3262
J. Phys. Chem. 1991, 95, 3262-3271
These conditions lead to
v2k
2kc1 e(k‘-k)hVIk = ke, + k’t2
In order to obtain an analytic expression for V(r)in the medium 1, we now replace v’lk by the following approximate form, having the same asymptotic value at small and large k , and the same derivative at k = 0: VjIk zz
[q + t 2
- 2e I ‘ + €
exp(
-T + xk ) l V l k 61
‘2
(A3) can then be completely integrated in the region z
(A7)
>0
A test charge qo in the medium 1 experiences the potential created by q and, within a good approximation, by two image charges ql and q2. q1 = q(tl - t 2 ) / ( t l + t2) is symmetrical to q with respect to the interface (dielectrical image). q2 = -2qel/(el + e2) is located at a distance h’below q1(Debye-Huckel effect). h’is of the order of fDH = I / & the effect of q2 on qo is mainly sensitive at a distance r >> 1DH from q, which is a quite reasonable result and confirms the above approximation. Moreover, we notice that q + q1 + q2 = 0, meaning that the charge q is exactly compensated by ql and qt. The field generated by q and its images is dipolar at long distance.
Catalytlc Partial Oxidation of Propylene to Acrolein over Copper( I 1)-Exchanged M-X and M-Y Zeolites Where M = Mg2+, Ca2+, Li+, Na+, K+, and H+: Evldence for Separate Pathways for Partial and Complete Oxidation Jong-Sung Yu and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: June 5, 1990)
The catalytic oxidation of propylene over copper(l1)-exchangedX and Y zeolites in the presence of the different major cocations, Mg2+,CaZ+,Li+, Na’, K’, and H’, was studied as a function of the oxygen/propylene mole ratio in a flow system at 350 OC. The yield of acrolein was optimized for a ratio of unity. The catalytic activities and the changes in cupric ion species were studied by gas chromatography and electron spin resonance. The catalytic activity for this reaction is shown to be due to copper species and is greatly dependent on the type of major cocation in the zeolites. Y zeolites are slightly more effective for the selective oxidation of propylene to acrolein than the corresponding X zeolites. The dependence of the product yields and the Cu( 11) concentration on the oxygen/propylene mole ratio indicates two parallel pathways for partial oxidation to acrolein and complete oxidation to carbon dioxide and water. It is suggested that partial oxidation is catalyzed by Cu(I), perhaps in a Cu20/Cu0 phase, and that complete oxidation is catalyzed by Cu(I1). An induction period for acrolein formation is observed. Catalyst deactivation is also observed and associated with coke formation which can be monitored by a singlet electron spin resonance signal.
Introduction Extensive studies on transition-metal-exchanged zeolites in the field of zeolite catalysis have been performed during the past two decades.’” A number of transition-metal-ion-exchanged zeolites have been shown to be catalytically active for various reactions. An important reaction among these is the oxidation of hydroc a r b o n ~ . ~The ~ ~conversion of hydrocarbons into products ( I ) Maxwell, 1. E. In Aduonces in Cotolysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press: New York, 1982; Vol. 31, p 2. (2) Lunsford, J. H. Cotol. Reo. 1975, 12, 13. (3) Ben Taarit, Y.; Che, M. In Cotolysis by Zeolite; Imelik, B., Naccache, C., Ben Taarit, Y.,Vedrine, C., Couduvier, G.,Praliaud. H.. Eds.; Elsevier: Amsterdam, 1980; pp 167-193. (4) Naccache. C.; Ben Taarit, Y. In Zeolite: Science ond Technology; Ribeiro, F. R.,Rodrigues, A. E., Rollman, L. D., Naccache, C., Eds.; Martius Nijhoff: Hague, 1984; p 373. ( 5 ) Naccache, C.; Ben Taarit, Y. Pure Appl. Chem. 1980, 52, 2175. (6) Rudham, R.; Sanders, M. K. J . Carol. 1972, 27, 287.
0022-3654/9 1/2095-3262$02.50/0
containing oxygen products valuable chemical intermediates for the petrochemical industry. (7) Mochida, I.; Yitsumatsu, T.; Kato, A.; Seiyama, T. J. Cotol. 1975, 36, 361. (8) Garten, R. L.; Boudart, M. Ind. Eng. Chem. 1973, 12, 299. (9) Mochida, I.; Hayata, S.; Kato, A.; Seiyama, T. Bull. Chem. Soc. Jpn. 1971, 44, 2282.
(IO) Uh, Y. S.;Chon, H. J . Korean Chem. Soc. 1979, 23, 80. (1 I ) (a) Mochida, 1.; Hayata, S.;Kato, A.; Seiyama, T. J . Cotol. 1969, 15. 314. (b) Mochida, I.; Hayata, S.; Kato, A,; Seiyama, T. J . Corol. 1970, 19, 405. (c) Mochida, I.; Hayata, S.;Kato, A.; Seiyama, T. J . Cotol. 1971, 23. 31. (12) Gentry, S.J.; Rudham, R.; Sanders, M. K. J . Cotol. 1974, 35, 376. (13) (a) Mahoney, F.;Rudham, R.; Stockwell, A. In The Properties and
Applications of Zeolites; Townsend, R. P., Ed.; The Chemical Society: London, 1980; p 329. (b) Bravo, F. 0.;Dwyer, J.; Zamboulis, D. In The Properties ond Applicotions of Zeolites; Townsend, R. P., Ed.; The Chemical Society: London, 1980; p 368. (14) (a) Lee, H.; Kevan, L. J. Phys. Chem. 1986, 90,5776. (b) Lee, H.; Kevan, L. J. Phys. Chem. 1986, 90. 5781.
