Hydroisomerization and Cracking of n-Octane and n-Hexadecane

Feb 1, 2006 - Hydroisomerization and Cracking of n-Octane and n-Hexadecane over Zirconia Catalysts. V. M. Benitez , J. C. Yori , J. M. Grau , C. L. Pi...
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Energy & Fuels 2006, 20, 422-426

Hydroisomerization and Cracking of n-Octane and n-Hexadecane over Zirconia Catalysts V. M. Benitez, J. C. Yori, J. M. Grau, C. L. Pieck, and C. R. Vera* Instituto de InVestigaciones en Cata´ lisis y Petroquı´mica, INCAPE (FIQ-UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina ReceiVed April 2, 2005. ReVised Manuscript ReceiVed December 19, 2005

C8-C16 n-paraffins can be upgraded to fuel-grade gasoline (C5-8) by means of simultaneous hydrocracking and hydroisomerization over bifunctional metal/acid catalysts. The hydroisomerization of n-paraffins over nickel/ sulfated-zirconia (NiSZ), nickel/tungsten-zirconia (NiWZ), platinum/sulfate-zirconia (PtSZ), and platinum/ tungsten-zirconia (PtWZ) “fresh” catalysts provides between 10 and 113 additional research octane number (RON) points over those of a model feedstock of n-octane. A close linear relation between the production of light gases (C1-C3) and the RON gain indicates that octane improvement can only be reached at the expense of the isomerizate yield. The RON gain stabilizes at a Ni-Pt content of 1% and varies slightly at higher loadings. PtSZ is the most active catalyst and provides a 100-113 RON upgrade depending on the Pt content. PtWZ can only provide 42 RON points at these reaction conditions. Though Ni-based catalysts provide a moderate RON increase of 79-83 points, they are cheaper than Pt catalysts and show an improved stability at high values of time-on-stream. The RON gain is better correlated to the total acidity than to the strong acidity, thus confirming that the production of branched isomers from long alkanes does not require highly acidic sites. On comparison, the reaction of n-hexadecane on the 0.1 and 1.0% Ni-promoted SZ and WZ catalysts indicates that the WZ catalysts provide the highest yield and selectivity to C5-C8 isomers. For the same amount of Ni, SZ catalysts have a higher activity than the WZ catalysts, but their selectivity for the production of light gases is too high.

1. Introduction Hydrocracking has been practiced in modern oil refineries for the production of light fuels from heavy distillate and residua since 1959 when Chevron announced its isocracking process. Both hydroisomerization and hydrocracking are able to convert medium- to heavyweight cuts into high-value streams, such as gasoline, jet fuel, other middle distillates, and lubricant oils. Hydroisomerization and selective hydrocracking of medium- and long-chain normal paraffins have been specially studied at both academic labs1-4 and industrial labs5-7 for the production of environmentally friendly high-quality transportation fuels. Usually the molecular weight of the feedstock has prompted a particular choice of conditions, catalysts, and end products. Medium-length C8-16 cuts have been targeted for extensive hydroisomerization and mild cracking to produce gasoline. Long paraffins (C16-C24) and waxes (C24-C60) have been hydrocracked and hydroisomerized for the production of diesel and gasoline. * Corresponding author. Phone: +54-342-4533858. Fax: +54-3424531068. E-mail: [email protected]. (1) Giannetto, G. E.; Perot, G. R.; Guisnet, M. R. Ind. Eng. Chem. Prod. Res. DeV. 1986, 25, 481. (2) Grau, J. M.; Vera, C. R.; Parera, J. M. Appl. Catal., A 1998, 172, 311. (3) Grau, J. M.; Yori, J. C.; Parera, J. M. Appl. Catal., A 2001, 213, 247. (4) Calemma, V.; Peratello, S.; Perego, C. Appl. Catal., A 2000, 190, 207. (5) Derr, W. R.; Garwood, W. E.; Kuo, J. C.; Leib, T. M.; Nace, D. M.; Tabak, S. A. U.S. Patent 4,684,756, 1987. (6) Hammer, G. P. U.S. Patent 4,832,819, 1989. (7) Venkatesh, K. R.; Hu, J.; Wang, W.; Holder, G. D.; Tierney, J. W.; Wender, I. U.S. Patent 6,184,430, 2001.

