AN AMERICAN CHEMICAL SOCIETY JOURNAL MAY/JUNE 1987
VOLUME 1, NUMBER 3 @Copyright 1987 by the American Chemical Society
Articles Characterization of Acidic Heteroatoms in Heavy Petroleum Fractions by Phase-Transfer Methylation and NMR Spectroscopy K. D. Rose* and M. A. Francisco Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received December 11, 1986
A characterization strategy has been developed for investigating the types and distribution of acidic heteroatom functionalities in heavy petroleum fractions. Two vacuum residua and the n-heptane asphaltenes from one of these residua have been studied by using a combination of phase-transfer-catalyzed (PTC) methylation chemistry and 13C and 2H nuclear magnetic resonance (NMR) spectroscopy. The derivatized heteroatoms generated by this chemistry are identified by characteristic changes in the N M R spectra of the methylated products. Since the concentration of acidic heteroatoms is often quite low compared to the total concentration of heteroatom present in the fraction, the incorporated methyl groups are isotopically enriched in the 13Cor 2Hisotope for easier spectroscopic detection. The methylated produds have also been examined for evidence of changes in their chemical and processing properties that could be attributed to the elimination of specific hydrogen-bonding interactions. The results indicate that these identified heteroatoms do not significantly contribute to the molecular mass characteristics and thermal coking tendencies of heavy petroleum fractions. Introduction The term petroleum heavy ends can be used to describe a variety of high molecular mass and high-boiling fractions derived from crude oil. Two of the most common representatives of petroleum heavy ends are petroleum residua, that fraction of crude oil which does not distill below 950 OF under atmospheric conditions, and the heptane-insoluble subfraction of residua, the heptane asphaltenes. These materials represent the least desirable portions of the crude oil barrel and an enormous challenge to describe a t a molecular level due to their hydrocarbon and heteroatom heterogeneity. Of special importance in this chracterization is an understanding of the molecular structures and concentrations of different oxygen, nitrogen, and sulfur species since these heteroatoms have been implicated in many of the undesirable physical and processing characteristics of petroleum heavy ends.' (1) (a) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1980, Chapter 7. (b) Speight, J. G. The Desulfurisation of Heauy Oils and Residw; Heinemann, H., Ed.; Marcel Dekker: New York, 1981; Vol. 4.
Certain acidic functional groups such as hydroxyl
(-OH), carboxylic acid (-COOH), imino (=NH), and thiol (-SH) as well as some basic functionalities have previously been identified in petroleum fractions. The impact of these groups on ,the physical properties of residua and asphaltenes has been suggested to result from their participation in intermolecular hydrogen-bonding interactions.2 Such intermolecular associations may increase the average molecular mass and may be responsible for the tendency of these materials to produce considerable quantities of gas and solid coke upon thermal decomposition. Unfortunately, in such complex materials, the quantitative identification of heteroatom functionalities is not an easy task and has usually been inferred from measurements of a less molecularly specific nature such as the effects of solvent fractionation or the strength of adsorbent interactions. In order to address these questions on petroleum residua and asphaltenes, a more specific and (2) Barbour, R. V.; Petersen, J. C. Anal. Chem. 1974,46, 273.
0881-062~/87/2501-0233$01.50/0 0 1987 American Chemical Society
234 Energy & Fuels, Vol. 1, No. 3, 1987
Rose and Francisco
Table 1. Composition Data on Derivatized Residua Products Resid A Resid B n-C, asphaltenes-Resid A before reacn after reacn before reacn after reacn before reacn after reacn carbon, mass % 83.37 9.87 hydrogen, mass % 0.44 nitrogen, mass % 5.47 sulfur, mass % 0.9' oxygen, mass % nickel, ppm 51 177 vanadium, ppm 1.42 H/C ratio 30.6 I3C aromaticity empirical formula C1COH142N0.5S2.500.8 By neutron activation analysis. By difference.
