J. Phys. Chem. 1996, 100, 7191-7199
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Catalytic Transformations of Olefins on HZSM5 Observed by Radiolysis/EPR† E. A. Piocos, P. Han, and D. W. Werst* Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: December 1, 1995; In Final Form: January 30, 1996X
A new method for elucidating elementary reaction steps in zeolite catalysis was demonstrated for reactions of isobutene and other monoolefins on HZSM5. Radiolysis was used to “spin label” reaction products, and EPR was used for product analysis. The evolution of products was revealed by quenching the reactions cryogenically in a series of equilibration experiments. Isobutene dimerization and isomerization occur on HZSM5 even at 77 K, and cracking occurs below room temperature. The radiolysis/EPR results allow a consistent interpretation of the literature on EPR signals that arise spontaneously upon interaction of acyclic olefins with air-activated H-Mordenite and HZSM5.
1. Introduction Zeolites are shape-selective catalysts widely used in the petrochemical industry and derive their catalytic activity from hydroxyl groups that bridge between Si- and Al-substituted tetrahedral lattice sites. Such sites exhibit Brønsted acidity, the strength of which varies with zeolite composition and structure.1 One of the most active zeolite catalysts, because of its strong Brønsted acidity, HZSM5, has found many critical applications in petroleum cracking, benzene alkylation, xylene isomerization, and aromatization of alkanes and alkenes.1-5 Although an adopted mechanistic formalism based on carbocation (pentavalent and trivalent) intermediates qualitatively describes reaction patterns and provides a rationale for the obtained product distributions in solid-acid catalysis in general,6-8 there has been a growing interest in direct experimental studies of reaction mechanisms and intermediates in zeolite catalysis. It is of particular interest to develop in situ spectroscopic techniques, such as infrared absorption, NMR, and EPR, to identify chemical products and/or intermediates involved in zeolite catalysis. Recent work using MAS NMR has demonstrated the need for variable-temperature capability to arrest the catalytic processes at early stages, as transformations can become very advanced after only a short time at ambient temperatures.9,10 For some time it has been known from EPR investigations that interactions of organic molecules with aluminosilicate catalysts can result in formation of paramagnetic species.11-14 In particular, calcining the H-form of ZSM5 or Mordenite to sufficiently high temperatures (g700 K) in an air or oxygen atmosphere converts Brønsted acid sites to Lewis acid sites that are capable of oxidizing adsorbates with low ionization potentials, and often radical cations have been detected which indicate complicated transformations of the starting compound.15-18 In this work we demonstrate the use of EPR to detect and identify radiolytically generated radical cations and neutral radicals of product molecules of hydrocarbon catalysis on HZSM5. That is, radiolysis is used to “spin label” reactant and product molecules by converting them to paramagnetic species. On HZSM5, this occurs by one-electron oxidation to form radical cations and by hydrogen atom addition to form neutral H-adduct radicals. These reactive species can be trapped in †Work at Argonne performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE under contract number W-31-109-ENG-38. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
0022-3654/96/20100-7191$12.00/0
the zeolite channels at low temperatures and observed by EPR. Thus the zeolite functions as catalyst and as isolation matrix. The radiolysis/EPR technique offers many advantages over the spontaneous oxidation technique for studying catalytic processes on zeolites. By using radiolysis, it is possible to label more compounds. The H-adduct radicals, for example, are only encountered in radiolysis experiments. Also, recent experiments with acetylene have shown that the radiolyzed zeolite is capable of ionizing compounds with gas-phase ionization potentials (IP) as high as 11.4 eV.19 More importantly, using radiolysis one can generate the paramagnetic species even at cryogenic temperatures. This is critical because the “labeled” species, especially radical cations, are very reactive, and their detection can be thwarted by reactive encounters with other molecules.20,21 In many cases, spontaneous development of EPR signals on air-activated zeolites only occurs at temperatures where many paramagnetic species would rapidly decay. Furthermore, the spontaneous EPR signal may only develop at the tail end of a complicated sequence of reaction steps. Radiolysis obviates the need to generate Lewis acid sites in the zeolite, and the revealed transformations should better reflect the catalytic activity of the Brønsted acid sites alone. It is of course desirable to address the possible involvement of radical cation intermediates as well. The inability to disentangle Brønsted acid-catalyzed processes from transformations stemming from oxidation and subsequent reactions of radical cations is a weak point of the spontaneous oxidation method, but by using radiolysis one can learn the intrinsic reactivity of radical cations by ionizing the appropriate compound on the inactive Na form of the zeolite. In situ product analysis at low temperatures makes it possible to carry out reaction studies whereby molecules are adsorbed onto the zeolite at a variable temperature and allowed to equilibrate for a given time after which the sample is quenched in liquid nitrogen to prevent any further reaction. Our approach, illustrated here for reactions of isobutene and related hydrocarbons on HZSM5, arrests the catalytic process at various stages and elucidates early steps in the catalysis mechanism. EPR provides superior sensitivity and chemical specificity for identifying the products that develop. Our results for isobutene and related olefins on HZSM5 reveal a pattern of dimerization, isomerization and cracking and a gradual trend toward formation of higher molecular weight species under increasingly severe reaction conditions, i.e., longer time, higher temperature and greater extensive acidity (smaller Si/Al ratio). Substantial convergence, after early differences, © 1996 American Chemical Society
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is obtained for different feed molecules after several reaction steps. Our findings explain the common shape of EPR spectra observed in many different laboratories for C3-C8 olefins on acidic zeolites.22-29 2. Experimental Section The zeolites HZSM5-X (X ) 50, 240, 400) and NaZSM5-X (X ) 170), where X denotes the Si/Al ratio, were kindly donated by Chemie Uetikon of Switzerland. All hydrocarbons were purchased from Aldrich or Wiley Organics. Purification was done by repeated freeze-pump-thaw cycles using liquid nitrogen. Zeolite samples were prepared on a glass vacuum manifold in 4 mm o.d. Suprasil sample tubes. The sample tube containing 50 mg zeolite powder was evacuated (e10-4 Torr) and heated to 450 °C for 4-6 h. A measured amount (0.5-20% by weight) of a given hydrocarbon was adsorbed onto the zeolite at room temperature or below, and then the tube was sealed under vacuum. The sealed samples were equilibrated at temperatures between 77 and 338 K for a specified time and then stored in liquid nitrogen until they were irradiated at 77 K with a 60Co source to a total dose of ∼0.3 Mrad. No EPR signals were detected in unirradiated samples. EPR spectra of the irradiated samples were collected at temperatures between 4 K and room temperature. Spectral changes were tested for temperature reversibility. Temperature control was accomplished with a LTR-3 liquid-helium-transfer Heli-Tran cryostat (APD) anchored in the cavity of the Varian E109 EPR spectrometer operating at microwave frequency of 9.31 GHz. Magnetic field control and data acquisition was accomplished by a LabVIEW (National Instruments) program on a Macintosh II computer.
