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ZSM-5 catalyst
APPLIED CATALYSIS A: GENERAL
ELSEVIER
Applied Catalysis A: General 166 (1998) 225-235
Conversion of ethane into benzene on Mo&/ZSM-5 F.
catalyst
Solymosi*, A. Sz6ke
Institute of Solid State and Radiochemistry, A. Jdzsef University, and Reaction Kinetics Research Group of the Hungarian Academy of Sciences, P.O. Bar 168. H-6701 Szeged, Hungaq
Received 24 January 1997; received in revised form 6 August 1997; accepted 6 August 1997
Abstract The reaction of ethane on unsupported and supported molybdenum compounds has been investigated at 773-973 K. ZSM-5 was used as support. Reaction products were analyzed using gas chromatography. Changes in the composition of catalyst samples were followed by X-ray photoelectron spectroscopy. Unsupported Moos is partially reduced with ethane above 800 K to give Hz0 and CO*. The formation of Hz, C2H4 and CH4 was also observed. MO-free ZSM-5 exhibited relatively high activity towards dehydrogenation, hydrogenolysis and aromatization of ethane above 773 K. Deposition of Moos on ZSM-5 significantly enhanced the conversion of ethane and also the selectivity of benzene production. Alteration of the catalytic behaviour of MoO,/ZSM-5 in time on-stream at 773-973 K was attributed to the reaction of MoOa, to carbon deposition and to the formation of MoaC. Unsupported Mo2C catalyzed the dehydrogenation of ethane without producing benzene. In contrast, Mo&/ZSM-5 was found to be an effective catalyst in the aromatization of ethane. At 973 K the conversion of ethane was -67% and the selectivity to benzene formation 31%. 0 1998 Elsevier Science B.V. Keywords:
MozC; ZSM-5; Ethane conversion;
Benzene formation
1. Introduction
After an extensive research on the oxidative coupling of methane [ 1,2], recently, increasing attention is being paid to the reaction of methane under nonoxidative conditions [3-91. Whereas the dominant route is the decomposition of CH4 to carbon, on supported metals, with the formation of minor amounts of C2 compounds [3-91, higher hydrocarbons and even aromatic compounds were also produced on oxide catalysts [lO,ll]. As regards the formation of benzene, MoOs/ZSM-5 proved to be the best catalyst
*Corresponding author. 0926-860X/98/$19.00 6’; 1998 Elsevier Science B.V. All rights reserved PII SO926-860X(97)00260-3
[12-l 81. The selectivity to benzene formation attained a value of 61-58% at 2.0-5.7% CH4 conversion. It is assumed that ethane and ethylene are the primary products, which - through several steps, some of them occurring on the support - are converted into benzene. Recent studies, however, revealed that Moos is transformed into MozC in the hightemperature introduction of CH4 with Moos, and the Mo2C formed is considered to be the active site for the production of CHa and CH2 fragments from methane [1.5-181. In order to establish the mechanism of benzene formation, it appeared inevitable to know more about the reaction of ethane on Mo~C, which - to our knowledge - has not been investigated, yet.
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2. Experimental Catalytic reactions were carried out at 1 atm C2H6 pressure in a fixed-bed continuous-flow reactor consisting of a quartz tube (17 mm id.) connected to a capillary tube [14,18]. The flow rate was 12 ml/min. Generally, 0.5 g of loosely compressed catalyst sample was used. Reaction products were analyzed with the help of a Hewlett-Packard 5890 gas chromatograph and a Porapak QS column. The ethane conversion was calculated from the hydrogen balance. The selectivity values of product formation represent the fraction of ethane that has been converted into specific products taking into account the number of carbon atoms in the molecules. The XPS measurements were performed in a Kratos XSAM 800 instrument at a base pressure of lop9 torr using MgKa primary radiation (15 kV, 5 mA). To compensate for possible charging effect, binding energies (BE) were normalized with respect to the position of the Si(2p) in SiOz (BE = 103.4) for supported samples and to Fermi-level for the MozC. The pass energy was set at 40 eV; and an energy step width of 50 meV and dwell time of 300 ms were used. Typically, 10 scans were accumulated for each spectrum. Fitting and deconvolution of the spectra were made using the VISION software (Kratos). Great care was taken to protect the samples from air during its transformation from the catalytic reactor to the XPS chamber. The gases used were of commercial purity (Linde). Ar (99.996) and H2 (99.999) were deoxygenated with an oxytrap. The other impurities were adsorbed by 5 A molecular sieve at the temperature of liquid nitrogen. The H-ZSM-5 support was obtained by five times repeated ion exchange of Na-ZSM-5 @i/Al = 55.0) with an aqueous solution of ammonium nitrate (1 N), and calcined in air at 863 K for 5 h. MoOs/ZSM-5 catalysts were prepared by impregnating H-ZSM-5 with a basic solution of ammonium paramolybdate to yield a nominal 2 wt% of Moos. The suspension was dried at 373 K and calcined at 873 K for 5 h. Before catalytic measurements each sample was oxidized in an O2 stream at 973 K for 30 min in situ and then flushed with Ar for 15 min. Hexagonal Mo2C was prepared by the method of Boudart et al. [19,20]. Briefly, about 0.5 g of Moos was heated in 1 : 4 methane-H2 mixture flowing at
300 ml (STP)/min in a quartz cell with two stopcocks. Preparation temperature was increased rapidly to 773 K and then at 30 K/h from 773 to 1023 K and maintained at 1023 K for 3 h. Following the suggestion of Boudart et al. [19,20], the sample was deactivated at 300 K with air or used in situ for catalytic studies. The BET surface area of this sample is 9.6 m*/g. Supported Mo2C catalysts were prepared by the carburation of calcined MoOs/ZSM-5 in the catalytic reactor, in a similar way as described above for the preparation of bulk MozC.
3. Results 3.1. Reactions
of ethane on H-ZSM-5
First the catalytic behaviour of the ZSM-5 support was tested. It exhibited relatively high activity (2225% conversion) towards dehydrogenation and hydrogenolysis of ethane at 973 K. There was practically no decay in the conversion and in the rate of formation of various products. The highest selectivity, 38~lO%, was measured for ethylene. This was followed by methane, S = 21-23%. Benzene was also produced; its selectivity varied in the 22-23% range. Lowering the temperature the conversion of ethane decreased and approached nearly zero valued around 773 K (Fig. 1). At the same time the selectivity to ethylene increased whereas that for benzene drastically decreased. No change was observed in the selectivity of methane formation (Fig. 1). 3.2. Interaction
of ethane with unsupported
Moo3
In the first part of our work examined, the interaction of ethane and Moos under the experimental conditions applied for the conversion of methane into benzene. Reaction of ethane was observed above 800 K. The primary products were H20, CO2 and CH+ In very small amounts, propane, butane and acetaldehyde were also detected. Above 950 K, Hz, CO and CH4 were the main products. 3.3. Reaction
of ethane with supported MoOj
Similar measurements have been performed with 2 wt% MoO,/ZSM-5 catalyst. The presence of ZSM-
E Solymosi, A. SzLike/Applied
80
loo
I
conv
WI
[i]
10
227
Catalysis A: General 166 (1998) 225-235 II
I
I:
; 923 K!
