Energy & Fuels 1994,8,487-497
487
Aqueous High-Temperature Chemistry of Carbo- and Heterocycles. 20.l Reactions of Some Benzenoid Hydrocarbons and Oxygen-Containing Derivatives in Supercritical Water at 460 "C Alan R. Katritzky,' Richard A. Barcock, Marudai Balasubramanian, and John V. Greenhill Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, Florida 32611-2046
Michael Siskin' and William N. Olmstead Corporate Research, Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received September 8, 1993. Revised Manuscript Received December I , 1 9 9 P
The reactions of a series of hydrocarbon- and oxygen-containing organic compounds have been studied in supercritical water, 15% aqueous formic acid, and 15% aqueous sodium formate a t 460 "C. For each substrate, a solution in cyclohexane at 460 "C was also examined in order to distinguish thermolytic (mainlyradical) reactions from the ionic reactions in the aqueoussystems. Moat substrates showed significant change within 1h. Biphenyl, 1,l'-binaphthyl, naphthalene, and phenanthrene were essentially unreactive. l-Benzylnaphthalene underwent only slow thermolysis, whereas l-benzyltetralin underwent rapid homolysis in all media. Cyclohexylbenzene and l-cyclohexylnaphthalene showed similar trends in reaction in all media, as did hexylbenzene and l-decylnaphthalene. The conversion rates for the n-alkyl-substituted aromatics were noticeably higher than those for the cyclohexyl-substituted aromatics. Dibenzofuran was unreactive, and diphenyl ether only underwent a slow base-catalyzed hydrolysis. Cyclohexyl phenyl ether was highly reactive at short reaction times, and l-naphthyl phenyl ether was noticeably less reactive. l-Naphthol and l-tetralone showed significant conversion in the basic media and there was evidence of reduction in the formate solutions. l-Octanol showed little reactivity after the shorter reaction time.
Introduction The reactions of a range of organic compounds in aqueous solutions at temperatures between 200 and 350 "Chave been described in recent series of publications by us.14 The significance of many of the results to our understanding of potential processes for the treatment of resource materials has stimulated us to extend the work to temperatures above the critical point of water (374.2 "C). There are potentially enormous environmental, social, and commercial advantages for the conversion and upgrading of fossil fuel resources by aqueous treatment rather than by conventional means. The conditions selected for the present studies were as follows: 460 "C; 7:l ratio of water to model compound; 0.2 g/mL water density; a short residence time (7 min) and a longer one (1h) were chosen to cover a range of processing possibilities and to investigate the mechanisms of the reactions. Little systematic work has been reported previously on the behavior of organic reactions in supercritical fluids, but the physical properties of supercritical water have led to Abatract publiahed in Advance ACS Abstracts, January 15, 1994. (1) Part 19: Katritzky, A. R.; Luxem, F. J.; Murugan,R Greenhill, J.V.; Siskin, M. Energy Fuels 1992,6,450. (2) Aqueoua Organic Chemistry. Parts 1-3. Siskin, Katritzky et al. Energy Fuels 1990,4,475. (3) AqueousHigh-TemperatureChemiatryof Carbo-and Heterocycles. Parts 1-15. Katritzky, Siskin et al. Energy Fuels 1990,4,493. (4) Katritzky, A. R.; Murugan, R.; Balasubramanian, M.;Greenhill, J.V.;Siskin, M.;Brons, G.Energy Fuels 1991,5, 823.
suggestionsthat different mechanisms may apply between sub- and supercritical conditions? Some of these physical properties of supercritical water include a decreased dielectric constant, an increased heat capacity, reduced density, and different solubility properties and lower polarity. Indeed, supercritical water, at high enough pressure, is miscible in all proportions with organic compounds. The temperature we have now employed (460 "C) should enable higher reaction rates and offer the possibility of converting compoundswhich were refractory at subcritical temperatures. The higher solubility of organics in supercritical fluids has spawned much current interest in supercritical water extraction of coals and other natural resources which in turn has stimulated several previous studies of reactions of model compounds in supercritical water. Published work on hydrocarbon-and oxygen-containingcompounds is compared with the observations we found in the Results and Discussion section of this report. We have now initiated a systematic study of the supercritical water reactions of typical examples of the main classes of organic compounds, with emphasis on structures likely to be present in natural fuel resources. Reactions were conducted at 460 "C and each compound was heated in water alone, in 15% aqueous formic acid, (5) Shaw,R. W.;Brill,T.B.;Clifford,A.A.;Eckert,C.A.;Fnmck,E.U. Chem. Eng. News 1991,69, 26-39.
0887-0624/94/2508-0487~04.5~/0 0 1994 American Chemical Society
Katritzky et al.
488 Energy 6 Fuels, Vol. 8, No. 2, 1994 and in 15% aqueous sodium formate. Formic acid was chosen t o enhance a n y acid-catalyzed reaction, because i t can act as a reducing agent by hydride ion donation, and
because it simulates CO/H20 treatments. Sodiumformate increases the pH of its aqueous solutions and is a powerful hydride donor. I n addition, each substrate was heated in cyclohexane in order to compare the thermal decompositions with t h e aquathermolyses. Initial runs were carried out for 7 min (i.e., 2 min heat-up time to 460 "C and 5 min residence time at that temperature), and if little or n o change was detected they were repeated over 1 h. In this, the first of a series of papers, we discuss our experience with simple benzenoid hydrocarbons and with oxygencontaining substrates in supercritical water. Experimental Section Diphenyl ether (47), dibenzofuran (54), l-octanol(29), l-naphtho1 (57), l-tetralone (45), hexylbenzene (37), diphenylmethane (52), naphthalene (34), phenanthrene (62), biphenyl (461, and 1,l'-binaphthyl (74) (all >98% purity) were obtained from Aldrich. Cyclohexylbenzene (42) (98% purity) was obtained from Kodak. Cyclohexyl phenyl ether (51), bp 125 "C/15 mm (lit.? 101-103 "C/3 mm); l-naphthyl phenyl ether (69), mp 53-55 "C; (lit.,'% "C); l-benzylnaphthalene (70), mp 56-57 "C (lit.: 55-56 "C); l-benzyltetralin (66), bp 110 "C/1 mm (lit.: 143-144 "C/1.4 mm); 1-cyclohexylnaphthalene (65), bp 141 "C/2 mm (lit.,lo 152 OC/3 mm), were all prepared according to the literature procedures and the correct spectral data was obtained for each compound. Benzylcyclohexane (49). Cyclohexyl phenyl ketone (8.00 g, 42.5 mmol), sodium hydroxide (4.00 g, 100 mmol), and hydrazine monohydrate (4 mL) were refluxed in diethylene glycol (80 mL) for 12 h. The reaction mixture was acidified with HCl(60 mL, 5% v/v) and extracted with diethyl ether (3 X 40 mL). The combined organic extracts were washed with water (2 X 50 mL), brine (1 X 30 mL), and dried (MgSOd) and the solvent was evaporated to give a yellow oil, which was purified by vacuum distillation (85 OC/2 mm) (lit.," 113-115 "C/5 mm) to give a colorless oil (7.08 g, 95%). l-Decylnaphthalene (73). l-Bromonaphthalene (15.00 g, 72.0 mmol) in anhydrous ether (80mL) was added to magnesium turnings (1.49 g, 62.0 mmol) in ether (20 mL) and the reaction was refluxed for 2 h. n-Decanenitrile (11.38 g, 74.0 mmol) was then added dropwise and on completion of the addition the reaction mixture was refluxed for 4 h. The organic layer was washed with water (3 X 30 mL), sodium bicarbonate (1X 50mL), and brine (1 X 30 mL) and dried (MgSOd) and the solvent evaporated to yield the crude 1-(1'-naphthy1)decanone as a brown oil. The product was purified by vacuum distillation (165 "C/4 mm) (lit.,12l4OoC/2mm) toyieldapaleyellowoil(12.0g,59%). l-(1'-Naphthy1)decanone (1.00 g, 3.55 mmol), sodium hydroxide (0.50 g, 12.5 mmol), and hydrazine monohydrate (0.5 mL) were refluxed together in diethylene glycol (10 mL) for 11 h. The reaction mixture was acidified (35 mL, 5 % v/v) and extracted with diethyl ether (3 X 30 mL). The combined organic extracts were washed with water (2 X 25 mL), brine (1 X 20 mL), and dried (MgSOd) and the solvent evaporated to give the crude l-decylnaphthalene (73) as a brown oil which was purified by vacuum distillation (150 OC/2 mm) (lit.:2 bp 200 "C/4 mm) to yield the product (73) as a colorless oil (0.93 g, 98%). (6) Abdurasuleva,A. R.; Akhemdov, K. N. Uzbeksk. Khim. Zh. 1964, 8, 31; Chem. Abstr. 1965,62, 10356b. (7) Bacon, R. G. R.; Stewart, 0. J. J. Chem. SOC1965, 4953. (8) Vingiello, F. A.; Quo, S.-G.; Sheridan, J. J. Org. Chem. 1961,26, 3202. (9) Chambers,Jr., R. R.; Collins, C. J.; Maxwell, B. E. J. Org. Chem. 1985, 50, 4960. (10) Orchin, M.; Reggel, L. J. Am. Chem. SOC.1947, 69, 505. (11) Soper,Q. F.;Buting, W. E.;Cochran,Jr., J. E.; Pohland,A. J. Am. Chem. SOC.1954, 76,4109. (12) Bannister, B.; Elmer, B. B. J. Chem. SOC.1951,1055.
