Ind. Eng. Chem. Res. 1987,26, 374-376
374
Discrimination between Molecular and Free-Radical Models of 1 -Phenyldodecane Pyrolysis Liquid-phase thermolyses of 1-phenyldodecane (PDD) with added tetralin-d12allowed discrimination between molecular mechanisms and stepwise free-radical mechanisms. The incorporation of a deuterium label into the major pyrolysis product, toluene, proved that free-radical steps were kinetically significant since an intramolecular mechanism would proceed independently of the added tetralin and yield only protonated products. Quantitation of the fraction (DI) of toluene that was singly deuteriated as a function of the ratio (R) tetra1in-dl2/PDD allowed convincing estimation 03. This demonstrated that at infinite dilution of PDD in tetralin-d12,all toluene of DI = 1.0 as R formed with a deuterium label, suggesting that PDD thermolysis was entirely free-radical.
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Both stepwise free-radical and concerted intramolecular reaction mechanisms have been set forth in explanation of the pyrolyses of hydrocarbons. The stepwise free-radical mechanisms involve the elementary steps of radical initiation, hydrogen abstraction, p-scission, and termination and are favored models of the pyrolyses of several highmolecular-weight a,w-diphenylalkanes (Miller and Stein, 1981; Poutsma and Dyer, 1982; Gilbert and Gajewski, 1982). However, a molecular mechanism was considered by Miller (1963) as a component of the gas-phase pyrolysis of a-olefins, and Rebick (1979) later determined that a concerted retro-ene mechanism, which followed the conservation of orbital symmetry rules of Woodward and Hoffmann (1970), accounted for between 5% and 22% of the conversion of 1-dodecene, depending on reaction conditions. Concerted mechanisms have also been considered for the pyrolysis of dibenzyl ether (Cronauer et al., 1979), diphenylethane (Virk, 1979), and phenethyl phenyl ether (Klein and Virk, 1983) and further noted as a possibility for the pyrolysis of n-pentadecylbenzene (Mushrush and Hazlett, 1984). Molecular mechanisms for the liquid-phase pyrolyses of n-hexadecane, 6-methyleicosane, and 1-phenyldodecane (PDD) have been delineated recently by Blouri et al. (1985). These workers found the first-order pyrolysis of 1-phenyldodecane to proceed with Arrhenius parameters [log A (s-l),E* (kcal/mol)] = [10.2,44.4] and yield toluene, styrene, n-decane, and 1-undeceneas major products. The results would be as expected from a free-radical chain. However, the low yields of ethylene and propylene and the high yields of liquid products observed were in conflict with the predictions of the classical Kossiakoff and Rice (1943) theory of free-radical thermal cracking, which led Blouri et al. (1985) to favor molecular mechanisms. These are illustrated in Figure lA, for the thermal cracking of PDD, and proceed through simultaneous intramolecular hydrogen transfer and carbon-carbon bond cleavage in a fourcentered transition state. This mechanistic model and quite reasonable estimates of the reactivities of primary, secondary, and tertiary C-H and C-C bonds allowed the formulation of a mathematical model that accurately predicted the observed product yields. However, the object of this communicationis to show that these kinetics results are not unique to a molecular mechanism and that the pyrolysis in fact proceeds through free-radical steps as illustrated in Figure 1B. Experimental Section Phenyldodecane and tetralin-d,, were both obtained from Aldrich and used as received. Pyrolyses, both neat and in tetralin-d,,, were performed in batch tubing bomb reactors comprising a nominal 1/4-in.stainless-steel Swagelok port connector and two caps. The reactors were each loaded with a measured amount of PDD (20-40 pL) and the amount of tetralin-d,, dictated by the desired molar
ratio (R) of tetralin to reactant. Several blank experiments wherein fully protonated toluene and tetralin-d,, were pyrolyzed allowed quantitation of demonstrably unimportant isotopic scrambling (see Results and Discussion section). Loaded reactors were purged with argon to provide an inert environment for reaction, sealed, and immersed for 30 min in an isothermal fluidized sandbath at 400 "C, which, according to the Arrhenius parameters of Blouri et al. (1985),effected approximately 10% reactant conversion. Vapor-liquid equilibrium calculations showed that for R > 3.0, greater than 90% of the 1-phenyldodecane was present in the liquid phase at reaction conditions. For R = 1.15 and 2.08, the liquid phase contained 78% and 86% of the substrate, respectively. Thus, as 1/R 0, all pyrolyses were completely in the liquid phase. Finally, the reaction was quenched by plunging the reactors into cold water immediately upon removal from the sandbath, and the reactor's contents were then extracted with acetone. Liquid products from the neat pyrolyses were analyzed via a Hewlett-Packard (HP) Model 5880 gas chromatograph equipped with a 50-m SE-54 WCOT capillary column and a flame ionization detector. GC/MS analysis with a HP Model 5890 gas chromatograph coupled to a Model 5970 mass-selective detector provided DI. In one instance, a complementary 13C NMR analysis (O'Malley et al., 1985) allowed a check of accuracy. A separately synthesized sample of singly deuteriated toluene (O'Malley et al., 1985) was quantitated by GC/MS and 13C NMR to have DI = 0.906 and 0.91, respectively.
