Anal. Chem. 7984, 56, 2740-2743
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LITERATURE CITED (1) Fred, M.; Nachtrleb, N. H.; Tomkins, F. S. J. Opt. SOC.Am. 1947, 37, 279-287. (2) Morris, J. M.; Pink, F. X. Am. SOC.Test. Mater., Spec. Tech. Pub/. 1957, 221, 39-46. (3) Nakajima, T.; Kawaguchl, H. Spectrochim. Acta 1982, 18, 1479-1486. (4) Szlvek, J.; Gilders, E. J.; Holt, J. M.; Valberg, L. S . Appl. Spectrosc. 1964, 18, 57-61. (5) Walters, J. P.; Malmstadt, H. V. Anal. Chem. 1965, 37, 1484-1503. (6) Bowmans, P. W. J. M. In "Analytical Emission Spectroscopy"; Grove, E. L., Ed.; Marcel Dekker: New York, 1972; Vol 1, Part 11, pp 117.
(7) Addink, N. W. H. Spectrocbim. Acta 1959, 15, 349-359. (8) DeGalan, L.; Smith, R.; Winefordner, J. D. Spectrochlm. Acta, Part 6 1988, 236, 521-525. (9) Wiese, W. L.; Smith, M. W.; Glennon, B. M. In "Atomic Transition Probabillties"; NSRDS-NBS4, Vol 1, pp 26. (IO) Zynger, J.; Crouch, S. R. Appl. Spectrosc. 1975, 29,244-255. (11) Engstrom, R. W. I n "Photomuklplier Handbook"; RCA: Lancaster, PA, 1980; pp 174. (12) Olson, K. W.; Haas, W. J., Jr.; Fassel, V. A. Anal. ch8m. 1977, 49, 632-637.
RECEIVED for review April 9,1984. Accepted August 7, 1984.
Hydroxyl Determination in Petroleum Oil Extracts by Fluorine- 19 and Silicon-29 Nuclear Magnetic Resonance Spectrometry Jean-Marie Dereppe and Bhuksndas Parbhoo* Department of Physical Chemistry, Catholic University of Louvain, 1348 Louvain-la-Neuve, Belgium
Qualitative and quantltatlve analyses of hydroxyl functions In two asphaltenes and In acidic extracts of two petroleum oils are determlned by fluorlne-19 and siilcon-29 NMR spectroscopy. Acidic hydrogens are speciflcaily substttuted elther by trlfluoroacetyl or by trlmethylsllyl groups wlh trtfluoroacetlc anhydride and hexamethyldlsilarane,respectlvely. Asphaltenes are extracted from treated beech and poplar wood. 'OF and *%I NMR signals are analyzed on the basls of chemlcal shlft and Integral measurements. Oxygen quantlty characterized by NMR together wtth total oxygen content determlned by elemental analysls glves an estlmatlon of non-hydroxyl oxygen atoms embedded In molecular structures of these petroleum oils extracts.
