28
Energy & Fuels 1987, 1, 28-31
which is responsible for particle agglomeration. Results of the present study suggest that more coke is formed by secondary reactions than during the conversion of kerogen to bitumen. The coke location (at the surface vs. in the particle body) and particle morphology help to better understand the coking mechanism and also explain the
observed differences in gasification reactivities of coke on spent shale particles.
Acknowledgment. The authors appreciate helpful discussions with their colleagues J. D. McCollum, J. T. Joseph and J. L. Taylor.
Transalkylation of Polycyclic Aromatics Catalyzed by Trifluoromethanesulfonic Acid1 Malvina Farcasiu,* T. R. Forbus, and B. R. Rubin Central Research Laboratory, Mobil Research and Development Corporation, Princeton, New Jersey 08540 Received June 9, 1986. Revised Manuscript Received August 29, 1986
Normal alkyl (C4-Cl0) and cycloalkyl (C6, C,) groups are transferred between aromatic rings of different degree of condensation (one to four rings) in the presence of trifluoromethanesulfonic acid. The structure of the compounds studied is relevant to chemical structures in fossil fuels. The rate of transalkylation exhibits an unexpected dependence upon the concentration of acid: for each system, a minimum concentration of acid is necessary to initiate the reaction; as the concentration is increased beyond this threshold, the reaction rate increases dramatically.
Introduction Structural studies of materials such as petroleum heavy ends, shale oils, and coal liquids are difficult because of the extreme complexity of these mixture^.^,^ Methods of separating these materials according to chemical class have been reported previously,z3but even within each separated group one normally finds a rather complex mixture of compounds. For many of these mixtures, spectroscopic measurements, especially 13C and 'H NMR, indicate the presence of both alkyl groups and aromatic and heterocyclic rings. However, obtaining more detailed structural information from NMR is difficult due to the large variety of compounds and superimposition of peaks from different chemical structures. The same structural complexity so far has frustrated attempts at establishing processes for conversion of these materials under relatively mild conditions. A few years ago, we began studying the potential of transalkylation reactions for structure identification and for processing of kerogens: and coal and coal liquids.' Previously, Heredy et aL8 and Ouchi et al.9 had (1)Fieported in part at the EUCHEM Conference on Superacids and Superbases, Liquid and Solid, Cirencester, U.K., Sept 11, 1984. (2)Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1980. (3)Farcasiu, M.Fuel 1977,56,9. (4)Farcasiu, M.; Forbus, T. R.; LaPierre, R. B. Prep.-Am. Chem. SOC.,Diu.Pet. Chem. 1983,28, 279. (5)Farcasiu, M. US.Patent 4317712,March 2, 1982. (6)Farcasiu, M.; Scott, E. J. Y.; LaPierre, R. B. Prepr.-Am. Chem. Sco., Diu. Pet. Chem. 1985,30, 672. (7)(a) Farcasiu, M. Presented at the Symposium on Chemistry of Coal
Liquefaction and Catalysis,Hokkaido University, Sapporo, Japan, March 17-20, 1985. (b) Farcasiu, M. J. Fuel Heat Technol., in press. (8)Heredy, L. A. Adu. Chem. Ser. 1982,No. 192, 179-190 and references therein.
0887-0624/87/2501-0028$01.50/0
Table I. Substrates Used in Transalkvlation Reactions donors" (Ar'R) acceptors ( A r " H )
?
1
pR Lv O
CHI
3
2
00
6
r
bCH3 &
5
d,R=(-,,L
studied reactions of coal with phenol in the presence of HF-BF3,' or p-toluenesulfonic acid, respectively. Under their reaction conditions, transalkylation occurred between coal and phenol. The system was quite complex, however, and other reactions took place as well.Io Therefore, we sought for our studies of the transalkylation reaction in fossil fuels a system of reactants that would minimize the importance of secondary reactions. We found that use of small aromatic hydrocarbons (benzene, toluene, xylene) as alkyl group acceptors satisfies this requirement. Early in our investigation, we found that there are no literature data on transalkylation reactions of compounds structurally relevant to fossil fuels. Such information is essential, however, in order to understand and optimize this reaction (9) Ouchi, K.; Imuta, K.; Yamashita, Y. Fuel 1973,52, 229.
