Energy & Fuels 1988,2,861-862
86 1
L'ommunications Definition of Reaction Sites in Coals by Double-Cross-Polarization Carbon-13 Nuclear Magnetic Resonance Spectroscopy
Sir: A recently developed 1H-13C-31P double-cross-polarization (DCP)/MAS 13CNMR technique allows selective, direct detection of the carbons in a 0.4-nm spherical volume element centered on 31P nuclei in organophosphorus substances.l Within this sensitive sphere, chemical shift labeled carbons are differentiated by their internuclear distance-dependent 31P-13C cross-polarization rates. This communication illustrates the use of this DCP NMR technique to derive a new operational classification of complex organic materials based on chemical reactivity. If a phosphorus moiety is chemically grafted into the organic matrix of such a material, the NMR technique delineates not the bulk carbon of the sample but that small fraction associated with the reactive centers in the material, as defined by the chemical procedure chosen to introduce the phosphorus functionality. The experiment reveals the carbon-bonding network in the microenvironment of the 31Patom. The definition of reactive sites by this methodology is illustrated for a low-volatile bituminous coal.2 The coal is first 0-methylated under conditions that convert phenols into aromatic ether^.^ This chemical step is performed to prevent oxygen chemistry from interfering in the process to be studied, namely, the generation of coal carbanions, R;, by treatment of the coal with the base (triphenylmethyl)lithium! This reaction introduces 7 methyls/1000 coal carbons when quenched with methyl iodide, or 7 tertiary phosphine oxide residues ((CH3)2R$O)/1000 coal carbons when quenched with dimethylphosphinic chloride. The conjugate acid of the base, with pKa = 31 in THF, the reaction solvent, places one bound on the functional group types that can form anions under these conditions; Le., the C-H acidities of the reactive sites have pKa < 31. In fact, the majority must have 19 < pK, < 22 since this 0methylated coal shows the same reactivity (7 sites/1000 coal C) with the base fluorenyllithium (pKa = 22 for the conjugate acid in THF) but undergoes little reaction (0.2 sites/ 1000 coal C) with (8phenylfluorenyl)lithium (pKa = 19 for the conjugate acid in THF).3 The DCP spectrum of this coal derivative, Figure lb, reveals three resonance bands; an intense signal at 14 ppm from the methyl resonances of the phosphine oxide group, a broad resonance band in the aliphatic region of the spectrum (30-60 ppm), and an aromatic resonance band with peak maximum near 140 ppm and width at half(1)Hagaman, E.W. J. Am. Chem. SOC.1988,110, 5594-5595. (2)The coal, PSOC-1197,was obtained through the Penn State Coal Sample Bank Program. Ita elemental analysis (Galbraith Laboratories, Knoxville, TN: C, 79.31 f 0.09;H, 4.22 & 0.06;N, 1.42 0.02;S,1.09 f 0.03;0,3.61f 0.26 (by difference);ash, 10.36 f 0.06)corresponds to the empirical formula CI&N&.~Os.~ (3)Chambers, R.R.,Jr.; Hagaman, E. W.; Woody, M. C. In Advances in Chemistry 21 7; Ebert, L., Ed.; American Chemical Society: Washington, DC, 1988; Chapter 15, pp 255-268. (4)The coal derivative was prepared by a published procedure with the substitution of dimethylphosphinic chloride for methyl iodide as the quench reagent. See: Chambers, R. R., Jr.; Hagaman, E. W.; Woody, M. C.; Smith, K. E.; McKamey, D. R. Fuel 1985,64, 1349-1354.
