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* E-mail: hrchen@mail.sic.ac.cn (Chen, Hangrong); jlshi@mail.sic.ac.cn (Shi, Jianlin); Tel.: 0086-021-52412706; Fax: 0086-021-52413122 Received August 22, 2010; revised September 26, 2010; accepted November 23, 2010. Project supported by the National Natural Science Foundation of China (Nos. 50872140 and 20633090).

Chin. J. Chem. 2011, 29, 483—488 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 483

Simultaneous Al2O3 Doping and Sulfation in Hierarchically Porous ZrO2 Solid Acids by an One-pot Synthesis for

Enhanced Recycling Catalytic Performances

Wang, Nan(王楠) Chen, Yu(陈雨) Ye, Zhengqing(叶真青) Chen, Hangrong*(陈航榕) Shi, Jianlin*(施剑林)

State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

Alumina doping and sulfation in hierarchically porous zirconia solid acids have been achieved simultaneously via one-pot and bi-surfactant-assisted self-assembly process, using aluminum sulfate as both Al and 2

4SO － sources. The prepared composite solid acids showed much enhanced acidity and recycling catalytic activity for an esterifica-tion reaction compared with sulfated zirconia without alumina doping and Al-doped sulfated zirconia without hier-archically porous structure.

Keywords one-pot synthesis, doping, Al-promoted sulfated zirconia, hierarchical porous structure, material sci-ence, self-assembly

Introduction

Since the discovery of the strong solid acidity, sul-fated zirconia (SZ) has attracted much attention as a promising solid acid catalyst because of its great poten-tial applications in industrial acid-catalyzed processes, such as the skeleton isomerization of alkanes1,2 and es-terification in biodiesel production.3 However, there are some critical problems of obtained SZ materials re-ported in literatures so far, such as poor stability in the presence of water because of the low crystallinity, quick loss of 2

4SO － and tendency to undergo deactivation, which severely limit its practical applications.4,5 Con-ventional methods of synthesizing SZ usually lead to lower surface area than 100 m2•g－1, which is also one main reason for its low catalytic activity. So it is still a great challenge to develop efficient synthetic protocols to prepare SZ materials with well-defined structural characteristics and highly preserved 2

4SO － to meet the requirements of highly active and recyclable super- strong acidic solid catalysts for practical applications.

In addition to sulfation, doping alumina into the framework of SZ to give stabilized and promoted activ-ity is an alternative route to enhance the catalytic prop-erty of SZ. Recently, Hwang et al.6-8 reported that the addition of an optimal amount of alumina into the framework of SZ catalyst with various morphologies gave rise to more enhanced catalytic activities than the corresponding one without promoting. In their work, aluminum source was impregnated into as-prepared

mesoporous zirconia by a post-treatment process. A hierarchically porous material of multiple length

scales has attracted much attention recently, because of its high surface area and regular porosity.9-15 However, most of the researches still focused on the synthesis and structure formation of the porous materials,16,17 the catalytic property is still a hypothesis.

Based on our recently developed synthetic strategy to prepare hierarchically porous zirconia with high thermal stability and high surface area,9 we doped alu-mina and sulfate radical simultaneously into the frame-work of hierarchically porous zirconia by a simple one-pot process, where composite surfactants were em-ployed as structural directing agents, and aluminum sulfate as both alumina and 2

4SO － sources was added into the surfactant solution during the synthetic process for porous zirconia. Furthermore, the synthetic parame-ters, which could influence the structural characteristics or catalytic activities of prepared alumina doped zirco-nia, were systematically investigated. Based on this novel and simple one-pot synthetic strategy, a series of alumina-doped and sulfated hierarchically porous zirco-nia solid acid materials with high surface area (ca. 234 m2•g－1), well-defined macro-/meso-porosity and crys-talline framework have been prepared. The material showed enhanced and recyclable catalytic performance as an esterification reaction compared with the refer-ences of sulfated zirconia without alumina doping and Al-doped sulfated zirconia without hierarchically po-rous structure.

