J. appl. Chem. Biotechnol. 1974,24,49-58

The Pyrolysis of Chlorocarbons

David Grant

I.C. I . Ltd, Central Instruments Research Laboratory, Pangbourne, Berks.' (Paper received 30 November 1973, and accepted 14 January 1974)

The formation of C,Cl,(bz), CCl, and C2CI6 by the sealed-tube pyrolysis of chlorocarbons, has been confirmed. The chlorinolysis of C6C16(bz), C4C16 (1,3 diene) and C2Cl4 yields CCl,.

1. Introduction

The pyrolysis chemistry of chlorocarbons is of interest for the possible use of chloro- carbon containing wastes as chemical feedstocks.

Convenient laboratory scale techniques in this field are sealed-tube pyrolysis at T = c 400 "C cf. (I), or, for 1 atm conditions, at T = c 600 "C. Aubrey and Van Wazer' observed, from several starting chlorocarbons, the establishment of an equilibrium :

C,Cl,(bz)+ 12CC14 ~t 9CZCls. (1 1 It was of particular interest to test the general occurrence of (1) and to investigate the kinetic details. The chlorinolysis of selected chlorocarbons was also studied.

2. Experimental and results 2.1. Sealed-tube studies The effects of heating chlorocarbons in sealed-tubes are summarised in Figure 1 (which gives the products present under "final equilibrium" conditions) and in Table 1, where the initial stages of pyrolysis are summarised.

The thermal stabilities of chlorocarbons are summarised in Table 2. Species having greater values of Cl/C (except CC14) or C-CCI2-C groups, tend to be less stable.

2.2. Chlorinolysis studies Chlorinolysis rate data (the principal products were CC14 and C,Cl,) and equilibria detected (at I atrn) are listed in Table 3.

The time required for establishment of (2) and (3) was 600, 150, 100 and 5 s at 533, 570, 600 and 690 "C, respectively, and for (4), 600 s at 900 "C.

' Present address: International Synthetic Rubber Company Ltd, Brunswick H o w , Brunswick Place, Southampton, SO9 3AT.

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D. Grant

TABLE 1. Initial stages of chlorocarbon pyrolysis (380 "C sealed-tube)

Max. amount Heating time Starting species Intermediate formed (mol- %) (h)

ClCl4

n-C4CII

c5cl6 (cyclodiene) C ,C1 (cycloene) r.t. 11 minb

r.t. 14.7 min r.t. 15.5 min'

ClClS

3 52 6

43

18 8

50

62 12 38

c 10 c 19 c l c 14 c 3 c 10

100 cu. 3000

2 5

cu. 40 cu. 40

120

1 1 30

80 80

100 120 150 150

-~ ~~ ~~

Results obtained at 290 "C. Tentative assignments (g.1.c. r.t. values shown). Hexachlorofulvcne. Octachloronaphthalene.

TABLE 2. The thermal stability of chlorocarbons

Half-life Time required to decr. to Spccies T "C 0) < 0.2 mol- % products (h)

380 380 380 380 380 290 380 3 80 380 4ooc 380

c 37ff 3 5Q 3 80 200

> 200 112. 64 53

110 2.6

0.3 0.3

c 0.3 t l < 1

tO.2 <0.1

-

(< O.l)-'

> 3ooob > 3oooh

150 150

> 3ooob

29 29

> 3 m

5' < I < 0.2

-

-

-

# Estimated from 1 atm expts, other data are from sealed-tube expts.

' Ref. 2. Ref. 3. Probable "final equilibrium" components. Ref. 1. Figures 1 and 2.

I

I .^ LI

2 3

CI/C

Figure I . Long term pyrolysis products oS chlorocarbons (35e405 ', sealed tubes) Starting species: 1. cc l4+c6cl6(bz~; 2. CICI6'; 3. C2CI4'; 4. C3C16'(e ne); 5. C3Cls (cyclo); 6. C,CIB'; 7. C4Cls (1.3-dieneY; 8. n-C4CI,; 9. C&i6 (cyclodiene); 10. C,CIa (cycloene); 11. C,oC1,a (PetltaCyClO); 12. c6c16 (bZ)+C1ze.

