Formation of aluminum diphosphonate mesostructures: The effect of aluminum source

Journal of Colloid and Interface Science 532 (2018) 718–726

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Formation of aluminum diphosphonate mesostructures: The effect of aluminum source Xiuzhen Lin a,b, Jun Xu c, Feng Deng c, Zhong-Yong Yuan b,⇑ a

School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, Guangdong, China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), National Institute for Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300071, China c Wuhan Center for Magnetic Resonance, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China b

g r a p h i c a l a b s t r a c t Lamellar, 2D-hexagonal and particle-packed mesostructures of aluminum phosphonate are synthesized by selecting aluminum source with different inorganic anions.

a r t i c l e

i n f o

Article history: Received 11 April 2018 Revised 2 August 2018 Accepted 9 August 2018 Available online 10 August 2018 Keywords: Aluminum phosphonate Mesostructure Mesoporous materials Formation mechanism Anion effect

a b s t r a c t Mesostructured aluminum phosphonates (AOP-x) bridging with 1,10 -hydroxyl ethylidene groups, including a lamellar mesostructure (AOP-N) with crystalline framework, a well-ordered 2D-hexagonal mesophase (AOP-Cl), and a particle-packed mesostructure (AOP-S), were simply synthesized in the presence of surfactant cetyltrimethylammonium bromide in the ethanol-water system, by choosing Al(NO3)3, AlCl3 and Al2(SO4)3 as the aluminum source, respectively. The crystallinity, morphology, mesophase, and skeletal structure of the as-prepared materials were characterized by XRD, TEM, SEM, 27Al, 31P and 13 C MAS NMR, and nitrogen sorption techniques. After calcination under N2 at 350 °C, the calcined AOP-x samples consist of aluminum phosphonate and phosphate, possessing desirable specific surface  2 areas of 116–585 m2/g. The effect of the inorganic counteranions (NO 3 , Cl and SO4 ) from the aluminum source on the formation of different AOP-x mesostructures was discussed in terms of their bind strength to the headgroups of the surfactant micelles, suggesting the potential for designed synthesis of nonsilica-based mesostructured organic-inorganic hybrid materials. Ó 2018 Elsevier Inc. All rights reserved.

⇑ Corresponding author. E-mail address: [email protected] (Z.-Y. Yuan). https://doi.org/10.1016/j.jcis.2018.08.030 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

X. Lin et al. / Journal of Colloid and Interface Science 532 (2018) 718–726

1. Introduction Since the discovery of the M41S family, the synthesis of mesostructured materials has been extended to various compositions such as metal oxides, carbons, metal phosphates and organophosphonates [1–4]. Metal organophosphonates are a typical class of non-silica-based organic-inorganic hybrid materials [5]. It is attractive to investigate the preparation of mesostructured and mesoporous metal (such as Ti, Al, Zr, Sn, etc.) organophosphonates with different mesophases and morphologies, because of their bifunctional and combined properties derived from both organic and inorganic species [4–13]. These structures have a broad range of potential applications, such as adsorbents, catalysts, electronic devices and optical materials [5,14–16]. Surfactant-templating strategies have been developed to efficiently fabricate mesostructured materials, and a variety of surfactants, including non-ionic block copolymers, cationic and anionic surfactants, are purposely selected for the preparation of a specific mesophase in a given system. In addition, it is reported that the synthesis factors, including the raw material precursors and the inorganic/organic additives, also have fundamental influences on the mesostructure and morphology of the final materials [17–19]. Such factors have been studied for silica-based materials, such as the variation of silica precursor (TEOS, TMOS and silicate clay) [20], the addition of inorganic (NaCl, Na2SO4 and NaI) [21–23] and organic additives (ethanol and butanol) [24–26] in the initial synthesis system. Sometimes the additives can give unexpected results [27,28]. For example, a new Ia3d cubic mesostructured silica was formed via triblock copolymer-assisted synthesis in the presence of NaI in acidic solution, and a structure with an appearance similar to multilamellar vesicle was formed without NaI addition [22]. However, the effect of the additives on the mesostructure formation for non-silica based materials was scarcely investigated, in particular for organic-inorganic hybrid metal organophosphonates. In the case of silica, only a single inorganic component is needed, while two or sometimes three additional components are required to prepare mesostructured organophosphonates [29–31]. The complexity of these synthetic systems makes it difficult to develop a mechanistic understanding of the mesostructure formation process and how the species in the reaction mixture interact with each other. While it is intriguing to study the influence of inorganic and/or organic additives on the mesophase formation of the hybrid materials, these additives would make the multi-component system of metal organophosphonates extremely complex such that the effect of the additives on the mesostructure and morphology of the final hybrid products would be all but impossible to explain. In this work, the synthesis of aluminum phosphonate mesostructures was selected as an example to explore the influence of inorganic additives on the formation of the mesostructures of the organic-inorganic hybrid materials. To simplify the inorganic additives, aluminum nitrate, chloride and sulfate were adopted both as aluminum sources and as inorganic anion additives, and 1-hydroxye thylidene-1,10 -diphosphonic acid (HEDP) was used as the phosphorus source. With the assistance of the surfactant cetyltrimethyl ammonium bromide (CTAB) as a templating agent, aluminum phosphonates (AOP-x) with different mesophases (lamellar, 2Dhexagonal and particle-aggregated mesophase) and morphologies (flower-like, block- and irregular-shaped particles) were obtained.

