- Email: [email protected]
Effect of added Sb2O4, BiPO4 or SnO2 on the catalytic properties of ZnFe2O4 in the oxidative dehydrogenation of butene to butadiene
Applied Catalysis, 51 (1989) 235-253 Elsevier Science Publishers B.V., Amsterdam -
235 Printed in The Netherlands
Effect of Added SbOOQ,BiP04 or SnO, on the Catalytic Properties of ZnFe20, in the Oxidative Dehydrogenation of Butene to Butadiene FENG-YAN QIU Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou (People’s Republic of China) LU-TAO WENG, E. SHAM, P. RUIZ and B. DELMON* Catalyse et Chimie des Matiriaux Divisb, Place Croix du Sud I,1348 Louvain-la-Neuue (Belgium) (Received 11 October 1988, revised manuscript received 6 February 1989)
ABSTRACT ZnFezOd is a moderately active and selective catalyst for the oxidative dehydrogenation of butene to butadiene. It can be deactivated at high temperatures ( > 38O”C), probably owing to the segregation of the phase. The properties of ZnFe,O, can be modified by mechanically mixing ZnFezO, with Sb204, BiPO, and SnOz. At low temperatures, the effect of these phases is negative and follows the order SnOz > BiPO, > Sb*O,. The effect of these phases appears to be positive at high temperatures (ca. 380°C for Sbz04 and BiPO, and ca. 420°C for SnOZ) and the positive effect decreases in the order BiPO, > Sb,O, > SnO,. This positive effect is explained as being the result of the stabilization of the ZnFe204 phase due to the presence of these external phases. The results are discussed by considering the migration of oxygen species by added phases and the nature of oxygen species.
INTRODUCTION
Ferrite catalysts have been widely used in the oxidative dehydrogenation of butene to butadiene [ 11. Much research has been devoted to the clarification of the structure of the active phase, the nature of the active sites and the effects of the additional ions, such as Cr in ZnCr,Fe,_,04. The analysis of the catalysts studied seems to show that a good catalyst usually contains two phases, one being pure ferrite with a spine1 structure and the other a-Fe,O, in small amounts [ 2,3]. Several studies have shown that ferrite catalysts can be deactivated irreversibly when they age. An interesting observation was that the deactivation of the catalyst occurred in parallel with the disappearance of aFezO,. Therefore, the presence of a-Fez03, even in small amounts, seems to protect the catalyst from deactivation [2,3]. It had been proposed that the
01669834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
236
cause of deactivation of the ferrite catalysts was the reduction of these catalysts in the course of reaction [ 21. However it seems that the role of cr-Fe,O, was not explained satisfactorily. We have been studying multi-phase catalysts in the selective oxidation of olefins for several years. The study of the cooperation between two metal oxides in the form of a mechanical mixture, for example MoO,-Sb,O, [4,5] and SnOz-SbpO, [6], in the oxidation of isobutene, has shown that the role of one oxide A (e.g., MOO, ) may be to carry catalytic sites necessary for oxidation while that of the other (e.g., Sbz04), namely donor D, is to activate the molecular oxygen into mobile oxygen species (called spillover oxygen), which can migrate over the surface of the former (the acceptor A) to improve its properties (creating the new catalytic sites and/or regenerating the deactivated sites). In this way, the catalytic activity of the first oxide A can be controlled by the second D through the spillover oxygen (Remote control mechanism). The migration of oxygen species from Sbz04 to MOO, was demonstrated recently by means of the “0 technique [ 71. Similarly, we speculated that it might be possible to stabilize the activity of ferrite catalysts by taking advantage of this kind of mechanism. This would be very interesting because it would provide new ways of improving industrial catalysts. In this work, we have attempted to consider this point, namely whether the catalytic properties of pure ZnFe,O, can be modified by the addition of an external oxide phase. Three oxides were used, Sbz04 and BiPO, (called donor), which have been proved (Sb,O,) or are believed to be capable of producing spillover oxygen, and SnO,, which is considered to be a solid with “intermediate properties”, i.e., it may be an acceptor or donor of spillover oxygen, depending on the conditions [ 81. Pure ZnFeaO, was prepared separately and then mixed mechanically with these oxides, which had also been prepared separately. The mechanical mixing procedure used minimizes the mutual contamination between two phases. The catalysts thus prepared were tested in the oxidative dehydrogenation of butene to butadiene. Three parameters were especially investigated: the composition of the catalyst, the react’ion temperature and the oxygen concentration. The fresh and used samples were characterized by X-ray diffraction, BET surface areas and electron microscopy. EXPERIMENTAL
Sample preparation and characterization Analytical-reagent grade chemicals were used. Oxide preparation ZnFe,O, was prepared by the citrate method [ 91. An aqueous solution of atomic ratio= 1:2) was prepared from Zn2+ and Fe3+ (Zn-to-Fe Zn(N0,)2*5H20 and Fe(N03),.9H20 (Merck). Addition of a small amount of nitric acid (6.5 M) was necessary to avoid precipitation or coprecipitation.
23’7
When the solution was homogeneous (transparent), citric acid was added in such a way that the number of moles of anions (citric acid) was equal to the total number of moles of Zn and Fe ions. The solution thus obtained was evaporated under reduced pressure in a rotary evaporator until the liquid was progressively converted into an amorphous solid precursor. This precursor was immediately decomposed at 300 ’ C for 16 h and finally calcined at 500 ’ C for 18 h. BiP04 was also prepared using the citrate method in a similar manner from Bi(NOs)3*5H20 and orthophosphoric acid. Sb,O, was prepared by calcination of Sbz03 (Merck) in air at 500°C for 20 h. SnO, was prepared by precipitation of tin (II) hydroxide from SnC12*2H,0 (Merck), followed by drying overnight at 110’ C and calcination at 600°C for 8 h. Mechanical mixtures Mechanical mixtures of two oxides were prepared by dispersing the powders under agitation in n-pentane for 10 min, followed by evaporation of the solvent and drying overnight at 80” C. No further calcination was carried out. The composition of mechanical mixtures was expressed as the mass ratio: R,=
weight of ZnFe, O4 weight of ZnFez 0, + weight of MO,
where MO, refers to Sbz04, BiP04 or SnOz. Characterization XRD. The crystallographic phases of the prepared samples were determined with a Kristalloflex 805 diffractometer (Siemens) using Cu Kcr radiation. For the samples treated under seven different reaction conditions, a more sensitive diffractometer (Siemens D-500) was also employed. BET surface areas. The BET surface areas of the samples were determined gravimetrically in a Setaram MTB 10-8 microbalance connected to a vacuum and gas handling system by nitrogen adsorption at 77 K. Electron microscopic measurements were carried out Electron microscopy. with a Jeol Temscan 100 CX electron microscope equipped with a Kevex 5100 C energy-dispersive spectrometer. Samples were ground, dispersed in a solvent (n-pentane) and deposited on a film of carbon supported on a copper grid. The samples were examined by means of the conventional transmission (CTEM ) mode. The nature of the particles was identified by analytical electron microanalysis (AEM), which principally permits mutual contamination between two oxides to be detected.
