Journal of Energy Chemistry 22(2013)769–777
Catalytic performance of hierarchical H-ZSM-5/MCM-41 for
methanol dehydration to dimethyl ether
Yu Sang,
Hongxiao Liu, Shichao He, Hansheng Li∗ , Qingze Jiao, Qin Wu,
Kening Sun
School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, China
[ Manuscript received January 3, 2013; revised February 22, 2013 ]
Abstract
Micro-mesoporous composite molecular sieves H-ZSM-5/MCM-41 were prepared by the hydrothermal technique with alkali-treated H-ZSM-5
zeolite as the source and characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy,
X-ray diffraction, N2 adsorption-desorption measurement and NH3 temperature-programmed desorption. The catalytic performances for the
methanol dehydration to dimethyl ether over H-ZSM-5/MCM-41 were evaluated. Among these catalysts, H-ZSM-5/MCM-41 prepared with
NaOH dosage (nNa /nSi ) varying from 0.4 to 0.47 presented excellent catalytic activity with more than 80% methanol conversion and 100%
dimethyl ether selectivity in a wide temperature range of 170−300 ◦ C, and H-ZSM-5/MCM-41 prepared with nNa /nSi = 0.47 showed constant
methanol conversion of about 88.7%, 100% dimethyl ether selectivity and excellent lifetime at 220 ◦ C. The excellent catalytic performances
were due to the highly active and uniform acidic sites and the hierarchical porosity in the micro-mesoporous composite molecular sieves. The
catalytic mechanism of H-ZSM-5/MCM-41 for the methanol dehydration to dimethyl ether process was also discussed.
Key words
hierarchical porosity; H-ZSM-5; composite molecular sieve; methanol dehydration; dimethyl ether
1. Introduction
Recently, environmentally benign and economical alternative fuel has received global attention because of the limited
oil reserves in the world and stringent environmental regulations. Dimethyl ether (DME) can be used as the most promising candidate for diesel engines due to its excellent behavior
in compression ignition combustion [1]. Accordingly, the synthesis of DME has drawn wide attention.
At present, there are two main methods to produce DME:
two-step DME synthesis with methanol as interim material
(MTD) [2] and direct DME synthesis from synthesis gas
(STD) over a hybrid catalyst comprising a methanol-synthesis
catalyst and a solid acid in a single reactor [3,4]. Both of these
two processes involve methanol dehydration, which plays a
key role in catalyst longevity, DME productivity and production costs. Besides, methanol dehydration prefers low temperature reaction environment, as it is an exothermic reaction.
[5]. Thus, in view of reaction thermodynamics and economy
of DME production, low-temperature, high activity and high
stability are essential for developing a solid acidic catalyst for
the MTD process or for the process which fits well with the
methanol-synthesis catalyst such as Cu/ZnO/Al2 O3 [6] in the
STD process.
∗
Up to now, many methanoldehydration catalysts have
been examined, for instance, γ-alumina [7−10], aluminasilica mixtures [11], aluminium phosphate [12], molecular
sieves [13], etc. It is generally accepted that Brönsted acid or
Lewis acid sites are the active sites for methanol dehydration
to DME [14]. The stronger the acidity of the active sites, the
higher the catalytic activity for methanol dehydration to DME.
However, many secondary reactions, especially coking reaction, usually take place on the sites with strong acidity during
the catalytic reaction of methanol to DME process, due to the
non-uniform distribution of acidic strength on the surface of
solid acids, and result in the decrease of DME selectivity and
catalyst deactivation [15,16].
Molecular sieves have been widely used in heterogeneous catalysis and H-ZSM-5 molecular sieves have been reported by many research groups to be excellent dehydration
catalysts with low-temperature activity superior to γ-Al2 O3 .
