Energy & Fuels 2006, 20, 1791-1798
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Hydroisomerization of Benzene-Containing Paraffinic Feedstocks over Pt/WO3-ZrO2 Catalysts Viviana M. Benitez, Javier M. Grau, Juan C. Yori, Carlos L. Pieck, and Carlos R. Vera* Instituto de InVestigaciones en Cata´ lisis y Petroquı´mica (FIQ-UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina ReceiVed April 20, 2006. ReVised Manuscript ReceiVed July 17, 2006
This report studies the feasibility of carrying out the elimination of benzene contained in paraffinic feedstocks (3-15%) with economy of equipment and sparing of pretreatment steps by hydrogenating benzene to cyclohexane and isomerizing partly the latter to methylcyclopentane in an isomerization reactor loaded with a Pt/WO3-ZrO2 catalyst. The results indicate that the temperature of calcination of the catalysts affects differently the acid and metal functions. The hydrogenating capacity of Pt mildly decreases at higher temperatures because of an increasing interaction with the support, while the isomerization capacity is enhanced because of the creation of strong acid sites. The optimum catalyst is the one calcined at 800 °C because of the formation of strong acid sites at this temperature.With respect to the reaction temperature, there exists a narrow range in which both the hydrogenation of benzene and the isomerization of n-paraffins are thermodynamically feasible with nonnegligible yield. At 200 °C, the conversion of benzene is greatly favored but the activity of the acid function for the acid-catalyzed reactions, i.e., the ring contraction of cyclohexane and the isomerization of n-hexane, is too small. At 300 °C, the acid activity is high but the conversion of benzene is low, even at high pressure, due to thermodynamic reasons. 250 °C seems to be the best temperature for performing both reactions simultaneously.The inhibition of the hydrogenolytic activity due to the interaction of Pt with the WO3-ZrO2 support suppresses the ring-opening activity of supported Pt; therefore ,benzene transforms only into cyclohexane and methylcyclopentane over Pt/WO3-ZrO2. Addition of Pt/Al2O3 to form a composite catalyst enhances the metal activity and ring-opening products appear. However, this is not considered convenient in this case, because methylcyclopentane has a conveniently high octane number and most ring-opening products have similar or lower values.The presence of benzene partly inhibits the conversion of n-hexane because of the adsorption over the strong acid sites of WO3-ZrO2. The selectivity is also modified because of the suppression of most of the cracking activity.
Introduction In the last few years, refineries have focused on the reduction of the content of benzene and sulfur in gasoline because of environmental and public health reasons. In the case of benzene, small percentages are present in the distillate virgin naphtha, but most benzene is produced in refinery units. The catalytic reformer is the main supplier of benzene, with 50-80% of the total, coming from the aromatization of C6 and aromatization/ dealkylation of heavier hydrocarbons. Smaller amounts are contained in the product streams of the coker, fluid catalytic cracking (FCC), and hydrocracking units. One usual solution for the reduction of benzene in the streams contributing to the gasoline pool is the diversion of the C6 fraction from the feed of the reformer to the feed of the isomerization unit.1 This prefractionation process scheme reduces benzene by preventing its production in the catalytic reforming unit. The benzene precursors are eliminated from the feed using a naphtha splitter. The relevant components are cyclohexane (CH), methylcyclopentane (MCP), and indigenous benzene (Bz). The naphtha splitter must now use a highertemperature cutting point to remove all the benzene precursors. Since the separation in this unit is not a sharp one, the new
feedstock for the isomerization unit will contain not only C6 cyclic molecules but also C7 paraffins boiling close to benzene. Another situation is that encountered in HDS (hydrodesulfurization)/octane recovery units for the treatment of FCC and pyrolysis naphtha.2 If a conventional HDS unit is used to eliminate sulfur compounds, the octane loss produced by the hydrogenation of the olefins must be recovered by isomerization of the hydrotreated FCC and pyrolysis naphtha. Once the sulfur is stripped away and sent to a sulfur recovery unit, the naphtha for isomerization still contains a relatively high content of aromatics. As can be seen, new refinery schemes aimed at reducing benzene and sulfur to acceptable levels generate uncommon feedstocks with relatively high benzene/ aromatics content that must enter the isomerization unit. This benzene should be fully transformed into other more environmentally friendly compounds of high octane number. The possible pathways of transformation of benzene on metal-acid bifunctional catalysts are depicted in Figure 1. Hydrogenation to cyclohexane (CH) on metal sites is fairly fast, and the concentration of intermediate unsaturated products is negligible. With respect to the acidcatalyzed reactions, the ring of cyclohexane can be contracted
* Corresponding author. (1) Gentry, J. C.; Lee, F.-M. Proceedings of the Annual Meeting of the National Petrochemical & Refiners Association; San Antonio, TX, 2000; Paper AM-00-35.