0 1991 American Chemical Society
Copper( 11)-Exchanged M-X and M-Y Zeolites The catalytic oxidation of propylene has been studied over metal oxide catalysts1628and transition-metal-ion-exchanged ~eolites.”~ Cupric ion has been shown to be involved for the complete oxidation of ~ r o p y l e n e . ’ ~ Most * ’ ~ of the investigations have been concerned with partial oxidation to the industrially important oxygenated compound acrolein (CH2=CH-CHO), mainly using metal oxide catalyst^.^^-^^ Cuprous oxide is the mast active catalyst among these oxides for the selective oxidation and has been investigated by several author^."-^ Acrolein has also been suggested as a reaction intermediate for the complete oxidation of propylene to carbon dioxide and water.21 More recently, the use of bismuth molybdate has enabled higher catalytic activity to be achieved, and the preparation of mixed oxide systems containing molybdate has been studied.28-31 Transition-metal-ion-exchanged zeolites are also active for propylene oxidation but generally give complete oxidation to carbon dioxide and Only a few studies of the selective oxidation of propylene to acrolein have been reported over transition-metal-ion-exchanged Mochida et al.llc reported that traces of acrolein were detected together with the complete oxidation products carbon dioxide and water for an oxygen/propylene mole ratio significantly greater than unity and that acrolein formation increased with decreasing oxygen/propylene mole ratio in Y zeolites. Uh et a1.I0 reported that it is the copper ion and not the Bronsted acid site which is primarily responsible for the formation of acrolein in the experiments on the CuH-Y zeolites. Copper(I1)-catalyzed propylene oxidation over Cu(I1)-exchanged X and Y zeolites has been also recently studied in this l a b o r a t ~ r y ~ ~where - ’ ~ the copper(I1) location, copper(I1) migration, and formation of new copper(I1) species were monitored by electron spin resonance, (ESR)combined with electron spin echo modulation (ESEM) spectroscopy. These results were correlated with the Si/A ratio and the catalytic activities at different reaction temperatures. In these studies, propylene was oxidized to completion under conditions of excess oxygen, and traces of acrolein were observed only in Y ~eolite.’~ (15) (a) Yu, J. S.;Lee, H.; Kevan, L. In Catalysis 1987; Ward, J. W., Ed.; Elsevier: Amsterdam, 1988; pp 273-279. (b) Yu, J. S.; Kevan, L. In Microsrrucrure and Properties of Caralysrs; Treacy, M. M. J., Thomas, J. M., White, J. M., Eds.; Materials Research Society: Pittsburgh, 1988; pp 245-429. (16) (a) Morooka, Y.; Ozaki, A. J . Catal. 1966,5, 116. (b) Davydov, A. A.; Mikhaltchenok, V. G.; Sokolovski, V. D.; Boreskov, G. K. J . C a r d 1978, 55, 299. (17) (a) Hearne, G. W.; Cerrito, FI; Adams, M. L. U S . Patent 2,451,485, 1948. (b) Hearne, G. W.; Adams, M. L. U S . Patent 2,486,842, 1955. (1 8) (a) Standard Oil Development Co. Chem. Abstr. 1953,47,4899d. (b) Ishikawa, T. Chem. Absrr. 1962, 56, 99451. (19) (a) Adams, C. R.: Jennings, T. J. J . Caral. 1963, 2,63. (b) Adams, C. R.; Jennings, T. J. J . Card. 1964, 3, 549. (20) (a) Garnish, A. M.; Shafranskii, L. M.; Skvortsov, N. P.; Zvezdina, E. A.; Stepanovskya, V. F. Kiner. Caral. 1962,3, 220. (b) Polkovnikova, A. G.; Kruzhalov, B. D.; Shatalova, A. N.; Tseitina, L. 1. Kiner. Catal. 1%2,3. 216. (c) Popova, E. N.; Gorokhovatskii, Ya, B. Proc. Acad. Sci. USSR., Chem. Secr. 1962, 145, 626. (d) Gorokhovatskii, Ya. B.; Vovyanko, I. I.; Rubanik, M. Ya. Kiner. Caral. 1964, 7, 65. (21) Voge. H. H.; Wagner, C. D.; Stevenson, D. P. J . Caral. 1963,2, 5 8 . (22) Wood, B. J.; Wise, H.; Yolles, R. S . J . Caral. 1969, 15, 355. (23) Holbrook, L. L.; Wise, H. J . Caral. 1971, 20, 367. (24) (a) Inui, T.; Ueda, T.; Suehiro, M. J . Caral. 1980,65, 166. (b) Aso, I.; Nakao, M.; Yamazoe, N.; Seiyama, T. J . Caral. 1979, 57, 287. (25) Sampson, R. J.; Shooter, D. Oxid. Combusr. Rev. 1965, I , 225. (26) Margolis, L. Ya. Adu. Caral. 1963, 14, 473. (27) Voge, H. H.; Adams, C. R. Adu. Caral. 1967, 17, 151. (28) Imachi, M.; Kuczkowski, R. L.; Gloves, J. T.; Cant, N. W. Kiner. Karal. 1983, 82, 355. (29) (a) Idol, J. D., Jr. U S . Patent 2,904,580, 1959. (b) Callhan, J. L.; Foreman, R.W.; Veatch, F. U S . Patent 2,904,580, 1959. (c) Batist, Ph. A,; Lippens, B. C.; Schuit, G. C. A. J. Catal. 1966, 5, 5 5 . (30) (a) Hucknall, D. J. Selecriue Oxidation ojHydrocarbons; Academic Press: New York, 1974; Chapter 3. (b) Alkhazov, T. G.; Adzhamov, K. Yu.; Khanmamedova, A. K. Run. Chem. Reu. 1982, 5, 542. (31) (a) Moro-Oka, Y.; Takita, Y. In Caralysis; Hightower, J. W., Ed.; Elsevier: New York, 1973; Vol. 2, p 1025. (b) Keulks, G. W.; Krenzke, L. D.; Notreman, T. M. Ado. Caral. 1978, 27, 183. (c) Keulks, G. W. Adu. Caral. 1978,27, 17. (d) Inui, T.; Ueda, T.; Suehiro, M.; Shingu, H. J . Chem. Soc., Faraday Trans. I 1989, 74, 2490. (32) Yu, J. S.; Kevan, L. J . Phys. Chem. 1990, 94, 5995. (33) Yu, J. S.; Kevan, L. J . Phys. Chem. 1990, 94, 7612. (34) Yu, J. S.; Kevan, L. J . Phys. Chem. 1990, 94, 7620.
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3263 The present work describes the first systematic studies of the catalytic partial oxidation of propylene over Cu(I1)-exchanged X and Y zeolites to optimize the yield of acrolein in the presence of different major cocations as a function of the oxygen/propylene mole ratio. The catalytic efficiency for the formation of acrolein is correlated with the major cocation, the oxygen/propylene mole ratio, the reaction temperature, the Si/AI ratio of the zeolite, the degree of Cu(I1) exchange, and ESR studies of the paramagnetic species present during the catalytic reaction. This is the first comprehensive study of the correlation of paramagnetic species present during the catalytic reaction with the reaction selectivity for the partial oxidation product of acrolein.
Experimental Section Materials and Cation Exchange. Linde Na-X (Si/Al = 1.2) zeolite from Alfa Chemical and Linde Na-Y (Si/AI = 2.4) zeolite from Union Carbide were exchanged with 0.1 M sodium acetate (Mallinckrodt Inc.) and were then exchanged with 0.1 M Li’, K+, and NH4+ with either nitrate or acetate as the counterion and Mg2+ and Ca2+with sulfate and dichloride as the counterion, respectively. To obtain maximum cation exchange, the exchange was carried out four times, each for 12 h at 70 OC. The filtered zeolites were washed repeatedly with distilled water to remove the counterions and dried in air at rmm temperature. Commercial analysis confirmed that the cation exchange was complete. These zeolites will be referred to as M-X and M-Y zeolites were M = Mg2+,CaZ+,Li+, Na+, K+, and NH4+. NH,+-exchanged zeolites were deammoniated by heating to 400 “C in flowing oxygen for 16 h to obtain H-X and H-Y zeolites. These H-X and H-Y and other M-X and M-Y zeolites were then further exchanged a t rwm temperature for 12 h with different volumes of 10 mM cupric nitrate (Alfa Products, Puratronic) per 1 g of each of the zeolites. These zeolites were then washed four times with hot triply distilled water and dried in air at room temperature. The cupric-ion-exchanged zeolites are denoted by Cu(a)M-X or Cu(a)M-Y where a is the number of Cu(I1) per unit cell determined by commercial analysis and M is the major cocation. Most experiments were done with zeolite samples exchanged with 15 mL of 10 mM cupric nitrate solution per 1 g of sample. This results in slightly different numbers of Cu(I1) per unit cell (2.6 f 0.3 per unit cell) depending upon the exchanged cocations and the type of zeolite. However, equal sample masses contain the same amount of cupric ion and can be intercompared for catalytic activity. Crystallinity of the exchanged zeolites was checked by X-ray diffraction using a Philips diffractometer scanning at 1O of 28 per min after dehydration and reaction. No loss of crystallinity was observed. Propylene (99.0% C P grade), oxygen, and helium gases were obtained from the Linde Division of Union Carbide Corp. Reactor System. Experiments were carried out in a fixed-bed type reactor with continuous gas flow at atmospheric pressure. The zeolite sample (0.16-0.21 g) was placed on a porous sintered glass disk fused across the body of an electrically heated 10mm4.d. Pyrex tube reactor of total internal volume of 30 cm3 with a connected Suprasil quartz ESR tube (2-mm i.d. by 3-mm 0.d.). The zeolite was heated in a steam of flowing oxygen while the temperature was slowly increased (- 100 OC/h) to some maximum temperature, usually 520 OC, at which heating was continued for 16 h and then decreased to the reaction temperature. This heat-treated sample is termed as a dehydrated zeolite catalyst. The reaction temperature was monitored by a thermocouple in a thermowell located at the center of the reactor tube. The reactant gas mixture consisted of oxygen, propylene, and helium. Propylene and oxygen were mixed first at a fixed ratio in a gas proportioner with high accuracy valves. This mixture was then mixed with helium gas in a mixing chamber before entering the reactor to maximize the homogeneity of the reactant mixture. The total flow rate was 22.5 cm3/min, and the flow rate of helium was fixed at 6.5 cm3/min. The oxygen/propylene mole ratio was varied from 5 to 0.4. The variation of partial pressures of oxygen and propylene was made without altering the flow rate of helium or the total flow rate. Propylene oxidation was studied on the dehydrated sample by flowing the reactant gas mixture
Yu and Kevan
3264 The Journal of Physical Chemistry, Vol, 95, No. 8,1991 PropyleneMole Fraction 0.3 0.5 0.7 A A Carbon Monldr
-
90 0 d
80
80
0
8
f
p y z ,Q;=l Acrolein
0.I
20
40
60 Time, hour
80
100
Figure 1. Variation of the yields of acrolein and carbon dioxide in terms of mole pcrcent propylene converted with reaction time over Cu(2.5)Ca-Y and Cu(2.7)Ca-X. Note that 1 mol of propylene converted gives 3 mol of carbon dioxide, and thus one-third of the actual yield of carbon dioxide is plotted. Conditions were as follows: catalyst, 0.16 g; reaction temperature, 350 OC; oxygen/propylene mole ratio, 1; and flow rate, 22.5 cm'/min. The curve is drawn through the points to indicate the trend.