973 K
-
,‘1 873 K
’ !
I
823 K
‘A
F
u -A+ 1
F +
O,l =
j I
0.01
I
0
’
1
13
50
I.1
I
100
ICI 150
1:
,
,ju,-
,
1
0
250
200
50
100
150
200
time [min]
time [min]
I. Conversion of ethane and the selectivity to methane, ethylene and benzene on ZSM-5 at different temperatures.
Fig.
80
r’
I
’
I
’
I
’
I
’
I
’
I’2
I
[&
+
800 60
600
v
methane
-
ethylene I
propa”e
40
6
+
benzene
*
conversion
3
400
20 200
I
0
0 0
20
40
60
80
100
120
time [min] Fig. 2. Conversion
of ethane, rates and selectivities
0
I
20
I
I,
I,
40
60
I
80
I
I
100
I
I
1
0
120
time [min] of the formation
of various products on 2 wt% MoOJZSM-5
at 773 K.
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Catalysis A: General 166 (1998) 225-235
5 exerted a dramatic influence on the product distribution. In flow experiments, a measurable reaction (2.54% CzH6 conversion) was observed even at 773 K (Fig. 2). At the beginning of the reaction, the reduction of MOO, proceeded yielding COz, CO and H20. The latter two components were identified up to 1OCL 120 min. The main products of ethane reaction were Hz, ethylene and methane. Propane, benzene, butane, and toluene also formed in smaller amounts. The rate of the formation of all these products increased in time on-stream (Fig. 2). The highest selectivity value (6768%) was obtained for ethylene. The selectivity of benzene production was relatively low, 45%. With the increase of the temperature the conversion increased, and at 973 K it attained an initial value of 95%, which decayed to ca. 50% at the steady state. The temporal dependence of the methane formation also changed at higher temperatures: in contrast to the other products its formation rate markedly decayed in time. Selectivity to benzene and methane increased, whereas that to ethylene, propane and toluene decreased with the rise of the temperature. In
loo,,
, , , , , , , ,
I,,
,
,
,
,
,
,
I
I
,,
Fig. 3, we plotted the selectivities to various products measured at the beginning of the reaction and at the steady state as a function of reaction temperature. The effect of supports on the reaction of ethane was examined at 873 K. The conversion of ethane was considerably less for other supported sample, particularly for the magnesia-supported catalyst (> 1%). The MoOs/SiOz sample resembled most to the MoOs/ZSM-5 catalyst. On alumina-, magnesia- and titania-supported Moos, the formation of ethylene occurred with highest selectivity. The selectivity to benzene was > 5% for Mo0s/A120s. On MoOs/Ti02 and MoOs/MgO, benzene was identified only in traces. Some important data are presented in Fig. 4. 3.4. Examination
of the used catalyst
An examination of the MoO,/ZSM-5 catalyst after reaction revealed the deposition of carbon on the catalyst. Its amount increased with the increase of the reaction time. At the same time, the reactivity of carbon also changed. TPR studies showed that the
100
s
S
conv
conv
[%I
WI
-C-
B
conversion
-A- methane
80
i
-A- ethylene f
propane
+
benzene
1
60
60
800
900
850 T [Kl
Fig. 3. Effects of temperature 15 min; and (B) 60 min.
on the conversion
950
800
900
850 T
950
WI
of ethane and on selectivities to various products on 2 wt% MoOx/ZSM-5.
Reaction time: (A)
E Solymosi, A. Szcke/Applied
229
Catalysis A: General 166 (1998) 225-235 100
100
ia conversion 80
80
_
B
? ?methane ethylene
60
60
7 @
-
F
40
40
20
20
0
0 ZSM-5
SQ
A12o3
TQ
ZSM-5
MgO
support
SO,
A$03
Ti02
MgO
support
Fig. 4. Effects of supports on the reaction of ethane on Moos at 973 K. Reaction time: (A) 15 min; and (B) 60 min
The interaction of CzH6 with Mo03/ZSM-5 has been also followed by XPS. Spectra obtained as a function of reaction time at 773 K are plotted in Fig. 5A. The binding energies (BE) for Mo(34 were at 232.7 and 235.7 eV for oxidized MoOa. A slight change in the spectrum occurred even after a short (~10 min) C2H6 treatment at 773 K. After 60 min, a new peak appeared at 228.6 eV. An increase in the reaction time caused an attenuation of the peak at 236.0 eV and the intensification of the new peak at 228.6 eV. The C(ls) signal exhibited less changes. At the very beginning, the signal appeared at
hydrogenation of surface carbon starts at 700 K. After 10 min reaction of ethane at 773 K, we obtained two peaks - at 830 and 930 K. On increasing the reaction time, the low-temperature peak disappeared and the carbon was hydrogenated only in the broad hightemperature peak, Tr = 930 K. When the reaction of ethane was carried out at 973 K, the amount of carbon deposited greatly increased and reacted with hydrogen at higher temperatures, Tr = 1000 and -1070 K. Note that the peak temperature of the hydrogenation of the pure MO& is -1000 K. Some characteristic data are listed in Table 1.
Table 1 Formation
of carbon in the reaction of ethane on 2 wt% MoOs/ZSM-5
Time of reaction (min)
Temperature
10 60 300 10
773 173 773 973
’
of the reaction (K)
Temperature
of C @mol/g)
77.0 294.7 541.6 489.9
a The amount of C was determined in TPR experiments with the reaction of hydrogen TP = peak temperature CH+ Note that the complete transformation of 2wt% Moo3 into MoaC requires 69.4 u mol C/g.
Tp (K) 830, 930 930 950 1000, -1070 of the hydrogenation
of C into
230
E Solymosi, A. SzcTke/Applied
Catalysis A: General 166 (1998) 225-235
:
L v
.I”1 331.
100 100 I
500 0 min
.,,. .P. .
I
‘.
0 240 Binding
,._r
238 236 Energy I eV
2%
232
230
to.5 ..-..
220
226
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,,,
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2’ ,, .,.02
. -_-
___.____.---.
,,I,
240 238 236 Binding Energy / eV
..,...,...,...,..,,;I 234
232
230
220
226
Fig. 5. XPS spectra of MO-containing catalysts. (A) Effects of reaction time with ethane at 773 K on the XPS of Mo07/ZSM-5. (B) 1, MO& 2, Mo03/ZSM-5 treated with CH4 at 973 K for 1 h; 3, MoO,/ZSM-5 treated with C2H, at 973 K for 10 min; 4, for 120 min; and 5, 2% Mo*C/ZSM-5 prepared from Moo3 by the method of Boudart et al. [19].