Since the early studies, not only the importance of hydroisomerization of long-chain linear alkanes for their upgrade to fuels oil has been pointed out but also the difficulty of achieving a high yield of isomerizate.8-11 Most catalysts display meaningful acid activity levels only at temperatures at which deep cracking and coking are also important, and these reactions decrease the global yield to isomers. Another concern has been the replacement of harmful and corrosive catalysts. Sulfuric acid, hydrofluoric acid, and halogenmodified metal oxides have been tried to be replaced by environmentally friendly solid catalysts. Potential substitutes have been molecular sieves or silica-alumina catalysts promoted with Pt, Pd, or Ni.1,4,12,13 High yields of branched isomers were found when using these catalysts for the isomerization of heavy n-paraffins. These catalysts, however, demanded temperatures above 300 °C and high H2 to hydrocarbon ratios and pressures to keep coking rates conveniently low. Hydroisomerization cracking catalysts contain two functions. The metallic function shows de/hydrogenation properties and is usually provided by a noble metal finely dispersed on the support. The acid function is the support itself, usually an oxide that has been modified to increase its acidity. Typical acidic supports in hydrocracking catalysts include amorphous and (8) Weitkamp, J. ACS DiV. Pet. Chem. Prepr. 1975, 20, 489. (9) Weitkamp, J.; Jacobs, P. A. ACS DiV. Pet. Chem. Prepr. 1981, 26, 9. (10) Barton, D. G.; Soled, S. L.; Meitzner, G. D.; Fuentes, G. A.; Iglesia, E. J. Catal. 1999, 181, 57. (11) Zhang, S.; Zhang, Y.; Tierney, J. W.; Wender, I. Appl. Catal., A 2000, 193, 155. (12) Weitkamp, J. Ind. Eng. Chem. Prod. Res. DeV. 1982, 21, 550. (13) Taylor, R. J.; Petty, R. Appl. Catal., A 1994, 119, 121.

10.1021/ef050092j CCC: $33.50 © 2006 American Chemical Society Published on Web 02/01/2006

RON Enhancement of Long Paraffins

crystalline silica-alumina, alumina, and zeolites. Typical de/hydrogenation components are noble metals (Pt, Pd) and sulfided nonnoble metals (Ni, Co, Mo). Weitkamp et al.12,14 have indicated that the relative strengths of the metal function and the acidic one determine the nature and distribution of the products. It has been made clear that “ideal” hydrocracking between n-alkanes and hydrogen should have the following features: low reaction temperatures, high selectivity for isomerization, and production of primary cracking products only.14 For ideal hydrocracking to occur both functions must be carefully balanced. Lately attention has been focused on the use of oxoanionpromoted zirconia catalysts that display high activity at low temperatures. Lower reaction temperatures favor the yield of branched isomers because the latter are more thermodynamically stable at these conditions. Lower coking rates can also be obtained, and the whole process can be driven at a lower hydrogen partial pressure with substantial cost savings. It has been suggested that these catalysts could have ideal hydrocracking properties if they were provided with a strong metal function. The most intensely studied catalysts have been SO42--ZrO2 and WO3-ZrO2 promoted with noble metals, such as Pt and Ni. The role of the metal promoters is still unclear. The isomerization of n-butane was studied by Yori et al.15 on Pt/WO3-ZrO2 catalysts and by Vera et al.16 on Pt/SO42--ZrO2 catalysts. Both found no correlation between chemisorption of hydrogen at room temperature, cyclohexane dehydrogenation activity, and the capacity to activate paraffins. The results seem to indicate that these catalysts do not operate with a conventional bifunctional mechanism involving hydrogenation/dehydrogenation. Addition of olefins has been reported to decrease the conversion of paraffins.17 This behavior is not expected for a classical bifunctional mechanism. Another strange feature is that in certain ranges the increase of hydrogen partial pressure enhances the reaction rate. To explain this effect, Shishido et al.18 and Parera and Fı´goli19 have postulated a nonconventional bifunctional mechanism for the transformation of alkanes on these catalysts: Atomic hydrogen activated on Pt would dissociate over strong Lewis acids forming H+ and H-, increasing the amount of surface Brønsted acid sites and enhancing the rates of proton-assisted carbenium ion formation and hydride transfer. In this work, the problem of balancing the strength of the metal and acid functions of hydroisomerization cracking catalysts of the oxoanion-promoted zirconia type was revisited. Promoted zirconia oxides of mild acidity (WO3-ZrO2) and strong acidity (SO42--ZrO2) were prepared and then promoted by adding a noble metal providing a strong metal function (Pt) or a nonnoble metal providing a weaker one (Ni). This series of catalysts was characterized by physicochemical techniques and tested in the hydroisomerization cracking of medium-length paraffins using n-octane and n-hexadecane as model compounds. The analysis of the reacted stream was focused on the calculation of its quality as contributor to the gasoline pool. The merit parameter used was the research octane number (RON), which (14) Martens, J. A.; Jacobs, P. A.; Weitkamp, J. Appl. Catal., A 1986, 20, 239. (15) Yori, J. C.; Pieck, C. L.; Parera, J. M. Appl. Catal., A 1999, 181, 5. (16) Vera, C. R.; Yori, J. C.; Pieck, C. L.; Parera, J. M. Appl. Catal., A 2003, 240, 161. (17) Venkatesh, K. R.; Hu, J.; Wang, W.; Holder, G. D.; Tierney, J. W.; Wender, I. Energy Fuels 1996, 10, 1163. (18) Shishido, T.; Tanaka, T.; Hattori, H. J. Catal. 1997, 172, 24. (19) Parera, J. M.; Fı´goli, N. S. Catalytic Naphtha Reforming, 2nd ed.; Marcel Dekker: New York, 2004; Chapter 3.

