Supercritical Fluid Engineering Science - ACS Publications - American

Chapter 30

Depolymerization Reactions of cis-Polyisoprene and Scrap Rubber in Supercritical Toluene 1

2

3

Jagdish C. Dhawan , Aladar F. Bencsath , and Richard C. Legendre 1

2

Department of Chemical Engineering, Department of Biochemistry, and Department of Chemistry, University of South Alabama, Mobile, A L 36688

Downloaded by UNIV OF NEW ENGLAND on February 10, 2017 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch030

3

cis-Polyisoprene was thermolysed in supercritical toluene at 349°C and 13.8 MPa in a 1-liter batch autoclave. The reaction products were examined by gas chromatography-mass spectrometry. A total of 171 components including isomers were identified. The extensive cracking of the polymer results in low molecular weight aromatics mainly consisting of xylenes, alkylbenzenes, and diphenylalkanes. Similar results were observed when scrap rubber from an aircraft tire is processed in toluene under identical operating conditions.

The problem of disposing of used tires has become acute in recent years simply because the discard rate of scrap tires in the United States approaches one per person per year. Obviously, scrap tires pose an important challenge from the material and energy conservation standpoint. Pyrolysis of scrap tires continues to be an area of intense investigation(7,2,3) However, the unfavorable economics of pyrolysis has dampened prospects for commercialization. In this chapter, we report on the GC-MS experimental results of the depolymerization of cis-polyisoprene in supercritical toluene. We will also discuss the results of our experiments with scrap rubber from an aircraft tire which is predominantly made of polyisoprene. These results show that depolymerization reactions under supercritical conditions indeed offer some advantages over conventional pyrolysis of scrap rubber. Pyrolysis of cis-Polyisoprene 6

Pyrolysis of polyisoprene^j in a high vacuum (~10" mmHg) begins at 300°C and is almost complete at 400°C. About 85 wt% of the resulting material is a non-volatile residue of 577 avg. molecular weight. The volatile material (-12 wt%

0097-6156/93/0514-0380$06.00/0 © 1993 American Chemical Society

Kiran and Brennecke; Supercritical Fluid Engineering Science ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

30.

DHAWAN ET AL.

Depolymerization of cis-Polyisoprene and Scrap Rubber 381


yield) consists of butanes, cyclopentadienes, isoprene monomer, pentenes, cyclohexadienes, hexadienes, hexenes, cycloheptadienes, heptenes, and dipentene. When natural rubber (cis-polyisoprene) is heated at 220-270°C, isoprene and its dimer, dipentene, are the main volatile products(5j. During the formation of these volatile products, the molten polymer becomes more viscous and finally sets to an insoluble solid. Sarfare et ai.(6) studied thermal degradation of natural rubber in Tetralin solution at 140°C. These investigators also concluded that degradation proceeds through a random chain scission process. However, the process of degradation was followed only viscometrically to follow the decrease in the molecular weight of the starting polymer; the resulting material was not otherwise characterized. Golub (7) examined the microstructural changes produced during the thermal degradation of 1,4-polyisoprene based on infrared and NMR-spectral measurements of the residues after about 60% weight loss in the temperature range 325-400°C, and reported thermal cyclization associated with the reaction of pendant vinyl groups by Scheme A as shown below: The use of a supercritical fluid (SCF) as a reaction medium can provide an alternative approach to lower the operating temperature of pyrolysis reactions. The lower operating temperature can minimize the formation of undesirable hydrocarbon gases and char. Improved yields and select!vities have been reported^ in an SCF reaction medium when compared with the results obtained under pyrolysis. Koll and Metzger(9) used supercritical acetone as the reaction medium for the thermal degradation of cellulose and found higher extraction yields at temperatures lower than those used for conventional pyrolysis. Metzger et al.(10) studied thermal intermolecular organic reactions in supercritical fluid media at pressures of up to 50.7 MPa and temperatures of up to 500°C and found that alkanes were coupled to alkenes, to 1,3-dienes, and to alkynes (acetylene). Blyumber et al.(ll) studied the oxidation of η-butane in both the liquid phase and an SCF-phase. The liquid-phase reaction products were predominantly acetic acid and methyl ethyl ketone, whereas the SCF-phase oxidation products were remarkably different and included formaldehyde, acetaldehyde, methyl-, ethyl-, and propyl alcohols, and formic acid. It is suggestedftfj that the difference in these two product groups is related to the types of free radicals that are formed under two different reaction conditions, indicating that in the SCF-phase the butane-derived free radicals have a higher probability to further decompose into methyl radicals rather than terminating the reaction by recombining. Pyrolysis reactions of hydrocarbon polymers can be divided into degradation (or primary) reactions and synthesis (or secondary) reactions. A l l degradation reactions are characterized by the cracking and dehydrogenation phenomena. In cracking reactions, paraffins decompose to one paraffin and one olefin. In dehydrogenation reactions, hydrogen is split off, causing the formation of a double bond without changing the chain length of the original paraffin. The pyrolytic cracking of a C - C or a C - H bond can take place by way of free-radical formations via homolytic decomposition or ion formation via heterolytic decomposition. It should be noted


