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Energy & Fuels 2007, 21, 1390-1395
Isomerization Cracking of n-Octane and n-Decane on Regulated Acidity Pt/WOx-SO4-ZrO2 Catalysts J. M. Grau,*,† V. M. Benitez,† J. C. Yori,† C. R. Vera,† J. F. Padilha,‡ L. A. Magalhaes Pontes,‡ and A. O. S. Silva‡ Instituto de InVestigaciones en Cata´ lisis y Petroquı´mica, INCAPE (FIQ, UNL-CONICET), Santiago del Estero 2654, S3000AOJ Santa Fe, Argentina, and UNIFACS, Laborato´ rio de Cata´ lise e Ambiente, Departamento de Engenharia, AV. Cardeal da SilVa 132, SalVador-BA, Brasil ReceiVed NoVember 17, 2006. ReVised Manuscript ReceiVed February 12, 2007
The effect of WO3 loading on the acidity and specific surface area of catalysts composed of Pt supported on ZrO2 and double-promoted with SO42- and WO3 catalysts (PtWSZ) was studied. The catalysts were tested in the isomerization cracking of heavy alkanes, and the objective was to assess their ability to produce branched shorter alkanes, contributing to the gasoline pool. As a consequence, the focus was put on the maximization of the yield of C4-C7 isomers (i-C4-7). The catalysts were characterized by several techniques. The crystalline structure was analyzed by XRD. The acidity of the catalysts was measured by thermal desorption of pyridine. A screening and first selection of the most promising catalysts was done by means of the reaction of n-octane at 300 °C, 0.1 MPa, WHSV ) 1 and H2/nC8 ) 6 mol/mol. A high yield of i-C8 was obtained with the Pt/ WO3-ZrO2 catalyst. The incorporation of SO42- as a promoter increased the acidity and the cracking activity. Pt/SO42--ZrO2 displayed very strong acid sites and generated the highest amount of light hydrocarbons. Catalysts of regulated acidity combining both promoters yielded the best results. The most promising PtWSZ catalyst was obtained with 5% W and 1.4% S. The test reaction of n-decane at near-industrial conditions (1.5 MPa, 300 °C, WHSV ) 4, H2/n-C10 ) 6 mol/mol) was used for a further assessment of the catalytic properties. This test confirmed that the double-promoted catalyst (PtWSZ, 5% W, 1.4% S) had high activity and stability and produced an isomerizate with the highest i-C4-7/i-Ctotal molar ratio in comparison to the sulfated zirconia (PtSZ) and tungstated zirconia (PtWZ) catalysts.
1. Introduction There is a growing trend in the refining industry concerning the use of a wider variety of petroleum feedstocks and the reprocessing of internal streams for the maximization of the production of middle distillates and gasoline. At the same time, environmental regulations are becoming stricter with respect to the content of aromatics, sulfur, and other nocive components in the fuels. These two trends have put a pressure on process developers to come up with new catalysts and processes to meet the needs of the market and the environmental regulations.1 The processing of heavier feedstocks not only demands an overload of hydrotreating units but also puts a stress on all units that contain catalysts sensitive to sulfur.2 Gasoline is one fuel that has been greatly modified because of environmental regulations.3 Most of the modifications have involved banning nocive compounds that greatly contributed to the octane number of the fuel. However, a high octane number is necessary for the efficient operation of the motor, and therefore, the banned compounds must be replaced by other ones with similar octane properties and lower environmental impact. * To whom correspondence should be addressed. Phone: +54-3424533858. Fax: +54-342-4531068. E-mail:
[email protected]. † Instituto de Investigaciones en Cata ´ lisis y Petroquı´mica. ‡ Laborato ´ rio de Cata´lise e Ambiente. (1) Seddon, D. Catal. Today 1992, 15, 1. (2) Speight, J. G. The Desulfurization of HeaVy Oils and Residua, 1st ed.; Marcel Dekker: New York, 1981. (3) Schuetzle, D; Siegl, W. O.; Jensen, T. E.; Dearth, M. A.; Kaiser, E. W.; Gorse, R.; Kreucher, W.; Kulik, E. EnV. Health Perspect. 1994, 102 (Suppl 4), 3.
