Znd. Eng. Chem. Res. 1995,34, 1074-1080
1074
Isomerization of n-Pentane and Other Light Hydrocarbons on Hybrid Catalyst. Effect of Hydrogen Spillover Aihua Zhang, Ikusei Nakamura, Kohjiroh Aimoto, and Kaoru Fujimoto* Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan Isomerization of n-pentane was carried out over Pt-containing zeolite catalyst with a flow-type reaction apparatus under atmosphere. The typical reaction conditions were 230-300 “C, 1atm, Hdn-CeH12 = 9/1 (mole ratio), W/F 10.0 g Wmol. Thermodynamic equilibrium conversion of about 70% at 270 “C could be reached on FWHZSM-5 (supported catalyst), Pt-hybrid (a physical mixture of Pt/SiOn and HZSM-5 powders) catalysts. The isomerization selectivity was higher for Pt-hybrid catalyst than for PWZSM-5. Pt-hybrid catalyst showed higher isomerization activity over wider temperature and pressure ranges than did Pt/HZSM-5. The effect of spilledover H is considered to play a n important role in the regeneration of Brgnsted acid site and the stabilization of an i-C5+ intermediate. A reasonable reaction mechanism is proposed. 1. Introduction
Isomerization of light paraffins is essential for the production of high-octane-number gasoline. According to a new published book (Huang et al., 19921, the earliest isomerization device was established in the 1930s with which n-butane was isomerized to isobutane that was used as the raw material of the consecutive alkylation process with butene. Fiedel-Crafts catalyst was used as one of the earliest isomerization catalyst. It showed high activity for n-butane isomerization at low reaction temperature below 90 “C, because of its low selectivity, especially for n-C5 and n-Cs isomerization, poor structure instability, and strong corrosivity, it could not meet the demand for modern isomerization processes. To solve these problem, bifunctional catalysts were developed, they were prepared by loading noble metals such as Pt and Pd on noncorrosive, high-surface-area solid support. It is known that Pt-AlzO3 was effective for the isomerization of n-pentane and n-hexane, but it has to be used at a high reaction temperature, usually between 455 and 510 “C. To improve the performance and to lower the operation temperature for bifunctional catalyst, lowtemperature type bifunctional catalysts, which were PtA1203 treated by AlCl3 and Pt-Al2O3 treated by organic chloride, were developed, and they are capable of isomerizating butane, pentane, and hexane and have been widely used in the isomerization industry. More recently, new kind of bifunctional catalysts of noble metaValuminosilicate and noble metauzeolite were developed (Ribeiro, 1983)and a high conversion which was near to equilibrium value was obtained upon them at medium temperatures of 260-315 “C. Reaction mechanisms which have been developed so far can be classified as follows: (1)isomerization over metal, especially platinum (Anderson, 1973; Gault, 19811, (2) bimolecular mechanisms providing dimerization followed by rearrangements and cleavage (Bolton and Lanewala, 1970; Karabatsos et al., 19611, and (3) monomolecular mechanism providing the formation of alkylcarbenium ions, skeletal rearrangements of the latter, and formation of product alkanes from rearranged carbenium ions (Weitkamp, 1982). In 1957 and 1964, a bifunctional mechanism which was composed of the dehydrogenation of normal paraffins to normal olefins on noble metal, its skeletal isomerization to branched olefins on acid site, and the hydrogenation of