Curves are for -loglo Kma, ( I ) = 10. Previous results', are included; pts, 405 "C, except C3ClB, (350 "C) (present results o pts,

380 "C). 8 h at 400 "C.

Steady-state concentrations of products were achieved at 380 "C for CaCI4, c3cl6 (cyclo), C3C16 (me), C4Cls (1,3diene) and n-C4C18 at 3000 h and for CICls (cyclodiene), C3CIB (cycloene) and CloClll (pentacyclo) at 400 h. The Cl/C ratio was calculated from the final g.1.c. analysia, omitting products with r.t. > 26min; the difference between starting and final value of Cl/C is attributed to these species.

52 D. Grant

TABLE 3. Rate and equilibrium results (1 atm experiments) well-aged inert wall conditions

P A E. kcal mol- * C2CL I9.(3) 51k1.5 C4Cls (1,3-diene) 20.(5) 58+3 CaC16 (bz) Y3). 69+5 CC14b 17.(0) 71 L 1.5

C.CI6 @z)+ 12CCL e 9C2CI6 ( 1 ) c2c14 + c1, i+ ClC16 (2) C2CI,+2CI, E? 2CCl4 (3 ) C6CI6 (bz)+9C12 Ti 6CC1, (4) -log,, K,, = B-AH/4.6T (2)' B = 8.8 (8.2): AH = 31 (31)d kcal mol-I (3)' B = 10.2 (8.8): AH = 46 (48)d kcal mol-'. ( 1 ) (4)

- b i o Km.1 rrmcilon (380°C) = 10. - loglo Kp (900 "C) = 7.

The rate law was observed for C2Cl4 and C4c16 (IJ-dicne). A was apparently less, e.g. 15.(9), when 60mol-% C12 was present intially. In agreement with ref. 4, slightly different from ref. 5. Values in brackets are calculated from thermodynamics, cf. 6. In agreement with ref. 7.

d/dr[CCI,] = kJP][C12]1.s' or - d/dr[CCI,] = k,[CCI,] log,, k , = A-E./4.6T, mol-' cm's or s- ' .

2.3. Sealed-tube experlments Previously described' techniques were used.

2.4. Atmospheric pressure experiments Preliminary experiments with stainless-steel reactors gave unsatisfactory results (due to the catalytic effects of metal chlorides formed at the vessel walls). Most results were obtained with c 2 cm3 SiO, or Pyrex-glass U-tubes, thermostated by an encasing steel block. The reagent gases or the g.1.c. He stream could be passed through the vessel or isolated in it for a measured time (under static reaction conditions), by the use of two joined 4-way taps at the tops of the U-tube arms. 25 cm3, fixed residence time, SiO, vessels were also employed. The chlorocarbon stream was diluted with N, and used to age the vessel walls; if ageing was carried out with Clz, or if packed vessels, or flowing gas reaction conditions were used, chlorinolysis rates were greater.

Reaction of C6Cl,(bz) and C1, was studied using a 135 cm3 Si02 vessel which was immersed in a furnace. Recordings of thermocouple readings were used to estimate the reaction conditions.

2.5. Reagents These were commercial samples or were prepared: C3C16 ( c y ~ l o ) ~ ; n-C4CI 8,9 by published methods. The effects of gaseous impurities on the reactivity of chlorocarbons was investigated for C2CI4. Samples which had been carefully distilled under N2 showed similar rates of chlorinolysis to samples which were diluted (in the gas stream) with various amounts of air or HCI.