2. Experimental section 2.1. Materials and synthesis CTAB, AlCl36H2O, Al(NO3)39H2O, and ethanol (95 wt%) were obtained from Tianjin Guangfu Fine Chemical Research Institute.

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Al2(SO4)318H2O was provided by Tianjin Huadong Reagent Co. HEDP (60 wt%) was donated by Henan Qingyuan Chemical Co. Distilled water was used all through. All chemicals were used as received without further purification. The synthesis of aluminum organophosphonate mesostructures was carried out according to the literature [8] with a small modification. In a typical synthesis procedure, 5.4 g of CTAB (0.015 mol) was dissolved in a solution of ethanol/water (30 mL/10 mL), followed by the addition of 6.87 g of HEDP (0.02 mol) under stirring. After the chemicals dissolved completely, 7.11 g of aluminum chloride (0.03 mol) was added to the solution very slowly under vigorous stirring and the stirring was maintained for 30 min. A clear solution (Al:HEDP:CTAB = 3:2:1.5) was obtained and emptied into a dish to volatilize the solvent at room temperature. After complete drying at 50 °C, the solid product was powdered for characterization. When aluminum nitrate and sulfate were used as aluminum sources, the syntheses were conducted under the same synthetic procedure as above. The obtained products were named as AOP-x, where x refers to Cl, N and S, respectively. Since the as-synthesized AOP-x samples was completely dissolved in distilled water and partly dissolved in alcohols such as methanol and ethanol, the removal of surfactant molecules was conducted by low-temperature calcination. The powder sample was heated to 300 °C at a heating rate of 2°/min in a N2 flow and held there for 4 h, followed by calcination at 350 °C for another 2 h. Additional calcinations were carried out under O2 to test the stability of the mesostructures. 2.2. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus diffractometer with Cu Ka radiation operated at 40 kV and 40 mA, and a Bruker Nanostar small angle X-ray scattering system, with a step width of 0.02° and 0.01° for wide-angle and small-angle XRD pattern collection, respectively. N2 adsorption– desorption isotherms were recorded on a Quantachrome NOVA 2000e sorption analyzer at liquid nitrogen temperature (77 K). The samples were degassed at 120 °C for 4 h prior to the measurement. The surface area was calculated by the multi-point Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the adsorption branch of the isotherms by the non-local density functional theory (NLDFT). Transmission electron microscopy (TEM) was carried out on a JEOL JEM-2100F microscope at 200 kV. Scanning electronic microscopy (SEM) was carried out on a Shimadzu SS-550 microscope at a working voltage of 15 kV. The materials in fine powder were stuck on the sample holder made by copper and then coated with a thin layer of gold by using the vacuum sputtering method. Solid state 27Al, 31P and 13 C MAS (magic angle spinning) NMR (nuclear magnetic resonance) spectra were measured on a Varian Infinity plus-400 spectrometer. The chemical shifts of the 31P MAS NMR spectra were referenced to 85% H3PO4. 3. Results and discussion 3.1. Characterization of the as-synthesized materials Fig. 1 shows the low- and wide-angle XRD patterns of the assynthesized AOP-x samples. The as-synthesized AOP-Cl sample shows a typical 2-D hexagonal mesophase (Fig. 1 (left)), as evidenced by a strong (1 0 0) diffraction peak at 2h = 1.8° with d spacing of 4.9 nm, together with two weak (1 1 0) and (2 0 0) reflections in the range of 34° (2h) with d spacing of 2.6 and pffiffiffi 2.3 nm, revealing a 1: 3:2 ratio of the three d-spacings. As to the sample AOP-N, two diffraction peaks at 2h = 2.7° and 5.4°,

