Catalytic cracking of n-hexane for producing propylene on MCM-22 zeolites

Article abstract of DOI:10.1016/j.apcata.2014.12.018

The catalytic cracking of n-hexane for producing propylene on MCM-22 catalysts was carried out as a model reaction of naphtha cracking. H-MCM-22 catalysts with different Si/Al ratios were prepared by direct or post-synthesis methods. The propylene selectivity was improved with an increase in the Si/Al ratio. Dealumination of H-MCM-22 with ammonium hexaflurosilicate (AHFS) (Del-Al-MCM-22) resulted in the increase in the catalytic life and propylene selectivity at a very high n-hexane conversion. This may be attributed to the decrease in the amount of Lewis acid sites which accelerate the hydride transfer and resultant coke formation. Del-Al-MCM-22 (Si/Al = 62) catalyst showed a high propylene selectivity (40 C-%) which was higher than H-ZSM-5 and H-Beta catalysts with similar Si/Al ratios at a high n-hexane conversion of 95%. Moreover, it showed a catalytic life comparable with H-ZSM-5 and longer than H-Beta catalyst.

Full text of DOI:10.1016/j.apcata.2014.12.018

Accepted Manuscript
Title: Catalytic Cracking of n-Hexane for Producing
Propylene on MCM-22 Zeolites
Author: Yong Wang Toshiyuki Yokoi Seitaro Namba Junko
N. Kondo Takashi Tatsumi
PII:
S0926-860X(14)00769-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.12.018
APCATA 15156
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
14-9-2014
5-12-2014
10-12-2014
Pleasecitethisarticleas:Y. Wang, T. Yokoi, S. Namba, J.N. Kondo, T. Tatsumi, Catalytic
Cracking of n-Hexane for Producing Propylene on MCM-22 Zeolites, Applied Catalysis
A, General (2014), http://dx.doi.org/10.1016/j.apcata.2014.12.018
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Catalytic Cracking of n-Hexane for Producing
Propylene on MCM-22 Zeolites
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Yong Wang, Toshiyuki Yokoi*, Seitaro Namba, Junko N. Kondo, Takashi Tatsumi
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-
ku, Yokohama 226-8503, Japan
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*E-mail yokoi.t.ab@m.titech.ac.jp (T. Yokoi), Fax +81 45 924 5282
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Abstract:
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The catalytic cracking of n-hexane for producing propylene on MCM-22 catalysts
was carried out as a model reaction of naphtha cracking. H-MCM-22 catalysts with
different Si/Al ratios were prepared by direct or post-synthesis methods. The propylene
selectivity was improved with an increase in the Si/Al ratio. Dealumination of H-MCM-
22 with ammonium hexaflurosilicate (AHFS) (Del-Al-MCM-22) resulted in the increase
in the catalytic life and propylene selectivity at a very high n-hexane conversion. This
may be attributed to the decrease in the amount of Lewis acid sites which accelerate the
hydride transfer and resultant coke formation. Del-Al-MCM-22 (Si/Al = 62) catalyst
showed a high propylene selectivity (40 C-%) which was higher than H-ZSM-5 and H-
Beta catalysts with similar Si/Al ratios at a high n-hexane conversion of 95%. Moreover,
it showed a catalytic life comparable with H-ZSM-5 and longer than H-Beta catalyst.
22 Keywords: Catalytic cracking; Dealumination of H-MCM-22; Lewis acid site
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1. Introduction
Light alkenes, in particular propylene, which are mainly supplied as a by-product of
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ethylene production through the thermal cracking of naphtha, are acquiring more and
more significance in chemical industry. It is known that the thermal cracking is an energy
intensive process, where the product distribution, especially the ratio of propylene to
ethylene, is hard to control. Therefore, catalytic cracking of naphtha has been drawing
increasing attention [1]. The catalytic cracking of alkanes over acidic zeolite catalysts
gives a high propylene/ethylene ratio, since the transformation of long-chain alkanes to
short-chain alkenes occurs at least partly via the carbenium ion/β-scission mechanism, i.e.
the classical bimolecular cracking [2]. Compared with the thermal steam cracking, the
catalytic cracking of naphtha which is carried out at lower temperatures would consume
20% less energy and reduce CO2 emission by approximately 20% [1].
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The catalytic cracking of light alkanes, such as pentane, hexane, heptane and octane,
has been studied over various zeolite catalysts as a test reaction of naphtha cracking to
clarify the performance of catalysts and the reaction mechanism [3]. There are many
reports on cracking of alkanes at relatively low temperatures to allow discussion on the
reaction mechanism and/or the effects of structure of zeolites [3-12]. However, there are
only a few papers dealing with the catalysts working at high temperatures above 873 K
which is necessary in order to obtain high alkene yields. Among the catalysts examined
for the catalytic cracking, ZSM-5 zeolite has been recognized as a prime candidate
because of its high thermal and hydrothermal stabilities and its considerable resistance to
deactivation caused by coking as well as its strong acidity [1, 13-14]. Yoshimura et al. [1]
have carried out the cracking of light naphtha in the presence of steam at 923 K and
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found that the addition of lanthanum (10 wt. %) to H-ZSM-5 (Si/Al = 100) enhanced the
yield of ethylene and propylene up to 61 C-% and that the ethylene/propylene ratio in the
product was approximately 0.7, which corresponds to a higher proportion of propylene
than the composition obtained by using the current steam cracking. Moreover, further
addition of phosphorus (2 wt. %) improved the stability. Recently, dealuminated MCM-
68 catalyst with multidimensional channels of 10-MR and 12-MR has also been reported
to exhibit a high propylene selectivity of 45 C-% [15]. However, the selectivity to
propylene is uncertain when the conversion approaches 100%.