Many research results [17−19] show that H-ZSM-5 with the
SiO2 /Al2 O3 ratio of 15−25 present a good catalytic activity
and stability for the MTD process. Besides, the catalytic activity decreases while DME selectivity shows an upward trend
as the Si/Al ratio increases. However, H-ZSM-5 has a certain
anti-coking capability due to the shape effect, secondary
Corresponding author. Tel/Fax: +86-10-68918979; E-mail: [email protected]
Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
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Yu Sang et al./ Journal of Energy Chemistry Vol. 22 No. 5 2013
products of hydrocarbons and coke are also generated when
H-ZSM-5 is used as the catalyst, lowering the DME selectivity to be less than 100%, due to the existing strong acid
sites on the surface. Selective poisoning H-ZSM-5 with Na
[20] or modifying it with Ti [21] or P [22] can effectively
improve the selectivity to DME. Na-modified H-ZSM-5 catalysts (0−80 mol%) prepared by the impregnation method
have higher activities than γ-Al2 O3 , and still present more
than 80% of methanol conversion, 100% DME selectivity
and a good water resistance at 230−340 ◦ C after 65 h, due to
the elimination of acid sites by partly replacing with Na and
avoiding the production of coke and/or hydrocarbons [20]. Timodified H-ZSM-5 catalyst (Ti-ZSM-5, Si/Al = 50−200 and
Si/Ti = 70, prepared by the sol-gel process associated with microwave radiation) shows a moderate activity and selectively
produces DME because of the insertion of Ti4+ and Al3+ into
the framework of Ti-ZSM-5 [21]. In P-modified H-ZSM-5
catalyst (P/ZSM-5), the interaction of P with the framework
of H-ZSM-5 results in decreasing the acid strength and generating new acid sites, and P/ZSM-5 shows lower acidity, higher
hydrothermal stability and improved DME selectivity. The
optimal activity was obtained at the P/Al ratio of 1.05 in respect to H-ZSM-5 [21]. However, the modified H-ZSM-5 are
still microporous zeolites like H-ZSM-5, and their small microporous channels in which the size of the reactants and the
micropore diameter are comparable and products like DME
does not diffuse quickly enough [23]. This causes modified
H-ZSM-5 to lose catalytic activity and selectivity quickly because many byproducts and carbon deposits are produced during the catalytic process.
For a given zeolitic material, the basic strategy to improve
diffusion is to shorten the length of the micropore channels or
to widen the pore diameter [24,25]. Zheng et al. [26] combine
β-zeolite and mordenite zeolite to form composite molecular
sieves (BMZ) with microporous and mesoporous structures,
along with the controllable Lewis/Brönsted acid by two-stage
hydrothermal crystallization to improve the deficiency of microporous molecular sieve. H-BMZ shows an excellent performance that: 90% methanol conversion and 100% DME selectivity are obtained during 197−275 ◦ C, and more than 80%
methanol conversion and 100% DME selectivity at 275 ◦ C are
achieved after 72 h. The authors attributed the high activity of
H-BMZ to the high amount of total acid and the secondary
mesoporous structure, and the high DME selectivity was ascribed to high Lewis/Brönsted acid ratio and microcrystals in
H-BMZ. Tang et al. [27] synthesize micro-mesoporous composite molecular sieves H-ZSM-5/MCM-41 by self-assembly
in nanoscale which show high methanol conversion, 100% selectivity and a long lifetime in a wide temperature range.
In this work, a series of micro-mesoporous composite
molecular sieves H-ZSM-5/MCM-41 were prepared by the
hydrothermal technique with alkli-treated H-ZSM-5 zeolite as
the source. The influences of the dosage of NaOH solution
on the structure and surface acidity of H-ZSM-5/MCM-41,
and the catalytic performance for the methanol dehydration
to DME were discussed. Besides, a catalytic mechanism for
methanol dehydration to DME over H-ZSM-5/MCM-41was
put forward based on the obtained results.
2. Experimental
2.1. Catalyst preparation
The micro-mesoporous H-ZSM-5/MCM-41 composite
molecular sieves were prepared by the hydrothermal technique using alkli-treated H-ZSM-5 zeolite as the source
[28,29]. Samples of 2.0 g H-ZSM-5 zeolite with SiO2 /Al2 O3
of 38 (The Catalyst Plant of Nankai University) were alkalitreated with 1.5 mol/L NaOH solution (Sinopharm Chemical Reagent Co., Ltd.) with the molar ratio nNa /nSi of 0.4,
0.47, 0.6, 0.8 and 1.0 at 40 ◦ C for 60 min. A zeolite solution
consisting of aluminosilicate fragments was formed. 25 mL
10 wt% cetrimonium bromide (CTAB, Sinopharm Chemical
Reagent Co., Ltd.) solution was added into the above solution and stirred for 60 min. Then, the resulting solution
was placed in an autoclave with trifluoroethanol lining and
crystallized at 110 ◦ C for 24 h. Further crystallization under
110 ◦ C for 24 h was carried out after cooling the reactor and
adjusting the pH value of the solution to 8.5. As crystallization was completed, Na-ZSM-5/MCM-41 composite molecular sieves were obtained after the solid product was filtered,
washed, dried, and calcined in air at 550 ◦ C for 6 h. Finally,
the Na-ZSM-5/MCM-41 was treated with 1.0 mol/L NH4 Cl
solution, and then filtered, washed, dried and calcined in air
at 550 ◦ C for 2 h to obtain the composite molecular sieves
H-ZSM-5/MCM-41.
