Ind. Eng. Chem. Prod. Res. D8V. 1986, 25,481-490
481
Hydroisomerization and Hydrocracking of n-Alkanes. 1. Ideal Hydroisomerization PtHY Catalysts Giuseppe E. Glannetto, Guy R. Perot, and Mlchel I?.Guisnet' UA CNRS 350, Cataiyse en Chimie Organique, UER Sciences, 86022 Poitiers Cedex, France
The transformation of n-heptane was studied at 250 OC, 1 atm, and pH,lpnheptane = 9 on a series of PtHY catalysts containing from 0.1 to 1.5 wt % platinum and having Si/AI atomic ratios of 3, 9, or 35. The activities, stabilities, and selectivities of the catalysts are clearly governed by the number of their acid sites (n,) and hydrogenating sites (nR). n, is defined here as the number of acid sites on which the adsorption heat of NH, is greater than 100 kJ mol-' and n R as the number of accessible platinum atoms. For n,/nR < 10 (Le.,with catalysts with about 1 wt % t'F and Si/AI atomic ratio of 9 or 35) the formation of monobranched, bibranched, and tribranched isomers and cracking products is a step by step process. Such catalysts present the best possible hydroisomerization selectivity, the cracking products appearing only at conversion rates of over 50% and the yield in isomers reaching 6 5 % for a total conversion rate of 75-80%. Thus they can be considered as "ideal hydroisomerization catalysts".
Introduction The use of heavy petroleum cuts as jet fuel, diesel oil, or lubricants is limited by their characteristics at low temperatures and especially by their pour point. These characteristics can be improved either with additives or by chemical treatments. The aim of the chemical treatment is to eliminate the long-chain n-paraffins having relatively high freezing points. This elimination can be carried out by hydroisomerization or by hydrocracking on bifunctional catalysts. In both cases the pouring properties at low temperature are much improved. Hydroisomerization is preferable, however, for it consumes no hydrogen and does not decrease the yield in liquids by the production of light fractions. Unfortunately, if we can carry out selectively the isomerization of light alkanes, it is not the case for that of alkanes with over six carbon atoms, which have a marked tendency to crack. Many works have shown that the activity and selectivity (isomerization/cracking selectivity, product distribution) of bifunctional catalysts in alkane transformation were governed by the characteristics of acid and hydrogenating sites (Jacobs et al., 1980; Steijns et al., 1981; Vansina et al., 1983; Weitkamp, 1982; Guisnet and Perot, 1984; Perot et al., 1984). The most detailed studies were carried out on Pt-Y zeolite catalysts. It is clearly demonstrated that n-alkane transformation occurs in successive steps, cracking being preceded by the branching of the skeleton. The distribution of the reaction products depends on the number of active acid sites that the olefinic intermediates can encounter during their diffusion between two hydrogenating sites (Guisnet and Perot, 1984) and therefore on the acid sites/hydrogenating sites ratio. The aim of this work is to define what are the ideal characteristics of a hydroisomerization catalyst and more particularly to define what must be its acid sites/hydrogenating sites ratio. With this in mind we determined on a series of PtHY catalysts the evolution of the product distribution of n-heptane transformation against the acid sites/hydrogenating sites ratios. Very different values of these ratios were obtained by modifying the platinum content of the catalyst or the Si/A1 atomic ratio of the HY zeolite used. Experimental Section The three zeolites were prepared from ultrastable NHIY zeolite (US YNH,, Union Carbide). The HY3 sample was 0196-432118611225-O48l$01.5OIO
obtained by calcination of US YNH, at 500 "C under a 10-h dry air flow. The HY9 sample was obtained by SiC14 dealumination of US YNH, (Beyer and Belenykaja, 1980) and the HY35 by treatment of HY9 with 1 N HC1 at 100 "C for 3 h. HY3, HY9, and HY35 had Si/A1 ratios of respectively 3,9, and 35. HY3 and HY9 had an adsorption capacity for N2and specific surface areas of about 0.28 cm3 g-' and 700 m2 g-', respectively; the values found for HY35 were slightly higher (0.30 cm3g-' and 730 m2 g-'). Moreover, the XRD patterns were the same as those reported in the literature (Van Ballmoos, 1984). The acidities of these catalysts were determined by adsorption of NH, followed by calorimetry (Cartraud et al., to be published). The number of acid sites decreased with dealumination: HY3, HY9, and HY35 had respectively 5.9, 2.6, and 1.0 acid sites per unit cell for which the heat of adsorption of NH3 was higher than 100 kJ mol-'. The PtHY catalysts were prepared by competitive ion exchange with Pt(NH3)4C12/NH4N03 molar ratios ranging from 1/50 to 1/1600 so as to obtain platinum loadings ranging from 1.5 to 0.07 wt % (Table I). The calcination and reduction of the samples were carried out at 300 and 500 OC, respectively, under the conditions already reported for PtHZSM-5 catalysts (Giannetto et al., 1985). The metal dispersion was determined by H2chemisorption and H2-02 titration in a pulse system (Duprez et al., 1983). The reactions were carried out in a flow reactor with a hydrogen/hydrocarbon molar ratio of 9. Benzene hydrogenation was carried out at 100 OC and n-heptane transformation at 250 "C. For both reactions the activities ((molar flow rate X conversion rate)/weight of catalyst) were measured below a 10% conversion rate. For n-heptane transformation different conversion rates were obtained by modifying contact time (50-200 mg of catalyst, 0.1-12 cm3 h-' n-heptane flow rate).