over the catalyst in the reactor tube at a specific reaction temperature. Gas Chromatographic Studies. Reactants and products were analyzed on-line at different reaction times by withdrawing aliquots for analysis with a six-port sampling valve to a Varian Model 1410 gas chromatograph with a thermal conductivity detector connected to an electronic integrator. The gas chromatographic column contained 100/120 mesh Haysep Q in 1/8-in.-o.d. by 6-ft-long stainless steel tubing and was maintained at 150 OC. This column can separate oxygen, methane, carbon dioxide, ethylene, ethane, water, formaldehyde, propylene, methanol, acetaldehyde, 1-butene, cis-2-butene, trans-Zbutene, acrolein, 2-propanol, and isopentane. Helium was used as a carrier gas. The propylene conversion percentage was reproducible to better than 10%average deviation and took into account that 3 mol of carbon dioxide is formed per mole of propylene. The selectivity percent for formation of a specific product is given by the moles of acrolein or one-third the moles of carbon dioxide times 100 divided by the moles of propylene reacted. Spectroscopic Studies. ESR measurements were carried out on catalyst samples treated at various temperatures for different periods of time to identify the cupric ion species and other paramagnetic species and to monitor their intensities during the reaction. At suitable points during reaction the sample was quenched to room temperature and transferred to the attached Suprasil quartz ESR tube. ESR spectra were recorded at -196 OC with Bruker ESP 300 and Varian E-4 ESR spectrometers. Results Catalytic Activity vs Reaction Time. The variation in catalytic activity with reaction time is illustrated for an oxygen/propylene mole ratio of 1 at 350 OC for Cu(2.5)Ca-Y and Cu(2.7)Ca-X in Figure 1. The yields of acrolein and carbon dioxide were determined as a function of reaction time. The yield of acrolein increases and reaches a plateau after about 30 h. Carbon dioxide shows a correlated behavior. The catalyst remains active beyond 100 h, and the total propylene conversion is about constant beyond 30 h. The relative catalytic activities for X and Y zeolites are determined after 40-h reaction when they reach a plateau unless otherwise mentioned. Effect of OxygenlPropylene Mole Ratio. The variation in catalytic activity with an oxygen/propylene mole ratio varied from 5 to 0.4 was measured for Cu(3.3)Mg-Y at 350 OC and is shown in Figure 2. At an oxygen/propylene mole ratio of 5 or higher, little acrolein yield was observed, even after 2 days of reaction, and only the complete oxidation products, carbon dioxide and water, were observed. The total propylene conversion is greater than 70%. The yield of acrolein increases with increasing pro-
-
Oxygen/Pmpylene Mole Ratio
Figure 2. Effect of the oxygen/propylene mole ratio or propylene mole fraction on the yields and selectivitiesof acrolein and carbon dioxide over Cu(3.3)Mg-Y. Conditions were as follows: catalyst, 0.21 g; reaction temperature, 350 OC; and reaction time, 48 h. The conversion of 1 mol of propylene to 3 mol of carbon dioxide was accounted for.
-
pylene mole fraction up to an oxygen/propylene mole ratio of 1.5 and then decreases at higher propylene mole fraction. The yield of carbon dioxide monotonically decreases with increasing propylene mole ratio as shown in the figure. The total propylene conversion also monotonically decreases with increasing propylene mole fraction along with the yield of carbon dioxide. At a mole fraction of 0.7 or more, no propylene conversion was observed. The selectivity for acrolein increases with increasing propylene mole fraction up to an oxygen/propylene mole ratio of 1 and then decreases with higher propylene mole fraction. The change in the selectivity for carbon dioxide is the inverse of that of acrolein because they are the two major oxidized products. When the selectivity of acrolein reaches a maximum, that of carbon dioxide reaches a minimum at an oxygen/propylene mole ratio of -1 as shown in the figure. The yield of acrolein did not exceed that of carbon dioxide at any oxygen/propylene mole ratio in the X and Y zeolites studied. Reaction Products. The main reaction products are carbon dioxide, water, and acrolein at propylene mole fractions between 0.15 and 0.7 over all the catalysts studied in the reaction temperature range 200-400 O C . Hydrocarbon compounds such as methane, ethylene, ethane, isobutene, 1-butene, cis-Zbutene, trans-Zbutene, and isopentane and some oxygenated products such as formaldehyde, methanol, acetaldehyde, and 2-propanol were also observed as initial products for the first 10-15 min of reaction. In particular, peaks of methanol and butene isomers are noticeable. The total butene isomers were about 70 mol % of the initial hydrocarbon products formed in the first 5 min of reaction. These initial products decrease with reaction time and are not detected after -20 min of reaction except for methane, ethylene, and acetaldehyde. Methane, ethylene, and acetaldehyde have a total yield of less than 7% of the converted propylene throughout the reaction for all the catalysts studied. Effect of the Degree of Ion Exchange. The yield and selectivity of acrolein produced are shown as a function of the number of cupric ions per unit cell in CuMg-Y zeolite with an oxygen/ propylene mole ratio of 1 at 350 OC in Figure 3. Both increase as the number of exchanged cupric ions increases but largely level off beyond three Cu(I1) per unit cell. The yield of acrolein in the absence of cupric ion was negligible at temperatures below
-
-
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3265
Copper( 11)-Exchanged M-X and M-Y Zeolites
r
e!
'1
ACuM-X
H Lit
X
Number of Cu2+perUnit Cell Figure 3. Effect of number of cupric ions per unit cell in CuMg-Y on the yield and selectivity of acrolein. Conditions were as follows: catalyst, 0.16 g; reaction temperature, 350 OC; oxygen/propylene mole ratio, 1; and reaction time, 48 h.
0 CUM-Y
k
- 20 Acrolein C d a ) Mg-Y/350°C
0
Acrolein
~ a +K'
Figure 4. Effect of zeolite cocation on the yields of acrolein over CUM-X and CUM-Y where M is the cocation. Conditions were as follows: catalyst, 0.16 g; reaction temperature, 350 OC; oxygen/propylenemole ratio, 1; and reaction time, 48 h.
