- Email: [email protected]
Deactivation Effects in the Synthesis of Methyl Ethyl Ketone by Selective Oxidation over Solid Wacker-type Catalysts
B. Delmon and G.F. Frornent (Eds.) Catalyst Deactivation 1994 Studies in Surface Science and Catalysis, Vol. 88 0 1994 Elsevier Science B.V. All rights reserved.
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Deactivation Effects in the Synthesis of Methyl Ethyl Ketone by Selective Oxidation over Solid Wacker-type Catalysts Gabriele Centi, Siglinda Perathoner and Giuseppina Stella Dept. of Industrial Chemistry and Materials, V.le Risorgimento 4,40136 Bologna, Italy,
fa:+39-51-6&-3680,E-mail: [email protected]
Solid Wacker-type catalysts (Pd-V20s or Pd-CeO2 supported on alumina and Pd-doped KH~PV1Mo11040) show a marked decline in activity in the gas phase selective oxidation of 1-butene to methyl ethyl ketone (ME& 2-butanone), whereas the selectivity passes through a maximum.ARer about 5-6hours on-stream nearly steady-state behavior is reached characterized by a slower deactivation rate and a slight decline in selectivity. The initial change in the catalytic behavior is discussed in terms of the formation of adsorbed species and changes in the valence state of V and Pd during the catalytic reaction. Both these deactivation effects can be regenerated by a n oxidizing treatment at 300°C.The second slow deactivation is instead related to the crystallization of V-oxide particles and is not reversible. It is also shown that Pd-doped V-heteropolyacid has a much higher initial activity and selectivity, but the catalyst rapidly deactivates.
INTRODUCTION The gas-phase oxidation of 1-butene to the methyl ethyl ketone (MEK, 2-butanone) by gaseous 0, and H20 using a solid Wacker-type heterogeneous catalyst such as Pd-V205 supported on alumina [l-71is an interesting alternative possibility to the synthesis of MEK in a liquid phase that has not been commercially applied due to corrosion problems and formation of chlorinated by-products. While the activity is rather stable in ethylene to acetaldehyde oxidation 11-43,these solid Wacker-type catalysts show a marked initial change in the activity in 1-butene oxidation to MEK The deactivation has been attributed to the loss of chlorine ions from the (PdC14),- active complex, suggesting that catalysts prepared from a PdS04 salt are more stable [51.However, the change in catalytic behavior with time-on-stream [6,71appears not to be connected only with this effect. Recent patents [8,9] have also proposed a new liquid-phase Wacker-type process using PdSO4 and H9[PM~6V6040] to prevent the formation of chlorinated products. The objective of the present study was to analyze the reasons for the deactivation effects observed in solid Wacker-type catalysts for 1-butene oxidation. For this purpose the catalytic behavior and characteristics of Pd-V20, on alumina catalysts, prepared using either a N%PdC14 or a PdSO, salt, were compared with those of alternative catalysts prepared by substituting the V-oxide with CeOz in order to obtain a better understanding of the role of V-oxide. In addition, the behavior of a Pd-doped V-heteropolyacid also is discussed to further extend the analysis.
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EXPERIMENTAL Pd-V20, on alumina was prepared by an incipient wet impregnation method using an aqueous solution of V02+-oxalate(obtained by reduction of NH4V03 with H2C202)and microspherical y-Al2O3 (Rhone-PoulencSpheralite 535) pellets. After drying and calcination a t 400°C for 5h, the supported vanadium samples were further impregnated with an aqueous solution containing PdC1, and NaCl (molar ratio 1:8) and then dried a t 120°C. Alternatively, impregnation was carried out using PdSO, dissolved in a few drops of concentrated H2S04 The final molar composition of the samples was 0.98% PdC1, (or equivalent moles of PdSO,), 7.63% V205,7,848 NaCl (% in moles), with the remainder being the support. Pd-CeO, on alumina was prepared in the same way, but using an aqueous solution of CeCI3. Pd-doped KH3PV1Mol1040 was prepared by adding the N+PdC14 salt to an aqueous solution containing the heteropoly acid and then evaporating the solvent in a rotavapor (temperature of 85°C). The P W molar ratio was 0.2 and Pd/Na = 0.5. The starting heteropoly acid was synthesized as reported elsewhere [lo]. Catalytic tests have been carried out in a continuous-flow fixed-bed glass microreactor at atmospheric pressure equipped with on-line gas chromatographs. Other details on the apparatus were reported previously [6,7]. Unless otherwise indicated, the standard reaction conditions were 0.8% 1-butene, 20% 0, and 20% H,O in helium. The total flow a t STP was 3.6 LA with 2.5 g of catalyst. The valence state of vanadium in the catalyst was determined by chemical analysis, as described previously [71. Fourier-transform (FT-IR)infrared spectra were recorded with a Perkin Elmer 1750 instrument in a quartz cell connected to grease-free evacuation and gas manipulation lines. The self-supporting disk technique was used. X-ray diffraction (XRD) patterns were recorded with a Perkin Elmer 1050/81diffiactometer using the powder technique and CUK, radiation. RESULTS AND DISCUSSION Catalytic Behavior. Reported in Fig. 1is the behavior in 1-butene oxidation to MEK at 120°C of Pd on alumina (A) and Pd-V206 on alumina catalysts (B and C). The latter two samples differ as regards the method of deposition of the Pd component; in catalyst B, Pd was deposited as a NazPdC14 salt and in C, as PdSO4. Apart from a different initial change in the selectivity to MEK (Fig. lb), the two preparation methods lead to relatively similar results, indicating that the nature of the Pd complex does not significantly modify the catalytic behavior. This shows that the deactivation pattern does not depend on the loss of chlorine ions as previously indicated (5).Both catalysts show sign5cantly better performances in comparison with the sample without the V-oxide component (A), indicating that the latter plays an effective role in the reoxidation of Pd. For all samples a marked decline of the 1-butene conversion is noted in the first 5 hours in time-on-stream (Fig. la), whereas for longer times-on-stream the activity declines at a much slower rate. The selectivity to MEK also changes considerably in the first 5 hours (Fig. lb). Initially, the selectivity of all samples is very low and significantly increases in the first 1-2 hours up to a maximum and then later declines further to a nearly constant value (activity and selectivity only decrease slightly) after around 5 hours. For the sample prepared from PdSO, (C),only an increase in the selectivity is ob-
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Fig. 1 Conversion of the hydrocarbon and selectivity to MEK in the oxidation of 1-butene at 120°C on: A Pd-A1203,B Pd-V20datUmina (Na2PdC14) and C Pd-V20dalumina (PdS04). B1 and B2 refer to the behavior of sample B after treatment at 300°C in a flow of NZ (BI) or 02 $2).
