Download pdf - Alumina-doped Pt/WOx/ZrO2 catalysts for n-heptane isomerization

Applied Catalysis A: General 232 (2002) 129–135

Alumina-doped Pt/WOx/ZrO2 catalystsfor n-heptane isomerization

Weiming Hua1, Jean Sommer∗Laboratoire de Physico-Chimie des Hydrocarbures, UMR 7513, Institut de Chimie, Université Louis Pasteur,

4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France

Received 17 December 2001; received in revised form 30 January 2002; accepted 31 January 2002

Abstract

n-Heptane isomerization over Al2O3-doped Pt/WOx /ZrO2 (Pt/WZA) in comparison with Pt/WOx /ZrO2 (Pt/WZ) wasstudied in the presence of H2 at 200◦C. Higher isomerization selectivity was observed for these catalysts. The catalytic activitydepends strongly on surface WOx loading and calcination temperature of WOx /Al2O3–ZrO2 (WZA) and WOx /ZrO2 (WZ)supports. The maximum activity for both Pt/WZA and Pt/WZ catalysts occurs at a WOx concentration of 10 wt.% W whichis slightly higher than the theoretical monolayer capacities of WZA and WZ supports calcined at 800◦C. Pt/WZA catalystexhibits the higher maximum activity than Pt/WZ. The loss in activity observed at high H2 pretreatment temperatures appearsto be due to the strong interaction between Pt and reduced WOx species. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Isomerization ofn-heptane; Alumina-doped Pt/WZ; Strong acids; H/D exchange

1. Introduction

Acid-catalyzed hydrocarbon conversions, such ascatalytic cracking, isomerization and alkylation, playan important role in the petrochemical industry[1].The use of strongly acidic catalysts can lower thereaction temperature, thus favoring the formationof branched paraffins with high octane numbers.There are increasing needs for replacing liquid acids(e.g. H2SO4, HF) or halogen-containing solids (e.g.Pt/Cl–Al2O3) by environmentally friendlier solidacids. Sulfated zirconia (SZ) catalysts, first developedby Holm and Bailey[2] in 1962 and later extensively

∗ Corresponding author. Tel.:+33-3-90-24-14-86;fax: +33-3-90-24-14-87.E-mail address: [email protected] (J. Sommer).

1 Present address: Department of Chemistry, Fudan University,Shanghai 200433, PR China.

studied by Japanese researchers[3–6], have attractedconsiderable attention because of their strong acid-ity as well as high activity and selectivity for theisomerization of lightn-alkanes at low temperatures(even at room temperature), particularly forn-butaneisomerization[7–10].

Tungstated zirconia (WOx /ZrO2, WZ) catalystswere first reported by Hino and Arata[11] in 1987,and they have drawn wide interest recently[12–19].Compared to SZ, WZ is less acidic, and thus lessactive. However, WZ is much more stable than SZat higher temperatures in H2, O2 or H2O atmo-spheres. Moreover, Iglesia et al.[12] reported thatPt/WOx /ZrO2 catalysts (Pt/WZ) catalyzedn-heptaneconversion with much higher isomerization selectivitythan Pt/SZ catalyst at similar turnover rates.

Gao et al.[20] found that the addition of smallamounts of Al2O3 to the SZ system enhanced signifi-cantly the catalytic activity and stability forn-butane

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-860X(02)00087-X

130 W. Hua, J. Sommer / Applied Catalysis A: General 232 (2002) 129–135

isomerization at 250◦C in the presence of H2. Thispromoting effect of Al on SZ was later confirmedby Olindo et al.[21], and was also observed for an-other strong acid-catalyzed reaction, i.e. benzoylationof toluene with benzoyl chloride[22]. The promotingmechanism of the main group element Al is differ-ent from that of transition metals, such as Fe, Mn andNi [20,23]. In this work,n-heptane isomerization overAl2O3-doped Pt/WZ catalysts (Pt/WOx /Al2O3–ZrO2,Pt/WZA) in comparison with Pt/WZ has been inves-tigated. The effects of reduction temperature, calcina-tion temperature and tungsten oxide loading on thecatalytic activity were also examined.

