Synthesis of aluminum-doped mesoporous zirconia with improved thermal stability

Microporous and Mesoporous Materials 186 (2014) 1–6

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Synthesis of aluminum-doped mesoporous zirconia with improved thermal stability Ruiping Liu, Chang-an Wang ⇑ School of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China

a r t i c l e

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Article history: Received 8 July 2013 Received in revised form 12 September 2013 Accepted 14 November 2013 Available online 23 November 2013 Keywords: Mesoporous zirconia Aluminum doping Textural properties Thermal stability

a b s t r a c t Mesoporous zirconia doped with varying amounts of aluminum are synthesized by the evaporation induced self-assembly (EISA) method, and their structure and textural properties are investigated extensively. The results show that in contrast to most other zirconia binary system, doping of zirconia with aluminum stabilizes tetragonal zirconia over the cubic phase, and the crystallization temperature of the as-obtained powders increases with increasing aluminum content, thereby improve the thermal stability of the mesoporous zirconia. The BET surface area and pore volume of the samples doped with the same amount of aluminum decreases with increasing calcination temperature, accompanied by an increase in the mean BJH pore size. Furthermore, the BET surface area and pore volume of the samples calcined at the same temperature increase with increasing amounts of aluminum. For sample with an Al/Zr molar ratio of 1, the disordered ‘‘wormhole-like’’ mesoporous structure remained even after calcination at 1000 °C for 1 h. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Since the first successful synthesis of well-ordered mesoporous silica materials in the 1990s such as the members of the M41S family [1] and SBA-15 [2], many efforts have been directed towards extending the group of ordered mesoporous transition metal oxides due to their potential applications in various areas such as catalysis, separation, sensors, lithium batteries, optoelectronics, solar cells and biotechnology [3–5]. Among these non-siliceous oxides, mesoporous zirconia is highly attractive in the separation and adsorption process, and as part of bifunctional catalysts support and catalysts in large-scale processes in the chemical and petrochemical industry, because of its high melting point, low thermal conductivity, high corrosion resistance, acidic–basic surface and unique red–ox properties [6–8]. In recent years, a lot of research work has been published on the preparation of mesoporous ZrO2 by using either a self-assembled surfactant or a structure-directing agent as a template. Mohamed [9] compared the synthesis methods for the textural properties of mesoporous zirconia and obtained samples with a high surface area of 213 m2/g after post-hydrothermal treatment at 100 °C. I-Ming Hung prepared hexagonal mesoporous 8YSZ using a block copolymer (F127) as the surfactant yielding a BET surface area of 124 m2/g with calcination at 500 °C, which drastically reduced to

⇑ Corresponding author. Tel./fax: +86 10 62785488. E-mail address: [email protected] (C.-a. Wang). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.11.024

29 m2/g after calcination at 800 °C [10]. In addition, I-Ming Hung also reported nano-structured mesoporous yttria-stabilized zirconia (YSZ) powders prepared by using cetyltrimethylammonium bromide (CTAB) as the surfactant and urea as the hydrolyzing agent. The sample had a surface area of 137 m2/g when calcined at 600 °C and drastically reduced to 40 m2/g after calcination at 900 °C [11]. In most cases, mesoporous structures will collapse to varying degrees upon the removal of the template or the subsequent calcination due to the phase transition at high temperatures. It has been shown that the thermal stability can be improved by doping sulfate or phosphate into zirconia [12–13] or post-treatment of the as-made mesoporous zirconia by supercritical carbon dioxide with the presence of small amounts of tetraethoxysilane (sc-CO2/TMOS treatment) [14–15] and alkaline solutions, such as ammonia [16], NaOH, and KOH solutions [17]. However, the maximum temperatures at which the mesoporous zirconia maintains stability do not exceed 850 °C [15]. There are many approaches to improve thermal stability of zirconia. The most effective way is by doping with different metal oxides as a stabilizing agent (MgO, CaO, Y2O3, Sc2O3, etc.). It is well known that the stabilization is achieved by the incorporation of divalent or trivalent cations in the fluorite-type structure by substitution of Zr4+ cations and creation of oxygen vacancies to maintain the local charge balance [18–19]. Al2O3 is usually added to zirconia to suppress grain growth during sintering [20]. Despite the rather limited equilibrium solubility due to the much smaller ionic radius of Al3+ (0.5 Å) compared to that of Zr4+ (0.8 Å), it is suggested that Al3+ ions substitute Zr4+ ions, creating oxygen

