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Oxidative dehydrogenation of n-butane on Cs doped nickel molybdate: Kinetics and mechanism
~
A PA LE IY D CP AT L SS I A: GENERAL
ELSEVIER
Applied Catalysis A: General 135 (1996) 137-153
Oxidative dehydrogenation of n-butane on Cs doped nickel molybdate: Kinetics and mechanism L.M. Madeira, F.J. M a l d o n a d o - H 6 d a r 1, M.F. Portela *, F. Freire, R.M. Martin-Aranda 2 M. Oliveira GRECAT~ Grupo de Estudos de Catdlise Heterogdnea, lnstituto Superior Tdcnico, Universidade Tdcnica de Lisboa, Av. Rovisco Pais, 1096 Lisbon Codex, Portugal
Received 25 April 1995; revised 2 August 1995; accepted 4 August 1995
Abstract The oxidative dehydrogenation of n-butane over a highly selective catalyst (3% Cs doped c~NiMoO4) has been studied in order to understand the mechanism of the reaction. The catalyst characterization was carried out by BET, X-ray diffraction ( X R D ) , coupled plasma spectroscopy (ICP), atomic absorption ( A A ) , X-ray fluorescence (XRF), IR and X-ray photoelectron spectroscopy (XPS). All the results showed an atomic ratio of M o / N i = 1 and a cesium surface concentration of 3%. The kinetic tests were carried out between 773 and 833 K changing the contact time and the partial pressure of both butane and oxygen. Only dehydrogenation products ( 1-butene, 2-butenes (cis and trans) and butadiene) and CO and COs were observed under all experimental conditions. The experimental results can be correlated using a redox model. Two active sites were observed by CO2TPD, which would be responsible for the formation of two different adsorbed intermediates of butane. It is proposed that in the more basic sites 1-butene would be formed which reacts partially to butadiene and in the less basic sites the formation of the 2-butene isomers would occur. Keywords: Butane dehydrogenation; Cesium doping; Kinetics; Nickel molybdate
1. Introduction The oxidative dehydrogenation of alkanes is an important altemative route for production of alkenes. However, only a small number of catalysts, mainly chro* Corresponding author. Tel. ( + 351-1 ) 8496446, fax. ( + 351-1 ) 8499242. On leave from: Departamento de Qu/mica Inorg~nica, Facultad de Ciencias, Universidad de Granada, 18071 Granada (Spain) 2 On leave from: Departamento de Qufmica Inorg~inica y Trcnica, UNED, C/ Senda del Rey, s/n, 2840Madrid, Spain. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI O 9 2 6 - 8 6 0 X ( 95 ) 00203-O
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mium or platinum compounds supported on silica or alumina, are industrially used for dehydrogenation of light hydrocarbons [ 1] since the 1930's. A considerable research effort around the world is leading to the development of different metal oxide catalysts for this reaction. Vanadium and molybdenum compounds, supported or unsupported, have been widely studied [2-6]. The reaction byproducts are carbon oxides and often oxygen-containing organic molecules (ketones, aldehydes, acids, etc.). Numerous kinetic studies, models and theories have been reported for the oxidative dehydrogenation of alkanes [7-11], but many important aspects of the reaction mechanism remain unclear and therefore factors that determine selectivity (the nature of active sites, the role of oxygen, etc. ) are far from being duly clarified. Of the Mo-compounds, the NiO-MoO3 system shows very interesting activity for butane selective dehydrogenation [6,12]. Previous work carried out in this laboratory [ 12] showed that important gains of selectivity could be obtained by doping NiMoO4 with alkali metals (Li, Na, K and Cs). The aim of this work is to study the oxidative dehydrogenation of butane over NiMoO4-3%Cs, attempting to clarify the nature and mechanism of this reaction over a very selective catalyst. The knowledge of these factors can contribute to a more efficient, economically favourable and environmentally safe utilization of the oxidative dehydrogenation of butane.
2. Experimental A stoichiometric c~-NiMoO4 catalyst was prepared by coprecipitation following the method described by Mazzocchia et al. [6]. The cesium doped catalyst was prepared by wet impregnation of the ol-NiMoO4 with cesium nitrate using the technique detailed elsewhere [ 12]. The catalyst characterization was carried out by BET surface area determination, X-ray diffraction (XRD), coupled plasma spectroscopy (ICP), atomic absorption (AA), X-ray fluorescence (XRF), IR and X-ray photoelectron spectroscopy (XPS). Experimental details and results were already reported [12]. All the data showed a Mo/Ni atomic ratio of I and a cesium surface concentration of 3%. The catalytic experiments were performed in a fixed-bed continuous flow tubular quartz reactor (I.D. 20 mm; length 600 mm) with a coaxially centred thermocouple. The catalyst charge was 0.3 g diluted with inert quartz (volume ratio of 1:2 catalyst to quartz). All the experiments were performed at 1.10 bar. The feed was a mixture of butane, oxygen and nitrogen with a molar ratio adjusted for each experiment. The reaction was studied at: 773,793,813 and 833 K. The experimental conditions used in this work were adjusted to obtain always differential conversions ( < 10%) to minimize the effects of products and of secondary reactions and to have a differential plug flow reactor. In this way, reaction
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
139
mixture composition could be considered practically constant along the catalytic bed [ 13]. This leads to: x rbutane = a w / F
(1)
where ?'butaneis global consumption rate of butane (mol/h m2); F is total feed flow of butane (mol/h); x is conversion; W is catalyst weight (g); and a is BET surface area (mZ/g). The formation rate for each product (ri in mol/h m 2) in terms of converted butane was obtained by the equation: ri
Yi aW/F
(2)
where Yi is the yield of product i. All the reaction rates were expressed on surface area basis. Blanks runs showed that under the experimental conditions used in this work the homogeneous reaction can be neglected. Analysis of the reactants and products were carried out with an on line Shimadzu GC-8A gas chromatograph with two columns: (i) molecular sieve 13X (60-80 mesh, 2 m × 1/8 in. SS); (ii) 33% dimethylsulpholane on Chromosorb P (60-80 mesh, 6 m × 1/8 in.), and hot-wire detector. The temperature-programmed desorption (TPD) experiments were performed with a hot-wire detector. After a thermal pretreatment in He, the samples were saturated in CO2 at 303 and 523 K, which was later on desorbed by heating at 10 K/rain in a He flow of 1 cm3/s up to 900 K.
