Aromatization of dimethyl and diethyl ethers on Mo2C-promoted ZSM-5 catalysts

Article abstract of DOI:10.1016/j.jcat.2008.06.003

The adsorption and reaction pathways of dimethyl and diethyl ethers have been investigated on pure and Mo₂C-containing ZSM-5. The catalysts have been characterized by XPS and surface acidity measurements. FTIR spectroscopic measurements indicated that a fraction of both ethers dissociates on pure and Mo₂C-promoted zeolites already at 180-200 K resulting in the formation of methoxy from dimethyl ether, and ethoxy from diethyl ether. TPD experiments following the adsorption at 300 K showed desorption profiles corresponding to starting compounds and their decomposition products (methane, ethene and propene). ZSM-5 effectively catalyzed the reaction of dimethyl and diethyl ether above 473 K to yield various olefins and aromatics. From dimethyl ether xylene and from diethyl ether toluene were the main aromatic compounds. Adding Mo₂C to the zeolites greatly promoted the formation of aromatics very likely by catalyzing the aromatization of olefins formed in the reaction of ethers on zeolites.

Full text of DOI:10.1016/j.jcat.2008.06.003

Journal of Catalysis 258 (2008) 111–120
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Journal of Catalysis
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Aromatization of dimethyl and diethyl ethers on Mo2C-promoted ZSM-5 catalysts
Anita Kecskeméti b, Róbert Barthos a, Frigyes Solymosi a,b,∗
a
Reaction Kinetics Research Group, Chemical Research Centre of the Hungarian Academy of Sciences, University of Szeged, P.O. Box 168, H-6701 Szeged, Hungary
Institute of Solid State and Radiochemistry, University of Szeged, P.O. Box 168, H-6701 Szeged, Hungary
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The adsorption and reaction pathways of dimethyl and diethyl ethers have been investigated on pure and
Mo2C-containing ZSM-5. The catalysts have been characterized by XPS and surface acidity measurements.
FTIR spectroscopic measurements indicated that a fraction of both ethers dissociates on pure and Mo2C-
promoted zeolites already at 180–200 K resulting in the formation of methoxy from dimethyl ether,
and ethoxy from diethyl ether. TPD experiments following the adsorption at 300 K showed desorption
profiles corresponding to starting compounds and their decomposition products (methane, ethene and
propene). ZSM-5 effectively catalyzed the reaction of dimethyl and diethyl ether above 473 K to yield
various olefins and aromatics. From dimethyl ether xylene and from diethyl ether toluene were the main
aromatic compounds. Adding Mo2C to the zeolites greatly promoted the formation of aromatics very
likely by catalyzing the aromatization of olefins formed in the reaction of ethers on zeolites.
© 2008 Published by Elsevier Inc.
Received 5 March 2008
Revised 19 May 2008
Accepted 2 June 2008
Available online 1 July 2008
Keywords:
Aromatization of dimethyl ether
Aromatization of diethyl ether
ZSM-5
Mo2C/ZSM-5
1. Introduction
reforming on ZSM-5 [40–45]. Even less attention was devoted to
the aromatization of DEE. Abdel–Ghaffar found that at lower tem-
An attractive example at the synergistic phenomenon is that
whereas neither Mo2C nor ZSM-5 alone can produce benzene from
methane, the combination of Mo2C with ZSM-5 provides an unique
and effective catalyst for the aromatization of methane [1–7]. Fur-
thermore, while ZSM-5 alone catalyzes only the aromatization re-
actions of C2–C9 alkanes with low selectivity and yield [8–12], the
deposition of Mo2C on this zeolite greatly enhances the extent of
the formation of aromatics [13–21]. Recently, it was found that
Mo2C also exhibits a marked promoting effect on the aromatiza-
tion of ethanol [22,23] and methanol [24].
peratures ethylene and ethanol are being formed in the reaction of
DEE on ZSM-5, but the production of aromatics was not reported
[46]. At higher temperatures additional reactions, such as addition
of C2 units, followed by isomerization, cracking and aromatization
are expected.
2. Experimental
2.1. Methods
In the present work an account is given on the reaction of
dimethyl ether (DME) and diethyl ether (DEE) on pure and Mo2C-
containing ZSM-5. The study of the reaction of DME was motivated
by several reasons. DME is a key compound in the conversion
of methanol to gasoline (MTG process) [25–33], and it is also a
reaction intermediate in the aromatization of methanol [24]. In ad-
dition, DME as an alternative fuel can replace diesel oil, as the
burning of DME produces much less pollutant [34–36]. The oxida-
tion of DME has been also studied [37–39]. In a comparative study
on its combustion on alumina-supported Pt metals Rh/Al2O3 ex-
hibited the highest specific activity [37]. In spite of the growing in-
terest towards the catalytic reactions of DME, its aromatization on
ZSM-5-based catalysts was relatively neglected. Most of the stud-
ies dealt with the synthesis of light olefins, hydrolysis and steam
Catalytic reaction was carried out at 1 atm of pressure in a
fixed-bed, continuous flow reactor consisting of a quartz tube [23,
24]. The flow rate was 40 ml/min (NTP). The carrier gas was Ar.
