Ind. Eng. Chem. Res. 1991,30, 1971-1976
1971
Measurement and Prediction of Methyl-Substituted Naphthalene Isomer Ratios in Liquid Hydrocarbon Fuels Veronica T. Borrett, John M. Charlesworth,* and Alan G. Moritz? Materials Research Laboratory, DSTO, P.O. Box 50, Ascot Vale, Victoria 3032, Australia
Results for the relative isomer abundance of di- and trimethylnaphthalenes in crude and processed products from petroleum, shale oil, and coal tar are reported, together with several procedures for the calculation of the isomer distribution a t equilibrium. The data suggest that the isomer distributions can approach the equilibrium values in certain circumstances. The mechanisms by which such equilibria are achieved may equally explain the distribution of the lower alkylbenzenes, with respect to both isomer distribution and equilibria involving trans-alkylation reactions. This is a t variance with the mechanism postulated for the acid-catalyzed isomerization of alkylnaphthalenes in which only intramolecular methyl migrations are thought to occur. The thermodynamic calculations employed here are easily extended to other polycyclic systems. Introduction Alkylnaphthalenes and polycyclic aromatic hydrocarbons (PAH) often occur in hydrocarbon fuels derived from petroleum, shale oil, or coal. While some reports have suggested that there may be geochemical significance in changes in the relative distribution of such compounds (Alexander et al., 1983,1984;Radke et al., 1982),there do not seem to be generally accepted thermodynamic equilibrium values or conditions under which equilibrium is achieved or approached (Bhattacharya, 1981). The situation is surprising since the distribution of C3-alkylbenzenes in crude petroleum has been known for some time to conform to that expected for thermodynamic equilibrium at approximately400 "C (Forziata and Rossini, 1947). While such observations might suggest that equilibration of isomers is to be expected for the di- and trimethylnaphthalenes or other methyl polycyclic aromatic hydrocarbons in crude or processed petroleum products, it is not expected on the basis of the known isomerization of dimethylnaphthalenes under acidic conditions, such as HF/BF3 (Suld and Stewart, 1964)or aluminosilicates at elevated temperatures (Ogasawara et al., 1974). The available evidence indicates that such acidic isomerizations are limited to intramolecular a-p type migrations. Polycyclic aromatic hydrocarbons and their alkyl derivatives are widespread contaminants in the environment (Lunde and Bjorseth, 1977). In view of their potential carcinogenic activity (Dipple, 1976)considerable attention has been paid to source identification and to modes of transport through the environment (Lee et al., 1977; Sporstol et al., 1983). Significant sources of such compounds arise from incomplete combustion of carbonaceous materials and from fugitive emissions (White et al., 1979). Therefore it is to be expected that the isomeric abundance of alkyl PAH will be dependent not only on a variety of combustion parameters, such as the fuel-to-air ratio and degree of mixing (Lee et al., 1977;Barbella et al., 1989)but also on the fuel characteristics. Relative to the parent PAH, alkyl derivatives are dominant in crude petroleum and relatively abundant in low-temperature (approximately lo00 K)combustion products of wood or coal (Lee et al., 1977). Conversely high-temperature (approximately 2000 K) soot may be devoid of these compounds (LaFlamme and Hites, 1978). In terms of more desirable properties, perhydro derivatives of methyl-substituted naphthalenes provide the
* To whom correspondence should be addressed. Deceased.
potential for high-energy-density, high-stability, low-volatility fuels for aviation turbine engines. A survey of 6000 compounds (Donath and Hem, 1960)highlighted the favorable ratio of physical density to heat of combustion for the fully saturated polycyclic hydrocarbons and pointed to the likelihood of excellent jet fuels made by hydrogenation of naphthalenic materials such as coal, coal tar,and the aromatic residues from the processing of petroleum. Knowledge of and the ability to predict the distribution of isomers of alkyl-substituted polycyclic aromatic material in fossil fuels derived from a variety of sources may therefore be an important factor in the ability to optimize the composition of the finished fuel. In this paper we report some results for the relative isomer abundance of di- and trimethylnaphthalenes in crude and processed products from petroleum, coal tar,and shale oil together with several procedures for calculation of the isomer distribution at equilibrium.