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Fig. 1. Low-angle (left) and wide-angle (right) XRD patterns of the as-synthesized AOP-x samples.

corresponding to the d-spacing of 3.27 and 1.63 nm, respectively, show their d-spacing ratio of 1:2, suggesting the (0 0 1) and (0 0 2) reflections of the lamellar mesophase. While for the sample AOP-S, there is only one broad peak at 2h = 2.2° observed, implying the ordered mesostructure from the aggregate of nanoparticles [32]. The pore walls for AOP-Cl and AOP-S are amorphous according to their wide-angle XRD patterns (Fig. 1 (right)). Interestingly, the pore walls for AOP-N are crystalline, although we could not define its exact phase yet. In addition, the striped features observed in the TEM images of AOP-N (Fig. 2A and B) and the crystalline pore

walls evidenced in Fig. 2B both demonstrate that the AOP-N sample has a lamellar mesophase and crystalline hybrid framework. Fig. 3 represents the SEM images of the AOP-x samples. Blocklike particles with roughly smooth surface are observed for AOPCl with particle sizes in the range of 9–71 lm (Fig. 3A and B). While the AOP-S particles have an irregular shape with particle sizes of 7–110 lm, which could be further accumulated by small-sized particles, giving an uneven surface (Fig. 3C and 3D). And the AOP-N sample has a perfect flower-like morphology with the size in the range of 0.8–2.0 lm (Fig. 3E and F).

A

20 nm

B

10 nm C

100 nm Fig. 2. TEM images of the as-synthesized AOP-N (A and B), the calcined AOP-S (C) and the calcined AOP-Cl (D).

D

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Fig. 3. SEM images of the as-synthesized AOP-x samples: AOP-Cl (A and B), AOP-S (C and D), and AOP-N (E and F).

The 27Al, 13C and 31P MAS NMR spectra of the as-synthesized AOP-x samples are shown in Figs. 4–6. The 27Al MAS NMR spectra of the as-synthesized materials show an asymmetric profile at around 11 ppm, assignable to Al atoms in six-coordinated species (Fig. 4 (left)). On the basis of the asymmetric profile, the Al atoms could be six-coordinated, bonding to the PO3 groups as well as to the retained OH and H2O [7,32–34]. The 13C CP/MAS NMR spectra in Fig. 5 (left) for the as-synthesized AOP-x samples show one characteristic peak at 69 ppm, which can be attributed to the P-C species from the organophosphonate groups integrated in the hybrid framework [35], in comparison with pure HEDP. The additional four peaks in the range of 15–55 ppm are assignable to the ACH3 and ACH2A moieties from CTAB [36], besides the ACH3 groups from organophosphonate. Since the AOP-x samples were achieved through solvent evaporation and self-assembly, all components should be included in the final hybrid materials. Taking AOP-Cl as an example, because the initial component molar ratio of AlCl3: HEDP:CTAB is 3:2:1.5, the weight for the hydrocarbon content from HEDP could be calculated to be 4.14 wt% and the surfactant CTAB at 47.6 wt%, without considering water molecules embedded in the final as-synthesized samples. Hence, it is reasonable for the

13

C CTAB peaks in the hybrid materials to be quite intense, and the relative peak intensity for the bridging carbon species (17 ppm) from the organophosphonate groups to be weakened and often difficult to discern due to the large content differences. The 31P MAS NMR spectra of the as-synthesized materials in Fig. 6 (left) show a broad signal centered at 8.0 or 5.5 ppm, which is assignable to the diphosphonate groups („PAC(OH)(CH3)AP„). The NMR results indicate that organophosphonate groups are preserved well during the synthesis. According to the above results, it can be concluded that through the solvent-evaporation and self-assembly, three aluminum phosphonate (AOP-x) mesostructures with different morphologies: flower-like AOP-N with lamellar mesophase, block-like AOP-Cl with 2-D hexagonal mesophase, and particle-aggregated AOP-S mesostructure, were synthesized with the use of Al(NO3)3, AlCl3 and Al2(SO4)3 as aluminum sources, respectively. 3.2. Characterization of the calcined AOP-x mesostructures Fig. 7 shows the low- and wide-angle XRD patterns of the calcined AOP-x samples. The main diffraction peak at low angle for

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Fig. 4.

