABr EO106PO70EO106 EO20PO70EO20 EO106PO70EO106 EO20PO70EO20
1.5 1.2 1.7 1.14 1.0 – 1.14 1.0
0.25 0.17 – 0.2 0.25 – 0.2 –
43.5, 1.2 43.8, 8.7 43, 1 42, 7 42, 8 42, 8, 5 50.3, 1.0 42, 11
0 20 – 0 19 0 30 0 30 2 18 15.3 25
1.0
–
38.4, 11.3
25.6
6. Unique synthetic methods of mesostructured and mesoporous AlPOs As an excellent synthetic pathway of hexagonal mesostructured AlPO-based materials, Holland and coworkers [80,81] succeeded in utilizing layered alumina-based intermediates like Keggin ions (AlO4 Al12 ðOHÞ24 7þ ðH2 OÞ12 ) containing dodecylsulfate (C12 H25 OSO 3 ). By the treatment of the intermediates with a buffered Na2HPO4/NaH2PO4 solution, the transformation into a hexagonal phase occurred at room temperature. Removal of the surfactant is carried out by anion-exchange with a sodium acetate/ethanol solution. However, calcination of the hexagonal materials at temperatures above 350 °C results in the collapse of the mesostructures. Cheng et al. [97] reported an interesting approach for the synthesis of mesoporous AlPO. A novel layered AlPO with a kanemite-like structure (AlPO-ntu) was synthesized and a hexagonal phase was obtained by the reaction of AlPO-ntu with C16TMACl, but the ordered structure was collapsed by calcination. Most of the surfactant molecules can be removed by ion-exchange with acetic acid in ethanol or NaCl in H2O/ethanol solution. The evidence for the formation of the hexagonal phase was not shown because of the low thermal stability. Fro¨ba and coworkers [98–100] have investigated a new synthetic route of mesostructured AlPOs by using dodecylphosphates (C12H25OPO(OH)2) as template and reactant. This strategy was originally conducted
Al
P
by Huo et al. [12]. Lamellar mesostructured AlPOs are obtained from the mixtures of C12H25OPO(OH)2 and Al(OiC3H7)3 with or without H3PO4 in aqueous systems [98,100]. The amounts of AlO4 and AlO6 species in the frameworks are variable depending on the Al/P molar ratios of the lamellar products. The formation of a hexagonal phase was also reported in an alcoholic system [99]. This phase was caused by the formation of reversed micelles of C12H25OPO(OH)2. AlPO nanorods are isolated by the presence of alkyl chains and 3-D inorganic networks were not present in the product. The insight is quite important for controlling the size and shape of not only AlPO but also metal phosphates in nanometer scale and such designed materials are potentially applicable to new functional materials. Interestingly, this type of materials can be obtained by using alkylphosphates with shorter alkyl chains such as C6H13OPO(OH)2 [101,102], whereas no mesostructured AlPOs are formed by using CnTMA surfactants with alkyl chains shorter than 10 [31].
7. Improvement of mesostructural orderings of mesoporous AlPO-based materials One of the possibilities to synthesize ordered mesoporous AlPO-based materials without deteriorating the framework structure is found in a previous paper on the synthesis of mesoporous AlPOs through neutral
T. Kimura / Microporous and Mesoporous Materials 77 (2005) 97–107
alkylamine templating [62]. Surfactant molecules can be removed by extraction without deformation of the mesostructures after post-treatment of the mesostructured materials with water vapor, though the original mesostructures become less ordered. Another approach to obtain ordered mesoporous AlPO-based materials has also been investigated. The lowering of the ordered mesostructure can be suppressed by the enhanced stabilities of the mesostructures after the organic modification with alkyltrimethoxysilanes [82]. Organic modification using bridged silsesquioxanes was conducted and the organic groups were present within the AlPO-based frameworks [103]. As a result of this capability incorporating organic fractions in the framework, a new route to synthesize mesoporous aluminum organophosphonates by using organically bridged diphosphonic acids has been reported recently [104], which is the first example of the synthesis of non-silica-based inorganic-organic hybrid mesoporous materials. Block copolymer templating syntheses lead to the formation of stable mesoporous AlPOs with uniform largepores based on the ‘‘acid-base pair’’ routes [105]. As shown in Scheme 3, an appropriate combination of inorganic sources such as AlCl3/H3PO4, AlCl3/OP(OCH3)3, and Al(OC4H9)3/PCl3 is quite important for the formation of Al–O–P bonds. The synthesis has been performed in ethanol by using triblock copolymers such as Pluronic P123 (EO20PO70EO20, EO; ethylene oxide unit, PO; propylene oxide unit) and F127 (EO106PO70EO106). In a typical synthesis, AlCl3 is added to a clear solution prepared by adding 85% H3PO4 to an ethanolic triblock copolymer solution. After the violent evolution of HCl gas, a highly acidic clear solution can be obtained and then the solution is transferred into a dish to evaporate the solvent. So far as I know from my experience, large-scale synthesis is quite difficult for obtaining ordered mesostructured precursors. Therefore, it is considered that the evaporation procedure is a key to the preparation of ordered materials. Metalincorporated mesoporous AlPOs can also be prepared under the same concept. The TEM images of ordered stable mesoporous Fe-AlPOs are shown in Fig. 5. This
Scheme 3. ‘‘Acid-base pair’’ routes for synthesizing mesostructured AlPOs. Reproduced with permission from [105].
