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Transcript of Effects of cage shape and size of 8-membered ring molecular sieves on their deactivation in...
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al 339 (2008) 36–44
Applied Catalysis A: GenerEffects of cage shape and size of 8-membered ring molecular sieves
on their deactivation in methanol-to-olefin (MTO) reactions
Ji Won Park a, Jae Youl Lee a, Kwang Soo Kim b, Suk Bong Hong b, Gon Seo a,*a School of Applied Chemical Engineering and the Center for Functional Nano Fine Chemicals, Chonnam National University,
Gwangju 500-757, Republic of Koreab School of Environmental Science and Engineering, POSTECH, Pohang, Gyeongbuk 790-784, Republic of Korea
Received 31 October 2007; received in revised form 28 December 2007; accepted 7 January 2008
Available online 17 January 2008
Abstract
Four kinds of 8-membered ring (8-MR) small-port molecular sieves with CHA (SAPO-34), ERI (UZM-12), LTA (UZM-9), and UFI (UZM-5)
topologies were prepared to investigate the effects of the cage shape and the cage size on their catalytic activities and on deactivation behaviors in
methanol-to-olefin (MTO) reactions. UZM-5, -9, and -12 zeolites with Si/Al molar ratios of 5–6 showed high initial activities in the MTO reaction
owing to their sufficient acidities, and they were highly selective to lower olefins such as SAPO-34 molecular sieve. However, they were rapidly
deactivated in the order of CHA < LTA < ERI < UFI. The UV–VIS and NMR spectroscopy examinations of the materials accumulated in the
cages of the 8-MR catalysts indicated that the concentration of large fused polycyclic aromatics was high in the cages of an easily deactivated 8-MR
catalyst. The selective and stable catalytic performance of SAPO-34 molecular sieve in the MTO reaction is explained by the suitable shape and
size of its cages, which are thus capable of preserving stably the active intermediates, multialkyl benzenes. They are also capable of suppressing the
formation of fused polycyclic aromatics that would cause its deactivation.
# 2008 Elsevier B.V. All rights reserved.
Keywords: Methanol to olefin; 8-MR molecular sieve; CHA; ERI; LTA; UFI; Deactivation; Cage shape and size
1. Introduction
The rapid rise of the crude oil price also causes the prices of
various petrochemical products to go up. The increase in the
prices of lower olefins such as ethylene, propylene and butenes,
therefore, is inevitable because they are mainly produced from
the thermal cracking of naphtha. Furthermore, the large
emission of carbon dioxide from thermal cracking of naphtha
due to its large consumption of energy drives the petrochemical
industries out to find new routes to produce lower olefins from
other carbon sources with lower energy requirement. Because
the methanol-to-olefin (MTO) process produces lower olefins
from natural gas or coal via methanol, it has been steadily
considered as a strong alternative, although the low crude oil
price has retarded its commercialization for the last decade [1].
Various catalysts such as ZSM-5 zeolites modified with
phosphorus and ferrosilicalite have shown high selectivity to
* Corresponding author. Tel.: +82 62 530 1876; fax: +82 62 530 1899.
E-mail address: [email protected] (G. Seo).
0926-860X/$ – see front matter # 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2008.01.005
lower olefins in the MTO reactions [2,3]. The suppression of
further reactions of lower olefins to paraffins and aromatics by
lowering the acidity of ZSM-5 zeolites with phosphorus
impregnation and iron substitution effectively improves the
selectivity to lower olefins. However, the SAPO-34 molecular
sieve employed in the UOP/Hydro MTO process as a catalyst
also has been known to be exceptionally selective to lower
olefins [4,5]. The small pore entrances of SAPO-34 molecular
sieve composed of 8-membered-rings (8-MR) allow only the
diffusion of linear hydrocarbons [6,7]. Since aromatics and
branched hydrocarbons are too large to transfer through its 8-
MR pore entrances, the lower olefins are naturally predominant
in product streams. In addition, the cage shape and the cage size
of SAPO-34 molecular sieves have been found suitable to
preserve reactive intermediates that produce selectively lower
olefins, resulting in the high selectivity [8].
The reaction mechanism for the selective MTO reaction over
SAPO-34 molecular sieve has been revealed by using in-situ
solid MAS NMR spectroscopy [9–12]. The formation of lower
olefins from methanol on hexamethyl benzenium ions located
in its cages has been confirmed. The hydrocarbon pool
J.W. Park et al. / Applied Catalysis A: General 339 (2008) 36–44 37
mechanism based on these experimental results provides
plausible explanations for its high selectivity to lower olefins in
the MTO reaction and for the induction period observed. The
same mechanism also suggested that the deactivation of SAPO-
34 molecular sieves in the MTO reaction is due to the
conversion of multialkyl benzenes to nonreactive polycyclic
aromatics which fill its cages and make it inactive by blocking
[13].
