Comparative catalytic studies on the conversion of 1-butene and n-butane to isobutene over MCM-22 and ITQ-2 zeolites

Article abstract of DOI:10.1016/j.jcat.2006.09.015

The catalytic properties of H-MCM-22 and Pt-MCM-22 for the skeletal isomerization of 1-butene and the dehydroisomerization of n-butane were compared with those obtained from their delaminated derivatives, H-ITQ-2 and Pt-ITQ-2. The overall results of our study strongly suggests that the desired 1-butene and n-butane conversions to isobutene in steady state occur mainly near the pore mouth inlets of the sinusoidal 10-ring channels retained in both of MCM-22 and ITQ-2 and inside this intralayer void space, respectively, which may be attributed to the unique geometric constraints imposed by the two-dimensionality of the pore system with a suitable degree of ellipticity of 10-ring channels. Whereas coke formation on the sinusoidal channels during the 1-butene skeletal isomerization can occur in such a way as to make most of the intrachannel Bronsted acid sites less available for nonselective dimerization-cracking reactions, it appears that the presence of H₂ in the n-butane dehydroisomerization stream would prevent extensive coke buildup, thereby allowing pore diffusion of reactants and products even in steady state.

Full text of DOI:10.1016/j.jcat.2006.09.015

Journal of Catalysis 245 (2007) 65–74
www.elsevier.com/locate/jcat
Comparative catalytic studies on the conversion of 1-butene and n-butane to
isobutene over MCM-22 and ITQ-2 zeolites
Hye Ja Jung a, Sang Soon Park a, Chae-Ho Shin b, Yong-Ki Park c, Suk Bong Hong a,∗
a
Division of Applied Chemistry and Biotechnology, Hanbat National University, Taejon 305-719, South Korea
Department of Chemical Engineering, Chungbuk National University, Chungbuk 361-763, South Korea
b
c
Division of Chemical Technology, Korea Research Institute of Chemical Technology, Taejon 305-606, South Korea
Received 18 July 2006; revised 14 September 2006; accepted 16 September 2006
Available online 25 October 2006
Abstract
The catalytic properties of H-MCM-22 and Pt-MCM-22 for the skeletal isomerization of 1-butene and the dehydroisomerization of n-butane
were compared with those obtained from their delaminated derivatives, H-ITQ-2 and Pt-ITQ-2. The overall results of our study strongly suggests
that the desired 1-butene and n-butane conversions to isobutene in steady state occur mainly near the pore mouth inlets of the sinusoidal 10-ring
channels retained in both of MCM-22 and ITQ-2 and inside this intralayer void space, respectively, which may be attributed to the unique geometric
constraints imposed by the two-dimensionality of the pore system with a suitable degree of ellipticity of 10-ring channels. Whereas coke formation
on the sinusoidal channels during the 1-butene skeletal isomerization can occur in such a way as to make most of the intrachannel Brønsted acid
sites less available for nonselective dimerization-cracking reactions, it appears that the presence of H in the n-butane dehydroisomerization
2
stream would prevent extensive coke buildup, thereby allowing pore diffusion of reactants and products even in steady state.
© 2006 Elsevier Inc. All rights reserved.
Keywords: MCM-22 and ITQ-2; 1-Butene skeletal isomerization; n-Butane dehydroisomerization; Location of reactions
1. Introduction
calcination between the layered sheets, to generate the so-called
MWW structure. This has prompted Corma and co-workers to
delaminate its lamellar precursor in a way similar to that applied
to the exfoliation of clays, to provide access for bulky molecules
to the catalytically active sites that would be otherwise excluded
or sieved out by the 10-ring pores of the MWW structure [3–5].
Indeed, the resultant material, termed ITQ-2, has been shown
to consist essentially of very thin (ca. 25 Å) sheets, the surface
of which is formed by a hexagonal array of 12-ring pockets that
penetrate into the sheet from both sides connected by a double
6-ring unit, but within which the large supercages no longer ex-
ist. But because the sinusoidal 10-ring channels are preserved
inside the ITQ-2 sheet, this delaminated material retains zeolitic
characteristics, together with a very high and structurally well-
defined external surface area.
MCM-22 (framework type MWW) is a high-silica zeolite
first described by Rubin and Chu in 1990 [1]. This synthetic
zeolite contains two independent pore systems, one defined
by two-dimensional sinusoidal 10-ring (4.1 × 5.1 Å) channels
and the other defined by large cylindrical supercages (7.1 Å in
diameter and 18.2 Å in height) that are circumscribed by 12-
ring channels but are accessible only through 10-ring (4.0 ×
5.5 Å) windows [2]. Furthermore, most of the external surface
of MCM-22 crystallites, which typically appear as very thin
plates, is terminated by 12-ring cups or pockets (7.1 Å in di-
ameter and 7.0 Å deep) that form as hemi-supercages, resulting
from the truncation of the supercages described above.
Another interesting feature of MCM-22 is that it first crystal-
lizes as a lamellar precursor that undergoes dehydroxylation on
Over the past decade, due to the peculiar crystal structure of
MCM-22, much attention has been directed to investigating its
catalytic properties for numerous petrochemical reactions, such
as isomerization, alkylation, and cracking [6–16]. Whereas the
protonic acid sites located in the external 12-ring pockets have
*
Corresponding author.