The purities of all starting materials were checked by GC prior
to use and were purified to >98% where necessary. Water, 15% aqueous formic acid, 15% aqueous sodium formate, and cyclohexane were deoxygenated with argon for 1h just before use. All the GC analyses were carried out on a Hewlett Packard 5890 gas chromatograph operated in the split injection mode (301 ratio) and equipped with a flame ionization detector (FID). A 15-m capillary column (SPB-1) was used and the oven temperature was programmed from 50 to 250 "C a t a rate of 10 "C/min. GC/ MS analyses of all compounds were performed on a Varian 3400 gas chromatograph and a Finnegan MAT 700 ion trap detector. General Procedure for Aquathermal Reactions. The modelcompound (0.16 g) and the solvent (1.14 mL) were charged into a nitrogen-blanketed stainless steel bomb which was then sealed. The reactors were kept, but not agitated, in a Techne fluidized sandbath (Model SBS-4) set a t 460 "C using a Techne temperature controller (TC-8D) for a time period of 7 min. If little or no reaction was observed, the run was repeated for 1 h. The temperature profile was measured by a Barnant 115 thermocouple thermometer (type J) placed in the sandbath adjacent to the reaction vessel. After the time period, the reaction was immediately quenched by cooling the bomb with cold air and then dry ice. The reaction mixture was worked up as previously de~cribed.~ The GC behavior of all the compounds included in the present paper (starting materials and products) are collated in Table 1. Table 2 records the source and mass spectral fragmentation pattern of the authentic compounds used, either as starting materials or for the identification of products. Tables 3 and 4 record the mass spectral fragmentation patterns of products for which authentic samples were not available and which were identified by comparison with published MS data (Table 3) or by deduction of their structure from the MS fragmentation pattern (Table 4). Tables 2, 3, and 4 along with the details of the mass spectral analyses have been deposited as supplementary material (see Supplementary Material Available paragraph at the end of this paper).
Results and Discussion All t h e results are collected in Tables 5-12 and are presented i n the same manner as described previously.3 Where appropriate,t h e results obtained from the reactions of some of the model compounds were placed in t h e same table so that the results could be compared more easily. The observed transformations are illustrated in t h e schemes. Compounds with numbers 1100 a r e postulated intermediates n o t detected i n the reaction mixture b y the
GUMS system. Hydrocarbon Compounds. Biphenyl (46) and 1,TBinaphthyZ(74. Both of these biaryls were found t o be unreactive after 1 h at 460 O C i n any of t h e media used (see ref 13a).
Naphthalene (34 and Phenanthrene (62). Our previous work demonstrated that phenanthrene (62) was unreactive in water at 315 "C for 3 days, whereas in 15% aqueous formic acid and 15 % sodium formate conversions of 0.5 and 1.076, respectively, t o 9,lO-dihydrophenanthrene (61) were observed for t h e same reaction conditions.13b (13) (a) Note: In some of the reactions in cyclohexane at 460 O C after 1 h, we did detect small quantities of products, typically ethyl-, propyl-, and butylcyclohexane, as well as bicyclohexyl, believed to arise solely from the solvent. Indeed, when cyclohexane waa heated alone at 460 OC for 1 h small traces of these products were generated; poeeibly other volatile products were also generated, but these were not detected in the GC/MS. When these products were clearly derived from the solvent and not fromthe starting material,they were omitted from the table of results. However,in ambiguouscases where these products could be derived from either cyclohexane or the starting materials, the results were included in the tables. (b) Siskin, M.;Katritzky, A. R.; Balasubramanian,M. Energy Fuels 1991, 5 , 770.
Chemistry of Carbo- and Heterocycles no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75
t& min
0.35 0.36 0.37 0.38 0.46 0.49 0.54 0.75 0.77 0.80 0.86 0.95 0.99 1.03 1.08 1.17 1.27 1.34 1.36 1.65 1.71 1.74 1.83 1.90 2.11 2.14 2.26 2.56 2.66 3.24 3.26 3.33 3.40 3.63 4.33 4.48 4.71 4.96 5.18 5.19 5.25 5.35 5.66 5.80 5.82 5.98 6.26 6.35 6.42 6.47 6.78 7.07 7.36 7.70 7.71 7.80 7.94 8.23 8.66 9.16 9.57 10.59 11.31 11.47 11.66 11.74 11.96 12.04 12.45 12.77 13.14 13.60 15.20 15.94 19.17
Energy & Fuels, Vol. 8,No. 2,1994 489
Table 1. Structure and Identification of Starting Materials and Products structure mol wt equiv w t identification basis 02 82 Table 3 1-methylcyclopentene 78 78 Table 3 benzene cyclohexane 84 84 Table 3 cyclohexene 82 82 Table 3 98 Table 3 98 1-heptane 100 100 Table 3 heptane 92 Table 3 toluene 92 1-octane 112 112 Table 3 112 112 Table 3 ethylcyclohexane 112 112 2-octane Table 3 Table 3 106 106 ethylbenzene 100 100 cyclohexanol Table 3 104 104 styrene Table 3 124 124 1-nonene Table 3 126 126 nonane Table 3 120 120 2-ethyltoluene Table 3 126 126 propylcyclohexane Table 3 120 120 propylbenzene Table 3 140 140 Table 4 butylcyclohexane Table 3 118 118 3-phenyl-1-propene 128 128 Table 3 l-OCtanal Table 2 94 94 phenol Table 3 120 120 1-propenylbenzene Table 3 140 140 1-decene Table 3 140 140 1-methylpropylcyclohexane Table 3 118 118 indan Table 3 134 134 butylbenzene Table 3 132 132 1-methylindane Table 2 130 130 1-octanol Table 4 152 152 1-pentenylcyclohexane Table 3 130 130 1,2-dihydronaphthalene Table 3 132 132 tetralin Table 3 148 148 pentylbenzene Table 2 128 128 naphthalene Table 4 1-hexenylbenzene 160 160 Table 3 162 162 (1-methylpenty1)benzene Table 3 162 162 hexylbenzene Table 3 144 144 1-cyclopentenylbenzene Table 3 83 166 bicyclohexyl 142 Table 3 142 1-methylnaphthalene Table 3 1-cyclohexenylbenzene 158 158 Table 2 160 160 cyclohexylbenzene Table 3 174 174 1-heptenylbenzene Table 3 174 174 2-heptanylbenzene Table 2 146 146 1-tetralone 77 Table 2 154 biphenyl Table 2 170 170 diphenyl ether Table 3 156 156 1-ethylnaphthalene 174 Table 2 174 benzylcyclohexane Table 3 154 1-ethenylnaphthalene 154 Table 2 176 176 cyclohexyl phenyl ether Table 2 168 168 diphenylmethane 172 Table 4 172 1-propylnaphthalene Table 2 168 168 dibenzofuran Table 3 141 242 dioctyl ether 188 Table 3 cyclohexylethylbenzene 188 144 144 Table 2 1-naphthol Table 3 182 182 2-methyldiphenylmethane Table 4 184 184 1-butylnaphthalene 182 91 Table 3 bibenzyl 180 180 Table 3 9,lO-dihydrophenanthrene 178 178 Table 2 phenanthrene Table 3 204 204 1-phenylnaphthalene Table 3 208 208 1-cyclohexen-1-ylnaphthalene 210 Table 2 210 1-cyclohexylnaphthalene 222 222 Table 2 1-benzyltetralin 220 Table 4 l-benzyl-3,4-dihydronaphthalene 220 210 210 Table 4 2-cyclohexylnaphthalene 220 220 Table 2 1-naphthyl phenyl ether Table 2 218 218 1-benzylnaphthalene 224 224 Table 4 1-methyl-2-cyclohexylnaphthalene 240 240 Table 3 1-octylnaphthalene Table 2 268 1-decylnaphthalene 268 254 Table 2 254 1,l’-binaphthyl Table 3 135 1,l’-binaphthyl ether 270
