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Intermolecular methyl group transfers in the xylene and xylene—benzene systems by means of anhydrous HF
JOURNAL OF
MOLECULAR CATALYSIS Journal of Molecular Catalysis
88 ( 1994) 205-2 12
Intermolecular methyl group transfers in the xylene and xylene-benzene systems by means of anhydrous HF LeRoy H. Klemm* Department
of Chemistry, University of Oregon, Eugene, OR 97403, USA
(Received January 18, 1993; accepted October 5, 1993)
Abstract Treatment of xylene with anhydrous HF (200 wt.%) at 180°C for 2 h in a sealed bomb gives disproportionation to toluene (A, 42%) and trimethylbenzenes (B, 32%). Extension to use of xylenebenzene mixtures for the hydrocarbon component gives transmethylation wherein yields of A and B (maxima of 30% and 27%, respectively) are found to vary with hydrocarbon composition, weight percentage of HF used, reaction temperature, reaction pressure, and addition of inorganic salts. Key words: benzene; hydrofluoric acid; methyl group transfer; xylene
1. Introduction Catalyzed disproportionation of xylene by intermolecular transfer of a methyl group to give toluene and trimethylbenzenes has been studied for more than a century. Catalysts used were AlCI, [ l-51, AlC&-HCl [ 61, AlBr,-HBr [ 71, BF,-HF [ 8-101, TiF,-HF [ 111, Pt-AIzO, [ 121, anhydrous HF (50-100 vol.%, as based on xylene used, at 160-175°C) [ 131, aluminosilicates [ 14,151, Nafion [ 16,171 and zeolites or mordenites [ 18-3 11. Less extensively studied is the process of transmethylation of benzene by means of xylene to yield toluene. Catalysts used were A1203-HCl [ 321, A&O,-HCl-ZnCl, [ 331, and aluminosilicates [ 34-371. The present paper describes the use of anhydrous HF at 180-253°C to effect both disproportionation of xylene and methyl group transfer from xylene to benzene. 2. Experimental
2.1. Apparatus, chemicals and procedure Reactions
were conducted
in a high pressure
Parr illium rocking
*Corresponding author. Fax. ( + l-503)3460487. 0304-5102/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO304-5102(93)EO260-N
bomb
(internal
206
L.H. Klemm/Joumal
of Molecular Catalysis 88 (1994) 205-212
volume, corrected to 180°C 744 ml), equipped with inlet and outlet ports bearing shut-off valves. The outlet port was attached to an internal monel steel tube reaching almost to the bottom of the bomb. The inlet port was connected, via a gas-oil-filled steel pressure tube, to a pressure gauge. The bomb was heated electrically. The temperature was controlled manually by a powerstat and read from a dial-type metal thermometer inserted into a thermal well protruding into the cavity of the bomb. The assembled bomb was evacuated by a water aspirator and cooled in an ice bath. Measured quantities of reagent grade xylene or xylene-benzene mixtures were introduced from an attached separatory funnel and commercial grade anhydrous I-IFfrom a steel sample bomb. In some experiments a plot of observed pressure (p) versus temperature (Z’) was made while the bomb plus contents were heated to the reaction temperature (usually 180°C) with or without rocking. The bomb was assumed to be filled with one or more liquid phase(s) in those cases where the pressure was unusually high and where p also attained linearity in T. The correctness of this assumption was checked in separate experiments where measured samples of pure methanol, heated in the same apparatus, exhibited analogous results [ 38,391. The reaction temperature was maintained for 2 h and then heating was discontinued. Contents of the cooled bomb were displaced (by spontaneously vaporizing I-IF) into ice and excess aqueous NaOH. The organic layer plus an isopentane extract of the aqueous layer were combined, washed, dried, and analyzed by Hypercal distillation for benzene, toluene, xylene, and trimethylbenzenes. Data for various experiments are presented in Tables 1 and 2. Distillation of a standard reference mixture indicated an absolute accuracy of Q 1 in each reported percentage of toluene and a relative accuracy of 6% in each reported percentage of trimethylbenzenes. Reported mol% yields are based on the assumed stoichiometry of 2 xylene + 1 toluene + 1 trimethylbenzene for use of xylene only. For xylene-benzene mixtures this same stoichiometry is used to calculate yields of trimethylbenzenes, but yields of toluene are based on the assumption of maximal attainability, as given by the equation 1 xylene + 1 benzene + 2 toluene. Only in expts. 7 and 8 was isomeric composition of the trimethylbenzene product investigated in detail, i.e. by careful fractional distillation, observation of boiling point curve, measurement of refractive index, and finally oxidation to separable benzenetricarboxylic acids. At least 90% of this product was pseudocumene, i.e., 1,2,4&imethylbenzene. Boiling point curves from various other experiments indicated a preponderance of pseudocumene there as well.