E-mail address: sbhong@hanbat.ac.kr (S.B. Hong).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.09.015

66
H.J. Jung et al. / Journal of Catalysis 245 (2007) 65–74
been shown to provide for the nest effect [17], making it possi-
ble to successfully commercialize the process for ethylbenzene
production in Chiba, Japan [18], the shape selective properties
of this zeolite have been considered generally intermediate be-
tween those of medium- and large-pore zeolites. In most of the
hydrocarbon conversions studied thus far, however, the location
of catalytically active sites responsible for the desired product
formation over MCM-22 remains to be well defined, despite
the lack of direct connection among its three pore systems that
have different shapes and sizes and hence should show differ-
ent shape selectivity effects. This also appears to be the case
for the skeletal isomerization of 1-butene and the dehydroiso-
merization of n-butane in which H-MCM-22 and its Pt-loaded
analog have been shown to be highly selective for isobutene
formation [11,19]. Here we compare the catalytic properties of
H-MCM-22 and Pt-MCM-22 zeolites for these two gas-phase
reactions with those obtained from the practically supercage-
free H-ITQ-2 and Pt-ITQ-2 materials, to elucidate the roles of
the three pore systems of MWW-type zeolites in the production
of isobutene, as well as in the formation of various byproducts
via undesired consecutive reactions like dimerization followed
by cracking into light hydrocarbons.
ter, and dried in air at room temperature. Subsequently, it was
heated in air to 450 ◦C at a ramp rate of 0.5 ◦C min−1 and kept
at the final temperature for 2 h.
2.2. Analytical methods
Powder X-ray diffraction (XRD) patterns were collected on
a Siemens D-5000 diffractometer with CuKα radiation. Ele-
mental analysis for Al, Si, and Pt was carried out at the An-
alytical Laboratory of Korea Institute of Science and Tech-
nology, using a Jarrell–Ash Polyscan 61E inductively coupled
plasma (ICP) spectrometer in combination with a Perkin–Elmer
5000 atomic absorption spectrophotometer. Thermogravimet-
ric analysis (TGA) was performed in air on a TA Instruments
SDT 2960 thermal analyzer, where the weight loss related to
the combustion of coke deposits formed during the 1-butene
skeletal isomerization or the n-butane dehydroisomerization
was further confirmed by differential thermal analysis (DTA)
using the same analyzer. The N2 sorption experiments were per-
formed on a Micromeritics ASAP 2010 analyzer. Temperature-
programmed desorption (TPD) of NH3 was recorded on a fixed-
bed, flow-type apparatus attached to a Balzers TCP 015 quadru-
ple mass spectrometer. About 0.1 g of sample was first activated
2. Experimental
in flowing He at 500 ◦C for 1 h, then pure NH3 (50 cm3 min−1
)
was passed over the sample at 150 ◦C for 0.5 h. The treated
sample was subsequently purged with He at the same tempera-
ture for 1 h to remove the physisorbed NH3. Finally, the TPD
profiles were obtained in flowing He (100 cm3 min−1) from 150
2.1. Catalyst preparation
An MCM-22 zeolite with a Si/Al ratio of 16.8 was syn-
thesized using hexamethyleneimine as an organic structure-
directing agent (SDA) according to the procedures described
in Ref. [20]. A portion of the crystallized product, frequently
referred to as MCM-22(P), was calcined in air at 550 ◦C for
12 h, giving MCM-22, and then converted into the proton form
by refluxing twice in 1.0 M NH4NO3 solutions (1.0 g solid per
100 ml solution) for 6 h, followed by calcination at 550 ◦C for
8 h. Another portion of MCM-22(P) was delaminated in accor-
dance with the procedures developed by Corma et al. [3–5],
to prepare the ITQ-2 sample. In a typical delamination ex-
periment, 5.0 g of MCM-22(P) was first mixed with 28.2 g
of hexadecyltrimethylammonium bromide (99%, Aldrich) and
60.1 g of tetrapropylammonium hydroxide (1.0 M aqueous so-
lution, Aldrich) in 59.5 g of water and refluxed at 80 ◦C for 16 h.
After the mixture was in an ultrasonic bath (100 W, 42 kHz) for
3 h, a few drops of 1.0 M HCl solution were added to adjust the
pH to <2. The solid product was recovered by centrifugation,
dried in air room temperature, and calcined at 550 ◦C for 6 h
to remove the occluded organic species. The resulting ITQ-2
sample was converted into its proton form by refluxing twice in
1.0 M NH4NO3 solutions (1.0 g of solid per 100 ml of solution)
for 6 h, followed by calcination at 550 ◦C for 8 h.
to 650 ◦C at a heating rate of 10 ◦C min−1
.
The IR spectra in the structural region were measured on
a Nicolet Magna 550 FT-IR spectrometer using the KBr pel-
let technique. The relative ratios of tetrahedral and octahedral
Al species in H-MCM-22 and H-ITQ-2 materials prepared here
were determined by 27Al MAS NMR spectroscopy. The exper-
iments were performed on a Bruker Avance 500 spectrometer
at a spinning rate of 10.0 kHz. The operating 27Al frequency
was 130.325 MHz, and the spectra were obtained with an ac-
quisition of ca. 1000 pulse transients, which were repeated with
a π/8 rad pulse length of 1.2 µs and a recycle delay of 1 s.