Experimental Section Analytical Procedures. All mass spectrometric measurements were made on a Vacuum Generators Model 7035 double-focusing mass spectrometer (MS) equipped with a laminated magnet, digital scanner, and computer acquisition system. The MS conditions used throughout the work were accelerating voltage 4 kV, source temperature 200 "C, ionising voltage 70 eV, emission current 0.1 mA, and scan speed 200 msldecade. The gas chromatograph (GC, Varian Model 3700)was equipped with a Scientific Glass Engineering (SGE) universal injection system and bonded-phase (BP1) fused silica column, 50 m X 0.2 mm (SGE) that was terminated directly in the ion source. Typical operating conditions were carrier gas UHP helium, split ratio 501,injector and interface temperature 250 "C, and linear temperature programming from 30 "C at 2 "C/min. Preconcentration of the aromatic fraction of all samples was achieved by column chromatography on silica gel (Davidson Grade 923)using AR grade petroleum ether (bp 40-60 "C) to elute the nonpolar material followed by stripping the aromatic compounds with 8% (v/v) methano1/40-60 "C petroleum ether. To ensure reproducibility of the retention volumes and optimum separation, the silica gel was predried for 3 h at 150 "C. Residual aromatic compounds in the petroleum ether were removed by passage through a column of silica gel. Both the petroleum ether and the methanol showed only tail-end absorption in the UV spectrum over the range 220-320 nm. Experiments using synthetic mixtures of aromatic compounds with a flow-through cell suggested that collection of the
0888-5885/91/2630-1971$02.50/00 1991 American Chemical Society
1972 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
t
5
c;, 10
I
I
Ci4
cis
15
R e t e n t i o n Time
cie
20
25
(min.)
Figure 1. Typical chromatogram illustrating the distribution of mono-, di-, and trimethylnaphthalenes in a sample of catalytically cracked middle distillate fuel from Bass Strait crude petroleum. In this example isothermal separation a t 130 O C was used in order to determine Kovats retention indexes. The retention times for CI3to CI8normal alkanes are indicated. Peak assignments are as follows: 1, 2-methylnaphthalene; 2, l-methylnaphthalene; 3, 2,6-dimethylnaphthalene; 4, 2,7-dimethylnaphthalene; 5, 1,3- and 1,7-dimethyl7 , 2,3-, 1,4-, and 1,5-dinaphthalene; 6, 1,6-dimethylnaphthalene; methylnaphthalene; 8, 1,2-dimethylnaphthalene;9, 1,3,7-trimethylnaphthalene; 10, 1,3,6-trimethylnaphthalene;11, 1,3,5- and 1,4,6trimethylnaphthalene; 12, 2,3,6-trimethylnaphthalene;13, 1,2,7-trimethylnaphthalene; 14, 1,6,7- and 1,2,6-trimethylnaphthalene;15, 1,2,5-trimethylnaphthalene;16, 1,2,4-trimethylnaphthalene;17, 1,2,3-trimethylnaphthalene.Peaks marked with an asterisk correspond to other alkyl-substituted naphthalenes.