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Experimental

Sample preparation

A micellar solution of 3.7 g Pluronic P-123 (BASF) and 4.5 g Brij35 (Fluka) was prepared in 100 g distilled water under stirring at 37 ℃ for more than 3 h. Then an appropriate amount of Al2(SO4)3•18H2O (Meixing Chemical Co. LTD, Shanghai) was added into the above solution. After stirring for 1 h, 10 mL zirconium pro-poxide [Zr(OC3H7)4] [Aldrich, 70% (w)] was added dropwise into the above solution. After further stirring for 12 h, the mixture was transferred into a Teflon-lined autoclave and heated at 150 ℃ for 24 h. After filtered and dried, the product was calcined at 550 ℃ for 6 h. The samples are hereafter labeled as ASZ-x (x＝0, 1, 2, 3, 4, 5, 6) corresponding to the different adding amounts of Al2(SO4)3•18H2O (0, 0.27, 0.54, 0.81, 1.08, 1.35, 1.62 g, respectively).

The reference sample without alumina doping R1 was also prepared via the same process by using 0.72 g H2SO4 instead of Al2(SO4)3•18H2O with the 2

4SO － content in the precursor solution equal to that in the sample ASZ-6. The reference Al-promoted sulfated zirconia without hierarchical porous structure R2 was prepared via the same process as ASZ-6 without Plu-ronic P-123 or Brij35.

Structural characterization and acidic measurement

The TG curves were recorded on a Netzsch STA 449C microanalyzer in nitrogen flow (20 mL/min) at 10 K/min. Scanning electron microscopy (SEM) analysis was carried out on a field emission JEOL JSM-6700F microscope. TEM images and SAED patterns were ob-tained by a JEOL JEM2010electrom microscope. XRD patterns were recorded on a Rigaku D/MAX-2250V diffractometer using Cu Kα radiation (40 kV and 40 mA). NH3-TPD curves were recorded on a TP-5080 automatic adsorption instrument of Tianjin Xianquan Industry and Trade Development CO., LTD.

Catalytic test

The esterification reaction was carried out using an appropriate amount (3 wt% of the reactants) of the cal-cined samples at 120 . The molar proportion of acetic ℃

acid to n-butanol was fixed at 1.5∶1.

Results and discussion

Structure analysis

Figure 1 shows the wide-angle XRD patterns of all samples. It can be seen that ASZ-0 sample shows the tetragonal ZrO2 phase combined with a small portion of monoclinic ZrO2 phase, while it presents the pure tetragonal ZrO2 phase once aluminum is doped into the framework of ZrO2. It indicates that doping a small amount of alumina into zirconia framework could effec-tively preserve the tetragonal phase of zirconia. With the increasing alumina doping amount (the contents of alumina were measured by XRFS and the results are listed in Table 1), the intensity of characteristic diffrac-tive peaks of the tetragonal phase decreased gradually, which indicated that the crystallization and crystallite growth of tetragonal zirconia were also effectively de-creased by the alumina incorporation.

Figure 1 XRD patterns of the prepared ASZ-x series samples.

The typical SEM images of ASZ-3 and ASZ-6 are

Table 1 BET, NH3-TPD and XRFS data of all samples

Integral area of NH3-TPDa Sample

BET area/ (m2•g－1)

Pore volume/ (cm3•g－1)

Pore diameter/ nm 120—500 ℃ 500—670 ℃

Alumina content (w)b/%

Sulfur trioxide content (w)b/%

ASZ-0 52 0.19 12.5 — — — —

ASZ-1 150 0.41 9.1 148.88 75.35 1.60 2.70

ASZ-2 174 0.35 6.8 373.38 159.23 2.96 5.33

ASZ-3 234 0.35 6.5 631.28 164.00 4.31 7.81

ASZ-4 220 0.37 6.2 754.06 312.21 5.44 9.91

ASZ-5 208 0.35 6.0 882.63 369.53 6.05 11.09

ASZ-6 203 0.33 5.7 1055.11 317.61 6.71 12.67

R1 132 0.47 13.8 722.28 395.76 — 12.03

R2 128 — — — — 6.40 12.60 a Generated by the analysis software of TP-5080 automatic multiple-use absorption instrument. b Estimated by XRFS.

