Effect of zeolite structure and acidity on the product selectivity and reaction mechanism for n-octane hydroisomerization and hydrocracking

Article abstract of DOI:10.1006/jcat.1998.2337

The activity, product selectivity, and stability of a series of bifunctional zeolite catalysts, primary ZSM-12, USY, and β-zeolite, with different Si/Al ratios were compared for the hydroisomerization and hydrocracking of n-octane. The performance of L-zeolite and mordenite was examined to a lesser extent as well. It was found that the activity per acidic site decreases at the initial stage (1 h on stream) in the following order: ZSM-12 > β-zeolite > mordenite > USY > L-zeolite. For extended periods of operation, the activity of ZSM-12 remains unchanged. The superior stability of ZSM-12 even under accelerating coking conditions results from its unique pore structure, which does not favor coke formation. Its one-dimensional noninterpenetrating puckered channels (5.5 × 6.1 A) act as perfect tubes, which do not trap coke precursors. The branched product selectivity increases with the increase in Bronsted acid site strength of the zeolite catalysts, and thus hydroisomerization is favored at the expense of cracking at a higher Bronsted acid strength. USY-5.8 (CBV-712) showed relatively high initial activity with respect to other USYs. This is probably related to its high surface Al content. The Bronsted acid strength of the USY zeolites decreases in the order USY-2.6 > USY-28 > USY-5.8. The 2,2-DMC₆ and 3,3-DMC₆ isomers are not favored as final products due to their bulky molecular size even in USY. In addition, the 2,2-DMC₆ species is more abundant than 3,3-DMC₆ because the rate of isomerization by PCP intermediates decreases in the following order: 2-MC₇ > 3-MC₇ > 4-MC₇. The 2,3-DMC₆ concentration is much higher than that predicted by equilibrium, which indicates that the interconversion of 2,3-DMC₆ to other dibranched isomers is not preferred. The i-C₄/n-C₄ ratio detected depends on both the reaction temperature and zeolite pore structure/acidity. Aluminium content determines the type of β-scission. For zeolites with a high concentration of acid sites (Si/Ai < about 30), type A β-scission dominates at low temperature, while at lower Al content, type A, B, and C β-scissions are equally important.

Full text of DOI:10.1006/jcat.1998.2337

Journal of Catalysis 182, 400–416 (1999)
Article ID jcat.1998.2337, available online at http://www.idealibrary.com on
Effect of Zeolite Structure and Acidity on the Product Selectivity
and Reaction Mechanism for n-Octane Hydroisomerization
and Hydrocracking
Wenmin Zhang and Panagiotis G. Smirniotis1
Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221–0171
Received June 17, 1998; revised November 18, 1998; accepted November 19, 1998
INTRODUCTION
The activity, product selectivity, and stability of a series of bifunc-
The hydroisomerization of n-paraffins is of considerable
interest and plays an important role in the petroleum in-
dustry, because it provides branched paraffins which in-
herently possess higher octane numbers than n-paraffins.
Bifunctional catalysts with an acidic function balanced by
the metal function have shown high efficiency in hydro-
isomerization of n-paraffins (1, 2). It is well established
that over zeolites the isomerization of n-paraffins proceeds
through consecutive branching reactions while cracking re-
actions occur in parallel with the isomerization; and multi-
branched paraffins are more susceptible to cracking than
monobranched ones (2, 3). However, it is still not clear
how the strength of the zeolite acidic site affects the re-
action mechanism and the product selectivities, especially
those of the branched products.
tional zeolite catalysts, primary ZSM-12, USY, and -zeolite, with
different Si/Al ratios were compared for the hydroisomerization and
hydrocracking of n-octane. The performance of L-zeolite and mor-
denite was examined to a lesser extent as well. It was found that the
activity per acidic site decreases at the initial stage (1 h on stream) in
the following order: ZSM-12 > -zeolite > mordenite > USY > L-
zeolite. For extended periods of operation, the activity of ZSM-12
remains unchanged. The superior stability of ZSM-12 even under
accelerating coking conditions results from its unique pore struc-
ture, which does not favor coke formation. Its one-dimensional
˚
noninterpenetrating puckered channels (5.5 6.1 A) act as perfect
tubes, which do not trap coke precursors. The branched product
selectivity increases with the increase in Brønsted acid site strength
of the zeolite catalysts, and thus hydroisomerization is favored at
the expense of cracking at a higher Brønsted acid strength. USY-
5.8 (CBV-712) showed relatively high initial activity with respect
to other USYs. This is probably related to its high surface Al con-
tent. The Brønsted acid strength of the USY zeolites decreases in
the order USY-2.6 > USY-28 USY-5.8. The 2,2-DMC6 and 3,3-
DMC6 isomers are not favored as final products due to their bulky
molecular size even in USY. In addition, the 2,2-DMC6 species is
more abundant than 3,3-DMC6 because the rate of isomerization
by PCP intermediates decreases in the following order: 2-MC7 > 3-
MC7 > 4-MC7. The 2,3-DMC6 concentration is much higher than
that predicted by equilibrium, which indicates that the interconver-
sion of 2,3-DMC6 to other dibranched isomers is not preferred. The
i-C4/n-C4 ratio detected depends on both the reaction temperature
and zeolite pore structure/acidity. Aluminium content determines
the type of -scission. For zeolites with a high concentration of acid
sites (Si/Al < about 30), type A -scission dominates at low temper-
ature, while at lower Al content, type A, B, and C -scissions are
c
The aim of this work is to investigate the effect of the
strength of acid sites and the pore structure of selected 12-
membered-ring (MR) large-pore zeolites, namely, ZSM-12,
-zeolite, USY, mordenite (8-MR pores exist as well), and
L-zeolite on the product selectivities, reaction mechanisms,
and time-on-stream stability.
EXPERIMENTAL
Catalyst Preparation
In this study, parent ZSM-12 (Si/Al = 35) and -zeolite
(Si/Al = 15) were hydrothermally synthesized from the cor-
responding aluminosilicate gels. Details about the prepara-
tion procedures can be found in our previous work (4). The
synthesized parent zeolites were calcined in air at 520 C
for 4 h to burn the occluded template, and then were de-
aluminated with hydrochloric acid. Different levels of de-
alumination were achieved by varying the concentrations
of HCl solutions in the range 1 to 7 N HCl in a 1-liter flask.
Two grams of zeolites was used in each batch of dealu-
mination. It should be noted that ZSM-12 does not lose
its crystallinity even at the highest dealumination severity.
equally important.
1999 Academic Press
Key Words: ZSM-12; USY; -zeolite; n-octane; hydroisomeriza-
tion; hydrocracking; time-stable catalysts.
1 To whom all correspondence should be addressed. E-mail: Panagiotis.
Smirniotis@Uc.Edu.
400
0021-9517/99 $30.00
c
Copyright
1999 by Academic Press
All rights of reproduction in any form reserved.