Figure 1. (a) EPR spectrum of the 2-methyl-2-pentene radical cation obtained at 70 K in NaZSM5-170 containing 2% 2-methyl-2-pentene. The sharp feature marked by * is due to the radiation-induced E′ centers in the cell walls and is a feature common to all spectra. (b) Simulation of a using aiso(CH2) ) 16 G, aiso(CH3) ) 17 G, ax(CH) ) 63 G, ay(CH) ) 60 G, az(CH) ) 45 G. (c) EPR spectrum obtained at 70 K in HZSM5-240 containing 2% 2-methyl-2-pentene.
3. Results Since it is our purpose to use radiolysis to label molecular products of acid catalysis on zeolites, it is essential to demonstrate that the radiolytically generated paramagnetic species, themselves reactive intermediates, are not involved in the transformations. To put it simply, what happens prior to radiolysis and what happens as a consequence of radiolysis? Fortunately, this can be determined in a variety of ways, the most direct way being comparison of results on the acidic zeolite to those on a catalytically inactive zeolite, NaZSM5. As an illustration, Figure 1 shows the EPR spectra observed after radiolysis of 2-methyl-2-pentene on both NaZSM5 and HZSM5. The EPR spectrum observed in NaZSM5 can be assigned to the parent radical cation and was simulated (Figure 1b) using the hyperfine coupling constants (hfcc) listed in the figure caption. Precedent exists for the very large doublet splitting of the HR, which reflects the nonplanar geometry about the double bond.35,36 By contrast, the EPR spectrum observed on HZSM5 exhibits no parent radical cation signal. The spectrum consists of a sharp multiplet, a ) 17.5 G, which is easily identified as the tetramethylethylene radical cation (TME•+),20,32,33 and a multiplet with an even number of broader lines and slightly smaller spacing. Thus radiolysis of the hydrocarbon on NaZSM5 provides one important control experiment to prove that the radiolytic labeling method does not intrude on the chemistry being probed. Similarly, results for isobutene on NaZSM5 reveal no significant chemistry from radiolysis of the parent compound. The dominant EPR signal observed following radiolysis of isobutene on NaZSM5 is the 2-methylallyl radical (Figure 2). The isotropic simulation (Figure 2b) is in very good agreement with the experimental spectrum, except for distortion caused by the
Figure 2. (a) EPR spectrum obtained at 110 K in NaZSM5-170 containing 1% isobutene and (b) simulation of the methylallyl radical EPR spectrum using the hfcc from the literature (Table 1).
presence of an underlying signal due to an unidentified species. The parent radical cation was not trapped at 77 K and is presumably converted into the 2-methylallyl radical by cationmolecule reaction, which is the usual fate of radical cations in the condensed phase.20,21 Evidently, isobutene is still mobile in ZSM5 at 77 K, and the isobutene radical cation does not survive long storage at this temperature. The results for isobutene on HZSM5 are complex and are the primary focus of the remainder of this study. A variety of products could be identified by the radiolytic labeling method. Further evidence that the observed transformations of isobutene are acid catalyzed and not caused by radiolysis is the strong dependence of the product yields on time, teq, and temperature, Teq, of sample equilibration prior to irradiation. Samples were equilibrated at 338, 295, 258, 235, 223 and 77 K. The equilibration times varied from 5 minutes to 16 h (overnight equilibration). Figure 3 shows the result for an HZSM5 sample containing 5% isobutene that was equilibrated overnight at room temperature. This spectrum consists almost entirely of two EPR
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TABLE 1: Isotropic Proton hfcc (G, 1 G ) 0.1 mT) of Radical Cations and Neutral Radicals matrix
T (K)
NaZSM5
70
HZSM5 CFCl3 MCH HZSM5 CF3C(O)OH HZSM5
70-230 77 190 150-230 295 150-230
CCl2FCClF2
103
HZSM5 Xe HZSM5
70 50 190-230
a(CH) a(CH2) a(CH3) ref 56
16
17
a
16 16.4 15
17.5 17.2 17.1 17.5 16.4 17
a 32 33 a 30 a
3.2
31
22 22.5 23
a 34 a
13.8 14.7 32
a This work. b Proposed assignments are R ) methyl, R′ ) isopropyl, or R ) R′ ) ethyl.
