Methanol to hydrocarbons reaction over HZSM-22 and SAPO-11: Effect of catalyst acid strength on reaction and deactivation mechanism

Article abstract of DOI:10.1016/S1872-2067(15)60953-6

The conversion of methanol to hydrocarbons has been investigated over HZSM-22 and SAPO-11. Both of these catalysts possess one-dimensional 10-ring channels, but have different acidic strengths. Comparison studies and ¹²C/¹³C isotopic switching experiments were conducted to evaluate the influence of the acidic strength of the catalyst on the conversion of methanol, as well as its deactivation mechanism. Although the conversion of methanol proceeded via an alkene methylation-cracking pathway over both catalysts, the acidity of the catalysts had a significant impact on the conversion and product distribution of these reactions. The stability of the catalysts varied with temperature. The catalysts were deactivated at high temperature by the deposition of graphitic coke on their outer surface. Deactivation also occurred at low temperatures a result that the pores of the catalyst were blocked by polyaromatic compounds. The co-reaction of ¹³C-methanol and ¹²C-1-butene confirmed the importance of the acidity of the catalyst on the distribution of the hydrocarbon products.

Full text of DOI:10.1016/S1872-2067(15)60953-6

Chinese Journal of Catalysis 36 (2015) 1392–1402
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Methanol to hydrocarbons reaction over HZSM-22 and SAPO-11:
Effect of catalyst acid strength on reaction and deactivation
mechanism
a,b,d
b
b
b
b,#
b
b,d
Jinbang Wang , Jinzhe Li , Shutao Xu , Yuchun Zhi , Yingxu Wei , Yanli He , Jingrun Chen ,
b,d
b
b,d
b,d
a,
b,c,$
Mozhi Zhang , Quanyi Wang , Wenna Zhang , Xinqiang Wu , Xinwen Guo *, Zhongmin Liu
a
State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology,
Dalian 116024, Liaoning, China
National Engineering Laboratory for Methanol to Olefins, State Energy Low Carbon Catalysis and Engineering R&D Center, Dalian National Laboratory
for Clean Energy, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian 116023, Liaoning, China
b
c
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
University of Chinese Academy of Sciences, Beijing 100049, China
d
A R T I C L E I N F O
A B S T R A C T
Article history:
The conversion of methanol to hydrocarbons has been investigated over HZSM-22 and SAPO-11.
Both of these catalysts possess one-dimensional 10-ring channels, but have different acidic
strengths. Comparison studies and 12C/ C isotopic switching experiments were conducted to eval-
uate the influence of the acidic strength of the catalyst on the conversion of methanol, as well as its
deactivation mechanism. Although the conversion of methanol proceeded via an alkene methyla-
tion-cracking pathway over both catalysts, the acidity of the catalysts had a significant impact on the
conversion and product distribution of these reactions. The stability of the catalysts varied with
temperature. The catalysts were deactivated at high temperature by the deposition of graphitic coke
on their outer surface. Deactivation also occurred at low temperatures a result that the pores of the
catalyst were blocked by polyaromatic compounds. The co-reaction of 13C-methanol and
12C-1-butene confirmed the importance of the acidity of the catalyst on the distribution of the hy-
drocarbon products.
Received 26 May 2015
Accepted 9 July 2015
Published 20 August 2015
13
Keywords:
HZSM-22
SAPO-11
Methanol-to-hydrocarbon
Acid strength
Mechanism
©
2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1
.
Introduction
increasing demand for fuels and light alkenes have stimulated
significant research efforts towards the development of new
processes for the production of these petrochemical products
from alternative and abundant resources, such as biomass, coal
and natural gas [1,2]. Among these non-petrochemical routes,
the methanol-to-hydrocarbon (MTH) route has become a suc-
Light alkenes and liquid hydrocarbon fuels are important
petrochemical commodities, and the majority of the com-
pounds belonging to these groups are produced through pet-
rochemical reactions. Dwindling oil supplies and the rapidly
*
Corresponding author. Tel: +86-411-84986133; Fax: +86-411-84986134; E-mail: guoxw@dlut.edu.cn
Corresponding author. Tel: +86-411-84379118; E-mail: weiyx@dicp.ac.cn
Corresponding author. Tel: +86-411-84379335; E-mail: liuzm@dicp.ac.cn
#
$
This work was supported by the National Natural Science Foundation of China (201473182, 21273005, 21273230, 21103180).
