Activity enhancement of ZSM-35 in dimethyl ether carbonylation reaction through alkaline modifications

Article abstract of DOI:10.1039/c3ra42197j

A series of ZSM-35 zeolites with different alkaline treatment degrees were prepared. The precise effects of alkaline treatment on composition, morphology, porosity, transportation and acidity of the samples were characterized by means of multiple techniques including N₂ sorption, transmission electron microscopy (TEM), intelligent gravimetric analyzer (IGA) and Fourier transform infrared spectroscopy (FTIR). ZSM-35 after moderate alkaline treatment exhibited enhanced carbonylation activity compared with the parent sample. As revealed by the N₂ adsorption and TEM results, alkaline treatment could induce the deaggregation of ZSM-35 clusters and remove the amorphous debris on the surface of ZSM-35 platelets. Furthermore, an improved diffusion behavior of the dimethyl ether reactant molecule was observed on the alkali-treated sample from IGA experiments which directly led to the better catalytic performance for the carbonylation of the dimethyl ether with carbon monoxide. Mesoporosity created by severe alkaline leaching does not enhance the catalytic properties of ZSM-35 in dimethyl ether carbonylation reaction. Especially, a decrease in reaction stability was observed due to the limitation effect of carbonaceous deposit formation. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42197j

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Activity enhancement of ZSM-35 in dimethyl ether
carbonylation reaction through alkaline modifications
Cite this: RSC Advances, 2013, 3,
16549
Xiujie Li, Xiaohang Liu, Shenglin Liu, Sujuan Xie, Xiangxue Zhu, Fucun Chen
and Longya Xu*
A series of ZSM-35 zeolites with different alkaline treatment degrees were prepared. The precise effects of
alkaline treatment on composition, morphology, porosity, transportation and acidity of the samples were
characterized by means of multiple techniques including N2 sorption, transmission electron microscopy
(TEM), intelligent gravimetric analyzer (IGA) and Fourier transform infrared spectroscopy (FTIR). ZSM-35
after moderate alkaline treatment exhibited enhanced carbonylation activity compared with the parent
sample. As revealed by the N2 adsorption and TEM results, alkaline treatment could induce the
deaggregation of ZSM-35 clusters and remove the amorphous debris on the surface of ZSM-35 platelets.
Furthermore, an improved diffusion behavior of the dimethyl ether reactant molecule was observed on
the alkali-treated sample from IGA experiments which directly led to the better catalytic performance for
the carbonylation of the dimethyl ether with carbon monoxide. Mesoporosity created by severe alkaline
leaching does not enhance the catalytic properties of ZSM-35 in dimethyl ether carbonylation reaction.
Especially, a decrease in reaction stability was observed due to the limitation effect of carbonaceous
deposit formation.
Received 3rd May 2013,
Accepted 8th July 2013
DOI: 10.1039/c3ra42197j
www.rsc.org/advances
common feature of these two zeolites is the existence of 8-MR
channels, in which the reaction mainly occurs.15,16 ZSM-35
1. Introduction
Methyl acetate (MA) and its derivative acetic acid are
important industrial chemicals which have been widely
applied in paints, plastics and solvents of many reactions.1–3
The traditional synthesis route is based on a methanol
carbonylation process utilizing noble metal rhodium (the BP
Monsanto process) or iridium (Cativa) homogeneous cata-
lysts.3–6 Heterogeneous catalysts, alternatively, have received
considerable interest due to their virtue of eliminating
corrosive halide promoters, and the possibility of continuous
plug-flow processing and simple product separation.7,8
Recently, Iglesia and co-workers reported a new series of acid
zeolite systems for the formation of methyl acetate through
catalytic carbonylation of dimethyl ether (DME) which
provided a promising non-halide and noble metal-free route
to produce acetic acid.9–12 H-mordenite (H-MOR) is proved to
be the most active and selective catalyst for the carbonylation
reaction of DME to MA. However, the catalytic stability of
mordenite was not satisfactory and the carbonylation activity
decreased sharply upon reaction time due to its high acid
density in a 12-member ring (12-MR) channel.13 Compared
with HMOR, ZSM-35 zeolite with FER structure exhibited good
consisted of a one-dimensional channel of 8-MR and a
perpendicularly intersected one-dimensional channel of 10-
MR which could well suppress the carbonaceous deposit
formation. However, the steric pore size of 10-MR may restrict
the entrance and diffusion of reactant molecules and lead to
low carbonylation activity in the pores.
