Conversion of methanol over H-ZSM-22: The reaction mechanism and deactivation

Article abstract of DOI:10.1016/j.cattod.2010.10.095

Conversion of methanol to olefins (MTO) over H-ZSM-22 was carried out in the pulse and the continuous-flow reaction systems. ¹³C labeling technique was used to reveal the reaction mechanism. The materials retained in the deactivated catalyst were detected by GC-MS after dissolving the catalyst framework and then extracting the organic species. The adsorption of these materials in ZSM-22 channels was computationally modeled. The results showed that the olefin methylation-cracking mechanism is the main route for the conversion of methanol over H-ZSM-22. It was suggested that the blockage of the zeolite pore openings by the coke species might be the reason of the relatively rapid deactivation of H-ZSM-22, instead of the prohibition of the formation of large transition-state intermediates involved in hydrocarbon pool mechanism.

Full text of DOI:10.1016/j.cattod.2010.10.095

Catalysis Today 164 (2011) 288–292
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Conversion of methanol over H-ZSM-22: The reaction mechanism and
deactivationଝ
Jinzhe Li, Yingxu Wei, Yue Qi, Peng Tian, Bing Li, Yanli He, Fuxiang Chang, Xinde Sun, Zhongmin Liu∗
National Engineering Laboratory for Methanol to Olefins, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2010
Received in revised form 21 October 2010
Accepted 21 October 2010
Available online 4 December 2010
Conversion of methanol to olefins (MTO) over H-ZSM-22 was carried out in the pulse and the continuous-
flow reaction systems. 13C labeling technique was used to reveal the reaction mechanism. The materials
retained in the deactivated catalyst were detected by GC–MS after dissolving the catalyst framework and
then extracting the organic species. The adsorption of these materials in ZSM-22 channels was computa-
tionally modeled. The results showed that the olefin methylation-cracking mechanism is the main route
for the conversion of methanol over H-ZSM-22. It was suggested that the blockage of the zeolite pore
openings by the coke species might be the reason of the relatively rapid deactivation of H-ZSM-22, instead
of the prohibition of the formation of large transition-state intermediates involved in hydrocarbon pool
mechanism.
Keywords:
ZSM-22
Methanol
Methylation
Propene
Ethene
© 2010 Elsevier B.V. All rights reserved.
MTO
1. Introduction
carbon pool mechanism [14,15], which was supported by Hunger
and co-workers [16,17]. Furthermore, Cui et al. [18] reported that
Methanol is an important platform product of coal chem-
istry. Light olefins can be obtained by conversion of methanol
over acidic zeolite catalysts through the process known as MTO
(methanol-to-olefins). Considerable effort has been devoted to
the optimization of the catalyst performance and process condi-
tions [1–3]. In parallel with that, numerous research works have
been done to elucidate the reaction mechanism of MTO conver-
sion [4–7] and more than 20 mechanisms have been proposed
by different researchers [1]. Among the proposed mechanisms,
the hydrocarbon pool mechanism attracted much more attention,
which is an indirect mechanism and first proposed by Kolboe and
co-workers based on the experiments of the MTO conversion over
SAPO-34 [8–10]. According to the hydrocarbon pool mechanism,
the reaction cycle involved the methylation of “hydrocarbon pool
intermediates” confined in the cages of SAPO-34 by methanol and
subsequent elimination of ethylene, propene, and butenes from the
intermediates. Later, detailed studies revealed that the polymethyl-
benzenes composed largest part of the materials retained in the
catalyst and that hexamethylbenzene was the most active species
for methanol to olefin conversion [11–13]. Haw et al. proposed that
the conversion of methanol over ZSM-5 also follows the hydro-
the MTO conversion could only take place on zeolites that allow
the hydrocarbon pool mechanism to work, and that due to the
transition-state shape selectivity, MTO conversion over ZSM-22 (a
zeolite which has one-dimensional channel with 10-member ring
opening) could only produce dimethylether as the product. They
also found that ZSM-22 displayed a low but appreciable produc-
tion of olefins at the beginning of methanol conversion, but they
believed that it was the result of the impurity phase (ZSM-11)
and/or the external acid sites [18].
However, Svelle and Bjorgen [19,20] pointed out that over
ZSM-5, ethene and propene maybe produced through dual-cyclic
reaction route. Ethene was formed through the hydrocarbon pool
mechanism with lower methylated benzene as reactive interme-
diates, and apart from that, the olefin methylation-cracking cycle
was also an effective route for the formation of propene, butenes
and higher olefins. This suggests that if the reactions follow-
ing the hydrocarbon pool mechanism were suppressed, methanol
might be converted into olefins through the methylation-cracking
route. In fact, our previous work presents a positive result of
methylation-cracking route in methanol conversion over ZSM-22.
