55
Ind. Eng. Chem. Fundam. 1985, 2 4 , 55-60 T a b l e V. F r e e Energy of C o n v e r s i o n C a l c u l a t i o n H y d r o x i d e to the O l e o p h i l i c P h a s e
of
AGdbt'
Fe A1 cu
Zn Ni Ag
log Kba -37.4 -32.7 -19.0 -16.4 -13.8 -7.7
log KPb 15.4 30.0 19.4 18.1 15.7 10.9
kJ/mol +125.0 +15.4 -2.3 -9.7 -18.2 -18.2
"From Banks (1956). bFrom DuRietz (1965). Estimated from this work: 2.303RT log (KbKp).
V provides AGdb or a number of metals. Negative values of AGdb predict the oleate soap to be stable. Hence, Ag, Ni, Zn, and Cu would appear in that order as decreasingly oleophilic materials. Under oxidizing conditions, A1 and Fe would be hydrophilic. Summary A modified Wilhelmy technique has been used to assess the surface tension at the o i ~ ~ c o p p e r ~ w ainterface ter as a function of electrochemical Dotential. DH. and concentration of oleic acid. The resilts sugg&t that species in the oil Dhase react with anodicallv formed comer oxides to produce an oleophilic surface. "Organic compounds in the oil have little influence on the oil/metal surface tension
but they dramatically change the metal/water surface tension as a result of chemisorption. The application of a potential across the metal-electrolyte interface can alter the activity of an oleophilic surface compound through a combination of electrochemical and electroosmotic effects. As a result, normally oleophilic copper becomes hydrophilic with the application of a cathodic potential. R e g i s t r y No. H3B03, 10043-35-3; NaClO,, 7601-89-0; Cu, 7440-50-8; oleic acid, 112-80-1.
Literature Cited Adams, R. A. C. Inst. Bull. Bur. Fed. Master Printers 1958, 73, 20. Banks, W. H. Int. Bull. Print. Allled Trades 1956, 73, 15. Briant. J. Rev. Inst. Fr. Pet. 1984, 79(6), 733. Bro, P.; Dey, A. N. AIChE J . 1060, 75(5), 293. Chandler S.; Fuerstenau, D. W. Int. J . of Miner. Process. 1975, 2 , 333. Chmieiewski H. J.; Bailey, A. I. Phil. Mag. 1981, A43(3), 739. Deinega, Yu, F.; Vinogradov, C. V.; Vovnenko, A. M. Kolloid. Zh. 1061, 23, 157. DuRietz, C. "Chemisorption of Collectors" in "Surface Chemistry Proceedings of the Second Scandinavian Symposium on Surface Activity", Ekwail, P.; Groth, K.; RunnstromReio V., Eds.; Academic Press: New York, 1965; p 9.i
L I.
Langmuir, I . Science 1038, 87(2266), 393. Lockwood, F.; Bridger, K.; Tadros, M. E. Preprint No. 83-AM-7C-1, 1983, to be published, A .S.L . E . Trans. Tritton. F. J. J . SOC. Chem. Ind. 1832. 57. T299-T307. Willman, K. W.; Murray, R. W. Anal. Chem. 1983, 55, 1139. Woods, R. J . Phys. Chem. 1971, 75, 354.
Received for review July 25, 1983 Accepted April 30, 1984
Free-Radical and Concerted Reaction Pathways in Dibenzyl Ether Thermolysis Mlchael B. Slmmons and Mlchael T. Klein' Department of Chemical Engineering, Unlversity of Delaware, Newark, Delaware 19716
Thermolyses in fully deuterated tetralin-d 12 probed the operative mechanisms of dibenzyl ether (DBE) pyrolysis. Quantification, by GC-MS and 'H FTNMR, of the incorporation of a deuterium label into the pyrolysis product toluene allowed discrimination between intermolecular and intramolecular reaction mechanisms. Reaction kinetics were essentially independent of the initial molar ratio tetralin to DBE, R , although deuterium incorporation, DZ = PhCH2D/(PhCH2D PhCH3), increased with R . Of three individual reaction mechanisms considered, a fraction M = 0.87 f 0.07 of a composite mechanism could be attributed to a free-radical chain and 1.00 - M = 0.13 to concerted, intramolecular reactions.
