Anal. Chem. 1989, 6 1 , 1821-1825 (22) Luffer, D. R.; Galante, L. J.; DavM. P. A,; Novotny, M.; Hieftje, G. M. Anal. Chem. 1988, 60, 1365. (23) Woolley, C . L.; Tarbet, B. J.; MarkMes, K. E.; Bradshaw, J. S.; Bartle, K. D.; Lee, M. L. HRC CC , J . H@h. Resoluf Chromatcgr . Chroma tcgr. Commun. 1988, 1 1 , 113. (24) Lee, M. L.; Xu, B.; Huang, E. C.; Djordjevic, N. M.; Tuominen, J. P.; Chang, H.G. K.; Markldes, K. E. J . Mlcrocol. Sep. 1989, 1 , 7. (25) Beenaker, C. I . M. Specfrochlm. Acfa 1977. 328, 173.
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(26) van D a b , J. P. J.; de Lezenne Coulander, P. A,; de Galan, L. Anal. Chim. Acfa 1977, 9 4 , 1.
.
REXEXVED for review October 21,1988. Accepted May 15,1989. This work was funded by the State of Utah, Centers of Excellence Program.
Spin Dynamics in the Analysis of Carbonaceous Deposits on Zeolite Catalysts by Carbon- 13 Nuclear Magnetic Resonance with Cross Polarization and Magic-Angle Spinning Benny R. Richardson and James F. Haw* Department of Chemistry, Texas A&M University, College Station, Texas 77843
The potentlal of solid-state 13C nuclear magnetlc resonance spectroscopy with cross poladzatton and magbangle splnnlng (CPIMAS) for the characterization of the carbonaceous deposits that form on zeolite catalysts during hydrocarbon processing Is explored. Partlcular attentlon Is given to the extent to which the results can be regarded as quantitative. The samples consldered In thls lnvestlgatlon were prepared by reactlon of butadiene on catalyst pellets containing zeolite HY In a flow reactor at temperatures between 150 and 600 OC. The NMR studies performed Included relaxation measurements to characterlze the spin dynamics relevant to quantltatlon and a variable-temperature 13C CP/MAS experlment. Comparlson of carbon spin counting results wlth carbon content from combustlon analysis revealed that atthough 78 % of the carbon was detected by NMR for the sample obtalned from the 150 OC reactor run, slgnlflcantly less carbon was detected for samples coked In the reactor at higher temperatures. Thls resuit correlated with the observation of organlc free radicals In the samples, but It could also be due In part to lnefflclent cross polarlratlon In hydrogen-deflclent regions.
INTRODUCTION Aluminosilicate catalysts such as zeolites are used in a number of important industrial processes including the cracking of fuel oil to yield gasoline-range products, hydrocarbon synthesis from methanol, and a number of isomerization and disproportionation reactions (1-3). A limiting factor in aJl of these processes is the formation of carbonaceous deposits (termed coke), which eventually deactivates the catalyst and necessitates regeneration (4-6).It has been stated that coke formation is one of the least understood phenomena in catalytic cracking (7). The chemical structure of coke deposits and the mechanisms by which they form have been, therefore, of considerable interest. As a result of the low solubility of coke deposits, especially those formed a t higher temperatures, most efforts to characterize the deposits have focused upon the coked catalyst particles themselves, without a prior attempt to separate the carbonaceous material from the inorganic catalyst and/or binder, although degradative methods involving either acid
* Author
to whom correspondence should be addressed. 0003-2700/89/0361-182 1$0 1.50/0
(8,9) or base (6)digestion of the catalyst matrix have also been proposed. Techniques previously applied to the characterization of coked catalyst samples have included elemental analysis (IO),electron microscopy and X-ray diffraction (11), and IR spectroscopy (12,13). There have also been several previous studies that have reported 13C CP/MAS spectra of carbonaceous residues on zeolites. For example, Derouane and co-workers have studied the residues formed in zeolites H-ZSM-5 and mordenite during the reactions of I3C-enriched methanol or 13C-enriched ethylene (14). The entrapped reaction products observed in that study were predominantly low molecular weight alkanes and simple alkylaromatics such as ethylbenzene and were therefore not properly termed carbonaceous deposits. Carlton and co-workers published 13C CP/MAS spectra of coked ZSM-5 samples that had each been subjected to one of several reactivation procedures (15). Weitkamp and Maixner studied the residues formed at relatively low temperatures by isobutanejbutene alkylation on a NaNH,Y zeolite (16). That study reported an increase in aromatics as the reaction temperature was increased from 80 to 314 OC. In none of those studies was there an investigation of the optimum conditions for the study of coke deposits on zeolites by 13CCP/MAS NMR, nor was there an investigation of the more complex deposits that are known to form on acidic Y zeolites at elevated temperatures (7, 8). At first glance, the application of I3C CP/MAS NMR to the characterization of coke deposits on oxide catalysts might appear to be a straightforward task. The experience of workers familiar with the application of CPjMAS NMR to coals (17, 18), lignins (19),and other