Alternative suppliers of octane number are the alkylate (isooctane obtained through the alkylation of isobutane with 1-butene), branched C5-C7 isoparaffins obtained by isomerization of light straight run naphtha, and branched C5-C8 obtained by cracking and branching of long paraffins. The latter have little value and are subproducts of the dewaxing of oils or come as the heavy fraction of Fischer-Tropsch products. Hydrocracking of heavy paraffins can be performed to produce diesel fuel or gasoline. In the first case, only an adjustment of the chain length is needed, and usually secondary cracking and isomerization are to be avoided. Branching reduces the cetane number. Catalysts for this task do not have great acidity. In the second case, deep cracking needs to be supressed to reduce the formation of light gases, and extensive branching is necessary to increase the octane number of the products. The catalysts in this case have a higher acidity, and several kinds of catalysts can be found in the literature: zeolites, oxoanionpromoted zirconia catalysts, silica-alumina catalysts, AlPO4 catalysts, etc. Most hydrocracking catalysts are zeolitic (e.g., HY, H-mordenites), but oxoanion-promoted zirconia catalysts have attracted much attention lately. These catalysts are composed of a zirconia support promoted with oxoanions (WO3, SO42-) and a Group VIII metal (Pt, Pd, Ni). Oxoanions promote the acidity, while noble metals increase the stability of the catalysts. Oxoanion-promoted zirconia catalysts can achieve activity values similar to those of zeolites at much lower temperatures. This is an advantage because at lower temperatures
10.1021/ef060583j CCC: $37.00 © 2007 American Chemical Society Published on Web 03/20/2007
Pt/WOx-SO4-ZrO2 Catalysts
branched isomers are thermodynamically favored. Some comparative reports illustrate this point.4 The acid strength needed for the isomerization of a n-paraffin depends on the length of the carbon chain. For example, the isomerization of light n-paraffins (butane, pentane) demands very strong acid sites such as those encountered in oxoanionpromoted zirconia catalysts. For much longer carbon chains, the acid strength required to perform the branching becomes lower. However, the need to adjust the average weight of the products to the boiling range of gasoline demands the cracking of long chains into smaller ones. Cracking reactions need a relatively high acid strength. This acidity must be regulated to prevent deep cracking and an excessive production of light gases (C1-C3) of low economical value. A high concentration of strong acid sites, like that of the Pt/SO42--ZrO2 catalysts, leads to deep cracking. A dominance of medium and weak acid sites, as in Pt/WO3-ZrO2 catalysts, selectively produces a majority of branched products but cannot meet the cracking activity needed to convert waxy feedstocks into compounds for the gasoline pool. A simple and efficient solution has been recently proposed.5-8 The combination of Pt/SO42--ZrO2 and Pt/WO3ZrO2 produces a hybrid catalyst with an average intermediate acid force with an improved yield of branched isoparaffins and a minimal production of light gases. These catalysts can be produced either by the mechanical mixing of a sulfate-promoted zirconia support and another tungsten-promoted zirconia support or by the double promotion of a single zirconia support with sulfate and tungsten. The manipulation of the synthesis parameters allows the modification of the acidity distribution, the textural properties of the support (specific surface area, crystalline phase, porosity), and the properties of the metal function (de/hydrogenating capacity, dispersion). The synthesis and characterization of double-promoted Pt/ sulfate-tungsten-zirconia catalysts were studied in this work. Their performance for the isomerization cracking of long paraffins was assessed using n-octane and n-decane as model compounds. 