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(30) Kotrily, S.; Sucha, L. Handbook of Chemical Equilibria in Analytical Chemistry; Ellis Horwood: Chichester, UK, 1985. (31) Veprek-Siska, J.; Wagnesova, D. M.; Eckschlangger, K. Collect. Czech. Chem. Commun. 1966, 31, 1248-1255. (32) Byerley, J. J.; Fouda, S. A.; Rempel, G. L. J. Chem. SOC., Dalton Trans. 1975, 1329-1338. (33) Graedel, T. E.; Weschler, C. J. Rev. Geophys. Space Phys. 1981, 19, 505-539. (34) Parks, G. A. Chem. Rev. 1965, 65, 177. (35) Schindler, P. W. In Adsorption of Inorganics at SolidLiquid Interfaces: Anderson, M. A., Rubin, A. J.,Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; pp 1-49. (36) Huang, C. P. In Adsorption of Inorganics at Solid-Liquid Interfaces;Anderson, M. A., Rubin, R. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; pp 183-217. (37) Sillen, L. G.; Martell, A. E. Stability Constants of Metal Ion Complexes;Special Publications; The Chemical Society: London, 1964; No. 17; 1970; No. 25. (38) Shenk, J. E.; Wefer, W. H., Jr. J.-Am. Water Works Assoc. 1968, 60, 199-203.
(39) Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper and Row: New York, 1987; pp 229-275. (40) Srivastava, R. D.; McMillan, A. F.; Harris, I. J. Can. J. Chem. Eng. 1968,46, 181-184. (41) Betterton, E. A.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5962-5964. (42) Berg, J. V. D.; Dillon, A. J. V.; Meijden, J. V. D.; Geng, J. W. In Surface Properties and Catalysis by Non-Metals; Bonnalle, J. P., Derauane, E.; Eds.; Reidel Publishing Co.: Drodrecht, The Netherlands, 1982; pp 493-532. (43) Raitert, V. A.; Golodets, G. J.; Pyatnitzkii, Yu. L. Proceedings of the 4th International Congress on Catalysis; Moscow Academiai Kiado: Budapest, 1971; p 466. (44) Prasad, D. S. N. Ph.D. Thesis, University of Rajasthan, Jaipur, India, 1990. (45) Berresheim, H.; Jaeschke, W. J. Atmos. Chem. 1986, 4, 311-333.
Received for review October 7,1991. Revised manuscript received November 15,1991. Accepted February%, 1992. This work was supported by an Indo-U.S. Subcommission Research Project.
Rare Earth Distributions in Catalysts and Airborne Particles Michael E. Kltto,+ David L. Anderson,*9$Glen E. Gordon, and Ilhan Olmez*
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 Zeolite cracking catalysts used by petroleum refineries were analyzed for 38 elements. Concentration patterns of rare earth elements (REEs) in 10 zeolite catalysts show an enhancement of light REEs relative to the crustal abundance pattern, resembling those measured in refineries emissions. Release of zeolite catalyst material from fluidized catalytic crackers and incorporation of zeolite catalysts into refined oil provide new atmospheric elemental signatures for tracing emissions from refineries and oil-fired power plants, respectively. Though both have enhanced La/REE ratios, emissions from these two sources can be distinguished by their La/V ratios. Three-way catalytic converters of newer automobiles contain REEs and may, thus, be a significant source in some cities.