The DME or DEE content was 8–10%. Generally 0.3 g of very loosely
compressed catalyst sample was used. Reaction products were an-
alyzed gas chromatographically using a Hewlett–Packard 5890 gas
chromatograph with PORAPAK Q + PORAPAK S columns. The con-
version of ethers was calculated taking into account its amount
consumed. The selectivity for reaction products, S; was defined as
xini
ꢀ
Si =
,
i xini
where xi is the mole fraction of product i, and ni is the number
of carbon atoms in each molecule of gaseous products. FTIR spec-
tra of adsorbed gases were recorded with a Biorad (Digilab. Div.
FTS 155) instrument with a wavenumber accuracy of 4 cm−1. All
the spectra presented are difference spectra. Thermal desorption
measurements (TPD) were carried out in the catalytic reactor. The
Corresponding author at: Institute of Solid State and Radiochemistry, University
of Szeged, P.O. Box 168, H-6701 Szeged, Hungary. Fax: +36 62 420 678.
*
E-mail address: fsolym@chem.u-szeged.hu (F. Solymosi).
0021-9517/$ – see front matter © 2008 Published by Elsevier Inc.
doi:10.1016/j.jcat.2008.06.003

112
A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
catalysts were treated with DME/Ar or DEE/Ar mixture containing
10% ethers at 313 K for 30 min, and flushed with Ar for 30 min.
The TPD was carried out in Ar flow (20 ml/min) with a tempera-
ture ramp of 5 K/min from 313 K to 1063 K. Desorbing products
were analyzed by gas chromatography. Samples were taken in ev-
ery 5 min.
alytic reactor. In order to remove the excess carbon deposited on
the Mo2C during the preparation, and the Mo–O species produced
during passivation, the Mo2C-containing catalyst has been reduced
before the catalytic measurements in situ at 873 K in H2 stream
for 60 min. DME was the product of Gerling Holz Comp. with 99%
purity. Other gases used were of commercial purity (Linde).
All of the catalysts used in this study have been character-
ized by XPS measurements. The binding energy for Mo(3d5/2) and
Mo(3d3/2) showed some slight variation with different samples,
but dropped to 227.8–228.2 and 230.7–231.1 eV, respectively; that
for C(1s) dropped to 283.8 eV. These values are consistent with
those attributed to Mo2C [47,48]. In terms of the acidity of the
catalysts, we refer to our previous work, which used the same
samples [49]. We found that during the preparation of Mo/ZSM-
5 catalyst, Mo compounds reacted with OH groups of ZSM-5. As
a result, the Brönsted sites decreased markedly in number, but
were not completely eliminated. The greater the carbide content,
the greater the decrease. At the same time, the Lewis acidity was
somewhat enhanced by Mo2C deposition.
The XPS measurements were performed in a Kratos XSAM 800
−8
instrument at a base pressure of 10
Torr using MgKα primary
radiation (14 kV, 10 mA). To compensate for possible charging ef-
fects, binding energies (BE) were normalized to the Fermi-level for
the Mo2C. The amount of coke deposited on the catalyst during
the reaction was determined by temperature-programmed reaction
(TPR). The catalyst was cooled down in flowing argon, then heated
in H2 stream with a rate of 5 K/min and the hydrocarbons formed
were measured.
2.2. Materials
Mo2C-containing catalysts prepared in the same way as de-
scribed in our previous papers [23,24]. The starting materials were
pure MoO3 or MoO3/support samples, the latter were prepared
by impregnating the supports with a basic solution of ammonium
heptamolybdate to yield the desired % of MoO3. The suspension
was dried and calcined at 863 K for 5 h. The following materials
were used as support ZSM-5 with SiO2/Al2O3 ratio at 80 and 280,
BET area: 425 and 400 m2/g. MoO3 samples were heated under
10% v/v C2H6/H2 gas mixture from room temperature to 900 K at a
heating rate of 0.8 K/min. Afterwards the sample was cooled down
to room temperature under argon. The carbide so formed was pas-
sivated in flowing 1% O2/He at 300 K, or used at once in the cat-
3. Results
3.1. Dimethyl ether
3.1.1. FTIR and TPD measurements
Fig. 1A shows the IR spectrum of ZSM-5(80) after adsorption of
DME at ∼180 K for 15 min. and subsequent degassing at differ-
ent temperatures. At 180–206 K vibrational modes are observed at
2997, 2972, 2945, 2927, 2898, 2878, 2840, 2831 and 2813 cm−1. In
the low frequency region weak absorption bands appeared at 1475,
(A)
(B)
Fig. 1. FTIR spectra of ZSM-5(80) (A) and 2% Mo2C/ZSM-5 (B) following the adsorption of dimethyl ether (1.5 Torr) at 180–190 K, and after heating the samples to different
temperatures during continuous degassing.

