Thermally Induced Phase Transformation on 12 ... - Springer Link

Journal of Sol-Gel Science and Technology 26, 213–216, 2003 c 2003 Kluwer Academic Publishers. Manufactured in The Netherlands.


Thermally Induced Phase Transformation on 12-Tungstophosphoric Acid/ZrO2 Sol-Gel ´ ´ T. LOPEZ, ´ ´ J.G. HERNANDEZ-CORT EZ, M.E. MANR´IQUEZ AND R. GOMEZ Departamento de Qu´ımica, Universidad Aut´onoma Metropolitana-Iztapalapa, A.P. 55-534, 09340 M´exico D.F., M´exico J. NAVARRETE Instituto Mexicano del Petr´oleo, Simulaci´on Molecular, Eje Central L´azaro C´ardenas 152, 007730, M´exico D.F., M´exico

Abstract. Zirconium (IV)-n-butoxide and tungstophosphoric acid (WP) were co-gelled at pH 3, 5 and 7 with HCl acid, C2 H4 O2 acid and NH4 OH, respectively. Pyridine adsorption bands at 1610 and 1442 cm−1 corresponding to Lewis acidic sites were observed by FTIR spectroscopy. Acidity determined by ammonia thermodesorption shows values around 1100 µmol of NH3 /g, which correspond to solids showing super acidity. It was found that the incorporation of WP to gelling zirconia delay the formation of tetragonal zirconia. Raman spectroscopy shows the stabilization of the Keggin structure on zirconia thermally treated at 400◦ C. Keywords: tungstophosphoric acid, sol-gel synthesis, ZrO2 sol-gel, ZrO2 NH3 -TPD, FTIR pyridine adsorption on ZrO2 , Raman spectroscopy on sol-gel ZrO2

1.

Introduction

Because of the recent environmental requirements it has been necessary to synthesize effective catalysts improving the gasoline environmental restrictions. Actually low concentration on aromatics is recommended, and olefins isomerization products became important components of ecological gasoline pools. Isomerization is a process, which requires strong acid catalysts to be carried out. That is because scientific laboratories around the world have focused its attention to the synthesis of superacid solids. In such a way modified zirconia or titania [1] have been reported as the catalysts with great possibilities to be used in alkylation units. On spite of the promising results obtained with modified ZrO2 , it sinters and its specific surface area drastically diminished by temperature effects. Alternative methods were used then to obtain stable zirconia, they include supporting zirconia

on stable oxides such as silica prepared by sol-gel methods [2]. Pure zirconia shows low acidity. However, when zirconia is prepared by sol-gel methods, it shows large hydroxylated surface in which acidity can be developed by chemical exchange reactions between the hy2− droxyls of its surface and oxo-anions (WO2− 4 , MoO4 2− and SO4 ) [3, 4] or transition metals cations [5, 6]. Heteropolyacids (HPAs) have also been reported as efficient polyoxometalates to develop acidity on the zirconia surface [7]. In this work WP/ZrO2 catalysts was prepared by adding tungstophosphoric acid (WP) (H3 PW12 O40 ) in gelling zirconia [8]. The method allowed us to control the variable synthesis and to analyze the effect of the gelling pH on the acidic sites. Samples were characterized by FTIR spectra pyridine adsorption, temperature-programmed desorption of ammonia and Raman spectroscopy.

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2.

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Experimental

Under continuous stirring, 0.162 mol of zirconium n-butoxide was dissolved in 50 ml of ethanol, and the solution was refluxed at 70◦ C. Concentrate hydrochloric acid, acetic acid or ammonia were used to adjust the gelling pH to 3, 5 and 7 respectively. The solution was stirred and refluxed for 10 min. Then it was cooled at 20◦ C and at this temperature, were added drop by drop 50 ml of WP dissolved in ethanol (0.00231 mol WP-tungstophosphoric for 25 wt%, and 0.00122 mol WP-acid for 15 wt%). Afterwards, 0.654 ml of water was added to complete hydrolysis and then the sol was stirred until gelling. Gels were isolated by filtration and dried in air at 100◦ C for 24 h, and eventually calcined in air at 400◦ C for 4 h. These solids were labeled xWP-Z-y (x = wt% of WP, Z = zirconia and y = hydrolysis pH). 2.1.