Introduction Catalysts. With the introduction of fluid catalytic crackers (FCCs) in the 1940s, traditional acid-treated clay catalysts were replaced by more stable synthetic silicaalumina gels. Recognizing the need for improved properties (activity, selectivity, and stability) in the catalysts, research efforts led to the invention of zeolite cracking catalysts in the late 1950s (1). Over 90% of the catalytic cracking units in the United States were using zeolite catalysts by 1969 (2). Cracking catalysts containing rare earth mixtures are consumed by the petroleum refining industry to produce light-weight hydrocarbons, such as gasoline and fuel oil (3). These zeolite cracking catalysts dominate the worldwide FCC demand, with the amorphous aluminosilicates still comprising a significant share of FCC usage in Europe and the Middle East ( 4 ) . The United States is responsible for -70% of global FCC zeolite consumption. Data from the ‘Present address: -Wadworth Center for Laboratories and Research, New York State Department of Health, Albany, NY 12201. Present address: FDA Laboratory, NIST, Building 235/B125 Gaithersburg, MD 20899. 8 Present address: Nuclear Reactor Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139. 1388
Environ. Sci. Technol., Vol. 26, No. 7, 1992
US.Bureau of Mines ( 5 , 6 )show the correlation between rare earth oxide (REO) demand and REO-containing FCC utilization (Figure 1). Petroleum catalysts have accounted for an average of 40% of the rare earth element (REE) consumption in the United States in the last 20 years, varying from a high (65%) in 1983 to a low (31%)in 1986. The recent decrease in use of REOs in petroleum catalysts is a result of the phase-out of leaded gasoline. Because of the need to replace alkyllead compounds with hydrocarbons of higher octane number, refiners are now using catalysts that contain reduced concentrations of REOs 0.5-2% (7). Zeolites typically comprise -15% of an FCC catalyst’s composition, the remainder being the aluminosilicate structure and an inert filler. A cavity-filled structure gives zeolites their cation-exchange and reversible dehydration properties (8). Synthetic zeolites used by the petroleum industry are somewhat distinguishable by their Si/ Al ratio, which is important in determining the hydrothermal stability for cracking (9, 20). Alkali and alkaline earth metals present in crude oils deactivate zeolite catalysts by poisoning (acid-site neutralization) and degradation (hightemperature fluxing). Sources of metal impurities and their effect on catalyst activities have been reviewed by Letzsch and Wallace (11). Recently developed synthetic zeolites have greater tolerances (up to several thousand ppm Na), while also upgrading octane numbers and limiting coke production (22). Cracking-catalyst activity levels depend on the accumulation of product coke on the catalyst. Thus, selective cracking and control of the zeolite’s pore size (to minimize coking deactivation) have dominated research interests. FCCs are circulated through a regeneration stage, using hot air and steam, to remove coke deposita prior to their reuse. However, they gradually lose their effectiveness and must be withdrawn from circulation and replaced with fresh catalyst. Also, as noted below, some of the catalyst material apparently gets into the fuel burned by oil-fired plants. Bastnasite and monazite ores are the major commercial sources of REEs (13,14). The former is enriched in light
0013-936X/92/0926-1368$03.00/0
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REEs relative to the Earth's crustal abundance pattern (15) and is primarily obtained from alkaline rocks and carbonatites. Monazite has a more normal REE abundance pattern and is obtained from placer deposits. Natural REE ratios in bastnasite and monazite ores, as well as their sources and consumption, have been summarized by Kilbourn (16). Concentration variations of the lanthanides have been reported for bastnasite (17)and monazite (18). Most multielement analyses of these minerals report only the REE composition. Similarly, we are aware of no analyses of zeolite cracking catalysts that incorporate both REE and non-REE results. Airborne Particles. Trace element compositions of airborne particles have been used to quantify source contributions to receptor sites (19), but as soil and other crustal sources have usually accounted for most of the REE concentrations, the REE patterns in aerosols have largely been ignored as atmospheric tracers. On the basis of the analysis of fine-fraction airborne particles collected in Camden, NJ (20),Olmez and Gordon (21) demonstrated that the REE ratios (La to other REEs and V) in emissions from petroleum-refining and oil-burning sources could be used to distinguish them from other sources. For atmospheric emissions from a refinery and a coal- and an oil-fired power plant, Olmez and Gordon (21) reported La/Sm ratios of 20, 5.2, and 28, respectively, in both fine and coarse size fractions. The La/V ratios can be used to distinguish refinery and oil-fired power plant emissions (15 and 2 mm) often match that found in crustal material, -5 (15), but the fine fraction can yield ratios close to that found in emissions from refineries and oil-fired power plants. From