Synthesis and Characterization of Titania-Based Ternary and Binary

Single oxides of titania, silica, alumina, and zirconia were synthesized by the sol−gel method, using a temperature treatment before hydrolysis. Bin...
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Ind. Eng. Chem. Res. 2007, 46, 1138-1147

Synthesis and Characterization of Titania-Based Ternary and Binary Mixed Oxides Prepared by the Sol-Gel Method and Their Activity in 2-Propanol Dehydration Ignacio R. Galindo,†,‡ Tomas Viveros,*,† and David Chadwick‡ Department of IPH, UniVersidad Autonoma Metropolitana, AVenida Michoac’an y la Purisima s/n, C.P. 09340, Mexico D.F., Mexico, and Department of Chemical Engineering, Imperial College London, Prince Consort Road, London SW72BY, U.K.

Single oxides of titania, silica, alumina, and zirconia were synthesized by the sol-gel method, using a temperature treatment before hydrolysis. Binary and ternary oxides of titania-alumina, titania-silica, titaniazirconia, titania-zirconia-alumina, and titania-zirconia-silica were similarly prepared. The synthesized oxides were analyzed by nitrogen adsorption, XPS, XRD, AAS, and TEM, to assess their physicochemical characteristics. The mixed oxides showed nearly identical textural properties, independent of the composition. Only the titania-silica mixed oxide presented anatase domains as determined by XRD. However, crystal segregation in some other mixed oxides was detected by TEM. XPS revealed that the surface atomic distribution in the solids was close to the bulk composition that was obtained by AAS. Furthermore, the binding energies determined by XPS were similar to those previously reported for homogeneous systems. The synthesized systems were tested in the 2-propanol dehydration reaction, where the silica-containing materials were found to present the highest activity. In the ternary systems, Al seemed to interact marginally with the Ti and Zr cations, whereas interaction with Si was evidenced by an increase in 2-propanol dehydration activity and by binding energy shifts for all of the cations. Introduction In the field of catalysis, there is a growing interest in the preparation of mixed oxide systems, either as catalysts or as catalytic supports.1-3 It is known that the synthetic procedure used to prepare the materials affects the degree of mixing and their textural properties.1 Among the known synthetic procedures, an approach that allows control of these properties and is able to ensure a high degree of mixing for bulk mixed oxides while keeping an anion-free resulting oxide is the sol-gel method. Using conventional methods, for instance, coprecipitation, anions such as Cl- or F- are frequently introduced in the oxide matrix from the metallic precursors (usually salts containing either of them) and are known to modify important catalytic functionalities (e.g., acidity, metallic dispersion).4-6 In this work, the sol-gel method was selected to avoid this concomitant anion contamination of the resulting materials, because other methods are unable to avoid this obstacle while producing a bulk homogeneous mixed oxide.1 In the realm of mixed oxides, several binary systems have been the subject of research for different applications. These binary oxides are prepared for several reasons: to improve the chemical properties of an “inert” support, to enhance a specific textural property, or to produce a solid with a tailored composition that is known to present distinguishing characteristics (larger acidity, larger surface areas, thermal stability, etc.). Of the pure oxides that have been mixed to improve their surface areas while trying to use their chemical properties, titania has been widely studied.7-9 It is a reducible oxide of great interest because it has proved useful for several applications,10 for example, hydrogenation reactions as a support or promoter, * To whom correspondence should be addressed. E-mail: tvig@ xanum.uam.mx. † Universidad Autonoma Metropolitana. ‡ Imperial College London.

photocatalytic applications as a catalyst, etc. Because of its versatility, titania has been mixed with other frequently used oxides (alumina,7 silica,8 zirconia,9 etc.), and certain compositions have been found to present characteristics that might be advantageous for some of the previously mentioned applications. From the reported results, it has been observed that a molar ratio of 1:1 in Zr-Ti mixed oxides produces systems with enhanced thermal stability, acceptable surface areas (250 m2/ g) for catalytic supports, and intermediate acidities when compared to those of the constituent single oxides.11,12 For the Al-Ti system, it has been observed that Al/Ti molar compositions of 2:1 present the highest acidity concentration in the series, that the acid strength increases as the Ti fraction increases, and that the textural properties for the oxides are essentially unaltered in the intermediate range of 85-25 mol % Ti.7 A composition of Al/Ti ) 1:1 was selected, so stronger acid sites could be present in the system with nearly no loss in the surface area of the oxide, even though the acid amount was not optimized at this composition. In the Ti-Si system, it has been observed that, at a composition of 10 mol % Ti, the titanium integrates in the silica matrix, inducing a modification of the acidity and presenting nearly no segregation. On the other hand, a 10 mol % Si composition produces the largest acidity in the series.13 However, after the usual calcination treatment at about 500 °C, the presence of TiO2 crystals in this composition cannot be avoided. Because this study focuses on the preparation of titania-based mixed oxides, the second composition, where Ti is the main component, was chosen. These binary oxides were synthesized as a reference for comparison with the behavior of silica-titania-zirconia and alumina-titaniazirconia systems. Whereas binary oxides have been studied thoroughly, ternary oxides have seldom been analyzed.14,15 In light of what has been reported for binary systems, it could now be possible to determine what kind of behavior ternary oxides would present.