DOI: 10.1016/S1872-2067(15)60953-6 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 8, August 2015

Jinbang Wang et al. / Chinese Journal of Catalysis 36 (2015) 1392–1402
1393
cessful commercial process [2–4]. Furthermore, a wide range of
different zeolites have been investigated as catalysts for the
MTH reaction during the past decades [2,3,5], including ZSM-5
and SAPO-34, which are believed to be the most effective cata-
lysts for this process. These catalysts have also been applied to
a variety of commercial processes, including methanol-to-olefin
determine the effects of the acidic strength of the catalyst on
MTO reactions over AFI and SAPO zeolite catalysts. The results
showed that the use of a catalyst with a low acidic strength
promoted the conversion of methanol via an alkene-mediated
mechanism. Taken together, these results show that it is of
critical importance to understand the detailed role played by
the acidic properties of these catalysts in the formation of al-
kenes. Furthermore, the performance characteristics of these
catalysts could be optimized by tuning their acidic properties,
representing an alternative approach to the optimization of
these processes, which should therefore be explored in greater
detail.
(MTO) [4,6], methanol-to-propene (MTP) [7] and metha-
nol-to-gasoline (MTG) [8–10] processes.
Considerable research efforts have also been devoted to
developing a deeper understanding of the mechanism of the
MTH reaction. The results of previous investigations have
shown that the detailed mechanism of the MTH reaction is very
complicated and strongly dependent on the topology of the
zeolite catalyst. For example, SAPO-34, which consists of large
supercages and small 8-ring windows, is currently regarded as
the best catalyst for MTO reactions [4,11]. It is noteworthy that
the high level of selectivity exhibited by this catalyst for ethene
and propene has been attributed to the hydrocarbon pool
mechanism [12–14]. Furthermore, polymethylbenzene and
polymethylcyclopentadiene, as well as their protonated ana-
logues, have been reported to be important reactive intermedi-
ates for the production of alkenes [15–22]. The formation of
these bulky intermediates was not only observed over SAPO-34
but was also detected in several other zeolites with wide or
intersectional channels, such as Hβ [23], ZSM-5 [20], SSZ-13
In this study, we have used two one-dimensional 10-ring
zeolites, HZSM-22 (TON, 0.46 × 0.57 nm) and SAPO-11 (AEL,
0.4 × 0.65 nm), to elucidate the role of their acidic strength on
the MTH reaction and their deactivation mechanism.
2. Experimental
2.1. Catalyst preparation
The K-ZSM-22 and SAPO-11 catalysts were supplied by
Group DNL0802 of the Dalian Institute of Chemical Physics,
Dalian, China. After being calcined at 600 °C for 10 h to remove
the organic template, K-ZSM-22 was converted to NH
4
-ZSM-22
[
[
19] and DNL-6 [24,25]. In a separate study, Svelle et al.
26–28] conducted a series of 12C/ C-methanol labeling ex-
by three ion-exchange processes in a NH NO solution (1
4
3
13
mol/L) at 80 °C for 6 h. The resulting catalyst was then washed
with deionized water, dried over night at 120 °C and calcined at
550 °C for 4 h to give protonated H-ZSM-22. The SAPO-11 sam-
ple was calcined at 550 °C for 4 h to give H-SAPO-11.
periments over the medium pore acidic zeolite ZSM-5 to show
that the generation of ethene occurred via an separated route,
independent of the one responsible for the formation of C
+
3
alkenes. The authors went on to propose a dual-cycle mecha-
nism for this transformation, which consists of an aro-
2.2. Catalyst characterization
matic-based cycle with xylene/triMB as reactive intermediates
+
for the production of ethene and a C
3
alkene-based cycle for
The structural properties of two catalysts were character-
ized using a PANalytical X’Pert PRO X-ray diffraction (XRD)
the formation of propene and higher alkenes. Based on this
mechanistic insight, there have been considerable researches
about whether the conversion of methanol could run in an in-
dependent manner while suppressing the formation of ethene
via an alkene cycle by carefully controlling the topology of the
catalyst [26]. This assumption was recently validated over the
system with Cu K radiation (λ = 0.154059 nm) at 40 kV and 40
α
mA. The chemical compositions of the catalysts were deter-
mined using a Philips Magix-601 X-ray fluorescence (XRF)
spectrometer. The morphological characteristics of the cata-
lysts were measured by field emission scanning electron mi-
croscopy (FE-SEM) on a Hitachi SU8020 system.
one-dimensional 10-ring zeolite, HZSM-22, which produces
+
hydrocarbons rich in C
5
branched alkenes and low in aromat-
The N physisorption isotherms of the samples were meas-
2
ics and ethene [29–32].
ured at –196 °C on a Micromeritics ASAP 2020 system. Fresh
samples of the catalysts were degassed under vacuum at 90 °C
for 1 h and then at 350 °C for 3 h before being analyzed. The
surface areas of the samples were calculated using the Brunau-
er-Emmett-Teller (BET) equation, and their micropore volumes
were evaluated using the t-plot method.
