Applied Catalysis A: General 193 (2000) 215–225

Thermal stability of 12-tungstophosphoric acid supported on zirconia

E. López-Salinasa,∗, J.G. Hernández-Cortéza, I. Schiftera, E. Torres-Garcıab,J. Navarretea, A. Gutiérrez-Carrillob, T. Lópezb, P.P. Lotticic, D. Bersanic

a Instituto Mexicano del Petróleo, Eje Central L. Cárdenas 152, 07730 Mexico D.F., Mexicob Universidad Autónoma Metropolitana-Iztapalapa, P.O. Box 55-534, 09340 Mexico D.F., Mexico

c Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Università degli Studi di Parma,Viale delle Scienze, 43100 Parma, Italy

Received 15 June 1999; received in revised form 15 September 1999; accepted 16 September 1999

Abstract

A freshly precipitated zirconia was impregnated with an ethanol solution of H3[W12PO40]·6H2O (TPA) in order to obtain0–25 wt.% TPA/ZrO2. The solids, heat-treated from 373 to 1073 K, were examined by means of differential thermal analysis,X-ray diffraction, surface area, infrared, Raman,31P nuclear magnetic resonance analyses, and used as catalysts in thedecomposition of isopropanol. Below 673 K, TPA on ZrO2 exist as distorted intact Keggin species interacting with≡Zr–OHor ≡Zr+ groups. However, at 773 K, bulk-like intact TPA species were detected. Above 773 K, the Keggin structure of TPAcollapse and transforms into WO3 and phosphorous oxides. The characteristic diffraction peaks of TPA cannot be observedeven with 20 wt.% TPA loading, indicating a high dispersion or a very small crystal size of TPA, while 20 wt.% TPA loadedon SiO2 clearly shows the presence of TPA particles.The heat-treatment temperature affects the catalytic activity of TPA onZirconia. The reaction rate was 3.3 times higher in a TPA/ZrO2 calcined at 773 than that at 473 K. ©2000 Elsevier ScienceB.V. All rights reserved.

Keywords:12-Tungstophosphoric acid; Heteropolyacid; ZrO2

1. Introduction

In recent years much attention has been focused onzirconium dioxide, ZrO2, as an outstanding catalystand/or catalyst support. The catalytic activity and se-lectivity of ZrO2 is greatly affected by the preparationmethods, the activating treatments and the modifyingagents. For instance, the addition of oxo-anions such as[SO4]2−, [WO4]2−, [MoO4]2− and [BO3]3− to freshlyprecipitated zirconia has yielded solid materials withoutstanding strong acid and catalytic properties, which

∗ Corresponding author. Tel./fax:+52-5-567-2927.E-mail address:esalinas@www.imp.mx (E. Lopez-Salinas).

are absent in each of the single components. SulfatedZrO2 shows the greatest acid strength (Ho = −16.04)among oxo-anion modified metal oxides [1]. The maindrawback of SO4/ZrO2 is its limited thermal stability,since sulfate anions are partially removed as SOx whencalcined at high temperatures (>873 K) and the catalystlooses acidity and activity [2]. Recently, WOx /ZrO2,may provide a good alternative to sulfated zirconia forsome isomerization and alkylation reactions [3,4].

The interest on developing solid superacids stemsfrom their potential use as catalysts to replaceenvironmentally hazardous liquid HF, H2SO4 orFriedel–Crafts type AlCl3 and BF3. A few processesthat may benefit from solid acids are: isomerization

0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved.PII: S0926-860X(99)00431-7

216 E. Lopez-Salinas et al. / Applied Catalysis A: General 193 (2000) 215–225

of linear C4–C6 paraffins, alkylation of isobutanewith C3 and C4 olefins to obtain high octane gasoline,alkylation of aromatics with olefins and a variety ofaromatic acylation processes.