Results Effect of the Platinum Content. Figure 1 shows the activity of PtHY3 catalysts for n-heptane conversion against the reaction time. All the samples had an initial aging period of 2-3 h at the end of which the activity decreased slowly and steadily. Both the initial and the final activities increase until the platinum content reaches 0.40 wt % (Figure 2), which corresponds to a benzene hydrogenation activity of 7.5 X lo-, mol h-' g-' (Table 11). Fouling obeys the law expressed by the equation A = A. 0 1986 American Chemical Society
482
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
Table I. Physicochemical Characteristics of the Catalystso catalyst 0.1 PtHY3 0.2 PtHY3 0.4 PtHY3 1 PtHY3 1.5 PtHY3 0.6 PtHY9 1 PtHY9 1 PtHY35
zeolite
Pt, wt % 0.07 0.2 0.35 0.9 1.5 0.6 0.85 0.8
N%.8H47.8A148.6Si143.403M Na0.8H47,8A148,6Sj143.40384 N%.8H47.8A148.6S1143.40384 N%.8H47.8A148.6Si143.40384
N%.8H47.8A148.6Si143.403M N%.8H18.1A118.9Si173.103&1 N~.8H18.1A118.9S~173.10384 N%.oo3H5.5A15,,S1182.503~
dispersion, % 90 90 80 90 70 90 90 90
nAl/nPt 1250 450 270 100 75 55 40 13
nA2/nPt
170 60 35 13 10 8.5 6 2.5
~ A I / ~ A Z
7.35 7.5 7.7 7.7 7.5 6.5 6.7 5.2
nnAl: total number of acid sites. nA2: number of acid sites for which adsorption heat of NH3 is greater than 100 kJ mol-'. npt: number of accessible P t atoms. Table 11. Activities mol h-' gl)for Benzene Hydrogenation (A ") and n -Heptane Transformation catalyst HY3 0.1 HtHY3 0.2 PtHY3 0.4 PtHY3 1 PtHY3 1.5 PtHY3 0.6 PtHY9 1 PtHY9 1 PtHY35 Initial activity.
100
An 1.3 3.2 7.5 20 28 13 40 60
AOa 9 48 75 96 98 97 12 12 2
Arb 0.3 22 38 70 76 72 12 12 2
Final activity.
I* Qs
0
1.0
1.5
2 . 0
Figure 2. Change of the initial activity (Ao, mol h-' g-l) and final activity ( A f , loe3 mol h-' g-') of PtHY3 catalysts vs. their platinum content.
a 0
0
0.
-0.25
Figure 1. n-Heptane transformation on PtHY catalysts. Change of the global activity (A, mol h-' g-') vs. time on stream (t,min): HY3 (+); 0.1 PtHY3 (*); 0.2 PtHY3 ( 0 ) ;0.4 PtHY3 (*); 1PtHY3 (a); 1.5 PtHY3 (A); 1 PtHY9 (A);1 PtHY35 (0). ea(t)l/z where A is the activity at time t , A, is the initial activity, and a is the deactivation coefficient. The greater
the platinum content, the lower is deactivation coefficient a (Figure 3). Table I11 gives for each catalyst an example of detailed product distribution at about 50% of the total conversion. The main cracking products are propane and isobutane. All the isomers are identified in variable quantities according to their structures. Figure 4 gives the distribution of three families of products: (a) methylhexanes and ethylpentane (monobranched isomers, M); (b) dimethylpentanes and trimethylbutane (multibranched isomers, B); and (c) cracked n-heptane (C) as functions of the total conversion for HY3 zeolite and all the PtHY3 catalysts. On HY3 (Figure 4a) as on 0.1 PtHY3 (Figure 4b), cracking is the main reaction even at low conversion rates; all the
-0.50
c
,
-2.70 W t % Pt
0 1.0 2:0 Figure 3. n-Heptane transformation on PtHY catalysts. Effect of the platinum content on the deactivation coefficient (a): PtHY3 (*); PtHY9 ( 0 ) ;PtHY35 (0).
