Structure and thermal stability of mesostructured zirconium oxophosphates

Microporous and Mesoporous Materials 100 (2007) 295–301 www.elsevier.com/locate/micromeso

Structure and thermal stability of mesostructured zirconium oxophosphates Yifeng Zhang a, Yongsheng Li a, Yasuhiro Sakamoto a

b,*

, Osamu Terasaki b, Shunai Che

a,*

School of Chemistry and Chemical Technology, State Key Laboratory of Composite Materials, Shanghai JiaoTong University, Shanghai 200240, PR China b Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-10691 Stockholm, Sweden Received 25 March 2006; received in revised form 3 November 2006; accepted 6 November 2006 Available online 26 December 2006

Abstract Highly ordered mesoporous zirconium oxophosphates (designated as ZOP) with hexagonal P63/mmc and cubic Fm 3m symmetries were firstly prepared by using gemini cationic surfactants as templates. It has been found that the thermal stability was elevated with the structure curvature order: cubic Ia3d < 2D-hexagonal p6mm < cubic Fm 3m, hexagonal P63/mmc and cubic Pm 3n. The ZOP mesoporous materials with cubic Fm3m and hexagonal P63/mmc structures were stable up to 800 C, which provides a new insight into the structural factors governing self-assembly of thermally stable mesoporous materials and would open up new possibilities of porous materials for advanced applications.  2006 Elsevier Inc. All rights reserved. Keywords: Mesoporous materials; Thermal stability; Zirconium oxophosphate; Self-assembly; Mesostructure

1. Introduction Non-siliceous materials with ordered structures are extensively recognized to be much more interesting and desirable in terms of their potential applications on chemical, electrical, magnetic, and optical properties [1–3]. To date, a variety of non-siliceous mesoporous materials have been synthesized based on titanium [1], niobium, zirconium, cerium, vanadium, aluminum, tantalum, and most of them are 2D-hexagonally ordered or disordered [3]. Due to the structural diversity of non-siliceous chemistry, it is difficult to directly and simply transfer the synthetic strategies from mesoporous silica to non-silica mesostructured materials [1]. The non-siliceous frameworks are usually more susceptible to lack of condensation, redox reactions or phase transitions accompanied by thermal breakdown of the structural integrity. The tendency for

*

Corresponding authors. E-mail addresses: [email protected] (Y. Sakamoto), chesa@sjtu. edu.cn (S. Che). 1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.11.014

structural collapse under thermal treatment has diminished their functionality and practical utility [1]. Consequently, improving thermal stabilities has become the major challenge in designing the synthesis of periodic mesoporous materials. As far as zirconium-based mesoporous materials are concerned, cationic [1], non-ionic [4] and anionic [4] surfactants have been used to synthesize its mesostructured materials following a mechanism of the surfactant self-assembly and subsequent and/or simultaneous condensation of inorganic precursors [1,4]. Since the birth of two-dimensional (2D)-hexagonal mesoporous zirconium oxophosphate (designated as ZOP) through a special post-synthetic treatment [5], two other cubic mesostructured ZOP with Ia 3d [6] and Pm3n [6] symmetries were synthesized in succession. In this work, we have for the first time explored the fabrication of well ordered 3D structured cubic Fm 3m and hexagonal P63/mmc mesostructured ZOP materials, and studied the correlations of the thermal stability with its mesostructure. It has been found that the thermal stability increases in the order: cubic Ia3d < 2D-hexagonal p6mm < cubic Pm3n, 3D-hexagonal P63/mmc and cubic Fm 3m.