Simultaneous Al2O3 Doping and Sulfation in Hierarchically Porous ZrO2 Solid Acids

Chin. J. Chem. 2011, 29, 483—488 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 485

shown in Figures 2a and 2b. It can be seen that the as-prepared alumina doped ZrO2 exhibits a macropor-ous-structured morphology with the macropore diame- ters of about 200—250 nm. Figures 2c and 2d show typical TEM images of ASZ-3 and ASZ-6 and their corresponding selected area electron diffraction patterns (SAED) (inset). The existence of the homogeneously distributed worm-like mesopores can be clearly ob-served. The SAED patterns reveal that ASZ-6 has less-crystallized tetragonal phase than ASZ-3, which is in agreement with the XRD result in Figure 1.

The N2 adsorption-desorption isotherms of obtained samples are shown in Figure 3, which can be attributed to type-IV behavior, indicating the presence of well-defined mesopores. The specific BET surface area, pore volume and the most probable pore sizes of the samples are summarized in Table 1. The pore sizes of alumina doped ZrO2 are narrowly distributed, and the most probable pore size is progressively shifted from 6.82 to 5.71 nm with the increment of concentrations of aluminum sulfate used in the synthetic process. As the doping amount of alumina increased, the surface area of alumina doped ZrO2 also increased gradually and reached the maximum at 4.31 wt% alumina sulfate doping amount. It also demonstrated that the mesopores of ZrO2 could be well preserved after alumina doping and high temperature calcination (550 ).℃

Thermal analysis

The TG curves of samples R1, ASZ-3 and ASZ-6 are shown in Figure 4. The curves of ASZ-3 and ASZ-6 present two weight loss steps at around 15—250 and ℃

690—900 over the whole temperature range used, ℃

while reference sample R1 shows two loss steps at 15—250 and 570—700 ℃, correspondingly. The lower temperature weight losses correspond to the desorption of adsorbed water, while those at higher temperatures are assigned to the decomposition of sulfate species. The increased sulfate decomposition temperature of Al-doped samples indicates that the addition of alumina has led to a significant increase in the thermal stability of the surface sulfate species.

Acidic characterization

The surface acidities of the samples were measured by NH3-TPD method. The NH3-TPD profiles of pre-pared samples are shown in Figure 5. The desorption peaks of the materials between 120 and 500 corr℃ e-spond to the weak and medium acidities while those between 500 and 670 correspond to strong and very ℃

strong (or super strong) acidities.17,18 The ASZ-0 sample presents no peak of desorption. The reference sample R1 presents a broad peak around 300 while the other ℃

alumina doped zirconia samples show a broad peak at

Figure 2 FE-SEM images of the ASZ-3 (a) and ASZ-6 (b) samples; TEM images of ASZ-3 (c) and ASZ-6 (d) samples. Insets in (c) and (d) are corresponding SAED patterns.

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Figure 3 Nitrogen adsorption-desorption isotherms of samples ASZ-0, ASZ-1, ASZ-3 and ASZ-6.

Figure 4 TG curves of samples R1, ASZ-3 and ASZ-6.

about 200 ℃ with varied peak intensities dependent on the alumina doping level. This demonstrates that the incorporation of alumina and 2

4SO － can generate a certain amount of acid sites. In addition, the ASZ-3, ASZ-4, ASZ-5 and ASZ-6 samples present a few small peaks of desorption in the range of 250—400 and ℃

650—710 , ℃ which suggest a broad distribution of het-erogeneous acidic sites of medium strength. The broad peaks at about 730 resulted from the decompos℃ ition of 2

4SO － . The NH3-TPD data including the integral areas of the peaks are also listed in Table 1. The integral

peak areas are calculated to estimate the number of total acid sites with different acid strength. As the adding amount of aluminum sulfate was increased, the number of total acid sites present on the samples increases ac-cordingly.