The pyrolysis of chlorocsrbons 53

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90

80

70

$ 50

3 40

30

20

10

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I

2.6. C.1.c. analysis Following the previous procedure' using a Pye-104 g.1.c. apparatus, temperature programmed at 50-250 "/12.5 min, the following retention times (min) were observed: N2, 0.37; Clz, 0.45; C2Cl,, 1.1 (prepared by passing CHCl : CClz through a KOH pre-column), CC14, 1.4; C2C14, 2.9; czcI6, 5.9; C,Cl, (cyclo), 6.2; c,cI6 (ene), 7.3; C4C16 (173-diene), 7.5; C,Cls (cyclodiene), 9.2; CSCle (cycloene), 10.5; C6Cls (bz), 12.4; C,C18, 13.8; n-C4Cle, 14.0 and ClOC1,, (pentacyclo), 26.0.

2.7. Details of individual pyrolysis and chlorinolysis experiments 2.7.1. C2C14, sealed-tube pyrolysis C,Cl, frequently occurred as an initial pyrolysis product of other chlorocarbons and its subsequent pyrolysis became, in these cases, the rate determining factor for the establishment of (1).

54 D. Grant

I I0 ?a Time ( h )

100

Figure 3. Sealed-tube pyrolysis of c5c16 (cyclodiene) (380 "c)..

tha1enc)l; K, Land M CloCllo and/or Cl0CII2 isomersb, r.t. values 14.5, 19 and 22 min respectively. Tentative assignments.

A time scale is used.'

'A, CCI4; C, ClCI6; E. C5CI6 (cyclodienc); F, CsCIe (cycloene); H, c6cll, (bz); J, CloCli (naph-

2.7.1 . I . Pyrolysis of C2CI4 with additives Inorganic additives altered the detailed pyrolysis behaviour but not the general behaviour. Various 240 h pyrolysis experiments at 310 "C with (a) no additive (b)-(f) with c 0.2 mol- % additive: (b) FeC13, (c) KI, (d) HgCI2, (e) AgN03, (f) K,Cr,O, and with no additives (g) at 380 "C and (h) at 210 "C, gave:

Products (mol- %) (a) (b) (c ) (4 (e) (f) (g) (h) CC14 32 74 20 25 29 22 68 11 CZC16 40 19 64 44 43 48 12 45

C6C16 (bd 12 7 10 13 10 9 18 5 C4Cl, (1,3-diene) _- - - 1 - 1 - 3

The pyrolysis of chlorocarbons 55

2.7.1.2. Pyrolysis of C2C14 at 1 atm Under 1 atm conditions C2CI4, heated at 580 "C in a new 2 cm3 SiOz vessel (aged under N2 at 500 "C for 2 days) for, consecutively, 5, 10, 15, 48 and 5 min gave (est.) 12.9, 13.4, 19.2, 25.2 and 6.7 mol-% decomposition. The 48 min products included C2CIz, 0.7; CCl,, 2.2; C4C16 (1,3-diene), 1.5 and C6Cl (bz), 0.8 mol-% species; the remainder was assigned as acetylenic and small ring species. The pyrolysis rates decreased with increased wall age.

2.7.2. Chlorinolysis of C2C14 (1 atrn) Typical results were:

Initial C12, 30 mol- %; 2 cm' vessels

Products Reaction time (s) T"C Vessel surface (rnol-%) 2.5 10 30 60

520 SiOp CCI4 2.4' 10 19 31 Clcl6 0.5' 1 1 1

520 Graphite chipsb CC14 0.8 5 14 CZClS 4.4 21 23

580 Si02 CCI4 5 35 50 C2Cl6 2 1.5 3

520 Pyrex-glass cc14 4 17 28 42' c2c16 0.9 2 5 3

' Under these conditions but in a flow system with a 25 cm3 vessel, the rno1-x products were CCI4, 7 and C2C16, 0.8.

Packed vessel, the others were empty.

These results suggest that C2C14 reacted with Clz, in empty vessels, to give CCI4 directly, not via C2C16. Surface participation was most evident for C2Cls formation.