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MCM-22, a kind of layered zeolite invented by Mobil in 1990 [16], belongs to the
MWW type zeolite and possesses two independent pore systems. One consists of two-
dimensional sinusoidal channels composed of slightly elliptical 10-MR, and the other is
characteristic of 12-MR supercages accessible by 10-MR windows [17]. In addition, half
supercages form surface pockets on the outer crystal surfaces. Because of its unique
structure, MCM-22 zeolite has been proved to be a shape-selective catalyst in the
alkylation [18, 19], disproportionation and isomerization [20-23], etc. Obviously, the acid
properties (amount, location, type and strength) can also play important roles in these
catalytic reactions. As we known, the acid properties are closely related to the Si/Al ratio
in the zeolite.
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A number of studies dealt with the synthesis of MCM-22 zeolite, concluding that
only a very narrow range of the Si/Al ratio of about 10 – 25 is suitable for the synthesis
of pure MCM-22 zeolite [24, 25]. Besides the direct synthesis methods, post-synthesis
methods are also frequently used for preparing high silica zeolites. Post-synthesis of
highly crystalline MWW zeolite with variable Si/Al ratios in a wide range (12 – 500)
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through the structural conversion of deboronated Si-MWW zeolite to lamellar Al-MWW
zeolite precursor has been studied by Liu et al. [26]. They found that the amount and
strength of Brønsted acid sites in H-MCM-22 zeolite both decreased with increasing
Si/Al ratios. Another method for obtaining high silica MCM-22 zeolites is dealumination
including acid leaching, hydrothermal treatment, combination of steaming and acid
leaching, ammonium hexafluorosilicate (AHFS) treatment [20, 27-30], etc. Obviously,
the acid properties in dealuminated MCM-22 are dependent on the different delamination
methods.
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MCM-22 has also proved to be a good cracking zeolite additive for FCC process [31,
32]. Corma et al. [32] found that when used as cracking additive, MCM-22 produced less
total gases, as well as less dry gases, with a lower loss of gasoline than ZSM-5. Moreover,
MCM-22 showed higher propylene/propane and butane/butane ratio and a good steam
stability which was even higher than that of the ZSM-5 zeolite. The catalytic performance
of MCM-22 in n-heptane cracking have been investigated by Meloni et al. [33], who
found that a very fast initial deactivation was occurred, due to trapping of carbonaceous
compounds (coke) in the large cages with small apertures. They suggested that n-heptane
cracking occurs through the classical carbenium ion chain mechanism in the supercages,
however it occurs through protolysis in the sinusoidal channels, the bimolecular reaction
of hydride transfer being impossible in the narrow space available near the protonic sites.
Though the role of shape selectivity on the catalytic performance was explained in the
above paper, the role of acid properties including the acid amount, strength and type was
seldom investigated.
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In this study, the catalytic performance of MCM-22 catalysts with different Si/Al
ratios synthesized and prepared by post-synthesis and treated with AHFS was
investigated for the catalytic cracking of n-hexane as a model reaction of naphtha
cracking to produce propylene. In addition, the relationship between the catalytic
performance and the acid properties of the catalysts was discussed.
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2. Experimental
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2.1. Catalyst preparation
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H-MCM-22 zeolite catalysts with different Si/Al ratios were prepared by two
methods according to literature [26, 34]. (1) Direct synthesis method was used for the
synthesis of zeolites with high Al contents. The MCM-22 lamellar precursors were
hydrothermally synthesized by using hexamethyleneimine (HMI) as a structure-directing
agent (SDA). Fumed silica (Cab-o-sil M5) and sodium aluminate (58.3% Al2O3, 32.3%
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14 Na2O) were used as Si and Al sources, respectively, and sodium hydroxide was used to
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adjust the gel pH. Deboronated MWW (Del-B-MWW) obtained by calcination of MWW
borosilicate zeolite at 873 K for 20 h, followed by refluxing in 6 M HNO3 at 393 K for 20
h, was used as seed to speed up the crystallization. The resulting mixture had a molar
composition of 1 SiO2 : 0.15 Na2O : (0.0125 - 0.025) Al2O3 : 0.9 HMI : 45 H2O. (2) The
MWW-type precursors with low Al contents were synthesized by a post-synthesis
method. Del-B-MWW zeolite was used as a Si source and added into an aqueous solution
of sodium aluminate and HMI under stirring. The resulting mixture had a molar
composition of 1 SiO2 : (0.005 - 0.007) Al2O3: 1 HMI: 30 H2O. The crystallization was
carried out in rotated (20 rpm) Teflon-lined stainless autoclaves under autogenous
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pressure at 423 K for 5 days. The samples were filtered, washed, and dried at 373 K to
produce lamellar precursors with a wide range of Si/Al ratios. Direct calcination of the
samples at 823 K for 10 h resulted in the products with the 3D MWW structure. The Na-
form MCM-22 was transformed into the H-form by treating it with 1 M NH4NO3 twice at
353 K for 2 h, followed by calcination in air at 773 K for 2 h. The H-form MCM-22
zeolites prepared by direct and post-synthesis methods were named H-MCM-22 (n) and
Post-H-MCM-22 (n), respectively, in which n denotes the Si/Al molar ratio determined
by ICP-AES analysis.
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Dealuminated MCM-22 (Del-Al-MCM-22) catalysts were obtained from H-MCM-
22 (Si/Al = 19) by the dealumination with aqueous solution of ammonium
hexaflurosilicate (AHFS) at 363 K for 3 h. The Del-Al-MCM-22 catalysts with different
Al contents were obtained by varying the concentration of AHFS solution from 0.005 to
0.05 M. For control, a small-sized H-ZSM-5 catalyst with Si/Al = 66 was synthesized
according to literature [13] and H-Beta catalyst with Si/Al = 67 was obtained from H-
Beta (Si/Al = 12) (Reference Catalyst, The Catalysis Society of Japan) by dealumination
with 1 M HNO3 solution at 393 K for 1 h.