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vacancies to maintain local charge balance analogous to other divalent and trivalent cations forming solid solutions with ZrO2 [21]. On the other hand, as a well-known industry catalyst and/ or catalyst support, mesoporous alumina has triggered enormous research activities regarding its rational synthesis and applications in the chemical and petrochemical industry, due to its favorable textural properties, abundant Lewis-acid sites and excellent thermal stability. Mesoporous alumina supports with large surface areas, large pore volumes, narrow pore size and suitable surface acidic–basic properties often result in favorable enhancements in catalytic performance. However, most of the works have been focused on the synthesis of the yttria-doped mesoporous zirconia [10–11], and up to now, only one paper reported the synthesis of aluminum-doped sulfated and oxophosphated mesoporous zirconia [22]. Based on the above reasons, we were focused on the preparation of aluminum doped mesoporous zirconia, which will combine the intrinsic properties of both zirconia and alumina, and meanwhile, the thermal stability of mesoporous zirconia will be improved. This newly mesoporous aluminum doped zirconia will be a promising material as the catalysts support in the chemical and petrochemical industry. In this paper, the mesoporous zirconia with varying amounts of doping aluminum were synthesized using the evaporation-induced self-assembly (EISA) method, and the phase transition and textural properties of the samples calcined at different temperatures were researched extensively. 2. Experiment procedure 2.1. Sample preparation The alumina doped mesoporous zirconia was prepared by evaporation induced self-assembly (EISA) method. In a typical synthesis, 10.45 g of F127 (EO106PO70EO106, EO = ethylene oxide, PO = propylene oxide) was dissolved in 200 ml anhydrous ethanol solution, followed by the addition of 48.3375 g of zirconium oxychloride in 41.76 g of de-ionized water while stirring, which produces a very acidic solution to prevent precipitation. The solutions contain 1Zr: 0.0047F127: 40EtOH: 20H2O (Solution A). Simultaneously, the required amount of aluminum isopropoxide (molar ratios of Al/Zr are 0.1, 0.2, 0.5, 0.8 and 1, respectively) was dissolved in 100 ml anhydrous ethanol solution containing different amounts of nitrate (1.8 molar ratio of Al3+/nitrate) (Solution B). Once dissolved completely, Solution B was added dropwise to Solution A under vigorous stirring at ambient temperature for 30 h. The resulting sol solution was gelled in an open Petri dish and underwent solvent evaporation at a relative humidity of 60% at 20 °C for 3 days followed by a thermal treatment at 100 °C for 24 h. The final products were calcined at 550 °C for 6 h to remove the template with a heating rate of 0.5 °C/min. The obtained samples were heat treated at 700–1000 °C for 1 h with a heating rate of 10 °C/min.

The thermal behavior of the precursor was assessed with TG–DSC analysis. To evaluate the extent of the phase transformations, wide-angle X-ray diffraction (XRD) analysis of the synthesized alumina doped mesoporous zirconia samples was conducted using a Bruker D8 advance instrument (Karlsruhe, Germany). Small angle X-ray diffraction was performed to evaluate the mesoporous structure (Rigaku D/max-2500PC, automatic powder X-ray diffraction). Textural properties of the prepared samples were determined from nitrogen adsorption/desorption isotherm measurements at 196 °C using a model NOVA 3200e

