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Structural changes during hydrogenation of lube distillates: n.m.r. studies
Short
Communication
Structural changes during hydrogenation of lube distillates: n.m.r. studies I. D. Singh,
M. K. S. Aloopwan,
G. S. Chaudhary
and Himmat
Singh
Indian Institute of Petroleum, Dehra Dun 248005, India (Received 26 April 1991; revised 4 November 7997)
13C and ‘H n.m.r. spectra of raw and hydrogenated lube distillates of intermediate and heavy viscosity ranges along with their aromatic concentrates were recorded. Structural changes occurring during hydrogenation were derived and used to explain the reactions taking place. Besides hydrogenation of aromatics, hydrodesulphurization and hydrodenitrogenation are also seen under the conditions used. While some hydrocracking/dealkylation of aromatic structures is observed in the intermediate viscosity lube distillate. conversion of higher condensed aromatics into hydroaromatics is predominant in the heavy viscosity lube distillate. (Keywords:
n.m.r.: lube distillates: hydrogenation)
The manufacture of quality lube oil base stocks (LOBS), which has previously been crude source dependent and followed a conventional processing sequence, has undergone significant changes during the last two decadesrm3. These include hydroprocessing3 under varying severities in one or two steps as well as deep hydrogenation depending upon the nature of feed and the required quality levels of base oils. Another scheme4 includes mild hydrogenation of raw lube distillates so as to improve their processibility before or after solvent extraction for better quality and yield of LOBS in the conventional processing scheme, thus improving the process economics. Such an approach results in partial saturation of aromatic ring structures and their conversion into molecular structures with improved properties desired in the lube base stocks. Recently, n.m.r. spectroscopy has been used to solve various industrial problems related to the petroleum industry5p9. Normally physico-chemical and hydrocarbon type compositional data obtained by mass spectrometry have been used to study the effectiveness of the hydrogenation step. But it is now well established” that the rate constant of hydrogenation of aromatic compounds strongly depends upon the alkyl side chains attached to the rings. The precise breakdown of feed carbon and hydrogen atoms obtained by n.m.r. techniques is being used to generate average structural data on aromatic rings as well as side chains which can be employed to explain the behaviour of feed under hydrogenation conditions. This study reports the observed structural changes obtained by n.m.r. spectrometry in an average hydrocarbon molecule of two raw lube distillates after their mild hydrogenation under similar operating conditions. The effect of structural parameters on the behaviour of lube
0016-1361.‘92,111335_03 r~ 1992 Bulterworth~Heinemann
Ltd.
distillates discussed.
during
hydrogenation
RESULTS
is also
EXPERIMENTAL Two raw lube distillates in intermediate (A) and heavy (B) viscosity ranges and their mild hydrogenated products (P, and P2 respectively) were obtained from an operating refinery. Reported typical hydrogenation conditions are: temperature:350~C:pressure:60kgcmm”:LSHV: 0.6 h; and a hydrogen partial pressure of Physico-chemical data on all 50kgcm-‘. the samples were generated using standard test methods. 13C and ‘H n.m.r. spectra of all the samples and their aromatic concentrates have been recorded in FT mode on a JEOL FX-lOO pulsed n.m.r. spectrometer at observing frequencies of 15.0 and 99.5 MHz respectively. The 13C n.m.r. spectra were recorded at room temperature under gated decoupling conditions with Cr (ac ac), as relaxation agent. All the measurements were made with CDCI, as solvent and TMS as internal standard. Other experimental and instrumental conditions were chosen to obtain quantitative spectra.
