474
Energy & Fuels 1989,3,474-480
Conclusions Sublimation of Re2(CO),,, onto NaY is a very effective way for the deposition of Re in the supercages of the zeolite; catalytic activity in n-heptane conversion is much higher than that reported for a conventional Re/NaY catalyst prepared by impregnation of (NH,) [Reo,]. In the presence of prereduced Pt clusters in the supercages, bimetallic PtRe clusters are selectively formed. Their catalytic signature is very high activity and selectivity for hydrogenolysis to methane of sulfur-free n-heph e . Deep hydrogenolysis of sulfur-free alkanes has been previously found to be a highly sensitive and direct probe for detecting formation of mixed PtRe cluster^.'^^^'^^ Bimetallic PtRe clusters have been claimed to be responsible for the unique properties of selectivity and stability of sulfided PtRe reforming catalysts, which are widely used in catalysis.M Addition of Re to Pt also lowers the rate of catalyst deactivation of the catalysts even in the absence of sulfur. No formation of bimetallic particles has been observed with catalysts that were prepared by solvent impregnation of PtRe2(CO)12followed by its reductive decomposition. Solvent impregnation does not appear to result in depositing the carbonyl into zeolite supercages. (34) Sachtler, W.M.H. J. Mol. Catal. 1984,25, 1-12.
On the basis of the present results, it is evident that only two synthetic routes appear promising for the preparation of mixed-transition-metal clusters in zeolites: (i) vapor deposition of mononuclear metal carbonyls in the supercages followed by thermal decomposition in the presence of prereduced clusters of the second metal;2f5(ii) ion exchange of the metal cations followed by in situ reductive condensation by means of a ”ship-in-bottle ~ynthesis”.~J’
Acknowledgment. We gratefully acknowledge support from the U.S.Department of Energy under Contract DEFG02-87ERA3654. We wish to thank G. D’Alfonso for providing a sample of PtRe2(CO)12. C.D. acknowledges receipt of financial support from the CNR Advanced Fellowship Program. Registry No. MCP, 96-37-7;eCP, 1640-89-7;Pt, 7440-06-4; Re, 7440-15-5;Rez(CO)lo,14285-68-8; PtRez(CO)lz,91443-96-8; heptane, 142-82-5;methane, 74-82-8; ethane, 74-84-0; isobutane, 75-28-5;propane, 74-98-6;butane, 106-97-8;isopentane, 18-78-4; pentane, 109-66-0;2-methylpentane, 107-83-5; 3-methylpentane, 96-14-0; hexane, 110-54-3;benzene, 71-43-2; toluene, 108-88-3; 2-methylhexane, 591-76-4; 3-methylhexane, 589-34-4. (35) Tri, T.;Candy, J. P.; Gallezot, P.; Massardier, J.; Primet, M.; Vedrine, J. C.; Imelik, B. J. Catal. 1983, 79, 396-409. (36) Sheu, L.-L.; KnGzinger, H.; Sachtler, W. M. H. Catal. Lett. 1989, 2,129-138. (37) Ichikawa, M.Polyhedron 1988, 7,2351-2367.
Supercritical Fluid Extraction of Coal Tar Contaminated Soil Samples Bob W. Wright,* Cherylyn W. Wright, and Jonathan S. Fruchter Chemical and Environmental Sciences Departments, Battelle Northwest Laboratories, Rich land, Washington 99352 Received December 6, 1988. Revised Manuscript Received April 18, 1989 Soil samples containing coal tar residues from manufactured gas (town gas) plants were extracted by carbon dioxide supercritical fluid extraction (SFE)and Soxhlet extraction techniques. The detection limits and the reproducibility of the SFE method were evaluated, and the comparability of measured concentrations of polycyclic aromatic hydrocarbons (PAH) by the two extraction methods was determined. Laboratory extracts were analyzed by high-resolution gas chromatography to quantify selected PAH. SFE recoveries of soil-spiked PAH compounds were quantitative within experimental error. The detection limits of the method were in the 50-100 ppb range. The reproducibility in PAH concentrations determined for three early SFE replicate extracts averaged f 2 5 % standard deviation of the mean. A later reproducibility study for 10 replicate SFE extracts of a different sample averaged i16% standard deviation of the mean (12% due to analysis; 4% due to SFE replication). Five Soxhlet extracts averaged f l l % standard deviation from the mean reproducibility (10% due to analysis; 1% due to replication). SFE extract PAH concentrations varied from Soxhlet extract PAH concentrations by an average f20%, i12%, and *lo% for three samples. Greater than 80% and 100% extraction efficiencies were achieved in the first 30 min of SFE when three successive extractions were performed on two samples, respectively. SFE tended to discriminate against high molecular weight PAH due to their lower solubilities in supercritical carbon dioxide. Overall, the SFE method provided rapid (30-min) extraction that may prove to be useful for in-the-field determinations of organic compounds in soils. The SFE method shows promise as an alternative to, or an addition to, traditional Soxhlet extraction.
