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The Dual CuII-PdII-Y Zeolite Analogue for the Wacker Catalyst in the Oxidation of Olefins
S. Kaliaguine and A. Mahay (Editors), Catalysis on the Ellergy Scene © 1984 Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands
139
THE DUAL CuII_PdII_y ZEOLITE ANALOGUE FOR THE WACKER CATALYST IN THE OXIDATION OF OLEFINS Boubaker ELLEUCH, Claude NACCACHE and Younes BEN TAARIT* and the Technical Assistance of Gerard WICKER Institut de Recherches sur la Catalyse, C.N.R.S., 2, Avenue Albert Einstein, 69626 -Villeurbanne Cedex - France.
ABSTRACT Previous spectroscopic studies of Pd I I_ and Cull exchanged Y zeolites were extended to investigate the behavior of each of these zeolites and coexchanged zeolites towards ethylene and oxygen and their mixture. In the light of the well known mechanism of ethylene oxidation to acetaldehyde, EPR observations and catalytic measurments were rationalized to interpret and account for the influence of such parameters as the Cu!Pd ratio, the zeolite Bronsted character and the presence of water on the activity, selectivity and stability of the CuII_PdII_y catalyst. The specific role of the zeolite was emphasized.
INTRODUCTION In recent years, zeolites have been often used as solid solvents to heterogenize soluble catalysts or analogue complexes (refs. 1, 2). In particular transition metal complexes have been anchored by Coulombic forces (ref. 3) or covalent bonds (ref. 4) to the zeolite lattice or simply occluded in the zeolite cavities (ref. 5). For example Rhodium- and Iridium- exchanged X and Y type zeolites have been used to carbonyl ate methanol into methyl acetate at atmospheric pressure and mild temperature after suitable activation of the exchange zeolites (refs. 6, 7). The active precursor was thought to be a monovalent metal dicarbonyl which was able to undergo a Redox cycle. Similarly copper - and Nickel- exchanged zeolites were reported to be active in isomerization of butene (ref. 7). Cupric zeolites have been shown to be active in the oxidation of olefins (ref. 8) and dehydrogenation of alcohol (ref. 9). Recently molybdenum and ruthenium loaded zeolites were investigated and their ability in epoxidation of olefins in the presence of tertiobutyl hydroperoxide demonstrated to proceed in a similar fashion to soluble molybdenum oxo species (ref. 10). A number of years ago, Tominaga and Arai (ref. 11) showed that palladium -and copper -exchanged Y zeolites produced acetaldehyde by ethylene oxidation
140
in a process similar to that described by Smidt (ref. 12). However no further investigation appeared as to the influence of various zeolitic and extra zeolitic parameters on the activity and stability of this zeolitic dual cationic catalyst. It is our purpose in this study to probe the influence of parameters such as the Copper/Palladium ratio and the acidity of the zeolite on the Redox properties of the exchanged cupric and palladic ions and investigate the relation between these and the activity and stability of the corresponding catalyst in ethylene conversion to acetaldehyde. We do not intend to establish the mechanism of this oxidation reaction. Experimental and materials Palladium II-V zeolite was obtained by ion exchange of NaV zeolite (Linde Division Union Carbide) using an aqueous solution of (Pd(NH3)4)2+ chloride. CuI I_V was obtained using an aqueous solution of copper sulfate. Various PdII_CuII_V samples were obtained by exchanging cupric ions first to the desired exchange level and finally palladium II ions. Samples with 2.3 PdI I ions per unit cell and Cu/Pd approximate atomic ratios of 0, 1, 4 and 5 were prepared. A sample of NaV was prelimin~ry exchanged by NH4Cl to a residual sodium content of 5 Na+ per unit cell and subsequently exchanged by palladium and copper respectively to achieve a Cu/Pd ratio of 4 and a palladium content of 2.3 Pd per unit cell. Actual copper and sodium contents were determined by flame photometry and Palladium content was determined by a colorimetric method. In all cases the investigated samples were heated in flowing oxygen. Temperature was increased at a rate of 0.5 K mn-1 up to 723 K. They were subsequently evacuated at the same temperature for 2 h and transferred to EPR quartz tubes equipped with break seals to allow adsorption of the desired reagent. Samples used in catalytic measurements were activated following the same procedure in a microreactor. Following activation in oxygen they were flushed with helium and they were cooled down to the desired reaction temperature and subsequently fulshed with the reaction mixture at a flow rate within 2 1 h- 1 for samples weighting 150-250 mg. Conversion levels were kept below 5 %. On line GL Chromatography provided for the products analysis. EPR spectra were recorded at 77 K on a Varian E9 spectrometer equipped with a dual cavity. DPPH was used as a g standard. RESULTS Activity measurements In agreement with Arai and coworkers findings (ref. 10) Cull V zeolite was essentially inactive in the conversion of various mixtures all of them include ethylene and oxygen C2H4 : 02. C2H4 : 02 : H20, C2H4 : 02 : H20: He at temperatures within the 363 - 573 K range. Only trace amounts of ethanol were observed at low temperature and low flow rate probably due to hydration of