A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
113
(A)
(B)
Fig. 2. TPD spectra following dimethyl ether adsorption on pure ZSM-5(80) (A) and 5% Mo2C/ZSM-5(80) (B) at 300 K.
Table 1
From pure ZSM-5 the major desorbing product is DME (Tp
=
Characteristic absorption bands of adsorbed dimethyl ether and its primary dissoci-
ation products
373 and 493 K). The desorption of a much smaller amounts of
ethene (Tp ∼ 583 K), and propene (Tp ∼ 583 K) were also ob-
served. From Mo2C containing ZSM-5 we found the same prod-
ucts with slightly different peak temperatures. A slight continuous
desorption of hydrogen and CO was also observed from both
samples. An interesting feature of TPD curves is that methane
was also released particularly from Mo2C-containing catalyst.
This is formed very likely in a surface reaction of carbonaceous
species.
Vibra-
tional
mode
DME(g)
[51]
DME(a)
alumina
[51]
CH3O(a)
alumina
[51]
CH3O(a)
silica
[52]
(CH3)2O/
Rh(111)
[53]
CH3(a)
Mo2C/Mo(100)
[50]
υa(CH3) 2996
2925
υs(CH3) 2817
2δ(CH3) 2887
2984
2922
2821
2890
1477
1459
1252
1116
2960
2954
2950
2930
2849
2858
δ(CH3)
1470
1456
1244
1179
1102
1475
1420
1081
1465
1175
1370
γ (CH3)
3.1.2. Catalytic reaction of DME
The reaction of DME on pure Mo2C started at 500–523 K and
the highest value, 15–17%, was achieved already at 673 K. Above
this temperature a rapid deactivation of the Mo2C occurred. The
major products were methane, CO and CO2 with selectivity of 50,
30 and 15%, respectively. Other carbon-containing compounds were
detected only in traces.
υas(CO)
δs(CH3)
δs(CO)
1092
1055
1180
885
450
δ(COC)
−1
1457 and 1428 cm
(not shown). Upon heating the sample under
Unpromoted ZSM-5 was found to be an effective catalyst for the
reaction of DME. Independently on the composition, reaction of
DME was observed above 473 K (Fig. 3A). The conversion was 10%
at 473 K, which grew to 95–97% at 573 K. An interesting feature of
the reaction on pure ZSM-5 is the formation of methanol in few %
at low temperature, 473–523 K. The aromatization of the DME was
observed already at 573 K, when benzene, toluene, xylene and C9
aromatics were produced. Raising the temperature was disadvanta-
geous for their production particularly for that of C9 aromatics, but
favored the formation of C2–C3 olefins. The selectivity of C4 olefins
also decreased at higher temperatures. Selectivity data obtained for
ZSM-5(80) as a function of reaction temperature are displayed in
Fig. 3A. Aromatics were produced with somewhat lower selectiv-
continuous degassing the intensities of the bands at 2927, 2898,
−1
2878, 1457 and 1428 cm
gradually decreased. At room tempera-
ture an IR spectrum with major vibrational modes at 2972, 2945,
−1
2840 and 1457 cm
2813 cm
is observed. The bands at 2898, 2878 and
−1
became very weak or not detectable. When the ad-
sorption was performed at 300 K we observed the same spectral
features. Very similar spectra were obtained for 5% Mo2C/ZSM-
5(80), but with much less intense bands (Fig. 1B). They exhibited
somewhat less stability than on the pure zeolite. In Table 1 the
characteristic vibrations of DME and its possible dissociation prod-
ucts are collected.
TPD spectra for various products after adsorption of DME on
pure and 5% Mo2C/ZSM-5(80) at 313 K are displayed in Fig. 2.

114
A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
(A)
(B)
Fig. 3. Selectivities of the products formed in the reaction of dimethyl ether over ZSM-5(80) (A) and 5% Mo2C/ZSM-5(80) (B) at different temperatures.
Table 2
Main results obtained in the reaction of dimethyl ether on ZSM-5-based catalysts
Catalyst
Temp.
(K)
Selectivity (%)
Ethylene
ꢀ
Propylene
C4 aliph.
C5 aliph.
C6 aliph.
Benzene
Toluene
Xylenes
C9+ arom.