10.1021/ie060539r CCC: $37.00 © 2007 American Chemical Society Published on Web 01/13/2007

Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1139 Table 1. Molar Ratios and Weight Percentages of the Prepared Mixed Oxide Supports molar ratioa

weight percentageb

components

sample

Ti/Me1

Ti/Me2

TiO2

Me1Ox

Me2Oy

titania-alumina titania-zirconia titania-silica titania-zirconia-alumina

TA1 TZ1 TS9 TZA15 TZA33 TZS15 TZS33

1 1 9 1 1 1 1

2.83 1 2.83 1

68.1 39.4 92.3 37.0 33.3 35.7 30.4

31.9 60.6 7.7 56.9 51.1 54.9 46.8

6.1 15.6 9.4 22.8

titania-zirconia-silica

a Me refers to Al for alumina, Zr for zirconia, and Si for the silica-containing support. In the ternary mixed oxides, Me refers to Zr, and Me refers to 1 1 2 Al or Si, as appropriate. b Me1Ox corresponds to the weight percentage of the oxides of the second kind of atom in the binary oxides and to ZrO2 in the ternary mixed oxides. Me2Oy corresponds to alumina or silica in the ternary mixed oxides.

Therefore, an attempt was made to correlate the behavior of the binary systems with those of the ternary oxides. It is well-known that, as long as molecular-scale mixing is achieved during the preparation of the mixed oxides, the acidity can be modified from that presented by the constituent single oxides.1 To assess the chemical properties of the surface and to determine whether any changes in the surface characteristics of the synthesized mixed oxides have occurred, the acidcatalyzed 2-propanol (2-PrOH) dehydration reaction was selected as probe reaction. Several authors have used this reaction to probe binary systems,2,12 so comparison with the literature can provide more information on the catalytic behavior of the ternary systems. Experimental Section Preparation of Single and Mixed Oxides. In the literature,1,2 several processing parameters have been modified to determine the conditions that favor a certain phase, certain textural property, or solid thermal stability, depending on the application of interest. The conditions selected for this work were based on the need to obtain homogeneous mixed oxides, that were thermally stable above 400 °C and preferably amorphous or with a microcrystalline structure. Aluminum tri-sec-butoxide (97%, Aldrich), titanium(IV) isopropoxide (97%, Aldrich), tetraethyl orthosilicate (98%, Aldrich), zirconium(IV) propoxide (70%, Sigma), and butanol (99.9%, Aldrich) were used as raw materials. The notation used in this work, the nominal molar ratios, and the corresponding weight percentages (wt %) of the prepared mixed oxide supports are indicated in Table 1. An alcohol/water/acid/alkoxide molar ratio of 65:30:0.2:1 was used to synthesize the single oxides, whereas an alcohol/water/ acid/alkoxide ratio of 85:35:0.3:1 was used in the mixed oxide syntheses. The silica (S100) synthesis required refluxing for 6 h to accelerate gel formation, because of the poor reactivity of TEOS. The alkoxides required for each synthesis were mixed with butanol and then added to a stirred glass reaction system, isolated from the atmosphere. This reaction mixture was stirred at 300 rpm throughout the process. First, the reaction system temperature was increased from ambient temperature to 70 °C and maintained at this temperature for 2 h to ensure complete dilution and interaction of all of the mixture components. Afterward, the solution was cooled to 0 °C, at which temperature the hydrolysis was performed. The hydrolysis mixture contained HNO3 (70%, Aldrich) and distilled water. HNO3 was used as a catalyst, and distilled water was the last reactant necessary to perform the hydrolysis reaction. This mixture was added into the system dropwise, at a constant rate of 0.3 cm3/min.

Once the hydrolysis solution had been completely added to the reaction system, a sol was formed, which, after a few minutes, turned into a gel. This gel was left still for a ripening period of 24 h, after which it was dried in flowing air at ambient temperature. After the gel had been dried, the hydroxide was formed, and after it had been calcined, the final oxide or mixed oxide was obtained. The calcination process used to prepare the oxides involved several heating ramps and holds. First, a heating rate of 1 °C/ min was applied to raise the temperature to 120 °C, where the materials were held for 1 h to allow for the slow desorption of free water and solvent. The second heating rate was 0.8 °C/ min to 300 °C, which was held for 2 h in an attempt to avoid hot spots in the reactor due to the combustion of organic carbon. The third and last stage for alumina (A100), titania (T100), and zirconia (Z100) consisted of a heating rate of 0.5 °C/min to 500 °C, where this samples were held for 4 h; for the rest of the materials, the hold lasted for 2 h. The last heating ramp was applied to all mixed oxides and to S100, in an attempt to minimize the characteristic microporosity of this solid. The samples were heated at 0.5 °C/min to 600 °C, where they were held for a 4-h period. Nitrogen Adsorption. Nitrogen physisorption was performed in a Micromeritics ASAP 2000 apparatus. The samples were degassed at 250 °C until a pressure of 0.1 Pa was achieved. The isotherms were measured at -195.9 °C (77.4 K). Mesopore size distributions were calculated from the desorption branch using the BJH method. Surface areas were determined by the BET method. The micropore assessment was performed using t-plot constructions. Transmission Electron Microscopy (TEM). Transmission electron microscopy was performed with a JEOL 2000FX Mk2 electron microscope, equipped with an emission source of electrons at 200 kV. The samples were ground in an agate mortar and suspended in ethanol. A few drops of this suspension were placed on carbon-coated copper grids and left to dry at ambient conditions in a closed environment. Atomic Absorption Spectroscopy (AAS). AAS was performed with a Perkin-Elmer AAS instrument. The supports were digested in hydrofluoric acid. The prepared solutions of known concentration were then analyzed by a standard procedure. A nitrous oxide flame was used to atomize the compounds. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were obtained using a VG Escalab MK II spectrometer equipped with a hemispherical electron energy analyzer and an Al KR (1486.6 eV) radiation source. It was operated at 10 kV and 20 mA. All spectra were obtained under vacuum at pressures below 10-8 mbar. The data were recorded in a computer interfaced to the spectrometer. The binding energies were corrected to the C 1s peak (C 1s ) 284.8 eV). The sensitivity factors for correlating