The spent catalysts were collected and analyzed by thermo-
gravimetric analysis (TGA) on a Q500 SDT thermogravimetric
analyzer. In a typical measurement, a small sample (10–14 mg)
In addition to their topological characteristics, the acidic
properties of zeolite catalysts can also have a significant impact
on their performances for the conversion of methanol. This
issue was first reported by Yuen et al. [33] over CHA and AFI
catalysts. The results of this study revealed that the acidity of
borosilicate sieves was too low to allow for the conversion of
methanol to hydrocarbons, and that SAPO-34 displayed lower
hydrogen transfer reactivity than SSZ-13. Most recently, Bleken
et al. [34] reported the systematic comparison of SAPO-34 and
SSZ-13 in the MTO reaction. The results showed that the more
acidic SSZ-13 catalyst exhibited higher activity, which leads to a
higher methanol conversion than that of the less acidic
SAPO-34 catalyst under the same operating conditions. West-
gård Erichsen et al. [35] also conducted a comparative study to
of spent catalyst was heated in an Al
2
O crucible from ambient
3
temperature to 900 °C at a heating rate of 10 °C/min under a
stream of air at a constant flow rate of 100 ml/min.
The acidity of the catalysts was determined by the temper-
3
ature programmed desorption of ammonia (NH -TPD) on a
Micromeritics AutoChem 2920 system. The samples were

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Jinbang Wang et al. / Chinese Journal of Catalysis 36 (2015) 1392–1402
loaded in a U-shaped micro-reactor and pretreated at 650 °C
for 30 min under an atmosphere of He. After cooling to 100 °C,
using an online GC-MS system.
3
the samples were saturated with NH , followed by purging with
2.4. In situ FTIR
3
helium to remove the physisorbed NH . The desorption ex-
periments were conducted under a stream of He (40 ml/min)
by increasing the temperature from 100 to 600 °C at a ramp
rate of 10 °C/min. The TPD signals were monitored simultane-
ously using a thermal conductivity detector (TCD).
1H MAS NMR experiments were carried out on a Bruker
Avance III 600 spectrometer equipped with a 14.1 T wide-bore
magnet and a 4 mm magic angle spinning (MAS) probe
The Brönsted acidities and characteristics of the surface
species during the MTH conversion over HZSM-22 were moni-
tored using in situ FTIR spectroscopy on a Bruker Vextex70
infrared spectrometer. The catalyst was activated at 500 °C for
1 h prior to the reaction, and the temperature was subsequent-
ly adjusted to the required reaction temperature of 400 °C.
After recording the spectrum of the activated catalyst, metha-
nol was fed into the cell by passing the carrier gas (He, 20
ml/min) through a saturator containing methanol at 5 °C. The
in situ FTIR spectra were recorded simultaneously with a reso-
(Bruker). The details of this procedure have been reported
elsewhere [36]. Briefly, the samples were dehydrated at 400 °C
for 20 h at a pressure of less than 10-3 Pa prior to the adsorp-
tion of perfluorotributylamine. The selective adsorption of per-
fluorotributylamine was performed by exposing the dehydrat-
ed samples to a saturated vapor at room temperature for 30
min. The samples were then degassed to remove any physical
adsorbates from their surfaces. Dehydrated samples both with
and without the perfluorotributylamine adsorption were then
–1
lution of 4 cm .
3. Results and discussion
3.1. Characterization of the catalysts
1
measured by H MAS NMR spectroscopy to determine their
The powder XRD patterns of the two samples (Fig. 1)
matched well with the simulated patterns of the TON and AEL
frameworks, which confirms that these catalysts possessed
good crystallinity and phase purity characteristic. The Si/Al
ratio of the HZSM-22 catalyst was determined to be 33 by XRF
acidic sites distributions.
2.3. Catalytic tests
200 mg samples of the catalysts were pressed, sieved
and the nsi/(nsi + nAl + n ) value of the calcined SAPO-11 mate-
p
through a 40–60 mesh and then loaded into a fixed-bed stain-
less tubular reactor with an inner diameter of 6 mm. Prior to
the catalytic measurements, the catalysts were activated at 500
rial was determined to be 0.13. Representative SEM micro-
graphs of the catalysts (Fig. 2) revealed that the HZSM-22 cata-
lyst existed as rod-like crystals with diameter and length
measurement of 40 and 300 nm, respectively. In contrast, the
SAPO-11 catalyst existed as flake-like crystals with dimensions
°C for 1 h, and the temperature was then adjusted to the ap-
propriate reaction temperature. All of the reactions were con-
ducted under atmospheric pressure. Saturated methanol vapor
was fed into the reactor by passing the carrier gas (He) through
a saturator, which was maintained at 40 °C. The effluent prod-
ucts inside the transfer line were kept at 120 °C and analyzed
by online gas chromatography (GC) on a Bruker GC450 system
equipped with a PoraPLOT Q-HT capillary column and a FID
detector. The conversion and selectivity of the catalysts were
of less than 2 μm. The N adsorption-desorption isotherms of
2
the two catalysts are shown in Fig. 3. Both catalysts gave a type
I isotherm, which is typical of microporous materials. The spe-
cific BET surface areas, microporous surface areas and mi-
croporous volumes of the two catalysts are summarized in
Table 1. These results show that HZSM-22 and SAPO-11 have
comparable surface areas and pore volumes.
