WOxâZrO2 Catalysts for the Production of

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Energy & Fuels 2008, 22, 1680–1686

Optimization of Pt/WOx-ZrO2 Catalysts for the Production of Reformulated Fuels by Isomerization-Cracking of Medium Length C8-C12 Paraffins J. M. Grau,* C. R. Vera, V. M. Benitez, and J. C. Yori Instituto de InVestigaciones en Catálisis y Petroquímica, INCAPE (FIQ-UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina ReceiVed NoVember 27, 2007. ReVised Manuscript ReceiVed February 12, 2008

The effect of the temperature of calcination of the support on the structural properties of ZrO2 and on the activity of the acid and metal functions of a Pt/WOx-ZrO2 catalyst used in the isomerization-cracking of medium length paraffinic C8-C12 cuts was studied. n-Octane was used as model molecule. The calcination temperature was varied in order to change the metal/acid balance and to increase the yield of the reaction to isoparaffins of high octane number. Four supports were prepared by impregnating Zr(OH)4 with ammonium metatungstate (15% W) and then they were calcined at 500, 600, 700, and 800 °C. These supports were then impregnated with H2Cl6Pt (1% Pt) and calcined in air at 500 °C. They were characterized by means of chemical analysis, XRD, N2 adsorption, pyridine TPD, hydrogen chemisorption, temperature-programmed reduction, and infrared spectroscopy of adsorbed CO. The catalytic activity of the catalysts was evaluated with the test reactions of n-octane (300 °C, 1 atm, WHSV ) 1, H2/n-C8 ) 6), n-butane (350 °C, WHSV ) 1, H2/n-C4 ) 6), and cyclohexane (300 °C, 1 atm, WHSV ) 12.6, H2/CH ) 1.4). The results reveal a strong influence of the calcination temperature on the final metal/acid balance of the catalysts. At 5 min time on stream, all catalysts produce a RON gain of 55 points. In general, the higher the calcination temperature the higher the promoting action of W for generating strong acid sites and the higher the concentration of Ptδ+ of the metal function. The highest liquid yield and isoparaffin yield were obtained with the sample calcined at 700 °C. The sample calcined at 800 °C had the highest cracking activity and the maximum yield of isobutane and propane.

Introduction High-octane hydrocarbons of low environmental impact, like alkylate and C5-C6 branched isomers, are growing in importance as octane number contributors to the gasoline pool because current regulations impose strict limits on the content of other nocive compounds like benzene, aromatic hydrocarbons, or tetraethyl lead.1 The transformation of C8-C12 paraffins into shorter branched isoparaffins appears attractive because it allows both a decrease of the content of undesirable aromatic hydrocarbons and an upgrading of a low-value refinery surplus.2 The production of light isoparaffins from these cuts requires an initial branching of the n-paraffin followed by a balanced cracking of the thus produced isoparaffin with minimal production of light gases (C1-C3). These two reactions are carried out by acid sites of different acid strength and an appropriate distribution of acid sites is needed in order to avoid secondary cracking and coke formation, reactions that compete for the same active sites.3 A metal function is needed in order to hydrogenate adsorbed coke precursors and to rapidly stabilize the catalyst.4 * To whom correspondence should be addressed. Telephone: +54-3424533858. Fax: +54-342-4531068. E-mail: [email protected]. (1) Redalieu, C. In Handbook of Air Pollution Control Engineering and Technology; Mycock, J. C., McKenna, J. D., Theodore, L., Eds.; Lewis Publishers: New York, 1995; Chapter 2, pp 11–27. (2) Weitkamp, J.; Jacobs, P. A.; Martens, J. A. Appl. Catal. 1983, 8, 123. (3) Arata, K. AdV. Catal. 1990, 37, 165. (4) Hosoi, T.; Shimidzu, T.; Itoh, S.; Baba, S.; Takaoka, H.; Imai, T.; Yokoyama, N. Proceedings of the American Chemical Society, Los Angeles; American Chemical Society: Washington, DC, 1998; p 562.

Previous research has shown that the appropriate catalysts for these reactions must have good isomerization and mild cracking activities. In previous articles, we have studied Pt supported on oxoanion promoted zirconia as a catalyst for the conversion of n-octane.5–9 The acid strength requirement to obtain isooctane from n-octane is low. The addition of SO42to zirconia and its subsequent calcination at 620 °C produces a solid acid with a high percentage of sites with strong acidity that are responsible for deep cracking and the production of light gases. The promotion of zirconia with tungstate anions and the calcination at 800 °C generates a milder solid acid than sulfated zirconia that is active for the skeletal isomerization of linear alkanes of medium length and for their cracking to shorter branched products.7 The acidic properties of tungstated zirconia can be regulated by manipulating preparation variables and pretreatment conditions. The addition of Pt enhances the acidity and the stability of the catalyst and increases the yield to light isoparaffins.5 The interaction of Pt with the support in these catalysts inhibits its metal properties. Its capacity for adsorbing hydrocarbons and hydrogen is decreased. We have found that in highly acidic Pt/SO42--ZrO2 catalysts with metal–support interaction both Ptδ+ and Pt0 coexist and that the crystalline (5) Grau, J. M.; Parera, J. M. Appl. Catal., A 1997, 162, 17. (6) Grau, J. M.; Vera, C. R.; Parera, J. M. Appl. Catal., A 1998, 198, 311. (7) Grau, J. M.; Yori, J. C.; Parera, J. M. Appl. Catal., A 2001, 213, 247. (8) Grau, J. M.; Vera, C. R.; Parera, J. M. Appl. Catal., A 2002, 227, 217. (9) Grau, J. M.; Yori, J. C.; Vera, C. R.; Lovey, F. C.; Condó, A. M.; Parera, J. M. Appl. Catal., A 2004, 265, 141.

