- Email: [email protected]
Determination of average molecular weights of high-boiling aromatic oil fractions by 13C and 1H nuclear magnetic resonance
Determination of average molecular weights of high-boiling aromatic oil fractions by 13C and ‘H nuclear magnetic resonance Bernard
Rousseau
and Alain
H. Fuchs
Laboratoire de Chimie-Physique des Matdriaux Amorphes (U.A. 1104 du C.N.R.S.), iJniversitr2 Paris-Sud, 97405 Orsay, France (Received 3 January 1989; revised 5 April 1989)
Average molecular weight (AMW) is a very important parameter in the thermodynamic simulation of crude oil. N.m.r. structural analysis models can be used to obtain AMWs of high-boiling aromatic fractions,
provided that the chemical shift range assumptions are chosen carefully and the number reduced to a minimum. A model using 13C spectra editing is particularly reliable, although it requires a long recording time. The models were tested on 17 narrow high-boiling aromatic fractions of a North Sea crude oil. The values obtained are in very good agreement with mass spectrometry data. (Keywords: n.m.r.; carbon-13 n.m.r.; aromatic)
Characterization of oil fractions using average molecular parameters obtained by quantitative n.m.r. analysis is a well-known method, introduced 30 years ago by Williams’. This method is often used to control the changes in composition at various stages of conversion processes. It can also be useful for simulating thermophysical properties of crude oil such as phase equilibria. In this case, accurate values for the average molecular weight (AMW) of the oil distillate fractions are required, to establish a relevant equation of state for the fluid. The accuracy of data obtained through vapour-pressure osmometry is sometimes questionable, especially for the high-boiling fractions. This paper therefore investigates ways of obtaining AMWs of aromatic oil fractions using n.m.r. only. The relevance of thermodynamic simulation has been proved to depend on precise characterization of the aromatic fractions. Many reported methods used for structural characterisation in terms of average parameters can be used, with very few additional assumptions, to obtain AMWs. However, the calculated values are sometimes noticeably model-dependent. Seventeen distillate aromatic fractions of a North Sea crude oil (named ‘Alwyn’) were studied to test different models. A comparison is made with mass-spectrometry data, and a reliable method combining various 13C and ‘H n.m.r. techniques is suggested. EXPERIMENTAL The aromatic fractions studied were obtained by liquid chromatography separation of 17 narrow high-boiling fractions issued from T.B.P. distillation of the Alwyn crude oil (see Table 1). They were prepared at the Centre de Recherches Total, Harfleur, France, which also supplied mass spectrometry data such as AMWs (M. Bouquet, personal communication). These data were 001~2361/89/091158~4$3.00 0 1989 Butterworth & Co. (Publishers) Ltd.
1158
FUEL, 1989, Vol 68, September
obtained by a method derived from the work of Fisher and Fischer*. Elemental analysis of the Alwyn fluid showed an atomic sulphur proportion of less than 0.05 wt%, which was neglected in the structural analysis. A Briiker AM 400 spectrometer equipped with Aspect 3000 software was used to record ‘H and 13C n.m.r. spectra at 400.13 and 100.60 MHz, respectively. For each aromatic fraction, a ‘H spectrum and a proton-decoupled inverse-gated 13C spectrum (decoupling during acquisition only) were recorded. Partial 13C spectra editing has also been undertaken, using the multiplet selection methods developed by Cookson and Smith3-‘. This was achieved by recording gated spin echo (GASPE) and conventional spin echo (CSE) spectra for different values of the parameter TJ, where z is the time delay between pulses and J the C-H coupling constant. The J values used were: J,,= 160 Hz (aromatic C-H) and Ja,= 125 Hz
Table 1
List of the 17 high-boiling fractions of Alwyn crude oil studied
Fraction number
Boiling point interval (“C)
F15 F16 F17 F18 F19 F20 F21 F22 F23 F24 F25 F26 F27 F28 F29 F30 F31
254.5-271.5 271 S-288 288 -303 303 -318 318 -332 332 -344 344 -357 357 -369 369 -381 381 -392 392 -402 402 413 413 -423 423 -432 432 -441 441 -450 450 -530
Determining
AMWs of high boiling aromatic oil fractions: B. Rousseau
