Conversion of dimethyl ether to toluene under an O2 stream over W/HZSM-5 catalysts

Article abstract of DOI:10.1039/c4cy01445f

The direct conversion of dimethyl ether (DME) to toluene without other aromatics is realized over W/HZSM-5 catalysts with high W contents. The influence of W content on the nature of tungsten species, the distribution of acid sites, the redox properties, and the subsequent catalytic performance of W/HZSM-5 in DME conversion was investigated. The introduction of a high content of W in HZSM-5 can bring about some new redox sites and acidic sites associated with W species, although they also cover some acidic sites in HZSM-5. In this way, the introduction of W interrupts the dual cycle of the DME-to-hydrocarbon reaction on HZSM-5, and effectively the formation of higher methylbenzenes is limited.

Full text of DOI:10.1039/c4cy01445f

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Conversion of Dimethyl Ether to Toluene under O2 stream over
W/HZSMꢀ5 catalyst
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Bo Wang a,b,1, Hui Wang a, , Guangbo Liu a, , Xuemin Li a,b, and Jinhu Wu a,
a Key Laboratory of Biofuels Qingdao Institute of Bioenergy and Bioprocess Technology
Chinese Academy of Sciences Qingdao, 266101, PR China
b University of Chinese Academy of Sciences, Beijing 100049, PR China
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Abstract:
The direct conversion of dimethyl ether (DME) to toluene without other aromatics
becomes true over W/HZSMꢀ5 catalysts with high W contents. The influence of W content on
the nature of tungsten species, the distribution of acid sites, the redox properties, and the
subsequent catalytic performance of W/HZSMꢀ5 in DME conversion was investigated. The
introduction of high content of W in HZSMꢀ5 can bring up some new redox sites and acidic
sites associated with W species, although they also cover some acidic sites in HZSMꢀ5. By
this way, the introduction of W interrupts the dual cycle of the DMEꢀtoꢀhydrocarbons reaction
on HZSMꢀ5, and the formation of higher methylbenzenes is limited effectively.
Keywords: Dimethyl ether, Toluene, tungsten, HZSMꢀ5
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1. Introduction
Dimethyl ether (DME) can be oneꢀstep synthesized from syngas with the elevated
maximum conversion, which makes it a promising candidate as clean and high efficiency
energy1ꢀ3. DME can replace liquefied petroleum gas, and it is also an attractive ecoꢀfriendly
∗ Corresponding authors. Tel.: +86 532 80662764; fax: +86 532 80662761.
Eꢀmail: huiwangsun@gmail.com (H. Wang), liugb@qibebt.ac.cn (G. Liu), and wujh@qibebt.ac.cn (J. Wu)
1 Present address: B. Wang, Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
East China Normal University, Shanghai, China
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fuel for vehicles due to its superior combustion properties4. Not only limited to energy
products, accompanied with the enlarged DME industry, DME is also an essential starting
material or intermediate for valueꢀadded chemicals. At present, some catalytic conversion
reactions of DME have been explored and progress has been achieved. Generally, DME
molecule can be easily activated to form CH3ꢀ, CH3Oꢀ and CH3OCH2ꢀ groups. Then the
active groups can form hydrocarbons like olefins, gasoline and aromatics57 through the
hydrocarbon pool (HP) mechanism, or they can form oxygenated compounds such as methyl
acetate8, formaldehyde (FA)9, dimethoxymethane (DMM)10,11, methyl formate (MF)12, and
ethanol13.
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Concerning the catalysts, DME catalytic conversion is usually conducted on solid acid
catalysts like zeolite. In particular, the zeolite ZSMꢀ5, distinguished by shape selectivity and
good acid properties, exhibits high activity in conversion of DME to hydrocarbons in a
fixedꢀbed reactor with a slow deactivation rate14ꢀ16. Moreover, transition metal oxides have
usually been used to modify zeolite property to enhance catalytic activity and selectivity,
because of the ability of accommodating acidity of zeolite and enhancement of the activation
of the DME molecule under O2 atmosphere at lower temperature, thus, adjusting product
distribution17. Among these transition metals, tungsten oxide gives the strong Brønsted acid
sites, either as bulk or supported oxides but their structure and catalytic properties are strongly
influenced by the support18. Several studies have explored how interactions between support
and tungsten oxide domains affect catalytic performance. Indeed, even for a given WOx
support system, WOx domains exhibit a range of surface structures that depend strongly on
synthetic protocols, such as the WOx precursor and content, as well as on the thermal history
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of these materials 19 ꢀ 21 . Yang et al. reported that W/HZSMꢀ5 catalyst was a good
dehydroaromatization catalyst in the presence of oxygen22. Anyway, Liu et al. obtained trace
toluene from DME oxidation to ethanol over HZSMꢀ5 catalysts with low tungsten contents
(<23 wt.%) without other aromatic products23. Reasonably, it is possible to gain toluene with
high selectivity from DME conversion through tuning the synergic effect of tungsten and
HZSMꢀ5. However, there have been no reports on direct conversion of DME to toluene over
W/HZSMꢀ5 catalysts.