105
Fig. 5. TEM images of calcined mesoporous Fe-AlPO prepared through block copolymer templating: (a) (0 0 1) and (b) (1 1 0) directions of a 2-D hexagonal phase. Reproduced with permission from [105].
new concept allows to synthesize a wide variety of ordered stable non-silica-based mesoporous materials [106,107].
8. Summary and future outlook The surfactant-templated synthesis is effective for the preparation of mesoporous AlPOs by keeping the AlPO frameworks amorphous. Crystallized AlPO frameworks have high charge densities, which results in the formation of lamellar phases containing a larger amount of ionic surfactants. The insights on the frameworks are useful for understanding the formation of non-silicabased mesostructured materials including their synthetic methods. However, the mesostructural control of AlPO-based materials is quite limited to lamellar and 2-D hexagonal phases, which may be concerned with the formation mechanism as the 2-D hexagonal phase is formed via layered intermediates [32]. Block copolymer templating approach has overcome this limit and a mesoporous AlPO with 3-D hexagonal phase in addition to lamellar and 2-D hexagonal phases can be prepared successfully [106]. This method should be investigated further for its applicability toward the formation of mesoporous AlPOs with different space groups. The periodicities of mesopores are lost for almost all the mesoporous AlPOs after the removal of surfactants though the uniformities of the mesopores are somehow retained. Recently, ordered mesoporous AlPOs prepared through block copolymer templating [106,107], organically modified materials with organosilanes [82,103], hybrid mesoporous AlPOs prepared by using organically bridged diphosphonic acids [104] have been developed. Morphological controls such as ordered mesoporous AlPO films based on the acid-base pair route [106] and mesoporous AlPO spheres by the nanocasting technique (hard templating) have been achieved recently [108]. As in the case of ordered mesoporous silicas, the catalytic performances and surface properties due to the ordered mesopores constructed by AlPO
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frameworks will be studied more extensively in many fields.
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Review
Surfactant-templated mesoporous aluminophosphate-based materials and the recent progress Tatsuo Kimura Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Shimoshidami, Moriyama-ku, Nagoya 463–8560, Japan Received 12 July 2004; received in revised form 20 August 2004; accepted 25 August 2004 Available online 11 November 2004
Abstract This review provides an overview of surfactant-templated mesoporous materials whose main frameworks are composed of aluminophosphates (AlPOs). Recent developments of the synthetic strategies are also summarized. Potential applications of mesoporous AlPO-based materials are also mentioned. The synthetic strategies to control the mesostructures and the relation to the structures of AlPO frameworks are discussed. Lamellar and hexagonal mesostructured AlPOs reported so far can be classified as crystalline and non-crystalline materials, respectively. Although hexagonal mesostructured AlPOs can be converted to mesoporous materials with amorphous walls, it is difficult to maintain the mesostructural orderings of hexagonal phases completely after the removal of surfactants. Due to strong requirements for obtaining ordered mesostructured AlPOs by further improvement of synthetic methods, the synthesis of highly ordered mesoporous AlPO-based materials has been realized through block copolymer templating, organic modification technique, and so on. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Aluminophosphate; Mesostructured material; Mesoporous material; Ordered mesopore; Surfactant-templated synthesis
Contents 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
2.
Mesostructural control of AlPO-based materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.
Morphological control of mesostructured AlPO-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.
Mesoporous AlPO-based materials and their properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.
Frameworks of mesostructured AlPO-based materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.
Unique synthetic methods of mesostructured and mesoporous AlPOs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.
Improvement of mesostructural orderings of mesoporous AlPO-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
E-mail address: [email protected] 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.08.023
98
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8.
Summary and future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
105
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Since the discovery of ordered mesoporous silica [1,2], surfactant assemblies have effectively been utilized to obtain several types of ordered mesoporous silicas [3–6]. Mesostructured precursors for ordered mesoporous silicas can be formed through cooperative organization of inorganic species and amphiphilic molecules such as surfactants [7] and block copolymers [8–10]. The selfassembling ability of amphiphilic molecules interacting with inorganic species leads to the formation of mesostructured materials [11,12]. The formed mesostructures have been directed by the geometrical packing of used surfactant molecules [5,6]. Non-silica-based mesoporous materials have also been prepared by a similar synthetic strategy [12]. Although almost all of the mesostructured materials reported at earlier stages were thermally unstable because of the lamellar structures, the syntheses of mesostructured materials with three-dimensional (3-D) inorganic networks and their derivative mesoporous materials were achieved through various synthetic routes [13–15]. Hydrogen-bonded and ligand-assisted synthetic pathways were proposed for the formation of various non-silica-based mesoporous materials and used surfactants were easily removed by extraction. A family of microporous crystalline AlPO4-n and SAPO-n molecular sieves is well known as isomorphous substituted materials of silicates (zeolites) [16,17]. AlPO4-n and SAPO-n including their transition metal doped materials have attracted much attention as heterogeneous catalysts for oxidation, methanol conversion, ethylene dimerization, and so on. [18]. However, the preparation of mesoporous AlPO-based materials was not successful so readily in the early stage of the development of mesoporous materials. One of the main difficulties came from the organization of two different units on the surface of surfactant assemblies. Aluminum species are able to behave as both four- (AlO 4 ) and six3þ coordinated species (AlðH2 OÞ6 ) depending on the pH values of aqueous solutions [19] though phosphate anions are always tetrahedral (PO3 4 ). Various complexes are present in aqueous solutions containing both aluminum and phosphorus species [20]. Accordingly, the advance in the fabrication of mesostructured and mesoporous AlPO-based materials is of practical use for overcoming the difficulty in the general synthesis of non-silica-based mesostructured materials by controlling the formation of inorganic frameworks. Only one short review focused on mesostructured and mesoporous AlPO-based materials was published in 2001 [21] though a large number of reviews on silica-based and
non-silica-based materials have been published so far [13–15,22–29]. However, new synthetic routes to obtain ordered mesoporous AlPO-based materials have been reported recently and their synthetic methods are widely applicable to the synthesis of ordered mesoporous materials composed of a wide variety of inorganic frameworks including inorganic-organic hybrid materials. Therefore, mesostructured and mesoporous AlPO-based materials are reviewed here for further understanding of surfactant-templated mesophase materials.