27

Al MAS NMR spectra of the as-synthesized (left) and calcined (right) AOP-Cl, AOP-S and AOP-N samples.

the calcined AOP-Cl is still preserved but widened with the disappearance of two weak reflections when compared with the assynthesized one, suggesting the collapse of mesostructure order during calcination. For the calcined AOP-N, all reflections, both at wide- and low-angle ranges, disappeared, demonstrating the collapse of lamellar structure and the decrease in crystallinity for the hybrid framework. As to the calcined AOP-S, no diffraction peaks were observed as well, implying the damage of mesostructure order from particle aggregation. Fig. 2C shows the TEM image of the calcined AOP-S sample, revealing the packing of nanoparticles. Mesovoids with sizes of mainly 3–10 nm as well as a small quantity of 50–100 nm voids are observed. While, uniform pores with diameters of about 2.0 nm are observed in the calcined AOP-Cl sample (Fig. 2D), having a somewhat ordered pore arrangement throughout the entire particle. Apparently, the calcination process caused great damage to the mesostructure order of the as-synthesized AOP-x materials. Fig. 8 shows the N2 adsorption-desorption isotherms of the calcined AOP-x materials and their corresponding pore size distribution curves. The isotherms of the calcined AOP-Cl are of type IVc with a sharp increase in N2 volume adsorbed at relative pressure P/P0 = 0.25–0.35, without visible hysteresis loop. This is indicative of reversible capillary condensation–evaporation in the mesopores, which is typical for some OOINs (ordered organic–inorganic nanocomposites) with accessible mesostructures [37]. The isotherms of AOP-N are of type I, characteristic of microporous materials, and a hysteresis loop of type H3 is observed, implying the occurrence of narrow slit-like pores [37]. AOP-S presents the isotherms of type IV with an N2 adsorption volume increase at a relative wide range of P/P0 = 0.4–0.8, suggesting the presence of mesopores with a broadening pore size distribution, and a

hysteresis loop close to type H1 indicates that the mesopores for AOP-S are from the assemblies of rigidly joint particles [38]. A narrow pore size distribution centered at 1.8 nm, calculated by the NLDFT method, is provided for AOP-Cl. Two pore sizes around 1.5 and 2.5 nm are determined for AOP-N, while AOP-S shows mesopores in the diameter range of 3.4–5.7 nm. The results obtained from N2 adsorption analysis are consistent with those determined from TEM images. The detailed textural properties of AOP-x samples are listed in Table 1, showing significant BET surface areas for AOP-S (585 m2/g), AOP-N (113 m2/g) and AOP-Cl (427 m2/g) as well as their large pore volumes of 0.55, 0.07 and 0.28 cm3/g, respectively. To test the mesostructure stability of AOP-Cl, further calcination was conducted under an O2 flow for another 4 h after heated in N2 at 350 °C for 2 h. Unfortunately, the mesostructure was destroyed completely with a specific surface area of only 52 m2/g. The complete collapse of mesostructure is probably caused by the combustion of organic moieties of the organophosphonate integrated into the hybrid framework. It is also suggested that calcination under N2 is a feasible way to remove the surfactant from the AOP-x materials to produce a significant level of porosity. The 27Al MAS NMR spectra of the calcined hybrid materials are shown in Fig. 4 (right), exhibiting resonances for four- and fivecoordinated Al species (38 and 8 ppm respectively), in addition to the six-coordinated Al species ( 14 ppm) that of the uncalcined materials [8,33–35]. During calcination, the six-coordinated AlO6 species could partly convert into four-coordinated (such as Al(OP)4 and/or Al(OP)4-xOHx) and five-coordinated (such as Al (OP)5 and/or Al(OP)5-xOHx) Al species through dehydration. 13 C CP/MAS NMR spectra of the calcined AOP-x samples are shown in Fig. 5 (right). In comparison with the as-synthesized

X. Lin et al. / Journal of Colloid and Interface Science 532 (2018) 718–726

Fig. 5.

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C MAS NMR spectra of the as-synthesized (left) and calcined (right) AOP-Cl, AOP-S and AOP-N samples.