calculated on CH
basis and methanol and dimethyl ether were
2
both considered to be reactants based on their rapid intercon-
version.
For the 12C/ C-methanol isotopic switching experiments,
12C-methanol was fed through a saturator maintained at 14 °C
for 15 min under a steady stream of He (12.4 ml/min). The
reactant was switched to 13C-methanol for 1 min and the reac-
tion was then stopped. The catalyst particles were subsequent-
ly transferred to a vessel containing liquid nitrogen for rapid
cooling. The discharged catalysts were dissolved in HF solution
13
HZSM-22
SAPO-11
(20%) and the organic materials trapped in the catalyst were
extracted with dichloromethane and subsequently analyzed by
GC-MS on an Agilent 7890A/5975C GC/MSD system [37].
In the co-reaction of 13C-methanol with 12C-alkenes, metha-
nol, which was fed through a saturator at 20 °C using a carrier
gas (17.6 ml/min) with a weight hourly space velocity (WHSV)
–1
10
20
30
2
40
50
60
of 3 h , was mixed with a stream of alkenes and introduced to
a reactor. The ratios of methanol to the different alkenes were
confirmed by GC analysis. The effluent products were analyzed
o
/( )
Fig. 1. XRD patterns of HZSM-22 and SAPO-11.

Jinbang Wang et al. / Chinese Journal of Catalysis 36 (2015) 1392–1402
1395
(
a)
HZSM-22
SAPO-11
1
00
200
300
Temperature ( C)
-TPD profiles of HZSM-22 and SAPO-11.
400
500
600
(
b)
o
Fig. 4. NH
3
According to the desorption areas measured under high tem-
perature conditions, the acid density of the HZSM-22 catalyst
was slightly higher than that of the SAPO-11 catalyst. Although
a large number of Si atoms were incorporated into the SAPO-11
framework, the contribution of these atoms to the generation of
Brönsted acidic sites was limited because of the formation of Si
islands in the SAPO-11 catalyst [38,39].
Fig. 2. SEM images of HZSM-22 (a) and SAPO-11 (b)
The Brönsted acidities of HZSM-22 and SAPO-11 were also
characterized by H MAS NMR spectroscopy, and the resulting
spectra are shown in Fig. 5. For HZSM-22, the signals at 3.9 and
.1 ppm were attributed to the bridging hydroxyl groups
140
120
100
H-ZSM-22
SAPO-11
1
6
(
Si(OH)Al) and the disturbed bridging hydroxyl groups
8
6
4
2
0
0
0
0
0
(Si(OH)Al), respectively, with the latter being influenced by an
additional interaction with the oxygen atoms in the framework.
Additional peaks were observed at 1.7 and 2.5 ppm, which
were attributed to SiOH and AlOH, respectively [40–42]. For
3.9
1.7
HZSM-22
0
.0
0.2
0.4
Relative pressure (p/p
adsorption-desorption isotherms of HZSM-22 and SAPO-11
0.6
0.8
1.0
0
)
2.5
Fig. 3. N
recorded at –196 °C.
2
6.1
(
1)
2)
Table 1
Textual properties and Brönsted acidities of HZSM-22 and SAPO-11.
(
S
BET
/
S
micro
/
V
micro
/
Brönsted acid
density (mmol/g)
Sample
2
2
3
(
m /g)
(m /g)
(cm /g)
3
.5
HZSM-22
SAPO-11
193
191
172
165
0.08
0.08
0.33
0.23
SAPO-11
4
.5
1.6
2.3
The acidic properties of HZSM-22 and SAPO-11 were char-
acterized by NH -TPD (Fig. 4). Both of these catalysts gave two
desorption peaks corresponding to two distinct acidic
strengths. For SAPO-11, the NH desorption peaks centered at
60 and 280 °C were attributed to weak and moderate acidic
(
1)
3
3
(2)
1
sites, respectively. For HZSM-22, the desorption peaks at 200
and 410 °C were attributed to weak and relatively strong acidic
sites, respectively. These results therefore revealed that the
HZSM-22 catalyst was more acidic than the SAPO-11 catalyst.
