Energy & Fuels 1992,6, 532-534
532
Table VI. Oxide Film Dispersion on Sodium Carbonate Promoted Chars sample 500 "C 500 "C 900 "C 900 " C a See
wt % Na
17.6 20.3 11.0 10.5
% Na oxidized to NazOza 20.6 17.4b 21.2 24.ab
text for explanation of assumptions employed. bRerun.
oxidized species. The heats observed are more consistent with a mixture of processes, all involving the oxidation of sodium metal. Again as has been pointed out, it is thermodynamicallyreasonable for sodium metal to form during the decomposition of the carbonate species above 500 "C. The X-ray diffraction data are also in accord with the formation of completely reduced sodium metal during pretreatment. Diffraction patterns for both the high- and low-temperaturesamples show evidence that some sodium metal still remains following calorimetric oxidation and air exposure. This suggests that a significant amount of sodium metal was present following the initial high temperature treatments. Some insight regarding morphological behavior is also available. Indeed, the moderately high dispersion (Table VI) suggests that the reduced sodium metal melts and spreads on the char surface to a considerableextent. This is supported by the normal melting point of sodium, 98 OC.= Moreover, there is ample evidence that sodium wets the surface of carbonaceous materials, especially coal (26) Barin, I. Thermochemical Data of Pure Substances; Verlagsgesellschaft: Weiskin, FRG, 1989.
cham2 It is also possible that some of the sodium on the high-temperature char evaporates during the final heat treatment step since the normal boiling point of sodium is about 900 "C. The sodium dispersion was eatimated as follows. First, the sodium content in each sample was determined by quantitative analysis. Oxidation stoichiometrythen provided the maximum amount of NazOzpossible. That is, it was assumed that all oxygen adsorbed by the samples participated in oxidation of reduced metallic sodium to Na202. Finally, the ratio of the amount of oxide formed to the maximum amount possible provided the fractional dispersion of sodium on the char surface. Table VI summarizes these results for each of the sodium promoted chars. Clearly, the dispersion is relatively high for all samples. However, it must be noted that the sodium dispersion is not as high as that found for potassium on the same char.*pg AB a final note, the results of this study suggest that the mechanism of char catalysis for sodium and potassium have both similarities and differences. They are similar in that in both cases gasification occurs via an oxidation/reduction process. They are different in that in the potassium case the metal is never fully reduced, whereas in the sodium case most of the metal apparently is fully reduced following the carbothermic reduction part of the cycle. The work done here suggests that additional work should be undertaken using different sodium and potassium precursors and perhaps the pure metals in order to more fully understand the role of alkali metal catalysts in gasification. Registry No. Na2C03,497-19-8; MgC03, 546-93-0;02,778244-7.
. . Communications N
Solvent Quality Assurance in Porphyrin Research Sir: Questions concerning the chemical stability of porphyrins in various solvents are important to porphyrin research in geochemistryand related analysis.'-7 Sporadic measurements of nickel porphyrin at low concentrations in dichloromethane suggested a question of solvent-related instability.6 Spectrophotometer source irradiation was found to cause a second problem, namely, photodegradation of nickel etioporphyrin-I. The effect is rapid in carbon tetrachloride: 119% loss during a 2-min scan. Under similar conditions, corresponding free-base porphyrin photodecomposes at about the same rate, while the vanadyl complex was 40-fold slower. Given the extent to which chlorinated solvents can enhance porphyrin research-as well as porphyrin decomposition-the present study was made to minimize such difficulties. (1) Symposium issue: Energy Fuels 1990, 4, no. 6. (2) Barwise, A. J. G.; Wolff, G. A.; Eglinton, G.;Maxwell, J. R. J. Chromatogr. 1986,368, 1-9. (3) Flynn, J. S.; Freeman, D. H. J. Chromatogr. 1987,386, 111-121. (4) Freeman, D. H.; Angeles, R. M.; Keller, S. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1988, 33, 231-8. (5) Boreham, C. J.; Fookes, C. R. J. Prep.-Am. Chem. SOC.,Diu. Pet. Chem. 1989,467, 195-208. (6) Freeman, D. H.; Swahn,I. D. Energy Fuels 1990,4,699-704. (7) Freeman, D. H.; OHaver, T. C. Energy Fuels 1990, 4, 695-99.
