2664 excellent agreement with that obtained under conditions of semi-infinite linear diffusion. l1 DH, Acknowledgment.-The author wishes to thank Taking 1 ) =~ 4.1 ~ X 10-6 cm.2/sec. (aso, H2O)’Y J. T. Adamchick for his valuable assistance with the approximate value obtained for Dco is 3.1 X the electronic instrumentation. cm.2/sec. (19) G. Tammann and V. Jessen, 2. U ~ O ~ allgem. Q . Chem., 179, 125 The value of De0 so obt,ained proves to be in (1929). Dco ~
= O.ij
(17)
THE CHEMISTRY OF SYLYLENES. SV. THE KINETICS OF FAST FLOW PYROLYSIS OF p-XYLENE BY L. A. ERI~EDE AND F. DEMARW Contribution N o . $33 frcm the Central Research Laboratories of the Minnesota Mining and Manzlfacturing Company,‘ St. P a d ,
Minn. Reeeiced May 29, 196.9
The fast flow pyrolysis of p-xylene was re-examined and the kinetics were followed by total analysis of the reaction products. Rate constants were determined for formation and subsequent destruction of p-methylbenzyl radicals and of toluene. These reactions are homogeneous and consequently unaffected by the type of surface in the pyrolysis system. The rate constant for formation of p-methylbenzyl radicals agrees with earlier results that were determined indirectly by analysis for HP. Equations were developed that give the conversions of p-xylene to p-methylbenzyl radicals and to toluene in terms of the pyrolysis temperature and residence time. When compared at like residence times, the conversions calculated by use of these equations agree within experimental error with results observed by other investigators. These equations also were used to calculate the amount of p-methylbenzyl radicals available for gas phase synthesis via coupling with other radicals.
Introduction When p-xylene is subjected to fast flow pyrolysis at low pressure, a mixture of products is obtained.2 Most of the components of this mixture can be traced to a common intermediate, p-methylbenzyl radical, as outlined in Fig. 1. This intermediate is formed by thermal rupture of the C-H bond (eq. 1 and 2) as the gas stream travels through the pyrolysis zone of the flow system. Most of these radicals then are dehydrogenated catalytically as the pyrolyzate streams away from the furnace3 (eq. 3).
Side reactions comprise coupling of p-methylbenzyl radicals to give 1,Zdi-p-tolylethane and p-ethyltoluene. If formed in the pyrolysis zone, (1) This work was done in the laboratories of the M. W. Kellogg Company. The data were acquired by the hfinnesota Mining and Manufacturing Company with the purchase of the Chemical Manufacturing Division of the M. W. Kellogg Company in March, 1957. (2) L. A. Errede and J. P. Cassidy, J . Am. Chem. Soe., 83, 3653 (1960). (3) L. A. Errede and J. P. Cassidy, Paper XVII, J . Phys. Ghern., in press8
these compounds can continue to react to give 4,4’-dimethylstilbene and p-methylstyrene. In addition the ditolylethane can rearrange to the o-methyldiphenylmethane, which in turn can be converted to the corresponding anthracene.2 When the pyrolysis conditions are severe, cyclooctatetraene and styrene are produced in appreciable amounts, presumably via successive thermal rea r r a n g e m e n t ~as ~ , indicated ~ in Fig. 1. Demethylation is a major competing reaction that consumes p-xylene2 and complicates the pyrolysis. Solutions of p-xylylene are prepared by collecting the p-xylene pyrolyzate in a solvent kept at -78°.5 When these solutions are warmed to room temperature, the accumulated p-xylylene polymerizes rapidly to give insoluble poly-(p-xylylene) easily separated by filtration. The other non-volatile but soluble products (low molecular weight polymer, the diarylethanes, the diarylmethanes, and the anthracenes) can be recovered from the mother liquor by evaporation to dryness.Zs6 It was shown2that 96-100Oj, of the phenyl units metered to the pyrolysis system as p-xylene can be accounted for if the non-volatile products are weighed as “p-xylyl equivalents”2 and the volatile components (p-xylene, toluene, styrene, and pethyltoluene) are determined by mass spectrometric analysis of an aliquot sample. Since all the reaction products containing phenyl units are formed via a common intermediate, p-methylbenzyl radical as shown in Fig. 1, the sum total moles of stable end products exclusive of toluene represents the number of moles of p-methylbenzyl radicals generated by pyrolysis of p-xylene. (4) E. J. Prosen, W. H. Johnson, and F. D. Xossini, J . Am. Chem. Soc., 69, 2068 (1947).