3. Results and discussion 3.1. Thermal behavior of the obtained precursor (Fig. 1) illustrates the TG–DSC patterns of the obtained precursor when the molar ratio of Al/Zr is 0.8. The endothermic peak at 167 °C is attributed to evaporation of water and ethanol in the gel. The endothermic peak at 335 °C is attributed to the removal of the template. The minor exothermic peak at 725 °C is associated with crystallization of ZrO2, and it should be noted that the crystallization temperature varies with different amounts of Al doping. 3.2. Structural aspects of the aluminum-doped mesoporous zirconia (Figs. 2 and 3) show the XRD patterns of the aluminum-doped mesoporous zirconia calcined at different temperatures. It can be seen that the XRD peaks become wider with increasing amounts of aluminum. Furthermore, the aluminum-doped mesoporous zirconia is amorphous when the molar ratio of Al/Zr is larger than 0.5 (Fig. 2(c)–(e)), whereas the powders with lower aluminum contents (Al/Zr 6 0.2) are crystalline, consisting only of t-ZrO2 phase (Fig. 2(b)) and mixtures of t-ZrO2 and m-ZrO2 phases (Fig. 2(a)) without any traces of crystalline alumina, indicating that the crystallization temperature increases with increasing aluminum dopant. It was also found that the XRD peaks of the mesoporous zirconia doped with 50 mol% aluminum (Al/Zr = 1) become narrower and more intense with increasing calcination temperatures, and the crystallized phases can be detected after calcination at 900 °C, indicating that increasing aluminum doping raises the crystallization temperature of the mesoporous zirconia. Due to the considerable broadening and overlapping of peaks corresponding to X-ray reflections, the structure of the zirconia phase can not be determined precisely by XRD patterns. Raman

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automated gas sorption system (Micromeritics ASAP 2020). The specific surface area was calculated by applying the Brunauer–Emmett–Teller (BET) equation. The textural properties, namely the total pore volume (Vp) and average pore diameter, were estimated from adsorption isotherm. Pore size distribution over the mesopore range was generated by the Barrett–Joyner–Halenda (BJH) analysis of the adsorption branches, and values of the average pore size were calculated. Transmission electron microscopy (TEM, Tecnai G2 20) was used to observe the morphology of some selected samples. In order to evaluate the state of Al3+ in the crystal lattice of zirconia, the 27Al MAS NMR of selected samples was tested with a 400 MHz WB Solid-State NMR Spectrometer.

TG (%)

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Fig. 1. TG–DSC curves of the obtained precursor when the molar ratio of Al/Zr is 0.8.

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t-ZrO2 m-ZrO2

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Fig. 2. XRD patterns of the mesoporous zirconia doped with different amounts of aluminum calcined at 550 °C. Al/Zr = 0.1 (a); Al/Zr = 0.2 (b); Al/Zr = 0.5 (c); Al/ Zr = 0.8 (d) and Al/Zr = 1 (e).

Fig. 4. Raman spectra for some selected samples. (a) Al/Zr = 0.1, calcination at 550 °C, (b) Al/Zr = 0.2, calcination at 550 °C, (c) Al/Zr = 0.5, calcination at 800 °C, (d) Al/Zr = 0.8, calcination at 1000 °C, (e) Al/Zr = 1, calcination at 1000 °C.

spectroscopy was used for further analysis of the initial crystallization phase since the cubic ZrO2 consists of a relatively weak peak at 680 cm 1 [23–24] while the tetragonal ZrO2 exhibits much stronger characteristic peaks at 148, 164, 264, 319, 461 and 643 cm 1 [21,25], and the monoclinic ZrO2 exhibits bands at 179, 190, 224, 235, 270, 305, 320, 334, 360, 375, 385, 476, 500, 553 and 636 cm 1 [26]. Raman spectra for some selected samples are shown in (Fig. 4). While the band at 263 cm 1, which is typical of the tetragonal phase, clearly appeared in all samples, only for the sample with lower aluminum content (Al/Zr = 0.1) was the tetragonal band accompanied by bands typical of the monoclinic phase (Fig. 4(a)). The Raman band and characteristic of the cubic structure located at approximately 680 cm 1 has not been observed in any of the samples. The results agree with the results of XRD analysis. According to the XRD and Raman results, in the doping range of this paper, it is clear that doping zirconia with aluminum stabilizes tetragonal zirconia, and the relative stabilization of the tetragonal zirconia over the cubic phase with aluminum additions is different from that observed in most other ZrO2 binary systems (CaO, MgO, Y2O3), in which the cubic phase is more stable. It can be explained by the formation of ZrO2/Al2O3 solid solutions previously discussed in the literatures [21,27]. According to Balmer’s research [21], as shown in (Fig. 5), in the Al2O3–ZrO2 binary system, crystallization of the tetragonal structure at T 6 1050 °C appears to always