complex carbonaceous materials (20-23), however, suggests that analogous studies of coke deposits be approached cautiously, especially if quantitative results are important. The cross polarization experiment is prone to errors in quantitation, especially for samples that are hydrogen deficient. Furthermore, the presence of paramagnetic sites can complicate efforts at quantitation by severely broadening resonances due to carbons in the vicinity of radical sites or by adversely affecting relaxation phenomena central to cross polarization dynamics. In this contribution, we have taken one catalyst (zeolite HY) and coked it with a single feed (butadiene) a t six different temperatures under otherwise identical conditions. We have then performed the detailed measurements necessary to evaluate the reliability of 13C CPjMAS NMR for these samples. We find that the 13C CPjMAS spectral intensities de0 1989 American Chemical Society
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termined for these samples are sufficiently quantitative to be useful, especially for samples coked a t the lower temperatures.
EXPERIMENTAL SECTION Sample Preparation. Type Y molecular sieves were obtained from Strem as extruded pellets consisting of 80% zeolite (in the sodium form as received) and 20% inorganic binder. The Si:A1 ratio of the zeolite component was 2.4, and the pore volume was reported to be 0.6 cm3/g. The catalyst was converted to the acidic HY form by ion exchange with 0.5 M ",NO3 followed by activation in a quartz flow reactor at 500 "C under a flow of dry
N2. Five grams of activated catalyst pellets was used in each reactor run. Following activation, the reactor temperature was changed to the desired reaction temperature, and 110 cm3/min of 1,3-butadiene (Matheson) was metered into the nitrogen flow stream, which was kept at 150 cm3/min. The reaction was allowed to proceed under these conditions for 1h, after which the sample was cooled under a N2 purge prior to grinding and storage in a glovebox under N2. Samples were prepared at reaction temperatures of 150, 200, 300, 400, 500, and 600 "C. Sample Characterization. Except where stated otherwise, all NMR experiments were performed on a Chemagnetics M-100s spectrometer operating at a 13C frequency of 25.02 MHz. Magic-angle spinning was carried out at a rate of 3-4 kHz. Kel-F spinners were loaded with approximately 0.4 g of sample in a glovebox under dry N2 gas. Chemical shifts were referenced externally to hexamethylbenzene and are reported relative to tetramethylsilane. Pulse delays of 1 s and contact times of 2 ms were used in all cross polarization experiments except for selected relaxation measurements. In general, 40000 scans were acquired per spectrum. Proton spin-lattice relaxation times (Tl(H))were determined by using the method described by Maciel et al. (24). Cross polarization time constants (TCH) were determined from variable-contact-time experiments, and rotating frame proton spin-lattice relaxation times (Tl,(H)) were determined by holding the proton magnetization in a spin-lock field for a variable delay prior to cross polarization. Generally, seven or more data points were used in all relaxation measurements. Carbon spin-counting experiments were performed by using the method previously described by Hagaman (20). Essentially, this method involves measuring spectra of a weighed amount of the sample of interest mixed with a known quantity of glycine. Integrated intensities, corrected for spectral overlap from the methylene carbon of glycine and Tl,(H) values provide a measure of the weight percent of NMR-observable carbon in the sample, which is then compared with the weight percent carbon determined by combustion analysis. Variable-temperature 13CCP/MAS experiments (25)were also performed on the 150 "C coked zeolite sample by using methods previously described (26). Electron spin resonance (ESR) measurements were performed on a Varian E-6S spectrometer. The spectrometer was standardized for quantitative measurements with a standard sample free radical (Aldrich). The of a,y-bis(dipheny1ene)-fl-phenylallyl standard material was diluted with dicarboxy-terminated polystyrene to a spin concentration of 2.02 X 1019spins/g. Standardization was performed at liquid nitrogen temperatures. All ESR sample spectra were also collected at liquid nitrogen temperatures, under vacuum, to minimize extraneous signals from molecular oxygen. Weight percent carbon was determined for all coked catalyst samples by combustion analysis using a Perkin-Elmer Model 240 elemental analyzer that was standardized with acetanilide. Weight percent hydrogen was also determined on that instrument, but blank determinations on uncoked zeolite samples gave variable percent hydrogen results that, in some cases, were comparable to those for the coked catalyst samples. Large variations were also observed for the coked catalyst samples. These problems were attributed to variable sorption of atmospheric water during sample loading. Therefore, the weight percent hydrogen data were concluded to be unreliable and are not reported.
