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The effect of electric type of platinum complex ion on the isomerization activity of Pt-loaded sulfated zirconia-alumina
Applied Catalysis A: General 251 (2003) 285–293
The effect of electric type of platinum complex ion on the isomerization activity of Pt-loaded sulfated zirconia-alumina Satoshi Furuta∗ Petroleum Refining Research & Technology Center, Japan Energy Corporation, Toda, Saitama 335-8502, Japan Received 7 September 2002; received in revised form 15 December 2002; accepted 17 April 2003
Abstract Six kinds of platinum-loaded sulfated zirconia-alumina (Pt-SZA) were prepared with the intention of investigating the effect of the electric types of platinum compounds, the amount of platinum, and the pH value of the impregnated material. For the effect of electric type, the platinum particles in catalysts prepared by using a cationic platinum compound, [(NH3 )4 Pt]2+ , showed much higher dispersion and more even distribution than those in catalysts prepared by an anionic compound, [PtCl6 ]2− , because of strong interaction of the cationic platinum compound with SZA. The above two catalysts showed almost the same isomerization activities in the synthesis of normal hexane, but the catalyst prepared by the cationic platinum compound showed higher isomerization activity when normal hexane containing 6% of benzene was used as feedstock. The catalysts with 0.33 and 0.50 wt.% amounts of Pt prepared by using the cationic platinum compound showed almost the same activities, even for the feedstock mixed with 6% of benzene. The catalyst with 0.11 wt.% of Pt also showed nearly the same activity as those two catalysts to the normal hexane, but its activity dropped remarkably with the feedstock with 6% of benzene. The reduction of activity was based on the amount of Pt, 0.11 wt.%, being insufficient for the ring-opening hydrogenation of the cycloalkane. As for the effect of pH of the cationic Pt complex ion solution, pH around 7 was desirable. The life of the catalyst (0.33 wt.% Pt) was evaluated for 1800 h by using commercial light-naphtha. The catalyst showed above 80 of research octane number (RON) and above 0.7 of iso-pentane/C5 alkane ratio of liquid product without showing any deactivation. The catalyst prepared by the anionic platinum compound also showed high performance. However, a much higher amount of coke was detected than for that prepared by the cationic one. The differences of the activity in existing benzene and the coke deposition of the catalyst in the life evaluation are due to the dispersion of platinum particles, which have influence upon the ability to supply spillover hydrogen. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Sulfated zirconia; Alumina; Platinum; Crystallite; Dispersion; Isomerization; Cation; Benzene; Hexane
1. Introduction Sulfated zirconia (SZ) is a novel solid acid catalyst that is well known as a solid superacid [1,2]. ∗ Tel.: +81-48-433-2105; fax: +81-48-441-4329. E-mail address: [email protected] (S. Furuta).
This catalyst shows excellent performance in various acid-catalyzed reactions (isomerization, esterification, acylation, alkylation, etherification, polymerization, cracking, glycosidation, etc.). Many researchers have reported the isomerization activity of SZ loaded with VIII group metals [3–9]. Among the results of research in our laboratory, platinum-loaded sulfated
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zirconia-alumina (Pt-SZA) pellets, prepared by our original method, showed excellent performance in light-naphtha isomerization, the isomerization activity being higher than that of Pt-loaded sulfated zirconia itself [10]. In order for producers to reduce the harmful components such as benzene in gasoline and to increase the octane number, the isomerization process of light-naphtha is a key technology. The catalysts of zeolite and chlorinated alumina are generally used for the isomerization of light-naphtha; the former needs higher temperature, which makes the octane number lower. Two of the attractive features of zeolite are that the catalysts are tolerant of contaminants and that they are regenerable. The chlorinated alumina catalysts are very sensitive to contaminants such as water, carbon oxides, oxygenates, and sulfur; a licenser comments that they are not regenerable. Thus, feeds and hydrogen must be hydrotreated and dried to remove water and sulfur. Furthermore, the chlorinated alumina catalysts require the addition of organic chloride to the feed in order to maintain their activities. This causes contamination in the waste gas of hydrogen chloride; a scrubber is needed to remove such contamination. The Pt-SZA catalyst can make full use of its ability as an isomerization catalyst at temperatures lower than zeolitic catalysts need, and it has good tolerance to contaminants so as to be regenerable. Therefore, this new catalyst is expected to be one of the best candidates for replacing the conventional catalysts. In this paper, the effects of electric type of Pt complex ions, [(NH3 )4 Pt]2+ or [PtCl6 ]2− are described, along with the Pt amount with respect to activity for the isomerization of light-naphtha.
Table 1 The differences of platinum impregnation Catalyst
Pt compound
Pt amount (wt.%)
pH of Pt solution
A B C D E F
[(NH3 )4 Pt]Cl2 H2 [PtC16 ] [(NH3 )4 Pt]Cl2 [(NH3 )4 Pt]Cl2 [(NH3 )4 Pt]Cl2 [(NH3 )4 Pt]Cl2
0.50 0.50 0.33 0.11 0.50 0.50
7.0 <1.0 7.0 7.0 9.6 1.8
form cylindrical pellets; these were dried at 130 ◦ C. The sulfated zirconia-alumina pellets were obtained (named “the support”) by calcining in air at 675 ◦ C for 1.5 h. Catalyst A: An aqueous solution of tetra-ammine platinum dichloride ([(NH3 )4 Pt]Cl2 ) was sprayed onto the support in such an amount as to give the platinum content of 0.50 wt.%. The sprayed pellets were dried and calcined in air at 680 ◦ C for 0.5 h to give a Pt-loaded sulfated zirconia-alumina pellet catalyst. Catalyst B: An aqueous solution of chloroplatinic acid (H2 [PtCl6 ]) was sprayed on the support and the above procedures were followed to give 0.50 wt.% Pt. Catalysts C and D: An aqueous solution of [(NH3 )4 Pt]Cl2 was sprayed onto the support and the above procedures were followed to give 0.33 and 0.11 wt.% Pt for catalysts C and D, respectively. Catalyst E and F: Aqueous solutions of [(NH3 )4 Pt] Cl2 , whose pH were adjusted to 9.6 with ammonia and 1.8 with hydrochloric acid for catalysts E and F, respectively, were sprayed on the support and the above procedures were followed to give 0.50 wt.% Pt. The differences of preparation are summarized in Table 1.