We have now treated these two compounds with aqueous formic acid at 460 OC for 7 min and for 1h to investigate
response facto9 1.00 0.97 0.97 0.97 0.96 0.96 0.96 0.96 0.97 0.96 0.96 0.95 0.96 0.95 0.95 0.95 0.95 0.95 0.94 0.95 0.58 0.79 0.95 0.95 0.95 0.95 0.95 0.95 0.89 0.94 0.95 0.95 0.94 0.95 0.94 0.94 0.94 0.93 0.94 0.95 0.93 0.94 0.93 0.93 0.79 0.94 0.71 0.94 0.93 0.93 0.71 0.93 0.94 0.74 0.72 0.93 0.77 0.93 0.93 0.93 0.93 0.93 0.92 0.92 0.92 0.91 0.91 0.92 0.84 0.91 0.91 0.94 0.90 0.90 0.70
whether they would hydrogenate in a reductive environment. Not surprisingly, little reaction was seen. From
Katritzky et al.
490 Energy &Fuels, Vol. 8, No. 2, 1994
Table 5. Products Obtained from Benzylcyclohexane (49) and Diphenylmethane (52) at 460 OC for 1 h no. 1 2 4 7
11 16 18 34 39 40 42 43 44 46 49 52 58 60
structure/solvent l-methylcyclopentane benzene cyclohexene toluene ethylbenzene 4-ethyltoluene propylbenzene naphthalene bicyclohexyl l-methylnaphthalene cyclohexylbenzene 1-heptenylbenzene 2-heptenylbenzene biphenyl benzylcyclohexane diphenylmethane (2-methy1)diphenylmethane bibenzyl
CeH12
9.8 6.6 36.8 1.6 0.1
benzylcyclohexane (49) H2O 15% HCOzH 15% HCOzNa 0.4 16.6 13.2 12.5 2.3 4.4 1.2 22.0 21.1 39.6 0.5 1.1 0.3
-
0.1
0.9 8.4
1.7
-
-
-
-
2.6 0.7
0.1 0.2
-
-
-
-
2.9
2.3 0.8
-
2.3 1.3
-
-
-
31.5 1.0
35.3 0.8
60.0 0.6
49.4 1.2
-
-
Scheme 1
8 39
the runs with naphthalene (34), after 7 min no conversion was detected and after 1h only 0.5% of tetralin (32) was generated; with phenanthrene (62) after 7 min there was no reaction and after 1 h, 2.0% of 9,lO-dihydrophenanthrene (61) was seen. Just as with biphenyl and binaphthyl, these compounds are resistant to significant reaction under these conditions. Benzylcyclohexane (49) (Table 5, Scheme 1 ) . This compound showed no reaction after 7 min, but significant reaction (40-79%) after 1h in all four media. The major products were similar in all cases, comprising toluene (7, 21.1-39.6% 1, benzene (2,9.8-16.6%), cyclohexene (4,1.26.6%), and l-heptenylbenzene (43, 2.3-2.9%), the latter arising from the ring-opening of the cyclohexyl portion of the substrate. The products, includingthe minor products, clearly result from radical reactions. Benzylcyclohexane (49) should readily homolyze to radicals 100 and 101, the former stabilized by the benzylic effect and the latter a secondary alkyl radical known to form even from cyclohexane at 460 "C (see ref 13a). It is well-known in the literature that the a-bonds of an aliphatic substituent of an aromatic compound are relatively fragile a t 400 "C(with respect to the carbon-carbon bonds of the aromatic nucleus and the remaining carbon-carbon bonds of the alkyl side chains).lPn Radical 100givestoluene (7). The cyclohexyl (14) Leigh, C. H.; Szwarc, M. J. Chem. Phys. 1952,20, 403. (15) Mushrush, G. W.; Hazlett, R. N. Ind. Eng. Chem. Fundam. 1984, 23,288.
-
-
-
-
0.7 -
-
4.6 2.2
-
5.6
-
2.0
-
1.0
-
-
diphenylmethane (52) 15% HCOzH 15% HCOzNa
0.2 3.7
-
0.9
C&2
-
radical 101 undergoes further reactions involving attack on the cyclohexane solvent to give bicyclohexyl (39) and loss of a hydrogen atom to give cyclohexene (4). The benzylic radical 102 should also be formed in quantity under these conditions in accordance with the free-radical chain reaction mechanism advanced for the pyrolysis of long-chain a l k y l b e n ~ e n e s . l 5 *Radical ~ ~ ~ ~ ~102 can ring-open to 103 which gives l-heptenylbenzene (43) by the take-up of a hydrogen atom. We also speculate that benzylcyclohexane (49) loses a hydrogen radical to give 104 which ring-opens to give intermediate 105which in turn gives 2-heptenylbenzene (44). 2-Heptenylbenzene (44) can give rise to the allylic radical 106 which then rearranges through a six-membered cyclic transition state to 107, and we suggest the further intermediate 108 to give naphthalene (34), from the elimination of propyl radical 109, as shown. Similarly, 108 could eliminate hydrogen to give 53which generates l-methylnaphthalene (40) from the loss of an ethyl radical 110. Diphenylmethane (62)(Table 5, Scheme 1 ) . Klein has reported that diphenylmethane was unaffected by water at 405 "C for 1h.23 We have now found this compound to be quite unreactive at 460 "C for 7 min in all four media. Over 1h, no change occurred in water and small conversions (ca.8.3-9.3%) were seen in aqueous 15% formic acid and aqueous 15% sodium formate to give mainly toluene (7, 2.0-2.2% ) and benzene (2,4.6-5.6 % ) probably from direct cleavage. Minor amounts of bibenzyl(60), from a radical reaction, and 2-methyldiphenylmethane (58) were also detected. Heating for 1 h in cyclohexane gave a 4.9% conversion to toluene (7, 0.7% 1, cyclohexylbenzene (42, 2.5%) and biphenyl (46, 1.7%). It is evident that some homolytic scission to phenyl 111 and benzyl 100 radicals occurs under these reaction conditions. However, the thermal process is slow due to the strength of the phenylalkyl bond. (16) Barton, B. D.; Stein, S. E. J. Phys. Chem. 1980,84,2141. (17) Savage, P. E.; Jacobs, G. E.; Javanmardian, M. Ind. Eng. Chem. Res. 1989, 28, 645. (18) Crowne, C . W. P.; Grigulis, V. J.; Throssel, J. J. Trans. Faraday SOC.1969,65,1051. (19) Billaud, F.;Chaverot,P.;Bertheh, M.;Freund, E. I d . Eng. Chem. Res. 1988,27, 1529. (20) Blouri, B.; Hamdan, F.; Herault,D. Znd. Eng. Chem. Process Des. Dev. 1986,24,30. (21) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987,26, 374. (22) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987,26,488. (23) Townsend, S. H.; Abraham, M. A.; Huppert, G. L.; Klein, M.T.; Paspek,S.C. Ind. Eng. Chem. Res. 1988,27, 143.