2. Experimental

2.1. Preparation of samples

The mixed hydroxide Al(OH)3–Zr(OH)4 (2.5 wt.%Al2O3, calculated on the basis of oxide weight)was obtained by hydrolysis of a mixed solution ofZrOCl2·8H2O and Al(NO3)3·9H2O using the con-trolled addition of aqueous ammonium hydroxideunder vigorous stirring until the final pH= 9–10.The precipitate was then filtered, washed and dried at110◦C for 24 h.

WOx /Al2O3–ZrO2 (WZA) was prepared by incip-ient wetness impregnation of Al(OH)3–Zr(OH)4 withan aqueous solution of ammonium metatungstate, fol-lowed by drying at 110◦C and then calcination in staticair at 800◦C (unless otherwise stated) for 3 h. WZ wasprepared in the same way as WZA with Zr(OH)4 re-placing Al(OH)3–Zr(OH)4. Nominal WOx concentra-tions (wt.% W) for WZA and WZ samples in the cal-cined state were calculated on the basis of the assump-tion that WOx was WO3. The tungsten oxide loadingwas 10 wt.% W, unless otherwise noted.

Pt/WZA and Pt/WZ were prepared by incipient wet-ness impregnation of WZA or WZ with an aqueoussolution of H2PtCl6, followed by drying at 110◦C andthen decomposition in static air at 500◦C for 3 h. ThePt content for all catalysts was 0.5 wt.%.

2.2. Surface area

Specific surface area measurements were per-formed by using the BET method based on the N2

physisorption capacity at 77 K on Coulter SA 3100apparatus.

2.3. Deuteration of the sample

Deuteration of the sample was carried out in anall-glass grease-free flow system described earlier[24]. The sample was activated in dry air at 450◦C for2 h to eliminate hydrocarbon contamination, followedby pretreatment in dry N2 at the same temperature foran additional 1 h. Then, it was deuterated at 200◦Cby sweeping D2O with N2 (40 ml min−1, ca. 3 mol%D2O in N2) for 1.5 h. Excess D2O was removed byflushing the sample at 450◦C with dry N2 for 1 h.

2.4. Brønsted acid sites

The determination of Brønsted acid sites in thesample was described in detail elsewhere[25]. Inbrief, 1 g of the aforementioned deuterated catalystwas titrated at 200◦C for 1.5 h with H2O (ca. 3 mol%H2O in 40 ml min−1 N2) in the same glass system.Excess water was then removed by flushing the sam-ple at 450◦C for 1 h with dry N2. During the H/Dexchange and flushing, the partially exchanged wa-ter (H2O/HDO/D2O mixture, named HxODy) wascollected in a cold trap. An excess of trifluoroaceticanhydride was used to transform trapped HxODy intoCF3COOH and CF3COOD. The acid solution thusobtained was analyzed by 400 MHz1H and2H NMRafter addition of a CDCl3/CHCl3 mixture used asinner reference. The Brønsted acid sites present onthe sample were calculated on the basis of H/D ratiodetermined by NMR and weight of HxODy collected.

2.5. Activity test

The isomerization ofn-heptane was carried out at200◦C in a flow-type fixed-bed reactor under ambi-ent pressure. A gas mixture ofn-heptane, H2 and N2(1:10:49.4 molar ratio) was fed atn-heptane weighthour space velocity of 0.59 h−1 (unless otherwisestated). Prior to the reaction, the catalyst was firstactivated in situ at 400◦C in dry N2 for 3 h and thenreduced in dry H2 at 200◦C (unless otherwise noted)for 1 h. The analysis of reactant and reaction prod-ucts was performed on a HP 5890 Series II equippedwith a FID detector and a 50 m HP fused silica

W. Hua, J. Sommer / Applied Catalysis A: General 232 (2002) 129–135 131

capillary column PONA. The oven temperature waskept at 70◦C. The catalytic activity was expressed asn-heptane conversion (%).

