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Langmuir 1999, 15, 5820-5824
Acidity of Tungstophosphoric Acid-Zirconia Catalysts Prepared by the Sol-Gel Method† T. Lo´pez and R. Go´mez* Department of Chemistry, Universidad Auto´ noma Metropolitana-Iztapalapa, A.P. 55-534, 09340 Me´ xico D. F., Mexico
J. G. Herna´ndez and E. Lo´pez-Salinas Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 125, 07730 Me´ xico D. F., Mexico
X. Bokhimi, A. Morales, J. L. Boldu´, E. Mun˜oz, and O. Novaro‡ Institute of Physics, The National University of Mexico (UNAM), P. 20-364, 01000 Me´ xico D. F., Mexico Received September 21, 1998. In Final Form: June 10, 1999 Zirconia was doped with tungstophosphoric acid (TPA) by co-geling the acid with zirconium n-butoxide. When samples were annealed at 400 °C, their specific BET areas were 164 and 74 m2/g for 15 and 25 wt % TPA, respectively. From the X-ray diffraction analysis, nanocrystalline tetragonal zirconia and highly dispersed nanosized WO3 were identified. Pyridine adsorption was characterized by FTIR spectroscopy: bands at 1610 and 1442 cm-1, which correspond to Lewis acidic sites, were observed. Pyridine desorption was not complete at 400 °C, because the catalysts had strong Lewis acid sites. The reaction for 2-propanol decomposition with the sample having 25 wt % TPA showed a large selectivity for isopropyl ether; when it was used for chain isomerization of 1-butene, isobutene and isobutane were important products. ESR spectra had signals that corresponded to free electrons.
Introduction
tion of two oxoanions; for example,
Although zirconia is an active catalyst for methanol synthesis and alcohol dehydration,1,2 its acidity is too weak to catalyze processes that require strong acidic sites. Zirconia, however, has strong acid sites when sulfate ions are incorporated on its surface and can catalyze hydrocarbon isomerization.3-6 Strong solid acids are potential catalysts for producing isomerized hydrocarbons and the branched alkyl ethers that are the bases of high-octaneindex gasoline. Besides sulfate ions, tungstate and molybdate oxoanions also produce strong acidic sites when they are deposited on hydroxylated zirconia.7 Therefore, we have studied the synthesis and catalytic properties of zirconia impregnated with tungstophosphoric acid H3[PW12O40], which is a heteropolyacid. These acids are catalysts used in liquid phase catalytic reactions such as hydrolysis, esterification, alkylation, and isomerization. Heteropolyacids are usually more active than solid acids such as zeolites or silicaalumina oxides8 and can be synthesized by the condensa-
12[WO4]2- + [HPO4]2- + 23H+ f
* To whom correspondence should be addressed: e-mail,
[email protected]; fax, (+52) 5 724 4666. † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998. ‡ Member of El Colegio de Me ´ xico. (1) Jackson, N. B.; Ekerdt, J. G. J. Catal. 1986, 101, 90. (2) Maruya, K.; Maehashi, T.; Haraoka, T.: Narui, S.; Domen, K.; Onishi, T. J. Chem. Soc., Chem. Commun. 1985, 1494. (3) Morterra, C.; Cerrato, G.; Bolis, V. Catal. Today 1993, 17, 505. (4) Parera, J. M. Catal. Today 1992, 15, 481. (5) Chen, F. R.; Coudurire, G.; Joly, J. F.; Vedrine, J. C. J. Catal. 1993, 143, 616. (6) Iglesia, E.; Soled, S. L.; Kramer, G. M. J. Catal. 1993, 144, 238. (7) Arata. K. Adv. Catal. 1990, 37, 165. (8) Tanabe K. in Solids Acids and Basis; Academic Press, New York, 1970; p 23.