n-OCTANE HYDROISOMERIZATION AND HYDROCRACKING
401
The USY samples were obtained from PQ Corporation. Acidity Measurement
Controlled steam treatment followed by lean acid leaching
The quantitative characterization of the zeolite surface
acidity (Brønsted and Lewis acid sites) was carried out with
NH3-stepwise temperature-programmed desorption (NH3-
STPD) experiments. The desorption curve consists of five
distinct peaks occurring at the following optimized temper-
ature ranges: 150 to 180 C, 180 to 250 C, 250 to 350 C, 350
to 440 C, 440 to 540 C. By coupling this technique with NH3
FT-IR, it was found that the first two peaks are mainly weak
and stronger Lewis acid sites, as well as weak Brønsted sites.
The other three peaks are Brønsted acid sites (IR stretching
frequency of NH+4 on AlO ) of increasing strength (5, 6).
The desorption started at 150 C to exclude any physisorbed
ammonia.
The sample of each protonated zeolite (50 mg) was held
by small plugs of Pyrex glass wool located in the middle of
a 6-mm-i.d. tube. Before starting the ammonia adsorption,
each sample was cleaned by purging He at 550 C for 1 h.
Then, anhydrous ammonia (4% in He) was admitted to the
sample for 1 h at 150 C to ensure the saturation of all acidic
sites of the zeolite. Helium was thereafter sent to the bed
to purge any physically adsorbed ammonia. This step was
maintained until no NH3 desorption was observed. Then,
the NH3-STPD profile, which consisted of isothermal steps
followed by steps of increasing temperature with a constant
rate, was started. The desorption experiment followed a
profile that started at 150 C and reached the targeted tem-
peratures (180, 250, 350, 440, and 540 C). After reaching
each temperature the bed temperature was held constant
for 60 min and then it was raised to the next temperature
within 30min. Blank experimentsconfirmed that at anydes-
orption temperature the background was stable. The STPD
experiments were repeated several times and good repro-
ducibility was obtained.
of the parent zeolite USY (CBV-100) was involved in the
synthesis of the dealuminated USY-5.8 (CBV-712) sample.
The CBV-712 sample was subject to severe acid leaching for
the synthesis of the USY-28 (CBV-760) sample. Mordenite
and L-zeolites were kindly donated to us from UOP. Am-
monium hexafluorosilicate was employed for the dealumi-
nation of mordenite and L-zeolite. An appropriate amount
of (NH4)2SiF6 dissolved in 200 ml H2O was added drop-
wise within 2 h, and kept at 90 C with stirring for 24 h. The
ammonium form of the zeolites was obtained by cation ex-
change of the dealuminated zeolites with a 2.0 N NH4Cl
solution at 90 C for 4 h. Finally, the zeolites were trans-
formed to their protonated form by calcination in air at
450 C for 1 h. The loading of Pt on the catalysts (0.5 wt% )
was conducted with wet impregnation of H2PtCl6 solution
(Aldrich, 8 wt% solution in water). The catalysts were dried
overnight in an oven at 120 C. The physical properties of
the zeolites are listed in Table 1. Each zeolite is designated
as its name followed by its bulk Si/Al ratio.
X-ray diffraction was employed for identification of the
synthesized zeolites and for determination of the crys-
tallinity of the dealuminated zeolites. The estimation of the
crystallinity was based on the areas under the main crys-
tallographic peaks of each zeolite. The bulk Si/Al ratios
were measured by wet analysis involving ICP spectroscopy.
For the analysis, lithium metaborate was used for the fu-
sion of zeolite powders (900 C), which subsequently were
dissolved in HNO3 solution. The sodium content was ex-
tremely low for the majority of the protonated zeolites.
TABLE 1
Zeolite Physical Properties
Dealumination agent/
method
Si/Al
(bulk)
Catalytic Experiments
Zeolite
ZSM-12-35
ZSM-12-40
ZSM-12-54
ZSM-12-58
ZSM-12-69
Crystallinity
n-Octane (Aldrich, >99.5% purity) wasused asthe probe
molecule. The impurities were mainly 3-methylheptane and
2,2,4-trimethylpentane, and they were excluded in calcu-
lating the product selectivities. The experiments were car-
ried out in a flow reactor system incorporating a 14 -in.-o.d.
stainless-steel reactor. The catalysts were activated in situ
by oxidation with high-purity oxygen for 1 h at 450 C, fol-
lowed by purging with ultrahigh-purity He (Wright Broth-
ers, 99.998% ) for 15 min. The reduction of the catalyst was
carried out at 450 C in high-purity H2 for 1 h at atmospheric
pressure. For the catalytic experiments, H2 was provided
from the gas cylinder to the reactor while the pressure reg-
ulation was achieved with a backpressure regulator placed
at the exit of the reactor tube. In all the experiments, 50 mg
of fresh catalyst was loaded on top of a glass wool plug. The
feed was introduced into the reactor at a predetermined
flow rate through a special septum injection port into the
As synthesized
1.0 N HCl
3.0 N HCl
4.0 N HCl
5.0 N HCl
35
40
54
58
69
100
100
100
100
100
Beta-15
Beta-85
Beta-132
As synthesized
5.0 N HCl
7.0 N HCl
15
85
132
100
79
90
USY-2.6(CBV-100)a
2.6
5.8
100
81
USY-5.8(CBV-712)a
Steaming + lean acid
leaching
USY-28(CBV-760)a
Steaming + strong acid
28.0
72
leaching
MOR-9.5b
MOR-36
LTL-3.8b
9.5
35.7
100
71
25 g (NH4)2SiF6
3.8
100
a Provided by PQ Corporation.
b Provided by UOP.