In this study, we developed an efficient alkaline treatment
method to improve the diffusion limitation of DME molecules
in the ZSM-35 channels. In previous studies, alkaline
treatment has been shown as an effective method to introduce
intra- or inter-mesopores in the zeolitic porous regular
structures, especially for that with MFI structure.17–20
Controlled extraction of silicon to produce extended meso-
porosity could be realized using proper alkali-treatment
conditions. Compared with ZSM-5 and Beta, ZSM-35 requires
harsher treatment conditions to desilicate due to the high
stability of its framework.21,22 In this contribution, we tried to
find an optimal treatment condition for ZSM-35 to enhance its
DME carbonylation activity. The influence of NaOH concen-
tration on the porous and mass transfer properties of ZSM-35
was characterized by N2 adsorption, SEM/TEM images, and
IGA measurements. The relevant acidity information was
obtained using NH3-TPD and IR spectra. Compared with the
parent sample, the modified ZSM-35 displayed high catalytic
activity in the DME carbonylation reaction on the premise of
catalytic stability in the DME carbonylation reaction.14
A
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China.
E-mail: xiujieli@dicp.ac.cn; lyxu@dicp.ac.cn; Fax: +86-411-8437 9279;
Tel: +86-411-8437 9279
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Scanning electron micrographs (SEM) were taken using an
FEI Quanta 200F field-emission microscope at an acceleration
voltage of 20 kV. Transmission electron microscopy images
were obtained with an FEI Tecnai G2 Spirit microscope at an
acceleration voltage of 100 kV.
good reaction stability. A good correlation between catalytic
performance and properties of the catalysts was proposed.
2. Experimental
NH3-TPD and TPO measurements. Temperature-pro-
grammed desorption of ammonium (NH3-TPD) measurements
were carried out in a conventional U-shaped stainless-steel
micro-reactor (i.d. = 4 mm) using flowing helium (He) as the
carrier gas. The NH3 concentration in the NH3-TPD process
was monitored by an on line gas chromatograph
(ShimadzuGC-8A) equipped with a TCD detector. Typically, a
140 mg sample was pretreated at 600 uC for 1 h in the flow of
He (25 mL min21), cooled to 150 uC and saturated with NH3.
Then, the sample was purged with the pure He stream for
certain time until a stable baseline was obtained. Subsequently
NH3-TPD experiment was carried out in the range of 150–600
2.1 Catalyst preparation
Commercially available ZSM-35 zeolites (Si/Al = 14.2) was
provided by Shanghai Novel Chemical Technology Co. Ltd.,
China. In the treatment process, ZSM-35 zeolites were treated
with NaOH solution at 80 uC for 2 h (solution/zeolite ratio of 6
cm3 21) with different concentrations. After the treatment,
g
zeolite suspension was cooled down and filtered. The resulting
solids were collected and washed with distilled water until
neutral pH was reached and dried at 100 uC for 8 h. After that,
the sample was exchanged with 0.5 M NH4NO3 solution at 80
uC for 2 h and calcined at 520 uC to make the zeolite to
transform from Na-form to H-form. According to the solution,
the alkali-treated samples were designated as AT02, AT04,
AT06 and AT08 corresponding to NaOH solution concentration
of 0.2, 0.4, 0.6 and 0.8 M L21. The parent ZSM-35 sample was
labeled as Z35.
uC at a heating rate of 18.8 uC min21
.
Temperature-programmed oxidation (TPO) experiments
were carried out in a quartz tube with 80 mg samples in it.
The temperature was increased linearly from room tempera-
ture to 850 uC under a 10% O2/He mixture (35 mL min21),
during which the m/z intensities for CO2(44), CO(28), O2(32),
and H2O(18) were recorded. The abundances of CO and CO2
were corrected by their respective response parameters.
Because CO and CO2 showed similar curve shapes, the
amount of CO was added to that of CO2 to simplify the
analysis, which was performed in terms of carbon oxides.