products (M, B, and C) appear to be primary products. This is also the case on 0.2 PtHY3 (Figure 4c), but isomerization here becomes the main reaction. With catalysts containing at least 0.4 w t % Pt (Figure 4d-f) the cracking
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986 483
Table 111. Product Distribution for a 50% Conversion of n-Heptane' catalyst c1 c2 c3
i-
HY3 0 0.1 35.2 43.7
0.1 PtHY3 0 0.1 45.0 40.6
0.4 PtHY3 0 0 20.8 19.1
1 PtHY3 0 0 12.2 11.3
1.5 PtHY3 0 0 10.4 9.5
1 PtHY9 0.2 0.2 2.2 1.7
1 PtHY35
1.0 1.2 5.4 2.7
c4
n-
i-
3.0 8.2
1.4 1.9
0.6 0.4
0.4 0.2
0.3 0.2
0.6 0.1
3.0 0.4
0.9 3.1
0.4 1.1
0.1 0.3
0.1 0.2
0.1 0.1
0.1
0.1
1.1 0.7
0.4 0.2 0.5 0.2 0.1 0.6 1.8 1.8 0.2 100
0.4 0.7 0.7 0.2 1.9 0.9 2.9 3.1 0.3 100
0.1 3.4 4.1 0.6 2.6 5.3 20.3 21.5 1.6 100
0.1 4.4 4.8 0.6 3.0 6.4 26.5 28.2 2.1 100
0.1 4.8 5.0 0.7 3.0 6.5 27.8 29.4 2.2 100
0.1 5.2 4.9 0.3 3.0 6.4 35.2 37.0 2.7 100
0.9 4.8 4.0 0.2 2.9 5.1 31.6 32.6 2.4 100
c5
n-
ic6
n2,2-DMCb 2,4-DMCS 2,2,3-TMC4 3,3-DMC5 2,3-DMC5 2-MCl3 3-MCl3 3-EC5 total (I
M, methyl; E, ethyl; DM, dimethyl; TM, trimethyl.
Table IV. Distribution of Multibranched Isomers moducts" 1 PtHY9, 5% convn 1 PtHY9,50% convn equilibrium
12 25 19.2
58 34 49.5
7 15 15.1
23 25 11.6
0 1 4.6
DM, dimethyl; TM, trimethyl.
products are no longer primary and the monobranched isomers are the main products in a wide range of conversion. The greater the platinum content, the more favored are these monobranched isomers. The bi- and tribranched isomers (B) seem also to be formed initially but in much lower quantities; the curves representing their formation are practically tangent to the x axis. Monobranched isomer distributions depend very little on the catalyst and conversion (Figure 5). Methylhexanes constitute over 97% of M, ethylpentane content being at best equal to half of its equilibrium value. 2- and 3methylhexanes are in a ratio close to equilibrium ( N 1). The distribution of multibranched isomers (B) changes with the conversion (Figure 6); the 2,3- and 2,4-dimethylpentanes which predominate see their quantities decrease in favor of 2,2- and 3,3-dimethylpentanes and 2,2,3-trimethylbutane. All the dimethylpentanes seem to be formed initially on all the catalysts; 2,2,3-trimethylbutane, however, is not a primary reaction product on either 1 PtHY3 or 1.5 PtHY3. In Figure 7 the isomerization/cracking activity ratios (I/C) are plotted against the conversion rates. I/C increases when the conversion decreases; the higher the platinum content, the higher is the I/C value, which becomes very high (>50) for a zero conversion rate with l PtHY3 and 1.5 PtHY3. Figure 8 shows the change of I/C measured at a 10% conversion rate against the platinum content of the catalysts. I t can be noticed that the I/C ratio, practically equal to zero for HY3, increases almost linearly until the platinum content reaches a value of 1, mol h-' g-l (Table 11). which corresponds to uH = 20 X For a wt % Pt of over 0.4 (aH value of over 7.5 X mol h-' g-l), the global activity remains constant (Figure 2), which means that above this value the isomerization activity increases at the expense of the cracking activity. Like I/C, the ratio of the monobranched/multibranched
formation rates (M/B) increases when the conversion decreases (Figure 9) and the platinum content increases (Figure 8). The cracking product distribution is in all the cases little different from that shown in Table 111. In particular, the isobutaneln-butane molar ratio is always high on these PtHY3 catalysts (-30 at 50% conversion) and varies little against both the conversion and the hydrogenation activity. Effect of the Si/Al Ratio in the Zeolite. The higher the Si/Al ratio, the more reduced is the activity. Thus while their hydrogenation activities are comparable, 1 PtHY35 and 1PtHY9 are respectively 30 and 6 times less active than 1PtHY3. 