1564
Anal. Chem. 1983, 55, 1564-1568
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LITERATURE CITED Seshadri, K. S.;Cronauer, D. C. Prepr. Pap.-Am. Chem. Soc., Dlv. Fuel Chem. 1982, 27(2), 64-75. Kuehn, D. W.; Davis, A.; Snyder, R. W.; Starslnic, M.; Painter, P. C. Prepr. Pap.-Am. Chem. SOC., Div. Fuel Chem. 1982, 27(2), 55-63. Ruberto, R. G.; Jewell, D. M.; Jensen, R. K.; Cronauer, D. C. Adv. Chem. Ser. 1976, No. 757, Chapter 3. Netzel, D. A.; McKay, D. R.; Heppner, R. A,; Guffey, F. D.; Cooke, S. D.; Varie, D. L.; Linn, D.E. Fue/ 1981, 60, 307-320. Brown, J. K.; Ladner, W. R. Fuel lS60, 3 9 , 87-96. Bartle, K. D.; Martin, T. G.; Williams, D. F. Fuel 1975, 5 4 , 226-235. Cantor. D. M. Anal. Chem. 1978, 50, 1185-1187. Charlesworth, J. M. Fuel 1980, 5 9 , 865-870. Clutter, D. R.; Petrakls, L.; Stenger, R. L.; Jensen, R. K. Anal. Chem. 1972. 44. 1395-1405. Dickinson, E. M. Fuel 1980, 5 9 , 290-294. Gillet, S.; Rublni, P.; Delpuech, J.; Escalier, J.; Valentin, p. Fuel 1 ~ 8 1 , 60, 254-262. Haley, G. A. Anal. Chem. 1972, 44, 580-585. Suzuki, T.; Itoh, M.; Yoshinobu, T.; Watanabe, Y. Fuel 1982, 6 7 , 402-4 IO.
(14) Tewarl, K. C.; Hara, T.; LI, N. C.; Fu, U. C. Fuell981, 60, 1137-1142. (15) Yokoyama, S.;Uchino, H.; Katoh, T.; Sanada, Y.; Yoshida, T. Fuel 1981, 60, 254-262. (16) Retcofsky, H. I,; Schweighardt, F, K.; ~ ~ ~ M,g Ah ~ , ~Chem, / , 1977, 49. 585-586. (17) Oka, M.; Chang, H.; Gavalas, G. R. Fuel 1977, 56, 3-8. (18) Swansiger, J. T.; Dlckson, F. E.; Best, H. T. Anal. Chem. 1974, 46, 730-734. (19) Solomon, P. R. Prepr. Pap.-Am. Chem. Soc., Dlv. Fuel Chem. 1979, 24(2), 184-195. (20) Petrakis, L.; Ruberto, R. G.; Young, D. C.; Gates, 8. C. Ind. Eng. Chem. Fundam. 1983, 22, 298. (21) Shih, S. S.; Katzer, J. R.; Kwart, H.; Stiles, A. B. Prepr., Div. Pet. Chem. Am. Chem. Soc. 1977, 22, 919-940. (22) Herod, A. A.; Ladner, W. R.; Snape, C. E. Phllos. Trans. R . SOC. London, Ser. A 1981, 300, 3-14. (23) Snape, C. E.; Ladner, W. E.; Bartie, K. D. “Coal Liquefaction; NMR Spectroscopic Characterizatlon and Production Processes”, in press, Chapter 5. (24) Luus, R.; Jaakola, T. H. I. AIChEJ. 1973, 19, 760-766.
RECEIVED for review January 8,1982. Resubmitted September 7,1982. Accepted May 10,1983. Support for this work was under Contracts 14940 provided by the Department Of and 790ET14880.
Analysis of Peroxytrifluoroacetic Acid Oxidation Products from Victorian Brown Coal T. Vincent Verheyen and R. B. Johns“ Organic Chemistry Department, University of Melbourne, Parkville, Victoria 3052, Australia
A method Is described for the detalled quantitative structural ldentlflcation of the components present In the oxidation product mlxture of a hlghiy allphatlc brown coal. Inltlal gross characterlratlon of the resldual ailphatlc structure present In the total reaction mlxture was achieved by ’H NMR spectrometry. Separation Into prlmary/secondary and tertlary protons was based on chemical shift limits. Further structural informatlon could be obtained vla separation of the complex product mixture Into neutral, acidic, and polyacldlc fractions, by solvent extractlon at basic and acldic pH comblned with resin (Amberlite XAD-8) adsorption/elutlon. These three fractlons were then derlvatlred and thelr CHCI,-soluble components Investigated by gas chromatography/mass spectrometry. The use of resin precluded the loss of components through Incomplete extractlon. Thls technique revealed that the fractlons were predomlnantly comprlsed of long chaln dlols, hydroxy aclds, dlcarboxyllc aclds, and short chaln polycarboxyllc aclds.