284.8 eV, and it shifted to 284.2 after 5 h. After the same time, the intensity of the signal became considerably higher. When the reaction was performed at 973 K, the aforementioned changes occurred much faster (Fig. 5B). For comparison, some XP spectra of different MO& samples are also shown in Fig. 5B. 3.5. Reactions supported
of ethane on MO& and Mo2C/ZSM-5
In the next experimental series, the reaction of ethane was investigated on unsupported and ZSM-5 supported MO&. On unsupported MO& the initial conversion was quite high (24%) which decreased to a low, constant value, 1.75% in 15 min (Fig. 6A). Besides Hz, ethylene (S = 90.0%), methane (S - 6.0%), propane (S = 2.0%) and butane (S N 1.0%) were found, but benzene was not identified. The product distribution suggests that the main process is the dehydrogenation of ethane.
We also measured high initial conversion (28%) for the ethylene decomposition on Mo$, which decayed to a constant value (-3%) in 15 min (Fig. 5B). The main products are methane, ethane, propane and butane in commensurable amounts. Benzene and toluene were produced in trace amounts (S - l-2%). A completely different picture was obtained for Mo&/ZSM-5. Results are presented in Fig. 7. Similar to the case of MoOs/ZSM-5 catalyst, the reaction of ethane was observed even at 773 K. The initial conversion was 11% which decreased to 4%. In this case, neither Hz0 nor CO2 was found in the reaction products, and CO was detected only at the very beginning of the reaction. The other products were Hz, methane, ethylene, propane and benzene. Traces of butane, pentane and toluene were also detected. Apart from the first values measured at 3 min, the rates of the formation of the products were practically constant. Selectivities of the main hydrocarbon products were Scud = 12-16%, Sc6u6 = 14-16% and Sc2s = 40-48%. When the calculation was based on
231
E Solymosi, A. Szc?ke/Applied Catalysis A: General 166 (1998) 225-235
t
-I ‘,’
\
f
II
propane
30 -C- conversion 4
-
-
-bmethane
L6
--C ethylene 120 3
-
t
20
propane -
I,5 10
1 1 0
I,4 20
40
60
80
0 u,
0
0
20
40
60
80
100
120
140
time [min]
time [min]
Fig. 6. (A) Conversion of ethane and selectivity of the formation of various products in the reaction of ethane on MozC at 973 K. (B) Conversion of ethylene and selectivity of the formation of various products in the reaction of ethylene on Mo?C at 873 K. Here, 0.5 g Mo2C was used.
Table 2 Some characteristic
data for the ethane reaction on different catalysts
Catalyst
Temperature
H-ZSM-5 MoOs/ZSM-5 MoaC/ZSM-5 H-ZSM-5 MoOs/ZSM-5 Mo&I/ZSM-5
873 873 873 913 973 913
(K)
a The yield value shows the percentage
Conversion
(%)
1.6 14.5 36.1 22.4 54.1 67.2 of the converted
Yield a (%) CH4
C2H.4
19.9 21.7 38.6 20.1 37.5 48.5
53.6 24.8 11.5 36.3 14.5 5.7
ethane that transforms
all products, i.e. hydrogen was also taken into account, we obtained the following values, Scn4 = lO-13.5%, s czH4 = 35--42%, SC6n6 = 12-1370. With the increase of temperature the conversion of ethane reached a constant value of SO%, the selectivity to methane significantly increased, whereas that of ethylene decreased. A moderate increase in the selectivity was measured for benzene formation: the highest value was 30% (Fig. 8).
CA
11.7 7.8 5.5 5.4 3.1 1.1
Wk,
3.2 22.2 25.2 20.5 26.6 28.3
C+b
2.3 12.9 10.2 10.1 7.8 8.2
into the specified product.
The effects of contact time on the reaction of ethane was determined by variation of the space velocity. Results are plotted in Fig. 9. We obtained the result that, with the increase of the contact time, the conversion and the selectivity to methane increased while the selectivity to ethylene decreased. No change occurred in the selectivity to benzene. Some important data for the transformation of ethane on different catalysts are shown in Table 2.
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Catalysis A: General 166 (1998) 225-235
2000
- conv
W
L-1
-
methfm
1500
+
ethylene
f
propane
+
benzene
-0-
conversion
[%I
mllOl g.sec
-
1000
9
-
-
6
20
500
0 0
40
60
80
100
120
0
20
Fig. 7. Conversion of ethane, rates and selectivities produced by the carburization of 2% MoOl/ZSM-5
of the formation
4. Discussion 4.1. Reactions
40
60
80
100
120
time [min]
time [min]
of ethane on ZSM-5 and Moo3
The aromatization of ethane over promoted ZSM-5 has been the subject of extensive research [2,21-281. The effects of several additives (Ga, Zn, Pt, etc.) have been examined and active and selective catalysts prepared [21-281. At high temperature, above 823 K, the aromatization of ethane proceeds on undoped ZSM-5 itself: the preparation, the composition of the ZSM-5 and the experimental conditions all influence the conversion and product distribution [21-281. The reaction of ethane was measurable at 873 K on our ZSM-5 sample (Fig. 1). The dehydrogenation reaction was the dominant pathway. The formation of benzene was limited, the selectivity was only 3-5%. At 973 K, this value increased to -22%. These characteristics are consistent with the results of Schultz and Baems [25] and differ from the behaviour reported recently by Wong et al. [28]. They found aromatics (with 51.8% selectivity) and methane
of various products
on 2 wt% Mo2C/ZSM-5
at 773 K. MozC was
(with 48.2% selectivity) at ethane conversion of 5.9% at 923 K. No data for ethylene formation was given. The reaction of ethane with unsupported Moos was detected above 850 K when H20, CO*, C2H4, Co and H2 and CH4 were produced. The conversion of ethane at 973 K approached the value of 10%. 4.2. Reactions
of ethane on MoOj/ZSM-5
The situation was different when Moos was deposited on ZSM-5 and calcined at 973 K. This treatment resulted in high dispersion of Moos and very likely in an interaction between Moos and ZSM-5. As a result, the reduction of Moos started at a higher temperature. The conversion of ethane significantly increased and the product distribution also differed (Fig. 3). Both the yield and the selectivity of benzene formation markedly increased (Table 2). Note that even the mechanical mixture of Moos with ZSM-5 exhibited an improved performance as regards the conversion of ethane and the formation of benzene. Whereas at
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s conv WI
WI
80
80
I -0-
B conversion
-L+ methane
60
60
+
ethylene
f
propane
P
40
20
0 800
850
900
950
800
T Kl Fig. 8. Effects of temperature 15 min; (B) 60 min.
lower temperatures, ethane
on the conversion
of ethane and selectivities
C2H(j = C2H4 + H2
of
(1) above 873 K the formation
C2Hfj --t CH4 + Hz + C
of
(21
or C2H6 + H2 = 2CH4
(31
3C2H6 = C6H6 + 6H2
(4)
come into prominence. In addition decomposition of ethylene C2H4 = CH4 + C
900 T
< 873 K, the dehydrogenation
is the main reaction, CH4 and C6H6
850
to reaction
2, the
(5)
also contributes to the deposition of surface carbon. Changes in the conversion and in the rates of product formation occurring in time on-stream suggest an alteration of Moos during the reaction. This may consist of (i) reduction of Moos, (ii) deposition of
950
WI
to various products on 2 wt% Mo*C/ZSM-5.