aromatic compounds could be easily achieved by monitoring the UV absorption of the eluant at 250 nm, and this procedure was followed with the fuel samples in order to define the appropriate fractions for collection. The bulk of the solvent from the collected fractions was removed by using a Nester-Faust spinning band distillation column. Examination of several samples, with and without column chromatography, established that there was no detectable fractionation of isomers. The relative concentration of isomers was estimated by using the existing software based on either the total ion current or selected ion summations, i.e., m / z = 141 + 156 or 155 + 170 for the di- and trimethylnaphthalenes, respectively. Selected ion experiments indicated that the quantitative accuracy was not limited by scan and cycle time considerations. The assignment of isomers for both the di- and trimethylnaphthalenes follows that reported by other workers using GC analysis on OV 101 coated columns (Alexander et al., 1983; Rowland et al., 1984). Reconstructed ion chromatograms were very similar to those previously reported (Alexander et al., 1983; Rowland et al., 19841, except that we were unable to satisfactorily resolve the 1,3- and 1,7-dimethyl isomers. A typical chromatogram, run under isothermal conditions to enable the determination of the Kovats retention indexes of the mono-, di-, and trimethylnaphthalenes, is shown in Figure 1. Thermodynamic Calculations. Equilibrium computations were carried out using the CHEMIX computer program which forms part of the CSIRO-SGTE THERMODATA software collection (Turnbull, 1977). The algorithm used in cHEMIX searches for the minimum value of the free energy of the system, defined by variables input by the user, according to the procedure formulated by Eriksson
(1971). The program is able to perform calculations on up to 100 species present simultaneously and has been applied to the prediction of product distributions in coal liquids (Linton and Turnbull, 1984), shale oils (Charlesworth, 1987; Charlesworth et al., 1987), and synthesis gas mixtures (Larkins and Khan, 1989). In the case of the dimethylnaphthalenes the computations were made using the enthalpy, entropy, and heat capacity values listed in the CSIRO Mineral Research Laboratories (MRL) databank provided for use with CHEMIX. The original source of these data is the work by Milligan et al. (1956). A search of the literature revealed no suitable thermodynamic information for the trimethylnaphthalenes; consequently values were calculated by using the group additivity method described by Benson et al. (1969). A modified version of the CHETAH Chemical Thermodynamic and Energy Release Evaluation Program (Seaton et al., 1973) was used for this purpose. The program simplifies the calculation of thermodynamic parameters for any given molecule by referring to a database of thermodynamic values for the groups that make up the molecule. These group values have been derived from known thermodynamic quantities measured for compounds in the gas phase under standard conditions. Nearest-neighbor effects are the prime interaction accounted for; however, corrections for more distant interactions can he made, as discussed in the following section. Results and Discussion Calculated Thermodynamic Values. In considering various methods for the calculation of the relative distribution of isomers, perhaps the simplest approach is the semiempirical method developed by Weitkamp (1968) for the dimethylcyclohexanes, methyldecalins, and dimethyldecalins. The advantages of this procedure are that few thermodynamic data are needed and the results are easily extended to the methyl and polymethyl derivatives of other systems, e.g., anthracene, phenanthrene, diphenyl, etc. The disadvantages include the limitation that all interactions between substituents are considered to be energetically equivalent, and no second-order effects can he taken into account. In contrast to the naphthenes, the calculations for the dimethylnaphthalenes are not complicated by conformational inversions or presence of dl or meso isomers. Thus, it is necessary to consider only the number of interactions, Le., an a-methyl group with a peri hydrogen or adjacent methyl groups, together with an allowance for the rotational entropies. With the same nomenclature as Weitkamp, the equilibrium constant k, is defined as the ratio of two isomers that differ by only one interaction, e.g., the monomethylnaphthalene isomers. For a pair of isomers that differ by n interactions
-A.C=RTlnK=RTlnk" (1) and in the absence of entropy terms, the relative yields of a pair of isomers would be l / k " and (k" - l ) / k " . For isomers that have axes of symmetry, the allowance for rotational entropies is straightforward. The symmetry numbers (u) for the compounds in the present study are either u = 1 or 2, i.e., no isomer has more than one %fold axis of rotational symmetry. The symmetry number can be incorporated into the calculation of isomer distribution as follows: AH = AG + T A S (2) -Si = R T In k n - T(-R In u )
(3)
- A H / R T = In (uk")
(4)
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1973 Table I. Calculated and Observed Distribution of Dimethylnaphthalenes calc
a isomer 2,6 2,7 1,3 1,7 1,6 1,4 1,5 2,3 182 198
300 "C 12.3 12.3 15.5 15.5 13.3 5.0 5.8 12.4 7.9 0.0
600 OC 12.2 12.2 14.9 14.9 13.5 5.2 5.8 12.4 9.0 0.0
re1
obs
b normalized
i
C
11.8 11.8 15.8 15.8 15.8 5.3 5.3 7.9 10.5 0.0
5.9 5.9 23.1 23.1 23.1 0.1 6.5 6.5 5.8 0.0
14.7 15.0 31.d
15.3 15.6 28.4
17.7 2.4 11.8'
19.8 4.6 13.1'
6.9 0.0
3.1
19.4 14.9 0.1 21.2 28.3 2.9 3.7 7.2 7.5
0.0
0.0
1/2k0
petroleum d e
tar
f
&?