2. Experimental 2.2. Measurement methods of catalyst properties 2.1. Catalyst preparation A mixture of 1860 g of hydrated zirconia (amorphous) powder, 1120 g of hydrated alumina (pseudoboehmite) powder, 575 g of ammonium sulfate, and deionized water were put into a kneader with stirring vanes. The mixture was kneaded for 45 min in the kneader. The kneaded product was extruded through a circular opening of 1.6 mm in diameter to
Sulfur amount was determined by combustion infrared absorptiometry using a HORIBA EMIA-610FA apparatus. BET surface area, pore volume, and average pore diameter were determined by N2 adsorption using a Micromeritics ASAP 2400 apparatus. Side crushing strength was measured by use of a tablet crushability tester (Toyama TH-203CP, measurement count = 20). Compacted bulk density was measured
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Table 2 Reaction conditions (simple once-through reaction)
Table 4 Composition and properties of light-naphtha
Catalyst Reactor Catalyst volume (ml) Reaction temperature (◦ C) Reaction pressure (MPa) H2 /feed ratio (mol/mol) LHSV (h−1 ) Hydrogen purity (vol.%)
by use of a tap denser (Seishin, KYT-3000, tapping count = 100). Distribution of platinum in the direction of the cross-section of the pellets was determined by EPMA (JOEL JXA-8900R). Crystallite diameter of platinum particles was determined by XRD (RIGAKU RAD-1C).
Composition (wt.%) C3 iso-C4 n-C4 iso-C5 n-C5 neo-C5 22DMB Cyclopentane 23DMB 2MP 3MP n-C6 MCP Benzene Cyclohexane C7+
2.3. Reaction of isomerization
Density at 20 ◦ C (kg/m3 ) RON (calculated) C5 + RON (calculated)
645 71.2 69.8
Impurity (ppmw) Sulfur Nitrogen Water
<1.0 <1.0 40
Pt-loaded sulfated zirconia-alumina Isothermal 4.0 180 0.98 5.0 1.5 99.99
Isomerization reaction was conducted in a fixed-bed flow reactor. The standard reaction conditions are presented in Table 2. Compositions of the product were analyzed by gas chromatography using a 100 m capillary column (CP-Squalane) with FID at 30–90 ◦ C. Prior to each reaction, catalysts were dried and reduced according to the procedure shown in Table 3. Table 3 Activation process of catalyst Step
Gas
Temperature (◦ C)
Pressure (MPa)
Time (min)
Purge reactor
N2
RT
atm
10
Leak check Pressure drop
H2 H2
RT RT
0.98 0.98–atm
30 5
Purge reactor
N2
RT
atm
10
Calcination Hold temperature Reduce temperature
Air Air Air
RT–400 400 400–200
atm atm atm
40 60 40
Purge reactor Reduction Reduce temperature and increase pressure Feed in
N2 H2 H2
200 200 200–180
atm atm atm–0.98
10 60 30
H2
180
0.98
N2 and H2 : 99.99 vol.% pure. Air: compressed air dried by silica gel. GHSV: 1200 h−1 for all gases.
0.0 0.4 5.4 23.6 35.2 0.0 0.4 2.3 1.5 10.3 6.0 11.5 2.2 1.0 0.3 0.0
Two kinds of feedstock were prepared. One was normal hexane (Kanto Chemical Co. Ltd.), and the other was normal hexane mixed with benzene (6% of benzene content). Generally, the catalysts show rapid deactivation in the first step; they become stabilized after 50 h. The activity of each catalyst was compared with the average values 70 h after start-up. Several catalysts were examined in the reaction of commercial light-naphtha in order to evaluate their catalyst lives; each composition and the reaction conditions are shown in Tables 4 and 5, respectively.
Table 5 Reaction conditions for life test (simple once-through reaction) Catalyst Reactor Catalyst volume (ml) Reaction temperature (◦ C) Reaction pressure (MPa) H2 /feed ratio (mol/mol) LHSV (h−1 ) Hydrogen purity (vol.%)
Pt-loaded sulfated zirconia-alumina Isothermal 30.0 200 1.96 2.0 2.0 99.99
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Table 6 Fundamental catalyst properties
Average pellet diameter (mm) Sulfur content (wt.%) Surface area (m2 /g) Pore volume (ml/g) Average pore diameter (nm) Side crushing strength (kg/4.5 mm)
Catalyst A
Catalyst B
Catalyst C
Catalyst D
Catalyst E
Catalyst F
1.5 2.8 162 0.31 5.8 4.0
1.5 2.8 161 0.31 5.6 4.2
1.5 2.7 158 0.31 5.9 3.7
1.5 2.7 163 0.31 5.7 4.5
1.6 2.8 159 0.31 5.9 3.2
1.6 2.7 159 0.31 5.9 3.1
3. Results 3.1. Catalyst properties 3.1.1. Fundamental catalyst properties Table 6 indicates the properties of the catalysts. BET surface area, pore volume, and average pore diameter were almost identical in all catalysts, and their side crush strength values were high enough to allow loading into the reactor in a refinery. The elemental analysis showed no chloride or nitrogen. 3.1.2. Pt distribution by EPMA and crystallite diameter by XRD The effect of electric type of Pt compound, [(NH3 )4 Pt]2+ or [PtCl6 ]2− , reflected the distribution and crystallite diameter of platinum. The Pt compounds interacted with the surface of the support by their electrical types, and the interaction had an effect on the distribution and crystallite diameter of platinum. Fig. 1 indicates the distribution of platinum in catalysts A and B as determined by EPMA. The result of catalyst B, prepared by the conventional method, shows a quite uneven distribution of platinum in the direction of the cross-section, while a quite even distribution is found for catalyst A. The results also reflect the standard deviation divided by average frequency, as is shown in Table 7; these values become smaller as the distribution of Pt become evener. From the results of catalyst D, we can see that 0.11 wt.% of platinum
Fig. 1. EPMA profiles of catalysts A ( ) and B ( ).
is not enough to make an even distribution. It is apparent from the results of catalysts A, E and F, that the pH value of the Pt solution has little effect on the distribution of platinum particles. The crystallite diameter of platinum particles determined by XRD is also shown in Table 7. The crystallite diameter of platinum on catalyst B, obtained by using H2 [PtCl6 ], was much bigger than the values on the catalysts prepared with [(NH3 )4 Pt]Cl2 . 3.2. The effect of electric type of Pt compound on the isomerization activity The interaction of Pt compounds with the support also has a big effect on the isomerization activity
Table 7 XRD and EPMA characterization factors
Pt distribution (standard deviation/average frequency) Crystallite (nm)
Catalyst A
Catalyst B
Catalyst C
Catalyst D
Catalyst E
Catalyst F
0.18 8
0.51 4 × 102
0.27 9
0.67 10
0.20 9
0.24 9
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Fig. 2. Isomerization of n-C6 over catalysts A and B (n-C6 conversion): (䊐) n-C6; (䊏) n-C6 + 6% of benzene.