(aliphatic C-H). GASPE and CSE spectra were recorded with zJ,,=O.5 for the total aromatic 13C editing (quantitative editing of the quaternary and tertiary aromatic carbon spectra). GASPE and CSE spectra were -recorded with rJa,= 1.0 and GASPE spectrum was recorded with tJ,,=0.5 for the partial aliphatic editing. Using complete spectra editing, we checked that the aliphatic primary and tertiary carbon signals did not overlap in the ‘CH + CH,’ partially edited spectrum. The last stage of the editing procedure was therefore substituted by a chemical shift range assignment (60 to 25Sppm for tertiary carbons and 25.5 to 5 ppm for primary carbons). This is in agreement with the works of Bouquet and Bailleul’ and Snape’. This procedure allows recording time corresponding to two GASPE spectra (rJ,,=0.25 and 0.75) to be saved. The analysis described is based on data from seven spectra recorded for each of the 17 distillate aromatic fractions of the Alwyn crude oil. The main acquisition parameters are given in Table 2, together with some information about the sample preparation. For all spectra, a digital phase correction, a baseline correction and a digital integration were performed. The analytical procedure used is in accordance with the ‘optimum experimental conditions’ described by Gillet et al. lo , for this kind of work. The accuracy of each measured quantity (e.g. % of quaternary, tertiary, aliphatic or aromatic carbon) is then assumed to be Q 1%. AMW CALCULATIONS Compactness assumption
c=G/f,
(1)
where C,, is the average number of aromatic carbons per molecule and f, is the aromaticity index
Main acquisition parameters Type of probe
A, obtained using r3C shift range using ‘H n.m.r. model derived from
(Ji = C,J{C,,+ C,,>). At this stage, a compactness assumption must be made to relate C,, to C,, the number of peripheral (non-bridged) carbon atoms. Ali” has pointed out that in a mixture of aromatic compounds, no significant difference occurs in the values of C&Z,, between ideal peri or cata-condensed systems, for a given value of C,,, as long as C,, does not exceed 15. This is typically the case for crude oil fractions. The simple relation between C,, and C,, which is valid for a cata-condensed aromatic system, can then be used. This can be written as follows: C,, = 6(2C,/C,, - 1)
In the absence of heteroatoms, the AMW determination involves calculating the average number of carbon and hydrogen atoms per molecule (C and R). The atomic ratio C/H is easily obtained (e.g. by elemental analysis), so attention is focussed on the most reliable way of obtaining C for an aromatic fraction. This quantity can be written as:
Table 2 The main acquisition preparation
Figure 1 C&, parameters: assignmentsr4; 0, calculated Brown and Ladner”
and A. H. Fuchs
parameters
‘H spectra
and conditions
for sample
r3C spectra (proton-decoupled and spectra editing) 1Omm 13C
Temperature (K) Spectral width (Hz) Digital resolution (Hz pts- ‘) n/2 pulse widths (ps) Number of scans Relaxation delay(s) Signal to noise ratio Memory capacity (K)
5 mm dual (‘H-=C) 300 4000 0.25 12 64 10 >I00 32
300 20000 0.62 9.5 2048-4096 10 >50 64
Sample preparation Solvent Reference Vol% of solute (approx.) Paramagnetic reagent
CDCI, T.M.S. 5 none
CDCL, T.M.S. 33 Cr(AcAc),,
3.10-‘M
(2)
CJC,, is now identified as being the crucial parameter for calculating AM W. CJC,, determination using chemical shijiirange assignments
Since the pioneer work by Williams’, many authors have based their structural analysis on the assignment of ‘H and/or 13C shift ranges to definite types of carbons or protons, e.g. H, (u to an aromatic ring), H,, H,‘“*‘2-‘6. Apart from the clear-cut separation between aromatic and aliphatic protons and carbons, any further distinction between subgroups (such as substituted and unsubstituted aromatic carbons) is sometimes doubtful because of the overlap of the shift ranges. The use of aromatic 13C shift range assignments provides a typical situation where the factor C,/C,, can be determined easily, but not accurately. The protondecoupled 13C spectra were analysed using the assignments for protonated (105-129.2 ppm), bridgehead (129.2-132.5 ppm) and substituted (132.5-149.2ppm) aromatic carbon atoms supplied by Yoshida et ~1.‘~. Figure I shows CJC,, values obtained by this method as a function of the fractions’ boiling points. It is clear that the assumed shift range distinction between the different types of carbons is erroneous, since CJC,, should decrease regularly as a function of the boiling point (this is confirmed by the mass spectrometry data). Because of this kind of problem, one has to be very careful when using shift range assumptions. A reasonable hypothesis seems to be the assumed separation between protons c( to an aromatic ring (H,), and all the other aliphatic protons (HP+). As seen in Figure 2, there is always a small overlap between H, and HP+, whatever
FUEL, 1989, Vol 68, September
1159
Determining
AMWs
t
of high boiling
aromatic
oil fractions:
and A. H. Fuchs
agrees with an expected parallel increase number of aromatic carbon atoms.