382

SUPERCRITICAL FLUID ENGINEERING SCIENCE

CH

CH

3

3

300°C

I V V

-CH C=CHCH ' -

/WCH C=CHCH 2

CH Downloaded by UNIV OF NEW ENGLAND on February 10, 2017 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch030

2

2

2

CH

3

I

3

•CH C=CHCH 2

r 2

/WCH C=CHCH < 2

2

II H C

I

3

N

CCH=CH H C 2

CH

CH

3

I

Isoprene

3

CH C=CHCH ~^ 2

^-^CH C=CHCH 2

2

2

IV III

CH

3

r

/ V V C H C - C H C H C H C=CHCH 2

"Cyclized

2

2

2

rubber" and H C 2

H C 3

N c j

,CH

XH

2

2

Dipentene

SCHEME A Reproduced with permissionfromreference 7. Copyright 1972 John Wiley & Sons.


2

30.

DHAWAN ET AL.


that homolytic decomposition (free-radical formation) is energetically favored over in heterolytic decomposition, due mainly to the threefold higher bond dissociation energy. Experimental A schematic diagram of the equipment used is shown in Figure 1. It consists of a 1-liter capacity (1 dm ) batch autoclave with a magnetically driven agitator (Autoclave Engineers, Inc.). The reactor is equipped with an electrical heater with temperature control, pressure gauge and product recovery system. The latter consists of a pressure let-down valve which allows release of part of the solvent from the pressurized reactor to a container at atmospheric pressure. A condenser is provided to ensure liquid recovery and gas separation.


3

Materials. The polymer used was cis-polyisoprene as supplied by Aldrich Chemical Co. The aircraft tire rubber was obtained from a used Goodyear tire (Flight Special II, part No. 156E 6 1-3, TSOC62b; 15 χ 6.00-6). Analytical grade toluene was obtained from Fisher Scientific Co. and was 99.9% pure as determined by the vendor's G C analysis. Procedure and Safety. The reactor was charged with 24.4 g of polymer (or 36.4 g tire rubber) and filled with toluene (~ 980 ml) at room temperature. Details of the experimental procedure have been discussed elsewhere(72,). For the safe operation of the reactor, polymer and liquid solvent were loaded and then compressed to the desired operating pressure of 13.8 MPa at room temperature. The compression energy stored in a liquid is several fold less than that in gases. Any air entrapped was released using bleed and vent valves before heating. During heating solvent volume was adjusted by discharging the solvent through the bleed valve so that the static pressure in the reactor could be maintained at 13.8 MPa during the entire operation. The components eluted from the chromatograph were identified by submitting the corresponding mass spectra to a library search.