3. Results and discussion

3.1. Surface area measurement

The addition of Pt (0.5 wt.%) does not changethe specific surface areas of WZA and WZ samples.Thus, BET surface areas of both Pt-free WZA andWZ samples with various WOx concentrations aftercalcination at 800◦C were measured, and the dataare presented inFig. 1. For both series of samples,the surface area depends on WOx coverage, and itpasses through a maximum at a WOx concentrationof, approximate 10 wt.% W. Similar phenomenon wasalso reported by Scheithauer et al.[26] and Naitoet al. [27] for zirconia-supported tungsten oxide sam-ples calcined at higher temperatures. The theoreticalmonolayer capacities of WZA and WZ samples cal-cined at 800◦C are 6.2 and 6.8 wt.% W, respectively,calculated from the model assuming that WO3 is dis-persed as a close-packed monolayer on the surface ofAl2O3–ZrO2 (2.5 wt.% Al2O3) and ZrO2. The sur-face area rises relatively steeply with increasing WOx

content below ca. monolayer capacity, and it variesweakly with surface WOx concentration beyond ca.monolayer capacity. Both WZA and WZ sampleshave similar surface areas at the same tungsten oxideloading.

Fig. 1. Specific surface area as a function of WOx concentrationon WZA (�) and WZ (�) samples calcined at 800◦C.

Surface WOx species are capable of reducing therate of ZrO2 surface diffusion, thus inhibiting the sin-tering of ZrO2 crystallite[28]. This is the reason whya relatively steep rise in the surface area up to ca.monolayer capacity for both WZA and WZ sampleswas observed, as shown inFig. 1. The decrease in sur-face areas of both WZA and WZ samples at higherWOx concentrations is due to a more pronounced con-tribution of tungsten oxide compound to the sampleweight which has lower surface area. Another reasoncould be that excess amorphous or microcrystallineWO3 narrows or plugs pores of the samples, as pro-posed by Scheithauer et al.[26]. WOx species are notthe only textural promoters of ZrO2. Other oxides,such as SO42−, CrOx and MoOx were also reportedto suppress ZrO2 crystallite sintering at higher calci-nation temperatures, thus stabilizing the surface area[29–31].

3.2. Catalytic selectivity and product distribution

Fig. 2 shows the typical time course ofn-heptaneisomerization over Pt/WZA and Pt/WZ catalysts at200◦C in the presence of H2. No catalyst deactivationwas observed during the test period of 90 min. BothPt/WZA and Pt/WZ catalysts isomerizen-heptanewith high selectivity. The variation of isomerizationselectivity with catalytic activity is shown inFig. 3. Toobtain different conversions we changed the reactionconditions only by increasing or decreasing the weightof catalyst used, i.e. varying the space velocity or bed

Fig. 2. Activity of Pt/WZA (�) and Pt/WZ (�) catalysts forn-heptane isomerization as a function of time at 200◦C.


Fig. 3. Evolution of isomerization selectivity and dimethyl isomerconcentration versus catalytic activity forn-heptane isomerizationover Pt/WZA catalyst at 200◦C.

residence time of the reactant, while keeping all theother reaction conditions constant. An increase in con-version results in a decrease in selectivity. Up to 65%conversion, 90% isomerization selectivity was stillobserved. At a conversion of 80%, it declines to 86%.Below 20% conversion, the selectivity exceeds 99%.

The main by-products are propane and isobutane,indicative of a cracking process. Similar observationwas reported by Iglesia et al.[32] for Pt/SZ catalyzedn-heptane isomerization. Minor amounts ofn-butanewas also detected during the catalytic reaction. Un-like n-hexane and smallern-alkanes, forn-heptane andhighern-alkanes, oligomerization–cracking pathwaysare no longer needed for�-scission reactions resultingin stable leaving groups in acid-catalyzed carbeniumion cracking chemistry. Cracking takes place primarilyby secondary reactions of isomerized products[32].