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Table 2. XPS- and AAS-Determined Compositions (mol %) of Mixed Oxides AAS (bulk composition)a

Table 3. Binding Energies of Calcined Samples Corrected for Charging with the C 1s Peak (284.8 eV)

XPS (surface composition)a

Ti 2p

sample

Ti

Me1

Me2

Ti

Me1

Me2

sample

O 1s

Al 2p

TA1 TS9 TZ1 TZA15 TZA33 TZS15 TZS33

0.60 0.77 0.47 0.45 0.36 0.41 0.34

0.40 0.23 0.53 0.40 0.30 0.34 0.28

0.15 0.35 0.25 0.38

0.57 0.82 0.41 0.38 0.31 0.34 0.28

0.43 0.18 0.59 0.48 0.44 0.48 0.38

0.15 0.24 0.18 0.34

A100 T100 S100 Z100 TA1 TS9 TZ1 TZA15 TZA33 TZS15 TZS33

531.0 530.1 532.6 530.9 530.5 530.5 530.1 530.2 530.2 530.6 531.0

74.1

a

Me1 refers to Al, Si, and Zr for TA1, TS9, and TZ1, respectively, and to Zr for the ternary mixed oxides. Me2 refers to Al and Si for TZA and TZS, respectively.

the areas under the peaks to atomic concentrations were obtained from Briggs and Seah.17 X-ray Powder Diffraction (XRD). A Philips PW1710 diffractometer operating at 40 kV and with a filament current of 50 mA with Ni-filtered Cu KR (λ ) 1.54056 Å) radiation was used. The samples were analyzed in the range of 2θ ) 10-90° using a step size of 0.02° and 4 s per step. The diffraction patterns were compared to JCPDS files. The wellknown Scherrer equation was applied to the samples that showed characteristic reflections, and the correction for instrumental broadening was made with reference to the silicon reflection (0.16°). 2-Propanol Dehydration. The reactions were carried out in a quartz fixed-bed microreactor at atmospheric pressure. The reactant (2-PrOH) was pumped into an evaporator, where the flow was mixed with helium as a carrier. The syringe pump that contained the liquid was set to a flow rate of 0.254 mL/h, and the carrier gas was set to a flow rate of 120 mL/min to obtain a 1 mol % concentration of 2-PrOH. To obtain good reproducibility, the catalysts were placed in the quartz reactor and dried at 450 °C in a 60 mL/min air flow for 1 h prior to reaction. The mass of catalyst was ca. 80 mg in each reaction. The reactions were carried out in a temperature-programmed reaction mode (TPR) by increasing the temperature of the reactor from ambient to 340 °C using a 1 °C/min heating rate. The products were analyzed online using a Shimadzu GC14B gas chromatograph equipped with a thermal conductivity detector (TCD) and a Porapak Q 870-100 column. Results The actual compositions of the supports were determined using AAS (Table 2). The results indicate a slight variation in each case with respect to the expected values indicated in Table 1 for the nominal compositions. The supports with the largest difference between the expected and obtained values are TS9 and TZS15, where Si was present in excess in both cases. Also, in Table 2, the surface compositions as determined by XPS are presented. From these data, it can be seen that TA1 is nearly homogeneous, with a slightly larger Al concentration on the surface. The binary mixed oxide TZ1 shows Zr surface enrichment, whereas TS9 shows Ti surface enrichment. Nonetheless, the compositions of the binary oxides are nearly the same in the bulk and on the surface. In the ternary mixed oxides, the surface concentration of Ti is always lower than that obtained from the AAS determination. In contrast, the concentration of Zr is always larger on the surface, reaching up to 50% Zr enrichment in the case of TZA33. Al and Si show similar values between the bulk and