There are other 8-MR small-port molecular sieves with LTA,
ERI, UFI and OFF topologies besides SAPO-34 molecular
sieve with CHA topology [14]. Since the pore entrances of these
molecular sieves are also small like that of SAPO-34 molecular
sieve, it may be reasonable to apply them as catalysts to the
MTO reaction. However, the deactivation of ERI zeolite has
been reported to be too rapid [15] and other 8-MR molecular
sieves with sufficient acidity for the MTO reaction have not
been prepared until the syntheses of UZM series zeolites were
reported [16–18]. UZM-5, -9, and -12 zeolites prepared using
tetraethylammonium hydroxide (TEAOH) and tetramethylam-
monium hydroxide (TMAOH) as the structure directing agents
are expected to be acidic due to their relatively high Si/Al molar
ratios.
Zeolite A prepared from alkaline synthetic mixtures has LTA
topology, while it is not active in the MTO reaction because of
its negligible acidity due to the too low Si/Al molar ratio of near
1. However, the UZM-9 zeolite with LTA topology has a higher
Si/Al molar ratio of 5–6 and shows a sufficient acidic property
[16]. Preparations of UZM-5 zeolite with UFI topology and of
UZM-12 zeolite with ERI topology with Si/Al molar ratios
above 5 have also been reported [17,18]. These UZM zeolites
are highly acidic owing to their high Si/Al molar ratios, and thus
are expected to be active for the MTO reaction. The UZM
zeolites have commonly the same 8-MR pore entrances as the
SAPO-34 molecular sieve, so they are expected to be selective
in the formation of lower olefins in the MTO reaction if the
shapes and sizes of their cages do not influence their activity
and deactivation behavior.
Even though these molecular sieves commonly have 8-MR
pore entrances, the shapes and sizes of their cages are not the
same [16–18]. In addition, their pore dimensionalities are
different. Spherical pores are connected three-dimensionally in
LTA topology by combining them through 8-MR cage
entrances. Each spherical cage of UFI topology has two small
Table 1
Synthesis conditions for the 8-MR small-port molecular sieves prepared in this stu
Molecular sieve Gel compositiona
CHA (SAPO-34) 0.5TEA2O�1.6DPA�1.0Al2O3�1.0P2O5�0.3SiO2�50
ERI (UZM-12) 6.5TEA2O�2HMBr2,�0.5K2O�0.5Al2O3�16SiO2�48
LTA (UZM-9) 4.0TEA2O�0.25TMA2O�0.25Na2O�0.5Al2O3�8SiO
UFI (UZM-5) 4.0TEA2O�0.5TMA2O�0.5Al2O3�8SiO2�240H2O
Crystallization was performed under rotation (60 rpm), unless otherwise stated.a Colloidal silica (Ludox AS-40, DuPont) was used as a silica source. Al[OC(CH3)
were used as aluminum and phosphorous sources, respectively. Other starting mater
Aldrich), tetramethylammonium chloride (TMACl, 97%, Aldrich), hexamethonium
(99%, Aldrich), and KCl (99%, Duksan).b Crystallization was performed under rotation (60 rpm), unless otherwise statedc Crystallized under static conditions.
half spheres composed of 5-MR above and another below it and
the spherical pores extend in two dimensions by connecting
them with 8-MR cage entrances. Cages of ERI and CHA
topologies are cylindrical, but their sizes are slightly different:
the length of ERI cages is 11 A and that of CHA cages is
relatively short 8.5 A. Since the cage shape and size of the 8-
MR molecular sieves influence not only the mass transfer of
reactants and products through the entrances of cages, but also
the formation of alkyl aromatic materials in cages, the 8-MR
molecular sieves consequently show different activities and
deactivation rates in the MTO reaction according to their
topologies.
We prepared four kinds of the 8-MR molecular sieves with
CHA, ERI, LTA and UFI topologies and investigated their
activities and deactivation behaviors in the MTO reaction.
Their selectivity to lower olefins and their deactivation behavior
are discussed in terms of their cage shapes and sizes. In
addition, the accumulated materials in their cages during the
MTO reaction were examined by using UV–VIS and NMR
spectroscopies to confirm the influence of the shape and size of
cages on the selectivity and deactivation of these catalysts.
2. Experimental
2.1. Preparation of catalysts
UZM-5, -9 and -12 zeolites were prepared using TEAOH,
tetramethyl chloride (TMACl), and hexamethoium bromide
(HMBr2) following the procedures in the literature [15–17].