2. Experimental Section Catalyst Preparation. Zr(OH)4 (ZH) was obtained by precipitation of ZrOCl2 (Strem Chem. 99,9998%), previously dissolved in water at a slightly acidic pH to promote the hydrolysis. Precipitation was produced by the dropwise addition of a NH4OH solution (Merck, 25% NH3) up to pH 10. The precipitate was then stabilized for 24 h, washed, and dried overnight at 110 °C. SO42--ZrO2 (SZ) was prepared by immersing the desired amount of ZH xerogel in a 1 M H2SO4 solution (10 mL g-1, 2 h), filtering, drying 12 h at 110 °C, and calcining in an air flow (3 h, 620 °C, 10 mL g-1 min-1). WOx-ZrO2 (WZ) was prepared by dipping the desired amount of ZH xerogels in a solution of ammonium metatungstate (AMT) (10 mL g-1, 12 h), which had been previously stabilized at pH 6 for a week to regulate the size of the promoter ion.9 Ammonium metatungstate was supplied by Fluka. The concentration of the solution was adjusted to obtain 15% W in the final catalyst. Then the solid was filtered, dried for 12 h at 110 °C, and calcined in an air flow (10 mL g-1 min-1) for 3 h at 800 °C. WOx-SO42--ZrO2 (WSZ) was prepared by immersion of a portion of the ZH xerogel in a stabilized solution of AMT (10 mL (4) Grau, J. M.; Parera, J. M. Appl. Catal. A 1997, 162, 17. (5) Corma, A. Chem. ReV. 1995, 95, 559. (6) Song, X.; Sayari, A. Catal. ReV. Sci. Eng. 1996, 38, 329. (7) Adeeva, V.; Liu, H. Y.; Xu, B. Q.; Sachtler, W. M. H. Top. Catal. 1998, 6, 41. (8) Ivanov, A. V.; Vasina, T. V.; Masloboishchikova, O. V.; Sergeeva, E. G. K.; Kustov, L. M.; Houzvicka, J. I. Catal. Today 2002, 73, 95. (9) Yori, J. C.; Vera, C. R.; Parera, J. M. Appl. Catal. A 1997, 163, 165.
Energy & Fuels, Vol. 21, No. 3, 2007 1391 of solution/g of xerogel, 12 h) with an adjusted concentration to obtain different W concentrations (1, 3, 5, or 10% W) in the final catalysts. These samples were then dried in a stove at 110 °C overnight. Then they were impregnated again, this time with a 1 M H2SO4 solution (10 mL of solution per gram of solid, 2 h), filtered, and dried for 12 h at 110 °C. Finally, the samples were calcined in an air flow (3 h, 620 °C, 10 mL g-1 min-1). These samples were named WxSZ (x indicates the tungsten loading expressed as a weight percentage). Pt was incorporated by impregnation to incipient wetness with a solution of H2PtCl6. The volume and concentration were adjusted to result in 1% Pt in each catalyst. Once impregnated, the catalysts were stabilized for 12 h at room temperature, dried at 110 °C, and calcined in an air flow for 3 h at 450 °C. The samples were pressed at 8 ton cm-2 and ground and sieved to 24-42 mesh to be used in the reaction tests. The Pt-loaded catalysts were named PtWZ, PtSZ, and PtWxSZ. Catalyst Characterization. Total reflection X-ray fluorescence (TXRF) analysis was employed to determine the composition of the catalysts. Measurements were performed in a Seifert Extra-II spectrometer (Rich Seifert & Co., Ahrensburg, Germany) equipped with an X-ray fine-focus line of the Mo anode and a Si (Li) detector. The X-ray diffraction spectra were measured on a Shimadzu XD-1 spectrometer, with Cu KR radiation, filtered with Ni. The spectra were recorded in the 2θ range between 20 and 65° and at a scanning rate of 1.2 °min-1. The percentage of the tetragonal phase of the samples was calculated using the method of Itoh.10 The peaks located at 2θ ) 28° and 2θ ) 31° were attributed to the monoclinic phase of zirconia, and those located at 2θ ) 30° were assigned to the tetragonal phase. The peaks located at 2θ ) 2325° were attributed to WO3 crystals. 