0887-0624/92/2506-0532$03.00/0
The presence of impurities in dichloromethanehas been the subject of numerous reports. To illustrate, chlorocarbons, phthalic acid and its esters? cyanogen chloride: hydrogen chloride, methyl chloride and chloroform,1°and tetrachloroethane" impurities have been identified. We have observed several instances where porphyrin losses were due to reactive impurities. These were traced to special lots of HPLC-grade solvent containing cyclohexane (C8H12:150 ppm) stabilizer where rapid partial degradation of nickel etioporphyrin-I, as well as free base, was found to occur in the absence of light. This is a plausible explanation for previously observed erratic spectral data? The use of dichloromethane containing cyclopentene was linked to certain HPLC difficulties12that were not found (8) Bowers, M. L.; Parsons, M. L.; Clement, R. E.; Eiceman, G.A,; Karasek, Jr. F. W. J. Chromatogr. 1981,206,279-288. (9) Franklyn, R. A.; Heatherington, K.; Morrison, B. J.; Sherren, P., Word, T. J. Analyst (London) 1978, 103, 662. (10) Sedivec, V.; Flek, J. Handbook of Annlysis of Organic Soluents; Halsted Press (Wiley): New York, 1976; p 146. (11) Bouis, P. A., J. T. Baker, Inc. Private Communication, 1991. (12) Pentene-stabilized dichloromethane caused anomalous HPLC peaks when an alcohol, either methanol or isopropyl alcohol, was introduced as the coeolvent, but not with hexane or ethyl acetate. The anomaly worsened with increasing air exposure. Cyclohexane-stabilized dichloromethane did not exhibit this property (Jarvis, B. B. Private Communication 1991).
0 1992 American Chemical Society
Communications
with cyclohexene (C6H16 48 ppm) stabilizer. Since the latter is unsuitable in other appli~ations,'~~'~ one can infer that dichloromethane may, on occasion, require end-use specific stabilizers. Dichloromethaneis often chosen for porphyrin manipulations because of its high solubility for nickel and vanadyl porphyrins6and its low boiling point. After extensive testing, we found dichloromethane reagent (Fisher Chemical, D-143) containing cyclohexene (Cd-I16 48 ppm) stabilizer was satisfactory for porphyrin research. The latter was used in the photochemical experiments to be discussed. However, it was also found that dichloromethane containing reactive impurities, as discussed earlier, showed no porphyrin degradation in the dark after passage through magnesium oxide (ca. 10 mL/g). The same was true for technical grade dichloromethane that was freshly distilled and for reagent grade preserved with cyclohexene. On the basis of limited tests, all three versions of this solvent (MgO filtered, fresh distilled, and reagent with cyclohexene) appeared to give equivalent results in photochemical experiments. Reagent grade solvents were used for photokinetic experiments. The initial porphyrin concentration was kept between 0.5 and 2 pg/mL. Photolysis was carried out in a simple optical train: quartz halogen lamp (Sylvania No. 58902-2,500W),a 5-cm quartz cell containing water as a heat filter, and a 2-cm-0.d. condensing lens (focal length 5 cm) focused inside a stoppered quartz sample cuvette. The distance between lamp and cuvette was 15 cm. All controls gave negative results when run in the dark (compared to positive results for reactive impurities, as stated above, in preliminary tests). Initial and post-irradiation concentrations were determined by derivative spectrophotometry? Preliminary sparging of sample solution with either nitrogen or oxygen showed no noticeable effect on photochemical reaction of nickel etioporphyrin-I in dichloromethane. Photochemical rate measurements were carried out with initial nickel porphyrin concentrationsof approximately 6 pg/mL. Solutions of nickel etioporphyrin-I in dichloromethane exhibit light absorbance at several wavelengths, most strongly with a molar extinction coeficient of e = 1.9 x 105 for the Soret (S)band, S, at 390 nm, and with lesser values of 1.1 X lo4 for the /3 band at 516 nm, and 3.04 X lo4 for Irradiation effects were measured the a band at 552 ma7 with quartz cuvettes at different wavelengths of (0> 316 nm, (11) > 460 nm and (111) 362-466 nm using Corning filters nos. 0160,3885, and 5113, respectively. Each filter behaved about the same and lessened the rate by about 15-fold. Since the three filters select for different absorption bands I(S,a,/3),II(a,/3), and III(S), the results suggest that photolysis in dichloromethaneis not specific to any one porphyrin absorption band. The use of a glass cuvette instead of quartz slowed the rate by about 1.4-fold. The photokinetic resulta, in general, obeyed the expression In (Co/C) = kqt (1) where Co is the initial concentration, C is concentration at time, t, where k = 0.693/t1/2,and q is an empirical constant that depends upon apparatus geometry, cell material, optical filters, source type, source intensity, etc. Results given in Table I were obtained under constant experimental conditions using stoppered quartz cuvettes, and q was assigned a value of unity. (13)Ibrahim, E. A.; Lippincott, R. L.; Brenner, L.; Suffet I. H. J. Chromatogr. 1987,393,237-253. (14)Campbell, J. A.;LaPack, M. A.; Peters, T. L.; Smock, T. A. Enuiron. Sci. Technol. 1987,21, 110-112.