+
Introduction Thermolyses of dibenzyl ether (DBE) (Brucker and Kolling, 1965; Cronauer et al., 1979; Kamiya et al., 1980; Schlosberg et al., 1981a) and other simple arylalkyl ethers and alkanes (Benjamin et al., 1978; Schlosberg et al., 198lb; Sweeting and Wilshire, 1962; Poutsma, 1980; Klein and Virk, 1983; Virk, 1979; Eckpenyong and Virk, 1982; Hellyar, 1982) have explored the reaction pathways, kinetics, and mechanisms underlying coal and lignin liquefaction. The relevance of such model reaction pathways to the reactions of moieties within macromolecules could be better ascertained with a more quantitative understanding of the operative pyrolysis mechanisms of the simple models than is presently available. The objective of the work reported herein was quantitative discrimination between the intermolecular and intramolecular reactions occurring during DBE pyrolysis. 0196-4313/85/1024-0055$01.50/0
Previous pyrolyses (Brucker and Kolling, 1965; Cronauer et al., 1979; Kamiya et al., 1980; Schlosberg et al., 1981a) show the major DBE fragmentationproducts to be toluene, benzaldehyde, and benzyl alcohol; products of lesser prevalence include CO, benzene, bibenzyl, biphenyl, and higher molecular weight species. Cronauer et al. (1979) found the reaction of DBE in tetralin to be first order in ether and slightly inhibited by the tetralin; an activation energy of 36.4 kcal/mol described the kinetics over a temperature range of 300-400 "C. The incorporation of deuterium into the reaction products when DBE was pyrolyzed in partially deuterated tetralin (38% available hydrogen) suggested that the overall reaction might involve both intermolecular and intramolecular components. Schlosberg et al. (1981a) interpreted neat DBE pyrolysis in terms of a set of free-radical reactions, although a pericyclic mechanism of the type first noted by Virk (1979) 0 1985 American Chemical Society
56
Ind. Eng. Chem. Fundam., Vol. 24, No. 1,
1985
Table I. ExDerimental Grid reactant setn A
B
C D E F
reactants tetralin (99% purity) DBE (99% purity), tetralin DBE, tetralin-& (99% atom D purity) DBE, toluene (99%) (PhCH3),tetralin-d12 toluene (PhCHB),tetralin-d12 DBE, tetralin-d,,
R , mol of tetralin/mol of reactant
holding time, min 117
5, 20, 60
30, 23, 40 18.2 17.5 (DBE) 20,11 0, 0.2, 0.5, 1.0, 2.0, 4.0, 8.0, 16.0
5.25, 10, 20, 30, 40
20 20, 120 5
"All pyrolyses at T = 400 "C.
in the context of coal liquefaction, was also considered. Finally, each of simple fission, radical chains, and concerted mechanisms have been proposed to explain the thermolyses of related compounds such as benzyl phenyl ether (Schlosberg et al. 1981b; Eckpenyong and Virk, 1982), phenethyl phenyl ether (Klein and Virk, 1983) and diphenylpropane (Sweeting and Wilshire 1962; Poutsma, 1980; Hellyar, 1982). Fully deuterated tetralin-d12 was used in the present examination of the DBE pyrolysis mechanisms illustrated in Figure 1. Quite like that suggested by Schlosberg et al. (1981a), the intermolecular mechanism considered was analogous to the classical Rice-Herzfeld chain, where hydrogen abstraction by the benzyl radical chain carrier results in the formation of the toluene product and a DBE radical which, upon @scission, both yields the benzaldehyde product and also regenerates the chain carrier. The relatively low activation energy of 36.4 kcal/mol determined by Cronauer et al. (1979) suggests that any free-radical reaction would involve a chain, and their observation of first-order kinetics requires that the predominant chain termination step be cross-coupling of benzylic and DBE radicals. Note that since tetralin also provides benzylic hydrogen, DBE pyrolysis in tetralin-d,, should yield both fully protonated (PhCH,) and singly deuterated (PhCH2D)toluene, the fraction of the latter increasing with the proportion of tetralin. The kinetics inhibition due to tetralin observed by Cronauer et al. (1979) can be addressed in a preliminary fashion by considering two limiting cases, namely, that tetralin could efficiently quench or propagate the chain. In contrast, the six-centered retroene (Hellyar, 1982; Klein and Virk, 1983) or the fourcentered Maccoll (Maccoll, 1969; Cronauer et al., 1979; Klein and Virk, 1983) intramolecular reactions of Figure 1 should be independent of tetralin and yield only fully protonated toluene. The foregoing provides the basis for the analysis that follows. Experimental methods, including a useful, simple analysis by 'H Fourier Transform NMR, are described first. The Results section delineates the products and kinetics of DBE pyrolysis, neat, in fully protonated tetralin, and in fully deuterated tetralin-d12,as well as blank experiments designed to examine tetralin pyrolysis and isotope exchange. The discussion is aimed at selection of the mechanisms that accord best with the present experimental results.