Introduction Prior to the widespread availability of natural gas, gas for fuel and light was manufactured by the high-temperature carbonization of bituminous coal, resulting in a product called “town gas” or ‘manufactured gas”. These early gasification activities, which date back to the MOOS, resulted in the formation of coal tar residues. These 0887~0624/89/2503-0474$01.50/0
residues were usually either burned on site as a supplemental fuel or land-disposed of near the plant.’ As a result of such practices, abandoned town gasification sites or (1) Villaume, J. F.Hazardous and Toxic Wastes: Technology, Management and Health Effects; Majumdar, s. K., Miller, E. W., Eds.; The Pennsylvania Academy of Science: Philadelphia,PA, 1984;pp 362-375.
1989 American Chemical Society
Supercritical Fluid Extraction
Energy & Fuels, Vol. 3, No. 4, 1989 475 1116" S.S. 118" S.S.
Pressure Controller
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Selection Valve 75 pm 1.d.
I Pumphead and Heated S.S.Syringe Tubing W
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Figure 1. Schematic diagram of the supercritical fluid extraction apparatus.
disposal sites are now becoming recognized as environmental trouble spots. Due to the widespread use of town gas in the late 19th and early 20th century, there are expected to be many such sites around the country. Consequently, it is necessary to have reliable methods for the analysis of soil samples that could potentially be contaminated with varying quantities of coal tar or other organic wastes. Most current methods for the measurement of organics in soils and sediments require lengthy solvent extraction. 1n.order to implement remedial measures and to carry out cost-effective site assessments, more rapid and fieldadaptable sample extraction methods are desirable. Supercritical fluid extraction (SFE) techniques provide a viable alternative with promising advantages over the current liquid extraction method^.^-^ The potential advantages of SFE accrue from the properties of a solvent at temperatures and pressures above its critical point. The liquidlike solvating power and rapid mass-transfer properties of a supercritical fluid provide the potential for more rapid extraction rates and more efficient extraction due to better penetration of the matrix than is feasible with liquids. The properties of a supercritical fluid are intermediate between those of the gas and those of the liquid phases. The compressibility of a supercritical fluid is large just above the critical temperature, and small changes in pressure result in large changes in the density of the fluid. The density of a supercritical fluid is typically 102-103 times greater than that of the gas. Molecular interactions increase at these higher densities because of shorter intermolecular distances, and solvating characteristics of the supercritical fluid approach those of a liquid. However, the diffusion coefficients and viscosity of the fluid remain intermediate between those of the gas and liquid phases, thus allowing rapid mass transfer of solutes compared to a liquid. The properties of a supercritical fluid are dependent on the fluid composition, pressure, and temperature. Many fluids have comparatively low critical temperatures that allow extractions to be conducted at relatively mild temperatures. For example, the critical temperature of carbon dioxide is only 31 "C. Density or solvating power of a supercritical fluid can be controlled by fluid pressure and/or temperature. In addition, various different fluids or fluid mixtures that exhibit different specific chemical interactions can be used to obtain the (2) Wright, B. W.; Wright, C. W.; Gale, R. W.; Smith, R. D. Anal. Chem. 1987,59, 38-44. (3) Schantz,M. M.; Cheder, S.N. J. Chromatogr. 1986,363,397-401. (4) Hawthorne, S.B.; Miller, D. J. Anal. Chem. 1987,59,1705-1708.
Table I. Particle Size and Total Organic Carbon Content of Soil Samples anal., wt % sand silt clay tot. organic sample (>53 um) (2-53 um) ( 6-1 > 3-1 > 5-3, 3-2 > 5-2. Although these samples were randomly chosen and were from independent sites, it appeared that the soils with higher clay contents (Table I) and lower sand and silt tended to contain higher concentrations of coal tar. Due to limited data available about the soil samples, it is not known if the PAH concentration ranges were due to soil type or sample location relative to the source of contamination, or both. The detection limits of the SFE method as applied were in the 50-100 ppb for PAH compounds. These low detection limits allow the SFE method (with subsequent gas chromatographic analysis) to be applicable to solid environmental samples such as soils and sediments. The reproducibility of the method (including sampling and analysis) during the development of SFE for the manufactured waste gas site samples was reasonable ( f a p proximately 25% standard deviation from the mean for three early determinations; f16% for ten later determinations). The greatest amount of variability of the method was the gas chromatographic analysis (CV of 12) due to the highly complex extract mixture and the limits of current electronic integration of chromatographic peaks. This study showed similar reproducibility (i.e., of the same order of magnitude) is possible with the SFE method compared to the more traditional Soxhlet extraction. The PAH extraction efficiencies of the SFE were comparable to those of the Soxhlet extraction when the concentrations of the PAH compounds in the manufactured (9) Czubryt, J. J.; Myers, M. N.; Giddings, J. C.J. Phys. Chem., 1970,
74,4260-4266.