served. Three different stages can thus be evidenced for Pd-V20, on alumina samples: a fist initial stage (fist hour of time-on-stream) of rapid decrease in 1-butene conversion with a parallel increase in the selectivity to MEK, an intermediate stage (from 1 to 5 hours) where both the activity and selectivity decrease and a h a l stage for the longer time-on-stream characterized by a slow decrease in both the activity and selectivity. Deactivated samples after around 5-6hours of time-on-stream can be regenerated by treatment in an 0,flow at 3OOOC (sample B2). The regeneration treatment can be repeated several times. However, when samples are regenerated after a longer time-onstream (sample C of Fig. 1, for example), the initial catalytic behavior can be only partially regenerated. A similar treatment, but in an inert flow (N,) (Bl) leads to a regeneration of the selectivity and initial activity (Fig. 1, Bl), but the 1-butene conversion decreases at a higher rate (Fig. la). Apparently this treatment leads to a higher selectivity to MEK, but this is due also to the lower conversion of 1-butene. Reported in Fig. 2a is the catalytic behavior of a sample analogous to B, but prepared substituting the V-oxide component with CeO,. Apart from the lower activity, but higher selectivity to MEK, the time-on-stream behavior of this sample is analogous to that of PdV205 based samples, showing that the effect of the change of activityhelectivity with time-on-stream is a general feature of these catalysts not specifically related to the presence of the V-oxide component. The data in Fig. 2a show also that the catalytic behavior of these solid Wacker-type catalysts is not related to a specific kind of Pd-V surface complex, but rather that the V acts only as the reoxidizing agent for reduced Pd similarly to the CuCl2 component in classical liquid-phase Wacker catalysts. The activity of this sample can also be regenerated by treatment in 0,a t 300°C. Reported in Fig. 2b is an example of the catalytic behavior of a Pd-doped V-phosphomolybdic acid catalyst. Differently hom the above samples, this catalyst shows a high in-
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Fig. 2 Oxidation of 1-butene to MEK at 120°C on Pd-CeOz/alumina (A) and on Pd-doped Vphosphomolybdic acid (B).
itial yield of MEK (around 70%) and selectivity (around 90%). The catalyst, however, rapidly deactivates and the yield of MEK becomes lower than 5% in around 6 hours . Adsorption of ReagentsIProducts of Reaction. Catalytic data show that for all supported samples there is an initial increase in the yield and selectivity to MEK. This increase, however, does not correspond to a parallel decrease in the formation of by-products indicating that it is due to adsorption of reagentdproducts of reaction. In the iirst hour a si@icant lack of carbon balance is in fact observed. In the Pd-doped V-phosphomolybdic acid (Fig. 2b),this effect is not present, probably due to its different surface acido-base characteristics and the lower surface area (around 10 m2/g versus 110 m2/g for alumina supported samples). Catalytic tests with a MEK feed in the presence of O@,O indicate a pronounced admoledg in 3 hours remain adsorbed as such or as its prosorption of MEK. About 5.6~10-~ ducts of conversion with respect to around 2.6.10'4 moles adsorbed per g of catalyst in 3 hours as estimated &om the lack of carbon balance using 1-butene. These results show that MEK considerably absorbs on the catalyst surface during the catalytic tests. The concentration of water in the feed also plays a role on the activity and rate of deactivation of these catalysts (6). The yield and selectivity to MEK passes through a maximum for an H20 concentrationin the feed of around 20% in nearly steady-state conditions, but in the absence of water in the feed an initial higher selectivity(over 80%)and yield of MEK (over 45%) is observed due to the adsorbed water present on the catalyst. Since this amount is limited, the catalyst rapidly loses its activity due to the absence of the water reagent, but these tests show that the presence of water in the gas phase influences the initial surface reactivity. The amount of water adsorbed on the catalyst during the catalytic reaction was estimated from thermobalance experiments of adsorption of water at 120°C (flow of He containing about 20% H20)on the Pd-V,O, on alumina catalyst, which show a weight increase of around 3.3%.Taking into account the mean area occupied by an H20 molecule and the surface area of the catalyst, it can be estimated that