Reaction time: (A)
carbon and (iii) transformation of Moos into Mo*C, as experienced in the cases of the conversion of CH4 on this catalyst [14,15,17,18]. A partial reduction of Moo3 is clearly indicated by the initial formation of Hz0 and CO. The deposition of carbon was also observed which increased with the time on-stream. XPS data also suggest the reduction of MoOa which, however, proceeds much slower than with CH4. The signal around 235.5-236.0 eV, indicative of the presence of Mo6+, is still strong even after a reaction of 2 h at 773 K. The characteristic binding energies of Mo(3d) in the Mo2C are at -231 .l and 227.8-228.3 eV [ 16,29-3 11. Deconvolution of the spectra suggest that - in the contrast to the CHq + MoOs/ZSMS system - the complete transformation of Moos into Mo2C does not occur even at 973 K, but MO& is produced in an increased amount with the increase of temperature and reaction time with &He+ Note that exact stoichiometry of carbide formed is not certain as the molybdenum carbide species having a different stoichiometry gives nearly the same XPS spectrum.
F: Solymosi, A. SzLfke/Applied Catalysis A: General 166 (1998) 225-235
234 120
I
S
+
conv PI c 90
t
6o t
0
I
I
,
I
I
I
,
I
I
I
conversion
-A- methane -L
ethylene
t
propane
-I
//
2
4
6
contact time [gsec/ml] Fig. 9. Effects of the contact time on the conversion of ethane and on the selectivities of the formation of various products on Mo&/ZSM-5 at 913 K.
4.3.
Reaction of ethane on MozC and Mo~CIZSM-5
Unsupported Mo2C catalysed the dehydrogenation of ethane at 973 K without production of benzene (Fig. 6A). Ethylene also reacted on MO& at this temperature, to give several hydrocarbons, but benzene formation was not observed (Fig. 6B). A significant improvement in the catalytic performance of Mo2C occurred when it was produced on ZSM-5. Apart from the very initial values, no changes in the conversion and selectivities to various products were experienced in time on-stream. Qualitatively, we observed the formation of the same C-containing compounds as in the case of MoO,/ZSM-5, which suggests the occurrence of similar reaction as was suggested for MoOa/ZSM-5. An important difference is that the products of the reduction of Moo3 (HZO, COz, CO) were missing. The advantageous properties of this catalyst were particularly revealed above 873 K, when the steady-state conversion reached a value of 65% with a 30% selectivity to benzene (Fig. 8). These results suggest that the highly dispersed Mo$/ZSM-
5 is a better catalyst for the transformation of ethane than the MoOJZSM-5 and the enhanced production of benzene in time on-stream is possibly due to the formation of Mo2C. Another difference is that, on Mo2C/ZSM-5, less ethylene and more methane are formed at the same ethane conversion. Accordingly, the aromatization of ethylene on Mo$/ZSM-5 is more favonred compared to MoO,/ZSM-5. It is important to emphasize that the role of the MO compounds is mainly the activation of ethane and the promotion of the formation of ethylene. The further reactions, namely the oligomerization and the aromatization of ethylene proceed on the acidic sites on ZSM-5. In accord with the literature data, we found that the latter reaction is rapid on the ZSM-5 used in the present study. The conversion of ethylene was > 90% and the selectivity of benzene formation attained a value of 30% even at 773 K. In the light of these results, it is interesting that a relatively large amount of ethylene is released from the ZSM-5-based catalysts or, in other words, a large fraction of ethylene produced escapes aromatization. This suggests that the MO& and/or the carbon formed in the reaction of ethane poisons the acidic sites of ZSM-5 and prevents the aromatization of ethylene. As the aromatization of methane was investigated on the same Mo2C/ZSM-5 catalyst, we may compare the results obtained in the two reactions. The remarkable difference is that the conversion of ethane is markedly higher than that of methane. As a result, the rates of the formation of all products were higher. Comparing the selectivities at nearly the same conversion, ca. &lo%, (which was measured at 773 K for the reaction of ethane, and at 973 K for the reaction of methane), we obtain that the selectivity of benzene formation is much less (15%) for ethane reaction than in the case of methane aromatization (-85%). We attribute this to the more extended formation of excess carbon, which may block some of the acidic sites of ZSM-5.
5. Conclusions 1. In the high temperature interaction of &He with MoO,/ZSM-5, we observed the reduction of Mo03, the deposition of carbon and the formation of MO&.
E Solymosi, A. SzcYke/Applied Catalysis A: General 166 (1998) 225-235
The aromatization of ethane on H-ZSM-5 occurred above 113 K. Deposition of Moos on ZSM-5 improved the catalytic performance of ZSM-5: the conversion of ethane and the selectivity of benzene formation increased, which was attributed to the formation of MO&. This idea is supported by the high activity of Mo2C/ZSM-5 in the production of ethylene which is aromatized on the acidic sites of ZSM-5.
Acknowledgements This work was supported by the Hungarian emy of Science and by OTKA No. 2038.
Acad-
References Cl1 D.M. Bibby, C.C. Chang, R.F. Howe, S. Yurchak (Eds.), Methane Conversion, Stud. Surf. Sci. Catal., 36 (1988). PI Y. Ono, Catal. Rev. Sci. Eng. 34 (1992) 179. [31 R.A. van Santen, A. De Koster, T. Koerts, Catal. Lett. 7 (1990) 1. [41 F. Solymosi. Gy. Kutsan, A. Erdiihelyi, Catal. Lett. 11 (1991) 149. 151 M. Belgued, P. Pareja, A. Amariglio, H. Amariglio, Nature 352 (1991) 789. [61 R.A. van Santen, A. De Koster, T. Koerts, Catal. Lett. 7 (199 I) 1; T. Koerts, R.A. van Santen, in: L. Guczi, F. Solymosi, P. Tetenyi (Eds.), Proc. 10th Int. Congr. on Catalysis, Akadimiai Kiado (Budapest 1993), p. 1065. L71 T. Koerts, M.J.A.G. Deelen, R.A. van Santen, .I. Catal. 138 (1992) 101. WI A. ErdBhelyi, J. Cserenyi, F. Solymosi, J. Catal., 141 (1993) 287; F. Solymosi, A. ErdBhelyi, J. Cserenyi, J. Catal., 147 (1994) 272.