r---F-I
HCI
B. Rousseau
CJC,,
determination
using spectra
of the average
editing
The multiplet selection method provides a quantitative and reliable means for determining the factor CH,,/C,, without the chemical shift range assignments hypothesis 3- *. No such method exists, however, to determine the factor Cqs/Car which is needed to compute CJCar, since :
c
b
C&a,
= W&r/G)
+ C,JGr)
(6)
where C,, is the number of substituted (quaternary) aromatic carbons. Again, instead of carbon shift range assumptions, the Brown-Ladner model was used to compute C,$C,,. For cata-condensed molecules, C,, = C,, and if x=y=R, then C, = (HJHJC,,
(7)
Therefore
(8) ,..,....,..,.,...!:....,....,, 3.0
3.s
2.5
PhO
1.5
1.0
’
Figure 2 Aliphatic parts of 400 MHz proton spectra: a, fraction F15; b, fraction F30. The shaded area indicates the separation between H, and H, + The separation value was 2.1 ppm
the boiling point of the fraction. The Brown and Ladner model17 is therefore used to show that this shift range assignment provides a simple and reliable way of obtaining AMWs. With the Brown-Ladner model, G/C,,
(3)
= (C, + CHJC,,
where C, is the number of aliphatic carbons CI to an aromatic ring. This can be calculated, provided that Equation (3) is rearranged C$C,,=
{(H,*/x)+H,*,)/((C/H)-
(HZ/x)-
(Hp+ */Y)) (4)
H,* is the percentage of protons c1 to an aromatic ring, H, + * is the percentage of protons fi or further to an aromatic ring, x = HJC,, and y = H, + /Ca + . In their original paper, Brown and Ladner17 set x=y=R=2, where R=(H/C),,. Equation (4) then becomes : WA,=
{K/W+
WkMWW-
W:IR))
(5)
Although the value of R can be determined through spectra editing, this very simple model (x = y = R = 2) was used to compute C,/C,, for these fractions. In agreement the H, shift range has been with previous authors”, assumed to be 2.1-5.0ppm, and the H,+ range to be 0.2-2.1 ppm. The value of 2.1 ppm corresponds to the minimum intensity of the proton absorption between 0.2 and 5.0ppm. A slightly different separation value (e.g. 1.85 ppm), leads to noticeably different values of H, (z 15% higher), but the decreasing tendency of H, with increasing boiling point remains. The 2.1 ppm separation value has been chosen mainly because lower values would values greater than 1.0 for the lighter yield CJC,, fractions. The overall decreasing tendency of C,/C,, with T, obtained with this model seems reasonable, since it
1160
FUEL, 1989, Vol 68, September
The factor Cal/Car is measured directly through the proton-decoupled, inverse-gated 13C spectrum, and HJH,, is determined as described previously. AMWs of the Alwyn aromatic fractions were determined using Equations (8), (6) and (2). They are shown in Figure 3 together with the value obtained using the ‘H n.m.r. model and the mass spectrometry data (M. Bouquet, personal communication). A fair agreement is observed between these sets of data, those from the proton model being somewhat more scattered and the values for the lighter fractions systematically too low. DISCUSSION We have shown that it is possible to obtain the AMW 9f a high-boiling aromatic fraction using n.m.r. only, provided that the chemical shift range assumptions are carefully chosen and the number reduced to a minimum. Two structural analysis methods were selected, which yield fairly precise AMW values (precision estimated at d 10%). A systematic comparison between this method and the usual VP0 scheme, for different kind of fluids, should now be undertaken.
AMW 450
t
t 350.