3. Results and discussion Observation of Table 1 shows that the highest yields of both toluene (A) and trimethylbenzenes (B) are obtained by disproportionation of xylene (expts. 7 and 8). The lower yield of B in these two experiments is ascribed to further methylation of B by xylene to yield tetramethylbenzenes, etc. [ 16,191. The pairs of experiments 3-4, 5-6, and 7-8 were made to ascertain the optimum weight percentage of HF to use. It is apparent that about 200 wt.% gives the highest yields of both A and B. In expt. 7 this corresponds to a molar ratio of HF : xylene of 10.6. In the various patents obtained by McCaulay and Lien [8-l 1,131 for the disproportionation of xylene by means of HF alone or usually with added catalyst ( BF3, TiF,, or ethylbenzene) this molar ratio varied from 3-20. As in expts. 7 and 8, these
L.H. Klemm/Joumal Table 1 Effect of variation in the composition the bomb rocking Expt. no.
1 2 3* 4d 5 6 I 8
Xylene” (mol%)
35 40 45 45 50 50 100 100
HP (wt.%)
200 200 139 191 110 220 200 395
of Molecular Catalysis 88 (1994) 205-212
of the charge to the bomb. All experiments
201
conducted at 180°C for 2 h with
Yield of producC (mol’%) Toluene
Trimethylbenzenes
(A)
(B)
24 30 17 29 4 21 42 44
21 26 8 23 6 14 32 32
‘In xylene-benzene mixture. bCompared to total weight of hydrocarbon used. ‘In expts. 7 and 8 the molar ratio of A to B produced=ratio of mol% yields of A and B shown. For all other experiments this molar ratio produced = twice the ratio of mol% yields shown. Data shown are corrected for product loss on workup and distillation. ?Shown in more detail in Table 2.
workers obtained principally pseudocumene as the trimethylbenzene product when using HF alone, but found mesitylene ( 1,3,5trimethylbenzene), instead, when 0.01-3 mol of BF, was added to the mixture [ 81. These results indicate that transmethylation is a FriedelCrafts reaction in which use of I-IF alone effects kinetic control to give pseudocumene; while the combination I-IF-BF, produces the most basic isomer, mesitylene, even to the exclusion of other isomers [ 40,4 1 ] . It was proposed that the strong acid, HBF,, present in I-IF-BF, mixtures, effects protonation of aromatic rings to give salts ArH+ BF; [ 91. These ArH+ ions then undergo intramolecular migrations of methyl groups, as in the conversion of initially formed pseudocumene into mesitylene. The poorer proton donor, I-IF, apparently is unable to cause such intramolecular rearrangement even at 180°C over a period of 2 h. The ortho-para orientation rules for methylation of any one of the three xylene isomers present in commercial xylene mixtures should favor formation of pseudocumene by an attacking electrophilic group such as a methyl carbocation or its chemical equivalent. It is suggested that this attack may occur via a transfer agent such as CH3F (known to give facile Friedel-Crafts alkylation [ 401) or perhaps more likely, by direct ring-to-ring transfer of an electron-deficient (i.e. positively charged) methyl group. This transmethylation process is catalyzed by I-IF, probably via initial complexation with the aromatic ring. Since basicity of the ring will, in general, increase with increasing numbers of methyl groups on the ring [41], it seems plausible that in benzene-xylene mixtures I-IF will preferentially complex with a xylene molecule and, thereby, foster transmethylation to a juxtaposed benzene molecule in the liquid phase (vide infra) . More generally, with sufficient reaction time, it is proposed that an array of products from benzene to hexamethylbenzene can be obtained, irrespective of whether xylene alone or xylene-benzene mixtures are used. The reaction is driven in opposing directions by two pertinent factors. Preferential complexation of the
208
LH. Klemm / Journal of Molecular Catalysis 88 (1994) 205-212
more basic aromatic rings fosters increasing ring methylation. Contrariwise, the more methyl substituents on the complexed ring, the more reactive the molecule becomes to transmethylating other rings. Some preliminary experiments (not reported here) indicated that a reaction time of two hours should give a maximum yield of toluene from use of a hydrocarbon mixture containing 40 mol% xylene. Expts. 1,2,4 and 6 were conducted with similar weight percentages of HF but varying xyleneknzene ratios. One notes that the yield of toluene attains a maximum for use of 40-45 mol% xylene, while the yield of trimethylbenzenes falls as one goes from use of 35 to use of 50 mol% xylene. Somewhat arbitrarily, 45 mol% xylene was chosen as the starting mixture for further investigation, as presented in Table 2. For expt. 4 (taken as a reference) the molar ratios of HF : xylene = 19.3 and HF : aromatic rings = 8.7. Comparison of expts. 3 and 4 shows that the larger quantity of HF used in 4 resulted not only in higher yields of A and B, but also a much larger reaction pressure at the same reaction temperature. As indicated in the Experimental part, in expt. 4 the plot of p versus Tduring heating showed a positive deviation from the curve found in expt. 3. The attainment of linearity in the graph for 4 heralded the filling of the bomb with one or more liquid phases only. Analogous pressure differences are found in expts. 10 and 11, albeit at 215°C which Table 2 Effect of variation in reaction temperature, xylene for reaction time 2 h Expt. no.
pressure,
and additive. All experiments
Wts. charged to bomb (g)
Reaction temp.