The 27Al chemical shifts are referenced to an Al(H2O)3+ so-
lution. Transmission electron microscopy (TEM) was a6pplied
to study the size and location of Pt clusters or particles. The
measurements were performed on a JEOL JEM-2010 electron
microscope operating at an acceleration voltage of 200 kV. The
samples were grounded and deposited from a suspension in
methanol onto a holey carbon grid.
2.3. Catalysis
All of the catalytic experiments were carried out under at-
mospheric pressure in a continuous-flow apparatus with a fixed-
bed microreactor. In the skeletal isomerization of 1-butene, the
catalyst was activated under flowing He (50 cm3 min−1) at
500 ◦C for 1 h and kept at 350 or 500 ◦C to establish a standard
operating procedure, allowing time for the product distribution
to stabilize. Then, a reactant stream of 10% 1-butene in He as
Pt loading onto the solid supports was done by ion exchange.
Tetraamineplatinum(II) chloride monohydrate (Pt(NH3)4Cl2·
H2O, 99%, Strem) was used as a Pt source. Typically, 40.9 mg
of Pt(NH3)4Cl2·H2O dissolved in 20 ml of water was added
dropwise to a slurry of the ammonium form (2.0 g) of MCM-
22 or ITQ-2 in 50 ml of water. After being stirred for 24 h,
the powder was carefully filtered, washed repeatedly with wa-

H.J. Jung et al. / Journal of Catalysis 245 (2007) 65–74
67
a balancing gas was fed into a quartz reactor containing 0.10–
0.22 g of zeolite catalyst at the same temperature. The total gas
nal SiOH groups in the latter zeolite, due to its delaminated
nature. The successful formation of ITQ-2 can be further sup-
ported by the N2 BET surface area data in Table 1. The fact
that the external (or mesoporous) surface area of H-ITQ-2 is
approximately six times larger than that of H-CM-22 suggests
that ITQ-2 prepared here is mostly formed by single layers.
The powder XRD patterns (not shown) of Pt-MCM-22 and Pt-
ITQ-2 after reduction were found to be essentially the same
as those of their proton form, respectively, except the minor
change in relative X-ray peak intensities. Elemental analysis in-
dicates that the Pt content (0.62 wt%) of Pt-MCM-22 is slightly
higher than that (0.51 wt%) of Pt-ITQ-2, although the identical
Pt2+ ion exchange conditions were applied to the preparation of
both materials. This can be rationalized by considering that the
bulk Si/Al ratio (16.8) of H-MCM-22 is slightly lower than that
(19.2) of H-ITQ-2 (Table 1). As described above, the prepara-
tion procedure of ITQ-2 includes the use of some concentrated
HCl to keep the pH of the sonicated mixture as low as 2 be-
fore separation. Therefore, it is not very difficult to predict that
a minor part of Al atoms has been extracted from the MCM-
22(P) framework during the delamination process, explaining
why H-ITQ-2 has a slightly lower Al content than H-MCM-22.
Fig. 3 shows the 27Al MAS NMR spectra of MCM-22(P),
H-MCM-22, and H-ITQ-2. The spectra of Pt-MCM-22 and
Pt-ITQ-2 are also given in Fig. 3. MCM-22(P) exhibits two
tetrahedral 27Al resonances around 50 and 56 ppm. In con-
flow at the reactor inlet was kept constant at 50 cm3 min−1
.
In the dehydroisomerization of n-butane, 0.1 g of the catalyst
was reduced in situ with flowing 20% H2 in He (50 cm3 min−1
)
at 500 ◦C for 2 h. The reaction was started by feeding a reac-
tant stream of 10% n-butane and 20% H2 in He as a balancing
gas, and the total gas flow was fixed to 50 cm3 min−1. The re-
action products from both reactions were analyzed on-line in
a Chrompack CP 9001 gas chromatograph equipped with an
Al2O3/KCl Plot capillary column (0.53 mm×50 m) and a flame
ionization detector, with the first analysis carried out after 3 min
on stream. Conversion to, selectivity to, and yield in each prod-
uct were calculated as described previously [19,21].
3. Results and discussion
Fig. 1 shows the powder XRD patterns of MCM-22(P), H-
MCM-22, swollen MCM-22(P), and H-ITQ-2 prepared in this
study. The XRD pattern of each material agrees well with that
of the corresponding material in Refs. [3,4]. The lack of (001)
and (002) reflections at 2θ = 3◦–7◦ in the pattern of H-ITQ-2
indicates a significant reduction of long-range order along the
c axis on exfoliation of MCM-22(P). Fig. 2 compares the IR
spectra in the structural region of H-MCM-22 and H-ITQ-2. As
reported previously [3,4], a band at around 960 cm−1, which
corresponds to SiOH groups and is hardly visible in the spec-
trum of H-MCM-22, is clearly resolved in that of H-ITQ-2.