Energy & Fuels, Vol. 6, No. 4,1992 533 Table I. Relative Half-Lives for Photodegradation of Nickel(I1) Etioporphyrin-Iin Various Solvents solvent CCl, CDC13 CHzCl2 C2F3C13 toluene diethyl ether
tll2 (h) 0.19 0.01 0.56 1.00
*
1.40 8.0
solvent tetrahydrofuran ethyl acetate cyclohexane dioxane methanol
tll2 (h) 50
140 170
220 290
35
The half-lives in Table I are averaged from the leastsquares analysis, using eq 1, of data taken in duplicate runs, except for degradation of nickel etioporphyrin-I in carbon tetrachloride where six runs provided a standard deviation of fO.O1 h. The resulting losses during sample exposure to the quartz halogen source were readily measured using a Hewlett-Packard 9834a diode array spectrophotometer. Spectrum acquisition was kept short enough (5 s or less) to preclude photodecompositionas a measurement error. Photoinstability of Ni porphyrin complexes in chlorinated solvents is not surprising. Chlorocarbons are good single electron acceptors. While our preliminary observations do not conclusively identify the decomposition mechanism, it is likely that the excited-state metalloporphyrin transfers an electron to chlorocarbons with subsequent cleavage of the carbon-chlorine bond. This process would generate chloride and a solvent radical.16 The latter would then initiate radical chain reactions which would destroy other NiP molecules. Consistent with this is our finding that triethylamine, a good single electron donor, inhibits NiP photodecomposition. Examples of electron transfer from porphyrins to other types of electron acceptors (e.g., peroxy radicals) have been documented. Yamamoto's study16 suggests that the porphyrin ring system undergoes a single electron transfer to solvent peroxyl radicals. Similarly,pulse radiolysis studies by Neta demonstrate that zinc tetratolyl porphyrin can undergo single electron transfer to chloromethylperoxyl and dichloromethylperoxylradicals.I7 Other photodegradation reactions of metalloporphyrins have been documented.18 Zinc and magnesium porphyrins undergo photooxygenation with disruption of the porphyrin ring system;19-21in some cases, singlet oxygen may be an intermediate.17 Photoreduction of zinc and magnesium porphyrins occurs when suitable hydrogen atom donors are present.22 In conclusion, the use of dilute nickel etioporphyrin is recommended to help assure solvent quality for porphyrin research. A failure to conserve mass during dilution with curtailed exposure to light provides an easily recognized indication of reactive impurities. Furthermore, nickel porphyrin solution can function as an a ~ t i n o m e t e to r~~ calibrate porphyrin photodegradationcaused by laboratory illumination. To illustrate, when samples were dissolved in carbon tetrachloride, placed in 10-mL glass volumetric (15)See for example: Eberson, L. Electron Transfer Reactions in Organic Chemistry; Springer-Verlag: Berlin, 1987,and referencestherein. (16)Yamamoto, K.; Hoshino, M.; Kohno, M.; Ohya-Nishiguchi, H. Bull. Chem. SOC.Jpn. 1986,59,351. (17)Alfasai, Z.B.;Mosseri, S.; Neta, P. J.Phys. Chem. 1989,93,1380. (18)Review: Hofp, F. R., Whitten, D. G. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978;Vol. IIB. (19)Fuhrhop, J.-H.; Kadish, K.; Davis, D. G. J.Am. Chem. SOC.1973, 95,5140. (20)Furhhop, J.-H.; Mauzerall, D. Photochem. Photobiol. 1971,13, 453. (21)Besecke, S.; Fuhrhop, J.-H. Angew. Chem., Int. Ed. Engl. 1974, 13,1, 50. (22)Seely, G.R.;Calvin, M. J. Chem. Phys. 1955,23,1068. (23)See, for example: Bowman, W. D.; Demas, J. N. J. Phys. Chem. 1976,80,2435.