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[9] F. Solymosi, J. Cserenyi, Catal. Today, 21 (1994) 561: Solymosi, J. Cserenyi, Catal. Lett., 34 (1995) 272. [lo] S.I. Abasov, EA. Babaeva, B.A. Dadashev, Kinet. Katal. 32 (1991) 202. [ll] J.B. Claridge, M.L.H. Green, S.C. Tsang, APE. York, Appl. Catal. A: General 89 (1992) 103. [W L. Wang, L. Tao, M. Xie, G. Xu, J. Huang, Y. Xu, Catal. Lett 21 (1993) 35. 1131 Y. Xu, S. Liu, L. Wang, M. Xie. X. Guo, Catal. Lett. 30 (1995) 135. [I41 F. Solymosi, A. ErdBhelyi, A. SzBke, Catal. Lett. 32 (1995) 43. [I51 A. Szbke, F. Solymosi, Appl. Catal. A: General 142 (1996) 361. IlfJl D. Wang, J.H. Lunsford, M.P. Rosynek, Topics in Catal. 3(4) (1996) 299. u71 F. Solymosi, A. SzBke, J. Cserenyi, Catal. Lett. 39 (1996) 157. [I81 F. Solymosi, J. Cserenyi, A. Sziike, T. Binsigi, A. Oszko, J. Catal., 165 (1997) 150. [I91 J.S. Lee, ST. Oyama, M. Boudart, J. Catal. 106 (1987) 125. PO1 L. Volpe, M. Boudart, J. Solid State Chem. 59 (1985) 348. 1211 M. Guisnet, N.S. Gnep, F. Alario, Appl. Catal. 89 (1992) 1. WI K.H. Steinberg, U. Mroczek, F. Roessner, Appl. Catal. 66 (1990) 37. S.A. Isaev, r231 O.V. Bragin, ES. Shpiro, A.V. Preobrazenskij, T.V. Vasina, B.B. Dyusenbina, G.V. Antoshin. Kh.M. Minachev, Appl. Catal. 27 (1986) 219. ~241 Y. Ono, H. Nakatami, H. Kitagawa, E. Suzuki, Stud. Surf. Sci. Catal. 44 (1988) 279. ~251 P. Schulz, M. Baems, Appl. Catal. 78 (1991) 15. WI O.V. Chetina, T.V. Vasina, V.V. Lunin. Appl. Catal. A: General 131 (1995) 7. ~271 O.P. Keipert, D. Wolf, P. Schulz, M. Baerns, Appl. Catal. A: General 131 (1996) 347. V81 S.T. Wong, Y. Xu, L. Wang, S. Liu, G. Li, M. Xie, X. Guo, Catal. Lett. 38 (1996) 39. P91 L. Leclercq, M. Provost, H. Pastor, J. Grimblot, A.M. Hardy, L. Gengembre, G. Leclercq, J. Catal. 117 (1989) 371. [301 M.J. Ledoux, C.P. Huu, J. Guille, H. Dunlop. J. Catal. 134 (1992) 383. [311 H.P. Bonzel, H.J. Krebs, Surf. Sci. 91 (1980) 499.
ELSEVIER
Applied Catalysis A: General 166 (1998) 225-235
Conversion of ethane into benzene on Mo&/ZSM-5 F.
catalyst
Solymosi*, A. Sz6ke
Institute of Solid State and Radiochemistry, A. Jdzsef University, and Reaction Kinetics Research Group of the Hungarian Academy of Sciences, P.O. Bar 168. H-6701 Szeged, Hungaq
Received 24 January 1997; received in revised form 6 August 1997; accepted 6 August 1997
Abstract The reaction of ethane on unsupported and supported molybdenum compounds has been investigated at 773-973 K. ZSM-5 was used as support. Reaction products were analyzed using gas chromatography. Changes in the composition of catalyst samples were followed by X-ray photoelectron spectroscopy. Unsupported Moos is partially reduced with ethane above 800 K to give Hz0 and CO*. The formation of Hz, C2H4 and CH4 was also observed. MO-free ZSM-5 exhibited relatively high activity towards dehydrogenation, hydrogenolysis and aromatization of ethane above 773 K. Deposition of Moos on ZSM-5 significantly enhanced the conversion of ethane and also the selectivity of benzene production. Alteration of the catalytic behaviour of MoO,/ZSM-5 in time on-stream at 773-973 K was attributed to the reaction of MoOa, to carbon deposition and to the formation of MoaC. Unsupported Mo2C catalyzed the dehydrogenation of ethane without producing benzene. In contrast, Mo&/ZSM-5 was found to be an effective catalyst in the aromatization of ethane. At 973 K the conversion of ethane was -67% and the selectivity to benzene formation 31%. 0 1998 Elsevier Science B.V. Keywords:
MozC; ZSM-5; Ethane conversion;
Benzene formation
1. Introduction
After an extensive research on the oxidative coupling of methane [ 1,2], recently, increasing attention is being paid to the reaction of methane under nonoxidative conditions [3-91. Whereas the dominant route is the decomposition of CH4 to carbon, on supported metals, with the formation of minor amounts of C2 compounds [3-91, higher hydrocarbons and even aromatic compounds were also produced on oxide catalysts [lO,ll]. As regards the formation of benzene, MoOs/ZSM-5 proved to be the best catalyst
*Corresponding author. 0926-860X/98/$19.00 6’; 1998 Elsevier Science B.V. All rights reserved PII SO926-860X(97)00260-3
[12-l 81. The selectivity to benzene formation attained a value of 61-58% at 2.0-5.7% CH4 conversion. It is assumed that ethane and ethylene are the primary products, which - through several steps, some of them occurring on the support - are converted into benzene. Recent studies, however, revealed that Moos is transformed into MozC in the hightemperature introduction of CH4 with Moos, and the Mo2C formed is considered to be the active site for the production of CHa and CH2 fragments from methane [1.5-181. In order to establish the mechanism of benzene formation, it appeared inevitable to know more about the reaction of ethane on Mo~C, which - to our knowledge - has not been investigated, yet.
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2. Experimental Catalytic reactions were carried out at 1 atm C2H6 pressure in a fixed-bed continuous-flow reactor consisting of a quartz tube (17 mm id.) connected to a capillary tube [14,18]. The flow rate was 12 ml/min. Generally, 0.5 g of loosely compressed catalyst sample was used. Reaction products were analyzed with the help of a Hewlett-Packard 5890 gas chromatograph and a Porapak QS column. The ethane conversion was calculated from the hydrogen balance. The selectivity values of product formation represent the fraction of ethane that has been converted into specific products taking into account the number of carbon atoms in the molecules. The XPS measurements were performed in a Kratos XSAM 800 instrument at a base pressure of lop9 torr using MgKa primary radiation (15 kV, 5 mA). To compensate for possible charging effect, binding energies (BE) were normalized with respect to the position of the Si(2p) in SiOz (BE = 103.4) for supported samples and to Fermi-level for the MozC. The pass energy was set at 40 eV; and an energy step width of 50 meV and dwell time of 300 ms were used. Typically, 10 scans were accumulated for each spectrum. Fitting and deconvolution of the spectra were made using the VISION software (Kratos). Great care was taken to protect the samples from air during its transformation from the catalytic reactor to the XPS chamber. The gases used were of commercial purity (Linde). Ar (99.996) and H2 (99.999) were deoxygenated with an oxytrap. The other impurities were adsorbed by 5 A molecular sieve at the temperature of liquid nitrogen. The H-ZSM-5 support was obtained by five times repeated ion exchange of Na-ZSM-5 @i/Al = 55.0) with an aqueous solution of ammonium nitrate (1 N), and calcined in air at 863 K for 5 h. MoOs/ZSM-5 catalysts were prepared by impregnating H-ZSM-5 with a basic solution of ammonium paramolybdate to yield a nominal 2 wt% of Moos. The suspension was dried at 373 K and calcined at 873 K for 5 h. Before catalytic measurements each sample was oxidized in an O2 stream at 973 K for 30 min in situ and then flushed with Ar for 15 min. Hexagonal Mo2C was prepared by the method of Boudart et al. [19,20]. Briefly, about 0.5 g of Moos was heated in 1 : 4 methane-H2 mixture flowing at
300 ml (STP)/min in a quartz cell with two stopcocks. Preparation temperature was increased rapidly to 773 K and then at 30 K/h from 773 to 1023 K and maintained at 1023 K for 3 h. Following the suggestion of Boudart et al. [19,20], the sample was deactivated at 300 K with air or used in situ for catalytic studies. The BET surface area of this sample is 9.6 m*/g. Supported Mo2C catalysts were prepared by the carburation of calcined MoOs/ZSM-5 in the catalytic reactor, in a similar way as described above for the preparation of bulk MozC.