In this work, a novel route for the synthesis of toluene from DME direct conversion over
W/HZSMꢀ5 in the presence of oxygen was studied. The catalysts with relative high W
contents were prepared and characterized by the Xꢀray diffraction (XRD), N2 adsorption,
ultraviolet visible spectroscopy (UVꢀvis), Xꢀray photoelectron spectroscopy (XPS),
temperatureꢀprogrammed desorption of ammonia (NH3ꢀTPD) and temperatureꢀprogrammed
reduction of hydrogen (H2ꢀTPR) in details. The influence of W content on the distribution of
acid sites, the redox properties, the nature of tungsten species, and the subsequent catalytic
performance of W/HZSMꢀ5 in DME conversion to toluene was investigated.
2. Experimental
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2.1 Catalyst preparation
The W/HZSMꢀ5 catalysts were prepared by a wet impregnation method, as described
elsewhere24. To obtain the catalysts with different tungsten contents, the HZSMꢀ5 powder
(supplied by Shandong Qilu Huaxin Highꢀtech Company) was first infiltrated in thermostat
water bath (80 oC) with aqueous solutions of ammonium metatungstate containing the desired
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amount of (NH4)6W12O40.xH2O. The resulting material was then dried at 110 C for 10 h and
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subsequent calcination at 500 C for 5 h. The concentration of W was adjusted with the
nominal value of W contents (25, 35, 45, 50 and 55 wt.%). The catalyst obtained was then
expressed as x W/HZSMꢀ5. The actual W contents x were determined by inductively coupled
plasmaꢀatomic emission spectrometry (ICPꢀAES), as shown in Table 1.
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2.2 Catalyst characterization
Xꢀray diffraction (XRD) analysis was carried out by using Bruker D8 Advance Xꢀray
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diffractometer with monochromatic highꢀintensity Cu Kα radiation (λ = 1.5418 Å) in the 2θ
range of 5ꢀ70° operated at 30 kV and 30 mA.
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Nitrogen adsorption/desorption isotherms were measured at ꢀ196 C on a Micromeritics
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ASAP 2020 adsorption apparatus. Before the measurements, the samples were degassed at
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300 C for more than 6 h under high vacuum conditions. The BrunauerꢀEmmettꢀTeller (BET)
method was utilized to calculate the specific surface areas (SBET). The pore size distribution
was derived from the adsorption branch by BarrettꢀJoynerꢀHalenda (BJH) method. The total
pore volumes were estimated from the adsorbed amount at a relative pressure P/P0 of 0.99.
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Diffuse reflectance UVꢀvis spectra were performed on
a
Hitachi Uꢀ4100
spectrophotometer equipped with a diffuse reflectance attachment. Spectra were recorded at
room temperature in the wavelength range 200ꢀ800 nm.
XPS was obtained on a Quantumꢀ2000 Scaning ESCA Microprobe instrument using Al
Kα Xꢀray radiation source. The pressure in the analysis equipment was maintained below 10ꢀ6
Pa during the data acquisition. The passing energy of the experiment was 46.95 eV, and all the
binding energy measured was calibrated with contaminated carbon (284.6 eV) as reference.
The spectra were decomposed by using XPSPEAK software (Ver. 4.1) after applying a
Shirley background subtraction and Gaussian (80%)ꢀLorentzian (20%) decomposition
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parameters. Atomic ratios were calculated from the peak areas that were calibrated by the
sensitive factors provided by the equipment manufacturer.
NH3ꢀTPD spectra were recorded on Micromeritica Auto Chem 2920. The sample (100
mg) was pretreated at 500 oC in an Ar flow of 20 ml/min for 1 h and then cooled down to 100
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oC to be introduced with NH3. The spectra were recorded from 100ꢀ700 C with a rate of 5
oC/min. The quantities of weak, medium, and strong acid sites are measured by the amounts
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of ammonia desorbed at 100ꢀ200, 200ꢀ300, and 300ꢀ600 C, respectively, through integrating
the NH3ꢀTPD profile in each temperature interval. The apparatus and pretreated condition in
H2ꢀTPR experiment were same with NH3ꢀTPD. After cooling down to room temperature and
introducing the reduction agent of 10% H2/Ar (30 ml/min), the sample was heated up to
900 °C from room temperature at a rate of 5 °C/min. The amount of H2 uptake during the
reduction was measured by a thermal conductivity detector (TCD), which was calibrated by
the quantitative reduction of CuO to the metallic copper25.