Zr 3d

3/2

1/2

458.8

464.2

Si 2p

5/2

3/2

183.0

185.5

182.4 182.4 182.4 182.5 182.9

184.8 184.8 184.8 184.9 185.3

103.4 74.4 74.3 74.2

458.9 459.0 458.6 458.6 458.6 458.8 459.0

464.6 464.5 464.1 464.1 464.1 464.3 464.5

102.8

102.2 102.7

the surface in all cases, yet tending toward lower surface concentrations than determined by AAS. The binding energies (BEs) of the single and binary oxides are similar to those previously reported for each system (Table 3). For γ-Al2O3, there are reports of an Al 2p BE similar17,18 to the BE obtained for A100. Also, in T100 and S100, the BEs of the corresponding cations agree with the previously reported values for TiO2 and SiO2.16 For Z100, the value of the Zr 3d5/2 BE shows a high-energy shift of 0.8 when compared to previously reported values for ZrO2.16 In the cases of the binary oxides, TZ1 shows BE values in good agreement with those observed on titania-zirconia prepared from metal chlorides by homogeneous coprecipitation with urea19 (i.e., O 1s ) 529.9, Ti 2p3/2 ) 458.5, and Zr 3d5/2 ) 182.3 eV). Also, reports on silica-titania20 with a composition similar to that of TS9 indicate BE values of O 1s ) 530.3, Ti 2p3/2 ) 459.0, and Si 2p ) 103.0 eV., which are similar to the values reported in Table 3 for the TS9 system. TA1 presents values close to those reported by Zhaobin et al.18 for grafted surface oxides, which are also similar to the values reported for coprecipitated mixed oxides17 (i.e., O 1s ) 530.2, Ti 2p3/2 ) 458.6, and Al 2p ) 74.5 eV). The BEs of the ternary mixed oxides cannot be compared to literature values, because no reports on these systems could be found. It is interesting to notice, however, that the TZA system shows the same values for the BEs of the different atoms, independent of the composition and that these values are similar to those of TZ1 for O 1s, Ti 2p, and Zr 3d, whereas the Al 2p value is similar to that measured for TA1. The TZS system shows a different behavior in that, as the concentration of silicon increases, the BEs of all atoms increase. The BE of Si 2p ) 102.2 in TZS15 is the lowest detected in all of the systems, and the value increases to nearly that observed in TS9 in the case of TZS33. The BEs of Ti 2p, O 1s, and Zr 3d increase from the values observed in TZ1 to those of TS9 for Ti 2p and Z100 for O 1s and Zr 3d as the silicon concentration increases. The XRD results are presented in Figures 1 and 2. The single oxides show reflections corresponding to mainly rutile for T100, tetragonal for Z100, δ- or γ-alumina for A100, and amorphous for S100. Assignment of the reflections of A100 is difficult, because the intensity of the bands is low. The peak at ca. 45° shows an intensity corresponding to δ-Al2O3, although this phase does not show reflections in the 55-60° region, whereas γ-alumina does show reflections in this region, but the reflection at 45° should be stronger in the γ phase. Z100 shows reflections that correspond only to the metastable tetragonal phase of zirconia. A crystal size of 12.0 nm for this sample was determined from the 011 reflection at 2θ ) 30.3°. T100 shows both anatase and rutile phases, but the amount of anatase is extremely small. By using the relationship described by Kumar et al.,21 it was determined that the weight fraction of rutile was

Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1141

Figure 1. XRD patterns of single oxides: (A) Z100, (B) T100, (C) A100, (D) S100*, calcined at (A-C) 500 or (D) 600 °C. (b) Tetragonal ZrO2 (50-1089), (2) anatase (21-1272), (1) rutile (21-1276), (f) γ-Al2O3 (500741), ([) δ-Al2O3 (46-1215).

Figure 3. TEM micrographs of binary oxides: (A) TA1, (B) TS9, and (C) TZ1. The circles mark some crystals, and the arrows indicate some positions where the edges start to be more defined.

Figure 2. XRD patterns of mixed oxides calcined at 600 °C: (A) TS9, (B) TZA33, (C) TZ1, and (D) TA1. (2) Anatase (21-1272).

0.84. The crystal sizes determined using the Scherrer equation on the reflections corresponding to the 101 plane of anatase and the 110 plane of rutile were 9.6 and 24.8 nm, respectively. In the mixed oxide systems, only TS9 presents sharp peaks, indicating this solid’s crystalline nature. TS9 shows the main reflections of the anatase phase from TiO2. The crystallite size as determined by the Scherrer equation was 7.9 nm. TZ1 presents two broad bands at 32° and 55° that cannot be assigned to either oxide or to zirconium titanate, and the same occurs in the TA1 case. This binary oxide presents three broad bands at approximately 30°, 45°, and 65°. The bands at 45° and 65° are close to the strongest bands in the γ-alumina system, but the band at 30° does not belong to either alumina, titania, or

aluminum titanate. It was concluded that TA1 and TZ1 are X-ray amorphous. All of the ternary mixed oxides were X-ray amorphous, presenting a pattern similar to that of TZ1, as can be observed in the case of TZA33 (Figure 2-B). TEM images were recorded for the binary (Figure 3) and ternary (Figure 4) oxides, in order to determine more precisely whether the oxides were truly amorphous or microcrystalline. In TA1, some scattered crystals that could be due to δ- or γ-alumina were identified, but the breadth of the diffraction pattern did not allow for a definite determination. TS9 presented a homogeneous distribution of anatase crystals in all of the particles that were observed. The statistical determination done by counting more than 300 anatase crystals indicated a mean particle size of 7.2 nm, which is close to that determined by the mean width broadening technique (7.9 nm). This indicates that the particles were homogeneously distributed over the whole solid and that their size was practically the same all over. TZ1 did not show scattered crystalline particles in the micrographs recorded. This solid shows a more densely packed configuration and seems prone to crystallization, as the particles show some sharp edges that are indicative of sintering to crystal formation. Regarding the morphology, the oxides seem to be configured by a stack of spherical particles. The ternary mixed oxides (Figure 4) show similar morphology to that determined for the binary mixed oxides. However, sharper edges can be observed in the TZA samples, probably

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Figure 5. N2 isotherms of supports after calcination at 600 or 500 °C. Table 4. BET Surface Areas, t-Plot Micropore Areas, Pore Volumes, and Average Pore Diameters of the Calcined Oxides

Figure 4. TEM micrographs of ternary mixed oxides: (A) TZA15, (B) TZA33, and (C) TZS33.