(10)Joseph, J. T.;Mahajan, 0. P. Fuel 1985,69, 1321.
0 1987 American Chemical Society
Transalkylation of Polycyclic Aromatics
Energy &Fuels, Vol. 1, No. 1, 1987 29
for structure identifi~ation?~J’-’~ as well as for conversion of fossil f ~ e l s . ~ i ~ J ~ Although known since 1882,15the transalkylation reaction had not been applied to polycyclic alkylaromatics prior to our earlier experiments4 We showed, in agreement with findings for the monocyclic analogues (alkylbenzenes),15J6 that no rearrangement of normal alkyl groups occurred upon their transfer. We also showed that the rate of nalkyl group transfer does not change with the size of the alkyl group (C2-Clo) but that it is determined by the aromatic moiety of the alkylaromatic ( d ~ n o r ) . ~ We report here on the continuation of our work on the tranalkylation reaction. We investigated for the first time the transfer of cycloalkyl groups. Transalkylation with cycloalkyl groups has not previously been studied even for monocyclics (cycloalkylbenzenes). We also examined the influence of the chemical structure of both donor and acceptor on the reaction rate, as well as the effect of the relative concentrations of donor, acceptor, and acid on this rate. As in our earlier work: trifluoromethanesulfonic acid was the catalyst in the current experiments. This is a rather mild superacid (Ho = -13)17 which has only Brransted acidity. It is therefore devoid of the complications inherent to defining the acidity of complex LewisBrransted acid systems previously employed in similar s t ~ d i e s . ’ ~ J Also, ~ J ~ trifluoromethanesulfonic acid does not react with the aromatics involved in transalkylation other than by proton transfer.
I
-
K
0
1
y
b
6
:c---“, 0.03
-
- 70
P
c
-60
6
V
8
$z4Vtuaec-
0.02 0.01
- 40
Results and Discussion The transalkylation reaction is expressed in a general manner in eq 1. We studied various combinations of the Ar’R (donor)
[H+I + (acceptor) Ar”H eAr’H + Ar”R
(1)
donors and acceptors shown in Table I. The reactions were usually run either at 80 or 25 “C. The reaction mixture was homogeneous at the reaction temperature in all experiments. The equilibrium constants of the transalkylation reactions as calculated from heats of formation for a sizable number of donor-acceptor pairs were found to be close to 1.6 Therefore, to achieve significant conversion, an acceptor-tudonor molar ratio of 101 was employed. The rate constants and reaction orders relative to the donor were then obtained by considering the acceptor concentration as constant. In all cases the reaction was second order with respect to donor concentration. The degree of condensation of the aromatic part of the donor significantly influences the rate of transfer of both linear4 and cycloalkyl groups (Figure 1). (11)Benjamin, B. M.; Douglas, E. C.; Canonico, D. M.Fuel 1984,63, 888. ...
(12)Benjamin, B. M.; Douglas, E. C.; Hershberger, P. M.; Gohdes, J. W . Fuel 1985,64,1340. (13)Roberts, R. M.; Sweeney, K. M. Fuel 1985,64,321. (14)(a) Yoneda, N.;Kumagai, H.; Sanada, Y. Presented at the Symposium on Chemistry of Coal Liquefaction and Catalysis, Hokkaido University, Sapporo, Japan, March 17-20, 1985. (b) Yoneda, N.;Kumagai, H.; Sanada, Y. J.Fuel Heat Technol., in press. (15)Heise, R.; Tohl, A. Justus Liebigs Ann. Chem. 1882,270, 150. (16)(a) Baddeley, G.;Kenner, J. J. Chem. SOC.1935, 303. (b) McCaulay, D. A.; Lien, A. P. J. Am. Chem. SOC.1953, 75, 2411. (c) Kinney, R. E.; Hamilton, L. A. J.Am. Chem. SOC.1954,76,786. (17)Kramer, G.M. J. Org. Chem. 1975,40,302. (18)(a) Lien, A.P.; McCauley, D. A. J.Am. Chem. SOC.1953,75,2407. (b) Burwell, R. L., Jr.; Shields, A. D. J.Am. Chem. SOC.1955,77,2766. (c) Brown, H. C.; Smoot, C. R. J . Am. Chem. SOC.1956,78, 2176. (d) Unseren, E.; Wolf, A. P. J. Org. Chem. 1962,27,1509. (e) Streitwieser, A.,Jr.; Reif, L. J. Am. Chem. SOC.1964,86,1988.