250
ml
1
Figure 3 Average molecular weight of Alwyn aromatic fractions: A, obtained through the initial proton n.m.r. model; *, obtained through the improved proton n.m.r. model; 0, obtained through the spectra editing model; W, obtained by mass spectrometry (M. Bouquet, personal communication). Error bars refer to the spectra editing model
b
Determining
AMWs
of high boiling
One of the models used is based on spectra editing. It requires a long recording time (80 h for each fraction), but provides most of the average structural parameters that are needed to simulate the fluid through the group contribution thermodynamic models. The other model, based on the analysis of the proton spectrum, is much simpler, but the gain in recording time is balanced by a loss of reliable structural information. Both models rely on the Brown-Ladner hypothesis, which can be summarized as follows: (9)
(H/C), = (H/C),, = 2
The first part of this expression cannot be tested directly through n.m.r. for the time being (the possible future existence of quantitative ‘D n.m.r. could provide a means to test this assumption), but the second part can be tested easily using spectra editing, since (H/C),, = (CH,, + 2CH, + 3CH3)/C,,
(10)
where CH,,, CH,, CH, are the number of primary, secondary and tertiary aliphatic carbons respectively. The (H/C),, values range from ~2.0 for the heavier fraction up to 2.2 for the lighter fraction. This has an important effect on the model in Figure4, which shows the aromaticity index calculated using the equation: f, = {(C/H) - (HZiIR))I(CIH)
(11)
with R=2 (the complete Brown-Ladner hypothesis), or R = (H/C),, determined experimentally through Equation (10). Also shown in Figure4 is the aromaticity index determined directly from the 13C spectrum (without any hypothesis). It is clear from Figure4 that the Brown-Ladner model is greatly improved when using the actual (H/C),, value. The second part of the hypothesis was tested through the aromaticity index. An indirect test of the validity of the first part of the Brown-Ladner assumption (H/C),= (H/C),,, is provided by comparing
aromatic
oil fractions:
B. Rousseau
and A. H. Fuchs
the AMWs computed through Equations (5), (2) and (1) using either R=2 or the actual (H/C),, value, and the AMWs data obtained independently through mass spectrometry. This is shown in Figure3. Provided that some faith can be placed on the mass spectrometry data, it seems that the first part of the Brown-Ladner hypothesis has been proved for the crude oil fractions. To sum up, the assumptions used in both models introduced in this work are: no heteroatoms; catacondensed aromatic systems only; (H/C), = (H/C),,; and the chemical shift range of the protons a to an aromatic ring is 2.1- 5.0 ppm. ACKNOWLEDGEMENTS The authors acknowledge the scientific and financial support from Total-Compagnie Francaise des Petroles, which supplied the n.m.r. spectrometer and a grant for Bernard Rousseau. They also thank Gerard Auxiette, project manager, and M. Bouquet and A. Bailleul of the Centre de Recherche Total, Harfleur. REFERENCES Williams, R. B. AS7M Spec. Tech. Publ. 1958, 224, 168 Fisher, I. P. and Fischer, P. Talenta 1974, 21, 867 Cookson, D. J. and Smith, B. E. Org. Mann. Res. 1981,16,111 Cookson, D. J. and Smith, B. E. Fuel i983, 62, 34 Cookson. D. J. and Smith. B. E. Fuel 1983.62. 986 Cookson; D. J. and Smith; B. E. Anal. Chem. i985, 57, 864 Cookson, D. J. and Smith, B. E. Energy & Fuels 1987, 1, 111 Bouquet, M. and Bailleul, A. Fue[ 1986, 65, 1240 Snape, C. E. Fuel 1982,61, 775 Gillet, S., Rubini, P., Delpuech, J. J., Escalier, J. C. and Valentin, P. Fuel 1981,60, 221 Ali, L. H. Fuel 1971, 50, 298 Knight, S. A. Chem. Ind. 1967, 1920 Clutter. D. R.. Petrakis. L., Stenaer. R. L. and Jensen. R. K. Anal. them. 1972,44, 1395’ Yoshida, T., Maekawa, Y., Uchino, H. and Yokoyama, S. Anal. Chem. 1980, 52, 817 Takagemi, Y., Watanabe, Y., Suzuki, T., Mitsudo, T. and Itoh, M. Fuel 1980, 59, 253 Leon, V. Fuel 1987,66, 1445 Brown, J. K. and Ladner, W. R. Fuel 1960,39, 79
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
1
NOMENCLATURE
\
6-
\ *
f
.
*
5.