This reveals the presence of a much larger amount of termi-
Fig. 1. Powder XRD patterns of (a) MCM-22(P), (b) H-MCM-22, (c) swollen
MCM-22(P), and (d) H-ITQ-2 prepared in this study.
Fig. 2. IR spectra in the structural region of (a) H-MCM-22 and (b) H-ITQ-2.
The band characteristic of SiOH groups is marked with an arrow.
Table 1
Characterization data for zeolite catalysts prepared in this study
a
a
b
2
−1
c
e
Catalyst
Si/Al ratio
Pt wt%
BET surface area (m g
)
Al /(Al + Al ) (%)
Oh
Oh
Td
d
Total
External
Microporous
H-MCM-22
Pt-MCM-22
H-ITQ-2
16.8
17.0
19.2
19.9
–
0.62
–
462
447
729
694
105
98
669
648
357
349
59
15
20
12
13
Pt-ITQ-2
0.51
46
a
Determined by elemental analysis.
b
c
d
e
Calculated from N adsorption data.
Calculated using the t-plot method.
2
Difference between the total and external surface areas.
27
Calculated from Al MAS NMR data.

68
H.J. Jung et al. / Journal of Catalysis 245 (2007) 65–74
27
Fig. 3. Al MAS NMR spectra of (a) MCM-22(P), (b) H-MCM-22, (c) H-ITQ-2, (d) Pt-MCM-22, and (e) Pt-ITQ-2.
trast, H-MCM-22 gives an additional shoulder around 61 ppm,
as well as a signal around 0 ppm attributable to octahedral Al.
These spectral data are in good agreement with those reported
previously [22–24]. Like H-MCM-22, H-ITQ-2 shows one oc-
tahedral 27Al resonance around 0 ppm. As shown in Fig. 3,
however, the tetrahedral resonance around 50 ppm that has been
tentatively assigned to Al atoms in sites T6 and T7 of the hexag-
onal model of the MCM-22 structure [22], where eight crys-
tallographically distinct tetrahedral sites (T sites) exist [2], is
hardly visible in the 27Al MAS NMR spectrum of H-ITQ-2, de-
spite the fact that both materials were prepared using the same
staring material, that is, MCM-22(P). Thus, if we assume that
the available T sites in the proposed structure of ITQ-2 are ex-
actly the same as those in the hexagonal MWW structure, then
the extent of dealumination from the ITQ-2 framework during
preparation of this laminated material should be considerably
higher on the Al atoms in sites T6 and T7 than those in the other
six T sites, unlike the case of the three-dimensional MCM-22
zeolite. This is somewhat unexpected, because the average T6-
O-T and T7-O-T angles (161.5◦ and 158.5◦) derived from the
Rietveld refinement [2] of the synchrotron powder XRD data
for a calcined borosilicate MCM-22 are higher than any of the
other six average Tn-O-T angles, where n = 1–5 and 8 [22], and
hence because sites T6 and T7 are relatively less strained. Fur-
thermore, sites T6 and T7 are not on the surface of the MWW
double layer consisting of two sheets of small {435663[43]}
cages. Thus, they can be expected to be sterically less accessi-
ble to HCl, added before separation of the sonicated mixture,
than the other sites, such as T1 or T2 exposed to the ITQ-2
sheet exterior, due to their location in the wall of the external
12-ring pockets after delamination. Although elucidating the
origin of the nonrandom nature of dealumination in ITQ-2 is
beyond the scope of this paper, the 27Al MAS NMR spectra
in Fig. 3 clearly show that the actual distribution of Al atoms
over the available T sites in H-ITQ-2 differs from that in H-
MCM-22. Assuming that all Al atoms in these two materials are
detected in their 27Al MAS NMR spectra and no intensity loss
is caused by the presence of NMR “invisible” extra-framework
Al species [25], curve deconvolution indicates that H-MCM-
22 and H-ITQ-2 have comparable fractions of octahedral Al
Fig. 4. NH TPD profiles from (a) H-MCM-22 and (b) H-ITQ-2.
3
(Table 1). We also note that the 27Al MAS NMR spectra of
Pt-MCM-22 and Pt-ITQ-2 in the tetrahedral framework region
are characterized by line shapes that are essentially identical to
those seen for H-MCM-22 and H-ITQ-2, respectively. It thus
appears that the Al distribution patterns in MCM-22 and ITQ-2
remain almost unchanged during the Pt loading step, that is, ion
exchange followed by calcination. Unlike Pt-ITQ-2, however,
Pt-MCM-22 gave a higher fraction of octahedral Al compared
with its proton form, which is consistent with the trend reported
by Lercher et al. [19]. This suggests that the extent of dealumi-
nation caused by incorporation of Pt is higher for Pt-MCM-22
than for Pt-ITQ-2, although the precise reason for this remains
unclear.