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this value corresponds to around 80-90% of monolayer capacity. The number of adsorbed species remaining on the catalyst after the catalytic tests was analyzed by infrared spectroscopy (Fig. 3). After vacuum treatment at room temperature (r.t.) a series of bands are observed in the 1000-1800 cm-' region which indicate the presence of adsorbed MJZK and acetic acid or acetaldehyde (vc,o at 1730 and 1700 cm-l, respectively) and acetate anions (vC,, and at 1575 and 1465 cm- , respectively) together with adsorbed water at 1630 cm-l). Other bands are due to bending 1800 1400 cm-1 1000 modes of methyl and methylic groups. Evacuation at higher Fig. 3 IR spectra of Pd-VzOdalumina after 6 hours of catalytic temperature(3000~) leads to tests in 1-butene oxidation at 120°C. (a) after evacuation at the disappearence of most of r.t, (b) after evacuation at 300°C and (c) after evacuation at 470°C. In the inset are compared the spectra after evacuation the bands of adsorbed species those Of the aceat 470°C of Pd-VzOdalumina after 6 hours (c) and 30 hours of apart catalytic tests (d) and consecutive reoxidation at 300°C. tate anion (probably bridging) and tentatively of an alkoxy species (v0-C at 1070 cm-'1 that can be removed only by vacuum treatment at higher temperature (470°C)(Fig. 3).This shows the strong interaction of these species with the catalyst surface. It should be noted that the gas chromatographic analysis of the products which desorb from the catalyst during the regenerative heat treatment at 300°C indicates the presence of MEK, acetic acid and acetaldehyde as the main organic desorption products, in agreement with IR results. The treatment at high temperature therefore leads to the desorption of water and adsorbed organic species, but acetate can be removed only by an oxidizingtreatment at 300°C. The comparison of the results of MEK and water adsorption with those of IR characterization (Fig. 3) and of the effect of higher temperature treatment with 0,and N2(Fig. 1) suggests that initially (clean surface) the supported catalysts are very active, but probably the MEK formed is rapidly consecutively converted to acetate species which remain on the surface together with the MEK formed. The progressive formation of a water layer on the surface as well as the effect of the catalytic reaction itself leads to an in-situ change in the surface reactivity with a decrease in the rate of 1-buteneconversion, but an J
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Fig. 4 Change of the conversion of hydrocarbon and of the valence state of V-oxide component during catalytic tests of oxidation of 1-butene at 120°C over Pd-VzO&lumina.
increase in the formation of MEK (increased yield). Together with this effect, the V- or Ce-oxide also reduce, possibly due to an inhibition of adspecies on the reoxidation of the catalyst by 0,.In Pd-doped V-heteropoly acid, due to its pseudo-liquid behavior (11) and higher retention capacity for water, the effect of the initial in-situ change in the surface reactivity is minimized and the catalyst shows a high rate of MEK synthesis (Fig. 2b). However, the catalyst rapidly deactivates. Reduction of the Catalyst. During the catalytic reaction, there is a progressive reduction of the catalyst. Summarized in Fig. 4 is the change in the conversion of hydrocarbon during the catalytic tests in 1-butene oxidation at 120°C over Pd-V205/alumina and the valence state of vanadium determined by chemical analysis. The nearly steady-state condition is characterized by a V5' t o v4' ratio in the V-oxide of about 1:l. This ratio corresponds to the formation of a V4Os-like phase. The data therefore show that V5'- oxide, which acts as the reoxidizing agent of the Pd reduced during oxidation of 1-butene, is reoxidized by gaseous oxygen at a lower rate than that of its reduction. A rapid reduction to a V4Os-like phase is thus observed. Since higher levels of reduction (for example, the formation of a V6013-like phase) were not observed, this phase corresponds to the equivalence between rates of reduction and reoxidation. The presence of this reduced phase cannot be confirmed by XRD analysis because the Pd-V205 on alumina is amorphous both before and after the catalytic tests. However, the XRD pattern of Pd-CeO, on alumina shows clearly the bands due to crystalline CeO, before the catalytic tests, even though the &&action lines are rather large due to microcrystallinity. After the catalytic tests, new narrow lines appear in the difiactograms due to the formation of large crystals of Ce601, (Fig. 5). It is worth noting that the reduced phase has larger crystal dimensions than the starting Ce-oxide crystals suggesting that together with the reduction, sintering of the oxide also occm.