Heteropolyacids (HPA) and their salts are anotherclass of compounds that may as well generate strongacidity once incorporated on ZrO2. HPA are poly-oxometalates made up of heteropolyanions havingmetal–oxygen octahedra as the basic structural unit[5]. The Keggin-type HPA are the most important incatalysis [6]. They can be represented by the generalformula: [XM12O40]x − 8 where X is the central het-eroatom (Si4+, P5+, etc.),x its oxidation state, and Mthe metal ion (W6+, Mo6+, V5+, etc.). The negativecharge can be compensated by hydrogen ions or awide variety of cations (e.g. HPA salts). The Kegginstructure is made up of a central XO4 tetrahedronsurrounded by twelve edge-sharing metal–oxygenoctahedra MO6. One of the main drawbacks of theKeggin-type HPA is their low specific surface areas(1–10 m2 g−1) [7,8] that limit their applications ingas–solid phase reactions. This disadvantage can beovercome by dispersing the HPA on high surface areasupports. The catalytic activity of supported HPA de-pends mainly on the HPA loading, the pretreatmentconditions and the type of carrier. Acidic or neutralsubstances such as: SiO2 [9,10], active carbon [11,12],acidic ion-exchange resin [13], TiO2 [14,15], are suit-able as supports. Metal oxides that exhibit basic prop-erties such as Al2O3 and MgO, tend to decomposethe HPA, causing a significant decrease in catalyticactivity [16,17]. So far, the addition of HPA on stabi-lized or hydrated ZrO2 has been scarcely examined[18,19]. We have reported recently that the additionof a Keggin-type HPA, 12-tungstophosphoric acid(TPA) H3[W12PO40], to freshly precipitated ZrO2yields strong solid acids (Ho ≥ −9.3) which showhigh catalytic activity in the skeletal isomerization of1-butene to isobutylene [19].

The chemical interactions between the HPA andthe support are a matter of great interest because astrong interaction could fix the HPA to the carrier,avoiding the leaching of HPA in liquid-phase reac-tions [11,12] or maintaining a high HPA dispersionin gas-phase reactions. The thermal stability of theHPA’s structure is also affected by the type of carrier.In silica-supported TPA,31P MAS-NMR studies in-dicate that silanol (Si–OH) groups may participate in

the HPA binding to form (≡SiOH2+)(H2PW12O40

−),though the mechanism is not yet clear [20]. In31PMAS-NMR spectra of carbon-supported TPA, a greatline broadening was observed, indicating a strong in-teraction between the TPA and carbon [21]. In TPAsupported on TiO2 and then calcined at 623 K, besidesintact Keggin units, a range of partially fragmentedclusters such as the lacunary [W11PO38]8− (e.g. a de-fect Keggin unit lacking one WO6 octahedron, weredetected by31P NMR and FT IR [15]. Similar to thecase of oxo-anion modified ZrO2, it is very likelythat bulky Keggin oxopolyanions on hydrated ZrO2may alter the structural transformation pathway upondifferent calcination conditions.

The purpose of this study was to examine the ef-fect of the calcination temperature on H3[W12PO40]supported on hydrated zirconia. Particularly, the struc-tural integrity of the TPA and the possible interactionsbetween TPA and ZrO2 were investigated.

2. Experimental

2.1. Preparation of xTPA/Z

The solids obtained from the precipitation of Zr saltsare complexes with general formula [ZrOx(OH)4− 2x ·yH2O]n where the relative amount of oxygen, hydrox-ide and water depends, among other parameters, onthe rate of precipitation [22] and the type of precur-sor salt [23,24]. For simplicity, these solids will behereinafter referred to as freshly precipitated or hy-drated zirconia. Hydrated zirconia was prepared byslow precipitation of an aqueous ZrO(NO3)2·xH2Osolution with aqueous NH4OH. The pH of the mix-ture was kept 10 by controlling the addition of thetwo solutions. The white gel obtained was repeat-edly washed with deionized water and separated in acentrifuge. After this, the white paste was dried in astove at 373 K for 12 h. The impregnation of the HPAwas carried out as follows. First an ethanolic solutionof 2.5× 10−3 M reagent grade TPA was prepared.Then, different amounts of this solution, in order toobtain 5.0, 10.0, 16.6, 20.0 and 25.0 wt.% on hydratedZrO2, were poured onto dried ZrO2. The materialswere transferred to a rotary evaporator where ethanolwas evaporated at ca. 353 K. After this, the solidswere stored in a dissicator containing both anhydrous

E. Lopez-Salinas et al. / Applied Catalysis A: General 193 (2000) 215–225 217

CaO and silica gel. These solids will be hereinafterreferred to asxTPA/Z (x= 5–25 wt.%).