Fig. 4 shows the NH3 TPD profiles obtained from H-MCM-
22 and H-ITQ-2 prepared here. The TPD profile of H-MCM-22
is characterized by one broad and asymmetric desorption peak
with maxima in the temperature region 370–380 ◦C, assignable
to NH3 desorption from strong acid sites. As seen in Fig. 4,
however, the desorption peak from H-ITQ-2 is located at a
lower temperature (maximum around 310 ◦C). Here we can-
not exclude the effects of the difficulty of trapping desorbed
NH3 molecules in the external pockets on the TPD profile of
H-ITQ-2, due to its delaminated nature. As expected from the
bulk Si/Al ratios given in Table 1, in addition, the total area

H.J. Jung et al. / Journal of Catalysis 245 (2007) 65–74
69
Fig. 5. (a) 1-Butene conversion, yields in (b) isobutene and (c) major by-products (i.e., propene and pentenes), and (d) coke formation as a function of time on
◦
stream in skeletal isomerization of 1-butene over H-MCM-22 (2) and H-ITQ-2 (") at 350 C and 10% 1-butene in the feed.
of NH3 desorption (i.e., the density of acid sites) was lower
for H-ITQ-2 and for H-MCM-22. It is well established that the
acidity and pore structure of zeolites are the two important fac-
tors determining their activity and selectivity during the skeletal
isomerization of 1-butene [26,27], which is also the case of vari-
ous zeolite-supported Pt catalysts for the dehydroisomerization
of n-butane [28]. Because the NH3 TPD results in Fig. 4 re-
veal that differences in the number and strength of acid sites
between H-MCM-22 and H-ITQ-2 prepared here are not so
large, comparison not only of the isomerization activities of
these two zeolites, but also of the dehydroisomerization activi-
ties of their Pt-loaded analogs with similar metal loading levels
will allow us to discriminate among the roles of three different
pore systems of MWW-type zeolites in the desired conversion
of 1-butene and n-butane to isobutene.
Fig. 5 shows 1-butene conversion and yields in isobutene
and major byproducts (i.e., propene and pentenes) as a func-
tion of time on stream (TOS) in the skeletal isomerization of
1-butene over H-MCM-22 and H-ITQ-2 measured at 350 ◦C
and 10% 1-butene in the feed. To make the comparison of the
product distributions in 1-butene skeletal isomerization over
these two catalysts as meaningful as possible, we attempted to
maintain their 1-butene conversions similar to one another dur-
ing the period of TOS studied here, by changing the amount
of catalyst in the reactor. The TOS effects on coke formation
over H-MCM-22 and H-ITQ-2 are also given in Fig. 5. Al-
though it was difficult to achieve similar conversions for the
first 10 min on stream, probably due to the difference in their in-
trinsic isomerization activities, both H-MCM-22 and H-ITQ-2
exhibited a low isobutene yield at the beginning of the reaction
while producing large amounts of byproducts (mainly propene
and pentenes). This indicates that the initial isobutene forma-
tion is dominated by nonselective bimolecular reactions, that
is, dimerization and subsequent cracking. As described above,
the large interconnecting supercages with smaller connecting
10-ring (4.0 × 5.5 Å) windows, typical for MCM-22 but no
longer for ITQ-2, are circumscribed by 12-rings so that they do
not have shape selectivity to effectively suppress nonselective
side reactions, which must also be the case for external 12-ring
pockets existing in a much greater quantity on ITQ-2. In ad-
dition, the free diameter (5.8 Å) at the intersection of the two
sinusoidal 10-ring channels retained in both of MCM-22 and
ITQ-2 is fairly larger than that (5.2 Å) at the intersection of
the 10-ring (4.2 × 5.4 Å) and 8-ring (3.5 × 4.8 Å) channels in
ferrierite (FER), which is the best catalyst among the zeolites
tested thus far for the skeletal isomerization of 1-butene but is
characterized by low initial isobutene selectivity [26,27]. This
indicates that the sinusoidal channel intersection in MWW-type
zeolites is also large enough to accommodate two n-butenes and
undergo nonselective dimerization-cracking reactions, like that
in FER, explaining the low isobutene yield observed for fresh
H-MCM-22 and H-ITQ-2.
The most interesting result obtained from Fig. 5 is that both
catalysts show a rapid increase in isobutene yield at early TOS,
together with a simultaneous decrease in formation of byprod-
ucts, and then a stable product distribution over the rest of the
TOS period studied. A quite similar trend was also observed
for the catalytic experiments carried out at the reaction temper-
ature (500 ◦C) of n-butane dehydroisomerization. This strongly
suggests that after some TOS, the prevailing mechanism for
isobutene formation over these two zeolites becomes altered,
which cannot be understood without considering the positive
effect of coke formation on their isomerization activities [26,
27,29–33]. Due to the reasons described above, one cannot

70
H.J. Jung et al. / Journal of Catalysis 245 (2007) 65–74
claim that the pore dimensions of supercages and external pock-
ets are properly altered by coke deposition to significantly en-
hance shape selectivity with TOS, although poisoning by coke
of the nonselective acid sites in these large cavities could lead to
some enhancement of isobutene selectivity. Nevertheless, Fig. 5
shows that coke formation on H-MCM-22 and H-ITQ-2 coin-
cides with their selective behavior in 1-butene skeletal isomer-
ization and that the difference in the amounts (∼9 wt%) of coke
deposited on these two catalysts after 480 min on stream of iso-
merization at 350 ◦C is essentially zero. It should also be noted
that this coke level is quite similar not only to the steady-state
amount (8–9 wt%) of coke on H-FER during the reaction at the
same temperature [27], but also to that (∼9 wt%) on the corre-
sponding catalysts at 500 ◦C, which led us to turn our attention
to the role of coke deposited within their sinusoidal 10-ring
(4.1 × 5.1 Å) channels.