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was calculated from chromatographic data and a reported correlation. 2. Experimental Section 2.1. Catalysts Preparation. Zirconium hydroxide, Zr(OH)4, was obtained by hydrolysis and precipitation of zirconium oxychloride (Strem, 99.9998%). Sulfated Zr(OH)4 was prepared by incipient wetness impregnation of Zr(OH)4 with a volume of H2SO4 2 N equal to the pore volume plus a 10% excess (0.4 mL/g). The material was then dried at 120 °C overnight. Sulfated zirconia, SO42--ZrO2 (SZ sample) was finally obtained by calcination in an air flow (10 mL min-1) at 620 °C for 3 h. Tungsten-zirconia, WO3-ZrO2 (WZ sample), was obtained by wet impregnation of Zr(OH)4 with 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 (to get 15% W on the final catalyst). After the impregnation, the catalyst was dried in a stove at 110 °C overnight. Then it was calcined for 3 h at 800 °C. WZ and SZ (both ground to 35-80 mesh) were impregnated by incipient wetness using Ni(NO3)2‚6H2O (Merck, >99.0%) aqueous solutions. The concentration of the impregnating solutions was adjusted to obtain catalysts with different Ni contents (0.1, 0.5, 1.0, and 5%). The catalysts were then dried at 120 °C overnight. Before the reaction test, the catalysts were calcined in an air flow (10 mL min-1) for 3 h at 450 °C. The catalysts thus obtained were called xNi/SZ and xNi/WZ (x ) 0.1, 0.5, 1.0, 5.0). Portions of the WO3-ZrO2 and SO42--ZrO2 batches were also impregnated with solutions of chloroplatinic acid (Strem Chem., 99.9%) and calcined in the same way as the Ni catalysts. Catalysts with x% Pt (x ) 0.1, 0.5, 1.0, and 5.0) were thus obtained (xPtSZ catalysts, xPtWZ catalysts). 2.2. Catalyst Characterization. Temperature-programmed reduction tests were performed in an Ohkura TP-2002-S apparatus. X-ray diffraction spectra were recorded in a Shimadzu XD-1 spectrometer. Hydrogen adsorption isotherms were taken to calculate the dispersion of the Pt metal particles. The specific surface area was measured by N2 adsorption at the temperature of liquid nitrogen after degassing in vacuo at 200 °C for 2 h. A Micromeritics 2100E equipment was used for both H2 and N2 adsorption measurements. The amount of acid was assessed by means of temperature-programmed desorption of pyridine in a flow apparatus equipped with a FID detector. 2.3. Reactions Tests. Isomerization Cracking of n-Octane. An amount of catalyst to keep a constant mass of 0.5 g of SO42-ZrO2 in each test was loaded in a plug-flow reactor connected to an online gas chromatograph. Reaction conditions were: 300 °C, 0.1 MPa, WHSV ) 4 and H2/n-C8 ) 6 (molar ratio), total timeon-stream ) 10 h. Before the reaction, the catalysts were heated from room temperature to 300 °C in hydrogen (2.3 °C min-1 heating rate) and kept for 1 h at 300 °C and 0.1 MPa. The n-C8 feedstock was Carlo Erba pro analysis, and the hydrogen was supplied by AGA. The reactor was an AISI 304 stainless steel tube with a wall thickness of 5 mm, a diameter of 5 mm, and a height of 20 cm. The catalyst was supported in the middle of the reactor with a glasswool plug. Stainless steel 1/8” tubing with Swagelok fittings was used for the inlet and outlet connections. The pressure inside the reactor was controlled by a dome-type back-pressure regulator. Hydrocracking/Isomerization of n-Hexadecane. n-Hexadecane has been previously used by many authors to study hydroisomerization/cracking over SO42--ZrO2 and WO3-ZrO2 catalysts. Temperatures lower than 250 °C have been generally used in the case of SO42--ZrO2 because it is the most active catalyst and toohigh temperatures promote excessive cracking.20,21 In the case of the Pt/WO3-ZrO2 catalyst, the temperature has been varied in the 200-350 °C range.11,22 A temperature of 250 °C was considered (20) Keogh, R. A.; Sparks, D.; Hu, J.; Wender, I.; Tierney, J. W.; Wang, W.; Davis, B. H. Energy Fuels 1994, 8, 755. (21) Wen, M. Y.; Wender, I.; Tierney, J. W. Energy Fuels 1990, 4, 372.

