ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
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Nuclear Magnetic Resonance Spectrometric Determination of Average Molecular Structure Parameters for Coal-Derived Liquids D. M. Cantor Phillips Petroleum Company, Research and Development, Bartlesville, Oklahoma 74004
'H and 13C Nuclear Magnetic Resonance (NMR) spectrometric techniques for the determination of a series of parameters whlch describe the average molecular structure of coll-derived liquids are presented. The methods are adaptations of establlshed technlques for aromatic fractions of petroleum. Average molecular structure parameters for six coal-derived llqulds are reported, and differences among the six samples are Interpreted.
Recent work in the development of alternate fuels technology has created a need for additional analytical methodology. Techniques for characterization of petroleum and petroleum derivatives have been developed over a 30-year period. Equivalent techniques for coal-derived liquids must now be devised. The development of suitable methods is simplified if experience gained in petroleum analysis can be applied to the new problems. Coal-derived liquids are extremely complex mixtures, often containing hundreds or thousands of components ( I ) . Highly aromatic condensed-ring structures predominate ( I , 2). Individual quantitation of each component is conceptually possible, but may be too detailed and time consuming for routine guidance of process development. An alternative approach involves characterization in terms of selected average properties of the sample. Such an analysis gives an easily interpreted picture of the sample, and yet contains significant quantitative information. Recent work on coal-derived liquids has resulted in NMR methods for the separate determination of C / H ratios for the aromatic and alkyl carbons of the sample (3, 4 ) , and for measurement of the ratio of aromatic to alkyl carbon (5,6), qualitative descriptions of coal liquid fractions have also been made (7). The approach taken in the current study is to use analytical data to calculate a series of parameters which describe an average molecule in the sample. This technique has been well developed for characterization of aromatic fractions of petroleum ( & I O ) , and is readily adapted to coal-derived liquids. Williams (8), using only l H NMR, originally defined the average molecular structure parameters shown in Table I. The method was extended and improved when Knight (9) applied 13C NMR to the problem. The nomenclature used in Table I is largely taken from a paper by Clutter et al. ( I O ) . The relatively high concentrations of fused-ring structures in coal-derived liquids means that some care must be taken in the interpretation of the parameters. For instance, the compound tetralin is considered to have two branches of two carbons each. Also, the calculations do not distinguish between saturate rings fused to aromatic rings, as in tetralin, and naphthene rings substituted on aromatic rings, as in cyclohexyl benzene. Thus the parameter R N was originally defined as the number of naphthene rings per average molecule (&IO), but, for coal liquids, is better defined as the 0003-2700/78/0350-1185$01.00/0
Table I. Average Molecular Structure Parameters: Definitions C = Weight fraction of carbon H = Weight fraction of hydrogen MW = Average molecular weight f a = Fraction of carbons which are aromatic f = C/H weight ratio in alkyl groups f , = C/H weight ratio in aromatic groups C, = Weight fraction of sample which is aromatic carbon ##Ca= Average number of aromatic carbons per molecule C , = Weight fraction of sample which is nonbridge aromatic carbon C I S= Portion of C, carbons which are alkyl substituted C I u = Portion of C, carbons which are unsubstituted # C , = Number of nonbridge aromatic carbons per molecule #C, = Number of saturate carbons per molecule %AS = Percent substitution of nonbridge aromatic carbon atoms r = Number of saturate rings per substituent R N = Number of saturate rings per molecule R , = Number of fused aromatic rings per molecule R , = Number of substituents per molecule n = Number of carbon atoms per substituent Table 11. Average Molecular Structure Parameters: Equations fa