2.2. Catalyst characterization
Scanning electron microscopy (SEM) was performed
with an FEI Quanta FEG 250 scanning electron microscope
operated at 20 KV to examine the surface topography of the
samples. Energy dispersive spectroscopy (EDS) equipped on
this instrument was used to obtain the SiO2 /Al2 O3 ratio of
H-ZSM-5 and H-ZSM-5/MCM-41 composite zeolites. Transmission electron microscopy (TEM) was operated at 120 kV
on a JEM-2010 transmission electron microscope.
X-ray diffraction (XRD) analysis was performed on an
X’Pert Pro MPD powder X-ray diffractometer system (40 kV,
40 mA) using a Cu Kα radiation source and a nickel filter in
the 2θ range of 0.5o –6o and 5o –80o.
Infrared spectroscopic (IR) analysis was carried out on
a PekinElmer spectrometer with a resolution of 4 cm−1 and
scanning range from 4000 cm−1 to 450 cm−1 . Before test, the
samples were mixed with KBr and then pressed into self supporting wafers.
N2 adsorption-desorption measurement was performed
on a Quantachrome autosorb iQ instrument at 77 K. The samples were degassed in vacuum for 3 h at 573 K. The total
surface area was determined by the BET method, based on
p/p0 data in the range of 0.05–0.3. The micropore volume
was obtained from the t-plot method. The non local density
functionol theory (NLDFT) model applied to the adsorption
branch of the isotherm was used to obtain the pore size distribution of composite molecular sieves and the Saito-Foley
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(SF) model was used to obtain the pore size distribution of
H-ZSM-5.
NH3 temperature-programmed desorption (NH3 -TPD)
was carried out using a Quantachrome ChemBET 3000 with
a thermal conductivity detector (TCD). 80 mg sample was
placed in a quartz tubular reactor and pretreated at 600 ◦ C with
a N2 flow of 30 mL/min for 1 h and then cooled to 100 ◦ C.
Ammonia diluted with Ar (5% v/v NH3 ) was then introduced
at a flow rate of 30 mL/min for 1 h at 100 ◦ C and then a He
stream was fed in until a constant signal of TCD was obtained.
NH3 -TPD was carried out with the reactor temperature at a
ramp rate of 10 ◦ C/min from 100 ◦ C to 700 ◦ C.
the components in the effluent with a sampling frequency of
0.05 min−1 . The atomic balances were satisfied with a deviation of less than 5%. The methanol conversion (Xmethanol ) and
DME selectivity (SDME ) was defined as follows:
2.3. Catalytic performance evaluation
3. Results and discussion
The performance of catalysts for methanol dehydration
was evaluated on a micro-reactor system. 0.5 g catalyst (about
1.5 cm height) was placed in the middle of a stainless steel
tubular reactor with quartz sand and glass beads packed at the
two ends. Thermocouple was placed in the middle of catalyst. The catalyst was pre-treated at 400 ◦ C in a N2 flow of
30 mL/min for 4 h. As the reactor temperature was cooled to
170 ◦ C, the methanol feed was input by a micro-liquid pump
(LabAlliance Series II, America) at a flow rate of 0.1 mL/min,
vaporized and then reacted on H-ZSM-5/MCM-41 or H-ZSM5 catalysts for 6 h at each set reaction temperature during the
range from 170 ◦ C to 300 ◦ C at 0.1 MPa. The lifetime of HZSM-5/MCM-41 was investigated at 220 ◦ C and the reaction
ran 500 h. An online Techcomp GC 7890T gas chromatograph equipped with a TCD detector and a Porapak T column (60−80 mesh, φ3×5000 mm) was connected to analyze
3.1. Structure of ZSM-5/MCM-41
Xm = (Fm,in − Fm,out) ÷ Fm,in
(1)
SDME = 2FD,out ÷ (Fm,in − Fm,out)
(2)
where, Fm,in , Fm,out and FD,out are the molar flow of methanol
at inlet, outlet and the molar flow of DME at outlet, respectively.