i
16.0 14.5 15.4 15.6 21.2 3.0 4.0 6.6 3.7 0.003
24.7k
11.0 12.7 17.3 17.0 12.6 0.3 9.7 10.0 9.4 0.0
15.9 13.8 19.0 5.9 5.9 7.4 7.5 0.0
shale oil h 14.2 14.4 19.9 38.0 2.1 8.6' 2.9 0.0
"Results obtained by calculation using CHEMIX; see text, (this work). bResulta obtained by calculation using the method described by Weitkamp (1968) with k = 1.5; see text (this work). 'Product from catalytic cracker unit, Bass Strait crude oil (this work). dLight gas oil, Bass Strait crude oil (this work). CAPIProject 6, data from Mair and Mayer (1964). /Barrow Island, Upper Jurassic crude oil, data from Alexander et al. (1983). #Data from Kabot and Ettre (1964). Jet Fuel from Paraho Shale Oil, Shale I, US Naval Research Laboratories (this work). 'Karr et al. (1967). j1,3 + 1,7. k2,6 + 2,7. '1,5 + 2,3. Table 11. Calculated and Observed Distribution of Trimethylnaphthalenes calc a
isomer'
300OC 17.6 17.6 7.8 7.4 12.4 9.0 6.7 9.0 4.2 4.2 4.2
600OC 15.1 15.1 8.7 7.9 12.9 9.1 7.1 9.1 5.3 4.8 4.8
re1 llk Ilk l/k2 l/k2 Ilk l/kz l/k2 l/k2 l/k3 l/k3 l/k3
b normalized 13.1 13.1 8.7 8.7 13.1 8.7 8.7 8.7 5.8 5.8 5.8
obs
C
d
22.1 24.6 9.7 9.4 13.4 5.7 7.5 5.4 1.2 0.8 0.3
16 19 14."
petroleum e f 25 16.7 20 22.8 20' 17.g
16 7 lBk
10 4 llk
3 6 3
1 7 1
14.6 3.0 10.8 5.0 5.6 4.0 0.0
g
17.3 27.1 17.0' 10.1 4.4 7.1 8.2 7.7 1.1 0.0
shale oil h 12 28 19i 11 3 1l k
4
8 4
aResultsobtained by calculation using CHEMIX; see text (this work). bResultaobtained by calculation using method described by Weitkamp (1968), with k = 1,5; see text (this work). 'Catalytic gas oil, data from Duswalt and Mayer (1971). "Product from catalytic cracker unit, Bass Strait crude oil, (this work). eKerosene fraction, Bass Strait crude oil, (this work). /Barrow Island, Upper Jurassic crude oil, data from Rowland et al. (1984). #North Apa, Miocene crude oil, data from Rowland et al. (1984). hParaho Shale Oil, US Naval Research Laboratories, (this work). 'Values for 1,2,8, 1,3,8, and 1,4,5 isomers assumed to be zero; see text. j1,3,5 + 1,4,6. k1,6,7 + 1,2,6.