branched olefins to isoparaffins was suggested by Weisz (Weisz and Sewgler, 1957), Coonradt and Garwork (Coonradt and Garwook, 1964). This mechanism dominated the explanation for hydroisomerization and hydrocracking for a long period. It is known that isomerization or hydrocracking of light paraffins occurs quickly on solid acid catalysts supported with hydrogen-dissociative metal under hydrogen atmosphere. In recent years, according to the reports of Steinberg (Steinberg et al., 1989) and Fujimot0 (Fujimoto et al., 19921, a physical mixture of a W Si02 and HZSM-5 hybrid catalyst showed catalytic activity similar to that of the supported one, whereas WSi02 and HZSM-5 showed poor isomerization activity when they are subjected to the reaction individually. From the point of stoichiometry, gaseous hydrogen is unnecessary to the reaction, but in fact, the catalysts exhibited poor isomerization activity in the absence of H2. This means that the function of Hz has to be taken into account. Fujimoto (Fujimoto et al., 1992)suggested that hydrogen (atoms or ions) migrated from noblemetal site to acid site during the reaction and promoted the reaction. A definition on spillover was proposed during The Second International Symposium on Spillover: “spillover involved the transport of an active species sorbed or formed on a first phase onto another phase that does not adsorb or form the species under the same condition”, and a comment was offered: The result may be the reaction of this species on the second phase with other sorbing gases andor reaction with andor activation of the second phase. The noble metals involved in spillover act as a “gate” or a “porthole” for spillover species, providing an easy way by which spilled-over species move in and out. The most important phenomena brought by spillover were summarized in several reviews (Sermon and Bond, 1973; Bond, 1983; Conner et ai., 1986; Teichner, 1993; Delmon, 1993; Fujimoto, 1993; Inui, 1993). They are (1)enhanced adsorption, (2) surface isotopic exchange, (3) bulk changes, and (4) strong metal-support interaction. The influence of spillover on catalytic process may be described as (1) spilled-over species keep catalyst clean, (2) create or regenerate selective sites through a remote controlling mechanism, and (3) as a result, catalytic reactions are accelerated and catalyst deactivation is inhibited effectively.
0888-588519512634-1074$09.0010 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1075 The objective of this work is to develop new excellent catalysts for hydroisomerization of lower paraffins and to confirm the role of hydrogen spillover on the isomerization reaction. Also, we try t o introduce some new concepts to the isomerization mechanism.
100
2. Experiment
2.1. Catalyst Preparation. PtJZSM-5 (0.5 wt %) catalyst was prepared using a commercially available ZSM-5 (Toso, HSZ-€340") with silicdalumina ratio of 44. Pt was introduced by the method of ion exchange with an aqueous solution of tetraammineplatinum(I1) chloride. The ion exchange was carried out a t 80 "C for 6 h with 0.1 w t % Pt(NH3)4Clz aqueous solution under stirring; the supported PtiHZSM-5 was washed with water until no chlorine ion was detected. Oxidesupported platinum (WSiOz, WAlzO3, pt/TiOz) was prepared by impregnating a commercial available Si02 (Aerosil380, BET specific surface area 380 m2/g). A1203 (Aerosil aluminium oxide C, BET specific surface area 100 mz/g), T i 0 2 (Aerosil titanium oxide P25, BET specific surface area 50 m2/g)with aqueous solution of HzPtCls. Hybrid catalyst was prepared by cogrinding the mixture of 4 weight parts of H-zeolite with one weight part of PtJSiOz (2.5 wt %) and pressure molding the mixture to granules to 20/40 mesh. Catalysts were activated in air a t 550 "C for 2 h and reduced in flow hydrogen at 400 "C for 1 h before use. 2.2. Reaction Apparatus and Procedure. The isomerization of n-pentane was conducted with a continuous-flow-type futed-bed reaction apparatus under atmosphere pressure and pressurized conditions. The catalysts were packed in a stainless steel tube with an inner diameter of 4 mm. n-Pentane was fed to the reactor by a carryover method using carrier gas such as hydrogen or nitrogen. A four-way switch valve was set upstream of the reactor, so that a quick response to any feed switch became available. The products analysis was performed on line with a HITACHI 163 gas chromatograph, equipped with a 2 m SE-30 column. 