RESULTS AND DISCUSSION Other solid-state NMR studies have shown that simple olefins such as propene (27) undergo rapid acid-catalyzed reactions on zeolite HY a t rmm temperature to give long-chain
I
I
I
I
loo
1
I
ppm spectra of samples of HY catalyst coked at various temperatures with 1,3-butadiene. The asterisks denote spinning sidebands. All spectra were obtained at room temperature. 200
Figure 1. 25.02-MHz
0
CP/MAS NMR
oligomeric products with primarily aliphatic carbons and also some olefinic functionality. Furthermore, a number of reaction pathways to aromatic products are available to hydrocarbon species on acidic catalysts a t the elevated temperatures used in this study (6, 28), and previous studies of coke deposits formed a t elevated temperatures have established their aromatic or even graphitic nature ( 9 , I I ) . Butadiene is expected to be even more reactive than propene, in accordance with the predicted order of stability of reactive intermediates. In addition to acid-catalyzed reactions, butadiene can also undergo thermally activated Diels-Alder cyclodimerization at temperatures of 350 OC or higher (28). Clearly, a complex spectrum of reaction pathways is available to butadiene and oligomeric coke precursors under the reaction conditions used in this study. The mechanisms by which butadiene reacts on zeolite catalysts will not be considered in this contribution; the focus of the present investigation is to ascertain the extent to which 13C CP/MAS spectra of coked catalyst samples can be regarded as quantitative. 13C CP/MAS spectra of the carbonaceous deposits formed by reaction of butadiene on zeolite HY catalyst at six different temperatures are shown in Figure 1. To a f i t approximation, these spectra are characterized by two broad spectral features: an aliphatic carbon resonance band (10-50 ppm) and an aromatic carbon resonance band (110-160 ppm). Olefinic carbons, if present, could also contribute to the latter band. The possibility of improving spectral resolution by operating a t a higher static magnetic field strength was assessed by obtaining a 13C CP/MAS spectrum of the sample coked a t 150 "C on a Bruker MSL-300 spectrometer a t a 13Cfrequency of 75.47 MHz (Figure 2). Only a very modest improvement in resolution was observed, in accordance with established ideas about spectral resolution in 13C MAS NMR (29). Inspecting the spectra in Figure 1, one notes that the relative intensity of the aromatic band is strongly dependent on the reactor temperature. The relative fraction of aromatic
ANALYTICAL CHEMISTRY, VOL. 61, NO.
I
300
200
loo
PPm
asterisks denote spinning sidebands in the hlgh-field spectrum.