141
ethylene catalyzed by Bronsted sites generated during the zeolite treatment. Likewise PdII also appeared as essentially inactive at mild temperatures and under the same reaction conditions. At higher temperatures 473 - SOD K a sudden changed occured and ethylene was converted to degradation products mainly. By contrast samples of coexchanged CuILpdILy zeolites exhibited a significant activity in selective ethylene conversion to acetaldehyde. This latter product could be detected at as mild temperatures as 363 K however for all samples whatever the Cu/Pd ratio the activity faded away in a matter of 4S-90 mn at the most in agreement with previous reports (ref. 11). The stability of the catalyst was improved by a significant rise in temperature up to 433 K, however COZ was then produced in significant yield depending on the Cu/Pd ratio the lower the ratio the higher the COZ yield (ZO - 10 %). This compound is presumbaly due to deep oxidation catalyzed by metallic Pd. The selectivity and stability improved not only be increasing the Cu/Pd ratio but also upon substitution of sodium ions by Brbnst ed sites. Such samples with a unit cell composition: NaSPdZ.3CulQHZ6 (SiOz1I56(A10Z)S6 could operate for over a week at 433 without alteration of its activity and selectivity and produced O.Z mole g- lh- 1 of acetaldehyde with a selectiVity of 90 %. EPR studies The mechanism of ethylene oxidation to acetaldehyde is a well established one in solution. However in zeolitic media such parameters as the presence of water, the Cu/Pd ratio play an important role in the catalytic reaction. EPR measurements, although performed at low temperature and at static conditions may help uncover the role and influence of these parameters in the Redox Cycle and electron transfer between cupric ions and reduced palladium. It is obvious that the EPR studies conditions are far remote from actual catalytic conditions but it would seem easier to track some of these processes at room temperature than at reaction temperature where rates of Redox reactions are far too rapid to be fully analyzed. The EPR signal of dehydrated Y zeolite containing a low cupric content has been already analyzed in a number of studies and was consistent with cupric ions located at two different sites (ref. 13). This signal was unaffected by adsorption of ethylene at room temperature. Higher cupric content samples were characterized by broad unresolved signals barely affected in their overall iontensity by addition of ethylene. PdII-Y zeolite activated as indicated in the experimental section exhibited a weak EPR signal at 9iso = Z.Z ascribed to Pd3+ (ref. 14). Addition of ethylene rapidly washed up this signal and generated a new signal with g" Z.33 and gJ. = Z.10 due to monovalent palladium d9 (ref. 14). This a clear indication that trivalent and bivalent palldium ions were reduced at least to
142
the monovalent state by ethylene as Pd(o) are unfortunatly not observable by EPR under the conditions we use. Removal of excess ethylene did not alter the magnetic parameters nor the intensity of the Pd+ signal. Prolonged contact with ethylene caused the Pd+ signal to reach a maximum intensity and to decay to zero eventually. Addition of 02 at room temperature slowly retored the Pd+ signal to an intensity level within the tenth of the total pall adium content indicating a rather uneasy reoxidation of the zerovalent palladium. In the presence of Cupric ions the behavior of palladium ions was drastically affected together with that of the cupric ions themselves. For a 1 : 1 ratio ethylene addition produced a dramatic decrease of the Cu 2+ signal intensity which then resulted in a well resolved signal because of the suppression of dipolar interactions. This clearly indicates a reduction of Cupric ions presumably to the cuprous state in the presence of palladium ions. As the Cu 2+ signal reached its minimum intensity about 10- 2 of the initial intensity, the Pd+ signal developped and reached a maximum value and finally decreased. Addition of 02 after removal of excess ethylene caused the Pd+ signal to increase again and level off at a value within the fifth of the total Pd content approximatly. The Cu 2+ signal also increased to about the tenth of its initial intensity. Addition of water vapor simultaneously to that of 02 decreased the Pd+ signal intensity and almost retored the Cu 2+ signal to almost its initial intensity thus indicating a more efficient oxidation of the cupreous ions and both zerovalent and monovalent palladium in the presence of the 02 : H20 mixture. For higher Cu/Pd ratios, the overall behavior is qualitatively similar in presence of etylene, oxygen or a mixture of the two. However the reman ant cupric ions signal in presence of ethylene was stronger and stronger as the Cu/Pd was