2. Mesostructural control of AlPO-based materials Preparation of lamellar mesostructured AlPO-based materials (SCS-22) was firstly reported in 1993 [30]. Diaminoalkanes (H2N(CH2)nNH2) were used as surfactants and SCS-22 materials were composed of crystalline AlPO frameworks. Surprisingly, the X-ray diffraction (XRD) peaks in low scattering angles remained slightly after calcination in spite of the lamellar structures. Later, Fyfe et al. succeeded in preparing lamellar mesostructured AlPOs by using alkyltrimethylammonium bromides (CnH2n+1(CH3)3NBr, CnTMABr) in 1995 [31]. The lamellar mesostructured AlPOs can be obtained when the alkyl chain length in CnTMABr was longer than 12. The pH values of the starting mixtures were adjusted by adding tetramethylammonium hydroxide (TMAOH). This finding is the beginning of the researches on the mesostructural control of AlPO-based materials because the CnTMA surfactants have often been used for the synthesis of ordered mesoporous silicas [1–4]. In the AlPO–C16TMACl–H2O system, the synthetic conditions of mesostructured AlPOs were investigated in detail [32]. Lamellar and hexagonal mesostructured AlPOs (named as APW-1 and APW-2, respectively) can be obtained without impurities and then their structures were characterized adequately. First successful preparation of hexagonal APW-2 was reported in 1996 [33]. The XRD pattern of lamellar APW-1 showed a (0 0 1) peak at the d-spacing of 3.1 nm and the higher order reflections. In addition to those peaks, some XRD peaks related to the crystalline framework were also observed in higher scattering angles. The 27Al and 31 P MAS NMR measurements of APW-1 showed that the crystalline AlPO framework is constructed by alternating AlO4 and PO4 units (Al/P = 0.75). In contrast, the XRD pattern of APW-2 showed only peaks due to the 2-D hexagonal mesostructural ordering (d100 = 4.0 nm). The 27Al and 31P MAS NMR spectra of APW-2 indicated that both AlO4 and AlO6 units are present in
T. Kimura / Microporous and Mesoporous Materials 77 (2005) 97–107
the hexagonal APW-2 and the units are bonded with various PO4 units (Al/P = 1.5) [32–34]. Accordingly, it is reasonable to describe that APW-2 is composed of amorphous and less condensed AlPO frameworks. Additionally, APW-2 was formed by transformation of a layered intermediate composed of AlPO oligomers and CnTMA cations. As described later, the removal of CnTMA surfactants from APW-2 is possible by calcination, yielding mesoporous AlPO-based materials [32,35,36]. Zhao and coworkers [37,38] synthesized not only mesoporous AlPO (UHM-1) but also mesoporous silicoaluminophosphate (SAPO) (UHM-3). The TEM images of UHM-1 and -3 showed disordered arrangements even before the removal of surfactants. The synthesis of UHM-1 and -3 was successful only at room temperature in the presence of TMAOH (pH > 8.5) under a wide gel composition such as Al2O3:0.6–3.4P2O5: 0.0–1.0SiO 2 :0.24–0.50C 16 TMACl:8.5–47.0TMAOH: 200–642H2O. TMA cations play an important role in modifying the strength of the electrostatic interaction between AlPO species and cationic surfactants. No mesostructured AlPOs are obtained in the presence of smaller Na+ cations because of the stronger ion-pair interaction of Na+ with the AlPO species. Khimyak et al. [39–41] also investigated the synthetic conditions of mesostructured AlPOs using C16TMACl by XRD and the local structures in the AlPO frameworks by solid-state NMR. Then, the presence of hexagonal mesostructured AlPOs with lesser condensed frameworks and lamellar mesostructured materials with highly condensed AlPO frameworks were found [42,43]. The above-mentioned studies have showed the relation between the formation of mesostructured AlPOs and the synthetic conditions. The synthetic conditions can be summarized simply through the investigation on the synthesis of mesostructured AlPOs at higher temperatures [32,43]. As shown in Scheme 1, the mesostructures of AlPO-based materials are changed systematically depending on the synthetic conditions. Several synthetic conditions are associated with one another. The pH value of the starting mixtures is changed according to the Al/P ratio and the amount of water in the starting mixtures. High temperature synthesis leads to the formation of lamellar mesostructured AlPOs because AlPO species are likely to be crystallized. In addition, the presence of four-coordinated species such as AlO4 species bonded to PO4 units in the starting mixtures also promotes the formation of lamellar mesostructured AlPOs. Such AlO4 species are likely to form at low Al/P molar ratios [20]. In the same manner, the addition of SiO4 species in the starting mixtures leads to the formation of lamellar phases. Thus, hexagonal mesostructured AlPOs are formed under restricted conditions. 2-D hexagonal mesostructured AlPOs are formed at room temperature under wider synthetic conditions
99
Scheme 1. Effects of the synthetic conditions on the mesostructural control of AlPO-based materials. Reproduced with permission from [32].