For comparison purposes, two additional sampleswere prepared in a similar manner as above: 20 wt.%TPA on a previously calcined ZrO2 (673 K, 4 h, air),denoted as 20TPA/Z*, and 20 wt.% TPA on a freshlyobtained SiO2, denoted as 20TPA/S. These two sam-ples were calcined at 673 K, for 4 h in air after theincorporation of the TPA, except where indicated.

2.2. Characterization techniques

xTPA/Z samples were water vapor-saturated atroom temperature for 24 h in a closed vessel beforethe thermal analysis. The thermal patterns of thesolids were recorded in a Netzch STA-409EP appa-ratus by linearly heating (10 K min−1) the sample ina static air atmosphere. The reference material wasZrO2 activated at 1473 K.

XRD analysis was carried out at room temperaturein a D-500 equipment from Siemens using monochro-matic Cu Ka1 radiation at 35 kV and 25 mA. Thesamples were previously calcined at indicated temper-atures in static air in a furnace for 4 h. The diffrac-tion patterns were compared with those included inthe JCPDS (Joint Committee of Powder DiffractionStandards) data base.

Specific surface areas using the BET method withN2 adsorption at 77 K and pore size distributionby means of N2 adsorption–desorption isothermswere measured in an ASAP-2000 apparatus fromMicromeritics.

The type of acid sites (Brønsted or Lewis) wasdetermined in a Fourier transform infrared (FT IR)Nicolet 710SX spectrometer by means of pyridineadsorption. The equipment was furnished with acell with interchangeable windows in which thesamples, previously pressed into thin self-supportedwafers (20 mg), were placed in order to be evacuated(1× 10−6 Torr) in situ at 673 K for 30 min, and then,after cooling to room temperature, to adsorb pyridine.The structural variations upon heating at differenttemperatures using self-supported samples (dried at352 K) were anlyzed at indicated temperatures underreduced conditions (1× 10−6 Torr) for 30 min.

Raman spectra were recorded at room temperatureon previously calcined samples in a nearly backscat-tering geometry using an ISA Labram micro-Raman

apparatus. The excitation line was the 632.8 nm of anHe–Ne laser. The laser power on the sample was keptlow (about 1mW) to avoid thermal effects.

31P magic angle spinning (MAS) NMR spectraof the solids were recorded in a 300 MHz BrukerASX-300 spectrometer. A 4.5ms pulse (90◦) was usedwith a repetition time of 5 s between pulses in order toavoid saturation effects. Zirconia cylindrical samplespinners with 5 mm i.d. were used, and the spinningfrequency was 5 kHz. The number of accumulationswas 500. All measurements were carried out at roomtemperature using H3PO4 as standard reference toobtain the chemical shift (d) of the solid materials.

xTPA/Z solids were tested as catalysts in a fixedbed flow-type microreactor connected to a gas chro-matograph equipped with a PONA capillary column(Crosslinked Methyl Silicone Gum, 50 m, 0.2 mm i.d.)and a flame ionization detector. The calcined catalystswere heat-treated in situ at indicated temperatures in astream of He for 1 h before reaction and then cooled toreaction temperature. Decomposition of isopropanol(IPA) was carried out with 0.05 g (150–200 mesh) ofcatalyst at 388 K and 60 mm Hg of IPA partial pres-sure. The catalytic evaluations were carried out at lowconversion in order to avoid diffusion effects. The re-action rate and conversion were calculated assuminga first order reaction.

3. Results and discussion

3.1. Thermal behavior of xTPA/Z

The DTA curves of hydrated ZrO2 and those of5–25TPA/Z from ambient temperature to 1273 Kare shown in Fig. 1. Curve a correspond to that ofhydrated ZrO2 and is made up of one endothermicpeak (Peak I) at 381 K and an exothermic one (PeakIV) at 711 K. The first peak has been ascribed to thedehydration and dehydroxylation of amorphous hy-drated ZrO2 and the second one to the crystallizationof microcrystalline hydrated ZrO2 into metastabletetragonal ZrO2 [23,25,26] in agreement with ourXRD results (see below). In Curve b, a small portionof TPA added to freshly precipitated ZrO2, such as in5TPA/Z, brought about important changes in the DTAcurve. Here, besides Peaks I and IV, two exothermicpeaks at 584 and 683 K (Peaks II and III, respectively)

218 E. Lopez-Salinas et al. / Applied Catalysis A: General 193 (2000) 215–225

Fig. 1. DTA curves of: (a) hydrated ZrO2 and hydrated ZrO2impregnated with (b) 5.0, (c) 10.0, (d) 16.6, (e) 20.0 and (f)25 wt.% TPA.