Experimental Section The reactants and solvents were available commercially (Aldrich) in the purities listed in Table I and were used as received. The batch pyrolysis reactors were stainless steel tubing bombs, each comprising a Swagelok port connector and two caps and fashioned from either or l/s-in. nominally sized parts. The tubing bombs were immersed in a constant-temperature fluidized sandbath to effect reaction, and products, collected with either methylene chloride or deuterated chloroform (for NMR
a
Figure 1. Possible dibenzyl ether pyrolysis mechanisms: (a) R i c e Herzfeld free-radical chain; (b) 6-centered retroene reaction; (c) 4centered Maccoll reaction.
analysis), were analyzed by GC, GC-MS, and Fourier Transform Proton NMR as follows. Product spectra, separated over a 65-m SE-54 capillary column and detected by flame ionization in an HP 5880 instrument, contained DBE and its fragmentationproducts toluene, benzene, benzaldehyde, and benzyl alcohol as well as tetralin and, in minor proportions, its reaction products including naphthalene, o-xylene, decalins, indans, and seven unidentified products separately determined (see below) to originate from tetralin decomposition. Quantitative product yields were calculated using separately determined response factors. Deuterated products were analyzed by GC-MS and FTNMR, principally the latter. The important GC-MS signals were at 93, 92, and 77 daltons, and assigned to singly deuterated toluene, fully protonated toluene, and a fully protonated phenyl fragment, respectively. Signals at 94 and 78 daltons, which might have indicated the presence of doubly deuterated toluene and a singly deuterated phenyl fragment, respectively, were conspicuously
Ind. Eng. Chem. Fundam., Vol. 24, No. 1, 1985
r
,-0012 lo 0018 ppm
I 00
238
236
234
595
590
585
c
I
I
I
I
I
57
I
I
4
2 3 2 p p m 580 Hz
CHEMICAL SHIFT
0
10
20
30
40
R e a c t i o n Time
0 016 ppm Ph CH3
l
240 600
,
l
238 595
l
- ~ Ph CHz D
l
236 590
l
l
l
l
l
l
234
232
230 ppm
585
580
575 nz
50
60
(min)
Figure 3. Temporal variations of major dibenzylether pyrolysis products.
the equal triplet peak areas being in proportion to the protons contained in PhCH2D methyl groups, nphCH&, the area A, would be in the same proportion to one-third of the PhCH2Dmethyl group protons plus all of the PhCH, methyl protons, nphCH3, i.e.
= c3A2
(1)
= c(A1 - A2)
(2)
nPhCHzD
CHEMICAL S H I F T
Figure 2. Idealized and actual FTNMR response to a mixture of toluene and singly deuterated toluene.