increased. Similarly the correspondig Pd+ signal also varied in the same fashion. In the presence of hydrated 02 introduced following removal of ethylene the rate of monovalent palladium reoxidation, as monitored by the decrease of the Pd+ signal intensity, also increased. One should also stress the faster regeneration of the Cupric ESR signal in presence of Bronsted acidity as opposed to that of Na+ ions. Finally, no significant alteration of the Cu 2+ signal was observed in presence of acetaldehyde apart from a slight g shift. DISCUSSION In agreement with previous findings, the dual component zeolite catalyst CuII_PdII_y exhibited a reasonable activity and a good selectivity over a significant range of temperature 363-453 K. The stabi 1ity of the catalyst improved as the reaction temperature was raised probalby due to the better evacuation of the reaction products from the zeolite pores, thus allowing
143
better access of the reagents to the active sites. However the prominent parameter governing the catalyst stability is probably the copper to palladium ratio (Cu/Pd). Indeed the requirements for an efficient Smidt type catalyst would be, as set up by Rabo and coworkers (ref. 15), a close contact between the active palladium center and the cocatalyst associated with the most absolute intertness of the latter towards the reagent and the products. The third requirement to be fulfilled is an easy oxidation of the active center by the cocatalyst. These conditions were set to ensure both a high activity and stability of the catalyst by rapid reoxidation of the zerovalent palladium. The selectivity itself was even more drastically dependent on this final condition. In line with this conception, it soon appeared that exchanged palladium zeolite failed to couvert ethylene for any reasonable length of time under mild temperatures compatible with selective oxidation to acetaldehyde. Used PdY catalysts were shown to be deeply reduced (beyond the monovalent state by ESR) and electron micrographs showe~ that such used samples exhibited large palladium crystallites (around 50 A) at the external surface of the zeolite particles. As the Cu/Pd increased both the activity and stability of the catalyst improved. This was ascribed to a more rapid reoxidation of the reduced palladium as the copper content increased. In fact, EPR measurements showed that reduction of bivalent to monovalent palladium took place readily as ethylene was admitted, even in presence of Cupric ions. These were however ultimatly reduced to the cuprous state. Whereas removal of excess ethylene and admission of oxygen resulted for the low Cu/Pd ratios in a low partial reoxidation of zerovalent palladium to essentially palladous ions, high ratios provided for a faster reoxidation of palladium partly to Pd I I species. Moreover on such high Cu/Pd ratio used samples, no metallic palladium was detected. Thus although this element was undergoing formal reduction to the zerovalent state, clustering to form metal particles was prevented presumably through too short life times of palladium atoms rapidly reoxidized by Cupric ions. Therefore it is concluded that a more efficient reoxidation of palladium atoms is taking place. This might be ascribed to two different factors. A temperature increase would accelerate electron transfer between Cupric ions and zero valent and/or monovalent palladium. This seams the least probable since the ~E is certainly small. Alternatively, at increased temperatures, the mobility of Cupric ions is certainly increased and optimum mutual location of palladium atoms and cupric ions may be achieved easily to facilitate the electron transfer between these two species. Also the mobility of negative charges throughout the lattice is increased contributing in turn to easy charge transfer between remorte palladium atoms and Cupric ions. It was also pinted out that Bronsted acidity improved the stability of the catalyst by comparison to predomina~tly Na+-balanced zeolite. This may be
144
attributed to a significant change in the Redox potential of Cupric ions, as we have observed using EPR technique that oxidation of naphtalene to its cationic radical vi a electron transfer to cupric ions was easiest in the presence of Bronsted acidity compared to that of Na+ ions (ref. 16). Conversely reoxidation of Cuprous ions by molecular oxygen was also favored in presence of acidic hydroxyl groups. This might be considered as proton assisted Redox process. The role of protons may be acounted for in view of the following reaction scheme (I)
Cupric ions would in turn oxidize palladium atoms or palladous ions via charge transfer (2) Pd(o) + 2(OLtLCu I I In the light of the mechanism of ethylene conversion into acetaldehyde, the beneficial influence of water may be accounted for as follows: Water molecule is ionized by bivalent Palladium ions according to (3) (OL"hPdII + H20 0L - Pd II - OH + 0LH (3) The hydroxide bound to the Palladium may then effect a nucleophilic attack on coordinated ethylene as depicted below:
o _ "Pel L
-u. C
- Ot4 t'?