402
ZHANG AND SMIRNIOTIS
heated line using a liquid infusion pump (Cole–Parmer) or
a syringe pump (ISCO Model 100DM) in the case of time-
on-stream experiments.
TABLE 2
Area Percentage of Different Ammonia Desorption Peaks That
Correspond to Lewis and Brønsted Acid Sites of Variable Strength
Determined by NH3-STPD Experiments
The product identification was accomplished by a high-
resolution gas chromatograph (Hewlett–Packard, 5890
Series II) equipped with a mass spectrometer (Hewlett–
Packard, 5972 Series II). The reactor effluent stream was
sent for analysis through a heated line (about 180 C) to
the gas chromatograph. The GC/MS unit was attached to
a PC unit for data acquisition and processing. The GC
was equipped with a high-performance capillary column
(SUPELCO Petrocol DH50.2, fused silica with bonded
dimethylsiloxane, 0.2-mm i.d., 50-m length, and 0.5- m film
thickness). By optimizing the flow of carrier gas in the
column and the temperature profile in the GC oven, we
achieved efficient separation of all the products of the re-
action.
Area (% )
TB
Zeolite Si/Al (bulk) Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 ( C)a
ZSM-12-35
ZSM-12-40
ZSM-12-54
ZSM-12-58
ZSM-12-69
35
40
54
58
69
11.3
7.5
6.7
6.1
6.6
21.0
16.4
18.8
19.6
19.3
42.0
48.9
48.8
54.3
51.7
13.3
10.7
8.7
8.0
9.5
12.4 404.4
16.6 405.4
16.7 403.5
12.1 391.7
12.9 395.9
Beta-15
Beta-85
Beta-132
15
85
132
10.3
7.0
8.4
22.2
23.3
25.6
27.8
27.8
22.0
19.2
19.1
20.6
20.5 436.1
22.7 439.4
23.5 448.7
USY-2.6
USY-5.8
USY-28
2.6
5.8
28.0
12.6
13.5
9.4
33.1
35.5
35.6
28.7
36.4
33.1
12.2
7.5
10.3
13.5 419.6
7.1 391.1
11.7 409.0
The reaction was carried out under a wide range of oper-
ating conditions: temperature, 180–450 C; liquid feed flow
rate, 0.03–8.0 cm3/h; H2 flow rate, 4–320 cm3/min; total pres-
sure, 100 psig. With the exception of the time-on-stream
experiments, all the data were collected at 1 h on stream.
The product selectivities (mol or wt% ) are defined as the
concentration of the individual component over the con-
centration of all products in the effluent stream.
MOR-9.5
MOR-36
9.5
35.7
7.0
5.5
9.5
9.0
23.1
41.7
34.6
30.9
25.9 446.1
12.9 414.8
LTL-3.8
3.8
10.0
33.3
37.1
9.8
9.9 400.4
a
Average NH3 desorption temperature from strong acid sites (last
three peaks).
Brønsted sites (peak 3 in Table 2). Moreover, the dealu-
mination results in an increase in the ratio of the total num-
ber of strong sites with respect to weak sites (Table 3). For
any extent of dealumination used in this study, the above
ratio reaches a value of about 3. The average NH3 des-
orption temperature from strong acid sites, TB, decreases
with dealumination over ZSM-12, which indicates a slight
decrease in the average acid site strength. In contrast, for
-zeolite, with an increase in the Si/Al ratio, the percent-
age of medium and strong Brønsted sites increases, and TB
RESULTS AND DISCUSSION
Zeolite Surface Acidity
NH3-STPD coupled with FT-IR enables us not only to
distinguish Brønsted and Lewis acidic sites, but also to
quantify the number of acid sites with different strength.
As proved in our previous work (5, 6), we can obtain up to
five distinct desorption peaks of NH3 chemisorbed on acid
sites in the range 150 to 540 C. According to FT-IR experi-
ments carried out at the same NH3 desorption temperature,
the first two peaks in the NH3-STPD response are believed
to be Lewis acid sites, and weak Brønsted sites. With the
exception of USY the other three peaks are Brønsted acid
sites. For the USY samples, due to the incomplete separa-
tion of Lewis and Brønsted sites for the first three peaks in
STPD, the deconvolution of STPD peaks based on the IR
spectra was used to obtain the numbers of Brønsted and
Lewis acid sites. The acidity characteristics of the different
zeolites used are presented in Tables 2 and 3. To quantita-
tively evaluate the average strength of the strong sites (last
three peaks in STPD experiments), we introduce, TB, the
average desorption temperature of NH3 from strong acid
sites, which is defined as
TABLE 3
Acidity Characteristics of the Zeolites as Determined
by NH3-STPD
Si/Al
(bulk)
ml NH3
(STP)/g
No. NH3/
No. Al
Strong/
weak
Zeolite
ZSM-12-35
ZSM-12-40
ZSM-12-54
ZSM-12-58
ZSM-12-69
35
40
54
58
69
10.4
8.8
6.8
6.9
5.7
0.98
0.96
0.99
1.09
1.06
2.09
3.19
2.91
2.89
2.87
Beta-15
Beta-85
Beta-132
15
85
132
24.6
4.0
2.7
1.01
0.91
0.94
2.08
2.30
1.94
USY-2.6
USY-5.8
USY-28
2.6
5.8
25.6
12.0
5.2
0.24
0.47
0.87
1.19
1.04
1.22
28.0
350 (area of third peak)+440 (area of fourth peak)+540 (area of fifth peak)
.
TB
=
total area of third, fourth, and fifth peaks
MOR-9.5
MOR-36
9.5
29.3
9.3
0.81
0.91
5.08
5.90
35.7
For ZSM-12, at any extent of dealumination, about 50%
of the total number of acid sites correspond to weak
LTL-3.8
3.8
28.2
0.27
1.31

n-OCTANE HYDROISOMERIZATION AND HYDROCRACKING
403
increases. Thus, the average Brønsted acid site strength in- were obtained by varying the reaction temperatures in the
creases with dealumination. On the other hand, the number range 195 to 415 C. The catalytic activity of ZSM-12 de-
of total acid sites decreases significantly with the increase creases with the increase in Si/Al ratio. This is as expected
in the Si/Al ratio as expected. Other researchers using CO since by increasing the Si/Al ratio the Al content of the ze-
as probe base found a similar trend, namely, an increase olites decreases; that is, the number of acid sites decreases.
in strength of Brønsted sites with the dealumination of
-
-Zeolite demonstrates behavior similar (Fig. 1b) to that of
zeolite (7). Regardless of the dealumination level, the ratio ZSM-12.
of strong to weak acid sites of -zeolite remained in the
In contrast, this monotonic decrease in activity was not
vicinity of 2. It is worthwhile to note that for both ZSM-12 observed over the dealuminated USY zeolites (Fig. 1c).
and -zeolites there is a one-to-one relation (Table 3) be- USY-5.8 demonstrates a higher activity than its parent ze-
tween the NH3 molecules chemisorbed on all acid sites and olite USY-2.6 at any temperature, although it possesses
the total number of Al atoms (nonframework and frame- less Al. Haag and Lago (9) observed a similar behavior,
work), which indicates that ammonia effectively probes all namely, steam dealuminated Y faujasite is more active than
the acid sites.
its parent zeolite for catalytic cracking reactions. The same
One, however, can see that the parent USY (USY-2.6) has trend in activity was also observed over steam dealumi-
the highest percentage of medium and strong Brønsted sites nated ZSM-5 for n-heptane cracking (10) and steam dealu-
(peak 4 and 5 in Table 2) among all the USYs tested. The minated mordenite for the disproportionation reaction of
increase in the severityofsteam treatment decreasesthe rel- o-xylene (11). The latter group attributed this increase in
ative population of medium and strong Brønsted acid sites disproportionation reactions to the formation of Lewis acid
(compare the USY-2.6 and USY-5.8 samples in Table 2). sites during the steam dealumination.
Other researchers also observed that by increasing steam
In fact, as one can see from our STPD results in Table 2,
severity, the strength of Brønsted acid sites and the ratio of USY-5.8 has the lowest average strong acid site strength
Brønsted/Lewis sites decrease (8). It is interesting to note among the three USY zeolites used. Thus, the enhancement
that after severe acid leaching of the steam dealuminated of activity cannot be attributed to the increased number or
USY sample (USY-5.8) to generate USY-28, the Brønsted strength of strong sites since both of them decrease with
acid site strength of the sample increases again (Table 2). steam dealumination. A similar conclusion was obtained
Thus, one may conclude that when steam treatment is in- recently by other researchers (12, 13).
volved in dealumination, Brønsted sites are preferentially
Our XPS investigations indicate that the USY-5.8 sam-
removed, while when acid leaching is used, Lewis acid sites ple possesses uneven Al distribution in comparison with
and weak Brønsted sites are extracted first. The decrease the other samples tested. Significantly higher Al concen-
in the average strong acid site strength index, TB, in the trations were detected on the surface of the USY-5.8 than
sequence USY-2.6 > USY-28 USY-5.8, is consistent with on other USY samples. We believe that the increased ac-
the above discussion.
tivity of USY-5.8 is due to this high surface Al content gen-
It seems that the ratio of strong to weak acid sites (B/L) erated by the steam treatment. The treatment with steam
is a characteristic property of different zeolites, and it does removed Al from framework tetrahedral positions, which
not change very much with the Si/Al ratio. As shown in migrated to the surface. Others (14) also found that steam
Table 3, very different values of the B/L ratio were ob- dealuminated USY samples (CBV families) have a much
served for each zeolite, i.e., about 2 for -zeolite, about 1.5 higher content of surface Al than the as-synthesized par-
for USY, about 3 for dealuminated ZSM-12, and about 5 for ent USY. They also found that steam-only dealuminated
mordenite. Moreover, the amount of NH3 adsorbed per Al Pt/HUSY catalysts were more active than either parent or
atom can be lower than one. We observed that for zeolites further acid-leached USYs for the hydroisomerization of
with inherently high aluminum content such as USY, mor- n-heptane and n-decane. This indicates that the intracrys-
denite, and L-zeolite, when the Si/Al ratio is below about 20, talline mass transfer resistance at this range of tempera-
the number of ammonia molecules chemisorbed per num- ture plays some role even with large-pore zeolites like USY
ber of Al atoms (bulk) is lower than unity. This trend was (15). It is also reported that the HY-zeolite crystallites are
observed over the above three zeolites reproducibly and severely diffusion limited for the cracking of long-chain
indicates that probably at high concentrations of Al, each paraffins; and the reduction of crystallite size increases the
ammonia molecule is associated with more than one acid cracking activity of HY and reduces isomerization (16, 17).
site and/or all the aluminum sites are not equally accessible Dadyburjor and Bellare (18) also found that there exists a
to ammonia molecules.
strong diffusion resistance for the cumene cracking reaction
over REY zeolites in the temperature range 225–575 C. It
is worth noting that further dealumination by acid leaching
(from CBV-712 to CBV-760) establishes uniformity of the
aluminum distribution in the zeolite crystals (14). Indeed,
our catalytic results (Fig. 1c) validate the above changes in
Catalyst Activity
The effect of the Si/Al ratio of different zeolites on the
catalytic activity is shown in Fig. 1. Different conversions