Β

C

Figure 1. Schematic diagram of the experimental apparatus. (A) autoclave; (B) agitator; (C) pressure gauge; (D) temperature indicator; (E) temperature controller; (F) electric furnace; (G) cooler; (H) sample collector; (I) gas chromatograph; (1-2) high-pressure valves. (Reprinted with permission from ref. 12. Copyright 1991 Polymer Research Associates, Inc.) Kiran and Brennecke; Supercritical Fluid Engineering Science ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

384



Results and Discussion Thermolysis of Toluene Under Supercritical Conditions. We conducted the thermolysis of supercritical toluene at 343°C and 13.8 MPa for 1-h. The total ion current chromatogram displaying separated compounds is shown in Figure 2. The results of the qualitative and semi-quantitative analyses are listed in Table I. We found that about 2% of the toluene reacts and forms various products with molecular weights ranging from 78 to 182. Diphenyl, diphenylmethane, diphenylethane and their methylated analogs are the major high molecular weight compounds. The low molecular weight products include benzene, ethylbenzene and xylenes. The thermolytic reaction products of supercritical toluene at 350°C and 17 MPa for 1-h conducted in a 1-liter rocking autoclave has been reported by Kçrshaw(75,). The yield of high boiling material was only about 0.03% in his experiment and diphenylethane was the major product. All of the compounds identified were the same as those in our work although the relative yields were different. We believe that the quantitative differences are owing to the difference in the experimental setup, method of solvent loading and the different reactor pressure (17 MPa vs 13.8 MPa). Thermolysis of cis-Polyisoprene in Supercritical Toluene. The G C - M S chromatogram of the reaction products from cis-polyisoprene in the presence of supercritical toluene is shown in Figure 3. 171 peaks were distinguished in the chromatogram and 26 of these were found in higher abundances than 0.5% (toluene-free basis). These major components (some of them include several isomers) are shown in Table II. The minor compounds including unidentified ones represent around 12% of the mixture and are listed in Table ΙΠ. As can be seen, the compounds in Table II consist of arylalkyl or diarylalkyl products. Besides a few low molecular weight (C3 to C9) aliphatics, arylalkyl and diarylalkyl are the major compounds. Diphenylmethane, methyldiphenyl and dimethyldiphenyl constitute 36% of the total products. Interestingly, about half of these diaryl compounds were also formed in the pyrolysis of pure supercritical toluene under identical experimental conditions. The amounts of benzene in both cases were comparable ( 7% vs 11% from pure toluene). However, there were significant differences in the concentrations of alkylated benzenes between the two experiments. Ethylbenzenes and xylenes were twice as abundant in the pure toluene experiment, and there were no detectable traces of higher alkylated benzenes. In the case of the polyisoprene experiment, the total concentration of higher alkylated benzene compounds was 23.3%. This is obviously due to the participation of the fragmented radicals from the polymer which is further supported by the insignificant amount of nonvolatile dry residue found in the reactor products (-0.6 g from the 24.4 g polymer charged). The presence of meaningful amounts of diphenyl alkanes having more than two methylene units and their alkyl-substituted analogs is additional evidence of polymer participation in the formation of these products. Further comparison of the diarylalkyl compounds reveals that the production of diphenylmethane was nearly the same in both experiments (12.3% vs 14.0% without polymer) which indicates that the dimerization of toluene is unaffected by the presence of the polymer. In contrast, tetrahydronaphthalene, dihydroindenes and their dehydrogenated derivatives



128

255

549

?1β

8 43

Ι θ 18

113?

Figure 2. Total ion chromatogram of supercritical toluene reaction products at 349°C and 13.8 MPa

422


1304

TÎHE



386

Table L GC-Mass Spectral Analysis of Supercritical Toluene Compound

Scan*

M.W . (%)** (%)***

1 Benzene 2 Ethylcyclohexane 3 Ethylbenzene 4 Xylene 5 Diphenyl 6 2-Methyldiphenyl 7 3-Methyldiphenyl 8 4-Methyldiphenyl 9 2,4-Dimethyldiphenyl 10 Diphenylmethane 11 2,2-Dimethyldiphenyl 12 2,3-Dimethyldiphenyl 13 Diphenylethane 14 4-Methyldiphenylmethane 15 l-Methyl-2-phenylmethylbenzene 16 l-Methyl-4-phenylmethylbenzene 17 3,3-Dimethyldiphenyl 18 4,4-Dimethyldiphenyl Unidentified compounds