indicating a more crystalline structure, nonetheless the samples remain amorphous. In the case of TZA33 (Figure 4b) some small-scattered particles were detected with a diffraction pattern corresponding to that of monoclinic zirconia. Also the TZS33 sample revealed the presence of small particles, with a diffraction pattern that was not conclusive. The texture of the solids was further determined by N2 adsorption measurements. The obtained isotherms are shown in Figure 5. It can be seen that most of the isotherms are similar and can be identified as type IV, with the exception of the S100, TZS15, and TZS33 isotherms, which show type I behavior.22 Type IV isotherms are characteristic of mesoporous solids, whereas type I isotherms correspond to microporous solids. The observation of type I isotherms in silicon-containing materials could be expected, because silica is known to exhibit a microporous structure. The fraction of micropores decreases with decreasing silica content. Furthermore, a small hysteresis loop can be detected for the TZS samples, whereas it was not observed for the S100 sample. Type E hysteresis loops were detected in most of the samples. This type of loop is characteristic of bottleneck pores and of solids composed by small spherical particles. These spherical particles were previously observed by the TEM measurements, so these results are consistent with each other. The BET specific surface areas, t-plot micropore areas, and average pore diameters of the different oxides are reported in Table 4. T100 presented the smallest surface area, as could be

sample

BET surf area (m2/g)

t-plot micropore area (m2/g)

pore vol (cm3/g)

avg pore diam (nm)

A100a T100a S100 Z100a TA1 TS9 TZ1 TZA15 TZA33 TZS15 TZS33

290 16.2 513 61 199 139 166 171 180 191 196

38 0 277 0.7 0 2.3 0 0 0 29.2 51.4

0.839 0.025 0.097 0.093 0.222 0.122 0.155 0.165 0.164 0.086 0.078

8.4 3.4 2.5 5.4 3.3 3.3 3.3 3.5 3.3 2.8 2.5

a Calcination temperature of 500 °C; unmarked materials calcined at 600 °C.

expected given that the high-temperature phase was its main component, as determined by XRD. This low value of surface area is characteristic of the rutile phase. Z100, which is composed of only the tetragonal phase, presented the second lowest surface area, which is also characteristic of the identified phase. A100 and S100, which were amorphous in nature, presented the largest surface areas of all of the synthesized oxides. However, S100, which is known to be microporous in nature, showed that about 50% of the area was located in micropores that are usually inaccessible for reaction or are prone to be blocked easily. All of the silica-containing mixed oxides presented micropores that could be considered to be inherited from the parent silica oxide. Of the mixed oxides, the one that showed the lowest surface area was TS9. This finding is mainly related to the fact that TS9 is the only one that presents crystalline particles homogenously distributed over the solid, although this surface area is still large, considering that the solid is mainly composed of the anatase phase. The rest of the mixed

Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1143 Table 5. Activation Energies and Onset Temperatures for 2-PrOH Dehydrationa

a

catalyst

Ea (kJ/mol)

onset temp (°C)

A100 T100 S100 Z100 TA1 TS9 TZ1 TZA15 TZA33 TZS15 TZS33

172 117 109 134 126 105 130 126 126 105 105

195 200 245 220 160 140 180 180 160 140 120

WHSV, 2.54 h-1; 2-PrOH flow, 1 mol % in He.

Figure 6. BJH desorption pore size distributions.

oxides presented areas in the range of 170-200 m2/g that are similar to those usually observed in commercially used catalytic supports. The average pore diameters of A100 and Z100 were the largest. The rest of the oxides presented average pore sizes of about 3.3 nm. The silica-containing materials showed the lowest pore sizes and values that might not be too reliable, because they are very close to the limits of the measurement technique. The pore size distributions are shown in Figure 6. Z100 and A100 showed bimodal distributions that were the broadest observed. The binary oxides and TZA15 and TZA33 showed well-defined peaks, indicating narrow distributions. On the other hand, as stated before, TZS15 and TZS33, which showed average pore sizes close to the limits of the technique, exhibited several peaks between 2 and 4 nm, so the determined values are not entirely reliable. In fact, problems for the assessment of micropores are well-known.23 The activities of the catalysts were determined at 220 °C using a pseudo-first-order reaction model and assuming that a steady state was reached at each temperature, because the reactor was run in a temperature-programmed mode. The rate of temperature increase was low (1 °C/min), so that this assumption could be justified, and some reactions at constant temperature were performed to verify the validity of the assumption at different temperatures. Assuming steady state allowed the determination of the activation energies for the different oxides. The linearity of the experimental points was high over a large range of temperatures, confirming the validity of the steady-state assumption. The main product observed was propene, and the absence of formation of acetone or diisopropyl ether is a common feature of the synthesized oxides. Only Z100 and T100 showed acetone formation at very high temperatures of 320 and 340 °C, respectively, and only in small amounts. The apparent activation energies (Table 5) varied from 109 kJ/mol for T100, S100, TS9, TZS15, and TZS33 to 172 kJ/mol for A100, whereas the rest of the catalysts showed apparent activation energies of ca. 126 kJ/mol. This observation could suggest that different active sites are present in the oxides, depending on the composition; that, in this reaction, there is a different heat of adsorption of the alcohol molecule for some solids; or that the mechanism or limiting step of the reaction changes with the solid. The catalysts that contain silicon showed the lowest activation energy, whereas the pure aluminum catalyst showed the highest activation energy. Mixtures of aluminum with titanium and zirconium showed intermediate activation energies, which could be related to a mixed process where more than one of the effects described

Figure 7. First-order specific activity for 2-PrOH dehydration at 220 °C.