Reaction pressure
Hydrocarbon
(“C)
(am0
HF
Other
conducted
Yield of product” (mol%) Toluene (A)
with 45 mol%
Remarks”
Trimethylbenzenes (B)
3’ 4’
160 159
222 304
none none
180 180
58 104
17 29
8 23
9 10 11
93 93 93
284 176 209
none none none
180 215” 215”
65 93 116
28 9 22
6 7 8
f. r d. s c. 8
12 13 14
93 93 150
189 178 286
none Ha gas’ NKBF,
253h 180 180
145 97 62
24 19 28
8 8 6
d.j f
15
150
279
29 g Hg(CN),
180
54
12
4
d
35 g “See footnote c, Table 1. bUnless otherwise noted, tbe bomb was rocked during the reaction. ‘Same experiment as in Table 1. “Bomb probably contains both liquid and gaseous phases. ‘There is experimental evidence that the bomb is filled with liquid phase(s) only (see Discussion). ‘Probably there is little or no gaseous phase in this reaction. gConducted without rocking the bomb. ‘The critical temperature of HF is 188 + 3’C [ 391. ‘The charged bomb was pressurized with H, gas to 2.7 atm at 25°C before heating. ‘No hydrogenated products were found.
d 5
L.H. Klemm /Journal of Molecular Catalysis 88 (1994) 205-212
209
is above the critical temperature of I-IF alone [ 391, but below that of benzene, 288.5”C [42]. Altogether expts. 4,9, 11, and 14, probably conducted with little or no gaseous phase present under the reaction conditions, gave exceptionally high yields (22-29 mol%) of toluene, believed to form in a liquid phase. A similar yield of toluene is produced in expt. 12, but it is not clear how much liquid phase is present there. The use of increased pressure to improve the yield of A from passing mixtures of benzene and xylene over an aluminosilicate catalyst in a vapor-phase reaction has been noted by others [ 35-371. They found a maximum yield of 25-30 mol% of A at 500-525°C and 10-12 atm for a single pass over the catalyst. These yields are comparable to those in expts. 1, 2, 4, 6, 9, 11, 12, and 14 which were carried out at much lower temperatures, but considerably higher pressures and for much longer reaction times. Expt. 13 was designed to increase the pressure on the liquid phase, while still retaining a gaseous phase, by introducing hydrogen gas (ostensibly chemically inert) into the bomb. Comparison of the results with expts. 10 and 11 shows that the total reaction pressure is similar to that in 10 (at a 35°C higher reaction temperature), but the yields of A and B are comparable to those in 11, where more I-IF was charged to the bomb. As noted previously, it seems likely that methyl group transfer occurs largely (or exclusively) in a liquid phase, where close association of two aromatic rings (one a methyl donor; the other a methyl acceptor) plus several molecules of HF, or more likely, one or more molecules of polymer (HF),, where n may well be greater than 6 [ 43,441, can occur. It is expected that the degree of polymerization of HF in the liquid phase will decrease as the hydrocarbon : I-F ratio increases, and vice versa. Thus, if maximal methyl transfer does occur only in the presence of molecules (I-IF),, where x has some minimal value, one can rationalize the observation that a large molar ratio of HF: aromatic rings may be optimal and that adding more HF will not improve the yields of products. In expt. 4, where one has both clear evidence that only a liquid phase is present and a!so obtains relatively high yields of A and B, x could be as large as 8.7 (vide supra) . Expts. 14 and 15 were made in order to test the possibility of catalysis by added salts. Thus, 14 introduced the fluoborate ion, proposed to be of significance in reactions with HFBF, (vide supra) ; while 15 was made with mercuric ion, known to complex fluoride ion. Both salts decreased the thermal expansion of HF and, thus, may be considered to increase the internal pressure on the liquid phase [45]. While yields in expt. 15 were low, the results from 14 are identical with those of 9 which was started with 57 fewer grams of hydrocarbon. It is believed that the bomb in both expts. 9 and 14 was virtually filled with liquid. The ratio of yields A/B varies widely for the various benzene-xylene reactions. It is not clear why this occurs.