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In the product oil formed, catalyst A showed higher conversion values of cyclic compounds (benzene, cyclohexane, and methylcyclopentane (MCP)) than catalyst B did. The total amount of cyclic compounds over catalyst A reduced to about a half compared with that of the feedstock, while the amount in product B was about 6%, with many more cyclic compounds remaining. The hydroisomerization of benzene is known to proceed through three steps: hydrogenation, isomerization, and ring-opening; the reactions of hydrogenation and ring-opening consume hydrogen. The benzene hydrogenation step was very fast over both catalysts under the conditions, but those catalysts showed differences in the isomerization and ring-opening steps. The ring-opening reaction of methylcyclopentane to C6 alkanes by catalyst A was much more advanced than that by B. 3.3. The effect of Pt amount on the isomerization activity
Fig. 3. Isomerization of n-C6 over catalysts A and B (22DMB/C6 alkane): (䊐) n-C6; (䊏) n-C6 + 6% of benzene.
under the conditions in Table 2. Figs. 2 and 3 show the conversions of normal hexane (n-C6) and 2,2-dimethylbutane (22DMB) content in the alkanes of carbon number 6, respectively. The amount of cyclic compounds in the feedstock and the amounts of products A and B formed over catalysts A and B, respectively, are shown in Table 8. Catalysts A and B showed identical activities to n-C6 feedstock, but catalyst B showed lower activity of the n-C6 conversion and formation of 22DMB with 6% of benzene.
Fig. 4 shows the conversions of n-C6 and 22DMB content in the alkanes of carbon number 6 over catalysts A, C and D. The isomerization activities (both n-C6 conversion and 22DMB contents) of three catalysts were almost the same when using n-C6 as feedstock. However, when benzene was added, n-C6 conversion and 22DMB contents decreased as expected. The activity of catalyst D was reduced remarkably, while catalyst C showed identical activity to that of A. Catalyst C (0.33 wt.% Pt) has the same
Table 8 Composition (wt.%) of cyclic compounds Feedstock
Product A
Product B
Benzene Cyclohexane MCP
6.1 0.0 1.9
0.0 1.6 2.7
0.0 2.4 3.8
Total
8.0
4.3
6.2
Fig. 4. Isomerization of n-C6 over the catalysts with 0.11, 0.33 and 0.50 wt.% Pt (catalysts A, C and D): n-C6 conversion—(䊊) n-C6, (䊐) n-C6 + 6% of benzene; 22DMB/C6 alkane—(䊉) n-C6, (䊏) n-C6 + 6% of benzene.
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Table 9 The amounts (wt.%) of cyclic compounds Feedstock
Product A
Product C
Product D
Benzene Cyclohexane MCP
6.1 0.0 1.9
0.0 1.6 2.7
0.0 1.6 2.7
0.0 2.8 3.8
Total
8.0
4.3
4.3
6.6
tolerance for 6% of benzene as that of catalyst A (0.50 wt.% Pt); catalyst D (0.11 wt.% Pt) is inferior to those cases, in spite of having the same activity as n-C6. The results indicate that the surplus platinum does not promote the tolerance to benzene; thus, there must be an optimized amount of platinum related to the amount of benzene in feedstock. Table 9 indicates the composition of cyclic compounds in the product oil. As expected, in the product oil formed over catalyst D (product D), large amounts of cyclic compounds remained, but all the benzene was converted. The amount of cyclic compounds in product C is identical to that in product A. This result proves that 0.33 wt.% of platinum is enough for the feedstock with 6% of benzene. 3.4. The effect of pH to the isomerization activity
We tried to adjust pH of the H2 [PtCl6 ] solution with ammonia to 7, but white precipitation was formed by the addition of ammonia. It might not be desirable for the acidic platinate to add such an alkaline compound. 3.5. Isomerization of commercial light-naphtha In order for us to evaluate catalyst life, catalysts B and C were examined in the reaction of commercial light-naphtha. Fig. 6 indicates the research octane number of liquid product (C5 + RON), along with the iso-pentane/C5 alkane ratio and the 22DMB/C6 alkane ratio. Both catalysts B and C showed quite high activity for the long duration of 1800 h, and both catalysts showed almost the same activity without any decrease of RON value. We also could not find any significant difference between the two catalysts as regards C5 + RON, iso-pentane/C5 alkane ratio, or 22DMB/C6 alkane ratio in the product. However, coke on the spent catalysts showed big differences. Fig. 7 shows the amount of coke accumulation on each catalyst. Spent catalysts were divided into five parts along the reactor axis and were analyzed. On catalyst C, the coke accumulation was much less than
Fig. 5 shows effects of pH of the Pt impregnating solution on the conversions of n-C6 and 22DMB content in the alkanes of carbon number 6. Catalyst A, in which the pH of the Pt solution was 7.0, showed activity higher than those of catalysts E and F did. It is considered that the desirable pH of an impregnate is around 7.
Fig. 5. Isomerization of n-C6, an effect of pH of Pt solution on the catalytic activity: (䊊) n-C6 conversion; (䊉) 22DMB/C6 alkane.
Fig. 6. Isomerization of light-naphtha: (䊊) C5 + RON, () iso-pentane/C5 alkane; (䊐) 22DMB/C6 alkane.
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Fig. 7. Coke on the spent catalysts B (䊏) and C (䊊).
that on catalyst B, and there was little coke from the second to the bottom layer. This result indicates that catalyst C has a much longer life than catalyst B. The distribution of coke was also characteristic; the coke was concentrated on the top layer of the catalyst.
4. Discussion 4.1. Catalyst properties The electrical type of Pt compound influences Pt distribution and its crystallite diameter. Chloroplatinic acid is in the anionic state, [PtCl6 ]2− , while tetra-ammine platinum dichloride is in the cationic state, [(NH3 )4 Pt]2+ , in an aqueous solution. The [(NH3 )4 Pt]2+ cation has electrical attraction to the surface of the support covered by SO4 2− anion and shows little migration during drying and calcination. On the other hand, [PtCl6 ]2− has electrical repulsion to the surface of the support; this gives rise to cohesion of platinum particles by migration. Furthermore, platinum crystals on the cross-section of catalyst B were observed by scanning electron microscopy (SEM), but platinum particles could not be observed on the cross-section of catalyst A. The results indicate that the platinum in catalyst A is dispersed more than that in catalyst B. The crystallite diameter of platinum showed little difference among catalysts A, C, D, E and F, indicating that platinum particles did not migrate by drying and calcining when the cationic species, [(NH3 )4 Pt]2+ , were impregnated to the support.