SAPO-34 molecular sieve was synthesized by the reported
method [19]. The chemical compositions of the synthetic
mixtures and the reaction conditions for the synthesis of 8-MR
molecular sieves are listed in Table 1. Hereafter, these
molecular sieves are denoted with their topology codes written
before ‘catalyst’ as follows: CHA (SAPO-34), ERI (UZM-12),
LTA (UZM-9), and UFI (UZM-5) catalysts.
2.2. Characterization of catalysts
The XRD patterns of the catalysts were recorded on a high
resolution X-ray diffractometer (HR-XRD, Rigaku D/MAX
Ultima III) using CuKa1 radiation (l = 1.54056 A) at 40 kV
and 40 mA condition. A scanning electron microscope (SEM,
dy
Temperature/timeb (8C/days) Ref.
H2O 175/6c [19]
0H2O 100/14 [17]
2�240H2O 100/14c [16]
150/10 [18]
3]3 (Aldrich) or pseudoboehmite (Catapal B, Vista) and o-H3PO4 (85%, Merck)
ials included tetraethylammonium hydroxide (TEAOH, 35% aqueous solution,
bromide (HMBr2, 98%, Acros), dipropylamine (DPA, 99%, Aldrich), NaCl
.
Fig. 1. X-ray diffraction patterns of the 8-MR catalysts.
J.W. Park et al. / Applied Catalysis A: General 339 (2008) 36–4438
Hitachi S-4700) was employed to investigate the shapes and
sizes of the catalysts prepared. The nitrogen adsorption
isotherms of the catalysts were obtained using an automatic
volumetric adsorption measurement system (Mirae SI nano-
Porosity-XG). Samples of the catalyst were evacuated at 200 8Cfor 2 h before exposing them to nitrogen gas at 77 K. Their
surface areas were calculated by the BET equation.
The temperature programmed desorption (TPD) profiles of
ammonia from the catalysts were recorded on a chemisorption
analyzer (BEL, BEL-CAT). The sample charged in a quartz
reactor was activated at 550 8C in the flow of helium gas. It was
saturated with ammonia at 150 8C by injecting pulses of
ammonia gas (Deokyang, 5% NH3 in He balance). After
purging them with a helium flow of 100 ml min�1 for 1 h to
remove physically adsorbed ammonia, the temperature of the
reactor increased to 600 8C with a ramping rate of 10 8C min�1.
The amount of desorbed ammonia was measured by a TCD
detector.
2.3. MTO reaction
An atmospheric flow microreactor was employed in the
investigation of MTO reaction on the 8-MR catalysts [20]. A
catalyst sample of 0.10 g was charged at the center of a 1/200
quartz reactor, and it was activated at 550 8C for 2 h in a flow of
nitrogen gas. Methanol (Duksan, 99.8%) reactant vaporized
in nitrogen gas was fed at a rate of 0.087 ml h�1
(WHSV = 0.70 h�1). The products of the MTO reaction were
analyzed using a directly coupled gas chromatograph (Donam,
DS-6200) equipped with a CP-Volamine capillary column and a
FID detector. The conversions of methanol were defined as the
percentages of methanol consumed during the MTO reaction.
Dimethyl ether was not considered as a product. The yield of a
given product was calculated as the percentage of the amount
(in moles) of methanol used in producing the product to the
amount of methanol fed.
The materials remaining in the cages of the 8-MR catalysts
during the MTO reaction were examined by ex-situ 13C MAS
NMR spectroscopy. A catalyst sample of 0.10 g was charged at
the center of a 1/200 quartz reactor and activated at 550 8C for
2 h in a flow of nitrogen gas. A vapor stream of 13C enriched
methanol (Aldrich, 13C 99%) diluted in nitrogen was fed at a
rate of 0.087 ml h�1 at 350 8C. The catalysts used were taken
after reacting for 3 and 90 min followed by purging with a flow
of nitrogen gas for 5 min to remove the low boiling materials
that remained in their cages and shapes. These catalysts were
carefully packed in NMR rotors and stored in a glove box
purged with nitrogen gas to prevent the further reaction of the
remaining materials with water and oxygen. 13C MAS NMR
spectra were recorded on an FT-NMR spectrometer (Varian,
Unity Solid Inova WB 200 MHz System) at a spinning rate of
5 kHz. The operating 13C frequency was 50.567 MHz with a p/
2 rad pulse length of 2 ms. Chemical shifts of 13C were
referenced to TMS.
