12
Ind. Eng. Chem. Fundam.
1985, 2 4 , 12-15
Thermal Decomposition of Benzyl Phenyl Ether and Benzyl Phenyl Ketone in the Presence of Tetralin Yoshlkl Sato' and Toshlo Yamakawa Coal and Carbon Division, National Research Institute for Pollution and Resources, Yatabe, Ibaraki 305, Japan
Rates of decomposition of benzyl phenyl ether and benzyl phenyl ketone have been measured in liquid phase over the temperature range 260-440 ' C in the presence of a large excess of Tetralin as a hydrogen donating agent. The decompositions obey the first-order rate law with the values of activation energy and A factor being 53.2 kcal/mol and s-l for benzyl phenyl ether and 60.1 kcal/mol and s-' for benzyl phenyl ketone, respectlvely. The kinetic parameters as well as product distributions can be interpreted by a consecutive mechanism in which the dissociation of the central C-0 or C-C bond contrds overall rate. The resonance stabilization energies of phenoxy and benzoyl radicals in conformity with the proposed mechanism are 10-17 and 13 kcal/mol, respectively. The effect of various solvents or additives on the decomposition has also been discussed.
Introduction Liquefaction of coal by the use of hydrogen donating agents such as Tetralin has been known as a solvent refining coal method. It has been reckoned, in this reaction, that radicals initially formed upon thermal dissociation of the linkages connecting fused aromatic clusters are effectively stabilized by hydrogen atoms supplied from the solvent and that Tetralin, in which naphthenic hydrogens are activated by an adjacent aromatic ring, is sufficiently reactive to minimize the possible occurrence of secondary recombinations leading to a coke formation. For further clarification of the mechanistic details of this reaction, we chose bibenzyl as a model compound and observed the thermal decomposition in the presence of Tetralin in both liquid and gas phases (Sato et al., 1978). The products consisted mainly of toluene, naphthalene, and hydrogen in amounta consistent with the stoichiometry
and the decomposition is found to obey the first-order rate law with respect to bibenzyl itself. The observed values of activation energy and A factor of 61 kcal/mol and l O I 4 s-l in both phases has been supported by Poutsma (1980) and Stein (1980) and has been in accord with the values measured in the gas-phase reaction by Stein et al. (1982) and estimated by Benson and O"eal(l970) and McMillen and Golden (1982). However, kinetic parameters obtained in the liquid phase reaction reported by Stein et al. (1982) are greater than ours. This difference is considered to be due to "cage" effects in the liquid phase. The following sequence of consecutive reactions has been proposed to account for the overall fiist reaction order, as well as the values of Arrheius kinetic parameters.
In terms of the proposed scheme and assuming reaction 2 to be a rate-controlling step, the resonance stabilization energy of benzyl radical has been estimated to be ap0198-43131851l024-OOl2$0l.5O/O
proximately 13 kcal/mol, in good agreement with 0.72 /3 deduced from the simple Hiickel calculation (Sato, 1979). The result not only provides a fundamental background for the solvent refined coal method but also indicates that the Tetralin carrier technique may be promising for measuring dissociation energy. The present work, as an extension of ow previous study, deals with the thermolysis of benzyl phenyl ether and benzyl phenyl ketone in the presence of a variety of hydrogen donating solvents to accumulate information as to the behavior of oxygen-containing aromatic compounds in coal liquefaction. The kinetics of oxygen-containingcompounds and related radicals have been discussed with interest by Cronauer et al. (1979), Schlosberg et al. (1981), Stein (1981), and McMillen and Golden (1982), but the reaction mechanism has not been established in detail. Experimental Section Commercial reagent grade Tetralin, 97.4 mol % purity, was used without further purification. Impurities consisted mainly of cis- and trans-decalins, naphthalene, and ethylbenzene. Commercial reagent grade benzyl phenyl ether, benzyl phenyl ketone, bibenzyl, and phenol were used as received. The liquid-phase reaction was