The relative contribution for any particular isomer is therefore expected to be l/ak". The calculated relative and normalized distributions as a function of 12 are given in Tables I and I1 for di- and trimethylnaphthalenes. The second approach that can be used to calculate the distribution of dimethylnaphthalene isomers at thermodynamic equilibrium has been described by Karr et al. (1967). This method is based on the free energy function (fef), -(Go - Iforel)/?',calculated for the various alkylnaphthalenes by Milligan et al. (1956), together with the assumption that -Horef/Tvalues are proportional to the difference in free energy function, A(fef), for the special case of closely related isomers. On this basis, an approximate equilibrium constant between a compound (A) and a more stable isomer (B) can be derived as follows: R In K = A(fef) + (-AHoref/T) (5)
R In (mol % B/mol % A)
A(fef) + cA(fef) (6)
where c is a constant derived from the experimental equilibrium concentrations of the monomethylnaphthalenes at the same temperature. The distribution at 800 K calculated by Karr et a1 (1967) for the dimethylnaphthalenes in this way is given in Table I. The only experimentalobservation to support these calculations given by Karr et al. (1967) was the result obtained by Mayer and Schiffner (1934) for an apparent equilibrium constant between 1,6- and 2,6-dimethylnaphthalene over silica gel at 420 OC.
The final method for calculating the isomer distribution involves the use of standard enthalpies, entropies, and heat capacities as input data for computations with the free energy minimization method. In the case of the trimethylnaphthalenes, the Benson group additivity technique was used for the reasons discussed above. A logical first step in this procedure is to commence with similar, less substituted molecules having defined thermodynamic properties. The dimethylnaphthalenes were therefore chosen, and the effect of including additional methyl substituents at the relevant ring positions was then calculated. Two effects can be visualized as important in this calculation: (a) the primary effect of replacing a hydrogen atom with a methyl group and (b) the secondary effects of ortho, meta, and para interactions. Denoting the primary effect as 4 and the secondary effects as 0,m, and p, a generalized equation for the substitution effects can be written as follows: F(a,b,c) = F(a,b) + 4
+ xo + ym + zp
(7)
where F is the enthalpy, entropy, or heat capacity, a, b, and c are the ring positions of the methyl Substituents, and IC, y, z are the number of additional ortho, meta, and para interactions, respectively, introduced by the replacement of hydrogen atoms with methyl groups in the c position. The methods of estimation and limits of reliability of data for the secondary substitution effect in aromatic
1974 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 Table 111. Group Additivity Relationships for Estimation of Thermodynamic Data entropy isomer F (enthaluv. entrouv. heat caoacitv) correction -R In 3 1,3,7-TMN F(1,3-DMN) + Q + m + P -R In 3 F(1,3-DMN) + Q + m + p 1,3,6-TMN -R In 3 F(1,B-DMN) + Q + o 1,3,5-TMN -R In 3 F(1,7-DMN) + Q + o + p 1,4,6-TMN -R In 1.5 F(2,6-DMN) + Q + o + m + p 2,3,6-TMN -R In 3 F(1,Z-DMN) + Q + m + p 1,2,7-TMN -R In 3 F(1,B-DMN) + Q + o + m + p 1,6,7-TMN -R In 3 F(1,Z-DMN) + Q + m + p 1,2,6-TMN -R In 3 F(1,Z-DMN) + Q + o + m 1,2,5-TMN -R In 3 F(1,Z-DMN) + Q + o + 2m + p 1,2,4-TMN -R In 3 F(1,S-DMN) + Q + o + 2m + p 1,2,3-TMN ~~