0.6 PtHY9 has the same activity as 1PtHY9. No deactivation is observed for these catalysts (a = 0 (Figure 3)). Figure 10 gives as an example the distribution for PtHY9 of M, B, and C as a function of the total conversion. Monobranched isomers (M) are the only primary products (maximum quantity over 40%). Multibranched isomers (B)appear only beyond a 5% total conversion. This is even more definitely so with cracking products, which appear in significant quantities only beyond a 30-40% total conversion. On 1PtHY35 a hydrogenolysisreaction (2% C1 + Cz at 50% conversion; low value of the isobutane to normal butane molar ratio (Table 111))can be observed. If this reaction is not taken into account, the n-heptane isomerization/cracking selectivity of 1 PtHY35 can be considered as intermediate between those of 1PtHY3 and 1PtHY9: I/C is higher for 1PtHY9 than for 1PtHY35 and 1PtHY3 (Figure 7). The ratio M/B increases from 1 PtHY3 to 1 PtHY9 to 1 PtHY35 (Figure 11). As is the case with PtHY3 catalysts, the monobranched isomer distribution changes very little with the conversion, the g-methyl/Qmethylhexane ratio remaining slightly higher than 1. For very low values of conversion, however, it increases significantly; thus it equals 1.4 on 1PtHY9 at
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
484
WT a
60.
40
20
80
40
h
20 r
WT
40
60
80
401I
I
20
0
%
60-
I
e
c
60-
WT
40
$0
80
% f
P
P
Figure 4. n-Heptane transformation on PtHY3 catalysts. Pc -centage of monobranched isomers (A),bibranched and tribranched isomers (e),and cracking products (A)as a function of the total conversion rate ( X I : (a) HY3; (b) 0.1 PtHY3; (c) 0.2 PtHY3; (d) 0.4 PtHY3; (e) 1 PtHY3; (f) 1.5 PtHY3.
1YO conversion (Figure 12). The multibranched isomer distribution changes also very noticeably with the conversion (Figure 13). Table IV allows comparison of the multibranched isomer distributions obtained on 1 PtHY9 for 5 and 50% conversions and their equilibrium distribution (Stull et al., 1969). 2,3-Dimethylpentane is initially highly favored but disappears for the benefit of 2,2- and 3,3-dimethylpentanes. The 2,4-dimethylpentane amount, however, remains practically constant. 2,2,3-Trimethylbutane, moreover, is only formed, like cracking products, for very high conversions.
Discussion With catalysts containing both a metal and an acid support, three reactional schemes can be considered: purely acid, purely metallic, and bifunctional. This latter mechanism implies the following five steps: (a) dehydrogenation of the alkane on the metal, (b) transport of olefins from the metal sites where they are formed to the acid sites; (c) isomerization or cracking of olefins on the acid sites through carbenium ion intermediates; (d) transport of the olefins produced from the acid to the metallic sites;
(e) hydrogenation of latter olefins on the metallic sites. The nature of the limiting step depends obviously on the characteristics of the acid and the hydrogenating sites, namely on their respective activities. Bifunctional catalysis is defined as being ideal when reaction c is kinetically limiting (Pichler et al., 1972; Weitkamp et al., 1978). After having determined quantitatively the necessary conditions for an ideal bifunctional catalysis, we will show that the selectivity of the catalysts depends here again on the relative concentrations of hydrogenating sites and active acid sites. Influence of Acid and Hydrogenating Site Concentrations on the Catalyst Characteristics. The number of hydrogenating sites (apt),considered as the number of accessible platinum atoms, was calculated for all the catalysts from the platinum content and dispersion. Two numbers of acid sites were considered: the theoretical number (aAl)drawn from the chemical formulas of the zeolites and the number of the acid sites for which the NH, adsorption heat (measured for a platinum-free zeolite) was ) . this value it is in greater than 100 kJ mol-' ( n ~ ~Below fact very likely that the acid sites are practically inactive
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
485
mole % a
-
40
40‘
20.