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2.2. Catalyst characterizations
The catalysts were characterized by various techniques. The Si/Al ratios were
determined by inductively coupled plasma-atomic emission spectrometer (ICP-AES) on a
Shimadzu ICPE-9000. The X-ray diffraction (XRD) patterns were recorded on a Rint-
Ultima III (Rigaku) diffractometry using a Cu Kα X-ray source. The N2 adsorption was
carried out at 77 K on a Belsorp-mini II (BEL Japan) instrument. The crystal morphology
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and size were examined with a Hitachi S-5200 field-emission scanning electron
microscope (FE-SEM). Thermogravimetric analyses (TGA) were carried out on a Mettler
TGA/SDT apparatus in air atmosphere. Temperature-programmed desorption of
ammonia (NH3-TPD) spectra were recorded on a Multitrack TPD equipment (Japan
BEL). IR spectra were collected on a Nicolet NEXUS-FTIR-670 spectrometer. The IR
spectra of adsorbed pyridine were recorded as follows: a self-supported wafer (9.6 mg
cm−2 in thickness and 2 cm in diameter) was set in a quartz IR cell, sealed with CaF2
windows, and then was evacuated at 723 K for 2 h before the pyridine adsorption. The
adsorption was carried out by exposing the wafer to pyridine vapor (1.3 kPa) at 423 K for
0.5 h. The physisorbed pyridine was then removed by evacuation at 423 K for 1 h. The IR
spectra were recorded at 423 K.
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2.3. Catalytic cracking
The catalytic cracking of n-hexane was carried out with a fixed-bed flow reactor
under atmospheric pressure. Typically, 0.1 g of catalyst (18 – 30 mesh) was put into a
tubular reactor (6 mm in i. d.) and activated in an air flow of 20 ml min-1 at 923 K for 1 h.
After the reactor temperature was adjusted to the desired, n-hexane (Wako Pure
Chemicals Ind.) vapor diluted in helium was fed into the reactor. The initial partial
pressure of n-hexane was set at 6 kPa. To investigate the effect of contact time, the ratio
of the catalyst weight to the n-hexane flow rate (W/Fn-hexane) was varied from 1.6 to 64 g-
cat h/mol-n-hexane by changing the catalyst amount.
The reaction products were analyzed with an on-line gas chromatograph (Shimadzu,
GC-14B) with an FID detector and a HP-AL/S column (50 m × 0.32 mm × 8 μm). The
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selectivities to the products were calculated based on the carbon numbers. The coke
amount was calculated by thermogravimetric analysis (TGA). The weight loss from 673
to 1073 K in each TG profile was defined as the contents of coke on the used catalyst.
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3. Results and discussion
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3.1. Physicochemical characteristics of the zeolites
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All of the H-MCM-22 zeolites synthesized in this study had the MWW topology and
a relatively high crystallinity, and contained no impurity of other phase as detected by
XRD patterns (Fig. 1A). As shown in Table 1, the H-MCM-22 samples had reasonably
high BET surface areas in the range of 448 - 540 m2 g-1 as determined by N2 adsorption,
which indicated that they were of good quality. The images of scanning electron
micrographs (SEM) revealed that both of H-MCM-22 (Si/Al = 19) and Post-H-MCM-22
(Si/Al = 100) samples were composed of flake-like crystals, approximately 500 nm in
length and 50 nm in thickness (Fig. 2a and 2b). Thus, the crystal morphology and size
were not dependent on the Si/Al ratio in the gels and the synthesis method. The Del-Al-
MCM-22 samples with different Al contents were further characterized by powder XRD
patterns (Fig. 1B). After dealumination by AHFS, little change was observed in their
diffraction patterns compared with those of the parent material. Namely, the crystal
structure of MCM-22 zeolite essentially remained.
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Table 1 lists the Si/Al ratios and textural properties of Del-Al-MCM-22 zeolite
catalysts. It can be seen that AHFS is an effective dealumination reagent for MCM-22
zeolite, and more than 40% of the aluminum atoms can be removed by the treatment with
0.005 M AHFS solution. With an increase in the AHFS concentration, the Si/Al ratio
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increased, which is consistent with the literature [30]. The specific surface area and
micropore pore volume of of Del-Al-MCM-22 samples are also listed in Table 1. It can
be seen that with an increase in the Si/Al ratio of the Del-Al-MCM-22 samples, viz. with
an increasing in the AHFS concentration, the surface area and the micropore volume of
the samples slightly decreased, while the external surface area was almost constant. On
the other hand, the AHFS treatment, especially with high AHFS concentration, resulted
in a significant fragmentation of crystal morphology to form a number of agglomerates
(Fig. 2d). The formation of agglomerates could happen through binding of the surface
hydroxyl groups (Si-OH) of neighboring crystallites, which can be facilitated in the
presence of a chemical agent like AHFS [35].
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Fig. 3 shows the NH3-TPD profiles of H-MCM-22 and Del-Al-MCM-22 catalysts
with different Al contents. As shown in Fig. 3, all these profiles were composed of two
13 desorption peaks so-called l-peak and h-peak. The l-peak corresponds to NH3 adsorbed
+
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on non-acidic –OH groups and on NH4 by hydrogen bonding, while the h-peak
corresponds to NH3 adsorbed on true acid sites [36, 37]. Thus, the acid amount was
calculated by the area of h-peak. The acid amount corresponded to only 63-77% of the
total Al content in H-MCM-22 zeolites (Table 1). The low concentration of acid relative
to that of Al are attributed to the fact that the portion of Al incorporated into the MCM-22
framework was not very high, regardless of the direct or post synthesis methods.
Obviously, the acid amount was decreased with the Si/Al ratio increased (Fig. 3A, Table
1). On the other hand, the acid amount was gradually decreased with an increase in the
AHFS concentration (Fig. 3B, Table 1), which means the AHFS treatment is an effective
dealumination method for MCM-22 zeolite.