[PW12O40]3- + 12H2O Heteropolyacids have the Keggin structure (Figure 1). They are thermodynamically stable and sinter easily, producing solids with low specific surface areas (1-10 m2/g).8 To stabilize heteropolyacids, they are frequently supported on high-specific surface area oxides such as SiO2, SiO2-Al2O3, TiO2, or activated carbon.9-11 Supported heteropolyacids act in gas-solid reactions as catalysts:12 olefin hydrolysis,13 alcohol dehydration,14,15 carboxylic acids decomposition,16 butene structural isomerization,17 and methanol conversion.18 Basic supports such as magnesia are not used because they are basic and decompose the heteropolyacid.19 Because acidity depends on sample preparation method, in the present work, we incorporated tungstophosphoric acid into zirconia by using the sol-gel technique.20,21 This (9) Izumi, Y.; Hasebe, R.; Urabe, K. J. Catal. 1983, 84, 402. (10) Damyanova, S.; Fierro, J. L. G. Appl. Catal. A 1996, 144, 59. (11) Chu, W.; Yang, X.: Ye, X.; Wu, Y. Appl. Catal. A 1996, 145, 125. (12) Izumi, Y.; Matsuo, K.; Urabe, K. J. Mol. Catal. 1983, 18, 299. (13) Muller, J.; Waterman, H. Y. Brennst. Chem. 1957, 38, 357. (14) Okuhara, T.; Hashimoto, T.; Hibi, T.; Misono, M. J. Catal. 1985, 93, 224. (15) Saito, Y.; Nam Cook, P.; Niiyama, H.; Echigoya, E.J. Catal. 1985, 95, 49. (16) Otake, M.; Onada, T. J. Catal. 1975, 38, 494. (17) John, T. P.; Thomas, S. P. Oil Gas J. 1993, May 24, 54. (18) Kasai, A.; Okumura, T.; Misono, M.; Yoneda, Y. Chem. Lett. 1981, 663. (19) Igarashi, S.; Matsuda, T.; Ogino, Y. J. Jpn. Petrol. Inst. 1979, 22, 331; 1980, 23, 30. (20) Bokhimi, X.; Morales, A.; Novaro, O.; Portilla, M.; Lo´pez, T.; Tzompanztzi, F.; Go´mez, R. J. Solid State Chem. 1998, 135, 28.
10.1021/la981294t CCC: $18.00 © 1999 American Chemical Society Published on Web 07/27/1999
Acidity of Tungstophosphoric Acid-Zirconia Catalysts
Langmuir, Vol. 15, No. 18, 1999 5821
Figure 1. Keggin structure of [PW12O40]3- anion.
method allowed us to control the synthesis variables and analyze the effect of pH of the hydrolysis reaction mixture on the development of strong acidic sites. Samples were characterized by nitrogen adsorption, pyridine infrared spectroscopy absorption, X-ray powder diffraction, and electron spin resonance and also by using 2-propanol decomposition and 1-butene isomerization as test reactions. Experimental Section Catalysts Preparation. The solution of zirconium n-butoxide (0.162 mol) in ethanol (50 mL) was stirred at 70 °C, and then it was slowly cooled to room temperature. Afterward, the pH of the solution was adjusted to the desired value by using the corresponding hydrolysis catalyst: hydrochloric acid (35.6% (volume) HCl in water) for pH 3, acetic acid (glacial) for pH 5, and ammonium hydroxide (35% (volume) NH3 in water) for pH 7. To the solution was dropwise added 50 mL of tungstophosphoric acid [H3PW12O40‚12H2O] dissolved in ethanol (0.00122 mol TPA for 15 wt %, and 0.00231 mol TPA for 25 wt %). Water (0.654 mol) was added to the solution, which was maintained under constant stirring until gelling. Gels were isolated by filtration, dried in air at 100 °C for 24 h, and eventually calcined at 400 °C for 4 h, also in air. Characterization. Mean pore size diameters and specific surface areas of the calcined samples at 400 °C were determined from the nitrogen adsorption isotherms by applying BJH and BET methods, respectively. Isotherms were measured with an automated Micromeritics ASAP-2000 apparatus. The acid sites of the samples calcined at 400 °C were determined by adsorbing pyridine. Self-supported sample wafers were put in a high vacuum cell; pyridine was then introduced at room temperature and monitored by Fourier transformed infrared spectroscopy (FTIR) with