carried out under pressures of 50-80 kg/cm2.G of nitrogen in a high-pressure 200-mL autoclave equipped with a stirrer and auxiliary devices for charging of reactants and sampling of products. In each run of the experiments, approximately 80% of solvent Tetralin and 20 kg/cm2.G of nitrogen were introduced into the autoclave and the autoclave was heated to the reaction temperature. Then, the decomposition was started by charging the mixture of reactant with the remaining Tetralin, which was warmed to around 200 "C, into the autoclave by higher pressure of nitrogen. About 5 mL of the products was collected periodically using a special sampling device and subjected to a gas chromatographic analysis. Initial Tetralin to reactant molar ratio was determined in the same manner by a gas chromatographic analysis for the sample collected at the reaction time of zero. Details of the apparatus and procedures were described in our previous work (Sato et al., 1978). The experimental conditions were as follows: temperature range, 260-320 "C and Tetralin to reactant molar ratio of 15-25 for benzyl phenyl ether and 380-440 "C and molar ratio of about 15 for benzyl phenyl ketone. Liquid products were subjected to a conventional gas chromato0 1985 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 24, No. 1, 1985
Table I. Composition of Liquid Products for the Decomposition of Benzyl Phenyl Ether at 300 "C reaction time, min 0 30 60 Tetralin/benzyl 15.30 15.30 15.30 phenyl ether molar ratio 1.22 2.43 conversion of benzyl phenyl ether mol 70 composition of liquid products, mol 70 toluene 0.08 0.15 phenol 0.04 0.12 Tetralin 93.28 93.20 93.26 naphthalene 0.46 0.44 0.44 benzyl phenyl ether 6.10 6.06 5.84 byproductsa 0.03 others 0.17 0.18 0.17
450
320
15.03 24.96 15.58 24.66 25.57
250
C'
3.88
0.23 0.23 93.30 0.43 5.42 0.22 0.18
Table 11. Rate Constants for the Decomposition of Benzyl Phenyl Ether in the Presence of Tetralin Tetralin/ benzyl phenyl ether reaction reaction molar ratio, rate mol/mol constant, s-' temp, OC time, min 480 180 90 90 90
300
350
90 15.30
1.3
Diphenyl ketone, diphenylmethanol, p- and o-benzylphenol.
260 280 300
400
13
1.4
1.5
1.8
l/T X I d
1.7
1.8
1.8
(l/'K)
Figure 1. Arrhenius plots of the first-order rate constants.
P "
0.230 X 10" 1.42 7.58 8.06" 363
a*3%.
graphic analysis using Apiezon Grease L (2 m) and Silicone SE-30 (2 and 5 m) packed columns and an Apizon Grease L capillary (45 m) column. Mass spectrometry was used for identification purposes. Rate constants were calculated by a least-squares method. Total material balance was more than 99 w t % in both experimental runs within our reaction conditions. Analytical data by gas chromatography for the products were checked by using 1-methylnaphthalene as an internal reference. Error limits for the analyses of Tetralin, benzyl phenyl ether, and benzyl phenyl ketone have been estimated within &3%. Results Benzyl Phenyl Ether. The liquid products consisted mainly of toluene, phenol, and naphthalene with trace amounts of diphenyl ketone, diphenylmethanol,and p - and o-benzylphenol. Very small amounts of hydrogen and carbon dioxide were detected in the gaseous product. The results obtained at 300 "C and Tetralin to benzyl phenyl ether molar ratio of about 15 are listed in Table I. The production of phenol and toluene appears to be almost the same at relatively lower levels of conversion but byproducts, such as isomers of benzyl phenyl ether, become larger in amount than that of toluene and phenol under more severe reaction conditions. Bibenzyl and benzyldihydronaphthalene were not produced in any runs of the experiments. It suggests that benzyl and phenoxy radicals, which are produced from primary cleavage at the central C-0 bond in benzyl phenyl ether, reaction 5, are consumed partly by the possible secondary reactions from byproducts. a C - O Q
+
Decomposition rates of benzyl phenyl ether to produce toluene and phenol were evaluated with the data at a low conversion level. The decomposition obeys the first-order rate law with the values of activation energy and A factor
01
0
60
120
R e a c t i o n time
180
(min)
Figure 2. Products distribution for the liquid-phase decomposition of benzyl phenyl ketone in the presence of Tetralin.