The catalysts used in the MTO reaction with the 13C
enriched methanol were also taken to investigate the chemical
species of the materials occluded in the cages of the 8-MR
catalysts. The collected catalysts that had been used in the MTO
reaction for 90 min of the MTO reaction were dissolved by 10%
solution of hydrogen fluoride (J.T. Baker, 48%) and neutralized
subsequently with potassium carbonate (Osaka Hayashi,
99.5%) [21]. Carbon tetrachloride (Duksan, 95%) was
employed to extract organic materials from the solutions.
Water contained in the organic phase was removed using
sodium sulfate (Duksan, 99%). Before they were used as
samples, the extracted organic phases were filtered by a syringe
filter (Advantec, DISMIC-13JP) to remove the carbon deposits.
The UV–VIS spectra of the extracted organic phases were
obtained using a UV–VIS spectrophotometer (Ocean Optics
Inc. USB2000). Pure carbon tetrachloride was used as a
reference material. 13C NMR spectra of the extracted organic
phases were recorded on a 300 MHz NMR spectrometer
(Bruker, AMX-300) at a spinning rate of 20 Hz.
The amounts of materials occluded in the cages of the
catalysts used were measured by a chemical balance after
removing carbon tetrachloride from the extracted organic
phases by purging them with a flow of nitrogen gas at ambient
temperature.
3. Results and discussion
3.1. Physico-chemical properties of 8-MR catalysts
Fig. 1 shows the XRD patterns of the 8-MR catalysts
prepared. Their diffraction patterns are precisely coincident
with those reported in the references [22], although their peak
intensities varied considerably according to their topologies.
The absence of peaks related to other topologies confirmed
their pure states. The ERI catalyst showed very small
Fig. 2. SEM images of the 8-MR catalysts.
J.W. Park et al. / Applied Catalysis A: General 339 (2008) 36–44 39
diffraction peaks because of its too small particle size.
Therefore, these distinct differences in the heights of diffraction
peaks indicate that their crystallite sizes might be largely
varied.
The crystallite sizes of the 8-MR catalysts prepared are
considerably different as shown in Fig. 2. The CHA catalyst is
composed of cuboids with different sizes ranging from 2 to
6 mm. The crystallites of the LTA catalyst are also small
cuboids, while their sizes are less than 1 mm. In contrast to the
CHA and LTA catalysts, the crystallite sizes of the ERI and UFI
catalysts are small; the ERI catalyst is composed of small rice-
grain type particles of 0.2 mm � 0.5 mm, and the UFI catalyst is
composed of thin platelets less than 0.05 mm � 0.3 mm. As
expected from their XRD patterns, the fact that the crystallites
of the ERI and UFI catalysts were rather small compared to
those of the CHA and LTA catalysts resulted in low intensity of
their diffraction peaks.
Table 2
Characterization data of the 8-MR small-port molecular sieves employed in this s
Catalyst Si/Al in the producta Crysta
CHA (SAPO-34) – Cuboi
ERI (UZM-12) 5.7 Rice-g
LTA (UZM-9) 5.1 Small
UFI (UZM-5) 6.9 Thin p
a Determined by elemental analysis.b Determined from N2 adsorption data for the proton form of each material.
Characterization data of the 8-MR catalysts prepared are
listed in Table 2. Si/Al molar ratios of the ERI (UZM-12), LTA
(UZM-9) and UFI (UZM-5) catalysts are around 6. The Si/Al
molar ratio of UZM-5 has been reported to be 8 [17], and those
of UZM-12 zeolites to be 5.6–7.4 [18]. The obtained Si/Al
molar ratios of the 8-MR catalysts prepared are similar to the
reported values of UZM zeolites. The surface area of the LTA
catalyst was a very high value of 620 m2 g�1, comparable to
that of zeolite A prepared from alkaline media [16]. However,
the surface area of the CHA catalyst is small compared to those
of other 8-MR catalysts. Its small surface area may be caused
by its small 8-ring cross-sectional area (11.3 A2) at pore
entrance and small cage volume (240 A3), as shown in Table 3.