involve a larger decrease in free energy than that associated with the formation of the cubic phase. The driving force for crystallization of the tetragonal phase decreases with increasing aluminum content, and crystallization within the fixed time would require an enhancement of the atomic mobility to compensate for the reduction of the driving force. That is, the crystallization temperature is expected to increase with the aluminum content. Analogous to Y3+ and other trivalent cations forming solid solutions with zirconia, the local charge balance can be maintained by creating one oxygen vacancy (Vo2 ) for every two aluminum ions introduced. Depending on the aluminum content, the as-received powders are either amorphous or crystallized. At lower aluminum content, Al3+ randomly substitute some of the Zr4+ creating oxygen vacancies to maintain the local charge balance. Thus the crystallized structure can be formed. In contrast, at higher aluminum content, a more disordered, amorphous structure of the as-obtained powders is produced, consisting of small scale of Al–O–Al units larger than those predicted statistically in the solid solution. In order to determine the local environment of the aluminum ion (i.e., the Al–O coordination), the 27Al MAS NMR of selected samples was tested with a 400 MHz WB Solid-State NMR Spectrometer, and the NMR spectra of the sample (Al/Zr = 1) with both amorphous (calcination at 550 °C) and crystallized (calcination at 1000 °C) states are presented in (Fig. 6). It can be seen that the spectra are characterized by three resonance maxima near 60, 30

t-ZrO2

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Fig. 3. XRD patterns of the aluminum doped mesoporous zirconia (Al/Zr = 1) calcined at 550 °C (a); 700 °C (b); 800 °C (c); 900 °C (d) and 1000 °C (e).

m

ZrO2

t

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mole percent

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Fig. 5. Schematic of the free energy vs composition functions for the ZrO2–Al2O3 binary system at a temperature below 1000 °C.

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1000°C 550°C

5.00E+008

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Al MAS NMR spectra obtained at 130 MHz for selected sample (Al/Zr = 1).

and 0 ppm, which are assigned to aluminum sites in 4-, 5- and 6fold coordination with oxygen [27]. The 5- and 6-fold coordination with oxygen can be attributed to the expected decrease in average coordination number due to the increase of the number of aluminum’s next-nearest aluminum neighbors increasing with increasing aluminum content. On the other hand, according to Balmer’s hypothesis [27], the occurrence of 4-fold coordination aluminum can be explained by the formation of Al2O3 clusters larger than those predicted by random distributions. The coordination number of atoms on the surface of the clusters is expected to be influenced by the neighboring zirconium atoms, whereas the atoms in the center of the clusters have a coordination number similar to amorphous Al2O3. It is also noted that the relative percentages of each type of aluminum coordination of the amorphous sample are different with that of the crystallized sample. The amorphous sample exhibits large amounts of 5- and 6-fold coordination aluminum and small amounts of 4-fold coordination aluminum, while the crystallized sample appears to have more 6-fold and less 4- and 5-fold coordination aluminum. According to the reference [27], the dramatic change in the aluminum coordination distribution in the crystallized samples is not simply an effect of calcination temperature, but specifically reflects the interactions between the ZrO2 and Al2O3 constituents in the crystalline state.

(e)

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Fig. 7. SXRD patterns pattern of the mesoporous zirconia doped with different amounts of aluminum calcined at 550 °C. Al/Zr = 0.1 (a); Al/Zr = 0.2 (b); Al/Zr = 0.5 (c); Al/Zr = 0.8 (d); Al/Zr = 1 (e).

1

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3 4 2 Theta (°)

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6

Fig. 8. SXRD pattern of the aluminum doped mesoporous zirconia (Al/Zr = 1) calcined at 550 °C (a); 700 °C (b); 800 °C (c); 900 °C (d) and 1000 °C (e).