3. Results 3.1. Reactions
of ethane on H-ZSM-5
First the catalytic behaviour of the ZSM-5 support was tested. It exhibited relatively high activity (2225% conversion) towards dehydrogenation and hydrogenolysis of ethane at 973 K. There was practically no decay in the conversion and in the rate of formation of various products. The highest selectivity, 38~lO%, was measured for ethylene. This was followed by methane, S = 21-23%. Benzene was also produced; its selectivity varied in the 22-23% range. Lowering the temperature the conversion of ethane decreased and approached nearly zero valued around 773 K (Fig. 1). At the same time the selectivity to ethylene increased whereas that for benzene drastically decreased. No change was observed in the selectivity of methane formation (Fig. 1). 3.2. Interaction
of ethane with unsupported
Moo3
In the first part of our work examined, the interaction of ethane and Moos under the experimental conditions applied for the conversion of methane into benzene. Reaction of ethane was observed above 800 K. The primary products were H20, CO2 and CH+ In very small amounts, propane, butane and acetaldehyde were also detected. Above 950 K, Hz, CO and CH4 were the main products. 3.3. Reaction
of ethane with supported MoOj
Similar measurements have been performed with 2 wt% MoO,/ZSM-5 catalyst. The presence of ZSM-
E Solymosi, A. SzLike/Applied
80
loo
I
conv
WI
[i]
10
227
Catalysis A: General 166 (1998) 225-235 II
I
I:
; 923 K!
973 K
-
,‘1 873 K
’ !
I
823 K
‘A
F
u -A+ 1
F +
O,l =
j I
0.01
I
0
’
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13
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I
100
ICI 150
1:
,
,ju,-
,
1
0
250
200
50
100
150
200
time [min]
time [min]
I. Conversion of ethane and the selectivity to methane, ethylene and benzene on ZSM-5 at different temperatures.
Fig.
80
r’
I
’
I
’
I
’
I
’
I
’
I’2
I
[&
+
800 60
600
v
methane
-
ethylene I
propa”e
40
6
+
benzene
*
conversion
3
400
20 200
I
0
0 0
20
40
60
80
100
120
time [min] Fig. 2. Conversion
of ethane, rates and selectivities
0
I
20
I
I,
I,
40
60
I
80
I
I
100
I
I
1
0
120
time [min] of the formation
of various products on 2 wt% MoOJZSM-5
at 773 K.
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Catalysis A: General 166 (1998) 225-235
5 exerted a dramatic influence on the product distribution. In flow experiments, a measurable reaction (2.54% CzH6 conversion) was observed even at 773 K (Fig. 2). At the beginning of the reaction, the reduction of MOO, proceeded yielding COz, CO and H20. The latter two components were identified up to 1OCL 120 min. The main products of ethane reaction were Hz, ethylene and methane. Propane, benzene, butane, and toluene also formed in smaller amounts. The rate of the formation of all these products increased in time on-stream (Fig. 2). The highest selectivity value (6768%) was obtained for ethylene. The selectivity of benzene production was relatively low, 45%. With the increase of the temperature the conversion increased, and at 973 K it attained an initial value of 95%, which decayed to ca. 50% at the steady state. The temporal dependence of the methane formation also changed at higher temperatures: in contrast to the other products its formation rate markedly decayed in time. Selectivity to benzene and methane increased, whereas that to ethylene, propane and toluene decreased with the rise of the temperature. In
loo,,
, , , , , , , ,
I,,
,
,
,
,
,
,
I
I
,,
Fig. 3, we plotted the selectivities to various products measured at the beginning of the reaction and at the steady state as a function of reaction temperature. The effect of supports on the reaction of ethane was examined at 873 K. The conversion of ethane was considerably less for other supported sample, particularly for the magnesia-supported catalyst (> 1%). The MoOs/SiOz sample resembled most to the MoOs/ZSM-5 catalyst. On alumina-, magnesia- and titania-supported Moos, the formation of ethylene occurred with highest selectivity. The selectivity to benzene was > 5% for Mo0s/A120s. On MoOs/Ti02 and MoOs/MgO, benzene was identified only in traces. Some important data are presented in Fig. 4. 3.4. Examination
of the used catalyst
An examination of the MoO,/ZSM-5 catalyst after reaction revealed the deposition of carbon on the catalyst. Its amount increased with the increase of the reaction time. At the same time, the reactivity of carbon also changed. TPR studies showed that the
100
s
S
conv
conv
[%I
WI
-C-
B
conversion
-A- methane
80
i
-A- ethylene f
propane
+
benzene
1
60
60
800
900
850 T [Kl
Fig. 3. Effects of temperature 15 min; and (B) 60 min.
on the conversion
950
800
900
850 T
950
WI
of ethane and on selectivities to various products on 2 wt% MoOx/ZSM-5.
Reaction time: (A)
E Solymosi, A. Szcke/Applied
229
Catalysis A: General 166 (1998) 225-235 100
100
ia conversion 80
80
_
B
? ?methane ethylene
60
60
7 @
-
F
40
40
20
20
0
0 ZSM-5
SQ
A12o3
TQ
ZSM-5
MgO
support
SO,
A$03
Ti02
MgO
support
Fig. 4. Effects of supports on the reaction of ethane on Moos at 973 K. Reaction time: (A) 15 min; and (B) 60 min
The interaction of CzH6 with Mo03/ZSM-5 has been also followed by XPS. Spectra obtained as a function of reaction time at 773 K are plotted in Fig. 5A. The binding energies (BE) for Mo(34 were at 232.7 and 235.7 eV for oxidized MoOa. A slight change in the spectrum occurred even after a short (~10 min) C2H6 treatment at 773 K. After 60 min, a new peak appeared at 228.6 eV. An increase in the reaction time caused an attenuation of the peak at 236.0 eV and the intensification of the new peak at 228.6 eV. The C(ls) signal exhibited less changes. At the very beginning, the signal appeared at
hydrogenation of surface carbon starts at 700 K. After 10 min reaction of ethane at 773 K, we obtained two peaks - at 830 and 930 K. On increasing the reaction time, the low-temperature peak disappeared and the carbon was hydrogenated only in the broad hightemperature peak, Tr = 930 K. When the reaction of ethane was carried out at 973 K, the amount of carbon deposited greatly increased and reacted with hydrogen at higher temperatures, Tr = 1000 and -1070 K. Note that the peak temperature of the hydrogenation of the pure MO& is -1000 K. Some characteristic data are listed in Table 1.