Figure 4. EPR spectra obtained at (a) 70, (b) 110, (c) 150, (d) 190, and (e) 230 K in HZSM5-240 containing 1% isobutene. Teq ) 235 K; teq ) overnight.
Figure 3. EPR spectra obtained at (a) 70 K and (b) 150 K in HZSM5240 containing 5% isobutene. Teq ) 295 K; teq ) overnight. (c) Simulation of the broad EPR component in b, a(9H) ) 17 G, a(2H) ) 15 G, assigned to the isobutene dimer radical cation (see text for details).
signals, the sharp multiplet of TME•+ and a set of broader lines that lie nearly equidistant between the TME•+ lines. The broad EPR component possesses an even number of lines and indicates coupling to an odd number of protons. This signal became sharper with increasing temperature (the change was reversible) and at 150 K was approximated by the isotropic simulation shown in Figure 3c calculated with a(9H) ) 17 G, a(2H) ) 15 G. For reasons discussed below, this species is assigned to the isobutene dimer radical cation, (CH3)2CdC(CH3)C3H7•+. Figure 4 shows the EPR results after isobutene was equilibrated overnight on HZSM5 at 235 K and then irradiated at 77 K. The EPR spectrum obtained at 70 K shows a prominent multiplet signal with at least eight lines and a constant 22 G spacing. The hfcc, narrow line width and even number of lines leave no question that this signal belongs to the tert-butyl radical.34 Although other H-adducts have been observed following radiolysis of guest molecules on H-zeolites, the ratio of H-adduct to radical cation can be very large or very small, depending on the adsorbate.20,37-39 In the present case, no isobutene radical cation EPR signal was observed. Therefore, the tert-butyl EPR signal was the only evidence of unreacted isobutene. However, it was not a very reliable measure of
unreacted isobutene because its intensity was not reproducible in similar samples. It is probable, since isobutene radical cations decay via ion-molecule reactions at 77 K, that the tert-butyl radicals also decayed slowly (via recombination) at 77 K. The sample storage time between irradiation and EPR measurements varied from hours to days and was not documented in our experiments. The poor reproducibility of the tert-butyl EPR intensity is attributed to this variation in storage time. Nevertheless, observation of the tert-butyl radical was clearly enhanced by low Teq. The tert-butyl radical signal decayed with increasing temperature and was essentially gone by 110 K. The next large change occurred at 150 K with the growth of the TME•+ signal (Figure 4c). This change was also irreversible. At anneal temperatures above 150 K, the spectrum became more resolved (reversible). At 230 K the EPR spectrum revealed three distinct species, TME•+, isobutene dimer•+ (broad lines) and an Hadduct radical of a coupling product which became more dominant at lower Teq and/or shorter teq. When the sample was annealed to 230 K and above, the EPR signal intensity of all species slowly decayed. The EPR spectrum of the neutral radical of Figure 4e is displayed more clearly in Figure 5, which shows the radiolysis/ EPR results after isobutene was equilibrated on HZSM5 for only 5 min at 258 K. Under these equilibration conditions it was the dominant signal and is seen to be a doublet (32 G) of heptets (23 G). The 23 G heptet splitting strongly identifies this radical as an alkyl-substituted isopropyl radical, (CH3)2C•CHRR′.40 The dependence of the doublet splitting on the twist angle about
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Figure 5. EPR spectra obtained at (a) 150 and (b) 230 K in HZSM5240 containing 1% isobutene. Teq ) 258 K; teq ) 5 min. (c) Simulation of (CH3)2C•CRR′ using the hfcc listed in Figure 1.
Figure 6. EPR spectra obtained in HZSM5-240 containing 1% isobutene at (a) 110 and (b) 110 K after the sample was annealed to 230 K. Teq ) 258 K; teq ) overnight.