above can take place. An apparent activation energy of 126 kJ/ mol for isopropanol dehydration over anatase has been previously reported.24 Other researchers25 have reported activation energies for TiO2 of 121-130 kJ/mol and for ZrO2 of 109113 kJ/mol in 2-PrOH dehydration, whereas, in the case of alumina, the observed apparent activation energy for dehydration of 2-propanol is within the previously reported26 range (167209 kJ/mol). The lowest onset temperatures (Table 5) of 120-140 °C were observed for the silicon-containing mixed oxide catalysts. The other mixed oxide catalysts showed onset temperatures of 160180 °C, whereas the single oxides started reacting at temperatures of 200 °C or above. In conclusion, the onset temperatures are related to the oxides composition and to whether they are pure or mixed oxides. In Figure 7, the first-order specific activity is reported. It was determined using a first-order reaction model, because most previous studies report a first-order kinetic dependence of the 2-PrOH dehydration rate of reaction on the alcohol.27,28 Only in the presence of large concentrations of water or in the presence of oxygen has a 1/2 order kinetic dependence been reported.24 It can be observed that the silica-containing mixed oxides presented the highest activities. TZ1 and TA1 presented activities that were between those presented by the two pure oxides that compose them. On the other hand, TZA15 and TZA33 presented lower activities than that presented by TZ1. Assuming that these ternary mixed oxides are homogeneous, heterolinkages between Al and Zr could be expected, because TA1 shows a larger activity than TZ1; the interaction between Al and Zr should be responsible for the observed activity decrease. 2-PrOH dehydration has generally been associated

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with acid site density because it can be realized even by the weakest acid sites.14 It has been observed that, for aluminazirconia systems, the acidity increases gradually from the less acidic zirconia toward the more acidic alumina values, showing no enhancement of the acidity due to heterolinkages between Al and Zr atoms.29 Discussion Single Oxides. To prepare single oxides with similar textural properties, variation of the synthesis conditions and reactants is necessary.30 In the present research, it was intended to relate the single oxide characteristics to the mixed oxide physicochemical properties; therefore, the syntheses were carried out using the same reactants and the same synthesis conditions, so the synthesis procedure would not become a variable. It was observed that the textural properties of the single oxides were in the expected range for each oxide. Even the pore size distributions followed what would be expected for each type of oxide. A100 is a microcrystalline mesoporous solid with a higher surface area than the usually observed values31 (e.g., 160-260 m2/g). Even when the XRD patterns were inconclusive in relation to the crystalline phase that is being detected in the A100 sample, it would be expected to be γ-alumina, because δ-alumina would be expected only at higher calcination temperatures and the full development of γ-alumina would be expected to occur before crystallization to the high-temperature phase was observed.32 Also, phase development retardation in alumina systems prepared by the sol-gel method has previously been reported.33 T100 contains nearly 84 wt % rutile phase, which is uncommon because the transition to this phase is expected to occur without additives at temperatures close to 800 °C10 and this sample was calcined at only 500 °C. It has been reported that hydrolysis of titanium alkoxides or chlorides at temperatures higher than 80 °C produces the titania rutile phase.10 In this research, the hydrolysis was carried out at 0 °C. However, the alkoxide solution was heated to 70 °C and stirred for 2 h before the addition of water. Self-condensation of the alkoxides as a result of the pretreatment could be the reason that this crystalline phase was obtained and, therefore, could be equivalent to performing the hydrolysis reaction at 80 °C. Similarly, Z100 presents the tetragonal phase, which is thermodynamically stable at temperatures above 1200 °C. The low-temperature synthesis of this phase has been reported several times,34,35 and explanations for its stability at low temperature have been advanced. There is still no agreement on the source of the stability of this phase; it has been related to several factors such as crystal size, lattice defects (oxygen vacancies), lattice strains, anionic impurities, etc.35,36 The crystal size obtained in the present work is small (ca. 12 nm), so a large amount of oxygen vacancies could be expected, and low stability of this phase would be predicted.36 However, Stefanic et al.35 also observed that there was no dependence on particle size and proposed that the presence of NO3- anions could stabilize the metastable tetragonal phase, especially when synthesized at low pH values, probably because of the surface charge.37 Therefore, the sample prepared in this work could be stabilized by these anions. Another option to synthesize pure microcrystalline tetragonal zirconia (without anion or cation doping) is to heat the zirconium alkoxides in organic solvents, as the formation of the oxides was reported even without the addition of water.38 This process produced nearly 100% tetragonal zirconia at 500 °C. In the present research, only

tetragonal zirconia was detected after calcination at the same temperature. The formation of only this phase could then be due to the pretreatment of the alkoxide solution or to the presence of NO3- anions in the oxide’s surface. The formation of the high-temperature phase of titania and the stabilization of the tetragonal phase of zirconia were not expected, but seem to be linked to the synthesis procedure. Refluxing of the solution prior to hydrolysis seems to be the key step for the high-temperature phases to be formed at low temperature. Mixed Oxides. In terms of textural properties, the nitrogen adsorption isotherms show that, independent of the composition, the mixed oxides have similar textures. The binary oxides and the TZA ternary oxides show narrow pore size distributions centered at 3.3 nm, with pore volumes of the same order of magnitude. The surface areas of all of the mixed oxides are close to that of the commonly used catalytic support γ-alumina (190 m2/g). The similar textures presented by all of the mixed oxides simplify the analysis of the kinetic measurements, because the effects of texture during reaction should be similar in all of the mixed oxides. More important than the textural properties of the mixed oxide systems is their homogeneity. The degree of interaction between cations can be analyzed by XPS, AAS, and reaction activity results. As mentioned earlier, 2-PrOH dehydration is a reaction catalyzed by even the weakest acid sites.14 According to Tanabe’s model,39 acidity development on mixed oxides can be predicted using two simple rules: (1) cations retain the coordination that they have in the single oxide, and (2) oxygen assumes the coordination it has in the host oxide. Strictly, this model cannot be applied at compositions that are near equimolar. Nonetheless, by associating acid sites with heterolinkages, this model establishes a conceptual relationship between molecular homogeneity and modification of acid site densities, which can be qualitatively determined via the 2-PrOH dehydration. (i) TS9. Comparing the AAS observations with the XPS measurements, it was determined that there was 5 mol % Ti enrichment in this solid’s surface. Literature reports indicate silicon surface enrichment of silica-titania materials.2,20 However, the titania-rich part was not analyzed in one case, and the actual concentration of Ti in the solid was not determined in the other. XRD and TEM indicate the presence of anatase crystals of similar size randomly distributed throughout the solid. It is not possible to determine whether these particles are covered with a titanium silicate or a silica layer as previously proposed.20,40 Nonetheless, a 5 mol % Ti surface enrichment could be produced by migration of the anatase crystals to the oxide’s surface to minimize the particle surface tension. Even when there is evidence of TiO2 segregation, this mixed oxide presents one of the highest activities in the 2-PrOH dehydration reaction. This high activity indicates that there is an interaction between the Ti and Si atoms and that this interaction modifies the acidity of the oxide. It is welldocumented2 that titania-silica mixed oxides develop mainly Brønsted acidity over the entire range of compositions, because of the Ti-O-Si interaction. This Brønsted site increase could account on its own for the large difference in 2-PrOH dehydration activities observed when comparing single oxide T100 with TS9. The pure oxide is mainly composed of rutile phase, whereas TS9 presents only the anatase phase. Therefore another contribution to the activity difference could be due to this phase modification, but it is clear that the large difference in activities cannot be justified only by this phase transition.