(4)
=- C Hz.
The intermedi ate a complex would then release the reduced Palladium, the acetaldehyde and a lattice hydroxyl group as indicated in (5). 0L-
8
i a-.<:
~,
c. _>\ C tl 1/ ...-' II
.~
4
o-H L.
... Cll,:=
c.l-lo 1-\
(c.
":3 LitO)
(5)
As to the influence of the Cu!Pd ratio, one could anticipate from equation (2) that higher ratios provide for rapid reoxidation of zerovalent palladium thus preventing the formation of metallic palladium particles difficult to oxidize, once formed. The optimum ratio would certainly vary upon the experimental conditions including temperature, and more importantly the ethylene partial pressure in the feed. Further experimental work is needed to probe this hypothesis. Also from the EPR observations and the reaction steps involved in the oxidation process, the zeolite lattice appeared to playa specific role in various ways: (i) as a simple counterion its provides the advantage of getting rid of chloride ions and corrosive HCl but also arises all diffusion problems. (ii) a more positive role is played by the zeolite in preventing clustering of
145
left over palladium atoms, as it was demonstrated that atomic species could well be prevented from clustering due to isolating matrix property of zeolite (ref. 17). (i i i ) Above all the specific zeolite role appears in the reaction schemes (1), (2), (3) and possibly (4) and (5), which indicate the active part of lattice hydroxyl groups in accelerating the reoxidation of the cocatalyst and allowing a reasonably rapid electron transfer from zerovalent palladium to lattice bound cupric ions. Further the dissociation of water (3) is dependent on the electrostatic fields prevailing in the zeolite cavities. Thus presumably the activity and stability of the Cu-Pd-zeolite catalvst would strongly depend on the nature of the zeolite and the major charge balancing cation within one type of zeolite. It would be certainly more difficult to weight the influence of the zeolite in effecting reactions (4) and (5), although at least a lattice oxide ligand is involved in interconverting complexes which make up the actual oxidation steps, on the activity and selectivity. Further studies involving X, Y and mordenite type zeolites are underway. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
M. Che and Y. Ben Taarit in K.L. Mittal and E.J. Fendler (Eds.), Solution Behavior of Sulfactants : Theoretical and Applied Aspects, Plenum Press, New York, 1982, pp. 189-214. C. Naccache and Y. Ben Taar t t in J.P. Bonnelle, B. Delmon and E. Derouane (Eds.), Surface Properties and Catalysis by non-metals, Reidel, Dordrecht, 1983, pp. 405-431. J.H. Lunsford, A.C.S. Symposium Series, 40, (1977), 473. M. Primet, J.C. V~drine and C. Naccache, J. Mol. Cat., 4 (1978), 411. G. Coudurier, P. Gallezot, H. Praliaud, M. Primet and B. Imelik, C.R. Acad. Sci., 282C (1976), 31l. P. Gelin, F. Lefebvre, B. Elleuch, C. Naccache and Y. Ben Taar i t , A.C.S. Symposium Series, 218 (1983) 455. J. Bandiera, M. Dufaux and C. Naccache, J. Chim. Phys., 70, (1973), 1096. 1. Mochida, S. Hayata, A. Kato and T. Seiyama, J. Catal., 23, (1971), 3l. S. Tsuruya, Y. Okamoto and T. Kuwada, J. Catal., 56 (1979), 52. P. Shing, E. Dai and J.H. Lunsford, J. Catal., 64 (1980) 184. H. Arai, T. Yamashiro, T. Kubo and H. Tominaga, Bul. Japan Petroleum Institute, 18 (1976) 39. J. Smidt, Angew. Chem., 71 (1959) 176. C. Naccache and Y. Ben Taarit, Chem. Phys. Letters, 11 (1971), 11. C. Naccache, J.F. Dutel and M. Che, J. Catal., 29 (1973) 179. A.B. Evnin, J.A. Rabo and P.H. Kasai, J. Catal., 30, (1973) 109. Y. Ben Taarit and C. Naccache, unpublished data M. Primet and Y. Ben Taarit, J. Phys. Chem., 81 (1977) 1317.