Table I. Percent Carbon Analysis and Apparent Aromaticities for Samples of HY Catalyst Coked with Butadiene at Various Temperatures
7% carbon' (by wt)
150 200 300 400 500 600
13.7 15.3 15.1 18.2 21.2 27.0
aromaticity,b% 31.5 44.3 50.7 57.2 >90 >90
Determined from the a Determined by combustion analysis. inteerated intensities of the NMR sDectra in Figure 1. ~
~~~
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I
0
Flgure 2. 13C CPlMAS NMR spectra of the sample of HY catalyst coked at 150 "C, obtalned at (a) 25.02 and (b) 75.47 MHz. The
coking temp, "C
17, SEPTEMBER 1, 1989
~
carbon signal intensity in I3C NMR spectra of coals and analogous samples is described quantitatively by the apparent aromaticity, which is calculated from the ratio of the integrated area of the aromatic peak to that of the total integrated carbon intensity. Apparent aromaticity values for the six coked catalyst samples are reported in Table I. The coke deposit formed at 150 "C has an apparent aromaticity of 0.31, and this ratio increased monotonically to approximately 0.90 for reactor temperatures of 500 "C or greater. The degree to which these apparent aromaticity values can be accepted as accurate will be discussed later in this contribution. The sample from the reactor run a t 150 "C was subjected to Soxhlet extraction using first methylene chloride and then toluene. The 13C CP/MAS spectrum of the sample following extraction was identical with that obtained prior to extraction. The failure of extraction to remove any significant fraction of the carbon deposit was confirmed by mass spectra of the concentrated extracts, which had insufficient signal-to-noise ratio for the detection of molecular ion peaks. These results suggest that the carbonaceous deposits are not simply low molecular weight products but are instead high molecular weight species that are either insoluble or are too large to be extracted through the pores of the zeolite. The presence of species with condensed aromatic rings in coked catalyst samples obtained from reactor runs a t higher temperatures can be inferred from the interrupted-decoupling (dipolar dephasing) spectra (30) reported in Figure 3. The interrupted-decoupling spectrum of the sample coked at 500 "C combined with chemical shift assignments strongly suggests that much of the carbon in this sample is at bridgehead sites in
Flgure 3. 25.02-MHz interrupteddecoupling spectra
(T
= 50 ps) of
HY catalyst coked with butadiene at various temperatures. The asterisks denote spinning sidebands. All spectra were obtained at room temperature.
highly condensed aromatic species. An important criterion for obtaining a quantitative response in cross polarization spectra is that it must be possible to choose a cross polarization contact time (t,) that satisfies the inequalities
for all of the signals. TCHis the time constant for transfer of magnetization from protons to 13C. Each 13Cenvironment will have its own characteristic TcH value, so in spectra of complex materials such as coked catalysts, the growth of any particular spectral feature will generally be a sum of contributions from different species with approximately the same chemical shift. The time constant for proton spin-lattice relaxation in the rotating frame, Tl,(H), characterizes the loss of spin-locked proton magnetization. For homogeneous, diamagnetic solids such as pure polycrystalline compounds, proton spin diffusion equalizes the proton relaxation time constants to common values for all protons in the sample, regardless of chemical environment. Such solids thus have a single Tl,(H) and a single Tl(H) for all protons. If a uniform Tl,(H) is observed, then the right side of the above inequality can be relaxed somewhat as the decay of spin-locked proton magnetization will affect all 13C cross polarization signals equally. It has previously been demonstrated (17,31)however that complex carbonaceous materials such as coals can have some degree of heterogeneity, which is reflected in small differences between T,,(H) values determined from the aromatic and aliphatic carbon signals, respectively. Measuring the time constant for proton spin-lattice relaxation in the laboratory frame, Tl(H),is also important for establishing the degree to which a 13C CP/MAS experiment is quantitative, since heterogeneous 'materials can also display a distribution of Tl(H) values, and a too-short pulse delay will result in the preferential saturation of the protons which relax more slowly. The results of these relaxation measurements are summarized in Table 11. These results are generally consistent with efficient proton spin diffusion in the samples studied, although the Tl,(H) data for the sample coked at 300 "C are suggestive of some degree of heterogeneity over the time scale of that measurement. From the results in Table 11, it is apparent that accurate measurements of the relative intensities of observable aromatic and aliphatic carbons can be obtained with cross
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Table 11. Summary of Relaxation Data for Samples of HY Catalyst Coked with Butadiene at Various Temperatures
TcH,ms
T,,(H), ms
Tl(H), ms
0.679 0.123
10.1 10.5
304 291
0.233 0.083
16.6 9.6
242 277
0.341
5.2
71
-loooc
150 "C run
aromatic signal aliphatic signal 300 O
C
run
aromatic signal aliphatic signal 500 "C run aromatic signal
n
23'C
Table 111. '*C CP/MAS Spin-Counting Results for Samples of HY Catalyst Coked with Butadiene reactor temp, "C
% carbon
150 300 500
69.5 42.9 31.3
% carbon,
correcteda
78.2 57.0 42.7
L
!