Figure 5 NH3-TPD profiles of all samples.

Catalytic analysis

The esterification reaction was chosen as an example, since it is an important unit process for the preparation of many industrial valuable chemicals. Here a typical esterification reaction of acetic acid with n-butanol was

Simultaneous Al2O3 Doping and Sulfation in Hierarchically Porous ZrO2 Solid Acids

Chin. J. Chem. 2011, 29, 483—488 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 487

used as a model reaction to test the catalytic activity and recycling catalytic properties of the prepared samples. This reaction is rather slow and needs to be activated either by high temperatures or appropriate catalysts.

Figure 6 shows the conversion rates of n-butanol re-acted with acetic acid in 4 h using prepared ASZ-x sam-ples and reference samples R1, R2 as catalysts, and that of a blank control reaction without using any catalyst is also presented. In the first cycle, the catalytic activities of all catalysts can be put in a descending order of ASZ-6＞ASZ-5＞ASZ-4＞R1＞ASZ-3＞ASZ-2＞ R2＞ASZ-1＞ASZ-0. The catalytic activity of the ASZ-x samples increases with the increase of the adding amounts of the aluminum sulfate. However, there are only very limited differences among the activities of ASZ-6, ASZ-5 and ASZ-4 samples due to their rela-tively high surface areas, and alumina and 2

4SO － in-corporating amounts. During coprecipitation and calci-nation, part of the Al3＋ ions could enter the lattice of zirconia19 and replace Zr4＋ ions. Since Al3＋ is trivalent and Zr4＋ is tetravalent, oxygen vacancies will be gen-erated to maintain the neutrality. Surrounded by cations and induced by the S＝O double bonds, the positively charged electricity of oxygen vacancy might be en-hanced, showing the strong Lewis acidity.

Figure 6 Conversion rates of esterifications in 4 h by using all ASZ-x samples and R1, R2.

Because the traditionally prepared sulfated zirconia solid acids inevitably suffered from the poor recycling capability due to the 2

4SO － loss in reaction media, few recycling data of such solid acid catalysts are available in the previous reports as far as we could find. In this research, four recycling experiments of the esterification reaction have been carried out, and the results are shown in Figure 6. It is clear that the ASZ-6, ASZ-5 and ASZ-4 samples can maintain their catalytic activity much better than the reference sample R1 after the four recycling reactions. This result demonstrates that the doping of alumina in the form of aluminum sulfate into the framework of zirconia can generate, and in the meantime, keep a considerable amount of acid sites on the catalysts, which is helpful to maintain their catalytic activity after esterification reactions for several cycles.

According to the principle of electronegativity equaliza-tion, since the electronegativity of Al3＋ is higher than that of Zr4＋, the positively charged electricity on Zr at-oms will be enhanced due to the electron attraction by neighboring Al atoms in Al—O—Zr bonds.20 The charge transferred from Zr atoms to the neighboring Al atoms strengthens the Al—O bonding between Al and the surface sulfate species. The strong Al—O bonding leads to an increase in the thermal stability of the sur-face sulfate species, and consequently the acidity of the catalysts is enhanced.

Therefore, the doping of alumina into zirconia framework would lead to the formation of oxygen va-cancies in tetragonal zirconia lattices, and consequently generate a high amount of Lewis acid sites. The forma-tion of Al—O—Zr bonds can increase the thermal sta-bility of the surface sulfate species.

The catalytic activity of the ASZ-6 is about 21% higher than that of R2 in the first cycle, though the con-tents of alumina and sulfur trioxide in the reference sample R2 are equal to those in ASZ-6. It is believed that the hierarchically porous structure contributes to the enhanced catalytic performance during the esterification reaction. The reactant diffusion in the pore structures can be well accessed by the activity centers on the sur-face of meso-/macro-porous channels owing to the in-terconnected pore network. Especially, the large pore size of macroporous channels in the samples makes the reactant/product molecules diffuse almost freely in the macropores, which decreases the maximum diffusion distance from half particle size in samples without macroporous structure, to half distance between macro-porous channels in meso-/macro-porous structured ma-terials, for reactant molecules to access the active sites in the center of catalytic particles.