4. Conclusions Anhydrous HF at 180°C or higher serves as a Friedel-Crafts catalyst to effect portionation of xylene to toluene and trimethylbenzenes (mainly pseudocumene) effect transmethylation from xylene to benzene. These reactions probably occur in phase and involve polymeric HF. Yields are comparable to those reported with other catalysts under a variety of conditions.
disproand to a liquid various
L.H. Klemm / Journal of Molecular Catalysis 88 (1994) 205-212
210
5. Acknowledgments
This work was conducted by the author while he was employed as a research chemist at Pan American Refining Corporation, Texas City, TX, during the period of 1944-1945. Preliminary studies (unpublished) were conducted by Dr. John Eibert and Dr. H.D. Radford.
6. References [ l] R. Anschiltz, AM., 235 ( 1886) 150. [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ 1 l] [ 121
[ 131 [ 141 [ 151 [ 161 [ 171
[ 181 [ 191 [20] [21] [22] [23] [24] [25] [ 261 [27] [28] [29]
F. Fischer and H. Niggemann, Berichte, 49 (1916) 1475. IJS Pat. 1 324 143 (1920) to B.T. Brooks; Chem. Abstr., 14 (1920) 415. A. Kobayashi and S. Akiyoshi, Kogyo Kagaku Zasshi, 59 (1956) 178; Chem. Abstr., 51 (1957) 11265. S. Akiyoshi, A. Kobayashi and M. Matsukane, J. Chem. Sot. Jpn., Ind. Chem. Sect. 59, (1956) 28; Chem. Abstr., 51 (1957) 1050. D.J. Collins, R.P. Scharff and B.H. Davis, Appl. Catal., 8 (1983) 273. A. Schriesheim, J. Org. Chem., 26 (1961) 3530. US Pat. 2 564 073 (1951) to A.P. Lien and D.A. McCaulay; Chem. Abstr., 46 (1952) 1750. A.P. Lien and D.A. McCaulay, J. Am. Chem. Sot., 75 (1953) 2407. US Pat. 2 722 560 ( 1955) to D.A. McCaulay and A.P. Lien; Chem. Abstr., 50 ( 1956) 5287. US Pat 2 683 761 ( 1954) to D.A. McCaulay and A.P. Lien; Chem. Abstr., 49 ( 1955) 6997. N.I. Shuikin, E.D. Tulupova and Z.P. Polyskova, Izv. Akad. Nauk SSSR, Otdel. Khim. Nat& ( 1958) 1476; Chem. Abstr., 53 ( 1959) 8025. US Pat 2 881 228 (1959) to D.A. McCaulay; Chem. Al&., 53 (1959) 16056. T. Amemiya, E. Tsunetomi, E. Nakamura and T. Nakazawa, Bull. Jpn. Petrol. Inst., 3 (1961) 14; Chem. Abstr., 56 (1962) 1672. SM. Aliev, R.G. Ismailov, G.M. Mamedaliev and N.L. Guseinov, Khim. Tekbnol. Topliv Masel, 11 ( 1966) 10; Chem. Abstr., 65 (1966) 12125. G.A. Olab and J. Kaspi, Nouv. J. Chim., 2 (1978) 581. P. Beltrame, P. Beltrame, P. Camiti and M. Magnoni, Gazz. Cbim. Ital., 108 (1978) 651; Chem. Abstr., 91 (1979) 20011. T. Hosoya, H. Takaya, H. Oshio, T. Minegishi and N. Todo, Nippon Kagaku Kaishi, (1972) 2448; Chem. Abstr., 78 (1973) 135776. R.M. Aliev, A.K. Zamanov, Yu.G. Kambarov and P.A. Ambartsumov, Neftepererab. Neftekhim. (Moscow), ( 1977) 30; Chem. Abstr., 87 (1977) 22591. USSR Pat. 566 813 (1977) to Kb.M. Minachev, ES. Mortikov, B.M. Kozlov, A.S. Leont’ev, N.F. Kononov and S.M. Lakiza; Chem. Abstr., 87 (1977) 167691. Yu.G. Egiazarov, M.F. Savchits, L.L. Potapova and B.V. Romanovskii, Neftekhimiya, 17 (1977) 837; Chem. Abstr., 88 (1978) 88745. W.M. Minaev, D.A. Kondrat’ev, A.A. Dergachev, LV. Mishin, I.N. Olesbko, B.K. Nefedov and T.V. Alekseeva, Izv. Akad. Nauk SSSR, Ser. Khim., (1982) 1076; Chem. Abstr., 97 (1982) 29146. N.S. Gnep, J. Tejada and M. Guisnet, Bull. Sot. Chim. Fr., l-2 ( 1) (1982) 5. K.H. Char and Y.G. Kim, Hwahak Konghak, 21 ( 1983) 283; Chem. Abstr., 100 ( 1984) 156290. T. Mai and Ch. Dimitrov, God. Sofii Univ. “Kliment Okbridski”, Kbim. Fak., 74 (1979-1980, published 1984) 108; Chem. Abstr., 101 (1984) 194116. N. Giordano, P. Vitarelli, S. Cavallaro, R. Ottana and R. Lembo, in D. Olson and A. Bisio (eds.) , Rot. Int. Zeolite Conf., 6th ( 1983). Butterworth, Guildford, 1984, pp. 331-336; Chem. Abstr., 103 (1985) 122920. I. Bankos, J. Papp and D. Kallo, Acta Chim. Hung., 119 (1985) 179; Chem. Abstr., 105 ( 1986) 208265. M. Chatterjee and D. Ganguli, Indian J. Technol., 23 (1985) 180. N. Naum, K.G. Wen, V. Ababi and N. Bilba, Rev. Chim. (Bucharest), 38 (1987) 389; Chem. Abstr., 108 (1988) 167026.