Energy & Fuels, Vol. 8, No. 2, 1994 491
Chemistry of Carbo- and Heterocycles
Table 6. Products Obtained from 1-Beneyltetralin (66) and 1-Beneylnaphthalene (70) at 460 OC benzyltetralin (66) no. 2
7 11 28
31 32 34 42 57 63 65 66 67 70
solvent etructure/time (min) benzene toluene ethylbenzene 1-methyliidane 1,2-dihydronaphthalene tetralin naphthalene cyclohexylbenzene 1-naphthol 1-phenylnaphthalene 1-cycohexylnaphthalene 1-benzyltetralii
C&lz . 7 ~~
52.5 0.2 2.3
-
29.9 7.5
0.1
-
0.1 0.5 1.8
l-benzyl-3,4-dihydronaphthalene
-
1-benzylnaphthalene
4.5
HzO 7 0.4 49.4
15% HCOZH
-
-
1.0 0.4 29.0 12.3
3.3
66
0.1 51.3
-
33.7 7.2
15% HCOzNa 7 1.9 52.0 0.6 2.2
-
-
0.5
0.6 0.1
-
1.4
1.3
0.6 4.5
- -
1-Benzyltetralin (66) (Table 6, Scheme 2). This compound was highly reactive with >98% conversions at 460 "C after only 7 min in all four media. The major thermal products are similar in all cases, including toluene (7,49.4-52.5%), 1-methylindane (28, 1.0-3.3%1, tetralin (32,29.0-33.7%), naphthalene (34,7.2-12.3% 1, and l-benzylnaphthalene (70, 2.64.5 % 1. 1-Benzyltetralin (66) reacts more rapidly than the other hydrocarbons examined presumably because it can undergo homolysis into two resonance stabilized radicals 100 and 112. The former gives toluene and the latter explains the formation of the observed produds 1-methylindane (28),1,2-dihydronaphthalene (31), tetralin (32), naphthalene (341, and l-cy-
-
2.6
15% HCOZH
15% HCOzNa
29.9 7.3
0.8 0.2
-
C6H12
-
-
Scheme 2
-
I
1-benzylnaphthalene (70)
-
1.1 0.4 3.9
clohexylnaphthalene (65), as shown. Hydrogen loss, as opposed to homolysis,would give radical 114which should give rise to l-benzyl-3,4-dihydronaphthalene(67) and 1-benzylnaphthalene (70). The sequence 114-118 is a possible explanation for the formation of l-phenylnaphthalene (63). It is interesting to note that tetralin has been shown to undergo aquathermolysis at 450 "C for 48 h.24 Tetralin (32) was completely consumed, about half of which formed naphthalene (34) along with indan (26) and 1-methylindane (28), while the other half underwent ring rupture to produce ethylbenzene (ll),toluene (7), and benzene (2). 1 -Benzylnaphthalene (70) (Table 6, Scheme 2). The results from this diarylmethane paralleled those of diphenylmethane (52). Even over 1h, it was completely inert to water alone. Heating in aqueous formic acid and aqueous sodium formate gave small (6-8 75 ) conversions to toluene (7) and naphthalene (34) whereas heating in cyclohexane gave a 17.7% conversion to toluene (7,9.3 75 1, cyclohexylbenzene (42,2.1% 1, and l-cyclohexylnaphthalene (65, 6.3%) generated from the solvent. 1-Benzylnaphthalene (70) as the starting material reacted much slower than 1-benzyltetralin (661, as seen above. This is to be expected, because the naphthalene radical (119) is less stable than the tetralin radical (112). However, some homolysis was observed to give a mixture of toluene (7) and naphthalene (34). Hexylbenzene (37)(Table 7,Scheme3). There has been previous little aquathermolysis work reported on alkylaromatics. We have previously reported that l-decylbenzene was unreactive a t 250 "C in either water or an aqueous sodium sulfite/bisulfite mixture.2s However, Houser found that ethylbenzene (11) underwent slight reaction in supercritical water a t 450 O C after 48 h, to give toluene (7) and benzene (2) as products.24 We now report that hexylbenzene (37) underwent moderate conversion in all four media within 7 min. Typical products included toluene (7, 3.3-7.3951, ethylbenzene (11,1.3-3.3%) andstyrene (13,2.8-7.0%). After 1h, a much greater conversion and more complex product slates were observed. Increased amounts of toluene (7, 22.3-38.7%), ethylbenzene (11,31.7-48.8% 1, and styrene (13, 2.5-11.1%), were formed along with quantities of propylbenzene (18,2.2-5.5% ) and butylbenzene (27,1.42.0% 1. (24) Houser, T. J.; Tiffany, D. M.;Li, Z.; M c C d e , M. E.; Houghton, M.E. Fuel 1986,65,827. (25) Katritzky, A. R.; Murugan, R.; Siskin, M. Energy Fuels 1990,4, 531.
492 Energy & Fuels, Vol. 8, No. 2, 1994
Kutritzky et al.
Table 7. Products Obtained from Hexylbenzeme (37) and Cyclohexylbenmsne (42) at 460 OC hexylbenzene (37) no. 2 6
7 9 11 13 17
18 19
20 23 25 27 30 33 34 35 36 37 38 39 41 42 46 56
solvent structure/time (min) benzene heptane toluene ethylcyclohexane ethylbenzene styrene propylcyclohexane propylbenzene butylcyclohexane 3-phenyl-l-propene l-propenylbenzene l-methylpropylcyclohexane butylbenzene l-pentenylcyclohexane pentylbenzene naphthalene l-hexenylbenzene (l-methylpenty1)benzene hexylbenzene l-cyclopentenylbenzene bicyclohexyl l-cyclohexenylbenzene cyclohexylbenzene biphenyl cyclohexylethylbenzene
CeHiz 7 60 2.0 7.3 36.3 7.8 1.8 2.8 11.1
-
0.6 0.4 -
7 4.4
-
1.3 7.0
-
-
2.2 4.8
2.1 1.6 1.7 2.3 2.4
1.1 76.3 13.8 4.2 6.6
H20
10.8
15% HCO2H 60 7 60 1.1 38.7 3.3 31.7 2.7 2.5 3.6
cyclohexylbenzene (42) 15% HCO2Na CeHI2 H 2 0 16% HCOzH 15% HCOlNa 7 60 60 60 60 60 5.8 0.6 -
-
-
-
26.0
-
5.5
-
-
39.3 3.0
2.8
-
-
-
0.3 0.9 -
5.5 0.5 4.5 2.0
0.6 0.3 -
5.1 0.3 6.0 1.4 -
-
-
86.1 -
12.5
-
-
1.0
-
-
-
-
-
-
89.5
-
-
-
-
-
The literature provides several accounts of the pyrolysis ~ J ~ ~ ~ ~ ~and ~~ of side-chain a l k y l a r o m a t i ~ s . ~ ~ JMushrush Hazlett reported that the neat pyrolysis of l-pentadecylbenzene at 450 "C for 0.25 h led to significant formation oftoluene (7),ethylbenzene (ll), andstyrene (13).15Klein also found similar results.22 Other work was done on butylbenzene (27)by Leigh and Szwarc14and by Barton and Stein;le on propylbenzene (18)by Crowne.18 These investigations revealed that the major primary product from the pyrolysis of an alkylbenzene with an aliphatic chain containing N carbon atoms were toluene plus an a-olefin with N - 1 carbon atoms and styrene plus an n-alkane with N - 2 carbon atoms. The appearance of these as major product pairs was found to be consistent with the relevant bond e n e r g i e ~ . ' ~Furthermore, *~~ Smith has reported toluene to be resistant to neat pyrolysis at temperatures as high as 900 0C,27and this accounts for its steady buildup during these reactions. Concerted retroene,16 and four-centered molecular schemes,20were all offered as possible reaction mechanisms, but Savage and Klein demonstrated that the pyrolyses were entirely via free-radical chains.21$22 The present results are interpreted along similar lines and due entirely to radical reactions (Scheme 3). Hexylbenzene (37)should readily give the benzylic radical (120)which is expected to proceed by chain fission to form styrene (13);subsequent reduction of (13)would yield ethylbenzene (11). Also, the stabilized radical 120 can give rise to l-hexenylbenzene (35). Aliphatic chains on aromatic nuclei can form radicals at any -CHz- group even at 400 OC and chain fission to olefins f 0 l l 0 ~ . ~The 9 formation in this way of radicals 121,122,and 123 accounts for the formation of the observed minor products 9,11,18, 20,23,27,and 33. The formation of toluene (7)from the benzyl stabilized radical 100 is also a favored process. However, this route cannot account for the amount of toluene (7)generated. We speculate that toluene (7)is (26) Allara, D. L.; Shaw, R. J. Phys. Chem. Ref. Data 1980, 9, 523. (27) Smith, R. D. Combust. Flame 1979,35, 179.