The TPD profiles of ammonia from the 8-MR catalysts
prepared are considerably different according to their
topologies as shown in Fig. 3. The desorption profiles of
ammonia could be considered as the sums of curves of two
tudy
l shape and average size (mm) BET surface areab (m2 g�1)
ds, 2.0–6.0 310
rains, 0.2 � 0.5 450
cuboids, 0.4 620
latelets, <0.05 � 0.3 510
Table 3
Pore characteristics and properties of molecular sieves used in this study
Catalyst Pore
dimensionality
8-MR pore size (A)
and areaa (A2)
Types of cages containing
8-MR windowsb,c
Cage dimensionsd
(A) and volumee (A3)
CHA (SAPO-34) 3 3.8 � 3.8, 11.3 20-hedral([4126286]) cha-cage 6.7 � 6.7 � 10.0, 240
ERI (UZM-12) 3 3.6 � 5.1, 14.4 23-hedral([4126586]) eri-cage 6.3 � 6.3 � 13.0, 270
LTA (UZM-900) 3 4.1 � 4.1, 13.2 26-hedral([4126886]) lta-cage 11.2 � 11.2 � 11.2, 740
UFI (UZM-5) 2 3.6 � 4.4, 12.4 26-hedral([4126886]) lta-cage 11.2 � 11.2 � 11.2, 740
a Calculated using the equation A = pab/4, where A, a, and b are the pore area and the shortest and longest 10-MR pore diameters, respectively. The 8-MR pores in
each material are assumed to be ideally circular or elliptical in shape.b In the notation [mn. . .], polyhedra are defined by the n number of faces with m T-O-T edges.c The 14-hedral ([45546481]) side-pocket (wbc-cage) in UZM-5 was excluded since its 8-MR size (3.2 � 3.2 A) and area (8.0 A2) are too small to host methanol
with a cross-sectional area of 11.3 A2.d Taken from Ref. [23].e Calculated using Marler’s equation V = pabc/6 [24], where V, a, b, and c are the pore volume and the width, length and height of the cage, respectively. All the
cages are assumed to be ideally ellipsoidal in shape.
J.W. Park et al. / Applied Catalysis A: General 339 (2008) 36–4440
different peaks called h-peaks and l-peaks [25]: the temperature
at peak maximum of h-peak was around 420 8C and that of l-
peak was around 250 8C. The areas of h-peak and l-peak
generally correspond to the amounts of strong and weak acid
sites, respectively [26]. The h-peak from the CHA catalyst was
large, while its l-peak was very small. On the other hand, the
ERI, LTA and UFI catalysts showed different TPD profiles of
ammonia compared to that obtained from the CHA catalyst:
their h-peaks are relatively small compared to the h-peak of
CHA catalyst, while their l-peaks are relatively large. The
heights of h-peaks on these catalysts decreased in the order of
LTA >UFI > ERI, indicating their amounts of strong acid
sites. The Si/Al molar ratios of these catalysts are around 5–6,
and thus, the closely located aluminum atoms in their cages
weakened the electrostatic field formed in the cages, decreasing
the number of strong acid sites.
Fig. 3. TPD profiles of ammonia from the 8-MR catalysts.
3.2. Deactivation of the 8-MR catalysts in the MTO
reaction
Methanol converts to various hydrocarbons via dimethyl
ether over acidic catalysts. However, the selectivity to lower
olefins and the deactivation behavior of the 8-MR catalysts in
the MTO reaction were strongly dependent on their acidities
and topologies. Fig. 4 shows the conversion profiles of
methanol along the time on stream over the 8-MR catalysts. At
the initial period of the MTO reaction (after 3 min), the
conversions were almost 100% over all catalysts, but the
decreasing tendencies of the conversion with the time on stream
were considerably different according to their topologies. The
conversion over the CHA catalyst remained 100%, even at
the time on stream of 240 min. Although the conversion over
the LTA catalyst also maintained 100% till the time on stream
Fig. 4. Conversion profiles on the 8-MR catalysts in the MTO reaction: reaction
temperature = 350 8C, WHSV = 0.70 h�1.
Fig. 5. Variation of the yields of lower olefins over 8-MR catalysts in the MTO
reaction with the time on stream: reaction temperature = 350 8C,
WHSV = 0.70 h�1.
Fig. 6. Ex-situ 13C MAS NMR spectra of materials present in the 8-MR
catalysts during the MTO reaction: reaction temperature = 350 8C, WHSV
= 0.70 h�1.
J.W. Park et al. / Applied Catalysis A: General 339 (2008) 36–44 41
of 80 min, the conversion started decreasing after that time.
However, the conversions over the ERI and UFI catalysts
decreased gradually from the beginning of the reaction. These
catalysts deactivate very rapidly, and thus, the conversion over
the UFI catalyst after 240 min was about 10%. The deactivation
rates of the 8-MR catalysts in the MTO reaction became severe
in the order of CHA� LTA < ERI < UFI.