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 treatment of the samples for 2 h at 200 °C in a vacuum. The amount and strength of the acid sites were assessed by means of the temperature-programmed desorption of a basic probe molecule. Pyridine (Merck, >98%) was used to test both Bro¨nsted and Lewis acid sites. The samples were calcined for 1 h at 450 °C in an air stream (10 mL min-1), cooled in dry nitrogen to 300 °C, and reduced in a hydrogen stream (10 mL min-1) for 1 h. They were finally cooled in hydrogen 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 °C at 10 °C min-1. The desorbed products were continuously analyzed in a flame ionization detector, and the signal was recorded by a connected computer. Catalytic Tests. Before the catalytic activity was measured, all samples were pretreated in situ in the reactors. They were calcined in air (1 h, 450 °C), flushed with nitrogen (0.5 h, 300 °C), and finally reduced in hydrogen (1 h, 300 °C). Hydroisomerization cracking of n-octane was performed in a stainless steel fixed-bed microreactor loaded with 0.25 g of the catalyst. The reaction was carried out at 0.1 MPa, 300 °C, WHSV ) 4, a molar ratio of H2/n-C8 ) 6, and total time on stream ) 6 h. The products were analyzed using a squalane-coated capillary column and a FID detector. n-C8 conversion, selectivities, and yields of the different products (on a carbon basis) were calculated from the chromatographic data according to ref 11. The analysis of the reacted stream was focused on the calculation of its contribution to the gasoline pool. Accordingly, the merit (10) Itoh, T. J. Mater. Sci. Lett. 1986, 5, 107. (11) Mazzieri, V. A.; Grau, J. M.; Vera, C. R.; Yori, J. C.; Parera, J. M.; Pieck, C. L. Appl. Catal. A 2005, 296, 216.
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Table 1. Chemical Composition and Specific BET Surface Area (Sg) of the Catalysts chemical composition (%) catalyst
Sg (m2 g-1)
Pt
PtSZ PtWZ PtW1SZ PtW3SZ PtW5SZ PtW10SZ
120 55 94 83 78 72
0.98 0.96 1.01 1.00 0.99 0.97
W
S 1.70
14.5 1.2 2.9 4.8 9.3
1.45 1.41 1.39 1.10
parameter used was the research octane number (RON), which was calculated from chromatographic data and a reported correlation.12 Hydroisomerization cracking of n-decane was performed in a stainless steel Berty catalytic microreactor with internal reflux. The reactor was loaded with 1 g of the catalyst diluted with 4 g of γ-alumina (inert at the reaction conditions) with the same mesh size. During the activation of the catalyst at atmospheric pressure, the reactor was continuously stirred at 2000 rpm. After this stage, the stirring rate was raised to 3000 rpm, and the reaction parameters were stabilized at 300 °C, 1.5 MPa, molar ratio of H2/nC10 ) 6, and WHSV ) 4. n-Decane was vaporized in a hydrogen stream fed to the reactor. The first sampling was done 30 min after the beginning of the reaction. The product was analyzed on-line with a Shimadzu gas chromatograph with an FID detector. To assess the performance of the metal function (Pt), reaction tests of cyclohexane dehydrogenation were also performed. Dehydrogenation of cyclohexane (CH) to benzene is a reaction that is insensitive to the structure of the metallic active site. The reaction was performed with the following conditions: catalyst mass ) 0.1 g, 300 °C, molar ratio of H2/CH ) 1.4, and WHSV ) 12.6. Cyclohexane was supplied by Merck (spectroscopy grade, 99.9% pure). The specified sulfur upper limit was 0.001%.