10.1021/ef700711q CCC: $40.75  2008 American Chemical Society Published on Web 04/15/2008

Optimization of Pt/WOx-ZrO2 Catalysts

Energy & Fuels, Vol. 22, No. 3, 2008 1681

structure of ZrO2 has a great influence on the acid and metal properties of the catalyst.9 Pt supported over oxoanion promoted tetragonal zirconia had a lower dehydrogenating activity than Pt supported over oxoanion promoted monoclinic zirconia. An opposite effect was found when the acid isomerization properties were analyzed. The objective of this work was to determine the best calcination temperature of the WOx-ZrO2 support in Pt/ WOx-ZrO2 catalysts used in the production of high-octane isomerizate by isomerization-cracking of medium length paraffinic C8-C12 cuts. This means that liquid yield, isomerizate yield, should be maximized while yield of light gases and aromatics should be minimized. A special focus was put on the study of the influence of the concentration of electron-deficient platinum (Ptδ+) on the yield to different reaction products and to the stability of the catalysts. n-Octane isomerization-cracking was used as the main test reaction. Experimental Section Catalysts Preparation. Zr(OH)4 (ZH sample) was obtained by hydrolysis of zirconium oxychloride (Strem Chem. 99.99%) in aqueous solution and precipitation with concentrated ammonium hydroxide. The precipitate was then washed and dried in a stove at 110 °C overnight. WOx-ZrO2 (WZ) was obtained by impregnation of ZH with a solution of ammonium metatungstate, which had been previously stabilized at pH ) 6 for a week. This procedure is known to enrich the solution in medium size tungstate anions.10 The volume and concentration of the solution were adjusted in order to get a 15% W in the final catalyst. After being impregnated the support was left 12 h at room temperature. Then it was dried at 110 °C and calcined in air for 3 h at 500, 600, 700, or 800 °C in order to obtain different materials identified as WZ500, WZ600, WZ700 and WZ800, respectively. Pt/WZTc catalysts (with Tc ) 500, 600, 700, and 800 °C) were obtained by incorporating Pt to the WZ supports by impregnation to incipient wetness with a solution of H2PtCl6. Volume and concentration were adjusted in order to obtain 1% Pt in each sample. Once impregnated, the catalysts were dried at 110 °C and then calcined in air for 2 h at 500 °C. Catalysts Characterization. The chemical analysis of Pt content of the solids was determined by atomic emission spectroscopy (ICPAES) using an ARL Model 3410 equipment. The solids were dissolved in a digestive pump with a mixture of 1 mL of sulfuric acid, 3 mL of chlorhydric acid, and 1 mL of nitric acid. The W content was determined by X-ray fluorescence. X-ray diffraction spectra were measured in a Shimadzu XD-1 equipment with Cu KR radiation filtered with Ni. The spectra were recorded in the 2θ range between 20° and 65° and with a scanning rate of 1.2° min-1. The percentage of tetragonal phase of the samples was calculated using eq 1.11 The peaks located at 2θ ) 28° and 2θ ) 31° were attributed to the monoclinic phase of zirconia and those located at 2θ ) 30° to the tetragonal phase. The peaks located at 2θ ) 23°–25° were attributed to WO3 crystals. Xt (%) ) 100RIt/(Im + RIt)

(1)

Xt (%) is the content of the tetragonal phase, R ) 0.81, It is the integrated intensity corresponding to the (111) tetragonal peak, and Im is the sum of the integrated intensities of the (111) and (11j1) monoclinic peaks. The quantification of the amount of each crystalline phase and the amount of amorphous matter was made by Rietveld quantitative analysis (RQA). From these values, the crystallinity percentage was calculated. Calibration constants were computed from reliable structural data. (10) Yori, J. C.; Parera, J. M. React. Kinet. Catal. Lett. 1998, 52, 227. (11) Itoh, T. J. Mater. Sci. Lett. 1986, 5, 107.