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(ii) TA1. For TA1, previous reports showed titanium surface enrichment,17 and ascribed this behavior to surface migration of TiO2, as suggested in this work for the TS9 mixed oxide. In contrast, aluminum enrichment has also been found in homogeneously precipitated urea samples synthesized at high temperature41 (100 °C); the Al surface enrichment was associated with a lower rate of Al precipitation. In the current research, TEM measurements indicate the presence of alumina crystals, so the expulsion of aluminum ions from a titania-rich matrix could be possible, thereby producing the small increase in Al surface concentration observed in the TA1. Previous reports on the acidity of this system indicate that titania-alumina presents mainly Lewis sites over the entire range of compositions, where the acidity strength is modified from that observed for Al3+ ions to that of Ti4+ ions.29 Because only a change in acidity strength is advanced for the Al2O3TiO2 mixed oxides according to the literature reports, a 2-PrOH activity intermediate between those presented by Al2O3 and TiO2 is expected in the mixed systems. This behavior was confirmed for the TA1 sample. (iii) TZ1. In contrast to what is observed in the titaniaalumina system, where segregation of the single oxides is always observed to occur as the calcination temperature increases,7,42 the equimolar composition of titania-zirconia always yields zirconium titanate.19,43 This phase transition has been observed to occur at temperatures between 600 and 670 °C depending on the synthesis method,19,43,44 with the solids prepared by the sol-gel method being the ones that present the highest thermal stability.44 The XPS-determined zirconium concentration is 10% larger than that observed in the bulk by AAS. This difference in concentration has previously been observed for solids of similar bulk composition.19,45 Reasons for the zirconium surface enrichment were not advanced in the previously mentioned reports. It seems that the surface enrichment in Zr is thermodynamically controlled, because, even when zirconium titanate is formed, surface Zr enrichment is still observed.43 2-PrOH activity intermediate between those presented by single oxides ZrO2 and TiO2 has been reported for the equimolar composition of zirconia-titania.12 That work determined, by pyridine adsorption, that the equimolar composition presents intermediate Brønsted acidity and intermediate 2-PrOH activity when compared to the single oxides. In this research, intermediate activity in 2-PrOH dehydration was observed, suggesting that a similar mixed oxide was obtained. Ternary Oxides. Ternary mixed systems have barely been analyzed. Therefore, to allow a thorough analysis of the ternary systems, some hypotheses need to be formulated as follows: (1) There are three types of heterolinkages that could be expected in the solids, i.e., Ti-O-Zr, Ti-O-X, and Zr-OX, if the cations are homogeneously distributed. (2) The probability of a triple connection, where all of the atoms of different natures are in contact with each other, is considered to be extremely low. (3) Long-range interactions do not affect the electronic states and activities of the cations as much as first neighbors do. (4) The mixed oxide with higher affinity (thermodynamically more stable) should make up the majority of the ternary mixed oxide, and interactions with the third cation are possible to a lower extent; the possibility of bonding to this third cation increases as the cation concentration in the solid increases. The ternary oxides were prepared from the most stable oxide, TZ1. As mentioned before, titania-zirconia crystallizes to zirconium titanate without the formation of segregated phases,19,43 in contrast to what is observed in titania-alumina, where the

