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Anal. Chem. 1990,62,2532-2536
this shift was due to the 1% lower water mass loading or to the presence of solid particles in the sample, an experiment was devised in which both the dust generator, operating at 1.3 L/min argon gas, and the Babington nebulizer, operating at 0.78 L/min argon gas, were connected over a Y-joint to the aerosol injector tube, thus obtaining about 2 L/min of total argon gas flow. The introduction of the dry powder led to a NPS with a af noise band at 260 Hz. Upon addition of a water aerosol (about 13 pL/L, assuming a 1% transport efficiency) via the nebulizer to the powder aerosol the peak did not shift significantly; however, when an A1 solution was nebulized, the af noise band was observed to have shifted about 10 Hz to lower frequencies. Thus, apparently, only the introduction of particles in the plasma induces a small increase in the af noise frequency. A further explanation of this phenomenon, however, could not be found.
CONCLUSIONS The use of slurries in ICP-OES is not hampered by major noise sources, different from those occurring when solutions are nebulized. Though small differences in the noise level have been observed when slurries are nebulized and a precision was obtained that is slightly smaller than that for solutions, it can be concluded that, for powders containing submicrometer particles, the slurry ICP-OES technique offers a promising approach for the analysis of powders.
ACKNOWLEDGMENT The authors thank R. Niessner of the Technical University of Munich for the loan of the rotating brush aerosol generator.
LITERATURE CITED (1) Broekaert, J. A. C.; Graule, T.; Jenett, H.; Tolg, G.; Tschopel, P. Fresenius' 2.Anal. Chem. 1989,332,825-838.
Raeymaekers, B.; Graule. T.; Broekaert, J. A. C.; Adams. F.; Tschopel, P. Specfrochlm. Acta, Part 8 1988, 438, 923-940. Walden, G. L.; Bower, J. N.; Nkdel, S.; W o n , D. 1.; Wlnefordner, J. D. Specfrochhn. Acta, Part 8 1980, 358, 535-546. Belchamber, R. M.; Horllck, G. Specfrochlm. Acta, Part8 1982,378, 17-27. Benettl, P.; Bonelll, A.; Cambiaghl, M.; Frigleri, P. Spectrochim. Acta, Part 8 1982,378,1047-1053. Davies, J.; Snook, R. D. J. Anal. At. Spectrom. 1987, 2, 27-31. Montaser, A.; Clifford, A. H.; Sinex, S. A,; Capar, S. G. J. Anal. At. Spectrom. 1989, 4 , 499-503. Ingle, J. D., Jr.; Crouch, S. R. Specfrochemical Analysis; PrenticeHall: London, 1988; Chapter 5. Winge, R. K.; Eckels, D. E.; DeKalb, E. L.; Fassel, V. A. J. Anal. At. Spectrom. 1988, 3,849-855. Olesik. J. W.; Smith, L. J.; Williamsen, E. J. Anal. Chem. 1989, 67, 2002-2008. Montaser, A.; Ishil, I.; Tan, H.; Clifford, R. H.; Golightly, D. W. Spectrochim. Acta, Part8 1989, 4 4 8 , 1163-1169. Graule, T.; von Bohlen, A.; Broekaert, J. A. C.; Grallath, E.; Klockenkamper, R.; Tschopel, P.; Tolg, G. Fresenlus' Z . Anal. Chem. 1989,335,637-642. Wendt, R. H.; Fassel, V. A. Anal. Chem. 1965, 37,920-922. Oteenfield, S.;Jones, I.LI.; Berry, C. T. Analyst 1964, 89, 713-720. Broekaert, J. A. C.; Hagenah, W.4.; Laqua. K.; Leis, F.; Stuwer, D. Specfrochim. Acta, Part 8 1986, 4 7 8 , 1357-1365. Van Bot", W. A.; Adams, F. C. Anal. Chim. Acta 1989, 278, 185-2 15. Vereln Deutscher Ingenieure Genefatlng of test aerosols wlth a rotating brush generator, VDI-Richtlinlen 3491, part 9; Beuth Verlag GmbH: Berlin, 1989. Salin, E. D.; Horllck, G. Anal. Chem. 1979, 57, 2284-2285. Scott, R. H. Specfrochim. Acta, Part 8 1978,338,123-124. Ng, K. C.; Zerezghl, M.; Caruso, J. A. Anal. Chem. 1984, 56, 417-421,
RECEIVED for review April 19,1990. Accepted August 17,1990. This work has been supported by the "Ministerium fur Wissenschaft und Forschung des Landes Nordrhein Westfalen", by the "Bundesministerium fur Forschung und Technologie", Bonn, and by the "Deutsche Forschungsgemeinschaft (DFG)".