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Diffuse reflectance infrared spectra were measured by a Bruker Tensor 27 with a MCT
detector (64 scans, 4 cmꢀ1). The catalyst was placed in an in situ IR cell equipped with KBr
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windows (Harrick). After heating at 400 C for 2 h and evacuating at 10−4 bar, the cell was
cooled down to reaction temperature and the spectrum was recorded. Next, the mixture gas of
DME and O2 (nDME/nO2=1:1) was introduced into the cell, and the spectra of catalyst
adsorption were recorded. Finally, the catalyst desorption spectra were recorded after 30 min
of reaction and the pressure of the cell was evacuated to 10ꢀ3 bar. Thermo
gravimetric/derivative thermoꢀgravimetry TG/DTG was measured on a NETZSCH STA 409
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PC analyser. The sample weight was about 10 mg. The temperature was ramped to 800 C at
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heating rate of 10 oC/min in an air flow (50 ml/min).
2.3 Catalytic tests and analytic procedures
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The catalytic test was performed in a continuous flow fixedꢀbed reactor with an inner
diameter of 10 mm. The catalyst (40ꢀ60 mesh) was diluted with SiO2 (20ꢀ40 mesh). Before
reaction, the catalyst was pretreated in oxygen (99.999%) flow (25 ml/min) at 400 oC for 1 h.
The reactant mixture consisted of DME and O2 (nDME/nO2 = 1:1), and the gas hourly space
velocity (GHSV) was 800 hꢀ1. The outlet stream line from the reactor to the gas
chromatograph was heated at 150 oC. The DME, toluene, MF, methanol, DMM, formaldehyde
products were analyzed by a gas chromatograph (GCꢀ2060) equipped with a thermal
conductivity detector (TCD) and a Porapak Q column (3 m x 3 mm), a gas chromatography
(Agilent 7890A) with an flame ionization detectors (FID) (HPꢀPLOT capillary column), and a
TCD (5 A molecular sieve) for analyzing alkanes, olefins, H2, CO and CO2.
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3. Results and discussion
3. 1 XRD and textural characterization
XRD patterns of HZSMꢀ5 and W/HZSMꢀ5 catalysts with different W contents are shown
in Fig. 1. The parent HZSMꢀ5 sample exhibits the characteristic peaks of MFI structure (at 2θ
of 7.9°, 8.9°, 23.07°, 23.6° and 24.4°) and quartz phase at 2θ of 26.6° (PDF number
89ꢀ1961)26. However, with the increase of the W content, these diffraction lines of HZSMꢀ5
decrease in intensity. Meanwhile new lines characteristic of cubic WO3 (at 2θ of 23.64°,
33.64°, 41.46° and 54.57°, PDF number 46ꢀ1096) and monoclinic WO3 (at 2θ of 23.12°,
23.59°, 24.38°, 34.16° and 49.95°, PDF number 43ꢀ1035) are observed for the W/HZSMꢀ5
catalysts. As for the 23.3 and 34.9 wt.% W/HZSMꢀ5 catalysts, the split of the peaks at 23.1o
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and 23.6o are highly likely to be caused by their overlapping with the HZSMꢀ5 zeolite
characteristic bands at 23.07o, 23.64o and 24.4o. Moreover, the overlapping peaks become less
obvious with the W content increasing from 46.1 to 56.0 wt.%, that is because the crystallinity
of HZSMꢀ5 zeolite decrease remarkably, pointing to a possible deterioration effect of tungsten
species on the framework of HZSMꢀ5 zeolite. Meanwhile, the intensity of the peaks at 23.6o
and 24.4o increase obviously, this suggests that the fraction of the monoclinic phase of WO3
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increases with the W content increasing.
{Fig. 1}
The surface area data of the HZSMꢀ5 and W/HZSMꢀ5 catalysts with different W
contents obtained by N2 adsorption method are shown in the Table 1. Obviously, the
introduction of W species leads to the decrease of surface area. With the W content from 23.3
to 56.0 wt.%, the surface area decreases from 216.35 to 111.56 m2/g. The porous structure of
the W/HZSMꢀ5 is also affected by introducing different W loadings. In general, the
microporous volume of W/HZSMꢀ5 zeolites is decreased by increasing W content. Although
most of WO3 particles located on the surface, as revealed by the XPS results, they still may
partially block the HZSMꢀ5 microchannels and reduce the accessibility of micropores (Table
1). Moreover, the mesoporous volume of W/HZSMꢀ5 increases first and then decreases with
the increasing of the W loading. These mesoporous may be some like interꢀcrystalline
mesopores built among the stacked nanoꢀcrystals, which is related to the agglomerative
morphology27. However, with the W content increasing to some extent, the interꢀcrystalline
voids may be reduced, and the mesoporous volume decreases.