The surface topography, chemical component, crystal
phase and pore size distribution of the composite molecular sieves H-ZSM-5/MCM-41 prepared with H-ZSM-5 were
characterized by SEM, TEM, EDS, XRD, IR analysis, and N2
adsorption-desorption measurement, in comparison with the
corresponding H-ZSM-5.
H-ZSM-5 has typical micropores with the characteristics
of being circular Z type with a crossover structure, but has
no ordered mesopores. Figure 1 and Figure 2 show the SEM
and TEM images of H-ZSM-5, respectively, and their corresponding H-ZSM-5/MCM-41 composite molecular sieves
prepared with nNa /nSi of 0.4, 0.47, 0.6, 0.8 and 1.0. Figure 2 also showed the TEM images of alkli-treated H-ZSM-5
prepared under the same conditions as the first crystallization
Figure 1. SEM images of (a) H-ZSM-5 and H-ZSM-5/MCM-41 prepared by alkli-treatment with nNa /nSi of (b) 0.4; (c) 0.47; (d) 0.6; (e) 0.8; and (f) 1.0
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Figure 2. TEM images of (a) H-ZSM-5, (b) alkli-treated H-ZSM-5; H-ZSM-5/MCM-41 prepared by alkli-treatment with nNa /nSi of (c) 0.4, (d) 0.47, (e) 0.6, (f)
0.8, and (g) 1.0; (h) MCM-41
process without CTAB for preparing H-ZSM-5/MCM-41 and
MCM-41. H-ZSM-5 molecular sieves are easily aggregated
to form cubic particles, as shown in Figure 1(a). As H-ZSM-5
was treated by NaOH, H-ZSM-5 cubic particles were disintegrated gradually with the increase of the dosage of NaOH
solution, and when nNa /nSi was equal to 1.0, most of the cubic particles were disintegrated and assembled into new particles as shown in Figure 1(b) to Figure 1(f). The difference in
the zeolite samples became more obvious from Figure 2(c) to
Figure 2(g) and the characteristics of MCM-41 became more
evident, reflecting the effect of the alkali-treatment. The pores
arrayed hexagonally along the direction of the pore while
one-dimensional lines were found in a regular arrangement in
the direction perpendicular to the pores in H-ZSM-5/MCM41, which is characteristic of the pores of MCM-41 [30].
Furthermore, the TEM image of the alkali-treated H-ZSM-5
displayed nanosized H-ZSM-5 nanoparticles. The H-ZSM5/MCM-41 composite molecular sieves and MCM-41 had a
similar morphology as shown in TEM images. These proved
that the alkali-treatment of H-ZSM-5 with the tested dosages
of NaOH solution can surely bring a change in the zeolites that
MCM-41 with a mesoporous structure is introduced around
H-ZSM-5 crystal particles.
Figure 3 shows the XRD patterns of H-ZSM-5, MCM-41
and H-ZSM-5/MCM-41 samples that were prepared by alkalitreatment with different dosages of NaOH solution. From
the high-angle XRD results as shown in Figure 3, the two
diffraction peaks between 7o and 10o and three diffraction
peaks between 22.5o and 25o were found in H-ZSM-5 and
H-ZSM-5/MCM-41 samples that were prepared by alkalitreatment with NaOH dosage increasing from 0.4 to 0.6,
which were the characteristic peaks of H-ZSM-5 molecular
sieve, corresponding to the (101), (020), (501), (151) and
(303) crystal faces [31], respectively. As NaOH dosage was
increased from 0.4 to 0.6, the diffraction peaks of the (101),
(020), (501), (151) and (303) crystal faces remained but became weaker. When NaOH dosage was 0.8, the diffraction
peaks of the (151) and (303) crystal faces disappeared. None
of the five diffraction peaks were seen as NaOH dosage was
1.0, indicating that the skeleton structure of ZSM-5 was destroyed completely by NaOH. From the low-angle XRD results as shown in Figure 3, when NaOH dosage increased from
0.4 to 1.0, the characteristic peak of the (100) crystal face assigned to the hexagonal mesopore structure of MCM-41 appeared [32]. In comparison with MCM-41, the diffraction
peak of the (100) crystal face of H-ZSM-5/MCM-41 samples shifted to the high angle and the diffraction peaks of the
(110) and (200) crystal faces disappeared. It is due to that
the alkali-treatment exerted certain influence on the assembling micelles in the hydrothermal process and the following
formation of mesopores. The results indicate that part of HZSM-5 were disintegrated into Si-Al nanoclusters and formed
the hexagonal mesopore structure in the presence of CTAB
templates [33]. It indicated that NaOH dosage had an important effect on the skeleton structure of H-ZSM-5 and the
proper NaOH dosage in alkali-treatment was favored to the
formation of composite molecular sieves.