83.49 9.99 0.42 4.92 1.2b 46 152 1.44 22.1
82.90 9.82 0.95 6.29 0.5a 151 430 1.42 30.0
81.97 9.72 0.98 5.89 1.44b 163 411 1.42 15.3
ClCOHl42N1,~2.8O0.5
quantitative probe of acidic heteroatoms has been developed and reported here. A mild and selective chemical reaction, methylation by phase-transfer catalysis (PTC), has been adapted from other applications3in order to convert hydroxyl, carboxylic acid, imino, and mercaptan groups in petroleum materials into easily identified products. The methyl derivatives of 0, N, and S are assigned by means of characteristic changes in the 13Cand 2HNMR spectra of the petroleum fraction after the methylation reaction has been completed. Since the quantities of acidic heteroatoms are often quite low compared to the total amount of heteroatom present, the incorporated methyl groups are isotopically enriched in an NMR-active stable isotope and detected in the derivatized products by 13C and 2HNMR and by FTIR spectroscopies. The isotopic enrichment achieves a substantial detection sensitivity benefit in the NMR experiment and allows the identification of acidic heteroatoms even at very low concentrations. The products and completeness of the chemical reactions are also monitored by characteristic changes in the FTIR spectra. The elimination of hydrogen-bonding interactions by using a mild chemical technique also alters the petroleum material without changing the bulk of the hydrocarbon structure. The methylated products, then, are useful materials for evaluating the importance of hydrogenbonding interactions on the average molecular mass of the petroleum fraction and its tendency to form thermal coke. The PTC-methylation chemistry and characterization strategies are demonstrated here for vacuum residua from two different crude oils and for the n-heptane asphaltenes derived from one of these residua. The average molecular mass and coking tendencies for the methylated and virgin fractions are also compared and the results discussed in terms of the importance of hydrogen-bonding interactions. Obviously, only those heteroatoms that are susceptible to this mild chemistry will be detected in the spectroscopic measurements as a methylated product. More severe chemical treatments are being evaluated to attack other heteroatom groups and will be reported a t a later date.
Experimental Section Materials. Residua fractions were obtained by vacuum distillation of two different crude oils. The distillation temperatures indicated represent the atmospheric equivalent temperatures of the vacuum distillation. The asphaltenes isolated from Resid A, 18 mass % of the virgin resid, were obtained by solvent precipitation with 50/1 mass/mass n-heptane. Composition data obtained on the petroleum fractions and their methylated products are summarized in Table I. Elemental oxygen contents were determined by neutron activation analysis on the unreacted samples. Molecular mass reported in Table I (3) (a) Liotta, R. Fuel 1979,58,724.(b) Liotta, R.;Rose, K.; Hippo, E.J. Org. Chem. 1981,46, 211.
83.61 8.08 1.00 7.59 2.71" 248 672 1.16 50.0
81.72 7.70 1.00 7.51 2.07b 211 676 1.13 23.7
CICOHllENl,OS3,402.4
was measured by vapor pressure osmometry (VPO) as dilute solutions in toluene at 50 "C and in o-dichlorobenzene at 130 "C. Major element analyses and molecular mass measurements were performed by Galbraith Laboratories, Inc., Knoxville, TN. Microcarbon residue (MCR) was used as a measure of a s a m ple's tendency to produce thermal coke: In a typical MCR measurement, a weighed sample of resid or derivatized product is rapidly heated to 500 "C and held at this temperature for 20 min and under a continuous nitrogen flow, allowing all volatiles to escape. The residue that remains is reported as a percentage of the weight of sample initially charged. Tetrahydrofuran (THF) solvent from Fisher Scientific was distilled from LiAlH, and maintained under a nitrogen atmosphere. Tetra-n-butylammonium hydrogen sulfate ((TBA)HS) was used as received from Aldrich Chemical Co. Aqueous sodium hydroxide (50mass %), sodium sulfate, and methylene chloride were used as received from Fisher Scientific. Carbon-13 enriched methyl iodide (90.1 atom % 13C)and deuterium enriched methyl iodide (99.5atom % 2H) were used as purchased from Merck Stable Isotopes. The purity and isotopic enrichment of the carbon-13 reagents were confirmed by mass spectrometry. Synthesis. In a typical methylation reaction, about 3 g of virgin resid or asphaltene was dissolved in 27 mL of THF followed by the addition of 1.95 g of (TBA)HS, 3 mL of aqueous NaOH, and 6.8 g of methyl iodide. The reaction flask was flushed with nitrogen and sealed, and the reaction mixture was stirred vigorously at 40-45 "C. After 48 h, the reaction mixture was cooled and the flask vented. The mixture was diluted with methylene chloride and distilled water and transferred to a separatory funnel. The aqueous and organic phases were shaken together and separated. The organic phase was washed with 10% methanol in