DME adsorption experiments. DME adsorption isotherms
were performed on an intelligent gravimetric analyzer (IGA
100) from Hiden Isochema Ltd. Usually, a 50 mg sample was
loaded into the bag. Prior to the sorption measurements,
samples were outgassed under pressure less than 1023 Pa at
400 uC for about 20 h. DME isotherms were obtained at 50 uC
and the equilibrium pressures were determined by high-
accuracy Baratron pressure transducers. Here, P0 represented
the saturated pressure of DME at 50 uC. The corresponding
curve of mass changes during the uptake process was recorded
at a DME pressure of 1000 Pa.
2.2 Catalyst characterization
XRD, N2 adsorption, ICP, MAS NMR, IR, SEM and TEM
measurements. X-ray diffraction (XRD) patterns were obtained
at room temperature on a Rigaku D/Max-RB diffractometer
using Cu-Ka radiation. Powder diffractograms of samples were
recorded over a range of 2h values from 5 to 50u under the
conditions of 40 kV and 100 mA at a scanning rate of 10u
min21. The relative crystallinity of the samples was calculated
according to the sum of the peak intensities at 2h values of 9.2u
and 25.7u.
Nitrogen sorption experiments were performed at 77 K on
an ASAP-2020 system in the static measurement mode. Tested
samples were outgassed at 350 uC for 10 h before the
measurements. Specific surface areas were calculated by the
BET method, the pore volume was determined by N2
adsorption at a relative pressure of 0.98.
Inductively Coupled Plasma (ICP) experiments were per-
formed on Shimadzu ICPS-8100 to determine the concentra-
tions of the silicon or aluminum in the solution.
2.3 Catalyst evaluation
DME carbonylation reaction was performed on a fixed bed
reactor. Typically, 7.0 g catalyst was loaded in the middle part
of the stainless reactor with 320 mm in length and 12 mm in
diameter. The reaction was performed at 220 uC, 2.0 MPa with
a DME weight hourly space velocity (WHSV) of 0.13 h21. Before
the reaction, the catalyst was pretreated at 500 uC for 2 h under
nitrogen flow to remove the water. DME feed (99.5%) was
introduced to the reactor with a plunger pump when the
temperature decreased to 220 uC. Partner reactant CO was
introduced though mass controller and the nCO/nDME (molar
ratio) was kept at 10.0. All the products were analyzed by an
Agilent-6890 gas chromatograph equipped with an FID
detector and a 100 m PONA capillary column. The activity of
catalysts was mainly evaluated by DME conversion, and the
methyl acetate selectivity with respect to the weight percent of
desired MA in the product.
NMR spectra were acquired on Varian infinity plus-400
spectrometer. 29Si MAS NMR spectra with high power proton
decoupling were recorded at 79.4 kHz using a 5.0 mm MAS
probe with a spinning rate of 6 kHz. 4,4-Dimethyl-4-silapen-
tane sulfonate sodium (DSS) was used as the chemical shift
reference. 1024 scans were accumulated with a p/4 pulse width
of 1 ms and a 4 s recycle delay. The Dmfit software was
employed for deconvolution using fitted Gaussian-Lorentzian
line shapes.23
Diffuse reflectance infrared Fourier transform (DRIFT)
spectra were recorded using Bruker Vertex 70 infrared
spectrometer in the range of 400–5000 cm21 with a resolution
of 4 cm21. Sample powder (50 mg) was charged in a DRIFT cell
with a KBr window and pretreated in dry N2 flow at 350 uC for 1
h. After the pretreatment, the sample was cooled to room
temperature to acquire spectra.
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Fig. 1 DME carbonylation activity over ZSM-35 zeolites with different alkaline treatments (reaction conditions: 220 uC, 2.0 MPa, nCO/nDME = 10/1, WHSV = 0.13 h21).
ZSM-35 should not be severely treated; 0.2–0.6 M NaOH
solutions performed preferentially.
3. Catalytic and characterization results
3.1 Catalytic performance
3.2 XRD and N2 adsorption results
Fig.