the central C-C bond accounts for the broadening of the spectrum at lower temperature. Comparison of Figure 5a to Figure 4b reveals that (CH3)2C•CHRR′ is a significant signal carrier in that spectrum as well. The development of the EPR signals from TME•+ and the dimer radical cation with increasing teq could be observed quite clearly at Teq ) 258 K. Figure 6 shows the radiolysis/EPR result after equilibrating isobutene on HZSM5 overnight at 258 K. As before, the TME•+ signal grows in only when the sample is annealed to ∼150 K. The EPR spectrum at 110 K, before annealing, is primarily a superposition of the EPR signals from (CH3)2C•CHRR′ and isobutene dimer•+. After annealing, the EPR signals from both TME•+ and dimer•+ are enhanced relative to (CH3)2C•CHRR′ (Figure 6b). There is clearly a net increase in the TME•+ intensity. It is difficult to judge if there is a net increase in the dimer•+ signal intensity because of strong overlap with the signal from the H-adduct. The EPR results displayed in Figures 3-6 are representative of the large matrix of data collected for isobutene samples in HZSM5. Above we have assigned four species that are generated by the radiolytic labeling method, two neutral radicals and two radical cations. The tert-butyl radical provided evidence of unreacted isobutene. The H-adduct radical, (CH3)2C•CHRR′, was more dominant at low Teq and/or short teq. It was supplanted gradually by TME•+ and C8H16•+ with increasing Teq and teq. While the two radical cations could be essentially
Piocos et al. “frozen out” by using Teq e 223 K, (CH3)2C•CHRR′ was not erradicated even for Teq ) 77 K. While there was some variation in the TME•+/C8H16•+ intensity ratio, their individual trends were in the same direction relative to (CH3)2C•CHRR′. Except for TME•+, all of the paramagnetic species were detected at 70 K, which indicates that they were formed at the temperature of irradiation (77 K) and did not depend in any way on postirradiation sample annealing. TME•+ was also detected at 70 K for Teq ) 295 K and teq g 30 min, but for milder equilibration conditions the TME•+ EPR signal developed only after annealing the sample. Interestingly, we found for one isobutene sample that equilibrating overnight at 258 K, after 1 h of equilibration at 295 K, also caused the TME•+ EPR signal to be “hidden” at 70 K following radiolysis. We suggest that radiolytic formation of TME•+ depends on the position of some conformational equilibrium (established at Teq and frozen in during fast-freezing in liquid nitrogen) in addition to the TME product yield. The concentration dependence was studied for 0.5-20% isobutene equilibrated overnight at 295 K on HZSM5. In general, an increase in concentration had the same effect as a small decrease in Teq and/or teq. The extensive acidity of the HZSM5, varied by changing the Si/Al ratio, also affected the product distribution for a given set of equilibration conditions. Trials for room-temperature equilibration of isobutene on HZSM5-400, HZSM5-240 and HZSM5-50 showed small differences between the two least acidic zeolites, whereas processes were accelerated on HZSM5-50 to the extent that the resulting EPR spectrum was weak and relatively unstructured. We interpet this to be a sign of further reaction and degradation of the hydrocarbon, similar to superambient equilibration of isobutene on HZSM5-240 (Vide infra). Trials on HZSM5-50 under milder equilibration conditions were not carried out. A variety of reference compounds were studied to try to further illuminate the catalytic processes observed for isobutene on HZSM5. First we will consider C6 olefins, including TME, 2-methyl-2-pentene, 2,3-dimethyl-1-butene, and 3,3-dimethyl1-butene. The results for 2-methyl-2-pentene (Figure 1) and 3,3-dimethyl-1-butene (Figure 7) are representative. The EPR spectra are combinations of signals from TME•+ and a second species with an even number of broader lines and slightly smaller spacing than the TME•+ spectrum. The changes in the broad EPR component with temperature (Figure 7) were reversible. The ratio of intensities, broad-to-sharp, depended on equilibration conditions, with TME•+ dominating at lower Teq and shorter teq, and the broad-to-sharp ratio was smaller in HZSM5-400 compared to HZSM5-240 for identical equilibration conditions. As the control experiment in NaZSM5 proves, TME is formed from 2-methyl-2-pentene prior to radiolysis on HZSM5. This is probably the case for all of the C6 olefins, although the radical cation also isomerizes to TME•+ in some cases and the parent radical cation would not be observed in any event.20 In general, the development of signal intensity in the broad EPR component follows the pattern of buildup of higher molecular weight species in the isobutene samples. The change in the broad-to-sharp ratio in favor of the broad component required relatively more severe conditions for TME itself compared to the other C6 olefins studied. The shape of the broad spectrum was not distinguishably different for the different C6 feed molecules. As models of potential isobutene dimers we ran trials for a few C8 olefins. We encountered difficulty adsorbing C8 compounds into ZSM5 at room temperature. The radiolysis/ EPR result for 2-methyl-3-ethyl-2-pentene on HZSM5 did not indicate any significant adsorption, and 2,3,4-trimethyl-2-
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Figure 8. EPR spectra obtained at (a) 70, (b) 150, and (c) 190 K in NaZSM5-170 containing 2% 2,3-dimethyl-2-pentene. Teq ) 338; teq ) overnight.
Figure 7. EPR spectra obtained at (a) 70, (b) 110, (c) 150, and (d) 190 K in HZSM5-240 containing 1% 3,3-dimethyl-1-butene. Teq ) 295 K; teq ) overnight.
pentene gave a very weak EPR signal with little resemblance to the EPR spectra from isobutene samples. Adsorption of an eight-carbon molecule from the gas phase into the ZSM5 channels is clearly less facile than the intrapore coupling of two four-carbon units. A stronger EPR signal was obtained after radiolysis of 2,5dimethyl-2-hexene on HZSM5-240 (Teq ) 295 K, teq ) overnight; not shown), which exhibited the same sharp (TME•+) and broad EPR components observed in isobutene samples (e.g., Figure 3b). We do not believe that the broad component is due to the parent radical cation because (1) a strong coincidence would be required to give an even number of lines and no resolved doublet splitting due to the HR and (2) charge transfer would easily occur41,42 from the radical cation of 2,5-dimethyl2-hexene (IP ≈ 8.6 eV) to nonionized TME molecules (IP ) 8.27 eV).43 Rather, we conclude that a combination of catalytic isomerization and cracking reactions gives rise to the same two species that were observed in the isobutene samples. Trials were run for one C7 olefin, 2,3-dimethyl-2-pentene, to model the potential isobutene dimer 2,3-dimethyl-2-hexene, which was not commercially available. Equilibration at 338 K on NaZSM5 resulted in a 2-3-fold enhancement of the resulting EPR intensity compared to room temperature equilibration. This result is shown in Figure 8. The hyperfine structure is temperature-dependent because of hindered rotation of the ethyl group. By 190 K the spectrum coalesces into a set of at least 10 equally spaced lines with a spacing of 17.5 G, in close agreement with the liquid-phase EPR spectrum of the 2,3dimethyl-2-pentene radical cation.30 Significantly, no TME•+ signal was observed following radiolysis of this compound in NaZSM5, which discounts the possibility of radiolytic fragmentation of higher molecular weight species to give TME•+. Figure 9 shows the EPR results for 2,3-dimethyl-2-pentene equilibrated overnight on HZSM5-400 at room temperature. The similarity to, for example, the isobutene experiment shown in
Figure 9. EPR spectra obtained at (a) 70, (b) 110, and (c) 150 K in HZSM5-400 containing 2% 2,3-dimethyl-2-pentene. Teq ) 295; teq ) overnight.