=AiI(Ai - A , )
n = (H, t H?)lH, Ca = faC C I S= A 2 C / n cIu= ~ ~ H A H
c , = c , s c,u f
%AS = 1OOCIs/C, f = A 2 C / ( H , + HpW
r = n 7 0.5 - 6n/f =Ca = CaMW/12 =C, =C,MW/12 =C, = A2CMW/12 R a = l - ( 7ca - =C,)/2 R , = %AS=C,/100 RN = rR,
f , = A,C/HAH
number of saturate rings per molecule, so that fused saturate rings are also included. The equations used to calculate the molecular structure parameters (8-10) are summarized in Table 11. The variables A , and .q2 are the normalized integrals for the aromatic (100-175 ppm vs. TAMS)and alkyl (5-50 ppm) regions of the 13C NMR spectrum, respectively. H A , H,, and Hj are the normalized integrals for the aromatic (6.0-9.0 ppm), a-alkyl (2.0-4.0 ppm), and other alkyl (0.3-2.0 ppm) regions of the proton NMR spectrum. T w o assumptions are made in the derivations. First, it is assumed that all alkyl groups are present as substituents on aromatic rings. This is not strictly true for coal-derived liquids, but the concentrations of saturate compounds are low enough that any errors introduced are minimal. Second, it is assumed 0 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
Table 111. Calculated Average Molecular Structure Parameters I anthracene coal anthracene parameter oil liquid oil n 1.34 1.34 1.39 4.58 f 6.03 6.04 18.8 f, 18.5 18.9 0.905 fa 0.889 0.908 15.9 19.1 12.7 #C, ifcI 11.8 13.5 9.1 2.0 2.0 1.4 #CS 12.6 10.6 10.6 %AS Ra 3.1 3.8 2.8 RN 0.8 0.8 0.1 C 0.916 0.916 0.906 H 0.061 0.058 0.062 MW 234 276 186 that the C / H ratio a t the 01 position is equal to the C / H ratio in the remainder of the alkyl groups. This assumption will break down when branching at the a-alkyl carbon is common, but the short-chain substituents found in this study preclude high concentrations of such structures.
EXPERIMENTAL Hydrogen was added to anthracene oil in a continuous reactor at 800-1000 psi and 750 OF. Anthracene oils I and I11 were prepared with different cobalt-molybdenum catalysts. A nickel-tungsten catalyst was used to hydrotreat anthracene oil 11. The anthracene oils were used as proton-donor solvents in identical extractions of an East Texas lignite. Each extraction took place in a stirred autoclave with continuously flowing hydrogen, at 800 O F and 1500 psi. Three coal liquids were produced, containing all of the original anthracene oils plus dissolved coal. Proton spectra were taken on a Varian Associates T-60 CW spectrometer. The high viscosity of the samples requires that they be diluted with a less viscous solvent to achieve acceptable line widths; 100.0 atom % pyridine-& (Aldrich Chemicals) was found to be a good solvent for all of the samples investigated. Dilution factors of approximately 2 to 5 were used, depending on the sample viscosity. 13C spectra were generated on a Varian Associates CFT-29 spectrometer. Deuterochloroform (Aldrich Chemicals) was used as a solvent and internal lock for four of the samples. Two of the coal liquids were not completely soluble in CDCl,-they were run neat, with a D20 external lock. Approximately 1.0 weight % trifluoroacetylacetonate iron(II1) was added to the samples to reduce the 13C relaxation times. Despite typical TI values of less than 0.2 s, it was found that very long delays between RF pulses were necessary to ensure consistent results. A delay of 20 s was used for all I3C NMR spectra. Each spectrum required the accumulation of 2000 to 6000 transients. To prevent systematic errors due to differences in the nuclear Overhauser enhancements (NOE), the proton noise decoupler was turned off for these experiments. To minimize baseline errors, the aromatic and alkyl regions of the spectra were integrated separately. Carbon and hydrogen elemental analysis was performed using a combustion technique. It is possible t o determine these parameters using only the NMR spectra ( 3 , 4 ) ,but the combustion method was found to be more reproducible for the samples used in this study. Molecular weights were determined by vaporpressure osmometry.
