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al 339 (2008) 36–44

Applied Catalysis A: Gener

Effects 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: gseo@chonnam.ac.kr (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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