~~
molecules have been dealt with in detail by Shaub (1982). Rather than use mean values for a very wide range of substituted aromatics, it was considered that the substitution effects could best be allowed for by using accepted reliable data for a smaller range of compounds. Consequently, the experimental thermodynamic values for the xylene isomers in the MRL database were substracted from the values calculated for the same molecules without accounting for the secondary effects using Benson's group additivity method. These values were then assumed to apply to both the secondary effects created by methylmethyl interactions, and interactions between methyl groups and aromatic carbon atoms at the CY position on the adjacent fused ring. The values used here are not greatly different to those previously reported (Shaub, 19821, for instance, AH for the 0,m, and p effects in this work are 0.32,-0.10, and 0.07 kcal/mol compared to 0.39, -0.03, and 0.14 kcal/mol respectively for Shaub's (1982) work. Table I11 provides complete details of the way in which the thermodynamic data for each of the 11 significant trimethyl isomers was calculated. The entropic term in Table I11 is included to allow for the change in external symmetry of a molecule and the extra three internal rotational states due to the additional methyl group. The final thermodynamic distributions calculated by using the tabulated free energy data for the dimethylnaphthalenes and the calculated data for the trimethylnaphthalenes, at two representative temperatures, are listed in Tables I and 11. Observed Equilibrium Constants. Comparison of calculated and observed equilibrium constants is limited to consideration of the dimethylnaphthalene isomers. This comparison is further restricted because in this series there exist a number of discrete equilibrium sets with no apparent interconversion between the members of the different sets. Suld and Stuart (1964) studied the problem using mono- and dimethylnaphthalenes isomerized in anhydrous HF/BF, mixtures. The sets of isomer groups within which interconversion was found to occur are listed in Table IV together with the measured equilibrium concentrations. The 1,2 isomer did not isomerize to any of the other nine dimethylnaphthalenes, and none of the other isomers gave rise to 1,2-dimethylnaphthalene.As might be expected from a consideration of steric effects, 1,8-dimethylnaphthalene is not expected to be observed in significant concentrations at equilibrium and is generally not observed in crude or refined products, shale oils, or coal tars. Similar observations apply to the trisubstituted naphthalenes where two of the substituents are in the adjacent a positions. The postulated first step in the isomerization under acidic conditions is the rapid reversible addition of a proton to the naphthalene ring to form the conjugate acid. This
Table IV. Distribution of DimethvlnaDhthalenes under Acidic Conditions 2.6-DMN F= 1.6-DMN * 1,bDMN calca 59 35 5 obsb 57 40 3 calco obsb
2,7-DMN =+ 1,7-DMN * 1,8-DMN 63 37 0 61 39 0
calca obsb
2,3-DMN 30 35
1,S-DMN 61 64
1,4-DMN 9 1
a Calculated values are based on the experimental equilibrium isomerization under constant for l-methyl-:2-methylnaphthalene the same conditions, k = 3.3 (Suld and Stuart, 1964); see text. *Observed values are for HF with catalytic amounts of BF3 a t 70 "C in the organic phase (Suld and Stuart, 1964).