Characterization

with a step program of 10◦ C/min. The estimation of adsorbed ammonia was made calculating the area of the desorption curves and compared with a calibration sample of known volume. 3. 3.1.

Results and Discussion Textural Properties

Specific surface areas depend on the hydrolysis catalyst and WP content [9]. In calcined samples (400◦ C) specific areas of 220 and 74 m2 /g were obtained with HCl acid, while with acetic acid the BET areas were 164 and 92 m2 /g for 25 and 15 WP wt% respectively. When the solids were prepared with ammonia as hydrolysis catalyst, the specific areas were found independent of the WP content, BET areas of 168 and 177 m2 /g were obtained for 25 and 15 WP wt% respectively.

2.1.1. FTIR Pyridine Adsorption. The type of the acid sites (Brønsted and Lewis) was determined with a Fourier transform infrared (FTIR) Nicolet 710SX spectrometer by means of pyridine adsorption. The equipment was furnished with a cell with interchangeable windows. The samples, previously pressed into thin self-supported wafers were placed in the cell and evacuated (1 × 10−6 Torr) in situ at 500◦ C for 30 min. After cooling to room temperature pyridine was adsorbed. The excess of pyridine was evacuated and then pyridine desorption was followed “in situ” by heating the sample. 2.1.2. Raman Spectroscopy. Raman spectra were recorded at room temperature on previously calcined samples in a nearly backscattering geometry using an ISA Labram micro-Raman apparatus. The excitation line was the 632.8 nm of a He-Ne laser. The laser power on the sample was kept low (about 1 mW) to avoid thermal effects. 2.1.3. NH3 -TPD. The thermal programmed desorption of NH3 was used to estimate the amount and strength of acid sites formed on the surface of WPZrO2 . The thermograms were carried out with an AMI3 apparatus from Altamira Instruments. The samples were heated in helium flow at 200◦ C for 2 hs. Then the samples were cooled at room temperature and exposed to flowing NH3 (20% in He). After saturation, ammonia desorption was carried out in helium flow

Figure 1. FTIR spectra of adsorbed pyridine on the samples with 25 wt% WP and prepared at different pH values: (a) pH 3, (b) pH 5, (c) pH 7, and at different desorption temperatures.

Thermally Induced Phase Transformation

3.2.

FTIR Studies

The samples were studied by FTIR pyridine adsorption spectroscopy to determine the presence of Br¨onsted and Lewis acid sites [10]. The thermodesorption spectra of pyridine from the solids are shown in Fig. 1. Absorption bands at 1610 and 1442 cm−1 assigned to the pyridine adsorbed on strong Lewis acid sites were observed. The band at 1488 cm−1 was assigned to Br¨onsted and Lewis total acid sites. At 1590 and 1577 cm−1 are observed the bands assigned to weak Lewis sites. The characteristic band at 1546 cm−1 assigned to Br¨onsted sites was also observed. This band was shifted toward high energy on function of the WP content. Adsorbed pyridine amount increases when hydrolysis pH decreases 2 (Fig. 2). At room temperature, the 25WP-Z-pH samples adsorb 407, 180 and 324 µmol of pyridine/g for pH 3, pH 5 and pH 7 preparations respectively. At a desorption temperature of 400◦ C the pyridine amount is diminished to 32, 22 and 21 µmol of pyridine/g on the samples prepared at pH 3, pH 5 and pH 7, respectively.

Figure 2.

3.3.

215

TPD Studies

NH3 -TPD was employed as a quantitative probe for the average acid site strength (acidity). The WP concentration has important effect on the total acidity of the solids. In pH 3 gelled samples the acidity increases from 949 to 1247 µmol of NH3 /g and in basic preparations from 686 to 1195 µmol of NH3 /g and for the samples prepared at pH5 the acidity varies from 607 to 1073 µmol of NH3 /g when the WP content increases from 15 to 25 WP wt% (Table 1). The lower values of acidity obtained for WP-ZrO2 will correspond to acidity values reported for acid zeolites (around 700 µmol of NH3 /g), while the higher values bigger than 1100 µmol of NH3 /g, will correspond to super acid solids. Moreover, we can observe that the density of site by m2 is practically constant in all the catalysts. Those the acidity is not a function of the specific surface area. 3.4.

Raman Spectroscopy

The Raman spectra of WP 25 wt% samples calcined at 400◦ are shown in Fig. 3. The spectrum of

Amount of adsorbed pyridine as a function of desorption temperature.