3. Results and discussion In a previous paper [ 12], the effect of Li, Na, K and Cs promoters at several concentrations were studied. The addition of the promoters provoke a decrease of the catalyst BET surface area and an increase of the molybdenum electron density as showed by XPS experiments. These results were correlated with the decrease in activity and selectivity increase for the dehydrogenation of n-butane. The 3%Cs-NiMoO4 catalyst BET surface area was 28.7 m2/g. The techniques used for measuring the composition showed an atomic ratio Mo/Ni = 1 in the bulk and on the surface, a cesium surface concentration of 3% and practically absence of cesium in the bulk. XRD spectra of the catalyst before and after impregnation were identical, showing only the pure ol-NiMoO4 peaks. Under all the experimental conditions used to test catalytic activity only butane dehydrogenation products and carbon oxides were found. The amounts of formed oxygen-containing organic molecules (aldehydes, ketones, acids, etc.) were neg-
140
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
Io T
x
9+ 8 x
7 6 5
.+
X •
X
X
•
•
m
2+ ol (?.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1,1
W/F (g.htmol)
8 X 7 6
gs w
X
•
4 X
3
X
2 1 0
i
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
10
1.1
W/F (g.h/mol)
Fig. 1. Conversion and dehydrogenation product (C4's) yields versus contact time. ( O ) 773 K; ( • ) 793 K; ( • ) 813 K; ( × ) 833 K; Phu,,.e=0.05 bar; Po~=0.10 bar.
ligible and the carbon unbalance was always less than 2%. The results showed also a good reproducibility. A preliminary study on the contact time influence was carried out at 1.10 bar maintaining constant the oxygen (Po2) and butane (Pbutane) pressures (0.10 and 0.05 bar respectively) and adjusting the total flow rate. The contact time was change from 0.28 to 1.04 g h/mol. In Fig. 1 are presented the conversion and yields of dehydrogenation products ( C a ' s ) as a function of contact time ( W / F ) . Both parameters increase with W/F. However, when the selectivity for each product is represented versus W/F, different trends were observed (Fig. 2). So, while the selectivities for 1-butene, cis-2-butene and trans-2-butene decrease with increase of contact time, for butadiene, CO and CO2 the opposite behaviour is observed. Moreover, it is also noteworthy that for l-butene and both 2-butene isomers the selectivity tends to decrease with the temperature for the same W/F, while in the second group of compounds selectivity increases with temperature.
0.2
.
04
08
0.9
(g.hhml,
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.
.
E
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.
.
:
10
X
l
. .
1.2
04
I
02
n
b
I/
XH
X .
Fig. 2. Influence of the contact time on the selectivity of reaction products.
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.
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.
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(Q.hlmol)
.
.
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.
. .
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.
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WIT 1Q.W) ( A ) 813 K; ( X ) 833 K; Phurane= 0.05 bar; PO, = 0.10 bar.
12
/
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X
. .
.
142
L.M. Madeira et al./Applied Catalysis A: General 135 (1996) 13~153
These results evidence that the first group of compounds ( l-butene and both 2butenes) are the primary reaction products, and they can undergo new dehydrogenation to butadiene and/or degradation to carbon oxides. Both these secondary reactions are favoured by increases of W/F and temperature, according the trends observed in Fig. 2. It is also noteworthy that at 773 K the CO and butadiene selectivities become null at short contact time. This is not observed for CO2 which always shows selectivity values greater than 3%. This can be due to the fact that CO and butadiene are fundamentally secondary products, i.e. they are formed from 1-butene and/or 2-butenes, but CO2 can also result directly from butane. The influence of the reactants' partial pressure on the product formation was also studied. First, the butane pressure was fixed at 0.05 bar and Po2 varied from 0.02 to 0.15 bar. Then, the Po2 was maintained at 0.05 bar and Pbutane changed from 0.02 to 0.25 bar. Nitrogen was added in both cases to keep the 1.10 bar total pressure. The presented formation rates of all products are based on surface area. Fig. 3 shows that CO2 formation rate, and CO rate at low temperature, are independent of Pbutane and tend to increase with Po2. CO rate increases with Pbutane at high temperatures. These data point out that the combustion processes must involve preferentially adsorbed hydrocarbons on the catalyst surface. In fact, the carbon-carbon bonds become weaker and so can be more easily attacked. The zeroorder in butane for combustion processes evidences that butane even at low pressures covers all the active sites on the catalyst surface, and the COx formation would result of the attack of such adsorbed hydrocarbon species by gaseous 02. The formation rates of Ca dehydrogenation products (regardless if they are primary or secondary) show an opposite behaviour, i.e. they are independent of Po2 (Fig. 4A) and increases with Pbutane (Fig. 4B). Therefore, it is now the dehydrogenation processes which do not appear to be influenced by the gaseous oxygen. These processes take place also on the catalyst surface, where the lattice oxygen plays an active role, possibly with formation o f - O H groups eliminated later on as H 2 (observed by gas chromatography) in small amount and mainly as H20. Some studies report that the - O H surface groups may have several functions: to facilitate the migration of hydrogen [ 14], to activate C - H bonds [ 15] or form - O - C bonds [16]. In such circumstances the catalyst surface would be reduced, but, at the same time, the gaseous oxygen reoxidizes it easily. Consequently, the catalyst surface does not need to undergo important transformations, because a high stability of C4 formation has also been observed. In order to obtain information about the redox nature of the reaction, an experiment without 02 was carried out (813 K, 10% butane and 90% He in the feed, W~ F: 0.524 g h/mol). The test was started by heating the catalyst in helium up to the reaction temperature. When butane was fed a smaller but measurable conversion was recorded. CO was not found and a strong decrease of CO2 formation was observed, probably due to the reduction of the more acids sites of the catalyst
6.OE~5
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X
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0.12
0.t4
CO and CO2 formation rates. ( O ) 773 K; (ram) 793 K; ( • ) 813 K; ( × ) 833 K.
~ 0.20
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•
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144
L.M. Madeira et al. /Applied
Catalysis A: General 135 (1996) 137-153
.