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Figure 1. 1H-13C-31PDCP/MAS 13C NMR spectra of a deriv-
atized coal (a, b) and dimethyl(9-methylfluoren-9-yl)phosphine oxide (c), recorded on a Nicolet NT-200 spectrometer (4.7 T) equipped with a Doty Scientific triply tuned single-coil probe employing alumina rotors. Spectra (2K FIDs; SW = a10 KHz; digital resolution = 20 Hz/point; line broadening of 50 Hz for the coal and 10 Hz for the model) were acquired b using a direct difference-Dulse seauences with lH-13C and 13C-gP cross-Dohization co&act times of 1 and 15 ms, respectively. Cross-polarization rf fields were 45 KHz. MAS = 4.0 0.05 KHz. Recycle delay: 1 s (coal) and 4 s (model). The number of acquisitions for parts a-c were 410K, 300K, and 400, respectively. A 40-ps dephasing interval was used to generate the dipolar-dephasedDCP spectrum (part a). Only carbons within three bond distances of the 31P atom contribute to the DCP spectrum of the model. Assignments are indicated on the spectrum; an asterisk indicates spinning sidebands.
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height of 30 ppm. If the possibility of one, two, and four coal carbon centers situated one, two, and three bonds distant from the 31Patom in this derivative is allowed for, the DCP spectrum contains chemical shift and intensity data on ca 5% of the coal carbon represented in the conventional CP/MAS 13CNMR spectrum of the sample (0.7 phosphine oxide groups/ 100 coal carbons X 7 carbons in the DCP sphere). Signal intensities from these sites are scaled by cross-polarization rates, which nominally have an inverse sixth power internuclear distance dependence. On the basis of similar 13C-31P cross-polarization rates for the methyl and aliphatic resonances bands, the latter is attributed to those centers that are bound to phosphorus, i.e., reaction centers with strong (one-bond)31P-13C dipolar interacti0ns.l Resonances from tetrahedral carbons in the second and third bonding sphere relative to the 31Patom may also contribute to this band. The aromatic resonances represent those carbons two and three bonds removed from the 31Patom. Hence, the NMR data specify the reaction site as a tetrahedral carbon bonded principally to trigonal carbon centers. Consideration of common functional group types with the requisite C-H acidity6 and the DCP reso0 1988 American Chemical Society
862 Energy & Fuels, Vol. 2, No. 6, 1988
nance distribution in Figure l b support the assignment of the reaction sites in the coal to moieties that form delocalized or aromatic anions upon proton removal, e.g., fluorenes, benzanthrenes, etc. Ketonic residues with acidic a-C-H bonds comprise a chemically reasonable functional group class that can be discounted as reactive site candidates on the basis of the DCP spectrum. The carbonyl carbon in such a potential site would occur in the second bonding sphere from the 31Patom in the derivative and would generate a resolved carbonyl resonance band in the spectrum. The DCP spectrum of dimethyl(9-methylfluoren-9-yl)phosphine oxide, a substance whose C(9)-H acidity in the parent molecule (9-methylfluorene) is in the appropriate pK, range, is shown in Figure IC. This spectrum is comparable to that of the coal derivative with respect to the area distribution between the methyl and aromatic resonances. The C(9) resonance in the spectrum of the model (53 ppm) is represented in the spectrum of the derivatized coal by the broad range of low-intensity signals in the 30-60 ppm region. This result points to the extreme structural heterogeneity of the reaction sites in this coal. This broad envelope can be partitioned into two components that represent protonated and nonprotonated fractions by combining the DCP and dipolar dephasing techniques! as shown in Figure la. The nonprotonated carbon signal intensity observed in the 50-60 ppm chemical shift range in this spectrum arises from methine sites (e.g., Ar,CHR) in the organic matrix of the precursor coal; the
Book Reviews