(19)Arnett, E. M.; Petro, C. J. Am. Chem. SOC.1978,100, 5408. (20)(a) Pokrovskaya, E. S.; Stepantseva, T. G. J. Gen. Chem. USSR 1939,9, 1953. (b) Friedman, B. S.; Kovach, S. M., unpublished data quoted by: Patinkin, S. H.; Friedman, B. S.In Friedel-Crafts and Related Reactions; Olah, G. A., Ed.; Interscience: New York, 1964; Vol. 2, p 60. (21)Olah, G. A.;Kaspi, J. Nouu. J . Chim. 1978,2, 585. (22)(a) Nenitzescu, C. D.; Necsoiu, I.; Glatz, A.; Zalman, M. Chem. Ber. 1959,92,lO.(b) Roberts, R. M. Intra-Sci. Chem. Rep. 1972,6,89 and references therein.
30 Energy &Fuels, Vol. 1, No. 1, 1987
Farcasiu et al. 40
Table 11. Rates of Transalkylation between 2-n -Butylnaphthalene (3a) and o-Xylene (8)"
no. 1
mol of CF,SO,H 1
krslb
2 3
1.5 2
1.3 7.1
L
"ru
I
800C .. CF.C~-CI
I
1
" 1 mol of 3a and 10 mol of 8 at 25 "C. Relative rate. Table 111. Rates of Transalkvlation between 3b and 8 or 9" ~~
no. 1
e
acceptor 8
mol of CF,S03H
kr*l
0.5
0.8
0.5 1.0 1.3 2.7
b 1.0
u,'-ip.. ..."m.. ... . .. .....m.. ... ......... 6'1'3-
2.4 27c
"1 mol of 3b and 10 mol of acceptor at 80 "C. bNo reaction. Approximate value; the reaction is quite fast.
n 0
I
I
1
I
I
I
1
2
3
4
5
6
..
1-
" %"13 ~~
1
7
Time [Hrsl
Table IV. Rates of Transalkylation between 3b and 8" no. mol of 3b mol of 8 mol of CF,SO,H k.-, lb 1 10 0.5 1 K0.5 2 5 5 0.5 c 1 0.5 3 10 4 1 10 1 3.1 5 2.5 >50 5 5 '
Figure 2. Transalkylation reaction between 2-n-hexvlnaphthalene - and o-xylene. 40
,
"At 80 O C . bSame as in Table 111, entry 1. cNo reaction.