1.6
-
-
-
-
-
-
-
0.4 4.7
-
11.1 -
-
3.1 69.8 0.4 -
-
-
-
13.1 -
-
-
8.4 3.3 87.3 1.0
-
Scheme 4
n
-
6- 8-8 .H'
129
133
41
IrH
rH.
also derived from the thermal cracking of the generated ethyl- (ll),propyl- (181,butyl- (27),and pentyl- (33) benzenes. Volatile hydrocarbon products, resulting from the side-chain fragmentation process, are lost as gases upon opening the reaction vessel. Cyclohexylbenzene (42) (Table 7, Scheme 4). This compound (42)was unreactive at 460 "C for 7 min in all four media. In the aqueous systems, after 1 h similar conversions (12.7-15.6% 1 were observed and all the product slates were also similar. In cyclohexane after 1 h, an increased conversion (30.25% ) was observed. The major products in all cases included l-hexenylbenzene (35, 1.6-4.7% 1, l-cyclopentenylbenzene (38,7.8-11.1%), and l-cyclohexenylbenzene (41,2.5-3.3 %). Again, the slate is explained entirely by thermal pathways (see Scheme 4). Cyclohexylbenzene (42) is expected to give up hydrogen to give cyclohexenylbenzene (41). The formation of the tertiary benzylic radical 129 is favored and can be expected to give rise to l-cyclopentenylbenzene (38),possibly uiu intermediates 133 and 134. Cyclohexylbenzene (42)also directly cleaves to 11 1 and
Energy & Fuels, Vol. 8, No. 2, 1994 493
Chemistry of Carbo- and Heterocycles
Table 8. Products Obtained from 1-Cyclohexylnaphthalene(65) at 460 O C no. 4 9
17 19
32 34 39 40 48 53 63 64 65 68
71
cyclohexane 7 60 0.1 3.8 1.2 3.0 1.4 2.7 0.1 1.2 4.1 0.1 0.1 2.6 6.0 2.8 99.8 63.5 1.9 5.6
solvent structurehime (min) cyclohexene ethylcyclohexane propylcyclohexane butylcyclohexane tetralin naphthalene bicyclohexyl 1-methylnaphthalene 1-ethylnaphthalene 1-propylnaphthalene 1-phenylnaphthalene 1-(cyclohexen-1-y1)naphthalene 1-cyclohexylnaphthalene 2-cyclohexylnaphthalene 1-methyl-2-cyclohexylnaphthalene
Scheme 5
n
A
0 0
I
~
I
0.
& 33
101. The cyclohexyl radical 101 is presumably involved in the generation of bicyclohexyl (39) and cyclohexene (4). The benzene radical 111 gives benzene (2). The generation of ethylbenzene (11) and toluene (7) in cyclohexane, probably results from ethyl- and methyl- radicals (from cracking of the solvent) attacking benzene (2). The secondary radicals 130 and 131 formed at the methylene groups in the cyclohexyl ring probably account for the generation of the minor products 11, 13, and 35. 1-Cyclohexylnaphthalene (65) (Table 8, Scheme 5 ) . 1-Cyclohexylnaphthalene (65) underwent very low conversions (50.3% ) in all four media after 7 min to give only cyclohexene (4) and naphthalene (34). When the three aqueous systems were run for 1h, conversion increased and they each gave similar product slates. In water, there
H2O 7 0.1
60 3.5
HCOzH (15%) 7 60 0.2 7.6
-
-
-
-
-
0.1
2.7 3.7
-
-
-
-
-
-
-
-
-
-
0.1
2.8 7.1
0.1
4.9 12.6
-
-
0.7
-
-
-
2.0
-
99.8
84.0
-
2.6 0.8
HCOZNa (15%) 7 60 0.2 6.1
99.7
-
-
-
-
1.2 2.2 76.4 2.7
-
-
99.7 -
-
2.2 0.9 73.3
-
was 16.0% conversion, in aqueous formic acid, 23.6% conversion and in aqueous sodium formate, a 26.7% conversion. The major products were small quantities of cyclohexene (4,3.5-7.6%), tetralin (32,2.7-4.9%), naphthalene (34, 3.7-12.6%), 1-phenylnaphthalene (63, 1.22.2 % ),and 1-(cyclohexen-1-yllnaphthalene(64,0.9-2.2 % 1. In the cyclohexane system a higher conversion (36.5%) was observed. A larger product slate was observed, with 1-phenylnaphthalene (63) (6.0 5% 1, and 1-methyl-2-cyclohexylnaphthalene (71,5.6%)as the major producta. Trace amounts of 1-methylnaphthalene (40, 0.1 % 1, l-ethylnaphthalene (48, 0.1 %), and 1-propylnaphthalene (53, 2.6%) were also detected. Our postulated mechanisms for the formation of these products consist entirely of thermal radical reactions. 1-Decylnaphthalene (73) (Table 9, Scheme 6 ) . Conversions of 6.9-12.7 % in all four media were achieved over 7 min and of 61.6-93.9% in the aqueous systems over 1 h. From the aqueous runs the major products obtained were 1-methylnaphthalene (40, 18.4-42.8% ) and l-ethylnaphthalene (48, 13.8-29.8% ) and various aliphatic hydrocarbons, including 1-nonene (14, 2.9-6.696 ) and 1-octene (8, 7.2-9.5%), were detected in lesser amounts. In cyclohexane after 1 h, a 93.6% conversion was seen giving a long product slate. The major products again included 1-methylnaphthalene (40,26.2 5%1, l-ethylnaphthalene (48,7.6%), 1-octene (8,10.8%) and 1-nonene (14, 2.3%) along with the products ethylcyclohexane (91, propylcyclohexane (17), and bicyclohexyl(39); the latter three products are all presumably derived from solvent participation. We postulate mechanisms for the formation of these products which follow from those seen for hexylbenzene (37) (Scheme 3), and which involve radical cleavage reactions as the key steps. 1-Decylnaphthalene (73) would be expected to cleave to the nonyl radical 138 and the stabilized radical 139. Radical 138eliminates a hydrogen radical to form 1-nonene (14) and radical 139 abstracts a hydrogen radical to give 1-methylnaphthalene (40). The generation of the stabilized radical 140 is also favored, which would be expected to eliminate an octyl radical 141 which eliminates a hydrogen to give 1-octene (81, to yield l-ethenylnaphthalene (50), which hydrogenates to 1-ethylnaphthalene (48). The secondary radicals 142 and 145, formed on the alkyl side chain, account for the generation of the small quantity of 1-propylnaphthalene (53) and 1-butylnaphthalene (59). Some cleavage of the strong naphthyl-CHz bond of 73 is also observed to give naphthalene (34) and 1-decene (24). Oxygen-ContainingCompounds. 1 -Naphthyl Phen-
Katritzky et al.