absent. This information indicated that the toluene was, at most, singly deuterated and that the deuterium was located at the benzylic and not a ring carbon. Deuterium incorporation was confirmed and quantified by proton NMR spectroscopy, with a Bruker 250 MHz Fourier transform instrument, by exploiting two effects (Bhacca and Williams, 1964; Ionin and Ershov, 1979; Macdondd et al., 1964; Tiers, 1958) of a deuterium atom on the signal of geminal benzylic protons. First, because the protons couple with the deuterium nucleus, which has three equally probable spin states, the resultant signal is a 1:l:l triplet where the peaks of equal intensity (Bhacca and Williams, 1964; Ionin and Ershov, 1979; Macdonald et al., 1964) are separated by 2.38 Hz (Tiers, 1958), the geminal coupling constant. Second, the deuterium atom also shifts the origin of the triplet peaks, i.e., the center peak, upfield from the fully protonated methyl group singlet peak by 0.012 to 0.018 ppm of the scanning frequency (Macdonald et al., 1964). The foregoing suggested that the response of the 250 MHz instrument to a mixture of PhCH2D and PhCH, would be like the superposition illustrated in Figure 2a. In anticipation of what follows, accounting for both the 0.015 ppm (average) isotope shift and the 2.38 Hz (0.0095 ppm) coupling constant, the most downfield (left-most in Figure 2) of the triplet peaks might overlap with the singlet due to PhCH,; thus of the peaks A,, A2, and A, shown in Figure 2a, A2 and A, would be of equal area and owe to the presence of PhCH2D, and Al would be of greater area on account of the presence of both PhCH, and PhCH2D. An actual FTNMR response to the present pyrolysis product spectra is shown in Figure 2b. In a general case three peaks were integrated, two of which (A2 and A,) were closely equal in area and third (A,) always greater than either of the other two. When the extent of deuterium incorporation was low (see below), only the predominant PhCH, singlet peak (A,) was integrated. Values of the isotopic shift and geminal coupling constant, 0.016 ppm and 2.45 Hz, respectively, accorded well with values reported elsewhere (Macdonald et al., 1964; Tiers, 1958). The fraction of deuterated toluene, DI = PhCH2D/ (PhCH2D + PhCH,), was estimated from the integrated peak areas A, and A2 (or A,) as follows. With the sum of
and nPhCH,
where c is some arbitrary NMR response factor to the benzylic protons that are twice and three times found on PhCH2D and PhCH,, respectively. Thus, on a molecular or molar basis = c3A2/2
(3)
= c(A1 - A2)/3
(4)
NPhCH2D
and NPhCH3
Equations 3 and 4 permit calculation of DI as DI = 9A2/(7A2 2A1)
+
(5)
Since actual integrated values of A2 and A, varied slightly, the average of two values of DI,separately determined using A2 and A,, respectively, is reported in the results that follow. Further details are available (Simmons, 1983).
Results DBE pyrolysis was probed through the experiments listed in Table I and described below. A single background pyrolysis of tetralin at 400 OC for 117 min resulted in less than 3% conversion to naphthalene, decalins, indans, o-xylene, toluene, benzene, and seven unidentified very minor products. Observed toluene and benzene product mole fractions of 4 X and 2 X respectively, were coupled with information from previous tetralin pyrolyses (Bredael and Vinh, 1979; Hooper et al., 1979; Benjamin et al., 1979; Ratto et al., 1980) to assure that any background tetralin decomposition would not interfere with the analysis of DBE pyrolysis. Pyrolysis of DBE in fully protonated tetralin at 400 OC yielded toluene, benzaldehyde, benzyl alcohol, and benzene as DBE fragmentation products and the other minor products of tetralin decomposition just noted. The temporal variations of the yields of DBE, toluene, benzaldehyde, benzyl alcohol, and benzene, shown in Figure 3, suggest the primary DBE pyrolysis pathway to be fragmentation to equimolar proportions of toluene and benzaldehyde, with the latter primary product capable of secondary reaction to benzyl alcohol and benzene. An apparent first-order rate constant of 7.8 X s-l was