L.H. Klemm/Joumal
of Molecular Catalysis 88 (1994) 205-212
211
[30] I. Bankos, A.L. Klyachko4T.R. Brueva, G.I. Kapustin and D. Kallo, React. Kinet. Catal. L&t., 33 ( 1987) 345. [31] J.A. Martens, J. Perez-Pariente, E. Sastre, A. Corma and P.A. Jacobs, Appl. Catal., 45 (1988) 85. [32] US Pat. 2 385 524 (1945) to W.J. Mattox; Chem. Abstr., 40 (1946) 710. [ 331 US Pat. 2 349 834 ( 1944) to L. Schmerling and V.N. Ipatieff; Chem. Abstr., 39 ( 1945) 1174. [34] R.C. Hansford, C.G. Myers and A.N. Sachanen, Ind. Eng. Chem., 37 ( 1945) 671. [35] L.N. Johnson and KM. Watson, Petrol Process., 1 (1946) 67. [36] Yu.G. Mamedaliev, A.V. Topchiev, G.M. Mamedaliev and G.N. Suleimanov, Dokl. Akad. Nauk SSSR, 106 (1956) 1027; Chem. Abstr., 50 ( 1956) 13768. [37] A.V. Topchiev, G.M. Mamedaliev and YuG. Mamedaliev, Izv. Akad. Nauk SSSR, Otdel. Khim. Nauk, (1956) 1390; Chem. Abstr., 51 (1957) 8025. [ 381 R.W. Gallant, Physical Properties of Hydrocarbons, Vol. 1, Gulf Publishing Co., Houston, Texas, 1968, pp. 62.65. [ 391 D. Koschel (chief ed.), Gmelin Handbook of Inorganic Chemistry, 8th Ed., Fluorine, Suppl. Vol. 3, Springer Verlag, New York, NY, 1982, pp. 87-88.93.95. [40] G.A. Olah, Friedel-Crafts and Related Reactions, Vol. 1, Interscience Publishers, New York, NY, 1963, pp. 36-43,70-72. [41] M. Kilpatrick and J.G. Jones, in J.J. Lagowski (ed.), The Chemistry of Non-Aqueous Solvents, Vol. II, Academic Press, New York, NY, 1967, pp. 51-58. [42] N.A. Lange, Handbook of Chemistry, 8th edn., Handbook Publishers, Inc., Sandusky, OH, 1952, p. 1464. See also R.W. Gallant and J.M. Railey, Physical Properties of Hydrocarbons, 2ndedn., Vol. 2, Gulf Publishing Co., Houston, TX, 1984, p. 143. [43] R.H. Maybury, S. Gordon and J.J. Katz, J. Chem. Phys., 23 (1955) 1277. [44] I. Sheft and A.J. Perkins, J. Inorg. Nucl. Chem., 38 ( 1976) 665. [45] R.L. Scott, in D. Henderson (ed.) , Physical Chemistry, An Advanced Treatise, Vol. VIIIA, Academic Press. New York, NY, 1971, pp. 68-80.