The specific surface area of the catalysts was measured by nitrogen adsorption in a Micromeritics 2100E equipment. The adsorption isotherm was measured at the temperature of liquid nitrogen and after treating the samples 2 h at 200 °C in a vacuum. The reducibility of the samples was analyzed by means of temperature-programmed reduction (TPR) in an Ohkura TP2002 apparatus equipped with a thermal conductivity detector. The samples were calcined in air for 1 h at 450 °C, cooled, and stabilized in Ar at 25 °C and then heated to 900 °C at a rate of 10 °C min-1 in a stream of 4.8% H2 in Ar. The capacity for chemisorbing hydrogen was measured in a Micromeritics 2100E equipment. The samples were first calcined in air, then reduced at 300 °C in hydrogen for 1 h, and finally degassed at 300 °C for 2 h in a vacuum of 10-5 Torr. The samples were cooled down to room temperature and then isotherms of total adsorption and reversible adsorption were measured. Irreversibly chemisorbed hydrogen was obtained by subtracting both isotherms and extrapolating to zero pressure. The amount and strength of the acid sites were assessed by means of temperature-programmed desorption of a basic probe molecule. Pyridine (Merck, >98%) was used to test both Brönsted and Lewis acid sites. Trimethylpyridine (Merck, 99%) was used to probe the Brönsted acid sites and to calculate the Brönsted/Lewis ratio. The samples were first calcined for 1 h at 450 °C and then they were cooled in dry nitrogen to room temperature and immersed in a vial containing pyridine. The vial was closed and left for 6 h. Then the vial was opened and the samples were filtered and dried in still air at room temperature. The samples were then placed in a quartz microreactor and stabilized in N2 for 1 h at 100 °C. Then they were heated from this temperature to 650 at 10 °C min-1. The desorbed products were continuously analyzed in a flame ionization detector and the signal recorded in a computer connected online. In order to study the electronic state of Pt, measurements of infrared absorption of adsorbed CO were performed. Self-supported wafers of 0.1–0.2 mm thickness (20 mg cm-2) were made by grinding and pressing the material with a die and a hydraulic press. These wafers were calcined in air at 450 °C for 30 min, degassed at 10-6 Torr at the same temperature for 1 h, reduced at 300 °C with 150 Torr of H2 for 30 min, and finally degassed again at 10-6 Torr and 450 °C for 1 h. A first spectrum of the sample was then taken. Then the sample was contacted with 30 Torr of CO for 10 min and a second spectrum recorded. Finally, the sample was degassed at 10-6 Torr at 25 °C and a last spectrum was taken. The spectrum of CO adsorbed over all the surface and of CO adsorbed only on the platinum sites were obtained by subtracting these spectra. In all cases the FTIR spectra were taken at room temperature with a Nicolet 5ZDX spectrometer, in the 400–4800 cm-1 range, with a resolution of 4 cm-1 and with a cumulative averaging of 128 spectra. Catalytic Tests. Before measuring the catalytic activity all samples were pretreated in situ in the reactors. First they were calcined in air (1 h, 450 °C) and then in hydrogen (1 h, 300 °C). At the pressure, temperature and flow rate conditions chosen for the three reactions, neither internal nor external mass transfer limitations in the catalyst particles were found, as confirmed by the calculation of the Weisz-Prater modulus (Φ < 0.01) and the Damköhler number (Da ≈ 0). No equilibrium limitations were observed either. Isomerization of n-Butane. 0.5 g of the catalysts was placed in a quartz reactor and the reaction was carried out under the following conditions: 0.1 MPa, 350 °C, WHSV ) 1, molar ratio H2/n-C4 ) 6. The separation of the products was performed with a 6 m long, 1/8 in. diameter copper column packed with 25% dimethylsulfolane on Chromosorb P. Cyclohexane Dehydrogenation. it was performed at 0.1 MPa, 300 °C, WHSV ) 12.6, molar ratio H2/HC ) 1.4 and with a catalyst mass of 0.1 g. The products were analyzed with a 2 m long, 1/8 in. diameter copper column, packed with FFAP supported on Chromosorb P. Hydroisomerization-Cracking of n-Octane. This reaction was performed in a stainless steel fixed bed reactor loaded with 0.25 g

1682 Energy & Fuels, Vol. 22, No. 3, 2008

Grau et al.

Table 1. Textural Properties of the Samples sample

W atoms, nm-2

Sg, m2 g-1

ZH Z620 WZ500 WZ600 WZ700 WZ800

4.5 6.2 7.8 12.6

262 37 110 80 63 39

a

crystal phase ZrO2

WO3

% crystallinity

amorphous 18% T, 82% Ma amorphous 100% T 100% T 100% T

M

89 81 88 91

T ) tetragonal, M ) monoclinic.

of the catalyst. The reaction was carried out at 0.1 MPa, T ) 300 °C, WHSV ) 4, and a molar ratio H2/n-C8 ) 6. The products were analyzed using a squalane-coated capillary column. In all cases, online chromatography was performed in a Shimadzu 4A GC equipped with a flame ionization detector. From these GC data, total conversion (X) and yield of each different reaction products (Yi) were calculated on a carbon basis according to the following formulas. The conversion of n-C8 was defined as X)

n-C8i - n-C8o n-C8i

(2)

Figure 1. Temperature-programmed reduction of Pt/WZ catalysts calcined at different temperatures (Tc).