Table 1 gives the typical isomer product distri-bution for n-heptane isomerization over Pt/WZAand Pt/WZ catalysts at 200◦C. 2-Methylhexane and3-methylhexane are the predominant isomerizationproducts. Small amounts of 2,2,3-trimethylbutane

Table 1Isomer distribution forn-heptane isomerization over Pt/WZA and Pt/WZ catalysts at 200◦C

Catalyst Conversion(%)

Isomerizationselectivity (%)

Isomer (%)

2MH 3MH 3EP 22DMP 23DMP 24DMP 33DMP 223TMB

Pt/WZA 51.9 96.8 36.37 34.56 2.43 5.71 9.57 8.66 1.95 0.75Pt/WZ 41.1 94.8 37.55 36.17 2.54 5.63 8.01 7.77 1.68 0.65

(triptane) was also observed duringn-heptane con-version. The variation of dimethyl isomer concen-tration (among all eight isomerized products) withcatalytic activity for Pt/WZA catalyzedn-heptaneisomerization at 200◦C is shown inFig. 3. Clearly,dimethyl isomers which have higher octane numbersthan monobranched isomers increase with a rise inn-heptane conversion. For example, at 20% conver-sion, dimethyl isomers occur at 16% concentration,and they are 35% concentration at 80% conversion. Inother words, longer bed residence time ofn-heptaneleads to more dimethyl isomer products. This canbe accounted for by thatn-heptane first isomerizedto monomethyl hexanes which then rearranged todimethyl pentanes. 3-Methylhexane is one of the twomain isomers duringn-heptane isomerization. Whenreplacingn-heptane with 3-methylhexane as reactantcatalyzed by Pt/WZA at 200◦C, at 70% conversiondimethyl isomers occur at 43% concentration of allisomerized products which is evidently higher thanusing n-heptane as reactant (32%). This supplemen-tary experiment also indicates that dimethyl pentanesoriginate from monomethyl hexanes.

In contrast to Pt/SZ catalyst[12,33], both Pt/WZAand Pt/WZ catalysts display much higher isomeriza-tion selectivity forn-heptane conversion in the pres-ence of H2, particularly at higher conversions. As sug-gested by Iglesia et al.[12], the reason is that on thelatter catalysts, hydride transfer step is faster whichleads to desorption of isomerized carbocations before�-scission. While on the former catalyst due to sulfurpoisoning or strong interaction with the support, Pt isless effective in H2 dissociation, and thus in hydridetransfer. Accordingly, the surface lifetime of isomer-ized carbocations and the possibility that they undergo�-scission is very high. This mechanistic aspect isconfirmed by the rapid isotopic exchange between D2and “unreacted”n-heptane over Pt/WZ catalyst duringn-heptane isomerization[12].


Pt-supported relatively low-acidity solids such asamorphous silica-alumina and zeolites behave as clas-sical bifunctional (metal-acid) catalysts. A monofunc-tional mechanism forn-alkane isomerization has beenproposed for the highly acidic Pt/SZ-based[32,34,35]and Pt/WZ-based catalysts[12,36]. In this case, one ofthe role of platinum in paraffin isomerization is to hy-drogenate coke precursors in the presence of H2, thuslimiting acid site deactivation. Another important roleis hydrogen activation generating H− and H+ via het-erolytic cleavage[32] of H2 on Pt. Pt-catalyzed hydro-gen activation is evidenced by extensive H/D exchangebetween D2 and “unreacted”n-heptane over Pt/WZ[12] catalyst as well as between D2 and “unreacted”n-hexane over Pt/FeOy /WOx /ZrO2 catalyst[36].