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However, the changes of the surface area and porous structure are more pronounced for
the W/HZSMꢀ5 with low W content from 23.3 to 46.1wt.% and the downward trends tend to
flat as the W content exceeding 46.1 wt.%. Furthermore, these changes are in agreement with
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the conversion of DME, which may be due to W domains partially covered the acid sites on
the surface. To compare the catalysts before and after reaction, BET surface area and
micropore volume decreases apparently, which may be a result that the conversion of DME
mainly occurs at micropore channel of zeolite, and excessive carbon deposits covers the
surface of the catalysts and in turn blocks the pores.
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3. 2 State of tungsten in HZSM-5
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To determine the structure and coordination states of tungsten species with different
contents, UVꢀvis DRS of W/HZSMꢀ5 catalysts were recorded. As shown in Fig. 2, all the
catalysts present three absorption bands at 230, 280 and 380 nm. The peak at 230 nm is
associated with tetrahedrally monomeric tungstate species ([WO4]2ꢀ), the band located at 280
and 380 nm can be assigned to octahedral of polytungstate species ([WO6]nꢀ), and WO3
crystallites respectively28. Therefore, three types of tungsten species exist in W/HZSMꢀ5
catalysts. With the W content increasing, the intensities of the three bands reinforce. When
the content is 23.3 wt. %, the intensity of the band at 230 nm is stronger than that of 280 nm,
and while the W content exceeds 34.9 wt.% the peak at 280 nm became the dominant band.
That is to say the tungsten species for the samples with low W contents (23.3ꢀ34.9 wt.%)
presents mainly as [WO4]2ꢀ tetrahedral species, and the tungsten domains for the samples
with high content (34.9ꢀ56.1 wt.%) are primarily [WO6]nꢀ octahedral polytungstate species.
These may be related to the change of the crystallite phase, as demonstrated by XRD.
{Fig. 2}
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The XPS survey spectra of W/HZSMꢀ5 with different W contents were carried out. Fig.
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contents. The binding energies of W6+, W5+ and W4+ are 37.3, 36.2 and 35.2 eV for 4f5/2 and
35.1, 33.6, and 32.9 eV for 4f7/2, respectively29,30. In the 23.3 wt.% W/HZSMꢀ5 catalyst, the
W species with about 93.5% W6+ and 6.5 % W5+ atoms detected. With the increasing of the W
content, the amount of W6+ increases and the W5+ decreases, when the W content reaches 56.0
wt.%, the WO3 becomes be completely oxidized with 100 % W6+, as shown in Table 2. It was
reported that the structure of monoclinic WO3 tended to be completely oxidized29, hence the
change in chemical states is related to the change of WO3 phase, as indicated by XRD.
The XPS spectra of O 1s shown in Fig. 4 show two BE peaks for all of the W/HZSMꢀ5
catalysts with different tungsten contents. The lower BE band at about 530.3 eV corresponds
to O2ꢀ anions of tungsten oxide, and the higher BE band at 532.5 eV corresponds to O2ꢀ anions
associated with the HZSMꢀ5 and surface hydroxides on the W/HZSMꢀ5 both in physically
adsorbed and chemically bonded forms29,31. With the increasing of W content, the amount of
the O2ꢀ anions of tungsten oxide increases to the maximum (38.04%) in 51.1W/HZSMꢀ5 and
then decreases to 31.03 % in the 56.0 W/HZSMꢀ5. As for the O2ꢀ species 532.5 eV, the
relative amount decreases from 82.72 % in 23.3W/HZSMꢀ5 to 61.96 % in 51.1 W/HZSMꢀ5
and then increases to 68.96 % in 56.0 W/HZSMꢀ5 catalyst. It is reasonable that the amount of
O2ꢀ anions associated with the HZSMꢀ5 decreases, because the content of HZSMꢀ5 decreases
with the increasing of the W content, and the later increase should be ascribed to the surface
hydroxides on the W/HZSMꢀ5, because the monoclinic WO3 contains more surface OH
groups and water molecules on the surface, compared with other WO3 phase, as reported by
Firkala29. This means that the surface OH species associated with tungsten oxide increases
with the increasing of the W contents, which can influence the acidic properties of the
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W/HZSMꢀ5 catalyst.
In summary, the results of UVꢀvis DRS and XPS demonstrate that the chemical state of
W species changes with the W content increasing. The tetrahedral [WO4]2ꢀ species changes to
[WO6]nꢀ octahedral polytungstate species, the W6+ is the main chemical state of tungsten in all
the W/HZSMꢀ5 catalysts. However there is more surface OH species appear on the
W/HZSMꢀ5 with higher W content, which can influence the acidic properties of W/HZSMꢀ5.