(2) Chitnis, G. K.; Dabkowski, M. J.; Sherif, M.; Richter, J. A.; AlKuwari, I.; Hilbert, T. L.; Al-Kuwari, N. Proceedings of the Annual Meeting of the National Petrochemical & Refiners Association; San Antonio, TX, 2003; Paper AM-03-125.
10.1021/ef060165e CCC: $33.50 © 2006 American Chemical Society Published on Web 08/08/2006
1792 Energy & Fuels, Vol. 20, No. 5, 2006
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Figure 2. Thermodynamic equilibrium molar ratios as calculated from ∆G°f values:11 MCP/CH ) methylcyclopentane/cyclohexane; i-C6/nC6 ) (2,2-dimethylbutane + 2,3-dimethylbutane + 2-methylpentane + 3-methylpentane)/n-pentane; CH/Bz ) cyclohexane/benzene.
Figure 1. Reaction pathways of benzene and C6 hydrocarbons.
to produce methylcyclopentane (MCP), and its ring might be further opened to produce acyclic C6. The latter can be further isomerized to produce any of the C6 acyclic isomers: the straight chain one, the two monomethylated pentanes, or the dimethylated butanes. The convenience of certain reaction pathways in comparison to others must be discussed with attention to the blending RON (research octane number). The octane numbers (blending RON) of cyclohexane and benzene are different (RONCH ) 84, RONBz ) 120), and the lower contribution of cyclohexane should be compensated by sequentially transforming cyclohexane to other high RON compounds. MCP has a blending RON of 96, so ring contraction is a desired reaction. If the ring were additionally open, the acyclic species produced should be isomerized to highly branched compounds because of the low RON value of n-hexane and the monomethylated pentanes (MP): RONn-hexane ) 31, RON2-MP ) 74, RON3-MP ) 76. Values for dimethyl butanes (DMB) are as follows: RON2.2-DMB ) 94, RON2,3-DMB ) 105.3 The convenience of a certain degree of transformation depends on many factors. Though the transformation to MCP could be considered sufficient from the point of view of the RON, the ring opening to hexanes is also very much desired because methylcyclopentane and, to a lesser extent, cyclohexane form very stable carbocations which occupy acid sites on the catalyst. Normally, the isomerization of paraffins is decreased as the level of naphthenes increases. Therefore, a catalyst that promotes the rapid reaction of cyclohexane to hexanes is also preferred, but as anticipated, this reaction must lead to a high yield of dibranched isomers. The management of benzene-containing paraffinic feedstocks (3) Anderson, G. C.; Rosin, R. R.; Stine, M. A.; Hunter, M. J. UOP Technical Bulletin AM-04-46; UOP: Des Plaines, IL.