a Nicolet 710SX spectrometer. Pyridine desorption was followed “in situ” by heating the sample. Decomposition of 2-propanol was performed in a “U” glass microreactor, with a volume of 20 cm3 and 1 atm continuous reactant flux; the reactor was coupled to a Varian 3700 gas chromatograph. Product analysis was made with a capillary Megaboro (GSQ) column and a flame ionization detector under the following conditions: helium carrier flow, reactant 2-propanol (Merck H.P), feed composition ratio He/2-propanol equal to 5, reaction temperature 150 °C, and a catalyst mass of 50 mg. The catalytic activity for 1-butene isomerization was performed in the same reactor system used for the 2-propanol dehydration. The reaction was carried out at atmospheric pressure using a molar ratio 1-isobutene/hydrogen of 1, at a reaction temperature of 400 °C. Product analysis was done with a Varian 3700 gas chromatograph having a flame ionization detector and a capillary column packed with KCl/Al2O3. Crystalline structures were measured by using X-ray powder diffraction and refined by the Rietveld method. Diffraction patterns were obtained at room temperature in a Siemens D-5000 diffractometer with Cu KR radiation; specimens were prepared by packing sample powder in a glass holder. Intensity was measured by step scanning in the 2θ range between 20 and 110°, with a step of 0.02° and a measuring time of 2 s per point. (21) Brinker, C. J.; Scherer, G. W. in Sol-Gel Science; Academic Press: Boston, MA, 1990.
Figure 2. Room-temperature X-ray diffraction patterns of the samples, calcined at 400 °C, with 25 wt % TPA: (A) sample prepared at pH 3; (B) at pH 5; (C) at pH 7. Lower tick marks correspond to tetragonal zirconia; upper, to WO3. Table 1. Average Crystallite Size of the Zirconia Tetragonal Phase sample
size (nm)
25% TPA/ZrO2 pH 3 15% TPA/ZrO2 pH 3 25% TPA/ZrO2 pH 5 15% TPA/ZrO2 pH 5 25% TPA/ZrO2 pH 7 15% TPA/ZrO2 pH7
36(8) 11(1) 71(1) 107(2) 11(1) 79(1)
Refinements were done with the DBWS-9411 program;20,22 peak profiles were modeled with a pseudo-Voigt function having average crystallite size as a fitting parameter. Standard deviations, showing the variation of the last figures of the corresponding number, are given in parentheses. When numbers corresponded to refined parameters, the standard deviations are not estimates of the analysis as a whole but only of the minimum possible probable error based on their normal distribution. Electron spin resonance (ESR) spectra were measured with a JEOL-RE3X spectrometer that had a cylindrical cavity (TE011 mode) operating at x-band frequencies (near 9.15 GHz) and 100 kHz field modulation. g-values were determined by measuring the resonance magnetic field with a NMR gaussmeter ES-FC5 (JEOL) and a frequency counter HP-5550B. Spectra were obtained by sweeping the static magnetic field and recording the first derivative of the absorption spectrum. Prior to measurements, samples were dried in air for 2 h at 400 °C; then immediately they were placed into the fused quartz tube that is introduced into ESR resonance cavity.
Results and Discussion X-ray diffraction patterns of the samples calcined at 400 °C showed the presence of three phases: one nanocrystalline and two amorphous ones (Figure 2). The nanocrystalline phase is tetragonal zirconia with an average crystallite size between 11.0(6) and 106(75) nm (Table 1). From the position of its main diffraction peak, it was determined that one of the amorphous phases had tetragonal zirconia local order; its concentration, determined by modeling it with a tetragonal zirconia having an average crystallite size of 1.2 nm, varied between 50 (22) Young, R. A.; Sarthivel, A.; Moss, T. S.; Paiva-Santos, C. O. J. Appl. Crystallogr. 1995, 28, 366.