1 illustrated the general DME conversion of the
Fig. 2 shows the XRD patterns of parent ZSM-35 and its
derivatives using different alkaline treatments. Typical diffrac-
tion peaks assigned to FER structure at 9.2, 25.2 and 25.7u
could be observed on the ZSM-35 sample. Mild alkaline
washing does not trigger the relative crystallinity. Interestingly,
AT04 exhibits an increasing trend and its relative crystallinity
increases to 105% compared with parent Z35 zeolite. The
diffraction pattern of AT08 shows a strong loss of peak
intensity and the crystallinity decreases to 82% indicating the
disappearance of the zeolitic phase due to partial dissolution
in strong alkaline solution media.
carbonylation reaction on ZSM-35 samples. For the parent
ZSM-35, an induction period of about 5 h is observed after
introduction of DME and CO. This result agrees well with the
catalytic behavior of MOR zeolite reported by Cheung et al.9,10
During the induction period, the DME molecule interacts with
the acidic proton of the zeolite to form a methoxy group and
one methanol molecule. Due to the side reaction yielding
methanol molecules, the desired product selectivity of MA was
slightly lower in the induction period. When the acid sites are
saturated with methyl groups, DME carbonylation reaction
kicks off in a steady state and CO insertion becomes the rate-
limiting step.10 DME conversion on ZSM-35 is 36.5% and it
remains at 32.3% after 66 h on stream, exhibiting robust
catalytic stability. The product selectivity for MA is higher than
98.0% during the steady state which is as good as the initial
MA selectivity on H-MOR. Besides methanol, the side products
include trace hydrocarbon and its content gradually amounts
to zero upon reaction time. After alkaline treatment, an
improved catalytic performance is observed on AT02 and the
DME conversion increases from 36 to 42% with a growth rate
of 17%. Usually, alkaline treatment could increase the catalytic
stability due to the improved molecular diffusion behavior
across zeolitic pores. Here, a pronounced increasing trend of
the catalytic activity is observed on AT02 indicating that mass
transfer constraints are important factors for the DME
carbonylation reaction on ZSM-35. The promoting effect of
alkaline modification is more apparent on the AT04 sample
and DME conversion substantially increases up to 43.6%.
Meanwhile, the catalytic stability is comparable to that of the
parent sample. However, the stronger severity of the alkaline
treatment could weaken the carbonylation activity as shown in
AT06. The activity of the AT08 sample dropped even faster
than AT06, and the conversion subsequently decreases from
38.6 to 32.0% after 30 h on stream. The MA selectivity on AT08
is lower than that of other samples. Hence, desilication on
The comparison of N2 adsorption-desorption isotherms are
depicted in Fig. 3. For the parent ZSM-35, its N2 adsorption
curve deviated from type I, especially at high relative pressure
Fig. 2 XRD patterns of the parent and alkaline-treated ZSM-35 samples.
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images also reveal the crystal deaggregation phenomena as
shown in Fig. 5. It is interesting that some tiny amorphous
debris exist on the surface of the ZSM-35 platelets on Z35-b.
These species disappear after alkaline washing as shown on
AT04-b. This provides direct evidence for the purification
effect of alkaline cleaning. Meanwhile, it could be clearly
observed that the attacking of NaOH starts from the edge of
the crystals and some fissures are formed which are labeled
using arrows in AT04-b. The external surface of AT08 is
significantly roughened compared with the parent sample.
Besides fissures, some small holes are present on the surface
due to harsher desilication conditions which are perpendicu-
lar to the crystal planes.
3.4 29Si MAS NMR spectra
29Si MAS NMR spectra were acquired to obtain more
information about the framework desilication. As shown in
Fig. 6, there exist obvious differences in the total peak
intensities of Z35, AT04 and AT08. This may be related to
the different relaxation time of Si atoms of the catalysts. It has
been proved that the existence of paramagnetic molecular
oxygen or water molecules in the zeolite channel could reduce
the 29Si spin–lattice relaxation time.24,25 Compared with Z35,
amorphous debris on AT04 is removed during the alkaline
treatment process and its micropore area also increased, as
evidenced by the TEM and N2 adsorption results, which may
increase the concentration of the adsorbed molecule. On
premise of the same amount of pulse delay during the NMR
experiment, 29Si MAS NMR peak intensity of AT04 is enhanced
due to its short relaxation time. In the case of AT08, its peak
intensity drops down sharply. This may be due to the
destruction of the microporous structure, which leads to a
longer relaxation time of silicon atoms.