2.3. Analysis of Carbon Deposit. The analysis of carbon deposit was performed using an element analysis equipment (MTZ CHZ CORDER). Carbon deposited on the samples was burned in 0 2 at high temperature into COS, which created a signal response on TCD. The results are expressed as the formula C% = (Wdw,,t.)%. 3. Results and Discussion 3.1. Isomerization of n-Paraffins on Pt-Based Zeolite Catalysts. 3.1.1. Isomerization of n-Pentane on Four Elementary Catalysts. Figure 1 shows the n-pentane conversions and isopentane selectivities and yields on the four catalsyts PtiHZSM-5, Pt-hybrid, WSiOz, and HZSM-5. The detailed products distribution at 270 "C for the four catalysts are shown in Table 1. The tests were performed in Hz a t temperatures ranging from 200 to 400 "C. It can be clearly seen that HZSM-5 and PtJSiOz showed poor catalytic activities either in n-CsH12 conversion or in i-C5Hlz selectivity, whereas FWHZSM-5 and Pt-hybrid showed high catalytic activities for the isomerization of n-pentane, particularly in the temperature range 220-350 "C. However, if Nz was used instead of Hz,as shown in the same table, PtiHZSM-5 and Pt-hybrid also showed low conversions and low isomerization selectivities. The data presented here were the values after reacting in Nz for 1h during which conversions and selectivities were kept
200
300
I
wsio2
I
400
300
400
Reaction Temperature ("C)
Figure 1. Isomerization of n-pentane on four catalysts. 1.0 atm, W/F 5.0 g Wmol, Hdn-C5 = 9:l (mole ratio). (W) conv %; ( 0 )sel %; (A)yield %. In all of the figures, mole conversion and mole selectivity are used. Table 1. Isomerization of n-Pentane at 270 "Ca ~
in H2
in NO
w
cat.
w
PtJ PtPtHZSM-5 Si02 HZSMB hybrid HZSM-5 hybrid ~~
conv, % yield, % sel % C1 C2
+
c3 c4
i-Cb cS+ a
3.4 0.7
1.8 1.2
66.9 61.5
55.8 54.9
13.8 1.4
4.9 1.1
1.8 24.4 40.6 20.6 12.6
13.3 8.3 3.9 66.7 10.5
3.5 2.2 1.9 91.9 0.4
0.4 0.4 0.4 98.4 0.5
1.4 43.8 32.8 10.1 11.4
0.8 27.1 37.9 22.4 12.0
1.0 atm, W/F 5.0 g Wmol, n-C5 90%.
stable. These results imply that HZhas some effect t o the selective formation of isopentane. From Table 1, it should be noted that when the reactions were carried out in Nz, the amounts of C3 and C4 cracking products were far higher than the amounts of CI and CZ. If monomolecular cracking occurred such as C5 CI Cq or C5 CZ+C3, equal mole amounts of CI and C4 or CZ and C3 would be formed. This suggested that the cracking reaction occurs not by a monomolecular mechanism but by a dimerizationcracking bimolecular process (Abbot and Wojciechowski, 1985). Furthermore, when the reaction was carried out in Nz,due to the shortage of Hz, dimerization also took place in company with the formation of C3 and C4 light paraffins resulting from cracking reaction. According to the results of carbon analysis done for the used catalysts, about 4.25 wt % and 3.42 wt % carbon deposited on the surfaces of catalysts after reacting in NZ for 1 h for PVHZSM-5 and Pt-hybrid catalysts, respectively, while no evident carbon depositing was detected for the catalysts after reacting in Hz. According t o the results in Hz, the maximum yields were obtained a t 270 "C for PtMZSM-5 and 300 "C for Pt-hybrid. It seems that the maximum yield had a little shift when Pt-hybrid was used instead of PtiHZSM-5. In contrast to PtiHZSM-5, Pt-hybrid catalyst maintained a high isomerization selectivity at a wider temperature range. The isomerization selectivity of PtJ HZSM-5 dropped drastically over 300 "C while Pt-hybrid catalyst kept the selectivity over 75% even at 350 "C.