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Results and Discussion Neat pyrolyses of PDD at 400 "C led to toluene, n-decane, 1-undecene,and styrene as the major products at low conversion, and the product summary in Table I, which lists product molar yields at representative holding times, shows that series of n-alkanes, a-olefins, alkylbenzenes, and phenylolefins were also produced. The temporal variation of the major products' yields is displayed in Figure 2 which shows these also to be primary products. Reactant conversion corresponded to a pseudo-first-order rate constant of O.OOO645 min-l, in good accord with the results of Blouri et al. (1985). Pyrolyses in tetralin-d,, probed the mechanism of the foregoing reaction. A radical chain carrier, such as pl, P,, or p3 in Figure lB, could abstract either protium from the substrate (PDD) or deuterium from the tetralin. In the limit of infinite substrate dilution in tetralin (tetralindl,/PDD = R m), the chain carrier would selectively abstract deuterium, and the free-radical mechanism would thus yield exclusively the singly deuteriated product P-D. In contrast, the products of the molecular mechanism, unaffected by the presence of tetralin, would be fully protonated P-H. Since toluene was the major product of PDD thermolysis, our focus was on the benzyl radical chain carrier (p2 in Figure 1B) and the deuterium incorporation
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0 1987 American Chemical Society 0888-588~/87/2626-03~4~0~.50~0
Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987 375 A . MOLECULAR MECHANISM Ph-CH;:CH,
~~
+ Ph-CHI + CHI=CH-(CHa),-CHI
i-l.&H-(CHI),-CHa Ph-CH-CHI l-.:l H'. 'CH2-(CH2),-CHa
+ PH-CHxCHz + CH~-(CH:).-CHI
E. FREE-RADICAL MECHANISM PDD
+
CHI-(CH,),-CH;
PDD
+ Ph-CH; + 'CH*-(CH,),-CH,
+ CH,-(CHa)I-CHa
+ PDD.
81 Ph-CH;
+ PDD + Ph-CHI + PDD.
8: Other Radicals
+ POD + Minor
Praductr
+ PDD
Ba Ph-CH -(CH~),O-CHI
Ph-CH:=CHr
PI
+ 'CHI-(CHI)I-CHI PI
+ Ph-CH; + CH,=CH-(CH,),-CH,
Ph-CHI-CH2-bH-(CH,),-CHa
82
k all other PDD'
+ Other Radicals + Minor
Products
P,
Ira
Ph-CH; + PDD' 4Termination Products Figure 1. PDD thermolysis mechanisms: (A) molecular mechanism; (BJ
28 24 -
2
Table I. Molar Yields of Products from PDD Thermolysis at 400 "C molar yield (%) at reaction times, min product 30 30 90 120 240 1.71 0.94 1.42 hexene 0.50 0.75 3.37 0.40 0.61 1.03 1.39 hexane 0.69 0.26 0.44 0.10 0.18 benzene 1.04 1.28 0.55 0.72 0.35 heptene 2.69 0.92 1.19 0.29 0.52 heptane 15.95 28.34 7.88 12.47 4.48 toluene 0.92 0.69 0.97 0.35 0.55 octene 1.61 0.12 0.26 0.50 0.64 octane 2.41 2.43 7.55 0.36 1.04 ethylbenzene 0.43 0.47 0.29 0.35 0.20 nonene 0.73 1.04 0.36 0.84 0.93 styrene 1.79 0.22 0.47 0.79 1.00 nonane 0.08 0.12 0.09 0.00 0.08 phenylpropene 1.49 0.48 0.62 0.14 0.25 propylbenzene 