step is then followed by the reversible methyl shift to an adjacent carbon atom shown as follows:
II other prdonated complexes
3& ;
0
0
It is evident that there exist migrational barriers between adjacent p positions as well as between the two rings of the naphthalene nucleus. Furthermore, with equimolar or excess BF, the equilibrium exists between the protonated complexes in the acid phase, while the measured equilibrium concentrations refer to the material sampled from the organic phase, benzene. The qualification must therefore be made that the attainment of true thermodynamic equilibrium is precluded by the limitations of the particular mechanism. Nevertheless, theoretical isomer ratios can be calculated for the sets of equilibria listed in Table IV by using as a reference point the experimental equilibrium constant (k = 3.3) for the 1-methyl- G 2methylnaphthalene isomerization under the same conditions. The calculated values listed in Table IV are based on the elementary assumption that the enthalpy change for an ortho methyl-methyl interaction is the same as that for an a-methyl peri-hydrogen interaction. Apart from the discrepancy between the calculated and observed values for the 1,Cdimethyl isomer, the data in Table IV show that even this most simple assumption is reasonable and confirm that the methods of calculation used here are valid. This conclusion is further reinforced by a comparison between the calculated and observed equilibrium constants in a drastically different experimental regime involving aluminosilicates, and similar catalysts, a t 300-550 "C (Bhattacharya, 1981; Ogasawara et al., 1974). Although the reaction network is also restricted to the same sets of isomer groups as in the acid-catalyzed system, the attainment of a true thermodynamic equilibrium is more closely approached. For example the measured equilibrium constant for the 1-methyl- s 2-methylnaphthalene isomerization in the range 390-527 OC over Si0,-A1203 is 2.0. This compares favorably with calculated values, 1.7-1.5 at 100-500 O C , by using the MRL database. The computed amounts for any of the dimethyl isomers under these conditions can be determined directly from the results in Table I. These values are in reasonable agreement with the equilibrium constants for the 1,7 2 2,7, 1,6 2 2,6, and 1,6 z 1,5 isomerizations reported in the literature
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1975 (Ogasawara et al., 1974), Le., k(expt) 1.1,1.0-1.1,0.2-0.4; cf. k(theory) 0.8-1.0, 0.9-1.0, 0.2-0.4, respectively. Isomer Distributions in Fuels. The distribution of alkylnaphthalenes, as well as alkylbenzenes, in coal tar products may be expected to depend on the structure of both the coal and process conditions. For instance, an extensive characterization of a low-temperature coal tar and comparison of isomeric distributions with thermodynamic equilibrium and kinetic distributions, for a variety of hydrocarbon types, led Karr et al. (1967) to conclude that the composition largely reflected the structure of the coal from which the tar was derived. The results obtained by Karr et al. (1967) for several of the substituted diaromatic compounds are, however, at odds with the overall trend and in fact show reasonable agreement with the revised thermodynamic distributions calculated here. The measured ratios of the dimethylnaphthalenes for a tar derived from Arkwright (W. Pa) high-volatility bituminous coal in a fluidized carbonization plant at 480-510 "C are listed in Table I. The value for the 2-methyl-:l-methylnaphthalene ratio (Karr et al., 1967) for this tar (1.3) is close to the result at 500 "C obtained by computation using the MRL database (1.5). Similarly, with the exception of the 1,4 and 1,5 isomers, the agreement between the measured and calculated values for the dimethylnaphthalenes is excellent. An analogous conclusion can be drawn from a comparison of the results of the analysis of coal tar by Kabot and Ettre (1964) with the values computed here (see Table I). There are other indications that a thermodynamic equilibrium can be approached for high-temperature coal tars. For example, the alkylbenzene distribution in a coal tar produced at about 600 "C by a Lurgi gasification plant using Yallourn Coal (Brown, 1960) is in reasonable agreement with the calculated values (Charlesworth, 1987). Furthermore there is evidence that equilibrium is approached for the Yallourn coal tar, not only with respect to the isomer distribution, but also for disproportionation reactions, i.e. benzene toluene
+ o-xylene
2 toluene
+ 1,2,4-trimethylbenzene
(8) 2 o-xylene (9)
The observed equilibrium constants for (8) and (9) (8.9 and 0.33, respectively) are in close agreement with the theoretical values (8.4 and 0.35, respectively) at 627 "C (Taylor et al., 1946). Similar comparisons have been reported for the cracking of cyclopentane and pentane (Kemball and Rooney, 1961) to support the facile transfer of methyl groups between aromatic nuclei over aluminosilicate catalysts. These considerations suggest that a thermodynamic equilibrium can be approached for coal tars under certain conditions, i.e., high temperature and comparatively long residence times or under fluidized bed conditions where there is a possibility of catalytic activity from mineral matter present. The relative distribution of the various dimethyl isomers for some petroleums and a shale oil are summarized in Table I. Apart from a few obvious exceptions, the distributions follow the trends expected on the basis of the equilibrium calculations. Similar behavior is apparent for the trimethyl isomers (Table 11). These observations agree with the predictions made by Rossini (1960) that the general distribution of alkylnaphthalenes would be independent of the source of petroleum. It seems probable that at least some of the discrepancies that appear in the data, for example, the concentration of 1,3-dimethylnaphthalene measured by Mair and Mayer (19641, can be attributed to limitations of the experimental
method employed. In terms of possible biological marker compounds, it should be noted that the 1,6-dimethyl isomer is generally more abundant in many cases than predicted. The observed value for this particular isomer in the shale oil studied here is not unique, as illustrated by the published chromatogram of West Wyoming crude oil (Alexanderet al., 1984),and may be indicative of common biological origins of these materials.