Table 1
AND DISCUSSION
Some of the relevant physico-chemical data of the feed materials and products are presented in Table 1. The structural parameters of raw lube distillates derived from 13C and ‘H n.m.r. spectrometry using procedures outlined by Gillet et (II.” are reported in Table 2. Similar data on the aromatic concentrates of all the four samples (lube distillates and their hydrogenated products) are presented in Table 3. Both the raw lube distillates are rich in sulphur (Table 1). The reduction in sulphur content during hydrogenation indicates the occurrence of a desulphurization reaction. This reaction is more pronounced in the intermediate viscosity distillate (66% reduction in sulphur) than in the heavy viscosity (58% reduction) stock. Further, the reduction in aromatic content during hydrogenation is more in A (16.5%) than in B (13.3%). These data along with other results presented in Table 1 conclusively show that the hydrogenation of intermediate viscosity lube distillate is more effective than that of heavy viscosity stock under similar operating conditions. The ‘“C n.m.r. data of both the feeds
Physico-chemical characteristics of lube distillates and their hydrogenated
Sample
API Kin. vis. cSt at 100 C Total sulphur. “1” wt Nitrogen. “%,wt Mol. wt Asphaltenes. “/u wt C ‘H atomic ratio Saturates, 0% wt Aromatics, % wt
A ‘2 9.0 1.9 0.14 410 0.10 0.60 43.5 56.5
PI
B
PZ
25 7.4
18 7, 3 _-,_
21 16.0
I .o 0.17
FUEL,
products
400 0.06 0.58 53.1 46.8
3.x 0.14 505 0.2 0.6’ 35.9 64.1
I .6 0.01 490
1992,
Vol 71, November
1335
0.1 0.60 44.4 55.6
Short
Communication
Table 2 Structural parameters distillates obtained from n.m.r.
of
lube
Sample Parameter
A
B
4: c,,, ?/o C n P O’”c, p “/” C
65.5 20.0 20.2 25.3 34.5 10.4 19.5 4.6 0.35 0.57 0.30 0.13 I I.7 7.5
67.1 22.6 19.7 24.8 32.9 10.2 16.8 5.9 0.33 0.51 0.31 0.18 IO.4 8.6
‘!‘UClr “41 C,,..,,, “‘0 C,,.,, “‘0 C,,,,, f, f.ir.tt frr.all f, “‘,u H,,, H.,, H,,
(T&/e 2) show that the per cent of saturated carbon atoms CC,,,) in A is lower than that in B, but after hydrogenation it is equal and higher than the feeds. The distribution of C,,, shows a higher amount of naphthenic carbons, C,. than those of normal and branched paraffinic carbons, Cn_, and Cipp; both being equal in A, while in B the amount decreases following the order C,, C,_, and Ci+ The aromatic carbons, C,,, are nearly one-third of the total carbons in the feeds and are present mainly in peripheral locations, indicating a low degree of condensation. The major portions of C,, in both the feeds are unsubstituted. The C,, at condensed points are slightly higher in B (18%) than in A (13%). An opposite trend in unsubstituted aromatic carbons is observed in these feeds. As aromatic compounds are the major constituents attacked during hydrogenation, it is thought to be worthwhile to generate structural parameters on the aromatic concentrates separated from
Table 3 Structural parameters hydrogenated products obtained
raw lube distillates and their corresponding hydrogenated products and to compare them. As is evident from Tuhle 3, there is no appreciable change in the percentage of C, of the aromatics from A during hydrogenation, but a significant increase (50%) in percentage of C, of aromatics from B is observed. This suggests that polyaromatics present in the aromatic concentrate from the base oil B are converted to hydroaromatics through hydrogenation of aromatic rings at terminal positions. This is corroborated by an increase in the C,/C, ratio (naphthenic to paraffinic carbon ratio) from 0.27 to 0.45 and reduction of the percentage of Har,poly in the case of B (aromatics). The comparison of the percentage increases in C, of P, (13.4%) and P, (aromatics) (3.5%) reveals that most of the hydrogenated aromatic compounds have become saturated. whereas in the case of the heavy viscosity distillate (B), a major portion of aromatics is partially hydrogenated. This is supported by the higher reduction in aromaticity (f,) of aromatic fractions. A relatively greater reduction in the average chain length of alkyl groups (ACL) on aromatic rings as well as in the percentage of Car,a,k (Table 3) in A and A (aromatics) indicates the occurrence of cracking/dealkylation to some extent under the conditions of hydrogenation used. This is also supported by a relatively higher increase in compactness factor (f,) as dealkylation renders aromatic nuclei more prone to condensation. Such reactions are insignificant in the case of the heavy viscosity distillate. The occurrence of hydrocracking has also been reported by Awadalla et al.” using GASPE 13C n.m.r. during a hydrogenation study of aromatic solvents. The decreasing trends in aromaticity (f,) of aromatic fractions indicate the conversion of aromatic carbons to naphthenic ones. The reduction in the percentage of H,, (particularly Har,poly) and increase in H,,,/H,, in the aromatics of
of the aromatic from n.m.r.