[37,38] because the room temperature synthesis suppresses the crystallization of AlPO-based frameworks. Even though the Al/P molar ratios in starting mixtures are less than 1.0, it is not difficult to prepare 2-D hexagonal mesostructured AlPOs at room temperature [39]. The formation diagrams of mesostructured AlPOs are shown in Fig. 1. The presence of TMAOH is indispensable for obtaining mesostructured AlPOs and 2-D hexagonal mesophases are formed at higher pH. 2-D hexagonal mesostructured AlPOs with lesser condensed frameworks can be obtained at much higher TMAOH contents. The condensation of the AlPO frameworks can be controlled during the drying process of as-synthesized materials at different temperatures [44]. Therefore, 2-D hexagonal mesostructured AlPOs described in different papers should be compared carefully. Feng et al. [45] prepared lamellar and hexagonal mesostructured AlPOs in the presence of fluoride anions. The organic fractions have not been removed yet to obtain mesoporous materials. Perez et al. [46] also investigated the synthesis of mesoporous AlPOs through a fluoride route. The formed 2-D hexagonal phase changed to a dense phase with a cristobalite-type structure by calcination. A lamellar mesostructured AlPO was obtained when the synthesis was conducted at 130 °C in a mixed solvent of water and methanol [47]. Yuan et al. [48] synthesized lamellar mesostructured AlPOs by using hexadecylpyridinium bromide (C16PBr) and the d001spacing was finely controlled in the mixed surfactant (C16PBr/C16TMABr or C16PBr/C14PBr) systems.
3. Morphological control of mesostructured AlPO-based materials In general, lamellar mesostructured AlPOs show plate-like morphologies (Fig. 2), reflecting the layered nature [44]. Nevertheless, Khimyak and Klinowski [42]
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condensed AlPO framework [42,43]. Several research groups have reported the preparation of lamellar mesostructured AlPOs by using neutral alkylamine (CnH2n+1NH2) surfactants under aqueous and nonaqueous conditions [49–58]. Oliver and coworkers [49– 51] reported that lamellar mesostructured AlPOs with various surface patterns can be prepared in the presence of tetraethyleneglycol (TEG) and the representative SEM pictures are shown in Fig. 3. The formation mechanism of the lamellar phase was proposed that the materials are formed via layered alkylammonium-phosphates [52,53]. The curvature of the surfactant assemblies increases by the presence of the TEG molecules that are penetrating into around the hydrophilic groups in the surfactant assemblies. Very recently, Shen also reported the synthesis of lamellar mesostructured AlPOs by using CnH2n+1NH2 [59,60]. When the alkyl chain length was 10, shell-like products were formed by the long-chain amine-stabilized oil-in-water emulsification methodology [59]. The use of a short-chain amine (C7H15NH2) provided the formation of cellular-like products through biomineralization reaction in interconnected water
Fig. 1. Formation diagrams of mesostructured AlPOs prepared at room temperature under (a) medium (1.00Al2O3:0.70–0.83C16TMACl: 325H2O) and (b) low water contents (1.00Al2O3:0.70–0.83C16TMACl: 200H2O). Reproduced with permission from [39].
Fig. 2. SEM image of the lamellar mesostructured AlPO (APW-2) with a plate-like morphology prepared using C16TMACl [44].
found the formation of a lamellar mesostructured AlPO prepared by using C16TMACl with a spherical aggregated morphology. The spherical aggregated morphology was observed in the lamellar phase with highly
Fig. 3. SEM images of lamellar mesostructured AlPOs with various macroscopic morphologies and surface patterning. Reproduced with permission from [49,51].
T. Kimura / Microporous and Mesoporous Materials 77 (2005) 97–107
droplet channels [60]. Sayari et al. [54] succeeded in preparing a vesicular mesostructured AlPO by using C12H25NH2 under the aqueous condition. Gao et al. [57,58] reported the synthesis of lamellar mesostructured AlPOs with a plate-like morphology by using CnH2n+1NH2 in the presence of alkyl alcohols and ethylene glycol (EG) was used as the solvent. Alkyl alcohols act as co-surfactants and then the basal spacing of the lamellar mesostructured AlPOs is changed according to the alkyl chain length of the alkyl alcohols. The AlPO framework was built from hydrated AlO4 units connected with PO4 units. Because of the presence of the hydrated AlO4 units, the material is useful as a good adsorbent for H2O molecules. Particles reflecting periodic mesostructures have not been obtained in the case of 2-D hexagonal mesostructured AlPOs. This is because hexagonal mesostructured AlPOs, whose frameworks are less condensed, are formed via layered intermediates [32]. Morphological variations of mesostructured AlPO-based materials have been presented only in the papers on lamellar mesostructured AlPOs with crystalline frameworks. Because particle morphologies of materials reflect the crystal structures, plate-like materials are likely to form whenever lamellar mesostructured materials are prepared. Exceptionally, morphologies of the particles are designed according to the presence of co-solvents [49–51,59,60], the formation of defects in the frameworks, and so on.