Isomerized carbocations desorbed from the cata-lyst surface as reaction products via hydride trans-fer reaction. Duringn-heptane isomerization catalyzedby Pt/WZ-based and Pt/SZ-based catalysts,n-heptane,isomerized products and H2 can act as hydride trans-fer agents. Compared ton-heptane, isomers are betterhydride transfer agents because these molecules formmore stable tertiary carbenium ions. In contrast to iso-mers, H2 has less steric hindrance during intermolec-ular hydride transfer. Santiesteban et al.[36] foundthat duringn-hexane isomerization in the presence ofH2 the addition of Pt to the FeOy /WOx /ZrO2 catalystfacilitated the formation of much less cracked prod-ucts and much more 2,2-dimethylbutane produced viathe most sterically hindered 3,3-dimethyl-2-butyliumcation precursor. The results presented in this studyand reported by Iglesia et al.[12] show that Pt/WZAand Pt/WZ catalysts exhibit much higher isomeriza-tion selectivity than Pt/SZ catalyst during the isomer-ization of n-heptane in the presence of H2. Theseresults suggest that Pt-catalyzed hydrogen activationgenerating a hydride source (H−) plays an impor-tant role in hydride transfer reaction duringn-hexaneandn-heptane hydroisomerization over Pt/WZ-basedcatalysts.

3.3. Effects of reduction and calcinationtemperatures on the catalytic activity

To impart the metallic property to Pt, reduction toplatinum-supported solid acids in H2 is generally re-quired. In this study, prior to the reaction the catalystwas first pretreated in situ at 400◦C in dry N2 for 3 h

Fig. 4. Influence of reduction temperature on the catalytic activityfor n-heptane isomerization over Pt/WZA catalyst at 200◦C.

and then reduced in dry H2 at different temperaturesfor 1 h. Fig. 4 depicts the effect of reduction temper-ature on the catalytic activity forn-heptane isomer-ization over Pt/WZA catalyst at 200◦C. At reductiontemperatures from 200 to 250◦C, no loss in activitywas observed. An activity decline occurs at 300◦C,and it is obvious at 350◦C.

During the reduction of Pt/WZA and Pt/WZ cata-lysts by H2, apart from platinum oxide, tungsten oxidealso underwent reduction. The extent of reduction forsurface tungsten oxide depends on the temperature.Slight reduction of WOx can occur during H2 pre-treatment at 200–250◦C, and Barton et al. have sug-gested that Brønsted acid sites are generated in H2 byslight reduction and delocalization of negative chargein WOx domains[37]. The partial loss in activity athigher reduction temperatures appears to be related tothe strong interaction between Pt and reduced WOx

species. Our result is in agreement with the substantialdecrease in H2 chemisorption observed on Pt/WZ cat-alyst when increasing the reduction temperature from200 to 400◦C [37].

Fig. 5 shows the effect of calcination temperatureof WZA support (10 wt.% W loading) on the catalyticactivity for n-heptane isomerization over Pt/WZA cat-alyst at 200◦C. The activity of Pt/WZA catalyst de-pends strongly on the calcination temperature. It in-creases with the calcination temperature from 700 to800◦C, followed by a decrease beyond 800◦C. Bar-ton et al.[37] reported that increasing surface WOx

concentration resulted in the lowering of optimum


Fig. 5. Influence of calcination temperature of WZA support onthe catalytic activity forn-heptane isomerization over Pt/WZAcatalyst at 200◦C.

calcination temperature of WZ catalyst foro-xyleneisomerization.