Figure 3 is striking. There is a strong TME•+ signal, which indicates catalytic cracking to form the C6 olefin, as was observed for 2,5-dimethyl-2-hexene. The broad EPR spectrum agrees with that of the parent radical cation observed at high temperature in NaZSM5, although the temperature dependence of the hfcc is not identical between 70 K and 150 K in the two matrices. We ascribe this to a matrix effect. The EPR intensity ratio, broad-to-sharp, is larger in HZSM5-400 than HZSM5240sopposite the trend observed for C6 feed molecules. Similar results were also obtained for a C5 olefin feed molecule, 2-methyl-1-butene. Because the contrast of sharp and broad components was muted in some samples, the formation of TME•+ on HZSM5 with this feed molecule only became apparent from variations in the intensities of the alternate lines as a function of loading and equilibration conditions. Finally, all of the C3-C8 feed molecules included in our study (Table 2) gave virtually identical EPR spectra after overnight equilibration on HZSM5 at 338 K. Figure 10 illustrates the
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4. Discussion
4.1. EPR Assignments. The TME•+ EPR spectrum is a prominent marker for the formation of tetramethylethylene. The sharp 17.5 G multiplet was observed on HZSM5 for feed molecules C3 through C8, including TME itself. In addition to dimerization and isomerization, catalytic cracking must occur to form TME from feed molecules other than C3 and C6. That TME•+ is generated from a genuine catalysis product, and is not formed by fragmentation of radical cations of higher molecular weight after radiolysis, is demonstrated by the absence of the TME•+ EPR signal after radiolysis of 2,3-dimethyl-2pentene in NaZSM5. Only the parent radical cation was observed, which conforms with the results of other experiments; i.e., olefin radical cations are not labile with respect to C-C bond scission in zeolite radiolysis.20 The formation of TME is consistent with the reaction types characteristic of trivalent carbenium ions. These are summarized in many reviews and include H-atom and C-atom shifts, polymerization, and β-scission reactions.6-8 Although the intermediates involved in zeolite catalysis are not believed to be free carbenium ions,44,45 the actual intermediates, which are probably hydrogen-bonded complexes, in many respects mimic the reaction patterns of carbenium ions. An unresolved detail in the isobutene results is the delayed development of the TME•+ EPR signal after certain samples were annealed to approximately 150 K. This was more noticeable for mild sample equilibration conditions. If TME is present before sample irradiation, the TME•+ signal is expected to be detectable at once, i.e., at the irradiation temperature, and without annealing. Nevertheless, the fully developed TME•+ intensity did correlate with equilibration conditions. It was possible to virtually eliminate the TME•+ signal at low equilibration temperatures, and room temperature equilibration followed by 77 K radiolysis gave rise to a prompt TME•+ signal without annealing. We propose that the delayed onset of the TME•+ signal upon annealing is caused by delayed charge transfer to the nascent precursor of TME•+ (Vide infra). The EPR multiplet with an even number of lines that was generated together with TME•+ in the isobutene samples on HZSM5 can be assigned to another substituted ethylene radical cation by its similar spacing to that of TME•+. It was formed under similar equilibration conditions that gave rise to TME•+, and it differs from the first coupling product formed from C6 feeds. Therefore, we conclude that it is derived from a dimer and not a trimer product. Other requirements include (1) low IP (the radical cation must coexist with that of TME) and (2) an odd number of hydrogens (9 or more) with nearly equivalent hfcc. The former requirement eliminates all C8H16 isomers with an HR.43 The remaining isomers that could give rise to the observed radical cation EPR spectrum are 1 and 2.
The radiolysis/EPR results for isobutene on HZSM5 reveal a gradual conversion of the monomer to C6, C8, and larger products. The products reflect a combination of oligomerization, isomerization and cracking reactions typical of Brønsted acidcatalyzed processes.6-8 There is no evidence that the radiolytic labeling method intrudes on the chemistry observed; i.e., all of the products are formed by catalysis prior to sample irradiation. Although the neutral radicals and radical cations observed by EPR are representative of genuine catalysis products, they are not necessarily an exclusive representation of all the transformations that occur on the zeolite. An important caveat is that charge-transfer reactions cause the radiolysis/EPR method to be biased toward detection of radical cations of species with lower ionization potentials.41,42 Therefore, the relative EPR intensity may not be a true measure of the relative yield of a given product molecule.
First, we can discount 2. The hfcc of the lone Ηβ of 2•+ in solution is 5.5 G, and the hfcc of the nine methyl hydrogens is 16.4 G.30 Although the configuration dependence of the lone Ηβ hfcc would cause it to be both temperature and matrix dependent, it would have to be on the order of 2 G or less to produce the line widths observed in Figure 3b. Perhaps this is too small to expect. Furthermore, the trial in HZSM5 after adsorbing 2 from the gas phase gave an EPR signal, albeit weak, that did not resemble the signal in question. Our assessment of 1 rests on the results for the close analog, 2,3-dimethyl-2-pentene, which gave rise to an EPR spectrum
Figure 10. EPR spectra obtained at 70 K on HZSM5-240 containing (a) 1% isobutene, (b) 1% 2,3-dimethyl-1-butene, and (c) propene. Teq ) 338 K; teq ) overnight.