4. Mesoporous AlPO-based materials and their properties Hexagonal mesostructured AlPOs (APW-2) can be converted to mesoporous AlPOs with tunable disordered mesopores (1.6–3.9 nm) upon calcination [32, 35,36]. Although the structure of APW-2 became disordered by substantial shrinkage during calcination, the mesoporous structure was confirmed by the N2 adsorption–desorption measurement (Fig. 4). Depending on the presence of uniform mesopore, no hysteresis loops were observed and the specific surface areas were higher than 700 m2 g1. In the case of UHM-1 with high specific surface area, a hysteresis loop was observed in the N2 isotherm [37,38]. Cabrera et al. [61] showed an interesting result for the synthesis of mesoporous AlPOs (ICMUV-3). The pore size (1.3–3.7 nm) was changed by different Al/P ratios in the starting mixtures without varying the chain length of C16TMA surfactants. Tiemann et al. [62] succeeded in preparing mesoporous AlPOs when alkylamine surfactants were used as the structure-directing agents. The stability and structural order were improved by postsynthetic thermal treatment of the as-synthesized materials and the surfactants were quickly removed by acidic solvent extraction. Kriesel et al. [63] reported that meso-
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Fig. 4. N2 adsorption–desorption isotherms of calcined APW-2 prepared by using (a) C16TMACl, (b) C22TMACl, and (c) C22TMACl with triisopropylbenzene. Reproduced with permission from [36].
porous AlPOs (UCB1-AlP) are successfully prepared by using triblock copolymers. The pore diameter (6.6–7.8 nm) and pore volumes (1.1 cm3 g1) of UCB1-AlP were quite larger than those reported for other mesoporous AlPOs. Ordered mesoporous silicas prepared by the surfactant-templated method have hydrophobic surfaces [3,4,64,65] that can be slightly weakened by incorporating AlO4 units [66]. Mesoporous aluminas also show hydrophobic nature because the inorganic frameworks are mainly composed of AlO6 units; water molecules are adsorbed on the surface physically [67]. In contrast, the water adsorption–desorption measurement for mesoporous APW-2 showed a type IV behavior, indicating that the surface has a hydrophilic nature [36]. Because the framework of mesoporous APW-2 consists of both AlO4 and PO4 units after calcination, water molecules are coordinated to the AlO4 units. Incorporation of metals into mesoporous AlPO frameworks has not been studied so extensively. Zhao and coworkers [68–70] tried to incorporate transition metals (Mn, V, Cu, Ni, Cr) into the frameworks of UHM-1 and UHM-3. Other research groups also reported the preparation of B-, Si-, Mg-, Ti-, Co-, and Fe-containing mesoporous AlPOs [71–77]. Chakraborty et al. [78,79] reported the synthesis of mesoporous SAPOs. Because of the lower stability of mesoporous AlPOs, there have been few reports on acidities and ion-exchange properties of metal-containing mesoporous AlPOs. Holland and coworkers [80,81] investigated
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anion-exchange properties of mesoporous AlPO formed through a layered intermediate composed of Keggin-like ions and the mesoporous AlPO showed the effective exchange property for several organic dyes. However, the mesoporous structure almost collapsed after the experiments. Recently, a novel strategy to improve the thermal stability of mesostructured AlPO-based materials by surface modification was reported [82]. Deformation of mesoporous AlPO-frameworks is suppressed according to enhanced hydrophobic nature due to the presence surface organic groups. Bae et al. studied the photoionization of N-alkylphenothiazines in UHM-3 and transition metal substituted UHM-3 [70,83]. Kapoor and Raj showed that the higher catalytic activity of Ti-containing mesoporous AlPO for epoxidation of some olefins using H2O2 in the presence of acetonitrile than that observed for Ti-MCM-41 [75]. Karthik et al. [77] showed the catalytic performance of Mg and Co substituted mesoporous AlPOs. Their mild acidities are useful for tert-butylation of m-cresol with tert-butyl alcohol. Selvam and co-workers have investigated the catalytic properties of metal species (Me=Fe, Co, Cr, and Ti) in the frameworks of mesoporous AlPOs (Me-AlPOs) [84–89]. Mesoporous Fe-AlPO exhibits a high catalytic activity for the oxidation of cyclohexene under mild conditions and the catalyst was repeatedly used without deactivation [84]. Mesoporous Fe-AlPO is a versatile heterogeneous catalyst for hydrogenation of nitro and carbonyl compounds and reductive cleavage of azo functions (Scheme 2) [85,86]. Other functional groups such as –CN, –CHO, –Cl, –CH3, –OCH3, and –NH2 groups in the reactants are not reacted during the reactions. Mesoporous Co-AlPO was also useful for selective hydrogenation of nitro and carbonyl compounds [87]. Cr- and Ti-AlPOs showed remarkable activity for the oxidation of cyclohexane [88] and alkyl substituted phen-
Scheme 2. (a) Hydrogenation of aromatic nitro and (b) carbonyl compounds and (c) reductive cleavage of azo functional groups [85,86].
ols [89] under mild conditions, respectively. As described above, several reports clearly show the catalytic properties of metal-doped mesoporous AlPOs. However, we are still in the very early stage in the studies on the catalytic performances of this class of materials. Comparative studies with those on other mesoporous materials are also a next step to be disclosed.