Measurement of catalytic actiuity Catalytic activities were measured in a conventional fixed-bed reactor system. The reactor was made of a Pyrex U-tube of 8 mm I.D. into which the catalyst was packed. A small tube of 4 mm I.D. was placed in the reactor, into which a thermocouple was inserted in order to measure the reaction temperature of the catalytic bed. The catalyst was screened and the fractions between 500 and 800 ,um were used. The catalyst (usually 500 mg) was diluted with glass balls of a similar size (5 mm). The entire catalytic bed was maintained at a height of 1.5 cm. The reactants (butene and oxygen) and nitrogen, whose flow-rates were controlled by mass flow controllers, passed through the catalytic bed. The remaining reactants and products were passed continuously into a gas chromatograph (Intersmat IGE 120ML) for analysis. A column containing 17% sebaconitrile supported on Chromosorb P AW was used. The standard reaction conditions were as follows: 76 mmHg butene; 152 mmHg 02;total feed, 100 ml/min; total pressure, atmospheric; and catalyst mass, 500 mg. In certain instances, the oxygen concentration could be modified while keeping the butene concentration and total feed constant. The catalytic activity results are expressed as the rate of butene conversion into each product, namely the number of millimoles of butene converted to product i per hour and per gram of catalyst:
where Fi is the total feed of butene (mmol/h), Xiis the conversion of butene to product i, which can be calculated according to the chromatographic peak area of product i, and g is the weight of catalyst in grams. The selectivity, Siis Ri/ CRi. RESULTS
Catalytic activity Catalytic activity of pure oxides Under our reaction conditions, the main reactions of butene are oxidative dehydrogenation to butadiene, isomerization to 2-butene and total oxidation to carbon dioxide and carbon monoxide. The latter could not be detected in the chromatographic analysis. Therefore, the results presented below will concern only the first three products, namely butadiene, 2-butene and carbon dioxide. However, the extent of carbon monoxide formation can be easily calculated.
239
ZnFe,O, Fig. la shows the influence of the reaction temperature on the catalytic activity of pure ZnFezO, at an oxygen-to-butene ratio of 2. The influence of the reaction temperature on the total butene transformation rate can be divided into three parts. At low temperatures, the butene conversion increases with increasing reaction temperature. Between 360 and 380’ C, a sharp increase in butene conversion is observed. Above 380 oC, the butene conversion increases slowly. The effect of the reaction temperature on butadiene selectivity is more complicated; it increases at low temperatures and then reaches a maximum at about 360°C. Between 360 and 380°C it decreases very quickly and finally reaches a plateau above 380°C. Fig. lb shows the influence of the oxygen-to-butene molar ratio on the conversion and formation of the various products at 400°C. The total butene transformation rate increases quickly with increasing oxygen-to-butene ratio when the latter is smaller than 2, and thereafter, the rate of increase is lower. The butadiene formation passes through a maximum at an oxygen-to-butene ratio of 2. The selectivity remains constant at a low ratio and decreases rapidly when the ratio is greater than 2.
500
-l
2‘ oi
b
P
s 2
.400-
I t
s P ?
300-
- 60
E 8
2 8
zoo-
lOC-
300
350
A
Fig. 1. Effects of (a) reaction temperature and (b) oxygen-to-butene molar ratio on the rates of n-butene conversion to products and on the selectivity to butadiene for ZnFe,O,. +, Total nbutene conversion; W, rate of n-butene conversion to butadiene; A, rate of n-butene conversion to carbon dioxide, 0 selectivity to butadiene.
240
Sb204. The effects of reaction temperature and oxygen-to-butene ratio on the catalytic properties of Sb,O, are presented on Fig. 2a and b, respectively. It can be seen that Sb,O, has a very poor activity (1% of ZnFe,04), even at high temperatures. The conversion of butene can be observed at temperatures above 370 oC. Formation of all the products increases with increasing reaction temperature (Fig. 2a) and appears to reach a maximum at an oxygen-to-butene ratio near 1 (Fig. 2b). Fig. 3a and b show the influence of reaction temperature and oxygento-butene molar ratio, respectively, on the catalytic properties of BiPO,. Compared with Sbz04, BiP04 is almost ten times more active (but ten times less active than ZnFezO,). The butadiene and carbon dioxide formation rates increase rapidly with increasing temperature whereas that of 2-butene decreases with increasing temperature. However, the increase in carbon dioxide formation is greater than that of butadiene. It follows that the butadiene selectivity (not shown) decreases with increasing temperature. Fig. 3b shows that the 2butene and carbon dioxide formation rates increase almost linearly with in-
BiPO,.
a-
_oxygen
molarratio
butene
Fig. 2. Effects of (a) reaction temperature and (b) oxygen-to- butene molar ratio on the rates of n-butene conversion to products for Sb,O,. Symbols as in Fig. 1; V, rate of n-butene conversion to 2-butene.
Temperature lW
butene
molar ratio
Fig. 3. Effects of ( a ) reaction temperature and (b) oxygen-to-butene molar ratio on the rates of n-butene conversion to products for BiPO,. Symbols as in Fig. 2.
creasing oxygen-to-butene ratio whereas that of butadiene is almost independent of the latter. It follows that the butadiene selectivity decreases quickly with increasing oxygen-to-butene ratio. SnO,. The influences of reaction temperature and oxygen-to-butene molar ratio on the catalytic properties of SnO, are reported in Fig. 4a and b, respectively. Compared with Sb204and BiPO,, SnO, produces much more butadiene and less isomerization product. The formation of all reaction products increases with increasing temperatures. The butadiene selectivity increases at low temperature and has a sharp increase at temperatures near 350"C, after which it remains constant. The reaction product formation rates depend to some extent on the oxygen-to-butene ratio and the butadiene selectivity decreases slowly with increase in the latter. Catalytic activity of mechanical mixtures We examined mixtures of ZnFe204with Sb20,, BiPO, and Sn02. For purposes of comparison, the butene transformation rates on mixtures (solid lines
2
-z
100
$5
a
a
25
E
a
1
9
i? Q L
P eo
60.
Q
$
5
z'== 08
60
3 3 .
..C
i! 20
-
0
-
-LO
LO
X)
20
a
300
1.0
Temperature l°CI
2.0
E%?C!mobr bukne
30 O mtio
Fig. 4. Effects of ( a ) reaction temperature and (b) oxygen-to-butene molar ratio on the rates or n-butene conversion to products and on the selectivity to butadiene for SnOz. Symbols as in Fig.
in the figures) were contrasted with those calculated supposing a simple addition of those of two pure oxides (dotted lines ), i.e., ri =RmriZnFe204(1R,)ri,ox. Hence the difference between the sound and dotted lines represents the magnitude of the cooperative effect between two oxides.
+
ZnFe20,-Sb,O, system. Fig. 5a presents the catalytic activity results as a function of the mass ratio, R,, a t 400" C. The total butene conversion is almost linearly dependent on R,. However, there is a small degree of synergy between two oxides on the butadiene formation rate. The maximum synergistic effect is observed at a mass ratio near 0.5. A strong synergy exists on butadiene selectivity and the maximum value remains almost the same in the range R,= 0.25-0.75. In order to investigate the possible effects of reaction temperature and oxygen concentration on the catalytic cooperative effect, a mechanical mixture of composition R, = 0.5 was taken as an example and the results are presented in Fig. 5b and c, respectively. Fig. 5b shows that Sb204has almost no effect at low temperatures (below 360°C) and has a negative effect a t temperatures between 360 and 380" C. The positive effect of Sb204on the catalytic properties of ZnFe,04 can be observed only when the temperature is higher than 380" C and the higher the temperature, the greater is the synergistic effect. For the
oxypen buleM, molar ratio
.,
Fig. 5. Effects of ( a ) composition (R,), (b) reaction temperature and (c) oxygen-to-butene molar rat0 on the rates of n-butene conversion to products and on the selectivity to butadiene for ZnTotal n-butene conversion; 0, rate of n-butene conversion to FezOl-SbzO, mixtures. +,0, butadiene; A , A rate of n-butene conversion to carbon dioxide, selectivity to butadiene.
whole range of reaction temperatures investigated, the butadiene selectivity is always greater than that of pure ZnFe,04, (compare Figs. l a and 5b). Fig. 5c shows that the cooperative effect between two oxides in terms of butadiene productivity can be observed only when the oxygen-to-butene ratio is greater than 1.5 and it increases with increase in oxygen concentration. However, the selectivity with respect to butadiene increases slightly with increasing oxygen concentration.