Petroleum oil genesis is a complex conversion process of organic materials into neutral and polar organic molecules. These latter contain oxygen, sulfur, and nitrogen atoms (1, 2 ) . The composition of oxygen in functionalized molecules is not very important (0-3% in weight) but it has marked consequences on oil secondary recovery, on reactions occurring during chemical treatments, and on geochemistry as geological tracers (2-4). Therefore analytical studies of these polar compounds appear necessary for the knowledge of fundamental and applied chemistry of petroleum oil. Heteroatom functionalities can be characterized by several techniques but the molecular complexity and physicochemical properties of coal and oil materials lead to many analytical difficulties (5, 9-15). Fortunately, in the particular case of protonated heteroatoms, NMR spectroscopy is a direct and reliable technique. The acidic proton of a particular heteroatom can be selectively replaced by the trifluoroacetyl or trimethylsilyl group (10, 13, 16-20). These derivatized heteroatoms can then be studied by 19F and 29SiNMR. Differences between molecular environments can be distinguished on the basis of chemical shifts whereas quantitative estimations can be calculated from relative measurements of signal integrals. Regarding functions of interest the wide observed chemical shift range (12 ppm) in 29SiNMR of Me&i derivatives relative to the one observed (2 ppm) in '9F NMR of TFA derivatives provides a good distinction between different environments (4,11,14,19).For this reason Me3Si derivatives
give the most qualitative information. Another specific advantage of Me3Si over TFA derivatized hydroxyls is the far removed chemical shift in 29Si NMR of the reagent hexamethyldisilazane and its byproduct hexamethyldisiloxane from the investigated range of chemical shifts. Therefore chemical separation of the studied compounds is no longer necessary and experimental conditions are more reliable, hydrolysis having been avoided and conversion having been total. In I9F NMR the signal of the byprodpct trifluoroacetic acid (TFAc) overlaps with the peaks of amine and acidic derivatives. Its elimination is necessary. Conditions are well documented by which the acidic proton of a particular heteroatom can be replaced either by the trifluoroacetyl (TFA) or by the trimethylsilyl (Me3Si)group. Introduction of fluorine probes into polar molecules can be realized by trifluoroacetic anhydride (TFAn)
RXH
+
il
lie
CF$O
= RXCCF3
CF,
0
II
t CF3COH
(1)
X=N, 0, S
Trifluoroacetylation is carried out in nonaqueous media such as halogenated hydrocarbon solvents (20). Although eq 1is equilibrated, primary and secondary alcohols as well as phenols are quasi-totally trifluoroacetylated whereas tertiary alcohols and sterically hindered phenols react in low yields or not at all (13,18,19,21). Many of the TFA derivatives are readily hydrolyzed by moisture. They are however stable when dry and pure. In the presence of P205as desiccating agent they can be stored for a few days (22). Introduction of silicon probes into molecules containing hydrogen active heteroatoms can be carried out by several reagents (23). We used hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) as catalyst 2RXH
+ (CH3),SiNHSi(CH3),
(CH,),SiCl
2RXSi(CH3), + NH, (2)
X = N,0 , S HMDS reacts rapidly and quantitatively with hydroxyl groups even if they are sterically hindered (10,23). However it seems that the conversion with aromatic thiols is not complete. We have applied these two reactions to two asphaltenes and to extracted acidic fractions of two petroleum oils. Correlation
0003-2700/84/0356-2740$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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of the hydroxyl content with its nature is presented.
EXPERIMENTAL SECTION Asphaltenes named in this work as SA 26 and SA 83 are extracted from treated beech and poplar wood, respectively. In a liquefaction process wood is hydrogenated under catalytic, pressure, and temperature conditions. A series of successive distillations enable the separation of light (35-95 "C) and heavy (230-280 "C) oil. Residue of distillation is then extracted with benzene. After evaporation, extraction with pentane gives the deasphaltened oil; the term "asphaltenes" is given to the residue which is left. These asphaltenes are bright dark brown solids. Acidic fractions are extracted from the scmdled Emeraude and Boscan petroleum oils free from compounds