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2. Experimental section 2.1. Synthesis of surfactant Gemini surfactants [CnH2n+1N(CH3)2(CH2)s(CH3)2NCm H2m+1]Br2 are kinds of two-chain dicationic surfactants, which can generally be designated as Cn-s-m, where n and m refer to the length of the alkyl tails and s is the number of methylene units of the alkyl spacer [7,8]. The gemini surfactants used in our synthesis are [CnH2n+1N(CH3)2(CH2)s (CH3)3N]Br2 (designated as Cn-s-1), [CnH2n+1N(CH3)2 (CH2)s(C2H5)3N]Br2 (designated as Cn-s-2). Typically, the gemini surfactant C18-3-1 and [C16H33N(CH3)2(CH2)3N (C2H5)3Br2] (C18-3-2) were synthesized by the reaction of N,N-dimethyloctadecylamine (C18H37N(CH3)2) (from TCI, Japan) with (3-Bromopropyl) trimethylammonium bromide (Br(CH2)3N(CH3)3)Br and (3-bromopropyl) triethylammonium bromide (Br(CH2)3N(C2H5)3)Br (both from Aldrich); C18H37N(CH3)2 (0.15 mol) was added to (Br(CH2)3N(CH3)3)Br (0.1 mol) and (Br(CH2)3N(CH3)3)Br (0.1 mol) in acetone (400 ml), respectively. The reaction mixture was stirred for 7 days under reflux conditions. The product was decanted and purified by recrystallization from an acetone solution. The resulting product was separated by filtration and dried under vacuum for several hours at 60 C. C14-3-1, C16-3-1, C16-3-2 and C18-2-18 were prepared through the same methods. 2.2. Synthesis of mesostructured ZOP High-quality mesoporous zirconium-based materials were prepared using the cationic gemini surfactants as templates, and zirconium sulfate as the inorganic precursor. In a typical synthesis, hexagonal P63/mmc mesostructured zirconium oxophosphate was synthesized as follows: 0.503 g surfactant C14-3-1 was dissolved in deionized 12 g water at room temperature, and 1.45 g zirconium sulfate (Zr(SO4)2 Æ 4H2O) (China Chemical Reagent Corporation) was added into the surfactant solution. After 3 h vigorous stirring, the mixture was transferred to a teflon bottle to cure for 48 h at 100 C. The product zirconium oxide sulfate was collected by filtering it and then rinsing it with deionized water. The mesostructured zirconium oxide sulfate was then put into an aqueous solution of 100 ml (0.5 M) H3PO4 and stirred for 10 h at ambient temperature. The resultant product was filtered, washed and dried at 100 C overnight and the surfactant was removed by calcination. The products were calcined at 300 C for 3 h. Their thermal stability was studied by heating to higher temperatures (600, 700, 800, 900 and 1000 C) for 6 h (the heating rate was 1 C/min for both steps). 2.3. Characterization The X-ray diffraction (XRD) measurements were performed on a Rigaku D/MAX-2200/PC powder X-ray diffractometer using Cu Ka radiation (40 kV and 20 mA).

High resolution transmission electron microscopy (HRTEM) was conducted with a JEOL-3010 microscope, ˚ ). Images operating at 300 kV (Cs 0.6 mm, resolution 1.7 A were recorded with a CCD camera (Gatan MSC model 794, 1024 · 1024 pixels, pixel size 24 · 24 lm) at 50,000– 100,000 k magnification under low-dose conditions. The thermogravimetric analyses were performed on a Netzsch STA 449 C thermobalance coupled with a Balzers Thermostar 442 mass spectrometer. The measurements were carried out under air with a heating rate of 5 K/min in all the cases. 3. Results and discussion The primary difficulty on obtaining thermally stable zirconium oxophosphate with various structures lies in choosing the appropriate surfactants to control surface curvatures of micelles in aqueous solutions [6]. Our approach to synthesize new mesostructured ZOP is based on selecting various structured gemini surfactant that form the high curvature lyotropic liquid crystal due to their larger head-groups. The surfactants and obtained mesophases are summarized in Table 1. Obviously, the effective headgroup area of the surfactants decreased in the following order: C16-3-2 > C16-3-1 > CTMABr and C18-3-2 > C18-3-1; and the chain length of the surfactant decreased in the order of C18-3-1 > C16-3-1 > C14-3-1. The overall mesostructure is determined by the geometry of the surfactant including chain length and head-group area. The mesostructures with decreasing curvature order of cubic Fm3m, hexagonal P63/mmc and cubic Pm3n > 2D-hexagonal p6mm have been synthesized according to these head-group and chain length changing order. C18-12-18 with a large effective volume has been found to be effective in forming the cubic Ia3d mesophases with lower curvature. Fig. 1 shows the XRD patterns of as-synthesized and calcined ZOP samples templated by gemini surfactant C18-3-2. The mesostructure has been confirmed to be cubic Fm3m by TEM observation (shown later). For as-synthesized sample, the first two well-resolved peaks in the range of 2h = 1.5–2.5 were indexed to 1 1 1 and 2 0 0 diffractions, and additional two weak peaks in the range of 3.5–6 were indexed to 2 2 0 and 3 1 1 diffractions [9], which is characteristic of cubic Fm3m mesophase with the unit cell parameter, Table 1 Surfactants and obtained ZOP mesophases Surfactant C18H37N(CH3)2(CH2)3N(C2H5)3Br2 (C18-3-2) C16H33N(CH3)2(CH2)3N(C2H5)3Br2 (C16-3-2) C14H29N(CH3)2(CH2)3N(CH3)3Br2 (C14-3-1) C16H33N(CH3)2(CH2)3N(CH3)3Br2 (C16-3-1) C18H37N(CH3)2(CH2)3N(CH3)3Br2 (C18-3-1) C16H33N(CH3)3Br (CTMABr) C18H37N(CH3)2(CH2)12N(CH3)2C18H37Br2 (C18-12-18)