The use of peroxytrifluoroacetic acid (perTFA) a8 an oxidation reagent for the structural investigation of coal related materials has been reported (1-4).The reagent differs from “classical” oxidizers such as Cfi and Mn7 in the following ways: (i) preferential attack on aromatic ring systems coupled with a reluctance to cleave aliphatic chains, (ii) insensitivity to minor variation in reaction conditions, and (iii) the ability to quantitate residual aliphatic protonated structures present in the total reaction product mixture by lH NMR spectrometry. A major problem associated with the quantitative structural determination of the polar perTFA oxidation product mixtures
(and from other oxidants) has been their incomplete isolation from the highly acidic aqueous solvent. This inability to completely extract the major polar products has hindered the utility of oxidative degradation techniques in the reconstruction of their parent structures. Neutral (nonacidic) compounds may also be expected to occur in the oxidation products derived from highly aliphatic macromolecules. However, they have been generally overlooked in the gas chromatographic analysis of oxidation product mixtures, due to their minor concentration compared with acidic moieties. This article reports on improvements to the l H NMR technique for quantitatively determining the residual aliphatic structure of perTFA oxidation products. The component isolation-derivatization scheme described in this article allows complete isolation of the organic components and also significantly improves their detection and quantitation by gas chromatography/mass spectrometry. The method also allows the structural investigation of the neutral (nonacidic) moieties produced during perTFA oxidation. In order to illustrate the effectiveness of the method, it has been applied to the perTFA oxidation product mixture from a highly aliphatic light lithotype (5,6) of low rank Victorian brown coal.
EXPERIMENTAL SECTION Coal Sample. The brown coal (A-2) is from the Flynn field (Latrobe Valley, Victoria) core LY-1276 at a depth of 50.0 to 50.5 m. The sample was classified as a “light” lithotype (5)with the following elemental composition expressed as a percent dry basis: C, 69.4; H, 5.8; N, 0.52; S (org), 0.25; and ash, 1.5. The A-2 brown coal contains an atypically high concentration of aliphatic structure as determined by solid-state 13C NMR (aromaticity- f(a) = 0.45) and infrared spectrometry. These data along with further structural information including solvent ex-
0003-2700/83/0355-1564$01.50/0 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
tractability has resulted in our reclassification of this A-2 brown coal as a pale lithotype. Oxidation. The expenmental procedure utilized for the brown coal oxidation has been (detailedby Den0 et al. (4). Triplicate oxidations were performed by using the following quantities of sample and reagents: coal, 100 mg; CF3C02H, 1 mL; 30% (v/v) H202,2 mL; and HzSO4, 1 mL. Upon completion, the products were centrifuged to remove undissolved material. ThEs solid residue was washed with distilled deionized water and dried at 100 "C prior to investigation by pyrolysis/gas chromatography (7). Proton Nuclear Magnetic Resonance Spectrometry ('B NMR). The 200 MHz, 'H NMR spectra of the total reaction product mixtures were obtained by using a Bruker WP 200 NMR spectrometer controlled by an Aspect 2000 computer. NMR tubes of 5 mm i.d., incorporating a calibrated reference capillary containing a solution of trimethylsilyl-l,l,2,2-tetradeuteriopropionate as the sodium salt in the oxidation mixture, were used for sample analysis. Gated homonuclear decoupling (medium power) of the huge resonance peak derived from the acidic solvent provided quantitative spectra of the aliphatic protons in the 0-4.4 ppm region. Electronic integration of the spectral regions (defined later) including the calibrated reference signal allowed quantitative assessment of their proton concentration present in the product mixtures. Spectrometer conditions are as follows: quadrature detection, 16K data acquisition points; pulse width, 3.7 ps (90" flip angle); spectral width, 3012.048 Hz; temperature, 300 K; decoupler offset frequency, 9825.66 Hz; observation offset frequency, 8320.00 Hz; relaxation delay, 0 e; acquisition time, 2.7197 s; dwell time, 166 rs; pulse repetition time, 8.0 s; recovery delay, 0.003 s; number of scans, 100; decoupler power, 15 db at 0.2 W setting. (The decoupler and observation offset frequency parameters were adjusted to place the decoupler at the center of the acidic proton resonance (-9 ppm) and place