The MTO reaction over the 8-MR catalysts produced lower
olefins selectively, regardless of their topologies. Fig. 5 shows
the changes in the yields of lower olefins over the 8-MR
catalysts with the time on stream. At the early stage of the MTO
reaction, the yield of lower olefins were very high over all the
catalysts, while its decreasing tendency was considerably
different according to each topology. The yield of lower olefin
remained about 90% over the CHA catalysts, but the yield
decreased gradually over the LTA, ERI and UFI catalysts at
different rates. The yield of propylene was the highest among
the lower olefins. The deactivation of the catalysts predomi-
nantly induced the decrease in the yield of butanes, and the
further deactivation subsequently caused the decrease in the
yield of ethylene. These results led us to suppose that the MTO
reaction over the 8-MR catalysts produces mainly propylene as
a primary olefin, and the re-equilibrium of lower olefins occurs
in cages when the conversion of methanol was high. The
lowering of the conversion reduced the concentration of lower
olefins in the cages, resulting in the suppression of re-
equilibrium among olefins and the increase in the concentration
of propylene. If the production paths for ethylene and propylene
were different as suggested by Kolboe and co-workers [27,28],
the active intermediates for producing ethylene and butenes
might deactivate more rapidly than that for propylene.
Although the major products of the MTO reaction over the
8-MR catalysts were the same at the initial stage, the
deactivation rates of catalysts were considerably different
according to the topologies. This means that the MTO reaction
over the 8-MR catalysts proceeds through the same reaction
path, but the materials produced in their cages were different
according to the cage shapes and sizes. The hydrocarbon pool
mechanism has emphasized the presence of hexamethyl
benzenium ions in the cages of CHA molecular sieve for the
selective formation of lower olefins [29]. In addition, the
formation of polycyclic aromatic materials from the mono-
aromatic ring materials in their cages induces its deactivation
by losing its catalytic activity in the side-chain alkylation or in
the steps for the release of lower olefins. Further accumulation
of large polycyclic aromatic ring materials in the cages finally
blocks their entrances, causing complete deactivation [13].
Fig. 6 shows the 13C MAS NMR spectra of materials present
in the cages of the 8-MR catalysts during the MTO reaction.
Although these spectra were not recorded in an in-situ
condition, the NMR spectra of the used catalysts taken after
3 min and after 90 min provided the information on the
materials present in their cages. Only a peak at 51 ppm assigned
to methanol appeared on the used CHA catalyst taken after
3 min. The additional large broad peaks at 10–30 and 120–
140 ppm appeared on the spectra obtained from the used CHA
catalyst taken after 90 min. The NMR spectrum was very
similar to that reported by Haw and co-workers [10,30]
recorded over SAPO-34 molecular sieve during the MTO
reaction by in-situ solid MAS NMR technique. The peaks at
Fig. 7. UV–VIS spectra of materials occluded in the cage of the 8-MR catalysts
in the MTO reaction: reaction temperature = 350 8C, WHSV = 0.70 h�1.
J.W. Park et al. / Applied Catalysis A: General 339 (2008) 36–4442
10–30 ppm and at 120–140 ppm are attributed to alkyl groups
and aromatic rings, respectively. Therefore, this spectrum
confirmed the presence of alkyl group-substituted benzenes in
the cages of the CHA catalyst during the MTO reaction which
were suggested to be active intermediates in the hydrocarbon
pool mechanism [29]. The 13C MAS NMR spectra of materials
present in the cages of the ERI, LTA, and UFI catalysts during
the MTO reaction were similar, regardless of their cage shapes
and sizes. The presence of the similar alkyl group-substituted
benzenes in their cages means that the reaction paths of the
MTO reaction over the 8-MR catalysts are the same. The higher
intensities of NMR peaks on the ERI, LTA, and UFI catalysts
compared to the CHA catalyst indicate the higher concentration
of alkyl aromatic compounds in their cages. However, the
formation of polycyclic aromatics in the cages was not certain
from these NMR spectra.
Since the molecular sizes of polycyclic aromatics are large,
and since they are occluded in the cages, the dissolution of the
catalyst framework is required to extract the organic materials
occluded in the cages. The spectroscopic examination of the
organic materials extracted from the used 8-MR catalysts
provides valuable information about the chemical species
formed in their cages during the MTO reaction. The amounts of
the organic phases extracted from the 8-MR catalysts used for
90 min in the MTO reaction varied according to their
topologies: the amount was as small as 0.7 mg per 100 mg
of the CHA catalyst, while the amounts were relatively large:
6.1, 6.1 and 5.7 mg per 100 mg of the ERI, LTA and UFI
catalysts, respectively. Since the volatile materials remaining in
the cages can be lost during the dissolution of the framework
with hydrogen fluoride followed by the extraction with carbon
chloride, the amounts of the extracted organic phases represent
the materials occluded in the cages with relatively high
molecular weights. Therefore, a comparison of the large
amounts of organic phases extracted from the cages of the ERI,
LTA and UFI catalysts to the CHA catalyst indicates that those
cages allowed the formation of alkyl aromatic compounds with
high molecular weights.