has motivated different solutions. A process patented by UOP4 accommodates variable amounts of benzene (2-25%) in a paraffinic stream and fed to a isomerization unit by means of a Pt/Al2O3 catalyst operated at a lower temperature and located in the entrance of the reactor (upper layer of the catalyst bed for a downflow arrangement). This catalyst produces most of the transformation of benzene, and the stream is further reacted in the main part of reactor, which is loaded with Pt/SO42-ZrO2. Shimizu et al.5 have indicated that this catalyst has a better performance than other metal/acid isomerization catalysts (Pt/ HY or Pt/H-mordenite) for the simultaneous hydrogenation of Bz and the isomerization of CH. Another process6 uses an additional hydrogenation reactor before the isomerization unit. The heat of hydrogenation is used to preheat the feed of the isomerization unit, where the ring opening and isomerization of the alkanes occur. The option of hydrogenation-isomerization in the same reactor eliminates one process unit step but requires catalysts that are not very sensitive to benzene. For example, the catalyst of the Penex process might be degraded by the exothermal hydrogenation of benzene if its concentration is high. The use of this catalyst requires the complete hydrogenation of benzene in a previous step. Zeolites and oxoanion-promoted zirconia catalysts (Pt/SO42--ZrO2, Pt/WO3-ZrO2) are less sensitive. In the case of the oxoanion-promoted zirconia catalysts, they have a metal function partially inhibited because of their interaction with the support and have a low hydrogenation capacity, though they can be also operated at low temperatures with a high isomerization activity.7 Another point to take into account is that of the thermodynamics of benzene transformation and C6 isomerization. Temperatures higher than 250-300 °C are prohibitive for the transformation of benzene to cyclohexane (see Figure 2), though they favor the MCP/CH molar ratio. Low temperatures also favor the molar ratio iso/normal of the acyclic C6 fraction and increase the yield of branched paraffins and the gain of RON points. The study of the simultaneous hydrogenation of benzene and the isomerization of short paraffins on these catalysts has both practical and theoretical interests. Arribas et al.8 have recently (4) Blommel, P. G.; Gosling, C. D.; Wilcher, S. A. U.S. Patent 5,962,755, 1999. (5) Shimizu, K.; Sunagawa, T.; Vera, C. R.; Ukegawa, K. Appl. Catal., A 2001, 206, 79. (6) Low, C. D.; Gembicki, V. A.; Hills, C.; Haizmann, R. S.; Meadow, R. U.S. Patent 5,003,118, 1991. (7) Grau, J. M.; Yori, J. C.; Vera, C. R.; Lovey, F. C.; Condo´, A. M.; Parera J. M. Appl. Catal., A 2004, 265, 141. (8) Arribas, M. A.; Marquez, F.; Martınez, A. J. Catal. 2000, 190, 309.
Hydroisomerization of Benzene-Containing Feedstocks
reported that benzene contained in high percentages (up to 25%) in a n-heptane feed can be totally converted to CH and MCP over a Pt/WO3-ZrO2 catalyst at industrial conditions (3.0 MPa, 200-300 °C) while keeping a high isomerization activity. This is an example of many reports currently exploring the occurrence of simultaneous acid- and metal-catalyzed reactions on oxoanion-promoted zirconia catalysts, like the HDS/isomerization of sulfur containing light naphtha over metal/SO42--ZrO2-Al2O3 9 or the hydrogenation/ring opening of aromatics over metal/ WO3-ZrO2.10 The transformation of benzene-containing paraffinic feedstocks over Pt/WO3-ZrO2 catalysts is studied in this work, using n-hexane as a model compound. The influence of different preparation, conditioning, and reaction conditions, such as the calcination and reaction temperatures, was assessed. Their role on the activity of the metal and acid functions, the conversion of benzene, and the yield to isoparaffins and MCP is specially discussed. Experimental