2.7.3. Thepyrolysis of CCI, (1 atm)

Typical results were: 2 cm3 Si02 vessels

Products Reaction time (s) T"C (mol-%) 1 5 15 120 240

533 CaCls 0.3 1.3 1.1 0.3 0.4 c~ch 0.1 0.4 1.6 8.8 14.9

570 c2cl6 1.4 0.9 0.8 0.6 1.5 CZCL 0.3 1.7 4.5 16.4 17.0

At heating times > 240s some "carbonisation" and the formation of acetylenic and/or small ring species, was apparent.

With 60 mol-% C12, initially present, the pyrolysis rate at 533 "C was reduced; after 5 s the amounts of C2C16 and C2C14 were, respectively, 0.6 and 0.1 mol-%; similar results were found after 1 s.

The main initial product was c2c16 (cf. CCI, + CCI, ++C2C16) in contrast to the results of C2CI4 chlorinolysis ()C2C14 -% CC14) where C2Cls was not involved

56 D. Grant

as an intermediate at 1 atm total pressure. This suggests that, under these conditions, the forward and reverse stages of (3) (Table 3). proceed by different mechanisms.

2.7.4. C3C16 (cyclo) The results, for the initial stages of the sealed-tube pyrolysis at 380 ", are shown in Figure 2. An early maximisation of CCI, and C2C14 occurred. A C, -> C2 t CI reaction may be of general occurrence for perchloro Cp molecules, e.g. it was also observed for C3ClS.'* lo

2.7.5. C3C16 (ene) This species was studied briefly at 380 "C by sealed-tube pyrolysis. It behaved similarly to C,CI 6 (cyclo).

2.7.6. C4C16 (1,3-Jiene) The sealed-tube pyrolysis at 380 "C showed the simultaneous initial formation of C2CI4, C,C14 and c6cI6 (bz); CC14 was subsequently produced, it increasing as C2C16 decreased.

The chlorinolysis of C4C16 (1,3-diene) at I atm gave the following typical results:

Initial CI2 50 mol- %; 2 cm' SOz vesstls

Products Reaction time (s) T"C (mol-%) 10 30 60

500 CCI, 0.9 1.7 2.1 C2Ch 0.2 0.4 0.7 CA 0.1 0.2 0.5

570 CCI, 19 33 44 3 1 2.5

- - ClCL c2c16 1

As with CzC14 chlorinolysis, a direct route to CCI4 was indicated.

2.7.7. n-C4ClS (The position of the double-bond has not been established). The sealed-tube pyrolysis at 380 "C showed an initial rapid generation of C2C14 and C4C16 (1,3-diene) mol ratio 5 : 1). Thereafter, C2C14 and c4c16 (1,3-diene) reacted as described above.

2.7.8. C,C16 (cyclodiene) The initial stages of the sealed-tube pyrolysis at 380 "C are given in Figure 3. A number of initial pyrolysis products were detected. Tentative assignments were made on the basis of g.1.c. and i.r. data (the latter were obtained from fractions separated in a n-hexane s o h on a Si02-gel column).

2.1.9. C,CI, (cycloene) The sealed-tube pyrolysis (380 "C), studied briefly, was qualitatively similar to that of CsCls (cyclodiene).

The pyrolysis of chlorocarbons 57

2.7.10. C6Cl6 ( b ~ ) 2.7.10.1. Chlorinolysis (sealed-tube) With initial CI/C = 1.05, the products after 8 h at 400 “C were: CCI,, 5.4; czCl6, 3.8 and c4c16 ( I ,3-diene), 0.4 mol- % species.

2.7.12.2. Chlorinolysis (1 atm) Typical results obtained were:

Initial C12, 90 mol-”/,; 135 (3131’ SiO, vessel.