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-ci8
LM. Madeira et al. /Applied
Catalysis A: General 135 (1996) 137-153
145
146
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 13~153
surface with loss of oxygen species. In fact, CO2 selectivity fell from 100% to 10% with simultaneous increase of the butenes selectivity from 0% to 90%. It is noteworthy that only the primary dehydrogenation products were obtained ( 1-butene and cis- and trans-2-butene) and a certain coke formation was also observed. Butadiene was not observed, possibly due to the low concentration of primary products. So, the catalyst surface is active, even without 02 in the gas phase, for formation of primary dehydrogenation products. When the 02 feed is reintroduced, the conversion increases again and now, CO and butadiene are formed with high yields as well all the primary products. Following the above mentioned results, the reaction would occur in a general way by a redox mechanism [ t7] according to: kr
butane + OC --* RC + products ko
RC + 02 ~ OC where OC and RC are oxidized and reduced active sites respectively and kr and ko kinetic constants with Arrhenius behaviour ( k = k'e ~-Eo/Rr~). The reduction rate of catalyst surface can be expressed as: rj = kr PbutaneO'Oc
( 3)
and the reoxidation rate: r 2 =
koPo2cr RC
(4)
where ~roc and O'RC are the fractions of oxidized and reduced sites, respectively. In equilibrium rl = r2. So, kr PbutaneO'OC = koPo2O'Rc
which leads to the total rate of consumption of butane: r-
koPbutanePo2
(5)
k o / k r P o 2 + Pbutane
When Eq. 5 was used to correlate the experimental rates of butane consumption, the following parameters were computed by nonlinear regression analysis: k'o=4.1.105 m o l / ( h m 2 bar), Eao= 1.1' 105 J/mol, k'r=307 m o l / ( h m 2 bar), Ear = 7.4.104 J/mol with a multiple correlation coefficient qt [ 18,19 ] equal to 0.98. In Fig. 5 are showed the experimental data (points) and the fitting curves (lines) obtained by Eq. 5. However, in order to clarify the formation mechanisms of the different reaction products it is necessary to analyze their experimental results independently. First, it is possible to separate the analysis of COx and Ca formation on the basis of their respective independence from Pbutane and Po2. But, moreover, when examining C4
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
147
1.0E-03
~x 8.0E-04 +
e" w
6.0E-04
4.0E..04
J
Jr
2.0E-04
O.OE+O0 0.00
0.05
0.10
0.15
0.20
0.25
P butane (bar)
4.0E-04
x x
3.0E-04
x J
i
x
•
2.0E-04 •
f
J 1.0E-04
0.00
•
j
•
J
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
P o2 (bar)
Fig. 5. Influence of Po_, variation at Pbutane = 0.05 bar and of Pbutane variation at Po,, = 0.05 bar on the butane consumption rates. ( 0 ) 773 K; (m) 793 K; ( • ) 813 K; ( X ) 833 K. Full lines obtained from Eq. 5 by nonlinear regression.
formation rates, the simple visualization of Fig. 4 shows differences in the formation of 2-butenes (cis and trans), and 1-butene and butadiene. In fact, while for both 2butene isomers the formation rates increase linearly with Pbut . . . . for 1-butene and butadiene a marked curvature is visible. When Eq. 5 is used to correlate the experimental results for each product, ~ = 0.99 for both 2-butene isomers and for 1-butene, while for butadiene ~ = 0.97. In Fig. 4a and 4b are showed together with the experimental data (points) the fitting curves (lines) obtained for each product by Eq. 5. The slightly worse fitting for
148
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153 3.0E-04
2.5E-04
2.0E-04
~ 1.5E434
] i.
1.1~i-04
5.0E..~ i
0.~+1~
I
I
0.001
0.002
i
i
0.003
0.004
q
0.005
I
0.006
I
0.007
I
0.008
P 1..but (b=r)
Fig. 6. Dependence of the butadiene formation rate of the 1-butene partial pressure. (O) 773 K; ( × ) 833 K; Po2 = 0.05 bar.
butadiene can be due to its secondary reaction product character mentioned above. However, when the butadiene rate formation is correlated with the concentration of primary products ( 1-butene and 2-butenes) previously formed by Eq. 5, still a worse correlation is obtained. When it is accepted that butadiene is formed only from 1-butene, i.e., replacing in Eq. 5 Pbutane by Pl-but, 9 = 0 . 9 9 . In Fig. 6 is represented the experimental formation rate of butadiene on such basis and the corresponding computed fitting curve versus the estimated Pl-butene.According with these results, the real rate formation of 1-butene must be the sum of the experimental 1-butene and butadiene formation rates; the correlation of the rate under such conditions by Eq. 5 also provides values of 9 = 0.99 (Fig. 7). The correlation of CO and CO2 formation rates by the proposed Eq. 5 leads to very poor correlation coefficients ( a/tCO2 = 0.74 and q/CO = 0.39). The poor fitting for CO and CO2 reflects the dispersion of experimental data (Fig. 3) with respect to Po2, but, moreover, the possible formation of these products from all the hydrocarbon species, while Eq. 5 is only sensitive to Pbutane" The above exposed results show that butadiene is a secondary reaction product. In an attempt to elucidate experimentally its provenance and the possible isomerization reactions between 1-butene and 2-butenes, an experiment was performed, feeding the reactor with 1-butene. However, due to the different surface conditions and the different reactivity of n-butane and 1-butene, the product distributions obtained in these experiments cannot provide conclusive proof. The experimental
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
149
150
L.M. Madeira et al. /Applied Catalysis A." General 135 (1996) 137-153 5000y
O0°
4r~O0"
i 35003000v 2~00 2OOO
•
•
0
:
IOO0
. . . .
200
300
400
500
4-------
600
I
~
I
[
700
800
900
1000
Tem~rature (K)
Fig. 8. C Q - T P D profile at ( e ) 303 K and (11) 523 K.
conditions used were: 793 K, 2% 1-butene and 10% 02, the remaining variables were fixed as for the previous experiments. The conversion obtained in this experiment was 85% with the following product distribution: COx selectivity: 50%, butadiene selectivity: 37% and 2-butenes (cis + trans) selectivity: 13%. The formation rate of butadiene in this experiment is approximately equal to the one computed from butane for the same experimental conditions, which confirms that butadiene proceeds exclusively from 1-butene previously formed. However, the values of the ratio 2-butenes/butadiene are ca. 2.0 for butane as feed and show a strong decrease when 1-butene is used as feed (0.35). This confirms that 2-butenes are in practice not formed from 1-butene via isomerization, because, with the same experimental conditions, the use of 1-butene as feed causes a strong increase of butadiene formation but not of the 2-butenes formation. Furthermore, the nature of the catalyst surface was tested by temperature-programmed desorption of CO2. CO2-TPD chromatograms show (Fig. 8) that the CO2 physisorption is negligible, in accord with the low surface area of the catalyst, because no noticeable detector response is observed below 373 K. However, the desorption curve of CO2 adsorbed at 303 K presents two maxima, the first at 470 K and the second one starting at about 770 K, but its exact position cannot be estimated because the TPD experiments were carried out only up to 900 K in order to avoid the transition from a to/3-phase of nickel molybdate. These results were confirmed by CO2 adsorption at 523 K, also illustrated in Fig. 8 with a similar curve but with the first peak logically lower and displaced to higher temperature. It is noteworthy that no cesium carbonates were detected by XRD and FTIR experiments and that the CO2-TPD profile of the unpromoted NiMoO4 also shows the existence
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
I* butane
~
~-
151
1-butene
butadiene
butane*
02
~
I
I~
2-butenes
•
COx Scheme 1.