fractions
of lube
distillates
and
lube distillates after hydrogenation corroborates the earlier inferences that partial conversion of condensed aromatics into hydroaromatics is more pronounced in B than in A. CONCLUSIONS From the comparison of structural parameters of raw lube distillates and their hydrogenated products under similar operating conditions. the following inferences can be drawn:
(i) both the feed stocks behave differ-
(ii)
(iii)
(iv)
(v)
ently under similar hydrogenation conditions depending upon their viscosity and compositional parameters; large condensed aromatic structures are converted into hydroaromatics. This change is more pronounced in the heavy viscosity lube distillate; conversion of aromatics into saturates leads to an increase in the lube base stock yield; the occurrence of cracking/dealkylation during hydrogenation is relatively higher in the intermediate viscosity lube distillate: desulphurization in both the feed stocks is significant, but quite appreciable in the intermediate viscosity raw lube distillate.
REFERENCES
5
6
their
Sample A (aromatics)
P,
Parameter
(aromatics)
B (aromatics)
PZ (aromatics)
“1” C,,, %1 C”_, “4, c,_, %I c, ACL % c,, % C,,,,,, “A C,,.,, %J C*,,, f, far.arlr f.U.,, f, % H,, H,,,;H.x
53.9 15.8 20.9 17.2 12.0 46.1 17.8 23.8 4.5 0.46 0.39 0.52 0.10 17.9 4.6
57.6 18.6 21.2 17.8 9.5 42.4 16.2 21.3 4.9 0.42 0.38 0.50 0.12 13.7 6.3
57.5 22.6 22.6 12.3 8.2 42.5 14.4 21.2 6.9 0.43 0.3J U.50 0.16 14.6 5.8
63.1 23.5 21.1 18.5 X.0 36.9 13.9 15.8 7.1 0.37 0.38 0.43 0.19 10.3 8.7
1336
FUEL,
1992,
Vol 71, November
Soudek, M. H~&ocrrrh. Ptw. 1974, 52. 59 Hournac, R. Ht&occ~+. Ptvc. 1981. 60, 207 Zakarian. J. A.. Robson. R. J. and Farsell, T. R. Errrry) Proyr. 1987. 7. 59 Bijwaard, H. M. J., Bremer. W. K. J. and Van Doorne. P. Proc. Pet. Rqf: Cmf: Jpn PC/. ht. 77-2X Oct., 1986 Mauleon, J. L.. Sigaud. J. B.. Biederman. J. M. and Heinrich, G. Proceedings of the 12th World Petrol Congress, Houston 1987. Vol. 4. p. 71 Bonquet. M., Colin, J. M.. Durand. J. P. and Boulet, R. An7. Chm. Sot. Dir. Pet. Chem. Preps. 1989, 34, 339 Vecchi, C. and Marengo. S. Fuel 1990, 69, 706 Singh. I. D.. Kothiyal. V.. Ramaswamy. V. and Krishna. R. Fuel 1990.69. 289 Singh. I. D., Kothiyal, V. and Ramaswamy, V. Erdorl Kohle Er&us Peiro(./1ew. 1991. 44, 22 Nate. D. M, Ind. E~J. Chcm. Prod. Res. Der. 1970. 9. 203 Gillet. S., Rubini. P., Delpuech. J. J.. Escalier, J. C. and Valentin, P. Fuel 1981, 60, 2’6 Awadalla. A. A.. Cookson. D. J. and Smith, B. E. Frrrl 1985. 64. 1097
NOMENCLATURE % C,,, % c, % c,_,
Percentage carbons Percentage carbons Percentage carbons
of total saturated of paraffinic of n-paraffinic
Short Communication
%
cimp
% c, % c,, % Cx.&
% CU.”