175 247 258 261 576 584 168 168 599 609 653 664 680 182 688 695 736 748 14.8 Total

78 112 106 106 154 168 1.3 0.5 182 168 182 182 182 1.1 182 182 182 182

10.9 6.3 8.0 7.4 0.2 1.1

0.3

1.1 5.0 11.4 4.4 14.0

71.9

1.6 1.0 10.3 17.3

4.8 1.6

100

98.7

2.4

•(Spectrum number corresponding to the chromatographic peak of the eluting compound) ••(Toluene-free basis) •••(Reference 13)


DHAWAN ET AL.



30.

Figure 3. Total ion chromatogram of supercritical toluene-cis-Polyisoprene reaction products at 349°C and 13.8 MPa



388


Table Π. GC-Mass Spectral Analysis of Supercritical Toluene-m Polyisoprene Reaction Products Above 0.5% Abundances COMPOUND

Scan** M.W.

1 Methylpropane(#137,#140,#146)* 2 Benzene 3 Xylene(#265,#267)* 4 l-Ethenyl-3-methylene-cyclopentene 5 Propylbenzene(#289,#303)* 6 Ethylmethylbenzene(#307,#313,#316)* 7 Trimethylbenzene(#309,#323)* 8 l-Methyl-4-l-methylethylbenzene (#337,#340)* 9 2-Methylstyrene(#299,#348)* 10 1,2-Diethylbenzene(#355,#361)* 11 2-Ethyl-1,4-dimethylbenzene (#367,#371,#391)* 12 2,3-Dihydro-5-methylindene(#374,#404)* 13 2,2-Dimethylpropylbenzene(#378,#410)* 14 Naphthalene 15 Dihydrodimethylindene (#424,429,432,439,482,496)* 16 3-Methylpentylbenzene(#447,#454)* 17 2-Methylnaphthalene(#509,#522)* 18 l-(2-Propenyl)naphthalene 19 Dimethylnaphthalene (#583,#589,#617,#621,#631)* 20 Diphenylethane(#681,#689)* 21 l-Methyl-2-phenylmethylbenzene 22 2-Ethyldiphenyl 23 Methyldiphenyl(#648,658)* 24 Diphenylpropane 25 Dimethyldiphenyl(#596,601,654, 665,722,737,748,#754,769,801)* 26 l-Methylfluorene(#837,#844)*

140 174 267 277 303 316 323 340

58 78 106 106 120 120 120 134

1.5 7.0 5.4 1.5 3.3 2.5 1.7 6.8

348 361 391

118 134 134

0.5 3.4 2.6

404 410 437 439

132 148 128 146

0.7 1.3 1.0 2.3

454 522 579 631

162 142 168 156

0.8 3.2 0.8 1.2

689 692 694 764 789 801

182 182 182 168 196 180

12.3 0.9 0.8 5.5 2.3 17.6

844 Total

180

1.3 88.2

(%)

•(Includes isomers at the scan numbers shown) **(Spectrum number corresponding to the chromatographic peak of the eluting compound)


30.

DHAWAN ET AL.

Depolymerization of cis-Polyisoprene and Scrap Rubber 3 8 9


Table ΠΙ. GC-Mass Spectral Analysis of Supercritical Toluenecis-Polyisoprene Reaction Products Below 0.5% Abundances COMPOUND

Scan**

M.W.

(%)