Catalysts Characterization. Table 1 contains data of the textural properties of the zirconia support and of the W content of the WZ samples. ZH, dried at 110 °C, was amorphous and had a specific surface area of 262 m2 g-1. The calcination at 620 °C of the W-free hydroxide produced zirconia (Z) with a very low surface area (37 m2 g-1) which was mainly monoclinic by XRD inspection. The addition of W to ZH and its subsequent calcination yielded samples with different crystal phases depending on the temperature of calcination (Tc). The sample calcined at 500 °C, WZ500, was mainly amorphous though the surface area was reduced to 110 m2 g-1 due to sintering and pore collapse. The other three samples had growing crystallinity and showed XRD peaks related to the tetragonal phase of zirconia. WZ600 had a surface area of 85 m2 g-1. WZ700 had a smaller surface area (63 m2 g-1), and WZ800 had the smallest surface area (39 m2 g-1). In WZ800, a segregated phase of monoclinic WO3 crystallites appeared in the XRD spectrum. The appearance of WO3 as a separated phase occurs for W surface concentrations between 7.8 and 12.6 W atoms nm-2, in coincidence with the reports of Barton et al.16 that indicate a value of 9.0 atoms W atoms nm-2 for the surface saturation

value. The stabilization of the tetragonal structure of zirconia by calcination of W-impregnated zirconium hydroxide coincides with previous reports.13,14 The nature of the phase stabilization phenomenon is still a matter of debate. Tetragonal zirconia crystals are stabilized in a tetragonal metastable state outside the temperature range of true thermodynamic stability when their size is small. This effect is related to the high surface energy of the monoclinic crystals that imposes a barrier to their growth. The interaction of WOx groups with the surface groups of ZH not only retards the crystallization process and stabilizes the tetragonal phase but also inhibits the sintering of the zirconia crystals, improving the specific surface area. The presence of Pt does not affect the specific surface area, the crystal phase or the crystal size of the support. Figure 1 displays the TPR traces of the Pt-containing catalysts, Pt/Z and Pt/WZTC, and of the Pt-free supports, WZ and Z. The TPR trace of the zirconia support without Pt and W shows no detectable peaks. When Pt was supported on Z there was a reduction peak of Pt to 160 °C. When Pt was supported on WZ a pattern develops that corresponds to the reduction of Pt species and to the different stages of WO3 or WOx reduction. The reduction profiles of WO3 species are broad and, according to Barton et al.,16 three different reduction peaks can be identified: at 300–500 °C (WO3 f WO2.9), 550–700 °C (WO2.9 f WO2), and 750–850 °C (WO2 f W). A fourth peak at a higher temperature (900–950 °C) corresponds to the reduction of WOx species strongly bonded to the support. These WOx species reduce at higher temperatures because they contain refractory oxygen bonds with the ZrO2 support. The sample calcined at 800 °C has four peaks that can be attributed to different W species present on the catalyst surface. The first peak at 380 °C corresponds to the first step of reduction of WO3 crystallites (WO3 f WO2.9). The presence of these crystallites is confirmed by the XRD spectra. The second peak with a maximum at 702 °C could be attributed to the WO2.9 f WO2 transition and the third peak centered at 770 °C could correspond to the WO2 f W transition. The second W reduction peak, at 702 °C, has been reported to be sensitive to the size of the WOx clusters. The bigger the cluster size the higher the proportion of W-O-W bonds and the lower the required temperature for their reduc-

(12) Nikolaou, N.; Papadopoulos, C. E.; Gaglias, I. A.; Pitarakis, K. G. Fuel 2004, 83, 517.

(13) Hino, M.; Arata, K. J. Chem. Soc. Chem. Commun. 1988, 1259. (14) Yori, J. C.; Parera, J. M. Catal. Lett. 2000, 65, 205.

where n-C8i is the number of n-C8 molecules at the reactor inlet and n-C8o is the number of n-C8 molecules at the reactor outlet. The selectivity to each product i was defined as Yi )

Aifini × 100 A ifini Mi Mi

(∑

)

(3)

where Ai is the area of the chromatographic peak of product i, fi is its response factor, ni is the number of carbon atoms of i and Mi is its molecular weight. The Research Octane Number (RON) of the product stream was calculated solely from compositional chromatographic data using a simple nonlinear method described elsewhere.12 The RON gain was defined as the difference between the RON of the mixture of products minus the RON of n-octane. The percentage of light gases was calculated as the weight fraction of the reaction products with carbon numbers between 1 and 4.