Mesophase  Cubic Fm3m Cubic Fm3m 3D-hexagonal P63/mmc Mixture of P63/mmc and Pm3n Cubic Pm3n 2D-hexagonal p6mm Cubic Ia3d

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1000 ºC

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111

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110 103 112

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311

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as-synthesized

as-synthesized

2

3

4

5

6

2θ / o Fig. 1. XRD patterns of as-synthesized and calcined ZOP mesoporous materials templated by C18-3-2. Insets are the calcination temperatures. The synthesis molar composition is C18-3-2:H2O:Zr(SO4)2 = 1:667:5.

2

3

4

2θ /

5

6

o

Fig. 3. XRD patterns of as-synthesized and calcined ZOP mesoporous materials templated by C14-3-1. Insets are the calcination temperatures. The synthesis molar composition is C14-3-1:H2O:Zr(SO4)2 = 1:667:4.

Fig. 2. HRTEM images taken along [1 1 0] zone axis and corresponding ED patterns of calcined ZOP mesoporous materials templated by C18-3-2 at (a) 600 C, (b) 800 C, (c) 900 C and (d) 1000 C.

a = 8.08 nm. This result indicates that the as-synthesized materials have a high degree of cubic Fm 3m mesoscopic

organization, which has not been reported before. The cubic Fm3m mesostructure was retained even after calcining

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ing after calcination even at 1000 C. The sample calcined at 1000 C was crystalline without mesostructure as indicated from long range XRD pattern (see Supporting information S2). These results sufficiently confirm that ZOP templated by C18-3-2 has an extraordinary high thermal stability up to 800 C which is the highest values so far in zirconium-based ordered mesostructured materials [4–6]. Fig. 3 shows the XRD patterns of the ZOP sample templated by C14-3-1. The as-synthesized sample showed the XRD pattern with three well-resolved peaks in the range of 2h = 1.5–3 indexed to 1 0 0, 0 0 2 and 1 0 1 diffractions, and additional three weak peaks in the range of 3.5–6 indexed 1 1 0, 1 0 3 and 1 1 2 diffractions, which are characteristic of the hexagonal P63/mmc mesophase with the unit cell parameters, a = 4.81 nm and c = 7.85 nm. This gives a

at 800 C, though the peaks intensity decreased with increasing calcination temperature. The unit cell parameter of a decreased from 7.01 nm (500 C) to 6.57 nm (800 C) indicating a substantial contraction in the lattice dimension occurred after calcination and the further densification of the frameworks, which can be explained by the condensation of Zr–OH groups [5,6]. HRTEM image (Fig. 2a) and corresponding electron diffraction pattern (ED, taken with [1 1 0] incidence) of the calcined ZOP sample at 600 C reveal large domain sizes of ordered pore networks. From the analysis of HRTEM images and ED patterns, the symmetry of the crystal was determined to be cubic Fm 3m. As demonstrated by the HRTEM image and their ED patterns in Fig. 2b–d, the cubic Fm 3m structure retained their well-defined order-

Fig. 4. HRTEM images taken along [1 1 0] zone axis and corresponding ED patterns of calcined ZOP mesoporous materials templated by C14-3-1 at (a) 500 C and (b) 900 C.

a

c

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Intensity

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Intensity

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2θ / o

5

6

2

3

4 5 2θ / o

6

Fig. 5. XRD patterns of as-synthesized and calcined ZOP mesoporous materials templated by (a) C18-3-1, (b) CTMABr and (c) C18-12-18. Insets are the calcination temperatures. The synthesis molar composition is (a) C18-3-1:H2O:Zr(SO4)2 = 1:1000:4, (b) CTMABr:H2O:Zr(SO4)2 = 1:1000:2.5 and (c) C18-12-18:H2O:Zr(SO4)2 = 1:884:3.7.