Section Catalysts. Zr(OH)4 was prepared by hydrolysis and precipitation of zirconium oxychloride (Strem Chem., 99.9998%) with concentrated NH4OH until a pH value of 10 was reached. The dried Zr(OH)4 xerogel was ground in a mortar and sieved to 35-80 meshes. Tungsten-zirconia, WO3-ZrO2 was obtained by impregnation with an excess of a solution of ammonium metatungstate [(NH4)6(H2W12O40‚nH2O)] (Fluka, 99.9%) previously stabilized at pH ) 6 during a week and with an adequate concentration in order to get 15% W in the final catalyst.11 After the impregnation, the catalyst was dried in a stove at 110 °C for 12 h. Portions of the catalyst were calcined at 500, 600, 700, and 800 °C for 2 h in air (10 mL min-1). The WO3-ZrO2 (WZ) support was then impregnated with a solution of chloroplatinic acid (Strem Chem., 99.9%). The volume and concentration of the impregnating solution were adjusted to get a final 1% Pt in the catalyst (PtWZ). Pt/Al2O3 (PtAl) samples were prepared by impregnating chloroplatinic acid by the incipient wetness method with a solution with an adjusted concentration in order to get a final 1% Pt over the final catalyst. γ-alumina was supplied by Ketjen (CK300, 200 m2 g-1). Composite PtWZ + PtAl catalysts were prepared by mixing powders of PtAl and PtWZ both ground to 200 meshes. PtAl and PtWZ powders of the same weigth were suspended in n-pentane and stirred for 1 h. The solvent was let to evaporate and then the mixture was pressed in order to form a solid pellet that was finally ground to a 35-80 meshes particle size. Before starting the test reactions, the catalysts were calcined at 500 °C in air (10 mL min-1). Then the temperature was reduced to 300 °C, and the catalyst was reduced in a hydrogen stream (10 mL min-1). Finally, the reactor was cooled to the reaction temperature (200-300 °C). The product stream was sampled after a time-on-stream of 1 h, and the products were analyzed in a chromatograph equipped with a 25 µm i.d., 100 m, ZB-1 (Phenomenex) capillary column. Hydrogenation-Isomerization of Benzene. The reaction was performed under the following conditions: pressure ) 0.1 MPa, reaction temperature ) 200-300 °C, catalyst mass ) 100 mg, (9) Watanabe, K.; Kawakami, T.; Baba, K.; Oshio, N.; Kimura, T. Appl. Catal., A 2004, 276 145. (10) Wakayama, T., Matsuhashi, H. J. Mol. Catal. A: Chem. 2005, 239, 32. (11) Rossini, M. American Petroleum Institute; Project 44.
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molar ratio H2/Bz ) 15, flowrate of Bz ) 0.5 mL h-1, and flowrate of H2 ) 30 mL min-1. Isomerization of Hexane/Benzene. This reaction was used with the objective of studying the catalytic activity and selectivity of the catalysts when performing the many reactions of interest simultaneously, the isomerization of short paraffins, the hydrogenation of Bz to CH, the ring contraction of CH to MCP, and the ring opening of MCP. A model mixture of n-hexane/benzene (85-95% vol/vol of n-hexane) was used. The reaction conditions were as follows: total pressure ) 0.1 MPa, reaction temperature ) 250-300 °C, catalyst mass ) 100 mg, molar ratio H2/HC ) 15, flowrate of benzene/hexane mixture ) 0.5 mL h-1, and flowrate of H2 ) 30 mL min-1. The catalyst mass was changed to 200 mg in the case of the PtAl + PtWZ composites. Temperature-Programmed Desorption (TPD) of CH, n-Hexane, Bz, and MCP. Catalyst (50 mg) calcined at 800 °C was dehydrated at 400 °C and cooled to room temperature in dry air (10 mL min-1). Then the sample was transferred to a closed vial where it was impregnated with the corresponding hydrocarbon. The sample was first stabilized in a nitrogen stream at 50 °C, and then it was heated to 500 °C with a heating rate of 10 °C min-1. The desorbed molecules were recorded in a flame ionization detector (FID) connected to the TPD cell. Temperature-Programmed Desorption of