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Langmuir 1997, 13, 970-973
Zirconia/Silica Sol-Gel Catalysts: Effect of the Surface Heterogeneity on the Selectivity of 2-Propanol Decomposition† R. Gomez,*,‡,§ T. Lopez,‡,§ F. Tzompantzi,‡ E. Garciafigueroa,‡ D. W. Acosta,| and O. Novaro| Department of Chemistry Universidad Autonoma Metropolitana-Iztapalapa, P.O. Box 55-534, Mexico 09340 D.F., Mexico, Universidad de Guanajuato, Lascurain de Retana s/n, Guanajuato 36000 Gto, Mexico, and Instituto de Fisica, Circuito de la Investigacion UNAM, Mexico 1000 D.F., Mexico Received September 28, 1995. In Final Form: September 12, 1996X Zirconia/silica mixed oxides were prepared by the sol-gel method at pH 3 and 9 using tetraethoxysilane and zirconium acetate at 1.3, 4.0, and 6.7 wt % ZrO2. In acidic preparations, high specific BET areas were obtained. 2-Propanol decomposition was used as the reaction test. It was found that the selectivity to propene or acetone is a function of the preparation medium. In acidic preparations, acid sites were developed and high selectivity to propene was observed; formation of Zr-O-Si bonds in bulk and surface was suggested. By the contrary in basic preparations, the main product was acetone. In such a case the formation of Zr-O-Si bonds and pure ZrO2 deposited on the silica surface is proposed.
Introduction Significant progress in the preparation of catalytic materials has been made using zirconia as the active composite. Zirconia by itself exhibits important catalytic properties. Hydrogenation reactions,1,2 aldol addition of acetone,3 and butene isomerization4,5 are examples of its reactivity. Additionally, zirconia was successful used as support catalysts. Rhodium supported on zirconia is a very active catalyst for CO hydrogenation,6 whereas copper supported on zirconia is an active catalyst for the carbon dioxide methanation.7 When zirconia is treated with sulfuric acid, the conversion of methanol to gasoline is achieved.8 Skeletal isomerization of hydrocarbons over zirconium oxide promoted by platinum and sulfated ion are the most promising reactions for the use of zirconia basis catalysts.9,10 However in spite of the great number of reactions which has important applications, the main problem of the use of zirconia as catalysts or support is its low thermal stability. Because of this, zirconia/silica mixed oxides are reported as alternative composites for the stabilization of zirconium oxide. The zirconia stabilization can be done by impregnation of thermally res* To whom correspondence should be addressed at Universidad Autonoma Metropolitana-I: e-mail,
[email protected]; fax, (+52) 57 24 46 66. † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. ‡ Universidad Autonoma MetropolitanasIztapalapa. § Universidad de Guanajuato. | Circuito de la Investigacion UNAM. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Tanabe, K. Mater. Chem. Phys. 1985, 13, 347. (2) Nakano, Y.; Yamaguchi, K.; Tanabe K. J. Catal. 1983, 80, 307. (3) Zhang, H.; Harrori, H.; Tanabe K. Appl. Catal. 1988, 36, 189. (4) Nakano, Y.; Iizuka, T.; Hattori, H; Tanabe K. J. Catal. 1979, 57, 1. (5) Pajonk, G. M.; El Tanany, A. React. Kinet. Catal. Lett. 1992, 47, 167. (6) Ichikawa, M. J. Chem. Soc., Chem. Commun. 1978, 566. (7) Denise, B.; Sneeden, R. P. A.; Beguin, B.; Cherifi, O. Appl. Catal. 1987, 30, 353. (8) Nitta, M.; Sakoh, H.; Aoimiera, J. Appl. Catal. 1984, 10, 215. (9) Garin, F.; Andriamassinoro, D.; Abdulsamad, A.; Sommer, J. J. J. Catal. 1991, 131, 199. (10) Chen, F. R.; Coudurier, G.; Joly, J. P.; Vedrine, J. C. J. Catal. 1993, 143, 616.