Figure 4 shows the IR spectra of H-ZSM-5, MCM-41
and H-ZSM-5/MCM-41 samples that were prepared by alkalitreatment with different dosages of NaOH solution. The absorption band appared at 1230 cm−1 as shown in Figure 4(a)–
Figure 4(f), which was assigned to the T–O–T (T is Al or
Si) asymmetric stretching mode. As shown in Figure 4(a),
the absorption band at 550 cm−1 appeared in ZSM-5, which
was the characteristic peak of the five-membered ring of HZSM-5 skeleton structure [34]. Comparing with H-ZSM5, as NaOH dosage increased from 0.4 to 0.8, the absorption band at 550 cm−1 as shown in Figure 4(a)–4(e) became weaker. It indicated that these samples contained the
Journal of Energy Chemistry Vol. 22 No. 5 2013
primary or secondary structural units of H-ZSM-5, but the
skeleton structure of H-ZSM-5 changed gradually with the
increase of NaOH dosage. When NaOH dosage was 1.0,
the absorption band at 550 cm−1 in Figure 4(f) disappeared,
773
which implied that the skeleton structure of H-ZSM-5 was
destroyed completely. These were consistent with the XRD
results.
Figure 3. Wide- (a) and small-angle (b) XRD patterns of (1) H-ZSM-5 and H-ZSM-5/MCM-41 prepared by alkli-treatment with nNa /nSi of (2) 0.4, (3) 0.47,
(4) 0.6, (5) 0.8, and (6) 1.0; and MCM-41 (7)
Figure 4. IR spectra of (1) H-ZSM-5 and H-ZSM-5/MCM-41 prepared by
alkli-treatment with nNa /nSi of (2) 0.4, (3) 0.47, (4) 0.6, (5) 0.8, and (6) 1.0;
and MCM-41 (7)
The N2 adsorption-desorption isotherms of H-ZSM-5 and
H-ZSM-5/MCM-41 and their pore size distributions obtained
based on the adsorption branch of isotherm were displayed
in Figure 5. The N2 adsorption-desorption isotherm of HZSM-5 as shown in Figure 5 was type I, which was typical
of microporous zeolites. The adsorbed amount of N2 kept at
the level of 100 cm3 /g, which was the value of the filling volume of micropores. The N2 adsorption-desorption isotherm
of H-ZSM-5/MCM-41 was type IV, which was typical of
mesoporous zeolites, and it indicated the existence of meso-
pores. In the low pressure range (p/p0<0.45), the adsorbed
amount of N2 increased linearly with pressure. This was due
to monolayer adsorption of N2 on the walls of the pores. In
range of 0.45<p/p0<0.95, there was a jump in the adsorbed
amount because N2 began filling the mesopores. Multilayer
adsorption of N2 in the mesopores occurred when p/p0 became higher. As showed in Figure 5, the average diameter of
the micropores in all samples was 0.8 nm, which was larger
than the actural pore size of H-ZSM-5 due to the calculation error. In the H-ZSM-5/MCM-41 samples, the micropores
still existed, but the microporous volume reduced with the increase of the dosages of the NaOH solution as nNa /nSi was
lower than 0.8. When nNa /nSi was equal to 1.0, the micropores disappeared, which indicated that original microporous
structure had been destroyed. All H-ZSM-5/MCM-41 composite zeolites had the dual mesopores with pore diameters of
3.5 nm and 5.1 nm. It further proved that the alkali-treatment
by NaOH solution with different dosages resulted in the destruction of microporous structure of H-ZSM-5, and the mesoporous structure of MCM-41 was formed in the following hydrothermal process. From the textural properties of H-ZSM-5
and H-ZSM-5/MCM-41 composite molecular sieves that were
prepared with different dosages of NaOH solution as shown in
Figure 6, it can be found that the total surface area increased,
while the micropore volume of H-ZSM-5/MCM-41 decreased
with increasing NaOH dosage. The total pore volume and
the mesopore volume showed an upward trend, followed by a
slight decrease trend as NaOH dosage continued to increase
and obtained the maxium pore volume at the nNa /nSi of 0.8.