Results and Discussion



Table 2. Results of Temperature-Programmed Desorption of Pyridine (Py) acidity, µmol Py g-1 catalysts

total

weak 110–300 °C

medium 300–500 °C

strong >500 °C

total surface acid density, µmol Py m-2

total Brönsted/Lewis ratio

Z Pt/Z WZ500 WZ600 WZ700 WZ800 Pt/WZ500 Pt/WZ600 Pt/WZ700 Pt/WZ800

15.4 22.3 27.9 79.0 97.1 115.5 33.2 79.3 104 120

12.8 10.7 2.5 21.5 25.7 38.8 2.7 19.7 30.1 36.5

2.2 11.6 25.3 57.4 71.1 73.5 31.7 59.8 73.6 78.0

0.0 0.0 0.1 0.1 0.3 3.2 0.3 0.3 0.7 5.4

0.39 0.87 0.25 0.99 1.54 2.95 -

0.13 0.25 0.81 1.33 0.40 0.34 0.79 1.60 0.62

tion.15 Barton and co-workers16 have found maximum isomerization activity for a high concentration of medium area size WOx clusters. They have suggested that this cluster size allows the reduction of WO2.9 into WO2. Iglesia et al.17 have further considered that these clusters are necessary for delocalizing unbalanced charges and for the formation of Brönsted sites in the presence of H2. They report that the active WOx species are those that have a highly distorted octahedral coordination. The fourth peak of the TPR trace (temperature higher than 900 °C) has the biggest area and corresponds to the reduction of WOx species strongly bonded to the support. The sample calcined at 700 °C has practically the same peaks, and therefore it should contain both WOx surface species and segregated WO3 crystals. The latter cannot be detected in the XRD spectrum of the sample and it can be further supposed that their crystal size is below the detection limit for the technique. The sample calcined at 600 °C has only a reduction peak related to the high temperature reduction of the WOx species in interaction with the support. In the case of the sample calcined at 500 °C, two reduction peaks are clearly distinguished at 634 and 870 °C. The last peak was attributed to the reduction of surface WOx species, and the first one was attributed to the WO2.9 f WO2 transition. Both peaks are shifted to lower temperatures when compared with the other catalysts. In this case the main peak of reduction of Pt is split into two different peaks at 106 and 203 °C that likely correspond to the reduction of Pt2+ species with different degrees of interaction with the support. The dynamic desorption of pyridine as a function of temperature gives information on the total amount of acid sites and their strength distribution. Table 2 shows the effect of the calcination temperature of the support on the total acidity and the distribution of acid strength of the catalysts. Zirconia has a low total acidity with a total absence of strong acid sites. Pt addition does not modify this situation, though a little increase in the amount of total acid sites is seen probably due to the incorporation of some chlorine ion remaining from the impregnation of chloroplatinic acid. Tungsten addition produces a small increase of the acidity at low calcination temperatures but a great one as the temperature is gradually increased. The acidity increase is noticeable. The acidity after calcining at 800 °C is 4 times that obtained when calcining at 500 °C. In all catalysts the sites of medium acid strength prevail but their relative importance decreases as the calcination temperature is increased. Weak acid sites are specially favored. Medium strength sites are 95% of the population at 500 °C and 65% at 800 °C. Weak (15) Falco, M. Doctoral Thesis, Universidad Nacional del Litoral, Argentina, 2002. (16) Barton, D. G.; Soled, S.; Meitzner, G.; Fuentes, G. A.; Iglesia, E. J. Catal. 1999, 181, 57. (17) Iglesia, E.; Barton, D. G.; Soled, S.; Miseo, S.; Baumgartner, J. E.; Gates, W. E.; Fuentes, G. A.; Meitzner, G. D. Stud. Surf. Sci. Catal. 1996, 101, 543.

Table 3. Metal Function Propertiesa νCO, cm-1/area, % sample

H/Pt

Lewis

Ptδ+

Pt0

Pt/Z Pt/WZ500 Pt/WZ600 Pt/WZ700 Pt/WZ800

0.38 0.23 0.07 0.00 0.00

2190/12 2190/15 2190/21 2190/26 2190/27

2115/9 2115/18 2115/19

2084/88 2089/85 2090/70 2094/56 2095/54

a H/Pt ) hydrogen/platinum atomic ratio, ν CO ) wavenumber of adsorbed CO, and area ) size of the absorption peak, expressed as a percentage of the total area in the 2050–2200 cm-1 range.

acid sites are 10% at 500 °C and 30% at 800 °C. Only the sample calcined at 800 °C has sites of strong acidity in meaningful concentrations. Clearly the calcination temperature of the support not only affects the total acidity but also the acid strength distribution. High calcination temperatures increase the total acidity. The concentration values of all the acid strength groups are increased, though speaking in relative terms weak acid sites are specially favored and only calcinations at 800 °C produces strong sites in meaningful amounts. The effect of calcination at high temperatures is more pronounced if analyzed from a surface density point of view. The surface acidity of WZ500 is 0.25 µmol Py m-2 while the surface acidity of WZ800 is almost 10 times this value, due to the simultaneous formation of acid sites during calcination and the sintering of the support. Finally, the data of the TPD of pyridine and TMP were used to determine the relative amounts of Brönsted and Lewis sites. As expected, the Brönsted/Lewis ratio of WZ is much higher than that of zirconia alone, indicating the effect of tungsten in enhancing the stability of protonic acid sites at all calcination temperatures. The Brönsted/Lewis is, however, decreased at high temperatures due to dehydration and transformation of Brönsted into Lewis acid centers. The presence of Pt and reduction of the support results in an increase in the amount of Brönsted acid sites as revealed by the data on the total amount of acid sites and the higher Brönsted /Lewis acid ratio for these samples. Table 3 shows the results of hydrogen chemisorption (H/Pt ratio) of the Pt/WZ catalysts as compared to the tungsten-free Pt/Z catalysts. The addition of W produces an important decrease of the H2 adsorption capacity. The H/Pt ratio of the W-free catalyst is 0.38, and this value is decreased to 0.23 in Pt/WZ500, to 0.07 in Pt/WZ600, and to 0.0 in the catalysts calcined at 700 and 800 °C. It can be seen that the decrease of the hydrogen chemisorption capacity increases drastically at temperatures higher than 500 °C. The reasons for the decrease in the chemisorption capacity of Pt supported over oxoanion-promoted zirconia catalysts have been a matter of debate for many years. In the case of the Pt/WZ catalysts, though an increase in particle size is partly responsible for the effect, the decoration of Pt particles by WOx species is most likely the main factor. In this