Carbon Dioxide
TABLE I: Induction Periods Observed for the Formation of Acrolein in X and Y Zeolites'
zeolites Cu(2.6)Mg-X Cu(2.7)Ca-X Cu(2.5)Li-X Cu(2.7)Na-X Cu(2.5)Mg-Y CU(13.3)Mg-Y Cu(2.5)Ca-Y Cu(2.4)Li-Y Cu(2.7)Na-Y Cu(2.4)K-Y
induction period? min 350 OC 400 OC 90 150
750 750 20 IO 20 20 40 60
90
420 15
LU
7
2
'Reaction conditions were 0.16 g catalyst at an oxygen/propylene
mole ratio of 1, Estimated errors are * 5 min for Y zeolites and 130 min for X zeolites. 400 OC. This demonstrates that a copper species is active for the
partial oxidation of propylene. Effect of SiIAI Ratio. The effect of the Si/Al ratio on acrolein production is examined by comparing Cu(2.S)Ca-Y and Cu(2.7)Ca-X in Figure 1. The yield of acrolein increases with reaction time for both X and Y type zeolites. CuCa-Y shows greater activity for the formation of acrolein than its corresponding X zeolite. The activity of CuCa-Y is 5 times greater after 5 h and 2.5 times greater after 10 h. After -48 h when a plateau yield of acrolein is reached, the activity of Cu(2S)Ca-Y is about 1.4 times that of Cu(2.7)Ca-X. Also notice that Cu(2.7)Ca-X shows an induction period of -2.5 h for the formation of acrolein while in Cu(2.5)Ca-Y the induction period is only -20 min. Both X and Y zeolites show some induction period for the formation of acrolein depending upon the major cocation, the extent of cupric ion exchange, and the reaction temperature. In general, X zeolite shows longer induction periods than the corresponding Y zeolite. Alkali-metal cocations show longer induction periods than alkaline-earth-metal cocations. For example, CuNa-X and CaLi-X show an induction period of 12.5 h. The induction periods observed for X and Y type zeolites as a function of major cocation, reaction temperature, and extent of copper ion exchange are summarized in Table I. Effect of Cocation. In order to compare the catalytic activities of catalysts ion-exchanged with the same amount of cupric ion (15 cm3 of 10 mM cupric nitrate solution per 1 g of each of the different zeolites so that 0.16 g of each sample placed in the reactor tube contains the same amount of cupric ion even though the number of cupric ions per unit cell for each of the zeolites is slightly different from each other) in the presence of different cocations, the relative catalytic activities under the same reaction conditions
0
Mget Ca2' H Lit Nat K t Figure 5. Effect of zeolite cocation on the yields of carbon dioxide over
CUM-X and CUM-Y where M is the cocation. Conditions were as follows: catalyst, 0.16 g; reaction temperature, 350 OC; oxygen/propylene mole ratio, 1; and reaction time, 48 h. The conversion of 1 mol of propylene to 3 mol of carbon dioxide was accounted for. a 0
Acrolein
k
%
0 CUM-Y A CUM-X
-
Figure 6. Effect of zeolite cocation on the selectivity for acrolein over CUM-X and CUM-Y where M is the cocation. Conditions were as follows: catalyst, 0.16 g; reaction temperature, 350 OC; oxygen/propylene mole ratio, 1; and reaction time, 48 h.
are measured for both X and Y type zeolites and shown in Figures 4-6. The relative yields of acrolein and carbon dioxide are shown
3266 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991
I8r
Yu and Kevan
0 OCU(13.3)MQ-Y \
Q
5
E
\
-
\
Dioxide
8o
8
3 60b
P
0
I
260
I
300
I
I
400
Temperoture,'C Figure 7. Effect of reaction temperature on the yields of acrolein and carbon dioxide over Cu(2S)Mg-Y and Cu( 13.3)Mg-Y. Conditions were as follows: catalyst, 0.16 g; reaction temperature, 350 OC; oxygen/propylene mole ratio, I ; and reaction time, 48 h. The conversion of 1 mol of propylene to 3 mol of carbon dioxide was accounted for. as a function of the cocations in X and Y zeolites in Figures 4 and 5 . The alkaline-earth cocation (Mg2+,Ca2+) zeolites show higher activity than the alkali-metal cocations (Li+, Na+, K+) and H+for the fomation of acrolein. Mg2+zeolite shows slightly better activity than Ca2+ zeolite. The same cocation size effect for the formation of acrolein is also oberved for the alkali-metal cocation-exchanged Y zeolites; the increasing activity order observed is Li+ > Na+ > K+ as shown in Figure 4. But for X zeolites the increasing activity order is Na+ > Li+ > K+. For the formation of carbon dioxide in Figure 5, rather different results are observed. Alkali-metal cocations show greater activity than alkaline-earth-metal cocations in both X and Y zeolites. CuLi-X shows the highest activity for complete oxidation among the X and Y zeolites. In K-X and proton-exchanged zeolites, the initial yield of carbon dioxide decreases rapidly with reaction time. From 1 to 48 h of reaction only a negligible yield of carbon dioxide is observed, and no acrolein is seen. In general, Y zeolites showed higher activity than the corresponding X zeolites in both selective oxidation to acrolein and complete oxidation to carbon dioxide and water. The activity of CuMg-Y was greater than that of other cocations for the formation of acrolein. The change in selectivity for acrolein in Figure 6 shows about the same trend for both X and Y zeolites. Due to the high activity for the formation of carbon dioxide in CuLi-X, the selectivity for acrolein decreases more compared to other zeolites. Alkalineearth-metal-cocation-exchanged zeolites show better selectivity than alkali-metal-cocation-exchanged ones. Effect of Reaction Temperature. The effect of the reaction temperature on the formation of acrolein and carbon dioxide was examined over Cu( 13.3)Mg-Y and Cu(2.5)mg-Y and is shown in Figure 7. The temperature that gives a maximum yield of acrolein at an oxygen/propylene mole ratio of 1 depends on the amount of cupric ion exchanged into the zeolites. CuMg-Y zeolite with high Cu(I1) content shows a plateau yield of acrolein at temperatures lower than CuMg-Y with lower Cu(I1) content. For example, a plateau yield occurred at 350 OC or above for Cu(2.5)Mg-Y and at 250 O C or above for Cu(13.3)Mg-Y. At temperatures lower than 250 OC, the initial yield of carbon dioxide rapidly decreases with reaction time. Only negligible propylene conversion is observed. No acrolein is produced. The yield of carbon dioxide increases with reaction temperature to above 400 OC. The yield of acrolein increases with reaction temperature up to 300 OC and then slightly decreases or plateaus. The selectivity for acrolein is shown in Figure 8. It is maximized at lower temperature for a higher degree of cupric ion exchange.
-
400
300
TemperaturePC Figure 8. Effect of reaction temperature on the selectivites for acrolein and carbon dioxide over Cu(2.5)Mg-Y and Cu( 13.3)Mg-Y. Conditions were as follows: catalyst, 0.16 g; reaction temperature, 350 OC; oxygen/propylene mole ratio, 1; and reaction time, 48 h. The conversion of 1 mol of propylene to 3 mol carbon dioxide was accounted for.
CU(2.5)Mg-Y a. Dehydrated/520aC
* XI28
x2
-
m
7
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2000 4-8x
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CuII(I1 992.33
VI
Figure 9. ESR spectra at -196 OC of Cu(2.5)Mg-Y evacuated for IO min at room temperature (a) after dehydration in flowing oxygen at -520 OC for 16 h, (b) after subsequent partial oxidation of propylene at an oxygen/propylene mole ratio of 1 at 350 OC for I5 min, (c) at 350 OC for 50 h, (d) after reoxidation in flowing oxygen at 530 O C for 18 h following reaction at 350 O C for -50 h, and (e) after partial oxidation of propylene at an oxygen/propylene mole ratio of 1 at 300 OC for 30 h.