-
+
-
1076 Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995
four weights part of HZSM-5 in which the two parts were kept in series, granular (20-40 mesh), powder contacts respectively. For the series, WSiO2 was placed upon HZSM-5. The contact conditions got poorer and poorer by the sequence of powder, granular series. As expected, high conversion and selectivity were obtained from the powder mixture. However, the result from the granular mixture was not very poor, and a fairly high selectivity (75%)was obtained from it at 300 "C. This is an interesting result because it cannot be explained in terms of classical bifunctional mechanism. From the results above, it can be concluded that the isomerization selectivity depends on the contact condition deeply. The results also implies the existence of synergetic action between WSi02 and HZSM-5. 3.1.4. Effect of Hydrogen Partial Pressure on Isomerization Reaction. The influence of hydrogen partial pressure on the reactivity is shown in Figure 4. The experiments were carried out on Pt-hybrid (WSiO2 HZSM-5, powder mixture). In the case of zero partial pressure of hydrogen, isomerization reaction scarcely took place. With the increasing of partial pressure, the selectivity increased gradually and came up to constant when H2 partial pressure exceeded 0.5 atm. This suggested the indispensability of Ha to the isomerization of n-pentane. 3.1.5. Isomerization of n-Pentane on Other Zeolites. Not only ZSM-5, but several other zeolites were used to the study. They were 5A, 13X, L, Mordenite, USY. These zeolites represented different pore sizes and acid strengths. The pore sizes and SiOdAl203ratio for these zeolites are listed in Table 2. No evident influences from the pore size were observed. However, the differences in acid strengths of the zeolites affected the activities significantly. As with ZSM-5, two types of Pt/H-zeolite and Pt/SiO2 H-zeolite (hybrid) were prepared. The results at 275 and 300 "C are listed in Table 3. 5A and 13X showed poor reactivity either in conversion or in selectivity. L, mordenite, USY showed an activity as high as ZSM-5, although the maximums of these differed from each other. 3.1.6. Isomerization of n-Butane and n-Hexane on Pt-HybridCatalyst. Figure 5 shows the isomerization reaction from n-butane, n-pentane, and n-hexane on Pt-hybrid. As with n-pentane, the n-hexane conver-
+
+
Figure 2. Effect of supporter in Wsupporter HZSM-5 hybrid catalyst on n-pentane isomerization. 250 "C, 1.0 atm, W/F 10.0 g Wmol, Hdn-C5 = 9:l (mole ratio).
This should be attributed to the lower hydrocracking activity of Pt particles supported on Si02 than that on HZSM-5. 3.1.2. Isomerization of n-Pentane on Pt-Hybrid with Different Supports. The isomerization activity of n-pentane on three Pt-hybrid catalysts with TiO2, SiO2, and A1203 supports are shown in Figure 2. Compared with HZSM-5 alone, all three catalysts exhibited high isomerization selectivities which were close to 100%. A relative lower conversion was observed on Pt/TiO2 hybrid than that on Pt/SiOa-hybrid and Pt/ Al203-hybrid. From the latter, a conversion approaching the thermodynamic equilibrium value could be attained. The poor metal dispersion resulting from the lower specific surface area of Ti02 (50 m2/g>might be the reason for this low activity. 3.1.3. Effect of Contact Condition between Pt/ Si02 and HZSM-5 on Isomerization Reaction. Figure 3 shows a group of results carried out with the mixtures of one weight part of Pt (2.5 wt %)/Si02 and
+
Conversion(%)
Series
9.9
I
IGranular
8.1
powder
71.5
0
20
40
60
80
100
Selectivity(%) Figure 3. Effect of contact condition on n-pentane isomerization. R (2.5 w t %)/SiOz:HZSM-5 = 1:4,300 "C, 1.0 atm, W/F 10.0 g h/mol, Hdn-Cs = 9:l (mole ratio).
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1077 100 Conv.%
80 ep $ 60 v) 4 €9 i40
8 20
2
Sel .%
0 0.4 0.6 0.8 H, partial pressure (atm)
0
1
0.2
500 t n - c 4 &
Figure 4. Effect of H2-partial pressure on n-pentane isomerization. Pt (2.5 w t %)/Si02:HZSM-5 = 1:19, 230 "C, 1.0 atm, W/F 10.0 g Wmol, (H2 N2)/n-C5 = 9:l (mole ratio).