The rate of transfer of n-alkyl groups does not change with the length of the alkyl group (see a b ~ v e ) .By ~ contrast, the rate of transfer of cycloalkyl groups changes with the size of the latter. Thus, in the transfer from alkyl- and cycloalkylnaphthalenes to o-xylene (1:10:0.5 donor:acceptor:acid; 80 "C),the relative rates are 1:1.53:6.1 for n-hexyl, cyclohexyl, and cyclopentyl, respectively. The difference between the cycloalkyl derivatives can be ascribed to the lesser steric congestion in the transition state for the transfer of the planar five-membered ring from the protonated donor to the acceptor. The observation is, therefore, consistent with the bimolecular mechanism1&Pd for the transfer of the cycloalkyl group. Role of the Acidity on the Rates of Transalkylation. As expected, the reaction rates increase with acid concentration. There is no simple relationship, however, between rates and stoichiometric concentration of acid (Table 11) since the effectiue acidity of the medium is determined by the relative basicities and concentrations of the donor and acceptor. For each particular composition there is a minimum concentration of acid below which the reaction does not occur at all. This is shown in Table 111, entries 2 and 3, for the n-hexyl group transfer from 3b to 1-methylnaphthalene (9). For the more weakly basic acceptor 8, the hexyl transfer takes place at an acid concentration that is ineffective for 9 (Table 111, entries 1and 2). Conversely, the hexyl group transfer from the weakly basic hexylbenzene (1mol) to the more basic 9 (10 mol) requires 10 mol of acid; at this high concentration of acid the side reactions become important (see below). The decrease in rate with the increase of the donor (3b):acceptor (8) ratio for the same amount of acid shown in Table IV (entries 1-3) can also be traced to the change in basicity of the hydrocarbon mixture as the latter becomes richer in the more basic 3b. On the other hand, the hexyl transfer from hexylbenzene to toluene (7) (1:lO mol), in which both compounds possess low but similar basicities, requires much more acid than the transfer from the more basic p-dihexylbenzene to 7 (1:lO molar). This is understandable since the reaction is
d
I
20
10
n 0
1
2
3
4
5
6
7
6
Time [Hrsl
Figure 3. Transalkylation reaction between 2-n-hexylnaphthalene and benzene.
favored by high donor protonation and low acceptor protonation. Finally, for the same hydrocarbon mixture, the rate increases much faster than the increase in concentration of acid (Table 11; Table I11 entries 4 and 5; Table IV entries 1 and 4 and 2 and 5 ) suggesting that the kinetic order of the transalkylation reaction with respect to acid concentration is greater than one. Side Reactions during Transalkylation. Disproportionation. Some disproportionation of 2-hexylnaphthalene (3b) in the reaction with 8 and with 6 can be seen in the formation of dihexylnaphthalenes (several isomers) in Figures 2 and 3, respectively. The isomer distribution changes during the reaction; therefore, only the sum of the isomer concentrations was plotted in the figures. It can be observed that somewhat more disproportionation product is formed when the acceptor is less basic (6 compared with 8). Disproportionation is mechanistically the same as transalkylation. Hydrogen Transfer and Cyclization. Small amounts of partially hydrogenated aromatics were observed in our experiments, but only from bi- and polycyclic precursors. Thus,alkyltetralins and tetralin were observed in reactions involving alkylnaphthalenes. The hydrogen source should be the alkyl group of another molecule of alkyl aromatics. Indeed, small amounts of ethyltetralin were observed in
Transalkylation of Polycyclic Aromatics