139
THE DUAL CuII_PdII_y ZEOLITE ANALOGUE FOR THE WACKER CATALYST IN THE OXIDATION OF OLEFINS Boubaker ELLEUCH, Claude NACCACHE and Younes BEN TAARIT* and the Technical Assistance of Gerard WICKER Institut de Recherches sur la Catalyse, C.N.R.S., 2, Avenue Albert Einstein, 69626 -Villeurbanne Cedex - France.
ABSTRACT Previous spectroscopic studies of Pd I I_ and Cull exchanged Y zeolites were extended to investigate the behavior of each of these zeolites and coexchanged zeolites towards ethylene and oxygen and their mixture. In the light of the well known mechanism of ethylene oxidation to acetaldehyde, EPR observations and catalytic measurments were rationalized to interpret and account for the influence of such parameters as the Cu!Pd ratio, the zeolite Bronsted character and the presence of water on the activity, selectivity and stability of the CuII_PdII_y catalyst. The specific role of the zeolite was emphasized.
INTRODUCTION In recent years, zeolites have been often used as solid solvents to heterogenize soluble catalysts or analogue complexes (refs. 1, 2). In particular transition metal complexes have been anchored by Coulombic forces (ref. 3) or covalent bonds (ref. 4) to the zeolite lattice or simply occluded in the zeolite cavities (ref. 5). For example Rhodium- and Iridium- exchanged X and Y type zeolites have been used to carbonyl ate methanol into methyl acetate at atmospheric pressure and mild temperature after suitable activation of the exchange zeolites (refs. 6, 7). The active precursor was thought to be a monovalent metal dicarbonyl which was able to undergo a Redox cycle. Similarly copper - and Nickel- exchanged zeolites were reported to be active in isomerization of butene (ref. 7). Cupric zeolites have been shown to be active in the oxidation of olefins (ref. 8) and dehydrogenation of alcohol (ref. 9). Recently molybdenum and ruthenium loaded zeolites were investigated and their ability in epoxidation of olefins in the presence of tertiobutyl hydroperoxide demonstrated to proceed in a similar fashion to soluble molybdenum oxo species (ref. 10). A number of years ago, Tominaga and Arai (ref. 11) showed that palladium -and copper -exchanged Y zeolites produced acetaldehyde by ethylene oxidation
140
in a process similar to that described by Smidt (ref. 12). However no further investigation appeared as to the influence of various zeolitic and extra zeolitic parameters on the activity and stability of this zeolitic dual cationic catalyst. It is our purpose in this study to probe the influence of parameters such as the Copper/Palladium ratio and the acidity of the zeolite on the Redox properties of the exchanged cupric and palladic ions and investigate the relation between these and the activity and stability of the corresponding catalyst in ethylene conversion to acetaldehyde. We do not intend to establish the mechanism of this oxidation reaction. Experimental and materials Palladium II-V zeolite was obtained by ion exchange of NaV zeolite (Linde Division Union Carbide) using an aqueous solution of (Pd(NH3)4)2+ chloride. CuI I_V was obtained using an aqueous solution of copper sulfate. Various PdII_CuII_V samples were obtained by exchanging cupric ions first to the desired exchange level and finally palladium II ions. Samples with 2.3 PdI I ions per unit cell and Cu/Pd approximate atomic ratios of 0, 1, 4 and 5 were prepared. A sample of NaV was prelimin~ry exchanged by NH4Cl to a residual sodium content of 5 Na+ per unit cell and subsequently exchanged by palladium and copper respectively to achieve a Cu/Pd ratio of 4 and a palladium content of 2.3 Pd per unit cell. Actual copper and sodium contents were determined by flame photometry and Palladium content was determined by a colorimetric method. In all cases the investigated samples were heated in flowing oxygen. Temperature was increased at a rate of 0.5 K mn-1 up to 723 K. They were subsequently evacuated at the same temperature for 2 h and transferred to EPR quartz tubes equipped with break seals to allow adsorption of the desired reagent. Samples used in catalytic measurements were activated following the same procedure in a microreactor. Following activation in oxygen they were flushed with helium and they were cooled down to the desired reaction temperature and subsequently