water (3X 500 mL) and extracted continuously with distilled water for 7 days to remove residual (TBA)HS. The organic phase was then dried over anhydrous sodium sulfate in a nitrogen atmosphere. The sodium sulfate was removed by fdtration, and the fdtrate was concentrated by rotary evaporation. The residue was dried of remaining solvents at 100 "C and pressure less than 0.1 mmHg for no less than 24 h. The reaction yields were 100.9,100.0,and 100.0% for Resids A and B and the n-heptane asphaltenes, respectively. The complete removal of reagents was also confirmed by FTIR and NMR spectroscopies. The quantities of reagents used in the methylation procedure are based on the total moles of 0, N, and S measured to be in the sample, assuming that each are in the acidic form. The molar excess of methyl iodide was greater than the molar excess of aqueous base in order to compensate for possible base-catalyzed hydrolysis of the methyl iodide reagent. Measurements. Carbon-13NMR spectra were accumulated on a JEOL GX-400spectrometer at 100.54 MHz for the 13C isotope. Each sample was dissolved in chloroformdl containing tetramethylsilane and chromium(II1)acetylacetonate relaxation pulse (60")and a 3-s repetition delay agent (about 0.2 M). A 10-j~~ were used throughout. Each spectrum was normally accumulated overnight in order to obtain the best possible signal-to-noiseratios. Gated hydrogen decoupling was used throughout to minimize residual NOE. (4)Noel, F.Fuel 1984, 63,931.
Heteroatoms in Heavy Petroleum Fractions
Energy & Fuels, Vol. 1, No. 3, 1987 235
Table 11. Properties of Petroleum Fractions Before and After Methylation Resid A Resid B n-C, asphaltenes-Resid A virgin CD3 13CH3 virgin CD3 13CH3 virgin CD3 13CH3
M,(toluene)' 1225 1225 1230 1254 1271 1270 8000 M,(ODCB)* 1049 1082 1046 1142 1197 4130 MCR, mass 70 22.8 21.6 22.4 21.8 21.1 22.0 55.3 'By vapor pressure osmometry at 50 "C. *By vapor pressure osmometry at 130 "C in o-dichlorobenzene. Resid A
7250 4031 54.3
Methylated Resid A R=H R=Alkyl
60
55
50
+ I I
I
I
I
I
I
I
PPM
LjppM
Figure 1. Aliphatic region of 13C NMR spectrum for Resid A before and after methylation with 13CH31. Deuterium NMR spectra were obtained on a JEOL FX-9OQ spectrometer operating at 13.75 MHz for the 2H isotope. Hydrogen decoupling was not used, and other acquisition parameters were chosen to provide quantitatively reasonable integral data. Methylated samples containing deuterium were prepared for measurement by accurately weighing together the sample, chloroform solvent, and a small amount of chloroform-d,. The amount of deuterated solvent was chosen such that the solvent integral was about one-fourth that of the derivatized sample. The spectrometer was externally locked to aqueous lithium chloride. Infrared spectra were recorded as dilute solutions in carbon tetrachloride on a Digilab FTS-20 spectrometer. Calculations. For the 13Cspectra, the aromaticity of virgin samples and effective aromaticity of methylated samples represent the fraction of the total integrated carbon intensity which appeared from 90-220 ppm with respect to Me4Si. Since there was no evidence in the N M R spectra of these materials for substantial acid or ketone carbonyl, the aromatic integral usually spanned the smaller range 90-160 ppm. The aliphatic carbon region as plotted in the figures covers chemical shift values of approximately 0-70 ppm. Since the fraction of substrate carbon in aromatic positions is not affected by the derivatization technique, the number of methyl groups incorporated per 100 mol of substrate carbon has been calculated by first determining the fraction of aliphatic carbons associated with added methyl carbon. This value was then divided by the fraction of the total integral due to aromatic and aliphatic substrate carbon, and the ratio was corrected for the isotopic enrichment of the methyl iodide reagent. Obviously, the reliability of the incorporated methyl group values by 13CNMR is highly dependent on the reproducibility of the aromaticity measurement. Since samples of very similar composition were being examined in this study, a * l % estimate of the carbon aromaticity is believed reasonable and within the reproducibility observed on repetitive measurements. The distribution of incorporated methyl groups as a function of specific aliphatic chemical shift regions was determined by measuring the excess aliphatic intensity after methylation in these regions. No spectral deconvolution was attempted at this stage due to the complexity of the spectra. For the 2H NMR spectra, the number of incorporated groups was determined by comparing the integral values for the incorporated methyl resonances and for the chloroform-d, solvent. Corrections were applied which took into account the number of deuterium atoms per mole of CDCl, and -CD3, the weights of the two materials, and the measured elemental carbon content. No correction is required, in this case, for isotopic enrichment since the *H contents of the CDC13 and CD31were identical.