Effects of Paramagnetic Lanthanides on the Study of Carbonaceous Deposits on Zeolite Catalysts by Carbon- 13 Solid-state Nuclear Magnetic Resonance Spectroscopy Eric J. Munson and James F.Haw*
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Relaxation and rpln-countlng measurements have been used to probe the extent to which lac NMR spectroscopy wlth crow polarization and maglc-angk sph#ting is a useful technique for characterizing carbonaceous (coke) deposits In zeoilte catalysts containing rare-earth cations. Such ions were antldpated to complicate this analysis as a result of paramagnetk shifts and/or rdaxatlon effects. Contrary to expectatlono, no paramagnetic shHts were observed for carbonaceous deposits in the rare-earth-exchanged Y-type zeoHtes examined. Even more surprtdng was thal the mixture of rare earths used in InduWal catalysts has neg4gWe effects upon the relaxation processes central to cross pdarlzatlon. Small concentrations of Ianthankles wlth long electron spinlattice relaxation times (in particular gadolinium), however, have a deleterious effect on characterizing carbonaceous deposlts by decreasing the proton rotating-frame spin-iattlce relaxation time.
* To whom correspondence should be addressed. 0003-2700/90/0362-2532$02.50/0
INTRODUCTION Zeolite catalysts are used in the petroleum industry for the catalytic cracking of crude oil into gasoline-range hydrocarbons (1,Z). A widely used industrial cracking catalyst is the synthetic zeolite Y, but a problem with this zeolite is structural stability a t high temperatures ( 3 , 4 ) . To improve stability, rare-earth (lanthanide) cations are often ion-exchanged for sodium or ammonium originally present in the zeolite. Such rare-earth-exchanged zeolites are much more stable at high temperatures. A significant problem in the operation of any type of cracking plant is the formation of carbonaceous deposits (termed coke) on the catalyat (5,6). These deposits eventually deactivate the catalyst and require removal of the coke before the catalyst can be reused. One approach to the study of coke deposits is the use of 13C NMR spectroscopy in conjunction with various line-narrowing techniques. Derouane and coworkers ( 7 ) used solid-state NMR spectroscopy to investigate the carbonaceous deposits formed on zeolites HZSM-5and H-Mordenite by small olefins and alcohols. 0 1990 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
Weitkamp and Maixner (8)studied the carbonaceous deposits formed on LaNaY coked with isobutane/butene a t various temperatures by l3C NMR spectroscopy with cross polarization and magic-angle spinning (CP/MAS). In a recent paper, Richardson and Haw (9) investigated the carbonaceous deposits formed by the reactions of butadiene on zeolite HY at various coking temperatures via 13C CP/MAS NMR spectroscopy. Particular attention was paid in that study to the dynamics of cross polarization and to the fraction of carbon spins observed by NMR spectroscopy. They found 13C CP/MAS NMR spectroscopy was a reliable technique for the study of carbonaceous deposits on zeolite HY, especially for samples coked at temperatures below 300 "C, for which spin-counting experiments showed that most of the 13Cnuclei were observed. The presence of paramagnetic lanthanide cations in rareearth-exchanged zeolite Y might be expected to affect the NMR signals from carbonaceous deposits in these catalysts. The experience of a number of workers familiar with NMR spectroscopy of paramagnetic lanthanide complexes in solution (10, 11) and in the solid state (12, 13) suggests that paramagnetic lanthanide cations in rare-earth-exchanged zeolite catalysts are capable of producing large paramagnetic shifts as well as dramatically reducing relaxation rates of the nuclei in the coke molecules. Previous studies (14,15) using spincounting techniques have shown that the presence of paramagnetic Fe3+ in various materials reduces the fraction of carbon spins observed by decreasing cross polarization efficiency and/or severely broadening the 13C resonances. Because of the importance of these rare-earth-exchanged catalysts in industry, we have undertaken a detailed study of the complicating effects of paramagnetism on NMR characterization of coke deposits in lanthanide-exchanged HY zeolites. For simplicity, a single feedstream and a fixed coking temperature were used. Five different lanthanides, which have a wide range of electron spin-lattice relaxation times (Tl(e)) and magnetic susceptibilities were used. Relaxation and spin-counting measurements were performed on selected coke samples, and the results were compared to those obtained from a coked zeolite prepared from a commercial mixture of lanthanides. The prospects appear favorable for characterization of commercial rare-earth-exchanged catalysts using 13C CP/MAS NMR spectroscopy, largely as a result of a favorable distribution of lanthanide elements in commercial rare-earth mixtures. Coke in zeolites containing small amounts of gadolinium, however, was difficult to detect with 13C CP/MAS NMR spectroscopy.