Grau et al.

Figure 2. Spectrum of CO-FTIR adsorbed on the different catalysts, after degassing to 10-6 Torr at room temperature.

sense the inhibition of the chemisorption capacity correlates with the increase in the surface concentration of W species with the calcination temperatura (Table 1). Indeed Figure 1 also reveals that the Pt reduction peaks were increasingly smaller as the calcinations temperature was augmented. The results of IR absorption of CO/Pt species are shown in Table 3 and Figure 2. The IR spectra of adsorbed CO over the reduced, undegassed catalysts (Table 3) have three absorption bands in the region of CO vibrations. The band at νCO ) 2190–2200 cm-1 can be assigned to the vibration of CO complexes adsorbed over Lewis acid sites.9 The broader band at νCO ) 2033–2120 cm-1 and the third band at νCO ) 1855 cm-1 correspond to the adsorption of the linear and bridge forms of CO over Pt.18 The band that brings more information with respect to the electronic state of Pt is the band of absorption of linear CO. The spectra of the four catalysts in the region of 2200–1800 cm-1 are shown in Figure 2. The spectra were taken after desorption at 25 °C and degassing in high vacuum. The desorption of the sample at room temperature swiftly eliminated the band at νCO ) 2190–2200 cm-1 due to the weakness of the bond of CO on the acid sites. The band of absorption of linear CO can be further decomposed into two smaller bands, one at a variable position in the range νCO ) 2070–2095 cm-1 corresponding to CO adsorbed over Pt0 and another band at a fixed wavenumber of νCO ) 2115 cm-1 that corresponds to CO adsorbed over electrodeficient Ptδ+.19 The variations of the position and relative intensity of these absorption bands reflect the variations of the electronic state of supported Pt. It must be remarked that, as the total acidity increases, the adsorption of linear CO is greatly decreased. The area of the corresponding peak decreases 1 order of magnitude indicating that the interaction of Pt with the support increases. These results coincide with the TPR ones; the peaks attributed to metallic Pt decrease in size as the calcination temperature is increased. The possible decoration or encapsulation of Pt particles by WOx species is an example of metal–support interaction related to the mobility of support surface species. Surface species would migrate onto Pt particles during heat treatments and would be stabilized there because of the lowering of their surface energy. The mobility of surface species is only important at high temperatures and it is therefore more likely that WOx migrate over Pt particles during the calcination of the Pt/WZ catalysts at 500 °C. This pretreatment step is necessary for the remotion of chlorine ligands of the Pt precursors. The blocking of Pt (18) Kustov, L. M.; Kazansky, V. B.; Figueras, F.; Tichit, D. J. Catal. 1994, 150, 143. (19) Ivanov, A. V.; Stakheev, A. Y.; Kustov, L. M. Russ. Chem. Bull. 1999, 48, 1255.

Figure 3. Catalytic activity of the samples in acid-catalyzed and metalcatalyzed reactions. Isomerization of n-C4 (time on stream ) 5 min; reaction products: i-C4 ) isobutane; C1-C3 ) methane, ethane, propane) and dehydrogenation of cyclohexane (time on stream ) 30 min; reaction products: MCP ) methylcyclopentane; Bz ) benzene).