3.3. Textural properties of the aluminum-doped mesoporous zirconia (Figs. 7 and 8) show the SXRD patterns of the selected samples. As can be noted from the figure, the small angle XRD patterns of all products, other than the lower aluminum containing sample calcined at 550 °C (Fig. 8(a)), show a clear diffraction peak, even though not sharp, located around 1–2°, which can be assigned to the disordered mesoporous structure. It also can be found that the sample with higher aluminum dopants calcined at 550 °C shows the sharpest peak located around 1°, and the diffraction peak is still preserved even after calcinations at 1000 °C, indicating that the aluminum doping can improve the thermal stability of the mesoporous zirconia effectively. For samples calcined at the same temperature, increasing the aluminum doping causing the diffraction peak first to widen and slightly shifted towards high angles, and then sharpen and shift to lower angles (Fig. 7). Similarly, with the elevation of calcination temperatures of samples with the same amount of aluminum dopants, the diffraction peak became wider and slightly shifted to high angles, and then shifted to low angles accompanied with a gradual disappearance of the peak (Fig. 8). It is due to the degree of crystallization of the samples. According to the XRD patterns shown in (Figs. 2 and 3), while all of the samples crystallize at higher temperatures, the samples with lower aluminum dopant contents began to crystallize at 550 °C, thereby destroying the mesopore structure. Meanwhile, the crystal lattice parameters of the corresponding sample decrease with increasing the amount of aluminum dopants, as indicated by the diffraction peak slightly shifting to higher angles in (Fig. 2). The higher amount of aluminum dopants added to the mesoporous zirconia raise the crystallization temperature of the amorphous mesoporous zirconia, leading to the following movement of the diffraction peak towards to lower angles. (Fig. 9) shows the nitrogen adsorption–desorption isotherms and BJH pore size distribution of some of the obtained aluminum-doped mesoporous zirconia calcined at different temperatures. The BET surface area, pore volume and BJH pore size of all samples are listed in Table 1. It can be seen that the isotherms of the samples doped with a lower content of aluminum show a poor hysteresis loop (Fig. 9(a)), suggesting a significant structure deterioration after calcinations. The BJH pore size distribution of the sample after calcination at 550 °C is relatively narrow, and with the increases of the calcination temperatures, the pore size distribution widens and the mean pore size grows (Fig. 9(b)). Whereas the isotherms of the samples doped with higher content of aluminum exhibit a distinct large hysteresis loop (Fig. 9(c)), which is the

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Fig. 9. Nitrogen adsorption–desorption isotherms and BJH pore size distribution of samples. (a) Al/Zr = 0.1, (b) Al/Zr = 0.1, (c) Al/Zr = 1, (d) Al/Zr = 1.

Table 1 Textural properties of the aluminum-doped mesoporous zirconia after calcination. Al/Zr (mol)

550 °C

BET surface area (m2/g) 0.1 66.460 0.2 76.110 0.5 130.036 0.8 148.440 1.0 172.690 Pore volume (cm3/g) 0.1 0.159 0.2 0.188 0.5 0.130 0.8 0.302 1.0 0.401 BJH pore size (nm) 0.1 9.440 0.2 12.186 0.5 12.139 0.8 7.716 1.0 9.423

700 °C

800 °C

900 °C

1000 °C

46.062 56.866 119.268 131.451 137.010

32.127 37.983 90.474 95.491 132.044

30.531 35.392 73.983 88.675 128.152

19.193 21.990 58.906 70.802 93.149

0.149 0.190 0.294 0.310 0.364

0.150 0.189 0.262 0.252 0.338

0.144 0.166 0.230 0.225 0.310

0.105 0.110 0.181 0.179 0.213

12.145 12.273 12.258 7.803 9.534

17.006 17.342 12.378 9.096 10.125

17.012 17.507 12.517 9.449 10.408

17.266 17.592 12.543 9.467 10.484

typical characteristic of mesoporous materials, namely a type IV isotherm with H1-shaped hysteresis loop assigned to mesoporous structure. The BJH pore size distribution of the samples calcined at different temperatures is narrower compared to that of the sample doped with lower content of aluminum after calcination at 550 °C (Fig. 9(d)). Neither the shape of the isotherm nor the distributions of pore size significantly vary. For instance, the BET surface area of aluminum-doped mesoporous zirconia (Al/Zr = 1) decreases 25.8%