AOP-Cl, the calcined samples exhibit a great reduction in intensity at 54.6 ppm, implying the breakage of C-N bond from the headgroup of CTAB, but the alkyl chains from CTAB molecules at 15– 30 ppm well remained during the calcination process at 350 °C under N2. The calcined AOP-S also exhibits the signal at 15–30 ppm with the intensity much lower than that of the calcined AOP-Cl, as well as a new broad peak in the range of 125–148 ppm. Similarly, the AOP-N sample exhibits a broad signal at 114– 148 ppm accompanied with a weak peak at 70 ppm from the organophosphonate and a weak broadened signal at 15–30 ppm. It is noteworthy that the residual alkyl groups in AOP-N almost disappeared. The reason for this is that the nitrate would decompose slowly under heating with the generation of oxygen, which accelerated the combustion of alkyl groups. Because the combustion of CTAB under N2 would first break the CAN bond with the generation of alkyl species, and their further decomposition would lead to the production of alkene-contained coke. So the new broad peak in AOP-S at 125–148 ppm and the one in AOP-N at 114–148 ppm should be ascribed to alkene-contained coke derived from the thermal decomposition of CTAB [39,40]. Among the three hybrids, AOP-Cl has the most amount of coke. And the presence of large amount of residual coke in the calcined AOP-x samples should be responsible for the difficult observation of P-C signal from HEDP in the 13C NMR spectra. The 31P MAS NMR spectrum of calcined AOP-Cl in Fig. 6 (right) shows a broadened peak centered at around 8.0 ppm with a range of 12.1 to 35.2 ppm, implying the complex chemical environment of organophosphorus. A new peak appears at around 24.7 ppm, which could be ascribed to phosphate groups (PO4) from the organophosphorus decomposition [41]. The phosphate species

were also found in the calcined AOP-N (25.3 ppm) and AOP-S (25.7 ppm), as well as a peak at 0.3 ppm and 2.2 ppm for phosphonate groups in these two samples, respectively. The peak shift for organophosphonate groups of AOP-x samples before and after calcination could be caused by the further condensation of hybrid aluminum organophosphonate framework, with the production of four- and five-coordinated Al species in the calcined materials (Fig. 4 (right)). Thus, during calcination, porous AOP-x materials consisting of aluminum organophosphonate and phosphate were produced with relatively high specific surface areas and narrow pore size distributions. 3.3. The effect of the aluminum source on the formation of the mesostructure phases Three mesostructured aluminum phosphonates, including lamellar, 2-D hexagonal and particle-aggregated mesophases, were generated with the use of Al(NO3)3, AlCl3 and Al2(SO4)3 as aluminum sources, respectively. Clearly, the inorganic aluminum sources play an important role in determining the final mesostructure of the prepared hybrid materials. It was reported that anions   3 (e.g. SO2 ) would be more or less hydrated in 4 , Cl , Br and NO solution and could be categorized into three types: fully hydrated (A), partially hydrated (B), or not hydrated at all (C) [19]. Less strongly hydrated anions had, in general, smaller ionic radii and bound more closely and strongly on the headgroups of surfactants such as CTA+. SO2 4 ions belong to type A, which was not effective in  neutralizing the surfactant positive charge, while NO 3 and Cl ions belonged to type B, and NO anions were less hydrated than Cl 3 ions.

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Fig. 6.

31

P MAS NMR spectra of the as-synthesized (left) and calcined (right) AOP-Cl, AOP-S and AOP-N samples.

Fig. 7. Low-angle (left) and wide-angle (right) XRD patterns of the calcined AOP-x samples.

The surfactant packing parameter g was used here: g = v/al, where v is the chain volume, a is the hydrophobic/hydrophilic interfacial area, and l is the chain length. The small anions have the strong electrostatic interaction with the cationic surfactant and the decrease in the effective area of surfactant a, resulting in  a significant increase in the g value. For the NO 3 and Cl anions, they are more likely to reduce the repulsive force between the

cationic surfactant headgroups, being easy to the formation of mesophase with a large g parameter [27]. Therefore, lamellar phase with g = 1 was obtained for AOP-N, and 1/3 < g < 1/2 for AOP-Cl with 2-D hexagonal mesophase. Che et al. reported that H2SO4 led to the facile formation of the 3D-hexagonal P63/mmc silica mesophase with a small g parameter, and HNO3 favored the forma tion of the Ia3d mesophase with a large g parameter [27]. The effect

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Fig. 8. N2 adsorption-desorption isotherms (left) of AOP-x samples, and their corresponding pore size distribution curves (right).