Nearly complete conversion of methanol could be obtained over
ZSM-22 at 450 ◦C (WHSV = 10 h−1) [21]. In a very recent report from
the group of Olsbye [22], they also found that under suitable con-
ditions ZSM-22 has a conversion capacity comparable to that of
SAPO-34. These results have been somewhat contradictive with the
report of Cui et al. [18].
ଝ
Student Award Paper.
Corresponding author. Tel.: +86 411 84685510; fax: +86 411 84691570.
∗
E-mail address: liuzm@dicp.ac.cn (Z. Liu).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.10.095

J. Li et al. / Catalysis Today 164 (2011) 288–292
289
Pulse No.2
Pulse No.1
Ethene
Pulse No.3
Pulse No.4
100
80
60
40
20
0
80
60
40
20
0
Propene
Butene
80
60
40
20
0
80
60
40
20
0
Pentene
Hexene
80
60
40
20
0
0
1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6
Number of13C atoms in molecule
Fig. 1. Isotopic distribution in the effluent products of successive 13C-methanol pulse reaction over ZSM-22 at 450 ◦C after the pre-reaction of 15 pulses of 12C-methanol,
CT = 0.08 s.
In the present study, methanol conversion over ZSM-22 was car-
ried out in the pulse and continuous-flow reaction systems. The
the sample was washed with deionized water, dried at 110 ◦C and
finally calcined at 550 ◦C for 4 h to achieve H-ZSM-22.
The crystallinity and phase purity of the samples was charac-
terized by powder X-ray diffraction (RIGAKU D/max-rb powder
diffractometer) with Cu K˛ radiation.
The acidity of the catalysts was determined by temperature pro-
grammed desorption of ammonia (NH3-TPD). A catalyst sample of
0.14 g was loaded into a U-shaped micro-reactor and pre-treated
at 650 ◦C for 30 min in a flow of helium. After the pre-treatment,
the sample was cooled to 100 ◦C and saturated with ammonia. The
temperature was increased from 100 to 600 ◦C at a constant heating
rate of 10 ◦C/min under a He flow of 40 ml/min. The concentration
of ammonia in the exit gas was monitored continuously with a TCD
detector.
reaction mechanism over ZSM-22 was elucidated by the aid of 13
C
labeling technique. It was demonstrated that the methanol could
be converted into olefins over ZSM-22 through olefin methylation
and cracking route. The blockage of the channel openings by coke
species was proved to be the main reason for the low efficiency
of methanol conversion over ZSM-22, instead of the prohibition of
hydrocarbon pool species formation associated with the transition-
state shape selectivity.
2. Experimental
2.1. Catalyst preparation and characterization
The sample of ZSM-22 (SiO2/Al2O3 = 69) was kindly provided by
another research group of Dalian Institute of Chemical Physics. The
NH4-ZSM-22 was obtained by ion-exchanging the calcined solid
with the solution of ammonium nitrate. After the ion-exchange,
2.2. Methanol conversion
Methanol conversion was performed in a fixed-bed quartz tubu-
lar reactor at atmospheric pressure. In the pulse reactions, a catalyst

290
J. Li et al. / Catalysis Today 164 (2011) 288–292
Table 1
100
Methanol conversion and product selectivity of MTO over ZSM-22.
95
90
85
80
75
70
65
Pulse number Conversion (%) Selectivity (C%)
0
=
0
=
0
+
CH3OH
C1
C2
C2
C3
C3
C4
C5
C6
Ethene
Propene
Butene
Pentene
Hexene
1
2
5
15
19
76.5
66.8
63.7
53.7
40.4
2.3 3.7 0.4 22.3 3.4 17.2 28.6 22.1
1.8 3.0 0.3 20.2 3.9 16.9 28.4 25.5
2.1 3.0 0.3 19.6 3.4 16.3 28.7 26.6
2.5 3.0 0.3 19.3 3.2 15.8 27.8 28.1
3.3 3.0 0.2 20.0 2.7 15.7 27.8 27.3
sample of 45 mg (60–80 mesh) was loaded into the reactor and acti-
vated at reaction temperature (450 ◦C) for 1 h before reaction. The
reactions were carried out at 450 ◦C with reactant-catalyst contact
time of 0.08 s. An injection of methanol of 1 ␮l was conducted onto
the catalyst, and the effluent was kept warm and analyzed by online
gas chromatography (Varian GC3800) equipped with a PoraPLOT
Q-HT capillary column and a FID detector. For the 13C labeling
experiments, pre-reaction of 15 pulses of 12C-methanol injections
was followed by successive 13C-methanol injections. The effluent
products of each 13C-methanol pulse reaction were collected and
analyzed by an Agilent 6890/5973N MSD GC–MS.