0.44 0.28 0.37 0.42 0.54 decene 7.76 3.26 4.85 5.50 2.18 decane 1.66 0.19 0.40 0.68 0.86 butylbenzene 2.27 3.86 4.54 3.28 4.03 undecene 1.22 0.24 0.41 0.56 0.18 undecane 0.09 0.12 0.14 0.09 0.12 phenylpentene 1.42 0.61 0.77 0.20 0.34 phenylpentane 0.30 0.09 0.12 0.00 0.04 dodecane 0.22 0.11 0.14 0.16 0.20 phenylhexene 0.90 0.42 0.51 0.15 0.24 phenylhexane 0.12 0.12 0.13 0.13 0.15 phenylheptene 0.72 0.38 0.44 0.13 0.21 phenylheptane 0.10 0.11 0.07 0.09 0.10 phenyloctene 0.71 0.41 0.47 0.16 0.25 phenyloctane 0.11 0.12 0.10 0.11 0.09 phenylnonene 0.73 0.18 0.28 0.45 0.51 phenylnonane 0.28 0.35 0.47 0.19 0.28 phenyldecene 0.41 0.09 0.17 0.28 0.38 phenyldecane 0.12 0.03 0.09 0.08 0.09 phenylundecene 0.33 0.15 0.21 0.23 0.26 phenylundecane phenyldodecane 93.26 75.48 54.25 50.67 22.22
20-
v
0
d>
16-
$
12-
2
-
-
L' /!r+/"_ 4
,**,I
-
4'
-v-
~:::a.,-,?>v
0 0
40
80
120
160
200
0.3
240
-
0.2 -
TIME (MINUTES)
0.1
Figure 2. Temporal variation of the yields of major products from PDD thermolysis at 400 "C.
1
0.04 0.0
(DI) into the toluene formed. The free-radical fraction of PDD thermolysis could then be inferred as the estimate of DI as R m. As a background experiment, the reaction of fully protonated toluene in tetralin-d,, was accomplished at 400 O C for 30 min and resulted in DI values of 0.024 and 0.027, respectively. Thus, isotope exchange was minimal and did not confound the analyses that follow. The results of the pyrolyses of PDD in tetralin-dI2 are summarized in Figure 3 as a plot of DI vs. 1/R. Extrapolated to DI = 1.0 as 1/R 0, these data convincingly demonstrate that PDD pyrolysis was free-radical; the molecular mechanism was not kinetically significant under these conditions. Note that DI, the fraction of toluene that contained a deuterium label, is independent of the total yield of toluene and its rate of formation. These conclusions were thus determined from a kinetics-free basis and
-
-+
I
I
0.2
I
, 0.4
I
I
0.6
,
I
0.8
,
1
1/R (Males PDD/Male tetralin-dl2)
Figure 3. Deuterium incorporation for mechanism discrimination.
-
accurately show the mechanism to be entirely free-radical as 1/R 0. The extrapolated value of DI for 1/R 0 provides a reliable quantitative estimate of the relative contributions of the free-radical and molecular mechanisms at other values of R only if the kinetics of both mechanisms are independent of R. Excepting the effect of reactant dilution with increasing R, which lowers the rate of all positiveorder reactions, the rate of the unimolecular reaction was independent of R. Although the kinetics of the pyrolyses in tetralin of dibenzyl ether and related compounds are not strongly dependent upon the solvent loading (Cronauer et al., 1979; Klein and Virk, 1983; O'Malley, 1985))it is instructive to examine this point in some detail.