+
Table 2. Pore Size and SiOdAlaOa Ratio zeolite pore size (A) 5A 5.2 13X 8-9 mordenite 6.6 L 7.1 DAF-3 8-9 ZSM-5 5.4-5.6
I
siOz/A1203 2 2.5 10 6 30 44
Table 3. Isomerization of n-Pentane on Various Zeolite Catalystk conv % sel % yield % cat. 275°C 300°C 275°C 300°C 275°C 300°C 1.8 51.8 52.7 0.6 0.9 W5A 1.2 1.0 2.5 72.3 72.5 5A-hybrid 1.3 3.5 2.7 50.0 45.9 0.8 1.2 Wl3X 1.7 13X-hybrid 2.2 3.5 71.6 73.1 1.6 2.6 50.8 91.7 92.2 21.1 46.8 Ptn 23.0 19.7 42.5 L-hybrid 20.7 45.0 95.4 94.6 84.1 71.5 28.3 34.4 33.7 48.2 Wmordenite 45.9 90.1 77.3 28.5 35.5 M-hybrid 31.3 FWHZSM-5 74.0 79.3 80.6 55.2 59.6 43.8 ZSM-5-hybrid 71.1 72.1 93.0 82.5 66.1 59.5 38.6 62.7 "339.0 63.8 99.1 98.2 a
1.0 atm, W/F 10.0 g Wmol, n-C5 lo%, H2 90%.
sion and isohexane selectivity were high and the selectivity decreased with the increase of reaction temperature. The cracking reaction was preferred at high temperature region. In the case of n-butane, the reaction activity was low, and the isobutane selectivity was also not very high. It is proposed that n-butane isomerization take place by different reaction mechanisms with n-Cs and n-Cs; according to Sie (Sie, 1992,19931,normal paraffins isomerize via a protonated dialkylcyclopropane carbonium ion intermediate mechanism, but this mechahism is not suitable for the isomerization of n-butane. From the results show above, it can be concluded that a hydrogen-dissociating metal, a relative strong acid site, and Hz atmosphere are the essential factors for the isomerization reaction. The site of H2 dissociation and acid centers could be a t one unit or at two separated units, but the two units should keep intimate contact. 3.2. Advantages of Pt-Hybridfor the Isomerization of n-Pentane. 3.2.1. Effect of Pt Loading on IsomerizationReactivity. The isomerization conversions and selectivities on PVHZSM-5 and Pt-hybrid are plotted in Figure 6 against the Pt contents in catalysts.
200
Conv.%
250 300 3% Reaction Temperature ("C)
400
Figure 5. Isomerization of n-butane, n-pentane, and n-hexane on Pt-hybrid (PUSiOdHZSM-5 = L4). 1.0 atm, W/F 10.0 g Wmol, Hdn-Cb = 9:l (mole ratio).
loo
)
- - - * &
-------
80
0
0.1
0.2 0.3 0.4 Pt-loading (\VI%)
0.5
Figure 6. Effect of Pt loading on n-pentane isomerization. 1.0 atm, W/F 10.0 g-hr/mol, Hdn-Cb = 9:l (mole ratio).
The starting point in this plot is the data of Pt-free HZSM-5. When 0.1 wt % Pt was introduced, the selectivity increased dramatically in both the cases W HZSM-5 and Pt-hybrid; from here, it can be said that the existence of Pt is necessary for the selective formation of &. Furthermore, in trying t o compare the two conversions at 0.1 wt % Pt loading, it is found that the conversion on Pt (0.1 wt %)/HZSM-5was far lower than that on Pt (0.1 wt %)-hybrid. Pt-hybrid showed high reaction activity even at low Pt content. As for why the catalyst Pt (0.1 wt %)/HZSM-5could not give high conversion, the following two reasons are suggested here: first is its weak H2 dissociation ability, usually when a small amount of metals is loaded on supports, they get well dispersed as tiny particles, but these microparticles usually have weak H2 dissociation ability compared with larger particles. The second reason is their reduction difficulty: when small amounts of Pt are exchanged to HZSM-5, they would first be bound a t some strongest acid sites, forming stable bonding between 0 and Pt2+,which results that these
1078 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 100 W H Z SM - 5
PI-tiybrid
2.30"~.w 1 ~ 2 . a
230°C WIE2.0
3
40
@>
g
1 1 1 1
20
U
0 200
300
400
300
Reaction Temperature ("C)
Figure 8. Isomerization of n-pentane on Pd/HZSM-5 and Pdhybrid. 1.0 atm, W/F 5.0 g Wmol, Hdn-C5 = 9:l (mole ratio). (M) conv % ( 0 )sel %; (A)yield %.