l%
Results and Discussion NMR Results on Methylated Residua. Two re-
sidua samples derived from different crude oils were investigated in this study. The H and C elemental analyses and carbon aromaticities (Table I) and VPO molecular masses and MCR values (Table 11) all suggest that there are not dramatic differences in the average hydrocarbon structure, the average molecular mass, or coking tendency between these two materials. This observation is also supported by the overall similarities of the NMR and IR spectra for these samples prior to methylation. There are, however, comparatively large differences in the total oxygen, nitrogen, and sulfur levels between Resids A and B as shown by the composition data in Table I. The proportion of each of these heteroatoms that bear an exchangeable hydrogen is one important aspect of a complete characterization that will be probed by the combination of methylation chemistry and NMR spectroscopy described here. Carbon-13 NMR spectra of Resid A before and after methylation with 90.1 atom 5% 13C methyl iodide are shown in Figure 1. Since there were no visible differences in the aromatic carbon envelope for any of the methylated products, only the aliphatic carbon regions have been plotted here and they are vertically adjusted to produce approximately the same peak height in each spectrum for the methyl group that terminates long aliphatic chains (6 = 14.2). This methyl group is not affected by the methylation chemistry, and its resonance is a useful guide for comparing virgin and methylated samples. New methyl resonances are clearly evident at chemical shift values characteristic of hindered (61 ppm) and unhindered (56 ppm) aromatic methyl ethers and methyl esters (51 ppm). Hindered phenols are defined to be those which possess two substituents (either other aromatic rings or alkyl attachments) ortho to the 0-CH3 group. Similar distributions of methyl ethers have previously been observed in methylated coal p r o d u ~ t s . ~ Although no other distinctive resonances are apparent upfield of this chemical shift region, the 30-46 ppm region ( 5 ) Stock, L.M.; Willia, R.S.P r e p . Pap-Am. Chem. Soc., Diu. Fuel Chem. 1985, 30, 21.
Rose and Francisco
236 Energy & Fuels, Vol. 1, NO.3, 1987 Resid B
Methylated Resid B
I
R=Alkyi
R=H
PPM
COOH
OH
-
H
+ R'-SH
Figure 2. Aliphatic region of 13C NMR spectrum for Resid B before and after methylation with 13CH31.
-
0-CH,
M'N-CHs
N-CHs C-CHs
%CHI 4 - 1 70
60
50 1%
40
30
20
10
0
Chemical Shift (ppm)
Figure 3. Typical 13C chemical shift ranges for 13CH3groups incorporated into petroleum fractions.
characteristic of N-methyl derivatives is enhanced in integrated intensity compared to the same region in the virgin resid. The region from about 10 to 29 ppm, that of S- or C-methyl derivatives, is also somewhat more intense. Carbon methylation is not expected to be a major product from such a mild chemical treatment, but the base-catalyzed addition of methyl groups to carbon sites activated by neighboring substituents has been reported to occur in fossil fuel samples.6 If all of the added intensity in the 20-29 ppm region were assigned to carbon methylation, the proportion of total resid carbon activated in this way is only about 0.1%. Typical chemical shift ranges for potential 0-,N-, S-, and C-methyl derivatives are summarized in Figure 3. The intensities of the incorporated methyl groups are 90 times enhanced compared with the intensities of resonances of the hydrocarbon structure itself due to the isotopic enrichment used. In view of this enhancement, the relative intensities of the methyl groups and inherent hydrocarbon resonances in these spectra do indicate that the total number of incorporated methyl groups must be very low. This example clearly emphasizes the need for isotopic enrichment and good spectral sensitivity in experiments of this sort. The I3C NMR spectra of the corresponding Resid B samples are shown in Figure 2. As seen previously in the spectrum of the Resid A methylated product, new methyl resonances appearing in the 50-65 ppm region can be assigned to methyl ethers and esters. One sharp resonance a t 58.5 ppm is certainly due to a 13C-enriched methyl product and is consistent with a derivatized alcohol although an impurity originating from the tetrahydrofuran solvent cannot be positively ruled out. Unlike the spectra for Resid A, however, additional methyl intensity and several new peaks are also apparent in both the 30-50 ppm (6)Liotta, R.;Brons, G . J. Am. Chem. SOC.1981,103, 1735.