Conclusions

A novel alumina-doped sulfated hierarchically po-rous zirconia solid acid material with high surface area and crystalline framework has been successfully pre-pared via a convenient one-pot bi-surfactant-assisted self-assembly by using aluminum sulfate as both Al and

24SO － sources. The tetragonal phase of zirconia was

well retained by small amount of alumina incorporation. With the increase of doping amount of aluminum sulfate, the surface area of hierarchically porous ZrO2 increases gradually and reaches the maximum (234 m2•g－1) at the alumina doping amount of 4.31 wt%, and the density of total acid sites also increases accordingly. The prepared AS-Z materials with alumina doping amount of 5.44—6.71 wt% showed much higher catalytic activity than the references of both sulfated zirconia without alumina doping and Al-doped sulfated zirconia without hierarchical porous structure, and more importantly, demonstrated enhanced recycling catalytic activity compared with the reference sulfated zirconia without alumina-doping.

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References

1 Hino, M.; Kobayashi, S.; Arata, K. J. Am. Chem. Soc. 1979, 101, 6439.

2 Hino, M.; Arata, K. J. Chem. Soc. Chem. Commun. 1980, 18, 851.

3 Chen, X. R.; Ju, Y. H.; Mou, C. Y. J. Phys. Chem. C 2008, 111, 18731.

4 Srinivasan, R.; Keogh, R. A.; Milburn, D. R.; Davis, B. H. J. Catal. 1995, 153, 123.

5 Yi, N.; Cao, Y.; Feng, W. L.; Dai, W. L.; Fan, K. N. Catal. Lett. 2005, 99, 73.

6 Wang, J. H.; Mou, C. Y. Appl. Catal. A 2005, 286, 128. 7 Wang, J. H.; Mou, C. Y. Microporous Mesoporous Mater.

2008, 110, 260. 8 Hwang, C. C.; Mou, C. Y. J. Phys. Chem. C 2009, 113,

5212. 9 Chen, H. R.; Gu, J. L.; Shi, J. L. Adv. Mater. 2005, 17,

2010. 10 Yuan, Z. Y.; Ren, T. Z.; Vantomme, A.; Su, B. L. Chem.

Mater. 2004, 16, 5096.

11 Kuang, D. B.; Brezesinski, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126, 10534.

12 Huang, P.; Zhang, X.; Wei, J. M.; Feng, B. X. J. Alloys. Compd. 2010, 489, 614.

13 Han, Q. F.; Sun, Y.; Wang, X.; Chen, L.; Yang, X. J. J. Al-loys. Compd. 2009, 481, 520.

14 Zhan, F. M.; Geng, B. Y.; Guo, Y. J. J. Alloys. Compd. 2009, 482, L1.

15 Smatt, J. H.; Weidenthaler, C.; Rosenholm, J. B.; Linden, M. Chem. Mater. 2006, 18, 1443.

16 Leonard, A.; Blin, J. L.; Su, B. L. Chem. Commun. 2003, 2568.

17 Yadav, G. D.; Murkute, A. D. Adv. Synth. Catal. 2004, 346, 389.

18 Benitez, V. M.; Yori, J. C.; Vera, C. R.; Pieck, C. L.; Grau, J. M.; Parera, J. M. Ind. Eng. Chem. Res. 2005, 44, 1716.

19 Shi, J. L.; Li, B. S.; Yuan, M. L.; Yen, T. S. J. Eur. Ceram. Soc. 1995, 15, 959.

20 Hua, W. M.; Xia, Y. D.; Yue, Y. H.; Gao, Z. J. Catal. 2000, 196, 104.

(E1008221 Zhao, C.; Fan, Y.)