Table 1 Textural properties for AOP-x samples calcined under N2.

a

Samples

SBET (m2/g)

Smicro (m2/g)

Vpore (cm3/g)

Vmicro (cm3/g)

DDFT (nm)

AOP-Cl AOP-N AOP-S AOP-Cla

427 113 585 52

354 74 – –

0.28 0.07 0.55 0.05

0.23 0.03 – –

1.8 1.5, 2.5 3.4–5.7 4.0–17.8

Calcined under O2 at 350 °C for 4 h.

of anions on the formation of aluminum phosphonate mesophases, in view of surfactant packing parameter g herein, is consistent with the reported previously in the silicate system. In addition, the effects of counteranion selection on the crystallinity of mesoporous silicas have been reported [27,28]. Yu et al. reported that the addition of K2SO4 and Na2SO4 to the non-ionic amphiphile F108-templated system increased the usually

weak interaction between the silicate species and the hydrophilic headgroup of the non-ionic block copolymer, favouring the formation of the facetted single-crystals of mesoporous silica [28]. In this work, influenced by the strong electrostatic interaction + between NO 3 and CTA , a lamellar crystalline aluminum phosphonate was obtained, and a possible formation route is proposed in Scheme 1. Firstly, all the ionic compounds would ionize in the

Scheme 1. A proposed formation process for lamellar AOP-N.

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mixed solution, e.g. R(PO3H2)2 (HEDP„R(PO3H2)2) would ionize into H+ cations and R(PO3H)2 anions, making the whole solution  acidic. Then NO and Br ions would compete for the 3 , RPO3H + CTA in the mixed solution, and finally NO 3 ions would be more adjacent to CTA+ micelles because of their relatively stronger binding force. Meanwhile, Al3+ cations would polymerize with R (PO3H)2 anions to form hybrid oligomers. The newly formed hybrid oligomers moved closer to micelle surface, which was surrounded by NO 3 anions, to further form monomers. And those monomers should be orderly arranged due to the strong electrostatic force from the NO 3 anions. With the evaporation of the solvent at room temperature, a white crystalline solid with a lamellar mesostructure was finally formed. In contrast, for the samples prepared with Cl or SO2 4 anions, due to their relatively weak binding force to CTA+, amorphous pore wall were obtained [42]. Recently Kimura and his coworkers reported the synthesis of crystalline lamellar-mesostructured aluminum phosphonate by using aluminum isopropoxide (Al(OiC3H7)3) as the aluminum source in alkaline solution templated with CTAB [43], and in early they found a 2-D hexagonal aluminum phosphonate with Al(OiC3H7)3 at pH = 6.7 [8]. It is indicated that the pH value plays an important role in determining the mesostructured phase as well as the crystallinity of the hybrid pore walls in their synthesis system. While, in our work, by selecting Al(NO3)3 as Al source, a lamellarmesostructured aluminum phosphonate with crystalline framework was simply produced in acidic solution.

(B12015), and the Fundamental Research Funds for the Central Universities (63185015). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

4. Conclusions Mesostructured aluminum phosphonates of flower-like AOP-N, block-like AOP-Cl and particle-aggregated AOP-S have been successfully synthesized through solvent-evaporation and selfassembly with the use of Al(NO3)3, AlCl3 and Al2(SO4)3 as aluminum sources, respectively, presenting lamellar mesophase with crystalline pore walls (AOP-N), 2-D hexagonal mesophase (AOP-Cl) and particle-aggregated mesostructure (AOP-S). The calcined products consist of aluminum phosphonate and phosphate, possessing desirable specific surface areas in the range of 116–585 m2/g. The presence of different inorganic counteranions in the reaction media, as introduced by selecting different aluminum sources, provides the opportunity to manipulate the mesophase structure of AOP-x, as well as their morphology. The effect of additives on the mesostructure formation for silica-based materials has been widely investigated [20–28]. For non-silica-based materials, however, it is scarcely reported. The present work extends the knowledge of the effect of inorganic additives on the formation of mesostructure from inorganic silicas to organic-inorganic hybrid materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (21421001, 21503041, 21573115), the 111 project

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