boiling under 210 "C. Extractions were carried out as follows: raw oil solubilized in toluene is treated with an alcoholic solution of potassium hydroxide. Potassium salts of acidic compounds are then isolated and acidified in aqueous phase. Two or three extractions produced a fragrant visquous black oil: the acidic fraction. For TFA derivatives, 50 mg of the studied compound is dissolved in 3 mL of dry chloroform. The solution is deoxygenated. One milliliter of TFAn is then added. The solution is kept in an hermetically closed flask and shaked overnight with a magnetic stirrer. After evaporation of excess reactive TFAn and byproduct TFAc, the residue is solubilized with CDC13and ready for NMR acquisitions. A weighted amount of trifluoroacetophenone as integral reference and a drop of difluorotetrachloroethane as chemical shift reference are added into the NMR tube. For Me3Si derivatives, a weighted amount (100-300 mg) of the studied compound is introduced into a 10-mm NMR tube. Benzoic acid or tert-butylcyclohexanol as integral reference, chromium(II1) acetylacetonate as relaxation reagent, and Me3& as chemical shift reference are aLS0 added. CDC13(1.5 mL), HMDS (1 mL), and TMCS (0.05 mL) are successively poured into the tube ( 4 ) . The solution is kept at room temperature for 1 h prior to NMR acquisition. NMR. All spectra were obtained at room temperature with a Bruker WM 250 FT NMR spectrometer operating at 250.13, 235.36, 62.90, and 49.70 MHz corresponding to the resonances of 'H, 19F,13C,and 29Si,respectively. An internal deuterium lock is used. Typical conditions are as follows: for ' 9 , 8 FID collected for a spectral width of 3000 Hz with an accumulation time of 6.36 s and a pulse width of 10 bs;for %i, zt1OOOO FID arg accumulated using inverse gated decoupling of protons. A spectral width of 2000 Hz is chosen while the pulse width is fixed at 15 bs (45"). Following a data acquisition period of 4.1 s, a 2.0-9 delay was used in the gated decoupling sequence. C r ( a ~ a cpermitted )~ full relaxation of 29Sinuclei without introducing observable shifts. RESULTS AND DISCUSSION Asphaltenes. Proton spectra of SA 26 and of SA 83 show a wide variety of hydrogen types. These asphaltenes consist on the average of condensed and uncondensed aromatic rings substituted by branched alkyl chains as indicated by the resonances between 6.3 and 8.4 ppm and 1.2 and 3.2 ppm. Cycloalkyl groups may also be present. Broad resanance around 5-6 ppm is assigned to phenolic protons and has been Confirmed by its disappearance following the addition of a drop of D20. Resonance around 4 ppm could be assigned to RCHZOX. Fluorine spectra of the TFA labeled asphaltenes (Figure 1) reveal resonances between -6.6 and -7.8 ppm upfield relative to difluorotetrachloroethane as reference. According to model compounds, signals from -7.0 to -7.4 ppm are presumably naphthol type TFA derivatives which are the major components of the asphaltenes. The shoulder from -6.7 to -7.0 ppm could be due to polyaromatic phenols while the one from -7.4 to -7.8 ppm has been attributed to substituted phenols (14, 19). This attribution is confirmed by the 29Si spectrum of the Me3Si derivatized asphaltenes (Figure 2). The wide range of chemical shifts in ?3i NMR relative to 19F NMR enables a more precise characterization of the OH functions. Resonances between 19.4 and 19.7 ppm are assigned to naphthols orland to polyhydroxybenzenes. The
Flgure 1. 'OF NMR spectra of beech wood SA 26; (B) poplar
trifluoroacetylated asphaltenes: wood SA 83.
(A)
Figure 2. %i NMR spectra of trimethylsiiylated asphaitenes: (A) beech wood SA 26; (B) poplar wood SA 83. Only asphaltenes signals are shown. Solvent was CDCI, and reference was Me,Si.
broad shoulder from 17.7 to 19.4 ppm is due to aromatic or nitrogen containing aromatic hydroxyls. A relatively less important broad signal a t 13-14 ppm could presumably be due to cycloalkylic alcohols (11). All NMR spectra of SA 26 and of SA 83 for a given nucleus are similar from a chemical shift point of view (Figures 1 and
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table I. Oxygen Quantitative Analyses by 19Fand 29Si NMR and Heteroatom (N, 0, S) by Elemental Analyses of Asphaltenesa % total hetero% oxygen atoms as aromatic by ele- % of nonacidic hydroxyls mental heteroatoms by NMR analysis by difference
asphaltene
derivatives
SA26
TFA Me,Si
6.2
'I
0.6
13.9
7.7
TFA Me,Si
7.7 9.7
i
1.0 0.5
14.7
5.0
SA 83 a
6.1 ~t 1.0
i
Percentage in weight.