Pt-hybrid 320°C. WIE5.0
0
I
1
I
0
3
6
I
9
I 11
1 3
1
I
I
6
9
11
Reaction Pressure (atm) Figure 7. Effect of reaction pressure on n-pentane isomerization. (M) conv %; ( 0 )sel %.
Pt ions become hard to be reduced. As a results, these Pt particles cannot exert H2-dissociation functions effectively. 3.2.2. Effect of Pressure on the Isomerization Reaction. The pressure effect tests were carried out under two conditions, which were low temperature, short contact time (230 "C, W/F 2.0 g Wmol) and higher temperature, longer contact time (320 "C, W/F 5.0 g Wmol). The pressure was changed from atmosphere pressure t o 11.0 atm during the reactions. The results are shown in Figure 7. In the case of 230 "C/ W/F 2.0 g Wmol, both of the two catalysts (F'tfHZSM-5 and Pthybrid) showed constant low conversions and high isomerization selectivities, and almost no pressure effects were observed for the two catalysts. However, when the tests were done at 320 "C (as shown in Figure 7 for WHZSM-B), the conversion increased a little with the increase of reaction pressure, while the selectivity of i-C5 dropped dramatically when pressure was increased, with a large amount of cracking products of C2H6, C3H8, and C4H10 (n-and iso-) being formed. The results obtained from Pt-hybrid were different from those of F'tfHZSM-5: there were no evident pressure effects observed either for the conversion or for the isomerization selectivity. The Pt particles supported on ZSM-5 are considered to act as cracking active sites a t high reaction pressure, while those on Si02 have weak cracking activity. This means that the formation of byproducts can be suppressed by separating metal and acid site as in the case of Pt-hybrid catalyst. 3.2.3. Isomerization of n-Pentane on Pd-Based Zeolite. The catalysts of Pd-containing Pd/HZSM-5, Pd-hybrid (Pd/SiOz HZSM-5),were prepared with the same method as Pt-based catalysts, and their isomerization activities were evaluated. The results are shown in Figure 8 and the reaction condition is listed below the figure. According t o Figure 8, both conversion and isomerization selectivity for Pd-hybrid are higher than that for Pd/HZSM-5over the whole temperature range, especially the conversion level of Pd/HZSM-5 was just about one-half of that for Pd-hybrid. This provided further evidence that the "hybrid" catalyst is better than the "direct" one.
+
Hybrid catalyst, obtained by mixing two powders in a very simple method, showed high catalytic activity even at low Pt loading and high selectivity in a wider temperature and pressure range for the isomerization of n-pentane. In addition, similar phenomena are observed in another isomerization system; in the case of the isomerization of cyclohexane methylcyclopentane, the difference between the two types of catalysts was observed more clearly. The results will be discussed in our following paper. This new type of catalyst, with its new concept, is showing its hopeful future for study and application. 3.3. Discussion on Isomerization Mechanism and Reaction Model. The classical bifunctional reaction scheme, proposed by Weisz (Weisz and Sewgler, 1957) and Coonradt and Garwook (Coonradt and Garwook, 19641, has been used to explain the mechanism of isomerization and hydrocracking of n-alkanes on bifunctional zeolite catalysts consisting of a noble metal on it. According t o it, n-alkane molecules are dehydrogenated on noble metal, protonation of alkenes on Bransted acid centers gives rise to the formation of alkylcarbenium ions, which undergo skeletal rearrangements and ,!I-scissionreactions; finally, the carbocations desorb from acid sites as isomerized alkenes and are hydrogenated over noble metal to paraffins. The occurrence of intermediate olefins is considered to be debatable (Steijins and Froment, 1981), and this classical mechanism also fails t o explain the reason the contact condition in Pt-hybrid affects isomerization activity. To explain the experiment results more reasonably, we present a new concept for the isomerization reaction on ZSM-5 catalyst, namely, that spilled-over hydrogen from the gas phase onto ZSM-5 zeolite plays an important role in generating hydroisomerization