399
The change in the oxidation state or coordination of Pd during the catalytic tests was monitored 500 by IR characterization of CO chemisorption a t r.t. CO, in fact, is a 400 sensitive probe to analyze possible d changes in the Pd active compom t 300 nent (12,13). Reported in Fig. 6 -E are the spectra observed for &esh 200 Pd-V20, on alumina prepared by Na2PdC1, deposition (a), after 3 100 hours in 1-butene oxidation a t 120°C (b) and after consecutive 0 15 25 35 45 55 65 75 treatment at 300°C in 0, flow (c). Z'theta The spectra show several analogies to those observed by Choi Fig. 5 X-ray diffractionpattern of Pd-CeOz on alumina and Vannice (13) for co samples after the catalytictests. tion on Pd-Al,O, and will be assigned accordingly. The &esh sample shows two main bands at 2150 & ; 1930 cm-' which can be assigned to a carbonyl coordinated on a Pd2+-chlorine complex such as Pd2(C0),C1, and bridged carbonyls in (Pd'COCl), species, respectively. Both these bands disappear during the catalytic reaction and after three hours only a band centred a t about 2120 cm-l can be observed. This band also can be assigned to a Pd2"chloro-carbonyl complex (131, but in a different coordination compound such as [Pd2+(CO)C1&. By treatment in oxygen at 300°C (c), the initial bands reform, even though a weak component at 2120 cm-I is still present. CO linearly coordinated on metallic Pd particles is expected to give rise to bands at about 2100 and 1980 cm-l, which are observed in our spectra even on the fresh sample and do not change signit ficantly after the catalytic tests or the reoxidation (Fig. 6). It is thus reasonable to conclude that these bands are only apparent and due to subtraction of the base sample, suggesting that they are not indicative of the real formation of Pdo species. 0 Small amounts of reduced palladium canFig. 6 IR spectra of CO adsorption (60ton)at not, however, be excluded. Results of CO r.t. On fresh Pd-VzOdalumina (a),after 3 hours chemkorption however suggest that durof catalytic reaction (b) and after a consecutive ing the catalytic reaction there is treatment with 0 2 at 3OOOC (c).
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a change in coordination of the active palladium complex which may be responsible for the initial change in surface reactivity. On the contrary, there is no evidence of the progressive reduction of Pd sites during the catalytic reaction parallel to the considerable reduction of V-oxide observed in Fig. 4. Changes after Long-Term Catalytic Tests. XRD data of Pd-V2O&unina samples prepared using both chlorine and sulphate complexes are amorphous even after long term catalytic tests. However, IR spectra after high temperature evacuation and consecutive reoxidation show the presence of a shoulder centred at around 1030 cm-lon the tail of the stronger skeletal vibrations of alumina. In the inset of Fig. 6 are compared the spectra in this reagion of the sample after 6 and 30 hours of catalytic tests. It should be noted that the VV,O in crystalline V205 shifis from 1025 to about 980 cm-' when the V-oxide is supported on an oxide such a8 T i 0 2 (14). The appearence of the shoulder in the sample after 30 hours of catalytic tests (inset of Fig. 3) therefore suggests that the initially amorphous V-oxide progressively aggregates forming particles of bulk V-oxide, even though they cannot be detected by XRD since they are still microcrystalline. Taking into account the results on Pd-CeOz/alumina samples (Fig. 5) it is reasonable to suggest that this aggregation process is caused by the reduction of V-oxide during the catalytic tests and the presence of a layer of water on the surface during the catalytic tests which favour the mobility of vanadium notwithstanding the low reaction temperature (120°C).It is also reasonable to suggest that the slow deactivation process observed for the longer time-on-stream is probably related to this slow process of agglomeration of V-oxide. ACKNOWLEDGEMENT This work was supported by KoninklijkdShell-Lab.,Amsterdam (The Netherlands). REFERENCES 1. AB. Evnin, J.A Rabo, P.H. Kasai,J. Catal., SO, 109 (1973). 2. L.Forni, G. Tenoni, Znd. Eng. Chem.Proc. Des. Dev., 16,288(1972). 3. L.Forni, G.G. Gilardi, J. Catul., 41,338(1976). 4. E. van der Heide, M. Zwinkels, A Gemtsen, J.J.F. Scholten,Appl. Catul. A: General, 86,181(1992). 5. E. van der Heide, JAM. Ammerlaan, A.W. Gerritsen, J.J.F. Scholten, J. Molec. Catul., 66, 320 (1989). 6. G. Centi, M. Malagutti, G. Stella , in New Developments i n Selective Oxidation, S. Vic and V. Corks Corberan Eds., Elsevier Pub.:Amsterdam 1994,in press. 7. G. Centi, G. Stella, in Catalysis i n Organic Reactions, Markel Dekker Pub., Proceedings 15th Conference on Catalysis, Phoenix May 1994,in press. 8. J. Vasilevskis, J.C. De Deken, R.J. Saxton, P.R. Wentrcek, J.D. Fellmann, L.S. Kipmis, PCT Znt. Appl. WO 870,615(1987). 9. Y.Saito, M. Tsusubi Europ. Pat.Appl. EP 418,764(1991). 10. G.Centi, V. Lena, F. Tritirb, D. Ghoussoub, C.F. Aissi, M. Guelton, J.P. Bonnelle, J. Chem. SOC.Faraday, 86,2775(1990). 11. M. Misono, Catul. Rev.-Sci. Eng., 29,269(1987). 12. A.A.Davydov, Infrared Spectroscopy ofAdsorbed Species on the Surface of Transition Metal Oxides, J. Wiley Pub Chichester 1990,p. 81. 13. K.J. Choi, M.A. Vannice, J. Catal., 127,465(1991). 14. G.Centi, D.Pinelli, F. Trifirb, D. Ghoussoub,M. Guelton ,J. Catal., 130,238(1991).
