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Concentration Estimate of Chemically Reactive Methylene and Methine Carbon Centers of Defined C-H Acidity in Coal by CP/MAS 13C NMR Spectrometry? Edward W. Hagaman,* R. Rife Chambers, Jr., and Madge C. Woody Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6201 Received February 17, 1987. Revised Manuscript Received April 27, 1987
The CP/MAS 13C NMR spectra of coal derivatives generated by serial carbon alkylations using methyl iodide reveal complex band shape changes in the methyl resonance region as a function of methylation step number. The spectra of coal derivatives serially prepared with methyl iodide whose specific 13Cenrichment is varied as a function of alkylation step number reveal a methyl group chemical shift reorganization that establishes repeated alkylation of some coal sites. From this result a minimum methylene/methine site ratio of 0.2 is estimated. Separate experiments that employ [9-13C]fluorene as the precursor for the alkylation base demonstrate that the reactive sites are a part of the native organic coal matrix and not artifacts of reagent incorporation in the coal.
Introduction Carbon-13 enriched reagents have been used previously in coal studies to label oxygen and/or carbon sites in coals.14 In the solid-state 13C NMR spectra of coal derivatives, resolution within the 0-methyl resonance band has permitted the identification of both hindered and unhindered phenolic functions2v3(via their labeled ethers) and carboxylic acids2 (via their labeled esters). The inherently larger chemical shift dispersion of methyl groups bound to carbon centers and the extreme structural diversity of coals have prevented detailed interpretation of the C-methyl resonance band in terms of the local methyl group environment. With regard to quantitative measurements from CP/ MAS 13CNMR experiments, previous work has established that “introduced” and “native” cod carbon populations are detected discriminately. This phenomenon is exhibited by the coal used in this study, PSOC-1197, a low-volatile bituminous coal, which gives response factors for 0-CH,, C-CH,, and coal carbon fractions similar to those for a previously investigated high-volatile bituminous coal, 11linois No. 6.6 The introduced C-methyl carbon and coal carbon fractions have responses that differ by 10%. Hence, as for Illinois No. 6, the NMR measurements of introduced methyl groups/ 100 coal carbons are overestimated by 10% when compared with the bench mark 14Cradioassay results determined from alkylations utilizing 14J3Cdoubly labeled methyl iodide. Derived parameters from both the NMR and 14C-radioassayanalyses are tabulated for comparison. It has been shown recently that methylation of an array of coal model compounds having doubly benzylic methylene carbons typically occurs in a stepwise fashion. Multiple alkylation is not a major competitive process in these systems even though excess base and alkylating reagent are present.’ The monomethyl derivatives undergo further alkylation when reacted a second time. These results suggest that exhaustive alkylation of coal under similar conditions will necessitate multiple treatments. This expectation has been born out in a study of Research sponsored by the Division of Chemical Sciences/Office of Basic Energy Sciences, U.S.Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
selective methylation of a low-volatile bituminous coal that was treated repetitively with a series of carbanion bases and then quenched with methyl iodide? The CP/MAS 13C NMR spectra of these materials show striking resonance band shape changes in the C-methyl region of the spectrum as a function of alkylation step number. This paper reports the results of studies that examine the evolution of the C-methyl resonance band shape by performing serial carbon alkylations of the coal with methyl iodide whose specific 13C enrichment is varied as a function of alkylation step number. Part 1of the Results and Discussion presents an analysis of the solid-state 13C NMR spectra of these C-alkylated coal derivatives. The serial spectra reveal a methyl chemical shift redistribution from which we draw our central result: the assignment of a minimum methylene/methine site ratio for those reactive carbon centers in the coal that have C-H bond acidities in the range 19 < pK, < 22.a Part 2 of the Results and Discussion discusses experiments that quantiatively estimate the number of methyl groups bound to these carbon sites in the coal. In addition, the chemical or physical retention and subsequent alkylation of fluorene itself, a reagent in contact with the coal, is quantitatively assessed to establish the extent to which the alkylated carbon sites are part of the coal network or an artifact derived from reagent incorporation in the coal. Results a n d Discussion
1. Evidence for Alkylation of Methylene and Methine Sites i n the Coal. The carbon alkylation experiments reported in this paper were performed on a low-volatile bituminous coal, PSOC-1197, which had been (1) Hagaman, E. W.; Woody, M. C. Proc.-Int. Kohlenwiss. Tag. 1981, 807-81 1. (2) Liotta, R.; Brons, G. J. Am. Chem. SOC.1981, 103, 1735-1742. (3) Hagaman, E. W.; Woody, M. C. Fuel 1982, 61, 53-57. (4) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Stock, L. M. Fuel 1984,63,513-521. ( 5 ) Chambers, R. R., Jr.; Hagaman, E. W.; Woody, M. C.; Smith, K. E.; McKamey, D. R. Fuel 1985,64, 1349-1354. (6) Hagaman, E. W.; Chambers, R. R., Jr.; Woody, M. C. Anal. Chem. 1986,58, 387-394. (7)Chambers, R. R., Jr.; Hagaman, E. W.; Woody, M. C. Fuel 1986, 65, 895-898.