3.7
4.5
aroma
ZSM-5(80)
723
773
20.8
20.5
33.8
33.0
16.4
15.7
3.5
3.9
0.9
0.8
0.6
0.5
5.0
4.9
7.7
8.5
16.9
18.3
2% Mo2C/ZSM-5(80)
5% Mo2C/ZSM-5(80)
10% Mo2C/ZSM-5(80)
ZSM-5(280)
723
773
12.7
14.1
21.0
19.2
3.6
12.9
7.8
5.0
2.8
1.4
0.7
0.3
5.4
6.9
14.6
17.5
8.2
11.9
28.9
36.6
723
773
11.2
7.1
21.0
6.5
11.7
3.2
5.1
1.7
1.4
1.1
0.8
1.0
5.7
5.5
18.9
27.3
12.6
27.1
37.9
60.8
723
773
11.3
8.2
14.6
4.7
8.7
2.1
4.2
1.2
2.3
0.9
1.0
0.5
4.7
3.4
22.3
22.5
14.4
20.5
42.5
46.9
723
773
9.3
14.5
45.7
49.3
25.4
21.7
9.5
6.6
2.1
0.4
2.1
1.1
1.4
1.8
2.4
2.5
1.1
0.5
6.9
5.9
2% Mo2C/ZSM-5(280)
5% Mo2C/ZSM-5(280)
10% Mo2C/ZSM-5(280)
723
773
8.7
11.4
35.9
35.7
21.8
17.3
8.9
7.2
3.7
2.0
1.8
1.2
3.7
5.9
8.8
10.3
3.6
3.1
17.8
20.4
723
773
11.5
8.2
22.7
8.2
11.7
3.7
5.0
2.2
0.4
0.1
2.3
0.2
5.7
5.0
17.8
25.3
12.2
24.9
38.0
55.5
723
773
9.1
11.9
35.6
31.6
21.4
15.5
10.5
7.9
4.3
2.5
1.5
1.0
3.0
4.9
8.2
11.3
2.7
4.0
15.4
21.2
ity on ZSM-5(280) compared to ZSM-5(80). Characteristic data for
both zeolites are collected in Table 2.
ZSM-5(80) raised total aromatic selectivity at 723 K from 16.9% to
28.9%. This value increased to 37.9% in case of 5% Mo2C/ZSM-5(80)
and reached value of 42.5% for 10% Mo2C/ZSM-5(80).
Selectivity data for 5% Mo2C/ZSM-5(80) are presented in Fig. 3B.
The effect of Mo2C content of ZSM-5 on the product distribution
is shown in Fig. 4A.The reaction of DME has been followed on
5% Mo2C/ZSM-5(80) in time on stream at 673 K. No decline in
the conversion of DME and in the selectivity of various aromat-
Deposition of Mo2C on zeolites influenced only little the con-
version of DME obtained for pure ZSM-5, but affected the prod-
uct distribution. The formation of aromatics, particularly that of
xylenes and C9, was clearly enhanced at the expense of olefins.
In contrast to the unpromoted ZSM-5 the selectivity of xylenes
and C9 increased with the temperature. Addition of 2% Mo2C to

A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
115
(A)
(B)
Fig. 4. Effects of Mo2C content of ZSM-5(80) on the selectivities of various products formed at 723 K (A). Formation of hydrocarbons in the TPR measurements following
dimethyl ether reaction on 5% Mo2C/ZSM-5(80) at 673 K for 17 h (B).
Table 3
Characteristic absorption bands of diethyl ether and its primary dissociation products
Vibrational
mode
(C2H5)2O(gas)
[50]
(C2H5)2O(a)
monolayer
Cu(111) [56]
(C2H5)2O(a) multilayer
Mo2C/Mo(100)
Ta = 100 K [50]
C2H5O(a)
alumina
[54]
C2HO5(a)
ZSM-5
[23]
C2H5(a)
Mo2C/Mo(100)
[55]
C2H4(a) ZSM-5
present study
νa(CH2)
νa(CH3)
2953
2981
2930
2961
2976
2986
2895
2970
2937
2984
2910
2937
2963
2938
2858
νs(CH3)
2900
2930
2870
1450
1390
2938
2860
νs(CH2) + ν(CH3)
+ 2*δ(CH3)
ν(CO)
2872
2870
δ(CH2)
1492
δa(CH3) in plane
δa(CH3) out of plane
δs(CH3) + ω(CH2)
τ(CH2)
1454
1453
1381, 1371, 1354
1278
1170
1478
1448
1470
1230
1442
1464
1390
1285
1280
1170
1157
1394
1478
ρ(CH2 + CH3)
ρ(CH2) + ν(CO)
1144
1170
+ ν(CC) + bends
ω(CH2)
ν(CO), νa(“COC”)
ρ(CH3), ν(CC)
ν(CC + CO) + ρ(CH3)
τ(CH2)
ν(CC), νs(CCO), ρ(CH3)
ρ(CH3 + CH2)
ρ(CH2)
1130
1047
923
1112
1044
1150
940
835
955
848
823
823
δ(CCO)
419
449
νs(MC)
ics occurred in 7 h. Positive effect of Mo2C was also observed
in case of ZSM-5(280). While over pure sample total aromatic
selectivity amounted to ∼7% at 723 K, this value raised to 18%
over 2% Mo2C/ZSM-5(280) and 38% over 5% Mo2C/ZSM-5(280). At
higher Mo2C loading (10%) the total aromatic fell to ∼15% (Ta-
ble 3).