Table 1 Formation
of carbon in the reaction of ethane on 2 wt% MoOs/ZSM-5
Time of reaction (min)
Temperature
10 60 300 10
773 173 773 973
’
of the reaction (K)
Temperature
of C @mol/g)
77.0 294.7 541.6 489.9
a The amount of C was determined in TPR experiments with the reaction of hydrogen TP = peak temperature CH+ Note that the complete transformation of 2wt% Moo3 into MoaC requires 69.4 u mol C/g.
Tp (K) 830, 930 930 950 1000, -1070 of the hydrogenation
of C into
230
E Solymosi, A. SzcTke/Applied
Catalysis A: General 166 (1998) 225-235
:
L v
.I”1 331.
100 100 I
500 0 min
.,,. .P. .
I
‘.
0 240 Binding
,._r
238 236 Energy I eV
2%
232
230
to.5 ..-..
220
226
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1,
,,,
J:
2’ ,, .,.02
. -_-
___.____.---.
,,I,
240 238 236 Binding Energy / eV
..,...,...,...,..,,;I 234
232
230
220
226
Fig. 5. XPS spectra of MO-containing catalysts. (A) Effects of reaction time with ethane at 773 K on the XPS of Mo07/ZSM-5. (B) 1, MO& 2, Mo03/ZSM-5 treated with CH4 at 973 K for 1 h; 3, MoO,/ZSM-5 treated with C2H, at 973 K for 10 min; 4, for 120 min; and 5, 2% Mo*C/ZSM-5 prepared from Moo3 by the method of Boudart et al. [19].
284.8 eV, and it shifted to 284.2 after 5 h. After the same time, the intensity of the signal became considerably higher. When the reaction was performed at 973 K, the aforementioned changes occurred much faster (Fig. 5B). For comparison, some XP spectra of different MO& samples are also shown in Fig. 5B. 3.5. Reactions supported
of ethane on MO& and Mo2C/ZSM-5
In the next experimental series, the reaction of ethane was investigated on unsupported and ZSM-5 supported MO&. On unsupported MO& the initial conversion was quite high (24%) which decreased to a low, constant value, 1.75% in 15 min (Fig. 6A). Besides Hz, ethylene (S = 90.0%), methane (S - 6.0%), propane (S = 2.0%) and butane (S N 1.0%) were found, but benzene was not identified. The product distribution suggests that the main process is the dehydrogenation of ethane.
We also measured high initial conversion (28%) for the ethylene decomposition on Mo$, which decayed to a constant value (-3%) in 15 min (Fig. 5B). The main products are methane, ethane, propane and butane in commensurable amounts. Benzene and toluene were produced in trace amounts (S - l-2%). A completely different picture was obtained for Mo&/ZSM-5. Results are presented in Fig. 7. Similar to the case of MoOs/ZSM-5 catalyst, the reaction of ethane was observed even at 773 K. The initial conversion was 11% which decreased to 4%. In this case, neither Hz0 nor CO2 was found in the reaction products, and CO was detected only at the very beginning of the reaction. The other products were Hz, methane, ethylene, propane and benzene. Traces of butane, pentane and toluene were also detected. Apart from the first values measured at 3 min, the rates of the formation of the products were practically constant. Selectivities of the main hydrocarbon products were Scud = 12-16%, Sc6u6 = 14-16% and Sc2s = 40-48%. When the calculation was based on
231
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t
-I ‘,’
\
f
II
propane
30 -C- conversion 4
-
-
-bmethane
L6
--C ethylene 120 3
-
t
20
propane -
I,5 10
1 1 0
I,4 20
40
60
80
0 u,
0
0
20
40
60
80
100
120
140
time [min]
time [min]
Fig. 6. (A) Conversion of ethane and selectivity of the formation of various products in the reaction of ethane on MozC at 973 K. (B) Conversion of ethylene and selectivity of the formation of various products in the reaction of ethylene on Mo?C at 873 K. Here, 0.5 g Mo2C was used.
Table 2 Some characteristic
data for the ethane reaction on different catalysts
Catalyst
Temperature
H-ZSM-5 MoOs/ZSM-5 MoaC/ZSM-5 H-ZSM-5 MoOs/ZSM-5 Mo&I/ZSM-5
873 873 873 913 973 913
(K)
a The yield value shows the percentage
Conversion
(%)
1.6 14.5 36.1 22.4 54.1 67.2 of the converted
Yield a (%) CH4
C2H.4
19.9 21.7 38.6 20.1 37.5 48.5
53.6 24.8 11.5 36.3 14.5 5.7
ethane that transforms
all products, i.e. hydrogen was also taken into account, we obtained the following values, Scn4 = lO-13.5%, s czH4 = 35--42%, SC6n6 = 12-1370. With the increase of temperature the conversion of ethane reached a constant value of SO%, the selectivity to methane significantly increased, whereas that of ethylene decreased. A moderate increase in the selectivity was measured for benzene formation: the highest value was 30% (Fig. 8).
CA
11.7 7.8 5.5 5.4 3.1 1.1
Wk,
3.2 22.2 25.2 20.5 26.6 28.3
C+b
2.3 12.9 10.2 10.1 7.8 8.2
into the specified product.
The effects of contact time on the reaction of ethane was determined by variation of the space velocity. Results are plotted in Fig. 9. We obtained the result that, with the increase of the contact time, the conversion and the selectivity to methane increased while the selectivity to ethylene decreased. No change occurred in the selectivity to benzene. Some important data for the transformation of ethane on different catalysts are shown in Table 2.