ZnFe,O,-BiPO, system. The influence of catalyst composition, reaction temperature and oxygen concentration are shown in Fig. 6a, b and c, respectively. Similarly to the ZnFe,04-Sb204 system, Fig. 6a shows that there is synergy between two oxides on butadiene formation rate and selectivity. The maximum synergistic effects are observed at R,=0.75 and 0.25 for the butadiene formation rate and its selectivity, respectively. However, for the present system, a maximum is also observed in the total butene conversion, which is not observed in the former system. The influence of reaction temperature on the cooperative effect between two oxides (Fig. 6b) is also similar to that for the ZnFe,04-Sb204 system. If we consider butadiene productivity, three regions of influence can be distinguished: at low temperature ( < 360" C ) BiPO, has no effect whereas between 360 and 380 C a negative effect is observed and finally, above 380" C, a positive effect is clearly observed which increases with increasing temperature. Compared with the ZnFe204-Sb204system, the synergistic effect is greater in the present instance. The butadiene selectivity is always superior to that of pure ZnFe204for the whole range of temperatures and it seems to decrease with increasing temperature. The effect of oxygen concentration on the catalytic behaviour of this system is also similar to that on the ZnFe204-Sb,O, system (Fig. 6c). However, the catalytic synergy on the rate of formation of butadiene can be observed a t lower oxygen-to-butene ratios ( > 1) and the influence of oxygen concentration is more pronounced for this system. ZnFe,O,-SnO, system. For this system, only the influence of reaction temperature was investigated. The results (Fig. 7 ) show that SnO, has a negative effect not only on butadiene formation but also on total butene conversion up to 420°C. Above 420°C, however, a remarkable synergistic effect is observed, which increases with increasing temperature. Physico-chemical characterization The physico-chemical characteristics of three systems (XRD phases and BET surfaces) are summarized in Table 1.I t can be observed that XRD phases of mechanical mixtures correspond to those observed in pure oxides. No new
Temperature
/TI
Fig. 6. Effects of ( a ) composition (R,), (b) reaction temperature and (c) oxygen-to-butene molar ratio on the rates of n-butene conversion to products and on the selectivity to butadiene for ZnFe20,BiPO, mixtures. Symbols as in Fig. 5.
Temperature
I°CI
Fig. 7. Effects of reaction temperature on the rates of n-butene conversion to products for ZnFe,O,-Sn02 mixtures. Symbols as in Fig. 5.
TABLE 1 XRD and BET surface area results Sample
ZnFe,O, ZnFe,04 Sb204 SbzO4 ZnFe,04 BiP040 BiPO, ZnFe,04 SnO, SnO,
+
+ +
Rm
1.0 0.5 0.0 0.5 0.0 0.5 0.0
XRD phase
BET surface area (m2/g)
Fresh
Used
Spinel Spinel Sb204 SbzO4 Spinel +BiP04 BiPO, Spinel SnO, SnO,
Spinel 9.4 Spinel 5.8 Sb2O4 Sb204 2.0 7.8 Spinel BiP04 4.3 BiPO, Spinel+ SnO, 13.0
+
+
+
+
" BiPO, was composed of two crystallographic forms.
Fresh
Used
phase was observed either during mixture preparation or during the catalytic test. The XRD spectra remain almost unchanged during the reaction, both for pure oxides and mechanical mixtures. The surface area of the fresh mechanical mixture is the simple sum of those of the pure oxides, taking into account the accuracy of the surface measurements. No change in the surface area was observed for pure oxides (ZnFe,04 and Sb204)and for mechanical mixtures between ZnFe,04 and SnO, or BiPO, after reaction. However, a small increase in surface area was obtained for the mechanical mixture between ZnFe,04 and Sb204after reaction. The ZnFe204-Sb20, system was characterized using electron microscopy. For pure ZnFe,O,, no change was observed with respect to the morphology and the size of the particles for the fresh and used samples. However, the formation of coke was observed for the used but not for the fresh sample. For the mechanical mixture with R,= 0.5, Figs. 8 and 9 present the CTEM micrographs and AEM spectra for fresh and used samples, respectively. In the CTEM micrographs, the larger particles correspond to Sb204and the smaller particles to ZnFe,04. Most ZnFe204particles are around Sb204particles. No difference was observed in either the morphology or the particles size for the fresh and used samples. In order to detect the existence of mutual contamination between two phases, AEM (using an electron probe microanalyser) was carried out at different positions in the samples, as shown in the CTEM micrographs. The corresponding energy peaks of these systems are presented in the EDS spectrum. It can be seen that only the Sb signal was detected when the analysis was made on
Fig. 8. ( a ) CTEM micrograph and (b) EDS spectrum for fresh ZnFe,O,-Sb,O, mechanical mixture with R, = 0.5.
Energy (KeV)
Fig. 9. (a) CTEM micrograph and (b) EDS spectrum for used ZnFe,O,-Sb,O, mechanical mixture with R, = 0.5.
Sb204particles, and only Zn and Fe signals were observed when ZnFez04particles were analysed. The Sb, Zn and Fe signals appeared at the same time only when the analysis probe was focused on the "contact" zone of the two oxide phases (e.g., position 6 in Fig. 8a). All these results indicate that no mutual contamination between ZnFe204and Sbz04takes place, either during mixture preparation or catalytic reaction. In order to investigate whether or not the spinel phase ZnFe,O, is segregated during reaction at high temperatures, two specially designed experiments were carried out. Pure ZnFez04 and a ZnFez04-Sb,04 mechanical mixture with R, = 0.5 were tested under two extreme reaction conditions for the same period of time (6 h): one at high temperature and high oxygen concentration (450"C and oxygen/butene = 3 ) and the other at high temperature and low oxygen concentration (450"C and oxygenlbutene = 1) .The catalysts thus tested were characterized by means of a more sensitive X-ray diffractometer (Siemens D500). The activity results are reported in Table 2. Pure ZnFe204was deactivated during the catalytic reaction, especially at high temperatures and low oxygen concentration. However, almost no deactivation was observed for the mechanical mixture under the same conditions. The presence of Sbz04protects ZnFe204from deactivation. The X-ray data are reported in Fig. 10, which clearly shows that ZnFe,04 was segregated into a spinel phase and ZnO after reaction and, at the same temperature for the experiments carried out, the lower the oxygen concentration, the greater was the segregation. The spectra of mechanical mixtures (Fig. 11), however, show only the presence of ZnFez04 and Sbz04.No segregation was observed. Hence the presence of Sbz04stabilizes the ZnFe,O, structure.
TABLE 2 Catalytic activity results for ZnFe204-Sb204systems under extreme conditions Reaction conditions
Sample
450" C, oxygenlbutene = 3 ZnFe204
Analysis Rate of butene converted to (mmol/g.h) time (h) C4H6 co2 Total 1
6 ZnFe204 1 +Sb2O4 6
29.8 27.7 53.9 59.4
62.5 63.5 48.8 35.7
94.9 92.1 106.9 104.2
0 . 0 ~ ' " " " " ~ 1.0 110 210 310 41.0 51.0 67.0
no 81.0 so lor.0 - theta (degrees) Fig.10. X-ray diffraction spectra for ZnFe204.(a) Fresh ZnFe,04; (b) ZnFe204treated at 450°C, oxygen/butene=3 for 6 h; (c) ZnFe20, treated at 450°C, oxygen/butene= 1for 6 h. two
DISCUSSION
The results show that the catalytic properties of ZnFe204can be affected by the addition of an external oxide phase (Sb20,, BiPO, or SnO,) . The effect of the added phase depends strongly on its nature and also on the reaction conditions. It is well known that several phenomena may take place when two oxide phases are mixed together, e.g., new phase formation and surface contamination of one phase by the other, in physical contact. In this way, the explanation of the results may be very different, depending on which phenomenon has occurred between two phases. In our case, mechanical mixtures of three systems, namely ZnFe204-Sb204,ZnFe204-BiPO, and ZnFe204-SnO,, were characterized by XRD and BET surface area measurements before and
r~~~
- THETA
( DEGREES I
Fig. 11. X-ray diffraction spectra for ZnFe20,-Sb20,. (a) Fresh ZnFe204;(b) ZnFe,O,-Sb,O, treated at 450°C,oxygen/butene=3 for 6 h; ( c ) ZnFe20,-Sb204treated at 450°C,oxygen/butene = 1 for 6h.