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Figure 1. XRD spectrum of the 0.5NiWZ catalyst.

appropriate to compare both kinds of catalysts in our case. The reaction was performed in a batch autoclave reactor, using a reaction volume of 100 mL. The reactant/catalyst ratio was 8:1 on a weight base, and the stirring rate was 1200 rpm. In a typical experiment, after being calcined and reduced at high temperature, the catalyst sample was placed in a desiccator and cooled to room temperature. Then the sample was quickly placed in the autoclave reactor that was previously dried at 110 °C for 2 h. The reactor was purged with hydrogen and pressurized to 0.3 MPa. Then it was heated to 250 °C, stirred at 1200 rpm, and kept at these conditions for 1 h. Then the reactor was quickly cooled to room temperature. The gas phase was collected in an ampule, and then the reactor was opened. For the analysis of the liquid and gas phases a Varian Star 3400CX chromatograph and a capillary, 50 m, DB-1 Zebron column (Phenomenex) were used. Conversion was defined as the difference between the n-C16 weight percentage of the feed (100%) and that present in the products. The identification of hydrocarbon components was done by comparing retention times with known standards.

3. Results and Discussion 3.1. Catalyst Characterization. The presence of Ni or Pt in the small amounts used did not affect the specific surface area of the base WZ or SZ samples which had an area of about 39 m2 g-1 (WZ) and 110-120 m2 (SZ). The chemical analysis indicated that part of the initial sulfate on the SZ catalysts was decomposed at high temperatures during calcinations. Final S contents were between 1.6 and 2.0%. The presence of surface sulfates and tungstates inhibited crystallization, favored the formation of the tetragonal phase of zirconia, and stabilized the specific surface area. Most catalysts had a 100% tetragonal crystal phase (main peak at 2θ ) 30°) with no peaks addressable to the monoclinic phase (main peak at 2θ ) 28°). Only in the case of the WZ catalysts did additional peaks due to small segregated WO3 crystals appear. A typical spectrum that corresponds to the 0.5NiWZ catalyst can be seen in Figure 1. Peaks due to WO3 crystals can be found in the 22-25° range. TPR traces of the WZ catalysts used are included in Figure 2. In the case of the sulfated catalysts, only smooth monotonic TPR traces were obtained during reduction. The TPR traces of PtSZ and NiSZ showed no peaks that could be addressed to the reduction of Pt or Ni species. Reduction cannot be ruled out, however, because Pt species interact strongly with the SZ support and the great size of the sulfate reduction peak precludes a clear discrimination of reducible species in small proportion such as Ni and Pt. The absence of Pt reduction peaks in the TPR trace has indeed been considered a misleading fact of the PtSZ system which has Pt species in the metal state as confirmed by other more accurate techniques such as EXAFS.18 The TPR test performed on the 1%Pt/WZ samples shows a Pt reduction peak at about 100-120 °C and three other peaks (22) Walendziewski, J.; Pniak, B.; Malinowska, B. Chem. Eng. J. 2003, 95, 113.