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Ind. Eng. Chem. Res. 1987,26, 376-378
376
For a classical Rice-Herzfeld (1944) free-radical chain reaction of a pure compound A in tetralin, the long-chain and steady-state approximations (Gavalas, 1966) facilitate development of an analytical expression for the rate of reaction, rR This is shown as eq 1,for the case of initiation
impact both molecular and free-radical fragmentation of carbon-carbon and carbon-hydrogen bonds. A molecular reaction might be viewed formally as the simultaneous occurrence of the free-radical reaction steps of @-scission and hydrogen abstraction. Thus, this model served as a quantitative assignment of the relative reactivities of primary, secondary, and tertiary bonds and not as a verification of a molecular mechanism. Literature Cited
by first-order fission, chain transfer by reaction of substrate-derived 0-radicals with tetralin and also the thusderived tetralyl radicals with substrate, and second-order chain termination by all possible combinations of p - , 0-, and y-radicals (tetralyl). The rate constants for termination by two different radicals were taken to be twice those for self-termination (k,) to reflect their statistically expected values (Pryor, 1966). The subscripts in eq 1 denote the elementary steps in the free-radical mechanism as follows: ki = chain initiation, k, = p-radical decomposition (@-scission),k, = H abstraction from A by y-radicals, kH = H abstraction from A by &radicals, kD = D abstraction from tetralin-d,, by 0,and k, = chain termination. Equation 1 shows that the primary effect of R on the rate of the free-radical reaction is through reactant dilution: in the liquid phase, A will approach zero as R m. In this limit, the reaction order in A approaches 1.5. On the other hand, the order of the molecular reaction remains 1.0 at all R, and thus, at very high tetralin loadings, reactant dilution decreases the rate of the radical reaction to a greater extent than the rate of the molecular reaction. Therefore, the value of DI as 1/R 0 is less than or equal to the free-radical component at all other values of R and hence serves as a lower bound for the free-radical fraction of neat pyrolysis. Since DI 1.0 as 1/R 0, these data suggest the PDD pyrolysis mechanism is entirely freeradical at all R. The accurate predictions of product profiles by Blouri et al, (1985) suggest that the same energetic fundamentals
-
-
-
-
Blouri, B.; Hamdan, F.; Herault, D. Ind. Eng. Chem. Process Des Dev. 1985, 24, 30. Cronauer, D. C.; Jewell, D. M.; Shah, Y. T.; Modi, R. J. Ind. Eng. Chem. Fundam. 1979, 18, 153. Gavalas, G. R. Chem. Eng. Sci. 1966, 21, 133. Gilbert, K. C.; Gajewski, F. J. J. Org. Chem. 1982, 47, 4899. Klein, M. T.; Virk, P. S. Ind. Eng. Chem. Fundam. 1983, 22, 35. Kossiakoff, A,; Rice, F. 0. J . Am. Chem. Soc. 1943, 65, 520. Miller, D. B. Ind. Eng. Chem. Product Res. Deu. 1963, 2, 220. Miller, R. E.; Stein, S. E. J . Phys. Chem. 1981, 85, 580. Mushrush, G. W.; Hazlett, R. N. Ind. Eng. Chem. Fundan. 1984,23. 288. O'Malley, M. M. BChE Thesis, University of Delaware, Newark, 1985. O'Malley, M. M.; Bennett, M. A.; Simmons, M. B.; Thompson. E. D.; Klein, M. T. Fuel 1985, 64, 1027. Poutsma, M. L.; Dyer, C. W. J . Org. Chem. 1982, 47, 4903. Pryor, W. A. Free Radicals; McGraw-Hill: New York, 1966; p 14. Rebick, C. Adu. Chem. Ser. 1979, 183, 1. Rice, F. 0.;Herzfeld, K. F. J. Am. Chem. SOC. 1944, 56, 284. Virk, P. S. Fuel 1979, 58, 149. Woodward, R. B.; Hoffmann, R. The Conservation of Orbrtal Symmetry; Academic: New York, 1970.
* Author to whom correspondence should he addressed. 'Present address: Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109.
Phillip E. Savage,+Michael T. Klein* Department of Chemical Engineering and Center for Catalytic Science and Technolog3 University of Delaware Newark, Delaware 19716 Received for review December 11, 1985 Accepted September 2, 1986
Optimization of Consecutive Reactions with Recovery and Reuse of Unconverted Reactant The operating conditions for consecutive reactions are determined, which give the maximum yield of intermediate in terms of the reactant recovery ratio m and t h e rate constant ratio k , / k , . The effectiveness of CSTR and PFR reactors in the recycle system (RS) is compared. The difference in general yield between CSTR and PFR in the RS can be reduced by raising the value of the recovery ratio m. For consecutive reactions such as k,
A-R-S
k,
the maximum amount of intermediate obtainable from a once through operation depends only on the ratio of the rate constants k,/k,. However, when some of the unconverted reactant can be separated from the exit stream and returned to the reactor, the yield of the intermediate can be increased. For brevity, the no-recycle system is called NRS, and the recycle system is called RS, in this paper. 0888-5885/87/2626-0376$01.50/0
I. Simplified Process and Optimum Equation The system studied consists of a reaction unit and a recovery unit. The reaction unit contains reactors and other auxiliary equipment. Reactors can be CSTR, PFR, or nonideal flow vessels modeled as tanks in series. The recovery unit includes all equipment needed to separate and purify various components from the mixture flowing from the reactor. Figure 1 shows the system considered with all streams labeled, where nAO= initial moles of the reactant A/unit time, nA = moles of component A at the outlet of the reactor/unit time, nR = moles of component R at the outlet 0 1987 American Chemical Society