appeared. These two exothermic peaks do not appearneither in pure TPA (Fig. 2c) nor in pure hydratedZrO2 (Fig. 1a), nor on TPA impregnated on a previ-ously stabilized ZrO2 at 773 K (Fig. 2d). These resultssuggest that Peaks II and III possibly result from theformation of Zr–O–W bonds between TPA’s termi-nal W=O oxygen atoms and surface≡Zr+ species.Considering that the Keggin polyoxoanion has a cu-booctahedral arrangement and that it will seat on aflat ZrO2 surface, a first stage (Peak II) may occurbetween the terminal oxygens initially adjacent to thesurface (four oxygen atoms at most) and a second one(Peak III) will take place on some of the rest of theterminal oxygen atoms as the crystallization of ZrO2proceeds. Nevertheless, Peak III could also be relatedto the removal of TPA protons, which may react with≡Zr–OH hydroxy groups to form water as reportedelsewhere [27]. Accordingly, in pure TPA protons

Fig. 2. DTA curves of: (a) freshly prepared SiO2, (b) SiO2 im-pregnated with 20 wt.% TPA, (c) bulk TPA and (d) 20TPA/Z*.

react with anionic oxygen atoms (from TPA itself) toform water between 673 to 773 K [27]. In all TPA/Zsamples, Peak I is accompanied by an endothermicpeak (Peak Ia) at about 473 K which appears partiallyoverlapped with Peak I. Peak Ia arises from the watermolecules bonded to TPA, e.g. H3[W12PO40]·6H2O,which are released near 473 K [28], in agreement withthe second endothermic peak in the DTA curve of TPAshown in Fig. 2a. In Figs. 2d–g, an exothermic peak(Peak V), appears at 993 K and becomes more intenseas the TPA content increases. This peak arises fromthe crystallization of tungstate species formed by thedecomposition of TPA, as discussed below. The tem-perature at which this transition occurs depends onthe nature of the support used. For instance, in Fig. 2,this transition in pure TPA or supported on a freshlyprepared silica occurs at 873 K (Curves a and b, re-spectively). These results indicate that the interactionof tungstate species with ZrO2 is much stronger thanthat with SiO2, resulting in a considerable delay onthe crystallization of tungstate species.

3.2. Evolution of TPA/Z structure uponheat-treatment

Fig. 3 shows the XRD patterns of 20TPA/Z calcinedat indicated temperatures for 4 h in air. Here, Pattern

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Fig. 3. XRD patterns of 20TPA/Z heat-treated at: (a) 373, (b) 773,(c) 923 and (d) 1023 K. Inset: (e) 20TPA/Z*, (f) 20TPA/Z and (g)20TPA/S.d bulk TPA, h tetragonal ZrO2, s monoclinic WO3.

a, which was recorded after drying at 393 K, showsvery wide peaks characteristic of microcrystalline hy-drated ZrO2. In Pattern b, recorded after calciningat 773 K, a portion of the amorphous material crys-tallizes into tetragonal ZrO2 (JCPDS card: 14–0534)showing no indication of any crystalline phases re-lated to TPA (the most intense peak of pure solidTPA is expected at about 2θ = 10◦) even though theamount of TPA impregnated is 20 wt.%. The reasonof these results may be that TPA particles are toosmall and/or too well dispersed and therefore unde-tectable by XRD. In order to clarify this aspect, theinset in Fig. 3 shows that in 20 wt.% TPA on freshlyprepared SiO2 and then calcined at 673 K, the TPA’scharacteristic peaks are clearly observed. In compar-ison, Patterns e and f from 20 wt.% TPA on stabi-lized (at 673 K) and hydrated ZrO2, respectively, showno indication of peaks from TPA’s particles. At same