EXPERIMENTAL SECTION Sample Preparation and Characterization. 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 form by ion exchange with 1M NH4N03followed by ion exchange twice with a 0.6 M solution of the rare-earth nitrate salt. The lanthanide content (Galbraith Laboratories) of each sample is shown in Table I. The commercial rare-earth-exchanged HY zeolite was obtained from Strem and used without further ion exchange. The commercial rare-earthexchanged zeolite contained (by weight) 6.5% lanthanum oxide, 3.5% neodymium oxide, and trace amounb of the other lanthanide oxides. Activation was performed in a quartz flow reactor at 500 "C by using 5 g of catalyst under a flow of dry nitrogen. Following activation, the reactor temperature was changed to 200 "C, and 110 cm3 of propene (Matheson) was metered into the nitrogen flow, which was maintained at 150 cm3/min. Propene was allowed to flow over the catalyst for 1 h, after which the sample was allowed to cool under a flow of dry nitrogen. The catalyst was then transferred to a glovebox under N2,where the sample was stored prior to grinding and packing in a rotor. Weight-percent
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Table I. Weight-Percent Carbon and Lanthanide Content of Catalysts Studied zeolite
W carbona
LaHY PrHY NdHY GdHY 6.0% GdHY 1.5% GdHY DyHY REHY
12.5 12.8 12.9 12.7 12.6
6.08 6.12 6.39 7.40 0.48c
13.0
O.llC
%
lanthanideb
10.6 13.0
Weight-percent carbon of coked rare-earth-exchangedcatalysts.
* Weight-percent lanthanide content for uncoked lanthanide-exchanged catalysts. Weight-percent Gd; these catalysts also contained La (see text).
carbon data (Galbraith Laboratories) for each sample studied are reported in Table I. NMR Experiments. All NMR experiments were performed on a Chemagnetics M-100s spectrometer operating at 25.02 MHz for 13C. Typically, spectra were acquired with sweep widths of 20 kHz. No additional signals were observed for any of the samples studied when the sweep width was increased to 40 kHz or when the transmitter frequency was varied. Kel-F rotors containing approximately 0.4 g of sample were spun at a rate of 3-4 kHz. Recycle delays of 1s and contact times of 2 ms were used unless otherwise stated. A total of 40000 transients was acquired per spectrum of coke sample, with 10000-12 000 transients acquired for relaxation measurements. Hexamethylbenzene was used as an external standard, and chemical shifts are reported relative to tetramethylsilane. All relaxation time constants were determined by methods reported previously (9), except carbon spin-lattice relaxation time constants (T,(C)),which were determined by the method described by Torchia (16). Eight or more data points were obtained for each relaxation measurement. Carbon spin-counting experiments were done as previously described (9). A spectrum is obtained of a sample containing glycine and coked zeolite, and the intergrated area of the glycine peaks is compared to the integrated area of the coked zeolite peaks. From the relative ratios of the peak areas and the moles of carbon in the zeolite (from carbon analyses),the percent carbon observed by NMR spectroscopy for each sample is calculated. A 2-me contact time was used for all experiments. A correction factor was used for the integrated intensities of the glycine and coke, since the inequality TcH