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Deactivation Effects in the Synthesis of Methyl Ethyl Ketone by Selective Oxidation over Solid Wacker-type Catalysts Gabriele Centi, Siglinda Perathoner and Giuseppina Stella Dept. of Industrial Chemistry and Materials, V.le Risorgimento 4,40136 Bologna, Italy,
fa:+39-51-6&-3680,E-mail: [email protected]
Solid Wacker-type catalysts (Pd-V20s or Pd-CeO2 supported on alumina and Pd-doped KH~PV1Mo11040) show a marked decline in activity in the gas phase selective oxidation of 1-butene to methyl ethyl ketone (ME& 2-butanone), whereas the selectivity passes through a maximum.ARer about 5-6hours on-stream nearly steady-state behavior is reached characterized by a slower deactivation rate and a slight decline in selectivity. The initial change in the catalytic behavior is discussed in terms of the formation of adsorbed species and changes in the valence state of V and Pd during the catalytic reaction. Both these deactivation effects can be regenerated by a n oxidizing treatment at 300°C.The second slow deactivation is instead related to the crystallization of V-oxide particles and is not reversible. It is also shown that Pd-doped V-heteropolyacid has a much higher initial activity and selectivity, but the catalyst rapidly deactivates.
INTRODUCTION The gas-phase oxidation of 1-butene to the methyl ethyl ketone (MEK, 2-butanone) by gaseous 0, and H20 using a solid Wacker-type heterogeneous catalyst such as Pd-V205 supported on alumina [l-71is an interesting alternative possibility to the synthesis of MEK in a liquid phase that has not been commercially applied due to corrosion problems and formation of chlorinated by-products. While the activity is rather stable in ethylene to acetaldehyde oxidation 11-43,these solid Wacker-type catalysts show a marked initial change in the activity in 1-butene oxidation to MEK The deactivation has been attributed to the loss of chlorine ions from the (PdC14),- active complex, suggesting that catalysts prepared from a PdS04 salt are more stable [51.However, the change in catalytic behavior with time-on-stream [6,71appears not to be connected only with this effect. Recent patents [8,9] have also proposed a new liquid-phase Wacker-type process using PdSO4 and H9[PM~6V6040] to prevent the formation of chlorinated products. The objective of the present study was to analyze the reasons for the deactivation effects observed in solid Wacker-type catalysts for 1-butene oxidation. For this purpose the catalytic behavior and characteristics of Pd-V20, on alumina catalysts, prepared using either a N%PdC14 or a PdSO, salt, were compared with those of alternative catalysts prepared by substituting the V-oxide with CeOz in order to obtain a better understanding of the role of V-oxide. In addition, the behavior of a Pd-doped V-heteropolyacid also is discussed to further extend the analysis.
394
EXPERIMENTAL Pd-V20, on alumina was prepared by an incipient wet impregnation method using an aqueous solution of V02+-oxalate(obtained by reduction of NH4V03 with H2C202)and microspherical y-Al2O3 (Rhone-PoulencSpheralite 535) pellets. After drying and calcination a t 400°C for 5h, the supported vanadium samples were further impregnated with an aqueous solution containing PdC1, and NaCl (molar ratio 1:8) and then dried a t 120°C. Alternatively, impregnation was carried out using PdSO, dissolved in a few drops of concentrated H2S04 The final molar composition of the samples was 0.98% PdC1, (or equivalent moles of PdSO,), 7.63% V205,7,848 NaCl (% in moles), with the remainder being the support. Pd-CeO, on alumina was prepared in the same way, but using an aqueous solution of CeCI3. Pd-doped KH3PV1Mol1040 was prepared by adding the N+PdC14 salt to an aqueous solution containing the heteropoly acid and then evaporating the solvent in a rotavapor (temperature of 85°C). The P W molar ratio was 0.2 and Pd/Na = 0.5. The starting heteropoly acid was synthesized as reported elsewhere [lo]. Catalytic tests have been carried out in a continuous-flow fixed-bed glass microreactor at atmospheric pressure equipped with on-line gas chromatographs. Other details on the apparatus were reported previously [6,7]. Unless otherwise indicated, the standard reaction conditions were 0.8% 1-butene, 20% 0, and 20% H,O in helium. The total flow a t STP was 3.6 LA with 2.5 g of catalyst. The valence state of vanadium in the catalyst was determined by chemical analysis, as described previously [71. Fourier-transform (FT-IR)infrared spectra were recorded with a Perkin Elmer 1750 instrument in a quartz cell connected to grease-free evacuation and gas manipulation lines. The self-supporting disk technique was used. X-ray diffraction (XRD) patterns were recorded with a Perkin Elmer 1050/81diffiactometer using the powder technique and CUK, radiation. RESULTS AND DISCUSSION Catalytic Behavior. Reported in Fig. 1is the behavior in 1-butene oxidation to MEK at 120°C of Pd on alumina (A) and Pd-V206 on alumina catalysts (B and C). The latter two samples differ as regards the method of deposition of the Pd component; in catalyst B, Pd was deposited as a NazPdC14 salt and in C, as PdSO4. Apart from a different initial change in the selectivity to MEK (Fig. lb), the two preparation methods lead to relatively similar results, indicating that the nature of the Pd complex does not significantly modify the catalytic behavior. This shows that the deactivation pattern does not depend on the loss of chlorine ions as previously indicated (5).Both catalysts show sign5cantly better performances in comparison with the sample without the V-oxide component (A), indicating that the latter plays an effective role in the reoxidation of Pd. For all samples a marked decline of the 1-butene conversion is noted in the first 5 hours in time-on-stream (Fig. la), whereas for longer times-on-stream the activity declines at a much slower rate. The selectivity to MEK also changes considerably in the first 5 hours (Fig. lb). Initially, the selectivity of all samples is very low and significantly increases in the first 1-2 hours up to a maximum and then later declines further to a nearly constant value (activity and selectivity only decrease slightly) after around 5 hours. For the sample prepared from PdSO, (C),only an increase in the selectivity is ob-
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Fig. 1 Conversion of the hydrocarbon and selectivity to MEK in the oxidation of 1-butene at 120°C on: A Pd-A1203,B Pd-V20datUmina (Na2PdC14) and C Pd-V20dalumina (PdS04). B1 and B2 refer to the behavior of sample B after treatment at 300°C in a flow of NZ (BI) or 02 $2).