Pyridine. The amount of acid sites and their acid strength were assessed by means of temperature-programmed desorption of pyridine. Pyridine was supplied by Merck (GC, >99.5%). The samples calcined in air at different temperatures (500, 600, 700, and 800 °C) were first immersed in an excess volume of pyridine at room temperature for 2 h. Then they were filtered and dried in still air in an open vial at room temperature. The samples were placed in a quartz microreactor and stabilized in N2 for 1 h at 110 °C. Then they were heated from this temperature to 650 °C in nitrogen (40 mL min-1) at a heating rate of 10 °C min-1. The desorbed products were sent to a flame ionization detector (FID) connected on-line, and the signal was recorded continuously in a computer. To assess the possible oxidation of pyridine by the support, another special TPD experiment was devised. The exhaust of the TPD cell was connected to a pyridine trap consisting of a fixed bed with 5 g of sulfated zirconia calcined at 600 °C in air and maintained at room temperature. The exhaust of the trap was connected to a methanator, and then the methanated products were sent to the FID. In this way, the amounts of CO and CO2 products formed by oxidation of pyridine by surface groups of the catalyst were measured. Hydrogen Chemisorption. Hydrogen adsorption isotherms were taken in order to measure the accessibility of the metal phase (Pt). First, samples were heated at 300 °C, reduced in H2 for 1 h, and degassed for 2 h. After cooling to room temperature, isotherms of total and reversible hydrogen adsorption were obtained. The amount of chemisorbed hydrogen was obtained by subtracting the two isotherms, and the H/Pt ratio was calculated assuming dissociative adsorption of hydrogen on the Pt atoms. Thermodynamic Equilibrium Calculations. Values of the molar ratios between key compounds in mixtures in thermodynamic equilibrium were calculated from reported values of the Gibbs free energy of formation of the compounds.11 Plots of these thermodynamic ratios are included in Figure 2.
∆G0f ) -RT ln K
(1)
1794 Energy & Fuels, Vol. 20, No. 5, 2006
K ) Πi pVi i
Benitez et al.
(2)
K is the equilibrium constant, and νi is the corresponding stoichiometric index of the ith component. In eq 2, partial pressures are used instead of activities, i.e., fugacity factors have been assumed to be equal to 1 because of the low pressure of the experiments. Both the equilibrium molar ratio between methylcyclopentane and cyclohexane (MCP/CH) and the molar ratio between branched hexanes and n-hexane (i-C6/n-C6) do not depend on the pressure because the isomerization reactions involved have no change in the total number of moles. In the case of the hydrogenation of benzene, hydrogen has a null contribution in the formula of the ∆Gf but its concentration appears in the ratio between products and reactants (formula 2). Values of the ratio between cyclohexane and benzene (CH/ Bz) were calculated at 0.1 and 5 MPa for comparison. In Figures 4-6, the equilibrium conversion of benzene at 0.1 MPa was calculated for reference. This conversion was calculated considering only the conversion to cyclohexane (CH) and disregarding the formation of methylcyclopentane (MCP) and ring-opening (ROP) products. In the figures and tables, the selectivities to CH, MCP, branched isohexanes (ISO), and cracking products (C1-C5) were calculated as
SCH ) xCH‚100/(xCH + xMCP) ) xCH‚100/(XBzx0Bz) SMCP ) xMCP‚100/(xCH + xMCP) ) xMCP‚100/(XBzx0Bz)
(3) (4)
SISO ) xISO‚100/(xISO + xC1-C5) ) 0 ) (5) xISO‚100/(Xn-hexanexn-hexane
Figure 3. TPD tests of Pt/WZ catalysts calcined at different temperatures: (a) pyridine TPDsthe trace labeled “oxidation” is the TP trace for the oxidation of pyridine on PtWZ800 with oxygen surface groups of the catalyst; (b) TPD of hydrocarbons adsorbed on the PtWZ800 catalyst.