TABLE 2: Olefins Included in This Work
remarkable similarity for three cases. The TME•+ signal is vanished, and what is left is an even number of broad lines with a constant spacing of ∼16 G. While this EPR spectrum may be a composite of olefin radical cations of different molecular weight, it bears a strong resemblance to the broad EPR component observed in experiments with C6 olefins equilibrated overnight at room temperature on HZSM5. As in the C6 experiments, the broad EPR spectra resulting from overnight equilibration of the various olefins at 338 K became less resolved with increasing anneal temperature.
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SCHEME 1: Reactions of Isobutene on HZSM5 (Species Observed after Radiolysis Are Shown at the Top)
that closely resembles the broad EPR component of isobutene samples. In solution the 11 Ηβ of 2,3-dimethyl-2-pentene•+ are equivalent,30 and this situation is also approached with increasing temperature in both NaZSM5 and HZSM5. Several observations allow us to equate the radical cation that gives rise to the broad component in HZSM5 (Figure 9) to that in NaZSM5 (Figure 8), i.e., 2,3-dimethyl-2-pentene•+, despite the spectral differences at lower temperatures. First, the intensity of the broad EPR component decreases relative to TME•+ upon increasing the catalytic severity, e.g., by increasing the extrinsic acidity of the zeolite. It is thus derived from a precursor to TME in the catalysis sequence, i.e., the parent molecule. Second, the broad EPR component sharpens with increasing temperature as do the spectra of 2,3-dimethyl-2-pentene•+ in NaZSM5 and the presumed isobutene dimer radical cation in HZSM5. In contrast, the EPR spectra of radical cations of coupling products with more than eight carbons become less resolved with increasing temperature. Finally, TME was found to be more resistant to oligomerization and required superambient temperatures to begin to dimerize. We believe that this behavior extends to other tetraalkyl-substituted ethylenes, such as the isobutene dimer and 2,3-dimethyl-2-pentene. We conclude that the most likely structure of the isobutene dimer radical cation observed on HZSM5 is 1•+. Although it has been tacitly assumed that this species is a radical cation, its EPR spectrum is inconsistent with either an alkyl radical (Vide infra) or an allylic radical. The latter type radical has been erroneously postulated before as a possible assignment of EPR signals generated spontaneously on zeolites in the presence of Lewis acid sites.22,24,25 One could consider an allylic radical such as 3•, formed either by
deprotonation of a dimer radical cation or by H addition to C8H14 derived from a disproportionation reaction involving H2 transfer. This radical has 11 nearly equivalent hydrogens (depending on the rotational dynamics of the ethyl group), but with hfcc in the range 13-15 G, and the allylic H would give a doublet splitting of 3-5 G.46 It therefore does not fit any of the observed EPR signals. Equilibration of isobutene on HZSM5 at 338 K led to another change in the resulting EPR spectrum that reflects a shift to higher molecular weight products. The TME•+ and 1•+ EPR signals were replaced by a new spectrum whose characteristic shape and temperature dependence was shared by the broad EPR component observed when C6 molecules were equilibrated at room temperature and higher on HZSM5. The most likely assignment of this species is a C12 olefin radical cation (not taking into account possible cracking reactions). Again it is possible that an artificially narrow distribution of species in this
mass range is reflected in the EPR spectrum because of charge transfer. The results for all feeds converged to this stage for samples equilibrated at 338 K on HZSM5-240. One species remains to be considered. The alkyl-substituted isopropyl radical with 32 G doublet splitting and 23 G heptet splitting, after eliminating C4 (tert-butyl) and C6 (tetramethylethyl20) candidates, could be assigned to one of the radicals 4•6•. It was generated in isobutene samples even for equilibration
conditions too mild to produce TME•+ and 1•+. Therefore, higher molecular weight species were not considered. Its EPR intensity tracked opposite those of TME•+ and 1•+ as a function of equilibration conditions, and thus we believe it is derived from a different precursor than the dimer radical cation. We conclude that this radical is the H adduct formed from a coupling product, either 5• or 6•, since the precursor to 4• has been shown to give a radical cation upon radiolysis. Although the mechanism of H-adduct formation has not been fully elucidated, many examples show that the relative yield of the H adduct is small or negligible when the radical cation is observed.20,39,47 Conversely, the H adduct is sometimes formed to the exclusion of the corresponding radical cation, as in the present case. Corroboration of the assignment to 5• or 6• by direct adsorption of the parent compounds was not feasible, because adsorption of the C8 molecules from the gas phase was not possible at temperatures sufficiently low to prevent further transformations. An alternative mechanism, addition of tert-butyl radical to unreacted isobutene after radiolysis, can be ruled out because addition would give the isooctyl radical, 7•.48 Reported EPR
spectra for the isooctyl radical exhibit a triplet splitting of approximately 12 G.49 In our study, no alkyl radical with a triplet splitting was observed. Isomerization of 7• to one of the radicals 4•-6• would not be expected, especially not at 77 K.50,51 4.2. Catalysis Mechanism. The results of the equilibration studies involving isobutene reactions on HZSM5 can be summarized approximately as in Scheme 1. There are at least two dimerization products, and the observation of TME•+ demonstrates the occurrence of cracking at very early stages in the catalysis. Whether 2,3-dimethyl-2-hexene and TME are formed sequentially as shown, or in parallel from the same or different precursors is uncertain. The precursor of the paramagnetic species produced in each case is depicted generically as a hydrogen-bonded adsorption