3.4. Effect of tungsten oxide loading on thecatalytic activity

The effect of WOx concentration on the catalyticactivity for n-heptane isomerization over Pt/WZA andPt/WZ catalysts at 200◦C is shown inFig. 6. The ac-tivity is strongly dependent on WOx concentration.Both catalysts are almost inactive below 5 wt.% Wloading. Increasing WOx concentration first leads toa large increase in catalytic activity, and then a smalldecline. The maximum value was found at a WOx

Fig. 6. Activity of Pt/WZA (�) and Pt/WZ (�) catalysts forn-heptane isomerization as a function of WOx concentration.

concentration of 10 wt.% W which is slightly higherthan the theoretical monolayer capacities of WZA andWZ supports calcined at 800◦C. Barton et al.[37]also observed that the maximum turn over rates inWZ-catalyzedo-xylene isomerization occurred at sur-face WOx density higher than the theoretical mono-layer capacity of WZ catalyst. It was reported thatthe acidity and number of Brønsted acid sites presenton WZ sample increased with the loading of tungstenoxide up to monolayer coverage[15,38]. As the sur-face WOx density increases above monolayer cover-age, formation of polytungstate species is responsiblefor generating Brønsted acid sites from H2 on WZ viathe partial reduction of W6+ Lewis acid centers anddelocalization of the negative charge, as suggested byBaertsch et al.[38]. These reported results could in-terpret the sharp increase of the catalytic activity withthe amount of tungsten oxide.

Comparing the maximum activity displayed by bothseries of catalysts (at 10 wt.% W loading), Pt/WZAis more active than Pt/WZ, which is probably causedby the more number of strong acid sites present largerthe WZA support than WZ. Based on the acid sitetitration performed by H/D exchange, Brønsted acidsites of WZA (0.0352 mmol g−1) is the same as thoseof WZ (0.0346 mmol g−1) within experimental devia-tion. However, these are the total Brønsted acid sites.The exact number of active acid sites should be lower,and difficult to know. Using 2,6-dimethylpyridine toselectively poison the Brønsted acid sites, Santieste-ban et al.[14] reported that the strong Brønsted acidsites present over WZ catalyst with 15 wt.% W load-ing were only 0.002 mmol g−1.

4. Conclusions

In the present work, we have shown that bothPt/WZA and Pt/WZ catalysts isomerizen-heptanewith high selectivity in the presence of H2. Hydrogenactivation on Pt generating a hydride source (H−)may play an important role in hydride transfer dur-ing n-paraffin hydroisomerization over Pt/WZ-basedcatalysts. The catalytic activity is strongly dependenton surface WOx loading and calcination tempera-ture of WZA and WZ supports. Both Pt/WZA andPt/WZ catalysts display the maximum activity atsurface WOx concentration slightly higher than the


theoretical monolayer capacities of WZA and WZsupports. Comparing the maximum activity displayedby both series of catalysts, Pt/WZA is more active thanPt/WZ, which is probably due to the larger number ofstrong acid sites present over the WZA support thanWZ. The loss in activity observed at high H2 pretreat-ment temperatures appears to be caused by the stronginteraction between Pt and reduced WOx species.

Acknowledgements

Financial support of our work by the Loker Hy-drocarbon Institute, Los Angeles, is kindly acknowl-edged. We also thank M. Bacri of ECPM at ULP forBET surface area measurement.

References

[1] H. Pines, The Chemistry of Catalytic HydrocarbonConversion, Academic Press, New York, 1981.

[2] V.C.F. Holm, G.C. Bailey, US Patent 3,032,599 (1962).[3] M. Hino, S. Kobayashi, K. Arata, J. Am. Chem. Soc. 101

(1979) 6439.[4] M. Hino, K. Arata, J. Chem. Soc. Chem. Commun. (1980)

851.[5] M. Nitta, H. Sakoh, K. Aomura, Appl. Catal. 10 (1984)

215.[6] T. Yamaguchi, K. Tanabe, J. Phys. Chem. 90 (1986) 4794.[7] K. Arata, Adv. Catal. 37 (1990) 165.[8] B.H. Davis, R.A. Keogh, R. Srinivasan, Catal. Today 20

(1994) 219.[9] X. Song, A. Sayari, Catal. Rev.-Sci. Eng. 38 (1996) 329.

[10] G.D. Yadav, J.J. Nair, Micropor. Mesopor. Mater. 33 (1999)1.

[11] M. Hino, K. Arata, J. Chem. Soc. Chem. Commun. (1987)1259.