after reaction. No new phase formation was observed with XRD. The surface areas of the mechanical mixtures are the simple sum of those of the two pure oxide components, indicating that the surface of the oxide is not modified by the preparation procedure. A change would be expected if some solid reaction between the oxides take place. The electron microscopic measurements showed that no mutual contamination was detected for ZnFe204-Sb204system. However, because of the lack of other sensitive surface characterization techniques (e.g., X-ray photoelectron or ion scattering spectroscopy), much care should be taken with the explanation of the results. However, a detailed analysis of our catalytic activity results indicates some common aspects observed for three added phases and suggests possible explanations. In the following, we shall focus our attention on three parameters: reaction temperature, oxygen concentration and nature of added phase. Effect of reaction temperature
Let us consider the influence of temperature on the properties of pure ZnFe204.At low temperatures, the activity of ZnFe204increases with increasing temperature, but an unexpected phenomenon is observed at high temperature ( > 3 8 0 ° C ) , namely that the activity is almost independent of temperature. This may be related to the stability of the spinel phase. The results in Fig. 10 clearly demonstrate that the spinel phase can be easily segregated into a spinel
phase and ZnO a t high temperature (450°C), although no new phase was detected for the sample tested under standard conditions (400°C and oxygen/ butene = 2). Therefore, we may conclude that ZnFe204is a good catalyst but a t high temperatures its structure is unstable. For the mechanical mixtures, the shapes of the curves of activity vs. temperature are similar for the three systems (see Figs. 5b, 6b and 7). At low temperatures, the effect of the added phase is negative or zero and a positive effect is observed a t high temperatures. However, several differences can be noted if we compare the curves carefully: ( i ) the magnitude of the negative effect of the added phase follows the order SnO, > BiP04> Sb204;(ii) the effect of SnO, appears positive a t around 420" C, which is much higher than that for Sb204and BiPO, (ca. 380" C ) ; (iii) a t high temperatures, the positive effect of the added phase follows the order BiP04> Sb2O4> > SnO,; and (iv) the butadiene selectivity of ZnFe204 is improved greatly by Sb204and BiPO, phases, even at low temperatures, which is not the case for SnO,. Based on these observations, we can divide these three added phases into two groups: ( i ) Sb204and BiPO,, which serve especially to improve the butadiene selectivity and appear to have a positive effect at relatively lower temperatures (ca. 380°C), and (ii) SnO,, which appears to have a positive effect a t higher temperatures (ca. 420" C ). If the structural stability of ZnFe,O, is taken into account, the positive effect of these phases a t high temperatures may be easily considered to be due to the stabilization of ZnFe,04. This is clearly demonstrated by Fig. 11,which shows that the presence of Sb204can protect the segregation of ZnFe204.But how does the added phase affect the properties of ZnFe204?Before answering this question, let us consider the influence of oxygen concentration.
Effect of oxygen concentration The influence of oxygen concentration on the catalytic properties of mechanical mixtures is similar for the two systems ZnFe204-Sb204(Fig. 5c) and ZnFe204-BiPO, (Fig. 6c). An increase in oxygen concentration favours the synergistic effect of the added phase. As mentioned in the Introduction, Sb204 and BiPO, may be considered to be the "donors" of ~ x y g e nspecies, as proposed previously [8]; hence it is reasonable to accept that an increase in gaseous oxygen concentration would favour the production of oxygen species (usually
o2-).
Interpretation of results based on "migration of oxygen species" For pure ZnFe,04, an increase in reaction temperature would favour the activity of lattice oxygen, which is why the activity of ZnFe,O, increases with an increase in temperature. After a certain temperature has been reached, how-
ever, the lattice oxygen is so active that its disappearance (to oxidation products) is much more rapid than the reoxidation rate of the catalyst by gaseous oxygen, resulting in a reduction of the catalyst (even if no ZnO is detected for the catalyst after reaction under standard conditions, defects might form a t the surface) and consequently a decrease of catalytic activity and selectivity. This is the case when the temperature rises above 380" C. If an external phase is present (Sb204or BiPO,), however, the oxygen produced by this phase can reoxidize ZnFe204in a more efficient way, which is why a positive effect of this phase at high temperatures was observed. The question is, why does the positive effect of SnO, appear a t higher temperatures (420" C ) ? It may be due to the difference in the nature of oxygen species produced by the external phase. It has been shown by ESR [ l o ] that oxygen species adsorbed on SnO, are mainly electrophilic (0,-, 0-) below 400" C, after which these species are progressively transformed to nucleophilic species ( 0 2 -) with increasing temperature. For the reoxidation of the catalyst, nucleophilic species are more suitable than the electrophilic ones. It may be expected that sufficient nucleophilic species are available only when temperatures are above 420°C, which is why the positive effect of S n 0 2 begins a t 420°C. It may therefore be assumed that the nature of the oxygen species produced by Sb204and BiPO, is nucleophilic, even at low temperatures. Now let us consider the effect of these external phases on the selectivity of butadiene. It is usually accepted that nucleophilic oxygen (0,- ) favours selective oxidation whereas electrophilic species favour total oxidation. The improvement in butadiene selectivity on addition of Sb,O, and BiPO, may confirm, in another way, the above assumption, namely that 02-is produced by these oxides.
Outlook Our results demonstrate clearly that the ferrite ZnFe,O, is an active and selective catalyst for the oxidative dehydrogenation of butene. However, it may be deactivated at high temperatures owing to the segregation of the phases. This segregation is presumably triggered by some reduction. The presence of an external phase, such as Sb204and BiPO,, capable of producing oxygen species (0'-) seems to stabilize the structure of ZnFe,O, and therefore to protect it from deactivation. This provides a new route to increasing the stability of industrial catalysts. These results could suggest that part of the role of a-Fe,03 in industrial catalysts might be similar to that of Sb204.However, this has yet to be proved and further investigations are necessary. ACKNOWLEDGEMENTS
The financial support of FNRS (Belgium) under an agreement with the Chinese Academy of Sciences for the stay of F.Y. Qiu in Belgium is greatly
acknowledged. The authors also thank Dr. J. Naud for his help with some of the XRD measurements. REFERENCES 1 H.H. Kung and M.C. Kung, Adv. Catal., 33 (1985 ) 159. 2 H.Q. Wang, L.P. Feng, Y.W. Chen, D.C. Miao, Q.Z. Cong and A.B. Jiang, Sci. Sin., 22, No. 12 (1979) 1398. 3 M.A. Gibson and J.W. Hightower, J. Catal., 41 (1976) 431. 4 P. Ruiz, B. Zhou, M. Remy, T. Machej, F. Aoun, B. Doumain and B. Delmon, Catal. Today, 1 (1987) 181.
5 L.T. Weng, B. Zhou, B. Yasse, B. Doumain, P. Ruiz and B. Delmon, in M.J. Phillips and M. Ternan (Editors), 9th International Congress on Catalysis, Calgary, Canada, 1988, Vol. 4, Chemical Institute of Canada, Ottawa, 1988, p. 1609. 6 L.T. Weng, N. Spitaels, B. Yasse, J . LadriBre, P. Ruiz and B. Delmon, to be published. 7 L.T. Weng, D. Duprez, P. Ruiz and B. Delmon, J. Mol. Catal., in press. 8 S. Ceckiewicz and B. Delmon, Bull. Soc. Chim. Belg., 93 (1984) 163. 9 Ph. Courty, H. Ajot, Ch. Marcilly and B. Delmon, Powder Technol., 7 (1973) 21. 10 S.C. Cheng, J. Vacuum Sci. Technol., 17 (1980) 366.