{Fig. 3 and Fig. 4}
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3. 3 Acidic properties
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The concentration and strength of acid sites in W/HZSMꢀ5 catalysts are determined by
NH3ꢀTPD, as presented in Fig. 5. Generally, a NH3ꢀTPD profile on HZSMꢀ5 zeolite shows
two desorption peaks, i.e. the highꢀtemperature one located above 300 oC and the
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lowꢀtemperature one situated below 200 C, which are ascribed to ammonia adsorbed on the
strong and weak acid sites, respectively27. In this work, the distribution of the acid sites across
the acidic strength is determined by integrating the NH3ꢀTPD profiles in different temperature
intervals, as listed in Table 2, where the quantities of weak, medium, and strong acid sites are
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measured by the amounts of ammonia desorbed at 100ꢀ200, 200ꢀ300, and 300ꢀ600 C ,
respectively. The W content exhibits a significant influence on the distribution of acid sites in
W/HZSMꢀ5. With the W content increasing, the total acid quantity decreases. It seems that the
amount of the strong acid sites is increased with the W content increasing from 23.3 to 34.9
wt.%, and then decreased with the W content increasing to 56.0 wt.%. As for the amount of
weak acid sites, it decreased with the W content increasing from 23.3 to 51.1 wt.%, and then it
increases from 0.18 to 0.24 mmol gꢀ1 with the W content increasing to 56.0 wt.%, as shown in
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Table 2. Undoubtedly, these are related to the cooperation between tungstate species and
HZSMꢀ5. As the XPS and UVꢀvis DRS results demonstrate that the octahedral polytungstate
domains could accommodate a proton by electron transfer and charge delocalization across an
extended WꢀO network and form some acidic sites32.
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{Fig. 5}
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3.4 Redox properties
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H2ꢀTPR profiles of W/HZSMꢀ5 with different W contents are shown in Fig. 6, and the
quantitative result is listed in Table 2. H2ꢀTPR has been extensively used to characterize the
reducibility of oxygen species in WO3 and WOx containing materials. It was found that bulk
WO3 can be reduced to W metal at temperatures below 927 oC. Three overlapping peaks were
detected for the reduction of bulk WO3 samples, which occurs via stepwise reduction:
WO3→WO2.9→WO2→W32. The reduction profiles for W/HZSMꢀ5 samples have three
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overlapping peaks (Fig. 6) that occur at similar temperatures (about 550, 730 and 810 C)
with all W concentrations. These three peaks correspond to the same three reduction steps
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found in bulk WO3 above. While the second peak at about 730 C becomes obvious with the
W concentration increasing. It was reported that the second reduction step (WO2.9→WO2) is
sensitive to the dispersion of WOx domains32. At low W concentration (23.3 wt.%), there are
not enough WOx neighbors in these highly dispersed domains to form the stable intermediate
WO2 species, a large fraction of the WOx domains bypass this second reduction step and
instead reduce directly to W metal (WO2.9→W) in the third reduction step. With the W
content increasing from 34.9 to 56.0 wt.%, the WOx surface densities become higher, the WOx
polyhedra are connected by WꢀOꢀW bonds, and there are enough WOx neighbors to share the
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reduced charge and the H2ꢀTPR profiles exhibit obvious reduction step from WO2.9 to WO2.
Anyway, the amount of redox sites of the W/HZSMꢀ5 catalysts is positive correlated with the
W content. The amount of redox sites increased from 3.75 to 10.25 mmol gꢀ1 with W content
increasing 23.3 to 56.0 wt.%, as shown in Table 2.
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{Fig. 6}
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3.5 Catalytic performance
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The catalytic performances of W/HZSMꢀ5 with different W contents in DME conversion
under O2 stream are illustrated in Fig.7. As for the conversion of DME, it decreases with the
W content increasing. When the W contents are 23.3 and 34.9 wt.%, the DME conversion is
above 99.0%. As the W content reaches 46.1 wt.%, the DME conversion decreases to 82.6%,
and then the conversion is almost unchanged although the W content increasing, DME
conversion keeps at 80.8% and 87.0% for 51.1 and 56.0 wt.% W/HZSMꢀ5 respectively.
Obviously, the W content has influence on the product distribution. The W/HZSMꢀ5 catalysts
with W content from 23.3 to 51.1 wt.% show high selectivity to toluene, however, when the
W content becomes 56.0 wt.%, the toluene selectivity decreases to about 29.6%. When the W
contents are 23.3 and 34.9 wt.%, there is nearly no methanol and CO, the main sideꢀproducts
are DMM, HCHO, MF and CxHy (CH4, C2H4, C3H6, C3H8). With the W content increasing
from 23.3 to 51.1 wt.%, the selectivity to CH3OH increases first and then decreases, and the
selectivity to HCHO decreases first and then increases, the highest CH3OH and HCHO
selectivity is about 23.2% and 24.4 %, which are achieved over the 46.1 wt.% and 56.0 wt.%
W/HZSMꢀ5 catalysts. Meanwhile, the selectivities to MF and DMM decrease with the
increasing of the W content. The 34.9 and 56.0 wt.% W/HZSMꢀ5 catalysts have relative high
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selectivity to the CxHy. However, for all of the W/HZSMꢀ5 catalysts with different W contents,
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there is no CO2 detected and small amounts of CO and ethanol formed.
