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DEPOLYM ERIZATION-LIQUEFACTION OF PLA STICS AND RUBBERS.1 POLYETHY LENE, POLYPROPYLENE AND POLYBU TADIENE.

Xin Xiao, Wlo dzim ien Zmierczak and Joseph ShabtaiDepartment of Chem ical and Fu els EngineeringUniversity of UtahSalt Lake C ity, Utah 84112ABSTRACTProcessing conditions were developed for high-yield depolymerization-liquefaction of isotacticpolypro pylene (M.W., -250 ,000) into a light, gasoline-like produc t. At 380 -420 "C, an initial H2pressure of 1200 psig, with 1 wt% of finely dispersed Fe203/SO;- or ZzCJSO, as solidsuperacid catalysts, the polypropylene is converted (yields, 72-83 wt%) into a liquid productconsisting predominantly of C j -C i2branched paraffins. The change in product composition as afunction sf reaction temperature, time, and catalyst concentration. was exam ined an d optimalconditions for production of gasoline-range branched paraffins determined. Depolymerization-liquefactio n of poly ethylen e with the same catalysts required higher processing temperature (420-450 "C) and longer reaction time . Liquid yields in the range of 78-85 wt% w ere obtained and theproduc t consisted of a mixture of C5-C30 (mostly C5-Clz) normal p araffins, accom panied by somebranched isomers. Polybutadiene (98 wt% &)was depolymerized-liquefied at 400 "C and 1200psig initial H2 pressure in -85 wt% liquid yield. The produ ct consisted of a mixture. of paraffinsand cyclic com pounds, including alkylcyclohexanes, alkylcyclopentanes, and alkylbenzenes withC I C J lkyl g roups.Keywords: depolymerization, liquefaction, plasticsINTRODUCTIONThe effective disposal of waste industrial polymers is now recognized to be a majorenvironmental problem in North America. Plastics and rubbers are troublesome components forlandfilling. inasm uch as they are not presently biodegradable. Their destruction by incinerationposes serious air pollution problems due to the release of airborne particles and carbon dioxideinto the atmosphere. An alternative would be true recycling, is ., conv ersion into monomers thatcan be reused. However, polyethylene and polypropylene do not depolymerize thermally toethylene or propylene with sufficient selectivity. On the other hand, waste plastics and rubberscan be regarded as a potentially cheap and abundant source for fuels. Thermodegradation ofpolyolefins has been investig ated extensiv ely since W orld War E'.', but relatively few studies onthe cataly tic conversion of the polymers have been carried out, especially for production of liquidfuels. Recen tly there have been rep orts on the p rolysis of polyolefins t aromatic hydrocarbonswith activated carbon-supported metal ca taly stJ7 ; cracking of polystyrene and polyethylene onsilica-alumna*;and reformin g of heavy oil fro m waste plastics using zeolite catalysts'. Most ofthese catalytic studies were conducted under nitrogen at am bient or low pressure. The presentpaper is concerned with an inves tigation of the catalytic depoly meriza tion-lique faction behav iorof three representative commercial polymers, is ., polypropylene. polyethylene and polybutadieneusing superac id catalysts1-i6 under high H2 pressure. The objective was to determine suitableconditio ns for conversion of such polym ers int: light liquid fuels, as well as to obtain dataneeded for predictive moldeling of waste polymers coprocessing with coal.EXPERIMENTALMaterials. High density polyethylene (d, 0.959 g/cm'; M.W.. 125,000 ) and isotacticpolyp ropylen e (d, 0.900 g/cm3; M.W., 250,000 ) were ob tained from Aldrich Chem ical Company;and polybutadiene (98% ;d, 0.910 g/cm'; M.W ., 1 97,000) from Scientific Polym ers Produ cts,InC.Catalysts. Three types of solid superac id catalysts. Le.. F e203 /S 02- , r O z / S O ~ -nd A I ~ O ~ S O ~ ~were synthesized. The preparation of the first two was the same as recently described in detailelsewh erei6. A1203/SO,2' was prepared by the follow ing procedure: 12.4 g of AI2(SO4),.(14-18)H20 was dissolved in 44ml of distilled water and subjected to hydrolysis at room temperatureby slowly adding 28-30% N b O H with vigorous mixing, until pH = 8.5 was reached. Theprecipitate was filtered, washed with distilled water, and then dried at I10 C or 2 h. Thesolid was pulverized and calcined at 5 50 "C for 2 h. T he resultant A1203,2.0 g, was treated with50 ml of an aqueous solution with concentrations of I S M (NH&SO4 and 1.0 M HzSO4 for 1 hwith contin uous stirring, then filtered , wash ed with -100 ml of w ater, dried at 110 "C for 2 h, andcalcined at 600 "C fo r 3 h.