494 Energy &Fuels, Vol. 8, No. 2, 1994 Table 9. Products Obtained from l-Decylnaphthalene (73) at 460 O C no. 5 6 7
8 9 14
15 17 24 26 34 39 40 48 SO 53 59 72 73
solvent structure/time (min) l-heptane heptane toluene l-octene ethylcyclohexane l-nonene nonane propylcyclohexane 1-decene indane naphthalene bicyclohexyl 1-methylnaphthalene 1-ethylnaphthalene 1-ethenylnaphthalene l-propylnaphthalene l-butylnaphthalene l-octylnaphthalene l-decylnaphthalene
HzO
cyclohexane 60 1.8 1.0 5.4 10.8 0.2 9.5 2.3 1.9 1.4 2.5 7.9 1.0 3.4 1.9 9.7 4.4 26.2 0.3 7.6 0.2 2.6 0.5 0.2 1.2 1.9 87.8 6.4
HCOzNa (15%) 7 60
HCOzH (15%) I 60
60
7
I
-
-
-
0.2
3.7 2.1
-
-
-
1.4
-
1.2
9.5
-
-
2.6
-
2.9
-
6.6
-
0.7
-
-
-
-
0.7
-
-
5.3
-
4.9 2.3 1.3
42.8 29.8 3.0
-
-
-
-
4.6
5.4 8.8 4.1 1.5
2.2
2.0
24.0 18.7
18.4 13.8
-
-
1.0
0.8 0.7
-
0.6
87.3
6.1
27.6
38.4
-
-
Table 10. Products Obtained from l-Naphthyl Phenyl Ether (69), l-Naphthol (67), and l-Tetralone (45) at 460 O C l-naphththyl phenyl ether (69) I-naphthol (57) l-tetralone (45) no. 7 11
18 22 23
26 28 32 34
39 45 57 65
69 75
15% 15% solvent C&IlZ HZ0 HCOzH HCO2Na structure/time(min) 60 60 60 60 toluene ethylbenzene propylbenzene phenol l-propenylbenzene indan 1-methylindane tetralin naphthalene bicyclohexyl 1-tetralone 1-naphthol l-cyclohexylnaphthalene l-naphthyl phenyl ether 1,l'-binaphthyl ether
Scheme 7
15% NazCos 7 60 3.6
-
yl Ether (69) (Table 10, Scheme 7). Our previous work
demonstrated that l-naphthylphenyl ether (69)undergoes virtually complete reaction in water, 15% aqueous formic acid, and 155% aqueous phosphoric acid during 3 days at 315 "C; l-naphthol(57) and phenol (22) were formed in high yields.13b A lower conversion (24.6%) was seen in 15% aqueous sodium formate, but once again phenol (22) and l-naphthol(57) were the major products; however, in this medium there were also significant amounts of reduction products naphthalene (34) and dihydronaph-
15% CaHlz H2O HCOZH 60 60 60
-
3.1 18.6 1.3
-
-
6.9
7.4
-
-
-
-
3.5
-
42.8 20.2
-
-
-
-
84.1 -
19.6
15.9
7.6 65.4
-
-
21.4 15.5 63.1
-
-
15% 15% 15% NaZCOs HCOnNa NaZCOs 60 60 60 12.1 17.0 2.3 2.1 10.2 3.2 9.1 16.1 17.1 0.9 3.9
15%
HCozNa 7 60
1.1 -
-
-
-
3.0
-
1.1
0.6
-
3.0
-
13.9 49.1
14.2
-
3.4 38.8
-
8.5
1.4
24.9 7.3
25.6 17.2
-
0.3
-
2.8 0.6
-
-
-
-
-
35.9
51.0
0.5
1.8
thalene (31). Reducing the reaction time to 2 h led to lower conversionsin both 15% formic acid and 15% sodium formate (36.6% and 4.5%, respectively), with product slates similar to those for the longer runs. l-Naphthyl phenyl ether (69) underwent a 91.6% conversion in 15% H3P04 in only 0.5 h at 315 "C. It is noteworthy that the presence of calcium carbonate inhibited the cleavage reaction of l-naphthyl phenyl ether. Also, Penninger and Kolmschate have demonstrated that 2-methoxynaphthalene (as an example of an alkoxynaphthalene) undergoes hydrolysis a t 390 "C to give 2-naphthol and methanol as the main products.28 In the present work, although l-naphthyl phenyl ether (69)was unchanged after 7 min at 460 "C in all four systems, there were significant conversions (21.3 5% in water, 24.6 5% in aqueous formic acid, 25.6 % in aqueous sodium formate, and 12.15% in cyclohexane)when the reactions were allowed to continue for 1h. Water caused hydrolysis to a mixture of phenol (22, 11.3%)and l-naphthol (57, 10.0%). The formic acid and formate treatments also gave mainly these two products, but the inherent reducing properties of the media were reflected in the production of small amounts of naphthalene (34). In marked contrast to the case of diphenyl ether (47) (discussed later), the small variation (28) Penninger, J. M. L.; Kolmschate, J. M. M. Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J. M. L., EdB.;ACS Symp. Series No. 406;American Chemical Society: Washington,DC, 1989, p 242.