Figure 2. Temperature-programmed reduction traces of the W catalysts. Table 1. Total Amount of Acid Sites and Acid Strength Distribution as Determined by Pyridine (Py) Desorption acidity distribution catalysts

total acidity (µmol Py/g)

150-300 °C (weak)

300-500 °C (mild)

500-600 °C (strong)

WZ 0.1NiWZ 0.5NiWZ 1.0NiWZ 5.0NiWZ 1.0PtWZ SZ 0.1NiSZ 0.5NiSZ 1.0NiSZ 5.0NiSZ 0.1PtSZ 0.5PtSZ 1.0PtSZ

90 57.6 137.6 166.4 120 238 44.4 147 188 218 280

37 3.07 2.28 4.6 44.4 57.2 1.4 52.4 118 129 56

43.2 28.6 78.8 97.8 63.58 83.4 24.6 76.8 52.0 75.0 184

9.36 28.7 59.4 78.6 10.3 97.0 18.4 17.8 17.0 14.0 106

corresponding to the successive reduction stages of WO3, with maxima at about 300-500 °C (WO3 f WO2.9), 700-750 °C (WO2.9 f WO2), and 820-900 °C (WO2 f W) [12]. The results corresponding to the Ni-containing samples show that, when Ni is deposited over WZ, different reduction patterns can be expected depending on the Ni content. Low Ni samples (0.1 and 0.5%) reduce differently than high Ni ones (1 and 5%). 0.1-0.5% Ni catalysts had a TPR trace with a small level of hydrogen consumption that started at 300 °C. This is also the temperature at which W species begin to reduce, and thus the consumption could not be safely addressed to the reduction of Ni species. Higher Ni contents, 1.0-5.0%, produced a change in the TPR curves which had a new bigger peak overimposed on the W reduction envelope. This peak at 250-400 °C can be attributed to the reduction of bulk NiO, known to occur at 250-320 °C. This peak was then attributed to the reduction of Ni oxidized species in a segregated state, with low interaction with the support. The absence of this peak in the 0.1-0.5% samples is thought to be related to a greater interaction of Ni with the support in the case of these samples. In summary, metallic Ni is only supposed to be found on the high Ni catalysts, 1.0-5.0%, and in a small fraction. Conversely, Pt is supposed to be in the metal state. Data of total acidity and distribution of acid strength are included in Table 1. The effect of Ni addition on the acidity is similar for both WZ and SZ catalysts. When a small amount of Ni is added (0.1%), a decrease in the total amount of acid sites of WZ and SZ occurs, though the effect is more drastic in the case of SZ. Further additions produce an increase in the total acidity, though the original level of the Ni-free sample cannot be restored in the case of SZ, even after 5.0% addition.

RON Enhancement of Long Paraffins

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Table 2. n-Octane and n-Hexadecane Hydroisomerization Crackinga n-octane catalysts C1-3 i-C4 n-C4 C5-6 C7-8 C1-3 0.1NiWZ 0.5NiWZ 1.0NiWZ 5.0NiWZ 1.0PtWZ 0.1NiSZ 0.5NiSZ 1.0NiSZ 5.0NiSZ 0.1PtSZ 0.5PtSZ 1.0PtSZ

0.4 2.8 11.9 14.6 4.5 8.2 12.2 12.1 8.9 16.0 18.1 17.8

1.2 8.0 27.5 28.8 9.8 28.8 38.4 37.6 31.6 32.8 33.6 33.7

0.6 4.1 15.0 16.0 5.1 4.5 6.7 6.3 5.2 18.0 21.3 25.4

n-hexadecane C4

C5-6 C7-8 C9-16

0.7 12.9 1.0 1.5 17.9 4.2 11.0 12.8 1.8 6.1 9.7 19.4 13.7 5.2 6.9 27.1 10.1 1.2 8.7 5.2 17.3 12.7 0.9 12.6 1.0 9.3 16.9 6.6 11.6 1.3 21.8 1.0 25.0 0.6 22.0 0.6 -

3.7 5.6 2.0 5.0 -

21.4 37.9 42.4 43.8 -

a Values of yield for some chosen products and carbon number ranges (n-octane and n-hexadecane not included).