Percentage of iso-paraffinic carbons Percentage of naphthenic carbons Percentage of aromatic carbons Percentage of aromatic carbons substituted by alkyl groups Percentage of protonated aromatic carbons
% c,, f, fdT.H
far,a,k
f,
Percentage of aromatic carbons at condensed points Aromaticity i.e. fraction of aromatic carbons Ratio of protonated aromatic carbons to total aromatic carbons Ratio of alkyl substituted aromatic carbons to total aromatic carbons Compactness factor i.e. ratio
% H,,
H nr.poly ACL
FUEL,
1992,
of bridge-head aromatic carbons to total aromatic carbons Percentage of protons attached with aromatic structures Percentage of protons attached with polyaromatic structures Average chain length of alkyl groups
Vol 71, November
1337
Communication
Structural changes during hydrogenation of lube distillates: n.m.r. studies I. D. Singh,
M. K. S. Aloopwan,
G. S. Chaudhary
and Himmat
Singh
Indian Institute of Petroleum, Dehra Dun 248005, India (Received 26 April 1991; revised 4 November 7997)
13C and ‘H n.m.r. spectra of raw and hydrogenated lube distillates of intermediate and heavy viscosity ranges along with their aromatic concentrates were recorded. Structural changes occurring during hydrogenation were derived and used to explain the reactions taking place. Besides hydrogenation of aromatics, hydrodesulphurization and hydrodenitrogenation are also seen under the conditions used. While some hydrocracking/dealkylation of aromatic structures is observed in the intermediate viscosity lube distillate. conversion of higher condensed aromatics into hydroaromatics is predominant in the heavy viscosity lube distillate. (Keywords:
n.m.r.: lube distillates: hydrogenation)
The manufacture of quality lube oil base stocks (LOBS), which has previously been crude source dependent and followed a conventional processing sequence, has undergone significant changes during the last two decadesrm3. These include hydroprocessing3 under varying severities in one or two steps as well as deep hydrogenation depending upon the nature of feed and the required quality levels of base oils. Another scheme4 includes mild hydrogenation of raw lube distillates so as to improve their processibility before or after solvent extraction for better quality and yield of LOBS in the conventional processing scheme, thus improving the process economics. Such an approach results in partial saturation of aromatic ring structures and their conversion into molecular structures with improved properties desired in the lube base stocks. Recently, n.m.r. spectroscopy has been used to solve various industrial problems related to the petroleum industry5p9. Normally physico-chemical and hydrocarbon type compositional data obtained by mass spectrometry have been used to study the effectiveness of the hydrogenation step. But it is now well established” that the rate constant of hydrogenation of aromatic compounds strongly depends upon the alkyl side chains attached to the rings. The precise breakdown of feed carbon and hydrogen atoms obtained by n.m.r. techniques is being used to generate average structural data on aromatic rings as well as side chains which can be employed to explain the behaviour of feed under hydrogenation conditions. This study reports the observed structural changes obtained by n.m.r. spectrometry in an average hydrocarbon molecule of two raw lube distillates after their mild hydrogenation under similar operating conditions. The effect of structural parameters on the behaviour of lube
0016-1361.‘92,111335_03 r~ 1992 Bulterworth~Heinemann
Ltd.
distillates discussed.
during
hydrogenation
RESULTS
is also
EXPERIMENTAL Two raw lube distillates in intermediate (A) and heavy (B) viscosity ranges and their mild hydrogenated products (P, and P2 respectively) were obtained from an operating refinery. Reported typical hydrogenation conditions are: temperature:350~C:pressure:60kgcmm”:LSHV: 0.6 h; and a hydrogen partial pressure of Physico-chemical data on all 50kgcm-‘. the samples were generated using standard test methods. 13C and ‘H n.m.r. spectra of all the samples and their aromatic concentrates have been recorded in FT mode on a JEOL FX-lOO pulsed n.m.r. spectrometer at observing frequencies of 15.0 and 99.5 MHz respectively. The 13C n.m.r. spectra were recorded at room temperature under gated decoupling conditions with Cr (ac ac), as relaxation agent. All the measurements were made with CDCI, as solvent and TMS as internal standard. Other experimental and instrumental conditions were chosen to obtain quantitative spectra.