TPA loading and calcination temperature, SiO2 shows2.6 times larger specific surface area than ZrO2 [19].Hence, a higher TPA’s particle dispersion would beexpected in the SiO2 than in ZrO2 (e.g. TPA’s XRDpeaks would be more intense in ZrO2 than in SiO2)at same TPA loading, which is not the case. These re-sults suggest a strong interaction between TPA parti-cles and ZrO2 surface, either in the hydrated or stabi-lized state. In agreement with these results, Y. Izumiet al. reported that the characteristic diffraction peaksof TPA in 20 wt.% TPA/SiO2 can be distinguishedonly above 20 wt.% TPA loading [9]. When the sam-ple is calcined at 923 K (Pattern c) practically all theamorphous material crystallizes into tetragonal ZrO2;this time including some peaks clearly associated toWO3 species. Further calcination at 1023 K (Patternd) brought about a crystal growth in both the tetrag-onal phase of ZrO2 and that of WO3. These resultspoint out that the Keggin heteropolyanion breaks apartat temperatures equal to or higher than 773 K. On theother hand, the absence of any peaks associated withthe monoclinic phase of ZrO2, even after calcining at1023 K (Pattern d) indicate that TPA and TPA’s frag-ments, i.e. tungsten oxide species, stabilize the tetrag-onal phase of ZrO2. Accordingly, ZrO2 obtained byprecipitation from aqueous ZrO(NO3)2·xH2O (in sim-ilar conditions to those used in this study) transformsfrom the tetragonal to the monoclinic phase at 823 K[29].

3.3. Stabilization of surface area upon TPA addition

Fig. 4 shows the effect of the TPA loading on thespecific surface area ofxTPA/Z. A ZrO2 calcinedat 673 K shows 132 m2 g−1, while in comparison20TPA/Z calcined at 673 K shows a 37% increase insurface area in comparison with that of single ZrO2. Asimilar stabilization trend can be observed in a seriesof xTPA/Z calcined at 773 K, in spite of the fact thatin comparison with those calcined at 673 K, a 40–60%reduction in specific area occurs. As a comparison, asimilar behavior takes place on a freshly precipitatedSiO2, dried at 393 K and later impregnated with theTPA. In this case, 16.6 wt.% of TPA on SiO2 shows22% higher surface area than that of single SiO2(383 m2 g−1). Accordingly, it has been reported thatthe addition of sulfate anions to freshly precipitated

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Fig. 4. Effect of the TPA loading and calcination temperature onthe specific surface area.

ZrO2 results in solids with much higher surface areaswhen calcined at 773–873 K in comparison with un-sulfated ZrO2 [2], thus showing an stabilizing effecton the structure of the final calcined zirconia.

3.4. Nature of acid sites

The impregnation of TPA on a freshly precipitatedZrO2 which upon further calcination undergoes dehy-droxylation and concomitant crystallization, implies

Fig. 5. Effect of the precursor support (Z: hydrated ZrO2, Z* pre-stabilized ZrO2) on the FTIR spectra of pyridine of supported TPA.(a) Room temperature, (b) 373, (c) 473, (d) 573, (e) 673 K. Shaded peaks indicate some of the bands ascribed to Brfnsted acid sites.

that some surface≡Zr–OH groups may react withTPA protons to form water. To clarify this behavior,a 20 wt.% supported on a previously calcined ZrO2 at673 K (Z*) was also examined by pyridine adsorptionFTIR. Fig. 5 (see Z* samples) shows that the bandsascribed to Brfnsted acid sites (1639 and 1533 cm−1)are not only more prominent than the correspondingones in Z-samples but also can be distinguished up to573 K, indicating that these Brfnsted sites are strong.In this case, the near absence of surface≡Zr–OHallowed protons to remain attached to the heteropoly-anion Keggin unit at higher temperatures.

3.5. Integrity of the Keggin structure on TPA/Z

Evidence for the retention of the Keggin ion struc-ture on the surface of ZrO2 was provided, in part, bythe FTIR spectra of 25TPA/Z heat-treated at differenttemperatures. The structure of the TPA is made up ofone PO4 tetrahedron surrounded by four trimetallicgroups of three edge-sharing WO6 octahedra, as re-ported extensively elsewhere [5]. As shown in Fig. 6,the group frequencies of the W3O13 triads for the im-pregnated TPA are very similar to those observed in thepure TPA [30] (compare Spectrum a and b). However,some of the bands of the TPA in the 1000–600 cm−1

region are partially overlapped with the bands of ZrO2(compare with Spectrum h). The band at 1627 cm−1

E. Lopez-Salinas et al. / Applied Catalysis A: General 193 (2000) 215–225 221

Fig. 6. FTIR spectra of: (a) pure TPA; those of 25TPA/Z heattreated at: (b) room temperature, (c) 393, (d) 473, (e) 573, (f) 673,(h) 773 K; and (h) that of ZrO2 calcined at 673 K.