RESULTS Average molecular structure parameters were calculated for three anthracene oils, and for the corresponding coal liquids produced when these solvents were used to extract coals. Multiple replicates were made on one pair of samples so that the precision of the method could be estimated. The parameters fall into four groups, depending on the estimated precisions: n, f,: k 5 % ; ifC,, ;Cl, BC,, fl: *lo%; f, %AS, R,, R,: *15%; and r , RN: f30%. The saturate ring parameter calculation involves the subtraction of two numbers of nearly equal magnitude, and
II1
I1 coal liquid 1.50 4.69 18.9 0.912 16.1 11.3 1.6 9.2 3.4 0.1 0.898 0.060 236
anthracene oil 1.45 4.38 19.3 0.899 12.4 8.7 1.5 11.1 2.9 0.0 0.916 0.064 181
coal liquid 1.53 5.47 18.3 0.897 14.3 10.5 1.6 10.3 2.9 0.4 0.856 0.058 224
the errors involved are relatively large. The parameters r and RN should therefore be regarded as estimates. The results of the calculations are summarized in Table 111. Several features are apparent. The coal liquids have higher molecular weights than the anthracene oils, and this is reflected in the number of aromatic carbon atoms per average molecule (#C,). The average number of alkyl carbons per molecule (fc,),on the other hand, changes very little between anthracene oils and coal liquids. Since #C, does change, i t may be inferred that the dissolved coal is less highly substituted than the solvent. The calculated values of %AS confirm this. One pair of samples (111)shows an interesting phenomenon: The C/H ratio in the aromatic groups (fl) is higher in the anthracene oil than in the coal liquid. A high value of fl implies fused-ring structures that are highly condensed. For example, when f l is large, compounds like pyrene
are more likely to be found than compounds like tetracene.
Thus Anthracene Oil I11 should contain more highly condensed fused-ring structures than Coal Liquid 111. This interpretation is further confirmed by noting that the average number of fused rings (R,) is the same in both samples, despite the fact that there are, on the average, more aromatic carbon atoms per molecule in the coal liquid. The variation in the parameter f (the C/H ratio in the alkyl groups) among the six samples is large. Measured values of f range from 6.0 to 4.4. A value of 6.0 implies that nearly all substituents are methylenes, while a value of 4.0 implies that nearly all substituents are methyls. The parameter f may be used as an estimate of the number of rings opened when hydrogen is added to the anthracene oils during hydrotreating, since every ring that is opened produces one or two methyl groups. Thus, Anthracene Oil I (f = 6.0) should contain more saturate rings (as in tetralin or acenaphthene) than Anthracene Oil I11 (f = 4.4),and fewer short-chain alkyl substituents. The absolute accuracy of the calculated average molecular structure parameters is probably not high. However, as the previous discussion has shown, the accuracy is sufficient to provide useful comparisons between samples. Because such comparisons can provide guidance to process studies, the analytical procedures described in this work are potentially very important to the development of alternate fuels tech-
ANALYTICAL CHEMISTRY, VOL. 50,
nology. However, the ultimate utility of these procedures will depend on whether or not correlations between the average molecular structure parameters and process conditions can be found. Studies to determine such correlations are in progress.
ACKNOWLEDGMENT T h e author expresses his appreciation to R. H. Pahl for preparing the samples and to Carol Collier for obtaining the proton NMR spectra. LITERATURE CITED (1) H. C. Anderson and W. R. K. Wu, Bull. U.S. Bur. Mines, No. 66 (1963). (2) A. G.Sharkey, Jr., J. L. Shultz, and R. A. Frledel, Fuel, 41,359 (1962).
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(3) D. L. Wooton, H. C. Dorn, L. T. Taylor, and W. M. Coleman, Fuel, 55, 224 (1976). (4) H. C.Dorn and D. L. Wooton, Anal. Chem., 48, 2146 (1976). (5) H. L. Retcofsky and R. A. Friedel, "Spectrometry of Fuels", R. A. Friedel, Ed., Plenum Press, New York, N.Y., 1970,p 70, p 99. (6) F. K. Schweighardt, H. L. Retcofsky, and R. A. Friedel, Fuel, 55, 313 (1976). (7) R. J. Pugmire, D. M. Grant, K. W. Zilm, L. L. Anderson, A. G. Oblad, and R. E. Wood, Fuel, 56,295 (1977). ( 8 ) R. B. Williams, "Symposium on Composition of Petroleum Oils, Determination and Evaluation", ASTM Spec. Tech. Pub/., 224, 168 (1958). (9) S.A. Knight, Chem. Ind., 1967,1920. (10) D. R. Clutter, L. Petrakls, R. L. Stenger, Jr., and R. K. Jensen, Anal. Chsm., 44, 1395 (1972).