being 53.2 kcal/mol and 10'5.2s-l, respectively. The results of the rate measurements are summarized in Table I1 and the Arrhenius plot of the first-order rate constants is given in Figure 1. Benzyl Phenyl Ketone. The liquid products from the decomposition consisted mainly of toluene, benzaldehyde, naphthalene, and 1-methylindane with small amounts of n-butylbenzene, benzene, and bibenzyl. The gaseous products consisted mainly of almost equivalent amounts of hydrogen and carbon monoxide to naphthalene and benzene produced by decomposition, respectively, with trace amounts of methane and carbon dioxide. The liquid products' distribution at 420 "C and Tetralin to benzyl phenyl ketone molar ratio of 15 is shown in Figure 2. 1-Methylindane and n-butylbenzene are assumed to be derived from Tetralin but not from benzyl phenyl ketone, as mentioned in our previous work and as reported on the pyrolysis of Tetralin by Nowak et al. (1975) and Penninger et al. (1973). The yields of toluene, bibenzyl, and benzene increase and, contrary to these, benzaldehyde decrease with a lapse of reaction time. It was found by an additional experiment on the decomposition of benzaldehyde at 420 "C in the presence of 15 parts of Tetralin that benzene and a trace of toluene were produced but bibenzyl was not detected. Benzyl alcohol was not detected in the products from the decomposition of benzyl phenyl ketone, as well as of benzaldehyde. Therefore, it has been shown that the central C-C bond in benzyl phenyl ketone cleaves to
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Ind. Eng. Chem. Fundam., Vol. 24, No. 1, 1985
Table 111. Rate Constant for the Decomposition of Benzyl Phenyl Ketone in t h e Presence of Tetralin Tetralin/ benzyl phenyl ketone rate reaction molar ratio, reaction constant, s-l time, min mol/mol temp, "C 0.610 X 10+ 380 360 14.94 15.01 2.76 400 360 7.24 420 180 15.25 24.4 440 90 15.03
,.
/
420
/
produce toluene and benzaldehyde in equivalent amounts in the amounts in the first-stage decomposition (reaction 6) followed by the hydrogen transfer from Tetralin. After 60
120
180
240
Reaction time
that benzaldehyde successively converts to mainly benzene with small amounts of toluene. Bibenzyl has been considered to be produced from a recombination of benzyl radicals, but there are some uncertainties on the formation of bibenzyl at a higher temperature of 380-440 "C. Conversions of the decomposition calculated by a decrease of benzyl phenyl ketone indicate the validity of the overall first reaction order as illustrated in Figure 3. The values of the rate constants listed in Table I11 are again fitted into the Arrhenius formulation which is described together with the result for benzyl phenyl ether in Figure 1. The values of activation energy and A factor have consequently been estimated to be 60.1 kcal/mol and 10'3.8 S-1.
Discussion Kinetics. Kinetic parameters for the decomposition of benzyl phenyl ether and benzyl phenyl ketone into the corresponding radicals are summarized in Table IV with the relevant values reported by several authors. The values of the A factor for the dissociation of the central C-0 as well as C-C bonds are in accord with the related reports to each bond, respectively. Evidently they indicate that the dissociation of these bonds takes place as the ratecontrolling role in the analogous consecutive scheme proposed for bibenzyl by us. The activation energy of 53.2 kcal/mol for the decomposition of benzyl phenyl ether was Table IV.