AMW C C,,
:
A
*
*
\
. ’
*
f, C,
z . . * ; \
0,4
I 300
1 *
I * i
400
4b& 1 Tb
Figure 4 Aromaticity index,f,, calculated using: *, the initial proton model with R = 2; A, the proton model with R = (H/C),, experimentally determined through spectra editing; 0, the inverse-gated, protondecoupled 13C spectrum (no structural hypothesis)
Cal, C,, CH,, CH, CH3 eels
Ha,
HP+
HT
G
average molecular weight average number of carbon atoms per molecule average number of aromatic carbon atoms per molecule aromaticity index number of peripheral (non-bridged) aromatic carbon atoms number of aliphatic/aromatic carbon atoms number of primary carbon atoms number of secondary carbon atoms number of tertiary carbon atoms number of substituted (quaternary) aromatic carbon atoms number of protons/carbons c( to an aromatic ring number of protons /I or further from an aromatic ring percentage of Hi
FUEL, 1989, Vol 68, September
1161
Rousseau
and Alain
H. Fuchs
Laboratoire de Chimie-Physique des Matdriaux Amorphes (U.A. 1104 du C.N.R.S.), iJniversitr2 Paris-Sud, 97405 Orsay, France (Received 3 January 1989; revised 5 April 1989)
Average molecular weight (AMW) is a very important parameter in the thermodynamic simulation of crude oil. N.m.r. structural analysis models can be used to obtain AMWs of high-boiling aromatic fractions,
provided that the chemical shift range assumptions are chosen carefully and the number reduced to a minimum. A model using 13C spectra editing is particularly reliable, although it requires a long recording time. The models were tested on 17 narrow high-boiling aromatic fractions of a North Sea crude oil. The values obtained are in very good agreement with mass spectrometry data. (Keywords: n.m.r.; carbon-13 n.m.r.; aromatic)
Characterization of oil fractions using average molecular parameters obtained by quantitative n.m.r. analysis is a well-known method, introduced 30 years ago by Williams’. This method is often used to control the changes in composition at various stages of conversion processes. It can also be useful for simulating thermophysical properties of crude oil such as phase equilibria. In this case, accurate values for the average molecular weight (AMW) of the oil distillate fractions are required, to establish a relevant equation of state for the fluid. The accuracy of data obtained through vapour-pressure osmometry is sometimes questionable, especially for the high-boiling fractions. This paper therefore investigates ways of obtaining AMWs of aromatic oil fractions using n.m.r. only. The relevance of thermodynamic simulation has been proved to depend on precise characterization of the aromatic fractions. Many reported methods used for structural characterisation in terms of average parameters can be used, with very few additional assumptions, to obtain AMWs. However, the calculated values are sometimes noticeably model-dependent. Seventeen distillate aromatic fractions of a North Sea crude oil (named ‘Alwyn’) were studied to test different models. A comparison is made with mass-spectrometry data, and a reliable method combining various 13C and ‘H n.m.r. techniques is suggested. EXPERIMENTAL The aromatic fractions studied were obtained by liquid chromatography separation of 17 narrow high-boiling fractions issued from T.B.P. distillation of the Alwyn crude oil (see Table 1). They were prepared at the Centre de Recherches Total, Harfleur, France, which also supplied mass spectrometry data such as AMWs (M. Bouquet, personal communication). These data were 001~2361/89/091158~4$3.00 0 1989 Butterworth & Co. (Publishers) Ltd.
1158
FUEL, 1989, Vol 68, September
obtained by a method derived from the work of Fisher and Fischer*. Elemental analysis of the Alwyn fluid showed an atomic sulphur proportion of less than 0.05 wt%, which was neglected in the structural analysis. A Briiker AM 400 spectrometer equipped with Aspect 3000 software was used to record ‘H and 13C n.m.r. spectra at 400.13 and 100.60 MHz, respectively. For each aromatic fraction, a ‘H spectrum and a proton-decoupled inverse-gated 13C spectrum (decoupling during acquisition only) were recorded. Partial 13C spectra editing has also been undertaken, using the multiplet selection methods developed by Cookson and Smith3-‘. This was achieved by recording gated spin echo (GASPE) and conventional spin echo (CSE) spectra for different values of the parameter TJ, where z is the time delay between pulses and J the C-H coupling constant. The J values used were: J,,= 160 Hz (aromatic C-H) and Ja,= 125 Hz
Table 1
List of the 17 high-boiling fractions of Alwyn crude oil studied
Fraction number
Boiling point interval (“C)
F15 F16 F17 F18 F19 F20 F21 F22 F23 F24 F25 F26 F27 F28 F29 F30 F31
254.5-271.5 271 S-288 288 -303 303 -318 318 -332 332 -344 344 -357 357 -369 369 -381 381 -392 392 -402 402 413 413 -423 423 -432 432 -441 441 -450 450 -530
Determining
AMWs of high boiling aromatic oil fractions: B. Rousseau