5822 Langmuir, Vol. 15, No. 18, 1999
Lo´ pez et al.
Table 2. Textural Properties of Calcined Samples at 400 °C sample
surface area (m2/g)
mean pore diameter (nm)
25% TPA/ZrO2 pH 3 15% TPA/ZrO2 pH 3 25% TPA/ZrO2 pH 5 15% TPA/ZrO2 pH 5 25% TPA/ZrO2 pH 7 15% TPA/ZrO2 pH 7
220 74 164 92 168 177
3.3 11.5 2.0 3.2 3.0 2.7
Figure 4. Amount of adsorbed pyridine as a function of desorption temperature. Table 3. Catalytic Activity for 2-Propanol Decomposition selectivity (% mol) sample
Xa (% mol)
-ra
propene
isopropyl ether
25% TPA/ZrO2 pH 3 25% TPA/ZrO2 pH 5 25% TPA/ZrO2 pH 7 15% TPA/ZrO2 pH 3 15% TPA/ZrO2 pH 5 15% TPA/ZrO2 pH 7
21.66 17.94 13.09 2.89 0.93 1.2
1.4 × 10-5 1.16 × 10-5 8.45 × 10-6 1.86 × 10-6 5.99 × 10-7 7.73 × 10-7
79.3 79.5 75.0 100 100 100
20.7 20.5 24.4 0 0 0
a Reaction temperature 150 °C, He/2-PrOH ) 5, sample calculated 400 °C; Xa total conversion; -ra rate (mol/g).
Figure 3. FTIR spectra of adsorbed pyridine on the samples with 25 wt % TPA and prepared at different pH values: (a) pH 3, (b) pH 5, (c) pH 7, and different desorption temperatures.
and 75 wt %, depending on the specific sample. The second amorphous phase had tungsten oxide local order, which was used for its modeling with an average crystallite size of 1.2 nm; this second amorphous phase and the nanocrystalline zirconia phase contributed with 50 wt % of the phases. Studies show a clear local order of the amorphous phases when samples were annealed at higher temperatures. Specific surface area depended on the hydrolysis catalyst and TPA content (Table 2). When hydrochloric or acetic acid was the hydrolysis catalyst, the specific area of the catalysts calcined at 400 °C was larger when WO3 content was increased. The catalysts prepared with ammonium hydroxide had specific areas almost independent of WO3 content; all catalysts had similar microporosity (Table 2). The FTIR spectra of the adsorbed pyridine showed absorption bands at 1610 and 1442 cm-1 (Figure 3), which are characteristic of strong Lewis acid sites. The bands were observed even after samples were annealed above 400 °C, showing the strong stability of these acidic sites. Another band at 1488 cm-1 (Figure 3) is assigned to Bro¨nsted and Lewis acid sites. The weak bands at 1590 and 1577 cm-1 are assigned to weak Lewis sites; the very weak bands at 1637, 1546, and 1533 cm-1 are characteristic
Figure 5. 2-Propanol conversion as a function of reaction time.