cascade-impactor samples collected in the Washington, DC, area during the summer of 1976, Kowalczyk et al. (22,231 determined an average La/Sm ratio of 17 on the fine fraction, but -7 on the coarse fraction. Also, the light REEs and T h had much greater fractions of their mass (31-4490) in the fine fraction than did the typical crustal elements, Al and Sc (10-12%). In air samples collected in Shenandoah Valley during the summer of 1980, Tuncel et al. (24) observed La/Sm ratios near 19 on fine-fraction samples collected when air masses were from the southeast and 10 in air masses from the other directions. Air masses from the southeast also contained the highest V concentrations, because of oil-fired power plants located along the East Coast. Loss of REE zeolites into the refined oils may have been responsible for the high La/Sm ratios (27 f 7) measured in the stack of an oil-fired power plant (25). Similarly high ratios (22 f 8) existed in 15 fuel oils burned during the sampling period. Upwind ambient air measurements had La/Sm ratios near crustal levels (9 f 3). Crude-oil con-
centration ratios near 12 have been reported (26), but few other data exist on REE concentrations in oil products. Mizohata (27) confirmed the REE results from the Philadelphia refinery by observing very high REE concentrations and La/Sm ratios in emissions from the catalyst-regeneration tower of an FCC at a Japanese refinery. Data from a sampling network in several Japanese cities showed a dramatic increase in the La/Sm ratio during the 1980s. For example, from 1984 to 1986, six sites throughout Osaka showed La/Sm ratios of 22-42 on fine particles (5 often being a result of dealumination of the catalysts. Eighteen of the 21 zeolites listed by Newsam (8) had %/A1 ratios of 15. Our 10 analyzed FCC zeolites had &/A1 ratios of 0.5-2.8, which are all below the crustal ratio (-3.4). The various catalysts are apparently not simply mixtures of the three raw materials in Table 11, as concentrations of elements in the catalysts are not linear combinations of concentration profiles of the raw materials, even considering just the REEs. Although the number of samples is too small to draw detailed conclusions, we subjected the concentrations to factor analysis in order to observe any common factors whose variations account for collective variations of groups of elements. We obtained five factors, each of which explain more than one unit of variance, the first having strong, positive loadings for Al, Sc, Ti, Cr, Co, Th, Ta, and Ga, and strong negative loadings for H and Si. This factor apparently represents the aluminosilicate matrix material. The negative loading for Si (and, perhaps, H) arises from the fact that the aluminium and silicon oxides typically account for 80% or more of the mass, so the Al and Si concentrations must be negatively correlated. The second factor has very strong loadings of Fe, C1, and heavy REEs (Gd, Tb, Yb, and Lu), and the third factor, Na, Ga, and the light REEs except Ce, i.e., La, Nd, and Sm. The fourth factor is loaded with only Eu and V (with negative Mn) and the fifth, Ce and B (with negative Hf). The fact that the fourth and fifth factors contain, respectively, Eu and Ce, the only REEs that have oxidation states other than 3+, suggests that some redox chemistry was involved in manufacture of the catalysts. To identify elements that are enriched in the FCC samples, relative to crustal averages, we calculated enrichment factors (EFcrust)using EFcrust = (X/Fe)s,,le/(X/Fe)cr"st (1) where (X/Fe)sample is the concentration ratio of element X to that of Fe in the sample and (X/Fe)crustis that of a representative crustal abundance pattern (15). Iron was chosen as the normalizing element because it is easily determined by INAA and less likely than A1 and Si to be enriched in the catalysts. A summary of the EF values is given in Table 111. The light REEs show the greatest enrichment, and the heavy REEs and T h are somewhat enriched. Although Sc is often classified as a REE, it is enriched by only a factor of 10 or less. Se and I are highly enriched on all the catalyst groups and are often enriched in ambient particles. The bastnasite ore and the concentrates are so different from normal crustal materials that EF values are not very helpful. Concentrations of typical crustal elements (Fe, Sc, Al) are so low that they cannot be used for normalization. When Cr is used for normalization, EF values for the two Sc concentrations in Table I1 are 1.24 and 1.33, again indicating that Sc is not strongly enhanced in these materials. However, alkaline earths, Sb, Zr, and T h are highly enriched, along with the REEs. It is probably more useful to consider the REE concentration patterns. In Table IV are listed the EF values of the REEs normalized to the concentration of Gd, which is fairly representative of the heavy REEs and measured generally more accurately than the others. In the lower part of the table are listed the ratios of the concentrations Environ. Scl. Technol., Vol. 26, No. 7, 1992 1371
Table 111. Enrichment Factors of Elements in Catalysts Relative to Taylor’s Crustal Abundances (Fe = 1.00) element B Na A1 Si
DA-25 19 0.4 25 6
c1
131
sc Ti V Cr Mn co Zn Ga Se Zr Sb I Ba La Ce Nd Sm Eu Gd Tb Yb Lu Hf Ta Th
6 17 6 7 0.1 2.2