MOLECULAR CATALYSIS Journal of Molecular Catalysis
88 ( 1994) 205-2 12
Intermolecular methyl group transfers in the xylene and xylene-benzene systems by means of anhydrous HF LeRoy H. Klemm* Department
of Chemistry, University of Oregon, Eugene, OR 97403, USA
(Received January 18, 1993; accepted October 5, 1993)
Abstract Treatment of xylene with anhydrous HF (200 wt.%) at 180°C for 2 h in a sealed bomb gives disproportionation to toluene (A, 42%) and trimethylbenzenes (B, 32%). Extension to use of xylenebenzene mixtures for the hydrocarbon component gives transmethylation wherein yields of A and B (maxima of 30% and 27%, respectively) are found to vary with hydrocarbon composition, weight percentage of HF used, reaction temperature, reaction pressure, and addition of inorganic salts. Key words: benzene; hydrofluoric acid; methyl group transfer; xylene
1. Introduction Catalyzed disproportionation of xylene by intermolecular transfer of a methyl group to give toluene and trimethylbenzenes has been studied for more than a century. Catalysts used were AlCI, [ l-51, AlC&-HCl [ 61, AlBr,-HBr [ 71, BF,-HF [ 8-101, TiF,-HF [ 111, Pt-AIzO, [ 121, anhydrous HF (50-100 vol.%, as based on xylene used, at 160-175°C) [ 131, aluminosilicates [ 14,151, Nafion [ 16,171 and zeolites or mordenites [ 18-3 11. Less extensively studied is the process of transmethylation of benzene by means of xylene to yield toluene. Catalysts used were A1203-HCl [ 321, A&O,-HCl-ZnCl, [ 331, and aluminosilicates [ 34-371. The present paper describes the use of anhydrous HF at 180-253°C to effect both disproportionation of xylene and methyl group transfer from xylene to benzene. 2. Experimental
2.1. Apparatus, chemicals and procedure Reactions
were conducted
in a high pressure
Parr illium rocking
*Corresponding author. Fax. ( + l-503)3460487. 0304-5102/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO304-5102(93)EO260-N
bomb
(internal
206
L.H. Klemm/Joumal
of Molecular Catalysis 88 (1994) 205-212
volume, corrected to 180°C 744 ml), equipped with inlet and outlet ports bearing shut-off valves. The outlet port was attached to an internal monel steel tube reaching almost to the bottom of the bomb. The inlet port was connected, via a gas-oil-filled steel pressure tube, to a pressure gauge. The bomb was heated electrically. The temperature was controlled manually by a powerstat and read from a dial-type metal thermometer inserted into a thermal well protruding into the cavity of the bomb. The assembled bomb was evacuated by a water aspirator and cooled in an ice bath. Measured quantities of reagent grade xylene or xylene-benzene mixtures were introduced from an attached separatory funnel and commercial grade anhydrous I-IFfrom a steel sample bomb. In some experiments a plot of observed pressure (p) versus temperature (Z’) was made while the bomb plus contents were heated to the reaction temperature (usually 180°C) with or without rocking. The bomb was assumed to be filled with one or more liquid phase(s) in those cases where the pressure was unusually high and where p also attained linearity in T. The correctness of this assumption was checked in separate experiments where measured samples of pure methanol, heated in the same apparatus, exhibited analogous results [ 38,391. The reaction temperature was maintained for 2 h and then heating was discontinued. Contents of the cooled bomb were displaced (by spontaneously vaporizing I-IF) into ice and excess aqueous NaOH. The organic layer plus an isopentane extract of the aqueous layer were combined, washed, dried, and analyzed by Hypercal distillation for benzene, toluene, xylene, and trimethylbenzenes. Data for various experiments are presented in Tables 1 and 2. Distillation of a standard reference mixture indicated an absolute accuracy of Q 1 in each reported percentage of toluene and a relative accuracy of 6% in each reported percentage of trimethylbenzenes. Reported mol% yields are based on the assumed stoichiometry of 2 xylene + 1 toluene + 1 trimethylbenzene for use of xylene only. For xylene-benzene mixtures this same stoichiometry is used to calculate yields of trimethylbenzenes, but yields of toluene are based on the assumption of maximal attainability, as given by the equation 1 xylene + 1 benzene + 2 toluene. Only in expts. 7 and 8 was isomeric composition of the trimethylbenzene product investigated in detail, i.e. by careful fractional distillation, observation of boiling point curve, measurement of refractive index, and finally oxidation to separable benzenetricarboxylic acids. At least 90% of this product was pseudocumene, i.e., 1,2,4&imethylbenzene. Boiling point curves from various other experiments indicated a preponderance of pseudocumene there as well.
3. Results and discussion Observation of Table 1 shows that the highest yields of both toluene (A) and trimethylbenzenes (B) are obtained by disproportionation of xylene (expts. 7 and 8). The lower yield of B in these two experiments is ascribed to further methylation of B by xylene to yield tetramethylbenzenes, etc. [ 16,191. The pairs of experiments 3-4, 5-6, and 7-8 were made to ascertain the optimum weight percentage of HF to use. It is apparent that about 200 wt.% gives the highest yields of both A and B. In expt. 7 this corresponds to a molar ratio of HF : xylene of 10.6. In the various patents obtained by McCaulay and Lien [8-l 1,131 for the disproportionation of xylene by means of HF alone or usually with added catalyst ( BF3, TiF,, or ethylbenzene) this molar ratio varied from 3-20. As in expts. 7 and 8, these
L.H. Klemm/Joumal Table 1 Effect of variation in the composition the bomb rocking Expt. no.