served. Three different stages can thus be evidenced for Pd-V20, on alumina samples: a fist initial stage (fist hour of time-on-stream) of rapid decrease in 1-butene conversion with a parallel increase in the selectivity to MEK, an intermediate stage (from 1 to 5 hours) where both the activity and selectivity decrease and a h a l stage for the longer time-on-stream characterized by a slow decrease in both the activity and selectivity. Deactivated samples after around 5-6hours of time-on-stream can be regenerated by treatment in an 0,flow at 3OOOC (sample B2). The regeneration treatment can be repeated several times. However, when samples are regenerated after a longer time-onstream (sample C of Fig. 1, for example), the initial catalytic behavior can be only partially regenerated. A similar treatment, but in an inert flow (N,) (Bl) leads to a regeneration of the selectivity and initial activity (Fig. 1, Bl), but the 1-butene conversion decreases at a higher rate (Fig. la). Apparently this treatment leads to a higher selectivity to MEK, but this is due also to the lower conversion of 1-butene. Reported in Fig. 2a is the catalytic behavior of a sample analogous to B, but prepared substituting the V-oxide component with CeO,. Apart from the lower activity, but higher selectivity to MEK, the time-on-stream behavior of this sample is analogous to that of PdV205 based samples, showing that the effect of the change of activityhelectivity with time-on-stream is a general feature of these catalysts not specifically related to the presence of the V-oxide component. The data in Fig. 2a show also that the catalytic behavior of these solid Wacker-type catalysts is not related to a specific kind of Pd-V surface complex, but rather that the V acts only as the reoxidizing agent for reduced Pd similarly to the CuCl2 component in classical liquid-phase Wacker catalysts. The activity of this sample can also be regenerated by treatment in 0,a t 300°C. Reported in Fig. 2b is an example of the catalytic behavior of a Pd-doped V-phosphomolybdic acid catalyst. Differently hom the above samples, this catalyst shows a high in-
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Fig. 2 Oxidation of 1-butene to MEK at 120°C on Pd-CeOz/alumina (A) and on Pd-doped Vphosphomolybdic acid (B).
itial yield of MEK (around 70%) and selectivity (around 90%). The catalyst, however, rapidly deactivates and the yield of MEK becomes lower than 5% in around 6 hours . Adsorption of ReagentsIProducts of Reaction. Catalytic data show that for all supported samples there is an initial increase in the yield and selectivity to MEK. This increase, however, does not correspond to a parallel decrease in the formation of by-products indicating that it is due to adsorption of reagentdproducts of reaction. In the iirst hour a si@icant lack of carbon balance is in fact observed. In the Pd-doped V-phosphomolybdic acid (Fig. 2b),this effect is not present, probably due to its different surface acido-base characteristics and the lower surface area (around 10 m2/g versus 110 m2/g for alumina supported samples). Catalytic tests with a MEK feed in the presence of O@,O indicate a pronounced admoledg in 3 hours remain adsorbed as such or as its prosorption of MEK. About 5.6~10-~ ducts of conversion with respect to around 2.6.10'4 moles adsorbed per g of catalyst in 3 hours as estimated &om the lack of carbon balance using 1-butene. These results show that MEK considerably absorbs on the catalyst surface during the catalytic tests. The concentration of water in the feed also plays a role on the activity and rate of deactivation of these catalysts (6). The yield and selectivity to MEK passes through a maximum for an H20 concentrationin the feed of around 20% in nearly steady-state conditions, but in the absence of water in the feed an initial higher selectivity(over 80%)and yield of MEK (over 45%) is observed due to the adsorbed water present on the catalyst. Since this amount is limited, the catalyst rapidly loses its activity due to the absence of the water reagent, but these tests show that the presence of water in the gas phase influences the initial surface reactivity. The amount of water adsorbed on the catalyst during the catalytic reaction was estimated from thermobalance experiments of adsorption of water at 120°C (flow of He containing about 20% H20)on the Pd-V,O, on alumina catalyst, which show a weight increase of around 3.3%.Taking into account the mean area occupied by an H20 molecule and the surface area of the catalyst, it can be estimated that