(8) Chambers, R. R., Jr.; Hagaman, E. W.; Woody, M. C. In Chemistry
of Polynuclear Aromatic Compounds; ACS Symposium Series: American
Chemical Society; in press.
0887-0624187 / 2501-0352$01.50/0 0 1987 American Chemical Society
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Figure 1. CP/MAS 13C NMR difference spectra (PMe),,"*(PH),: (A) n = 1; (B)n = 2; (C) n = 3.
Figure 2. CP/MAS 13C NMR difference spectra: (A) (PMe)l* - (PMe)lo;(B) (PMe)2**- (PMe)2*o; (C) (PMe)3*** - (PMe)3**o.
pretreated to convert hydroxylic functions into O-methyl derivatives as described in the Experimental Section. This O-methylated coal, PSOC-1197-OCH3, is designated P. Much of the spectral interpretation presented in this paper is of difference spectra generated from serially alkylated derivatives of P, which often differ by virtue of the percent abundance of 13C contained in the alkylating reagent, CH31, used in their preparation. In order to facilitate discussion of various difference spectra, we use the following notation: (PMe),,*O-, where PMe indicates a carbon-alkylated derivative of P produced by using the fluorenyllithium/methyl iodide system described in the Experimental Section. The subscript gives the number, n, of completed serial alkylations the material has undergone. The superscript symbols specify, in order from left to right, that the first to nth serial alkylation has been performed by using 13C-enriched (asterisk) or naturalabundance (degree) CHJ. Thus, (PMe)3**oindicates a coal derivative obtained after three serial alkylations, the first and second performed by using enriched 13CH31and the third performed by using natural abundance 13CH31. The formalism that indicates the difference of two coal derivatives is to be interpreted as the difference spectrum derived from subtraction of the spectra of the indicated coal derivatives. In order to monitor the accumulating number of methyl groups introduced into the coal in consecutive alkylations, the CP/MAS 13C NMR spectra of the products can be subtracted from that of the O-methylated coal, P. However since we know the coal products are not completely free of reagents used in the derivatization (vide infra), in practice the spectrum of P is replaced with that of (PH),, i.e., P that has been subjected to the methylation reaction procedure but with the methyl iodide addition replaced with a protonic quench. This blank and the methylated products have more similar reagent impurity levels, and
the quality of the difference spectra generated from these materials is improved. In most cases the subtraction criterion that is applied is nulling the aromatic resonance area in the difference spectrum. If the methyl resonance area is evaluated by the integral over the entire aliphatic resonance band, this procedure is equivalent to the numerical evaluation from the change in aromaticity, Af,, between the samples. We analytically show in part 2 that any systematic errors that arise from inexact cancellation of the aromatic area are minor. Figure 1 displays the aliphatic region of three difference spectra, (PMe),"* (PH), (n = 1,2,3), whose areas are normalized to constant aromatic area in the common, blank spectrum. The spectra depict the accumulating sum of methyl groups, 0.8, 1.8, and 2.0 CH3/100 coal carbons, found in the organic matrix of the coal following the first, second, and third alkylations, respectively (vide infra). To a good first approximation the carbon resonances of the native coal cancel in these spectra. In a strict sense exact cancellation is not expected for either the aliphatic or aromatic resonance bands since the introduced methyl groups will alter the chemical shifts of carbons a t and within three bond distances of the substitution site. This chemical shift reorganization between the alkylated coal and the blank is reduced to a small perturbation in the difference spectrum of Figure 1 by the use of high 13C isotopic enrichment in the methylation reagent. The 54.6 atom % 13CH31used to prepare these materials yields an intensity advantage of 50.6 (54.6/1.08) for the introduced methyls relative to that of the native coal carbon (1.08 atom % 13C). The total area shift due to the resonances of the coal carbons represents a 2% error in the difference spectrum on a 1:l carbon basis. The fact that several carbon chemical shifts will be perturbed by the introduction of a single methyl group could inflate this error.