S0743-7463(95)00804-3 CCC: $14.00
sitance supports using ZrOCl solutions11-13 or by gels synthesis using alkoxides.14,15 Zirconia/silica mixed oxides are the composites which attract major attention of different research groups. Bulk characterization, crystalline phase identification, and thermal stability have been successfully done using X-ray diffraction (XRD), transmission electron microscopy (TEM), and DTA (differential thermal analysis).16-18 However, zirconia/silica mixed oxides are surface heterogeneous composites; they show important surface heterogeneity. Diffusion of zirconium oxide in the silica support or the reverse phenomenum can occur producing important heterogeneity, of which quantification is extremely difficult. Sophisticated physical analytical methods like X-ray photoelectron spectroscopy (XPS) or LEED (low energy electron diffraction) have found use to characterize the surface composition of mixed oxides. Nevertheless, such techniques, if some of them are considered as surface techniques, give results far from the real composition. The analysis of the surface by such methods is limited to a very small exposed area, usually on the order of square nanometers, and hence not representative of large surface areas. Physical and chemical adsorption or spectroscopic studies of adsorbed molecules are suitable methods to determine the surface heterogeneity, since the adsorption phenomenum is limited to the surface atoms. Wherever possible, a catalytic test is largely recommended to study the surface atom distribution. The catalytic test will be useful for the quantitative analysis of the segregation occurring on mixed oxides and hence for the determination of the surface heterogeneity. In the present work the quantitative analysis of zirconia exposed in ZrO2/SiO2 catalysts is reported. The catalysts were prepared by the sol-gel method by the addition of (11) Davis, B. H. J. Am. Ceram. Soc. 1984, 67, c168. (12) Singh, P.; Date, S. K. J. Mater. Sci. Lett. 1987, 6, 621. (13) Kokubo, T.; Teranishi, Y.; Maki, T. J. Non-Cryst. Solids 1983, 56, 411. (14) Debsikdar, J. C. J. Non-Cryst. Solids 1986, 86, 231. (15) Kundu, P.; Pal, D.; Sen, S. J. Mater. Sci. 1988, 23, 1539. (16) Lopez, T.; Gomez, R., Ferrat, G.; Dominguez, J,M,; Schifter, I. Chem. Lett. 1992, 1941. (17) Acosta, D. R.; Novaro, O.; Lopez, T.; Gomez, R. J. Mater. Res. 1995, 10, 1397. (18) Lopez, T.; Asomoza, M.; Gomez, R. Thermochim. Acta 1993, 223, 233.
© 1997 American Chemical Society
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Table 1. Characterization of ZrO2/SiO2 Sol-Gel Oxides
ZrO2 wt % BET areaa pore diameterb acidityc a
1
acid 2
3
4
basic 5
6
1.3 620 55 0.079
4.0 741 56 0.155
6.7 805 45 0.162
1.3 34 179 0.023
4.0 47 154 0.039
6.7 23 155 0.056
ref 19 7 100 300
m2/g. b in Å. c mequiv of NH3/g.
zirconium acetate to a basic or acid alcoholic solution containing tetraethoxysilane (TEOS). The overall reaction of 2-propanol decomposition is used as a chemical test to quantify the zirconium oxide exposed. The results show that the zirconium oxide exposed strongly depends on the preparation method and on the amount of zirconium acetate. Experimental Section The preparation of the solids was achieved by the cogelation of tetraethoxysilane and zirconium acetate. Details of the preparation method have been reported.19 In brief, the cogelation was made in an acid (HCl, pH 3) and basic (NH4OH, pH 9) medium. The general procedure consisted of the addition of an aqueous solution of zirconium acetate to an alcoholic solution containing TEOS and the hydrolysis catalysts (HCl or NH4OH). The mixed solution was maintained under reflux until the gel was formed. Afterward, the gel is dried at 70 °C and then calcined for 12 h at 500 °C. Solids at 1.3, 4.0, and 6.7 wt % ZrO2 were prepared. For samples calcined at 500 °C for 4 h in flowing N2, the surface areas and pore size distribution form were obtained from N2 isotherms determined on an automated analyzer ASAP 2000 from Micromeritics. The BET surface areas were calculated from these isotherms using values for P/P0 in the range 0.05-0.25. The mean pore size distribution was calculated using the classical BJH method. The total acidity was calculated in a conventional flow adsoprtion system using N2 as carrier. The adsorption of NH3 was made at 200 °C using a calibrated loop coupled to a six-way valve. The saturation of the NH3 adsorption was recorded using a conductivity detector for quantification. Conventional scanning transmission electron microscopy (STEM) observations were carried out in a single entry JEOL 100CX electron microscope. Powder samples were ground in each case in a agate mortar and dispersed in water in a ultrasonic bath for several minutes. Some drops were deposited for STEM observations on 200 mesh cooper grids covered with carbon holey film. The catalytic activity was determined in a flow reactor coupled to a gas chromatograph for the analysis of reactants and products. 2-Propanol was passed to the reactor through a saturator using nitrogen as carrier. The partial pressure of 2-propanol used was 22 Torr, and the reaction temperature was 200 °C. Typically, tests were made using 20-50 mg of catalyst sample and a carrier flow of 3.6 L/h. Under these conditions the detected products were, propene, acetone, and isopropyl ether.