Products Reaction time (s) T°C (mol-%) 30 120 I80 360

- 6 9 630 ccl4 - 2 3

690 CCI, 6 8 16 18 czc14 2 2 15 12

890 CCI, 27 52 51 C2CI4 2 9 4

- - czc14

2.7.1 1. C,,Cl12 (penracyclo) m 485 “C This insecticide” has a (5.3.0.02* 6.04* l o .Os* 9, probable ring structure.”

The sealed-tube pyrolysis at 380 “C proceeded by the initial formation of a number of species of unknown structure. None of these was however present after prolonged heating. The slow initial rate of pyrolysis was considered to be due to the initial presence of a crystalline phase. The structural reorganisation at 380 “C did not how- ever require the presence of a fluid chlorocarbon phase. A steady increase in the amount of c6c16 (bz), was observed throughout the initial stages of the pyrolysis. CC14 and cZc16 were formed, mainly as secondary products.

3. Discussion

The presence of small amounts of Cl,, particularly in the initial stages of sealed-tube pyrolysis, was apparent from the g.1.c. results. The reaction sequences observed also suggested a simultaneous occurrence of dechlorination, chlorination and chlorinolysis. Cl, reacted with C6cI6 (bz), under sealed-tube conditions to give CC14 and C,CI, which were also formed by chlorinolysis of C2C14, C4C16 (lJ-diene) and c&l6 (bz) at I atm. C2C16 had been previously reported4- to yield CI, during the initial stages of pyrolysis. It is therefore likely that the reactions represented by (1) occur principally via dechlorination, chlorination and chlorinolysis sequences.

The effects of heating C,C14 with additives and the effects of wall activities on the chlorinolysis reaction rates indicated that the initial dechlorination and CI, dissocia- tions reactions were heterogeneous in character.

Starting chlorocarbons, as well as the others formed during pyrolysis, all decom- posed into the components of the suggested equilibrium ( I ) with inappreciable formation of “char”. The present results therefore confirm the previous hypothesis’

51 D. Crnnt

that all chlorocarbons undergo the rearrangement reactions required to give equilibrium (I). In addition, it is likely that the equilibrium (2), (Table 3) occurs, giving small amounts of C,CI, in the “final equilibrium” composition. C,CI, was, however, not detected during the pyrolysis of C,C16 (cyclodiene) or C,,CI,, (penta- cyclo). In these cases the CI, concentration may have been greater than with the other starting chlorocarbons (g.1.c. results supported this), the equilibrium (2) being displaced towards C,CI,.

4. Conclusions

The question as to whether the observed reactions ( I ) are in true thermodynamic equilibrium cannot be established by the present results. It is noted that the apparent equilibrium constants, calculated from the 1 atm chlorinolysis results, are significantly different from those calculated from the accepted thermodynamic data. On the other hand, the general occurrence of the process ( I ) , has been established.

Acknowledgements Thanks are due to Dr S. F. Bush and Dr J. E. White for helpful discussions and suggestions and to the management of 1.C.T. Ltd for permission to publish this work.

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1963,256, 3471. 5. Chiltz, G.; Goldfinger, P.; Huybrcchts, G.; Martens, G.; Verbeke, G. Chem. Rev. 1963,63,355. 6. Goldfinger. P.; Martens, G. Trans. Fara&y Soc. 1961,57, 2220. 7. Obrrrhl. R. P.; Bender. H., U.S. Pat. 2,857.438 (to the Stauffer Chemical Company). 8. Fields, E. K.; Meyerson, S . J. Org. Chem. 1963, 28, 1915. 9. Koedig, A. Ann. 1951, 574, 122.

10. Prins, H. J. J. prukt. Chem. 1914, 89, 414. 11. Chem. Week 1972, 111, (4), 25. 12. Sundaram, S . Dev. Appl. Spectroscopy 1964, 4, 179. 13. Dainton, F. S.; Ivin, K. J. Trans. Furrrdoy Soc. 1950, 46, 295.