Conclusions On the basis of the thermodynamic calculations and experimental data considered here, the results suggest that the isomer distribution in di- and trimethylnaphthalenes can approach equilibrium values in certain circumstances. Presumably, the mechanisms by which such equilibria are achieved could equally explain the distribution of the lower alkylbenzenes with respect to both isomer distribution and equilibria involving trans-alkylation reactions. This is at variance with the mechanism postulated for the acidcatalyzed isomerization of alkylnaphthalenes in which only intramolecular methyl migrations are thought to occur. It has been suggested on the basis of a detailed isomer analysis of the C1,-alkylnaphthalenes in catalytic gas oil and comparison with those found in the API-6 studies of crude petroleum (Duswalt and Mayer, 1971) that the dinuclear aromatics are relatively unaffected by catalytic cracking. Such a result could be anticipated given that the isomer distribution of alkylnaphthalenes and alkylbenzenes in crude petroleum approximates to equilibrium conditions at 450-550 "C and that selective dealkylation is not a significant process. The thermodynamic calculations reported here are easily extended to other polycyclic systems. For example, as a first approximation the expected relative equilibrium concentrations of the 1-, 2-, 3-, and 9-methylphenanthrenes are l / k , l/k", l/ko, and l / k , respectively, and for the 1-, 2-, and 4-methylpyrenes,l/k, 1/2k0, and l / k , respectively. Better approximations can be obtained using thermodynamic values calculated by using the group additivity method. At present, quantitative experimental data for the isomeric distribution of such systems in hydrocarbon fuels are not available for comparison. Acknowledgment We would like to thank Dr. Robert Hazlett, US Naval Research Laboratories, for valuable discussions and advice during the course of this work. Registry No. 2,6-Dimethylnaphthalene, 581-42-0; 2,7-di575-41-7; methylnaphthalene, 582-16-1; 1,3-dimethylnaphthalene, 1,7-dimethylnaphthalene,575-37-1; 1,6-dimethylnaphthalene, 575-43-9; 1,4-dimethylnaphthalene,571-58-4; 1,bdimethylnaphthalene, 571-61-9; 2,3-dimethylnaphthalene,581-40-8; 1,2dimethylnaphthalene, 573-98-8; l&dimethylnaphthalene, 56941-5; 1,3,7-trimethylnaphthalene,2131-38-6; 1,3,btrimethylnaphthalene, 3031-08-1; 1,3,5-trimethylnaphthalene,2131-39-7; 1,4,Gtrimethylnaphthalene, 2131-42-2; 2,3,6-trimethylnaphthalene, 829-26-5; 1,2,7-trimethylnaphthalene,486-34-0; 1,6,7-trimethylnaphthalene, 2245-38-7; 1,2,64rimethylnaphthalene,3031-05-8; 1,2,5-trimethylnaphthalene,641-91-8; 1,2,4-trimethylnaphthalene, 2717-42-2; 1,2,3-trimethylnaphthalene,879-12-9; l-methylphenanthrene, 832-69-9; 2-methylphenanthrene, 2531-84-2; 3methylphenanthrene, 832-71-3; 9-methylphenanthrene, 883-20-5; l-methylpyrene, 2381-21-7; 2-methylpyrene, 3442-78-2; 4methylpyrene, 3353-12-6.
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