291
Fig. 8. Reaction image of benzene, cyclohexane, and methylcyclopentane.
The pH value of Pt solution has no effect on the dispersion and crystallite size of platinum particles on the catalyst surface. 4.2. The effect of electric type of Pt compound on the isomerization activity The difference of tolerance to benzene is presumably due to the contact probability of the spillover hydrogen and benzene or other cyclic compounds. The effect of the spillover hydrogen molecules in the catalytic reaction has been proposed by many researchers [11–14]. Fig. 8 shows the conceptual pictures of the reaction of cyclic compounds. Catalyst A has higher hydrogenation activity because of its high dispersion of platinum. In this case, it is considered that the spillover hydrogen exists evenly on the catalyst surface, and the cyclic compounds adsorbed on the catalyst contact with spillover hydrogen more easily. On the other hand, catalyst B cannot exhibit enough hydrogenation activity enough for the ring-opening reaction of methylcyclopentane, because of the low dispersion of platinum. In this case, the spillover hydrogen presumably is present unevenly. The ring-opening reaction, which needs hydrogen, is controlled because of the decline of the contact probability to the spillover hydrogen. As a result, the isomerization of cyclohexane is controlled by the equilibrium. The conversion of benzene to C6 alkanes via hydrogenation, isomerization, and ring-opening reactions is known [15]. The quantitative comparison of reactivity among cyclohexane, MCP, and n-C6 is
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not clear, but the adsorption of n-C6 is obstructed by hydrogenation or isomerization of these cyclic compounds. 4.3. The effect of Pt amount on the isomerization activity The catalytic activity also relates to the amount of platinum. The reason for a remarkable drop in activity for catalyst D is coincident with the case of catalyst B; that is, the platinum particles cannot supply enough spillover hydrogen to methylcyclopentane. However, the size of platinum particles in catalyst D is slightly different from that in catalyst A. The number of platinum particles on the surface of catalyst D is fewer, and the distances of platinum particles are longer than those of catalyst A. It is considered that 0.33 wt.% of platinum is enough amount of platinum for 6% of benzene, but 0.50 wt.%, the surplus platinum, also has some ability to supply spillover hydrogen, and 0.50 wt.% of platinum will surely show higher tolerance to the high content of cyclic compounds. 4.4. Isomerization of commercial light-naphtha Catalyst C showed almost the same activity as catalyst B, on account of the small amount (3.5 wt.%) of C6 cyclic compounds in the feedstock. Even when one uses this feedstock, the distribution of platinum has an effect on the amount of coke on catalyst. These results support the hypothesis in Section 4.2. Catalyst B should have almost the same amount of active sites as catalyst C, and the same amount of cyclic compounds in the feedstock should adsorb to these active sites. In the case of catalyst C, spillover hydrogen is supplied to the cyclic compounds that adsorbed to the active sites, and the cyclic compounds desorbed easily, due to the even distribution of the platinum. However, on catalyst B, the only cyclic compounds that adsorb to the neighborhood of the platinum have high probability of contact with the spillover hydrogen, and the others could make the coke because of the much slower desorption. The reason of coke deposition on the catalyst in the top layer is presumably due to the rapid exothermal reactions (hydrogenation of benzene, cracking of branched alkane, ring-opening reaction, etc.).
5. Conclusion Two kinds of platinum compounds were used for preparing Pt-loaded sulfated zirconia-alumina catalyst. The cationic compound of [(NH3 )4 Pt]Cl2 showed dispersion of the Pt particles higher than that produced by the anionic compound of H2 [PtCl6 ], together with higher isomerization activity to a feedstock mixed with 6% of benzene. The difference of activity was brought about by the difference in ability to supply spillover hydrogen based on the platinum dispersion, owing to the electric interaction of platinum compounds with the support. When [(NH3 )4 Pt]Cl2 , was used, 0.11, 0.33 and 0.50 wt.% Pt-supported catalysts showed nearly the same isomerization activities to normal hexane as a feedstock. However, to the feedstock of synthesized normal hexane containing 6% of benzene, the activities of all of the three catalysts declined because of the shortage of their ability to supply spillover hydrogen owing to the amount of platinum, though 0.33 and 0.50 wt.% Pt-supported catalysts experienced smaller drops in activities compared with that of the 0.11 wt.% one. As for the pH value of the Pt solution, the catalytic activity was affected a little, and a pH around 7 was desirable. The results of the 1800 h life test with commercial light-naphtha indicated that the catalyst impregnated with [(NH3 )4 Pt]Cl2 (0.33 wt.% of Pt) gave quite high isomerization activity with much smaller amount of coke compared with the catalyst impregnated with H2 [PtCl6 ] (0.50 wt.% of Pt), resulting in the long life of the former catalyst. These results can be also explained by the ability to supply spillover hydrogen owing to the state and amount of the platinum. References [1] X. Song, A. Sayari, Catal. Rev. Sci. Eng. 38 (1996) 329. [2] K. Arata, M. Hino, Appl. Catal. A 146 (1996) 3. [3] R.A. Comelli, S.A. Canavese, S.R. Vaudagna, N.S. Figoli, Appl. Catal. A 135 (1996) 287. [4] M. Hino, K. Arata, J. Chem. Soc., Chem. Commun. 7 (1995) 789. [5] H. Matsuhashi, H. Shibata, K. Arata, Prepr. Am. Chem. Soc., Div. Pet. Chem. 42 (1997) 748. [6] J.M. Grau, J.C. Yori, J.M. Parera, Appl. Catal. A 213 (2001) 247. [7] S.R. Vaudagna, R.A. Comelli, N.S. Figoli, React. Kinet. Catal. Lett. 58 (1996) 111.
S. Furuta / Applied Catalysis A: General 251 (2003) 285–293 [8] H. Liu, V. Adeeva, G.D. Lei, W.M.H. Sachtler, J Mol. Catal. A: Chem. 100 (1995) 35. [9] F. Garin, D. Andriamasinoro, A. Abdulsamad, J. Sommer, J. Catal. 131 (1991) 199. [10] K. Matsuzawa, Patent WO9703092 (1997), to Japan Energy Corporation. [11] K. Fujimoto, in: Proceedings of the Third International Conference on Spillover, Kyoto, Elsevier, Amsterdam, 1993, p. 9.