from 172.69 m2/g to 128.152 m2/g with an increase in calcination temperature from 550 °C to 900 °C. The BET surface area of the sample after calcination at 1000 °C is 93.149 m2/g, which is much higher than that of the sample after calcination at 800 °C previously reported [10–11]. It is also found that the BET surface area and pore volume of the samples doped with the same amount of aluminum decrease with increasing calcination temperatures, while the mean BJH pore size increases. The evolution of pore size is attributed to the partial degradation of the walls surrounding mesopores leading to an extended network of pores. Due to the increase of the crystallization temperature, the BET surface area and pore volume of the samples calcined at the same temperature increase with increasing the amount of aluminum, and the change of mean BJH pore size of the samples with increasing of aluminum doping calcined at the same temperature is not regular. The evolution of the mesoporous structure of the zirconia was further examined by TEM. TEM images of mesoporous zirconia calcined at 550 °C and 1000 °C with varying amounts of aluminum added are shown in (Fig. 10(a)–(c)). It can be seen that the mesoporous zirconia with a lower aluminum doping content is already crystallized after calcination at 550 °C (Fig. 10(a)), the average crystallite size was found to increase to 2030 nm, and the mesoporous structure is no longer observed. With an increase in aluminum doping, the pore architecture of the sample is a disordered ‘‘wormhole-like’’ variety (Fig. 10(b)), and after calcinations at 1000 °C, the sample is partially crystallized and the mesoporous structures begin to collapse gradually. The TEM observations are consistent well with the results of XRD, BET and SXRD.

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Fig. 10. TEM images of some selected samples. (a) Al/Zr = 0.1, calcination at 550 °C, (b) Al/Zr = 1, calcination at 550 °C, (c) Al/Zr = 1, calcination at 1000 °C.