activity. The new reaction mechanism and spillover model can be expressed as in Figures 9 and 10. Hydrogen gas is dissociated on the noble metal and spills over onto the zeolite. The spilled-over hydrogen presumably exists on the zeolite surface as proton and hydride. The protons act as the acid and abstract hydride from n-pentane or attack the carbon atom directly to promote isomerization or cracking. The n-C5Hl1+is isomerized to i-C5Hllf and then i-C5H11+is stabilized by hydride addition. The B-acid site is regenerated by Hsp+. At step 2, nonclassical five-valent carbonium is considered to be a transient state. At step 3, Sie (Sie, 1993,1992) suggested that n-C5+ is isomerized to i-Cs+ via a dialkylcyclopropane carbenium ion mechanism. According to this mechanism, the smallest
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Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1079 4. Conclusions I
",e" (2)
(4)
C-C-C-C-CHB-
c-6-c-c + C
(51
B +
HFp'
c-e-c-c-c
I,;
Hsp-
HB
I
L c-E-c-c-c +
B + H2
I deprotonation
c-7-c-c
4
+
polymerization
C
cracking pmducts and coke
Figure 9. Suggested isomerization reaction mechanism.
H2
11
In summarizing the results shown above, the following conclusions can be deduced. It was found that a hybrid catalyst composed of the physical mixture of Pt/SiO2 or Pd/SiOz and HZSM-5 is effective for the isomerization of n-pentane and nhexane under hydrogen atmosphere. Compared t o supported FtA-€ZSM-5,Pt-hybrid catalyst showed a higher isomerization activity a t low Pt loading and higher isomerization selectivity over a wider temperature and pressure range. Other zeolites, such as USY, Mordenite, L, are also effective for the reaction. It is suggested that hydrogen-spillover effects promote the reaction via the regenerating Bransted acid site and the stabilizing i-C5+ intermediate.
Acknowledgment
H++H'
The authors thank Shingo Hattori, who took part in some of this work.
Literature Cited
i-Cs Figure 10. Suggested spillover model.
carbon number for hydrocarbon is 5; therefore, butane is unfavorable, in agreement with the result shown in butane isomerization. If reaction pressure is added, the cracking Of C5+ occurring on the Pt site will be promoted, in particular for the PVHZSM-5 catalyst. The increased hydrogen pressure brings about the decrease in isomerization, accompanied by a large amount of cracking products being formed (C2 and (23). If the supply of hydrogen from the gas phase is insufficient, as in the case of reaction in Nz, the deficiency of H+ and H- on the ZSM-5 surface may cause deprotonation of carbenium ion to form olefins (CsHll+ C5H10 H+),which may polymerize and be subjected t o cracking or cause coke formation. For hybrid catalyst, its performance depends on the contact condition deeply. As shown in Figure 10, if the two parts (WSi02 and HZSM-5)remain in good contact just like the powder hybrid, the reaction cycle could proceed smoothly accompanied by the supply of Hsp+ and Hsp-. However, if the granular mixture hybrid catalyst in which the two parts are in poor contact is used, the supply of spillover species will be low and the number supplied will be insufficient, so that the catalytic activity cannot be high. But in another way, if i-C5+ is formed, it can be stabilized by Hsprapidly; this differs from HZSM-5 and series mixture (Pt/SiOz HZSM-5) catalysts essentially. As a result, the isomerization selectivity of granular mixture could remain high. This hydrogen spillover concept is considered to be the most acceptable one for explaining the high selectivity and high activity of the hybrid catalyst. With regard t o the generation of an active site by spillover species, Ebitani (Ebitani et al., 1991, 1992) presented evidence that spillover H migrating from metal to solid superacid of S022--Zr02 to initiate a Bransted acid site. Zhou (Zhou et al., 1991) also presented the fact that an active site was generated by spilled-over oxygen species.
-
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Received for review September 1, 1994 Revised manuscript received December 14, 1994 Accepted December 30, 1994@
IE940522L Abstract published in Advance A C S Abstracts, March 1, 1995. @