ITQ-2 for isobutene formation may originate mainly from the
unique pore structure of their intralayer channel system, within
which coke formation occurs in such a way as to effectively
suppress nonselective dimerization-cracking reactions, but for
which most of the 10-ring pore mouth inlets with a suitable
degree of ellipticity are still accessible for 1-butene molecules
even after the buildup of a considerable amount of coke inside
the sinusoidal channels, as well as inside the large supercages or
on the external pockets. If this were the case, then the catalytic
results in Fig. 5 could then provide another piece of experi-
mental evidence in support of our recent claim that the positive
effect of coke deposits on the isobutene selectivity is not unique
only with H-FER among the already known medium-pore zeo-
lites [32].
Fig. 6 shows n-butane conversion and yields in isobutene
and various major byproducts as a function of TOS in the
dehydroisomerization of n-butane over 0.6% Pt-MCM-22 and
0.5% Pt-ITQ-2 measured at 500 ◦C, 7.8 h−1 WHSV, and 10%
n-butane and 20% H2 in the feed. Pt-MCM-22 exhibits a sig-
nificantly higher initial n-butane conversion (87% vs 51%) than
Pt-ITQ-2, whereas the opposite is observed for the yield in
isobutene (3% vs 7%). As a result, their initial byproduct pat-
terns differ significantly. For the former, methane is the most
dominant byproduct; for the latter, however, the main byprod-
ucts are linear butenes and propane and, to a lesser extent,
isobutene, methane, and ethane. We also note that both Pt-
MCM-22 and Pt-ITQ-2 hardly show any production of pentene,
indicating a negligible contribution of dimerization of butenes
(formed by dehydrogenation over Pt) followed by cracking
to overall byproduct formation at the beginning of the reac-
tion. This is not unexpected, because 1-butene dimerization
is not thermodynamically favored at higher reaction temper-
atures [11]. In fact, we found that the initial molar ratios of
propene to pentenes in the skeletal isomerization of 1-butene
over H-MCM-22 and H-ITQ-2 measured at 500 ◦C (i.e., the re-
action temperature of n-butane dehydroisomerization) are one
order of magnitude larger than the values over the correspond-
ing catalysts at 350 ◦C. Then three types of side reactions can be
proposed for the initial byproduct formation in dehydroisomer-
ization over Pt-MCM-22 and Pt-ITQ-2: protolytic cracking of
n-butane, hydrogenolysis of n-butane, and cracking of butenes
followed by hydrogenation [35]. Given that Pt-MCM-22 pro-
duces a much larger amount of methane compared with the
sum of propane and propene, hydrogenolysis over the metal in
this catalyst appears to be the major source of methane. More-
over, the fact that the yields in methane and ethane decease
rapidly with TOS suggests that they are formed by the same
reaction (i.e., hydrogenolysis). In contrast, the high initial yield
in propane observed for Pt-ITQ-2 implies that hydrogenation
of propene, a butene-cracking product, over Pt in this layered
zeolite is the main side reaction. We speculate that such a no-
table difference in the initial hydrogenolysis activities of these
two catalysts may originate mainly from the difference in their
metal dispersions (see below). But on the other hand, as shown
in Fig. 6, Pt-MCM-22 shows a rapid decrease in n-butane con-
version with increasing TOS, along with a significant increase
in isobutene and n-butene yields, in good agreement with the
Undoubtedly, the pore size of zeolites and related microp-
orous materials is the most important feature with regard to
coke formation and consequently for isobutene selectivity and
stability as well, because all microporous materials reported as
selective isomerization catalysts have 10-ring channels. How-
ever, not all medium-pore materials are selective for this reac-
tion. In particular, the high selectivity together with fairly good
stability is observed only over the aged H-FER catalyst with
coke deposits present. Although the prevailing mechanism in
the selective formation of isobutene over aged H-FER is still
subject to vigorous debate [26,27,29–33], there is a general
consensus that after some TOS, the H-FER pores are largely
filled by coke, with catalysis occurring primarily near the 10-
ring pore mouth inlets. We believe that this may also be the
case for aged H-MCM-22 and H-ITQ-2. Our recent studies
on the isomerization activities of the proton form of a num-
ber of medium-pore zeolites, including clinoptilolite (HEU),
FER, ZSM-22 (TON), SUZ-4, ZSM-57 (MFS), ZSM-5 (MFI),
and TNU-10 (STI), with different structures have shown that in
general, the more elliptical 10-ring channels the zeolite has, the
lower the 1-butene conversion but the higher initial isobutene
selectivity the zeolite shows [32,34]. Such a trend is also ob-
served when the catalytic data are obtained after 8 h on stream,
and hence after coke formation at certain levels on this series
of zeolites. In contrast, no general relationship between the iso-
merization activity of each zeolite and its 10-ring pore area,
dimensionality, or amount of coke deposited was found. These
results have led us to focus on the pore shape of the 10-ring
channels in medium-pore zeolites as a more crucial parameter
substantially influencing the chemisorption of 1-butene mole-
cules near the pore mouths and thus the isomerization activity.