RECEIVED for review January 23, 1978.
Accepted April 28,
1978.
Analysis of Commercial Chloroform for Chloromethyl Methyl Ether by Fourier Transform Infrared Spectrometry S. R. Lowry" and J. D. Banzer T. R. Evans Research Center, Diamond Shamrock Corporation, Painesvllle, Ohio 44077
Chloroform solutions have been analyzed for trace amounts of chloromethyl methyl ether (CME) using Fourier transform Infrared (FTIR) spectrometry. The signal averaging Capability and high sensitivity of the FTIR system were essentlal in overcoming the strong absorbance of the chloroform. CME was detected In standard solutions contalnlng as little as 1 ppm of CME. NOCME could be detected in samples of cornmerclal chloroform.
amounts of commercial CME (Eastman Organic Chemicals). Solutions in the low ppm region were obtained by diluting diquots from a 2000-ppmstock The were run in a sealed liquid cell with a -2-mm path length. All the infrared spectral data described in this study were obtained with a Nicolet 7199 FTIR system; 4096 data points were acquired for each interferogram, and an 8192 point transform was calculated utilizing the Happ-Geyzel apodization function shown below.
0.54 + 0.46 There is evidence in the literature ( I ) that chloromethyl methyl ether (CME) is carcinogenic. This is of concern because CME could possibly be formed in the production of methyl chloride by chlorination of dimethyl ether, a byproduct. When methyl chloride is subsequently chlorinated to produce chloroform, CME will codistill and contaminate the finished chloroform. A method was needed for the detection and measurement of possible trace levels of CME in chloroform. Although chromatographic procedures have been reported for determining CME ( 2 , 3 ) ,major instrument modifications would be required to prevent interference from the chlorinated solvent. In order to avoid these difficulties, other methods of analysis were sought. Because of the high sensitivity and spectral subtraction capability of Fourier transform infrared spectrometry (FTIR), a simple method utilizing this technique appeared feasible ( 4 , 5 ) . A study of this nature also offered an opportunity to determine the limitations of the instrument in the detection of trace components. This paper discusses the results of the analysis of CME in chloroform by FTIR spectrometry.
EXPERIMENTAL Reference chloroform was prepared by washing propellant-grade chloroform (inhibited with 300 ppm 2-pentene) with dilute KOH solution, drying with anhydrous sodium sulfate, and storing over a molecular sieve. This procedure will hydrolyze any CME which might be present and will remove residual water. Standard solutions were prepared with this reference chloroform and known 0003-2700/78/0350-1187$01 .OO/O
COS
where ni is the ith data point from the zero path difference point and N is the total number of data points after the one at zero path difference. This apodization function yields a final spectrum with smaller side bands than the boxcar function and narrower peak widths than the triangular apodization function. The signal-to-noise ratio was improved in the spectra by co-adding 400 interferograms before performing the Fourier transform. In order to eliminate any discrepancies due t o a mismatch of sample and reference cells, all spectra were obtained with a single cell. A spectrum of the reference chloroform was subtracted from each sample spectrum using the software supplied with the system. This difference mectrum is calculated as shown below.
where D(i)is the ith value of the difference spectrum, A,(i) and A,(i) are the ith values of the sample and reference spectra in absorbance units, and x, is a scaling factor. For the work reported here, x , was always close to 1 because the small amounts of CME in the solutions produced a negligible concentration effect. However, small adjustments in x, frequently improved the baseline in the region of interest. The criteria for subtraction were a flat baseline and the disappearance of the peaks around 2000 cm-'.
RESULTS AND DISCUSSION Spectra were obtained from a sample of reference chloroform and from a 2000-ppm solution of CME in chloroform. Although the spectra are dominated by peaks corresponding to the chloroform vibrational modes, measurable differences can be seen in the two spectra. Figure 1 shows these two spectra and the difference spectrum that resulted from 0 1976 American Chemical Society