Electron Spin Resonance Measurements. ESR was used to identify the cupric ion species developed during the catalytic reaction. Figure 9 shows the ESR spectra of Cu(2.5)Mg-Y before and after reaction. It is clear that the ESR spectra of CuMg-Y zeolites change dramatically during the reaction. Zeolites were first dehydrated in a stream of flowing oxygen for 16 h at -520 O C . Figure 9a shows the ESR spectrum of Cu(I1) in CuMg-Y after oxygen gas was evacuated or removed by flushing with helium gas for 10 min a t room temperature following the dehydration process at -520 OC. Two different cupric ion species, Cu( 1) and Cu(2), are observed. This indicates that cupric ions
-
Copper(I1)-Exchanged M-X and M-Y Zeolites
The Journal of Physical Chemistry, Vol, 95, No.8, 1991 3267
Cu(2.6)Mg-X a. Dehvdrated/500°C/ 16h
ar
-
I
fOOG
"
"1
c.Reacted/3OdC, 4 8 h
XI28
I
~
0.
Rooetod/460.C. 4 8 h
-
200 0
'
cull'I)
I
Figure 11. ESR spectra at -196 O C of Cu(13.3)Mg-Y evacuated for 10 min at room temperature (a) after dehydration in flowing oxygen at
-520 OC for 16 h, (b) after subsequent partial oxidation of propylene at an oxygen/propylene mole ratio of 1 at 300 "C for 26 min, (c) at 300 O C for 48 h, (d) at 250 OC for 50 h, and (e) at 400 O C for 48 h.
(35) (a) Jacobs. P. A.; Tielen, M.; Linart, J. P.; Uytterhoeven, J. B.;Beyer, H.J. Chem. Soc., Faraday Tram. I 1976,72,2793. (b) Beyer, H.;Jacobs, P. A.; Uytterhoeven, J. B. J. Chem. Soc., Faraday Trans. I 1976, 72, 614. (36) Ayscough. P.B. Electron Spin Resonance in Chemistry; Methuen & Co.: London, 1967; p 374. (37) Mann. R. S.; Khulbe, K. C. J. Catal. 1976,12, 115.
and no detectable acrolein yield is seen for up to 48 h a t this temperature. Double integration of the ESR spectrum shows that 38% of the initial cupric ion is still present after reaction for 30 h. This amount is much more than that present after reaction at 350 "C. A peak a t g = 2.001 due to carbon char is also observed. The ESR spectra after reaction show only one species that corresponds to Cu( 1). Figure 10a shows the ESR spectrum of dehydrated Cu(2.6)Mg-X where Cu(1) and Cu(2) are observed. After reaction a t 350 OC for 48 h, the cupric ion intensity decreases with only Cu(1) weakly observable as shown in Figure lob. The development of a strong singlet at g = 2.001 due to carbon char is also seen. These observations are the same as observed for the corresponding Y zeolite. Figure 11 shows the ESR spectra of dehydrated Cu( 13.3)Mg-Y before and after reaction. Before reaction, Cu(1) and Cu(2) are again probably present although most of the hyperfine peaks are not resolved in this highly exchanged zeolite (Figure 1la). Figure 1 1b-e shows ESR spectra after reaction at different reaction temperatures for different periods of time. The cupric ion intensity decreases during reaction. Figure 1 1, b and c, shows ESR spectra after reaction at 350 "C for 26 min and 48 h, respectively. The ESR spectrum after -48 h reaction did not change much from that after 26 min of reaction in terms of the shape and intensity of the cupric ion. Only Cu(1) in site SI is observed after -48 h of reaction a t 300 or 400 OC together with a carbon char peak in the ESR spectra in Figure 1ld,e. In general, the cupric ion intensity decreases more at higher reaction temperature. The relative cupric ion intensities determined by double integration after about 48 h of reaction were 32% at 250 OC, 21% a t 300 O C , and only 1% at 400 OC of the initial cupric concentration in the dehydrated state. Figure 12 shows the ESR spectra of Cu(2S)Ca-Y and Cu(2.7)Ca-X before and after reaction. The different Si/AI ratio between X and Y zeolite results in quite different ESR spectra. The cupric ions are initially distributed into two different sites in dehydrated CuCa-Y zeolite (Figure 12a), while there seem to be three different sites with a very low relative intensity of Cu( 1) compared to the other two species,in dehydrated CuCa-X (Figure
3268 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 Cu(2.5)Ca-Y a. Dehydrated/520°C
r4
vu
b. Reacted/350eC. 48 h
*
'I1
'
Yu and Kevan TABLE 11: ESR Parametersa of Cu(11) at -196 OC in Dehydrated X and Y Zeolites probable zeolite species gb gl A, siteb CuMg-Y 1 2.33 2.07 175 SI 2 2.38 140 SII' CuCa-Y 1 2.32 2.07 174 SI 2.37 2 138 SII' I CuLi-Y 2.34 2.07 173 SI L 2.36 143 SI' CuNa-Y I 2.35 2.07 175 SI 2.40 2 130 SI' CUK-Y 1 178 2.33 2.07 SI 137 2.39 L SI' CUH-Y 1 182 2.32 2.07 SI 2 138 2.38 SI' 2.34 1 CuMg-X 160 2.07 SI 131 2.39 2 SII' 2.34 1 CuCa-X 2.07 159 SI 142 2.37 2 2.07 SII' 3 2.45 95 SII* 2.33 2.07 1 CuLi-X 155 SI 2.38 L 134 SI' 2.07 2.35 158 CuNa-X 1 SI 2.38 138 2 SI' 2.35 153 CUK-X 1 2.06 SI 2.38 L 135 SI' 2.34 CUH-X I 2.07 SI 161 2.38 2 135 SI'
+!-
c. Dehydmted/520%
d. Reackd/350°C, 48 h x16
Figure 12. ESR spectra at -196 OC of Cu(2.5)Ca-Y and Cu(2.7)Ca-X evacuated for IO min at room temperature after dehydration in flowing oxygen at 520 OC for 16 h over (a) Cu(2.5)Ca-Y and (c) Cu(2.7)Ca-X, and after subsequent partial oxidation of propylene at an oxygen/propylene mole ratio of 1 at 350 OC for 48 h over (b) Cu(2.5)Ca-Y and (d) Cu(2.7)Ca-X, respectively.
"The estimated errors in g values are 10.01 and in A values are 1 3 IO-" cm-I; the unit of A is 1 X IO-" cm-I. bThe zeolite sites are defined in the t e x t . X
Cu (2.4)Li-Y a. Dehydrated/52$C
-r
P-cage
a-cage
W
'tiexagonal prism Figure 14. Schematic structure of X zeolite where the center of each line represents an oxygen and each vertex represents AI or Si. Possible cation positions are shown by Roman numerals.
Cu(2.5) Li-X c. Reackd/350°C, 49h
I
Cu(27)Na-X
Cut((')g ~2.35
Figure 13. FSR spectra at -196 OC of Cu(2.4)Li-Y. Cu(2.5)Li-X and Cu(2.7)Na-X evacuated for IO min at room temperature (a) after dehydration in flowing oxygen at 520 OC for 16 h Over Cu(2.4)Li-Y and (b), (c), and (d) after subsequent partial oxidation of propylene at an oxygen/propylene mole ratio of 1 at 350 OC for -2 days over Cu(2.4)Li-Y, Cu(2.5)Li-X, and Cu(2.7)Na-X, respectively.