235 Printed in The Netherlands
Effect of Added SbOOQ,BiP04 or SnO, on the Catalytic Properties of ZnFe20, in the Oxidative Dehydrogenation of Butene to Butadiene FENG-YAN QIU Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou (People’s Republic of China) LU-TAO WENG, E. SHAM, P. RUIZ and B. DELMON* Catalyse et Chimie des Matiriaux Divisb, Place Croix du Sud I,1348 Louvain-la-Neuue (Belgium) (Received 11 October 1988, revised manuscript received 6 February 1989)
ABSTRACT ZnFezOd is a moderately active and selective catalyst for the oxidative dehydrogenation of butene to butadiene. It can be deactivated at high temperatures ( > 38O”C), probably owing to the segregation of the phase. The properties of ZnFe,O, can be modified by mechanically mixing ZnFezO, with Sb204, BiPO, and SnOz. At low temperatures, the effect of these phases is negative and follows the order SnOz > BiPO, > Sb*O,. The effect of these phases appears to be positive at high temperatures (ca. 380°C for Sbz04 and BiPO, and ca. 420°C for SnOZ) and the positive effect decreases in the order BiPO, > Sb,O, > SnO,. This positive effect is explained as being the result of the stabilization of the ZnFe204 phase due to the presence of these external phases. The results are discussed by considering the migration of oxygen species by added phases and the nature of oxygen species.
INTRODUCTION
Ferrite catalysts have been widely used in the oxidative dehydrogenation of butene to butadiene [ 11. Much research has been devoted to the clarification of the structure of the active phase, the nature of the active sites and the effects of the additional ions, such as Cr in ZnCr,Fe,_,04. The analysis of the catalysts studied seems to show that a good catalyst usually contains two phases, one being pure ferrite with a spine1 structure and the other a-Fe,O, in small amounts [ 2,3]. Several studies have shown that ferrite catalysts can be deactivated irreversibly when they age. An interesting observation was that the deactivation of the catalyst occurred in parallel with the disappearance of aFezO,. Therefore, the presence of a-Fez03, even in small amounts, seems to protect the catalyst from deactivation [2,3]. It had been proposed that the
01669834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
236
cause of deactivation of the ferrite catalysts was the reduction of these catalysts in the course of reaction [ 21. However it seems that the role of cr-Fe,O, was not explained satisfactorily. We have been studying multi-phase catalysts in the selective oxidation of olefins for several years. The study of the cooperation between two metal oxides in the form of a mechanical mixture, for example MoO,-Sb,O, [4,5] and SnOz-SbpO, [6], in the oxidation of isobutene, has shown that the role of one oxide A (e.g., MOO, ) may be to carry catalytic sites necessary for oxidation while that of the other (e.g., Sbz04), namely donor D, is to activate the molecular oxygen into mobile oxygen species (called spillover oxygen), which can migrate over the surface of the former (the acceptor A) to improve its properties (creating the new catalytic sites and/or regenerating the deactivated sites). In this way, the catalytic activity of the first oxide A can be controlled by the second D through the spillover oxygen (Remote control mechanism). The migration of oxygen species from Sbz04 to MOO, was demonstrated recently by means of the “0 technique [ 71. Similarly, we speculated that it might be possible to stabilize the activity of ferrite catalysts by taking advantage of this kind of mechanism. This would be very interesting because it would provide new ways of improving industrial catalysts. In this work, we have attempted to consider this point, namely whether the catalytic properties of pure ZnFe,O, can be modified by the addition of an external oxide phase. Three oxides were used, Sbz04 and BiPO, (called donor), which have been proved (Sb,O,) or are believed to be capable of producing spillover oxygen, and SnO,, which is considered to be a solid with “intermediate properties”, i.e., it may be an acceptor or donor of spillover oxygen, depending on the conditions [ 81. Pure ZnFeaO, was prepared separately and then mixed mechanically with these oxides, which had also been prepared separately. The mechanical mixing procedure used minimizes the mutual contamination between two phases. The catalysts thus prepared were tested in the oxidative dehydrogenation of butene to butadiene. Three parameters were especially investigated: the composition of the catalyst, the react’ion temperature and the oxygen concentration. The fresh and used samples were characterized by X-ray diffraction, BET surface areas and electron microscopy. EXPERIMENTAL
Sample preparation and characterization Analytical-reagent grade chemicals were used. Oxide preparation ZnFe,O, was prepared by the citrate method [ 91. An aqueous solution of atomic ratio= 1:2) was prepared from Zn2+ and Fe3+ (Zn-to-Fe Zn(N0,)2*5H20 and Fe(N03),.9H20 (Merck). Addition of a small amount of nitric acid (6.5 M) was necessary to avoid precipitation or coprecipitation.
23’7
When the solution was homogeneous (transparent), citric acid was added in such a way that the number of moles of anions (citric acid) was equal to the total number of moles of Zn and Fe ions. The solution thus obtained was evaporated under reduced pressure in a rotary evaporator until the liquid was progressively converted into an amorphous solid precursor. This precursor was immediately decomposed at 300 ’ C for 16 h and finally calcined at 500 ’ C for 18 h. BiP04 was also prepared using the citrate method in a similar manner from Bi(NOs)3*5H20 and orthophosphoric acid. Sb,O, was prepared by calcination of Sbz03 (Merck) in air at 500°C for 20 h. SnO, was prepared by precipitation of tin (II) hydroxide from SnC12*2H,0 (Merck), followed by drying overnight at 110’ C and calcination at 600°C for 8 h. Mechanical mixtures Mechanical mixtures of two oxides were prepared by dispersing the powders under agitation in n-pentane for 10 min, followed by evaporation of the solvent and drying overnight at 80” C. No further calcination was carried out. The composition of mechanical mixtures was expressed as the mass ratio: R,=
weight of ZnFe, O4 weight of ZnFez 0, + weight of MO,
where MO, refers to Sbz04, BiP04 or SnOz. Characterization XRD. The crystallographic phases of the prepared samples were determined with a Kristalloflex 805 diffractometer (Siemens) using Cu Kcr radiation. For the samples treated under seven different reaction conditions, a more sensitive diffractometer (Siemens D-500) was also employed. BET surface areas. The BET surface areas of the samples were determined gravimetrically in a Setaram MTB 10-8 microbalance connected to a vacuum and gas handling system by nitrogen adsorption at 77 K. Electron microscopic measurements were carried out Electron microscopy. with a Jeol Temscan 100 CX electron microscope equipped with a Kevex 5100 C energy-dispersive spectrometer. Samples were ground, dispersed in a solvent (n-pentane) and deposited on a film of carbon supported on a copper grid. The samples were examined by means of the conventional transmission (CTEM ) mode. The nature of the particles was identified by analytical electron microanalysis (AEM), which principally permits mutual contamination between two oxides to be detected.