the reaction of 3b and 6, probably formed by the dehydrocyclization of the hexylbenzene product. Such hydride transfers and cyclizations were reported by others as the major pathway for reactions in a significantly stronger ~uperacid.~~ Formation of Diarylalkylmethanes: The arylalkyl cation formed by hydride transfer can undergo not only intra- but also intermolecular alkylation. Indeed, Figure 3 shows that about 6% of the hexyl groups end up as 1,l-diphenylhexane (10). It is noteworthy that 1,l-diarylalkanes have been proposed as obligate intermediates in transalkylation.’” The kinetics we determined (second order in donor) are also compatible with such a mechanism. Phenylalkyl cations are not expected to be formed by a hydride abstraction by the mild Brernsted superacid employed in our experimenta. Instead, if the diarylalkane mechanism were correct, it should be initiated by a side reaction (hydride transfer from the alkylaromatics to the protonated aromatic). The method of preparation of our substrates4 renders improbable the alternative pathway to arylalkyl cations, namely protonation of an arylalkene. Moreover, in the literature experiments in which the strong Lewis acid catalyst GaBra was in excess over the Brernsted acid HBr, the rate increase with aging of the catalyst-benzene mixture indicated the formation of a hydride acceptor in the mixture. The authors also noted some oxidations forming bromine, which could also lead to increased levels of hydride acceptors, which catalyze the formation of catalytic carbocations.lse We chose our acid catalyst such as to avoid similar sources of uncertainty. The formation of a diarylalkane was observed in the experiment described in Figure 3, in which about 4% 1,l-diphenylhexane was formed after 1 h, and then its concentration changed very little to the end of the experiment. It should be noted, however, that the fastest reaction in that experiment was the hexyl transfer between two molecules of hexylnaphthalene. Clearly, 1,l-diphenylhexane is not the intermediate in this fastest process, and no intermediate for the latter was evidenced, even at very small conversion. Dihexylnaphthalene is, however, an intermediate of the overall process, because its con-
Energy & Fuels, Vol. 1, No. 1, 1987 31
centration goes through a maximum; it transfers the hexyl group(s) to benzene. By contrast, 1,l-diphenylhexane concentration increases slower than the concentration of dihexylnaphthalene and then remains constant. Our results thus indicate that 1,l-diarylalkanes may contribute to the overall transalkylation reaction only to a small extent. The major pathway seems to be the bimolecular alkyl transfer from a protonated alkylarene to an unprotonated one.
Experimental Section Materials and Analytical Procedures. The n-alkylaromatia used in the study have been prepared as described p r e v i ~ u s l y . ~ The cycloarenes were obtained from Professor J. K. Stille of Colorado State University; they were obtained by the reactions of the cycloalkylmagnesium bromides with chloroarenes in the presence of dichloro[1,3-bis(diphenylphosphino)ethane]ni~kel(II).~ in. Product analysis was accomplished by GLC on a 6 f t X 0.d. stainleas-steel column, with 3% OV17 on SO/lOO Supelcoport, and by GLC-MS on a Hewlett-Packard 5992A instrument, equipped with a 50-m OV-101 SCOT column. Transalkylation Reactions. The desired quantities of reactantswere mixed at 25 “C in a round-bottomed flask, equipped with an immersed thermometer, a reflux condenser topped with a drying tube (Drierite), and a magnetic stirring bar. For the reactions conducted a t 80 “C the flask was heated quickly to that temperature and after that (5-10 min) the first sample was withdrawn. The composition a t that moment was taken as the starting point for the kinetic study. In fact, very little conversion occurred during heating; mixing a t room temperature allowed a more accurate determination of the quantities of reactants actually introduced. The aliquot samples were quenched in aqueous sodium hydroxide, and the hydrocarbon phase was isolated and dried (CaS04) before analysis. Registry No. la, 104-51-8; lb, 1077-16-3; IC, 827-52-1; Id, 700-88-9; 2a, 1634-09-9 2b, 2876-53-1; 2c, 3042-69-1; 2d, 9257867-1; 3a, 1134-62-9;3b, 2876-46-2; 3c, 42044-07-5; 3d, 3042-68-0; 4a, 10394-57-7; 4b, 23921-09-7; 4c, 792-06-3; 4d, 17024-08-7; 5a, 104715-86-8; 5b, 104715-87-9; 5c, 104715-88-0; 5d, 104715-89-1; 6,71-43-2; 7,108-88-3; 8,95-47-6; 9,90-12-0;CF,SO,H, 1493-13-6; l-cyclohexyl-3,4-dimethylbenzene,4501-53-5; l-cyclohexylnaphthalene, 3042-69-1; naphthalene, 91-20-3; n-hexylbenzene, 1077-16-3; di-n-hexylnaphthalene, 100766-33-4; 1,l-diphenylhexane, 1530-04-7;3,4-dimethyl-l-n-hexylbenzene, 104715-90-4; ethyltetralin, 81598-29-0.
(23) Farcasiu, D.; Siskin, M.; Rhodes, R. P. J.Am. Chem. SOC.1979, 101,1611.
(24) Stille, J. K., Colorado State University, personal communication.