13c). The ESR spectra in Figure 12b,d show that only one species is weakly observed together with a singlet peak at g = 2.001 after reaction at 350 O C for about 48 h. Figure 13 shows the ESR spectra of Cu(2.4)Li-Y, Cu(2.5)Li-X, and Cu(2.7)Na-X zeolites. The cupric ion intensity is
markedly decreased after reaction with only Cu(1) in site SI weakly observable together with the development of a strong single peak due to carbon char. The relative cupric ion intensity in Figure 14c determined by double integration after 48 h of reaction at 350 O C was 8% of the initial cupric ion concentration in dehydrated Cu(2.5)Li-X. This amount is quite significant compared to 3% after reaction at 350 OC in both Cu(2.4)Li-Y and Cu(2.7)Na-X. The ESR parameters of Cu(I1) at -196 O C in dehydrated X and Y zeolites in the presence of different major cocations are summarized in Table 11. Some of the assignments were made based upon earlier work in our l a b ~ r a t o r y . ' ~ ~ ~ ~ * - ~ ~ Discussion X and Y zeolites consist of a three-dimensional network of AlO, and Si04 tetrahedra with %/AI ratios of about 1.2 and 2.4, respectively. These tetrahedral units are linked to each other by sharing all of the oxygens to form truncated octahedra called sodalite or P-cages. The sodalite cages are tetrahedrally interconnected through a double six-ring (hexagonal prism) to form a larger cage called a supercage or a-cage. The free apertures to the a-cage and 8-cage are 0.74 and 0.22 nm, respectively. Figure 14 shows a schematic structure of X and Y type zeolites. Due to the excess negative charge present on the A102units, charge-compensating cations are present to neutralize the negative framework charge. Various cation sites have been characterized
The Journal of Physical Chemistry, Vol. 95, No. 8, I991 3269
Copper(I1)-Exchanged M-X and M-Y Zeolites
TABLE III: Comparison of Catalytic Activities a d Related Properties for Propylene Oxidation at Different Owypen/Propylene Mole Ratios over 0.16 g of Cu(2.4)Li-Y Zeolite at 350 O C for 48 h oxygen/propyiene mole ratio 5 major products" selectivity, % total propylene conversion, 5% rei Cu(I1) conc after rxn, 3'% coking carbon char ESR peak
CO1, H20
acrolein c2
93
1
2 COz, Hz0 82
70 30 f 5 weak weak
acrolein 12
COz, HzO 73
42 7 f 2 severe strong
acrolein 21 20
3f1 severe strong
@Acetaldehydeis observed to be about 2% of the products independent of the oxygen/propyiene mole ratio.
by X-ray cry~tallography~~ and are indicated in Figure 14. Sites S U and SV are at the centers of the &cage and a-cage, respectively. SI is at the center of the hexagonal prism. SI' is a site displaced into the @-cagefrom the hexagonal prism. SI1 is at the center of the hexagonal window between the &cage and the a-cage. SII' and SIl* correspond to displacement from site SI1 into the 0-cage and into the a-cage, respectively, along an axis perpendicular to the hexagonal window. Finally, SI11 is used in a broad sense to cover sites near four-rings in the a-cage. X and Y zeolites are structurally similar to each other, but they have different ratios of silicon to aluminum atoms. This results in differences in the possible Si-A1 ordering in the tetrahedral sites and in the cation composition and distribution in Y compared to X zeolite. Since A102 has an effective negative charge, a stronger electric field occurs in X zeolite because of its higher AI content than in Y zeolite. Thus, the interaction of cupric ions in Y zeolite may be different from that in X zeolite. Decrease in Propylene Conversion. Figure 2 shows that the total propylene conversion decreases monotonically as the oxygen/propylene mole ratio decreases. Since the steady-state level of Cu(1I) observed by ESR also decreases as the oxygen/propylene mole ratio decreases, the conversion decrease seems partly due to the relative amount of Cu(I1) present. It may also be partially due to coke formation which is indicated by the singlet ESR peak at g = 2.001. Cu(l1) and Cu(1) may be restricted from interacting with oxygen by the coke which partially blocks the six-ring. Similar coke-induced deactivation has been observed in other reactions studied by several authors.3942 Active Species. The yield of acrolein over zeolites exchanged with different major cocations without cupric ion exchange is negligible at 400 OC or lower reaction temperature. The yield of acrolein increases with the degree of copper(I1) exchange as shown in Figure 3. The selectivity for acrolein also increases slightly even though the yield of carbon dioxide also increases with the amount of copper(I1) loading. This indicates that a copper species is responsible for the selective oxidation of propylene to acrolein. It took 30-40 h for the formation of acrolein to reach a constant plateau as shown in Figure 1. For about the same period of time, the yield of carbon dioxide decreases. This seems to indicate that the active species for the formation of acrolein is different from that for the formation of carbon dioxide and that the increase in the acrolein yield corresponds to an increase of the active species responsible for acrolein formation. Thus, two parallel reaction pathways are indicated for partial oxidation to acrolein and for complete oxidation to carbon dioxide and water. More direct evidence for the species active for the formation of acrolein is seen in the product formation at different oxygen(38) Smith, J. V. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1076; Chapter 1. (39) (a) Jacobs, P. A.; Declerck, L. J.; Vandamme, L. J.; Uytterhoeven, J. B. J. Chem. Soc.. Faraday Trans. I 1975, 71. 1545. (b) Jacobs, P. A.; Leeman, H. E.; Uytterhovenn, J. B. J. Catal. 1974, 33, 17. (40) Pine, H. The Chemistry of Catalytic Hydrocarbon Conversions; Academic Press: New York, 1981; p 87. (41) Marczewski. M.; Wojciechowski, B. W. Can. J. Chem. Eng. 1982.60, 617. (42) (a) Ghosh, A. K.; Kydd, R. J. Catal. 1986,100, 185. (b) Ghosh, A. K.; Kydd. R.J. Caral. 1987, 103. 399.
to-propylene mole ratios together with the ESR intensities of Cu(I1) in Cu(2.4)Li-Y as summarized in Table 111. Complete oxidation of propylene to carbon dioxide and water is predominant at an oxygen/propylene mole ratio of 5 or higher. Under this condition 30% of Cu(I1) is still observed after 48 h of reaction. There is little coke formation with only a weak carbon char ESR peak. In our earlier work with CuK-X and CuK-Y ~ e o l i t e s ~ ~ - ~ ~ after only 1 h of reaction more than 80% of Cu(I1) is still observed. This is attributed to the excess oxygen which helps to maintain the copper in the cupric ion form and assists oxidation of any coke formed.40 Thus, the initial step in the complete oxidation of propylene is suggested to be propylene coordination to Cu(I1). In contrast, the yield of acrolein is negligible at an oxygen/propylene mole ratio of 5 . With decreasing oxygen/propylene mole ratio, the reduction of cupric ion becomes greater as shown by the ESR measurements during the reaction in Table 111. In parallel with this, the yields of the complete oxidation products, carbon dioxide and water, decrease and the yield of acrolein increases with a maximum at an oxygen/propylene mole ratio of -1. Thus, a reduced copper species, Cu(1) and/or Cu(O), is implicated in a separate reaction pathway leading to acrolein. The initial step in this separate reaction pathway is suggested to be propylene coordination to Cu(1). Evidence that a reduced copper species actually forms during propylene oxidation is as follows. After reaction at 350 OC for 50 h followed by reoxidation of the zeolite in flowing oxygen at 530 "C, the ESR spectrum recovered much of the cupric ion intensity as shown in Figure 9d. The carbon char ESR peak disappeared. However, the zeolite did not recover the original light blue color of the dehydrated state. It turned light gray after reoxidation compared to dark brown during propylene oxidation. The light gray color may indicate some formation of copper oxides, CuO and Cu20, which are not detected by ESR. This is consistent with the doubly integrated ESR spectrum which shows that only about 58% of the initial cupric ion concentration is recovered. There is evidence that some of the reduced copper species form CuO and/or Cu20 as a separate phase in the zeolite lattice.35*43*44 The reduced copper species is suggested to be the active species for partial oxidation of propylene to acrolein. The time required for the catalytic activity to reach a plateau is considered to correspond to the formation time for the reduced copper species. This is consistent with the work of Ben Taarit et al.,3 who reported that Cu(I1) in Y zeolite was reduced to metallic copper during propylene oxidation to acrolein. They suggested that a Cuo/Cu20/Cu0 mixture is active for the partial oxidation of propylene. This conclusion also agrees with others who have investigated partial oxidation of propylene on copper Cuprous oxides have been shown to be the most active species for the selective oxidation of propylene to acrolein. Even though there are still some c o n t r o ~ e r s i e sas ~ ~to* the ~ ~ oxidation state of the active species, cuprous ion is most commonly suggested as the active species for the selective formation of acrolein.22-2k Reducibility ofCu(1r). The reduction of transition-metal ions in dehydrated zeolites depends on the zeolite structure, the exThe exchanged changed cocation, and the metal ion
-
(43) Naccache, C.; Ben Taarit, Y. J . Cafal. 1961, 22, 171. (44) Herman, R. G.; Lunsford, J. H.; Beyer, H.;Jacobs, P. A.; Utterhoeven, J. B. J . Phys. Chem. 1975, 79, 2388.