In the continuous flow reactions, a catalyst sample of 120 mg
was loaded into the reactor and activated at the reaction tem-
perature (450 ◦C) for 1 h. The methanol was fed by passing the
carrier gas (8.5 ml/min) through a saturator containing methanol at
24 ◦C, which gave a WHSV of 1.0 h−1. The effluent was analyzed by
online gas chromatography. After the reaction, the discharged cat-
alyst was dissolved and the hydrocarbons retained in the catalyst
was extracted and determined by GC–MS, following the method
introduced by Guisnet [23].
1
2
3
4
13C-methanol pulse
Numbers of
Fig. 2. Total 13C content in the effluent products of successive 13C-methanol pulse
reaction over ZSM-22 at 450 ◦C after the pre-reaction of 15 pulses of 12C-methanol,
CT = 0.08 s.
distribution in the effluent of each pulse was analyzed by GC–MS
and displayed in Fig. 1.
Isotopic distribution indicated that the product molecules con-
taining only 13C atoms was predominant (>80%) in the effluent and
the proportion of 13C-atoms into the products increased with the
following 13C-methanol injection.
Different from the publications [19,20], in which the reac-
tions were carried out under the continuous-flow conditions, the
isotopic switch experiments in this study were performed on a
pulse reaction system. In this setup, the mixing of 12C-methanol
and 13C-methanol was avoided and the switch of 12C/13C could
be clear-cut and an immediate products analysis after isotopic
switch could be realized. This also makes it possible to correlate
the differences in the 13C distribution to the reaction mech-
anism. 12C-methanol pre-reaction possibly generated 12C-coke
species, such as polymethylbenzenes, in the cages or channels
of the catalysts. The incorporation of 12C atom into the products
would predict that the reaction followed hydrocarbon pool mech-
anism [4,5] and these 12C-coke species may work as the reaction
centers of methanol conversion. Through another reaction route
proposed for methanol conversion, olefin methylation-cracking
[19,20,25–27] without involving the 12C-polymethylbenzenes
retained in the catalysts, the products generation would be inde-
pendent of the scrambling of isotopic distribution. However, if
both of the two reaction mechanisms were allowed to oper-
ate on a specific catalyst, the products from the former reaction
route would possibly serve as the initial olefins for the later
reaction route, which makes the reaction route determination
2.3. Computational modeling
Computational modeling and geometry optimization of the
selected materials within the zeolite channels were performed
using FORCITE program in MATERIAL STUDIO. For simplicity, the
ZSM-22 was modeled as pure silica TON type zeolite. The unit
cell and atomic coordinates for the framework was taken from the
known crystal structure of the TON structure [24]. The zeolite lat-
tice was held fixed during the simulation and the organic species
was allowed to move freely in the channels. All of the calculations
were performed using Universal Forcefield.
3. Results and discussion
3.1. Pulse reaction of methanol using 13C labeling
Methanol conversion over ZSM-22 was performed on the pulse
reaction system using 13C labeling technique and the results are
displayed in Table 1 and Figs. 1 and 2. The conversion in the context
was referred to the percent of methanol which were converted into
hydrocarbons, that is to say, dimethylether was also considered as
reactant in the following discussion.
The methanol conversion was 76.5% for the first methanol injec-
tion and decreased in the following injections. After 19 injections
of methanol were conducted, methanol conversion was 40.4%.
Propene was one of the main products with a selectivity of ∼20%,
while very low ethene selectivity (only 3%) was observed. The selec-
+
tivities for C5 and C6 were in the range of 22–29% and most of the
+
C6 were olefins. It was noticeable that the deactivation appeared
over ZSM-22 since the second methanol injection. This means that
no induction period was observed over ZSM-22.
In the 13C labeling experiments, after 15 times of 12C-methanol
pulse reactions were performed, 4 pulses of the 13C labeling
methanol were injected into reactor successively, and the isotopic
Fig. 3. Conversion of methanol and product selectivities of MTO reaction over
HZSM-22 in fixed-bed reactor at 450 ◦C, WHSV = 1.0 h−1, He/MeOH (mol) = 5.4.