1 Pentane(#154,#159)* 2 Hexane 3 Methylpentene(#162,#172)* 4 2,4-Hexadiene 5 1,2-Dimethylcyclohexene 6 Ethylcyclohexane 7 Propylcyclohexane 8 Methylstyrene 9 Butylbenzene 10 l-Methyl-2-propylbenzene 11 2-Methylbutylbenzene 12 1,3-Dimethyl-5- 1-raethylethylbenzene 13 Diethylmethylbenzene(#383,#402)* 14 1,2,3,4-Tetrahydronaphthalene 15 l,2-Diethyl-3,4-dimethylbenzene 16 Cyclopentylbenzene 17 l,3-Dimethyl-2-butenylbenzene 18 Methylphenylpentene 19 1,2,3,4-Tetrahydro-5-methylnaphthalene 20 3-Methylcyclopentylbenzene 21 Dihydro-l,l,5-trimethylindene (#465,#480,#500)* 22 l-2-Butenyl-2,3-dimethylbenzene 23 Tetrahydro-2,7-dimethylnaphthalene (#534,#540)* 24 Cyclohexylmethylbenzene(#537,#543)* 25 2,4-Dimethylcyclopentylbenzene (#559,#564)* 26 Diphenyl 27 Diphenylmethane 28 l,2,3,4-Tetrahydro-2 5,8-trimethyl naphthalene 29 1,4,6-Trimethylnaphthalene 30 1,2-Dihydro-1,1,6-trimethylnaphthalene 31 2,3,6-Trimethylnaphthalene 32 l-Methyl-7(l-methylethyl)naphthalene 33 1,2-Dimethyl-4-(phenylmethyl)-benzene 34 1-Methyldiphenylethane

159 166 172 192 252 255 282 299 329 351 388 397 402 420 443 457 462 472 485 491 500

72 84 84 82 106 112 126 118 134 134 148 148 148 132 162 146 160 162 146 160 160

0.3 0.3 0.2 0.1 0.1 0.1 0.1 ~0 0.1 0.3 0.4 0.1 0.5 0.4 ~0 0.2 0.1 0.3 0.3 0.3 0.4

526 540

160 160

0.4 0.2

543 564

174 174

0.4 0.2

567 607 613

154 168 174

0.3 0.3 0.1

626 636 705 717 827 856

170 172 170 184 196 194

0.1 ~0 0.4 0.1 -0 0.2

>

Continued on next page


390



Table ΠΙ. Continued COMPOUND

Scan**

M.W.

(%)

35 Diphenylbutane (#814,#873)* 36 l-Methyl-3-(4-methylphenyl)methylbenzene 37 2-Ethyldiphenyl 38 Phenylmethylphenylbutane 39 Phenylbutylphenylmethane 40 Phenanthrene 41 l-Methyl-3-(2-methylphenyl)methylbenzene 42 2,3-Dihydro-1 -methyl-3-phenylindene 43 1,1 '-(3-Methyl-l-propene- 1,3-diyl) bisbenzene 44 2-Phenylnaphthalene 45 2-Methylphenanthrene(#1036,#1039)* 46 Dimethylphenanthrene (#1077,1126,1133,1144)* 47 2-Phenylmethylnaphthalene Unidentified compounds

873 884

210 196

0.4 0.1

887 891 896 914 936

182 224 224 178 194

0.1 0.2 0.1 0.4 0.1

942 950

208 208

0.1 ~0

972 1039 1144

204 192 206

0.1 0.2 0.1

1169

218

-0 2.6 11.8

Total

•(Includes isomers at the scan numbers shown) ••(Spectrum number corresponding to the chromatographic peak of the eluting compound) are only found when polymer was present in the reactor. Naphthalene can be produced by the pyrolytic dehydrogenation of butylbenzene(14). Similar dehydrogenation mechanisms of butyl- or higher substituted alkylbenzenes can be considered behind the formation of Tetralin and methylindenes. Figure 4 shows the GC-MS chromatogram obtained from an experiment with scrap rubber of an aircraft tire in supercritical toluene. The elution pattern of the products is remarkably similar to those obtained in the model experiment with cis-polyisoprene. The products corresponding to the chromatographic peaks were identical in these two chromatograms. The weight of filtered and dried carbon residue from extraction of 36.4g scrap aircraft tire rubber was 13.2g (36.3%). It should be noted that the similarities in the results (Figure 2 and 3) exist in spite of the fact that in the scrap rubber the polyisoprene is present in a vulcanized (cross-linked) state. In conclusion, our results demonstrate that under supercritical conditions, the reaction of toluene with cis-polyisoprene results in an extensive cracking of polymer. The pool of free-radicals thus generated undergo several types of radical combination reactions with the radicals from toluene such that the stabilized products are low molecular weight aromatic compounds. Secondary reactions leading to