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A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
In one case after catalytic run at 673 K for 17 h we car-
ried out temperature-programmed reaction (TPR) with H2. The re-
sults plotted in Fig. 4B, show the formation ethylene, propylene
(Tp = 598 K) ethane (Tp = 773 K) and a much larger amount of
methane (Tp = 923 K). The sum of the hydrocarbons, expressed
in surface carbon is 23.47 mgC/gcat. The carbon content of DME
passed through the catalyst in 17 h is 8.32 gC/gcat. It means that
only its 0.28% underwent total decomposition to surface carbon.
This means that the carbon deposition cannot influence apprecia-
bly the selectivities of various compounds.
3.2. Diethyl ether
3.2.1. FTIR and TPD measurements
FTIR spectrum of adsorbed DEE at ∼186 K on ZSM-5(80) is
presented in Fig. 5. In the C–H stretching region intense absorp-
tion bands were registered at 2981, 2934, 2908, 2870, 2806 and
2776 cm−1. In the C–H deformation mode five weak peaks were
detected at 1475, 1458, 1446, 1417 and 1384 cm−1. The spectra
remained practically unaltered up to 263 K. A dramatic change oc-
curred when the adsorbed layer was heated to and above 300 K,
−1
when vibrational frequencies at 2930, 2870, 2806 and 2776 cm
−1
disappeared, a shoulder developed at 2997 cm
and new weak
appeared. These spec-
−1
absorption bands at 2908 and 2883 cm
tral features were eliminated or reduced markedly in intensities at
523 K. In the case of 5% Mo2C/ZSM-5(80) we obtained the same
absorption bands of lower intensities and at slightly different po-
sitions. In Table 3 the absorption bands of DEE and its possible
primary dissociation products are shown.
TPD spectra following the adsorption of DEE on ZSM-5(80)
and 5% Mo2C/ZSM-5(80) at 300 K are shown in Figs. 6A and 6B.
The weakly adsorbed DEE desorbed from undoped zeolite with
Tp = 463 K. In addition the desorption of ethylene (Tp = 523 K)
and a very small amount of ethane (Tp = 463 K) also occurred. The
deposition of Mo2C on this zeolite resulted in the release of same
compounds with slightly different peak temperatures (Fig. 6B).
Fig. 5. FTIR spectra of ZSM-5(80) following the adsorption of diethyl ether (1.5 Torr)
at 180–190 K, and after heating the samples to different temperatures during con-
tinuous degassing.
A new feature for Mo2C/ZSM-5 is the desorption of methane (Tp
=
493 K).
(A)
(B)
Fig. 6. TPD spectra following diethyl ether adsorption on pure ZSM-5(80) (A) and 5% Mo2C/ZSM-5(80) (B) at 300 K.

A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
117
(A)
(B)
Fig. 7. Selectivities of the products formed in the reaction of diethyl ether over ZSM-5(80) (A) and 5% Mo2C/ZSM-5(80) (B) at different temperatures.
3.3. Catalytic reaction
ZSM-5(280) based catalysts exhibited weaker catalytic performance
than ZSM-5(80) supported materials. The total aromatic selectiv-
ity varied between 31–40% at 573–773 K. Besides aromatics, C2–C4
olefins and C5–C4 hydrocarbons were the main reaction products.
Adding 5% Mo2C to ZSM-5(280) resulted in the 6–12% growth in
the aromatic selectivity. Data for catalytic studies for this sample
are also summarized in Table 4.
As in the previous case, pure Mo2C catalyzed effectively the de-
composition of DEE above 523 K to give H2, ethane, ethylene, CO
and methane as major products. Total conversion was approached
at 673 K. Aromatic compounds were not identified in the temper-
ature range of 523–773 K.
TPR measurements were also carried out for the 5% Mo2C/ZSM-
5(80) catalyst used for 5 h at 723 K. Ethylene, ethane, propane
all with Tp ≈ 798 K, and methane (Tp ≈ 973 K) were released.
Calculation based on their amounts showed that appr. 0.9% of
DEE transferred decomposed completely to carbonaceous de-
posit.