arises from the bending mode of adsorbed water andis present in 25TPA/Z at evacuation treatment be-low 473 K. Accordingly, temperature-programmeddesorption experiments on TPA indicate that the wa-ter molecules, hydrogen-bonded to the protons andto the polyoxoanions, desorb from the solid TPA attemperatures near 473 K. For [W12PO40]3− anions,an IR absorption band at 1080 cm−1 in Spectrum ais well separated from the other bands of the spectra,which allows a clear assignment of then3 vibrationof the PO4 central tetrahedron [30]. However, whenthe TPA is impregnated on hydrated ZrO2, this band

splits into two components at 1087 and 1055 cm−1

(Spectrum b). Accordingly, this splitting is due to thesymmetry decrease of the central PO4 tetrahedron,similar to that reported for lacunary [W11PO39]8−Keggin anions, i.e. a defect Keggin structure lackingone WO3 unit [31]. The band at 956 cm−1 in Spec-trum b arises from thenas vibration of W=O (terminaloxygen in the Keggin structure), which in comparisonwith that in TPA (Spectrum a) is shifted towards thelower energy region, indicating a weakening of theanion cohesion as reported elsewhere [30]. Thenasvibration of W–O–W (oxygen connecting two WO6octahedra in a triad) bonds is responsible for theband at 810 cm−1 in the TPA Spectrum a, which isclearly present in that of 25TPA/Z but its frequencyhas lowered to 797 cm−1, for the same reason ex-plained above. Upon increasing the heat-treatmenttemperature to 373 and 473 K, the split band of thenas P–O vibration broadens and only one band canbe observed at about 1085 cm−1. Concomitantly, theband at 956 cm−1 broadens and becomes less intense.The dehydration and/or dehydroxylation of hydratedZrO2, which occurs between 373 to 473 K may beresponsible for the TPA rearrangement on the sur-face of ZrO2. Proton-containing molecules like H2Oand surface≡Zr–OH are likely to strongly interactwith Keggin anions due to the high polar nature ofthe later. At 573 K the band at 956 cm−1 completelydisappeared, suggesting the cleavage of some oxoW=O bonds and possibly their transformation intoW–O–Zr bonds. Each TPA Keggin unit contains 12terminal oxygen atoms and only the oxo W=O bondsadjacent to the zirconia surface are likely to partici-pate in the formation of W–O–Zr bonds. Noteworthyto mention, the intensity of the band at 956 cm−1

decreases and finally disappears at 573 K, along withthe band at 1630 cm−1 from water. Accordingly,the water molecules in TPA are hydrogen-bondedto some of the terminal oxygen atoms of the TPA[32]. In this regard, the band at 956 cm−1 may beassociated with the terminal W=O bonds interactingwith water. On the other hand, a weak band at about1000 cm−1, ascribed to the stretching mode of W=Ogroups, becomes more prominent above 573 K andmay be associated to the W=O groups non-interactingwith water. Above 573 K and below 773 K, the bandspreviously ascribed to the Keggin unit in 25TPA/Zclearly remain distinguishable. In bulk TPA, it has

222 E. Lopez-Salinas et al. / Applied Catalysis A: General 193 (2000) 215–225

Fig. 7. Raman spectra of 25TPA/Z heat-treated in air at: (a) 373,(b) 523, (c) 623, (d) 673, (e) 773, (f) 923, (g) 1023 and (h) 1073 K.

been demonstrated by means of Photoacoustic FTIRthat the Keggin structure remains up to 723 K [33].

The Raman spectra of 25TPA/Z calcined atindicated temperatures are shown in Fig. 7. Themost intense bands in pure TPA at 1012, 1007 and990 cm−1, have been ascribed to the symmetric (thefirst two bands) and asymmetric stretching modes ofW=O (terminal oxygen bond) [30]. In the 25TPA/Zmaterial dried at 373 K three bands at 997, 981and 950 cm−1 indicate the preservation of the Keg-gin structure of the impregnated TPA. The first twobands appear red-shifted in comparison with thoseof the pure TPA (1012 and 990 cm−1) [28], point-ing out a weakening of W=O bonds, possibly due tohydrogen interactions with surface Zr–OH groups.The broad band at about 950 cm−1 may arise fromthe stretching mode in W–O–W bonds in pure TPA[30]. When 25TPA/Z is heat treated between 523 and673 K (Spectra b–d in Fig. 5), the two narrow bandsfrom the W=O bonds disappear at 523 K and only thebroad band at 950 cm−1 remains. These results indi-cate that W=O double bonds break apart, suggestingthe formation of a W–O–Zr bond. In fact, dehydroxy-flation of hydrated ZrO2 to form one water molecule