balance of the broad resonance in the DCP spectrum arises from corresponding methylene sites (e.g., ArzCH2).’ This example illustrates the level of structural detail that is accessible with the use of this general methodology on complex organic mixtures. It demonstrates solid material characterization based on the selective identification of a unique set of carbon centers that determine the material’s chemical behavior in a specific reaction system. This definition can be elaborated by changing the reaction system. The new structure and/or reactivity information are realized by detection of carbon resonances one or more bonds removed from the reaction center but within 0.4 nm of the 31Patom. While there are some intermolecular 31P-13Ccontacts near this soft boundary, the DCP spectrum reflects almost exclusively intramolecular carbons.’ Hence, the resonances of reagent and solvent impurities that can confound conventional CP/MAS 13C NMR studies of complex organic mixtures are suppressed in DCP spectra. Acknowledgment. I thank R. R. Chambers, Jr., and M. C. Woody for the coal derivative used in this study. This work was supported by the Division of Chemical Sciences/Office of Basic Energy Sciences, US.Department of Energy, under Contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Inc. Registry No. Dimethyl(9-methylfluoren-9-yl)phosphineoxide, 113844-58-9. (7) Work is in progress to place these observatiionson a quantitative
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(5) Streitwieser,A., Jr.; Ciuffarin, E.; Hammons, J. H. J. Am. Chem. SOC.1967,89,63-67. Streitwieser,A., Jr.; Word, J. M.; Guibe, F.; Wright, J. S. J. Org. Chem. 1981,46,2588-2589. Kaufman, M. J.; Gronert, S.; Streitwieaer,A., Jr. J. Am. Chem. SOC.1988, 110, 2829-2835. (6) Opella, S. J.; Frey, M. H. J. Am. Chem. SOC.1979,101,5854-5856. Murphy, P. D.; Cassady, T. J.; Gerstein, B. C. Fuel 1982,61,1233-1240.
Murphy, P. D.; Gerstein, B. C.; Weinberg, V. L.; Yen, T. F. Anal. Chem. 1982,54,522-525. Wilson, M. A.; Pugmire, R. J.; Karas, J.; Alemany, L. B.; Woolfenden, W. R.; Grant, D. M.; Given, P. H. Anal. Chem. 1984,56,
933-943.
basis.
(8) Stejjskal,E. 0.;Schaefer, J.; McKay, R. A. J. Magn. Reson. 1984, 57,471-485.
Edward W.Hagaman Chemistry Division, Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6201 Received J u n e 29, 1988 Revised Manuscript Received August 26, 1988
Book Reviews Innovation in Process Energy Utilization. Edited by A. Rogers et al. The Institution of Chemical Engineers Symposium Series No. 105. Hemisphere Publishing Corp.: New York. 1988. 445 pp. $98.50. This book is the documentation of a 3-day symposium on the title subject organized jointly by The Institution of Chemical Engineers (South Western Branch) and The Institute of Energy (South Wales and West England Section) and held a t the University of Bath during Sept 16-18, 1987. The objective of the conference and this book is to highlight innovations in the utilization of energy in the process industries “with the express purpose of cross-fertilizing ideas between industries”. I believe that, by emphasizing unit operations and cost reduction in a large number of case studies, this objective has been achieved. Admittedly, most of the case studies are site-specific and therefore not quantitatively transferable to other sites, but the approaches are stimulative and generate a large number of options that may not be immediately obvious to someone wishing to improve the economics of energy utilization. One of the strengths of the book is the breadth of topics covered, which are divided into six categories: drying and evaporation;
energy control systems; applications of thermodynamics; process heating and waste heat recovery; energy from wastes; funding of innovative projects. In each of these categories, except the last, which has only one paper, there are from three to eleven different presentations covering case studies varying from development of computer models, through control strategies, to pilot plant and full-scale plant test results. For example, the drying and evaporation category includes use of heat pumps, vapor compression, adsorption, and membrane technology to achieve results. The process heating and waste heat recovery category covers fuel cells, fluid-bed boilers, fluid-bed heat exchangers, regenerative heaters, pump-around heat recovery systems, submerged combustion, coal-fired process heaters, energy efficient cyclones for classification, and design considerations to conserve heat. From the wide range of topics covered in just these two categories, it is clear that the book presents much food for thought for anyone contemplating ways to reduce energy consumption. However, that is really what it is, food for thought, because the examples are so site-specific that similar studies must be made for the actual site for which an application is being considered before decisions about implementing the application can be reliably made.