I
Corrected for the differences in T1,(H) values (ref 31) between the internal standard (glycine) and the coke sample.
100 0 PPm Figure 4. Variable-temperature I3C CP/MAS NMR spectra of the sample of HY catalyst coked at 150 "C. Spectra were recorded at 23 and -100 'C. The asterisks denote spinning sidebands.
polarization time constants of approximately 2 ms and pulse delays of greater than or equal to 1 s. But it is known that in complex carbonaceous materials such as coal a significant fraction of the total carbon can be unobservable in cross polarization experiments (20, 21). For coals, unobservable carbon is believed to be the result of two different problems. Carbon in hydrogen-deficient domains (e.g., graphitic phases) will have minimal dipolar coupling to remote protons and hence will not cross polarize efficiently. Secondly, a fraction of the carbon in coals is believed to be near free radical sites which either broaden the resonances due to nearby 13Cnuclei beyond detectability or greatly reduce the T,,(H) in the vicinity of the radical. Since pseudographitic structures have been proposed for some coked catalyst samples (9) and previous ESR measurements have shown that organic free radicals can exist in coke deposits (32,33),both possibilities must be considered for our samples. The spin-counting procedure of Hagaman and co-workers (20) was used in order to quantify the fraction of detectable carbon in selected coked-catalyst samples. Basically, this procedure involves measuring a 13C CP/MAS spectrum of a weighed quantity of the sample of interest co-ground with a weighed quantity of a suitable quantitation standard, glycine in this case. From the weight percent carbon (from combustion analysis of the coked catalyst) and the integrated peak intensities, it is possible to calculate the fraction of detectable carbon. The integrated intensities used in the calculations were corrected for their T,,(H) values. Samples from three reactor runs were selected for 13C spin counting. The results (Table 111) show that 78% of the carbon was observed for the sample from the reactor run a t 150 "C, while a lower percentage of carbon was observed for the 300 "C sample (57%). This trend continued for the sample coked at 500 "C, for which only 43% of the carbon was observed. These values are similar to those found in analogous spin-counting experiments on coals of various rank (31). The similarities between the above results and those from analogous experiments on coals motivated an examination of the ESR spectra of several of the coked catalyst samples. Quantitative ESR spectroscopic measurements (34) revealed that the sample coked a t 150 "C had a free-radical content of 9.5 X lo'* spins/g while that which was coked a t 500 "C had a radical content of 7.3 X 1020spins/g. These values are similar to those obtained for coals of varying rank (35) and account, at least in part, for a fraction of unobserved carbon in the 13C CP/MAS spectra.