of two types of active sites. Consequently, the second peak found at high temperature can not be assigned to carbonate species. An overall analysis of the above mentioned results shows that the possibility of formation of two different intermediates from adsorbed butane on the catalyst surface and the formation of butadiene from 1-butene, is suggested mainly by the - - existence of two very different types of sites evidenced by CO2-TPD; - - similar very good linearity of cis- and trans-2-butene formation rates versus the butane partial pressure, while 1-butene and butadiene formation rates exhibit identical behaviour but different of the 2-butenes; - - less isomerization recorded when 1-butene was used as feed; - - very poor correlations found when the intervention of 2-butenes was tested for the butadiene formation. In this way, the reaction would occur according to Scheme 1. It is well known that the catalyst acid sites favour COx and 2-butenes formation [20,21 ]. On the less basic sites the intermediate T would be formed, leading to 2butenes, and on the more basic sites the intermediate 'I*' which produces 1-butene. In this way, Eq. 5 provides a very good correlation (~b=0.99) for the consumption of the I* intermediate (sum of the experimental rates of formation of 1-butene and butadiene, if the COx formation by this pathway is negligible). For the I intermediate, the addition of the experimental rates of formation for both 2butene isomers is also well expressed by Eq. 5 with ~b= 0.99. Experimental data for both intermediates and the computed curves by Eq. 5 are showed in Fig. 7. The computed values of k'o, Eao, k'r and E~ are presented in Table 1. As would be expected, the activation energies of formation of both intermediates (Ear) are not very different, however, their values also agree with the hypothesis previously advanced. The 1-butene formation and the subsequent dehydrogenation to butadiene take place in the more basic active sites of the catalyst surface. Due to Table 1 Parameters computed by fitting Eq. 5 to experimental data
rb0 r(~-but+bd) = r~* rie_but) =r~
k'o ( m o l / h m 2 bar)
k'~ ( m o l / h m 2 bar)
E~o ( J / m o l )
1.1 ' 102 1.9' 108 1.6' 105
0.3 18.0 5.7
4.6.10 4
1.6.10 4
1.6" 105 2.5.105
6.1 ' 104 5.8. 104
E~ ( J / m o l )
152
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 13~153
a greater electronic density their reduction must be more difficult and E~ of formation of the I* intermediate must be greater than for the I intermediate. On the other hand, the reoxidation of these sites must be easier (Eao lower). Both these facts are confirmed by the values of Table 1. Furthermore, the dehydrogenation of 1-butene to butadiene is energetically easy [ 3] and, in fact, the Ea obtained for this process is relatively small (Table 1 ).
4. Conclusions The oxidative dehydrogenation of butane over 3% Cs doped NiMoO4 was systematically studied over a large range of experimental conditions. Only dehydrogenation products (Ca's) and COx were found, the formed oxygen-containing organic molecules being negligible. The observed influence of contact time confirms that 1-butene and 2-butenes are formed as primary reaction products while butadiene and COx are secondary ones. The formation rates of COx are independent of Pbutane while the selective dehydrogenation rates are independent of Po2. However, while the formation rate for 2butenes increases linearly with Pbut . . . . this increase presents a decreasing slope in the case of 1-butene and butadiene. A redox reaction mechanism with two different intermediates is proposed. One intermediate leads to 1-butene which is partially transformed to butadiene. The other one produces 2-butenes. COx would proceed from all the present hydrocarbons species. The CO2-TPD experiments confirmed the existence of two types of basic sites that would be responsible for the formation of both intermediates. The values of activation energies computed by the proposed model indicate that in the more basic sites 1-butene is formed and in the less basic sites 2-butenes. The formation of butadiene exclusively from 1-butene and the independence of 1-butene and 2butenes (practically no isomerization occurs) is also evidenced using 1-butene as feed.
Acknowledgements F.J.M.H. and R.M.M.A. thank the European Community for financial support. L.M.M. also thanks PRAXIS XXI program from J.N.I.C.T. (Junta Nacional de Investiga9fio Cientifica e Tecnol6gica) for financial support. Support of this work by European Community (contract n ° CHRX-CT92-0065) is gratefully acknowledged.
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
153
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10] [ 11 ] [ 12] [ 13] [ 14] [ 15] [ 16] [ 17] [ 18] [ 19] [20] [ 21 ]
D.E. Resasco and G.L. Hailer, Catalysis Vol. 11, Royal Society of Chemistry, 1994, p. 379. A. Corma, J.M. L6pez Nieto and N. Paredes, J. Catal., 144 (1993) 425. F. Cavani and F. Trifir6, Catalysis Vol. 11, Royal Society of Chemistry, 1994, p. 246. M.A. Chaar, D. Patel, M.C. Kung and H.H. Kung, J. Catal., 105 (1987) 483. C. Mazzocchia, A. Kaddouri, R. Anouchinsky, M. Sautel and G. Thomas, Solid State Ionics, 63-65 (1993) 731. C. Mazzocchia, R. Del Rosso and P. Centola, An. Quim., 79 (1983) 108. S.L.T. Andersson, Appl. Catal. A, 112 (1994) 209. H.H. Kung, P. Michalakos, L. Owens, M. Kung, P. Andersen, O. Owen and I. Jahan, Am. Chem. Soc. Symp. Ser., 523 (1993) 389. G. Centi, F. Trifiro, J.R. Ebner and V.M. Franchetti, Chem. Rev., 88 (1988) 55. M.A. Chaar, D. Patel and H.H. Kung, J. Catal., 109 (1988) 463. A.M. Efstathiou, D. Boudouvas, N. Vamvouka and X.E. Verykios, J. Catal., 140 (1993) 1. R.M. Martin-Aranda, M.F. Portela, L.M. Madeira, F. Freire and M. Oliveira, Appl. Catal. A, 127 (1995) 201. G.F. Froment and K.B. Bischoff, Chemical Reactor Analysis and Design, Wiley, New York, 1979, Ch. 9. J.R. Ebner and M.R. Thompson, Catal. Today, 16 (1993) 51. N.H. Batis, H. Batis, A. Ghorbel, J.C. Vedrine and J.C. Volta, J. Catal., 128 ( 1991 ) 248. G. Centi, F. Trifiro, G. Busca, J.R. Ebner and J. Gleaves, Faraday Disc. Chem. Soc., 87 (1989) 215. P. Mars and D.W. van Krevelen, Chem. Eng. Sci. Spec. Suppl., 3 (1949) 41. C. Fourgeaud and A. Fuchs, in Dunod (Collection Universitaire de Math6matiques, 24), Statistique, Chapter 6, Paris, 1967. I.M. Chakravarti, R.G. Laha and J. Roy, Handbook of Methods of Applied Statistics, Vol. I, Wiley, New York, 1967, Chapter 5~6. P. Forzatti, F. Trifiro and P.L. Villa, J. Catal., 52 (1978) 389. P. Concepci6n, A. Dejoz, J.M. L6pez Nieto and M.I. Vazquez, XIV Simposio Iberoamericano de Catalisis, Vol. 2, Concepcion, Chile, 1994, p. 769.