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[12] A.M. Stumbo, P. Grange, B. Delmon, in: Proceedings of the 11th International Congress on Catalysis, Baltimore, United States, 1996, p. 97. [13] B. Delmon, Bull. Soc. Chim. Belg. 104 (1995) 173. [14] H. Hattori, T. Yamada, T. Shishido, Res. Chem. Intermed. 24 (4) (1998) 439. [15] K. Shimizu, T. Sunagawa, C.R. Vera, K. Ukegawa, Appl. Catal. A 206 (2001) 79.
The effect of electric type of platinum complex ion on the isomerization activity of Pt-loaded sulfated zirconia-alumina Satoshi Furuta∗ Petroleum Refining Research & Technology Center, Japan Energy Corporation, Toda, Saitama 335-8502, Japan Received 7 September 2002; received in revised form 15 December 2002; accepted 17 April 2003
Abstract Six kinds of platinum-loaded sulfated zirconia-alumina (Pt-SZA) were prepared with the intention of investigating the effect of the electric types of platinum compounds, the amount of platinum, and the pH value of the impregnated material. For the effect of electric type, the platinum particles in catalysts prepared by using a cationic platinum compound, [(NH3 )4 Pt]2+ , showed much higher dispersion and more even distribution than those in catalysts prepared by an anionic compound, [PtCl6 ]2− , because of strong interaction of the cationic platinum compound with SZA. The above two catalysts showed almost the same isomerization activities in the synthesis of normal hexane, but the catalyst prepared by the cationic platinum compound showed higher isomerization activity when normal hexane containing 6% of benzene was used as feedstock. The catalysts with 0.33 and 0.50 wt.% amounts of Pt prepared by using the cationic platinum compound showed almost the same activities, even for the feedstock mixed with 6% of benzene. The catalyst with 0.11 wt.% of Pt also showed nearly the same activity as those two catalysts to the normal hexane, but its activity dropped remarkably with the feedstock with 6% of benzene. The reduction of activity was based on the amount of Pt, 0.11 wt.%, being insufficient for the ring-opening hydrogenation of the cycloalkane. As for the effect of pH of the cationic Pt complex ion solution, pH around 7 was desirable. The life of the catalyst (0.33 wt.% Pt) was evaluated for 1800 h by using commercial light-naphtha. The catalyst showed above 80 of research octane number (RON) and above 0.7 of iso-pentane/C5 alkane ratio of liquid product without showing any deactivation. The catalyst prepared by the anionic platinum compound also showed high performance. However, a much higher amount of coke was detected than for that prepared by the cationic one. The differences of the activity in existing benzene and the coke deposition of the catalyst in the life evaluation are due to the dispersion of platinum particles, which have influence upon the ability to supply spillover hydrogen. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Sulfated zirconia; Alumina; Platinum; Crystallite; Dispersion; Isomerization; Cation; Benzene; Hexane
1. Introduction Sulfated zirconia (SZ) is a novel solid acid catalyst that is well known as a solid superacid [1,2]. ∗ Tel.: +81-48-433-2105; fax: +81-48-441-4329. E-mail address: [email protected] (S. Furuta).
This catalyst shows excellent performance in various acid-catalyzed reactions (isomerization, esterification, acylation, alkylation, etherification, polymerization, cracking, glycosidation, etc.). Many researchers have reported the isomerization activity of SZ loaded with VIII group metals [3–9]. Among the results of research in our laboratory, platinum-loaded sulfated
0926-860X/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(03)00360-0
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zirconia-alumina (Pt-SZA) pellets, prepared by our original method, showed excellent performance in light-naphtha isomerization, the isomerization activity being higher than that of Pt-loaded sulfated zirconia itself [10]. In order for producers to reduce the harmful components such as benzene in gasoline and to increase the octane number, the isomerization process of light-naphtha is a key technology. The catalysts of zeolite and chlorinated alumina are generally used for the isomerization of light-naphtha; the former needs higher temperature, which makes the octane number lower. Two of the attractive features of zeolite are that the catalysts are tolerant of contaminants and that they are regenerable. The chlorinated alumina catalysts are very sensitive to contaminants such as water, carbon oxides, oxygenates, and sulfur; a licenser comments that they are not regenerable. Thus, feeds and hydrogen must be hydrotreated and dried to remove water and sulfur. Furthermore, the chlorinated alumina catalysts require the addition of organic chloride to the feed in order to maintain their activities. This causes contamination in the waste gas of hydrogen chloride; a scrubber is needed to remove such contamination. The Pt-SZA catalyst can make full use of its ability as an isomerization catalyst at temperatures lower than zeolitic catalysts need, and it has good tolerance to contaminants so as to be regenerable. Therefore, this new catalyst is expected to be one of the best candidates for replacing the conventional catalysts. In this paper, the effects of electric type of Pt complex ions, [(NH3 )4 Pt]2+ or [PtCl6 ]2− are described, along with the Pt amount with respect to activity for the isomerization of light-naphtha.
Table 1 The differences of platinum impregnation Catalyst
Pt compound
Pt amount (wt.%)
pH of Pt solution
A B C D E F
[(NH3 )4 Pt]Cl2 H2 [PtC16 ] [(NH3 )4 Pt]Cl2 [(NH3 )4 Pt]Cl2 [(NH3 )4 Pt]Cl2 [(NH3 )4 Pt]Cl2
0.50 0.50 0.33 0.11 0.50 0.50
7.0 <1.0 7.0 7.0 9.6 1.8
form cylindrical pellets; these were dried at 130 ◦ C. The sulfated zirconia-alumina pellets were obtained (named “the support”) by calcining in air at 675 ◦ C for 1.5 h. Catalyst A: An aqueous solution of tetra-ammine platinum dichloride ([(NH3 )4 Pt]Cl2 ) was sprayed onto the support in such an amount as to give the platinum content of 0.50 wt.%. The sprayed pellets were dried and calcined in air at 680 ◦ C for 0.5 h to give a Pt-loaded sulfated zirconia-alumina pellet catalyst. Catalyst B: An aqueous solution of chloroplatinic acid (H2 [PtCl6 ]) was sprayed on the support and the above procedures were followed to give 0.50 wt.% Pt. Catalysts C and D: An aqueous solution of [(NH3 )4 Pt]Cl2 was sprayed onto the support and the above procedures were followed to give 0.33 and 0.11 wt.% Pt for catalysts C and D, respectively. Catalyst E and F: Aqueous solutions of [(NH3 )4 Pt] Cl2 , whose pH were adjusted to 9.6 with ammonia and 1.8 with hydrochloric acid for catalysts E and F, respectively, were sprayed on the support and the above procedures were followed to give 0.50 wt.% Pt. The differences of preparation are summarized in Table 1.