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Table 1.

Total amount and density of acid sites in WP/ZrO2 sol-gel. Total quantity of sites (mol NH3 /g cat.)

Density of sites (mol NH3 /m2 )

25WP/ZrO2 pH 3

1247

5.6

15WP/ZrO2 pH 3

949

5.8

25WP/ZrO2 pH 5

1073

6.4

15WP/ZrO2 pH 5

607

8.2

25WP/ZrO2 pH 7

1195

6.7

15WP/ZrO2 pH 7

686

7.4

Catalysta

a Thermal

treatment at 400◦ C, 4 h in flowing air.

are indicative of the slow transformation of amorphous zirconium oxyhydroxide to crystalline tetragonal zirconia [12]. These results are in good agreement with that obtained from XRD reported by Hern´andez-Cort´ez et al. [13]. 4.

Conclusions

The addition of tungstophosphoric acid to zirconia by the sol-gel process induces the formation of strong acid Lewis sites on the samples. Zirconia prepared by the sol-gel method was found highly hydroxylated producing an effect of stabilization of the Keggin structure after high temperature treatments. The formation of tetragonal zirconia was delayed by the incorporation of WP to ZrO2 gel. From NH3 -TPD acidity values higher than 1100 µmol of NH3 /g were obtained showing that the WP-ZrO2 sol-gel derived solids can be considered as super acid solids. Acknowledgments We thanks CONACyT (M´exico) for financial support (Beca No. 111633) and Instituto Mexicano del Petroleo. References

Figure 3. Raman spectra of 25WP-Z heat-treated in air at 400◦ C, obtained at: (a) pH 3, (b) pH 5 and (c) pH 7.

tungstophosphoric acid (WP) shows intense bands at 1012, 1007 and 990 cm−1 . These bands have been assigned to the symmetric and assymmetric stretching modes of W O (terminal oxygen bond) [11]. In the sample gelled at pH 3, the Raman spectra is completely covered by the luminescence of the sample. At pH 5 and pH 7 three bands at 997, 981 and 950 cm−1 can be seen suggesting the stabilization of the WP Keggin structure on the sol-gel zirconia. The 997 and 981 cm−1 bands are shifted to high energy region in comparison with those of pure WP (1012 and 990 cm−1 ), since the W O bonds were strongly tied due to the hydroxyl interactions with the phosphate groups. The absence of sharp bands in the spectra shows that the samples are partially amorphous. However, bands at 146, 259, 307, 461, 595 and 632 cm−1 ,

1. T. Lopez, J. Navarrete, R. Gomez, H. Armend´ariz, and F. Figueras, Appl. Catal. A: General 125, 217 (1995). 2. J.L. Sohn and H.J. Jang, J. Mol. Catal. 64, 349 (1991). 3. K. Arata, Advances in Catalysis 37, 165 (1990). 4. C.Y. Hsu, C.R. Heimbuch, C.T. Armes, and B.C. Gates, J. Chem. Soc., Chem. Comm. 1645 (1992). 5. Y. Amenomiya, Appl. Catal. 30, 57 (1987). 6. T. Iizuka, M. Kojima, and K. Tanabe, J. Chem. Soc., Chem. Comm. 638 (1983). 7. M. Misono, Catal. Rev. Sci. Eng. 29, 269 (1987), 30, 339 (1988). 8. X. Bokhimi, A. Morales, O. Novaro, M. Portilla, T. Lopez, F. Tzompantzi, and R. Gomez, J. Solid State Chem. 135, 28 (1998). 9. T. Lopez, R. Gomez, J.G. Hern´andez, E. L´opez-Salinas, X. Bokhimi, A. Morales, J.L. Boldu, E. Mu˜nos, and O. Novaro, Langmuir 15, 5820 (1999). 10. E.P. Parry, J. Catal. 2, 374 (1963). 11. U.B. Mioc, R.Z. Dimitrijevic, M. Dividovic, Z.P. Nedic, M.M. Mitrovicand, and P.H. Colomban, J. Mater. Sci. 29, 3705 (1994). 12. P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Burgraaf, and J.R. Ross, Appl. Catal. 57, 127 (1990). 13. J.G. Hern´andez-Cort´ez, T. L´opez, R. Gomez, E. Torres Garcia, P.P. Lottici, and D. Bersani, J. Solid State Chem., to be published.