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Catalysis A: General 166 (1998) 225-235
2000
- conv
W
L-1
-
methfm
1500
+
ethylene
f
propane
+
benzene
-0-
conversion
[%I
mllOl g.sec
-
1000
9
-
-
6
20
500
0 0
40
60
80
100
120
0
20
Fig. 7. Conversion of ethane, rates and selectivities produced by the carburization of 2% MoOl/ZSM-5
of the formation
4. Discussion 4.1. Reactions
40
60
80
100
120
time [min]
time [min]
of ethane on ZSM-5 and Moo3
The aromatization of ethane over promoted ZSM-5 has been the subject of extensive research [2,21-281. The effects of several additives (Ga, Zn, Pt, etc.) have been examined and active and selective catalysts prepared [21-281. At high temperature, above 823 K, the aromatization of ethane proceeds on undoped ZSM-5 itself: the preparation, the composition of the ZSM-5 and the experimental conditions all influence the conversion and product distribution [21-281. The reaction of ethane was measurable at 873 K on our ZSM-5 sample (Fig. 1). The dehydrogenation reaction was the dominant pathway. The formation of benzene was limited, the selectivity was only 3-5%. At 973 K, this value increased to -22%. These characteristics are consistent with the results of Schultz and Baems [25] and differ from the behaviour reported recently by Wong et al. [28]. They found aromatics (with 51.8% selectivity) and methane
of various products
on 2 wt% Mo2C/ZSM-5
at 773 K. MozC was
(with 48.2% selectivity) at ethane conversion of 5.9% at 923 K. No data for ethylene formation was given. The reaction of ethane with unsupported Moos was detected above 850 K when H20, CO*, C2H4, Co and H2 and CH4 were produced. The conversion of ethane at 973 K approached the value of 10%. 4.2. Reactions
of ethane on MoOj/ZSM-5
The situation was different when Moos was deposited on ZSM-5 and calcined at 973 K. This treatment resulted in high dispersion of Moos and very likely in an interaction between Moos and ZSM-5. As a result, the reduction of Moos started at a higher temperature. The conversion of ethane significantly increased and the product distribution also differed (Fig. 3). Both the yield and the selectivity of benzene formation markedly increased (Table 2). Note that even the mechanical mixture of Moos with ZSM-5 exhibited an improved performance as regards the conversion of ethane and the formation of benzene. Whereas at
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s conv WI
WI
80
80
I -0-
B conversion
-L+ methane
60
60
+
ethylene
f
propane
P
40
20
0 800
850
900
950
800
T Kl Fig. 8. Effects of temperature 15 min; (B) 60 min.
lower temperatures, ethane
on the conversion
of ethane and selectivities
C2H(j = C2H4 + H2
of
(1) above 873 K the formation
C2Hfj --t CH4 + Hz + C
of
(21
or C2H6 + H2 = 2CH4
(31
3C2H6 = C6H6 + 6H2
(4)
come into prominence. In addition decomposition of ethylene C2H4 = CH4 + C
900 T
< 873 K, the dehydrogenation
is the main reaction, CH4 and C6H6
850
to reaction
2, the
(5)
also contributes to the deposition of surface carbon. Changes in the conversion and in the rates of product formation occurring in time on-stream suggest an alteration of Moos during the reaction. This may consist of (i) reduction of Moos, (ii) deposition of
950
WI
to various products on 2 wt% Mo*C/ZSM-5.
Reaction time: (A)
carbon and (iii) transformation of Moos into Mo*C, as experienced in the cases of the conversion of CH4 on this catalyst [14,15,17,18]. A partial reduction of Moo3 is clearly indicated by the initial formation of Hz0 and CO. The deposition of carbon was also observed which increased with the time on-stream. XPS data also suggest the reduction of MoOa which, however, proceeds much slower than with CH4. The signal around 235.5-236.0 eV, indicative of the presence of Mo6+, is still strong even after a reaction of 2 h at 773 K. The characteristic binding energies of Mo(3d) in the Mo2C are at -231 .l and 227.8-228.3 eV [ 16,29-3 11. Deconvolution of the spectra suggest that - in the contrast to the CHq + MoOs/ZSMS system - the complete transformation of Moos into Mo2C does not occur even at 973 K, but MO& is produced in an increased amount with the increase of temperature and reaction time with &He+ Note that exact stoichiometry of carbide formed is not certain as the molybdenum carbide species having a different stoichiometry gives nearly the same XPS spectrum.
F: Solymosi, A. SzLfke/Applied Catalysis A: General 166 (1998) 225-235
234 120
I
S
+
conv PI c 90
t
6o t
0
I
I
,
I
I
I
,
I
I
I
conversion
-A- methane -L
ethylene
t
propane
-I
//
2
4
6
contact time [gsec/ml] Fig. 9. Effects of the contact time on the conversion of ethane and on the selectivities of the formation of various products on Mo&/ZSM-5 at 913 K.
4.3.
Reaction of ethane on MozC and Mo~CIZSM-5
Unsupported Mo2C catalysed the dehydrogenation of ethane at 973 K without production of benzene (Fig. 6A). Ethylene also reacted on MO& at this temperature, to give several hydrocarbons, but benzene formation was not observed (Fig. 6B). A significant improvement in the catalytic performance of Mo2C occurred when it was produced on ZSM-5. Apart from the very initial values, no changes in the conversion and selectivities to various products were experienced in time on-stream. Qualitatively, we observed the formation of the same C-containing compounds as in the case of MoO,/ZSM-5, which suggests the occurrence of similar reaction as was suggested for MoOa/ZSM-5. An important difference is that the products of the reduction of Moo3 (HZO, COz, CO) were missing. The advantageous properties of this catalyst were particularly revealed above 873 K, when the steady-state conversion reached a value of 65% with a 30% selectivity to benzene (Fig. 8). These results suggest that the highly dispersed Mo$/ZSM-
5 is a better catalyst for the transformation of ethane than the MoOJZSM-5 and the enhanced production of benzene in time on-stream is possibly due to the formation of Mo2C. Another difference is that, on Mo2C/ZSM-5, less ethylene and more methane are formed at the same ethane conversion. Accordingly, the aromatization of ethylene on Mo$/ZSM-5 is more favonred compared to MoO,/ZSM-5. It is important to emphasize that the role of the MO compounds is mainly the activation of ethane and the promotion of the formation of ethylene. The further reactions, namely the oligomerization and the aromatization of ethylene proceed on the acidic sites on ZSM-5. In accord with the literature data, we found that the latter reaction is rapid on the ZSM-5 used in the present study. The conversion of ethylene was > 90% and the selectivity of benzene formation attained a value of 30% even at 773 K. In the light of these results, it is interesting that a relatively large amount of ethylene is released from the ZSM-5-based catalysts or, in other words, a large fraction of ethylene produced escapes aromatization. This suggests that the MO& and/or the carbon formed in the reaction of ethane poisons the acidic sites of ZSM-5 and prevents the aromatization of ethylene. As the aromatization of methane was investigated on the same Mo2C/ZSM-5 catalyst, we may compare the results obtained in the two reactions. The remarkable difference is that the conversion of ethane is markedly higher than that of methane. As a result, the rates of the formation of all products were higher. Comparing the selectivities at nearly the same conversion, ca. &lo%, (which was measured at 773 K for the reaction of ethane, and at 973 K for the reaction of methane), we obtain that the selectivity of benzene formation is much less (15%) for ethane reaction than in the case of methane aromatization (-85%). We attribute this to the more extended formation of excess carbon, which may block some of the acidic sites of ZSM-5.
5. Conclusions 1. In the high temperature interaction of &He with MoO,/ZSM-5, we observed the reduction of Mo03, the deposition of carbon and the formation of MO&.
E Solymosi, A. SzcYke/Applied Catalysis A: General 166 (1998) 225-235
The aromatization of ethane on H-ZSM-5 occurred above 113 K. Deposition of Moos on ZSM-5 improved the catalytic performance of ZSM-5: the conversion of ethane and the selectivity of benzene formation increased, which was attributed to the formation of MO&. This idea is supported by the high activity of Mo2C/ZSM-5 in the production of ethylene which is aromatized on the acidic sites of ZSM-5.
Acknowledgements This work was supported by the Hungarian emy of Science and by OTKA No. 2038.
Acad-
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