2. Experimental 2.2. Measurement methods of catalyst properties 2.1. Catalyst preparation A mixture of 1860 g of hydrated zirconia (amorphous) powder, 1120 g of hydrated alumina (pseudoboehmite) powder, 575 g of ammonium sulfate, and deionized water were put into a kneader with stirring vanes. The mixture was kneaded for 45 min in the kneader. The kneaded product was extruded through a circular opening of 1.6 mm in diameter to
Sulfur amount was determined by combustion infrared absorptiometry using a HORIBA EMIA-610FA apparatus. BET surface area, pore volume, and average pore diameter were determined by N2 adsorption using a Micromeritics ASAP 2400 apparatus. Side crushing strength was measured by use of a tablet crushability tester (Toyama TH-203CP, measurement count = 20). Compacted bulk density was measured
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Table 2 Reaction conditions (simple once-through reaction)
Table 4 Composition and properties of light-naphtha
Catalyst Reactor Catalyst volume (ml) Reaction temperature (◦ C) Reaction pressure (MPa) H2 /feed ratio (mol/mol) LHSV (h−1 ) Hydrogen purity (vol.%)
by use of a tap denser (Seishin, KYT-3000, tapping count = 100). Distribution of platinum in the direction of the cross-section of the pellets was determined by EPMA (JOEL JXA-8900R). Crystallite diameter of platinum particles was determined by XRD (RIGAKU RAD-1C).
Composition (wt.%) C3 iso-C4 n-C4 iso-C5 n-C5 neo-C5 22DMB Cyclopentane 23DMB 2MP 3MP n-C6 MCP Benzene Cyclohexane C7+
2.3. Reaction of isomerization
Density at 20 ◦ C (kg/m3 ) RON (calculated) C5 + RON (calculated)
645 71.2 69.8
Impurity (ppmw) Sulfur Nitrogen Water
<1.0 <1.0 40
Pt-loaded sulfated zirconia-alumina Isothermal 4.0 180 0.98 5.0 1.5 99.99
Isomerization reaction was conducted in a fixed-bed flow reactor. The standard reaction conditions are presented in Table 2. Compositions of the product were analyzed by gas chromatography using a 100 m capillary column (CP-Squalane) with FID at 30–90 ◦ C. Prior to each reaction, catalysts were dried and reduced according to the procedure shown in Table 3. Table 3 Activation process of catalyst Step
Gas
Temperature (◦ C)
Pressure (MPa)
Time (min)
Purge reactor
N2
RT
atm
10
Leak check Pressure drop
H2 H2
RT RT
0.98 0.98–atm
30 5
Purge reactor
N2
RT
atm
10
Calcination Hold temperature Reduce temperature
Air Air Air
RT–400 400 400–200
atm atm atm
40 60 40
Purge reactor Reduction Reduce temperature and increase pressure Feed in
N2 H2 H2
200 200 200–180
atm atm atm–0.98
10 60 30
H2
180
0.98
N2 and H2 : 99.99 vol.% pure. Air: compressed air dried by silica gel. GHSV: 1200 h−1 for all gases.
0.0 0.4 5.4 23.6 35.2 0.0 0.4 2.3 1.5 10.3 6.0 11.5 2.2 1.0 0.3 0.0
Two kinds of feedstock were prepared. One was normal hexane (Kanto Chemical Co. Ltd.), and the other was normal hexane mixed with benzene (6% of benzene content). Generally, the catalysts show rapid deactivation in the first step; they become stabilized after 50 h. The activity of each catalyst was compared with the average values 70 h after start-up. Several catalysts were examined in the reaction of commercial light-naphtha in order to evaluate their catalyst lives; each composition and the reaction conditions are shown in Tables 4 and 5, respectively.
Table 5 Reaction conditions for life test (simple once-through reaction) Catalyst Reactor Catalyst volume (ml) Reaction temperature (◦ C) Reaction pressure (MPa) H2 /feed ratio (mol/mol) LHSV (h−1 ) Hydrogen purity (vol.%)
Pt-loaded sulfated zirconia-alumina Isothermal 30.0 200 1.96 2.0 2.0 99.99
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Table 6 Fundamental catalyst properties
Average pellet diameter (mm) Sulfur content (wt.%) Surface area (m2 /g) Pore volume (ml/g) Average pore diameter (nm) Side crushing strength (kg/4.5 mm)
Catalyst A
Catalyst B
Catalyst C
Catalyst D
Catalyst E
Catalyst F
1.5 2.8 162 0.31 5.8 4.0
1.5 2.8 161 0.31 5.6 4.2
1.5 2.7 158 0.31 5.9 3.7
1.5 2.7 163 0.31 5.7 4.5
1.6 2.8 159 0.31 5.9 3.2
1.6 2.7 159 0.31 5.9 3.1
3. Results 3.1. Catalyst properties 3.1.1. Fundamental catalyst properties Table 6 indicates the properties of the catalysts. BET surface area, pore volume, and average pore diameter were almost identical in all catalysts, and their side crush strength values were high enough to allow loading into the reactor in a refinery. The elemental analysis showed no chloride or nitrogen. 3.1.2. Pt distribution by EPMA and crystallite diameter by XRD The effect of electric type of Pt compound, [(NH3 )4 Pt]2+ or [PtCl6 ]2− , reflected the distribution and crystallite diameter of platinum. The Pt compounds interacted with the surface of the support by their electrical types, and the interaction had an effect on the distribution and crystallite diameter of platinum. Fig. 1 indicates the distribution of platinum in catalysts A and B as determined by EPMA. The result of catalyst B, prepared by the conventional method, shows a quite uneven distribution of platinum in the direction of the cross-section, while a quite even distribution is found for catalyst A. The results also reflect the standard deviation divided by average frequency, as is shown in Table 7; these values become smaller as the distribution of Pt become evener. From the results of catalyst D, we can see that 0.11 wt.% of platinum
Fig. 1. EPMA profiles of catalysts A ( ) and B ( ).
is not enough to make an even distribution. It is apparent from the results of catalysts A, E and F, that the pH value of the Pt solution has little effect on the distribution of platinum particles. The crystallite diameter of platinum particles determined by XRD is also shown in Table 7. The crystallite diameter of platinum on catalyst B, obtained by using H2 [PtCl6 ], was much bigger than the values on the catalysts prepared with [(NH3 )4 Pt]Cl2 . 3.2. The effect of electric type of Pt compound on the isomerization activity The interaction of Pt compounds with the support also has a big effect on the isomerization activity
Table 7 XRD and EPMA characterization factors
Pt distribution (standard deviation/average frequency) Crystallite (nm)
Catalyst A
Catalyst B
Catalyst C
Catalyst D
Catalyst E
Catalyst F
0.18 8
0.51 4 × 102
0.27 9
0.67 10
0.20 9
0.24 9
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Fig. 2. Isomerization of n-C6 over catalysts A and B (n-C6 conversion): (䊐) n-C6; (䊏) n-C6 + 6% of benzene.