The resolution of five different Si sites, characteristic of the
siliceous ferrierite, is not achieved because of the incorpora-
tion of aluminum into the framework. Two groups of T sites in
the Si(0Al) region could be differentiated at around 2112 and
2115 ppm. And the peak intensity loss is observed more
obviously at the position of 2115 ppm. For comparison, all the
spectra were deconvoluted and listed in the left upper corner
of Fig. 6. According to ref. 26 and 27, group TA comprises the
sites Si2 + Si3 and the group of type TB comprises Si5 + Si4 +
Si1. The ratio of group TA in the 29Si MAS NMR is all around
30% before and after modification. The ratio of TB decreases
from 46.9 to 42.3% after NaOH treatment indicating the
preferential extraction of Si atoms on special T sites. The
framework Si/Al ratio values are calculated and listed in
Fig. 3 N2 adsorption isotherms of the parent and alkaline-treated ZSM-35
samples. BJH pore size distributions derived from the adsorption branch are
depicted at the upper left corner.
due to its small thickness of the platelet crystals and the
existence of secondary cumulative pores. After alkaline
treatment, an increase in nitrogen uptake is observed between
intermediate and high pressure regions. As summarized in
Table 1, the microporous area increases from 263 to 289 m2
g21 after treatment with 0.4 M L21 NaOH solution which may
be attributed to the purification of the zeolite channel by
cleaning the amorphous particles. The micropore volume
follows the same trend. The observed tendency of increasing
adsorption amount of nitrogen is even noticeable on AT08 and
its hysteresis loop begins at a P/P0 value of 0.45. A substantial
increase in the external surface is also observed on AT08,
indicating the introduction of mesopores. From the BJH pore
distribution profiles listed in the left upper corner of Fig. 3, it
seems that a broad distribution of mesopores is observed.
3.3 SEM and TEM images
Scanning and transmission electron microscopes were applied
to study the morphological changes in the alkaline-treated
ZSM-35 samples. As shown in Fig. 4, NaOH washing leads to
an impressive size decrease of the aggregated particles. For
ZSM-35, the original particle size is around 8–12 mm and it
decreases to 5–8 mm in the case of AT02. The particle size
becomes much smaller upon the NaOH solution concentration
of alkaline treatment. At high magnifications, SEM images of
ZSM-35 with different treatment conditions are similar and no
obvious platelet solution phenomena are observed. TEM
Table 1 Textural property of ZSM-35 zeolites with different alkaline treatments
BET surface
area (m2 g21
Micropore
External surface
Pore volume
Micropore
Sample
)
area (m2 g21
)
area (m2 g21
)
(cm3 g21
)
volume (cm3 g21
)
HZ35
AT02
AT04
AT06
AT08
350
361
380
363
371
263
289
289
276
254
87
72
91
87
117
0.4393
0.4638
0.3981
0.3829
0.4205
0.1193
0.1311
0.1312
0.1252
0.1152
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observed on the parent ZSM-35 sample. The band at 3745
cm21 is assigned to the isolated silanol groups and the 3600
cm21 band is from the contribution of framework aluminum
hydroxyl i. e. Brønsted acid sites.21 Besides, a broad side peak
at around 3645 cm21 could be discriminated due to the
existence of extra-framework aluminum hydroxyl groups.28
Upon mild alkaline treatment, an increase in peak intensity at
3600 cm21 is discerned. This may be correlated with the
removal of amorphous Si(Al) debris from the zeolite channel.
Moderate alkaline washing could increase acid density per
unit mass of the alkali-treated samples. This conclusion is
further supported by the NH3-TPD results. As shown in
Fig. 7(b), the total uptake of ammonia increases from 0.760
on Z35 to 0.915 mmol g21 on AT02. The desorption peak
intensity at around 500 uC becomes stronger after alkaline
treatment, suggesting the increasing number comes from
strong acid sites which is in accordance with the IR results.
Meanwhile, the band intensity of the isolated silanol groups at
3745 cm21 of AT08 is obviously enhanced due to the severe
silicon extraction.