58
Ind. Eng. Chem. Fundam., Vol. 24, No. 1, 1985
Table 11. Evaluation of Possible Limiting Reaction Mechanisms in Light of Experimental Observations" experimental observations 011 positive deuterium as pyrolysis rate limiting reaction R' independent of R mechanism considered incorporation, DI X d 1. unimolecular (UNI) only X 2. free-radical (FR) only t t' X (a) chain quenched by tetralin (b) chain propagated by tetralin t/ d t/ 3. fraction M FR, fraction (1 - M) UNI V (a) chain quenched by tetralin (b) chain propagated by tetralin \
possible reaction mechanism? no Yes no Yes yes no Yes
v' indicates that limiting reaction mechanism is consistent with the experimental observation; x indicates that limiting reaction mechanism is inconsistent with the experimental observation. (I
independent of R (initial moles of tetralin to initial moles of DBE) over a range R = 23 to 40, indicating that the pyrolysis kinetics were essentially unaffected by tetralin dilution in the range studied. Cronauer et al. (1979) reported both Arrhenius parameters that predict k = 4.8 X s-l at 400 "C and also a slight reduction in the rate of DBE decomposition as its concentration in tetralin dropped from 50 to 10%. These apparent discrepancies may well be attributed to differences in the reactors used and their associated heat-up times, but are, in any event, not of determining consequence in the analysis to follow. This is because the relative proportions of singly deuterated and fully protonated toluene are of interest and not the absolute rate of toluene formation. GC analyses of the product spectra obtained from DBE pyrolyses in tetralin-d,, at R = 18.2 and from t = 5 to 40 min showed toluene, benzaldehyde, benzyl alcohol, and benzene to be the only significant products formed; the addition of toluene to the reactant mixtures revealed no new information. Kinetics results indicated a first-order rate constant of k = 12.3 x 10"' s-l that was unaffected by the addition of toluene and is in closer agreement to the value due to Cronauer et al. (1979) noted above. The fate of the isotope label was determined by FTNMR analysis. The separate "blank" experiments E of Table I showed exchange, or scrambling, of hydrogen between toluene and tetralin-dlz to be negligible. In contrast, reaction of DBE in tetralin-dlz resulted in the deuterium incorporation into the toluene that was apparent by the splitting of the toluene benzylic proton signal; proton incorporation into the deuterated tetralin at, predominantly, a positions, was also observed. The fraction DI of the toluene that contained the singly deuterated methyl groups was estimated as described above, and as illustrated in Figure 4, DI 0.86, essentially independent of conversion for R = 18.2. The influence of dilution level on D I was examined through a set of pyrolyses, each for 5 min at 400 OC, where R ranged from 0 to 16. For experiments where R I 2, the FTNMR integrated only a single methyl group peak because of the predominance of fully protonated toluene. In all other cases DI increased with increasing values of R . The plot of D I vs. 1/R in Figure 5 allows a simple linear extrapolation of DI to conditions of infinite dilution (1/R = 0) and suggests that DI 0.87 f 0.07 as R a. The points in Figure 5 are averages of the two values of DI calculated from eq 5 using A2 and As, respectively, which individually provide the indicated limits of uncertainty on the least-squares estimate of DI at 1 / R = 0. Discussion The present discussion is aimed at deduction of the operative DBE pyrolysis mechanisms and is organized in terms of the matrix Table 11, where the individual mechanisms and combinations thereof head the rows and are
-
-
loo
I
I
I
l
1
I
I
050
075
1
I
R=18
0
9 1 0
025
I00
Froctionol Converslon, X
Figure 4. Influence of dibenzyl ether conversion on deuterium incorporation.
--
----
I
i
: 025L 0
01
0
I/R
02
( mols DBE/mols tetralin)
Figure 5. Extrapolation of deuterium incorporation to conditions of infinite dibenzyl ether dilution in tetralin.