2). Sharp signals in SA 83 spectra are probably due to more abundant specific functions distributed in the complex mixtures of organic molecules which are asphaltenes. By integrating NMR signals of a derivatized oil extract relative to a standard signal, one can calculate the relative weight of oxygen involved in a specific type of oxygen functionality 100Mo ms I A % oxygen = N (3) ~
mA
MS
%
where mA is the weight of oil extract, ms is the weight of standard, M Sis the molecular weight of standard, Mo is the atomic weight of oxygen, IA is the NMR signal integral of derivatized oil extract, 1s is the NMR signal integral of the standard, N = 1 for OH derivatives, and N = 2 for COOH derivatives. Results are summarized in Table I. Oxygen content is systematically less abundant when determined by 19FNMR probably because sterical effects play an important role in the conversion process of trifluoroacetylation (eq 1) (4,10,11, 18,19). Furthermore only nonsterically hindered phenols derivatives can be stored and conserved against moisture. As trimethylsilylation with HMDS is quantitative even for highly hindered phenols and the reaction is carried out in situ, it is reasonable to state that 29SiNMR provides a good quantitative measure of the oxygen content found in both OH and COOH functions. NMR comparison between the two nuclei naturally leads to the conclusion that in SA 26 all the OH groups are sterically nonhindered phenols whereas in SA 83 more or less 20% of the OH groups are in hindered sites. Alkylic alcohol, thiol, carboxylic, and imidazol functions may be present but would not be detectable if their concentrations do not exceed -2% for the same distribution of chemical shifts. From the carbon and hydrogen elemental analyses (C 79.3% and H 6.8% for SA 26; C 78.1% and H 7.2% for SA 83) one can reasonably suppose that oxygen, sulfur, and nitrogen are the missing links in the atomic composition. Thus from NMR and elemental analyses, nonacidic oxygen, sulfur, and nitrogen atoms could be evaluated (Table I). Acidic Extracts of Petroleum Oils. The proton NMR spectrum of the acidic fraction of Emeraude petroleum consists, on the average, of moderate paraffinic chains with a small amount of aromatic rings which are mostly uncondensed. Some chains are attached to the rings.' There is not a measurable amount of olefins. The carbon-13 NMR spectrum confirms the above conclusions. Furthermore it shows around 180 ppm (Me3Si as reference) carbonyl signals of acidic functions. The IR spectrum displays the carbonyl band and the oxygen-hydrogen stretching band. The fluorine spectrum of TFA derivatives is reproduced in Figure 3. The large broad resonance at -8.47 ppm is attributed to CF,COOH resulting from partial hydrolysis of
Figure 3. "F spectra of trifluoroacetylated acidic fractions of petroleum oils: (A) Emeraude, OH and COOH derivatives; (B) Boscan, OH derivatives only are seen. Mixed anhydrides (COOH derivatives) have
been hydrolyzed. Solvent was CDCI, and reference was C,F,CI,. Table 11. Oxygen Qualitative and Quantitative Analyses by l9F and 29SiNMR of Two Petroleum Acidic Extracts extract
derivatives
functions
% oxygen
Emeraude
TFA MesSi
PhOH + PhSH RCOOH
0.5 f 0.1 0.5 f 0.1
Boscan
TFA Me3Si
PhOH + PhSH PhCOOH RCOOH
0.6 f 0.1 0.4 f 0.1 5.8 f 0.6