/
1 2 1 0
I
l
8
,
l
6
l
l
4
l
l
2
,
l
0
,
-
2
ZH Chemical Shift (ppm)
Figure 4. Deuterium NMR spectra of Resid A and B after methylation with CDJ.
(nitrogen) and 10-30 ppm (sulfur and carbon) regions. Two resonances, in particular, at 16 and 13 ppm that are not detected in the spectra of the virgin Resid B or methylated Resid A must be due to thioether derivatives since potential carbon reaction products generated by this acid-base chemistry would not be expected to produce a methyl resonance upfield of about 20 ppm. Although these two resonances contribute only a small fraction to the overall integral, the presence of these peaks in Figure 2 (and their absence in Figure 1)suggest that free aliphatic or aromatic mercaptans are present in Resid B that are not present in Resid A. The sensitivity of this technique to low-level variations from resid to resid is nicely demonstrated. 2HNMR Results on Methylated Residua. Deuterium NMR spectra of both CD,-methylated residua are compared in Figure 4. The large singlet a t 7.25 ppm is due to the chloroform-d, internal standard. Contributions to the NMR spectrum of natural abundance deuterium originating either from the CHC1, solvent or resid molecules are negligibly small. A small resonance appearing at 4.6 ppm is due to natural abundance deuterium from the aqueous lock solution, and this resonance was excluded from the integral calculations. Typical chemical shift ranges for CD3 derivatives are summarized in Figure 5; these have been compiled from the well-documented correspondence between 2Hand 'H chemical shift values. It is clear from the distribution of the deuterium intensity in the methylated resid spectra
-
Heteroatoms in Heavy Petroleum Fractions
Energy & Fuels, Vol. 1, No. 3, 1987 237
0-CDa
Table 111. Incorporated Methyl Groups per 100 mol of Carbon Determined by NMR Techniaues by "c NMR by 2~ NMR 0 N S + C tot. tot. Resid A o.io 0.1~ 0.2~ 0.5 0.4 Resid B 0.26 0.48 0.4, 1.2 0.8 n-C, asphaltenes from 0.32 0.43 0.66 1.4 1.6
ON-CD3
N-CDI C-COS
c--C S-CDa 5
3
4
2
1
0
Resid A
ZH Chemical Shift (ppm)
Figure 5. Typical 2H chemical shift ranges for CD3 groups incorporated into petroleum fractions.
0
3615
d
'E
1770
1660
AI CO,CH,
C
C
L
1650
a? Mathyirted Redd A 3600 3400 3W0 2600 2200
1MM 1MO
3600 3400 3001 2MM 2200
1800
1MIo
Wavenumbers
. Figure 6.
Dilute-solution FTIR spectra of Resid A before and after methylation with 13CHaI.
u
L
z Resid B I
,
I
I
I
3600 3400 3Doo 2MM 2200
I
I
I
.