fulshed with the reaction mixture at a flow rate within 2 1 h- 1 for samples weighting 150-250 mg. Conversion levels were kept below 5 %. On line GL Chromatography provided for the products analysis. EPR spectra were recorded at 77 K on a Varian E9 spectrometer equipped with a dual cavity. DPPH was used as a g standard. RESULTS Activity measurements In agreement with Arai and coworkers findings (ref. 10) Cull V zeolite was essentially inactive in the conversion of various mixtures all of them include ethylene and oxygen C2H4 : 02. C2H4 : 02 : H20, C2H4 : 02 : H20: He at temperatures within the 363 - 573 K range. Only trace amounts of ethanol were observed at low temperature and low flow rate probably due to hydration of
141
ethylene catalyzed by Bronsted sites generated during the zeolite treatment. Likewise PdII also appeared as essentially inactive at mild temperatures and under the same reaction conditions. At higher temperatures 473 - SOD K a sudden changed occured and ethylene was converted to degradation products mainly. By contrast samples of coexchanged CuILpdILy zeolites exhibited a significant activity in selective ethylene conversion to acetaldehyde. This latter product could be detected at as mild temperatures as 363 K however for all samples whatever the Cu/Pd ratio the activity faded away in a matter of 4S-90 mn at the most in agreement with previous reports (ref. 11). The stability of the catalyst was improved by a significant rise in temperature up to 433 K, however COZ was then produced in significant yield depending on the Cu/Pd ratio the lower the ratio the higher the COZ yield (ZO - 10 %). This compound is presumbaly due to deep oxidation catalyzed by metallic Pd. The selectivity and stability improved not only be increasing the Cu/Pd ratio but also upon substitution of sodium ions by Brbnst ed sites. Such samples with a unit cell composition: NaSPdZ.3CulQHZ6 (SiOz1I56(A10Z)S6 could operate for over a week at 433 without alteration of its activity and selectivity and produced O.Z mole g- lh- 1 of acetaldehyde with a selectiVity of 90 %. EPR studies The mechanism of ethylene oxidation to acetaldehyde is a well established one in solution. However in zeolitic media such parameters as the presence of water, the Cu/Pd ratio play an important role in the catalytic reaction. EPR measurements, although performed at low temperature and at static conditions may help uncover the role and influence of these parameters in the Redox Cycle and electron transfer between cupric ions and reduced palladium. It is obvious that the EPR studies conditions are far remote from actual catalytic conditions but it would seem easier to track some of these processes at room temperature than at reaction temperature where rates of Redox reactions are far too rapid to be fully analyzed. The EPR signal of dehydrated Y zeolite containing a low cupric content has been already analyzed in a number of studies and was consistent with cupric ions located at two different sites (ref. 13). This signal was unaffected by adsorption of ethylene at room temperature. Higher cupric content samples were characterized by broad unresolved signals barely affected in their overall iontensity by addition of ethylene. PdII-Y zeolite activated as indicated in the experimental section exhibited a weak EPR signal at 9iso = Z.Z ascribed to Pd3+ (ref. 14). Addition of ethylene rapidly washed up this signal and generated a new signal with g" Z.33 and gJ. = Z.10 due to monovalent palladium d9 (ref. 14). This a clear indication that trivalent and bivalent palldium ions were reduced at least to
142
the monovalent state by ethylene as Pd(o) are unfortunatly not observable by EPR under the conditions we use. Removal of excess ethylene did not alter the magnetic parameters nor the intensity of the Pd+ signal. Prolonged contact with ethylene caused the Pd+ signal to reach a maximum intensity and to decay to zero eventually. Addition of 02 at room temperature slowly retored the Pd+ signal to an intensity level within the tenth of the total pall adium content indicating a rather uneasy reoxidation of the zerovalent palladium. In the presence of Cupric ions the behavior of palladium ions was drastically affected together with that of the cupric ions themselves. For a 1 : 1 ratio ethylene addition