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c/a ratio of 1.63, which is close to the ideal c/a ratio of 1.633 for the hexagonal close-packed (hcp) structure of hard spheres [10]. Similar to the cubic Fm 3m structure, the characteristic XRD diffractions of this hexagonal P63/mmc mesophase remain even when this material has been subjected to a calcination temperature of 800 C. The d0 0 2 spacing of this ZOP decreases by only 0.36 nm on heating from 500 C to 900 C. The HRTEM image of this sample taken along the [1 0 0] zone axis is presented in Fig. 4a. Apparently, cages are stacked along the c-axis merely in the ‘‘ABAB. . .’’ sequence of the hcp structure [10]. The corresponding ED pattern (inset) supports extinction conditions of the reflection for the hexagonal symmetry. The 3D-hexagonal P63/mmc ZOP has also not been reported before. The HRTEM micrograph (Fig. 4b) obviously reveals that precise 3D pores and orientational ordering were achieved after high temperature treatments at 900 C. The large domains of ordering are characteristic of the structural fidelity with high degree of periodic arrangements in the framework. When the C18-3-1, CTMABr and C18-12-18 surfactant were used as templates, ZOP mesoporous materials with cubic Pm 3n, 2D-hexagonal p6mm and cubic Ia 3d symmetry were formed, respectively [5,6]. They are confirmed by combining analysis of the XRD patterns (Fig. 5a–c) and HRTEM images (Fig. 6a, c and e). After calcining at various temperatures, three types of ZOP exhibit different thermal stabilities. Similar to other two micellar cubic Fm 3m and hexagonal P63/mmc mesophases, calcined Pm 3n mesostructured ZOP [6] also exhibited a high thermal stability up to 900 C as evidenced by XRD patterns (Fig. 5a) and HRTEM images (Fig. 6b). However, the 2D-hexagonal mesostructured ZOP sample treated at 700 C lost its periodicity in mesophase. Surprisingly, the Ia 3d ZOP changed its d value at a much lower temperature of 600 C implying a complete collapse of mesostructures. These results could all be further verified by corresponding HRTEM images (Fig. 6d and f). Specifically, it has been found that with the surfactant C16-3-1, a mesostructure of mixed Pm 3n (Fig. 7a) and P63/ mmc (Fig. 7b) was obtained as confirmed by TEM images. It is clear that upon increasing the chain length of gemini surfactants (from n = 14, n = 16 to n = 18), hexagonal (P63/mmc symmetry), mixed and a cubic (Pm 3n symmetry) mesostructured ZOP can be obtained, respectively, implying that from C14-3-1 through C16-3-1 to C18-3-1, a phase transfer occurs. This is reasonable that a mixed mesostructure of the two symmetries would be formed for the interface curvature of the C16-3-1 surfactant in aqueous solution being between that of C14-3-1 and C18-3-1. After the calcination temperature was increased to 1000 C, the XRD measurement shows lower and broadened reflections or even no peaks absolutely signifying the destruction of the mesostructures. Typical HRTEM images (Supporting information S1) for the cubic Fm3m mesostructured ZOP sample calcined at 1000 C suggest the crystallization of the zirconium oxophosphate and

299

Fig. 6. HRTEM images and ED patterns of ZOP mesoporous materials synthesized with: (a) C18-3-1, calcined at 500 C, HRTEM taken along [1 0 0] zone axis; (b) C18-3-1, calcined at 900 C, HRTEM taken along [1 1 0] zone axis; (c) CTMABr, calcined at 500 C, HRTEM taken along [1 0 0] zone axis; (d) CTMABr, calcined at 700 C, HRTEM taken along [1 0 0] zone axis; (e) C18-12-18, calcined at 500 C and (f) C18-12-18, calcined at 600 C.