A PA LE IY D CP AT L SS I A: GENERAL
ELSEVIER
Applied Catalysis A: General 135 (1996) 137-153
Oxidative dehydrogenation of n-butane on Cs doped nickel molybdate: Kinetics and mechanism L.M. Madeira, F.J. M a l d o n a d o - H 6 d a r 1, M.F. Portela *, F. Freire, R.M. Martin-Aranda 2 M. Oliveira GRECAT~ Grupo de Estudos de Catdlise Heterogdnea, lnstituto Superior Tdcnico, Universidade Tdcnica de Lisboa, Av. Rovisco Pais, 1096 Lisbon Codex, Portugal
Received 25 April 1995; revised 2 August 1995; accepted 4 August 1995
Abstract The oxidative dehydrogenation of n-butane over a highly selective catalyst (3% Cs doped c~NiMoO4) has been studied in order to understand the mechanism of the reaction. The catalyst characterization was carried out by BET, X-ray diffraction ( X R D ) , coupled plasma spectroscopy (ICP), atomic absorption ( A A ) , X-ray fluorescence (XRF), IR and X-ray photoelectron spectroscopy (XPS). All the results showed an atomic ratio of M o / N i = 1 and a cesium surface concentration of 3%. The kinetic tests were carried out between 773 and 833 K changing the contact time and the partial pressure of both butane and oxygen. Only dehydrogenation products ( 1-butene, 2-butenes (cis and trans) and butadiene) and CO and COs were observed under all experimental conditions. The experimental results can be correlated using a redox model. Two active sites were observed by CO2TPD, which would be responsible for the formation of two different adsorbed intermediates of butane. It is proposed that in the more basic sites 1-butene would be formed which reacts partially to butadiene and in the less basic sites the formation of the 2-butene isomers would occur. Keywords: Butane dehydrogenation; Cesium doping; Kinetics; Nickel molybdate
1. Introduction The oxidative dehydrogenation of alkanes is an important altemative route for production of alkenes. However, only a small number of catalysts, mainly chro* Corresponding author. Tel. ( + 351-1 ) 8496446, fax. ( + 351-1 ) 8499242. On leave from: Departamento de Qu/mica Inorg~nica, Facultad de Ciencias, Universidad de Granada, 18071 Granada (Spain) 2 On leave from: Departamento de Qufmica Inorg~inica y Trcnica, UNED, C/ Senda del Rey, s/n, 2840Madrid, Spain. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI O 9 2 6 - 8 6 0 X ( 95 ) 00203-O
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mium or platinum compounds supported on silica or alumina, are industrially used for dehydrogenation of light hydrocarbons [ 1] since the 1930's. A considerable research effort around the world is leading to the development of different metal oxide catalysts for this reaction. Vanadium and molybdenum compounds, supported or unsupported, have been widely studied [2-6]. The reaction byproducts are carbon oxides and often oxygen-containing organic molecules (ketones, aldehydes, acids, etc.). Numerous kinetic studies, models and theories have been reported for the oxidative dehydrogenation of alkanes [7-11], but many important aspects of the reaction mechanism remain unclear and therefore factors that determine selectivity (the nature of active sites, the role of oxygen, etc. ) are far from being duly clarified. Of the Mo-compounds, the NiO-MoO3 system shows very interesting activity for butane selective dehydrogenation [6,12]. Previous work carried out in this laboratory [ 12] showed that important gains of selectivity could be obtained by doping NiMoO4 with alkali metals (Li, Na, K and Cs). The aim of this work is to study the oxidative dehydrogenation of butane over NiMoO4-3%Cs, attempting to clarify the nature and mechanism of this reaction over a very selective catalyst. The knowledge of these factors can contribute to a more efficient, economically favourable and environmentally safe utilization of the oxidative dehydrogenation of butane.
2. Experimental A stoichiometric c~-NiMoO4 catalyst was prepared by coprecipitation following the method described by Mazzocchia et al. [6]. The cesium doped catalyst was prepared by wet impregnation of the ol-NiMoO4 with cesium nitrate using the technique detailed elsewhere [ 12]. The catalyst characterization was carried out by BET surface area determination, X-ray diffraction (XRD), coupled plasma spectroscopy (ICP), atomic absorption (AA), X-ray fluorescence (XRF), IR and X-ray photoelectron spectroscopy (XPS). Experimental details and results were already reported [12]. All the data showed a Mo/Ni atomic ratio of I and a cesium surface concentration of 3%. The catalytic experiments were performed in a fixed-bed continuous flow tubular quartz reactor (I.D. 20 mm; length 600 mm) with a coaxially centred thermocouple. The catalyst charge was 0.3 g diluted with inert quartz (volume ratio of 1:2 catalyst to quartz). All the experiments were performed at 1.10 bar. The feed was a mixture of butane, oxygen and nitrogen with a molar ratio adjusted for each experiment. The reaction was studied at: 773,793,813 and 833 K. The experimental conditions used in this work were adjusted to obtain always differential conversions ( < 10%) to minimize the effects of products and of secondary reactions and to have a differential plug flow reactor. In this way, reaction
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
139
mixture composition could be considered practically constant along the catalytic bed [ 13]. This leads to: x rbutane = a w / F
(1)
where ?'butaneis global consumption rate of butane (mol/h m2); F is total feed flow of butane (mol/h); x is conversion; W is catalyst weight (g); and a is BET surface area (mZ/g). The formation rate for each product (ri in mol/h m 2) in terms of converted butane was obtained by the equation: ri
Yi aW/F
(2)
where Yi is the yield of product i. All the reaction rates were expressed on surface area basis. Blanks runs showed that under the experimental conditions used in this work the homogeneous reaction can be neglected. Analysis of the reactants and products were carried out with an on line Shimadzu GC-8A gas chromatograph with two columns: (i) molecular sieve 13X (60-80 mesh, 2 m × 1/8 in. SS); (ii) 33% dimethylsulpholane on Chromosorb P (60-80 mesh, 6 m × 1/8 in.), and hot-wire detector. The temperature-programmed desorption (TPD) experiments were performed with a hot-wire detector. After a thermal pretreatment in He, the samples were saturated in CO2 at 303 and 523 K, which was later on desorbed by heating at 10 K/rain in a He flow of 1 cm3/s up to 900 K.