{Fig. 7}
These catalytic results suggest that the W content not only influences the selectivity to
5
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toluene but also the selectivities to oxygen containing chemicals such as HCHO, DMM and
MF. It is understandable that the change of DME conversion has similar tendency to the total
amount of acid sites of W/HZSMꢀ5 with different W contents, because the acid sites are the
main active sites for DME activation. DME molecule could be easily activated to generate
CH3·, CH3O·and CH3OCH2· group in the presence of O2. Then these active groups can form
toluene and CxHy through acidic catalysis process, meanwhile, they also can form DMM, MF
and HCHO over redox sites. However, with the W content increasing, the number of the
redox sites increases to some extent, the DMM and MF may be oxidized further to CO, hence
the selectivities to MF and DMM slightly decrease and small amount of CO produced with
the W content increasing to 56.0 wt.%. Meanwhile, with the increasing of W content, the WO3
chemical state change. And when the W content exceeds 46.1 wt.%, monoclinic WO3
becomes the dominant phase, which makes the nature of the acid sites change, this may be the
main reason for the variation in the product slate, especially for the change of CH3OH
selectivity. As indicated by Table 2, although the total acid sites decreases generally, however
the relative ratio of the amount of medium strong acid sites is similar, they are 30%, 30%,
28%, 28% and 28% for 23.3, 34.9, 46.1, 51.1 and 56.0 wt.% W/HZSMꢀ5 respectively. This
may be the reason why the former four catalysts have similar toluene selectivity. Moreover, as
for the 56.0 wt.% W/HZSMꢀ5, the acid sites redistribution cannot be omitted, the ratio of the
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amount of weak acid sites increases and the amount strong acid sites decreases, besides that
the amount of redox sites increases to maximum, this kind of chemical environment may be
favor for the formation of HCHO and CxHy, therefore, the toluene selectivity decreases and
the selectivities to HCHO and CxHy increase in the 56.0 W/HZSMꢀ5 catalyst.
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5
3.6 Catalytic mechanism of DME to toluene over W/HZSM-5
6
Fig. 8 depicts the IR spectra of DME/O2 adsorption and desorption on the 46.1 wt.%
W/HZSMꢀ5 catalyst. After purification under vacuum condition, infrared spectra of the
catalysts show some bands in the OH stretching region at 3200ꢀ3800 cmꢀ1. They are
associated with the framework aluminum [Si(OH)Al] (3610 cmꢀ1), the isolated external
silanol groups (3745 cmꢀ1), free internal silanol groups (3728 cmꢀ1), and delocalized
hydrogenꢀbonded groups (3500 cmꢀ1) of lattice defects, partially hydrolyzed framework Al
(around 3660 cmꢀ1) and the band of 3338 cmꢀ1 could be ascribed to the OH stretching
associated with W species3334. As for the adsorption spectra, it shows the typical DME
adsorption bands with CꢀOꢀC stretching vibration at 1100ꢀ1200 cmꢀ1, CꢀH stretching vibration
at 2800ꢀ3000 cmꢀ1 and CꢀH deformation vibration at about 1400ꢀ1600 cmꢀ1. In addition, the
bands at 900ꢀ1000 cmꢀ1 appear which can be ascribed to the heterocyclic ꢀCꢀOꢀCꢀ asymmetric
stretching35. However, in the desorption spectra, the bands at 900ꢀ1000 cmꢀ1 disappear, which
suggests that this kind of species should be attributed to the intermediate reactant, formed
through the adsorption of DME on the catalyst surface. Meanwhile, the bands of OꢀH
stretching region, which are related with the Brønsted acid groups in W/HZSMꢀ5 reduce
greatly, and strong multiple absorption bands at 2800ꢀ3100 cmꢀ1 and 1400ꢀ1600 cmꢀ1 are
observed. These two bands are related to CꢀH and CꢀC stretching of the multisubstituted
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benzene35. The band at 1739 cmꢀ1 can be ascribed to the C=O groups, this may be related with
the formation of oxygen containing chemicals such as HCHO, DMM and HCOOCH3. These
results may demonstrate that the formation of toluene over W/HZSMꢀ5 catalyst may be
accordance with the MTH reaction process on HZSMꢀ5 with the acidic sites involved.
Meanwhile, the deactivated catalysts with different W contents were investigated by TG.