The results of the temperature-programmed desorption of pyridine can be found in Figure 3a and Table 1. The acidity of the PtWZ catalysts increases monotonically with the calcination temperature. WZ calcined at 800 °C (PtWZ800) has the highest total amount of acid sites and also the highest amount of strong acid sites. These strong acid sites are those desorbing pyridine at a temperature >500 °C. The evolution of the acidity with the temperature confirms a similar trend observed in other reports.12-15 For example, Santiesteban et al.13 found that WZ catalysts, with 15% W and calcined at 800 °C, were the most active in isomerization of hydrocarbons, and they related this activity to the maximization of the amount of Brønsted acid sites and the concentration of strong acid sites. Boyse and Ko14
studied commercial tungstated zirconia gels with 10 and 12% W and found that those with 12% W and calcined at 800 °C had the highest activity for the conversion of n-butane. Vaudagna et al.15 studied the conversion of n-hexane over WZ catalysts and found that catalysts with 12-15% W calcined at 750-850 °C had the highest concentration of strong Brønsted acid sites and the highest catalytic activity. It is important to remark that the total concentration of acid sites is not reduced by the area decrease that accompanies the sintering of the zirconia gel (Table 1). Most likely, the acidic and catalytic properties at 12-15% W loading and at a calcination temperature of ∼800 °C are related to the formation of a material at the point of saturation coverage by 1-2 monolayers of tungstate, as previously and extensively reported.16 Formation of these monolayers during the calcination at 800 °C may be favored by the differential wetting of tungsten species. It is a known phenomenon that, at high temperatures, vanadium, tungsten, molybdenum, and other heavy anions tend to cover support surfaces by surface diffusion and to form layers. The spontaneous formation of monolayers of polyanions has been studied in the past,17 and the driving force for the process has been claimed to be the lowering of the surface energy of the M2Ox oxide. The wetting of the support depends on its specific surface energy. The difference of surface energy of ZrO2 with respect to WO3 greatly favors the wetting over the zirconia particles.18 This phenomenon has been recently used to prepare WZ catalysts with no impregnation procedure.19 By dividing the amount of W (15%) by the specific surface of
(12) Yori, J. C.; Vera, C. R.; Parera, J. M. Appl. Catal., A 1997, 163, 165. (13) Santiesteban, J. G.; Vartuli, J. C.; Han, S.; Chang, C. D. J. Catal. 1997, 168, 431. (14) Boyse, R. A.; Ko, E. I. Appl. Catal., A 1999, 177, 131. (15) Vaudagna, S. R.; Canavese, S. A.; Comelli, R. A.; Fı´goli, N. S. Appl. Catal., A 1998, 168, 93.
(16) Boyse, R. A.; Ko, E. I. J. Catal. 1997, 171, 191. (17) Leyrer, J.; Margraf, R.; Taglauer, K.; Kno¨zinger, H. Surf. Sci. 1998, 201, 603. (18) Overbury, S. H.; Bertrand, P. A.; Somorjai, A. Chem. ReV. 1975, 75, 547. (19) Niwa, M.; Habuta, Y.; Okumura, K.; Katada, N. Catal. Today 2003, 87, 213.
SC1-C5 ) xC1-C5‚100/(xISO + xC1-C5) ) 0 ) (6) xC1-C5‚100/(Xn-hexanexn-hexane
where X ) conversion and x ) molar fraction. In the previous formulas, n-hexane and benzene are supposed to react independently, i.e., the extent of ring opening is considered negligible. Therefore, the selectivities to MCP and CH are based solely on the conversion of Bz, and the selectivities to branched isohexanes and cracking products are based on the conversion of n-hexane only. Results and Discussion
Hydroisomerization of Benzene-Containing Feedstocks
Energy & Fuels, Vol. 20, No. 5, 2006 1795
Table 1. Specific Surface Area, H/Pt Surface Atomic Ratio, and Distribution of Acid Strength of the Catalysts as Measured by Temperature-Programmed Desorption of Pyridine acidity, micromol g-1 catalyst
100 × (H/Pt)
surface area, m2 g-1
total
weak 110-300 °C
medium 300-500 °C
strong >500 °C
PtAl PtWZ500 PtWZ600 PtWZ700 PtWZ800 PtAl + PtWZ800
60.1 22.8 0.0 0.0 0.0 29.5
195 110 89 63 39 115
33.2 79.3 104 120 61.2
2.7 19.2 30.1 36.5 18.1
31.7 59.8 73.6 78.0 40.5
0.3 0.3 0.7 5.4 2.6