3. Results and discussion In a previous paper [ 12], the effect of Li, Na, K and Cs promoters at several concentrations were studied. The addition of the promoters provoke a decrease of the catalyst BET surface area and an increase of the molybdenum electron density as showed by XPS experiments. These results were correlated with the decrease in activity and selectivity increase for the dehydrogenation of n-butane. The 3%Cs-NiMoO4 catalyst BET surface area was 28.7 m2/g. The techniques used for measuring the composition showed an atomic ratio Mo/Ni = 1 in the bulk and on the surface, a cesium surface concentration of 3% and practically absence of cesium in the bulk. XRD spectra of the catalyst before and after impregnation were identical, showing only the pure ol-NiMoO4 peaks. Under all the experimental conditions used to test catalytic activity only butane dehydrogenation products and carbon oxides were found. The amounts of formed oxygen-containing organic molecules (aldehydes, ketones, acids, etc.) were neg-
140
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
Io T
x
9+ 8 x
7 6 5
.+
X •
X
X
•
•
m
2+ ol (?.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1,1
W/F (g.htmol)
8 X 7 6
gs w
X
•
4 X
3
X
2 1 0
i
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
10
1.1
W/F (g.h/mol)
Fig. 1. Conversion and dehydrogenation product (C4's) yields versus contact time. ( O ) 773 K; ( • ) 793 K; ( • ) 813 K; ( × ) 833 K; Phu,,.e=0.05 bar; Po~=0.10 bar.
ligible and the carbon unbalance was always less than 2%. The results showed also a good reproducibility. A preliminary study on the contact time influence was carried out at 1.10 bar maintaining constant the oxygen (Po2) and butane (Pbutane) pressures (0.10 and 0.05 bar respectively) and adjusting the total flow rate. The contact time was change from 0.28 to 1.04 g h/mol. In Fig. 1 are presented the conversion and yields of dehydrogenation products ( C a ' s ) as a function of contact time ( W / F ) . Both parameters increase with W/F. However, when the selectivity for each product is represented versus W/F, different trends were observed (Fig. 2). So, while the selectivities for 1-butene, cis-2-butene and trans-2-butene decrease with increase of contact time, for butadiene, CO and CO2 the opposite behaviour is observed. Moreover, it is also noteworthy that for l-butene and both 2-butene isomers the selectivity tends to decrease with the temperature for the same W/F, while in the second group of compounds selectivity increases with temperature.
0.2
.
04
08
0.9
(g.hhml,
06
w/F
.
.
E
(Q.Ml
1 .o
.
.
:
10
X
l
. .
1.2
04
I
02
n
b
I/
XH
X .
Fig. 2. Influence of the contact time on the selectivity of reaction products.
5
I
’
.
0
A
Am
m15t
10
06
04
02
wfl
+t
xl/
X
A
.
.
l
AZ g * X X
x.
I
06
(0)
WR
0.8
08
l
.
.
X
10
10
773 K; ( W) 793 K;
(Q.hlmol)
.
.
A
X
wff lQ.hhnol)
0.6
X
.
. .
0.2
Am
XA
a=
X
04
i
l
n
xA
0.6
06
.
.
.
X
WR 1Q.W)
06
06
10
10
.
.
.
X
X
A
.
.
12
,
12
WIT 1Q.W) ( A ) 813 K; ( X ) 833 K; Phurane= 0.05 bar; PO, = 0.10 bar.
12
/
12
X
. .
.
142
L.M. Madeira et al./Applied Catalysis A: General 135 (1996) 13~153
These results evidence that the first group of compounds ( l-butene and both 2butenes) are the primary reaction products, and they can undergo new dehydrogenation to butadiene and/or degradation to carbon oxides. Both these secondary reactions are favoured by increases of W/F and temperature, according the trends observed in Fig. 2. It is also noteworthy that at 773 K the CO and butadiene selectivities become null at short contact time. This is not observed for CO2 which always shows selectivity values greater than 3%. This can be due to the fact that CO and butadiene are fundamentally secondary products, i.e. they are formed from 1-butene and/or 2-butenes, but CO2 can also result directly from butane. The influence of the reactants' partial pressure on the product formation was also studied. First, the butane pressure was fixed at 0.05 bar and Po2 varied from 0.02 to 0.15 bar. Then, the Po2 was maintained at 0.05 bar and Pbutane changed from 0.02 to 0.25 bar. Nitrogen was added in both cases to keep the 1.10 bar total pressure. The presented formation rates of all products are based on surface area. Fig. 3 shows that CO2 formation rate, and CO rate at low temperature, are independent of Pbutane and tend to increase with Po2. CO rate increases with Pbutane at high temperatures. These data point out that the combustion processes must involve preferentially adsorbed hydrocarbons on the catalyst surface. In fact, the carbon-carbon bonds become weaker and so can be more easily attacked. The zeroorder in butane for combustion processes evidences that butane even at low pressures covers all the active sites on the catalyst surface, and the COx formation would result of the attack of such adsorbed hydrocarbon species by gaseous 02. The formation rates of Ca dehydrogenation products (regardless if they are primary or secondary) show an opposite behaviour, i.e. they are independent of Po2 (Fig. 4A) and increases with Pbutane (Fig. 4B). Therefore, it is now the dehydrogenation processes which do not appear to be influenced by the gaseous oxygen. These processes take place also on the catalyst surface, where the lattice oxygen plays an active role, possibly with formation o f - O H groups eliminated later on as H 2 (observed by gas chromatography) in small amount and mainly as H20. Some studies report that the - O H surface groups may have several functions: to facilitate the migration of hydrogen [ 14], to activate C - H bonds [ 15] or form - O - C bonds [16]. In such circumstances the catalyst surface would be reduced, but, at the same time, the gaseous oxygen reoxidizes it easily. Consequently, the catalyst surface does not need to undergo important transformations, because a high stability of C4 formation has also been observed. In order to obtain information about the redox nature of the reaction, an experiment without 02 was carried out (813 K, 10% butane and 90% He in the feed, W~ F: 0.524 g h/mol). The test was started by heating the catalyst in helium up to the reaction temperature. When butane was fed a smaller but measurable conversion was recorded. CO was not found and a strong decrease of CO2 formation was observed, probably due to the reduction of the more acids sites of the catalyst
6.OE~5
•
•
X
0.02
•
I 0.05
•
X
I
0,04
X
i
q
O.OB p (~(bar)
I--
0.10
•
X
Pbutane on
4 ; 0.10 015 P t ~ t a l ~ (bar)
•
X
0.06
Fig. 3. Influence o f P o , and
0.OE+O0 - O.00
2.OE~5
~ 4.0E~5
• I
8.OE4J5
1.0E4~4
O.OE+O0 H O.OD
2.0E..05
.~ 6.0E-05
8.0E-OE;
4•
1.0E~4 -
1.2E-04
I
l
014
--~
0.25
0,16
8.