(aliphatic C-H). GASPE and CSE spectra were recorded with zJ,,=O.5 for the total aromatic 13C editing (quantitative editing of the quaternary and tertiary aromatic carbon spectra). GASPE and CSE spectra were -recorded with rJa,= 1.0 and GASPE spectrum was recorded with tJ,,=0.5 for the partial aliphatic editing. Using complete spectra editing, we checked that the aliphatic primary and tertiary carbon signals did not overlap in the ‘CH + CH,’ partially edited spectrum. The last stage of the editing procedure was therefore substituted by a chemical shift range assignment (60 to 25Sppm for tertiary carbons and 25.5 to 5 ppm for primary carbons). This is in agreement with the works of Bouquet and Bailleul’ and Snape’. This procedure allows recording time corresponding to two GASPE spectra (rJ,,=0.25 and 0.75) to be saved. The analysis described is based on data from seven spectra recorded for each of the 17 distillate aromatic fractions of the Alwyn crude oil. The main acquisition parameters are given in Table 2, together with some information about the sample preparation. For all spectra, a digital phase correction, a baseline correction and a digital integration were performed. The analytical procedure used is in accordance with the ‘optimum experimental conditions’ described by Gillet et al. lo , for this kind of work. The accuracy of each measured quantity (e.g. % of quaternary, tertiary, aliphatic or aromatic carbon) is then assumed to be Q 1%. AMW CALCULATIONS Compactness assumption
c=G/f,
(1)
where C,, is the average number of aromatic carbons per molecule and f, is the aromaticity index
Main acquisition parameters Type of probe
A, obtained using r3C shift range using ‘H n.m.r. model derived from
(Ji = C,J{C,,+ C,,>). At this stage, a compactness assumption must be made to relate C,, to C,, the number of peripheral (non-bridged) carbon atoms. Ali” has pointed out that in a mixture of aromatic compounds, no significant difference occurs in the values of C&Z,, between ideal peri or cata-condensed systems, for a given value of C,,, as long as C,, does not exceed 15. This is typically the case for crude oil fractions. The simple relation between C,, and C,, which is valid for a cata-condensed aromatic system, can then be used. This can be written as follows: C,, = 6(2C,/C,, - 1)
In the absence of heteroatoms, the AMW determination involves calculating the average number of carbon and hydrogen atoms per molecule (C and R). The atomic ratio C/H is easily obtained (e.g. by elemental analysis), so attention is focussed on the most reliable way of obtaining C for an aromatic fraction. This quantity can be written as:
Table 2 The main acquisition preparation
Figure 1 C&, parameters: assignmentsr4; 0, calculated Brown and Ladner”
and A. H. Fuchs
parameters
‘H spectra
and conditions
for sample
r3C spectra (proton-decoupled and spectra editing) 1Omm 13C
Temperature (K) Spectral width (Hz) Digital resolution (Hz pts- ‘) n/2 pulse widths (ps) Number of scans Relaxation delay(s) Signal to noise ratio Memory capacity (K)
5 mm dual (‘H-=C) 300 4000 0.25 12 64 10 >I00 32
300 20000 0.62 9.5 2048-4096 10 >50 64
Sample preparation Solvent Reference Vol% of solute (approx.) Paramagnetic reagent
CDCI, T.M.S. 5 none
CDCL, T.M.S. 33 Cr(AcAc),,
3.10-‘M
(2)
CJC,, is now identified as being the crucial parameter for calculating AM W. CJC,, determination using chemical shijiirange assignments
Since the pioneer work by Williams’, many authors have based their structural analysis on the assignment of ‘H and/or 13C shift ranges to definite types of carbons or protons, e.g. H, (u to an aromatic ring), H,, H,‘“*‘2-‘6. Apart from the clear-cut separation between aromatic and aliphatic protons and carbons, any further distinction between subgroups (such as substituted and unsubstituted aromatic carbons) is sometimes doubtful because of the overlap of the shift ranges. The use of aromatic 13C shift range assignments provides a typical situation where the factor C,/C,, can be determined easily, but not accurately. The protondecoupled 13C spectra were analysed using the assignments for protonated (105-129.2 ppm), bridgehead (129.2-132.5 ppm) and substituted (132.5-149.2ppm) aromatic carbon atoms supplied by Yoshida et ~1.‘~. Figure I shows CJC,, values obtained by this method as a function of the fractions’ boiling points. It is clear that the assumed shift range distinction between the different types of carbons is erroneous, since CJC,, should decrease regularly as a function of the boiling point (this is confirmed by the mass spectrometry data). Because of this kind of problem, one has to be very careful when using shift range assumptions. A reasonable hypothesis seems to be the assumed separation between protons c( to an aromatic ring (H,), and all the other aliphatic protons (HP+). As seen in Figure 2, there is always a small overlap between H, and HP+, whatever
FUEL, 1989, Vol 68, September
1159
Determining
AMWs
t
of high boiling
aromatic
oil fractions:
and A. H. Fuchs
agrees with an expected parallel increase number of aromatic carbon atoms.
