Energy & Fuels 1987,1, 280-286
280
Density Separation of Alkylated Coal Macerals Chol-yoo Choi, Gary R. Dyrkacz,* and Leon M. Stock* Argonne National Laboratory, Argonne, Illinois 60439, and Department of Chemistry, T h e University of Chicago, Chicago, Illinois 60637 Received November 24, 1986. Revised Manuscript Received February 18, 1987
A two-dimensional approach involving chemical alkylation and density gradient centrifugation has been investigated for the separation of coal and its petrographically pure macerals. The approach was tested by the study of two whole bituminous coals and several isopycnic maceral fractions from each of these coals. Alkylation of the whole coal changes the density distribution in a complex way. Alkylation of the individual maceral fractions also changes the density distribution. The specific changes are discussed in this report. The density variations that are observed as a consequence of alkylation suggest that isopycnic maceral fractions often contain several particulate constituents. Introduction The heterogeneous nature of coal presents a very serious obstacle to the definition of the structures of its components. The separation of a coal into pure macerals provides an important first step in the reduction of the complexity of the structural problem. This task can be most effectively accomplished by density gradient centrifugation as demonstrated by Dyrkacz and his co-workers.'V2 However, the densities of some maceral groups are not sufficiently different to enable a complete separation. For example, the densities of certain vitrinites and inertinites are so similar that centrifugation is unsuitable for their resolution. Moreover, there may be differences in the chemical compositions of materials in narrow density fractions within one maceral group. I t seemed appropriate to consider a new approach to maceral separation based simultaneously upon differences in density and in chemical reactivity. Particles that fortuitously have the same density but different chemical structures such as vitrinite and inertinite might be derivatized to different degrees and subsequently separated by density centrifugation. Even the constituents within one maceral group may also exhibit differences in their chemical behavior and be converted to more readily separable substances. There are significant variations in the oxygen ~ o n t e n t ~and - ~ the hydroxyl c0ntent~9~ of macerals. Therefore, chemical transformations that exploit the hydroxyl groups were considered. A~etylation,~ silylation; and alkylationg methods have been used to selectively derivatize the hydroxylic groups in coals. The conditions for the acetylation and silylation are harsh, and the results are not always reliable.8r10 An alternative procedure, the base-catalyzed alkylation reaction developed by L i ~ t t a transforms ,~ hydroxylic and carboxylic acid groups to ether and ester groups under mild conditions. The coal is first treated with tetrabutylammonium hydroxide, which serves as a base and a phase-transfer catalyst (eq 1). Reaction with a primary coal-OH
+ (1-C4HJ4NOH
-+
coal-0-
(1-C4Hg),N++ H 2 0 (1)
alkyl halide yields the 0-alkyl derivative (eq 2). coal-0-
+ RI
-
coal-OR
+ I-
The (2)
hydroxyl and carboxyl groups in coal are quantitatively
* T o whom correspondence should be addressed at Argonne National Laboratory (G.R.D. and L.M.S.) or The University of Chicago (L.M.S.). 0887-0624/ 87 / 2501-0280$01.50/0
Table I. Distribution of Added Methyl Groups in Alkvlated Coal As Estimated bv Solid 13CNMR SDectraa coal PSOC-726 PSOC-732
tot. 2.3 f 0.2 4.8 f 0.2
methyl group/100 C on oxygenb on carbonb 1.6 0.7 3.8 1.0
"Reference 16. *The results are based upon observed integrated areas. Recent results suggest that the quantity of C-methyl products may be underestimated.