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H-ZSM-5 and H-Beta catalysts had relatively high crystallinities as indicated by
XRD patterns (not shown) and reasonably high BET surface areas (Table 1). As shown in
Fig. 2, H-ZSM-5 (Si/Al = 66) and H-Beta (Si/Al = 67) were composed of coffin-like
crystals with an average size of about 200 nm and sphere-like crystals with an average
size of about 100 nm, respectively (Figs. 2e and 2f). In addition, the Si/Al ratio and acid
amount of H-ZSM-5 and H-Beta catalysts were similar to those of Del-Al-MCM-22
catalyst (Table 1).
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3.2. Catalytic cracking of n-hexane over H-MCM-22 catalysts
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3.2.1. Effect of reaction temperature
The effect of the reaction temperature on the catalytic cracking of n-hexane over H-
MCM-22 (Si/Al = 19) is shown in Fig. 4. The initial conversion of n-hexane increased
with an increase in the temperature, achieving nearly 100% at 923 K. The selectivity to
propylene was almost constant in the temperature ranging between 723 and 923 K. With
an increase in the reaction temperature, apparently butenes once formed were gradually
transformed into aromatic compounds, resulting in the decrease in the selectivity to
butenes accompanied with the increase in the selectivity to BTX (benzene, toluene, and
xylenes). The selectivity to ethylene was low compared with those to other alkenes at low
temperature; however, the value increased steeply with an increase in the reaction
temperature. Ethylene is formed via primary carbenium ions, regardless of whether the
monomolecular mechanism or the bimolecular mechanism operates [5, 7] and therefore
the apparent activation energy for ethylene formation should be high. Thus, the higher
reaction temperature is of benefit to the higher ethylene yield. Moreover, the cracking
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via the monomolecular mechanism may be of more advantage in the ethylene formation.
The selectivities to propane and butanes decreased with an increase in the reaction
temperature, because the subsequent reactions of propane and butanes take place at high
temperatures, resulting in the formation of alkenes, BTX and so on [38]. In addition, the
selectivities to methane and ethane, which form solely via the monomolecular mechanism,
are higher at the higher reaction temperatures. These findings clearly indicate that the
cracking via the monomolecular mechanism is more predominant at the high temperature.
According to the product distributions at different reaction temperatures, we
conclude that higher reaction temperature is beneficial to n-hexane conversion and
propylene production. Kubo et al. reported similar results in n-heptane cracking over H-
ZSM-5 [14].
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3.2.2. Effect of contact time
The effect of the contact time on the reaction was investigated. The contact time
expressed as the ratio of the catalyst weight to the n-hexane flow rate (W/Fn-hexane) was
changed by the change of catalyst weight to keep the contribution of the thermal cracking
constant. As shown in Fig. 5, the conversion of n-hexane increased with an increase in
W/Fn-hexane, achieving nearly 100% at the W/Fn-hexane of 64 g-cat h/mol-n-hexane. Fig. 5
indicates that the decrease in the selectivities to propylene and butenes is accompanied
with the increase in the selectivity to BTX, suggesting that BTX is formed mainly from
propylene and butenes. On the other hand, primary carbenium ions would be relatively
easy to produce at the high reaction temperature of 923 K [5, 7]. Therefore, ethylene
would be formed not only by n-hexane cracking but also by butanes and/or propane
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cracking, resulting in an increase in the ethylene selectivity [38]. Meanwhile, the
selectivities to propane and butanes decreased with an increase in the conversion, while
those to methane and ethane simultaneously increased.
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3.2.3. Effect of Al content
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It is widely known that acidity of zeolites is largely dependent on their Si/Al molar
ratios. In order to elucidate the effect of the acidity on the catalytic cracking of n-hexane,
H-MCM-22 zeolites with different Si/Al ratios were employed as catalysts. First, effect
of the Al content on catalytic activity was investigated. We consider that n-hexane
cracking at such high temperatures is the sum of the catalytic cracking and the thermal
cracking since the contribution of thermal cracking is not negligible. If both the catalytic
cracking and the thermal cracking at high temperatures obey the first-order kinetics, the
following equation is derived;
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kc W/F = ln(1/1-x) –ln(1/1-xp).
eq. (1)
15 where kc is the rate constant for the catalytic cracking, x and xp are the conversion for the
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n-hexane cracking with and without catalyst, respectively [14]. Fig. 6 shows the
dependence of kc on the acid amount of H-MCM-22 catalysts at different reaction
temperatures. The linear relationship between log kc and log acid amount of H-MCM-22
catalyst was observed at every reaction temperature. As shown in Fig. 6, the slopes of the
straight line observed in the log kc vs. log (Acid amount) plots are 1.8, 1.6, and 1.3 at 823,
873, and 923 K, respectively. The kc is almost proportional to the acid site density at 923
K; however, at lower temperatures the effect of the acid site density is enhanced with a
decrease in reaction temperature. Haag et al. reported that the catalytic activity in the
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catalytic cracking of n-hexane at 811 K over H-ZSM-5 is proportional to the acid site
density [39-41]. A similar dependence of n-hexane and n-heptane cracking on the acid
density over H-ZSM-5 with various Si/Al ratios has been reported by Post et al. and
Kubo et al. [6, 14].
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Then, the effect of the Al content on products distribution was investigated. Fig. 7
shows the relationship between the selectivities and the acid amount of H-MCM-22
catalysts at an n-hexane conversion of ca. 80%. It can be seen that, with a decrease in the
acid amount, propylene and butenes selectivities slightly increased while ethylene and
BTX selectivities slightly decreased. These findings suggest that the decrease in the acid
amount suppressed the secondary reactions (hydride transfer and cyclization-
aromatization) of propylene and butenes to form BTX. Thus the H-MCM-22 catalysts
with the higher Si/Al ratios showed the higher propylene selectivity in the catalytic
cracking of n-hexane. Meanwhile, the subsequent cracking reaction of butanes and/or
propane to form ethylene at high reaction temperature of 923 K was reduced with a
decrease in the acid amount, resulting in the decrease in the ethylene selectivity
accompanied with the increase in the selectivties to butanes and propane (not shown
here).