of Bro¨nsted acidity. The weak bands disappeared when samples were annealed at 100 °C. Adsorbed pyridine amount increased when hydrolysis pH decreased. At room temperature, the samples containing 25 wt % TPA adsorbed 407, 324, and 180 µmol of pyridine/g for pH 3, pH 5, and pH 7, respectively (Figure 4). Pyridine adsorption diminished when sample temperature was increased. At 400 °C pyridine concentration was reduced to 32, 22, and 21 µmol of pyridine/g in the samples prepared at pH 3, pH 5, and pH 7, respectively. Bro¨nsted acidity was not observed, although TPA is a polytungstate characterized by having this kind of acidity. This absence may be caused by the reaction of zirconia hydroxyls with TPA protons. The large number of acid sites favored 2-propanol decomposition (Table 3). In this reaction, the conversion increased when TPA content was higher, which is in agreement with the large number of strong acidic sites observed from pyridine adsorption analysis. At 150 °C, the catalysts with 25 wt % TPA had conversions of 21.6, 17.9, and 9.2% for pH 3, pH 5, and pH 7 preparations, respectively; propene and isopropyl ether were the reaction products. Isopropyl ether was an important product, because its selectivity was 20.7 mol % for pH 3, 20.5 mol
Acidity of Tungstophosphoric Acid-Zirconia Catalysts
Langmuir, Vol. 15, No. 18, 1999 5823
Table 4. Conversion and Selectivity for 1-Butene Isomerization selectivity (mol %) catalyst
Xa* (mol %)
C1-C3
iC4
nC4
iC42-
t2C42-
c2C42-
other
c/t
15% TPA/ZrO2 pH 3 15% TPA/ZrO2 pH 5 15% TPA/ZrO2 pH 7 25% TPA/ZrO2 pH 3 25% TPA/ZrO2 pH 5 25% TPA/ZrO2 pH 7
69.0 66.5 70.4 73.9 75.3 74.7
0.5 3.2 0.8 2.5 2.8 2.5
0.0 0.0 0.0 1.2 1.2 1.0
0.0 0.0 0.7 1.3 1.6 1.4
6.8 2.5 7.5 12.9 14.4 15.0
46.6 49.2 47.3 42.3 40.3 40.3
42.9 45.1 43.7 36.7 36.0 36.5
0.0 0.0 0.0 3.1 3.8 3.2
0.9 0.9 0.9 0.9 0.9 0.9
a Conversion after 5 min of reaction: temperature 400 °C; 0.1 g mass catalyst; 1-C42-/H (molar) ) 1. Nomenclature: C1-C3 fraction; 2 iC4 isobutane; nC4 n-butane; iC42- isobutene; t2C42- trans-2-butene; c2C42- cis-2-butene.
% for pH 5, and 24.4 mol % for pH 7. The catalysts with 15 wt % TPA had lower conversions and produced only propene. Ether production is a reaction that needs strong acidic sites; it involves a pair of 2-propanol molecules; in contrast, the formation of olefin is a monomolecular reaction.23-25 The high selectivity for isopropyl ether observed in the catalysts prepared with 25 wt % TPA was more probably an effect of the strong acidic sites. At the low TPA content, site acidity was expected to be low, but large enough to carry out 2-propanol dehydratation. Catalyst deactivation depended on pH value (Figure 5). For long reaction times, 2-propanol total conversion reduced was to half of its initial value for the samples prepared at pH 3, to 30% for those prepared at pH 5, and 20% for those prepared at pH 7. Alcohol conversion was almost constant after 2 h. A great number of factors determine the catalyst deactivation; one of the most important is carbonaceous deposits. At low conversion, however, deactivation will be low. When olefins react with strong acid sites, the catalyst enhances polymer and oligomer formation. The activity of the analyzed catalysts depended on time (Figure 5), which is consistent with the FTIR spectra of the adsorbed pyridine. Isomerization of 1-butene was nearly independent of TPA concentration (Table 4) and the pH used in the synthesis. High TPA concentrations, however, favored isobutene and isobutane formation. For 15 wt % TPA, the conversion was 69.0, 66.5, and 70.4 mol % for pH 3, pH 5, and pH 7, respectively; the selectivity to isobutene was 6.8, 2.5, and 7.5 mol %. For 25 wt % TPA, the conversion values were 73.9, 75.3, and 74.7 mol % for pH 3, pH 5, and pH 7, respectively; the