7198 J. Phys. Chem., Vol. 100, No. 17, 1996 complex associated with a Brønsted acid site. It is assumed that all of these species exist at least in equilibrium with such complexes, which constitute the reactive intermediates in the catalytic reactions. Why some species give rise to radical cations and others to H adducts upon radiolysis is not yet understood. Radical cation formation is clearly a hole-trapping process, and we view H-adduct formation as an electron-trapping process (formally equivalent to one-electron reduction of the protonated adsorbate). In most cases one process occurs to the exclusion of the other.39 An alternative H-adduct mechanism, radiolytic liberation of H atoms from bridging hydroxyl or from silanol groups which then undergo addition reactions with olefins, would not be expected to show the observed dependence on adsorbate structure. This issue deserves further study as it may shed light on the nature of the adsorption complexes involved in zeolite catalysis. We propose that the growth of the TME•+ EPR intensity upon annealing of certain isobutene samples is somehow related to a change in conformation or shift in the position of the equilibrium between the adsorbed and desorbed TME precursor, and that such change is necessary to allow hole transfer from the matrix to TME. This subtle effect was not observed when TME was generated from other feed molecules. Our conclusion that TME is a genuine catalysis product is further supported by the detection of TME•+ when olefins are adsorbed on air-activated zeolites (Vide infra). Our conclusions are in general agreement with previous studies of oligomerization of small olefins on HZSM5 by NMR,10,52 thermogravimetric analysis,53 and temperatureprogramed desorption.54 Using MAS NMR, Lazo et al. observed dimers of isobutene on HZSM5 at temperatures as low as 143 K and found an onset for trimerization at about 273 K.10 The work of van den Berg et al. also estimated an average size range of C8-C12 for reaction of various small olefins on HZSM5 at 300 K.52 Neither of these groups found cracking to be important at low temperature, however, and van den Berg et al. report the onset for cracking is 400 K.53 There is some disagreement between the NMR studies and our radiolysis/EPR results regarding the structural assignments of oligomeric species. A strong preference for linear structures in reactions at room temperature is cited in one case,52 a tendency not observed in our study. Reasons for the disagreement might include both the superior sensitivity and structural specificity of EPR vs NMR and the previously mentioned charge-transfer bias of the EPR technique. Several authors have discussed the spontaneous development of EPR signals on activated H-Mordenite exposed to small olefins.22-27,29 Another channel-type zeolite, Mordenite is second only to ZSM5 in Brønsted acid strength and exhibits similar catalytic activity.1 While opinions were varied whether the observed EPR signals in Mordenite reflected one or multiple species or were due to neutral radical or radical cation species, the appearance of the EPR spectra obtained in each study was quite similar and was not strongly dependent on the olefin. The hyperfine structure consisted of 15 or more lines with an average spacing of 8 G. Although there was not a strong alternation in line widths in the H-Mordenite EPR spectra, Lange et al. measured a slight asymmetry which they interpreted as possible evidence that the spectrum was the superposition of EPR signals with both even and odd numbers of hyperfine components and each with an approximate spacing of 16 G.29 The first work to assign one of the EPR components generated from olefins adsorbed on H-Mordenite to the radical cation of TME was by Ichikawa et al.27 They proposed that a second species is the TME dimer radical cation (π-complex), a ) 8 G,
Piocos et al. formed by cation-molecule reactions of TME•+. Although the dimer radical cation is not formed in ZSM5,20 perhaps it is formed in the relatively larger Mordenite channels (the largest channels in Mordenite are 6.5 × 7.0 Å, compared to 5.3 × 5.6 Å in ZSM5).52 However, it is doubtful that (TME)2•+ can fully account for the EPR intensity at the half-spacing positions judging from the comparison of EPR spectra obtained on H-Mordenite with adsorbed butenes and TME, respectively.27 One work stands out from the rest of the EPR studies in H-Mordenite. Roduner et al. observed the EPR spectrum of the 2,5-dimethyl-2,4-hexadiene radical cation after absorption of isobutene or 2,3-dimethyl-1-butene on H-Mordenite at room temperature.16 This result implies a disproportionation reaction involving H2 transfer, in addition to dimerization, isomerization and cracking. Of all the H-Mordenite works, these authors used the highest calcining temperature (870 Ksthis is 150 degrees higher than any previous experiment) when activating the Mordenite. This may account for the different outcome in their experiment. Development of any significant concentration of the hexadiene product would dictate against detection of the monoolefinic radical cations because of charge transfer to 2,5dimethyl-2,4-hexadiene (IP ) 7.67 eV).43 Clearly, there is strong similarity between the EPR spectra that develop from olefins adsorbed on air-activated H-Mordenite (calcined at 670-730 K) and combinations of EPR signals of radical