10
8
6
4

2
0
-2
1
Fig. 5.
H MAS NMR of HZSM-22 and SAPO-11 before (1) and after (2)
the adsorption of perfluorotributylamine.

1396
Jinbang Wang et al. / Chinese Journal of Catalysis 36 (2015) 1392–1402
SAPO-11, the signals at 3.5 and 4.5 ppm were attributed to the
bridging hydroxyl groups (Si(OH)Al), whereas the signals at 1.6
and 2.3 ppm were assigned to SiOH and AlOH, respectively
500 °C. However, further increasing the temperature up to 550
°C resulted in a reduction in the duration with 100% methanol
conversion.
[
38,42–44]. The signals of the bridging hydroxyl groups were
Compared with HZSM-22, SAPO-11 showed inferior reactiv-
ity for the conversion of methanol. Increasing the temperature
from 350 to 500 °C led to an increase in the initial methanol
conversion over SAPO-11 from 23 to 98%. It is noteworthy that
the deactivation of the SAPO-11 catalyst was much slower at
lower temperatures, even though the activity of this catalyst
was also lower at lower temperatures. A sharp decrease was
observed in the methanol conversion with time on stream
above 400 °C, which indicates the rapid deactivation of the
catalyst.
indicative of the characteristic Brönsted acidic sites of the
strongly acidic aluminosilicate zeolite (HZSM-22), as well as
those of silicoaluminophosphate molecular sieves with medium
strong acidity (SAPO-11). The results of the quantitative analy-
sis given in Table 1 show that the concentration of Brönsted
acidic sites in HZSM-22 (0.33 mmol/g) was a little higher than
that of SAPO-11 (0.23 mmol/g). Moreover, perfluorotributyla-
mine is a useful basic probe for distinguishing between the
1
internal and external acidic sites in zeolites with the aid of H
MAS NMR [45]. Given that the diameter of perfluorotributyla-
mine is 0.94 nm, and therefore much larger than the pore sizes
of HZSM-22 and SAPO-11, this probe molecule could only be
adsorbed by the acidic sites located on the external surfaces of
It was assumed that the significant differences observed in
the initial conversion profiles of methanol over HZSM-22 and
SAPO-11 could be correlated with their acidic properties. Fur-
thermore, the experimental observations described above in-
dicate that high acidic strength leads to high reactivity, which is
in agreement with the results observed over SSZ-13 and
SAPO-34 [34]. From the perspective of catalytic stability,
HZSM-22 displayed a higher optimum temperature (500 °C)
than that of SAPO-11 (400 °C).
Figure 7 shows the product distributions obtained over
HZSM-22 and SAPO-11 at different reaction temperatures. Alt-
hough both of these catalysts possess analogous topological
structures, there were significant differences in the product
1
these catalysts. The differences in the H MAS NMR spectra of
these catalysts both before and after the adsorption of per-
fluorotributylamine could be used to estimate the amount of
acidic sites located on the external surfaces of the catalysts. The
variations in the intensities of these spectra were very small, as
shown in Fig. 5, which indicates that the majority of the acidic
sites on both HZSM-22 and SAPO-11 were located on the in-
ternal surface, allowing for the meaningful comparison of the
reactions carried out over these two catalysts.
distributions formed over the two catalysts. At 350–500 °C, C
5
3
.2. Catalytic performances of HZSM-22 and SAPO-11 for the
and higher hydrocarbons were formed as the major products
over SAPO-11. For HZSM-22, ethene and propene were gener-
ated as the major products, especially at relatively high reac-
tion temperature of 450–550 °C. Ignoring the impact of the
topology of the zeolite, the differences in the product distribu-
tions over the two catalysts could stem from the differences in
the acidities of the catalysts. The stronger acidity of HZSM-22
compared with SAPO-11 would favors the generation of light
alkenes and other hydrocarbon products, especially at higher
conversion of methanol
3.2.1. Methanol conversion and product distribution data
Figure 6 shows the methanol conversion profile with re-
spect to time on stream over HZSM-22 and SAPO-11 at tem-
peratures in the range of 350 to 550 °C. At the beginning of the
reaction, methanol was completely converted to hydrocarbons
over HZSM-22 at all of the reaction temperatures investigated.
The time required for the complete conversion of methanol
could be increased by increasing the temperature from 350 to
temperature, whereas the lower acidity of SAPO-11 would fa-
+
vor the formation of higher hydrocarbons, such as C
5
hydro-
(a)
(b)
1
00
o
3
4
50 C
o
00 C
o
8
6
4
2
0
0
0
0
0
450 C
o
5
5
00 C
o
50 C
0
30
60
90
120
0
40
80
120
160
200
Time on stream (min)
Fig. 6. Methanol conversion over HZSM-22 (a) and SAPO-11 (b) as a function of time on stream at various reaction temperatures with a methanol
WHSV of 3.0 h–1 and a He/methanol ratio of 1.9.