Fig. 9 shows the profiles of the deactivated catalysts with 34.9, 46.1 and 51.1 wt.% W
2
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4
5
6
o
7
contents. There are two peaks of weight loss at 150 and 450 C, which are attributed to the
8
coke formation on the external surface and inside the zeolite voids respectively36. Obviously,
the weight loss of the deactivation catalyst decreased with the W content increasing. The BET
results (Table 1) show that the fresh catalyst surface area decreases with tungsten content
increasing, which indicates that the W content affects the external surface of the catalyst, the
more the W content, the less the coke formation. This may be due to the introduction of
tungsten covering the surface acid sites, as demonstrated by NH3ꢀTPD.
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{Fig. 8 & Fig. 9}
The characteristic pore structure of HZSMꢀ5 zeolite makes it the archetypical catalyst for
the conversion from methanol/DME to hydrocarbon, hence tremendous efforts have been
spent on elucidating the mechanism, varying from autocatalytic mechanism to
hydrocarbonꢀpool mechanism. The dualꢀcycle concept, first introduced by Olsbye’s group37,
demonstrates the formation of ethylene and C3+ olefins more reasonably, being acknowledged
as a refinement of hydrocarbonꢀpool mechanism. It consists of the olefinꢀbased methylations/
interconversions cycle mainly producing C3+ alkenes and aromaticꢀbased cycle involved the
formation of ethylene and aromatics. Though it is argued whether the dual cycle operates
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independently or mutually dependently, the fact that the aromatics is constantly formed in
olefinꢀbased cycle through the aromatization of higher alkenes, and also ethylene is generated
from aromaticꢀbased cycle, is proved by numerous kinetic experimental data38. Therefore, it
can be imagined target product could be obtained through facilitating or suppressing the
operation of certain cycle by selecting a catalyst with special topology and acidity.
Combined with the product distribution, high selectivity to toluene was obtained and
ethylene occupies the largest proportion among CxHy, and the FTꢀIR results, the conversion of
DME to toluene over W/HZSMꢀ5 should obey to the famous dual cycle of the MTH reaction
on HZSMꢀ5, and the aromaticꢀbased cycle may be promoted with the synergistic effect of
tungsten and ZSMꢀ5 zeolites in the W/HZSMꢀ5 catalyst. While the introduction of amounts of
W cover the surface acidic sites may break the cycle at toluene partially, and the formation of
higher methylbenzenes is limited effectively. Based on these results, a possible transformation
pathway of DME conversion over W/HZSMꢀ5 zeolite is deduced preliminarily, as shown in
Fig.10. For one side, the oxygenic groups might be converted to methanol, DMM, ethanol and
MF over the redox sites of W/HZSMꢀ5. For the other side, the groups might undergo acid
catalysis reaction to produce hydrocarbonꢀpool species under the cooperation of acids site and
special pore channel, and toluene obtained.
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{Fig. 10}
19
4. Conclusions
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A novel synthesis route of toluene from dimethyl ether has been efficiently performed
over W/HZSMꢀ5. The nature of tungsten species, the distribution of acid sites, the redox
properties, and the subsequent catalytic performance of W/HZSMꢀ5 in DME conversion to
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toluene was influenced by the W content. Obviously, the introduction of W in HZSMꢀ5 can
bring up new redox sites which are relevant to the formation of the oxygen containing
chemicals such as HCHO, DMM and MF and so on. With the W content increasing, the
octahedral polytungstate domains become the primary species, which are favor for the
formation of new acidic sites associated with W, although the introduction of high content of
W can cover lots of acidic sites in HZSMꢀ5. By this way, the existence of W prevents the coke
formation and may break the dual cycle of the DMEꢀtoꢀhydrocarbons reaction in HZSMꢀ5 at
toluene, and the formation of higher methylbenzenes is limited effectively. When the W
content is up to 51.1 wt.% the selectivity of toluene reaches 45% with the DME conversion
decreasing from about 99% to about 80%, and the W content increases further, the selectivity
of toluene decreases to about 29.3%. These may be related with the redistribution of the active
acidic sites with the high content of W.
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Acknowledgments
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The authors are grateful for financial support by the Chinese Academy of Sciences
Research Program (No. XDA07070301), the National Science Foundation of China (No.
21403264) and the Ministry of Science and Technology of the People’s Republic of China
(No. 2011BAD22B06).