404
ZHANG AND SMIRNIOTIS
FIG. 1. n-Octane conversion versus temperature over (a) Pt/HZSM-12, (b) Pt/HBeta, and (c) Pt/HUSY at WHSV = 7 h 1, H2/n-C8 (mole) = 16,
total pressure of 100 psi, and Pt loading level of 0.5 wt% .
the Al distribution of USY as a function of the method and tion temperature <250 C) play no role in the n-hexane iso-
extent of dealumination. USY-5.8 is the most active cata- merization at the reaction temperature of 200 C. From our
lyst among all the USYs used at any reaction temperature, work we observed that at comparable Si/Al ratios and 1 h on
followed in activity by USY-2.6 (even though the latter one stream, the zeolite activity decreases in the following order:
is richer in Al). USY-28 acquires the lowest activity as ex-
ZSM-12 > -zeolite > mordenite > USY > L-zeolite.
pected. USY-28 is also the least active at any temperature
in comparison even with the most severely dealuminated
It is worth noting that ZSM-12 with Si/Al = 35 is much
ZSM-12 and -zeolite, though they possesses much higher
more active than USYs although the latter ones contain
Si/Al ratios (69 and 132, respectively).
significantly higher number of Al atoms. The activity of
A comparison between different types of zeolite cata-
ZSM-12, however, decreases rapidly with dealumination.
lysts based on turnover frequencies at 285 C (after 1 h on
The TOF of ZSM12-69 is lower than that of Beta-85
stream, Table 4) reveals that ZSM-12 is generally more ac-
(Table 4).
tive than all other zeolites. One reason for this behavior is
that the ZSM-12 samples have relatively high strong/weak
acid site ratiosasshown in Table 3. Ifthe strongsitesare con-
Product Distribution versus Zeolite Pore Structure
sidered as being the active sites for the reaction, the differ-
ence in TOF values becomes smaller. This would agree with
the general belief that only Brønsted acid sites are strong
enough to catalyze the skeletal isomerization and cracking
reactions of alkanes at current mild reaction temperatures.
Additional supportive evidence indicates (19) that Lewis
acid sites (acid sites corresponding to NH3-TPD desorp-
The product distribution over the zeolite catalysts inves-
tigated at comparable n-octane conversion in the vicinity
of about 50% is shown in Table 5. The same types of prod-
ucts are obtained over all catalysts, which include in general
(a) isooctanes (monobranched and dibranched C8 paraffins,
and trace amounts of trimethylpentanes), (b) cracked prod-
ucts (light hydrocarbons) with carbon number less than

n-OCTANE HYDROISOMERIZATION AND HYDROCRACKING
405
(2,2-DMC6), 3,3-dimethylhexane (3,3-DMC6), and even 3-
ethyl-2-methylpentane (3-E, 2-MC5), were observed in the
products. This is not as expected because their sizes are
comparable to those of tribranched isomers as suggested
in the references (22, 23). Indeed, we found (24) that 2,2,4-
trimethylpentane in thistemperature range can diffuse even
in the pores of ZSM-12 and further cracked to lighter
hydrocarbons. This means that the effective pore size of
ZSM-12 is larger than its crystallographic pore diameter.
Other researchers (25) suggested that the openings of
TABLE 4
Turnover Frequency (s 1) of Different Zeolite Catalysts for the
n-Octane Conversion at 285 C Based on Different Definitionsa
Based on all
Based on strong
Based on all
acid sitesb
sitesc
10
Al atomsd
3
3
3
Zeolite
(
10
)
(
)
(
10
)
ZSM-12-35
ZSM-12-54
ZSM-12-69
28.3
12.8
8.9
39.4
16.7
12.2
27.7
12.7
9.4
Beta-15
Beta-85
Beta-132
8.6
11.7
9.4
12.7
16.7
14.4
8.7
10.6
8.8
˚
ZSM-12 are larger (5.6 7.7 A) than what is commonly
believed. Similarly, it was also reported (26) that the effec-
˚
tive pore dimensions of mordenite were 8.2–8.6 A, which is
USY-2.6
USY-5.8
USY-28
5.0
18.9
3.3
13.3
37.2
6.1
1.1
8.9
2.9
˚
1.5–1.6 A larger than the calculated crystallographic values.
It is well established that the hydroisomerization re-
actions proceed through successive mono-, di-, and tri-
branched intermediates formed by type A (methyl shift)
and B [via protonated cyclopropane (PCP)] isomerization
mechanisms. The former type of isomerizations results in
products with the same number of branches as the reac-
tant. Isomerizations via PCP intermediates are responsible
for the net increase or decrease in the extent of branching
of the reaction products. At the same time, hydrocrack-
ing follows type A (tertiary carbenium ion to tertiary), B1
(secondary carbenium ion to tertiary), B2 (tertiary to sec-
ondary), C (secondary carbenium ion to secondary), and D
(secondary carbenium ion to primary) -scissions (1, 16, 27,
28). Our experiments agree with the above reaction mech-
anisms even for zeolites with high Si/Al ratios, which were
not commonly discussed in the past. As shown in Fig. 2, at
a very short space time, isomers are the only primary prod-
ucts over USY-28. The selectivity of isomers passes through
a maximum, which is a typical behavior of consecutive re-
actions. The rate of hydrocracking, as a secondary reaction,
increases rapidly with the space time. Other zeolites fol-
lowed a similar behavior.
MOR-9.5
MOR-36
8.7
5.5
10.2
6.4
7.0
5.0
LTL-3.8
2.7
4.8
0.7
a Catalyst weight, 50 mg; n-octane feed rate, 0.5 cm3/h; H2/n-C8, 16;
total pressure, 100 psig; 1 h on stream.
b Based on the NH3-STPD experiments (all desorption peaks were
included).
c Based on the acid sites with NH3 desorption from last three peaks.
d Based on Al amount from elemental analysis (ICP spectroscopy).
8, and (c) trace amounts of toluene, xylene, and C5 and
C6 naphthenes at relatively high temperatures. Products
with more than eight carbon numbers were not observed.
Methane was not detected under a wide range of the oper-
ating conditions involved, thus excluding the existence of
hydrogenolysis reactions of the feed occurring over metal
sites.
As expected, USY and -zeolites, due to their relatively
large pore sizes, allow to a higher extent the branching
of C8 paraffins at the same level of n-octane conversion
in comparison with ZSM-12 (Table 5). Trace amounts of
tribranched C8 as final products are observed over USY
and -zeolite catalysts. USY has a tridimensional channel
˚
structure with an aperture size of 7.4 A, and -zeolite con-
sists of 12-membered-ring pores of linear channels of about
˚
˚
7.3 6.5 A and tortuous channels of 5.5 5.5 A. Its channel
˚
intersections of about 10 A and render the structure tridi-
mensional (20). ZSM-12 possesses one-dimensional nonin-
˚
terconnecting tubular-like channels (5.5 6.1 A) (21). Tri-
branched C8 paraffins with a critical molecular diameter
˚
of about 7.0 A (22) were not detected as final products
over ZSM-12 at the current reaction temperatures used.
This is partially because of the relatively high tempera-
tures, which do not favor thermodynamically the formation
of tribranched isomers. Moreover, the fast cracking of the
latter hydrocarbons is responsible for the absence of tri-
branched hydrocarbons in the final products. Dibranched
alkane molecules, even those with two branches on the
FIG. 2. n-Octane conversion versus space time over Pt/HUSY-28 at
330 C, H2/n-C8 (mole) = 16, total pressure of 100 psi, and Pt loading level
same carbon of the main chain, such as 2,2-dimethylhexane of 0.5 wt% .