Figure 3. RON gain as a function of the Ni and Pt content. n-Octane hydroisomerization.

Conversely, in the case of NiWZ, the total amount of acid sites is almost doubled after addition of 1% Ni. With respect to the relative strength of the original and new acid sites, the acidity distribution reveals different results for SZ and WZ after Ni addition. This addition produces mainly medium and strong acid sites on WZ and mainly weak sites on SZ. The influence of Pt addition is different. Pt addition increases the total amount of acid sites both on SZ and on WZ and always enhances the fraction of mild strength acid sites irrespective of the support. 3.2. Hydroisomerization Cracking Tests. Table 2 contains the values of the yield to different products for the reactions of hydroisomerization of n-octane and n-hexadecane. In the case of the n-octane reaction, alkanes with carbon numbers exceeding the n-C8 of the feed were absent. This result favors a unimolecular mechanism for hydrocracking of n-C8 alkanes over metal-promoted oxoanion-promoted zirconia. This observation would suggest that the mechanism for isomerization and cracking of n-C4 is different from the mechanism by which the same reactions take place in the case of higher alkanes. n-C5 is an abundant species found among the products of n-butane isomerization, and it comes from the cracking of an adsorbed C8 dimer.23 Although dimerization does not occur, cracking of an adsorbed C8 species seems to be also the dominant route for n-C8 reaction over oxoanion-promoted catalysts in this case. The molar ratio C5-7/C1-3 varies between 1 and 1.3 for all the catalysts, indicating that the fractions lower and higher than four carbon atoms seemingly come from the dismutation of the C8 species. In any case, isobutane is the most abundant product of the n-C8 reaction. A close inspection of the n-butane and isobutane yields indicates that the iso/normal ratio is about 2.0 for all NiWZ and PtWZ catalysts, 5.5 for the NiSZ catalysts, and 1.5 for the PtSZ catalysts. The high iso/normal ratio in the case of the NiSZ catalyst could be related to the low fraction of very strong acid sites of these catalysts, a fact that would prevent the secondary cracking of isobutene to lighter gases. The comparison between the Ni- and the Pt-promoted catalysts discloses also a basic distinction: SZ is more suited to isocracking of alkanes, which is reflected in the case of the n-C8 test in the high selectivity to isobutane and isopentane (4548% yield). WZ is more selective to isomerization products. The yield of methylheptanes on 1%PtWZ is 17% and is less than 1% on any PtSZ catalyst. The yield of dimethylhexanes on 1%PtWZ is 4.3% and practically zero on the PtSZ catalysts.

In the case of the reaction of n-hexadecane on the 0.1 and 1.0% Ni-promoted SZ and WZ catalysts, it can be seen that isobutane is no longer the product with the highest yield. Hydrocarbons in the C9-C16 range now provide 20-40% of the yield. C5-6 hydrocarbons are produced with a selectivity of 20-40% in all cases and the major products of the C5-8 carbon range. These compounds as well as the C7-8 ones are important for their contribution to the gasoline pool if their degree of branching is adequate. WZ catalysts provide the highest selectivity to C5-C8 isomers. An analysis of the products of the C6 fraction gives an idea of the skeletal isomerization activity of the Ni catalysts. No trimethylated species were detected, and the yield of mono/dimethyl species for this fraction was: 0.1NiSZ ) 83/17, 0.1NiWZ ) 78/22, 1.0 NiSZ ) 71.2/28.8, and 1.0 NiWZ ) 77.4/22.6. For the same amount of Ni, SZ catalysts have a higher activity than the WZ catalysts, but their selectivity for the production of light gases is too high, with i-C4 being the major product. All the results indicate that 1.0NiWZ is the best catalyst of the Ni-tested catalysts for the hydroisomerization of n-hexadecane. The antiknocking quality of the product in the reaction of n-C8 was assessed by means of the calculation of the RON and is included in Figures 3-5. A chromatographic nonlinear estimation method was used.24 The results of the runs with n-C8 were chosen because all compounds could be accurately measured and identified, something that was more difficult in the case of the n-hexadecane reaction. The total RON of the mixture was calculated as the sum of the weighed contributions of the individual components. Only liquid products entered the

(23) Vera, C. R.; Pieck, C. L.; Shimizu, K.; Querini, C. A.; Parera, J. M. J. Catal. 1999, 187, 39.