converted to methyl ethers and methyl esters with excess methyl iodide in the presence of tetrabutylammonium hydroxide. l1 Several lines of reasoning suggested that the densities of the coal macerals, which are typically between 1.1 and 1.7 g cm-3,12would be altered significantly by alkylation. First, empirical concepts such as Traube's rules predict that the addition of alkyl fragments to heteroatomic substituents should reduce the density.13 Second, the alkylation reactions of the macerals of high-volatile bituminous coals occur almost exclusively on phenolic groups: and the densities of phenyl alkyl ethers are known to be less than those of the corresponding phen01s.l~ Third, the helium densities of alkylated Illinois No. 6 coals differ from the density of the original coal, and the change in density depends upon the structure of the alkyl group intr~duced.'~ Accordingly, we studied the impact of methylation and butylation on the behavior of two bituminous coals in density gradient centrifugation. (1) Dyrkacz, G. R.; Horwitz, E. P. Fuel 1982, 61, 3-12. (2) Dyrkacz, G. R.; Bloomquist, C. A. A.; Ruscic, L. Fuel 1984, 63, 1166-1173. (3) Dyrkacz, G. R.; Bloomquist, C. A. A.; Ruscic, L. Fuel 1984, 63, 1367-1373. (4) Dyrkacz, G. R.; Bloomquist, C. A. A.; Ruscic, L.; Horwitz, E. P. In Chemistry and Characterization o f Coal Macerals; Winans, R. E., Crelling, J. C., Eds.; American Chemical Society: Washington, DC, 1984. (5) Given, P. H.; Peover, M. E.; Wyss, W. F. Fuel 1960,39, 323-340. ( 6 ) Given, P. H.; Peover, M. E.; Wyss, W. F. Fuel 1965,44,425-435. (7) Bloom, L.; Edelhausen, L.; Van Krevelen, D. W. Fuel 1957, 36, 135-153. (8) Friedman, S.; Kaufman, M. L.; Steiner, W. A.; Wender, I. Fuel 1961, 40, 33-46. (9) Liotta, R. Fuel 1979, 58, 724-728. (IO) Ruberto, R. G.; Cronauer, D. C. In Organic Chemistry o f Coal; Larsen, J. W., Ed.; ACS Symposium Series 71; American Chemical Society: Washington, DC, 1978. (11) Liotta, R.; Brons, G. J. A m . Chem. SOC.1981, 103, 1735-1742. (12) Franklin, R. E. Trans. Faraday SOC.1949, 25, 274-286. (13) Van Krevelen, D. W. Coal; Elsevier: New York, 1961. (14) Shriner, R. L.; Fuson, R. C.; Curtin, D. Y. T h e Systematic Identification of Organic Compounds, 5th ed.; Wiley: New York, 1964; pp 46-50. (15) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 227-283.
0 1987 American Chemical Society
Energy & Fuels, Vol. 1, No. 3, 1987 281
Separation of Coal Macerals
..
1.1
1.2
1.3
1.4
1.5
Density (g ~ m - ~ )
Figure 1. Density distribution patterns of the alkylated PSOC-726 coals. The height of the largest peak in each pattern has been arbitrarily scaled to 1.0 in this figure and in the other figures.
Results and Discussion The term “maceral” in the following discussion refers to “maceral group” unless specific maceral types are mentioned. Two demineralized high-volatile A bituminous coals, PSOC-726 and PSOC-732 were alkylated by using Liotta’s m e t h ~ d These .~ coal products were characterized by elemental analyses and by solid-state 13CNMR spectroscopy. The results are presented in Table I. Independent replicate microanalyses of methylated PSOC-726 suggest that the degree of methylation has been reliably established. Solid 13C NMR spectra of the products obtained by using methyl-13C or butyl-lJ3C iodide clearly show that C- and 0-alkylation both occur.16 Limitations in quantitative 13C NMR analyses” prevent an accurate determination of the C-alkylation/O-alkylation ratio; however, our observations suggest that approximately 20-30% of the added alkyl groups are bonded to carbon atoms within the coal. The density spectra of the original coals and the alkylated coals were recorded by using density gradient centrifugation. The density distribution patterns were determined by measurement of the absorbance of each density fraction at 660 nm. The results, which are shown in Figures 1and 2, reveal that the density spectra of the coals are indeed altered by alkylation. These changes do not result from the treatment of the coal with tetrabutylammonium hydroxide in tetrahydrofuran. Control experiments carried out with the base, but without the alkylating agents, showed little change in the density spectra of the coals. However, in certain instances, long exposure of the coal or the maceral to tetrabutylammonium hydroxide can alter the density pattern. This aspect of the work will be discussed more fully in a forthcoming report. The butylated coals obtained from both PSOC-726 and PSOC-732 are less dense than the methylated coals, which are, in turn, less dense than the unalkylated coals. This apparently general trend is in accord with the helium densities of alkylated coals reported by Liotta and cow o r k e r ~ .Many ~ ~ other features of the density distribution patterns observed for the alkylated coals are surprising. The alkylated coal density patterns have little similarity to the unalkylated coal density patterns and do not exhibit the changes anticipated for a uniform density shift of each
1.0
1.1
1.2
1.3
1.4
1.5
Density (g cmm3)
Figure 2. Density distribution patterns of the alkylated PSO(2-732coals.