3210 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 cocations determine the overall chemical properties of the zeolites through their average ele~tronegativity.~’Thus, the degree of reduction of coexchanged transition-metal ions in alkalineearth-metal-exchanged zeolites generally exceeds that found in alkali-metal-exchanged zeolites.46 The lower reducibility of cupric ion in alkali-metal-exchanged zeolites is consistent with a lower activity for acrolein formation in these zeolites as shown in Figure 4. However, CuLi-X shows an anomalously high activity and CuH-X, CuH-Y, and CuK-X show little or no activity. The reasons for these specific monovalent cation effects may be as follows. In CuH-X and CuH-Y zeolites, the propylene conversion decreases at an early stage of reaction and becomes very small. No acrolein is observed in these H-exchanged zeolites for -48 h of reaction with negligible propylene conversion. Propylene reacts on acid sites of these zeolites and polymerizes to form an oligomer that is responsible for part of the coke formation.4w2*4&w This coke can block the entrance of the various rings in the zeolite framework to prevent cupric ions from catalyzing the oxidation of propylene. Such coke-induced deactivation is observed particularly over proton-exchanged In the case of CuK-X, K+ is the largest cation of those studied and may simply cause steric blocking of propylene diffusion in the zeolite. This is not expected to be as severe in CuK-Y because only about half as many cations are present due to the lower framework negative charge in Y vs X zeolites. This explanation is not very satisfying since in earlier work,’5b CuK-X zeolite showed the best catalytic activity among several monovalent cations for the complete oxidation of propylene with an oxygen-to-propylene ratio of - 5 . The complete oxidation products, carbon dioxide and water, were observed, and no acrolein was formed. Here, only negligible catalytic activity for CuK-X is observed at an oxygen-to-propylene ratio of 1. Further investigation of this striking cation effect seems warranted. The cation effects on complete oxidation in Figure 5 are generally consistent with those for partial oxidation in Figure 4. For complete oxidation the monovalent cations show higher activity than divalent ions consistent with Cu(I1) reducibility arguments advanced above. The CuH-X, CuH-Y, and CuK-X systems also show little or no reactivity for complete oxidation as they did for partial oxidation. For both partial and complete oxidation the Y zeolites generally show greater reactivity than the X zeolites. This is attributed to more efficient diffusion in the Y zeolites because of their approximately 2-fold lower cation loading. Ben Taarit et aL3 have also reported the Y zeolite is more effective for partial oxidation of propylene than X zeolite. The Induction Period. It is known that the nonacidic catalytic properties of zeolites are largely dependent on the type, amount, and distribution of the catalytically active cations in the zeolite structure.’-’ The location of the catalytically active species in the zeolite structure is a key aspect for controlling the catalytic efficiency. It has been shown that the location and distribution of a small amount of Cu(I1) in zeolites can be controlled by the charge and size of the more abundant cocation and the Si/AI ratio of the zeolite structure.51 At low cupric ion exchange, cupric ions usually preferentially occupy relatively inaccessible sites in or near the hexagonal prism
-
(45) (a) Briend-Faure, M.; Jeanjean, J.; Kermarec, M.; Delafosse, D. J . Chem. Soc., Furuduy Truns. I 1978,74. 1538. (b) Briend-Faure, M.; Jeanjean, J.; Delafosse, D.; Gallezot, P. J . Phys. Chem. 1980, 84, 875. (46) Suzuki, M.; Tsutsumi, K.; Takahashi, H. zeolites 1982, 2, 51. (47) Mortier, W.; Schoonheyt, R. A. frog. Solid Srure Chem. 1985, 16,
Yu and Kevan in dehydrated X and Y zeolites (see Table 11). Since propylene, with a kinetic diameter of 0.43 nm, is constrained to the a-cage and cannot enter the @-cageor the hexagonal prism, the cupric ion must migrate toward the a-cage in order to interact with The induction period observed for the formation of acrolein can be considered to be the time needed for cupric ion migration toward the a-cage and reduction to form the active species. X zeolite has more aluminum tetrahedra in the zeolite lattice and thus needs more charge-balancing cations than Y zeolite. This causes crowding in the X zeolite cages which may inhibit cupric ion migration. In addition, X zeolite has a stronger internal electrostatic field. Thus, cupric ion is more strongly coordinated to the zeolite lattice in X zeolite than Y zeoliteSZwhich can also inhibit migration. This is consistent with the longer induction periods observed in X zeolites. The induction period observed in alkaline-earth-metal-exchanged zeolites is shorter than that in alkali-metal-exchanged zeolites. Possible reasons for this are as follows. The former has half as many cocations as the latter. The cupric ions are preferentially located in or near the hexagonal prism sites in alkalimetal-exchanged zeolites while some cupric ions occupy more accessible SII’ sites near the six-rings in alkaline-earth-metalexchanged zeolites since the alkaline-earth-metal cations themselves have a strong preference for the hexagonal prism sites.’J6 Thus, the migration of cupric ion toward the a-cage should be facilitated in alkaline-earth-metal-exchanged zeolites. The induction period decreases with the Cu(I1) exchange level in the zeolite and with the reaction temperature. At higher exchange levels cupric ions occupy more than one site, and their activation energy for migrating to the a-cage decreases. Catalyst Deactivation. Methane, ethylene, butenes, and isopentane are formed as initial hydrocarbon products only during the first 10-15 min of reaction, and butenes and isopentane disappear after 20 min of reaction. Methane and ethane decrease after 20 min of reaction, but they are observed throughout the reaction, even though their yields are small compared to those of carbon dioxide and acrolein. X and Y zeolites exchanged with alkali-metal or alkaline-earth-metal ions have Bronsted acid sites.53 Most industrial applications of zeolites exploit the acid sites for cracking, hydrocracking, and i s o m e r i z a t i ~ n .It~ is ~ ~also ~ ~known that olefins react on the acid sites of zeolites to oligomerize?wz*w The butene isomers and isopentane observed are suggested to come from such dimerization or oligomerization products. C1 and Cz hydrocarbons such as methane, ethylene, and ethane are considered to be products of cracking reactions. Thus, the these initially formed hydrocarbons are considered to arise from the reaction of propylene on acid sites. The subsequent decrease of these hydrocarbons may correspond to poisoning of the acid sites during the reaction. The carbon char peak seen by ESR increased over the first 15-20 min of reaction, and then its intensity was almost constant. This peak is also observed over X and Y zeolites containing no cupric ions. The carbon char is believed to arise from the reaction propylene on the acid sites of the zeolites. Uh et a1.I0 also observed nearly the same hydrocarbons during the initial stages of propylene oxidation on Cu(I1) Y zeolite. They reported that no hydrocarbon was observed in a Cu(I1) A zeolite with no BrBnsted acidity and attributed the formation of hydrocarbons to the Bronsted acidity associated with Y zeolites. This clearly demonstrates that acid sites are not active species for the formation of acrolein. Catalyst deactivation is also observed at low reaction temperature for high cupric ion loading. Cu(2.5)Mg-Y zeolite was
1.