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this value corresponds to around 80-90% of monolayer capacity. The number of adsorbed species remaining on the catalyst after the catalytic tests was analyzed by infrared spectroscopy (Fig. 3). After vacuum treatment at room temperature (r.t.) a series of bands are observed in the 1000-1800 cm-' region which indicate the presence of adsorbed MJZK and acetic acid or acetaldehyde (vc,o at 1730 and 1700 cm-l, respectively) and acetate anions (vC,, and at 1575 and 1465 cm- , respectively) together with adsorbed water at 1630 cm-l). Other bands are due to bending 1800 1400 cm-1 1000 modes of methyl and methylic groups. Evacuation at higher Fig. 3 IR spectra of Pd-VzOdalumina after 6 hours of catalytic temperature(3000~) leads to tests in 1-butene oxidation at 120°C. (a) after evacuation at the disappearence of most of r.t, (b) after evacuation at 300°C and (c) after evacuation at 470°C. In the inset are compared the spectra after evacuation the bands of adsorbed species those Of the aceat 470°C of Pd-VzOdalumina after 6 hours (c) and 30 hours of apart catalytic tests (d) and consecutive reoxidation at 300°C. tate anion (probably bridging) and tentatively of an alkoxy species (v0-C at 1070 cm-'1 that can be removed only by vacuum treatment at higher temperature (470°C)(Fig. 3).This shows the strong interaction of these species with the catalyst surface. It should be noted that the gas chromatographic analysis of the products which desorb from the catalyst during the regenerative heat treatment at 300°C indicates the presence of MEK, acetic acid and acetaldehyde as the main organic desorption products, in agreement with IR results. The treatment at high temperature therefore leads to the desorption of water and adsorbed organic species, but acetate can be removed only by an oxidizingtreatment at 300°C. The comparison of the results of MEK and water adsorption with those of IR characterization (Fig. 3) and of the effect of higher temperature treatment with 0,and N2(Fig. 1) suggests that initially (clean surface) the supported catalysts are very active, but probably the MEK formed is rapidly consecutively converted to acetate species which remain on the surface together with the MEK formed. The progressive formation of a water layer on the surface as well as the effect of the catalytic reaction itself leads to an in-situ change in the surface reactivity with a decrease in the rate of 1-buteneconversion, but an J
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Fig. 4 Change of the conversion of hydrocarbon and of the valence state of V-oxide component during catalytic tests of oxidation of 1-butene at 120°C over Pd-VzO&lumina.
increase in the formation of MEK (increased yield). Together with this effect, the V- or Ce-oxide also reduce, possibly due to an inhibition of adspecies on the reoxidation of the catalyst by 0,.In Pd-doped V-heteropoly acid, due to its pseudo-liquid behavior (11) and higher retention capacity for water, the effect of the initial in-situ change in the surface reactivity is minimized and the catalyst shows a high rate of MEK synthesis (Fig. 2b). However, the catalyst rapidly deactivates. Reduction of the Catalyst. During the catalytic reaction, there is a progressive reduction of the catalyst. Summarized in Fig. 4 is the change in the conversion of hydrocarbon during the catalytic tests in 1-butene oxidation at 120°C over Pd-V205/alumina and the valence state of vanadium determined by chemical analysis. The nearly steady-state condition is characterized by a V5' t o v4' ratio in the V-oxide of about 1:l. This ratio corresponds to the formation of a V4Os-like phase. The data therefore show that V5'- oxide, which acts as the reoxidizing agent of the Pd reduced during oxidation of 1-butene, is reoxidized by gaseous oxygen at a lower rate than that of its reduction. A rapid reduction to a V4Os-like phase is thus observed. Since higher levels of reduction (for example, the formation of a V6013-like phase) were not observed, this phase corresponds to the equivalence between rates of reduction and reoxidation. The presence of this reduced phase cannot be confirmed by XRD analysis because the Pd-V205 on alumina is amorphous both before and after the catalytic tests. However, the XRD pattern of Pd-CeO, on alumina shows clearly the bands due to crystalline CeO, before the catalytic tests, even though the &&action lines are rather large due to microcrystallinity. After the catalytic tests, new narrow lines appear in the difiactograms due to the formation of large crystals of Ce601, (Fig. 5). It is worth noting that the reduced phase has larger crystal dimensions than the starting Ce-oxide crystals suggesting that together with the reduction, sintering of the oxide also occm.