5. Frameworks of mesostructured AlPO-based materials Mesostructured and mesoporous AlPO-based materials have been developed by surfactant-templated synthetic strategies and those synthetic conditions are listed in Table 1. Low Al/P ratios in starting mixtures and higher reaction temperatures, which can allow AlPO frameworks to crystallize, are required for the synthesis of lamellar mesostructured AlPOs. Hexagonal mesostructured AlPOs are prepared at lower reaction temperatures under conditions with higher pH values adjusted by the addition of TMAOH. Mesostructural orders of lamellar and hexagonal mesostructured AlPOs are usually characterized by XRD in low scattering angles and observed by TEM directly. Electron diffraction data are also important for defining the mesostructures. The crystallinity of AlPO frameworks is confirmed by XRD and solid-state NMR measurements. However, such results are not discussed adequately in almost all of the papers. This is because the synthesis of mesoporous AlPOs has been a main topic in this research field and the mesostructured precursors are always composed of amorphous AlPO frameworks. In contrast, only lamellar mesostructured AlPOs can be composed of crystalline inorganic frameworks. On the basis of the compositions, structural features and properties, KraushaarCzarnetzki et al. [30] proposed that the lamellar SCS-22 prepared by using H2N(CH2)nNH2 in an aqueous system takes an intermediate position between 3-D AlPOs and 2-D zirconium phosphates. Feng et al. [90] identified the crystal structures of lamellar mesostructured materials prepared by using H2N(CH2)nNH2 (n = 9–12) in a non-aqueous system (UCSB-50, 51, 52, 53). The formula and the space group were [H2N(CH2)nNH2]7(H2O)8[Al13(PO4)18H] and R3, respectively. The inorganic framework consists of both fourand six-membered rings and the periodicity of the hexagonal c axis is unusually long and varies at around 5.1–6.2 nm. Adjacent crystalline AlPO sheets, which are separated one another by the presence of layered H2N(CH2)nNH2 assemblies, can be arranged periodically according to the interaction between the AlPO sheets and the amino groups at the both ends of the hydrocarbon chain. On the basis of the XRD and NMR results, other lamellar mesostructured AlPOs are also thought to be composed of crystalline AlPO
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Table 1 Synthetic conditions of mesostructured AlPO-based materials Surfactant (S)
Typical composition of starting mixture
Temperature, time, pH
Lamellar Kraushaar-Czarnetzki et al. [30] Fyfe et al. [31] Fyfe et al. [31] Kimura et al. [32] Oliver and coworkers [49–51] Sayari and coworkers [54–56] Gao et al. [57,58]
H2N(CH2)nNH2 (n = 8–12) H2N(CH2)nNH2 (n = 8) CnTMABr (n = 12–18) CnTMACl (n = 16) CnH2n+1NH2 (n = 6–18) CnH2n+1NH2 (n = 8–12) CnH2n+1NH2 (n = 6–12)
120 180 150 150 180 100 180
Khimyak and Klinowski [42,43]
CnTMACl (n = 16)
Yuan et al. [48]
CnPBrc/CnTMACl (n = 14, 16) H2N(CH2)nNH2 (n = 9–12)
0.5Al2O3:1.0P2O5:1.0S:80H2O 1.0Al2O3:1.4P2O5:0.7S:50H2O 0.3Al2O3:1.0P2O5:1.0S:1.6TMAOH:15H2O 0.5Al2O3:1.0P2O5:1.0S:2.0TMAOH:65H2O 0.9Al2O3:1.8P2O5:3.0S:2.5H2O:14TEGa 1.0Al2O3:1.0P2O5:1.0S:60H2O 0.5Al2O3:0.9P2O5:3.4S: 1.7H2O:3.4n-CmH2m+1OH:13.8EGb 0.83Al2O3:1.0P2O5:0.8S: 1.10TMAOH:190H2O(L1) 0.83Al2O3:1.0P2O5:0.6S: 0.75TMAOH:110H2O(L2) 1.0Al2O3:1.8P2O5:1.0S:4.0TMAOH:335H2O
100 °C, 2 d
0.5Al2O3:1.0P2O5:0.32 S:0.95H2O:13.5 EG
180 °C, 4 d (pH 7.0)
1.0Al2O3:1.0P2O5:1.0S:2.0TMAOH:65H2O 0.38Al2O3:1.0P2O5:0.014SiO2:0.44S:85.4H2O 1.0Al2O3:1.3P2O5:0.26S:1.7TMAOH: 0.8HF:117H2O:8.2EtOH 1.24Al2O3:1.0P2O5:1.0S:1.3TMAOH:196H2O 1.0Al2O3:1.0P2O5:1.0S:64 EtOH:6H2O 1.0Al2O3:1.0P2O5:0.5S:32EtOH Using a single source precursor such as [Al(PO4)(HCl)(C2H5OH)4]4 1.0Al2O3:1.0P2O5:0.35S:2.0–4.0TMAOH:335H2O 0.5Al2O3:0.75P2O5:0.2SiO2:1.0S: 1.4TMAOH:8.0EtOH:138H2O Using a molecular precursor such as [Al(OiPr)2O2P(OtBu2)]4 Using inorganic acid-base pair sources such as AlCl3 and H3PO4
<130 °C (pH 10) 110 °C, 12 h (pH 2.5) <70 °C (pH 8.3)
Feng et al. [90] 2-D Hexagonal (or disordered) Kimura et al. [43–47] Chakraborty et al. [78,79] Feng et al. [45]
CnTMACl (n = 12–22) CnTMABr (n = 16) CnTMABr (n = 16)
Zhao and coworkers [37,38] Tiemann et al. [62] Tiemann and Fro¨ba [96]
CnTMACl (n = 16) CnH2n+1NH2 (n = 12–16) CnH2n+1NH2 (n = 12–16)
Yuan et al. [72] Zhao and Lu [74]
CnTMABr (n = 16) CnTMABr (n = 14)
Kriesel et al. [63]
EO106PO70EO106 EO20PO70EO20 EO106PO70EO106 EO20PO70EO20
Wang and coworkers [105,106] a b c
°C, °C, °C, °C, °C, °C, °C,
24 h 16 h 5d 5 d (pH 8.5) 3d 24 h (pH 2.5–3.5) 8d
130 °C, 2 d
RT (pH 9.5) 90 °C, 2 d 90 °C, 2 d
RT, 24 h 70 °C, 2 d (pH 8.5) 135 °C, 12 h RT
TEG: tetraethylene glycol. EG: ethylene glycol. C16PBr: cetylpyridinium bromide.