406
ZHANG AND SMIRNIOTIS
TABLE 5
Product Selectivities (mol%) of n-Octane over Different Zeolite Catalystsa
Productb
ZSM-12-35
ZSM-12-69
Beta-15
Beta-132
USY-2.6
USY-28
MOR-9.5
MOR-36
LTL-3.8
C1
C2
C3
i-C4
n-C4
i-C5
n-C5
0.2
15.8
15.5
11.5
9.1
0.1
21.7
16.8
14.8
11.2
8.0
0.3
24.6
35.0
14.8
13.4
2.9
17.1
32.4
18.5
11.6
3.1
11.5
12.4
8.6
6.8
3.0
7.1
8.3
4.7
4.6
0.5
3.5
8.2
3.4
2.8
0.4
16.9
24.2
11.8
9.5
14.5
20.4
11.7
9.6
4.8
2.0
1.6
DMC4
2-MC5
3-MC5
n-C6
0.02
0.07
0.04
0.05
0.05
0.4
0.2
0.03
0.02
0.05
0.02
0.01
0.06
0.06
0.04
0.06
0.1
0.2
0.1
0.01
0.02
0.04
0.2
DMC5
2-MC6
3-MC6
n-C7
0.02
0.01
0.05
13.6
13.7
5.0
1.7
0.8
1.5
2.7
2.4
0.4
0.8
0.2
0
0.01
0.03
0.02
0.04
4.1
5.5
2.2
0.8
0.5
0.3
1.4
0.9
0.3
0.4
0.1
trace
0.01
0.10
0.09
0.09
1.5
2.3
0.9
0.5
0.3
0.4
0.7
0.5
0.1
0.3
0.1
0.04
0.2
0.04
0.03
0.07
11.2
12.6
5.9
3.1
0.8
1.5
2.7
1.9
0.5
1.5
0.3
0
0.00
0.03
9.7
9.2
3.6
1.3
0.2
0.6
1.1
0.9
0.1
0.4
0.06
0
0.07
20.4
25.4
10.3
4.8
1.5
2.1
4.2
2.6
1.0
1.9
0.3
Trace
0
0.04
21.1
23.2
10.8
5.9
2.9
3.1
5.9
3.7
2.1
2.4
0.5
Trace
0
0.06
8.2
11.9
4.7
2.5
0.8
1.4
2.5
1.6
0.5
2-MC7
3-MC7
4-MC7
3-EC6
2,2-DMC6
2,3-DMC6
2,4-DMC6
2,5-DMC6
3,3-DMC6
3,4-DMC6
3-E,2-MC5
TMC5
Others
17.5
17.9
8.9
4.8
0.8
1.4
2.7
2.2
0.3
1.1
0.3
Trace
0.01
1.1
0.3
Trace
0
0.02
0
Conversion (% )
50.5
55.2
52.3
47.6
52.0
55.7
51.4
50.5
49.7
a Catalyst weight, 50 mg; varying n-octane feed rate with the same H2/n-C8 = 16; total pressure, 100 psig; temperature, 290 C; 1 h on stream.
b C1–C7 stand for alkanesfrom methane to heptanes;MC5, methylpentanes;DMC4, dimethylbutanes;DMC5, dimethylpentanes;MC6, methylhexanes;
MC7, methylheptanes; 3-EC6, 3-ethylhexane; DMC6, dimethylhexanes; 3-E,2-MC5, methylethylpentane; TMC5, trimethylpentanes; others, mainly
alkylcycloalkanes.
The transformation of n-octane via various isomerization product distribution of methylheptanes we observed de-
and cracking steps can be shown in Scheme 1.
viation from the predicted one based on the PCP mecha-
The rates of the various isomerization and cracking steps nism (Table 6). This is because the products of the B-type
were originally studied by Brower (28) for superacid homo- isomerization of n-octane (monobranched C8 isomers) are
geneous catalysts (HF) at relatively low temperature. Fur- further transformed via type A isomerization reactions and
ther study was carried out for metal-loaded zeolites and finally approach the thermodynamic equilibrium (Table 6).
a refined reaction rate relation was proposed as follows Equilibrium compositions at 290 C in Table 6 are inter-
(29, 30):
polated values based on calculation from thermodynamic
data reported in the literature (31). It is worth noting that
the above observation is valid in a wide range of Si/Al ra-
tios (compare Beta-15 with Beta-132). The zeolite struc-
ture can impose steric constraints even on monobranched
isomers. ZSM-12, possessing channels of smaller diame-
hydride shift > type A cracking > type A isomeri-
zation (alkyl shift) > type B isomerization (PCP
branching) > type B1 and B2 cracking > type C
cracking > type D cracking.
Type B isomerization (PCP mechanism), which proceeds ter in comparison with -zeolite and USY, imposes transi-
via protonated cyclopropane intermediates, predicts the tion state-type shape selectivity, thusfavoringthe formation
following relative ratio among the three methylheptane of the less bulky 2-methyl and 3-methyl isomers. In addi-
isomers:2-MC7/3-MC7/4-MC7 = 1/2/1from n-octane hydro- tion, smaller amounts of 4-MC7 and 3-EC6 (3-ethylhexane)
isomerization (3). This product distribution is expected to are observed over ZSM-12 than over USY and -zeolites
occur only if the PCP mechanism is taking place. From the (Table 6).