will eventually generate some≡Zr+ species whichmay react with W=O bonds. At 773 K (Spectrume) the band at 997 cm−1 reappeared (from TPA’sW=O oxo groups), pointing out that at this tempera-ture the interaction between the Keggin ion and theZrO2 disappeared (see NMR discussion below). At923 K (Spectrum f) new bands at 265, 322, 717 and807 cm−1 typical of WO3 [34] and that of zirconiaat about 650 cm−1, appeared. Here, the broad bandat about 950 cm−1 indicate that some Keggin unitsstill remain (see NMR discussion below). At 923 and1073 K (Spectra g and h), predominantly the bandsfrom the tungsten oxide species can be observed.

Fig. 8 shows the31P MAS-NMR spectra of bulkTPA (broken line) and those of 25TPA/Z as a function

Fig. 8. 31P MAS NMR spectra of bulk TPA (broken line) andthose of 25TPA/Z heat-treated at: (a) 523, (b) 623, (c) 673, (d)773, (e) 923, (f) 1023 and (g) 1073 K.

E. Lopez-Salinas et al. / Applied Catalysis A: General 193 (2000) 215–225 223

of the heat treatment temperature. The bulk TPAshows a sharp signal at−14 ppm characteristic ofphosphorus in the central position of a Keggin unit ofbulk solid TPA, as reported elsewhere [15]. However,in 25TPA/Z pretreated at 523 K, a broader signal canbe observed at−8 ppm, indicating that TPA is presentin some other form. The TPA derivative species thatmay give rise to such chemical shift could be: (1)Partially fragmented Keggin units, and/or (2) Keg-gin units strongly interacting with surface≡Zr–OHgroups (e.g. distorted intact Keggin units). In the firstcase, lacunary [W11O39P–OH] Keggin units and in thesecond, either (≡Zr–OH2)n+ [H3− nW12PO40]n − 3 or(≡Zr–O)n–[H3− nW12PO40− n]. The former speciesare likely to exist at low heat-treatment tempera-ture, when≡Zr–OH groups abound, and the laterat high temperatures, when dehydroxylation is ex-tensive. However, the partial charge compensationof polyoxoanions by≡Zr+ cation-species as in(≡Zr+)n[H3− nW12PO40], cannot be ruled out. In aprevious section, FT IR results of adsorbed pyridineshowed that Brfnsted acidity is almost negligiblewhen TPA is supported on hydrated ZrO2 and thencalcined at 673 K, in spite of the fact that bulk TPAshows considerable Brfnsted acidity. In other words,TPA’s acidic hydrogen atoms are presumably re-moved as water when reacting with surface≡Zr–OHgroups. Then, it is more likely that the species respon-sible for the−8 ppm signal be distorted intact Kegginunits (This assumption will become evident below).Spectra b and c from samples calcined at 623 and673 K are essentially similar to Spectrum a. However,in Spectrum d from 25TPA/Z calcined at 773 K thereis a considerable change in peak shape. Here, the sig-nal from the distorted TPA species (−8 ppm) almostdisappeared and a sharp signal appeared (−14 ppm),

Table 1Effect of the heat-treatment temperature of 20TPA/Z on the IPA conversiona, reaction rate and selectivity to products

Catalyst Heat-treatment temperature (K) Xa (mol%) −rab (10−7) (mol s−1 g−1) Selectivity (mol%)

C3= DIPE

20TPA/Z 673 1.4 10.3 42.0 58.020TPA/Z 773 4.8 34.2 38.6 61.420TPA/Z 873 3.1 22.5 33.7 66.320TPA/Z* 473 25.5 165.0 71.9 28.120TPA/Z* 673 4.8 23.5 55.7 44.3

a Taken after 15 min of reaction. Reaction conditions: 388 K, 0.05 g cat,Xa: IPA conversion.b −ra: rate of reaction, C3=: propene, DIPE: diisopropyl ether.