Table 1
AND DISCUSSION
Some of the relevant physico-chemical data of the feed materials and products are presented in Table 1. The structural parameters of raw lube distillates derived from 13C and ‘H n.m.r. spectrometry using procedures outlined by Gillet et (II.” are reported in Table 2. Similar data on the aromatic concentrates of all the four samples (lube distillates and their hydrogenated products) are presented in Table 3. Both the raw lube distillates are rich in sulphur (Table 1). The reduction in sulphur content during hydrogenation indicates the occurrence of a desulphurization reaction. This reaction is more pronounced in the intermediate viscosity distillate (66% reduction in sulphur) than in the heavy viscosity (58% reduction) stock. Further, the reduction in aromatic content during hydrogenation is more in A (16.5%) than in B (13.3%). These data along with other results presented in Table 1 conclusively show that the hydrogenation of intermediate viscosity lube distillate is more effective than that of heavy viscosity stock under similar operating conditions. The ‘“C n.m.r. data of both the feeds
Physico-chemical characteristics of lube distillates and their hydrogenated
Sample
API Kin. vis. cSt at 100 C Total sulphur. “1” wt Nitrogen. “%,wt Mol. wt Asphaltenes. “/u wt C ‘H atomic ratio Saturates, 0% wt Aromatics, % wt
A ‘2 9.0 1.9 0.14 410 0.10 0.60 43.5 56.5
PI
B
PZ
25 7.4
18 7, 3 _-,_
21 16.0
I .o 0.17
FUEL,
products
400 0.06 0.58 53.1 46.8
3.x 0.14 505 0.2 0.6’ 35.9 64.1
I .6 0.01 490
1992,
Vol 71, November
1335
0.1 0.60 44.4 55.6
Short
Communication
Table 2 Structural parameters distillates obtained from n.m.r.
of
lube
Sample Parameter
A
B
4: c,,, ?/o C n P O’”c, p “/” C
65.5 20.0 20.2 25.3 34.5 10.4 19.5 4.6 0.35 0.57 0.30 0.13 I I.7 7.5
67.1 22.6 19.7 24.8 32.9 10.2 16.8 5.9 0.33 0.51 0.31 0.18 IO.4 8.6
‘!‘UClr “41 C,,..,,, “‘0 C,,.,, “‘0 C,,,,, f, f.ir.tt frr.all f, “‘,u H,,, H.,, H,,
(T&/e 2) show that the per cent of saturated carbon atoms CC,,,) in A is lower than that in B, but after hydrogenation it is equal and higher than the feeds. The distribution of C,,, shows a higher amount of naphthenic carbons, C,. than those of normal and branched paraffinic carbons, Cn_, and Cipp; both being equal in A, while in B the amount decreases following the order C,, C,_, and Ci+ The aromatic carbons, C,,, are nearly one-third of the total carbons in the feeds and are present mainly in peripheral locations, indicating a low degree of condensation. The major portions of C,, in both the feeds are unsubstituted. The C,, at condensed points are slightly higher in B (18%) than in A (13%). An opposite trend in unsubstituted aromatic carbons is observed in these feeds. As aromatic compounds are the major constituents attacked during hydrogenation, it is thought to be worthwhile to generate structural parameters on the aromatic concentrates separated from
Table 3 Structural parameters hydrogenated products obtained
raw lube distillates and their corresponding hydrogenated products and to compare them. As is evident from Tuhle 3, there is no appreciable change in the percentage of C, of the aromatics from A during hydrogenation, but a significant increase (50%) in percentage of C, of aromatics from B is observed. This suggests that polyaromatics present in the aromatic concentrate from the base oil B are converted to hydroaromatics through hydrogenation of aromatic rings at terminal positions. This is corroborated by an increase in the C,/C, ratio (naphthenic to paraffinic carbon ratio) from 0.27 to 0.45 and reduction of the percentage of Har,poly in the case of B (aromatics). The comparison of the percentage increases in C, of P, (13.4%) and P, (aromatics) (3.5%) reveals that most of the hydrogenated aromatic compounds have become saturated. whereas in the case of the heavy viscosity distillate (B), a major portion of aromatics is partially hydrogenated. This is supported by the higher reduction in aromaticity (f,) of aromatic fractions. A relatively greater reduction in the average chain length of alkyl groups (ACL) on aromatic rings as well as in the percentage of Car,a,k (Table 3) in A and A (aromatics) indicates the occurrence of cracking/dealkylation to some extent under the conditions of hydrogenation used. This is also supported by a relatively higher increase in compactness factor (f,) as dealkylation renders aromatic nuclei more prone to condensation. Such reactions are insignificant in the case of the heavy viscosity distillate. The occurrence of hydrocracking has also been reported by Awadalla et al.” using GASPE 13C n.m.r. during a hydrogenation study of aromatic solvents. The decreasing trends in aromaticity (f,) of aromatic fractions indicate the conversion of aromatic carbons to naphthenic ones. The reduction in the percentage of H,, (particularly Har,poly) and increase in H,,,/H,, in the aromatics of
of the aromatic from n.m.r.