n-OCTANE HYDROISOMERIZATION AND HYDROCRACKING
407
SCHEME 1. General isomerization and hydrocracking schemes of n-octane.
Regarding the dibranched octane isomer distributions, posed by the zeolite structure, their formation is not favored
isomers with two branched alkyl groups on the same car- (see rows 1 and 5 in Table 7). Hence, 4-MC7 is preferably
bon of the main chain, i.e., 2,2-DMC6 (2,2-dimethylhexane) transformed to the less bulky 2,3-DMC6 via B-type isomer-
and 3,3-DMC6, are not favored in zeolites with relatively ization (reaction 2.9); and 3-MC7 is preferentially trans-
small openings. Indeed, we observed that the selectivities formed to 3,4-DMC6 and 2,4-DMC6 (reactions 2.6 and 2.7 in
of the above dimethylhexanes are significantly lower than Scheme 2). Our product distribution agrees very well with
the values predicted by the thermodynamic equilibrium earlier findings (32). The present data indicate that the
(Table 7). It should be noted that for zeolites with rela- transformation of 2,3-DMC6 to other dimethylhexanes is
tively large pores such as -zeolite and USY the concentra- relatively slow. One should note that 2,4-DMC6 can be
tions of the above isomers approach the equilibrium values. generated by reactions 2.3 and 2.7 of Scheme 2, from 2-
The selectivity of 2,3-DMC6 is significantly higher than the methylheptane and 3-methylheptane, while 2,5-DMC6 is
equilibrium composition. The latter isomer is readily pro- produced only by reaction 2.4 from 2-methylheptane. 2,4-
duced from several B-type isomerization reactions (PCP DMC6 follows the equilibrium compositions closely. All
mechanism) of methylheptanes (Scheme 2, reactions 2.2 other dibranched isomers (2,5-DMC6, 3,4-DMC6, and 3-
and 2.9). Due to the bulkier transition state intermediates E,2-MC5) are near equilibrium distribution. Another rea-
of 2,2-DMC6 and 3,3-DMC6 and the space constraint im- son for the preferential formation of dibranched products
TABLE 6
Monobranched Product Isomer Distributions
Zeolite
ZSM-12-35
ZSM-12-69
Beta-15
Beta-132
USY-2.6
USY-28
MOR-9.5
MOR-36
LTL-3.8
Equilibrium
2-MC7
3-MC7
4-MC7
3-EC6
39.9
40.3
14.7
5.1
40.9
38.6
15.2
5.4
32.6
43.7
17.5
6.2
35.6
36.5
18.1
9.8
33.5
41.3
17.1
8.3
34.6
38.1
17.6
9.7
28.6
44.6
17.8
8.9
29.9
43.7
17.2
9.2
34.2
38.3
18.1
9.4
32.5
37.6
12.5
17.4
Note. n-Octane conversion: about 50% at 290 C (see Table 5).

408
ZHANG AND SMIRNIOTIS
TABLE 7
Dibranched Product Isomer Distributions
Zeolite
ZSM-12-35
ZSM-12-69
Beta-15
Beta-132
USY-2.6
USY-28
MOR-9.5
MOR-36
LTL-3.8
Equilibrium
2,2-DMC6
2,3-DMC6
2,4-DMC6
2,5-DMC6
3,3-DMC6
3,4-DMC6
3-E,2-MC5
9.6
17.2
30.4
26.9
4.9
6.8
17.3
31.3
26.8
3.9
12.1
17.9
31.0
19.5
6.4
8.7
16.5
31.4
24.9
3.1
11.1
15.7
30.5
19.2
7.5
14.2
14.8
28.6
18.0
10.4
11.7
2.4
12.8
15.7
30.8
19.8
5.3
9.6
17.3
30.8
19.8
5.7
9.5
17.1
30.5
21.2
5.7
15.2
7.1
27.6
24.8
12.9
9.7
9.1
2.0
11.3
2.7
9.9
3.1
12.1
3.3
13.9
2.5
11.8
3.9
13.4
3.5
12.2
3.8
2.7
Note. n-Octane conversion: about 50% at 290 C (see Table 5).
SCHEME 2. Formation of dimethylhexanes from monometylheptanes by protonated cyclopropane carbenium ions.

n-OCTANE HYDROISOMERIZATION AND HYDROCRACKING
409
at the 2-position is the relatively faster type B isomeriza-
tion of 2-MC7 in comparison with other methylheptanes. It
is reported that for methylheptanes, the rate of type B iso-
merization decreases in the order 2-MC7 > 3-MC7 > 4-MC7
(33). This also explains why 2,2-DMC6 is more abundant
than 3,3-DMC6. In other words, we observed a higher rel-
ative deviation from the equilibrium concentration for 3,3-
DMC6 than for 2,2-DMC6. For the dibranched isomers con-
taining ethyl branching, only 3-E,2-MC5 is observed, and its
concentration follows that predicted by equilibrium.
Lumped Product C8 Isomer Selectivity versus
Zeolite Acidity
C8 isomer product selectivity is plotted versus n-octane
conversion, and reveals the capability of each catalyst to
favor the formation of branched isomers. The increase in
the conversion is achieved by increasing the reaction tem-
perature, while keeping the other operating conditions the
same. Over -zeolite catalysts the product C8 isomer se-
lectivities increase with the increase in Si/Al ratio (Fig. 3).
Wang et al. (34) also observed a similar trend in a relatively
narrow range of Si/Al ratios (from 18 to 30). An explana-
tion for the increased product isomer selectivity is offered
and it is based on the strength of the Brønsted acid sites.
The average Brønsted acid site strength of -zeolite cata-
lysts increases with the increase in the Si/Al ratio as verified
by the NH3-STPD experiments presented in Table 2. Since
the isomerization proceeds through successive branching
from normal to mono- to di-, and possibly to tribranched
C8 paraffins, high acid strength favors more branching. This
is because high acid strength allows relatively longer res-
idence time of the intermediate carbenium ions on the
acid sites, thus providing sufficient time for the latter in-
termediates to be isomerized further. It should be noted
that this is the case at relatively low temperatures where
FIG. 4. Branched C8 product selectivities versus conversion over
Pt/HUSY at WHSV = 7 h 1, H2/n-C8 (mole) = 16, total pressure of 100 psi,
and Pt loading level of 0.5 wt% .
the cracking rate of the surface intermediates does not in-
crease when the residence time on the acid site is relatively
long. However, when the temperature increases above a
certain level, the rate of the cracking increases. This can
be seen in Fig. 3 where the isomerization selectivity drops
significantly at high conversions (>70% ) or equally high
temperatures. Jacobs et al. (35) also found that high-acid-
strength sites and a smaller number of weak sites reduce the
rate of type A isomerization (via alkyl shift) and increase
the rate of chain branching via protonated cyclopropane
(PCP mechanism) intermediates at the same temperature.
In this manner, more multibranched intermediates can be
produced. It is also interesting to note that with the increase
in the average acid site strength, the selectivity of cracked
light paraffinic products decreases in the temperature range
involved in this study. Chao et al. (36) also observed a simi-
lar decrease in cracking products with the increase in Si/Al
ratio at the same level of conversion over Pt/HMOR and
Pt/HBeta catalysts. Conclusively, under the mild reaction
conditions involved in our study, hydroisomerization is fa-
vored at the expense of hydrocracking when Brønsted acid
site strength is high.
Similarly, over USY catalysts, we observe the same re-
lation between C8 isomer selectivity and Brønsted acid
strength. More specifically, the C8 isomer product selectiv-
ity does not change monotonically with Si/Al ratio (Fig. 4).
One can observe that the selectivity for isomers decreases
in the order USY-2.6 > USY-28 > USY-5.8 at any n-octane
conversion. This trend indicates that the isomerization ca-
pability of USYs increases with the increase in the avera-
ge Brønsted acid site strength. Our STPD experiments
(Table 2) verified that the acid strength decreases in the
same order: USY-2.6 > USY-28 > USY-5.8.
Over ZSM-12, the isomer product selectivities do not
increase with the increase in Si/Al ratio (Fig. 5). Its acid-
ity characteristics, presented in Table 2, indicate that the
FIG. 3. Branched C8 product selectivities versus conversion over
Pt/HBeta at WHSV = 7 h 1, H2/n-C8 (mole) = 16, total pressure of 100 psi,
and Pt loading level of 0.5 wt% .