Results and Discussion The textural properties of the solids are reported in Table 1. At pH 3 preparation the ZrO2/SiO2 mixed oxides show high surface areas between 620 and 805 m2/g and small pore diameter (45-56 Å). On the other hand the pH 9 preparations low BET areas between 23 and 47 m2/g and large pore diameter (155-179 Å) are determined. The preparation method used in the synthesis of zirconiasilica oxides has important effects on the texture of the solids. Acid preparation forms solids of high specific surface area, whereas basic ones form solids of low specific surface area. Table 1 shows that the addition of zirconium acetate during the gelation of the TEOS has a positive (19) Lopez, T.; Navarrete, J.; Gomez, R.; Novaro, O.; Figueras, F.; Armendariz, H. Appl. Catal. 1995, 125, 217.
Figure 1. SEM micrographs of ZrO2/SiO2 at 6.7 wt % ZrO2 prepared at pH 9.
Figure 2. SEM micrographs of ZrO2/SiO2 at 6.7 wt % ZrO2 prepared at pH 3.
effect on the specific surface area when the samples are prepared in acid pH. However, in basic preparations only a slightly effect is observed. The hydrolysis of TEOS in acid medium is faster than those occurring in basic medium. Fast hydrolysis forms solids showing high hydroxylated samples. The effect of hydrolysis on specific area can be understood taking in to account the hydrolysis/ condensation ratio as a function of the pH.20 Moreover, the effect of the pH on the gelation of ZrO2/ SiO2 mixed oxides is of importance on the textural and shape of the solid particles. Small spherical particles are obtained in basic pH, and cubic particles are observed when the catalysts were prepared in acid medium. The shape of the particles is illustrated in the SEM (scanning electron microscopy) micrographs of Figures 1 and 2. The gelations of tetraethoxysilane and zirconium acetate produce ZrO2/SiO2 mixed oxides. X-ray diffraction spectra show only amorphous materials even when the samples were treated at 1000 °C. No characteristic peaks of tetragonal or monoclinic zirconia can be detected by this technique. At low zirconia concentration, X-ray diffraction is not sensitive enough to characterize crystalline zirconia phases. However, microcrystalline structure was observed by STEM and the coexistence of tetragonal and monoclinic phases were observed.17 (20) Henry, M.; Joviet, J. P. Livage, J. In Aqueouse Chemistry of metal Cations: Hydrolysis, Condensation and Complexation in Structure and Bonding; Springer-Verlag: Berlin, 1992.
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Table 2. Activity and Selectivity in the 2-Propanol Decomposition of ZrO2/SiO2 Sol-Gel Catalysts
conversion, % activitya propene, % acetone, % isopropyl ether, % a
1
acid 2
3
4
0.9 0.90 67 31 2
6.0 2.89 79 16 5
6.6 3.21 89 6 5
1.1 0.56 19 79 2
basic 5
6
0.36 0.17 25 75
0.17 0.08 47 53
Table 3. Propene and Acetone Rate Formation and Estimated ZrO2 Surface Area in ZrO2/SiO2 Oxides acid
ref 19 7 80 320 100
10-6 mol/g.
ratea
propene acetone ratea ZrO2 area m2/gb
basic
1
2
3
4
5
6
0.60 0.27 0.27
2.28 0.46 0.46
2.85 0.19 0.19
0.10 0.44 0.44
0.04 0.13 0.13
0.04 0.04 0.04
a 10-6 mol/g. b From acetone activity of ZrO /SiO oxides/activity 2 2 of high surface area pure ZrO2 (∼1.0 × 10-6 mol/m2).
Figure 4. Modeling of ZrO2 on SiO2 sol-gel catalysts: (a) pH 3; (b) pH 9.
Figure 3. Propene selectivity (%) as function of total acidity of ZrO2/SiO2 catalysts.
The pH medium and the addition of zirconium acetate also have an important effect on the total acidity of the mixed oxides. In acid preparations the total acidity increases from 0.079 to 0.162 mequiv of NH3/g and in basic preparations from 0.023 to 0.056 mequiv of NH3/g when the zirconia content increases from 1.3 to 6.7 wt %. In pure silica prepared from TEOS at pH 3 and pH 9, the acidities determined by NH3 adorption were less than 0.01 mequiv of NH3/g. The acidity behavior as a function of the zirconia content suggests that acid sites are created by the incorporation of zirconium in the silica structure, forming probably ZrO-Si bonds. The number of new acid sites should increase with the zirconia content. 2-Propanol decomposition was studied on zirconia/silica mixed oxides. The results show that the overall rate of 2-propanol decomposition is higher in acid preparations than in basic ones (Table 2). As a reference, in the last column is reported the activity of tetragonal pure ZrO2 showing a BET area of 300 m2/g. In Table 2, selectivities to propene, acetone, and isopropyl ether are reported. The selectivity to propene is a function of the overall acidity of the samples, i.e., is a determination of the total acidity.21 In Figure 3 propene selectivity on function of total acidity is represented. When acidity is increased, the propene selctivity increases. Concerning the selectivity to acetone, it can be seen in Table 2 that the selectivity of pure zirconia is 100% to acetone. Zr-O-Si bonds in bulk and surface as well as ZrO2 supported (by segregation) on the silica surface are expected to be formed during the catalyst preparation. 2-Propanol decompositon is a reaction which essentially (21) Ai, M.; Ikawa, T. J. Catal. 1975, 40, 327.