3. Results and Discussion Table 1 shows data of the BET specific surface area (Sg) and of the chemical composition of the catalysts. It can be seen that the measured amount of each element is very close to the theoretical value. The support promoted with sulfate and calcined to 620 °C had the highest surface area. The catalyst promoted with WO3 and calcined at 800 °C had the lowest surface area. The double-promoted catalysts (promoted with SO42- and WO3) that were calcined at 620 °C had intermediate values. For this set of catalysts, the BET area decreases as the tungsten content increases indicating that some surface area is lost by pore plugging with tungsten oxide agglomerates. Figure 1 shows the X-ray diffraction spectra of the PtWZ, PtSZ, and PtWSZ catalysts. For these three catalysts, both tetragonal (T) and monoclinic (M) crystals can be detected, but the materials crystallize mainly in the tetragonal phase. The stabilization of the T structure is necessary to generate sites of strong acidity in the support, as has been reported in previous works.13 In the sulfate-containing catalysts, PtSZ and PtW5SZ, the tetragonality values are T ) 92 and 88%, respectively, expressed in volume percent. The tetragonality of PtWZ is lower, T ) 74%, indicating that WO3 is less effective than SO42in the stabilization of the tetragonal structure. The spectrum of this catalyst, PtWZ, also has signals corresponding to WO3 crystals at θ ) 28°, indicating that the promoter was partially segregated on the surface of this catalyst. Table 2 shows the values of total acidity and the distribution of acid strength of the samples, with and without Pt. The (12) Nikolaou, N.; Papadopoulos, C. E.; Gaglias, I. A.; Pitarakis, K. G. Fuel 2004, 83, 517. (13) Grau, J. M.; Yori, J. C.; Vera, C. R.; Lovey, F. C.; Condo´, A. M.; Parera, J. M. Appl. Catal. A 2004, 265, 141.
Figure 1. X-ray diffraction patterns of samples promoted with sulfur, tungsten, or both. Table 2. Acidic Properties of the Catalysts as Measured by Temperature Programmed Desorption of Pyridine acidity (µmol Py g-1) catalyst
total
weak 150-300 °C
medium 300-500 °C
strong 500-650 °C
SZ PtSZ WZ PtWZ W1SZ W3SZ W5SZ W10SZ PtW1SZ PtW3SZ PtW5 SZ PtW10SZ
238 280 90 120 220 205 190 106 262 245 212 168
57 56 37 44 49 50 47 33 67 87 75 58
83 151 43 64 84 80 75 56 105 96 95 97
97 73 9 12 90 75 68 17 87 62 42 13
distribution of acid strength was calculated by measurement of the area under the signal of desorption of pyridine in three different temperature intervals. SZ mainly has strong acid sites that according to other reports are mainly of the Lewis type.14 The incorporation of Pt dimminishes the concentration of strong acid sites possibly by blocking Lewis centers on the surface. The total acidity is however increased. Pt in the presence of hydrogen can generate Bro¨nsted acid sites as reported by other authors.15 Tungsten promotion of zirconia produces catalysts of milder acidity which are more effective for isomerization of long normal paraffins but that are less effective for cracking.16 In the case of the WSZ series, the combined effect of both promoters generates catalysts of intermediate acidity. The relative concentration of weak, mild, and strong acid sites depends on the amount of W and S on the support. In the WSZ catalysts, both Bro¨nsted and Lewis coexist, as reported by Ivanov et al.,8 on the basis of tests of chemisorption and FTIR absorption using CD3CN and C2D4 molecular probes. Table 3 shows the results of dehydrogenation of cyclohexane (CH) at 30 min time on stream (TOS) for all the tungsten- and sulfate-promoted zirconia catalysts. The products obtained were always benzene (Bz) and methylcyclopentane (MCP). The formation of MCP during cyclohexane/benzene de/hydrogenation on PtWZ and PtSZ catalysts has also been reported by (14) Sun, Y.; Zhu, L.; Lu, H.; Wang, R.; Lin, S.; Jiang, D.; Xiao, F. Appl. Catal. A 2002, 237, 21. (15) Tichit, D.; El Alami, D.; Figueras, F. J. Catal. 1996, 163, 18. (16) Iglesia, E.; Barton, D. G.; Soled, S. L.; Miseo, S.; Baumgartner, J. E.; Gates, B. C.; Fuentes, G. A.; Meitzner, G. D. Stud. Surf. Sci. Catal. 1996, 101, 533.