When the ellipticity (ε), defined by {(b2 − a2)/b2}0.5, where a
and b are the shortest and longest pore diameters, respectively,
is taken as a quantitative measure of differences in the 10-ring
pore shape, the ε value of the sinusoidal 10-ring channels in H-
MCM-22 or H-ITQ-2 was calculated to be 0.595. This value is
essentially the same as that (0.591) calculated for H-ZSM-22,
which is also among the most active zeolite catalysts for the
isomerization reaction [26,27], and is slightly lower than that
(0.629) of the analogous channels in H-FER [32]. Therefore, we
believe that the selective behavior of aged H-MCM-22 and H-

H.J. Jung et al. / Journal of Catalysis 245 (2007) 65–74
71
Fig. 6. (a) n-Butane conversion and yields in (b) isobutene, (c) n-butenes, (d) isobutane, (e) propene, (f) propane, (g) ethane, and (h) methane as a function of time
◦
−1
on stream in dehydroisomerization of n-butane over 0.6% Pt-MCM-22 (2) and 0.5% Pt-ITQ-2 (") at 500 C, 7.8 h WHSV, and 10% n-butane and 20% H in
2
the feed.
findings of Lercher et al. [19]. A similar TOS behavior can
also be observed for Pt-ITQ-2. However, note that this cata-
lyst gives much lower decreases in n-butane conversion and
in methane and ethane yields at early TOS than Pt-MCM-22.
Consequently, not only the n-butane conversions of these two
catalysts, but also their isobutene yields become almost identi-
cal after 100 min on stream.
as a function of n-butane conversion at 500 ◦C and 100 min on
stream. It can be seen that the isobutene selectivities of these
two catalysts decreases with increasing n-butane conversion but
are comparable over the conversion range studied here. A quite
similar result was obtained from their selectivities to n-butenes.
ꢀ
When the i-C=4 / C=4 ratio is taken as a measure of the isomer-
ization activity, Pt-MCM-22 and Pt-ITQ-2 exhibit essentially
identical activities (∼37%), slightly lower than the thermody-
namic limit of 43%, which are invariable with n-butane con-
version. This clearly shows that after stabilization, Pt-MCM-22
Fig. 7 shows the steady-state selectivities of Pt-MCM-22 and
Pt-ITQ-2 to isobutene and n-butenes and their molar fractions
ꢀ
of isobutene to all butenes (i.e., the i-C=4 / C4= ratio) recorded

72
H.J. Jung et al. / Journal of Catalysis 245 (2007) 65–74
ꢀ
=
=
Fig. 7. (a) Selectivities to isobutene (closed symbols) and n-butenes (open symbols) and (b) i-C
/
C
ratio as a function of n-butane conversion in dehy-
4
4
◦
droisomerization of n-butane over Pt-MCM-22 (squares) and Pt-ITQ-2 (circles) measured at 500 C, 100 min on stream, and 10% n-butane and 20% H in the
2
feed.
and Pt-ITQ-2 behave in a comparable manner. Fig. 8 compares
their stabilities during n-butane dehydroisomerization under the
conditions described in Fig. 6. The initial dehydrogenation ac-
ꢀ
tivity (i.e., the yield in C4=) of Pt-MCM-22 was significantly
lower than that of Pt-ITQ-2, possibly related to its higher ini-
tial hydrogenolysis activity (see above). As shown in Fig. 8,
however, the dehydrogenation activity of the former increases
rapidly for the first 50 min on stream. As a result, Pt-MCM-
22 and Pt-ITQ-2 have nearly the same dehydrogenation activity
over the rest period of TOS studied, although the latter shows a
slight deactivation. Hydrogenolysis is a structure-sensitive reac-
tion that competes with structure-insensitive dehydrogenation
over the metal [36]. Therefore, the rapid increase in dehydro-
genation activity with increasing TOS observed for Pt-MCM-
22 can be rationalized by suggesting that some of its Pt particles
are small enough to easily sinter under the reaction conditions
used here, leading to decreased formation of hydrogenolysis
products. Whereas the isomerization activity of Pt-MCM-22
is larger by a narrow margin than that of Pt-ITQ-2 from the
beginning of the reaction, almost no TOS dependence on iso-
merization activity is observed. Fig. 8 also shows the influence
of TOS on coke formation over Pt-MCM-22 and Pt-ITQ-2. We
note that the amount of coke deposited after 380 min on stream
of dehydroisomerization at 500 ◦C, as well as the coke forma-
tion pattern, is nearly the same for these two catalysts.