Since the color of polycyclic aromatic compound varies with
the number of aromatic rings fused, the UV–VIS adsorption
spectra of the extracted organic phases make it possible to
identify the basic structures of polycyclic aromatic compounds
occluded in the cages. Fig. 7 shows the UV–VIS absorption
spectra of the organic phases extracted from the 8-MR catalysts
used for 90 min in the MTO reaction. The concentrations of the
organic phases extracted from the ERI, LTA and UFI catalysts
were adjusted to be 130 ppm, while that from the CHA catalyst
was adjusted to be 260 nm to obtain similar levels of absorbance.
The spectrum of the organic phase extracted from the CHA
catalyst showed a narrow band at 260–300 nm without any
absorption at longer wavelengths. The absorption maximum of
the organic phases extracted from the ERI catalyst appeared at
265 nm, but considerable absorption was still observed even at
300–400 nm. However, the spectra of the organic phases
extracted from the LTA and UFI catalysts had two distinct
absorption bands with their absorption maxima at 280 and
340 nm. A shoulder at 410 nm was additionally observed.
Benzene and naphthalene showed their absorption band at
260–300 nm. Absorption bands of anthrancene and phenan-
threne composed of three fused benzene rings appear at 300–
400 nm, while those of pyrene composed of four benzene fused
rings appear above 400 nm [31]. Therefore, the organic phase
extracted from the CHA catalyst used for 90 min in the MTO
reaction contained mainly the alkyl derivatives of benzene and
naphthalene. The formation of large polycyclic aromatics such
as phenanthrene and pyrenes was reported from the SAPO-34
catalyst used for a long time in the MTO reaction [13].
However, the formation of such large polycyclic aromatics in
the cages of the CHA catalyst for a short time was not
detectable. On the other hand, the presence of polycyclic
aromatics including anthracene and phenanthrene was obvious
in the organic phases extracted from the LTA and UFI catalysts
because of their distinct absorption bands at 360 nm. The
shoulder at 410 nm observed on these spectra led us to think of
the possibility of the formation of polycyclic aromatics
composed of four fused benzene rings in their cages.
The 13C NMR spectra of the organic phases extracted from
the cages of the 8-MR catalysts used in the MTO reaction also
support the presence of polycyclic aromatics in their cages, as
shown in Fig. 8. These spectra are similar to the solid MAS
NMR spectra shown in Fig. 6 which indicates the materials
presented in the cages of the 8-MR catalysts. However, the
spectra of Fig. 8 showed peaks more clearly than those of Fig. 6,
because they were recorded in liquid state. The loss of volatile
materials during the extraction makes it easy to identify
materials remaining in the cages. The low intensities of NMR
peaks of the organic phases extracted from the CHA catalyst
indicated the small amounts of occluded materials in its cages.
Fig. 8. 13C NMR spectra of materials occluded in the cage of the 8-MR
catalysts in the MTO reaction: reaction temperature = 350 8C, WHSV
= 0.70 h�1.
Fig. 9. Comparison of the cage shapes and size of the 8-MR catalysts.
J.W. Park et al. / Applied Catalysis A: General 339 (2008) 36–44 43
The peaks observed at 16 and 132 ppm were in good
coincidence with those of hexamethyl benzene. The other
peaks might be attributed to alkyl-substituted benzenes and
naphthalenes. On the other hand, the organic phase extracted
from the UFI catalyst showed several intense peaks at 15–30
and 115–140 ppm. The peaks at the former ranges indicated the
presence of methyl and ethyl groups substituted on benzene and
polycyclic aromatics. The wide peak around 115–140 ppm was
attributed to the carbon atoms of polycyclic aromatics
composed of several benzene rings. The NMR spectra of
Fig. 8 strongly indicate that the formation of large polycyclic
aromatics was more rapid in the cages of the ERI. LTA and UFI
catalysts compared to the cage of the CHA catalyst.
The 8-MR catalyst showed almost the same product
composition of lower olefins at the initial stage of the MTO
reaction, regardless of their topologies. Their high selectivity to
lower olefins was maintained during the MTO reaction, and the13C NMR spectra of organic phases extracted from the cages of
the used 8-MR catalysts consistently showed the presence of
alkyl aromatics in their cages. However, the UV–VIS spectra of
the organic phases extracted clearly indicated the preferable
formation of heavy polycyclic aromatics in the cages of the
LTA and UFI catalysts rather than in that of the CHA catalyst.