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Besides activity and selectivity, stability is another important factor for applications
of catalysts in industry. Fig. 8 shows the change in n-hexane conversion with time on
stream (TOS) for H-MCM-22 catalysts with different Si/Al ratios. Obviously, the initial
activity and deactivation rate closely depended on the Si/Al ratio. The H-MCM-22
catalyst with higher Al contents (Si/Al = 19 and 34) gave nearly 100% n-hexane
conversion at the initial stage, whereas the catalyst with a lower Al content (Si/Al = 100)
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showed only ca. 80% conversion of n-hexane. A sharp decrease in the n-hexane
conversion with TOS was observed for the H-MCM-22 catalyst with high Al contents.
For example, the n-hexane conversion of H-MCM-22 (Si/Al = 19) decreased to ca. 60%
at TOS of 210 min. This deactivation was probably due to the deposition of a large
amount of coke (95 mg/g-catalyst). The formation of large amount of coke can be
explained by two possible reasons. One reason may be related to the high amount of acid
sites, which would accelerate the secondary reactions including hydride transfer,
cyclization-aromatization and dehydrogenating coupling, etc. and resultant coke
formation. Another reason may be related to the special structure of MCM-22 zeolite.
The nonconnected property between 12- and 10-MR channels and the 10-MR window
openings accessible to the 12-MR supercages would increase the diffusion limitation,
leading to the high coke formation. The coke composition and the mode of deactivation
in n-heptane cracking over MCM-22 zeolite had been investigated and established by
Meloni et al. [33]. In the case of Post-H-MCM-22 (Si/Al = 100) catalyst, the deactivation
rate was relative slow, due to the smaller amount of acid sites leading to the suppression
of hydride transfer and coke formation (14 mg/g-catalyst). These results indicate that H-
MCM-22 catalysts with lower Al contents exhibit a better stability than the catalyst with
higher Al contents.
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In addition, the crystallite size strongly might affect the catalytic stability. Mochizuki
et al. reported the small-sized H-ZSM-5 catalyst exhibits a higher stability in n-hexane
catalytic cracking, owing to large external surface area and short diffusion path length
[13]. However, in our present study, MCM-22 catalysts with different Al contents by
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direct or post-synthesis method had similar crystallite morphology and sizes (Fig 2a – 2d).
Thus, the influence of crystallite size on catalytic stability could not be examined.
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3.3. Catalytic cracking of n-hexane over Del-Al-MCM-22 catalysts
5
From the results reported above, H-MCM-22 catalysts with lower Al contents
prepared from the post-synthesis method exhibited the higher propylene selectivity.
However, the catalytic stability was still not satisfactory. Thus, the catalytic performance
of several high-silica MCM-22 zeolites prepared by the dealumination was further
investigated.
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Fig. 9 shows the change in n-hexane conversion with TOS for H-MCM-22 (Si/Al =
19) and Del-Al-MCM-22 catalysts with different Si/Al ratios. The stability of Del-Al-
MCM-22 catalysts was greatly improved by the dealumination, though the initial n-
hexane conversion slightly decreased. In the case of Del-Al-MCM-22 (Si/Al = 62), the
initial n-hexane conversion was about 95%, and only slightly decreased to 88% at 210
min. This also may be due to the smaller amount of acid sites in the Del-Al-MCM-22
catalyst, resulting in suppression of the hydride transfer and coke formation (22 mg/g-
catalyst). It is noted that the catalytic stabilities of Post-H-MCM-22 (Si/Al = 66) and Del-
Al-MCM-22 (Si/Al = 62) catalysts was different, though they had similar Si/Al ratios and
acid amounts. This finding indicates that some other factor than the acid amount affects
the catalytic stability, which is to be discussed in Section 3.5. In addition, the amount of
coke on the Post-H-MCM-22 (Si/Al = 66) and Del-Al-MCM-22 (Si/Al = 62) catalysts
was nearly the same, however the stabilities was different, indicating the different coke
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composition and location on the two catalysts. This may be due to the difference in the
type, strength and location of acid sites.
3
Fig. 10 shows the changes in selectivities to alkenes and BTX in the n-hexane
conversion on H-MCM-22 (Si/Al = 19) and Del-Al-MCM-22 catalysts with different
Si/Al ratios. As shown in Fig. 10, for H-MCM-22 (Si/Al = 19) catalyst, the selectivities
to propylene and butenes were decreased along with increasing n-hexane conversion,
while those to ethylene and BTX were increased. The selectivity to propylene was
decreased from 41 to 30 C-% when the n-hexane conversion was increased from 60% to
nearly 100%. Compared with the parent H-MCM-22(Si/Al = 19) catalyst, Del-Al-MCM-
22 catalysts showed high selectivities to propylene and butenes and simultaneously low
selectivities to ethylene and BTX at high n-hexane conversions. In addition, Del-Al-
MCM-22 catalysts showed higher selectivities to butanes and propane at high n-hexane
conversions (not shown here). For example, in the case of Del-Al-MCM-22(Si/Al = 62),
the propylene selectivity was kept almost constant (ca. 40 C-%) even at a high n-hexane
conversion of 95%. These findings indicate that the secondary reactions to form BTX
from propylene and butenes and the cracking reaction of butanes and/or propane to form
ethylene at high reaction temperature of 923 K were suppressed when the acid amount of
catalyst was decreased.