8
i -
0.00
2.0E-05 T
E ;
6,0E-05 J-
I
8.0E-05 T E
0.00
1 ,OIE..~
2.~4~
•
X
o.o2
X
0.05
•
0.04
•
o.oe p oz (bar)
0.~0
0.10 0.15 P butane (bar)
X
0.06
020
0.12
0.t4
CO and CO2 formation rates. ( O ) 773 K; (ram) 793 K; ( • ) 813 K; ( × ) 833 K.
~ 0.20
0.12
•
3.0E-05
4.0E4~
•
025
•
X
016
ox
e~
144
L.M. Madeira et al. /Applied
Catalysis A: General 135 (1996) 137-153
.
I’
I,,’
i \ i
-ci8
LM. Madeira et al. /Applied
Catalysis A: General 135 (1996) 137-153
145
146
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 13~153
surface with loss of oxygen species. In fact, CO2 selectivity fell from 100% to 10% with simultaneous increase of the butenes selectivity from 0% to 90%. It is noteworthy that only the primary dehydrogenation products were obtained ( 1-butene and cis- and trans-2-butene) and a certain coke formation was also observed. Butadiene was not observed, possibly due to the low concentration of primary products. So, the catalyst surface is active, even without 02 in the gas phase, for formation of primary dehydrogenation products. When the 02 feed is reintroduced, the conversion increases again and now, CO and butadiene are formed with high yields as well all the primary products. Following the above mentioned results, the reaction would occur in a general way by a redox mechanism [ t7] according to: kr
butane + OC --* RC + products ko
RC + 02 ~ OC where OC and RC are oxidized and reduced active sites respectively and kr and ko kinetic constants with Arrhenius behaviour ( k = k'e ~-Eo/Rr~). The reduction rate of catalyst surface can be expressed as: rj = kr PbutaneO'Oc
( 3)
and the reoxidation rate: r 2 =
koPo2cr RC
(4)
where ~roc and O'RC are the fractions of oxidized and reduced sites, respectively. In equilibrium rl = r2. So, kr PbutaneO'OC = koPo2O'Rc
which leads to the total rate of consumption of butane: r-
koPbutanePo2
(5)
k o / k r P o 2 + Pbutane
When Eq. 5 was used to correlate the experimental rates of butane consumption, the following parameters were computed by nonlinear regression analysis: k'o=4.1.105 m o l / ( h m 2 bar), Eao= 1.1' 105 J/mol, k'r=307 m o l / ( h m 2 bar), Ear = 7.4.104 J/mol with a multiple correlation coefficient qt [ 18,19 ] equal to 0.98. In Fig. 5 are showed the experimental data (points) and the fitting curves (lines) obtained by Eq. 5. However, in order to clarify the formation mechanisms of the different reaction products it is necessary to analyze their experimental results independently. First, it is possible to separate the analysis of COx and Ca formation on the basis of their respective independence from Pbutane and Po2. But, moreover, when examining C4
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
147
1.0E-03
~x 8.0E-04 +
e" w
6.0E-04
4.0E..04
J
Jr
2.0E-04
O.OE+O0 0.00
0.05
0.10
0.15
0.20
0.25
P butane (bar)
4.0E-04
x x
3.0E-04
x J
i
x
•
2.0E-04 •
f
J 1.0E-04
0.00
•
j
•
J
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
P o2 (bar)
Fig. 5. Influence of Po_, variation at Pbutane = 0.05 bar and of Pbutane variation at Po,, = 0.05 bar on the butane consumption rates. ( 0 ) 773 K; (m) 793 K; ( • ) 813 K; ( X ) 833 K. Full lines obtained from Eq. 5 by nonlinear regression.
formation rates, the simple visualization of Fig. 4 shows differences in the formation of 2-butenes (cis and trans), and 1-butene and butadiene. In fact, while for both 2butene isomers the formation rates increase linearly with Pbut . . . . for 1-butene and butadiene a marked curvature is visible. When Eq. 5 is used to correlate the experimental results for each product, ~ = 0.99 for both 2-butene isomers and for 1-butene, while for butadiene ~ = 0.97. In Fig. 4a and 4b are showed together with the experimental data (points) the fitting curves (lines) obtained for each product by Eq. 5. The slightly worse fitting for
148
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153 3.0E-04
2.5E-04
2.0E-04
~ 1.5E434
] i.
1.1~i-04
5.0E..~ i
0.~+1~
I
I
0.001
0.002
i
i
0.003
0.004
q
0.005
I
0.006
I
0.007
I
0.008
P 1..but (b=r)
Fig. 6. Dependence of the butadiene formation rate of the 1-butene partial pressure. (O) 773 K; ( × ) 833 K; Po2 = 0.05 bar.
butadiene can be due to its secondary reaction product character mentioned above. However, when the butadiene rate formation is correlated with the concentration of primary products ( 1-butene and 2-butenes) previously formed by Eq. 5, still a worse correlation is obtained. When it is accepted that butadiene is formed only from 1-butene, i.e., replacing in Eq. 5 Pbutane by Pl-but, 9 = 0 . 9 9 . In Fig. 6 is represented the experimental formation rate of butadiene on such basis and the corresponding computed fitting curve versus the estimated Pl-butene.According with these results, the real rate formation of 1-butene must be the sum of the experimental 1-butene and butadiene formation rates; the correlation of the rate under such conditions by Eq. 5 also provides values of 9 = 0.99 (Fig. 7). The correlation of CO and CO2 formation rates by the proposed Eq. 5 leads to very poor correlation coefficients ( a/tCO2 = 0.74 and q/CO = 0.39). The poor fitting for CO and CO2 reflects the dispersion of experimental data (Fig. 3) with respect to Po2, but, moreover, the possible formation of these products from all the hydrocarbon species, while Eq. 5 is only sensitive to Pbutane" The above exposed results show that butadiene is a secondary reaction product. In an attempt to elucidate experimentally its provenance and the possible isomerization reactions between 1-butene and 2-butenes, an experiment was performed, feeding the reactor with 1-butene. However, due to the different surface conditions and the different reactivity of n-butane and 1-butene, the product distributions obtained in these experiments cannot provide conclusive proof. The experimental
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
149
150
L.M. Madeira et al. /Applied Catalysis A." General 135 (1996) 137-153 5000y
O0°
4r~O0"
i 35003000v 2~00 2OOO
•
•
0
:
IOO0
. . . .