considered in the light of the experimental observations that head the columns. The likely mechanisms are divided into three major categories: (1)fully concerted, (2) the fully free-radical mechanism of Figure 1,and (3) a combination of a fraction M free-radical and (1.00 - M) concerted mechanisms. The concerted reaction may be either the pericyclic retroene or the Maccoll elimination. In the former, the rate-determining step is fragmentation to isotoluene and benzaldehyde through a coiled six-centered transition state; the isotoluene quickly undergoes a hydrogen shift to form toluene. The rate-limiting reaction would be unimolecular and thus occur with first-order kinetics, independent of tetralin concentration, and yield only fully protonated toluene. Likewise, in the Maccoll(l969) mechanism where DBE eliminates toluene and benzaldehyde through a four-centered transition state, fully protonated toluene should appear with first-order kinetics independent of tetralin. Termination of the free-radical chain of Figure 1 and Table I1 involving the benzyl radical and the ether radical
Ind. Eng. Chem. Fundam., Vol. 24,
would require the reaction to be first order in DBE. The influence of tetralin on reaction kinetics can be examined in two limiting cases. In the first, motivated in part by the inhibition observed by Cronauer et al. (1979), the tetralin could intercept and quench the propagation cycle; at infinite dilution the rate of reaction would be reduced to that of the initiation step. In the second extreme, the tetralin would intercept the propagation but in turn act as an efficient chain carrier through hydrogen abstraction from DBE. It is noteworthy, however, that in either case, and strictly independent of the kinetics, singly deuterated toluene (PhCH,D) would result in proportions that should increase with the available deuterated tetralin. The combination of concerted and free-radical mechanisms indicated in Table I1 encompasses two possibilities in order to account for the influence of tetralin on the propagation cycle of the radical chain. Note that in a simple case where the proportions of both components of a composite mechanism are unaffected by the tetralin dilution level, a constant fraction M , independent of R, would occur by the free-radical mechanism and 1.00 - M by concerted mechanisms. Three pertinent experimental observations head the columns in Table 11. First, and quite simply, singly deuterated toluene was observed from pyrolysis of DBE in tetralin-dI2. Second, DI increased with R and, third, the reaction rate was insensitive to R. With regard to the entries of Table 11, a "v"' in a particular row and column indicates that the mechanistic possibility heading the row is consistent with the experimental observation heading the column; an "x" indicates that the mechanistic possibility has been eliminated because it is inconsistent with the experimental observation. In sum, a plausible mechanism must be consistent with each experimental observation. The discussion to follow will examine the implications of each experimental observation in turn. A positive DI requires an interaction between the tetralin and either DBE or one of its fragmentation intermediates, e.g., a benzyl radical. On this basis the singly operative concerted (or M = 0) mechanistic possibility was eliminated, which is indicated in Table I1 by the x in the 1,l position. Possibilities involving free-radical reactions are consistent with DI > 0, and this is indicated by the v " s in the appropriate rows of Table I1 and column 1. The second relevant experimental observation of Table I1 concerns the increase of DI to high values (0.87 f 0.07) as 1/R 0. The fully concerted mechanism possibility is clearly inconsistent with this observation. With regard to the radical chain, the initial selectivity S of hydrogen abstraction by the benzyl radical from deuterated tetralin relative to that from DBE would be proportional to R since S = {kd[PhCH2-][tetralin])/{k2[PhCH2~][DBE]J = (k,'/k2)R (6) The singly operative free-radical mechanisms 2a and 2b accord well with this observation, especially in light of the uncertainty inherent in experiments and the linear extrapolation to 1/R 0. In mechanism 3a, where both a quenched free-radical chain and a concerted reaction are operative, the rate of the free-radical component would diminish in the approach 0, and the concerted reto the initiation rate as 1/R action would thus, by difference, selectively predominate. The corresponding behavior of DI could be complicated. When R = 0, the free-radical reaction rate would be at its maximum value but DI would be identically zero since no deuterium would be present. The addition of tetralin-dI2 would simultaneously introduce deuterium, which would
-
-
-
No. 1, 1985 59