Jinbang Wang et al. / Chinese Journal of Catalysis 36 (2015) 1392–1402
1397
HZSM-22
SAPO-11
100
Aromatics
+
5
C
C
4
80
60
40
20
0
Propene
Ethene
C C alkane
1 3
350
400
450
500
550
350
400
450
500
o
Reaction temperature ( C)
Fig. 7. Product selectivity of the methanol conversion process over HZSM-22 and SAPO-11 at various reaction temperatures with a methanol WHSV of
.0 h–1 and a He/methanol ratio of 1.9.
3
carbons. As well as the acidities of the catalysts, the tempera-
ture of the reaction also had a significant impact on the product
there did not appear to be a significant variation in the product
distribution over SAPO-11 with reaction temperature. Fur-
thermore, increasing the temperature from 350 to 500 °C led to
+
distribution. Over HZSM-22, C
5
hydrocarbons were formed as
–C hydrocarbons,
+
the main products below 400 °C, while C
2
4
a slight decrease in the C
5
selectivity, as well as an increase in
especially ethene, were formed as the major products at the
propene selectivity.
+
expense of the C
5
hydrocarbons when the temperature was
increased. Increasing the temperature from 350 to 550 °C led
3.2.2. Deactivation of methanol conversion over HZSM-22 and
SAPO-11
+
to a decrease in the selectivity for C
5
hydrocarbons from 53%
to 6%, with a concomitant increase in the selectivity for ethene
from 1.9% to 26.6%) and propene (from 16.1% to 36.2%).
Although the temperature had influence on the product distri-
bution over SAPO-11 in a manner consistent with HZSM-22,
Figure 8 shows the GC-MS chromatograms and the amounts
of deposited coke determined by TGA on HZSM-22 and
SAPO-11 following their reactions at 350, 400, 450 and 500 °C.
Naphthalene, phenanthrene, 9H-fluorene and tetraphene, as
(
(a)
(b)
o
o
3
(
50 C
3
(
50 C
Magnified 4 times
1.3%)
3.2%)
(CH3)1-2
(CH3)4
(CH3)1-4
(
CH3)3
(CH3)6
(
CH3)1-4
o
o
4
(
00 C
400 C
(1.9%)
3.4%)
(CH3)1-4
(CH3)1-3
(CH3)2-4
o
o
4
(
50 C
4
(
50 C
3.9%)
3.4%)
(CH3)1-3
o
o
5
(
00 C
500 C
(5.2%)
3.9%)
10
20
30
40
50
60
10
20
30
40
50
60
Retention time (min)
Retention time (min)
Fig. 8. GC-MS chromatograms of the organic species retained in HZSM-22 (a) and SAPO-11 (b) after the MTO reactions at various reaction tempera-
–1
tures with a methanol WHSV of 3.0 h and a He/methanol ratio of 1.9. The coke amounts are shown in the brackets.

1398
Jinbang Wang et al. / Chinese Journal of Catalysis 36 (2015) 1392–1402
well as their methyl-substituted derivatives were identified as
the main species to have been retained on deactivated
HZSM-22. The main species retained on deactivated SAPO-11
were naphthalene, phenanthrene and their methyl-substituted
derivatives. It can be seen from Fig. 8 that the concentration of
the retained coke species detected by GC-MS varied with the
reaction temperature. The concentrations of detected organic
coke species increased to their maximum values at 400 and
0.1
3740
3585
3136
60
450 °C over HZSM-22 and 400 °C over SAPO-11. The concen-
trations then decreased when the temperature was further
enhanced up to 500 °C. However, the amounts of deposited
coke determined by TGA increased steadily with increasing
temperature, which indicates that graphitic coke species were
being gradually formed and becoming dominant at high tem-
peratures over both catalysts. It was subsequently concluded
that these graphitic deposits, which were not soluble in di-
chloromethane and therefore not detected by GC-MS, were the
leading cause of the deactivation of both catalysts at high tem-
peratures. In contrast, the deactivation of both catalysts at low
temperatures could be attributed to the blocking of the zeolite
pores by condensed aromatic compounds based on the analysis
of the concentrations of the retained coke species analyzed by
GC-MS, as discussed above. The formation of more heavy hy-
drocarbons, including methyl-substituted phenanthrene and
tetraphene, as the coke species over HZSM-22 compared with
SAPO-11 could be the primary reason for the rapid deactivation
of HZSM-22 relative to SAPO-11. It is noteworthy that the rapid
deactivation behavior of methanol conversion over HZSM-22
relative to SAPO-11 at 400 °C could be related to the stronger
acidic strength of HZSM-22 compared with SAPO-11.