Measurement of catalytic actiuity Catalytic activities were measured in a conventional fixed-bed reactor system. The reactor was made of a Pyrex U-tube of 8 mm I.D. into which the catalyst was packed. A small tube of 4 mm I.D. was placed in the reactor, into which a thermocouple was inserted in order to measure the reaction temperature of the catalytic bed. The catalyst was screened and the fractions between 500 and 800 ,um were used. The catalyst (usually 500 mg) was diluted with glass balls of a similar size (5 mm). The entire catalytic bed was maintained at a height of 1.5 cm. The reactants (butene and oxygen) and nitrogen, whose flow-rates were controlled by mass flow controllers, passed through the catalytic bed. The remaining reactants and products were passed continuously into a gas chromatograph (Intersmat IGE 120ML) for analysis. A column containing 17% sebaconitrile supported on Chromosorb P AW was used. The standard reaction conditions were as follows: 76 mmHg butene; 152 mmHg 02;total feed, 100 ml/min; total pressure, atmospheric; and catalyst mass, 500 mg. In certain instances, the oxygen concentration could be modified while keeping the butene concentration and total feed constant. The catalytic activity results are expressed as the rate of butene conversion into each product, namely the number of millimoles of butene converted to product i per hour and per gram of catalyst:
where Fi is the total feed of butene (mmol/h), Xiis the conversion of butene to product i, which can be calculated according to the chromatographic peak area of product i, and g is the weight of catalyst in grams. The selectivity, Siis Ri/ CRi. RESULTS
Catalytic activity Catalytic activity of pure oxides Under our reaction conditions, the main reactions of butene are oxidative dehydrogenation to butadiene, isomerization to 2-butene and total oxidation to carbon dioxide and carbon monoxide. The latter could not be detected in the chromatographic analysis. Therefore, the results presented below will concern only the first three products, namely butadiene, 2-butene and carbon dioxide. However, the extent of carbon monoxide formation can be easily calculated.
239
ZnFe,O, Fig. la shows the influence of the reaction temperature on the catalytic activity of pure ZnFezO, at an oxygen-to-butene ratio of 2. The influence of the reaction temperature on the total butene transformation rate can be divided into three parts. At low temperatures, the butene conversion increases with increasing reaction temperature. Between 360 and 380’ C, a sharp increase in butene conversion is observed. Above 380 oC, the butene conversion increases slowly. The effect of the reaction temperature on butadiene selectivity is more complicated; it increases at low temperatures and then reaches a maximum at about 360°C. Between 360 and 380°C it decreases very quickly and finally reaches a plateau above 380°C. Fig. lb shows the influence of the oxygen-to-butene molar ratio on the conversion and formation of the various products at 400°C. The total butene transformation rate increases quickly with increasing oxygen-to-butene ratio when the latter is smaller than 2, and thereafter, the rate of increase is lower. The butadiene formation passes through a maximum at an oxygen-to-butene ratio of 2. The selectivity remains constant at a low ratio and decreases rapidly when the ratio is greater than 2.
500
-l
2‘ oi
b
P
s 2
.400-
I t
s P ?
300-
- 60
E 8
2 8
zoo-
lOC-
300
350
A
Fig. 1. Effects of (a) reaction temperature and (b) oxygen-to-butene molar ratio on the rates of n-butene conversion to products and on the selectivity to butadiene for ZnFe,O,. +, Total nbutene conversion; W, rate of n-butene conversion to butadiene; A, rate of n-butene conversion to carbon dioxide, 0 selectivity to butadiene.
240
Sb204. The effects of reaction temperature and oxygen-to-butene ratio on the catalytic properties of Sb,O, are presented on Fig. 2a and b, respectively. It can be seen that Sb,O, has a very poor activity (1% of ZnFe,04), even at high temperatures. The conversion of butene can be observed at temperatures above 370 oC. Formation of all the products increases with increasing reaction temperature (Fig. 2a) and appears to reach a maximum at an oxygen-to-butene ratio near 1 (Fig. 2b). Fig. 3a and b show the influence of reaction temperature and oxygento-butene molar ratio, respectively, on the catalytic properties of BiPO,. Compared with Sbz04, BiP04 is almost ten times more active (but ten times less active than ZnFezO,). The butadiene and carbon dioxide formation rates increase rapidly with increasing temperature whereas that of 2-butene decreases with increasing temperature. However, the increase in carbon dioxide formation is greater than that of butadiene. It follows that the butadiene selectivity (not shown) decreases with increasing temperature. Fig. 3b shows that the 2butene and carbon dioxide formation rates increase almost linearly with in-
BiPO,.
a-
_oxygen
molarratio
butene
Fig. 2. Effects of (a) reaction temperature and (b) oxygen-to- butene molar ratio on the rates of n-butene conversion to products for Sb,O,. Symbols as in Fig. 1; V, rate of n-butene conversion to 2-butene.
Temperature lW
butene
molar ratio
Fig. 3. Effects of ( a ) reaction temperature and (b) oxygen-to-butene molar ratio on the rates of n-butene conversion to products for BiPO,. Symbols as in Fig. 2.
creasing oxygen-to-butene ratio whereas that of butadiene is almost independent of the latter. It follows that the butadiene selectivity decreases quickly with increasing oxygen-to-butene ratio. SnO,. The influences of reaction temperature and oxygen-to-butene molar ratio on the catalytic properties of SnO, are reported in Fig. 4a and b, respectively. Compared with Sb204and BiPO,, SnO, produces much more butadiene and less isomerization product. The formation of all reaction products increases with increasing temperatures. The butadiene selectivity increases at low temperature and has a sharp increase at temperatures near 350"C, after which it remains constant. The reaction product formation rates depend to some extent on the oxygen-to-butene ratio and the butadiene selectivity decreases slowly with increase in the latter. Catalytic activity of mechanical mixtures We examined mixtures of ZnFe204with Sb20,, BiPO, and Sn02. For purposes of comparison, the butene transformation rates on mixtures (solid lines
2
-z
100
$5
a
a
25
E
a
1
9
i? Q L
P eo
60.
Q
$
5
z'== 08
60
3 3 .
..C
i! 20
-
0
-
-LO
LO
X)
20
a
300
1.0
Temperature l°CI
2.0
E%?C!mobr bukne
30 O mtio
Fig. 4. Effects of ( a ) reaction temperature and (b) oxygen-to-butene molar ratio on the rates or n-butene conversion to products and on the selectivity to butadiene for SnOz. Symbols as in Fig.
in the figures) were contrasted with those calculated supposing a simple addition of those of two pure oxides (dotted lines ), i.e., ri =RmriZnFe204(1R,)ri,ox. Hence the difference between the sound and dotted lines represents the magnitude of the cooperative effect between two oxides.
+
ZnFe20,-Sb,O, system. Fig. 5a presents the catalytic activity results as a function of the mass ratio, R,, a t 400" C. The total butene conversion is almost linearly dependent on R,. However, there is a small degree of synergy between two oxides on the butadiene formation rate. The maximum synergistic effect is observed at a mass ratio near 0.5. A strong synergy exists on butadiene selectivity and the maximum value remains almost the same in the range R,= 0.25-0.75. In order to investigate the possible effects of reaction temperature and oxygen concentration on the catalytic cooperative effect, a mechanical mixture of composition R, = 0.5 was taken as an example and the results are presented in Fig. 5b and c, respectively. Fig. 5b shows that Sb204has almost no effect at low temperatures (below 360°C) and has a negative effect a t temperatures between 360 and 380" C. The positive effect of Sb204on the catalytic properties of ZnFe,04 can be observed only when the temperature is higher than 380" C and the higher the temperature, the greater is the synergistic effect. For the
oxypen buleM, molar ratio
.,
Fig. 5. Effects of ( a ) composition (R,), (b) reaction temperature and (c) oxygen-to-butene molar rat0 on the rates of n-butene conversion to products and on the selectivity to butadiene for ZnTotal n-butene conversion; 0, rate of n-butene conversion to FezOl-SbzO, mixtures. +,0, butadiene; A , A rate of n-butene conversion to carbon dioxide, selectivity to butadiene.
whole range of reaction temperatures investigated, the butadiene selectivity is always greater than that of pure ZnFe,04, (compare Figs. l a and 5b). Fig. 5c shows that the cooperative effect between two oxides in terms of butadiene productivity can be observed only when the oxygen-to-butene ratio is greater than 1.5 and it increases with increase in oxygen concentration. However, the selectivity with respect to butadiene increases slightly with increasing oxygen concentration.