18
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Table 1
Textural and structural properties of different catalysts
W content (wt.%)
SBET
Vmicro
Vmeso
Catalyst
(m2/g) (cm3/g) (cm3/g)
ICP
23.3
34.9
46.1
51.1
56.0
XPS
25.4
36.6
46.2
52.7
56.9
23.3W/HZSMꢀ5
34.9W/HZSMꢀ5
46.1W/HZSMꢀ5
51.1W/HZSMꢀ5
56.0 W/HZSMꢀ5
216.35 0.086
177.88 0.066
121.73 0.048
113.33 0.045
111.56 0.042
0.054
0.072
0.034
0.032
0.022
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Table 2
NH3ꢀTPD, H2ꢀTPR and XPS results for the W/HZSMꢀ5 catalysts
Acid sitesa
(mmol gꢀ1)
Redox sitesb
(mmol gꢀ1)
Chemical statec
(%)
Catalysts
Weak Medium Strong Total Low High Total W6+ W5+ OꢀW OꢀHZSMꢀ5
23.3W/HZSMꢀ5 0.64
34.9W/HZSMꢀ5 0.39
46.1W/HZSMꢀ5 0.27
51.1W/HZSMꢀ5 0.18
56.0W/HZSMꢀ5 0.24
0.63
0.59
0.43
0.28
0.27
0.86
1.04
0.83
0.54
0.48
2.13 0.55 3.20 3.75 93.5 6.5 17.28
2.02 1.0 5.20 6.20 95.9 4.1 19.35
1.53 1.44 6.58 8.02 97.6 2.4 32.46
1.0 1.75 8.13 9.88 98.7 1.3 38.04
82.72
80.65
67.54
61.96
68.96
0.99 1.77 8.48 10.25 100
0
31.03
a
Density of the acid sites, assorted according to the acidic strength, determined by
NH3ꢀTPD.
b “Low” and “High” represent respectively the low temperature and high temperature
band in H2ꢀTPR profiles shown in Fig. 5, density of the redox sites, determined by
H2ꢀTPR.
c
The change of the relative amounts of W and O species of different chemical state,
“OꢀW” and “OꢀHZSMꢀ5” represent the O2ꢀ anions of tungsten oxide and the O2ꢀ
anions associated with the OꢀSi, OꢀAl and OꢀH on the W/HZSMꢀ5.
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Figure Caption
•
•
•
•
∗
•
a
♦
♦
♦
♦
♦
♦
♦
♦
b
c
♦
d
e
f
10
20
30
40
50
60
2θ (°)
Fig. 1 The XRD patterns of catalysts: (a) HZSMꢀ5, (b) 23.3W/HZSMꢀ5, (c) 34.9
W/HZSMꢀ5, (d) 46.1W/HZSMꢀ5, (e) 51.1W/HZSMꢀ5, (f) 56.0W/HZSMꢀ5.
Crystalline phases detected: •, HZSMꢀ5, ♦,WO3
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e
d
c
b
a
200
300
400
500
600
700
Wavelength (nm)
Fig. 2 UVꢀvis spectra of W/HZSMꢀ5 catalysts with different W contents: (a) 23.3
wt.%, (b) 34.9 wt.%, (c) 46.1 wt.%, (d) 51.1 wt.%, and (e) 56.0 wt.%.
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d
c
b
a
40
39
38
37
36
35
34
33
Binding energy(eV)
Fig. 3 XPS spectra of the W4f region for different W/HZSMꢀ5 catalysts: (a) 23.3 wt.%,
(b) 34.9 wt.%, (c) 46.1 wt.%, (d) 51.1 wt.%, and (e) 56.0 wt.%.
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a
b
c
d
e
536 535 534 533 532 531 530 529 528
Binding energy (eV)
Fig. 4 XPS spectra of the O1s region for different W/HZSMꢀ5 catalysts: (a) 23.3 wt.%,
(b) 34.9 wt.%, (c) 46.1 wt.%, (d) 51.1 wt.%, and (e) 56.0 wt.%.
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a
b
c
d
e
100
200
300
400
500
600
700
Temperature (oC)
Fig. 5 NH3ꢀTPD spectra of W/HZSMꢀ5 with different W contents (a) 23.3 wt.%, (b)
34.9 wt.%, (c) 46.1 wt.%, (d) 51.1 wt.%, and (e) 56.0 wt.%.
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d
c
b
a
200
400
600
800
Temperature (oC)
Fig. 6 H2ꢀTPR profiles of W/HZSMꢀ5 catalysts with different W contents (a) 23.3
wt.%, (b) 34.9 wt.%, (c) 46.1 wt.%, (d) 51.1 wt.%, and (e) 56.0 wt.%.
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100
80
60
40
20
0
CH3OH
MF
HCHO
CH3CH2OH
DMM
CxHy
Toluene
CO
DME Conversion
25
30
35
40
45
50
55
W Content (wt.%)
Fig. 7 Catalytic performance of DME conversion over W/HZSMꢀ5 catalysts with
different W contents.
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0.05
Desorption
Adsorption
4000
3000
2000
1000
Wavenumber (cm-1)
Fig. 8 DRIFTS spectra of DME/O2 coꢀadsorption in (1% DME + 1% O2)/Ar flow on
the 46.1 wt.%W/HZSMꢀ5 catalyst.
29