r---F-I
HCI
B. Rousseau
CJC,,
determination
using spectra
of the average
editing
The multiplet selection method provides a quantitative and reliable means for determining the factor CH,,/C,, without the chemical shift range assignments hypothesis 3- *. No such method exists, however, to determine the factor Cqs/Car which is needed to compute CJCar, since :
c
b
C&a,
= W&r/G)
+ C,JGr)
(6)
where C,, is the number of substituted (quaternary) aromatic carbons. Again, instead of carbon shift range assumptions, the Brown-Ladner model was used to compute C,$C,,. For cata-condensed molecules, C,, = C,, and if x=y=R, then C, = (HJHJC,,
(7)
Therefore
(8) ,..,....,..,.,...!:....,....,, 3.0
3.s
2.5
PhO
1.5
1.0
’
Figure 2 Aliphatic parts of 400 MHz proton spectra: a, fraction F15; b, fraction F30. The shaded area indicates the separation between H, and H, + The separation value was 2.1 ppm
the boiling point of the fraction. The Brown and Ladner model17 is therefore used to show that this shift range assignment provides a simple and reliable way of obtaining AMWs. With the Brown-Ladner model, G/C,,
(3)
= (C, + CHJC,,
where C, is the number of aliphatic carbons CI to an aromatic ring. This can be calculated, provided that Equation (3) is rearranged C$C,,=
{(H,*/x)+H,*,)/((C/H)-
(HZ/x)-
(Hp+ */Y)) (4)
H,* is the percentage of protons c1 to an aromatic ring, H, + * is the percentage of protons fi or further to an aromatic ring, x = HJC,, and y = H, + /Ca + . In their original paper, Brown and Ladner17 set x=y=R=2, where R=(H/C),,. Equation (4) then becomes : WA,=
{K/W+
WkMWW-
W:IR))
(5)
Although the value of R can be determined through spectra editing, this very simple model (x = y = R = 2) was used to compute C,/C,, for these fractions. In agreement the H, shift range has been with previous authors”, assumed to be 2.1-5.0ppm, and the H,+ range to be 0.2-2.1 ppm. The value of 2.1 ppm corresponds to the minimum intensity of the proton absorption between 0.2 and 5.0ppm. A slightly different separation value (e.g. 1.85 ppm), leads to noticeably different values of H, (z 15% higher), but the decreasing tendency of H, with increasing boiling point remains. The 2.1 ppm separation value has been chosen mainly because lower values would values greater than 1.0 for the lighter yield CJC,, fractions. The overall decreasing tendency of C,/C,, with T, obtained with this model seems reasonable, since it
1160
FUEL, 1989, Vol 68, September
The factor Cal/Car is measured directly through the proton-decoupled, inverse-gated 13C spectrum, and HJH,, is determined as described previously. AMWs of the Alwyn aromatic fractions were determined using Equations (8), (6) and (2). They are shown in Figure 3 together with the value obtained using the ‘H n.m.r. model and the mass spectrometry data (M. Bouquet, personal communication). A fair agreement is observed between these sets of data, those from the proton model being somewhat more scattered and the values for the lighter fractions systematically too low. DISCUSSION We have shown that it is possible to obtain the AMW 9f a high-boiling aromatic fraction using n.m.r. only, provided that the chemical shift range assumptions are carefully chosen and the number reduced to a minimum. Two structural analysis methods were selected, which yield fairly precise AMW values (precision estimated at d 10%). A systematic comparison between this method and the usual VP0 scheme, for different kind of fluids, should now be undertaken.
AMW 450
t
t 350.