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3.4. Comparison of the catalytic performance of Del-Al-MCM-22 with H-ZSM-5 and H-
Beta catalysts
In the present study, the catalytic performance of Del-Al-MCM-22 was also
compared with that of H-ZSM-5 and H-Beta catalysts, which are the catalyst mostly used
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in FCC process [31, 32]. Fig. 11 shows the change in n-hexane conversion with TOS for
H-ZSM-5, H-Beta and Del-Al-MCM-22 catalysts with similar Si/Al ratios and acid
amounts. Among these catalysts investigated, the H-ZSM-5 (Si/Al = 66) catalyst gave a
high initial n-hexane conversion of nearly 100% and the highest stability. The amount of
coke formed during the reaction for 210 min was very low (29 mg/g-catalyst). This may
be related to the tridirectional 10-MR channels in ZSM-5 zeolite, which is beneficial to
the mass transfer, leading to very low deactivation. The Del-Al-MCM-22 (Si/Al = 62)
catalyst showed a little lower initial n-hexane conversion (95%); however the stability
was still good. The resistivity of Del-Al-MCM-22 (22 mg/g-catalyst) to coke formation
was as high as that of H-ZSM-5, probably due to its acid strength and density of acid sites
are similar to those of H-ZSM-5 [36]. Although H-Beta (Si/Al = 67) catalyst showed a
high initial n-hexane conversion (nearly 100%), the amount of coke formed thereon
during the reaction for 210 min (107 mg/g-catalyst) was much higher than those on the
other two catalysts, leading to a significant deactivation. Despite the milder acidity of H-
Beta catalyst compared with H-MCM-22 and H-ZSM-5 catalysts [42], the polymerization
of olefins and the successive formation of coke components in the 3-dimensional 12-MR
micropores and their wide intersections may predominantly occur [15]. A similar result
has been reported by Corma et al., who suggest that the stability of H-MCM-22 zeolite is
intermediate between those of H-Beta and H-ZSM-5 in n-heptane cracking at the lower
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20 temperature of 723 K, mainly because MCM-22 zeolite has larger pores and/or cavities
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than ZSM-5 and smaller ones than Beta zeolite [31]. In addition, the crystallite size and
morphology of zeolite also have influenced on the mass transfer. The flake-like crystal of
MCM-22 zeolite is beneficial to the mass transfer, compared to the coffin-like crystal
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ZSM-5 and sphere-like crystal Beta zeolite (Fig. 2).
Fig. 12 shows the changes in selectivities to alkenes and BTX with n-hexane
conversion on H-ZSM-5, H-Beta and Del-Al-MCM-22 catalysts. As shown in Fig. 12, for
H-ZSM-5 and H-Beta catalyst, the selectivities to propylene and butenes steeply
decreased with an increase in n-hexane conversion, while those to ethylene and BTX
increased drastically. However, in the case of Del-Al-MCM-22 (Si/Al = 62) catalyst, the
propylene selectivity was kept almost constant (ca. 40 C-%), even at an n-hexane
conversion as high as 95%. Moreover, when the n-hexane conversion was higher than
95%, it showed a lower BTX selectivity than either H-ZSM-5 or H-Beta catalysts, which
could be due to the inferior hydride transfer ability of MCM-22 catalyst at high n-hexane
conversions [43]. These findings indicate that the catalytic performance of Del-Al-MCM-
22 catalyst is superior to that of H-ZSM-5 and H-Beta catalysts in the catalytic cracking
of n-hexane to produce propylene under these reaction conditions.
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3.5 Investigation of the relationship between catalytic performance and acidity of MCM-
22 catalysts
As shown in Figs. 6-10, the catalytic performance depended closely on the Si/Al
ratio of H-MCM-22 and Del-Al-MCM-22 catalysts. With increasing Si/Al ratio of H-
MCM-22 or Del-Al-MCM-22 catalysts, the acid amount was decreased (Fig. 3, Table 1),
resulting in the decrease in the cracking activity of catalysts. H-MCM-22 catalysts with
the higher Si/Al ratios and Del-Al-MCM-22 catalysts showed a higher propylene
selectivity and longer catalytic life than H-MCM-22 catalysts with high Al contents. This
could be ascribed to the decrease in the acid amount, which suppressed the secondary
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reaction of propylene and butenes and the hydride transfer to form coke. Similarly, Zhu et
al. also reported the ZSM-5 zeolites with lower acid amount (higher Si/Al ratio) exhibited
better performance in butene catalytic cracking to propylene and ethylene [44].
It is well know that not only the acid amount but also the acid strength and type of
acid site, Brønsted or Lewis acid site affect the catalytic performance. However, the
results of NH3-TPD only showed the change of the acid amount. So, the pyridine-IR
spectroscopy was investigated to provide more detailed information on the type and
amount of acid sites and their strength.
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Fig. 13 shows the IR spectra of H-MCM-22 catalysts with different Si/Al ratios
before (A) and after (B) pyridine was absorbed and desorbed at 423 K. As shown in Fig.
13A, in the region of hydroxyl stretching vibration, the H-MCM-22 (Si/Al = 19) catalyst
exhibited two strong bands at 3743 and 3615 cm-1, which are attributed to external
silanols and structural Si(Al)OH hydroxyls, respectively. In addition, two weak bands
appeared at 3727 and 3663 cm-1, which are assigned to internal silanols and hydroxyls
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15 related to extra-framework Al, respectively [20, 34]. The 3615 cm-1 band gradually
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decreased in intensity with a decrease in Al content, indicating a decrease in the amount
of Brønsted acid sites. In the case of Post-H-MCM-22 catalysts, the 3727 cm-1 band
increased significantly in intensity and a broad band around 3500 cm-1 attributed to
hydrogen bonded silanols appeared when the Al content was decreased. These facts
indicate that Post-H-MCM-22 catalysts have more defect sites than the catalysts prepared
by the direct synthesis method.