634 Energy & Fueb, Vol. 6, No.4, 1992
flasks and positioned 2 m below two fluomcent lamps (GE Cool White 34 W), the measured tl,2 was 72 h. In dichloromethane, based on eq 1and Table I, a tolerance of 1%degradation is calculated as 4-h exposure, or 160 h for the vanadyl complex. For nickel etioporphyrin-I the tolerance increased 30-fold by substituting two GE Fluorescent Gold FG 40 W lamps. The use of 50 ppm triethylamine was found experimentally to lower the photodecomposition rate of Ni etioporphyrin-I by 12-fold, making this additive appropriate for routine metalloporphyrin transfers. The rate for free-base etioporphyrin-I was not affected by added amine. In sum, porphyrin research can be affected, even plagued, by solvent-related artifacts, degradation, and photodecomposition. To avoid such problems, a quality assurance protocol is highly recommended,ui26including (24) Freeman, D. H.; Phillip, W.; Taylor, J. K.; Crummett, W. B.; Amore, F. J.; Laitinen, H. k ;Reddy, M. Anal. C k m . 1980,52,2242-2249.
Book Reviews testa for reactive impurities and properly limited exposure to light.
Acknowledgment. We express our appreciation to the Amoco Production Co., Marathon Oil Co., and Unocal Corp., sponsors of Porphyrin Standards Research Consortium at the University of Maryland. (25) Keith, L. H.; Crummett, W.; Deegan, Jr., J.; Libby, R. A.; Taylor, J. K.; Wentler, G. Anal. Chem. 1983,55,2210-2218.
David H.Freeman,* Delphine Castres Saint Martin Daniel E.Falvey Department of Chemistry and Biochemistry University of Maryland College Park, Maryland 20742 Received January 28,1992 Revised Manuscript Received May 11, 1992
Book Reviews Coal Gasification for IGCC Power Generation. Edited by Toehi Ichi Takematsu and Chris Maude. IEACR/37. International Energy Agency Coal Research London, U.K., 1991. 80 PP. The IEA Coal Research branch produces a series of reports that represent technical assesementa and economic evaluations on a series of technicaltopics concerned with coal production and use. This report is devoted to coal gasification technologies and providea technical information on their applicability for producing electric power in integrated gasification combined cycle (IGCC) systems. These systems integrate coal gasification technologies with combined cycle equipment that feature combustion turbines as topping cycles and steam turbines as bottoming cycles. The report covers (1) IGCC power generation features, (2) coal gasification technologies, (3) status of hot dry cleaning technologies, and (4)status of IGCC dhrelopment worldwide by country. It represents an authoritative and very readable treatment of the subject and ita potential for being used to generate electric power with minimum environmentalimpact The report covers the topic up to 1989. It represents a useful reference document and provides an assessment of the prospects for commercial deployment and application. The chapter on dry hot gas clean up provides a realistic appraisal of the status and the many challenges that remain in removing particulate and capturing sulfur,alkali metal, and trace metal elements at elevated preesure and tempemture from product gases produced from coal gasification. Remaining problems include the demonstration of the technology, and the report discusses the complexities of the chemistry, (e.g., alkali and trace metal removal) that need solutions before the hot gas clean up gas purification step can be considered to replace conventional
gas clean up equipment. A number of large-scale trials of hot gas clean up systems have been recently announced for planta that are being constructed in the U.S.under the Department of Energy's Clean Coal program. Work on this topic is also being performed in Japan and Europe. Recent progrese hae been made on other more promiaing aspects of IGCC systems which are only mentioned or partially covered. They include the following: 1. commercial introduction of high-temperature (2300 OF), large-size turbines (250 m e ) , and progress in designing more efficient topping cycles based on even higher temperature gas turbines, and molten carbonate fuel cella which eliminate the need for steam bottoming cycles; 2. improvements in plant integration leading to high-efficiency systems (4346%) for generating electsic power from coal involving efficient utilization of air plant compressors, and integration of high-pressure nitrogen from the air separation plant; 3. coproduction opportunities in efficiently using synthesis gas (carbon monoxide and hydrogen) to produce high-value chemicale and fuels; 4.innovations in cycles that use low-level heat to simplify the IGCC system and lower costa. In addition, plant configurations and incorporation of storage promise low cost and high efficiency. Thew directions are more important for emerging commercial applications of IGCC systems that follow ita successful introduction in the U.S.and Europe based on ita success at the Cool Water station in the late 1980s. The quality of the IEA Coal Research report is good. The technical details are authoritative and represent a balanced but limited treatment and discussion. The format and illustrations provide an easily readable introduction to the topic. Seymour B. Alpert, Electric Power Research Institute