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In the product oil formed, catalyst A showed higher conversion values of cyclic compounds (benzene, cyclohexane, and methylcyclopentane (MCP)) than catalyst B did. The total amount of cyclic compounds over catalyst A reduced to about a half compared with that of the feedstock, while the amount in product B was about 6%, with many more cyclic compounds remaining. The hydroisomerization of benzene is known to proceed through three steps: hydrogenation, isomerization, and ring-opening; the reactions of hydrogenation and ring-opening consume hydrogen. The benzene hydrogenation step was very fast over both catalysts under the conditions, but those catalysts showed differences in the isomerization and ring-opening steps. The ring-opening reaction of methylcyclopentane to C6 alkanes by catalyst A was much more advanced than that by B. 3.3. The effect of Pt amount on the isomerization activity
Fig. 3. Isomerization of n-C6 over catalysts A and B (22DMB/C6 alkane): (䊐) n-C6; (䊏) n-C6 + 6% of benzene.
under the conditions in Table 2. Figs. 2 and 3 show the conversions of normal hexane (n-C6) and 2,2-dimethylbutane (22DMB) content in the alkanes of carbon number 6, respectively. The amount of cyclic compounds in the feedstock and the amounts of products A and B formed over catalysts A and B, respectively, are shown in Table 8. Catalysts A and B showed identical activities to n-C6 feedstock, but catalyst B showed lower activity of the n-C6 conversion and formation of 22DMB with 6% of benzene.
Fig. 4 shows the conversions of n-C6 and 22DMB content in the alkanes of carbon number 6 over catalysts A, C and D. The isomerization activities (both n-C6 conversion and 22DMB contents) of three catalysts were almost the same when using n-C6 as feedstock. However, when benzene was added, n-C6 conversion and 22DMB contents decreased as expected. The activity of catalyst D was reduced remarkably, while catalyst C showed identical activity to that of A. Catalyst C (0.33 wt.% Pt) has the same
Table 8 Composition (wt.%) of cyclic compounds Feedstock
Product A
Product B
Benzene Cyclohexane MCP
6.1 0.0 1.9
0.0 1.6 2.7
0.0 2.4 3.8
Total
8.0
4.3
6.2
Fig. 4. Isomerization of n-C6 over the catalysts with 0.11, 0.33 and 0.50 wt.% Pt (catalysts A, C and D): n-C6 conversion—(䊊) n-C6, (䊐) n-C6 + 6% of benzene; 22DMB/C6 alkane—(䊉) n-C6, (䊏) n-C6 + 6% of benzene.
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Table 9 The amounts (wt.%) of cyclic compounds Feedstock
Product A
Product C
Product D
Benzene Cyclohexane MCP
6.1 0.0 1.9
0.0 1.6 2.7
0.0 1.6 2.7
0.0 2.8 3.8
Total
8.0
4.3
4.3
6.6
tolerance for 6% of benzene as that of catalyst A (0.50 wt.% Pt); catalyst D (0.11 wt.% Pt) is inferior to those cases, in spite of having the same activity as n-C6. The results indicate that the surplus platinum does not promote the tolerance to benzene; thus, there must be an optimized amount of platinum related to the amount of benzene in feedstock. Table 9 indicates the composition of cyclic compounds in the product oil. As expected, in the product oil formed over catalyst D (product D), large amounts of cyclic compounds remained, but all the benzene was converted. The amount of cyclic compounds in product C is identical to that in product A. This result proves that 0.33 wt.% of platinum is enough for the feedstock with 6% of benzene. 3.4. The effect of pH to the isomerization activity
We tried to adjust pH of the H2 [PtCl6 ] solution with ammonia to 7, but white precipitation was formed by the addition of ammonia. It might not be desirable for the acidic platinate to add such an alkaline compound. 3.5. Isomerization of commercial light-naphtha In order for us to evaluate catalyst life, catalysts B and C were examined in the reaction of commercial light-naphtha. Fig. 6 indicates the research octane number of liquid product (C5 + RON), along with the iso-pentane/C5 alkane ratio and the 22DMB/C6 alkane ratio. Both catalysts B and C showed quite high activity for the long duration of 1800 h, and both catalysts showed almost the same activity without any decrease of RON value. We also could not find any significant difference between the two catalysts as regards C5 + RON, iso-pentane/C5 alkane ratio, or 22DMB/C6 alkane ratio in the product. However, coke on the spent catalysts showed big differences. Fig. 7 shows the amount of coke accumulation on each catalyst. Spent catalysts were divided into five parts along the reactor axis and were analyzed. On catalyst C, the coke accumulation was much less than
Fig. 5 shows effects of pH of the Pt impregnating solution on the conversions of n-C6 and 22DMB content in the alkanes of carbon number 6. Catalyst A, in which the pH of the Pt solution was 7.0, showed activity higher than those of catalysts E and F did. It is considered that the desirable pH of an impregnate is around 7.
Fig. 5. Isomerization of n-C6, an effect of pH of Pt solution on the catalytic activity: (䊊) n-C6 conversion; (䊉) 22DMB/C6 alkane.
Fig. 6. Isomerization of light-naphtha: (䊊) C5 + RON, () iso-pentane/C5 alkane; (䊐) 22DMB/C6 alkane.
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Fig. 7. Coke on the spent catalysts B (䊏) and C (䊊).
that on catalyst B, and there was little coke from the second to the bottom layer. This result indicates that catalyst C has a much longer life than catalyst B. The distribution of coke was also characteristic; the coke was concentrated on the top layer of the catalyst.