250
ml
1
Figure 3 Average molecular weight of Alwyn aromatic fractions: A, obtained through the initial proton n.m.r. model; *, obtained through the improved proton n.m.r. model; 0, obtained through the spectra editing model; W, obtained by mass spectrometry (M. Bouquet, personal communication). Error bars refer to the spectra editing model
b
Determining
AMWs
of high boiling
One of the models used is based on spectra editing. It requires a long recording time (80 h for each fraction), but provides most of the average structural parameters that are needed to simulate the fluid through the group contribution thermodynamic models. The other model, based on the analysis of the proton spectrum, is much simpler, but the gain in recording time is balanced by a loss of reliable structural information. Both models rely on the Brown-Ladner hypothesis, which can be summarized as follows: (9)
(H/C), = (H/C),, = 2
The first part of this expression cannot be tested directly through n.m.r. for the time being (the possible future existence of quantitative ‘D n.m.r. could provide a means to test this assumption), but the second part can be tested easily using spectra editing, since (H/C),, = (CH,, + 2CH, + 3CH3)/C,,
(10)
where CH,,, CH,, CH, are the number of primary, secondary and tertiary aliphatic carbons respectively. The (H/C),, values range from ~2.0 for the heavier fraction up to 2.2 for the lighter fraction. This has an important effect on the model in Figure4, which shows the aromaticity index calculated using the equation: f, = {(C/H) - (HZiIR))I(CIH)
(11)
with R=2 (the complete Brown-Ladner hypothesis), or R = (H/C),, determined experimentally through Equation (10). Also shown in Figure4 is the aromaticity index determined directly from the 13C spectrum (without any hypothesis). It is clear from Figure4 that the Brown-Ladner model is greatly improved when using the actual (H/C),, value. The second part of the hypothesis was tested through the aromaticity index. An indirect test of the validity of the first part of the Brown-Ladner assumption (H/C),= (H/C),,, is provided by comparing
aromatic
oil fractions:
B. Rousseau
and A. H. Fuchs
the AMWs computed through Equations (5), (2) and (1) using either R=2 or the actual (H/C),, value, and the AMWs data obtained independently through mass spectrometry. This is shown in Figure3. Provided that some faith can be placed on the mass spectrometry data, it seems that the first part of the Brown-Ladner hypothesis has been proved for the crude oil fractions. To sum up, the assumptions used in both models introduced in this work are: no heteroatoms; catacondensed aromatic systems only; (H/C), = (H/C),,; and the chemical shift range of the protons a to an aromatic ring is 2.1- 5.0 ppm. ACKNOWLEDGEMENTS The authors acknowledge the scientific and financial support from Total-Compagnie Francaise des Petroles, which supplied the n.m.r. spectrometer and a grant for Bernard Rousseau. They also thank Gerard Auxiette, project manager, and M. Bouquet and A. Bailleul of the Centre de Recherche Total, Harfleur. REFERENCES Williams, R. B. AS7M Spec. Tech. Publ. 1958, 224, 168 Fisher, I. P. and Fischer, P. Talenta 1974, 21, 867 Cookson, D. J. and Smith, B. E. Org. Mann. Res. 1981,16,111 Cookson, D. J. and Smith, B. E. Fuel i983, 62, 34 Cookson. D. J. and Smith. B. E. Fuel 1983.62. 986 Cookson; D. J. and Smith; B. E. Anal. Chem. i985, 57, 864 Cookson, D. J. and Smith, B. E. Energy & Fuels 1987, 1, 111 Bouquet, M. and Bailleul, A. Fue[ 1986, 65, 1240 Snape, C. E. Fuel 1982,61, 775 Gillet, S., Rubini, P., Delpuech, J. J., Escalier, J. C. and Valentin, P. Fuel 1981,60, 221 Ali, L. H. Fuel 1971, 50, 298 Knight, S. A. Chem. Ind. 1967, 1920 Clutter. D. R.. Petrakis. L., Stenaer. R. L. and Jensen. R. K. Anal. them. 1972,44, 1395’ Yoshida, T., Maekawa, Y., Uchino, H. and Yokoyama, S. Anal. Chem. 1980, 52, 817 Takagemi, Y., Watanabe, Y., Suzuki, T., Mitsudo, T. and Itoh, M. Fuel 1980, 59, 253 Leon, V. Fuel 1987,66, 1445 Brown, J. K. and Ladner, W. R. Fuel 1960,39, 79
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
1
NOMENCLATURE
\
6-
\ *
f
.
*
5.
AMW C C,,
:
A
*
*
\
. ’
*
f, C,
z . . * ; \
0,4
I 300
1 *
I * i
400
4b& 1 Tb
Figure 4 Aromaticity index,f,, calculated using: *, the initial proton model with R = 2; A, the proton model with R = (H/C),, experimentally determined through spectra editing; 0, the inverse-gated, protondecoupled 13C spectrum (no structural hypothesis)
Cal, C,, CH,, CH, CH3 eels
Ha,
HP+
HT
G
average molecular weight average number of carbon atoms per molecule average number of aromatic carbon atoms per molecule aromaticity index number of peripheral (non-bridged) aromatic carbon atoms number of aliphatic/aromatic carbon atoms number of primary carbon atoms number of secondary carbon atoms number of tertiary carbon atoms number of substituted (quaternary) aromatic carbon atoms number of protons/carbons c( to an aromatic ring number of protons /I or further from an aromatic ring percentage of Hi
FUEL, 1989, Vol 68, September
1161