Chemistry of Carbo- and Heterocycles of the hydrolysis rates with pH (cf. the reaction which occurs in aqueous sodium carbonate) probably indicates that the cleavage of the 1-naphthyl phenyl ether (69) involvesboth acid and base catalysis: either the protonated form of 1-naphthyl phenyl ether is attacked by water or the ether (69) is attacked directly by hydroxide to generate 1-naphthol (57) and phenol (22).'3b Phenol (22) does not undergo any further reaction (we have confirmed this by experiments in which neither aqueous sodium formate nor aqueous formic acid reacted with phenol (22) a t 460 "Cover 1h). However, 1-naphthol(57)doesundergosome reduction by formic acid or sodium formate to generate naphthalene (34) and other products (see Scheme 7 and the following section). The significantly increased rate of acid-catalyzed hydrolysis of 1-naphthyl phenyl ether (69) over diphenyl ether (47) (only base-catalyzed hydrolysis) is probably due to the easier protonation of the naphthalene ring as compared to the benzene ring. Evidence for this comes from independent 'H NMR studies which have demonstrated that 1-naphthyl phenyl ether (69) is deuterated faster at the 2-positionof the naphthalene ring than at the two ortho-positions of the phenyl ring; and in turn, these positions are deuterated at least an order of magnitude faster than at any other position.13b In cyclohexane, a 12.1 % conversion was seen, with the generation of phenol (22,4.8%), naphthalene (34,2.9% ), and 1-cyclohexylnaphthalene(65,4.4 % ). Radicalcleavage may initially give phenoxy and naphthyl radicals, which readily abstract hydrogen radicals from the solvent: the cyclohexyl radicals thus formed then attack the 1-position of the naphthalene ring to give 1-cyclohexylnaphthalene (65) and more phenoxy radicals. 1-Naphthol (57)(Table 10, Scheme 7). Previously, we have demonstrated that at 315 "C for 3 days, although conversion is low (110%) in cyclohexane, water, and aqueous formic acid, in 15% aqueous sodium formate 1-naphthol (57) undergoes a substantial conversion (81.0%).m Product distribution included naphthalene (34) (38.8%), tetralin (32, 23.7%), 1,l'-binaphthyl ether (75, 8-57?,),and 1-methylnaphthalene (40, 2.2% ). We have now found that at 460 "C only the run in aqueous sodium formate showed any change over 7 min, with a 4.2 % conversion into small quantities of 1-tetralone (45, 1.8%) and 1,l'-binaphthyl ether (75, 2.4%). The tetralone (45) arose from a reduction, and the ether (75) from a condensation reaction of two molecules of l-naphthol (57). Extending the reaction time to 1 h, a high conversion (91.5%) was seen in aqueous sodium formate and the products now included 1,l'-binaphthyl ether (75, 35.9%),naphthalene (34,14.2%),and tetralin (32,3.0%), as well as the ring-opened products, propylbenzene (18, 16.1%),ethylbenzene (11,g a l % ) ,and toluene (7,12.1%). Areaction of 1-naphthol (57) in aqueous sodium carbonate at 460 O C for 1 h gave a 98.6% conversion. The major products included 1,l'-binaphthyl ether (75,51.0%) along with toluene (7, 17.0%),ethylbenzene (11, 10.2%) and propylbenzene (18, 17.1%). In formic acid after 1 h, a 36.9% conversion was seen with the generation of naphthalene (34,21.4%) and tetralone (45,15.5%). In water, a 15.9% conversion was seen with 1,l'-binaphthyl ether (75) as the sole product. Cyclohexane gave a 34.6% conversion into 1,l'-binaphthyl ether (75, 19.6%) and 1-tetralone (45, 7.6% ) as major products. (29)Siskin, M.;Brons, G.;Vaughn, S. N.; Katritzky, A. R.; Balasubramanian, M.;Greenhill, J. V. Unpublished results.
Energy & Fuels, Vol. 8, No. 2,1994 496 Scheme 8
156
22
The reaction mechanisms postulated are shown in Scheme 7, and many have been discussed previously.lsb In previous papers we have observed the conversion of alcohols into ethers in water at high temperature.mlm For example, benzyl ether was readily generated from benzyl alcohol in water at 250 OC.3O This is now shown to occur also for at least some phenols under supercritical conditions. The simultaneous formation of 1-tetralone and naphthalene in aqueous formic acid indicates reduction. The formation of toluene (7) and ethylbenzene (11) presumably from propylbenzene (18) follows from the above discussions about radical cleavage of the alkyl chains (see section on hexylbenzene, 37). These products were only seen in the basic media. Propylbenzene (18)probably results from the base-induced ring-opening of the l-naphthol tautomer 152 to give 154 uia 153, which undergoes subsequent decarboxylation to give 23 and reduction to give 18; the results in the next section indicate that 1-tetralone'isnot the main precursor. In cyclohexane, the generation of 1-tetralone (45) indicates some thermal reduction of 1-naphthol by the solvent. 1 -Tetralone (45) (Table 10,Scheme 7). This compound was run in aqueous sodium formate and aqueous sodium carbonate solutions for 1h a t 460 OC to compare the results with those obtained with 1-naphthol (57) in these media. In the sodium formate, 1-tetralone (45)was highly reactive with a 75.1 % conversion giving mainly naphthalene (34, 49.1%)and tetralin (32,13.9%)alongwithsmallamounts of 1-methylindane (28, 1.1%) and propylbenzene (18, 0.9% ). In aqueous sodium carbonate, a similar conversion (74.4%)gave mainly naphthalene (34,38.8%) and l-naphthol (57, 17.2%). Less tetralin (32, 3.4%) was observed in this solvent than in aqueous sodium formate attesting to the reductive properties of the latter solvent. An increase in the amount of propylbenzene (18,3.9 % ) and the presence of 1-propenylbenzene (23, 2.8%) and ethylbenzene (11,3.2%)indicates that the base-induced ringopening reaction of 1-tetralone (45) occurs more readily in aqueous 15% sodium carbonate (stronger base) than in aqueous 15% sodium formate solution. Diphenyl Ether (47) and Dibenzofuran (54) (Scheme 8). Previous work in our laboratories has shown that diphenyl ether (47) at 315 OC for 3 days gives a 92% conversion to phenol (22) in 15% phosphoric acid and a 6.6% conversion, also to phenol (221, in 15% aqueous sodium formate.lSb Diphenyl ether (47) was unreactive in cyclohexane and water at 250 OC or 315 OC for 3 days and gave very low conversions in aqueous sodium bisulfite/ sodium sulfite, aqueous 10% phosphoric acid a t 250 OC for 3 days, and in 15% aqueous formic acid at 315 "C for 3 days.31 Klein observed no change on treatment of diphenyl ether (47) and dibenzofuran (54) in supercritical water (at 405 and 500 OC,respectively),for 1h.= Phenethyl phenyl ether was readily hydrolyzed at 374-400 O C to give mainly phenethyl alcohol, phenol (221, and styrene (13). Similarly,benzyl phenyl ether was also shown to hydrolyze readily a t 332 OC to give benzyl alcohol and phenol (22).23 (30) Katritzky, A. R.; Balasubramanian,M.;Siskin, M.Energy Fuels 1990,4, 499. (31) Katritzky, A. R.; Murugan, R.; Balasubramanian,.M.;Siskin, M. Energy Fuekr 1990,4,543.
Katritzky et al.
496 Energy & Fuels, Vol. 8, No. 2, 1994 Table 11. Products Obtained from Cyclohexyl Phenyl Ether (81) at 460 O C 15%
no. 4 1 12 22 39 42 51
15%
15%
solvent cyclohexane H20 HCO2H HCOlNa NazCOs structure/time (min) I I I I I cyclohexene 49.3 1-methylcyclo53.9 52.4 55.8 54.0 pentane cyclohexanol 1.9 1.8 0.6 phenol 46.8 44.2 45.8 43.0 46.0 bicyclohexyl 1.2 cyclohexyl0.6 benzene cyclohexyl 2.1 phenyl ether
Scheme 9 12
I
I+.,
OH
39
I2
We have now confirmed that diphenyl ether (47) is remarkably unreactive even at 460 "C. No change was observed in any of the four media tested after 7 min and after 1 h the only conversion was 7.0% into phenol (22) in the aqueous sodium formate. However, a run in aqueous sodium carbonate (15%) at 460 "C for 1 h gave a 32.9% hydrolysis into phenol (22). We speculate that the reaction is a simple nucleophilic attack of hydroxide anion on (47) to give intermediate 156,with subsequent loss of phenolate. All attempts at reacting diphenyl ether (47) with aqueous sodium hydroxide (15%) at 460 "C failed due to the corrosion of the reaction vessel seal and the subsequent escape of the contents. Dibenzofuran (54) showed no reaction under any of the conditions used at 460 "C, even after 1 h. Cyclohexyl Phenyl Ether (61) (Table 1 1 , Scheme 9). Our previous studies at 250 "C for 5.5 days showed that cyclohexyl phenyl ether (51), under both thermolysis (in nonane, 8.9% conversion) and aquathermolysis (15.1% conversion), formed 1-methycyclopentene (1) (0.8 and 0.6%, respectively) and phenol (22) (5.6 and 12.096, respectively) as major products.32 It was also reported that both thermolysis and aquathermolysis reactions were very much accelerated in the presence of an acidic clay (calcium montmorillonite) but that in the presence of a weak base, calcium carbonate, the reaction was strongly inhibited. It is clear that, at 250 "C this ether underwent cleavage faster under aqueous reaction conditions than under thermal conditions. We also found that higher temperatures, (343 "C for 2 h) increased conversion of (32) Siskin, M.; Brons, G.; Katritzky, A. R.; Murugan, R. Energy Fuels 1990, 4, 482.