Pt/WOx-SO4-ZrO2 Catalysts
Energy & Fuels, Vol. 21, No. 3, 2007 1393
Table 3. Properties of the Metal Function of the Catalysts as Measured by a Reaction Test of the Dehydrogenation of Cyclohexanea Conversion at 30 min TOS (%)
a
catalyst
total
to Bz
to MCP
PtSZ PtW1SZ PtW3SZ PtW5 SZ PtW10SZ PtWZ
21 20 21 21 22 31
2 3 6 8 11 19
19 17 15 13 11 12
Bz ) benzene; MCP ) methylcyclopentane.
Arribas et al.17 and Falco et al.18 Total conversion, as well as MCP conversion, decreases during the first minutes on the stream and then remain constant to the end of the run. The yield of Bz is practically constant all throughout the run. It can be considered that the isomerization of CH to MCP is produced on the acid sites of the catalyst and that the initial deactivation of the acid function is caused by coke deposition. At 30 min TOS, the catalyst reaches a pseudostationary state. The production of benzene is lower when using PtSZ or PtWSZ. The selectivity of the two reaction products remains constant after 30 min TOS and depends on the acidity and the SO42- content of the catalysts. A comparison of the results obtained with PtSZ, PtWSZ, and PtWZ suggests that Pt gets more electrodeficient or sulfur poisoned in the case of the PtSZ and PtWSZ catalysts that show a lower metallic activity. Using Pt/ZrO2, a neutral catalyst, a relatively long residence time, the same reaction temperature and pressure, and a higher hydrogen partial pressure, benzene is the only product with a conversion of 80%.18 The thermodynamic conversion value for the current experimental conditions should therefore be higher because in our case the partial pressure of hydrogen is lower. The fact that benzene production is much higher on PtZ than on PtWZ, PtWxSZ, or PtSZ is an indication of a strong interaction with the support in the latter catalysts because of the presence of WOx and SO42species. The inhibition of the de/hydrogenation properties of the PtSZ and PtWZ catalysts by interaction between the Pt metal and the support has been extensively reported and has been attributed to different causes depending on the research group. Paa´l and co-workers19,20 have postulated that, in the case of PtSZ catalysts, the support partial encapsulates the Pt particles. Pt particles would be metallic, although mostly buried in the first surface layers of the support. The model was based on the results of ion scattering spectra (ISS) of fresh and calcined PtSZ xerogels. Hattori and co-workers21,22 proposed that Pt particles in PtSZ had a metallic inner core and an oxidized surface. Vera et al.23 and Grau et al.13 postulated that Pt particles were electron deficient because of electronic induction from the highly electronegative support. In any case, the metal particles keep a minimum activity for activation of hydrogen that is enough for stabilization of the catalyst against deactivation during the isomerization reaction, as can be seen from the initial (30 min) and final (300 min) values of conversion of n-decane in Table 4. (17) Arribas, M. A.; Marquez, F.; Martinez, A. J. Catal. 2000, 190, 309. (18) Falco, M. G.; Grau, J. M.; Fı´goli, N. S. Appl. Catal. A 2004, 264, 183. (19) Paa´l, Z.; Wild, U.; Muhler, M.; Manoli, J.-M.; Potvin, C.; Buchholz, T.; Sprenger, S.; Resofszki, G. Appl. Catal. A 1999, 188, 257. (20) Manoli, J.-M.; Potvin, C.; Muhler, M.; Wild, U.; Resofszki, G.; Buchholz, T.; Paa´l, Z. J. Catal. 1998, 178, 338. (21) Ebitani, K.; Tanaka, T.; Hattori, H. Appl. Catal. 1993, 102, 79. (22) Shishido, T.; Tanaka, T.; Hattori, H. J. Catal. 1997, 172, 24. (23) Vera, C. R.; Yori, J. C.; Pieck, C. L.; Parera, J. M. Appl. Catal. A 2003, 240, 161.