J. Li et al. / Catalysis Today 164 (2011) 288–292
291
(CH3)2
(CH3)5
(CH3)4
CH3
(CH3)2
(CH3)2
CH3
(CH3)3
(CH3)3
*
5
7.5
10
12.5
15
17.5
20
22.5
25
Retention time (min)
Fig. 4. GC–MS chromatogram of the compounds retained in the deactivated H-ZSM-22 catalyst of MTO conversion in the fixed bed reactor, * internal standard.
more difficult from the isotopic distribution. Anyway, the obser-
vations mentioned above in the 13C switch experiments is still
helpful for distinguishing the reaction route of methanol conver-
sion over ZSM-22, even the conclusion is difficult to be drawn
exactly.
issue, methanol conversion over ZSM-22 was also carried out in a
fixed bed reactor at 450 ◦C with the methanol WHSV of 1.0 h−1. The
conversion and product selectivity were plotted against the reac-
tion time on stream in Fig. 3. Methanol conversion was 100% before
160 min of time on stream under present reaction conditions. These
results further confirmed that under the continuous-flow condition
with an appreciable contact time, the high reactivity over ZSM-22
could be kept for a long period, and suggest that the deactivation
over ZSM-22 in pulse reactions might be associated with the quick
diffusion of olefins from the catalyst channels and/or the depres-
sion of the olefins cracking by active sites coverage caused by the
gradually deposited coke species.
The total 13C contents of the products were calculated from
the detailed isotopic distribution and plotted in Fig. 2 against the
pulse number of 13C methanol. The incorporation of 12C atoms
into the products over ZSM-22 was less obvious than that over
SAPO-34 [28] and ZSM-5 [19,20], but ethene shows apprecia-
ble incorporation of 12C atoms. At the 2nd to 4th pulse of 13C
methanol, the total 13C contents of propene, butene, pentene and
+
hexene were higher than 95%. As shown in Table 1, C3 olefins
In the continuous-flow conversion of methanol, before the cat-
alyst became deactivated, propene was the main product and its
selectivity increased from 35% to about 40%. At the same time, the
ethene selectivity was in the range of 17–10%. During this period,
the selectivities for ethene and propene were higher than that
were the main products of methanol conversion over ZSM-22. So
it was reasonable to propose that methanol conversion over ZSM-
22 mainly follows the olefin methylation-cracking mechanism. The
incorporation of 12C atoms into ethene over ZSM-22 was higher
+
+
than that into C3 olefins. This suggested that the ethene forma-
obtained in the pulse reaction. While the selectivities to C5 and C6
tion over ZSM-22 might be relevant with the hydrocarbon pool
mechanism, as its formation over ZSM-5 [19]. However, lower
selectivity for ethene was observed over ZSM-22 than that over
ZSM-5.
were lower than that obtained in the pulse reaction. This result sug-
gests that the higher olefins might be cracked into lighter olefins
under the condition of lower space velocity and longer reaction
contact time in the continuous-flow fixed bed reactor.
The differences in the product distribution over ZSM-22 and
ZSM-5 can be explained by taking the zeolite topology into account.
ZSM-22 has only one dimensional 10 member ring channels with
no intersections, so the largest space which ZSM-22 can offer was
It was noticeable that when the methanol conversion decreased
to lower than 100% with prolonging the reaction time, the selec-
tivities of ethene, propene and C4 decreased rapidly, meanwhile
+
the C5 and C6 selectivities increased. After ∼190 min on stream,
2
˚
5.7 × 4.6 A . ZSM-5 also has the 10 member ring channels but
methanol conversion decreased sharply from 95% to ∼10% and
it contains channel intersections which can accommodate cyclic
species as hydrocarbon pool. Actually, large amount of toluene and
C10–C30 alkanes selectivity increased. During this period, the selec-
+
tivities of C5 and C6 increased to a maximum and then decreased
+
xylene were detected among C6 products over ZSM-5 [1]. Com-
(they were still higher than the selectivity for ethene, propene
and C4).
+
parably, olefins accounted for the largest part of C6 over ZSM-22.
Considering the channel of ZSM-22 was too narrow for the reac-
tion following the mechanism of hydrocarbon pool, the methanol
conversion more possibly went through the olefin methylation
and cracking route. This could explain the generation of large
amount of high olefins (>C3=) and small amount of ethene over
ZSM-22.