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However, to the extent that resonance reorganization in the derivative is self-compensating, i.e., occurs in both field directions over a limited chemical shift domain to yield a small net area change, the error increase from this effect i-, minimized. 'I v;o striking observations can be made from Figure 1. IPS the number of added methyl groups increases with wid alkylation, the peak methyl resonance chemical shift moves to lower field and the full width a t half-height of the nretliyl resonance band, vlI2, monotonically decreases. Peak chemical shifts (and for the first, second, and third serial methylations are 19 (420 Hz), 25 (380 Hz), and 20 pprn (360 Hz), respectively. The observation of this uncomnion evolution in line shape as a function of seqliential chemical modification of a coal is unprecedented. 'I he line shape of the methyl resonance band following the first alkylation is asymmetric with a peak maximum ot 19 ppm and a prominent low-field shoulder a t 25 ppm (k'igure 111). Major spectral contributions occur from 11-27 ppm (19 f 8 ppm; peak maximum f0.5~~~~). The large distribution of methyl chemical shifts implicate the formation of both -C(H)CH3- and -C(R)CH3- functional groups. These two functionalities derive from the methylation of methylene and methine sites in the coal, respsctively. The products of reaction at tertiary carbon cmters will yield methyl resonances in the low-field methyl region that are not directly influenced by chemical mod; f k a t i m in successive methylation steps. The narrowed resonance band centered a t 26 ppm that develops with repetitive treatments (Figure IC) is associated with the formation of -C(R)CH3- where R is either a substituent native t o the coal or a methyl group introduced in a prior methylation step. For the special case where R = I3CH3,the NMR spectra display a reorganization of the high-field (19 ppm) methyl resonance component in the spectra of serially alkylated products such that a fraction of these introduced methyl resonances attain chemical shift values centered at 26 ppm. The spectroscopic result is consistent either with carbon substitution on the methyl group (a-effect) or on the carbon to which the methyl group is bonded ( P - e f f e ~ t ) . The ~ first alternative i s an unlikely chemical event given our reaction system and is eliminated by dipolar dephasing experiments, which show that the difference signals in Figure 1have associated dipolar dephasing times characteristic of methyl group^.^ The remaining option represents the chemically probable alkylation of carbon sites that have reacted similarly in a previous step. The evolution in local structure that accounts for the fraction of reorganized ~ i g 1 ) 3 in 1 Figure 1 is -CH2- --* -CHCH3-C(CH3)2... 1 ii 111 The resonance of the methyl group in ii is shifted downfield in iii by the introduction of the second methyl group. 'l'he 9 ppm geminal carbon P-effect found between 9methyl- and 9,9-dimethylfluorene (Table I) is a typical example. The magnitude of the effect is not strongly dpperident on a more distant structure.lOJ1 litter three methylation passes a rather symmetrical resonarm band with contributions from 19-33 ppm (26 f '7 ppm; peak maximum f 0 . 5 ~emerges. ~ ~ ~ ) It is apparent from the conversion i iii that this chemical shift re+
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(9) Stot,hers,J. B. Carbon-19 N M R Spectroscopy; Academic: New York! 297% Chapter 3. (10)Dal!ing, D. K.; Grant, D. M. J. Am. Chem. SOC.1972, 94, 6318--5.324. (11) Ualling, D. K.; Zilm, K. W.; Grant, D. M.; Heeschen, W. A.; Hextm, 'Q'. :J.: Pugmire, R. J. J . Am. Chem. SOC.1981, 103, 4817-4824.