It is known that the thermal stability of the pure mesoporous zirconia does not exceed 400 °C [17], otherwise the mesostructure tend to disappear due to the crystallization of zirconia. In this paper, the calcination temperature of removal of the template was 550 °C for 6 h, when the content of aluminum doping is lower, the as-received mesoporous zirconia was well crystallized after removal of the template (Fig. 2(a and b) and Fig. 4(a and b)), result in the deterioration of the mesostructure (Figs.7, 9(a) and 10(a)). On the other hand, although the crystallization temperature of the as-received mesoporous zirconia increases with increasing aluminum dopant (Figs. 2 and 3), meanwhile, the ordering degree of the mesopores will be reduced with increasing aluminum dopant, and thus the disordered ‘‘worm-like’’ mesopores are present (Fig. 10 (b and c)). 4. Conclusions The mesoporous zirconia doped with different amounts of aluminum were synthesized by the evaporation induced self-assembly (EISA) method, and preparing the mesoporous materials with an Al/Zr molar ratio of 1 resulted in a disordered ‘‘wormhole-like’’ mesoporous structure with high thermal stability, which is essential for further application. In contrast to most other ZrO2 binary systems, doping of zirconia with aluminum stabilizes tetragonal zirconia over the cubic phase. The structure and textural properties can be controlled effectively by adjusting the amount of aluminum doping. By increasing the aluminum content, the crystallization temperature of as-obtained powders increases and the BET surface area and pore volume of the samples calcined at the same temperature increase, thereby improving the thermal stability of the mesoporous zirconia. Acknowledgements The authors would like to thank the financial support from the National Natural Science Foundation of China (NSFC-No. 51172119 and 51202117). The authors also would like to thank Dr. Yutao Li (Tsinghua University) for the in-depth discussion of the MAS NMR results and Mr. Halperin Shakked O (University of Missouri) for the assistance in the polish of the paper.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.micromeso.2013.11.024. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [2] Dongyuan Zhao, Jianglin Feng, Qisheng Huo, Nicholas Melosh, Glenn H. Fredrickson, Bradley F. Chmelka, Galen D. Stucky, Science 279 (1998) 548–552. [3] Fei Huang, Ying Zheng, Guohui Cai, Yong Zheng, Yihong Xiao, Kemei Wei, Scr. Mater. 63 (2010) 339–342. [4] Weiquan Cai, Yu Jiaguo, Chokkalingam Anand, Ajayan Vinu, Mietek Jaroniec, Chem. Mater. 23 (2011) 1147–1157. [5] Krisztian Niesz, Peidong Yang, Gabor A. Somorjai, Chem. Commun. (2005) 1986–1987. [6] M. Mamak, N. Coombs, G.A. Ozin, Chem. Mater. 13 (2001) 3564–3570. [7] M. Mamak, N. Coombs, G.A. Ozin, Adv. Mater. 12 (2000) 198–202. [8] E.L. Crepaldi, Angew. Chem. Int. Ed. 42 (2003) 347–351. [9] Mohamed Mokhtar, Sulaiman N. Basahel, Tarek T. Ali, J. Mater. Sci. 48 (2013) 2705–2713. [10] I.-Ming Hung, Kuan-Zong Fung, De-Tsai Hung, Min-Hsiung Hon, J. Eur. Ceram. Soc. 28 (2008) 1161–1167. [11] I.-Ming Hung, Kuan-Zong Fung, De-Tsai Hung, Min-Hsiung Hon, J. Eur. Ceram. Soc. 26 (2006) 2627–2632. [12] U. Ciesla, M. Froba, G. Stucky, F. Schuth, Chem. Mater. 11 (1999) 227–234. [13] Y. Liu, J. Chen, Y. Sun, Stud. Surf. Sci. Catal. 156 (2005) 249–256. [14] D.J. Suh, T.J. Park, Chem. Mater. 14 (2002) 1452–1954. [15] Kaixue Wang, Michael A. Morris, Justin D. Holmes, Yu Jihong, Xu Ruren, Microporous Mesoporous Mater. 117 (2009) 161–164. [16] K. Cassiers, T. Linssen, V. Meynen, P. Van Der Voort, P. Cool, E.F. Vansant, Chem. Commun. 21 (2003) 1178–1179. [17] S.G. Liu, H. Wang, J.P. Li, N. Zhao, W. Wei, Y.H. Sun, Mater. Res. Bull. 42 (2007) 171–176. [18] P. Li, I.-W. Chen, J.E. Penner-Hahn, Phys. Rev. B48 (1993) 10074–10081. [19] P. Li, I.-W. Chen, J.E. Penner-Hahn, J. Am. Ceram. Soc. 77 (1994) 118–128. [20] F.F. Lange, M.M. Hirlinger, J. Am. Ceram. Soc. 70 (1987) 827–830. [21] Mari Lou Balmer, Fred F. Lange, Carlos G. Levi, J. Am. Ceram. Soc. 77 (1994) 2069–2075. [22] E. Zhao, S.E. Hardcastle, G. Pacheco, A. Garcia, A.L. Blumenfeld, J.J. Fripiat, Microporous Mesoporous Mater. 31 (1999) 9–21. [23] C.M. Phillippi, K.S. Mazdiyasni, J. Am. Ceram. Soc. 54 (1971) 254–258. [24] V.G. Keramidas, W.B. White, J. Am. Ceram. Soc. 57 (1974) 22–24. [25] R. Srinivasan, L. Rice, B.H. Davis, J. Am. Ceram. Soc. 73 (1990) 3528–3530. [26] T. Yamamoto, T. Tanaka, S. Takenaka, S. Yoshida, T. Onari, Y. Takahashi, T. Kosaka, S. Hasegawa, M. Kudo, J. Phys. Chem. B 103 (1999) 2385–2393. [27] Mari Lou Balmer, J. Am. Ceram. Soc. 79 (1996) 321–326.