Table 4. n-Decane Reaction Results and Conversion and Selectivity Valuesa catalyst TOS (min) conv (%) S(C1) S(C2-C4) S(C5-C9) S(iC4) S(iC5) S(iC6) S(iC7) S(iC9) S(iC10) S(iCTotal) iC4-7/iCTotal
PtSZ 30 89.3 0.3 39.1 20.7 0.0 21.6 8.4 0.0 4.8 3.9 38.7 0.78
PtW5SZ 300 69.2 0.4 21.8 27.1 0.0 17.7 18.2 2.9 0.0 10.9 49.7 0.78
30 85.3 0.3 6.2 28.8 18.3 19.0 17.9 2.1 1.6 5.5 64.4 0.89
300 72.2 0.4 4.8 27.8 17.0 17.7 18.0 3.0 0.0 10.9 66.6 0.84
PtWZ 30 79.9 0.0 6.4 12.9 11.7 19.1 16.2 4.0 0.7 28.6 80.3 0.64
300 74.9 0.0 5.9 12.1 10.7 17.8 15.4 4.1 1.0 32.7 81.7 0.59
a Reaction conditions: 300 °C, 1.5 MPa, WHSV ) 4, H /nC 2 10 ) 6. Abbreviations: TOS ) time-on stream; conv ) conversion of n-decane; S(Cx) ) selectivity to normal paraffin of x carbon number; S(iCx) ) selectivity to isoparaffins of x carbon number.
Figure 2. Conversion of n-octane as a function of the total acidity of catalysts with supported Pt (9, b) and without Pt (0, O) at two values of time on stream: 10 min (s) and 240 min (- - -).
Figure 2 shows the results of n-octane conversion as a function of the total acidity of the catalysts at 5 and 240 min time on stream. In the latter case, a pseudostationary state is reached, and the conversion reaches a stable value. If two catalysts of equal acidity, with and without Pt, are compared, an important increase of the conversion can be seen in the Ptcontaining one. The activity drop before the stationary state is reached is also smaller in these catalysts. These results indicate that the Pt particles on the support play an important role in keeping the surface free of coke. This is done by activating hydrogen, and hydrogenating the adsorbed coke precursors. The increase in conversion can be explained by the onset of a new reaction mechanism in the presence of Pt. Fı´goli and Parera24 have indicated that on Pt-containing oxoanion-promoted zirconia catalysts a nonclassical bifunctional mechanism takes place. Atomic hydrogen is transformed into protons over surface Lewis sites, and new Bro¨nsted acid sites appear. The hydride ions also formed help to enhance the hydride transfer rates. In all the sulfated catalysts, the conversion at 5 min (before visible deactivation by coke) is greater than 40% in the absence of Pt and greater than 90% when this metal is present. After the (24) Parera, J. M.; Fı´goli, N. S. In Catalytic Naphtha Reforming, 2nd. ed.; Antos, G. J., Aitani, A. M., Eds.; Marcel Dekker: New York, 2004; Chapter 3.
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Figure 4. Gain in research octane number of the feed (n-decane) upon reaction over three selected catalysts.
Figure 3. Yields of each product as a function of the total acidity at 5 min time on stream: iCX ) isoparaffins of x carbon number (x ) 4, 5, 8); iCT ) total isoparaffins; nCx ) n-paraffins; C3 ) propane.