the sample calcined at 800 °C (39 m2 g-1), a theoretical surface density of 20 (µmol of W) m-2 is obtained. This is twice the value of the ideal monolayer density of polytungstate, 10 µmol m-2. Results of the deconvolution of the pyridine TPD peaks were included in the last column of Table 1. Three peaks can be recognized, and their variation with the calcination temperature is clear if the sample calcined at 600 °C is not considered for the analysis. The first two main peaks contribute to 95-100% of all the pyridine TPD trace. The first peak located at 229251 °C contributes to 13-31% of the total area, and upon calcination, its position is shifted to higher temperatures and its relative size is increased. The second peak is located at 371380 °C and contributes to 86-74% of all acid sites. Its position is shifted to higher temperatures and its size is decreased upon calcination. A third small peak at 455-503 °C contributes to 1-5% the population of the acid sites. Its size is increased and its position is shifted to higher temperatures as the calcination temperature is increased. In summary, the three main peaks are shifted to higher temperatures, i.e., they increase their acid strength, as the calcination temperature is increased. The size of the last peak is specially affected by the calcination treatment. This is a peak related to the strong acid sites. The thus-descripted pyridine TPD results correspond to the desorption of pyridine with no parallel oxidation by the catalyst surface. During the TPD of organic bases adsorbed on SO42-ZrO2, oxidation of the base proceeds helped by the reduction of the sulfate groups, mainly to SO2.20 In the case of WZ, oxidation can only occur by reduction of surface WO3. Bulk WO3 is an n-type oxide which tends to lose oxygen atoms and form nonstoichiometric phases. At 400-450 °C in hydrogen, the reduction of WO3 to the WO2.9 stable phase occurs.21 In the case of the reduction of surface W species on WZ with 1520% W, reduction in hydrogen begins at ∼450 °C and most of the W is reduced between 500 and 650 °C.22 Our pyridine TPD data were taken between 100 and 550 °C in flowing nitrogen, and during the desorption of pyridine in this range, practically no oxidation occurs. The TP trace labeled “oxidation” in Figure 3a indeed indicates that the oxidation of pyridine over WZ is negligible in the temperature range scanned. Another set of data included in Table 1 is that of the chemisorption capacity of the Pt particles supported over tungsten-zirconia calcined at different temperatures. The H/Pt ratio is practically zero for all catalysts calcined at temperatures > 500 °C, indicating that the capacity for adsorbing hydrogen is highly inhibited in these catalysts. Previous reports have indicated that the H chemisorption capacity of oxoanionpromoted supports depends both on the temperature of calcina(20) Milburn, D. R.; Saito, K.; Keogh, R. A.; Davis, B. H. Appl. Catal., A 2001, 215, 191. (21) Bigey, C.; Hilaire, L.; Maire, G. J. Catal. 1999, 184, 406. (22) Di Gregorio, F.; Keller, V. J. Catal. 2004, 225, 45.
acidity, position (°C), and area (%) of deconvoluted peaks 229 (13%); 388 (86%); 455 (1%) 306 (64%); 369 (36%); 232 (26%); 371 (71%); 485 (3%) 251 (31%); 389 (64%); 503 (5%)
tion of the support7 and the temperature of reduction23 of the final catalyst. High calcination temperatures increase the strong metal-support interaction in these catalysts and decrease the hydrogen chemisorption capacity but are completely necessary for the conditioning of the acid function.7 On the other side, Santiesteban et al.23 have shown that increasing prereduction temperatures decreases the chemisorption capacity in highly dispersed PtWZ catalysts. The authors reported H/Pt ) 1-1.4 for reduction temperatures of 200-250 °C and H/Pt ) 0.27 at 450 °C (WZ precalcined at 800 °C). The catalysts in this work were reduced at 300 °C, and therefore, the low chemisorption capacity of the samples can only be the result of the great interaction with the support. Figure 3b contains the results obtained in the temperatureprogrammed desorption tests of CH, MCP, and Bz using PtWZ800 as support. Previous results have indicated that benzene is strongly adsorbed on oxoanion-promoted zirconia catalysts and that complete desorption is not completed at temperatures