this irradiated band at the extreme left of the collected spectra. The resultant FID was subjected to a 1 Hz line broadening function prior to Fourier transformation. Fractionation. A sunimary of the method developed in order to isolate the Neutral, Acids 1,and Acids 2 fractions investigated is illustrated in Figure 1. One milliliter from each of the three A-2 brown coal product imixtures were combined and the pH of the mixture raised to pH 12 by the slow addition of KOH (20% (w/w)). The potassium srdts formed at alkaline pH were dissolved upon the addition of excess H20,this prevented their interference in subsequent extractions. The Neutral products were isolated from the alkaline solution by triple extraction with 10 mL of n-pentane/dichloromethane ( 4 1 (v/v)). Solvent was removed from the combined extracts by using a N2stream prior to weighing. Acidic products (Acids 1)were isolated by triple dichloromethane (10 mL) extraction after the aqueous alkaline layer had been reacidified to pH 2 with HC1. The combined extracta were again concentrated by removal of solvent, and the residue was weighed prior to methylation with BF3/MeOH (8). ]Excesstrifluoroacetic acid and water were removed from the residual aqueous layer by rotary evaporation. The residue which included precipitated potassium salts was then sonicated in methanol to extract the nonvolatile polar acidic products (Acids 2) and residual HzS04. This crude extract was then methylated with BF3/MeOH. The excess methylating agent was subsequently destroyed with a &fold excess of water. The total mixture was then centrifuged and the aqueous supernatant solution twice eluted through a column packed with Amberlite XAD-8 polymeric adsorbent resin (9,10). Residual H2S04 and salts were then eluted with water until the pH of the eluant was neutral. Methanol was then used to elute the Acids 2 fraction. The methanol/water meotrope was removed by rotary evaporation prior to the now concentrated, polar acids fraction being dissolved in acetone. Residual water was removed via CaSO, (anhydrous) addition and filtration. Gas Chromatography/Mass Spectrometry (GC/MS). Prior to GC/MS analysis!all three fractions1 (Neutrals and Acids 1and 2) were derivatized to their trimethylsilyl ethers by reaction with N,O-bis(trimethylsily1)trifluoroacetarnide(BSTFA), using acetone as a solvent. The chloroform-soluble fraction of these produds was then analyzed with a Varian 3700 GC equipped with a S.G.E. Unijector and 25 m X 0.25 mm SE 30 fused silica WCOT
1565
--
TOTAL REACTION PRODUCT MIXTURE KOH 20% (w/w)
pH. 12
extract,n-pentane/CH2CI2 (4:l vlv) 10 ml x 3
/\
extractable
NEUTRALS
non extractable
Aqueous Acid Mixture
/
conc. HCI
pH. 2 extract, CH2C12 1 0 ml x 3
/ \
extractable
Aqueous Polar Mixture
14% BF,/MeOH
.I=
ACIDS 1 (methyl esters)
1
evaporate
extract MeOH 10 ml x4
14% BF3/MeOH
SALTS
I
crude ACIDS 2.(methyl esters)
I
1 XAD-8
t
Amberlite XAD-8 elute H 2 0
~
ACIDS 2 (methyl esters)
I
SALTS
elute MeOH
ACIDS 2 (methyl esters) Flgure 1. Fractionation scheme for peroxytrifluoroacetic acid oxidation product mixture.
column and flame ionization detector. The column was temperature programmed from 50 "C to 80 OC at 28 "C/min, wait (5 min), and then at 4 O C/min to 300 "C. Integration was performed with a Hewlett-Packard 3390 electronic integrator calibrated with external standard compounds: 1,6-hexanediol, 1,6-hexanedicarboxylic acid, 1,2,3-propanetricarboxylic acid, hexadecanoic acid, 1,16-hexadecanedicarboxylicacid, and octadecanol, all present as their corresponding methyl esters or trimethyl silyl ethers. The chromatographic weight percentage data were calculated in absolute terms and describe the mass of the particular oxidation product relative to the mass of the total fraction. A Hewlett-Packard 5995A GC/MS system fitted with a open split Interface (S.G.E. Australia)and 50 m X 0.25 mm, SE30 fused silica WCOT column was used to analyze the three fractions. Identical temperature programming was employed as listed for GC with the MS operating at 70 eV. Contamination. All solvents employed were redistilled and checked by GC prior to use. The reagents employed were the highest grade commercially available and preextracted with chloroform where possible. The resin was preconditioned and extracted with the eluting solvents (10). Identification. Resonance peaks in the 'H NMR spectra were identified by comparison of their chemical shift with previously prepared standard mixtures. Chromatographic peaks were tentatively identified by comparison of their mass spectral fragmentation and chromatographic retention with those of available standards.