These results therefore indicate that the rapid deactivation
of HZSM-22 at relatively low temperatures, such as 400 °C,
could be attributed to the blockage of the channel openings of
the catalyst. With this in mind, we conducted in situ FTIR spec-
troscopy experiments to provide evidence to support this find-
ing by monitoring changes in the Brönsted acidic sites during
the conversion of methanol over HZSM-22. The spectra col-
lected at 400 °C with time on stream are shown in Fig. 9. The
two characteristic bands of the activated sample at 3740 and
0
4000 3800 3600 3400 3200 3000 2800 2600
1
Wavenumber (cm )
Fig. 9. In situ FTIR spectra for the conversion of methanol over
HZSM-22 at 400 °C with a He/CH OH ratio of 18.
3
deactivation behavior of HZSM-22 during the MTH conversion.
The blocking of the channels with polyaromatic compounds
would lead to the rapid deactivation of the catalysts at low
temperatures, which would prevent the methanol molecules
from coming into contact with the remaining acidic sites inside
the channel after the deactivation of the catalysts.
1
2
13
3.3.
C/ C-methanol switch experiments over HZSM-22 and
SAPO-11
1
2
13
C/ C-Methanol switch experiments were performed to
elucidate the mechanism of the conversion of methanol over
the two catalysts, HZSM-22 and SAPO-11, and the results of the
isotopic distribution are shown in Fig. 10. Despite the differ-
ences in acid strength of the two catalysts, the isotopic switch-
ing experiments showed very similar isotopic distributions in
the alkene products and the retained materials (methylated
benzene) for both catalysts. The much higher 13C content in the
alkene products compared with the retained materials implied
that 13C atoms could be incorporated into alkenes with greater
ease than heavy hydrocarbons, which suggests that the alkene
3
585 cm-1 were assigned to terminal silanol groups (Si(OH))
located on the external surface and framework bridging hy-
droxyl groups (Si(OH)Al), respectively, with the latter acting as
the Brönsted acidic sites [46]. A new band appeared during the
cycle was dominant over the aromatic cycle in both catalysts.
The slightly lower 13C content in ethene compared with the C
+
3
–1
progress of the methanol reaction at 3136 cm , which is as-
signed to the C-H stretching vibrations of the aromatic species
and gradually increased in intensity with time on stream [47].
At the same time, there was a slight decrease in the intensity of
the band derived from the bridging hydroxyl group at 3585
alkenes could be related to the contribution of the arene cycle
to the formation of ethene, although this pathway would make
only a minor contribution to the formation of ethene [30].
Among the retained organics, the pentamethyl benzene (Pen-
taMB) and hexamethyl benzene (HexaMB) materials found
within SAPO-11 and HZSM-22, exhibited higher total 13C con-
tents than any of the other organic materials. However, these
two polymethylated benzene molecules cannot act as efficient
hydrocarbon pool species because of the limited chemical en-
vironment exerted by the 1-dimensional and 10-ring channel
structures of the catalysts. Taken together, these results pro-
vide conclusive proof that the alkene methylation and cracking
route is the dominating mechanism for the formation of al-
kenes over HZSM-22 and SAPO-11. These results are in good
–1
cm , which indicates that the Brönsted acidic sites were being
lost with the progress of the methanol reaction. After a reaction
time of 15 min, the intensity of the peak at 3136 cm–1 reached a
plateau, which implies that the reaction had ended because of
the deactivation of the catalyst. However, the peak at 3585 cm–1
from the bridging hydroxyl groups could still be observed quite
clearly. The consequences of these changes in the spectra for
the aromatics formed during the reaction and the uncovered
Brönsted acidic sites provide direct evidence to confirm the

Jinbang Wang et al. / Chinese Journal of Catalysis 36 (2015) 1392–1402
1399
40
30
20
10
0
1
00
(a)
Methanol + 1-Butene
8
6
4
2
0
0
0
0
0
HZSM-22
SAPO-11
HZSM-22
SAPO-11
100
(b)
80
60
40
20
0
Fig. 11. Effluent product distributions for the conversion of methanol
over HZSM-22 and SAPO-11 at 400 °C with a He/CH OH ratio of 7.
HZSM-22
SAPO-11
3
over HZSM-22 from neat methanol, as well as the co-reaction of
methanol with 1-butene. However, the conversion of methanol
over SAPO-11 was much lower than it was over HZSM-22, alt-
hough the feeding of 1-butene into the reaction mixture as a
co-reactant led to an increase in the conversion from 30.5% to
45.6%. The distribution of effluent products for the co-reaction
of methanol with 1-butene over HZSM-22 and SAPO-11 is
shown in Fig. 11. It is clear from these results that the majority
Fig. 10. Total 13C contents in the effluent products (a) and retained
12
of the effluent products formed over HZSM-22 were C
drocarbons. However, the product distribution shifted to heav-
ier hydrocarbons (i.e., C –C hydrocarbons) over SAPO-11.