surface sites would produce two immediate consequences: a decrease of the accessible surface Pt sites (geometric) and an increase of the electronic interaction between Pt atoms and W and O atoms (electronic effect). Both effects might be responsible for the decrease in the hydrogen chemisorption capacity while the second should be reflected in the appearance of electrodeficient Ptδ+ in the IR absorption spectra. From this point of view there would not be a direct relation between the inhibition of the metal properties and the increase of the acidity of the support, but a parallel growth of both properties as the calcinations temperature is increased. In order to assess the state of each catalytic function (acid or metal) and correlate its activity level with some properties of the catalyst, catalytic activity measurements were performed using two different test reactions at atmospheric pressure: n-butane isomerization (InC4) (350 °C, WHSV ) 1, molar ratio H2/n-C4 ) 6) and cyclohexane dehydrogenation (DCH) (300 °C, WHSV ) 12.6, molar ratio H2/HC ) 1.4). Figure 3 shows the obtained results. In metal-acid bifunctional catalysts showing poor or no metal–support interaction, test reactions are commonly used to characterize the metal or acid function of the catalysts. It is commonly found that n-butane isomerization is a useful test of the presence of strong acid sites20 while the dehydrogenation of cyclohexane is a structure-insensitive reaction for which the activity is proportional to the number of exposed active metal atoms.21 If we analyze the results we can see that in the case of the unpromoted Pt/Z catalyst the metal activity measured with the reaction of DCH is very high. The conversion is 61.5% and the selectivity to benzene is 100%. For the same catalyst the yield to isobutane in the InC4 reaction is null. There is a main yield of 37.0% of C1-C3 light gases. Specially, the presence of C1 in important amounts among the reaction products indicates that the metal function is fully functional and has a meaningful hydrogenolytic capacity. In the case of the Pt/WZ catalysts calcined at different temperatures, there exists a different interaction between the metal and acid functions. Both test reactions, DCH and InC4, (20) Parera, J. M.; Fígoli, N. S. Catal., Spec. Period. Rep. 1992, 9, 65. (21) Boudart, M.; Aldag, A.; Benson, J. E.; Dougherty, N. A.; Harkins, C. G. J. Catal. 1966, 6, 92.



Figure 4. n-Octane reaction. Conversion as a function of time on stream.

change their selectivity as the activity of each function changes. At low calcination temperatures (500 °C) the concentration of acid sites, specially those of high strength, is very low. The activity in the n-butane reaction is therefore very low. The interaction between the acid support and the metal function is also very low, and the catalyst has maximum metal activity, with a high yield of C1-C3 products (29.0%) and a very low yield of isobutane (2.5%). As the number of acid sites grows, both the conversion to isobutane and the metal–support interaction are increased. The latter produces a decrease of the metal activity. At calcination temperatures of 500 °C the formation of acid sites is low and the metal–support interaction is weak. The reaction is then 100% selective to benzene. As the calcination temperature is increased, both the concentration of acid sites and their strength are increased, and methylcyclopentane (MCP) is formed due to the acid-catalyzed ring contraction of cyclohexane. As the MCP yield increases, a decrease in the conversion of Bz occurs, indicating that the higher the acid activity the lower the metal activity. These results coincide with those obtained in the TPR and CO-FTIR experiments (Figure 1 and Table 3). The role of W is to stabilize the catalytically active tetragonal phase of zirconia and to create surface acid sites. The role of the temperature of calcination is to create surface WOx species of high acid strength. The electronic state of Pt changes with both the addition of W and the increase of the calcination temperature. In all cases, a decrease is produced in the intensity of the absorption bands corresponding to CO linearly adsorbed over Pt0, a shift to higher frequencies (νCO ) 2070 cm-1 to νCO ) 2095 cm-1) and a splitting of the band. A shoulder appears at νCO ) 2115 cm-1 that corresponds to CO adsorbed over Ptδ+. These results suggest that both Pt forms, Pt0 and Ptδ+, can coexist on the surface of WO42--ZrO2. These results are similar to those published by Ivanov and Kustov22 and by our group9 for the Pt/SZ system. The integrated intensity level of each signal depends on the degree of the metal–support interaction. Hydroisomerization-Cracking of n-Octane. Figures 4, 5, and 6 show the results corresponding to the reaction of n-octane (300 °C, 0.1 MPa, WHSV ) 4, molar ratio H2/n-C8 ) 6). Figure 4 shows the evolution of the total conversion as a function of the time on stream. At the beginning of the reaction, the catalyst surface is free of coke and has maximum conversion. The catalyst shows a marked deactivation during the first 2 h of reaction. Then it reaches a stable value of conversion which corresponds to an equilibrium between the coking rate and the rate of hydrogenation of coke precursors. After 240 min of time on stream, the conversion of the catalysts stabilizes at 26–31%. (22) Ivanov, A. V.; Kustov, L. M. Russ. Chem. Bull. 1998, 47, 1061.

Figure 5. n-Octane reaction. Yield of different reaction products at 5 min of time on stream.

Figure 6. n-Octane reaction. RON gain and gases percentage at 5 min of time on stream as a function of the calcination temperature of the zirconia support.

Pt/WZ800 is the most active. The activity of the stabilized catalysts indicates that in spite of the interaction of Pt with the acid sites of the support a small percentage of Pt0 remains that is able to generate enough activated hydrogen to hydrogenate coke precursors and stabilize the catalyst during reaction. Figures 5 and 6 show the yield values at 5 min time on stream (coke-free catalysts). In all cases, isooctanes are the main products (Figure 5). The maximum yield is displayed by the Pt/WZ700 catalyst. While in the individual reaction tests of the acid (n-butane isomerization) and metal functions (cyclohexane dehydrogenation) the sensitivity to the calcination temperature was remarkable, this effect seemed to be less important when both functions were tested simultaneously in the n-C8 hydroisomerizationcracking test. However, there were important differences in distribution of products in the isomerizate as a function of this temperature. The mechanism of isomerization and cracking of a linear paraffin involves a first step of formation of a branched compound that is afterward cracked on the acid site. In order to be cracked the alkane must be adsorbed on an acid site of sufficiently high acid strength.23–25 The n-C8 molecule may be either isomerized to i-C8 or it can be cracked to yield the following pairs: i-C4 + n-C4, i-C5 + C3, i-C6 + C2. The latter pair is the least feasible. The two first ones are more likely because long molecules tend to be broken in middle positions. (23) Martens, J. A.; Vanbutsele, G.; Jacobs, P. A.; Denayer, J.; Ocakoglu, R.; Baron, G.; Muñoz, J. A.; Thybaut, J.; Marin, G. B. Catal. Today 2001, 65, 111. (24) Deldari, H. Appl. Catal., A 2005, 293, 1. (25) Rezgui, Y.; Guemini, M. Appl. Catal., A 2005, 282, 45.