RESULTS AND DISCUSSION Experimental Method. The oxidation product mixtures were clear pale yellow solutions after the removal of insoluble material. Examination of the pyrogram produced by this insoluble material (by pyrolysis/gas chromatography) gave evidence for the presence of mineral residue only. This result
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
Table I. Quantitative ‘HNMR Analysis of A-2 Brown Coal Oxidation Product Mixture
a
6 (PPm) from Me,Si
assignment
structure a
primary and secondary protons tertiary protons acetic acid methanol succinic acid benzylic acids and esters carbonyl compounds malonic acid
(CH,),Si(CO,),CO,-Na’ RCH,, RCH,R R,CHR CH,CO,H CH,OH HO,C( CH,),CO,H ArCH RO,CCR, H HCR,CR=O HO,CCH,CO,H
Italic H denotes proton being observed.
0 0.3-1.45 1.45-2.20 2.14b 2.2b 2.13b
T,
wt
3.165 0.627
14.5 10.5 3.4
0.303
2.3
2.2-3.34 3.59b
%d
9.0 0.338
0.5
Chemical shift position determined by comparison with standard compound.
T, spin-lattice relaxation time for brown coal perTFA oxidation product. wt %, weight percentage defined as the fraction of hydrogen present in the specified aliphatic structure in terms ol the total hydrogen present in the unreacted A-2
brown coal. 2
Table 11. Composition of Brown Coal perTFA Oxidation Product Mixturea fraction residue acetic acid succinic acid malonic acid Neutrals Acids 1 Acids 2
wt % of initial coal 0.91 i 0.04 3.98 i 0.23 3.91 .c 0.21 0.0s * 0.01 0.17 ?: 0.05 3.22 i 0.19 16.27 k 1.32 29.20 i 2.11
a Wt = weight. Fractions are defined in Table I and Figure 1.
Chemical Shift (ppm)
Figure 2. Allphatlc region of 200.00 MHz ‘H NMR spectrum of A-2 brown coal oxidation product mixture: (1) (CH,),Si(CD2),C0,-Na+; (2) CH3COpH; (3) CH3OH; (4) HO,CCH&H2CO,H; (5) H02CCH2C02H.
is consistent with the low rank of the brown coal investigated (11).
Initial lH NMR analysis of the total reaction mixture (after mineral matter removal) allowed quantitative data to be obtained regarding the level of volatile products, including acetic, malonic, and succinic acids, along with general aliphatic resonance bands as defined in Table I. The major problems in the ‘H NMR experiments were the dilute nature of the samples and the enormous concentration of acidic protons present in the solvent. The application of decoupling irradiation (to reduce the intensity of the acidic proton resonance along with signal averaging) overcame these problems. A minimum level of decoupler power was applied to prevent possible distortion of intensities in the aliphatic region of the lH NMR spectra. The curving effect of the remaining acidic proton intensity on the spectral base line was virtually eliminated by adjusting the spectral width to place the irradiated peak at the extreme left of the spectrum. However the acidic solvent masked the aromatic chemical shift region of the lH NMR spectra. Inversion recovery T1(spin-lattice relaxation time experiments were performed to calculate the TI values for the individual spectral resonances depicted in Figure 2. The relaxation times are included in Table I and resulted in a pulse delay of 8 s (5 X TI)being employed to ensure full relaxation and quantification. Reproducibility, as gauged by lH NMR band areas was better than k5% for the three oxidation experiments. The polar acids (Acids 2) fraction was methylated prior to loading on the XAD-8 resin (Figure 1) in order to increase the efficiency of the adsorption/desorption process. The inclusion
of a methylation step prior to XAD-8 treatment also avoids a second resin treatment. This second passage would be required in order to separate the aqueous phase produced during the BF,/MeOH methylation procedure. All three fractions were soluble in chloroform after derivatization. lH NMR Spectrometry. The averaged lH NMR data for the product mixtures are expressed on a weight percentage basis in Table I. These were derived by comparing the calculated mass of hydrogen present in the defined aliphatic regions and products with the total mass of atomic hydrogen present in the unoxidized A-2 coal. These data in Table I indicate the preservation of over 40% of the hydrogen content of the coal in the aliphatic oxidation products. The major individual products formed comprise acetic, succinic, and malonic acids. These three compounds are indicative ( I , 4)of arylmethyl, Ar(CH2)2Ar,and ArCHzAr structures, respectively, where the latter two can be in cyclic as well as noncyclie structures. The dominance of acetic and succinic acids suggests the primary aliphatic structure of the light lithotype contains high concentrations of arylmethyl and phenylalkane species. The small peak attributed to methanol (Table I, Figure 2) suggests that aryl methoxy groups are not a major contributor to the structure of the brown coal. The predominant unresolved aliphatic resonances (0.3-1.45, 1.45-2.20,2.2&3.34 ppm) are present (Figure 2) as three broad resonance envelopes or bands. The dominant resonance band centered at 1.2 ppm is indicative of a large contribution by polymethylene (CH,), structures to the product mixture. The nature and origin of the structures contributing to these resonance envelopes will be discussed further in the GC/MS analysis of the fractionated product mixture. Fractionation. The quantitative data for the fractional composition of the oxidation product mixture obtained from A-2 brown coal are listed in Table 11. As expected, the predominant reaction products are polar acidic moieties. The
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983 X
X
NEUTRALS
3
Table IV. Comparison of Product Yields from Oxidation vs. Extraction for A-2 Brown Coal oxidation w/w % coal (DAF) basis
extractiona
w -hydroxymonocar-
0.2 1.1 0.8
0.4 0.04 0.3
boxylic acids alcohols
0.6
0.1
class C, -C, monocarboxylic acids a ,w -dicarboxylic acids I
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ACIDS 1
Includes a saponification step (6). Data obtained from ref 13. a
ACIDS 2
I
I
I
I
:a 0
10
I
1
-
40
30
ELUTION TIME ( m i d I
- 1
80
120
1
160
1
-
200
240
COLUMN TEMPERATURE (OC) Flgure 3. Identical sections of capillary gas chromatograms illustrating the variation in product type and distribution for the three fractions isolated from A-2 brown coal perTFA oxldaticm. The labeled products are identified and quantified in Table I I I (Supplementary material). Peaks marked with an X ,are procedural contaminants. The short bars and numbers under the neutrals chromatogram refer to the elution of w-6 through to w-1 diols of specifled carbon chain length.
level of insoluble residue obtained is similar to the ash content of the parent coal. The high level of vollatile products (Table 11) including C02(-70%), which were not measured, suggests that the perTFA reagent is very efficient in the destruction of the aromatic coal structure. The presience of a high degree of aromatic ring substiitution by alkyl and oxygenated moieties produces a less condensed, more open macromolecular structure characteristic of low rank cocils. The accessibility of the reagent (OH+)to the brown coal skeletal structure and activation of aromatic rings (12) by heteroatomic substitution allow efficient electrophilic attack producing only a 30% yield of detectable oxidation products. GC/MS Analysis. Identical sections of the gas chromatograms obtained for the three fractions are presented in Figure 3. GC/MS analysis of the fractions resulted in the identification of over one hundred different oxidation products. The identification and quantification of these chromatograms are available as supplement,ary material which is listed as Table 111. See paragraph at end of paper for information on obtaining supplementary material. The Neutrals fraction was found to be dominated by a mixture of straight chiain (Cg-C18) alcohols with diols being the major class of components. Evidence was found for low levels of triterpenoid ntructures in the 300 "C elution region of the chromatogram; however, these were not investigated any further. The chain length distribution for the alcohols maximizes at C12(Table 111,Figure 3) for all classes identified. Associated with the dominant terminal hydroxylated (w) diols are minor series of diols reporting hydroxyl substitution at intermediate positions w-1 through to w-6, based on their mass of unsaturated spectral fragmentation. A short series (Clo-Clz)
diols (position of double bond unknown) are also present. The less polar Acids 1 fraction is composed of several compound classes. These include straight chain (>Cg) dicarboxylic acids and monocarboxylic acids exhibiting terminal hydroxyl substitution. Low levels of straight chain monocarboxylic acids are also reported, however, these are expected to derive from impurities in the methylation reagent. The carbon chain length distribution of the authentic Acids 1 products reproduces the pattern reported for the Neutrals in exhibiting a maximum at Clz. Several positional isomers of the hydroxyl substitutent are present in the long chain hydroxy acids. These are principally at the w-1 and C4 positions. The C4 position is significant due to its cyclization under the acidic reaction conditions to produce a minor series of ylactones characterized by their intense m / e 85 ion. The major classes of compounds present in the dominant polar Acids 2 fraction include (1)short chain (