cyclohexyl phenyl ether (51) to 67% in water. The fact that only 39.5 % conversion was obtained thermally again clearly illustrated the dominance of aqueous ionic pathways over thermal radical pathways. The products indicate that, at 343 "C, cyclohexyl phenyl ether (51) underwent ionic reaction in water and that the water acted as an acid catalyst producing 1-methylcyclopentene (1, 35.9 % ) and phenol (22,28.8 % ) as the major products and cyclohexylbenzene (42, 0.9 % ) and 2-cyclohexylphenol (1.0%) as minor products.33 In the present work at 460 "C, cyclohexyl phenyl ether (51) was completely converted in each of the media within 7 min. In the three aqueous systems, phenol (22) and 1-methylcyclopentene (1) were again the major products along with a small quantity of cyclohexanol (12). A separate run in 15% aqueous sodium carbonate also gave a complete conversion to phenol (22) and l-methylcyclopentene (1). In cyclohexane, a 97.3 5% conversion was seen to give phenol (22) and cyclohexene (4) as products. Some bicyclohexyl(39) was also formed from either the solvent or the substrate or a combination of both. The hydrolysis of cyclohexyl phenyl ether (51) occurs readily in the aqueous systems to give phenol (22)and a cyclohexylcation 159, which undergoes a rearrangement to the tertiary 1-methylcyclopentylcation 160, which subsequently eliminates a hydride to give 1-methylcyclopentene (1). However, as this compound is readily cleaved in cyclohexane, we cannot rule out the possibility of radical mechanisms in the aqueous phase as well. In the thermal system, we would expect a homolysis of 51 initially to give phenoxy radical 158 and cyclohexyl radical 101 which would lead to the generation of phenol (22) and cyclohexene (4), respectively. The mechanisms for the aquathermolysis and thermolysis of (51) are given in Scheme 9 and have been discussed previ0usly.3~ The preceding results extend earlier studies carried out on alkyl and aryl ethers. Thus, Townsend and Klein observed rapid hydrolysis of dibenzyl ether in water at 374-412 0C:34the benzyl alcohol produced gave small amounts of toluene and benzaldehyde as secondary products. Tsao and Houser found that benzyl alcohol at 400 "C (for 3 h) gave substantial amounts of toluene (7) and benzyltoluenes and lesser quantities of benzene and benzaldehdye,while benzaldehyde as starting material (400 "C for 1h) gave benzene (21, toluene (71, benzyl alcohol, and benzoic a ~ i d . 3These ~ results are similar to those we had obtained at 250 "Cover 5 days,3O so there is probably no change in mechanism here. However, benzoic acid in supercritical water (400 "C for 3 h) was reported to undergo almost complete decarboxylati~n,~~ while at 250 "C we detected no change after 5 days.30 1-Octanol(29) (Table 12, Scheme 10). 1-Octanol(29) showed relatively low reactivity (513.6% in cyclohexane, water, and aqueous formic acid after 7 min a t 460 "C,but a 26.2 % conversion was seen in aqueous sodium formate. Similar product slates were seen in all four cases with the generation of 1-heptene (5, 1.3-8.1 % 1, 1-octene (8, 0.72.6%), and octanal (21, 2.6-14.2%). Surprisingly, a significant amount (14.2%) of octanal(21) was generated (33) Siskin, M.; Brons, G.; Vaughn, S. N.; Katritzky, A. R.; Balasubramanian, M. Energy Fuels 1990,4, 488. (34) Townsend, S. H.; Klein, M. T. Fuel 1985, 64,635. (35) Tsao, C. C.; Houser, T. J. Prep. Pap.-Am. Chem. SOC.,Diu. Fuel
Chem. 1990,35,442. (36) Mueumarra, G.; Pismo, D.; Katritzky, A. R.; Lapucha, A. R.; Luxem, F. J.; Murugan, R.; Siskin, M.; Brons, G. Tetrahedron Comput. Method. 1989,2, 17.
Chemistry of Carbo- and Heterocycles Table 12. Products Obtained from 1-Octanol (29) at 460 O C
no. 6 8 10 21 29 56
15% 15% solvent cvclohexane HoO HC09H HCOvNa structure/time (min) 7 7 7 7 l-heptene 2.3 1.3 6.1 8.1 l-octene 1.1 0.7 2.6 1.5 2-octene 1.9 octanal 3.7 2.6 3.0 14.2 l-o~tanol 92.9 95.4 86.4 73.8 dioctyl ether 2.4 S c h e m e 10
in aqueous sodium formate, suggesting an oxidation reaction, as well as a small quantity (2.4% ) of dioctyl ether (55) derived from a base-catalyzed condensation of octanol (29). l-Octene (8) is generated from an acid-catalyzed dehydration.
Conclusions In this paper, the reactivities of a series of benzenoid hydrocarbons and oxygen-containing compounds in supercritical aqueous and thermal media are discussed. The aryl hydrocarbons biphenyl, 1,l'-binaphthyl, naphthalene, and phenanthrene were essentially unreactive at 460 "C. l-Benzylnaphthalene showed only a slow thermolysis to toluene and naphthalene whereas l-benzyltetralin underwent rapid homolysis in both aqueous and nonaqueous environments after short reaction times (7 min). Similarly, diphenylmethane was essentially inert under all reaction conditions but benzylcyclohexane underwent significant reaction after 1 h at 460 "C. l-Cyclohexylnaphthalene was more reactive than cyclohexylbenzene, although a somewhat similar trend in reaction mechanisms was
Energy & Fuels, Vol. 8, No. 2, 1994 497 observed. l-Decylnaphthalene and hexylbenzene were similarly reactive and showed parallel trends in reaction pathways. The conversionrates for the n-alkyl-substituted aromatic rings were noticeably higher than those for the cyclohexyl-substituted aromatics. Nearly all the reaction pathways for the hydrocarbons are radicaloid, and qualitatively the product slates are usually similar for all media. However, the quantitative composition of the products varies with the differing ability of the medium to intervene in radical reactions, either by healing radical chains or by starting them. Cyclohexane can clearly act as a radical source itself and many products are found containing a cyclohexyl group. Aqueous formate and formic acid are both more active than just water. The only reaction observed for diphenyl ether was a slow base-catalyzed hydrolysis. After 1 h at 460 "C in 159% sodium formate, this conversion was still only 7.0 7%. The mechanism was supported by a run in 15% aqueous sodium carbonate which gave an increase in conversion (32.9%)to phenol. Dibenzofuran showed no change in any medium, including aqueous sodium carbonate. For l-naphthyl phenyl ether conversion rates were higher in the aquathermolyses than the thermolysis reaction. There is evidence that the hydrolysis is both acid- and basecatalyzed. Cyclohexyl phenyl ether also showed radical cleavage in the nonaqueous medium and presumably hydrolysis in the aqueous solutions. l-Naphthol gave mainly 1,l'-binaphthyl ether in cyclohexane and water, but in the formate solutions reductions were strongly in evidence. 1-Octanolgave l-heptene, l-octene, and octanal in all four systems. The cyclohexane run undoubtedly gave radical reactions, but the aqueous solutions possibly induced both radical and ionic reactions. The reactions studied in this work showed quantitative rather than qualitative differences from previous reactions in subcritical water.293 There was no clear evidence from these reactions of hydrocarbons and oxygen-containing compounds for any major change of mechanism between subcritical and supercritical conditions, except that a t 460 "C there is an obvious increase in the involvement of radical pathways. Supplementary Material Available: Mass spectral assignmenta of the structures and Tables 2,3, and 4 listing mass spectral fragmentation patterns (9 pages). Ordering information is given on any masthead page.