Of the possible sources of unobserved signal intensity in 13C CP/MAS spectra, the presence of both hydrogen-deficient graphitic domains and organic free radicals would be expected to result in inaccurately low estimates of the fraction of aromatic carbons. To test this possibility, a Bloch decay spectrum of the sample from the 200 "C reactor run was obtained by using 90" 13C pulses (8600 scans). A pulse delay of 10 s was used to ensure that saturation of the 13C magnetization did not occur (previous studies on coals suggested that this delay value would be sufficient). The resulting spectrum (not shown) was similar to the cross polarization spectrum of that material (Figure 1)with the exception that the aromaticity determined from the Bloch decay spectrum (0.49) was slightly higher than that from the cross polarization spectrum (0.44). This result is consistent with the view that aromatic carbon is more likely to be underestimated in cross polarization spectra of coked catalyst samples than aliphatic carbon as a result of inefficient cross polarization in graphitic domains as well as the presence of free radicals. The above sources of uncertainty in the quantitative significance of 13C CP/MAS spectra of coked catalyst samples are similar to those encountered in analogous studies of coals and other complex carbonaceous materials. There is, however, an additional source of uncertainty that might be encountered for coked catalyst samples. Molecular motion of oligomeric compounds on or in the catalyst framework (e.g., in the supercages of a Y-type zeolite) could result in an attenuated signal due to inefficient cross polarization dynamics, or a very broad resonance for mobile components in the special case (36)of molecular motion at the rotating-frame precessional frequency of the 'H spin-lock field (7B1 = 45 kHz in this experiment). The possibility that these effects might have a significant effect on the 13C CP/MAS spectra of coked catalyst samples was investigated by obtaining a low-temperature spectrum of the sample coked a t 150 "C, which was selected on the premise that it was more likely to contain mobile, oligomeric species than samples coked a t higher temperatures. Indeed, the somewhat long TCHvalue for the aromatic carbons in this sample (Table 11) is suggestive of some degree of molecular motion. Figure 4 compares the 13C CP/MAS NMR spectrum obtained at -100 "C with the room-temperature spectrum. No significant differences are observed between the two spectra. In particular, the aromaticities obtained from the two spectra are identical within experimental error when contributions from spinning sidebands are taken into account for the low-temperature spec-
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17. SEPTEMBER 1, 1989
trum, which was obtained a t a slower spinning speed. We conclude that either the oligomeric coke molecules do not possess enough molecular motion in the sample matrix over the temperature range studied to effect the appearance of 13C CP/MAS spectra or the distribution of correlation times for molecular motion is so broad as to preclude a noticeable appearance in the spectral band shapes.
CONCLUSIONS The coke deposit formed by the reaction of butadiene on zeolite catalyst HY has an aromaticity that is strongly dependent upon reaction temperature. Samples coked a t 500 "C and above are almost completely aromatic and/or graphitic. Since negligible organic matter was extractable from even the sample prepared with the lowest reactor temperature (precluding high-resolution solution-state NMR studies), the application of 13C CP/MAS NMR to the characterization of coked catalysts is appropriate. The results of this study indicate that 13C CP/MAS spectra of coked catalyst samples can be regarded as quantitative, especially for samples prepared at lower reactor temperatures. Cross polarization contact times of 2 ms and pulse delays of 1 s are generally acceptable for accurate measurement of relative signal intensities of observable carbons. For samples coked at higher temperatures, however, the fraction of aromatic carbon will be underestimated due to the presence of organic free radicals and hydrogen-deficient regions. Molecular motion of oligomeric coke molecules in the catalyst is apparently not s i g nificant enough in these samples to visibly affect the spectral band shapes or relative intensities.