Then, the IR spectra of adsorbed pyridine in the range of pyridine ring-stretching
modes after desorption at 423 K was measured, as shown in Fig. 13B. The bands at 1543
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and 1454 cm-1 were attributed to Brønsted acid sites (BAS) and Lewis acid sites (LAS),
respectively. With a decrease in Al content, the intensity of the 1543 and 1454 cm-1 band
decreased simultaneously, indicating not only Brønsted acid but also Lewis acid
decreased in amount. However, the ratio of BAS/LAS, which was calculated by using the
area of peaks ascribed to the Brønsted and Lewis acid sites, for Post-H-MCM-22
catalysts was lower than that for H-MCM-22 catalysts; e.g., the BAS/LAS ratios for Post-
H-MCM-22 (Si/Al = 100) and H-MCM-22 (Si/Al = 19) were calculated to be 1.9 and 2.6,
respectively. Thus, the Post-H-MCM-22 catalysts have higher proportion of Lewis acid
sites than the H-MCM-22 catalysts prepared by the direct synthesis method. Based on the
catalytic performance results shown in Figs. 8 and 9, Section 3.2.3, although the Post-H-
MCM-22 catalysts exhibited better stabilities than the direct synthesized H-MCM-22
catalysts, their stabilities were not so high compared to the dealuminated MCM-22
catalysts. The possible reason may be due to the higher proportion of Lewis acid sites in
the post synthesized MCM-22 zeolites. Since the Lewis acid sites interact with olefins to
enhance their oligomerization and subsequent hydride transfer reactions, leading to the
formation of coke products [45].
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Similarly, Fig.14 shows the IR spectra of Del-Al-MCM-22 catalysts with different
Si/Al ratios before (A) and after (B) pyridine was absorbed and desorbed at 423 K. The
3615 cm-1 band gradually decreased in intensity; however the 3743 cm-1 band increased
with a progress of dealumination (Fig. 14A). Fig. 14B showed that the intensity of the
1543 and 1454 cm-1 bands ascribed to Brønsted and Lewis acid sites, respectively,
decreased with a progress of dealumination. These findings imply that aluminum atoms
not only in the framework but also of extra-framework are removed by the AHFS
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treatment. Moreover the BAS/LAS ratio increased after the dealumination, which means
that the Lewis acid sites are removed more readily than the Brønsted acid sites. Thus, it
was deduced not only the acid amount but also the acid type was changed with
dealumination using AHFS treatment. The BAS/LAS ratios for dealuminated MCM-22
catalysts were higher than that for H-MCM-22 (Si/Al = 19) catalyst; e.g., the BAS/LAS
ratios for Del-Al-MCM-22 (Si/Al = 62) and H-MCM-22 (Si/Al = 19) were calculated to
be 4.3 and 2.6, respectively. Thus, the Del-Al-MCM-22 catalysts have lower proportion
of Lewis acid sites than the parent H-MCM-22 catalyst. The superiority of Del-Al-MCM-
22 catalysts in the catalytic life and propylene selectivity could be due to their smaller
amount of Lewis acid sites which accelerate the hydride transfer and coke formation.
The distribution of the acid strength of Brønsted and Lewis acid sites can be
measured from the thermal desorption of pyridine [46]. Fig. 15 shows the IR spectra in
the pyridine vibration region of H-MCM-22 (Si/Al = 19) (A), Post-H-MCM-22 (Si/Al =
66) (B) and Del-Al-MCM-22 (Si/Al = 62) (C) catalysts after pyridine adsorption and
following desorption at different temperatures. It is apparent that for all the catalysts, the
intensity of the band ascribed to Brønsted acid sites decreased with desorption
temperature increasing from 423 K to 723 K. For H-MCM-22 and Post-H-MCM-22
catalysts, the intensity of the band ascribed to Brønsted acid sites was reduced by ca. 75%
and 90%, respectively when the temperature of desorption increased from 423 K to 723
K. No band was observed in the case of Del-Al-MCM-22 catalyst when the desorption
temperature was increased to 723 K. These facts demonstrate that the Brønsted acid
strength of Del-Al-MCM-22 catalyst is relatively low compared with that of parent H-
MCM-22 and Post-H-MCM-22 catalysts. On the other hand, for H-MCM-22 and Del-Al-
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MCM-22 catalysts, the intensity of band ascribed to Lewis acid sites also decreased with
desorption temperature increased. When the desorption temperature was increased from
423 K to 723 K, the intensity of the band ascribed to Lewis acid sites was reduced by ca.
10% and 50% respectively, which suggests that the strength of Lewis acid is higher than
that of Brønsted acid in these two catalysts. It is noted that the intensity of the band
ascribed to Lewis acid sites was almost unchanged with desorption temperature increased
in the case of Post-H-MCM-22 (Si/Al = 66), indicating that the strength of Lewis acid
sites on Post-H-MCM-22 is higher than that of H-MCM-22 and Del-Al-MCM-22
catalysts. This could be a possible reason for the lower stability of Post-H-MCM-22
compared with Del-Al-MCM-22 catalyst in n-hexane cracking. According to the above
findings, it is also concluded that the decrease in the strength of acid sites would be
another reason for the higher propylene selectivity and stability of Del-Al-MCM-22
catalysts, since strong acid sites play a significant role in coke formation and coke formed
on strong acid sites is much heavier than that formed on weak acid sites [47].
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4. Conclusions
H-MCM-22 catalysts with higher Si/Al ratios prepared by the post-synthesis method
showed higher propylene selectivity in catalytic cracking of n-hexane. This could be
attributed to the smaller acid amount, which resulted in suppression of the secondary
reaction of propylene and butenes. However, the catalytic stability was still not
satisfactory. This can be explained by the fact that the Post-H-MCM-22 catalysts had
more strong Lewis acid sites, which accelerate the hydride transfer and coke formation.
The catalytic life and propylene selectivity at high n-hexane conversions were improved
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by dealumination using AHFS. This could be due to the smaller amount of Lewis acid
sites and the decrease in the strength of acid sites. Del-Al-MCM-22 (Si/Al = 62) showed
a higher propylene selectivity (40 C-%) than H-ZSM-5 (Si/Al = 66) and H-Beta (Si/Al =
67) catalysts with similar Si/Al ratios at a high n-hexane conversion of 95%. Moreover,
the catalyst stability was comparable to that of H-ZSM-5 and higher than that of H-Beta
catalyst.