produced a dramatic decrease of the Cu 2+ signal intensity which then resulted in a well resolved signal because of the suppression of dipolar interactions. This clearly indicates a reduction of Cupric ions presumably to the cuprous state in the presence of palladium ions. As the Cu 2+ signal reached its minimum intensity about 10- 2 of the initial intensity, the Pd+ signal developped and reached a maximum value and finally decreased. Addition of 02 after removal of excess ethylene caused the Pd+ signal to increase again and level off at a value within the fifth of the total Pd content approximatly. The Cu 2+ signal also increased to about the tenth of its initial intensity. Addition of water vapor simultaneously to that of 02 decreased the Pd+ signal intensity and almost retored the Cu 2+ signal to almost its initial intensity thus indicating a more efficient oxidation of the cupreous ions and both zerovalent and monovalent palladium in the presence of the 02 : H20 mixture. For higher Cu/Pd ratios, the overall behavior is qualitatively similar in presence of etylene, oxygen or a mixture of the two. However the reman ant cupric ions signal in presence of ethylene was stronger and stronger as the Cu/Pd was increased. Similarly the correspondig Pd+ signal also varied in the same fashion. In the presence of hydrated 02 introduced following removal of ethylene the rate of monovalent palladium reoxidation, as monitored by the decrease of the Pd+ signal intensity, also increased. One should also stress the faster regeneration of the Cupric ESR signal in presence of Bronsted acidity as opposed to that of Na+ ions. Finally, no significant alteration of the Cu 2+ signal was observed in presence of acetaldehyde apart from a slight g shift. DISCUSSION In agreement with previous findings, the dual component zeolite catalyst CuII_PdII_y exhibited a reasonable activity and a good selectivity over a significant range of temperature 363-453 K. The stabi 1ity of the catalyst improved as the reaction temperature was raised probalby due to the better evacuation of the reaction products from the zeolite pores, thus allowing
143
better access of the reagents to the active sites. However the prominent parameter governing the catalyst stability is probably the copper to palladium ratio (Cu/Pd). Indeed the requirements for an efficient Smidt type catalyst would be, as set up by Rabo and coworkers (ref. 15), a close contact between the active palladium center and the cocatalyst associated with the most absolute intertness of the latter towards the reagent and the products. The third requirement to be fulfilled is an easy oxidation of the active center by the cocatalyst. These conditions were set to ensure both a high activity and stability of the catalyst by rapid reoxidation of the zerovalent palladium. The selectivity itself was even more drastically dependent on this final condition. In line with this conception, it soon appeared that exchanged palladium zeolite failed to couvert ethylene for any reasonable length of time under mild temperatures compatible with selective oxidation to acetaldehyde. Used PdY catalysts were shown to be deeply reduced (beyond the monovalent state by ESR) and electron micrographs showe~ that such used samples exhibited large palladium crystallites (around 50 A) at the external surface of the zeolite particles. As the Cu/Pd increased both the activity and stability of the catalyst improved. This was ascribed to a more rapid reoxidation of the reduced palladium as the copper content increased. In fact, EPR measurements showed that reduction of bivalent to monovalent palladium took place readily as ethylene was admitted, even in presence of Cupric ions. These were however ultimatly reduced to the cuprous state. Whereas removal of excess ethylene and admission of oxygen resulted for the low Cu/Pd ratios in a low partial reoxidation of zerovalent palladium to essentially palladous ions, high ratios provided for a faster reoxidation of palladium partly to Pd I I species. Moreover on such high Cu/Pd ratio used samples, no metallic palladium was detected. Thus although this element was undergoing formal reduction to the zerovalent state, clustering to form metal particles was prevented presumably through too short life times of palladium atoms rapidly