1800 1800
Wavenumbers
Figure 7. Dilute-solution FTIR spectra of Resid B before and after methylation with 13CH31. that the major reactive heteroatoms in both samples are oxygen and/or nitrogen (2.3-5.0 ppm) while sulfur and/or carbon (1.0-2.5 ppm) are less significant. This observation supports the observations made from the 13C spectra. FTIR Results on Methylated Residua. Infrared spectra are shown in Figures 6 and 7 for Resids A and B both before and after methylation with 13CH31. The FTIR spectra were acquired on dilute solutions of the resid fractions, a sample preparation technique that helps to minimize absorption broadening in the 3100-3700-cm-' region due to intermolecular hydrogen bonding. In this case, the use of the 13C-enrichedmethyl group does not significantly add to or detract from the normal detection and assignment of IR absorptions. Infrared spectra of the deuteriated samples are somewhat more distinctive. Both spectra of unmethylated resids show a broad absorption in the region 3100-3500 cm-' attributed to residual hydrogen-bonding interactions, presumably intrarather than intermolecular. Sharper bands a t 3475 and 3615 cm-' are assigned to NH and OH stretches of nonhydrogen-bonded imino and phenol groups, respectively. The imino band appears to be somewhat stronger for Resid B than for Resid A, but in other details, the infrared spectra of these materials are very similar. Dilute solutions of the methylated resids, on the other hand, show no evidence for hydrogen-bonded or non-hy-
drogen-bonded interactions, indicating that the methylation of all hydroxyl and imino groups is complete. A new absorption a t about 1705-1710 cm-' appears in the spectra of both methylated samples and is consistent with the carbonyl absorption of carboxylic acid methyl esters. The carbonyl stretch of the original carboxylic acid, generally near 1690 cm-' or less for aromatic carboxylic acids, is probably obscured by the broad 1600-cm-' band. Similar spectra were obtained for the CD3-methylatedsamples. In view of the FTIR evidence, a quantitative interpretation of the NMR integral data in terms of heteroatom functionalities seems justified.
Comparison of Incorporated Methyl Groups by NMR Techniques. By an accurate measurement of the change in '3c aromaticity as described in the Experimental Section, the total number of incorporated methyl groups were calculated for Resids A and B. These results and the distribution of methyl group in specific chemical shift regions are summarized in Table 111. About 0.5 methyl group in Resid A, compared to 1.2 groups in Resid B, is added by the PTC-derivatization chemistry per 100 mol of substrate carbon. Derivatized oxygen in Resid A accounts for only about 20% of the total 0.5 group and only about 14% of the total organic oxygen measured to be present in the sample by neutron activation analysis. In Resid B, although 21% of the total 1.2 groups can once again be assigned to oxygen derivatives, this value corresponds to a significantly higher proportion of the total oxygen in the sample, about 66%. The percentage of total oxygen, sulfur, and carbon attacked by this technique is overall very similar for the two samples. The proportion of total nitrogen converted to N-methyl derivatives seems rather different between samples, however. In Resid B for example, about 46% of total nitrogen is derivatized by this technique while only 33% is converted in Resid A. The availability of an independent means for evaluating both the reliability of the 13C counting technique and the repeatability of the chemical derivatization was especially valuable in these experiments. The number and distribution of incorporated CD3 groups were determined by 2H NMR as described in the Experimental Section. Most importantly, the reliability of the 2H counting procedure relies on entirely different spectral properties and potential sources of error than the estimation technique by 13C NMR. For example, although relaxation times and strong hydrogen couplings must be correctly accounted for in order to obtain quantitative 13C NMR data, these effects are not significant in deuterium NMR measurements. Furthermore, changes in aromaticity are used to calculate the incorporation of methyl groups by 13CNMR while a comparison of integrals and sample weights is used by 2H NMR. The number of incorporated methyl groups calculated from the 2H integration data are compared in Table I11 with those obtained by 13C NMR. With the exception of the low value obtained by 2H NMR for Resid B, the comparison of data is favorable. No explanation can be offered for the difference observed in this case and insufficient data has been accumulated thus far to suggest the most
Rose and Francisco
238 Energy &Fuels, Vol. 1, No. 3, 1987 I
I
Methylated Asphaltenes
70
OH
60
50
40
30
20
10
0
'PM
COOH
bb&-&)" Figure 8. Aliphatic regions of the I3C NMR spectra for the heptane asphaltenes of Resid A before and after methylation.