3
–C hy-
5
materials (b) after a 15 min C-methanol reaction followed by a 1 min
13
C-methanol reaction over HZSM-22 and SAPO-11 at 400 °C, with a
WHSV of 2.0 g/(g·h) and a He/methanol ration of 10.1.
4
6
Furthermore, the selectivity for propene over HZSM-22 was
much higher than it was over SAPO-11, whereas the selectivity
+
agreement with those of several previously published reports
concerning the formation of alkenes over HZSM-22 [29–32].
for C
3
hydrocarbons over HZSM-22 was lower than it was over
SAPO-11. These differences indicated that the cracking reaction
for the formation of light alkenes, especially propene, was fa-
vored over HZSM-22 compared with SAPO-11, despite the fact
that the methylation of 1-butene occurs for the generation of
higher alkenes.
The distribution of the 12C content in the effluent products
was determined by GC-MS, and the results are shown in Table
3. For the reaction over HZSM-22, the 12C atoms originating
3
.4. Co-reaction of 13C-methanol and 12C-1-butene over
HZSM-22 and SAPO-11
As discussed above, the results of the 12C/ C-methanol
switching experiments demonstrated that the alkene-mediated
mechanism can run as an independent process over HZSM-22
and SAPO-11. However, the effect of the Brönsted acidic
strength of these catalysts on the conversion of methanol via
the alkene methylation and cracking route remains unclear. We
then proceeded to investigate the co-feeding of 13C-methanol
and 12C-1-butene over HZSM-22 and SAPO-11 to determine the
impact of the Brönsted acidic strengths of these catalysts on the
methylation and cracking reaction and product distribution
characteristics.
13
from 12C-1-butene were mainly located in the C
bon products, which accounts for 31.85%, 22.78% and 24.47%
of the carbon atoms based on 12C-1-butene. For SAPO-11, the
3
–C hydrocar-
5
C
4
–C
hydrocarbons contained the majority of the 12C atoms
6
and accounted for 38.68%, 37.59% and 15.27% of the 12C at-
oms, respectively. These findings therefore provided further
evidence that the conversion of methanol occurred via the al-
kene-mediated pathway and indicated that the acidic strength
had a significant impact on the cracking reactivity of heavy
Methanol can be completely converted to hydrocarbons
Table 3
1
2C distribution of co-feeding experiments of 13C-methanol and 12C-Butene (5.3 based on C atom) over HZSM-22 and SAPO-11.
12C distribution in reactor outlet products (%)
Catalyst
0
=
0
=
0
3
C
1
C
2
C
2
C
3
C
C
4
C
5
C
6
C
7
C
8
Toluene
0.48
Xylene
0.56
HZSM-22
SAPO-11
0
0
4.26
0
0
0
31.85
3.10
2.38
0.18
22.78
38.68
24.47
37.59
9.70
2.70
4.08
0.83
0.95
15.27
0.06
0.08

1400
Jinbang Wang et al. / Chinese Journal of Catalysis 36 (2015) 1392–1402
hydrocarbons, leading to the differing product distributions.
Based on these results, we can conclude that the acidic proper-
ties of zeolite catalysts have been identified as a key factor for
their reactivity and product distribution in the MTH reaction,
and that the flexible adjustment of these acidic properties could
facilitate the optimization of catalytic performance.
that the cracking of higher hydrocarbons was promoted over
HZSM-22 because of its higher acidic strength compared with
SAPO-11. This resulted in C
products for HZSM-22, while C
3
–C
5
hydrocarbons as the major
hydrocarbons were formed as
+
5
the major products for SAPO-11.
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Graphical Abstract
Chin. J. Catal., 2015, 36: 1392–1402 doi: 10.1016/S1872-2067(15)60953-6
Methanol to hydrocarbons reaction over HZSM-22 and SAPO-11: Effect of catalyst acid strength on reaction and deactivation
mechanism
Jinbang Wang, Jinzhe Li, Shutao Xu, Yuchun Zhi, Yingxu Wei*, Yanli He, Jingrun Chen, Mozhi Zhang, Quanyi Wang, Wenna Zhang,
Xinqiang Wu, Xinwen Guo *, Zhongmin Liu*
Dalian University of Technology; Dalian Institute of Chemical Physics, Chinese Academy of Sciences; University of Chinese Academy of Sciences
HZSM-22
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cracking route.

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