reoxidized by Cupric ions. Therefore it is concluded that a more efficient reoxidation of palladium atoms is taking place. This might be ascribed to two different factors. A temperature increase would accelerate electron transfer between Cupric ions and zero valent and/or monovalent palladium. This seams the least probable since the ~E is certainly small. Alternatively, at increased temperatures, the mobility of Cupric ions is certainly increased and optimum mutual location of palladium atoms and cupric ions may be achieved easily to facilitate the electron transfer between these two species. Also the mobility of negative charges throughout the lattice is increased contributing in turn to easy charge transfer between remorte palladium atoms and Cupric ions. It was also pinted out that Bronsted acidity improved the stability of the catalyst by comparison to predomina~tly Na+-balanced zeolite. This may be
144
attributed to a significant change in the Redox potential of Cupric ions, as we have observed using EPR technique that oxidation of naphtalene to its cationic radical vi a electron transfer to cupric ions was easiest in the presence of Bronsted acidity compared to that of Na+ ions (ref. 16). Conversely reoxidation of Cuprous ions by molecular oxygen was also favored in presence of acidic hydroxyl groups. This might be considered as proton assisted Redox process. The role of protons may be acounted for in view of the following reaction scheme (I)
Cupric ions would in turn oxidize palladium atoms or palladous ions via charge transfer (2) Pd(o) + 2(OLtLCu I I In the light of the mechanism of ethylene conversion into acetaldehyde, the beneficial influence of water may be accounted for as follows: Water molecule is ionized by bivalent Palladium ions according to (3) (OL"hPdII + H20 0L - Pd II - OH + 0LH (3) The hydroxide bound to the Palladium may then effect a nucleophilic attack on coordinated ethylene as depicted below:
o _ "Pel L
-u. C
- Ot4 t'?
(4)
=- C Hz.
The intermedi ate a complex would then release the reduced Palladium, the acetaldehyde and a lattice hydroxyl group as indicated in (5). 0L-
8
i a-.<:
~,
c. _>\ C tl 1/ ...-' II
.~
4
o-H L.
... Cll,:=
c.l-lo 1-\
(c.
":3 LitO)
(5)
As to the influence of the Cu!Pd ratio, one could anticipate from equation (2) that higher ratios provide for rapid reoxidation of zerovalent palladium thus preventing the formation of metallic palladium particles difficult to oxidize, once formed. The optimum ratio would certainly vary upon the experimental conditions including temperature, and more importantly the ethylene partial pressure in the feed. Further experimental work is needed to probe this hypothesis. Also from the EPR observations and the reaction steps involved in the oxidation process, the zeolite lattice appeared to playa specific role in various ways: (i) as a simple counterion its provides the advantage of getting rid of chloride ions and corrosive HCl but also arises all diffusion problems. (ii) a more positive role is played by the zeolite in preventing clustering of
145
left over palladium atoms, as it was demonstrated that atomic species could well be prevented from clustering due to isolating matrix property of zeolite (ref. 17). (i i i ) Above all the specific zeolite role appears in the reaction schemes (1), (2), (3) and possibly (4) and (5), which indicate the active part of lattice hydroxyl groups in accelerating the reoxidation of the cocatalyst and allowing a reasonably rapid electron transfer from zerovalent palladium to lattice bound cupric ions. Further the dissociation of water (3) is dependent on the electrostatic fields prevailing in the zeolite cavities. Thus presumably the activity and stability of the Cu-Pd-zeolite catalvst would strongly depend on the nature of the zeolite and the major charge balancing cation within one type of zeolite. It would be certainly more difficult to weight the influence of the zeolite in effecting reactions (4) and (5), although at least a lattice oxide ligand is involved in interconverting complexes which make up the actual oxidation steps, on the activity and selectivity. Further studies involving X, Y and mordenite type zeolites are underway. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
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