----f
C,H,OC,H, C,H,OC,H, C,H,OCH,
380
(mln)
Figure 3. First-order plots for the liquid-phase decomposition of benzyl phenyl ketone in the presence of Tetralin.
compared with the normal C-0 bond dissociation energy of 76-83 kcal/mol in aliphatic ethers reported by Kerr (1966), McMillen and Golden (1982), Pacey (19751, and Aronowitz and Naegeli (1977). The difference between these was 20-30 kcal/mol, being reasonably accounted for by the sum of the resonance stabilization energies of benzyl and phenoxy radicals. Using the known value of the benzyl resonance energy of 13 kcal/mol, the phenoxy resonance energy has been deduced as being 10-17 kcal/mol. Colussi et al. (1977) reported 17.5 kcal/mol for the stabilization energy of phenoxy radical. On the other hand, activation energy and the A factor for benzyl phenyl ketone obtained in this study are lower in value than those estimated by Stein (1981). Our estimation of the activation energy on the basis of an additivity rule proposed by Benson (1976) also reveals a higher value of 68 kcal/mol. It seems to be necessary to examine in detail the experimental procedure and kinetic analysis. The value of activation energy for benzyl phenyl ketone calculated in conformity to a first-order rate law is nearly the same as that for bibenzyl. Effect of Solvent on the Decomposition. The effect of various solvents on the rate constant has been investigated for the decomposition of benzyl phenyl ether at 300 O C and the reaction time of 90 min. The first-order rate
Activation Energies and A Factors for the Decomposition act. energy, kcalimol
reactions CH,OCH,
300
CH,O. t CH;
----f
---f
--f
C,H,CH,OC,H,
C,H,O,
-
C,H;
C,H,O. t C,H;
C,H,CH; --+
+ CH,O.,
C,H,CH;
t C,H,O.
80 83.3 76 76.6 79 82.1 60.4 63.0 65.2 70.5 53.2 50.8 86 61.5 60.4 56.8 62.3 66.81 61.6 82 81.2 60.1 72 ~~
CH,CH, 3 2CH; C,H,CH,CH,C,H,
2C,H,CHZ.
logA,s-'
phase
15.0 15.33
gas gas
15.3
gas
14.5
gas
15.2 14.7 16 14.4 14.8 14.4 15.25 16.58 15.5
13.8 16.0
gas liquid gas gas gas liquid
reference Kerr (1966) McMillen and Golden (1982) Pacey (1975) Aronowitz and Naegeli (1977) Kerr (1966) McMillen and Golden (1982) Colussi et al. (1977) McMillen and Golden (1982) Colussi et al. (1977) McMillen and Golden (1982) this study Stein (1981) Lin and Back (1966) Sat0 et al. (1978) Sat0 et al. (1978) Benson and O'Neal (1970) Stein et a]. (1982) Stein et al. (1982) McMillen et al. (1981) Kerr (1966) McMillen and Golden ( 1 9 8 2 ) this study Stein (1981)
Ind. Eng. Chem. Fundam., Vol. 24, No. 1, 1985
Table V. Effect of Solvent on the Decomposition Rate of Benzyl Phenyl Ether (Reaction Temperature, 300 OC; Reaction Time, 90 min) rate solvent/reactant constant, molar ratio solvent 5-l 25.1 8.06 X 10” Tetralin 6.16 15.8 trans-decalin 8.06 13.2 Tetralin 17.2 cyclohexanol 22.1 8.19 Tetralin 2.1 9,lO-dihydroanthracene 18.9 9.37 Tetralin 11.3 phenol 15.4 13.5 Tetralin 18.1 phenol Table VI. Effect of Solvent on the Decomposition Rate of Benzyl Phenyl Ketone and Bibenzyl (Reaction Temperature, 420 OC; Reaction Time, 180 min) solvent/ reactant rate constant, molar reactant solvent ratio S-1 benzyl phenyl ketone Tetralin 15.2 0.724 X Tetralin 14.6 1.19 phenol 9.7 1.52 Tetralin 15.3 phenol 17.8 bibenzyl 1.11 x 10-6 Tetralin 15.0 1.11 Tetralin 15.0 phenol 9.5 1.23 Tetralin 14.5 phenol 17.4