fractions
of lube
distillates
and
lube distillates after hydrogenation corroborates the earlier inferences that partial conversion of condensed aromatics into hydroaromatics is more pronounced in B than in A. CONCLUSIONS From the comparison of structural parameters of raw lube distillates and their hydrogenated products under similar operating conditions. the following inferences can be drawn:
(i) both the feed stocks behave differ-
(ii)
(iii)
(iv)
(v)
ently under similar hydrogenation conditions depending upon their viscosity and compositional parameters; large condensed aromatic structures are converted into hydroaromatics. This change is more pronounced in the heavy viscosity lube distillate; conversion of aromatics into saturates leads to an increase in the lube base stock yield; the occurrence of cracking/dealkylation during hydrogenation is relatively higher in the intermediate viscosity lube distillate: desulphurization in both the feed stocks is significant, but quite appreciable in the intermediate viscosity raw lube distillate.
REFERENCES
5
6
their
Sample A (aromatics)
P,
Parameter
(aromatics)
B (aromatics)
PZ (aromatics)
“1” C,,, %1 C”_, “4, c,_, %I c, ACL % c,, % C,,,,,, “A C,,.,, %J C*,,, f, far.arlr f.U.,, f, % H,, H,,,;H.x
53.9 15.8 20.9 17.2 12.0 46.1 17.8 23.8 4.5 0.46 0.39 0.52 0.10 17.9 4.6
57.6 18.6 21.2 17.8 9.5 42.4 16.2 21.3 4.9 0.42 0.38 0.50 0.12 13.7 6.3
57.5 22.6 22.6 12.3 8.2 42.5 14.4 21.2 6.9 0.43 0.3J U.50 0.16 14.6 5.8
63.1 23.5 21.1 18.5 X.0 36.9 13.9 15.8 7.1 0.37 0.38 0.43 0.19 10.3 8.7
1336
FUEL,
1992,
Vol 71, November
Soudek, M. H~&ocrrrh. Ptw. 1974, 52. 59 Hournac, R. Ht&occ~+. Ptvc. 1981. 60, 207 Zakarian. J. A.. Robson. R. J. and Farsell, T. R. Errrry) Proyr. 1987. 7. 59 Bijwaard, H. M. J., Bremer. W. K. J. and Van Doorne. P. Proc. Pet. Rqf: Cmf: Jpn PC/. ht. 77-2X Oct., 1986 Mauleon, J. L.. Sigaud. J. B.. Biederman. J. M. and Heinrich, G. Proceedings of the 12th World Petrol Congress, Houston 1987. Vol. 4. p. 71 Bonquet. M., Colin, J. M.. Durand. J. P. and Boulet, R. An7. Chm. Sot. Dir. Pet. Chem. Preps. 1989, 34, 339 Vecchi, C. and Marengo. S. Fuel 1990, 69, 706 Singh. I. D.. Kothiyal. V.. Ramaswamy. V. and Krishna. R. Fuel 1990.69. 289 Singh. I. D., Kothiyal, V. and Ramaswamy, V. Erdorl Kohle Er&us Peiro(./1ew. 1991. 44, 22 Nate. D. M, Ind. E~J. Chcm. Prod. Res. Der. 1970. 9. 203 Gillet. S., Rubini. P., Delpuech. J. J.. Escalier, J. C. and Valentin, P. Fuel 1981, 60, 2’6 Awadalla. A. A.. Cookson. D. J. and Smith, B. E. Frrrl 1985. 64. 1097
NOMENCLATURE % C,,, % c, % c,_,
Percentage carbons Percentage carbons Percentage carbons
of total saturated of paraffinic of n-paraffinic
Short Communication
%
cimp
% c, % c,, % Cx.&
% CU.”
Percentage of iso-paraffinic carbons Percentage of naphthenic carbons Percentage of aromatic carbons Percentage of aromatic carbons substituted by alkyl groups Percentage of protonated aromatic carbons
% c,, f, fdT.H
far,a,k
f,
Percentage of aromatic carbons at condensed points Aromaticity i.e. fraction of aromatic carbons Ratio of protonated aromatic carbons to total aromatic carbons Ratio of alkyl substituted aromatic carbons to total aromatic carbons Compactness factor i.e. ratio
% H,,
H nr.poly ACL
FUEL,
1992,
of bridge-head aromatic carbons to total aromatic carbons Percentage of protons attached with aromatic structures Percentage of protons attached with polyaromatic structures Average chain length of alkyl groups
Vol 71, November
1337