4. Discussion 4.1. Catalyst properties The electrical type of Pt compound influences Pt distribution and its crystallite diameter. Chloroplatinic acid is in the anionic state, [PtCl6 ]2− , while tetra-ammine platinum dichloride is in the cationic state, [(NH3 )4 Pt]2+ , in an aqueous solution. The [(NH3 )4 Pt]2+ cation has electrical attraction to the surface of the support covered by SO4 2− anion and shows little migration during drying and calcination. On the other hand, [PtCl6 ]2− has electrical repulsion to the surface of the support; this gives rise to cohesion of platinum particles by migration. Furthermore, platinum crystals on the cross-section of catalyst B were observed by scanning electron microscopy (SEM), but platinum particles could not be observed on the cross-section of catalyst A. The results indicate that the platinum in catalyst A is dispersed more than that in catalyst B. The crystallite diameter of platinum showed little difference among catalysts A, C, D, E and F, indicating that platinum particles did not migrate by drying and calcining when the cationic species, [(NH3 )4 Pt]2+ , were impregnated to the support.
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Fig. 8. Reaction image of benzene, cyclohexane, and methylcyclopentane.
The pH value of Pt solution has no effect on the dispersion and crystallite size of platinum particles on the catalyst surface. 4.2. The effect of electric type of Pt compound on the isomerization activity The difference of tolerance to benzene is presumably due to the contact probability of the spillover hydrogen and benzene or other cyclic compounds. The effect of the spillover hydrogen molecules in the catalytic reaction has been proposed by many researchers [11–14]. Fig. 8 shows the conceptual pictures of the reaction of cyclic compounds. Catalyst A has higher hydrogenation activity because of its high dispersion of platinum. In this case, it is considered that the spillover hydrogen exists evenly on the catalyst surface, and the cyclic compounds adsorbed on the catalyst contact with spillover hydrogen more easily. On the other hand, catalyst B cannot exhibit enough hydrogenation activity enough for the ring-opening reaction of methylcyclopentane, because of the low dispersion of platinum. In this case, the spillover hydrogen presumably is present unevenly. The ring-opening reaction, which needs hydrogen, is controlled because of the decline of the contact probability to the spillover hydrogen. As a result, the isomerization of cyclohexane is controlled by the equilibrium. The conversion of benzene to C6 alkanes via hydrogenation, isomerization, and ring-opening reactions is known [15]. The quantitative comparison of reactivity among cyclohexane, MCP, and n-C6 is
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not clear, but the adsorption of n-C6 is obstructed by hydrogenation or isomerization of these cyclic compounds. 4.3. The effect of Pt amount on the isomerization activity The catalytic activity also relates to the amount of platinum. The reason for a remarkable drop in activity for catalyst D is coincident with the case of catalyst B; that is, the platinum particles cannot supply enough spillover hydrogen to methylcyclopentane. However, the size of platinum particles in catalyst D is slightly different from that in catalyst A. The number of platinum particles on the surface of catalyst D is fewer, and the distances of platinum particles are longer than those of catalyst A. It is considered that 0.33 wt.% of platinum is enough amount of platinum for 6% of benzene, but 0.50 wt.%, the surplus platinum, also has some ability to supply spillover hydrogen, and 0.50 wt.% of platinum will surely show higher tolerance to the high content of cyclic compounds. 4.4. Isomerization of commercial light-naphtha Catalyst C showed almost the same activity as catalyst B, on account of the small amount (3.5 wt.%) of C6 cyclic compounds in the feedstock. Even when one uses this feedstock, the distribution of platinum has an effect on the amount of coke on catalyst. These results support the hypothesis in Section 4.2. Catalyst B should have almost the same amount of active sites as catalyst C, and the same amount of cyclic compounds in the feedstock should adsorb to these active sites. In the case of catalyst C, spillover hydrogen is supplied to the cyclic compounds that adsorbed to the active sites, and the cyclic compounds desorbed easily, due to the even distribution of the platinum. However, on catalyst B, the only cyclic compounds that adsorb to the neighborhood of the platinum have high probability of contact with the spillover hydrogen, and the others could make the coke because of the much slower desorption. The reason of coke deposition on the catalyst in the top layer is presumably due to the rapid exothermal reactions (hydrogenation of benzene, cracking of branched alkane, ring-opening reaction, etc.).
5. Conclusion Two kinds of platinum compounds were used for preparing Pt-loaded sulfated zirconia-alumina catalyst. The cationic compound of [(NH3 )4 Pt]Cl2 showed dispersion of the Pt particles higher than that produced by the anionic compound of H2 [PtCl6 ], together with higher isomerization activity to a feedstock mixed with 6% of benzene. The difference of activity was brought about by the difference in ability to supply spillover hydrogen based on the platinum dispersion, owing to the electric interaction of platinum compounds with the support. When [(NH3 )4 Pt]Cl2 , was used, 0.11, 0.33 and 0.50 wt.% Pt-supported catalysts showed nearly the same isomerization activities to normal hexane as a feedstock. However, to the feedstock of synthesized normal hexane containing 6% of benzene, the activities of all of the three catalysts declined because of the shortage of their ability to supply spillover hydrogen owing to the amount of platinum, though 0.33 and 0.50 wt.% Pt-supported catalysts experienced smaller drops in activities compared with that of the 0.11 wt.% one. As for the pH value of the Pt solution, the catalytic activity was affected a little, and a pH around 7 was desirable. The results of the 1800 h life test with commercial light-naphtha indicated that the catalyst impregnated with [(NH3 )4 Pt]Cl2 (0.33 wt.% of Pt) gave quite high isomerization activity with much smaller amount of coke compared with the catalyst impregnated with H2 [PtCl6 ] (0.50 wt.% of Pt), resulting in the long life of the former catalyst. These results can be also explained by the ability to supply spillover hydrogen owing to the state and amount of the platinum. References [1] X. Song, A. Sayari, Catal. Rev. Sci. Eng. 38 (1996) 329. [2] K. Arata, M. Hino, Appl. Catal. A 146 (1996) 3. [3] R.A. Comelli, S.A. Canavese, S.R. Vaudagna, N.S. Figoli, Appl. Catal. A 135 (1996) 287. [4] M. Hino, K. Arata, J. Chem. Soc., Chem. Commun. 7 (1995) 789. [5] H. Matsuhashi, H. Shibata, K. Arata, Prepr. Am. Chem. Soc., Div. Pet. Chem. 42 (1997) 748. [6] J.M. Grau, J.C. Yori, J.M. Parera, Appl. Catal. A 213 (2001) 247. [7] S.R. Vaudagna, R.A. Comelli, N.S. Figoli, React. Kinet. Catal. Lett. 58 (1996) 111.
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