pseudostationary state is reached, the catalysts with Pt keep a high level of activity, whereas the samples without metal are deactivated almost totally, showing a conversion lower than 5% in all cases. For the double-promoted PtWSZ samples, the conversion closely follows the pattern of the acidity values: the higher the acidity, the higher the conversion. The conversion is practically complete over the catalyst with 212 mmol Py g-1 of acid sites (PtW5SZ) when this catalyst is free of coke. This conversion is the highest when the values in the pseudostationary state are compared. For greater acidity, values the curve begins to decay slightly. These results indicate that the PtW5SZ catalyst has the optimal balance between the activity of the metal and acid functions. If we recall the values in Tables 1, 2, and 3, PtSZ is the most acidic catalyst. It is also the catalyst with the highest sulfur content. This high sulfur content is detrimental to the activity of the metal function because the poisoning of Pt is closely related to the amount of sulfur on the surface. Accordingly, PtSZ has the smallest metallic activity. In contrast, the PtWZ catalyst is the material with the lowest acidity and the highest metal activity. The double-promoted PtWSZ catalysts should have intermediate values of metal activity. It can therefore be expected that the PtW5SZ catalyst has the most convenient metal/acid ratio. With respect to the product distribution obtained in each case, the acid catalysts without Pt show an important initial activity needed for the formation of octane branched isomers. As the acid content increases, the rate of the cracking reactions, the yield of shorter isomers (C4-C7), and the yield of light gases (C1-C3) are also increased. In the absence of a metal function, the olefins formed by the cracking reactions react over the acid sites and rapidly deactivate the catalysts by forming coke. Figure 3 shows the yields of different products obtained at 5 min time on stream in the reaction of n-octane with different catalysts and as a function of the total acidity. Dependent on the acid and metal activity of each catalyst, a different product distribution is obtained. When the acidity is mainly of the weak and mild types, heavy isomers dominate the product distribution. When the acidity is concentrated in the mild and strong fractions, the yield of light gases is higher. This high acidity is also linked to a high concentration of sulfate species on the support;
therefore, the metal function is highly inhibited, and the rate of deactivation by coke formation is high. The results indicate that the PtW5SZ catalyst (212 µmol Py g-1) is the one that produces the highest isobutane yield. This catalyst was further tested in the isomerization-cracking reaction of n-decane at high pressure, under near industrial conditions. The n-decane reaction results are detailed in Table 4 for the three catalysts of interest: PtWZ, PtSZ, and PtW5SZ. The three catalysts reach a stable state fairly rapidly. PtWZ displays the highest activity at the end of the experiment. With respect to their initial activity, the catalyst displayed the same pattern as that shown in the case of the reaction with n-octane at atmospheric pressure. The hydrogenolytic activity of the catalysts is supposed to be well represented by the yield of methane in the products. The hydrogenolysis contribution was therefore smaller than 0.5% for the sulfated catalysts, and it was not detected in PtWZ. With respect to the formation of gases (SC2-C4), PtSZ displayed the highest yield. A remarkable reduction in the production of gases was achieved with the WSZ catalysts. If we analyze the selectivity to total isoparaffins, it can be seen that the most selective catalyst is PtWZ. Despite this, the results obtained with the PtW5SZ are more attractive. This catalyst produces an isomerizate with the highest iC4-7/iCtotal ratio, and therefore, the production of valuable components for the gasoline pool is maximized in this catalyst. Finally in Figure 4, we can see a bar plot indicating the RON gain achieved by isomerization of n-decane over PtW5SZ, PtSZ, and PtWZ. The gain is 85-90 for all catalysts, but the best results are obtained with the PtW5SZ catalyst. 4. Conclusions The promotion of zirconia hydroxide with SO42- and WO3 produces an acid material of mainly tetragonal structure and with an intermediate acidity between that of standard sulfatezirconia and tungsten-zirconia catalysts. This double-promoted catalyst has a higher specific area than that of tungsten zirconia and can be advantageously used for the hydroisomerization and cracking of long chain n-paraffins. The acidity of the double-promoted tungsten-sulfatezirconia catalyst (WSZ) can be regulated by adjustment of the WO3 content. The addition of Pt to WSZ generates a stable catalyst with good isomerization capacity and low cracking activity. These
Pt/WOx-SO4-ZrO2 Catalysts
Energy & Fuels, Vol. 21, No. 3, 2007 1395
features maximize the production of the medium length isomers (C4-C8) of interest for its use in reformulated gasolines.
specially indebted to UNIFACS for financing his stay at Salvador (Bahia, Brazil).
Acknowledgment. This work was supported by UNIFACS (Brazil), CONICET, ANPCyT, and UNL (Argentina). V.B. is
EF060583J