[12] E. Iglesia, D.G. Barton, S.L. Soled, S. Miseo, J.E.Baumgartner, W.E. Gates, G. Fuentes, G. Meitzner, Stud.Surf. Sci. Catal. 101 (1996) 533.

[13] G. Larsen, E. Lotero, R.D. Parra, Stud. Surf. Sci. Catal. 101(1996) 543.

[14] J.G. Santiesteban, J.C. Vartuli, S. Han, R.D. Bastian, C.D.Chang, J. Catal. 168 (1997) 431.

[15] M. Scheithauer, T.K. Cheung, R.E. Jentoft, R.K. Grasselli,B.C. Gates, H. Knözinger, J. Catal. 180 (1998) 1.

[16] R.D. Wilson, D.G. Barton, C.D. Baertsch, E. Iglesia, J. Catal.194 (2000) 175.

[17] S.G. Zhang, Y.L. Zhang, J.W. Tierney, I. Wender, Appl. Catal.A 193 (2001) 155.

[18] J.M. Grau, J.C. Yori, J.M. Parera, Appl. Catal. A 213 (2001)247.

[19] S. Kuba, B.C. Gates, R.K. Grasselli, H. Knözinger, J. Chem.Sco. Commun. (2001) 321.

[20] Z. Gao, Y.D. Xia, W.M. Hua, C.X. Miao, Top. Catal. 6 (1998)101.

[21] R. Olindo, F. Pinna, G. Strukul, P. Canton, P. Riello, G.Cerrato, G. Meligrana, C. Morterra, Stud. Surf. Sci. Catal.130 (2000) 2375.

[22] Y.D. Xia, W.M. Hua, Z. Gao, Catal. Lett. 55 (1998) 101.[23] W.M. Hua, Y.D. Xia, Y.H. Yue, Z. Gao, J. Catal. 196 (2000)

104.[24] A. O’Cinneide, F.G. Gault, J. Catal. 37 (1975) 311.[25] R. Olindo, A. Goeppert, D. Habermacher, J. Sommer, F.

Pinna, J. Catal. 197 (2001) 344.[26] M. Scheithauer, R.K. Grasselli, H. Knözinger, Langmuir 14

(1998) 3019.[27] N. Naito, N. Katada, M. Niwa, J. Phys. Chem. B 103 (1999)

7206.[28] P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J.

Burggraaf, J.R.H. Ross, Appl. Catal. A 71 (1991) 363.[29] T. Yamaguchi, K. Tanabe, Mater. Chem. Phys. 16 (1986) 67.[30] M. Valigi, A. Cimino, D. Cordischi, S. De Rossi, C. Ferrari,

G. Ferraris, D. Gazzoli, V. Indovina, M. Occhiuzzi, SolidState Ionics 63–65 (1993) 136.

[31] M. Valigi, D. Gazzoli, in: P. Barret, L.C. Dufour (Eds.),Reactivity of Solids, Elsevier, Amsterdam, 1985, p. 1081.

[32] E. Iglesia, S.L. Soled, G.M. Kramer, J. Catal. 144 (1993) 238.[33] Ü.B. Demirci, F. Garin, Catal. Lett. 76 (2001) 45.[34] K. Ebitani, J. Konishi, H. Hattori, J. Catal. 130 (1991) 257.[35] J.C. Yori, M.A. D’Amato, G. Costa, J.M. Parera, J. Catal.

153 (1995) 218.[36] J.G. Santiesteban, D.C. Calabro, C.D. Chang, J.C. Vartuli,

T.J. Fiebig, R.D. Bastian, J. Catal. 202 (2001) 25.[37] D.G. Barton, S.L. Soled, G.D. Meitzner, G.A. Fuentes, E.

Iglesia, J. Catal. 181 (1999) 57.[38] C.D. Baertsch, S.L. Soled, E. Iglesia, J. Phys. Chem. B 105

(2001) 1320.