Figure 4. Total ion chromatogram of supercritical toluene-aircraft tire rubber reaction products at 349°C and 13.8 MPa


w


392


oligo- and polycondensates are remarkably suppressed under the experimental conditions of 343°C temperature and 13.8 MPa pressure. As a result, less than 1% of the product consists of three-ring fused aromatics such as phenanthrene and its alkylated derivatives. It is recognized that the polymer in the supercritical phase apparentiy undergoes several types of free-radical rearrangement reactions such as chain-initiation, hydrogen abstraction, free-radical decomposition and concerted-molecular reactions. Some pathways likely to result in the type of reaction products involving the radicals from toluene and from the polymer are postulated below. In these examples, a toluene radical attacks a double bond in the isoprene chain forming a C-C bond (Markovnikov's rule). A subsequent β-scission, thermolysis and cyclization results in the formation of: (I) dihydromethylindene isomers and (II) methylnaphthalene isomers.


30.

DHAWAN E T AL.

Depolymerizatiott of cis-Polyisoprene and Scrap Rubber

393

REACTION I

CH*

CH-i

I.

CH

I


2

2

2

CH

3

~CH -CH -C-CH=CH

- CH -CH -C-CH-CH -CH -C=CH-CH 2

I

β-scission 2

2

2

+

3

• CH -C=CH-CH 2

2

2

thermolysis

REACTION Π

ÇH

ÇH

CH,

3

β-scission - CH -CH -Ç-CH-CH -CH -C=CH-CH * 2

2

2

PH

2

2

~CH -CH -C-CH=CH 2

CH

3

2

2

+

CH

3

· CH -C=CH-CH -

2

2


2

2

394


ACKNOWLEDGMENT The authors gratefully acknowledge the assistance of Mr. L . E . McCormick of the College of Engineering for the building of the experimental apparatus.

Literature Cited


1.

Williams, P. T.; Besler, S.; Taylor, D. T. FUEL, 1990, 2, pp 1474-1479 2. Lucchesi, Α.; Maschio, G.; Conservation & Recycling, 1983, 6, pp 85-90 3. Kawakami, S.; Inoue, K.; Tanaka, H.; Sakai, T. ACS Symp. Ser., 1980, 130, pp 557-572 4. Madorsky, S. L.; Straus, S.; Thompson, D.; Williamson, L. J. Research NBS, 1949, 42, pp 499-514 5. Grassie, N. Chemistry of High Polymer Degradation Processes; Butterworths Scientific Publications, London, 1956,p83 6. Sarfare, P. S.; Bhatnagar, H. L.; Biswas, A.B. J. Appl. Polym. Sci., 1963, 7,p2199 7. Golub, Μ. Α.; Gargiulo, R. J.; Polymer Letters, 24, 1972, 10, pp 41-49 8. McHugh, M.; Krukonis, V. Supercritical Fluid Extracnon-Principles and Practice, Butterworths, Stoneham, Massachusetts, 1986, pp 199-215 9. Koll, P.; Metzger, J. Angew. Chenu Int. Ed. Engl., 1978, 77,p754 10. Metzger, J. O.; Hartmans, H.; Malwitz, D.; Koll, P. In Thermal Organic Reactions in Supercritical Fluids; Chemical Engineering at Supercritical Conditions,p515 Paulaitis, M. E.; Penninger, J. M. L.; Gray, R. D.; Davidson, P., Eds.; Ann Arbor Science, Ann Arbor, Michigan, 1983 11. The Oxidation of Hydrocarbons in the Liquid Phase; Emanuel, N. M.; McMillan, New York, 1985 12. Dhawan, J. C.; Legendre, R. C.; Bencsath, A. F.; Davis, R. M. J. Supercrit. Fluids, 1991, 4, pp 160-165 13. Kershaw, J. R. So. Af. J. of Chem., 1978, 31, pp 15-18 RECEIVED June 12, 1992


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