Fig. 4 SEM images of the parent and alkaline-treated ZSM-35 samples.
3.6 Diffusive properties
It is generally accepted that alkaline treatment could improve
the mass transfer ability of the parent zeolite.29 Here, DME was
chosen as the probe molecule to measure the adsorption
behavior of ZSM-35 samples under different alkali-treatment
conditions as it is the reactant molecule in the carbonylation
reaction. Intelligent gravimetric analysis (IGA), which shows
high sensitivity and mass resolution, is applied to detect the
mass changes during the adsorption process.30,31
Table 2. It is clear that part of the framework Si is extracted
from the framework of the AT08 sample.
3.5 Acidity information: NH3-TPD and IR results
The acidity of the ZSM-35 samples was studied by FT-IR and
NH3-TPD spectra. Fig. 7(a) shows the infrared spectra of the
alkali-treated ZSM-35 samples in the OH-stretching region.
Two characteristic peaks at 3745 and 3600 cm21 could be
Fig. 5 TEM images of the parent and alkaline-treated ZSM-35 samples.
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multistep post-synthesis route (including NaAlO2, HCl and
NaOH) to prepare hierarchical ferrierite porous materials with
an Si/Al ratio of 27.22 In the case of ZSM-35 with an Si/Al ratio
of 14, the mechanism of introduction of intracrystalline
mesoporosity is similar to that of ferrierite zeolite. The
pronounced effect of alkaline treatment under mild treatment
conditions (0.2–0.4 M) is the deaggregation of the large
particles and cleaning of zeolite channels. The amorphous Si
(Al) debris is removed from the surface of the thin crystal sheet
which may increase the internal surface area and pore volume
of Z35. Such cleaning effect was also observed on MCM-49
zeolites in our group and Xu et al. found that its relative
crystallinity increased under mild alkaline treatment condi-
tions.33 Compared with MCM-49, higher NaOH concentration
is needed to obtain the cleaning effect on ZSM-35. NaOH
solution washing was also applied to large ferrierite crystals by
Weitkamp et al. to obtain a clean surface.34 From the chemical
composition analysis of the filtrates shown in Table 2, the
main composition of the washing debris is silicon and the
aluminum content is below 5% on AT02. The removal of
amorphous Si debris in the zeolite channel leads to an
increase in micropore area and acidity density which may
contribute to the improvement of the carbonylation activity.
As pointed out by Iglesia et al., the DME carbonylation
reaction was directly related to the number of proton sites
within the 8-MR channel and the CO molecule preferentially
attacked the methoxy groups.11,12 Only the surface acetyl
species in the 8-MR channel could react with DME to form the
objective MA molecule.35 The T3-O33 site inside the 8-MR
pockets of MOR was selective for the carbonylation reaction
due to its beneficial special orientation for attacking CO.15,16
For ZSM-35, its 8-MR channel is interconnected with a 10-MR
channel. The small size of 10-MR may suppress the formation
of carbonaceous species leading to better catalytic stability
compared with MOR zeolite whose 8-MR channel is connected
with a 12-MR. The 10-MR channel in ZSM-35 also constrains
the diffusion of the MA product molecule which dramatically
suppresses its carbonylation activity. Removal of Si debris and
formation of mesopores facilitates the transportation of
reactant DME and product MA molecules. The pronounced
increase in carbonylation activity of AT04 is due to the
enhanced number of adsorption sites and an improved
diffusion of DME as shown in Fig. 7.
Fig. 6 29Si MAS NMR spectra of the parent and alkaline-treated ZSM-35
samples.
Fig. 8(a) illustrates the adsorption isotherms of DME on
ZSM-35 samples. The isotherm of Z35 shows typical Langmuir
Type-I behavior, which is saturated at low pressure and then
increases slightly with further increase of pressure. The
isotherm for AT04 is similar to that of Z35 except for a small
increment in DME uptake. AT06 and AT08 also show a higher
adsorption capacity of DME than that of Z35. The time
dependence of DME sorption on parent ZSM-35 and alkali-
treated samples are compared in Fig. 8(b). Under fixed certain
pressure, the uptake amount of AT-Z35 is higher than that of
Z35. Meanwhile, the adsorbing uptake rate of DME is highly
enhanced on AT02 and AT04, especially during the initial 15
min. The fast diffusion rate in AT04 may be attributed to
deaggregation of large particles and elimination of amorphous
Si (or Al) debris on the crystal surface.