399
The change in the oxidation state or coordination of Pd during the catalytic tests was monitored 500 by IR characterization of CO chemisorption a t r.t. CO, in fact, is a 400 sensitive probe to analyze possible d changes in the Pd active compom t 300 nent (12,13). Reported in Fig. 6 -E are the spectra observed for &esh 200 Pd-V20, on alumina prepared by Na2PdC1, deposition (a), after 3 100 hours in 1-butene oxidation a t 120°C (b) and after consecutive 0 15 25 35 45 55 65 75 treatment at 300°C in 0, flow (c). Z'theta The spectra show several analogies to those observed by Choi Fig. 5 X-ray diffractionpattern of Pd-CeOz on alumina and Vannice (13) for co samples after the catalytictests. tion on Pd-Al,O, and will be assigned accordingly. The &esh sample shows two main bands at 2150 & ; 1930 cm-' which can be assigned to a carbonyl coordinated on a Pd2+-chlorine complex such as Pd2(C0),C1, and bridged carbonyls in (Pd'COCl), species, respectively. Both these bands disappear during the catalytic reaction and after three hours only a band centred a t about 2120 cm-l can be observed. This band also can be assigned to a Pd2"chloro-carbonyl complex (131, but in a different coordination compound such as [Pd2+(CO)C1&. By treatment in oxygen at 300°C (c), the initial bands reform, even though a weak component at 2120 cm-I is still present. CO linearly coordinated on metallic Pd particles is expected to give rise to bands at about 2100 and 1980 cm-l, which are observed in our spectra even on the fresh sample and do not change signit ficantly after the catalytic tests or the reoxidation (Fig. 6). It is thus reasonable to conclude that these bands are only apparent and due to subtraction of the base sample, suggesting that they are not indicative of the real formation of Pdo species. 0 Small amounts of reduced palladium canFig. 6 IR spectra of CO adsorption (60ton)at not, however, be excluded. Results of CO r.t. On fresh Pd-VzOdalumina (a),after 3 hours chemkorption however suggest that durof catalytic reaction (b) and after a consecutive ing the catalytic reaction there is treatment with 0 2 at 3OOOC (c).
L
400
a change in coordination of the active palladium complex which may be responsible for the initial change in surface reactivity. On the contrary, there is no evidence of the progressive reduction of Pd sites during the catalytic reaction parallel to the considerable reduction of V-oxide observed in Fig. 4. Changes after Long-Term Catalytic Tests. XRD data of Pd-V2O&unina samples prepared using both chlorine and sulphate complexes are amorphous even after long term catalytic tests. However, IR spectra after high temperature evacuation and consecutive reoxidation show the presence of a shoulder centred at around 1030 cm-lon the tail of the stronger skeletal vibrations of alumina. In the inset of Fig. 6 are compared the spectra in this reagion of the sample after 6 and 30 hours of catalytic tests. It should be noted that the VV,O in crystalline V205 shifis from 1025 to about 980 cm-' when the V-oxide is supported on an oxide such a8 T i 0 2 (14). The appearence of the shoulder in the sample after 30 hours of catalytic tests (inset of Fig. 3) therefore suggests that the initially amorphous V-oxide progressively aggregates forming particles of bulk V-oxide, even though they cannot be detected by XRD since they are still microcrystalline. Taking into account the results on Pd-CeOz/alumina samples (Fig. 5) it is reasonable to suggest that this aggregation process is caused by the reduction of V-oxide during the catalytic tests and the presence of a layer of water on the surface during the catalytic tests which favour the mobility of vanadium notwithstanding the low reaction temperature (120°C).It is also reasonable to suggest that the slow deactivation process observed for the longer time-on-stream is probably related to this slow process of agglomeration of V-oxide. ACKNOWLEDGEMENT This work was supported by KoninklijkdShell-Lab.,Amsterdam (The Netherlands). REFERENCES 1. AB. Evnin, J.A Rabo, P.H. Kasai,J. Catal., SO, 109 (1973). 2. L.Forni, G. Tenoni, Znd. Eng. Chem.Proc. Des. Dev., 16,288(1972). 3. L.Forni, G.G. Gilardi, J. Catul., 41,338(1976). 4. E. van der Heide, M. Zwinkels, A Gemtsen, J.J.F. Scholten,Appl. Catul. A: General, 86,181(1992). 5. E. van der Heide, JAM. Ammerlaan, A.W. Gerritsen, J.J.F. Scholten, J. Molec. Catul., 66, 320 (1989). 6. G. Centi, M. Malagutti, G. Stella , in New Developments i n Selective Oxidation, S. Vic and V. Corks Corberan Eds., Elsevier Pub.:Amsterdam 1994,in press. 7. G. Centi, G. Stella, in Catalysis i n Organic Reactions, Markel Dekker Pub., Proceedings 15th Conference on Catalysis, Phoenix May 1994,in press. 8. J. Vasilevskis, J.C. De Deken, R.J. Saxton, P.R. Wentrcek, J.D. Fellmann, L.S. Kipmis, PCT Znt. Appl. WO 870,615(1987). 9. Y.Saito, M. Tsusubi Europ. Pat.Appl. EP 418,764(1991). 10. G.Centi, V. Lena, F. Tritirb, D. Ghoussoub, C.F. Aissi, M. Guelton, J.P. Bonnelle, J. Chem. SOC.Faraday, 86,2775(1990). 11. M. Misono, Catul. Rev.-Sci. Eng., 29,269(1987). 12. A.A.Davydov, Infrared Spectroscopy ofAdsorbed Species on the Surface of Transition Metal Oxides, J. Wiley Pub Chichester 1990,p. 81. 13. K.J. Choi, M.A. Vannice, J. Catal., 127,465(1991). 14. G.Centi, D.Pinelli, F. Trifirb, D. Ghoussoub,M. Guelton ,J. Catal., 130,238(1991).