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Experimental procedure. A mixture of the polymer, 10.0 g, and catalyst, 0.1 or 0.2 g (withoutany solvent), was introduced in a 5 ml Microclave reactor (Autoclave Engineers). Th e latter wasclosed, purged with nitrogen, and then pressurized with hydrogen to a selected initial pressure.The reactor was heated to the desired temperature in 12-15 min, and stirring (500 rpm) waspressures from ambien t to 2000 psig resulted in reaction pressures between 350-3600 psig in thereaction tem perature range of 38 046 5 'C.Analytical Methods. At the end of each experiment, the reactor was cooled down and the gasproduct was passed through a stainless steel trap kept at liquid nitrogen temperature. Afterweighing, the condensed gas was analyzed by GC. It consisted mostly of C1-C4components,accompanied by some C5, C6. and traces of C,. CS compounds. In runs with partial conversion,the liquid and solid products were removed from the reactor and weighed. The liquid wasseparated by decantation and filtration. The solid was rinsed with a little of n-hexane, dried, andweighed. The solid was then washed with n-hexadecane (-80 "C) and n-hexane (roomtemperature), dried and weighed in order to determine the weight of recovered catalyst. In thisway, the product was separated and the weight of gas, liquid, solid and recovered catalyst wasdetermined, The m ass balance of the runs was 90-95% (relative to the weight of the feed). Gasand liquid products were identified mainly by G C , GC/MS and FTIR, and qu antitatively analyzedby gas chromatography and simulated distillation (SIMD). Columns used for gas products: 4 m x0.3 cm 0.d. stainless steel packed with Chromosorb 102; for liquid products: 4 m x 0.3 cm 0.d.stainless steel packed with 10 OV-17 on Chromosorb W-HP; for SIMD: .5 m x 0.3 cm 0.d.stainless steel, Supelco PETROCOLm B column.RESULTS AND DISCUSSIONS1. Polypropylene. At 390 - 420 "C and an initial H2 pressure of I500 psig, with 1 0wt% ofZrOz/SO:~ as catalyst, and a reaction time of 2.0 hours, the polypropylene was convetted in veryhigh yield (ov er 90 wt%) into a low-boiling liquid product. B ranched C5-Cjo paraffins (and s omeolefins) were predominant components of the product. Results on the change in productcomposition as a function of reaction temperature are given in Figure I . As seen, the gasolinerange fraction (Cs-CIz) eached a maximum (-64.5 wt%) at 400 410 "C, then decreased slowly athigher temperature. The CI3+ omponents decreased and the C I-C 4gas increased with increase intemperature. At 400 C,about 6 wt% of gasoline range, 29 wt% of higher hydrocarbons and 7wt% of gas are produced. The depolymerization-liquefaction of polypropylene was investigatedalso as a function of reaction time. The change in product composition showed the same trendsas those indicated above for the temperature effect. This demonstrated the potential of acontrollable stepwise depolymerization of polypropylene into light liquid hydrocarbons. The Hzpressure effect was smaller compared with those of reaction temperature and time. Increase in Hzpressure from 15 to 500 1500 psig suppressed gas formation. decreased the amount of C,,,products, and increased gasoline boiling range production. A comparative study of the threedifferent types of solid superacid, Le., FezOdSO,2', Zr02/SO: and AIZ0JSO:- (seeExperimental) was also performed, keeping other processing variables constant (reactiontemperature, 410 "C, time 1.0 h, initial H2 pressure 1500 psig, catalyst amount, 1.0 wt%). Forcomparison, a run without catalyst was also carried out. The extent of depolymerization of thefeed into gasoline range hydrocarbons was significantly high er in the catalytic runs as comparedwith that in the thermal (non-catalytic) run. Amon the catalysts examin ed, the order ofproduct composition as a function of reaction temperature and reaction time, a plausiblecarbonium ion mech anism for depolymerization of polypropylene can be considered (see Figure4).2. Polyethylene. Liquid yields in the range of 76-87 wt% were found for polyethylene withZr02/SO:- as catalyst at reaction temperatures in the range o f42 0-45 0 "C. The product consistedof a mixture of C5 -C30 (mostly CS-CIZ) ormal paraffins and smaller amounts of branchedisomers. Results on he change in product composition as a function of reaction temperature aregiven in Figure 2. The gasoline range fraction increased to a maximum of 63 wt% at 45 0 "C andthen decreased at 465 'C. The C13+ components decreased while CI-C4 gas increased withincrease in reaction temperature. The change in product composition as a function of reactiontime (between 0.5-3.0) showed trends similar to those of the temperature effect. This was ingood agreement with the above results for polypropylene and again demonstrated the controllablestepwise break-down of the polymer. The effect of HZ ressure (500-2000 psig) on the productcomposition was relatively weak. As the H2 pressure was increased from 500 to 1500 psig, thegasoline boiling range fraction increased while the C I ~ +raction decreased. However, for H2pressures higher than 1500 psig , the conce ntrations of gasoline ra nge and C I3 + fractionsremained relatively stable.