in the same position as the bulk TPA signal (see bro-ken line). These results, which are in agreement withthe Raman determinations (Figs. 7a and e), can be ex-plained assuming that TPA’s distortion was removed.The driving force of this phenomenon may be thecrystallization of ZrO2, which occurs at about 723 K(see DTA section). In Spectrum e after calcination at923 K, the sharp signal at−14 ppm smeared out, in-dicating the collapse of the Keggin ion arrangement,and a new signal at−30 ppm, typical from P–O–Plinkages, appeared [35]. This signal is probably asso-ciated to phosphorous oxide, by-product of the TPAtransformation. Notwithstanding, a small amount ofthe Keggin structure from TPA is present (smearedout signal at−14 ppm) even after calcining at 1023 K,along with the phosphorous oxide species signal.

3.5.1. Decomposition of isopropanolIn order to examine whether the structural trans-

formations of TPA on Zirconia have an influence onthe catalytic activity, a 20TPA/Z catalyst was evalu-ated in the decomposition of IPA. The reaction rateof 20TPA/Z calcined at 773 K is 3.3 times higher(4.5 times higher based on unit surface area) thanthat calcined at 673 K (Table 1). This activity increasemay be due in part to the reformation of the Kegginstructure, as discussed above. The activity increasecould be ascribed to an increase of Brfnsted and/orLewis acid site amount and/or strength. There are sev-eral possible explanations: (1) As dehydroxylation ofhydrated ZrO2 (and concomitant crystallization) pro-ceeds, some≡Zr–OH groups may convert themselvesinto ≡Zr+ species and act as Lewis acid sites (simi-lar to the sulfated ZrO2 case [2]) and the charge com-pensating cation of adjacent Keggin polyoxoanions.(2) ≡Zr–OH groups could also form Zr–O–Zr bonds

224 E. Lopez-Salinas et al. / Applied Catalysis A: General 193 (2000) 215–225

and some protons may be released and used to com-pensate the reformed Keggin polyoxo anion, acting asBrfnsted acid sites. At present we are carrying out1HMAS NMR studies to clarify this aspect.

When 20TPA/Z was calcined at 873 K, the reactionrate dropped about 34% in comparison with that at773 K. This is a consequence of the collapse of theTPA’s Keggin structure which occurs at 873 K (seeRaman and NMR discussion). On the other hand,20TPA/Z* heat-treated at 473 K, that is, at conditionswhere initial amount of TPA’s acid protons is highest,showed the highest activity. In comparison, the activ-ity of 20TPA/Z* heat-treated at 673 K was seven timeslower. In this case of TPA supported on a previouslystabilized zirconia (Z*), there is a clear indicationthat TPA’s acid hydrogen ions react with remain-ing ≡Zr–OH groups resulting in a catalytic activitydecrease.

4. Conclusions

TPA impregnated on hydrated ZrO2 exists asdistorted intact Keggin TPA species at temper-atures below 673 K. The nature of this speciesis either (≡Zr–OH2)n+ [H3− nW12PO40]n − 3 or(≡Zr–O)n–[H3− nW12PO40− n]. At 773 K however,the interaction between TPA species and ZrO2 sur-face disappeared, yielding undistorted bulk-like TPAspecies. The crystallization of ZrO2 into the tetrago-nal phase, which occurs at 723 K, may be the causeof such phenomenon. The collapse of the TPA’s Keg-gin structure occurs at temperatures above 773 K,which yields WO3 and phosphates. A comparisonbetween the XRD patterns of 20 wt.% TPA loadedon ZrO2 and SiO2 indicates that the interaction ofTPA with SiO2 surface is much weaker than thatwith ZrO2. On the other hand, the addition of TPAto hydrated zirconia delays the crystallization of theamorphous phase into the tetragonal phase. That is,when pure hydrated ZrO2 is heat-treated at 1073 Kpredominantly monoclinic ZrO2 is expected, whilethat containing 20 wt.% TPA and calcined at 1073 Kyields predominantly the tetragonal phase with TPA’scollapse products. 20TPA/Z calcined at 773 K shows3.3 times higher catalytic activity in the decompo-sition of IPA than that at 673 K. It is suggested thatthe activity increase is caused by an increase in the

amount/strength of acid sites in connection with theTPA’s Keggin structure reformation.

Acknowledgements

We greatly appreciate the funding of this study bya FIES 95-02-III project.

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