likely origin for the difference. Considering, however, that there are substantial differences in the factors contributing to accurate counting by these NMR techniques as well as the necessity to investigate independently methylated samples, the comparison of 2H and 13C results is overall quite acceptable. In our experience, the comparison of groups in this concentration range by the two NMR techniques has been consistently better than data obtained by other methods, H / D ratios by combustion analysis, for example. Methylation using a I4C or 3H radiolabel is another alternative that was not easily available to us. NMR Results on the Heptane Asphaltenes from Resid A. The n-heptane asphaltenes represent that fraction of the residua sample which precipitates from heptane solution. Although asphaltene precipitation has been used for many years in both refinery and laboratory applications, the mechanism by which heptane precipitation of the most aromatic- and heteroatom-rich components occurs is still unclear. I t is possible, for example, that intermolecular hydrogen bonding may be important or alternatively that the polarity imparted to certain asphaltene molecules by localized concentrations of heteroatom functionalities may be a contributing factor. In order to gain some insight into the characteristics of the precipitated fraction, the n-heptane asphaltenes from Resid A were also characterized in this study and compared with data collected on the unfractionated resid. Carbon-13 NMR spectra (Figure 8), and composition data (Tables I and 11) were collected on the n-heptane asphaltenes from Resid A. The concentration of oxygen, nitrogen, and sulfur are considerably higher in the asphaltene fraction than in the virgin resid. The 13C aromaticity and molecular mass are also considerably different. As described for the residua samples, the methylation reaction produces characteristic changes in the FTIR spectra, suggesting that complete conversion of OH, NH, and COOH functional groups in the asphaltenes has occurred. The number of incorporated groups and the distribution of these groups were determined as previously described. Although the n-heptane asphaltene fraction is only 18 mass % of the virgin Resid A, the methylated asphaltenes, as one might expect from the composition data, has more than three times the total number of incorporated methyl groups as found in the methylated resid. About 1.4 groups by 13C NMR (1.6 groups by 2H NMR) are incorporated into the asphaltene structure, distributed as shown in Table 111. The distribution of methyl groups and the fraction of total oxygen, nitrogen, and sulfur identified by methylation are remarkably similar between the virgin resid and asphaltenes samples.
Since nearly half of the total acidic functional groups can be found in the 18 mass % asphaltenes fraction, the methylation and NMR results suggest that the polarity of resid molecules identified in part by this strategy is a driving force for precipitation from nonpolar solvents. The importance of high molecular masses also cannot be overlooked. Effect of Methylation Chemistry on M , and Coking Tendency. One of our principal interests in undertaking this study was to relate changes in the properties of residua and asphaltenes to acidic heteroatom groups identified by spectroscopic means. For this purpose, the methylated products are especially useful samples because they can be prepared in sufficient quantity to permit a variety of measurements other than spectroscopic to be carried out on the product. In contrast to the NMR and FTIR spectral data obtained on these two resids and asphaltenes, the composition data (Table I) show very little change in elemental analysis or metals contents as a consequence of the methylation reaction. This, in itself, is not surprising since the total number of methyl groups added to each sample is not large. The similarity of the metals contents before and after methylation provides additional evidence that the base treatment does not significantly alter the hydrocarbon or porphyrin structure. More surprising, perhaps, is the observation that the average molecular mass by VPO (Table 11) for residua and asphaltenes is not different among the CD3- and 13CH3methylated and virgin samples and changes in the same direction and magnitude with the solvent used in the VPO measurement. Similar observations to these have been reported on the basis of nonselective trimethylsilylation s t u d i e ~ .There ~ are also no significant differences between the two resid samples that can be ascribed to the total number or proportion of heteroatoms identified by the methylation strategy. There may be several explanations for these observations. First, the absolute level of these functionalities available to participate in hydrogen-bonding interactions may simply be too low. Although hydrogen bonding is known to affect the measured molecular mass in Athabasca asphaltenes, for example, the fraction of total oxygen and the fraction of oxygen that is in the OH or COOH form are both considerably higher in the tar sands sample than measured here for petroleum residua.8 Second, any hydrogen-bonding interactions interrupted in these experiments may be predominantly intra- rather than intermolecular. The apparent high proportion of highly hindered (7) Gould, K. A. Fuel 1979,58, 550. (8) Ignasiak, T.; Straw, 0.P.;Montgomery, D.S.Fuel 1977,56,359.