maceral group to lower densities. The vitrinites appear to shift to a lower density region, but the density behavior of the alkylated liptinites and inertinites cannot be ascertained. Specifically, the main bands, which are centered at 1.262 g cm-3 for unalkylated PSOC-726 and at 1.292 g for unalkylated PSOC-732, arise from pure vitrinite. The main bands observed in the alkylated coals are presumed to be the corresponding alkylated vitrinite. The main bands for the methylated coals are at 1.261 g for PSOC-726 and 1.250 g for PSOC-732. The main bands appear at 1.237 g cm-3 for butylated PSOC-726 and 1.168 g for butylated PSOC-732. Thus, the density of the alkylated vitrinite decreases as the length of the alkyl chain increases. The liptinites, which *arewell separated from the vitrinites in the unalkylated coals, are not resolved from the other macerals in the alkylated coals. The shoulders on the main bands for the methylated coals may be due to the superposition of methylated liptinites and vitrinites, but there is no obvious band that can be assigned to the liptinites in the density spectra of the butylated coals. The effects of alkylation on the inertinites are also unclear. There is only a small maximum at 1.312 g in the spectrum of the butylated PSOC-726. For PSOC-732, the inertinite maximum, which appears at 1.347 g cm-3 in the original coal, is absent in the methylated derivative but may be present at 1.249 g cm-3 in the butylated derivative. These observations indicated that alkylation did not uniformly affect the densities of each maceral group. It was evident that petrographic examination would be necessary to define the maceral distributions in the isopycnic fractions of the alkylated coals. Therefore, larger quantities (2 g) of methylated and butylated PSOC-726 were prepared and separated into narrow density fractions by density gradient centrifugation. The results of these larger scale separations are shown in Figures 3 and 4. Petrographic examination revealed that the morphology and the texture of the alkylated macerals appeared unchanged, but that their reflectances and fluorescence properties were altered. Methylated and butylated macerals that appeared to be vitrinites under white light fluoresced under blue light. In contrast, the macerals of a coal that had been treated with tetrabutylammonium hydroxide, but not alkylated, showed no unusual changes in fluorescence properties. Usually, only liptinites fluoresce under blue light while vitrinites and inertinites show little or no fluorescence.18 Although the change in fluorescence
(16) Choi, C. Y.Ph.D. Dissertation, The University of Chicago, Chi-
cago, IL,1986.
(17) Haaaman, E. W.;Chambers, R. R., Jr.; Woody, M. D. Anal. Chem. 1986,58,337-394.
(18) Ting,F.T.C.; In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic: New York, 1978; Vol I.
282 Energy &Fuels, Vol. 1, No. 3, 1987 260
Choi et al.
,
1
Table 11. Results of Methylation for the Macerals density, g cm-3 methy1/100 C" O/lOO C PSOC-726 liptinite 1.193-1.201 2.0 4.6 vitrinite 1.258-1.271 3.2 7.6 inertinite 1.348-1.356 1.0 6.3 maceral
7
601
0
iis
'O
0
LIP~I~;~. ,
A
Vltrlnlle
,
?"
,
!n-tr!n!?
,
1
liptinite vitrinite inertinite inertinite
PSOC-732 1.189-1.195 1.292-1.295 1.372-1.378 1.420-1.426
4.4 5.2 3.2 1.8
5.9 9.2 7.7 10.0
aThe precision of these values is hO.2.
20 AA
0 10
11
12
13
14
16
Denslty (g c ~ n - ~ )
Figure 3. Density distribution pattern and maceral analysis of the methylated PSOC-726 whole coal. 200
,
1.10
1.16
1.20
1.25
1.30
1. 5
Denslty (g ~ m - ~ )
Figure 5. Density distribution patterns of the alkylated vitrinites (1.258-1.276 g ~ m - from ~ ) PSOC-726 coal.
1.0
1.1
1.2
1.3
1.4
Denslty (g cm-s)
Figure 4. Density distribution pattern and maceral analysis of
the butylated PSOC-726 whole coal.
is clearly a consequence of alkylation, the phenomenon is complex, depending in part upon the immersion oil and the light intensity as well as the occurrence of alkylation. In summary, the petrographic work with the alkylated PSOC-726 coals suggested that the alkylated liptinites had higher density than the original liptinite and that the alkylated vitrinites and the alkylated inertinites had lower densities than the unalkylated macerals. The observed density shifts are complex and cannot be explained in terms of a uniform effect of alkylation upon the macerals. Thus, it was clear that the objectives of this study could not be realized by the alkylation of whole coals. Therefore, pure macerals were alkylated to establish the effect of alkylation on the density. Alkylation of Pure Maceral Fractions. Preliminary experiments showed that small quantities (20 mg) of macerals could be alkylated reproducibly. Accordingly, macera1 fractions of PSOC-726 and PSOC-732 coal were separated by density gradient centrifugation. Petrographic analyses indicated that the selected samples contained more than 98% of a single maceral group. Seven fractions were alkylated, and the degree of alkylation was determined by a comparison of elemental content of the macerals and the alkylated macerals. The results are summarized in Table 11. Fewer alkyl groups were added to the macerals of PSOC-726 than the macerals in PSOC-732. This is expected since PSOC-726 is a higher rank coal than PSOC-732 and, also, the total oxygen content of PSOC-726 is
less than that of PSOC-732. The number of added alkyl groups per 100 carbon atoms in the macerals of both coals decreased in the following order: vitrinite > liptinite > inertinite. Density Behavior of Alkylated Macerals. The influences of added alkyl groups on the density of the macerals were determined by density gradient centrifugation. The maxima and widths of the density distributions for the alkylated macerals were defined by absorption measurements as previously described. Absorbance measurements were necessary because of the very small amounts of material (99 0 0
63 99