480 Energy & Fuels, Vol. 3, No. 4, 1989
gas waste site samples were in parts per million. The only exception was the higher molecular weight compounds tended to have lower extraction efficiencies in the 30-min SFE versus the overnight Soxhlet extraction. If a sample is known to have concentrations greater than about 50 ppm each of higher molecular weight PAH or if those compounds are of particular interest, an SFE time of greater than 30 min or increased flow rates over the same amount of time would be necessary under the conditions used. Both options would be practical. No attempts were made to fractionate the samples by controlling the solvating power of the supercritical carbon dioxide in this study, although this potential does exist. An important consideration in the development of a viable SFE method is the provision taken to ensure efficient collection of the analytes. It is possible for the analytes to form small aerosol particles with the expanding carbon dioxide that can then be easily lost to the atmosphere.2 This behavior can be a particular problem when rapid fluid flow rates are used and a large volume of expanding gas is produced. Such a problem was overcome in this work (as evidenced by the high extraction efficiencies) by expanding the extraction effluent through a relatively long solvent path (see Figure 3). Significant Joule-Thompson cooling occurs during expansion of carbon dioxide, which cooled the solvent and minimized evaporation of both the solvent and analytes. However, with the high gas flow, it was also necessary to utilize a condenser to prevent rapid loss of the collection solvent. The cooling from the expansion also requires that extra heat (see Figure 1, heated hypodermic tubing) be added to the fluid prior to expansion to prevent plugging of the restrictor. Detection limits, reproducibility, and extraction efficiencies of the SFE show that this method may be a viable alternative to the traditional Soxhlet extraction for some applications. In addition, SFE has some advantages over Soxhlet extraction. First, the SFE method is rapid; the extraction can be done in about 30 min. Second, fewer contaminants were present in the SFE extracts as compared to the Soxhlet extracts. The solvating power of supercritical carbon dioxide was sufficient to extract the analytes of interest, but did not provide sufficient solvating power to extract any of the polar and high molecular weight organic soil materials that could interfere with subsequent analyses. And third, the SFE method lends itself to in-the-field extraction of soil samples. Large quantities of glassware are not needed, the apparatus is self-cleaning by purging the system with supercritical carbon dioxide while the extraction cell is empty, and large
Wright et al. volumes of solvent are not required to be used or concentrated. In addition, it is not necessary to dry the soil samples prior to extraction. Water (up to approximately 1.5 mol % at the conditions used for extractionlo)is soluble in supercritical carbon dioxide and is carried over to the collection vessel without interfering with analyte extraction from the matrix. The apparatus described in this manuscript has been successfully used for two in-the-field demonstrations of the characterization of contaminated soil samples. The apparatus was packaged, shipped, and reassembled in-the-field after which soil samples were successfully extracted and analyzed. The total weight of the prototype extraction apparatus was approximately 100 lbs excluding the carbon dioxide tank. Improvements in portability are expected with refinement of design. Inthe-field SFE could be used to characterize sample sites that have been contaminated with organic wastes with a rapid turnaround time. Analyses of the soil samples from manufactured gas sites have shown the application of SFE to environmental samples.
Acknowledgment. We thank A. J. Kopriva for technical assistance with the supercritical fluid extractions, R. W. Sanders for the atomic absorption spectrophotometry, and R. L. Buschbaum for statistical nested analysis of variance. We also thank I. Murarka for encouraging this project and providing the samples. This work was supported by the Electric Power Research Institute, Land and Water Quality Studies Program, under Contract RP2879-04. Registry No. Indene, 95-13-6; methylindenes, 29036-25-7; naphthalene, 91-20-3; benzo[b]thiophene, 95-15-8; 2-methylnaphthalene, 91;57-6; l-methylnaphthalene, 90-12-0; biphenyl, 92-52-4; dimethylnaphthalene, 28804-88-8; acenaphthalene, 208-96-8;acenaphthene, 83-32-9;dibenzofuran, 132-64-9;fluorene, 86-73-7;methylfluorenes, 26914-17-0;dibenzothiophene,132-65-0; phenanthrene, 85-01-8;anthracene, 120-12-7; carbazole, 86-74-8; 3-methylphenanthrene,832-71-3;2-methylphenanthrene,253184-2; 4H-cyclopenta[deflphenanthrene,203-64-5; 9-methylphenanthrene, 883-20-5; 1-methylphenanthrene, 832-69-9;fluoranthene, 206-440; acephenanthrylene,201-06-9;pyrene, 129-00-0; benzo[a]fluorene, 30777-18-5;benzo[blfluorene, 30777-19-6;methylpyrene, 27577-90-8; benzo[b]naphtho[1,2-d]thiophene,20543-6; benz[a]anthracene, 56-55-3; chrysene, 218-01-9; benzofluoranthenes,56832-73-6;benzo[e]pyrene,192-97-2; benzo[a]pyrene, 50-32-8; perylene, 198-55-0; indeno[l,2,3-~d]pyrene, 193-39-5;benzo[ghi]perylene, 191-24-2; triphenylene, 217-59-4; 4-methylphenanthrene, 832-64-4; carbon dioxide, 124-38-9. (IO) Smith, R. D.; Udseth, H. R.; Wright, B. W. In Supercritical Fluid Technology; Penninger, J. M. L., Radosz, M., McHugh, M. A., Krukonis, V. J., Eds.; Elsevier: Amsterdam, 1985; p 191.