These results may be attributed to the simultaneous existence
of H-ZSM-5 nanoparticles and MCM-41 in H-ZSM-5/MCM41 composite molecular sieves.
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Yu Sang et al./ Journal of Energy Chemistry Vol. 22 No. 5 2013
Figure 5. N2 adsorption-desorption isotherms and pore size distributions of (1) H-ZSM-5 and H-ZSM-5/MCM-41 prepared by alkli-treatment with nNa /nSi of
(2) 0.4, (3) 0.47, (4) 0.6, (5) 0.8, and (6) 1.0
Figure 6. Textural properties of H-ZSM-5 and H-ZSM-5/MCM-41 prepared
by alkli-treatment with different dosages of NaOH solution. (1) BET specific
surface area obtained from N2 adsorption isotherms, (2) total pore volume
obtained from NLDFT method, (3) micropore volume obtained from t-plot
method, (4) mesopore volume obtained by total pore volume minus micropore volume
3.2. Surface acidity of H-ZSM-5/MCM-41
NH3 -TPD characterization was used to measure the distribution of surface acidity and strength of acid sites in HZSM-5 and H-ZSM-5/MCM-41. The NH3 -TPD curves, and
the relative amounts and the absolute amounts and the acidic
strength of acidic sites for H-ZSM-5 and the corresponding
H-ZSM-5/MCM-41 were displayed in Figure 7 and Table 1.
In the NH3 -TPD curve (1) in Figure 7, there were two desorption peaks for H-ZSM-5. The lower-temperature peak (P1 )
was due to NH3 desorption from moderate acidic sites while
the high-temperature peak (P2 ) was due to NH3 desorption
from strong acidic sites [35]. P1 was due to NH3 desorption
from moderate acid sites which were in favor of methanol dehydration to DME at low temperatures. In contrast, P2 was
due to NH3 desorption from strong acid sites which were in
favor of secondary reactions, for example, methanol cracking and coking reaction at high temperature. For H-ZSM5/MCM-41, as NaOH dosage was increased from nNa /nSi = 0
to 0.8, the amounts of moderate acidic sites (A1 ), strong acidic
sites (A2 ) and total acidic sites (A) of H-ZSM-5/MCM-41
decreased but A1 /A2 ratio of H-ZSM-5/MCM-41 increased.
When NaOH dosage was increased to nNa /nSi = 1.0, the two
types of acidic sites disappeared. In addition, the peak temperatures of moderate acidic sites (TP1 ) and strong acidic sites
(TP2 ) of H-ZSM-5/MCM-41 shifted to lower temperature with
increasing NaOH dosage. The changes herein were made by
the changes of some strong acid sites to moderate acid sites
as H-ZSM-5 had been treated during the preparation of HZSM-5/MCM-41, in which MCM-41 instead of Na-ZSM-5
was favorable to form. It suggested that the ratio of moderate acidic sites to strong acidic sites of H-ZSM-5/MCM-41
could be adjusted to meet the needs of reactions by different
dosages of NaOH solution. In addition, the strength of moderate acidic sites and strong acidic sites decreased with increasing NaOH dosage, which indicated that a change in the distribution of acidity was good for methanol dehydration because
its total amount of acid sites was enough for effective catalysis
while the decrease in strength of acid sites could restraint byproduct formation and improve selectivity. The molar ratios
of Si/Al on the surface of H-ZSM-5 and H-ZSM-5/MCM-41
which were obtained from EDS were showed in Table 1. It
can be seen that molar ratios of Si/Al on the surface of HZSM-5/MCM-41 obviously decreased with NaOH dosage increasing to nNa /nSi = 0.8. It indicated that the alkali-treatment
is easier to remove the Si ions from the surface of H-ZSM-5.