titanate is not formed directly,7,42 and also to what is observed in titania-silica, where the solubility range is below 15 mol % Ti.2 The ternary oxides were prepared so as to keep the same degree of interconnectivity between Ti and Zr, so the variable to be taken into account is the interaction with the third metal ion. (i) TZA. TZA33 presents 37% less aluminum than what is observed in the bulk measurements. The Zr/Ti surface ratio increases to nearly that observed in the TZ1 oxide. This sample presents crystals corresponding to monoclinic zirconia. The increase in Zr surface concentration and the decrease in surface Al could be related to this crystal formation, which somehow preferentially obscures the Al atoms. Another possibility is that the sample is starting to form zirconium titanate, while the aluminum atoms remain in the core. TEM measurements show that some sections of this sample start to show sharp edges, allowing this second possibility. Zr segregation could be caused by the higher solubility of Ti than Zr in alumina. A higher interaction between two of these cations could induce the remaining one to segregate. The BEs of Zr 3d and Ti 2p remain constant and equal to those detected in the TZ1 solid. The increase in Al concentration does not seem to modify the environments of Ti and Zr. The Al 2p BE is also approximately constant. Assuming that the probability of Ti-O-Al heterolinkages increases from TZA15 to TZA33, and based on the TA1 measurements, the BEs of Al 2p and of Ti 2p are expected to show high-energy shifts as the Al concentration increases. Bohemite presents octahedral coordination of all of the Al cations and a BE of 74.2 eV, whereas γ-Al2O3, with a known defective spinel structure, presents a BE of 73.7 eV. The presence of one-third tetrahedral Al cations could account for the 0.5 eV difference in BE.16,32 It has been shown that, at low concentrations, Al assumes a pentahedral coordination at the expense of Al tetrahedral cations in aluminazirconia systems.46 As the concentration of aluminum increases, the pentacoordinated cations disappear, acquiring the expected Al tetrahedral coordination. An increase in the Al BE would therefore be expected when the Al concentration is low in alumina-zirconia mixed oxides, and the BE value should approach that of Al2O3 as the Al concentration increases. On this basis, the apparent insensitivity of the Al BE to the surroundings could be due to counteracting effects in the BEs of the Al-O-Zr and Al-O-Ti heterolinkages and not to a lack of interactions between cations. It is known that alumina, titania, and zirconia are mainly Lewis acids. The acidity of the cations decreases in the order Al . Ti > Zr. Alumina-titania and alumina-zirconia have been observed to exhibit only Lewis acidity that shifts from the less acidic to the most acidic cation.12,29 Hence, when introducing Al into the titania-zirconia system, an increase in Lewis acidity together with a decrease in Brønsted acidity would be expected, because, as indicated before, titania-zirconia interactions produce only Brønsted acidity.12,29 Also, aluminatitania and alumina-zirconia are more active in 2-PrOH dehydration than titania-zirconia at similar molar ratios. The reaction results indicate that the specific activity decreases in the order TZ1 > TZA33 > TZA15. As postulated in the hypotheses, the strongest interaction present in this solids should occur between Ti and Zr, the introduction of small amounts of Al induces a decrease in the activity, and further addition of Al recovers part of this activity. From the determined BEs, Al does not seem to interact strongly with Ti and Zr, so the activity recovery could be due to the direct action of the Al cations on the reaction, that is, Al acting on its own as in A100, which

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exhibits higher activity than TZ1. The activity decrease in the ternary systems when compared to TZ1 might be caused by Brønsted acidity diminution, resulting from the marginal interaction between Al and the other cations. (ii) TZS. The AAS and XPS composition determinations indicate surface Zr enrichment of approximately 32% in both cases, with a 19% surface depletion of Ti. The Si surface and bulk contents are in good agreement with each other. The Zr/ Ti surface ratios in the silicon-containing samples and TZ1 are similar. Therefore, there is an apparently homogeneous distribution of cations. The Ti 2p BE increases from what is observed in TZ1 to the value obtained in TS9, suggesting an increase of Ti-O-Si heterolinkages with increasing Si content. BEs of 103.4 for Si 2p and 183.4 eV for Zr 3d in ZrO2-SiO2 mixed oxides have been reported.47 Therefore, an increase in the Zr BE from the value observed in TZ1 is also expected if Zr-O-Si heterolinkages are increasing. The Si BE increases if Ti-O-Si or if ZrO-Si bonds are formed, so the observed increase in the Si 2p BE is consistent with what would be expected in a homogeneous oxide where all possible interactions between cations are taking place. The reaction results show a large increase in the 2-PrOH dehydration activity from what is observed in the TZ1 system. All possible cation interactions generate Brønsted acidity. The interactions of Zr and Ti with Si have been found to produce a large amount of Brønsted acidity even when the concentration of Si might be small.2,48 That is, even when the main structure might be similar to that of TZ1, the presence of small amounts of Si can generate a large increase in acidity that would be reflected directly in the 2-PrOH dehydration reaction, as observed in the present research. Conclusions The synthesized single oxides present textures similar to what is reported in the literature for each system. The titania and zirconia structures seem to be affected by the alkoxide solution heat treatment prior to hydrolysis, generating the high-temperature rutile phase and stabilizing the metastable tetragonal zirconia at low calcination temperature (500 °C) without the addition of additives. All of the mixed oxides show similar pore size distributions and textural properties. The largest difference in texture is detected for the silicon-containing materials. The presence of silicon induces the formation of micropores and causes a reduction in average pore diameters. Of the binary oxides, only titania-silica presents the anatase crystalline phase, but comparison of XPS and AAS data indicates a homogeneous distribution of Si and Ti throughout the solid. Titania-zirconia and titania-alumina are also homogeneous, as verified by the binding energies and reaction results that are consistent with what has previously been reported in the literature. The interaction between Zr and Ti is strong and does not seem to be markedly affected by the presence of Al or Si. Al seems to interact marginally with Ti and Zr in the ternary mixed systems, as observed from the reaction results and the binding energies, which showed no modification. On the other hand, Si appears to have more interactions, as the acid-catalyzed 2-PrOH dehydration shows that the silica-containing ternary systems present higher activities, and the binding energies also show that silica induces important changes in the electronic states of the Ti and Zr cations.

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ReceiVed for reView April 27, 2006 ReVised manuscript receiVed September 5, 2006 Accepted October 11, 2006 IE060539R