simultaneously the loss of the porosity [11,12]. This result could be further demonstrated by wide-angle XRD patterns (Supporting information S2). Mesostructured ZOP frameworks have been characterized by 31P MAS NMR. All of the mesostructured ZOP calcined at 500 C show three resonance peaks centered at 12.8, 21.3, and 28.6 ppm similar to the cubic Fm3m mesostructured ZOP as shown in Fig. 8a, which can be attributed to tetrahedral phosphorus-connected two ((HO)2P(O–Zr)2), three (HOP(O–Zr)3) and four zirconia (P(O–Zr)4) corresponding to surface, respectively [6]. As shown in Fig. 8b, after calcined at 700 C, the resonance peaks of (HO)2P(O–Zr)2 and HOP(O–Zr)3 are dramatically decreased and the main peak of P(O–Zr)4 is chemically shifted to higher magnetic fields implying the changing compositions of inorganic walls of the mesostructures. There are no great changes in NMR results for the samples calcined at higher than 700 C. The P/Zr molar ratios for the ZOP mesostructures calcined at 500, 700 and 900 C

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Fig. 7. HRTEM images and ED patterns of calcined (500 C) ZOP sample templated by C16-3-1. HRTEM taken along (a) [1 0 0] and (b) [1 1 0] zone axis.

b

should be attributed to desorption of organic species. Comparing to the 2D-hexagonal and cubic Ia3d ZOP samples deduced by TGA weight loss from 100 C, the cage-type 3n mesocubic Fm3m, hexagonal P63/mmc and cubic Pm structured ZOP samples with the similar higher thermal stability show similar lower template contents. It is evident that the higher the mesostructure curvature, the lower the surfactant content per unit weight and higher the thermal stability. The surface areas of calcined cubic Fm3m, hexagonal P63/mmc and cubic Pm3n mesostructured ZOP samples were very low similar to the Pm3n mesostructured ZOP and TOP reported previously [6b,13]. It is supposed that 3D-hexagonal mesoporous ZOP and its cubic Fm 3m mesostructured counterpart have a cage-type structure with small pore windows, which is easy to close upon calcination with induction of pore shrinkage process. However, their mesostructures could still be retained leading to the resolved XRD patterns and well-ordered HRTEM images. 4. Conclusions

a -60

-40

-20

0

20

40

60

ppm Fig. 8. 31P MAS NMR spectra of cubic Fm3m structured ZOP calcined at 500 C (a) and 700 C (b).

were calculated to be 0.97, 0.87 and 0.79, respectively, by energy dispersive X-ray (EDX) analysis. From the TGA results (Fig. 9), it is established that the textural properties of the samples, subsequently the thermal stability depends on the ratio of zirconium oxophosphate/surfactant. The TG curves show up to two different weight losses. The one below 100 C corresponds to desorption of water, whereas those centered at 400 C

Weight (%)

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30

 and 3DHighly ordered mesostructured cubic Fm3m hexagonal P63/mmc zirconium oxophosphate have been successfully prepared by cationic gemini surfactants. Both Fm3m, P63/mmc ZOP and Pm3n mesostructures are thermally stable up to 800 C, which is exceptionally high thermal stability among mesoporous transition metal oxides prepared via surfactant controlled synthesis so far. Furthermore, it has been demonstrated that the higher the organic–inorganic interface curvature of the mesostructure is, higher the thermal stability of the sample is. The thermal stability of ZOP samples could be sequenced in the following order: cubic Fm3m  3D-hexagonal P 63 =mmc  cubic Pm3n  2D-hexagonal p6mm > cubic Ia3d, which would give new insights in attempts to extend the applications of mesoporous materials to fields such as sensors, optical devices and electrode materials. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 20425102 and 20521140450) and the China Ministry of Education. O.T. thanks Swedish Science Research Council (VR) and Japan Science and Technology Agency (JST) for financial support.

Fm3m P63/mmc Pm3n

Appendix A. Supporting information available

p6mm Ia3d

200

400 600 Temperature (oC)

800

1000

Fig. 9. TGA profiles of as-synthesized mesostructured ZOP with the symmetry of cubic Fm 3m, hexagonal P63/mmc, cubic Pm3n, 2D-hexagonal p6mm and cubic bicontinuous Ia3d.

Typical HRTEM images (Supporting information S1) for the sample calcined at 1000 C suggest the crystallization of the zirconium oxophosphate and simultaneous loss of the porosity [11]. This result could be further demonstrated by wide-angle XRD patterns (Supporting information S2). These materials are available free of charge via the Internet at http://pubs.acs.org. Supplementary data

Y. Zhang et al. / Microporous and Mesoporous Materials 100 (2007) 295–301

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