sheets [32,42,43]. However, the crystal structures of these materials have not been proved yet. The structural information (compositions and coordination numbers) of the inorganic frameworks in the mesostructured AlPOs is summarized in Table 2. The Al/P ratios observed for almost all lamellar mesostructured AlPOs are less than unity as well as those reported for layered crystalline AlPOs (Al/P = 0.75) [91–94]. Judging from the 27Al and 31P MAS NMR results including XRD data, the lamellar mesostructured AlPOs are composed of crystalline AlPO frameworks with alternating AlO4 and PO4 units. The reasonable explanation for the interactions of AlPO frameworks with positively charged surfactants is possible because of the formation of negatively charged sheets ([Al3(PO4)4]3). In the case of UCSB-50, 51, 52, and 53, aluminophosphate sheets ([Al13(PO4)18H]14) are also negatively charged and then can be interacted with protonated H2N(CH2)nNH2 molecules.
Hexagonal mesostructured AlPOs have unusual Al/P ratios higher than unity, being related to less condensed AlPO frameworks containing AlO6, AlO4, and PO4 units. However, all the aluminum species are converted to AlO4 species in mesoporous AlPOs after calcination at 600 °C [36], meaning that an Al2O3 impurity is not present in the mesoporous AlPOs. Acidic P–OH and Al–OH groups are exposed at the surface of mesopores [95]. In addition, the compositional control of mesoporous AlPOs is quite difficult. According to pH values and Al/P ratios, the composition and structure of AlPO oligomers are variable. Recently, the stoichiometrically controlled mesoporous AlPOs can be successfully obtained from single source precursors such as [Al(OiPr)2O2P(OtBu2)]4 through triblock copolymer templating [63] and ([Al(PO4)(HCl)(C2H5OH)4]4) through alkylamine templating [96]. These studies will open a new step for the formation of well designed AlPO frameworks retaining mesostructures.
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Table 2 Structural information on mesostructured AlPO-based materials Surfactant (S)
Lamellar Kraushaar-Czarnetzki et al. [30] Fyfe et al. [31] Kimura et al. [32] Oliver and coworkers [49–51] Sayari and coworkers [54–56] Gao et al. [57,58] Khimyak and Klinowski [42,43]
Yuan et al. [48] Feng et al. [90] 2-D Hexagonal (or disordered) Kimura et al. [32–36] Feng et al. [45] Zhao et al. [37,38] Tiemann et al. [62] Tiemann and Fro¨ba [96] Yuan et al. [72] Zhao and Lu [74] Kriesel et al. [63] Wang and coworkers [105,106]
Composition (molar ratio)
MAS NMR (ppm)
Al/P
S/(Al + P)
27
31
H2N(CH2)nNH2 H2N(CH2)nNH2 CnTMACl CnH2n+1NH2 CnH2n+1NH2 CnH2n+1NH2 CnTMACl(L1) CnTMACl(L2)
0.75 – 0.75 0.5 0.79 2.0 0.68 0.62
0.21 – 0.34 0.67 – 0.33 0.41 0.33
44.8 45.5 43.1 – 46.8, 10.4, 8.7 4.3 43.2 44.1, 36.5
CnPBrc/CnTMACl H2N(CH2)nNH2
– 0.72
– 0.23
40 –
8.4, 16.2 18.6 17.1, 20.3 – 13.0 7.8 17.5, 21.1 18.0, 20.7, 22.9, 30.8 20.4 –
CnTMACl CnTMABr CnTMACl CnH2n+1NH2 CnH2n+1NH2 CnTMABr CnTMABr EO106PO70EO106 EO20PO70EO20 EO106PO70EO106 EO20PO70EO20
1.5 1.2 1.7 1.14 1.0 – 1.14 1.0
0.25 0.17 – 0.2 0.25 – 0.2 –
43.5, 1.2 43.8, 8.7 43, 1 42, 7 42, 8 42, 8, 5 50.3, 1.0 42, 11
0 20 – 0 19 0 30 0 30 2 18 15.3 25
1.0
–
38.4, 11.3
25.6
6. Unique synthetic methods of mesostructured and mesoporous AlPOs As an excellent synthetic pathway of hexagonal mesostructured AlPO-based materials, Holland and coworkers [80,81] succeeded in utilizing layered alumina-based intermediates like Keggin ions (AlO4 Al12 ðOHÞ24 7þ ðH2 OÞ12 ) containing dodecylsulfate (C12 H25 OSO 3 ). By the treatment of the intermediates with a buffered Na2HPO4/NaH2PO4 solution, the transformation into a hexagonal phase occurred at room temperature. Removal of the surfactant is carried out by anion-exchange with a sodium acetate/ethanol solution. However, calcination of the hexagonal materials at temperatures above 350 °C results in the collapse of the mesostructures. Cheng et al. [97] reported an interesting approach for the synthesis of mesoporous AlPO. A novel layered AlPO with a kanemite-like structure (AlPO-ntu) was synthesized and a hexagonal phase was obtained by the reaction of AlPO-ntu with C16TMACl, but the ordered structure was collapsed by calcination. Most of the surfactant molecules can be removed by ion-exchange with acetic acid in ethanol or NaCl in H2O/ethanol solution. The evidence for the formation of the hexagonal phase was not shown because of the low thermal stability. Fro¨ba and coworkers [98–100] have investigated a new synthetic route of mesostructured AlPOs by using dodecylphosphates (C12H25OPO(OH)2) as template and reactant. This strategy was originally conducted
Al
P
by Huo et al. [12]. Lamellar mesostructured AlPOs are obtained from the mixtures of C12H25OPO(OH)2 and Al(OiC3H7)3 with or without H3PO4 in aqueous systems [98,100]. The amounts of AlO4 and AlO6 species in the frameworks are variable depending on the Al/P molar ratios of the lamellar products. The formation of a hexagonal phase was also reported in an alcoholic system [99]. This phase was caused by the formation of reversed micelles of C12H25OPO(OH)2. AlPO nanorods are isolated by the presence of alkyl chains and 3-D inorganic networks were not present in the product. The insight is quite important for controlling the size and shape of not only AlPO but also metal phosphates in nanometer scale and such designed materials are potentially applicable to new functional materials. Interestingly, this type of materials can be obtained by using alkylphosphates with shorter alkyl chains such as C6H13OPO(OH)2 [101,102], whereas no mesostructured AlPOs are formed by using CnTMA surfactants with alkyl chains shorter than 10 [31].