ZSM-22 was a TON type zeolite with only 1-dimensional chan-
nels and the channel openings of ZSM-22 were easy to be blocked
by the coke species. The increase of alkanes selectivity after 190 min
on stream indicates that the hydride transfer reaction became the
main reaction, which led to heavier coking of the catalyst. This made
the diffusion of the products out of the channels more difficult
and further reaction more possibly occur. With coke deposition,
the stronger acidic sites were easier to be covered by the coke
and as a result the cracking of higher olefins was suppressed.
These were responsible for the increase in heavier olefin pro-
duction and the low efficiency in methanol conversion and the
production of lighter olefin. The catalyst was completely deacti-
vated when most of the channels of ZSM-22 catalyst were blocked.
The materials retained in the discharged catalyst after reaction
were analyzed by GC–MS and the chromatogram in Fig. 4 shows the
retained organics were dominated by a large amount of aromatics,
including polymethylbenzenes, naphthalenes, and phenanthrenes
3.2. Continuous-flow reaction of methanol over ZSM-22
As shown in Table 1, in the pulse reactions deactivation of
methanol conversion over ZSM-22 occurred very quickly. As men-
tioned above, the olefin methylation-cracking mechanism was the
main reaction route for methanol conversion over ZSM-22, so the
deactivation may stem from the quick diffusion of the olefins out of
the catalyst channels, which makes the reaction cycle discontinued.
At the same time, the deactivation from the blockage of the chan-
nels by coke species could still not be excluded. To shed light on this

292
J. Li et al. / Catalysis Today 164 (2011) 288–292
Fig. 5. Equilibrium geometry of p-xylene (a), 1,2,4-trimethylbenzene (b) and 2,6-dimethylnaphthalene (c) adsorbed in the channels of TON type pure silica zeolite.
Table 2
Adsorption
to-olefins conversion. Furthermore, the GC–MS analysis of the
retained coke species and computational modeling of them in
ZSM-22 channels suggested that these coke species (such as poly-
methylated benzenes and naphthalenes) were responsible for the
deactivation of ZSM-22 by blocking the zeolite channel openings,
rather than acting as the active centrals for olefins production.
energy
of
p-xylene,
1,2,4-trimethylbenzene
and
2,6-
dimethylnaphthalene in the channels of TON type pure silica zeolite.
Adsorbed molecule
PX
TMB
DMN
−54.4
Adsorption energy (kcal/mol)
−42.9
−43.0
Acknowledgments
(or anthracenes). These compounds with big size were possi-
bly responsible for the deactivation of methanol conversion over
ZSM-22.
The authors thank the Natural Science Foundation of China (Nos.
20903091 and 20973164) for financial support. The authors thank
Prof. Tao Zhang for his valuable help in theoretical calculation.
The location of these coke species in the catalyst was still
unknown. Some researchers [20,29,30] argued that ZSM-5 (MFI
type with 10 member ring and intersections) cannot provide
enough spaces for the formation of coke species larger than tetram-
ethylbenzenes, and this was believed to be the reason of the long
life time of ZSM-5 for methanol conversion. The deactivation of
ZSM-5 was caused by the coke on the external surface of the zeo-
lite crystals [29]. The diameter of ZSM-22 channels was close to that
of ZSM-5, so the formation of the large coke species in the channels
might also be suppressed. But the large species was detected in the
coke analysis by GC–MS. They may form on the external surface or
near the pore mouth. These species could age to insoluble graphitic
species or be adsorbed in the channels (or near the pore mouth) and
then blocked the pore openings. The adsorption of these species
in the channels was confirmed by computational modeling of the
selected materials within the pure silica TON type zeolite (the
topology which ZSM-22 belongs to, for detailed structure informa-
tion see [24]) using FORCITE program in MATERIAL STUDIO. Fig. 5
shows the energy minimized adsorption complexes of p-xylene
(PX), 1,2,4-trimethylbenzene (TMB) and 2,6-dimethylnaphthalene
(DMN) within the TON channels, and the adsorption energy is
shown in Table 2. It can be seen that the adsorption energy was in
the order of PX ≈ TMB < DMN. This indicates that the DMN molecule
size was more comparable with the TON channel than PX and TMB.
The nearest distances of the adsorbed molecules to the zeolite wall
were also displayed in Fig. 5. These results show that these large
coke species could be adsorbed in the TON channels, but the void
spaces might be too small for further reactions.
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4. Conclusions
In summary, under the pulse reaction conditions, the methanol
conversionoverH-ZSM-22showed highselectivityforolefins heav-
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that the conversion mainly followed the olefin methylation-
cracking reaction cycle, which was different from the report by Cui
et al. [18]. The results obtained under the continuous-flow reaction
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