410
ZHANG AND SMIRNIOTIS
centrations of C3 and C5 hydrocarbons are almost equal
(Table 5) indicates that they are the result of primary crack-
ing of either linear or branched C8 carbenium ions. An
inspection of the C5 product distribution indicates that ze-
olites with large pore openings such as USY favor the pro-
duction of i-C5. The very small concentration of C2 hydro-
carbon supports the fact that secondary cracking reactions
do not take place except at very high reaction tempera-
ture. From the above discussion, one can conclude that the
cracked products over all zeolites result from the primary
hydrocracking of C8 linear and branched carbenium ions.
Several possible -scission routes exist for the further
transformation of a branched intermediate carbenium ion.
For instance, a 2-methylheptyl ion may crack according to
the routes shown in Scheme 3. However, route 3.3 is the
only feasible one since all the other routes involve a primary
carbenium ion, which has a high activation energy barrier
and is not favored (37). Based on this fact, the following
analysis is limited to the most energetically favored reaction
routes.
The most possible -scission routes occurring for all nor-
mal and branched octyl carbenium ions on zeolites via type
A, B1, B2, C, and D -scissions are given in Scheme 4. Exam-
ining all the possible products of -scission of C8 carbenium
ions with variable branching, one can conclude that only
A and B types of -scission are responsible for the direct
production of i-butane. More specifically, type A -scission
(route 4.1) produces only i-C4; type C and D scissions can-
not produce any i-C4. Routes 4.1 (from 2,2,4-TMC+5 ), 4.2
(from 2,2,3-TMC+5 ), 4.5 (from 2,2-DMC6+), and 4.6 (from
2,4-DMC+6 ) are responsible for the production of i-C4 after
the hydrogenation of the produced olefin. One should not
forget that i-C4 carbenium ions could finally desorb and re-
sult in i-butane. Routes 4.2 (from 2,2,3-TMC+5 ), 4.5 (from
2,2-DMC+6 ), 4.6 (from 2,4-DMC6+), 4.12 (from 3,4-DMC+6 ),
4.15 (from 3-MC+7 ), and 4.20 (from n-C8+) will lead to n-C4.
All other scission reactions produce neither i-C4 nor n-C4.
The interconversion of isobutane and n-butane via the di-
rect transformation of i-butane to n-butane through uni-
molecular isomerization of tert.-butyl carbenium ion fol-
lowed by chain hydride transfer is minimal (38). This
reaction could occur only at very high temperatures or con-
centrations. This is because it involves the formation of pri-
mary carbenium ions, which are energetically unstable. An-
other route for the interconversion ofisobutane to n-butane
goesthrough a bimolecular mechanism, which requiresvery
strong Brønsted acid sites, long contact time, and relatively
high temperature (39). The presence of hydrogen (hydro-
genation of intermediate olefins) further inhibits the above
bimolecular pathway, since the C4 carbenium ions required
for the alkylation step are in very low concentration (40).
Indeed, from our experiments we found that the hydrocon-
version of isobutane over the zeolite catalysts used in the
current study is negligible, unless the reaction temperature
FIG. 5. Branched C8 product selectivities versus conversion over
Pt/HZSM-12 at WHSV = 7 h 1, H2/n-C8 (mole) = 16, total pressure of
100 psi, and Pt loading level of 0.5 wt% .
average strong site strength slightly decreases with dealu-
mination. These observations reinforce the general trend
observed in our study with other zeolites that the hydroi-
somerization capability of a bifunctional catalyst increases
with an increase in the average Brønsted strength.
A small number of experiments were also carried out
at the same temperature (290 C) to verify that the selec-
tivity differences discussed above are indeed due to the
change in the acidic properties of the catalyst and not to
the difference in temperature. The same trends as above
are observed. More specifically, the increase in the Si/Al
ratio of -zeolite results in an increase in the selectivity of
the branched octanes. On the other hand, for ZSM-12, as
discussed earlier, the trend is reversed with the increase
in Si/Al ratio. Thus, the effect of temperature on the total
isooctane product selectivity is almost comparable to that
of all zeolites with the same structure but different acidity.
The different selectivity truly resulted from the difference
in acidic site strength.
Distribution of Cracked Products
-Scission of carbenium ions generated by the protona-
tion of the parent and isomerized paraffins is the main re-
action path for the fragmentation of alkanes over ideal bi-
functional catalysts. The rate of cleavage of carbenium ions
depends on the reaction temperature, acidity, and structure
of the individual zeolite. No CH4 was produced (Table 5),
which indicates that hydrogenolysis does not occur over
the catalysts tested under the reaction conditions discussed.
The concentrations of hydrocarbons with six and seven car-
bon atoms were very low. This is as expected because the D
type of -scission (secondary to primary carbenium ions),
which will produce C1 and C2 hydrocarbons by side cleav-
age of linear and branched carbenium ions, is energetically
unfavorable. The majority of the cracked products possess
three, four, and five carbon atoms. The fact that the con-

n-OCTANE HYDROISOMERIZATION AND HYDROCRACKING
411
SCHEME 3. Possible -scission routes of 2-methylheptyl ion.
is above 350 C (Table 8). Considering the coexistence pounds. Therefore, one can conclude that the produced
of other high-molecular-weight paraffins, the reaction of normal and isobutanes result from the primary -scission
i-butane is even lower. Other researchers (41) found that modes, unless either the conversion or temperature is high.
the longer alkanes in mixtures are preferentially converted
With the increase in reaction temperature, the i-C4/
due to competitive adsorption favoring the heavy com- n-C4 ratio significantly decreases for high-Al-content ze-
olites (USY and nondealuminated -zeolite), and it does
not change that much for low-Al-content zeolite catalysts
such as Beta-132 (Fig. 6). Other researchers also found that
with the increase in Si/Al ratio, the i-C4/n-C4 ratio decreases
TABLE 8
Product Yields of i-C4 Hydroconversion over Zeolite Catalystsa
for n-hexane isomerization over mordenite (42), and for
Isomeriza- Disproportiona-
Temperature Conversion tion yield
tion yield
(% )c
Catalyst
( C)
(% )
(% )b
ZSM-12-35
290
350
400
0.9
4.2
13.9
0.5
1.3
6.8
0.4
2.9
5.3
Beta-15
USY-2.6
MOR-9.5
290
350
400
450
0.2
3.7
12.8
33.7
0.2
3.3
9.7
0.0
0.3
1.5
6.4
24.7
290
350
400
450
0.2
1.0
7.7
0.1
0.3
4.0
0.1
0.2
0.8
2.4
22.4
14.9
290
350
400
0.9
18.7
49.1
0.6
12.3
21.2
0.3
6.4
27.3
a Reaction conditions: H2/i-C4 (mole) = 6, WHSV = 10 h 1, Pt loading
level of 0.5 wt% on all catalysts, 10 min on stream.
b n-C4 paraffin and olefin.
FIG. 6. i-C4/n-C4 ratio of cracked products from n-octane hydrocon-
version versus temperature at WHSV = 7 h 1, H2/n-C8 (mole) = 16, total
pressure of 100 psi, and Pt loading level of 0.5 wt% .
c C3, C5 paraffins and olefins.