constants obtained using trans-decalin went down in comparison with the values measured using Tetralin, Tetralin-cyclohexanol, and Tetralin-9,lO-dihydroanthracene systems as shown in Table V. These findings are reasonably accounted for by the difference in the hydrogen donating ability between these solvents. But the rate constants observed by adding phenyl to the reaction system became considerably larger. Similar behavior due to phenol was observed for benzyl phenyl ketone but not for bibenzyl, as shown in Table VI. These present observations cannot be explained by the difference of the hydrogen
15
donating ability, but they can be caused by the ionic character of phenol. Incidentally, it has been reported by several authors (Pott and Broche, 1934; Orchin and Storch, 1948) that higher conversion of coal liquefaction was obtained by adding phenol derivatives to the reaction system for the following reason: the cleavage of the ether bond in coal, which is considered to be the first-step reaction of liquefaction, will be accelerated by the interaction with the OH group in phenol. Acknowledgment The authors wish to acknowledge Professor A. Amano and H. Kameyama of Tohoku University for helpful discussions. Registry No. C6H6CH2OCsH,946-80-5; C6H5COCH2C6H, 451-40-1; CsH&H2., 2154-56-5; C6H50., 2122-46-5; Tetralin, 119-64-2.
Literature Cited Aronowitz. D.; Naegell, D. Int. J . Chem. Klnet. 1977, 9 , 471. Benson, S. W.; O’Neai, H. E. “Kinetic Data on Gas Phase Unimolecuiar Reactions”, No.404; National Bureau of Standards: Washington, DC, 1970.
Benson, S.W. “ThermochemicalKinetics”; Wiley: New York, 1976. Colussi, A. J.; Zabei, F.; Benson, S. W. Int. J . Chem. Klnet. 1977, 9 , 161. Cronauer, D. C.; Jewell, D. M.; Shah, Y. T.; Modi, R. J. Ind. Eng. Chem. Fundam. 1979, 18, 153.
Kerr, J. A. Chem. Rev. 1968. 66,465. Lin, M. C.; Back, M. H. Can. J . Chem. 1966, 4 4 , 2357. McMillen, D. F.; Goiden, D. M. “AnnualReview of Physical Chemistry”; Annual Reviews, Inc.: Menlo Park, CA, 1982. McMillen, D. F.; Ogier, W. C.; Ross, D. S. J . Org. Chem. 1981, 46, 3322. Nowak, S.;Keil, G.; Gunschel, H.; Pechstein, G. WorMfet. Cong., 9th 1975, 19(4), 1.
Orchin, M.; Storch, H. H. I d . Eng. Chem. 1948, 4 0 , 1385. Pacey, P. D. Can. J. Chem. 1975, 53, 2742. Penninger, J. M. L.; Slotboom, H. W. Recl. Trav. Chlm. 1973, 92, 513. Pott, A.; Broche, H. Fuel Sci. Prectlce 1934, 13(4), 125. Poutsme, M. L. Fuel 1960, 59, 335. Sato, Y.; Yamakawa, T.: Onishi, R.; Kameyama, H.; Amano, A. J . Jpn. Pet. Inst. 1978, 21(2), 110. Sato, Y. Fuel 1979, 58, 318. Schlosberg, R. H.; Davis, W. H., Jr.; Ashe, T. R. Fuel 1981, 60, 201. Stein, S. E. Fuel 1960, 59, 900. Stein, S. E.; Robaugh, D. A.; Alfierl, A. D.; Miller, R. E. J . Am. Chem. Soc. 1982. 104. 6587.
Stein, S.’ E. ACS Symp. Ser. 1981, 169
Receiued for reuiew February 8, 1983 Reuised manuscript received February 9, 1984 Accepted April 23, 1984