Depending on the amount of acid sites and the residence time, the molecule can react consecutively on more than one site. In this way, cracked paraffins can be reacted again. If one acid site is strong enough to retain the molecule after cracking, consecutive reactions may take place on the same site. The acidity values included in Table 2 show that the concentration of sites of weak and medium acidity increases as the temperature is raised. Up to 700 °C the concentration of strong acid sites, responsible for cracking, remains negligible. For Pt/WZ800 the concentration of strong acid sites is already important as it is the cracking of the i-C8 products. It can be seen that as the calcination temperature is decreased, the metal activity increases. At low calcination temperatures, 500 °C, the yield of hydrogenolysis (C1) and aromatization products is the highest (1.7 and 7.2%, respectively). When the temperature is raised, the metal activity is greatly reduced. At 800 °C the yield of methane and aromatic compounds is 0.1 and 2.2%, respectively. As the acidity of the support is increased, the metal-acid interaction is also increased, thus weakeaning the metal activity. This interaction results in the formation of electrodeficient Pt. The acid activity is weak as revealed by the low iso/normal butane ratio (see Figure 5). As the temperature is increased the ratio is also increased (0.57, 0.96, 1.57, and 4.64). This confirms the results of pyridine TPD. The formation of i-C4 not only comes from the cracking of i-C8 but also from the isomerization of n-C4, a product of the cracking of i-C8. In this sense the Pt/WZ800 sample displays the maximum yield of i-C4. The pattern of i-C5 is similar to that of i-C4. The RON gain, i.e., the difference between the RON of the isomerizate and the RON of the model n-C8 feedstock, and the percentage of light gases, were calculated at 5 min time on stream. These values are plotted in Figure 6 as a function of the calcination temperature of the support. It can be seen that the RON gain is approximately equal in the four samples despite the differences in the composition of the products as seen in Figure 5. Each compound contributes differently to the total RON. Aromatic compounds contribute with RON values greater than 100. C6 mono- and dibranched isomers contribute with 85 points on average. C7 isomers contribute with 70 points and C8 isomers with less than 30 points. Therefore, we can see that after a modification of the balance between the metal and acid functions we can obtain similar RON gain values but with a

Grau et al.

liquid product richer in light isoparaffins and with fewer aromatic compounds. With respect to the yield of light gases Pt/WZ700 is the catalyst that produces the lowest amount of them. The catalyst calcined at 700 °C had the maximum liquid yield and displayed a similar RON gain as the other studied samples. As was pointed out when discussing the TPD results, these catalysts present mainly acid sites of medium and weak strength, thus confirming that the production of branched isomers from long alkanes does not require highly acidic sites. The catalysts calcined at 700 °C also had the maximum amount of Brönsted acid sites and a Brönsted/Lewis ratio of 1.3–1.6. Greater calcination temperatures (800 °C) produced a conversion of Brönsted into Lewis acid sites with a decrease of the total amount of Brönsted acid sites and a decrease of the Brönsted/ Lewis ratio. The amount of strong acid sites is also increased. All these changes result in a catalyst with enhanced cracking activity. The final consequence is that the feedstock becomes more degraded to light gases when reacted over this catalyst. Conclusions As in the case of Pt/SO42--ZrO2 catalysts there exists a strong interaction in WO3-ZrO2 catalysts between the support and the metal Pt particles, which is seemingly related to the decoration of Pt particles by WOx species during heat treatments. The electronic density of Pt is depleted, and Ptδ+ domains are generated that coexist with Pt0 on the surface. This phenomenon weakens the activity of the metal function for H adsorption and cyclohexane dehydrogenation. In spite of this, the metal activity level is high enough to enable the hydrogenation of coke precursors and the stabilization of the catalyst. The optimum calcination temperature of Pt/WO42--ZrO2 catalysts is located at about 700 °C. At this temperature the liquid yield and the selectivity to isoparaffins are maximized and the conversion, the stability and the RON gain are relatively good. This maximum coincides with the maximum concentration of Brönsted acid sites and a Brönsted/Lewis ratio of 1.3–1.6. Acknowledgment. We are indebted to CONICET (PIP 5423) and Universidad Nacional del Litoral for financially supporting this work. EF700711Q

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