Completely different products were formed on ZSM-5(80). At
573 K, when the conversion was more than 80%, propane and
butenes were the main reaction products, C2–C3 olefins and alka-
nes formed only with low selectivities. Whereas the selectivity of
butenes decreased that of C2–C3 olefins increased with the tem-
perature. The formation of aromatic compounds started occurred
even at 573 K with 29% total selectivity. Upon increasing the re-
action temperature this value slowly rises and at 773 K 41.5%
was measured. Selectivity data are shown in Fig. 7A. Over 5%
Mo2C/ZSM-5(80) catalyst the conversion changed similarly as in
case of non-doped catalyst. In respect of aromatic formation the
positive effect of Mo2C additive was experienced in whole tem-
perature range. The growth of aromatic selectivity was 16–20%
compared to non doped sample (Fig. 7B). Over 10% Mo2C/ZSM-5
catalyst only a slight growth (2–5%) in the selectivity of aromatics
was detected compared to 5% Mo2C/ZSM-5. Whereas adding Mo2C
to ZSM-5 increased the total selectivity of aromatics, the distribu-
tion of aromatic compounds depended on the Mo2C content. At
723 K the selectivity of benzene and toluene gradually increased
with the loading of Mo2C: this value for xylene and particularly for
C9 aromatics varied only little. This is well illustrated by the data
plotted in Fig. 8A. The effect of Mo2C content on the product distri-
bution of aromatics was somewhat different at 773 K (Table 4). The
4. Discussion
4.1. Dimethyl ether
4.1.1. Adsorption and desorption
The absorption bands 2997, 2927, 2898, 2878, 2831 and 2813
−1
cm
determined following the adsorption of DME on pure ZSM-
5(80) at ∼180 K (Fig. 1) correspond very well to the vibrational
motions of the molecularly adsorbed DME (Table 1). In addition,
however, the appearance of spectral features at 2972, 2945 and
−1
2840 cm
suggests the occurrence of a dissociation in the ad-
sorbed layer even at the temperature of adsorption. This process
very likely involves the scission of one of the C–O bonds of DME
(CH3)2O(a) → CH3O(a) + CH3(a)
(1)
and the formation of Al–OCH3. The generation of Si-OCH3 is
less likely as it requires high temperature and high pressure

118
A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
(A)
(B)
Fig. 8. Effects of Mo2C content of ZSM-5(80) on the selectivities of various products formed at 723 K (A). Formation of hydrocarbons in the TPR measurements following
diethyl ether reaction on 5% Mo2C/ZSM-5(80) at 723 K for 5 h (B).
Table 4
Main results obtained in the reaction of diethyl ether on ZSM-5-based catalysts
Catalyst
Temp.
(K)
Selectivity (%)
Methane
ꢀ
Ethylene
Ethane
Propylene
C4 aliph.
Benzene
Toluene
Xylenes
C9+ arom.
5.5
3.1
aroma
ZSM-5(80)
673
773
1.1
7.7
3.9
13.8
2.9
9.9
3.3
9.8
22.0
5.0
4.6
10.3
13.9
17.3
12.3
10.9
36.1
41.5
2% Mo2C/ZSM-5(80)
5% Mo2C/ZSM-5(80)
10% Mo2C/ZSM-5(80)
ZSM-5(280)
673
773
0.4
2.4
6.2
29.5
1.2
4.7
6.9
10.9
15.8
2.8
4.0
13.2
15.5
20.0
24.5
10.9
13.4
2.6
57.2
46.7
673
773
0.5
3.5
7.7
26.6
2.2
8.4
7.3
5.3
19.2
1.1
4.8
17.0
16.9
19.2
18.7
11.6
7.3
3.8
47.8
51.6
673
773
1.3
4.5
4.5
16.1
4.3
15.0
3.5
3.3
10.7
0.6
9.6
24.5
22.1
19.4
17.1
7.6
4.9
2.6
53.8
54.2
673
773
0.4
4.2
12.2
29.7
1.5
4.1
11.9
19.2
26.4
7.7
2.9
6.0
9.1
12.4
13.6
9.0
9.9
2.8
35.6
30.1
5% Mo2C/ZSM-5(280)
673
773
0.4
2.7
13.0
33.9
1.9
6.5
10.7
7.6
21.2
2.2
2.6
13.0
12.1
17.4
17.4
10.1
9.1
3.1
41.2
43.6
[52]. The spectral features detected at 300 K can readily be as-
signed to the normal modes of the surface methoxy species (Ta-
ble 1). As we found no vibrational modes for adsorbed CH3
we assume that it also interacted with the surface oxygen of
the zeolite yielding CH3O species. Similar spectral features were
measured for DME adsorbed on the Mo2C-promoted ZSM-5(80).
This is not surprised if we take into account the large sur-
face area of the zeolite. The fact that the intensities of vibra-
tional modes were lower than in the previous case indicates
that Mo2C occupies some of the adsorption sites on the ZSM-
5, which also leads to the decrease of the number of Brönstead
sites [49].