Fig. 9 shows the TEM pictures of Pt-MCM-22 and Pt-ITQ-
2 before and after their use in n-butane dehydroisomerization
under the conditions described above. In general, Pt particles
cannot be observed in fresh Pt-MCM-22, suggesting that they
are well dispersed over the MCM-22 crystallites. The average
Pt particle size of used Pt-MCM-22 must be close to that of the
fresh catalyst, because no large Pt particles are observed even
after exposure to reaction conditions. This indicates that Pt par-
ticles in MCM-22 do not migrate to the surface of the catalysts
Fig. 8. (a) Dehydrogenation and (b) isomerization stabilities and (c) coke for-
mation as a function of time on stream in dehydroisomerization of n-butane
at reaction conditions (500 ◦C) in a period of about 6 h. Because
there is a trend of rapidly increasing dehydrogenation activity
at early TOS (Fig. 8), however, we cannot rule out the possi-
bility that the sintering of very small Pt particles (<10 Å) that
remain within the micropores of MCM-22, especially within its
supercages, occurs during the reaction. It has been repeatedly
shown that the particle size distribution of highly dispersed Pt
particles in zeolites determined by TEM is very sensitive to ex-
over Pt-MCM-22 (2) and Pt-ITQ-2 ("). The reaction conditions are the same
as those given in Fig. 6.
perimental conditions, because the zeolite framework is easily
damaged when the incident beam is sufficiently strong (200 kV)
[37,38]. For fresh Pt-ITQ-2, in contrast, Pt particles with diame-
ters of 50–100 Å were found. This suggests that Pt is distributed

H.J. Jung et al. / Journal of Catalysis 245 (2007) 65–74
73
tem may be ultimately responsible for the high dehydroisomer-
ization activity observed for Pt-MCM-22 and Pt-ITQ-2 after
some TOS. We should note here that the steady-state amounts
(6 wt%) of coke formed on Pt-MCM-22 and Pt-ITQ-2 dur-
ing the dehydroisomerization at 500 ◦C are fairly smaller than
those (9 wt%) of coke on H-MCM-22 and H-ITQ-2 during
1-butene isomerization at the same temperature, due mainly to
the presence of H2 in the former reaction stream. Given the two-
dimensionality of the sinusoidal pore system, this suggests that
diffusion of the reactants and products imposed by coke forma-
tion would not be so severe, as in the case of Pt-ZSM-5 with
two intersecting 10-ring channels, which is among the most ac-
tive catalysts for n-butane dehydroisomerization [28].
4. Conclusion
Skeletal isomerization of 1-butene and dehydroisomeriza-
tion of n-butane at atmospheric pressure have been investigated
over the proton form of MCM-22 and ITQ-2 and their Pt-loaded
analogs. The two-dimensional sinusoidal 10-ring channels of
these zeolites were found to play a much more crucial role in
the selective production of isobutene than the large supercages
or the 12-ring external pockets. The rate of coke formation ap-
pears to be much faster on the protonic sites of the latter two
pore systems, probably due to the greater coke-forming ten-
dency, leading to suppression of undesired consecutive reac-
tions such as dimerization cracking at early TOS in these larger
cavities. The overall results of our study lead us to conclude that
isobutene formation over stabilized H-MCM-22 and H-ITQ-2
occurs mainly near the pore mouth inlets of sinusoidal 10-ring
channels, whereas the selective sites for n-butane dehydroiso-
merization over Pt-MCM-22 and Pt-ITQ-2 in steady state are
located inside this intralayer pore system.
Fig. 9. Transmission electron micrographs of (a) Pt-MCM-22 and (b) Pt-ITQ-2
before (left) and after (right) their use in dehydroisomerization of n-butane at
◦
−1
500 C, 7.8 h WHSV, and 10% n-butane and 20% H in the feed for 380 min.
2
randomly throughout the intracrystalline pores as well as on the
external surface of the delaminated crystallites. Because there
are no noticeable differences in the average particle size of fresh
and used Pt-ITQ-2 catalysts, like the case of Pt-MCM-22, it ap-
pears that the reduction of Pt-ITQ-2 at 500 ◦C may result in
the migration and agglomeration of Pt(NH3)2+ ions exchanged
onto its external 12-ring pockets, where the4re is no beneficial
effect of the micropores.
With bifunctional metal-loaded zeolite catalysts for the de-
hydroisomerization of n-butane, in general, the dehydrogena-
tion over the metal is the first step, followed by isomerization
over the Brønsted acid sites in zeolite supports [28,35]. The
TEM results in Fig. 9 reveal that most, if not all, of the Pt par-
ticles in the used MCM-22 catalyst are still intrazeolitic. Thus,
the main location of selective dehydroisomerization sites can-
not be on its external 12-ring pockets. We believe that this is
also the case of Pt-ITQ-2 containing much larger Pt particles
(50–100 Å) compared with Pt-MCM-22, because the steady-
state catalytic behavior is essentially the same for these two
catalysts (Figs. 6–8). In fact, it has been shown that dehydro-
genation activity of Pt-loaded zeolite catalysts is not directly
related to the metal dispersion [28]. Due to the inherently
shape-selective nature of zeolites, on the other hand, coke for-
mation on these microporous materials is also a shape-selective
process that depends highly on pore architecture [39,40]. Be-
cause the rates of coke formation over zeolites containing large
cavities with windows of smaller size are normally higher than
those over medium-pore zeolites with 10-ring pores of uniform
dimension, leading to rapid deactivation, it is not difficult to
infer that the selective dehydroisomerization sites, especially
in steady state, cannot be located within the large supercages.
Therefore, it is most likely that the intralayer 10-ring pore sys-
Acknowledgments
Funding for this work was provided by the Korea Research
Foundation (grant 2004-D00172 to S.B.H.). The authors thank
Professor G. Seo, Chonnam National University, for the NH3
TPD measurements.
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