Table I. Selected Chemical S h i f t s of Fluorenes" compd a(CH4 b(C-9) fluorene E F1 36.7 9-methylF1 17.9 42.2 9,9-dimethylF1 27.0 46.7 9-phenylF1 54.4 9-methyl-9-phenylF1 25.2 54.5 9-ethyl-9-phenylF1 59.2 9,9'-bifluorene biFl 49.8 g-methyl(biFl)* 26.1 55.8 9,9'-dimethyl(biFl)* 20.9 55.3 9-hydroxy-9-phenylF1 83.4 9-methoxy-9-phenylFl' 89.0 Chemical shifts are in ppm downfield from Me4Si, G(Me,Si) = 6(CDC13) 76.9 ppm. All are solution values measured in deuteriochloroform. bComplete I3C NMR spectra are reported in the Experimental Section. Methoxy resonance occurs at 51.3 ppm.
+
organization is a measure of the concentration of reactive methylene sites in the coal. Methyl Resonance Band Shape Deconvolution: An Evaluation of Chemical Selectivity and Estimate of Reactive Methylene/Methine Sites in the Coal. The methyl resonance band shape can be used to assess site selectivity for each methylation step and estimate the relative concentration of reactive methylene and methine sites in the coal. This information is not directly accessible by the difference spectra shown in Figure 1 or by sequential difference spectra of the type (PMe),+l(n+l)*(PMe)nn*. Each of these difference spectra result from the summation of two resonance contributions of undefined band shape, i.e., that from the introduction of new methyl groups in the last methylation step, and that from earlier introduced methyl groups that assume new chemical shifts due to a change in their chemical environment, effected in the last methylation pass. The separation of these effects can be accomplished in an experiment that uses advantageously the low natural abundance of the 13C isotope. Consider the difference spectrum (PMe)2**- (PMe)2*0. The isotopic difference in the methylation reagent in the second serial alkylation between these samples constitutes the only difference between the samples. Hence the chemical shift reorganization of first-pass methyls that occurs in the second pass identically cancels in the difference spectrum, which becomes an accurate monitor of the resonance band shape and concentration of the methyl groups added in the second alkylation step. Analogous difference spectra can be made for the first [(PMe)l* - (PMe)lo] and third [(PMe)3*** - (PMe)3**o]alkylation steps, though the difference spectrum for the first alkylation pass is used solely to correctly subtract the native coal carbon contribution. The aliphatic resonance region of the difference spectra (PMe)nn*- (PMe)n(n-l)*rofor the three alkylation steps are presented in Figure 2. The aromatic carbon resonance null subtraction criterion used to generate these spectra works well for these samples that have essentially identical chemical histories. The first-pass methyl resonances (Figure 2A) display a band shape which is essentially that given in Figure 1A for (PMe)l* - (PHI,. The dominant 19 ppm peak is evident. In contrast to Fig. l B , the difference spectrum (PMe)***- (PMe)**" (Figure 2B) has a clear shoulder at 19 ppm showing that additional methylene sites are alkylated in the second pass. On the other hand, no high-field shoulder is found in the third-pass difference spectrum (Figure 2C), indicating that reactive methylene sites have been exhausted by this pass. The net effect is that the reaction shows a methine site selectivity as more alkylation treatments are performed.
CPIMAS I3C NMR Spectra of Coals
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Energy & Fuels, Vol. 1, No. 4, 1987 355
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Figure 3. CP/MAS 13C NMR spectra: of (A) (PMe)l*; (B) (PMe)2*o;(C)the difference (PMe)l*- (PMe)2*o.Spectrum C depicts the redistribution of a fraction of the first-pass methyl resonances shown in spectrum A, which results from a change in the methyl group chemical environment effected in the second-pass methylation step.
An estimate of the concentration of reacted methylene and methine sites in the coal can be made from the set of difference spectra (PMe),"* - (PMe)n+ln**o. The large 13C enhancement factor gained by the use of 13CH31for these samples (asterisk/degree = 54.6/1.108 = 49.3) means that to a good approximation the natural-abundance methyl groups added in the last-pass alkylation can be considered transparent. Within this approximation, the difference spectrum monitors the reorganization of nth-pass methyl resonances as effected by the ( n 11th-pass alkylation. The aliphatic region of the difference spectrum (PMe)l* - (PMe)2*ois shown in Figure 3C. The integral over the aliphatic spectral region is, by definition, zero and should ideally occur a t the aromatic carbon resonance null condition. In fact for these samples the null aliphatic integral is achieved with a slight (