ZnFe,O,-BiPO, system. The influence of catalyst composition, reaction temperature and oxygen concentration are shown in Fig. 6a, b and c, respectively. Similarly to the ZnFe,04-Sb204 system, Fig. 6a shows that there is synergy between two oxides on butadiene formation rate and selectivity. The maximum synergistic effects are observed at R,=0.75 and 0.25 for the butadiene formation rate and its selectivity, respectively. However, for the present system, a maximum is also observed in the total butene conversion, which is not observed in the former system. The influence of reaction temperature on the cooperative effect between two oxides (Fig. 6b) is also similar to that for the ZnFe,04-Sb204 system. If we consider butadiene productivity, three regions of influence can be distinguished: at low temperature ( < 360" C ) BiPO, has no effect whereas between 360 and 380 C a negative effect is observed and finally, above 380" C, a positive effect is clearly observed which increases with increasing temperature. Compared with the ZnFe204-Sb204system, the synergistic effect is greater in the present instance. The butadiene selectivity is always superior to that of pure ZnFe204for the whole range of temperatures and it seems to decrease with increasing temperature. The effect of oxygen concentration on the catalytic behaviour of this system is also similar to that on the ZnFe204-Sb,O, system (Fig. 6c). However, the catalytic synergy on the rate of formation of butadiene can be observed a t lower oxygen-to-butene ratios ( > 1) and the influence of oxygen concentration is more pronounced for this system. ZnFe,O,-SnO, system. For this system, only the influence of reaction temperature was investigated. The results (Fig. 7 ) show that SnO, has a negative effect not only on butadiene formation but also on total butene conversion up to 420°C. Above 420°C, however, a remarkable synergistic effect is observed, which increases with increasing temperature. Physico-chemical characterization The physico-chemical characteristics of three systems (XRD phases and BET surfaces) are summarized in Table 1.I t can be observed that XRD phases of mechanical mixtures correspond to those observed in pure oxides. No new
Temperature
/TI
Fig. 6. Effects of ( a ) composition (R,), (b) reaction temperature and (c) oxygen-to-butene molar ratio on the rates of n-butene conversion to products and on the selectivity to butadiene for ZnFe20,BiPO, mixtures. Symbols as in Fig. 5.
Temperature
I°CI
Fig. 7. Effects of reaction temperature on the rates of n-butene conversion to products for ZnFe,O,-Sn02 mixtures. Symbols as in Fig. 5.
TABLE 1 XRD and BET surface area results Sample
ZnFe,O, ZnFe,04 Sb204 SbzO4 ZnFe,04 BiP040 BiPO, ZnFe,04 SnO, SnO,
+
+ +
Rm
1.0 0.5 0.0 0.5 0.0 0.5 0.0
XRD phase
BET surface area (m2/g)
Fresh
Used
Spinel Spinel Sb204 SbzO4 Spinel +BiP04 BiPO, Spinel SnO, SnO,
Spinel 9.4 Spinel 5.8 Sb2O4 Sb204 2.0 7.8 Spinel BiP04 4.3 BiPO, Spinel+ SnO, 13.0
+
+
+
+
" BiPO, was composed of two crystallographic forms.
Fresh
Used
phase was observed either during mixture preparation or during the catalytic test. The XRD spectra remain almost unchanged during the reaction, both for pure oxides and mechanical mixtures. The surface area of the fresh mechanical mixture is the simple sum of those of the pure oxides, taking into account the accuracy of the surface measurements. No change in the surface area was observed for pure oxides (ZnFe,04 and Sb204)and for mechanical mixtures between ZnFe,04 and SnO, or BiPO, after reaction. However, a small increase in surface area was obtained for the mechanical mixture between ZnFe,04 and Sb204after reaction. The ZnFe204-Sb20, system was characterized using electron microscopy. For pure ZnFe,O,, no change was observed with respect to the morphology and the size of the particles for the fresh and used samples. However, the formation of coke was observed for the used but not for the fresh sample. For the mechanical mixture with R,= 0.5, Figs. 8 and 9 present the CTEM micrographs and AEM spectra for fresh and used samples, respectively. In the CTEM micrographs, the larger particles correspond to Sb204and the smaller particles to ZnFe,04. Most ZnFe204particles are around Sb204particles. No difference was observed in either the morphology or the particles size for the fresh and used samples. In order to detect the existence of mutual contamination between two phases, AEM (using an electron probe microanalyser) was carried out at different positions in the samples, as shown in the CTEM micrographs. The corresponding energy peaks of these systems are presented in the EDS spectrum. It can be seen that only the Sb signal was detected when the analysis was made on
Fig. 8. ( a ) CTEM micrograph and (b) EDS spectrum for fresh ZnFe,O,-Sb,O, mechanical mixture with R, = 0.5.
Energy (KeV)
Fig. 9. (a) CTEM micrograph and (b) EDS spectrum for used ZnFe,O,-Sb,O, mechanical mixture with R, = 0.5.
Sb204particles, and only Zn and Fe signals were observed when ZnFez04particles were analysed. The Sb, Zn and Fe signals appeared at the same time only when the analysis probe was focused on the "contact" zone of the two oxide phases (e.g., position 6 in Fig. 8a). All these results indicate that no mutual contamination between ZnFe204and Sbz04takes place, either during mixture preparation or catalytic reaction. In order to investigate whether or not the spinel phase ZnFe,O, is segregated during reaction at high temperatures, two specially designed experiments were carried out. Pure ZnFez04 and a ZnFez04-Sb,04 mechanical mixture with R, = 0.5 were tested under two extreme reaction conditions for the same period of time (6 h): one at high temperature and high oxygen concentration (450"C and oxygen/butene = 3 ) and the other at high temperature and low oxygen concentration (450"C and oxygenlbutene = 1) .The catalysts thus tested were characterized by means of a more sensitive X-ray diffractometer (Siemens D500). The activity results are reported in Table 2. Pure ZnFe204was deactivated during the catalytic reaction, especially at high temperatures and low oxygen concentration. However, almost no deactivation was observed for the mechanical mixture under the same conditions. The presence of Sbz04protects ZnFe204from deactivation. The X-ray data are reported in Fig. 10, which clearly shows that ZnFe,04 was segregated into a spinel phase and ZnO after reaction and, at the same temperature for the experiments carried out, the lower the oxygen concentration, the greater was the segregation. The spectra of mechanical mixtures (Fig. 11), however, show only the presence of ZnFez04 and Sbz04.No segregation was observed. Hence the presence of Sbz04stabilizes the ZnFe,O, structure.
TABLE 2 Catalytic activity results for ZnFe204-Sb204systems under extreme conditions Reaction conditions
Sample
450" C, oxygenlbutene = 3 ZnFe204
Analysis Rate of butene converted to (mmol/g.h) time (h) C4H6 co2 Total 1
6 ZnFe204 1 +Sb2O4 6
29.8 27.7 53.9 59.4
62.5 63.5 48.8 35.7
94.9 92.1 106.9 104.2
0 . 0 ~ ' " " " " ~ 1.0 110 210 310 41.0 51.0 67.0
no 81.0 so lor.0 - theta (degrees) Fig.10. X-ray diffraction spectra for ZnFe204.(a) Fresh ZnFe,04; (b) ZnFe204treated at 450°C, oxygen/butene=3 for 6 h; (c) ZnFe20, treated at 450°C, oxygen/butene= 1for 6 h. two
DISCUSSION
The results show that the catalytic properties of ZnFe204can be affected by the addition of an external oxide phase (Sb20,, BiPO, or SnO,) . The effect of the added phase depends strongly on its nature and also on the reaction conditions. It is well known that several phenomena may take place when two oxide phases are mixed together, e.g., new phase formation and surface contamination of one phase by the other, in physical contact. In this way, the explanation of the results may be very different, depending on which phenomenon has occurred between two phases. In our case, mechanical mixtures of three systems, namely ZnFe204-Sb204,ZnFe204-BiPO, and ZnFe204-SnO,, were characterized by XRD and BET surface area measurements before and
r~~~
- THETA
( DEGREES I
Fig. 11. X-ray diffraction spectra for ZnFe20,-Sb20,. (a) Fresh ZnFe204;(b) ZnFe,O,-Sb,O, treated at 450°C,oxygen/butene=3 for 6 h; ( c ) ZnFe20,-Sb204treated at 450°C,oxygen/butene = 1 for 6h.