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9
Acknowledgements
This work was partially supported by the green sustainable chemistry project of New
Energy and Industrial Technology Development Organization (NEDO).
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Figure captions
Fig. 1 XRD patterns of H-MCM-22 samples (A) with Si/Al ration of 19 (a), 34 (b), 66 (c)
and 100 (d) and Del-Al-MCM-22 samples (B) with Si/Al ratio of 34 (b), 56 (c) and 62 (d).
Fig. 2 SEM images of H-MCM-22(Si/Al = 19) (a), Post-H-MCM-22(Si/Al = 100) (b),
Del-Al-MCM-22(Si/Al = 34) (c), Del-Al-MCM-22(Si/Al = 62) (d), H-ZSM-5(Si/Al = 66)
(e) and H-Beta(Si/Al = 67) (f) samples.
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Fig. 3 NH3-TPD profiles of H-MCM-22 (A) and Del-Al-MCM-22 (B) samples with
different Si/Al ratios.
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Fig. 4 Effect of reaction temperature on the catalytic cracking of n-hexane on H-MCM-
22(Si/Al = 19). Reaction conditions: Cat., 0.1 g; Pn-hexane = 6 kPa; W/F n-hexane = 64 g-cat
h/mol-n-hexane; temp., 723 - 923 K, TOS = 15 min.
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Fig. 5 Effect of contact time on the catalytic cracking of n-hexane on H-MCM-22(Si/Al =
19). Reaction conditions: W/F
others see Fig. 4.
= 6.4 - 64 g-cat h/mol-n-hexane; temp., 923 K,
n-hexane
Fig. 6 Dependence of kc on the acid amount of H-MCM-22 catalysts at different reaction
temperatures of 723 K (a), 823 K (b) and 923 K (c).
Fig. 7 Relationship between product selectivities and acid amount of H-MCM-22
catalysts at an n-hexane conversion of ca. 80%. Reaction conditions: Pn-hexane = 6 kPa;
temp., 923 K, TOS = 15 min.
Fig. 8 Change in n-hexane conversion with time on stream (TOS) for H-MCM-22
catalysts with different Si/Al ratios. Reaction conditions: temp., 923 K, others see Fig. 4.
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1
2
Fig. 9 Change in n-hexane conversion with time on stream (TOS) for H-MCM-22(Si/Al
= 19) and Del-Al-MCM-22 catalysts with different Si/Al ratio. Reaction conditions:
temp., 923 K, others see Fig. 4.
3
=
=
=
4
Fig. 10 Changes in selectivities to C3 (A), C2 (B), C4 (C) and BTX (D) with n-hexane
conversion on H-MCM-22(Si/Al = 19) and Del-Al-MCM-22 catalysts with different
Si/Al ratios. Reaction conditions: see Fig. 5.
5
6
7
Fig. 11 Change in n-hexane conversion with time on stream (TOS) for H-ZSM-5 (Si/Al =
66), H-Beta (Si/Al = 67) and Del-Al-MCM-22 (Si/Al = 62) catalysts. Reaction conditions:
temp., 923 K, others see Fig. 4.
8
9
=
=
=
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Fig. 12 Changes in selectivities to C3 (A), C2 (B), C4 (C) and BTX (D) with n-hexane
conversion on H-ZSM-5 (Si/Al = 66), H-Beta (Si/Al = 67) and Del-Al-MCM-22 (Si/Al =
62) catalysts. Reaction conditions: see Fig. 5.
Fig. 13 IR spectra of H-MCM-22 catalysts with Si/Al ratio of 19 (a), 34 (b), 66 (c) and
100 (d) before (A) and after (B) pyridine was absorbed and desorbed at 423 K for 1 h.
Fig. 14 IR spectra of H-MCM-22 (Si/Al = 19) (a) and Del-Al-MCM-22 catalysts with
Si/Al ratio of 39 (b), 56 (c) and 62 (d) before (A) and after (B) pyridine was absorbed and
desorbed at 423 K for 1 h.
Fig. 15 IR spectra in the pyridine vibration region of H-MCM-22 (Si/Al = 19) (A) Post-
H-MCM-22 (Si/Al = 66) (B) and Del-Al-MCM-22 (Si/Al = 62) (C) catalysts after
pyridine adsorption and desorption at 423 K (a), 523 K (b), 623 K (c), and 723 K (d) for 1
h, respectively.
28
Page 28 of 46

1
2
3
Table 1 Physicochemical properties of various catalysts
Al contentb Acid contentc SBETd/ S Exte/ V micro
/
e
Cat.
CAHFS/ M Si/Ala
/mmol g-1 /mmol g-1
m2 g-1 m2 g-1 cm3 g-1
H-MCM-22 (19)
H-MCM-22 (34)
Post-H-MCM-22 (66)
-
-
-
19
34
66
0.84
0.48
0.25
0.53
0.37
0.17
0.12
0.31
0.23
0.16
0.19
0.18
517 92
412 74
594 96
562 92
497 97
0.17
0.14
0.20
0.19
0.16
Post-H-MCM-22 (100) -
100 0.17
Del-Al-MCM-22 (34)
Del-Al-MCM-22 (56)
Del-Al-MCM-22 (62)
H-ZSM-5 (66)
0.005
34
56
62
66
67
0.48
0.29
0.26
0.25
0.25
0.02
466 112 0.14
450 127 0.13
0.05
-
-
445 24
0.18
H-Beta (67)
587 201 0.15
4
5
a Molar ratio determined by ICP.
b Calculated by ICP result.
c Determined by NH3-TPD.
6
7
8
9
d Measured by N2 adsorption at 77 K.
e Calculated by t-plot method.
10
11
30
Page 29 of 46