1 2 3* 4d 5 6 I 8
Xylene” (mol%)
35 40 45 45 50 50 100 100
HP (wt.%)
200 200 139 191 110 220 200 395
of Molecular Catalysis 88 (1994) 205-212
of the charge to the bomb. All experiments
201
conducted at 180°C for 2 h with
Yield of producC (mol’%) Toluene
Trimethylbenzenes
(A)
(B)
24 30 17 29 4 21 42 44
21 26 8 23 6 14 32 32
‘In xylene-benzene mixture. bCompared to total weight of hydrocarbon used. ‘In expts. 7 and 8 the molar ratio of A to B produced=ratio of mol% yields of A and B shown. For all other experiments this molar ratio produced = twice the ratio of mol% yields shown. Data shown are corrected for product loss on workup and distillation. ?Shown in more detail in Table 2.
workers obtained principally pseudocumene as the trimethylbenzene product when using HF alone, but found mesitylene ( 1,3,5trimethylbenzene), instead, when 0.01-3 mol of BF, was added to the mixture [ 81. These results indicate that transmethylation is a FriedelCrafts reaction in which use of I-IF alone effects kinetic control to give pseudocumene; while the combination I-IF-BF, produces the most basic isomer, mesitylene, even to the exclusion of other isomers [ 40,4 1 ] . It was proposed that the strong acid, HBF,, present in I-IF-BF, mixtures, effects protonation of aromatic rings to give salts ArH+ BF; [ 91. These ArH+ ions then undergo intramolecular migrations of methyl groups, as in the conversion of initially formed pseudocumene into mesitylene. The poorer proton donor, I-IF, apparently is unable to cause such intramolecular rearrangement even at 180°C over a period of 2 h. The ortho-para orientation rules for methylation of any one of the three xylene isomers present in commercial xylene mixtures should favor formation of pseudocumene by an attacking electrophilic group such as a methyl carbocation or its chemical equivalent. It is suggested that this attack may occur via a transfer agent such as CH3F (known to give facile Friedel-Crafts alkylation [ 401) or perhaps more likely, by direct ring-to-ring transfer of an electron-deficient (i.e. positively charged) methyl group. This transmethylation process is catalyzed by I-IF, probably via initial complexation with the aromatic ring. Since basicity of the ring will, in general, increase with increasing numbers of methyl groups on the ring [41], it seems plausible that in benzene-xylene mixtures I-IF will preferentially complex with a xylene molecule and, thereby, foster transmethylation to a juxtaposed benzene molecule in the liquid phase (vide infra) . More generally, with sufficient reaction time, it is proposed that an array of products from benzene to hexamethylbenzene can be obtained, irrespective of whether xylene alone or xylene-benzene mixtures are used. The reaction is driven in opposing directions by two pertinent factors. Preferential complexation of the
208
LH. Klemm / Journal of Molecular Catalysis 88 (1994) 205-212
more basic aromatic rings fosters increasing ring methylation. Contrariwise, the more methyl substituents on the complexed ring, the more reactive the molecule becomes to transmethylating other rings. Some preliminary experiments (not reported here) indicated that a reaction time of two hours should give a maximum yield of toluene from use of a hydrocarbon mixture containing 40 mol% xylene. Expts. 1,2,4 and 6 were conducted with similar weight percentages of HF but varying xyleneknzene ratios. One notes that the yield of toluene attains a maximum for use of 40-45 mol% xylene, while the yield of trimethylbenzenes falls as one goes from use of 35 to use of 50 mol% xylene. Somewhat arbitrarily, 45 mol% xylene was chosen as the starting mixture for further investigation, as presented in Table 2. For expt. 4 (taken as a reference) the molar ratios of HF : xylene = 19.3 and HF : aromatic rings = 8.7. Comparison of expts. 3 and 4 shows that the larger quantity of HF used in 4 resulted not only in higher yields of A and B, but also a much larger reaction pressure at the same reaction temperature. As indicated in the Experimental part, in expt. 4 the plot of p versus Tduring heating showed a positive deviation from the curve found in expt. 3. The attainment of linearity in the graph for 4 heralded the filling of the bomb with one or more liquid phases only. Analogous pressure differences are found in expts. 10 and 11, albeit at 215°C which Table 2 Effect of variation in reaction temperature, xylene for reaction time 2 h Expt. no.
pressure,
and additive. All experiments
Wts. charged to bomb (g)
Reaction temp.