after reaction. No new phase formation was observed with XRD. The surface areas of the mechanical mixtures are the simple sum of those of the two pure oxide components, indicating that the surface of the oxide is not modified by the preparation procedure. A change would be expected if some solid reaction between the oxides take place. The electron microscopic measurements showed that no mutual contamination was detected for ZnFe204-Sb204system. However, because of the lack of other sensitive surface characterization techniques (e.g., X-ray photoelectron or ion scattering spectroscopy), much care should be taken with the explanation of the results. However, a detailed analysis of our catalytic activity results indicates some common aspects observed for three added phases and suggests possible explanations. In the following, we shall focus our attention on three parameters: reaction temperature, oxygen concentration and nature of added phase. Effect of reaction temperature
Let us consider the influence of temperature on the properties of pure ZnFe204.At low temperatures, the activity of ZnFe204increases with increasing temperature, but an unexpected phenomenon is observed at high temperature ( > 3 8 0 ° C ) , namely that the activity is almost independent of temperature. This may be related to the stability of the spinel phase. The results in Fig. 10 clearly demonstrate that the spinel phase can be easily segregated into a spinel
phase and ZnO a t high temperature (450°C), although no new phase was detected for the sample tested under standard conditions (400°C and oxygen/ butene = 2). Therefore, we may conclude that ZnFe204is a good catalyst but a t high temperatures its structure is unstable. For the mechanical mixtures, the shapes of the curves of activity vs. temperature are similar for the three systems (see Figs. 5b, 6b and 7). At low temperatures, the effect of the added phase is negative or zero and a positive effect is observed a t high temperatures. However, several differences can be noted if we compare the curves carefully: ( i ) the magnitude of the negative effect of the added phase follows the order SnO, > BiP04> Sb204;(ii) the effect of SnO, appears positive a t around 420" C, which is much higher than that for Sb204and BiPO, (ca. 380" C ) ; (iii) a t high temperatures, the positive effect of the added phase follows the order BiP04> Sb2O4> > SnO,; and (iv) the butadiene selectivity of ZnFe204 is improved greatly by Sb204and BiPO, phases, even at low temperatures, which is not the case for SnO,. Based on these observations, we can divide these three added phases into two groups: ( i ) Sb204and BiPO,, which serve especially to improve the butadiene selectivity and appear to have a positive effect at relatively lower temperatures (ca. 380°C), and (ii) SnO,, which appears to have a positive effect a t higher temperatures (ca. 420" C ). If the structural stability of ZnFe,O, is taken into account, the positive effect of these phases a t high temperatures may be easily considered to be due to the stabilization of ZnFe,04. This is clearly demonstrated by Fig. 11,which shows that the presence of Sb204can protect the segregation of ZnFe204.But how does the added phase affect the properties of ZnFe204?Before answering this question, let us consider the influence of oxygen concentration.
Effect of oxygen concentration The influence of oxygen concentration on the catalytic properties of mechanical mixtures is similar for the two systems ZnFe204-Sb204(Fig. 5c) and ZnFe204-BiPO, (Fig. 6c). An increase in oxygen concentration favours the synergistic effect of the added phase. As mentioned in the Introduction, Sb204 and BiPO, may be considered to be the "donors" of ~ x y g e nspecies, as proposed previously [8]; hence it is reasonable to accept that an increase in gaseous oxygen concentration would favour the production of oxygen species (usually
o2-).
Interpretation of results based on "migration of oxygen species" For pure ZnFe,04, an increase in reaction temperature would favour the activity of lattice oxygen, which is why the activity of ZnFe,O, increases with an increase in temperature. After a certain temperature has been reached, how-
ever, the lattice oxygen is so active that its disappearance (to oxidation products) is much more rapid than the reoxidation rate of the catalyst by gaseous oxygen, resulting in a reduction of the catalyst (even if no ZnO is detected for the catalyst after reaction under standard conditions, defects might form a t the surface) and consequently a decrease of catalytic activity and selectivity. This is the case when the temperature rises above 380" C. If an external phase is present (Sb204or BiPO,), however, the oxygen produced by this phase can reoxidize ZnFe204in a more efficient way, which is why a positive effect of this phase at high temperatures was observed. The question is, why does the positive effect of SnO, appear a t higher temperatures (420" C ) ? It may be due to the difference in the nature of oxygen species produced by the external phase. It has been shown by ESR [ l o ] that oxygen species adsorbed on SnO, are mainly electrophilic (0,-, 0-) below 400" C, after which these species are progressively transformed to nucleophilic species ( 0 2 -) with increasing temperature. For the reoxidation of the catalyst, nucleophilic species are more suitable than the electrophilic ones. It may be expected that sufficient nucleophilic species are available only when temperatures are above 420°C, which is why the positive effect of S n 0 2 begins a t 420°C. It may therefore be assumed that the nature of the oxygen species produced by Sb204and BiPO, is nucleophilic, even at low temperatures. Now let us consider the effect of these external phases on the selectivity of butadiene. It is usually accepted that nucleophilic oxygen (0,- ) favours selective oxidation whereas electrophilic species favour total oxidation. The improvement in butadiene selectivity on addition of Sb,O, and BiPO, may confirm, in another way, the above assumption, namely that 02-is produced by these oxides.
Outlook Our results demonstrate clearly that the ferrite ZnFe,O, is an active and selective catalyst for the oxidative dehydrogenation of butene. However, it may be deactivated at high temperatures owing to the segregation of the phases. This segregation is presumably triggered by some reduction. The presence of an external phase, such as Sb204and BiPO,, capable of producing oxygen species (0'-) seems to stabilize the structure of ZnFe,O, and therefore to protect it from deactivation. This provides a new route to increasing the stability of industrial catalysts. These results could suggest that part of the role of a-Fe,03 in industrial catalysts might be similar to that of Sb204.However, this has yet to be proved and further investigations are necessary. ACKNOWLEDGEMENTS
The financial support of FNRS (Belgium) under an agreement with the Chinese Academy of Sciences for the stay of F.Y. Qiu in Belgium is greatly
acknowledged. The authors also thank Dr. J. Naud for his help with some of the XRD measurements. REFERENCES 1 H.H. Kung and M.C. Kung, Adv. Catal., 33 (1985 ) 159. 2 H.Q. Wang, L.P. Feng, Y.W. Chen, D.C. Miao, Q.Z. Cong and A.B. Jiang, Sci. Sin., 22, No. 12 (1979) 1398. 3 M.A. Gibson and J.W. Hightower, J. Catal., 41 (1976) 431. 4 P. Ruiz, B. Zhou, M. Remy, T. Machej, F. Aoun, B. Doumain and B. Delmon, Catal. Today, 1 (1987) 181.
5 L.T. Weng, B. Zhou, B. Yasse, B. Doumain, P. Ruiz and B. Delmon, in M.J. Phillips and M. Ternan (Editors), 9th International Congress on Catalysis, Calgary, Canada, 1988, Vol. 4, Chemical Institute of Canada, Ottawa, 1988, p. 1609. 6 L.T. Weng, N. Spitaels, B. Yasse, J . LadriBre, P. Ruiz and B. Delmon, to be published. 7 L.T. Weng, D. Duprez, P. Ruiz and B. Delmon, J. Mol. Catal., in press. 8 S. Ceckiewicz and B. Delmon, Bull. Soc. Chim. Belg., 93 (1984) 163. 9 Ph. Courty, H. Ajot, Ch. Marcilly and B. Delmon, Powder Technol., 7 (1973) 21. 10 S.C. Cheng, J. Vacuum Sci. Technol., 17 (1980) 366.