200
300
400
500
4-------
600
I
~
I
[
700
800
900
1000
Tem~rature (K)
Fig. 8. C Q - T P D profile at ( e ) 303 K and (11) 523 K.
conditions used were: 793 K, 2% 1-butene and 10% 02, the remaining variables were fixed as for the previous experiments. The conversion obtained in this experiment was 85% with the following product distribution: COx selectivity: 50%, butadiene selectivity: 37% and 2-butenes (cis + trans) selectivity: 13%. The formation rate of butadiene in this experiment is approximately equal to the one computed from butane for the same experimental conditions, which confirms that butadiene proceeds exclusively from 1-butene previously formed. However, the values of the ratio 2-butenes/butadiene are ca. 2.0 for butane as feed and show a strong decrease when 1-butene is used as feed (0.35). This confirms that 2-butenes are in practice not formed from 1-butene via isomerization, because, with the same experimental conditions, the use of 1-butene as feed causes a strong increase of butadiene formation but not of the 2-butenes formation. Furthermore, the nature of the catalyst surface was tested by temperature-programmed desorption of CO2. CO2-TPD chromatograms show (Fig. 8) that the CO2 physisorption is negligible, in accord with the low surface area of the catalyst, because no noticeable detector response is observed below 373 K. However, the desorption curve of CO2 adsorbed at 303 K presents two maxima, the first at 470 K and the second one starting at about 770 K, but its exact position cannot be estimated because the TPD experiments were carried out only up to 900 K in order to avoid the transition from a to/3-phase of nickel molybdate. These results were confirmed by CO2 adsorption at 523 K, also illustrated in Fig. 8 with a similar curve but with the first peak logically lower and displaced to higher temperature. It is noteworthy that no cesium carbonates were detected by XRD and FTIR experiments and that the CO2-TPD profile of the unpromoted NiMoO4 also shows the existence
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 137-153
I* butane
~
~-
151
1-butene
butadiene
butane*
02
~
I
I~
2-butenes
•
COx Scheme 1.
of two types of active sites. Consequently, the second peak found at high temperature can not be assigned to carbonate species. An overall analysis of the above mentioned results shows that the possibility of formation of two different intermediates from adsorbed butane on the catalyst surface and the formation of butadiene from 1-butene, is suggested mainly by the - - existence of two very different types of sites evidenced by CO2-TPD; - - similar very good linearity of cis- and trans-2-butene formation rates versus the butane partial pressure, while 1-butene and butadiene formation rates exhibit identical behaviour but different of the 2-butenes; - - less isomerization recorded when 1-butene was used as feed; - - very poor correlations found when the intervention of 2-butenes was tested for the butadiene formation. In this way, the reaction would occur according to Scheme 1. It is well known that the catalyst acid sites favour COx and 2-butenes formation [20,21 ]. On the less basic sites the intermediate T would be formed, leading to 2butenes, and on the more basic sites the intermediate 'I*' which produces 1-butene. In this way, Eq. 5 provides a very good correlation (~b=0.99) for the consumption of the I* intermediate (sum of the experimental rates of formation of 1-butene and butadiene, if the COx formation by this pathway is negligible). For the I intermediate, the addition of the experimental rates of formation for both 2butene isomers is also well expressed by Eq. 5 with ~b= 0.99. Experimental data for both intermediates and the computed curves by Eq. 5 are showed in Fig. 7. The computed values of k'o, Eao, k'r and E~ are presented in Table 1. As would be expected, the activation energies of formation of both intermediates (Ear) are not very different, however, their values also agree with the hypothesis previously advanced. The 1-butene formation and the subsequent dehydrogenation to butadiene take place in the more basic active sites of the catalyst surface. Due to Table 1 Parameters computed by fitting Eq. 5 to experimental data
rb0 r(~-but+bd) = r~* rie_but) =r~
k'o ( m o l / h m 2 bar)
k'~ ( m o l / h m 2 bar)
E~o ( J / m o l )
1.1 ' 102 1.9' 108 1.6' 105
0.3 18.0 5.7
4.6.10 4
1.6.10 4
1.6" 105 2.5.105
6.1 ' 104 5.8. 104
E~ ( J / m o l )
152
L.M. Madeira et al. /Applied Catalysis A: General 135 (1996) 13~153
a greater electronic density their reduction must be more difficult and E~ of formation of the I* intermediate must be greater than for the I intermediate. On the other hand, the reoxidation of these sites must be easier (Eao lower). Both these facts are confirmed by the values of Table 1. Furthermore, the dehydrogenation of 1-butene to butadiene is energetically easy [ 3] and, in fact, the Ea obtained for this process is relatively small (Table 1 ).
4. Conclusions The oxidative dehydrogenation of butane over 3% Cs doped NiMoO4 was systematically studied over a large range of experimental conditions. Only dehydrogenation products (Ca's) and COx were found, the formed oxygen-containing organic molecules being negligible. The observed influence of contact time confirms that 1-butene and 2-butenes are formed as primary reaction products while butadiene and COx are secondary ones. The formation rates of COx are independent of Pbutane while the selective dehydrogenation rates are independent of Po2. However, while the formation rate for 2butenes increases linearly with Pbut . . . . this increase presents a decreasing slope in the case of 1-butene and butadiene. A redox reaction mechanism with two different intermediates is proposed. One intermediate leads to 1-butene which is partially transformed to butadiene. The other one produces 2-butenes. COx would proceed from all the present hydrocarbons species. The CO2-TPD experiments confirmed the existence of two types of basic sites that would be responsible for the formation of both intermediates. The values of activation energies computed by the proposed model indicate that in the more basic sites 1-butene is formed and in the less basic sites 2-butenes. The formation of butadiene exclusively from 1-butene and the independence of 1-butene and 2butenes (practically no isomerization occurs) is also evidenced using 1-butene as feed.
Acknowledgements F.J.M.H. and R.M.M.A. thank the European Community for financial support. L.M.M. also thanks PRAXIS XXI program from J.N.I.C.T. (Junta Nacional de Investiga9fio Cientifica e Tecnol6gica) for financial support. Support of this work by European Community (contract n ° CHRX-CT92-0065) is gratefully acknowledged.
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