Energy & Fuels 1987,1, 239-242 aromatic methyl ether derivatives and the hydrogenbonding interactions observed by FTIR that are not eliminated by dilute solution measurement support this explanation. Finally, the extent of intermolecular hydrogen bonding may be attenuated by the amphotericity of petroleum molecule^.^ Comparing the microcarbon residue values on the methylated and virgin materials, it is also clear that the MCR values, and therefore coking tendencies, are not sensitive to the presence or absence of hydrogen-bonding interactions involving 0, N, or S atoms. During the measurement of MCR, it is possible that the cleavage and evolution of incorporated methyl groups proceeds faster than the volatilization of resid molecules. Although this possibility was suggested by a recent pyrolysis study conducted on alkylated coal,l0 the time scale of the sample’s temperature rise in the MCR measurement does suggest that the thermal evolution of incorporated methyl groups and resid structures will occur simultaneously. Unlike the comparatively slow thermal ramping used in the pyrolysis study, the MCR test produces a nearly instantaneous jump to a holding temperature of 500 OC. However, even if these methyl-heteroatom bonds are more thermally labile than other resid bonds, any resulting intermolecular interactions involving thermally exposed heteroatoms must be free radical as opposed to hydrogen (9) (a) Seifert, W. K.; Teeter, R. M. Anal. Chem. 1970, 42, 750. (b) Zumer, M.; Such, C.; Brule, B. Analusis 1981, 9, 348. (10) Vasollo, A. M. Fuel 1984, 63, 1236.
239
bonding in nature. Thus, regardless of the time scale for volatilization, it is apparent that the molecular aggregation by means of hydrogen-bonding interactions is not a significant contributor to growth reactions leading to coke.
Conclusions The methylation chemistry developed in this study is effective for “tagging” acidic oxygen, nitrogen, and sulfur functional groups in heavy petroleum fractions with isotopically enriched methyl groups. The methyl derivatives of these heteroatoms are identified by characteristic changes in 13Cand 2H NMR spectra while the completeness and products of the methylation reaction are confirmed by FTIR. Due to the isotopic enrichment used in the methylation chemistry, the assignment and measurement of classes of acidic functionalities, even when present at very low concentrations, is possible. The remainder of the hydrocarbon structure is not altered. Since the presence or absence of these exchangeable hydrogens apparently has no influence on the average molecular mass and tendency to produce thermal coke, intermolecular hydrogen bonding is not an important factor in the molecular aggregation often observed in resid or asphaltene solutions.
Acknowledgment. The authors acknowledge the capable assistance of D. Danik for synthesizing these materials, of C. F. Pictroski and J. V. Vieira for obtaining the NMR spectra, and of K. R. Graf for obtaining the FTIR spectra. Registry No. CHJ, 74-88-4.
Alkaline Cleavage of Model Lignin Compounds David A. Nelson,* William D. Samuels, and Richard T. Hallen Chemical Sciences Department, Pacific Northwest Laboratory, Richland, Washington 99352 Received December 1, 1986. Revised Manuscript Received January 20, 1987
The alkaline cleavage of lignin models 2a-h, based upon 2-phenoxy-1-phenyl-1-propanol with electron-donating and -withdrawing groups on the phenoxy ring, proceeds through an internal displacement of the aryloxy group to provide an epoxide intermediate that opens to the glycol. The epoxide (2-methyl-3-phenyloxirane) was observed only after the alkaline cleavage of 2h, 244-nitrophenoxy)-1-phenylpropanol,since the observed rate of disappearance of the epoxide is faster than that of all the lignin models except 2h. The observed rate of reaction was found to be sensitive to steric bulk and polar substituents in the ortho position of the aryloxy ring. Another product, 1phenyl-Zpropanone, is present in the product mixture, which suggests the occurrence of a concurrent pyrolysis mechanism.
Introduction Lignin is a phenolic copolymer whose most important type of linkage is the alkyl aryl ether, with the a- and /3-alkyl aryl ethers composing approximately 54-68 % of the total interunit 1inkages.l Veratrylglycol-/3-guaiacyl ether or 2-(2-methoxyphenoxy)-l-(3,4-dimethoxyphenyl)ethanol (1)and related analogues are excellent models for (1) Evans, R. J.; Milne, T. A.; Soltys, M. N. J.Anal. Appl. Pyrolysis 1986,9, 207-236.
the lignin @-ethermonomers. The thermal decomposition of lignin models, such as 1 and others, has been examined under pyrolytic as well as alkaline pulping conditions. For instance, the major pathway for the pyrolysis of (1) involves the elimination of water to produce the vinyl ether (Scheme I).283 Both 3,4-dimethyloxyacetophenoneand (2) McDermott, J. B; Klein, M. T.;Obst, J. R. Ind. Eng. Chem. Process Res. Deu. 1986, 25, 885-889. ( 3 ) McDermott, J. B.; Klein, M. T. Chem. Eng. Sci. 1986, 41, 1053-1060.
0887-0624/87/2501-0239$01.50/00 1987 American Chemical Society