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

Figure 4. Linear correlation between the production of light gases and the increase in RON points.

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Figure 5. RON gain as a function of the number of acid sites. (a) All acid sites. (b) Strong acid sites.

summation. The RON gain in the hydroisomerization reactor was calculated as the difference between the RON of the products mixture and the RON of n-octane (-19). A plot of the RON gain as a function of the metal promoter content is included in Figure 3. From a RON gain base level corresponding to the 0.1%Ni and 0.1%Pt catalysts, further metal additions increased the RON gain in the case of the PtSZ, NiWZ, and PtWZ catalysts. Only NiSZ did not follow this trend, and the RON gain in the reactor had a maximum at 0.5%Ni and decreased at higher loads. PtSZ had the best performance of all catalysts, and the RON gain was 100-112 for all of the samples tested. The effect of the metal promotion was, however, more important in the case of the NiWZ catalysts. The low activity of the 0.1%NiWZ was increased 8-fold after increasing the Ni content to 5% and the RON gain reached 83. The production of light gases (not included) closely mimicks the pattern of the RON gain. The reasons become clear when the yields to each product are examined (Table 2). SZ catalysts are more active but are less selective to C7-8 isomers and have an important cracking contribution. WZ catalysts are less active but are more selective to isomers and produce fewer cracking products. In other words, the increase in the isomerizate yield goes parallel with the increase in the cracking yield. This is better analyzed by pairing the data of the yield to light gases and the RON gain (Figure 4). The linear plot resembles the common pattern in many refinery units: higher severity conditions demanded to increase the yield accelerate the irreversible production of unwanted byproducts. The higher severity is associated here with catalysts of higher activity. The key intrinsic feature to be analyzed to correlate the octane gain seems to be the acidity. Correlation plots between RON gain and acidity measures are included in Figure 5. Figure 5b contains too many outliers, and for this reason a relation between the RON gain and the amount of strong acid sites seems inappropriate. The RON gain is better correlated to the total amount of acid sites, as indicated in Figure 4a. This is not surprising because the reactivity of the paraffins increases with their length, and the hydroisomerization of long n-paraffins such as C8-16 can be expected to proceed on sites of both low and high acid strength. It has been propos25,26 that the reactivity of n-paraffins is expected to be proportional to the number of

secondary carbon atoms per molecule or to Cn-6 for hydrocracking and Cn-4 for hydroisomerization. 4. Conclusions Hydroisomerization cracking over nickel/sulfated-zirconia (NiSZ), nickel/tungsten-zirconia (NiWZ), platinum/sulfate-zirconia (PtSZ), and platinum/tungsten-zirconia (PtWZ) “fresh” catalysts provides a variable gain of 10-113 RON points over those of a model feedstock of n-octane. This octane improvement is fairly insensitive to the metal load in the case of the PtSZ and NiSZ catalysts and is highly dependent on the Ni content in the case of the NiWZ catalyst. A close linear relationship between the production of light gases (C1-C3) and the RON gain indicates that octane number improvement can only be reached at the expense of the isomerizate yield. PtSZ catalysts are the most active, and depending on the Pt content they provide a 100-113 RON upgrade when isomerizing n-octane. These catalysts are especially selective to isocracking and are not suitable for reacting n-hexadecane because of their relatively high cracking activity and formation of light gases. PtWZ catalysts are especially selective in isomerization. In the reaction of n-octane, they provide 42 RON points at the reaction conditions used (300 °C, 0.1 MPa, WHSV ) 4 h-1, H2/n-C8 ) 6) while Ni-based catalysts provide a moderate RON increase of 79-83 points. In the reaction of n-hexadecane, NiWZ catalysts provide the highest selectivity to the C5-C8 fraction and a low selectivity to light gases, though monobranched isomers are the main species formed. Acknowledgment. This research effort was financed with the support of the National Research Council (CONICET, Argentina), through PEI Grants 6544, 6546, and 6550, and the support of the National Agency for the Promotion of Science and Technology (ANPCyT, Argentina) through 2002 PICT Grant 14-08282. EF050092J (25) Sie, S. T. Ind. Eng. Chem. Res. 1993, 32, 403. (26) Carabineiro, H.; Pinheiro, C. I. C.; Lemos, F.; Ramoˆa Ribeiro, F. Chem. Eng. Sci. 2004, 59, 1221.