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Journal of Energy Chemistry Vol. 22 No. 5 2013
Table 1. The acid sites on H-ZSM-5 and H-ZSM-5/MCM-41 alkali-treated with different dosages of NaOH solution
Samples
nNa /nSi
H-ZSM-5
H-ZSM-5/MCM-41
H-ZSM-5/MCM-41
H-ZSM-5/MCM-41
H-ZSM-5/MCM-41
H-ZSM-5/MCM-41
−
0.4
0.47
0.6
0.8
1.0
Si/Al
ratio
19
16
18
18
11
9
Relative amount of acidic sitesa
A1
A2
A
A1 /A2
41.7
58.3
100
0.72
34.8
44.7
79.5
0.78
32.3
40.0
72.3
0.81
21.8
24.0
45.8
0.91
7.22
7.68
14.9
0.94
−
−
−
−
Absolute amount of
total acidic sitesb (µmol/g)
0.24
0.19
0.17
0.11
0.04
−
Peak temperature (◦ C)
TP1
TP2
242.5
462.6
216.2
444.9
216.0
428.0
215.6
389.4
210.5
369.3
−
−
a
Relative amount of acidic sites were calculated based on the ammonia desorption area in respect to H-ZSM-5; b Absolute amount of acidic sites were
calculated based on the ammonia peak area
Figure 7. NH3 -TPD profiles of (1) H-ZSM-5 and H-ZSM-5/MCM-41 prepared by alkli-treatment with nNa /nSi of (2) 0.4, (3) 0.47, (4) 0.6, (5) 0.8 and
(6) 1.0
3.3. Catalytic performance and mechanism
High activity, high selectivity and long lifetime are the
three essential requirements for an excellent catalyst. The catalytic performances of H-ZSM-5/MCM-41 composite molecular sieves were tested for methanol dehydration to DME.
The catalytic activities of H-ZSM-5 and H-ZSM-5/MCM41 were shown in Figure 8. Pure H-ZSM-5 had a high activity.
When the temperature was increased to 200 ◦ C, the methanol
conversion exceeded 80%, and achieved the highest conversion of 89.6% at 220 ◦ C, although a slight decrease of conversion to 88% appeared along with the increase of temperature
to 250 ◦ C, due to the exothermic charateristics of methanol
dehydration. However, pure H-ZSM-5 showed DME selectivity less than 100% as the reaction temperature was increased
from 220 ◦ C. Activities of H-ZSM-5/MCM-41 prepared with
dffierent dosages of NaOH solution were lower than that of
H-ZSM-5 due to the decrease of some Brönsted acid. When
NaOH dosage was increased to 0.47, the sample showed good
methanol conversion and DME selectivity (100%) in a wide
temperature range of 170 ◦ C to 300 ◦ C, owing to the increase
of the amount of moderate acids relative to strong acids and
the hierarchical pore structures which shortened DME residence time in the narrow channels of H-ZSM-5 [26]. However, the further stronger alkali-treatments (nNa /nSi = 0.6 and
0.8) caused an evident decrease of activity. It resulted from
the loss of a large amount of acidity which was proved by the
NH3 -TPD data. When NaOH dosage was 1.0, the conversion
of methanol was nearly zero because of the transformation of
all H-ZSM-5 to MCM-41.
By integrating the above results, H-ZSM-5/MCM-41 that
was prepared with nNa /nSi of 0.47 was selected to test lifetime
in methanol dehydration as shown in Figure 9. Over a period
of 500 h, the activity of H-ZSM-5/MCM-41 (nNa /nSi = 0.47)
remained about 88.7% methanol conversion and 100% DME
selectivity. In contrast, pure H-ZSM-5 produced byproducts (hydrocarbons such as light olefins) and the catalytic
activity and selectivity of DME decreased at 220 ◦ C at the
initial stage. The low selectivity of H-ZSM-5 was due to
two reasons. Firstly, the surface acidity of H-ZSM-5 was
too strong; secondly, microporous structure was unfavourable
to the diffusion of products, which gave more byproducts. From the XRD and NH3 -TPD characterization discussed above, H-ZSM-5/MCM-41 (nNa /nSi = 0.4−0.47) had
the proper microporous-mesoporous structure, acid amount
and acid distribution needed for effective catalysis. Meanwhile the mesoporous structure could improve the diffusion
of reactants and products, resulting in high activity and selectivity.