cations observed in the present radiolysis/EPR study. (As expected, the H-adduct radicals are only formed by radiolysis.) It can reasonably be expected that there is considerable overlap between the radical cations observed in both types of experiment. Finally, the resemblance between our radiolysis/ EPR results and the EPR signals that develop when isobutene is adsorbed on air-activated HZSM5 is incontestable. This experiment was carried out by Slinkin et al.28 The chief difference between the H-Mordenite and HZSM5 EPR spectra, apart from slight matrix shifts in hfcc, is that the TME•+ spectrum is sharper in the latter, and thus the contrast in line width of the EPR components with even and odd numbers of lines is obtained. The assignments made by Slinkin et al.28 do not stand up in light of the radiolysis/EPR work. On the basis of our results, the EPR signals in their HZSM5 study and those observed in H-Mordenite can all be interpreted as composite spectra of two or more radical cations, including TME•+ and radical cations of larger, C8-C12, branched olefins, the principal one being 1•+ as discussed above. The gradual increase in the broad-to-sharp intensity ratio observed by Slinkin et al.28 is due to loss of TME and buildup of higher molecular weight species with time. As shown in our radiolysis experiments, the radical cation EPR signals rapidly decay at room temperature. In contrast to the radiolysis technique used here, the spontaneous oxidation of adsorbates by the activated zeolite involves the continuous generation and decay of radical cations. Therefore, the EPR spectrum observed by spontaneous oxidation changes with time and reflects the changing product distribution. Slinkin et al.28 also failed to equate an identical multiplet EPR signal generated from 2,4,4-trimethyl-1-pentene to that observed when isobutene was adsorbed on HZSM5. That is, they also generated TME•+ from this isobutene dimer, which reflects a combination of the isomerization and cracking reactions that have been characterized in our radiolysis/EPR study. Their result is comparable to ours for 2,5-dimethyl-2hexene and 2,3-dimethyl-2-pentene. Thus with the new understanding provided by the radiolysis/ EPR technique, a fully consistent explanation has been determined for this long-standing problem of EPR signals generated
Catalytic Transformations of Olefins on HZSM5 from small olefins adsorbed on acidic zeolites. The observed paramagnetic species give an important, if not comprehensive, view of the catalytic transformations that occur on these widely used catalysts. 5. Conclusions The radiolysis/EPR technique is a promising method for elucidating mechanisms of zeolite catalysis. For isobutene on HZSM5, dimerization and isomerization occur even at cryogenic temperatures. Some cracking occurs below room temperature, and the onset for trimerization is above room temperature. Our results require a reassessment of previous EPR studies of radicals that form from olefin reactions in the presence of Lewis acid sites on dehydroxylated H-Mordenite and HZSM5, and which may be involved in processes of aromatization and coking.29 The EPR signals observed in such systems are composite spectra of radical cations, including TME•+ and radical cations of larger, branched olefins. A charge-transfer bias, favoring detection of species with lower ionization potential, may be partly responsible for the similarity of results for different feed molecules; however, a large degree of convergence of the hydrocarbon composition, irrespective of the starting material, is highly probable after equilibration at temperatures much above room temperature. This view is supported by our study. Acknowledgment. We acknowledge Chemie Uetikon (Switzerland) for the gift of the zeolites, J. Gregar for constructing the glass vacuum manifold and supplying the EPR tubes, and A. Svirmickas for carrying out the 60Co irradiations of our samples. References and Notes (1) Gajda, G. J.; Rabo, J. A. Acidity and Basicity of Solids; Fraissard, J., Petrakis, L., Eds.; Kluwer Academic Publishers: Netherlands, 1994; p 127. Corma, A. Chem. ReV. 1995, 95, 559. (2) Kerr, G. T. Sci. Am. 1989, July, 100. (3) Ribeiro, F. R.; Alvarez, F.; Henriques, C.; Lemos, F.; Lopes, J. M.; Ribeiro, M. F. J. Mol. Catal. A 1995, 96, 245. (4) Derouane, E. G. Catalysis by Zeolites; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 5. (5) Derouane, E. G. Catalysis on the Energy Scene; Kaliaguine, S., Mahay, A., Eds.; Elsevier: Amsterdam, 1984; p 1. (6) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979; Chapter 1. (7) Poutsma, M. L. Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; American Chemical Society: Washinton, D. C., 1976; Chapter 8. (8) Jacobs, P. A.; Martens, J. A. Stud. Surf. Sci. Catal. 1991, 58, 445. (9) Haw, J. F.; Richardson, B. R.; Oshiro, I. S.; Lazo, N. D.; Speed, J. A. J. Am. Chem. Soc. 1989, 111, 2052. (10) Lazo, N. D.; Richardson, B. R.; Schettler, P. D.; White, J. L.; Munson, E. J.; Haw, J. F. J. Phys. Chem. 1991, 95, 9420. (11) Dollish, F. R.; Hall, W. K. J. Phys. Chem. 1967, 71, 1005. Sagert, N. H.; Pouteau, R. M. L.; Bailey, M. G.; Sargent, F. P. Can. J. Chem. 1972, 50, 2041. (12) Vedrine, J. C.; Auroux, A.; Bolis, V.; Dejaifve, P.; Naccache, C.; Wierzchowski, P.; Derouane, E. G.; Nagy, J. B.; Gilson, J.-P.; van Hoof, J. H. C.; van den Berg, J. P.; Wolthuizen, J. J. Catal. 1979, 59, 248. (13) Kurita, Y.; Sonoda, T.; Sato, M. J. Catal. 1970, 19, 82. (14) Corio, P. L.; Shih, S. J. Catal. 1970, 18, 126. Corio, P. L.; Shih, S. J. Phys. Chem. 1971, 75, 3475. (15) Rhodes, C. J. J. Chem. Soc., Faraday Trans. 1991, 87, 3179. (16) Roduner, E.; Wu, L.-M.; Crockett, R.; Rhodes, C. J. Catal. Lett. 1992, 14, 373.
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