( 5 ) L. A. Errede and B. F. Landrum, ibid., 79, 4952 (1957). (6) L. A. Errede, R. Y. Gregorian, and J. hf. Hoyt, %bid..82, 5215 (1960).
KINETICS OF FAST FLOW PYROLYSIS OF P-XYLESE
Dec., 1962
n
c RACK1NC
2665
W
t CHZQ
I
CH2.
i
Q Fig. 1.-Reactions that account for the products isolated when p-xylene is subjected to fast flow pyrolysis a t low pressure. Bold faced arrows indicate the main reaction paths and smaller arrows indicate less important side reactions.
These results suggested that the over-all pyrolysis kinetics could be followed by total analysis of the reaction products using a combination of gravimetric analysis for non-volatile products and mass spectrometric analysis for volatile products. It was of interest to compare these results with those reported by earlier workers7.8 mho studied the pyrolysis of p-xylene indirectly by measuring the rate of hydrogen and methane formation. Results and Discussion Since most of the products of p-xylene pyrolysis can be traced to p-methylbenzyl radicals as shown in Fig. 1, the pyrolysis picture can be simplified for the purpose of the kinetic studies to the following indicated reactions
dC - = icaB dt where the letters A , B, and C refer to the mole fraction of the components in question. Equation 5 can be integrated to give
A
= ~~~-(ki+ks)t
(8)
Substitution of (8) into eq. 6 gives
Equation 9 is a linear equation of the first order and it can be shown that the integrated form of eq. 9 is
(D)
non-isolated volatile products of thermal decomDosition (C)
+
we let K = lcl lc3 and divide both sides by t It was shown that kl and k3 are first-order re- If one obtains a c t i o n ~ . ~If~ one ~ makes the assumption that kz IS also a first-order reaction and that kl, R,, and 1ca - are the only reactions of importance that consume t t kp - K p-xylene (A) and p-methylbenzyl radicals (B), to give toluene (D) and non-isolated volatile products of thermal decomposition (C), respectively, one where F9 is the fraction of p-xylene converted to then can write the rate equations p-methylbenzyl radicals. Equation 11 can be rewritten as eq. 12. b5) (9) F = Zn;/n,. where Zni is the moles of pmetbylbenryl radical~
(7) M. Szwarc, J . Chem. Phgs.. 16, 128 (1948). (6) J . R. Bchaefgen, .I. Polymer Sei., 16, 203 (1955).
produced and isolated as “p-xylyl equivalents,”%styrene, and p ethyltoluene, and where nx is the number of moles of p-xylene metered to the pyrolysis system.
L. A. ERREDE . 4 m F. DELIARIA
"66
Vol. 66
fC.3
._
The last term of eq. 12 can be expressed in the form of a power series which, if divided by t and if the terms for In (IC2 - K ) are combined, gives eq. 13. In (F'lt)= In k , - k2t = In [1 + (k2 . . . . . . [(k* - K ) n - l P - l ] ] 2! ?a!
m+
(13)
Fig. 2.-Pyrolysis system used for kinetic study of fast flow pyrolysis of p-xylene. Procedure and definition of numbered articles are given in the Experimental section.
When LZ - K is small the entire third term of eq. 13 converges to zero as t approaches zero and for pyrolysis temperatures below 1050" can be neglected when t is