20-
A
XC%)
A - L
4 20
0
60
40
80
0
60{
10
iI-==-
60
40
%
-
20
-
.
40
60
80
0
X(X,
*--A0
40
1
m
Yt
--*---
A
80
C
I 20
i
Figure 5. n-Heptane transformation on PtHY3 catalysts. Distribution of monobranched isomers as a function of the total conversion rate (XI: 2-methylhexane ( 0 ) ;3-methylhexane (A);3-ethylpentane (A);(a) HY3; (b) 0.1 PtHY3; (c) 0.2 PtHY3; (d) 0.4 PtHY3; (e) 1 PtHY3; (f) 1.5 PtHY3.
product distribution of the platinum-containing catalysts for the skeletal isomerization or for the cracking of alkenes is quite different from that of the pure zeolite (Table 111). in step c of the n-heptane transformation (Bourdillon, 2. All the platinum-containing catalysts (except 1 1985). PtHY35) show a very low hydrogenolysis activity (very low Dealuminated samples have obviously a proportion of C1 and C2content). This means that the contributions of “strong” acid sites greater than HY3; actually nAl/nA2 a metal-catalyzed reaction to cracking can be considered decreases as the degree of dealumination increases (Table negligible. Moreover, since the Pt catalysts have small I). However, nA2cannot be considered as the number of metallic particles, one could expect the cyclic isomerization active sites as it includes both Lewis and Brtansted acid mechanism to occur on these particles (Corolleur et al., sites, while alkene skeletal isomerization, in particular, 1972), which is not the case since no cyclopentanic or involves only the latter (Irvine et al., 1980). Moreover, all aromatic compounds are found among the products (exthe accessible platinum atoms are likely to show the same activity, but for acid sites, the stronger the site, the greater cept on 1 PtHY35). Effect of n A / n P ton the Activity. The activity is its activity (Bourdillon, 1985). Therefore, nA2/nPt(and changes with nA/npt as can be predicted for bifunctional hence nAl/nPt)cannot be directly correlated to the ratio between the acid and hydrogenating activities of the catcatalysis. alysts. nAl/nPtvaries from 13 to 1250 and n ~ ~ from / n ~ ~ For nA2/npt> 35 (nAl/npt> 270) the higher the value of this ratio, the lower is the activity of the PtHY3 cata2.5 to 170 (Table I). lysts. This means that the metallic sites are not sufficiently Before discussion of the effect of the relative concennumerous or active for all the acid sites to be used at the trations of hydrogenating and acid sites, a few preliminary maximum of their activity; part of them are not fed with remarks should be made. olefins. This is all the more so as npt decreases. 1. All the platinum-containing catalysts except 1 PtHFor nA2/nPt< 35 (nAl/nR< 270) PtHY3 catalyst activity Y35 are considerably more active than HY3, so that in is constant. This means that the metallic sites are suffimost cases the contribution to the reaction of the purely ciently numerous (or active) to feed all the acid sites with acid-catalyzed process can be neglected; moreover, the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
486
mole
%
"1
60
mole %
.
40-*'\.
-
+-----A-
,c4-
yl 0
10
mole
40
60
&-*
*-*
20-
-
A
*,
o~~-o----oL
-
G
XK
-
A
1
80
c
10.
-
60
b
X % 2b
0
40
60
BO BO
0
mole K 60
m
C
so
-
20
mole
40
BO
60
%
t
20
0
'
40
60
80
Figure 6. n-Heptane transformation on PtHY3 catalysts. Distribution of bi- and tribranched isomers a8 a function of the total conversion (X): 2,2-dimethylpentane (*); 2,3-dimethylpentane (0);2,4-dimethylpentane (A); 3,3-dimethylpentane (0); 2,2,3-trimethylbutane(A);(a) HY3; (b) 0.1 PtHY3; (c) 0.2 PtHY3; (d) 0.4 PtHY3; (e) 1 PtHY3; (0 1.5 PtHY3.