tend to increase DI,but also inhibit the radical chain; the increase in selectivity of the concerted reaction, which generates only fully protonated toluene, would tend to suppress DI. Depending on the details of the quenching, either a maximum in DI might occur at a balance between the availability of deuterium and the kinetic selectivity of the free-radical to the concerted reaction, or an asymptotic upper-limit of DI might be attained that would correspond to the rate of DBE initiation divided by the rate of DBE initiation plus the rate of the concerted reactions. Although DI increased to high values (-0.87) and the implied predominant mechanism of simple bond fission seems unlikely, the mechanistic possibility could be strictly consistent with this particular experimental observation in Table 11. Finally, with regard to the second experimental observation of Table 11, DI would increase with R if the freeradical component of a composite mechanism were efficiently propagated by tetralin. The final pertinent experimental observation in Table I1 is that the pyrolysis reaction was substantially independent of R; a previous DBE pyrolysis (Cronauer et al., 1979) suggests the kinetics to be slightly diminished with increases in R. The concerted reactions and the efficiently propagated free-radical chain are in substantial accord, whereas the tetralin-quenched free-radical chain mechanism is in conflict with this experimental result. The rate of a reaction occurring via combination 3a, i.e., the quenched free-radical mechanism and concerted mechanisms, would decrease as R until the overall rate was the rate of the concerted reactions plus the rate of the initiation step of the chain. This could be qualitatively consistent with the experimental observation if the concerted reactions were predominant, but since DI 0.87 as 1/R 0, mechanistic possibility 3a was eliminated. Neither of the free-radical nor concerted components of mechanism 3b would be kinetically sensitive to R, and so this composite mechanism was considered possible. The operative mechanism must be consistent with each of the experimental observations of Table 11, inspection of which shows reaction mechanisns 2b and 3b to be possible. A decisive selection of the operative mechanism requires further analysis of, at least, the kinetic isotope effect and influence of tetralin on the pyrolysis kinetics. On the basis of the present results, however, mechanism 3b, more general than 2b and in fact containing possibility 2b with a limiting value of M , is a good preliminary selection for the operative DBE pyrolysis mechanism. These conclusions are in substantial accord with the suggestion of Schlosberg et al. (1981a). Since the deduced composite mechanism contains free-radical and unimolecular components that are substantially unaffected by dilution in tetralin, it follows that, at a given temperature, M should be constant and, in particular, independent of R. The ratio of the moles of toluene (NTol) formed by the radical pathway to those by the unimolecular pathway, or
-
0)
-
-
NF3/NYJ1 = M / (1 - M) (7) allows M to be derived from the dependence of DI on R (or S ) , since
NF3 = NphCH2D + N&H, = @F~H,(S-k 1) Nk#l = Nk?l%H3
(8)
(9)
and NPhCHs
= NIlhNdH, + N!kH3
(10)
where the superscripts FR and UNI refer to the free-rad-
60
Ind. Eng. Chem. Fundam., Vol. 24, No. 1, 1985
ical and unimolecular pathways, respectively. Equations 8-10 indicate that toluene formed by the free-radical mechanism can be either fully protonated or singly deuterated, while only fully protonated toluene results from unimolecular reactions. Rearrangement of eq 7-10 shows N r f c ~ , / N k @ ~ ,= M/[(1 - M)(1 - S ) ]
(11)
Ni = moles of i PhCHzD = singly deuterated toluene R = Ntetrdin/NDBE = ratio of initial mol of tetralin to mol of
DBE S = (k,’/k,)R = initial hydrogen abstraction selectivity
Subscripts 0 = initial conditions
To1 = toluene
and
DI = MS/(1
+ S)
(12)
Thus, DI approaches M as R, and hence S, approaches large values. The linear extrapolation of DI vs. 1/R to 1/R = 0 in Figure 5 suggests M = 0.87 f 0.07. Further investigation is required for discrimination between the unimolecular mechanisms. Summary and Conclusions Important conclusions dervied from this work follow. 1. The mechanism of dibenzylether (DBE) thermolysis in fully deuterated tetralin was investigated by quantifying the incorporation of a deuterium label into the pyrolysis product toluene. The use of a simple FTNMR technique that exploited the influence of a deuterium atom on the proton NMR signal of geminal benzylic protons is noted. 2. Reaction kinetics were substantially insensitive to the molar ratio tetralin to DBE, R, although the deuterium incorporation, DZ = PhCH,D/(PhCH2D PhCH3), increased with R such that, by simple linear extrapolation from a plot of DZ vs. 1/R, DZ 0.87 f 0.07 as 1/R 0. 3. Of three individual reaction mechanisms considered, a fraction M = 0.87 f 0.07, independent of R, of a composite mechanism, was attributed to a free-radical chain and 1.00 - M = 0.13 was attributed to concerted intramolecular reactions. Acknowledgment This work was supported by the National Science Foundation under Grant CPE-820440. The authors are grateful to Susan H. Townsend, Margaret O’Malley, Karyn Thompson, and Roger Crecely for technical collaboration. Nomenclature D I = N p h c H 2 D / ( N p h c H 2 D + N P h c H 3 ) = extent of deuterium incorporation n, = benzylic protons of i
-
+
-
Superscripts FR = free radical UNI = unimolecular Registry No. 108-88-3.
DBE, 103-50-4; tetralin, 119-64-2; toluene,
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Received for reuiew October 6 , 1983 Accepted July 19, 1984