corresponding selectivities was 12.9, 14.4, and 15.0 mol %. In the isomerization of olefins, double bond migration is faster than chain isomerization.26-27 The high selectivity for isobutene and isobutane of the catalysts with 25 wt % TPA could be related to the strong acidity. The constant cis-trans ratio evidenced that bond shift isomerization is a reaction that does not depend on acidity strength or acidic site density.27 When isobutane was formed, the samples with 25 wt % TPA had a high deactivation rate; its conversion significantly diminished with time (Figure 6) caused by secondary reactions: cracking, oligomeration, and coke formation. This is in agreement with their high acidity. During hydrolytic alkoxide condensation, vacancies were formed, which played an important role in the formation of acid Lewis sites. Vacancies are proposed to (23) Berteau, P.; Delmon B. Appl. Catal. 1991, 70, 307. (24) Shi, B.; Davis, B. H. J. Catal. 1995, 157, 359. (25) De Boer, J. H.; Fahim, R. B.; Lisen, B. G.; Viseren, W. J.; de Vleesschauwer, W. F. J. Catal. 1967, 7, 163. (26) Guisnet, M. in Catalysis by Acids and Bases; Elsevier Science Publishers B. V.: Amsterdam, 1985; p 283. (27) Matsuda, T.; Sato, M.; Kanno, T.; Miura, H.; Sugiyama, K. J. Chem. Soc., Faraday Trans. 1 1981, 77, 3107.
Figure 6. 1-Butene conversion as a function of reaction time.
Figure 7. ESR spectra of the samples with 15 wt % TPA and calcined at 400 °C.
Figure 8. ESR spectra of the samples with 25 wt % TPA and calcined at 400 °C.
be centers that trap free electrons.28-29 The evidence for the formation of free electrons from alkoxides is shown in Figures 7 and 8. The ESR absorption curves of the samples annealed in air at 400 °C were located in the region of free electron with g values between 2.0014 and 2.0022. The number of spins was about 7 × 1016. The number was obtained by a double numerical integration of the ESR curve and compared with a known number of Mn2+ spins in a NaCl monocrystal. The spectra of the catalysts with 25 wt % TPA had one more signal that is interpreted as a second neighbor perturbation, probably produced by a strong interaction between the TPA and zirconia. This interaction is responsible for the high acidity. Samples contained also nanosized WO3 particles. Therefore, it cannot be ruled out that highly dispersed WO3 alone could contain Lewis acidic sites. (28) Go´mez, R.; Lo´pez, T.; Bokhimi, X.; Mun˜oz, E.; Boldu´, J. L.; Novaro O. J. Sol-Gel Sci. Technol. 1988, 11, 1. (29) Sa´nchez, E.; Lo´pez, T.; Go´mez, R.; Boldu´, J. L.; Mun˜oz, E.; Novaro, O. Ceram. Trans. 1995, 55, 391.
5824 Langmuir, Vol. 15, No. 18, 1999
Conclusions Addition of tungstophosphoric acid to zirconia produced strong acidic Lewis sites. The hydroxyls liberated when samples were annealed reacted with the protons associated to TPA, destroying its characteristic Bro¨nsted acid sites. Samples were composed of nanocrystalline tetragonal zirconia amorphous zirconia and nanosized WO3 particles. The TPA-doped zirconia catalysts had good activity for 2-propanol dehydration, producing propene. The samples with 25 wt % TPA generated large quantities of isopropyl ether, in addition to propene; this corresponds to a high density of acidic sites. Lewis sites became more numerous
Lo´ pez et al.
when TPA concentration was increased, which favored the selectivity to isobutene and promoted the chain isomerization that forms isobutane. The ESR spectra had absorption lines with g values between 2.0014 and 2.0022, which correspond to trapped free electrons. This suggests the formation of vacancies during zirconium alkoxide decomposition. Acknowledgment. We thank Mr. M. Aguilar for technical assistance. The financial support given by CONACYT and FIES-IMP programs is acknowledged. LA981294T