mixed anhydrides while peaks at -8.23 and -8.27 ppm are attributed to trifluoroacetylated acids. Mixed anhydrides are very sensitive to hydrolysis. The range of resonances between -7.0 and -7.4 ppm corresponds to aromatic hydroxyls and thiols. After hydrolysis of mixed anhydrides in mild conditions and further elimination of trifluoroacetic acid, a fluorine NMR spectrum is acquired from which an oxygen content (as phenols) is estimated to be around 0.5%. I t is assumed that no important hydrolysis of aromatic hydroxyl derivatives has occurred during the process. This is confirmed by the fact that the spectrum shape in the chemical shift range attributed to phenolic derivatives has not changed after hydrolysis of mixed anhydrides. The silicon NMR spectrum of Emeraude Me3% derivatives clearly shows that MesSi groups are mainly attached to aliphatic carboxylic acids in the resonance range of 22.0-22.8 ppm ( 4 ) (Figure 4A). This is confirmed by a previous work based on TLC and IR techniques which showed that cyclic saturated acids constitute the most important class of acids present in Emeraude oil within the distillation interval of 210-370 OC (16). Saturated carboxylic acids have also been found. Principal classes are linear, iso, anteiso, isoprenoid, alicyclic, and aromatic acids. From expression 3, oxygen weight percentage in Emeraude acidic fraction is calculated and is attributed to carboxylic functions (Table 11). The Boscan fraction is treated with trifluoroacetic anhydride. The excess reagent and byproduct are eliminated by evaporation under vacuum. After hydrolysis in mild conditions of mixed anhydrides, the I9F spectrum is acquired (Figure 3B). The result shows specific phenolic hydroxyls. Naphthol type hydroxyls seem to be the most abundant hydroxyl
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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Acidic extracts of Emeraude and Boscan involve oxygen as aromatic and aliphatic carboxylic acids and, in a less extent, as phenols. Sulfur is mainly present as thioethers. This example of oil extracts illustrates the complementarity of the two NMR probes in analyzing the OH functions. Whereas OH functions are not easily detectable as their Me@ derivatives because of their less abundance (2%), they are however observable as stable TFA derivatives.
CONCLUSIONS We have applied 19Fand 29SiNMR in a study of derivatized asphaltenes and oil acidic extracts in order to determine qualitatively and quantitatively the oxygen atoms contained in these complex mixtures of organic molecules. On the basis of the chemical shift parameter, different types of phenols and carboxylic acids have been assessed whereas signal integral measurements have provided estimation of oxygen contents in these specific functions. 29Si NMR is more convenient compared to 19FNMR for the qualitative assignation of the chemical environments of hydroxyl oxygen groups. Both silylation and trifluoroacetylation methods give complementary information: for asphaltenes sterically hindered phenols could have been evaluated whereas for petroleum extracts I9F and 29Si NMR have been applied to correlate the oxygen content involved in hydroxyl and carboxylic functions, respectively. At last nonacidic oxygen relative quantities have been estimated by comparison between NMR and elemental analyses results for both petroleum extracts.
I h
26
25
24
23
22
21
pprn
Figure 4. *'Si NMR spectra of trimethylsiiylated acidic extracts of petroleum oils: (A) Emeraude; (6) Boscan. Solvent was CDCI, and reference was Me,%
Table 111. Heteroatom Elemental Analyses of Emeraude and Boscan Petroleum Oil Extracts (1 ) extract
% oxygen
% sulfur
% nitrogen
Emeraude Boscan
11.3 9.6
1.8
5.3
0.3 0.8
functions with a fraction of substituted phenols or alkylic alcohols. Oxygen weight percentage as hydroxyl in the acidic fraction is estimated at 0.6%. The 29Sispectrum of MeaSi derivatives of the Boscan acidic extract reveals types of naphthoic (24.4 ppm), alicyclic, and aliphatic carboxylic (22.0-23.5 ppm) acids as major signals (Figure 4b). By use of expression 3 oxygen concentrations are calculated. Results are reported in Table 111. On comparison of oxygen content in the Boscan acidic extract determined by NMR measurements (6.8%) and by elemental analysis (9.6%), it seems that 70% of the oxygen atoms are distributed as RCOOH, PhCOOH (