Reaction pressure
Hydrocarbon
(“C)
(am0
HF
Other
conducted
Yield of product” (mol%) Toluene (A)
with 45 mol%
Remarks”
Trimethylbenzenes (B)
3’ 4’
160 159
222 304
none none
180 180
58 104
17 29
8 23
9 10 11
93 93 93
284 176 209
none none none
180 215” 215”
65 93 116
28 9 22
6 7 8
f. r d. s c. 8
12 13 14
93 93 150
189 178 286
none Ha gas’ NKBF,
253h 180 180
145 97 62
24 19 28
8 8 6
d.j f
15
150
279
29 g Hg(CN),
180
54
12
4
d
35 g “See footnote c, Table 1. bUnless otherwise noted, tbe bomb was rocked during the reaction. ‘Same experiment as in Table 1. “Bomb probably contains both liquid and gaseous phases. ‘There is experimental evidence that the bomb is filled with liquid phase(s) only (see Discussion). ‘Probably there is little or no gaseous phase in this reaction. gConducted without rocking the bomb. ‘The critical temperature of HF is 188 + 3’C [ 391. ‘The charged bomb was pressurized with H, gas to 2.7 atm at 25°C before heating. ‘No hydrogenated products were found.
d 5
L.H. Klemm /Journal of Molecular Catalysis 88 (1994) 205-212
209
is above the critical temperature of I-IF alone [ 391, but below that of benzene, 288.5”C [42]. Altogether expts. 4,9, 11, and 14, probably conducted with little or no gaseous phase present under the reaction conditions, gave exceptionally high yields (22-29 mol%) of toluene, believed to form in a liquid phase. A similar yield of toluene is produced in expt. 12, but it is not clear how much liquid phase is present there. The use of increased pressure to improve the yield of A from passing mixtures of benzene and xylene over an aluminosilicate catalyst in a vapor-phase reaction has been noted by others [ 35-371. They found a maximum yield of 25-30 mol% of A at 500-525°C and 10-12 atm for a single pass over the catalyst. These yields are comparable to those in expts. 1, 2, 4, 6, 9, 11, 12, and 14 which were carried out at much lower temperatures, but considerably higher pressures and for much longer reaction times. Expt. 13 was designed to increase the pressure on the liquid phase, while still retaining a gaseous phase, by introducing hydrogen gas (ostensibly chemically inert) into the bomb. Comparison of the results with expts. 10 and 11 shows that the total reaction pressure is similar to that in 10 (at a 35°C higher reaction temperature), but the yields of A and B are comparable to those in 11, where more I-IF was charged to the bomb. As noted previously, it seems likely that methyl group transfer occurs largely (or exclusively) in a liquid phase, where close association of two aromatic rings (one a methyl donor; the other a methyl acceptor) plus several molecules of HF, or more likely, one or more molecules of polymer (HF),, where n may well be greater than 6 [ 43,441, can occur. It is expected that the degree of polymerization of HF in the liquid phase will decrease as the hydrocarbon : I-F ratio increases, and vice versa. Thus, if maximal methyl transfer does occur only in the presence of molecules (I-IF),, where x has some minimal value, one can rationalize the observation that a large molar ratio of HF: aromatic rings may be optimal and that adding more HF will not improve the yields of products. In expt. 4, where one has both clear evidence that only a liquid phase is present and a!so obtains relatively high yields of A and B, x could be as large as 8.7 (vide supra) . Expts. 14 and 15 were made in order to test the possibility of catalysis by added salts. Thus, 14 introduced the fluoborate ion, proposed to be of significance in reactions with HFBF, (vide supra) ; while 15 was made with mercuric ion, known to complex fluoride ion. Both salts decreased the thermal expansion of HF and, thus, may be considered to increase the internal pressure on the liquid phase [45]. While yields in expt. 15 were low, the results from 14 are identical with those of 9 which was started with 57 fewer grams of hydrocarbon. It is believed that the bomb in both expts. 9 and 14 was virtually filled with liquid. The ratio of yields A/B varies widely for the various benzene-xylene reactions. It is not clear why this occurs.
4. Conclusions Anhydrous HF at 180°C or higher serves as a Friedel-Crafts catalyst to effect portionation of xylene to toluene and trimethylbenzenes (mainly pseudocumene) effect transmethylation from xylene to benzene. These reactions probably occur in phase and involve polymeric HF. Yields are comparable to those reported with other catalysts under a variety of conditions.
disproand to a liquid various
L.H. Klemm / Journal of Molecular Catalysis 88 (1994) 205-212
210
5. Acknowledgments
This work was conducted by the author while he was employed as a research chemist at Pan American Refining Corporation, Texas City, TX, during the period of 1944-1945. Preliminary studies (unpublished) were conducted by Dr. John Eibert and Dr. H.D. Radford.
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