412
ZHANG AND SMIRNIOTIS
SCHEME 4. Energetically favored -scission modes of various octyl carbenium ions.
decane hydroisomerization–hydrocracking reaction over generating potentially two i-butane molecules. Type B scis-
some USY zeolites (43). By looking at all the possible sion is also responsible for the production of i-butane. Ac-
-scissions of C8 carbenium intermediates (reactions 4.1 cording to the relative order of rates for isomerization and
to 4.21), one can observe that i-C4 can be produced as a cracking presented earlier in this paper, type A scission is
primary product only by A and B types of scission. More significantly faster than B, C, and D types of cracking. The
specifically, type A scission (reaction 4.1) is responsible for latter two typesofscission can generate onlynormalbutane.

n-OCTANE HYDROISOMERIZATION AND HYDROCRACKING
413
SCHEME 4—Continued
From our experiments, it seems that for high-Al-content ze- tion of 2,2,4-trimethylpentane decreases significantly, and
olites (USY-2.6, -5.8, -28, and Beta-15), type A cracking thusthe contribution oftype A scission (reaction 4.1), which
(reaction 4.1) dominates over the other scission mecha- produces isobutane, diminishes. Moreover, the difference
nisms. The transformation of 2,2,4-trimethylpentane ion between the reaction rates of types A, B, C, and D decrease
via reaction 4.1 will shift the equilibrium among all the with increasing temperature (3, 27, 44), thus resulting in the
trimethylpentane carbenium ions toward the former isomer decrease in the relative production of i-C4. It should also
(2,2,4-TMC+5 ). On the other hand, 2,2,4-trimethylpentane is be noted that with the increase in temperature, the equilib-
the most abundant tribranched isomer thermodynamically rium concentrations of 3-MC7 and 3,4-DMC6 increase sig-
(Fig. 7). In this way, the i-C4/n-C4 ratio acquires high values. nificantly (Fig. 7). This means that relatively more n-butane
By increasing the temperature, the equilibrium concentra- can be produced via reactions 4.12 and 4.15, respectively.

414
ZHANG AND SMIRNIOTIS
are responsible for hydrocracking, are deactivated. USY
˚
˚
has supercages of 12.2 A connected with channels of 7.4 A,
favoring the accumulation of coking precursors, which re-
sult in fast deactivation of the above catalyst.
It is remarkable to note that ZSM-12 does not lose any
of its initial activity over 1100 h on stream under the same
severe conditions. The product selectivities and yields are
also very stable as a function of time. In our previous inves-
tigation (4, 45), we observed this superior performance of
ZSM-12 for the reforming of naphthenic mixtures at higher
temperatures (vicinity of 470 C). The superior stability of
ZSM-12 even under accelerating coking conditions results
from its unique pore structure, which does not favor coke
formation. Its unidimensional noninterpenetrating puck-
˚
ered channels (5.6 6.1 A) act as perfect tubes, which do
not trap coke precursors (Fig. 9). This unique pore struc-
ture renders ZSM-12 an excellent zeolite to withstand coke
deactivation. We also found that L-zeolite, although it also
˚
possesses a one-dimensional configuration of pores (7.1 A),
deactivates fast. This is because of the existence of periodic
expansions along the channels with dimensions of about
˚
13 A (46). Coke precursors probably tend to accumulate in
these expansions of the pores.
CONCLUSION
Both the zeolite acidity (number and strength distribu-
tion) and the pore structure play very important roles in the
conversion and product selectivity for the hydroconversion
of n-paraffins. Catalytic activity per acidic site (TOF) at
the initial stages of the reaction (1 h on stream) and for
comparable Si/Al ratios decreases in the following order:
FIG. 7. Thermodynamic equilibrium of normal, monobranched, di-
branched, and tribranched octanes versus temperatures.
On the other hand, for zeolite catalystswith relativelylow ZSM-12 > -zeolite > mordenite > USY > L-zeolite.
aluminum content (ZSM12-35, -69, Beta-85, -132), we ob-
ZSM-12 demonstrated superior stability in the acceler-
serve that the i-C4/n-C4 ratio at low temperature decreases ated coking experiments. The superior behavior of ZSM-12
significantly in comparison with the Al-rich zeolites. The in-
crease in temperature does not alter significantly the above
ratio. This is evidence that the role of type A scission (re-
action 4.1) diminishes while B1, B2, and C types of scission
become dominant.
Time-on-Stream Behavior
Accelerated coking experiments were carried out to test
the resistance of the zeolites to deactivation. Significantly
higher WHSV and lower H2/n-octane ratios than in the
usual experiment were used to expedite the deactivation of
the catalysts in the accelerated coking experiments. USY-
2.6 lost almost 50% of its initial activity in about 60 h
on stream due to the deactivation resulting from carbona-
ceous deposit (Fig. 8). The selectivities of isomers and light
cracked products also change significantly with time. More
specifically, the isomerization capability of USY increases
while that of cracking decreases. This is because during the
initial stages of the reaction, the strongest acid sites, which
FIG. 8. Time-on-stream behavior of n-octane hydroconversion over
ZSM-12(290 C) and USY (320 C) zeolitesat the acceleratingdeactivation
conditions (WHSV = 11.2 h 1, H2/n-C8 = 4, total pressure of 100 psig, and
Pt loading level of 0.5 wt% ).

n-OCTANE HYDROISOMERIZATION AND HYDROCRACKING
415
FIG. 9. Schematic of the ZSM-12 pore structure.
even under accelerated coking conditions results from its
ACKNOWLEDGMENTS
unique pore structure, which doesnot favor coke formation.
Its one-dimensional noninterpenetrating puckered chan-
The authors acknowledge financial support from the National Science
Foundation through a CAREER award (CTS-9702081). Acknowledg-
ment is also made to the donors of the Petroleum Research Fund (ACS)
for the support of this research through Grant ACS-PRF 316065-G5.
˚
nels (5.5 6.1 A) act as perfect tubes, which do not trap
coke precursors.
Branched product selectivityincreaseswith an increase in
Brønsted acid site strength of the zeolite catalysts, and thus
hydroisomerization is favored at the expense of cracking at
a higher Brønsted acid strength in the temperature range
studied. 2,2-DMC6 and 3,3-DMC6 are not favored in final
products due to their bulky molecular size even in USY. In
addition, the 2,2-DMC6 species is more abundant than 3,3-
DMC6 because the rate of isomerization by PCP intermedi-
ates decreases in the following order: 2-MC7 > 3-MC7 > 4-
MC7. The concentration of 2,3-DMC6 is much higher than
equilibrium, which indicates that the interconversion of 2,3-
DMC6 to other dibranched isomers is not preferred. The
i-C4/n-C4 ratio detected depends on both the reaction tem-
perature and zeolite pore structure/acidity. The aluminum
content determines the type of -scission reactions. For ze-
olites with high concentration of acid sites (Si/Al < about
30), type A -scission dominates at low temperature, while
at lower Al content type A, B, and C -scissions are equally
important.
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