Figure 8. Catalytic activities of (1) H-ZSM-5 and H-ZSM-5/MCM-41 prepared by alkli-treatment with nNa /nSi of (2) 0.4, (3) 0.47, (4) 0.6, (5) 0.8, (6)
1.0; and (7) equilibrium conversion
776
Yu Sang et al./ Journal of Energy Chemistry Vol. 22 No. 5 2013
the micropores of H-ZSM-5, and then avoided the occurrence
of the secondary reactions. Last but not least, controllable
acidic distribution by alkali-treatment provided proper acid
strength and acid amount for methanol dehydration to DME.
The three aspects described above jointly contribute to the
high catalytic activity, high DME selectivity, and high stability of H-ZSM-5/MCM-41 composite molecular sieves.
4. Conclusions
Figure 9. Lifetime of H-ZSM-5/MCM-41 prepared by alkli-treatment with
nNa /nSi of 0.47 in methanol dehydration. (a) methanol conversion; (b) DME
selectivity
Based on the above results, the catalytic performance of
H-ZSM-5 and H-ZSM-5/MCM-41 was discussed as follows.
In H-ZSM-5, methanol firstly diffused into the micropores of
H-ZSM-5 from the gas phase, then adsorbed and reacted at
the acidic sites DME was finally generated and diffused out
of the micropores of H-ZSM-5 catalyst. The small pore size
and long distance of micropores in H-ZSM-5 led to diffusion
limitations on reaction rates, thus products (HCs and coke)
produced, which resulted in the deactivation of catalyst and
the decrease of DME selectivity. In H-ZSM-5/MCM-41 catalyst, methanol diffused into micropores of H-ZSM-5/MCM41 from mesopores of composite molecular sieves and produced DME. Then, DME spread out from the mesopores without any byproducts. The mesoporous structure improved the
diffusion of reactants and products; The H-ZSM-5 nanoparticles were dispersed uniformly in MCM-41 matrix, which
made catalysts possess more proper acid amount and acid distribution and shorter channels than pure H-ZSM-5. These resulted in the high activity and excellent DME selectivity of HZSM-5/MCM-41. Zheng et al. [23] prepared H-BMZ composite molecular sieves which had micro-mesoporous structure and regulation of L/B acid ratio. The methanol conversion reached 90% and the selectivity of DME reached 100%
due to high acidity and micro-mesoporous structure of composite molecular sieves. Rownaghi et al. [28,36–38] found
that the metanol conversion on nano-ZSM-5 and meso-ZSM5 which have a small crystal size and a high mesoporosity, was
easier to occur inside the channels than on the larger crystal
size as well as less mesoporosity, and that uniform H-ZSM5 nanocrystals were the most active and selective catalyst for
the methanol dehydration to DME. In this work, firstly, the
nanosized H-ZSM-5 particles embeded in the MCM-41 matrix which had shorter-distance micropores and large specific
surface area became more selective than pure H-ZSM-5 [39].
Secondly, the ordered mesopores of MCM-41 were more favorable to the diffusion of DME than that in H-BMZ which
was random. The combination of nanosized H-ZSM-5 particles and MCM-41 to form H-ZSM-5/MCM-41 composite
molecular sieves shortened the distance of DME diffusion in
Micro-mesoporous
H-ZSM-5/MCM-41
composite
molecular sieves were successfully synthesized by the hydrothermal technique using alkali-treated H-ZSM-5 zeolite
as the source. They had a micropore and mesopore dual
pore size distribution, which combined the channel advantage of a mesoporous molecular sieve with the acidity of a
microporous zeolite. Compared with H-ZSM-5, the catalysts
prepared in this study have relatively favorable acid strength
and acid sites distributions, higher BET surface area and special hierarchial porosity, which greatly improved the reaction
and diffusion of reactants in the pores. H-ZSM-5/MCM-41
composite molecular sieves prepared by NaOH dosage varying from 0.4 to 0.47 showed high activity, and high DME
selectivity for methanol dehydration to DME in the wide
temperature range of 170−300 ◦ C. Highly active nanosized
H-ZSM-5 particles uniformly dispersed in MCM-41 matrix
with controllable acidic distribution and amount resulted in
the high activity, high DME selectivity and high stability of
H-ZSM-5/MCM-41.
Acknowledgements
This work is supported by the National Nature Science Foundation of China (No: 20976013) and International Science & Technology Cooperation Program of China (No: 2012DFR40240).
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