(48) (a) Liengme, B. V.; Hall, W. K. Trans. Furuduy Soc. 1966,62,3229. (b) Weeks, T. J.; Angell, C. L.; Ladd, I. R.; Botton, A. B. J . CUlUl. 1974.33, 256. (c) Hassan. S. M.; Panchenkov, G . M.; Kuznetsov, 0. I. Bull. Chem. Soc. Jpn. 1977,50, 2597. (49) (a) Haber, J.; Komorek-Hlodzik,J.; Romotowski, T. Zeolites 1982, 2, 179. (b) Kiricsi, 1.; Wrster, H. J . Chem. Soc. Furuduy Trans. I 1988,84,
..
A 0, 1 -
(50) Coudurier, G.; Decamp, T.; Praliaud, H. J . Chem. Soc., Furuduy Trans. I 1982, 78, 2661. (51) Kevan, L. Arc. Chem. Res. 1987, 20, 1.
(52) Heilbron, M. A.; Vickerman, J. C. J. Cural. 1974, 33, 434. (53) (a) Ward, J. W. J . Curul. 1969, 14, 365. (b) Ward, J. W. J . Coral. 1971, 22, 237.
(54) (a) Eastwood, S.C.; Drew, R. D.; Hartzell, F. D. Oil Gus J . 1962,
60, 152. (b) Elliott, K. M.; Eastwood, S. C. Oil Gus J . 1962, 60. 142. (55) (a) Demmel, E. J.; Perella, A. V.; Stover, W. A.; Shambaugh, J. P. Oil Gas J . 1966, 64, 178. (b) Ward, J. W.; Hansford, R. D.; Reichle, A. D.; Sosnowski, J. Oil Gas J . 1973, 71, 69.
(56) Breck, D. W. Zeolite Moleculur Sieves; John Wiley & Sons: New York, 1973; Chapter 2.
J. Phys. Chem. 1991,95, 3271-3277 active only at 350 "C or above. With Cu(13.3)Mg-Y, good catalytic activity was observed at temperatures as low as 250 OC. At reaction temperatures lower than these, propylene conversion drastically decreased and the catalyst was quickly deactivated, Slow Cu( 11) migration from the fl-cage at low reaction temperature and the formation of carbon deposits which prevent Cu(I1) from interacting with oxygen inhibit the oxidation reaction.
Conclusions The present work describes copper-species-catalyzed partial oxidation of propylene over copper(I1)-exchanged X and Y zeolites in the presence of different major cocations as a function of the oxygen/propylene mole ratio in a flow system. The yield of acrolein is dependent upon the oxygen/propylene mole ratio, the nature of the major cocation, the reaction temperature, the Si/Al ratio of the zeolite structure, and the extent of Cu(I1) exchange. Y zeolites show slightly better catalytic activity for the formation of acrolein than the corresponding X zeolites. The maximum yield of acrolein was obtained for an oxygen/propylene mole ratio of -1 with CuMg-Y zeolite. The dependence of acrolein and carbon dioxide yields on reaction time and on the oxygen/propylene mole ratio clearly in-
3271
dicate two parallel reaction pathways for partial and complete oxidation of propylene, The correlation of acrolein formation with decreased Cu(I1) observed by ESR suggests that acrolein formation is catalyzed by Cu(I), perhaps in a Cu2O/CuO phase, while complete oxidation to carbon dioxide seems catalyzed by Cu(I I). The formation of acrolein in X and Y zeolites also shows an induction period associated with cupric ion migration in the zeolite lattice and reduction to form an active copper species. The alkali-metal-exchanged X zeolites show longer induction periods than alkaline-earth-metal-exchanged zeolites, and X zeolites show longer induction periods than Y zeolites. Catalyst deactivation is most prominently observed in K-X, H-X, and H-Y zeolites. Deactivation is attributed to the formation of coke which restricts cupric ion migration and/or propylene diffusion.
Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Research Program. Registry No. Cu, 7440-50-8; Mg2+,7439-95-4; Ca2+,7440-70-2; Li+, 7439-93-2; Na+, 7440-23-5; K+,7440-09-7; propylene, 11 5-07-1.
General Approach to the Interpretation of Electrochemical Quartz Crystal Microbalance Data. 1. Cyclic Voltammetry: Kinetic Subtleties in the Electrochemical Doping of Polybithiophene Films A. Robert Hillman,* Marcus J. Swann, School of Chemistry, University of Bristol. Bristol BS8 1 TS, England
and Stanley Bruckenstein* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214 (Received: June 19, 1990)
Electrochemical quartz crystal microbalance (EQCM) data can be used to demonstrate the existence of a global equilibrium during redox state changes in electroactive polymer films. In the absence of a global equilibrium, the rate-limiting process can be associated with the motion of electrons, ions, or neutral species. Criteria are developed for using EQCM data to distinguish between all these possibilities. The criteria necessitate consideration of charge, mass, and a new function, @ (derived from EQCM data), as functions of potential and mass and as functions of charge. A systematic procedure for applying these criteria to cyclic voltammetric studies is given. An illustration of these general principles is provided by their application to the simple case of polybithiophene doping in a single symmetrical electrolyte,tetraethylammonium tetrafluoroborate/CH,CN. In this particular case, the approach to global equilibrium is governed by neutral species transfer.
Introduction Electrochemical quartz crystal microbalance (EQCM) experiments provide a direct measure of changes in the mass rigidly attached to an electrode.'-3 The EQCM experiment measures resonant frequency f + Af, where f is the resonant frequency at the start of a particular experiment, and Af is the instantaneous change in frequency caused by the electrochemical process. Af can be related to the mass attached to the electrode. In particular we consider mass changes caused by mobile species transfer into/out of surface-immobilized redox polymer ~
~~~~
~
( I ) Bruckenstein, S.;Shay, M. Electrochim. Acra 1985, 30, 1295. (2) Kanazawa. K. K.; Gordon, J. G. Anal. Chim. Acra 1985, 175, 99. (3) Deakin, M. R.; Buttry, D. A. Anal. Chem. 1989.61, 1147A. (4) Ward, M. D. J . Phys. Chem. 1988, 92, 2049. ( 5 ) Varineau, P. T.; Buttry, D. A. J . Phys. Chem. 1987, 91, 1292. (6) Ward, M. D. J . Eledrochem. Soc. 1988, 135. 2747. (7) Hillman, A. R.; Loveday, D. C.; Bruckenstein, S. J. Electroanal. Chem. 1989. 274. 157. ( 8 ) Bruckenstein, S.;Wilde, C. P.; Shay, M.; Hillman, A. R. J . Phys. Chem. 1990, 94, 787.
In this paper we present a novel analysis of EQCM data and illustrate its merits through a quantitative study of neutral species transport in polybithiophene (PBT) films. This system superficially appears uncomplicated when customary data treatments are used, but our more sophisticated analysis reveals unsuspected phenom(9) Orata, D.; Buttry, D. A. J . Am. Chem. Soc. 1987, 109, 3574. (IO) Kaufman, J. H.; Kanazawa, K. K.; Street, G. B. Phys. Rev. Lerr. 1984,53,2461. ( 1 1) Baker, C. K.; Reynolds, J. R. J . Elecrroanal. Chem. 1988,251,307. (12) Baker, C. K.; Reynolds, J. R. Synrh. Me?. 1989, 28, C21. (13) Reynolds, J. R.; Sundaresan, N. S.; Pomerantz, M.; Basak, S.; Baker, C. K. J . Elecrroanal. Chem. 1988, 250, 355. (14) Hillman, A. R.; Eales, R. M.; Loveday. D. C.; Swann, M. J.; Hamnett, A.; Higgins, S.J.; Bruckenstein, s.;Wildc, C. P. J . Chem. Soc., Faraday Discuss. 1989, 88, 1 15 1. (15) Borjas, R.; Buttry, D. A. Absrracrs of Papers; 198th American Chemical Society Meeting, Miami Beach, FL; Abstract Number ANYL 0093, 1989. (16) Hillman, A. R.; Swann, M. J.; Bruckenstein, S. J. Elecrmnal. Chem., in press.
0022-3654/91/2095-327 1%02.50/0 0 1991 American Chemical Society