4.1.2. Catalytic reactions
ZSM-5 zeolites effectively catalyzed the reaction of DME. As
shown in Fig. 3, whereas at lower temperatures C4 olefins are
the main products, at higher temperature the formation of C2H4
and C3H6 became the dominant compounds. Aromatics, benzene,
toluene, xylene and C9 are also produced: their total selectivity
is decreased with the temperature (Fig. 3A, Table 2). The conver-
sion of DME to olefins (DTO process) is closely related to methanol
to olefins (MTO) process, which is well studied due to its great
technological importance. The most effective catalysts for the MTO
process are the zeolites, which due to their acidic sites can pro-
mote the formation of C–C bond, which is the vital step in the

A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
119
MTO reaction and also in the subsequent reactions [25–28]. De-
spite the intensive research, however, many details of this very
complex transformation has been the subject of a great debate.
As regards the C–C formation more than 20 different proposals
may be discerned. The formation of free carbene radicals [26], for-
mation of trimethyloxonium and ylide intermediates [57], carboca-
tions [27], and mechanisms involving ketene intermediate and CO
as the active catalyst [58] have been proposed. Intermediate sur-
face methoxy species formation has also been suggested [59]. The
hydrocarbon-pool mechanism has also gained increasing accep-
tance [60]. It proceeds via repeated methylations and dealkylations
of organic reaction centers trapped inside the zeolite voids. Methyl
benzenes, and the higher homologues have been identified as key
intermediates [60]. According to Oláh and Molnár [57] it is now
generally accepted that methanol undergoes preliminary Brönsted
acid catalyzed dehydration step, and DME, or equilibrium mixture
of DME and methanol, acts as the precursor to hydrocarbon for-
mation. An important contribution to this subject was provided
by the use of density functional theory (DFT). Blaszkowszki and
van Santen [61] analyzed several reaction paths for C–C formation
bond with this method. They confirmed the experimental results of
Manson et al. [62], namely that ethanol and ethyl methyl ether are
formed from surface methoxy groups, methanol, and/or dimethyl
ether. Formation of trimethyl oxonium, proposed to be an inter-
mediate in initial C–C bond formation, was also considered as a
possible intermediate.
ing the same products with a small amounts of C2 compounds.
There was, however, no sign of the formation of aromatics. We
cannot rule out the decomposition of DME proceeds in this way
on Mo2C when it supported by ZSM-5. As Mo2C reduces the num-
ber of Brönsted sites of ZSM-5 [49], we have to assume that it
opens a new route for the activation of DME and/or for the fur-
ther reaction. As was mentioned above pure Mo2C catalyzes the
decomposition of DME by facilitating its dissociation to the reac-
tive CH3 and CH3O species (Eq. (1)). Another role of the Mo2C very
likely is to activate the light olefins formed in the early reaction
of DME on ZSM-5. Previous works disclosed Mo2C can promote
effectively the aromatization of olefins by providing active dehy-
drogenation centers for them resulting in different CxHy products,
which are converted into aromatics on the acidic sites of the zeo-
lite [4,13,23].
4.2. Diethyl ether
4.2.1. Adsorption and desorption
Taking into account the similarity of IR spectra of DEE and its
possible dissociation products (Table 3), it is not easy to distinguish
between these compounds. The intense absorption bands at 2981,
−1
2930 and 2870 cm
observed on the IR spectra of adsorbed DEE
on ZSM-5(80) at 180–200 K correspond very likely to the vibration
of the molecularly adsorbed DEE. The presence of weak spectral
−1
features at 2939 and 2908 cm
suggests that dissociation of DEE
Depending on the nature of the catalysts the light olefins
formed can be also activated on the Brönsted sites of ZSM-5 to give
higher olefins, aromatics, alkanes and naphthalenes. As regards the
production of aromatics the extent of the activation, however, is
limited, as a result the yield of aromatics from methanol was only
25.1% on pure ZSM-5(80) and 7.0% on ZSM-5(280) [24]. Similar
processes will occur on zeolites when the starting compound is
DME. The Brönsted sites of ZSM-5 obviously play a decisive role
both in the activation of DME and also in the further reaction of
light olefins into aromatics via hydrogen transfer, alkylation and
polycondensation. The yields of aromatics was, however, somewhat
less compared to those measured in the reaction of methanol (Ta-
ble 2). When Mo2C was added to ZSM-5 the product distribution
changed, and the formation of aromatics was clearly enhanced. At
723 K the total selectivity of aromatics increased with the rise of
the amount of Mo2C on ZSM-5(80) (Fig. 4A). This result is mainly
due to the higher formation of xylenes and C9 aromatics. Similar
feature was experienced in the case of ZSM-5(280). In this case the
optimum amount of Mo2C is 5%, as at higher loading the formation
of aromatics falls down (Table 2).