4. Discussions
Alkaline treatment has been proved to be an efficient
modification method to improve the mass transfer properties
of microporous zeolites. For ZSM-35 zeolites with FER
structure, relatively harsh conditions were needed for meso-
pore formation due to its special sub-unit of 5-MR and high
stability of the framework aluminum.21,32 In order to control
the introduction of mesoporosity, Perez-Ramirez proposed a
When severe alkaline treatment conditions (e.g. AT08) are
applied to ZSM-35 zeolite, a large amount of silicon is
Table 2 Chemical compositions of the alkali-treated ZSM-35 zeolites and resulting filtrates
Sample
Si/Ala
Si/Alb
[Si]a
(ppm)
[Al]a
(ppm)
filtrate
Yieldc (%)
solid
framework
filtrate
Z35
14
18.3
—
17.7
14.5
—
742
6 028
13 003
—
33
48
36
—
AT02
AT04
AT08
13.9
12.9
10.2
94.7
91.8
85.4
a
b
c
Determined from ICP measurements. Determined from 29Si MAS NMR spectra. Grams of solid after alkaline treatment per gram of
parent zeolite.
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Fig. 7 (a) DRIFT spectra and (b) NH3-TPD profiles of the parent and alkaline-treated ZSM-35 samples.
Fig. 8 (a) DME adsorption isotherms and (b) adsorption rate on the parent and alkaline-treated ZSM-35 samples at 50 uC detected by IGA.
extracted from the framework. Its Si/Al ratio decreases from 14
to 10.2 and the product yield after treatment is around 85%, as
shown in Table 2. An obvious increase in external surface area
is observed. From the pore distribution curve, a broad
distribution of large mesopores is observed on AT08 which is
quite different from alkali-treated MFI zeolites. This is related
to its particular crystal morphology consisting of thin platelets
and Bonilla et al. also observed such phenomena.21 The
relative crystallinity of AT08 also decreases under such
conditions, as shown in the XRD patterns. Under mild
treatment conditions, fissures and cracks are first observed
on AT04 due to the preferential silicon extraction at the edge of
the platelets. Harsher treatment (AT08) results in the
appearance of perforated ZSM-35 platelets. Thus, the dissolu-
tion of silicon occurs from surface to the core of the platelets.
The formation of mesopores also leads to an increase in the
external surface area and total pore volume compared with
that of AT04. It should be pointed out that the excessive
extraction of silicon is not beneficial for the stabilization of the
carbonylation intermediate. The limitation effect on the
Fig. 9 TPO profiles of the spent ZSM-35 catalysts (in 35 mL min21 10% O2/He
mixture, heating rate 10 uC min21).
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carbonaceous deposits is weakened, which is directly linked
with its poor catalytic stability.
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carbonylation, TPO profiles of the spent ZSM-35 samples were
acquired and shown in Fig. 9. Two main peaks at around 320
and 600 uC are observed on Z35. According to Shen et al.,13,14
the oxidation peak at 320 uC is mainly from the surface-bound
methyl and acetyls related to the formation of MA. The high
temperature peak at around 550 uC may be associated with the
heavy coke formed during the course of reaction. The hard
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´
´
´
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5. Conclusions
Through varying the NaOH concentration in the range of 0.1–
0.8 M, a series of ZSM-35 zeolites with different porous
structures were prepared. The mildly alkali-treated ZSM-35
samples exhibited higher carbonylation activity than the
parent sample due to the impressive size decrease of the
deaggregated particles and removal of amorphous Si(Al) debris
on the crystal surface. An enhanced mass transfer behavior of
DME molecule was observed on the AT04 sample. Severe
treatment (AT08) leads to the excessive extraction of frame-
work silicon which weakens the transition state shape selective
and coke suppression functions. An enhanced number of
accessible acid sites is more important than the formation of
mesopores on ZSM-35 zeolites in the DME carbonylation
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