gives propene and acetone as main products. The formation of propene is a reaction which strongly depends on the acidity of the catalyst,22-24 while the formation of acetone depends of acid and basic sites.25,26 In ZrO2/SiO2 mixed oxides the 2-propanol decomposition will ocurr on the mixed surface and on the zirconia oxide phase. The two phases will act as competitive phases for the overall 2-propanol decomposition. In Table 3, it can be seen that the propene rate formation cannot be related exlusively on function of the acidity. Nevertheless, when the selectivity to propene is given as a function of the acidity, a good correlation is obtained. Figure 3 represents that the number of acid sites determined by NH3 adsorption are the sites active for 2-propanol dehydration. The overall reaction depends of the sum of the activities of the ZrO2 and ZrO2/SiO2 phases. If we assume that the formation of acetone is a reaction which only depends of the exposed ZrO2, the formation of acetone will be an estimation of it. In Table 3, the estimated exposed ZrO2 areas for the various mixed oxides are reported. It was calculated from the rate of acetone formation divided by the activity (1.0 × 10-6 mol/m2) of a high surface area pure ZrO2. The good correlation obtained between the propene formation and the total acidity shows that the formation of Zr-O-Si acid sites is more important in acidic preparations than in basic ones. Thus the zirconium atoms on the silica surface of acidic preparations are preferentially found as Zr-O-Si sites. By contrast, the high selectivity to acetone (100% in pure zirconia) in basic preparations suggests that a great amount of zirconium should be ZrO2 deposited on silica (Figure 4). One of the most important applications of zirconia or mixed zirconia oxides is the development of superacid catalysts. In a recent paper is reported that zirconia/ silica mixed oxides prepared at acid pH develop strong acidity when they are sulfated with sulfuric acid. Basic preparations only develop weak acidity.19 Such results show that the assumed Zr-O-Si sites formed in acid preparations are easily hydrolyzed and hence they are (22) Carrinzosa, Y.; Munuera, G. J. Catal. 1975, 49, 189. (23) Knozinger, H. Adv. Catal. 1975, 25, 184. (24) DeBoer, J.; Visserer, W. Catal. Rev. 1971, 5, 55. (25) Tanabe, K. In Catalysis and Basis; Imelik, B., et al., Eds.; Elsevier Sci. Pub. B.V.: Amterdam, 1985; p 1. (26) Szabo, Z.; Jover, B.; Ahmscht, R. J. Catal. 1975, 39, 225.
Zirconia/Silica Sol-Gel Catalysts
easily sulfated. The acidity developed in such sulfated samples is of the same order of the acidity of zeolites, 1.5 mequiv of NH3/g. The low sulfation in samples prepared at basic pH suggests that the hydrolysis of ZrO2 on a zirconia structure is more difficult. Analytical electron microscopy (AEM) was used to estimate the ZrO2 concentration of samples 3 and 6.17 It was found that the concentration of zirconia is 8.0 and 6.7 wt % ZrO2, respectively. The values are close between them and close to the nominal composition. In contrast the estimated values determined from chemical reactivity show that the ZrO2 exposed differs from 0.19 to 0.04 m2/g for samples 3 and 6, respectively. Such a comparative result suggests that AEM analysis can be far from the surface composition of ZrO2/SiO2 mixed oxides. Conclusions The sol-gel preparation method used in the present work to obtain zirconia/silica mixed oxides is an interesting
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method to modify the textural and catalytic properties of the mixed oxides. Is shown that a chemical reactivity test could be used as a very sensitive technique to determine the surface composition of zirconia/silica oxides. The characterization and catalytic test of sol-gel prepared samples suggest that in acid preparations, substantial substitution of silicon atoms by zirconium to form ZrO-Si bonds could be occurring. On the other hand, in basic preparations it is suggested that large amounts of high dispersed zirconia will be found on the silica surface. The relative concentrations of such species can be controlled by changing the initial conditions used during their preparation. Acknowledgment. The authors acknowledge support from CONACYT-Mexico and CNRS-France. E. Garciafigueroa to Instituto Mexicano del Petroleo for helpful aid. LA9508046