The question to be answered is the way of the effect of Mo2C.
Previous studies on Mo2C/Mo(100) surface showed that the Mo2C,
or more precisely the metal-terminated carbide is a reactive sur-
face [63]. It can activate the C–O bond in the adsorbed CO and
CO2 [64] and even the C–I bond in various alkyl iodides by form-
ing a strong bond with the oxygen and iodine [65]. The release
of iodine from Mo2C proceeds above 900 K with Tp ∼ 950 K [65].
Mo2C/Mo(100) model catalyst also exhibited a high reactivity to-
wards DME under UHV conditions [50]. The extensive lowering of
the work function of Mo2C/Mo(100) following the adsorption of
DME indicated a strong interaction consisting of an electron dona-
tion from the oxygen long pair of DME to the carbide surface [50].
As a result one of the C–O bonds is cleaved yielding CH3O(a) and
CH3(a) fragments (Eq. (1)) as the primary products. HREEL spec-
troscopic measurements provided spectroscopic evidences for the
formation of these species. Both intermediates, however, are un-
stable on Mo2C and decompose to hydrogen, CO, CH4 and surface
carbon. The formation of C2 compounds was not detected.
(C2H5)2O(a) = C2H5O(a) + C2H5(a)
(3)
already occurred at 180–200 K, and they reflect the formation of
Al–OC2H5 surface species. The vibration of C2H5 formed may also
contribute to these spectral features (Table 3). Upon heating the
adsorbed layer the vibrational modes of DEE gradually attenuated,
while those belonging to ethoxy species, 2984, 2939–2943, 2908
and 2883 cm−1, remained almost unaltered up to 423 K.
4.2.2. Catalytic reaction
The reaction of DEE on ZSM-5 catalyst produced mainly C2–C4
olefins and various aromatics at 523–773 K. For the description of
the formation of these compounds it should be pointed out an im-
portant difference between DME and DEE. In the latter case the
C–C bond already exists in the molecule, consequently there is no
need to consider and assume the complex reaction sequence de-
scribed for the activation and reaction of DME. This is probably the
reason that the formation of aromatics from DEE proceeds more
easily compared to the case of DME. Apart from this the acidic
sites of pure ZSM-5 still play an important role in the activation of
the molecule, and in the production of aromatics (Fig. 7).
In addition to the rupture of C–O bond (Eq. (4)) we may count
with the dehydration of DEE
(C2H5)2O(a) = 2C2H4(g) + H2O(g)
or
(4)
(C2H5)2O(a) = C4H8(a) + H2O(a)
.
(5)
This process occurs readily in the case of ethanol over zeolites of
high Brönsted acidity [22,23]. The fact that at low temperature,
523–573 K, butanes are the main products suggests that the cou-
pling of C2H5 groups in the adsorbed DEE on ZSM-5(80) (Eq. (5))
is more favorable than the simple dehydrogenation step (Eq. (4)).
As in the previous case Mo2C exerted a considerable influence
on the product distribution and enhanced the formation of aromat-
ics. In the study of the interaction of DEE with Mo2C/Mo(100) sur-
face it was found that the adsorption of DEE also causes a marked
decrease (maximum 2.8 eV) in the work function of Mo2C/Mo(100)
at 100 K [50], which was again attributed to the electron donation
In harmony with this behavior unsupported polycrystalline
Mo2C catalyzed the decomposition of DME above 523 K yield-

120
A. Kecskeméti et al. / Journal of Catalysis 258 (2008) 111–120
from oxygen lone pair of DEE to the carbide. This electronic in-
teraction may lead to an increased activation of C–O bond in the
adsorbed DEE on Mo2C. Analysis of the HREEL spectra of adsorbed
and annealed layer led to the assumption of the occurrence of pri-
mary process (Eq. (3)) followed by the decomposition of adsorbed
reaction intermediates mainly to CO, H2, methane and ethylene
[50]. Similar features and products were found in the present case
on polycrystalline Mo2C without any traces of aromatics. How-
ever, the situation is different, when Mo2C is deposited on ZSM-5
containing Brönsted sites. The carbide changed the product dis-
tribution and promoted the aromatization of DEE. Following the
previous discussion (Section 4.1), the effect of Mo2C can be again
attributed to the dehydrogenation property of Mo2C, which is the
first step in the oligomerization, cyclization, and hydrogen transfer
of olefins [66,67]. As was reported before Mo2C markedly facil-
itates the aromatization of butanes and butenes on ZSM-5 zeo-
lites [68].
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Acknowledgments
This work was supported by OTKA under contract number NI
69327 and by the Hungarian National Office of Research and Tech-
nology (NKTH) and the Agency for Research Fund Management and
Research Exploitation (KPI) under contract no. RET-07/2005 and by
the Office of Mayor for Hódmezo˝vásárhely.
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