The formation of heavy polycyclic aromatics in the cages must
be related to the cage shapes and sizes of the catalysts because
their cages determine the allowable size of polycyclic aromatic
molecules.
The cages of the 8-MR catalysts used in this study are shown
in Fig. 9. The cage characteristics such as pore size and cage
volume are listed in Table 3. The CHA catalyst has the smallest
pore entrances among the 8-MR catalysts, and they are close to
the cross-sectional area of methanol. On the other hand, the ERI
catalyst has the largest pore entrance. The pore entrances of the
CHA and LTA catalysts are circular, while those of the ERI and
UFI are elliptical. The cage volumes calculated using Marler’s
equation [23] were also different according to the topology of
the catalysts: the cage volumes of the LTA and UFI catalysts are
large, while those of CHA and ERI are small.
Based on the structural information on the cages of the 8-MR
catalysts, one can explain their catalytic selectivity and
deactivation in the MTO reaction. Their cages provide enough
space for the formation of multimethyl benzenes which are
known as active intermediates producing lower olefins.
Furthermore, the small pore entrances of these cages restrict
their migration and preserve them in cages to work as active
intermediates, resulting in high selectivity to lower olefins. As
suggested by Haw and Marcus [13], the deactivation of the
SAPO-34 catalyst with CHA topology is caused by the
formation of polycyclic aromatics from the active multimethyl
benzenes. Since the cages of the LTA and UFI catalysts are
relatively large compared to that of the CHA catalyst, the cages
J.W. Park et al. / Applied Catalysis A: General 339 (2008) 36–4444
of the former catalysts do not restrict the formation of
polycyclic aromatics in their cages, resulting in rapid
deactivation. The UFI catalyst deactivates rapidly because its
two-dimensionally-connected cages are relatively easy to be
blocked compared to the three-dimensionally-connected LTA
cages. However, the cages of the ERI catalyst are not large
enough, and thus, they are easily blocked even by a small
amount of alkyl naphthalene. Different materials are generated
inside the cages by their spatial restrictions as the shape and size
of the cage vary, which causes the cages to be blocked to
different degrees. This explains why the deactivation speed in
MTO reaction changes according to the structure of the cages.
This report may be the first attempt to explain the
deactivation behavior of molecular sieve catalysts in terms
of their cage shapes and sizes in the experimental results. Zhu
et al. [25] supposed the dependence of the deactivation of CHA
and MTF zeolites in the MTO reaction on their cage shape and
size. However, this paper provides a distinct experimental
example clearly showing the effects of cage shape and size of
catalysts on their deactivation rate. Furthermore, these results
also provide plausible explanations for the high selectivity and
relatively long catalyst life of SAPO-34 molecular sieve by
investigating its suitable cage shape and cage size. Its cages
allow the formation of multialkyl benzenes, but its small cage
volume compared to the ERI, LTA, and UFI molecular sieves
suppresses the formation of polycyclic aromatics, making its
catalyst life longer. The preparation of 8-MR molecular sieves
which have sufficient acidity and a slightly smaller cage
volume than the CHA topology may open the path to the
development of more efficient catalysts for the MTO reaction
showing high selectivity to lower olefins and negligible
deactivation.
4. Conclusions
Four kinds of the 8-MR small-port molecular sieves with
CHA (SAPO-34), ERI (UZM-12), LTA (UZM-9), and UFI
(UZM-5) topologies showed high activities in the MTO
reaction owing to their sufficient acidity. The selectivity to
lower olefins was also high over the 8-MR catalysts, regardless
of their topologies. However, the ERI, LTA, and UFI catalysts
deactivated rapidly in the order of LTA < ERI < UFI, while
the CHA catalyst showed a stable conversion at this
experimental condition. The presence of similar alkyl aromatic
compounds in their cages during the MTO reaction suggests
the same reaction path over the 8-MR catalysts. The occlusion
of polycyclic aromatics fused of 3–4 benzene rings was
observed in the cages of the LTA and UFI catalysts, even for as
long as 90 min of the reaction. The rapid deactivation of the
UFI catalyst in the MTO reaction was explained by its large
cages that allow the formation of large polycyclic aromatics
and the easy blocking of its pores because of its two-
dimensionally-connected cages. The selective and stable
catalytic performance of the CHA catalyst in the MTO
reaction can be explained by its cages being of suitable shape
and size to preserve stably the active intermediates, multialkyl
benzenes, in them.
Acknowledgement
This work was supported by a grant-in-aid for Next-
Generation New Technology Development Programs from the
Korea Ministry of Commerce, Industry and Energy (no.
0028414-2006-11) through the Korean Research Institute of
Chemical Technology.
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