LITERATURE CITED Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalyiic Processes; McGraw-HIII: St. Louis, MO 1979. Satterfield, C. N. Heterogeneous Catalysis in Practice ; McGraw-Hill: St. Louis, MO 1980. Venuto, D. B.; Habib, E. T. Fluid Cata/ytic Cracking with Zeolite Catalysts; Marcel Dekker: New York, 1979. Wolf, E. E.; Aifanl, F. Catal. Rev. Sci. Eng. 1982, 2 4 , 329-371. Derouane, E. G. I n Proceedlngs of the International Symposium on Catalysis by Acids and Bases; Imeilk, B. et al., Eds.; Elsevier Scientific: New York, 1985; pp 221-240. Appleby, W. G.; Gibson, J. W.; Good, G. M. Ind. Eng. Chem. Process Des. Dev. 1982, 1 , 102-110. Langner, B. E. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 326-33 1. Magnoux, P.; Roger, P.; Canaff, C.; Fouche, V.; Gnep, N. S.; Guisnet, M. I n Proceedings of fhe Fourth Infernational Symposium on Catalysf
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Deactivation; Delmon, B., Froment, G. F., Eds.; Eisevier Scientific: New York, 1987; pp 317-331. (9) Venuto, P. B.; Hamilton, L. A. Ind. Eng. Chem. Prod. Res. D e v . 1987, 6 , 190-192. (10) Rollmann, L. D.; Waish, D. E. I n Proceedings of the NATO Advanced Study Institote on Catalysf Deactivation; Figueiredo, J. L., Ed.; Martinus Nijhoff: Boston, 1982; pp 81-91. (11) Haldeman, R. 0.; Bow, M. C. J. Phys. Chem. 1959, 63, 489-496. (12) Eisenbach, D.; Gallei, E. J. Catal. 1979. 56, 377-389. (13) Blackmond, D. G.; Goodwin, J. G.; Lester, J. E. J. Cafai. 1982, 78, 34-43. (14) Derouane, E. G.; Giison, J. P.; Nagy, J. B. Zeol/fes1982, 2 , 42-46. (15) Cartton, L.; Copperthwaite, R. G.; Hutchings, G. J.; Reynhardt, E. C. J. Chem. SOC., Chem. Commun. 1986. 13, 1008-1009. (16) Wekamp, J.; Maixner, S.Zeolites 1987, 7 , 6-8. (17) Sullivan, M. J.; Maciel, 0. E. Anal. Chem. 1982, 54, 1615-1623. (18) Dudley, R. L.; Fyfe, C. A. Fuel 1982, 6 1 , 651-657. (19) Hatfield, G. R.; Maciei, 0. E.; Erbatur, 0.; Erbatur, G. Anal. Chem. 1987, 59, 172-179. (20) Hagaman, E. W.; Chambers, R. R.; Woody, M. C. Anal. Chem. 1986, 58, 387-394. (21) Vassalio, A. M.; Wilson, M. A.; Collin, P. J.; Oades, J. M.; Waters, A. G.; Malcolm, R. L. Anal. Chem. 1987, 59, 558-562. (22) Wilson, M. A. NMR Techniques and Applications in Geochemistry and Soil Chemistry; Pergamon Press: New York, 1987; pp 75-77. (23) Axeison, D. E. Solid State Nuclear Magnetic Resonance of Fossil Fuels ; Muttiscience Publications: Montreal, 1985. (24) Maclel, G. E.; Sullivan, M. J.; Szeverenyi, N. M.; Miknis, F. P. I n Chemistry and Physics of Coal Utilization- 1980; Cooper, 8. R., Petrakis, L., Eds.; American Institute of Physics: New York, 1981; pp 66-81. (25) Haw, J. F. Anal. Chem. 1988, 60, 559A-570A. (26) Haw, J. F.; Campbell, G. C.; Crosby, R. C. Anal. Chem. 1986, 5 8 , 3172-3177. (27) Haw, J. F.; Richardson, B. R.; Oshiro, I.S.; Lazo, N. D.; Speed, J. A. J. Am. Chem. SOC.1989, 1 7 1 , 2052-2058. (28) Langner, B. E.; Meyer, S. I n P r d i n g s of the International Symposium on Catalyst Deactivation; Deimon, B., Froment, G. F., Eds.; Eisevier Scientific: New York, 1980; pp 91-102. (29) VanderHart, D. L.; Earl, W. L.; Garroway, A. N. J. Magn. Reson. 1981, 44, 361-401. (30) Opeila. S. J.; Frey, M. H. J. Am. Chem. Soc.1979, 701, 5854-5856. (31) Botto, J. V.; Wilson, R.; Wlnans, R. E. Energy Fuels 1987, 1 , 173-181. (32) Kaiinina, N. G.; Poluboyarov, V. A.; Anufrienko, V. F.; Ione, K. G. Kinet. Katal. 1986, 2 7 , 237-240. (33) Kaiinina, N. G.; Ryabov, Y. V.; Korobitsyna, L. L.; Poiuboyarov, V. A.; Erofeev, V. I.; Kurina, L. N.; Anufrienko, V. F. Kinet. Katal. 1988, 2 7 , 240-242. (34) Alger, R. S. Electron Paramgnetlc Resonance: Techniques and Applications; Interscience Publishers: New York, 1968; p 213. (35) Retcofsky, H. L.; Stark, J. M.; Friedei, R. A. Anal. Chem. 1988, 40, 1699-1 704. (36) Rothweii, W. P.; Waugh, J. S. J. Chem. Phys. 1981, 74, 2721-2732.
RECEIVED for review March 6,1989. Accepted May 15,1989. The support of the National Science Foundation through Grant CHE 87-00667 is gratefully acknowledged.