I started after reaching the melting or softening point of the polymer (130-189C). Initial Hz

depolymerization activity was A120JS042' > ZrOz/SO, > Fez03/SO:-. Based on the change in

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3. Polybutadiene.Polybutadiene was smooth1 depolymerized-liquefied at 400 C nd 1200 psiginitial H2 pressure, w ith I wt% of Fe20&SO~ as catalyst. The liquid yield was about 85 wtW.Figure 3 shows the GC/MS of the gasoline boiling range product. As seen, the product consistsof a mixture of paraffins and cyclic compounds, including alkylcyclohexanes,alkylcyclopentanes. and alkylbenzenes with CI-C3alkyl groups (CI-C3 ndicating either single ortwo alkyl substituents). The formation of cyclic hydrocarbons from polybutadiene can beexplained as follows. Bu tadi ene obtained by depolymerization of polybutadiene, can undergo fastcyclodimerization to form 4-vinylcyclohexene. which undergoes a sequen ce of rearrangementand aromatization (or rin hydrogenation) reactions to yield a full range of alkylsubstitutednaphthenes and benzen esCONCLUSIONSIt is found that representative polyolefins, e.g., polypropylene. p jieihyIen e and polybutadiene,unde rgo high-yield depolymerization- iqddaction n the temperature range of 380-450 C, nderH2 p r e s s u ~ s f iii30-2ooO psig, and in the presence. of catalytic amounts of finely dispersed solidsuperacids, is., Al203/SO2 , Zr02/S0, or Fe203/SO:-. Th e depo lyme rization -liquefactionprocess is easily controllable for preferential formation of gasoline-range hydrocarbons.Production of the latter can be rationalized in terms of step wis e breakd own of the poly mericchains by a carbonium ion mechanism. The data obtained can be used fo r predictive modeling ofcoal coprocessing with waste polymers.A C K N O W L E D G M E N TThe authors wish to thank the US. Department of Energy for financial support through theConsortium for Fossil Fuel Liquefaction Science (DB-FC22-90PC 90029).Thanks are also due toProfessor H. L. C . M e u z e lm fo r h elp fu l d is cu ss io n o f G W S data.R E F E R E N C E S(1) Jellinek, H. H. G. J. Polym. Sci. 1949.4. 13.(2) Madorsky, L. J Polym. Sei. 1952 .9, 133.(3) Inaba, A.; Inou e. H. Kagaku K ogak u Ronbunshu 1980.6.95.(4) Kuroki, T.; Honda, T.; Sekigu chi, Y.; O gaw a, T.; Saw aguch i, T.; Ikem ura, T. Nippon( 5 ) Mu rata, K.; Sa to, K. Kagaku Kogaku Ronbunshu 1981.7.64.6) Uemichi, Y.; Makino. Y.; Kanazuka. T. J AM^. Appl. Pyrolysis 1989. 16,229-238.(7) Scott, D. S.: Czemik. S.R.; Piskorz, J.; Radlein. D. St. A. G. Energy Fuels 1990.4.407-

(8) Yam amo to, M.; Takarniya, N. Bulletin of science andEngineering Research Loboratory(9) Songip, A. R.; Masud a, T.; Kuw ahara, H.; Hashimoto, K. Applied Catalysis B:IO) Yam aguch i, T.; Jin, T.; Tana be, K. J. Phys. Chem. 1986,90,3148.

(11) Hin o, M.; A rata, K. Chemistry Lett. 1979,477., (12) Hin o, M.; A rata, K. Chemistry Lett. 1979, 1259.(13) Wen, M. Y.; Wender. I.; Tiem ey, J. W. Energy Fuels 1990.4.372-379.

(14) Arata . K.: H ino , M. Applied Catalysis 1990 ,59. 197-204.(15) Garin, F.; And riama sinoro . D.; A bduls amad , A.; Sommer J. J Catalysis 1991. 131,(16) Zmierczak. W.; Xiao, X ; Shab tai, J. Energy Fueh 1994.8, 113-116.(17) Gil-Av, E.; Shabta i, J.; Steckel, F. I n d Eng. Chem. 1960,52,31.

Y

B

Ka go h Kaish i 1977,894.

411.Waseda Universify 1985, 111,8-14.Environmental 1993.2, 153-164.

199-203.

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loo -wt .

80

60 -40 -

20 -

0 -360

- - ..-*--- - - - - Comrs ion-/

( SG12

I380 400 420 440 460

o . , . , . , . , . , .410 420 430 440 450 460 470

100 -wt .

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Fig. 2 Change in Product Composition from Depolymerizationof Polyethylene as a Function of Reaction Temperature, "C.

, - - - - - - - - * - - - - a Comrsion.

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1

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Fig. 3 GC/MS of Liquid Product (Gasoline-Range Fraction)from Depolymerization-Liquefaction of Polybutadiene, 98% .

cid catalyst

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Fig. 4 Example of Proposed Mechanism for PolypropyleneDepolymerization into Branched Paraffins and Olefins.

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