7. Improvement of mesostructural orderings of mesoporous AlPO-based materials One of the possibilities to synthesize ordered mesoporous AlPO-based materials without deteriorating the framework structure is found in a previous paper on the synthesis of mesoporous AlPOs through neutral
T. Kimura / Microporous and Mesoporous Materials 77 (2005) 97–107
alkylamine templating [62]. Surfactant molecules can be removed by extraction without deformation of the mesostructures after post-treatment of the mesostructured materials with water vapor, though the original mesostructures become less ordered. Another approach to obtain ordered mesoporous AlPO-based materials has also been investigated. The lowering of the ordered mesostructure can be suppressed by the enhanced stabilities of the mesostructures after the organic modification with alkyltrimethoxysilanes [82]. Organic modification using bridged silsesquioxanes was conducted and the organic groups were present within the AlPO-based frameworks [103]. As a result of this capability incorporating organic fractions in the framework, a new route to synthesize mesoporous aluminum organophosphonates by using organically bridged diphosphonic acids has been reported recently [104], which is the first example of the synthesis of non-silica-based inorganic-organic hybrid mesoporous materials. Block copolymer templating syntheses lead to the formation of stable mesoporous AlPOs with uniform largepores based on the ‘‘acid-base pair’’ routes [105]. As shown in Scheme 3, an appropriate combination of inorganic sources such as AlCl3/H3PO4, AlCl3/OP(OCH3)3, and Al(OC4H9)3/PCl3 is quite important for the formation of Al–O–P bonds. The synthesis has been performed in ethanol by using triblock copolymers such as Pluronic P123 (EO20PO70EO20, EO; ethylene oxide unit, PO; propylene oxide unit) and F127 (EO106PO70EO106). In a typical synthesis, AlCl3 is added to a clear solution prepared by adding 85% H3PO4 to an ethanolic triblock copolymer solution. After the violent evolution of HCl gas, a highly acidic clear solution can be obtained and then the solution is transferred into a dish to evaporate the solvent. So far as I know from my experience, large-scale synthesis is quite difficult for obtaining ordered mesostructured precursors. Therefore, it is considered that the evaporation procedure is a key to the preparation of ordered materials. Metalincorporated mesoporous AlPOs can also be prepared under the same concept. The TEM images of ordered stable mesoporous Fe-AlPOs are shown in Fig. 5. This
Scheme 3. ‘‘Acid-base pair’’ routes for synthesizing mesostructured AlPOs. Reproduced with permission from [105].
105
Fig. 5. TEM images of calcined mesoporous Fe-AlPO prepared through block copolymer templating: (a) (0 0 1) and (b) (1 1 0) directions of a 2-D hexagonal phase. Reproduced with permission from [105].
new concept allows to synthesize a wide variety of ordered stable non-silica-based mesoporous materials [106,107].
8. Summary and future outlook The surfactant-templated synthesis is effective for the preparation of mesoporous AlPOs by keeping the AlPO frameworks amorphous. Crystallized AlPO frameworks have high charge densities, which results in the formation of lamellar phases containing a larger amount of ionic surfactants. The insights on the frameworks are useful for understanding the formation of non-silicabased mesostructured materials including their synthetic methods. However, the mesostructural control of AlPO-based materials is quite limited to lamellar and 2-D hexagonal phases, which may be concerned with the formation mechanism as the 2-D hexagonal phase is formed via layered intermediates [32]. Block copolymer templating approach has overcome this limit and a mesoporous AlPO with 3-D hexagonal phase in addition to lamellar and 2-D hexagonal phases can be prepared successfully [106]. This method should be investigated further for its applicability toward the formation of mesoporous AlPOs with different space groups. The periodicities of mesopores are lost for almost all the mesoporous AlPOs after the removal of surfactants though the uniformities of the mesopores are somehow retained. Recently, ordered mesoporous AlPOs prepared through block copolymer templating [106,107], organically modified materials with organosilanes [82,103], hybrid mesoporous AlPOs prepared by using organically bridged diphosphonic acids [104] have been developed. Morphological controls such as ordered mesoporous AlPO films based on the acid-base pair route [106] and mesoporous AlPO spheres by the nanocasting technique (hard templating) have been achieved recently [108]. As in the case of ordered mesoporous silicas, the catalytic performances and surface properties due to the ordered mesopores constructed by AlPO
106
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frameworks will be studied more extensively in many fields.
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