olefins. A single metallic site is sufficient to feed 35 A2 or 270 Al sites with olefins. As the activity now depends only on the acidity, the more acid the zeolite, the more active is the catalyst, 1 PtHY3 > 1 PtHY9 > 1 PtHY35. Effect of n A/n on the Product Distribution. Table V shows the changes of the apparent reactional scheme of n-heptane transformation (i.e., the reactional scheme that can be deduced from the distribution of the products in the gas phase at low conversion rates) with the nA/npt values. For nA2/nR> 35 (nAl/nR > 270) all the isomers and all the cracking products are formed even a t very low nheptane conversion. This does not mean, however, that the cracking products are the result of the direct scission of linear C7 olefins since isobutane and not n-butane is produced. During their diffusion between two metallic sites, part of the intermediate olefins encounter enough acid sites (even more than necessary) for their ultimate transformation into cracking products or into coke, which produces in this case a significant deactivation. For nA2/npt< 35 (nAl/nR < 270) cracking follows isomerization. If the activity remains constant, the selectivity continues to change significantly with nA/nPt.
Table V. Apparent Reactional Schemes of n -Heptane Transformation nAl/nPt nA2/nPt apparent reactional scheme" >270
I(M
>35
7 , n-C,
' c
75-270 (75
10-35 lo%. The low value of the ratio found on the other catalysts like PtCaY is therefore due to a rapid interisomerization by .(.ethyl shift (reaction 14) of methylC
+f-
C-C-C-C-C-C
C
----
I +
C-C-C-C--C-C
(14)
hexenes formed by reactions 7 and 8. This methyl shift is definitely slower than the ethyl shift (reaction 13) since the percentage of ethylpentane this latter reaction produces does not decrease with the conversion even at low values. The conclusion that the ethyl shift is faster than the methyl shift has already been drawn (Le Normand et al., 1982; Fajula, 1985). However, the reason for which the 2-methylalkane/3methylalkane ratio tends toward the kinetic ratio (value expected for a protonated cyclopropane mechanism) when the length of the chain increases is not clear. It must be admitted that the q~tesoL rearrangements by methyl shifts in methylalkanes compared with their rate of formation by protonated cyclopropane intermediates from the linear alkane decrease when the length of the chain increases. It is true that statistically the longer the chain, the more unfavored will be the formation of the 2-methylalkane: one carbocation out of four can lead to 2-methylhexane from 3-methylhexane, wheL ~3 with methylnonanes only one out of seven will do so. Reactions 9-1 1 in which protonated cyclopropanes intervene can explain the isomerization of methylhexanes into dimethylpentanes. To account for the distribution of dimethylpentanes it is not possible to reason statistically in the same way as in the case of methylhexanes since the carbocations are not all secondarv but in part tertiary (hence highly favored). It can be predicted, however, that 2,3-dimethylpentane will be highly favored since (1)it can be formed through reactions 9-11, whereas the other dimethylpentanes are formed only by one of these three reactions, and ( 2 ) 2,3-dimethylpentanic carbocations formed by reactions 9 and 11 are tertiary ones, whereas the other dimethylpentanic carbocations formed are secondary. In agreement with this 2,3-dimethylpentane is highly favored at low conversion rate on 1PtHY9, and the 2,2- and 2,3-dimethylpentanes are very unfavored (Figure 13). Curiously, however, 2,4-dimethylpentane formation is relatively fast in spite of the intervention of secondary
carbocations. This could be explained by supposing that trialkyl (or tmaalkyl) protonated cyclopropanes are fairly unstable in comparison with dialkyl ones (Chevalier, 1979). Reaction 10 would then be favored compared to reactions 9 and 11. This would account for the fact that the formation of both 2,3- and 2,4-dimethylpentanes is favored while that of 2,2- and 3,3-dimethylpentanes is inhibited. It can be noted, moreover, that on all the catalysts the dimethylpentanes formed isomerize between themselves very rapidly by alkyl shift as do methylhexanes.
Conclusion This paper brings out quantitatively the relation that exists between the activity, the stability, and, more psrticularly, the selectivity of bifunctional catalysts on one hand and on the other the relative concentration in acid sites and hydrogenating sites. Thus in the case of Y zeolites an “ideal hydroisomerization catalyst” must have at least one metallic site per 10 acid sites. Such a catalyst should be particularly well adapted for n-pentane and n-hexane isomerization and should also be of interest as ideal hydrocracking catalyst because it should be possible with such a catalyst to avoid at best the production of light products (gas) by secondary cracking. Acknowledgment G.G. thanks the “Consejo de !’esarollo Cientifico y Humanistic0 de la Universidad Central de Venezuela” for grants. Registry No. Pt, 7440-06-4; n-heptane, 142-82-5.
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Received for review July 2, 1985 Revised manuscript received February 5, 1986 Accepted March 31, 1986
